DESCRIPTION
Title of Invention
MEDIUM CARBON STEEL SHEET FOR COLD WORKTNG AND METHOD FOR
MANUFACTURTNG THE SAME
Technical Field
[OOOl]
The present invention relates to a medium carbon steel sheet for cold working,
10 which has excellent cold workability and which has a strength that may be increased
even in a quenching treatnient, represented by high-frequency quenching, in a short
heat-treatment time, and a nlethod for manufacturing the same.
Background All
15 [0002]
A ~nediunic arbon steel sheet has been widely used as a niaterial for a chain, a
gear, a clutch, a saw,, a blade, and the like. To make the material into a product, a
process of shaping the material into a predetermined shape and hardening the niaterial
using a heat treatment such as quenching and tempering is cornnionly performed.
20 Therefore, both workability and hardenability are required for niediu~nc arbon steel
sheets. Particularly, in recent years, a working tecl~nologyh as been developed, wherein
a shaping method in which compression working and tension working are perfomled at
the same time, and the working ratio is higher compared to the related art has been
adopted. Therefore, it is necessary for a medium carbon steel sheet to have shaping
25 properties capable of enduring hard working. Furthermore, according to recent
2
detnatids for energy conservation, there are tnovetnetlts to change the quenching atid
tetilpering process from a furnace heating type in the related art to a high-frequency
heating type. To adapt the changing needs, it is necessaly to develop a medium carbon
steel sheet that is soft before cold working, endures working during the cold working,
5 and has an excelletit liardenability after high-frequency heating (hereinaftel; referred to as
high-frequency hardenability. In addition, quenching after high-frequency heating is
referred to simply as "high-freqoency quenching).
Citation List
10 Patent Doctnnent
[0003]
[Patent Document I] Japanese Unesatnined Patent Application, First
Publication No. H11-80884
[Patent Doct~tnen2t 1 Japanese Unesamined Patetit Application, First
15 Publication No. H09-268344
[Patent Docutilet~3t 1 Japanese Unesatnined Patetit Application, First
Publication No. 2001-329333
[Patent Document 41 Japanese Unexamined Patent Application, First
Publicatioti No. 2001-355047
20
Sotiitnary of Invention
Technical Problem
[0004]
In the related art, various researches on a relationship between workability and
25 high-frequency liardenability of the mediutn carbon steel sheet have been conducted (for
3
example, refer to Patent Document 1 to Patent Document 4). Howevel; it is considered
that an exatiiple which has an excellent cold workability, and in which the hardenability
can be sufficiently secured at a heating rate of 10O0C/second or more has not been
reported.
[0005]
For exatnple, Patent Document 1 discloses ti~ediu~ainl d high carbon steel sheets
consist of hypo-eutectoid steel that contains 0.1 to 0.8 inass% of C and 0.0 1 mass% or
less of S. In these medium and high carbon steel sheets, carbide is dispersed in ferrite in
such a manner that the carbide spheroidizit~gra tio becomes 90% or more. The average
10 particle size ofthe carbide is 0.4 to 1.0 pm, and tlie ferrite grain size is adjusted to be 20
Fun or more as necessary. However, in the medium and high carbon steel sheet, local
ductility is improved by appropriately controlling the shape of tlie carbide as described
above, thereby improving the stretch-flange property. However, it is considered
working characteristics of these sheets regarding both cotl~pressionw orking and tension
15 working are not sufficient.
In addition, Patent Document 2 discloses a high strength steel for
high-frequency quenching, which is excellent in static strength, betiding fatigue strength,
and rolling contact fatigue strength as well as in cold forgeability. In tlie steel for
high-frequency quenching, a specific shape of the carbide, which is necessary to obtain
20 forgeability, is not clearly disclosed, and specific conditions such as tlie heating
temperature and the holding time during quenching are not specified.
In addition, Patent Document 3 discloses steel for high-frequency quenching,
which is excellent in cold forgeability and in which a limit upsetting rate is high.
Ho\vevet; specific conditions such as the heating temperature and the holding titile during
25 the quenching are not specified, and it is not clear that tlie hardenability is actually
4
excellent.
Furthermore, Patetit Document 4 discloses a carbon steel tube excellent in cold
workability and high-frequency hardenability. It is considered that the high carbon steel
tube is suitable for a working method such as swaging and tube expansion that depend on
5 local ductility, but cold forgeability in terms of punching, drawing, bending, burring,
upsetting, ironing, extrusion, and the like, which are the targets of the present invention,
is not sufficient.
[0006]
The cold working that is a target of the present invention represents various
10 kinds of working such as punching, drawing, bending, burring, ironing, and extrusion,
and hard compression and tension are applied during these kinds of working. In a case
where tlie above-described cold orkin king is applied to a medium carbon steel sheet, it is
considered that a crack due to interfacial peeling is generated atid propagates behveeti a
ferrite phase and carbide, atid thus cracking occurs. Therefore, adjustment of tlie
15 chemical compositions and shape control of the carbide are important to prevent
interfacial peeling during tlie working.
111 addition, a material, which is cold worked, is frequently subjected to a
quenching treatment. However, in the high-frequency quenching treatment in which a
lieat treattne~it ime is short, the carbide in the material is not sufficiently dissolved
20 during tlie heating, and thus it is difficult to obtain stable hardenability. Therefore,
shape control of tlie carbide in tlie material is important to sofficietitly dissolve the
carbide during tlie high-frequency quenching.
However, it is considered that points of proble~nsin a case where cold working
is applied to tlie ~nediutnc arbon steel sheet atid the high-frequency quenching is
25 perfomled are not clear until now.
5
In addition, in the present invention, a medium carbon steel sheet represents a
steel sheet which contains 0.30 to 0.60% of C, and which has a sheet thickness of 1.6 to
20 inin.
[0007]
The present invention has been made in consideration of the above-described
circumstances, and an object thereof is to provide a ~nediunic arbon steel sheet wliich has
excellent cold workability and thus has sufficient quenching hardenability even in a
high-frequency quenching treat~nent,a nd which has excellent high-fieqoency
hardenability, and a manufacturing lnetliod of the same.
10
Solution to Problein
[0008]
The present inventors conducted tliorough research looking for a method of
accomplishing tlie above-described object. As a result, it is foulid that in addition to
15 adjustment of chemical composition of a steel sheet, \\,lien the average diameter of
carbide and the spheroidizing ratio of the carbide are controlled to satisfy predetermined
conditions, a medit~mc arboii steel sheet, in \vhich the hardness during cold working
decreases and thus the cold workability beco~iiese xcellent, and which has snfficient
quenching hardenability even io a high-frequency quenching treatment at an average
20 heating rate of 10O0C/second or more, inay be provided.
[0009]
The present invention has been made based on this finding, and tlie gist thereof
is as follows.
roo 101
(1) According to an aspect ofthe present invention, there is provided a medium
6
carbon steel sheet for cold working that has a hardness of 500 HV to 900 HV in a case of
being subjected to high-frequency quenching in which a temperature is raised at an
average heating rate of 10O0C/second, the temperature is held at 1,00O0C for 10 seconds,
and a quick cooling to a room te~nperatureis carried out at an average cooling rate of
5 20O0C/second. The medium carbon steel sheet includes, by mass%, C: 0.30 to O.60%,
Si: 0.06 to 0.30%, Mn: 0.3 to 2.0%, P: 0.030% or less, S: 0.0075% or less, Al: 0.005 to
0.10%, N: 0.001 to 0.01%, and Cr: 0.001 to 0.10%, the balance cotnposed of Fe and
inevitable ialpurities. An average diameter d of a carbide is 0.6 pn or less, a
spheroidizing ratio p of the carbide is equal to or more than 70% and less than 90%, and
10 the average diameter d (p~no) f the carbide and the spheroidizing ratio p % of the carbide
satisfy dS0.04xp-2.6.
[OOll]
(2) In the medium carbon steel sheet for cold working according to (I), fi~rther
includes one or more of, by inass%, Ni: 0.01 to 0.5%, Cu: 0.05 to 0.5%, Mo: 0.01 to
15 0.5%,Nb: 0.01 to 0.5%,Ti: 0.001 to 0.05%, V: 0.01 to 0.5%,Ta: 0.01 to0.5%, B: 0.001
to O.Ol%, W: 0.01 to 0.5%, Sn: 0.003 to 0.03%, Sb: 0.003 to 0.03%, and As: 0.003 to
0.03%,
(3) In the medium carbon steel sheet for cold working according to (2), a Cr
content [Cr] and a Mo content [Mo] lnay satisfy [Cr]+[Mo]/lO < 0.10.
20 (4) 111 the mediom carbon steel sheet for cold working according to (I) 01. (2),
the hardness before the cold workitlg may be equal to or more than 120 HV and less than
170 HV.
[OO 121
(5) In the medianl carbon steel sheet for cold working according to (1) or (2),
7
the medium carbon steel sheet may fi~rtherin clude a sorface treatment fill11 that contains
each of chemical co~npositionsd erived from a silanol bond that contains a metal
conlponent X and is expressed by Si-0-X, a heat-resistant resin, an inorganic acid salt,
and a lubricant on at least one surface, the surface treatment filtn may have a
5 concentration gradient for each of the chemical compositions in a film thickness direction,
and have three layers including an adhesion layer, a base layer, wherein tlie three layers
are positioned in order from an interface between the surface treatment film and the
nlediu~nc arbon steel sheet for cold working of the adhesion layet; the base layer and the
lubricant layet; the adhesion layer may contain largest amount of the che~llical
10 cornposition derived from the silanol bond among the three layers, and nlay have a
thickness of 0.1 nm to 100 nm; the base layer may contain largest amount of the
heat-resistant resin and tlie inorganic acid salt among the three layers, niay contain 0.01
to 10 parts by mass ofthe inorganic acid salt to 100 palls by Illass of tlie heat-resistant
resin, and may have a thickness of 0.1 pm to 15 pm; the lobricant layer may contain
15 largest atnount ofthe lubricant among the three layers, and has a thickness of 0. I pm to
10 pm; and a ratio of the thickness of the base layer to the thickness of the lubrica~ilta yer
map be 0.2 to 10.
[00 131
(6) In the inedium carbon steel sheet for cold working according to (5), the
20 inorganic acid salt may be at least one of compound selected from a group consisting of a
pllosphate, a borate, a silicate, a molybdate, and a tuogstate.
(7) In tlie medium carbon steel sheet for cold working according to (5), tlie
heat-resistant resin may be at least one resin selected from a group consisting of a
polyi~nider esin, a polyester resin, an epoxy resin, and a fluorine resin.
25 (8) In the medil~mc arbon steel sheet for cold working according to (5), the
8
lubricant inay be at least one conlpound selected from a group consisting of a
polytetrafluoroetliylene, a molybdenum disulfide, a tungsten disulfide, a zinc oxide, and a
graphite.
[0014]
(9) According to another aspect of tlie present invention, there is provided a
method for manufacturing medium carbon steel slieet for cold working, the method
including: a first process of holding a temperature of a cast slab having a chemical
composition according to (1) or (2) at 1,050 to 1,300°C; a second process ofperforming
a hot rolling in which rolling is terminated at 750 to l,OOO°C for the cast slab to obtain a
10 steel sheet after the first process; a third process of cooling the steel sheet to a first
cooling termination ternperattire of 500 to 700°C at a first average cooling rate of 20 to
50°C/second after the second process; a fourth process of cooling the steel sheet to a
second cooling teniliaation temperature that is equal to or higher than 400°C and equal to
or lower tlian a temperature that is lower than tlie first cooling termination tetnperatore
15 by 50°C at a second average cooling rate of 5 to 3O0C/second, and coiling the steel sheet
after the third process; a fifth process of holding the steel slieet so that a tinle held at a
temperature range of 400°C to the second cooling terinination teniperature is limited to
30 hours or less after tlie fouilh process; and a sixth process of performing annealing by
heating the steel sheet to a temperature of 600°C to A,, point-iO°C and holding the steel
20 slieet in this temperature for a time equal to or more tlian 5 hours and less than 100 hours
after tlie fifth process.
[OO 151
(10) In tlie tnetliod for manufacturing the medium carboll steel slieet for cold
working according to (9), in the sixth process, a dew point at 400°C or lower niay be less
9
that1 -20°C, the dew point at a telnperature higher than 400°C may be less that1 -40%
and a concentration of hydrogen may be 95% or more.
(11) In the method for manufacturing the medium carbon steel sheet according
to (9) or (lo), a water-based surface treatment liquid, which contains a water-soluble
5 silane coupling agent, a water-soluble inorganic acid salt, a water-soluble heat-resistant
resin, and a lubricant, may be applied onto at least one surface of the nledit~inc arbon
steel sheet for cold working, and the surface treatment liqnid may be dried to forin the
surface treatnlent fihn on at least one surface of the medium carbon steel sheet for cold
working after tlie sixth process.
Advantageous Effects of Invention
[00 161
According to the present invention, it is possible to provide a inedium carbon
steel slieet for cold working, which has lo\\? Iiardness (soft) before cold working and has
15 excellent workabilitp for both compression \\,orking and tension working, and thus has
sufficient quenching hardenability even in a Ilig11-frequency quenching treattilent at an
average heating rate of 10O0C/second or inore after the cold working, and co~npatibility
between cold workability atid high-frequency hardenability, which is capable of securing
high strength, is realized; and a method for manufacturing the satlie.
20
Brief Description of Drawings
[00 1 71
FIG. 1 is a diagram illustrating an effect of the average diameter of carbide atid
the spheroidizing ratio of the carbide on quenching liardtiess and cold workability.
25 FIG. 2 is a diagram illustrating a relationship between the Si content and the
10
number of cracks at a carbide interface and in a grain after cold working.
FIG. 3 is a diagram illustrating a relationship between [Cr]+[Mo]/lO and
quenching hardness.
FIG. 4 is a diagram illustrating a relationship between the spheroidizing ratio of
5 carbide and the tmmber of cracks starting from the carbide.
FIG. 5 is a diagraun illustrating a relationship between the S content and the
number of cracks starting fro111 sulfide.
FIG. 6 is a longitudinal cross-section diagram schenlatically illustrating a
configuration of a steel sheet for cold working, which is according to a modified example
10 of an embodiment of the present invention.
FIG. 7A is a schematic diagram illustrating a spike test method.
FIG. 7B is a schematic diagram illustrating shapes of a spike test specimen
before and after working.
FIG. 8 is a flowchart sclieniatically illustrating an outline of a lnethod of
15 ~nanufacturingth e inedium carbon steel sheet for cold working of the present invention.
Description of Embodinlents
1001 81
Hereinafter, tile present invelltiotl will be described in detail.
[00 191
First, a description will be provided with respect to the reason for limitation
regarding chemical co~npositioo~fl a steel sheet for cold working according to an
embodiment of the present invention (hereinaftel; may be referred to as a "steel sheet of
this embodiment"). In addition, " % represents "mass% in the following description.
[0020]
C is an important element to secure the quenching strength of a steel sheet.
Therefore, C is added in the steel in an amount of 0.30% or~noreto secure necessary
strength. When the C content is less than 0.30%, the hardenability decreases, and the
5 strength needed for a high-strength steel sheet to be used in mechanical structure may not
be obtained, and thus the lower limit of the C content is 0.30%. When the C content
exceeds 0.60%, the percentage of carbide, which acts as a starting point of fracture,
increases, and cold \vorkability deteriorates, and thus the upper limit of the C content is
0.60%. In a case where it is necessary to further secure hardenability, it is preferable
10 that the lower lirnit of the C content be 0.35%, more preferably be 0.37%, and still Inore
preferably be 0.40%. In addition, to f~~rtheears ily co~itrolth e shape of tlie carbide, it is
preferable that tlie upper linlit of the C content be 0.55%, tilore preferably be 0.52%, and
still more preferably be 0.50%.
[002 I ]
Si: 0.06 to 0.30%
Si is an element that acts as a deoxidizer and is effective for sc~ppressing
interfacial peeling behveen ferrite and carbide during \\lorking and for improvi~lg
hardenability. When the Si content is less than 0.06%, this addition effect tnay not be
obtained, and thus the lower limit ofthe Si content is 0.06%. On the other hand, \\!hen
20 the Si content exceeds 0.30%, since the frequency of crack occurrence (a frequency of
transgranular crack occurrence) in a ferrite phase increases doe to solid solution
strengthening, and the surface testore deteriorate due to scale defects during hot rolling,
the upper limit oftlie Si content is 0.30%. In tlie case of further reducing of the peeling
at the interface of between the ferrite and the carbide, it is preferable that the lower limit
25 ofthe Si content be O.lO%, more preferably be 0.13%, and still more preferably be
12
0.15%. In addition, to fi~rther educe the generation of cracks (transgranular crack) in
the ferrite phase, it is preferable that the apper litnit of the Si content be 0.26%.
[0022]
Mn: 0.3 to 2.0%
Mn is an element that acts as a deoxidizes and is effective at improving
hardenability. When the Mn content is less than 0.3%, this addition effect may not be
obtained, and thus the lower limit of the Mn content is 0.3%. When the Mn content
exceeds 2.0%, dissolution of the carbide during high-frequency heating becomes slow,
and the hardenability (quenching hardness) decreases, and tli~lsth e upper limit of the Mn
10 content is 2.0%. In the case of further increasing of the liardenability, it is preferable
that the lower limit of the MI] content be 0.5%, more preferably be 0.55%, and still lnore
preferably be 0.65% or 0.73%. In addition, to fi~rthers ecure the high-frequency
hardenability, it is preferable that tlie upper litnit of the Mn content be 1.6%, inore
preferably be 1.4%, and still lnore preferably be 1.2% or I .O%.
15 [0023]
P: 0.030% or less
P is a solid solution strengthening element, and is an element that is effective for
increasing the strength of the steel sheet. When P is excessively contained in steel,
toughness decreases, and thus the upper litnit of the P content is 0.030%. P is an
20 inevitable itnpurity. When tlie P content is reduced to be less than 0.005%, the refining
cost increases, and thus tlie P content does not need to be reduced to less than 0.005%.
In a case where relatively higher toughness is necessary, it is preferable that the upper
limit of tlie P content be 0.020%.
[0024]
S: 0.0075% or less
13
S forms a non-nletallic inclasion (sulfide) and becomes a cause of deterioration
of workability and tooghness after a heat treatment, and thus the upper litnit of the S
content is 0.0075% or less. FIG. 5 shows a relationship between the S content and the
number of cracks in which the sulfide acts as the starting point (cracks starting from the
5 sulfide) during cold working. As can be seen fiotom FIG. 5, when the S content is
0.0075% or less, the number of the cracks starting hotn the sulfide largely decreases. In
addition, S is an inevitable impurity. When the S content is reduced to be less than
0.0001%, the refining cost greatly increases, and thus the S content does not need to be
reduced to less than 0.0001% or equal to or less than 0.001%. In addition, in a case
10 where it is necessary to secure relatively higher workability and toughness, it is
preferable that the upper limit of the S content be 0.007%, and more preferably be
0.005%.
[0025]
Al: 0.005 to 0.10%
15 A1 is an element that acts as a deoxidizer and is effective for fixation of N.
When the Al content is less than 0.005%, this addition effect tnay not be soficiently
obtained, and thus the lower litnit of the Al content is 0.005%. When the Al content
exceeds 0.10%, the addition effect is saturated, and there is a tetlde~lcyfo r a surface
defect to occur, and thus the lrpper litnit of the Al cotltettt is 0.10%. To further
20 sufficiently fix N, it is preferable that the lower litnit ofthe Al content be 0.01%. In
addition, to further reliably suppress the occurrence of surface defects, the upper lititit of
the Al content may be set to 0.07% or 0.05%.
[0026]
N: 0.001 to 0.01%
N is a nitride forming elentent. In curved-type continuous casting, when the
14
nitride precipitates during bending correction of cast slab, cracking may occur in the cast
slab, and thus the upper limit of the N content is 0.01%. N is an inevitable i~npurity.
As for the N content of steel, smaller is tnore preferable. However, when the N content
is reduced to be less than 0.0010%, the refining cost increases, and thus the lower limit of
5 the N content is 0.0010%. In a case where it is desirable to reduce the refining cost, it is
preferable that the lower limit of the N content be 0.002%. In a case where it is
necessary to fitrther suppress generation of the nitride or coarsening, it is preferable that
the upper litnit of the N content be 0.008%, and Inore preferably be 0.006%.
[0027]
Cr: 0.001 to 0.10%
Cr is an elelllent that increases the stability of carbide during high-frequency
heating. When the Cr content exceeds 0.10% due to the addition of Cr into steel, the
stability of the carbide greatly increases, dissolution of the carbide is suppressed during
high-frequency heating, and the hardenability decreases. Therefore, the upper linlit of
15 the Cr content is 0.10%. As the Cr content in the steel is decreased, the high-frequency
hardenability increases. However, when the Cr content is reduced to 0.001% or less, the
refining cost greatly increases, and thus the lower limit of the Cr content is 0.001%. In
a case where it is preferable to fi~rtherin crease the dissolution rate of the carbide during
the high-frequency heating, it is preferable that the upper litnit of the Cr content be
20 0.080%, and tnore preferably be 0.070%. In addition, in a case of ftn-ther reducing the
refining cost, it is preferable that the lower limit ofthe Cr content be 0.010%.
[0028]
To strengthen mechanical properties of the steel sheet, one or more of Ni, Cu,
and Mo [nay be added into the steel in the required amoant.
[0029]
Ni: 0.01 to 0.5%
Ni is an element that is effective for improving toughness atid the hardenability.
When the Ni content is less than 0.01%, this addition effect tnay not be obtained, and
thus the lower li~iiiot f the Ni content is 0.01%. When the Ni content exceeds 0.5%, the
5 effect is saturated, and the cost increases, and thus the upper limit of the Ni cotitelit is
0.5%. From the viewpoints of strength, it is preferable that the lower limit of the Ni
content be 0.05%. In addition, from tlie viewpoint of the cost, it is preferable that the
upper limit of the Ni content be 0.3%, Inore preferably be 0.2%, and still Inore preferably
be 0.15%.
[0030]
Cu: 0.05 to 0.5%
Cu is an element that is effective for securing hardenability. When tlie Cu
content is less than 0.05%, this addition effect is not de~nonstratesu fficiently, and thus
the lower li~iliot f tlie Cu content is 0.05%. When the Cu content exceeds 0.5%,
15 stiffiiess excessively increases, and cold workability deteriorates, and thus the upper litiiit
of the Co content is 0.5%. From the viewpoint of strength, it is preferable that the lower
limit oftbe Cu content be 0.08%. In addition, from the viewpoint of workability, it is
preferable that tlie upper liniit oftlie Cu content be 0.3%, inore preferably be 0.2%, and
still more preferably be 0.15%.
20 [003 I]
Mo: 0.01 to 0.5%
Mo is an element that is effective for i~nprovingth e hardenability. When the
Mo content is less than 0.01%, this addition effect decreases, and thus the lower litnit of
the Mo content is 0.01%. When tlie Mo content exceeds 0.5%, Mo-based carbide
25 precipitates niuch in the steel. Since the Mo-based carbide is not s~~fficie~ditilsyso lved
16
in tlie high-frequency quenching, the hardenability of a ~iiateriadl eteriorates, and thns the
upper liniit of the Mo content is 0.5%. In a case where a relatively higher hardenability
is necessary, it is preferable that the upper limit of the Mo content be 0.3%, and Inore
preferably be 0.1%.
[0032]
To further strengthen the mechanical properties of the steel sheet, one or Inore of
Nb, V, Ta, B, and W may be added into tlie steel in a required arnount.
[0033]
Nb: 0.01 to 0.5%
10 Nb is an element that fonus a carbonitride and is effective for preventing
coarsening of a crystal grain and for improving tougliness. When the Nb content is less
than O.OI%, tliis additio~ei ffect is not sufficiently exhibited, atid thus the lower limit of
the Nb content is 0.01%. Wlie~til ie Nb content exceeds 0.5%, the addition effect is
saturated, and thus the upper limit of tlie Nb content is 0.5%. To effectivelp use tlie
15 addition effect, it is preferable that the Nb content be 0.07 to 0.4%. According to
necessity, the lower limit of the Nb colitelit may be li~iiitedt o 0.09% or 0.14%, and the
upper limit thereof liiay be limited to 0.35% or 0.3%.
[0034]
Ti: 0.001 to 0.05%
Ti is added into the steel from the viewpoint of fixatioli of N and contributes to
st~ppressioo~fi embrittlement of cast slab and stabilization of a material quality. When
Ti is added into the steel and the Ti content exceeds 0.05%, tliis effect is saturated, and
when tlie Ti content is 0.001% or less, tliis effect may not be obtained. Therefore, the
range of the TI content is 0.001 to 0.05%. To effectively use the above-described effect,
25 it is preferable that the upper limit of the Ti content be 0.20%, Inore preferably be 0.10%,
and still more preferably be 0.06%.
[0035]
V: 0.01 to 0.5%
Similarly to Nb, V is an element that forms a carbonitride and is effective for
5 preventing coarsening of a c~ystagl rain and for iinproving toughness. When the V
content is less than 0.0 I%, this addition effect is small, and thus the lower litnit of the V
content is 0.01%. When the V content exceeds 0.5%, carbide is generated, and the
quenching hardness decreases, and thos the upper litnit of the V conte~dis 0.5%. To
effectively use the above-described effect, it is preferable that the V content be 0.07 to
10 0.2%.
[0036]
Ta: 0.01 to 0.5%
Sinlilarly to Nb and V, Ta is an elenlent that forills a carbonitride and is effective
for preventing coarsening of a crystal grain and for itnproving toughness. When the Ta
15 content is less than O.OI%, this addition effect is not sufficiently exhibited, and thus the
lower linlit of the Ta content is 0.01%. When the Ta content exceeds 0.5%, carbide is
generated, and the quenching hardness decreases, and thus the upper limit of the Ta
content is 0.5%. To effectively use the above-described effect, it is preferable that the
Ta content be 0.07 to 0.2%.
20 100371
B: 0.001 to 0.01%
B is an ele~nentth at is effective for improving the hardenability by addition of
an extremely sillall amount. When the B content is less than 0.001%, this addition
effect is not obtained, and thus the lower limit of the B content is 0.001%. When the B
25 content esceeds 0.01%, castability decreases, a B-based compound is generated, and
18
tougliness decreases. Therefore, the upper limit of the B content is 0.01%. In a case
where relatively higher hardenability is necessary, it is preferable that the lower litnit of
the B content be 0.003%. In addition, in a case where it is necessary to suppress
generation of the B-based compound, it is preferable that the upper limit of the B content
5 be 0.007%, and more preferably be 0.005%.
[0038]
W: 0.01 to 0.5%
W is an ele~netlth at is effective for strengthening of the steel sheet. When the
W content is less than 0.01%, this addition effect is not exhibited, and thus the lower
10 litnit ofthe W content is 0.01%. When the W content exceeds 0.5%, wvorkability
decreases, and thus the upper limit of the W content is 0.5%. Fro111 the viewpoillt of
strength, it is preferable that the lancer limit oftbe W content be 0.04%. From tlie
viewpoint of workability, it is preferable that tlie upper litilit of tlie W content be 0.2%.
[0039]
15 In a case of using scrap as a raw tnaterial of the steel sheet, one or more of Sn,
Sb, and As inay be unavoidably mixed into the steel. Howevel; when the content
thereof is 0.03% or less, tlie high-frequency hardenability and the hardenability do liot
deteriorate. Accordingly, one or inore of Sn: 0.03% less, Sb: 0.03% or less, and As:
0.03% or less niay be cotitaitied in the steel. Commonly, these chemical compositions
20 are cotitailled as impmities in a content of 0.003% or more, respectively. However, it is
preferable that the amount of these chemical co~npositionsis small.
[0040]
The 0 content in the steel sheet is not defined. However; when oxides
aggregate and coarsen, the cold workability decreases, and tlins the 0 content is
25 preferably 0.0025% or less. As for the 0 content, less is inore preferable. I-lowever, it
19
is technically difficult to reduce the content of 0 that is unavoidably cotitailled therein to
be less than 0.0001%, and thus 0.0001% or more of 0 may be contained therein.
[0041]
In a case of using the scrap as an ingot material of the steel sheet, elements soch
5 as Zn and Zr are mixed in as an inevitable impurities, but the above-described element
may be mixed into tlie steel within a range which does not deteriorate the properties of
the steel sheet. In addition, elements other than Zn and Zr may be mixed into the steel
witliili a range not deteriorating the properties of the steel sheet.
[0042]
10 As described above, both Cr atid Mo suppress the supply (solid solution) of C
fro111 carbide to a parent phase at a high temperature, and decrease hardenability. That
is, Cr is solid-soluted in cementite, and suppresses the solid solution of C from tlie
cementite to the paretit phase during high-frequency heating, and thus decreases
hardenability. in addition, when being excessively contained in tlie steel, Mo forms
15 Mo-based carbide. In this case, a solid solution of C fro111 tlie Mo-based carbide to the
parent phase is suppressed during the high-frequency heating, and thus liardenability
decreases. Therefore, in a case where Cr and Mo are contained in the steel, it is
preferable that tlie Cr content [Cr] and the Mo content [Mo] satisfy the following
Expression (1).
20 [Cr]+[Mo]/lO i 0.10 ... (I)
As described above, the medium carbon steel sheet of this embodiment bas a
chemical composition that contains the above-described basic elements, the remainder
being Fe and inevitable impurities, ora cheiiiical composition that contains the
above-described basic elements and at least one selected fro111 the selective elements, the
25 balance composed of Fe and inevitable impurities.
[0043]
Furthennore, in this embodiment, it is necessaty to control the shape of the
carbide in addition to the above-described clietnical composition. Hereinafter, the shape
of the carbide will be described in detail.
Specifically, the average diameter of the carbide is 0.6 pm or less, the
spheroidizing ratio of the carbide is equal to or more than 70% and less than 90%, and
the average diameter d (pm) of the carbide and the spheroidizing ratio p (%) of the
carbide satisfy the following Expression (2).
d<0.04xp-2.6 . . . (2)
[0044]
It is preferable to use a scanning electron nlicroscope for the observation of a
structure (carbide). Four or nlore sites of visual fields (regions), in which 500 or more
carbides are contained on a structure observation surface at a tnagnification of 3,000
times, are selected, and an area of each carbide contained in the regions is measured.
15 Here, carbide having the area of 0.01 pm2 or less is excluded from an object to be
evaluated to suppress an effect of a measurenlent error due to a noise. The diameter
(equivalent circle diameter), which is obtained by approxinlating an average area (an
average value of the area) of the carbide that was measure to a circle, is defined as an
average diameter (average carbide diameter). Carbide in which the ratio of the long
20 axial length to the short axial length (aspect ratio) of each carbide is 3 or more is defined
as acicular carbide, and carbide in which the ratio is equal to or more than I and less than
3 is defined as spherical carbide. In addition, a value, which is obtained by dividing the
tiun~bero f the spherical carbides by the nlnnber of total carbides, is defined as the
spheroidizing ratio of the carbide.
It is necessary for the average diameter of the carbide to be set to 0.6 pm or less.
Since it takes long time to co~npleted issolution of coarse carbide, there is a te~idelicyf or
tlie hardenability to deteriorate. Particularly, in a case where the average diameter of
5 tlie carbide is larger than 0.6 pm, the quenching hardenability during high-frequency
quenching at an average heating rate of 1OOoC/second decreases. In addition, it is
preferable that the average diameter of the carbide be controlled to be 0.55 Fun or less
accordi~igto co~lditiotlso f the high-frequency quellchitig and the chea~icacl omposition,
and Inore preferably 0.5 Fun or less. In addition, in the above-described lneasuretnent
10 method, the average diameter of the carbide having an area exceeding 0.01 pm2 niay be
set to a range exceeding 0.1 1 (= 0.21dn) bun and equal to or less tlla~i0 .6 lun.
[0046]
The spheroidizing ratio of the carbide is equal to or more than 70% and less than
90%. There is a tendency for stress during cold working to be localized at the periphery
15 of the acicular carbide, atid there is a tendency for the periphery to acts as a starting point
of cracking. Particularly, when the splleroidizing ratio is less that1 70%, the cold
workability deteriorates, and thus the spheroidizing ratio of the carbide is set to be 70%
or more. In addition, in a case where relatively higher cold workability is necessary, it
is preferable that tlie spheroidizing ratio of tlie carbide be 73% or more, and more
20 preferably be 75% or inore. On the other hand, in the spherical carbide, the surface area
at which the steel atid the parent phase come into contact with each other is stualler, and
the emission and diffi~siop~ait h of carbon fiom the carbide to the parent phase is
liarrower conlpared to the acicular carbide. Particularly, in a case where tlie
spheroidizing ratio is 90% or niore, tlie quenching hardenability in the high-frequency
22
quenching at an average heating rate of 100°C/second is not sofficient. In addition, it is
preferable that the spheroidizing ratio of the carbide be controlled to be less tlian 85%
according to the conditions of tlie high-frequency qaenching. In addition, in the
above-described measurement method, the spheroidizing ratio of tlie carbide having an
5 area exceeding 0.01 pm2 may be set to a range equal to or tilore tlian 70% and less than
90%.
[0047]
In addition to the above-described conditions (the average diameter and the
spheroidizing ratio), it is necessary that the average diameter d (pn) of the carbide and
10 the splieroidizing ratio p (%) of the carbide satisfy the above-described Expression (2).
That is, when the spheroidizing ratio ofthe carbide is equal to or more than 70% and less
than 80%, and tlie acicular carbide is rich, the absolute value of tlie long axial lengtli of
the acicular carbide has an effect on cold workability. Tlierefore, tlie relationship of
Expression (2) is necessary between the spheroidizing ratio of the carbide and the
15 average diameter of the carbide. Hereinaftel; Expression (2) will be described.
The cold \vorkability has a close relationship with the number of cracks during
cold working. The larger the nu~nbero f cracks is, the lower the cold workability is. It
is considered that each crack during the cold working is generated from a void (atomic
vacancy), which is generated due to entanglenient of dislocation introduced by tlie
20 working or cutting of the dislocation, as a nucleus. Therefore, it is possible to secure
cold workability by suppressing the concentration of working strain.
When the carbide has tlie acicular shape, a dimensional difference between the
short axial lengtli and the long axial length is large, and stress is concentrated on ends
(ends of the long axis) in a long axial direction of tlie acicular carbide, and thus a
25 difference in stress between a stress field in the ends of the long axis and a stress field ill
23
the ends (ends of short axis) in a short axial direction increases. Dislocation (strain) is
introduced to solve unevenness of this stress field. Therefore, it is considered that a
number of voids are generated in the vicinity of the acicular carbide during the cold
working, and the crack is generated. On the other hand, when the carbide has the
5 spherical shape, the di~nensionald ifference behveen the short axial length and the long
axial length is small, and thus tlie unevenness of the stress field is less. Therefore, it is
considered that the dislocatio~(ls train) in the vicinity ofthe carbide is not likely to be
localized, and thus generation of cracks is suppressed.
In addition, not only the aspect ratio of the carbide but also the absolute value of
10 the long axial length of the acicular carbide has an effect on the stress concentration at
the ends of tlie lotig axis. The larger the long axial length is, the more the stress
concentration to the elids of the long axis increases. Therefore, there is a telidellcy for
dislocations (strain) to occur. Accordingly, in a case \vliere the sphcroidizitlg ratio of
the carbide is not high (in a case where the acicular carbide is rich), it is necessary to
15 make the average diameter of tlie carbide small to secure cold workability. That is, in a
case where the spheroidizing ratio of the carbide is equal to or more than 70% and less
than SO%, it is necessary for the average diameter d (lun) of the carbide and the
spheroidizing ratio p (%) of the carbide to satisfy the above-described Expression (2).
In this matuner, the present inventors found that when appropriate predetermined
20 conditions are satisfied with regard to the average diameter of tlie carbide and the
spheroidizing ratio of the carbide, cold workability may be increased while securing
high-frequency hardenability.
[004S]
Furthermore, in this embodiment, in addition to the above-described chemical
25 composition and the shape of the carbide, it is preferable to control the hardness before
cold working.
When the hardness before the cold working is less than 170 HV, sufficient
ductility may be obtained, and thus a sufficient amount of shaping niay be secured during
the working. To secure a relatively larger atnount of shaping, it is preferable that the
5 hardness before the cold working be less than 165 HV, more preferably be less than 160
HV, and still more preferably be less than 155 HV. When the steel sheet is soft, ductility
is improved, and thus the steel slieet may also endure hard working, but there is a
tendency for sagging to occur during punching working. Therefore, it is preferable that
the hardness before the cold ivorking be 120 HV or more. In recent years, a cold
10 working technology in which puncliing, bending, and drawing \\lorking are conibined has
also spread, and thus it is preferable that the hardness before tlie cold working be
appropriately controlled according to the combination of these ~nanufacturingp rocesses.
[0049]
The teclinology, in which in addition to the chemical compositiotl, the
15 above-described predetermined conditions are satisfied \\,it11 regard to the average
diameter of the carbide and the splieroidizing ratio of the carbide so as to realize
cotnpatihility between the cold workability and the high-frequency hardenability of the
steel slieet, is a new finding of the present inventors. The present inventors have also
found tliat when the hardness before the cold working is controlled to be less than 170
20 HV, the steel slieet may be appropriately used for cold working.
[0050]
It is preferable that the quenching hardness after the high-frequeicy quenching
be 500 HV or inore. When the quenching hardness is 500 HV or more, abrasion
resistance accompanying high-strength of quenched steel is improved. Particularly, in a
25 nietnber such as a clutch plate and a gear tliat are colnponents of a vehicle, hardening of
25
500 HV or more is preferable to obtain abrasion resistance. When the quenching
hardness is too high, toughness of a quenched portion greatly decreases, and thus a
fi~tictiotai s a member for a mechanical structure may be lost. Therefore, it is preferable
tliat the quenching hardness after the high-freqoency quenching be 900 HV or less, tnore
5 preferably be 800 HV or less, and still tilore preferably be 750 HV or less.
[005 I]
Here, to define a standard of the quenching liardness that is necessary for a part,
high-frequency quetichilig is performed in such a llianlier tliat heating is performed from
rootn temperature to l,OOOoC at an average heating rate of 100"C/second, holditig is
10 carried out for 10 seconds, and quick cooling to roo111 tetnperature is immediately carried
out at an average cooling rate of 20O0C/second or more. Specifically, the test conditions
of the high-frequency quenching in the present invention are as follows. The
temperature is raised from room temperatore to L,000+20°C at an average heating rate set
to 100i15"C/second in a temperature range of 750°C or highel; holding at 1,000i20°C is
15 carried out for 10zh0.5 seconds, and quick coolitig is perfomled to room tetllperatltre at an
average cooling rate set to 200k10°C between 800°C and 400°C. A steel sheet, which
has a Vickers hardness of 500 or tnore (that is, 500 HV or more) after the high-frequency
quenching under these conditiotls, is a target of the present invention.
[0052]
20 In addition, although the sheet thicktiess is not particularly limited, it is
preferable that the slieet thickness of the steel slieet be 20 tiitn or less or 16 mm or less,
more preferably be 14 tiitn or less, still more preferably be 12 111111 or less or 9 mm or less
from the viewpoint of workability. 111 addition, from the viewpoint of strength, it is
preferable that the slieet thickness be I tnni or more or 2 mm 01. more, Inore preferably
26
2.5 mm or more, and still riiore preferably be 3 mm or more.
[0053]
Furthennore, an important concept of the steel sheet of this embodiment will be
described referring to FIGS. 1 to 5.
[0054]
FIG. 1 shows an effect of tlie average diameter of the carbide and the
spheroidizing ratio of the carbide on the quenching hardness and the cold workability.
In addition, the cold workability is evaluated by a flat-sheet bending test using a test
specitlien (flat-sheet bending test specimen) having a \vidth of 30 nxn and a length of 100
10 mm. In this bending test, compression stress arid tensile stress are applied to an inner
surface (inner circumferential surface) and an outer surface (outer circu~nferential
surface) of the bended sample (flat-sheet bending sample), respectively, and thas
workability due to the compression stress and workability due to the tensile stress can be
simultaneoasly measured by evaluating cracks on an inner surface side and on an outer
15 surface side of tlie sample. In tlie present invention, the bending radius is set to 112
times tlie sheet thickness, and the bending angle is set to 90'. As shown in FIG. 1, in a
steel sheet in which the average diameter d of the carbide is 0.6 pm or less, the
spheroidizing ratio p of the carbide is equal to or more than 70% and less than 90%, and
the average diameter d (pm) of the carbide and the spheroidizing ratio p (%) of the
20 carbide satisfy the above-described Expression (2) (a white circle in FIG. I), a qoenching
hardness of 500 HV or more is obtained after tlie high-frequency quenching, and
cracking does not occur during working. Conversely, in a steel sheet in which the
spheroidizing ratio of tlie carbide is equal to or Inore than 70% and less than 80%, and
the average dianieter d (11111) of tlie carbide and the spheroidizing ratio p (%) of the
27
carbide do not satisfy the above-described Expression (2) (a black square in FIG. I), or a
steel sheet in which the spheroidizing ratio of the carbide is less than 70% (a black
square in FIG. I), the percentage of the carbide that acts as a starting point of fiacture
increases, and thus cracking occurs during the working. In addition, in a steel sheet in
5 which the average diameter of the carbide exceeds 0.6 pm, and the spheroidizing ratio of
the carbide is equal to or Inore than 80% and less than 90% (a black triangle in FIG. 1) or
a steel sheet in which the spheroidizing ratio of the carbide is 90% or inore (a black
triangle in FIG. I), the qoenching hardness is not sufficient.
[OOSS]
10 Here, as long as not pal-ticularly stated additionally, the quenching hardness that
is used as a standard is defined as hardness measured by the following conditions.
Specifically, the high-frequency quenching is performed in such a manner that a salllple
is heated fro111 room telnperatiire to l,OOO°C at a fi'equency of 78 kHz at an average
heating rate of 10O0C/second, the sample is held for I0 seconds, and then the sample is
15 quickly cooled to room temperature at an average cooling rate of 20O0C/second or more,
and then Vickers hardness is measured. The Vickers hardness after the high-frequency
quenching is the quenching hardness of this embodiment. As the spheroidizing ratio
decreases, it is easy for the carbide to be dissolve. Therefore, there is a tendency for the
quetlchitig hardness to increase. Similarly, as the average diameter of the carbide
20 decreases, it is easy for the carbide to be dissolved. Therefore, there is a tendency for
the quenching hardness to increase. On the other hand, as the spheroidizing ratio
increases, the workability increases. As described above, when the shape of the carbide
is controlled, the workability and the quenching hardenability, which are contrary to each
other, may be co~npatiblew ith each other.
28
[0056]
FIG. 2 shows a relationship between the Si content and the cold workability (the
number of cracks at a carbide interface and in a grain after the cold working). The
unmnber of cracks at regions of 118 to 318 and 518 to 718 of the sheet thickness in a sheet
5 thickness cross-section (cross-section including a sheet thickness direction and a
longitodinal direction) of a bending angle portion (maximum curvature portion) of the
flat-sheet bending sample is nieasured by a scanning electron niicroscope at a
tliagnification of 3,000 times. In a case where the number of cracks is within 20 per 1
~n~ni't, may be determined that occurrence of cracks caused by interfacial peeling during
10 the cold working is suppressed, and thus the cold workability is evaluated as "good". In
addition, in a case where the number of cracks exceeds 20, the cold workability is
evaluated as "poor". As sliowtl in FIG. 2, in a steel sheet in \vliicli the Si content is
0.06% or more, the cold workability is good. Furthennore, until the Si content reaches
0.3%, the Si content increases and the nu~nbero f cracks decreases. In this case, the
15 cracks are generated due to peeling at an interface between ferrite and carbide.
Furtherunore, when the Si content exceeds 0.3%, the Si content increases, and the number
of cracks increases. In this case, cracks are generated in a ferrite phase. In addition,
when the Si content increases and is 0.06 to 0.1%. a rate which the nomber of cracks are
decreased is large, and the cold workability is largely improved. Furthermore, when the
20 Si content is 0.15 to 0.26%, the effect of improving the cold workability due to Si is
~nasimallyo btained. In addition, in FIG. 2, after a saniple in which the C content is
0.40 to 0.45% is annealed at 680°C for 30 hours, evaluation of the cold workability to be
described later is performed.
[0057]
FIG. 3 shows a relationship between [Cr]+[Mo]/lO and the quenching hardness.
29
As sliown in FIG. 3, when [Cr]+[Mo]/lO is less than 0.10, the quenching hardness is
fi~rtherim proved. Therefore, it is preferable that [Cr]+[Mo]/lO be less than 0.10. In
addition, in FIG. 3, after the sample in which the C content is 0.40 to 0.45% is annealed
at 680°C for 30 hours, the above-described high-frequency quetiching is perfonned, and
5 then the quenching hardness is measured.
[0058]
FIG. 4 shows a relationship between the spheroidizing ratio of the carbide and
tlie ~iutnheor f cracks that are generated from the carbide as a starting point (cracks
starting from the carbide) daring tlie cold working. As can be seen fro111 FIG. 4, when
10 the splieroidizing ratio of tlie carbide is less than 70%, the number of cracks rapidly
increases.
In addition, FIG. 5 shows a relationship between the S content and the number of
cracks starting fro111 sulfide during tlie cold working. As can be seen frotn FIG. 5, when
the S content is less than 0.0075%, the number of cracks starting frotn the sulfide greatly
15 decreases.
In addition, an energy dispersion X-ray spectroscope (EDS) that is attached to a
scanning electron tnicroscope is used to detertiline tlie cracks caused by tlie sulfide and
the cracks caused by the carbide.
LO0591
Next, a method for nianufacturing the mediom carbon steel sheet for cold
working according to an euiibodit~ienot f the present invention (hereinaRer, referred to as
"a tnanofacturing inethod of this emboditnetit") will be described. The manufacturing
method of this embodiment has a technical idea of cotllbining high-temperature coiling
atid low-temperature annealing, and a specific example thereof is shown below. In
25 addition, tlie tiian~tfacturingm ethod to be showti below is an example, and other
30
lnaril~facturingm ethod tnay be adapted as long as a necessary structure ]nay be obtained.
[0060]
First, continuous casting slab (cast slab) having a chemical cotnpositioll of the
above embodiment is heated. The heating te~ilperatureis set to 1,050 to 1,300°C. In
5 addition, to suppress decarbonization and nitrogen absorption during heating, it is
preferable that the soaking time be 150 ~ninuteso r less, and Inore preferably be 90
minutes or less. Here, the soaking time represents the time taken fro111 a time at which a
surface temperature of the cast slab reaches a te~nperat~ilroew er than a target heating
tetnperature by 20°C to a time at which the cast slab is taken out from a heating furnace.
10 When the heating tetnperature exceeds 1,300°C or' tlie soaking time is excessively long,
decarbonization at a surface layer portion of the slab beco~nesi gnificant during the
heating process, and thus liardenability of the surface of the steel sheet deteriorates. In
addition, when the cast slab is heated at 1,050°C or highel; a structure of an
approximately austenite single phase lllay be obtained. Froill the viewpoint of
15 sappressing decarbonization, it is preferable that tlie heating temperature be 1,280°C or
lower, Inore preferably be 1,24O0C or lower, and still more preferably be 1,200°C or
lower. Similarly, it is preferable that the soaking time be 60 ininutes or less. In
addition, the lower limit of the soaking time is not particularly liinited.
[0061]
20 In addition, the coiltinuous casting slab tnay be directly provided to hot rolling,
or the continuous cast slab may be reheated after being cooled and may then be provided
to the hot rolling. A difference in properties of a steel sheet hardly occurs between tlie
former and tlie latter.
[0062]
31
As the hot rolling, any cotntnon hot rolling or cotltit~uoush ot rolling in which
slab is joined at finish rolling tnay be adapted. The termination temperature of the hot
rolling (hot-rolling termination temperature) is determined kom the viewpoints of
productivity, accuracy of sheet thickness, improvement of anisotropy, and suppression of
5 a surface defect. When the hot-rolling ternlination temperature is lower than 750°C,
lots of surface defects occur due to seizure during finish rolling. In addition, when the
hot-rolling ternlitlation temperature is higher than 1,00O0C, the occurrence frequency of
the defects caused by scale increases, and thus a yield ratio decreases, and the cost
increases. Therefore, the hot-rolling termination tenlperature is set to 750 to l,OOO°C.
[0063]
Furthennore, the steel sheet after the hot rolling (finish rolling) is cooled down
to a cooling temperature (first cooling termination temperature) of 500 to 700°C at an
average cooling rate (a first average cooling rate) of 20 to 50°C/second imniediately after
the hot rolling. In this case, a lamella spacing of pearlite that is generated in the steel
15 sheet is decreased while litniting generation and growth of pro-eutectoid ferrite, and thus
the carbide in the steel sheet after annealing may be made fine. When the average
cooling rate fro111 the hot-rolling termination temperature to the cooling tetnperature (the
first cooling tertnination temperature) is 20°C or lowet; since the generation and growth
of the pro-eutectoid ferrite are not limited, a pearlite band caused by segregation tnay be
20 generated. Therefore, there is a tendency for coarse carbide to be generated after
annealing, and tluts hardenability tnay decrease. In addition, when the average cooling
rate is 5O0Clsecond or tnore, it is difficult to control the temperature of the steel sheet.
Furthermore, the pearlite, which is necessary to secure cementite having the
above-described spheroidizing ratio, is not sufficiently generated. As described above,
32
the above-described cooling temperature is controlled to 500 to 700°C in consideration of
a transformation initiation temperatnre and a transformation termination temperature of
pearlite to appropriately perfonn a strocture control of the pearlite.
[0064]
Then, the steel sheet is cooled to a coiling tetnperature (a second cooling
ter~ninatioitie mperature) that is equal to or higher than 400°C and lower than the first
cooling tennination temperature by at least 50°C (that is, a temperatnre at which a
difference between the first cooling termination temperature and the second cooling
ter~~ii~iattieomllp erature is 50°C or higher, and which is equal to or lower than a
10 temperature that is lower than the first cooling ter~ninationt emperature by 50°C at an
average cooling rate (a second average cooling rate) of 5 to 30°C/second, and then the
steel sheet is wound. 111 this case, the lainella spacing of tlie pearlite that is generated in
the steel sheet may be reduced while securing the amount of pearlite in tlie steel sheet,
and thus the carbide in the steel sheet after annealing may be made fine. In a case
15 where the steel sheet is \w,ound at a temperature range from the first cooling terinination
temperature to a teniperature that is lower than the first cooling termination temperature
by 50°C), pearlite having a rough lamella spacing is generated, and it is difficult for the
carbide after the annealing to be splleroidized, and thus cold workability deteriorates.
Therefore, the coiling is performed at a teolperature that is lower than the first cooling
20 tennination temperature by at least 50°C). In addition, when the average cooling rate in
a range from the above-described cooling teniperature to the coiling telllperature is
S0C/second or less, a pearlite band caused by segregation may be generated, or tlie
lamella spacing of pearlite may greatly increase. Therefore, there is a tendency for
coarse carbide to be generated after annealing, and the hardenability of the steel sheet
33
may decrease. In addition, wllen the average cooling rate is 3O0C/second or more, the
pearlite, which is necessaty to secure cementite having tlie above-described spheroidizing
ratio, is not sufficiently generated.
[0065]
In addition, as described above, the steel sheet after the cooling is wound at the
coiling temperature which is from 400°C to a temperatnre that is lower than the first
cooling termination telnperahlre by at least 50°C. When the coiling temperature is
lower than 400°C, partial mattensite transfonnation occurs, and the strength of the steel
sheet increases, and thus handling nlay be difficult. In addition, a tnicrostructure
10 becoilles non-uniform, and gauge hunting occurs during cold rolling, and thus a yield
ratio limy decrease. On the other hand, when the coiling is perfornled at a temperature
higher than a te~nperatarelo wer than the first cooling ternlinatioti temperature by 5O0C,
as described above, pearlite having a rough lamella spacing is generated, and the
splieroidizing ratio after annealing decreases, and thus cold workability deteriorates.
[0066]
Furthennore, the steel sheet is held at a temperature in a range of 400°C to the
second cooling termination te~nperaturein such a manner that a time for which a wound
steel sheet (coil) is held at 400°C or higher is linlited to 30 hours or less. Then, the steel
sheet is cooled to a temperature of 400°C or lower (for example, room temperature, or a
20 teniperature at which acid washing is possible). Here, in a case of perfonning tlie
holding and the cooling as the same process, the time at which the steel sheet is held at
400°C or higher during tlie cooling, is lilnited to 30 hours or less. In this case,
decarbonization is suppressed, and thus tlie C content on a surface may be sufficiently
secured. When the time for which the steel sheet is held at 400°C or higher exceeds 30
34
hours, at1 oxygen sonrce (for example, air) and carbon react with each other on the
surface of the steel sheet, and it is difficult to secure the carbon content on the surface of
the steel sheet, which is necessaly for the high-frequency quenching.
The cooled steel sheet is performed pickling to clean the surface, and then the
5 steel sheet is subjected to softetiitlg annealing. It1 this embodiment, tlie steel sheet is
subjected to the softening box annealing to improve the workability of the steel sheet.
[0067]
In the softet~itiga nnealing (annealing), the steel sheet is heated from roolll
temperature to an annealing temperature of 600°C to A,!-10°C, and then the steel sheet is
10 held for a time eq~catl o more than 5 hours and less than 100 hours. By holding for a
time equal to or more than 5 lioors and less than 100 hours, the steel sheet is annealed in
such a manner that a ferrite grain is coarsened, and the steel sheet becomes soft, and the
spheroidizing ratio of the carbide is not too lligh. When the annealing te~nperaturei s
A,I-10°C or higher (particularly, A,l0C or higher), since the carbide is rapidly
15 spheroidized, the hartlenability during subsequent high-frequency quenching decreases.
On tlie other hand, when the annealing temperature is 600°C to A,I-10°C, the diffi~sion
rate of elemelits in the steel sheet (particularly, C) is optimized, and thus tlie
spheroidizitlg ratio of the carbide may be appropriately controlled.
[0068]
20 In the above-described box annealing, it is preferable that the hydrogen
concentratioti is 95% or more, the dew point at 400°C or lower is less than -20°C, and the
dew point at a temperatore higher than 400°C is less than -40°C.
[0069]
Wlieti the annealing is performed in an atmosphere in which tlie hydrogen
3 5
concentration is 95% or inore, tlie temperature distribution in the coil may be controlled
in a relatively uniform mannel; and in addition to this, a decrease in hardenability due to
intrusion of nitrogen may be suppressed. In addition, when the dew point at 400°C or
lower is controlled to be less than -20°C, and the dew point at a temperature higher than
5 400°C is controlled to be less than -40°C, decarbonization during the annealing may be
sufficiently suppressed.
[0070]
With regard to other treatments, there is no particular li~nitationth ereto as long
as the shape of the carbide satisfies the conditions of the steel sheet of tlie
10 above-described embodiment. For exaniple, cold rolling and subsequent softening
annealing may be performed. Here, for the easy anderstanding, FIG. 8 shows a
flowcha~stc hematically illustrating an outline of a method of manufactoring tlie steel
slieet for cold working of the present invention, which includes this embodinient and tlie
following niodified examples.
[0071]
In addition, a surface treatment filni may be fornied on a surface of the steel
slieet of the above-described e~nbodi~nefnrot m a viewpoint of lubricity to further
improve tlie cold workability. For exaniple, as a modified esaniple of the steel sheet of
the above-described embodiment, it is preferable to fomi thefollowing surface treattilent
20 film 011 the snrface of the steel sheet of the above-described embodiment.
[0072]
In this niodified example, a gradient-type surface treatment film including an
adhesion layer that secures adhesiveness with the steel slieet that is a base material, a
base layer that suppo~tsa lubricant, and a lubricant layer that i~iiprovesln bricity is
25 provided on tlie surface of the steel sheet, and tlie thickness of each layer is controlled.
36
Accordingly, a surface treatment film, which is veiy suitable from the viewpoint of
global enviro~line~clotn servation, may be formed on the surface of the steel sheet with a
simple treatment process. In addition, lubricity, a seizure prevention perfor~nancea, nd a
galling prevention perfor~nanceth at are excellent may be given to the steel sheet.
[0073]
Hereinafter, a steel sheet for cold working (steel sheet) according to this
modified example will be described in detail while referring to FIG. 6.
In addition, in the present specificatio~al nd the above drawings, like reference
numerals will be given to like parts having substantially same functions, and redundant
10 description thereof will be omitted here.
[0074]
[Configuration of Steel Sheet according to This Modified Example]
First, a configuration of a steel sheet (hereinafter, referred to as a "surface
treated steel sheet") according to this modified example will be described while referring
15 to FIG. 6. FIG. 6 sho~vas longitudinal cross-sectional diagram schematically illustrating
a co~ifigilrationo f the surface treated steel sheet.
[0075]
As shown in FIG. 6, a surface treated steel sheet 1 inclitdes a steel sheet 10 that
is a base material, and a surface treatment film (film) 100 that is forriled on at least one
20 surface ofthe steel sheet 10.
[0076]
(Steel Sheet 10)
As the steel sheet 10 that is a base niaterial of the surface treated steel sheet 1,
the steel sheet of the above-described e~nbodi~nemnta y be used as is. However, the
25 steel sheet of the above-described embodituent tnay be subject to plating. For example,
37
the steel sheet of the above-described embodiment tnap be subjected to plating using one
or more tnetals of zinc, nickel, iron, alu~ninuint,i tanium, n~agnesium,c hrome,
manganese, and tin.
[0077]
(Surface Treatment Filnl 100)
In the surface treatment filtn 100, each chetnical colnpositiotl in the film has a
concelltration gradient in a filtn thickness direction. Therefore, the surface treatlnetlt
filtn 100 that is a gradient-type fill11 inay be divided into three layers. That is, in the
surface treat~netltf ihn 100, three layers including an adhesion layer 110, a base layer 120,
10 and a lubricant layer 130 are formed in this order fro111 an interface between the surface
treatment fill11 100 and the steel sheet 10 toward a surface of the surface treatmetit fill11
100.
[0078]
Here, tlie "gradient-type" in this modified example represents that the chemical
15 coinpositions that are contained in tlie surface treatment fill11 100 have a concentration
gradient in the film thickness directioll ofthe filtn as described above. That is, tnain
chemical colnpositions in the surface treatment film 100 include a chemical cotnpositio~l
that is derived from a silanol bond (details thereofwill be described later) formed
behveetl the film and a tiletal otl the surface of the steel sheet 10 that is a base material, a
20 heat resistant resin, an inorganic acid salt, and a lubricant. These clie~nical
compositions have the concentratioil gradient in the film thickness direction of the
surface treattnent film 100. More specificall): the concentration of a lubricant 13 1
increases fro111 the interface between tlie surface treat~nentf ilm 100 atld the steel sheet 10
toward the surface of the surface treatinent film 100. Conversely, a concentration of
25 each of the heat resistant resin and the inorganic acid salt decreases. In addition, as it
3 8
closes to the interface between the surface treatment film 100 and tlie steel sheet 10, the
clie~nicacl ornposition derived from the silanol bond increases. Accordingly, it does not
mean that the adhesion layer 110, the base layer 120, and the lubricant layer 130, which
are included in the surface treatment film 100, are cotnpletely separated from each other
5 and form three layers (a chemical cotnposition in an arbitra~yla yer that is not present in
another layer).
[0079]
Hereinafter, a configuratioti of each layer configuring the surface treatment film
100 will be described in detail.
[0080]

The adhesion layer 110 secures adliesiveness during the cold working between
the surface treatment film 100 and the steel slieet 10 that is a base material, and has a
f~~nctiofn p reventiiig seizure between the surface treated steel sheet 1 and a inold.
15 Specificallj: tlie adliesio~l layer 110 is located at a side close to tlie interface between tile
s~~rfatcrea tment film 100 and the steel sheet 10, and contains the largest amount of a
chemical composition derived from tlie sila~iobl ond alllong the three layers configuring
the surface treatlne~itfi ll11 100.
[0081]
Here, the silanol bond in this modified exa~ilpleis expressed by Si-0-X (X
represents a ~netatlh at is a constituent cliemical cotllposition of the steel slieet (base
material)), and is for~nedin the vicinity of the interface between the surface treatment
film 100 and the steel sheet 10. It is assi~~nethda t tliis sila~iobl ond corresponds to a
covalent bond between a silane coupling agent contained in a sarface treatment liquid
25 that forms the surface treatment film 100, and an oxide of a metal (for exaluple, in a case
39
where the steel sheet 10 is subjected to plating, metal species (Zn, AI, and the like)
contained in a plating material, and in a case where the steel sheet 10 is not subjected to
the plating, Fe) on the surface of the steel sheet 10. In addition, whether or not the
silanol bond is present may be confirn~edb y a method capable of performing element
5 analysis in a depth direction of a sample (for example, whether or not the sila~iobl ond is
present may be confir~nedb y determining the quantity of each element from a spectrum
intensity of chemical composition (Si, X, 0) element derived from the silanol bond in the
fill11 tllick~lessd irection of the surface treatment film 100 using a high-frequency glow
discharge emissio~sip ectrometric analyzer (high-fi'equency GDS). In addition, whether
10 or not the silanol bond is present may be confirmed by directly observing a cross-section
of the sample (for esa~nplea, n observation tnethod nsing a field emission-type
transmissiotl electrot1 microscope (FE-TEM)) and by an elementary a~lalysiso f a micro
part (for example, an analysis method using an energy-dispersive X-ray spectroscope
(EDS)).
15 [OOS2]
In addition, it is necessaly for the adhesion layer 110 to have a thickness of 0.1
nln to I00 am. Wben the thickness of the adhesion layer 110 is less than 0.1 Inn, the
formation of the sila~lobl ond is not sufficient, and thus st~fficie~aldt hesivity betweet1 the
surface treatment film 100 and tlie steel sheet 10 may not be obtained. On the other
20 hand, when the thickt~esso f the adhesion layer 1 I0 exceeds 100 nm, the number of the
silanol bonds becomes excessive, and internal stress inside the adhesion layer 110
increases during the working of the surface treated steel sheet 1, and thus the fill11
becomes brittle. As a result, the adhesivity between the surface treatment film 100 and
the steel sheet 10 decreases. Fro111 the viewpoint of further reliably securing tlie
25 adhesivity between the surface treatment fihn 100 and the steel sheet 10, it is preferable
40
that the thickness ofthe adhesion layer 110 be 0.5 to 50 nm.
[0083]

The base layer 120 itnproves steel-sheet follow-up properties during the cold
5 working, and gives hardness and strength against the seizure with the tnold to the surface
treated steel sheet 1. In addition, the base layer 120 suppot-ts the lubricant 13 1 and the
lubricant layer 130. Specifically, the base layer 120 is positioned between the adhesion
layer 110 and the lubricant layer 130 as an intermediate layel; and contains the largest
amount of the heat resistant resin and the inorganic acid salt among the three layers
10 configuring the surface treatment fill11 100.
[0084]
Whet1 the inorganic acid salt is selected as a chemical composition contained in
tlie base layer 120, the fill11 having the gradient-type three-layer structure sinlilar to this
nlodified exalnple may be formed, and the above-described fi~~lctiootfl the base layer
15 120 may be surticiently carried out. In addition, in this ~nodifiede xample, since the
surface treatment filtn 100 is for~nedu sing a water-based sitrface treatnlent liquid, it is
preferable that the inorganic acid salt in this modified exanlple be water-soluble in
consideration of stability of the surface treatment liquid. However, even when the
inorganic acid salt is a salt that is insoluble or poorly soluble in water, for example, in a
20 case of a salt that is soluble in an acid, it is not necessary to consider the stability of the
surface treatment liquid. When an inorganic acid salt that is soluble in water (zinc
nitrate) and an acid (for example, phosphoric acid) are used in combination, a film
containing a salt that is insoluble or poorly soluble in water (for example, zinc phosphate)
may be formed.
25 [OOSS]
41
Fro111 the above-described fi~nctiona, s the inorganic acid salt in this modified
example, for exa~nplep, liospliate, borate, silicate, sulfate, molybdate, and tungstate may
be used alone or in combination of two or more kinds. More specifically, as the
inorganic acid salt, for example, zinc phosphate, calcium phosphate, sodium sulfate,
5 potassium sulfate, potassium silicate, sodium borate, potassium borate, amtnonium
borate, potassiu~nm olybdate, sodium molybdate, potassium tungstate, sodi~~tumn gstate,
or the like may be used. I-Iowevel; alllong these, it is more preferable that the inorganic
acid salt be at least one compound selected from a group consisting of phosphate, borate,
and silicate in consideration of convenience and the like during measurement of the
10 thickness of each of tlie adhesion layer 110, the base layer 120, and the lubricant layer
130.
[0086]
In addition, the licat resistant resin as a main cl~emicacl omposition is contained
in the base layer 120. During the cold working, since the surface treated steel sheet I
15 becomes a relative high te~nperaturec lue to a frictional force between the surface treated
steel sheet 1 that is a base material and the mold, it is necessary for the surface treatment
film 100 to ~naintaina shape as a film even under tlie high-temperature working
conditions. From this viewpoint, it is preferable tliat the heat resistant resin in this
modified example have heat resistance that is capable of maintaining the film shape at a
20 telnperature (for example, a predetermined tenlperature higher than 200°C and equal to
or lower than 400°C) exceeding an arrival te~nperature( approximately, 200°C) during the
cold working. In addition, in this niodified exa~nples, ince the surface treatment filnl
100 is formed using the water-based surface treatment liquid, it is preferable tliat the heat
resistant resin in this nlodified example be water-soluble in consideration of stability of
25 tlie surface treatment liquid.
[0087]
Fro111 tlie above-described fi~nction,a s the heat resistant resin in this nlodified
example, for example, a polyimide resin, a polyester resin, an epoxy resin, a fluorine
resin, or tlie like may be used. To secure relatively higher heat resistance and water
5 solubility, particularly, it is preferable to use at least one resin selected from a group
co~isistiligo f the polyi~iiidere sin and the fluorine resin as the heat resistant resin.
F~irtliennore, it is more preferable to use the polyitnide resin as the heat resistant resin.
[0088]
In addition, a coinposition of the base layer 120 also has an effect oti a
10 coinpositioti of the surface treated steel sheet 1. Therefore, in this modified example,
the base layer 120 contains the heat resistant resin as a main chemical composition so as
to give a working follow-up property and heat resistance to the surface treat~nenft ilm
100. Furtherinore, in the base layer 120, for example, a contained aniount of an
inorganic component such as phosphate, borate, silicate, molybdate, and tiiligstate is
15 adjusted to be snialler than a contained amount of tlie heat resistant resin. Specifically,
the base layer 120 contains 0.1 to 10 parts by mass of the inorgatiic acid salt to 100 parts
by Inass of the heat resistant resin. When tlie contained amount of the inorganic acid
salt is less than 0.1 parts by mass, the coefficient of frictioli of the surface treat~ilentf ilm
100 increases, and thus sufficient lubricity may not be obtained. On the other hand,
20 when tlie contained amount of the inorganic acid salt exceeds 100 parts by mass, the
performance of the base layer 120 that suppot-ts tlie lubricant 13 I is not st~ff~ciently
exhibited.
[0089]
In addition, it is necessary for the base layer 120 to have a thickness of 0.1 [itn
25 to 15 pm. When tlie thickness ofthe base layer 120 is less than 0.1 ~1111tl,i e
43
perfor~nanceo f the base layer 120 that supports the lubricant 131 is not sufficiently
exhibited. On the other hand, when the thickness of the base layer 120 exceeds 15 Iun,
since the thickness of the base layer 120 excessively increases, there is a tendency for
indentation scratching to occur during the cold working. From the viewpoint of
5 improving the perfortnance of the base layer 120 that supports the lubricant 13 1, it is
preferable that the thickness ofthe base layer 120 be 0.5 pm or more. In addition, kotn
the viewpoint of reliably preventing the indentation scratching during the working, it is
preferable that the thickness ofthe base layer 120 be 3 pln or less.
[0090]

The lubricant layer 130 has a fi~nctiono f reducing the coeff~cietiot f friction by
improving lubricity of the surface treatment film 100. Specifically, the lubricant layer
130 is located on a side close to the outernlost surface ofthe surface treatment film 100,
and contains tlie largest amount oftlie lt~bricant 131 atiiotig the three layers configuring
15 the surface treatment film 100.
[009 I]
In this tnodified example, the lubricant 131 is not particularly limited as long as
the surface treatment fill11 100 having the gradient-type three-layer structure may be
fomied, and the lubricity of tlie surface treatment fil~n 100 may be sufficiently improved.
20 For example, as the lubricant 13 1, at least one colnpound selected fiom a groitp
consisting of pol~etrafluoroetliyle~ime,o lybdenum disulfide, tungsten disulfide, zinc
oxide, and graphite may be used.
[0092]
In addition, it is necessary for tlie lubricant layer 130 to have a thickness of 0.1
25 Ftm to 10 ltni. When the thickness of tlie lubricant layer 130 is less than 0.1 bun,
44
sufficient lubricity niay not be obtained. On the other hand, when the thickness of tlie
lubricant layer 130 exceeds 10 Inn, a slurplus residue is generated during the working,
and there is a proble~ni n that this surplus residue is attached to a niold or tlie like. From
tlie viewpoint of finther improving the lubricity, it is preferable that the thickness of tlie
5 lubricant layer 130 be 1 pm or more. In addition, from the viewpoint of fu~.tlierre liably
preventing the generation of the surplus residue during tlie working, it is preferable that
the thickness of tlie lubricant layer 130 is 6 pin or less.
[0093]
Furthermore, a thiclaiess ratio between tlie lubricant layer 130 and the base layer
10 120 is also important to carry out tlie function of tlie base layer 120 and tlie filnction of
tlie lubricant layer 130. Specifically, the ratio of tlie thickness of tlie lubricant layer 130
to tlie thickness of the base layer 120, that is, it is necessary for (tlie tliickness of tlie
lubricant layer/the thickness ofthe base layer) to be 0.2 to 10. When the (the thickness
of tlie lubricant layer/tlie thickness of the base layer) is less than 0.2, the surface
15 treatment film 100 (the entirety of tlie film) becomes too hard, and thus tlie lubricity niay
not be sufficiently obtained. On the otlier hand, when the (the thickness of tlie lubricant
layedthe thickness of the base layer) esceeds 10, a property of supporting the lubricant
131 becomes inferior, and thus a working follow-np property of the entirety of tlie film is
deficient.
20 [0094]

As described above, in the surface treated steel sheet 1 according to this
modified example, it is important that three layers, which include tlie adhesion layer 110
25 close to tlie steel sheet 10, the lubricant layer 130 close to tlie film surface, and the base
45
layer 120 positioned therebetween, are made to be present. When any one of these
layers is omitted, it is difficult to exhibit the lubricity capable of enduring the cold
working, which is intended in this modified example. In addition, in a case where the
thickness of each layer of the adhesion layer 110, the base layer 120, and the lubricant
5 layer 130 is not within the above-described range, it is also difficult to exhibit the
lubricity capable of endurirlg the cold working, which is intended in this ~nodified
esatnple. Accordingly, in this modified example, a method of confirming whether or
not each of the adhesion layer 110, the base layer 120, and the lubricant layer 130 are
formed, and a neth hod of measuring the thickness of each of the layers is also important.
[0095]
First, with regard to the method of coafirming the formation of each layer of the
adhesion layer 110, the base layer 120, and the lubricant layer 130, the fomlation of each
of the layers may be cotlfirnled by a quantitative analysis of elements in the fihn
thickness direction (depth direction) of the surface treatment film 100 using the
15 Iligh-freqoency GDS. That is, first, in the main chemical co~upositionsth at are
contained in the surface treated film 100 (the chemical composition derived from the
silanol bond, the inorganic acid salt, the heat resistant resin, and the labricant),
representative elenlents (characteristic eletnents in chemical compositions) are set in each
layer. For example, with regard to the chemical co~npositiond erived from the sila~iol
20 bond, Si is appropriately set as the representative element. In addition, for esample,
with regard to the lubricant, in a case where the Iltbricant is polytetrafluoroethylene, F is
appropriately set as the representative element, and in a case where the lubricant is
molybdenum disulfide, Mo is appropriately set as the representative element. Nest,
from a high-frequency GDS measurement chart, the intensity of a peak corresponding to
25 each of the representative elecnents is obtained, and the concentration of each cheinical
46
composition is calculated from the peak intensity that is obtained for each position in the
film thickness direction.
[0096]
The nlethod of deterniining the thickness of each layer in this modified example
5 is defined as follows.
First, the thicluiess of the lubricant layer 130 is a distance fro111 the outennost
surface of the surface treatment film 100 to a position (depth) in the film thickness
direction at which peak intensity is I12 times the nlaxinllnn value of tlie peak intensity
with regard to tlie representative element (for example, F, Mo, W, Zn, or C) of the
10 lubricant that is set as described above in the high-frequency GDS measurement chart.
That is, the interface bchveen tlie lubricant layer 130 and the base layer 120 is consistent
\\,it11 a position in the film thickness direction at which the peak intensity of the
representative elenlent of the lubricant becomes 112 times the maximom value of the
peak intensity thereof.
15 [0097]
In addition, the thickness of the adhesion layer 110 is the distance fronl the
interface between the surface treatment film 100 and the steel sheet 10 to a position
(depth) in the film thickness direction at which the peak intensity is 112 times the
maxitnmn value of the peak intensity with regard to the representative element (Si) of the
20 chemical conlposition derived frotn the silanol bond in the high-frequency GDS
measurement cllart. That is, tlie interface between the adhesion layer 110 and the base
layer 120 is consistent with a position in the film thickness direction at which tlie peak
intensity of the representative element (Si) of the chenlical conlposition derived fro111 the
silanol bond becomes 112 times the tnaxi~nulnv alue of the peak intensity.
[0098]
47
Fu~lher~norteli,e thickness of the base layer 120 is a distance fiom the position
at which peak intensity is 112 times tlie maximum value of the peak intensity with regard
to tlie representative element of the lubricant to a position at which peak intensity is 112
times tlie maximum value of the peak intensity with regard to tlie representative element
5 (Si) of tlie cheliiical composition derived from the silanol bond. In addition, for
example, a cross-section of tlie surface treatment film 100 may be observed by a
lnicroscope to obtain a total thickness of tlie surface treatment film 100, and this total
tliicluiess of the surface treated filtii 100 may be subtracted by a total thickness of tlie
adhesion layer 110 and the lubricant layer 130 to obtain the thickness of the base layer
10 120.
LO0991
However, in a case of using graphite as tlie lubricant 131, when carbon (C) is set
as tlie representative element that deteniiines tlie interface betnreen the lubricant layer
130 and tlie base layer 120, it is difficult to distingoisli C in the lubricant 131 and C
15 originating from tlie heat resistant resin or the like. Therefore, tlie thickness of tlie
lubricant layer 130 is obtained using a representative element (for example, P, B, or Si) of
the inorganic acid salt. In this case, the interface between the lubricant layer 130 and
tlie base layer 120 is co~lsistetitw ith a position in the film thickness direction at which
the peak intensity of the represetitative element of the inorganic acid salt becomes 112
20 times the maximu111 value of the peak intensity thereof.
[OlOO]
In addition, i l l a case of using silicate as the inorganic acid salt of the base layer
120, wlien silicon (Si) is set as the representative element that determines the interface
between the base layer 120 and the adhesion layer 110, it is difficult to distinguish Si
25 originating from silicate (inorganic acid salt) and Si originating fro111 tlie clie~iiical
48
cotiiposition derived from the silanol bond ofthe adhesion layer 110. Therefore, the
tliich~esso f each of the adhesion layer 110 and the base layer 120 are obtained using
carbon (C) originating from the heat resistant resin co~nponenot f the base layer 120 as
the representative element. Furthermore, in a case of using tnolybdate or hlngstate as
. 5 tlie inorganic acid salt of the base layer 120, when molybdenum (Mo) or tungsten (W) is
set as the representative elenlent that deter~ninesth e interface between tlie lubricant layer
130 and the base layer 120, it is difficult to distinguish Mo or W originating fro111 the
inorganic acid salt and Mo or W originating from tlie lubricant 13 1. Therefore, the
thickness of each oftlie base layer 120 and the lubricant Layer 130 are obtained using
10 sulfi~(rS ) originating froni the lubricant 13 1 as the representative element.
[OlOI]
In additio~iw, ith regard to a calculation method of tlie thickoess ofeach layel; a
position at which tlie peak intensity is 112 ti~ilestl ie masimutn value ofthe peak intensity
in the representative element of each chemical composition, that is, a position in tlie film
15 thickness direction ofthe surface treatment film 100 limy be obtained from a sputteritig
time by tlie high-frequency GDS (in a case of this modified example, a time in terms of a
sputtering rate of SOz) by tlie high-frequency GDS.
[O 1021
Furthennore, in this modified example, tlie base layer 120 contains 0.01 to 10
20 parts by mass of the inorganic acid salt to 100 parts by mass of the heat resistant resin.
A method ofmeasuring the Inass of the heat resistant resin and tlie inorganic acid salt in
the base layer 120 is as follows. A fill11 is gro~111idn a tllicktiess direction by a
~nicrotornet o cut out the base layer. This fill11 is collected in an amount with which
analysis may be performed, and the11 tliis collected film is crushed with an agate mortar.
25 After the crushing, an initial \\,eight of the collected film is measured, and water is added
49
thereto to dissolve the inorganic acid salt (inorganic cotnpound). After dissolving the
inorganic acid salt, the film is sufficiently dried. The weight ofthe film after being
dried is deter~lii~leads parts by mass of the heat resistant resin, and the difference
between the initial weight and the weight of the film after being dried is regarded as parts
5 by Illass of the inorganic acid salt.
[0103]
[Method of Manufactoring Surface Treated Steel Sheet]
Hereinbefore, the configuration of the surface treated steel sheet has been
described in detail. Sabsequently, a method of manufacturing the surface treated steel
10 sheet having this configl~ration\\ ,ill be described.
[0 1 041
In the tnethod of alanufacturing the surface treated steel sheet, a water-based
surface treatment liquid, which contains a water-soluble silatle coupling agent, a
water-soluble inorganic acid salt, a water-soluble heat resistant resin, and the lubricant, is
15 applied to at least one surface of the steel sheet 10 (the steel sheet of the above-described
embodiment), and then this surface treatment liquid is dried, whereby the surface
treatment film 100 is formed 011 at least one surface of the steel sheet 10.
[0 1051
(With Regard to Surface Treatment Liquid)
20 The surface treatment liquid that is used in the method of manofacturing the
surface treated steel sheet includes the water-soluble silane coupling agent, the
water-soluble inorganic acid salt, the water-soluble heat resistant resin, and the lubricant,
since the details of the inorganic acid salt, the heat resistant resin, and the lubricant were
described above, a descriptio~tlh ereofwill be omitted here.
[0 1 061
50
The water-soluble silane coupling agent is not particularly limited, and may be a
silatie coupling agent in the related art. For example, 3-aminopropyl trimethoxy silane,
N-2-(aminomethyl)-3-aminoyropyl methyl dimethoxy silane, 3-glycidoxypropyl
tritnethoxy silane, 3-glycidoxypropyl triethoxy silane, or the like may be used.
[0107]
In addition, various additives tilay be added to the surface treatment liquid.
[0108]
As the surface treatment liquid that is used in tlie iiiethod of manufacturing the
surface treated steel sheet, a leveling agent that itliproves coating properties, a
10 water-soluble solvent, a tnetal stabilizitig agent, an etching inhibitol; a regulatitlg agent,
atid the like may be used within a range not deteriorating the effect oftlie modified
example. As the leveling agent, a nonionic or cationic surfactant may be used.
Exalliples of tlie leveling agelit include polyethylene oxide, a polypropylene oxide
additive, and acetylene glycol cotnpound, and tlie like. Examples of the water-soluble
15 solvent include alcohols such as ethanol, isopropyl alcohol, t-butyl alcohol, and
propyletle glycol; cellosolves such as ethylene glycol mot~obutpel ther and ethylene
glycol monoethyl ether; esters such as ethyl acetate and butyl acetate; ketones such as
acetone, methyl ethyl ketone, atid methyl isobutyl ketone; atid tlie like. Examples ofthe
metal stabilizing agent include a chelate compound snch as EDTA and DTPA.
20 Esat~ipleso f the etching inhibitor include atnitie compoiitids such as ethylene diamine,
triethylene pentarnine, gt~aoidinea, nd pyrimidine. Howevel; particularly, sitice an
amine compound having two or more amino groups in one molecule also has an effect as
the metal stabilizitig agent, it is more preferable to use the amine compound as the
etching inhibitor. Example of pH adjusting agent itlclude organic acids such as acetic
25 acid and latic acid; inorganic acids such as hydrofluoric acid; all aaimonium salt; amines;
and the like.
[0 1091
By dissolving or dispersing each of tlie above-described chemical co~npositioli
uniformly in watel; it is possible to prepare the surface treatment liquid that is used in the
5 method of tnatmfacturing the surface treated steel sheet.
[OIIO]
(Application and Drying of Surface Treatnient Liquid)
As a method of applying the surface treatment liquid onto the steel sheet 10, for
example, a method of immersing the steel sheet 10 in the surface treattilent liquid, or the
10 like may be used. In this case, it is necessary to warm the steel sheet 10 in advance to a
teniperatc~reth at is higher than the temperature of the surface treatment liquid or to dry
the steel sheet 10 with warm air during drying thereof. Specifically, for example, the
steel sheet 10 is immersed in warn1 water of approxiniately SO°C for approxi~iiately1
minute, and then is iniriiersed in tlie surface treatment liquid of 40 to 60°C for
15 approximately 1 second, and then is dried at room temperature for approxiniately 2
minutes. According to this method, the gradient-type surface treatment film 100 having
three-layer structilre ofthe adhesion layer 110, tlie base layer 120, and the lubricant layer
130 may be fomied.
[Olll]
(Method of Controlling Fill11 Thickness of Each Layer)
The fihn thickness of each of tlie layers configuring the surface treatment film
100 may be adjusted to be withiti the above-described film thickness range by
appropriately controlling the application amount of the surface treatment liquid, the
concentration of each chemical composition in the surface treatment liquid, the reactivity
25 bet\veeti tlie surface treatment liquid and the steel slieet 10 that is a base material, and the
52
I~ydropl~iliciatyn d hydrophobicity of the surface treatment liquid.
[0112]
(Reason Why Gradient-type Film is Fomied)
As described above, with regard to tlie reason why the gradient-type surface
5 treatment fill11 100 is forliied when the surface treatment liquid obtained by dissolvitig or
dispersilig the water-soluble silaoe couplillg agent, the water-soluble inorganic acid salt,
the water-soluble heat resistatit resin, and the lubricant in water is applied outo the steel
sheet 10, and is dried. The present inventor assumed the reason of tlie above as follows.
First, as described above, the steel sheet 10 is warnied in advance to a telnperature higher
10 than that of the surface treatment liquid, since the temperature of the steel sheet I0 is
Iiigher than that of the surface treatment liquid, in the thin fihn that is forliled after the
surface treatment liquid is applied on the steel sheet 10, a temperature of a solid-liquid
interface is high, and a tetliperature of the gas-liquid interface is low. Therefore, a
temperature difference occclrs in the thin film, \\rater that is a solvent is vaporized, and
15 convection slightly occurs in the thin fihn. In the case of drying the thin film, which is
forliied by applying the rootu-temperature surface treatment liquid onto the
rooln-temperature steel sheet 10, is dried with wann ail; tlie temperature of the gas-liquid
interface is increased, the surface tension at the gas-liquid interface decreases, and thus a
variation it1 temperature and a variation in surface tension are mitigated. As a result,
20 convection slightly occurs in the thin film. In ally application and drying tnethod
described above, the surface treatment liquid is separated into a component having high
affinity with air (for example, the lubricant) and a component having high affinity with a
metal or water (for example, the inorganic acid salt or the heat resistant resin)
sitnultatleously with occurrence of the convection. Then, \\cater is gradually vaporized,
25 atid the surface treatment liquid has a fihn shape, and the gradient-type filtn having a
53
concentration gradient for each chemical co~npositionis formed.
[0113]
In addition, in this tnodified exatnple, the silane coupling agetit has high affinity
with the tnetal on the surface of the steel sheet 10, and thus the silane coupling agent
5 diffi~sesto the vicinity of the steel sheet 10 in the thin film. Then, it is considered that
the silane coupling agent that reaches the vicinity of the steel sheet 10 forms a covalent
bond with a tiietal oxide (for example, zinc oxide in a case where the steel sheet 10 is
plate with zinc) that is present on the surface ofthe steel sheet 10, atid tlie silanol botid
expressed by Si-0-X is fomied. As described above, when the silatiol bond is formed
10 in tlie vicinity ofthe steel sheet 10, adhesiveness between the surface treated fill11 100
alld the steel sheet 10 is significantly improved, and thus occurrence of seizure and
galling is prevented.
[0114]
[Comparison \vith Other Surface Treatment Methods and Summery of Modified
15 Example]
In addition, in the cold working, a temperature of contact portions of the steel
sheet and the mold is relatively raised (to approxitnately 300°C higher) due to friction
behveen tlie steel sheet and the mold. Therefore, when a steel sheet to which any
surface treatment is not applied is subject to the cold working, in a case where lubricity
20 behveen the steel sheet and the tnold is not sufficient, there is a tendency for the seizure
or galling to occor between the steel sheet and the mold. In this case, tlie mold is
locally broken, or abrasioo occurs rapidly, and thus an operational lifespati of the Inold
may be significantly shortened.
[0115]
To prevent the seizure or galling, commonly, a surface treatment (liereinaftel;
54
may be referred to as a "lubricant treatinent"), which gives lubricity to the surface of the
steel sheet that is to be subjected to the cold working, is performed to the steel sheet.
As this lobricaiit treatment, a phosphate treatment (bonderizing treatinent), which is
performed to form a phosphate filtli formed from a phosphate compound (zinc phosphate,
5 lnaiigatiese phosphate, calcium phosphate, iron phosphate, or the like) on tlie surface of
tlie steel sheet, has been known in the related art.
[0116]
The phosphate-treated steel sheet has a relatively higher seizure prevention
perforlnaiice arid galling prevention performance. However, tratisitioti fro111 a working
10 field, such as hot forging and a cutting process accoinpanied with large shape
deforn~ationt,o cold working is in progress in the background of recent environinental
measures, atid thus there is demand to perfor111 Inore sever plastic working 011 tlie steel
sheet for cold working. Frotii this viewpoint, a cotnposite filiii, which is obtained by
laminating a layer formed froin a nietal soap (for example, sodiom stearate) on the
15 pliospliate film, lias beeti widely used. This composite fill11 lias an excellent seizure
prevention fbnction and a galling prevelltioii function even under hard friction co~iditions
due to high-surface-pressure pressing during the cold working.
[OI 171
When the cotnposite fil~nis forined by this lubricant treatment, the illeta1 soap
20 reacts \vitIi the phosphate film, and thus high lubricity is exhibited. Ho\vever, since this
lubricant treatment needs various complicated treatment processes such as a ~vasliing
process and a reaction process that allows the ~iietals oap and the pliospliate filiii to react
with each other (process manageliient socli as treatment liquid management or a
temperature management daring reaction is also necessary), and the lubricant treatment is
25 a batch process, there is a problem in that productivity decreases. In addition, in the
55
lubricant treatment using the co~npositefi ltn, there is also a probletn of disposal of waste
liquid that is generated daring the treatment, atid thus this treattlletlt is not preferable also
from the viewpoint of environment conservation.
[0118]
On the other hand, in this modified example, the surface treated steel sheet can
be manufactured with a convenient treatment process and with a manufacturitlg tnethod
that is also very suitable from the global e~iviron~nec~onits ervation, and has excellent
lubricity. Therefore, the working method may tratisfer from a workitlg field
accompanied with large shape deformation such as hot forging in which energy
10 consumption is large and cutting in which a large amount of material loss occurs to cold
working on the background of recent environmental countermeasures. Furthermore,
when the above-described surface treated steel sheet is used, even when relatively Illore
hard plastic working or fi~rtherc otnplicate working is required, the material (steel sheet)
may be worked withoat any problem while not generating the seizure or galling \\,it11 the
15 mold. Particularly, wlien the surface treated film, which may be appropriately used for
tlie hard cold working, is for~iledo n the surface of the steel slieet of the above-described
embodiment that tnay be appropriately used for the hard cold working, a synergistic
effect (integral workability) between the workability of the steel sheet that is a base
~nateriala nd tlie steel sheet follow-up property ofthe sorface treated film tnay be
20 obtained. Accordingly, even wlien the cold working is performed with respect to the
steel slieet, sufficient workability may be secured without decreasing all operational
lifespan ofthe mold. Fullhermore, when the ~ i l e d i ~ca~rb~onn steel sheet for cold
\\,orking of this modified example is applied to the cold working and the high-frequency
quenching, a component having excellent tilecha~licalp roperties due to the synergistic
25 effect ins)' be inanufactured with a high yield ratio, and resource saving and saving of
energy may be accomplished.
[0119]
Hereinbefore, tlie very suitable elnbodimelit of tlie present itivention has been
described in detail while refening to the drawings, but the present invention is not limited
5 to the example. It should be understood by a person having ordinary skill in the art that
various modified examples and variation examples may be made without departing from
tlie tecliliical idea described in the attached claims, and these naturally belong to the
technical scope of the present invention.
Examples
[O 1201
Next, examples of the present itiveiitio~\i\ ,ill be described, but conditions of the
examples are one conditional example adapted to confirm an execution possibility atid an
effect of tlie present invention, and the present invention is tiot limited to tlie one
conditional example. The present ilivetitioti adapt various conditions as long as the
15 object of tlie present invention is accomplished n4thout departing fiom tlie gist of the
present invention.
[0121]
Steel having the above-described chemical coliiposition shown in Table 1 was
dissolved, was hot-rolled, and was atniealed to manufactnre each steel sheet having a
20 hardness, a diameter of carbide, atid a spheroidizing ratio of tlie carbide that are different
in each steel sheet, and the cold workability and tlie high-frequency quenching liardtiess
were observed. Hereinafter, a method of manufacturing tlie steel sheet will be
described.
A steel ingot (cast slab) having a sheet thickness of 150 mm was held at 1,220°C
25 for 2 hot~rsa, nd then was hot-rolled under a condition in which a rollirig termination
57
te~iiperaturei s 870°C to obtain a hot-rolled steel slieet having a sheet thickness of 6 mm.
Then, this hot-rolled steel sheet was cooled to a first cooling temperature at a first
average cooling rate sliown in Tables 2 to 7, and was cooled to a second cooli~ig
temperature at a second average cooling rate sliown ill Tables 2 to 7, and then the
5 resultant steel sheet was cooled with air after being wound. hi addition, it was
confirmed tliat an interval from 550 to 40OoC was held for 30 hours or less.
Samples (corresponding to steel Nos. A, B, C, K, and L) having a sheet
thickness of 2 mtn were obtained from each hot-rolled steel sheet by cutting a surface
layer of 2.0 lnln and a rear surface layer of 2.0 mm in a sheet thickness direction. In
10 addition, a surface layer of 0.5 mlli atid a rear surface layer of 0.5 mm were cut from each
hot-rolled steel slieet manufactured under tlie same conditio~isto obtain samples
(corresponding to steel Nos. D, E, M, N, 0, P, and Q) having a slieet tliickness of 5 mm.
Similarly, a steel ingot that was casted under a vacuum atmosphere and has a
sheet thickness of 150 tnm was held at 1,240°C for 1.5 hours, and then was hot-rolled
15 under a condition in \\~liich the rolling termination temperature is 920°C to obtain a
hot-rolled steel sheet having a slieet thickness of 16 mni. Then, this steel slieet was
cooled to the first cooling temperature at tlie first average cooling rate shown in Tables 2
to 7, and then was cooled to the second cooling temperature at the second average
cooling rate shown in Tables 2 to 7, and theti was cooled with air after being wound. In
20 addition, it was co~ifinnedtl iat an interval fro111 550 to 400°C was held for 30 liours or
less.
A surface layer of 3.5 nim and a rear surface layer of 3.5 mm \\,ere cut from each
hot-rolled steel sheet described above to obtain samples (correspotiding to sheet Nos. F,
R, U, and V) having a sheet tllick~iesso f 9 m~n. 111a ddition, a surface layer of 2.0 tnm
5 8
was cut from the hot-rolled steel sheet iiiannfach~redu nder the same conditions to obtain
samples (corresponding to steel Nos. H, W, X, and Y) having a sheet thickness of 12 mm.
Furthennore, hot-rolled steel sheets having a sheet thickness of 16 mm, which were not
subjected to the cutting, were used as samples (corresponding to steel Nos. I, J, 2, AA,
5 atid AB).
[O 1221
An Acl temperature of each sample was measured by a thermal expansion test.
Here, in this thennal expansion test, a temperature at which austenite transformation
initiates during the heating at an average heating rate of 3O0C/hour close to that of a box
10 annealing furnace of a real machine was determined as the Acl temperature.
Each of the sa~ilples( corresponding to steel Nos. A to AB) was a~liiealedi n a
hydrogen 95% atmosphere under six conditions soch as at 680°C for 3 hours
(corresponding to Table 2), at 680°C for 30 hours (corresponding to Table 3), at 700°C
for 30 hours (corresponding to Table 4), at 740°C for 10 hours (corresponding to Table 5),
15 at 700°C for 90 hours (corresponditig to Table 6), and at 700°C for 60 hours
(corresponding to Table 7). The satnples, which were annealed at 680°C and 700°C,
were subjected to fiirnace cooling after retention (annealing) was temiinated. The
sample, which was annealed at 740°C, was cooled to 700°C at an average cooling rate of
2°C/seco~id after the retention was completed, and then was subjected to the furllace
20 cooling. In addition, for example, the salnples (steel sheet Nos. A-l to AB-I), which
were atutiealed at 680°C for 3 liours, are shown in Table 2, atid the saiiiple of steel sheet
Nos. A-1 to AB-1 \),ere prepared f?om satnples having chemical compositions of steel
Nos. A to AB, respectively.
In a high-frequency quenching test, each of the samples (steel sheet Nos. A-1 to
59
B-6) was heated at a freqoency of 78 kHz from room te~nperatureto 1,000+20°C at an
average heating rate set to l O O i 15"CIsecond in a temperature range of 750°C or higher:
was held at 1,000+20°C for 10i0.5 seconds, was quickly cooled to rooin teunperature at
an average cooling rate set to 200+10°C/second between 800°C and 400°C, and Vickers
5 hardness (quenching hardness) of the quenched material was measured. In addition, L
flat-sheet bending test specinien having a width of 30 lnln and a length of 100 lnln was
prepared from each sample, and a bending test was carried out under conditions at which
the bending radius was set to 112 titnes the sheet thickness, and the bending angle was set
to 90'. Then, the number of cracks at regions of 118 to 318 and 518 to 718 of the sheet
10 thickness in a sheet thickness cross-section of a bending angle portion (maximum
curvature portion) of the flat-sheet bending sanlple \\,as measured by a scanning electron
microscope at a magnification of 3,000 times. In a case where the nutnber of cracks
was \\,ithin 20 per I mm2, it was determined that the occurrence of cracks caused by
interfacial peeling during the cooling working was suppressed, and thos the cooling
15 workability was evaluated as "good". In addition, in a case where the number of cracks
exceeded 20, the cold workability was evaluated as "poor". In addition, these cracks
were classified for each kind (a crack starting fro111 cementite, a crack starting from
sulfide, and an transgranular crack) and then \Irere counted. An energy dispersion X-ray
spectroscope (EDS) attached to a scanning electron ~nicroscopew as used to distinguish
20 the cracks starting from ce~nentitea nd the cracks starting froin sulfide. In addition, the
average diameter ofthe carbide and the spheroidizing ratio ofthe carbide were measured
using the above-described method.
[0123]
[Table I ]

67 74
[0130]
In steel sheet Nos. A-2 to D-2, F-2, 5-2, A-3, C-3, E-3, G-3 to 5-3, B-5, C-5, E-5,
F-5, H-5,I-5, E-6, and G-6 to 1-6 in Tables 3,4,6 and 7, the average diameter of the
carbide and the spheroidizing ratio of the carbide were appropriately controlled, and thus
5 the cold workability and the high-frequency quenching hardenability (quenching
hardness) were excellent.
[0131]
On the other hand, in steel sheet Nos. A-1 to AB-I in Table 2, the annealing time
was shall, and tlie spheroidizing ratio of the carbide was less than 70%, and thus the cold
10 workability was not sufficient. In addition, in steel sheets Nos. A-4 to AB-4 in Table 5,
the spheroidizing ratio of the carbide esceeded 9O%, and thus tlie high-frequency
quenching hardenability \\,as not sufficient. In steel sheet Nos. D-3 and F-3 in Table 4,
since the first average cooling rate esceeded 50°C/second, carbide having a spheroidizing
ratio of 90% or inore was generated from bainite in the hot-rolled steel sheet, and thus the
15 high-frequency quenching hardenability was not sufficient. Furthemiore, in steel sheet
Nos. A-5 and B-6 in Tables 6 and 7, since tlie first average cooling rate was less than
2O0C/second, the average diameter of the carbide esceeded 0.6 Cun, and thus the
high-frequency quenching hardenability was not sufficient. In steel sheet Nos. H-4 a~id
A-6 in Tables 5 and 7, since tlie first cooling tennination temperature exceeded 700°C,
20 defects due to scale occurred. In steel sheet Nos. H-2 and G-5 in Tables 3 and 6, since
the first cooling temiination temperature exceeded 700°C, the average diameter of the
carbide and tile spheroidizing ratio of the carbide did not satisfy Expression (2) described
above, and thus the cold workability was not sufficient. In steel sheet Nos. G-2 and D-6
in Tables 3 and 7, since the first cooling termination teniperature was less than 500°C, the
ss7T
average dianieter of the carbide exceeded 0.6 pm, and thus the high-frequency quenching
hardenability was not sufficient. In this case, it was considered that austenite to wliicli
working strain applied after hot rollitig was rich, and thus coarse pearlite was
preferentially generated from the austenite during cooling. In steel sheet Nos. B-3 and
5 F-6 in Tables 4 and 7, since tlie second average cooling rate exceeded 30°C, carbide
having the spheroidizing ratio of 90% or tilore was generated, and thus the
high-frequency quenching liardenability was not sufficient. In steel sheet Nos. D-5 and
J-5 in Table 6, since the second average cooling rate was less than 5"C/second, the
average diameter of the carbide exceeded 0.6 pm, and thus the high-frequency quenching
10 hardenability was not sufficient. In steel sheet Nos. E-2 and 1-2 in Table 3, since the
second cooling termination temperature was higher tlian a tenlperature lower than the
first cooling termination temperature by 50°C, the spheroidizing ratio of the carbide was
less tlian 70%, and thus tlie cold workability was not sufficient. In steel sheet No. C-6
in Table 7, since the second cooling termination teniperature was higher than a
15 temperature lower tlian the first cooling termination temperature by 50°C, the average
dianieter of the carbide and the spheroidizing ratio did not satisfy Expression (2)
described above, and thus the cold workability was not sufficient. In steel sheet No. 5-6
in Table 7, since the second cooling termination temperature was less than 400°C,
carbide having spheroidizing ratio of 90% or more \was generated, and thus the
20 high-frequency quenching hardenability was not sufficient.
[0 1321
I11 steel sheet Nos. K-2, K-3, K-5, and K-6 in Tables 3, 4, 6, and 7, since the Mo
content exceeded 0.5 mass%, tlie carbide was not sufficiently dissolved during the
high-frequency heating, and the high-frequency quenching hardenability was not
6 9 7 6
sufficient. In steel sheet Nos. L-2, L-3, L-5, and L-6, since tlie Mn content was less
than 0.3 mass%, the liardenability of the steel decreased, and thus the high-frequency
quenching hardenability was not sufficient. In steel sheet Nos. M-2, 4-2, M-3, 4-3,
M-5, Q-5, M-6, and 4-6, since the Si content was less than 0.06%, the above-described
5 interfacial peeling occurred, and thus the cold workability was not sufficient. In steel
sheet Nos. N-2,Y -2, N-3, Y-3, N-5, Y-5, N-6, and Y-6, since the C content was less than
0.3%, tlle liardenability of the steel decreased, and thus high-frequency quenching
hardenability was not sufficient. In steel sheet Nos. 0-2, P-2, 0-3, P-3, 0-5, P-5, 0-6,
and P-6, since the Mn content exceeded 2.0%, the high-frequency quenching
10 hardenability was not sufficient. In steel sheet Nos. R-2, AA-2, R-3, AA-3, R-5, AA-5,
R-6, and AA-6, since tlie Cr content exceeded 0.10%, tlie carbide was not sufficiently
dissolved during tlie high-frequency heating, and thus the high-freqoency quenching
liardenability was not sufficient. 111 steel sheet Nos. U-2, W-2, U-3, W-3, U-5, W-5, U-6,
and LV-6, since the C content exceeded 0.6%, the cold workability decreased. In steel
15 sheet Nos. V-2, X-2, V-3, X-3, V-5, X-5, V-6, and X-6, since the S content exceeded
0.0075%, the cold workability was not sufficient. In steel sheet Nos. 2-2,Z-3,Z-5, and
Z-6, since the S content exceeded 0.30%, and the P content exceeded 0.03%, the cold
workability was not sufficient. In steel sheet Nos. AB-2, AB-3, AB-5, and AB-6, since
the V content exceeded 0.5%, the high-freqaeticy quenching liardenability was not
20 sufficient.
[0133]
Furthermore, a surface treatment film appropriately used for cold working (tlie
medit~iicl arbon steel sheet for cold working that includes the surface film) will be
described in detail using examples.
[0 1341
20-7 7
(Preparation of Surface Treatment Liqoid)
First, surface treatment liquids (chemical agents) a to q, which contained
cliemical co~npositionsh own in Table 8 to be described below, were prepared. In
addition, in Table 8, the reason why a combination of zinc nitrate and phosphoric acid is
5 used as the inorganic acid salt is because zinc phosphate is hardly dissolved in watel; and
is dissolved in an acid. As described above, when the zinc nitrate that is soluble in
water and the phosphoric acid are used in combination, zinc phosphate that is poorly
soluble in water is generated to be present in the surface treatment liquid.
[0135]
[Table 81

-
-L d
Kind -
VlOS,
-
bloS,
-
\l0S,
-
PTFE
-
Zno
-
bloS*
-
bloS2
-
mphi
-
hloS,
-
MoS,
-
hloS,
-
iraplli
-
imphi
-
hloS; -
MoS,
-
MoS,
-
MoS,
-
Zr so
101361
(Manufacturing of Surface Film Steel Sheet)
Nest, st~rfacetr eated steel sheets (Nos. 1 to 29), in which a gradient-type surface
treatment fihn having three-layer structure was fonned on both surfaces of tlie sheets,
5 were ~nanufacturedb y the following method by using the surface treatment liquids a to q
that were prepared as described above (refer to Table 10 to be described below).
[0137]
The method of manufacturing tlie surface treated steel sheet will be described in
detail. Steel having clie~nicacl ot~lpositionss hown in Table 9 was casted by a conltnon
10 convettcr and a vacuun~d egassing treat~iienta nd a slab was prepared. Furthennore, the
cast slab was held at 1,220°C for 1 Iioul; and was hot-rolled under conditions in whicli
the rolling tern~inatiout emperature was 870°C to obtain a hot-rolled steel sheet having a
sheet thickt~esso f 8 mtn. Then, this hot-rolled steel sheet was cooled to 670°C at an
average cooling rate of 3O0C/second, was cooled to 560°C at a cooling rate of
15 15"C/second, and tlien was wound. The wountl hot-rolled steel sheet was fnrther cooled
to 400°C for 20 hours. The obtained hot-rolled steel sheet was antlealed under a
hydrogen 95% atmosphere at 700°C for 30 hours, and the11 was subjected to f~~rnace
cooling. The surface treatment liquids a to q were applied onto the annealed hot-rolled
steel sheet (annealed steel slieet) with coating #3 bar (coating bar). The fihn thickness
20 of the surface treatment filni W ~cSon trolled tl~roughth e concentration of the surface
treatment liquid. Furtliermore, the annealed steel slieet onto wliicli the surface treatment
liquid was applied was dried in a hot air drying furnace of 3 0 0 " ~un der conditions in
which an arrival slieet temperature became 150"~. After the dlying, the steel slieet was
cooled with air to prepare a surface treated steel slieet.
73 81
In addition, a quenching sample having ditnetisions of sheet thickness: 8
mmxsheet width: 15 mmxslieet length: 100 lnln was collected from the annealed steel
sheet before being subjected to the surface treatment, and this sample was heated at a
freq~~encoyf 78 kHz from room temperature to l,OOO°C at an average heating rate of
5 10OoC/second, was held at l,OOO°C for 10 seconds, and was rapidly cooled to room
telnperature at an average cooling rate of 200°C or higher. Then, the Vickers hardness
(quenching hardness) of the quenched material was measured. Furthennore, the
average diameter of tlie carbide of the annealed steel sheet and the spheroidizing ratio of
the carbide were measured using tlie above-described method. As a result, it was
10 cotifirlned that the average diameter of the carbide \\,as 0.3 1 {un, the sphemidizi~lgra tio
was 85.7%, atid the hardness after the high-frequency quenching was 638.7 HV.
[0138]
[Table 91
[0139]
(Measurement of Film Thickness)
Measarement of tlie film thickness \\,as carried out using the high-frequency
GDS with respect to tlie surface treated steel sheet that was obtained. Specifically, the
distance fiom the outerlnost surface ofthe surface treatment film to a position (depth) in
tlie fill11 thickness direction, at \\,liicli peak intensity was 112 times the masi~nutn value of
20 the peak intensity with regard to tlie representative element (for example, Mo or C) ofthe
lubricatit in the high-frequency GDS measure~iienct hall, was measured to deterliiine the
thickness ofthe lubricant layer. 111 addition, a distance from the interface between the
surface treatment film and the steel sheet to a position (depth) in tlie fill11 thickness
direction, at which peak intensity was 112 times the maximum value of tlie peak intensity

74- *a3
with regard to the representative elenlent (Si) of the cheinical conlposition derived from
the silanol bond in the high-frequency GDS measurement chat, was measured to
determine the thickness of the adhesion layer. F~utthermore, the distance fioin the
position at which peak intei~sityw as 112 tiiues the lnaximtltn value of the peak intensity
5 with regard to the representative element (Mo) of the lubricant to a position, at which
peak intensity was 112 times the tnaxitnum value of the peak iiitetlsity with regard to the
representative elelnetit (Si) of the chemical colllposition derived from the silanol bond,
was measured (calculated) to deterlnine the thickness of the base layer. In addition, the
measurement was carried out osing elements that were different from each other as the
10 representative element so that the representative elelllent of the lubricant layer (lobricant
component) and the base layer (inorganic acid salt component), and the representative
eleinent of the base layer (inorganic acid salt component) and the adhesion layer
(chemical coinposition derived from the silallol bond) were not the sanle as each other.
[0 1401
15 For example, in a case of using graphite as the lubricant, the thickness of the
lubricant layer and the base layer was obtained using peak intensity of the representative
element (P, Si, Mo, or W) ofthe inorganic acid salt.
[0141]
(Evaluation Method and Evaluation Staiidard)
Furthermore, the film adhesiveness and workability of the surface treated steel
sheet, which was maoufactc~red as described above, were evaluated based on the
followitlg evaluation method and evaluation standard.
[0 1421

The fill11 adhesivetiess was evaluated by a drawing sliding test using a flat bead
75- 8'1
mold. In this drawing sliding test, a test specimen (sample), from which shearing burr
was removed and which had a size of 30x200 mln, was collected fro~nth e surface treated
steel sheet and was used. In addition, the intensity (intensity before test) of a tilain
constitilent ele~ne~init t he filtn was measured by a fluorescent X-ray analysis device
5 before carrying out tlie sliding test with respect to the sample.
[0 1431
As the flat bead mold, a pair of ~uoldsw, hich had a length of 40 inm, a width of
60 mm, and a thickness of 30 mm, ofwhich material was SKDl1, and of which surface
was polished with eliiety paper of #1,000, were prepared. Next, the sample was
10 interposed between the molds, was pressed with 1,000 kg by an air cylindel; and then the
sa~nplew as drawn by a drawing tester. With respect to the sa~npleaf ter the drawing,
intensity (intensity after test) of the above-described element was measured by tlie
fluorescent X-ray analysis device, and a remaining rate (intensity after testlintensity
before test)xl00 [%I was calculated.
[O 1441
With regard to evaluation standard of the filtii adhesiveness, a case where the
re~iiainitlgra te was less than 70% was evaluated as "poor", a case where the remaining
rate was equal to or more than 70% and less than 90% was evaluated as "good", and a
case where the re~nainingr ate was 90% or more was evaluated as "excellent."
20 [0145]

The workability was evaluated by a spike test. In this spike test, first, a
columnar spike test speci~nen 1A (spike test specimen IA before working ofFIG. 78)
prepared from the surface treated steel sheet was mounted on a die 3 having a filnnel-like
25 internal shape shown in FIG. 7A. Then, load was applied to the spike test specinien IA
?77 95
through a plate 2 show11 in FIG. 7A to insert the test specimen 1A into a die 3, whereby
the spike test speci~nen1 A was shaped into to have a shape of a spike test specimen 1B
after working as shown in FIG. 7B. The spike colifortning to a die shape was formed
with this method, and lubricity was evaluated by tlie height ofthe spike at this time.
5 Therefore, tlie higher the height (mm) of the spike is, the further the lubricity is excellent.
In addition, co~iditiotlso fthe spike test were conformed to a tnethod disclosed in
Japanese Unexa~nined Patent Application, First Publication No. H05-7969.
[0146]
With regard to evaluation standards of the workability, evaluation was carried
10 out using the height oftlie spike. A case where the height of the spike was less than
12.5 lilln was evaluated as "poor", a case where tlie height of the spike was 12.5 to 13.5
llnn was evaluated as "good", and a case where the height of the spike exceeded 13.5 mrn
was evaluated as "excellent". In addition, the evaluation as "good" corresponds to the
perforn~atlceo f the sa~ilpleth at was prepared by forming tlie cotnposite film (chemical
15 reactionlsoap treatment) oil the salile steel sheet in tlie related art.
[0147]
Measurement results of the thickness of each layer, the fill11 adhesiveness, and
the workability, which were obtained by performing the measurement as described above,
are shown in Table 10.
20 [0 1481
[Table 101
[0149]
As shown in Table 10, in the surface treated steel sheets of Nos. 1 to 19, the film
adhesiveness and the workability were excellent. On the other hand, in the surface
25 treated steel slieets of Nos. 24 and 25, since the thickness of tlie adhesion layer was not

77 87
optimized, the fill11 adhesiveness was inferior to the surface treated steel sheets ofNos. 1
to 19. Furthennore, in the surface treated steel sheets of Nos. 20 to 29, since one of the
conditions of each layer was not optimized, the workability (lobricity) was inferior to the
surface treated steel sheets of Nos. 1 to 19.
5 IndustrialApplicability
[Ol SO]
As described above, according to tlie present invention, a tnediutil carbon steel
sheet for cold working, which is excellent in high-frequency Iiardetiability, and a
nianufacturing i~ietliodth ereof tilay be provided. Accordingly, the present invention has
10 an important role ofgreatly enlarging a use of the medium carbon steel sheet in which
the high-frequency quenching is used, and thus applicability ofthe present invention is
high in tlie steel product manufacturing industry.
2- BZ
CLAlMS
1. A medium carbon steel sheet for cold working that has a hardness of 500
HV to 900 HV ia a case of being sobjected to high-frequency quenching in which a
5 te~nperatureis raised at an average heating rate of 100°C/second, the temperature is held
at 1,00O0C for 10 seconds, and a quick cooling to a room temperature is carried out at an
average cooli~lgra te of 200°C/secotld, the medium carbon steel sheet comprises, by
mass%,
C: 0.30 to 0.60%,
Si: 0.06 to 0.30%,
Mn: 0.3 to 2.0%
P: 0.030% or less
S: 0.0075% or less,
Al: 0.005 to 0.10%,
N: 0.001 to 0.0 I%, and
Cr: 0.001 to 0.10%,
and a balance co~nposedo f Fe and inevitable impurities, wherein
an average diameter d of a carbide is 0.6 pol or less, a spheroidizing ratio p of
the carbide is equal to or Inore than 70% and less than 90%, and the average diameter d
20 (pm) of the carbide and the spheroidizing ratio p % of the carbide satisfy dS0.04~~-2.6.
2. The tnedii~~cnar bon steel sheet for cold working according to claim 1,
further co~npriseso ne or more of, by tilass%,
Ni: 0.01 to 0.5%,
Cu: 0.05 to 0.5%,
Mo: 0.01 to 0.5%,
Nb: 0.01 to 0.5%,
Ti: 0.001 to 0.05%,
V: 0.01 to 0.5%,
Ta: 0.01 to 0.5%,
B: 0.001 to 0.01%,
W: 0.0 I to 0.5%,
Sn: 0.003 to 0.03%,
Sb: 0.003 to 0.03%, and
As: 0.003 to 0.03%.
3. The inedium carbon steel sheet for cold working according to claim 2,
wherein a Cr content [Cr] and a Mo content [Mo] satisfy [Cr]+[Mo]/lO < 0.10.
4. The medinm carbon steel sheet for cold working according to claim I or 2,
wlierein the hardness before the cold tvorking is equal to or liiore than 120 HV
and less than 170 HV.
5. The medium carbon steel sheet for cold working according to 1 or 2,
wherein the medium carbon steel sheet ~LII-thienrc ludes a surface treatment film
that contains a chelnical composition derived from a silanol bond that contains a metal
co~npotieliXt and is expressed by Si-0-X, a heat-resistant resin, an inorganic acid salt
and a lubricant on at least one surface,
the surface treatment fill11 has a concentration gradient for each of the chemical
ss 40
co~npositionin a film thickness direction, and has three layers including an adhesion
layel; a base layer, and a lubricant layel; wherein the three layers are positioned in order
frotll an interface between the surface treatment film and the tiiediunl carbon steel sheet
for cold working of the adhesion layer, the base layer and tlie lubricant layer,,
the adhesion layer coiltains largest amount of the chemical cotilpositioli derived
from the silanol bond alnolig the three layers, and has a thickness of 0.1 nm to 100 11111,
the base layer contains largest amount of the heat-resistant resin and tlie
inorganic acid salt alilolig the three layers, contains 0.01 to 10 parts by Inass of the
inorganic acid salt to 100 parts by Inass of the heat-resistant resin, and has a thickness of
10 0.1 pui to 15 pm,
the lubricant layer contains largest a~nounot f tlie l~tbrica~ailtll ong the three
layers, and has a thickness of 0.1 ptu to 10 LLIIIa, nd
a ratio of the thickness of the base layer to the thickness of tlie lubricant layer is
0.2 to lo.
15
6. The ~nedii~ca~rbno n steel sheet for cold working according to claim 5,
wherein the inorganic acid salt is at least one of compound selected from a
group consisting of a phosphate, a borate, a silicate, a molybdate, and a tungstate.
7. The medium carbon steel sheet for cold working according to claim 5,
wherein the heat-resistant resin is at least one resin selected froin a group
consisting of a polyi~nider esin, a polyester resin, an epoxy resin, and a fluorine resiti.
8. The medilutil carbon steel sheet for cold working according to clai~il5 ,
wherein the lubricant is at least one co~npounds elected from a group consisting
%(41iof
a pol~.tetraflooroethylene, a molybdenum disolfide, a tungsten disulfide, a zinc oxide,
and a graphite.
9. A method for manufacturing medium carbon steel sheet for cold working,
5 the method comprising:
a first process of holding a temperahlre of a cast slab having a chemical
cotnpositiot~a ccording to claim 1 or 2 at 1,050 to 1,300°C;
a second process of performing a hot rollit~gin which rolling is terminated at
750 to l,OOO°C for the cast slab to obtain a steel sheet after the first process;
10 a third process of cooling the steel sheet to a first cooling terminatiotl
temperature of 500 to 700°C at a first average coolit~gra te of 20 to 50°C/second after the
second process;
a foutth process of cooling the steel sheet to a second cooling termination
temperature that is equal to or higher than 400°C and equal to or lower than a
15 tenlperature that is lowver than the first cooling ter~ninatiot~elm perature by 50°C at a
second average cooling rate of 5 to 30°C/second, and coiling the steel sheet after the third
process;
a fifth process of holding the steel sheet so that a time held at a temperatore
range of 400°C to the second cooling termination temperature is litnited to 30 hours or
20 less after the fourth process; and
a sixth process of performing annealing by heating the steel sheet to a
tetnperature of 600°C to A,, poiat-10°C and holding the steel sheet in this temperature
for a time equal to or more than 5 hours and less than 100 hours after the fifth process.
ST qL
10. The method for manufacti~rhigtl ie ti1edi11111c arbon steel sheet for cold
\\,orking according to claim 9,
\vherein in the sixth process, a de\v point at 400°C or lov~eris less than -20°C,
the tle\vpoint at a tetnperature higher than 400°C is less tlian -40°C, and a concentratioli
5 of hyclrogen is 95% or more.
11. The tnetllod for ma~~ufacturiuthge 1i1ediuiu carbon steel sheet for cold
working according to clailn 9 or 10,
\\'herein a \\cater-based surface treatment liquid, \vl~icIic ontailis a water-soluble
10 silanc coupliilg agent, a water-soluble inorganic acid salt, a water-soluble heat-resistant
resin, and a lubrica~iti, s applietl onto at least one stl~faceo f tlie medium carbon stcel
slieet for cold \\,orking, and the surface treatment liquid is dried to form the surface
treatmetit film on at least one surface of the mediuin carbon steel sheet for cold working
after the sixth 11rocess.