U.S. patent application number 11/922250 was filed with the patent office on 2009-05-21 for high carbon hot rolled steel sheet and method for manufacturing same.
Invention is credited to Takeshi Fujita, Norio Kanamoto, Nobusuke Kariya, Yoshiharu Kusumoto, Hidekazu Ookubo.
Application Number | 20090126836 11/922250 |
Document ID | / |
Family ID | 37595206 |
Filed Date | 2009-05-21 |
United States Patent
Application |
20090126836 |
Kind Code |
A1 |
Kariya; Nobusuke ; et
al. |
May 21, 2009 |
High Carbon Hot Rolled Steel Sheet and method for manufacturing
same
Abstract
A high carbon hot-rolled steel sheet, as a hot-rolled
spheroidizing annealed material, having both excellent
stretch-flange formability and excellent homogeneity of hardness in
the sheet thickness direction is provided by a manufacturing method
having the steps of: hot-rolling a steel containing 0.2 to 0.7% C
by mass at finishing temperatures of (A.sub.r3 transformation
point-20.degree. C.) or above to prepare a hot-rolled sheet;
cooling the hot-rolled sheet to temperatures of 650.degree. C. or
below at cooling rates from 60.degree. C./s or larger to smaller
than 120.degree. C./s; coiling the hot-rolled sheet after cooling
at coiling temperatures of 600.degree. C. or below; and annealing
the coiled hot-rolled sheet at annealing temperatures from
640.degree. C. or larger to A.sub.c1 transformation point or
lower.
Inventors: |
Kariya; Nobusuke; (Kanagawa,
JP) ; Kanamoto; Norio; (Chiba, JP) ; Ookubo;
Hidekazu; (Chiba, JP) ; Kusumoto; Yoshiharu;
(Chiba, JP) ; Fujita; Takeshi; (Hiroshima,
JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 Fifth Avenue, 16TH Floor
NEW YORK
NY
10001-7708
US
|
Family ID: |
37595206 |
Appl. No.: |
11/922250 |
Filed: |
June 19, 2006 |
PCT Filed: |
June 19, 2006 |
PCT NO: |
PCT/JP2006/312670 |
371 Date: |
October 29, 2008 |
Current U.S.
Class: |
148/602 ;
148/330; 148/333; 148/336; 148/337 |
Current CPC
Class: |
C21D 9/46 20130101; C21D
8/0226 20130101; C22C 38/02 20130101; C22C 38/18 20130101; C21D
8/0205 20130101; C22C 38/04 20130101; C22C 38/40 20130101; C22C
38/44 20130101; C22C 38/48 20130101; C21D 6/004 20130101; C22C
38/58 20130101 |
Class at
Publication: |
148/602 ;
148/337; 148/330; 148/333; 148/336 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/00 20060101 C22C038/00; C22C 38/18 20060101
C22C038/18; C22C 38/08 20060101 C22C038/08 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 2005 |
JP |
2005-189578 |
Claims
1. A method for manufacturing high carbon hot-rolled steel sheet,
comprising the steps of: hot-rolling a steel containing 0.2 to 0.7%
C by mass at a finishing temperature of (A.sub.r3 transformation
point-20.degree. C.) or above to prepare a hot-rolled sheet;
cooling the hot-rolled sheet to a temperature of 650.degree. C. or
below at a cooling rate ranging from 60.degree. C./s or larger to
smaller than 120.degree. C./s; coiling the hot-rolled sheet after
cooling at a coiling temperature of 600.degree. C. or below; and
annealing the coiled hot-rolled sheet at an annealing temperature
ranging from 640.degree. C. or larger to A.sub.c1 transformation
point or lower.
2. The method for manufacturing high carbon hot-rolled steel sheet
according to claim 1, wherein the step of cooling conducts cooling
the hot-rolled sheet to a temperature of 600.degree. C. or below at
a cooling rate ranging from 80.degree. C./s or larger to smaller
than 120.degree. C./s, and the step of coiling conducts coiling the
sheet at a temperature of 550.degree. C. or below.
3. A high carbon hot-rolled steel sheet which is a hot-rolled
spheroidizing annealed material, comprising 0.2 to 0.7% C, 2% or
less Si, 2% or less Mn, 0.03% or less P, 0.03% or less S, 0.08% or
less Sol.Al., and 0.01% or less N, by mass, the content of carbide
having smaller than 0.5 .mu.m of particle size being 15% or less by
volume to the total amount of carbide, and the difference between
the maximum hardness H.sub.V max and the minimum hardness H.sub.V
min, .DELTA.Hv (=H.sub.V max-H.sub.V min), in the sheet thickness
direction being 10 or smaller.
4. The high carbon hot-rolled steel sheet according to claim 3,
wherein the content of carbide having smaller than 0.5 m of
particle size is 10% or less by volume to the total amount of
carbide, and the difference between the maximum hardness Hv max and
the minimum hardness Hv min, .DELTA.Hv (=H.sub.V max-H.sub.V min),
in the sheet thickness direction is 8 or smaller.
5. The high carbon hot-rolled steel sheet according to claim 3,
further comprising at least one element of: about 0.005% or less B,
about 3.5% or less Cr, about 3.5% or less Ni, about 0.7% or less
Mo, about 0.1% or less Cu, about 0.1% or less Ti, about 0.1% or
less Nb, and about 0.1% or less of the total of W, V, and Zr, by
mass.
6. The high carbon hot-rolled steel sheet according to claim 4,
further comprising at least one element of: about 0.005% or less B,
about 3.5% or less Cr, about 3.5% or less Ni, about 0.7% or less
Mo, about 0.1% or less Cu, about 0.1% or less Ti, about 0.1% or
less Nb, and about 0.1% or less of the total of W, V, and Zr, by
mass.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high carbon hot-rolled
steel sheet having excellent workability and a method for
manufacturing thereof.
BACKGROUND ART
[0002] Users of high carbon steel sheets as tools, automotive parts
(gear and transmission), and the like request excellent workability
because these steel sheets are formed in various complex shapes. In
recent years, on the other hand, requirement of reduction in the
cost for manufacturing parts increases. Responding to the
requirement, some working processes are eliminated and working
methods are changed. For example, as the forming technology of
automobile driving system parts using high carbon steel sheets,
there was developed a double-acting forming technique which allows
applying thickness-additive forming process and realizes
significant shortening of manufacturing process, and the technique
has been brought into practical applications in a part of
industries, (for example, refer to Journal of the JSTP, 44, pp.
409-413, (2003)).
[0003] Along with that movement, the high carbon steel sheets face
ever-increasing request of workability to attain higher ductility
than ever. Since some of the parts are often subjected to
hole-expansion (burring) treatment after punching, they are wanted
to have excellent stretch-flange formability.
[0004] Furthermore, from the viewpoint of cost reduction
accompanied with increase in the product yield, these steel sheets
are strongly requested to have homogeneous mechanical properties.
In particular, the homogeneity of hardness in the sheet thickness
direction is keenly desired because large differences of hardness
in the steel sheet thickness direction between the surface portion
and the central portion significantly deteriorate the punching tool
during punching.
[0005] To answer these requests, several technologies were studied
to improve the workability and homogeneous mechanical properties of
high carbon steel sheets.
[0006] For example, JP-A-3-174909, (the term "JP-A" referred to
herein signifies the "Unexamined Japanese Patent Publication"),
proposed a method for manufacturing stably a high carbon hot-rolled
steel strip having excellent homogeneous mechanical properties in
the longitudinal direction of coil by the steps of: [0007] dividing
a hot-run table (or run-out table) into an accelerated cooling zone
and an air-cooling zone; [0008] applying accelerated cooling to a
finish-rolled steel strip to a specific temperature or below
determined by the length of cooling zone, the transfer speed of
steel sheet, the chemical composition of the steel, and the like;
and then [0009] applying air-cooling to the steel strip. The
cooling rate in the accelerated cooling zone according to
JP-A-3-174909 is about 20 to about 30.degree. C./s suggested by
FIG. 3 in the disclosure.
[0010] As another example, JP-A-9-157758 proposed a method for
manufacturing high carbon workable steel strip having excellent
structural homogeneity and workability (ductility) by the steps of:
[0011] hot-rolling a high carbon steel having a specified chemical
composition, followed by descaling therefrom; [0012] annealing the
steel in a hydrogen atmosphere (95% or more of hydrogen by volume)
while specifying heating rate, soaking temperature (A.sub.c1
transformation point or above), and soaking time depending on the
chemical composition; and [0013] cooling the annealed steel at
cooling rates of 100.degree. C./hr or smaller.
[0014] As further example, JP-A-5-9588 proposed a method for
manufacturing high carbon steel thin sheet having good workability
by the steps of: [0015] rolling a steel at finishing temperatures
of (A.sub.c1 transformation point+30.degree. C.) or above to
prepare a steel sheet; [0016] cooling the steel sheet to
temperatures from 20.degree. C. to 500.degree. C. at cooling rates
from 10 to 100.degree. C./s; [0017] holding the steel sheet for 1
to 10 seconds; [0018] reheating the steel sheet to temperatures
from 500.degree. C. to (A.sub.c1 transformation point+30.degree.
C.), followed by coiling the steel sheet; and [0019] soaking the
steel sheet, at need, at temperatures from 650.degree. C. to
(A.sub.c1 transformation point+30.degree. C.) for 1 hour or
more.
[0020] As still another example, JP-A-2003-13145 proposed a method
for manufacturing high carbon steel sheet having excellent
stretch-flanging formability by the steps of: [0021] using a steel
containing 0.2 to 0.7% C by mass; [0022] hot-rolling the steel at
finishing temperatures of (A.sub.r3 transformation point-20.degree.
C.) or above; [0023] cooling the steel sheet at cooling rates of
higher than 120.degree. C./s and at cooling-stop temperatures of
not higher than 650.degree. C.; [0024] coiling the steel sheet at
temperatures of 600.degree. C. or below; and then [0025] annealing
the steel sheet at temperatures from 640.degree. C. or larger to
A.sub.c1 transformation point or lower.
[0026] Although the object does not agree with that of above
examples, JP-A-2003-73742 disclosed a technology for manufacturing
high carbon hot-rolled steel sheet which satisfies the above
requirements except for selecting the cooling-stop temperature of
620.degree. C. or below.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0027] The related art, however, cannot assure the homogeneous
mechanical properties including that homogeneity in the sheet
thickness direction, and fails to assure that homogeneity and the
stretch-flange formability.
[0028] The above related art also has the problems described
below.
[0029] For the method disclosed in JP-A-3-174909, the obtained
steel sheet is what is called the "as hot-rolled" steel sheet
without subjected to heat treatment after hot-rolling. Accordingly,
the manufactured steel sheet not necessarily attains excellent
elongation and stretch-flange formability.
[0030] Regarding the method disclosed in JP-A-9-157758, a
microstructure composed of pro-eutectoid ferrite and pearlite
containing lamellar carbide is formed depending on the hot-rolling
condition, and the succeeding annealing converts the lamellar
carbide into fine spheroidal cementite. Thus formed fine spheroidal
cementite becomes the origin of voids during hole-expansion step,
and the generated voids connect with each other to induce fracture
of the steel. As a result, no excellent stretch-flange formability
is attained.
[0031] According to the method disclosed in JP-A-5-9588, the steel
sheet after hot-rolling is cooled under a specified condition,
followed by reheating thereof by direct electric heating process
and the like. As a result, a special apparatus is required and a
vast amount of electric energy is consumed. In addition, since the
steel sheet coiled after reheating likely forms fine spheroidal
cementite, there are often failed to obtain excellent
stretch-flange formability owing to the same reason to that given
above.
[0032] An object of the present invention is to provide a high
carbon hot-rolled steel sheet having excellent stretch-flange
formability and excellent homogeneity of hardness in the sheet
thickness direction, and a method for manufacturing thereof.
Means to Solve the Problems
[0033] The inventors of the present invention conducted detail
study of the effect of microstructure on the stretch-flange
formability and the hardness of high carbon hot-rolled steel sheet,
and found that it is extremely important to adequately control the
manufacturing conditions, specifically the cooling condition after
hot-rolling, the coiling temperature, and the annealing
temperature, thus found that the stretch-flange formability is
improved and the hardness in the sheet thickness direction becomes
homogeneous by controlling the volume percentage of carbide having
smaller than 0.5 .mu.m of particle size to the total carbide in the
steel sheet, determined by the method described later, to 15% or
less.
[0034] Furthermore, the inventors of the present invention found
that further excellent stretch-flange formability and homogeneous
distribution of hardness are attained by controlling more strictly
the cooling condition after hot-rolling and the coiling
temperature, thereby controlling the volume percentage of the
carbide to 10% or less.
[0035] The present invention has been perfected on the basis of
above findings, and the present invention provides a method for
manufacturing high carbon hot-rolled steel sheet having excellent
workability, by the steps of: hot-rolling a steel containing 0.2 to
0.7% C by mass at finishing temperatures of (A.sub.r3
transformation point-20.degree. C.) or above to prepare a
hot-rolled sheet; cooling thus hot-rolled sheet to temperatures of
650.degree. C. or below, (called the "cooling-stop temperature"),
at cooling rates from 60.degree. C./s or larger to smaller than
120.degree. C./s; coiling the hot-rolled sheet after cooling at
coiling temperatures of 600.degree. C. or below; and annealing the
coiled hot-rolled sheet at annealing temperatures from 640.degree.
C. or larger to A.sub.c1 transformation point or lower, (called the
"annealing of hot-rolled sheet).
[0036] According to the method of the present invention, it is more
preferable that, for the above manufacturing method, the cooling
step and the coiling step are conducted by cooling the hot-rolled
sheet to temperatures of 600.degree. C. or below at cooling rates
from 80.degree. C./s or larger to smaller than 120.degree. C./s,
and then coiling the sheet at temperatures of 550.degree. C. or
below.
[0037] Generally the coiled hot-rolled sheet is subjected to
descaling such as pickling before applying annealing of hot-rolled
sheet.
[0038] The present invention provides a high carbon hot-rolled
steel sheet which is a hot-rolled spheroidizing annealed material,
which steel sheet contains 0.2 to 0.7% C, 2% or less Si, 2% or less
Mn, 0.03% or less P, 0.03% or less S, 0.08% or less Sol.Al, and
0.01% or less N, by mass, in which the quantity of carbide having
smaller than 0.5 .mu.m of particle size is 15% or smaller by volume
to the total amount of carbide, further the difference between the
maximum hardness H.sub.V max and the minimum hardness H.sub.V min,
.DELTA.Hv (=H.sub.V max-H.sub.V in), in the sheet thickness
direction is 10 or less.
[0039] It is more preferable that the above volume percentage of
carbide having smaller than 0.5 .mu.m in particle size is 10% or
less, and that above .DELTA.Hv is 8 or smaller.
BRIEF DESCRIPTION OF THE DRAWING
[0040] FIG. 1 shows the relation between .DELTA.Hv (vertical axis)
and volume percentage (horizontal axis) of carbide having smaller
than 0.5 .mu.m of particle size.
BEST MODE FOR CARRYING OUT THE INVENTION
[0041] The high carbon hot-rolled steel sheet and the method for
manufacturing thereof according to the present invention are
described below in detail.
<Steel Composition>
(1) C Content
[0042] Carbon is an important element of forming carbide and
providing hardness after quenching. If the C content is less than
0.2% by mass, formation of pre-eutectoid ferrite after hot-rolling
becomes significant, and the volume percentage of carbide having
smaller than 0.5 .mu.m of particle size after annealing of
hot-rolled sheet, (the volume percentage to the total carbide in
the steel sheet), increases, thereby deteriorating the
stretch-flange formability and the homogeneity of hardness in the
sheet thickness direction. In addition, even after quenching,
satisfactory strength as the machine structural parts cannot be
attained. On the other hand, if the C content exceeds 0.7% by mass,
sufficient stretch-flange formability cannot be attained even if
the volume percentage of carbide having smaller than 0.5 .mu.m of
particle size is 15% or less. In addition, the hardness after
hot-rolling significantly increases to result in inconvenience in
handling owing to the brittleness of the steel sheet, and also the
strength as the machine structural parts after quenching saturates.
Therefore, the C content is specified to a range from 0.2 to 0.7%
by mass.
[0043] For the case that the hardness after quenching is
emphasized, it is preferable to specify the C content to above 0.5%
by mass. For the case that the workability is emphasized, it is
preferable to specify the C content to 0.5% or less by mass.
(2) Other Steel Compositions
[0044] Although there is no specific limitation on the elements
other than C, elements such as Mn, Si, P, S, Sol.Al, and N can be
added within ordinary respective ranges. Since, however, Si likely
converts carbide into graphite, thus interfering the hardenability
by quenching, the Si content is preferably specified to 2% or less
by mass. Since excess amount of Mn likely induces the decrease in
ductility, the Mn content is preferably specified to 2% or less by
mass. Since excess amount of P and S decreases ductility and likely
induces cracks, the content of P and S is preferably specified to
0.03% or less by mass, respectively. Since excess amount of Sol.Al
deteriorates the hardenability by quenching owing to the
precipitation of AlN in a large amount, the Sol.Al content is
preferably specified to 0.08% or less by mass. Since excess amount
of N deteriorates ductility, the N content is preferably specified
to 0.01% or less by mass. Preferable respective contents of these
elements are: 0.5% or less Si, 1% or less Mn, 0.02% or less P,
0.05% or less Sol.Al, and 0.005% or less N, by mass. For improving
the stretch-flange formability, the S content is preferably
reduced. For example, the stretch-flange formability is further
significantly improved by specifying the S content to 0.007% or
less by mass. When each of these elements is decreased to less than
0.0001% by mass, the cost increases so that the content thereof is
preferably accepted by amounts of 0.0001% by mass or more.
[0045] Depending on the objectives of improvement in hardenability
by quenching and/or improvement in resistance to temper softening,
the effect of the present invention is not affected by the addition
of at least one of the elements such as B, Cr, Cu, Ni, Mo, Ti, Nb,
W, V, and Zr within ordinarily adding ranges to the high carbon
hot-rolled steel sheet. Specifically for these elements, there can
be added: B in amounts of about 0.005% or less by mass, Cr about
3.5% or less by mass, Ni about 3.5% or less by mass, Mo about 0.7%
or less by mass, Cu about 0.1% or less by mass, Ti about 0.1% or
less by mass, Nb about 0.1% or less by mass, and W, V, and Zr, as
the total, about 0.1% or less by mass. On adding Cr and/or Mo, it
is preferable to add Cr in amounts of about 0.05% or more by mass
and Mo about 0.05% or more by mass.
[0046] Balance of above composition is preferably iron and
inevitable impurities. For example, even if elements such as Sn and
Pb entered the steel composition as impurities during the
manufacturing process, they do not affect the effect of the present
invention.
<Hot-Rolling Conditions>
(3) Finishing Temperature of Hot-Rolling
[0047] If the finishing temperature is below (A.sub.r3
transformation point-20.degree. C.), the ferrite transformation
proceeds in a part, which increases the volume percentage of
carbide having smaller than 0.5 .mu.m of particle size, thereby
deteriorating both the stretch-flange formability and the
homogeneity of hardness in the sheet thickness direction.
Accordingly, the finishing temperature of hot-rolling is specified
to (A.sub.r3 transformation point-20.degree. C.) or above. The
A.sub.r3 transformation point may be the actually determined value,
and may be the calculated value of the following formula (1).
A.sub.r3 transformation point=910-203[C].sup.1/2+44.7[Si]-30[Mn]
(1)
where, [M] designates the content (% by mass) of the element M.
[0048] Responding to the additional elements, correction terms such
as (-1.[Cr]), (+31.5[Mo]), and (-15.2[Ni]) may be added to the
right-hand member of the formula (1).
(4) Condition of Cooling after Hot-Rolling
[0049] If the cooling rate after hot-rolling is smaller than
60.degree. C./s, the supercooling of austenite becomes small, and
the formation of pre-eutectoid ferrite after hot-rolling becomes
significant. As a result, the volume percentage of carbide having
smaller than 0.5 .mu.m of particle size exceeds 15% after annealing
of hot-rolled sheet, thereby deteriorating both the stretch-flange
formability and the homogeneity of hardness in the sheet thickness
direction.
[0050] If the cooling rate exceeds 120.degree. C./s, the
temperature difference in the sheet thickness direction, between
the surface portion and the central portion, increases, and the
formation of pre-eutectoid ferrite becomes significant at the
central portion. As a result, both the stretch-flange formability
and the homogeneity of hardness in the sheet thickness direction
deteriorate, similar to above. The tendency becomes specifically
large when the sheet thickness of hot-rolled steel sheets becomes
4.0 mm or larger.
[0051] That is, to specifically homogenize the hardness in the
sheet thickness direction, there exists an adequate cooling rate,
and excessively large or excessively small cooling rates cannot
attain the desired homogeneity of hardness. In related art,
particularly the optimization of cooling rate is not attained so
that the homogeneity of hardness cannot be assured.
[0052] Consequently, the cooling rate after hot-rolling is
specified to a range from 60.degree. C./s or larger to smaller than
120.degree. C./s. Furthermore, if the volume percentage of carbide
having smaller than 0.5 .mu.m of particle size is to be brought to
10% or less, the cooling rate is specified to a range from
80.degree. C./s or larger to smaller than 120.degree. C./s. It is
more preferable to specify the upper limit of the cooling rate to
115.degree. C./s or smaller.
[0053] If the end point of the cooling of hot-rolled steel sheet
with that cooling rates, or the cooling-stop temperature, is higher
than 650.degree. C., the pre-eutectoid ferrite is formed, and the
pearlite containing lamella carbide is formed during the cooling
step before coiling the hot-rolled steel sheet. As a result, the
volume percentage of carbide having smaller than 0.5 .mu.m of
particle size exceeds 15% after annealing of hot-rolled sheet,
thereby deteriorating the stretch-flange formability and the
homogeneity of hardness in the sheet thickness direction.
Therefore, the cooling-stop temperature is specified to 650.degree.
C. or below, and more preferably to 600.degree. C. or below.
[0054] To bring the volume percentage of the carbide having smaller
than 0.5 .mu.m of particle size to 10% or less, there are
specified, as described above, the cooling rate in a range from
80.degree. C./s or larger to 120.degree. C./s or smaller,
(preferably 115.degree. C./s or smaller), and the cooling-stop
temperature of 600.degree. C. or below.
[0055] Since there is a problem of accuracy of temperature
measurement, the cooling-stop temperature is preferably specified
to 500.degree. C. or above.
[0056] After reaching the cooling-stop temperature, natural cooling
may be applied, or forced cooling may be continued with a weakened
cooling force. From the viewpoint of homogeneous mechanical
properties of the steel sheet, however, forced cooling to a degree
of suppressing the reheating is preferred.
(5) Coiling Temperature
[0057] The hot-rolled steel sheet after cooling is coiled. If the
coiling temperature exceeds 600.degree. C., pearlite containing
lamella carbide is formed. As a result, the volume percentage of
carbide having smaller than 0.5 .mu.m of particle size exceeds 15%
after annealing of hot-rolled sheet, thereby deteriorating the
stretch-flange formability and the homogeneity of hardness in the
sheet thickness direction. Therefore, the coiling temperature is
specified to 600.degree. C. or below. The coiling temperature is
selected to a temperature below the above cooling-stop
temperature.
[0058] From the viewpoint of the homogeneity of hardness, it is
preferable that the above cooling-stop temperature is specified to
600.degree. C. or below, and that the coiling temperature is
specified to 550.degree. C. or below.
[0059] For bringing the volume percentage of carbide having smaller
than 0.5 .mu.m of particle size to 10% or less, there are
specified, as above, the cooling rate to a range from 80.degree.
C./s or larger to 120.degree. C./s or smaller, (preferably
115.degree. C./s or smaller), the cooling-stop temperature to
600.degree. C. or below, and the coiling temperature to 550.degree.
C. or below.
[0060] To prevent the deterioration of shape of the hot-rolled
steel sheet, the coiling temperature is preferably specified to
200.degree. C. or above, and more preferably to 350.degree. C. or
above.
(6) Descaling (Pickling and the Like)
[0061] The hot-rolled steel sheet after coiling is generally
subjected to descaling before applying annealing of hot-rolled
sheet. Although there is no specific limitation on the
scale-removal method, it is preferably to adopt ordinary
pickling.
<Condition of Annealing of Hot-Rolled Sheet>
[0062] (7) Temperature of annealing of hot-rolled sheet The
hot-rolled sheet after pickling is subjected to annealing of
hot-rolled sheet to spheroidize the carbide. If the temperature of
annealing of hot-rolled sheet is below 640.degree. C., the
spheroidization of carbide becomes insufficient or the volume
percentage of carbide having smaller than 0.5 .mu.m of particle
size increases, which deteriorates the stretch-flange formability
and the homogeneity of hardness in the sheet thickness direction.
On the other hand, if the annealing temperature exceeds the
A.sub.c1 transformation point, the austenite formation proceeds in
a part, and the pearlite again forms during cooling, which
deteriorates the stretch-flange formability and the homogeneity of
hardness in the sheet thickness direction. Accordingly, the
temperature of annealing of hot-rolled sheet is specified to a
range from 640.degree. C. to (A.sub.c1 transformation point). To
attain further excellent stretch-flange formability, the
temperature of annealing of hot-rolled sheet is preferably
specified to 680.degree. C. or above.
[0063] The A.sub.c1 transformation point may be the actually
determined value, and may be the calculated value of the following
formula (2).
A.sub.c1 transformation point=754.83-32.25[C]+23.32[Si]-17.76[Mn]
(2)
where, [M] designates the content (% by mass) of the element M.
[0064] Responding to the additional elements, correction terms such
as (+17.13[Cr]), (+4.51[Mo]), and (+15.62[V]) may be added to the
right-hand member of the formula (2).
[0065] The annealing time is preferably between about 8 hours and
about 80 hours. By applying the annealing for spheroidization, the
obtained hot-rolled steel sheet becomes a hot-rolled spheroidizing
annealed material. The carbide treated by spheroidizing annealing
gives about 5.0 or smaller average aspect ratio, (determined at a
depth of about one fourth in the sheet thickness direction).
<Other>
[0066] For steel making of the high carbon steel according to the
present invention, either converter or electric furnace can be
applied. Thus made high carbon steel is formed into slab by
ingoting and blooming or by continuous casting.
[0067] The slab is normally heated, (reheated), and then treated by
hot-rolling. For the slab manufactured by continuous casting may be
treated by hot direct rolling directly from the slab or after
heat-holding to prevent temperature reduction. For the case of
hot-rolling the slab after reheating, the slab heating temperature
is preferably specified to 1280.degree. C. or below to avoid the
deterioration of surface condition caused by scale.
[0068] The hot-rolling can be given only by finish rolling
eliminating rough rolling. To assure the finishing temperature, the
material being rolled may be heated during hot-rolling using a
heating means such as sheet bar heater. To enhance spheroidization
or to decrease hardness, the coiled sheet may be thermally
insulated by a slow-cooling cover or other means.
[0069] Although the thickness of the hot-rolled sheet is not
specifically limited if only the manufacturing conditions of the
present invention are maintained, a particularly preferable range
of the thickness thereof is from 1.0 to 10.0 mm from the point of
operability.
[0070] The annealing of hot-rolled sheet can be done either by box
annealing or by continuous annealing. After annealing of hot-rolled
sheet, skin-pass rolling is applied, at need. Since the skin-pass
rolling does not affect the hardenability by quenching, there is no
specific limitation of the condition of skin-pass rolling.
[0071] Regarding the amount of carbide having 0.5 .mu.m or coarse
particle size in the steel sheet, there raises no problem if only
the amount is within that corresponding to the C content according
to the present invention.
EXAMPLES
Example 1
[0072] Continuously cast slabs of Steels A to E having the
respective chemical compositions shown in Table 1 were heated to
1250.degree. C. Thus heated slabs were treated by hot-rolling and
annealing of hot-rolled sheet under the respective conditions given
in Table 2 to form the Steel sheets Nos. 1 to 19, having a sheet
thickness of 5.0 mm. The annealing of hot-rolled sheet was given in
a non-nitrizing atmosphere, (Ar atmosphere).
[0073] Steel sheets Nos. 1 to 10 are Examples of the present
invention, and Steel sheets Nos. 11 to 19 are Comparative Examples.
The following methods were adopted to determine the particle size
and volume percentage of carbide, the hardness in the sheet
thickness direction, and the hole-expansion rate .lamda.. The
hole-expansion rate .lamda. was adopted as an index to evaluate the
stretch-flange formability.
(i) Determination of Particle Size and Volume Percentage of
Carbide
[0074] A cross section of steel sheet parallel to the rolling
direction was polished, which section was then etched at a depth of
one fourth of sheet thickness using a Picral solution (picric
acid+ethanol). The microstructure on the etched surface was
observed by a scanning electron microscope (.times.300
magnification).
[0075] The particle size and volume percentage of carbide were
quantitatively determined by image analysis using the image
analyzing software "Image Pro Plus ver.4.0.TM." manufactured by
Media Cybernetics, Inc. That is, the particle size of each carbide
was determined by measuring the diameter between two point on outer
peripheral circle of the carbide and passing through the center of
gravity of an equivalent ellipse of the carbide, (an ellipse having
the same area to that of carbide and having the same first moment
and second moment to those of the carbide), at intervals of 2
degrees, and then averaging thus measured diameters.
[0076] Furthermore, for all the carbides within the visual field,
the area percentage of every carbide to the measuring visual field
was determined, which determined value was adopted as the volume
percentage of the carbide. For the carbides having smaller than 0.5
.mu.m of particle size, the sum of volume percentages, (cumulative
volume percentage), was determined, which was then divided by the
cumulative volume percentage of all carbides, thus obtained the
volume percentage for every visual field. The volume percentage was
determined on 50 visual fields, and those determined volume
percentages were averaged to obtain the volume percentage of
carbide having smaller than 0.5 .mu.m of particle size.
[0077] In the above image analysis, the average aspect ratio
(number average) of carbide was also calculated, and the
spheroidizing annealing was confirmed.
(ii) Hardness Determination in the Sheet Thickness Direction
[0078] The cross section of steel sheet parallel to the rolling
direction was polished. The hardness was determined using a
micro-Vickers hardness tester applying 4.9 N (500 gf) of load at
nine positions: 0.1 mm depth from the surface of the steel sheet;
depths of 1/8, 2/8, 3/8, 4/8, 5/8, 6/8, and 7/8 of the sheet
thickness; and 0.1 mm depth from the rear surface thereof.
[0079] The homogeneity of hardness in the sheet thickness direction
was evaluated by the difference between maximum hardness Hv max and
the minimum hardness Hv min, .DELTA.Hv (=H.sub.V max-H.sub.V min).
When .DELTA.Hv.ltoreq.10, the homogeneity of hardness was evaluated
as excellent.
(iii) Determination of Hole-Expansion Rate .lamda.
[0080] The steel sheet was punched using a punching tool having a
punch diameter of 10 mm and a die diameter of 12 mm (20% of
clearance). Then, the punched hole was expanded by pressing-up a
cylindrical flat bottom punch (50 mm in diameter and 8 mm in
shoulder radius). The hole diameter d (mm) at the point of
generating penetration crack at hole-edge was determined. Then, the
hole-expansion rate .lamda. (%) was calculated by the formula
(3).
.lamda.=100.times.(d-10)/10 (3)
[0081] Similar tests were repeated for total six times, and the
average hole-expansion rate .lamda. was determined.
[0082] Table 3 shows the result. Steel sheets Nos. 1 to 10, which
are Examples of the present invention, gave 15% or smaller volume
percentage of carbide having smaller than 0.5 .mu.m of particle
size, and, compared with Steel sheets Nos. 11 to 19, which are
Comparative Examples with the same chemical compositions,
respectively, the hole-expansion rate .lamda. was large, and the
stretch-flange formability was superior. A presumable cause of the
high hole-expansion rate .lamda. is that, as described above,
although the fine carbide having smaller than 0.5 .mu.m of particle
size acts as the origin of voids during hole-expansion step, which
generated voids connect with each other to induce fracture, the
quantity of that fine carbide decreases to 15% or less by
volume.
[0083] FIG. 1 shows the relation between the .DELTA.Hv (vertical
axis) and the volume percentage of carbide having smaller than 0.5
.mu.m of particle size, (horizontal axis). As in the case of Steel
sheets Nos. 1 to 10, which are Examples of the present invention,
when the volume percentage of the carbide having smaller than 0.5
.mu.m of particle size is brought to 15% or less, .DELTA.Hv becomes
10 or less, adding to the excellent stretch-flanging formability as
described above, thereby providing excellent homogeneity of
hardness in the sheet thickness direction, (black circle in FIG.
1). A presumable cause of the effect of fine carbide on the
homogeneity of hardness is that the fine carbide likely segregates
into a zone where pearlite existed.
[0084] Steel sheets Nos. 2, 4, 6, 8, and 10, which are Examples of
the present invention, having 10% or less of volume percentage of
carbide having smaller than 0.5 .mu.m of particle size, prepared
under the conditions of 600.degree. C. or below of cooling-stop
temperature and 550.degree. C. or below of coiling temperature,
provided not only more excellent stretch-flange formability but
also more excellent homogeneity of hardness, of .DELTA.Hv of 8 or
smaller, in sheet thickness direction.
TABLE-US-00001 TABLE 1 A.sub.r3 A.sub.c1 Composition (mass %)
transformation transformation Steel C Si Mn P S Sol. Al N point*
(%) point** (%) A 0.26 0.22 0.83 0.010 0.0025 0.037 0.0031 791 737
B 0.34 0.20 0.74 0.015 0.0018 0.026 0.0033 778 735 C 0.35 0.02 0.15
0.009 0.0030 0.034 0.0036 786 741 D 0.49 0.19 0.76 0.011 0.0027
0.036 0.0032 754 730 E 0.66 0.21 0.75 0.014 0.0045 0.027 0.0030 732
725 *Calculated by the formula (1). **Calculated by the formula
(2).
TABLE-US-00002 TABLE 2 Hot-rolling conditions Steel Finishing
Cooling-stop Coiling sheet temperature Cooling rate temperature
temperature Annealing of No. Steel (.degree. C.) (.degree. C./s)
(.degree. C.) (.degree. C.) hot-rolled sheet Remark 1 A 801 110 620
550 700.degree. C. .times. 40 hr Example 2 A 811 95 560 510
720.degree. C. .times. 40 hr Example 3 B 788 115 610 540
680.degree. C. .times. 40 hr Example 4 B 808 85 570 520 710.degree.
C. .times. 40 hr Example 5 C 801 75 610 590 670.degree. C. .times.
40 hr Example 6 C 806 105 580 490 720.degree. C. .times. 40 hr
Example 7 D 774 90 620 580 710.degree. C. .times. 40 hr Example 8 D
784 100 550 500 720.degree. C. .times. 40 hr Example 9 E 752 65 600
570 700.degree. C. .times. 40 hr Example 10 E 772 100 540 490
720.degree. C. .times. 40 hr Example 11 A 801 80 680 580
700.degree. C. .times. 40 hr Comparative example 12 A 751 100 610
570 700.degree. C. .times. 40 hr Comparative example 13 B 798 110
620 560 600.degree. C. .times. 40 hr Comparative example 14 B 793
90 600 630 690.degree. C. .times. 40 hr Comparative example 15 C
816 150 580 520 720.degree. C. .times. 40 hr Comparative example 16
C 806 55 630 550 710.degree. C. .times. 40 hr Comparative example
17 D 794 115 670 590 720.degree. C. .times. 40 hr Comparative
example 18 D 719 95 610 580 680.degree. C. .times. 40 hr
Comparative example 19 E 752 130 590 550 710.degree. C. .times. 40
hr Comparative example
TABLE-US-00003 TABLE 3 Volume percentage of carbide having Steel
smaller than 0.5 .mu.m sheet No. of particle size (%) .DELTA.Hv
.lamda. (%) Remark 1 13 9 111 Example 2 9 7 128 Example 3 12 9 72
Example 4 8 8 83 Example 5 13 10 69 Example 6 10 7 86 Example 7 14
10 48 Example 8 9 7 56 Example 9 12 9 36 Example 10 10 8 42 Example
11 28 14 75 Comparative Example 12 21 15 69 Comparative Example 13
19 16 44 Comparative Example 14 24 13 37 Comparative Example 15 21
12 53 Comparative Example 16 30 18 39 Comparative Example 17 20 12
22 Comparative Example 18 23 13 17 Comparative Example 19 26 17 13
Comparative Example
Example 2
[0085] Continuous casting was applied to the steels given below to
form the respective slabs:
[0086] Steel F (0.31% C, 0.18% Si, 0.68% Mn, 0.012% P, 0.0033% S,
0.025% Sol.Al, and 0.0040% N, by mass; 785.degree. C. of A.sub.r3
transformation point; and 737.degree. C. of A.sub.c1 transformation
point);
[0087] Steel G (0.23% C, 0.18% Si, 0.76% Mn, 0.016% P, 0.0040% S,
0.025% Sol.Al, 0.0028% N, and 1.2% Cr, by mass; 785.degree. C. of
A.sub.r3 transformation point; and 759.degree. C. of A.sub.c1
transformation point);
[0088] Steel H (0.32% C, 1.2% Si, 1.5% Mn, 0.025% P, 0.010% S,
0.06% Sol.Al, and 0.0070% N, by mass; 804.degree. C. of A.sub.r3
transformation point; and 746.degree. C. of A.sub.c1 transformation
point); Steel I (0.35% C, 0.20% Si, 0.68% Mn, 0.012% P, 0.0038% S,
0.032% Sol.Al, 0.0033% N, 0.98% Cr, and 0.17% Mo, by mass;
773.degree. C. of A.sub.r3 transformation point; and 754.degree. C.
of A.sub.c1 transformation point); and Steel E given in Table
1.
[0089] These slabs were heated to 1230.degree. C., which were then
treated by hot-rolling and annealing of hot-rolled sheet under the
respective conditions shown in Table 4, thus manufactured the Steel
Sheets Nos. 20 to 36, having 4.5 mm in sheet thickness. The
annealing of hot-rolled sheet was given in a non-nitrizing
atmosphere (H.sub.2 atmosphere).
[0090] To thus prepared hot-rolled steel sheets, similar method to
that in Example 1 was applied to determine the particle size and
volume percentage of carbide, the hardness in the sheet thickness
direction, and the hole-expansion rate .lamda.. The results are
given in Table 5.
[0091] Among Steel sheets Nos. 20 to 26 in which the conditions
other than the cooling rate were kept constant, Steel sheets Nos.
21 to 25 in which the cooling rate was within the range of the
present invention showed significantly excellent stretch-flange
formability and homogeneity of hardness in the sheet thickness
direction. Steel sheets Nos. 22 to 25 showed further significant
improvement in these characteristics, giving maximum values thereof
at around 100.degree. C. (for Steel sheets Nos. 23 to 25).
[0092] As for Steel sheets Nos. 27 to 32 which were treated by a
constant cooling rate, Steel sheets Nos. 29 to 32 which are within
the range of the present invention in both the cooling-stop
temperature and the coiling temperature gave significantly
excellent values in the stretch-flange formability and the
homogeneity of hardness in the sheet thickness direction. For the
case of satisfying 600.degree. C. or lower cooling-stop temperature
and of 550.degree. C. or lower coiling temperature, (Steel sheet
No. 32), the volume percentage of fine carbide became 10% or less,
thus further significantly excellent stretch-flange formability and
homogeneity of hardness in the sheet thickness direction were
attained.
[0093] Steels E to I which have the steel compositions within the
range of the present invention showed excellent stretch-flange
formability and excellent homogeneity of hardness in the sheet
thickness direction, including the cases of adding alloying
elements other than the basic components, (Steel G and Steel I).
When, however, Steel F, Steel G, and Steel I gave further and
significantly excellent absolute values of hole-expansion rate
compared with the case of large quantity of other basic elements,
(Steel H).
TABLE-US-00004 TABLE 4 Hot-rolling conditions Steel Finishing
Cooling-stop Coiling sheet temperature Cooling rate temperature
temperature Annealing of No. Steel (.degree. C.) (.degree. C./s)
(.degree. C.) (.degree. C.) hot-rolled sheet 20 F 820 50 560 530
700.degree. C. .times. 30 hr 21 F 820 70 560 530 700.degree. C.
.times. 30 hr 22 F 820 85 560 530 700.degree. C. .times. 30 hr 23 F
820 95 560 530 700.degree. C. .times. 30 hr 24 F 820 105 560 530
700.degree. C. .times. 30 hr 25 F 820 115 560 530 700.degree. C.
.times. 30 hr 26 F 820 140 560 530 700.degree. C. .times. 30 hr 27
F 820 105 660 530 700.degree. C. .times. 30 hr 28 F 820 105 630 610
700.degree. C. .times. 30 hr 29 F 820 105 630 560 700.degree. C.
.times. 30 hr 30 F 820 105 630 530 700.degree. C. .times. 30 hr 31
F 820 105 580 560 700.degree. C. .times. 30 hr 32 F 820 105 580 530
700.degree. C. .times. 30 hr 33 E 790 105 560 530 715.degree. C.
.times. 60 hr 34 G 800 105 560 530 720.degree. C. .times. 50 hr 35
H 810 105 560 530 700.degree. C. .times. 30 hr 36 I 820 105 560 530
700.degree. C. .times. 30 hr
TABLE-US-00005 TABLE 5 Volume percentage of carbide having Steel
smaller than 0.5 .mu.m sheet No. of particle size (%) .DELTA.Hv
.lamda. (%) 20 22 15 42 21 13 10 70 22 10 9 78 23 8 9 84 24 6 7 93
25 7 8 88 26 23 17 38 27 26 16 45 28 23 17 39 29 11 9 70 30 13 10
74 31 12 10 75 32 7 7 89 33 9 7 50 34 8 9 95 35 9 7 67 36 9 9
80
INDUSTRIAL APPLICABILITY
[0094] The present invention has realized the manufacture of high
carbon hot-rolled steel sheet which gives excellent stretch-flange
formability and excellent homogeneity of hardness in the sheet
thickness direction without adding special apparatus.
* * * * *