JP5825199B2 - Method of manufacturing age-hardening steel and machine parts - Google Patents

Description

  The present invention relates to an age-hardening steel and a method for producing a machine part using the steel. More specifically, in the present invention, after being processed into a predetermined shape by hot forging and cutting, an age hardening treatment (hereinafter simply referred to as “aging treatment”) is performed, and the desired strength is obtained by the aging treatment. The present invention relates to steel for manufacturing machine parts such as automobiles, industrial machines, and construction machines to be secured, and a method of manufacturing such machine parts using the steel.

  High fatigue strength is required for machine parts such as automobiles, industrial machines, and construction machines from the viewpoints of higher engine output and lighter weight aimed at improving fuel efficiency. If the steel only has high fatigue strength, it can be easily achieved by increasing the hardness of the steel by using an alloy element and / or heat treatment. However, in general, the above machine parts are formed by hot forging and then finished into a predetermined product shape by cutting. For this reason, the steel used as the material for the machine parts must have a high fatigue strength and sufficient machinability at the same time.

  Therefore, in order to achieve both fatigue strength and machinability, it is possible to keep the hardness low in the molding stage where good machinability is required. Various techniques that can increase the hardness in the product stage are disclosed.

  For example, Patent Document 1 discloses a machine structural steel excellent in age hardening and a method for manufacturing a machine structural forged member using the steel.

  Specifically, in terms of mass%, C: 0.05 to 0.25%, Si: 0.10 to 1.00%, Mn: 0.30 to 0.60%, Cu: 1.00 to 2. 00%, Ni: 0.50 to 2.00%, Al: 0.10 to 0.70%, B: 0.0010 to 0.0030%, Ti: 0.015 to 0.050%, Further, S: 0.005 to 0.080%, Pb: 0.005 to 0.300%, Bi: 0.005 to 0.200%, Te: 0.003 to 0.020%, Ca: 0.00. "Mechanical structural steel excellent in age hardening" containing one or two or more selected from 0005 to 0.0050% and the balance being substantially Fe, and a member by hot forging using this steel And then cold working locally or locally, and at a temperature of 450 ° C. or higher and 550 ° C. or lower, 30 to 120 ° C. And performing aging treatment of holding between "manufacturing method of the plastic deformation resistance excellent mechanical structural forged member" it is disclosed.

  Patent Document 2 discloses the following age hardened steel.

That is, in mass%, C: 0.11 to 0.60%, Si: 0.03 to 3.0%, Mn: 0.01 to 2.5%, Mo: 0.3 to 4.0%, V: 0.05-0.5% and Cr: 0.1-3.0%, Al: 0.001-0.3%, N: 0.005-0.025 as necessary %, Nb: 0.5% or less, Ti: 0.5% or less, Zr: 0.5% or less, Cu: 1.0% or less, Ni: 1.0% or less, S: 0.01-0. 20%, Ca: 0.003 to 0.010%, Pb: 0.3% or less and Bi: 0.3% or less, and the balance is Fe and inevitable impurities. In between
4C + Mn + 0.7Cr + 0.6Mo−0.2V ≧ 2.5,
C ≧ Mo / 16 + V / 5.7,
V + 0.15Mo ≧ 0.4
Is satisfied, and after rolling, forging, or solution treatment, it is cooled at an average cooling rate of 0.05 to 10 ° C./sec between 800 ° C. and 300 ° C., and before aging treatment, An “age hardening steel” is disclosed, characterized in that the area ratio of the bainite structure is 50% or more and the hardness is 40 HRC or less, and the aging treatment makes the hardness 7 HRC or more higher than the hardness before the aging treatment. Has been.

  Patent Document 3 discloses the following bainite steel.

That is, in mass%, C: 0.14-0.35%, Si: 0.05-0.70%, Mn: 1.10-2.30%, S: 0.003-0.120%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.40%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.30%, and V: 0.0. Containing 0.5 to 0.45%, and optionally containing one or more selected from Ti: 0.001 to 0.100% and Ca: 0.0003 to 0.0100%, the balance Consists of Fe and inevitable impurities,
13 [C] +8 [Si] +10 [Mn] +3 [Cu] +3 [Ni] +22 [Mo] +11 [V] ≦ 30,
5 [C] + [Si] +2 [Mn] +3 [Cr] +2 [Mo] +4 [V] ≦ 7.3,
2.4 ≦ 0.3 [C] +1.1 [Mn] +0.2 [Cu] +0.2 [Ni] +1.2 [Cr] +1.1 [Mo] +0.2 [V] ≦ 3.1 ,
2.5 ≦ [C] + [Si] +4 [Mo] +9 [V],
[C] ≧ [Mo] / 16 + [V] / 3
“Bainitic steel” characterized by satisfying the above requirements is disclosed.

JP 2001-107176 A JP 2006-37177 A JP 2011-236451 A

  The machine structural steel disclosed in Patent Document 1 has good machinability before aging treatment, but in Examples 1 to 8 disclosed as specific examples, the hardness after aging is less than 325 HV. It is. In addition, in order to obtain high fatigue strength and high 0.2% proof stress from improved plastic deformation resistance, cold working must be performed before aging treatment.

  It is an essential requirement for the steel disclosed in Patent Document 2 to contain a large amount of Mo of 0.3 to 4.0% by mass. However, Mo is an expensive element. Therefore, it is not a technology that can meet the demand of the industry to reduce the material cost by reducing the Mo content as much as possible.

  The steel disclosed in Patent Document 3 has a hardness before the aging treatment (after hot forging) of 300 HV or less by adjusting the content of the alloy element so as to satisfy a specific parameter formula. However, many of the steels of Examples 1 to 21 disclosed as specific examples of bainite steel according to Patent Document 4 are specifically 15 steel types out of 21 steel types having a hardness of 290 HV or more. Yes, it can barely be cut, but it cannot be said that the hardness of mass-produced products is sufficiently low. Also, for those whose hardness before aging treatment (after hot forging) is less than 290 HV, the hardness after aging treatment is less than 325 HV, and a sufficiently high hardness is obtained for parts that require high fatigue strength. It cannot be said that it is done.

  Accordingly, an object of the present invention is to provide an age hardenable steel that satisfies the following <1> and <2>.

  <1> When forging into a predetermined shape by performing cutting after hot forging, even if there is a difference in the cooling rate after hot forging, hot forging that is the hardness before cutting The hardness afterwards is sufficiently low and excellent in machinability.

  <2> Even if there is little content of Mo, it can be hardened | cured by the aging treatment given after cutting, and a machine part can be made to have desired fatigue strength.

  Specifically, even if the object of the present invention contains Mo, the amount thereof is less than 0.30% by mass, and without hot quenching such as oil cooling after hot forging, for example, Hardness in a state cooled at an average cooling rate of 0.4 to 2 ° C./second such as air cooling or air cooling (that is, hardness before aging treatment) is 290 HV or less, and after aging treatment It is to provide an age-hardenable steel having a hardness of 325 HV or higher and a fatigue strength described later of 390 MPa or higher.

  Furthermore, it is also an object of the present invention to provide a method for producing a machine part using the above age-hardening steel.

  In order to solve the above-mentioned problems, the present inventors first conducted a basic investigation on elements to be precipitated during aging treatment using steels having various chemical compositions. As a result, the following findings (a) to (f) were obtained.

  (A) The element for generating a precipitate during the aging treatment has a strong ability to form a compound (secondary phase) at the aging treatment temperature, and is in a state of being dissolved in a matrix during hot forging. Must exist in Therefore, an element having a very high precipitation start temperature is inappropriate, and V should be used.

  (B) In the case of V, the precipitation peak of carbide during cooling from a high temperature is about 750 to 700 ° C., which is lower than other carbide forming elements such as Nb and Ti, for example. Moreover, for example, in a steel containing 0.3% by mass of V and 0.1% by mass of C, V does not precipitate until around 850 ° C. once dissolved in the matrix. It is relatively easy to suppress.

  (C) Like V, Mo is an element that has a relatively low carbide precipitation temperature and is easy to use for age hardening. However, in order to harden steel with precipitates mainly composed of Mo, it is necessary to contain a large amount of Mo exceeding 0.30% by mass, for example. However, if Mo is compounded and contained in steel containing 0.20% by mass or more of V, Mo contributes to improvement of age-hardening ability even if the Mo content is less than 0.30% by mass. become.

  (D) One of the matters necessary for securing the solid solution V at the stage before the aging treatment is to reduce the solid solution N at the time of hot forging. When a large amount of solute N is present during hot forging, the solute N is combined with V and precipitated as VN during forging and cooling after forging. N is an element inevitably mixed in the steel, and the temperature at which VN starts to precipitate is higher than VC. For example, the precipitation start temperature of VN in steel containing 0.2% by mass of V and 0.015% by mass of N is about 1150 to 1100 ° C. when calculated using a known solubility product of VN. This temperature corresponds to a general hot forging temperature. Therefore, in order to reduce the solute N at the time of hot forging, it is necessary to contain Ti.

  (E) Another matter necessary for securing the solid solution V in the stage before the aging treatment is to use the main phase of the structure after hot forging (that is, a phase having an area ratio of 70% or more) as bainite. It is to be. The area ratio means the ratio of the phase identified from the two-dimensional photograph of the cross section of the steel material in the steel structure by the method described later.

  (F) VC tends to precipitate at the phase interface when austenite is transformed into ferrite. Therefore, when a large amount of proeutectoid ferrite is generated during cooling after hot forging, VC precipitates at the phase interface and the amount of solute V decreases, so that it precipitates during the aging treatment to be performed later. Thus, it becomes impossible to secure the amount of the solid solution V necessary for curing to a desired hardness of 325 HV or higher.

  Then, next, the present inventors changed the chemical composition of the steel variously with respect to the steel containing 0.20% by mass or more of V, and even when a difference occurs in the cooling rate after hot forging, the structure The conditions for stably increasing the area ratio of bainite were investigated. Furthermore, the age hardening ability when aging treatment was given to those steels was investigated. As a result, the following findings (g) to (i) were obtained.

  (G) After hot forging, the structure cooled at an average cooling rate of 0.4 to 2 ° C./second has a close correlation with the contents of C, Mn, Cr, Mo and B. In other words, if the content of the above element is within a specific range of the value expressed by the formula (1) or (1 ′), which indicates a hardenability index described later, pre-deposition that is harmful to securing the solid solution V A large amount of ferrite precipitation can be suppressed, and a large amount of martensite precipitation adversely affecting the machinability can also be suppressed. For this reason, it easily becomes a structure having bainite as the main phase, particularly a structure in which 70% or more of the area ratio is bainite, and the average cooling rate after hot forging is in the range of 0.4 to 2 ° C./second. If there is, even if the cooling rate changes within the range, the hardness is stably reduced.

  (H) If the contents of C, Mn, Cr, Mo and B satisfy the specific range of the expression (1) or (1 ′) described in the above (g), Since the hardness before the aging treatment is increased, the machinability may be lowered.

  (I) On the other hand, the content of C, Si, Mn, Cr, V and Mo indicates the hardness index before aging treatment, which will be described later, and the value represented by the formula (2) or (2 ′) is specified. Since the hardness before the aging treatment can be kept low, good machinability can be obtained.

  Therefore, the present inventors further include the condition that the content of C, Si, Mn, Cr, Mo, V and B is 0.20% by mass or more and the contents of (g) and (i) described above. The steel satisfying the above conditions was hot forged, cooled at various cooling rates, and then subjected to an aging treatment, and the conditions under which the hardness after the aging treatment was 325 HV or higher were investigated. As a result, the following findings (j) to (l) were obtained.

  (J) Hardness after aging treatment is obtained by adding “(b) Hardness before aging treatment” to “(b) Precipitation strengthening during aging treatment” and “(c) Matrix during aging treatment”. This is the value obtained by subtracting “No annealing fee”.

  (K) The value represented by the expression (1) or (1 ′), which is an index of the hardenability described in (g) above, is in a specific range, and the hardness before aging treatment described in (i) By setting the value represented by the expression (2) or (2 ') as a specific range within a specific range, while suppressing that the annealing cost of the matrix during the aging treatment of (c) does not increase, ) Can be increased before the aging treatment, but by itself, the hardness after the aging treatment cannot be increased to 325 HV or more.

  (L) Another condition necessary for setting the hardness after the aging treatment to 325 HV or higher is to increase the precipitation strengthening allowance during the aging treatment of (b). For this purpose, the content of V, Ti, Nb, and Mo needs to satisfy the conditions represented by the formula (3) or (3 ′) indicating the age-hardening ability index described later.

  The present invention has been made on the basis of the above knowledge, and the gist of the invention is the age-hardening steel shown in the following (1) and (2) and the machine part using the age-hardening steel shown in (3). It is in the manufacturing method.

(1) By mass%, C: 0.05 to 0.25%, Si: 0.05 to 0.50%, Mn: 1.2 to 2.5%, P: 0.05% or less, S: 0.10% or less, Cr: 0.20 to 1.5%, Al: 0.06% or less, Ti: 0.005 to 0.20%, V: 0.20 to 0.60%, and N: 0 .15% or less, and the balance is Fe and impurities, F1 represented by the following formula (1) is 0.83 to 1.10, and F2 represented by the formula (2) is 0.72 An age-hardenable steel having a chemical composition of ˜0.90 and F3 represented by the formula (3) of 0.33 or more.
F1 = C + 0.3Mn + 0.25Cr (1)
F2 = C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V (2)
F3 = V + Tieff (3)
The element symbols in the above formulas (1) to (3) mean the content in mass% of the element.
Tieff in the equation (3) indicates {Ti- (48/14) N} or the larger value of 0. However, said Ti and N mean content in the mass% of the element.

(2) By mass%, C: 0.05 to 0.22 %, Si: 0.05 to 0.50%, Mn: 1.2 to 2.5%, P: 0.05% or less, S: 0.10% or less, Cr: 0.20 to 1.5%, Al: 0.06% or less, Ti: 0.005 to 0.20%, V: 0.20 to 0.60%, and N: 0 0.15% or less and at least one element belonging to any one of <1> to <4> below, the balance being Fe and impurities, and the following (1 ′) F1 ′ represented by the formula is 0.83 to 1.10, F2 ′ represented by the formula (2 ′) is 0.72 to 0.90, and F3 ′ represented by the formula (3 ′) is 0.33 or more. Age-hardenable steel, characterized by having a chemical composition of
<1> Mo: Less than 0.30% <2> B: 0.005% or less <3> Cu: 0.6% or less, Ni: 0.6% or less and Nb: 0.1% or less and <4> Ca: 0.005% or less and Bi: 0.4% or less F1 ′ = C + 0.3Mn + 0.25Cr + 0.6Mo + Beff (1 ′)
F2 ′ = C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo (2 ′)
F3 ′ = V + Tieff + 0.35Mo + 0.5Nb (3 ′)
The element symbols in the above formulas (1 ′) to (3 ′) mean the content in mass% of the element.
Beff in the equation (1 ′) indicates the following value.
When the content of B is less than 0.0005%: Beff = 0,
When the content of B is 0.0005 to 0.005%: Beff = 0.05.
Tieff in the expression (3 ′) indicates a larger value of {Ti− (48/14) N} or 0. However, said Ti and N mean content in the mass% of the element.

(3) A method for manufacturing a machine component, wherein the age-hardenable steel according to (1) or (2) is 1000 ° C. or higher, or the Tieff in the steel is {Ti- (48/14) N}. In this case, heat is applied at a temperature not lower than 1000 ° C. or T ° C. determined by the following formula (4), which is higher, and a hot forging with a finishing temperature of 900 ° C. or higher is applied. A method for producing a machine part, comprising cooling to at least 500 ° C. at an average cooling rate of 2 ° C./second, and further performing aging treatment at a temperature of 560 to 700 ° C. after cutting.
T = [− 10475 / {log (Tieff × C) −5.33}] − 273 (4)
C in said (4) Formula means content of C in the mass%.

  “Heating temperature” means the average temperature in the furnace. Similarly, the temperature of the “aging treatment” means an average value of the furnace temperature of the heating furnace.

  The “finishing temperature” of hot forging refers to the temperature of the surface of the material to be processed when it is formed into a predetermined shape by hot forging. Similarly, the temperature at which cooling is performed at an average cooling rate of 0.4 to 2 ° C./second after hot forging also refers to the temperature of the surface of the workpiece.

  “Average cooling rate” up to 500 ° C. after hot forging is obtained by dividing the temperature difference between the hot forging finishing temperature and 500 ° C. by the time required for cooling to 500 ° C. after hot forging. Point to.

  The age-hardening steel of the present invention can be used as a material for machine parts such as automobiles, industrial machines, and construction machines. In addition, when a machine part is manufactured by the method of the present invention using the steel of the present invention, hot forging that is the hardness before cutting is performed when hot forging and cutting are performed to obtain a predetermined shape. A machine part having a desired fatigue strength of 390 MPa or more is obtained by being hardened to 325 HV or more by aging treatment performed after cutting and having low hardness afterwards and excellent in machinability.

It is a figure which shows the shape of the uniaxial tension compression type fatigue test piece used in the Example. The numerical value in a figure shows a dimension (unit: mm).

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

(A) Chemical composition:
C: 0.05-0.25%
C combines with V to form carbides and strengthens the steel. However, if the C content is less than 0.05%, the VC deposition driving force becomes small and VC becomes difficult to precipitate, so the desired strengthening effect cannot be obtained. On the other hand, when the content of C exceeds 0.25%, C that does not bond with V forms a carbide with Fe and strengthens the structure before aging treatment. Further, the C concentration concentrated in the austenite during the transformation from austenite to bainite also increases, and martensite is partially mixed into the structure after transformation. As described above, when C forms a carbide with Fe or martensite is partially mixed, the machinability deteriorates. Therefore, the content of C is set to 0.05 to 0.25%. The C content is preferably 0.07% or more. On the other hand, the C content is preferably 0.22% or less, and more preferably less than 0.20%.

Si: 0.05 to 0.50%
Si is useful as a deoxidizing element at the time of steel making, and at the same time, has an action of improving the strength of the steel by dissolving in a matrix. In order to sufficiently obtain these effects, Si needs to be contained in an amount of 0.05% or more. However, when the Si content is excessive, the hot workability and machinability of the steel are deteriorated. In particular, when the content exceeds 0.50%, the hot workability and machinability of the steel are caused. The reduction of the becomes remarkable. Further, since Si promotes the formation of proeutectoid ferrite, it is not preferable to contain it excessively in order to stably obtain bainite. Therefore, the Si content is set to 0.05 to 0.50%. The Si content is preferably 0.08% or more. On the other hand, the Si content is preferably 0.35% or less, and more preferably less than 0.30%.

Mn: 1.2 to 2.5%
Mn has the effect of improving hardenability and making the main phase of the structure bainite. Regardless of the cooling rate after hot forging, in order to keep the hardness within a certain range, it is necessary to reduce the amount of pro-eutectoid ferrite as much as possible even when the cooling rate is slow. An amount of Mn must be included. Moreover, Mn has the effect | action which forms MnS in steel and improves machinability. In order to obtain these effects sufficiently, the Mn content needs to be at least 1.2%. However, if the Mn content exceeds 2.5%, the hardenability becomes excessive and martensite is generated. Furthermore, since Mn is an element that easily segregates during solidification of steel, its content increases, especially when it exceeds 2.5%, avoids an increase in hardness variation in parts after hot forging. I can't. Therefore, the Mn content is set to 1.2 to 2.5%. The Mn content is preferably 1.3% or more, and more preferably 1.5% or more. On the other hand, the Mn content is preferably 2.3% or less.

P: 0.05% or less P is an element that is inevitably contained as an impurity, and lowers toughness. When the P content exceeds 0.05%, the toughness is extremely lowered. Therefore, the content of P is set to 0.05% or less. The P content is preferably 0.04% or less.

S: 0.10% or less S is an element inevitably contained as an impurity. On the other hand, S combines with Mn in steel to form MnS to improve machinability, so it is actively contained when importance is attached to machinability. In order to sufficiently obtain the effect of improving machinability, the S content is desirably set to 0.01% or more. However, as the S content increases, the solid solution amount of Mn decreases, and the coarsened MnS serves as a starting point for fatigue fracture, resulting in a decrease in fatigue strength. In particular, when the S content exceeds 0.10%, the fatigue strength is significantly reduced. Therefore, the content of S is set to 0.10% or less. When higher fatigue strength is required, the S content is preferably less than 0.05%, and more preferably 0.02% or less. On the other hand, in the case where the effect of improving the machinability of S is given priority over the fatigue strength, it is preferable to contain 0.05% or more of S.

Cr: 0.20 to 1.5%
Cr has the effect of increasing hardenability and making the main phase of the structure bainite. Regardless of the shape and size of hot-forged parts, in order to keep the hardness within a certain range, it is necessary to reduce the amount of pro-eutectoid ferrite as much as possible even when the cooling rate is slow. Must contain a sufficient amount of Cr. In addition, having the effect of improving hardenability itself is substantially the same between Cr and Mn. However, when only Mn is included and the hardenability is increased, the segregation of Mn causes each part part. Hardness variation may increase. Therefore, it is necessary to contain 0.20% or more of Cr in addition to the above-mentioned amount of Mn as an element responsible for hardenability. However, if the Cr content exceeds 1.5%, the hardenability becomes excessive, resulting in the formation of martensite and the machinability deteriorates. Therefore, the content of Cr is set to 0.20 to 1.5%. The Cr content is preferably 0.25% or more, and more preferably 0.30% or more. The Cr content is preferably 1.3% or less, and more preferably less than 1.0%.

Al: 0.06% or less Al is an element inevitably contained as an impurity. In some cases, Al is intentionally contained for the purpose of deoxidation. In order to obtain a sufficient deoxidation effect, the Al content is preferably 0.005% or more. However, if the residual amount of Al in the steel exceeds 0.06%, the toughness decreases. Therefore, the Al content is set to 0.06% or less. The Al content is preferably 0.05% or less.

Ti: 0.005 to 0.20%
Ti suppresses precipitation of VN during hot forging and during cooling after forging by fixing N as TiN. Since V precipitated as VN does not contribute to age hardening, suppressing the precipitation of VN contributes to an increase in age hardening ability. In order to sufficiently obtain this effect, Ti needs to have a content of 0.005% or more. Moreover, since Ti in a solid solution state is combined with V by aging treatment, it has an effect of increasing hardness. In addition, said effect can be stably acquired by containing more Ti used for fixing the quantity of N mentioned later. However, if the Ti content is more than 0.20%, coarse TiC precipitates even during heating during forging, thereby reducing toughness. Therefore, the content of Ti is set to 0.005 to 0.20%. The Ti content is preferably 0.15% or less. On the other hand, the Ti content is preferably 0.010% or more, and more preferably 0.020% or more.

V: 0.20-0.60%
V is the most important element in the steel of the present invention. V has the effect of increasing strength by being combined with C to form fine VC during aging treatment. V also has the effect of further aging hardening ability by being combined with Mo, Nb and Ti by aging treatment. In order to sufficiently obtain these effects, V needs to be 0.20% or more. However, when the content of V is excessive, undissolved carbonitride tends to remain even during heating during hot forging, leading to a decrease in toughness. In particular, when the content exceeds 0.60%. , The toughness is significantly reduced. Moreover, if the V content exceeds 0.60%, the machinability is also significantly reduced. Therefore, the content of V is set to 0.20 to 0.60%. When importance is attached to machinability, the V content is preferably 0.55% or less, and more preferably 0.50% or less. On the other hand, the V content when strength is important is preferably 0.22% or more, and more preferably 0.25% or more.

N: 0.015% or less N is an undesirable element because V is fixed as VN in the steel of the present invention. That is, V precipitated as VN does not contribute to age hardening, so the content of N must be lowered in order to suppress the precipitation of VN. When the content is excessively large, it is necessary to increase the content of Ti in order to fix N. However, TiN produced in association with it becomes coarse and lowers toughness. Therefore, an upper limit is set and the N content is set to 0.015% or less. The N content is preferably 0.012% or less, and more preferably less than 0.010%.

  One of the age-hardening steels of the present invention contains the elements from C to N described above, the balance is Fe and impurities, and F1 represented by the above formula (1) is 0.83 to 0.83. 1.10, a steel having a chemical composition in which F2 represented by formula (2) is 0.72 to 0.90 and F3 represented by formula (3) is 0.33 or more.

  In addition, an impurity refers to what mixes from raw materials ore, scrap, refractory, etc. or from a manufacturing environment when manufacturing steel materials industrially.

  F1 to F3 will be described later together with F1 ′ to F3 ′.

  Another one of the age-hardening steels of the present invention contains the above-described elements C to N and one or more elements belonging to any one of the above <1> to <4>. The balance consists of Fe and impurities, and F1 ′ represented by the formula (1 ′) is 0.83 to 1.10, and F2 ′ represented by the formula (2 ′) is 0.72 to 0. 90 and steel having a chemical composition in which F3 ′ represented by the formula (3 ′) is 0.33 or more.

  Hereinafter, the effect of the arbitrary element belonging to each of <1> to <4> and the reason for limiting the content will be described.

<1> Mo: Less than 0.30% Mo has an effect of increasing hardenability, increasing the area ratio of bainite as the main phase of the structure after hot forging. Mo has a function of increasing age-hardening ability by forming carbide in a composite with V in steel containing 0.20% or more of V. For this reason, you may contain Mo as needed. However, since Mo is a very expensive element, if the content increases, the manufacturing cost of steel increases. Therefore, the amount of Mo in the case of inclusion is set to less than 0.30%. When Mo is contained, the amount of Mo is preferably 0.25% or less.

  On the other hand, in order to stably obtain the effect of Mo described above, the amount of Mo in the case of inclusion is preferably 0.05% or more, and more preferably 0.10% or more.

<2> B: 0.005% or less B has an effect of increasing the hardenability of the steel of the present invention and increasing the area ratio of bainite which is the main phase of the structure after hot forging. For this reason, you may contain B as needed. However, when the B content exceeds 0.005%, the above effect is saturated. Therefore, the amount of B when contained is set to 0.005% or less. When B is contained, the amount of B is preferably 0.003% or less.

  On the other hand, in order to stably obtain the effect of increasing the hardenability of B and increasing the area ratio of bainite, the amount of B when contained is preferably 0.0005% or more.

<3> Cu: 0.6% or less, Ni: 0.6% or less, and Nb: 0.1% or less Cu, Ni, and Nb all have an effect of increasing fatigue strength. For this reason, when it is desired to obtain a greater fatigue strength, these elements may be contained within the range described below.

Cu: 0.6% or less Cu has an effect of improving fatigue strength. For this reason, you may contain Cu as needed. However, when the Cu content exceeds 0.6%, the hot workability decreases. Therefore, the amount of Cu in the case of inclusion is set to 0.6% or less. When Cu is contained, the amount of Cu is preferably 0.5% or less.

  On the other hand, in order to stably obtain the effect of increasing the fatigue strength of Cu described above, the amount of Cu in the case of inclusion is preferably 0.1% or more.

Ni: 0.6% or less Ni has an effect of improving fatigue strength. Furthermore, Ni also has the effect | action which suppresses the fall of the hot workability by Cu. For this reason, you may contain Ni as needed. However, if the Ni content exceeds 0.6%, the above effect is saturated in addition to the increase in cost. Therefore, the amount of Ni in the case of inclusion is set to 0.6% or less. When Ni is contained, the amount of Ni is preferably 0.5% or less.

  On the other hand, in order to stably obtain the effect of Ni described above, the amount of Ni in the case of inclusion is preferably 0.1% or more.

Nb: 0.1% or less Nb has the effect of improving the age-hardening ability of steel and improving the fatigue strength by forming carbides in combination with V. For this reason, you may contain Nb as needed. However, if the Nb content exceeds 0.1%, coarse Nb carbonitrides are generated during heating during forging, and the toughness is deteriorated. Therefore, the amount of Nb in the case of inclusion is set to 0.1% or less. When Nb is contained, the amount is preferably 0.05% or less, and more preferably 0.03% or less.

  On the other hand, in order to stably obtain the above-described effect of Nb, the amount of Nb when contained is preferably 0.005% or more.

  Said Cu, Ni, and Nb can be contained only in one of them, or 2 or more types of composites. The total content of the above elements in the case of inclusion may be 1.3% in the case where the contents of Cu, Ni and Nb are the respective upper limit values, but is preferably 1.0% or less. More preferably, the content is 0.6%.

<4> Ca: 0.005% or less and Bi: 0.4% or less Both Ca and Bi have an effect of improving machinability. For this reason, when it is desired to obtain better machinability, these elements may be contained within the range described below.

Ca: 0.005% or less Ca has an effect of improving machinability. For this reason, you may contain Ca as needed. However, when the Ca content exceeds 0.005%, the hot workability is deteriorated. Therefore, the Ca content in the case of inclusion is set to 0.005% or less. When Ca is contained, the amount of Ca is preferably 0.0035% or less.

  On the other hand, in order to stably obtain the above-described effect of improving the machinability of Ca, the amount of Ca in the case of inclusion is preferably 0.0005% or more.

Bi: 0.4% or less Bi has an effect of improving machinability. For this reason, you may contain Bi as needed. However, when the Bi content exceeds 0.4%, hot workability is deteriorated. Therefore, the amount of Bi when contained is set to 0.4% or less. When Bi is contained, the amount of Bi is preferably 0.3% or less.

  On the other hand, in order to stably obtain the above-described effect of improving the machinability of Bi, the amount of Bi in the case of inclusion is preferably 0.03% or more.

  Said Ca and Bi can be contained only in any 1 type or 2 types of composites. When included, the total content of these elements may be 0.405% when the Ca and Bi contents are the respective upper limit values, but is preferably 0.3% or less.

F1 or F1 ′: 0.83 to 1.10
When the age-hardenable steel of the present invention does not contain any element from Mo to Bi,
F1 = C + 0.3Mn + 0.25Cr (1)
When F1 represented by the formula contains one or more optional elements from Mo to Bi,
F1 ′ = C + 0.3Mn + 0.25Cr + 0.6Mo + Beff (1 ′)
F1 ′ represented by the following must be 0.83 to 1.10.

  As already stated, the element symbols in the above formulas (1) and (1 ′) mean the content of the element in mass%. Beff in the formula (1 ′) is “Beff = 0” when the B content is less than 0.0005%, while “Beff” when the B content is 0.0005 to 0.005%. = 0.05 ".

  F1 and F1 ′ are indices for hardenability, and if the above conditions are satisfied, the structure after hot forging becomes bainite-based, and the cooling rate after hot forging is 0.4-2. Even if it changes in the range of ° C./second, the effect on the hardness is reduced.

  When F1 or F1 ′ is less than 0.83, in a place where the cooling rate after hot forging is relatively slow, ferrite is mixed in the structure, the hardness before aging treatment is increased, and the age hardening ability is reduced. . On the other hand, if F1 or F1 'exceeds 1.10, martensite is mixed in the structure at a place where the cooling rate after hot forging is relatively fast, so that machinability deteriorates.

  F1 and F1 'are preferably 0.84 or more, and more preferably 0.85 or more. On the other hand, F1 and F1 'are preferably 1.05 or less, and more preferably 1.00 or less.

F2 or F2 ′: 0.72 to 0.90
When the age-hardenable steel of the present invention does not contain any element from Mo to Bi,
F2 = C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V (2)
In the case where F2 represented by the formula contains one or more optional elements from Mo to Bi,
F2 ′ = C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo (2 ′)
F2 ′ represented by the formula (1) must be 0.72 to 0.90, respectively.

  As already stated, the element symbols in the above formulas (2) and (2 ′) mean the content of the element in mass%.

  F2 and F2 'are indices indicating the hardness before aging treatment. If the age-hardenable steel of the present invention merely satisfies the above-mentioned conditions of F1 or F1 ′, the hardness before aging treatment is too low, and it is difficult to increase the strength even after aging treatment, or aging In some cases, the hardness before treatment becomes too high, and good machinability cannot be secured.

  That is, even if the age-hardenable steel of the present invention satisfies the above F1 or F1 ′ conditions, if F2 or F2 ′ is less than 0.72, the hardness after hot forging becomes too low, Even when the structure is bainite and sufficient age-hardening ability is imparted, it is difficult to increase the hardness after aging treatment to 325 HV or higher. On the other hand, if F2 or F2 ′ exceeds 0.90, the hardness of the bainite structure becomes too high even if F1 or F1 ′ satisfies the above-described conditions and martensite is suppressed from being mixed into the structure. Therefore, machinability deteriorates.

  However, if the age-hardenable steel of the present invention satisfies F1 of 0.83 to 1.10 and F2 of 0.72 to 0.90, or F1 ′ of 0.83 to 1.10. If F2 ′ satisfies 0.72 to 0.90 after satisfying 10, the hardness before aging treatment is 260 to 260 ° C. even if a difference occurs in the cooling rate range of 0.4 to 2 ° C./second. Since it is in the range of 290 HV, machinability does not deteriorate, and it is not difficult to increase the strength after aging treatment.

  F2 and F2 'are preferably 0.73 or more, and more preferably 0.74 or more. On the other hand, F2 and F2 'are preferably 0.88 or less, and more preferably 0.86 or less.

F3 or F3 ′: 0.33 or more The age-hardenable steel of the present invention does not contain any element from Mo to Bi.
F3 = V + Tieff (3)
When F3 represented by the formula contains one or more optional elements from Mo to Bi,
F3 ′ = V + Tieff + 0.35Mo + 0.5Nb (3 ′)
Each of F3 ′ expressed by the following must be 0.33 or more.

  As already described, the element symbols in the above formulas (3) and (3 ′) mean the content of the element in mass%. Further, Tieff indicates a larger value of {Ti− (48/14) N} or 0. However, said Ti and N mean content in the mass% of the element.

  The above Tieff specifically means the amount of solid solution Ti immediately before the aging treatment, that is, the amount of Ti that can be utilized for aging precipitation of carbides (hereinafter referred to as “effective Ti amount”). This is a value obtained by subtracting the amount of Ti fixed by N from the amount of Ti. TiN has a very low solubility, and once precipitated TiN cannot be re-dissolved at a normal solution treatment temperature. Therefore, the amount of Ti that can be used to age precipitate carbide is an amount obtained by subtracting the amount fixed as TiN from the total amount of Ti. Note that since TiC also has a high melting temperature, if Tieff is greater than 0, that is, if Tieff is {Ti- (48/14) N}, depending on the heating conditions during hot forging, TiC may be heated during heating. May precipitate. However, when Tieff is greater than 0, the heating condition during hot forging satisfies, for example, 1000 ° C. or T ° C. determined by the formula (4) described later, whichever is higher. By controlling in this way, precipitation of TiC can be suppressed. Therefore, the influence of the C content on the effective Ti amount may be ignored.

  F3 and F3 'are indexes indicating age hardening ability, that is, an increase in hardness due to aging treatment.

  If F3 and F2 satisfy the above-mentioned conditions and F3, or F1 ′ and F2 ′ satisfy the above-mentioned conditions and F3 ′ is 0.33 or more, the increase in hardness due to aging treatment Even if the hardness before aging treatment is 290 HV or less, the hardness after aging treatment can be 325 HV or more.

  However, even if the above F1 or F1 'condition and F2 or F2' condition are satisfied, the age-hardening ability becomes small if F3 or F3 'is less than 0.33. For this reason, especially the hardness after an aging treatment at the time of carrying out the aging treatment of the components whose hardness before an aging treatment is 290 HV or less does not become 325 HV or more.

  F3 and F3 'are preferably 0.34 or more, and more preferably 0.35 or more.

  If F1 is 0.83 to 1.10 and F2 is 0.72 to 0.90, F3 can be as large as possible, the V content is 0.60% of the upper limit, and Ti content It may be a value close to 0.80 when the N content is close to 0% at the upper limit of 0.20%.

  Similarly, if F1 ′ is 0.83 to 1.10 and F2 ′ is 0.72 to 0.90, F3 ′ may be as large as possible, and the V content is 0.60% of the upper limit. , When the Ti content is 0.20% of the upper limit, the Nb content is 0.10% of the upper limit, the N content is close to 0%, and the Mo content is close to 0.30%. A value close to 96 may be used.

  The method for producing the age-hardening steel of the present invention is not particularly limited, and the chemical composition may be adjusted by melting by a general method.

(B) Manufacturing method of machine parts according to the present invention:
In order to sufficiently obtain the effect of precipitation strengthening of V, the mechanical component according to the present invention is subjected to hot forging and subsequent cooling by a predetermined method, and further subjected to aging treatment after cutting. This will be described below.

  As materials used for hot forging (hot forging materials), billets obtained by ingot rolling ingots, billets obtained by rolling ingots from continuous casting materials, or steel bars obtained by hot rolling or hot forging these billets, What is good. However, the chemical composition of the material for hot forging must be as described in the above section (A).

(B-1) Hot forging and subsequent cooling:
When the Ti content is increased beyond that fixed as TiN and precipitated as a composite carbide with V at the time of aging treatment, Ti in a solid solution state must be present in the heating stage during hot forging. is there. Therefore, in the case of a steel containing Ti in an amount larger than that fixed as TiN, that is, in the case of a steel having Tieff of {Ti- (48/14) N}, the heating temperature during hot forging is TiC. It is necessary to exceed the solid solution temperature. Specifically, it is necessary to heat at a temperature equal to or higher than the higher one of 1000 ° C. or T ° C. obtained by the following formula (4) obtained by modifying the solubility product of TiC.
T = [-10475 / {log (Tieff * C) -5.33}]-273 ... (4).

  If the heating temperature during hot forging falls below T ° C, undissolved TiC exists and a sufficient amount of effective Ti cannot be secured, so the amount of Ti deposited by aging also decreases, and sufficient age-hardening ability is obtained. I can't. On the other hand, if the heating temperature during hot forging is below 1000 ° C., the forging load increases and the burden on the mold increases. For this reason, when the Tieff of the steel is {Ti- (48/14) N}, it must be heated at a temperature equal to or higher than 1000 ° C or T ° C determined by the above equation (4). I must.

  On the other hand, in the case of steel where Ti is consumed for N fixation, that is, in the case where Tieff is 0, it is not necessary to dissolve TiC, and only the burden on the mold needs to be considered. For this reason, the heating temperature at the time of hot forging should just be a temperature of 1000 degreeC or more.

  When the heating temperature is excessively high, the energy cost is increased and the scale loss is increased. For this reason, the heating temperature at the time of hot forging is preferably 1300 ° C. or lower in both cases of steels having Tieff of {Ti- (48/14) N} and 0 steel.

  Although there is no restriction | limiting in particular in the holding time in said heating temperature, For example, it is preferable to set it as 30 to 1000 minutes.

  After heating to the above temperature range, hot forging is performed. In order to cut into a predetermined shape by cutting from a state of being cooled after hot forging, in particular, the finishing temperature of hot forging must be 900 ° C. or higher. When the finishing temperature of hot forging is less than 900 ° C., recrystallization will occur at a low temperature. Therefore, the generated austenite grains become small and hardenability cannot be ensured, and the structure having bainite as the main phase is present. I can't get it. Accordingly, the finishing temperature for hot forging is set to 900 ° C. or higher. The finishing temperature for hot forging is preferably 950 ° C. or higher. On the other hand, the finishing temperature of hot forging is preferably 1250 ° C. or lower.

  In order to cut and process into a predetermined shape after being cooled after hot forging, after finishing hot forging at the above finishing temperature, an average cooling rate of 0.4 to 2 ° C./sec. It must be cooled to at least 500 ° C.

  If the average cooling rate up to 500 ° C. is too small after hot forging is completed, the structure may not be bainite and productivity may be reduced. On the other hand, when the average cooling rate exceeds 2 ° C./second, even if the chemical composition of the steel satisfies the above-described conditions, a large amount of martensite is generated and machinability is lowered. For this reason, the average cooling rate to 500 ° C. after the end of hot forging must be 0.4 to 2 ° C./second. The average cooling rate up to 500 ° C. after completion of hot forging is desirably 0.5 ° C./second or more, and more desirably 0.6 ° C./second or more. Moreover, it is preferable that the average cooling rate to 500 degreeC after completion | finish of hot forging is less than 1.8 degreeC / second, and it is still more preferable that it is less than 1.6 degreeC / second.

  As a specific method for setting the average cooling rate up to 500 ° C. after hot forging to 0.4 to 2 ° C./second, for example, a member having a shape corresponding to a round bar having a diameter of 20 to 55 mm This can be achieved by cooling in the air or by cooling with a fan.

  The cooling rate in the temperature range below 500 ° C. does not affect the function and effect of the present invention. Therefore, if the average cooling rate up to 500 ° C. after the end of hot forging is 0.4 to 2 ° C./second, the cooling rate from 500 ° C. to room temperature does not need to be particularly controlled.

  In the case of the steel having the chemical composition described in the section (A), by performing hot forging under the above-described conditions and subsequent cooling, a structure having bainite as the main phase is obtained, and the hardness is the shape of the part. Regardless, it easily becomes 290 HV or less.

(B-2) Cutting:
Under the conditions described in the section (B-1), the steel that has been subjected to hot forging and the subsequent cooling is then subjected to cutting to be processed into a predetermined machine part shape.

  In the case of the steel having the chemical composition described in the section (A), the hardness becomes 290 HV or less by performing hot forging and subsequent cooling under the conditions described in the section (B-1). It can be cut into a predetermined machine part shape. The conditions for this cutting process are not particularly limited, as long as they can be machined into machine parts having a predetermined shape.

(B-3) Aging treatment:
The aging treatment must be performed at a temperature of 560 to 700 ° C. When the aging treatment temperature exceeds 700 ° C., the structure itself is tempered and softened. On the other hand, when the aging treatment temperature is lower than 560 ° C., the diffusion of V is slowed down, so that the time required for the aging treatment becomes long, so that the productivity is lowered and the heat treatment cost is increased.

  The aging treatment temperature is preferably 580 ° C. or more, and the aging treatment temperature is preferably 680 ° C. or less.

  In addition, although it changes with the size (mass) of machine parts, it is preferable that the retention time of said aging treatment shall be 30-1000 minutes, for example.

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

[Example 1]
Steel A and steels C to Y having chemical compositions shown in Tables 1 and 2 were melted in a 50 kg vacuum melting furnace. Steel B was melted by a 180 kg vacuum melting furnace.

  Steels A to O in Tables 1 and 2 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, the steels P to Y are steels whose chemical compositions deviate from the conditions defined in the present invention.

  In Tables 1 and 2, Tieff in the expression (3) or (3 ′), that is, the larger value of ({Ti− (48/14) N} or 0) is also shown.

  Tables 1 and 2 also show the value of T ° C. determined by the equation (4) as the melting temperature of TiC in steels with non-zero Tieff, ie, steels of {Ti- (48/14) N}. . Accordingly, “-” in the “TiC melting temperature T” column indicates that the steel has a Tieff of 0 and does not require solid dissolution of TiC.

  Each steel ingot was heated at 1250 ° C. and then hot forged into a steel bar having a diameter of 60 mm and 50 mm. Each hot forged steel bar was once allowed to cool in the atmosphere and cooled to room temperature. Thereafter, further heating to 1250 ° C., assuming forging to a part shape, finishing temperature is 950 ° C., a steel bar with a diameter of 60 mm is a steel bar with a diameter of 50 to 40 mm, and a steel bar with a diameter of 50 mm is 35 to 25 mm in diameter. Each steel bar was hot forged. The temperature of the finishing temperature was measured using a radiation thermometer on a portion of the steel bar surface where there was no scale. In addition, after hot forging, all were left to cool in the air and cooled to room temperature.

  The measurement of the cooling rate at the time of cooling in the air after the above hot forging was performed using a radiation thermometer on a portion of the steel bar surface where there was no scale. The average cooling rate to 500 ° C. after forging was 0.4 to 1.8 ° C./second.

  A part of steel B having a diameter of 60 mm was also used for the investigation in Example 2 described later.

  For each test number, some of the steel bars cooled to room temperature after finishing to the above-mentioned sizes by hot forging were not subjected to aging treatment (that is, in the cooled state), and both ends of the steel bars were After cutting off each 100 mm, a test piece was cut out from the remaining central portion, and the hardness before aging treatment and the area ratio of bainite in the structure were examined.

  On the other hand, for each test number, the rest of the steel bar cooled to room temperature after finishing the above-mentioned sizes by hot forging was subjected to an aging treatment of 60 to 180 minutes at 610 to 630 ° C., and both ends of the steel bar were 100 mm. After cutting off each, a test piece was cut out from the remaining central portion, and the hardness after aging treatment was investigated. Further, for each test number, a test piece was cut out from a steel bar having a larger diameter, which is expected to have a lower cooling rate and lower strength, and the fatigue strength after aging treatment was investigated.

  Hardness measurement was performed as follows. First, a test piece was prepared by crossing a steel bar, filling the resin so that the cut surface became the test surface, and mirror polishing. Next, in accordance with “Vickers hardness test—test method” in JIS Z 2244 (2009), the test force is set to 9 for 10 points near the R / 2 part (“R” represents a radius) of the test surface. The hardness was measured as 8N. The above 10 points were arithmetically averaged to obtain Vickers hardness.

  The measurement of the area ratio of the bainite in the structure was performed as follows. The test piece, which was embedded in the resin used for the hardness measurement and was mirror-polished, was etched by night. With respect to the test piece after etching, the structure | tissue was image | photographed by 200 time magnification using the optical microscope. The area ratio of bainite was measured by image analysis from the photographed photographs.

The fatigue strength was investigated by collecting a uniaxial tension-compression type fatigue test piece. That is, the diameter and length of the parallel portion shown in FIG. 1 are 3.4 mm and 12.7 mm, respectively, and a smooth fatigue test piece is parallel to the forging direction from the R / 2 portion of the steel bar (longitudinal direction of the steel bar). The samples were collected and subjected to a fatigue test under the conditions of room temperature, air, stress ratio 0.05, test speed 10 Hz. Under the above conditions, the maximum stress that did not break at the stress addition repetition number of 10 ⁷ times was defined as fatigue strength. When the fatigue strength was 390 MPa or more, it was evaluated that the fatigue strength was sufficiently high, and this was the target.

  Table 3 shows the results of the above investigations, along with the details of the average cooling rate up to 500 ° C. after the hot forging and the aging treatment conditions for each test number.

  As apparent from Table 3, in the case of “Examples of the present invention” of test numbers A1 to A15 having the chemical composition defined in the present invention, the hardness before aging treatment is 290 HV or less, and the hardness after aging treatment is 325 HV. As described above, the fatigue strength is 390 MPa or more and the target is achieved, and it can be seen that both strength and machinability before aging treatment are compatible.

  On the other hand, in the case of “Comparative Examples” with test numbers A16 to A25 that are out of the definition of the present invention, the target performance is not obtained.

  In test number A16, since F3 of steel P used is as small as 0.30, age hardening ability, that is, an increase in hardness due to aging treatment is small. Although the hardness before the aging treatment of the steel bar with a diameter of 42 mm and the steel bar with a diameter of 30 mm is 279 HV and 284 HV, which are similar to the test number A2 of the example of the present invention, respectively, the hardness after the aging treatment is 309 HV and 310 HV. The fatigue strength is as low as 375 MPa.

  In test number A17, since the F2 ′ of the steel Q used is as large as 0.93, the hardness before aging treatment is as large as 321 HV (steel with a diameter of 42 mm) and 339 HV (bar with a diameter of 30 mm), and the machinability is inferior. It is expected that.

  In test number A18, since the F2 'of the steel R used was as small as 0.65, the hardness before aging treatment was very low, such as 251 HV (a steel bar having a diameter of 42 mm) and 259 HV (a steel bar having a diameter of 30 mm). In addition, since F3 'of steel R satisfies the provisions of the present invention at 0.33, the age-hardening ability is not low, but the hardness before aging treatment is too low, so the hardness after aging treatment is 291HV (Bar steel with a diameter of 42 mm) and 299 HV (bar steel with a diameter of 30 mm) did not reach 325 HV, and the fatigue strength was as low as 350 MPa.

  In test number A19, the F1 ′ of the steel S used is as large as 1.21, so that the hardenability is too high and martensite is mixed in the structure. As a result, it is expected that the hardness before aging treatment is as large as 311 HV (steel with a diameter of 42 mm) and 352 HV (bar with a diameter of 30 mm) and machinability is inferior even though F2 ′ satisfies the provisions of the present invention. Is done.

  In test number A20, the F1 of the steel T used was as small as 0.75, so that the hardenability was too small, and the average cooling rate up to 500 ° C after hot forging was 0.7 ° C / second and a relatively slow diameter of 42 mm. In the case of steel bars, the hardness before aging treatment is as high as 291HV. This was because the hardenability was reduced, and pro-eutectoid ferrite was generated. When the pro-eutectoid ferrite is formed, V carbide is precipitated at the phase interface between the pro-eutectoid ferrite and austenite, and thus the hardness increases. In addition, even when aging treatment is performed on steel on which V carbide has already been precipitated, coarsening of the precipitated V carbide occurs. For this reason, since age hardening is difficult, in the case of a steel bar having a diameter of 42 mm, the hardness after the aging treatment is 323 HV and does not reach the target 325 HV, and the fatigue strength is also low as 375 Pa.

  Test No. A21 has a high C content of 0.26% in the steel U used. For this reason, in a steel bar having a diameter of 42 mm, which has a slow cooling rate, C that does not bond with V forms Fe and carbide, strengthens the structure before aging treatment, and the hardness before aging treatment is as large as 294HV. In addition, in a steel bar having a diameter of 30 mm with a high cooling rate, not only C forms Fe carbide, but also martensite is mixed in the structure, the hardness is as large as 299 HV, and machinability is expected to be inferior. The

  In Test No. A22, since the steel V used does not contain Ti, N cannot be fixed as TiN, and it is inevitable that VN precipitates during hot forging and during cooling after forging. For this reason, the amount of solute V that contributes to age hardening decreases, the hardness after aging treatment does not reach 325 HV at 319 HV (steel bar with a diameter of 42 mm) and 323 HV (steel bar with a diameter of 30 mm), and fatigue strength is also high As low as 385 MPa.

  In test number A23, the V content of the steel W used is as low as 0.15%. For this reason, the amount of VC deposited during the aging treatment is reduced, and the aging treatment is combined with Mo, Nb and Ti to precipitate, and the effect of V for further enhancing the age hardening ability is not sufficiently exerted, and the diameter is 42 mm. In both the steel bar and the steel bar with a diameter of 30 mm, the hardness after aging treatment is 304 HV and does not reach 325 HV, and the fatigue strength is as low as 365 MPa.

  In the case of test number A24, since the S content of the used steel X is as high as 0.151%, coarsening of MnS that adversely affects fatigue fracture occurs. For this reason, the steel bar with a diameter of 42 mm has a fatigue strength as low as 350 MPa, and has not reached the target, even though the hardness after aging is 325 HV and the target is achieved.

  In test number A25, the N content of the steel Y used is as high as 0.0181%. For this reason, when the Ti content is 0.025%, free N exists, and it is inevitable that VN precipitates during hot forging and during cooling after forging. For this reason, the amount of solid solution V that effectively works for age hardening decreases, and the hardness after aging treatment does not reach 325 HV at 299 HV (steel bar with a diameter of 42 mm) and 306 HV (steel bar with a diameter of 30 mm), The fatigue strength is as low as 350 MPa.

[Example 2]
As steel with Tieff of {Ti- (48/14) N}, hot forging conditions are characteristic for steel B shown in Table 1 above, using a part of the 60 mm diameter steel bar produced in Example 1. The effects on the environment were investigated. In addition, as shown in Table 1, the specific value of Tieff of steel B is 0.084,
T = [− 10475 / {log (Tieff × C) −5.33}] − 273 (4)
The T (° C.) calculated by 1 is 1176 ° C.

  Under the conditions shown in Table 4, a steel bar having a diameter of 60 mm was hot forged into a steel bar having a diameter of 45 mm. After hot forging, the steel bar was allowed to cool in the air and cooled to room temperature. The finishing temperature of hot forging and the cooling rate at the time of cooling in the air after hot forging were measured by the same method as in Example 1.

  For each test number, some of the steel bars cooled to room temperature after finishing to 45 mm in diameter by hot forging were not subjected to aging treatment (that is, in a cooled state), and both ends of the steel bars were 100 mm each. After cutting off, the test piece was cut out from the remaining central portion, and the hardness before the aging treatment was investigated by the same method as in Example 1.

  On the other hand, for each test number, the rest of the steel bar cooled to room temperature after finishing to the above sizes by hot forging was subjected to aging treatment for 60 to 120 minutes at 620 to 630 ° C., and both ends of the steel bar were 100 mm. After cutting off each, a test piece was cut out from the remaining central portion, and the hardness and fatigue strength after the aging treatment were investigated by the same method as in Example 1.

  Table 4 shows the results of the above investigations, along with details of the average cooling rate up to 500 ° C. and the aging treatment conditions after the completion of hot forging, for each test number.

  As apparent from Table 4, all of the test numbers B1 to B5 having the chemical composition defined in the present invention have a hardness before aging treatment of 290 HV or less, a hardness after aging treatment of 325 HV or more, and a fatigue strength. The target was achieved at 390 MPa or more, and it can be seen that both strength and machinability before aging treatment are compatible.

  Among the above, test numbers B1 and B2 satisfying the production conditions defined in the present invention can secure a sufficient amount of solute V in the stage before the aging treatment, and therefore the hardness after the aging treatment is 339 HV. The fatigue strength is as high as 440 MPa.

  The age-hardening steel of the present invention can be used as a material for machine parts such as automobiles, industrial machines, and construction machines. In addition, when a machine part is manufactured by the method of the present invention using the steel of the present invention, hot forging that is the hardness before cutting is performed when hot forging and cutting are performed to obtain a predetermined shape. A machine part having a desired fatigue strength of 390 MPa or more is obtained by being hardened to 325 HV or more by aging treatment performed after cutting and having low hardness afterwards and excellent in machinability.

Claims (3)

In mass%, C: 0.05 to 0.25%, Si: 0.05 to 0.50%, Mn: 1.2 to 2.5%, P: 0.05% or less, S: 0.10 %: Cr: 0.20 to 1.5%, Al: 0.06% or less, Ti: 0.005 to 0.20%, V: 0.20 to 0.60%, and N: 0.015% The remainder is composed of Fe and impurities, and F1 represented by the following formula (1) is 0.83 to 1.10, and F2 represented by the formula (2) is 0.72 to 0. An age-hardenable steel having a chemical composition of 90 and F3 represented by the formula (3) being 0.33 or more.
F1 = C + 0.3Mn + 0.25Cr (1)
F2 = C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V (2)
F3 = V + Tff (3)
The element symbols in the above formulas (1) to (3) mean the content in mass% of the element.
Tieff in the equation (3) indicates {Ti- (48/14) N} or the larger value of 0. However, said Ti and N mean content in the mass% of the element. In mass%, C: 0.05 to 0.22 %, Si: 0.05 to 0.50%, Mn: 1.2 to 2.5%, P: 0.05% or less, S: 0.10 %: Cr: 0.20 to 1.5%, Al: 0.06% or less, Ti: 0.005 to 0.20%, V: 0.20 to 0.60%, and N: 0.015% In addition to the following, it contains one or more elements belonging to any of the following <1> to <4>, the balance is composed of Fe and impurities, and is represented by the following formula (1 ′): F1 ′ is 0.83 to 1.10, F2 ′ represented by the formula (2 ′) is 0.72 to 0.90, and F3 ′ represented by the formula (3 ′) is 0.33 or more. An age-hardening steel, characterized by having a composition.
<1> Mo: Less than 0.30% <2> B: 0.005% or less <3> Cu: 0.6% or less, Ni: 0.6% or less and Nb: 0.1% or less and <4> Ca: 0.005% or less and Bi: 0.4% or less F1 ′ = C + 0.3Mn + 0.25Cr + 0.6Mo + Beff (1 ′)
F2 ′ = C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo (2 ′)
F3 ′ = V + Tieff + 0.35Mo + 0.5Nb (3 ′)
The element symbols in the above formulas (1 ′) to (3 ′) mean the content in mass% of the element.
Beff in the equation (1 ′) indicates the following value.
When the content of B is less than 0.0005%: Beff = 0,
When the content of B is 0.0005 to 0.005%: Beff = 0.05.
Tieff in the expression (3 ′) indicates a larger value of {Ti− (48/14) N} or 0. However, said Ti and N mean content in the mass% of the element. A method of manufacturing a machine component, wherein the age-hardenable steel according to claim 1 or 2 is 1000 ° C or higher, or when the Tieff in the steel is {Ti- (48/14) N}, 1000 ° C or Heat at a higher temperature of T ° C determined by the following formula (4), perform hot forging with a finishing temperature of 900 ° C or higher, and then average at 0.4 to 2 ° C / sec. A method for manufacturing a machine part, characterized by cooling to a temperature of at least 500 ° C at a cooling rate, and further performing an aging treatment at a temperature of 560 to 700 ° C after performing a cutting process.
T = [− 10475 / {log (Tieff × C) −5.33}] − 273 (4)
C in said (4) Formula means content of C in the mass%.

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