U.S. patent application number 14/765029 was filed with the patent office on 2016-07-14 for age-hardenable steel.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Tatsuya HASEGAWA, Masashi HIGASHIDA, Taizo MAKINO, Hitoshi MATSUMOTO, Yutaka NEISHI, Masato YUYA.
Application Number | 20160201175 14/765029 |
Document ID | / |
Family ID | 52778739 |
Filed Date | 2016-07-14 |
United States Patent
Application |
20160201175 |
Kind Code |
A1 |
YUYA; Masato ; et
al. |
July 14, 2016 |
AGE-HARDENABLE STEEL
Abstract
An age-hardenable steel consists of: 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%, the balance
being Fe and impurities. Within impurities, P .English Pound.
0.03%, Ti<0.005%, and N<0.0080%, and
[C+0.3Mn+0.25Cr+0.6Mo.sup.3 0.68],
[C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo .English Pound. 0.85], and
[-4.5C+Mn+Cr-3.5V-0.8Mo.sup.3 0.00]. The hardness before aging is
not more than 290 HV, a quantity of hardening by aging being not
less than 25 HV, fatigue strength is not less than 350 MPa, and
absorbed energy at 20.degree. C. after aging is not less than 16
J.
Inventors: |
YUYA; Masato; (Tokyo,
JP) ; HIGASHIDA; Masashi; (Tokyo, JP) ;
MATSUMOTO; Hitoshi; (Tokyo, JP) ; HASEGAWA;
Tatsuya; (Tokyo, JP) ; NEISHI; Yutaka; (Tokyo,
JP) ; MAKINO; Taizo; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Family ID: |
52778739 |
Appl. No.: |
14/765029 |
Filed: |
October 1, 2014 |
PCT Filed: |
October 1, 2014 |
PCT NO: |
PCT/JP2014/076260 |
371 Date: |
July 31, 2015 |
Current U.S.
Class: |
148/328 |
Current CPC
Class: |
C21D 8/06 20130101; C22C
38/22 20130101; C22C 38/50 20130101; C22C 38/44 20130101; C22C
38/58 20130101; C22C 38/02 20130101; C22C 38/06 20130101; C22C
38/24 20130101; C22C 38/001 20130101; C21D 1/18 20130101; C21D 1/06
20130101; C22C 38/002 20130101; C22C 38/46 20130101; C22C 38/20
20130101; C22C 38/42 20130101; C22C 38/60 20130101; C22C 38/38
20130101; C21D 2211/002 20130101; C22C 38/28 20130101; C21D 9/0075
20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/42 20060101 C22C038/42; C22C 38/38 20060101
C22C038/38; C22C 38/28 20060101 C22C038/28; C22C 38/24 20060101
C22C038/24; C22C 38/22 20060101 C22C038/22; C22C 38/20 20060101
C22C038/20; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 1/06 20060101
C21D001/06; C21D 1/18 20060101 C21D001/18; C22C 38/50 20060101
C22C038/50 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 2013 |
JP |
2013-207125 |
Claims
1. An age-hardenable steel, comprising a chemical composition
consisting of: by mass %, C: 0.05 to 0.20%, Si: not less than 0.01%
and less than 0.35%, Mn: 1.5 to 2.05%, 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;
F1=C+0.3Mn+0.25Cr+0.6Mo (1), F2=C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo
(2), and F3=-4.5C+Mn+Cr-3.5V-0.8Mo (3), where each symbol of
element in the Formulas (1) to (3) means the content of the element
in mass %.
2. 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%.
3. The age-hardenable steel according to claim 1, wherein the steel
contains bainite as a main phase, and an average block size of the
bainite is 15 to 60 .mu.m.
4. The age-hardenable steel according to claim 1, wherein hardness
is not more than 290 HV.
5. The age-hardenable steel according to claim 2, wherein the steel
contains bainite as a main phase, and an average block size of the
bainite is 15 to 60 .mu.m.
6. The age-hardenable steel according to claim 2, wherein hardness
is not more than 290 HV.
7. The age-hardenable steel according to claim 3, wherein hardness
is not more than 290 HV.
8. The age-hardenable steel according to claim 4, wherein hardness
is not more than 290 HV.
Description
TECHNICAL FIELD
[0001] 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
[0002] 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.
[0003] 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.
[0004] For example, Patent Document 1 discloses the following
age-hardening steel.
[0005] 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:
4C+Mn+0.7Cr+0.6Mo-0.2V.gtoreq.2.5,
C.gtoreq.Mo/16+V/5.7,
V+0.15Mo.gtoreq.0.4
and wherein after rolling, forging, or solution treatment, the
steel is cooled at an average cooling velocity of 0.05 to
10.degree. C./sec in a temperature range of 800.degree. C. to
300.degree. 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.
[0006] Patent Document 2 discloses the following bainite steel.
[0007] 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:
13[C]+8[Si]+10[Mn]+3[Cu]+3[Ni]+22[Mo]+11[V].ltoreq.30,
5[C]+[Si]+2[Mn]+3[Cr]+2[Mo]+4[V].ltoreq.7.3,
2.4.ltoreq.0.3[C]+1.1[Mn]+0.2[Cu]+0.2[Ni]+1.2[Cr]+1.1[Mo]+0.2[V].ltoreq.-
3.1,
2.5.ltoreq.[C]+[Si]+4[Mo]+9[V], and
[C].gtoreq.[Mo]/16+[V]/3.
[0008] 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.
LIST OF PRIOR ART DOCUMENTS
Patent Document
[0009] Patent Document 1: JP2006-37177A
[0010] Patent Document 2: JP2011-236452A
[0011] Patent Document 3: WO2010/090238
[0012] Patent Document 4: WO2011/145612
[0013] Patent Document 5: WO2012/161321
[0014] Patent Document 6: WO2012/161323
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0015] 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.
[0016] 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.
[0017] Moreover, once a fatigue crack occurs in steel with low
toughness, the propagation of the crack becomes faster, and
fracture becomes large scaled.
[0018] 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.
[0019] 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 BRE, they contain not less than 1.4% of
Mo, and in addition to that, their toughness is not taken into
consideration at all.
[0020] 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 ITV, 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.
[0021] Therefore, it is an object of the present invention to
provide an age-hardenable steel which satisfies the following items
<1> to <3>.
[0022] <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".
[0023] <2> It is possible to impart desired fatigue strength
to a mechanical part through hardening by aging treatment.
[0024] <3> Toughness after aging treatment is high.
[0025] 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.degree. 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
[0026] 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.
[0027] (a) V exhibits a precipitation peak of carbide at about 750
to 700.degree. 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.degree. C., suppressing precipitation during hot forging is
relatively easy.
[0028] (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.
[0029] (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.
[0030] 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.
[0031] (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.
[0032] (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.
[0033] (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.
[0034] Further, the present inventors investigated conditions to
obtain absorbed energy of not less than 16 J at 20.degree. 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.
[0035] (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.
[0036] 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.
[0037] (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.
[0038] (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.
[0039] The present invention has been made based on the above
described findings, and its gist is an age-hardenable steel
described below.
[0040] (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
[0041] 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;
F1=C+0.3Mn+0.25Cr+0.6Mo (1),
F2=C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo (2), and
F3=-4.5C+Mn+Cr-3.5V-0.8Mo (3),
[0042] where each symbol of element in the Formulas (1) to (3)
means the content of the element in mass %.
[0043] (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>;
[0044] <1> Mo: 0.05 to 1.0%,
[0045] <2> Cu: 0.1 to 0.3%, and Ni: 0.1 to 0.3%, and
[0046] <3> Ca: 0.0005 to 0.005%, and Bi: 0.03 to 0.4%.
[0047] (3) The age-hardenable steel according to the above
described (1) or (2), wherein the steel contains bainite as a main
phase, and an average block size of the bainite is 15 to 60
.mu.m.
[0048] (4) The age-hardenable steel according to any of the above
described (1) to (3), wherein hardness is not more than 290 HV.
Advantageous Effects of the Invention
[0049] 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.degree. 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
[0050] FIG. 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
[0051] Hereafter, each requirement of the present invention will be
described in detail. Note that "%" of the content of each element
means "mass %".
[0052] C: 0.05 to 0.20%
[0053] 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%.
[0054] Si: 0.01 to 0.50%
[0055] 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%.
[0056] Mn: 1.5 to 2.5%
[0057] 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%.
[0058] S: 0.005 to 0.08%
[0059] 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%.
[0060] Cr: 0.03 to 0.50%
[0061] 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%.
[0062] Al: 0.005 to 0.05%
[0063] 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%.
[0064] V: 0.25 to 0.50%
[0065] 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%.
[0066] Mo: 0 to 1.0%
[0067] 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%.
[0068] 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%.
[0069] 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.
[0070] Cu: 0 to 0.3%
[0071] 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%.
[0072] 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%.
[0073] Ni: 0 to 0.3%
[0074] 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%.
[0075] On the other hand, to stably achieve the above described
effects of Ni, its content is desirably not less than 0.1%.
[0076] 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.
[0077] 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.
[0078] Ca: 0 to 0.005%
[0079] 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%.
[0080] 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%.
[0081] Bi: 0 to 0.4%
[0082] 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%.
[0083] 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%.
[0084] 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%.
[0085] 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.
[0086] 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.
[0087] P: not more than 0.03%
[0088] 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%.
[0089] Ti: less than 0.005%
[0090] 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%.
[0091] N: less than 0.0080%
[0092] 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%.
[0093] F1: not less than 0.68
[0094] 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:
F1=C+0.3Mn+0.25Cr+0.6Mo (1),
[0095] As already described, each symbol of element in the Formula
(1) means the content of that element in mass %.
[0096] 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.
[0097] 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.
[0098] 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.
[0099] F2: not more than 0.85
[0100] 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:
F2=C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo (2)
[0101] As already described, each symbol of element in the Formula
(2) means the content of that element in mass %.
[0102] 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.
[0103] 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.
[0104] 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.
[0105] 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:
F3=-4.5C+Mn+Cr-3.5V-0.8Mo (3)
[0106] As already described, each symbol of element in the Formula
(3) means the content of that element in mass %.
[0107] 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.
[0108] 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.
[0109] F3 is preferably not less than 0.01.
[0110] 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.
[0111] The age-hardenable steel of the present invention preferably
has an average block size of bainite of 15 to 60 .mu.m. 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.degree. 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 .mu.m. Moreover, the
average block size is more preferably not more than 45 .mu.m, and
further preferably not more than 30 .mu.m.
[0112] 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.
[0113] 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.
[0114] 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.
[0115] 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.
[0116] 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.
[0117] Note that in the above described hot forging, for example,
the material for hot forging is heated at 1100 to 1350.degree. C.
for 0.1 to 300 minutes and thereafter forged such that the surface
temperature after finish forging is not less than 900.degree. C.,
thereafter being cooled to the room temperature with an average
cooling velocity in a temperature range of 800 to 400.degree. C.
being 10 to 90.degree. C./min (0.2 to 1.5.degree. C./sec). After
being cooled in this way, the material is further subjected to
cutting process to be finished into a predetermined part shape.
[0118] The faster the average cooling velocity in the temperature
range of 800 to 400.degree. C., the smaller the average block size
of bainite becomes. The lower limit of this average cooling
velocity is preferably 20.degree. C./min, and the upper limit is
preferably 80.degree. C./min
[0119] 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.
[0120] Note that the above described aging treatment is performed,
for example, in a temperature range of 540 to 700.degree. C., and
preferably in a temperature range of 560 to 680.degree. 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.
[0121] Hereafter, the present invention will be described in
further detail utilizing examples.
EXAMPLE
Example 1
[0122] Steels A to AG having chemical compositions shown in Tables
1 and 2 were melted with a 50 kg vacuum furnace.
[0123] 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.
[0124] 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-US-00001 TABLE 1 Chemical composition (mass %) Balance: Fe
and Impurities Steel C Si Mn P S Cr Al Ti V N Mo Cu Ni Ce 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-US-00002 TABLE 2 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 1.50 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 0.0048 -- --
-- -- -- 0.78 0.69 0.01 T 0.20 0.15 2.00 0.009 0.018 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 0.25 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 0.28 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.
[0125] The ingot of each steel was heated at 1250.degree. 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.degree. 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.degree. 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.degree. C. after forging was about
40.degree. C./min (0.7.degree. C./sec).
[0126] 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).
[0127] 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.degree. 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.
[0128] 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 ".DELTA.HV") between before and after
the aging treatment became not less than 25, and this was set as a
target.
[0129] 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.
[0130] It was judged that toughness was sufficiently high when the
absorbed energy at 20.degree. 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.
[0131] 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 FIG. 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 10.sup.7 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.
[0132] The results of the investigations are shown in Table 3. Note
that symbols "O" and "X" 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-US-00003 TABLE 3 Before aging After aging treatment Quantity
treatment Charpy absorbed of Hardness Hardness Fatigue strength
energy hardening Test No. Steel [HV] Bainitization [HV] (MPa) (J)
[.DELTA.HV] Remarks A1 A 249 .largecircle. 286 360 64 37 Inventive
A2 B 245 .largecircle. 287 385 70 42 Example A3 C 247 .largecircle.
295 375 58 48 A4 D 263 .largecircle. 306 380 46 43 A5 E 262
.largecircle. 295 365 53 33 A6 F 260 .largecircle. 299 370 46 39 A7
G 277 .largecircle. 315 400 41 38 A8 H 245 .largecircle. 286 355 38
41 A9 I 281 .largecircle. 337 420 30 56 A10 J 260 .largecircle. 299
370 50 39 A11 K 285 .largecircle. 339 415 31 54 A12 L 279
.largecircle. 323 405 36 44 A13 M 250 .largecircle. 287 360 55 37
A14 N 281 .largecircle. 316 390 48 35 A15 O 283 .largecircle. 325
395 38 42 A16 P 252 .largecircle. 287 355 48 35 A17 Q 285
.largecircle. 341 415 22 56 A18 R 265 .largecircle. 310 390 36 45
A19 S 259 .largecircle. 297 360 36 38 A20 T 265 .largecircle. 326
395 36 41 A21 U 260 .largecircle. 295 385 41 35 A22 V 283
.largecircle. 321 385 46 38 A23 W 290 .largecircle. 352 420 16 62
B1 *X 256 # X 274 # 340 43 # 18 Comparative B2 *Y # 311
.largecircle. 359 430 # 10 48 Example B3 *Z 284 .largecircle. 322
395 # 14 38 B4 *AA 285 .largecircle. 328 405 # 13 43 B5 *AB 266 # X
289 350 # 15 # 23 B6 *AC 248 .largecircle. 286 # 335 # 12 38 B7 *AD
261 .largecircle. 280 # 340 72 # 19 B8 *AE 252 .largecircle. 290
355 # 10 38 B9 *AF 250 .largecircle. 273 # 340 88 # 23 B10 *AG 241
.largecircle. 272 # 340 89 31 *mark indicates deviation from the
chemical composition condition defined in the present invention.
"#" mark indicates failure to reach target.
[0133] As obvious from Table 3, in the case of "Inventive Examples"
of Test Nos. Al 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.
[0134] 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.
[0135] 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.
[0136] 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.
[0137] 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.
[0138] 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.
[0139] 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.
[0140] 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.
[0141] 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.
[0142] 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.
[0143] 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.
[0144] 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
[0145] 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.degree. 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.degree.
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.degree.
C. at various cooling velocities.
[0146] 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.degree. C. by using a blower and mist
and further cooled to room temperature.
[0147] On the other hand, for each Test No., the rest of the
hot-forged steel bars was subjected to aging treatment at
630.degree. 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.
[0148] 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.
[0149] 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.degree. was defined as a "block", and the area of
each block was determined by image analysis.
[0150] 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.
[0151] 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 .mu.m.sup.2 analyzed by the EBSD method, an average
block size was calculated.
[0152] 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 (.mu.m), and the areas thereof are S1,
S2, . . . Sn (.mu.m.sup.2), the average block size was determined
as (D1.times.S1+D2.times.S2+ . . . +Dn.times.Sn)/30000. The target
of average block size was set to be 15 to 60 .mu.m.
[0153] 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.degree. 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-US-00004 TABLE 4 Average Before aging After aging treatment
Quantity cooling treatment Charpy absorbed Bainite average of
velocity Hardness Hardness Fatigue strength energy block size
hardening Test No. Steel (.degree. C./s) [HV] [HV] (MPa) (J)
(.mu.m) [.DELTA.HV] Remarks C1 P 0.7 252 287 355 48 37.4 35
Inventive C2 P 0.8 253 288 355 49 32.5 35 Example 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.
[0154] 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 .mu.m, 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.
[0155] 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.degree. C./min, that is, 0.2 to
1.5.degree. 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.
[0156] 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 .mu.m 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
[0157] 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.degree. 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.
* * * * *