JP5131770B2 - Non-tempered steel for soft nitriding - Google Patents

Description

  The present invention relates to a non-tempered steel for soft nitriding, and more specifically, an automobile having high fatigue strength and excellent bend straightening even if soft nitriding treatment is performed without performing tempering treatment (quenching-tempering treatment), The present invention relates to a low-cost non-refined steel for soft nitriding with a low V content, which is a material for soft nitriding machine parts such as crankshafts for industrial machinery and construction machinery.

  Conventionally, crankshafts for automobiles, industrial machinery and construction machinery, etc. are hot-worked into a desired shape by a method such as hot forging, then tempered to refine the structure, It has been manufactured by applying a soft nitriding treatment mainly for the purpose of increasing fatigue strength.

  However, from the viewpoint of cost reduction and energy saving, it is desired to omit the tempering process, and in recent years, the demand is particularly strong.

  In general, if the tempering treatment is omitted, the crystal grain size is not adjusted, so that the crystal structure developed in the cooling process during hot working and after hot working is passed on to the final product as it is. For example, a normal crankshaft is finished by hot forging several times after holding the material in an austenite single phase region of about 1250 ° C. Generally, the forging end temperature exceeds 1000 ° C. In this case, the austenite grain size becomes coarse due to the retention in the austenite single-phase region, and as a result, the ferrite nucleation sites are reduced during the transformation to ferrite during cooling, which is brought to a low temperature. Austenite increases. And if the above-mentioned austenite undergoes eutectoid transformation, it becomes pearlite, but since the prior austenite grain size is large, the so-called “pearlite grain size” when the pearlite colony group surrounded by ferrite is made “pearlite grain” becomes large. End up. For this reason, it is often impossible to ensure high fatigue strength even by soft nitriding.

  When soft nitriding is performed, distortion occurs and the dimensional accuracy of the component decreases. Therefore, especially in parts such as a crankshaft, bending straightening is often performed after soft nitriding, and therefore, the steel for soft nitriding is also required to have excellent bending straightening after soft nitriding. However, the bending straightness when the tempering treatment is omitted is extremely inferior.

  For this reason, there is a great demand for non-tempered steel for soft nitriding having high fatigue strength and excellent bend straightening properties even if the tempering treatment is omitted.

  Here, the “excellent bend straightening property” indicates that the surface of the part does not crack until a large bending displacement amount, or that the crack length is sufficiently short. Specifically, in a three-point bending correction test with a distance between support points of 200 mm using a test piece having a diameter of 20 mm and a length of 300 mm, which will be described later, a stroke amount that does not crack (does not result in gauge out) is 3.0 mm or more. Refers to that.

  The “soft nitriding treatment” is generally a technique in which N and C simultaneously enter and diffuse in a temperature range of 500 to 600 ° C. to harden the surface. In contrast to “nitriding” which mainly aims to improve wear resistance, “soft nitriding” is particularly excellent as a technique for improving fatigue strength, and is rapidly spreading.

  In order to meet the above-described demands, various soft nitriding steels are proposed in Patent Documents 1 to 3. Patent Documents 4 and 5 propose non-tempered steel for nitriding.

  Specifically, in Patent Document 1, in terms of weight percent, C: 0.20 to 0.60%, Si: 0.05 to 1.0%, Mn: 0.3 to 1.0%, P: 0 0.05% or less, S: 0.005 to 0.10%, Cr: 0.3% or less, Al: 0.08% or less, Ti: 0.03% or less, N: 0.008 to 0.020% , Ca: 0.005% or less, Pb: 0.30% or less, Cu: 0.30% or less, Ni: 0.30% or less, Mo: 0.30% or less, V: 0.20% or less, Nb : A specific element such as 0.05% or less, and the value of the formula represented by [221C (%) + 99.5Mn (%) + 52.5Cr (%) − 304Ti (%) + 577N (%) + 25] is 150 The remainder is the chemical composition of Fe and inevitable impurities, and the structure is composed of ferrite and pearlite and the ferrite Wherein the fraction is 10% or more "soft nitriding steel materials" is disclosed.

  Patent Document 2 discloses “non-tempered steel for soft nitriding” in which the content of specific elements such as C, Si, Mn, Cr, Mo, acid-soluble Al and N is optimized.

  Further, Patent Document 3 proposed by the present inventors includes C: 0.30 to 0.50%, Si: 0.05 to 0.5%, Mn: 0.2 to 0.8%, P: 0.005 to 0.05%, S: 0.005 to 0.1%, V: more than 0.2% and 0.3% or less and N: 0.005 to 0.030%, the balance Is made of Fe and impurities, and Cr in the impurities is less than 0.10%, “non-tempered steel for soft nitriding” is disclosed.

  In Patent Document 4, in mass%, C: 0.15 to 0.40%, Si ≦ 0.50%, Mn: 0.20 to 1.50%, Cr: 0.05 to 0.50%, etc. It contains a specific element, consists of the remainder Fe and inevitable impurities, and the structure after hot working is substantially a ferrite-pearlite structure, that is, a mixed structure of “ferrite” and “pearlite”, and the ferrite area ratio is 30%. “Nitride steel” is disclosed in which the ferrite grain size number is 5 or more and the pearlite average dimension is 50 μm or less.

In Patent Document 5, the mass% is C: 0.30 to 0.50%, Cr: 0.1 to 1.5%, V: 0.09 to 0.25%, and the atomic% ratio is Mn / S. : Specified elements such as 0.6 to 1.4, the balance Fe and inevitable impurities, tempering after hot working is omitted, and 5000 sulfide inclusions mainly composed of fine MnS "High-strength non-tempered steel for nitriding" containing at least / mm ² is disclosed.

Japanese Patent Laid-Open No. 9-324241 JP 2000-309846 A JP 2007-2292 A Japanese Patent Laid-Open No. 9-291339 JP 2005-113163 A

  According to the technique disclosed in Patent Document 1 described above, a steel material for nitrocarburizing excellent in tensile strength, fatigue strength and bending characteristics can be provided by optimizing the steel components and setting the ferrite fraction to 10% or more. The fatigue strength disclosed in this technique was 381.8 to 440.9 MPa.

  According to the technique disclosed in Patent Document 2, “non-refining for soft nitriding has excellent bending straightening property that can be corrected after soft nitriding treatment and exhibits excellent fatigue strength by soft nitriding treatment. Although “steel” can be provided, the bending straightness is only evaluated by a three-point bending test using a plate having a cross-sectional area of 5 mm × 10 mm and a distance between supporting points of 100 mm. For this reason, it cannot necessarily be said that sufficient bending correction property is obtained.

  The non-tempered steel for soft nitriding proposed by the present inventors in Patent Document 3 has high fatigue strength and good bend straightening properties. However, the increase in alloy cost is significant, and V is contained in mass%, exceeding 0.2% and not more than 0.3%. For this reason, the demand of the industry for reducing the V content as much as possible to suppress the alloy cost cannot be sufficiently met.

  According to the technique disclosed in Patent Document 4, a nitrided steel that has improved fatigue characteristics and bending characteristics by optimizing the contents of C, Mn, and Cr to increase the ferrite area ratio and miniaturizing is provided. Although it can be provided, the bending straightness (bending characteristics) is only evaluated by a three-point bending test with a distance between supporting points of 100 mm using a cylindrical test piece having a diameter of 10 mm, and cracking occurs. The maximum displacement amount up to 598 μm. For this reason, it cannot necessarily be said that it has sufficient bending correction property.

  According to the technique disclosed in Patent Document 5, the Mn / S ratio is regulated and the amount of fine sulfide inclusions in the steel is controlled to suppress the crack depth during correction. However, the steel proposed in Patent Document 5 is a nitriding steel and has not been studied for nitrocarburizing treatment or fatigue strength.

  The present invention has been made in view of the above-described situation. Even when soft nitriding is performed without tempering, low fatigue soft nitriding with high fatigue strength and excellent bend straightening and low V content is achieved. Non-heat treated steel, specifically, a specimen having a fatigue strength of 460 MPa or more and a diameter of 20 mm × length of 300 mm when an Ono-type rotary bending fatigue test is performed at room temperature using a specimen having the shape shown in FIG. In a three-point bending straightening test using a fulcrum distance of 200 mm, the crack is not cracked (it does not become gauge-out) and the stroke amount is 3.0 mm or more. The purpose is to provide tempered steel.

  Here, “becomes gauge out” means that the strain gauge peels off or exceeds the measurable range when a crack enters a position where the strain gauge is attached to the test piece. When a crack is entered, the gauge is always physically gauged out. Therefore, the occurrence of gauge-out is synonymous with the occurrence of a crack in the test piece.

  In order to solve the above-mentioned problems, the present inventors made various soft nitriding steels and investigated fatigue strength and bend straightening after soft nitriding, and also performed detailed research on the microstructure, The effects on fatigue strength and bend straightening were investigated. As a result, the following basic findings (a) and (b) were obtained.

  (A) Since the non-tempered steel has a “ferrite / pearlite structure” containing soft ferrite, the strength of the parent phase is low and high fatigue strength cannot be obtained. Moreover, since it is a coarse “ferrite / pearlite structure”, good bending straightness cannot be obtained.

  (B) After soft nitriding, deterioration of bending straightness also occurs when the ferrite ground is excessively strengthened and the hardness of the surface layer portion is too high.

  Based on the above findings, we conducted detailed studies on the effects of various alloy elements on fatigue strength and bend straightening. As a result, important findings shown in the following (c) to (h) were obtained.

  (C) When V is 0.05% or more by mass%, V carbide is finely precipitated in the cooling process after hot working, and the hardness of the base material (dough) is enhanced by the precipitation strengthening action of this V carbide. Increases, and high fatigue strength can be secured.

  (D) On the other hand, V is bent in order to excessively increase the hardness of the surface layer portion by forming fine nitride on the surface layer portion during soft nitriding like Al, Cr, Mn and the like having high nitride forming ability. It has been thought to harm the correctiveness.

  (E) However, if the Cr content in the impurities is limited to less than 0.10% and the Al content is limited to 0.005% or less, V excessively strengthens the ferrite ground by soft nitriding. And has the effect of appropriately increasing the hardness of the surface layer portion. For this reason, even if 0.05% by mass or more of V is contained, the bending straightness does not significantly decrease.

  (F) Furthermore, in the case of containing Ti with a content lower by about one order with respect to V, a composite carbonitride of V and Ti with the same atomic ratio of V and Ti (hereinafter, “ (V + Ti) carbonitride ”) is formed in the cooling process after solidification of the steel. This (V + Ti) carbonitride does not dissolve even when heated in the temperature range exceeding 1000 ° C. during hot working, not only pinning austenite grains to refine the structure, but also from the Baker-Nutting relationship Since it can be a competent ferrite nucleation site, the ferrite formation site can be increased, and the “ferrite / pearlite structure” of the resulting base material can be further refined, thereby reducing the bending straightness. Further suppression is possible. In order to obtain the above effect, it is necessary to contain Ti in an amount of 0.005% or more in combination with V in mass%. However, in some cases, it is not possible to achieve both excellent fatigue strength and bend straightening property by simply adding 0.05% by mass or more of V and 0.005% by mass or more of Ti in combination.

(G) When mass% contains 0.05% or more of V and 0.005% or more of Ti, (V + Ti) carbonitride as described in (f) above makes the “ferrite / pearlite structure” fine. In order to obtain the effect of achieving the above and ensure high bending straightness, the value of fn1 expressed by the following equation (1) needs to satisfy “0 ≦ fn1”. That is, in the case of “fn1 <0”, N is insufficient and a sufficient amount of (V + Ti) carbonitride is not generated, so that “ferrite / pearlite structure” cannot be made fine, and good bending is achieved. Correctiveness cannot be obtained.
fn1 = N−0.63 × Ti (1)
Here, the element symbol in the formula (1) represents the content in mass% of the element.

  (H) On the other hand, in order to ensure high fatigue strength by the precipitation strengthening action by the V carbide of (c), the value of fn1 represented by the above formula (1) satisfies “fn1 ≦ 0.020”. It is necessary to satisfy. That is, in the case of “0.020 <fn1”, the amount of N becomes excessive, so that not only (V + Ti) carbonitride but also V nitride is generated in the cooling process after solidification of the steel. Since this V nitride does not form a solid solution even when heated at a temperature exceeding 1000 ° C. during hot working, the amount of V contributing to the precipitation of fine V carbide having a precipitation strengthening action is insufficient. The precipitation amount of V carbide that finely precipitates during the cooling process decreases. For this reason, sufficient hardness of the base material (fabric) is not ensured, and the fatigue strength decreases.

  The present inventors further examined in detail, in terms of microstructure and hardness profile, the effects of other elements on bend straightening and fatigue strength when 0.05% V is contained in mass%. . As a result, the following important findings (i) and (j) were obtained.

  (I) When V of 0.05% or more is contained in mass%, even if a small amount of Al and Cr, the hardness of the surface layer is increased and the bending straightness is remarkably lowered. . For this reason, the Al content must be limited to 0.005% or less, and the Cr content must be limited to less than 0.10%.

  (J) When 0.05% or more of V is contained by mass%, if Pb exceeding 0.02% is contained, a soft part is formed in the base material (fabric). Since fatigue failure occurs starting from the soft part, the fatigue strength decreases.

  This invention is completed based on said knowledge, The summary exists in the non-tempered steel for soft nitriding shown to following (1)-(3).

(1) By mass%, C: 0.30-0.60%, Si: 0.05-0.80%, Mn: 0.20% or more and less than 0.80%, P: 0.050% or less, S: 0.005 to 0.10%, V: 0.05 to 0.20%, Ti: 0.005 to 0.030% and N: 0.007 to 0.030%, the balance being Fe And Cr, Al and Pb in the impurities are Cr: less than 0.10%, Al: 0.005% or less and Pb: 0.02% or less, respectively, and represented by the following formula (1) The non-tempered steel for soft nitriding characterized in that the value of fn1 satisfies 0 ≦ fn1 ≦ 0.020.
fn1 = N−0.63 × Ti (1)
Here, the element symbol in the formula (1) represents the content in mass% of the element.

  (2) The non-tempered steel for soft nitriding according to the above (1), which contains Mo: 0.30% or less instead of part of Fe.

  (3) The non-tempered steel for soft nitriding as described in (1) or (2) above, wherein Ca is contained in an amount of 0.0050% or less instead of part of Fe.

  The non-tempered steel for nitrocarburizing of the present invention has high fatigue strength and excellent bend straightening even when subjected to nitrocarburizing treatment without tempering treatment, so that it is used for automobiles, industrial machinery and construction machinery. It can be used as a material for soft nitriding machine parts such as crankshafts. Since the non-tempered steel for soft nitriding has a small V content, the alloy cost can be suppressed.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

C: 0.30 to 0.60%
C combines with V in the cooling process after hot working to form fine V carbide, and contributes to precipitation strengthening and has the effect of improving fatigue strength. C is an element effective for imparting wear resistance to mechanical parts such as a crankshaft. Further, C combines with V and Ti together with N in the cooling process after solidification to form (V + Ti) carbonitrides, and these (V + Ti) carbonitrides are austenite grains during heating during hot working. In addition to its pinning action, it becomes a ferrite nucleation site in the cooling process after hot working, so the "ferrite-pearlite structure" can be refined, and this structure refinement ensures high bending straightness. It is an element that also has an effect. In order to obtain these effects, a C content of 0.30% or more is necessary. However, when C is contained excessively, the amount of pearlite is increased and the bend straightening property is impaired. In particular, when the C content exceeds 0.60%, the bend straightening property is significantly deteriorated. Therefore, the content of C is set to 0.30 to 0.60%. The desirable lower limit of the C content is 0.35%, and the desirable upper limit is 0.55%.

Si: 0.05 to 0.80%
Si has a deoxidizing action and has a solid solution strengthening action by being dissolved in ferrite. However, if the content is less than 0.05%, the effect is poor. On the other hand, when Si is contained excessively, the bending straightening property is impaired, and particularly when the Si content exceeds 0.80%, the bending straightening property is significantly deteriorated. Therefore, in the present invention, 0.80% necessary and sufficient for solid solution strengthening of ferrite is set as the upper limit of the content, and the Si content is set to 0.05 to 0.80%. The desirable lower limit of the Si content is 0.10%, and the desirable upper limit is 0.60%.

Mn: 0.20% or more and less than 0.80% Mn is a solid solution strengthening element, and has the effect of increasing the base material hardness and improving the fatigue strength. In order to obtain this effect, a Mn content of 0.20% or more is necessary. However, when Mn is contained excessively, the bend straightening property is impaired. In particular, when the Mn content is 0.80% or more, the bend straightening property is significantly deteriorated. Therefore, the Mn content is set to 0.20% or more and less than 0.80%. The desirable lower limit of the Mn content is 0.30%, and the desirable upper limit is 0.75%.

P: 0.050% or less P is an impurity inevitably contained. Moreover, it also has an effect as a strengthening element. When obtaining such an effect, it is desirable to contain 0.005% or more. However, excessive P segregates at the grain boundary and promotes embrittlement cracking at the grain boundary. Particularly, when the content exceeds 0.050%, embrittlement cracking at the grain boundary becomes remarkable. Therefore, the content of P is set to 0.050% or less. A desirable upper limit of the P content is 0.045%.

S: 0.005-0.10%
S is an element effective for improving the machinability of steel, and in order to obtain this effect, it is necessary to contain 0.005% or more. However, when the content of S is too large, the hot workability and fatigue strength are reduced. In particular, when the content exceeds 0.10%, the hot workability and fatigue strength are significantly reduced. Therefore, the content of S is set to 0.005 to 0.10%. A desirable lower limit of the S content is 0.010%, and a desirable upper limit is 0.080%.

V: 0.05-0.20%
V is one of the most important elements in the present invention. That is, V combines with C in the cooling process after hot working and has the effect of ensuring high fatigue strength through precipitation strengthening action of V carbide. V also combines with C and N together with Ti in the cooling process after solidification to form (V + Ti) carbonitrides, and these (V + Ti) carbonitrides become austenite grains during heating during hot working. In addition to its pinning action, it becomes a ferrite nucleation site in the cooling process after hot working, so the structure ("ferrite pearlite structure") can be refined, and this microstructure refinement improves bending straightness. It is an element that also has an enhancing effect. In order to obtain such an effect, it is necessary to contain V in an amount of 0.05% or more. However, excessive addition of V leads to increased alloy costs. Therefore, in the present invention, the upper limit of the content is 0.20%, and the V content is 0.05 to 0.20%. The desirable lower limit of the V content is 0.08%.

Ti: 0.005-0.030%
Ti is one of the most important elements in the present invention. That is, Ti combines with C and N together with V in the cooling process after solidification to form (V + Ti) carbonitrides, and these (V + Ti) carbonitrides become austenite grains during heating during hot working. In addition to its pinning action, it becomes a ferrite nucleation site in the cooling process after hot working, so the "ferrite-pearlite structure" can be refined, and this structure refinement ensures high bend straightening. It is an element that has an effect. In order to obtain such an effect, a Ti content of 0.005% or more is necessary. On the other hand, when the Ti content increases, especially when it exceeds 0.030%, (V + Ti) carbonitride is coarsened and its effect is saturated, and it becomes a starting point of fatigue fracture as an inclusion, resulting in fatigue. May reduce strength. Therefore, the content of Ti is set to 0.005 to 0.030%. The desirable lower limit of the Ti content is 0.010%, and the desirable upper limit is 0.025%.

N: 0.007 to 0.030%
N combines with V and Ti together with C in the cooling process after solidification to form (V + Ti) carbonitride, and this (V + Ti) carbonitride is a pin of austenite grains during heating during hot working In addition to having a stopping action, it becomes a ferrite nucleation site in the cooling process after hot working, so that the “ferrite / pearlite structure” is refined and has an effect of improving the bending straightness. In order to acquire the said effect, N content of 0.007% or more is required. However, it is not only industrially difficult to make N more than 0.030% in content, but, for example, bubble defects may be generated in the ingot to damage the material. For this reason, the N content is set to 0.007 to 0.030%. A desirable lower limit of the N content is an amount exceeding 0.010%, and a desirable upper limit is 0.025%.

  In the present invention, the contents of Cr, Al and Pb in the impurities are further defined as follows.

Cr: Less than 0.10% In the case of the non-tempered steel for soft nitriding according to the present invention containing V of 0.05% or more, Cr causes a significant decrease in bending straightness even in a small amount, and in particular, its content is 0 When it is 10% or more, the bending straightness is significantly deteriorated. Therefore, the content of Cr in the impurities is set to less than 0.10%.

Al: 0.005% or less In the case of the non-tempered steel for soft nitriding according to the present invention containing V of 0.05% or more, Al causes a significant decrease in bending straightness even in a small amount. If it exceeds 0.005%, the bend straightening property is remarkably deteriorated. Therefore, the Al content in the impurities is set to 0.005% or less.

Pb: 0.02% or less In the case of the non-tempered steel for soft nitriding according to the present invention containing V of 0.05% or more, Pb forms a soft part in the base material (fabric) and reduces fatigue strength. In particular, when the content exceeds 0.02%, the fatigue strength is significantly reduced. Therefore, the content of Pb in the impurities is set to 0.02% or less.

  In the present invention, the value of fn1 represented by the above formula (1) relating to the contents of Ti and N, that is, the formula of [fn1 = N−0.63 × Ti] is further defined as follows. This will be described below.

0 ≦ fn1 ≦ 0.020
In the case of containing Ti with a content of about one order lower than V, a composite carbonitride of V and Ti in which the atomic ratio of V and Ti is substantially equal in the cooling process after solidification of the steel, that is, , (V + Ti) carbonitride is produced. This (V + Ti) carbonitride does not dissolve even when heated in the temperature range exceeding 1000 ° C. during hot working, and not only pinning austenite grains to refine the structure but also cooling after hot working In the process, the generation site of ferrite can be increased, and the “ferrite / pearlite structure” of the obtained base material can be further refined.

  In the non-tempered steel for soft nitriding of the present invention in which the V content is about one order higher than that of Ti, in order to obtain such (V + Ti) carbonitride, the amount of the same order with respect to the Ti content. N is required to form (V + Ti) carbonitride composed of V. (V + Ti) carbonitride is obtained by replacing part of Ti in Ti carbonitride with V. When the amount of V contained is maximum, the atomic ratio is V: Ti = 1: 1. Therefore, the amount of N necessary to make all the contained Ti into (V + Ti) carbonitride can be calculated as follows.

  When the atomic weights of V, Ti, N, and (V + Ti) are represented by M (V), M (Ti), M (N), and M (V + Ti), respectively, M (V) / M (Ti) = 50.94 Since /47.88≈1.1, M (V + Ti) is 2.1 × M (Ti). Similarly, M (N) / M (Ti) = 14.01 / 47.88≈0.3.

  Therefore, the amount of N required to make all of (2.1 × Ti) nitride in mass% is (2.1 × Ti) × 0.3, and the amount of N is at least (2. If there is more than 1 * Ti) * 0.3 = 0.63 * Ti, all Ti is sufficient to be (V + Ti) carbonitride.

  That is, if the value of fn1 represented by the formula (1) is smaller than 0, a sufficient amount of (V + Ti) carbonitride cannot be obtained, whereas if the value of fn1 is 0 or more, it is desirable. Since (V + Ti) carbonitride is obtained, it is possible to ensure high bending straightness.

  On the other hand, when the value of fn1 represented by the above formula (1) exceeds 0.020, the amount of N becomes excessive, so not only (V + Ti) carbonitride during the cooling process after solidification of steel. V nitride is produced. Since this V nitride does not form a solid solution even when heated at a temperature exceeding 1000 ° C. during hot working, the amount of V contributing to the precipitation of fine V carbide having a precipitation strengthening action is insufficient. The precipitation amount of V carbide that finely precipitates during the cooling process decreases. For this reason, sufficient hardness of the base material (fabric) is not ensured, and the fatigue strength decreases.

  For the above reasons, it was defined that the value of fn1 represented by the above formula (1) satisfies 0 ≦ fn1 ≦ 0.020.

  One of the non-tempered steels for soft nitriding of the present invention is composed of Fe and impurities in the balance in addition to the above elements. In addition, about Cu and Ni in impurities in the non-tempered steel for soft nitriding according to the present invention, Cu is acceptable within a range of 0.3% or less and Ni within a range of 0.2% or less.

  Another one of the non-tempered steels for soft nitriding of the present invention contains one or more elements selected from Mo and Ca in addition to the above elements. Hereinafter, the effect of these elements and the reason for limiting the content will be described.

Mo: 0.30% or less Mo has the effect of increasing the strength of ferrite as a solid solution strengthening element and thereby increasing the fatigue strength. Therefore, Mo may be contained in order to obtain such an effect. However, if the Mo content exceeds 0.30%, the cost increases. Therefore, the Mo content in the case of addition is set to 0.30% or less. Note that the Mo content is preferably 0.20% or less.

  On the other hand, in order to surely obtain the effect of improving the fatigue strength of Mo described above, the lower limit of the Mo content is preferably 0.01%, and more preferably 0.03%.

Ca: 0.0050% or less Ca has an effect of enhancing the machinability of steel. However, excessive addition of Ca causes a decrease in hot workability and fatigue strength. In particular, when the Ca content exceeds 0.0050%, the hot workability and fatigue strength are significantly reduced. Therefore, when Ca is added, the content of Ca is set to 0.0050% or less. The Ca content is preferably 0.0045% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the machinability of Ca, the lower limit of the Ca content is preferably 0.0005%, and more preferably 0.0008%.

  Soft nitriding machine parts such as crankshafts for automobiles, industrial machines and construction machines are cast slabs or ingots made of non-tempered steel for soft nitriding of the present invention, or slabs made from the slabs or ingots After hot working into a desired shape, it can be obtained by subjecting it to soft nitriding treatment without performing tempering treatment.

  In addition, it is not necessary to prescribe | regulate especially the hot processing conditions to the said desired shape. However, the heating temperature before hot working into a desired shape is set to 1100 to 1250 ° C., the working temperature is set to 900 ° C. or more at which V carbide does not precipitate during the working, and the cooling after the hot working is allowed to cool in the air. It is preferable that

  Further, the soft nitriding conditions do not need to be specified, and gas soft nitriding, salt bath soft nitriding, plasma soft nitriding, or the like may be used as appropriate. In any treatment, a compound layer having a thickness of approximately 20 μm and a diffusion layer immediately below the compound layer can be stably and uniformly formed on the surface. For example, in the case of gas soft nitriding, as is normally performed, the processing may be performed for about 3 hours in an atmosphere in which RX gas and ammonia gas are mixed at 1: 1 at a temperature of 570 ° C. The above “RX gas” is a kind of modified gas and is a trade name of gas.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels 1 to 20 having the chemical composition shown in Table 1 were melted in a vacuum furnace to produce a 180 kg steel ingot.

  Steels 1 to 10 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, the steels 11 to 20 are steels of comparative examples in which the content of any of the component elements or the value of fn1 represented by the above formula (1) deviates from the conditions defined in the present invention.

  The steel ingot thus obtained was heated to 1200 ° C., and then hot forged so that the temperature of the steel material did not fall below 1000 ° C. to obtain a round bar having a diameter of 60 mm. Cooling after hot forging was allowed to cool in the atmosphere.

  From the R / 2 part ("R" means radius) of each round bar of steel 1 to 20 with a diameter of 60 mm, an Ono-type rotating bending fatigue test piece shown in FIG. 1 and a bending of 20 mm in diameter and 300 mm in length Orthodontic specimens were collected.

  Next, each test piece was soft-nitrided by holding RX gas and ammonia gas in a 1: 1 mixture at a temperature of 570 ° C. for 3 hours, and then cooled in 100 ° C. oil.

  Using the test piece having the shape shown in FIG. 1 subjected to soft nitriding treatment, an Ono-type rotary bending fatigue test was performed in the atmosphere at room temperature, and the fatigue strength was measured.

The Ono-type rotating bending fatigue test was performed with the number of tests being 8 and the number of rotations being 3000 rpm, and the highest stress that did not break until the number of repetitions of 1.0 × 10 ⁷ times was defined as “fatigue strength”. When fatigue strength is obtained, the fatigue resistance is considered excellent.

  In addition, a bending straightness test was performed using a soft nitriding-treated test piece having a diameter of 20 mm and a length of 300 mm to investigate the bending straightness.

  The bending straightness test is performed by a three-point bending method with a fulcrum distance of 200 mm, a strain gauge is attached to the center of the test piece, and the stroke amount when the gauge is out is recorded. When the thickness is 0 mm or more, the bending straightness is considered good. When the stroke amount reached 3.3 mm, the test was terminated.

  As already explained, “being gauge-out” means that the strain gauge is peeled off due to a crack at the position where the strain gauge is attached to the test piece or exceeds the measurable range, that is, the test piece. Indicates that a crack has entered.

  Table 2 summarizes the stroke amounts as evaluation criteria for fatigue strength and bending straightness for each test steel.

From Table 2, in the case of test numbers 1 to 10 using the non-tempered steel for soft nitriding of steels 1 to 10 according to the present invention, the stroke when the bending straightening test is performed is 3.0 mm or more and good It is clear that it has excellent bend straightening properties. Moreover, it is clear that the fatigue strength is a value exceeding 460 MPa and has high fatigue strength.
On the other hand, in the case of the test numbers 11-20 using the steel of the comparative example which deviated from the conditions prescribed | regulated by this invention, either fatigue strength or bend straightening property is inferior.

  That is, in the case of test number 11, since the Si content of steel 11 exceeds 0.80%, the stroke does not reach 3.0 mm at 2.9 mm, and the bending straightness is poor.

  In the case of the test number 12, since the V content of the steel 12 is less than 0.05%, the fatigue strength is as low as 439 MPa and does not reach the target.

  In the case of the test number 13, since the Cr content of the steel 13 is 0.10% or more, the stroke is as small as 2.3 mm and the bending straightness is inferior.

  In the case of the test number 14, since the value of fn1 represented by the formula (1) of the steel 14 is less than 0, the stroke is as small as 2.2 mm and the bending straightness is inferior.

  In the case of test number 15, since the C content of steel 15 exceeds 0.60%, the stroke does not reach 3.0 mm at 2.9 mm, and the bending straightness is poor.

  In the case of test number 16, since the Al content of steel 16 is 0.015% and exceeds 0.005%, the stroke is as small as 2.1 mm and the bending straightness is inferior.

  In the case of test number 17, since the value of fn1 represented by the formula (1) of steel 17 exceeds 0.020, the fatigue strength is as low as 450 MPa and does not reach the target.

  In the case of test number 18, since the Ti content of steel 18 is less than 0.005%, the stroke is 2.8 mm and does not reach 3.0 mm, and the bending straightness is poor.

  In the case of the test number 19, since the Mn content of the steel 19 is 0.80% or more, the stroke is as small as 2.5 mm and the bending straightness is inferior.

  In the case of test number 20, since the Pb content of steel 20 exceeds 0.02%, the fatigue strength is as low as 395 MPa and the target has not been reached.

  The non-tempered steel for nitrocarburizing of the present invention has high fatigue strength and excellent bend straightening even when subjected to nitrocarburizing treatment without tempering treatment, so that it is used for automobiles, industrial machinery and construction machinery. It can be used as a material for soft nitriding machine parts such as crankshafts. Since the non-tempered steel for soft nitriding has a small V content, the alloy cost can be suppressed.

It is a figure which shows the shape of the Ono type | formula rotation bending fatigue test piece used in the Example.

Claims (3)

In mass%, C: 0.30 to 0.60%, Si: 0.05 to 0.80%, Mn: 0.20% or more and less than 0.80%, P: 0.050% or less, S: 0 0.005 to 0.10%, V: 0.05 to 0.20%, Ti: 0.005 to 0.030% and N: 0.007 to 0.030%, the balance being Fe and impurities Fn1 represented by the following formula (1): Cr, Al and Pb in the impurities are Cr: less than 0.10%, Al: 0.005% or less and Pb: 0.02% or less, respectively. The non-tempered steel for soft nitriding is characterized by satisfying 0 ≦ fn1 ≦ 0.020.
fn1 = N−0.63 × Ti (1)
Here, the element symbol in the formula (1) represents the content in mass% of the element.   The non-tempered steel for soft nitriding according to claim 1, characterized by containing Mo: 0.30% or less in place of a part of Fe.   The non-tempered steel for soft nitriding according to claim 1 or 2, characterized by containing Ca: 0.0050% or less in place of part of Fe.

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