In order to meet the needs, it is preferentially required that a medium carbon
steel sheet which is soft and has sufficient cold workability, excellent hardenability and
accuracy of sheet thickness. Here, the workability is based on bending, thickening,
tie-dyed, or the like. With regards to a relationship between the workability and
5 quenching characteristic of the medium carbon steel sheet, many researches have been
conducted (For example, refer to Patent Documents 1 to 6).
[OOC'41
For example, Patent Document 1 discloses a medium-high carbon steel sheet
which is hypo-eutectoid steel containing C: 0.1 to 0.8 by mass% and S: 0.01 by mass%
10 or less, in which carbides are dispersed in a ferrite so that a spheroidizing ratio of the
carbides is 90% or more, an average grain size of the carbides is 0.4 to 1.0 pm, and as
necessary, a ferrite crystal grain size is controlled to be 20 pm or more. However,
according to the steel sheet described above, a research for an influence of the structure
based on the local process such as a stretch flangeability is conducted and the
15 relationship above-mentioned is suggested as a main agent, and thus, it could have been
hardly found that a main agent in which a specific mechanical properties such as a yield
rate YR and tensile strength TS for cold working is suggested. In addition, it could have
been hardly found that a manufacturing method for the steel sheet which is used to obtain
a structuring stability during annealing before using as a member.
20 melated Art Documents]
patent Documents]
[OOOS]
[Patent Document 11 Japanese Unexarnined Patent Application, First
Publication No. Hl l-80884
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2003-89846
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. H9-268344
5 [Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. 2001 -329333
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. 2001 -355047
[Patent Document 61 Japanese Unexamined Patent Application, First
10 Publication No. 2004- 137527
[Summary of the Invention]
[Problems to be Solved by the Invention]
[0006]
The present invention has been made in consideration of the above-described
15 circumstances and an object thereof is to provide a medium carbon steel sheet excellent
in cold workability and quenching stability which are applied to an automotive industry
or the like, and methods for manufacturing the same, and a member thereof.
[Means for Solving the Problem]
[0007]
An intensive research regarding a method for solving the above-described
problems has been conducted by the inventors. As a result, it has been found that, since
cold workability is effectively improved by ensuring uniformity in strain propagation so
as to prevent fine cracking, it is important to control an average ferrite grain size to be 10
pm or more, an average size of cementites to be 0.4 pm or less, and a spheroidizing ratio
25 of the cementites to be 90% or more. It has been also found that, since the steel sheet in
4
which the workability is improved has remarkable features such that the average size of
the cementites is exceedingly fine and a fraction of the coarse cementites is suppressed, it
is possible to stabilize hardenability of the steel sheet under any quenching condition.
[OOOS]
In addition, it has been found fiom further various researches that the method for
manufacturing steel sheet satisfying the above-described conditions is hardly
manufactured even if individual manufacturing such as rolling conditions, annealing
conditions or the like are merely controlled and that the steel sheet can be manufactured
only if plural manufacturing conditions of interactional processes such as hot-rolling,
10 cold-rolling, or annealing are simultaneously optimized. As a result, the present
invention has been accomplished.
[0009]
An aspect of the present invention employs the following.
[Effect of the Invention]
[OOl 01
(1) A medium carbon steel sheet excellent in cold workability includes, by
mass%, C: 0.10 to 0.80%, Si: 0.01 to 0.3%, Mn: 0.3 to 2.0%, P: 0.001 to 0.03%, S:
0.0001 to 0.01%, Al: 0.005 to 0.10%, N: 0.001 to 0.01%, and a balance consisting of Fe
and unavoidable impurities, in which an average size of cementites is 0.4 pm or less, a
20 number fraction of cementites whose sizes are 1.5 times or more of the average size of
the cementites in a total number of the carbides is 30% or less, spheroidizing ratio of the
cementites is 90% or more, an average grain size of a ferrite is 10 pm or more.
[OO 1 11
(2) In the medium carbon steel sheet excellent in cold workability according to
5
(1) further includes at least one or two selected from, by mass%, Cr: 0.01 to 1.5%, B:
0.001 to 0.01%, Nb: 0.01 to 0.5%, Mo: 0.01 to 0.5%, V: 0.01 to 0.5%, Ti: 0.01 to 0.3%,
Cu: 0.05 to 0.5%, W: 0.01 to 0.5%, Ta: 0.01 to 0.5%.
COO1 21
5 (3) In the medium carbon steel sheet excellent in cold workability according to
(1) or (2) further includes at least one or two selected from, by mass% Ni: 0.01 to 0.5%,
Mg: 0.0005 to 0.003%, Ca: 0.0005 to 0.003%, Y: 0.001 to 0.03%, Zr: 0.001 to 0.03%,
La: 0.001 to 0.03%, Ce: 0.001 to 0.03%.
[00 1 31
10 (4) In the medium carbon steel sheet excellent in cold workability according to
(1) to (3), in which a yield ratio (YR) before cold-working may be 60% or less.
[00 141
(5) In the medium carbon steel sheet excellent in cold workability according to
(1) to (4), in which strength thereof before cold-working is TS550 MPa or less.
15 [00 1 51
(6) A manufacturing method for a medium carbon steel sheet excellent in cold
workability includes: when continuous casting slab of components according to any one
of the above (1) to (5) is directly hot-rolled or is hot-rolled after heating, coiling is
conducted in a temperature range of 400°C to 580°C by pearlitic transformation, and one
cold-rolling and one annealing is conducted on a hot-rolled sheet in which an average
lamellar thickness of cementites in the pearlite is 0.02 to 0.5pm.
[00 1 61
(7) The manufacturing method for a medium carbon steel sheet excellent in
cold workability according to (6), in which the cold-rolling reduction of cold-rolling after
before quenching and an area fraction % (an area fraction% of abnormal austenite) of
crystal grains having the grain-size number which is different by two or more
corresponding to the average grain size in the austenite structure at quenching.
Embodiment of the Invention]
[002 11
Hereinafter, the present invention will be described in detail.
[0022]
The manufacturing features according to the embodiment will be described in
detail as follow.
10 [00231
Features of hot-rolling sheet: when researching a cooling pattern, it makes in a
range of 400°C to 580°C such that an average lamellar thickness of a pearlite is adjusted
in a range of 0.02 to 0.5 pm and then pickling and cold-rolling are performed
sequentially on the hot rolled sheet.
[0024]
Features of cold-rolling sheet: it makes under the lower pressure in a range of
5% to less than 30%; since the thickness of the cold-rolling sheet may be adjusted better
than the hot-rolling sheet by cold-rolling and the number fraction of coarse cementites
can be reduced, the cold-rolling is necessary.
[0025]
In a condition in which the steel sheet is annealed in an annealing temperature of
Acl or less during relatively short time, by only once annealing, it is possible to grow the
ferrite to 10 pm or more in a state in which the spheroidal carbides are finely and
uniformly dispersed, and it is possible to ensure the workability of the obtained medium
carbon steel sheet. At the same time, it is possible to control the coarsening ratio of the
grain size of the ferrite, and it is possible to improve the quenching stability of the
obtained medium carbon steel sheet.
[0026]
A quenching stability may be varied according to the degree of duplex grain of
austenites at heating. The more uniform grain size of the austenites at heating, the
better quenching stability in the steel sheet. Since there are differences of deformation
start time and hnish time according to respective the austenites grain during cooling at
quenching, when the degree of duplex grain is high, a structure uniformity of quenched
10 material is deteriorated, heating is distorted, and shape deformation may be occurred.
Unless, the uniformity of the quenching structure is improved by increasing the cooling
speed so as to avoid the shape deformation, a residual stress in the steel sheet is uniform.
In the quenching stability of the present invention, it is referred that the uniformity of the
quenching structure, heating control, residual stress in the steel sheet. Accordingly, the
15 above-mentioned problems may be solved by control of the austenites grain distribution
through a controlling of the ferrite grain size and cementite grain distribution.
[0027]
Hereinafter, each configuration of the present invention will be described.
[0028]
20 Each of the following embodiments has been obtained by repeatedly
investigating a steel sheet and a method for manufacturing the same on the basis of a
composition provided in JIS G 405 1 (carbon steel for machine structural use), JIS G
4401 (carbon tool steel), or JIS G 4802 (cold-rolled steel strip for springs). Accordingly,
the basic components of the carbon steel sheet in the present invention are made by
25 following above-mentioned description, however, reasons for limitation about
9
composition of those components will be described. Here, "%" in the components,
indicates by mass %.
[0029]
(C: 0.10 to 0.80%)
C is an important element for ensuring the strength after quenching of a steel
sheet when C content is added 0.10% or more, an intended strength is ensured. When C
content is less than 0.1 0%, the ferritic transformation is promoted during hot-rolling and
coiling, and it is difficult to uniformly disperse the cementites in the steel material.
Therefore, a lower limit of C content is to be 0.10%. On the other hand, when C
10 content is more than 0.80%, a lamellar thickness of cementites in the pearlite included in
the hot-rolled sheet is more than 0.5 pm. Therefore, in the case, the cementites are
hardly spheroidized, a ratio of coarse cementites is increased, and thus, the quenching
stability deteriorates. Therefore, an upper limit of C content is to be 0.80%, and
preferably 0.20% or 0.60%,
[0030]
(Si: 0.01 to 0.3%) .
Si acts as a deoxidizing agent and is an element effective in improving
hardenability and in increasing strength. When Si content is less than 0.01%, the above
effect cannot be obtained. Therefore, a lower limit of Si content is to be 0.01%. On
20 the other hand, when Si content is more than 0.3%, since strength is increased and the
tensile strength TS is increased, the increasing of a formation weight tends to occur.
Therefore, an upper limit of Si content is to be 0.3%, and preferably 0.10% or 0.25%.
[003 11
(Mn: 0.3 to 2.0%)
Mi is an important element for controlling the thermal stability of the
10
cementites. When Mn content is less than 0.3%, the above effect cannot be obtained.
Therefore, a lower limit of Mn content is to be 0.3%. On the other hand, when Mn
content is more than 2.0%, an amount of MnS is increased and the fracture tends to occur
easily during cold-working. In addition, in the case, the cementites tend to remain at
quenching and the degree of austenite tends to be increased. Therefore, an upper limit
of Mn content is to be 2.0%, and more preferably 0.5% to 1.5%.
[0032]
(P: 0.001% to 0.03%)
P is an element which enhances the strength. When an excessive amount of P
is contained in the steel, the tensile strength TS increases, the toughness deteriorates, and
the cold workability deteriorates. Thus, an upper limit of P content is to be 0.03%.
However, when P is reduced to less than 0.001 %, refuring costs are greatly increased.
Thus, a lower limit of P content may be 0.00 1 %.
[0033]
(S: 0.0001 to 0.01%)
S foms non-metallic inclusions such as MnS and deteriorates the cold
workability. Since the growth of the austenites grain at heating is stopped by pinning
effect, it is caused to increase the degree of the duplex grains. Therefore, an upper limit
of S content is preferably to be 0.01%. However, when S is reduced to less than
0.0001%, refining costs are greatly increased. Thus, a lower limit of S content may be
0.0001%.
[0034]
(Al: 0.001 to 0.10%)
A1 acts as a deoxidizing agent, and is an element having a solid-solution
strengthening, and is effective in fixing N. When A1 content is less than 0.001%, the
addition effect is not sufficiently obtained. Thus, a lower limit of A1 content is to be
0.001 %. On the other hand, when A1 content is more than 0.10%, the above effect is
saturated, and the increasing of formation weight which has a remarkably increasing
strength tends to occur. Therefore, an upper limit of A1 content is to be 0.10%.
LO03 51
(N: 0.001 to 0.01%)
N is an element which forms nitrides. When an excessive amount of N is
contained in the steel, the cold workability deteriorates. Also, the grain growth of the
austenite during heating before quenching is suppressed, and thus, the degree of the
10 duplex grained structure is increased. Thus, an upper limit of N content is to be 0.01%.
It is preferable that N content is as small as possible. However, when N content is
reduced to less than 0.001%, refining costs are increased. Thus, a lower limit of N
wntent is to be 0.001%.
LO0361
15 Specifically, the medium carbon steel sheet may include, as the optional
elements, in order to enhance mechanical characteristic of the steel sheet according to the
embodiment, at least one or two selected from the group consisting of Cr, B, Nb, Mo, V,
Ti, Cu, W, and Ta, these elements may be added the steel sheet, as needed..
[0037]
(Cr: 0.010 to 1.5%)
Cr is an element effective in enhancing the strength of the steel sheet and in
controlling the thermal stability of the cementites. When Cr wntent is less than 0.01%,
the above effect is insufficient. Thus, a lower limit of Cr content may be 0.010%.
However, when Cr content is more than IS%, by suppressing the growth of cementites,
25 the yield ratio YR may be increased or the austenite may be changed to be a duplex
grained structure during heating, Therefore, an upper limit of Cr content is to be 1.5%.
[003 81
(B: 0.001 to 0.01%)
B is an element effective in improving the hardenability with a small amount of
5 addition. When B content is less than 0.001%, the above effect may not be obtained.
Thus, a lower limit of B content may be 0.001%. However, when B content is more
than 0.01%, casting property is deteriorated and segregation may cause to generate
coarse carbides. Thus, an upper limit of B content is to be 0.01%.
[003 91
(Nb: 0.01 to 0.5%)
Nb is an element which forms carbonitrides and is effective in preventing the
excessive coarsening of the austenite grains. When Nb content is less than 0.01 %, the
addition effect may not be sufficiently obtained. Thus, a lower limit of Nb content may
be 0.01%. However, when Nb content is more than 0.5%, the yield ratio YR may be
15 increased and the ratio of the fine austenite grains may be increased. Thus, an upper
h i t of I% content is to be 0.5%, and preferably 0.07% to 0.2%.
[0040]
(Mo: 0.01 to 0.5%)
Mo is an element which forms carbides and is effective in preventing the
20 excessive coarsening of the austenite grains. When Mo content is less than 0.01%, the
above effect may not be sufficiently obtained. Thus, a lower limit of Mo content may
be 0.01%. However, when Mo content is more than 0.5%, the yield ratio YR may be
increased and the ratio of the fine austenite grains may be increased. Thus, an upper
limit of Mo content is to be 0.5%.
25 [004 11
13
(V: 0.01 to 0.5%)
V fonns carbonitrides, and is effective in preventing the excessive coarsening of
the austenite grains, which is similar to Nb. When V content is less than 0.01%, the
above effect may not be sufficiently obtained. Thus, a lower limit of V content may be
5 0.01%. However, when V content is more than 0.5%, the yield ratio YR may be
increased and the ratio of the fine austenite grains may be increased. Thus, an upper
limit of V content is to be 0.5%, and preferably 0.07 to 0.2%.
[0042]
(Ti: 0.01 to 0.3%)
10 Ti forms carbonitrides, and is effective in preventing the excessive coarsening of
the austenite grains, which is similar to V. When Ti content is less than 0.01%, the
above effect may not be sufficiently obtained. Thus, a lower limit of Ti content may be
0.01%. However, when Ti content is more than 0.3%, the yield ratio YR may be
increased and the ratio of the fine austenite grains may be increased. Thus, an upper
15 limit of Ti content is to be 0.3%.
COO431
(Cu: 0.05 to 0.5%)
Cu is an element contaminated from scrap or the like. When Cu is contained in
the steel, it is caused to enhancing the strength and hot-rolling. Thus, an upper limit of
20 Cu content is to be 0.5%. It is preferable that Cu content is as small as possible.
However, when Cu content is reduced to less than 0.05%, refining costs are greatly
increased. Thus, a lower limit of Cu content may be 0.05%.
/0044]
(W: 0.01 to 0.5%)
25 W forms carbides, and is effective in preventing the excessive coarsening of the
austenite grains, which is similar to Mo. When W content is less than 0.01%, the above
effect may not be sufficiently obtained. Thus, a lower limit of W content may be 0.01%.
However, when W content is more than 0.5%, the yield ratio YR may be increased and
the ratio of the fine austenite grains may be increased. Thus, an upper limit of W
5 content is to be 0.5%.
[0045]
(Ta: 0.01 to 0.5%)
Ta forms carbonitrides, and is effective in preventing the excessive coarsening of
the austenite grains, which is similar to Ti. When Ta content is less than 0.01%, the
10 above effect may not be sufficiently obtained. Thus, a lower limit of Ta content may be
0.01%. However, when Ta content is more than 0.5%, the yield ratio YR may be
increased and the ratio of the fine austenite grains may be increased. Thus, an upper
limit of Ta content is to be 0.5%.
[0046]
In order to fiu?.her enhance the mechanical properties of the steel sheet of the
present invention, at least one or two selected from Ni, Mg, Ca, Y, Zr, La and Ce may be
added in the steel as the optional element as needed.
[0047]
(Ni: 0.01 to 0.5%)
Ni is an element effective in improving the toughness and the hardenability.
When Ni content is less than 0.01 %, the above effect may not be obtained. Thus, a
lower limit of Ni content may be 0.01%. However, when Ni content is more than O.5%,
the above effect may be saturated and costs are increased. Thus, an upper limit of Ni
content is to be 0.5%.
25 [0048]
15
(Mg: 0.0005 to 0.003%)
Mg is an element effective in controlling the shape of sulfides with a small
amount of addition. As necessary, Mg may be included in the steel, When and Mg
content is less than 0.0005%, the effect may not be obtained. Thus, a lower limit of Mg
5 content may be 0.0005%. In addition, Mg easily forms oxides and the compounds
including the oxides suppress the growth of the austenite grains. When Mg content is
more than 0.003%, Mg may not be uniformly distributed in the steel. Accordingly,
during heating before quenching, an area where the growth of the austenite grains is
suppressed and an area where the growth of the austenite grains is not suppressed may be
10 unevenly formed. Thus, it may be difficult to obtain an austenite structure having a
uniform grain size at the quenching. Therefore, an upper limit of Mg content is to be
0.003%.
[0049]
(Ca: 0.0005 to 0.003%)
Ca is an eIement effective in controIling the shape of sulfides with a small
amount of addition, which is similar to Mg. As necessary, Ca may be included in the
steel. When Ca content is less than 0.0005%, the effect may not be obtained. Thus, a
lower limit of Ca content may be 0.0005%. In addition, Ca easily forms oxides and the
compounds including the oxides suppress the growth of the austenite grains. When Ca
20 content is more than 0.003%, Ca may not be uniformly distributed in the steel.
Accordingly, during heating before quenching, an area where the growth of the austenite
grains is suppressed and an area where the growth of the austenite grains is not
suppressed may be unevenly formed. Thus, it may be difficult to obtain an austenite
structure having a uniform grain size at the quenching. Therefore, an upper limit of Ca
25 content is to be 0.003%.
[OOSO]
(Y 0.001 to 0.03%)
Y is an element effective in controlling the shape of sulfides with a small
amount of addition, which is similar to Ca and Mg. As necessary, Y may be included in
5 the steel. When Y content is less than 0.001%, the effect may not be obtained. Thus, a
lower limit of Y content may be 0.001%. In addition, Y easily forms oxides and the
compounds including the oxides suppress the growth of the austenite grains. When Y
content is more than 0.03%, Y may not be uniformly distributed in the steel.
Accordingly, during heating before quenching, an area where the growth of the austenite
10 grains is suppressed and an area where the growth of the austenite grains is not
suppressed may be unevenly formed. Thus, it may be difficult to obtain an austenite
structure having a uniform grain size at the quenching. Therefore, an upper limit of Y
content is to be 0.03%.
COOS 11
(Zr: 0.001 to 0.03%)
Zr is an element effective in controlling the shape of sulfides with a small
amount of addition, which is similar to Y, Ca and Mg. As necessary, Zr may be
included in the steel. When Zr content is less than 0.001%, the effect may not be
obtained. Thus, a lower limit of Zr content may be 0.001%. In addition, Zr easily
20 forms oxides and carbides and the compounds including the oxides and the carbides
suppress the growth of the austenite grains. When Zr content is more than 0.03%, Zr
may not be uniformly distributed in the steel. AccordingIy, during heating before
quenching, an area where the growth of the austenite grains is suppressed and an area
where the growth of the austenite grains is not suppressed may be unevenly formed.
25 Thus, it may be difficult to obtain an austenite structure having a uniform grain size at the
quenching. Therefore, an upper limit of Zr content is to be 0.03%.
[0052]
(La: 0.001 to 0.03%)
La is an element effective in controlling the shape of sulfides with a small
5 amount of addition, which is similar to Zr, Y, Ca and Mg. As necessary, La may be
included in the steel. When La content is less than 0.001%, the effect may not be
obtained. Thus, a lower limit of La content may be 0.001%. In addition, La easily
forms oxides and the compounds including the oxides suppress the growth of the
austenite grains. When La content is more than 0.03%, La may not be uniformly
10 distributed in the steel. Accordingly, during heating before quenching, an area where
the growth of the austenite grains is suppressed and an area where the growth of the
austenite grains is not suppressed may be unevenly formed. Thus, it may be difEcult to
obtain an austenite structure having a uniform grain size at the quenching. Therefore,
an upper limit of La content is to be 0.03%.
[0053]
(Ce: 0.001 to 0.03%)
Ce is an element effective in controlling the shape of sulfides with a small
amount of addition, which is similar to La, Zr, Y, Ca and Mg. As necessary, Ce may be
included in the steel. When Ce content is less than 0.001%, the effect may not be
20 obtained. Thus, a lower limit of Ce content may be 0.001%. In addition, Ce easily
forms oxides and the compounds including the oxides suppress the growth of the
austenite grains. When Ce content is more than 0.03%, Ce may not be uniformly
distributed in the steel. Accordingly, during heating before quenching, an area where
the growth of the austenite grains is suppressed and an area where the growth of the
25 austenite grains is not suppressed may be unevenly formed. Thus, it may be difficult to
obtain an austenite structure having a uniform grain size at the quenching. Therefore,
an upper limit of Ce content is to be 0.03%.
[0054]
In a case where scrap is used as the raw material of the steel sheet, at least one or
5 two of Sn, Sb, and As may be unavoidably included in the steel sheet of the present
invention, and the each content thereof may be 0.003% or more. When each amount of
the elements is 0.03% or less, the hardenability of the medium carbon steel sheet does not
deteriorate. Therefore, at least one or two of 0.003% to 0.03% of Sn, 0.003%,to 0.03%
of Sb, and 0.003% to 0.03% of As may be included in the steel.
10 [0055]
Although 0 (oxygen) content is not particularly limited in the steel sheet of the
present invention, when oxides agglomerate to be coarsened, the cold workability
deteriorates. Thus, 0 content is to be 0.0025% or less. It is preferable that 0 content
is as small as possible. However, it is technically difficult to reduce 0 content to less
15 than 0.0001%. Thus, 0 content may be 0.0001% or more.
[0056]
The steel sheet of the present invention in addition to the above-mentioned
components composition, includes the characteristics of YR before cold-working is 60%
or less and strength is TS 550 MPa or less, average diameter of the cementites is 0.4 pm
20 or less, the number fraction of cementites whose sizes are 1.5 times or more of the
average size of the cementites is 30% or less, a spheroidizing ratio of the carbides is 90%
or more, and an average grain size of a ferrite is 10 pm or more. After forming member
shape using the steel sheet formed thereon, when quenching, the sum of area fractian
which has the grain-size number by two or more from the average grain size is in a range
of 30% or less in the austenites structure at quenching, it is possible to obtain quenching
stability. Here, the strength before cold-working indicates strength before working.
The present invention in addition to the above-mentioned components composition
includes the properties of YR before cold-working is less than 60% and strength is less
5 than TS 550 MPa, average diameter of the cementites is 0.4 pm or less and the number
fraction of cementites whose sizes are 1.5 times or more of the average size of the
cementites is 30% or less, a spheroidizing ratio of the carbides is 90% or more, and an
average grain size of a ferrite is 10 pm or more. Accordingly, the cold workability of
the steel sheet is improved. Further, it is a knowledge having novelty that acquisition of
10 the quenching stability by satisfying the structure shape has been found by the skilled in
the art.
[0057]
The strength is to be TS550 MPa or less before cold-working. When the
tensile strength TS is more than 550 MPa and YR is in a range, it is not possible to ensure
15 sufficient deformation volume during working because ductility thereof decreases.
When the value is decreased, the ductility is improved and the workability is improved.
However, a case where sagging is increased at the time of punching is present. In recent
years, since a technique of forging a steel sheet, which combines punching, bending, and
thickening, has been widely used, a lower limit of the tensile strength TS may be 400
20 MPa or more depending on an intended working method
[OOS8]
When the yield ratio YR is decreased to 60% or less, the strain concentration
during working may be prevented. In Particular, in a case where the sheet is formed,
since the material is formed while being adapted to a die, strain propagation and material
20
flow may be uniform during the deformation so that the material can be uniformly
formed in synchronization with the motion of the die. A press method in which a
compressive load is simultaneously applied from multiple directions to improve working
accuracy and to shorten processing time is enumerated, it is required that the material can
5 be uniformly deformed and the fracture is not occurred. Therefore, it has been
additionally found that the yield ratio YR may be controlled to be low so as to be
uniformly work-hardened. When the yield ratio YR is more than 60%, the strain is
easily propagated, the strain concentration at local area during cold-working is prevented,
and therefore, it is possible to preferably prevent insufficient flow or the fracture. In
addition, when the yield ratio YR is less than 30%, since the tensile strength is reduced,
and the defects on the surface of the steel sheet caused by friction or the like with a tool
during cold-working are generated, and disfiguration is occurred, a lower limit may be
30% or more.
[0059]
An average size of cementites is 0.4 pm or less, a number fraction of cementites
whose sizes are 1.5 times or more of the average size of the cementites is 30% or less, a
spheroidizing ratio of the carbides is 90% or more.
[0060]
FIG 1 shows a relationship of the cold workability and, the average size of the
20 cementites before working and the number fraction of coarse carbides. When the
average size of the cementites is 0.4 pm or less and the number fraction of coarse
cementites is 30% or less, it is possible to ensure the cold workability (refer to "0:op en
circle" of developed steel).
Since the fracture during cold-working tends to initiate from the coarse
cementites, when the average size of the cementites is more than 0.4 p, the cold
workability may not be ensured. In particularly, the inventors have been found that,
when the number hction of coarse carbides is more than 30%, the cold workability may
not be ensured (refer to "X: cross mark" of comparative steel).
[006 11
That is, coarse cementites may cause to generate fraction during cold-working
and to increase the degree of duplex grain of the austenites at quenching. The research
,for the cementites diameter and spheroidizing ratio in stretch flangeability has been
conducted, however, there is no such a research for an evaluation of the cementites
10 diameter and an influence of a spheroidizing ratio when a local deformation is not carried
out in high workability. The inventors has been researched for influence investigation
and improvement means, as a result, occurring of micro-voids, features without local
deformation by connection have been found. In addition, in order to avoid the fiacture
without occurring due to the local stress concentration, it has been found that the lower
15 YR is effective. Further, the microvoids are generated near the cementites, the
workability is effectively improved by finely dispersing the carbides in the steel as
described above in order to suppress the strain concentration, it is required that
spheroidizing process is carried out to disperse the cementites having a size of 0.4 pm or
less smaller than that of the related art, in consideration. ~hen.thaev erage size of the
20 cementites is less than 0.10 pm, the lower limit is more than 0.10 pm because the grain
dispersion is enhanced and TS is higher. Particularly, in the periphery of the acicular
cementites, the stress may be localized during the cold-working and the fracture tends be
initiated.
[0062]
When the spheroidizing ratio of the carbides is less than 90% and grain size of
carbides is less than 0.4 pm, since there is a case of fixture origin due to the local stress
concentration, the cold workability deteriorates. Thus, the spheroidizing ratio of the
carbides is to be 90% or more. The cold workability is improved with an increase in the
5 spheroidizing ratio of the carbides. Ideally, it is most preferable to increase the
spheroidizing ratio of the carbides to 100%. On the other hand, in order to control all
the cementites to be spheroidal, the manufacturing processes need to be severely
controlled. Thus, in order to suppress a decrease in yieId, the upper limit of the
spheroidizing ratio of the carbides may be 99.5% or less.
10 [0063]
As explained above, during heating at quenching, the coarse cementites requires
time until dissolving is finished, and act as suppress the growth of the austenites grain.
Particularly, in a case in which the cementites whose sizes are 1.5 times or more of the
average size of the carbides are present 30% or more in number fraction, pinning effect
15 of the cementites is remarkable in the growth of the austenites grain and local grain
growth is suppressed, or the like, it is caused to increase the degree of the duplex grains.
The smaller ratio of the coarse cementites, the better quenching ability. Thus, the
optimum limit may be 0%. On the other hand, in order to control the number fraction of
coarse carbides to 0%, the manufacturing processes require to be severely controlled,
20 Therefore, in order to reduce the manufacturing costs, the lower limit may be 5% or
more.
[0064]
When the austenites diameter is not uniform immediately before quenching, in
order to start the deformation of the austenites having small diameter during cooling at
25 quenching, the deformation distortion is not uniformed in the steel sheet and heating
23
distortion is increased, and quenching stability is deteriorated. In order to satisfy both
of workability and stability, it is necessary that the controlling of average size of the
cementites of the steel sheet is 0.4 pm or less, the number fraction of cementites whose
sizes are 1.5 times or more of the average size of the cementites is 30% or less, and
5 spheroidizing ratio is 90% or more. It is considerable that the fraction of coarse
austenites is preferable small as possible, more preferably, 0%. Meanwhile, in order to
make the ratio of coarse austenites 0%, more severe control for the ferrite grain size and
the cementites grain distribution are required. However, in order to reduce the
manufacturing cost, a lower limit is 1% or more, preferably, 3% or more.
10 [0065]
It is necessary that the average ferrite grain size of the steel sheet is 10 pm or
more.
COO661
The less the ferrite grain size, the higher yield strength, since the YR is increased,
15 the ferrite average grain size may be 10 pm or more. FIG. 3 is a diagram illustrating a
relationship between the number fractions of cementites 1.5 times more than average size
of cementites before quenching and an area fraction % (an area fraction% of abnormal
austenite) of grains having the grain-size number which is different by two or more from
the average grain size in the austenite structure at quenching. When the ferrite grain
20 size is lower than 10 pm, the number fraction of the coarse austenites during heating at
quenching is increased. In addition, when the number fraction of the cementites having
a grain size 1.5 times more than the average cementite diameter is excessive 30% even
the ferrite grain size is 10 pm or more, the number fraction of coarse austenites is
increased during heating at quenching. In the coarse austenites structure during heating
at quenching, the sum of the crystal grains having a grain-size number which is different
by two or more corresponding to the average grain size is more than 30%, the degree of
the duplex grain is increased. Since there are differences of deformation start time and
finish time according to respective the austenites grain size during cooling at quenching,
5 a structure uniformity of quenched material is deteriorated, a defective shape is brought
about due to an increase in heat treatment distortion the quenching stability is
deteriorated. As described above, it is important to control the ferrite and the carbides
in order to ensure the quenching stability.
[0067]
In addition, it is also important to control the ferrite for ensure the quenching
stability. When heating at quenching, nucleation of the austenites mainly occurs on the
grain boundary, According to increasing of heating temperature or maintain time, the
austenites become growth in the ferrites grain. As the nucleation site of the austenites is
increased, the fraction of the fine austenites is increased. In order to increase the degree
15 of duplex grain, it is preferable that the nucleation site of the austenites be small.
Particularly, when the grain size of the ferrite of the steel sheet is less than 10 pm, the
nucleation site of the austenites during heating at quenching is increased. In order to
increase the degree of the duplex grain, the ferrite grain size is required to be controlled
10 pm or more. Meanwhile, the ferrite grain size of the steel sheet is coarsen, surface
20 roughness easily occurs during cold-working, and product appearance may deteriorate.
Therefore, the upper limit of the average ferrite grain size may be 100 pm, and more
preferably, 80 pm or less.
[0068]
The structure of the hot-rolled sheet and annealed sheet are observed by the
scanning electron microscope. Since the lamellar thickness of cementites in the pearlite
after the hot-rolling is fine in a range of 0.02 to 0.5 pm, the pearlite is observed at a
magnification of at least 3000-fold to 10000-fold. An average thickness of larnellae
perpendicular to the observed section is measured. The lamellar thickness is measured
5 by a linear interception method and an average lamellar thickness of cementites is
calculated by averaging measurement values obtained fkom measured areas of 30 places
or more which are randomly selected.
[0069]
In addition, in the structure observation of the annealed sheet each area of
10 carbides is measured in detail in the regions which are at least 16 selected visual fields
where the carbides (cementites) of 500 pieces or more are included on each
micxostnrctural observed section by using a scanning electron microscope at a
magnification of 3000-fold to 10000-fold in order to influence to quenching stability by
the number of finely segregated cementites, and in some cases, at a magnification of
1 5 30000-fold. Thereafter, from an average area per carbide, a diameter is calculated as
the average size of carbides by hypothesizing that the shape is a circle. In addition, a
carbide in which a ratio of a major axis to a minor axis is 3 or more is regarded as an
acicular carbide, a carbide in which the ratio is less than 3 is regarded as a spheroidal
carbide, and thereby, the number of the acicular carbides and the number of the
20 spheroidal carbides are measured.
It is preferable that the ferrite gain size of the steel sheet is also measured by
using the scanning electron microscope. Micrographs are taken fkom at least 5 visual
fields at a magnification where the ferrite gains of 200 pieces or more are included. The
25 number of the ferrite grains included in the micrographs is counted. Here, a ferrite grain
which is entirely included in the micrographs is regarded as 1 piece and a ferrite grain
which is partially included in the micrographs is regarded as 0.5 pieces. An area per
ferrite grain is obtained by dividing the observed area by the counted number of the
ferrite grains. A square root of the obtained area is regarded as a ferrite grain size and
5 the average thereof is regarded as an average ferrite grain size.
The austenites grain size at quenching is preferably uniformed. When the
duplex grains and quenching stability are deteriorated, such as shape distortion is not
uniformed in the steel material or heat treatment distortion is occurred, in order to start
10 formation of the austenites grain size having a small grain size. Particularly, when the
total area fractions having grain-size number different by two or more of the average
grain size is more than 30% in the austenites structure at quenching, the deformation is
increased at quenching. Therefore, in order to ensure the quenching stability, it is
necessary to control that the sum of crystal grain having grain-size number different by
15 two or more of the average grain size is less than 30% in the austenites structure at
quenching.
[0072]
Next, a method for manufacturing a steel sheet according to an embodiment of
the present invention will be described.
[0073]
In the embodiment, by using a material which includes the component
composition satisfying the range described in the above embodiment and by researching
conditions at the cooling after finish-rolling in hot-rolling, the optimum hot-rolled sheet
is accomplished. As explained above, a medium carbon steel sheet which is particularly
25 excellent in cold workability and excellent in quenching stability can be manufactured by
cold-rolling under small cold-rolling reduction and by annealing at a low temperature for
a short time (for example, one cold-rolling and one annealing). Hereinafter, the detailed
description of method for manufacturing the same will be described.
[0074]
(Hot-Rolling)
In hot-rolling, the casting which has the composition satisfying the range
described in the above embodiment may be directly hot-rolled or may be hot-rolled after
heating to form pearlite by pearlitic transformation. Thereafter, coiling is conducted in
a temperature range of 400°C to 580°C in order to obtain a hot-rolled sheet in which an
10 average lamellar thickness of cementites in the pearlite is 0.02 to 0.5pm. The
cementites tend to be spheroidized during the annealing after cold- rolling with a
decrease in the lamellar thickness. Thus, it is preferable that the lamellar thickness of
cementites is as thin as possible. On the other hand, when the lamellar thickness is less
than 0.02 pm, the pearlite is excessively hardened, and thus, strain is hardly introduced
15 into the pearlite during the cold-rolling. As a result, the ferrite is hardly grown during
the subsequent annealing so that the lower YR may not be accomplished. Therefore, a
lower limit of the lamellar thickness may be 0.02 pm. In addition, when the lamellar
thickness is more than 0.5 pm, the spheroidization of the cementites is not promoted
during the annealing after cold-rolling and a lower YR may not be accomplished. Thus,
20 an upper limit of the lamellar thickness may be 0.5 pm. In order to obtain the
microstructure after the hot-rolling, the cooling after the finish-rolling is controlled as
follows and a coiling temperature is controlled as follows. Specifically, the steeI is
air-cooled for 2 seconds or longer and 10 seconds or shorter immediately after the
finish-rolling (hot-rolling). Subsequently, the steel is cool to a pearlite range of 480 to
600°C fiom the finish temperature of the air-cooling at an average cooling rate of 10 to
80 "Us. Subsequently, the steel is coiled in a temperature range of 400°C to 580°C.
By controlling the cooling and by controlling the coiling temperature, the
above-described microstructure after the hot-rolling can be stably obtained.
[0075]
(Cold-rolling reduction)
Cold-rolling is performed on the hot-rolling sheet prepared by above-mentioned
intended hot-rolling in a range of 5 to less than 30% of cold-rolling reduction.
[0076]
10 In the cold-rolling, by applying strain to the microstructure of the hot-rolled
sheet, a strain difference between each structure is increased. By the strain difference,
grain growth and recrystallization are promoted during the annealing. In order to obtain
an effect of the strain introduction, the cold-rolling reduction is to be 5% or more.
[0077]
In addition, by increasing the cold-rolling reduction, it is possible to refine the
carbides. When the cold-rolling reduction is less than 5%, the carbides precipitated in
the hot-rolling are not sufficiently fractured. Thus, the carbides may remain in the
microstructure, the average size of the carbides may increase. On the other hand, when
the cold-rolling reduction is 30% or more, the strength is excessively increased by
20 refining the ferrite due to the recrystallization and therefore, an upper limit of the
cold-rolling reduction is to be less than 30%.
[0078]
(Annealing temperature and time of cold-rolling sheet annealing)
The steel sheet which was hot-rolled, picked, and cold-rolled is annealed and is
aim to be obtained carbides structure and the low YR type steel sheet.
[0079]
In the annealing, pro-eutectoid ferrite is recrystallized by using the difference in
an amount of strain introduced in each structure, and coarse grains are formed by grain
5 growth. Thus, the steel sheet is soften.
[0080]
In addition, the carbides before the annealing are uniformly dispersed in the
microstructure by a relatively low coiling temperature and also are controlled to be
exceedingly fine after finishing the cold-rolling by the cold-rolling. As a result,
10 immediately after starting the annealing, the carbides begin to dissolve and the
spheroidiiation is promoted. As described above, since the fine carbides are quite
uniformly dispersed by the above-described manufacturing conditions, the
spheroidization of the carbides immediately progresses, and a large number of
exceedingly fine spheroidal carbides are simultaneously formed. In the annealing, the
15 annealing conditions such as a lower temperature and a shorter time are preferable. The
optimum annealing conditions are to be at a temperature of 690°C and for a time of 20 to
40 hours.
[0081]
It is preferable that the annealing is conducted by means of box-annealing from
20 a viewpoint of controllability of an annealing atmosphere. AIthough the annealing
atmosphere is not particularly limited, the annealing atmosphere may be under a
hydrogen concentration of 95% or more, a dew point in a range of up to 400°C of lower
than -20°C, and a dew point in a range of higher then 400°C of lower than -40°C. In the
case, it is possible to further suppress unevenness in the properties in the width direction
1
[Document Type] Specification
[Title of the Invention] STEEL SHEET EXCELLENT WORIGWILITY AND
METHOD FOR MANUFACTURING THE SAME, AND MEMBER OBTAINED BY
QUENCHING THE STEEL SHEET
5 [Technical Field]
[OOOl]
The present invention relates to a medium carbon steel sheet excellent in cold
workability, particularly in cold forgeability, and a method for manufacturing the same,
and a member obtained by quenching the steel sheet.
10 [Background Art]
[0002]
A medium carbon steel sheet is widely used for a material of a chaii, a gear, a
clutch, cutlery and the like. In a use of the medium carbon steel sheet as a material,
since the medium carbon sheet is formed on a predetermined member, formability is
15 required. Recently, a manufacturing technique is improved such that formation method
having higher formability than the related art, such as forging steel sheet, is adopted. A
difficult process or the like is mainly performed on a sheet material which has thickness
of 2 mm or more, it is required that a sheet has extremely high degree of entire width and
length. In addition, the sheet is a member which is hardened by a heating process from
20 quenching and heating at quenching afier forming. In recent years, energy saving has
been socially anticipated to consider the environment, a movement in which a quenching
process is carried out at a low temperature and in a short time has been promoted.
Accordingly, afier cold-working, the steel sheet to be quenched is required to have both
of workability and hardenability.
25 [0003]
30
of the steel sheet. In addition, under a nitrogen atmosphere, it is possible to
manufacture the steel sheet having the intended properties.
Example
[0082]
Next, Examples will be described.
[0083 ]
The condition in the examples is an example condition employed to confirm the
operability and the effects of the present invention, and therefore, the present invention is
not limited to the example condition. The present invention can employ various
10 conditions as long as the conditions do not depart from the scope of the present invention
and can achieve the object of the present invention.
[0084]
Steel pieces having compositions shown in Tables 1 were hot-rolled, coiled at
500°C, annealed for 24 hours at 680°C and cooled to room temperature in order to obtain
15 hot-rolled sheets. In addition, the obtained hot-rolled sheets were cold-rolled under a
cold-rolling reduction of 15%, and thereafter, the steel sheets were evaluated in order to
obtain various steel sheets having microstructures shown in Table 2. In addition, by
using the steel pieces having the components composition shown in Table 1, steel sheets
were prepared under conditions of hot-rolling and cold-rolling of Tables 3 and 4. A
20 lamellar thickness of cementites in pearlite of the hot-rolled sheets, a size distribution of
the carbides of the annealed sheets, and a ferrite grain size of the annealed sheets were
measured, and strength tests were conducted. The steel sheets after holding in the
annealing were furnace-cooled.
[0085]
[Table I]

[0086]
In the cold workability test, a circular plate having a diameter of 100 mm and
thickness 4 rnm t as shown in FIG 2 was formed into a cup shape having a diameter of
5 60 mm and a height of 28 mm, the formation load or the presence of the fracture was
evaluated. When the formation load is more than 30 t or the fracture is occurred, the
steel was judged as "X" which indicated inferior cold workability. On the other hand,
when the formation load is less than 30 t and no fixture occurred, the steel was judged
as "0" which indicated superior cold workability.
10 [0087]
In the annealing test, a test material having a sheet width of 5 t and was heated
from room temperature at a heating rate of 100°C Is by a frequency of 78 KHz, was held
at 950°C for 10 seconds, and immediately, was rapid-cooled to room temperature at a
cooling rate of 1 OO°C Is or faster. The grain distribution of austenitei of the annealing
15 material is measured in austenites structure at annealing, the sum of crystal grain having
size different by two or more according to the grain-size number based on the average
grain size indicates as an index of duplex grains. Therefore, when the index of duplex
grain was more than 30%, the quenching stability was judged inferior so that shown as a
comparative steel.
20 [0088]
In steel sheets Nos. C-0, E-0, H-0, L-0, and N-0 in Table 2, the average size of
the cementites of the present invention was 0.4 pm or less, the number fraction of
carbides whose sizes were 1.5 times or more of the average size of the cementites was
30% or less, the invention steel satisfies the spheroidizing ratio of the cementites was
90% or more, and the average ferrite grain size was 10 pm or more. In the invention
steel, the workability and the quenching stability were excellent.
In steel sheets Nos. A-0, B-0, D-0, F-0, G-0, 1-0, J-0, K-0, M-0 and 0-0 in Table
5 2, at least one condition of the composition, the average size of the cementites (pm), the
number fraction of coarse carbides (%), the spheroidizing ratio of the cementites (Oh), and
the average ferrite grain size (pm) of the steel sheet was insufficient. Therefore, in the
comparative steel, the cold workability was insufficient due to the fracture initiated from
cluster of cementites, the fracture initiated from cementites, the fracture initiated from
10 MnS, the intragranular fixture, increasing of the formation load or the like.
[0090]
C-1 , C-2, E-1 , H-1 , L-2, N-1 , Z-1,Z-2, AA-2, AB-2, AC-2, AD-1, AD-2, AE-2,
AF-1, AG-1, AG-2, AH-2, AI-1, AJ-2, AK- 1, AL-1, AM-1, and AM-2 in Tables 3 (3-1
and 3-2) and 4 (4-1 and 4-Z), the cold workability was excellent and the quenching
15 stability was excellent such that the index of duplex grain was 30% or less.
[009 11
D-1, D-2, and K-2 are comparative steel that the tensile strength (TS) is more
than SSOMPa, and the cold workability was insufficient.
' J-1,J-2, K- 1, P-1 , and P-2 are comparative steel that the tensile strength is more
20 than 550MPa, the yield rate (YR) is more than 60 %, and the cold workability was
insufficient.
[0092]
E-2 is comparative steel that the spheroidizing ratio of the cementites is 90% or
less and workability is insufficient.
F- 1, F-2, G- I, G-2, H-2, M-1 ,M -2,X - 1, X-2, Y- 1, and Y-2 are comparative steel
that the index of duplex grain is more than 30% after quenching and the quenching
stability is insufficient,
V-2, W- 1, W-2, AA-1, AB-1, AC-1, AE-1, AF-2, AH-1, A1 -2, AK-2, AL-2, AN- 1 and
AN-2 are comparative steel that the YR is more than 60% and the workability is
insuacient, and after quenching, the index of duplex grain is more than 30% such that
1 0 quenching stability is insmcient.
[Table 21
Note
'The bolLIlalicr ln the table arc out ofrange.
annealing is less than 5 to 30%.
[00 1 71
(8) The manufacturing method for a medium carbon steel sheet excellent in
cold workability according to (6) or (7), in which, after cold-rolling, annealing the steeI
5 in a time range of 5 to 40 hours at temperature of 650 to 720°C.
[OO 181
(9) A member that performs quenching on a medium carbon steel sheet
according to the Claims 1 to 5 after cold-working, the member includes: when quenching,
austenites structure immediately before quenching in which the sum of area fraction of
10 crystal grains having a size different by two or more from the grain-size number based on
the average grain size is in a range within 30%.
[00 191
According to the present invention, there is to provide the medium carbon steel
sheet excellent in cold workability in severe cold-working and in thickness accuracy, and
15 the method for manufacturing the same, and to provide the quenched member using the
steel sheet.
[Brief Description of the Drawings]
[0020]
[Figure 11 FIG 1 is a diagram illustrating a relationship of cold workability, an
20 average size of cementites before cold-working and a number fraction of coarse
cementites.
[Figure 21 FIG 2 is a diagram illustrating a shape of a test piece cold-working
to evaluate cold workability.
[Figure 31 FIG 3 is a diagram illustrating a relationship between the number
25 fraction of cementites grain size which is 1.5 times more than average size of cementites
[0096]
[Table 3-11
688 31 a2 12.1 957 396
699 8 013 181 917 576
741 19 0.19 16.8 EUIZ 38e
--
602 37 CUB 97.4 91.1 746
658 31 Q13 18.2 91.4 95.3
697 37 0,lS 16.7 92.8 1 381
656 28 (128 20,6 91.2 [ 279
[Table 3-21
Steel
'SPe
B n d m ~ ~ p l n d L I c l c r ladox d
\'P TS
m gr~tiusi ze v i md i.&r II.~ I ~ I I H NOPLDbU15 pk) Notc
(UPa) (h4Ps) h 2 m m
(r m) (s m)
858 bW / 84 17 1 x wMhv.w
3@ S l l j & ? 5 169 8 4 38 8 x $,Q ~-.*hrrhd
* Tl~ch old-Itnlics in thc tnhlc nrc out ofrnngc.
[Table 4- 11
' The bold-Ilnlica in tbe (able are out 01 rnoge.
[0099]
[Table 4-21
* Tbe bold-Hnlkt in tbr ublo am out or mge.
[Document Type] Claims
[Claim 11
A medium carbon steel sheet excellent in cold workability comprising, by
mass%,
5 C: 0.10 to 0.80%,
Si: 0.01 to 0.3%,
Mn: 0.3 to 2.0%,
P: 0.001 to 0.03%,
S: 0.0001 to 0.01%,
10 AI: 0.005 to 0.10%,
N: 0.001 to 0.01%,
and a balance consisting of Fe and unavoidable impurities,
wherein an average size of cementites is 0.4 pm or less, a number fraction of
cementites whose sizes are 1.5 times or more of the average size of the cementites in a
15 total number of the carbides is 30% or less, spheroidizing ratio of the cementites is 90%
or more, an average grain size of a ferrite is 10 pm or more.
[Claim 21
The medium carbon steel sheet excellent in cold workability according to Claim
1 further including at least one or two selected from, by mass%,
20 Cr: 0.01 to IS%,
B: 0.001 to 0.01%,
Nb: 0.01 to 0.5%,
Mo: 0.01 to 0.5%,
V 0.01 to 0.5%,
Ti: 0.01 to 0.3%,
Cu: 0.05 to 0.5%,
W: 0.01 to 0.5%,
Ta: 0.01 to 0.5%.
5 [Claim 31
The medium carbon steel sheet excellent in cold workability according to Claim
1 or 2 fhther including at least one or two selected from,
Ni: 0.01 to 0.5%,
Mg: 0.0005 to 0.003%,
10 Ca: 0.0005 to 0.003%,
Y 0.001 to 0.03%,
Zr: 0.001 to 0.03%,
La: 0.001 to 0.03%,
Ce: 0.001 to 0.03%.
15 [Claim 41
The medium carbon steel sheet excellent in cold workability according to any
one of Claims 1 to 3,
wherein a yield ratio (YR) before cold-working may be 60% or less.
[Claim 51
20 The medium carbon steel sheet excellent in cold workability according to any
one of Claims 1 to 4,
wherein a strength thereof before cold-working is TS 550 MPa or less.
[Claim 61
A manufacturing method for a medium carbon steel sheet excellent in cold
25 workability comprising:
4 1
when continuous casting slab of components according to any one of the Claims
1 to 5 is directly hot-rolled or is hot-rolled after heating, coiling is conducted in a
temperature range of 400°C to 580°C by pearlitic transformation, and one cold-rolling
and one annealing is conducted on a hot-rolled sheet in which an average lamellar
5 thickness of cementites in the pearlite is 0.02 to 0.5 ym.
[Claim 71
The manufacturing method for a medium carbon steel sheet excellent in cold
workability according to Claim 6,
wherein the cold-rolling reduction of cold-rolling after annealing is in a range of
10 5 to less than 30%.
[Claim 81
The manufacturing method for a medium carbon steel sheet excellent in cold
workability according to Claim 6 or 7,
wherein, afier cold-rolling, annealing the steel in a time range of 5 to 40 hours at
15 temperature of 650 to 720°C.
[Claim 93
A member that perfoms quenching on a medium carbon steel sheet according to
the Claims 1 to 5 afier cold-working, the member comprising:
when quenching, austenites structure immediately before quenching in which
20 the sum of area fraction of crystal grains having a size different by two or more from the
grain-size number based on the average grain size is in a range within 30%.