CASE HARDENED STEEL HAVING REDUCED THERMAL TREATMENT
DISTORTION
Technical Field
[OOOl]
The present invention relates to a case hardened steel having a surface layer
portion hardened through quenching processes using carburizing, carbonitriding, or
carburizing nitriding (hereinafter, also referred to as carburizing and nitriding). This
case hardened steel is useful for using as a material for components, especially
mechanical ones such as gears, shafts, and constant velocity universal joints in
automobiles for which a high level of wear resistance or fatigue resistance is required.
The present application claims priority based on Japanese Patent Application
No. 2012-014474 filed in Japan on January 26,2012, the disclosures of which are
incorporated herein by reference in their entirety.
Background Art
[0002]
In recent years, from the viewpoint of reducing CO2 emissions as well as
further advancing energy conservation, there has been demand for transportation
devices including automobiles and motorcycles having a vehicle body with reduced
weights, thereby reducing the energy consumption. As part of the weigh reduction of
the vehicle body, the size or weight of mechanical components such as gears and shafts
has been reduced. This leads to the fact that these mechanical components are
required to have improved wear resistance or fatigue resistance.
[0003]
Conventionally, the wear resistance or fatigue resistance of these mechanical
components such as gears has been usually improved by applying case-hardening
processes typified by quenching processes using carburizing and nitriding. However,
in the case of mechanical components which have case-hardened processes applied
thereto with the aim of achieving improved smoothness and quietness thereof during
operation, in order to respond to technical demands for improvements in the accuracy of
the dimensions of these mechanical components, it is extremely important to reduce
distortion occurring during the case-hardening processes (hereinafter, also referred to as
thermal treatment disto~tiona) s much as possible.
[0004]
As for a measure for reducing the thermal treatment distortion, Patent
Documents 1 and 2 disclose the example of a method of adjusting intelnal structures so
as to have an austenite + ferrite layer after a thermal treatment using carburizing and
nitriding, and quenching the steel, thereby manufacturing a high-strength gear having
reduced distortion.
[OOOS]
However, with this method, resistance to softening is low due to a small
amount of Si in the steel used. This leads to a problem in that, in an application where
the produced gear is used at a high rotational speed, temperatures on the surface
increase, and the surface is softened, whereby pitting resistance reduces.
[0006]
Patent Document 3 discloses a case hardened steel having thermal treatment
distortion reduced in a similar manner. However, this case hardened steel has a large
amount of C, and hence, has a problem of deterioration in machinability, cold working
characteristics, toughness or other characteristics.
[0007]
Patent Document 4 discloses a steel for gears, in which an ideal critical
diameter after carburizing processes is defined, and an inner portion of the metal where
carburizing and nitriding are not applied after carburizing and quenching has a
carburized and quenched structure having reduced distortion with ferrite: 10 to 70%.
However, this steel for gears has a problem of deterioration in characteristics related to
carburizing due to a large amount of Si, and deterioration in machinability and a cold
working characteristic.
[OOOS]
Patent Document 5 discloses a method of reducing the thermal treatment
distortion by appropriately adjusting chemical components in steel, and employing
appropriate conditions for carburizing processes. Further, Patent Document 6
discloses a method of reducing distortion after thermal treatments by controlling critical
cooling rates using the amount of C or the amount of Mn in steel.
[0009]
Patent Documents 7 and 8 disclose a method of applying quenching processes
after case-hardening processes by setting a quenching starting temperature depending on
chemical conlponents, thereby adjusting an area fraction of pro-eutectoid ferrite in a
st~uctureo f a core portion after the case-hardening process, in other words, a structure
of a non-carburized layer so as to fall in the range of 20 to 80%.
[OO 101
Patent Document 9 discloses a method of reducing an amount of distortion by
applying processes of carburizing, cooling, reheating, and quenching, thereby reducing
the thermal treatment distortion and improving bending fatigue strength. However,
with this method, it is not possible to prevent deterioration in productivity and increase
in costs of thermal treatments resulting from reheating and quenching.
[OOl I]
Patent Document 10 discloses a steel for nitriding that does not have any
substantial white band, in which pressure is applied to unsolidified regions under
specific conditions, electro-magnetic stil~ingis not perfosmed at solidification end
positions so as not to generate any white band, the degree of segregation C/Co at a Dl4
portion is set so as to fall in a range of 0.99 to 1.01.
[0012]
Patent Document 11 discloses a case hardened steel in which a difference
between the maximum value and the minimum value of the degree of micro-segregation
of C and Mn in a cross section of bloom in a radial direction is not more than 0.03%,
and a difference in contents adjacent to each other is not more than 0.02%. Further,
Patent Document 12 discloses a case hardened steel having reduced distortion and
manufactured from a bloom having a degree of segregation of C at the center in a range
of 1.1 to 1.0.
[0013]
However, in reality, any of the methods and steels described above cannot
achieve the reduction in distortion that satisfies the severe demands made by recent
consumers.
Related Art Document(s)
Patent Document
[0014]
Patent Document 1: Japanese Unexamined Patent Application, First
Publication No. H05-070924
Patent Document 2: Japanese Unexamined Patent Application, First
Publication No. H05-070925
Patent Document 3: Japanese Unexamined Patent Application, First
Publication No. S58-113316
Patent Document 4: Japanese Unexamined Patent Application, First
PublicationNo. H08-109435
Patent Document 5: Japanese Unexamined Patent Application, First
Publication No. H02-298250
Patent Document 6: Japanese Unexamined Patent Application, First
Publication No. S61-210154
Patent Document 7: Japanese Unexamined Patent Application, First
Publication No. H09-137266
Patent Document 8: Japanese Unexamined Patent Application, First
PublicationNo. H10-147814
Patent Document 9: Japanese Unexarnined Patent Application, First
Publication No. H05-148535
Patent Document 10: Japanese Unexamined Patent Application, First
PublicationNo. 2000-343191
Patent Document 11: Japanese Unexamined Patent Application, First
Publication No. 2006-097066
Patent Document 12: Japanese Unexamined Patent Application, First
Publication No. S58-052459
Disclosure of the Invention
Problems to be Solved by the Invention
[0015]
In view of the circumstances described above, the present invention has a
problem of, in quenching processes using carburizing and nitriding applied to a case
hardened steel, reducing, as much as possible, the thermal treatment distortion that is
caused through the quenching processes. An object of the present invention is to solve
this problem, and to provide a case hardened steel product exhibiting excellent wear
resistance and fatigue strength, and having high dimensional accuracy.
Means for Solving the Problem
[0016]
The following are main points of the present invention.
[0017]
(1) The first aspect of the present invention provides a case hardened steel having
a cross section having a macrostructure including an equiaxed zone and a columnar
zone disposed around the equiaxed zone. The case hardened steel has a composition
including, inmass%: C: 0.05 to 0.45%; Si: 0.01 to 1.0%; Mn: over 0 to 2.0%; Al: 0.001
to 0.06%; N: 0.002 to 0.03%; S: over 0 to 0.1%; P: over 0 to 0.05%; and balance: Fe
and inevitable impurities, in which Equation (a) described below and Equation (b)
described below are satisfied in the equiaxed zone, or Equation (c) described below is
satisfied in the columnar zone.
Re = (AeIAo) x 100 5 30% Equation (a)
(Cmin, 11Co) 2 0.95 Equation (b)
(Cmin, 21Co) 2 0.95 Equation (c)
where,
Re: area fraction (%) of the equiaxed zone,
Ae: area (%) of the equiaxed zone,
Ao: area (%) of the cross section,
Co: average concentration (mass%) of C in the cross section, or concentration
(mass%) of C in molten steel in a ladle or continuous casting tundish,
Cmin, 1: minimum concentration (mass%) of C in the equiaxed zone, and
Cmin, 2: minimum concentration (mass%) of C in the columnar zone.
(2) In the case hardened steel according to (1) described above, Equation (a) and
Equation (b) may be satisfied in the equiaxed zone, and Equation (c) may be satisfied in
the columnar zone.
(3) In the case hardened steel according to (1) or (2) described above, at least one
of Equation (d) described below and Equation (e) described below may be satisfied in
the equiaxed zone.
(LIF) ? 0.6 Equation (d)
(LIS) 2 0.6 Equation (e)
L: distance (mm) from the center of a cross section to a position closest to the
center of the cross section and located on a periphery of the equiaxed zone,
F: distance (mm) from the center of the cross section to a position located on
the periphery of the equiaxed zone and in a direction opposed, with respect to the center
of the cross section, to the position closest to the center of the cross section and located
on the periphery of the equiaxed zone, and
S: larger distance (mm) from among distances from the center of the cross
section to positions at which the periphery of the equiaxed zone crosses a line passing
through the center of the cross section of all lines perpendicular to a line connecting the
center of the cross section and a position closest to the center of the cross section and
located on the periphery of the equiaxed zone.
(4) In the case hardened steel according to (3) described above, Equation (d) and
Equation (e) may be satisfied in the equiaxed zone.
(5) In the case hardened steel according to any one of (1) to (4) described above,
the composition may fiutlier include at least one of, in mass%, Mo: over 0 to 1.5%; V:
over 0 to 1.5%; Nb: over 0 to 1.5%; Cu: over 0 to 1.0%; Ni: over 0 to 2.5%; Cr: over 0
to 2.0%; and Sn: over 0 to 1.0%.
(6) In the case hardened steel according to any one of (1) to (5) described above,
the composition may further include at least one of, in mass%: Ca: over 0 to 0.01%; Zr:
over 0 to 0.08%; Pb: over 0 to 0.4%; Bi: over 0 to 0.3%; Te: over 0 to 0.3%; Rem: over
0 to 0.1%; and Sb: over 0 to 0.1%.
(7) In the case hardened steel according to any one of (1) to (6) described above,
the composition may further include at least one of, in mass%: Ti: over 0 to 0.30%; and
B: over 0 to 0.005%.
(8) In the case hardened steel according to any one of (1) to (7) described above,
the composition may hther include, in mass%, W: over 0 to 2.0%.
(9) The second aspect of the present invention provides a mechanical component
obtained by machining the case hardened steel according to any one of (1) to (8)
described above, and applying a thermal treatment to the machined case hardened steel.
Effects of the Invention
[0018]
According to the present invention, it is possible to provide a case hardened
steel product having reduced thermal treatment distortion caused through the quenching
processes using carburizing and nitriding, having high dimensional accuracy, and
exhibiting excellent fatigue characteristics. Further, by machining the case hardened
steel described above and applying thermal treatments to this case hardened steel, it is
possible to provide mechanical components having reduced noise and vibration, and
having improved fatigue life.
Brief Description of the Drawings
[0019]
FIG. 1 is a diagram schematically illustrating imbalance of equiaxed zone in a
macrost~ucturein a cross section of a steel.
FIG. 2 is a diagram illustrating conditions for carburizing and quenching
applied in Example.
Embodiments of the Invention
[0020]
In this specification, the present invention will be described with focus placed
on application of the present invention to gears. However, the application target of a
case hardened steel according to the present invcntion is not limited to the gears. The
case hardened steel according to the present invention can be used for mechanical
componelits having a surface layer portion hardened through the quenching processes,
in particular, for mechanical components required to have a reduced amount of
distaltion after quenclling processes using carburizing and nitriding.
[0021]
In order to solve the problems of the present invention and achieve the object
of the present invention described above, the present inventors first carried out a study
of factors affecting the thermal treatment distortion. As a result, they found that the
factors that largely affect the thermal treatment distortion include, in a macrostructure
(solidification structure) in a cross section of steel:
(a) reduction in the concentration of C;
(b) the area of and the area fraction of an equiaxed zone in which concentrations of
dissolved matters are more likely to be nonuniform, and
(c) reduction in the concentration of C in the equiaxed zone and a columnar zone around
the equiaxed zone.
[0022]
Further, the present inventors continued the thorough investigation, and as a
result, found that the thermal treatment distortion can be reduced to the level that
satisfies the recent consumers' severe demand, by performing the following, in the
macrostructure (solidification structure) in a cross section of steel:
(x) reducing the size of the equiaxed zone and then, suppressing a reduction in the
concentration of C in the equiaxed zone;
(y) suppressing a reduction in the concentration of C in the columnar zone around the
equiaxed zone; or
(z) bringing the distribution of the equiaxed zone closer to axial symmetry in a cross
section in steel, or by combining two or more of (x), (y), and (z).
[0023]
In the equiaxed zone in the macrostructure in a cross section of steel, the
concentration of C or other elements tends to decrease from the outer peripheral portion
toward the center of the cross section. Thus, if the equiaxed zone is not in axial
symmetry in the cross section, the thermal treatment distortion increases because of:
(A) nonuniformity of the amount of swelling resulting from martensite transformation
occurring through quenching processes using carburizing and nitriding;
(B) time lag between occurrences of martensite transformations; and
(C) nonunifo~mity of mechanical properties in the circumferential direction after the
martensite transformation.
[0024]
On the other hand, by bringing the distribution of the equiaxed zone close to
the axial symmetry in the macrostructure in the cross section in the steel, the points (A),
(B), and (C) described above are corrected, and hence, the thermal treatment distortion
is reduced.
[0025]
Further, by reducing the equiaxed zone in the macrost~ucturein the cross
section of the steel, preventing the concentration of C in the equiaxed zone from
decreasing, or suppressing the reduction in the concentration of C in the columnar zone
around the equiaxed zone, it is possible to, in the equiaxed zone or columnar zone
around the equiaxed zone, reduce the amount of swelling resulting from transformation
occurring through the quenching processes using carburizing and nitriding, the time lag
between the times when respective martensite transformations occur, and nonuniformity
of mechanical properties in the circumferential direction after the martensite
transformation, thereby reducing the thermal treatment distortion.
[0026]
More specifically, the thermal treatment distortion can be effectively reduced,
by, in the macrostructure in the cross section of the steel, setting an area fraction (Re =
Ae/Ao) of an area (Ae) of the equiaxed zone relative to an area (Ao) of the cross section
so as to be not more than 30%; and setting a ratio (Cmin, l/Co) of the minimum
concentration of C (Cmin, 1) (mass%) in the equiaxed zone in the cross section of the
steel relative to the average concentration of C (Co) (mass%) in the cross section of the
steel or the concentration of C (Co) (mass%) in the molten steel in a ladle or a
continuous casting tundish so as to be not less than 0.95.
[0027]
Further, the thermal treatment distortion can be further reduced, by
quantitatively identifying the degree of imbalance (see FIG. 1) of the equiaxed zone in
the macrostructure in the cross section of the steel as indices (LIF) and (LIS), where L,
F, and S are defined below, and maintaining the (LIF) andlor the (LIS) to be not less
than 0.6.
[0028]
L: distance (mm) from the center of a cross section of steel to a position
closest to the center of the cross section of the steel and located on the periphery of the
equiaxed zone in the macrostructure in the cross section of the steel.
F: distance (nun) from the center of a cross section of steel to a position
located on the periphery of the equiaxed zone and in a direction opposed, with respect to
the center of the cross section, to the position closest to the center of the cross section
and located on the periphery of the equiaxed zone in the macrostructure in the cross
section of the steel.
S: larger distance (mm) among distances from the center of the cross section
of steel to positions at which the periphery of the equiaxed zone crosses a line passing
through the center of the cross section of all lines perpendicular to a line connecting the
center of the cross section and the position closest to the center of the cross section and
located on the periphery of the equiaxed zone in the macrostructure in the cross section
of the steel.
[0029]
Further, the thermal treatment distortion can be fi~rthere duced, by
maintaining a ratio (Cmin, 21Co) of Cmin, 2 (mass%), which represents the minimum
concentration of C (mass%) in the columnar zone around the equiaxed zone in the
macrostructure in the cross section of the steel, relative to the average concentration C
(Co) (mass%) in the cross section of the steel or the concentration of C (Co) (mass%) in
a molten steel in a ladle or a continuous casting tundish so as to be not less than 0.95.
[0030]
As described above, the thermal treatment distortion can be stably reduced, if:
(a) Equation (1) and Equation (2) described below are satisfied; or
(b) Equation (3) described below is satisfied. Further, the thermal treatment distortion
can be reduced in various applications, if:
(c) all of Equation (1) to Equation (3) described below are satisfied.
Furthel; the thermal treatment distortion can be further stably reduced for
mechanical conlponents having various shapes, if:
(d) either of or both of Equation (4) and Equation (5) described below are satisfied.
[003 11
Re = (AeIAo) x 100 5 30% Equation (1)
(Cmin, 1/Co) 1 0.95 Equation (2)
(Crnin, 2/Co) 1 0.95 Equation (3)
(LJF) > 0.6 Equation (4)
(LIS) 1 0.6 Equation (5)
[0032]
The measurement of L, F, and S in the equiaxed zone in the macrostructure in
the cross section of the steel, the measurement of the minimum concentration of C in
the equiaxed zone, and the measurement of the minimum concentration of C in a
columnar band zone may be performed for any form of the following steel: bloom, a
steel piece, rolled steel, and a mechanical conlponent obtained by machining rolled
steel.
[0033]
The equiaxed zone or the columnar zone in the macrostructure in the cross
section of the steel may be made appear through etching using hydrochloric acid-based
etching reagent, picric acid-based etching reagent, or Oberhofer's reagent, or may be
made appear through a sulfur print method or an etch print method. Alternatively, the
equiaxed zone or the colutnnar zone may be identified through elemental mapping (area
analysis) for a solidification structure using EPMA or other various types of electron
microscopes.
[0034]
Cmin, 1 of the equiaxed zone and Cmin, 2 of the columnar zone are evaluated,
by, after the macrostructure is examined, chemically analyzing cuttings obtained from
each of the zones, for example, through drilling or step cutting method, or measuring
the distribution of the concentration of C in each of the zones through Quantvac method,
or tneasuring the distribution of the concentration of C through EPMA or other
elemental mapping, or line analysis method.
100351
Co may be obtained by measuring the average concentration of C in the cross
section of the steel through tlie metl~odsd escribed above, or may be obtained through
chemical analysis applied to samples of molten steel taken with a ladle or continuous
casting tundish, or through analysis using the Quantvac method.
100361
According to the present invention, it is possible to reduce circumferential
non-uniformity of hasdenability and mechanical properties in the cross section of the
case hardened steel, by limiting the area fraction of the equiaxed zone in the cross
section of the case hardened steel subjected to the quenching processes using
carburizing and nitriding, suppressing formation of negative segregation in the equiaxed
zone or the columnar zone around the equiaxed zone, and correcting imbalance of the
distribution or shape of the equiaxed zone in the cross section. Accordingly, it is
possible to provide case hardened steel products having the reduced thermal treatment
distortion caused through the quenching processes using carburizing and nitriding,
having improved dimensional accuracy, and exhibiting excellent fatigue characteristics.
100371
Next, descriptions will be made of reasons for limiting chemical components
in the case hardened steel according to the present invention. Note that "%" means
mass%.
[0038]
C: 0.05 to 0.45%
C is an element essential for securing internal strength sufficient to make the
steel function when used as mechanical components. If the amount of C is less than
0.05%, the sufficient internal strength cannot be obtained. Thus, the lower limit is set
to 0.05%. If the amount of C exceeds 0.45%, toughness deteriorates, and
machinability or cold forgeability deteriorates, whereby workability deteriorates. Thus,
the upper limit is set to 0.45%.
Preferably, the lower limit of the amount of C is set to 0.10%. More
preferably, the lower limit is set to 0.20%.
Preferably, the upper limit of the amount of C is set to 0.30%. More
preferably, the upper limit is set to 0.25%.
[0039]
Si: 0.01 to 1.0%
Si functions as a deoxidizing agent at the time of smelting, and has a function
of increasing a transformation point, and enhancing the internal strength. Further, Si
has a function of separating the internal structure into two phases at normal quenching
temperatures (800 to 1050°C) and suppressing the thermal treatment distortion.
[0040]
The amount of Si added is set to 0.01% or more to obtain an additive effect.
However, if the amount of Si contained exceeds 1.0%, intergranular oxidation advances,
bending fatigue strength deteriorates, and cold forgeability or machinability deteriorates.
Thus, the upper limit is set to 1.0%. In the case where a gas carburizing and nitriding
method is used as a case hardening method, carburizing and nitriding are hindered if the
amount of Si exceeds 1.0%. Thus, for this reason, the upper limit is set to 1.0%.
Preferably, the lower limit of the amount of Si is set to 0.15%, and more
preferably, the lower limit is set to 0.30%.
Preferably, the upper limit of the amount of Si is set to 0.7%, and more
preferably, the upper limit is set to 0.6%.
[0041]
Mn: over 0 to 2.0%
Mn is an element that functions as a deoxidizing agent, and contributes to
improving strength and hardenability. However, if the amount of Mn exceeds 2.096,
cold working characteristics deteriorate, and the amount of segregation to grain
boundaries increases, which results in a deterioration in bending fatigue characteristics.
Thus, the upper limit is set to 2.0%. Preferably, the upper limit is set to 1.5% or less.
The lower limit is set to over 0%. However, in order to reliably obtain the additive
effect, it is preferable to set the lower limit to 0.3% or more.
[0042]
Al: 0.001 to 0.06%
A1 is an element that functions as a deoxidizing agent, bonds to N in the steel
to form AIN, and has a function of preventing crystal grains from coarsening. In order
to obtain the deoxidizing effect, the amount of A1 added is set to 0.001% or more. If
the amount of A1 exceeds 0.06%, the additive effect saturates, and A1 bonds to oxygen
to form non-metal-based inclusions that adversely affect impact characteristics. Thus,
the upper limit is set to 0.06%.
Preferably, the lower limit of the amount of A1 is set to 0.005%, and more
preferably, the lower limit is set to 0.01%.
Preferably, the upper limit of the amount of A1 is set to 0.04%, and more
preferably, the upper limit is set to 0.03%.
[0043]
N: 0.002 to 0.03%
N is an element that bonds, for example, to Al, V, Ti, and Nb in the steel, and
forms nitrides that suppress coarsening of crystal grains. In order to obtain the
additive effects, the amount of N added is set to 0.002% or more. Preferably, the
amount of N added is set to 0.007% or more. If the amount of N exceeds 0.03%, the
additive effects saturate, and the formed nitrides serve as inclusions and have adverse
effects on characteristics. Thus, the upper limit of N is set to 0.03%. Preferably, the
upper limit is set to 0.01% or less.
[0044]
P: over 0 to 0.05%
P is an element that is segregated in grain boundaries, and deteriorates
toughness. Thus, the upper limit of P is set to 0.05%. Preferably, the upper limit is
set to 0.03% or less. It is preferable that P is as low as possible, and the lower limit is
over 0%. However, in general, approximately 0.001% of P inevitably exists.
[0045]
S: over 0 to 0.1%
S is an element that suppresses decarbonization in the surface layer during
thelmal treatments, and improves machinability. If the amount of S exceeds 0.1%, hot
workability or fatigue characteristics deteriorate. Thus, the upper limit is set to 0.1%.
In the case of gears, attention should be paid not only to vertical-impact characteristics
but also to transverse-impact characteristics. Thus, in order to enhance the
transverse-impact characteristics by reducing anisotropy, it is preferable to set the
amount of S to 0.03% or less. More preferably, the amount of S is set to 0.01% or
less.
[0046]
The balance of the case hardened steel according to the present invention
includes Fe and inevitable impurities. However, it is possible to improve the
characteristics by further adding, as selective elements, at least one of the following:
Mo: over 0 to 1.5%.
V: over 0 to 1.5%,
Nb: over 0 to 1.5%,
Cu: over 0 to 1.0%,
Ni: over 0 to 2.5%,
Cr: over 0 to 2.0%, and
Sn: over 0 to 1 .O%
[0047]
Mo, V, and Nb are elements that each have functions of increasing the
transfonnation points, enabling the internal structure to be separated into two phases
even at normal quenching temperatures (800 to 1050°C), and suppressing the thermal
treatment distortion. Mo is an element that contributes to improving grain boundary
strength, reducing an imperfectly quenched structure, and improving hardenability.
However, if the amount of Mo exceeds 1.5%, the additive effects saturate. Thus, the
upper limit is set to 1.5%, preferably 1.0% or less.
[0048]
V and Nb are elements that each bond to C or N to folm carbonitrides and
make crystal grain finer, and contribute to improving toughness. However, if the
amount of V exceeds 1.5%, machinability deteriorates. Thus, the upper limit of V is
set to 1.5%. If the amount of Nb exceeds 1.5%, workability deteriorates. Thus, the
upper limit ofNb is set to 1.5%.
Preferably, the lower limit of each of Mo, V, and Nb is set to 0.005%.
Preferably, the upper limit of each of Mo, V, and Nb is set to 1.0%.
to0491
Cu, Ni, Cr, and Sn are elements that each contribute to separating the internal
structure into two phases. Cu and Sn are elements that contribute to improving a
corrosion resistance. If each of Cu and Sn exceeds 1.0%, the additive effects saturate,
and hot workability deteriorates. Thus, the upper limit of each of Cu and Sn is set to
1.0%. Preferably, the upper limit of each of Cu and Sn is set to 0.6% or less.
[OOSO]
It should be noted that addition of Cu alone, or addition of Cu and Sn in a
combined manner has a significant adverse effect on the hot workability. Thus, in the
case where Cu is added alone, or Cu and Sn are added in a combined manner, it is
preferable to add Ni approximately equal to or more than the amount of Cu added.
[OOSl]
Ni is an element that makes the structures finer after quench hardening to
enhance toughness, contributes to improving workability, and contributes to stably
securing internal hardness. If the amount of N exceeds 2.5%, the additive effects
saturate. Thus, the upper limit is set to 2.5%. Preferably, the upper limit is set to
2.0% or less.
[0052]
Cr is an element that provides a function of enhancing hardenability to
increase the internal hardness. However, if the amount of Cr exceeds 2.0%, carbides
precipitate at grain boundaries, the strength at grain boundaries deteriorates, and
toughness deteriorates. Thus, the upper limit is set to 2.0%. Preferably, the upper
limit is set to 1.5% or less.
[0053]
In order to improve characteristics, the case hardened steel according to the
present invention may further contain, as a selective element, at least one of the
following:
Ca: over 0 to 0.01%,
Zr: over 0 to 0.08%,
Pb: over 0 to 0.4%,
Bi: over 0 to 0.3%,
Te: over 0 to 0.3%,
Rem (rare earth metal such as Ce, La, and Nb): over 0% to 0.1%, and
Sb: over 0 to 0.1%.
[0054]
Ca is an element that softens hard oxide to enhance machinability. However,
if the amount of Ca exceeds 0.01%, the additive effects saturate. Thus, the upper limit
is set to 0.01%. Preferably, the upper limit is set to 0.007% or less. Zr is an element
that makes MnS have a spherical shape to improve anisotropy, and enhances
machinability. However, if the amount of Ca exceeds 0.08%, the additive effects
saturate. Thus, the upper limit is set to 0.08%. Preferably, the upper limit is set to
0.05% or less.
[0055]
Pb, Bi, Te, Rem (rare earth metal such as Ce, La, and Nb), and Sb are
elements that each contribute to improving machinability, prevent sulfides from
elongating, thereby suppressing deterioration in toughness or other mechanical
properties resulting from sulfides, or increase in anisotropy. The excessive amount of
these elements causes a significant adverse effect on pitting life or fatigue strength.
Thus, the amount of Pb is set to 0.40% or less, the amount of each of Bi and Te is set to
0.3% or less, and the amount of each of Rem and Sb is set to 0.1% or less. Preferably,
the amount of Pb is set to 0.30% or less, the amount of each of Bi and Te is set to 0.2%
or less, and the amount of each of Rem and Sb is set to 0.06% or less.
[0056]
In order to improve characteristics, the case hardened steel according to the
present invention may further contain at least one of the following:
Ti: over 0% to 0.3%, and
B: over 0% to 0.005% or less.
[0057]
Ti is an element that bonds to N to form nitrides, make crystal grains finer,
and contributes to improving toughness. The excessive amount of Ti causes an
adverse effect on pitting life or machinability. Thus, the upper Iimif is set to 0.1%.
Preferably, the lower limit of Ti is set to 0.005%. More preferably, the Iower
limit of Ti is set to 0.010%.
Preferably, the upper limit of Ti is set to 0.05%. More preferably, the upper
limit of Ti is set to 0.02%.
B is an element that contributes to improving hardenability. However, the
additive effects saturate if the amount of B reaches 0.005%. Thus, the upper limit of B
is set to 0.005%. Preferably, the upper limit of B is set to 0.002% or less.
[OOSS]
W: over 0% to 2.0%
In order to improve characteristics, the case hardened steel according to the
present invention may further contain W: over 0% to 2.0%.
An appropriate amount of W added is effective in improving hardenability and
improving strength through strengthening of ferrite. However, the additive effects
saturate if tlie amount of W reaches 2.0%. Thus, the upper limit is set to 2.0%.
Preferably, the upper limit is set to 1.5% or less.
[0059]
The case hardened steel according to the present invention is a steel having the
chemical components described above, in which the area fraction of the equiaxed zone
in the cross section of the steel, the degree of negative segregation of the equiaxed zone,
the shape or imbalance of the equiaxed zone, and the degree of negative segregation of
the colun~nazr one satisfy Equation (1) and Equation (2), or Equation (3), and further
satisfy Equation (4) andlor Equation (5) as needed. Thus, by applying the quenching
processes using carburizing and nitriding to the steel that has been formed into
mechanical components, it is possible to obtain mechanical components having high
dimensional accuracy, having improved surface hardness, and exhibiting excellent wear
resistance.
[0060]
The quenching processes using carburizing and nitriding employed in the
present invention are not limited to specific processes, and it may be possible to employ,
for example, known gas carburizing (or carbonitriding), pack carburizing (or
carbonitriding), salt bath carburizing (or carbonitriding), plasma carburizing (or
carbonitriding), or vacuum carburizing (or carbonitriding). Note that, in the case of
obtaining case hardened steel products having a significantly high-level toughness, it is
desirable to apply the quenching processes using carburizing and nitriding, and then,
apply a tempering process at temperatures in the range of approximately 100 to 200°C.
[0061]
The fatigue strength can be further improved by applying a shot peening
process to the case hardened steel product to provide compressive residual stress to the
surface thereof, after the quenching processes using carburizing and nitriding are
applied or after the quenching processes using carburizing and nitriding are applied and
then a tempering process is applied. Preferably, conditions for the shot peening
process are set, for example, such that shot particles having shot hardness of HRC 45 or
more and particle size in the rage of 0.04 to 1.5 mm are used, and an arc height (value
indicating a height of deformation of the surface resulting from shot peening) is set to
0.2 to 1.2 l d .
[0062]
If the hardness of the shot particles is less than HRC 45 or the arc height is
less than 0.2 mmA, it is not possible to provide a sufficient compressive residual stress
to the surface of the case hardened steel product. On the other hand, if the arc height
exceeds 1.2 mmA, over shot peening occurs, which has an adverse effect on the fatigue
characteristics. The upper limit of the hardness of the shot particles is not specifically
limited. However, practically, the upper limit is approximately HRC 65. Although
no specific limitation is applied to the particle size of the shot particles, the particle size
is set preferably to fall in a range of 0.04 to 1.5 mm, more preferably 0.3 to 1.0 mm.
100631
In general, the shot peening process is performed once sufficiently. However,
the shot peening process may be repeated for two or more times depending on
applications.
Examples
[0064]
Next, by giving Examples, the configuration and operational effects of the
present invention will be described more specifically. However, there is no limitation
applied to the present invention with Examples described below, and any modifications
may be applied and performed, provided that such modifications conform to the scope
of the present invention. Further, such modifications are included in the technical
scope of the present invention.
[0065]
(Examples)
Steels having the chemical composition shown in Tables 1 to 4 and 7 to 10
were casted through normal continuous casting using a mold having a square shaped in
cross section with thickness 220 mnl x width 220 mrn, or a mold having a rectangle
shape in cross section with thickness 350 mm x width 560 mm. Tables 1 to 4 show
Examples according to the present invention, and Tables 7 to 10 show Comparative
Examples. In the tables, chemical components together with Re (%), (Cmin, lICo),
(Cmin, 2/Co), (LIF) and (LIS) are shown. Further, in the tables, "tr" means that the
amount of a corresponding element is extremely small to the extent that the amount of
the corresponding element contained can be ignored.
[0066]
Re, (Cmin, lICo), (Cmin, 2/Co), (LIF), and (LIS) of the steels according to the
present invention and steels according to comparative examples were adjusted in the
following manner.
[0067]
For example, (a) vary the superheat of molten steel in a tundish; (b) vary the
strength of electro-magnetic stirring in the mold; and (c) vary the casting speed.
Further, for some of the blooms, soft reduction at the late stage of solidification was
applied to suppress negative segregation in the equiaxed zone, thereby varying the area
of and the area fraction of the equiaxed zone in the cross section of the steel, the shape
and the imbalance of the equiaxed zone in the cross section, the concentration of C in
the equiaxed zone, and the distribution of the concentration of C in the columnar zone
around the equiaxed zone.
[0068]
With decrease of the superheat of the molten steel in the tundish, the area
fraction of the equiaxed zone increases. With increase in the strength of
electro-magnetic stirring in the mold, the area fiaction of the equiaxed zone increases.
Further, in the case where casting is performed with a mold having a flattened
rectangular cross section, the cross-sectional shape of the equiaxed zone is more likely
to be flattened, as compared with the case where a mold having a square cross section is
used.
[0069]
With increase in the casting speed in the continuous casting process, equiaxed
grains are more likely to move down toward the lower surface of the bloom, whereby
the equiaxed zone is positioned in an imbalanced manner toward the lower surface of
the bloom in the cross section. With increase in the strength of the electro-magnetic
stirring in the mold, the concentration of C in the columnar zone on the surface layer
side decreases. By applying soft reduction at the late stage of solidification, it is
possible to suppress centerline segregation or formation of negative segregation in the
surrounding zone, whereby it is possible to suppress the reduction in the concentration
of C within the equiaxed zone.
[0070]
The blooms obtained by casting under various casting conditions were
subjected to billet mill to form steel pieces with a 162 mm square, and then were formed
into steel bars with 25 mmcp and 48 mmcp through hot rolling. The steel bars with 25
mmcp were maintained at 900°C for one hour, then were subjected to a normalizing
process with air cooling, and were cut into pieces each having a length of 200 mm.
Then, the surface layers of the pieces thus obtained were cut, and then were machined
into test pieces with a bar shape with 22 mmcp x length 200 mm.
[0071]
The steel bars with 48 mmcp were maintained at 900°C for one hour, then were
subjected to a normalizing process with air cooling, and were cut into pieces each
having a length of 15 mm. Then, the surface layers of the pieces thus obtained were
cut, and then were machined to obtain pieces having an outside diameter of 45 mmcp.
The center portion of each of the pieces thus obtained was hollowed to obtain
ring-shaped test pieces having an inside diameter 26 nuncp x an outside diameter 45
mmcp x a height 15 mm.
LO0721
These test pieces were used to perform a carburizing and quenching test under
conditions shown in FIG. 2. The number of test pieces used for performing the
carburizing and quenching test were five for each condition. Then, the degree of
off-center rotation and the roundness of each of the test pieces were measured to
evaluate the thermal treatment distortion, and calculate the average value of the five test
pieces.
to0731
For the carburizing and quenching, one test piece was processed at a time.
Note that, at the time of oil quenching, the bar-shaped test pieces were immersed in a
vertical position relative to the oil surface and the ring-shaped test pieces were
immersed in a position in which the upper and the lower surfaces of each of the test
pieces were parallel to the oil surface, so that variation in the methods or conditions of
carburizing and quenching does not affect the thermal treatment distortion.
LO0741
Before and after the carburizing and quenching test, for the bar-shaped test
pieces with 22 mmcp x length 200 mm, test pieces were rotated in the circumferential
direction with the cross-sectional center of both ends of each of the test pieces serving
as a center; measurement was made of the amount of bending, which corresponds to the
degree of off-center rotation at the center in the longitudinal direction; and the average
value of the results was calculated. For the ring-shaped test pieces, the roundness was
measured at three points in the height direction of each of the test pieces along the inner
circumference and the outer circumference to calculate the average value of the results.
The average values were calculated with n = 5.
LO0751
Tables 5,6,1 I, and 12 show the average values of the maximum amount of
bending of the bar-shaped test pieces, and the average values of the maximum values of
roundness of the ring-shaped test pieces.
[0076]
Further, samples for observing structures were taken from the test pieces after
the carburizing and quenching, and were etched with a picric acid-based etching reagent
to make macrostructures appear. Then, Ae, L, F, and S were measured to calculate Re,
L/F, and LIS. Elemental mapping was applied to the samples with EPMA, and Cmin,
1 in the equiaxed zone and Cmin, 2 in the columnar zone were obtained. Then, the
concentration Co of C of the molten steel in the tundish was obtained to calculate (Cmin,
1ICo) and (Cmin, 2/Co). The calculation results are shown in Tables 5,6, 11, and 12.
[0077]
For Examples (Ex. 1 to Ex. 100) shown in Tables 1 to 6, the maximum amount
(average value in the case of n = five test pieces) of bending measured afler the
bar-shaped test pieces were subjected to carburizing and quenching is reduced to 15 ym
or less, and the maximum value (average value in the case of n = five test pieces) of
roundness measured after the ring-shaped test pieces were subjected to carburizing and
quenching is reduced to 10 pm or less.
[0078]
On the other hand, for Comparative Examples (Comp. Ex. 1 to Comp. Ex. 79)
shown in Tables 7 to 12, the maximum amount of bending measured after the
bar-shaped test pieces were subjected to carburizing and quenching results in 20 pm or
more, and the maximum value of roundness measured after the ring-shaped test pieces
were subjected to carburizing and quenching results in 15 pm or more, each of which is
greater than values of examples according to the present invention by 5 pm or more.

[0082]
[Table 41
NO.
Example Mo V Nb Cu Ni Cr Sn Ca Zr Pb Bi Te Rem Sb Ti B W
Maximum roundness value
(Ring-shaped test piece)
p 1

[0086]
Table 81
Comparative
Example
C Si Mn P S Al N

Industrial Applicability
[0091]
As described above, according to the present invention, it is possible to
provide the case hardened steel product having reduced thermal treatment distortion
caused through the quenching processes using carburizing and nitriding, having
improved dimensional accuracy, and exhibiting excellent fatigue characteristics. Thus,
the present invention is highly applicable in the indnstly where mechanical components
are manufactured.
Brief Description of the Reference Symbols
[0092]
L: distance (mm) from the center of a cross section of steel to a position
closest to the center of the cross section of the steel and located on the periphery of the
equiaxed zone in the macrostructure in the cross section of the steel.
F: distance (mm) from the center of a cross section of steel to a position
located on the periphery of the equiaxed zone and in a direction opposed, with respect to
the center of the cross section, to the position closest to the center of the cross section
and located on the periphery of the equiaxed zone in the macrostructure in the cross
section of the steel.
S: larger distance (mrn) from among distances fiom the center of the cross
section of steel to positions at which the periphery of the equiaxed zone crosses a line
passing through the center of the cross section of all lines perpendicular to a line
connecting the center in the cross section and the position closest to the center of the
cross section and located on the periphery of the equiaxed zone in the macrostructure in
the cross section of the steel.
Document A
(Amendment to claims under PCT Art. 19)
CLAIMS
1. A case hardened steel having a cross section having a macrostructure including an
equiaxed zone and a columnar zone disposed around the equiaxed zone, tlie case hardened
steel having a conlposition conlprising, in mass%:
C: 0.05 to 0.45%;
Si: 0.01 to 1.0%;
Mn: more than 0 to 2.0%;
Al: 0.001 to 0.06%;
N: 0.002 to 0.03%;
S: more than 0 to 0.1%;
P: more than 0 to 0.05%; and
balance: Fe and inevitable impurities, wherein
Equation ( 1 ) described below and Equation (2) described below are satisfied in the
equiaxed zone, or
Equation (3) described below is satisfied in the colu~linarz one.
Re = (AeIAo) x 100 < 36% Equation ( 1 )
(Cmin, 11Co) > 0.95 , Equation (2)
(Cmit~2,t Co) l0.95 Equation (3)
where,
Re: area fraction (Yo) of the equiaxed zone,
Ae: area (96) of the equiaxed zone,
Ao: area (%) of tlie cross section,
Co: average concentration (mass%) of C in the cross section, or concentration
(mass%) of C in molten steel in a ladle or conti~luousc asting tundish,
Cmin, 1: minimum concentratioli (mass%) of C in the equiaxed zone, and
Cmin, 2: minimum concentration (mass%) of C in the columnar zone.
2. The case hardened steel according to clain~I , wherein
Equation (1) and Equation (2) are satisfied in the equiaxed zone, and
Equation (3)i s satisfied in the colum~~azorn e.
3. The case hardened steel according to claim 1 or 2, wherein
at least one of Equation (4) described below and Equation (5) described below is
satisfied in the equiaxed zone.
(LtF) 2 0.6 Equation (4)
(LIS) > 0.6 Equation (5)
where,
L: distance (mm) from a.center of the cross section to a position closest to the center
of the cross section and located on the periphery of the equiaxed zone,
F: distance (mm) from tlie center of the cross section to a position located on the
periphery of the equiaxed zone and in a direction opposed, with respect to the center of the
cross section, to the position closest to the center of the cross section and located on the
periphery of the equiaxed zone, and
S: larger distance (mnl) from among distances from the center of the cross section
to positions at which the periphery of the equiaxed zone crosses a line passing tlxough the
center of the cross section of all lines perpendicular to a line connecting the center of tlie
cross section and a position closes\ to tlie center of the cross section and located on the
periphery of the equiaxed zone.
4. The case hardened steel according to clairn 3, wherein
Equation (4) and Equation (5) are satisfied in the equiaxed zone.
5. The case hardened steel according to any one of claims 1 to 4, wherein
tlie coniposition of the steel fnrther comprises at least one of, in mass%:
Mo: niore than 0 to 1.5%;
V: more than 0 to 1.5%;
Nb: nnore than 0 to 1.5%;
Cu: more than 0 to 1.0%;
Ni: more than 0 to 2.5%;
Cr: more than 0 to 2.0%; and
Sn: more than 0 to 1.0%.
6. The case hardened steel according to any one of claims 1 to 5, wherein
the composition of the steel fnrther comprises at least one of, in mass%:
Ca: more than 0 to 0.01%;
Zr: more than 0 to 0.08%;
Pb: more than 0 to 0.4%;
Bi: more than 0 to 0.3%;
Te: more than 0 to 0.3%;
Rem: more than 0 to 0.1%; and
Sb: more than 0 to 0.1%.
7. The case hardened steel according to any one of claims 1 to 6, wherein
the conlposition of the steel fii~therc omprises at least one of, in mass%:
Ti: more than 0 to 0.30%; and
B: more than 0 to 0.005%.
8. The case hardened steel according to any one of claims 1 to 7, wherein
the cotnposition of the steel further comprises, in mass%:
W: more than 0 to 2.0%.
9. A mechanical coniponent obtained by machining the case hardened steel according to
any one of claims 1 to 8, and applying a thermal treatment to the machined case hardened
steel.