JP3567713B2 - Steel with excellent ductility and excellent surface hardness and internal hardness after nitrocarburizing - Google Patents

Description

【０００９】
Ｓｉは微量では問題はないが、０．０５０％を超えると延性を低下させる。したがって、Ｓｉは０．０５０％以下でなければならない。
Ｍｎは鋼中のＳと反応し、ＭｎＳを形成することで、鋼を製造する際の高温での割れを防止する役割をはたす。そのためには、０．１％以上の添加が必要である。しかし、０．５％を超えると、延性を低下させる。したがって、Ｍｎ量を０．１〜０．５％に限定した。
【００１０】
Ｐはその添加量を増すことにより鋼板の強度を高めることができるが、本発明においては積極的に添加される必要のない元素である。含有量が０．０３５％を超えると延性の低下が大きくなる。したがって、Ｐは０．０３５％以下でなければならない。
Ｓは多いほど鋼の延性は低下し、鋼を製造する際に高温で加工を行った場合の割れの原因ともなる。したがって、Ｓは少ないほどよく、０．０３０％以下でなければならない。
【００１１】
Ａｌは、Ｖ、Ｃｒが添加された鋼の延性の低下を防止するのに有効な元素であり、その効果を発揮させるためには０．０１５％以上が必要である。また、その添加に伴い、窒化によって鋼中に侵入するＮと反応し、ＡｌＮを形成することによって表面硬さを向上させる役割がある。しかし、添加量が多すぎても延性が低下し、その上限は１．５０％である。したがって、Ａｌを０．０１５〜１．５０％に限定した。
【００１２】
Ｎは延性を低下させるので、その量は低いほど好ましく、０．００５０％以下でなければならない。
Ｔｉは鋼中のＣおよびＮと結び付いて固溶Ｃおよび固溶Ｎを低減させることで、延性を確保するとともに、後述するＶやＣｒに軟窒化後の硬度上昇作用を持たせるために必要な元素である。また、ＣおよびＮと反応した後、鋼中に固溶状態になっているＴｉは軟窒化において、窒化物を形成し、表面付近の硬さを向上させるのに必要な元素である。これらの効果を発揮させるには、０．０３０％以上が必要である。しかし、０．５００％を超えると延性を低下させる。したがって、Ｔｉは０．０３０〜０．５００％に限定した。
【００１３】
Ｃｕは軟窒化処理において鋼の硬さを上昇させるのに必要な元素である。この元素を添加することにより、軟窒化時の表面および表面近傍への窒素の拡散による硬さ向上のみでなく、この部分以外の部分の硬さを向上させることができる。その効果を発揮させるには０．８％以上が必要であり、さらに軟窒化処理前には０．８％以上が固溶状態にあることが必要である。しかし、２．２％を超えると延性が低下し、さらに軟窒化処理による鋼の内部強度向上の効果が減少する。したがって、Ｃｕ量を０．８〜２．２％に限定し、さらに軟窒化前には０．８％以上のＣｕが固溶状態にあることが必要である。
【００１４】
Ｎｉは鋼を製造する際に高温で加工を行った場合のＣｕによる割れを防止するために必要な元素である。その効果を発揮させるためには、重量比にてＣｕの０．５倍以上の添加が必要である。しかし、１．５％を超える量が添加されると、延性の低下につながる。したがって、Ｎｉは０．５×（Ｃｕ）〜１．５％に限定した。
【００１５】
Ｖは軟窒化において窒化物を形成させ、表面付近を硬化させることができる。本発明では低いＣ量を規定しており、さらにＴｉを添加することによって固溶Ｃを極めて低く抑えているため、添加されたＶの大部分が固溶Ｖとなり、０．２％以上の少量の添加で軟窒化時の表面硬化に寄与することが本発明者らの検討によって見出された。しかし、Ｖが１．０％を超えると延性の低下が大きくなる。したがって、Ｖは０．２〜１．０％に限定した。なお、Ｖを無添加とし、Ｃｒの添加によっても表面付近の硬さ向上を図ることができる。あるいは、ＶとＣｒの両方を添加して表面付近の硬さ向上を図ることもできる。
【００１６】
Ｃｒも軟窒化において窒化物を形成させ、表面付近を硬化させることができる。本発明では低いＣ量を規定しており、さらにＴｉを添加することによって固溶Ｃを極めて低く抑えているため、添加されたＣｒの大部分が固溶Ｃｒとなり、Ｃｒを本発明範囲内で含有する場合に、０．２％以上の少量の添加で軟窒化時の表面硬化に寄与することが本発明者らの検討によって見出された。しかし、Ｃｒが１．５％を超えると延性の低下が大きくなる。したがって、Ｃｒは０．２〜１．５％に限定した。
【００１７】
以上示した鋼を製造する手段は、熱間圧延、鍛造、鋳造あるいは冷間圧延後の焼鈍などいずれの方法でもよいが、高い延性と軟窒化処理における表面付近以外の部分の硬さ向上を実現させるためには、添加したＣｕを固溶状態に保つ必要があり、そのためには、製造工程において８００℃以上の温度に加熱し、８００℃以上の温度から５０℃／ｓ以上の冷却速度で３４０℃以下に急冷する必要がある。また、軟窒化処理によって高い表面硬さとそれ以外の部分の硬さを向上させるためには、５００〜６５０℃で処理する必要があり、好ましくは５５０〜６００℃での処理である。
【００１８】
以下に本発明の実施例を比較例と共に示す。
【００１９】
【実施例】
表２、表３（表２のつづき−１）、表４（表２のつづき−２）に示す化学成分の鋼塊を製造し、熱間圧延によって薄鋼板とした。鋳片の加熱温度は１２００℃とし、熱延においては仕上温度を９００℃として３．５ｍｍの厚みに仕上げた後、１２０℃まで１００Ｃ／ｓで急冷し、添加しているＣｕを固溶状態とした。得られた熱延鋼板の硬さおよび伸びを測定した。伸びはＪＩＳ５号試験片に加工して測定した。ここで、硬さＨｖ：１６０以下、かつ伸び２５％以上のものは、冷間加工性が優れていると判断し、いずれか一方または両方が外れているものは冷間加工性が劣ると判断した。
【００２０】
さらに、熱延鋼板をアンモニアガスを含んだ雰囲気において、５８０℃で８０ｍｉｎの軟窒化処理を施し、板厚方向の硬さ分布を測定し、表面硬さ、内部硬さなどを求めた。ここで、表面硬さＨｖ：６００以上かつ内部硬さＨｖ：１９０以上のものは、窒化部品として強度が優れていると判断し、いずれか一方または両方が外れているものは、強度が劣ると判断した。これらの測定結果を表５、表６（表５のつづき）に示す。
【００２１】
図２は、試料番号３０の軟窒化前後の板厚方向の硬さ分布を示したものである。
【００２２】
【表２】

【００２３】
【表３】

【００２４】
【表４】

【００２５】
【表５】

【００２６】
【表６】

【００２７】
表５、表６から明らかなように、本発明範囲の組成のいずれか一つあるいは複数が外れている鋼は、軟窒化前の鋼板の伸び、硬さ、および軟窒化後の表面硬さ、内部硬さのいずれか一つあるいは複数が本発明範囲内の鋼により得られた値よりも劣っていることがわかる。
【００２８】
【発明の効果】
以上のように、鋼の組成を特定範囲とすることにより、２５％以上の延性とＨｖ：１６０以下の特性を持たせることができ、さらに、軟窒化後には表面硬さＨｖ：６００以上で内部硬さＨｖ：１９０以上の特性を得ることができる。
【図面の簡単な説明】
【図１】延性に及ぼすＣ量の影響を示した図である。
【図２】鋼板の板厚方向の硬さ分布を示した図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel for nitrocarburizing, by setting the composition of the steel to a specific range, excellent ductility, having a hardness of a certain value or less, the hardness of the surface and the internal hardness after nitrocarburizing treatment. The present invention relates to steel having improved hardness.
[0002]
[Prior art]
Various techniques for improving the surface hardness and the internal hardness of steel by nitrocarburizing have been proposed before, for example, as disclosed in JP-A-3-122254, a hot-rolled steel sheet containing Cu and Ti. There is. In this technique, the surface hardness after nitrocarburizing is targeted at Hv: 350 or more, and the results are shown in Table 2 and FIG. 1 of JP-A-3-122254. 410. However, this hardness may not be enough to be applied to transportation equipment parts and machine parts. For example, as shown in FIG. 4 of “Special Steel” 41 (1992) P28, Hv: 600 or more is required. It cannot be applied to the use described.
[0003]
As a means of increasing the surface hardness by nitrocarburizing other than the above technique, there is a method of adding Cr, Mo, V, Al, etc. to steel as shown in “Special Steel” 41 (1992) P11. However, these steels have a problem of low elongation as shown in P15 of the same document. Therefore, when manufacturing parts using these steels, it is often difficult to perform cold forging or cold press forming, so hot forging with a low material yield or cold forging Heat treatment such as spheroidizing annealing of the material is required before forging, and heat treatment steps such as quenching and tempering are also required after plastic working to increase the internal hardness to the required hardness. Therefore, there is a problem that the manufacturing cost of parts using these steels increases.
[0004]
In addition, since steel containing V, Cr, etc. is hard, heat treatment such as annealing may be required before the cutting process to create the shape of the part before nitrocarburizing, which also increases manufacturing costs. It has become.
The hot forging performed in the component manufacturing process is cold forging or cold press forming, improving the yield of materials and omitting heat treatment before and after plastic working, eliminating the problem of cost increase. In order to solve the problem, it is necessary to give an elongation of 25% or more. Further, in order to improve the machinability, the material before nitrocarburizing must be soft, and Hv: 160 or less is required. Therefore, it is necessary to improve the hardness of the steel surface to Hv: 600 or more, and furthermore, after nitrocarburizing, it is necessary to have strength in order to use these as strength parts. For that purpose, the internal hardness after soft nitriding also needs to be Hv: 190 or more.
[0005]
[Problems to be solved by the invention]
The present invention increases the ductility of steel to 25% or more, sets the hardness before nitrocarburizing to 160 or less in Hv, and improves the hardness of the surface portion by nitrocarburizing treatment to 600 or more in Hv, It is an object of the present invention to provide a steel having a hardness of 190 or more in Hv.
[0006]
[Means for Solving the Problems]
The present inventors have found that the above problems can be solved by setting the steel composition to a specific range.
That is, the gist of the present invention is that, by weight ratio, C: 0.0003 to 0.010%, Si: 0.050% or less, Mn: 0.10 to 0.50%, P: 0. 035% or less, S: 0.030% or less, N: 0.0050% or less, Al: 0.015 to 1.50%, Ti: 0.030 to 0.500%, Cu: 0.8 to 2. 2%, Ni: 0.5 × (Cu) -1.5%, V: 0.2-1.0%, Cr: 0.2-1.5%, 1 or 2 types , The balance being Fe and unavoidable impurities, and further containing 0.8% or more of Cu as a solid solution, and having excellent ductility and excellent surface hardness and internal hardness after nitrocarburizing. In steel.
[0007]
Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the chemical components of steel in the present invention will be described.
As C in the steel increases, the ductility of the steel decreases. In particular, when V, Cr, or the like is added, if a certain amount or more of C is present, the decrease in ductility becomes extremely significant. This is presumably because V, Cr, etc. react with C in the steel to form carbides and increase the strength, but also decrease the ductility. In this case, the machinability also decreases at the same time. FIG. 1 shows that, as shown in Table 1, a steel slab containing V, Cr and the like and having various C contents was heated to 1200 ° C. and hot rolled to 4.3 mm at a finishing temperature of 900 ° C. The relationship between the ductility and the C content of a hot-rolled steel sheet manufactured by rapid cooling to room temperature is shown below. As is apparent from FIG. 1, the C content needs to be 0.010% or less in order to ensure high ductility even when V, Cr, or the like, which is a nitride-forming element, is added. If C is less than 0.0003%, the cost for decarburization increases, which is not preferable. Therefore, the amount of C was limited to 0.0003 to 0.010%.
[0008]
[Table 1]

[0009]
Although there is no problem with a small amount of Si, if it exceeds 0.050%, the ductility is reduced. Therefore, Si must be 0.050% or less.
Mn reacts with S in steel to form MnS, thereby playing a role in preventing cracking at a high temperature when manufacturing steel. For that purpose, addition of 0.1% or more is necessary. However, if it exceeds 0.5%, the ductility is reduced. Therefore, the amount of Mn was limited to 0.1 to 0.5%.
[0010]
P can increase the strength of the steel sheet by increasing the amount of P added, but is an element that does not need to be positively added in the present invention. If the content exceeds 0.035%, the ductility is significantly reduced. Therefore, P must be less than 0.035%.
As the amount of S increases, the ductility of the steel decreases, which causes cracking when the steel is processed at a high temperature. Therefore, S should be as small as possible, and should be 0.030% or less.
[0011]
Al is an element that is effective in preventing a decrease in ductility of steel to which V and Cr are added, and 0.015% or more is required to exert its effect. Further, with the addition, it has a role of improving the surface hardness by forming a AlN by reacting with N penetrating into the steel by nitriding. However, the ductility is reduced even if the addition amount is too large, and the upper limit is 1.50%. Therefore, Al was limited to 0.015 to 1.50%.
[0012]
N lowers the ductility, so the lower the amount, the better, and it should be 0.0050% or less.
Ti binds to C and N in the steel to reduce solid solution C and solid solution N, thereby ensuring ductility and making V and Cr, which will be described later, have a hardness increasing effect after nitrocarburizing. Element. Further, Ti which is in a solid solution state in steel after reacting with C and N is an element necessary for forming a nitride and improving the hardness near the surface in soft nitriding. To achieve these effects, 0.030% or more is required. However, when it exceeds 0.500%, the ductility decreases. Therefore, Ti was limited to 0.030 to 0.500%.
[0013]
Cu is an element necessary for increasing the hardness of the steel in the nitrocarburizing treatment. By adding this element, it is possible not only to improve the hardness by diffusion of nitrogen to the surface and the vicinity of the surface during soft nitriding, but also to improve the hardness of portions other than this portion. 0.8% or more is required to exhibit the effect, and more than 0.8% must be in a solid solution state before the nitrocarburizing treatment. However, if it exceeds 2.2%, the ductility decreases, and the effect of improving the internal strength of the steel by the nitrocarburizing treatment decreases. Therefore, it is necessary to limit the amount of Cu to 0.8 to 2.2%, and it is necessary that 0.8% or more of Cu is in a solid solution state before nitrocarburizing.
[0014]
Ni is an element necessary for preventing cracking due to Cu when processing at a high temperature when producing steel. In order to exhibit the effect, it is necessary to add 0.5 times or more of Cu in a weight ratio. However, the addition of more than 1.5% leads to reduced ductility. Therefore, Ni is limited to 0.5 × (Cu) to 1.5%.
[0015]
V can form nitrides in nitrocarburizing and harden near the surface. In the present invention, a low C content is specified, and furthermore, since the dissolved C is extremely low by adding Ti, most of the added V is dissolved V, and a small amount of 0.2% or more is added. It has been found by the present inventors that the addition contributes to the surface hardening during nitrocarburizing. However, when V exceeds 1.0%, the ductility is greatly reduced. Therefore, V was limited to 0.2 to 1.0%. The hardness near the surface can also be improved by adding Cr without adding V and adding Cr. Alternatively, both V and Cr can be added to improve the hardness near the surface.
[0016]
Cr can also form nitrides in soft nitriding and harden near the surface. In the present invention, a low C content is specified, and furthermore, since the dissolved C is extremely low by adding Ti, most of the added Cr becomes solid-dissolved Cr, and Cr is contained within the range of the present invention. It has been found by the present inventors that addition of a small amount of 0.2% or more contributes to the surface hardening during nitrocarburizing. However, if Cr exceeds 1.5%, the ductility is greatly reduced. Therefore, Cr was limited to 0.2 to 1.5%.
[0017]
Means for producing the steel shown above may be any method such as hot rolling, forging, casting or annealing after cold rolling, but it realizes high ductility and improvement in hardness of parts other than near the surface in soft nitriding treatment. In order to achieve this, it is necessary to keep the added Cu in a solid solution state. To this end, the Cu is heated to a temperature of 800 ° C. or more in the manufacturing process and is cooled from a temperature of 800 ° C. or more at a cooling rate of 50 ° C./s or more to 340 It is necessary to quench below ℃. Further, in order to improve the high surface hardness and the hardness of the other parts by the nitrocarburizing treatment, it is necessary to perform the treatment at 500 to 650 ° C., preferably at 550 to 600 ° C.
[0018]
Hereinafter, examples of the present invention are shown together with comparative examples.
[0019]
【Example】
Steel ingots having the chemical components shown in Tables 2 and 3 (continuation-1 in Table 2) and Table 4 (continuation-2 in Table 2) were manufactured, and were thinned by hot rolling. The heating temperature of the slab was 1200 ° C, and in hot rolling, the finishing temperature was 900 ° C and after finishing to a thickness of 3.5 mm, it was rapidly cooled to 120 ° C at 100 C / s to bring the added Cu into a solid solution state. did. The hardness and elongation of the obtained hot-rolled steel sheet were measured. The elongation was measured by processing a JIS No. 5 test piece. Here, those having a hardness Hv of 160 or less and an elongation of 25% or more are judged to be excellent in cold workability, and those having one or both of them deviated are judged to be poor in cold workability. did.
[0020]
Further, the hot rolled steel sheet was subjected to a soft nitriding treatment at 580 ° C. for 80 minutes in an atmosphere containing ammonia gas, and the hardness distribution in the thickness direction was measured to determine the surface hardness, the internal hardness and the like. Here, those having a surface hardness of Hv: 600 or more and an internal hardness of Hv: 190 or more are judged to have excellent strength as nitrided parts, and those having one or both of them deviated have poor strength. It was judged. The measurement results are shown in Tables 5 and 6 (continuation of Table 5).
[0021]
FIG. 2 shows the hardness distribution in the thickness direction of sample No. 30 before and after soft nitriding.
[0022]
[Table 2]

[0023]
[Table 3]

[0024]
[Table 4]

[0025]
[Table 5]

[0026]
[Table 6]

[0027]
As is evident from Tables 5 and 6, the steel having one or more of the compositions falling outside the range of the present invention is the elongation and hardness of the steel sheet before nitrocarburizing, and the surface hardness after nitrocarburizing, It can be seen that any one or more of the internal hardnesses are inferior to those obtained with steels within the scope of the present invention.
[0028]
【The invention's effect】
As described above, by setting the composition of the steel to a specific range, it is possible to impart a ductility of 25% or more and a property of Hv: 160 or less, and further, after soft nitriding, the surface hardness Hv: 600 or more and the internal Hardness Hv: A characteristic of 190 or more can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing the effect of the amount of C on ductility.
FIG. 2 is a diagram showing a hardness distribution of a steel sheet in a thickness direction.

Claims (1)

By weight ratio,
C: 0.0003-0.010%,
Si: 0.050% or less,
Mn: 0.10 to 0.50%,
P: 0.035% or less,
S: 0.030% or less,
Al: 0.015 to 1.50%,
N: 0.0050% or less,
Ti: 0.030 to 0.500%,
Cu: 0.8 to 2.2%,
Ni: 0.5 × (Cu) to 1.5%
And V: 0.2 to 1.0%;
Cr: 0.2-1.5%
One or two of the above, the balance being Fe and unavoidable impurities, and further having a solid solution of 0.8% or more of Cu, which is excellent in ductility and surface hardness after soft nitriding. Steel with excellent hardness and internal hardness.

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