U.S. patent application number 15/129520 was filed with the patent office on 2017-05-04 for high-carbon hot-rolled steel sheet and method for manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Takashi KOBAYASHI, Yuka MIYAMOTO, Kaneharu OKUDA.
Application Number | 20170121787 15/129520 |
Document ID | / |
Family ID | 54194717 |
Filed Date | 2017-05-04 |
United States Patent
Application |
20170121787 |
Kind Code |
A1 |
MIYAMOTO; Yuka ; et
al. |
May 4, 2017 |
HIGH-CARBON HOT-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING THE
SAME
Abstract
A high-carbon hot-rolled steel sheet having a chemical
composition containing, by mass %, C: 0.20% or more and 0.40% or
less, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S:
0.010% or less, sol.A1: 0.10% or less, N: 0.0050% or less, B:
0.0005% or more and 0.0050% or less, and at least one of Sb, Sn,
Bi, Ge, Te, and Se in an amount of 0.002% or more and 0.030% or
less in total. The steel sheet has a microstructure including
ferrite and cementite, in which the density of cementite in the
ferrite grains is 0.08 pieces/m.sup.2 or less. Additionally, the
steel sheet has a hardness of 73 or less in terms of HRB and a
total elongation of 39% or more.
Inventors: |
MIYAMOTO; Yuka; (Kawasaki,
JP) ; KOBAYASHI; Takashi; (Chiba, JP) ; OKUDA;
Kaneharu; (Sendai, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
54194717 |
Appl. No.: |
15/129520 |
Filed: |
March 26, 2015 |
PCT Filed: |
March 26, 2015 |
PCT NO: |
PCT/JP2015/001712 |
371 Date: |
September 27, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 9/46 20130101; C21D
8/0226 20130101; C22C 38/06 20130101; C22C 38/002 20130101; C22C
38/60 20130101; C21D 2211/009 20130101; C22C 38/001 20130101; C22C
38/02 20130101; C22C 38/04 20130101; C22C 38/18 20130101; C22C
38/08 20130101; C22C 38/32 20130101; C21D 8/0263 20130101; C22C
38/00 20130101; C21D 2211/005 20130101; C21D 2211/003 20130101;
C22C 38/12 20130101; C22C 38/008 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/60 20060101 C22C038/60; C22C 38/32 20060101
C22C038/32; C22C 38/18 20060101 C22C038/18; C22C 38/00 20060101
C22C038/00; C22C 38/08 20060101 C22C038/08; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C21D 8/02 20060101 C21D008/02; C22C 38/12 20060101
C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2014 |
JP |
2014-068737 |
Claims
1. A high-carbon hot-rolled steel sheet having a chemical
composition-containing, by mass %, comprising: C: 0.20% or more and
0.40% or less, by mass %; Si: 0.10% or less, by mass %; Mn: 0.50%
or less, by mass %; P: 0.03% or less, by mass %; S: 0.010% or less,
by mass %; sol.Al: 0.10% or less, by mass %:, N: 0.0050% or less,
by mass %; B: 0.0005% or more and 0.0050% or less, by mass %; at
least one of Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% or
more and 0.030% or less in total, by mass %; and Fe and inevitable
impurities, wherein: a proportion of a content of a solid solution
B to a content of B is 70% or more, the steel sheet has a
microstructure including ferrite and cementite, the density of
cementite in ferrite grains is 0.08 pieces/.mu.m.sup.2 or less, the
steel sheet has a hardness of 73 or less in terms of HRB, and the
steel sheet has a total elongation of 39% or more.
2. The high-carbon hot-rolled steel sheet according to claim 1,
wherein the chemical composition of the steel sheet further
comprises at least one of Ni, Cr, and Mo in an amount of 0.50% or
less in total, by mass %.
3. The high-carbon hot-rolled steel sheet according to claim 1,
wherein: an average grain diameter of all the cementite in the
steel sheet is 0.60 .mu.m or more and 1.00 .mu.m or less, and an
average grain diameter of the cementite in the ferrite grains is
0.40 .mu.m or more.
4. A method for manufacturing a high-carbon hot-rolled steel sheet,
the method comprising: performing hot rough rolling on steel, the
steel having a chemical composition comprising: C: 0.20% or more
and 0.40% or less, by mass %. Si: 0.10% or less, bv mass %, Mn:
0.50% or less, by mass %, P: 0.03% or less, by mass %, S: 0.010% or
less, by mass %, sol.Al: 0.10% or less, by mass %, N: 0.0050% or B:
0.0005% or more and 0.0050% or less, by mass %, at least one of Sb,
Sn, Bi, Ge, Te, and Se in an amount of 0.002% or more and 0.030% or
less in total, by mass %, and and inevitable impurities, then,
after the hot rough rolling, performing finish rolling with a
finishing delivery temperature equal to or higher than the Ar.sub.a
transformation temperature and 870.degree. C. or lower:, then,
after the finish rolling, cooling the hot-rolled steel sheet to a
temperature of 700.degree. C. at an average cooling rate of
25.degree. C./s or more and 150.degree. C./s or less; then, after
the cooling, coiling the cooled steel sheet at a coiling
temperature of 500.degree. C. or higher and 700.degree. C. or lower
in order to obtain a steel sheet having a microstructure including
pearlite and, in terms of volume fraction, 5% or more of
pro-eutectoid ferrite: and then, after the coiling, annealing the
steel sheet at a temperature equal to or lower than the Ac.sub.1
transformation temperature.
5. The method for manufacturing a high-carbon hot-rolled steel
sheet according to claim 4, wherein the chemical composition of the
steel further comprises at least one of Ni, Cr, and Mo in an amount
of 0.50% or less in total. total, by mass %.
6. The high-carbon hot-rolled steel sheet according to claim 1,
wherein the density of the cementite in the ferrite grains is 0.06
pieces/.mu.m.sup.2 or less,
7. The high-carbon hot-rolled steel sheet according to claim 1,
wherein the microstructure of the steel sheet further includes
pearlite and, in terms of volume fraction, 5% or more of
pro-eutectoid ferrite.
8. The method for manufacturing a high-carbon hot-rolled steel
sheet according to claim 4, wherein: the steel sheet has a
microstructure including ferrite and cementite, and the density of
cementite in ferrite grains is 0.08 pieces/.mu.m.sup.2 or less.
9. The method for manufacturing a high-carbon hot-rolled steel
sheet according to claim 8, wherein the density of the cementite in
the ferrite grains is 0.06 pieces/.mu.m.sup.2 or less.
10. The method for manufacturing a high-carbon hot-rolled steel
sheet according to claim 4, wherein the steel sheet has a hardness
of 73 or less in terms of HRB.
11. The method for manufacturing a high-carbon hot-rolled steel
sheet according to claim 4, wherein the steel sheet has a total
elongation of 39% or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-carbon hot-rolled
steel sheet and a method for manufacturing the steel sheet, and, in
particular, to a high-carbon hot-rolled steel sheet excellent in
terms of workability and hardenability to which B is added and
which is highly effective for inhibiting nitrogen ingress in a
surface layer thereof and a method for manufacturing the steel
sheet.
BACKGROUND ART
[0002] Nowadays, automotive parts such as gears, transmission
parts, and seat recliner parts are manufactured by forming a
hot-rolled steel sheet, which is carbon steel material for machine
structural use prescribed in JIS G 4051, into desired shapes by
using a cold forming method and by performing a quenching treatment
on the formed steel sheet in order to achieve a desired hardness.
Therefore, a hot-rolled steel sheet, which is a raw material for
the parts, is required to have excellent cold workability and
hardenability, and various steel sheets have been proposed to
date.
[0003] For example, Patent Literature 1 discloses a medium-carbon
steel sheet to be subjected to cold forming, the medium-carbon
steel sheet having a hardness of 500 HV or more and 900 HV or less
in the case where the steel sheet is subjected to an induction
hardening treatment in which the steel sheet is heated at an
average heating rate of 100.degree. C./s, then held at a
temperature of 1000.degree. C. for 10 seconds, and then rapidly
cooled to room temperature at an average cooling rate of
200.degree. C./s, having a chemical composition containing, by mass
%, C: 0.30% to 0.60%, Si: 0.06% to 0.30%, Mn: 0.3% to 2.0%, P:
0.030% or less, S: 0.0075% or less, Al: 0.005% to 0.10%, N: 0.001%
to 0.01%, Cr: 0.001% to 0.10%, and, optionally, one or more of Ni:
0.01% to 0.5%, Cu: 0.05% to 0.5%, Mo: 0.01% to 0.5%, Nb: 0.01% to
0.5%, Ti: 0.001% to 0.05%, V: 0.01% to 0.5%, Ta: 0.01% to 0.5%, B:
0.001% to 0.01%, W: 0.01% to 0.5%, Sn: 0.003% to 0.03%, Sb: 0.003%
to 0.03%, and As: 0.003% to 0.03%, a microstructure, in which the
average grain diameter d .mu.m of carbides is 0.6 .mu.m or less, in
which the spheroidizing ratio P % of carbides is 70% or more and
less than 90%, and in which the average grain diameter d pm of the
carbides and the spheroidizing ratio P % of the carbides satisfy
the relationship d.ltoreq.0.04.times.P-2.6, and, optionally, a
hardness of 120 HV or more and less than 170 HV before cold forming
is performed. In addition, Patent Literature 1 discloses a method
for manufacturing such a medium-carbon steel sheet to be subjected
to cold forming in which steel having the chemical composition
mentioned above is held at a temperature of 1050.degree. C. to
1300.degree. C., then subjected to hot rolling in which rolling is
finished at a temperature of 750.degree. C. to 1000.degree. C.,
then cooled to a temperature of 500.degree. C. to 700.degree. C. at
a cooling rate of 20.degree. C./s to 50.degree. C./s, then cooled
to a specified temperature at a cooling rate of 5.degree. C./s to
30.degree. C./s, then coiled, then held under specified conditions,
and then annealed at a temperature of 600.degree. C. or higher and
equal to or lower than the Ac.sub.t-10.degree. C.
[0004] In addition, Patent Literature 2 discloses a boron-added
steel sheet having a chemical composition containing, by mass %, C:
0.20% or more and 0.45% or less, Si: 0.05% or more and 0.8% or
less, Mn: 0.5% or more and 2.0% or less, P: 0.001% or more and
0.04% or less, S: 0.0001% or more and 0.006% or less, Al: 0.005% or
more and 0.1% or less, Ti: 0.005% or more and 0.2% or less, B:
0.001% or more and 0.01% or less, N: 0.0001% or more and 0.01% or
less, and, optionally, one, two, or more of Cr: 0.05% or more and
0.35% or less, Ni: 0.01% or more and 1.0% or less, Cu: 0.05% or
more and 0.5% or less, Mo: 0.01% or more and 1.0% or less, Nb:
0.01% or more and 0.5% or less, V: 0.01% or more and 0.5% or less,
Ta: 0.01% or more and 0.5% or less, W: 0.01% or more and 0.5% or
less, Sn: 0.003% or more and 0.03% or less, Sb: 0.003% or more and
0.03% or less, and As: 0.003% or more and 0.03% or less, in which
an average concentration of a solid solution B in a region from the
surface to a depth of 100 .mu.m is 10 ppm or more. In addition,
Patent Literature 2 discloses that, in the case where annealing is
performed in an atmosphere mainly containing nitrogen, since a
phenomenon called nitrogen absorption occurs, B, which is an
important chemical element from the viewpoint of hardenability,
combines with N in steel to form BN in an annealing process, which
results in the effect of increasing hardenability through the use
of B not being realized due to a decrease in the amount of a solid
solution B. Patent Literature 2 discloses that, in order to achieve
satisfactory hardenability, it is necessary to control the
concentration of a solid solution B in a region from the surface to
a depth of 100 .mu.m to be 10 ppm or more, and that, therefore, it
is important to suppress the influence of the atmosphere of a
heating process and an annealing process included in a
manufacturing process. In addition, Patent Literature 2 discloses a
method for manufacturing such a boron-added steel sheet in which
steel having the chemical composition mentioned above is heated to
a temperature of 1200.degree. C. or lower, then subjected to hot
rolling with a finishing delivery temperature of 800.degree. C. to
940.degree. C., then cooled to a temperature of 650.degree. C. or
lower at a cooling rate of 20.degree. C./s or more, then cooled at
a cooling rate of 20.degree. C./s or less, then coiled at a
temperature of 400.degree. C. to 650.degree. C., then pickled, and
then annealed at a temperature of 660.degree. C. or higher and
equal to or lower than the Ac.sub.1 in an atmosphere in which
hydrogen concentration is 95% or more, the dew point in a
temperature range lower than 400.degree. C. is -20.degree. C. or
lower, and the dew point in a temperature range of 400.degree. C.
or higher is -40.degree. C. or lower. In addition, Patent
Literature 2 discloses, for example, a method in which the
above-mentioned pickled steel sheet is further subjected to cold
rolling and a method in which the above-mentioned annealed steel
sheet is subjected to cold rolling, then further subjected to
annealing in a temperature range from the Ac.sub.1 to the
Ac.sub.1+50.degree. C., and then subjected to slow cooling to a
temperature of the Ac.sub.1-30.degree. C.
CITATION LIST
Patent Literature
[0005] PTL 1: Japanese Patent No. 5048168
[0006] PTL 2: Japanese Patent No. 4782243
SUMMARY OF INVENTION
Technical Problem
[0007] From the viewpoint of achieving good cold workability, a
high-carbon hot-rolled steel sheet is required to have a
comparatively low hardness and a high elongation. For example, in
the case where, for example, automotive parts, which have been
manufactured by performing plural processes such as hot forging,
machining, and welding, are integrally molded by performing cold
press forming, the automotive parts are required to have properties
such as a hardness of 73 or less in terms of Rockwell hardness HRB
and a total elongation (El) of 39% or more. In addition, a
high-carbon hot-rolled steel sheet having a comparatively low
hardness and a high elongation in order to achieve good workability
as described above is required to have excellent hardenability such
that, for example, the steel sheet has a Vickers hardness of HV440
or more after water quenching has been performed.
[0008] In the case of the technique according to Patent Literature
1 where the average grain diameter of carbides is controlled to be
0.6 .mu.m or less in order to achieve quenching hardenability to be
realized in an induction hardening treatment which is performed at
an average heating rate of 100.degree. C./s, since the average
grain diameter of carbides is controlled to be 0.6 .mu.m or less in
steel having a high C content of 0.3% to 0.6%, there is a tendency
for strength to increase due to high density of carbides, which
raises a risk of a decrease in workability. In addition, since, in
the manufacturing method according to Patent Literature 1, two-step
cooling control, in which cooling is performed to a temperature of
500.degree. C. to 700.degree. C. at a cooling rate of 20.degree.
C./s to 50.degree. C./s after hot rolling has been performed, and
then cooling is performed at a cooling rate of 5.degree. C./s to
30.degree. C./s, is performed, there is a problem in that it is
difficult to control cooling.
[0009] Also, in the case of the technique according to Patent
Literature 2 where two-step cooling control, in which cooling is
performed to a temperature of 650.degree. C. or lower at a cooling
rate of 20.degree. C./s or more after hot rolling has been
performed, and then cooling is performed at a cooling rate of
20.degree. C./s or lower, is performed, there is a problem in that
it is difficult to manage a cooling control. Moreover, in the case
of the technique according to Patent Literature 2, Mn is added in
an amount of 0.5% or more in order to increase hardenability.
Although Mn increases hardenability, since there is an increase in
the strength of a hot-rolled steel sheet through solid solution
strengthening, there is an increase in the hardness of the
hot-rolled steel sheet.
[0010] On the other hand, B is known as chemical element that
increases hardenability when added in minute amounts, however, as
described in Patent Literature 2, in the case where annealing is
performed in an atmosphere containing mainly nitrogen, which is
generally used as an atmospheric gas, there is a problem in that it
is not possible to realize the effect of increasing hardenability
caused by adding B due to a decrease in the amount of a solid
solution B. Although, in Patent Literature 2, such a problem is
solved by performing annealing in an atmosphere containing 95% or
more of hydrogen or in an atmosphere in which an inert gas such as
Ar is used instead of hydrogen, there is an increase in cdst in the
case of a heat treatment in which such a gas is used. In addition,
it is not clear whether or not it is possible to inhibit nitrogen
absorption in an annealing process performed in a nitrogen
atmosphere only with this technique.
[0011] An object of the present invention is, in order to solve the
problems described above, to provide a high-carbon hot-rolled steel
sheet whose raw material is B-added steel having a lower Mn content
than conventional steel, with which it is possible to stably
achieve excellent hardenability even if annealing is performed in a
nitrogen atmosphere, and which has excellent workability
corresponding to a hardness of 73 or less in terms of HRB and to a
total elongation of 39% or more before a quenching treatment is
performed and a method for manufacturing the steel sheet.
Solution to Problem
[0012] The present inventors diligently conducted investigations
regarding the relationship between manufacturing conditions and
workability and hardenability, in the case of a B-added high-carbon
hot-rolled steel sheet having lower Mn content than conventional
steel, that is, a Mn content of 0.50% or less, and, as a result,
obtained the following knowledge.
[0013] i) The hardness and total elongation (hereafter, also simply
referred to as elongation) of a high-carbon hot-rolled steel sheet
before a quenching treatment is performed are strongly influenced
by the density of cementite in ferrite grains. In order to obtain a
steel sheet having a hardness of 73 or less in terms of HRP and a
total elongation (El) of 39% or .sub.more it is necessary that the
density of cementite in ferrite grains be 0.08 pieces/.mu.m.sup.2
or less.
[0014] ii) The density of cementite in ferrite grains is strongly
influenced by the finishing delivery temperature of finish rolling
included in hot rolling and a cooling rate down to a temperature of
700.degree. C. after finish rolling has been performed. In the case
where the finishing delivery temperature is excessively high or
where the cooling rate is excessively low, since it is not possible
to obtain a steel sheet having a microstructure including pearlite
and a specified volume fraction of pro-eutectoid ferrite phase
fraction after hot rolling has been performed, it is difficult to
decrease the density of cementite after spheroidizing annealing has
been performed.
[0015] iii) By adding at least one of Sb, Sn, Bi, Ge, Te, and Se to
steel, since it is possible to prevent nitrogen ingress even if
annealing is performed in a nitrogen atmosphere, it is possible to
achieve high hardenability by inhibiting a decrease in the amount
of a solid solution B.
[0016] The present invention has been completed on the basis of
such knowledge, and the subjective matter of the present invention
is as follows.
[0017] [1] A high-carbon hot-rolled steel sheet having a chemical
composition containing, by mass %, C: 0.20% or more and 0.40% or
less, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S:
0.010% or less, sol.Al: 0.10% or less, N: 0.0050% or less, B:
0.0005% or more and 0.0050% or less, one or more of Sb, Sn, Bi, Ge,
Te, and Se in an amount of 0.002% or more and 0.030% or less in
total, and the balance being Fe and inevitable impurities, in which
the proportion of the content of a solid solution B to the content
of B is 70% or more, a microstructure including ferrite and
cementite, in which the density of cementite in the ferrite grains
is 0.08 pieces/.mu.m.sup.2 or less, a hardness of 73 or less in
terms of HRB, and a total elongation of 39% or more.
[0018] [2] The high-carbon hot-rolled steel sheet according to item
[1] above, the steel sheet having the chemical composition further
containing, by mass %, one or more of Ni, Cr, and Mo in an amount
of 0.50% or less in total.
[0019] [3] The high-carbon hot-rolled steel sheet according to item
[1] or [2] above, the steel sheet having the microstructure
including ferrite and cementite, in which the average grain
diameter of all the cementite is 0.60 .mu.m or more and 1.00 .mu.m
or less, and in which the average grain diameter of cementite in
ferrite grains is 0.40 .mu.m or more.
[0020] [4] A method for manufacturing a high-carbon hot-rolled
steel sheet, the method including performing hot rough rolling on
steel having a chemical composition containing, by mass %, C: 0.20%
or more and 0.40% or less, Si: 0.10% or less, Mn: 0.50% or less, 2:
0.03% or less, S: 0.010% or less, sol.Al: 0.10% or less, N: 0.0050%
or less, B: 0.0005% or more and 0.0050% or less, one or more of Sb,
Sn, Bi, Ge, Te, and Se in an amount of 0.002% or more and 0.030% or
less in total, and the balance being Fe and inevitable impurities,
then performing finish rolling with a finishing delivery
temperature equal to or higher than the Ar.sub.a transformation
temperature and 870.degree. C. or lower, then cooling the
hot-rolled steel sheet to a temperature of 700.degree. C. at an
average cooling rate of 25.degree. C./s or more and 150.degree.
C./s or less, then coiling the cooled steel sheet at a coiling
temperature of 500.degree. C. or higher and 700.degree. C. or lower
in order to obtain a steel sheet having a microstructure including
pearlite and, in terms of volume fraction, 5% or more of
pro-eutectoid ferrite, and then annealing the steel sheet at a
temperature equal to or lower than the Ac.sub.1 transformation
temperature.
[0021] [5] The method for manufacturing a high-carbon hot-rolled
steel sheet according to item [4] above, the steel having the
chemical composition further containing, by mass %, one or more of
Ni, Cr, and Mo in an amount of 0.50% or less in total.
ADVANTAGEOUS EFFECTS OF INVENTION
[0022] According to the present invention, it is possible to
manufacture a high-carbon hot-rolled steel sheet excellent in terms
of hardenability and workability. The high-carbon hot-rolled steel
sheet according to the present invention can preferably be used for
automotive parts such as gears, transmission parts, and seat
recliner parts whose raw material steel sheet is required to have
satisfactory cold workability.
Description of Embodiments
[0023] Hereafter, the high-carbon hot-rolled steel sheet and the
method for manufacturing the steel sheet according to the present
invention will be described in detail. Here, "%", which is the unit
of the content of a constituent chemical element, refers to "mass
%", unless otherwise noted.
[0024] 1) Chemical composition
[0025] C: 0.20% or more and 0.40% or less
[0026] C is a chemical element which is important for achieving
strength after quenching has been performed. In the case where the
C content is less than 0.20%, it is not possible to achieve the
desired hardness by performing a heat treatment after a part has
been formed, or, specifically, it is not possible to achieve a
hardness of HV440 or more after water quenching has been performed.
Therefore, it is necessary that the C content be 0.20% or more. On
the other hand, in the case where the C content is more than 0.40%,
since there is an increase in the hardness of a steel sheet, there
is a decrease in cold workability. Therefore, the C content is set
to be 0.40% or less. It is preferable that the C content be 0.26%
or more in order to achieve a high quenched hardness. It is more
preferable that the C content be 0.32% or more, because it is
possible to stably achieve a hardness of HV440 or more after water
quenching has been performed.
[0027] Si: 0.10% or less
[0028] Si is a chemical element which increases strength through
solid solution strengthening. Since hardness increases with
increasing Si content, there is a decrease in cold workability.
Therefore, the Si content is set to be 0.10% or less, preferably
0.05% or less, or more preferably 0.03% or less. Although it is
preferable that the Si content be as small as possible because Si
decreases cold workability, since there is an increase in refining
costs in the case where the Si content is excessively decreased, it
is preferable that the Si content be 0.005% or more.
[0029] Mn: 0.50% or less
[0030] Although Mn is a chemical element which increases
hardenability, but, on the other hand, Mn is also a chemical
element which increases strength through solid solution
strengthening. In the case where the Mn content is more than 0.50%,
there is a decrease in cold workability due to an excessive
increase in the hardness of a steel sheet. In addition, since a
band structure grows due to the segregation of Mn and a non-uniform
microstructure is formed, there is a tendency for a variation in
hardness and elongation to increase. Therefore, the Mn content is
set to be 0.50% or less, preferably 0.45% or less, or more
preferably 0.40% or less. Here, although there is no particular
limitation on the lower limit of the Mn content, it is preferable
that the Mn content be 0.20% or more in order to achieve the
specified quenched hardness by allowing all the C in a steel sheet
to form a solid solution in a heating process for a quenching
treatment as a result of inhibiting the precipitation of
graphite.
[0031] P: 0.03% or less
[0032] P is a chemical element which increases strength through
solid solution strengthening. In the case where the P content is
more than 0.03%, there is a decrease in cold workability due to an
excessive increase in the hardness of a steel sheet. In addition,
since there is a decrease in the strength of grain boundaries,
there is a decrease in toughness after quenching has been
performed. Therefore, the P content is set to be 0.03% or less. It
is preferable that the P content be 0.02% or less in order to
achieve excellent toughness after quenching has been performed.
Although it is preferable that the P content be as small as
possible because P decreases cold workability and toughness after
quenching has been performed, since there is an increase in
refining costs in the case where the P content is decreased more
than necessary, it is preferable that the P content be 0.005% or
more.
[0033] S: 0.010% or less
[0034] Since S forms sulfides and decreases the cold workability of
a high-carbon hot-rolled steel sheet and toughness after quenching
has been performed, S is a chemical element whose content should be
decreased. In the case where the S content is more than 0.010%,
there is a significant decrease in the cold workability of a
high-carbon hot-rolled steel sheet and toughness after quenching
has been performed. Therefore, the S content is set to be 0.010% or
less. It is preferable that the S content be 0.005% or less in
order to achieve excellent cold workability and excellent toughness
after quenching has been performed. Although it is preferable that
the S content be as small as possible because S decreases cold
workability and toughness after quenching has been performed, since
there is an increase in refining costs in the case where the S
content is decreased more than necessary, it is preferable that the
S content be 0.0005% or more.
[0035] sol.Al: 0.10% or less
[0036] In the case where the sol.Al content is more than 0.10%,
since there is an excessive decrease in austenite grain diameter
due to the formation of AlN in a heating process for a quenching
treatment, a microstructure including ferrite and martensite is
formed as a result of promoting the formation of a ferrite phase in
a cooling process, which results in a decrease in hardness after
quenching has been performed. Therefore, the sol.Al content is set
to be 0.10% or less, or preferably 0.06% or less. Here, Al is
effective for deoxidation, and it is preferable that the sol.Al
content be 0.005% or more in order to sufficiently perform
deoxidation.
[0037] N: 0.0050% or less
[0038] In the case where the N content is more than 0.0050%, since
an excessive amount of BN is formed, there is a decrease in the
amount of a solid solution B. In addition, since BN and AlN are
formed in amounts larger than necessary, there is an excessive
decrease in austenite grain diameter in a heating process for a
quenching treatment, the formation of a ferrite phase is promoted
in a cooling process, which results in a decrease in hardness after
quenching has been performed. Therefore, the N content is set to be
0.0050% or less, or preferably 0.0045% or less. Here, although
there is no particular limitation on the lower limit of the N
content, N forms BN and AlN as described above. In the case where
appropriate amounts of BN and AlN are formed, since such nitrides
suitably inhibit an increase in austenite grain diameter in a
heating process for a quenching treatment, there is an increase in
toughness after quenching has been performed. Therefore, it is
preferable that the N content be 0.0005% or more.
[0039] B: 0.0005% or more and 0.0050% or less
[0040] B is an important chemical element which increases
hardenability. Under the condition regarding the cooling rate after
finish rolling has been performed in hot rolling according to the
present invention, in the case where the B content is less than
0.0005%, since there is an insufficient amount of a solid solution
B, which delays ferrite transformation, it is not possible to
realize sufficient effect of increasing hardenability. Therefore,
it is necessary that the B content be 0.0005% or more, or
preferably 0.0010% or more. On the other hand, in the case where
the B content is more than 0.0050%, the recrystallization of
austenite after finish rolling has been performed is delayed. As a
result, since the rolled texture of a hot-rolled steel sheet grows,
there is an increase in the in-plane anisotropy of the mechanical
properties of a steel sheet after annealing has been performed.
Therefore, since earing tends to occur and there is a decrease in
roundness when drawing is performed, problems tend to occur when
forming is performed. Therefore, it is necessary that the B content
be 0.0050% or less. It is preferable that the B content be 0.0035%
or less from the viewpoint of increasing hardenability and of
decreasing anisotropy. Therefore, the B content is set to be
0.0005% or more and 0.0050% or less, or preferably 0.0010% or more
and 0.0035% or less.
[0041] The proportion of the content of a solid solution B to the
content of B: 70% or more
[0042] In the present invention, in addition to the optimization of
the B content described above, the control of the amount of a solid
solution B, which contributes to an increase in hardenability, is
important. In the case where the proportion of the amount of B
present in a solid solution state to the amount of B contained in a
steel sheet is 70% or more, that is, in the case where the
proportion of the content of a solid solution B to the total
content of B (B content) in a steel sheet is 70% or more, it is
possible to achieve excellent hardenability targeted in the present
invention. Therefore, the proportion of the content of a solid
solution B to the content of B is set to be 70% or more, or
preferably 75% or more. Here, "the proportion of the content of a
solid solution B to the content of B" refers to {(content of a
solid solution B (mass %))/(total B content (mass %))}.times.100
(%).
[0043] One or more of Sb, Sn, Bi, Ge, Te, and Se: 0.002% or more
and 0.030% or less in total
[0044] Sb, Sn, Bi, Ge, Te, and Se are all chemical elements which
are effective for inhibiting nitrogen ingress through the surface
of a steel sheet, and it is necessary that one or more of Sb, Sn,
Bi, Ge, Te, and Se be added in the present invention. In addition,
in the case where the total content of these chemical elements is
less than 0.002%, sufficient effect of inhibiting nitrogen ingress
is not realized. Therefore, one or more of Sb, Sn, Bi, Ge, Te, and
Se is added in an amount of 0.002% or more in total, or preferably
0.005% or more in total. On the other hand, in the case where the
total content of these chemical elements is more than 0.030%, the
effect of inhibiting nitrogen ingress becomes saturated. In
addition, since these chemical elements tend to be segregated at
grain boundaries, grain boundary embrittlement may occur in the
case where the total content of these chemical elements is more
than 0.030%. Therefore, in the present invention, one or more of
Sb, Sn, Bi, Ge, Te, and Se is added in an amount of 0.030% or less
in total, or preferably 0.020% or less.
[0045] As described above, by controlling the N content to be
0.0050% or less, and by adding one or more of Sb, Sn, Bi, Ge, Te,
and Se in an amount of 0.002% or more and 0.030% or less in total,
since it is possible to inhibit an increase in nitrogen
concentration in the surface layer of a steel sheet by inhibiting
nitrogen ingress through the surface of the steel sheet even in the
case where annealing is performed in a nitrogen atmosphere, it is
possible to control the difference between an average nitrogen
concentration in a region from the surface to a depth of 150 .mu.m
in the thickness direction of the steel sheet and an average
nitrogen concentration in the whole steel sheet to be 30 mass ppm
or less. In addition, since it is possible to inhibit nitrogen
ingress as described above, it is possible to control the
proportion of the content of a solid solution B to the content of B
to be 70% or more in a steel sheet after annealing has been
performed even if annealing is performed in a nitrogen
atmosphere.
[0046] In the case where the difference between an average nitrogen
concentration in a region from the surface to a depth of 150 .mu.m
in the thickness direction of the steel sheet and an average
nitrogen concentration in the whole steel sheet is more than 30
mass ppm, there is an increase in the difference between the
amounts of BN and AIN formed in the surface layer, of the steel
sheet and the amounts of BN and AlN formed in the vicinity of the
central portion in the thickness direction of the steel sheet. In
this case, there is a problem such as one in that it is not
possible to achieve uniform hardness distribution after a quenching
treatment has been performed. Therefore, it is necessary to
suppress the difference between an average nitrogen concentration
in a region from the surface to a depth of 150 .mu.m the thickness
direction of the steel sheet and an average nitrogen concentration
in the whole steel sheet to be 30 mass ppm or less.
[0047] Although remainder other than those above is Fe and
inevitable impurities, one or more of Ni, Cr, and Mo may be added
in order to further increase hardenability. In order to realize
such an effect, it is preferable that one or more of Ni, Cr, and Mo
be added and that the total content of these chemical elements be
0.01% or more. On the other hand, since these chemical elements are
expensive, in the case where one or more of Ni, Cr, and Mo are
added, it is necessary that the total content of these chemical
elements be 0.50% or less, or preferably 0.20% or less.
[0048] 2) Microstructure
[0049] In the present invention, in order to increase cold
workability, it is necessary that a microstructure including
ferrite and cementite be formed by performing annealing
(spheroidizing annealing), in which spheroidal cementite is formed,
after hot rolling has been performed. Here, "spheroidal" refers to
a case where the proportion of the amount of cementite having an
aspect ratio (the length of major axis/the length of minor axis) of
3 or less to the total amount of cementite is 90% or more in terms
of volume fraction. In particular, in order to achieve a Rockwell
hardness of 73 or less in terms of HRB and a total elongation of
39% or more, it is necessary that the density of cementite in
ferrite grains be 0.08 pieces/.mu.m.sup.2 or less. Hereinafter,
"the density of cementite" is also referred to as "the number
density of cementite grains".
[0050] Number density of cementite grains in ferrite grains: 0.08
pieces/.mu.m.sup.2 or less
[0051] The steel sheet according to the present invention has a
microstructure including ferrite and cementite. In the case where
the number density of cementite grains in ferrite grains is high,
there is an increase in hardness due to dispersion strengthening
and there is a decrease in elongation. In order to control hardness
to be equal to or less than the specified value and in order to
control elongation to be equal to or more than the specified value,
it is necessary that the number density of cementite grains in
ferrite grains be 0.08 pieces/.mu.m.sup.2 or less, preferably 0.07
pieces/.mu.m.sup.2 or less, or more preferably 0.06
pieces/.mu.m.sup.2 or less. Since the length of the major axis of
cementite grains in ferrite grains is about 0.15 .mu.m to 1.8
.mu.m, the sizes of cementite grains slightly contributes to
precipitation strengthening of a steel sheet. Therefore, it is
possible to decrease strength by decreasing the number density of
cementite grains in ferrite grains. Since cementite grains existing
at ferrite grain boundaries scarcely contribute to dispersion
strengthening, the number density of cementite grains in ferrite
grains is set to be 0.08 pieces/.mu.m.sup.2 or less. Here, it is
acceptable that remaining microstructures such as pearlite other
than ferrite and cementite described above be inevitably formed in
the case where the total volume fraction of the remaining
microstructures be about 5% or less, because the effects of the
present invention are not decreased.
[0052] Average grain diameter of all the cementite: 0.60 .mu.m or
more and 1.00 .mu.m or less and average grain diameter of cementite
in ferrite grains: 0.40 .mu.m or more
[0053] In the case of a steel sheet in which the average grain
diameter of cementite in ferrite grains is less than 0.40 .mu.m,
since there is an increase in the number density of cementite
grains in ferrite grains, there is a case where there is an
increase in the hardness of the steel sheet after annealing has
been performed. In order to control hardness to be equal to or less
than the desired value, it is preferable that the average grain
diameter of cementite in ferrite grains be 0.40 .mu.m or more, or
more preferably 0.45 .mu.m or more.
[0054] Since the grain diameter of cementite at ferrite grain
boundaries is more likely to increase than that of cementite in
ferrite grains, it is necessary that the average grain diameter of
all the cementite be 0.60 .mu.m or more, or preferably 0.65 .mu.m
or more, in order to control the average grain diameter of
cementite in ferrite grains to be 0.40 .mu.m or more. On the other
hand, in the case where the average grain diameter of all the
cementite is more than 1.00 .mu.m, since cementite is not
completely dissolved in a short-time heating such as heating for an
induction hardening treatment, there is a case where it is not
possible to control hardness to be equal to or less than the
desired value. Therefore, it is preferable that the average grain
diameter of all the cementite be 1.00 .mu.m or less, or more
preferably 0.95 .mu.m or less. Regarding the average grain diameter
of cementite described above, it is possible to determine the
average grain diameter of all the cementite and the average grain
diameter of cementite in ferrite grains by observing the
microstructure by using a SEM and by determining the lengths of the
major axis and minor axis of cementite grains.
[0055] Here, in the case where the grain diameter of ferrite is
excessively large, although there is a decrease in hardness, since
there is a case where the effect of increasing elongation becomes
saturated, it is preferable that the average grain diameter of
ferrite be 12 .mu.m or less, or more preferably 9 .mu.m or less, in
the microstructure including ferrite and cementite described above.
On the other hand, in the case where the average grain diameter of
ferrite is less than 6 .mu.m, there is a case where there is an
increase in the hardness of a steel sheet. Therefore, it is
preferable that the average grain diameter of ferrite be 6 .mu.m or
more. It is possible to determine the grain diameter of ferrite
described above by observing the microstructure by using a SEM.
[0056] 3) Mechanical Properties
[0057] In the case of the steel sheet according to the present
invention, since automotive parts such as gears, transmission
parts, and seat recliner parts are formed by performing cold press
forming, excellent workability is required. In addition, it is
necessary to provide abrasion resistance to the parts by increasing
hardness by performing a quenching treatment. Therefore, in
addition to increasing hardenability, it is necessary that the
hardness of a steel sheet is decreased to 73 or less in terms of
HRB and total elongation (El) of a steel sheet is increased to 39%
or more. Although it is preferable that the hardness of a steel
sheet be as low as possible from the viewpoint of workability,
since some parts are partially subjected quenching, the strength of
a raw material steel sheet may influence fatigue characteristics.
Therefore, it is preferable that the hardness of a steel sheet be
more than 60 in terms of HRB. Here, it is possible to determine
hardness in terms HRB described above by using a Rockwell hardness
meter (B scale). In addition, it is possible to determine total
elongation by performing a tensile test at a tensile speed of 10
mm/min on a JIS No. 5 tensile test piece which has been taken in a
direction (L-direction) at an angle of 0.degree. to the rolling
direction by using tensile test machine AG-10TB AG/XR produced by
SHIMADZU CORPORATION and by butting the pieces of a broken
sample.
[0058] 4) Manufacturing Condition
[0059] The high-carbon hot-rolled steel sheet according to the
present invention is manufactured by using raw material steel
having the chemical composition described above, by performing hot
rolling including performing hot rough rolling and then performing
hot finish rolling with a finishing delivery temperature equal to
or higher than the Ar.sub.3 transformation temperature and
870.degree. C. or lower in order to obtain a desired thickness, by
then cooling the hot-rolled steel sheet to a temperature of
700.degree. C. at an average cooling rate of 25.degree. C./s or
more and 150.degree. C./s or less, by then coiling the cooled steel
sheet at a coiling temperature of 500.degree. C. or higher and
700.degree. C. or lower in order to obtain a steel sheet having a
microstructure including pearlite and, in terms of volume fraction,
5% or more of pro-eutectoid ferrite, and by then performing
spheroidizing annealing on the steel sheet at a temperature equal
to or lower than the Ac.sub.1 transformation temperature. Here, it
is preferable that the rolling reduction of finish rolling be 85%
or more.
[0060] Hereafter, the reasons for limitations on the method for
manufacturing a high-carbon hot-rolled steel sheet according to the
present invention will be described.
[0061] Finishing delivery temperature: equal to or higher than the
Ar.sub.3 transformation temperature and 870.degree. C. or lower
[0062] In order to control the number density of cementite grains
in ferrite grains to be 0.08 pieces/.mu.m.sup.2 or less after
annealing has been performed, it is necessary to perform
spheroidizing annealing on a hot-rolled steel sheet having a
microstructure including pearlite and, in terms of volume fraction,
5% or more of pro-eutectoid ferrite. In the case where the
finishing delivery temperature is higher than 870.degree. C. in hot
rolling in which finish rolling is performed after hot rough
rolling has been performed, since there is a decrease in the
proportion of pro-eutectoid ferrite, it is not possible to achieve
the specified number density of cementite grains after
spheroidizing annealing has been performed. And there is a tendency
for cementite grain diameter and ferrite grain diameter to increase
after spheroidizing annealing has been performed. Therefore, the
finishing delivery temperature is set to be 870.degree. C. or
lower. In order to sufficiently increase the proportion of
pro-eutectoid ferrite, it is preferable that the finishing delivery
temperature be 850.degree. C. or lower. On the other hand, in the
case where the finishing delivery temperature is lower than the
Ar.sub.1 transformation temperature, since ferrite grains having a
large grain diameter are formed after hot rolling or annealing has
been performed, there is a significant decrease in elongation.
Therefore, the finishing delivery temperature is set to be equal to
or higher than the Ar.sub.3 transformation temperature, or
preferably 820.degree. C. or higher. Here, "finishing delivery
temperature" refers to the surface temperature of a steel
sheet.
[0063] Average cooling rate from finishing delivery temperature to
700.degree. C.: 25.degree. C./s or more and 150.degree. C./s or
less
[0064] In order to control the number density of cementite grains
in ferrite grains to be 0.08 pieces/.mu.m.sup.2 or less after
annealing has been performed, it is necessary to perform
spheroidizing annealing on a hot-rolled steel sheet having a
microstructure including pearlite and, in terms of volume fraction,
5% or more of pro-eutectoid ferrite. Since a temperature range down
to a temperature of 700.degree. C. after finish rolling included in
hot rolling has been performed is a temperature range in which
ferrite transformation start temperature and pearlite
transformation start temperature exist, the cooling rate from the
finishing delivery temperature to 700.degree. C. is an important
factor in order to control a pro-eutectoid ferrite phase fraction
in a steel sheet after hot rolling has been performed to be 5% or
more in terms of volume fraction. In the case where the average
cooling rate in a temperature range from the finishing delivery
temperature to 700.degree. C. is less than 25.degree. C./s, since
ferrite transformation is less likely to progress in a short time,
which results in an increase in pearlite phase fraction more than
necessary, it is not possible to form, in terms of volume fraction,
5% or more of pro-eutectoid ferrite. In addition, since pearlite
having a large grain diameter is formed, it is difficult to form
the desired steel sheet microstructure after spheroidizing
annealing has been performed. Therefore, the average cooling rate
in a temperature range down to a temperature of 700.degree. C.
after finish rolling has been performed is set to be 25.degree.
C./s or more. In addition, since it is preferable that the
pro-eutectoid ferrite phase fraction be 10% or more in terms of
volume fraction in order to control the number density of cementite
grains in ferrite grains to be 0.07 pieces/.mu.m.sup.2 or less
after spheroidizing annealing has been performed, it is preferable
that the average cooling rate be 30.degree. C./s or more, or more
preferably 40.degree. C./s or more, in this case. On the other
hand, in the case where the average cooling rate is more than
150.degree. C./s, it is difficult to form pro-eutectoid ferrite.
Therefore, the average cooling rate down to a temperature of
700.degree. C. after finish rolling has been performed is set to be
150.degree. C./s or less, preferably 120.degree. C./s or less, or
more preferably 100.degree. C./s or less. Here, this "temperature"
refers to the surface temperature of a steel sheet.
[0065] Coiling temperature: 500.degree. C. or higher and
700.degree. C. or lower
[0066] The steel sheet which has been subjected to finish rolling
is wound in a coil shape at a coiling temperature of 500.degree. C.
or higher and 700.degree. C. or lower after cooling has been
performed as described above. It is not preferable that the coiling
temperature be higher than 700.degree. C., because it is not
possible to form the desired steel sheet microstructure after
annealing has been performed due to an increase in the grain
diameter of the microstructure of a hot-rolled steel sheet, and
because, from the viewpoint of operational efficiency, there is a
case where coil deforms under its own weight due to an excessive
decrease in the strength of a steel sheet when the steel sheet is
wound in a coil shape. Therefore, the coiling temperature is set to
be 700.degree. C. or lower, or preferably 650.degree. C. or lower.
On the other hand, in the case where the coiling temperature is
lower than 500.degree. C., since there is an increase in the
hardness of a steel sheet due to a decrease in the grain diameter
of the steel sheet microstructure, there is a decrease in
workability due to a decrease in elongation. Therefore, the coiling
temperature is set to be 500.degree. C. or higher, or preferably
550.degree. C. or higher. Here, "coiling temperature" refers to the
surface temperature of a steel sheet.
[0067] Steel sheet microstructure after hot rolling has been
performed: including pearlite and, in terms of volume fraction, 5%
or more of pro-eutectoid ferrite
[0068] In the present invention, after spheroidizing annealing has
been performed as described below, a steel sheet having a
microstructure which includes ferrite and cementite and in which
the number density of cementite grains in the ferrite grains is
0.08 pieces/.mu.m.sup.2 or less is obtained. The microstructure
after spheroidizing annealing has been performed is strongly
influenced by the steel sheet microstructure after hot rolling has
been performed. By forming a steel sheet microstructure including
pearlite and, in terms of volume fraction, 5% or more of
pro-eutectoid ferrite after hot rolling has been performed, since
it is possible to form the desired microstructure after
spheroidizing annealing has been performed, it is possible to
obtain steel having high workability. In addition, in the case of a
steel sheet having a microstructure which does not include pearlite
or in which a pro-eutectoid ferrite phase fraction is less than 5%
in terms of volume fraction, since it is not possible to achieve
the specified number density of cementite grains after
spheroidizing annealing has been performed at a temperature equal
to or lower than the Ac.sub.1 transformation temperature, there is
an increase in the strength of a steel sheet. Therefore, the
microstructure of a steel sheet (hot-rolled steel sheet) obtained
by performing hot rolling, cooling, and coiling under the
conditions described above is a microstructure including pearlite
and, in terms of volume fraction, 5% or more of pro-eutectoid
ferrite, or preferably, pearlite and, in terms of volume fraction,
10% or more of pro-eutectoid ferrite. Here, in order to achieve a
higher level of uniformity in a microstructure after annealing has
been performed, it is preferable that the pro-eutectoid ferrite
phase fraction be 50% or less in terms of volume fraction.
[0069] Annealing temperature: equal to or lower than the Ac.sub.1
transformation temperature
[0070] The hot-rolled steel sheet obtained as described above is
subjected to annealing (spheroidizing annealing). In the case where
the annealing temperature is higher than the Ac.sub.1
transformation temperature, since austenite is formed, a pearlite
structure having a large grain diameter is formed in a cooling
process following the annealing process, which results in a
non-uniform microstructure being formed. Therefore, the annealing
temperature is set to be equal to or lower than the Ac.sub.1
transformation temperature. Here, although there is no particular
limitation on the lower limit of the annealing temperature, it is
preferable that the annealing temperature be 600.degree. C. or
higher, or more preferably 700.degree. C. or higher, in order to
control the number density of cementite grains in ferrite grains to
be the desired value. Here, as an atmospheric gas, any of nitrogen,
hydrogen, and a mixed gas of nitrogen and hydrogen may be used,
and, although, it is preferable to use such gases, Ar may also be
used without any particular limitation. In addition, it is
preferable the annealing time be 0.5 hours or more and 40 hours or
less. By controlling the annealing time to be 0.5 hours or more,
since it is possible to stably form the desired microstructure, it
is possible to control the hardness of a steel sheet to be equal to
or lower than the desired value, and it is possible to control
elongation to be equal to or more than the desired value.
Therefore, it is preferable the annealing time be 0.5 hours or
more, or more preferably 8 hours or more. In addition, in the case
where the annealing time is more than 40 hours, there is a decrease
in productivity, and there is tendency for manufacturing costs to
excessively increase. Therefore, it is preferable that the
annealing time be 40 hours or less. Here, "annealing temperature"
refers to the surface temperature of a steel sheet. In addition,
"annealing time" refers to a period of time during which the
specified temperature is maintained.
[0071] Here, in order to prepare the molten material of the
high-carbon steel according to the present invention, any of a
converter and an electric furnace may be used. In addition, the
molten material of the high-carbon steel prepared as described
above is made into a slab by using an ingot casting-slabbing method
or a continuous casting method. The slab is usually heated and then
subjected to hot rolling. Here, in the case of a slab manufactured
by using a continuous casting method, hot direct rolling, which is
performed on the slab in the cast state or after heat retention has
been performed in order to inhibit a fall in temperature, may be
performed. In addition, in the case where slab is subjected to hot
rolling after heating has been performed, it is preferable that the
slab heating temperature be 1280.degree. C. or lower in order to
inhibit a deterioration in surface quality due to scale. In hot
rolling, in order to perform finish rolling at a specified
temperature, the material to be rolled may be heated by using a
heating means such as a sheet bar heater in a hot rolling
process.
EXAMPLE 1
[0072] By preparing molten steels having the chemical compositions
corresponding to steel codes A through H given in Table 1, and by
then performing finish rolling, cooling, and coiling under the hot
rolling conditions given in Table 2, hot rolled steel sheets were
obtained. Here, the cooling rates given in Table 2 were the average
cooling rates down to a temperature of 700.degree. C. after finish
rolling has been performed. Subsequently, by performing pickling,
and by performing annealing (spheroidizing annealing) in a nitrogen
atmosphere (atmospheric gas: nitrogen) under the annealing
conditions given in Table 2, hot-rolled steel sheets (hot-rolled
and annealed steel sheets) having a thickness of 4.0 mm and a width
of 1000 mm were manufactured. The hardness, elongation, and
microstructure of the hot-rolled and annealed steel sheets
manufactured as described above were investigated. In addition, the
microstructures of the hot-rolled steel sheets before annealing was
performed were also investigated. The results are given in Table 2.
Here, the Ar.sub.3 transformation temperatures and the Ac.sub.1
transformation temperatures given in Table 1 were derived by using
a formaster.
[0073] Hardness (HRB) of Hot-Rolled and Annealed Steel Sheet
[0074] By taking a sample from the central portion in the width
direction of the annealed steel sheet, and by determining hardness
at five points by using a Rockwell hardness meter (B scale), an
average value was derived.
[0075] Total elongation (El) of Hot-Rolled and Annealed Steel
Sheet
[0076] By performing a tensile test at a tensile speed of 10 mm/min
on a JIS No. 5 tensile test piece which had been taken from the
annealed steel sheet in a direction (L-direction) at an angle of
0.degree. to the rolling direction by using tensile test machine
AG-10TB AG/XR produced by SHIMADZU CORPORATION, and by butting the
pieces of a broken sample, elongation (total elongation) was
derived.
[0077] Microstructure
[0078] By observing the microstructure of the hot-rolled steel
sheet before annealing was performed (the microstructure of the
hot-rolled steel sheet) by using a SEM, the kinds of the
microstructures were identified, and a pro-eutectoid ferrite phase
fraction was derived. By distinguishing the area of ferrite from
the area of other phases, and by deriving the proportion of the
area of ferrite in order to deriving an area fraction, the volume
fraction of pro-eutectoid ferrite was determined as the obtained
area fraction thereof. Here, it was confirmed that pearlite existed
in the hot-rolled steel sheet before annealing was performed given
in Table 2 in the SEM observation described above.
[0079] The microstructure of the hot-rolled steel sheet after
annealing had been performed (the microstructure of the hot-rolled
and annealed steel sheet) was observed by using microstructure
photographs which were captured by using a scanning electron
microscope at a magnification of 3000 times at five positions
located at a depth of 1/4 in the thickness direction of a sample
which had been prepared by taking the sample from the central
portion in the width direction of the steel sheet, by performing
cutting and polishing, and by performing nital etching. By
identifying the kinds of the microstructures of the sample, by
counting the number of cementite grains which did not exist at
grain boundaries and which had a major axis of 0.15 .mu.m or more,
and by dividing the number by the area of the fields of view of the
photographs, the density of cementite in ferrite grains (the number
density of cementite grains in ferrite grains) was derived. By
determining the lengths of the major axis and minor axis of each of
the cementite grains by using the microstructure photographs
described above, the average grain diameter of all the cementite
and the average grain diameter of cementite in grains were derived.
Ferrite grain diameter was derived by determining grain size by
using the microstructure photograph described above, and then
average ferrite grain diameter was calculated.
[0080] In addition, with respect to the steel sheet after annealing
had been performed (hot-rolled and annealed steel sheet), the
difference between N content in a region from the surface to a
depth of 150 .mu.l of the surface layer and the average N content
of the steel sheet and the proportion of the content of a solid
solution B to the content of B were derived by using the following
methods. The results are given in Table 2.
[0081] Difference Between Average N Content within 150 .mu.m of the
Surface Layer and the Average N Content of the Steel Sheet
[0082] With respect to a sample taken from the central portion in
the width direction of the steel sheet after annealing had been
performed, average N content within 150 .mu.m of the surface layer
and the average N content of the steel sheet were determined, and
then the difference between the average N content within 150 .mu.m
of the surface layer and the average N content of the steel sheet
was derived. Here, "average N content within 150 .mu.m of the
surface layer" refers to average N content in a region from the
surface of the steel sheet to a depth of 150 .mu.m in the thickness
direction. In addition, the average N content within 150 .mu.m of
the surface layer was derived by using the following method. That
is, by starting machining from the surface of a taken sample steel
sheet, and by machining the steel sheet to a depth of 150 .mu.m
from the surface thereof, the produced cutting chips were collected
as samples. The average N content within 150 .mu.m of the surface
layer was defined as the N content of the samples. The average N
content within 150 .mu.m of the surface layer and the average N
content of the steel sheet were determined by using an inert gas
fusion-thermal conductivity method. A case where the difference
between the average N content within 150 .mu.m of the surface layer
(N content in a region from the surface to a depth of 150 .mu.m
from the surface) and the average N content of the steel sheet (N
content in the steel) determined as described above was 30 mass ppm
or less may be judged as a case where nitrogen ingress was
inhibited.
[0083] Proportion of the Content of a Solid Solution B to the
Content of B
[0084] A sample was taken from the central portion in the width
direction of the steel sheet after annealing had been performed. By
extracting BN in steel by using 10 vol. %-Br-methanol, by
subtracting the content of B which was precipitated in the form of
BN from the total content of B in steel, the amount of a solid
solution B was derived. The proportion of the content of a solid
solution B to the total content of B (B content) in steel was
calculated to be equal to {(content of a solid solution B (mass
%))/(total B content (mass %))}.times.100 (%). A case where this
proportion was 70 (%) or more may be judged as a case where a
decrease in the content of a solid solution B was inhibited.
[0085] Hardness (Quenched Hardness) of a Steel Sheet After
Quenching has Been Performed
[0086] In addition, by using the steel sheet after annealing had
been performed as a raw material steel sheet, by performing three
kinds of quenching treatments as described below, and by
investigating the hardness (quenched hardness) of the steel sheet
after quenching had been performed, hardenability was evaluated.
The results are given in Table 2.
[0087] By taking a flat-sheet-type test piece (having a width of 15
mm, a length of 40 mm, and a thickness of 4 mm) from the central
portion in the width direction of the steel sheet (raw material
steel sheet) after annealing had been performed, a quenching
treatment was performed on the flat-sheet-type test piece by using
a method in which cooling (water cooling) was performed with water
immediately after the test piece had been held at a temperature of
870.degree. C. for 30 seconds or a method in which cooling
(120.degree. C.-oil cooling) was performed with oil having a
temperature of 120.degree. C. immediately after the test piece had
been held at a temperature of 870.degree. C. for 30 seconds. By
measuring the hardness at five points in the cut surface of the
test piece which had been subjected to the quenching treatment by
using a Vickers hardness meter with a load of 1 kgf, and by
deriving an average hardness, quenched hardness was defined as the
average hardness.
[0088] In addition, by taking a disc-type test piece (having a
diameter of 55 mm.phi. and a thickness of 4 mm) from the central
portion in the width direction of the steel sheet (raw material
steel sheet) after annealing had been performed, a quenching
treatment was also performed by using an induction hardening method
(heating the test piece to a temperature of 1000.degree. C. at a
heating rate of 200.degree. C./s and then cooling the test piece
with water). At this time, by measuring the hardness at two points
in the cut surface of the test piece at the outermost periphery of
the test piece by using a Vickers hardness meter with a load of 0.2
kgf, and by deriving an average hardness, quenched hardness was
defined as the average hardness.
[0089] A case where all of the criteria for satisfactory quenched
hardness given in Table 3 in the case of water cooling after
holding at a temperature of 870.degree. C. for 30 seconds, in the
case of 120.degree. C.-oil cooling after holding at a temperature
of 870.degree. C. for 30 seconds, and in the case of induction
hardening were satisfied was judged as satisfactory (O), that is,
the case of excellent hardenability. A case where one of the
criteria for satisfactory quenched hardness given in Table 3 in the
case of water cooling after holding at a temperature of 870.degree.
C. for 30 seconds, in the case of 120.degree. C.-oil cooling after
holding at a temperature of 870.degree. C. for 30 seconds, and in
the case of water cooling in induction hardening was not satisfied
was judged as unsatisfactory (x), that is, the case of poor
hardenability. Here, Table 3 indicates the empirical values of
quenched hardness corresponding to sufficient hardenability in
accordance with C content.
[0090] As Table 2 indicates, it is clarified that the hot-rolled
steel sheets of the examples of the present invention had a
microstructure which included ferrite and cementite and in which
the number density of cementite grains in the ferrite grains was
0.08 pieces/.mu.m.sup.2 or less, a hardness of 73 or less in terms
of HRB, and a total elongation of 39% or more, which means these
hot-rolled steel sheets were excellent in terms of cold workability
and hardenability.
TABLE-US-00001 TABLE 1 Ac.sub.1 Ar.sub.3 Transfor- Transfor- mation
mation Chemical Composition (mass %) Temper- Temper- Steel Sb, Sn,
Bi, ature ature Code C Si Mn P S sol. Al N B Ge, Te, Se Other
(.degree. C.) (.degree. C.) Note A 0.35 0.02 0.36 0.01 0.003 0.038
0.0035 0.0032 Sb: 0.009 -- 719 801 Within Scope of Invention B 0.36
0.01 0.34 0.01 0.003 0.033 0.0038 0.0028 Sb + Sn: 0.015 -- 719 797
Within Scope of Invention C 0.35 0.01 0.35 0.01 0.003 0.035 0.0039
0.0019 Sb: 0.010 -- 719 799 Within Scope of Invention D 0.20 0.01
0.39 0.01 0.003 0.033 0.0036 0.0035 Sb: 0.010 Cr: 0.21 719 827
Within Scope of Invention E 0.38 0.02 0.34 0.01 0.003 0.038 0.0038
0.0025 Sb + Ge + Mo: 0.02 719 796 Within Scope Te + Se: 0.010 of
Invention F 0.40 0.01 0.33 0.01 0.003 0.042 0.0040 0.0015 Sb + Bi:
0.015 Ni: 0.05 719 794 Within Scope of Invention G 0.28 0.01 0.40
0.01 0.003 0.037 0.0035 0.0038 Sb: 0.009 Cr: 0.22 719 811 Within
Scope of Invention H 0.35 0.01 0.36 0.01 0.003 0.038 0.0038 0.0028
Sb + Sn + -- 719 800 Comparative Bi + Ge + Example Te + Se:
0.001
TABLE-US-00002 TABLE 2 Micro- Microstructure of Hot-rolled
structure of and Annealed Steel Sheet Hot-rolled Spheroidizing
Average Hot Rolling Condition Steel Sheet Annealing Cementite
Finishing Volume Condition Cementite Average Grain Average Delivery
Coiling Fraction Annealing Density Cementite Diameter Ferrite Sam-
Temper- Cooling Temper- of Pro- Temper- in Ferrite Grain in Ferrite
Grain ple Steel ature Rate ature eutectoid ature Annealing Grain
Diameter Grain Diameter No. Code (.degree. C.) (.degree. C./s)
(.degree. C.) Ferrite (%) (.degree. C.) Time (h) Phase
(piece/.mu.m.sup.2) (.mu.m) (.mu.m) (.mu.m) 1 A 860 80 620 31 715
30 Ferrite + 0.05 0.80 0.58 7 Cementite 2 A 870 40 610 20 715 30
Ferrite + 0.07 0.77 0.55 8 Cementite 3 A 860 29 620 9 715 30
Ferrite + 0.08 0.80 0.60 9 Cementite 4 A 870 10 620 3 715 30
Ferrite + 0.10 0.58 0.37 10 Cementite 5 A 870 160 620 3 715 30
Ferrite + 0.10 0.57 0.35 6 Cementite 6 A 870 50 750 15 720 50
Ferrite + 0.03 1.40 0.70 14 Cementite 7 B 850 50 580 15 715 30
Ferrite + 0.07 0.78 0.50 9 Cementite 8 B 830 90 630 25 710 25
Ferrite + 0.08 0.73 0.53 9 Cementite 9 B 950 50 580 4 715 30
Ferrite + 0.14 0.50 0.30 10 Cementite 10 B 830 125 630 8 710 25
Ferrite + 0.08 0.65 0.48 9 Cementite 11 C 850 80 590 27 715 30
Ferrite + 0.06 0.78 0.47 8 Cementite 12 C 850 32 590 13 715 30
Ferrite + 0.07 0.77 0.50 9 Cementite 13 D 870 75 600 28 715 30
Ferrite + 0.07 0.74 0.55 12 Cementite 14 E 830 50 650 20 715 30
Ferrite + 0.06 0.72 0.51 9 Cementite 15 F 820 80 630 23 715 30
Ferrite + 0.06 0.94 0.80 8 Cementite 16 G 870 80 610 29 715 30
Ferrite + 0.07 0.95 0.81 11 Cementite 17 H 860 50 610 20 715 30
Ferrite + 0.07 0.75 0.55 10 Cementite Difference In Average N
Concentration Property of between Hot-rolled Region (Content and
Annealed within 150 .mu.m of Solid Hardness of Quenched Steel Sheet
of Surface Content Solution B)/ Steel Sheet (HV)* Sam- Hard- Elon-
Layer and of Solid (Total B 120.degree. C.- Evaluation Evaluation
ple ness gation Steel Sheet Solution B Content) .times. Water Oil
Induction of Cold of Harden- No. (HRB) (%) (mass ppm) (mass %) 100
(%) Cooling Cooling hardening Workability ability* Note 1 63 43 20
0.0026 81 610 552 605 .smallcircle. .smallcircle. Example 2 67 41
20 0.0025 78 603 548 597 .smallcircle. .smallcircle. Example 3 73
39 20 0.0026 81 610 552 600 .smallcircle. .smallcircle. Example 4
75 38 20 0.0025 78 604 547 598 x .smallcircle. Comparative Example
5 75 38 20 0.0025 78 615 555 610 x .smallcircle. Comparative
Example 6 60 45 20 0.0021 66 610 540 550 .smallcircle. x
Comparative Example 7 69 40 20 0.0023 82 615 560 607 .smallcircle.
.smallcircle. Example 8 72 40 20 0.0024 86 618 563 610
.smallcircle. .smallcircle. Example 9 78 36 20 0.0024 86 605 540
603 x .smallcircle. Comparative Example 10 73 39 20 0.0024 86 619
562 614 .smallcircle. .smallcircle. Example 11 64 41 20 0.0016 84
605 558 600 .smallcircle. .smallcircle. Example 12 72 40 20 0.0016
84 604 556 599 .smallcircle. .smallcircle. Example 13 60 45 20
0.0031 89 452 370 447 .smallcircle. .smallcircle. Example 14 69 40
20 0.0019 76 620 560 610 .smallcircle. .smallcircle. Example 15 71
39 20 0.0011 73 624 565 618 .smallcircle. .smallcircle. Example 16
66 42 20 0.0031 82 560 470 550 .smallcircle. .smallcircle. Example
17 68 40 190 0.0004 14 605 410 570 .smallcircle. x Comparative
Example *refer to Table 3 for information about quenching condition
and the evaluation criteria for satisfactory hardenability
TABLE-US-00003 TABLE 3 Hardness Hardness Hardness of Sample of
Sample of Sample Water-cooled 120.degree. C.-oil-cooled
Water-cooled in after Holding after Holding Induction C content at
870.degree. C. at 870.degree. C. Hardening (mass %) for 30 s (HV)
for 30 s (HV) (HV) 0.20 or more and .gtoreq.440 .gtoreq.360
.gtoreq.435 less than 0.35 0.35 or more and .gtoreq.600 .gtoreq.530
.gtoreq.595 less than 0.38 0.38 or more and .gtoreq.610 .gtoreq.540
.gtoreq.605 less than 0.40 0.40 .gtoreq.620 .gtoreq.550
.gtoreq.615
* * * * *