JPWO2015108067A1 - Age-hardening steel - Google Patents

Abstract

In mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to 2.50%, S: 0.005 to 0.08%, Cr: 0.03 to 1.60%, Al: 0.005 to 0.05%, V: 0.25 to 0.50% and Nb: 0.01 to 0.10%, further if necessary <a> Mo ≦ 1.0%, <b> One or both of Cu ≦ 0.3% and Ni ≦ 0.3%, and <c> Ca ≦ 0.005% and Bi ≦ 0.4% Satisfying at least one composition condition selected from one or both of them, limiting P ≦ 0.03%, Ti <0.005% and N <0.0080%, the balance consisting of Fe and impurities, [C + 0.3 × Mn + 0.25 × Cr + 0.6 × Mo ≧ 0.68], [C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V 0.2 × Mo ≦ 0.85] and [−4.5 × C + Mn + Cr−3.5 × V−0.8 × Mo ≧ 0.00] .

Description

The present invention relates to an age-hardenable steel, and more particularly to an age-hardenable steel that is processed into a predetermined shape by cutting or the like and then subjected to an age-hardening treatment (hereinafter simply referred to as “aging treatment”).
This application claims priority on January 15, 2014 based on Japanese Patent Application No. 2014-005137 for which it applied to Japan, and uses the content here.

  High fatigue strength is required for machine parts such as automobiles, industrial machines, and construction machines from the viewpoint of increasing engine output and reducing the weight of parts aimed at improving fuel efficiency. The problem of imparting high fatigue strength to steel can be easily achieved by increasing the hardness of the steel by addition of alloy elements and heat treatment. However, when machine parts are first formed by hot forging and then manufactured into a predetermined product shape by cutting, steel is also required to have sufficient machinability. That is, the machinability of the steel is required in the mechanical part forming stage, and the fatigue strength is required of the steel in the final product stage.

  In response to such demands, there has been proposed an age-hardening steel that can keep the hardness low in the molding stage and then can increase the hardness in the final product stage by applying an aging treatment. (For example, see Patent Documents 1 to 4).

Patent Documents 1 and 2 control the cooling rate after forming by hot forging, suppress the formation of structures other than bainite, suppress the precipitation amount of VC during cooling, and secure the solid solution V amount. A production method that makes it possible to obtain sufficient age-hardening ability is disclosed. However, in the manufacturing methods described in Patent Documents 1 and 2, in the cooling process after hot forging, it is necessary to control the cooling rate for each specific temperature range, and there are restrictions on facilities and devices, etc. Since there are cases where rapid cooling cannot be performed in the production line, it has been difficult to stably produce age-hardened steel.
Therefore, in Patent Documents 3 and 4, it is not necessary to set strict conditions in the cooling process after hot forging, and age-hardening for materials of machine parts that can be manufactured by cooling with air cooling or air blowing. Steel has been proposed.

International Publication No. 2010/090238 Japanese Unexamined Patent Publication No. 2012-246527 Japanese Unexamined Patent Publication No. 2011-241441 Japanese Unexamined Patent Publication No. 2012-193416

  As described above, steel used as a material for machine parts is required to have excellent machinability at the manufacturing stage of machine parts and excellent fatigue strength after completion of the machine parts. When manufacturing a machine part by a manufacturing method including an aging treatment, the above-described requirements can be achieved by using steel having a low hardness before the aging treatment and a high hardness after the aging treatment. A large difference between the hardness before the aging treatment and the hardness after the aging treatment (that is, the age-hardening ability is high) is preferable in order to obtain a machine part excellent in both productivity and fatigue strength.

  However, the manufacturing method for obtaining age-hardening steel according to the prior art needs to include a step of quenching the steel. This rapid cooling step increases the manufacturing cost of age hardenable steel.

  Further, it is known that a steel whose strength is increased by dispersing fine precipitates in the steel by aging treatment is greatly deteriorated in toughness. When the toughness of steel deteriorates, the notch sensitivity of the steel increases, so if surface flaws occur in the steel for some reason, the low cycle fatigue strength of the steel decreases. The low cycle fatigue strength is a characteristic required for steel that is assumed to be subjected to stress exceeding the elastic range. The methods for producing age-hardenable steel disclosed in Patent Documents 3 and 4 do not require increasing the cooling rate after hot forging, but it is difficult to obtain steel that does not lack toughness after aging treatment. .

  The present invention has been made in view of such circumstances, production conditions are not particularly limited, excellent machinability before aging treatment, fatigue strength can be improved by hardening by aging treatment, aging It is an object of the present invention to provide age-hardening steel that does not lack toughness after processing.

  In order to ensure sufficient hardness, fatigue strength, and low cycle fatigue strength after aging treatment, the amount of precipitates such as carbides and carbonitrides precipitated by aging treatment depends on the type of precipitate. It needs to be properly controlled.

  Here, the inventors paid attention to the matters described below. V is present as a solid solution in steel during hot forging performed at a general temperature. This is because the production start temperature (precipitation temperature) of V carbide or V carbonitride is low. On the other hand, V is an element effective for hardening by aging treatment because it has a strong ability to form precipitates (V carbide or V carbonitride) by aging treatment. However, if the N content is large, V nitride is generated during cooling after hot forging and before aging treatment, the hardness increases before aging treatment, and the machinability is impaired. Based on these findings, the inventors tried to promote the formation of V carbide or V carbonitride after aging treatment and to suppress the formation of V nitride before aging treatment.

  Further, Ti combines with N to form coarse TiN and greatly deteriorates toughness even if the content is as small as about 0.005%. The inventors tried to reduce the Ti content of steel based on these findings.

  In addition, Nb precipitates in steel as carbide or carbonitride during heating and processing during hot forging, refines the austenite grain size by the pinning effect, and refines the bainite structure in the subsequent bainite transformation. There is an effect to. Furthermore, some Nb in steel does not precipitate as carbide or carbonitride during hot forging, but exists as solute Nb. This solute Nb precipitates as Nb carbide or Nb carbonitride during the aging treatment after hot forging, and increases hardness without reducing toughness, thereby improving low cycle fatigue strength and fatigue strength. There is an effect to achieve. Based on these findings, the inventors tried to suppress toughness reduction due to aging treatment using Nb.

  The present invention has been made on the basis of such knowledge, and the gist thereof is as follows.

(1) The age-hardening steel according to an embodiment of the present invention is in mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to 2. 50%, S: 0.005 to 0.08%, Cr: 0.03 to 1.60%, Al: 0.005 to 0.05%, V: 0.25 to 0.50%, Nb: 0 0.01 to 0.10%, P: 0.03% or less, Ti: less than 0.005%, N: less than 0.0080%, the balance consisting of Fe and impurities, F1 represented by the formula (A) is 0.68 or more, F2 represented by the formula (B) is 0.85 or less, and F3 represented by the formula (C) is 0.00 or more.
F1 = C + 0.3 × Mn + 0.25 × Cr (A)
F2 = C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V (B)
F3 = −4.5 × C + Mn + Cr−3.5 × V (C)
The element symbol in the above formulas (A) to (C) means the content in mass% of the element.

(2) Age-hardening steel according to another embodiment of the present invention is mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to 2. .50%, S: 0.005-0.08%, Cr: 0.03-1.60%, Al: 0.005-0.05%, V: 0.25-0.50%, Nb: 0.01 to 0.10% is satisfied, and any one or more of the compositional conditions shown in the following <a> to <c> are satisfied, P: 0.03% or less, Ti: 0.00. Less than 005%, N: limited to less than 0.0080%, the balance is made of Fe and impurities, and F1 ′ represented by the following formula (A ′) is 0.68 or more, and the formula (B ′) F2 ′ represented has a chemical composition of 0.85 or less and F3 ′ represented by the formula (C ′) is 0.00 or more.
<a> Mo: 1.0% or less <b> Cu: 0.3% or less and Ni: 0.3% or less <c> Ca: 0.005% or less and Bi: 0.4% or less One or both of F1 ′ = C + 0.3 × Mn + 0.25 × Cr + 0.6 × Mo (A ′)
F2 ′ = C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V + 0.2 × Mo (B ′)
F3 ′ = − 4.5 × C + Mn + Cr−3.5 × V−0.8 × Mo (C ′)
The element symbol in said (A ')-(C') type | formula means content in the mass% of the element.

  According to the above aspect of the present invention, the production conditions are not particularly limited, the machinability before the aging treatment is excellent, the fatigue strength can be improved by hardening by the aging treatment, and the toughness is not insufficient after the aging treatment. Hardenable steel can be provided.

  In addition, by using the age-hardening steel of the present invention as a raw material, it is possible to provide a machine part that is excellent in productivity, excellent in fatigue strength, and does not have insufficient toughness.

  The age-hardening steel of the present invention has a Vickers hardness before aging treatment, which is an index of cutting resistance, of 290 Hv or less. Further, the age-hardening steel of the present invention will be described using the characteristics after the aging treatment performed under the condition that the steel has a substantially cylindrical shape with a diameter of 35 mm and the temperature of the steel is held at 620 ° C. for 120 minutes. It is as follows. The amount of increase in Vickers hardness (age hardening ability, ΔHv) due to aging treatment of the age hardening steel of the present invention is 30 Hv or more, and the fatigue strength of the age hardening steel of the present invention after aging treatment is 350 MPa or more. .

  Further, the absorbed energy at 20 ° C. evaluated by the Charpy impact test conducted using a standard test piece with a U-notch having a notch depth of 2 mm and a notch bottom radius of 1 mm of the age-hardening steel of the present invention after the aging treatment was obtained. It is 16 J or more, and the low cycle fatigue strength is 510 MPa or more.

  Thus, the age-hardening steel of the present invention can be used very suitably as a material for machine parts such as automobiles, industrial machines, and construction machines, and its industrial contribution is extremely remarkable.

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

First, elements important from the viewpoint of age hardening in the chemical composition of the age hardening steel according to the present embodiment will be described.
Machines manufactured by a manufacturing method including hot forging, cutting, aging treatment, and the like are mainly used for age-hardening steel according to this embodiment (hereinafter may be abbreviated as steel according to this embodiment). The material of the part. Therefore, in order to describe the characteristics of the steel according to the present embodiment, the characteristics of the steel after being subjected to hot forging, cutting, and aging treatment may be referred to. However, the steel according to the present embodiment does not necessarily require such a treatment. That is, the use of the steel according to the present embodiment is not limited to hot forging and cutting.

  First, the present inventors have found that in the steel according to the present embodiment, the V content needs to be 0.25% by mass or more. By setting the V content to 0.25% by mass or more, the amount of V carbide or V carbonitride produced by aging treatment is increased, the hardness after aging treatment is increased, and fatigue strength is ensured. can do. V, once dissolved in steel, does not precipitate until the steel is cooled to around 850 ° C., and has a strong ability to form carbide or carbonitride at the age hardening temperature. Further, in the steel according to the present embodiment, similarly to V, Mo may be added which has a relatively low carbide precipitation temperature and can be easily used for age hardening. If steel is further added to steel containing 0.25% by mass or more of V, a composite carbide of V and Mo or a composite carbonitride of V and Mo is formed. Increased further.

  As described above, V is an element that, once solid-dissolved in steel, has the property of not precipitating until the steel is cooled to around 850 ° C., and thus can be stably present in the steel in a solid solution state. . However, V carbide tends to precipitate at the phase interface when austenite is transformed into ferrite. When the precipitation amount of V carbide increases, the solid solution V amount decreases. That is, if a large amount of pro-eutectoid ferrite is generated during cooling after hot forging, V carbide precipitates at the phase interface, so that it is impossible to secure the amount of solid solution V necessary for precipitation hardening by the subsequent aging treatment. Therefore, in order to secure a sufficient amount of solute V in the age-hardening steel before the aging treatment, the phase ratio of the area ratio of 70% or more in the steel structure after the hot forging and before the aging treatment (below) , "Main phase") needs to be bainite. In order to prevent an increase in the manufacturing cost of machine parts, it is necessary that such structure control be performed not by controlling hot forging conditions but by controlling the composition of steel.

The structure after hot forging has a close correlation with the contents of C, Mn, and Cr that improve hardenability, and further with the content of Mo. The present inventors have included the content of elements in these formulas so that the values of F1 and F1 ′, which are hardenability indexes represented by formula (1) or formula (1 ′), are equal to or greater than a specific numerical value. Is controlled, a large amount of pro-eutectoid ferrite, which is harmful to securing solid solution V, is suppressed in the cooling process after normal hot forging (cooling rate 15 ° C./min to 60 ° C./min). I found out. That is, by controlling F1 and F1 ′, the steel structure easily becomes a structure containing bainite as a main phase, that is, a structure containing bainite having an area ratio of 70% or more. The present inventors have found that V can be secured.
F1 = C + 0.3 × Mn + 0.25 × Cr (1)
F1 ′ = C + 0.3 × Mn + 0.25 × Cr + 0.6 × Mo (1 ′)

However, even if a sufficient amount of solid solution V is secured by making the steel structure a bainite main phase (70% or more in area ratio), the hardness before aging treatment (the bainite is the main phase). The hardness of the tissue) may increase. In this case, the cutting resistance of the steel after hot forging is increased, and the machinability may be lowered. The present inventors examined a method for solving this problem. As a result, the contents of C, Si, Mn, Cr, V, and Mo are F2 and F2 ′, which are indices of hardness before aging treatment expressed by the following formula (2) or (2 ′): By controlling the chemical composition of the steel according to the present embodiment so that the value is equal to or less than a specific numerical value, it is possible to keep the hardness before aging treatment low and to suppress an increase in cutting resistance. The inventors have found.
F2 = C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V (2)
F2 ′ = C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V + 0.2 × Mo (2 ′)

  In addition, the inventors include 0.25% by mass or more of V, and the contents of C, Si, Mn, Cr, Mo, and V are the above formulas (1) and (2) or the above formula (1). A steel whose composition is adjusted so that F1 and F2 or F1 'and F2' determined by ') and (2') meet a specific numerical range is manufactured, and the steel is subjected to aging treatment after hot forging. A sample was prepared and the toughness of this sample was investigated. Specifically, after hot forging and aging treatment of the steel described above, a standard test piece with a U notch having a notch depth of 2 mm and a notch bottom radius of 1 mm is prepared, and a Charpy impact test is performed on the test piece. The effects of components on toughness after aging treatment were investigated.

As a result of the above investigation, in order to obtain a steel capable of suppressing a decrease in toughness due to aging treatment, F3 indicates an index of toughness after aging treatment represented by the formula (3) or (3 ′) described later. It has been found that the contents of C, Mn, Cr, V and Mo in the steel need to be controlled so that the values of F3 ′ and F3 ′ are not less than a specific value.
F3 = −4.5 × C + Mn + Cr−3.5 × V (3)
F3 ′ = − 4.5 × C + Mn + Cr−3.5 × V−0.8 × Mo (3 ′)

  When F3 and F3 'are large, the toughness of the steel after aging treatment is not insufficient. Also, increasing the content of C, V, and Mo decreases F3 and F3 '. Therefore, the formula (3) or the formula (3 ′) is used to suppress the toughness reduction due to the aging treatment with respect to the contents of C, V and Mo necessary for improving the hardness and fatigue strength after the aging treatment. This means that it needs to be reduced.

  Furthermore, in order to achieve both strength and toughness, it is necessary to improve the strength by increasing the hardness after aging treatment by utilizing elements other than C, V, and Mo.

  In order to suppress a decrease in toughness after aging treatment, it is effective to refine the structure. In order to refine the bainite structure as the main phase, it is effective to refine the austenite grain size before the bainite transformation. In order to refine the austenite grain size, it is generally effective to contain Ti, but this means cannot be used in the steel according to the present embodiment. Since the present inventors form coarse TiN that degrades the toughness of the steel according to the present embodiment, even if the Ti content is as small as about 0.005%, the Ti is after the aging treatment. It has been found that the toughness of the steels in the steel is greatly deteriorated. Therefore, it is necessary to limit the Ti content of the steel according to the present embodiment to zero or a specific value as much as possible.

  Further, if inclusions that adversely affect toughness are present in steel, sufficient toughness cannot be obtained. In order to suppress the presence of inclusions in steel harmful to toughness, the S content needs to be a specific value or less. Further, S is an element that forms coarse MnS by bonding with Mn and deteriorates toughness, so excessive addition of S must be avoided. On the other hand, MnS is an essential inclusion in order to ensure sufficient machinability. Therefore, it is not preferable that the S content is completely zero. In order to increase the machinability of the steel before aging treatment and to suppress the deterioration of the toughness of the steel due to aging treatment, it is necessary to appropriately control the S content so that the amount of MnS does not become excessive. is there.

  The present inventors have confirmed that the inclusion of Nb is effective as a means for sufficiently increasing the machinability before the aging treatment and the low cycle fatigue strength after the aging treatment, and suppressing the toughness reduction due to the aging treatment. I found out. Nb has the effect of refining the austenite grain size before the bainite transformation, similar to Ti.

  Nb is an element that has the effect of reducing the austenite grain size and also has the ability to form a compound (secondary phase) at the aging temperature. This is because Nb has a higher deposition temperature than V and Mo. That is, since the precipitation temperature of Nb is relatively high, a part of the contained Nb is precipitated as carbide or carbonitride during hot forging, and this Nb precipitate such as carbide contributes to refinement of the austenite grain size. To do.

  Solid steel that satisfies the condition that the formula (1) or the formula (1 ′) falls within a specific range contains solute Nb. This is because, as described above, the main phase of steel satisfying the formula (1) or (1 ′) has a bainite structure, and Nb easily dissolves in the bainite structure. Therefore, Nb carbide or Nb carbonitride can be precipitated by aging treatment in steel in which formula (1) or (1 ') falls within a specific range. Further, the present inventors have found that the hardness of the steel after aging treatment can be increased without causing a decrease in toughness even if these Nb precipitates are precipitated. In addition, the present inventors have also found that by containing Nb, a steel capable of obtaining an excellent low cycle fatigue strength by refining the bainite structure and precipitation strengthening can be realized.

  The present embodiment relates to the age-hardenable steel made based on the above-described investigation results of the present inventor and the obtained knowledge. Hereinafter, each requirement of age hardening steel which is one embodiment of the present invention is explained in detail.

  First, components of the age-hardening steel according to this embodiment will be described. In addition, “%” of the content of each element means “mass%”.

C: 0.05-0.20%
C is an important element in the present embodiment. C combines with V by aging treatment to form V carbide and strengthens the steel. However, if the C content is less than 0.05%, the V carbide precipitation driving force becomes small and V carbide is difficult to precipitate, so that the desired strengthening effect cannot be obtained. On the other hand, if the C content exceeds 0.20%, C that does not bond with V is combined with Fe to form carbide (cementite), and the toughness of the steel is significantly deteriorated. Further, when the content of C exceeds 0.20%, the C concentration concentrated in the austenite during the transformation from austenite to bainite also increases, and martensite is partially mixed in the structure after the bainite transformation. It becomes like. When cementite and / or martensite is contained in steel, cutting resistance increases and machinability decreases. Therefore, the content of C is set to 0.05 to 0.20%. The C content is preferably 0.08% or more, and more preferably 0.10% or more. The C content is preferably 0.18% or less, and more preferably 0.16% or less.

Si: 0.01 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 0.01% or more. However, when the Si content is excessive, the hot workability of the steel is lowered and the cutting resistance is raised, leading to a decrease in machinability. In particular, when the Si content exceeds 0.50%, the hot workability of steel and the cutting resistance increase remarkably. Furthermore, since Si may promote the generation of proeutectoid ferrite and reduce the amount of bainite, it is not preferable to contain Si excessively in order to stably obtain bainite. When pro-eutectoid ferrite is produced in the steel production stage, as described above, V carbide precipitates at the phase interface between pro-eutectoid ferrite and austenite, which may reduce the age hardening ability of the steel. Therefore, the Si content is set to 0.01 to 0.50%. The Si content is preferably 0.06% or more. The Si content is preferably 0.45% or less, and more preferably less than 0.35%.

Mn: 1.50 to 2.50%
Mn has the effect of improving hardenability and making the main phase of the structure bainite. Furthermore, Mn has the effect of lowering the bainite transformation temperature, and by doing so, it also has the effect of refining the structure and increasing the toughness of the matrix. In addition, the structure which occupies most of the volume of steel is called a matrix, and the matrix of the steel which concerns on this embodiment is a bainite. Moreover, Mn has the effect | action which forms MnS in steel and reduces cutting resistance, and thereby improves machinability. Moreover, when the amount of Mn is less than 1.50%, the generation of pro-eutectoid ferrite is promoted and there is a risk of causing a decrease in the amount of bainite and a decrease in age hardening ability as described above. In order to obtain these effects sufficiently, the Mn content needs to be at least 1.50%. However, since Mn is an element that easily segregates during solidification of the steel, its content increases, especially when it exceeds 2.50%, it is avoided to increase the hardness variation in the parts after hot forging. I can't. Therefore, the Mn content is 1.50 to 2.50%. The Mn content is preferably 1.60% or more, and more preferably 1.70% or more. Further, the Mn content is preferably 2.30% or less, and more preferably 2.10% or less.

S: 0.005-0.08%
S combines with Mn in steel to form MnS, thereby reducing the cutting resistance and improving the machinability of the steel. In order to obtain sufficient machinability, it is necessary to contain 0.005% or more of S. However, when the S content is excessively high, coarse MnS increases, which may deteriorate the toughness and fatigue strength of the steel. In particular, when the S content exceeds 0.08%, the toughness and fatigue strength are significantly reduced. Therefore, the content of S is set to 0.005 to 0.08%. The S content is preferably 0.01% or more. The S content is preferably 0.05% or less, and more preferably 0.03% or less.

Cr: 0.03 to 1.60%
Cr has the effect of increasing hardenability and making the main phase of the structure bainite. Furthermore, Cr has the effect of lowering the bainite transformation temperature, and by doing so, it also has the effect of refining the structure and increasing the toughness of the matrix. However, if the Cr content exceeds 1.60%, the hardenability becomes too high, and the hardness before aging treatment may exceed 290 Hv in terms of Vickers hardness depending on the size and part of the part. The cutting resistance may increase and the machinability may decrease. Therefore, the Cr content is 0.03 to 1.60%. The lower limit of the Cr content is preferably 0.05% or more, and more preferably 0.10% or more. The upper limit of the Cr content is preferably 1.00% or less, and more preferably 0.50% or less.

Al: 0.005 to 0.05%
Al is an element having a deoxidizing action, and in order to exert this action, the content needs to be 0.005% or more. However, if the Al content exceeds 0.05%, coarse oxides are generated, and the toughness and fatigue strength of the steel are reduced. Therefore, the Al content is 0.005 to 0.05%. The Al content is preferably 0.04% or less.

V: 0.25 to 0.50%
V is the most important element in the steel according to the present embodiment. V is the strength of steel after aging treatment by combining with C during aging treatment to form fine V carbide, or by combining with C and N to form fine V carbonitride. There is an action to increase. V also has the effect of further increasing the age-hardening ability of steel by being combined with Mo and precipitated by aging treatment. In order to sufficiently obtain these effects, V needs to be a content of 0.25% or more. However, if the V content is excessive, undissolved V carbide or V carbonitride tends to remain even during heating during hot forging, leading to a decrease in toughness. If it exceeds%, the toughness is significantly reduced. In addition, if the V content exceeds 0.50%, the cutting resistance increases with the remaining undissolved V carbide or V carbonitride, and the machinability of the steel may be significantly reduced. . Therefore, the content of V is set to 0.25 to 0.50%. The V content is preferably less than 0.45%, and more preferably 0.40% or less. The V content is preferably 0.27% or more.

Nb: 0.01 to 0.10%
Nb is one of important elements in the steel according to the present embodiment. Part of Nb contained in the steel precipitates in the steel as Nb carbide or Nb carbonitride during the heating and processing during hot forging, and austenite grains are refined by the pinning effect. The refinement of austenite crystal grains during hot working has the effect of refining the bainite structure in the bainite transformation after completion of hot working. Refinement of the bainite structure improves the toughness of the steel after age hardening. Furthermore, a part of Nb in the steel at the time of hot forging is present as solute Nb, and this solute Nb is precipitated as Nb carbide or Nb carbonitride during the aging treatment after hot forging, There is an effect of increasing hardness and achieving low cycle fatigue strength and improved fatigue strength without incurring toughness reduction. In order to sufficiently obtain these effects, Nb needs to have a content of 0.01% or more. However, when the content of Nb is excessive, undissolved carbide or carbonitride tends to remain even during heating during hot forging, and the effect of increasing the hardness after aging treatment and / or after aging treatment The effect of improving fatigue strength is saturated. In addition, if the Nb content exceeds 0.10%, the cutting resistance increases with the remaining undissolved carbide or carbonitride, and the machinability of the steel may be significantly reduced. Therefore, the Nb content is set to 0.01 to 0.10%. The Nb content is preferably less than 0.08%, and more preferably 0.05% or less. The Nb content is preferably 0.02% or more.

  The age-hardening steel of this embodiment is composed of the above-mentioned C, Si, Mn, S, Cr, Al, V, and Nb, and the balance is Fe and impurities, and P, Ti, and N in impurities described later are included. , P: 0.03% or less, Ti: less than 0.005% and N: 0.020% or less, and F1 represented by the formula (1) is 0.68 or more, It is a steel having a chemical composition in which F2 represented by the formula (2) is 0.85 or less and F3 represented by the formula (3) is 0.00 or more. In addition, an impurity refers to the thing mixed from the ore as a raw material, a scrap, or a manufacturing environment, when manufacturing steel materials industrially.

P: 0.03% or less P is contained as an impurity and is an undesirable element in the steel according to the present embodiment. That is, P segregates at the grain boundaries to lower toughness, and particularly when its content exceeds 0.03%, the toughness is extremely lowered. Therefore, the P content is limited to 0.03% or less. The P content is preferably limited to 0.025% or less.
In addition, the effect of the steel which concerns on this embodiment is exhibited, without setting the minimum of content of P in particular. The lower limit value of the P content may be 0%. However, excessively reducing P causes an extreme increase in de-P cost and is economically disadvantageous, so the lower limit of the P content is preferably 0.005%.

Ti: Less than 0.005% Ti is contained as an impurity and is an undesirable element in the steel according to the present embodiment. That is, Ti combines with N to form coarse TiN, leading to a decrease in toughness. In particular, when its content is 0.005% or more, the toughness is greatly deteriorated. Therefore, the Ti content is limited to less than 0.005%. In order to suppress a decrease in toughness due to the aging treatment, the Ti content is preferably limited to 0.0035% or less, and more preferably 0.0015% or less. The lower limit value of the Ti content may be 0%.

N: Less than 0.0080% N is contained as an impurity. In the steel according to the present embodiment, N is an undesirable element that fixes V as VN. That is, V precipitated as VN does not contribute to age hardening, so the content of N must be lowered in order to suppress the precipitation of VN. In order to suppress the precipitation of VN and ensure a sufficient amount of solute V in the stage before the aging treatment, it is necessary to limit the N content to less than 0.0080%. The upper limit of the N content is preferably 0.0070%, 0.0060%, or 0.0050%. The lower limit of the N content is 0%.

  Another embodiment of the age-hardening steel according to this embodiment is a composition satisfying any one or more of the above-described elements <C> to <C>, and the balance being Fe. And P, Ti and N in the impurity are limited to P: 0.03% or less, Ti: less than 0.005% and N: 0.020% or less, and F1 ′ represented by the formula (1 ′) is 0.68 or more, F2 ′ represented by the formula (2 ′) is 0.85 or less, and F3 ′ represented by the formula (3 ′) is 0.00 or more. Is a steel having a chemical composition of

  Hereinafter, the effects of the optional elements shown in <a> to <c> that are selectively added in other embodiments of the age-hardening steel according to this embodiment and the reasons for limiting the contents thereof will be described.

<a> Mo: 1.0% or less Since containing Mo is not essential, the lower limit of the Mo content is 0%. On the other hand, Mo enhances the hardenability of steel, has the effect of increasing the area ratio of bainite, while making the main phase of the steel structure after hot forging bainite. Mo also has the effect | action which forms a carbide | carbonized_material with V and enlarges the age hardening ability of steel in steel containing 0.25% or more of V. For this reason, you may contain Mo as needed. However, since Mo is a very expensive element, if the content is increased, the manufacturing cost of steel increases and the toughness also decreases. Therefore, the Mo content when contained is 1.0% or less. When Mo is contained, the content of Mo is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably less than 0.30%. On the other hand, in order to stably obtain the effect of Mo described above, the Mo content in the case of inclusion is preferably 0.03% or more, more preferably 0.05% or more, 0 10% or more is more desirable.

<B> Cu: 0.3% or less and Ni: 0.3% or less Both Cu and Ni have an effect of increasing the fatigue strength of steel 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: 0.3% or less Since Cu is not essential, the lower limit of the Cu content is 0%. On the other hand, Cu has the effect | action which improves the fatigue strength of steel after an aging treatment. For this reason, you may contain Cu as needed. However, if the Cu content exceeds 0.3%, the hot workability of the steel decreases. Therefore, the Cu content when contained is set to 0.3% or less. When Cu is contained, the content of Cu is preferably 0.25% or less. On the other hand, in order to stably obtain the effect of increasing the fatigue strength of Cu described above, the Cu content in the case of inclusion is preferably 0.03% or more, and more preferably 0.05% or more. More desirably, the content is more preferably 0.1% or more.

Ni: 0.3% or less Since the Ni content is not essential, the lower limit of the Ni content is 0%. On the other hand, Ni has the effect | action which improves the fatigue strength of steel after an aging treatment. 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 0.3%, the above effect is saturated in addition to the increase in cost. Therefore, the Ni content when contained is set to 0.3% or less. When Ni is contained, the content of Ni is preferably 0.25% or less. On the other hand, in order to stably obtain the above-described effect of Ni, the Ni content in the case of inclusion is preferably 0.03% or more, more preferably 0.05% or more, and 0% More preferably, the content is 1% or more.

  In addition, said Cu and Ni can be contained only in any 1 type among them, or 2 types of composites. The total content of the above elements when contained may be 0.6%, which is the total value of the upper limit value of the Cu content and the upper limit value of the Ni content.

<C> Ca: 0.005% or less and Bi: 0.4% or less Both Ca and Bi have the effect of reducing the cutting resistance of the steel before aging treatment and improving the 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 Since Ca is not essential, the lower limit of Ca content is 0%. On the other hand, Ca has the effect of spheroidizing the sulfide form, and has the effect of reducing the cutting resistance of the steel before aging treatment without reducing toughness. For this reason, you may contain Ca as needed. However, when the Ca content exceeds 0.005%, the form of the sulfide is changed, but a coarse oxide containing Ca is formed, and the toughness of the steel is deteriorated. Therefore, if Ca is included, the Ca content is 0.005% or less. When Ca is contained, the content of Ca is preferably 0.0035% or less. On the other hand, in order to stably obtain the effect of reducing the cutting resistance due to the effect of changing the shape of sulfide by Ca, the Ca content when contained is preferably 0.0005% or more. It is further desirable to set it to 0007% or more, 0.0010% or more, or 0.0015% or more.

Bi: 0.4% or less Since the Bi content is not essential, the lower limit of the Bi content is 0%. On the other hand, Bi has the effect | action which reduces the cutting resistance of steel before an aging treatment, and the effect | action which improves chip-treatment property. For this reason, you may contain Bi as needed. However, when the Bi content exceeds 0.4%, hot workability is deteriorated. Therefore, the Bi content when contained is 0.4% or less. When Bi is contained, the Bi content is preferably 0.3% or less. On the other hand, in order to stably obtain the above-described effect of reducing the cutting resistance of Bi and the effect of improving chip disposal, the Bi content in the case of inclusion should be 0.01% or more. Desirably, 0.03% or more is more desirable.

  In addition, said Ca and Bi can be contained only in any 1 type among them, or 2 types of composite. The total content of these elements when contained may be 0.405%, which is the sum of the upper limit value of Ca content and the upper limit value of Bi content, but may be 0.3% or less. preferable.

  In the present embodiment and other embodiments described above, the balance other than the above-described elements is substantially Fe and impurities.

  Next, F1 to F3 represented by the expressions (1) to (3) and F1 'to F3' represented by the expressions (1 ') to (3') will be described.

<F1 or F1 ′: 0.68 or more>
When the age-hardenable steel according to this embodiment does not contain Mo,
F1 = C + 0.3 × Mn + 0.25 × Cr (1)
F1 represented by the following must be 0.68 or more.
On the other hand, when age-hardenable steel according to this embodiment includes the Mo,
F1 ′ = C + 0.3 × Mn + 0.25 × Cr + 0.6 × Mo (1 ′)
F1 ′ represented by the following must be 0.68 or more.

  As already stated, the element symbols in the above formulas (1) and (1 ') mean the content in mass% of the element.

  F1 and F1 'are indicators for hardenability. Even if accelerated cooling such as water cooling is not performed after hot forging if the content of each alloy element contained in the steel satisfies the above-described range and F1 and F1 ′ satisfy the above conditions, The structure after forging has bainite as the main phase.

  When F1 or F1 ′ is less than 0.68, proeutectoid ferrite is mixed in the structure after hot forging, and VC precipitates at the phase interface, so the hardness of the steel before aging treatment increases or age hardening occurs. The performance becomes smaller.

  F1 and F1 'are preferably 0.70 or more, and more preferably 0.72 or more. On the other hand, an excessive increase in hardenability may lead to a reduction in the toughness of the steel, so F1 and F1 'are preferably 1.00 or less, and more preferably 0.98 or less.

<F2 or F2 ′: 0.85 or less>
When the age-hardenable steel according to this embodiment does not contain Mo,
F2 = C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V (2)
F2 represented by the following must be 0.85 or less.
On the other hand, when age-hardenable steel according to this embodiment includes the Mo,
F2 ′ = C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V + 0.2 × Mo (2 ′)
F2 ′ represented by the following must be 0.85 or less.

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

  F2 and F2 'are indices indicating the hardness before aging treatment. Even if the steel satisfies the above F1 or F1 ′ conditions, if F2 or F2 ′ is not within an appropriate range, the steel before aging treatment becomes too hard, and the cutting resistance increases. Good machinability may not be ensured. That is, when F2 or F2 'exceeds 0.85, the hardness of the bainite structure becomes too high. Therefore, an increase in cutting resistance is unavoidable, and good machinability may not be ensured.

  F2 and F2 'are preferably 0.82 or less, and more preferably 0.80 or less. On the other hand, if F2 and F2 'are too low, the hardness after age hardening may be insufficient. Therefore, F2 and F2' are preferably 0.55 or more, and more preferably 0.60 or more.

<F3 or F3 ′: 0.00 or more>
When the age-hardenable steel according to this embodiment does not contain Mo,
F3 = −4.5 × C + Mn + Cr−3.5 × V (3)
F3 represented by must be greater than or equal to 0.00.
On the other hand, when the age-hardenable steel according to the embodiment contains the Mo,
F3 ′ = − 4.5 × C + Mn + Cr−3.5 × V−0.8 × Mo (3 ′)
F3 ′ represented by must be greater than or equal to 0.00.

  As already described, the element symbols in the above formulas (3) and (3 ′) mean the content of the element in mass%.

  F3 and F3 'are indices indicating toughness after aging treatment. That is, even if the steel satisfies the conditions of F1 or F1 ′ and F2 or F2 ′, if F3 or F3 ′ is not within an appropriate range, the toughness of the steel after aging treatment is lowered and the target toughness May not be secured. That is, when F3 or F3 'is less than 0.00 (negative number), the toughness of the steel after aging treatment is lowered. F3 and F3 'are preferably 0.01 or more.

  If F1 is 0.68 or more and F2 is 0.85 or less, there is no need to particularly limit the upper limit of F3. Similarly, if F1 'is 0.68 or more and F2' is 0.85 or less, there is no need to particularly limit the upper limit of F3 '.

  Next, the steel structure (microstructure) of the age-hardening steel according to this embodiment will be described.

  As described above, a large amount of pro-eutectoid ferrite is not preferable before the aging treatment. Furthermore, the production of a large amount of martensite is not preferable from the viewpoint of machinability. For this reason, it is important that the main phase before the aging treatment of the age-hardening steel according to the present embodiment is bainite. That is, in order to ensure sufficient machinability and solid solution V, the structure before the aging treatment needs to have an area ratio of bainite of 70% or more. The area ratio of bainite is preferably 80% or more, and the area ratio of bainite single phase, that is, bainite, is most preferably 100%. When the area ratio of bainite is less than 100%, the phases other than bainite as the main phase are a ferrite phase, a pearlite phase, a martensite phase, and the like. The smaller the number of these phases, the better.

  When the amount of increase in the Vickers hardness of the steel is defined as the age-hardenability of the steel when the steel has a substantially cylindrical shape with a diameter of 35 mm and the temperature of the steel is held at 620 ° C. for 120 minutes, The lower limit of the age hardening ability of the steel according to the embodiment is preferably 30 Hv, more preferably 33 Hv, 35 Hv, or 40 Hv. The treatment for holding the temperature of steel at 620 ° C. for 120 minutes is a general aging treatment condition for producing a machine part by subjecting the steel according to this embodiment to age hardening treatment. When the age hardening ability is 30 Hv or more, the steel according to the present embodiment has good machinability before the aging treatment and has a good fatigue strength after the aging treatment.

  The method for producing the age-hardening steel of the present embodiment is not particularly limited, and the chemical composition may be adjusted by melting by a general method. Below, an example of the method of manufacturing machine parts, such as a motor vehicle, an industrial machine, and a construction machine, is shown using the age hardening steel which concerns on this embodiment manufactured as mentioned above as a raw material.

First, an age-hardening steel (hereinafter referred to as “intermediate material”) to be used for component molding is produced from steel whose chemical composition is adjusted to the above-mentioned range.
The intermediate material may be any billet, such as a billet obtained by ingot-rolling an ingot, a billet obtained by subjecting a continuous cast material to rolling, or a steel bar obtained by hot-rolling or hot-forging these billets. However, if the temperature is maintained under conditions where V carbide is likely to precipitate during the production of the intermediate material, the age hardening ability may be lost. For example, when the temperature of the intermediate material is maintained within the range of 540 to 700 ° C. for 30 minutes or more after the batch rolling, or after the hot rolling or hot forging, the age hardening ability may be lost. However, such a situation does not occur if the intermediate material is left in the room temperature environment after the partial rolling or after the hot rolling or hot forging according to a general method.

  Next, the intermediate material is hot forged and further cut to finish a predetermined part shape. In the hot forging described above, for example, after the hot forging material is heated at 1200 to 1250 ° C. for 5 to 60 minutes, forging is performed so that the finishing temperature becomes 1100 ° C. or higher, and then to 800 to 400 ° C. The temperature is cooled to room temperature at an average cooling rate of 15 to 60 ° C./min. Such an average cooling rate can be easily obtained by leaving the forged steel in a room temperature environment. However, when the cooling rate is less than 15 ° C./min, V carbide precipitates during cooling, and the age hardening ability may be 30 Hv or less. After cooling in this way, further cutting is performed to finish a predetermined part shape.

  Finally, an aging treatment is performed on the rough shaped material formed into a predetermined part shape to obtain machine parts such as automobiles, industrial machines, and construction machines having desired characteristics. The aging treatment is performed, for example, in a temperature range of 540 to 700 ° C., preferably in a temperature range of 560 to 680 ° C. The holding time of the aging treatment is appropriately adjusted depending on the size (mass) of the machine part, for example, 30 to 1000 minutes.

  As described above, the age-hardening steel according to this embodiment and the machine part using the same can be manufactured.

  Hereinafter, the present invention will be described in more detail with reference to examples. The conditions in the following examples are one condition example adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  Steels 1-31 having chemical compositions shown in Table 1-1 and Table 1-2 were melted in a 50 kg vacuum melting furnace. Steels 1 to 20 in Table 1-1 and Table 1-2 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, the steels 21 to 31 in Table 1-1 and Table 1-2 are steels whose chemical compositions deviate from the conditions defined in the present invention. Note that “<0.001” in the column of Ti indicates that the content of Ti as an impurity was less than 0.001%, which is the detection lower limit value of the emission spectroscopic analysis.

  Each steel ingot was heated in an atmosphere of 1250 ° C. and then hot forged into a steel bar having a diameter of 60 mm. Each hot forged steel bar was once allowed to cool in the atmosphere and cooled to room temperature. Then, the steel bar further cooled to room temperature was heated to 1250 ° C. as an intermediate material, the finishing temperature was set to 950 ° C. or higher, and the steel bar having a diameter of 35 mm was hot forged again. This second hot forging was performed to simulate forging into a component shape. The steel bar after the second hot forging was allowed to cool to room temperature in the air. The cooling rate during the second hot forging was measured using a radiation thermometer. The average cooling rate in the temperature range from 800 to 400 ° C. after the second hot forging was 50 ° C./min in any steel bar.

  In the obtained steels 1 to 31, after hot-forging a part of the steel bar having a diameter of 35 mm, after cutting off both ends of the steel bar by 100 mm in a state where aging treatment is not performed (that is, in a cooled state) The test piece was cut out from the remaining central portion, and the Vickers hardness before aging treatment and the area ratio of bainite in the structure were examined. On the other hand, the hot-forged steel bar with a diameter of 35 mm is subjected to an aging treatment that is held at 620 ° C. for 120 minutes, and then both ends of the steel bar are cut off by 100 mm each, and then a test piece is cut out from the remaining central portion and subjected to aging treatment Later Vickers hardness was investigated. Moreover, in the obtained steels 1-31, the test piece was cut out from the bar steel after an aging treatment, and the absorbed energy in the Charpy impact test after an aging treatment, and low cycle fatigue strength and fatigue strength were investigated.

  The Vickers hardness measurement was performed as follows. First, the steel bar was crossed, the steel bar crossed so that the cut surface became the test surface was filled with resin, and the test surface was 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 value obtained by arithmetically averaging the measured hardness values at the 10 points was defined as the Vickers hardness of the steel bar. In addition, when the Vickers hardness before an aging treatment is 290 Hv or less, it judged that it was sufficiently low, and made this into the target value. Further, when the difference between the Vickers hardness after the aging treatment and the Vickers hardness before the aging treatment (hereinafter referred to as “ΔHv”) is 30 Hv or more, it is determined that the age hardening ability is sufficiently high, and this is the target. Value.

  The measurement of the area ratio of the bainite in the structure was performed as follows. First, the test piece which carried out resin embedding used for the hardness measurement and mirror-polished was etched using the nital. With respect to the test piece after etching, the structure | tissue was image | photographed by 200-times multiplication factor using the optical microscope. The area ratio of bainite was measured by image analysis from the photographed photographs. When the area ratio of bainite was 70% or more, it was determined that the structure was sufficiently bainite, and this was set as a target value.

  Toughness was performed using a standard specimen with a U-notch with a notch depth of 2 mm and a notch bottom radius of 1 mm. When the absorbed energy at 20 ° C. after the aging treatment evaluated by this Charpy impact test was 16 J or more, it was judged that the toughness deterioration due to the aging treatment was sufficiently suppressed, and this was set as a target value.

The fatigue strength was investigated by collecting a uniaxial tension-compression type fatigue test piece. That is, a smooth fatigue test piece having a shape with a parallel part diameter of 3.4 mm and a parallel part length of 12.7 mm shown in FIG. 1 is parallel to the forging direction from the R / 2 part of the steel bar (the length of the steel bar). Direction) and a fatigue test was conducted under conditions of room temperature, air, stress ratio 0.05, test speed 10 Hz. Under the above conditions, the maximum stress which does not break in stressing repeated several 10 ⁷ times was fatigue strength. When the fatigue strength was 350 MPa or more, it was judged that the fatigue strength was sufficiently high, and this was set as a target value.

The low cycle fatigue strength was determined by the following method. First, a rectangular parallelepiped having a length and width of 13 mm in a longitudinal section and a length of 100 mm is parallel to the forging direction (longitudinal direction of the steel bar) so that the sampling site is R / 2 part of the steel bar. From. Thereafter, a semicircular notch having a radius of 2 mm was provided at a central portion in the length direction of one surface of the rectangular parallelepiped (that is, a surface having a fatigue evaluation region), thereby obtaining a four-point bending test piece. The low cycle fatigue test was performed at room temperature and in the air by performing a 4-point bending fatigue test under the conditions of a stress ratio of 0.1, a fulcrum distance of 45 mm, and a test frequency of 5 Hz. Under the above conditions, the stress load is repeated, and the strength is evaluated by defining the strength 5 × 10 ³ times as the low cycle fatigue strength. When the low cycle fatigue strength is 510 MPa or more, the low cycle fatigue strength is sufficient. Judged as high and set this as the target value.

  Table 2 shows the results of the above investigations. In the “bainiteization” column, “GOOD” indicates that the area ratio of bainite is 70% or more, and “BAD” indicates that the target is not reached when it is less than 70%. . In Table 2, “absorbed energy in the Charpy impact test” is expressed as “Charpy absorbed energy”.

  As is clear from Table 2, in the case of “Examples of the present invention” of test numbers A1 to A20 having the chemical composition defined in the present invention, the Vickers hardness before the aging treatment is 290 Hv or less, and the Vickers hardness is reduced by the aging treatment. Cured by 30Hv or more, absorbed energy in Charpy impact test is also 16J or more, fatigue strength is 350MPa or more, low cycle fatigue strength is 510MPa or more, achieving the target, strength and toughness after aging treatment and aging It can be seen that the machinability before processing is compatible.

  On the other hand, in the case of “Comparative Examples” with test numbers B1 to B11 that are not defined in the present invention, the target performance is not obtained.

  Test No. B1 has a small Nb content of the steel 21 used in the present invention and is small, so that the effect of refining the austenite grain size due to precipitation of Nb carbides during hot forging is small, and the low cycle fatigue strength is low. Low.

  In test number B2, the Nb content of the steel 22 used is large outside the scope of the present invention, so that undissolved Nb carbide remains during hot forging, and aging is an index of machinability, particularly cutting resistance. Hardness before treatment exceeds the target value, and hardness after aging treatment is difficult to increase (age hardening ability is insufficient).

  In test number B3, since the C content of the steel 23 used is too much outside the scope of the present invention, cementite increases, the absorbed energy in the Charpy impact test after aging treatment is small, and the toughness is inferior.

  In test number B4, since the Si content of the steel 24 used was too much outside the definition of the present invention, proeutectoid ferrite was precipitated and the bainite area ratio was outside the specification. Thereby, in test number B4, the precipitation amount of V carbide before an aging treatment is increasing. Therefore, in test number B4, the machinability, particularly the hardness before aging treatment, which is an index of cutting resistance, exceeds the target value, and the age hardening ability is insufficient.

  In Test No. B5, since the Mn content of the steel 25 used is low from the definition of the present invention, pro-eutectoid ferrite is precipitated and the bainite area ratio is out of the specification. When this pro-eutectoid ferrite is generated, V carbide is precipitated at the phase interface with austenite, and V carbide is already precipitated before the aging treatment. Therefore, since the aging treatment causes coarsening of the already precipitated V carbide, test number B5 is difficult to age harden and has low cycle fatigue strength after aging treatment. Test number B5 has low absorbed energy in the Charpy impact test and is inferior in toughness.

  In Test No. B6, since the S content of the steel 26 used deviates from the definition of the present invention, coarse MnS increases and the deterioration of toughness is remarkable. Therefore, test number B6 has low absorbed energy and poor toughness in the Charpy impact test after aging treatment. Test number B6 has low fatigue strength and low cycle fatigue strength.

  In test number B7, the Ti content of the steel 27 used was too high outside the scope of the present invention, so that coarse TiN increased and the toughness deterioration was remarkable. Therefore, test number B7 has low absorbed energy and poor toughness in the Charpy impact test after aging treatment.

  In test number B8, since the V content of the steel 28 used is low from the definition of the present invention, the amount of V carbide precipitated by the aging treatment is small. Therefore, test number B8 is hard to age harden, and the fatigue strength after aging treatment is also low.

  In test number B9, since F1 of steel 29 used is small and deviates from the definition of the present invention, the hardenability is small and proeutectoid ferrite is generated. When pro-eutectoid ferrite is formed, V carbide precipitates at the phase interface with austenite. If the steel on which V carbide has already precipitated before aging treatment is aged, the precipitated V carbide is coarsened, so test number B9 is hard to age harden, and the fatigue strength after aging treatment is low.

  In Test No. B10, F2 ′ of the steel 30 used was large and deviated from the definition of the present invention, so that the machinability, in particular, the hardness before aging treatment, which is an index of cutting resistance, exceeded the target value.

  In Test No. B11, F3 ′ of the steel 31 used is small and deviates from the definition of the present invention. Therefore, the absorbed energy in the Charpy impact test after aging treatment is small and the toughness is inferior.

  The age-hardenable steel of the present invention has a machinability, particularly a hardness before aging treatment, which is an index of cutting resistance, of 290 Hv or less, and good machinability can be expected. In addition, when the age-hardenable steel of the present invention is used, a Vickers hardness of 30 Hv or more is cured by an aging treatment that is performed after cutting, so that a fatigue strength of 350 MPa or more can be obtained. In addition, with the age-hardening steel of the present invention, at 20 ° C. after aging treatment evaluated by a Charpy impact test conducted using a standard test piece with a U-notch having a notch depth of 2 mm and a notch bottom radius of 1 mm. The absorbed energy is 16 J or more, and the toughness reduction due to the aging treatment is sufficiently suppressed. Furthermore, if the age-hardening steel of the present invention is used, the low cycle fatigue strength can be made 510 MPa or more. For this reason, the age-hardening steel of the present invention can be used very suitably as a material for machine parts such as automobiles, industrial machines, and construction machines.

Claims (2)

% By mass
C: 0.05-0.20%,
Si: 0.01 to 0.50%,
Mn: 1.50 to 2.50%,
S: 0.005 to 0.08%,
Cr: 0.03 to 1.60%,
Al: 0.005 to 0.05%,
V: 0.25 to 0.50%,
Nb: 0.01 to 0.10%
Containing
P: 0.03% or less,
Ti: less than 0.005%,
N: limited to less than 0.0080%, the balance consists of Fe and impurities,
Further, the following chemistry in which F1 represented by the formula (1) is 0.68 or more, F2 represented by the formula (2) is 0.85 or less, and F3 represented by the formula (3) is 0.00 or more. An age-hardening steel, characterized by having a composition.
F1 = C + 0.3 × Mn + 0.25 × Cr (1)
F2 = C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V (2)
F3 = −4.5 × C + Mn + Cr−3.5 × V (3)
The element symbol in the above formulas (1) to (3) means the content in mass% of the element. % By mass
C: 0.05-0.20%,
Si: 0.01 to 0.50%,
Mn: 1.50 to 2.50%,
S: 0.005 to 0.08%,
Cr: 0.03 to 1.60%,
Al: 0.005 to 0.05%,
V: 0.25 to 0.50%,
Nb: 0.01 to 0.10%
Further satisfying any one or more of the composition conditions shown in the following <a> to <c>,
P: 0.03% or less,
Ti: less than 0.005%,
N: limited to less than 0.0080%, the balance consists of Fe and impurities,
Further, F1 ′ represented by the following formula (1 ′) is 0.68 or more, F2 ′ represented by the formula (2 ′) is 0.85 or less, and F3 ′ represented by the formula (3 ′) is 0. Age-hardenable steel having a chemical composition of 0.000 or more.
<a> Mo: 1.0% or less <b> Cu: 0.3% or less and Ni: 0.3% or less <c> Ca: 0.005% or less and Bi: 0.4% or less One or both of F1 ′ = C + 0.3 × Mn + 0.25 × Cr + 0.6 × Mo (1 ′)
F2 ′ = C + 0.1 × Si + 0.2 × Mn + 0.15 × Cr + 0.35 × V + 0.2 × Mo (2 ′)
F3 ′ = − 4.5 × C + Mn + Cr−3.5 × V−0.8 × Mo (3 ′)
The element symbol in said (1 ')-(3') formula means content in the mass% of the element.

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