JP5664371B2 - 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. 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 the following manufacturing method.

  Specifically, it is a method for producing an age-hardening machine part, and in mass%, C: 0.10 to 0.25%, Si: 0.10 to 0.50%, Mn: 0.60 to 1. 0%, S: 0.01-0.10%, Ti: 0.005- (4 × N)%, V: 0.10-0.25% and Mo: 0.05-0.60%, 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. The hot forging is finished at a temperature T1 of 900 ° C. or higher by heating, and is cooled at a cooling rate of 10 ° C./second or more from at least 900 ° C. to a temperature T2 in the range of 550 to 450 ° C. Next, hold it in the temperature range of 550 to 450 ° C. for 1 minute or more, cool it to room temperature, and perform cutting. After, characterized by age hardening treatment "method for producing age hardened mechanical parts" is disclosed at a temperature of 560-650 ° C..

  Patent Document 2 discloses the following age hardened steel.

Specifically, by 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 to 0.5% and Cr: 0.1 to 3.0%, and if necessary, Al: 0.001 to 0.3%, N: 0.005 -0.025%, 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: 0.0. One or more of 01 to 0.20%, Ca: 0.003 to 0.010%, Pb: 0.3% or less and Bi: 0.3% or less, with the balance being 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
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, “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 increases the hardness by 7 HRC or more than the hardness before the aging treatment. Has been.

JP 2008-88508 A JP 2006-37177 A

  In the case of the technique disclosed in Patent Document 1 described above, in order to increase the age-hardening ability, it is necessary to hold each part for a predetermined time in a specific temperature range 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.

  In the technique disclosed in Patent Document 2, the hardness before aging treatment is 40 or less in HRC, but the hardness of many of the disclosed steels before aging treatment is 32 or more in HRC, which is a high workability. In some cases, it cannot be used for members that require high performance. Further, Patent Document 2 discloses steels having a hardness before age hardening of 27.9 to 28.5 in HRC, but these steels contain much greater than 1.0% Mo, In terms of cost, it was not practical.

  Accordingly, an object of the present invention is to provide an age-hardening steel that satisfies all the following <1> to <3>.

  <1> When cutting into a predetermined shape by performing cutting after hot forging, the hardness after hot forging, which is the hardness before cutting, is 315 in terms of Vickers hardness (hereinafter referred to as “HV”). Excellent in machinability as follows.

  <2> The fatigue strength after aging treatment is 510 MPa or more.

  <3> A dedicated soaking furnace is not required in the cooling process after hot forging.

  Specifically, the object of the present invention is to perform an average cooling rate of less than 5 ° C./second (hereinafter referred to as “comparison”) without performing rapid cooling such as oil cooling after hot forging. The hardness in the cooled state is 315 or less in HV, and the steel can be cut in the cooled state after hot forging, and the aging treatment is performed after the cutting. Is to provide an “age-hardening steel” capable of obtaining a fatigue strength of 510 MPa or more.

  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 forming the compound to be precipitated during the aging treatment has a strong ability to form a compound (secondary phase) at the aging treatment temperature, and in a state of being dissolved in the matrix during hot forging. Must exist.

  (B) In the steel of the present invention, it is necessary to obtain a high fatigue strength of 510 MPa or more by aging treatment. Mo is an element that forms and precipitates carbides by aging treatment. Moreover, since the carbide of Mo has high solubility, Mo can be easily dissolved in the matrix in a state before aging treatment. Therefore, when high fatigue strength is required, high fatigue strength can be stably obtained by adding Mo to the steel. However, in order to obtain a desired fatigue strength by precipitation of carbides of Mo, it is necessary to contain a large amount of Mo in the steel, and a significant increase in cost is inevitable. Therefore, in order to obtain a practical steel, the content of Mo should be allowed in terms of cost, and other carbide-forming elements should be included in a complex manner, and precipitation hardening by the carbide-forming elements that have been incorporated in a complex manner. It is necessary to make it effective.

  (C) Examples of carbide forming elements other than Mo include Ti, Nb, and V. However, Ti carbide and Nb carbide are less soluble in steel than V carbide. For this reason, in the case of Ti and Nb, the amount that can be dissolved in the matrix in the state before the aging treatment is small. On the other hand, in the case of V, the solubility of V carbide in steel is larger than that of Ti carbide and Nb carbide, and the precipitation peak temperature of V carbide when cooled from a high temperature is 750. It is about -700 degreeC, Comprising: It is low compared with the temperature of the precipitation peak of the carbide | carbonized_material of Ti and the carbide | carbonized_material of Nb. For example, in a steel containing 0.3% by mass of V and 0.1% by mass of C, V carbides do not precipitate until around 850 ° C. once they are dissolved in the matrix. For this reason, in the case of V, it is relatively easy to suppress precipitation during hot forging. And since V forms a carbide | carbonized_material compositely with Mo, even if it does not contain Mo exceeding a practical amount by including V further in steel containing Mo, big age hardening ability can be anticipated. .

  (D) In order to effectively exhibit precipitation hardening due to the compositely contained V, it is necessary to keep V in a solid solution state before the aging treatment.

  (E) The first means for keeping V in a solid solution state before the aging treatment is to reduce the amount of N in the solid solution state during hot forging. When the amount of N in a solid solution state is large during hot forging, N is combined with V during precipitation and cooling after forging, and precipitates as a nitride of V. N is an element inevitably mixed in the steel, and the temperature at which the V nitride starts to precipitate is higher than that of the V carbide. For example, the precipitation start temperature of the nitride of V 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. is there. This temperature corresponds to a general hot forging temperature. Therefore, in order to reduce the amount of N in a solid solution state during hot forging, it is necessary to contain Ti having a higher nitride forming ability than V.

  (F) A second means for keeping V in a solid solution state before the aging treatment is to use bainite as the main phase of the structure after hot forging (that is, a phase ratio of 60% or more). It is. The carbide of V precipitates at the phase interface when austenite is transformed into ferrite. Therefore, if a large amount of proeutectoid ferrite is generated during cooling after hot forging, the amount of V in a solid solution state is reduced. Therefore, it is necessary to prevent ferrite from being precipitated as much as possible during cooling after hot forging.

  Then, next, the present inventors investigated the conditions for making the main phase of a structure | tissue into bainite by changing a steel component variously about the steel containing 0.20 mass% or more of V. Moreover, the hardness before aging treatment was investigated. Furthermore, the conditions under which the fatigue strength after aging was 510 MPa or more were investigated.

  As a result, the following findings (g) to (j) were obtained.

  (G) Although machinability decreases when the structure becomes martensite, Mo is a strong bainite stabilizing element, and bainite can be stably generated without making the structure martensite compared to Cr and Mn. Can do. After hot forging, the structure cooled at the relatively slow cooling rate has a close correlation with the contents of C, Mn, Cr, Mo and B. That is, if the content of the above element satisfies the conditions represented by the formula (1) or (1 ′) described later, proeutectoid ferrite does not precipitate in a large amount. Moreover, a large amount of martensite does not precipitate at the relatively slow cooling rate described above. For this reason, it can be easily made into a structure having bainite as the main phase.

  (H) However, if the contents of C, Mn, Cr, Mo and B satisfy only the conditions described in the above (g), the hardness before aging treatment is increased by the action of solid solution strengthening, etc. The machinability is reduced. On the other hand, if the content of C, Si, Mn, Cr, Mo and V satisfies another condition represented by the formula (2) described later, the hardness before the aging treatment can be kept low, which is good Machinability is obtained.

  (I) A steel in which the contents of C, Si, Mn, Cr, Mo, V and B satisfy both the conditions described in (g) and (h) is a structure whose main phase is bainite. Since it is excellent in machinability, it can be easily cut into a predetermined shape. In addition, this steel can be easily strengthened because Mo and V in a solid solution precipitate carbide and age harden by aging treatment.

(J) In the case of Ti as described above, the amount that can be dissolved in the matrix in the state before the aging treatment is smaller than that in the case of V. However, the remaining Ti that forms nitrides and sulfides, that is, Ti in a solid solution state before the aging treatment, can further increase the fatigue strength after the aging treatment, and contains V, Ti, N, S, and Mo. If the amount further satisfies the condition expressed by the following formula (3) , the fatigue strength after the aging treatment can be set to 510 MPa or more.

  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 0.28%, Si: 0.05 to 0.50%, Mn: 0.50 to 2.5%, P: 0.05% or less, S: 0.10% or less, Cr: 0.05 to 2.5%, Al: 0.06% or less, Ti: 0.005 to 0.20%, V: 0.20 to 0.75%, Mo: 0 30 to 0.70 % and N: 0.020% or less, the balance being Fe and impurities, and satisfying the following formulas (1) to (3) steel.
C + 0.3Mn + 0.25Cr + 0.6Mo ≧ 0.65 (1)
C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo ≦ 0.82 (2)
V + 0.8Tieff + 0.35Mo> 0.35 (3)
The element symbols in the above formulas (1) to (3) mean the content (% by mass) of the element.
Further, Tieff in the expression (3) indicates {Ti- (48/14) N- (48/32) S} or 0, whichever is larger.

(2) By mass%, C: 0.05 to 0.28%, Si: 0.05 to 0.50%, Mn: 0.50 to 2.5%, P: 0.05% or less, S: 0.10% or less, Cr: 0.05 to 2.5%, Al: 0.06% or less, Ti: 0.005 to 0.20%, V: 0.20 to 0.75%, Mo: 0 .30 to 0.70 % and N: 0.020% or less, and at least one element belonging to any one of <1> to <3> below, and the balance from Fe and impurities An age-hardening steel characterized by satisfying the following formulas (1 ′), (2) and (3):
<1> B: 0.005% or less <2> Cu: 0.6% or less and Ni: 0.6% or less under <br/><3> Ca: 0.005% or less and Bi: 0.4% C + 0.3Mn + 0.25Cr + 0.6Mo + Beff ≧ 0.65 (1 ′)
C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo ≦ 0.82 (2)
V + 0.8Tieff + 0.35Mo> 0.35 (3)
The element symbols in the above formulas (1 ′), (2) and (3) mean the content (mass%) of the element, and Beff in the formula (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.
Further, Tieff in the expression (3) indicates {Ti- (48/14) N- (48/32) S} or 0, whichever is larger.

  (3) A method of manufacturing a machine part, wherein the steel described in (1) or (2) is heated at a temperature of 1000 ° C. or higher, hot forging with a finishing temperature of 900 ° C. or higher is performed, and then 5 A method for producing a machine part, comprising cooling to at least 500 ° C. at an average cooling rate of less than ° C./second, and further performing aging treatment at a temperature of 560 to 700 ° C. after cutting.

  “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 for cooling at an average cooling rate of less than 5 ° C./second after hot forging also refers to the temperature of the surface of the material to be treated.

  The “average cooling rate” when cooling to at least 500 ° C. after hot forging is the time required to cool the temperature difference between the hot forging finishing temperature and 500 ° C. to 500 ° C. after hot forging. Refers to the one that is divided.

  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 the hot forging and cutting are performed to form a predetermined shape, the cutting that is in a cooled state after the hot forging is performed. A machine part having a desired fatigue strength of 510 MPa or more can be obtained by curing with an aging treatment performed after cutting and having a low previous hardness of 315 or less and excellent in machinability.

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: 0.05 to 0.28%
C combines with V, Mo and Ti to form carbides and strengthens the steel. However, if the C content is less than 0.05%, the precipitation driving force of the carbide becomes small and it becomes difficult for the carbide to precipitate, so the desired strengthening effect cannot be obtained. On the other hand, if the C content exceeds 0.28%, C forms Fe and carbides and strengthens the structure before the aging treatment, so that machinability is lowered. Therefore, the content of C is set to 0.05 to 0.28%. The C content is preferably 0.10% or more, and preferably 0.24% 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 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. Therefore, the Si content is set to 0.05 to 0.50%. The Si content is preferably 0.10% or more, and preferably 0.40% or less.

Mn: 0.50 to 2.5%
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 obtain these effects sufficiently, the Mn content must be at least 0.50%. However, if the content of Mn exceeds 2.5%, the hardenability becomes excessive, and the amount of solid solution strengthening by Mn increases, so that the machinability and hot workability are greatly reduced. Therefore, the Mn content is set to 0.50 to 2.5%. The Mn content is preferably 0.60% or more, and 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 the machinability, so it may be 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, when positively containing the machinability improving effect of S over fatigue strength, it is more preferable to contain 0.05% or more of S.

Cr: 0.05-2.5%
Cr increases the hardenability, has the effect of increasing the area ratio of bainite as the main phase of the structure after hot forging. In order to sufficiently obtain this effect, the Cr content needs to be 0.05% or more. However, if the Cr content exceeds 2.5%, the hardenability becomes excessive, and the amount of solid solution strengthening due to Cr increases, so that the machinability and hot workability are greatly reduced. Therefore, the Cr content is set to 0.05 to 2.5%. The Cr content is preferably 2.3% or less, and more preferably 2.0% or less.

Al: 0.06% or less Al is an element inevitably contained as an impurity. Al may be 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 Al content 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 precipitation of VN during hot forging and during cooling after forging. In order to sufficiently obtain this effect, Ti needs to have a content of 0.005% or more. Since Ti in solid solution precipitates carbide in combination with V by aging treatment, it has the 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 0.06% or less, and more preferably 0.04% or less, when it is desired to reduce the hardness before aging treatment. On the other hand, even if the hardness before the aging treatment is somewhat increased, when it is desired to improve the fatigue strength by obtaining a high hardness after the aging treatment, it is preferable 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: 0.20 to 0.75%
V is an important element in the steel of the present invention. V combines with C during aging treatment to form fine carbides, and thus has the effect of significantly increasing the strength. In order to sufficiently obtain this effect, 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 0.75%. , The toughness is significantly reduced. If the V content exceeds 0.75%, the machinability is also significantly reduced. Therefore, the content of V is set to 0.20 to 0.75%. When importance is attached to machinability, the V content is preferably 0.70% or less, and more preferably 0.65% or less. On the other hand, the V content when importance is placed on strength is preferably 0.23% or more, and more preferably 0.25% or more.

Mo: 0.30 to 0.70 %
Mo enhances hardenability and has the effect of stably forming the main phase of the structure into bainite and increasing the area ratio without converting the structure after hot forging into martensite. Moreover, Mo can make age-hardening ability larger than the bainite strengthened with Cr and Mn. Therefore, there is an effect of increasing the fatigue strength after the aging treatment without greatly impairing the machinability. Furthermore, Mo also has the effect | action which enlarges age hardening ability by forming a carbide | carbonized_material compositely with V in the steel containing 0.20% or more of V. In order to sufficiently obtain these effects, Mo needs to be contained in an amount of 0.30% or more. However, since Mo is a very expensive element, if the content increases, the manufacturing cost of steel increases. Therefore, an upper limit is provided, and the Mo content is set to 0.30 to 0.70 %. The Mo content is preferably 0.32% or more .

N: 0.020% 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 content of Ti for fixing N increases, and the TiN produced in association with it increases to reduce toughness. Therefore, an upper limit is set and the N content is set to 0.020% or less. The N content is preferably 0.015% or less, and more preferably 0.012% or less.

  One of the age-hardening steels of the present invention is a steel that contains the above-described elements from C to N, the balance is Fe and impurities, and satisfies the above formulas (1) to (3). .

  The “impurities” in the remaining “Fe and impurities” refer to materials mixed from ores, scraps, or production environments as raw materials when industrially producing steel materials.

(1) For ~ (3), described below together with (1 ') below.

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 <3>. The balance is made of Fe and impurities, and satisfies the above formulas (1 ′), (2) and (3) .

  Hereafter, the effect of the element which belongs to each of <1>-<3> and the reason for limitation of content are demonstrated.

<1> 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, 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 included, the amount of B is preferably 0.003% or less.

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

<2> Cu: 0.6% or less and Ni: 0.6% or less under <br/> Cu and Ni are both has an effect of improving the fatigue strength after aging treatment. 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 a function of improving fatigue strength by solid solution strengthening if the amount is small, and precipitation strengthening at the time of aging treatment if the amount is large. 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 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 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 surely obtain the effect of Ni described above, the amount of Ni in the case of inclusion is preferably 0.1% or more.

Said Cu and Ni can be contained only in any 1 type or 2 types of composites. The total content of the above elements when contained may be 0.6% for Cu and Ni, but is preferably 1.1% or less.

<3> 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 surely 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 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 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. .

[C + 0.3Mn + 0.25Cr + 0.6Mo] or [C + 0.3Mn + 0.25Cr + 0.6Mo + Beff]: 0.65 or more When the age-hardening steel of the present invention does not contain any element from B to Bi, the following When the formula (1) includes one or more optional elements from B to Bi, the following formula (1 ′) must be satisfied.
C + 0.3Mn + 0.25Cr + 0.6Mo ≧ 0.65 (1),
C + 0.3Mn + 0.25Cr + 0.6Mo + Beff ≧ 0.65 (1 ′).

  This is because, if the above conditions are satisfied, even in a state of cooling at a relatively slow cooling rate as described above after hot forging, it becomes a structure in which 60% or more of the area ratio is bainite easily. .

  As already described, the element symbols in the above formulas (1) and (1 ′) mean the content (% by mass) of the element.

  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 ".

  The value on the left side of the formula (1) is preferably 0.68 or more, and more preferably 0.70 or more. On the other hand, the value on the left side of the equation (1) is preferably 1.00 or less.

  Similarly, the value on the left side of the expression (1 ′) is preferably 0.68 or more, and more preferably 0.70 or more. On the other hand, the value on the left side of the expression (1 ′) is preferably 1.00 or less.

[C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo]: 0.82 or less The age-hardenable steel of the present invention needs to satisfy the following formula (2).
C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo ≦ 0.82 (2).

  The age-hardening steel of the present invention has high hardness before aging treatment only by satisfying the above formula (1) or (1 ′). For this reason, good machinability cannot be ensured. However, if the age-hardenable steel of the present invention satisfies the formula (2) in addition to the formula (1) or (1 ′), the aging is performed even though the main phase of the structure is bainite. The hardness before processing becomes a relatively low value of 315 or less in HV, and good machinability is obtained. For this reason, it can be easily cut into a predetermined shape.

  As already stated, the element symbol in the above formula (2) means the content (% by mass) of the element.

The value on the left side of equation (2) is 0 . 80 or less is more preferable. On the other hand, the value on the left side of the equation (2) is preferably 0.50 or more.

[V + 0.8Tieff + 0.35Mo]: 0.35 age hardenable steel super present invention, even if they do not contain any element to Bi from the B, include any element to Bi 1 or more from the previous SL B case also, it is necessary to meet the following equation (3).
V + 0.8Tieff + 0.35Mo> 0.35 ··· (3).

Tieff in the formula (3) means the remaining Ti content that forms nitrides and sulfides, that is, the Ti content (% by mass) that acts to form carbides, and {Ti− (48/14 ) N- (48/32) S} or 0, whichever is larger.

  In addition to the above formula (1) or (1 ′), the age-hardening steel of the present invention may satisfy the formula (2) and the fatigue strength after aging treatment may be insufficient. That is, in some cases, a machine part after aging treatment cannot be provided with a high fatigue strength of 510 MPa or more.

However, if the age-hardenable steel of the present invention satisfies the above formulas (1) to (3), or if the above formulas (1 ′), (2), and (3) are satisfied, an aging treatment is performed. Although the previous hardness is 315 or less in HV, a high fatigue strength of 510 MPa or more is stably obtained after the aging treatment, and therefore, it can be applied to a part with a large load.

As already described , the element symbol in the above formula (3) means the content (% by mass) of the element.

  The value on the left side of the formula (3) is preferably 0.37 or more, and more preferably 0.39 or more. On the other hand, the value on the left side of equation (3) is preferably 2.00 or less.

  The method for producing the age-hardening steel of the present invention is not particularly limited. However, in order to make maximum use of VC, a large amount of pro-eutectoid ferrite is not preferred before aging treatment. Furthermore, the production of a large amount of martensite is not preferable from the viewpoint of machinability. Therefore, the age-hardening steel of the present invention is, for example, a steel having the above-described chemical composition heated at a temperature of 1000 ° C. or higher, and held for 30 to 1000 minutes, for example, with a finishing temperature of 900 ° C. or higher. It is preferable to manufacture by forging and then cooling to at least 500 ° C. at an average cooling rate of less than 5 ° C./second.

  In addition, the above-mentioned temperature of “heating the steel at a temperature of 1000 ° C. or higher and holding it for 30 to 1000 minutes” also means an average value of the furnace temperature of the heating furnace.

  If the steel having the above-described chemical composition is produced as described above, it is possible to stably and surely prevent the generation of a large amount of pro-eutectoid ferrite and martensite, and a structure having bainite as a main phase, in particular, , 60% or more of the area ratio becomes a bainite structure.

(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 The heating temperature of the raw material for hot forging needs to be 1000 degreeC or more. 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.

  In addition, although it changes with the size (mass) of the raw material for hot forging, it is preferable that the holding time at said heating temperature shall be 30-1000 minutes, for example.

  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 into a predetermined shape after being cooled after hot forging, at least 500 ° C. with an average cooling rate of less than 5 ° C./second after finishing hot forging at the above finishing temperature. Must be cooled to When the average cooling rate up to 500 ° C. after the completion of hot forging is 5 ° C./second or more, a large amount of martensite is generated and machinability is lowered. Therefore, the average cooling rate up to 500 ° C. after completion of hot forging must be less than 5 ° C./second. The average cooling rate up to 500 ° C. after completion of hot forging is preferably less than 4 ° C./second, and more preferably less than 3 ° C./second. If the above average cooling rate is too small, the structure may not be bainite and the productivity may be lowered. Therefore, the average cooling rate is preferably 0.2 ° C./second or more, and more preferably 0.4 ° C./second or more.

  Specific methods for setting the average cooling rate up to 500 ° C. to less than 5 ° C./second after the completion of hot forging include, for example, cooling in the air and 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 completion of hot forging is less than 5 ° 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 above section (A), by performing hot forging under the above-described conditions and subsequent cooling, a structure having bainite as a main phase is obtained, and the hardness is easily increased to HV. 315 or less.

(B-2) Cutting The steel subjected to hot forging and subsequent cooling under the conditions described in the above section (B-1) is then subjected to cutting to be processed into a predetermined machine part shape. The

  In the case of steel having the chemical composition described in the section (A), the hardness becomes 315 or less in HV by performing hot forging and subsequent cooling under the conditions described in the section (B-1). 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.

(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, so that the time required for the aging treatment becomes long, resulting in a decrease in productivity and an increase in heat treatment cost.

  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.

Steels A to G, I to K, and M to S having chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace.

Steels A to G, I to K, and MP in Table 1 are steels whose chemical compositions are within the range defined in the present invention. On the other hand, steels Q to S are steels whose chemical compositions deviate from the conditions defined in the present invention.

  Table 1 shows the value on the left side of equation (1) when no arbitrary element from B to Bi is included, and the left side of equation (1 ') when one or more optional elements from B to Bi are included. The value of is indicated as “value of F1”.

  Similarly, in Table 1, the value on the left side of equation (2) is shown as “value of F2.”

Further, in Table 1, the value on the left side of the expression (3) is shown as “value of F3”.

  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 having a diameter of 45 mm was once cooled in the atmosphere and cooled to room temperature. Thereafter, the steel was further heated to 1250 ° C., and forging into a part shape was assumed. The finishing temperature was set to 950 ° C., and hot forging into a plurality of steel bars having a diameter of 30 mm and a length of 1000 mm. 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 0.8 ° C./second.

  For each test number, a portion of the 30 mm diameter steel bar cooled to room temperature is not subjected to aging treatment (that is, in a cooled state), and after cutting off both ends of the steel bar by 100 mm, the remaining central part Specimens were cut out and examined for hardness and structure before aging treatment.

  On the other hand, for each test number, the remainder of the steel bar with a diameter of 30 mm cooled to room temperature was subjected to an aging treatment that was held at 600 to 630 ° C. for 1 to 3 hours, and both ends of the steel bar were cut off by 100 mm from the remaining central part. A test piece was cut out and examined for hardness and fatigue strength after aging treatment.

  Hardness measurement was performed as follows.

  First, a steel bar having a diameter of 30 mm was crossed, the resin was embedded so that the cut surface became the test surface, and mirror-polished to prepare a test piece. 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 determine the hardness at HV.

  The organization survey before aging treatment was conducted as follows.

  That is, first, the test piece used for the hardness measurement was mirror-polished again to prepare a test piece corroded with nital.

  Next, using an optical microscope, photographs of three fields of view were randomly taken in the vicinity of R / 2 at a magnification of 200 times and the size of the field of view of 220 μm × 180 μm to identify phases in the tissue. Moreover, the image analysis was performed about said each visual field, and the area ratio of bainite was calculated | required.

  The fatigue strength was investigated by collecting a smooth Ono type rotating bending fatigue test piece.

  That is, a smooth Ono-type rotating bending fatigue test piece (parallel portion length 20 mm) having the shape shown in FIG. 1 was taken in parallel to the forging direction from the center of the steel bar, and was subjected to Ono under conditions of room temperature, air, and rotation speed of 3400 rpm. Type rotary bending fatigue test was conducted.

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.

  Table 2 shows, for each test number, the structure before the aging treatment (that is, as-cooled), the hardness before and after the aging treatment, and the fatigue strength after the aging treatment.

  In Table 2, “phase” in the structure is expressed as “B” for bainite, “F” for proeutectoid ferrite, and “M” for martensite.

As is clear from Table 2, in the case of “Examples of the present invention” of test numbers 1 to 7, 9 to 11 and 13 to 16 having the chemical composition defined in the present invention, a dedicated leveling is performed in the cooling process after hot forging. Even without using a heat furnace, the hardness before aging treatment in the state of cooling is 315 or less in HV, and the fatigue strength after aging treatment is 510 MPa or more, and the machinability before aging treatment is It can be seen that the fatigue strength after aging treatment is compatible.

  On the other hand, in the case of “Comparative Examples” with test numbers 17 to 19 that deviate from the provisions of the present invention, the hardness before aging treatment exceeds 315 in HV, or the fatigue strength after aging treatment is It does not reach 510 MPa, and the machinability before aging treatment and the fatigue strength after aging treatment are not compatible.

  In the case of the test number 17, the “F1 value” of the steel Q is as small as 0.57 and does not satisfy the formula (1). For this reason, the amount of ferrite produced increases, and V precipitates as carbide before the aging treatment. Therefore, the amount of carbide precipitated by the aging treatment is small, and the fatigue strength after the aging treatment is as low as 480 MPa.

  In the case of the test number 18, the “value of F2” of the steel R is as large as 0.91 and does not satisfy the formula (2). For this reason, the hardness before aging treatment is as high as 341 in HV, and it is expected that the machinability is inferior.

  In the case of the test number 19, the “value of F3” of the steel S is as small as 0.33 and does not satisfy the expression (3). For this reason, age hardening ability is small and the fatigue strength after an aging treatment is as low as 490 MPa.

  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 (3)

In mass%, C: 0.05 to 0.28%, Si: 0.05 to 0.50%, Mn: 0.50 to 2.5%, P: 0.05% or less, S: 0.10 %: Cr: 0.05-2.5%, Al: 0.06% or less, Ti: 0.005-0.20%, V: 0.20-0.75%, Mo: 0.30 Age-hardenable steel containing 0.70 % and N: 0.020% or less, the balance being Fe and impurities, and satisfying the following formulas (1) to (3).
C + 0.3Mn + 0.25Cr + 0.6Mo ≧ 0.65 (1)
C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo ≦ 0.82 (2)
V + 0.8Tieff + 0.35Mo> 0.35 (3)
The element symbols in the above formulas (1) to (3) mean the content (% by mass) of the element.
Further, Tieff in the expression (3) indicates {Ti- (48/14) N- (48/32) S} or 0, whichever is larger. In mass%, C: 0.05 to 0.28%, Si: 0.05 to 0.50%, Mn: 0.50 to 2.5%, P: 0.05% or less, S: 0.10 %: Cr: 0.05-2.5%, Al: 0.06% or less, Ti: 0.005-0.20%, V: 0.20-0.75%, Mo: 0.30 0.70 % and N: not more than 0.020% and at least one element belonging to any of <1> to <3> below, the balance being Fe and impurities, and An age hardenable steel characterized by satisfying the following formulas (1 ′), (2) and (3):
<1> B: 0.005% or less <2> Cu: 0.6% or less and Ni: 0.6% or less under <br/><3> Ca: 0.005% or less and Bi: 0.4% C + 0.3Mn + 0.25Cr + 0.6Mo + Beff ≧ 0.65 (1 ′)
C + 0.1Si + 0.2Mn + 0.2Cr + 0.35V + 0.2Mo ≦ 0.82 (2)
V + 0.8Tieff + 0.35Mo> 0.35 (3)
The element symbols in the above formulas (1 ′), (2) and (3) mean the content (mass%) of the element, and Beff in the formula (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.
Further, Tieff in the expression (3) indicates {Ti- (48/14) N- (48/32) S} or 0, whichever is larger.   It is a manufacturing method of machine parts, Comprising: The steel of Claim 1 or 2 is heated at the temperature of 1000 degreeC or more, the hot forging whose finishing temperature is 900 degreeC or more is given, and the average of less than 5 degree-C / sec after that is performed. A method for manufacturing a machine part, comprising 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 cutting.

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