TECHNICAL FIELD
[0001] The present invention is concerned with a steel sheet suitable for applications to
automobile parts or the like and in particular, relates to a steel sheet excellent
in fine blanking performance suitable for the uses to which fine blanking working
(hereinafter also referred to as "FB working") is applied.
BACKGROUND ART
[0002] In manufacturing complicated mechanical parts, from the viewpoints of an improvement
in dimension precision, a reduction in manufacturing process, and the like, it is
known that fine blanking working is an extremely advantageous working method as comparing
with machining working.
[0003] In usual blanking working, a tool-to-tool clearance is from approximately 5 to 10
% of a thickness of a metal sheet as a material to be blanked. However, the fine blanking
working differs from the usual blanking working and is a blanking working method of
not only setting up the tool-to-tool clearance extremely small as substantially zero
(actually, not more than approximately 2 % of the thickness of the metal sheet as
a material to be blanked) but also making a compression stress act on a material in
the vicinity of a tool cutting blade. Then, the fine blanking working has the following
characteristic features.
- (1) The generation of a crack from the tool cutting blade is inhibited, and a fracture
surface seen in usual blanking working becomes substantially zero, whereby a smooth
worked surface (blanked end surface) in which the worked surface is a substantially
100 % shear surface is obtained.
- (2) The dimensional precision is good.
- (3) A complicated shape can be blanked by one process. However, in the fine blanking
working, a working ratio which the material (metal sheet) receives is extremely severe.
Also, in the fine blanking working, since the working is carried out at a tool-to-tool
clearance of substantially zero, there is involved a problem that a load to a mold
becomes excessive so that a mold life is shortened.
[0004] For that reason, materials to which the fine blanking working is applied are required
to not only have excellent fine blanking performance but also prevent a reduction
in mold life.
[0005] In response to these requirements, for example, Patent Document 1 proposes a high
carbon steel sheet excellent in fine blanking performance, which has a composition
containing from 0.15 to 0.90 % by weight of C, not more than 0.4 % by weight of Si
and from 0.3 to 1.0 % by weight of Mn, has a microstructure with a cementite having
a spheroidization ratio of 80 % or more and an average grain size of from 0.4 to 1.0
µm scattered in a ferrite matrix and has a notch tensile elongation of 20 % or more.
According to a technology described in Patent Document 1, it is described that the
fine blanking performance is improved and that the mold life is also improved.
[0006] However, the high carbon steel sheet described in Patent Document 1 involved a problem
that fabrication performance after the fine blanking working is inferior.
[0007] Also, Patent Document 2 proposes a steel sheet for fine blanking prepared by applying
proper hot rolling to a billet containing from 0.08 to 0.19 % of C and proper amounts
of Si, Mn and Al and containing from 0.05 to 0.80 % of Cr and from 0.0005 to 0.005
% of B into a steel sheet. It is described that the steel sheet described in Patent
Document 2 is a steel sheet which is low in a yield strength, high in an impact value,
excellent in fine blanking performance, high in an n-value in a low strain region,
excellent in combined formability and excellent in quenching property at short-time
rapid heating. However, Patent Document 2 does not show concrete evaluation regarding
the fine blanking performance. Also, the steel sheet described in Patent Document
2 involved a problem that fabrication performance after the fine blanking working
is inferior.
[0008] Also, Patent Document 3 proposes a high carbon steel sheet excellent in flow forming
and fine blanking working, which has a composition containing from 0.15 to 0.45 %
of C, with the contents of Si; Mn, P, S, Al and N being adjusted at proper ranges
and has a structure having a fractional ratio of (pearlite + cementite) of not more
than 10 % and an average grain size of ferrite grain of from 10 to 20 µm. It is described
that the high carbon steel sheet described in Patent Document 3 is excellent in fine
blanking performance and is improved in mold life in the fine blanking working. However,
the high carbon steel sheet described in Patent Document 3 involved a problem that
fabrication performance after the fine blanking working is inferior.
[0009] Furthermore, it is hard to say that all of the steel sheets described in Patent Document
1, Patent Document 2 and Patent Document 3 are not provided with satisfactory and
thorough fine blanking performance in the fine blanking working under a recent severe
working condition. Also, problems that the mold life is not thoroughly improved and
that fabrication performance after the fine blanking working is inferior still remained.
[0010] At the beginning, the fine blanking working had been applied to parts to which working
is not applied after fine blanking working even among gear parts and the like. However,
recently, the application of fine blanking working to automobile parts (for example,
reclining parts) tends to expand, and its application to parts which require stretch
flanging working, bulging, etc. is investigated. For that reason, steel sheets which
are not only excellent in fine blanking performance but also excellent in fabrication
performance after fin blanking working in stretch flanging working, bulging, etc.
are eagerly desired as automobile parts.
[0011] As a technology for improving stretch flanging workability, there have been made
a number of proposals up to date. For example, Patent Document 4 proposes a wear resistant
hot rolled steel sheet excellent in stretch flanging property, which has a composition
containing from 0.20 to 0.33 % of C, with the contents of Si, Mn, P, S, sol. Al and
N being adjusted at proper ranges and further containing from 0.15 to 0.7 % of Cr
and has a ferrite-bainite mixed structure which may contain pearlite. In the hot rolled
steel sheet described in Patent Document 4, it is described that by taking the foregoing
structure, a hole expansion ratio becomes high, whereby the stretch flanging property
is improved. Also, Patent Document 5 proposes a high carbon steel sheet excellent
in stretch flanging property, which has a composition containing from 0.2 to 0.7 %
of C and has a structure in which a cementite average particle size is 0.1 µm or more
and less than 1. 2 µm and a volume ratio of a cementite-free ferrite grain is not
more than 15 %. In the high carbon steel sheet described in Patent Document 5, it
is described that the generation of a void on an end surface at the time of blanking
is inhibited, that the growth of a crack in hole expansion working can be made slow
and that the stretch flanging property is improved.
[0012] Also, Patent Document 6 proposes a high carbon steel sheet excellent in blanking
performance and quenching property, which has a composition containing 0.2 % or more
of C and has a structure composed mainly of ferrite and a cementite and having a cementite
particle size of not more than 0.2 µm and a ferrite grain size of from 0.5 to 1 µm.
It is described that according to this, both blanking performance and quenching property
which are determined by a burr height and mold life are improved.
Patent Document 1: JP-A-2000-265240
Patent Document 2: JP-A-59-76861
Patent Document 3: JP-A-2001-140037
Patent Document 4: JP-A-9-49065
Patent Document 5: JP-A-2001-214234
Patent Document 6: JP-A-9-316595
DISCLOSURE OF THE INVENTION
[0013] However, all of the technologies described in Patent Document 4 and Patent Document
5 are those made on the assumption that the conventional blanking working is applied
but not those made taking into consideration the application of fine blanking working
in which the clearance is substantially zero. Accordingly, it is difficult to ensure
similar stretch flanging property after the severe fine blanking working, and even
when the stretch flanging property can be ensured, there is encountered a problem
that the mold life is short.
[0014] Also, in the technology described in Patent Document 6, it is necessary that the
ferrite grain size is in the range of from 0.5 to 1 µm; and it is difficult to stably
manufacture a steel sheet having such a ferrite grain size on an industrial scale,
resulting in a problem that the product yield is reduced.
[0015] In view of the foregoing problems of the conventional technologies, the invention
has been made, and an object thereof is to provide a steel sheet excellent in fine
blanking performance and also excellent in fabrication performance after fine blanking
working and a manufacturing method of the same.
[0016] In order to achieve the foregoing object, the present inventors made extensive and
intensive investigations regarding influences of a metallographic structure against
fine blanking performance (hereinafter abbreviated as "FB performance"), especially
influences against morphology and distribution state of ferrite and a cementite.
[0017] As a result, it has been found that the FB performance and the mold life are closely
related with a particle size of a cementite present in a ferrite grain and a ferrite
grain size. Then, it has been found that when a raw steel material having a composition
of a prescribed range is formed into a hot rolled steel sheet having a substantially
100 % pearlite structure by making a finish rolling condition of hot rolling and a
condition of subsequent cooling proper, which is then subjected to hot rolling annealing
under a proper condition, thereby converting the metallographic structure into a (ferrite
+ cementite) (spherical cementite) structure in which a cementite amount in ferrite
grain is controlled such that an average ferrite grain size is not more than 10 µm,
a spheroidization ratio of a cementite is 80 % or more and a ratio of an area of a
cementite present on a ferrite grain boundary to an area of the whole of cementites
is 40 % or more, the FB performance and the mold life are remarkably improved. Also,
it has been newly found that when the cementite amount in ferrite grain is controlled,
the fabrication property after the FB working is remarkably improved.
[0018] In the FB working, the material is worked in a state of zero clearance and compression
stress. For that reason, after receiving large deformation, a crack is generated in
the material. When a number of cracks are generated during large deformation, the
FB performance is largely reduced. In order to prevent the generation of a crack,
it is said that spheroidization of a cementite or miniaturization of a cementite particle
size is important. However, in the FB working, in the case where even a 100 % spheroidized
fine cementite is present in the ferrite grain, the generation of a fine crack is
unavoidable. For that reason, the present inventors thought that in the case where
stretch flanging working is further applied after the FB working, fine cracks generated
at the time of the FB working are connected to each other, leading to a reduction
in the stretch flanging property. Also, with respect to the mold life, the present
inventors assumed that when a number of cementites are present in the ferrite grain,
wear of a cutting blade is accelerated, leading to a reduction in the mold life.
[0019] First of all, the experimental results on a basis of which the invention has been
made are described.
[0020] A high steel slab (corresponding to S35C) containing 0.34 % of C, 0.2 % of Si and
0.8 % of Mn in terms of % by mass was heated at 1,150°C and then subjected to hot
rolling consisting of rough rolling of 5 passes and finish rolling of 7 passes, thereby
preparing a hot rolled steel sheet having a thickness of 4.2 mm. Incidentally, a rolling
termination temperature was set up at 860°C; a coiling temperature was set up at 600°C;
and after the finish rolling, the steel sheet was cooled while changing a cooling
rate from 5°C/s to 250°C/s. Incidentally, in the case where cooling (forced cooling)
other than air cooling was carried out, a cooling stopping temperature was set up
at 650°C. Subsequently, the hot rolled steel sheet was subjected to pickling and then
to batch annealing (at 720°C for from 5 to 40 hours) as hot rolled sheet annealing.
With respect to the steel sheet to which the hot rolled sheet annealing had been thus
applied, not only its metallurgical structure was observed, but also its FB performance
was evaluated.
[0021] In the observation of the metallurgical structure, a specimen was collected from
the obtained steel sheet; a cross section parallel to a rolling direction of the subject
specimen was polished and corroded with nital; and with respect to a position of 1/4
of the sheet thickness, the metallurgical structure was observed by a scanning electron
microscope (SEM), thereby measuring a ferrite grain size and a spheroidization ratio
of a cementite.
[0022] With respect to the ferrite grain size, an area of each ferrite grain was measured,
and a circle-corresponding size was determined from the resulting area and defined
as a grain size of each ferrite grain. The thus obtained respective ferrite grain
sizes were arithmetically averaged, and its value was defined as a ferrite average
grain size of that steel sheet. Incidentally, the number of measured ferrite grains
was 5,000 for each.
[0023] Also, a maximum length
a and a minimum length
b of each cementite were determined in each field of the structure observation (magnification:
3,000 times) by using an image analyzer; its ratio a/b was computed; and the number
of cementite grains with a/b of not more than 3 was expressed by a proportion (%)
against the total number of measured cementites, thereby defining it as a spheroidization
ratio (%) of cementite. Incidentally, the number of measured cementites was 9,000
for each.
[0024] Also, in each field of the structure observation, a cementite present on the ferrite
grain boundary and a cementite present in the ferrite grain were discriminated from
each other; with respect to the cementites present per unit area, an occupied area
S
on of a cementite present on the ferrite grain boundary and an occupied area S
in of a cementite present in the ferrite grain were measured by using an image analyzer;
and an amount (S
gb) of a ferrite intergranular cementite as defined by the following expression:

was computed. Incidentally, the area of the cementite particle was measured in 30
fields (magnification: 3,000 times) for each.
[0025] Also, a specimen (size: 100 × 80 mm) was collected from the obtained steel sheet
and subjected to a fine blanking test (FB test). The FB test was carried out by blanking
a sample having a size of 60 mm × 40 mm (corner radius R: 10 mm) from the specimen
by using a 110t hydraulic press machine under a lubricious condition of a clearance
of 0.060 mm (1.5 % of the sheet thickness) and a working pressure of 8.5 tons. With
respect to an end surface (blanked surface) of the blanked sample, a surface roughness
(ten-point average roughness Rz) was measured, thereby evaluating the FB performance.
Incidentally, with respect to the specimen, in order to eliminate influences of a
deviation in sheet thickness against the clearance, the both surfaces were equally
ground in advance, thereby regulating the sheet thickness at 4.0 ± 0.010 mm.
[0026] With respect to the measurement of the surface roughness, as illustrated in Fig.
4, in each of four end surfaces (sheet thickness surfaces) other than R parts, a region
within a range of from 0.5 mm to 3.9 mm of the surface in the punch side in the sheet
thickness direction and 10 mm in parallel to the surface (X direction) was scanned
35 times at a pitch of 100 µm in the sheet thickness direction (t direction) by using
a contact probe profilometer, and a surface roughness Rz in each scanning line was
measured according to JIS B 0601-1994. Furthermore, with respect to the surface roughness
Rz on the measured surface, Rzs in the respective scanning lines were summed up, and
an average value thereof was employed. The four end surfaces were measured in the
same method as described above, and an average surface roughness Rz ave (µm) defined
according to the following expression: Rz ave = (Rz 1 + Rz 2 + Rz 3 + Rz 4)/4 (wherein
Rz 1, Rz 2, Rz 3 and Rz 4 each represents Rz on each surface) was computed.
[0027] In general, the case where the appearance of the fracture surface on the blanked
surface is not more than 10 % is defined as "excellent in FB performance". However,
in the invention, the case where the average surface roughness Rz ave is small as
10 µm or less is defined as "excellent in FB performance". Incidentally, in the case
of measuring a surface roughness of a specimen having a sheet thickness different
from the foregoing, the measurement may be carried out by repeatedly performing scanning
in a pitch of 100 µm in a sheet thickness direction in a region within a range of
approximately { (sheet thickness (mm)) - 0.1 mm} in the sheet thickness direction
of 0.5 mm from the surface and 10 mm in parallel to the surface to determine Rz on
each surface, thereby determining Rz ave from Rzs of the respective surfaces.
[0028] The obtained results are shown in Figs. 1 and 2.
[0029] From a relationship between an average surface roughness Rz ave and a spheroidization
ratio of a cementite as shown in Fig. 2, it is noted that when the spheroidization
ratio is 80 % or more, Rz ave is not more than 10 µm, and the FB performance is abruptly
improved. Incidentally, the data shown in Fig. 2 is concerned with the case where
the average ferrite grain size is from approximately 3 to 8 µm. Furthermore, it was
acknowledged that when the spheroidization ratio is 80 % or more and the amount of
an intergranular cementite increases, Rz ave becomes smaller and the FB performance
is remarkably improved. From a relationship between the surface roughness (average
surface roughness: Rz ave) and the amount (S
gb) of a ferrite intergranular cementite as shown in Fig. 1, when a proportion of the
intergranular cementite of the cementites increases such that the amount of a ferrite
intergranular cementite is 40 % or more, Rz ave is not more than 10 µm and the FB
performance is abruptly improved.
[0030] As a result of further extensive and intensive investigations on the basis of the
foregoing knowledge, the invention has been accomplished. That is, the gist of the
invention is as follows.
- (1) A steel sheet excellent in fine blanking performance, which is characterized by
having a composition containing from 0.1 to 0.5 % of C, not more than 0.5 % of Si,
from 0.2 to 1.5 % of Mn, not more than 0.03 % of P and not more than 0.02 % of S in
terms of % by mass, with the remainder being Fe and unavoidable impurities and having
a structure mainly composed of ferrite and cementites, wherein the foregoing ferrite
has an average grain size of from 1 to 10 µm, the foregoing cementite has a spheroidization
ratio of 80 % or more, and of the foregoing cementites, an amount Sgb of a ferrite intergranular cementite which is an amount of a cementite present on
a crystal grain boundary of ferrite and which is defined by the following expression
(1) is 40 % or more:

(wherein Son represents a total occupied area of a cementite present on the ferrite grain boundary
of the cementites present per unit area; and Sin represents a total occupied area of a cementite present in a ferrite grain of the
cementites present per unit area.)
- (2) The steel sheet as set forth in (1), which is characterized in that the cementite
present on the crystal grain boundary of the foregoing ferrite has an average particle
size of not more than 5 µm.
- (3) The steel sheet as set forth in (1) or (2), which is characterized in that in
addition to the foregoing composition, the composition further contains not more than
0.1 % of A1 in terms of % by mass.
- (4) The steel sheet as set forth in any one of (1) to (3), which is characterized
in that in addition to the foregoing composition, the composition further contains
one or two or more members selected from not more than 3.5 % of Cr, not more than
0.7 % of Mo, not more than 3.5 % of Ni, from 0.01 to 0.1 % of Ti and from 0.0005 to
0.005 % of B in terms of % by mass.
- (5) A manufacturing method of a steel sheet excellent in fine blanking performance
including successively applying hot rolling by heating and rolling a raw steel material
to form a hot rolled sheet and hot rolled sheet annealing by applying annealing to
the subj ect hot rolled sheet, which is characterized in that the foregoing raw steel
material is a raw steel material having a composition containing from 0.1 to 0.5 %
of C, not more than 0.5 % of Si, from 0.2 to 1.5 % of Mn, not more than 0.03 % of
P and not more than 0.02 % of S in terms of % by mass, with the remainder being Fe
and unavoidable impurities; and the foregoing hot rolling is a treatment in which
a termination temperature of finish rolling is set up at from 800 to 950°C, after
completion of the subject finish rolling, cooling is carried out at an average cooling
rate of 50°C/s or more, the subject cooling is stopped at a temperature in the range
of from 500 to 700°C, and coiling is carried out at from 450 to 600°C.
- (6) The manufacturing method of a steel sheet as set forth in (5), which is characterized
in that in addition to the foregoing composition, the composition further contains
not more than 0.1% of Al in terms of % by mass.
- (7) The manufacturing method of a steel sheet as set forth in (5) or (6), which is
characterized in that in addition to the foregoing composition, the composition further
contains one or two or more members selected from not more than 3.5 % of Cr, not more
than 0.7 % of Mo, not more than 3.5 % of Ni, from 0.01 to 0.1 % of Ti and from 0.0005
to 0.005 % of B in terms of % by mass.
- (8) The manufacturing method of a steel sheet as set forth in any one of (5) to (7),
which is characterized in that the foregoing hot rolled sheet annealing is carried
out at an annealing temperature of from 600 to 750°C.
[0031] According to the invention, a steel sheet which is not only excellent in FB performance
but also excellent in fabrication property after the FB working can be easily and
cheaply manufactured, thereby giving rise to remarkable effects in view of the industry.
Also, according to the invention, there are brought effects that a steel sheet excellent
in FB performance is provided; an end surface treatment after the FB working is not
necessary; a time of completion of manufacture can be shortened; the productivity
is improved; and the manufacturing costs can be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032]
Fig. 1 is a graph to show a relationship between an FB performance (surface roughness
on a blanked surface) and an amount of a ferrite intergranular cementite.
Fig. 2 is a graph to show a relationship between an FB performance (surface roughness
on a blanked surface) and a spheroidization ratio of a cementite.
Fig. 3 is a graph to show a relationship between an FB performance (surface roughness
on a blanked surface) and an average ferrite crystal grain size.
Fig. 4 is an explanatory view to schematically show a measurement region of surface
roughness on a blanked surface after FB working.
BEST MODES FOR CARRYING OUT THE INVENTION
[0033] First of all, the reasons why the composition of the steel sheet of the invention
is limited are described. Incidentally, the "% by mass" in the composition is expressed
merely as "%" unless otherwise indicated.
C: from 0.1 to 0.5 %
[0034] C is an element influencing the hardness after hot rolling annealing and quenching,
and in the invention, C is required to be contained in an amount of 0.1 % or more.
When the content of C is less than 0.1 %, the hardness required as automobile parts
cannot be obtained. On the other hand, since C is contained in a large amount exceeding
0.5 %, the steel sheet becomes hard, an industrially sufficient mold life cannot be
ensured. For that reason, the content of C was limited to the range of from 0.1 to
0.5 %.
Si: not more than 0.5 %
[0035] Si is an element not only acting as a deoxidizing agent but also increasing the strength
(hardness) due to solution hardening. However, when Si is contained in a large amount
exceeding 0.5 %, ferrite becomes hard, thereby reducing the FB performance. Also,
when Si is contained in an amount exceeding 0.5 %, a surface defect called as red
scale is generated at the hot rolling stage. For that reason, the content of Si was
limited to not more than 0.5 %. Incidentally, the content of Si is preferably not
more than 0.35 %.
Mn: from 0.2 to 1.5 %
[0036] Mn is an element not only increasing the strength of steel due to solution hardening
but also acting effectively in improving the quenching property. In order to obtain
such an effect, it is desirable that Mn is contained in an amount of 0.2 % or more.
However, when Mn is contained excessively in an amount exceeding 1.5 %, the solution
hardening becomes excessively strong so that the ferrite becomes hard, thereby reducing
the FB performance. For that reason, the content of Mn was limited to the range of
from 0.2 to 1.5 %. Incidentally, the content of Mn is preferably from 0.2 to 1.0 %,
and more preferably from 0.6 to 0.9 %.
P: not more than 0.03 %
[0037] Since P segregates on the grain boundary or the like and reduces the performance,
in the invention, it is desirable that P is reduced as far as possible. However, the
content of P of up to 0.03 % is tolerable. For such a reason, the content of P was
limited to not more than 0.03 %. Incidentally, the content of P is preferably not
more than 0.02 %.
S: not more than 0.02 %
[0038] S is an element which forms a sulfide such as MnS and exists as an inclusion in the
steel, thereby reducing the FB performance, and it is desirable that S is reduced
as far as possible. However, the content of S of up to 0.02 % is tolerable. For such
a reason, the content of S was limited to not more than 0.02 %. Incidentally, the
content of S is preferably not more than 0.01 %.
[0039] The foregoing components are a basic composition. However, in the invention, in addition
to the foregoing basic composition, Al and/or one or two or more members selected
from Cr, Mo, Ni, Ti and B can be contained.
Al: not more than 0.1 %
[0040] Al is an element not only acting as a deoxidizing agent but also binding with N to
form AlN, thereby contributing to prevention of an austenite grain from coarseness.
When Al is contained together with B, Al fixes N and B forms BN, thereby bringing
an effect for preventing a reduction of the content of B effective for improving the
quenching property. Such effects become remarkable when the content of Al is 0.02
% or more. However, when the content of Al exceeds 0.1 %, an index of cleanliness
of steel is reduced. For that reason, when Al is contained, it is preferable that
the content of Al is limited to not more than 0.1 %. Incidentally, the content of
Al as an unavoidable impurity is not more than 0.01 %.
[0041] All of Cr, Mo, Ni, Ti and B are an element contributing to an improvement in quenching
property and/or an improvement in resistance to temper softening and can be selected
and contained as the need arises.
Cr: not more than 3.5 %
[0042] Cr is an element effective for improving the quenching property. In order to obtain
such an effect, it is preferable that Cr is contained in an amount of 0.1 % or more.
However, when the content of Cr exceeds 3.5 %, not only the FB performance is reduced,
but also an excessive increase of the resistance to temper softening is brought. For
that reason, when Cr is contained, it is preferable that the content of Cr is limited
to not more than 3.5 %. Incidentally, the content of Cr is more preferably from 0.2
to 1.5 %.
Mo: not more than 0.7 %
[0043] Mo is an element acting to effectively improve the quenching property. In order to
obtain such an effect, it is preferable that Mo is contained in an amount of 0.05
% or more. However, when the content of Mo exceeds 0.7 %, the steel becomes hard,
thereby reducing the FB performance. For that reason, when Mo is contained, it is
preferable that the content of Mo is limited to not more than 0.7 %. Incidentally,
the content of Mo is more preferably from 0.1 to 0.3 %.
Ni: not more than 3.5 %
[0044] Ni is an element effective for improving the quenching property. In order to obtain
such an effect, it is preferable that Ni is contained in an amount of 0.1 % or more.
However, when the content of Ni exceeds 3.5 %, the steel becomes hard, thereby reducing
the FB performance. For that reason, when Ni is contained, it is preferable that the
content of Ni is limited to not more than 3.5 %. Incidentally, the content of Mo is
more preferably from 0.1 to 2.0 %.
Ti: from 0.01 to 0.1 %
[0045] Ti is easy to bind with N to form TiN and is an element effectively acting to prevent
coarseness of a γ grain at the time of quenching. Also, when Ti is contained together
with B, since Ti reduces N which forms BN, it has an effect for minimizing the addition
amount of B necessary for improving the quenching property. In order to obtain such
effects, it is required that the content of Ti is 0.01 % or more. On the other hand,
when the content of Ti exceeds 0.1 %, the ferrite is subjected to precipitation strengthening
due to precipitation of TiC or the like and becomes hard, thereby reducing the mold
life. For that reason, when T is contained, it is preferable that the content of Ti
is limited to the range of from 0.01 to 0.1 %. Incidentally, the content of Ti is
more preferably from 0.015 to 0.08 %.
B: from 0.0005 to 0.005 %
[0046] B is an element which segregates on an austenite grain boundary and when contained
in a trace amount, improves the quenching property. In particular, the case where
B is compositely added together with Ti is effective. In order to improve the quenching
property, it is required that the content of B is 0.0005 % or more. On the other hand,
even when B is contained in an amount exceeding 0.005 %, the effect is saturated and
an effect that corresponds to the content cannot be expected, and therefore, such
is economically disadvantageous. For that reason, when B is contained, it is preferable
that the content of B is limited to the range of from 0.0005 to 0.005 %. Incidentally,
the content of B is more preferably from 0.0008 to 0.004 %.
[0047] The remainder other than the foregoing components is Fe and unavoidable impurities.
Incidentally, as the unavoidable impurities, for example, not more than 0.01 % of
N, not more than 0.01 % of O and not more than 0.1 % of Cu are tolerable.
[0048] Next, the reasons why the structure of the steel sheet of the invention is limited
are described.
[0049] The steel sheet of the invention has a structure composed mainly of ferrite and a
cementite. The "structure composed mainly of ferrite and a cementite" as referred
to herein means a structure in which ferrite and a cementite account for 95 % or more
in terms of a volume ratio.
[0050] In the invention, the grain size of ferrite is from 1 to 10 µm in terms of an average
crystal grain size. When the average ferrite crystal grain size is less than 1 µm,
not only the steel sheet is remarkably hardened, but also the cementite amount in
ferrite grain increases, whereby the fabrication property such as hole expansion property
after the FB working as well as the FB performance and the mold life are reduced.
On the other hand, when the grain size of ferrite exceeds 10 µm, though the steel
sheet is softened, thereby improving the mold life, the FB performance is reduced
as shown in Fig. 3. For that reason, the average ferrite crystal grain size was limited
to the range of from 1 to 10 µm. Incidentally, the average ferrite crystal grain size
is preferably from 1 to 5 µm.
[0051] In the steel sheet of the invention, a spheroidization ratio of the cementite is
80 % or more. When the spheroidization ratio is less than 80 %, not only the steel
sheet becomes hard, but also the deformability is small and the FB performance is
reduced. As shown in Fig. 2, when the spheroidization ratio is less than 80 %, Rz
ave exceeds 10 µm and becomes large, and the FB performance is abruptly reduced. For
that reason, in order to ensure a sufficient FB performance, the spheroidization ratio
of a cementite was limited to 80 % or more. Incidentally, in order to make the spheroidization
ratio large, since long-term annealing is required, the spheroidization ratio is preferably
from 80 to 85 %.
[0052] Also, in the steel sheet of the invention, an amount S
gb of a ferrite intergranular cementite is 40 % or more. The amount S
gb of a ferrite intergranular cementite is a ratio of an occupied area of a cementite
present on the ferrite crystal grain boundary to an occupied area of the whole of
cementites and is a value as defined by the following expression (1):

(wherein S
on represents a total occupied area of a cementite present on the ferrite crystal grain
boundary of the cementites present per unit area; and S
in represents a total occupied area of a cementite present in a ferrite grain of the
cementites present per unit area.) When the amount S
gb of a ferrite intergranular cementite is less than 40 %, the amount of the cementite
present in the ferrite grain is large; Rz ave exceeds 10 µm and becomes large as shown
in Fig. 1; and the FB performance is abruptly reduced. It is considered that this
is caused due to the matter that when even a fine and spheroidized cementite is present
in the ferrite grain, fine cracks are generated in the periphery of the cementite
at the time of FB working and connected to each other, thereby reducing the FB performance.
It is also considered that when fine cracks are generated in the periphery of the
cementite at the time of FB working and remain, these cracks are connected to each
other in the subsequent fabrication, leading to a reduction of the fabrication property.
Also, when the cementite is present in the ferrite grain, the ferrite grain itself
becomes hard, thereby reducing the mold life. For that reason, in the invention, the
amount S
gb of a ferrite intergranular cementite was limited to 40 % or more. Incidentally, the
amount S
gb of a ferrite intergranular cementite is preferably 50 % or more.
[0053] Also, in the steel sheet of the invention, it is preferable that the cementite present
on the crystal grain boundary of ferrite has an average grain size of not more than
5 µm. This is because it has been newly found that in the case where the amount S
gb of a ferrite intergranular cementite is 40 % or more, with respect to the cementite
present on the ferrite grain boundary, the smaller the particle size, the more improved
the FB working and the larger the contribution to an improvement in mold life. Also,
when the particle size of the cementite, in short-time heating in high-frequency quenching,
it is possible to easily dissolve the cementite in the austenite, whereby it is easy
to ensure a desired quenching hardness. For these reasons, it is preferable that the
average particle size of a cementite present on the ferrite crystal grain boundary
is limited to not more than 5 µm.
[0054] Next, a preferred manufacturing method of the steel sheet of the invention is described.
[0055] It is preferable that a molten steel having the foregoing composition is molten by
a common melting method using a converter or the like and formed into a raw steel
material (slab) by a common casting method such as a continuous casting method.
[0056] Subsequently, the obtained raw steel material is subjected to hot rolling to form
a hot rolled sheet by heating and rolling.
[0057] The hot rolling is preferably a treatment in which a termination temperature of finish
rolling is set up at from 800 to 950°C, after completion of the finish rolling, cooling
is carried out at an average cooling rate of 50°C/s or more, the cooling is stopped
at a temperature in the range of from 500 to 700°C, and coiling is carried out at
from 450 to 600°C. The hot rolling in the invention is characterized by adjusting
the termination temperature of finish rolling and the subsequent cooling condition.
Thus, a hot rolled steel sheet having a substantially 100 % pearlite structure is
obtained.
Termination temperature of finish rolling: from 800 to 950°C
[0058] It is preferable that the termination temperature of finish rolling is a temperature
in the range of from 800 to 950°C, which is a termination temperature region of usual
finish rolling. When the termination temperature of finish rolling exceeds 950°C and
becomes high, not only a generated scale becomes thick so that the pickling property
is reduced, but also a decarburized layer may possibly be formed in the steel sheet
surface layer. On the other hand, when the termination temperature of finish rolling
is lower than 800°C, an increase in the rolling load becomes remarkable, and an excessive
load against a rolling mill becomes problematic. For that reason, it is preferable
that the termination temperature of finish rolling is a temperature in the range of
from 800 to 950°C.
Average cooling rate after completion of finish rolling: 50°C/s or more
[0059] After completion of the finish rolling, cooling is carried out at an average cooling
rate of 50°C/s or more. Incidentally, the subject average cooling rate is an average
cooling rate of from the termination temperature of finish rolling to a stopping temperature
of the subject cooling (forced cooling). When the average cooling rate is less than
50°C/s, cementite-free ferrite is formed during cooling, and the structure after cooling
is a heterogeneous structure of (ferrite + pearlite), whereby a homogeneous structure
composed of substantially 100 % pearlite cannot be ensured. When the hot rolled sheet
structure is a heterogeneous structure of (ferrite + pearlite), whatever the subsequent
hot rolled sheet annealing is devised, the amount of the cementite present in the
grain increases, and the amount of the cementite present on the grain boundary decreases.
Thus, the FB performance is reduced. For these reasons, it is preferable that the
average cooling rate after completion of finish rolling is limited to 50°C/s or more.
Incidentally, for the purpose of preventing the formation of bentonite, it is more
preferable that the average cooling rate after completion of finish rolling is not
more than 120°C/s.
Cooling stopping temperature: from 500 to 700°C
[0060] It is preferable that a temperature at which the foregoing cooling (forced cooling)
is stopped is from 500 to 700°C. When the cooling stopping temperature is lower than
500°C, there are caused problems in operation such as a problem that hard bentonite
or martensite is formed, whereby the hot rolled sheet annealing takes a long time;
and the generation of a crack at the time of coiling. On the other hand, when the
cooling stopping temperature exceeds 700°C and becomes high, since a ferrite transformation
noise is present in the vicinity of 700°C, ferrite is formed during standing for cooling
after stopping of cooling, whereby a homogeneous structure composed of substantially
100 % pearlite cannot be ensured. From these matters, it is preferable that the cooling
stopping temperature is limited to a temperature in the range of form 500 to 700°C.
Incidentally, the cooling stopping temperature is more preferably from 500 to 650°C,
and further preferably from 500 to 600°C.
[0061] After stopping the cooling, the hot rolled sheet is immediately coiled in a coil
state. The coiling temperature is preferably from 450 to 600°C, and more preferably
from 500 to 600°C.
[0062] When the coiling temperature is lower than 450°C, a crack is formed in the steel
sheet at the time of coiling, resulting in a problem in operation. On the other hand,
where the coiling temperature exceeds 600°C, there is a problem that ferrite is formed
during the coiling.
[0063] The thus obtained hot rolled sheet (hot rolled steel sheet) is then subjected to
removal of an oxidized scale of the surface by pickling or shot blasting and subsequently
to hot rolled sheet annealing. By applying proper hot rolled sheet annealing to the
hot rolled sheet having a substantially 100 % pearlite structure, not only the spheroidization
of a cementite is accelerated, but also the grain growth of ferrite is inhibited,
whereby a large amount of the cementite can be made present on the ferrite crystal
grain boundary.
[0064] Incidentally, in the hot rolled sheet annealing, the annealing temperature is a temperature
in the range of from 600 to 750°C. When the annealing temperature is lower than 600°C,
spheroidization of the cementite cannot be sufficiently achieved. On the other hand,
where the annealing temperature exceeds 750°C and becomes high, pearlite is regenerated
during cooling, and the fine blanking performance and other fabrication property are
reduced. Incidentally, though a holding time of the hot rolled sheet annealing is
not required to be particularly limited, in order to sufficiently spheroidize the
cementite, it is preferable that the holding time is 8 hours or more. Also, when it
exceeds 80 hours, since the ferrite grain becomes excessively coarse, the holding
time is preferably not more than 80 hours.
EXAMPLES
[0065] A raw steel material (slab) having a composition as shown in Table 1 was subjected
to hot rolling and hot rolled sheet annealing as shown in Table 2, thereby forming
a hot rolled steel sheet (thickness: 4.3 mm).
[0066] The obtained hot rolled steel sheet was examined with respect to the structure, FB
performance and stretch flanging property after the FB performance. The examination
methods are as follows.
(1) Structure:
[0067] A specimen for structure observation was collected from the obtained steel sheet.
A cross section parallel to a rolling direction of the specimen was polished and corroded
with nital; and with respect to a position of 1/4 of the sheet thickness, a metallurgical
structure was observed (field number: 30 places) by a scanning electron microscope
(SEM) (magnification, ferrite: 1,000 times, cementite: 3,000 times); and a volume
ratio of ferrite and a cementite, a ferrite grain size, a spheroidization ratio of
a cementite, an amount of ferrite intergranular cementite and an average particle
size of a cementite on the ferrite grain boundary were measured.
[0068] With respect to the volume ratio of ferrite and a cementite, the metallurgical structure
was observed (field number: 30 places) by SEM (magnification: 3,000 times); an area
ratio obtained by dividing an area resulting from summing up an area of ferrite and
an area of a cementite by a total field area; and this value was judged as a volume
ratio of ferrite and a cementite.
[0069] With respect to the ferrite grain size, an area of each ferrite grain was measured,
and a circle-corresponding size was determined from the resulting area and defined
as a grain size of each ferrite grain. The thus obtained respective ferrite grain
sizes were arithmetically averaged, and its value was defined as a ferrite average
grain size of that steel sheet.
[0070] With respect to the spheroidization ratio of a cementite, a maximum length
a and a minimum length
b of each cementite were determined in each field (field number: 30 pieces) of the
structure observation (magnification: 3,000 times) by using an image analyzer; its
ratio a/b was computed; and the number of cementite grains of a/b with not more than
3 was expressed by a proportion (%) against the total number of measured cementites,
thereby defining it as a spheroidization ration (%) of cementite.
[0071] With respect to the amount of (S
gb) of a ferrite intergranular cementite, in each field (field number: 30 pieces) of
the structure observation (magnification: 3,000 times), a cementite present on the
ferrite grain boundary and a cementite present in the ferrite grain were discriminated
from each other; an occupied area S
on of a cementite present on the ferrite grain boundary and occupied area S
in of a cementite present in the ferrite grain were measured by using an image analyzer;
and an amount (S
gb) of a ferrite intergranular cementite was computed according to the following expression
(1).

[0072] Also, with respect to each cementite present on the ferrite grain boundary, a diameter
passing through two points on the periphery of the cementite and a center of gravity
of a corresponding oval of the cementite (an oval having the same area as the cementite
and having a primary moment and a secondary moment equal to each other) was measured
at every 2° to determine a circle-corresponding size, thereby defining it as a grain
size of each cementite. The thus obtained respective cementite particle sizes were
averaged, and its value was defined as a cementite average particle size in ferrite
grain.
(2) FB performance:
[0073] A specimen (size: 100 × 80 mm) was collected from the obtained steel sheet and subjected
to an FB test. The FB test was carried out by blanking a sample having a size of 60
mm × 40 mm (corner radius R: 10 mm) from the specimen by using a 110t hydraulic press
machine under a lubricious condition of a tool-to-tool clearance of 0.060 mm (1.5
% of the sheet thickness) and a working pressure of 8.5 tons. With respect to an end
surface (blanked surface) of the blanked sample, a surface roughness (ten-point average
roughness Rz) was measured, thereby evaluating the FB performance. Incidentally, with
respect to the specimen, in order to eliminate influences of a deviation in sheet
thickness against the clearance, the both surfaces were equally ground in advance,
thereby regulating the sheet thickness at 4.0 ± 0.010 mm.
[0074] That is, with respect to the measurement of the surface roughness, as illustrated
in Fig. 4, in each of four end surfaces (sheet thickness surfaces) other than R parts,
a region within a range of from 0.5 mm to 3.9 mm of the surface in the punch side
in the sheet thickness direction and 10 mm in parallel to the surface (X direction)
was scanned 35 times at a pitch of 100 µm in the sheet thickness direction (t direction)
by using a contact probe profilometer, and a surface roughness Rz in each scanning
line was measured according to JIS B 0601-1994. Furthermore, with respect to the surface
roughness Rz on the measured surface, Rzs in the respective scanning lines were summed
up, and an average value thereof was employed. The four end surfaces were measured
in the same method as described above, and an average surface roughness Rz ave (µm)
defined according to the following expression was computed.

(wherein Rz 1, Rz 2, Rz 3 and Rz 4 each represents Rz on each surface.)
[0075] Also, the life of the used tool (mold) was evaluated. A surface roughness (ten-point
average roughness Rz) of the sample end surface (blanked surface) at the point of
time when the number of blanking in the FB working reached 30,000 times was measured,
thereby evaluating the mold life. Incidentally, the measurement method the surface
roughness was the same as described above. The case where the average surface roughness
Rz ave of the sample end surface is not more than 10 µm is defined as "○"; the case
where it is more than 10 µm and not more than 16 µm was defined as "Δ"; and the case
where it is more than 16 µm was defined as "×".
(3) Stretch flanging property after FB working:
[0076] A specimen (size: 100 mm × 100 mm) was blanked from the obtained steel sheet by FB
working, thereby examining a stretch flanging property. Incidentally, the FB working
was carried out under a lubricious condition of a tool-to-tool clearance of 0.060
mm (1.5 % of the sheet thickness) and a working pressure of 8.5 tons.
[0077] The stretch flanging property was evaluated by carrying out a hole expansion test
to determine a hole expansion ratio λ. The hole expansion test was carried out by
a method in which a punch hole of 10 mmφ (do) was blanked in a specimen and expanding
the subject hole by a tool; a hole size
d at the point of time when a through thickness crack was generated in a flange of
the punch hole was determined; and a hole expansion ratio λ (%) as defined by the
following expression was determined.

[0078] The obtained results are shown in Table 2, too.
[0079] In all of the examples of the invention, the average surface roughness Rz ave on
the blanked surface is not more than 10 µm; the FB performance is excellent; the blanked
surface at the time of 30,000 times in blanking number is smooth (evaluation: ○);
and a reduction in mold life is not acknowledged. Also, the examples of the invention
are excellent in the stretch flanging property after FB working. Incidentally, the
volume ratio of ferrite and a cementite was confirmed in the foregoing method. As
a result, in all of the examples of the invention, it was confirmed that the sum of
volume ratio of the ferrite and cementite is 95 % or more, thereby forming a structure
composed mainly of ferrite and a cementite. Also, the particle size of a cementite
present on the ferrite crystal grain boundary was confirmed by the foregoing method.
As a result, in all of the examples of the invention, the average particle size was
not more than 5 µm.
[0080] On the other hand, in the examples of comparison falling outside the scope of the
invention, the surface roughness Rz on the blanked surface exceeds 10 µm and becomes
coarse, whereby a reduction of the FB performance is confirmed, the mold life is reduced,
and the stretch flanging property is reduced. Incidentally, in the steel sheet No.
15, since a crack was generated at the time of coiling, the hot rolled sheet annealing
and subsequent treatments were not carried out.
Table 1
Steel No. |
Chemical components (% by mass) |
Remark |
C |
Si |
Mn |
P |
S |
Al |
N |
Cr |
Mo |
Ni |
Ti |
B |
A |
020 |
018 |
0.83 |
0.011 |
0.006 |
0.044 |
0.0035 |
- |
- |
- |
- |
- |
Invention |
B |
0.35 |
0.18 |
0.79 |
0.014 |
0.006 |
0.028 |
0.0038 |
- |
- |
- |
- |
- |
Invention |
C |
0.44 |
0.22 |
0.76 |
0.012 |
0.007 |
0.021 |
0.0042 |
- |
- |
- |
- |
- |
Invention |
D |
0.20 |
0.17 |
0.74 |
0.017 |
0.009 |
0.025 |
0.0028 |
1.21 |
- |
- |
- |
- |
Invention |
E |
0.16 |
0.17 |
0.88 |
0.012 |
0.008 |
0.024 |
0.0034 |
- |
0.27 |
- |
- |
- |
Invention |
F |
0.21 |
0.21 |
0.73 |
0.014 |
0.005 |
0.022 |
0.0031 |
- |
- |
1.48 |
- |
- |
Invention |
G |
0.19 |
0.16 |
0.74 |
0.015 |
0.007 |
0.023 |
0.0029 |
- |
- |
- |
0.04 |
- |
Invention |
H |
0.21 |
0.18 |
0.73 |
0.014 |
0.007 |
0.029 |
0.0044 |
- |
- |
- |
- |
0.0024 |
Invention |
I |
0.23 |
0.22 |
0.81 |
0.012 |
0.006 |
0.035 |
0.0035 |
- |
- |
- |
0.02 |
0.0016 |
Invention |
J |
0.22 |
0.23 |
0.69 |
0.017 |
0.006 |
0.026 |
0.0037 |
0.79 |
0.41 |
0.72 |
- |
- |
Invention |
K |
0.21 |
0.24 |
0.71 |
0.015 |
0.006 |
0.025 |
0.0042 |
0.78 |
0.28 |
1.23 |
0.02 |
0.0019 |
Invention |
L |
0.34 |
0.65 |
0.83 |
0.015 |
0.006 |
0.017 |
0.0044 |
- |
- |
- |
- |
- |
Comparison |
M |
0.36 |
0.23 |
1.67 |
0.014 |
0.008 |
0.022 |
0.0033 |
- |
- |
- |
- |
- |
Comparison |
N |
0.36 |
0.18 |
0.72 |
0.011 |
0.006 |
- |
0.0042 |
- |
- |
- |
- |
- |
Invention |
O |
0.35 |
0.05 |
0.75 |
0.013 |
0.007 |
0.031 |
0.0040 |
- |
- |
- |
- |
- |
Invention |
P |
0.22 |
0.20 |
0.70 |
0.013 |
0.006 |
- |
0.0038 |
0.80 |
0.25 |
1.05 |
- |
- |
Invention |
