DESCRIPTION
TITLE OF THE INVENTION
High-Strength Hot-Dip Galvanized Steel Sheet
5
TECHNICAL FIELD
[OOOl]
The present invention relates to a hot-dip
galvanized steel sheet. More specifically, the present
10 invention relates to a high-strength hot-dip galvanized
steel sheet, which can easily realize a high strength
(for example, a tensile strength of 980 MPa or more), is
excellent in the formability thereof, and is suitably
usable as a member in the automotive field, home
15 appliance field, building material field and the like.
BACKGROUND ART
[0002]
Heretofore, a hot-dip galvanized steel sheet has
20 been used mainly in the automotive field. However, in
the case of a hot-dip galvanized steel sheet using, as
the substrate, a high-strength (for example, a tensile
strength of 980 MPa or more) steel sheet, a crack may
readily occur particularly in the underlying steel sheet
25 during severe working thereof such as bending, and a
stress concentration on that portion may cause a fracture
in many cases.
From this standpoint, for example, Patent Document 1
has proposed to reduce the amount of Si enrichment in the
3 0 surface of a steel sheet before the immersion thereof in
a plating bath, to a certain value or less, by
controlling an annealing atmosphere. However, the
control itself to such an annealing atmosphere has been
difficult.
3 5 Also, Patent Document 2 describes a high-strength
steel sheet where Si+A1 satisfies 0.7% or more and where
as the steel sheet structure, the area ratio of the total
amount of lower bainite and total martensite to the whole
steel sheet structure is from 10 to 90%, the amount of
retained (or residual) austenite is from 5 to 50%, and
the area ratio of bainitic ferrite in upper bainite to
5 the whole steel sheet structure is 5% or more. Patent
Document 3 describes an alloyed hot-dip galvanized steel
sheet having a microstructure containing, in terms of
area ratio, from 20 to 75% of ferrite and from 5 to 25%
of retained austenite, where the average crystal grain
10 size of the ferrite is 10 pm or less. Patent Document 4
describes an alloyed hot-dip galvanized steel sheet
having formed in the surface thereof, starting from the
steel sheet side, a r phase, a mixed layer of T1 phase
and 61 phase, and a 61 phase, or a 61 phase and a 4 phase,
15 wherein the average thickness of r phase is 1.5 pm or
less, the average thickness of the mixed layer of T1
phase and 81 phase is less than two times the average
thickness of T phase, and the average aspect ratio (ratio
of long side to short side in cross-sectional
20 observation) of r1 crystal is 2 or more.
Further, Patent Document 5 describes an alloyed hotdip
galvanized steel sheet where the alloyed hot-dip
galvanized layer has a chemical composition containing,
in mass%, Fe: from 10 to 15% and Al: from 0.20 to 0.45,
25 with the balance being Zn and impurities, and the
interface adhesion strength between the steel sheet and
the alloyed hot-dip galvanized layer is 20 MPa or more.
Patent Document 6 describes an alloyed hot-dip galvanized
steel sheet excellent in impact resistance and adhesion,
30 having a coating weight of 20 to 100 g/mZ on one surface
or both surfaces, wherein the average Fe content of the
plating layer is from 8 to 16% and the thickness of r
phase in the plating layer is from 0.2 to 1.5 pm. Patent
Document 7 describes a hot-dip galvanized steel sheet
3 5 having a galvanized film in which an Fe-Al-based alloy
layer, an Fe-Zn-based alloy layer and a zinc plating
layer are present in this order starting from the base
steel sheet side, wherein the A1 content in the Fe-A1-
based ally layer is from 10 to 300 mg/m2 and the thickness
5 of the Fe-Zn-based alloy layer is 1/2 or less of the
thickness of the galvanized film. Patent Document 8
describes an alloyed hot-dip galvanized steel sheet,
wherein the number of iron-zinc alloy crystals in contact
with the plating film/base iron interface is 5.5 or more
10 per 1 pm of the interface
However, with respect to a high-strength steel sheet
having a high strength (for example, a tensile strength
of 980 MPa or more), a hot-dip galvanized steel sheet
exhibiting a sufficient effect may not be known.
15 RELATED ART
PATENT DOCUMENTS
[0003]
[Patent Document 11 JP-A (Japanese Unexamined Patent
Publication; KOKAI) No. 4-211887
20 [Patent Document 21 JP-A No. 2010-65273
[Patent Document 31 JP-A No. 2011-17046
[Patent Document 41 JP-A No. 10-306360
[Patent Document 51 JP-A No. 2006-97102
[Patent Document 61 JP-A No. 6-93402
[Patent Document 71 JP-A No. 2006-307302
[Patent Document 81 JP-A No. 2000-144362
SUMMARY OF THE INVENTION
PROBLEM TO BE SOLVED BY THE INVENTION
30 [0004]
An object of the present invention is to provide a
hot-dip galvanized steel sheet comprising, as the
substrate, a high-strength steel sheet having a high
strength (for example, a tensile strength of 980 MPa or
35 more), which is a high-strength hot-dip galvanized steel
sheet which is capable of effectively suppressing a crack
or a fracture and is excellent in formability.
MEANS FOR SOLVING THE PROBLEM
[OOOS]
As a result of earnest study, the present inventors
5 have found that an intermetallic compound composed of
mainly Fe, A1 and Zn is deposited to a thickness in a
predetermined range on an interface between the hot-dip
galvanized layer and an underlying steel sheet, and
further, the contents of Ra and RSm in the steel sheet
10 surface after the removal of the hot-dip galvanized layer
are controlled to fall in a predetermined range, whereby
the formability can be enhanced, while suppressing a
crack or a fracture of the steel sheet during severe
working.
15 Thus, the present invention relates to a highstrength
alloyed hot-dip galvanized steel sheet, which is
excellent in formability. The present invention may
include, for example, the folloriing embodiments.
[0006]
2 0 [I] A hot-dip galvanized steel sheet, which is a
steel sheet comprising, in mass%,
C: from 0.10 to 0.4%,
Si: from 0.01 to 0.5%,
Mn: from 1.0 to 3.0%,
0: 0.006% or less,
P: 0.04% or less,
S: 0.01% or less,
Al: from 0.1 to 3.0%, and
N: 0.01% or less, with the balance being Fe and
3 0 unavoidable impurities,
wherein the structure of the steel sheet further
comprises, in terms of volume fraction, 40% or more of
the total content of bainite and martensite, from 8 to
60% of retained austenite, and less than 40% of ferrite,
35 with the balance being an unavoidable structure, and
alloying hot-dip galvanization is applied to the steel
sheet surface, and
the hot-dip galvanized steel sheet has a layer of an
intermetallic compound composed of Fe, Al, Zn and
unavoidable impurities at the interface between the hotdip
galvanized layer and the underling steel sheet, the
5 average thickness of the intermetallic compound is 0.1 to
2 pm or less, and the crystal grain size of the
intermetallic compound is 0.01 or more to 1 pm or less,
and
the arithmetic mean roughness Ra of the underlying
10 steel sheet surface after removing the hot-dip galvanized
layer is 0.1 or more to 2.0 pm or less, and the average
length RSm of the contour curve element in the roughness
curve is 5 or more to 300 pm or less.
[2] The hot-dip galvanized steel sheet according to
15 [I], wherein the steel sheet further comprises one member
or two or more members of, in mass%,
Cr: from 0.05 to 1.0%,
Ni: from 0.05 to 1.0%,
Cu: from 0.05 to 1.0%,
Nb: from 0.005 to 0.3%,
Ti: from 0.005 to 0.3%,
V: from 0.005 to 0.5%,
B: from 0.0001 to 0.01%,
Ca: from 0.0005 to 0.04%,
Mg: from 0.0005 to 0.04%,
La: from 0.0005 to 0.04%,
Ce: from 0.0005 to 0.04%, and
Y: from 0.0005 to 0.04%.
[3] A process for producing a hot-dip galvanized
30 steel sheet, comprising:
heating a steel material comprising, in mass%,
C: from 0.10 to 0.4%,
Si: from 0.01 to 0.5%,
Mn: from 1.0 to 3.0%,
0: 0.006% or less,
P: 0.04% or less,
S: 0.01% or less,
Al: from 0.1 to 3.0%, and
N: 0.01% or less, with the balance being Fe and
unavoidable impurities, at 1,100 to 1,300°C and then
5 subjecting the steel sheet to a hot rolling treatment at
a finish rolling temperature of Ar3 temperature or more;
taking up the hot-rolled steel sheet at a take-up
temperature of 700°C or less and then cold-rolling the
steel sheet;
10 annealing the cold-rolled steel sheet at a maximum
heating temperature of 750 to 900°C;
cooling the annealed steel sheet to a plating bath
immersion temperature at a cooling rate of 3 to 200°C/sec
in the range of 500 to 750°C and then holding the steel
15 sheet at 350 to 500°C for 10 to 1,000 seconds;
performing a plating treatment by immersing the
steel sheet in a hot-dip galvanizing bath having an A1
concentration WAI and an Fe concentration Wre satisfying,
in mass%, the following relational expressions (1) and
2 0 (2), at a steel sheet temperature ranging, on immersion
in a plating bath, from a temperature 40°C lower than the
hot-dip galvanizing bath temperature to a temperature 50°C
higher than the hot-dip galvanizing bath temperature, in
a nitrogen atmosphere having a nitrogen content of 95
25 mass% or more, in which the logarithm log(PHzo/PHz) value
of the ratio between hydrogen partial pressure PH2 and
water vapor partial pressure PHZO is from -5 to -2:
0. OliWF,SO. 05 (1)
0. O~<(WAI-WF1~0). 30 (2)
30 on the roll surface of the final stand at the cold
rolling, the arithmetic mean roughness Ra is 0.1 or more
to 8.0 pm or less, and the average length RSm of the
contour curve element in the roughness curve is 5 or more
to 1,200 pm or less,
35 wherein Ar3=901-325xC+33xSi92x(
Mn+Ni/2+Cr/2+Cu/2+Mo/2), wherein C, Si, Mn, Ni, Cr,
Cu and Mo indicate the contents (mass%) of respective
components and take 0 when the component is not
contained.
5 141 The process for producing a high-strength hotdip
galvanized steel sheet according to [3], wherein on
the surface of a roll in one stage before final stand at
the cold rolling, the arithmetic mean roughness Ra is 0.1
or more to 8.0 pm or less, and the average length RSm of
10 the contour curve element in the roughness curve is 5 or
more to 1,200 pm or less.
EFFECT OF THE INVENTION
[0007]
15 The present invention can provide a hot-dip
galvanized steel sheet which is excellent in formability.
The production of the high-strength hot-dip galvanized
steel sheet according to the present invention may be
relatively easy and can be performed stably. Therefore,
20 the high-strength hot-dip galvanized steel sheet may be
optimally usable particularly as a steel sheet for
automobiles in recent years, which is intended for
attaining weight reduction. As a result, the industrial
value thereof may be remarkably high.
25
BRIEF DESCRIPTION OF THE DRAWINGS
[OOOS]
[Fig. 11 Fig. 1 is a graph showing a relationship
among the thickness and particle size of an intermetallic
30 compound, and the formability. In the figure, plots of
A, B, C and D represent the evaluation results relating
to the later-described formability; all of A, B and C
show Examples where the formability pass, and D shows
Comparative Examples where the formability fail.
35 [Fig. 21 Fig. 1 is a graph showing a relationship
between the roughness of an underlying steel sheet, and
the formability. In the figure, plots of A, B, C and D
represent evaluation results relating to the laterdescribed
formability; all of A, B and C show Examples
where the formability pass, and D shows Comparative
5 Examples where the formability fail.
MODES FOR CARRYING OUT THE INVENTION
[00091
Hereinbelow, the present invention is described in
sequence.
10 First, the reasons for the limitation on the
components are described. In this connection, "%" means
mass%.
[00101
C:
15 C may be an element capable of increasing the
strength of the steel sheet. However, if its content is
less than 0.1%, it may be difficult to satisfy both of
the tensile strength of 980 MPa or more, and the
workability. On the other hand, if the content exceeds
20 0.40%, spot weldability can be hardly ensured. For this
reason, the content is set to be from 0.1 to 0.40% or
less. The C content may preferably be from 0.12 to 0.3,
more preferably from 0.13 to 0.28%
[OOll]
2 5 Si :
Si may be an alloying (or strengthening) element and
may be effective in increasing the strength of the steel
sheet. Also, this element may suppress the precipitation
of cementite and in turn, contribute to stabilization of
30 retained austenite, and therefore, its addition may be
indispensable. If its content is less than 0.01%, the
effect of increasing the strength may be small. On the
other hand, if the content exceeds 0.5%, the workability
may be reduced. For this reason, the Si content is set
3 5 to be from 0.01 to 0.5%. The Si content may preferably
be from 0.05 to 0.45%, more preferably from 0.15 to
0.42%.
Mn:
Mn may be an alloying element and may be effective
in increasing the strength of the steel sheet. However,
5 if its content is less than 1.0%, the tensile strength of
980 MPa or more may be difficult to obtain. On the other
hand, if the content is large, co-segregation with P or S
may be promoted to involve significant deterioration of
the workability and therefore, an upper limit of 3.0% is
10 specified. For this reason, the Mn content is set to be
from 1.0 to 3.0%. The Mn content may preferably be from
2.0 to 2.7%, more preferably from 2.03 to 2.6%.
[00131
0 :
15 0 may form an oxide and deteriorate the elongation,
bendability or hole expandability and therefore, the
amount added of this element must be kept low. Among
others, an oxide may often exist as an inclusion and when
the oxide is present in the punched edge face or cut
20 surface, a notched flaw or a coarse dimple may be formed
on the end face to invite stress concentration during
hole expansion or severe working and serve as an origin
of crack formation, giving rise to significant
deterioration of the hole expandability or bendability.
25 If the content of 0 exceeds 0.006%, the above-described
tendency may be conspicuous, and therefore, the 0 content
is specified to an upper limit of 0.006% or less. That
is, 0 is limited as an impurity to 0.006% or less. The 0
content may preferably be 0.004% or less, more preferably
3 0 0.003% or less. On the other hand, an 0 content of less
than 0.0001% may be economically disadvantageous because
of involving an excessive rise in the cost, and
therefore, this value may be substantially the lower
limit.
35 [00141
P :
P may tend to be segregated at the center in the
sheet thickness of the steel sheet and bring about
embrittlement of a welded part. If its content exceeds
0.04%, significant embrittlement of the welded part may
occur, and formability is also reduced. Therefore, a
5 proper content range of 0.04% or Less is specified. That
is, P is limited as an impurity to 0.04% or less. The P
content may preferably be 0.03% or less, more preferably
0.025% or less. The lower limit of P content may not be
particularly specified, but a content of less than
10 0.0001% may be economically disadvantageous, and
therefore, this value may preferably be set as the lower
limit.
[0015]
S:
15 S may adversely affect the weldability and
manufacturability during casting and hot rolling. For
this reason, the upper limit of its content is set to
0.01% or less. That is, S is limited as an impurity to
0.01% or less. The S content may preferably be 0.006% or
20 less, more preferably 0.005% or less. The lower limit of
S content may not be particularly specified, but a
content of less than 0.0001% may be economically
disadvantageous, and therefore, this value may preferably
be set as the lower limit. In addition, since S may be
2 5 bound to Mn to form coarse MnS and deteriorate the
formability such as bendability or hole expandability,
the content of this element should be set as low as
possible.
[00161
30 Al:
A1 may promote ferrite formation to enhance the
ductility and therefore, may be added. This element may
also be utilized as a deoxidizing agent. If its content
is less than 0.1%, the effect of the element may be
3 5 insufficient. On the other hand, its excessive addition
may lead to an increase in the number of Al-based coarse
inclusions and give rise to deterioration of hole
expandability or cause a surface flaw. For this reason,
the upper limit of A1 content is set to be 3.0%. The A1
content may preferably be from 0.2 to 1.5%, more
preferably from 0.3 to 1.0%.
5 [0017]
N:
N may form a coarse nitride to deteriorate the
bendability or hole expandability and therefore, the
amount added thereof must be kept low. If the N content
10 exceeds 0.01%, the tendency above may be conspicuous, and
therefore, the range of the N content is set to 0.01% or
less. The N content may preferably be 0.007% or less,
more preferably 0.005% or less. From the standpoint of
reducing the formation of a blow hole during welding, the
15 N content may be smaller. Although the effects according
to the present invention can be achieved without
particularly specifying the lower limit, an N content of
less than 0.0005% may involve a great increase in the
production cost, and therefore, this value may be
20 substantially the lower limit.
[OOlS]
Cr:
Cr may be an alloying element and at the same time,
may be important in enhancing the quenchability.
25 However, if its content is less than 0.05%, these effects
may not be obtained, and therefore, a lower limit of
0.05% is specified. On the contrary, containing this
element in excess of 1.0% may adversely affect the
manufacturability during production and hot rolling, and
30 therefore, an upper limit of 1.0% is specified. The Cr
content may preferably be 0.6% or less, more preferably
0.5% or less.
[00191
Ni :
35 Ni may be an alloying element and at the same time,
may be important in enhancing the quenchability. In
addition, this element may enhance the wettability of
molten metal or promote a reaction and therefore, may be
added. However, if its content is less than 0.05%, these
effects may not be obtained, and therefore, a lower limit
of 0.05% is specified. On the contrary, containing this
5 element in excess of 1.0% may adversely affect the
manufacturability during production and hot rolling, and
therefore, an upper limit of 1.0% is specified. The Ni
content may preferably be 0.7% or 1-ess, more preferably
0.6% or less.
10 [0020]
Cu :
Cu may be an alloying element and at the same time,
may be important in enhancing the quenchability. In
addition, this element may enhance the wettability of
15 molten metal or promote a reaction and therefore, may be
added. However, if its content is less than 0.05%, these
effects may not be obtained, and therefore, a lower limit
of 0.05% is specified. On the contrary, containing this
element in excess of 1.0% may adversely affect the
2 0 manufacturability during production and hot rolling, and
therefore, an upper limit of 1.0% is specified. The Cu
content may preferably be 0.6% or less, more preferably
0.5% or less.
[00211
25 Nb:
Nb may be an alloying element and may contribute to
increase in the strength of the steel sheet by
precipitation strengthening, fine grain strengthening
through suppressing growth of a ferrite crystal grain,
3 0 and dislocation strengthening through suppressing
recrystallization. If the amount added thereof is less
than 0.005%, these effects may not be obtained, and
therefore, a lower limit of 0.005% is specified. If this
element is contained in excess of 0.3%, the amount of
35 carbonitride precipitated may be increased to deteriorate
the formability, and therefore, an upper limit of 0.3% is
specified. The Nb content may preferably be 0.25% or
less, more preferably 0.20% or less.
[0022]
Ti :
Ti may be an alloying element and may contribute to
5 increase in the strength of the steel sheet by
precipitation strengthening, fine grain strengthening
through suppressing growth of a ferrite crystal grain,
and dislocation strengthening through suppressing
recrystallization. If the amount added thereof is less
10 than 0.005%, these effects may not be obtained, and
therefore, a lower limit of 0.005% is specified. If this
element is contained in excess of 0.3%, the amount of
carbonitride precipitated may be increased to deteriorate
the formability, and therefore, an upper limit of 0.3% is
15 specified. The Ti content may preferably be 0.25% or
less, more preferably 0.20% or less.
[0023]
v:
V may be an alloying element and may contribute to
20 increase in the strength of the steel sheet by
precipitation strengthening, fine grain strengthening
through suppressing growth of a ferrite crystal yrain,
and dislocation strengthening through suppressing
recrystallization. If the amount added thereof is less
25 than 0.005%, these effects may not be obtained, and
therefore, a lower limit of 0.005% is specified. If this
element is contained in excess of 0.5%, the amount of
carbonitride precipitated may be increased to deteriorate
the formability, and therefore, an upper limit of 0.5% is
3 0 specified. The V content may preferably be 0.45% or
less, more preferably 0.3% or less.
[0024]
Addition of B in an amount of 0.0001% or more may be
effective in strengthening a grain boundary or increasing
35 the strength of the steel material, but if the amount
added exceeds 0.01%, not only the effect may be saturated
but also the manufacturability during hot rolling may be
reduced, and therefore, an upper limit of 0.01% is
specified.
[0025]
One member or two or more members selected from Ca,
5 Mg and REM may be added in a total amount of 0.0005 to
0.04%. Ca, Mg and REM may be an element used for
deoxidization, and it may be preferred to contain one
member or two or more members thereof in a total amount
of 0.0005% or more. Here, REM is Rare Earth Metal.
10 However, if the total content of Ca, Mg and REM exceeds
0.04%, degradation of forming workability may be caused.
For this reason, the total content thereof is set to be
from 0.0005 to 0.04%.
Incidentally, in the present invention, REM may be
15 added in the form of misch metal in many cases, and there
may be a case in which a combination of elements in the
lanthanoid series is contained in addition to La and Ce.
Even when such elements in the lanthanoid series other
than La and Ce are contained as unavoidable impurities,
20 the effects according to the present invention may be
brought out. In this connection, the effects according
to the present invention may also be brought out even
when metallic La and Ce are added.
[0026]
25 The structure of the steel material may be described
below.
In the steel sheet according to the present
invention, the total content of bainite and martensite
should be 40% or more. The bainite and martensite may be
30 necessary to ensure elongation and strength. The lower
limit of their total content percentage is set to 40%,
because if the volume fraction of total content is less
than 40%, the effect thereof is poor.
[0027]
35 The steel sheet according to the present invention
needs to contain, in terms of volume fraction, from 8 to
60% of retained austenite. By containing retained
austenite, increase in the strength and further
improvement of ductility may be achieved at the same
time. If the volume fraction is less than 8%, the effect
above can be hardly obtained, and therefore, a lower
5 limit of 8% or more is specified. An upper limit of 60%
or less is specified, because if its volume fraction
exceeds 60%, the volume fraction of bainite or martensite
may be less than 40%, and sufficient elongation and
strength may not be secured. The volume fraction of
10 retained austenite (y) may preferably be from 9 to 40%,
more preferably from 10 to 30%.
[0028]
The ferrite content must be less than 40%. Ferrite
may enhance the ductility, but if the content is 40% or
15 more, the strength cannot be secured. As the form of
ferrite, acicular ferrite may be incorporated other than
polygonal ferrite.
[00291
Also, the unavoidable structure of the balance as
20 used in the present invention indicates a pearlite
structure.
[0030]
With respect to the above-described microstructure
phases, ferrite, martensite, bainite, austenite, pearlite
25 and the balance structure, the identification,
observation of existing position, and measurement of area
ratio can be performed by using a nital reagent and a
reagent disclosed in JP-A No. 59-219473 to etch the steel
sheet in the rolling direction cross-section or the
3 0 cross-section in the direction perpendicular to the
rolling direction and effecting a quantitative
determination by observation through an optical
microscope at 1,000 times and scanning and transmission
electron microscopes at 1,000 to 100,000 times. After
35 observation of 20 or more visual fields for each, the
area ratio of each structure can be determined by a point
counting method or image analysis.
[00311
The constitution of the hot-dip galvanized layer may
be described below.
In the hot-dip galvanized steel sheet of the present
5 invention, an intermetallic compound composed of Fe, Al,
Zn and unavoidable impurities is present in an average
thickness of 0.1 to 2 pm at the interface between the
hot-dip galvanized layer and the underling steel sheet.
Further, the crystal grain size of the intermetallic
10 compound is from 0.01 to 1 pm, the arithmetic mean
roughness Ra of the underlying steel sheet surface after
removing the hot-dip galvanized layer is from 0.1 to 2.0
pm, and the average length RSm of the contour curve
element in the roughness curve is from 5 to 300 pm.
15 [0032]
The intermetallic compound composed of Fe, Al, Zn
and unavoidable impurities may be necessary for
suppressing a fracture of the underlying steel sheet.
The reason therefor may not be necessarily clear, but
20 according to the knowledge of the present inventors, it
may be presumed that the intermetallic compound cracks in
a finely dispersed manner during bending work and thereby
produces an effect of preventing stress concentration on
the cracked part of the underlying steel sheet.
2 5 As shown in Fig. 1, if the thickness of the
intermetallic compound is less than 0.1 pm, the effect of
the compound may be poor, whereas if the thickness
exceeds 2 pm, the intermetallic compound itself may not
be finely cracked and a local crack may be generated,
30 making it impossible to relieve the stress concentration.
For this reason, the thickness of the intermetallic
compound is from 0.1 to 2 pm. The thickness may be
preferably from 0.2 to 1.5 pm, more preferably from 0.4
to 1 ym.
3 5 Also, as seen similarly from Fig. 1, the average
crystal grain size of the intermetallic compound must be
from 0.01 to 1 pm. If the average crystal grain size
exceeds 1 pm, the intermetallic compound may not form a
fine crack but may form a local cleavage, and stress
concentration on that portion may readily occur. The
5 grain size may be preferably smaller, but if the grain
size is less than 0.01 pm, not only the effect of the
compound may be saturated but also the production load
for ensuring a thickness of 0.1 pm or more may be
increased. For this reason, the average crystal grain
10 size of the intermetallic compound is from 0.01 to 1 pm.
The average crystal grain size may be preferably from
0.01 to 0.8 pm, more preferably from 0.01 to 0.5 pm.
[00331
The process for measuring the thickness of the
15 intermetallic compound may include va,rious methods and
include, for example, "Microscopic Cross-Sectional Test
Method" (JIS H 8501). This may be a method where the
cross-section of the sample is embedded, polished and
then, if desired, etched with an etchant and the polished
20 surface is analyzed by an optical microscope, a scanning
electron microscope (SEM), an electron beam microanalyzer
(EPMA) or the like to determine the thickness.
In the present invention, the sample was embedded in
Technovit 4002 (produced by Maruto Instrument Co., Ltd.),
25 polished with polishing papers (JIS R 6001) H240, #320,
H400, #600, #800 and #I000 in this order, and a portion
of the polished face corresponding to the plating layer
to the depth of 5pm of the underlying steel sheet is
observed with EPMA by using a line analysis along a
3 0 perpendicular direction to the interface between the
plating layer and the underlying steel sheet, to thereby
determine the thickness.
In this connection, the thickness of the
intermetallic compound as used herein may mean a value
35 obtained by determining the thickness of the
intermetallic compound in the plating layer at arbitrary
loportions spaced apart from one another by 1 mm or more
and averaging the determined thicknesses of the
intermetallic compound. The composition and structure of
the intermetallic compound may be different from those of
5 I; phase (FeZnl,), 61 phase (FeZn7), phase (Fe5Zn21) and r
phase (Fe3Znlo) which are Fe, Zn and Fe-Zn alloy phase.
Therefore, the intermetallic compound can be identified
by the analysis using EPMA, an X-ray diffraction method
(XRD), a transmission electron microscope (TEM) or the
10 like.
In the present invention, the kind of each alloy
phase was identified by TEM analysis (the same analysis
as those described, for example, in Hong, M.N., and Saka,
H., Proc. 4th Intern. Conf. On Zn and Zn Alloy Coated
15 Steel Sheet, Galvatech '98, p. 248, 1998; and Kato, T.,
Hong, M.H., Nunome, K., Sasaki, K., Kuroda, K., and Saka,
H., Thin Solid Films, 319, 132, 1998). For details of
the analysis method by TEM, these publications can be
referred to, if desired.
20 [0034]
The crystal grain size of the intermetallic compound
may be measured by various methods, but in the present
invention, the crystal grain size may be measured by the
following method. First, a sample may be produced by
2 5 immersing the steel sheet of the present invention in
fuming nitric acid to dissolve and remove the hot-dip
galvanized layer. The time for which the steel sheet is
immersed may vary depending on the thickness of the
plating layer, but a bubble may be generated by a
30 corrosion reaction of the plating layer during
dissolving, and the bubble may stop occurring when the
dissolving is terminated. Therefore, the immersion may
be stopped upon disappearance of the bubble. Since the
intermetallic compound may be difficult to dissolve in
35 fuming nitric acid, the crystal grain size of the
remaining intermetallic compound may be measured by
observing the surface of the sample after dissolving and
removal of the plating layer by SEM at 50,000 times.
However, the crystal grain size of the intermetallic
compound as used herein may mean an average value of
diameters obtained by determining the diameter of a
5 crystal grain (when the crystal grain is a polygonal
grain, the diameter of a circle assuming a minimum circle
including the crystal grain) on arbitrary 10 crystal
grains of intermetallic compound in the visual field of
SEM observation).
10 [0035]
Also, the underlying steel sheet surface after
removing the hot-dip galvanized layer must have a certain
degree of roughness. The reason therefor may be because
when the underlying steel sheet surface is nearly smooth,
15 the anchor effect of anchoring the intermetallic compound
to the underlying steel sheet surface may be
insufficient, and the intermetallic compound may be
readily separated during work, failing in offering any
effect on fracture prevention of the underlying steel
20 sheet. This effect may be exerted when, as shown in Fig.
2, the arithmetic mean roughness Ra of the underling
steel sheet surface is 0.1 pm or more. On the other
hand, if Ra exceeds 2 pm, the unevenness may conversely
produce a portion on which the stress is concentrated, as
25 a result, a fracture may readily occur. For this reason,
Ra is from 0.1 to 2.0 pm. Ra may be preferably from 0.2
to 1.0 pm.
[0036]
In the present invention, the average length RSm of
3 0 the contour curve element in the roughness curve must
fall in a certain range. As shown in Fig. 2, if RSm
exceeds 300 pm, the surface may become nearly smooth, and
the anchor effect may be insufficient. RSm may be
preferably smaller, but if it is less than 5 pm, the
3 5 effect thereof may be saturated, and only a needless
increase in the production cost may be involved. For
this reason, RSm is from 5 to 300 pm. RSm may be
preferably from 10 to 200 pm.
[0037]
Incidentally, Ra and RSm as used herein may mean the
5 arithmetic mean roughness and the average length of
roughness curve element, respectively, which are defined
in JIS B 0601.
In the measurements of these, the plating layer may
be removed by treating the hot-dip galvanized steel sheet
10 with an inhibitor-containing hydrochloric acid to expose
the underlying steel sheet surface, and the roughness of
the steel sheet surface may be then measured. For
example, in the present invention, the hot-dip galvanized
steel sheet was immersed in a solution prepared by adding
15 0.02% of IBIT 700A (produced by Asahi Chemical Co., Ltd.)
as an inhibitor to an aqueous 5% hydrochloric acid
solution, whereby the underlying steel sheet surface was
exposed.
The time for which the steel sheet is immersed may
2 0 vary depending on the thickness of the plating layer, but
a bubble may be generated by a corrosion reaction of the
plating layer during dissolving, and the bubble may stop
occurring when the dissolving is terminated. Therefore,
the immersion may be stopped upon disappearance of the
25 bubble. Since the underlying steel sheet surface may be
difficult to be dissolved due to the action of inhibitor,
a sample with the underlying steel sheet surface being
exposed can be obtained by the method above.
Subsequently, the surface was measured to determine Ra
30 and RSm by Handy Surf E-40A (manufactured by Tokyo
Seimitsu Co., Ltd.) under the conditions of an evaluation
length of 4 mm and a cutoff value of 0.8 mm.
[0038]
The production process of the high-strength hot-dip
35 galvanized steel sheet with excellent formability
according to the present invention may be described
below. In the present invention the production process
preceding hot rolling may not be particularly limited.
Namely, various kinds of secondary refining may be
performed subsequently to smelting in a blast furnace, an
electric furnace or the like, and thereafter, casting may
5 be performed by normal continuous casting, casting by an
ingot method, thin slab casting or other methods. In the
case of continuous casting, the steel may be once cooled
to a low temperature, again heated and then hot-rolled,
or the cast slab may be continuously hot-rolled. Scrap
10 may be used for the raw material.
[0039]
The effects according to the present invention can
be brought out without particularly specifying the hotrolled
slab heating temperature. However, an excessively
15 high heating temperature may not be preferred from an
economical point of view, and therefore, the upper limit
of the heating temperature may preferably be less than
1,300°C. Also, if the heating temperature is excessively
low, the finish rolling temperature can be hardly
20 controlled to Ar3 temperature or more, and therefore, the
lower-limit temperature may preferably be l,lOO°C.
[0040]
If the finish rolling temperature enters the twophase
region of austenite + ferrite, the structural non-
25 uniformity in the steel sheet may be increased to
deteriorate the formability after annealing. For this
reason, the finish rolling temperature may preferably be
Ar3 temperature or more. Incidentally, the Ar3
temperature map be calculated according to the following
30 formula:
Cooling after rolling may not be particularly
specified, and the effects according to the present
35 invention can be obtained even when a cooling pattern for
performing structure control matching respective purposes
is employed.
too421
The take-up temperature must be 700°C or less. If
the take-up temperature exceeds 700°C, not only a coarse
5 ferrite or pearlite structure may be allowed to exist in
the hot-rolled structure, giving rise to a failure in
keeping retained austenite to fall in the range according
to the present invention and in turn, obtaining an
underlying steel sheet in the scope according to the
10 present invention, but. also the structure non-uniformity
after annealing may tend to become large, leading to an
increase in material anisotropy of the final product. In
the present invention, it may be preferred to enhance the
strength-ductility balance by making the structure after
15 annealing fine. Also, a take-up temperature exceeding
700°C may not be preferred, because the thickness of an
oxide formed on the steel sheet surface may be
excessively increased and in turn, the pickling effect
may be poor. Although the effects according to the
2 0 present invention can be brought out without particularly
specifying the lower limit, taking up at a temperature
not more than room temperature may be technically
difficult and therefore, this temperature may be
substantially the lower limit. Incidentally, at the hot
25 rolling, finish rolling may be continuously performed by
splicing crude rolled sheets together. Also, the crude
rolled sheet may be once taken up.
[0043]
The steel sheet after hot rolling may be usually
30 subjected to removal of scale on the surface by a
pickling treatment. Pickling may be performed once, or
pickling may be performed in a plurality of parts.
[0044]
The hot-rolled steel sheet after pickling may be
35 usually cold-rolled. The rolling reduction ratio may
preferably be from 40 to 80%. If the rolling reduction
ratio is less than 40%, the shape can be hardly kept flat
or the ductility of the final product may become bad. On
the other hand, in the case of cold ro1Lling at a
reduction ratio in excess of 80%, the cold-rolling load
5 may be excessively large, and the cold rolling may become
difficult. The efrecLs according to the present
invention can be brought out without particularly
specifying the number of rolling passes and the rolling
reduction ratio of each pass. However, the steel sheet
10 surface after cold rolling must be in a state where the
arithmetic mean roughness Ra is from 0.1 to 2.0 pm and
the average length RSm of the contour curve element in
the roughness curve is from 5 to 300 pm. To create this
state, the roll surface of the final stand of cold
15 rolling may be preferably worked such that the arithmetic
mean roughness Ra becomes from 0.1 to 8.0 pm and the
average length RSm of the contour curve element in the
roughness curve becomes from 5 to 1,200 pm. It may be
more preferred that a roll in one stage before final
20 stand is also worked to have Ra and RSm in the same
ranges.
[0045]
In the present invention, the cold-rolled steel
sheet may be usually subjected to annealing and plating
25 in a continuous annealing and plating line. Although the
effects according to the present invention can be brought
out without particularly specifying the heating rate
during passing through the line, a heating rate of less
than 0.5OC/sec may not be preferred, because the
30 productivity may be greatly impaired. On the other hand,
a heating rate exceeding 100°C may involve excessive
capital investment and may not be economically preferred.
roo461
In the present invention, the maximum heating
35 temperature (annealing temperature) must be from 750 to
900°C. If the maximum heating temperature is less than
750°C, it may take too much time for the carbide formed
during hot rolling to again enter a solid solution state,
and a carbide or a part thereof may remain, as a result,
a strength of 980 MPa or more can be hardly secured,
5 failing in obtaining an underlying steel sheet within the
scope according to the present invention. For this
reason, the lower limit of the maximum heating
temperature may be 750°C. On the other hand, excessively
high-temperature heating may not only involve a rise in
10 the cost and be disadvantageous from an economical point
of view but also may induce a trouble such as
deterioration of a sheet shape during passing of the
sheet through the line at a high temperature or decrease
in life of the roll. For this reason, the upper limit of
15 the maximum heating temperature may be 900°C.
[0047]
The heat treatment time in this temperature region
may not be particularly limited, but for achieving
dissolution of carbide, a heat treatment for 10 seconds
20 or more may be preferred. On the other hand, if the heat
treatment time exceeds 600 seconds, a rise in the cost
may be involved, and therefore, such a heat treatment
time may be not preferred from an economical point of
view. Also in the heat treatment, isothermal holding may
25 be performed at the maximum heating temperature, and even
when gradient heating is performed and after reaching the
maximum heating temperature, cooling is immediately
started, the effects according to the present invention
may be brought out.
3 0 [0048]
After the completion of annealing, the steel sheet
may be usually cooled to the plating bath immersion
temperature. The average cooling rate from the maximum
heating temperature to 750°C may preferably be from 0.1 to
3 5 200°C/sec. A cooling rate of less than O.l0C/sec may be
not preferred, because the productivity may be greatly
impaired. An excessive increase in the cooling rate may
involve a rise in the production cost, and therefore, the
upper limit may preferably be 200°C/sec.
100491
5 In the present invention, the cooling rate in the
range of 500 to 750°C must be from 3 to 200°C/sec. If the
cooling rate is too low, austenite may transform to a
pearlite structure in the cooling process, and the
austenite volume fraction of 8% or more can be hardly
10 secured. For this reason, the lower limit may be 3OC/sec
or more. Even if the cooling rate is increased, there
may be no problem in terms of steel quality, but an
excessive increase in the cooling rate may involve a rise
in the production cost, and therefore, the upper limit
15 may preferably be 200°C/sec. The cooling method may be
any process for roll cooling, air cooling, water cooling,
and a combination thereof.
[0050]
Thereafter, in the present invention, the steel
2 0 sheet may be held at a temperature of 350 to 500°C for 10
to 1,000 seconds to cause bainite transformation and
stabilize the retained austenite. The upper limit of the
holding temperature may be set to 500°C, because bainite
transformation may occur at not more than that
25 temperature. Incidentally, if the steel sheet is held at
a temperature of less than 350°C, the bainite
transformation may spend a long time and in turn,
excessively large equipment may be required, giving rise
to poor productivity. For this reason, the holding
30 temperature must be from 350 to 500°C. The lower limit
may be set to 10 seconds, because holding for less than
10 seconds may not allow bainite transformation to
proceed sufficiently, making it impossible to stabilize
the retained austenite and obtain excellent formability.
35 On the other hand, holding for more than 1,000 seconds
may cause reduction in the productivity and may be not
preferred. Incidentally, holding may not indicate only
isothermal holding but may encompass gradual cooling or
heating in this temperature region.
[0051]
5 The sheet temperature on immersion in the plating
bath may preferably be from a temperature 40°C lower than
the hot-dip galvanizing bath temperature to a temperature
50°C higher than the hot-dip galvanizing bath temperature.
If the bath-immersion sheet temperature is less than
10 (hot-dip galvanizing bath temperat~re-40)~Cn,o t only the
heat extraction at immersion and entry in the plating
bath may be large, causing partial solidification of the
molten zinc to deteriorate the plating appearance, but
also the intermetallic compound is less liable to be
15 produced, which is an essential feature of the present
invention. For this reason, the lower limit is set to
(hot-dip galvanizing bath temperature-40)OC. However,
even when the sheet temperature before immersion is below
(hot-dip galvanizing bath temperature-40)OC, the steel
2 0 sheet may be reheated before immersion in the plating
bath to a sheet temperature of (hot-dip galvanizing bath
temperature-40)OC or more and then be immersed in the
galvanizing bath. On the other hand, if the plating bath
immersion temperature exceeds (hot-dip galvanizing bath
25 temperature+50)OC, a problem in the operation may be
caused, along with the increase in the plating bath
temperature. A preferred range may have a lower limit of
(hot-dip galvanizing bath temperature-20)OC and an upper
limit of (hot-dip galvanizing bath temperature+30)'C, and
3 0 amore preferred range may have a lower limit of (hot-dip
galvanizing bath temperature-10)'C and an upper limit of
(hot-dip galvanizing bath temperature+20)OC.
[0052]
In addition to pure zinc, A1 must be added to the
3 5 plating bath. By virtue of adding Al, an intermetallic
compound composed of Fe, A1 and Zn, which is an essential
requirement of the present invention, can be produced.
Incidentally, the plating bath may contain Fe, Al, Mg,
Mn, Si, Cr and the like, in addition to pure zinc.
5 to0531
The atmosphere at the time of immersing the steel
sheet in the plating bath is a nitrogen atmosphere having
a nitrogen content of 95 vol.% or more, in which the
logarithm log (PHzo/PHz) value of hydrogen partial pressure
10 PH2 to water vapor partial pressure PHzO is from -5 to -2.
If the log(PHzo/PH2) value is less than -5, this may not
preferred from the economical view of point and in
addition, the reactivity on the steel sheet surface or
plating bath surface may be increased to allow thick
15 formation of brittle Fe-Zn alloy layer, and the plating
adhesion at during working may be poor. On the other
hand, if the log(PHzo/PHz) value exceeds -2, a Zn oxide may
be formed on the plating bath surface and inhibit
formation of the intermetallic compound composed of Fe,
20 A1 and Zn may be insufficient, and as a result, not only
a plating within the scope according to the present
invention may not be obtained but also the plating may
not adhere to the steel sheet, giving rise to unplating.
If the nitrogen content is less than 95 vol.%, the
25 proportion of water vapor and hydrogen in the atmosphere
may be increased, which may not be preferred in view of
profitability and safety. An increase in the proportion
of hydrogen in the atmosphere may cause embrittlement of
the steel sheet and reduction in the ductility and not be
30 preferred. The atmosphere at the time of immersing the
steel sheet in the plating bath as used herein may mean
an atmosphere in the furnace at least 10 seconds or more
before immersion in the plating bath, based on the time
at which the steel sheet is immersed in the plating bath,
3 5 and may mean the whole atmosphere in the time period
maximally from annealing to immersion in the plating bath
in a continuous annealing and plating line.
[0054]
In order to control the properties of the plating
layer, the hot-dip galvanizing bath is a plating bath
having an A1 concentration WAI and a Fe concentration WFe
5 satisfying, in mass%, the following relational
expressions (1) and (2):
0.011WF,10.05 (1)
0.072 (WAI-WF5~0). 30 (2)
If WFe is less than 0.01, a brittle Zn-Fe alloy
10 layer, may be formed thick at the interface between the
plating layer and the steel sheet, and the plating
adhesion at during working may be poor. If WFe exceeds
0.05, a thick layer of an intermetallic compound composed
of Fe, A1 and Zn may be formed, and a crack is liable to
15 be produced in the intermetallic compound in itself, and
in addition, top dross of Fe2A15 may be formed in the
plating bath so as to cause an indentation mark or an
unplating portion, whereby the appearance after the
plating is deteriorated.
2 0 The reason why (WA1-WFe)i s set to be 0.07 or more to
0.30 or less is because if (WAL-WF~i)s less than 0.07, a
brittle Zn-Fe alloy layer, may be formed thick at the
interface between the plating layer and the steel sheet,
and the plating adhesion at during working may be poor.
25 On the other hand, if (WAl-WFee)x ceeds 0.30, a thick layer
of an intermetallic compound composed of Fe, A1 and Zn
may be formed, and a crack is liable to be produced in
the intermetallic compound in itself.
[0055]
30 The material of the high-strength hot-dip galvanized
steel sheet excellent in formability according to the
present invention may be, in principle, produced through
normal iron making steps of refining, steelmaking,
casting, hot rolling and cold rolling, but the effects
35 according to the present invention can be obtained even
with a material produced by partially or entirely
omitting these steps, as long as the conditions according
to the present invention may be satisfied.
EXAMPLES
[0056]
5 Hereinbelow, the present invention is described in
more detail.
A slab having the components shown in Table 1 was
heated at 1,200°C, water-cooled in a water-cooling zone,
and then taken-up at the temperature shown in Table 2.
10 The thickness of the hot-rolled sheet was set to fall in
the range of 2 to 4.5 mm.
The hot-rolled sheet was pickled and then coldrolled
to have a sheet thickness of 1.2 mm after cold
rolling, whereby a cold-rolled sheet was obtained. The
15 following Table 2 shows Ra and RSm of the surface of a
roll in the final stand, and the surface of a roll, which
was just prior to the final stand, to be used in this
operation.
Thereafter, the cold-rolled sheet was subjected to a
20 heat treatment and a hot-dip galvanization treatment
under the conditions shown in Table 2 in a continuous
alloying hot-dip galvanization line, and the steel sheet
was cooled at the cooling rate shown in Table 2 from the
annealing temperature to a temperature of 500 to 750°C,
25 then held at a temperature of 350 to 500°C from 5 to 300
seconds, immersed in a galvanizing bath controlled to
predetermined conditions, and subsequently cooled to room
temperature. Finally, the obtained steel sheet was
skin-pass rolled at a rolling reduction ratio of 0.4%.
30 At this time, the plating weight was set to about 45 g/mZ
on both surfaces.
[0057]

[Table 2-11
Table 2

(continued)
The letter underlined in bold denotes outside the scope of the present invention.
(continued)
I Sten, I . . ! I Surface I I I Underlying Steel Sheet Structure
-----
Species Particle
I Retained STternesniglteh ITnhtiecrkmneetsasl loifc IDnitaemremteetra lolfi c Formability Remarks I'%I'%I Is I l , M P a c o r n p o ~ d ~ l - , l f ~ 1 7 ~ l I
F: Ferrite, B: bainite, y: austenite, M: martensite, P: pearlite.
Coating Profile
Underlying
Steel Sheet
[Table 2-21
The letter underlined in bold denotes outside the scope of the present invention.
Steel
Species
Hot Rolling
Take-Up
Temperature
Remarks
Cold Rolling
RSm of Roll
surface in one
Staqe Before
Cold
Rolllng
R P r i , l p t , on
RSm Of
Stand Roll
Ra Of
Stand Roll
Ra of Roll
surface in one
Staae Before
(continued)
The letter underlined in bold denotes outside the scope of the present invention.
(continued)
The letter underlined in bold denotes outside the scope of the present invention.
F: Ferrite, B: bainite, y: austenite, M: martensite, P: pearlite.
[0059]
In the tensile test, a JIS No. 5 test piece was
sampled in the directions perpendicular and parallel to
the rolling direction of the 1.2 mm-thick sheet and
5 evaluated for tensile properties. From the obtained
elongation value, the difference (AEl) between elongation
(L-El) when a tensile test was performed in the direction
parallel to the rolling direction, and elongation (C-El)
when the tensile test was performed in the direction
10 perpendicular to the rolling direction, was calculated.
In each sample, the tensile test was performed on 5
sheets and by determining the average of the values, the
tensile strength (TS) was calculated from the average
value. Incidentally, with respect to the steel sheet
15 having a large material anisotropy, the elongation value
tended to vary.
As for the formability, a steel sheet cut into
40x100 mm at an arbitrary position in an arbitrary
direction was bent at 120' (bending radius R = 3 mm), a
20 region of 200 pm x 200 pm on the convex side surface of
the bent part was observed by a scanning electron
microscope at 10 portions differing from each other, and
evaluation was made according to the following criteria
by counting, out of 10 portions, the number of portions
25 where a crack was observed. "A", "8" and "C" were
Example, and "D" was Comparative Example.
A: A crack in 0 portion (pass)
B: A crack in 1 to 2 portions (pass)
C: A crack in 3 to 5 portions (pass)
30 D: A crack in 6 to 10 portions (fail)
INDUSTRIAL APPLICABILITY
[0060]
According to the present invention, a high-strength
hot-dip galvanized steel sheet excellent in formability
35 may be provided. The production of the high-strength
hot-dip galvanized steel sheet may be relatively easy and
can be performed stably. Therefore, the high-strength
hot-dip galvanized steel sheet according to the present
invention may be optimal particularly as a steel sheet
for automobiles pursuing weight reduction in recent
5 years, and its industrial value may be remarkably high.
CLAIMS
[Claim 11
A hot-dip galvanized steel sheet, which is a steel
sheet comprising, in mass%,
5 C: from 0.10 to 0.4%,
Si: from 0.01 to 0.5%,
Mn: from 1.0 to 3.0%,
0: 0.006% or less,
P: 0.04% or less,
S: 0.01% or less,
Al: from 0.1 to 3.0%, and
N: 0.01% or less, with the balance being Fe and
unavoidable impurities,
wherein the structure of the steel sheet further
15 comprises, in terms of volume fraction, 40% or more of
the total content of bainite and martensite, from 8 to
60% of retained austenite, and less than 40% of ferrite,
with the balance being an unavoidable structure, and
alloying hot-dip galvanization is applied to the steel
20 sheet surface, and
the hot-dip galvanized steel sheet has a layer of an
intermetallic compound composed of Fe, Al, Zn and
unavoidable impurities at the interface between the hotdip
galvanized layer and the underling steel sheet, the
25 average thickness of the intermetallic compound is 0.1 to
2 pm or less, and the crystal grain size of the
intermetallic compound is 0.01 or more to 1 pm or less,
and
the arithmetic mean roughness Ra of the underlying
30 steel sheet surface after removing the hot-dip galvanized
layer is 0.1 or more to 2.0 pm or less, and the average
length RSm of the contour curve element in the roughness
curve is 5 or more to 300 pm or less.
[Claim 21
3 5 The hot-dip galvanized steel sheet according to
claim 1 or la, wherein the steel sheet further comprises
one member or two or more members of, in mass%,
Cr: from 0.05 to 1.0%,
Ni: from 0.05 to 1.0%,
Cu: from 0.05 to 1.0%,
Nb: from 0.005 to 0.3%,
Ti: from 0.005 to 0.3%,
V: from 0.005 to 0.5%,
B: from 0.0001 to 0.01%,
Ca: from 0.0005 to 0.04%,
Mg: from 0.0005 to 0.04%,
La: from 0.0005 to 0.04%,
Ce: from 0.0005 to 0.04%, and
Y: from 0.0005 to 0.04%.
[Claim 31
15 A process for producing a hot-dip galvanized steel
sheet, comprising:
heating a steel material comprising, in mass%,
C: from 0.10 to 0.4%,
Si: from 0.01 to 0.5%,
Mn: from 1.0 to 3.0%,
0: 0.006% or less,
P: 0.04% or less,
S: 0.01% or less,
Al: from 0.1 to 3.0%, and
N: 0.01% or less, with the balance being Fe and
unavoidable impurities, at 1,100 to 1,300°C and then
subjecting the steel sheet to a hot rolling treatment at
a finish rolling temperature of Ar3 temperature or more;
taking up the hot-rolled steel sheet at a take-up
30 temperature of 700°C or less and then cold-rolling the
steel sheet;
annealing the cold-rolled steel sheet at a maximum
heating temperature of 750 to 900°C;
cooling the annealed steel sheet to a plating bath
35 immersion temperature at a cooling rate of 3 to 200°C/sec
in the range of 500 to 750°C and then holding the steel
sheet at 350 to 500°C for 10 to 1,000 seconds;
performing a plating treatment by irnmersi-ng the
steel sheet in a hot-dip galvanizing bath having an Al
concentration Wni and an Fe concentration W F ~ satisfying,
5 in mass%, the foll.owing relational expressions (1) and
(2), at a steel sheet temperature ranging, on immersion
in a plating bath, from a temperature 40°C lower than the
hot-dip galvanizing bath temperature to a temperature 50°C
higher 'than the hot-dip galvanizing bath temperature, in
10 a nitrogen atmosphere having a nitrogen content of 95
mass% or more, in which the logarithm ~ o ~ ( P H ~ ~va/lPue~ ~ )
of the ratio between hydrogen partial pressure PH2 and
water vapor partial pressure PHZO is from -5 to -2:
0. O1