TECHNICAL FIELD
[0001] The present invention relates to a plated
steel sheet suitable for application such as a
vehicle body of an automobile in which it is
subjected to press forming.
BACKGROUND ART
[0002] In recent years, it has been required to
improve fuel economy of an automobile for the purpose
of global environment conservation, and needs for a
high-strength steel sheet have been increasing in
order to reduce weight of a vehicle body and to
secure safety of a passenger. It is insufficient
that a steel sheet used for a member for automobile
has only high strength, and the steel sheet is
required to have high corrosion resistance, good
press formability, and good bendability.
[0003] As a hot-dip galvanized steel sheet having
good elongation, a steel sheet utilizing TRIP
(Transformation Induced Plasticity) effect of
retained austenite is known. For example, Patent
Literature 1 discloses a high-tensile hot-dip
galvanized steel sheet made for the purpose of
improving strength and ductility. However, if hard
martensite is contained in a steel sheet for the
purpose of high-strengthening, formability of the
steel sheet deteriorates.
[0004] Other than the Patent Literature 1, Patent
- 1 -
Literatures 2 to 14 disclose techniques for the
purpose of improving mechanical properties of a steel
sheet such as performing tempering of martensite.
However, even with these conventional techniques, it
is difficult to improve Ll1e elongation property and
the formability of a plated steel sheet while
obtaining high strength. Specifically, although the
formability may be improved by performing the
tempering, it is not possible to avoid reduction in
strength caused by the tempering.
CITATION LIST
PATENT LITERATURE
[0005] Patent Literature 1: Japanese Laid-open
Patent Publication No. 11-279691
Patent Literature 2: Japanese Laid-open Patent
Publication No. 6-93340
Patent Literature 3: Japanese Laid-open Patent
Publication No. 6-108152
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2005-256089
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2009-19258
Patent Literature 6: Japanese Laid-open Patent
Publication No. 5-195149
Patent Literature 7: Japanese Laid-open Patent
Publication No. 10-130782
Patent Literature 8: Japanese Laid-open Patent
Publication No. 2006-70328
Patent Literature 9: Japanese Laid-open Patent
- 2 -
Publication No. 2011-231367
.Patent Literature 10: Japanese Laid-open Patent
Publication No. 2013-163827
Patent Literature 11: International Publication
No. WO 2013/047760
Patent Literature 12: International Publication
No. WO 2013/047821
Patent Literature 13: Japanese Laid-open Patent
Publication No. 2014-19905
Patent Literature 14: Japanese Laid-open Patent
Publication No. 2008-255441
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006] The present invention has an object to
provide a plated steel sheet capable of improving an
elongation property and bendability while obtaining
high strength.
SOLUTION TO PROBLEM
[0007] The present inventors conducted earnest
studies in order to improve an elongation property
and bendability of a plated steel sheet having high
strength, and as a result of this, they found out
that the elongation property is improved when a form
of martensite and retained austenite is a M-A
(Martensite-Austenite constituent, also known as
island martensite). Here, as described in the
Literature "Journal of the JWS Vol. 50 (1981), No. 1,
pp. 37-46", the M-A indicates a region of complex of
martensite and retained austenite generated in
- 3 -
martensite transformation during cooling after
concentration of C in non-transformed austenite is
caused in ferrite transformation or bainite
transformation, and is dispersed in an island form in
a matrix.
[0008] Meanwhile, excessively hard marLenslLe
deteriorates bendability. Accordingly, the present
inventors further conducted earnest studies
repeatedly for improving the bendability. As a
result, they found out that when a decarburized
ferrite layer is formed before causing the generation
of M-A, and after the generation of M-A, the M-A is
tempered at a temperature at which the retained
austenite is remained, it is also possible to improve
the bendability while maintaining good elongation
property. Further, the inventors of the present
application arrived at various embodiments of the
invention to be described below. Note that the
concept of plated steel sheet includes a plated steel
strip as well.
[0009] (1) A plated steel sheet, comprising:
a steel sheet; and
a plating layer on the steel sheet, wherein:
the plating layer is a hot-dip galvanizing layer
or an alloyed hot-dip galvanizing layer;
the steel sheet comprises:
a base material; and
a decarburized ferrite layer on the base
material;
- 4 -
the base material includes a chemical composition
represented by, in mass%:
C: 0.03% to 0.70%;
Si: 0.25% to 3.00%;
Mn: 1.0% to 5.0%;
P: 0.10% or less;
S: 0.0100% or less;
sol. Al: 0.001% to 1.500%;
N: 0.02% or less;
Ti: 0.0% to 0.300%;
Nb: 0.0% to 0.300%;
V: 0.0% to 0~300%;
Cr: 0% to 2.000%;
Mo: 0% to 2.000%;
Cu: 0% to 2.000%;
Ni: 0% to 2. 000%;
B: 0% to 0.0200%;
Ca: 0.00% to 0.0100%;
REM: 0.0% to 0.1000%;
Bi: 0.00% to 0.0500%; and
the balance: Fe and impurities;
the base material includes a structure, at a
position at which a depth from a surface of the steel
sheet corresponds to 1/4 of a thickness of the steel
sheet, represented by, in volume fraction:
tempered martensite: 3.0% or more;
ferrite: 4.0% or more; and
retained austenite: 5.0% or more;
an average hardness of the tempered martensite in
- 5 -
the base material is 5 GPa to 10 GPa;
a part or all of the tempered martensite and the
retained austenite in the base material form an M-A;
a volume fraction of ferrite in the decarburized
ferrite layer is 120% or more of the volume fraction
of the ferrite in the base material at the position
at which the depth from the surface of the steel
sheet corresponds to 1/4 of the thickness of the
steel sheet;
an average grain diameter of the ferrite in the
decarburized ferrite layer is 20 pm or less;
a thickness of the decarburized ferrite layer is
5 pm to 200 pm;
a volume fraction of tempered martensite in the
decarburized ferrite layer is 1.0 vol•1me% or more;
a number density of the tempered martensite in
the decarburized ferrite layer is O.Ol/pm2 or more;
and
an average hardness of the tempered martensite in
the decarburized ferrite layer is 8 GPa or less.
[0010] (2) The plated steel sheet according to (1),
wherein, in the chemical composition,
Ti: 0.001% to 0.300%,
Nb: 0.001% to 0.300%, or
V: 0.001% to 0.300%,
or any combination thereof is satisfied.
[0011] (3) The plated steel sheet according to (1)
or (2), wherein, in the chemical composition,
Cr: 0.001% to 2.000%, or
- 6 -
Mo: 0.001% to 2.000%,
or both of them is satisfied.
[ 0012 J
one of
( 4)
( 1)
The plated steel sheet according to any
to ( 3), wherein, in the chemical
composition,
Cu: 0.001% to 2.000%, or
Ni: 0.001% to 2.000%,
or both of them is satisfied.
[0013] (5) The plated steel sheet according to any
one of ( 1) to ( 4), wherein, in the chemical
composition, B: 0.0001% to 0.0200% is satisfied.
[0014] (6) The plated steel sheet according to any
one of (1) to (5), wherein, in the chemical
composition,
Ca: 0.0001% to 0.0100%, or
REM: 0.0001% to 0.1000%,
or both of them is satisfied.
[0015] (7) The plated steel sheet according to any
one of (1) to (6), wherein, in the chemical
composition, Bi: 0.0001% to 0.0500% is satisfied.
ADVANTAGEOUS EFFECTS OF INVENTION
[0016] According to the present invention, a base
material and a decarburized ferrite layer includes a
configuration, so that it is possible to improve an
elongation property and bendability while obtaining
high strength.
BRIEF DESCRIPTION OF DRAWINGS
[ 0017] [Fig. 1] Fig. 1 is a sectional view
illustrating a plated steel sheet according to an
- 7 -
embodiment of the present invention;
[Fig. 2] Fig. 2 is a chart illustrating an
outline of a distribution of volume fraction of
ferrite in a steel sheet;
[Fig. 3] Fig. 3 is a flow chart illustrating a
first example of a metl1od of manufacturing a plated
steel sheet; and
[Fig. 4] Fig. 4 is a flow chart illustrating a
second example of a method of manufacturing a plated
steel sheet.
DESCRIPTION OF EMBODIMENTS
[0018] Hereinafter, a plated steel sheet according
to embodiments of the present invention will be
described while referring to the attached drawings.
Fig. 1 is a sectional view illustrating a plated
steel sheet according to an embodiment of the present
invention.
[0019] As illustrated in Fig. 1, a plated steel
sheet 1 according to the present embodiment includes
a steel sheet 10, and a plating layer 11 on the steel
sl1eet 10. The sLeel ~heeL 10 includes a base
material 13, and a decarburized ferrite layer 12 on
the base material 13. The plating layer 11 is a hotdip
galvanizing layer or an alloyed hot-dip
galvanizing layer. The decarburized ferrite layer 12
is between the base material 13 and the plating layer
11.
[0020] Here, a chemical composition of the base
material 13 and a raw material steel sheet used for
- 8 -
manufacturing the plated steel sheet 1 will be
described. Although details will be described later,
the plated steel sheet 1 is manufactured by making a
raw material steel sheet to be subjected to heating,
annealing, first cooling, second cooling, hot-dip
galvanizing, third cooling, and the like. Alloying
may be performed between the plating and the third
cooling. Therefore, the chemical composition of the
base material 13 and the raw material steel sheet
takes not only properties of the plated steel sheet 1
but also these treatments into consideration. In the
description hereinbelow, ~%" being a unit of content
of each element contained in the base material 13 and
the raw material steel sheet means ~mass%"r unless
otherwise specified. The base material 13 and the
raw material steel sheet includes a chemical
composition represented by C: 0.03% to 0.70%, Si:
0.25% to 3.00%, Mn: 1.0% to 5.0%, P: 0.10% or less,
S: 0.0100% or less, acid-.soluble Al (sol. Al): 0.001%
to 1.500%, N: 0.02% or less, Ti: 0.0% to 0.300%, Nb:
0.0% to 0.300%, V: 0.0% to 0.300%, Cr: 0% to 2.000%,
Mo: 0% to 2.000%, Cu: 0% to 2.000%, Ni: 0% to 2.000%,
B: 0% to 0. 0200%, Ca: 0. 00% to 0. 0100%, rare earth
met a l (REM) : 0 . 0% to 0 . 1 0 0 0%, B i : 0 . 0 0% to 0 . 0 50 0%,
and the balance: Fe and impurities. As the impurity,
one contained in a raw material such as ore or scrap
and one contained in a manufacturing process may be
exemplified.
[ 0 021 J (C: 0.03% to 0.70%)
-· 9 -
C contributes to improvement of tensile strength.
If the C content is less than 0.03%, it is not
possible to obtain sufficient tensile strength.
Therefore, the C content is 0.03% or more, and
preferably 0.05% or more. On the other hand, if the
C content exceeds 0.70%, weldabilily of the plated
steel sheet 1 is lowered. Therefore, the C content
is 0.70% or less, and preferably 0.45% or less.
[0022] (Si: 0.25% to 3.00%)
Si suppresses precipitation of cementite and
makes it easy for austenite to be retained, to
thereby contribute to improvement of elongation. Si
also contributes to strengthening of ferrite,
uniformization of structure, and improvement of
strength. If the Si content is less than 0.25%,
these effects cannot be sufficiently obtained.
Therefore, the Si content is 0.25% or more, and
preferably 0.40% or more. Si also contributes to
generation of austenite and growth of the
decarburized ferrite layer 12. In order to
sufficiently obtain this effect, the Sl conLeJJL ls
more preferably 0.60% or more. On the other hand, if
the Si content exceeds 3.00%, plating defect may
occur in hot-dip galvanizing. Therefore, the Si
content is 3.00% or less, and preferably set to 2.50%
or less.
[0023] (Mn: 1.0% to 5.0%)
Mn makes tempered martensite sufficiently
disperse in the decarburized ferrite layer 12, to
- 10 -
thereby contribute to improvement of number density
of the tempered martensite in the decarburized
ferrite layer 12. Mn suppresses precipitation of
cementite to facilitate generation of M-A, and
contributes also to improvement of strength and
elongation. If the Mn content is less than 1.0%,
these effects cannot be sufficiently obtained.
Therefore, the Mn content is 1.0% or more, and
preferably 1.9% or more. On the other hand, if the
Mn content exceeds 5.0%, the weldability of the
plated steel sheet 1 is lowered. Therefore, the Mn
content is 5.0% or less, preferably 4.2% or less, and
more preferably set to 3.5% or less.
[0024] (P: 0.10% or less)
P is not an essential element, and is contained
in the steel as an impurity, for example. P
deteriorates the weldability, so that the lower the P
content, the better. In particular, if the P content
exceeds 0.10%, the weldability is significantly
lowered. Therefore, the P content is 0.10% or less,
and preferably 0.02% or less.
[0025] (S: 0.0100% or less)
S is not an essential element, and is contained
in the steel as an impurity, for example. S forms
MnS in the steel to deteriorate hole expandability,
so that the lower the S content, the better. In
particular, if the S content exceeds 0.0100%, the
hole expandability is significantly lowered.
Therefore, the S content is 0.0100% or less,
- 11 -
preferably 0.0050% or less, and more preferably
0.0012% or less.
[0026] (sol. Al: 0.001% to 1.500%)
Sol. Al has a deoxidation effect, suppresses
generation of surface flaw, and improves
productivity. If the sol. Al content is less than
0.001%, these effects cannot be sufficiently
obtained. Therefore, the sol. Al content is 0.001%
or more. Similar to Si, sol. Al suppresses the
precipitation of cementite to make it easy for
austenite to be retained. In order to sufficiently
obtain this effect, the sol. Al content is preferably
0.200% or more. On the other hand, if the sol. Al
content exceeds 1.500%, an inclusion increases to
deteriorate the hole expandability. Therefore, the
sol. Al content is 1.500% or less, and preferably
1.000% or less.
[0027] (N: 0.02% or less)
N is not an essential element, and is contained
in the steel as an impurity, for example. N forms a
niL.tide during continuous casting in forming the raw
material steel sheet, which sometimes causes
occurrence of crack in a slab, so that the lower the
N content, the better. In particular, if the N
content exceeds 0.02%, the crack in the slab easily
occurs. Therefore, the N content is 0.02% or less,
and preferably 0.01% or less.
[ 0028] Ti, Nb, V, Cr, Mot Cu, Ni, B, Ca, REM, and Bi
are not essential elements, and are optional elements
- 12 -
which may be appropriately contained in a steel sheet
and a slab in an amount up to a specific amount as a
limit.
[0029] (Ti: 0.0% to 0.300%, Nb: 0.0% to 0.300%, V:
0.0% to 0.300%)
Ti, Nb, and V generate precipitates to be nuclei
of grains, and thus contribute to refinement of
grains. The refinement of grains leads to
improvement of strength and toughness. Therefore,
Ti, Nb, or V, or any combination thereof may also be
contained. In order to sufficiently obtain this
effect, each of the Ti content, the Nb content, and
the V content is preferably 0.001% or more. On the
other hand, if one of the Ti content, the Nb content,
and the V content exceeds 0.300%, the effect is
saturated and the cost is unnecessarily increased.
Therefore, each of the Ti content, the Nb content,
and the V content is 0.300% or less. Specifically,
it is preferable to satisfy the condition of QTi:
0.001% to 0.300%," "Nb: 0.001% to 0.300%,'' or ~v:
0.001% to 0.300%," or any combination thereof. Ti
and Nb facilitate the concentration of C in austenite
caused by the generation of ferrite, in first
cooling, in a raw material steel sheet in which at
least a part of a structure is transformed into
austenite in annealing, so that the M-A is easily
generated. In order to sufficiently obtain this
effect, Ti or Nb, or both of them is/are more
preferably contained in an amount of 0.010% or more
- 13 -
in total, and still more preferably contained in an
amount of 0.030% or more in total.
[0030] (Cr: 0% to 2.000%, Mo: 0% to 2.000%)
Cr and Mo stabilize austenite to contribute to
improvement of strength owing to the generation of
martensite. Therefore, Cr or Mo, or both of them may
also be contained. In order Lo sufficiently obtain
this effect, the Cr content is preferably 0.001% or
more, and more preferably 0.100% or more, and the Mo
content is preferably 0.001% or more, and more
preferably 0.050% or more. On the other hand, if the
Cr content or the Mo content exceeds 2.000%, the
effect is saturated and the cost is unnecessarily
increased. Therefore, the Cr content is 2.000% or
less, and preferably 1. 000% or less, and the Mo
content is 2.000% or less, and preferably 0.500% or
less. Specifically, it is preferable to satisfy the
condition of ''Cr: 0.001% to 2.000%,u or nMo: 0.001%
to 2.000%,n or both of them.
[0031] (Cu: 0% to 2.000%, Ni: oe, to 2.000%)
Cu and Nl suppress corrosion of the plated steel
sheet 1, and concentrate in a surface of the plated
steel sheet 1 to suppress entrance of hydrogen into
the plated steel sheet 1, thereby suppressing delayed
fracture of the plated steel sheet 1. Therefore, Cu
or Ni, or both of them may also be contained. In
order to sufficiently obtain this effect, each of the
cu content and the Ni content is preferably 0.001% or
more, and more preferably 0.010% or more. On the
- 14 -
other hand, if the Cu content or the Ni content
exceeds 2.000%, the effect is saturated and the cost
is unnecessarily increased. Therefore, each of the
Cu content and the Ni content is 2.000% or less, and
preferably 0.800% or less. Specifically, it is
preferable to satisfy the condition of "Cu: 0.001% to
2.000%,u or
[0032] (B:
"Ni: 0.001% to
0% to 0.0200%)
2.000%,u or both of them.
B suppresses nucleation of ferrite from a grain
boundary, and enhances hardenability of the plated
steel sheet 1, to thereby contribute to highstrengthening
of the plated steel sheet 1. B also
contributes to improvement of elongation of the
plated steel sheet 1 by effectively generating the MA.
Therefore, B may also be contained. In order to
sufficiently obtain this effect, the B content is
preferably 0.0001% or more. On the other hand, if
the B content exceeds 0.0200%, the effect is
saturated and the cost is unnecessarily increased.
Therefore, the B content is 0.0200% or less.
Specifically, it is preferable to satisfy the
condition of "B: 0.0001% to 0.0200%.u
[0033] (Ca: 0.00% to 0.0100%, REM: 0.0% to 0.1000%)
Ca and REM spheroidize a sulfide to improve
expandability of the plated steel sheet 1.
Therefore, Ca or REM, or both of them may also be
contained. In order to sufficiently obtain this
effect, each of the Ca content and the REM content is
preferably 0.0001% or more. On the other hand, if
- 15 -
the Ca content exceeds 0.0100% or if the REM content
exceeds 0.1000%, the effect is saturated and the cost
is unnecessarily increased. Therefore, the Ca is
0.0100% or less, and the REM content is 0.1000% or
less. Specifically, it is preferable to satisfy the
condiLlor1 of "Ca: 0.0001% to 0.0100%," or ~REM:
0.0001% to 0.1000%," or both of them.
[ 0034] REM indicates 17 kinds of elements in total
of Sc, Y, and lanthanoide series, and "REM content"
means a total content of these 17 kinds of elements.
Industrially, the lanthanoide series are added in a
form of misch metal, for example.
[0035] (Bi: 0.00% to 0.0500%)
Bi concentrates in a solidification interface to
narrow a dendrite interval, to thereby suppress
solidifying segregation. When micro-segregation of
Mn or the like occurs, there is a chance that a band
structure with nonuniform hardness develops, and
workability lowers, and Bi suppresses reduction of
properties caused by such micro-segregation.
Therefore, Bl 1nay also be contained. In order to
sufficiently obtain this effect, the Bi content is
preferably 0.0001% or more, and more preferably
0.0003% or more. On the other hand, if the Bi
content exceeds 0.0500%, a surface quality
deteriorates. Therefore, the Bi content is 0.0500%
or less, preferably 0.0100% or less, and more
preferably 0.0050% or less. Specifically, it is
preferable to satisfy the condition of "Bi: 0.0001%
- 16 --
to 0.0500%.n
[0036] Next, the base material 13 will be described.
A position at which a structure of the base material
is defined is a position at which a depth from a
surface of the steel sheet 10 corresponds to 1/4 of a
thickness of the steel sheet 10. This position is
sometimes referred to as "1/4 sheet thickness
position,n hereinafter. This is because the 1/4
sheet thickness position is generally considered to
be a position at which average configuration and
properties of the steel sheet are exhibited.
Normally, a structure at a position other than the
1/4 sheet thickness position of the base material 13
is substantially the same as the structure at the l/4
sheet thickness position. In the description
hereinbelow, "%" being a unit of volume fraction of
each structure contained in the base material 13
means "volume%,'' unless otherwise specified. The
base material 13 includes, at the position at which
the depth from the surface of the steel sheet 10
corresponds to 1/4 of the thickness of the steel
sheet 10, a structure represented by, in volume
fraction, 3.0% or more of tempered martensite, and
5.0% or more of retained austenite. An average
hardness of the tempered martensite in the base
material 13 is 5 GPa to 10 GPa, a part or all of the
tempered martensite and the retained austenite in the
base material 13 form the M-A. In order to obtain
the plated steel sheet 1 having good workability and
- 17 -
tensile strength of 780 MPa or more, it is effective
to make the structure of the base material 13 to be a
structure obtained by performing tempering on the
structure containing the M-A at a temperature at
which the retained austenite remains. When the base
material 13 has such a structure, local elongation is
improved while maintaining good total elongation
realized by the M-A.
[0037] (Tempered martensite: 3.0% or more)
The tempered martensite contributes to
improvement of bendability. If the volume fraction
of the tempered martensite is less than 3.0%, it is
not possible to obtain sufficient bendability.
Therefore, the volume fraction of the tempered
martensite is 3.0% or more, and preferably 5.0% or
more. The tempered martensite also contributes to
improvement of strength, and in order to obtain
higher strength, the volume fraction of the tempered
martensite is preferably 8.0% or more.
[0038] (Retained austenite: 5.0% or more)
The retained austenite contributes to improvement
of elongation. If the volume fraction of the
retained austenite is less than 5.0%, it is not
possible to obtain sufficient elongation. Therefore,
the volume fraction of the retained austenite is 5.0%
or more. The retained austenite also contributes to
improvement of strength, and in order to obtain
higher strength, the volume fraction of the retained
austenite is preferably 8.0% or more.
- 18 -
[0039] (Average hardness of tempered martensite: 5
GPa to 10 GPa)
If the average hardness of the tempered
martensite is less than 5 GPa, it is not possible to
obtain sufficient strength, for example, tensile
strength of 780 MPa or more. Therefore, the average
hardness of the tempered martensite in the base
material 13 is 5 GPa or more. On the other hand, if
the average hardness of the tempered martensite
exceeds 10 GPa, a crack easily occurs when bending is
applied, resulting in that excellent bendability
cannot be achieved. Therefore, the average hardness
of the tempered martensite in the base material 13 is
10 GPa or less. The average hardness of the tempered
martensite can be measured by a nano-indentation
method. In the measurement, for example, an indenter
having a shape of cube corner is used, and an
indentation load is 500 pN.
[0040] (M-A)
In the present embodiment, a part or all of the
tempered martensite and the retained austenite in the
base material 13 form the M-A. The M-A contributes
to improvement of total elongation (T. El). In order
to obtain further excellent bendability, the entire
martensite contained in the base material 13 is
preferably the tempered martensite.
[ 0041] (Balance)
It is preferable that the balance of the base
material 13 is mainly composed of ferrite or of
- 19 -
ferrite and bainite. If the volume fraction of
ferrite is less than 4.0%, there is a chance that
sufficient elongation property and bendability cannot
be obtained. Therefore, the volume fraction of
ferrite in the bae material 13 is 4.0% or more from a
viewpoint of mechanical property such as tensile
strength. On the other hand, if the volume fr~~tion
of ferrite exceeds 70%, there is a chance that
sufficient strength cannot be obtained. Therefore,
the volume fraction of ferrite in the base material
13 is preferably 70% or less. It is preferable that
no cementite having a circle-equivalent diameter of 5
pm or more exists in a grain of ferrite and a grain
of martensite in the base material 13. This is for
facilitating the generation of M-A.
[0042] Next, the decarburized ferrite layer 12 will
be described. The decarburized ferrite layer 12 is a
layer formed on the base material 13 as a result of
making a surface of the raw material steel sheet to
be subjected to decarburization during annealing, and
in which a volume frdction of ferrite is 120% or more
of a volume fraction of ferrite in the base material
13 at the 1/4 sheet thickness position.
Specifically, in the present embodiment, the volume
fraction of ferrite is measured at intervals of 1 pm
from the surface of the steel sheet 10, and it is
defined that an interface between the decarburized
ferrite layer 12 and the base material 13 exists at a
position at which the measurement result shows 120%
- 20 -
of the volume fraction of ferrite at the 1/4 sheet·
thickness position of the steel sheet 10, and
accordingly, a portion on a surface side of the steel
sheet 10 with respect to the interface can be
regarded as the decarburized ferrite layer 12. Fig.
2 illustrates an outline of a distribution of the
volume fraction of ferrite in the steel sheet 10. A
vertical axis in Fig. 2 indicates a proportion when
the volume fraction of ferrite at the 1/4 sheet
thickness position is set to 100%.
[0043] The decarburized ferrite layer 12 is softer
than the base material 13 since the decarburized
ferrite layer 12 contains C in an amount smaller than
that of the base material 13, so that even if the
plated steel sheet 1 is bent, a crack is difficult to
occur in the decarburized ferrite layer 12. Further,
since the decarburized ferrite layer 12 is easily
deformed uniformly, constriction is difficult to
occur in the decarburized ferrite layer 12.
Therefore, the decarburized ferrite layer 12 improves
bendability of the plated steel sheet 1.
[0044] The present inventors repeatedly conducted
earnest studies by focusing attention on the fact
that although decarburization of a raw material steel
sheet is performed also in a conventional plated
steel sheet,
bendability.
it is not possible to achieve sufficient
As a result, it was clarified that in
the conventional plated steel sheet, an average grain
diameter of ferrite in the decarburized ferrite layer
- 21 -
is large to be 20 ~m or more and a fine crack occurs
in a decarburized ferrite layer since deformation
intensively occurs in a grain boundary of ferrite
when bending deformation of the steel sheet occurs.
Further, the present inventors found out that in
order to solve this problem, it is effective to
reduce the average grain diameter of ferrite in the
decarburized ferrite layer, and to disperse tempered
martensite provided with the specifi average hardness
in the decarburized ferrite layer. In the present
embodiment, an average grain diameter of ferrite in
the decarburized ferrite layer 12 is 20 ~m or less, a
thickness of the decarburized ferrite layer 12 is 5
~m to 200 pm, a volume fraction of the tempered
martensite in the decarburized ferrite layer 12 is
1.0 volume% or more, a number density of the tempered
martensite in the decarburized ferrite layer 12 is
0.01/pm2 or more, and an average hardness of the
tempered martensite in the decarburized ferrite layer
12 is 8 GPa or less.
[0045]
less)
(Average grain diameter of ferrite: 20 ~m or
The volume fraction of ferrite in the
decarburized ferrite layer 12 is 120% or more of the
volume fraction of ferrite in the base material 13 at
the 1/4 sheet thickness position. If the average
grain diameter of ferrite in the decarburized ferrite
layer 12 exceeds 20 pm, a total area of the grain
boundary of ferrite is small, and deformation
- 22 -
intensively occurs in a narrow region, resulting in
that excellent bendability of the plated steel sheet
1 cannot be obtained. Therefore, the average grain
diameter of ferrite is 20 pm or less. The smaller
the average grain diameter of ferrite, the more
preferable, but, it is difficult to make the average
grain diameter of ferrite 0.5 pm or less under the
current technical level.
[0046] (Thickness: 5 pm to 200 pm)
If the thickness of the decarburized ferrite
layer 12 is less than 5 pm, it is not possible to
sufficiently achieve the effect of improvement of
bendability realized by the decarburized ferrite
layer 12. For this reason, when the plated steel
sheet 1 is bent, the base material 13 whose strength
is higher than that of the decarburized ferrite layer
12 is deformed to cause a microcrack. Therefore, the
thickness of the decarburized ferrite layer 12 is 5
pm or more. If the thickness of the decarburized
ferrite layer 12 exceeds 200 ym, it is not possible
to obtain sufficient tensile strength. Therefore,
the thickness of the decarburized ferrite layer 12 is
200 pm or more.
[0047] (Volume fraction of tempered martensite: 1.0
volume% or more)
If the volume fraction of the 12 tempered
martensite in the decarburized ferrite layer is less
than 1.0 volume%, nonuniform deformation easily
occurs in the plated steel sheet 1, resulting in that
- 23 -
excellent bendability cannot be obtained. Therefore,
the volume fraction of the tempered martensite in the
decarburized ferrite layer 12 is 1.0 volume% or more.
The decarburized ferrite layer 12 is formed through
the decarburization ot the raw material steel sheet,
so Ll1at there is no chance that the volume fraction
of the tempered martensite in the decarb1Jri~ed
ferrite layer 12 exceeds the volume fraction of the
tempered martensite in the base material 13. If the
volume fraction of the tempered martensite in the
decarburized ferrite layer 12 exceeded the volume
fraction of the tempered martensite in the base
material 13, this would mean that no decarburization
occurred in the decarburized ferrite layer 12.
Therefore, the volume fraction of the tempered
martensite in the decarburized ferrite layer 12 is
equal to or less than the volume fraction of the
tempered martensite in the base material 13. In the
present embodiment, the martensite contained in the
decarburized ferrite layer 12 is not fresh martensite
(untempered n1artensite) but the tempered martensite,
so that it is possible to suppress occurrence of
crack at an interface between ferrite and martensite.
[0048] The balance of the structure of the
decarburized ferrite layer 12 is mainly composed of
ferrite. As described above, the area fraction of
ferrite in the decarburized ferrite layer 12 is 120%
or more of the area fraction of ferrite in the base
material 13 at the 1/4 sheet thickness position. The
- 24 -
balance of the structure of the decarburized ferrite
layer may also contain, for example, bainite,
pearlite, and the like, within a range of exerting no
influence on the properties of the plated steel sheet
l according to the present embodiment, for example,
within a range of 5 volume% or less.
[0049] (Number density of tempered martensite:
0.01/pm2 or more)
If the number density of the tempered martensite
in the decarburized ferrite layer 12 is less than
0.01/pm2
, nonuniform deformation easily occurs in the
plated steel sheet 1, resulting in that excellent
bendability cannot be obtained. Therefore, the
number density of the tempered martensite in the
decarburized ferrite layer 12 is 0.01/pm2 or more.
The higher the number density of the tempered
martensite, the better, but, it is difficult to make
the number density l/pm2 or more, under the current
technical level.
[0050] (Average hardness of tempered martensite: 8
GPa or less)
If the average hardness of the tempered
martensite in the decarburized ferrite layer 12
exceeds 8 GPa, a crack easily occurs in the
decarburized ferrite layer 12 when the plated steel
sheet 1 is bent, and thus excellent bendability
cannot be obtained. Therefore, the average hardness
of the tempered martensite in the decarburized
ferrite layer 12 is 8 GPa or less. Although a lower
- 25 -
limit of the average hardness of the tempered
martensite in the decarburized ferrite layer 12 is
not limited, when tempering is performed to a degree
at which high strength of the plated steel sheet 1 is
secured, the average hardness of the tempered
martensite in the decarburized ferrite layer 12 does
not become less than 4 GPa. The average hardness of
the tempered martensite in the decarb11rized ferrite
layer 12 is smaller than the average hardness of the
tempered martensite in the base material 13.
[0051] With the use of the plated steel sheet 1
according to the present embodiment, it is possible
to improve the elongation property and the
bendability while obtaining high strength. For
example, in a tensile test in which a sheet width
direction (a direction perpendicular to a rolling
direction) is set as a tensile direction, it is
possible ·to obtain tensile strength (TS) of 780 MPa
or more, yield strength (YS) of 420 MPa or more, and
total elongation (T. El) of 12% or more. Further,
for example, in a hole expansion test, it is possible
to obtain a hole expansion ratio of 35% or more, and
regarding the bendability, it is possible to obtain a
result such that in a 90-degree V-shaped bending
test, no crack occurs and no constriction of 10 ~m or
more occurs.
[0052] Next, description will be made on examples of
a method of manufacturing the plated steel sheet 1
according to the embodiment of the present invention.
- 26 -
In a first
(stepS2),
(step S4),
example, heating (step Sl), annealing
first cooling (step S3), second cooling
hot-dip galvanizing (step S5), third
cooling (step S6), and tempering (step S7), of a raw
material steel sheet, are performed in this order, as
illustrated in Fig. 3. In a second example, heating
(step Sl), annealing (step S2), first cooling (step
S3), second cooling (step S4), hot-dip galvanizing
(step S5), alloying (step S8), third cooling (step
S6), and tempering (step 87), of a raw material steel
sheet, are performed in this order, as illustrated in
Fig. 4. As the raw material steel sheet, a hotrolled
steel sheet or a cold-rolled steel sheet is
used, for example.
[0053] (Heating)
In the heating (step Sl) of the raw material
steel sheet, an average heating rate in a temperature
range of lOO"C to 720°C is l"C/second to 50°C/second.
The average heating rate indicates a value obtained
by dividing a difference between a heating start
temperature and a heating finish temperature by a
heating time. If the average heating rate is less
than l°C/second, cementite in the raw material steel
sheet is not dissolved in the heating of the raw
material steel sheet, resulting in that the tensile
strength of the plated steel sheet 1 reduces. If the
average heating rate is less than l"C/second, it is
difficult to disperse the tempered martensite in the
decarburized ferrite layer 12, and the number density
- 27 -
of the tempered martensite in the decarburized
ferrite layer 12 becomes less than 0.01/pm2
•
Therefore, the average heating rate is 1"C/second or
more. On the other hand, if the average heating rate
exceeds SO"C/second, coarse ferrite is generated in
the raw material steel sheet in the heating of the
raw material steel sheet. Also, when the averHge
heating rate exceeds SO"C/second, it is difficult to
disperse the tempered martensite in the decarburized
ferrite layer 12, and the number density of the
tempered martensite in the decarburized ferrite layer
12 becomes less than 0.01/pm2
. Therefore, the average
heating rate is SO"C/second or less.
[0054] (Annealing)
In the annealing (step S2), the raw material
steel sheet is held at 720"C to 950"C for 10 seconds
to 600 seconds. The austenite is generated in the
raw material steel sheet in the annealing. If an
annealing temperature is less than 720"C, the
austenite is not generated, and it is not possible to
generaLe Ll1e Lempered martensite after that.
Therefore, the annealing temperature is 720"C or more.
In order to make the structure of the base material
13 to be a more uniformized structure to obtain
further excellent bendability, the annealing
temperature is preferably an Ac 3 point or more
(austenite single-phase region). In this case, it is
preferable that it takes 30 seconds or more for
increasing temperature from 720"C to the Ac 3 point.
- 28 -
This is because the decarburized ferrite layer 12
having an average grain diameter of 10 pm or less can
be stably generated on the surface of the raw
material steel sheet. On the other hand, if the
annealing temperature exceeds 950"C, it is difficult
to set the number density of the tempered martensite
in the decarburized ferrite layer 12 to O.Ol/~m 2 or
more, or the austenite is grown during the annealing,
resulting in that the volume fraction of ferrite in
the decarburized ferrite layer becomes too small.
Therefore, the annealing temperature is 950"C or less.
Note that if the holding time in the annealing is
less than 10 seconds, the thickness of the
decarburized ferrite layer 12 becomes less than 5 ~m.
Therefore, the holding time is 10 seconds or more.
On the other hand, if the holding time in the
annealing exceeds 600 seconds, the thickness of the
decarburized ferrite layer 12 exceeds 200 pm, or the
effect of annealing is saturated to lower the
productivity. Therefore, the holding time is 600
seconds or less.
[0055] The annealing is performed under an
atmosphere in which a hydrogen concentration is 2
volume% to 20 volume%, and a dew point is -30"C to
20"C. If the hydrogen concentration is less than 2%,
it is not possible to sufficiently reduce an oxide
film on the surface of the raw material steel sheet,
and it is not possible to obtain sufficient plating
wettability at the time of performing the hot-dip
- 29 -
galvanizing (step 85). Therefore, the hydrogen
concentration is 2 volume% or more. On the other
hand, if the hydrogen concentration is less than 20
volume%, it is not possible to maintain the dew point
to 2o•c or less, resulting in that dew condensation
occurs in a facility to hinder operation of the
facility. Therefore, the hydrogen concentration is
20 volume% or more. If the dew point is less than -
Jo•c, the thickness of the decarburized ferrite layer
12 becomes less than 5 ym. Therefore, the dew point
is -30°C or more. On the other hand, if the dew point
exceeds 2o•c, dew condensation occurs in a facility to
hinder operation of the facility. Therefore, the dew
point is 2o•c or less.
[0056] (First cooling)
In the first cooling (step 83), an average
cooling rate from 720°C to 650°C is o.s•cfsecond to
lo.o•c;second. The average cooling rate indicates a
value obtained by dividing a difference between a
cooling start temperature and a cooling finish
temperature by a cooling time. In the first cooling,
the martensite is generated in the decarburized
ferrite layer 12, C is concentrated in nontransformed
austenite, and a part or all of the
martensite and the retained austenite form the M-A.
If the average cooling rate js less than 0.5"C/second,
cementite is precipitated in the first cooling,
resulting in that it becomes difficult for the
martensite to be generated in the decarburized
- 30 -
ferrite layer 12. Therefore,
rate is 0.5"C/second or more,
the average cooling
preferably 1.0"C/second
or more, and more preferably 1.5"C/second or more. On
the other hand, if the average cooling rate exceeds
10.0"C/second, Cis difficult to be diffused, and thus
a concentration gradient of C in the austenite is not
sufficiently provided. For this reason, the retained
austenite is difficult to be generated, and thus the
M-A is difficult to be generated in the base material
13. Therefore, the average cooling rate is
10.0"C/second or less, preferably 8.0"C/second or
less, and more preferably 6.0"C/second or less.
[0057] (Second cooling)
In the second cooling (step S4), an average
cooling rate from 650"C to 500"C is 2.0"C/second to
100.0"C/second. If the average cooling rate is less
than 2.0"C/second, pearlite is precipitated to
suppress the generation of retained austenite.
Therefore, the average cooling rate is 2.0"C/second or
more, preferably 5.0"C/second or more, and more
preferably 8.0"C/second or more. On the other hand,
if the average cooling rate exceeds 100.0"C/second,
flatness of the steel sheet 10 deteriorates, and a
thickness of the plating layer 11 varies greatly.
Therefore, the average cooling rate is 100.0"C/second
or less, preferably 60.0"C/second or less, and more
preferably 40"C/second or less.
[0058] (Hot-dip galvanizing, alloying)
A bath temperature and a bath composition in the
- 31 -
hot-dip galvanizing (step S5) are not limited, and
general ones may be employed. A coating weight is
also not limited, and a general one may be employed.
For example, the coating weight per one side is 20
g/m2 to 120 g/m2
• When an alloyed hot-dip galvani~ing
layer is formed as the plating layer 11, the alloying
(step SB) is performed following the hot-dip
galvanizing treatment. The alloying is preferably
performed under a condition in which an Fe
concentration in the plating layer 11 becomes 7 mass%
or more. In order to make the Fe concentration 7
mass% or more, for example, a temperature in the
alloying is 4 90°C to 560°C, and a period of time of
the treatment is 5 seconds to 60 seconds, although
depending also on the coating weight. When a hot-dip
galvanizing layer is formed as the plating layer 11,
the alloying is not performed. In t.his case, the Fe
concentration in the plating layer 11 may also be
less than 7 mass%. The weldability of the hot-dip
galvanized steel sheet is lower than the weldability
of Lhe alloyed hot-dip galvanized steel sheet.
However, the corrosion resistance of the hot-dip
galvanized steel sheet is good.
[0059] It is also po~sible to perform isothermal
holding and cooling of the raw material steel sheet,
according to need, between the second cooling (step
S4) and the hot-dip galvanizing treatment (step SS).
[0060] (Third cooling)
In the third cooling (step S6), an average
- 32 -
cooling rate from the alloying temperature in the
case of performing the alloying or the bath
temperature in the hot-dip galvanizing in the case of
performing no alloying to a temperature of 200°C or
less is 2°C/second or more. In the third cooling,
stabilized austenite is generated. Almost all of the
stabilized austenite remains as it is as austenite
even after being subjected to the tempering (step
s 7 I . In the third cooling, hard martensite may be
generated other than the stabilized austenite, and
the hard martensite is turned into the tempered
martensite having ductility by being subjected to the
tempering (step S7). If the average cooling rate is
less than 2°C/second, it is not possible to
sufficiently obtain the stabilized austenite, and the
volume fraction of the retained austenite in the base
material 13 becomes less than 5.0%. There fore, the
average cooling rate is 2°C/second or more, and
preferably 5°C/second or more. Although an upper
limit of the average cooling rate is not limited, it
is preferably 500°C/second or less, from a viewpoint
of economic efficiency. Although a cooling stop
temperature of the third cooling is not limited, it
is preferably a temperature of 100°C or less.
[0061] (Tempering)
In the tempering (step S7), the raw material
steel sheet is held at 100°C or more and less than
200°C for 30 seconds (0.5 minutes) to 48 hours (1152
minutes). The effect of tempering is exhibited more
- 33 -
significantly in the decarburized ferrite layer 12
than in the base material 13. Specifically, at the
tempering temperature of less than 200"C, the degree
of softening of martensite in the base material. 13 is
low, and meanwhile, in the decarburized ferrite layer
12, the C concentration is lower than that in the
base material 13, and thus surface diffusion easily
occurs, which leads to significant softening. The
easiness of occurrence of crack in the vicinity of
the surface of the steel sheet 10 exerts a large
influence on the bendability, and it is possible to
appropriately reduce the hardness of the tempered
martensite in the decarburized ferrite layer 12 while
maintaining a high average hardness of the tempered
martensite in the base material 13. Therefore, it is
possible to improve the bendability and the
elongation while securing high tensile strength. In
addition, by performing the tempering, C is
concentrated not only in the non-transformed retained
austenite but also in the ferrite when the raw
Ittalerial steel sheet contains the ferrite. Further,
because of the concentration of C, the retained
austenite and the ferrite are hardened, resulting in
that uniform elongation (U. Ell of the plated steel
sheet 1 is improved.
[0062] If the tempering temperature is less than
100"C, the tempering of martensite in the decarburized
ferrite layer 12 is insufficient, and the average
hardness of the tempered martensite in the
- 34 -
decarburized ferrite layer 12 exceeds 8 GPa.
Therefore, the tempering temperature is 100°C or more,
and preferably 120°C or more. On the other hand, if
the tempering temperature is 200°C or more, the
retained austenite in the base material 13 and the
decarburized ferrite layer 12 is decomposed, and the
average hardness of the tempered martensite in the
base material 13 becomes less than 5 GPa. As a
result, the tensile strength lowers, and the
elongation deteriorates. Therefore, the tempering
temperature is less than 200°C. If a tempering time
is less than 30 seconds, the tempering of martensite
in the decarburized ferrite layer 12 is insufficient,
and the average hardness of the tempered martensite
in the decarburized ferrite layer 12 exceeds 8 GPa.
Therefore, the tempering time is 30 seconds or more.
On the other hand, if the tempering time exceeds 48
hours, the effect is saturated and the productivity
is unnecessarily lowered.
time is 48 hours or less.
Therefore, the tempering
In the tempering, it is
preferable to suppress temperature fluctuation to
keep a certain temperature, in order to suppress
variation of properties of the steel sheet 10. It is
preferable that the entire martensite of the M-A in
the base material 13 is tempered by the tempering.
[0063] After the tempering, it is also possible to
perform correction of flatness by using a leveler,
and it is also possible to perform oil coating or
provide a coating film having a lubrication action.
- 35 -
[ 0 0 64] It is possible to manufacture the plated
steel sheet l according to the present embodiment in
a manner as described above.
f0065] Although the mechanical properties of the
plated steel sheet 1 are not limited, in the tensile
test in which the sheet width direction is set as the
tensile direction, the tensile strength (TS) is
preferably 780 MPa or more, more preferably 800 MPa
or more, and still more preferably 900 MPa or more.
If, in this tensile test, the tensile strength is
less than 780 MPa, it is sometimes difficult to
secure sufficient shock absorbency when the plated
steel sheet 1 is used as automotive parts. When
considering the application to the automotive parts
with respect to which a high degree of strength when
plastic deformation starts at a time of collision is
required, the yield strength (YS) in this tensile
test is preferably 420 MPa or more, and more
preferably 600 MPa or more. When considering the
application to the automotive parts with respect to
which the formability is required, the total
elongation is preferably 12% or more, and the hole
expansion ratio is preferably 35% or more. In
addition, regarding the bendability, it is preferable
to provide characteristics such that in the 90-degree
V-shaped bending test, no crack occurs and no
constriction of 10 ~m or more occurs.
[0066] Note that the above-described embodiments
merely illustrate concrete examples of implementing
- 36 -
the present invention, and the technical scope of the
present invention is not to be construed in a
restrictive manner by these embodiments. That is,
the present invention may be implemented in various
forms without departing from the technical spirit or
main features thereof.
EXAMPLE
[0067] Next,
be described.
examples of the present invention will
A condition of the examples is one
condition example which is adopted in order to
confirm a possibility of implementation and an effect
of the present invention, and the present invention
is not limited to this one condition example. The
present invention allows an adoption of various
conditions as long as an object of the present
invention is achieved without departing from the gist
of the present invention.
[0068] Steels having chemical compositions presented
in Table 1 were smelted in an experimental furnace to
produce slabs each having a thickness of 40 mm. The
balance of the chemical composition presented in
Table 1 is composed of Fe and impurities. An
underline in Table 1 indicates that a numeric value
to which the underline is applied is out of the range
of the present invention. Then, hot rolling, cooling
using a water spray, and first heat treatment were
performed on the slabs. In the cooling using the
water spray, an average cooling rate was about
30°C/second. A finish temperature of the hot rolling,
- 37 -
a thickness after the hot rolling (a thickness of a
hot-rolled steel sheet), and a cooling stop
temperature are presented in Table 2 and Table 3. In
the first heat treatment, the hot-rolled steel sheet
was charged into a furnace, held in the furnace at
the cooling stop temperaLure for 60 minutes, and
cooled in the furnace to 100°C or less at a cooling
rate of 20°C/hour. The cooling stop temperature is
set by assuming a coiling temperature, and the first
heat treatment simulates a thermal history during
coiling the hot-rolled steel sheet. After the first
heat treatment, a scale was removed through pickling,
and cold rolling was performed. A thickness after
the cold rolling (a thickness of a cold-rolled steel
sheet) is presented in Table 2 and Table 3.
[0069] Thereafter, test materials for heat treatment
were collected from the cold-rolled steel sheets, and
heating, annealing, first cooling, second cooling,
second heat treatment which simulates hot-dip
galvanizing, third cooling, and tempering were
performed. Some of the test malerlals were subjected
to third heat treatment which simulates alloying
between the second heat treatment and the third
cooling. An average heating rate from 10o•c to 720"C
in heating each of the test materials is presented ln
Table 2 and Table 3. In the annealing, the test
materials were held at temperatures presented in
Table 2 and Table 3 for periods of time presented in
Table 2 and Table 3. A dew point and a hydrogen
- 38 -
concentration in the atmosphere at that time are
presented in Table 2 and Table 3. An average cooling
rate from 720°C to 650°C of the first cooling and an
average cooling rate from 650°C to 500°C of the second
cooling are presented in Table 4 and Table 5.
Between the second cooling and the second heat
treatment, the test materials were held at 460°C to
500°C for periods of time presented in Table 4 and
Table 5, the test materials were held at 460°C for 3
seconds in the second heat treatment, and the test
materials were held at 510°C for 3 seconds in the
third heat treatment. A cooling stop temperature of
the third cooling, an average cooling rate from the
temperature of the third heat treatment to the
cooling stop temperature regarding the test material
which was subjected to the third heat treatment, and
an average cooling rate from the temperature of the
second heat treatment to the cooling stop temperature
regarding the test material which was not subjected
to the third heat treatment are presented in Table 4
and Table 5. A maximum attained temperature of the
tempering and a period of time of holding at the
temperature are presented in Table 4 and Table 5. A
rate of heating to the maximum attained temperature
was 20°C/second. An underline in Table 2 to Table 5
indicates that a numeric value to which the underline
is applied is out of the desirable range.
[0070] [Table 1]
- 39 -
TABLE 1
---~---.. --·--·
STEEL CHEMICAL COMPOSITION (MASS%)
SYMBOL c Si Mn p s soi.AI N OTHERS --
A 0.235 1.46 2.12 0.005 0.0008 0.048 0.0022
B 0.211 0.21 2.26 0.006 0.0011 0.045 0.0024
c 0.188 1.82 2.53 0.005 0.0012 0.046 0.0034
D O.i75 1.24 0.82 0.005 0.0012 0.047 0.0036 Mo: 0.5
E 0.191 1.61 2.88 0.005 0.0011 0.045 0.0033 Ti:0.012
f 0.183 1.37 2.85 0.006 0.0009 0.048 0.002) Nbo 0.018
--j
G 0.202 1.50 -2.-54- 0.005 -0.0008 0.046 0.0035 Ti: 0.025, 8: 0.0019
H 0.227 1.32 2.06 0.004 0.0008 0.045 0.00.2 6 Cu: 0.28, Ni: 0.16
[ 0.177 1.63 2.51 0.006 0.0008 O.D47 0.0038 Mo: 0.17, B: 0.0015
J 0.182 1.65 2.70 0J}Q5 0.0012 0.048 0.0031 Cr: 0.32, Mo: O.OH
K 0.183 1.52 2.54 0.005 0.0011 0.047 0.0026 Ca: 0.0008, Mg: 0.0007
L 0.188 1.00 2.97 0.000 0.0012 0.046 0.0029 Bi: 0.0030, REM: 0.0005
-~""
M 0.220 1.47 2.03 0.004 0.0011 0.045 0.0032 Ti: 0.047 -
N 0.299 1.64 3.07 0.004 0.0009 0.049 0.0025 Cr: 0.55
··-
0 0.297 1.67 2.55 0.004 0.0008 0.048 0.0023
p 0.365 1.83 2.76 0.004 00008 0.047 0.0023
Q 0.024 1.65 4.33 O.OQ5 0.0008 0.043 0.0029
R 0.180 1.31 2.23 0.0[3 0.0006 0.021 0.0046
s omo 1.01 2.04 0.004 0.0009 0.023 0.0036
~- 0.062 0.65 1.57 0.005 0.0011 0.034 0~0036
~- 0.140 1.88 1.60 0.012 0.0007 0.036 0.0041
v 0.081 1.16 2.83 0.011 0.0044 0.020 0.0019
w 0.255 !]!l_ 2.01 0.008 0.0014 0.053 0.0052 Nbo 0.015
i( ot_JI 1.09 U7 0.014 0.0059 0.069 0.0033 Ni: 1.13
y 0.130 1.30 2.50 0.006 0.0057 0.051 0.0027 W: 0.2500
z 0.195 0.27 2.72 0.011 0.0037 0.041 0.0027 Ti: 0.081
- 40 -
"f-'"
0
0
.__J
"'
>-3
J)J
C5
f-'
(])
w
TABLE L
I
I l HOI ROLUNG COLD ROLUNG HEA11NO ANNEAUNG I
SAMPLE I STEEL I THJCKNCSS OF FINISH COJUNG THJCKNESS OF AVEAAO DEW HYDROGEN
No. [l'sYMBOL1 HOT-,ROLLED TEMPERATURE TEMPERATURE\ O~LD-ROLLED E. TEMPE:~~TURE r;ME POINT OONCENTRATlDNI REMARKS
STEE .... SHEET roc' (oC) S 1 EEL SHEET HEA TlNO ( CJ ,s) (~C) 'VOLUMI=Ol(mm) ' ' (mm) PAll ' · ---
1 I A 2.5 ' 960 ()50 1.2 ll 820 30 10 4
2 I A 2.5 .950 550 1_2 8 820 30 -10 4
3 1 A 2.[5 980 550 1.2 8 820 30 10 4
4 I A j 2.5 960 ~0 1.2 8 820 30 • -10 4
5 I 0 I 2.5 ' 9-40 BOO 1.2 8 840 30 10 4
6 .l Q J 2.5 ) 940 i 6QQ 1_2 I 8 840 3Q -10 4
c i 2.5 i 940 600 1.2 $ 840 30 -10 4
8 0 __ t 2.5 940 000 1.2 ' 8 840 30 10 4
9 I E ! 2.5 940 600 1.2 8 840 30 -10 4
10 I E 25 940 600 1.2 8 840 30 10 4
F 2.5 B40 600 1.2 B 840 30 -10 4
1z I G z.s sso 1 eoo 1.2 a 840 so -10 4
13 i H l 3.0 I 950 I 550 1.6 I 8 -·-··- 820 30 -10 4
14 ! ! 2.5- ---------gso- 550 1.2 s -840-"I aa·-- -10 4
15 i J 2.5 940 BOO 1.2 S SSO 1 30 -10 4
16 J 2..5 940 SOD 1.2 13 850 30 -10 ""
17 K I 3.0 950 600 I 1.6 S 840 30 -10 4
18 L 3.0 I 940 I 600 1,6 8 840 30 10 4
lS M 3.0 950 550 i.e 8 820 30 10 4
zo N 2.s ' eso MD 1.2 a ?eo 3D ;o 4
N 2.5 950 640 1.2 S 790 30 -10 4
2:2 I 0 2.5 94Q j 640 1.2 8 780 30 -jQ 4
23 p 2.5 840 640 1.2 8 780 30 -10 4
24 p 2.5 940 640 1.2 8 820 30 10 4
25 R 2.5 i 880 530 1.2 5 860 50_1_ 10 4
~ _ ?_ __ z.s , s1o s2o 1.2 i e sao so 1 -1o 4
N EXAMPLE
N EXAMPLE
!INVENTION EXAMPU
j INVENT!O!'J EXAMPLE
N EXAMPLE
N EXAMPLE
~EXAMPLE
0
0
--J
f-"
>-3
J)J
C5
I-'
(])
N
"N '
TABLE 3
I I HOi-ROLLING GOLD-ROLLING HEATING ArJNEAL!NG ---i I
!sAMPLE I STEE' iH!CKNESS OF FINr~·H I '"'OILING THICKNESS OF AVERAO I DEW I HYDROGEN I
i No. SYMBOL ~~T~ROLLE~ TEMPEAAruREITEM'-'PERATURE CCLD-RO'::-ED t: E iEMPE~R~TURE T!ME POiNT CONCENTRATION I REMARKS I
1 : ,__ l EE1- SHEE 1 (oC) I COc) STEEL SHt:ET H...,A11NG ( C; (s) (C) (VOLUME%) I 1
I 1 (mm) I I (mm) KA.ts
1 21 I A I~ 2.5 I "" i 550 1.2 i e I e2o 1 3o 1 -10 I 4 I coMPARATIVE Exi\i4eLEI
! za I A 1 2.s -- ---r soo sso I 1.2 I s soo 1 20 I -1o I 4 I coMPAR.A.nvE EXAMPLE]
2e A 2.s I s6o sso I u: I e I 1QQ I 3o I -10 I 4 I coMPARATIVE EXAMPLE
3o A 2.s sao ! sso 1 1.2 1 e a2o 1 so i -1o I 4 I coMPARATIVE EXAMPLE
31 A 2.s 960 soo i 1.2 1 e s2o I 20 I -10 I 4 ! coMPARATrvE EXAMPLE
32 A 2.5 96(1 550 i 12 I s I 820 I 30 I -10 I 4 I COMPAAATIVE EXAMPLE
33 .A 2.5 960 55.0 I 1.2 I" s "] 820 I 30 I -10 I 4 !COMPARATIVE EXAMPLE
34 -~ 3.0 I 90(1 I 500 T- 1.6 i 880 I 30 I -10 I 4 I OOMPARATNE EXAMPLE
35 I c 2.o ·: e4o 1.2 1 e s4o 1 3o 1 -10 I 4 !coMPARATIVE EXAMPLE
36 _Q 3.0 96(1 I 880 -- T---·1:6--1 8 84D i 30 I -10 I 4 I COMPARATIVE EXAMPLE
37 1 e 1 ----Ts-- ---T----~11 eoo 8 1 lU.Q 1 30 1 -10 1 4 1 coMPARATIVE EXAMPLE
l 38 I f I '~' ~~ --,,,, ~ I 600 1.2 I 8 I 840 I 30 I -10 I 4 I COMPARATIVE EXAMPLE
l 39 1 Q 1 z.s 940 1 e4o - -1--- -u·- 1 s azo 1 ao r-::.-1-o 1 4 1 coMPARATIVE EXAMPLE
i 40 l R I 2.5 1 87o i 580 I 1.2 I a I 870 I 10 I -s I 3 I coMPARATrvE EXAMPLE
I 41 I R I 2.5 I 890 ! 070--l- 1-.,--r -~.ill I 849 I 80 I 0 i 2 I COMPARATIVE EXAMPLE
I 42 1- R I 2.5 I '"' I 520 I 1.2 I 8 860 70 ::ill 4 GOMPAR' TIVE EXAMPLE
1 43 , R , z.s 1 89CI 52o I 1.2 I e 8so 2 20 4 coMPARATIVE EXAMPLE
44 R [ 2.5 I 880 530 1.2 s 860 I 50 -10 I 4 I COMPARATIVE EXAMPLE " I s '~' I 980 I 700 I 1.2 I 8 I 800 I= I -10 i 4 COMPARATIVE EXAMPLE
46 s 2.5 : 89G 520 1,2 I M, 7SO 50 -10 4 COMPARATIVE EXAMPLE
I~ N I 2.5 I 920, I 570 1.2 i 8 I 790 I 30 ! -10 I 4 I COMPARATIVE EXAMPLE
i 48 I T I 2~5 I 94C i 580 I 1.2 I 8 I 880 I 80 I -10 I 4 I COMPARATIVE EXAMPLE
! " 1 --u--r 2.s 1 9o 1 soo 1 1~' 1 s---~---~iso- 1 so r -1o 1 ' I coMPARATIVE EXAMPLE
i 56--T-----:,,r-r- 2.s 903 e1o 1 1.2 1 s.a J s24 1 .so 1 -10 I 4 I coMPARATIVE EXAMPLE
L__51 ~-,Yl '~' 94·7 I 660 I 12 I 5.3 I 877 I 30 I -10 I 4 I COMPARATIVE EXAMPLE
I 52 : X ! 2.5 960 I 640 j L2 5.4 I 857 ! 30 l -10 I 4 i COMPARATIVE EXAMPLE
I 53 .---y I 2.5 1 932 aao 1.2 I 11A I 763 I 20 ) -10 I 4 !coMPARATIVE EXAMPLE
i 54 : z ! 2.5 I 950 I 690 I 1.2 I 3.9 I 883 I 30 I -10 I 4 I COMPARATIVE EXAMPLE
w"'
TABLE 4
SECOND
RRST I SECOND ! THERMAL
I
' ICOOUNG ODOUNGI T.• REATMENT
,SAMPLE! STEEL 1 (HOT-DIP
Nc. ISYM80Lt--- GAl Vl.~lf7!hlr>'
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
11
2C
21
22
23
24
25
26
: IAVEAAGE I COOUNG I COOUNG
A
A
A
A
c
G
0
c
E
E
F
G
H
i
J
J
K
L
N
RATE RA.TE
(oC/s) (C/s)
5
5
5
5
2
2
2
2
5
5
2
2
5
5
2
10
2
2
5
2
2
2
2
2
5
5
30
30
30
30
4
4
4
4
30
30
4
4
30
30
4
10
4
4
30
4
4
4
4
4
30
30
TIME
(,)
24
24
24
78
19
19
19
19
19
--- 14
19
24
19
19
19
19
19
20
20
TEMPERING
THIRD THERMAL .~~ -------r------:::·-----t~~~~::~::l
!TREATMENT AVERAGE STOP
(ALLOYING) OOOUNG TEMPERA TUR!.t
THIRD
OOOUNG
TUREITJME
REMARKS
WITH
WITH
VVITHOUT
VVITH
WITH
WITHOUT
WITH
WITH
WITH
WITH
WITH
WITH
WITH
WiTH
VviTH
WITH
WITH
WITH
NITHOUT
WITH
WITH
WITH
WITH
WITH
RATE (s)
('C/s)
14
14
14
14
12
12
14
14
14
12
14
14
14
14
12
14
14
14
14
14
12
12
I ROO
100
ROOM
I ROOM
iROOM
ruRE
~
"URE
cURE
TURE
tURE
~UR.I:
"URE
~R~I
{eC) (s)
190 15
140 1000 1NVENTI'2.!'-
190 3 !NVENTlO
180 60 INVENTlm
190 80
190 360 CAMF
1 so 2$0
190 200 ~
190 200 INVENT10 :.XAMP
150 100 INVENTIO
190 1.5 lNVENTIO.
190 24 lNVENilO~
170 100 INVENTION
160 200
150 200._t.JNVENT10~
160 400
190 200 ! JNVENTI_9N E
90 20
IH
30C
180 20
190 40
130 800 i
120 900
0
0
--.]
w
>-3
OJ
t1
~
(1)
"'
""""''
u
(i)
::J
r:L
f-'·
::J
cQ
ct
(i)
GO
ct
:s
(i)
Cj
(i)
'D
(i)
Cj
en
0
Cj
8
m
r:L
0
::J
m
("l' ::r
0
H1
ct
::r
m
ct
(i)
Vl
ct
8
"c't
(i)
Cj
f-'·
"' 1-"
(/)
:s
"(/')
0
u
GO
m
Cj
~
m
r:L
I1J
::J
r:L
"'
ct
m
::J
GO
f-'·
1-"
(i)
ct
(i)
GO
ct
"::J'
r:L
"'
0
0
._J
();
>-j
::r
m
::J
\lJ
GO
ct
Cj
c
(l
ct
c
Cj
(i)
0
H1
(i)
("l'
::r
0
H1
ct
::r
(i)
ct
(])
GO
ct
TABLES
I I I SEOOND
IC OOUNG COOLING i I REA .!_MEN' i COOUNO i tEMPERING FiRST II S<::COND I - Ti!E~lJAL- ! L THIRD :
(HOi -Dr~G) I THIRD i
SAMPLE STEE:... " GAL'v'ANIZI - THERMAL ,
No. SYMtlOLIAVEPJIGEIAV-""RAG" iTREA7MENTIAVERAG~ I I I
cooUNG! c60UNG T!ME CALLOYJNG)] coouNG _ s~TO~ lrt:MPEMTURI£1. TIME
I PATE I RATE r~l I RATE •EMP-R;-\,URE I ("Cl (sl
("C/s) 1' ('Cis) ( i ~'Ch/ (s, I
___!::. 5 .' 30
A
29 A :;~· 30
"' ill ~"
32. A 30
2 I
~ I , , I
E i s 30 I
!~oiR ~~3~~
~--41 R ~13ol
42 R 5'30 1
43 R_ Sj3DI
!4.i R ;E8FPRMED
li TEMPERATURE 1BO
'II TEMPERATUI'\E 1$0
.\ TEMPERATl.IF!e 21Q
14. ROOM TEMPERATU"E 18\1
RATtJf\E N¢1 PERFORMED
'"
-~"-
12 ROOM TEMPECATHI:IC::
~PE :;p.;
i. 202_
53.9 ROOM
"'1 P.EFERE,'IClAL VAWE {:>DOLING STA'1.7 "'O!NT WAS 700'"C)
RSJARKS
~
~
OMPARP
~
~
>L£
g_
'E EXAMPLE
""-
IE EXAMPLE
/E EXAMPLE
- ~u"'"'LE
0
0
._J
"'
>-j
I1J
tY
I-'
(i)
();
materials.
[ 007 6] It is important whether or not the martensite
is tempered, and in this determination, a cross
section of each of the test materials was subjected
to nital corrosion, and observed with a scanning
electron microscope (SEM). Further, it was
determined that the martensite was tempered in the
test material having a carbide, and the martensite
was not tempered in the test material having no
carbide.
[0077] In the observation of the structure of the
base material, image analysis of electron microscope
observation images of a cross section perpendicular
to a rolling direction and a cross section
perpendicular to a sheet width direction (a direction
perpendicular to the rolling direction) was
performed, and a volume fraction of M-A at a 1/4
sheet thickness position in each of the cross
sections was measured. Further, an average value of
the volume fractions was defined as a volume fraction
of the M-A of the base material in the test material.
Further, volume fractions of retained austenite in
the above-described two cross sections were measured
through X-ray diffraction, and an average value of
the volume fractions was defined as a volume fraction
of the M-A of the base material. Furthermore, a
value obtained by subtracting the volume fraction of
the retained austenite from the volume fraction of
the M-A was defined as a volume fraction of the
- 45 -
tempered martensite. In addition, an average
hardness of the tempered martensite was measured by
the nano-indentation method. In this measurement, an
indenter having a shape of cube corner was used, and
an indentation load was 500 pN. Results therP-of are
presented in Table 6 and Table 7. Note that the
volume fraction of ferrite of the base material in
each of th~ samples was 4.0% or more.
[0078] In the observation of the decarburized
ferrite layer, an area ratio of ferrite was measured
at intervals of 1 pm from the surface of each of the
test materials, and a position at which the
measurement value indicated 120% of the volume
fraction of ferrite of the base material at the l/4
sh~~t thickness position was defined as an interface
between the decarburized ferrite layer and the base
material. Further, a distance from the surface of
the test material to the interface was defined as a
thickness of the decarburized ferrite layer at the
cross section. The observation as described above
was performed on the above-described two cross
sections, and an average value in the observation was
defined as a thickness of the decarburized ferrite
layer in the test material. Further, by the
aforementioned image analysis, a grain diameter of
ferrite, a volume fraction of the tempered
martensite, and a number density of the tempered
martensite were calculated. Also in this
calculation, an average value of the above-described
- 46 -
two cross sections was determined. In addition, an
average hardness of the tempered martensite was
measured by the nano-indentation method. In this
measurement, an indenter having a shape of cube
corner was used, and an indentation load was 500 pN.
Results thereof are presented in Table 6 and Table 7.
An underline in Table 6 and Table 7 indicates that a
numeric value to which the underline is applied is
out of the range of the present invention.
[0079] In the tensile test, a JIS No. 5 tensile test
piece was collected from each of the test materials
so that the sheet width direction (the direction
perpendicular to the rolling direction) corresponded
to the tensile direction, and the yield strength
(YS), the tensile strength (TS), and the total
elongation (T. El) were measured. In the bending
test, the 90-degree V-shaped bending test with a bend
radius corresponding to twice the sheet thickness was
conducted, in which the test piece with no crack and
no constriction of 10 pm or more was determined as
"goodu, and the test piece other than the above was
determined as "poor.u Results thereof are presented
in Table 6 and Table 7. An underline in Table 6 and
Table 7 indicates that an item to which the underline
is applied is out of the desirable range.
[0080] [Table 6]
- 47 -
"'" Q)
0
0
Q)
~
>-:3
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TABLE 6
DEOAR...,...,,."4.'-Vr<;;C\Nik'-"''"'" lii!""IQh;\YlMio;,...,.,.,_._ • L () ~,..,.,..,_,
I 1 t-ERRITE TEMPERED. MARTENStTE VOLUMr! TEMPE~ED MECHAN!CA PR P:::r~.t'
SA~M,0P LE I,S SYTMEBEOLL .,,.H !CKN,ESS I VOLUME II AVGER RAAiNG : VOLUME NUMBER AVERAGE o=-!o(EiA!NED VD1-UME AVERAGE~! vg TS TEl REMARKS I' F"RAOT10N I ' I I i
(j.lm; I FRA?T!ON DIAMETER! FRA?T!ON D~NS~ HARDNESS AUSTENITE FRACTION I HA~DNES$1 (MPa) I (MPa) ri~) SENDABL..Jif
1 ,%) 1 IIJ;) · ' Jh) ll!-lm) (GPa) (%) (%) , ,GPa) I ;; i' , )
I A S ! 5Sd I 4 , 11 Z 0081 6$ 1~1 138 , 83 1 S40 11055 21.2 GOOD /INVENTION EXAMPLE
! 2 A 8 I 74.2 ! 5 i 9.3 0.054 5.4 10.2 13.5 I 7.9 i 626 11024 23.5 GOOD !NVEI\'TION EXAMPLE
I 3 )\ s I 7/.4 6 10.4 0.036 6.5 13.2 13.2 i 8.4 646 ' I 057 21.7 GOOD I INVENTION EXAMPLE
i 4 A 13 6£-5 5 I 10.3 0.052 5.7 12.2 12.6 I 8.2: 517 102B 22.5 GOOD I !NVEI\:TlON- EXAMPLE I 5 c 13 I 5!,8 4 23,0 0,081 ,,, "' 49A I 8,8 I 913 ! 1275 14,8 GOOD INVENTION EXAMPLE
' 6 C 9 70.3 6 21.3 0.036 5,4 S.O 49.5 8.5 933 ' 1246 114,3 GOOD jiNVENTION EXAMPLE
I 7 C 11 £9.5 6 16.4 O.o36 5.6 e,5 49.0 i 8.4 1 92:1 i2S1 14.9 GOOD I INVENTION EXAMPL
e c 9 I ns s 22.1 o.osz M 12.4 43.5 1 s.2 I ars nos 16.7 aooo lrNVENTION EXAMPLE
9 E 11 I 63.8 5 24.3 0.052 5.3 9.2 54.8 8.5 i 948 , 1262 15.2 GOOD !INVENTION EXAMPLE
10 E s I 72_1 s s.a oms 6.4 ' 1o.a 45.e 1 9.2 774 1412 14-.5 1 nooo 1 iNVENTION EXAMPLE
11 F 10 65.6 7 ! 25.1 0.027 6.2 10,3 52.7 I 8.5 i 890 1256 15.8 GOOD I INVENTION EXAMPLE
12: G -ro I so.7 1 s J 23.1 o.052 6.2 9.7 ss.o 1 s:r 913 1122s 16.4 GOOD IINVENllON EXAMPLE
13 H 11 74.1 6 I 10.3 i D.o36 6.1 i 13.3 14.9 I HI I 645 ( 1046 2:4.2 GOOD !NVENI!ON EXAMPLE
14 ! 1 9 64.7 __ i ____ 3 22.8 : 0.144 5.9 i 8.7 49.0 I 8.5 891 I 1238 15.8 GOOD lNVENIJON EXAMPLE
15 J 11 60.3 8 23.5 0.020 6.3 1 11.1 60.2 8.6 1012 i 1336 10.2 GOOD lNVENTION EXAMPLE
16. J 12 54.2 ; 5 30.9 0.052 6.1 itS 85.4 8.7 1033 i 1343 14.5 GOOD iNVENTION EXAMPLE
17 ! K I 8 60.1 8 I 22.4 0.020 5.9 8.1 66.1 7.9 875 i 1219 1&.1 GOOD !NVENitON EXAMPLE'
iB .. ! l I 11 61.3 : 4 22.7 0.081 6.4 8.8 54.3 3.6 884 1273 15.4 GOOD INVENTION EXAMPLS)
"19 i M 14 70.5 7 10.7 0.02.7 5.8 1.2..9 13.8 I ?.8 629 i 1043 23.2 GOOD iNVENTION EXAMPL
20 I N I g 57.5 8 24.5 0.02.0 5.6 14.1 57.2 i S.3 1143 1548 14.8 GOOD INVENTION EXAMPLE:
21 j N I 12 I 52.4 3 28.3 0.144 6.1 16.7 57.4 I 8.3 I 1120 1486. 14.4 GOOD
22 I 0 I 10 I 68.1 5 13.7 0.052 5.8 15.5 55.8 I 8.7 11150 1.481 15.1 GOOD
23 ! p I 8 I 55.7 7 22.4 0.027 6.4 16.8 57.0 8.4 i 1146 1536 115.7 GOOD
24 I p 10 ' 54.6 8 28.4 0.020 6.2 .22.5 55.9 I 8.5 I 1072 Hl32 15.4 GOOD
25 I R i 12 84.3 8 4.2 0.020 5.8 1U: I 5.3 j 7.0 . I 650 1091 < 20.7 QOOD
2€ s I 13 I 77.9 12. 12:.0 0,016 6.3 $.4 I S.3 i 8.2 ! 531 i 846_f34.1 GOOD
N EXAMPLE
N EXAMPLE
"' '.0
TABLE 7
I I i DECARBURIZED FERRITE LAYER BASE MATERIAL i FERRITE TEMPE.REO MARTENSITE MECHANJOAL PROPERTY VOLUME TEMPERED
SAMPLE STEEL r • FRACTION
No. 1sYMBOL TH!?KN,ESSI VD!..~ME !A6~~E VOLUME NUMBER AVERAGE or: RETAINED VOLUME AVERAGE
i 1 ,~m, FRAC"fiON DIAMETER FP.AC;!ON DENS~ HA~DNESS AUSTENITE FRACTION HARDNESS YS rs T.EI Is e:NDAB!UTY ' I , (MP.a) (MPa) (%) I ... (%, ( m: (%, (/!Jm ; 1.GPa) (%) (%) (GPa;
27 A 9 84.5 6 = 9.5 ~l 15.0 I = 1~2~2 514 1103 14.8 EQQ!l
28 A 10 SS.tl 6 7.2 .L o.oas 3.6 4.3 I 13.1 ~ 672 176 .1.J.g GOOD
29 A 9 94.,. 7 NONE NONE i = NONE 472 791 11.1 POOR
30 A ' 11 84A 4 0.8 0.023 ' 5.3 I 12.5 i 15.2 8.2 631 I 1142 14.6 EQQ!l
31 A 1 12 80,~ 6 3.2 0.036 I 6.1 4.3 I 3.9 6.4 450 971 11.0 GOOD
32 A 10 7s.t ' 6.1 0.019 5.4 4.6 10.5 s.o 506 1006 116 GOOD
33 A 11 69.tl 6 NONE I 9.8 .. 1 15.3 = 10.3 *~ 503 1125 14.2 POOR
34 I " " 82.'4 8 11.1 1 o.o2.3 5.4 1.0 B2.1 7.5 753 1035 10.9 GOOD
35 c 10 71.5 5 NONE 8.9 ~l 9.4 llilli!l 10.2.-1 895 1391 11.8 EQQ!l
" Q 19 so.s 9 0.5 0.015 7.4 5.5 15.1 8.2 842 1025 14.7 = 37 E I 17 66.3 8 1.5 MQ4 6.4 10.6 43.9 9.7 758 1402 13.0 EQQ!l
3B , I 9 60.6 ' 22.4 i 0.036 4.3 4.8 52.9 ,.,. 651 1175 . .1.1..2. GOOD
" g L 9 62.8 4 24.5 1 o.os1 5.5 9.4 44.3 6.5 509 ru 2.2.9 GOOD
40 R ' 64.1 10 NONE 10.3 •l 10.3 = 1.Q.4 ·~ 515 1139 18.2. EQQ!l
41 I R 45 I 80.5 24 1.5 1Mll2 6.2 9.7 4.6 8.1 784 1145 16.5 =
" R 0 NONE 11.4 4.8 8.4 725 1132 17.8 = 43 R Q llillif 1o.3 5.2 8.2 '" 1073 16.4 PQQR
I 44 R • 08.2 ,.
" 3.8 0.015 6.5 u 0.4 7.9 883 1082 ill GOOD
45 s 220 73.1 I 1! 25.6 0.021 8.2 8.1 32.4 6.6 575 772 23.2. GOOD
45 s 1 13 80.7 I 12 3.4 MQ4 6.1 5.3 8.3 7.1 525 ill 33.2 . i?9Qil
47 N 13 63.2. 3 22.3 0.02.0 M 14.7 59.4 9.7 895 1572 13.5 E'QQB
46 T 15 48.8 6 43.2 0.02.5 3.3 5.2 64.3 y 702 5 13,6: GOOD
49 u 14 87.4 8 13.5 0.021 8.7 6.3 14.2. 1M '" 825 25.2. P.QQJJ
50 1 v 9 80.2. 5 3.8 0.032 4.2 Q 5.2 ll 712 798 11.3 OOOD
51 w 10 63.4 5 24.3 0.042 6.3 1 37.6 6.5 1053 116"2. 1M GOOD
52 X 9 68.5 7 11.6 0.022 4.5 g 10.4 ti 723 $53 .1M GOOD
53 y a 67.1 5 14.7 O.D27 4.8 1 22.8 5.5 102.7 1123 M GOOD
54 z a 73.1 ! 5 8.5 0.Q39 5.3 i Q 7.6 5.7 905 $52 1QJ. GOOD
*1 REFERENOlAL VALUE (HARDNESS OF FRESH MARTENSITE)
*2 REFERENO!AL VALUE (HARDNESS OF FRESH MARTENSITE)
REMARKS
'COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
OOMPARATtVE EXAMPLE
COMPARATtvE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATtVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARA T!VE EXAMPLE
COMPARATJVt:. EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPAAATNE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATfVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
[0082] As presented in Table 6 and Table 7, in the
samples No. 1 to No. 26 within the range of the
present invention, it was possible to obtain high
tensile strength of 780 MPa or more, good elongation
of 12% or more, and good bendability.
[0083] In the sample No. 27, the temperature of the
tempering was excessively low, so that the martensite
in the rlecarburized ferrite layer was not tempered.
For this reason, the volume fraction and the number
density of the tempered martensite in the
decarburized ferrite layer were insufficient, and the
bendability was not good.
In the sample No. 28, the temperature of the
tempering was excessively high, so that the austenite
was decomposed. For this reason, the volume fraction
of the retained austenite in the base material was
insufficient, and the elongation and the tensile
strength were low.
In the sample No. 29, the annealing temperature
was excessively low, so that it was not possible to
obtain the retained ausLer1ite. For this reason, the
volume fraction of the retained austenite in the base
material was insufficient, and the elongation was
low.
In the sample No. 30, the average cooling rate of
the first cooling was excessively low, so that the
martensite was not sufficiently generated. For this
reason, the volume fraction of the tempered
martensite in the decarburized ferrite layer was
- 50 -
insufficient, and the bendability was not good.
In the sample No. 31, the average cooling rate of
the second cooling was excessively low, so that the
pearlite was generated, and the generation of
austenite was suppressed. For this reason, the
volume fraction of the retained austenite in the base
material was insufficient, and the elongation was
low.
In the sample No.
the third cooling was
32, the average cooling rate of
excessively low, so that the
austenite was decomposed. For this reason, the
volume fraction of the retained austenite in the base
material was insufficient, and the elongation was
In the samples No. 33, No. 35, and No. 40, the
tempering was omitted, so that the martensite in the
decarburized ferrite layer was not tempered. For
this reason, the volume fraction of the tempered
martensite in the decarburized ferrite layer was
insufficient, and the bendability was not good.
In the sample No. 34, the Si content was
excessively low, so that the volume fraction of the
retained austenite in the base material was
insufficient, and the elongation was low.
In the sample No. 36, the Mn content was
excessively low, so that the volume fraction of the
tempered martensite in the decarburized ferrite layer
was insufficient, and the bendability was not good.
In the sample No. 37, the annealing temperature
- 51 -
was excessively high, so that the tempered martensite
in the decarburized ferrite layer was not
sufficiently refined. For this reason, the number
density of the tempered martensite in the
decarburized ferrite layer was insufficient, and the
bendability was not good.
In the sample No. 38, the temperature of the
tempering was excessively high, so that the austenite
was decomposed. For this reason, the volume fraction
of the retained austenite in the base material was
insufficient, and the elongation was low.
In the sample No. 39, the C content was
excessively low, so that the tensile strength was
low.
In the sample No. 41, the average heating rate of
the heating was excessively high, so that the ferrite
in the decarburized ferrite layer became coarse, and
the tempered martensite was not sufficiently
dispersed. For this reason, the average grain
diameter of ferrite in the decarburized ferrite layer
becaJue excessively large, and the number density of
the tempered martensite was insufficient, resulting
in that the bendability was not good.
In the sample No. 42, the dew point in the
annealing atmosphere was excessively low, so that the
decarburized ferrite layer was not generated. For
this reason, the thickness of the decarburized
ferrite layer was insufficient, and the bendability
was not good.
- 52 -
In the sample No. 43, the annealing time was
excessively short, so that the decarburized ferrite
layer was not generated. For this reason, the
thickness of the decarburized ferrite layer was
insufficient, and the bendability was not good.
In the sample No. 44, the average cooling rate of
the first cooling was excessively high, so that the
retained austenite was not sufficiently generated.
For this reason, the volume fraction of the retained
austenite in the base material was insufficient, and
the elongation was low.
In the sample No. 45, the annealing time was
excessively long, so that the decarburized ferrite
layer was excessively grown. For this reasonf the
thickness of the decarburized ferrite layer became
excessively large, and the tensile strength was low.
In the sample No. 46, the average heating rate of
the heating was excessively low, so that the tempered
martensite was not dispersed in the decarburized
ferrite layer. For this reason, the volume fraction
and the number density of the tempered martensite in
the decarburized ferrite layer were insufficient, the
tensile strength was low, and the bendability was not
good.
In the sample No. 47, the temperature of the
tempering was excessively low, so that the martensite
in the decarburized ferrite layer was not
sufficiently tempered. For this reason, the hardness
of the tempered martensite in the decarburized
- 53 -
ferrite layer became excessively large, and the
bendability was not good.
In the sample No. 48, the temperature of the
tempering was excessively high, so that the
martensite in the base material was excessively
tempered. For this reason, although the bendability
was good, the average hardness of the tempered
martensite in the base material was insufficient, and
the tensile strength was low.
In the sample No. 49, the period of time of the
tempering was excessively short, so that the
martensite in the base material was not sufficiently
tempered. For this reason, the average hardness of
the tempered martensite in the base material became
excessively large, and the bendability was not good.
In each of the samples No. 50 to No. 54, the
temperature of the tempering was excessively high, so
that the austenite was decomposed. For this reason,
the volume fraction of the retained austenite in the
base material was insufficient, and the elongation
was low.
INDUSTRIAL APPLICABILITY
[0084] The present invention can be utilized for
industry associated with a plated steel sheet
suitable for automotive parts, for example.

CLAIMS
[Claim 1] A plated steel sheet, comprising:
a steel sheet; and
a plating layer on the steel sheet, wherein:
the plating layer is a hot-dip galvanizing layer
or an alloyed hot-dip galvanizing layer;
the steel sheet comprises:
a base material; and
a decarburized ferrite layer on the base
material;
the base material includes a chemical composition
represented by, in mass%:
C: 0.03% to 0.70%;
Si: 0.25% to 3.00%;
Mn: 1.0% to 5.0%;
P: 0.10% or less;
S: 0.0100% or less;
sol. Al: 0.001% to 1.500%;
N: 0.02% or less;
Ti: 0 9 0% to 0.300%;
Nb: 0.0% to 0.300%;
V: 0.0% to 0.300%;
Cr: 0% to 2.000%;
Mo: 0% to 2.000%;
Cu: 0% to 2.000%;
Ni: 0% to 2.000%;
B: 0% to 0.0200%;
Ca: 0.00% to 0.0100%;
REM: 0.0% to 0.1000%;
-· 55 -
Bi: 0.00% to 0.0500%; and
the balance: Fe and impurities;
the base material includes a structure, at a
position at which a depth from a surface of the steel
sheet corresponds to l/4 of a thickness of the steel
sheet, represented by, in volume fraction:
tempered martensite: 3.0% or more;
ferrite: 4.0% or more; and
retained austenite: 5.0% or more;
an average hardness of the tempered martensite in
the base material is 5 GPa to 10 GPa;
a part or all of the tempered martensite and the
retained austenite in the base material form an M-A;
a volume fraction of ferrite in the decarburized
ferrite layer is 120% or more of the volume fraction
of the ferrite in the base material at the position
at which the depth from the surface of the steel
sheet corresponds to 1/4 of the thickness of the
steel sheet;
an average grain diameter of the ferrite in the
decarburized ferrlle layer ls 20 ~m or less;
a thickness of the decarburized ferrite layer is
5 ~m to 200 ~m;
a volume fraction of tempered martensite in the
decarburized ferrite layer is 1.0 volume% or more;
a number density of the tempered martensite in
the decarburized ferrite layer is O.Ol/~m 2 or more;
and
an average hardness of the tempered martensite in
- 56 -
the decarburized ferrite layer is 8 GPa or less.
[Claim 2] The plated steel sheet according to claim
1, wherein, in the chemical composition,
Ti: 0.001% to 0.300%,
Nb: 0 . 0 0 1% to 0. 3 0 0%, or
V: 0.001% to 0.300%,
or any combination thereof is satisfied.
[Claim 3] The plated steel sheet according to claim
1 or 2, wherein, in the chemical composition,
Cr: 0. 001% to 2. 000%, or
Mo: 0.001% to 2.000%,
or both of them is satisfied.
[Claim 4] The plated steel sheet according to any
one of claims 1 to 3, wherein, in the chemical
composition,
Cu: 0.001% to 2.000%, or
Ni: 0.001% to 2.000%,
or both of them is satisfied.
[Claim 5] The plated steel sheet according to any
one of claims 1 to 4, wherein, in the chemical
composition, B: 0.0001% to 0.0200% is satisfied.
[Claim 6] The plated steel sheet according to any
one of claims 1 ·to 5, wherein, in the chemical
composition,
Ca: 0.0001% to 0.0100%, or
REM: 0.0001% to 0.1000%,
or both of them is satisfied.
[Claim 7] The plated steel sheet
one of claims 1 to 6, wherein, in
- 57 ..
according to any
the chemical
composition, Bi: 0.0001% to 0.0500% is satisfied.