JP5034803B2 - Steel sheet for soft nitriding treatment and method for producing the same - Google Patents
Steel sheet for soft nitriding treatment and method for producing the sameInfo
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Description
本発明は、加工後、軟窒化処理を施して、工具や機械構造用部品、自動車部品等の耐摩耗性や耐疲労特性が要求される部品に用いる軟窒化処理用鋼板に関し、特に、加工性に優れるとともに、加工後、ガス軟窒化処理を施すことにより優れた疲労特性を付与することができる軟窒化処理用鋼板とその製造方法に関するものである。 The present invention relates to a steel sheet for nitrocarburizing treatment that is subjected to soft nitriding after processing to be used for parts requiring wear resistance and fatigue resistance, such as tools, machine structural parts, and automobile parts, and in particular, workability. The present invention relates to a steel sheet for nitrocarburizing treatment and a method for producing the same, which can give excellent fatigue characteristics by performing gas soft nitriding after processing.
表面硬化処理は、鋼の表面を硬化させるのと同時に、鋼の表面に残留応力を生じさせ、耐摩耗性や疲労特性を向上させる処理である。現在実用化されている代表的な表面硬化処理の方法としては、浸炭処理と窒化処理を挙げることができる。 The surface hardening treatment is a treatment that hardens the surface of the steel and at the same time generates residual stress on the surface of the steel to improve the wear resistance and fatigue characteristics. As typical surface hardening treatment methods currently in practical use, carburizing treatment and nitriding treatment can be mentioned.
このうち、窒化処理は、活性窒素の拡散により、鋼表面に高硬度の拡散層を得る処理であり、その窒化機構は、
＜２ＮＨ３ → ２Ｎ＋３Ｈ２＞
の反応でＮＨ３ガスの分解によって生じた活性窒素Ｎを鋼表面に拡散させて、高硬度の拡散層（窒化層）を得る技術である。この窒化処理では、Ａ１点以下で窒素を拡散・浸透させるため、処理温度が５００〜５５０℃と低いのが特徴である。そのため、加熱による相変態が起こらないので、浸炭処理のように鋼に歪みが生じることはない。しかし、処理時間が５０〜１００時間と長く、処理後も表面に生成した脆い化合物層を除去する必要があるなどの問題点がある。
Among these, the nitriding treatment is a treatment for obtaining a high hardness diffusion layer on the steel surface by the diffusion of active nitrogen, and the nitriding mechanism is
<2NH 3 → 2N + 3H 2 >
This is a technique for diffusing active nitrogen N generated by the decomposition of NH 3 gas in the reaction to the steel surface to obtain a diffusion layer (nitriding layer) with high hardness. This nitriding treatment is characterized in that the treatment temperature is as low as 500 to 550 ° C. in order to diffuse and permeate nitrogen at A 1 point or less. Therefore, no phase transformation due to heating does not occur, and the steel is not distorted unlike carburizing treatment. However, the treatment time is as long as 50 to 100 hours, and there is a problem that it is necessary to remove the brittle compound layer formed on the surface after the treatment.
そこで、ガス軟窒化処理と呼ばれる方法が開発されている。このガス軟窒化処理は、主として浸炭性ガス（具体的には、急熱型変性ガスあるいは有機溶剤の熱分解ガスなどの浸炭性ガス）または窒素ガス雰囲気中にＮＨ３ガスを３０〜５０ｖｏｌ％添加し、５５０〜６００℃の温度で１〜５時間加熱保存することにより、窒素と炭素を同時に侵入拡散させ、表面に炭窒化物を形成させる方法である。また、浸炭性ガスの他に、Ｎ２・ＮＨ３・ＣＯ２ガスの混合雰囲気を使用した方法も開発されている。これらの処理によって、表面近傍（表層）には、Ｆｅを主成分とするε（Ｆｅ２−３Ｎ）相およびＦｅ３Ｃの混合相を含む化合物層が、また、その内部には拡散層としてν´（Ｆｅ４Ｎ）相が形成されて、表面の硬度を高める作用を発現する。なお、以降、上記ガス軟窒化処理を、「軟窒化処理」ともいう。 Therefore, a method called gas soft nitriding has been developed. This gas soft nitriding treatment is mainly performed by adding 30 to 50 vol% of NH 3 gas in a carburizing gas (specifically, a carburizing gas such as a rapid-heating modified gas or a pyrolysis gas of an organic solvent) or a nitrogen gas atmosphere. In this method, nitrogen and carbon are simultaneously penetrated and diffused to form carbonitrides on the surface by heating and storing at a temperature of 550 to 600 ° C. for 1 to 5 hours. In addition to carburizing gas, a method using a mixed atmosphere of N 2 · NH 3 · CO 2 gas has been developed. By these treatments, a compound layer containing an ε (Fe 2-3 N) phase mainly composed of Fe and a mixed phase of Fe 3 C is formed in the vicinity of the surface (surface layer), and a diffusion layer is formed therein. The ν ′ (Fe 4 N) phase is formed, and the effect of increasing the surface hardness is exhibited. Hereinafter, the gas soft nitriding treatment is also referred to as “soft nitriding treatment”.
このような軟窒化処理を施して用いられる鋼としては、例えば、特許文献１には、冷間鍛造性および疲労特性に優れた軟窒化処理用鋼の製造方法が、特許文献２には、熱処理歪みの少ない軟窒化処理用鋼の製造方法が開示されている。 As steels used after such soft nitriding treatment, for example, Patent Document 1 discloses a method for producing a soft nitriding steel excellent in cold forgeability and fatigue characteristics, and Patent Document 2 discloses heat treatment. A method for producing a soft nitriding steel with less strain is disclosed.
ところで、軟窒化処理は、予め鋼板を所定の形状に成形加工した後に施されることが多いため、斯かる用途に用いられる鋼板には、加工性、特にプレス成形性に優れることが求められる。この点、特許文献１や２の鋼板は、Ｃ含有量が０．１０％以上と高く、他の添加元素の量も多いため、伸びなどの加工特性に劣るものである。 By the way, since the nitrocarburizing treatment is often performed after the steel sheet is formed into a predetermined shape in advance, the steel sheet used for such applications is required to have excellent workability, particularly press formability. In this respect, the steel sheets of Patent Documents 1 and 2 have a high C content of 0.10% or more and a large amount of other additive elements, so that the processing characteristics such as elongation are inferior.
一方、特許文献３や特許文献４には、良好なプレス加工性を有する軟窒化処理用鋼板が開示されている。しかし、これらの軟窒化処理用鋼板は、鋼の炭素含有量を０．０１％以下と極めて少なくした上で、さらにＶ等の高価な添加元素の多量添加を必要とするため、製造コストが増大する。そこで、安価でプレス加工性に優れる軟窒化処理用鋼板が提案されている。例えば、特許文献５には、ＴｉやＶを含有しない安価でプレス成形性のよい軟窒化処理用鋼が提案されている。 On the other hand, Patent Document 3 and Patent Document 4 disclose a steel sheet for soft nitriding treatment having good press workability. However, these steel sheets for nitrocarburizing treatment require a large amount of expensive additive elements such as V after the carbon content of the steel is extremely reduced to 0.01% or less, which increases the manufacturing cost. To do. Therefore, a steel sheet for nitrocarburizing treatment that is inexpensive and excellent in press workability has been proposed. For example, Patent Document 5 proposes a soft nitriding steel that does not contain Ti or V and is inexpensive and has good press formability.
また、耐摩耗性および耐疲労強度などが強く要求される用途では、従来から知られている低炭素鋼や極低炭素鋼などの鋼板を適用した場合には、十分な表面硬さが得られないという問題がある。この問題に対しては、例えば、特許文献６には、低炭素系の鋼に、０．０１〜１．０％のＶ，Ｔｉ，Ｎｂの１種または２種以上を含有させた軟窒化処理用鋼板が提案されている。そして、これら元素の添加により、ガス軟窒化処理後における鋼板の表面硬度を高めることができるので、寸法精度、強度、耐久性に優れた一般構造用部品や自動車部品を低コストで製造できるとしている。
しかしながら、特許文献５に記載された技術は、軟窒化処理後の表面硬さ、硬化深さ、密着曲げ性について検討しているものの、実部品における疲労特性については何ら考慮していないため、耐久性に問題を抱えている。また、特許文献６に記載の技術は、耐久性の向上を意図したものであるが、表面硬さ、硬化深さのみの評価をしており、疲労特性については十分に考慮していない。さらに、特許文献６のように、軟窒化処理後の硬度上昇に寄与する成分を多く含む鋼板では、プレス成形性の劣化が懸念されるだけでなく、ガス軟窒化後の表面硬度の上昇に伴い、表層部のクラック感受性が大きくなる。また、母材内に析出した窒化物も粗大なものとなるため、実部品では、使用中の応力集中により亀裂が発生しやすいという問題がある。したがって、上記特許文献５や６に記載された鋼板は、加工性と疲労特性を兼備したものではないというのが実情である。 However, although the technique described in Patent Document 5 examines the surface hardness after nitrocarburizing treatment, the curing depth, and the adhesion bendability, it does not consider the fatigue characteristics of actual parts at all. Have a problem with sex. Moreover, although the technique described in Patent Document 6 is intended to improve durability, only the surface hardness and the curing depth are evaluated, and fatigue characteristics are not sufficiently considered. Furthermore, as in Patent Document 6, in steel sheets containing a large amount of components that contribute to the increase in hardness after soft nitriding, not only is there concern about deterioration of press formability, but also with an increase in surface hardness after gas soft nitriding. The crack sensitivity of the surface layer portion is increased. In addition, since the nitride deposited in the base material becomes coarse, the actual part has a problem that cracks are likely to occur due to stress concentration during use. Accordingly, the actual situation is that the steel sheets described in Patent Documents 5 and 6 do not have both workability and fatigue characteristics.
そこで、本発明の目的は、上記の従来技術では十分に考慮されていない疲労特性の改善を意図し、加工性と疲労特性とを兼備した軟窒化処理用鋼板と、その製造方法を提案することにある。 Accordingly, an object of the present invention is to provide a steel sheet for nitrocarburizing treatment having both workability and fatigue characteristics, and a method for producing the same, with the aim of improving fatigue characteristics that are not sufficiently considered in the above-described prior art. It is in.
発明者らは、上記課題を解決するために鋭意検討を重ねた。その結果、部品形状に成形加工後、軟窒化処理を施し、表層部を高強度化して用いる軟窒化処理用鋼板の疲労特性を向上するためには、軟窒化処理後の母材の強度（板厚中央部硬さ）と表層部の強度（鋼板表面硬さ）とのバランスを適正化することが重要であること、つまり、部品に求められる強度を確保するためには、プレス成形性を損なわない範囲で母材を強化することが重要であり、一方、疲労特性を改善する観点からは、軟窒化処理後の表層部の硬さが高くなりすぎないよう調整して、表層部と母材の強度差を小さくすることが重要であること、また、その実現のためには、従来から強化元素として多量に添加されてきたＮｂを、極微量添加してやることが極めて有効であることを見出し、本発明を完成させた。 Inventors repeated earnest examination in order to solve the said subject. As a result, in order to improve the fatigue characteristics of the steel sheet for nitrocarburizing treatment, which has been subjected to soft nitriding treatment after forming the part shape and the surface layer portion is strengthened, the strength of the base material after nitronitriding treatment (plate It is important to optimize the balance between the thickness (thickness center hardness) and the surface layer strength (steel sheet surface hardness), that is, to ensure the strength required for the parts, press formability is impaired. It is important to reinforce the base material within the range, but from the viewpoint of improving fatigue characteristics, the surface layer portion and the base material are adjusted so that the hardness of the surface layer portion after nitrocarburizing treatment does not become too high. It is important to reduce the difference in strength of Nb, and to realize that, it has been found that it is extremely effective to add a very small amount of Nb that has been conventionally added in a large amount as a strengthening element, The present invention has been completed.
すなわち、本発明は、Ｃ：０．０３〜０．１０ｍａｓｓ％、Ｓｉ：０．５ｍａｓｓ％以下、Ｍｎ：０．１〜０．６ｍａｓｓ％、Ｐ：０．０４ｍａｓｓ％以下、Ｓ：０．０４ｍａｓｓ％以下、Ａｌ：０．００５〜０．０８ｍａｓｓ％、Ｃｒ：０．４〜１．２ｍａｓｓ％、Ｎｂ：０．００２ｍａｓｓ％以上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である。 That is, the present invention is C: 0.03-0.10 mass%, Si: 0.5 mass% or less, Mn: 0.1-0.6 mass%, P: 0.04 mass% or less, S: 0.04 mass% Hereinafter, Al: 0.005 to 0.08 mass%, Cr: 0.4 to 1.2 mass%, Nb: 0.002 mass% or more and less than 0.01 mass%, and N: 0.01 mass% or less, the balance being A steel sheet for soft nitriding treatment comprising Fe and inevitable impurities.
また、本発明は、Ｃ：０．０３〜０．１０ｍａｓｓ％、Ｓｉ：０．５ｍａｓｓ％以下、Ｍｎ：０．１〜０．６ｍａｓｓ％、Ｐ：０．０４ｍａｓｓ％以下、Ｓ：０．０４ｍａｓｓ％以下、Ａｌ：０．００５〜０．０８ｍａｓｓ％、Ｃｒ：０．４〜１．２ｍａｓｓ％、Ｎｂ：０．００２ｍａｓｓ％以上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鋼素材を、加熱温度：１１００〜１２５０℃、仕上圧延終了温度：Ａｒ３変態点〜９８０℃、巻取温度：５００〜７４０℃とする熱間圧延する軟窒化処理用鋼板の製造方法を提案する。 Moreover, this invention is C: 0.03-0.10 mass%, Si: 0.5 mass% or less, Mn: 0.1-0.6 mass%, P: 0.04 mass% or less, S: 0.04 mass% Hereinafter, Al: 0.005 to 0.08 mass%, Cr: 0.4 to 1.2 mass%, Nb: 0.002 mass% or more and less than 0.01 mass%, and N: 0.01 mass% or less, the balance being For nitrocarburizing treatment of hot rolling a steel material comprising Fe and inevitable impurities at a heating temperature of 1100 to 1250 ° C., a finish rolling finishing temperature: Ar 3 transformation point to 980 ° C., and a winding temperature of 500 to 740 ° C. A method for manufacturing steel sheets is proposed.
本発明によれば、プレス成形性等の加工性に優れ、かつガス軟窒化処理後の疲労耗性にも優れる鋼板を安価に得ることができる。斯かる特性を有する本発明の軟窒化処理用鋼板は、所定の形状に加工後、ガス軟窒化処理を施して、工具や自動車部品の一般構造用部品等に用いる軟窒化処理用鋼板として好適である。 ADVANTAGE OF THE INVENTION According to this invention, the steel plate which is excellent in workability, such as press formability, and is excellent also in the fatigue wear after gas soft nitriding can be obtained at low cost. The steel sheet for soft nitriding treatment of the present invention having such characteristics is suitable as a steel sheet for soft nitriding treatment used for general structural parts such as tools and automobile parts by performing gas soft nitriding treatment after processing into a predetermined shape. is there.
発明者らは、部品形状に成形加工後、ガス軟窒化処理を施して用いる軟窒化処理用鋼板について、素材における加工性と、軟窒化処理を施した後の疲労特性とを両立させる観点から、当該鋼板が具備すべき特性について鋭意検討を重ねた。その結果、軟窒化処理後の疲労特性を向上するためには、母材の強度（板厚中央部硬さ）と窒化処理後の表層部の強度（鋼板表面硬さ）とのバランスを適正化することが重要であることを見出した。つまり、母材の強度が高すぎるとプレス成形性が低下し、逆に、母材の強度が低すぎると、部品自体に求められる強度（剛性や耐久性）を確保できない。一方、軟窒化処理後に鋼板表層が硬くなり過ぎると、応力集中部で脆化を起こし、耐久性を満足できなくなる。とりわけ、トルクコンバータ等の自動車部品のように、使用時に応力集中が発生する部品では、表層の硬さが高くなり過ぎるのは避ける必要がある。 From the viewpoint of achieving both the workability in the material and the fatigue characteristics after the soft nitriding treatment, the steel sheet for soft nitriding treatment, which is used after being subjected to gas soft nitriding treatment after being molded into a part shape, The earnest examination was repeated about the characteristic which the said steel plate should have. As a result, in order to improve the fatigue characteristics after nitrocarburizing, the balance between the strength of the base metal (sheet thickness center hardness) and the strength of the surface layer after nitriding (steel sheet surface hardness) is optimized. I found it important to do. That is, if the strength of the base material is too high, the press formability deteriorates. Conversely, if the strength of the base material is too low, the strength (rigidity and durability) required for the component itself cannot be ensured. On the other hand, if the steel sheet surface layer becomes too hard after the soft nitriding treatment, embrittlement occurs at the stress concentration portion, and the durability cannot be satisfied. In particular, it is necessary to avoid that the hardness of the surface layer becomes too high in a part where stress concentration occurs during use, such as an automobile part such as a torque converter.
そこで、斯かる観点から、軟窒化処理用鋼板の成分組成について検討した結果、従来の軟窒化処理用鋼板では高強度化成分として多量に添加されていたＮｂを、極微量添加してやることが母材の強度と軟窒化処理後の表層部の強度とのバランスを適正化する上で極めて有効であるとの新規知見に基き、本発明を開発した。 Therefore, as a result of examining the component composition of the nitrocarburizing steel sheet from such a viewpoint, it is possible to add a very small amount of Nb, which has been added in a large amount as a strengthening component in the conventional nitrocarburizing steel sheet. The present invention was developed based on the novel finding that it is extremely effective in optimizing the balance between the strength of the surface layer and the strength of the surface layer portion after the soft nitriding treatment.
以下、本発明を開発する契機となった実験について説明する。
Ｃ：０．０４５〜０．０６５ｍａｓｓ％、Ｓｉ：０．０１〜０．０３ｍａｓｓ％、Ｍｎ：０．２〜０．４ｍａｓｓ％、Ｐ：０．０１〜０．０３ｍａｓｓ％、Ｓ：０．００１〜０．００５ｍａｓｓ％、Ａｌ：０．０４〜０．０６ｍａｓｓ％、Ｎ：０．００２０〜０．００４０ｍａｓｓ％、Ｃｒ：０．７５〜０．９５ｍａｓｓ％を含有し、Ｎｂの含有量を０〜０．０３ｍａｓｓ％の範囲で種々のレベルに変化させた鋼素材を、条件Ａ（加熱温度：１１９０℃、仕上圧延終了温度：８８０℃、巻取温度：５９０℃）および条件Ｂ（加熱温度：１２２０℃、仕上圧延終了温度：９２０℃、巻取温度：５００℃）の２条件で熱間圧延し、板厚が２ｍｍの熱延鋼板とした。
In the following, experiments that triggered the development of the present invention will be described.
C: 0.045-0.065 mass%, Si: 0.01-0.03 mass%, Mn: 0.2-0.4 mass%, P: 0.01-0.03 mass%, S: 0.001- 0.005 mass%, Al: 0.04 to 0.06 mass%, N: 0.0020 to 0.0040 mass%, Cr: 0.75 to 0.95 mass%, and the content of Nb is 0 to 0.0. The steel material changed to various levels in the range of 03 mass% was subjected to condition A (heating temperature: 1190 ° C., finish rolling finishing temperature: 880 ° C., winding temperature: 590 ° C.) and condition B (heating temperature: 1220 ° C., Finishing rolling finish temperature: 920 ° C., coiling temperature: 500 ° C.) was hot-rolled to obtain a hot-rolled steel sheet having a thickness of 2 mm.
この熱延鋼板から、図１に示したように中央部に開口のある形状、寸法を有する疲労試験片を、圧延方向が長さ方向と平行となるように採取した後、（ＲＸ：５０％＋ＮＨ３：５０％）混合ガス中で、５８０℃×２ｈｒのガス軟窒化処理を施して表面硬化させてから、応力振幅が３５０ＭＰａの平面曲げ疲労試験に供して、破断するまでのサイクル数を測定した。 From this hot-rolled steel sheet, a fatigue test piece having a shape and size having an opening at the center as shown in FIG. 1 was collected so that the rolling direction was parallel to the length direction, and (RX: 50% + NH 3 : 50%) In a mixed gas, after surface hardening by gas soft nitriding treatment at 580 ° C. × 2 hr, the number of cycles until it breaks is measured in a plane bending fatigue test with a stress amplitude of 350 MPa. did.
図２は、上記疲労試験の結果を、破断までのサイクル数とＮｂ含有量との関係として示したものである。この図から、熱延鋼板の疲労特性は、Ｎｂ含有量と相関があり、Ｎｂ含有量が従来技術の添加量より低い０．０１ｍａｓｓ％以下の領域で向上する傾向にあり、この傾向は熱延条件（Ａ，Ｂ）によって余り影響されないこと、そして、熱延条件によらず、破断までのサイクル数が１０５以上の優れた疲労特性を示すＮｂ含有量は、０．００２ｍａｓｓ％以上０．０１ｍａｓｓ％未満の範囲であることがわかる。ここで、応力振幅３５０ＭＰａでの破断までのサイクル数：１０５サイクルは、実部品への適用可否を判断するのに用いられている疲労特性の評価基準値でもある。
FIG. 2 shows the results of the fatigue test as a relationship between the number of cycles until fracture and the Nb content. From this figure, the fatigue properties of the hot-rolled steel sheet have a correlation with the Nb content, and the Nb content tends to improve in the region of 0.01 mass% or less, which is lower than the additive amount of the prior art. condition (a, B) that is not much affected by, and, irrespective of the hot rolling condition, Nb content number of cycles to failure indicates 10 5 or more excellent fatigue characteristics than 0.002% 0.01 mass It can be seen that the range is less than%. Here, the number of cycles to failure at a
Ｎｂの極微量添加で、疲労寿命が向上するメカニズムは十分に明らかになってはいないが、Ｎｂを微量添加した場合には、表層中に存在するＦｅ２ＮやＦｅ３Ｎ等の化合物相の硬さを過剰に上昇させることなく、軟窒化処理後の鋼板の最表層の硬さを適度に高めることができ、しかも、母材の表層から０．２５ｍｍ以上の深さのフェライト中に、微細なＮｂ炭窒化物を析出して、母材の強度（硬さ）を適度に高めることができるので、表層部と母材との強度（硬さ）のバランスが改善されて、応力集中部における耐久性（疲労強度）の向上に有利に働くためと考えられる。なお、Ｎｂ以外の添加成分として、ＴｉおよびＶについても同様の実験を行なったが、Ｔｉは添加すると、窒化後の表層近傍の硬度が著しくばらつくという問題があり、また、Ｖは、母材硬度を高める効果が小さいという問題があり、いずれも疲労特性の改善効果がＮｂと比較して小さいことがわかった。そこで、本発明では、疲労特性が特に要求される軟窒化処理用鋼板には、Ｎｂを０．００２ｍａｓｓ％以上０．０１ｍａｓｓ％未満の範囲で微量添加することとした。 The mechanism by which fatigue life is improved by adding a very small amount of Nb has not been fully clarified, but when a small amount of Nb is added, the compound phase such as Fe 2 N or Fe 3 N present in the surface layer Without excessively increasing the hardness, the hardness of the outermost surface layer of the steel sheet after the soft nitriding treatment can be increased moderately, and in the ferrite having a depth of 0.25 mm or more from the surface layer of the base material, Since the Nb carbonitride can be precipitated and the strength (hardness) of the base material can be increased moderately, the balance of strength (hardness) between the surface layer portion and the base material is improved, and the stress concentration portion This is considered to be advantageous for improving durability (fatigue strength). The same experiment was conducted for Ti and V as additive components other than Nb. However, when Ti is added, there is a problem that the hardness in the vicinity of the surface layer after nitriding varies significantly, and V is the hardness of the base material. It was found that the effect of improving fatigue characteristics was small compared to Nb. Therefore, in the present invention, a small amount of Nb is added in the range of 0.002 mass% or more and less than 0.01 mass% to the steel sheet for soft nitriding that requires particularly fatigue characteristics.
次に、本発明に係る軟窒化処理用鋼板が有すべき成分組成について説明する。
Ｃ：０．０３〜０．１０ｍａｓｓ％
Ｃは、鋼板の強度および加工性に大きな影響を及ぼす成分である。Ｃの含有量が０．０３ｍａｓｓ％未満の場合には、工具や機械構造用部品、自動車部品等に要求される所望の強度が得られない。一方、Ｃ含有量が０．１０ｍａｓｓ％を超えると、加工性が劣化し、所望の加工性を確保できなくなる。よって、本発明では、Ｃの含有量は０．０３〜０．１０ｍａｓｓ％の範囲とする。好ましくは、０．０４〜０．０８ｍａｓｓ％の範囲である。
Next, the component composition that the steel sheet for nitrocarburizing treatment according to the present invention should have will be described.
C: 0.03-0.10 mass%
C is a component that greatly affects the strength and workability of the steel sheet. If the C content is less than 0.03 mass%, the desired strength required for tools, machine structural parts, automobile parts, etc. cannot be obtained. On the other hand, if the C content exceeds 0.10 mass%, the workability deteriorates and the desired workability cannot be ensured. Therefore, in the present invention, the C content is in the range of 0.03 to 0.10 mass%. Preferably, it is the range of 0.04-0.08 mass%.
Ｓｉ：０．５ｍａｓｓ％以下
Ｓｉは、脱酸剤として、また、鋼板の強度を高めるために添加される成分である。しかし、Ｓｉの含有量が０．５ｍａｓｓ％を超えると、熱延鋼板の加工性が低下するだけでなく、表面性状が悪化して、窒化層の形成が不均一となり、結果として、疲労特性に劣る部品しか得られなくなる。よって、Ｓｉ含有量は０．５ｍａｓｓ％以下とする。好ましくは、０．１ｍａｓｓ％以下である。
Si: 0.5 mass% or less Si is a component added as a deoxidizer and to increase the strength of the steel sheet. However, when the Si content exceeds 0.5 mass%, not only the workability of the hot-rolled steel sheet is deteriorated, but also the surface properties are deteriorated and the formation of the nitrided layer becomes uneven, resulting in fatigue characteristics. Only inferior parts can be obtained. Therefore, Si content shall be 0.5 mass% or less. Preferably, it is 0.1 mass% or less.
Ｍｎ：０．１〜０．６ｍａｓｓ％
Ｍｎは、鋼の強度を高めるとともに、不純物として含まれるＳによる熱間脆性を防止する効果を有する成分である。しかし、Ｍｎ含有量が０．１ｍａｓｓ％未満の場合には、上記効果を得られず、特に、所望の強度が得られない。一方、Ｍｎ含有量が０．６ｍａｓｓ％を超えると、強度が過度に上昇し、加工性が低下する。よって、Ｍｎ含有量は０．１〜０．６ｍａｓｓ％とする。好ましくは０．２〜０．５ｍａｓｓ％の範囲である。
Mn: 0.1 to 0.6 mass%
Mn is a component that has the effect of increasing the strength of steel and preventing hot brittleness due to S contained as impurities. However, when the Mn content is less than 0.1 mass%, the above effect cannot be obtained, and in particular, a desired strength cannot be obtained. On the other hand, if the Mn content exceeds 0.6 mass%, the strength increases excessively and the workability decreases. Therefore, Mn content shall be 0.1-0.6 mass%. Preferably it is the range of 0.2-0.5 mass%.
Ｐ：０．０４ｍａｓｓ％以下
Ｐは、不純物として含まれる成分であり、また、加工性を劣化させることなく強度を高める成分でもある。しかし、Ｐは粒界に偏析し易い元素であるため、特に、Ｐの含有量が０．０４ｍａｓｓ％を超えると、粒界偏析に起因して脆性を引き起こし易くなる。よって、本発明では、Ｐの含有量は０．０４ｍａｓｓ％以下とする。好ましくは、０．０３ｍａｓｓ％以下である。
P: 0.04 mass% or less P is a component contained as an impurity, and is also a component that increases strength without degrading workability. However, since P is an element that easily segregates at grain boundaries, in particular, when the P content exceeds 0.04 mass%, brittleness is likely to occur due to grain boundary segregation. Therefore, in this invention, content of P shall be 0.04 mass% or less. Preferably, it is 0.03 mass% or less.
Ｓ：０．０４ｍａｓｓ％以下
Ｓは、不純物として含まれる成分であり、熱間脆性により表面性状を劣化させたり、加工性を低下させたりする成分である。特に、Ｓ含有量が０．０４ｍａｓｓ％を超えると、粗大な硫化物を生成して熱間延性を低下し、表面性状の劣化を招くおそれがある。よって、Ｓ含有量は０．０４ｍａｓｓ％以下とする。好ましくは、０．０１ｍａｓｓ％以下である。
S: 0.04 mass% or less S is a component contained as an impurity, and is a component that deteriorates surface properties or deteriorates workability due to hot brittleness. In particular, when the S content exceeds 0.04 mass%, coarse sulfides are generated, hot ductility is lowered, and surface properties may be deteriorated. Therefore, the S content is set to 0.04 mass% or less. Preferably, it is 0.01 mass% or less.
Ａｌ：０．００５〜０．０８ｍａｓｓ％
Ａｌは、脱酸剤として添加される成分であり、十分な脱酸効果を得るためには０．００５ｍａｓｓ％添加する必要がある。一方、Ａｌは、安定な窒化物を生成しやすい元素であり、含有量が０．０８ｍａｓｓ％を超えると、軟窒化処理後の表層硬さが過剰に上昇し、応力集中部における疲労強度を低下させる。よって、Ａｌの含有量は０．００５〜０．０８ｍａｓｓ％の範囲とする。好ましくは、０．０３〜０．０７ｍａｓｓ％である。
Al: 0.005-0.08 mass%
Al is a component added as a deoxidizer, and 0.005 mass% needs to be added in order to obtain a sufficient deoxidation effect. On the other hand, Al is an element that is likely to generate stable nitrides. If the content exceeds 0.08 mass%, the surface layer hardness after nitrocarburizing treatment increases excessively, and the fatigue strength in the stress concentration part decreases. Let Therefore, the Al content is in the range of 0.005 to 0.08 mass%. Preferably, it is 0.03-0.07 mass%.
Ｎ：０．０１ｍａｓｓ％以下
Ｎは、軟窒化熱処理前に０．０１ｍａｓｓ％を超えて含まれると、軟窒化処理する前にＡｌやＮｂと窒化物を形成して素材強度を高め、加工性を低下させるため、好ましくない。よって、本発明では、Ｎ含有量は０．０１ｍａｓｓ％以下とする。好ましくは０．００６０ｍａｓｓ％以下である。
N: 0.01 mass% or less If N is contained in an amount exceeding 0.01 mass% before the soft nitriding heat treatment, it forms a nitride with Al or Nb before the soft nitriding treatment to increase the material strength and improve the workability. Since it reduces, it is not preferable. Therefore, in this invention, N content shall be 0.01 mass% or less. Preferably it is 0.0060 mass% or less.
Ｃｒ：０．４〜１．２ｍａｓｓ％
Ｃｒは、軟窒化処理により窒化物を形成して表面硬度を高める効果を有する成分であり、本発明の鋼板においては、重要な成分の１つである。Ｃｒの含有量が０．４ｍａｓｓ％未満では、上記効果が十分ではなく、軟窒化処理後に所望の硬さが得られない。一方、Ｃｒの含有量が１．２ｍａｓｓ％を超えると、却って疲労特性に劣るようになる。よって、Ｃｒの含有量は０．４〜１．２ｍａｓｓ％の範囲とする。好ましくは、０．５５〜１．０ｍａｓｓ％の範囲である。
Cr: 0.4-1.2 mass%
Cr is a component that has the effect of increasing the surface hardness by forming a nitride by soft nitriding, and is one of the important components in the steel sheet of the present invention. When the Cr content is less than 0.4 mass%, the above effect is not sufficient, and the desired hardness cannot be obtained after the soft nitriding treatment. On the other hand, if the Cr content exceeds 1.2 mass%, the fatigue characteristics are inferior. Therefore, the Cr content is in the range of 0.4 to 1.2 mass%. Preferably, it is the range of 0.55-1.0 mass%.
Ｎｂ：０．００２ｍａｓｓ％以上０．０１ｍａｓｓ％未満
Ｎｂは、本発明においては、軟窒化処理後の表面硬さと母材強度のバランスを確保する上で極めて重要な成分である。Ｎｂの含有量が、０．００２ｍａｓｓ％未満の添加では、所望の母材強度が得られず、一方、Ｎｂの含有量が０．０１ｍａｓｓ％以上となると、表面硬さが高くなりすぎて、母材の強度と表面硬さとのバランスがくずれて、応力集中部における疲労特性が低下する。また、母材の強度も高くなりすぎるため加工性も低下する。よって、本発明では、Ｎｂは０．００２ｍａｓｓ％以上０．０１ｍａｓｓ％未満の範囲で添加する。好ましくは、０．００３〜０．００９ｍａｓｓ％の範囲、より好ましくは、０．００５〜０．００９％の範囲である。
Nb: 0.002 mass% or more and less than 0.01 mass% Nb is an extremely important component in the present invention to ensure the balance between the surface hardness after soft nitriding and the strength of the base material. If the Nb content is less than 0.002 mass%, the desired base material strength cannot be obtained. On the other hand, if the Nb content is 0.01 mass% or more, the surface hardness becomes too high, The balance between the strength of the material and the surface hardness is lost, and the fatigue characteristics at the stress concentration portion are reduced. Moreover, since the strength of the base material becomes too high, the workability is also lowered. Therefore, in this invention, Nb is added in 0.002 mass% or more and less than 0.01 mass%. Preferably, it is in the range of 0.003 to 0.009 mass%, more preferably in the range of 0.005 to 0.009%.
本発明における軟窒化処理用鋼板は、上記成分以外の残部は、Ｆｅおよび不可避的不純物である。ただし、本発明の作用効果を害しない範囲であれば、他の成分を含むことを拒むものではない。 In the steel sheet for soft nitriding in the present invention, the balance other than the above components is Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, it does not refuse to contain other components.
次に、本発明に係る軟窒化処理用鋼板の製造方法について説明する。
本発明では、鋼の製造は、通常公知の方法で行なうことができ、例えば、転炉、電気炉等で鋼を溶製後、必要に応じて、真空脱ガス処理等の２次精錬を経て上記成分組成を有する溶鋼とし、その後、造塊−分塊圧延法あるいは連続鋳造法で鋼スラブ（鋼片）とするのが好ましい。
Next, a method for producing a nitrocarburizing steel sheet according to the present invention will be described.
In the present invention, steel can be produced by a generally known method. For example, after steel is melted in a converter, electric furnace, etc., it is subjected to secondary refining such as vacuum degassing as necessary. It is preferable to use molten steel having the above component composition, and then form a steel slab (steel slab) by ingot-bundling rolling or continuous casting.
加熱温度：１１００〜１２５０℃
上記のようにして得た鋼スラブは、次いで、熱間圧延に供するが、この際の加熱炉におけるスラブ加熱温度は、１１００〜１２５０℃の範囲とする必要がある。加熱温度が１１００℃未満では、スラブ製造後の冷却段階で生成したＮｂ炭窒化物を完全に再溶解することが難しく、一方、加熱温度が１２５０℃を超えると、結晶粒が粗大化したり、表面性状を損ねたりするため、疲労特性を確保する上で好ましくないからである。好ましくは、加熱温度は１１４０〜１２４０℃の範囲である。
Heating temperature: 1100-1250 ° C
The steel slab obtained as described above is then subjected to hot rolling, and the slab heating temperature in the heating furnace at this time needs to be in the range of 1100 to 1250 ° C. When the heating temperature is less than 1100 ° C, it is difficult to completely redissolve Nb carbonitride generated in the cooling stage after slab production. On the other hand, when the heating temperature exceeds 1250 ° C, the crystal grains become coarse, This is because the properties are impaired, and this is not preferable for ensuring fatigue characteristics. Preferably, the heating temperature is in the range of 1140 to 1240 ° C.
仕上圧延終了温度：Ａｒ３変態点〜９８０℃
スラブ加熱後の熱間圧延は、仕上圧延終了温度を、Ａｒ３変態点〜９８０℃の範囲とする必要がある。仕上圧延終了温度が、Ａｒ３変態点未満、即ち、（α＋γ）域での圧延となると、加工性が低下し、好ましくない。一方、仕上圧延終了温度が９８０℃を超えると、結晶粒の粗大化により所望の強度が得られなくなるばかりか、パーライトが粗大化するため、加工性や疲労特性に悪影響を及ぼすようになる。よって、仕上圧延終了温度は、Ａｒ３変態点〜９８０℃の範囲とする。好ましくは、８４０〜９２０℃の範囲である。
Finish rolling temperature: Ar 3 transformation point ~980 ℃
In the hot rolling after slab heating, it is necessary that the finish rolling end temperature be in the range of Ar 3 transformation point to 980 ° C. If the finish rolling end temperature is less than the Ar 3 transformation point, that is, rolling in the (α + γ) region, the workability is lowered, which is not preferable. On the other hand, when the finish rolling finish temperature exceeds 980 ° C., not only the desired strength cannot be obtained due to the coarsening of the crystal grains, but also the pearlite becomes coarse, which adversely affects workability and fatigue characteristics. Accordingly, the finish rolling end temperature is in the range of Ar 3 transformation point ~980 ℃. Preferably, it is the range of 840-920 degreeC.
巻取温度：５００〜７４０℃
熱間圧延後の巻取温度は、５００〜７４０℃の範囲とする。巻取温度が５００℃未満では、熱間圧延組織が残留し、所望の加工性が得られない。一方、巻取温度が７４０℃を超えると、Ｎｂ炭窒化物およびパーライトがともに粗大化して、疲労亀裂が伝播し易くなり、疲労特性が低下するからである。好ましい巻取温度は５５０〜６３０℃の範囲である。
Winding temperature: 500-740 ° C
The coiling temperature after hot rolling is in the range of 500 to 740 ° C. When the coiling temperature is less than 500 ° C., the hot rolled structure remains and desired workability cannot be obtained. On the other hand, when the coiling temperature exceeds 740 ° C., both Nb carbonitride and pearlite are coarsened, fatigue cracks are easily propagated, and fatigue characteristics are deteriorated. A preferable winding temperature is in the range of 550 to 630 ° C.
上記のようにして得た本発明の熱延鋼板は、熱間圧延まま、あるいは、必要に応じて調質圧延を施してから酸洗等で脱スケールしたのち、軟窒化処理用鋼板として用いることができる。さらに、本発明の鋼板は、上記熱延鋼板を、酸洗後、通常公知の条件で冷間圧延し、再結晶焼鈍し、あるいはさらに必要に応じて調質圧延することにより冷延鋼板とし、軟窒化処理用鋼板とすることもできる。 The hot-rolled steel sheet of the present invention obtained as described above is used as a steel sheet for nitrocarburizing treatment as it is hot-rolled or after descaling by pickling after performing temper rolling if necessary. Can do. Furthermore, the steel sheet of the present invention is a cold-rolled steel sheet obtained by pickling the hot-rolled steel sheet, cold-rolling under known conditions, recrystallization annealing, or further temper rolling as necessary, A steel sheet for nitrocarburizing treatment can also be used.
なお、上述したように、本発明の軟窒化処理用鋼板は、成形加工と軟窒化処理を施されたのち、工具や機械構造部品として用いられることが多い。したがって、本発明の鋼板には、加工性や軟窒化特性（特に、疲労特性）に優れること以外に、素材自体も高強度であること、好ましくは引張強さが、３４０〜４８０ＭＰａ、より好ましくは３６０〜４５０ＭＰａの範囲にあることが望ましい。引張強さが３４０ＭＰａ未満では、部品等に求められる強度が得られず、一方、４８０ＭＰａを超えると加工性が低下するようになるからである。 As described above, the soft nitriding steel sheet of the present invention is often used as a tool or a machine structural component after being subjected to forming and soft nitriding. Therefore, in the steel sheet of the present invention, in addition to excellent workability and soft nitriding properties (particularly fatigue properties), the material itself is also high strength, preferably a tensile strength of 340 to 480 MPa, more preferably It is desirable to be in the range of 360-450 MPa. This is because if the tensile strength is less than 340 MPa, the strength required for the parts and the like cannot be obtained, while if it exceeds 480 MPa, the workability decreases.
また、本発明の鋼板が軟窒化処理後に優れた疲労特性を発現するためには、母材強度と軟窒化処理後の表層部の強度とのバランスを適正化することが重要である。このバランスは、軟窒化処理後の板厚中央部の硬さと鋼板表面硬さの比（鋼板表面硬さ／板厚中央硬さ）として表した場合、４．４〜５．７の範囲であることが好ましい。４．４未満では、軟窒化処理による表面硬化が不十分であり、一方、５．７超えでは、表層部が硬くなりすぎて、応力集中部における疲労特性が低下するようになるからである。より好ましくは、４．７〜５．６の範囲である。なお、上記の鋼板表面硬さとは、鋼板表面から深さ５０μｍの位置にて測定したビッカース硬さのことである。 In addition, in order for the steel sheet of the present invention to exhibit excellent fatigue characteristics after the soft nitriding treatment, it is important to optimize the balance between the base material strength and the strength of the surface layer portion after the soft nitriding treatment. This balance is in the range of 4.4 to 5.7 when expressed as a ratio of the hardness of the sheet thickness center after soft nitriding to the surface hardness of the steel sheet (steel sheet surface hardness / sheet thickness central hardness). It is preferable. If it is less than 4.4, surface hardening by soft nitriding is insufficient, while if it exceeds 5.7, the surface layer portion becomes too hard and fatigue characteristics at the stress concentration portion are lowered. More preferably, it is the range of 4.7-5.6. In addition, said steel plate surface hardness is Vickers hardness measured in the position of 50 micrometers deep from the steel plate surface.
表１に示した成分組成を有する記号Ａ〜Ｍの鋼を溶製し、スラブとした後、該スラブを１１５０〜１２３０℃に加熱し、Ａｒ３変態点以上である８５０〜９１０℃の仕上圧延終了温度で熱間圧延を終了し、５６０〜６２０℃の温度で巻取り、板厚２ｍｍの熱延鋼板とした。次いで、上記熱延鋼板から、圧延方向が引張方向となるようにＪＩＳ５号引張試験片を採取し、ＪＩＳ Ｚ２２４１に準拠して引張試験を行ない、引張強さ（ＴＳ）と破断伸び（Ｅｌ）を測定した。なお、本実施例においては、引張試験における伸びＥｌが４０％以上を加工性良と評価し、引張強さは３４０〜４８０ＭＰａの範囲を目標とした。 After melting steels of symbols A to M having the composition shown in Table 1 to form slabs, the slabs were heated to 1150 to 1230 ° C., and finish rolling at 850 to 910 ° C. which is not lower than the Ar 3 transformation point. Hot rolling was completed at the end temperature, and the sheet was wound at a temperature of 560 to 620 ° C. to obtain a hot-rolled steel sheet having a thickness of 2 mm. Next, a JIS No. 5 tensile test piece is taken from the hot-rolled steel sheet so that the rolling direction is the tensile direction, and a tensile test is performed according to JIS Z2241, and the tensile strength (TS) and breaking elongation (El) are determined. It was measured. In this example, an elongation El in the tensile test of 40% or more was evaluated as good workability, and the tensile strength was targeted in the range of 340 to 480 MPa.
次いで、上記熱延鋼板から、硬さ測定用の試験片と、図１に示した形状、寸法の平面曲げ疲労試験片を採取し、これらの試験片に、ＮＨ３：ＲＸ＝５０：５０の雰囲気ガス中で、５８０℃×２ｈｒのガス軟窒化処理を施した。そして、硬さ測定用試験片については、軟窒化処理後の鋼板表面および板厚中央部のビッカース硬さを、それぞれ１０箇所ずつ測定し（測定荷重：表面は５０ｇ、板厚中央部は１００ｇ）、それらの平均値の比を算出した。ここで、上記鋼板表面硬さとは、鋼板表面から深さ５０μｍの位置にて測定したビッカース硬さである。また、疲労試験片については、応力振幅が３５０ＭＰａの疲労試験に供し、破断するまでのサイクル数を測定した。なお、本実施例においては、破断までのサイクル数が１０５サイクル以上を疲労特性良（○）、１０５サイクル未満を疲労特性劣（×）と評価した。 Next, test pieces for hardness measurement and plane bending fatigue test pieces having the shape and dimensions shown in FIG. 1 were taken from the hot-rolled steel sheet, and NH 3 : RX = 50: 50 was collected on these test pieces. A gas soft nitriding treatment at 580 ° C. × 2 hr was performed in an atmospheric gas. And about the test piece for hardness measurement, the Vickers hardness of the steel plate surface after soft nitriding treatment and the plate thickness center part is measured 10 each (measurement load: surface is 50 g, plate thickness center part is 100 g) The ratio of their average values was calculated. Here, the steel sheet surface hardness is a Vickers hardness measured at a position of a depth of 50 μm from the steel sheet surface. Moreover, about the fatigue test piece, it used for the fatigue test whose stress amplitude is 350 Mpa, and measured the cycle number until it fractures. In the present embodiment, the number of cycles fatigue properties good for more than 10 5 cycles to failure (○), was less than 10 5 cycles was evaluated as fatigue poor (×).
上記測定の結果を、表１中に併記した。表１から、本発明の成分組成を有する鋼を用いて、本発明の条件を満たす方法で製造した鋼板（鋼板Ｅ〜Ｇ）は、微量のＮｂ添加によって、Ｎｂ炭窒化物の微細析出による母材強化と軟窒化処理による表面硬化とが適度にバランスしているため、軟窒化処理用鋼板として適度の強度と加工性を有するとともに、軟窒化処理後の疲労特性にも優れていることがわかる。
これに対して、本発明の成分組成を外れている比較例の鋼板はいずれも、鋼板強度、加工性、疲労特性のいずれか１以上の特性が劣っている。図３は、鋼Ａ〜Ｅから得られた鋼板における応力振幅と破断発生までのサイクル数との関係を示したものであるが、Ｎｂを微量添加した本発明の鋼板Ｅのみが、破断までのサイクル数が１０５サイクルを超えており、疲労特性に優れていることが明らかである。
これに対して、Ｎｂを含有していない鋼板Ａ、Ｎｂを含有せず、ＣｒやＡｌを過剰に含有する鋼板ＢおよびＣは、鋼板強度および伸びは良好であるが、軟窒化処理後の素材強度と表面硬さとのバランスが良くないため、破断までのサイクル数が１０５サイクル未満であり、疲労特性が劣っている。また、Ｎｂを過剰に含有する鋼板Ｄは、鋼板の強度が高過ぎて伸びが低下し、加工性の劣化が大きいことに加えて、軟窒化処理後の鋼板表面の硬さが、著しく高くなるため、疲労強度が低下している。
また、ＣおよびＭｎ含有量が高い鋼板ＨおよびＪは、鋼板の強度が高くなり過ぎて、伸びの低下が大きく、いずれも加工性が不足している。また、Ｓｉを過剰に含有する鋼板Ｉは、表面性状が劣るため、十分な疲労強度が得られていない。また、Ｐ含有量が高い鋼板Ｋは、鋼板強度が比較的高く、伸びの低下も大きいが、Ｐの粒界への偏析により、部品成形過程で割れを引き起こすおそれがある。また、Ｓの含有量が高い鋼板Ｌは、一見、加工性は良好であるが、多量の硫化物が粒界に析出するため、疲労強度が劣るだけでなく、熱間での延性低下により割れを引き起こすおそれがある。Ｎの含有量が高い鋼板Ｍは、ガス軟窒化処理前に、既に、鋼中のＮが窒化物となって多量に析出しているため、強度上昇、伸び低下により成形性が劣る。
The results of the above measurements are also shown in Table 1. From Table 1, the steel plates (steel plates E to G) manufactured by the method satisfying the conditions of the present invention using the steel having the component composition of the present invention are the mother by fine precipitation of Nb carbonitride by adding a small amount of Nb. It is understood that the steel strengthening and surface hardening by soft nitriding have an appropriate balance, so that it has moderate strength and workability as a steel sheet for soft nitriding, and also has excellent fatigue properties after soft nitriding. .
On the other hand, all the steel plates of comparative examples that deviate from the component composition of the present invention are inferior in any one or more of steel plate strength, workability, and fatigue properties. FIG. 3 shows the relationship between the stress amplitude and the number of cycles until breakage in steel plates obtained from steels A to E, but only the steel plate E of the present invention to which a small amount of Nb was added until breakage. number of cycles is above 10 5 cycles, it is clear to have excellent fatigue properties.
On the other hand, the steel plates A and N that do not contain Nb, and steel plates B and C that do not contain Nb and contain excessive amounts of Cr and Al have good steel plate strength and elongation, but are materials after soft nitriding treatment. Since the balance between strength and surface hardness is not good, the number of cycles to break is less than 10 5 cycles, and the fatigue properties are inferior. In addition, the steel sheet D containing Nb excessively has a steel sheet whose strength is too high, the elongation is lowered, the workability is greatly deteriorated, and the hardness of the steel sheet surface after the nitrocarburizing treatment is remarkably increased. For this reason, the fatigue strength is reduced.
Further, the steel plates H and J having a high C and Mn content are too strong in the steel plate, resulting in a large decrease in elongation, both of which are insufficient in workability. Moreover, since the steel sheet I containing excessive Si has inferior surface properties, sufficient fatigue strength is not obtained. The steel plate K having a high P content has a relatively high steel plate strength and a large decrease in elongation, but may cause cracking in the part forming process due to segregation of P to the grain boundaries. In addition, the steel sheet L with a high S content has a good workability at first glance, but a large amount of sulfide precipitates at the grain boundaries, so that not only the fatigue strength is inferior, but also cracking due to a decrease in hot ductility. May cause. The steel sheet M having a high N content is inferior in formability due to an increase in strength and a decrease in elongation because N in the steel is already nitrided in large amounts before the gas soft nitriding treatment.
表１に示した本発明の成分組成を満たす鋼Ｅのスラブを、加熱温度、仕上圧延終了温度および巻取温度を表２に示したように種々に変化させて熱間圧延し、板厚が２ｍｍの熱延鋼板とした。これらの熱延鋼板について、実施例１と同様の条件で、引張特性、硬さ測定および疲労試験に供し、その結果を表２中に併記して示した。なお、加工性および疲労特性の評価は、実施例１と同様、引張試験における伸びＥｌが４０％以上を加工性良、引張強さは３４０〜４８０ＭＰａの範囲を目標とした。また、疲労特性の評価は、破断までのサイクル数が１０５サイクル以上を良（○）、１０５サイクル未満を劣（×）とした。 The steel E slab satisfying the composition of the present invention shown in Table 1 is hot-rolled with various changes in the heating temperature, finish rolling finish temperature and winding temperature as shown in Table 2, and the sheet thickness is A 2 mm hot-rolled steel sheet was used. These hot-rolled steel sheets were subjected to tensile properties, hardness measurement, and fatigue tests under the same conditions as in Example 1. The results are also shown in Table 2. In addition, the evaluation of workability and fatigue characteristics was conducted in the same manner as in Example 1, with an elongation El in the tensile test of 40% or more as a good workability and a tensile strength in the range of 340 to 480 MPa. The fatigue characteristics were evaluated as good (◯) when the number of cycles until breakage was 10 5 cycles or more, and poor (X) when less than 10 5 cycles.
表２から、Ｎｏ．１の鋼板は、本発明の成分組成を満たす鋼Ｅを本発明に適合する条件で熱間圧延した例であり、十分な加工性と疲労特性を有していることがわかる。
これに対して、Ｎｏ．２の鋼板は、加熱温度が本発明の上限値を外れた例であり、表面性状の劣化やスケール性の表面欠陥を引き起こすのに加えて、フェライト粒が粗大化するため疲労特性が低下している。一方、Ｎｏ．３の鋼板は、加熱温度が本発明の下限値を外れた例であり、粗大なフェライトとパーライトの組織となるため、やはり、疲労特性が低下している。また、Ｎｏ．４の鋼板は、仕上圧延終了温度が上限値を外れた例であり、鋼板組織が粗大なフェライトとパーライトとなるため、疲労特性が劣る。一方、Ｎｏ．５の鋼板は、仕上圧延終了温度が下限値を外れた例であり、ポリゴナルフェライトが得られないため、加工性が低下している。また、Ｎｏ．６は、巻取温度が上限値を外れた例であり、組織の粗大化に加え、母材中のＮｂ炭窒化物が粗大化してしまい、疲労特性が劣る。一方、Ｎｏ．７は、巻取温度が下限値を外れた例であり、組織が針状のフェライトとなるため、鋼板強度が上昇し、伸びが低下するため、所望の加工性が得られていない。
From Table 2, no. Steel plate No. 1 is an example in which steel E satisfying the component composition of the present invention is hot-rolled under conditions suitable for the present invention, and it can be seen that it has sufficient workability and fatigue characteristics.
In contrast, no. Steel plate No. 2 is an example in which the heating temperature deviates from the upper limit of the present invention. In addition to causing surface property deterioration and scale surface defects, the ferrite grains are coarsened and fatigue characteristics are reduced. Yes. On the other hand, no. Steel plate No. 3 is an example in which the heating temperature deviates from the lower limit of the present invention, and has a coarse ferrite and pearlite structure. No. Steel plate No. 4 is an example in which the finish rolling finish temperature deviates from the upper limit value, and since the steel plate structure becomes coarse ferrite and pearlite, fatigue properties are inferior. On the other hand, no. Steel plate No. 5 is an example in which the finish rolling finish temperature is out of the lower limit value, and polygonal ferrite cannot be obtained, so the workability is lowered. No. No. 6 is an example in which the coiling temperature deviates from the upper limit value, and in addition to the coarsening of the structure, the Nb carbonitride in the base metal becomes coarse and the fatigue characteristics are inferior. On the other hand, no. No. 7 is an example in which the coiling temperature deviates from the lower limit value, and since the structure is acicular ferrite, the strength of the steel sheet is increased and the elongation is decreased, so that the desired workability is not obtained.
本発明の技術は、ガス軟窒化処理用鋼板に限定させるものではなく、例えば、窒化処理用鋼板としても好適に用いることができる。 The technique of the present invention is not limited to the steel sheet for gas soft nitriding treatment, and can be suitably used, for example, as a steel plate for nitriding treatment.
Claims (2)
Ｓｉ：０．５ｍａｓｓ％以下、
Ｍｎ：０．１〜０．６ｍａｓｓ％、
Ｐ：０．０４ｍａｓｓ％以下、
Ｓ：０．０４ｍａｓｓ％以下、
Ａｌ：０．００５〜０．０８ｍａｓｓ％、
Ｃｒ：０．４〜１．２ｍａｓｓ％、
Ｎｂ：０．００２ｍａｓｓ％以上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。 C: 0.03-0.10 mass%,
Si: 0.5 mass% or less,
Mn: 0.1 to 0.6 mass%,
P: 0.04 mass% or less,
S: 0.04 mass% or less,
Al: 0.005 to 0.08 mass%,
Cr: 0.4 to 1.2 mass%
Nb: 0.002 mass% or more and less than 0.01 mass% and N: 0.01 mass% or less,
A steel sheet for soft nitriding treatment, the balance being Fe and inevitable impurities.
Ｓｉ：０．５ｍａｓｓ％以下、
Ｍｎ：０．１〜０．６ｍａｓｓ％、
Ｐ：０．０４ｍａｓｓ％以下、
Ｓ：０．０４ｍａｓｓ％以下、
Ａｌ：０．００５〜０．０８ｍａｓｓ％、
Ｃｒ：０．４〜１．２ｍａｓｓ％、
Ｎｂ：０．００２ｍａｓｓ％以上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鋼素材を、加熱温度：１１００〜１２５０℃、仕上圧延終了温度：Ａｒ３変態点〜９８０℃、巻取温度：５００〜７４０℃とする熱間圧延する軟窒化処理用鋼板の製造方法。 C: 0.03-0.10 mass%,
Si: 0.5 mass% or less,
Mn: 0.1 to 0.6 mass%,
P: 0.04 mass% or less,
S: 0.04 mass% or less,
Al: 0.005 to 0.08 mass%,
Cr: 0.4 to 1.2 mass%
Nb: 0.002 mass% or more and less than 0.01 mass% and N: 0.01 mass% or less,
Soft nitriding is performed by hot rolling a steel material having the balance of Fe and inevitable impurities to a heating temperature of 1100 to 1250 ° C., finish rolling finishing temperature: Ar 3 transformation point to 980 ° C., and winding temperature: 500 to 740 ° C. Manufacturing method of steel plate for processing.
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| JP3153108B2 (en) * | 1995-07-12 | 2001-04-03 | 新日本製鐵株式会社 | Steel sheet for nitriding excellent in deep drawability and its pressed product |
| JP3991884B2 (en) * | 2003-02-24 | 2007-10-17 | Ｊｆｅスチール株式会社 | Steel for nitriding with excellent magnetic properties after nitriding and molded body thereof |
| JP4561136B2 (en) * | 2004-03-17 | 2010-10-13 | Ｊｆｅスチール株式会社 | Steel sheet for nitriding treatment |
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2007
- 2007-09-12 JP JP2007236222A patent/JP5034803B2/en active Active
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| JP2009068057A (en) | 2009-04-02 |
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