[Designation of Document] SPECIFICATION
[Title of the Invention] HOT-ROLLED STEEL SHEET FOR GAS
NITROCARBURIZING AND MANUFACTURING METHOD THEREOF
[Technical Field]
[OOO 11
The present invention relates to a hot-rolled steel sheet for gas
nitrocarburizing having improved isotropic workability and a manufacturing method
thereof. Priority is claimed on Japanese Patent Application No. 20 1 1-08949 1, filed
on April 13,201 1, and the contents of which are incorporated herein by reference.
[Background Art]
[0002]
Recently, in order to achieve a weight saiving of various members for
improving fuel consumption of an automobile, thinning by high-strengthening of a
steel sheet such as an iron alloy or application of a light metal such as A1 alloy has
been developed. Compared to a heavy metal such as steel, the light metal such as Al
alloy has an advantage such as having high specific strength, but there is a
disadvantage such as having significantly high costs. Thereby, the application of the
light metal is limited to a specific use. Accordingly, in order to promote weight
reduction of various members at lower cost and in a wider range, the thinning by highstrengthening
of the steel sheet is needed.
[0003]
In general, due to the high-strengthening of the steel sheet, deterioration of
material characteristics such as formability (workability) is accompanied. Thereby,
improvement of the high-strengthening without deterioration of the material
characteristics is important in the development of a high-strength steel sheet.
Particularly, a steel sheet, which is used as a vehicle member such as an inner sheet
member, a structural member, a suspension member, or a transmission, requires
bendability, stretch-flange workability, burring workability, ductility, fatigue durability,
impact resistance (toughness), corrosion resistance, or the like according to the use.
Accordingly, having an improved balance of material characteristics at a high level and
high standard is important.
[0004]
Particularly, in automobile parts, a part in which a sheet metal is processed as
a material and functions as a rotating body, for example, a drum, a carrier, or the like
configuring an automatic transmission is an important part which transmits engine
output to an axle shaft. The part requires circularity or uniformity of a sheet thickness
in a circumferential direction as a shape for decreasing friction or the like. In addition,
since a forming type such as burring processing, drawing, ironing, or stretch forming is
used when the part is formed, ultimate deformability which is represented by local
elongation is significantly important.
[OOOS]
Moreover, it is preferable to improve impact resistance, that is, toughness in
the steel sheet used for the member, in which the impact resistance is a characteristic in
which the member is not easily broken even though the member receives impact due to
collision or the like after the formed member is mounted to an automobile as a part of
the automobile. Particularly, when use of the member under a cold climate is
considered, it is preferable to improve the toughness at low temperature (lowtemperature
toughness) in order to secure the impact resistance at low temperature.
Thereby, it is important to increase the impact resistance of the steel. In addition, the
impact resistance (toughness) is defined by vTrs (Charpy fracture appearance transition
temperature) or the like.
That is, in a steel sheet for a part including the above-described part which
requires uniformity of a sheet thickness, satisfying both of plastic isotropy and impact
resistance (toughness) is required in addition to improve/d workability.
[0006]
For example, in Patent Document 1, in order to satisfy both of high strength
and various material characteristics which particularly contribute to formability, a
manufacturing method of the steel sheet, which satisfies high strength, ductility, and
hole expansibility by including a steel structure which has ferrite of 90% or more and
the balance consisting of bainite, is disclosed.
However, in the steel sheet which is manufactured by applying the technique
disclosed in Patent Document 1, the plastic isotropy is not disclosed at all. Thereby,
for example, if it is assumed that the steel sheet of Patent Document 1 is applied to a
part such as a gear which requires circularity or uniformity of the sheet thickness in the
circumferential direction, unfair vibration due to eccentricity of the part or a decrease
in the output due to friction loss is concerned.
[0007]
Moreover, for example, in Patent Documents 2 and 3, a hot-rolled high tensile
steel sheet, which has high strength and improved stretch flangeability by adding Mo
and refining precipitates, is disclosed.
However, in the steel sheet to which the above-described technique disclosed
in Patent Documents 2 and 3 is applied, since it is essential to add Mo, which is an
expensive alloy element, by 0.07% or more, there is a problem that the manufacturing
costs are increased. Moreover, in the technique disclosed in Patent Documents 2 and
3, the plastic isotropy is not disclosed at all. Thereby, if it is assumed that the steel
sheet of Patent Documents 2 and 3 is applied to a part which requires circularity or
uniformity of the sheet thickness in the circumferential direction, unfair vibration due
to eccentricity of the part or a decrease in the output due to friction loss is concerned.
[0008]
On the other hand, for example, in Patent Document 4, with respect to
improvement in plastic isotropy of the steel sheet, that is, a decrease of the plastic
anisotropy, a technique is disclosed which makes texture at austenite of a surface shear
layer be adequate by combining endless rolling and lubricant rolling and decreases inplane
anisotropy of a r value (Lankford value).
However, the endless rolling is needed for preventing defective biting caused
by slip between a roll caliber tool and a rolled material during rolling in order to
perform the lubricant rolling having a small friction coefficient over the full length of a
coil. Thereby, since equipment investment such as a rough bar joining device or a
high-speed crop shear is accompanied to apply the technique of Patent Document 4, a
burden is large.
[0009]
In addition, for example, in Patent Document 5, a technique is disclosed
which satisfies both of stretch flangeability and deep drawability by decreasing
anisotropy of a r value in a steel sheet having strength level of 780 MPa or more which
is obtained by compositely adding Zr, Ti, and Mo and ending finish rolling at high
temperature of 950°C or more.
However, since adding Mo, which is an expensive alloy element, of 0.1% or
more is essential, there is a problem that the manufacturing costs are increased.
[OO 1 01
Research for improving toughness of a steel sheet has been advanced than
conventional. However, a hot-rolled steel sheet for gas nitrocarburizing having high
strength, improved plastic isotropy and toughness is not disclosed in the abovedescribed
Patent Documents 1 to 5.
[Prior Art Document]
[Patent Document]
[OO 1 11
[Patent Document I] Japanese Unexamined Patent Application, First
Publication No. H6-293910
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2002-322540
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2002-32254 1
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. H10-183255
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. 2006-124789
[Disclosure of the Invention]
[Problem to be solved by the Invention]
[OO 121
The present invention is made in consideration of the above-described
problems. That is, an object of the present invention is to provide a hot-rolled steel
sheet for gas nitrocarburizing which has a high strength of 440 MPa or more in tensile
strength, can be applied to a member which requires ductility and strict uniformity of a
sheet thickness, circularity, and impact resistance after processing, has improved
isotropic workability (isotropy) and hole expansibility, and exhibits sufficient chipping
resistance and rolling fatigue resistance after gas nitrocarburizing treatment, and a
manufacturing method which can inexpensively and stably manufacture the steel sheet.
[Means for Solving the Problems]
[00 131
In order to solve the above-described problems and achieve the related object,
the present invention adopts the following measures.
[00 141
(I) According to an aspect of the present invention, there is provided a hotrolled
steel sheet, by mass%, C content [C]: C of more than 0.07% and equal to or less
than 0.2%, Si content [Si]: Si of 0.001% or more and 2.5% or less, Mn content [Mn]:
Mn of 0.01% or more and 4% or less, and A1 content [All: A1 of 0.001% or more and
2% or less, P content [PI limited to 0.15% or less, S content [S] limited to 0.03% or
less, and N content [N] limited to 0.01% or less, Ti content [Ti] which satisfies the
following Equation (a), the balance consisting of Fe and unavoidable impurities, in
which an average pole density of an orientation group of {100)<011> to (22314 lo>,
which is represented by an arithmetic average of pole density of each orientation of
{100}<011>, {116)<110>, {114)<110>, {112)<110>, and {223)<110> is 1.0 or more
and 4.0 or less, a pole density of a crystal orientation of (3321-4 13> is 1 .O or more and
4.8 or less, in a center portion of a sheet thickness which is a range of the sheet
thickness of 518 to 318 from a surface of the steel sheet, and in which an average grain
size in a center in the sheet thickness is lOpm or less; and a microstructure includes, by
a structural fraction, pearlite of more than 6% and ferrite in the balance.
0.005 + Ir\r] x 48 I 14 + [S] x 48 / 32 5 Ti 5 0.015 + [N] x 48 / 14 + [S] x 48 /
32 . .. (a)
[00 1 51
(2) In the hot-rolled steel sheet for gas nitrocarburizing according to (I), the
average pole density of the orientation group of {100)<01l> to {223)<110> may be
2.0 or less and the pole density of the crystal orientation of {332)<113> may be 3.0 or
less.
[00 1 61
(3) In the hot-rolled steel sheet for gas nitrocarburizing according to (I), the
average grain size may be 7 pm or less.
[00 1 71
(4) The hot-rolled steel sheet for gas nitrocarburizing according to any one of
(1) to (3), may further include any one or two or more of, by mass%, Nb content [Nb]:
Nb of 0.005% or more and 0.06% or less, Cu content [Cu]: Cu of 0.02% or more and
1.2% or less, Ni content [Nil: Ni of 0.01% or more and 0.6% or less, Mo content [Mo]:
Mo of 0.01% or more and 1% or less, V content [V]: V of 0.01% or more and 0.2% or
less, Cr content [Cr]: Cr of 0.01% or more and 2% or less, Mg content [Mg]: Mg of
0.0005% or more and 0.01% or less, Ca content [Ca]: Ca of 0.0005% or more and
0.01% or less, REM content [REM]: REM of 0.0005% or more and 0.1% or less, and
B content [B]: B of 0.0002% or more and 0.002% or less.
[00 1 81
(5) According to another aspect of the present invention, there is provided a
manufacturing method of a hot-rolled steel sheet for gas nitrocarburizing, including:
performing a first hot rolling, which includes one of more of rolling reduction having a
rolling-reduction ratio of 40% or more at a temperature range of 1000°C or more and
1200°C or less, with respect to a steel ingot or a slab which includes, by mass%, C
content [C]: C of more than 0.07% and equal to or less than 0.2%, Si content [Si]: Si of
0.001% or more and 2.5% or less, Mn content [Mn]: Mn of 0.01% or more and 4% or
less, and A1 content [All: A1 of 0.001% or more and 2% or less, and P content [PI
limited to 0.15% or less, S content [S] limited to 0.03% or less, and N content [N]
limited to 0.01% or less, Ti content [Ti] contains Ti which satisfies the following
Equation (a), and the balance consists of Fe and unavoidable impurities; starting a
second hot rolling at a temperature range of 1000°C or more within 150 seconds after a
completion of the first hot rolling, performing rolling includes one or more of rolling
reduction having a rolling-reduction ratio of 30% or more in a temperature range of Tl
+ 30°C or more and TI + 200°C or less when temperature determined by a component
of the steel sheet in the following Equation (b) is defined as TI°C in the second hot
rolling and a total of the rolling-reduction ratio is 50% or more; performing a third hot
rolling, in which a total of the rolling-reduction ratio is 30% or less, at a temperature
range equal to or more than an Ar3 transformation point temperature and less than T1
+ 30°C; ending the hot rollings at the Ar3 transformation point temperature or more;
when a pass having rolling-reduction ratio of 30% or more at the temperature range of
TI + 30°C or more and T1 + 200°C or less is a large rolling-reduction pass, performing
a cooling, in which a cooling temperature change is 40°C or more and 140°C or less
and a cooling end temperature is T1 + 100°C or less, at a cooling rate of 50°C/second
or more so that a waiting time t second from a completion of a final pass of the large
rolling-reduction passes to a start of cooling satisfies the following Equation (c); and
coiling the steel sheet at more than 550°C.
0.005+[N] x 4 8 / 1 4 + [ S ] x 4 8 / 3 2 I T i 1 0 , 0 1 5 + ~ ] x 4 8 / 1 4 + [ S ] x 4 8 /
32 . .. (a)
T1 =850+ 10 x ([C] + [N]) x [Mn] +350 x [Nb] +250 x [Ti] +40 x [B]+
10 x [Cr]+ 100 x [Mo] + 100 x [V] ... (b)
t 52.5 x tl ... (c)
Here, tl is represented by the following Equation (d).
tl =0.001 x ((Tf-TI) x PI /loo)*-0.109 x ((Tf-T1) x P1 / 100)+3.1 ... (d)
Here, Tf is a temperature (OC) after the final pass rolling reduction of the large
rolling-reduction passes and PI is a rolling-reduction ratio (%) of the final pass of the
large rolling-reduction passes.
[00 1 91
(6) In the manufacturing method of a hot-rolled steel sheet for gas
nitrocarburizing according to (9, the primary cooling may perform cooling between
rolling stands.
[0020]
(7) In the manufacturing method of a hot-rolled steel sheet for gas
nitrocarburizing according to (5) or (6), the waiting time t second may further satisfy
the following Equation (e).
tl I t I 2 . 5 x tl ... (e)
[002 11
(8) In the manufacturing method of a hot-rolled steel sheet for gas
nitrocarburizing according to (5) or (6), the waiting time t second may further satisfy
the following Equation (f).
t 5 tl ... (f)
[0022]
(9) In the manufacturing method of a hot-rolled steel sheet for gas
nitrocarburizing according to any one of (5) to (8), a temperature increase between the
respective passes in the second hot rolling may be 18°C or less.
[0023]
(10) In the manufacturing method of a hot-rolled steel sheet for gas
nitrocarburizing according to (9), the slab or the steel ingot may further include any
one or two or more of, by mass%, Nb content [Nb]: Nb of 0.005% or more and 0.06%
or less, Cu content [Cu]: Cu of 0.02% or more and 1.2% or less, Ni content [Nil: Ni of
0.0 1% or more and 0.6% or less, Mo content [Mo]: Mo of 0.01% or more and 1% or
less, V content [V]: V of 0.01% or more and 0.2% or less, Cr content [Cr]: Cr of 0.01%
or more and 2% or less, Mg content [Mg]: Mg of 0.0005% or more and 0.01% or less,
Ca content [Ca]: Ca of 0.0005% or more and 0.01% or less, REM content [REM]:
REM of 0.0005% or more and 0.1% or less, and B content [B]: B of 0.0002% or more
and 0.002% or less.
[0024]
(1 1) In the manufacturing method of a hot-rolled steel sheet for gas
nitrocarburizing according to any one of (5) to (8), the slab or the steel ingot may
further include any one kind or two or more kinds of, by mass%, Nb content [Nb]: Nb
of 0.005% or more and 0.06% or less, Cu content [Cu]: Cu of 0.02% or more and 1.2%
or less, Ni content [Nil: Ni of 0.01% or more and 0.6% or less, Mo content [Mo]: Mo
of 0.01% or more and 1% or less, V content [V]: V of 0.01% or more and 0.2% or less,
Cr content [Cr]: Cr of 0.01% or more and 2% or less, Mg content [Mg]: Mg of
0.0005% or more and 0.01% or less, Ca content [Ca]: Ca of 0.0005% or more and
0.01% or less, REM content [REM]: REM of 0.0005% or more and 0.1% or less, and
B content [B]: B of 0.0002% or more and 0.002% or less.
[Advantage of the Invention]
[0025]
According to the present invention, a high strength hot-rolled steel sheet for
gas nitrocarburizing which can be applied to a member which requires ductility and
strict uniformity of a sheet thickness, circularity, and impact resistance after processing
and has improved isotropic workability, hole expansibility, and toughness, is obtained.
In addition, the above-described hot-rolled steel sheet for gas nitrocarburizing can be
inexpensively and stably manufactured. Therefore, the present invention has a high
industrial value.
[Brief Description of the Drawing]
[0026]
FIG. 1 is a view showing a relationship between average pole density of an
orientation group of (100)<011> to (223)<110> and isotropy.
FIG. 2 is a view showing a relationship between a pole density of a crystal
orientation of (3321-4 13> and isotropy.
FIG. 3 is a flowchart showing a manufacturing method of a hot-rolled steel
sheet according to the present embodiment.
[Best Mode for Carrying Out the Invention]
[0027]
Hereinafter, an embodiment of the present invention will be described in
detail. Moreover, hereinafter, mass% in a composition is simply described as %.
Moreover, in the present embodiment, a hot-rolled steel sheet for gas nitrocarburizing
having improved isotropic workability may be simply referred to as a hot-rolled steel
sheet.
[0028]
The inventors have diligently repeated research to satisfy both of isotropy and
impact resistance in addition to workability with respect to a hot-rolled steel sheet for
gas nitrocarburizing which is suitably applied to a member which requires ductility and
strict uniformity of a sheet thickness, circularity, and impact resistance after processing.
In addition, in the hot-rolled steel sheet for gas nitrocarbuirzing, it is assumed
that gas nitrocarburizing treatment is performed when the steel sheet is used as a part.
Therefore, not only toughness of an original sheet (a hot-rolled steel sheet in which the
gas nitrocarburizing treatment is not performed) but also sufficient impact resistance
(toughness) after the gas nitrocarburizing treatment (may be simply referred to as after
nitriding treatment) are required. In general, due to influences such as a compound
phase generated on a surface, in the hot-rolled steel after the gas nitrocarburizing
treatment, compared to the hot-rolled steel sheet before the gas nitrocarburizing
treatment, impact resistance is deteriorated. In the hot-rolled steel sheet according to
the present embodiment, by setting the toughness of the original sheet to be greater
than or equal to a target value and controlling a nitride layer, it is investigated that the
toughness of the hot-rolled steel sheet after the gas nitrocarburizing treatment is also
set to be a target value or more.
In addition, in the present embodiment, a case, which is simply referred to as
impact resistance or toughness, indicates impact resistance or toughness of both of the
original sheet and the sheet after nitriding treatment.
As a result of the investigation, the following new findings are obtained.
[0029]
In order to improve isotropy (decrease anisotropy), avoiding formation of
transformation texture from non-recrystallization austenite which is a cause of the
anisotropy is effective. Thus, it is preferable to promote recrystallization of austenite
after finish rolling. In addition, as the measures for the promotion, an optimum
rolling pass schedule at the finish rolling and an increase of rolling temperature are
effective.
[0030]
On the other hand, also before the nitriding treatment and after the nitriding
treatment, in order to improve impact resistance (toughness), refining of a fracture unit
of a brittle fracture face, that is, grain refining of a microstructure unit is effective.
For the grain refining, increasing a nucleation site of a at the time of transformation of
y (austenite) + a (ferrite) is effective. Accordingly, it is preferable to increase grain
boundaries or dislocation density of the austenite which can be the nucleation site. In
order to increase the grain boundaries or the dislocation density, it is preferable that the
rolling is performed at greater than or equal to y+a transformation point temperature
and at temperature as low as possible. In other words, it is preferable to perform the
y+a transformation in a state where austenite is non-recrystallized and anonrecrystallization
ratio is high. This is because growth of austenite grains after the
recrystallization is fast at recrystallization temperature, and thus, the austenite grains
coarsen for a very short time and grain coarsening occurs even at a phase after the
y+a transformation.
[003 11
The inventors considered that it was difficult to satisfy both the isotropy and
the toughness since preferable conditions are contrary to each other in the abovedescribed
general hot rolling measures. Whereas, the inventors found a new hot
rolling method capable of obtaining a steel sheet which balances the isotropy and the
impact resistance in a high standard.
[0032]
The inventors obtain the following findings with respect to a relationship
between the isotropy and the texture.
When a steel sheet is processed to a part which requires circularity or
uniformity of a sheet thickness in a circumferential direction, in order to obtain the
uniformity of the sheet thickness and the circularity which satisfy characteristics of a
part as processed by omitting a process of trimming or cutting, it is preferable that an
isotropy index lllAr( which is an index of the isotropy is 3.5 or more. As shown in
FIG. 1, in order to make the isotropy index be 3.5 or more, average pole density of a
orientation group of { 100)<011> to {223)<110> in a center portion of a sheet
thickness which is a range of the sheet thickness of 518 to 318 from a surface of the
steel sheet is 4.0 or less in the texture of the steel sheet. If the average pole density is
more than 4.0, anisotropy becomes significantly strong. On the other hand, the
average pole density is less than 1.0, there is a concern that hole expansibility is
deteriorated due to deterioration of local deformability. In order to obtain further
improved isotropy index 6.0, it is more preferable that the average pole density of the
orientation group of {100)<011> to (2231-4 10> be 2.0 or less. When the isotropy is
6.0 or more, even in a case where dispersion in a coil is considered, the uniformity of
the sheet thickness and the circularity, which sufficiently satisfy part characteristics as
processed, are obtained. Here, the average pole density of the orientation group of
{ 100)<0 1 1> to {223)<110> is an orientation group which is represented by an
arithmetic average of each orientation of { 100)<01 1>, (1 16)<1 lo>, (1 14)<1 lo>,
(1 1214 lo>, and (2231-4 lo>. Therefore, the average pole density of the orientation
group of { 100)<011> to {223)<110> can be obtained by arithmetically averaging the
pole density of each orientation of {100)<011>, (1 16)<1 lo>, (1 14)<1 lo>,
{112)<110>, and {223)<110>.
[0033]
The isotropy index is obtained according to a test method described in JIS Z
2241 by processing No. 5 test piece described in JIS Z 2201 and testing. In 11lArl
which is the isotropy index, if plastic strain ratios (r values) of a rolling direction, and
45" direction and 90" direction (sheet width direction) with respect to the rolling
direction are defined as rO, r45, and 190 respectively, IArl is defined as Ar = (rO - 2 x
r45 + r90) 12. Moreover, 1Ar1 indicates an absolute value of Ar.
[0034]
The pole density of each orientation is measured using a method such as
Electron Back Scattering Diffraction Pattern (EBSP method). Specifically, the pole
density may be obtained from a three-dimensional texture which is calculated by a
vector method based on a (1 10) pole figure or a three-dimensional texture which is
calculated by a series expansion method using a plurality of pole figures (preferably,
three or more pole figures) of { 1 OO), { 1 101, (2 1 1 ), and (3 10) pole figures.
[003 51
Similarly, as shown in FIG. 2, in order to make the isotropy index l/lArl be 3.5
or more, the pole density of the crystal orientation of (33214 13> in the center portion
of the sheet thickness which is a range of the sheet thickness of 518 to 318 from a
surface of the steel sheet is set to 4.8 or less in the texture of the steel sheet. If the
pole density is more than 4.8, anisotropy becomes significantly strong. On the other
hand, the pole density is less than 1 .O, there is a concern that hole expansibility is
deteriorated due to deterioration of the local deformability. In order to obtain 6.0 or
more which is further improved isotropy index, it is more preferable that the pole
density of the crystal orientation of {332)<113> is 3.0 or less. When the value of the
isotropy index is 6.0 or more, even in a case where dispersion in a coil is considered,
since the uniformity of the sheet thickness and the circularity, which sufficiently satisfy
part characteristics as processed, are obtained, it is more preferable that the value of the
isotropy index is 6.0 or more.
In addition, the average pole density of the orientation group of {100)<011>
to (2231-4 1 O> and the pole density of the crystal orientation of (33214 13> are
increased in a case of intentionally making a ratio of grains toward the crystal
orientation be higher than other orientations.
In addition, if the average pole density and the pole density are decreased,
workability such as the hole expansibility is improved. In addition, it is preferable
that the hole expansibility is 70% or more.
[0036]
The above-described pole density is synonymous with an X-ray random
intensity ratio. The X-ray random intensity ratio is a value which is obtained by
measuring X-ray intensity of a standard sample which does not have integration in a
specific orientation and a sample material in the same conditions by X-ray diffraction
method or the like, and by dividing the X-ray intensity of the standard sample by the
obtained X-ray intensity of the sample material. The pole density can be measured by
any method of an X-ray diffraction, an EBSP method, or an Electron Channeling
Pattern (ECP) method. For example, the pole density of the orientation group
{100)<011> to {223)<110> is obtained by obtaining the pole density of each
orientation of {100)<011>, {116)<110>, {114)<110>, {112)<110>, and {223)<110>
from the three-dimensional texture (ODF) which is calculated by a series expansion
method using a plurality of pole figures of { 1 lo), { loo), (21 1 ), and (3 10) pole
figures measured by the above-described methods, and by arithmetically averaging the
pole density. To perpare the sample which is supplied to the EBSP or the like, the
thickness of the steel sheet is decreased to a predetermined sheet thickness fiom the
surface by mechanical polishing or the like. Subsequently, strain is removed by
chemical polishing, electrolytic polishing, or the like, and the sample may be adjusted
and measured according to the above-described methods so that a proper surface at the
range of 518 to 318 of the sheet thickness is the measurement surface. In a sheet width
direction, it is preferable that the sample is collected at a position of 114 or 314 from an
end of the steel sheet. In addition, the pole density is not changed before and after the
gas nitrocarburizing treatment.
[003 71
Of course, when the above-described limitation of the pole density satisfies
not only the center portion of the sheet thickness but also thickness, as much as
possible, the local deformability is hrther improved. However, since the orientation
integration in the sheet thickness of 318 to 5/8 from the surface of the steel sheet most
largely influences the anisotropy of a product, performing the measurement of the
center portion of the sheet thickness which is the range of the sheet thickness of 518 to
318 fiom the surface of the steel sheet can approximately represent material
characteristics of the entire steel sheet. Therefore, the average pole density of the
orientation group of {100)<011> to {223)<110> and the pole density of the crystal
orientation of {332)<113>, in the center portion of the sheet thickness which is the
range of the sheet thickness of 5/23 to 3/8 from the surface of the steel sheet, are defined.
[003 81
Here, {hkl) indicates that a normal direction of the sheet surface is
parallel to {hkl) and the rolling direction is parallel to when the sample is
collected by the above-described method. In addition, generally, in the orientation of
the crystal, an orientation perpendicular to the sheet surface is represented by [hkl] or
{hkl) and an orientation parallel in the rolling direction is represented by (uvw) or
. {hkl) and are collective terms of equivalent planes, and [hkl] and
(uvw) indicate respective crystal planes. That is, for example, since the present
embodiment has a body-centered cubic structure as a target, (I 1 I), (-1 1 I), (1-1 I), (1 1 -
I), (-1-11), (-11-I), (1-1-I), and (-1-1-1) planes are equivalent and are not classified.
In this case, the orientation is referred to as (1 11) as the collective term. In the ODF
display, since the orientation of the crystal is used for orientation displays of other
crystal structures having low symmetry, generally, each orientation is represented by
[hkl](uvw). However, in the present embodiment, [hkl](uvw) and {hkl) are
synonymous with each other.
[0039]
Next, the inventors examine impact resistance (toughness).
The temperature of vTrs of the original sheet and vTrs after nitriding
treatment is decreased with decreases in the average grain sizes. That is, toughness is
improved. Moreover, the vTrs after nitriding is affected by a pearlite fraction or the
like in addition to the average grain size. In the hot-rolled steel sheet according to the
present embodiment, when the vTrs after nitriding is -20°C or less which is a
temperature capable of enduring as a nitrided part under a cold climate, it is found that
the hot-rolled steel sheet preferabry includes a composition range described in the
present embodiment, in the hot-rolled steel sheet in which the pearlite fraction is
preferabry 6% or more, and the average grain size in the center portion of the sheet
thickness is preferably 10 pm or less. In addition, when it is assumed that the steel
sheet is used in a strict environment and thus, the vTrs after nitriding is -40°C or less, it
is preferable that the average grain size in the center portion of the sheet thickness be 7
pm or less.
The impact resistance (toughness) is evaluated by vTrs (Charpy fracture
appearance transition temperature) which is obtained by V notch Charpy impact test.
Here, in the V notch Charpy impact test, a test piece is manufactured based on JIS Z
2202, the Charpy impact test is performed to the test piece according to the content
defined in JIS Z 2242, and thus, the vTrs is measured.
[0040]
As described above, the average grain size in the center portion of the sheet
thickness of the structure largely influences the impact resistance (toughness). The
measurement of the average grain size in the center portion of the sheet thickness is
performed as follows. A micro-sample is cut from near the center portion in the sheet
thickness direction of the steel sheet, and grain sizes are measured using an EBSP-OIM
(registered trademark) (Electron Back Scatter Diffraction Pattern-Orientation Image
Microscopy). The micro-sample is ground for 30 to 60 minutes using colloidal silica
abrasives, and the EBSP measurement is performed under a measurement condition of
a magnification of 400, an area of 160 pm x256 pm, and a measurement step of 0.5
CLm.
[004 11
The EBSP-OIM (registered trademark) method measures the crystal
orientation of an irradiation point for a short waiting time by radiating electron beams
to a largely inclined sample in a scanning electron microscope (SEM), photographing a
Kikuchi pattern, which is backscattered and formed, by a high sensitive camera, and by
performing a computer image processing to the pattern.
In the EBSP method, a microstructure of and the crystal orientation of a bulk
sample surface can be quantitatively analyzed, and an analysis area can be analyzed by
resolution of the SEM or resolution of minimum 20 nm in an area which can be also
observed by the SEM. The analysis is performed by mapping the area to be analyzed
according to tens of thousands of points in a grid shape with equal intervals for several
hours. In a polycrystalline material, the crystal orientation distribution or sizes of the
grains in the sample can be viewed.
100421
In the present embodiment, 15", which is a threshold of a high angle grain
boundary which is generally recognized as a grain boundary in orientation differences
of the grains, is defined as a grain boundary, and the average grain size is obtained by
visualizing the grains from the mapped image. That is, the "average grain size" is a
value which can be obtained by EBSP-OIM (registered trademark).
[0043]
As described above, the inventors clarified each condition for obtaining the
isotropy and the impact resistance.
That is, the average grain size, which is directly related to the impact
resistance, is decreased with a decrease of finish rolling ending temperature.
However, the average pole density of the orientation group of {100)<011> to
{223)<110> which is represented by an arithmetic average of the pole density of each
orientation of {100)<011>, {116)<110>, {114)<110>, {112)<110>, and {223)<110>,
and the pole density of the crystal orientation of {332)<113>, in a center portion of the
sheet thickness which is a range of the sheet thickness of 518 to 318 from the surface of
the steel sheet, which are controlling factors of the isotropy, have a reverse correlation
with the average grain size with respect to the finish rolling temperature. Thereby, a
technique which satisfies both the isotropy and the impact resistance has not been
shown at all until now.
[0044]
Thus, the inventors searched hot rolling methods and conditions which
simultaneously improve the isotropy and the impact resistance by sufficiently
recrystallizing austenite after the finish rolling for the isotropy and suppressing growth
of the recrystallized grains as much as possible.
[0045]
In order to recrystallize the austenite grains which become a worked structure
by the rolling, it is preferable that the finish rolling is performed at an optimum
temperature range and by a large rolling-reduction ratio of 50% or more in total. On
the other hand, in order to perform grain refining to the microstructure of a product
sheet, it is preferable to suppress the grain growth after the recrystallization of
austenite grains as much as possible by starting cooling of the sheet within a fixed
period of time after the finish rolling ends.
[0046]
Thus, temperature which is determined by the component of the steel sheet
represented by the above-described Equation (b) is Tl("C), the hot rolling of total
rolling-reduction ratio R is performed at a temperature range of TI + 30°C or more and
TI + 200°C or less, and a waiting time t second until cooling, in which cooling
temperature change is 40°C or more and 140°C or less by a cooling rate of
50°C/second or more and the cooling ending temperature becomes TI + 100°C or less,
is performed from the hot rolling ending is obtained. In addition, a relationship
between the waiting time and "the average pole density of the orientation group of
{100)<01 I> to {223)<110> in a center portion of the sheet thickness which is the
range of the sheet thickness of 518 to 318 from the surface of the steel sheet in the
texture of the steel sheet and the average grain size at the center of the sheet thickness",
which are requirements of the hot-rolled steel sheet according to the present
embodiment, is examined. In addition, al1.R is 50% or more. The total rollingreduction
ratio (total of the rolling-reduction ratios) is synonymous with a so-called
accumulated rolling-reduction ratio, and is a percentage of accumulated rollingreduction
ratio (a difference between the inlet sheet thickness before the initial pass in
the rolling at each temperature range and an outlet sheet thickness after the final pass
in the rolling at each temperature range) with respect to a reference based on an inlet
sheet thickness before an initial pass in the rolling at each temperature range.
[0047]
As represented by the above-described Equation (c), when the waiting time t
until performing of the cooling by the cooling rate of 50°C/second or more after
ending the hot rolling of the total rolling-reduction ratio R in the temperature range of
TI + 30°C or more and TI + 200°C or less is within tl x 2.5 seconds, in a case where
the cooling temperature change is 40°C or more and 140°C or less and the cooling
ending temperature is TI + 100°C or less, "the average pole density of the orientation
group of { 100)<011> to {223)<110> is 1.0 or more and 4.0 or less and the pole
density of the crystal orientation of (332)<113> is 1.0 or more and 4.8 or less, in the
texture of the steel sheet, and in the center portion of the sheet thickness which is the
range of the sheet thickness of 518 to 318 from the surface of the steel sheet", and "the
average grain size at the center in the sheet thickness is lOpm or less" are satisfied.
That is, it is considered that the isotropy and the impact resistance, which are the object
of the present embodiment, are satisfied.
This indicates that the range which improves both the isotropy and the impact
resistance, that is, the range, which satisfies both sufficient recrystallization and grain
refining of the austenite, can be achieved by a hot rolling method which is specified by
the present embodiment described in detail below.
In addition, when the average grain size is 7pm or less with an object of
further improving the toughness, it is found that the waiting time t second is preferably
less than tl, and when the average pole density of the orientation group of { 100)<011>
to {223)<1 lO> is 2.0 or less with an object of further improving the isotropy, it is
found that the waiting time t second is preferably tl or more and 2.5 x tl or less.
[004 81
Moreover, based on the findings obtained by the basic research described as
above, the inventors have diligently investigated with respect to a hot-rolled steel sheet
for gas nitrocarburizing which is suitably applied to the member which requires
ductility and strict uniformity of the sheet thickness, the circularity, and the impact
resistance after processing and a manufacturing method of the hot-rolled steel sheet.
As a result, the hot-rolled steel sheet including the following conditions and the
manufacturing method thereof are conceived.
Limitation reasons of chemical composition in the present embodiment will
be described.
[0049]
C content [C]: more than 0.07% and equal to or less than 0.2%
C is an element which largely influences strength and pearlite fraction of a
base metal. However, C is also an element which generates iron-based carbide such
as cementite (Fe3C) which becomes origins of cracks at the time of hole expansion.
When the C content [C] is 0.07% or less, effects of improvement in strength achieved
by structure strengthening due to a low-temperature transformation forming phase
cannot be obtained. On the other hand, when the C content is more than 0.2%, center
segregation is remarkably generated, and thus, the iron-based carbide such as
cementite (Fe3C), which becomes origins of cracks of a secondary shear surface at the
time of punching, is increased, and punching quality or hole expansibility is
deteriorated. Thereby, the C content [C] is limited to a range of more than 0.07% and
equal to or less than 0.2%. When balance between ductility and strength in addition
to the improvement in the strength is considered, the C content [C] is preferably 0.15%
or less.
[0050]
Si content [Si]: 0.001% or more and 2.5% or less
Si is an element which contributes an increase in strength of the base metal.
Moreover, Si has a role as a deoxidizer material of molten steel. The effects are
exerted when the Si content [Si] is 0.001% or more. However, even when the Si
content is more than 2.5%, the effect contributing the increase in the strength is
saturated. Si is an element which largely influences transformation point temperature,
when the Si content [Si] is less than 0.001% or is more than 2.5%, there is a concern
that generation of pearlite may be suppressed. Thereby, the Si content [Si] is limited
to a range of 0.001% or more and 2.5% or less. In addition, from the viewpoint of the
improvement in the strength and improvement in the hole expansibility, Si is added to
be more than 0. I%, and thus, according to the increase of the Si content, precipitation
of the iron-based carbide such as cementite in the structure of the steel sheet is
suppressed, which contributes the improvement in the strength and improvement in the
hole expansibility. On the other hand, if the added amount is more than I%, the effect
which suppresses the precipitation of the iron-based carbide is saturated. Accordingly,
a preferable range of the Si content [Si] is more than 0.1% and equal to or less than 1%.
[005 11
Mn Content [Mn]: 0.01% or more and 4% or less
Mn is an element which contributes the improvement in the strength by solute
strengthening and quenching strengthening. However, if the Mn content [Mn] is less
than 0.01%, the effect cannot be obtained. On the other hand, the effect is saturated if
the Mn content is more than 4%. Moreover, Mn is an element which largely
influences the transformation point temperature, and when the Mn content [Mn] is less
than 0.01% or more than 4%, there is a concern that generation of pearlite may be
suppressed. Thereby, the Mn content [Mn] is limited to a range of 0.01% or more and
4.0% or less. When elements other than Mn are not sufficiently added to suppress
occurrence of hot cracks due to S, it is preferable that the Mn content [Mn] and the S
content [S] satisfy, by mass%, [Mn]/[S] 2 20. In addition, Mn is an element which
improves hardenability by enlarging austenite region temperature to a low temperature
side according to the increase of the Mn content, and makes a continuous cooling
transformation structure having an improved burring property is easily formed. Since
this effect is not easily exerted when the Mn content [Mn] is less than I%, it is
preferable that the Mn content be added 1% or more.
[0052]
P content [PI: more than 0% and equal to or less than 0.15%
P is impurity contained in molten iron, and is an element which is segregated
on grain boundaries and decreases toughness according to an increase in the content.
Therefore, it is desirable that the P content be as low as possible. If the P content is
more than 0.15%, P adversely affects workability or weldability, and thus, the P
content is limited so as to be 0.15% or less. Particularly, considering hole
expansibility or weldability, the P content is preferably 0.02% or less. Since it is
difficult that the content of P becomes 0% because of operational problems, the content
[PI of P does not include 0%.
[0053]
S content [S]: more than 0% and equal to or less than 0.03%
S is impurity which is contained in molten iron, and is an element which not
only decrease toughness or generates cracks at the time of hot rolling but also
generates A type inclusion which deteriorates hole expansibility if the content is too
large. Thereby, the S content should be decreased as much as possible. However,
since the S content of 0.03% or less is an allowable range, the S content is limited to be
0.03% or less. In addition, in a case where some extent of hole expansibility is
needed, the S content [S] is preferably 0.01% or less, and more preferably 0.005% or
less. Since it is difficult that the content of S becomes 0% because of operational
problems, the content [S] of S does not include 0%.
[0054]
A1 content [All: 0.00 1 % or more and 2% or less
A1 of 0.001% or more is added for deoxidation of molten steel in a refining
process of steel. However, since a large amount of addition increase costs, the upper
limit is 2%. Moreover, if too large of an amount of A1 is added, nonmetallic inclusion
is increased, and ductility and toughness are deteriorated. Therefore, fi-om the
viewpoint of the ductility and the toughness, the A1 content is preferably 0.06% or less.
More preferably, the A1 content is 0.04% or less. Similar to Si, in order to obtain the
effect which suppresses the precipitation of iron-based carbide such as cementite in the
material structure, it is preferable that the A1 content of 0.016% or more is contained.
Accordingly, it is more preferable that the A1 content [All is 0.016% or more and
0.04% or less.
[0055]
N content [N]: more than 0% and equal to or less than 0.01%
N generates coarse TiN with Ti at the time of casting, and decreases a surface
hardness improvement effect by Ti at the time of gas nitrocarburizing. Therefore, N
should be decreased as much as possible. However, the N content of 0.01% or less is
an allowable range. From the viewpoint of aging resistance, it is more preferable that
the N content be 0.005% or less. Since making the N content be 0% is difficult in the
operational aspect, 0% is not included.
[0056]
Ti content [Ti]: 0.005 + [N] x 48 / 14 + [S] x 48 I32 r [Ti] 10.015 + [N] x 48
/ 14+ [S] x 48/32 ... (a)
Ti added to be precipitated as Tic after ferrite transformation, and is added to
suppress growth of a grains by a pinning effect during cooling or after coiling.
However, Ti is precipitated and fixed as TiN, TiS, or the like in high temperature range
of an austenite phase. Therefore, in order to secure Ti effective in the pinning in a a
phase, the Ti content is added to be greater than or equal to 0.005 + [N] x 48 / 14 + [S]
x 48 / 32. On the other hand, even when the Ti content is added to be more than
0.015 + [N] x 48 / 14 + [S] x 48 132, the effect is saturated, and thus, 0.015 + [N] x 48
/ 14 + [S] x 48 1 32 is the upper limit. In addition, since Ti fixes C with Tic, if Ti is
excessively added, there is a concern that generation of pearlite may be suppressed.
Moreover, Ti is bonded to N in gas nitrocarburizing treatment after forming
and has an effect which increases hardness. Therefore, Ti is added to be greater than
or equal to 0.005 + [N] x 48 1 14 + [S] x 48 / 32. If the Ti content [Ti] is less than
0.005 + [Nl x 48 I 14 + [S] x 48 132, since chipping resistance and rolling fatigue
resistance are decreased after the gas nitrocarburizing treatment, thererore, even though
the steel sheet has a sufficient mechanical characteristics as an original sheet, the steel
sheet is insufficient as the hot-rolled steel sheet for gas nitrocarburizing.
[0057]
The above-described chemical elements are basic components (basic
elements) of the steel in the present embodiment, and a chemical composition, in
which the basic elements are controlled (contained or limited) and the balance consists
of Fe and unavoidable impurities, is the basic composition of the present embodiment.
However, in the present embodiment, in addition to (instead of a portion of Fe of the
balance) the basic components, if necessary, one kind or two or more kinds of Nb, Cu,
Ni, Mo, V, Cr, Ca, Mg, REM, and B may be further contained. In addition, even
when the selective elements are inevitably (for example, amount less than the lower
limit of the amount of each selective element) mixed into the steel, the effects in the
present embodiment are not damaged. Hereinafter, limitation reasons of the
component of each element will be described.
[OOSS]
Nb, Cu, Ni, Mo, V, and Cr are elements having an effect which improves
strength of the hot-rolled steel sheet by precipitation strengthening or solute
strengthening. However, when the Nb content [Nb] is less than 0.005%, the Cu
content [Cu] is less than 0.02%, the Ni content [Nil is less than 0.01%, the Mo content
[Mo] is less than 0.01%, the V content [V] is less than 0.01%, and the Cr content [Cr]
is less than 0.01%, the effect cannot be sufficiently obtained. Moreover, even when
the Nb content w ] is added to be more than 0.06%, the Cu content [Cu] is added to
be more than 1.2%, the Ni content [Nil is added to be more than 0.6%, the Mo content
[Mo] is added to be more than I%, the V content [V] is added to be more than 0.2%,
and the Cr content [Cr] is added to be more than 2%, the effect is saturated, and
economic efficiency is decreased. Accordingly, when Nb, Cu, Ni, Mo, V, and Cr are
contained if necessary, it is preferable that the Nb content [Nb] is 0.005% or more and
0.06% or less, the Cu content [Cu] is 0.02% or more and 1.2% or less, the Ni content
[Nil is 0.01% or more and 0.6% or less, the Mo content [Mo] is 0.01% or more and 1%
or less, the V content [V] is 0.01% or more and 0.2% or less, and the Cr content [Cr] is
0.01% or more and 2% or less.
[0059] -
Mg, Ca, and REM (Rare Earth Element: Rare Earth Metal) are elements
which improve workability by controlling the shape of nonmetallic inclusion which
becomes origins of breaks and causes deterioration of workability. If Ca, REM, and
Mg are added less than 0.0005% respectively, the effect is not exerted. In addition,
even when the Mg content [Mg] is added to be more than 0.01%, the Ca content [Ca]
is added to be more than 0.01%, and the REM content [REM] is added to be more than
0.1%, the effect is saturated, and economic efficiency is decreased. Accordingly, it is
preferable that the Mg content [Mg] is added 0.0005% or more and 0.01% or less, the
Ca content [Ca] is added 0.0005% or more and 0.01% or less, and the REM content
[REM] is added 0.0005% or more and 0.1% or less.
[0060]
B content [B]: 0.0002% or more and 0.002% or less
B is bonded to N in gas nitrocarburizing treatment after forming and has an
effect which increases hardness. However, if B is added to be less than 0.0002%, the
effect cannot be obtained. On the other hand, if B is added to be more than 0.002%,
the effect is saturated. Moreover, since B is an element which suppresses
recrystallization of austenite in the hot rolling, if a large amount of B is added, y+a
transformation texture is strengthened from non-recrystallization austenite, and thus,
there is a concern that isotropy may be deteriorated. Thereby, the B content @3] is
0.0002% or more and 0.002% or less. On the other hand, from the viewpoint of slab
cracks in the cooling process after continuous casting, the [B] is preferably 0.0015% or
less. That is, the B content [B] is more preferably 0.001% or more and 0.0015% or
less.
[006 11
Moreover, in the hot-rolled steel sheet which has the above-described
elements as main components, Zr, Sn, Co, Zn, and W may be contained to 1% or less
in total as unavoidable impurities. However, since there is a concern that scratches
may occur due to Sn at the time of the hot rolling, Sn is preferably 0.05% or less.
[0062]
Next, metallurgical factors such as microstructure in the hot-rolled steel sheet
according to the present embodiment will be described in detail.
The microstructure of the hot-rolled steel sheet according to the present
embodiment includes, by structural fraction, pearlite more than 6% and ferrite in the
balance. The limitation of the structural configuration is related to toughness after
nitriding treatment, that is, impact resistance when is used as a part after the gas
nitrocarburizing treatment.
[0063]
The gas nitrocarburizing treatment is performed at relatively low temperature
of approximately 570°C which is less than or equal to the a+y transformation point
temperature. That is, unlike quenching processing, the gas nitrocarburizing treatment
is not the processing which strengthens the structure by quenching using phase
transformation, and is the processing which is remarkably hardened by forming nitride
having high hardness.
When a cross-section of a material which is subjected to the gas
nitrocarburizing treatment, is observed by a microscope, a compound layer (white
layer: E nitride Fe2-3N) having thickness of approximately 10 to 20 pm and a diffusion
layer having thickness of approximately 100 to 300 pm in the deep portion can be
confirmed. Moreover, a base metal structure, which is not almost changed compared
to before the treatment, exists in the further deep portion. In addition, the compound
layer is a brittle layer, and since there is a concern that toughness after nitriding
treatment may be decreased if the compound layer is too deep, the compound layer is
preferably 20 pm or less.
Moreover, in order to satisfy chipping resistance and rolling fatigue resistance
in the part which is subjected to the gas nitrocarburizing treatment, average Vickers
hardness Hv (0.005 kg0 in the position of Opm to 5 pm from the surface in the
compound layer after the gas nitrocarburizing requires hardness of 350 Hv or more.
From the viewpoint of abrasive resistance, the average Vickers hardness is more
preferably 400 Hv or more.
[0064]
In the gas nitrocarburizing treatment,
N which is obtained from a reaction of 2NH3 H 2N + 3H2 is diffused on the
surface of the steel sheet and forms nitride. At this time, in the compound of Fe and
N, there are two kinds of y' phase (Fe4N) of a face-centered cubic lattice and I; phase
(FezN) of a closed-packed hexagonal lattice, and the phase is generated if N
concentration is more than 11%. The I; phase deteriotrate the toughness after the
nitriding treatment significantly.
[0065]
In order to satisfy both of wear resistance, seize resistance, fatigue resistance,
corrosion resistance, or the like which is obtained by the gas nitrocarburizing treatment
and toughness after nitriding treatment, generation of the I; phase should be avoided by
controlling the diffusion of N.
The inventors have diligently repeated research with respect to a method,
which avoids generation of the I; phase if possible by suppressing the diffusion of N,
from the viewpoint of metallography. As a result, the inventors newly found that the
diffusion of N is suppressed and generation of the I; phase can be avoided if pearlite
more than 6% by structural faction exists in the microstructure.
Although this mechanism has not been clear, it is considered that this is
because C exists much in Fe lattices in ferrite which exits in a state which is
sandwiched to band-like cementite lamellars forming a pearlite structure, C occupies
invasion sites of N which is to be diffused into Fe lattices at the gas nitrocarburizing
treatment, and thus, the diffusion of N is suppressed.
[0066]
The upper limit of the structural fraction of pearlite in the hot-rolled steel
sheet according to the present embodiment is not particularly limited. However, since
the composition range of the hot-rolled steel sheet according to the present
embodiment is a range which becomes hypo-eutectoid steel, 25% becomes the upper
limit.
Lamellar spacing of pearlite in the hot-rolled steel sheet according to the
present embodiment is not particularly limited. However, when the lamellar spacing
is more than 2 pm, concentration of C, which exists in Fe lattice of the ferrite existing
in a state sandwiched to the cementite lamellar, is decreased, and the effect which
suppresses the diffusion of N may be decreased. Therefore, the lamellar spacing of
pearlite is preferably 2 pm or less, more preferably 1.5 pm or less, and still more
preferably 1.0 pm or less.
[0067]
A measurement of the lamellar spacing is performed as follows. After the
steel sheet is etched by NITAL, the sheet is observed at least 5 or more fields at a
magnification of 5,000 times or more by SEM, and thus, the lamellar spacing of the
pearlite structure is measured. The lamellar spacing in the present embodiment
indicates the average value.
[0068]
Next, the reasons for limitation of a manufacturing method of the hot-rolled
steel sheet according to the present embodiment will be described in detail below
(hereinafter, referred to as a manufacturing method according to the present
embodiment).
In the manufacturing method according to the present embodiment, a steel
piece such as a slab including the above-described components is manufactured prior
to the hot rolling process. The manufacturing method of the steel piece is not
particularly limited. That is, as the manufacturing method of the steel piece including
the above-described components, a melting process is performed at a blast furnace,
converter, an electric furnace, or the like, subsequently, component adjustment is
performed by various secondary refining processes to obtain the intended component
content, subsequently, a casting process may be performed by a method such as thinslab
casting in addition to casting by general continuous casting or an ingot method.
When the slab is obtained by the continuous casting, the slab may be sent to a hot
rolling mill in a state of a high temperature cast slab, and the slab is reheated in the
heating furnace afier being cooled to room temperature and thereafter, hot rolling may
be performed to the slab. Scraps may be used for a raw material.
[0069]
The slab which is obtained by the above-described manufacturing method is
heated in a slab heating process before the hot rolling process. In the manufacturing
method according to the present embodiment, the heating temperature is not
particularly limited. However, if the heating temperature is more than 1260°C, since
yield is decreased due to scale-off, the heating temperature is preferably 1260°C or less.
Moreover, in the heating temperature which is less than 1 150°C, since operation
efficiency in a schedule is significantly damaged, the heating temperature is preferably
11 50°C or more.
Heating time in the slab heating process is not particularly limited. However,
from the viewpoint of avoiding center segregation or the like, it is preferable that the
heating of the slab is maintained for 30 minutes or more after reaching the abovedescribed
heating temperature. However, the heating time is not applied to a case
where the cast slab after casting is directly sent in a high temperature state and is rolled.
[0070]
Without waiting in particular after the slab heating process, for example, a
rough rolling process, which performs rough rolling (first hot rolling) to the slab which
is extracted from the heating furnace within 5 minutes, starts, and thus, a rough bar is
obtained.
Due to the reasons described below, the rough rolling (first hot rolling),
includes once or more of reduction with reduction ratio of 40% or more at a
temperature range of 1000°C or more and 1200°C or less. When the rough rolling
temperature is less than 1000°C, hot deformation resistance is increased in the rough
rolling, and there is a concern that the operation of the rough rolling may be damaged.
On the other hand, when the rough rolling temperature is more than 1200°C,
the average grain size is increased, and toughness is decreased. Moreover, a
secondary scale which is generated in the rough rolling is too grown, and thus, there is
a concern that the scale may be not easily removed by descaling or the finish rolling
which is performed later. When rough rolling ending temperature is more than
11 50°C, inclusion extends, and thus, hole expansibility may be deteriorated.
Therefore, the rough rolling ending temperature is preferably 1150°C or less.
[007 11
In addition, if the rolling-reduction ratio is small in the rough rolling, the
average grain size is increased, and thus, toughness is decreased. Preferably, if the
rolling-reduction ratio is 40% or more, the grain size is more uniform and fine. On
the other hand, when the rolling-reduction ratio is more than 65%, the inclusion
extends, and thus, hole expansibility may be deteriorated. Therefore, the upper limit
is preferably 65%.
[0072]
In order to refine the average grain size of the hot-rolled steel sheet, the
austenite grain size after the rough rolling, that is, before finish rolling (second hot
rolling) is important. Therefore, the austenite grain size is preferably 200 pm or less.
Refining and homogenization of grains of the hot-rolled steel sheet are largely
promoted by decreasing the sizes of the austenite grains before the finish rolling. In
order to make the austenite grain size be is 200 pm or less, rolling reduction of 40% or
more is performed once or more.
In order to more efficiently obtain the effects of the grain refining and the
homogenization, the austenite grain size is preferably 100 pm or less. Thereby, it is
preferable that the rolling reduction of 40% or more is performed twice or more in the
rough rolling (first hot rolling). However, if a number of the rolling reduction is more
than ten times, there is a concern that a decrease in the temperature or excessive
generation of the scales may occur.
[0073]
In this way, decreasing the austenite grain size before the finish rolling is
effective for promotion of recrystallization of austenite in the finish rolling later. It is
assumed that this is because austenite grain boundaries after the rough rolling (that is,
before the finish rolling) function as one of recrystallized nuclei during the finish
rolling. In this way, appropriately controlling the time until the finish rolling and
cooling starting after decreasing the austenite grain size as described below is effective
for the refining of the average grain size in the steel sheet.
In order to confirm the austenite grain size after the rough rolling, it is
preferable to cool the steel sheet as rapidly as possible before the sheet enters the finish
rolling. That is, the steel sheet is cooled at a cooling rate of 10°C/s or more, the
austenite grain boundaries stand out by etching the structure of the cross-section, and
thus, the steel sheet is measured by an optical microscope. At this time, 20 or more
fields are measured at magnification of 50 times or more by image analysis or a
intercept method.
[0074]
In the rolling (a second hot rolling and a third hot rolling) which is performed
after the rough rolling completion, endless rolling may be performed in which the
rolling is continuously performed by joining the rough bars, which are obtained after
the rough rolling process ends, between the rough rolling process and the finish rolling
process. At this time, the rough bars are temporarily coiled in a coil shape, the coiled
rough bar is stored in a cover having a thermal insulation function if necessary, and the
joining may be performed by recoiling the rough bar.
[0075]
Moreover, when the finish rolling (a second hot rolling) is performed, it may
be preferable that dispersion of temperature in a rolling direction, a sheet width
direction, and a sheet thickness direction of the rough bar is controlled to be decreased.
In this case, if necessary, the rough bar may be heated by a heating device which can
control the dispersion of the temperature in the rolling direction, the sheet width
direction, and the sheet thickness direction of the rough bar between a rough rolling
mill of the rough rolling process and a finish rolling mill of the finish rolling process,
or between respective stands in the finish rolling process.
[0076]
As heating measures, various heating measures such as gas heating, electrical
heating, or induction heating is considered. However, if the dispersion of the
temperature in the rolling direction, the sheet width direction, and the sheet thickness
direction of the rough bar can be controlled to be decreased, any well-known measures
may be used. As the heating device, an induction heating device having industrially
improved control responsiveness of temperature is preferable. Particularly, in the
induction heating device, if a plurality of transverse type induction heating devices
which can be shifted in the sheet width direction are installed, since the temperature
distribution in the sheet width direction can be arbitrarily controlled according to the
sheet width, the transverse induction heating devices are more preferable. As the
heating device, a device, which is configured by combining the transverse induction
heating device and a solenoid induction heating device which excellently heats the
overall sheet width, is most preferable.
[0077]
When temperature is controlled using the above-described heating devices, it
is preferable to control a heating amount by the heating device. h this case, since the
temperature of the inner portion of the rough bar cannot be actually measured, the
temperature distribution in the rolling direction, the sheet width direction, and the sheet
thickness direction when the rough bar reaches the heating device is assumed using
previously measured results data such as the temperature of a charged slab, staying
time in the furnace of the slab, heating furnace atmosphere temperature, heating
furnace extraction temperature, and transportation time of a table roller. In addition,
it is preferable to control the heating amount by the heating device based on the
respective assumed values.
[0078]
For example, the control of the heating amount by the induction heating
device is performed as follows.
As properties of the induction heating device (transverse type induction
heating device), when alternating current flows to a coil, a magnetic field is generated
in the inner portion. Moreover, in a conductor disposed in the coil, an eddy current in
a direction opposite to the coil current is generated in a circumferential direction
perpendicular to a magnetic flux by electromagnetic induction action, and the
conductor is heated by Joule heat. The eddy current is most strongly generated on the
surface of the inside of the coil and is exponentially decreased toward the inside (this
phenomenon is referred to as skin effect).
[0079]
Therefore, a current penetration depth is increased with a decrease in
frequency, and thus, a uniform heating pattern can be obtained in the thickness
direction. Conversely, the current penetration depth is decreased with an increase in
frequency, and it is known that an excessively heated small heating pattern, which has
the surface in the thickness direction as the peak, is obtained.
Therefore, the heating in the rolling direction and the sheet width direction of
the rough bar can be performed similar to the conventional method by the transverse
induction heating device.
[OOSO]
In the heating in the sheet thickness direction, homogenization of the
temperature distribution can be performed by changing a penetration depth by the
frequency change of the transverse induction heating device and operating the heating
pattern in the sheet thickness direction.
In this case, a frequency variable induction heating device is preferably used.
However, the frequency change may be performed by adjusting a capacitor. In the
control of the heating amount by the induction heating device, a plurality of inductors
having different frequencies are disposed, and allocation of each heating amount may
be changed to obtain the required heating pattern in the thickness direction. In the
control of the heating amount by the induction heating device, the frequency is
changed when an air gap between a material to be heated and the heating device is
changed. Therefore, desired frequency and heating pattern may be obtained by
changing the air gap.
[OOS 11
In addition, for example, as described in Metal Material Fatigue Design
Manual (edited by Soc.of Materials Sci., Japan), there is a correlation between fatigue
strength of the steel sheet which is hot-rolled or pickled and a maximum height Ry of
the steel sheet surface. Therefore, it is preferable that the maximum height Ry
(corresponding to Rz defined in JIS B0601:2001) of the steel sheet surface after the
finish rolling is 15 pm (15 pmRy, 1 2.5 mm, In 12.5 mm) or less. In order to obtain
the surface roughness, it is preferable that a condition of collision pressure P of highpressure
water on the steel sheet surface x a flow rate L 2 0.003 is satisfied in the
descaling. In order to prevent scales from occurring again, it is preferable that the
subsequent finish rolling is performed within 5 seconds after the descaling.
[0082]
After the rough rolling (the first hot rolling) process ends, the finish rolling
(the second hot rolling) process starts. Here, the time from the ending of the rough
rolling to the starting of the finish rolling is set to 150 seconds or less. If the time
from the ending of the rough rolling to the starting of the finish rolling is more than
150 seconds, the average grain size in the steel sheet is increased, and thus, toughness
is decreased. The lower limit of the time is not particularly limited. However, when
recrystallization is completely completed after the rough rolling, the time is preferably
5 seconds or more. Moreover, in a case where a temperature decrease of the rough
bar surface due to roll contact and influence to the material due to unevenness of the
temperature in the sheet thickness direction of the rough bar by generation of heat at
the time of processing are concerned, the time is preferably 20 seconds or more.
[0083]
In the finish rolling, a starting temperature of the finish rolling is set to
1000°C or more. If the starting temperature of the finish rolling is less than 1000°C,
the rolling temperature of the rough bar to be rolled is decreased in each finish rolling
pass, the rolling reduction is preformed at a non-recrystallization temperature range,
the texture is developed, and isotropy is deteriorated.
The upper limit of the starting temperature of the finish rolling is not
particularly limited. However, if the starting temperature is more than 11 50°C or
more, there is a concern that blisters which become origins of scale-like spindle scale
defects may occur between ferrite of the steel sheet and the surface scale before the
finish rolling and between passes. Therefore, it is preferable that the starting
temperature of the finish rolling is less than 1150°C.
[0084]
In the finish rolling, when temperature determined by components of the steel
sheet is represented by Tl("C), the rolling reduction of 30% or more by one pass is
performed at least once in a temperature range of T1 + 30°C or more and T1 + 200°C
or less, and total of the rolling-reduction ratio at the temperature range is set to 50% or
more, and the hot rolling ends at Tl + 30°C or more. Here, T1 is temperature which
is calculated by the following Equation (b) using the content of each element.
TI = 850 + 10 x ([C] + M) x [Mn] + 350 x [Nb] + 250 x [Ti] -t 40 x [B] +
lox [Cr]+ 100x [Mo]+ 100x [V] ... (b)
The TI temperature itself is obtained empirically. The inventors empirically
found that recrystallization is promoted at an austenite range of each steel based on the
T1 temperature in an experiment. However, an amount of chemical elements
(chemical composition) which are not included in Equation (b) is regarded as 0%, and
the calculation is preformed.
[0085]
If the total rolling-reduction ratio is less than 50% at the temperature range of
T1 + 30°C or more and TI + 200°C or less, since rolling strain accumulated in the hot
rolling is not sufficient and recrystallization of austenite does not sufficiently proceed,
the grain size is coarsened, texture is developed, and thus, isotropy is deteriorated.
Therefore, the total rolling-reduction ratio in the finish rolling is set to 50% or more.
If the total rolling-reduction ratio is preferably 70% or more, sufficient isotropy is
obtained even if dispersion due to temperature change or the like is considered.
On the other hand, if the total rolling-reduction ratio is more than 90%, due to
generation of heat at the time of processing or the like, it is difficult to maintain the
temperature range of Tl + 200°C or less. Therefore, the total rolling-reduction ratio
of 90% or more is not preferable. In addition, if the total rolling-reduction ratio is
more than 90% a rolling load increased, and thus, the rolling may not be easily
performed.
In addition, in order to promote uniform recrystallizatoin by opening of the
accumulated strain, after total of the rolling-reduction ratio at TI + 30°C or more and
T1 + 200°C or less is set to 50% or more, the rolling reduction of 30% or more by one
pass is performed at least once during the rolling.
[0086]
After the second hot rolling ends, in order to promote uniform
recrystallization, it is preferable that a processing amount at a temperature range equal
to or more than the Ar3 transformation point temperature and less than T1 + 30°C is
suppressed to be decreased if possible. Therefore, a total of the rolling-reduction
ratio in the rolling (third hot rolling) at the temperature range equal to or more than the
Ar3 transformation point temperature and less than TI + 30°C is limited to 30% or less.
From the viewpoint of accuracy of the sheet thickness or the sheet shape, a rollingreduction
ratio of 10% or less is preferable. However, when isotropy is further
required, the rolling-reduction ratio of 0% is more preferable.
[0087]
The first rolling to the third hot rolling is needs to be ended at the Ar3
transformation point temperature or more. In the hot rolling of less than the Ar3
transformation point temperature, the hot rolling becomes dual phase rolling, and
isotropy and ductility are decreased due to residual of the processing ferrite structure.
In addition, rolling ending temperature is preferably Tl°C or more.
[OOSS]
Moreover, in order to suppress growth of recrystallized grains, when a pass
having rolling-reduction ratio of 30% or more at temperature range of Tl + 30°C or
more and T1 + 200°C or less is defined as a large rolling-reduction pass, and a primary
cooling, in which the cooling temperature change is 40°C or more and 140°C or less
and the cooling stop temperature is T1 + 100°C or less, is preformed at a cooling rate
of 50°C/second or more so that a waiting time t (second) from completion of the final
pass of the large rolling-reduction passes to start of the cooling satisfies the following
Equation (c).
If the waiting time t until the cooling is more than 2.5 x tl seconds, since the
recrystallized austenite grains are maintained at high temperature, the grains are
significantly grown, and as a result, toughness is deteriorated. In addition, in order to
water-cool the steel sheet rapidly, if possible, after the rolling, it is preferable that the
primary cooling is performed between rolling stands. In addition, when an
instrumental device such as a thermometer or a sheet thickness meter is installed on a
rear surface of a final rolling stand, since the measurement is difficult due to steam or
the like which is generated when cooling water is applied, it is difficult to install a
cooling device immediately behind the final rolling stand.
t52.5 x tl ... (c)
tl = 0.001 x ((Tf-TI) x PI / 100)~- 0.109 x ((Tf-T1) x P1 / 100) + 3.1 ... (d)
Here, Tf is the temperature (OC) after the final pass rolling reduction of the
large rolling-reduction passes and PI is the rolling-reduction ratio (%) of the final pass
of the large rolling-reduction passes.
In addition, the waiting time t is not the time from ending of the hot rolling,
and it is found that setting the waiting time as described above is preferable since a
preferable recrystallization ratio and recrystallized grain size can be obtained.
Moreover, if the waiting time until the start of the cooling is set as described above,
either the primary cooling or the third hot rolling may be performed in advance.
[0089]
By limiting the cooling temperature change to 40°C or more and 140°C or
less, the growth of recrystallized austenite grains can be further suppressed. In
addition, by more efficiently controlling variant selection (avoidance of variant
limitation), the development of the texture can be further suppressed. If the
temperature change of the primary cooling is less than 40°C, the recrystallized
austenite grains are grown, and toughness is deteriorated. On the other hand, if the
temperature change is more than 140°C, there is a concern that the temperature may be
overshot to the Ar3 transformation point temperature or less, and in this case, the
variant selection is rapidly performed even at transformation from the recrystallized
austenite, and as a result, texture is formed and isotropy is decreased. Moreover,
when the cooling stop temperature is the Ar3 transformation point temperature or less,
a bainite structure is generated, and there is a concern that generation of ferrite and
pearlite may be suppressed.
If the cooling rate during cooling is less than 50°C/second, the recrystallized
austenite grains are grown and toughness is deteriorated. The upper limit of the
cooling rate is not particularly limited. However, from the viewpoint of the sheet
shape, it is properly considered that the upper limit is 200°C/second or less. In
addition, if the steel sheet temperature at the end of cooling ending is more than T1 +
100°C, cooling effects cannot be sufficiently obtained. For example, this is because
even though the primary cooling is performed under appropriate conditions after the
final pass, there is a concern that grain growth may occur and the austenite grain size
may be significantly coarsened when the steel sheet temperature after the end of
primary cooling is more than T1 + 100°C.
[0090]
Moreover, when the waiting time t until the start of cooling is limited to be
less than tl, the grain growth is hrther suppressed, and more improved toughness can
be obtained.
[009 11
On the other hand, the waiting time t until the start of the cooling is further
limited to satisfy tl I t 5 2.5 x tl, randomization of grains is sufficiently promoted, and
a stable and further improved pole density and isotropy can be obtained.
[0092]
Moreover, in order to suppress the grain growth and obtain improved
toughness, in the rolling of a temperature range of T1 + 30°C or more and T1 + 200°C
or less, it is preferable that temperature increase between respective finish rolling
passes is 18°C or less. For example, in order to suppress the temperature increase, a
cooling device between passes or the like may be used.
[0093]
Regarding whether or not the rolling specified as above is performed, a
rolling-reduction ratio can be obtained from actual results or calculation from
measurements of the rolling load and the sheet thickness, or the like. In addition, the
temperature can be measured if the thermometer between stands is provided, or since
calculation simulation which considers generation of heat at the time of processing
from a line speed, the rolling-reduction ratio, or the like can be performed, whether or
not the rolling defined as above is performed can be obtained from either the rolling
ratio or the temperature or both.
[0094]
In the manufacturing method according to the present embodiment, rolling
speed is not particularly limited. However, if the rolling speed at the final finishing
stand is less than 400 mpm, y grains tend to be grown and coarsened. Accordingly,
regions capable of performing precipitation of ferrite to obtain ductility are decreased,
and thus, there is a concern that ductility may be deteriorated. Moreover, effects can
be obtained even if the upper limit of the rolling speed is not particularly limited. For
installation limitation, 1800 mpm or less is reasonably practical. Accordingly, it is
preferable that the rolling speed in the finish rolling process be 400 mpm or more and
1800 mpm or less if necessary.
Moreover, after the primary cooling, before the coiling process and after
passing through the rolling stand, the secondary cooling may be performed. The
cooling pattern is not particularly limited and may be appropriately set according to the
line speed or coiling temperature in a range which satisfies the coiling temperature
described below.
[0095]
Subsquently, in the coiling process, the coiling temperature is more than
550°C. If the coiling temperature is 550°C or less, the coiling temperature becomes
Bs point or less, bainite is mixed into the microstructure, and there is a concern that
impact resistance after the nitriding treatment may be deteriorated. Moreover, after
the coiling, the pearlite transformation does not sufficiently proceed. The upper limit
of the coiling temperature is not particularly limited. However, the upper limit is not
higher than the rolling ending temperature. Moreover, when the upper limit is more
than 850°C, since there is a concern that steel sheet surface characteristics may be
deteriorated due to oxidation of the outermost circumference of the coil, the upper limit
is preferably 850°C or less. The upper limit is more preferably 800°C or less.
However, when the lamellar spacing of the pearlite structure is set to 2 pm or
less, the coiling temperature is preferably 800°C or less. When the lamellar spacing
is 1.5 pm or less, the coiling temperature is more preferably 700°C or less. The
pearlite structure is mainly generated in the coiling process, and the lamellar spacing of
the pearlite is largely affected by diffusion distances of Fe and C.
[0096]
In addition, with an object of improving the ductility by correction of the steel
sheet shape or introduction of moving dislocation, after all rolling processes end, skin
pass rolling having the rolling-reduction ratio of 0.1% or more and 2% or less may be
performed. In addition, after all processes end, with an object of removing scales
attached to the surface of the obtained hot-rolled steel sheet, pickling may be
performed to the obtained hot-rolled steel sheet if necessary. Moreover, after the
pickling, a skin pass or cooling rolling having the rolling-reduction ratio of 10% or less
may be performed to the obtained hot-rolled steel sheet at an in-line or an off-line.
[0097]
In the hot-rolled steel sheet according to the present embodiment, even in any
case after the casting, the hot rolling, and the cooling, heat treatment may be performed
to the steel sheet at a hot-dip plating line, and a separate surface processing may be
performed to the hot-rolled steel sheet. By performing the plating at the hot-dip
plating line, corrosion resistance of the hot-rolled steel sheet is improved. When
galvanizing is performed to the hot-rolled steel sheet after pickling, the obtained steel
sheet is immersed in a galvanizing bath, and alloying treatment may be performed if
necessary. By performing the alloying treatment, in the hot-rolled steel sheet, the
corrosion resistance is improved and weld resistance with respect to various welding
such as spot welding is improved.
For reference, FIG. 3 is a flowchart showing an outline of the manufacturing
method according to the present embodiment.
[0098]
In addition, gas nitrocarburizing treatment is performed to the obtained hotrolled
steel sheet after the processes are completed, and thus, a nitrided part is obtained.
[Example]
[0099]
Hereinafter, the present invention is fbrther described based on Example.
Theast slabs of A to A1 having chemical compositions shown in Table 1 were
manufactured via a converter, a secondary refining process, and continuous casting.
Then, the cast slabs were reheated, were rolled to a sheet thickness of 2.0 mm to 3.6
mm at the finish rolling continuous to the rough rolling, were subjected to the primary
cooling, and were coiled after being subjected to the secondary cooling if necessary,
and thus, hot-rolled steel sheets were manufactured. More specifically, according to
manufacturing conditions shown in Tables 2 to 7, the hot-rolled steel sheets were
manufactured. In addition, gas nitrocarburizing treatment, which is heated and
maintained for 5 hours at 560°C to 580°C in atmosphere of ammonia gas + Nz +C02,
were performed to the hot-rolled steel sheet. Moreover, all indications of the
chemical compositions in Tables are mass%.
In addition, the balance of components in Table 1 indicate Fe and unavoidable
impurities, and "0%" or "-" indicates that Fe and unavoidable impurities are not
detected. Moreover, underlines in Tables indicate ranges out of the range of the
present invention.
[O 1001
Here, a "component" represents the steels including the component
corresponding to each symbol shown in Table 1, "Ar3 transformation point
temperature" represents the Ar3 temperature (OC) which is calculated by the following
Equation (g), and "Tl" represents the temperature which is calculated by the Equation
(b), and "tl" represents the times which is calculated by the Equation (d).
Ar3=910-310 x [C]+25 x [Sil-80 x [Mne ql... (g)
Here, [Mneq] is indicated by the following Equation (h) when B is not added
and by the following Equation (i) when B is added.
[Mneq] = [Mn] + [Cr] + [Cu] + [Mo] + mi] 1 2 + 10 x ([Nb] - 0.02). . . (h)
[Mneq] = Fin] + [Cr] + [Cu] + [Mo] + [Nil / 2 + 10 x ([Nb] - 0.02) + 1.. . (i)
Here, [component element] is amount of a component element which is
represented by mass%.
[OlOl]
"Heating temperature" represents the heating temperature in the cast slab
heating process, "holding time" represents the holding time at a predetermined heating
temperature in the heating process, the "number of times of rolling reduction of 40% or
more at 1000°C or more" or a "rolling-reduction ratio of 40% or more at 1000°C or
more" represents the rolling-reduction ratio or the number of times of rolling reduction
of a pass of 40% or more in a temperature range of 1000°C or more and 1200°C or less
in the rough rolling, "time until starting of finish rolling" represents the time from the
rough rolling process ending to the finish rolling process starting, and "total rollingreduction
ratio" represents the total rolling-reduction ratio in the hot rolling of each
temperature range. In addition, "Tf' represents the temperature after the final pass
rolling reduction of the large rolling-reduction pass, "Pl" represents the rollingreduction
ratio of the final pass of the large rolling reduction pass", "maximum
temperature increase between passes" represents a maximum temperature which is
increased by the generation of heat at the time of processing or the like between passes
at the temperature range of Tl + 30°C or more and TI + 200°C or less. In addition,
in the Example, the finish rolling ended at the final rolling reduction of 30% or more
except for a case where PI was "-" Tf is the finish rolling ending temperature except
for the case where P1 was "-"
Moreover, "waiting time until primary cooling starting" represents the waiting
time from completion of the final pass of the large rolling-reduction passes to start of
cooling when the pass having rolling-reduction ratio of 30% or more at the temperature
range of Tl + 30°C or more and TI + 200°C or less is set to a large rolling-reduction
pass, "primary cooling rate" represents an average cooling rate from primary cooling
temperature starting to the completion of the primary cooling, "primary cooling
temperature change" represents a difference between the starting temperature of
primary cooling and the ending temperature of primary cooling, and "coiling
temperature" represents the temperature when the steel sheet is coiled by a coiler in the
coiling process.
[O 1 021
Evaluation results of the obtained steel sheets are shown in Tables 8 to 10.
Among mechanical properties, with respect to tensile properties, isotropy, and hole
expansibility, evaluation was performed to an original sheet. With respect to
toughness, evaluation was performed to both the original sheet and the hot-rolled steel
sheet after nitriding treatment. Moreover, as evaluations of the chipping resistance
and the rolling fatigue resistance after gas nitrocarburizing treatment, average hardness
(Hv(0.005 kgf)) from the surface of the compound layer after the gas nitrocarburizing
to 5 pm was examined. An evaluation method of the steel sheet is the same as the
above-described method. Here, "pearlite fraction" indicates an area fraction of the
pearlite structure which is measured by a point counter method from an optical
microscope structure, "average grain size" indicates the average grain size which is
measured by EBSP-OIMTM, "average pole density of orientation group of
{ lOO) to (223 )<1 lo>" indicates the pole density of the orientation group of
{ 100)<011> to {223)<110> parallel to the rolling surface, "pole density of crystal
orientation of {332)<1 13>" indicates the pole density of the crystal orientation of
- 50 -
{332)<113> parallel to the rolling surface, "compound layer depth after gas
nitrocarburizing" indicates the depth (thickness) of a compound layer (white layer: E
nitride Fe2-3N) which collects a cross-section micro-sample from the surface, observed
by a microscope, and measures after performing the gas nitrocarburizing treatment
which heated and maintained for 5 hours at 560°C to 580°C in atmosphere of ammonia
gas + N2 +C02. In addition, the pearlite fraction indicates the approximately same
value even when the fraction is measured in the surface portion and the center portion
of the sheet thickness.
[0 1031
Results of "tensile test" indicate results of C direction using JIS No.5 test
piece. In Tables, "YP indicates a yield point, "TS" indicates tensile strength, and
"El" indicates elongation respectively. "Isotropy" has a reciprocal of lArl as the index.
Results of "hole expansion" indicate the results which can be obtained by a hole
expansion test method described in JFS T 1001: 1996. "Toughness" indicates a
transition temperature (vTrs) which is obtained by a subsize V-notch Charpy test.
[0 1041
The hot-rolled steel sheets according to the present invention are steel Nos. 8,
13, 15, 16,24 to 28,30,31,34 to 37,40 to 42,56,61, 63,64,72 to 76,78,79, 82 to 85,
and 88 to 90. The steel sheets contain a predetermined amount of steel component
and hot-rolled steel sheets for gas nitrocarburizing in which the average pole density of
the orientation group of {100)<011> to {223)<110> is 1.0 or more and 4.0 or less and
the pole density of the crystal orientation of {332)<113> is 1.0 or more and 4.8 or
less ,in the texture of the steel sheet in the center portion of the sheet thickness which is
the range of the sheet thickness of 518 to 318 from the surface of the steel sheet, and the
average grain size at the center in the sheet thickness is 10 pm or less, and the hotrolled
steel sheets are microstructures which include, by structural fraction, pearlite
more than 6% and ferrite in the balance, and have tensile strength 440 MPa or more.
Moreover, the hot-rolled steel sheets have improved isotropy, toughness after nitriding
treatment, toughness of the original sheet and the average hardness from the surface of
the compound layer after gas nitrocarburizing to 5 pm, and hole expansibility.
TABLE 1-1 Mass%
STEEL
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0
P
Q
Nb
0.020
0.01 7
0.041
-
-
-
0.00
0.025
-
-
-
-
0.01 1
-
-
C
0.069
0.071
0.036
0.043
-0.73
0.089
0.180
0.022
o.004
0.230
0.091
0.100
0.081
0.090
0.087
0.220
Cu
0.00
-
-
-
-
-
0.00
0.00
0.06
-
-
-
-
Si
1.20
1.1 7
0.0670.141.980.007
0.94
0.98
0.91
0.03
0.05
0.12
0.18
0.02
0.03
0.01
0.02
0.02
0.12
Ni
0.00
-
-
-
-
-
0.00
0.00
0.03
0.03
-
-
-
Mn
2.51
2.46
1.34
0.98
1.20
0.72
1.12
1.61
0.74
1.50
1.45
1.51
1.55
1.52
1.25
P
0.016
0.01 1
0.008
0.010
1.040.011
0.008
0.017
0.009
0.080
0.01 7
0.007
0.008
0.010
0.01 1
0.008
0.012
Mo
0.00
-
-
-
-
-
- - - - -
- - - - -
0.00
0.00
- - - - -
-
-
0.48
-
-
- - - - -
Cr
0.00
-
-
-
-
-
0.00
0.00
-
-
-
-
0.91
V
0.00
-
-
-
-
-
0.00
0.00
-
-
-
0.10
-
S
0.003
0.002
0.001
0.001
0.001
0.001
0.001
0.004
0.004
0.002
0.002
0.001
0.001
0.001
0.001
0.001
0.005
B
0.0014
-
-
-
-
-
-
-
0.001 1
0.001 1
-
-
-
-
-
-
-
Al
0.023
0.029
0.011
0.020
0.036
0.024
0.033
0.01 1
0.025
0.041
0.005
0.01 1
0.020
0.036
0.020
0.033
0.026
Mg
0.0022
-
0.0019
-
-
-
-
-
-
-
-
-
-
-
-
-
-
N
0.0026
0.0040
0.0046
0.0028
0.0034
0.0041
0.0038
0.0035
0.0047
0.0027
0.0051
0.0046
0.0028
0.0034
0.0041
0.0038
0.0041
Ca
-
0.0024
-
-
0.0021
-
0.0022
-
-
-
-
-
-
-
-
-
-
Ti
0.144
0.179
0.0910.038
0.126
0.099
0.0350.019
0.000
0.025
0.102
0.025
0.000
0.026
0.020
0.022
0.024
0.023
0.028
Rem
-
-
-
-
-
0.0018
-
-
0.0020
0.0020
0.0020
-
-
-
-
-
-
OTHERS
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
REMARKS
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
-
THEPRESENT
INVENTION
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
THEPRESENT
INVENTION
THEPRESENT
INVENTION
THEPRESENT
INVENTION
THEPRESENT
INVENTION
THEPRESENT
INVENTION
COMPARATIVE
STEEL
TABLE 1-2
STEEL
R
S
T
U
V
W
X
Y
Z
AA
AB
AC
AD
AE
AF
AG
AH
A1
Si
0.15
0.18
0.24
2.65
2.42
0.95
0.11
0.01
o.00
0.12
0.14
0.14
0.1 1
0.18
0.1 6
0.15
0.13
0.12
C
0.145
0.075
0.067
0.142
0.144
0.151
0.146
0.143
0.149
0.144
0.145
0.146
0.139
0.141
0.144
0.145
0.149
0.141
Mn
1.22
1.24
1.28
1.25
1.22
1.24
1.28
1.22
1.24
4.60
3.80
1.10
0.02
0.00
1.22
1.24
1.24
1.22
Ti
0.025
0.036
0.025
0.018
0.021
0.020
0.019
0.027
0.020
0.025
0.024
0.016
0.018
0.021
0.020
0.078
0.040
0.020
P
0.01 1
0.010
0.009
0.007
0.008
0.010
0.011
0.008
0.01 2
0.01 2
0.01 1
0.010
0.009
0.007
0.200
0.002
0.01 1
0.011
Nb
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
S
0.004
0.010
0.003
0.001
0.001
0.001
0.001
0.004
0.004
0.002
0.002
0.001
0.001
0.001
0.001
0.040
0.005
0.004
Cu
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
A1
0.024
0.030
0.022
0.036
0.020
0.033
0.026
0.024
0.030
0.036
0.020
0.033
0.026
0.024
0.030
0.022
0.023
0.026
N
0.0040
0.0044
0.0043
0.0034
0.0041
0.0038
0.0035
0.0047
0.0027
0.0051
0.0046
0.0028
0.0034
0.0041
0.0038
0.0037
0.0042
0.0045
Ni
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Mo
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
B
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
OTHERS
-
-
-
-
-
-
CO:O-Ool
-
-
-
-
Zr:0.002
-
-
-
-
-
-
REMARKSTHEPRESENT
INVENTION
THE PRESENT
INVENTION
COMPARATIVE
STEEL
COMPARATIVE
STEEL
THEPRESENT
INVENTION
THEPRESENT
INVENTION
THEPRESENT
lNVENT,ON
THEPRESENT
INVENTION
COMPARATIVE
STEEL
COMPARATIVE
STEEL
THEPRESENT
INVENTION
THEPRESENT
INVENTION
THEPRESENT
INVENTION
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
COMPARATIVE
STEEL
V
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Mg
0.0012
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Cr
-
-
2.40
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Ca
-
-
-
-
0.0022
-
-
-
-
-
-
-
-
-
-
-
-
-
Rem
-
0.0020
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
TABLE 2- 1
(1)COMPONENT (2)Ar3 TRANSFORMATION POINT TEMPERATURErC) (3)HEATING TEMPERATURE("C) (4)HOLDING TIME (MINUTE)
(5)NUMBER OF TIMES OF ROLLING REDUCTION OF 40% OR MORE AT 1000°C OR MORE
(6)ROLLING-REDUCTION RATIO OF 40% OR MORE AT 1000°C OR MORE (%) (7) y GRAIN SIZE(fl m)
(8)ROLLING ENDING TEMPERATURE("C) (9)TIME UNTIL FINISH ROLLING STARTING (SECOND) (10)ROLLING STARTING TEMPERATURE ("C)
(1 1)TOTAL ROLLING-REDUCTION RATIO (%) (12)NUMBER OF TIMES OF PASS HAVING 30% OR MORE BY ONE PASS
(1 3)MAXIMUM TEMPERATURE INCREASE BETWEEN PASSES r C )
STEEL
NO.
MANUFACTURING CONDITION
METALLURGICAL
FACTOR
HEATING TEMPERATURE
CONDITION
(1) (3)
FIRST HOT ROLLING
(2) (4) (5)
SECOND HOT ROLLING
T I
(oC)
(6) (7) (10) (11)
Tf
(8) (9)
(%)
(12) (13)
TABLE 2-2
(1 )COMPONENT (2)Ar3 TRANSFORMATION POINT TEMPERATURE("C) (3)HEATING TEMPERATURE("C) (4)HOLDING TIME (MINUTE)
(5)NUMBER OF TIMES OF ROLUNG REDUCTION OF 40% OR MORE AT 1000°C OR MORE
(6)ROLUNG-REDUCTION RATIO OF 40% OR MORE AT 1000°C OR MORE (%) (7) y GRAIN SIZE(,U rn)
(8)ROLUNG ENDING TEMPERATURE("C) (9)TIME UNTIL FINISH ROLLING STARTING (SECOND) (10)ROLLING STARTING TEMPERATURE ("C)
(1 1)TOTAL ROLLING-REDUCTION RATIO (%) (12)NUMBER OF TIMES OF PASS HAVING 30% OR MORE BY ONE PASS
(13)MAXIMUM TEMPERATURE INCREASE BETWEEN PASSES ("C)
THE PRESENT INVENTION
STEEL
NO.
16
METALLURGICAL
FACTOR
MANUFACTURING CONDITION
(1)
H
(2)
813
HEATING TEMPERATURE
CONDITION
TI
858
(3)
1200
(4)
60
FIRST HOT ROLLING
(5)
1
SECOND HOT ROLLING
(10)
1020
(6)
50
(1 1)
93
(7)
150 980
(8)
1030
(9)
60 35
(12)
2
(13)
15
TABLE 3-1
ETALLURGICA
FIRST HOT ROLLING SECOND HOT ROLUNG
COMPARATIVE STEEL ,
COMPARATIVE STEEL ( 44 ( AE ( 886 1 855 ( 1250 1 30 121 45/45 (100(1080(120(1070(89(930(32(3 110
COMPARATIVE STEEL 1 45 1 AF 1 787 1 857 1 1250 1 30 121 45/45 ~ 1 0 0 ~ 1 0 8 0 ~ 1 2 0 ~ 1 0 7 0 ~ 8 9 ~3 913100~ 3 2 ~
(1)COMPONENT (2)Ar3 TRANSFORMATION POINT TEMPERATURE("C) (3)HEATING TEMPERATURE("C) (4)HOLDING TIME (MINUTE)
(5)NUMBER OF TIMES OF ROLLING REDUCTION OF 40% OR MORE AT 1000°C OR MORE
(6)ROLLING-REDUCTION RATIO OF 40% OR MORE AT 1000°C OR MORE (%) (7) y GRAIN SIZE(fl rn)
(8)ROLUNG ENDING TEMPERATURE("C) (9)TIME UNTIL FINISH ROLLING STARTING (SECOND) (10)ROLLING STARTING TEMPERATURE ("C)
(1 1)TOTAL ROLLING-REDUCTION RATIO (%) (12)NUMBER OF TIMES OF PASS HAVING 30% OR MORE BY ONE PASS
(13)MAXIMUM TEMPERATURE INCREASE BETWEEN PASSES ("C)
TABLE 3-2
TEMPERATURE FIRST HOT ROLLING SECOND HOT ROLLING
(1)COMPONENT (2)Ar3 TRANSFORMATION POINT TEMPERATURE(%) (3)HEATING TEMPERATURE("C1 (4)HOLDING TIME (MINUTE)
(5)NUMBER OF TIMES OF ROLUNG REDUCTION OF 40% OR MORE AT 1000°C OR MORE
(6)ROLLING-REDUCTION RATIO OF 40% OR MORE AT 1000°C OR MORE (%) (7) y GRAIN SIZE(fl m)
(8)ROLLING ENDING TEMPERATURE("C) (9)TIME UNTIL FINISH ROLLING STARTING (SECOND) (10)ROLLING STARTING TEMPERATURE ("C)
(1 1)TOTAL ROLLING-REDUCTION RATIO (%) (12)NUMBER OF TIMES OF PASS HAVING 30% OR MORE BY ONE PASS
(13)MAXIMUM TEMPERATURE INCREASE BETWEEN PASSES ("C)
TABLE 4- 1
(1)COMPONENT (2)Ar3 TRANSFORMATION POINT TEMPERATURErC) (3)HEATING TEMPERATURE(%) (4)HOLDING TIME (MINUTE)
(5)NUMBER OF TIMES OF ROLUNG REDUCTION OF 40% OR MORE AT 1000°C OR MORE
(6)ROLUNG-REDUCTION RATIO OF 40% OR MORE AT 1000°C OR MORE (%) (7) y GRAIN SIZE(fl rn)
(8)ROLUNG ENDING TEMPERATURE("C) (9)TIME UNTIL FINISH ROLLING STARTING (SECOND) (10)ROLLING STARTING TEMPERATURE ("C)
(1 1)TOTAL ROLLING-REDUCTION RATIO (%) (12)NUMBER OF TIMES OF PASS HAVING 30% OR MORE BY ONE PASS
(13)MAXIMUM TEMPERATURE INCREASE BETWEEN PASSES ("C)
THE PRESENT INVENTION
MANUFACTURING CONDITION
STEEL
NO.
61
THE PRESENTINVENTION
COMPARATIVE STEEL
' HEATING TEMPERATURE
CONDITION
(3)
1200
76
77
TI
(OC)
858
(4)
60
(1)
H
FIRST HOT ROLLING
(2)
813
(5)
1
SECOND HOT ROLUNG
P
Q
(10)
1020
(6)
50
705
761
(11)
93
(7)
150
866
860
TF
1050
(8)
1030
1180
1250
(9)
30
PI
35
90
30
(12)
2
(13)
15
3
1
40/40/40
50
80
160
1010
1080
90
120
1000
1070
89
90
940
950
32
40
3
1
12
11
TABLE 4-2
THE PRESENTINVENTION
(1)COMPONENT (2)Ar3 TRANSFORMATION POINT TEMPERATURE("C) (3)HEATING TEMPERATURE("C1 (4)HOLDING TIME (MINUTE)
(5)NUMBER OF TIMES OF ROLUNG REDUCTION OF 40% OR MORE AT 1000°C OR MORE
(6)ROLLING-REDUCTION RATIO OF 40% OR MORE AT 1000°C OR MORE (%) (7) y GRAIN SIZE(/d rn)
(8)ROLUNG ENDING TEMPERATURE(OC) (9)TIME UNTIL FINISH ROLLING STARTING (SECOND) (10)ROLLING STARTING TEMPERATURE ("C)
(1 1)TOTAL ROLLING-REDUCTION RATIO (%I (12)NUMBER OF TIMES OF PASS HAVING 30% OR MORE BY ONE PASS
(1 3)MAXIMUM TEMPERATURE INCREASE BETWEEN PASSES ("C)
STEEL
NO.
78
FACTOR
MANUFACTURING CONDITION
TI
("(.)
858
(1)
R
(2)
787
HEATING TEMPERATURE
CONDITION
(3)
1250
(4)
30
FIRST HOT ROLUNG
(5)
1
SECOND HOT ROLLING
(10)
1070
(6)
50
(1 1)
90
(7)
160
(8)
1080
(9)
120
TF
(oc)
950
(12)
1
PI
(s)
40
(13)
11
TABLE 5

TABLE 7- 1
TEMPERATURE
TABLE 7-2
WAITING TIME UNTIL

TABLE 8-2
MICROSTRUCTURE
(1)PEARLITE FRACTION (%I (2)LAMELLAR SPACING (fl rn) (3)AVERAGE CRYSTAL GRAIN SIZE(fl rn)
(4)AVERAGE POLE DENSITY OF ORIENTATION GROUP OF [100]<011> TO [2231<110>
(5)POLE DENSITY OF CRYSTAL ORIENTATION OF [332)<113>
(6)AVERAGE HARDNESS IN 0 TO 5 fl m OF COMPOUND LAYER AFTER GAS NITROCARBURIZING (Hv(0.005 kgf))
(7)COMPOUND LAYER DEPTH AFTER GAS NITROCARBURIZING (fl m)
TABLE 9-1
(1)PEARUTE FRACTION (%) (2)LAMELLAR SPACING (fl m) (3)AVERAGE CRYSTAL GRAIN SIZE(p m)
(4)AVERAGE POLE DENSITY OF ORIENTATION GROUP OF [100]<011> TO [223)<110>
(5)POLE DENSITY OF CRYSTAL ORIENTATION OF [332)<113>
(6)AVERAGE HARDNESS IN 0 TO 5 fl rn OF COMPOUND LAYER AFTER GAS NITROCARBURIZING (Hv(0.005 kgD)
(7)COMPOUND LAYER DEPTH AFTER GAS NITROCARBURIZING (fl m)
TABLE 9-2
(1)PEARLITE FRACTION (%) (2)LAMELLAR SPACING (fl rn) (3)AVERAGE CRYSTAL GRAIN SIZE(fl m)
(4)AVERAGE POLE DENSITY OF ORIENTATION GROUP OF (100]<011> TO [223]<110>
(5)POLE DENSITY OF CRYSTAL ORIENTATION OF [332]<113>
(6)AVERAGE HARDNESS IN 0 TO 5 fl m OF COMPOUND LAYER AFTER GAS NITROCARBURIZING (Hv(0.005 kgf))
(7)COMPOUND LAYER DEPTH AFTER GAS NITROCARBURIZING (fl m)
MICROSTRUCTURE
55
56
57
58
59
60
7.5
15.2
13.5
14.0
15.2
15.1
0.8
1.1
1.1
1.1
1.1
1.1
9.5
8.0
12.5
12.5
12.0
5.0
2.0
2.0
2.0
2.1
4.2
5.3
3.0
2.9
3.0
3.2
4.9
5.4
300
450
450
450
450
450
19
12
13
13
12
12
340
362
346
335
368
386
448
476
455
441
484
499
33.5
31.5
33.0
34.0
31.0
26.0
6.5
6.5
6.5
5.9
3.2
3.0
89
84
7 6
7 9
60
63
-24
-48
10
10
6
-1 25
-20
-48
15
15
10
-67
TABLE 10-1
(1)PEARUTE FRACTION (%) (2)LAMELLAR SPACING (fi rn) (3)AVERAGE CRYSTAL GRAIN SIZE(fi rn)
(4)AVERAGE POLE DENSITY OF ORIENTATION GROUP OF [100]<011> TO [223]<110>
(5)POLE DENSITY OF CRYSTAL ORIENTATION OF [332]<113>
(6)AVERAGE HARDNESS IN 0 TO 5 fi rn OF COMPOUND LAYER AFTER GAS NITROCARBURIZING (Hv(0.005 kgD)
(7)COMPOUND LAYER DEPTH AFTER GAS NITROCARBURIZING (fi rn)
TABLE 10-2
(1)PEARLITE FRACTION (%) (2)LAMELLAR SPACING ( p m) (3)AVERAGE CRYSTAL GRAIN SIZE(/.f rn)
(4)AVERAGE POLE DENSITY OF ORIENTATION GROUP OF {100)<01 1> TO {223}<110>
(5)POLE DENSITY OF CRYSTAL ORIENTATION OF [332]<113>
(6)AVERAGE HARDNESS IN 0 TO 5 p rn OF COMPOUND LAYER AFTER GAS NITROCARBURIZING (Hv(0.005 kgf))
(7)COMPOUND LAYER DEPTH AFTER GAS NITROCARBURIZING ( p m)
MICROSTRUCTURE
TOUGHNESS
95
96
97
7.0
4.2
13.9
1.8
1.8
1.8
9.0
9.0
13.0
2.0
2.0
1.7
2.9
3.0
2.4
350
450
300
17
25
8
346
363
361
455
477
475
26.0
31.4
31.6
6.5
6.1
12.5
6 1
84
84
-1 6
-3 1
15
-7
-5
20
[Industrial Applicability]
[0115]
According to the present invention, a hot-rolled steel sheet for gas
nitrocarburizing, which includes improved isotropic workability capable of being
applied to a member which requires ductility and strict uniformity of a sheet thickness,
circularity, and impact resistance after processing, is obtained. The steel sheet, which
is manufactured by the present invention, can be used in a vehicle member such as an
inner sheet member, a structural member, a suspension arm, or a transmission which
requires ductility and strict uniformity of a sheet thickness, circularity, and impact
resistance after processing, and can be used in every use such as shipbuilding,
buildings, bridges, offshore structures, pressure vessels, line pipes, and machine parts.
Therefore, the present invention has high industrial value.

Claims
[claim 11
A hot-rolled steel sheet for gas nitmcarburizing comprising, by mass%,
C content LC]: C of more than 0.07% and equal to or less than 0.2%,
Si content [Si]: Si of 0.001% or more and 2.5% or less,
Mn content [Mn]: Mn of 0.01% or more and 4% or less, and
A1 content M:Al of 0.001% or more and 2% or less,
P content [PI limited to 0.15% or less,
S content [S] limited to 0.03% or less, and
N content [Nl limited to 0.01% or less,
Ti content [Td which satisfies the following Equation 1, and
the balance consisting of Fe and unavoidable impurities,
wherein an average pole density of an orientation group of {100)<011> to {223}<110>,
which is represented by an arithmetic average of a pole density of each orientation of {100)<011>,
{116)<110>, {114)<110>, {112)<110>, and {223)<110> is 1.0 or more and 4.0 or less, and a pole
density of a crystal orientation of {332)<113> is 1.0 or more and 4.8 or less, in a center portion of a
sheet thickness which is a range of the sheet thickness of 518 to 318 h m a surface of the steel sheet,
wherein an average grain size in a center in the sheet thickness is 10pm or less, and
wherein a mimstructure includes, by a structural &adion, pearlite of more than 6% and
ferrite in the balance.
0.005+[N]~48/14+[S]~48/321Tir0.015+[N]~48/14+[S]..~. (41)8 /32
[claim 21
The hot-rolled steel sheet for gas nitrocarburizing amrdmg to claim 1,
wherein the average pole density of the orientation group of {100)<011> to {223)<110> is
2.0 or less and the pole density of the crystal orientation of {332)<113> is 3.0 or less.
The hot-rolled steel sheet for gas nitrcarbwizing according to claim 1,
wherein the average grain size is 7pm or less.
[Claim 41
The hot-rolled steel sheet for gas nitrocarburizing according to any one of claims 1 to 3,
furether comprises any one or two or more of, by mass%,
Nb content [Nb]: Nb of 0.005% or more and 0.06% or less,
Cu content [Cd: Cu of 0.02% or more and 1.2% or less,
Ni content [Nil: Ni of 0.01% or more and 0.6% or less,
Mo content [Mo]: Mo of 0.01% or more and 1% or less,
V content [Vl: V of 0.01% or more and 0.2% or less,
Cr content [Cr]: Cr of 0.01% or more and 2% or less,
Mg content [Mg]: Mg of 0.0005% or more and 0.01% or less,
Ca content [Ca]: Ca of 0.0005% or more and 0.01% or less,
REM content kEd: R;EM of 0.0005% or more and 0.1% or less, and
B content [B]: B of 0.0002% or more and 0.002% or less.
[Clam 51
A manufacturing method of a hot-rolled steel sheet for gas nitrocarburking, the method
comprising:
performing a &st hot rolling, which includes one or more of rolling reduction having a
rolling-reduction ratio of 40% or more at a temperature range of 1000°C or more and 1200°C or less, ,
with respect to a steel ingot or a slab which includes, by mass%,
C content LC]: C of more than 0.07% and equal to or less than 0.2%,
Si content [Sd: Si of 0.001% or more and 2.5% or less,
Mn content [Mn]: Mn of 0.01% or more and 4% or less, and
Al content M1:Al of 0.001% or more and 2% or less, and
P content [PI limited to 0.15% or less,
S content [Sl limited to 0.03% or less, and
N content [N] limited to 0.01% or less,
content [Ti] contains Ti which satisfies the following Equation 1, and
the balance consists of Fe and unavoidable impurities;
starting a second hot rolling at a temperature range of 1000°C or more w i h 150 seconds
after a completion of the first hot rolling;
wherein the second rolling includes one or more of rolling reduction having a
mhg-reduction ratio of 30% or more in a temperature range of T1+ 30°C or more and T1+ 200°C
or less when temperature determined by a component of the steel sheet in the following Equation 2
is defined as Tl°C in the second hot rolling and a total of the rollingreduction ratio is 50% or more;
performing a third hot rohg, in which a total of the rollingredudion ratio is 30% or less,
at a temperature range equal to or more than an Ar3 transformation point temperature and less
than T1+ 30°C;
ending the hot rolling3 at theAr3 transformation point temperature or more;
when a pass having rolling-reduction ratio of 30% or more at the temperature range of T1
+ 30°C or more and T1+ 200°C or less is a large mhg-reduction pass, performing a cooling, in
which a cooling temperature change is 40°C or more and 140°C or less and a cooling end
temperature is T1+ 100°C or less, at a cooling rate of 50°C/second or more so that a waiting time t
second from a completion of a hd pass of the large rolling-reduction passes to a start of the cooling
sati&es the following Equation 3; and
coding the steel sheet at more than 550°C.
0.005+[N]x48/14+[S]x48/32~Ti~0.015+[N]x48/14+[S]x48/32...(1)
T1=850+10x([C]+[N])x~+350x[Nb]+250x[~+40x[B]+10x[Cr]+100x
Here, tl is represented by the following Equation (4).
tl =O.OOl x ((Tf-TI) x PI/ 100)2-0.109 x ((Tf-T1) x P11100) +3.1... (4)
Here, Tf is a temperature PC) after the final pass rolling reduction of the large
rolling-reduction passes and P1 is a rolling-reduction ratio ('?of ?theI final pass of the large
rolling-reduction passes.
[Claim 61
The manufactwmg method of a hot-rolled steel sheet for gas nitmarburizing according to ,
claim 5,
wherein the primary cooling performs cooling between rolling stands.
[Claim 71
The manufacturing method of a hot-rolled steel sheet for gas nitrocarburizing according to
claim 5 or 6,
wherein the waiting time t second further satisfies the following Equation 5.
tl