EP2985361B1

EP2985361B1 - Age-hardening steel

Info

Publication number: EP2985361B1
Application number: EP14850316.2A
Authority: EP
Inventors: Masato Yuya; Masashi Higashida; Hitoshi Matsumoto; Tatsuya Hasegawa; Yutaka Neishi; Taizo Makino
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2013-10-02
Filing date: 2014-10-01
Publication date: 2018-03-14
Anticipated expiration: 2034-10-01
Also published as: JP5880795B2; HUE037998T2; WO2015050151A1; CN105164297A; EP2985361A1; US20160201175A1; KR20150110804A; CN105164297B; EP2985361A4; JPWO2015050151A1; KR101709883B1

Description

TECHNICAL FIELD

The present invention relates to an age-hardenable steel. More specifically, the present invention relates to a steel which is processed into a desired shape by hot forging and cutting process, and is thereafter subjected to age-hardening treatment (hereafter, simply referred to as "aging treatment") to ensure desired strength and toughness by the aging treatment, and which is quite suitably used as a starting material for producing mechanical parts such as for automobiles, industrial machinery, construction machinery, and the like.

BACKGROUND ART

From the viewpoint of weight reduction for the purpose of increasing output power of engines and fuel economy, high fatigue strength is required for mechanical parts such as for automobiles, industrial machinery, construction machinery, and so on. Simply imparting high fatigue strength to steel can be easily achieved by increasing the hardness of steel by utilizing alloying elements and/or heat treatment. However, in general, the above described mechanical parts are formed by hot forging and thereafter finished into a predetermined product shape by cutting process. For this reason, the steel to be used as the starting material for the above described mechanical parts must have high fatigue strength and satisfactory machinability at the same time. In general, as the hardness of the starting material increases, the fatigue strength increases. On the other hand, regarding machinability, as the hardness of the starting material increases, cutting resistance and tool life tend to deteriorate.
Accordingly, to achieve fatigue strength and machinability at the same time, various techniques have been disclosed which allow the hardness to be suppressed to a low level in a forming stage in which high machinability is required and, on the other hand, allow the hardness to be increased by thereafter performing aging treatment in a final product stage in which strength is required.
For example, Patent Document 1 discloses the following age-hardening steel.
That is, there is disclosed an "age-hardening steel" containing: by mass%, C: 0.11 to 0.60%, Si: 0.03 to 3.0%, Mn: 0.01 to 2.5%, Mo: 0.3 to 4.0%, V: 0.05 to 0.5%, and Cr: 0.1 to 3.0%, and further containing, as needed, one or more kinds of Al: 0.001 to 0.3%, N: 0.005 to 0.025%, Nb: 0.5% or less, Ti: 0.5% or less, Zr: 0.5% or less, Cu: 1.0% or less, Ni: 1.0% or less, S: 0.01 to 0.20%, Ca: 0.003 to 0.010%, Pb: 0.3% or less and Bi: 0.3% or less, with the balance being Fe and inevitable impurities, wherein the following relationships are established among each component: $4 C + Mn + 0.7 Cr + 0.6 Mo - 0.2 V \geq 2.5,$
$C \geq Mo / 16 + V / 5.7,$
$V + 0.15 Mo \geq 0.4$
and wherein after rolling, forging, or solution treatment, the steel is cooled at an average cooling velocity of 0.05 to 10°C/sec in a temperature range of 800°C to 300°C so that before the aging treatment, an area fraction of bainite structure is not less than 50%, hardness thereof is not more than 40 HRC, and the hardness becomes 7 HRC or more higher than that before the aging treatment, due to the aging treatment.
Patent Document 2 discloses the following bainite steel.
That is, there is disclosed a "bainite steel", containing: by mass%, C: 0.14 to 0.35%, Si: 0.05 to 0.70%, Mn: 1.10 to 2.30%, S: 0.003 to 0.120%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.40%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.30%, and V: 0.05 to 0.45% and further containing, as needed, one or more kinds selected from Ti: 0.001 to 0.100%, and Ca: 0.0003 to 0.0100%, with the balance being Fe and inevitable impurities, wherein the following relationships are satisfied: $16 [C] + 8 [Si] + 10 [Mn] + 3 [Cu] + 3 [Ni] + 22 [Mo] + 11 [V] \leq 30,$
$5 [C] + [Si] + 2 [Mn] + 3 [Cr] + 2 [Mo] + 4 [V] \leq 7.3,$
$2.4 \leq 0.3 [C] + 1.1 [Mn] + 0.2 [Cu] + 0.2 [Ni] + 1.2 [Cr] + 1.1 [Mo] + 0.2 [V] \leq 3.1,$
$2.5 \leq [C] + [Si] + 4 [Mo] + 9 [V],$
and $[C] \geq [Mo] / 16 + [V] / 3.$
Further, Patent Documents 3 and 4 disclose age-hardenable steels having a predetermined chemical composition or micro structure, and Patent Documents 5 and 6 disclose, as a method for obtaining steel parts for mechanical structures, a method of performing aging treatment, in which steel material is cooled at a predetermined cooling velocity after hot forging, and thereafter is subjected to aging treatment in a predetermined temperature range.
Patent Document 7 provides a steel for nitriding with a composition including, by mass%: C: 0.10% to 0.20%; Si: 0.01% to 0.7%; Mn: 0.2% to 2.0%; Cr: 0.2% to 2.5%; Al: 0.01% to less than 0.19%; V: over 0.2% to 1.0%; Mo: 0% to 0.54%; N: 0.001% to 0.01%; P limited to not more than 0.05%; S limited to not less than 0.2%; and a balance including Fe and inevitable impurities, the composition satisfying 2 ≤ [V]/[C] ≤ 10, where [V] is an amount of V by mass% and [C] is an amount of C by mass%, in which the steel for nitriding has a microstructure containing bainite of 50% or more in terms of an area percentage.
Patent Document 8 is directed to surface of a steel, as a material for a crankshaft, that is nitrocarburized. The steel contains, as alloy elements C having a content 0.10 mass % or more 0.30 mass % or less, Si having a content 0.5 mass % or more and 0.3 mass % or less, Mn having a content 0.3 mass % or more and 1.5 mass % or less, Mo having a content 0.8 mass % or more and 2.0 mass % or less, Cr having a content 0.1 mass % or more and 1.0 mass % or less, and V having a content 0.1 mass % or more and 0.5 mass % or less, with a remainder consisting of Fe and inevitable impurities. The contents of the alloy elements fall within ranges: 2.0 mass % ≤ Mn+Cr+Mo ≤ 3.0 mass %, 2.3 mass % ≤ C+Mo+5V ≤ 3.7 mass %, and 2.7 mass % ≤ 2.16 Cr+Mo+2.54V ≤ 4.0 mass %. If a steel sample extracted from a central portion of the nitrocarburized steel free from an influence of the nitrocarburizing treatment is austenitized at 1200 DEG C. for one hour, and cooled to a room temperature so that a cooling rate at which the steel sample passes through a temperature range between 900 DEG C. and 300 DEG C. is 0.5 DEG C./second, then an area percentage of a bainite structure in steel structures is 80% or more and a Vickers hardness measured at a cross section is 260 Hv or more and 330 Hv or less. A surface hardness of a nitrocarburized layer is 650 Hv or more, a formation depth of the nitrocarburized layer is 0.3 mm or more, and a hardness of the central portion is 340 Hv or more.
Patent Document 9 describes a process for producing a hot forged steel of ferrite+bainite structure type which includes: applying hot forging to a steel product of a composition by weight of C: 0.10-0.35%, Si: 0.15-2.00%, Mn: 0.40-2.00%, S: 0.03-0.10%, Al: 0.0005-0.050%, Ti: 0.003-0.050%, N: 0.0020-0.0070%, V: 0.30-0.70%, and further containing one or more of Cr, Mo, Nb, Pb and Ca in a specified amount; cooling thereafter so that 80% or more of the metallographic structure after finishing of the transformation is ferrite+bainite structure; and further applying aging treatment at a temperature of 200 DEG -700 DEG C.

LIST OF PRIOR ART DOCUMENTS

PATENT DOCUMENT

Patent Document 1: JP2006-37177A
Patent Document 2: JP2011-236452A
Patent Document 3: WO2010/090238
Patent Document 4: WO2011/145612
Patent Document 5: WO2012/161321
Patent Document 6: WO2012/161323
Patent Document 7: EP 2 578 717 A1
Patent Document 8: US 2006/225814 A
Patent Document 9: US 5 601 667 A

DISCLOSURE OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

However, attempting to achieve higher strength by causing a fine secondary phase to precipitate in steel by aging treatment will result in deterioration of toughness of steel.
A steel whose toughness has deteriorated has an increased notch susceptibility. With a higher notch susceptibility, the fatigue strength of steel becomes more likely to be affected by fine surface flaws.
Moreover, once a fatigue crack occurs in steel with low toughness, the propagation of the crack becomes faster, and fracture becomes large scaled.
Further, when an attempt is made in the cold to correct distortion which has occurred during hot forging, the correction may become difficult even in a cold condition when toughness of the steel is excessively reduced.
Since the steel disclosed in Patent Document 1 is permitted to have a hardness before aging treatment of up to 40 HRC and thus a very high hardness, it is difficult to ensure machinability, specifically, cutting resistance is high so that tool life is decreased, thereby increasing cutting cost. While steels disclosed as a specific example include those whose hardness before aging treatment is less than 40 HRC, they contain not less than 1.4% of Mo, and in addition to that, their toughness is not taken into consideration at all.
In the steel disclosed in Patent Document 2, the contents of alloying elements are adjusted so as to satisfy a particular parametric formula so that while the content of Mo is kept to be relatively low, the hardness before aging treatment (after hot forging) is not more than 300 HV, and the hardness after aging treatment is not less than 300 HV. However, sufficient efforts have not been made to increase toughness after aging treatment.
Therefore, it is an object of the present invention to provide an age-hardenable steel which satisfies the following items <1> to <3>.

<1> Hardness after hot forging which relates to cutting resistance and tool life is sufficiently low. Note that in the following description, the hardness after hot forging is referred to as "hardness before aging treatment".
<2> It is possible to impart desired fatigue strength to a mechanical part through hardening by aging treatment.
<3> Toughness after aging treatment is high.

Specifically, it is an object of the present invention to provide an age-hardenable steel which has a chemical composition containing not more than 1.0 mass% of Mo and in which hardness before aging treatment is not more than 290 HV, hardness increases by 25 in HV by aging treatment, and the below-described fatigue strength is not less than 350 MPa, as well as absorbed energy at 20°C after aging treatment is not less than 16 J when evaluated by a Charpy impact test performed by using a standard specimen with a U-notch having a notch depth of 2 mm and a notch bottom radius of 1 mm according to JIS Z 2242.

MEANS FOR SOLVING THE PROBLEMS

To solve the above described problems, the present inventors have conducted investigation by using steels whose chemical compositions are varied. As a result of that, the following findings (a) to (c) have been obtained.

(a) V exhibits a precipitation peak of carbide at about 750 to 700°C when cooled from a high temperature. For example, in a steel containing 0.3 mass% of V and 0.1 mass% of C, since once resolved into the matrix, V will not precipitate until around 850°C, suppressing precipitation during hot forging is relatively easy.
(b) V carbide is likely to precipitate at phase boundaries when austenite transforms into ferrite. Therefore, when a large amount of pro-eutectoid ferrite is generated during cooling after hot forging, since V carbide precipitates at phase boundaries thereby reducing the amount of dissolved V, it becomes not possible to secure an amount of dissolved V necessary for precipitating and hardening during aging treatment.
(c) Therefore, to secure dissolved V in a stage before aging treatment, it is necessary that the micro-structure after hot forging contains bainite as a main phase.
Next, the present inventors have investigated conditions for stably obtaining a high area-fraction of bainite in the micro-structure, by varying the chemical composition of steel for a steel containing not less than 0.25 mass% of V. Further, they also investigated the age hardenability of those steels when they are subjected to aging treatment. As a result of that, the following findings (d) to (f) have been obtained.
(d) The micro-structure after hot forging has close correlation with the contents of C, Mn, Cr and Mo. That is, if the contents of the above described elements are controlled such that the value represented by Formula (1), which is to be described below and shows an index of hardenability, falls within a specific range, precipitation of a large amount of pro-eutectoid ferrite, which is harmful for ensuring dissolved V, is suppressed. For this reason, a micro-structure containing bainite as a main phase, that is, a micro-structure containing not less than 70% in area fraction of bainite is obtained with ease so that it is possible to secure a sufficient amount of dissolved V.
(e) When the contents of C, Mn, Cr and Mo satisfy only the condition that Formula (1) described in the above described (d) falls within a specific range, there may a case in which the cutting resistance during cutting increases, thereby reducing tool life since the hardness before aging treatment increases due to working of solid solution strengthening.
(f) On the other hand, if the contents of C, Si, Mn, Cr, V and Mo are controlled such that the value represented by Formula (2) to be described below falls within a specific range, it is possible to maintain the hardness before aging treatment to a low level.
Further, the present inventors investigated conditions to obtain absorbed energy of not less than 16 J at 20°C after aging treatment evaluated by a Charpy impact test performed by using a standard specimen with a U-notch having a notch depth of 2 mm and a notch bottom radius of 1 mm, by preparing steels containing not less than 0.25 mass% of V, in which contents of C, Si, Mn, Cr, Mo, and V satisfy both conditions as described in above (d) and (f), and which is subjected to hot forging and thereafter to aging treatment. As a result of that, the following findings (g) to (i) have been obtained.
(g) Elements that deteriorate toughness after aging treatment are C, V, Mo, and Ti. Among those, Ti combines with N and/or C to form TiN and/or TiC. Precipitation of TiN and/or TiC may increase fatigue strength, but it significantly deteriorates toughness. The intensity of action of Ti to deteriorate toughness is very high compared with V and Mo which are similar precipitation strengthening elements. For that reason, the content of Ti must be restricted as much as possible. C forms cementite in steel, and may act as a starting point of cleavage fracture. Even when a steel which contains excess amounts of V and Mo with respect to C is subjected to aging treatment, some part of cementite remains. V and Mo cause carbide to precipitate in the same crystal plane of matrix as a result of aging treatment, thereby accelerating the progress of cleavage fracture and deteriorating toughness. Therefore, to improve toughness, it is necessary to decrease the contents of C, V, and Mo.
(h) Moreover, to improve toughness, it is necessary to refine bainite structure. Refining of bainite structure can be achieved by decreasing the transformation temperature from austenite to bainite. Decreasing of the transformation temperature of bainite can be achieved by increasing the contents of Mn and Cr which decrease the start temperature of bainite transformation.
(i) From what has been described so far, to impart sufficient toughness to an age-hardenable steel having high strength, it is necessary to control the contents of C, Mn, Cr, V, and Mo such that the value represented by Formula (3) showing an index of toughness after aging treatment to be described later is not less than a certain value, and further to control the content of Ti to be not more than a specific value such that inclusions and precipitates which are harmful for toughness are not included in steel.

The present invention has been made based on the above described findings, and its gist is an age-hardenable steel described below.

(1) An age-hardenable steel, having a chemical composition consisting of: by mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.5 to 2.5%, S: 0.005 to 0.08%, Cr: 0.03 to 0.50%, Al: 0.005 to 0.05%, V: 0.25 to 0.50%, Mo: 0 to 1.0%, Cu: 0 to 0.3%, Ni: 0 to 0.3%, Ca: 0 to 0.005%, and Bi: 0 to 0.4%,
with the balance being Fe and impurities, wherein
- P, Ti, and N included in the impurities are: P: 0.03% or less, Ti: less than 0.005%, and N: less than 0.0080%, and further wherein
- F1 represented by the following Formula (1) is not less than 0.68, F2 represented by the following Formula (2) is not more than 0.85, and F3 represented by the following Formula (3) is not less than 0.00; $F 1 = C + 0.3 Mn + 0.25 Cr + 0.6 Mo$
  $F 2 = C + 0.1 Si + 0.2 Mn + 0.15 Cr + 0.35 V + 0.2 Mo$
  and $F 3 = - 4.5 C + Mn + Cr - 3.5 V - 0.8 Mo$
  - where each symbol of element in the Formulas (1) to (3) means the content of the element in mass%,
  - wherein the steel contains bainite as a main phase, and
  - wherein hardness is not more than 290 HV.
(2) The age-hardenable steel according to the above described (1), wherein the chemical composition contains, by mass%, one or more kinds selected from elements shown in the following <1> to <3>;
1. <1> Mo: 0.05 to 1.0%,
2. <2> Cu: 0.1 to 0.3%, and Ni: 0.1 to 0.3%, and
3. <3> Ca: 0.0005 to 0.005%, and Bi: 0.03 to 0.4%.
(3) The age-hardenable steel according to the above described (1) or (2), wherein an average block size of the bainite is 15 to 60 µm.

ADVANTAGEOUS EFFECTS OF THE INVENTION

The age-hardenable steel of the present invention has hardness before aging treatment of not more than 290 HV. Furthermore, using the age-hardenable steel of the present invention causes the hardness to increase by not less than 25 in HV through aging treatment performed after cutting process, and can ensure a fatigue strength of not less than 350 MPa, and an excellent toughness, that is, absorbed energy at 20°C after aging treatment of not less than 16 J when evaluated by a Charpy impact test performed by using a standard specimen with a U-notch having a notch depth of 2 mm and a notch bottom radius of 1 mm. Therefore, the age-hardenable steel of the present invention can be quite suitably used as a starting material for mechanical parts such as for automobiles, industrial machinery, construction machinery, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[Figure 1] Figure 1 shows the shape of a uniaxial tension-compression type fatigue test specimen used in Examples. Numerical values in the figure represent dimensions (unit: mm).

MODE FOR CARRYING OUT THE INVENTION

Hereafter, each requirement of the present invention will be described in detail. Note that "%" of the content of each element means "mass%".

C: 0.05 to 0.20%

C is a crucial element in the present invention. C combines with V and forms a carbide, thereby strengthening the steel. However, when C content is less than 0.05%, the carbide of V becomes not likely to precipitate, and therefore desired strengthening effect cannot be achieved. On the other hand, when C content is excessively large, the amount of C which does not combine with V and Mo, but combines with Fe to form carbide (cementite) increases, thereby deteriorating the toughness of steel. Therefore, C content is specified to be 0.05 to 0.20%. The C content is preferably not less than 0.08%, and more preferably not less than 0.10%. Moreover, the C content is preferably not more than 0.18%, and more preferably not more than 0.16%.

Si: 0.01 to 0.50%

Si is useful as a deoxidizing element during steel making, and also has an effect of dissolving into matrix and thereby increasing the strength of steel. To achieve such effects satisfactorily, Si content of not less than 0.01% is required. However, when the Si content is excessive, hot workability of steel is deteriorated and its hardness before aging treatment increases. Therefore, Si content is specified to be 0.01 to 0.50%. The Si content is preferably not less than 0.06%. Moreover, the Si content is preferably not more than 0.45%, and more preferably less than 0.35%.

Mn: 1.5 to 2.5%

Mn has effects of improving hardenability, and causing the micro-structure to contain bainite as a main phase. Further, Mn also has an effect of decreasing the bainite transformation temperature, thereby refining the bainite structure and improving toughness of the matrix. Further, Mn has an effect of forming MnS in steel, thereby improving chip treatability during cutting. To achieve such effects satisfactorily, Mn content needs to be at least 1.5%. However, since Mn is an element which is likely to segregate during solidification of steel, when its content is excessive, it is inevitable that variation of hardness increases within a steel part after hot forging. Therefore, Mn content is specified to be 1.5 to 2.5%. The Mn content is preferably not less than 1.6%, and more preferably not less than 1.7%. Moreover, the Mn content is preferably not more than 2.3%, and more preferably not more than 2.1%.

S: 0.005 to 0.08%

Since S combines with Mn to form MnS in steel, thereby improving chip treatability during cutting, S content needs to be not less than 0.005%. However, when S content increases, coarse MnS increases thereby deteriorating toughness and fatigue strength. Therefore, S content is specified to be 0.005 to 0.08%. The S content is preferably not less than 0.01%. Moreover, the S content is preferably not more than 0.05%, and more preferably not more than 0.03%.

Cr: 0.03 to 0.50%

Cr, as well as Mn, has effects of improving hardenability, and causing the micro-structure to contain bainite as a main phase. Further, Cr also has an effect of decreasing the bainite transformation temperature, thereby refining the bainite structure and improving toughness of matrix. However, when Cr content is more than 0.50%, hardenability increases so that hardness before aging treatment may be more than 290 HV depending on the size and region of a steel part. Therefore, Cr content is specified to be 0.03 to 0.50%. The Cr content is preferably not less than 0.05%, and more preferably not less than 0.15%.

Al: 0.005 to 0.05%

Al is an element having a deoxidizing effect, and to achieve such an effect, Al content needs to be not less than 0.005%. However, when Al content is excessive, coarse oxides are likely to be produced, thereby deteriorating toughness. Therefore, the Al content is specified to be 0.005 to 0.05%. The Al content is preferably not more than 0.04%.

V: 0.25 to 0.50%

V is the most crucial element in the steel of the present invention. V has an effect of combining with C to form fine carbides during aging treatment, thereby increasing fatigue strength. Moreover, when Mo is contained in steel, V has an effect of being compounded with Mo and precipitated by aging treatment, further increasing age hardenability. To achieve such effects satisfactorily, V content needs to be not less than 0.25%. However, when V content is excessive, undissolved carbonitrides are likely to remain even during heating for hot forging, thereby causing deterioration of toughness. Further, when V content is excessive, the hardness before aging treatment may increase. Therefore, V content is specified to be 0.25 to 0.50%. The V content is preferably less than 0.45%, and more preferably not more than 0.40%. Moreover, the V content is preferably not less than 0.27%.

Mo: 0 to 1.0%

Mo, as well as V, has a relatively low precipitation temperature of carbide, and is an element which can be readily utilized for age-hardening. Mo has effects of improving hardenability, causing the micro-structure after hot forging to contain bainite as a main phase, and increasing its area fraction. Mo is compounded with V to form a carbide, thereby increasing age-hardenability. For that purpose, Mo may be contained as needed. However, since Mo is a very expensive element, an increase in its content will cause an increase in steel manufacturing cost, and also deterioration of toughness. Therefore, when Mo is contained, its content is specified to be not more than 1.0%. The content of Mo is preferably not more than 0.50%, more preferably not more than 0.40%, and further preferably less than 0.30%.
On the other hand, to stably achieve the above described effects of Mo, its content is preferably not less than 0.05%, and more preferably not less than 0.10%.
Each of Cu and Ni has an effect of increasing fatigue strength. Therefore, when higher fatigue strength is desired, these elements may be contained in the following range.

Cu: 0 to 0.3%

Cu has an effect of increasing fatigue strength. Therefore, Cu may be contained as needed. However, when Cu content increases, hot workability deteriorates. Therefore, when Cu is contained, its content is specified to be not more than 0.3%. The Cu content is preferably not more than 0.25%.
On the other hand, to stably achieve the above described effect of Cu of increasing fatigue strength, its content is preferably not less than 0.1%.

Ni: 0 to 0.3%

Ni has an effect of increasing fatigue strength. Moreover, Ni also has an effect of suppressing the deterioration of hot workability due to Cu. Therefore, Ni may be contained as needed. However, increase of Ni content causes saturation of the above described effect in addition to increase of cost. Therefore, when Ni is contained, its content is specified to be not more than 0.3%. The Ni content is preferably not more than 0.25%.
On the other hand, to stably achieve the above described effects of Ni, its content is desirably not less than 0.1%.
As for the above described Cu and Ni, only one of them, or two of them in combination may be contained. The total content of the above described elements, when they are contained, may be 0.6% at which each of Cu and Ni contents has its upper limit value.
Each of Ca and Bi has an effect of prolonging tool life during cutting. Therefore, when further prolonged tool life is desired, these elements may be contained within the following range.

Ca: 0 to 0.005%

Ca has an effect of prolonging tool life. Therefore, Ca may be contained as needed. However, when Ca content increases, coarse oxides are formed, thereby deteriorating toughness. Therefore, when Ca is contained, its content is specified to be not more than 0.005%. The Ca content is preferably not more than 0.0035%.
On the other hand, to stably achieve the above described effect of Ca for prolonging tool life, the Ca content is desirably not less than 0.0005%.

Bi: 0 to 0.4%

Bi has an effect of reducing cutting resistance and thereby prolonging tool life. Therefore, Bi may be contained as needed. However, when Bi content increases, hot workability deteriorates. Therefore, when Bi is contained, its content is specified to be not more than 0.4%. The Bi content is preferably not more than 0.3%.
On the other hand, to stably achieve the above described effect of Bi for prolonging tool life, the Bi content is preferably not less than 0.03%.
As for the above described Ca and Bi, only one of them, or two of them in combination may be contained. The total content of these elements, when they are contained, may be 0.405% at which each of Ca and Bi contents has its upper limit value, but is preferably not more than 0.3%.
The age-hardenable steel of the present invention is a steel having a chemical composition consisting of the above described elements, with the balance being Fe and impurities, wherein P, Ti, and N included in the impurities are: P: 0.03% or less, Ti: less than 0.005%, and N: less than 0.0080%, and further wherein, F1 represented by the above described Formula (1) is not less than 0.68, F2 represented by the above described Formula (2) is not more than 0.85, and F3 represented by the above described Formula (3) is not less than 0.00.
Note that impurities refer to those which are mixed from ores as the raw material, scrap, or manufacturing environments when steel material is industrially manufactured.

P: not more than 0.03%

P is contained as an impurity and is an undesirable element in the present invention. That is, P segregates at grain boundaries, and thereby deteriorates toughness. Therefore, the P content is specified to be not more than 0.03%. The P content is preferably not more than 0.025%.

Ti: less than 0.005%

Ti is contained as an impurity and is a particularly undesirable element in the present invention. That is, Ti combines with N and/or C to form TiN and/or TiC, thereby causing deterioration of toughness, and particularly when its content is not less than 0.005%, toughness is significantly deteriorated. Therefore, Ti content is specified to be less than 0.005%. To ensure excellent toughness, the Ti content is preferably not more than 0.0035%.

N: less than 0.0080%

N is contained as an impurity, and is an undesirable element which immobilizes V as a nitride in the present invention. That is, since V which has precipitated as a nitride will not contribute to age hardening, the N content needs to be kept low to suppress precipitation of nitride. For that purpose, the N content needs to be less than 0.0080%. The N content is preferably not more than 0.0070%, and more preferably less than 0.0060%.

F1: not less than 0.68

The age-hardenable steel of the present invention must satisfy the condition that F1 represented by the following Formula (1) is not less than 0.68: $F 1 = C + 0.3 Mn + 0.25 Cr + 0.6 Mo$
As already described, each symbol of element in the Formula (1) means the content of that element in mass%.
F1 is an index for hardenability. Provided that the amount of each alloying element contained in steel is within the range described above, if F1 satisfies the above described condition, the micro-structure after hot forging will contain bainite as a main phase.
When F1 is less than 0.68, since pro-eutectoid ferrite is mixed in the micro-structure after hot forging, and carbide of V will precipitate at phase boundaries, the hardness before aging treatment may increase and age hardenability may decrease.
F1 is preferably not less than 0.70, and more preferably not less than 0.72. Moreover, F1 is preferably not more than 1.0, and more preferably not more than 0.98.

F2: not more than 0.85

The age-hardenable steel of the present invention must satisfy the condition that F2 represented by the following Formula (2) is not more than 0.85: $F 2 = C + 0.1 Si + 0.2 Mn + 0.15 Cr + 0.35 V + 0.2 Mo$
As already described, each symbol of element in the Formula (2) means the content of that element in mass%.
F2 is an index showing hardness before aging treatment. When the age-hardenable steel of the present invention only satisfies the above described condition of F1, there may be a case that the hardness before aging treatment becomes excessively high and the cutting resistance during cutting process increases, thereby shortening tool life.
That is, if F2 is more than 0.85, the hardness before aging treatment will become excessively high. To make the hardness before aging treatment not more than 290 HV, it is necessary that the above described content of each alloying element is within the specified range, and the condition of F2 is satisfied after the condition of F1 is satisfied.
F2 is preferably not more than 0.82, and more preferably not more than 0.80. Moreover, F2 is preferably not less than 0.55, and more preferably not less than 0.60.

F3: not less than 0.00

The age-hardenable steel of the present invention must satisfy the condition that F3 represented by the following Formula (3) is not less than 0.00: $F 3 = - 4.5 C + Mn + Cr - 3.5 V - 0.8 Mo$
As already described, each symbol of element in the Formula (3) means the content of that element in mass%.
F3 is an index showing toughness after aging treatment. That is, only satisfying the conditions of F1 and F2 may result in deterioration of toughness after aging treatment, making it impossible to ensure targeted toughness.
That is, when F3 is less than 0.00, toughness after aging treatment will deteriorate. To ensure targeted toughness, it is necessary that the above described content of each alloying element is within the specified range, and the condition of F3 is satisfied after the conditions of F1 and F2 are satisfied.
F3 is preferably not less than 0.01.
Note that if F1 is not less than 0.68 and F2 is not more than 0.85, there is no need of setting a limit on the upper limit of F3.
The age-hardenable steel of the present invention preferably has an average block size of bainite of 15 to 60 µm. The term "block" of bainite as used in the present invention refers to a region surrounded by boundaries with an orientation difference of not less than 15° when orientation analysis of the micro-structure is performed by an EBSD (Electron BackScatter Diffraction) method. As the average block size of bainite increases, the hardness before aging decreases, and therefore good machinability is obtained. On the other hand, if the average block size is excessively large, toughness will deteriorate. The average block size is more preferably not less than 20 µm. Moreover, the average block size is more preferably not more than 45 µm, and further preferably not more than 30 µm.
The manufacturing method of the age-hardenable steel of the present invention will not be particularly limited, and it may be melted by a general method to adjust the chemical composition.
Hereafter, an example of the method for manufacturing a mechanical part such as for automobiles, industrial machinery, construction machinery, and the like using an age-hardenable steel of the present invention manufactured as described above as the starting material, will be described.
First, from a steel whose chemical composition has been adjusted to be within the above described range, material to be subjected to hot forging (hereafter, referred to as "material for hot forging") will be made.
The above described material for hot forging may be of any kind such as a billet obtained by blooming an ingot, a billet obtained by blooming a continuous casting material, or a steel bar obtained by hot rolling or hot forging those billets.
Next, the above described material for hot forging is subjected to hot forging and further to cutting process to be finished into a predetermined part shape.
Note that in the above described hot forging, for example, the material for hot forging is heated at 1100 to 1350°C for 0.1 to 300 minutes and thereafter forged such that the surface temperature after finish forging is not less than 900°C, thereafter being cooled to the room temperature with an average cooling velocity in a temperature range of 800 to 400°C being 10 to 90°C/min (0.2 to 1.5°C/sec). After being cooled in this way, the material is further subjected to cutting process to be finished into a predetermined part shape.
The faster the average cooling velocity in the temperature range of 800 to 400°C, the smaller the average block size of bainite becomes. The lower limit of this average cooling velocity is preferably 20°C/min, and the upper limit is preferably 80°C/min.
Finally, the material is subjected to aging treatment to obtain a mechanical part such as for automobiles, industrial machinery, construction machinery, and the like, which have desired properties.
Note that the above described aging treatment is performed in a temperature range of 540 to 700°C, and preferably in a temperature range of 560 to 680°C. The retention time of this aging treatment is appropriately adjusted to be, for example, 30 to 1000 minutes depending on the size (mass) of the mechanical part.
Hereafter, the present invention will be described in further detail utilizing examples.

EXAMPLE

Example 1

Steels A to AG having chemical compositions shown in Tables 1 and 2 were melted with a 50 kg vacuum furnace.
Steels A to W in Tables 1 and 2 are steels whose chemical compositions are within the range defined in the present invention. On the other hand, Steels X to AG in Table 2 are steels whose chemical compositions are out of the conditions defined in the present invention.
Note that the expression "<0.001" in the column of Ti indicates that the content of Ti as an impurity was less than 0.001%.

[Table 1]

Table 1

Steel	Chemical composition (mass%) Balance: Fe and impurities
Steel	C	Si	Mn	P	S	Cr	Al	Ti	V	N	Mo	Cu	Ni	Ca	Bi	F1	F2	F3
A	0.10	0.19	1.79	0.008	0.019	0.20	0.025	<0.001	0.30	0.0048	-	-	-	-	-	0.69	0.61	0.49
B	0.10	0.05	1.80	0.013	0.019	0.25	0.022	0.001	0.29	0.0046	0.01	0.01	0.01	-	-	0.71	0.61	0.58
C	0.10	0.19	1.80	0.008	0.018	0.31	0.025	<0.001	0.31	0.0041	0.20	-	-	-	-	0.84	0.67	0.42
D	0.11	0.20	1.80	0.020	0.020	0.29	0.020	<0.001	0.41	0.0044	0.02	-	-	-	-	0.73	0.68	0.14
E	0.13	0.20	1.80	0.018	0.022	0.30	0.019	<0.001	0.33	0.0043	-	-	-	-	-	0.75	0.67	0.36
F	0.13	0.21	1.92	0.022	0.022	0.05	0.019	<0.001	0.31	0.0045	0.01	0.01	0.01	-	-	0.72	0.65	0.29
G	0.13	0.19	1.94	0.009	0.011	0.25	0.022	<0.001	0.33	0.0049	0.15	0.25	0.28	-	-	0.86	0.72	0.33
H	0.13	0.14	1.78	0.015	0.010	0.05	0.011	<0.001	0.25	0.0079	0.22	-	-	-	-	0.81	0.64	0.19
I	0.13	0.21	1.82	0.011	0.014	0.39	0.026	<0.001	0.40	0.0041	0.20	-	-	-	-	0.89	0.75	0.06
J	0.13	0.20	1.82	0.005	0.005	0.39	0.032	<0.001	0.33	0.0040	0.01	0.01	0.01	-	-	0.78	0.69	0.46
K	0.13	0.03	2.05	0.015	0.021	0.39	0.020	<0.001	0.49	0.0039	-	-	-	-	-	0.84	0.77	0.14
L	0.13	0.45	1.83	0.014	0.019	0.20	0.022	<0.001	0.33	0.0041	0.28	-	-	-	-	0.90	0.74	0.07
M	0.13	0.20	1.82	0.015	0.049	0.21	0.006	<0.001	0.31	0.0042	-	-	-	-	-	0.73	0.65	0.36
N	0.15	0.20	2.42	0.010	0.021	0.05	0.015	<0.001	0.31	0.0048	-	-	-	-	-	0.89	0.77	0.71
O	0.14	0.02	1.95	0.008	0.019	0.47	0.013	<0.001	0.41	0.0078	0.01	-	-	-	0.015	0.85	0.75	0.35
P	0.15	0.19	1.80	0.009	0.019	0.08	0.026	<0.001	0.30	0.0068	-	-	-	-	-	0.71	0.65	0.16
F1=C+0.3Mn+0.25Cr+0.6Mo
F2=C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo
F3=-4.5C+Mn+Cr-3.5V-0.8Mo

[Table 2]

Table 2

Steel	Chemical composition (mass%) Balance: Fe and impurities
Steel	C	Si	Mn	P	S	Cr	Al	Ti	V	N	Mo	Cu	Ni	Ca	Bi	F1	F2	F3
Q	0.15	0.28	1.88	0.014	0.021	0.28	0.025	0.003	0.27	0.0041	0.46	0.10	0.10	-	-	1.06	0.78	0.17
R	0.15	0.25	1.90	0.015	0.019	0.07	0.025	<0.001	0.30	0.0051	0.18	0.10	0.10	-	-	0.85	0.71	0.10
S	0.19	0.22	1.80	0.016	0.010	0.11	0.024	<0.001	0.30	0.0046	-	-	-	-	-	0.76	0.69	0.01
T	0.20	0.15	2.00	0.009	0.016	0.31	0.020	<0.001	0.32	0.0068	-	-	-	-	-	0.88	0.77	0.29
U	0.15	0.29	1.52	0.013	0.021	0.39	0.008	<0.001	0.30	0.0042	-	-	-	-	-	0.70	0.65	0.19
V	0.15	0.20	1.85	0.011	0.020	0.48	0.023	<0.001	0.32	0.0046	0.01	0.01	0.01	0.0005	-	0.83	0.73	0.53
W	0.13	0.08	1.95	0.007	0.011	0.45	0.015	<0.001	0.33	0.0072	0.62	0.01	-	-	-	1.20	0.84	0.16
X	0.12	0.20	1.55	0.011	0.018	0.06	0.018	<0.001	0.29	0.0042	-	-	-	-	-	*0.60	0.56	0.06
Y	0.20	0.39	2.20	0.010	0.015	0.35	0.022	<0.001	0.35	0.0040	0.49	0.10	0.10	-	-	1.24	*0.95	0.03
Z	0.19	0.35	1.75	0.016	0.022	0.20	0.022	<0.001	0.31	0.0078	0.10	0.10	0.10	-	-	0.83	0.73	*-0.07
AA	*0.23	0.05	1.88	0.014	0.020	0.35	0.022	<0.001	0.30	0.0044	0.13	0.10	0.10	-	-	0.96	0.79	0.04
AB	0.19	0.21	*1.44	0.015	0.012	0.48	0.022	<0.001	0.28	0.0054	0.05	-	-	-	-	0.77	0.68	0.04
AC	0.13	0.20	1.80	0.015	*0.095	0.30	0.025	<0.001	0.33	0.0050	-	-	-	-	-	0.75	0.67	0.36
AD	0.14	0.10	2.25	0.013	0.022	0.29	0.019	<0.001	*0.23	0.0066	0.10	-	-	-	-	0.95	0.74	1.03
AE	0.13	0.20	1.80	0.015	0.020	0.30	0.025	*0.014	0.33	0.0050	-	-	-	-	-	0.75	0.67	0.36
AF	0.11	0.20	1.78	0.015	0.025	0.44	0.029	<0.001	0.26	*0.0188	-	-	-	-	-	0.75	0.64	0.82
AG	0.10	0.25	1.74	0.015	0.029	0.26	0.029	0.001	0.25	*0.0122	-	-	-	-	-	0.69	0.60	0.68
F1=C+0.3Mn+0.25Cr+0.6Mo
F2=C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo
F3=-4.5C+Mn+Cr-3.5V-0.8Mo
"*" mark indicates deviation from the condition defined in the present invention.

The ingot of each steel was heated at 1250°C and thereafter hot forged into a steel bar having a diameter of 60 mm. Each of the hot-forged steel bars was temporarily allowed to cool to room temperature in the atmosphere. Thereafter, the steel bar was further heated at 1250°C for 30 minutes and hot forged into a steel bar having a diameter of 35 mm with the surface temperature of the forged material at the time of finishing being kept at 950 to 1100°C supposing that it is forged into a part shape. After hot forging, each steel bar was allowed to cool to room temperature in the atmosphere. The cooling velocity during cooling in the atmosphere was measured by embedding a thermocouple at a depth of around R/2 ("R" indicates a radius of steel bar) in a steel bar, and reheating the steel bar, which had been hot-forged at the above described condition, up to around the finishing temperature for hot forging and thereafter allowing it to be cooled in the atmosphere. Thus measured average cooling velocity in a temperature range of 800 to 400°C after forging was about 40°C/min (0.7°C/sec).
For each Test No., some of the steel bars which had been finished to the above described diameter of 35 mm by hot forging and cooled to room temperature were subjected to the investigation of the hardness before aging treatment and the area fraction of bainite in the micro-structure by cutting off both end portions of the steel bar, each having a length of 100 mm, and thereafter cutting out a specimen from the remaining middle portion in a state without aging treatment (that is, as-cooled state).
On the other hand, for each Test No., the rest of the hot-forged steel bars were subjected to aging treatment at 610 to 630°C for 60 to 180 minutes, and were subjected to the investigation of hardness after the aging treatment by cutting off both end portions of the steel bar, each having a length of 100 mm, and thereafter cutting out a specimen from the remaining middle portion. Moreover, for each Test No., a specimen was cut out from each steel bar and was subjected to the investigation of absorbed energy in a Charpy impact test and fatigue strength after the aging treatment.
The hardness measurement was conducted in the following way. First, a specimen was prepared by transecting a steel bar, embedding it in a resin such that the cut plane became the surface to be inspected, and thereafter mirror-polishing it. Next, hardness measurement was conducted with the testing force being 9.8 N at 10 points around R/2 portion ("R" represents radius) in the surface to be inspected conforming to the "Vickers hardness test - testing method" in JIS Z 2244 (2009). Vickers hardness was determined by arithmetically averaging the values of the 10 points. It was judged that the hardness before the aging treatment was sufficiently low when the hardness was not more than 290 HV, and this was set as a target. It was also judged that the quantity of hardening was sufficiently large, when the difference of hardness in HV (hereafter, referred to as "ΔHV") between before and after the aging treatment became not less than 25, and this was set as a target.
The measurement of area fraction of bainite in the micro-structure was conducted in the following way. The specimen which was embedded in resin and mirror-polished for hardness measurement was etched with NITAL. Micro-structure of the specimen after etching was photographed at a magnification of 200 by using an optical microscope. The area fraction of bainite was measured by image analysis from a photographed picture. It was judged that the micro-structure became fully bainitic when the area fraction of bainite was not less than 70%, and this was set as a target.
It was judged that toughness was sufficiently high when the absorbed energy at 20°C after the aging treatment was not less than 16 J when evaluated by a Charpy impact test performed by using a standard specimen with a U-notch having a notch depth of 2 mm and a notch bottom radius of 1 mm, and this was set as a target.
The fatigue strength was investigated by sampling a uniaxial tension-compression type fatigue test specimen. That is, a smooth fatigue test specimen of a shape whose parallel portion, as shown in Figure 1, has a diameter of 3.4 mm and a length of 12.7 mm was taken in parallel with the forging direction (in the longitudinal direction of steel bar) from a R/2 portion of the steel bar, and was subjected to a fatigue test under conditions of room temperature, the atmosphere, a stress ratio of 0.05, and a test speed of 10 Hz. The fatigue strength was determined as the maximum stress applied to the specimens which have not ruptured up to a number of stress repetition of 10⁷ under the conditions described above. It was judged that the fatigue strength was sufficiently high when the fatigue strength was not less than 350 MPa, and this was set as a target.
The results of the investigations are shown in Table 3. Note that symbols "○" and "×" in the column of "Bainitization" respectively indicate that the area fraction of bainite was not less than 70%, thus achieving the target, and that the same was less than 70%, thus failing to achieve the target. Moreover, the "absorbed energy in Charpy impact test" is denoted as "Charpy absorbed energy" in Table 3.

[Table 3]

Table 3

Test No	Steel	Before aging treatment		After aging treatment			Quantity of hardening	Remarks
		Hardness	Bainitization	Hardness	Fatigue strength	Charpy absorbed energy	Quantity of hardening
		[HV]		[HV]	(MPa)	(J)	[ΔHV]
A1	A	249	○	286	360	64	37
A2	B	245	○	287	365	70	42
A3	C	247	○	295	375	58	48
A4	D	263	○	306	380	46	43
A5	E	262	○	295	365	53	33
A6	F	260	○	299	370	46	39
A7	G	277	○	315	400	41	38
A8	H	245	○	286	355	38	41
A9	I	281	○	337	420	30	56
A10	J	260	○	299	370	50	39
A11	K	285	○	339	415	31	54
A12	L	279	○	323	405	36	44	Inventive Example
A13	M	250	○	287	360	55	37
A14	N	281	○	316	390	48	35
A15	O	283	○	325	395	38	42
A16	P	252	○	287	355	48	35
A17	Q	285	○	341	415	22	56
A18	R	265	○	310	390	36	45
A19	S	259	○	297	360	36	38
A20	T	285	○	326	395	36	41
A21	U	260	○	295	365	41	35
A22	V	283	○	321	395	46	38
A23	W	290	○	352	420	16	62
B1	* X	256	# ×	274	# 340	43	# 18	Comparative Example
B2	* Y	# 311	○	359	430	# 10	48
B3	* Z	284	○	322	395	# 14	38
B4	*AA	285	○	328	405	# 13	43
B5	*AB	266	# ×	289	350	# 15	# 23
B6	*AC	248	○	286	# 335	# 12	38
B7	*AD	261	○	280	# 340	72	# 19
B8	*AE	252	○	290	355	# 10	38
B9	*AF	250	○	273	# 340	88	# 23
B10	*AG	241	○	272	# 340	89	31
"*" mark indicates deviation from the chemical composition condition defined in the present invention.
"#" mark indicates failure to reach target.

As obvious from Table 3, in the case of "Inventive Examples" of Test Nos. A1 to A23 which each had a chemical composition defined in the present invention, the hardness before the aging treatment was not more than 290 HV, and the hardness increased by not less than 25 in HV, the fatigue strength increased to not less than 350 MPa, and further the absorbed energy in the Charpy impact test increased to not less than 16 J as the result of aging treatment, thus respectively achieving the targets so that strength and toughness after the aging treatment were successfully achieved at the same time. Further, the fact that the hardness before the aging treatment was low revealed that reduction of cutting resistance and prolongation of tool life can be expected.
In contrast to this, in the case of "Comparative Examples" of Test Nos. B1 to B10 which were out of the definition of the present invention, the target performances have not been achieved.
In Test No. B1, since Steel X was used in which F1 was small deviating from the definition of the present invention, hardenability was low, more than 30% in area fraction of pro-eutectoid ferrite was produced, and the area fraction of bainite was less than 70%. Therefore, age hardening was not likely to occur, and the fatigue strength after the aging treatment was low.
In Test No. B2, since Steel Y was used in which F2 was large deviating from the definition of the present invention, the hardness before the aging treatment became as high as 311 HV, which was hard.
In Test No. B3, since Steel Z was used in which F3 was small deviating from the definition of the present invention, the absorbed energy in the Charpy impact test after the aging treatment was small, indicating poor toughness.
In Test No. B4, since Steel AA was used in which although F3 satisfied the definition of the present invention, C content was excessively large deviating from the definition of the present invention, deterioration of toughness was remarkable. Therefore, the absorbed energy in the Charpy impact test after the aging treatment was small, indicating poor toughness.
In Test No. B5, since Steel AB was used in which Mn content was excessively low deviating from the definition of the present invention, pro-eutectoid ferrite precipitated, and the bainite portion of the micro-structure was not sufficiently refined. For that reason, age hardening was not likely to occur, and the fatigue strength after the aging treatment was low. Moreover, the absorbed energy in the Charpy impact test was small, indicating poor toughness.
In Test No. B6, since Steel AC was used in which S content was excessively large deviating from the definition of the present invention, coarse MnS increased, and deterioration of toughness was remarkable. For that reason, the absorbed energy in the Charpy impact test after the aging treatment was small, indicating poor toughness. Moreover, fatigue strength was low as well.
In Test No. B7, since Steel AD was used in which V content was excessively low deviating from the definition of the present invention, the amount of V carbide precipitated by the aging treatment was small. For that reason, age hardening was not likely to occur, and also the fatigue strength after the aging treatment was low.
In Test No. B8, since Steel AE was used in which Ti content was excessively high deviating from the definition of the present invention, the amount of coarse TiN increased, and deterioration of toughness was remarkable. For that reason, the absorbed energy in the Charpy impact test after the aging treatment was small, indicating poor toughness.
In Test No. B9, since Steel AF was used in which N content was excessively high deviating from the definition of the present invention, V nitride precipitated during hot forging. For that reason, age hardening was not likely to occur, and also the fatigue strength after the aging treatment was low.
In Test No. B10, since Steel AG was used in which N content was as high as exceeding the definition of the present invention, nitride of V precipitated during hot forging. Therefore, the fatigue strength after the aging treatment was low. However, since the N content was smaller compared with Steel AF, the amount of V nitride that precipitated during hot forging was smaller, and age hardening progressed further than in Steel AF.

Example 2

A part of a steel bar having a diameter of 60 mm of each of Steels P and Y, which were fabricated by being hot forged and thereafter being cooled to room temperature in Example 1, was cut out. The cut out steel bar was further heated at 1250°C for 30 minutes and was hot forged into a steel bar having a diameter of 35 mm with the surface temperature of the forged material at the time of finishing being kept at 950 to 1100°C supposing that it is forged into a part shape. After the hot forging, the steel bar was allowed to cool in the atmosphere, or by using a blower and mist to a temperature not more than 400°C at various cooling velocities.
For each Test No., the hardness before aging treatment was measured by using some of the steel bars which, after being finished into a diameter of 35 mm by hot forging, was cooled to a temperature not more than 400°C by using a blower and mist and further cooled to room temperature.
On the other hand, for each Test No., the rest of the hot-forged steel bars was subjected to aging treatment at 630°C for 60 minutes. By using specimens sampled from the steel bars which had been subjected to the aging treatment, the hardness after the aging treatment, the absorbed energy in the Charpy impact test, the fatigue strength, and the block size of bainite structure were investigated.
The investigations of the hardnesses before and after the aging treatment, the absorbed energy in the Charpy impact test, and the fatigue strength were conducted under the same conditions as in Example 1. Moreover, target values of these were the same as in Example 1.
Measurement of the block size of bainite structure was conducted in the following way. The specimens embedded in resin and used for hardness measurement were polished again by using colloidal silica. The polished specimens were subjected to orientation analysis of micro-structure by the EBSD method. A region surrounded by boundaries with an orientation difference of not less than 15° was defined as a "block", and the area of each block was determined by image analysis.
An interface between blocks has a complicated shape with unevenness. For that reason, when an observation surface of micro-structure is created in such a way to cut off the vicinity of an uneven end part of a block, it may be observed as if a block enclosed in another block was present. In such a case, measurement accuracy of the area of block will deteriorate. To eliminate such effects, when a certain block was fully enclosed in another block in a cross-sectional image, they were regarded as one block, and the area was determined from a larger block alone, neglecting the enclosed smaller block.
For each block which was subjected to measurement of area as described above, the size of block was defined as the diameter of a circle which has the same area. From the size of each block in region of 30000 µm² analyzed by the EBSD method, an average block size was calculated.
When calculating an average block size, the size of each block was weighted according to the area of the block. That is, for n blocks 1 to n in an analysis region, supposing that the sizes of each block are D1, D2, ..., Dn (µm), and the areas thereof are S1, S2, ..., Sn (µm²), the average block size was determined as (D1 × S1 + D2 × S2 + ...+ Dn × Sn)/30000. The target of average block size was set to be 15 to 60 µm.
Table 4 shows results of each investigations described above. Test No. C1 corresponds to Test No. A16 of Table 3. The cooling velocity shown in Table 4 is an average cooling velocity in a temperature range of 800 to 400°C during cooling after hot forging the steel bar having a diameter of 35 mm. The measurement method of the average cooling velocity was the same as that in Example 1.

[Table 4]

Table 4

Test No.	Steel	Average cooling velocity	Before aging treatment	After aging treatment				Quantity of hardening	Remarks
		Average cooling velocity	Hardness	Hardness	Fatigue strength	Charpy absorbed energy	Bainite average block size	Quantity of hardening
		(°C/s)	[HV]	[HV]	(MPa)	(J)	(µm)	[ΔHV]
C1	P	0.7	252	287	355	48	37.4	35	Inventive Example
C2	P	0.8	253	288	355	49	32.5	35
C3	P	1.0	255	288	355	51	27.7	33
C4	P	1.5	256	290	360	52	24.2	34
C5	P	2.0	264	295	365	55	18.6	31
C6	P	3.0	276	301	370	55	16.5	25
D1	* Y	1.5	# 320	364	440	# 12	# 9.9	44	Comparative Example
"*" mark indicates deviation from the chemical composition condition defined in the present invention.
"#" mark indicates failure to reach target.

As obvious from Table 4, in the case of "Inventive Examples" of Test Nos. C1 to C6 which each had a chemical composition defined in the present invention, the average block size of bainite was within a target range of 15 to 60 µm, and the hardness before the aging treatment was not more than 290 HV. Therefore, excellent machinability can be expected. As a result of the aging treatment, the hardness increased by not less than 25 in HV, the fatigue strength became not less than 350 MPa, and further the absorbed energy in the Charpy impact test became not less than 16 J, achieving each target so that the strength and toughness after the aging treatment were successfully achieved at the same time. Note that in Test Nos. C1 to C6, the area fraction of bainite before the aging treatment was not less than 70%, thus achieving its target.
Among Inventive Examples of the present invention, in Test Nos. C1 to C4, the average cooling velocity satisfied the average cooling velocity (10 to 90°C/min, that is, 0.2 to 1.5°C/sec) shown as one example of the method for manufacturing an age-hardenable steel of the present invention described above. In Test Nos. C5 and C6, the average cooling velocity was faster than the aforementioned example of average cooling velocity. Comparing Test Nos. C1 to C6 to each other, it is seen that the slower the average cooling velocity, the larger the average block size of bainite becomes. Moreover, it is seen that the larger the average block size of bainite, the lower the hardness before the aging treatment becomes, and thus better machinability can be expected.
In contrast to this, in the case of "Comparative Example" of Test No. D1 which was deviated from the definition of the present invention, the target performance has not been achieved. That is, in Test No. D1, Steel Y was used in which F2 was large deviating from the definition of the present invention. For that reason, the average block size of bainite was as small as 9.9 µm and the hardness before the aging treatment was 320 HV, which was hard. Therefore, the machinability was considered to be poor. Moreover, the absorbed energy in the Charpy impact test was as low as 12 J, thus indicating poor toughness.

INDUSTRIAL APPLICABILITY

The age-hardenable steel of the present invention has hardness before aging treatment of not more than 290 HV, and therefore can be expected to have reduced cutting resistance and a prolonged tool life. Furthermore, using the age-hardenable steel of the present invention causes the hardness to increase by not less than 25 in HV through aging treatment performed after cutting process, and can ensure a fatigue strength of not less than 350 MPa, and an excellent toughness, that is, absorbed energy at 20°C after aging treatment of not less than 16 J when evaluated by a Charpy impact test performed by using a standard specimen with a U-notch having a notch depth of 2 mm and a notch bottom radius of 1 mm. Therefore, the age-hardenable steel of the present invention can be quite suitably used as a starting material for producing mechanical parts such as for automobiles, industrial machinery, construction machinery, and so on.

Claims

An age-hardenable steel, comprising a chemical composition consisting of: by mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.5 to 2.5%, S: 0.005 to 0.08%, Cr: 0.03 to 0.50%, Al: 0.005 to 0.05%, V: 0.25 to 0.50%, Mo: 0 to 1.0%, Cu: 0 to 0.3%, Ni: 0 to 0.3%, Ca: 0 to 0.005%, and Bi: 0 to 0.4%,
with the balance being Fe and impurities, wherein
P, Ti, and N included in the impurities are: P: 0.03% or less, Ti: less than 0.005%, and N: less than 0.0080%, and further wherein

F1 represented by the following Formula (1) is not less than 0.68, F2 represented by the following Formula (2) is not more than 0.85, and F3 represented by the following Formula (3) is not less than 0.00; $F 1 = C + 0.3 Mn + 0.25 Cr + 0.6 Mo$
$F 2 = C + 0.1 Si + 0.2 Mn + 0.15 Cr + 0.35 V + 0.2 Mo$
and $F 3 = - 4.5 C + Mn + Cr - 3.5 V - 0.8 Mo$

where each symbol of element in the Formulas (1) to (3) means the content of the element in mass%,

wherein the steel contains bainite as a main phase, and

wherein hardness is not more than 290 HV.
The age-hardenable steel according to claim 1, wherein the chemical composition contains, by mass%, one or more kinds selected from elements shown in the following <1> to <3>;
<1> Mo: 0.05 to 1.0%,

<2> Cu: 0.1 to 0.3%, and Ni: 0.1 to 0.3%, and

<3> Ca: 0.0005 to 0.005%, and Bi: 0.03 to 0.4%.
The age-hardenable steel according to claim 1 or 2, wherein an average block size of the bainite is 15 to 60 µm.
Use of the age-hardenable steel according to any of the preceding claims, whereby the steel is subjected to age-hardening treatment, wherein the age-hardening treatment is performed in a temperature range of 540 to 700°C and has a retention time being adjusted to be 30 to 1000 minutes.
Use according to claim 4, wherein the steel is used as a starting material for producing mechanical parts such as for automobiles, industrial machinery, construction machinery.
Use according to claim 4 or 5 of the steel according to claim 4, wherein the hardness of not more than 290 HV is the hardness before aging treatment.
Use according to claim 6, wherein after aging treatment,
- hardness increases by 25 HV, and

- the fatigue strength is not less than 350 MPa, and

- absorbed energy at 20°C is not less than 16 J when evaluated by a Charpy impact test performed by using a standard specimen with a U-notch having a notch depth of 2 mm and a notch bottom radius of 1 mm according to JIS Z 2242.