U.S. patent application number 16/340867 was filed with the patent office on 2021-07-08 for steel sheet for carburizing, and method for manufacturing steel sheet for carburizing.
This patent application is currently assigned to NIPPON STEEL CORPORATION. The applicant listed for this patent is NIPPON STEEL CORPORATION. Invention is credited to Motonori HASHIMOTO, Kazuo HIKIDA, Yuri TODA.
Application Number | 20210207235 16/340867 |
Document ID | / |
Family ID | 1000005519658 |
Filed Date | 2021-07-08 |
United States Patent
Application |
20210207235 |
Kind Code |
A1 |
HIKIDA; Kazuo ; et
al. |
July 8, 2021 |
STEEL SHEET FOR CARBURIZING, AND METHOD FOR MANUFACTURING STEEL
SHEET FOR CARBURIZING
Abstract
[Object] To provide a steel sheet for carburizing having further
improved formability and toughness after carburizing, and a method
for manufacturing the same. [Solution] A steel sheet consisting of,
in mass %, C: more than or equal to 0.02%, and less than 0.30%, Si:
more than or equal to 0.005%, and less than or equal to 0.5%, Mn:
more than or equal to 0.01%, and less than or equal to 3.0%, P:
less than or equal to 0.1%, S: less than or equal to 0.1%, sol. Al:
more than or equal to 0.0002%, and less than or equal to 3.0%, N:
more than or equal to 0.0001, and less than or equal to 0.035%, and
the balance: Fe and impurities, in which average crystal grain size
of ferrite is smaller than 10 .mu.m, average equivalent circle
diameter of carbide is 5.0 .mu.m or smaller, percentage of number
of carbides with an aspect ratio of 2.0 or smaller is 80% or larger
relative to the total carbides, percentage of number of carbides
present in ferrite crystal grain is 60% or larger relative to the
total carbides, and average nitrogen concentration in a region
ranging from topmost surface of steel sheet to a depth of 50 .mu.m
is 0.040 mass % or higher and 0.200 mass % or lower.
Inventors: |
HIKIDA; Kazuo; (Tokyo,
JP) ; TODA; Yuri; (Tokyo, JP) ; HASHIMOTO;
Motonori; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL CORPORATION
Tokyo
JP
|
Family ID: |
1000005519658 |
Appl. No.: |
16/340867 |
Filed: |
October 2, 2018 |
PCT Filed: |
October 2, 2018 |
PCT NO: |
PCT/JP2018/036950 |
371 Date: |
April 10, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/001 20130101;
C21D 8/0263 20130101; C21D 1/06 20130101; C21D 6/008 20130101; C23C
8/02 20130101; C21D 6/007 20130101; C22C 38/06 20130101; C22C 38/54
20130101; C21D 9/46 20130101; C22C 38/04 20130101; C21D 8/0205
20130101; C22C 38/52 20130101; C22C 38/002 20130101; C22C 38/50
20130101; C23C 8/22 20130101; C21D 6/004 20130101; C22C 38/02
20130101; C21D 8/0226 20130101; C22C 38/46 20130101; C22C 38/42
20130101; C22C 38/44 20130101; C22C 38/48 20130101; C21D 6/005
20130101; C21D 8/0236 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00; C21D 1/06 20060101 C21D001/06; C23C 8/22 20060101
C23C008/22; C23C 8/02 20060101 C23C008/02; C22C 38/54 20060101
C22C038/54; C22C 38/52 20060101 C22C038/52; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00 |
Claims
1. A steel sheet for carburizing consisting of, in mass %, C: more
than or equal to 0.02%, and less than 0.30%, Si: more than or equal
to 0.005%, and less than or equal to 0.5%, Mn: more than or equal
to 0.01%, and less than or equal to 3.0%, P: less than or equal to
0.1%, S: less than or equal to 0.1%, sol. Al: more than or equal to
0.0002%, and less than or equal to 3.0%, N: more than or equal to
0.0001%, and less than or equal to 0.035%, and the balance: Fe and
impurities, wherein average crystal grain size of ferrite is
smaller than 10 average equivalent circle diameter of carbide is
5.0 .mu.m or smaller, percentage of number of carbides with an
aspect ratio of 2.0 or smaller is 80% or larger relative to the
total carbides, percentage of number of carbides present in ferrite
crystal grain is 60% or larger relative to the total carbides, and
average nitrogen concentration in a region ranging from topmost
surface of steel sheet to a depth of 50 .mu.m is 0.040 mass % or
higher and 0.200 mass % or lower.
2. The steel sheet for carburizing according to claim 1, further
comprising, in place of part of the balance Fe, one of, or two or
more of, in mass %, Cr: more than or equal to 0.005%, and less than
or equal to 3.0%, Mo: more than or equal to 0.005%, and less than
or equal to 1.0%, Ni: more than or equal to 0.010%, and less than
or equal to 3.0%, Cu: more than or equal to 0.001%, and less than
or equal to 2.0%, Co: more than or equal to 0.001%, and less than
or equal to 2.0%, Nb: more than or equal to 0.010%, and less than
or equal to 0.150%, Ti: more than or equal to 0.010%, and less than
or equal to 0.150%, V: more than or equal to 0.0005%, and less than
or equal to 1.0%, and B: more than or equal to 0.0005%, and less
than or equal to 0.01%.
3. The steel sheet for carburizing according to claim 1, further
comprising, in place of part of the balance Fe, at least either one
of, in mass %, W: less than or equal to 1.0%, or Ca: less than or
equal to 0.01%.
4. A method for manufacturing the steel sheet for carburizing
according to claim 1, the method comprising: a hot-rolling step, in
which a steel material having the chemical composition of said
steel sheet is heated, hot finish rolling is terminated in a
temperature range of 800.degree. C. or higher and lower than
920.degree. C., followed by winding at a temperature of 700.degree.
C. or lower; and an annealing step, in which the steel sheet
obtained by the hot-rolling step, or, the steel sheet having been
cold-rolled subsequently to the hot-rolling step is heated in an
atmosphere with nitrogen concentration controlled to 25% or higher
in volume fraction, at an average heating rate of 5.degree. C./h or
higher and 100.degree. C./h or lower, up into a temperature range
not higher than point Ac.sub.1 defined by equation (1) below,
annealed in the temperature range not higher than the point
Ac.sub.1 for 10 h or longer and 100 h or shorter, and then cooled
at an average cooling rate of 5.degree. C./h or higher and
100.degree. C./h or lower in a temperature range from a temperature
at the end of annealing down to 550.degree. C., in the hot-rolling
step, cooling being started within one second after end of the hot
finish rolling, at an average cooling rate of higher than
50.degree. C./s, and an average grain size of ferrite after the
annealing being controlled to smaller than 10 .mu.m, where in
equation (1) below, notation [X] represents the content of element
X (in mass %), which is substituted by zero if such element X is
absent, Ac 1 = 750.8 - 26.6 [ C ] + 17.6 [ Si ] - 11.6 [ Mn ] -
22.9 [ Cu ] - 23 [ Ni ] + 24.1 [ Cr ] + 22.5 [ Mo ] - 39.7 [ V ] -
5.7 [ Ti ] + 232.4 [ Nb ] - 169.4 [ Al ] - 894.7 [ B ] . Equation (
1 ) ##EQU00004##
5. A method for manufacturing the steel sheet for carburizing
according to claim 4, further comprising: a continuous casting step
for obtaining the steel material to be subjected to the hot-rolling
step, in which at least either soundness enhancing treatment of the
steel material, namely production of a predetermined inclusion, or
reduction of center segregation of a predetermined element, is
carried out.
6. A steel sheet for carburizing comprising, in mass %, C: more
than or equal to 0.02%, and less than 0.30%, Si: more than or equal
to 0.005%, and less than or equal to 0.5%, Mn: more than or equal
to 0.01%, and less than or equal to 3.0%, P: less than or equal to
0.1%, S: less than or equal to 0.1%, sol. Al: more than or equal to
0.0002%, and less than or equal to 3.0%, N: more than or equal to
0.0001%, and less than or equal to 0.035%, and the balance
comprising Fe and impurities, wherein average crystal grain size of
ferrite is smaller than 10 .mu.m, average equivalent circle
diameter of carbide is 5.0 .mu.m or smaller, percentage of number
of carbides with an aspect ratio of 2.0 or smaller is 80% or larger
relative to the total carbides, percentage of number of carbides
present in ferrite crystal grain is 60% or larger relative to the
total carbides, and average nitrogen concentration in a region
ranging from topmost surface of steel sheet to a depth of 50 .mu.m
is 0.040 mass % or higher and 0.200 mass % or lower.
7. The steel sheet for carburizing according to claim 6, further
comprising, in place of part of the balance Fe, one of, or two or
more of, in mass %, Cr: more than or equal to 0.005%, and less than
or equal to 3.0%, Mo: more than or equal to 0.005%, and less than
or equal to 1.0%, Ni: more than or equal to 0.010%, and less than
or equal to 3.0%, Cu: more than or equal to 0.001%, and less than
or equal to 2.0%, Co: more than or equal to 0.001%, and less than
or equal to 2.0%, Nb: more than or equal to 0.010%, and less than
or equal to 0.150%, Ti: more than or equal to 0.010%, and less than
or equal to 0.150%, V: more than or equal to 0.0005%, and less than
or equal to 1.0%, and B: more than or equal to 0.0005%, and less
than or equal to 0.01%.
8. The steel sheet for carburizing according to claim 6, further
comprising, in place of part of the balance Fe, at least either one
of, in mass %, W: less than or equal to 1.0%, or Ca: less than or
equal to 0.01%.
9. A method for manufacturing the steel sheet for carburizing
according to claim 6, the method comprising: a hot-rolling step, in
which said steel material having the chemical composition of said
steel sheet is heated, hot finish rolling is terminated in a
temperature range of 800.degree. C. or higher and lower than
920.degree. C., followed by winding at a temperature of 700.degree.
C. or lower; and an annealing step, in which the steel sheet
obtained by the hot-rolling step, or, the steel sheet having been
cold-rolled subsequently to the hot-rolling step is heated in an
atmosphere with nitrogen concentration controlled to 25% or higher
in volume fraction, at an average heating rate of 5.degree. C./h or
higher and 100.degree. C./h or lower, up into a temperature range
not higher than point Ac.sub.1 defined by equation (1) below,
annealed in the temperature range not higher than the point
Ac.sub.1 for 10 h or longer and 100 h or shorter, and then cooled
at an average cooling rate of 5.degree. C./h or higher and
100.degree. C./h or lower in a temperature range from a temperature
at the end of annealing down to 550.degree. C., in the hot-rolling
step, cooling being started within one second after end of the hot
finish rolling, at an average cooling rate of higher than
50.degree. C./s, and an average grain size of ferrite after the
annealing being controlled to smaller than 10 .mu.m, where in
equation (1) below, notation [X] represents the content of element
X (in mass %), which is substituted by zero if such element X is
absent, Ac 1 = 750.8 - 26.6 [ C ] + 17.6 [ Si ] - 11.6 [ Mn ] -
22.9 [ Cu ] - 23 [ Ni ] + 24.1 [ Cr ] + 22.5 [ Mo ] - 39.7 [ V ] -
5.7 [ Ti ] + 232.4 [ Nb ] - 169.4 [ Al ] - 894.7 [ B ] . Equation (
1 ) ##EQU00005##
10. A method for manufacturing the steel sheet for carburizing
according to claim 9, further comprising: a continuous casting step
for obtaining the steel material to be subjected to the hot-rolling
step, in which at least either soundness enhancing treatment of the
steel material, namely production of a predetermined inclusion, or
reduction of center segregation of a predetermined element, is
carried out.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel sheet for
carburizing, and a method for manufacturing the steel sheet for
carburizing.
BACKGROUND ART
[0002] In recent years, mechanical and structural parts such as
automotive gear, clutch plate and damper have been required to be
highly durable, and in addition to be manufacturable at low costs.
These parts have widely been manufactured by cutting and
carburizing using hot-forged materials. However, in response to
increasing need for cost reduction, having been developed are
technologies by which hot-rolled steel sheet or cold-rolled steel
sheet, employed as a starting material, is cold-worked into shapes
of the parts, followed by carburizing. In the cold-working,
components are formed by punching materials, followed by bending,
drawing, hole expansion or the like. In this process, a steel sheet
for carburizing to be worked is required to have good bendability
which relates to a most basic deformation mode. In addition,
automotive components such as damper for torque converter are
required to have excellent impact resistant characteristics
including toughness. From this point of view, a variety of
technologies have recently been proposed.
[0003] For example, Patent Literature 1 listed below proposes a
technology for forming a structure of a hot-rolled steel sheet with
ferrite and pearlite, and then spherodizing carbide by spherodizing
annealing.
[0004] Meanwhile, Patent Literature 2 listed below proposes a
technology for improving impact characteristics of a carburized
member, by controlling particle size of carbide, as well as
controlling percentage of the number of carbides at ferrite crystal
grain boundaries relative to the number of carbides within ferrite
particles, and further by controlling crystal size of the ferrite
matrix.
[0005] Moreover, Patent Literature 3 listed below proposes a
technology for improving cold workability, by controlling particle
size and aspect ratio of carbide, as well as controlling crystal
size of ferrite matrix, and further by controlling aspect ratio of
ferrite.
CITATION LIST
Patent Literature
[0006] Patent Literature 1: JP 3094856B
[0007] Patent Literature 2: WO 2016/190370
[0008] Patent Literature 3: WO 2016/148037
SUMMARY OF INVENTION
Technical Problem
[0009] The aforementioned mechanical and structural parts are
required to be hardenable for enhanced strength. That is, the
materials used for mechanical and structural parts are required to
satisfy formability, while keeping hardenability. In addition, the
mechanical and structural parts after carburized are required to
have impact resistance characteristics (particularly, toughness
after carburizing).
[0010] With the manufacturing method disclosed in Patent Literature
1, mainly relying upon control of a microstructure of carbide, is
however not expected to effectively improve the toughness after
carburizing, although the method might improve impact resistance
characteristics originated from cracks that may be introduced
during the cold-working. Meanwhile, the manufacturing method
proposed in Patent Literature 2, mainly relying upon control of
microstructures of carbide and ferrite, might improve the
formability, but still have room for improvement in pursuit of more
advanced toughness, if intended to be applied to specific
automotive components such as damper for automotive torque
converter, for which a high level of impact resistance is required.
Furthermore, use of the technology proposed in Patent Literature 3
might improve the formability, but still have room for improvement
in pursuit of more advanced toughness, if intended to be applied to
specific automotive components such as damper of automotive torque
converter, for which a high level of impact resistance is required.
As described above, the ever-proposed technologies still have room
for improvement in an effort to obtain a sufficient level of
toughness after carburizing, while keeping formability and
hardenability of the steel sheet for carburizing. Hence, there has
been desired the steel sheet for carburizing, which is more
suitably applicable to specific automotive components such as
damper of torque converter, for which a high level of impact
resistance is required.
[0011] The present invention was made in consideration of the
aforementioned problems, an object of which is to provide a steel
sheet for carburizing further improved in the formability, and
toughness after carburizing, and a method for manufacturing the
same.
Solution to Problem
[0012] The present inventors made thorough investigations into
methods of solving the aforementioned problems, and consequently
reached an idea that, as detailed later, the formability during
cold-working and the toughness after carburizing may be improved
while keeping the hardenability, by appropriately controlling
position of production of carbides in ferrite crystal grain, and
nitrogen concentration in a skin layer of the steel sheet, to
complete the present invention.
[0013] Summary of the present invention reached on the basis of
such idea is as follows. [0014] [1]
[0015] A steel sheet for carburizing consisting of, in mass %,
[0016] C: more than or equal to 0.02%, and less than 0.30%, [0017]
Si: more than or equal to 0.005%, and less than or equal to 0.5%,
[0018] Mn: more than or equal to 0.01%, and less than or equal to
3.0%, [0019] P: less than or equal to 0.1%, [0020] S: less than or
equal to 0.1%, [0021] sol. Al: more than or equal to 0.0002%, and
less than or equal to 3.0%, [0022] N: more than or equal to 0.0001,
and less than or equal to 0.035%, and [0023] the balance: Fe and
impurities,
[0024] in which average crystal grain size of ferrite is smaller
than 10 .mu.m,
[0025] average equivalent circle diameter of carbide is 5.0 .mu.m
or smaller,
[0026] percentage of number of carbides with an aspect ratio of 2.0
or smaller is 80% or larger relative to the total carbides,
[0027] percentage of number of carbides present in ferrite crystal
grain is 60% or larger relative to the total carbides, and
[0028] average nitrogen concentration in a region ranging from
topmost surface of steel sheet to a depth of 50 .mu.m is 0.040 mass
% or higher and 0.200 mass % or lower. [0029] [2]
[0030] The steel sheet for carburizing according to [1], further
including, in place of part of the balance Fe, one of, or two or
more of, in mass %, [0031] Cr: more than or equal to 0.005%, and
less than or equal to 3.0%, [0032] Mo: more than or equal to
0.005%, and less than or equal to 1.0%, [0033] Ni: more than or
equal to 0.010%, and less than or equal to 3.0%, [0034] Cu: more
than or equal to 0.001%, and less than or equal to 2.0%, [0035] Co:
more than or equal to 0.001%, and less than or equal to 2.0%,
[0036] Nb: more than or equal to 0.010%, and less than or equal to
0.150%, [0037] Ti: more than or equal to 0.010%, and less than or
equal to 0.150%, [0038] V: more than or equal to 0.0005%, and less
than or equal to 1.0%, and [0039] B: more than or equal to 0.0005%,
and less than or equal to 0.01%. [0040] [3]
[0041] The steel sheet for carburizing according to [1] or [2],
further including, in place of part of the balance Fe, at least
either one of, in mass %, [0042] W: less than or equal to 1.0%, or
[0043] Ca: less than or equal to 0.01%. [0044] [4]
[0045] A method for manufacturing the steel sheet for carburizing
according to any one of [1] to [3], the method including:
[0046] a hot-rolling step, in which a steel material having the
chemical composition according to any one of [1] to [3] is heated,
hot finish rolling is terminated in a temperature range of
800.degree. C. or higher and lower than 920.degree. C., followed by
winding at a temperature of 700.degree. C. or lower; and
[0047] an annealing step, in which the steel sheet obtained by the
hot-rolling step, or, the steel sheet having been cold-rolled
subsequently to the hot-rolling step is heated in an atmosphere
with nitrogen concentration controlled to 25% or higher in volume
fraction, at an average heating rate of 5.degree. C./h or higher
and 100.degree. C./h or lower, up into a temperature range not
higher than point Ac.sub.1 defined by equation (1) below, annealed
in the temperature range not higher than the point Ac.sub.1 for 10
h or longer and 100 h or shorter, and then cooled at an average
cooling rate of 5.degree. C./h or higher and 100.degree. C./h or
lower in a temperature range from a temperature at the end of
annealing down to 550.degree. C.,
[0048] in the hot-rolling step, cooling being started within one
second after end of the hot finish rolling, at an average cooling
rate of higher than 50.degree. C./s, and
[0049] an average grain size of ferrite after the annealing being
controlled to smaller than 10 .mu.m. [0050] [5] A method for
manufacturing the steel sheet for carburizing according to [4],
which further includes a continuous casting step for obtaining the
steel material to be subjected to the hot-rolling step, in which at
least either soundness enhancing treatment of the steel material,
namely production of a predetermined inclusion, or reduction of
center segregation of a predetermined element, is carried out.
[0050] [ Math . 1 ] Ac 1 = 750.8 - 26.6 [ C ] + 17.6 [ Si ] - 11.6
[ Mn ] - 22.9 [ Cu ] - 23 [ Ni ] + 24.1 [ Cr ] + 22.5 [ Mo ] - 39.7
[ V ] - 5.7 [ Ti ] + 232.4 [ Nb ] - 169.4 [ Al ] - 894.7 [ B ]
Equation ( 1 ) ##EQU00001##
[0051] In equation (1) above, notation [X] represents the content
of element X (in mass %), which is substituted by zero if such
element X is absent.
Advantageous Effects of Invention
[0052] As explained above, according to the present invention, it
now becomes possible to provide a steel sheet for carburizing
having further improved formability and toughness after
carburizing.
DESCRIPTION OF EMBODIMENTS
[0053] Preferred embodiments of the present invention will be
detailed below.
(Details of Examination Made by Present Inventors, and Reached
Idea)
[0054] Prior to description on the steel sheet for carburizing and
the method for manufacturing the same according to the present
invention, the examination made by the present inventors, aimed at
solving the aforementioned problems, will be detailed below.
[0055] In the examination, the present inventors started first by
examining a method for improving formability (particularly,
bendability) before carburizing.
[0056] In order to improve the formability (particularly,
bendability) before carburizing, it is important to suppress
cracking during bending deformation, and further to suppress, if
the cracking once occurred, propagation of the produced crack.
Control of the aspect ratio (long axis/short axis) of carbide
produced in the steel sheet is effective to suppress the cracking,
posing importance of reduction of the aspect ratio of carbide by
spherodizing annealing. Meanwhile, suppression of production of
coarse carbide, and control of position of precipitation of carbide
are effective to suppress propagation of the crack. That is, since
carbide produced in the grain boundary of ferrite can promote the
crack to propagate while routed through the grain boundary, so that
it is important to produce carbide inside crystal grains of
ferrite. Such propagation of crack through the grain boundary is
considered to be suppressed by producing carbide inside the crystal
grains of ferrite.
[0057] After employing such structural control, the present
inventors further focused on improvement of toughness through
condensation of nitrogen in a skin layer of the steel sheet for
carburizing, in pursuit of a method for improving the impact
resistance characteristics after carburizing, and made thorough
investigations and researches on operations and effects of the
nitrogen condensation in the skin layer of the steel sheet. The
present inventors consequently found that the toughness after
carburizing (particularly, impact value at room temperature) may be
dramatically improved by controlling the nitrogen concentration in
the skin layer of the steel sheet. More specifically, it was found
that nitrogen, in the process of annealing a hot-rolled steel sheet
or a cold-rolled steel sheet, was successfully condensed in the
skin layer of the steel sheet for carburizing, by controlling the
nitrogen concentration in an atmosphere at a level not lower than a
predetermined threshold value, and that the impact value at room
temperature of carburized member made of the steel sheet for
carburizing was dramatically improved as a consequence.
[0058] A possible mechanism of improvement of toughness after
carburizing is as follows. By annealing the steel sheet in a
nitrogen-rich atmosphere, the atmospheric nitrogen enters the steel
sheet to form nitride in the skin layer of the steel sheet. The
thus formed nitride is mainly composed of fine AIN, and can
demonstrate an effect of suppressing growth of grains of prior
austenite during carburizing heat treatment. Grain size of prior
austenite and grain size of transformed martensite are in a
proportional relationship. It is therefore contemplated that if the
grains of prior austenite are suppressed from growing by such fine
AlN, also the grain size of martensite in a structure of carburized
member will be micronized, and the impact value dramatically
increased as a consequence. Extensive investigations by the present
inventors revealed that fine AIN was found to be produced in the
skin layer of the steel sheet for carburizing, resulted in increase
of impact value in the carburized member.
[0059] Note that the aforementioned bendability and toughness after
carburizing will become inferior as the strength of steel sheet
increases. Meanwhile from the viewpoint of satisfying a necessary
level of hardenability for the steel sheet for carburizing, the
steel sheet is desired to be strengthened. In order to balance
these contradictory characteristics, the key is to satisfy the
hardenability by way of the above-outlined structural control, as
well as to improve the bendability and toughness after carburizing.
Hence, through the above-outlined structural control, obtainable is
the steel sheet for carburizing that is well balanced among the
hardenability, bendability, and toughness after carburizing.
[0060] The present inventors have succeeded, by the aforementioned
structural control of steel sheet, in improving the bendability
during cold-working and the toughness after carburizing, while
keeping the hardenability. In this way, it now becomes possible to
obtain the steel sheet for carburizing well balanced among the
hardenability, formability, and toughness after carburizing.
[0061] The steel sheet for carburizing and the method for
manufacturing the same according to embodiments of the present
invention, as detailed later, have been reached on the basis of the
aforementioned findings. Paragraphs below will detail the steel
sheet for carburizing and the method for manufacturing the same
according to the embodiments reached on the basis of the
findings.
(Steel Sheet for Carburizing)
[0062] First, the steel sheet for carburizing according to the
embodiment of the present invention will be detailed.
[0063] The steel sheet for carburizing according to the embodiment
has a predetermined chemical composition detailed below. In
addition, the steel sheet for carburizing according to this
embodiment has a specific microstructure featured by that the
average equivalent circle diameter of carbide is 5.0 .mu.m or
smaller; that the percentage of the number of carbides with an
aspect ratio of 2.0 or smaller is 80% or larger relative to the
total carbides; that the percentage of the number of carbides
present in ferrite crystal grain is 60% or larger relative to the
total carbides; and that the nitrogen concentration in a region
ranging from the topmost surface of the steel sheet to a depth of
50 .mu.m is 0.040 mass % or higher and 0.200 mass % or lower. Hence
the steel sheet for carburizing according to this embodiment will
have further improved formability and toughness after carburizing,
while keeping the hardenability.
<Chemical Composition of Steel Sheet for Carburizing>
[0064] First, chemical ingredients at the middle-thickness portion
of the steel sheet for carburizing according to this embodiment
will be detailed. Note that in the following description, notation
"%" relevant to the chemical components means "mass %", unless
otherwise specifically noted. [0065] [C: More than or Equal to
0.02%, and Less than 0.30%]
[0066] C (carbon) is an element necessary for keeping strength at
the center of thickness of a finally obtainable carburized member.
In the steel sheet for carburizing, C is also an element
solid-soluted into the grain boundary of ferrite to enhance the
strength of the grain boundary, to thereby contribute to
improvement of the bendability.
[0067] With the content of C less than 0.02%, the aforementioned
effect of improving the bendability will not be obtained. Hence the
content of C in the steel sheet for carburizing according to the
embodiment is specified to be more than or equal to 0.02%. The
content of C is preferably more than or equal to 0.05%. Meanwhile,
with the content of C more than or equal to 0.30%, carbide will
have an average equivalent circle diameter exceeding 5.0 .mu.m,
thereby the bendability will degrade. Hence the content of C in the
steel sheet for carburizing according to the embodiment is
specified to be less than 0.30%. The content of C is preferably
less than or equal to 0.20%. Note that, taking a balance between
bendability and hardenability into account, the content of C is
further preferably be less than or equal to 0.10%.
[Si: More than or Equal to 0.005%, and Less than or Equal to
0.5%]
[0068] Si (silicon) is an element that acts to deoxidize molten
steel to improve soundness of the steel. With the content of Si
less than 0.005%, the molten steel will not thoroughly be
deoxidized. Hence the content of silicon in the steel sheet for
carburizing according to the embodiment is specified to be more
than or equal to 0.005%. The content of Si is preferably more than
or equal to 0.01%. Meanwhile, with the content of Si more than
0.5%, Si having been solid-soluted in carbide will stabilize the
carbide and will allow the carbide to have an average equivalent
circle diameter exceeding 5.0 .mu.m, degrading the bendability.
Hence the content of Si in the steel sheet for carburizing
according to the embodiment is specified to be less than or equal
to 0.5%. The content of Si is preferably less than or equal to
0.3%.
[Mn: More than or Equal to 0.01%, and Less than or Equal to
3.0%]
[0069] Mn (manganese) is an element that acts to deoxidize molten
steel to improve soundness of the steel. With the content of Mn
less than 0.01%, the molten steel will not thoroughly be
deoxidized. Hence the content of Mn in the steel sheet for
carburizing according to the embodiment is specified to be more
than or equal to 0.01%. The content of Mn is preferably more than
or equal to 0.1%. Meanwhile, with the content of Mn more than 3.0%,
Mn having been solid-soluted in carbide will stabilize the carbide
and will allow the carbide to have an average equivalent circle
diameter exceeding 5.0 pm, degrading the bendability. Hence the
content of Mn is specified to be less than or equal to 3.0. The
content of Mn is more preferably less than or equal to 2.0%, and
even more preferably less than or equal to 1.0%.
[P: Less than or Equal to 0.1%]
[0070] P (phosphorus) is an element that segregates in the grain
boundary of ferrite to degrade the bendability. With the content of
P exceeding 0.1%, the grain boundary will have considerably reduced
strength, and thereby the bendability will degrade. Hence, the
content of P in the steel sheet for carburizing according to the
embodiment is specified to be less than or equal to 0.1%. The
content of P is preferably less than or equal to 0.050%, and more
preferably less than or equal to 0.020%. Note that the lower limit
of the content of P is not specifically limited. The content of P
reduced below 0.0001% will however considerably increase cost for
dephosphorization, causing economic disadvantage. Hence the lower
limit of content of P will substantially be 0.0001% for practical
steel sheet.
[S: Less than or Equal to 0.1%]
[0071] S (sulfur) is an element that can form an inclusion to
degrade the bendability. With the content of S exceeding 0.1%, a
coarse inclusion will be produced, and thereby the bendability will
degrade. Hence the content of S in the steel sheet for carburizing
according to the embodiment is specified to be less than or equal
to 0.1%. The content of S is preferably less than or equal to
0.010%, and more preferably less than or equal to 0.008%. Note that
the lower limit of content of S is not specifically limited. The
content of S reduced below 0.0005% will however considerably
increase cost for desulfurization, causing economic disadvantage.
Hence, the lower limit of content of S will substantially be
0.0005% for practical steel sheet.
[sol. Al: More than or Equal to 0.0002%, and Less than or Equal to
3.0%]
[0072] Al (aluminum) is an element that acts to deoxidize molten
steel to improve soundness of the steel. With the content of Al
less than 0.0002%, the molten steel will not thoroughly be
deoxidized. Hence the content of Al (in more detail, the content of
sol. Al) in the steel sheet for carburizing according to the
embodiment is specified to be more than or equal to 0.0002%. The
content of Al is preferably more than or equal to 0.0010%, more
preferably more than or equal to 0.0050%, and even more preferably
more than or equal to 0.010%. Meanwhile, with the content of Al
exceeding 3.0%, coarse oxide will be produced, and thereby the
bendability will degrade. Hence the content of Al is specified to
be less than or equal to 3.0%. The content of Al is preferably less
than or equal to 2.5%, more preferably less than or equal to 1.0%,
even more preferably less than or equal to 0.2%, and yet more
preferably less than or equal to 0.05%.
[N: More than or Equal to 0.0001%, and Less than or Equal to
0.035%]
[0073] The content of N (nitrogen) in the steel sheet for
carburizing according to this embodiment need be less than or equal
to 0.035%. Note that the content of N defined now is an average
value of N present throughout the thickness direction of the steel
sheet (an average value of the content of N in the thickness
direction). With the content of N exceeding 0.035%, a large amount
of nitride will be precipitated throughout the thickness direction
of the steel sheet for carburizing, making it difficult to obtain
desired bendability. Hence, the content of N in the steel sheet for
carburizing according to the embodiment is specified to be less
than or equal to 0.035%. The content of N is preferably less than
or equal to 0.030%, more preferably less than or equal to 0.020%,
and even more preferably less than or equal to 0.010%. The lower
limit of content of N is not specifically limited. The content of N
reduced below 0.0001% will however considerably increase cost for
denitrification, causing economic disadvantage. Hence, the lower
limit of content of N will substantially be 0.0001% for practical
steel sheet. Alternatively, in consideration of fully introducing
nitrogen into the skin layer of the steel sheet, the content of N
may be specified to be 0.0020% or larger.
[Cr: More than or Equal to 0.005%, and Less than or Equal to
3.0%]
[0074] Cr (chromium) is an element having an effect of increasing
the hardenability of the finally obtainable carburized member, and
is also an element, for the steel sheet for carburizing, having an
effect of micronizing ferrite crystal grains to further improve the
toughness after carburizing. Hence in the steel sheet for
carburizing according to the embodiment, Cr may be contained as
needed. In order to obtain more enhanced effect of toughness after
carburizing, the content of Cr, if contained, is preferably
specified to be more than or equal to 0.005%. The content of Cr is
more preferably more than or equal to 0.010%. Further, in
consideration of the effects of production of carbide and nitride,
the content of Cr is preferably less than or equal to 3.0%, in view
of obtaining more enhanced effect of toughness after carburizing.
The content of Cr is more preferably less than or equal to 2.0%,
and even more preferably less than or equal to 1.6%.
[Mo: More than or Equal to 0.005%, and Less than or Equal to
1.0%]
[0075] Mo (molybdenum) is an element having an effect of increasing
the hardenability of the finally obtainable carburized member, and
is also an element, for the steel sheet for carburizing, having an
effect of micronizing ferrite crystal grains to further improve the
toughness after carburizing. Hence in the steel sheet for
carburizing according to the embodiment, Mo may be contained as
needed. In order to obtain more enhanced effect of toughness after
carburizing, the content of Mo, if contained, is preferably
specified to be more than or equal to 0.005%. The content of Mo is
more preferably more than or equal to 0.010%. Further, in
consideration of the effects of production of carbide and nitride,
the content of Mo is preferably less than or equal to 1.0%, in view
of obtaining more enhanced effect of toughness after carburizing.
The content of Mo is more preferably less than or equal to
0.8%.
[Ni: More than or Equal to 0.010%, and Less than or Equal to
3.0%]
[0076] Ni (nickel) is an element having an effect of increasing the
hardenability of the finally obtainable carburized member, and is
also an element, for the steel sheet for carburizing, having an
effect of micronizing ferrite crystal grains to further improve the
toughness after carburizing. Hence in the steel sheet for
carburizing according to the embodiment, Ni may be contained as
needed. In order to obtain more enhanced effect of toughness after
carburizing, the content of Ni, if contained, is preferably
specified to be more than or equal to 0.010%. The content of Ni is
more preferably more than or equal to 0.050%. Further, in
consideration of the effects of segregation of Ni in the grain
boundary of ferrite, the content of Ni is preferably less than or
equal to 3.0%, in view of obtaining more enhanced effect of
toughness after carburizing. The content of Ni is more preferably
less than or equal to 2.0%, even more preferably less than or equal
to 1.0%, and yet more preferably less than or equal to 0.5%.
[Cu: More than or Equal to 0.001%, and Less than or Equal to
2.0%]
[0077] Cu (copper) is an element having an effect of increasing the
hardenability of the finally obtainable carburized member, and is
also an element, for the steel sheet for carburizing, having an
effect of micronizing ferrite crystal grains to further improve the
toughness after carburizing. Hence in the steel sheet for
carburizing according to the embodiment, Cu may be contained as
needed. In order to obtain more enhanced effect of toughness after
carburizing, the content of Cu, if contained, is preferably
specified to be more than or equal to 0.001%. The content of Cu is
more preferably more than or equal to 0.010%. Further, in
consideration of the effects of segregation of Cu in the grain
boundary of ferrite, the content of Cu is preferably less than or
equal to 2.0%, in view of obtaining more enhanced effect of
toughness after carburizing. The content of Cu is more preferably
less than or equal to 0.80%.
[Co: More than or Equal to 0.001%, and Less than or Equal to
2.0%]
[0078] Co (cobalt) is an element having an effect of increasing the
hardenability of the finally obtainable carburized member, and is
also an element, for the steel sheet for carburizing, having an
effect of micronizing crystal grains to further improve the
toughness after carburizing. Hence in the steel sheet for
carburizing according to the embodiment, Co may be contained as
needed. In order to obtain more enhanced effect of toughness after
carburizing, the content of Co, if contained, is preferably
specified to be more than or equal to 0.001%. The content of Co is
more preferably more than or equal to 0.010%. Further, in
consideration of the effects of segregation of Co in the grain
boundary of ferrite, the content of Co is preferably less than or
equal to 2.0%, in view of obtaining more enhanced effect of
toughness after carburizing. The content of Co is more preferably
less than or equal to 0.80%.
[Nb: More than or Equal to 0.010%, and Less than or Equal to
0.150%]
[0079] Nb (niobium) is an element that contributes to micronize
ferrite crystal grains to further improve the toughness after
carburizing. Hence in the steel sheet for carburizing according to
the embodiment, Nb may be contained as needed. In order to obtain
more enhanced effect of toughness after carburizing, the content of
Nb, if contained, is preferably specified to be more than or equal
to 0.010%. The content of Nb is more preferably more than or equal
to 0.035% Further, in consideration of the effects of production of
carbide and nitride, the content of Nb is preferably less than or
equal to 0.150%, in view of obtaining more enhanced effect of
toughness after carburizing. The content of Nb is more preferably
less than or equal to 0.120%, even more preferably less than or
equal to 0.100%, and yet more preferably less than or equal to
0.050%.
[Ti: More than or Equal to 0.010%, and Less than or Equal to
0.150%]
[0080] Ti (titanium) is an element that contributes to micronize
ferrite crystal grains to further improve the toughness after
carburizing. Hence in the steel sheet for carburizing according to
the embodiment, Ti may be contained as needed. In order to obtain
more enhanced effect of toughness after carburizing, the content of
Ti, if contained, is preferably specified to be more than or equal
to 0.010%. The content of Ti is more preferably more than or equal
to 0.035% Further, in consideration of the effects of production of
carbide and nitride, the content of Ti is preferably less than or
equal to 0.150%, in view of obtaining more enhanced effect of
toughness after carburizing. The content of Ti is more preferably
less than or equal to 0.120%, even more preferably less than or
equal to 0.050%, and yet more preferably less than or equal to
0.020%.
[V: More than or Equal to 0.0005%, and Less than or Equal to
1.0%]
[0081] V (vanadium) is an element that contributes to micronize
ferrite crystal grains to further improve the toughness after
carburizing. Hence in the steel sheet for carburizing according to
the embodiment, V may be contained as needed. In order to obtain
more enhanced effect of toughness after carburizing, the content of
V, if contained, is preferably specified to be more than or equal
to 0.0005%. The content of V is more preferably more than or equal
to 0.0010% Further, in consideration of the effects of production
of carbide and nitride, the content of V is preferably less than or
equal to 1.0%, in view of obtaining more enhanced effect of
toughness after carburizing. The content of V is more preferably
less than or equal to 0.80%.
[B: More than or Equal to 0.0005%, and Less than or Equal to
0.01%]
[0082] B (boron) is an element that segregates in the grain
boundary of ferrite to enhance strength of the grain boundary, to
thereby further improve the toughness after carburizing. Hence in
the steel sheet for carburizing according to the embodiment, B may
be contained as needed. In order to obtain more enhanced effect of
toughness after carburizing, the content of B, if added, is
preferably specified to be more than or equal to 0.0005%. The
content of B is more preferably more than or equal to 0.0010% Note
that, such more enhanced effect of toughness after carburizing will
saturate if the content of B exceeds 0.01%, so that the content of
B is preferably specified to be less than or equal to 0.01%. The
content of B is more preferably less than or equal to 0.0075%, even
more preferably less than or equal to 0.0050%, and yet more
preferably less than or equal to 0.0020%.
[W: Less than or Equal to 1.0%]
[0083] W (tungsten) is an element that acts to deoxidize molten
steel to improve soundness of the steel. Hence in the steel sheet
for carburizing according to the embodiment, W may be contained as
needed at a maximum content of 1.0%. The content of W is more
preferably less than or equal to 0.5%.
[Ca: Less than or Equal to 0.01%]
[0084] Ca (calcium) is an element that acts to deoxidize molten
steel to improve soundness of the steel. Hence in the steel sheet
for carburizing according to the embodiment, Ca may be contained as
needed at a maximum content of 0.01%. The content of Ca is more
preferably less than or equal to 0.005%.
[Balance: Fe and Impurities]
[0085] The balance of the component composition at the center of
thickness includes Fe and impurities. For example, the impurities
are exemplified by elements derived from the starting steel or
scrap, and/or incorporated in the process of steel making, which
are acceptable so long as characteristics of the steel sheet for
carburizing according to the embodiment will not be adversely
affected.
[0086] Chemical components contained in the steel sheet for
carburizing according to the embodiment have been detailed.
<Microstructure of Steel Sheet for Carburizing>
[0087] Next, the microstructure that makes up the steel sheet for
carburizing according to the embodiment will be detailed.
[0088] The microstructure of the steel sheet for carburizing
according to the embodiment is substantially composed of ferrite
and carbide. In more detail, the microstructure of the steel sheet
for carburizing according to the embodiment is composed so that the
average crystal grain size of ferrite is smaller than 10 .mu.m, the
percentage of area of ferrite typically falls in the range from 80
to 95%, the percentage of area of carbide typically falls in the
range from 5 to 20%, and the total percentage of area of ferrite
and carbide will not exceed 100%.
[0089] Such percentages of area of ferrite and carbide are measured
by using a sample sampled from the steel sheet for carburizing so
as to produce the cross section to be observed in the direction
perpendicular to the width direction. A length of sample of 10 mm
to 25 mm or around will suffice, although depending on types of
measuring instrument. The surface to be observed of the sample is
polished, and then etched using nital. The surface to be observed,
after etched with nital, is observed in regions at a quarter
thickness position (which means a position in the thickness
direction of the steel sheet for carburizing, quarter thickness
away from the surface), at a 3/8 thickness position, and at the
half thickness position, under a thermal-field-emission type
scanning electron microscope (for example, JSM-7001F from JEOL,
Ltd.).
[0090] Each sample is observed for the regions having an area of
2500 .mu.m.sup.2 in ten fields of view, and percentages of areas
occupied by ferrite and carbide relative to the area of field of
view are measured for each field of view. An average value of
percentages of area occupied by ferrite, being averaged from all
fields of view, and, an average value of percentages of area
occupied by carbide, being averaged from all fields of view, are
respectively denoted as the percentage of area of ferrite, and, the
percentage of area of carbide.
[0091] Now the carbide in the microstructure according to the
embodiment is mainly iron carbide such as cementite which is a
compound of iron and carbon (Fe.sub.3C), and, .epsilon. carbide
(Fe.sub.2-3C). Alternatively, besides the aforementioned iron
carbide, the carbide in the microstructure occasionally contains a
compound derived from cementite having Fe atoms substituted by Mn,
Cr and so forth, and alloy carbides (such as M.sub.23C.sub.6,
M.sub.6C and MC, where M represents Fe and other metal element).
Most part of the carbide in the microstructure according to the
embodiment is composed of iron carbide. Hence, focusing now on the
later-detailed number of such carbides, the number may be the total
number of the aforementioned various carbides, or may be the number
of iron carbide only. That is, the later-described various
percentages of the number of carbides may be defined on the basis
of a population that contains various carbides including iron
carbide, or may be defined on the basis of a population that
contains iron carbide only. The iron carbide may be identified
typically by subjecting the sample to diffractometry or EDS (Energy
Dispersive X-ray spectrometry).
[0092] In bending deformation, deformation stress is concentrated
at the interface between a soft structure and a hard structure. It
is therefore desired to reduce as possible difference of hardness
between the soft structure and hard structure, or, to control
geometry of the hard structure so as to relieve the stress
concentration. Now the cracks may be suppressed from generating by
reducing the aspect ratio of carbide through spherodizing
annealing. As the bending deformation further proceeds, the
produced cracks may extend. Since the cracks propagate through
regions where fracture is likely to occur, grain boundary of
ferrite, and, interface between ferrite and carbide may serve as
routes for propagation. In this process, since the carbide if
produced in the grain boundary of ferrite can assist extension of
the cracks while routed through the grain boundary, the carbide is
desired to be produced within crystal grains of ferrite.
Propagation of cracks through the grain boundary is considered to
be suppressible, by producing the carbide within the ferrite
crystal grains.
[0093] The carburized member will have carbon introduced by
carburizing in the skin layer, so that the member will have high
strength in the skin layer, whereas the steel material as a
starting material for the carburized member will become brittle as
the strength increases. Hence, the toughness of the skin layer
holds the key for the steel sheet for carburizing as the starting
material. Regarding this point, the toughness is improved by
micronizing crystal grains in the skin layer of the steel sheet. As
will be detailed below, by annealing the steel sheet in a
nitrogen-rich atmosphere, the atmospheric nitrogen enters the steel
sheet to form nitride in the skin layer of the steel sheet. The
thus formed nitride is mainly composed of fine AlN, and can
demonstrate an effect of suppressing growth of grains of prior
austenite during carburizing heat treatment. Grain size of prior
austenite and grain size of transformed martensite are in a
proportional relationship. It was therefore made clear that if the
grains of prior austenite are suppressed from growing by such fine
AIN, also the grain size of martensite in a structure of carburized
member can be micronized.
[0094] Reasons for limitations of the microstructure that composes
the steel sheet for carburizing according to this embodiment will
be detailed below.
[Average Crystal grain size of Ferrite: Smaller than 10 .mu.m]
[0095] In the microstructure of the steel sheet for carburizing
according to this embodiment, the average crystal grain size of
ferrite is specified to be smaller than 10 .mu.m as described
above. With the average crystal grain size of ferrite specified to
be smaller than 10 .mu.m, the aforementioned effect through
micronization of crystal grains may be demonstrated, and the impact
value after carburizing may be improved. With the average crystal
grain size of ferrite set to 10 .mu.m or larger, the aforementioned
effect through micronization of crystal grains will not be
obtained, failing in improving the impact value after carburizing.
The average crystal grain size of ferrite is preferably smaller
than 8 .mu.m. The lower limit value of the average crystal grain
size of ferrite is not specifically limited. Since, however, it is
difficult to control the average crystal grain size of ferrite
smaller than 0.1 .mu.m in practical operation, 0.1 .mu.m is
understood as a substantial lower limit.
[Percentage of Number of Carbides with Aspect Ratio of 2.0 or
Smaller, Relative to Total Carbides: 80% or Larger]
[0096] As described previously, the carbide according to the
embodiment is mainly composed of iron carbides such as cementite
(Fe.sub.3C) and, c carbide (Fe.sub.2-3C). Investigation by the
present inventors revealed that good bendability is obtainable, if
the percentage of the number of carbides with an aspect ratio of
2.0 or smaller, relative to the total carbides, is 80% or larger.
With the percentage of the number of carbides with an aspect ratio
of 2.0 or smaller relative to the total carbides fallen below 80%,
good bendability will not be obtained due to accelerated cracking
during bending deformation. Therefore in the steel sheet for
carburizing according to the embodiment, the lower limit value of
the percentage of the number of carbides with an aspect ratio of
2.0 or smaller, relative to the total carbides, is specified to be
80%. The percentage of the number of carbides with an aspect ratio
of 2.0 or smaller relative to the total carbides is more preferably
85% or larger, for further improvement of the bendability. Note
that there is no special limitation on the upper limit of the
percentage of the number of carbides with an aspect ratio of 2.0 or
smaller relative to the total carbides. Since, however, it is
difficult to achieve 98% or larger in practical operation, 98% will
be a substantial upper limit.
[Percentage of Number of Carbides Present in Ferrite crystal grain,
Relative to Total Carbides: 60% or Larger]
[0097] Investigations by the present inventors revealed that good
bendability is obtainable, if the percentage of the number of
carbides present in ferrite crystal grain, relative to total
carbides, is 60% or larger. With the percentage of the number of
carbides present in ferrite crystal grain relative to total
carbides fallen under 60%, good bendability will not be obtained
due to accelerated cracking during bending deformation. Therefore
in the steel sheet for carburizing according to the embodiment, the
lower limit value of the percentage of the number of carbides
present in ferrite crystal grain, relative to total carbides, is
specified to be 60%. The percentage of the number of carbides
present in ferrite crystal grain relative to total carbides is more
preferably 65% or larger, for further improvement of the
bendability. Note that there is no special limitation on the upper
limit of the percentage of the number of carbides present in
ferrite crystal grain relative to the total carbides. Since,
however, it is difficult to achieve 98% or larger in practical
operation, 98% will be a substantial upper limit.
[Average Equivalent Circle Diameter of Carbide: 5.0 .mu.m or
Smaller]
[0098] In the microstructure of the steel sheet for carburizing
according to the embodiment, the average equivalent circle diameter
of carbide need be 5.0 .mu.m or smaller. With the average
equivalent circle diameter of carbide exceeding 5.0 .mu.m, good
bendability will not be obtained due to cracking that occurs during
bending deformation. The smaller the average equivalent circle
diameter of carbide is, the better the bendability. The average
equivalent circle diameter is preferably 1.0 .mu.m or smaller, more
preferably 0.8 .mu.m or smaller, and even more preferably 0.6 .mu.m
or smaller. The lower limit value of the average equivalent circle
diameter of carbide is not specifically limited. Since, however, it
is difficult to achieve an average equivalent circle diameter of
carbide of 0.01 .mu.m or smaller in practical operation, 0.01 .mu.m
will be a substantial lower limit.
[0099] Next, methods for measuring the average grain size of
ferrite in the microstructure, and, various percentages of the
number of carbides and the average equivalent circle diameter of
carbide will be detailed. Note that the measurement below employed
fixed positions of observation of samples, but there is no large
difference between the states of ferrite and carbide measured in
the samples, and the states of ferrite and carbide in the skin
layer (nitrogen-rich region) of the steel sheet according to this
embodiment.
[0100] First, a sample is cut out from the steel sheet for
carburizing, so as to produce a cross section to be observed, which
is perpendicular to the surface (thickness-wise cross section). A
length of sample of 10 mm or around will suffice, although
depending on types of measuring instrument. The cross section is
polished and corroded, and is then subjected to measurement of
position of precipitation, aspect ratio, and average equivalent
circle diameter of carbide. For the polishing, it suffices for
example to polish the surface to be measured using a 600-grit to
1500-grit silicon carbide sandpaper, and then to specularly finish
the surface using a liquid having diamond powder of 1 .mu.m to 6
.mu.m in diameter dispersed in a diluent such as alcohol or in
water. The corrosion is not specifically limited so long as the
shape and position of precipitation of carbide can be observed. In
order to corrode the grain boundary between carbide and matrix
iron, it is suitable to employ, for example, etching using a
saturated picric acid-alcohol solution; or a method for removing
the matrix iron to a depth of several micrometers typically by
potentiostatic electrolytic etching using a nonaqueous
solvent-based electrolyte (Fumio Kurosawa et al., Journal of the
Japan Institute of Metals and Materials (in Japanese), 43, 1068,
(1979)), so as to allow the carbide only to remain.
[0101] The average crystal grain size of ferrite is estimated by
photographing a 2500 .mu.m.sup.2 area at around a quarter thickness
position of the sample under a thermal-field-emission type scanning
electron microscope (for example, JSM-7001F from JEOL, Ltd.), and
by applying the line segment method to the captured images.
[0102] The aspect ratio of carbide is estimated by observing a
10000 .mu.m.sup.2 area at around a quarter thickness position of
the sample, under a thermal-field-emission type scanning electron
microscope (for example, JSM-7001F from JEOL, Ltd.). All carbides
contained in an observed field of view are measured regarding the
long axes and the short axes to calculate aspect ratios (long
axis/short axis), and an average value of the aspect ratios is
determined. Such observation is made in five fields of view, and an
average value for these five fields of view is determined as the
aspect ratio of carbide in the sample. Referring to the thus
obtained aspect ratio of carbide, the percentage of the number of
carbides with an aspect ratio of 2.0 or smaller relative to the
total carbides is calculated, on the basis of the total number of
carbides with an aspect ratio of 2.0 or smaller, and the total
number of carbides present in the five fields of view.
[0103] The position of precipitation of carbide is confirmed by
observing a 10000 .mu.m.sup.2 area at around a quarter thickness
position of the sample, under a thermal-field-emission type
scanning electron microscope (for example, JSM-7001F from JEOL,
Ltd.). All carbides contained in an observed field of view are
measured regarding the position of precipitation, and percentage of
carbides that precipitated within the ferrite crystal grain,
relative to the total number of carbides, is calculated. The
observation is made in five fields of view, and an average value
for these five fields of view is determined as the percentage of
carbides formed within the ferrite crystal grain, among from the
carbides (that is, the percentage of the number of carbides present
within the ferrite crystal grain, among from the total
carbides).
[0104] The average equivalent circle diameter of carbide is
estimated by observing a 600 .mu.m.sup.2 area at around a quarter
thickness position of the sample in four fields of view, under a
thermal-field-emission type scanning electron microscope (for
example, JSM-7001F from JEOL, Ltd.). For each field of view, the
long axes and the short axes of captured carbides are individually
measured, using image analysis software (for example, IMage-Pro
Plus from Media Cybernetics, Inc.). For each carbide in the field
of view, the long axis and the short axis are averaged to obtain
the diameter of carbide, and the diameters obtained from all
carbides captured in the field of view are averaged. The thus
obtained average values of the diameter of carbides from four
fields of view are further averaged by the number of fields of
view, to determine the average equivalent circle diameter of
carbide.
[0105] The microstructure possessed by the steel sheet for
carburizing according to the embodiment has been detailed.
[Average Nitrogen Concentration of Skin Layer of Steel Sheet: 0.040
mass % or Higher, and 0.20 mass % or Smaller]
[0106] Next, the average nitrogen concentration in the skin layer
of the steel sheet for carburizing will be explained.
Investigations by the present inventors revealed that, with the
average nitrogen concentration in the skin layer of the steel sheet
for carburizing controlled to 0.040 mass % or higher, the
carburized members made of the steel sheet for carburizing were
successful in obtaining good toughness. Such findings will be
detailed below.
[0107] The present inventors sampled a thin film sample of 40 .mu.m
long and 25 .mu.m deep from a region around the skin layer of a
carburized member which was found to show good toughness, using a
focused ion beam processing/observation apparatus, and observed the
microstructure under a transmission electron microscope. As a
consequence, fine AlN with an average diameter of 50 nm or smaller
was found to be produced in the thin film sample.
[0108] The present inventors further made an analysis as described
below to investigate correlation between the position of production
of AlN and a matrix structure. That is, a thin film sample of 100
.mu.m long and 100 .mu.m deep, sampled using the focused ion beam
processing/observation apparatus was fixed on a mesh holder made of
copper, and analyzed using a transmission electron backscatter
diffractometer equipped on a thermal-field-emission type scanning
electron microscope (JSM-6500F, from JEOL, Ltd.). A crystal
orientation map of prior austenite was reorganized referring to the
measurement results obtained from the electron backscatter
diffractometry, and compared with an image obtained under the
transmission electron microscope. It was consequently made clear
that the fine MN resides at around grain boundary of prior
austenite, and that the grain boundary of prior austenite in which
the fine AlN precipitated was found to reside over a range from the
topmost surface of steel sheet to a depth of 50 .mu.m or around.
More specifically, it was contemplated that the fine AlN, produced
in the skin layer of the steel sheet (a region ranging from the
topmost surface of steel sheet to a depth of 50 .mu.m) suppressed
the prior austenite grains from growing during carburization heat
treatment, so that the grain size of martensite was micronized in
the structure of carburized member, and the impact value
dramatically increased. Note that the topmost surface of the steel
sheet in this context means the surface of the base material of
steel sheet, while excluding various layers including a scale layer
which possibly resides on the surface of the base material of steel
sheet.
[0109] The present inventors further analyzed the carburized member
whose toughness was found to be good, regarding a profile of
nitrogen concentration over a range from the surface of steel sheet
up to the center of steel sheet, using an electron probe
microanalyzer equipped with a wavelength dispersive X-ray
spectrometer and a field-emission electron gun. As a consequence,
the skin layer of the steel sheet (that is, the region ranging from
the topmost surface of steel sheet to a depth of 50 .mu.m) was
confirmed to have an average nitrogen concentration of 0.040 mass %
or higher.
[0110] The present inventors confirmed after thorough
investigations that the skin layer of the steel sheet will have an
average nitrogen concentration of 0.040 mass % or higher and 0.200
mass % or lower, by using as a material the steel sheet with the
average nitrogen concentration in the middle-thickness portion (in
more detail, the average nitrogen concentration over a range from
the middle-thickness portion up to 100 .mu.m away towards the
surface) controlled to 0.2 mass % or lower, by heating the steel
sheet used as the material in an atmosphere with the nitrogen
concentration controlled to 25% or higher in volume fraction, at an
average heating rate of 5.degree. C./h or higher and 100.degree.
C./h or lower, up into a temperature range not higher than point
Ac.sub.1; by keeping the steel sheet in the temperature range not
higher than the point Ac.sub.1 for 10 h or longer and 100 h or
shorter; and then by cooling the steel sheet at an average cooling
rate of 5.degree. C./h or higher and 100.degree. C./h or lower.
That is, by heating the steel sheet in an atmosphere with the
nitrogen concentration controlled to 25% or higher in volume
fraction, at an average heating rate of 5.degree. C./h or higher
and 100.degree. C./h or lower, up into a temperature range not
higher than point Ac.sub.i; by keeping the steel sheet in the
temperature range not higher than the point Ac.sub.i for 10 h or
longer and 100 h or shorter, and then by cooling the steel sheet at
an average cooling rate of 5.degree. C./h or higher and 100.degree.
C./h or lower, the skin layer of the steel sheet will have produced
therein fine AlN of 50 nm or smaller. As a consequence, the skin
layer of the steel sheet is understood to have an average nitrogen
concentration of 0.040 mass % or higher and 0.200 mass % or lower.
Note that the aforementioned structure of fine AlN produced by
annealing will remain almost unmodified throughout cold-working,
and will contribute to suppress the prior austenite grains from
growing during carburization heat treatment.
[0111] As described above, the thorough investigations by the
present inventors revealed that, with the average nitrogen
concentration controlled to 0.040 mass % or higher in the skin
layer of steel sheet (the region ranging from the topmost surface
of steel sheet to a depth of 50 .mu.m) of the steel sheet for
carburizing, the skin layer of the steel sheet will have produced
therein fine AIN, and the impact value will be improved in the
carburized member. The average nitrogen concentration in the skin
layer of the steel sheet is preferably 0.045 mass % or higher.
Meanwhile, with the average nitrogen concentration exceeding 0.200
mass % in the skin layer of the steel sheet, coarse nitride will be
produced to degrade the toughness. The average nitrogen
concentration in the skin layer of the steel sheet is therefore
specified to be 0.200 mass % at maximum. The average nitrogen
concentration in the skin layer of the steel sheet is preferably
0.150 mass % or lower.
[0112] Next, a method for determining the average nitrogen
concentration on the surface of steel sheet will be explained.
[0113] As mentioned previously, the structure of fine AIN produced
by annealing will remain almost unchanged throughout cold-working,
and contributes to suppress the prior austenite grains from growing
during carburization heat treatment. Hence, it suffices to examine
the nitrogen profile, using the steel sheet for carburizing
obtained after annealing a hot-rolled steel sheet or a cold-rolled
steel sheet.
[0114] More specifically, a sample is cut out from the steel sheet
for carburizing, so as to produce a cross section to be observed,
which is perpendicular to the surface (thickness-wise cross
section). A length of sample of 10 mm to 25 mm or around will
suffice, although depending on types of measuring instrument. The
surface to be measured is prepared under argon ion beam so as not
to produce streak-like irregularity over the surface to be
measured, using a cross section polisher from JEOL, Ltd. and a
sample rotating holder from JEOL, Ltd. Thereafter by using an
electron probe microanalyzer equipped with a wavelength dispersive
X-ray spectrometer and an field-emission electron gun, the nitrogen
concentration profile is measured over a range from the topmost
surface of the steel sheet up to the middle-thickness portion
(half-thickness position) at 50 nm intervals. An average value of
the nitrogen concentration (in mass %) over a range from the
topmost surface of the steel sheet up to a 50 .mu.m deep point is
then calculated, and is specified to be aforementioned average
nitrogen concentration of the skin layer of the steel sheet. In
addition, the average value of the nitrogen concentration (in mass
%) over a range from the middle-thickness portion up to 100 .mu.m
away towards the surface is specified to be the average nitrogen
concentration in the middle-thickness portion. Note that the amount
of intrusion of nitrogen in the annealing step does not largely
differ between the top and back surfaces of a coil, so that the
measurement made only either on the top or back surface of the
steel sheet will suffice.
<Thickness of Steel Sheet for Carburizing>
[0115] The thickness of the steel sheet for carburizing according
to the embodiment is not specifically limited, but is preferably 2
mm or larger, for example. With the thickness of the steel sheet
for carburizing specified to be 2 mm or larger, difference of
thickness in the coil width direction may further be reduced. The
thickness of the steel sheet for carburizing is more preferably 2.3
mm or larger. Further, the thickness of the steel sheet for
carburizing is not specifically limited, but is preferably 6 mm or
smaller. With the thickness of the steel sheet for carburizing
specified to be 6 mm or smaller, load of press forming may be
reduced, making forming into components easier. The thickness of
the steel sheet for carburizing is more preferably 5.8 mm or
smaller.
[0116] The steel sheet for carburizing according to the embodiment
has been detailed.
(Method for Manufacturing Steel Sheet for Carburizing)
[0117] Next, a method for manufacturing the above-explained steel
sheet for carburizing according to the embodiment will be
detailed.
[0118] The method for manufacturing the above-explained steel sheet
for carburizing according to the embodiment includes (A) a
hot-rolling step in which a steel material having the chemical
composition explained above is used to manufacture the hot-rolled
steel sheet according to predetermined conditions, and (B) an
annealing step in which the thus obtained hot-rolled steel sheet,
or the steel sheet having been cold-rolled subsequently to the
hot-rolling step is annealed according to predetermined heat
treatment conditions.
[0119] The hot-rolling step and the annealing step will be detailed
below.
<Hot-Rolling Step>
[0120] The hot-rolling step described below is a step in which a
steel material having the predetermined chemical composition is
used to manufacture the hot-rolled steel sheet according to the
predetermined conditions.
[0121] Steel billet (steel material) subjected now to hot-rolling
may be any billet manufactured by any of usual methods. For
example, employable is a billet manufactured by any of usual
methods, such as continuously cast slab and thin slab caster.
[0122] In addition, from the viewpoint of improving the toughness
and regarding the inclusions such as MnS or center segregation of
Mn in the steel material to be hot-rolled, the fewer the better.
Hence, for example, in the continuous casting step for obtaining a
billet to be hot-rolled, it is preferable to carry out soundness
enhancing treatment of the steel material, such as producing a
predetermined inclusion by controlling the amount of pouring of
molten steel per unit time, or such as reducing the center
segregation before the billet completely solidifies.
[0123] In more detail, using the steel material having the
aforementioned chemical composition, the steel material is heated
and subjected to hot-rolling, the hot finish rolling is terminated
in the temperature range of 800.degree. C. or higher and lower than
920.degree. C., and then wound up at a temperature of 700.degree.
C. or lower, to thereby manufacture the hot-rolled steel sheet. In
this process, the cooling after the hot finish rolling is started
within one second after the end of the hot finish rolling, and the
average cooling rate after the hot finish rolling is specified to
be higher than 50.degree. C./s.
[Rolling Temperature in Hot Finish Rolling: 800.degree. C. or
Higher, and Lower than 920.degree. C.]
[0124] In the hot-rolling step according to this embodiment,
rolling in the hot finish rolling need take place at a rolling
temperature of 800.degree. C. or higher. With the rolling
temperature during the hot finish rolling (that is, the finish
rolling temperature) dropped below 800.degree. C., also temperature
at which ferrite transformation starts will drop, to thereby
coarsen the carbides to be precipitated. As a consequence, these
coarse carbides are acceleratingly grown in the annealing step in
the succeeding stage, to degrade the bendability. The finish
rolling temperature in the hot-rolling step according to this
embodiment is therefore specified to be 800.degree. C. or higher.
The finish rolling temperature is preferably 830.degree. C. or
higher. Meanwhile, with the finish rolling temperature reached
920.degree. C. or higher, austenite grains will be distinctively
coarsened to reduce sites of nucleation of ferrite, the temperature
at which ferrite transformation starts will thus be lowered, making
the carbides to be precipitated more easily be coarsened. In this
case, theses coarse carbides are acceleratingly grown in the
annealing step in the succeeding stage, to degrade the bendability.
The finish rolling temperature in the hot-rolling step according to
this embodiment is therefore specified to be lower than 920.degree.
C. The finish rolling temperature is preferably lower than
900.degree. C.
[Winding Temperature: 700.degree. C. or Lower]
[0125] As mentioned previously, the microstructure of the steel
sheet for carburizing need be featured by that the percentage of
the number of carbides with an aspect ratio of 2.0 or smaller
relative to the total carbides is 80% or larger; that the
percentage of the number of carbides present in ferrite crystal
grain relative to the total carbides is 60% or larger; that the
average equivalent circle diameter of carbide is 5.0 .mu.m or
smaller; and that the average nitrogen concentration in the skin
layer of the steel sheet is 0.040 mass % or higher and 0.200 mass %
or lower. Accordingly, the steel sheet before being subjected to
the annealing step in the succeeding stage (in more detail,
spherodizing annealing) preferably has a structure (hot-rolled
steel sheet structure) that mainly includes 10% or more and 80% or
less, in percentage of area, of ferrite, and 10% or more and 60% or
less, in percentage of area, of pearlite, totaling 100% or less in
percentage of area, and the balance that preferably includes at
least any of bainite, martensite, tempered martensite and residual
austenite.
[0126] If the winding temperature in the hot-rolling step according
to the embodiment exceeds 700.degree. C., ferrite transformation
will be excessively promoted to suppress production of pearlite,
making it difficult to control, in the steel sheet for carburizing
after the annealing, the percentage of number of carbides with an
aspect ratio of 2.0 or smaller, among from the total carbides, to
80% or larger. Hence in the hot-rolling step according to the
embodiment, the upper limit of the winding temperature is specified
to be 700.degree. C. The lower limit of the winding temperature in
the hot-rolling step according to the embodiment is not
specifically limited. Since, however, winding at room temperature
or below is difficult in practical operation, room temperature will
be a substantial lower limit. Note that the winding temperature in
the hot-rolling step according to the embodiment is preferably
400.degree. C. or higher, from the viewpoint of further reducing
the aspect ratio of carbide in the annealing step in the succeeding
stage.
[Cooling Start Time after Hot Finish Rolling: within One Minute
after End of Hot Finish Rolling] [Average Cooling Rate after Hot
Finish Rolling: Higher than 50.degree. C./s]
[0127] In the hot-rolling step according to this embodiment,
cooling at an average cooling rate of higher than 50.degree. C./s
is started within one second after the end of the hot finish
rolling. In this way, austenite grains after the hot finish rolling
may be micronized. With the austenite grains micronized after the
hot finish rolling, it now becomes possible to control the average
grain size of ferrite, after the annealing step (in more detail,
spherodizing annealing) in the succeeding stage to smaller than 10
.mu.m.
[0128] With the cooling start time fallen behind one second after
the end of hot finish rolling, the austenite grains will be
coarsened, so that the average crystal grain size of ferrite after
spherodizing annealing will exceed 10 .mu.m, making it unable to
exhibit the effect of micronizing the crystal grains. The cooling
start time after the hot finish rolling preferably falls within 0.8
seconds after the end. The lower limit of the cooling start time is
not specifically limited. Note however that it is difficult to
allow the cooling start time to fall within 0.01 seconds after the
end in practical operation, so that 0.01 seconds is understood as a
substantial lower limit.
[0129] Meanwhile, with the average cooling rate after the hot
finish rolling fallen to 50.degree. C./s or lower, the austenite
grains will be coarsened, so that the average crystal grain size of
ferrite after spherodizing annealing in the succeeding stage will
exceed 10 .mu.m. The average cooling rate after the hot finish
rolling is preferably 55.degree. C./s or higher. The upper limit of
the average cooling rate is not specifically limited. Note however
that it is difficult to control the average cooling rate to
300.degree. C./s or higher in practical operation, so that
300.degree. C./s is understood as a substantial upper limit.
[0130] Alternatively, the steel sheet thus wound up in the
aforementioned hot-rolling step (hot-rolled steel sheet) may be
unwound, pickled, and then cold-rolled. Through removal of oxide on
the surface of steel sheet by pickling, the hole expandability may
further be improved. The pickling may be carried out once, or may
be carried out in multiple times. The cold-rolling may be carried
out at an ordinary draft (30 to 90%, for example). The hot-rolled
steel sheet and cold-rolled steel sheet also include steel sheet
temper-rolled under usual conditions, besides the steel sheets that
are left unmodified after hot-rolled or cold-rolled.
[0131] The hot-rolled steel sheet is manufactured as described
above, in the hot-rolling step according to the embodiment. The
thus manufactured hot-rolled steel sheet, or, the steel sheet
having been cold-rolled subsequently to the hot-rolling step may
further be subjected to specific annealing in the annealing step
detailed below, to obtain the steel sheet for carburizing according
to the embodiment.
<Annealing Step>
[0132] The annealing step detailed below is a step in which the
hot-rolled steel sheet obtained in the aforementioned hot-rolling
step, or, the steel sheet having been cold-rolled subsequently to
the hot-rolling step is subjected to annealing (spherodizing
annealing) under predetermined heat treatment conditions. Through
the annealing, pearlite having been produced in the hot-rolling
step is spherodized, and the average crystal grain size of ferrite
after spherodizing annealing is controlled to smaller than 10
.mu.m.
[0133] In more detail, the hot-rolled steel sheet obtained as
described above, or, the steel sheet having been cold-rolled
subsequently to the hot-rolling step is heated in an atmosphere
with nitrogen concentration controlled to 25% or higher in volume
fraction, at an average heating rate of 5.degree. C./h or higher
and 100.degree. C./h or lower, up into a temperature range not
higher than point Ac.sub.i defined by equation (101) below,
annealed in a temperature range not higher than the point Ac.sub.i
for 10 h or longer and 100 h or shorter, and then cooled at an
average cooling rate of 5.degree. C./h or higher and 100.degree.
C./h or lower in a temperature range from a temperature at the end
of annealing down to 550.degree. C.
[0134] Now in the equation (101) below, the notation [X] represents
the content of element X (in mass %), which will be substituted by
zero if such element X is absent.
[ Math . 2 ] Ac 1 = 750.8 - 26.6 [ C ] + 17.6 [ Si ] - 11.6 [ Mn ]
- 22.9 [ Cu ] - 23 [ Ni ] + 24.1 [ Cr ] + 22.5 [ Mo ] - 39.7 [ V ]
- 5.7 [ Ti ] + 232.4 [ Nb ] - 169.4 [ Al ] - 894.7 [ B ] Equation (
101 ) ##EQU00002##
[Annealing Atmosphere: Atmosphere with Nitrogen Concentration
Controlled to 25% or Higher in Volume Fraction]
[0135] In the aforementioned annealing step, the annealing
atmosphere will be controlled to have a nitrogen concentration of
25% or higher in volume fraction. With the nitrogen concentration
fallen below 25% in volume fraction, the average nitrogen
concentration in the skin layer of the steel sheet will no longer
be controlled to 0.040 mass % or higher and 0.200 mass % or lower.
Hence, in the annealing step according to this embodiment, the
nitrogen concentration in the annealing atmosphere is specified to
be 25% or higher in volume fraction. The nitrogen concentration in
the annealing atmosphere is preferably 75% or higher in volume
fraction, and even more preferably 80% or higher in volume
fraction. Note that the higher the nitrogen concentration, the
better. Since it is, however, not cost-effective to control the
nitrogen concentration to 99% or higher in volume fraction, 99% in
volume fraction is understood as a substantial upper limit.
[0136] In the annealing step according to this embodiment, the heat
treatment is carried out while introducing, as the atmospheric gas,
a gas that is composed of a molecule containing nitrogen atom,
while controlling the annealing atmosphere. For example, it
suffices to control the annealing atmosphere typically by
regulating flow rate of the atmospheric gas to be introduced into a
heating furnace used for the annealing step, using a gas
concentration gauge installed in an annealing furnace.
[0137] Note that the balance of the atmospheric gas may be mainly
composed of any inert gas other than nitrogen, allowing appropriate
use of reducing gas such as hydrogen and argon, for example. More
specifically, the annealing atmosphere may have a nitrogen
concentration of 75% or higher in volume fraction, with the balance
of hydrogen. Alternatively, the atmospheric gas may contain a gas
such as oxygen if the content is not so large.
[Heating Condition: at Average Heating Rate of 5.degree. C./h or
Higher and 100.degree. C./h or Lower, up into Temperature Range not
Higher than Point Ac.sub.1]
[0138] In the annealing step according to the embodiment, the
aforementioned hot-rolled steel sheet, or, the steel sheet having
been cold-rolled subsequently to the hot-rolling step need be
heated at an average heating rate of 5.degree. C./h or higher and
100.degree. C./h or lower, up into a temperature range not higher
than point Ac.sub.1 defined by the equation (101) above. With the
average heating rate set lower than 5.degree. C./h, the average
equivalent circle diameter of carbide will exceed 5.0 .mu.m,
degrading the bendability. Meanwhile, with an average heating rate
exceeding 100.degree. C./h, spherodizing of carbide will not be
fully promoted, making it difficult to control the percentage of
the number of carbides with an aspect ratio of 2.0 or smaller,
among from the total carbides, to 80% or larger. Further, at a
heating temperature exceeding point Ac.sub.1 defined by the
equation (101) above, the percentage of the number of carbides
formed within the ferrite crystal grains among from the total
carbides will fall under 60%, making it unsuccessful to obtain good
bendability. Note that the lower limit of the temperature range of
heating temperature is not specifically limited. However, in the
temperature range of heating temperature below 600.degree. C.,
retention time in annealing process will become longer, making the
process not cost-effective. Hence, the temperature range of heating
temperature is preferably specified to be 600.degree. C. or higher.
For more proper control of the state of carbide, the average
heating rate in the annealing step according to the embodiment is
preferably specified to be 20.degree. C./h or higher. Further, for
more proper control of the state of carbide, the average heating
temperature in the annealing step according to the embodiment is
preferably specified to be 50.degree. C./h or lower. For more
proper control of the state of carbide, the temperature range of
heating temperature in the annealing step according to the
embodiment is more preferably specified to be 630.degree. C. or
higher. Furthermore, for more proper control of the state of
carbide, the temperature range of heating temperature in the
annealing step according to the embodiment is more preferably
specified to be 670.degree. C. or lower.
[Retention Time: in Temperature Range not Higher than Point
Ac.sub.1, for 10 h or Longer and 100 h or Shorter]
[0139] In the annealing step according to the embodiment, the
aforementioned temperature range not higher than point Ac.sub.1
(preferably, 600.degree. C. or higher and point Ac.sub.1 or lower)
need be kept for 10 h or longer and 100 h or shorter. With the
retention time set shorter than 10 h, spherodizing of carbide will
not be fully promoted, making it difficult to control the
percentage of the number of carbides with an aspect ratio of 2.0 or
smaller, among from the total carbides, to 80% or larger.
Meanwhile, with the retention time exceeding 100 h, the average
equivalent circle diameter of carbide will exceed 5.0 .mu.m,
degrading the bendability. For more proper control of the state of
carbide, the retention time in the annealing step according to the
embodiment is preferably 20 h or longer. Further, for more proper
control of the state of carbide, the retention time in the
annealing step according to the embodiment is preferably 80 h or
shorter.
[Cooling Conditions: Cooled at Average Cooling Rate of 5.degree.
C./h or Higher and 100.degree. C./h or Lower]
[0140] In the annealing step according to the embodiment, the steel
sheet after the aforementioned retention under heating, is cooled
at an average cooling rate of 5.degree. C./h or higher and
100.degree. C./h or lower. Now the average cooling rate in this
context means an average cooling rate over the range from the
temperature of retention under heating (in other words, the
temperature at the end of annealing) down to 550.degree. C. With
the average cooling rate set below 5.degree. C./h, the carbide will
be excessively coarsened, degrading the bendability. Meanwhile,
with the average cooling rate exceeding 100.degree. C./h,
spherodizing of carbide will not be fully promoted, making it
difficult to control the percentage of the number of carbides with
an aspect ratio of 2.0 or smaller, among from the total carbides,
to 80% or larger. For more proper control of the state of carbide,
the average cooling rate over the range from the temperature of
retention under heating down to 550.degree. C. is preferably
specified to be 20.degree. C./h or higher. Further, for more proper
control of the state of carbide, the average cooling rate over the
range from the temperature of retention under heating down to
550.degree. C. in the annealing step according to the embodiment is
preferably specified to be 50.degree. C./h or lower.
[0141] Note that, in the annealing step according to the
embodiment, the average cooling rate in a temperature range below
550.degree. C. is not specifically limited, allowing cooling at a
freely selectable average cooling rate down into a predetermined
temperature range. The lower limit of temperature at which the
cooling is terminated is not specifically limited. Since, however,
cooling below room temperature is difficult in practical operation,
room temperature will be a substantial lower limit.
[0142] The annealing step according to the embodiment has been
detailed.
[0143] By carrying out the aforementioned hot-rolling step and
annealing step, the above-explained steel sheet for carburizing
according to the embodiment may be manufactured.
[0144] Note that, prior to the above-explained annealing step, the
hot-rolled steel sheet may be retained in the atmospheric air
within the temperature range of 40.degree. C. or higher and
70.degree. C. or lower, for 72 h or longer and 350 h or shorter.
Through such retention, it now becomes possible to form an
aggregate of carbon solid-soluted in the ferrite crystal grain. The
aggregate of carbon is an article formed by several carbon atoms
aggregated in the ferrite crystal grain. Formation of such
aggregate of carbon can further promote formation of carbide in the
annealing step in the succeeding stage. As a consequence, mobility
of dislocation in the annealed steel sheet may further be improved,
and thereby formability of the annealed steel sheet may further be
improved.
[0145] Moreover, the thus obtained steel sheet for carburizing may
be, for example, subjected to cold working as a post-process.
Further, the thus cold-worked steel sheet for carburizing may be
subjected to carburization heat treatment, typically within a
carbon potential range of 0.4 to 1.0 mass %. Conditions for the
carburization heat treatment are not specifically limited, and may
be appropriately controlled so as to obtain desired
characteristics. For example, the steel sheet for carburizing may
be heated up to a temperature that corresponds to the austenitic
single phase, carburized, and then cooled naturally down to room
temperature; or may be cooled once down to room temperature,
reheated, and then quickly quenched. Furthermore, for the purpose
of controlling the strength, the entire portion or part of the
member may be tempered. Alternatively, the steel sheet may be
plated on the surface for the purpose of obtaining a rust-proofing
effect, or may be subjected to shot peening on the surface for the
purpose of improving fatigue characteristics.
EXAMPLES
[0146] Next, examples of the present invention will be explained.
Note that conditions described in examples are merely exemplary
conditions employed in order to confirm feasibility and effects of
the present invention. The present invention is not limited to
these exemplary conditions. The present invention can employ
various conditions without departing from the spirit of the present
invention, insofar as the purpose of the present invention will be
achieved.
TEST EXAMPLES
[0147] Steel materials having chemical compositions listed in Table
1 below were hot-rolled (and cold-rolled) according to conditions
listed in Table 2, and then annealed, to obtain the steel sheets
for carburizing. The hot-rolling according to the conditions listed
in Table 2 below was followed by retention in the atmospheric air
at 55.degree. C. for 105 hours, and by annealing according to
conditions listed in Table 2. Now in exemplary conditions listed in
Table 2 below, in the continuous casting step for obtaining the
steel material to be subjected to hot-rolling, the soundness
enhancing treatment of the steel material was carried out by
controlling the amount of pouring of molten steel per unit time.
Note that in Table 1 and Table 2, the underlines are used to
indicate deviation from the scope of invention.
TABLE-US-00001 TABLE 1-1 Chemical Ingredients of Matrix Steel Sheet
(in mass %, Balance is Fe and Impurities.) No. C Si Mn P S sol. Al
N Cr Mo Ni Cu Co Nb Ti 1 0.03 0.020 0.21 0.018 0.0040 0.0180 0.0101
0.030 0.836 0.000 0.000 0.000 0.000 0.008 2 0.08 0.010 0.38 0.014
0.0051 0.0230 0.0064 0.020 0.855 0.000 0.000 0.000 0.000 0.009 3
0.14 0.010 0.66 0.014 0.0038 0.0530 0.0051 0.020 0.018 0.000 0.000
0.000 0.000 0.004 4 0.05 0.110 1.54 0.014 0.0041 0.0190 0.0116
0.250 0.654 0.000 0.000 0.000 0.000 0.005 5 0.08 0.100 2.10 0.017
0.0051 0.0260 0.0050 0.000 0.000 0.000 0.000 0.000 0.000 0.000 6
0.13 0.030 0.76 0.013 0.0052 0.0110 0.0057 1.510 0.017 0.000 0.000
0.000 0.000 0.004 7 0.01 0.010 0.50 0.013 0.0038 0.0370 0.0088
0.000 0.000 0.000 0.000 0.000 0.000 0.000 8 0.18 0.020 0.54 0.016
0.0049 0.0340 0.0035 0.000 0.000 0.000 0.000 0.000 0.000 0.000 9
0.22 0.030 0.44 0.018 0.0045 0.0320 0.0111 0.000 0.000 0.000 0.000
0.000 0.000 0.000 10 0.27 0.010 0.39 0.018 0.0054 0.0460 0.0116
0.000 0.000 0.000 0.000 0.000 0.000 0.000 11 0.41 0.030 0.49 0.018
0.0053 0.0150 0.0089 0.000 0.000 0.000 0.000 0.000 0.000 0.000 12
0.07 0.001 0.45 0.015 0.0055 0.0220 0.0097 0.000 0.000 0.000 0.000
0.000 0.000 0.000 13 0.08 1.540 0.53 0.017 0.0038 0.0140 0.0113
0.000 0.000 0.000 0.000 0.000 0.000 0.000 14 0.06 0.020 0.002 0.014
0.0050 0.0440 0.0097 0.000 0.000 0.000 0.000 0.000 0.000 0.000 15
0.08 0.020 3.55 0.016 0.0046 0.0150 0.0115 0.000 0.000 0.000 0.000
0.000 0.000 0.000 16 0.06 0.020 0.48 0.015 0.0037 0.0320 0.0062
1.520 0.000 0.000 0.000 0.000 0.000 0.000 17 0.07 0.010 0.51 0.016
0.0050 0.0310 0.0071 0.000 0.540 0.000 0.000 0.000 0.000 0.000 18
0.08 0.030 0.54 0.015 0.0047 0.0290 0.0033 0.000 0.000 0.390 0.000
0.000 0.000 0.000 19 0.08 0.030 0.54 0.016 0.0039 0.0130 0.0104
0.000 0.000 0.000 0.700 0.000 0.000 0.000 20 0.08 0.030 0.43 0.017
0.0036 0.0320 0.0066 0.000 0.000 0.000 0.000 0.550 0.000 0.000 21
0.06 0.020 0.39 0.017 0.0045 0.0420 0.0078 0.000 0.000 0.000 0.000
0.000 0.032 0.000 22 0.07 0.010 0.49 0.015 0.0046 0.0370 0.0098
0.000 0.000 0.000 0.000 0.000 0.000 0.018 23 0.06 0.020 0.58 0.014
0.0051 0.0490 0.0063 0.000 0.000 0.000 0.000 0.000 0.000 0.000 24
0.07 0.010 0.54 0.014 0.0046 0.0350 0.0043 0.000 0.000 0.000 0.000
0.000 0.000 0.000 25 0.08 0.010 0.48 0.013 0.0050 0.0360 0.0043
0.000 0.000 0.000 0.000 0.000 0.000 0.000 26 0.07 0.020 0.58 0.015
0.0038 0.0140 0.0056 0.000 0.000 0.000 0.000 0.000 0.000 0.000 27
0.07 0.020 0.50 0.018 0.0038 0.0460 0.0065 0.000 0.000 0.000 0.000
0.000 0.000 0.000 28 0.06 0.030 0.47 0.017 0.0048 0.0400 0.0119
0.000 0.000 0.000 0.000 0.000 0.000 0.000 29 0.08 0.006 0.38 0.016
0.0056 0.0153 0.0048 0.000 0.000 0.000 0.000 0.000 0.000 0.008 30
0.07 0.460 0.40 0.018 0.0058 0.0150 0.0043 0.000 0.000 0.000 0.000
0.000 0.000 0.000 Chemical Ingredients of Matrix Steel Sheet (in
mass %, Balance is Fe and Impurities.) Ac.sub.1 No. V B W Ca
(.degree. C.) Remark 1 0.0000 0.0003 0.00 0.000 764 2 0.0000 0.0004
0.00 0.000 760 3 0.0000 0.0001 0.00 0.000 731 4 0.0000 0.0002 0.00
0.000 751 5 0.0000 0.0000 0.00 0.000 722 6 0.0000 0.0002 0.00 0.000
774 7 0.0000 0.0000 0.00 0.000 739 Comparative steel 8 0.0000
0.0000 0.00 0.000 734 9 0.0000 0.0000 0.00 0.000 735 10 0.0000
0.0000 0.00 0.000 731 11 0.0000 0.0000 0.00 0.000 732 Comparative
steel 12 0.0000 0.0000 0.00 0.000 740 Comparative steel 13 0.0000
0.0000 0.00 0.000 767 Comparative steel 14 0.0000 0.0000 0.00 0.000
742 Comparative steel 15 0.0000 0.0000 0.00 0.000 705 Comparative
steel 16 0.0000 0.0000 0.00 0.000 775 17 0.0000 0.0000 0.00 0.000
750 18 0.0000 0.0000 0.00 0.000 729 19 0.0000 0.0000 0.00 0.000 725
20 0.0000 0.0000 0.00 0.000 739 21 0.0000 0.0000 0.00 0.000 745 22
0.0000 0.0000 0.00 0.000 737 23 0.0610 0.0000 0.00 0.000 732 24
0.0000 0.0015 0.00 0.000 736 25 0.0000 0.0000 0.00 0.000 737 26
0.0000 0.0000 0.22 0.000 740 27 0.0000 0.0000 0.00 0.004 736 28
0.0000 0.0000 0.00 0.000 738 29 0.0000 0.0003 0.000 0.000 741 30
0.0000 0.0003 0.000 0.000 749
TABLE-US-00002 TABLE 1-2 Chemical Ingredients of Matrix Steel Sheet
(in mass %, Balance is Fe and Impurities.) No. C Si Mn P S sol. Al
N Cr Mo Ni Cu Co Nb Ti 31 0.05 0.012 0.02 0.018 0.0056 0.0146
0.0043 0.000 0.000 0.000 0.000 0.000 0.000 0.008 32 0.09 0.008 2.88
0.016 0.0052 0.0152 0.0046 0.000 0.000 0.000 0.000 0.000 0.000
0.008 33 0.06 0.012 0.39 0.089 0.0053 0.0147 0.0049 0.000 0.000
0.000 0.000 0.000 0.000 0.008 34 0.08 0.009 0.37 0.019 0.0920
0.0154 0.0043 0.000 0.000 0.000 0.000 0.000 0.000 0.008 35 0.09
0.009 0.42 0.019 0.0051 0.0004 0.0046 0.000 0.000 0.000 0.000 0.000
0.000 0.008 36 0.05 0.012 0.41 0.019 0.0059 2.9000 0.0048 0.000
0.000 0.000 0.000 0.000 0.000 0.008 37 0.08 0.010 0.38 0.014 0.0051
0.0230 0.0002 0.020 0.855 0.000 0.000 0.000 0.000 0.009 38 0.08
0.010 0.38 0.014 0.0051 0.0230 0.0310 0.020 0.855 0.000 0.000 0.000
0.000 0.009 39 0.05 0.010 0.42 0.016 0.0052 0.0153 0.0044 0.007
0.000 0.000 0.000 0.000 0.000 0.008 40 0.09 0.010 0.42 0.019 0.0057
0.0151 0.0049 2.940 0.000 0.000 0.000 0.000 0.000 0.008 41 0.08
0.010 0.36 0.018 0.0051 0.0150 0.0043 0.000 0.007 0.000 0.000 0.000
0.000 0.008 42 0.07 0.008 0.38 0.016 0.0057 0.0151 0.0047 0.000
0.920 0.000 0.000 0.000 0.000 0.008 43 0.08 0.010 0.36 0.017 0.0051
0.0148 0.0048 0.000 0.000 0.030 0.000 0.000 0.000 0.008 44 0.08
0.009 0.44 0.017 0.0051 0.0153 0.0043 0.000 0.000 2.880 0.000 0.000
0.000 0.008 45 0.06 0.011 0.42 0.018 0.0059 0.0152 0.0046 0.000
0.000 0.000 0.002 0.000 0.000 0.008 46 0.09 0.012 0.37 0.019 0.0053
0.0146 0.0048 0.000 0.000 0.000 1.920 0.000 0.000 0.008 47 0.06
0.012 0.37 0.018 0.0052 0.0147 0.0049 0.000 0.000 0.000 0.000 0.002
0.000 0.008 48 0.05 0.009 0.43 0.015 0.0052 0.0147 0.0044 0.000
0.000 0.000 0.000 1.980 0.000 0.008 49 0.05 0.012 0.44 0.018 0.0051
0.0149 0.0044 0.000 0.000 0.000 0.000 0.000 0.016 0.008 50 0.06
0.010 0.43 0.015 0.0052 0.0154 0.0046 0.000 0.000 0.000 0.000 0.000
0.140 0.008 51 0.05 0.010 0.43 0.018 0.0058 0.0148 0.0043 0.000
0.000 0.000 0.000 0.000 0.000 0.012 52 0.05 0.012 0.42 0.019 0.0055
0.0150 0.0047 0.000 0.000 0.000 0.000 0.000 0.000 0.130 53 0.05
0.011 0.40 0.016 0.0059 0.0153 0.0048 0.000 0.000 0.000 0.000 0.000
0.000 0.008 54 0.06 0.010 0.39 0.018 0.0051 0.0146 0.0049 0.000
0.000 0.000 0.000 0.000 0.000 0.008 55 0.09 0.012 0.37 0.017 0.0054
0.0152 0.0049 0.000 0.000 0.000 0.000 0.000 0.000 0.008 56 0.07
0.012 0.40 0.018 0.0055 0.0153 0.0044 0.000 0.000 0.000 0.000 0.000
0.000 0.008 57 0.06 0.008 0.41 0.015 0.0055 0.0146 0.0047 0.000
0.000 0.000 0.000 0.000 0.000 0.008 58 0.09 0.011 0.42 0.018 0.0058
0.0153 0.0045 0.000 0.000 0.000 0.000 0.000 0.000 0.008 59 0.08
0.010 0.38 0.014 0.0051 0.0230 0.0890 0.020 0.855 0.000 0.000 0.000
0.000 0.009 Chemical Ingredients of Matrix Steel Sheet (in mass %.
Balance is Fe and Impurities.) Ac.sub.1 No. V B W Ca (.degree. C.)
Remark 31 0.0000 0.0003 0.000 0.000 747 32 0.0000 0.0003 0.000
0.000 713 33 0.0000 0.0003 0.000 0.000 742 34 0.0000 0.0003 0.000
0.000 742 35 0.0000 0.0003 0.000 0.000 743 36 0.0000 0.0003 0.000
0.000 252 37 0.0000 0.0004 0.000 0.000 760 38 0.0000 0.0004 0.000
0.000 760 39 0.0000 0.0003 0.000 0.000 742 40 0.0000 0.0003 0.000
0.000 812 41 0.0000 0.0003 0.000 0.000 742 42 0.0000 0.0003 0.000
0.000 762 43 0.0000 0.0003 0.000 0.000 741 44 0.0000 0.0003 0.000
0.000 675 45 0.0000 0.0003 0.000 0.000 742 46 0.0000 0.0003 0.000
0.000 699 47 0.0000 0.0003 0.000 0.000 742 48 0.0000 0.0003 0.000
0.000 742 49 0.0000 0.0003 0.000 0.000 746 50 0.0000 0.0003 0.000
0.000 772 51 0.0000 0.0003 0.000 0.000 742 52 0.0000 0.0003 0.000
0.000 741 53 0.0006 0.0003 0.000 0.000 742 54 0.9100 0.0003 0.000
0.000 707 55 0.0000 0.0007 0.000 0.000 741 56 0.0000 0.0092 0.000
0.000 734 57 0.0000 0.0003 0.960 0.000 742 58 0.0000 0.0003 0.000
0.008 741 59 0.0000 0.0004 0.000 0.000 760 Comparative steel
TABLE-US-00003 TABLE 2-1 Spherodizing annealing Continuous casting
Nitrogen Soundness enhancing Hot-rolling Cold-rolling concentration
treatment of Finish rolling Winding Average Draft in in annealing
Steel steel material temperature temperature Cooling start cooling
rate cold-rolling atmosphere No. No. Yes/No (.degree. C.) (.degree.
C.) time (s) (.degree. C./s) (%) (%) 1 1 No 868 595 0.7 99 -- 84 2
2 No 884 596 0.8 57 -- 76 3 3 No 856 593 0.7 80 -- 85 4 4 No 880
565 0.8 80 -- 70 5 5 No 879 469 0.6 61 -- 17 6 6 No 908 504 0.4 99
-- 19 7 7 No 851 585 0.4 64 -- 83 8 8 No 871 608 0.5 99 -- 72 9 9
No 865 504 0.4 58 -- 73 10 10 No 879 491 0.7 56 -- 75 11 11 No 871
595 0.6 86 -- 71 12 12 No 874 610 0.7 99 -- 82 13 13 No 852 489 0.5
98 -- 81 14 14 No 848 564 0.8 96 -- 84 15 15 No 868 546 0.4 96 --
85 16 16 No 851 604 0.5 60 -- 79 17 17 No 857 598 0.4 96 -- 74 18
18 No 877 502 0.7 56 -- 80 19 19 No 840 611 0.5 70 -- 71 20 20 No
866 612 0.4 59 -- 81 21 21 No 866 541 0.8 90 -- 73 22 22 No 848 599
0.6 70 -- 84 23 23 No 841 544 0.5 61 -- 81 24 24 No 846 501 0.6 82
-- 81 25 25 No 853 490 0.6 58 -- 70 26 26 No 862 480 0.8 58 -- 85
27 27 No 874 582 0.8 59 -- 72 28 28 No 867 515 0.6 87 -- 70 29 2 No
922 608 0.6 93 -- 83 30 2 No 847 453 0.6 55 -- 85 31 2 No 782 475
0.8 94 -- 85 32 2 No 851 756 0.6 64 -- 80 33 2 No 871 560 0.7 84 --
75 Spherodizing annealing Average Heating Retention Average Steel
heating rate temperature time cooling rate Thickness No. No.
(.degree. C./h) (.degree. C.) (h) (.degree. C./h) (mm) Remark 1 1
16 659 66 45 5.5 Example 2 2 47 662 47 35 5.5 Example 3 3 31 640 20
40 5.4 Example 4 4 50 648 78 43 4.6 Example 5 5 11 731 4 11 5.2
Comparative Example 6 6 99 720 33 84 5.3 Comparative Example 7 7 26
664 27 27 5.1 Comparative Example 8 8 48 659 45 20 5.3 Example 9 9
27 645 60 27 5.5 Example 10 10 25 645 22 23 5.3 Example 11 11 35
645 74 34 5.0 Comparative Example 12 12 29 661 71 44 5.7
Comparative Example 13 13 20 684 38 34 4.6 Comparative Example 14
14 22 665 34 31 5.1 Comparative Example 15 15 18 625 68 20 4.5
Comparative Example 16 16 41 685 48 28 4.2 Example 17 17 24 675 60
19 5.6 Example 18 18 31 611 66 19 4.3 Example 19 19 50 639 36 40
5.2 Example 20 20 20 655 45 19 4.0 Example 21 21 24 682 50 48 4.6
Example 22 22 15 658 44 35 5.1 Example 23 23 23 656 26 30 5.2
Example 24 24 28 661 63 31 5.6 Example 25 25 28 661 23 43 5.4
Example 26 26 41 655 37 37 5.3 Example 27 27 19 655 30 29 4.4
Example 28 28 24 668 47 26 5.5 Example 29 2 49 673 34 26 4.1
Comparative Example 30 2 47 673 65 27 4.7 Example 31 2 34 653 62 18
4.5 Comparative Example 32 2 32 663 27 28 4.9 Comparative Example
33 2 49 661 60 35 5.7 Example
TABLE-US-00004 TABLE 2-2 Continuous casting Soundness Spherodizing
annealing enhancing Nitrogen treatment Hot-rolling Cold-rolling
concentration of steel Finish rolling Winding Cooling Average Draft
in in annealing Steel material temperature temperature start
cooling rate cold-rolling atmosphere No. No. Yes/No (.degree. C.)
(.degree. C.) time (s) (.degree. C./s) (%) (%) 34 2 No 861 451 0.6
73 51 74 35 2 No 861 580 0.4 78 -- 19 36 2 No 861 520 0.5 79 -- 74
37 2 No 857 522 0.4 99 -- 88 38 2 No 862 515 0.5 74 -- 97 39 2 No
860 450 0.6 55 -- 78 40 2 No 883 583 0.7 80 -- 83 41 2 No 882 553
0.4 56 -- 77 42 2 No 864 450 0.4 90 -- 78 43 2 No 860 459 0.4 84 --
71 44 2 No 863 575 0.5 68 -- 84 45 2 No 876 553 0.6 100 -- 77 46 2
No 883 535 0.4 79 -- 82 47 2 No 870 502 0.7 84 -- 79 48 2 No 868
471 0.6 85 -- 81 49 2 No 863 487 0.4 63 -- 85 50 2 No 867 518 0.6
97 -- 80 51 2 Yes 865 501 0.8 65 -- 84 52 29 No 889 595 0.5 91 --
76 53 30 No 875 600 0.4 71 -- 77 54 31 No 891 592 0.5 95 -- 79 55
32 No 880 597 0.4 57 -- 81 56 33 No 889 603 0.4 91 -- 72 57 34 No
882 599 0.5 81 -- 81 58 35 No 890 605 0.6 63 -- 75 59 36 No 892 594
0.7 94 -- 81 60 37 No 894 606 0.8 67 -- 76 61 38 No 882 598 0.5 55
-- 80 62 39 No 890 587 0.5 57 -- 75 63 40 No 884 590 0.4 91 -- 76
64 41 No 886 606 0.7 96 -- 71 65 42 No 876 593 0.6 69 -- 72 66 43
No 879 602 0.4 64 -- 75 Spherodizing annealing Average Heating
Retention Average Steel heating rate temperature time cooling rate
Thickness No. No. (.degree. C./h) (.degree. C.) (h) (.degree. C./h)
(mm) Remark 34 2 19 659 51 20 2.8 Example 35 2 33 661 67 49 4.5
Comparative Example 36 2 38 679 72 49 4.6 Example 37 2 34 674 59 41
4.6 Example 38 2 38 671 73 26 4.7 Example 39 2 130 666 55 37 4.8
Comparative Example 40 2 45 664 35 39 4.2 Example 41 2 2 668 30 39
4.9 Comparative Example 42 2 24 773 26 42 4.6 Comparative Example
43 2 37 662 78 25 4.5 Example 44 2 41 617 43 44 4.7 Example 45 2 40
679 146 22 5.8 Comparative Example 46 2 20 662 24 41 5.3 Example 47
2 27 664 2 23 5.6 Comparative Example 48 2 48 659 31 130 4.5
Comparative Example 49 2 32 657 71 21 4.9 Example 50 2 44 668 22 2
5.4 Comparative Example 51 2 30 667 67 19 4.8 Example 52 29 51 667
43 30 5.6 Example 53 30 47 657 48 32 5.3 Example 54 31 45 665 48 35
5.3 Example 55 32 55 660 42 36 5.5 Example 56 33 38 663 46 35 5.6
Example 57 34 38 658 51 34 5.3 Example 58 35 56 657 47 34 5.7
Example 59 36 57 662 42 33 5.3 Example 60 37 44 662 48 40 5.3
Example 61 38 48 666 45 34 5.4 Example 62 39 44 658 47 38 5.3
Example 63 40 40 658 52 37 5.6 Example 64 41 41 661 43 39 5.4
Example 65 42 47 663 50 33 5.6 Example 66 43 39 666 44 35 5.3
Example
TABLE-US-00005 TABLE 2-3 Continuous casting Soundness enhancing
Hot-rolling Cold-rolling treatment of Finish rolling Winding
Average Draft in Steel steel material temperature temperature
Cooling start cooling rate cold-rolling No. No. Yes/No (.degree.
C.) (.degree. C.) time (s) (.degree. C./s) (%) 67 44 No 878 601 0.4
70 -- 68 45 No 889 591 0.7 65 -- 69 46 No 881 590 0.7 58 -- 70 47
No 887 597 0.5 75 -- 71 48 No 881 587 0.4 59 -- 72 49 No 886 592
0.4 83 -- 73 50 No 883 589 0.7 61 -- 74 51 No 890 593 0.4 60 -- 75
52 No 887 590 0.4 74 -- 76 53 No 890 598 0.6 80 -- 77 54 No 879 604
0.7 60 -- 78 55 No 882 592 0.8 62 -- 79 56 No 886 604 0.5 93 -- 80
57 No 878 602 0.5 89 -- 81 58 No 893 593 0.7 56 -- 82 2 No 809 606
0.5 67 -- 83 2 No 877 691 0.8 61 -- 84 2 No 892 603 0.6 88 -- 85 2
No 877 603 0.5 98 -- 86 2 No 881 586 0.5 98 -- 87 2 No 881 593 0.7
55 -- 88 2 No 894 597 0.4 83 -- 89 2 No 893 587 0.7 98 -- 90 2 No
893 605 0.5 75 -- 91 2 No 895 612 1.8 74 -- 92 2 No 880 560 0.9 57
-- 93 2 No 881 577 0.1 57 -- 94 2 No 888 587 0.5 43 -- 95 2 No 889
588 0.8 151 -- 96 2 No 876 591 0.8 203 -- 97 2 No 884 596 0.8 57 --
98 59 No 882 600 0.5 56 -- Spherodizing annealing Nitrogen
concentration in annealing Average Heating Retention Average Steel
atmosphere heating rate temperature time cooling rate Thickness No.
No. (%) (.degree. C./h) (.degree. C.) (.degree. C.) (.degree. C./h)
(mm) Remark 67 44 77 47 659 48 40 5.6 Example 68 45 78 37 667 47 37
5.6 Example 69 46 79 40 659 42 39 5.4 Example 70 47 76 49 662 51 31
5.3 Example 71 48 75 47 658 49 38 5.6 Example 72 49 75 47 657 44 36
5.6 Example 73 50 80 43 662 45 40 5.3 Example 74 51 75 40 658 47 36
5.7 Example 75 52 73 54 667 45 37 5.5 Example 76 53 76 40 661 47 32
5.4 Example 77 54 81 42 665 42 36 5.7 Example 78 55 80 57 657 44 31
5.5 Example 79 56 80 52 661 52 39 5.3 Example 80 57 75 46 666 48 33
5.4 Example 81 58 73 38 659 51 30 5.6 Example 82 2 81 53 659 47 30
5.7 Example 83 2 79 38 665 43 30 5.5 Example 84 2 28 57 665 45 30
5.7 Example 85 2 75 7 660 42 32 5.3 Example 86 2 74 94 664 52 32
5.6 Example 87 2 73 55 660 12 38 5.5 Example 88 2 81 37 667 97 35
5.6 Example 89 2 73 53 666 46 8 5.5 Example 90 2 75 51 662 42 94
5.5 Example 91 2 77 48 660 42 31 5.5 Comparative Example 92 2 77 37
651 48 30 5.7 Example 93 2 71 39 649 48 36 5.4 Example 94 2 70 39
643 40 30 5.5 Comparative Example 95 2 75 42 653 47 34 5.6 Example
96 2 76 40 655 49 31 5.5 Example 97 2 96 47 660 47 35 5.5 Example
98 59 82 51 666 48 34 5.4 Comparative Example
[0148] Each of the obtained steel sheets for carburizing was
measured regarding (1) percentage of the number of carbides with an
aspect ratio of 2.0 or smaller, among from the total carbides, (2)
percentage of the number of carbides produced in the ferrite
crystal grains, among from the total carbides, (3) average
equivalent circle diameter of carbides, (4) average nitrogen
concentration in the skin layer of the steel sheet, and, (5)
average crystal grain size of ferrite after spherodizing annealing,
according to the methods described previously. Note that the
average crystal grain size of ferrite after spherodizing annealing
is understood to be the average crystal grain size of ferrite of
the obtained steel sheet for carburizing.
[0149] In addition, in order to evaluate bendability of the
obtained individual steel sheets for carburizing, specimens were
sampled from freely selectable positions of the steel sheets for
carburizing, and measured regarding bendability under the following
conditions, in compliance with the VDA Standards (VDA238-100)
specified by Verband der Automobilindustrie e.V. In this test
example, dislocation under maximum load obtainable in the bend test
was converted to angle according to the VDA Standards, to determine
maximum angle of bend (in degree). [0150] Size of test specimen: 30
mm (rolling direction).times.60 mm (direction perpendicular to
rolling direction) [0151] Bending ridge: laid in parallel with
rolling direction [0152] Test method: roll-supported, punch-pressed
[0153] Roll diameter: .phi. 30 mm [0154] Punch shape: end with
R=0.4 mm [0155] Roll-to-roll distance: 2.0.times.sheet thickness
(mm)+0.5 mm [0156] Pressing velocity: 20 mm/min [0157] Tester:
Shimadzu Autograph (registered trademark) 20 kN
[0158] Also in order to evaluate toughness after carburizing of the
obtained individual steel sheets for carburizing, each of the thus
obtained steel sheets for carburizing was carburized as described
below. That is, each of the steel sheets for carburizing was
carburized while being kept in a gas atmosphere with a carbon
potential of 0.8 mass % at 900.degree. C. for 2.5 hr, and further
being kept at 850.degree. C. for 0.5 hr, and then oil-quenched at
100.degree. C. The steel sheet was then kept at 160.degree. C. for
2.0 hr for tempering, and cooled down to room temperature. A 2 mm
V-notched Charpy test piece was sampled from a freely selectable
position of the steel sheet after carburizing heat treatment, and
subjected to Charpy test at room temperature in compliance with a
method specified in JIS Z2242, to measure the impact value
(J/cm.sup.2).
[0159] As a reference, also ideal critical diameter, which is an
index for hardenability after carburizing, was calculated. The
ideal critical diameter D.sub.i is an index calculated from
ingredients of the steel sheet, and may be determined using the
equation (201) according to Grossmann/Hollomon, Jaffe's method. The
larger the value of ideal critical diameter D, the more excellent
the hardenability.
[ Math . 3 ] D i = ( 6.77 .times. [ C ] 0.5 ) .times. ( 1 + 0.64
.times. [ S i ] ) .times. ( 1 + 4.1 .times. [ Mn ] ) .times. ( 1 +
2.83 .times. [ P ] ) .times. ( 1 - 0.62 .times. [ S ] ) .times. ( 1
+ 0.27 .times. [ Cu ] ) .times. ( 1 + 0.52 .times. [ Ni ] ) .times.
( 1 + 2.33 .times. [ Cr ] ) .times. ( 1 + 3.14 .times. [ Mo ] )
.times. X For [ B ] = 0 : X = 1 For [ B ] > 0 : X = 1 + 1.5
.times. ( 0.9 - [ C ] ) Equation ( 201 ) ##EQU00003##
[0160] In this test example, the cases where the maximum bending
angle of the steel sheet for carburizing is 100.degree. or larger,
and the impact value after carburizing is 60 J/cm.sup.2 or larger
were judged to show high bendability during cold-working and high
toughness after carburizing, and were accepted as "examples".
[0161] Microstructures and characteristics of the individual steel
sheets for carburizing thus obtained were collectively summarized
in Table 3 below.
TABLE-US-00006 TABLE 3-1 Microstructure Average nitrogen Average
circle concentration Percentage of number Percentage of number
equivalent Average crystal grain in skin layer of carbides with
aspect of carbides within diameter of size of ferrite after Steel
of steel sheet ratio of 2.0 or smaller ferrite crystal grain
carbide spherodizing annealing No. No. (mass %) (%) (%) (.mu.m)
(.mu.m) 1 2 0.051 97 75 0.54 7.7 2 2 0.056 97 81 0.61 7.0 3 3 0.051
86 83 0.47 7.1 4 4 0.045 91 87 0.60 6.8 5 5 0.006 89 31 0.46 4.8 6
6 0.007 91 89 0.56 7.2 7 7 0.051 82 45 0.61 4.1 8 8 0.054 92 69
0.35 5.5 9 9 0.053 86 68 0.36 6.0 10 10 0.054 85 86 0.45 4.0 11 11
0.060 84 68 7.58 7.0 12 12 0.058 85 86 6.85 6.6 13 13 0.051 96 68
6.46 8.0 14 14 0.056 97 87 6.95 5.6 15 15 0.050 90 83 6.99 8.0 16
16 0.055 84 77 0.49 4.5 17 17 0.049 94 74 0.37 5.3 18 18 0.049 88
89 0.57 6.4 19 19 0.048 92 83 0.58 6.0 20 20 0.051 96 82 0.65 6.1
21 21 0.054 84 78 0.59 4.2 22 22 0.049 83 71 0.47 4.3 23 23 0.045
90 76 0.52 7.5 24 24 0.061 97 78 0.42 6.1 25 25 0.061 86 88 0.39
5.4 26 26 0.056 89 86 0.42 4.0 27 27 0.053 82 68 0.48 6.2 28 28
0.053 90 78 0.53 4.7 29 2 0.058 49 78 0.35 4.4 30 2 0.047 84 80
0.35 4.8 31 2 0.060 98 73 7.25 4.3 32 2 0.047 66 69 0.55 7.2 33 2
0.045 84 89 0.59 5.5 Mechanical characteristics Maximum Impact
value Hardenability bending after Ideal critical Steel angle
carburizing diameter No. No. (deg) (J/cm.sup.2) (--) Remark 1 2 120
82 20.7 Example 2 2 113 85 43.9 Example 3 3 108 66 23.2 Example 4 4
120 78 135.1 Example 5 5 88 81 20.5 Comparative Example 6 6 72 64
108.5 Comparative Example 7 7 69 63 1.5 Comparative Example 8 8 105
64 9.7 Example 9 9 106 66 9.5 Example 10 10 101 71 9.6 Example 11
11 79 67 13.9 Comparative Example 12 12 67 85 5.3 Comparative
Example 13 13 69 83 12.6 Comparative Example 14 14 77 83 1.8
Comparative Example 15 15 85 83 31.4 Comparative Example 16 16 114
80 23.5 Example 17 17 118 75 15.7 Example 18 18 113 82 7.8 Example
19 19 115 84 7.8 Example 20 20 111 80 5.6 Example 21 21 113 76 5.6
Example 22 22 120 83 5.6 Example 23 23 111 80 5.9 Example 24 24 117
81 13.5 Example 25 25 110 82 5.9 Example 26 26 116 81 6.4 Example
27 27 114 85 5.8 Example 28 28 113 82 5.2 Example 29 2 77 81 43.9
Comparative Example 30 2 110 81 43.9 Example 31 2 79 80 43.9
Comparative Example 32 2 86 85 43.9 Comparative Example 33 2 119 82
43.9 Example
TABLE-US-00007 TABLE 3-2 Microstructure Average nitrogen Average
circle concentration Percentage of number Percentage of number
equivalent Average crystal grain in skin layer of carbides with
aspect of carbides within diameter of size of ferrite after Steel
of steel sheet ratio of 2.0 or smaller ferrite crystal grain
carbide spherodizing annealing No. No. (mass %) (%) (%) (.mu.m)
(.mu.m) 34 2 0.047 85 73 0.58 4.5 35 2 0.007 96 84 6.37 7.0 36 2
0.055 86 80 0.35 4.6 37 2 0.188 86 80 0.35 6.9 38 2 0.181 86 80
0.35 5.0 39 2 0.057 41 75 0.51 7.1 40 2 0.048 90 71 0.54 7.5 41 2
0.047 92 72 6.69 6.5 42 2 0.055 85 36 0.43 6.8 43 2 0.057 96 85
0.58 5.7 44 2 0.061 96 78 0.57 7.9 45 2 0.056 88 68 5.95 7.2 46 2
0.055 92 86 0.59 7.2 47 2 0.050 65 70 0.48 5.0 48 2 0.049 71 68
0.49 5.9 49 2 0.048 95 82 0.42 7.2 50 2 0.054 88 79 7.61 5.0 51 2
0.055 96 81 0.41 5.9 52 29 0.056 88 75 0.63 5.6 53 30 0.050 90 80
4.70 7.7 54 31 0.061 91 80 0.71 5.9 55 32 0.050 88 77 4.81 7.8 56
33 0.064 91 78 0.64 4.3 57 34 0.059 90 77 0.51 7.5 58 35 0.061 88
74 0.65 7.2 59 36 0.046 87 77 0.62 5.9 60 37 0.057 87 72 0.62 7.8
61 38 0.052 89 82 0.58 5.8 62 39 0.061 92 81 0.67 9.5 63 40 0.056
90 79 0.56 5.1 64 41 0.061 92 73 0.60 9.7 65 42 0.047 92 81 0.66
8.0 66 43 0.066 89 76 0.55 9.1 Mechanical characteristics Maximum
Impact value Hardenability bending after Ideal critical Steel angle
carburizing diameter No. No. (deg) (J/cm.sup.2) (--) Remark 34 2
110 82 43.9 Example 35 2 71 55 43.9 Comparative Example 36 2 114 85
43.9 Example 37 2 117 94 43.9 Example 38 2 113 87 43.9 Example 39 2
81 76 43.9 Comparative Example 40 2 111 79 43.9 Example 41 2 86 75
43.9 Comparative Example 42 2 78 80 43.9 Comparative Example 43 2
113 79 43.9 Example 44 2 112 83 43.9 Example 45 2 87 85 43.9
Comparative Example 46 2 117 77 43.9 Example 47 2 71 85 43.9
Comparative Example 48 2 74 78 43.9 Comparative Example 49 2 116 84
43.9 Example 50 2 82 79 43.9 Comparative Example 51 2 131 95 43.9
Example 52 29 103 90 11.4 Example 53 30 102 89 14.4 Example 54 31
104 81 5.9 Example 55 32 101 76 60.3 Example 56 33 105 78 12.2
Example 57 34 101 87 10.7 Example 58 35 102 80 12.9 Example 59 36
104 85 9.8 Example 60 37 128 82 43.9 Example 61 38 102 81 43.9
Example 62 39 114 64 9.9 Example 63 40 115 71 101.6 Example 64 41
113 62 11.4 Example 65 42 116 70 41.9 Example 66 43 110 63 11.3
Example
TABLE-US-00008 TABLE 3-3 Microstructure Average nitrogen Average
circle concentration Percentage of number Percentage of number
equivalent Average crystal grain in skin layer of carbides with
aspect of carbides within diameter of size of ferrite after Steel
of steel sheet ratio of 2.0 or smaller ferrite crystal grain
carbide spherodizing annealing No. No. (mass %) (%) (%) (.mu.m)
(.mu.m) 67 44 0.050 90 77 0.58 4.6 68 45 0.059 91 80 0.64 9.4 69 46
0.058 90 73 0.57 4.1 70 47 0.050 92 78 0.70 9.4 71 48 0.064 92 80
0.66 6.1 72 49 0.059 92 76 0.55 9.1 73 50 0.059 91 77 0.55 5.1 74
51 0.064 90 78 0.51 9.3 75 52 0.055 91 83 0.57 4.0 76 53 0.055 90
74 0.63 9.5 77 54 0.056 91 75 0.63 7.0 78 55 0.066 91 79 0.64 6.4
79 56 0.052 90 81 0.61 6.6 80 57 0.058 89 92 0.70 5.1 81 58 0.046
88 80 0.69 7.6 82 2 0.052 87 75 4.66 6.2 83 2 0.058 81 75 0.68 6.8
84 2 0.041 92 62 0.60 5.4 85 2 0.066 90 79 4.71 4.8 86 2 0.052 82
77 0.63 5.5 87 2 0.046 81 71 0.69 7.6 88 2 0.057 91 82 4.81 6.8 89
2 0.048 92 71 4.71 4.6 90 2 0.057 81 73 0.63 6.4 91 2 0.064 92 71
0.71 13.0 92 2 0.066 92 79 0.54 9.1 93 2 0.054 89 81 0.56 2.3 94 2
0.060 92 73 0.64 12.5 95 2 0.058 89 79 0.60 3.2 96 2 0.065 92 77
0.68 1.8 97 2 0.050 91 76 0.70 4.1 98 59 0.289 89 81 0.54 5.7
Mechanical characteristics Maximum Impact value Hardenability
bending after Ideal critical Steel angle carburizing diameter No.
No. (deg) (J/cm.sup.2) (--) Remark 67 44 111 65 31.4 Example 68 45
117 63 10.8 Example 69 46 116 62 18.2 Example 70 47 118 65 10.0
Example 71 48 115 62 9.9 Example 72 49 101 81 10.2 Example 73 50
103 80 10.8 Example 74 51 103 87 10.0 Example 75 52 105 84 9.9
Example 76 53 103 78 9.5 Example 77 54 104 76 10.3 Example 78 55
102 81 11.9 Example 79 56 125 75 11.2 Example 80 57 121 84 10.5
Example 81 58 118 78 12.9 Example 82 2 103 83 43.9 Example 83 2 101
79 43.9 Example 84 2 110 62 43.9 Example 85 2 103 76 43.9 Example
86 2 104 79 43.9 Example 87 2 103 88 43.9 Example 88 2 101 83 43.9
Example 89 2 101 85 43.9 Example 90 2 102 76 43.9 Example 91 2 110
51 43.9 Comparative Example 92 2 118 62 43.9 Example 93 2 118 93
43.9 Example 94 2 107 52 43.9 Comparative Example 95 2 108 91 43.9
Example 96 2 109 97 43.9 Example 97 2 114 99 43.9 Example 98 59 54
82 43.9 Comparative Example
[0162] As is clear from Table 3 above, the steel sheets for
carburizing that correspond to the examples of this invention were
found to have good formability and toughness after carburizing,
showing maximum bending angles of the steel sheet for carburizing
of 100.degree. or larger, and impact values after carburizing of 60
J/cm.sup.2 or larger. Also the ideal critical diameter, described
for reference, was found to be 5 or larger, teaching that the steel
sheets for carburizing that come under examples of the present
invention also excel in hardenability.
[0163] Meanwhile, as is clear from Table 3 above, the steel sheets
for carburizing that correspond to comparative examples of this
invention were found to be ill-balanced between the formability and
the toughness after carburizing, showing at least either of maximum
bending angle or impact value after carburizing dropped below the
standard values.
[0164] Although having detailed the preferred embodiments of the
present invention, the present invention is not limited to these
examples. It is obvious that those having general knowledge in the
technical field to which the present invention pertains will easily
arrive at various modified examples or revised examples within the
scope of technical concept described in claims, and also these
examples are naturally understood to come under the technical scope
of the present invention.
* * * * *