DESCRIPTION
GALVANNEALED LAYER AND STEEL SHEET COMPRISING THE SAME,
AND METHOD FOR PRODUCING THE SAME
Technical Field
[OOOl] The present invention relates to a galvannealed
layer and a plated steel sheet, the galvannealed layer
being formed on a surface of a high-strength steel sheet
as a base material. In particular, the present invention
relates to a galvannealed layer and a plated steel sheet,
improved in the adhesiveness of a galvanized layer with a
base steel sheet, and to a method for producing the
galvannealed layer.
Background Art
[0002] In recent years, the higher strength of steel
sheets used in various components and structures, such as
automotive outside sheets (body sheets), construction
20 machines, and, in addition, building and civil
engineering structures, has been increasingly demanded,
and a high-strength steel sheet with a maximum tensile
stress of 900 MPa or more has also been used. Further,
the steel sheets having such uses usually need excellent
corrosion resistance because of often being used
outdoors.
Conventionally, as steel sheets having such uses,
hot dip galvanized steel sheets subjected to hot dip
galvanizing have been widely used. Recently, there a
galvannealed steel sheet subjected to alloying treatment
including hot dip galvanizing, thereafter heating a
plated layer to a temperature that is not less than the
melting point of Zn to diffuse Fe from a base steel sheet
into the plated layer, and forming the plated layer as a
layer based on a Zn-Fe alloy has also been widely used.
Such a galvannealed steel sheet is known to have
excellent surface appearance and corrosion resistance,
compared with a hot dip galvanized steel sheet that is
not subjected to the alloying treatment.
[0003] However, in uses for automotive outside sheets
and the like, the periphery of a sheet is usually
subjected to severe bending working (hemming) by press
working; and, in not only the automotive outside sheets
but also in other uses, such a sheet, subjected to severe
bending working, bore-expanding working, or the like by
press working, is often used. In addition, when a
conventional galvannealed steel sheet is subjected to
severe bending working, bore-expanding working, or the
like, a plated layer might peel from a base steel sheet
in a portion worked in such a manner. When the plated
layer peels off in such a manner, there is a problem that
the corrosion resistance of an area where the plated
layer peels off is lost to early corrode and rust the
base steel sheet. Even when the plated layer does not
peel off, lose of adhesiveness between the plated layer
and the base steel sheet to produce even a few voids in
an area where the adhesiveness is lost causes outside air
and moisture to enter the voids, the anticorrosion
function of the plated layer to be lost, and the base
steel sheet to be early corroded and rusted in the same
manner as described above. Thus, there has been a strong
25 desire to develop a galvannealed layer and a plated steel
sheet, having the excellent adhesiveness of the plated
layer with a base steel sheet, for uses in which such
severe bending working or the like is performed.
[0004] There have already been proposed various ways
for improving the adhesiveness of a plated layer with a
base steel sheet in a galvannealed steel sheet, and some
examples thereof are described in Patent Literatures 1 to
8.
35 Citation List
Patent Literature
[0005] [Patent Literature 11 Japanese Laid-open Patent
Publication No. 2009-68061
[Patent Literature 21 Japanese Laid-open Patent
Publication No. 2008-26678
[Patent Literature 31 Japanese Laid-open Patent
Publication No. 2005-256041
[Patent Literature 41 Japanese Laid-open Patent
Publication No. 2002-173756
[Patent Literature 51 Japanese Laid-open Patent
Publication No. 9-13147
[Patent Literature 61 Japanese Laid-open Patent
Publication No. 6-235077
[Patent Literature 71 Japanese Laid-open Patent
Publication No. 2002-146503
[Patent Literature 81 Japanese Laid-open Patent
Publication No. 5-311371
SUMMARY OF INVENTION
Technical Problem
[0006] As mentioned above, a galvannealed layer and a
plated steel sheet, subjected to bending working or the
like and used, desirably have the excellent adhesiveness
of a plated layer with a base steel sheet; however,
conventional manners for improving adhesiveness, as
described in Patent Literatures 1 to 8, have been still
insufficient, and it has been difficult to reliably and
stably prevent the plated layer from peeling off
particularly when the galvannealed layer and the plated
steel sheet are subjected to very severe working such as
hemming working or bore-expanding working and are used.
For example, Patent Literature 7 describes that the
recesses and projections of a plating coating can be
eliminated by bending working or the like prior to hot
dip galvanizing. This is presumed to be because a large
number of preferred nucleation sites are generated in a
base material interface by bending working or the like
prior to plating, to accelerate alloying. However, there
is neither a description nor a suggestion that the
concentration of Fe in a plated layer is controlled by
bending working after a plating treatment step.
Further, Patent Literature 8 describes that an
alloying rate can be improved by bending working in
alloying by heating after plating. This is because Fe-
Al-Zn which decreases a Fe-Zn alloying rate is cracked by
bending working, to accelerate Fe-Zn alloying. However,
temperature in alloying by heating is not described at
all, and there is nether description nor suggestion that
the concentration of Fe in a plated layer is controlled
by adjusting temperature.
The present invention was accomplished with respect
to the above circumstances as a background and is
directed at providing a galvannealed layer and a plated
steel sheet, reliably and sufficiently improved in the
adhesiveness of a plated layer with a base steel sheet,
as a galvannealed layer and a plated steel sheet,
prepared by using a high-strength steel sheet as a base
material and at providing a method for producing the
galvannealed layer.
Solution to Problem
[0007] As a result of repeating various experiments
and examinations of the adhesiveness of a plated layer in
a galvannealed steel sheet, the present inventors found
that in a hot dip galvanized layer that is alloyed, the
concentration gradient of the amount of Fe in the
thickness direction of the plated layer has a great
influence on the adhesiveness of the plated layer with a
base steel sheet. In other words, when the hot dip
galvanized layer is subjected to alloying treatment, Fe
diffuses from the inside of the base steel sheet into the
plated layer and the plated layer has a Zn-Fe alloy-based
structure; however, in this case, since the diffusion of
Fe proceeds from a side closer to the base steel sheet,
the concentration of Fe in the plated layer after the
alloying treatment is usually higher in the side closer
to the base steel sheet and lower in a side closer to the
external surface of the plated layer. On the other hand,
the Zn-Fe alloy that forms the alloyed galvanized layer
is softer with decreasing the concentration of Fe but is
5 brittler with increasing the concentration of Fe.
Therefore, by decreasing the concentration of Fe in the
vicinity of the external surface due to the concentration
gradient of Fe as mentioned above, the external surface
is softened during press working and therefore adheres to
a die causing flaking. In contrast, when the
concentration of Fe is increased in the vicinity of an
interface with the base steel sheet due to the abovementioned
Fe concentration gradient to make the vicinity
brittle, the plated layer is fractured in the region by
severe working, easily causing powdering.
[ 0 0 0 8 ] As a result of further pursuing experiments and
examinations based on such findings, it was found that,
by performing treatment in which Fe in a plated layer is
diffused in the layer while preventing Fe from diffusing
from a base steel sheet into the plated layer as much as
possible after alloying treatment of a hot dip galvanized
layer, the concentration gradient of Fe in the plated
layer can be reduced (the gradient of the concentration
of Fe is flattened) to equalize the concentration of Fe
in the plated layer to optimal concentration (around
lo%), at which peeling resistance is excellent, in any
portion in the thickness direction thereof, to thereby
more greatly improve the adhesiveness of the galvannealed
layer with the base steel sheet than ever before, and the
present invention was thus accomplished.
[0009] The present invention was accomplished based on
such novel findings as described above and is to
basically provide a galvannealed layer and a plated steel
sheet, improved in the adhesiveness of the plated layer
with a base steel sheet, by flattening the concentration
gradient of Fe in the plated layer of the plated steel
sheet in which the galvannealed layer is formed on a
surface of a high-strength steel sheet as a base
material. Further, the present invention is to provide a
method for producing a galvannealed layer, including a
treatment step for reducing the concentration gradient of
5 Fe in a hot dip galvanized layer.
[ 0 0 10 ] Accordingly, the present invention is
summarized as follows:
(1) A galvannealed layer formed on a surface of a
base steel sheet, wherein the average amount of Fe in the
10 galvannealed layer is in a range of 8.0 to 12.0%; and the
absolute value of a difference AFe between the amount of
Fe at a position of 1/8 of the thickness of the plated
layer (the amount of Fe in the vicinity of an internal
side) and the amount of Fe at a position of 7/8 of the
thickness of the plated layer (the amount of Fe in the
vicinity of an external side) in the galvannealed layer,
the thickness being from an interface between the
galvannealed layer and the base steel sheet to the
external surface of the plated layer, is in a range of
0.0 to 3.0%.
[OOll] (2) A galvannealed steel sheet, wherein the
galvannealed layer according to (1) is formed on a
surface of a base steel sheet including, in mass%,
C: 0.050 to 0.300%,
Si: 0.10 to 2.50%,
Mn: 0.50 to 3.50%,
P: 0.001 to 0.030%,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.500%,
0: 0.0001 to 0.0100%,
N: 0.0001 to 0.0100%, and
the balance of Fe and unavoidable impurities.
[0012] (3) The galvannealed steel sheet according to
the above (2), wherein the base steel sheet further
includes, in mass%, one or two or more selected from
Cr: 0.01 to 2.008,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005 to 0.150%,
Mo: 0.01 to 1.00%, and
B: 0.0001 to 0.0100%.
[0013] (4) The galvannealed steel sheet according to
the above (2) or ( 3 ) , wherein the base steel sheet
further includes 0.0001 to 0.5000% in total of one or two
10 or more selected from Ca, Ce, Mg, Zr, Hf, and REM.
[ 0 0 14 ] (5) The galvannealed steel sheet according to
any of the above (2) to (4), wherein a coating including
a P oxide and/or a complex oxide containing P is formed
on a surface of the galvannealed layer.
15 [ 00151 (6) A method for producing a galvannealed
layer, including: in mass%,
a hot dip galvanizing step for subjecting a surface
of a base steel sheet to hot dip galvanizing to obtain a
hot dip galvanized steel sheet;
20 an alloying treatment step for heating a hot dip
galvanized layer, formed in the hot dip galvanizing step,
to a temperature in a range of 470 to 650°C to form a
galvannealed layer and to produce a galvannealed steel
sheet; and
25 an intra-galvannealed-layer diffusion treatment step
for, after the alloying treatment step, allowing the
galvannealed steel sheet to stay at a temperature in a
range of 250 to 450°C and subjecting the galvannealed
steel sheet to one or more times of bending-unbending
30 working in the temperature range to diffuse Fe in the
galvannealed layer.
[0016] (7) The method for producing a galvannealed
layer according to the above (6), wherein the
galvannealed steel sheet is obtained, in which, after the
35 intra-galvannealed-layer diffusion treatment step, the
average amount of Fe in the galvannealed layer is in a
range of 8.0 to 12.0%; and the absolute value of a
difference AFe between the amount of Fe at a position of
1/8 of the thickness of the plated layer (the amount of
Fe in the vicinity of an internal side) and the amount of
Fe at a position of 7/8 of the thickness of the plated
5 layer (the amount of Fe in the vicinity of an external
side) in the galvannealed layer, the thickness being from
an interface between the galvannealed layer and the base
steel sheet to the external surface of the plated layer,
is in a range of 0.0 to 3.0%.
10 [0017] (8) The method for producing a galvannealed
layer according to the above (6) or ( 7 ) , wherein in the
intra-galvannealed-layer diffusion treatment step, the
bending working is performed so that a maximum tensile
strain amount in a surface of the steel sheet ranges from
15 0.0007 to 0.0910.
[0018] (9) The method for producing a galvannealed
steel sheet according to any of the above (6) to (8),
wherein a surface of the galvannealed layer is subjected
to phosphate coating treatment for forming a coating
20 including a P oxide and/or a complex oxide containing P
after the intra-galvannealed-layer diffusion treatment
step.
[0019] (10) The method for producing a galvannealed
layer according to any of (6) to (9), wherein a base
steel sheet including, in mass%,
C: 0.050 to 0.300%,
Si: 0.10 to 2.50%,
Mn: 0.50 to 3.50%,
P: 0.001 to 0.030%,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.500%,
0: 0.0001 to 0.0100%,
N: 0.0001 to 0.0100%, and
the balance of Fe and unavoidable impurities
is used as the base steel sheet.
[0020] (11) The method for producing a galvannealed
layer according to the above (lo), wherein the steel
sheet further including, in mass%, one or two or more
selected from
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005 to 0.150%,
Mo: 0.01 to 1.00%, and
B: 0.0001 to 0.0100%
is used as the base steel sheet.
[0021] (12) The method for producing a galvannealed
layer according to any of the above (10) to (ll), wherein
the steel sheet further including, in mass%, 0.0001 to
0.5000% in total of one or two or more selected from Ca,
Ce, Mg, Zr, Hf, and REM is used as the base steel sheet.
Advantageous Effects of Invention
[0022] In accordance with the present invention, a
20 galvanized layer and a plated steel sheet, reliably and
sufficiently improved in the adhesiveness of a plated
layer with a base steel sheet, can be obtained as a
galvannealed layer and a plated steel sheet, in which a
steel sheet, particularly a high-strength steel sheet is
25 used as a base material, and therefore, the plated layer
can be effectively prevented from being fractured and
peeling even in uses subjected to severe working such as
bending working or bore-expanding working.
30 BRIEF DESCRIPTION OF DRAWINGS
[0023] [Figure 11 Figure 1 is a graph that indicates a
relationship of the average amount of Fe and the absolute
value of the amount of AFe in a plated layer and the
appearance of the plated layer.
35 [Figure 21 Figure 2 is a graph that indicates a
relationship between tensile strength and an elongation
in the galvannealed steel sheet according to the present
invention.
DESCRIPTION OF EMBODIMENTS
100241 The present invention will be explained in
detail below.
[0025] In the galvannealed layer and plated steel
sheet of the present invention, basically, a highstrength
steel sheet having a predetermined component
composition is used as a base material and a galvannealed
layer is formed on the surface of the base steel sheet.
In addition, particularly as for the galvannealed layer,
not only the average amount of Fe in the plated layer is
specified but also a Fe concentration distribution (Fe
concentration gradient) in the thickness direction of the
plated layer is specified.
In other words, the galvannealed layer is an alloy
layer formed by alloying treatment including forming a Zn
plated layer on the surface of the base steel sheet by
hot dip galvanizing and thereafter reheating the plated
layer to a temperature that is not less than the melting
point of Zn to diffuse Fe in the base steel sheet in the
plated layer and has a structure based on a Zn-Fe alloy.
In the present invention, the average amount of Fe in the
galvannealed layer is, in mass%, in the range of 8.0 to
12.0%, and the absolute value of the difference AFe
between the amount of Fe in the vicinity of the external
side and the amount of Fe in the vicinity of the internal
side is specified in the range of 0.0 to 3.0% as a Fe
concentration gradient condition in the thickness
direction in the galvannealed layer. Thus, the
limitation reasons of the conditions will be explained.
LO0261 [Average Amount of Fe in Plated Layer: 8.0 to
12.0 % ]
When the average amount of Fe in the galvannealed
layer is less than 8.0%, the plated layer becomes soft
and easily adheres to a die for press working, and
therefore, flaking (flaky peeling) easily occurs during
press working. Thus, the average amount of Fe in the
plated layer is preferably 8.0% or more from the
viewpoint of flaking resistance. Preferably, the average
amount of Fe is 9.0% or more. On the other hand, the
5 average amount of Fe in the galvannealed layer is more
than 12.0%, the plated layer becomes brittle and is
easily fractured, and powdering (powdery peeling) easily
occurs during press working. Thus, the average amount of
Fe in the plated layer is preferably 12.0% or less from
10 the viewpoint of powdering resistance. Preferably, the
average amount of Fe is 11.0% or less. Thus, an average
Fe concentration in a range of 8.0 to 12.0%, preferably
in a range of 9.0 to 11.0%, causes both flaking and
powdering to hardly occur and the adhesiveness of the
15 plated layer to become good.
100271 [Condition of Fe Concentration Gradient in
Plated Layer: Absolute Value of AFe of 0.0 to 3.0 %]
As mentioned above, in a hot dip galvanized layer
subjected to alloying treatment, a Fe concentration
20 gradient usually exists in the thickness direction
thereof. In the Fe concentration gradient, there is a
general tendency for the concentration of Fe to be high
in the vicinity of an interface with a base steel sheet
and for the concentration of Fe to be low in the vicinity
25 of the external surface of the plated layer. In a region
in the vicinity of the surface, in which the
concentration of Fe is low, the plated layer becomes soft
to adhere to a die during press working and flaking
peeling easily occurs. On the other hand, in the
30 vicinity of the interface with the base steel sheet, in
which the concentration of Fe is low, the plated layer
becomes brittle and powdering peeling easily occurs.
Accordingly, in any case, peeling of the plated layer
easily occurs when severe working is performed. Thus, in
35 the present invention, the Fe concentration gradient in
the plated layer is reduced to specify a Fe concentration
gradient condition so that the optimal concentration of
Fe (8.0 to 12.0%, preferably 9.0 to 11.0%) at which
flaking or powdering hardly occurs in any portion in the
thickness direction thereof is achieved. In other words,
it is specified that the absolute value of the difference
5 AFe between the amount of Fe in the vicinity of the
interface with the base steel sheet (the amount of Fe in
the vicinity of the internal side) and the amount of Fe
in the vicinity of the external surface of the plated
layer (the amount of Fe in the vicinity of the external
10 side) is in the range of 0.0 to 3.0%. The amount of Fe
in the vicinity of the internal side means the amount of
Fe at the position of 1/8 of the total thickness of the
plated layer from the interface with the base steel sheet
to the external surface of the plated layer, while the
15 amount of Fe in the vicinity of the external side means
the amount of Fe at the position of 7/8 of the total
thickness of the plated layer from the interface with the
base steel sheet to the external surface of the plated
layer (i-e., the position of 1/8 of the total thickness
20 of the plated layer from the external surface of the
plated layer to the interface with the base steel sheet).
The absolute value of AFe of more than 3.0% results
in the insufficient effect of improving the adhesiveness
of the plated layer. Thus, it is specified that the
25 absolute value of AFe is in the range of 0.0 to 3.0%.
The absolute value of AFe of 3.0% or less results in less
possibility that peeling due to flaking or powdering
occurs in the plated layer even when severe working is
performed, so that the adhesiveness of the plated layer
30 is improved. In addition, for more reliably obtaining
the adhesiveness improvement effect, the absolute value
of AFe is preferably 2.0% or less, further more
preferably 1.5% or less.
[0028] In addition, the coating amount of the
35 galvannealed layer is not particularly limited but is
desirably 20 g/m2 or more from the viewpoint of corrosion
resistance and 150 g/m2 or less from the viewpoint of
economical efficiency.
Furthermore, the galvannealed layer is prepared by
alloying Fe based on Zn; however, even when the
galvannealed layer contains Zn and Fe as well as a small
amount of one or two or more of Al, Pb, Sb, Si, Sn, Mg,
Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I, Cs, and
REM, the effects of the present invention are not
deteriorated and a preferable effect such as improvement
of corrosion resistance or workability may be provided
depending on the amount.
[0029] The limitation reasons of the component
composition of the steel sheet used as the base material
for the galvannealed steel sheet of the present invention
will be explained below. In the following description,
all " % " represents mass%.
[0030] [C: 0.050 to 0.300%]
C is contained for increasing the strength of a
high-strength steel sheet. However, a C content of more
than 0.300% results in insufficient weldability. The
content of C is preferably 0.250% or less, more
preferably 0.220% or less, from the viewpoint of
weldability. On the other hand, a C content of less than
0.050% results in decreased strength and precludes
securing of a maximum tensile strength of 900 MPa or
more. The content of C is preferably 0.075% or more,
more preferably 0.100% or more, for more further
enhancing strength.
[ 0 0 3 1 ] [Si: 0.10 to 2.50%]
Si is an element that suppresses generation of an
iron-based carbide in a steel sheet and enhances strength
and moldability. However, a Si content of more than
2.50% results in embrittlement of the steel sheet and in
deterioration of ductility. The content of Si is
preferably 2.20% or less, more preferably 2.00% or less,
from the viewpoint of ductility. On the other hand, a Si
content of less than 0.10% results in generation of a
large amount of coarse iron-based carbide during alloying
treatment of a plated layer and in deterioration of
strength and moldability. From the viewpoint of the
above, the lower limit of Si is preferably 0.30% or more,
more preferably 0.45% or more.
[0032] [Mn: 0.50 to 3.5081
Mn is added for enhancing the strength of a steel
sheet. However, a Mn content of more than 3.50% results
in generation of a coarse Mn-concentrated unit in the
center in the sheet thickness of the steel sheet to cause
embrittlement to easily occur and to cause a trouble such
as braking of a cast slab to easily occur. A Mn content
of more than 3.50% also results in deterioration of
weldability. Accordingly, the content of Mn is
preferably 3.50% or less. The content of Mn is
preferably 3.20% or less, more preferably 3.00% or less,
from the viewpoint of weldability. On the other hand,
since a Mn content of less than 0.50% results in
formation of a large amount of soft structure during
cooling after annealing, to thereby preclude securing of
a maximum tensile strength of 900 MPa or more, the
content of Mn is preferably 0.50% or more. The content
of Mn is preferably 1.50% or more, more preferably 1.70%
or more, for further enhancing strength.
[0033] [P: 0.001 to 0.030 %]
P tends to segregate in the center in the sheet
thickness of a steel sheet and embrittles a weld. Since
a P content of more than 0.030% results in great
embrittlement of the weld, the upper limit of the content
of P is 0.030%. On the other hand, since a P content of
less than 0.001% results in a greatly increased
production cost, the lower limit thereof is 0.001%.
[0034] [S: 0.0001 to 0.0100%]
S adversely affects weldability as well as
productability in casting and hot rolling. Thus, the
upper limit of the content of S is 0.0100% or less.
Further, S is bound to Mn to form MnS which is coarse and
to deteriorate ductility and stretch flanging properties
and is therefore preferably 0.0050% or less, more
preferably 0.0025% or less. On the other hand, since a S
content of less than 0.0001% results in a greatly
increased production cost, the lower limit thereof is
0.0001%.
COO351 [Al: 0.005 to 1.500%]
A1 suppresses generation of an iron-based carbide
and enhances the strength and moldability of a steel
sheet. However, an A1 content of more than 1.500%
results in deteriorated weldability and the upper limit
of the content of A1 is therefore 1.500%. From the
viewpoint of the above, the content of A1 is preferably
1.200% or less, more preferably 0.900% or less. A1 is
also an element that is effective as a deoxidation
material; however, since an A1 content of less than
0.005% results in an insufficient effect as the
deoxidation material, the lower limit of the content of
A1 is 0.005%. The amount of A1 is preferably 0.010% or
more for more sufficiently obtaining a deoxidation
effect.
100361 [N: 0.0001 to 0.0100%]
Since N forms a coarse nitride and deteriorates
ductility and stretch flanging properties, the amount of
added N is preferably reduced. Since a N content of more
than 0.0100% results in the significant tendency of the
above, the upper limit of the content of N is 0.0100%.
Since N also causes generation of blowholes in welding,
the lower content thereof is better. The effects of the
present invention are exerted even when the lower limit
of the content of N is not particularly specified;
however, since a N content of less than 0.0001% results
in a greatly increased production cost, the N content is
0.0001% or more.
35 [0037] [O: 0.0001 to 0.0100%]
Since 0 forms an oxide and deteriorates ductility
and stretch flanging properties, the content thereof is
preferably reduced. Since an 0 content of more than
0.0100% results in significant deterioration of stretch
flanging properties, the upper limit of the content of 0
is 0.0100%. Furthermore, the content of 0 is preferably
0.0080% or less, more preferably 0.0060% or less. The
effects of the present invention are exerted even when
the lower limit of the content of 0 is not particularly
specified; however, since an 0 content of less than
0.0001% results in a greatly increased production cost,
the lower limit is 0.0001%.
[0038] In addition, elements described below may be
optionally added to the base steel sheet for the
galvannealed steel sheet of the present invention.
[0039] [Cr: 0.01 to 2.00%]
Cr is an element that suppresses phase
transformation at high temperature and is effective for
higher strength and may be added instead of part of C
and/or Mn. Since a Cr content of more than 2.00% results
in deteriorated hot workability to reduce productivity,
the content of Cr is 2.00% or less. Although the effects
of the present invention are exerted even when the lower
limit of the content of Cr is not particularly specified,
the content of Cr is preferably 0.01% or more for
sufficiently obtaining the effect of achieving higher
strength by adding Cr.
[0040] [Ni: 0.01 to 2.00%]
Ni is an element that suppresses phase
transformation at high temperature and is effective for
higher strength and may be added instead of part of C
and/or Mn. Since a Ni content of more than 2.00% results
in deteriorated weldability, the content of Ni is 2.00%
or less. Although the effects of the present invention
are exerted even when the lower limit of the content of
Ni is not particularly specified, the content of Ni is
preferably 0.01% or more for sufficiently obtaining the
effect of achieving higher strength by adding Ni.
[0041] [Cu: 0.01 to 2.00%]
Cu is an element that exists as fine particles in a
steel to thereby enhance strength and can be added
instead of part of C and/or Mn. Since a Cu content of
more than 2.00% results in deteriorated weldability, the
content of Cu is 2.00% or less. Although the effects of
the present invention are exerted even when the lower
limit of the content of Cu is not particularly specified,
the content of Cu is preferably 0.01% or more for
sufficiently obtaining the effect of achieving higher
strength by adding Cu.
[0042] [Ti: 0.005 to 0.150%]
Ti is an element that contributes to increase in the
strength of a steel sheet by precipitate strengthening,
fine grain strengthening due to suppression of ferrite
crystal grain growth, and dislocation strengthening
through suppression of recrystallization. However, since
a Ti content of more than 0.150% results in increase in
precipitated carbonitrides to deteriorate moldability,
the content of Ti is 0.150% or less. The content of Ti
is more preferably 0.100% or less, further preferably
0.070% or less, from the viewpoint of moldability.
Although the effects of the present invention are exerted
even when the lower limit of the content of Ti is not
particularly specified, the content of Ti is preferably
0.005% or more for sufficiently obtaining the effect of
increasing strength by adding Ti. The content of Ti is
more preferably 0.010% or more, further preferably 0.015%
or more, for further achieving the higher strength of the
steel sheet.
[ 0 0 4 3 ] [Nb: 0.005 to 0.150%]
Nb is an element that contributes to increase in the
strength of a steel sheet by precipitate strengthening,
fine grain strengthening due to suppression of ferrite
crystal grain growth, and dislocation strengthening
through suppression of recrystallization. However, since
a Nb content of more than 0.150% results in increase in
precipitated carbonitrides to deteriorate moldability,
the content of Nb is 0.150% or less. The content of Nb
is more preferably 0.100% or less, further preferably
0.060% or less, from the viewpoint of moldability.
Although the effects of the present invention are exerted
5 even when the lower limit of the content of Nb is not
particularly specified, the content of Nb is preferably
0.005% or more for sufficiently obtaining the effect of
increasing strength by adding Nb. The content of Nb is
more preferably 0.010% or more, further preferably 0.015%
10 or more, for further achieving the higher strength of the
steel sheet.
[0044] [V: 0.005 to 0.150%]
V is an element that contributes to increase in the
strength of a steel sheet by precipitate strengthening,
15 fine grain strengthening due to suppression of ferrite
crystal grain growth, and dislocation strengthening
through suppression of recrystallization. However, since
a V content of more than 0.150% results in an increase in
precipitated carbonitrides to deteriorate moldability,
20 the content of V is 0.150% or less. Although the effects
of the present invention are exerted even when the lower
limit of the content of V is not particularly specified,
the content of V is preferably 0.005% or more for
sufficiently obtaining the effect of increasing strength
25 by adding V.
[0045] [Mo: 0.01 to 1.00%]
Mo is an element that suppresses phase
transformation at high temperature and is effective for
higher strength and may be added instead of part of C
30 and/or Mn. Since a Mo content of more than 1.00% results
in deteriorated hot workability to reduce productivity,
the content of Mo is 1.00% or less. Although the effects
of the present invention are exerted even when the lower
limit of the content of Mo is not particularly specified,
35 the content of Mo is preferably 0.01% or more for
sufficiently obtaining the effect of achieving higher
strength by adding Mo.
[0046] [W: 0.01 to 1.00%]
W is an element that suppresses phase transformation
at high temperature and is effective for higher strength
and may be added instead of part of C and/or Mn. Since a
W content of more than 1.00% results in deteriorated hot
workability to reduce productivity, the content of W is
preferably 1.00% or less. Although the effects of the
present invention are exerted without particularly
specifying the lower limit of the content of W, the
content of W is preferably 0.01% or more for sufficiently
obtaining higher strength due to W.
[0047] [B: 0.0001 to 0.0100%]
B is an element that suppresses phase transformation
at high temperature and is effective for higher strength
and may be added instead of part of C and/or Mn. Since a
B content of more than 0.0100% results in deteriorated
hot workability to reduce productivity, the content of B
is 0.0100% or less. The content of B is more preferably
0.0050% or less, further preferably 0.0030% or less, from
the viewpoint of productivity. Although the effects of
the present invention are exerted even when the lower
limit of the content of B is not particularly specified,
the content of B is preferably 0.0001% or more for
sufficiently obtaining the effect of achieving higher
strength by adding B. The content of B is more
preferably 0.0003% or more, more preferably 0.0005% or
more, for further higher strength.
100481 Furthermore, 0.0001 to 0.5000% in total of one
or two or more of Ca, Ce, Mg, Zr, Hf, and REM as
additional elements may be added to the base steel sheet
in the galvannealed steel sheet of the present invention.
The reason of adding the elements is as described below.
[0049] Ca, Ce, Mg, Zr, Hf, and REM are elements
effective for improving moldability and one or two or
more thereof can be added. However, since a total of the
contents of one or two or more of Ca, Ce, Mg, Zr, Hf, and
REM of more than 0.5000% may rather result in
deteriorated ductility, the total of the contents of the
respective elements is preferably 0.5000% or less.
Although the effects of the present invention are exerted
even when the lower limit of the content of one or two or
5 more of Ca, Ce, Mg, Zr, Hf, and REM is not particularly
specified, the total of the contents of the respective
elements is preferably 0.0001% or more for sufficiently
obtaining the effect of improving the moldability of a
steel sheet. The total of the contents of one or two or
10 more of Ca, Ce, Mg, Zr, Hf, and REM is preferably 0.0005%
or more, further preferably 0.0010% or more, from the
viewpoint of moldability. REM is an abbreviation for
Rare Earth Metal and refers to an element belonging to
the lanthanide series. In the present invention, REM and
Ce are often added in a misch metal, which may contain a
lanthanide series elem-ent as well as La and Ce in a
complex. The effects of the present invention are
exerted even when the lanthanide series element as well
as La and Ce described above are contained as unavoidable
impurities. Further, the effects of the present
invention are exerted even when metal La and Ce are
added.
[0050] The balance other than the above respective
elements may be Fe and unavoidable impurities. In
addition, each of Cr, Ni, Cu, Ti, Nb, V, Mo, W, and B
mentioned above may be contained as an impurity in a
minute amount less than the lower limit thereof. Ca, Ce,
Mg, Zr, Hf, and REM may also be contained as impurities
in a trace amount less than the lower limit of the total
amount thereof.
[0051] The structure of the high-strength steel sheet
used as the base material for the galvannealed steel
sheet of the present invention will be explained below.
The high-strength steel sheet used as the base
material for the galvannealed steel sheet of the present
invention preferably includes, by volume fraction,
ferrite: 10 to 758, bainitic ferrite and/or bainite: 10
to 50%, tempered martensite: 10 to 50%, fresh martensite:
15% or less, and retained austenite: 20% or less as the
microstructures thereof in the range of 1/8 to 3/8 of a
sheet thickness assuming that 1/4 of the thickness is a
5 center. When the high-strength steel sheet as the base
material has such structures, the galvannealed steel
sheet having superior moldability is made. Thus, the
preferred conditions of each of the structures will be
explained below.
10 [0052] [Ferrite: 10 to 75%]
Ferrite is a structure effective for improving
ductility and is preferably contained in a volume
fraction of 10 to 75% in a steel sheet structure. A
volume fraction of ferrite of less than 10% may result in
15 insufficient ductility. Ferrite is more preferably
contained in the steel sheet structure in a volume
fraction of 15% or more, further preferably 20% or more,
from the viewpoint of ductility. On the other hand,
since ferrite is a soft structure, a volume fraction of
20 ferrite of more than 75% may result in insufficient
strength. The volume fraction of ferrite contained in
the steel sheet structure is preferably 65% or less,
further preferably 50% or less, for enhancing the tensile
strength of the steel sheet.
25 [0053] [Bainitic Ferrite and/or Bainite: 10 to 50%]
Bainitic ferrite and/or bainite are structures
excellent in balance between strength and ductility and
are preferably contained in a volume fraction of 10 to
50% in a steel sheet structure. Further, bainitic
30 ferrite and/or bainite are microstructures having
strength between those of soft ferrite and hard
martensite and between those of tempered martensite and
retained austenite and are more preferably contained in
15% or more, further preferably contained in 20% or more,
35 from the viewpoint of bendability and stretch flanging
properties. On the other hand, a volume fraction of
bainitic ferrite and/or bainite of more than 50% is not
preferred because of resulting in excessively increased
yield stress and deteriorated shape fixability.
[0054] [Tempered Martensite: 10 to 50%]
Tempered martensite is a structure that greatly
improves tensile strength and may be contained in a
volume fraction of 50% or less in a steel sheet
structure. The volume fraction of tempered martensite is
preferably 10% or more from the viewpoint of tensile
strength. On the other hand, a volume fraction of
tempered martensite contained in the steel sheet
structure, of more than 50%, is not preferred because of
resulting in excessively increased yield stress and
deteriorated shape fixability.
[0055] [Fresh Martensite: 15% or less]
Fresh martensite greatly improves tensile strength
but becomes a fracture origin to greatly deteriorate
bendability, and the volume fraction thereof is therefore
preferably limited to 15% or less in a steel sheet
structure. The volume fraction of fresh martensite is
more preferably 10% or less, further preferably 5% or
less, for enhancing bendability and stretch flanging
properties.
[0056] [Retained Austenite: 20% or less]
Retained austenite greatly improves strength and
ductility and may be therefore contained in an amount
having an upper limit of 20% in a steel sheet. On the
other hand, retained austenite becomes a fracture origin
to greatly deteriorate stretch flanging properties, and
the volume fraction thereof is therefore preferably 17%
or less, more preferably 15% or less.
[0057] [Other Structures]
The steel sheet structure of the high-strength steel
sheet as the base material in the present invention may
contain structures other than the above, such as pearlite
and/or coarse cementite. However, bendability is
deteriorated when there is a large amount of pearlite
and/or coarse cementite in the steel sheet structure of
the high-strength steel sheet. Thus, the total volume
fraction of pearlite and/or coarse cementite contained in
the steel sheet structure is preferably 10% or less, more
preferably 5% or less.
[0058] The volume fraction of each structure contained
in the steel sheet structure of the high-strength steel
sheet used as the base material in the present invention
can be measured, e.g., by a method described below.
For the volume fraction of retained austenite, X-ray
analysis is performed with a surface that is parallel to
the sheet surface of the steel sheet and is at 1/4 of the
thickness thereof as an observation surface, to calculate
an area fraction, which can be regarded as the volume
fraction.
For the volume fraction of each of the structures,
i.e., ferrite, bainitic ferrite, bainite, tempered
martensite, and fresh martensite, a sample is collected
with a cross section in the sheet thickness parallel to
the direction of rolling the steel sheet as an
20 observation surface, the observation surface is polished
and nital-etched, and the range of 1/8 to 3/8 of a the
thickness, assuming that 1/4 of the thickness is a
center, is observed with a field emission scanning
electron microscope (FE-SEM) to measure an area fraction,
25 which can be regarded as the volume fraction.
[0059] The method for producing a galvannealed layer
and a plated steel sheet of the present invention will be
explained below.
In the production method of the present invention,
30 steps before obtaining the base steel sheet are not
particularly limited, and thus, each step for forming the
galvannealed layer on the base steel sheet having a
predetermined sheet thickness is first explained.
However, each step for forming the galvannealed layer can
35 also be incorporated into an annealing step after cold
rolling in a process for producing the base steel sheet,
particularly a cooling process thereof, and the above
viewpoint of corrosion resistance and preferably 150 g/m2
or less from the viewpoint of economical efficiency, and
dipping time (sheet leaping rate), bath temperature, and
the like may be appropriately adjusted so that such a
coating weight is achieved.
[0062] [Alloying Treatment Step]
The alloying treatment step is a step for diffusing
Fe from the base steel sheet into the hot dip galvanized
layer formed on the surface of the base steel sheet in
10 the preceding step and may include heating to a
temperature in a range of 470 to 650°C to maintain the
temperature in the range or heating to a temperature in a
range of 470 to 650°C to perform annealing to the
solidification temperature (about 420°C) of Zn. When the
15 heating temperature for the alloying treatment is less
than 470°C, it becomes difficult to sufficiently diffuse
Fe in the base steel sheet into the plated layer or long
time is needed for diffusing a sufficient amount of Fe,
to deteriorate productivity. On the other hand, when the
20 heating temperature for the alloying treatment is more
than 650°C, a problem that a coarse iron-based carbide is
generated in the steel sheet occurs. Thus, the heating
temperature for alloying treatment is specified in the
range of 470 to 650°C. When the alloying treatment is
25 performed by maintaining the temperature in the range of
470 to 650°C by heating, time for the maintenance is
desirably in a range of 10 to 120 seconds. Further,
annealing time in the case of heating to the temperature
in the range of 470 to 650°C to perform annealing to the
30 solidification temperature (about 420°C) of Zn is
preferably 15 to 200 seconds.
[0063] [Intra-Plated Layer Diffusion Treatment Step]
The hot dip galvanized layer subjected to the
alloying treatment in the preceding step is subjected to
35 diffusion treatment for diffusing Fe into the plated
layer to reduce the concentration gradient of the amount
of Fe in the plated layer, i.e., treatment for achieving
the absolute value of a difference AFe between the amount
of Fe in the vicinity of the interface with the base
steel sheet (the amount of Fe in the vicinity of an
5 internal side) and the amount of Fe in the vicinity of
the external surface of the plated layer (the amount of
Fe in the vicinity of the external side) in the range of
0.0 to 3.0%. The intra-plated-layer diffusion treatment
includes allowing the hot dip galvanized steel sheet
10 subjected to the alloying treatment to stay at a
temperature in a range of 250 to 450°C and subjecting the
hot dip galvanized steel sheet to one or more times of
bending-unbending working in the temperature range. By
bending-unbending working one or more times at the
15 temperature in the range of 250 to 450°C in such a manner,
Fe can be easily diffused in the plated layer while
suppressing diffusion of Fe from the base steel sheet
into the plated layer, to thereby reduce the
concentration gradient of Fe in the plated layer. The
20 reason why Fe in the plated layer can be easily diffused
while suppressing the diffusion of Fe from the base steel
sheet in the bending-unbending working at the temperature
in the above range can be considered as follows: a defect
such as an atomic vacancy and/or a dislocation is
25 introduced mainly into the plated layer by the bendingunbending
working, to activate the diffusion of Fe atoms
in the plated layer, while the diffusion of Fe atoms in
the base steel sheet does not occur due to sufficiently
low temperature, and therefore, the diffusion of Fe from
30 the base steel sheet into the plated layer can only
restrictively occur.
When the temperature in the intra-plated-layer
diffusion treatment is less than 250°C, the diffusion of
Fe in the plated layer does not sufficiently proceed;
35 while, at the temperature of more than 450°C, melting of
the plated layer may be started to rapidly diffuse Fe
from the base steel sheet into the plated layer and to
conversely increase the Fe concentration gradient, and
hot dipping metal simultaneously adheres to a roll for
bending-unbending working due to the melting of the
5 plated layer, to make it practically impossible to
perform the bending-unbending working. Thus, the
temperature in the intra-plated-layer diffusion treatment
is in the range of 250 to 450°C.
One bending working is preferably performed so that
a maximum tensile strain amount on the surface of the
steel sheet ranges from 0.0007 to 0.0910. A maximum
tensile strain amount of less than 0.0007 results in an
insufficient alloying acceleration effect. The maximum
tensile strain amount is preferably 0.0010 or more for
sufficiently accelerating alloying. On the other hand, a
maximum tensile strain amount of more than 0.0910 results
in impossible keeping of the shape of the steel sheet to
deteriorate flatness. The maximum tensile strain amount
is preferably 0.0500 or less, further preferably 0.0250
20 or less, for well keeping the shape of the steel sheet.
The sheet thickness of the steel sheet of the
present invention is 0.6 mm to 10.0 rnrn. This is because
the thickness of less than 0.6 mm results in impossible
sufficient keeping of the flat shape of the sheet while
25 the thickness of more than 10.0 mm results in difficult
temperature control to make it impossible to obtain
predetermined characteristics.
A roll diameter can be selected depending on the
steel sheet so that a strain amount in bending working
30 has an appropriate value and preferably ranges from 50 mm
to 800 mrn in consideration of a maintenance cost. The
maximum tensile strain amount introduced into the surface
of the steel sheet is a value obtained by dividing the
thickness t of the sheet by the sum (D+t) of the roll
35 diameter D and the thickness t of the sheet.
Such a galvannealed steel sheet of which the
alloying treatment has been finished can be subjected as
a product sheet without processing the sheet to coating
or press working for an automotive outside sheet or the
like and may be further subjected to phosphate coating
treatment as described below.
100641 [Phosphate Coating Formation Step]
The phosphate coating formation step is a step for
forming a coating including a P oxide and/or a complex
oxide containing P on the surface of the galvannealed
layer subjected to the intra-plated-layer diffusion
treatment. In other words, in some cases, an oxide layer
containing P (phosphate coating) has been conventionally
formed by treating the plated surface of the steel sheet
with a treatment liquid including phosphoric acid or a Pcontaining
oxide in order to enhance the press
15 moldability and deep drawability of the galvannealed
steel sheet, to thereby impart the die of the steel sheet
with lubricity and adhesion prevention properties; and
the galvannealed steel sheet of the present invention may
also be subjected to treatment for forming such a
20 coating, and the effects of the present invention are not
deteriorated even in the case. The specific conditions
of the phosphate coating treatment step are not
particularly limited, but the step may be performed under
the same conditions as conventional ones.
25 [0065] A desirable embodiment of the method for
producing a high-strength steel sheet which becomes the
base material for the galvannealed steel sheet of the
present invention will be explained below. As mentioned
above, hot dip galvanizing on the surface of the steel
30 sheet, alloying treatment, and, in addition, intraplated-
layer diffusion treatment can be incorporated into
a step for producing a base steel sheet, particularly
into a cooling process in an annealing step after cold
rolling, and the plating-related steps in the case will
35 also be explained together. In addition, various
conditions described in the explanation of the method for
producing the base steel sheet below are described
strictly as desirable conditions, and the method for
producing the base steel sheet is not limited to the
conditions.
[0066] For producing the high-strength steel sheet as
the base steel sheet, first, a slab having the abovementioned
chemical components (composition) is cast and
the slab is hot-rolled.
As the slab subjected to the hot rolling, a
continuously cast slab or a slab produced by a thin slab
caster or the like can be used. The method for producing
the high-strength steel sheet of the present invention is
adapted to a process such as continuous casting-direct
rolling (CC-DR) in which hot rolling is performed
immediately after casting.
15 [0067] In the hot rolling step, slab heating
temperature is 1050°C or more. When the slab heating
temperature is excessively low, finishing rolling
temperature is less than the Ar3 transformation
temperature to result in dual-phase rolling of ferrite
20 and austenite, the structure of the hot-rolled sheet
becomes a heterogeneous duplex grain structure, the
heterogeneous structure does not disappear even through
cold rolling and annealing steps, and ductility and
bendability are poor. Since there is a concern that
25 reduction in finishing rolling temperature results in
excessive increase of a rolling load to preclude rolling
or to result in the defect of the shape of the rolled
steel sheet, the slab heating temperature is preferably
1050°C or more. The effects of the present invention are
30 exerted without particularly specifying the upper limit
of the slab heating temperature; however, since the
excessively high heating temperature is economically
unfavorable, the upper limit of the slab heating
temperature is desirably 1350°C or less.
35 [0068] The above Ar3 transformation temperature is
calculated by the following expression:
92x(Mn+Ni/2+Cr/2+Cu/2+M0/2)+52x~l
In the above expression, each of C, Si, Mn, Ni, Cr,
Cu, Mo, and A1 represents the content [mass%] of each
element.
[0069] The lower and upper limits of the finishing
rolling temperature of the hot rolling are the higher one
of 800°C and the Ar3 temperature and 1000°C, respectively.
There is a concern that a finishing rolling temperature
of less than 800°C results in increase of a rolling load
in finishing rolling to preclude hot rolling or to result
in the defect of the shape of the hot-rolled sheet steel
obtained after hot rolling. When finishing rolling
temperature is less than the Ar3 temperature, hot rolling
15 may become dual-phase rolling of ferrite and the
austenite and the structure of the hot-rolled steel sheet
may become a heterogeneous duplex grain structure.
On the other hand, the effects of the present
invention are exerted without particularly specifying the
20 upper limit of the finishing rolling temperature;
however, when the finishing rolling temperature is
excessively high temperature, excessively high slab
heating temperature is preferred for securing the
temperature. Thus, the upper limit temperature of the
finishing rolling temperature is desirably 1000°C or less.
[0070] The finishing-rolled steel sheet (hot-rolled
steel sheet) is usually immediately wound up in coil
form. Since the winding up at a temperature of more than
800°C results in the excessively increased thickness of an
30 oxide formed on the surface of the steel sheet to
deteriorate pickling properties, the winding temperature
is 750°C or less. The winding temperature is preferably
720°C or less, further preferably 700°C or less, for
enhancing pickling properties. On the other hand, since
35 the winding temperature of less than 500°C results in the
excessively increased strength of the hot-rolled steel
sheet to preclude cold-rolling, the winding temperature
is 500°C or more. The winding temperature is preferably
550°C or more, more preferably 600°C or more, for reducing
a cold-rolling load.
[0071] The hot-rolled steel sheet produced in such a
manner is pickled. Pickling makes it possible to remove
an oxide on the surface of the steel sheet and is
therefore important for the hot dipping properties of the
steel sheet as the base material for the galvannealed
steel sheet. Further, the pickling may be performed once
or several times.
[0072] Although the pickled steel sheet may be
subjected to an annealing step without being processed,
the steel sheet having high sheet thickness accuracy and
15 an excellent shape is obtained by being subjected to cold
rolling at a rolling reduction of 35 to 75%. Since the
rolling reduction of less than 35% makes it difficult to
keep the flat shape to deteriorate the ductility of a
final product, the rolling reduction is 35% or more. On
20 the other hand, cold rolling at a rolling reduction of
more than 75% results in an excessively large coldrolling
load to preclude cold rolling. Thus, the upper
limit of the rolling reduction is 75% or less.
In addition, the effects of the present invention
25 are exerted without particularly limiting the number of
times of roll passing and a rolling reduction at each
passing.
[0073] Then, the obtained cold-rolled steel sheet is
subjected to annealing treatment. Hot dip galvanizing
30 treatment, alloying treatment, and, in addition, intraplated-
layer diffusion treatment of the surface of the
steel sheet are desirably incorporated into a cooling
process in the annealing step. Thus, the annealing
treatment of the base steel sheet, into which the
35 plating-related steps are incorporated, will be
explained.
[0074] It is desirable to heat the steel sheet so that
maximum heating temperature is in a range of 740 to 870°C
and to then cool the steel sheet so that an average
cooling rate is 1.0 to 10.O°C/sec until 680°C and an
average cooling rate is 5.0 to 200.0°C/sec in a range of
500°C to 680°C, in the annealing treatment. The maximum
heating temperature of more than 870°C results in
significantly deteriorated plating properties.
Preferably, the maximum heating temperature is 850°C or
less. Further, the maximum heating temperature of less
than 740°C causes a large amount of molten coarse ironbased
carbide to remain to deteriorate bendability.
Preferably, the maximum heating temperature is 760°C or
more. When a cooling rate condition after heating to the
maximum heating temperature deviates from the above
range, it may be impossible to obtain the steel sheet
that satisfies the preferred microstructure conditions of
such a base steel sheet as mentioned above.
[0075] After cooling so that the average cooling rate
in the range of 500°C to 680°C is 5.0 to 200.0°C/sec as
mentioned above, cooling is temporally performed to 350
to 450°C and reheating is then performed or the steel
sheet is dipped without being processed in a hot dip
galvanizing tank to perform hot dip galvanizing
treatment. The hot dipping treatment may be performed
under the conditions described in the above-mentioned
section [Hot Dip Galvanizing Step].
[0076] After the hot dip galvanizing treatment,
cooling is performed to a temperature that is lower than
the solidification temperature of Zn to solidify Zn
adhering to the surface of the steel sheet, followed by
performing alloying treatment of the hot dip galvanized
layer. In other words, reheating is performed to 470 to
650°C, and annealing is performed to 420°C for 15 to 200
seconds to promote alloying of the plated layer.
Alternatively, alloying of the plated layer may also be
promoted by performing reheating to a temperature in a
range of 470 to 650°C and maintaining a temperature in the
range for 10 to 120 seconds. Conditions on the alloying
treatment are the same as those described in the abovementioned
section [Alloying Treatment Step].
100771 Subsequently, diffusion treatment for
flattening the concentration gradient of Fe in the plated
layer is performed. In other words, staying is performed
for 60 to 1000 seconds at a temperature in a range of 250
to 420°C in the cooling process after the alloying
treatment or cooling to room temperature or around room
temperature is temporally performed after the alloying
treatment, reheating is then performed to a temperature
in a range of 250 to 420°C, and staying is performed at a
temperature in the range for 60 to 1000 seconds. In
addition, repeated bending-unbending transformation is
performed one or more times in the temperature range.
For the repeated bending-unbending transformation in the
diffusion treatment, it is desirable to use a roll having
a radius in a range of 50 to 800 mm, e.g., a roll having
a radius of 800 rnrn as mentioned above.
[0078] In the above-mentioned annealing step, surface
modification and improvement of plating properties may be
attempted by controlling an atmosphere in a furnace,
disposing an oxidizing zone and a reducing zone, and
causing an oxidation-reduction reaction of Fe and alloy
elements in the surface layer of the steel sheet.
Specifically, plating treatment can be performed while
making Si inhibiting plating properties remain in the
30 steel by forming an external oxidizing zone mainly
including Fe in the oxidizing zone at a combustion air
ratio of 0.9 or more and 1.2 or less, further making Si
participate therein to fix Si in the steel, and then
performing reduction in the reducing zone in an
35 atmosphere in which the logarithm log (PH20/PH2) of a
water partial pressure and a hydrogen partial pressure is
-3.0 or more and 0.0 or less, to reduce only an iron
oxide in the surface layer.
[0079] After the annealing treatment also serving as
each step for plating treatment, cooling may be performed
to room temperature, followed by performing cold-rolling
again at 0.05 to 3.00% for correcting a shape.
Furthermore, a coating including a P oxide and/or a
complex oxide containing P can be formed by such
phosphate coating formation treatment as mentioned above.
[0080] The present invention is specifically explained
below with reference to examples. It will be appreciated
that the examples described below are intended to
describe the specific effects by the present invention
and conditions described in the examples do not limit the
technical scope of the present invention.
Examples
[ 0 0 8 1 ] Slabs having the chemical components
(compositions) of A to BD listed in Table 1 and Table 2
(note: the left-side end of Table 2 follows the rightside
end of Table 1 in Table 1 and Table 2 indicating
each chemical component) were cast and subjected to hot
rolling, cooling, winding, and pickling under conditions
listed in Table 3 to Table 5 immediately after the
25 casting. Then, Experiment Examples 3, 9, 27, 32, 35, and
44 were not processed but the other Experiment Examples
were cold-rolled at rolling reductions listed in Table 3
to Table 5, followed by annealing the examples under
conditions listed in Table 6 to Table 8 to make steel
30 sheets in Experiment Examples 1 to 83 and 101 to 116.
Sheet thicknesses after the cold-rolling are 1.0 mm
in Experiment Examples 1 to 29 and 81 to 83, 2.4 mm in
Experiment Examples 30 to 48, 0.8 mm in Experiment
Examples 49 to 66, and 1.6 mm in Experiment Examples 67
35 to 80. Sheet thicknesses in Experiment Examples 101 to
116 are listed in Table 8.
Heating was performed to maximum heating
temperatures listed in Table 6 to Table 8 in the
annealing step after the cold-rolling, and, in the
subsequent cooling process, cooling was performed at
"cooling rates 1" in Table 6 to Table 8 from the maximum
heating temperatures to 680°C, cooling was performed at
"cooling rates 2" from 680°C to 500°C, and cooling was
further performed to " cooling stop temperatures". When
the cooling stop temperature was less than 430°C,
reheating was performed to 430°C or more. Furthermore,
dipping was performed in a galvanizing bath to perform
hot dip galvanizing treatment, heating was then performed
to alloying temperatures listed in Table 6 to Table 8 as
an alloying treatment step, and annealing was performed
to 420°C for treatment times listed in Table 6 to Table 8.
Then, staying was performed at average temperatures
listed in Table 6 to Table 8 in a range of 250 to 420°C
for times listed in Table 6 to Table 8 as an intraplated-
layer diffusion treatment step, during which
bending-unbending working with rolls having radii listed
in Table 6 to Table 8 was performed at strain amounts and
the number of times of working listed in Table 6 to Table
8, followed by performing cooling to room temperature.
After the cooling to the room temperature, coldrolling
at 0.15% was performed on conditions 7 to 24,
cold-rolling at 0.60% was performed on conditions 25 to
44, and cold-rolling at 0.25% was performed on conditions
45 to 83.
In addition, the condition 26 or 31 is an example in
which a coating including a P-based complex oxide was
applied to the surface of a plated layer and provides
good characteristics.
100821 The analysis results of the microstructures of
the steel sheets in Experiment Examples 1 to 83 and 101
to 116 are listed in Table 9 to Table 11. In
35 microstructure fractions, the amount of retained
austenite (retained y) was measured by X-ray diffraction
on a plane at a 1/4 thickness parallel to a sheet
surface. The others, which were the results of measuring
the fractions of microstructures in the range of a 1/8
thickness to a 3/8 thickness, were measured by cutting a
sheet thickness cross section parallel to a rolling
direction, nital-etching the cross section polished to be
a mirror surface, and observing the cross section using a
field emission scanning electron microscope (FE-SEM).
[ 0 0 8 3 ] The evaluation results of the plated layers and
characteristics of the steel sheets in Experiment
Examples 1 to 83 and 101 to 116 are listed in Table 12 to
Table 14. For Fe% of the plated layer, Fe% was measured
in the range of (1/8 x plated layer thickness) to (7/8 x
plated layer thickness) starting from a ferrite/plated
layer interface using EDX, to determine the average
amount of Fe, and the absolute value of a difference AFe
between the amount of Fe at a position of (1/8 x plated
layer thickness) and the amount of Fe at a position of
(7/8 x plated layer thickness), i.e., the value of JAFe%l
was determined. In addition, a relationship of the value
of the average amount of Fe, the value of IAFe%l, and the
appearance of the plated layer in each experiment example
is indicated in Figure 1.
[0084] Tensile test pieces according to JIS Z 2201
were collected from the steel sheets in Experiment
Examples 1 to 83 and 101 to 116 and were subjected to a
tensile test according to JIS Z 2241 to measure the yield
strengths, tensile strengths, and total elongations
thereof.
Further, a 90-degree V-bending test was conducted.
Test pieces of 35 mrn x 100 rnrn were cut from the steel
sheets in Experiment Examples 1 to 83 and 101 to 116, the
shear cutting planes thereof were mechanically ground to
make a bend radius twice a sheet thickness, and a test
piece in which broking and/or necking did not occur at
all was evaluated as accepted (0) while a test piece in
which any thereof was observed was evaluated as rejected
(XI
In addition, for the tensile strength TS, the case
of TS 2 900 MPa can be evaluated as accepted, and for the
ductility, the case of TS x EL 2 15000 MPa.% can be
evaluated as accepted.
Furthermore, as a test for evaluating the appearance
of a plated layer, unbending of a test piece was
performed, an adhesion tape (cellophane tape) was stuck
to the test piece and was removed, and the degree of
peeling of plating adhering to the adhesion tape was
visually observed. A test piece in which a plated layer
was not peeled was evaluated as accepted (0), while a
test piece in which plating was considerably peeled was
evaluated as rejected ( x ) .
100851 In Experiment Examples 1 to 83 and 101 to 116,
Experiment Examples 1 to 3, 5 to 9, 11 to 14, 19, 20, 23,
25 to 64, 67, 68, 73 to 80, 101 to 102, 104 to 105, 107
to 108, 110 to 111, and 113 to 116 are present invention
examples. All the invention examples were confirmed not
only to be excellent in mechanical performance but also
to have good workability, particularly good bendability
and to have the good peeling resistance of a plated
layer.
On the other hand, in each experiment example
corresponding to a comparative example, poor performance
was exhibited as described below.
[0086] In other words, Experiment Example 16 is a
comparative example in which the completion temperature
30 of hot rolling was low and bendability was poor because a
microstructure elongated in one direction and became
heterogeneous.
[0087] Experiment Example 15 is a comparative example
in which winding temperature was high after hot rolling,
a pickling property was deteriorated, and the peeling
resistance of a plated layer thus became poor.
[0088] Experiment Examples 4 and 69 are comparative
examples in which annealing after cold-rolling was
performed under the condition of high maximum heating
temperature and the peeling resistance of each plated
layer was poor.
[0089] Experiment Example 5 is a comparative example
in which annealing after cold-rolling was performed under
the condition of low maximum heating temperature, a
coarse iron-based carbide was present, and the
bendability of the steel sheet was poor since a large
amount of the coarse iron-based carbide which became a
fracture origin was contained. However, a plated layer
was not peeled to provide a good appearance.
[0090] Experiment Example 11 is a comparative example
in which a cooling rate 1 was low in a cooling process in
annealing, a coarse iron-based carbide was generated, and
the bendability of the steel sheet was poor. However, a
plated layer was not peeled to provide a good appearance.
[0091] Experiment Example 12 is a comparative example
in which a cooling rate 1 was high in a cooling process
in annealing, a soft structure was not sufficiently
generated, and the ductility and stretch flanging
property of the steel sheet were poor. However, a plated
layer was not peeled to provide a good appearance.
[0092] Experiment Example 6 is a comparative example
in which a cooling rate 2 was low in a cooling process in
annealing, a coarse iron-based carbide was generated, the
stretch flanging property of the steel sheet was poor,
and the bendability thereof was thus poor. However, a
plated layer was not peeled to provide a good appearance.
[0093] Experiment Example 10 is a comparative example
in which the temperature of alloying treatment of a hot
dip galvanized layer was high, the plated layer was
excessively alloyed, the amount of Fe in the plated layer
was excessive, a coarse iron-based carbide was generated
in the steel sheet, bendability was poor, and the peeling
resistance of the plated layer was also poor.
[0094] Experiment Example 70 is a comparative example
in which alloying treatment temperature was low, alloying
of a plated layer did not proceed, and the peeling
resistance of the plated layer was poor.
[0095] Experiment Example 17 is a comparative example
in which alloying treatment time was short, alloying of a
plated layer did not proceed, and the peeling resistance
of the plated layer was poor.
[0096] Experiment Example 18 is a comparative example
in which alloying treatment temperature was long, a
plated layer became an excessive alloy, a coarse ironbased
carbide was generated in the steel sheet,
bendability was poor, and the peeling resistance of the
plated layer was poor.
COO971 Experiment Examples 21 and 65 are comparative
examples in which staying temperature was low in the
intra-plated-layer diffusion treatment step, flattening
of Fe% did not proceed in a plated layer, and the peeling
resistance of the plated layer was poor.
[0098] Experiment Examples 22 and 72 are comparative
examples in which staying time was short in the intraplated-
layer diffusion treatment step, flattening of Fe%
did not proceed in a plated layer, and the peeling
resistance of the plated layer was poor.
COO991 Experiment Example 23 is a comparative example
in which staying time was too long in the intra-platedlayer
diffusion treatment step, a coarse iron-based
carbide was generated in the steel sheet, and the
bendability of the steel sheet was poor. However, a
plated layer was not peeled to provide a good appearance.
[OlOO] Experiment Examples 24, 66, and 71 are
comparative examples in which the number of times of
working was insufficient in the intra-plated-layer
diffusion treatment step, flattening of Fe% in a plated
layer did not proceed, and the peeling resistance of the
plated layer was poor.
[OlOl] Experiment Examples 81 to 83 are examples in
which chemical components deviated from the predetermined
ranges and any sufficient characteristics were not
obtained in all the examples.
[0102] Experiment Examples 103 and 112 are comparative
examples in which the strain amount of working in the
intra-plated-layer diffusion treatment step was large,
the shape of the steel sheet was not flat, it was
impossible to conduct a tensile test, a bending test and
an unbending test, and the examples were inadequate as
products.
[0103] Experiment Examples 106 and 109 are comparative
examples in which the strain amount of working in the
intra-plated-layer diffusion treatment step was small,
flattening of Fe% in a plated layer did not proceed, and
the peeling resistance of the plated layer was poor.
[ 0 10 4 ] Accordingly, it is clear from the above
experimental results that the present invention is
effective for improving the adhesiveness of the
galvannealed layer with the base steel sheet.
- 41 -
[Table 11

C01071 [Table 31
Experiment
Example
1
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2 1
22
23
24
25
2 6
27
28
2 9
30
Chemical
Component
A
A
A
A
A
A
B
B
B
B
B
B
C
C
C
C
C
C
D
D
D
D
D
D
E
E
E
F
F
G
Slab
Heating
Temperature
"C
1230
1190
1185
1175
1170
1255
1190
1255
1190
1215
1215
1230
1245
11 90
1210
1255
1260
1205
1270
1225
1240
1230
1265
1230
1195
1180
1215
1240
11 95
1230
Ar3
Transformation
Temperature
O C
792
792
792
792
792
7 92
712
712
712
712
712
712
678
678
678
678
678
678
790
790
790
790
790
790
659
659
659
772
772
618
Hot
Completion
Temperature
OC
950
957
961
910
888
950
907
92 6
947
895
928
920
922
923
961
7 90 -
948
954
930
922
889
925
934
893
932
894
895
943
918
886
Temperature
OC
64 4
607
665
569
654
683
670
64 9
680
584
681
627
566
660
815 -
590
597
653
580
593
629
648
569
560
683
604
589
568
598
62 6
Cold
Reduction
'i,
50
50
O
50
50
5 0
67
47
4 7
4 7
4 7
54
71
54
54
5 4
54
67
44
4 4
4 4
4 4
4 4
50
50
38
54
60
Present
Invention Example
Present
Invention Example
Present
Invention Examplt
Comparative
Example
Comparative
Example
Comparative
Example
Present
Invention Example
Present
Invention Examplt
Present
Invention Examplt
Comparative
Example
Comparative
Example
Comparative
Example
Present
Invention Example
Present
Invention Example
Comparative
Example
Comparative
Example
Comparative
Example
Comparative
Example
Present
Invention Example
Present
Invention Example
Comparative
Example
Comparative
Example
Comparative
Example
Comparative
Example
Present
Invention Example
Present
Invention Example
Present
Invention Example
Present
Invention Example
Present
Invention Example
Present
Invention Example

7 8 Z 1190 709 916 645 60 Present
Invention Example
7 9 AA 1170 635 926 680 47 Present
InventionExample
8 0 A?. 1195 635 94 1 572 67 Present
Invention Example
8 1 AB 1230 751 912 609 5 0 Comparative
Example
8 2 AC 691 828 887 648 5 0 Comparative
Example
83 AD 1185 916 910 628 5 0 Comparative
Example
101 B A 1215 663 899 585 67 Present
Invention Example
102 BA 1230 663 908 513 67 Present
Invention Example
103 B A 1205 663 920 570 67 Comparative
Example
104 BB 1210 686 911 563 67 Present
Invention Example
105 BB 1210 686 895 527 67 Present
Invention Example
106 BB 1215 686 900 600 67 Comparative
Example
107 BC 1220 754 918 561 50 Present
Invention Example
108 BC 1205 754 908 593 50 Present
Invention Example
109 BC 1220 754 895 567 5 0 Comparative
Example
110 BD 1215 600 868 600 50 Present
Invention Example
111 BD 1205 600 904 558 50 Present
Invention Example
112 BD 1225 600 8 97 556 5 0 Comparative
Example
113 BA 1190 663 942 624 67 Present
Invention Example
114 BB 1210 686 891 624 67 Present
Invention Example
115 BC 1230 754 905 594 50 Present
Invention Example
116 BD 1190 600 92 6 634 50 Present
Invention Example

[0112] [Table 81
- 49 -
[0113] [Table 91
[0114] [Table 101
[0115] [Table 111
[0116] [Table 121
Chemical Component
[0117] [Table 131
Chernlcal Component

INDUSTRIAL APPLICABILITY
[0119] The present invention can be preferably applied
to components in uses for hot dip galvanizing and working
such as bending working, among components needing
strength, such as structural members and reinforcement
members for automobiles, construction machines, and the
like and can be applied particularly to components
needing the excellent adhesiveness of a plated layer.

- 56 -
CLAIMS 0
[Claim 11
A galvannealed layer formed on a surface of a base
steel sheet, wherein the average amount of Fe in the
galvannealed layer is in a range of 8.0 to 12.0%; and the
absolute value of a difference AFe between the amount of
Fe at a position of 1/8 of the thickness of the
galvannealed layer and the amount of Fe at a position of
7/8 of the thickness of the galvannealed layer in the
galvannealed layer, the thickness being from an interface
between the galvannealed layer and the base steel sheet
to the external surface of the galvannealed layer, is in
a range of 0.0 to 3.0%.
[Claim 21
A galvannealed steel sheet, wherein the galvannealed - '
layer according to claim 1 is formed on a surface of a
base steel sheet comprising, in mass%,
C: 0.050 to 0.300%,
Si: 0.10 to 2.50%,
Mn: 0.50 to 3.50%,
P: 0.001 to 0.030%,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.500%,
0: 0.0001 to 0.0100%,
N: 0.0001 to 0.0100%, and
the balance of Fe and unavoidable impurities.
[Claim 31
The galvannealed steel sheet according to claim 2,
wherein the base steel sheet further comprises, in mass%,
one or two or more selected from
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005 to 0.150%,
Mo: 0.01 to 1.00%, and
- 57 -
B: 0.0001 to o . ~ ~ ~ ~ % . 2.5 \\!$!\ 4
[Claim 41 3 1 JAN 2011
The galvannealed steel sheet according to any one of
claim 2 and claim3, wherein the base steel sheet further
comprises 0.0001 to 0.5000% in total of one or two or
more selected from Ca, Ce, Mg, Zr, Hf, and REM.
[Claim 51
The galvannealed steel sheet according to any one of
claim 2 to claim 4, wherein a coating comprising a P
oxide and/or a complex oxide containing P is formed on a
surface of the galvannealed layer.
[Claim 61
A method for producing a galvannealed layer,
comprising:
a hot dip galvanizing step for subjecting a
surface of a base steel sheet to hot dip galvanizing to
obtain a hot dip galvanized steel sheet;
an alloying treatment step for heating a hot
dip galvanized layer, formed in the hot dip galvanizing
step, to a temperature in a range of 470 to 650°C to form
a galvannealed layer and to produce a galvannealed steel
sheet; and
an intra-galvannealed-layer diffusion treatment
step for, after the alloying treatment step, allowing the
25 galvannealed steel sheet to stay at a temperature in a
range of 250 to 450°C and subjecting the galvannealed
steel sheet to one or more times of bending-unbending
working in the temperature range to diffuse Fe in the
galvannealed layer.
30 [Claim 71
The method for producing a galvannealed layer
according to claim 6, wherein the galvannealed steel
sheet is obtained, in which, after the intragalvannealed-
layer diffusion treatment step, the average
35 amount of Fe in the galvannealed layer is in a range of
8.0 to 12.0%; and the absolute value of a difference AFe
Q luG i-J AL
-'" 00 2;!j% @4between the amount of Fe at a position of 1/8 of the 3 1 JAN j O l l -
thickness of the galvannealed layer and the amount of Fe
at a position of 7/8 of the thickness of the galvannealed
layer in the galvannealed layer, the thickness being from
an interface between the galvannealed layer and the base
steel sheet to the external surface of the galvannealed
layer, is in a range of 0.0 to 3.0%.
[Claim 81
The method for producing a galvannealed layer
according to any one of claim 6 and claim 7, wherein in
the intra-galvannealed-layer diffusion treatment step,
the bending working is performed so that a maximum
tensile strain amount in a surface of the steel sheet
ranges from 0.0007 to 0.0910.
[Claim 91
The method for producing a galvannealed layer
according to any one of claim 6 to claim 8, wherein a
surface of the galvannealed layer is subjected to
phosphate coating treatment for forming a coating
20 comprising a P oxide and/or a complex oxide containing P
after the intra-galvannealed-layer diffusion treatment
step.
[Claim 101
The method for producing a galvannealed layer
25 according to any one of claim 6 to claim 9, wherein a
base steel sheet comprising, in mass%,
C: 0.050 to 0.300%,
Si: 0.10 to 2.50%,
Mn: 0.50 to 3.50%,
P: 0.001 to 0.030%,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.500%,
0: 0.0001 to 0.0100%,
N: 0.0001 to 0.0100%, and
the balance of Fe and unavoidable impurities is used
as the base steel sheet.
[Claim 111
~pdi;IFdAt
A
k - 59 -
The method for producing a galvannealed layer 3 1 JAN
according to claim 10, wherein the steel sheet further
comprising, in mass%, one or two or more selected from
Cr: 0.01 to 2.00%,
5 Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005 to 0.150%,
10 Mo: 0.01 to 1.00%, and
B: 0.0001 to 0.0100%
is used as the base steel sheet.
[Claim 121
The method for producing a galvannealed layer
15 according to any one of claims 10 to 11, wherein the
steel sheet further comprises, in mass%, 0.0001 to
0.5000% in total of one or two or more selected from Ca,
Ce, Mg, Zr, Hf, and REM is used as the base steel shst.
-- . -
Dated this 3 1 st day of January, 20 14
(SMTI PAHUJA)
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]