AGE-HARDENABLE STEEL, AND METHOD FOR MANUFACTURING COMPONENTS USING AGE-HARDENABLE STEEL

Abstract

Age hardenable steel is low in hardness after hot forging, providing a machine part with the desired fatigue strength and yield strength by aging treatment, and high in toughness after aging treatment, comprising 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%, and a balance of Fe and impurities, where an area rate of bainite structures is 80% or more, an effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more, a P and Ti in the impurities is P: 0.03% or less and Ti: less than 0.005%, and the chemical composition is one where the following F1 is 1.00 or less and the F2 is 0.30 or more:

F1=C+0.1Si+0.2Mn+0.15Cr+0.35V

F2=4.5C+Mn+Cr3.5V

Claims

1. 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%, and a balance of Fe and impurities, the P and Ti in this 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 (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:
F1=C+0.1Si+0.2Mn+0.15Cr+0.35V(1)
F2=4.5C+Mn+Cr3.5V(2) where, in the above formulas (1) and (2), the element symbols mean the contents of the elements by mass %.

2. 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%, Mo: 0.9% or less, N: over 0.0080 to 0.0170%, and a balance of Fe and impurities, the P and Ti in this 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 (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:
F1=C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo(1)
F2=4.5C+Mn+Cr3.5V0.8Mo(2) where, in the above formulas (1) and (2), the element symbols mean the contents of the elements by mass %.

3. The age hardenable steel according to claim 1 further comprising one or more of Cu: 0.3% or less and Ni: 0.3% or less.

4. The age hardenable steel according to claim 1, further comprising one or more of Ca: 0.005% or less and Bi: 0.4% or less.

5. A method of production of part using age hardenable steel comprising: a forging step of heating age hardenable steel according to claim 1 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.

6. The age hardenable steel according to claim 2 further comprising one or more of Cu: 0.3% or less and Ni: 0.3% or less.

7. The age hardenable steel according to claim 2, further comprising one or more of Ca: 0.005% or less and Bi: 0.4% or less.

8. The age hardenable steel according to claim 3, further comprising one or more of Ca: 0.005% or less and Bi: 0.4% or less.

9. A method of production of part using age hardenable steel comprising: a forging step of heating age hardenable steel according to claim 2 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.

10. A method of production of part using age hardenable steel comprising: a forging step of heating age hardenable steel according to claim 3 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.

11. A method of production of part using age hardenable steel comprising: a forging step of heating age hardenable steel according to claim 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.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0058] FIG. 1 A view showing the correlation between a steel material hardness before aging and an F1 value

[0059] 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

[0060] Below, the requirements of the present invention will be explained in detail. Note that the % of the contents of the elements mean mass %.

[0061] Age Hardenable Steel

Essential Components

[0062] C: 0.09 to 0.20%

[0063] 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.

[0064] Si: 0.01 to 0.40%

[0065] 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.

[0066] Mn: 1.5 to 2.5%

[0067] 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.

[0068] S: 0.001 to 0.045%

[0069] 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.

[0070] Cr: Over 1.00% to 2.00%

[0071] 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.

[0072] Al: 0.001 to 0.060%

[0073] 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.

[0074] V: 0.22 to 0.55%

[0075] 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.

[0076] N: Over 0.0080 to 0.0170%

[0077] 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.

[0078] 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.

[0079] Impurities

[0080] 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.

[0081] P: 0.03% or Less

[0082] 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.

[0083] Ti: Less than 0.005%

[0084] 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.

[0085] Structure

[0086] 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 depth to 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.

[0087] Effective V Ratio

[0088] 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.

[0089] Optional Components

[0090] Next, the optional components able to be contained in the age hardenable steel of the present invention will be referred to.

[0091] Mo: 0.9% or Less

[0092] 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.

[0093] Cu: 0.3% or Less

[0094] 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.

[0095] Ni: 0.3% or Less

[0096] 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.

[0097] 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.

[0098] Ca: 0.005% or Less

[0099] 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.

[0100] Bi: 0.4% or Less

[0101] 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.

[0102] 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.

[0103] Values Calculated by Formulas Using Contents of Specific Elements: F1 (F1) and F2 (F2)

[0104] 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.

[0105] First, the value F1 (F1) calculated by a formula using contents of specific elements will be explained.

[0106] That is, when optional elements from Mo to Bi are not contained, F1 expressed by

F1=C+0.1Si+0.2Mn+0.15Cr+0.35V(1)

is 1.00 or less and when one or more optional elements from Mo to Bi are contained, F1 expressed by

F1=C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo(1)

is 1.00 or less.

[0107] Note that, in the above formula (1) and formula (1), the element symbols mean the contents of those elements in mass %.

[0108] 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.

[0109] 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.

[0110] 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 F11.00 or less, it is judged that the hardness before aging 340 HV.

[0111] Next, the value F2 (F2) calculated by a formula using contents of specific elements will be explained.

[0112] That is, when optional elements from Mo to Bi are not contained, F2 expressed by

F2=4.5C+Mn+Cr3.5V(2)

is 0.30 or more and when one or more optional elements from Mo to Bi are contained, F2 expressed by

F2=4.5C+Mn+Cr3.5V0.8Mo(2)

is 0.30 or less.

[0113] Note that, in the above formula (2) and formula (2), the element symbols mean the contents of those elements in mass %.

[0114] 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.

[0115] 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.

[0116] F2 and F2 preferably are 0.45 or more, more preferably are 0.60 or more.

[0117] 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.

[0118] 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.

[0119] Method of Production of Age Hardenable Steel

[0120] 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.

[0121] Method of Production of Part Using Age Hardenable Steel

[0122] 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.

[0123] 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.

[0124] 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.

[0125] Furthermore, the thus cooled worked material was further machined to finish it into a predetermined part shape.

[0126] 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

[0127] The Steels 1 to 27 of the chemical compositions shown in Table 1 were smelted with a 50 kg vacuum melting furnace. The Steels 1 to 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-US-00001 TABLE 1 Steel Components type mass % (balance: Fe and impurities) name C Si Mn P S Cu Ni Cr Al 1 0.13 0.11 1.63 0.012 0.018 <0.01 <0.01 1.11 0.021 2 0.12 0.06 2.16 0.010 0.015 0.11 0.13 1.22 0.022 3 0.10 0.30 2.00 0.011 0.013 <0.01 <0.01 1.20 0.018 4 0.13 0.34 1.85 0.012 0.016 <0.01 <0.01 1.35 0.025 5 0.13 0.20 1.81 0.012 0.015 0.01 0.01 1.38 0.025 6 0.16 0.06 1.55 0.008 0.023 <0.01 <0.01 1.52 0.005 7 0.16 0.10 1.55 0.009 0.022 0.21 0.11 1.23 0.016 8 0.12 0.19 1.71 0.006 0.006 0.01 0.01 1.20 0.019 9 0.12 0.20 1.72 0.005 0.005 0.01 0.01 1.21 0.018 10 0.13 0.20 1.75 0.011 0.016 0.01 0.01 1.20 0.022 11 0.10 0.30 1.81 0.011 0.015 0.01 0.01 l.03 0.03 12 0.10 0.20 1.77 0.010 0.014 <0.01 <0.01 1.25 0.026 13 0.11 0.20 1.78 0.010 0.016 <0.01 <0.01 1.50 0.029 14 0.14 0.15 1.65 0.010 0.014 <0.01 <0.01 1.11 0.031 15 0.11 0.20 1.77 0.010 0.014 <0.01 <0.01 1.49 0.026 16 0.12 0.06 2.25 0.022 0.024 0.01 0.01 1.12 0.011 17 0.12 0.31 1.86 0.011 0.020 <0.01 <0.01 1.75 0.023 18 0.16 0.34 2.25 0.015 0.015 <0.01 <0.01 1.70 0.024 19 0.14 0.20 1.59 0.015 0.033 <0.01 <0.01 1.02 0.029 20 0.16 0.16 1.55 0.021 0.029 <0.01 0.11 1.03 0.011 21 0.15 0.19 1.82 0.013 0.019 <0.01 <0.01 1.29 0.020 22 0.08 0.20 1.75 0.010 0.019 <0.01 <0.01 1.39 0.026 23 0.23 0.05 1.79 0.010 0.021 <0.01 <0.01 1.49 0.026 24 0.13 0.10 1.61 0.010 0.021 <0.01 <0.01 1.13 0.026 25 0.13 0.11 1.65 0.005 0.018 <0.01 <0.01 1.11 0.022 26 0.11 0.10 1.59 0.010 0.024 <0.01 <0.01 1.02 0.025 27 0.11 0.33 1.55 0.01 0.020 <0.01 <0.01 0.20 0.019 Area rate Steel Components of bainite Eff. F1 P2 type mass % (balance: Fe and impurities) structures V or or name V N Mo Ti Others (%) ratio F1 P2 1 0.31 0.0130 0.10 <0.001 +0.005Nb 100 0.99 0.76 0.99 2 0.35 0.0161 0.05 0.001 100 0.98 0.87 1.58 3 0.45 0.0090 <0.01 <0.001 100 0.99 0.87 1.18 4 0.39 0.0099 <0.01 <0.001 100 0.99 0.87 1.25 5 0.32 0.0115 0.39 <0.001 100 0.99 0.91 1.17 6 0.29 0.0089 0.35 <0.001 100 0.99 0.88 1.06 7 0.30 0.0126 0.05 0.001 100 0.98 0.78 0.97 8 0.31 0.0126 0.30 0.001 +0.001Ca 100 0.98 0.83 1.05 9 0.30 0.0146 0.30 0.001 +0.015Bi 100 0.98 0.83 1.10 10 0.32 0.0146 0.30 0.001 100 0.98 0.85 1.01 11 0.40 0.0155 0.29 <0.001 +0.018Nb 100 0.98 0.84 0.78 12 0.35 0.0151 0.14 0.002 +0.018Nb 100 0.97 0.81 1.23 13 0.42 0.0149 0.15 <0.001 100 0.98 0.89 1.20 14 0.23 0.0111 0.49 <0.001 100 0.99 0.83 0.93 15 0.32 0.0158 0.20 <0.001 +0.018Nb 100 0.98 0.86 1.49 16 0.35 0.0133 0.01 <0.001 100 0.99 0.87 1.60 17 0.42 0.0129 0.32 <0.001 100 0.98 1.00 1.34 18 0.41 0.0126 0.12 <0.001 48 0.98 1.07 1.70 19 0.45 0.0126 0.16 <0.001 100 0.98 0.82 0.28 20 0.45 0.0110 <0.01 <0.001 100 0.98 0.80 0.29 21 0.39 0.0111 <0.01 0.012 100 0.97 0.86 1.07 22 0.30 0.0129 0.11 <0.001 100 0.99 0.79 1.64 23 0.23 0.0103 0.15 <0.001 100 0.98 0.93 1.32 24 0.30 0.0021 0.09 <0.001 +0.005Nb 100 0.99 0.75 1.03 25 0.30 0.0229 0.08 <0.001 +0.005Nb 100 0.89 0.76 1.06 26 0.30 0.007 0.06 <0.001 100 0.99 0.71 1.02 27 0.25 0.0135 0.01 <0.001 69 0.86 0.57 0.37

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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 TestTest 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.

[0132] 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 25 J or more, it was judged sufficiently high. This was made the target.

[0133] 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 1.010.sup.7 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.

[0134] A tensile test was conducted using a tensile test piece of 14A of JIS having a 6 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-US-00002 TABLE 2 Before After aging Steel aging Fatigue Yield Impact Test type Hardness Hardness strength strength value no. name HV HV MPa MPa J A1 1 265 301 510 815 66 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 A11 11 294 336 540 889 36 A12 12 291 316 525 862 75 A13 13 300 332 540 915 58 A14 14 295 323 535 861 39 A15 15 296 335 525 890 80 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 24 266 292 485 775 72 B8 25 263 282 470 744 66 B9 26 260 280 475 748 81 B10 27 261 273 460 715 65

[0135] As clear from Table 2, in the case of the invention examples of Test Nos. A1 to 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 36 J 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.

[0136] 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

[0137] 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 (25 J 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.