[Name of Document] DESCRIPTION
[Title of the Invention] HOT-ROLLED.STEEL SHEET FOR NITRIDING,
COLD-ROLLED STEEL SHEET FOR NITRIDING EXCELLENT IN
FATIGUE STRENGTH, MANUFACTURING METHOD THEREOF, AND
AUTOMOBILE PART EXCELLENT IN FATIGUE STRENGTH USING
THE SAME
[Technical Field]
[0001] The present invention relates to a steel sheet for nitriding excellent
in fatigue strength that secures workability and is capable of obtaining a hard
nitrided layer by an nitriding treatment such as gas nitriding, gas
nitrocarburizing, or salt-bath nitrocarburizing, a manufacturing method
thereof, and an automobile part excellent in fatigue property having a hard
nitrided layer on its surface.
This application is based upon and claims the benefit of priority of the
prior Japanese Patent Application No. 2011-253677, filed on November 21,
2011, the entire contents of which are incorporated herein by reference.
[Background Art]
[0002] For automobiles and respective machine parts, many surface
hardening treated parts are used. The surface hardening treatment is
performed with the aim of improving abrasion resistance and fatigue strength,
and as a representative surface hardening treatment method, carburizing,"
nitriding, induction heating, and the like can be cited. Nitriding treatments
such as gas nitriding, 'gas nitrocarburizing. and salt-bath nitrocarburizing are
performed' at a transformation point to austenite or lower unlike other
methods, to thus need a treatment time for several hours but has an advantage
of capable of making heat treatment strain small.
[0003] Thus, the nitnding is a surface hardening treatment suitable for
high-precision worked parts such as a crankshaft and a transmission gear in
terms of automobile members or members requiring product shape accuracy
after a hardening treatment of a damper disc and a damper plate formed by
being pressed. Regarding the nitriding treatment, gas nitrocarburizing,
salt-bath nitrocarburizing, and so on can be cited, but gas nitriding to be
performed in an ammonia atmosphere makes it possible to obtain high surface
hardness but generally needs a treatment time of 20 hours or longer because
diffusion of nitrogen is slow. On the other hand, a nitrocarburizing treatment
to be performed in a bath or an atmosphere containing carbon together with
nitrogen such as gas nitrocarburizing or salt-bath nitrocarburizing makes it
possible to accelerate diffusion speed of nitrogen. As a result, the
nitrocarburizing treatment makes it possible to obtain a part having an
increased surface hardened layer depth for several hours. By such a
nitriding treatment, it is possible to form a surface hardened layer having an
increased surface hardening depth," suppress fatigue crack initiation in the
surface of apart, and improve fatigue endurance.
[0004] For increasing the surface hardened layer depth and surface
hardness, a steel containing nitride forming alloys has been proposed to be
disclosed in Patent Document 1, for example. Further, regarding a part
obtained by pressibrming a. hot-rolled steel sheet or a cold-rolled steel sheet,
a gas nitrocarburizing treated steel sheet having improved workability at the
time of press forming before a nitriding treatment and having an improved
part surface hardness property after the nitriding treatment has been proposed
-to bc-discloscd in Patent Documents 2-and 3, for example. In each of the
previously described well-known documents, for the improvement of surface
hardness by the gas nitrocarburizing treatment, elements such as Al, Cr, and V
being nitride forming elements are effective to be contained, as alloying
elements of a steel sheet for gas nitrocarburizing. -
[Prior Art Document]
[Patent Document]
[0005] Patent Document 1: Japanese Laid-open Patent Publication No.
2007-162138
Patent Document 2: Japanese Laid-open Patent Publication No. •
2005-264205
Patent Document 3: Japanese Laid-open Patent Publication No. Hei
9-25544
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0006] In the case of a gas nitrocarburized part formed by pressing a
hot-rolled steel sheet or a cold-rolled steel sheet, for example, alloy
component designing of a steel sheet achieving workability before a gas
nitrocarburizing treatment and a fatigue property after the treatment is
required.
[0007] For the fatigue property after the gas nitrocarburizing treatment, it
is necessary to increase the surface hardness and the depth by nitrides of Al,
Cr,_and V. Particularly, V promotes diffusion of N to thereby increase the
hardened layer depth, and Cr and Al are effective for increasing the surface
hardness, but regarding Al and V, fine nitrides precipitate linearly at austenite
grain boundaries to significantly deteriorate burring .formability and stretch
-flangcability. Further, regarding V, in a cooling step after a hot finish rolling
step and in a coiling step of a hot-rolled sheet, high strengthening by
precipitation of V and C is promoted and workability deteriorates. In order
to avoid such precipitation strengthening of V and.C, it is effective to set a
cooling stop temperature after hot roiling to 500°C or lower, but lower bainite
or martensite transformation is promoted and ductility decreases significantly.
Thus, it is necessary to suppress a strength increase in a steel sheet for gas
nitrocarburizing by decreasing the content of V as much as possible, but when
V is decreased, there is caused a problem that it becomes difficult to increase
the surface hardening depth after the gas nitrocarburizing treatment.
[0008] The present invention makes it possible to provide a hot-rolled
steel sheet for nitriding, a cold-rolled steel sheet for nitriding excellent in
fatigue strength that are capable of making a surface hardened layer deep for
excellent workability before a gas nitrocarburizing treatment and fatigue
strength improvement after the treatment, a manufacturing method thereof,
and an automobile part excellent in fatigue strength having a nitrided layer
with increased hardness in its surface layer.
[Means for Solving the Problems]
[0009] The present inventors examined a steel sheet alloy composition
capable of obtaining a surface hardening depth without impairing formability
of an automobile part by an nitriding treatment such as gas nitrocarburizing or
salt-bath nitrocarburizing, a manufacturing method, and further hardness of
the part.
[0010] As a result, it was found that an appropriate amount of B is_
contained in a steel containing appropriate amounts of Cr and V, a skin pass'
reduction ratio range is defined in a manufacturing step, and TVT, being a ratio
of a line load F (kg/mm) of a rolling mill load of the skin pass reduction
I divided by a sheet width of a steel sheet and a load T (kg/mm ) per unit area
5
at the rolling outlet side being a load to be applied in the longitudinal
direction of the steel sheet, is set to be in a predetermined range, and thereby a
dislocation density in the sheet thickness direction of the steel sheet is defined
and a hardening depth after nitriding is increased, and thereby it is possible to,
while suppressing strength moderately, suppress a decrease in ductility caused
by dislocation introduction, decrease roughness of a fracture surface of a
sheared end surface, and secure a sufficient surface hardening depth after
nitriding, and reached the present invention.
[0011] That is, the present invention is as follows.
(1) A steel sheet for nitriding excellent in fatigue strength, includes:
in mass%, C of not less than 0.0002% nor more than 0.07%; Si of not
less than 0.0010% nor more than 0.50%; Mn of not less than 0.10% nor more
than 1.33%; P of not less than 0.003% nor more than 0.02%; S of not less than
0.001% nor more than 0.02%; Cr of greater than 0.80% and 1.20% or less; Al
of not less than 0.10% nor more than 0.50%; V of not less than 0.05% nor
more than 0.10%; Ti of not less than 0.005% nor more than 0.10%; B of not
less than 0.0001% nor more than 0.0015%; and a balance being composed of
Fe and inevitable impurities, in which a dislocation density within 50 u,m in
the sheet thickness direction from the surface of the steel sheet is not less than
2.0 times nor more than 10.0 times as compared to a dislocation density at the
position of 1/4 in the sheet thickness direction.
[0012] (2) The steel sheet for nitriding excellent in fatigue strength
according to (1), further includes:
in mass%, one or both of Mo of not less than 0.001% nor more than
0.20%; and Nb of not less than 0.001% nor more than 0.050%. -
[0013J (3) A manufacturing method of a hot-rolled steel sheet for
6
nitriding excellent in fatigue strength, includes:
on a steel billet containing, in mass%, C of not less than 0.0002% nor
more than 0.07%, Si of not less than 0.0010% nor more than 0.50%, Mn of
not less than 0.10% nor more than 1.33%, P of not less than 0.003% nor more
than 0.02%, S of not less than 0.001% nor more than 0.02%, Cr of greater
than 0.80% and 1.20% or less, Al of not less than 0.10% nor more than 0.50%,
V of not less than 0.05% nor more than 0.10%, Ti of not less than 0.005% nor
more than 0.10%, B of not less than 0.0001% nor more than 0.0015%, and a
balance being composed of Fe and inevitable impurities, performing hot
rolling; performing pickling; and then performing skin pass rolling under the
condition that a reduction ratio is 0.5 to 5.0% and F/T, being a ratio of a line
load F (kg/mm) of a rolling mill load divided by a sheet width of the steel
sheet and a load T (kg/mm2) per unit area to be applied in the longitudinal
direction of the steel sheet, is 8000 or more.
[0014] (4) A manufacturing method of a cold-rolled steel sheet for
nitriding excellent in fatigue strength, includes:
on a steel billet containing, in mass%, C of not less than 0.0002% nor
more than 0.07%, Si of not less than 0.0010% nor more than 0.50%, Mn of
not less than 0.10% nor more than 1.33%, P of not less than 0.003% nor more
than 0.02%, S of not less than 0.001% nor more than 0.02%, Cr of greater
than 0.80% and 1.20% or less, Al of not less than 0.10% nor more than 0.50%,
V of not less than 0.05% nor more than 0.10%, Ti of not less than 0.005% nor
more than 0.10%, B of hot less than 0.0001% nor more than 0.0015%, and a
balance being composed of Fe and inevitable impurities, performing hot
rolling; performing pickling, cold rolling, and annealing; and then'performing
skin pass rolling under the condition that a reduction ratio is 0.5 to 5.0% and
7
F/T (mm), being a ratio of a line load F (kg/mm) of a rolling mill load divided
by. a sheet, width of the steel sheet and a Joad T (kg/mm2) per unit area tq.be
applied in the longitudinal direction of the steel sheet, is 8000 or more. -
[0015] ' (5) An automobile part excellent in fatigue strength, in which
a steel sheet that contains, in mass%, C of not less than 0.0002% nor
more than 0.07%, Si of not less than 0.0010% nor more than 0.50%, Mn of
not less than 0.10% nor more than 1.33%, P of not less than 0.003% nor more
than 0.02%, S of not less than 0.001% nor more than 0.02%, Cr of greater
than 0.80% and 1.20% or less, Al of not less than 0.10% nor more than 0.50%,
V of not less than 0.05% nor more than 0.10%, Ti of not less than 0.005% nor
more than 0.10%, B of not less than 0.0001% nor more than 0.0015%, and a
balance being composed of Fe and inevitable impurities and in which a
dislocation density within 50 urn in the sheet thickness direction from the
surface of the steel sheet is not less than 2.0 times nor more than 10.0 times as
compared to a dislocation density at the position of 1/4 in the sheet thickness
direction is formed to then be nitriding treated.
[Effect of the Invention]
[0016] According to the present invention, it becomes possible to provide
a steel sheet having excellent press formability before a nitriding treatment
and capable of obtaining a surface hardened layer with a deep depth by the
nitriding treatment and further an automobile part having a surface hardened
layer with a deep depth. As a result, industrial contributions such as small
heat treatment strain and capability of obtaining a nitriding treated part high
in fatigue strength arc extremely prominent.
[Brief Description of the Drawings] - . -
" [0017] - [FIG. 1]'FIG. 1 is a graph showing the relationship between 'F/T,
8
being a ratio of a line load F (kg/mm) of a skin pass rolling mill load divided
by a sheet width of a steel.sheet and a load T (kg/mm2) per unit area to be._.
applied in the longitudinal direction of the steel sheet and a ratio of
dislocation densities of a'surface of the steel sheet and at 50 jam from the
surface;
[FIG 2] FIG. 2 is a graph showing the relationship between F/T
described previously and a dislocation density at the position of 1/4 sheet
thickness of the steel sheet;
[FIG. 3] FIG. 3 is a graph showing the relationship between a ratio of
dislocation densities at the position of 50 u,m from the surface and at 1/4 sheet
thickness and a surface hardening depth;
[FIG 4] FIG. 4 is a graph showing the relationship between a surface
hardening depth and a fatigue strength at 105 cycles of the surface of the steel
sheet;
[FIG. 5] FIG. 5 is a plane bending fatigue test piece shape for
evaluating a fatigue strength at 105 cycles of the surface of the steel sheet after
nitriding; and
[FIG. 6] FIG. 6 is a plane bending fatigue test piece shape for
evaluating a fatigue strength at 105 cycles of a sheared end surface after
nitriding.
[Mode for Carrying out the Invention J
[0018] In the present invention, a hot-rolled steel sheet for nitriding and a
cold-rolled steel sheet for nitriding each arc a_steel sheet to be used as a
material;. of .a nitriding treated _ part. Incidentally, the steel sheet is
manufactured by a -later-described manufacturi ng -method. An -automobile -
part is an automobile part using the hot-rolled steel sheet for nitriding or the
9
cold-rolled steel sheet for nitriding of the present invention as a material and
having been subjected to a jiitriding treatment after being formed. The
hot-rolled steel sheet for nitriding or the cold-rolled steel sheet for nitriding of
the present invention is press-formed in cold working to be subjected to
cutting, sharing, punching, and the like according to need to a final product
shape, and then is subjected to a nitriding treatment to thereby be an
automobile part excellent in fatigue strength.
[0019] In the present invention, the "nitriding treatment" means a
treatment to diffuse nitrigen into a surface layer of an iron and steel to harden
the surface layer, and a treatment to diffuse nitrogen and carbon into a surface
layer of an iron and steel to harden the surface layer is called a
"nitrocarburizing treatment." As representative ones, gas nitriding, gas
nitrocarburizing, salt-bath nitrocarburizing, and the like can be cited, and
among them, the gas nitrocarburizing and the salt-bath nitrocarburizing are a
nitrocarburizing treatment. Further, when a product is a nitriding treated part,
it is possible to confirm that by the nitriding treatment, the surface of a steel
sheet is hardened as compared to before the nitriding treatment and the
concentration of nitrogen of a surface layer of the steel sheet increases.
[0020] First, in the present invention, there will be explained reasons for
limiting chemical components of a steel. The limitation of chemical
components is applied to each of, of the present invention, the hot-rolled steel
sheet for nitriding, the cold-rolled steel sheet for nitriding, and the automobile
part.using the same. ' r " [0021] C is an element effective for improving strength by precipitating
carbide of another-, carbide-forming clement, - and-is-an element—that- -
precipitates alloy carbide during a nitriding treatment and contributes also to
10
precipitation strengthening to increase the surface hardness after the nitriding
treatment; _When C exceeds 0,07%, a precipitation density of cementite
increases to thereby impair burring formability. Further, when C is less than
0.0002%, grain boundary strengthening decreases, and thereby secondary
working brittleness deteriorates and further the cost of decarburizing in
steelmaking increases too much, which is not preferable. Thus, the content
of C is set to not less than 0.0002% nor more than 0.07%.
[0022] Si is a useful element as a deoxidizer, but does not contribute to
improvement of the surface hardness in the nitriding treatment to make a
surface hardening depth shallow. Therefore, the content of Si is preferably
limited to 0.50% or less. On the other hand, when Si is decreased
significantly, the cost is increased at the time of manufacture, so that the
content of Si is preferably 0.001% or more. Thus, the content of Si is set to
( not less than 0,001 nor more than 0.50%. For obtaining a deeper surface
hardening depth, the upper limit of the content of Si is more preferably 0.1%
or less.
[0023] Mn is a useful element for delaying pearlite transformation in a
temperature region of Acl or lower. When Mn is less than 0.10%, the above
effect cannot be obtained. Further, when Mn exceeds 1.33%, a band
structure of MnS is formed prominently, and thereby roughness of a sheared
end surface increases, resulting in that an extreme deterioration of fatigue
property of the sheared end surface is exhibited. Thus, the content of Mn is
set to not less than 0.10% nor more than 1.33%. - -
[0024] P exhibits a prominent decrease in toughness caused by grain
boundary segregation when exceeding 0.02%. When P-is less than 0.003%,
an effect that meets the cost of dephosphorization in steelmaking cannot be
11
obtained. Thus, the content of P is set to not less than 0.003% nor more than
0.02%. ;.;.., ; ____ _
[0025] When S exceeds 0.02%, red shortness is exhibited, and further the
density of MnS inclusions increases, and thereby formability is deteriorated.
When S is less than 0.001%, an effect that meets the cost of desulfurization in
steelmaking cannot be obtained. Thus, the content of S is set to not less than
0.001% nor more than 0.02%.
[0026] Cr is an element extremely effective for improving the surface
hardness by forming carbonitride with N to enter at the time of the nitriding
treatment and C in the steel. When the content of Cr is 0.8% or less,
sufficient surface hardness cannot be obtained. On the other hand, when the
content of Cr exceeds 1.20%, an effect is saturated. Thus, the content of Cr
is set to greater than 0.8% and 1.20% or less.
[0027] Al forms nitrides with N to enter at the time of nitriding and is an
element effective for increasing the surface hardness. However, when Al is
contained excessively, an effective hardening depth is sometimes made
shallow. When Al is less than 0.10%, sufficient surface hardness is not
exhibited. When greater than 0.50% of Al is contained, diffusion of nitrogen
in the depth direction is suppressed because of a high affinity for N, and
thereby the surface hardening depth is decreased, Thus, the content of Al is
set to not less than 0.10% nor more than 0.50%. Incidentally, when 0.30%
or more of Al is contained, the surface hardness increases prominently, so that
the content of Al is preferably 0.30% or more. _ " _ -
[0028] V is an clement that contributes to strength of the steel by forming
- carbonitride in a hot rolling step. Further, in-the present invention, similarly -
to Mo and Nb, V forms complex carbonitride with Cr and Al to be extremely
12
effective for hardening of a nitrided layer. When 0.05% or- more of V is
contained, the surface hardness and the surface hardening depth improve
prominently. On the other hand, when the content of V is greater than 0.10%,
a significant increase in strength of the steel sheet caused by structure
strengthening by hardenability improvement and caused by precipitation
strengthening is exhibited and a deterioration of formability caused by a
decrease in elongation is exhibited. Further, when V is contained
excessively, a prominent decrease in toughness and a prominent deterioration
of fatigue property of the sheared end surface that are caused by nitride
formation in a hot rolling step are exhibited. Thus, the content of V is set to
not less than 0.05% nor more than 0.10%. A more preferable range of the
content is 0.07% or more.
[0029] Regarding the range of Ti, its range is determined by the balance
with Al. As described previously, Al is an element extremely effective for
increasing the surface hardness by forming nitrides after the nitriding
treatment. On the other hand, Al is arranged in a punctate manner and
precipitates at crystal grain boundaries in a y region. Therefore, when
nitrides of Al precipitate before the nitriding treatment, the end surface
roughness at the time of shearing is increased to deteriorate the fatigue
propeity of the sheared end surface. Ti has an affinity for nitrogen higher
than that of Al, and nitrides of Ti are formed by priority to Al. . Therefore,
containing Ti makes it possible to suppress the deterioration of the fatigue
property of the sheared end surface caused _ by the previously described
nitrides of Al. However, when Ti is less than 0.005%, an Al nitride
formation suppressing effect obtained by forming nitrides of Ti is not
exhibited. On the other hand, when Ti exceeds 0.10%, due to a decrease in
13
toughness of a cast slab, slab cracking during air cooling is caused. Thus,
the content of Ti is..set. to not less than 0.005% nor more than 0.10%. The
previously described sheared end surface roughness is surface roughness of an
end surface at the time of shearing and indicates average roughness, and when
this roughness increases, in the sheared end surface during fatigue
deformation, excessive stress concentration occurs, and the fatigue property
tends to deteriorate. Incidentally, for the previously described roughness, a
measurement value in the sheet thickness direction of a sheared fracture
surface is used.
[0030] B solid-dissolves at crystal grain boundaries, to thereby suppress
grain boundary segregation of P being a grain boundary embrittling element
and improve the secondary working brittleness. Further, B decreases the end
surface roughness at the time of shearing to improve the fatigue property of
the sheared end surface. When the content of B is less than 0.0001%, the
above effect is not exhibited. Further, when greater than 0.0015% of B is
contained, ferrite transformation is delayed, so that elongation of the steel
sheet is decreased. Thus, the content of B is set to not less than 0.0001% nor
more than 0.0015%.
[0031] Mo and Nb form complex carbonitride with Cr and Al and are
extremely effective for hardening of the nitrided layer. When each content
of Mo and Nb is less than 0.001%, the above effect is not exhibited. When
the content of Mo exceeds 0.20%, the effect of improving the surface
hardness obtained by forming carbonitride of Mo deteriorates and the
ductility decreases. Therefore, the content of Mo is set to 0.01% to 0.20%. -
Further,-- when -greater- than -0.050%-of Nb-is contained, y
recrystallization during hot rolling of the steel sheet is delayed, so that
14
extremely high anisotropy is caused and thereby the burring formability
deteriorates. Thus, the content of Nb is set to not less than 0,001% nor more
than 0.05%. "--,
[0032] Next, there will be explained a dislocation density of the steel
sheet characterizing the present invention.
The dislocation promotes diffusion in the steel. During the nitriding
treatment, the_ dislocation promotes the diffusion of nitrogen to make the
surface hardening depth deep. It was newly found in the present invention
that when a dislocation density within 50 u,m in the sheet thickness direction
from the surface of the steel sheet is 2.0 times or more as compared to a
dislocation density at the position of 1/4 in the sheet thickness direction, the
above effect is exhibited. On the other hand, when the dislocation density
within 50 jam in the sheet thickness direction from the surface exceeds 10.0
times as compared to the dislocation density at the position of 1/4 in the sheet
thickness direction, a prominent decrease in ductility caused by dislocation
strengthening is exhibited. Incidentally, the sheet thickness of the steel sheet
is 1.6 to 5.0 mm, and the present inventors found that in the case of the sheet
thickness being 2.3 mm or more, in particular, a prominent effect is obtained.
[0033] A measurement value of this dislocation density is preferably
obtained from a full width at half maximum by X-ray diffraction typified by
the Williamson-Hall method. This is because in measurement by direct
observation at a TEM, a measurement range is limited, and in fabricating an
observation sample, strain is - introduced and" thereby a decrease in
measurement accuracy is concerned. Incidentally, the obtaining method
from a full width at half maximum by X-ray diffraction is described in, for
example, "Evaluation method of dislocation density using X-ray diffraction"
15
(NAKASHIMA et al. CAMP-ISIJ Vol. 17 (2004) p. 396).
[0034] The size of a measurement sample is preferably set to a size of 10
mm square or more. The surface of the measurement sample is preferably
electropolished to be decreased in thickness by 50 urn or more. Thus, when
a predetermined position of the sheet thickness is tried to be measured, it is
necessary to consider a decreased amount of the thickness by the
electropolishing and to perform mechanical polishing. Incidentally, the
intact'surface obtained after the mechanical polishing is not enough, and thus
an accurate dislocation density cannot be obtained due to working strain,
Further, for the foil width at half maximum of an X ray, diffraction peaks of
(110), (112), and (220) are preferably used. For example, when diffraction
peaks of (200) and (311) are included, the full width at half maximum is
estimated to be high extremely to make accurate measurement difficult to be
performed.
[0035] Next, there will be explained a desired microstructure of the steel
sheet of the present invention.
The present invention preferably has a metal structure constituted of
90% or more in total of ferrite and bainite in area ratio. When the total area
ratio of the other metal structures exceeds 10%, it becomes difficult to
achieve the ductility and the bulling formability. Here, the other metal
.structure's indicate austenite, martensite, andpearlite.
[0036] Identification of the metal structures of the steel can be performed
by an optical microscope by nital corrosion and by a crystal structure of an X
ray or a diffraction pattern. Further, discrimination using a corrosion
solution other than nital may also be performed. In the-case of the nital
corrosion/ after mirror polishing, etching is performed with a nital solution,
16
five visual fields are observed at 500 magnifications by an optical microscope
to take photographs,.a portion_is determined by visual observation, and the
portion determined by visual observation is image-analyzed to be obtained.
[0037] Next, there will be explained a manufacturing method of the steel
sheet of the present invention.
There will be explained a manufacturing method from hot rolling to
pickling when the steel sheet of the present invention is a hot-rolled steel
sheet. A slab being a steel billet having-the previously described steel
component is preferably set to a pre-rolling heating temperature of 1200°C or
higher in a heating furnace. This is to sufficiently solve contained
precipitation elements, and when the heating temperature exceeds 1300°C,
austenite grain boundaries become coarse, so that the heating temperature is
preferably 1300°C or lower. A hot rolling temperature is preferably 900°C
or higher. When it is lower than 900°C, deformation resistance "increases,
and further the formability deteriorates due to anisotropy by formation of a
rolled texture. Further, for prevention of a decrease in martensite fraction, a
coiling temperature is preferably 450°C or higher after hot rolling. As long
as the coiling temperature is 600°C or higher, precipitation of carbide of Ti
and V is promoted, so that the coiling temperature is between 550°C and
600°C. A cooling rate only needs to be in a range where ferrite
transformation and bainite transformation occur during cooling, and the upper
limit value is preferably set to. 10°C/s or less. This is because when the
cooling is stopped at a codling rate at which fertile transformation and bainite
transformation do not occur, after performing coiling into a coil shape, for
example, transformations are promoted and a steel sheet coil is deformed.
Incidentally, intermediate air cooling may also be performed until the
17
temperature reaches the coiling temperature. After hot rolling is finished,
pickling is performed by an .ordinary, method, to remove scales on the surface
of the steel sheet.
[0038] There will be explained a manufacturing method from hot rolling
to pickling when the steel sheet of the present invention is a cold-rolled steel
sheet. It is preferable that the previously described hot-rolled steel sheet
should be pickled to then be subjected to cold rolling to a predetermined sheet
thickness, and then should be heated in such a manner that the maximum
heating temperature becomes a temperature obtained by subtracting 50°C
from an Ar3 point or higher and should be subjected to an annealing process
in which cooling is performed down to a cooling stop temperature of 550°C
or lower from the previously described maximum heating temperature.
[0039] Next, there will be explained skin pass rolling. It is
characterized in that the previously described pickled hot-rolled steel sheet or
cold-rolled steel sheet is subjected to skin pass rolling under the condition that
a reduction ratio is not less than 0.5% nor more than 5% and F/T, being a ratio
of a line load F (kg/mm) of a rolling mill load divided by a sheet width of the
steel sheet and a load T (kg/mm2) per unit area to be applied in the
longitudinal direction of the steel sheet, is 8000 or more.
[0040] The purpose of the previously described skin pass rolling is to
introduce a mobile dislocation to thereby suppress yield elongation, but it was
found that in addition to just'-setting the reduction ratio to a predetermined
value, as long as the condition is set that F/T described previously is 8000 or
more, it is possible to increase the dislocation density of the surface of the
steel sheet and to manufacture the hot-rolled steel sheet or the cold-rolled -
steel sheet in which the dislocation density within 50 urn in the "sheet
18
thickness direction from the surface of the steel sheet is not less than 2.0 times
nor more than 10.0 times as compared to the dislocation density at the
position of 1/4 in the sheet thickness direction. Hereinafter, (the dislocation
density within 50 \xm in the sheet thickness direction from the surface of the
steel sheet)/(the dislocation density at the position of 1/4 in the sheet
thickness direction) is set to a "dislocation density ratio."
[0041] In FIG. 1, there are shown results obtained by examining the
relationship between the skin pass condition F/T and the dislocation density
ratio of hot-rolled steel sheets and cold-rolled steel sheets having components
shown in Table 1. When the skin pass condition F/T was less than 8000, the
dislocation density ratio was less than 2.0. Further, when F/T was not less
than 8000 nor more than 14000, the dislocation density ratio was not less than
2.0 nor more than 10.0. When F/T was greater than 14000, ones each having
the dislocation density ratio of greater than 10.0 appeared. In FIG. 2, there
are shown effects of F/T on the dislocation density at the position of 1/4 sheet
thickness. When F/T exceeded 14000, the dislocation density at the position
of 1/4 sheet thickness increased.
[0042] When F/T is less than 8000, tension in the longitudinal direction
of the steel sheet is strong, and by uniaxial tension stress, a dislocation is
introduced into the whole surface of a cross section in the sheet thickness
direction of the steel sheet, which is not desirable as the manufacturing
method of the steel sheet of the present invention. Incidentally, as a
condition of allowinga dislocation to be introduced only into the surface of
the steel sheet. F/T is preferably 14000 or less. Incidentally, when the
reduction ratio exceeds 5%, the dislocation is introduced down to the center in
the sheet thickness direction, and thereby the ductility decreases. On the
19
other hand, when the reduction ratio is less than 0.5%, it is found that it is not
possible to suppress the. yield elongation and .further it becomes difficult to
stably secure 8000 or more of F/T described previously. Thus, the range of
the reduction ratio is set to 0.5 to 5%. Incidentally, when reduction greater
than 5% is added, it is only necessary to perform an annealing step for
dislocation recovery and to thereafter perform cold rolling at a reduction ratio
of not less than 0.5% nor more than 5%. In this case, when an annealing
temperature is 200°C or lower, the dislocation does not recover, so that the
annealing temperature is preferably 200°C or higher,
[0043] When the steel sheet satisfying the skin pass reduction ratio, F/T,
and the dislocation density ratio is nitriding treated, dislocation is introduced
into the surface, and thereby diffusion of nitrogen during the nitriding
treatment is promoted to make the surface hardening depth after the nitriding
deep. In a nitriding treated steel sheet having this deep surface hardening
depth, a crack initiation life is improved, propagation resistance of fatigue
microcracking is excellent, and not only the fatigue strength but also stress at
which fracture occurs at a predetermined number of cycles, namely fatigue
strength at finite life is improved.
[0044] In FIG. 3, the relationship between, of the present invention, the
dislocation density ratio and the surface hardening depth is shown. When
the dislocation density ratio is 2.0 or less, the. surface hardening depth
decreases prominently. On the other hand, in the present invention range,
the deep surface hardening depth is. ^stably exhibited, and in the
.implementation range, the surface hardening depth is 425 um or more.
-Further, the surface hardening depth is deep by about 50 |im on average with
respect to the case of the dislocatibii density ratio being 2.0 or less. From
20
this result, the surface hardening depth is preferably 425 u,m or more.
Incidentally, the .surface hardening depth is. set to the distance from, the
surface to the position where HV starts to increase with reference to
JIS-G-0557.
[0045] As one evaluation of the fatigue property, the relationship between
the surface hardening depth after the nitriding and a fatigue strength at 105
cycles of the surface of the steel sheet is shown in FIG. 4. Incidentally,
comparative steels are plotted according to the dislocation density ratio falling
within the range of the present invention and the dislocation density ratio
falling outside the range. The relationship between the fatigue strength at
10 cycles of the surface of the steel sheet and the surface hardening depth has
a positive correlation, and when the surface hardening depth is 425 u,m or
more in particular, the fatigue strength at 105 cycles of the surface of the steel
sheet increases prominently with respect to the surface hardening depth. It is
found that when the surface hardening depth becomes 425 u,m or more by the
present invention, the fatigue strength at 10f cycles of the surface of the steel
sheet by the surface hardening depth improves greatly. Further, in each of
the steel sheets of the present invention, appropriate components are selected
and appropriate ranges are set, and thereby the fatigue strength at 105 cycles
of the surface of the steel sheet becomes 400 MPa or more. Incidentally, for
a fatigue test, a Schenck type fatigue test was employed, and stress at which
fracture occurs at 105 cycles, namely the fatigue strength at TO5 cycles was
examined. The frequency of the fatigue "test was set to 25 Hz constantly and
the fatigue test was performed under a test condition of displacement control.
Regarding acceptance or rejection, when the surface hardening depth becomes
425 urn or more, the fatigue strength at 105 cycles of the surface of the steel
21
sheet increases prominently to be 400 cr/MPa or more, so that this is set to a
_._ threshold value.. ;-:
[0046] Next, there will be explained characteristics of an automobile part
obtained by nitriding treating the hot-rolled steel sheet or the cold-rolled steel
sheet of the present invention. The hot-rolled steel sheet or the cold-rolled
steel sheet of the present invention, as described previously, can be formed
into an intended automobile part shape without impairing formability by
dislocation introduction. Here, forming means press - forming or bending
forming after performing shearing. Further, the automobile part is a driving
system part or a structural part formed from the steel sheet. The nitriding
treatment is performed after forming to thereby form a nitrided layer having a
deep surface hardening depth on the surface, and thereby an excellent fatigue
property is exhibited. Further, the end surface roughness at the time of
shearing is decreased, so that the fatigue property of the sheared end surface is
also excellent. As the nitriding treatment, gas nitriding, plasma nitriding,
gas nitrocarburizing, and salt-bath nitrocarburizing can be cited. When the
gas nitriding is performed, for example, the automobile part is retained for 20
hours or longer in an ammonia atmosphere at 540°C. Particularly, as long as
the nitriding treatment is a general gas nitrocarburizing treatment with a N2 +
NH3 + C02 mixed gas at 570°C, for example, the previously described
nitrided layer can be obtained for a treatment time of about five hours or
longer.
[Example] . r :
[0047] I lereinafler, there will be described examples of the present
- invention. — —; - :
: /•--
[0048] [Table 1] :'~\0:WB
25
[0051] Steels of 28 kinds having chemical components shown in Table 1
were melted. Incidentally, Steel types. 1 to 12 are in the component range of
the present invention and Steel types 13 to 28 are comparative components
each deviating from the component of the present invention. Further, C was
excluded from the implementation because the component of less than
0.0002% was melted and an extremely high cost was required. Some of
these steels were each hot rolled to be fabricated into a rough-rolled material
having a sheet thickness of 25 mm by way of trial. The rough-rolled
materials were heated to 1200 to 1250°C to be subjected to finish rolling at a
finish rolling temperature of 950°C to then be cooled at an average cooling
rate of 5°C/s in a cooling zone, and steel sheets were each coiled into a coil
shape at a coiling temperature of 550°C to thereby manufacture steel sheets
each having a sheet thickness of 2.3 mm, and in a 7% hydrochloric acid
aqueous solution, scales on each surface were removed, and under skin pass
conditions in Table 2, rolling was performed and hot-rolled steel sheets for
nitriding were obtained.
Further, hot-rolled steel sheets before skin pass rolling were each
subjected to cold rolling at a cold-rolling ratio of 60%, retained for a
maximum heating temperature retention time of 30 (sec) at a heating rate of
10(°C/sec), subjected to an annealing process in which cooling is performed
down to 550°C to be stopped, and rolled under the skin pass conditions in
Table 2 to manufacture cold-rolled steel sheets for nitriding. In Table 2, Test
numbers 1 to 12 each have the steel sheet component and the manufacturing
condition falling within the ranges, Test numbers 13 to 28 each have cither the
steel sheet component or the manufacturing- condition falling outside the
range, and Test numbers 29 to 33 each have the skin pass rolling condition
falling outside the range.
[0052] . Of the steel sheets..of .all. Test numbers, a full width at half
maximum of X-ray diffraction was measured and a dislocation density was
measured by a Williamson-Hall method. Incidentally, for the full width at
half maximum of an X ray, diffraction peaks of (110), (112), and (220) were
used. Incidentally, in order to measure the dislocation density at the position
of 50 jam from the surface and the dislocation density at the position of 1/4
sheet thickness, a sample having a size of 25 mm length x 15 mm-width was
cut out from each Steel type to be decreased in thickness to a predetermined
measurement position by electropolishing.
[0053] Measurement results are as shown in Table 2, and in Test numbers
1 to 28 falling within the manufacture range of the present invention, the ratio
of the dislocation densities at the position of 50 u.m from the surface and at
the position of 1/4 sheet thickness was not less than 2.0 nor more than 10.0.
In Test number 29 with the skin pass reduction ratio falling below 0.5%, F/T
was 8000 or less, so that the dislocation density ratio fell below 2.0. Further,
in Test number 30, the skin pass reduction ratio was 5% or more and tension
was increased significantly, resulting in that in addition to the dislocation
density at the position of 50 jim from the surface, the dislocation density at
the position of 1/4 sheet thickness increased significantly and the dislocation
density ratio fell below 2.0. Further, in Test number 31, a line load at the
time of skin pass rolling was increased, resulting in that the dislocation
density ratio exceeded 10.0. Incidentally, as compared to Test number 2, the.
dislocation density at the position of 1/4 sheet thickness also increased
prominently. - —_--- - -
[0054] - Next, on all Steel types, a gas riitriding treatment was performed
under the following condition. The condition of the gas nitriding treatment
was set that an atmosphere is a mixed gas of NH3: N2: C02 = 50: 45: 5 in
volume fraction, a temperature is 570°C, and a retention time is five hours.
Tensile strength TS and ductility El before the nitriding treatment were
evaluated in accordance with a test method described in JIS-Z2241 by
fabricating a No. 5 test piece described in JIS-Z2201. Further, burring
formability X before the nitriding was evaluated in accordance with a test
method described in JIS-Z2256. Roughness of a sheared end surface before
the nitriding was measured by using a contact type surface roughness tester
after punching and shearing were performed by using a die having a
cylindrical punch with 10 mm § and 15% of a clearance. Incidentally,
regarding the sheared end surface roughness, a fracture surface was measured
in the sheet thickness direction and average roughness was employed. The
steel sheets of all Test numbers were each worked into a plane test piece
shown in FIG. 5 in order to examine a fatigue property of the surface of the
steel sheet after the nitriding, and were each worked into a test piece shown in
FIG. 6 under the previously described punching condition in order to examine
a fatigue property of the sheared end surface, and nitrided fatigue test pieces
that underwent the nitriding treatment under the previously described
nitriding treatment condition were each fabricated and had the previously
described fatigue test performed thereon. The hardness after the nitriding
treatment was measured in accordance with JIS-Z-2244. Regarding a
measurement place, each lest piece was cut so that its T- cross section could
appear and was polished and HV0.3(2.9N) was measured at intervals of 10
ujtn from 1/4 of the diameter to the surface,
[0055] ^ There are shown material properties before the nitriding treatment
in Table 3.
In terms of comparison of Test numbers .2, 18, and 24 different in the
content of Si, in Test number 18 having the content of Si being greater than
0.5%, the surface hardening depth decreased prominently. Further, in Test
number 24 having the content of Si being less than 0.001%, the surface
hardening depth slightly increased with respect to Test number 2, which was
not a prominent effect. In terms of comparison of Test numbers 2, 20, and
21 different in the content of Mn, in Test number 20 having the content of Mn
being greater than 1.33%, a prominent increase in the sheared end surface
roughness was confirmed. In terms of comparison of the surface hardness of
Test numbers 2, 4, 14, and 15 different in the content of Cr, the hardness after
the nitriding was secured stably in the component range of the present
invention and the hardness hardly changed even though the content of Cr
exceeded 2.0%.
[0056] In terms of comparison of Test numbers 2, 6, 7, 16, and 25
different in the content of Al, in the case of the content of Al being 0.10% or
more, prominent surface hardening was able to be confirmed. Further, when
greater than 0.5% of Al was contained, an increase in the surface hardness
was confirmed, but a prominent decrease in the surface hardening depth was
confirmed. In terms of comparison of Test numbers 2, 3, 13, and 17
different in the content of V,-when V exceeded 0.1%, El (%) being an index of
the ductility decreased prominently. Regarding the surface hardening depth
after the nitriding, whcn_the content of V was 0.05% or more" the surface
hardening depth increased prominently, but when the content of V exceeded
0.10%, the surface hardening depth tended to be saturated, and in Test number -
13, the surface hardening depth rather decreased. Further, it was found that
the present invention steels each contain B to thereby suppress a prominent
increase in.the sheared end surface roughness and are each in an appropriate
range where B is not contained excessively. In terms of comparison of Test
numbers 2, 22, and 26 different in the content of Ti, in Test number 22 having
the content of Ti greater than 0.1%, a prominent increase in the sheared end
surface roughness was confirmed. Further, also in Test number 26 having
the content of Ti being less than 0.005%, a prominent increase in the sheared
end surface roughness was confirmed. In terms of comparison of Test
numbers 2, 23, and 24 different in the content of B, in Test number 23 not
containing B, a prominent increase in the sheared end surface roughness was
confirmed. Further, in Test number 24 containing greater than 0.0015% of B,
an effect of decreasing the sheared end surface roughness equal to or more
than the result of Test number 2 was not confirmed. In Test numbers 1 and 5
each containing Mo and Nb, an improvement of the surface hardness was
confirmed. However, in Test number 27 having the content of Mo being
greater than 0.20%, an improvement of the surface hardness was not
confirmed, and in Test number 28 having the content of Nb being greater than
0.05%, a prominent deterioration of the burring formability A- was confirmed. .
[0057] In Test number 29 having the skin pass reduction ratio of 0.4%,
the dislocation density ratio fell below 2.0, and as compared to the result of
Test number 2 with the same steel sheet number, an effect of improving the
surface hardening depth was not confirmed. Further, in Test number 30, the
reduction ratio was 5.1% and the dislocation density ratio fell below 2.0, and
as compared to the result of Test number 2 with the same steel sheet number,
a prominent decrease-in the ductility was con firmed. Further, in Test number
,31 having the dislocation density; ratio being greater than 10.0, a more
prominent decrease in the ductility was confirmed. Further, in Test numbers
29 to 31, a .decrease in the;.surface, hardening depth was also confirmed. In
Test number 32, the skin pass reduction ratio was in the appropriate range, but
F/T described previously was less than 8000, so that the dislocation density
ratio was less than 2.0. Therefore, the surface hardening depth after the
nitriding in Test number 32 was extremely low as compared to Test number 2.
Further, in Test number 33, F/T described previously and the dislocation
density ratio were satisfied, but the skin pass reduction ratio was 0.4%, so that
it was confirmed that an upper yield point-a lower yield point occurred and
yield elongation was not able to be suppressed.
[0058] Finally, fatigue property results of the steel sheets of the present
invention are shown in Table 3. In each of the steel sheets of the present
invention, the fatigue strength at 105 cycles of the surface of the steel sheet
was 400 MPa or more. Incidentally, in Test number 15, greater than 2.0% of
Cr was contained, and as compared to Test number 4 having the content in the
appropriate range, the previously described fatigue strength rather decreased,
the surface hardness improved but the surface hardening depth decreased, and
the fatigue strength at 105 cycles of the surface of the steel sheet was 400 MPa
or less. Similarly also to Test number 16 having the content of Al being
greater than 0.50% and Test number 13 having the content of V being greater
than 0.10%, the surface hardening depth decreased and the fatigue strength at
105 cycles of the surface of the steel sheet was 400 MPa or less. Further, in
Test number 23 containing "greater than 0.0015% of B, a prominent decrease
in the fatigue strength at 105 cycles of the sheared end surface was able to be
suppressed, but B was contained excessively, so that die fatigue strength al
105 cycles of the surface of the steel sheet was 400 MPa or less. It is
considered that this is ascribable to delay of diffusion of atomic vacancies
caused by B being contained excessively. It was found that the range of the
present invention is set to the appropriate component range, and thereby the
fatigue strength at 105 cycles of the sheared end surface and the fatigue
strength at 105 cycles of the surface of the steel sheet are achieved.
[0059] From the above, it was found that the steel sheet of the present
invention having the appropriate component range and manufactured by the
appropriate manufacturing method is used, thereby making it possible tomake
the surface hardening depth after the nitriding deep and to exhibit an
extremely excellent fatigue property after the nitriding without deteriorating
the formability before the nitriding.
[Name of Document] What is claimed is
[Claim 1 ] . A steel sheet for nitriding excellent in fatigue strength,
comprising:
inmass%,
C: not less than 0.0002% nor more than 0.07%;
Si: not less than 0.0010% nor more than 0.50%;
Mn: not less than 0.10% nor more than 1.33%;
P: not less than 0.003% nor more than 0.02%;
S: not less than 0.001% nor more than 0.02%;
Cr: greater than 0.80% and 1.20% or less;
Al: not less than 0.10% nor more than 0.50%;
V: not less than 0.05% nor more than 0.10%;
Ti: not less than 0.005% nor more than 0.10%;
B: not less than 0.0001 % nor more"than 0.0015%; and
a balance being composed of Fe and inevitable impurities, wherein
a dislocation density within 50 um in the sheet thickness direction
from the surface of the steel sheet is not less than 2.0 times nor more than
10.0 times as compared to a dislocation density at the position of 1/4 in the
sheet thickness direction.
[Claim 2] The steel sheet for nitriding excellent in fatigue strength
according to claim 1, further comprising:
one or both of, in mass%5
Mo: not less than 0.001 nor more than 0.20%; and
Nb: not less than 0.001 nor more than 0.050%.
[Claim3] -A manufacturing method of a hot-rolled steel sheet-for
nitriding excellent in fatigue strength, comprising:
on a steel billet containing, in mass%, C of not less than 0.0002% nor
more than 0.07%, Si of not less than.0.0010% nor more than 0.50%, Mn of
not less than 0.10% nor more than 1.33%, P of not less than 0.003% nor more
than 0.02%, S of not less than 0.001% nor more than 0.02%, Cr of greater
than 0.80% and 1.20% or less, Al of not less than 0.10% nor more than 0.50%,
V of not less than 0.05% nor more than 0.10%, Ti of not less than 0.005% nor
more than 0.10%, B of not less than 0.0001% nor more than 0.0015%, and a
balance being composed of Fe and inevitable impurities, performing hot
rolling; performing pickling; and then performing skin pass rolling under the
condition that a reduction ratio is 0.5 to 5.0% and F/T (mm), being a ratio of a
line load F (kg/mm) of a rolling mill load divided by a sheet width of the steel
sheet and a load T (kg/mm2) per unit area to be applied in the longitudinal
direction of the steel sheet, is 8000 or more.
[Claim 4] A manufacturing method of a cold-rolled steel sheet for
nitriding excellent in fatigue strength, comprising:
on a steel billet containing, in mass%, C of not less than 0.0002% nor
more than 0.07%, Si of not less than 0.0010% nor more than 0.50%, Mn of
not less than 0.10% nor more than 1.33%, P of not less than 0.003% nor more
than 0.02%, S of not less than 0.001% nor more than 0.02%, Cr of greater
than 0.80% and 1.20% or less, Al of not less than 0.10% nor more than 0.50%,
V of not less than 0.05% nor more than 0.10%, Ti of not less than 0.005% nor.
more than 0.10%, B of not less than 0.0001% nor more than 0.0015%, and a
balance being composed of Fe and inevitable impurities, performing hot
rolling: performing pickling, cold rolling, and annealing; and then performing
skin pass rolling under the condition that a reduction ratio is 0.5 to 5.0% and
F/T (mm), being a ratio of a line load F (kg/mm) of a rolling mill load divided
^by a sheet width of-the steel sheet and a load T (kg/mm2) per unit area to be
iappjied in the longitudinal direction_of the st_e.el sheet, is. 8000 or more,
[Claim 5] -;-;-;. An automobile part excellent in fatigue strength, wherein
a steel sheet that contains, in mass%, C of not less than 0.0002% nor
more than 6.07%, Si of not less than 0.0010% nor more than 0.50%, Mn of
not less than 0.10% nor more than 1.33%, P of not less than 0.003% nor more
than 0.02%, S of.not.less than 0.001% nor more than 0.02%, Cr of greater
than 0.'80% and 1.20% or less, Al of not less than 0.10% nor more than 0.50%,
V of not less than 0.05% nor more than 0.10%, Ti of not less than 0.005% nor
more than 0.10%, B of not less than 0.0001% nor more than 0.0015%, and a
balance being composed of Fe and. inevitable impurities and in which a
dislocation density within 50 urn in the sheet thickness direction from the
surface of the steel sheet is not less than 2.0 times nor more than 10.0 times as
compared to a dislocation "density at the position of 1/4 in the sheet thickness
direction is formed to then be nitriding treated.