WO2017056896A1

WO2017056896A1 - Preform for crankshaft, nitride crankshaft, and manufacturing method for same

Info

Publication number: WO2017056896A1
Application number: PCT/JP2016/076345
Authority: WO
Inventors: 将人祐谷; 裕章多比良; 基成西原
Original assignee: 新日鐵住金株式会社
Priority date: 2015-10-01
Filing date: 2016-09-07
Publication date: 2017-04-06
Also published as: JP6477904B2; JPWO2017056896A1

Abstract

Provided is a preform for a crankshaft which has excellent fatigue strength and straightening capabilities. The preform for a crankshaft has the following chemical composition, in mass%: 0.35-0.70% of C; 0.01-0.45% of Si; 1.3-3.0% of Mn; not more than 0.05% of P; 0.005-0.100% of S; 0.05-0.90% of Cr; 0.001-0.080% of Al; 0.003-0.025% of N; 0-0.05% of Ti; 0-0.05% of Nb; 0-0.50% of Mo; 0-0.50% of Cu; 0-0.50% of Ni; and 0-0.005% of Ca with the remainder Fe and impurities. A structure at a depth of 10 mm from the surface comprises, in volume%, not less than 50% in total of tempered martensite and tempered bainite, not more than 10% of proeutectoid ferrite, and not more than 40% of pearlite.

Description

Crankshaft rough profile, nitrided crankshaft, and method of manufacturing the same

The present invention includes a crankshaft rough shape material that has been subjected to quenching and tempering treatment after forming a steel material into a rough shape of a crankshaft hot, and a nitrided crankshaft that has been subjected to nitriding treatment by machining the rough shape material. And a manufacturing method thereof. The nitrided crankshaft according to the present invention has excellent fatigue strength and bending straightness, and is suitable for use as a machine part such as an automobile, an industrial machine, and a construction machine.

¡Fatigue strength is an important mechanical property for crankshafts. The crankshaft may be hot-forged into a desired shape for machine structural steel, subjected to hot forging or heat treatment, then machined and subjected to nitriding to further improve fatigue strength. is there. When the nitriding treatment is performed, the fatigue strength of the crankshaft is improved, but the crankshaft is slightly warped. The generated warpage must be eliminated by applying bending stress. Therefore, bend straightening, which is easy to correct warping, is one of the important characteristics for nitrided parts together with fatigue strength. A problem in the manufacture of a nitrided crankshaft is that the harder the surface layer after nitriding, the better the fatigue strength and the worse the bend straightening properties. As a technique for achieving both fatigue strength and bend straightening, for example, there is one disclosed in Japanese Patent Application Laid-Open No. 2004-162161.

In JP 2004-162161 A, the steel composition is optimized, the hardness distribution of the nitrided layer after nitriding, and the hardness of the core that is not affected by nitriding are controlled, so that fatigue strength and bend straightening are improved. A technique for achieving both is disclosed.

Generally, by performing pre-heat treatment such as quenching and tempering or normalizing on steel before nitriding, the bend straightening and fatigue strength after nitriding are improved. In particular, when the nitriding treatment is performed after quenching and tempering treatment before nitriding, the bending straightness and fatigue strength are improved as compared with the case where the nitriding treatment is performed on the steel as hot forged.

Japanese Unexamined Patent Application Publication Nos. 2009-167505 and 2011-42846 disclose techniques for achieving both fatigue strength after nitriding and bending straightening by performing quenching and tempering before nitriding. Japanese Patent Application Laid-Open No. 2009-167505 discloses a nitrocarburized rough product which is soft nitrided after quenching and tempering a steel containing V and further containing Cr and Al, and a crankshaft. In this technique, the pinning action of V carbonitrides refines the austenite grain size at the time of quenching and improves the bending straightness. Furthermore, by making the steel structure a pearlite-based structure, excellent fatigue strength can be achieved.

Japanese Patent Application Laid-Open No. 2011-42846 uses steel with optimized components to control the hardness in the vicinity of the surface layer after nitriding and to reduce the compound layer depth to achieve high fatigue strength and bending straightening. A nitrocarburized part that can be obtained is disclosed.

The technique described in the aforementioned Japanese Patent Application Laid-Open No. 2004-162161 controls the hardness distribution of the nitrided layer after nitriding and the hardness of the core that is not affected by nitriding by optimizing the steel components. However, since the steel structure has not been optimized, it is difficult to say that both fatigue strength and bend straightening can be achieved at a sufficiently high level.

In the technique disclosed in Japanese Patent Application Laid-Open No. 2009-167505, the pinning effect of V carbonitride is utilized, and the austenite grain size at the time of quenching is refined to achieve both fatigue strength and bend correction. However, since the general heating temperature in the quenching and tempering treatment is close to the solid solution temperature of V carbonitride, the mechanical characteristics vary when the heating temperature during quenching changes.

In the technique described in Japanese Patent Application Laid-Open No. 2011-42846, in order to make the compound layer thin, it is necessary to control the gas composition during nitriding using a special apparatus and to perform nitriding under special conditions. In addition, the cost increases because the processing for removing the compound layer is performed after nitriding.

In order to provide steel with sufficiently excellent bend straightening properties, it is necessary that the surface hardness after nitriding does not become too high. In order to increase the fatigue strength without excessively increasing the surface hardness, the present inventors consider that it is only necessary to increase the compressive residual stress introduced during nitriding, and have studied a method for increasing the compressive residual stress. went. As a result, the following findings (a) to (g) were obtained.

(A) One method for increasing the residual stress is to increase the content of nitride-forming elements and increase the amount of alloy nitride after nitriding. On the other hand, the greater the content of the nitride-forming element, the higher the surface hardness after nitriding, and at the same time the bending straightness deteriorates.

(B) The finer the alloy nitride, the greater the strengthening ability for steel. On the other hand, the magnitude of the residual stress resulting from the precipitation of alloy nitride is determined by the total amount regardless of the size of the alloy nitride. That is, when a predetermined amount of hardening is obtained, the residual stress introduced is larger when steel is strengthened using a large amount of coarse alloy nitride than when a small amount of fine alloy nitride is used.

(C) Another method for increasing the residual stress is to disperse cementite in the steel. When nitriding steel in which cementite is dispersed, the amount of nitrogen introduced increases and a large residual stress can be introduced.

(D) Among alloy elements that can form alloy nitrides in steel, Mn, which is an element having relatively weak interaction with nitrogen, tends to form moderately coarse alloy nitrides. On the other hand, if the content of Cr, Al, V or the like, which is an element having a relatively strong interaction with nitrogen, is excessive, fine alloy nitrides are likely to be generated.

Next, the inventors examined a base material structure suitable for increasing the fatigue strength while appropriately bending the alloy nitride and securing the bending straightness. As a result, the following findings (e) to (g) were obtained.

(E) In order to improve the fatigue strength while ensuring the bending straightness, the structure of the nitride layer is preferably as homogeneous as possible. Mixing proeutectoid ferrite is not preferred. If a part of the structure is pro-eutectoid ferrite, the hardness of the steel becomes non-uniform and the fatigue characteristics deteriorate. On the other hand, when the structure before nitriding is tempered martensite or tempered bainite obtained by quenching and tempering, the fatigue characteristics are improved.

(F) The structure of the crankshaft rough profile before nitriding should not be mainly composed of as-quenched martensite or bainite. In these structures, the amount of inherent strain increases due to supersaturated carbon and high dislocation density. Therefore, in these structures, alloy nitrides tend to precipitate finely. On the other hand, if tempering is performed after quenching to reduce the amount of inherent strain, the alloy nitride can be easily coarsened appropriately.

(G) The structure of the crankshaft rough profile before nitriding should not contain residual austenite. Residual austenite may expand due to transformation during nitriding. Since the retained austenite becomes more difficult to be stably transformed as the nitrogen concentration increases, the amount of expansion due to transformation of the retained austenite during nitriding is small in the nitrided layer and large in the core portion that is not affected by nitriding. This reduces the compressive residual stress of the nitride layer.

The present invention has been completed based on the above findings, and the gist of the present invention resides in a crankshaft rough profile, a nitrided crankshaft, and a nitrided crankshaft manufacturing method shown in the following (1) to (6). .

(1) Chemical composition in mass%, C: 0.35 to 0.70%, Si: 0.01 to 0.45%, Mn: 1.3 to 3.0%, P: 0.05% Hereinafter, S: 0.005 to 0.100%, Cr: 0.05 to 0.90%, Al: 0.001 to 0.080%, N: 0.003 to 0.025%, Ti: 0 to 0.05%, Nb: 0 to 0.05%, Mo: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Ca: 0 to 0.005%, The rest: Fe and impurities, the structure of 10 mm depth from the surface is volume%, the total of tempered martensite and tempered bainite is 50% or more, pro-eutectoid ferrite is 10% or less, pearlite is 40% or less, Crankshaft rough profile.

(2) The chemical composition contains one or two selected from the group consisting of Ti: 0.005 to 0.05% and Nb: 0.005 to 0.05% by mass%. The crankshaft rough profile according to (1) above.

(3) The chemical composition is, in mass%, Mo: 0.03-0.50%, Cu: 0.05-0.50%, and Ni: 0.05-0.50%. The crankshaft rough profile according to (1) or (2) above, containing one or more selected.

(4) The crankshaft rough profile according to any one of (1) to (3), wherein the chemical composition contains Ca: 0.0001 to 0.005% by mass.

(5) The dough has a chemical composition of mass%, C: 0.35 to 0.70%, Si: 0.01 to 0.45%, Mn: 1.3 to 3.0%, P: 0.00. 05% or less, S: 0.005 to 0.100%, Cr: 0.05 to 0.90%, Al: 0.001 to 0.080%, N: 0.003 to 0.025%, Ti: 0 to 0.05%, Nb: 0 to 0.05%, Mo: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Ca: 0 to 0.005 %, Balance: Fe and impurities, the structure 10 mm deep from the surface is volume%, the total of tempered martensite and tempered bainite is 50% or more, proeutectoid ferrite is 10% or less, and pearlite is 40% or less. Yes, the surface layer has a cured layer whose Vickers hardness is 50HV or more higher than the Vickers hardness of the fabric, and the thickness of the cured layer Is 200μm or more, nitrided crankshaft.

(6) A step of preparing the crankshaft rough profile according to any one of (1) to (4), a step of machining the coarse profile, and a nitriding atmosphere for the machined coarse profile And a nitriding treatment step of holding at 540 to 620 ° C. for 30 to 360 minutes.

According to the present invention, there are provided a crankshaft rough shape material having excellent fatigue characteristics and bending straightening, a nitrided crankshaft, and a method for manufacturing the same. Machines other than the nitrided crankshaft, such as automobiles, industrial machines, and construction machines It can also be applied when manufacturing parts.

FIG. 1 is a front view of an Ono rotary bending fatigue test piece. FIG. 2 is a front view of a four-point bending test piece. FIG. 3 is a diagram for explaining the four-point bending test.

Hereinafter, the crankshaft rough profile and the nitrided crankshaft according to an embodiment of the present invention will be described in detail. “%” Of the content of each element means “mass%”.

[Chemical composition]
The chemical composition of the crankshaft rough profile according to this embodiment contains the following elements.

C: 0.35 to 0.70%
Carbon (C) increases the hardness and fatigue strength of the steel material. Furthermore, C is present in the steel as cementite and has the effect of increasing the amount of nitrogen introduced during nitriding and increasing the residual stress. If the C content is too low, the above effect cannot be obtained. On the other hand, if the C content is too high, the volume fraction of cementite in the steel material becomes too high, and the machinability decreases. Therefore, the C content is 0.35 to 0.70%. The minimum with preferable C content is 0.40%, More preferably, it is 0.45%. The upper limit with preferable C content is 0.65%, More preferably, it is 0.60%, More preferably, it is 0.58%.

Si: 0.01 to 0.45%
Silicon (Si) dissolves and strengthens the steel (solid solution strengthening). If the Si content is too low, the above effect cannot be obtained. On the other hand, if the Si content is too high, the hardness of the surface layer becomes excessively high during nitriding. Therefore, the bend straightening property of the steel material is lowered. Furthermore, the diffusion of nitrogen during nitriding is hindered. Therefore, the Si content is 0.01 to 0.45%. The minimum with preferable Si content is 0.05%, More preferably, it is 0.10%, More preferably, it is 0.15%. The upper limit with preferable Si content is 0.40%, More preferably, it is 0.35%, More preferably, it is 0.30%.

Mn: 1.3 to 3.0%
Manganese (Mn) is the most important alloying element in the present invention. Mn combines with N introduced into the steel material by nitriding treatment to form a nitride, and introduces a large compressive residual stress while appropriately increasing the hardness of the surface layer. Further, Mn forms MnS in the steel material to enhance the machinability of the steel material. If the Mn content is too low, the above effect cannot be obtained. On the other hand, if the Mn content is too high, the variation in hardness increases due to segregation. Therefore, the Mn content is 1.3 to 3.0%. The minimum with preferable Mn content is 1.4%, More preferably, it is 1.5%, More preferably, it is 1.6%. The upper limit with preferable Mn content is 2.5%, More preferably, it is 2.2%, More preferably, it is 2.0%.

P: 0.05% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and causes grain boundary embrittlement cracking. Therefore, the P content is preferably as low as possible. The upper limit of the P content is 0.05%. A preferable upper limit of the P content is 0.02%.

S: 0.005 to 0.100%
Sulfur (S) combines with Mn in the steel material to form MnS and enhances the machinability of the steel material. If the S content is too low, the above effect cannot be obtained. On the other hand, if S content is too high, coarse MnS will be formed and the fatigue strength of steel materials will fall. Therefore, the S content is 0.005 to 0.100%. The minimum with preferable S content is 0.010%, More preferably, it is 0.020%, More preferably, it is 0.030%. The upper limit with preferable S content is 0.080%, More preferably, it is 0.070%, More preferably, it is 0.060%.

Cr: 0.05-0.90%
Chromium (Cr) combines with N introduced into the steel material by nitriding treatment to form CrN in the nitrided layer, strengthens the nitrided layer, and introduces compressive residual stress. If the Cr content is too low, the above effect cannot be obtained. On the other hand, if the Cr content is too high, the nitride layer is excessively cured, and the bending straightness deteriorates. Therefore, the Cr content is 0.05 to 0.90%. The minimum with preferable Cr content is 0.08%, More preferably, it is 0.12%, More preferably, it is 0.15%. The upper limit with preferable Cr content is 0.70%, More preferably, it is 0.50%, More preferably, it is 0.40%.

N: 0.003 to 0.025%
Nitrogen (N) is dissolved in the steel material to increase the strength of the steel material. If the N content is too low, the above effect cannot be obtained. On the other hand, if the N content is too high, bubbles are generated in the steel material. Since bubbles become defects, it is preferable to suppress the generation of bubbles. Therefore, the N content is 0.003 to 0.025%. A preferable lower limit of the N content is 0.005%. The upper limit with preferable N content is 0.020%, More preferably, it is 0.018%.

Al: 0.001 to 0.080%
Aluminum (Al) is an element that deoxidizes steel. On the other hand, if the Al content is too high, fine nitrides are formed, the steel is excessively hardened, and the bending straightness is deteriorated. Therefore, the Al content is 0.001 to 0.080%. The minimum with preferable Al content is 0.005%, More preferably, it is 0.010%. The upper limit with preferable Al content is 0.060%, More preferably, it is 0.050%, More preferably, it is 0.040%.

The balance of the chemical composition of the crankshaft rough profile according to the present embodiment is Fe and impurities. Here, the impurities are mixed from ore as a raw material, scrap, or production environment when industrially producing steel materials, and have an adverse effect on the crankshaft rough profile according to the present embodiment. It means what is allowed in the range.

[Arbitrary elements]
Among the arbitrary elements in the present embodiment, the group consisting of Ti and Nb has an effect of preventing coarsening of austenite crystal grains, and may contain one or two kinds.

Ti: 0 to 0.05%
Titanium (Ti) forms nitrides and carbonitrides, and suppresses coarsening of crystal grains during hot forging and quenching. However, if the Ti content is too high, TiC is generated and the hardness of the steel material varies greatly. Therefore, the Ti content is 0 to 0.05%. The preferable lower limit of the Ti content when Ti is contained is 0.005%, more preferably 0.010%. The upper limit with preferable Ti content is 0.04%, More preferably, it is 0.03%.

Nb: 0 to 0.05%
Niobium (Nb) forms nitrides and carbonitrides, and suppresses coarsening of crystal grains during hot forging and quenching. Nb further delays recrystallization during hot forging and quenching and tempering, and suppresses coarsening of crystal grains. However, if the Nb content is too high, carbides are generated and the hardness of the steel material varies greatly. Therefore, the Nb content is 0 to 0.05%. When Nb is contained, the preferable lower limit is 0.005%, more preferably 0.010%. The upper limit with preferable Nb content is 0.04%, More preferably, it is 0.03%.

Among the optional elements in this embodiment, the group consisting of Mo, Cu and Ni has an effect of increasing the strength of the crankshaft rough profile, and may contain one or more kinds.

Mo: 0 to 0.50%
When molybdenum (Mo) is contained, it increases the strength of the steel material by increasing the hardenability of the steel. As a result, the fatigue strength of the steel material is increased. However, if the Mo content is excessively increased, the effect is saturated and the cost of the steel material is increased. Therefore, the Mo content is 0 to 0.50%. The minimum with preferable Mo content is 0.03%, More preferably, it is 0.05%. The upper limit with preferable Mo content is 0.40%, More preferably, it is 0.30%, More preferably, it is 0.20%.

Cu: 0 to 0.50%
When copper (Cu) is contained, it dissolves to increase the strength of the steel material. Therefore, the fatigue strength of the steel material is increased. However, if the Cu content increases excessively, it segregates at the grain boundaries of the steel during hot forging and induces hot cracking. Therefore, the Cu content is 0 to 0.50%. The minimum with preferable Cu content is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Cu content is 0.30%, More preferably, it is 0.20%.

Ni: 0 to 0.50%
When nickel (Ni) is contained, it dissolves to increase the strength of the steel material. Therefore, the fatigue strength of the steel material is increased. Ni further suppresses hot cracking caused by Cu when the steel material contains Cu. However, if there is too much Ni content, the effect will be saturated and manufacturing cost will become high. Therefore, the Ni content is 0 to 0.50%. The minimum with preferable Ni content is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Ni content is 0.30%, More preferably, it is 0.20%.

The chemical composition of the crankshaft rough profile according to the present embodiment may further contain Ca as an optional element.

Ca: 0 to 0.005%
When calcium (Ca) is contained, it enhances the machinability of the steel material. However, if the Ca content is too high, coarse Ca oxide is generated, and the fatigue strength of the steel material is reduced. Therefore, the Ca content is 0 to 0.005%. The minimum with preferable Ca content for acquiring the said effect stably is 0.0001%, More preferably, it is 0.0003%. The upper limit with preferable Ca content is 0.003%, More preferably, it is 0.002%.

[Microstructure of crankshaft rough profile]
The crankshaft shaft rough profile is a steel material roughly formed into a crankshaft by hot forging. The rough shape of the crankshaft according to the present embodiment makes the hardness in the nitrided layer uniform after nitriding, and the surface layer portion affected by nitriding in order to appropriately coarsen the alloy nitride generated in the nitrided layer. It is necessary to make this structure into a structure mainly composed of tempered martensite and tempered bainite. More specifically, the total volume fraction of tempered martensite and tempered bainite in a structure 10 mm deep from the surface needs to be 50% or more. A preferred lower limit of the total volume fraction of tempered martensite and tempered bainite is 60%, more preferably 70%, and even more preferably 80%.

Proeutectoid ferrite is the cause of variation in the hardness of the nitride layer, so less is better. The crankshaft rough profile according to this embodiment needs to have a volume fraction of pro-eutectoid ferrite of 10% or less in a structure having a depth of 10 mm from the surface. A preferable upper limit of the volume fraction of pro-eutectoid ferrite is 8%, more preferably 6%.

Perlite, which is a mixed structure of ferrite and cementite, is a preferable structure for coarsening the alloy nitride, but the lower structural unit called a block constituting pearlite has the crystal orientation of ferrite aligned in a certain direction. For this reason, the fatigue characteristics may be deteriorated because they are easily coarsened as compared with blocks in tempered martensite and tempered bainite. Therefore, it is preferable that the crankshaft rough profile according to the present embodiment has a small pearlite volume fraction in a structure 10 mm deep from the surface. A preferable upper limit of the pearlite volume fraction is 40%, more preferably 30%, and further preferably 20%.

Since the crankshaft rough profile according to the present embodiment is a steel material that has been quenched and tempered, there may be unavoidable residual austenite, but residual austenite is not preferable because it deforms parts during nitriding. Therefore, the volume fraction of retained austenite in the structure 10 mm deep from the surface contained in the steel is preferably 5% or less, more preferably 0%.

[Microstructure of nitrided crankshaft]
The nitrided crankshaft according to the present embodiment is obtained by nitriding after machining the above-described crankshaft rough profile. The nitrided crankshaft has a hardened layer formed by nitriding as a surface layer using the steel material having the above-described chemical composition and microstructure as a base material. The dough is a portion of the nitriding crankshaft that is not affected by nitrogen introduced from the surface by nitriding, that is, a portion having a constant nitrogen concentration. Specifically, the nitrided crankshaft has a hardened layer with a Vickers hardness of 50 HV or more higher than the hardness of the dough on the surface, and the thickness of the hardened layer is 200 μm or more.

As will be described later, the nitriding treatment is performed at 540 to 620 ° C. In this temperature range, the steel structure is not affected. Therefore, the nitrided crankshaft has substantially the same structure as that of the crankshaft rough profile except for the surface layer portion where nitrogen is diffused at a high concentration.

[Production method]
An example of a method for manufacturing a crankshaft rough profile, a nitrided crankshaft, and a nitrided crankshaft according to the present embodiment will be described.

The method for manufacturing a nitrided crankshaft according to the present embodiment includes a steel material preparation process, a crankshaft forming process, a quenching and tempering process, a machining process, and a nitriding process. Hereinafter, each process is demonstrated.

[Steel material preparation process]
A molten steel that satisfies the chemical composition described above is manufactured. The produced molten steel is used to make a slab (slab, bloom) by a general continuous casting method. Alternatively, the molten steel is used to make an ingot by the ingot-making method. A billet is manufactured by hot working a slab or an ingot. The hot working may be hot rolling or hot forging. Further, the billet is heated, rolled, and cooled under general conditions to produce a steel bar, which is used as the material for the crankshaft.

[Crankshaft molding process]
The manufactured steel bar is formed into a crankshaft rough shape by hot forging to produce an intermediate crankshaft profile. If the heating temperature for hot forging is too low, an excessive load is applied to the forging device. On the other hand, if the heating temperature is too high, the scale loss is large. Therefore, the preferred heating temperature is 1000 to 1300 ° C.

The preferred finishing temperature for hot forging is 900 ° C. or higher. This is because if the finishing temperature is too low, the burden on the mold increases. On the other hand, the preferable upper limit of the finishing temperature is 1250 ° C.

[Quenching and tempering]
The intermediate product after hot forging is quenched and tempered to produce a crankshaft rough profile. At this time, the quenching temperature is at least 10 ° C. lower than the A ₃ point represented by the formula (1), that is, (A ₃ −10) ° C. or more. Further, the tempering temperature is 550 ° C. or higher, and (2) is not more than A ₁ point of the formula. The tempering time is preferably 30 minutes or longer.
A ₃ = 910-203C + 44.7Si-30Mn-11Cr (1)
A ₁ = 723-10.7Mn + 29.1Si-16.9Ni + 16.9Cr (2)

In order to make the pro-eutectoid ferrite 10% or less, the quenching temperature needs to be (A ₃ -10) ° C. or more. In order to make the structure immediately before quenching an austenite single phase, it is preferable that the quenching temperature be A ₃ point or higher. The quenching temperature is preferably 1000 ° C. or less. The quenching temperature is more preferably 950 ° C. or lower, and further preferably 900 ° C. or lower.

By quenching, the structure of the crankshaft rough profile is mainly composed of martensite and bainite. Martensite and bainite are in a carbon supersaturated state. In order to precipitate the supersaturated carbon of martensite and bainite by tempering treatment, and to coarsen the alloy nitride appropriately in the subsequent nitriding treatment, it is preferable to temper at a temperature of 550 ° C. or higher. The tempering temperature is more preferably 600 ° C. or higher, and further preferably 620 ° C. or higher. On the other hand, in order to suppress reverse transformation during tempering, the tempering temperature needs to be A ₁ point or less.

[Machining]
The aforementioned crankshaft rough profile is machined into a predetermined crankshaft shape.

[Nitriding treatment]
Nitriding is performed on the machined crankshaft rough profile. In this embodiment, a well-known nitriding process is employed. The nitriding treatment is, for example, gas nitriding, salt bath nitriding, ion nitriding or the like. The gas introduced into the furnace during nitriding may be NH ₃ alone or an air-fuel mixture containing NH ₃ and N ₂ and / or H ₂ . In addition, a carburizing gas may be contained in these gases to perform soft nitriding. Therefore, “nitriding” in this specification includes “soft nitriding”.

When performing the gas soft nitriding treatment, for example, in an atmosphere in which endothermic modified gas (RX gas) and ammonia gas are mixed in a 1: 1 ratio, the soaking temperature is set to 540 to 620 ° C. and the soaking is performed for 30 to 360 minutes. do it.

The nitrided crankshaft manufactured by the above manufacturing process has excellent fatigue strength and excellent bending straightness.

A 150 kg ingot of steels A and I having the chemical composition shown in Table 1 was manufactured using a vacuum melting furnace. Also, 50 kg ingots of steels B to H and J to R were manufactured using a vacuum melting furnace. “-” In the table indicates that the element is not added.

In the “A1” and “A3” columns in Table 1, A ₁ point (° C.) defined by Formula (2) and A ₃ point (° C.) defined by Formula (1) are described, respectively. Yes.

Each ingot was heated to 1250 ° C. The heated ingot was hot forged to produce a steel bar having a diameter of 55 mm. Using bar steel as a raw material, heating at 1200 ° C. and air cooling were performed to simulate the hot forging process of the crankshaft rough profile. The round bar that was allowed to cool was subjected to heat treatment under the conditions described in the first heat treatment column in Table 2, and after cooling to near room temperature, the heat treatment was performed under the conditions described in the second heat treatment column. . In the case where the first stage heat treatment column is still hot forged, the material after hot forging was not subjected to heat treatment, and a test piece was processed from the raw material as it was and subjected to nitriding treatment.

[Evaluation test]
The following test was implemented using the round bar of each test number.

[Hardness measurement and tissue volume ratio measurement]
From the R / 2 position of the round bar after the two-stage heat treatment of Test Nos. 1 to 18 and 21 to 24, samples having a cross section as the test surface were collected. Further, samples having a cross section as a test surface were taken from the R / 2 position of the round bars after hot forging of test numbers 19 and 20. The position of R / 2 is 13.75 mm deep from the surface of a round bar having a diameter of 55 mm. Vickers hardness (HV) based on JIS Z 2244 was measured at an arbitrary 7 points of the collected samples. The test force was 9.8N. The average value of the obtained seven Vickers hardnesses was defined as the hardness before nitriding of each test number. When the hardness was 300 HV or less, it was judged that the machinability was excellent.

The sample after the hardness measurement was corroded with the nital to reveal the structure. Thereafter, an optical micrograph at a magnification of 1000 was taken, and the fraction of each tissue was determined from image analysis. The structure having a depth of 13.75 mm from the surface is a shallower position when the total of tempered martensite and tempered bainite is 50% or more, pro-eutectoid ferrite is 10% or less, and pearlite is 40% or less. The structure having a depth of 10 mm was determined that the total of tempered martensite and tempered bainite was 50% or more, pro-eutectoid ferrite was 10% or less, and pearlite was 40% or less.

[Preparation of Ono type rotating bending fatigue test piece and 4-point bending test piece]
A plurality of Ono-type rotary bending fatigue test pieces shown in FIG. 1 were collected from the R / 2 position of the round bar of each test number along the longitudinal direction of the round bar. The length L1 in the figure was 80 mm, and the diameter D1 was 12 mm. The radius of curvature R1 of the notch at the center of the test piece was 3 mm, and the diameter of the cross section of the test piece at the notch bottom was 8 mm.

Furthermore, from the R / 2 position of the round bar of each test number, a 4-point bending test piece shown in FIG. 2 was collected along the longitudinal direction of the round bar. The length L2 of the 4-point bending test piece was 180 mm, and the diameter D2 was 15 mm. The radius of curvature R2 of the notch at the center of the test piece was 4 mm, and the diameter of the cross section of the test piece at the notch bottom was 12 mm.

The nitriding treatment was performed on the collected Ono-type rotating bending fatigue test piece and the four-point bending test piece under the conditions (heat treatment temperature and soaking time) shown in Table 2. Specifically, the test pieces are inserted into a heat treatment furnace, and the flow rate of ammonia gas and RX gas is 1: 1 while the temperature inside the furnace is raised to the heat treatment temperature (° C.) in the column “Nitriding conditions” in Table 2. In this way, it was introduced into the furnace. Then, nitriding was performed at the heat treatment temperature (° C.) and the holding time (h) shown in the “nitriding conditions” column of Table 2. After the holding time had elapsed, the test piece was taken out of the heat treatment furnace and quenched with 100 ° C. oil.

[Hardened layer depth measurement]
A sample having a cross section as a test surface was taken from the parallel part of the four-point bending test piece subjected to the above nitriding treatment. The Vickers hardness at the center of the collected sample was measured with a measurement number of 7 and a test force of 9.8 N. The Vickers hardness distribution of the sample nitride layer was measured. The measurement range was from the outermost surface to a depth of 1.20 mm, and measurement was performed at a pitch of 50 μm. The measurement at each depth position was performed at 3 measurement points and the test force was 2.9N. As a result of measurement, the Vickers hardness with a depth of 1.20 mm from the surface and the Vickers hardness at the center were not changed. Therefore, the Vickers hardness at the center is defined as the hardness of the fabric, and the region from the surface that is 50 HV or higher than the hardness at the center is defined as the cured layer, and the cured layer depth of each test number was calculated. In addition, the hardness distribution between each measurement depth position approximated with the straight line which connected the measured value of both sides, and calculated the hardened layer depth.

[Ono type rotating bending fatigue test]
Using the Ono rotary bending fatigue test piece subjected to the above nitriding treatment, an Ono rotary bending fatigue test was performed. A rotating bending fatigue test in accordance with JIS Z2274 (1978) was performed in an air atmosphere at room temperature (25 ° C.). The test was carried out under a double swing condition with a rotational speed of 3000 rpm. Of the test pieces that did not break until the number of repetitions of 1.0 × 10 ⁷ times, the highest stress was defined as the fatigue strength (MPa) of that test number. When the fatigue strength was 570 MPa or more, it was judged that the fatigue strength was excellent.

[4-point bending test]
A four-point bending test was performed in the atmosphere at room temperature using the above-mentioned nitriding-treated four-point bending test piece. The arrangement of the 4-point bending test is schematically shown in FIG. The distances d1 to d5 in FIG. 3 are 21.5 mm, 51.0 mm, 17.5 mm, 90 mm, and 180 mm, respectively. The fulcrum during the test was set at a position of 21.5 mm from each end of the test piece in the axial direction, and at a position of 72.5 mm. In order to measure the amount of strain at the notch bottom of the test piece, a strain gauge was attached to the center of the notch bottom in parallel with the axial direction of the test piece. For two fulcrums near the center in the axial direction, a pushing stroke of a pushing speed of 0.5 mm / min was given. As the indentation stroke increases, the amount of strain at the notch bottom increases, and when a harmful crack occurs in the notch bottom, the increment of strain with respect to the indentation stroke increases abruptly. Therefore, if the indentation stroke is increased at the above indentation speed and the increment of the strain gauge value when the indentation stroke increases by 0.01 mm becomes 2400 με or more, it is assumed that a crack has occurred in the test piece. Was defined as the correctable strain (με). When the correctable strain amount was 12,500 με or more, it was evaluated that the bending straightness was excellent.

[Test results]
Table 2 shows the test results. “Structure fraction” in Table 2 means the fraction (volume%) of each structure constituting the steel. “Tempered structure” means the sum of the fraction of tempered martensite and the fraction of tempered bainite. “Fatigue strength” means the fatigue strength (MPa) obtained in the Ono rotary bending test.

Referring to Table 2, in test numbers 1 to 15, the chemical composition and steel microstructure are within the scope of the present invention. Those having these test numbers have a fatigue strength of 570 MPa or more and a strain amount of 13659 με or more, indicating that both the fatigue strength and the bending straightness are compatible. On the other hand, in the case of test numbers 16 to 24 that are out of the definition of the present invention, the target performance is not obtained.

Although the chemical compositions of Test Nos. 16 to 18 were within the scope of the present invention, the steel microstructure was outside the scope of the present invention. Therefore, the fatigue strength is as low as 535 MPa or less.

Although the chemical compositions of Test Nos. 19 and 20 were within the scope of the present invention, the steel microstructure was out of the scope of the present invention because it was hot forged and no heat treatment was performed. Therefore, the correctable strain amount is 9001 με or less, and the bending straightness is inferior.

Although the steel structure of test number 21 was within the scope of the present invention, the C content was lower than the scope of the present invention, so the fatigue strength was lower than the target of 560 MPa, the correctable strain amount was 10182 με, and the bending straightness was poor. .

Although the steel structure of the test number 22 was within the scope of the present invention, the amount of C is higher than the scope of the present invention, so the hardness before nitriding is as high as 302 Hv and the machinability is poor.

Although the steel structure of the test number 23 was within the scope of the present invention, the fatigue strength was as low as 515 MPa because the amount of Mn was lower than the scope of the present invention.

Although the steel structure of test number 24 was within the range of the present invention, the amount of Cr was higher than the range of the present invention, so the strain amount was as low as 7512 με and the bending straightness was poor.

The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims

A crankshaft rough profile for a nitriding crankshaft, the chemical composition of which is mass%,
C: 0.35 to 0.70%,
Si: 0.01 to 0.45%,
Mn: 1.3 to 3.0%,
P: 0.05% or less,
S: 0.005 to 0.100%,
Cr: 0.05-0.90%
Al: 0.001 to 0.080%,
N: 0.003 to 0.025%,
Ti: 0 to 0.05%,
Nb: 0 to 0.05%,
Mo: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Ca: 0 to 0.005%,
Balance: Fe and impurities,
A crankshaft rough profile in which the structure 10 mm deep from the surface is volume%, the total of tempered martensite and tempered bainite is 50% or more, pro-eutectoid ferrite is 10% or less, and pearlite is 40% or less.
The chemical composition is mass%,
Ti: 0.005 to 0.05%, and Nb: 0.005 to 0.05%,
The crankshaft rough profile according to claim 1, comprising one or two selected from the group consisting of:
The chemical composition is mass%,
Mo: 0.03-0.50%,
Cu: 0.05 to 0.50%, and Ni: 0.05 to 0.50%,
The crankshaft rough profile according to claim 1 or 2, comprising one or more selected from the group consisting of:
The chemical composition is mass%,
Ca: 0.0001 to 0.005%,
The crankshaft rough profile according to any one of claims 1 to 3, comprising:
The chemical composition of the dough is
C: 0.35 to 0.70%,
Si: 0.01 to 0.45%,
Mn: 1.3 to 3.0%,
P: 0.05% or less,
S: 0.005 to 0.100%,
Cr: 0.05-0.90%
Al: 0.001 to 0.080%,
N: 0.003 to 0.025%,
Ti: 0 to 0.05%,
Nb: 0 to 0.05%,
Mo: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Ca: 0 to 0.005%,
Balance: Fe and impurities,
The structure of 10 mm depth from the surface is volume%, the total of tempered martensite and tempered bainite is 50% or more, pro-eutectoid ferrite is 10% or less, pearlite is 40% or less
The surface layer has a cured layer whose Vickers hardness is 50HV or more higher than the Vickers hardness of the fabric,
A nitrided crankshaft, wherein the thickness of the hardened layer is 200 μm or more.
Preparing the crankshaft rough profile according to any one of claims 1 to 4,
Machining the rough profile;
And a nitriding treatment step of holding the machined rough profile at 540 to 620 ° C. for 30 to 360 minutes in a nitriding atmosphere.