JP5257460B2 - 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 also 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, construction machines, and the like, 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. In general, the mechanical parts are formed by hot forging and then finished into a predetermined product shape by cutting. Therefore, the steel used as the material for the machine parts must have good machinability.

  In order to improve the machinability of steel, the S content may be increased. However, when the S content increases, the size of inclusions in the steel increases, resulting in a decrease in fatigue strength. 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 technologies that can increase the hardness in the product stage have been proposed.

  For example, Patent Document 1 discloses the following manufacturing method.

  Specifically, “A method for producing an age-hardening machine component, in mass%, C: 0.10 to 0.25%, Si: 0.10 to 0.50%, Mn: 0.60 to 1 0.0%, S: 0.01 to 0.10%, Ti: 0.005 to (4 × N)%, V: 0.10 to 0.25% and Mo: 0.05 to 0.60% And V + 0.5Mo: less than 0.50%, N: 0.010 to 0.030% and Ca: 0.0001 to 0.005%, with the balance being steel made of Fe and impurities at 1000 to 1300 ° C. And the hot forging is finished at a temperature T1 of 900 ° C. or higher, and is cooled at a cooling rate of 10 ° C./second or higher from at least 900 ° C. to a temperature T2 in the range of 550 to 450 ° C. After that, hold it in the temperature range of 550 to 450 ° C for 1 minute or longer, cool it to room temperature, and perform further cutting. And then, the production method of age hardening machine parts which comprises treating age hardening at a temperature of five hundred and sixty to six hundred and fifty ° C. "is disclosed.

  Patent Document 2 discloses the following age hardened steel.

That is, “C: 0.11 to 0.60 mass%, Si: 0.03 to 3.0 mass%, Mn: 0.01 to 2.5 mass%, Mo: 0.3 to 4.0 mass% , V: 0.05 to 0.5% by mass, Cr: 0.1 to 3.0% by mass, the balance is composed of Fe and inevitable impurities, and between each component,
4C + Mn + 0.7Cr + 0.6Mo−0.2V ≧ 2.5,
C ≧ Mo / 16 + V / 5.7, V + 0.15Mo ≧ 0.4
A relationship that satisfies
After rolling, forging, or solution treatment, cooling is performed at an average cooling rate of 0.05 to 10 ° C./second between a temperature of 800 ° C. and 300 ° C.,
Before the aging treatment, the area ratio of the bainite structure is 50% or more and the hardness is 40 HRC or less,
An age-hardening steel whose hardness is higher by 7 HRC or more than the hardness before the aging treatment is disclosed.

The steel may further contain an element selected from at least one element group of the following four element groups as necessary.
(1) Al: at least one selected from the group of 0.001-0.3 mass%, N: 0.005-0.025 mass%,
(2) at least one selected from the group consisting of Nb: 0.5% by mass or less, Ti: 0.5% by mass or less, and Zr: 0.5% by mass or less,
(3) Cu: 1.0 mass% or less, Ni: at least one selected from the group of 1.0 mass% or less,
(4) S: 0.01 to 0.20 mass%, Ca: 0.003 to 0.010 mass%, Pb: 0.3 mass% or less, and Bi: 0.3 mass% or less At least one.

JP 2008-88508 A JP 2006-37177 A

  In the case of the technique proposed in the above-mentioned Patent Document 1, it is necessary to hold for a predetermined time in a specific temperature range for each part in the cooling process after hot forging. For this reason, in order to enable management of the holding time in a specific temperature range, there may be a restriction on equipment in manufacturing, for example, using a dedicated soaking furnace.

  When steel disclosed in Patent Document 2 is used, according to the results of the example described in paragraph [0031], the hardness at the stage of cutting is 27.9 or more in HRC. Therefore, the steel proposed in Patent Document 2 is difficult to use as a material for parts that require high machinability.

  The steels proposed in both patent documents contain V for the purpose of age hardening. However, V is a very expensive element.

  V easily forms carbides in steel, and can be easily precipitated as VN and / or VC even by performing normal hot forging and cooling after forging without additional heat treatment such as aging treatment. is there. On the other hand, it is the solid solution V that affects the amount of curing during the aging treatment. For this reason, when V is contained in age-hardenable steel, it is important that V is not precipitated during hot forging and during cooling after hot forging.

  However, in the case of the inventions disclosed in the above-mentioned Patent Documents 1 and 2, it cannot be said that the above-described device has been made. Furthermore, the steel proposed in the above-mentioned patent document is required to contain Mo. In addition to being an expensive element, Mo may need to contain a larger amount of Mo than V in order to obtain age hardening ability by Mo carbide. Therefore, it is desirable not to add Mo from the viewpoint of cost. However, Mo has the effect of increasing the age-hardening ability by VC by being compounded with V. For this reason, if the Mo content is reduced, sufficient age-hardening ability may not be obtained.

  Therefore, an object of the present invention is to perform cutting after hot forging, and when processing into a predetermined shape, the content of V is not excessively increased, and even if Mo is not actively contained as a component, cutting is performed. Hardness after hot forging, which is the hardness before processing, is sufficiently low and excellent in machinability. Further, it can be cured by an aging treatment performed after cutting, and a machine part can have a desired strength. It is to provide age hardenable steel.

  Specifically, the object of the present invention is that the hardness before aging treatment is 280 or less in HV, the fatigue strength is improved by 50 MPa or more by the aging treatment, and the fatigue strength after aging treatment is S content. It is to provide an age-hardenable steel that is 340 MPa or more when the content is 0.05% or more and 370 MPa or more when the S content is less than 0.05%. 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 (d) were obtained.

  (A) The elements precipitated during the aging treatment are present in a solid solution state in the matrix during hot forging, in addition to the strong ability to form a compound (secondary phase) at the aging treatment temperature. There must be. Therefore, an element having a very high precipitation start temperature is inappropriate, and V should be used.

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

  (B) One of the matters necessary for securing the solid solution V at the stage before the aging treatment is to fix the solid solution N at the time of hot forging. If solid solution N exists during hot forging, the solid solution 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, when the precipitation start temperature of VN in a steel containing 0.2% by mass of V and 0.015% by mass of N is calculated using the known solubility product of VN, it becomes about 1150 to 1100 ° C. This temperature corresponds to a general hot forging temperature.

  (C) Another matter necessary for securing the solid solution V at the stage before the aging treatment is that the main phase of the structure after hot forging (that is, the phase ratio of 50% or more) is bainite. It is to be. VC tends to precipitate at the phase interface when austenite transforms into ferrite. Therefore, when proeutectoid ferrite is generated during cooling after hot forging, the amount of solute V decreases, so it is necessary to precipitate during subsequent aging treatment and harden the steel to the desired hardness. A sufficient amount of solute V cannot be secured. The area ratio referred to here is a phase fraction of the steel structure measured from a two-dimensional photograph of the cross section of the steel material by the method shown in the examples described later.

  (D) Ti and Nb are elements that form and precipitate carbides by aging treatment. However, since TiC and NbC are less soluble in steel than VC, the amount of solute Ti and solute Nb in the state before aging treatment is less than the amount of solute V. Therefore, sufficient age-hardening ability cannot be obtained even if Ti and Nb are contained alone. However, when Ti and Nb are contained at the same time as V, they are precipitated in a composite manner, so that when compared with steel containing Ti and Nb alone, it is compared with steel containing V alone. However, a large age-hardening ability can be obtained.

  Then, next, the inventors hot forged steel containing V, changed the cooling conditions after hot forging in various ways, and investigated the conditions under which the main phase of the structure could be bainite without containing Mo. did. Furthermore, the age hardening ability when aging treatment was given to those steel was investigated. As a result, the following findings (e) to (i) were obtained.

  (E) During hot forging, heating to 1000-1300 ° C. to perform hot forging, and if the hot forging is finished at a specific temperature or higher, the austenite grain size before transformation does not become too small. Sex can be secured.

  (F) After cooling at an arbitrary rate from the hot forging end temperature to 900 ° C., cooling is performed under the condition that the average cooling rate in the temperature range of 900 to 500 ° C. is a specific value or more. Can be suppressed. Therefore, precipitation of VC during cooling can be suppressed.

  (G) When cooling during hot forging, if the average cooling rate in the temperature range of 500 to 150 ° C. is below a certain value, martensitic transformation during cooling can be suppressed. Therefore, the hardness after cooling does not become excessively high.

  (H) The steel material that has undergone the cooling process as described above has excellent machinability since the hardness after hot forging (that is, the hardness before aging treatment) is low. For this reason, it can be easily cut into a predetermined shape.

  (I) The steel material subjected to the above-described controlled cooling is easily strengthened because V in a solid solution is precipitated as VC and age hardened by aging treatment.

  The present invention has been made on the basis of the above knowledge, and the gist thereof is the manufacture of machine parts using the age-hardening steel (1) below and the age-hardening steels (2) and (3) below. Is in the way.

(1) Age-hardenable steel in mass%, C: 0.025 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.8%, P: 0.05% or less, S: 0.10% or less, Cr: 0.05-0.6%, Al: 0.06% or less, Ti: 0.005-0.20%, V: 0.10 0.60% and N: 0.012% or less, and if necessary, Cu: 0.6% or less, Ni: 0.6% or less, Mo: 0.1% or less, Nb: 0 1% or less, B: 0.005% or less, Ca: 0.005% or less, and Bi: 0.4% or less, and the balance is Fe and impurities. An age-hardening steel having a chemical composition satisfying the formulas (1) and (2) and having a structure with an area ratio of bainite of 50% or more.
C / V ≧ 0.20 ... (1)
N / Ti ≦ 0.60 (2)
The element symbols in the above formulas (1) and (2) mean the content (% by mass) of the element.

  (2) A method for manufacturing a machine part, wherein the steel described in (1) above is heated at a temperature of 1000 ° C or higher, subjected to hot forging with a finishing temperature of 950 ° C or higher, and then 900 to 500 ° C. Cooling is performed under the condition that the average cooling rate in the temperature range is 5 ° C./second or more and the average cooling rate in the temperature range of 500 to 150 ° C. is 20 ° C./second or less, and after performing cutting, 560 to 700 ° C. A process for producing a machine part, characterized in that an aging treatment is carried out at a temperature of

  (3) A method of manufacturing a machine part, wherein after hot forging the steel described in (1) above, solution treatment is performed at a temperature of 950 ° C. or higher, and an average cooling rate in the temperature range of 900 to 500 ° C. Is 5 ° C./second or more, and is cooled under the condition that the average cooling rate in the temperature range of 500 to 150 ° C. is 20 ° C./second or less. Further, after performing the cutting process, aging treatment is performed at a temperature of 560 to 700 ° C. A method for manufacturing a machine part, characterized by comprising:

  The average cooling rate in the temperature range of 900 to 500 ° C. is obtained by dividing the temperature difference of 400 ° C. between 900 ° C. and 500 ° C. by the time required for cooling from 900 ° C. to 500 ° C. Similarly, the average cooling rate in the temperature range of 500 to 150 ° C. means a temperature difference of 350 ° C. between 500 ° C. and 150 ° C. divided by the time required for cooling from 500 ° C. to 150 ° C.

  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, if a machine part is manufactured by the method of the present invention using the steel of the present invention, when performing hot forging and cutting into a predetermined shape, the hardness before cutting is low and machinability is achieved. Further, it is cured by an aging treatment performed after cutting, and a mechanical part having a desired strength can be obtained.

It is a figure which shows the shape of the smooth Ono type | formula rotation bending 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: 0.025 to 0.25%
C combines with V to form carbides and strengthens the steel. However, if the C content is less than 0.025%, the VC deposition driving force becomes small and VC is 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 Fe and carbides and strengthens the structure before aging treatment, so that machinability is lowered. Therefore, the content of C is set to 0.025 to 0.25%. The C content is preferably 0.20% or less.

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 must have a content 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. Therefore, the Si content is set to 0.05 to 0.50%. The Si content is preferably 0.10% or more. On the other hand, the Si content is preferably 0.40% or less.

Mn: 0.50 to 1.8%
Mn is an element that improves the strength and hardenability, and at the same time has the effect of improving the machinability by forming MnS in the steel. In order to sufficiently obtain these effects, the Mn content needs to be at least 0.50%. However, if the content of Mn exceeds 1.8%, the machinability and hot workability are significantly lowered due to excessive solid solution strengthening of Mn. Therefore, the content of Mn is set to 0.50 to 1.8%. The Mn content is preferably 0.60% or more. On the other hand, the Mn content is preferably 1.7% or less, and more preferably 1.5% 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 positively incorporated when machinability is required. 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 desirable to contain 0.05% or more of S.

Cr: 0.05-0.6%
Cr increases the fatigue strength by increasing the hardenability and increasing the area ratio of bainite. In order to obtain this effect, the Cr content needs to be 0.05% or more. However, if the Cr content exceeds 0.6%, the hardenability becomes too high and the machinability at the time of cutting is lowered, and further, the precipitation of VC during the aging treatment is delayed by stabilizing the cementite. End up. Therefore, the Cr content is set to 0.05 to 0.6%. The Cr content is preferably less than 0.45%, and more preferably less than 0.30%.

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 fixes N as TiN, thereby suppressing the precipitation of VN during forging and cooling after forging. In order to sufficiently obtain this effect, Ti needs to have a content of 0.005% or more. Ti in an excessive solid solution state with respect to the amount of N precipitates in combination with V by aging treatment, and thus has an effect of increasing the hardness. 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 0.01% or more. When it is desired to reduce the hardness before the aging treatment, the Ti content is preferably 0.06% or less, and more preferably 0.04% or less. On the other hand, even if the hardness before the aging treatment is somewhat high, when it is desired to improve the fatigue strength by obtaining a high hardness after the aging treatment, it is desirable to contain Ti exceeding 0.06%. When it is desired to obtain a high hardness after aging treatment, the Ti content is preferably less than 0.15%.

V: 0.10 to 0.60%
V is an 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. In order to sufficiently obtain this effect, V needs to have a content of 0.10% 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, and the machinability is also significantly reduced. Therefore, the content of V is set to 0.10 to 0.60%. The content of V in the case where the machinability is regarded as important may be small within the above range. On the other hand, the content of V in the case where the strength is important even if the machinability is sacrificed to some extent may be set within the above range.

N: 0.012% or less N is an undesirable element because V is fixed as VN in the steel of the present invention. Moreover, when the content increases too much, the amount of Ti for fixing N increases, and TiN produced along with it becomes coarse and lowers toughness. Therefore, the N content is set to 0.012% or less. The N content is preferably 0.010% or less, and more preferably 0.008% or less.

C / V: 0.20 or more The content of C and V needs to satisfy the following formula (1).
C / V ≧ 0.20 ... (1)
However, C and V in the formula (1) represent respective contents (mass%).

  Since C and V precipitate as VC during the aging treatment, the ratio of their contents, that is, “C / V” greatly affects the precipitation behavior of VC. When the value of “C / V” is less than 0.20, a part of V is left over and does not contribute to age hardening, which is uneconomical. Further, in the state where V is surplus, the age-hardening ability is lowered because precipitates are likely to be coarser than in the state where C is surplus. Therefore, the value of “C / V” is set to 0.20 or more. The value of “C / V” is preferably 0.25 or more. On the other hand, “C / V” may be 2.5 when C is 0.25% and V is 0.10%.

N / Ti: 0.60 or less The content of N and Ti must satisfy the following formula (2).
N / Ti ≦ 0.60 (2)
However, N and Ti in the formula (2) represent respective contents (mass%).

  If solid solution N exists during forging and cooling after forging, since solid solution N fixes V as VN, a part of V is not used for age hardening. Therefore, in order to suppress solid solution N from fixing V as VN, the ratio of the content of N and Ti, that is, the value of “N / Ti” is set to 0.60 or less. The value of “N / Ti” is preferably 0.50 or less, and more preferably 0.40 or less. The value of “N / Ti” is more preferably 0.35 or less. On the other hand, since the value of “N / Ti” can be as small as possible, no lower limit is provided.

  One of the chemical compositions of the age-hardenable steel of the present invention is that the balance is Fe and impurities in addition to the elements described above. 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.

  Another chemical composition of the age-hardenable steel of the present invention contains one or more elements of Cu, Ni, Mo, Nb, B, Ca and Bi instead of part of Fe It is. Hereinafter, the effect of these arbitrary elements and the reason for limiting the content will be described.

  Cu, Ni, Mo 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. Hereinafter, Cu, Ni, Mo and Nb will be described.

Cu: 0.6% or less Cu has a function of improving fatigue strength by solid solution strengthening if it is a small amount and by precipitation strengthening at the time of aging treatment if it is a large amount. Good. However, when the Cu content exceeds 0.6%, the hot workability decreases. Therefore, the Cu content 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 reliably 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 action of improving fatigue strength, and further has an action of suppressing a decrease in hot workability due to Cu, and may be contained as necessary. However, if the Ni content exceeds 0.6%, the above effect is saturated in addition to the increase in cost. Therefore, the Ni content 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 reliably obtain the effect of Ni described above, the amount of Ni in the case of inclusion is preferably 0.1% or more.

Mo: 0.1% or less Mo has a function of improving age-hardening ability of steel by forming carbides in combination with V, so that fatigue strength is improved. Therefore, Mo may be included as necessary. . However, since Mo is a very expensive element, if the content increases, the manufacturing cost of steel increases. Therefore, the Mo amount in the case of inclusion is set to 0.1% or less. When Mo is contained, the Mo amount is preferably 0.08% or less. On the other hand, in order to reliably obtain the effect of Mo described above, the amount of Mo in the case of inclusion is preferably 0.01% or more.

Nb: 0.1% or less Nb has the effect of improving the age-hardening ability of steel by forming carbides in combination with V, so that fatigue strength is improved. . However, if the Nb content exceeds 0.1%, coarse Nb carbonitrides are generated during heating during forging, and the toughness is deteriorated. Therefore, the Nb content in the case of inclusion is set to 0.1% or less. When Nb is contained, the Nb content is preferably 0.05% or less. On the other hand, in order to surely obtain the effect of Nb described above, the amount of Nb in the case of inclusion is preferably 0.005% or more.

  Said Cu, Ni, Mo, and Nb can be contained only in any 1 type or 2 or more types of composites. When included, the total content of the above elements may be 1.4% when Cu and Ni are each 0.6% and Mo and Nb are each 0.1%, but 1.2% The following is preferable.

B: 0.005% or less B has the effect of increasing the hardenability of the steel of the present invention and increasing the area ratio of bainite, and may be contained as necessary. However, the above effect is saturated even if the B content exceeds 0.005%. Therefore, the B content when contained is set to 0.005% or less. When B is contained, the amount of B is preferably 0.003% or less, and more preferably 0.0025% or less. On the other hand, in order to surely obtain the effect of increasing the quenching of B and increasing the area ratio of bainite, the B content in the case of inclusion is preferably 0.0005% or more.

  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. Hereinafter, Ca and Bi will be described.

Ca: 0.005% or less Ca has an effect of improving machinability, and may be contained as necessary. 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 Ca content is preferably 0.0035% or less. On the other hand, in order to reliably obtain the above-described effect of improving the machinability of Ca, the Ca content when contained is preferably 0.0005% or more.

Bi: 0.4% or less Bi has an effect of improving the machinability and may be contained as necessary. However, when the Bi content exceeds 0.4%, hot workability is deteriorated. Therefore, the Bi content in the case of inclusion is set to 0.4% or less. When Bi is included, the amount of Bi is preferably 0.35% or less. On the other hand, in order to surely obtain the above-described effect of improving the machinability of Bi, the amount of Bi when contained 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 Ca is 0.005% and Bi is 0.4%, but is preferably 0.3% or less. .

  It is necessary to satisfy the above formulas (1) and (2) even in the steel containing optional components from Cu to Bi.

(B) Organization:
In order to make maximum use of VC, a large amount of pro-eutectoid ferrite is not preferable before aging treatment. Furthermore, the production of a large amount of martensite is not preferable from the viewpoint of machinability. For this reason, the main phase of the structure before the aging treatment of the age-hardening steel of the present invention needs to be bainite which is an intermediate structure. That is, in order to ensure sufficient machinability and solid solution V, the area ratio of bainite needs to be 50% or more in the structure before the aging treatment. The area ratio of the bainite is preferably 70% or more, and most preferably the bainite single phase, that is, the area ratio of bainite is 100%. When the area ratio of bainite is less than 100%, phases other than bainite which is the main phase are pro-eutectoid ferrite, pearlite, and martensite. From the viewpoint of machinability, the smaller the martensite area ratio, the better.

The structure having bainite as the main phase can be obtained by, for example, subjecting the steel having the chemical composition described in the above section (A) to the following treatment (i) or (ii).
(I) Heat at a temperature of 1000 ° C. or higher, perform hot forging with a finishing temperature of 950 ° C. or higher, and then average cooling rate in the temperature range of 900 to 500 ° C. is 5 ° C./second or higher and 500 to 150 ° C. The temperature is cooled under the condition that the average cooling rate in the temperature range is 20 ° C./second or less.
(Ii) After hot forging, solution treatment is performed at a temperature of 950 ° C. or higher, and the average cooling rate in the temperature range of 900 to 500 ° C. is 5 ° C./second or higher, and the average cooling in the temperature range of 500 to 150 ° C. Cooling is performed under the condition that the speed is 20 ° C./second or less.

(C) Manufacturing method of machine parts according to the present invention:
The mechanical component according to the present invention is subjected to an aging treatment after forging or solution treatment by a predetermined method in order to sufficiently obtain the effect of precipitation strengthening of V. 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).

(C-1) Hot forging and subsequent cooling:
The heating temperature of the hot forging material needs to be 1000 ° C. or higher. This is because when the heating temperature is below 1000 ° C., undissolved VC does not dissolve. When the heating temperature is excessively high, the energy cost is increased and the scale loss is increased. For this reason, the heating temperature is desirably 1300 ° C. or lower.

  After heating to the above temperature range, hot forging is performed. When cutting into a predetermined shape by cutting from a state of being cooled after hot forging, the hot forging finishing temperature must be 950 ° C. or more. When the finishing temperature of hot forging is less than 950 ° C., recrystallization occurs at a low temperature. Therefore, the generated austenite grains become small and hardenability cannot be secured, and the area ratio of bainite is 50% or more. The organization of can not be obtained. Therefore, the finishing temperature for forging is 950 ° C. or higher. The finishing temperature for hot forging is preferably 1000 ° C. or higher. On the other hand, the finishing temperature of hot forging is preferably 1250 ° C. or lower.

  In the case of cutting into a predetermined shape after being cooled after hot forging, in the cooling after finishing forging at the finishing temperature of the above hot forging, in the temperature range of 900 to 500 ° C. The average cooling rate must be at least 5 ° C / second. When the average cooling rate in the temperature range of 900 to 500 ° C. is less than 5 ° C./second, a large amount of proeutectoid ferrite is generated and the amount of solid solution V is also reduced. Therefore, the average cooling rate in this temperature range must be 5 ° C./second or more. As the average cooling rate in this temperature range is increased, the constraints on the facility increase. Therefore, the average cooling rate is desirably 100 ° C./second or less.

  When cutting into a predetermined shape after being cooled after hot forging, the average cooling rate in the temperature range of 500 to 150 ° C. needs to be 20 ° C./second or less. . When the average cooling rate in the temperature range of 500 to 150 ° C. is fast and exceeds 20 ° C./second, a large amount of martensite is mixed in the structure, the hardness is increased, and the machinability is lowered. Therefore, the average cooling rate in the above temperature range must be 20 ° C./second or less. On the other hand, if the cooling rate becomes too slow, the productivity decreases. Therefore, the average cooling rate is desirably 0.01 ° C./second or more.

  Specific methods for setting the average cooling rate in the temperature range of 900 to 500 ° C. and 500 to 150 ° C. to the above ranges are, for example, oil cooling treatment, a method of cooling while spraying water by spraying, and the like. can give. Any method may be used as long as the above average cooling rate can be achieved.

  The cooling rate in the temperature range below 150 ° C. does not affect the function and effect of the present invention. Therefore, if the average cooling rate in the temperature range of 900 to 500 ° C and the temperature range of 500 to 150 ° C is the above range, the cooling rate from 150 ° C to room temperature does not need to be controlled.

  In the case of the steel having the chemical composition described in the item (A), the hardness can be easily reduced to 280 or less in HV by performing hot forging and subsequent cooling under the above-described conditions.

(C-2) Solution treatment and subsequent cooling:
When it is difficult to satisfy the above-mentioned conditions in hot forging, hot forging is performed without satisfying the above-mentioned conditions, and further, the same condition as when the above-mentioned conditions are satisfied by performing a predetermined solution treatment Performance can be obtained. Here, the conditions for solution treatment and subsequent cooling are described.

  When solution treatment is performed, there are no special restrictions on hot forging conditions, forging methods under general conditions that do not cause problems in equipment, for example, hot forging after heating to 1200 ° C. The method of standing to cool to room temperature may be used. However, when the heating temperature is excessively high, the energy cost is increased and the scale loss is increased. For this reason, it is desirable to perform hot forging with the upper limit of the heating temperature being 1300 ° C. or lower.

  The solution treatment temperature needs to be 950 ° C. or higher. This is because undissolved VC does not dissolve when the solution treatment temperature falls below 950 ° C. When the solution treatment temperature is excessively high, the energy cost is increased and the scale loss is increased. For this reason, the solution treatment temperature is desirably 1300 ° C. or lower.

  In cooling after solution treatment at the above temperature, the average cooling rate in the temperature range of 900 to 500 ° C. must be 5 ° C./second or more. When the average cooling rate in the temperature range of 900 to 500 ° C. is less than 5 ° C./second, a large amount of proeutectoid ferrite is generated and the amount of solid solution V is also reduced. Therefore, the average cooling rate in this temperature range must be 5 ° C./second or more. As the average cooling rate in this range is increased, the facility restrictions increase. Therefore, the average cooling rate is desirably 100 ° C./second or less.

  Furthermore, the average cooling rate in the temperature range of 500 to 150 ° C. needs to be 20 ° C./second or less. When the average cooling rate in the temperature range of 500 to 150 ° C. is fast and exceeds 20 ° C./second, a large amount of martensite is mixed in the structure, the hardness is increased, and the machinability is lowered. Therefore, the average cooling rate in the above temperature range must be 20 ° C./second or less. On the other hand, if the cooling rate becomes too slow, the productivity decreases. Therefore, the average cooling rate is desirably 0.01 ° C./second or more.

  As described above, specific methods for bringing the average cooling rate in the temperature ranges of 900 to 500 ° C. and 500 to 150 ° C. into the above ranges are, for example, oil cooling treatment, water by spraying, and the like. A method of cooling while spraying is mentioned. However, any method may be used as long as the above average cooling rate can be achieved. Furthermore, the cooling rate in a temperature range of less than 150 ° C. does not affect the operational effects of the present invention. Therefore, if the average cooling rate in the temperature range of 900 to 500 ° C and the temperature range of 500 to 150 ° C is the above range, the cooling rate from 150 ° C to room temperature does not need to be controlled.

  In the case of steel having the chemical composition described in (A) above, after hot forging, solution treatment and subsequent cooling are performed under the conditions described above to easily reduce the hardness to 280 or less in HV. Can do.

(C-3) Cutting:
Steel subjected to hot forging and subsequent cooling under the conditions described in the section (C-1), or solution treatment after hot forging under the conditions described in the section (C-2) and thereafter Next, the steel subjected to the above cooling is subjected to cutting and processed into a predetermined machine part shape.

  In the case of steel having the chemical composition described in the section (A), hot forging and subsequent cooling are performed under the conditions described in the section (C-1), or the conditions described in the section (C-2). Thus, by performing solution treatment after hot forging and subsequent cooling, the hardness becomes 280 or less in HV, so that it can be easily 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.

(C-4) Aging treatment:
The aging treatment must be performed at a temperature of 560 to 700 ° C. When the aging 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 productivity is lowered and heat treatment cost is increased.

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

[Example 1]
Example 1 is an example for steel with a low S content.

  Steels A1 to T1 having chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace.

  Steels A1 to O1 and Steel R1 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steel P1, steel Q1, steel S1, and steel T1 are steels whose chemical compositions deviate from the conditions defined in the present invention.

  Each steel ingot was heated at 1250 ° C. and hot forged into a steel bar having a diameter of 45 mm. The hot-forged steel bar with a diameter of 45 mm is once cooled in the atmosphere and cooled to room temperature, then heated to 1250 ° C., assuming forging into a part shape, the finishing temperature is 1100 ° C., and the diameter is 30 mm. Hot forged into steel bars.

  After performing the above hot forging, for each test number shown in Table 2, cooling while spraying a small amount of water in the shower (test number 2), forced air cooling with a blower (test number 23), water cooling (test number) 24), air cooling (test number 5) and oil cooling (test numbers 1, 3, 4, 6-22, 25, and 26) were performed in the atmosphere, and the mixture was cooled to room temperature. Test No. 5 was further subjected to solution treatment after being allowed to cool in the air. The solution treatment was heated at 1100 ° C. for 30 minutes and then cooled to room temperature by oil cooling.

  The average cooling rate for each test number in Table 2 is prepared separately from commercially available S25C steel, processed into a test piece with a diameter of 30 mm and a length of 100 mm, and then at the bottom of a hole with a depth of 50 mm provided on the bottom surface. Measured with a thermocouple attached. That is, after heating a test piece having a diameter of 30 mm and a length of 100 mm of the S25C steel to 1100 ° C., cooling while spraying a small amount of water in a shower, forced air cooling by a blower, water cooling, and oil cooling each cooling method It was measured by cooling with.

  For each test number, a part of the steel bar after cooling was not subjected to an aging treatment (that is, in a cooled state), and both ends of the steel bar were cut off by 100 mm each, and then a test piece was cut out to obtain a structure, a hard The thickness and fatigue strength were investigated. On the other hand, for each test number, the rest of the steel bar after cooling was subjected to an aging treatment that was held at 580 to 650 ° C. for 1 to 4 hours. A strength study was conducted.

  The organizational survey was conducted as follows. That is, after cutting out a cross section perpendicular to the longitudinal direction of the steel bar and including R / 2 part (“R” represents the radius of the steel bar), a specimen that was mirror-polished and corroded with nital was prepared. Using an optical microscope, photographs of three fields of view were randomly taken at a magnification of 200 times and a field size of 220 μm × 180 μm to identify phases in the tissue. In addition, image analysis was performed for each of the above fields of view, and the area ratios of bainite, proeutectoid ferrite, pearlite, and martensite were obtained.

  Hardness measurement was performed as follows. A test piece was prepared by crossing a steel bar with a diameter of 30 mm, filling the resin so that the cut surface became the test surface, and mirror polishing. In accordance with “Vickers hardness test-test method” in JIS Z 2244 (2009), the test force was set at 9.8 N for 10 points near the R / 2 part of the test surface. Obtained by arithmetic averaging.

The fatigue strength was investigated by collecting a smooth Ono type rotating bending fatigue test piece. That is, the smooth ono-type rotating bending fatigue test pieces having a diameter and length of 8 mm and 18 mm, respectively, shown in FIG. An Ono type rotating bending fatigue test was performed under the condition of several 3400 rpm. Under the above conditions, the maximum stress which does not break in stressing repeated several 10 ⁷ times was fatigue strength.

  Table 2 shows the structure of each test number in the cooled state (that is, before the aging treatment), and the hardness and fatigue strength before and after the aging treatment. In Table 2, “phase” in the structure is expressed as “B” for bainite, “P” for pearlite, “F” for proeutectoid ferrite, and “M” for martensite. Furthermore, the amount of age hardening, which is the amount of increase in hardness due to aging, was expressed as “ΔHV”, and the amount of increase in fatigue strength due to aging was expressed as “Δσw”.

  As is clear from Table 2, in the case of test numbers 1 to 20 having the chemical composition and structure defined in the present invention, the hardness before aging treatment is 280 or less in HV and excellent in machinability, fatigue due to aging treatment The increase in strength is 50 MPa or more, and the fatigue strength after aging treatment is excellent at 410 MPa or more.

  On the other hand, in the case of test numbers 21 to 26 that are out of the definition of the present invention, the target performance is not obtained.

  In the case of test number 21, in addition to steel P1 not containing V, the area ratio of bainite in the structure before aging treatment is as low as 12%. For this reason, the amount of increase in hardness due to aging treatment (hereinafter referred to as “age hardening”) is small, and the fatigue strength after aging treatment is low.

  In the case of test number 22, the C content of steel Q1 is small, and the value of “C / V” is also small. Therefore, since the precipitation amount of VC decreases, the age hardening amount is small, and the fatigue strength after the aging treatment is low.

  In the case of test number 23, although the chemical composition of steel A1 satisfies the provisions of the present invention, the average cooling rate in the temperature range of 900 to 500 ° C. is slow, so the area ratio of bainite in the structure before aging treatment is 27%. Low. For this reason, the age hardening amount is small, and the fatigue strength after the aging treatment is low.

  In the case of test number 24, although the chemical composition of the steel R1 satisfies the provisions of the present invention, the average cooling rate in the temperature range of 500 to 150 ° C. is too high, so the martensite area ratio in the structure before aging treatment is 90 %, And the area ratio of bainite is as low as 10%. For this reason, the hardness before an aging treatment is 329 in HV, and is too high.

  In the case of the test number 25, the N content of the steel S1 is high and the value of “N / Ti” is too large. Therefore, since V precipitates as VN before the aging treatment, the amount of age hardening is small and the fatigue strength after the aging treatment is low.

  In the case of test number 26, the value of “N / Ti” of steel T1 is too large. Therefore, since V precipitates as VN before the aging treatment, the amount of age hardening is small and the fatigue strength after the aging treatment is low.

[Example 2]
Example 2 is an example for steel with a high S content.

  Steels A2 to Q2 having chemical compositions shown in Table 3 were melted in a 50 kg vacuum melting furnace.

  Steels A2 to L2 in Table 3 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, the steels M2 to Q2 are steels whose chemical compositions deviate from the conditions specified in the present invention.

  Each steel ingot was heated at 1250 ° C. and hot forged into a steel bar having a diameter of 45 mm. The hot-forged steel bar with a diameter of 45 mm is once cooled in the atmosphere and cooled to room temperature, then heated to 1250 ° C., assuming forging into a part shape, the finishing temperature is 1100 ° C., and the diameter is 30 mm. Hot forged into steel bars.

  After performing the above hot forging, for each test number shown in Table 4, cooling while spraying a small amount of water in a shower (test number 29), oil cooling (test numbers 27, 28, 30-35, 37- 41 and 43 to 47), forced air cooling with a blower (test number 42) and air cooling (test number 36) were performed, and the mixture was cooled to room temperature. Test number 36 was further subjected to a solution treatment after being allowed to cool in the air. The solution treatment was heated at 1100 ° C. for 30 minutes and then cooled to room temperature by oil cooling.

  Similarly to the case of Example 1, the average cooling rate for each test number in Table 4 was prepared separately from commercially available S25C steel, processed into a test piece having a diameter of 30 mm and a length of 100 mm, and then provided on the bottom surface. It is measured by attaching a thermocouple to the bottom of a hole having a depth of 50 mm. That is, after heating the test piece of S25C steel with a diameter of 30 mm and a length of 100 mm to 1100 ° C., cooling with a small amount of water in a shower, cooling with oil cooling and forced air cooling with a blower. And measured.

  For each test number, a part of the steel bar after cooling was not subjected to an aging treatment (that is, in a cooled state), and both ends of the steel bar were cut off by 100 mm each, and then a test piece was cut out to obtain a structure, a hard The thickness and fatigue strength were investigated. On the other hand, for each test number, the rest of the steel bar after cooling was subjected to an aging treatment that was held at 580 to 650 ° C. for 1 to 4 hours. A strength study was conducted.

  The organizational survey was conducted as follows. That is, after cutting out a cross section perpendicular to the longitudinal direction of the steel bar and including R / 2 part (“R” represents the radius of the steel bar), a specimen that was mirror-polished and corroded with nital was prepared. Using an optical microscope, photographs of three fields of view were randomly taken at a magnification of 200 times and a field size of 220 μm × 180 μm to identify phases in the tissue. In addition, image analysis was performed for each of the above fields of view, and the area ratios of bainite, proeutectoid ferrite, pearlite, and martensite were obtained.

  Hardness measurement was performed as follows. A test piece was prepared by crossing a steel bar with a diameter of 30 mm, filling the resin so that the cut surface became the test surface, and mirror polishing. In accordance with “Vickers hardness test-test method” in JIS Z 2244 (2009), the test force was set at 9.8 N for 10 points near the R / 2 part of the test surface. Obtained by arithmetic averaging.

The fatigue strength was investigated by collecting a smooth Ono type rotating bending fatigue test piece. That is, the smooth ono-type rotating bending fatigue test pieces having a diameter and length of 8 mm and 18 mm, respectively, shown in FIG. An Ono type rotating bending fatigue test was performed under the condition of several 3400 rpm. Under the above conditions, the maximum stress which does not break in stressing repeated several 10 ⁷ times was fatigue strength.

  Table 4 shows the structure of each test number in the cooled state (that is, before the aging treatment), and the hardness and fatigue strength before and after the aging treatment. In Table 4, “phase” in the structure is expressed as “B” for bainite, “P” for pearlite, “F” for proeutectoid ferrite, and “M” for martensite. Furthermore, the amount of age hardening, which is the amount of increase in hardness due to aging, was expressed as “ΔHV”, and the amount of increase in fatigue strength due to aging was expressed as “Δσw”.

  As apparent from Table 4, in the case of Test Nos. 27 to 41 having the chemical composition and structure defined in the present invention, the hardness before aging treatment is 280 or less in HV, which is excellent in machinability, and fatigue due to aging treatment. The increase in strength is 50 MPa or more, and the fatigue strength after aging treatment is excellent at 340 MPa or more.

  On the other hand, in the case of the test numbers 42 to 47 that are out of the definition of the present invention, the target performance is not obtained.

  In the case of test number 42, although the chemical composition of steel B2 satisfies the provisions of the present invention, the average cooling rate in the temperature range of 900 to 500 ° C. is slow, so no bainite is generated in the structure before aging treatment. For this reason, the age hardening amount is small, and the fatigue strength after the aging treatment is low.

  In the case of test number 43, since the S content of steel M2 is too high, the fatigue strength after aging treatment is low.

  In the case of test number 44, in addition to the C content of steel N2 being too high, the N content is high and the value of “N / Ti” is too large. For this reason, the hardness before an aging treatment is 305 in HV, and is too high.

  In the case of test number 45, since steel O2 does not contain V, the amount of age hardening is small and the fatigue strength after aging treatment is low.

  In the case of test number 46, the N content of steel P2 is high, and the value of “N / Ti” is too large. Therefore, since V precipitates as VN before the aging treatment, the amount of age hardening is small and the fatigue strength after the aging treatment is low.

  In the case of test number 47, the value of “N / Ti” of steel Q2 is too large. Therefore, since V precipitates as VN before the aging treatment, the amount of age hardening is small and the fatigue strength after the aging treatment is low.

  The age-hardening steel of the present invention is extremely useful as a material for machine parts such as automobiles, industrial machines, and construction machines. In addition, according to the manufacturing method of the present invention, it is possible to manufacture a machine part having a desired strength by performing cutting in a state where the hardness of the steel is low, adjusting the shape of the part, and thereafter curing by aging treatment. .

Claims (7)

Age-hardenable steel in mass%, C: 0.025 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.8%, P: 0.05 %: S: 0.10% or less, Cr: 0.05 to 0.6%, Al: 0.06% or less, Ti: 0.005 to 0.20%, V: 0.10 to 0.60 % And N: 0.012% or less, with the balance being Fe and impurities, having a chemical composition that satisfies the following formulas (1) and (2), and an area ratio of bainite of 50% An age-hardening steel characterized by having the above structure.
C / V ≧ 0.20 ... (1)
N / Ti ≦ 0.60 (2)
The element symbols in the above formulas (1) and (2) mean the content (% by mass) of the element.   The age-hardenable steel according to claim 1, wherein the Mn content is 0.50 to 1.5%, the Ti content is 0.005 to 0.06%, and N / Ti is 0.40 or less.   Age-hardenable steel according to claim 1 or 2, wherein the S content is 0.05 to 0.10%.   Age-hardenable steel according to claim 1 or 2, wherein the S content is less than 0.05%. It replaces with a part of Fe and contains the element chosen from at least 1 element group in the element group of following <1>-<3> by the mass%. The age-hardening steel according to any one of the above.
<1> Cu: 0.6% or less, Ni: 0.6% or less, Mo: 0.1% or less, and Nb: 0.1% or less <2> B: 0.005% or less <3> One or more of Ca: 0.005% or less and Bi: 0.4% or less   It is a manufacturing method of machine parts, Comprising: The steel in any one of Claim 1-5 is heated at the temperature of 1000 degreeC or more, the hot forging whose finishing temperature is 950 degreeC or more is given, and 900-500 after that The cooling rate is 560 ° C. after cooling under the condition that the average cooling rate in the temperature range of 5 ° C. is 5 ° C./second or more and the average cooling rate in the temperature range of 500 to 150 ° C. is 20 ° C./second or less. A method for manufacturing a machine part, characterized by performing an aging treatment at a temperature of 700 ° C.   It is a manufacturing method of machine parts, Comprising: After hot forging the steel in any one of Claim 1-5, it solution-treats at the temperature of 950 degreeC or more, and is the average of the temperature range of 900-500 degreeC Cooling is performed under conditions where the cooling rate is 5 ° C./second or more and the average cooling rate in the temperature range of 500 to 150 ° C. is 20 ° C./second or less, and after aging, aging is performed at a temperature of 560 to 700 ° C. A method of manufacturing a machine part, characterized by performing processing.

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