Superfine grain steel having nitrided layer
An ultra fine grain steel has a ferrite grain structure having an average grain size of 3 μm or less and has a nitride layer formed on a surface of the steel. The nitride layer is formed without adding an alloy element such as Cr or Mo that is expensive and detrimental to recycling. Fatigue strength is enhanced by the nitride layer.
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The present invention relates to an ultra fine grain steel having a nitride layer. More particularly, the present invention relates to an ultra fine grain steel having a nitride layer that is formed without adding alloy elements such as Cr and Mo, which are expensive and detrimental at the time of recycling and to the steel whose fatigue strength is enhanced.
BACKGROUND ARTIn a metal part such as a rotary shaft that receives a bending stress and a twisting stress, a fatigue crack is generated in a surface subjected to high stress and the metal part is finally brought to fatigue fracture. For this reason, hardening of a surface and thereby imparting high fatigue strength to the surface is effective for imparting high fatigue strength to an entire metal part. Moreover, to harden a surface is also effective from a viewpoint of wear resistance and corrosion resistance. This is similar to an ultra fine grain steel having high strength and high toughness in which ferrite grains are extremely small in size (for example, see patent document 1).
Conventionally, in order to harden a surface, nitrifying of a surface has been thought to be effective but in order to nitride a surface, it is necessary to produce nitrides of alloy elements such as Cr, Mo, Ti, or Nb by adding the alloy elements and by holding temperatures from 450° C. to 590° C. for from 0.5 to 100 hours (for example, see non-patent document 1). Actually, when pure iron is nitrified by use of ammonia gas, an iron nitride film having a thickness of several μm to several tens μm is only formed on a surface and hardness of surface becomes, at most, about 250 Hv. Iron nitride hardly precipitates inside or even if an iron nitride slightly precipitates, the iron nitride hardly contributes to hardening of a surface.
Since the alloy elements such as Cr and Mo are expensive and are detrimental at the time of recycling, it is desirable to avoid addition of the alloy elements.
The present invention has been made in view of these circumstances. An object of the present invention is to provide an ultra fine grain steel having a nitride layer that is formed without adding alloy elements such as Cr and Mo, which are expensive and detrimental at the time of recycling, and the steel whose fatigue strength is enhanced.
- Patent document 1: Japanese Patent Publication No. 2000-309850
- Non-patent document 1: Doctoral Dissertation, Hideyuki Kuwahara, November in 1992, Kyoto University
The present invention provides an ultra fine grain steel having a nitride layer, wherein the steel has a ferrite grain structure having an average grain size of 3 μm or less and a nitride layer is formed on a surface of the steel (claim 1).
Further, the present invention provides the following modes: grain growth at the time of nitrifying is suppressed by precipitation of carbide or addition of a solid solute element or both (claim 2); the amount of C is 0.01 mass % or more (claim 3); at least one element selected from the group consisting of Mn, Cr, Mo, Ti, Nb, V and P is added (claim 4); the amount of Mn is 0.4 mass % or more (claim 5); the amount of P is 0.035 mass % or more (claim 6); the steel is a carbon steel and the total amount of Cr, Mo, Ti, Nb, and V is 0.1 mass % or less (claim 7); a fatigue limit is 1.6 times larger than Vickers hardness of a base material (claim 8); a molded part, a part, or a member which is formed from an ultra fine grain steel having a nitride layer as described above (claim 9).
Hereafter, an ultra fine grain steel having a nitride layer of the present invention will be described in more detail while showing embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 9(a) and 9(b) are FE-SEM photographs showing a base metal structure before nitrifying of a Fe—C—Mn—Si ultra fine grain steel containing 0.15 mass % C and a Fe—C—Mn—Si ultra fine grain steel containing 0.45 mass % C, respectively.
When an ultra fine steel having a simple composition and a ferrite grain structure having an average grain size of 3 μm or less is held in an ammonia gas atmosphere or an atmosphere containing ammonia gas at temperatures from 450° C. to 590° C. for from 0.5 to 100 hours, a nitride layer is formed on a surface, hardening of the surface occurs, and high fatigue strength is obtained.
Here, a ferrite grain structure means a structure in which ferrite grains are predominant. In this meaning, a ferrite grain structure may be a ferrite single phase structure or may contain carbide, pearlite, martensite, and austenite as a second phase.
In an ultra fine grain steel having a nitride layer in accordance with the present invention, it is preferable that the amount of Mn is 0.4 mass % or more. In a Fe—C—Mn or Fe—C—Mn—Si ultra fine grain steel containing 0.37 mass % Mn, by nitrifying under a condition of 500° C., 16 hours, a surface is hardened but a deep hardened layer is hardly formed. In a case where the amount of Mn is 0.83 mass %, a surface of an ultra fine grain steel is hardened and a deep nitride layer is formed. By the way, in an ordinary steel, Mn of 0.4 mass % is added to at the minimum as measures against MnS. In consideration of the above-described circumstances, it is desirable that the amount of Mn is 0.4 mass % or more in an ultra fine grain steel having a nitride layer in accordance with the present invention.
In an ultra fine grain steel containing C from 0.05 mass % to 0.15 mass %, when the steel is held at temperatures around 550° C. for a long time, grains grow and an ultra fine grain structure is apt to collapse. Grain growth can be prevented or suppressed by increasing the amount of C to precipitate carbide such as Fe3C, NbC, and TiC, or by adding a solid solution element such as P (phosphor) and V (vanadium), or by both of them. For example, nitrifying can be performed at temperatures around 500° C. for as long a time as 26 hours. By this long-time nitrifying, a deeper nitride layer effective for high fatigue strength is formed. The preferable amount of C is 0.05 mass % or more and the amount of P is preferably 0.035 mass % or more.
Usually, the thickness of a layer hardened by nitrifying, that is, a nitride layer is from 0.5 mm to 1.0 mm in view of fatigue, and in the case of hardening a surface sufficiently, in a coarse grain steel, fatigue fracture occurs in a matrix just below the nitride layer. The fatigue strength of an entire nitrified material is generally determined by strength determined from stress at a starting point of fatigue fracture and the fatigue strength of a matrix. However, in an ultra fine grain steel having a nitride layer in accordance with the present invention, fatigue strength extremely higher than fatigue strength expected from the above-described law is obtained. This is because a nitride layer formed in an ultra fine grain steel by nitrifying is hardened more heavily than the nitride layer of a coarse grain steel. Therefore, an ultra fine grain steel not only has high strength and high toughness but also has high fatigue strength by nitrifying.
Increasing surface hardness leads to enhancing wear resistance. That is, large compressive stress is given to a nitride layer and remaining compressive stress cancels tensile stress caused by sliding to relatively reduce the tensile stress applied to a surface of an ultra fine grain steel. Moreover, in an ultra fine grain steel in accordance with the present invention, grain growth is prevented or suppressed at nitrifying temperatures and hence properties are less prone to be degraded by heat generated by friction. Therefore, an ultra fine grain steel having a nitride layer in accordance with the present invention shows excellent wear resistance.
An ultra fine grain steel having a nitride layer in accordance with the present invention can be formed in the shape of a bulk or powder.
When a Fe—C—Mn or Fe—C—Mn—Si steel powder is nitrified, the powder shows the same strength as a nitride layer formed on a bulk of an ultra fine grain steel. Therefore, a bulk obtained by sintering nitride powders can become a high-strength material. In this case, sintering can be performed in an atmosphere, which includes single gas of nitrogen gas or ammonia gas, or a mixture of them, or either of them to which hydrogen gas is added, in total pressure of 2 atmospheric pressure or less under compressive stress of 0.1 MPa or more in a temperature range of 1200° C. or less. Further, at the time of sintering, all the powder does not necessarily need to be nitrified and, for example, pure iron powders as a sintering-promoting agent may be mixed with. Furthermore, in the case of using steel powder whose grain size is 20 μm or less, in order to present grain growth suppressing effect, ceramic powders of TiN and TiC whose grain size is 2 μm or less can be added to steel powders.
An ultra fine grain steel having a nitride layer in accordance with the present invention is given not only high strength and high toughness but also high fatigue strength and excellent wear resistance and hence is expected to be put into practical use as a material expanding an application field to various kinds of molded parts, parts, and members.
In Table 1 are shown chemical components and ferrite grain sizes of materials used in examples and in Table 2 are shown mechanical properties. Each of the Fe—C—Mn coarse grain steel containing 0.002 mass % C and the Fe—C—Mn—Si coarse grain steel containing 0.25 mass % C has a ferrite grain size of about 20 μm and has a ferrite pearlite structure. The Fe—C—Mn—Si coarse grain steel containing 0.45 mass % C has a tempered martensite structure. The ultra fine grain steel has a ferrite structure in which fine ferrite grains and carbides are dispersed. The ultra dine grain steel was formed into a square bar of 18 mm ×18 mm by rolling with grooved rolls.
As shown in Table 2, in the Fe—C—Mn ultra fine grain steel containing 0.05 mass % C and Fe—C—Mn—Si coarse grain steel containing 0.15 mass % C, 0.45 mass % C, 0.75 mass % C, and 0.90 mass % C, as shown in FIGS. 9(a) and 9(b), grain-shaped carbides increase with the amount of carbon and hence tensile strength and Vickers hardness increase by precipitation strengthening of carbides. In the Fe—C—Mn—Si ultra fine grain steels containing 0.15 mass % C and 0.1 mass % P and Fe—C—Mn—Si ultra fine grain steels containing 0.45 mass % C and 0.1 mass % P, as compared with the P-added ultra fine grain steel containing the same amount of C, tensile strength and Vickers hardness are larger. This is due to the solid solution strengthening of P.
The Fe—C—Mn coarse grain steel containing 0.002 mass % C and the Fe—C—Mn ultra fine grain steel containing 0.05 mass % C were nitrified by plasma under a condition of 550° C.×26 hours.
Fatigue limit =1.6 ×Vickers hardness,
a fatigue limit of the base material is estimated to be 1.6×160=256 [MPa].
Assuming that the thickness of the nitride layer is 1 mm, and considering a stress gradient in a rotary bending test of the test piece having a diameter φ of 6 mm (radius of 3 mm), a stress amplitude σa′ applied to a portion just below the nitride layer becomes σa′/σa=(3-1)/3 ≈0.67 where σa is a nominal stress amplitude at the surface. Therefore, the fatigue limit estimated from the hardness of the matrix and the stress gradient is about 256/0.67 =382 [MPa], but the fatigue limit of the actual nitrified material is 640 MPa as described above. It is recognized that the nitrified material has much more enhanced fatigue strength as compared with a fatigue limit expected from the hardness of the matrix. If the same estimation as described above is done by use of the fatigue limit of 376 MPa measured on the raw material, from the result shown in
As described above, in an ultra fine grain steel, even if grains are coarsened in some degree by nitrifying, a large effect of nitrifying with respect to fatigue strength is obtained. However, when considering actual parts and members, it is desirable that even if a base material is nitrified, it keeps an ultra fine grain structure and retains strength.
Then, results of examination on a grain growth suppressing effect by Fe3C are shown in
FIGS. 9(a) and 9(b) are FE-SEM photographs showing a base material structures before nitrifying of the Fe—C—Mn—Si ultra fine grain steel containing 0.15 mass % C and the Fe—C—Mn—Si ultra fine grain steel containing 0.45 mass % C, respectively. Many Fe3Cs (white speckles) precipitated in the Fe—C—Mn—Si ultra fine grain steel containing 0.45 mass % C. It is estimated that grains of the Fe—C—Mn—Si ultra fine grain steel containing 0.45 mass % C were not coarsened by grain growth suppressing effect of the Fe3C precipitates. Moreover, it was recognized that ferrite grain size was 1 μm or less in both of the ultra fine grain steels.
From the results described above, it is concluded that the grain growth suppressing effect of a carbide or solid solute element prevents or suppresses grain coarsening to maintain an ultra fine grain structure and that nitrifying of a long time is possible while preserving high strength.
In order to further examine effectiveness of nitrifying a ultra fine grain steel by use of the grain growth suppressing effect, a Fe—C—Mn—Si ultra fine grain steel containing 0.45 mass % C that utilizes Fe3C precipitates, a Fe—C—Mn—Si ultra fine grain steel containing 0.15 mass % C and 0.1 mass % P that utilizes solid solution of P, and a Fe—C—Mn—Si based ultra-refined steel containing 0.45 mass % C and 0.1 mass % P that utilizes both of the Fe3C precipitate and the solid solution of P were actually nitrified and the fatigue test was conducted. The nitrifying was performed by plasma under a condition of 500° C.×16 hours. In the fatigue test, stepwise tests one unit of which was 107 tests per one test piece having a test portion of 6 mm in diameter and formed into a shape of a sand clock were conducted by use of the Clauze type rotary bending test apparatus and only fatigue limits were measured.
The fatigue limits after nitrifying were 700 MPa for the ultra fine grain steel containing 0.45 mass % C, 780 MPa for the ultra fine grain steel containing 0.15 mass % C and 0.1 mass % P, and 700 MPa for the ultra fine grain steel containing 0.45 mass % C and 0.1 mass % P. As shown in Table 2, the Vickers hardness of the matrixes of the respective steels are 300, 308, and 339, so that the ratios of fatigue limit/Vickers hardness of a matrix are 2.33, 2.53, and 2.06 and all of them are larger than 1.6.
By the way, fatigue fracture started from a surface in the ultra fine grain steel containing 0.45 mass % C and the ultra fine grain steel containing 0.15 mass % C and 0.1 mass % P, whereas fatigue fracture started from inside inclusions in the ultra fine grain steel containing 0.45 mass % C and 0.1 mass % P. It is understood from these facts that an intrinsic fatigue limit of the nitrified structure was obtained in the ultra fine grain steel containing 0.45 mass % C and the ultra fine grain steel containing 0.15 mass % C and 0.1 mass % P but that a fatigue fracture decreases in the ultra fine grain steel containing 0.45 mass % C and 0.1 mass % P because fatigue fracture starts from the inclusions. It is expected that if the inclusions are reduced in size, for example, by use of a cleaning technology to prevent fatigue fracture starting from the inclusions, the ultra fine grain steel containing 0.45 mass % C and 0.1 mass % P can provide a fatigue limit beyond 780 MPa of the ultra fine grain steel containing 0.15 mass % C and 0.1 mass % P because the ultra fine grain steel containing 0.45 mass % C and 0.1 mass % P has higher hardness.
As hardness becomes higher, wear resistance becomes more excellent. As shown in
In addition, as shown in
Of course, the present invention is not limited to the above-described embodiments and examples. Needless to say, the invention can be modified in the details such as chemical components of steel, nitrifying conditions, and nitrifying method.
Industrial ApplicabilityAs described above in detail, the present invention can provide an ultra fine grain steel having a nitride layer that is formed without adding alloy elements such as Cr and Mo, which are expensive and detrimental, at the time of recycling, and the steel whose fatigue strength is enhanced.
Claims
1. An ultra fine grain steel having a nitride layer, wherein the steel has a ferrite grain structure having an average grain size of 3 μm or less and the nitride layer is formed on a surface of the steel.
2. The ultra fine grain steel having a nitride layer as claimed in claim 1, wherein grain growth at a time of nitrifying is suppressed by precipitation of carbide or addition of a solid solute element or both of them.
3. The ultra fine grain steel having a nitride layer as claimed in claim 1, wherein the amount of C is 0.01 mass % or more.
4. The ultra fine grain steel having a nitride layer as claimed in claim 1, wherein at least one element selected from a group consisting of Mn, Cr, Mo, Ti, Nb, V and P is added.
5. The ultra fine grain steel having a nitride layer as claimed in claim 4, wherein the amount of Mn is 0.4 mass % or more.
6. The ultra fine grain steel having a nitride layer as claimed in claim 4, wherein the amount of P is 0.035 mass % or more.
7. The ultra fine grain steel having a nitride layer as claimed in claim 4, wherein the steel is a carbon steel and the total amount of Cr, Mo, Ti, Nb, and V is 0.1 mass % or less.
8. The ultra fine grain steel having a nitride layer as claimed in claim 1, wherein a fatigue limit is 1.6 times larger than Vickers hardness of a base material.
9. A molded part, a part, or a member which is formed from the ultra fine grain steel having a nitride layer as claimed in claim 1.
Type: Application
Filed: Oct 17, 2003
Publication Date: Nov 3, 2005
Applicant: National Institute for Materials Science (Ibaraki)
Inventors: Saburo Matsuoka (Ibaraki), Yoshiyuki Furuya (Ibaraki), Hisashi Hirukawa (Ibaraki), Shiro Torizuka (Ibaraki)
Application Number: 10/531,319