EP3272896B1

EP3272896B1 - Age-hardenable steel, and method for manufacturing components using age-hardenable steel

Info

Publication number: EP3272896B1
Application number: EP16765035.7A
Authority: EP
Inventors: Masato Yuya; Motoki Takasuga; Taizo Makino; Masashi Higashida; Tatsuya Hasegawa; Tadashi Nishiwaki; Kouji Morita; Toshimasa Ito; Yoshiro TANIMURA; Tomomitsu Fukuoka
Original assignee: Denso Corp; Nippon Steel Corp
Current assignee: Denso Corp; Nippon Steel Corp
Priority date: 2015-03-16
Filing date: 2016-03-16
Publication date: 2020-01-08
Anticipated expiration: 2036-03-16
Also published as: ES2769257T3; US20180245172A1; EP3272896A1; EP3272896A4; JP6465959B2; JPWO2016148206A1; WO2016148206A1

Description

Technical Field

The present invention relates to age hardenable steel. More specifically, it relates to steel for production of a machine part for automobiles, industrial machinery, and construction machinery which is worked into a predetermined shape by hot forging and machining, is treated for age hardening (below, simply referred to as "aging treatment"), and has the desired strength and toughness secured by this aging treatment. Further, the present invention relates to such a method of production of a part using age hardenable steel.

Background Art

From the viewpoints of raising the engine output, lightening the weight aiming at improvement of the fuel economy, etc., machine parts for automobiles, industrial machinery, construction machinery, etc. are required to be high in fatigue strength. If just providing steel with a high fatigue strength, it is possible to easily achieve this by utilizing alloy elements and/or heat treatment to raise the hardness of the steel. However, in general, machine parts are formed by hot forging, then are machined to finish them to predetermined product shapes. For this reason, the steel used as a material for machine parts must be provided with high fatigue strength together with sufficient machinability simultaneously.
In general, the fatigue strength becomes better the higher the hardness of the material. On the other hand, in the machinability, the machining resistance and the tool life tend to become more inferior the higher the hardness of the material. Furthermore, among the parts forming the engine, precision shaped machine parts are required to remain unchanged in dimensions during use. Depending on the environment of use, these precision shaped machine parts can be instantaneously subjected to higher loads compared with the loads of the extents of usual use. Yield strength is also required so that the dimensions do not change in the face of such loads.
Therefore, various arts have been disclosed which are able to provide fatigue strength, yield strength, and machinability all together by keeping the hardness low at the shaping stage where good machinability is required while raising the hardness by aging treatment at the final product stage where strength is required.
For example, Japanese Patent Publication No. 2006-37177A (PLT 1) discloses "age hardening steel" obtained by rolling, forging, or solutionization of steel in which the precipitation strengthening elements of Mo and V are contained in amounts limited by specific formulas, and cooling between a temperature of 800°C to 300°C by an average cooling rate of 0.05 to 10°C/sec, having an area rate of bainite structures of 50% or more and a hardness of 40 HRC or less before aging treatment, and, after aging treatment, having a hardness of 7 HRC or more higher than the hardness before aging treatment.
Japanese Patent Publication No. 2011-236452A (PLT 2) discloses, as steel excellent in hot forgeability and machinability after hot forging and able to be raised in strength by age hardening after machining, bainite steel containing specific amounts of Mo and V as precipitation hardening elements.
Japanese Patent Publication No. 2000-17374A (PLT 3) proposes, as age hardening type high strength bainite steel for hot forging use, age hardening type high strength bainite steel characterized by having a yield point or 0.2% yield strength of 900 MPa or more obtained by hot rolling or hot forging steel containing Mo and V, then cooling it according to the steel components, making the hardness 400 HV or less, making the structure a bainite rate of 70% or more, making the old austenite grain size 80 µm or less, then machining or plastic forming the steel according to need and applying aging treatment.
Japanese Patent Publication No. 2013-245363A (PLT 4) describes steel promising both a high machinability and a high fatigue strength which is adjusted in the contents of the alloy elements to satisfy specific parameter formulas and thereby relatively reduce the content of Mo while making the hardness before aging treatment after hot forging 290 HV or less and making the hardness after aging treatment 325 HV or more.
WO2012/161323A (PLT 5) discloses a steel part for machine structure use using cooling and heat treatment after hot forging to optimize the shapes of V carbonitrides and shapes of bainite structures having a precipitation strengthening ability and provide machinability, fatigue strength, and toughness all together.
Japanese Patent Publication No. 2013-213254A (PLT 6) discloses steel for cold forging and nitriding use excellent in cold forgeability and chip removal ability after cold forging and able to provide a cold forged nitrided part with a high core hardness, high surface hardness, and deep effective hardened layer depth. FR 2990218 A1 discloses an age hardenable microalloyed bainitic steel.

Summary of Invention

Technical Problem

By using aging treatment to cause fine secondary phase particles to precipitate in steel, it is possible to obtain a high fatigue strength and yield strength. In this regard, the steel strengthened by aging treatment is lowered in toughness.
Steel lowered in toughness rises in notch sensitivity. If the notch sensitivity rises, the fatigue strength of the steel becomes easily affected by fine surface flaws.
Further, steel low in toughness suffers from faster progression of fractures and larger scale fractures once fatigue cracks occur.
Furthermore, if steel becomes too low in toughness, it becomes difficult to correct strain caused in the hot forging by cold working.
The steel disclosed in PLT 1 can be adjusted in the contents of the alloy elements so as to satisfy specific parameter formulas to obtain a high age hardening ability, but the toughness is not considered at all.
The steel disclosed in PLT 2 adjusts the contents of the alloy elements to satisfy specific parameter formulas so as to relatively reduce the content of Mo while making the hardness before aging treatment 300 HV or less after hot forging and making the hardness after aging treatment 300 HV or more. In this regard, however, the steel is not sufficiently designed to be raised in toughness after aging treatment.
The steel disclosed in PLT 3 has a C content kept low at 0.06 to 0.20%, but the V content is an extremely high 0.51 to 1.00%, so the steel is remarkably strengthened by age hardening but is not excellent in toughness.
The steel disclosed in PLT 4 is not sufficiently designed to raise the toughness and the yield strength after aging treatment.
The steel disclosed in PLT 5 is not sufficiently designed to raise the yield strength after aging treatment.
The steel disclosed in PLT 6 is low in N content, so nitrides are insufficiently produced and as a result an excellent yield strength is not obtained.
Therefore, an object of the present invention is to provide age hardenable steel satisfying the following <1> to <3>:

<1> A low hardness after hot forging related to the machining resistance and tool life. Note that, in the following explanation, the hardness after the above hot forging will be referred to as the "hardness before aging treatment".
<2> The ability to provide a machine part with a desired fatigue strength and yield strength by aging treatment.
<3> A high toughness after aging treatment.

Specifically, an object of the present invention is to provide age hardenable steel having a hardness before aging treatment of 340 HV or less, a fatigue strength explained later after aging treatment of 480 MPa or more, a 0.2% yield strength, found by the offset method using a prescribed plastic strain amount of 0.2% in a tensile test conducted using a tensile test piece of 14A of the JIS having a φ6 parallel part, of 800 MPa or more, and further having an absorption energy at 20°C after aging treatment, evaluated by a Charpy impact test conducted using a U-notched standard test piece having a notch depth of 2 mm and a notch bottom radius of 1 mm described in JIS Z 2242, of 25J or more.

Solution to Problem

Findings (a) to (d)

The inventors engaged in surveys and studies relating to the chemical composition, structure, and effective V ratio (amount of dissolved V/total amount of V) and the values calculated by formulas using contents of specific elements so as to solve the above problem. Specifically, they investigated the conditions for obtaining good toughness even with steel giving a high fatigue strength and yield strength by causing the precipitation of fine secondary phase particles in the steel due to aging. As a result, they obtained the following findings (a) to (d).

(a) Limitation of Chemical Composition (C, V, Mo, and Ti)

The elements for causing deterioration of toughness after aging treatment are C, V, Mo, and Ti. Among these, Ti bond with N and/or C to form TiN and/or TiC. If TiN and/or TiC precipitates, the fatigue strength sometimes becomes higher, but the toughness is made to greatly fall. The intensity of the action of Ti in lowering the toughness is extremely large compared with those of V and Mo which are the elements contributing to precipitation strengthening as V. For this reason, Ti must be limited as much as possible.
C forms cementite in steel and can become the starting point for cleavage fracture. Even if treating steel containing an amount of V or Mo excessive with respect to the amount of C by aging, part of the cementite remains. Both V and Mo cause precipitation of carbides at the same crystal planes of the matrix along with aging treatment and thereby assist the progression of cleavage fractures and cause deterioration of the toughness. Therefore, to raise the toughness, it is necessary to reduce the contents of C, V, and Mo.

(b) Limitation of Structure

To raise the toughness, it is necessary to make the majority of the structure fine bainite. Furthermore, making the difference in orientation between blocks forming the bainite greater is also essential for improving the toughness. If the difference in orientation between blocks is small, even if the size of the blocks is refined, the effect of raising the toughness is not sufficiently obtained. To enlarge the difference in orientation between blocks, it is necessary to enlarge the driving force at the time of bainite transformation and promote the formation of nuclei of blocks with large differences in orientation. To obtain these effects, the contents of C, Mn, Cr, and Mo have to be increased.
However, C and Mo have the effect of refining the structure and raising the toughness and the action of precipitating as cementite or carbides and lowering the toughness. Overall, C greatly lowers the toughness and Mo slightly lowers the toughness.

(c) Limitation of Effective V Ratio

To utilize the precipitation strengthening by V to the maximum extent, it is necessary to limit the effective V ratio, defined as the amount of dissolved V to the total amount of V. The effective V ratio being small means the ratio of the amount of V contributing to precipitation strengthening is small and the strengthening ability is small and is not preferable. There is no upper limit of the effective V ratio. The closer to 1, the better.

(d) Limitation of Values Calculated by Formulas Using Contents of Specific Elements and Limitation of Amount of Ti

To impart sufficient toughness to age hardenable steel having a high strength, the contents of C, Mn, Cr, V, and Mo have to be controlled so that the value expressed by (2) or (2') showing an indicator of toughness after aging treatment explained later becomes a specific value or more. Furthermore, the content of Ti has to be made a specific value or less so that inclusions and precipitates harmful to toughness are not contained in the steel.

Findings (e) to (g)

Next, the inventors engaged in further surveys and studies relating to the values calculated by the formulas using the chemical composition and contents of specific elements. Specifically, they adjusted the components of steel able to secure toughness after aging and investigated the conditions relating to the hardness before aging and the hardness after aging and the age hardening ability expressed by the difference of the same. As a result, they obtained the findings of the following (e) to (g).

(e) Limitation of Values Calculated by Formulas Using Contents of Specific Elements

If the contents of C, Si, Mn, Cr, V, and Mo are controlled so that the value expressed by the later explained formula (1) or the formula (1') becomes a specific range, the hardness before the above aging treatment can be kept from excessively rising. For this reason, when machined under various conditions, machinability enabling industrial mass production can be expected.

(f) Limitation of Chemical Composition (Mo, V, C)

If reducing the contents of Mo, V, and C so as to raise the toughness after aging, the driving force in precipitation of V carbonitrides at the time of aging becomes smaller. For this reason, the fine precipitates formed due to aging become fewer and the hardness and yield strength after aging become lower.

(g) Limitation of Chemical Composition (Mn, Cr)

If increasing the contents of Mn and Cr so as to raise the toughness after aging, the hardenability becomes higher and the hardness before aging becomes harder. With such a structure, the structure easily recovers at the time of aging, so the margin of increase of the hardness due to aging easily becomes smaller. If the hardenability becomes higher, the mobile dislocation density remaining in the matrix after aging also easily becomes greater, so obtaining a high yield strength becomes difficult.

Findings (h) to (j)

Next, the inventors engaged in further surveys and studies relating to the chemical composition. Specifically, they focused on the fact that even if raising the toughness after aging by reducing the contents of C, V, and Mo and increasing the contents of Mn and Cr, to cause sufficient amounts of precipitation strengthening particles to precipitate and obtain a sufficient age hardening ability and high yield strength, it is necessary to increase the precipitation strengthening ability per unit V amount. Further, the inventors engaged in various studies on techniques for increasing the precipitation strengthening ability of V and obtained the following findings (h) to (j).

(h) Limitation of Chemical Composition (C, N)

By utilizing precipitation strengthening by V to the maximum extent, it is sufficient to raise the driving force for precipitation of V carbonitrides. For this purpose, it is necessary to sufficiently secure the amount of C and amount of N able to be utilized for precipitation of V carbonitrides in a range not obstructing toughness.

(i) Limitation of Chemical Composition (N)

The bonding force between N and V is larger than the bonding force between C and V, so the effect of promoting the precipitation of V carbonitrides is larger with N than with C.

(j) Limitation of Chemical Composition (C, N)

If the contents of C and N become too great, V does not enter a solution even by heating at the time of hot forging or ends up precipitating in the austenite region during forging. For this reason, if overly increasing the contents of C and N, conversely the precipitation strengthening ability falls.
The present invention was made based on the above findings (a) to (j) and is defined in the appended claims.

Advantageous Effects of Invention

The age hardenable steel of the present invention has a hardness before aging treatment of 340 HV or less. Further, a machine part using the age hardenable steel of the present invention has a fatigue strength of 490 MPa or more due to aging treatment performed after machining. Further, the machine part has a toughness (absorption energy at 20°C after aging treatment evaluated by a Charpy impact test performed using a standard test piece with a U-notch of a notch depth of 2 mm and a notch bottom radius of 1 mm) of 25J or more. Furthermore, the machine part has a yield strength of 800 MPa or more. For this reason, the age hardenable steel of the present invention can be extremely suitably used as a material for a machine part of automobiles, industrial machinery, construction machinery, etc.

Brief Description of Drawings

[FIG. 1] A view showing the correlation between a steel material hardness before aging and an F1 value [FIG. 2] A view showing the relationship between a Charpy impact value of a steel material after aging and an F2 value.

Description of Embodiments

Below, the requirements of the present invention will be explained in detail. Note that the "%" of the contents of the elements mean "mass%".

Age Hardenable Steel

Essential Components

C: 0.09 to 0.20%

C is an important element in the present invention. C bonds with V to form carbides and strengthen the steel. However, if the content of C is under 0.09%, carbides of V become harder to precipitate, so the desired strengthening effect cannot be obtained. On the other hand, if the content of C becomes too great, the amount of C not bonding with V or Mo forming carbides with Fe (cementite) increases, so the toughness ends up being degraded. Therefore, the content of C was made 0.09 to 0.20%. The content of C preferably is made 0.10% or more, more preferably made 0.11% or more. Further, the content of C preferably is made 0.18% or less, more preferably is made 0.16% or less.

Si: 0.01 to 0.40%

Si is useful as a deoxidizing element at the time of steelmaking and simultaneously has the action of improving the strength of the steel by dissolving in the matrix. To sufficiently obtain these effects, Si has to be made 0.01% or more in content. However, in steel containing Mn and Cr in large amounts, if the content of Si becomes excessive, sometimes the amount of residual austenite of the structure after hot forging becomes too great and deformation becomes greater during aging treatment. Therefore, the content of Si was made 0.01 to 0.40%. The content of Si preferably is made 0.05% or more. Further, the content of Si preferably is made 0.35% or less, more preferably 0.30% or less.

Mn: 1.5 to 2.5%

Mn has the effect of improving the hardenability and making the structure bainite. Furthermore, it has the effect of lowering the bainite transformation temperature and thereby refining the bainite structure to improve the toughness of the matrix. Further, Mn has the action of forming MnS in steel to improve the chip removal ability at the time of machining. To sufficiently obtain these effects, Mn has to be made at least 1.5% in content. However, Mn is an element which easily segregates at the time of solidification of the steel, so if the content becomes too great, the fluctuation in hardness in a part after hot forging unavoidably becomes larger. Therefore, the content of Mn was made 1.5 to 2.5%. The content of Mn preferably is made 1.6% or more, more preferably is made 1.7% or more. Further, the content of Mn preferably is made 2.3% or less, more preferably is made 2.1% or less.

S: 0.001 to 0.045%

S bonds with Mn in the steel to form MnS and improves the chip removal ability at the time of machining, so has to be made 0.001% or more. However, if the content of S becomes greater, coarse MnS increase and the toughness and fatigue strength are degraded. In particular, if the content of S exceeds 0.045%, the fall in toughness and fatigue strength becomes remarkable. Therefore, the content of S was made 0.001 to 0.045%. The content of S preferably is made 0.005% or more, more preferably is made 0.010% or more. Further, the content of S preferably is made 0.040% or less, more preferably is made 0.035% or less.

Cr: Over 1.00% to 2.00%

Cr, like Mn, has the effect of raising the hardenability and making the structure bainite. Furthermore, it lowers the bainite transformation temperature to refine the bainite structure. Furthermore, it has the effect of lowering the ease of movement of grain boundaries to refine the austenite grain size at the time of hot forging and as a result refine the bainite structure after transformation. Cr has the effect of raising the toughness of the matrix through the effects of these in refining the bainite structure. To sufficiently obtain these effects, it must be included in over 1.00%. However, if the content of Cr is over 2.0%, the hardenability becomes larger and the hardness before aging treatment sometimes exceeds 340 HV. Therefore, the content of Cr was made over 1.00% to 2.00%. The content of Cr preferably is made 1.10% or more. Further, the content of Cr preferably is made 1.80% or less, more preferably is made 1.60% or less.

Al: 0.001 to 0.060%

Al is an element having a deoxidizing action. To obtain this effect, 0.001% or more in content is required. However, if Al is excessively contained, coarse oxides are formed and the toughness falls. Therefore, the content of Al was made 0.001 to 0.060%. The content of Al preferably is made 0.050% or less.

V: 0.22 to 0.55%

V is the most important element in the steel of the present invention. At the time of aging treatment, V bonds with C to form fine carbides and thereby has the action of raising the fatigue strength. Further, when the steel contains Mo, V has the effect of combining with Mo and precipitating due to aging treatment and further raising the age hardening ability. To obtain these effects, V has to be made 0.22% or more in content. However, if the content of V becomes excessive, even in the heating at the time of hot forging, undissolved carbonitrides easily remain inviting a drop in toughness. Further, if the content of V becomes excessive, sometimes the hardness before aging treatment ends up becoming higher. Therefore, the content of V was made 0.22 to 0.55%. The content of V preferably is under 0.45%, more preferably is made 0.40% or less. Further, the content of V preferably is made 0.25% or more, more preferably is made 0.27% or more.

N: Over 0.0080 to 0.0170%

N has the effect of promoting the precipitation of V carbonitrides at the time of aging and raising the yield strength. To sufficiently obtain this effect, the content of N has to be made over 0.0080%. However, if the content of N exceeds 0.0170%, at the time of hot forging, the V carbonitrides fail to enter a solution and at the next time of aging and precipitation of a sufficient amount of fine V carbonitrides becomes difficult, so the yield strength falls. Therefore, the content of N was made over 0.0080 to 0.0170%. The content of N preferably is made 0.0090% or more, more preferably is made 0.0100% or more. Further, the content of N preferably is made 0.0160% or less, more preferably is made 0.0150% or less.
The age hardenable steel of the present invention is comprised of the above elements from C to N and a balance of Fe and impurities, the P and Ti in the impurities are P: 0.03% or less and Ti: less than 0.005%, the area rate of the bainite structure is 80% or more, and the effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more.

Impurities

The "impurities" indicate elements which enter from the starting materials of the ore and scraps and the manufacturing environment etc. when industrially producing ferrous metal materials.

P: 0.03% or less

P is an element contained as an impurity and not preferable in the present invention. That is, P segregates at the grain boundaries to thereby cause a drop in toughness. Therefore, the content of P was made 0.03% or less. The content of P preferably is made 0.025% or less.

Ti: Less than 0.005%

Ti is an element contained as an impurity and is particularly not preferable in the present invention. That is, Ti bonds with N and/or C to form TiN and/or TiC to invite a drop in toughness. In particular, if the content becomes 0.005% or more, the toughness greatly deteriorates. Therefore, the content of Ti was made less than 0.005%. To secure a good toughness, the content of Ti preferably is made 0.0035% or less.

Structure

In the age hardenable steel of the present invention, the area rate of the bainite structure is 80% or more. Here, the area rate of the bainite structure means the area rate in the case of observing a metal structure at a position from 1/3 depth to 1/2 depth of thickness from the surface of the steel material with an optical microscope. If making the area rate of the bainite structure 80% or more, the precipitation of V is suppressed, the effective V ratio becomes larger, and a high fatigue strength and 0.2% yield strength can be obtained.

Effective V Ratio

The effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more. Here, the "effective V ratio" means the amount of dissolved V in the total amount contained in the steel. If the effective V ratio is 0.9 or more, the amount of V carbonitrides precipitating during the aging treatment becomes greater and a high fatigue strength and 0.2% yield strength can be obtained.

Optional Components

Next, the optional components able to be contained in the age hardenable steel of the present invention will be referred to.

Mo: 0.9% or less

Mo, like V, is an element with a relatively low precipitation temperature of carbides and suitable for age hardening. Mo has the action of raising the hardenability and making the structure after hot forging bainite and of increasing the area rate. Mo has the action of forming carbides together with V to increase the age hardening ability in steel containing 0.22% or more of V. For this reason, Mo may be included in accordance with need. However, Mo is an extremely expensive element, so if the content becomes greater, the cost of manufacture of the steel increases and the toughness also falls. Therefore, the amount of Mo when included was made 0.9% or less. The amount of Mo when included preferably is made 0.75% or less, more preferably is made 0.60% or less, and still more preferable is less than 0.50%. On the other hand, to stably obtain the above effect of Mo, the amount of Mo when included desirably is made 0.05% or more, more desirably is made 0.10% or more.

Cu: 0.3% or less

Cu has the action of improving the fatigue strength. For this reason, Cu may be included according to need. However, if the content of Cu becomes greater, the hot workability falls. Therefore, the amount of Cu when included was made 0.3% or less. The amount of Cu when included preferably is made 0.25% or less. On the other hand, to stably obtain the effect of raising the fatigue strength of Cu, the amount of Cu when included is desirably made 0.1% or more.

Ni: 0.3% or less

Ni has the action of improving the fatigue strength. Furthermore, Ni also has the action of suppressing the drop in hot workability due to Cu. For this reason, Ni may be included in accordance with need. However, if the content of Ni becomes greater, the cost swells and, in addition, the above effect is also saturated. Therefore, the amount of Ni when included was made 0.3% or less. The amount of Ni when included preferably is made 0.25% or less. On the other hand, to stably obtain the above effects of Ni, the amount of Ni when contained desirably is made 0.1% or more.
The above Cu and Ni may be included as just one type of either of the same or as two types combined. The total content of the elements when included can be made 0.6% of the case where the contents of Cu and Ni are at their respective upper limit values.

Ca: 0.005% or less

Ca has the action of lengthening the tool life. For this reason, Ca may be included in accordance with need. However, if the content of Ca becomes larger, coarse oxides are formed and the toughness is lowered. Therefore, the amount of Ca when included was made 0.005% or less. The content of Ca when included is preferably made 0.0035% or less. On the other hand, to stably obtain the effect of Ca on increasing tool life, the amount of Ca when included is desirably made 0.0005% or more.

Bi: 0.4% or less

Bi has the action of lowering the machining resistance and increasing the tool life. For this reason, Bi may be included in accordance with need. However, if the content of Bi becomes greater, it causes a drop in the hot workability. Therefore, the amount of Bi when included was made 0.4% or less. The amount of Bi when included is preferably made 0.3% or less. On the other hand, to obtain the effect of Bi in prolonging the tool life, the amount of Bi when included is desirably made 0.03% or more.
The above Ca and Bi may be included as just one type of either of the same or as two types combined. The total content of the elements when included can be made 0.405% of the case where the contents of Ca and Bi are at their respective upper limit values, but is preferably made 0.3% or less.

Values Calculated by Formulas Using Contents of Specific Elements: F1 (F1') and F2 (F2')

The age hardenable steel of the present invention satisfies the conditions of the above-mentioned chemical composition (essential components and optional components), structure, and effective V ratio. Further the values F1 (F1') and F2 (F2') calculated by formulas using contents of specific elements has to be 1.00 or less and 0.30 or more respectively.
First, the value F1 (F1') calculated by a formula using contents of specific elements will be explained.
That is, when optional elements from Mo to Bi are not contained, F1 expressed by $F 1 = C + 0.1 \times Si + 0.2 \times Mn + 0.15 \times Cr + 0.35 \times V$
is 1.00 or less and when one or more optional elements from Mo to Bi are contained, F1' expressed by $F 1' = C + 0.1 \times Si + 0.2 \times Mn + 0.15 \times Cr + 0.35 \times V + 0.2 \times Mo$
is 1.00 or less.
Note that, in the above formula (1) and formula (1'), the element symbols mean the contents of those elements in mass%.
F1 and F1' are indicators showing the hardness before aging treatment. If the age hardenable steel of the present invention satisfies the conditions relating to the above F1 or F1', the hardness before aging treatment does not become too high, the machining resistance at the time of machining does not become large, and longer tool life is realized.
F1 and F1' are preferably 0.97 or less, more preferably 0.95 or less. Further, F1 and F1' are preferably 0.60 or more, more preferably 0.65 or more.
FIG. 1 is a graph showing the relationship between the hardness before aging (ordinate; HV) and the F1 values of various types of steel (abscissa). As clear from the graph of FIG. 1, a strong primary positive correlation is found between the two. If F1≤1.00 or less, it is judged that the hardness before aging ≤340 HV.
Next, the value F2 (F2') calculated by a formula using contents of specific elements will be explained.
That is, when optional elements from Mo to Bi are not contained, F2 expressed by $F 2 = - 4.5 \times C + Mn + Cr - 3.5 \times V$
is 0.30 or more and when one or more optional elements from Mo to Bi are contained, F2' expressed by $F 2' = - 4.5 \times C + Mn + Cr - 3.5 \times V - 0.8 \times Mo$
is 0.30 or less.
Note that, in the above formula (2) and formula (2'), the element symbols mean the contents of those elements in mass%.
F2 and F2' are indicators showing the toughness after aging treatment. That is, by just satisfying the condition of F1 or F1', sometimes the toughness after aging treatment falls and the targeted toughness cannot be secured, so it is necessary to separately prescribe F2 and F2'.
FIG. 2 is a view showing a relationship between a Charpy impact value of a steel material after aging and an F2 value. As shown in this figure, a positive correlative relationship is observed between the Charpy impact value (J) after aging treatment and the F2 value (abscissa). When F2 or F2' is less than 0.30, toughness after aging treatment is not sufficiently obtained. To obtain a yield strength of 800 MPa or more while securing the targeted toughness, it is necessary to make the contents of the above alloy elements within the prescribed ranges, satisfy the conditions of F1 or F1', and satisfy the conditions of F2 or F2'.
F2 and F2' preferably are 0.45 or more, more preferably are 0.60 or more.
Note that, if F2 becomes larger, often the hardness before aging also becomes larger. However, so long as F1 is controlled to 1.00 or less, even if F2 is large, the hardness before aging will not become too large and the machinability will not be degraded. Accordingly, there is no need to particularly set an upper limit for F2. Similarly, if F1' is 1.00 or less, there is no need to particularly set an upper limit for F2'.

Method of Production of Age Hardenable Steel

The method of production of the age hardenable steel of the present invention is not particularly limited. A general method may be used to smelt the steel and adjust the chemical composition.

Method of Production of Part Using Age Hardenable Steel

Below, one example of the method of production of a machine part for an automobile, industrial machinery, construction machinery, etc. using as a material the age hardenable steel of the present invention produced in the following way will be shown.
First, a material used for hot forging (below, referred to as a "material for hot forging use") is prepared from steel with a chemical composition adjusted to the above-mentioned range. As the material for hot forging use, a billet obtained by blooming from an ingot, a billet obtained by blooming from a continuously cast material, or steel rods obtained by hot rolling or hot forging these billets etc. can be used.
Next, the material for hot forging use is hot forged and further is machined to finish the worked material to a predetermined part shape. Note that the hot forging is for example performed by heating the material for hot forging use to 1100 to 1350°C for 0.1 to 300 minutes, then allowing the surface temperature after the finish forging to fall to 900°C or more, then cooling down to room temperature by an average cooling rate of 10 to 90°C/min in the temperature region from 800 to 400°C.
Furthermore, the thus cooled worked material was further machined to finish it into a predetermined part shape.
Finally, the worked material was supplied to aging treatment to obtain a machine part for an automobile, industrial machinery, construction machinery, etc. provided with the desired characteristics. The aging treatment is, for example, performed in the temperature region from 540 to 700°C, preferably from 560 to 680°C. The holding time of the aging treatment is adjusted by the size (mass) of the machine part for soaking, but can be made 30 to 1000 minutes.

Example 1

The Steels 2-10, 13, 14 and 16 to 27 of the chemical compositions shown in Table 1 were smelted with a 50 kg vacuum melting furnace. The Steels 2 to 10, 13, 14, 16 and 17 in Table 1 are steels with chemical compositions within the ranges prescribed in the present invention. On the other hand, the Steels 18 to 27 in Table 1 are steels with chemical compositions outside the conditions prescribed by the present invention.

Table 1

Steel type name	Components														Area rate of bainite structures (%)	Eff. V ratio	F1 or	P2 or P2'
	mass% (balance: Fe and impurities)
	C	Si	Mn	P	S	Cu	Ni	Cr	Al	V	N	Mo	Ti	Others

2	0.12	0.06	2.16	0.010	0.015	0.11	0.13	1.22	0.022	0.35	0.0161.	0.05	0.001		100	0.98	0.87	1.58
3	0.10	0.30	2.00	0.011	0.013	<0.01	<0.01	1.20	0.018	0.45	0.0090	<0.01	<0.001		100	0.99	0.87	1.18
4	0.13	0.34	1.85	0.012	0.016	<0.01	<0.01	1.35	0. 025		0.0099	<0.01	<0.001		100	0.99	0.87	1.25
5	0.13	0.20	1.81	0.012	0.015	0.01	0.01	1.38	0.025	0.32	0.0115	0.39	<0.001		100	0.99	0.91	1.17
6	0.16	0.06	1.55	0.008	0.023	<0.01	<0.01	1.52	0.005	0.29	0.0089	0.35	<0.001		100	0.99	0.88	1.06
7	0.16	0.10	1.55	0.009	0.022	0.21	0.11	1.23	0.016	0.30	0.0126	0.05	0.001		100	0.98	0.78	0.97
8	0.12	0.19	1. 71	0.006	0.006	0.01	0.01	1.20	0.019	0.31	0.0126	0.30	0.001	0.001Ca	100	0.98	0.83	1.05
9	0.12	0.20	1.72	0.005	0.005	0.01	0.01	1.21	0.018	0.30	0.0146	0.30	0.001	+0.015Bi	100	0.98	0.83	1.10
10	0.13	0.20	1.75	0.011	0.016	0.01	0.01	1.20	0.022	0.32	0.0146	0.30	0.001		100	0.98	0.85	1.01

13	0.11	0.20	1.78	0.010	0.016	<0.01	<0.01	1.50	0.029	0.42	0.0149	0.15	<0.001		100	0.98	0.89	20
14	0.14	0.15	1.65	0.010	0.014	<0.01	<0.01	1.11	0.031	0.23	0.0111	49	<0.001		100	0.99	0.83	0.93

16	0.12	0.06	2.25	0.022	0.024	0.01	0.01	1.12	0.011	0.35	0.0133	0.01	<0.001		100	0.99	0.87	1.60
17	0.12	0.31	1.86	0.013	0.020	<0.02	<0.01	1.75	0.022	0.42	0.0129	0.32	<0.001		100	0.98	1.00	1.34
18	0.16	0.34	2.25	0.015	0.015	<0.01	<0.01	1.70	0. 024	0.41	0.0126	0.12	<0.001		48	0.98	1.07	1.70
19	0.14	0.20	1.59	0.01:	0.033	<0.0]	<0.01	1. 02	0.029	0.45	0.0126	0.16	<0.001		100	0.98	0.82	0.28
20	0.16	0.16	1.55	0.021	0.029	<0. 01	0.11	1.03	0.011	0.45	0.0110	<0.01	<0. 001		100	0.98	0.80	0.29
21	0.15	0.19	1.82	0. 13	0.019	<0. 01	<0.01	1.29	0.020	0.39	0.0111	<0.01	0.012		100	0.97	0.86	1.07
22	0.08	0.20	1.75	0.010	0.019	<0.01	<0.01	1.39	0.26	0.30	0.0129	0.11	<0.001		100	0.99	0.79	1.64
23	0.23	0.05	1.79	0.010	0.021	<0. 01	<0.01	1.49	0.026	0.23	0.0103	0.15	<0.001		100	0.98	0.93	1.32
24	0.13	0.13	1.61	0.010	0.021	<0.01	<0.01	1.13	0.026	0.30	0. 0021	0.09	<0.001	+0.005Nb	100	0.99	0.75	1.03
25	0.13	0.11	1.65	0.005	0.018	<0.01	1 <0.01	1.11	0.022	0.30	0.0229	0.08	<0.001	+0.005Nb	100	0.89	0.76	1.06
26	0.11	0.10	1.59	0.010	0.024	<0.01	<0.01	1.02	0.025	0.30	0.007	0.06	<0.001		100	0.99	0.71	1.02
27	0.11	0.33	1.55	0.01	0.020	<0.01	<0.01	0.20	0.019	0.25	0.135	0.01	<0.001		69	0.86	0.57	0.37

The ingots of the various steel were heated at 1250°C, then were hot forged to steel rods of diameters of 60 mm. The hot forged steel rods were cooled to room temperature in the atmosphere. After that, these were heated at 1250°C for 30 minutes, then, envisioning forging to part shapes, were hot forged to steel rods with a diameter of 35 mm while surface temperatures of the forging rods at the time of finishing was kept from 950 to 1100°C. After the hot forging, all of the rods were cooled to room temperature in the atmosphere. The cooling rate at the time of allowing the rods to cool in the atmosphere was measured after by burying a thermocouple near R/2 of the steel rods hot forged under the above conditions ("R" indicates the radius of the steel rods), again raising the temperature to near the finish temperature in hot forging, then allowing the rods to cool in the atmosphere. The average cooling rate in the temperature region from 800 to 400°C after forging measured in this way was about 40°C/min.
For each steel, from part of the steel rods hot forged to a diameter of 35 mm, then cooled down to room temperature, in the state not subjected to aging treatment (that is, in the state as cooled), the two end parts of the steel rods were cut off by 100 mm in length, then test pieces were cut out from the remaining center parts and were investigated for hardness before aging treatment.
On the other hand, for each steel, the remainder of the hot forged steel rods were treated for aging by holding them at 600 to 630°C for 60 to 180 minutes, the two end parts of the steel rods were cut off by 100 mm in length, then test pieces were cut out from the remaining center parts and were investigated for hardness after aging treatment. Further, for each steel, test pieces were cut out from the steel rods and were investigated for absorption energy in a Charpy impact test, fatigue strength, and yield strength after aging treatment.
The hardness was measured in the following way. First, a steel rod was cross-cut, was buried in resin so that the cut surface became the measured surface, then was polished to a mirror finish to prepare a test piece. Next, based on "Vickers Hardness Test - Test Method" in JIS Z 2244 (2009), 10 points near the R/2 Part of the measured surface ("R" indicating the radius) were measured for hardness with a test force of 9.8N. The values of the above 10 points were arithmetically averaged to obtain the Vickers hardness. When the hardness before the aging treatment was 340 HV or less, it was judged that mass production was industrially possible even with parts machined under various conditions. This was made the target. The test piece after measurement of the hardness was corroded with Nital and observed for structure, whereupon the structure of the test piece of each steel was also mainly bainite with some MA structures mixed in.
The toughness after aging treatment was evaluated by a Charpy impact test conducted using a U-notched standard test piece with a depth of notch of 2 mm and notch bottom radius of 1 mm. When the absorption energy at a test temperature of 20°C was 25J or more, it was judged sufficiently high. This was made the target.

The fatigue strength was investigated by fabricating an Ono type rotating bending fatigue test piece with a diameter of the parallel part of 8 mm and length of 106 mm. That is, the above test piece was taken so that the center of the fatigue test piece becomes the R/2 part of a steel rod. An Ono type rotating bending fatigue test was conducted eight times under conditions of room temperature, the atmosphere, and a stress ratio of -1. The maximum value of the stress amplitude up to 1.0×10⁷ repetitions while not fracturing was made the fatigue strength. If the fatigue strength was 490 MPa or more, it was judged that the fatigue strength was sufficiently high and this was made the target.
A tensile test was conducted using a tensile test piece of 14A of JIS having a ϕ6 parallel part, the 0.2% yield strength was found by the offset method using a prescribed plastic strain of 0.2%, and the yield strength was made equal to this. When the yield strength was 800 MPa or more, it was judged sufficiently high and this was made the target. Table 2 shows the results of the surveys.

Table 2

Test no.	Steel type name	Before aging	After aging
		Hardness	Hardness	Fatigue strength	Yield strength	Impact value
		HV	HV	MPa	MPa	J

A2	2	309	330	530	885	77
A3	3	300	335	530	925	61
A4	4	293	329	520	884	50
A5	5	319	353	565	955	49
A6	6	320	352	550	940	38
A7	7	310	350	545	941	40
A8	8	299	331	535	886	50
A9	9	295	330	540	883	45
A10	10	300	333	540	899	42

A13	13	300	332	540	915	58
A14	14	295	323	535	861	39

A16	16	305	330	520	890	66
A17	17	335	364	545	960	54
B1	18	351	360	560	978	46
B2	19	291	344	535	940	22
B3	20	310	355	560	955	19
B4	21	301	349	535	953	9
B5	22	280	293	485	790	84
B6	23	326	339	520	895	16
B7	24	266	292	485	715	72
B8	25	263	282	470	744	66
B9	26	260	280	475	748	81
B10	27	261	273	460	715	65

As clear from Table 2, in the case of the "invention examples" of Test Nos. A2 to A10, A13, A14, A16 and A17 having the chemical composition, structure, and effective V ratio (amount of dissolved V/total amount of V) prescribed in the present invention and the values calculated using the formulas using the contents of specific elements, the hardness before aging treatment becomes 340 HV or less, while due to aging treatment, the fatigue strength becomes 510 MPa or more, the yield strength becomes 815 MPa or more, and the absorption energy in the Charpy impact test becomes 36J or more. For this reason, all of the target values are achieved, so both strength and toughness can be realized after aging treatment and the hardness before aging treatment is also low, so a fall in the machining resistance and a longer tool life can be expected.
As opposed to this, in the case of the "comparative examples" of the Test Nos. B1 to B10 outside that which is prescribed in the present invention, at least one of the targeted performances cannot be obtained.

Industrial Applicability

The age hardenable steel of the present invention can secure a suitable hardness before aging treatment (340 HV or less) and promises a drop in machining resistance and longer life of tools. Further, if using the age hardenable steel of the present invention, due to the aging treatment performed after machining, a suitable fatigue strength (490 MPa or more), yield strength (800 MPa or more), and impact value (25J or more) can be secured together. For this reason, the age hardenable steel of the present invention can be extremely suitably used as a material for a machine part in automobiles, industrial machinery, construction machinery, etc.

Claims

Age hardenable steel comprising, by mass%, C: 0.09 to 0.20%, Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over 1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, N: over 0.0080 to 0.0170%, optionally Mo: 0.9% or less, further optionally one or more of Cu: 0.3% or less and Ni: 0.3% or less, and further optionally one or more of Ca: 0.005% or less and Bi: 0.4% or less, and a balance of Fe and impurities, the P and Ti in these impurities being P: 0.03% or less and Ti: less than 0.005%, wherein
an area rate of bainite structures is 80% or more,

an effective V ratio which is amount of dissolved V/total amount of V, is 0.9 or more, and

a chemical composition is one where the F1 expressed by the following formula (1') is 1.00 or less and the F2 expressed by the following formula (2') is 0.30 or more: $F 1 = C + 0.1 \times Si + 0.2 \times Mn + 0.15 \times Cr + 0.35 \times V + 0.2 \times Mo$
$F 2 = - 4.5 \times C + Mn + Cr - 3.5 \times V - 0.8 \times Mo$
where, in the above formulas (1') and (2'), the element symbols mean the contents of the elements by mass%.
The age hardenable steel according to claim 1, comprising Mo: 0.9% or less.
The age hardenable steel according to claim 1 or 2 comprising one or more of Cu: 0.3% or less and Ni: 0.3% or less.
The age hardenable steel according to any one of claims 1 to 3, comprising one or more of Ca: 0.005% or less and Bi: 0.4% or less.
A method of producing a part using an age hardenable steel comprising:
a forging step of heating the age hardenable steel according to any one of claims 1 to 4 at 1100 to 1350°C for 0.1 to 300 minutes, then forging it so that a surface temperature after finish forging becomes 900°C or more, then cooling it down to room temperature while making the average cooling speed in a temperature region from 800 to 400°C a speed of 10 to 90°C/min,

a machining step machining the steel after forging, and

an aging treatment step holding the steel after machining in the temperature region from 540 to 700°C for 30 to 1000 minutes.