[Title of the Invention] HIGH-STRENGTH GALVANIZED STEEL SHEET .. -
EXCELLENT IN BENDABILITY AND MANUFACTURING METHOD
THEREOF
5 [Technical Field]
[0001] The present invention relates to a high-strength galvanized steel
sheet and a manufacturing method thereof, and particularly relates - . to a
high-strength galvanized steel sheet having excellent bendability and a
manufacturing method thereof. This application is based upon and claims
10 the benefit of priority from Japanese Patent Application No. 201 1-1 67436,
filed in Japan on July 29, 201 1; the entire contents of which are incorporated
herein by reference.
[Background Art]
[0002] In recent years, there have been increasing demands for
15 high-strength plated steel sheets used for automobiles or the like, and
high-strength plated steel sheets with maximum tensile stress of 900 MPa or
more is started to be used. As a method for forming vehicles or members of
automobiles using such high-strength plated steel sheets, there may be
mentioned bending such as press forming. Generally, the more the strength
20 of a steel sheet is increased, the more the bendability worsens. Accordingly,
when bending is performed on a high-strength plated steel sheet, there have
- been problems which occur easily, such as a crack inside a steel sheet at a
deformation part, necking in an interface between a steel sheet surface and a
plating layer, and destruction or peeling of a plating layer.
25 [0003] As a technique for improving bendability of steel sheet, Patent
Document 1 proposes a high-tension hot-dip galvanized steel sheet in which a
chemical composition of a steel sheet contains, in mass%, C: more than
0.02% and 0.20% or less, Si: 0.01 to 2.0%, Mn: 0.1 to 3.0%, P: 0.003 to
0.10%, S: 0.020% or less, Al: 0.001 to 1.0%, N: 0.0004 to 0.015%, and Ti:
0.03 to 0.2%, a balance includes Fe and impurities, a metal structure of the
5 steel sheet contains 30 to 95% of ferrite by an area ratio, a second phase of the
balance is constituted of one or more of martensite, bainite, perlite, cementite,
I
and retained austenite, an area ratio of the martensite is 0 to 50% when the
-" -. -
martensite is contained, and the steel sheet contains Ti-based carbonitride
precipitates with a grain diameter of 2 to 30 nm and an average interparticle
10 distance of 30 to 300 nm and contains a crystallized TiN with a grain diameter
of 3 pm or more and an average interparticle distance of 50 to 500 pm.
[0004] Further, Patent Document 2 describes, as a hot-dip galvanized
steel sheet excellent in bendability, a hot-dip galvanized steel sheet which has
a chemical composition containing, in mass%, C: -0.03 to O.11%, Si: 0.005 to
N: 0.01% or less and fbrther containing one or both of Ti: 0.50% or less and
Nb: 0.50% or less in the range satisfying Ti + Nbl2 2 0.03 (Ti and Nb in this
expression indicate the contents of respective elements (unit: mass%)) with a
balance being constituted of Fe and impurities, has a steel structure having an
20 average interval of 300 pm or less in a sheet width direction of an Mn
concentrated portion extending in a rolling direction at the position of 1120t
depth (t: sheet thickness of the steel sheet) from a surface, an area ratio of
ferrite of 60% or more, and an average grain diameter of ferrite of 1.0 to 6.0
pm and containing 100 or more precipitates per pm2 with a grain diameter of
25 1 to 10 nm in ferrite, and has tensile strength of 540 MPa or more.
[0005] Further, Patent Document 3 describes, as a hot-dip plated steel
sheet having both ductility and bendability, a hot-dip plated steel sheet having
a plating layer containing zinc on a surface of a cold-rolled steel sheet which
has a chemical composition containing, in mass%, C: 0.08 to 0.25%, Si: 0.7%
or less, Mn: 1.0 to 2.6%, Al: 1.5% or less, P: 0.03% or less, S: 0.02% or less
and N: 0.01% or less and having a relation between Si and A1 satisfying 1 .O%
5 Si + A1 5 1.8% with a balance being constituted of Fe and impurities, and
has mechanical characteristics satisfying TS 2 590 (TS: tensile strength
(MPa)), TS x El 2 17500 (El: total elongation (%)), and p 5 1.5 x t (p: limit
bend radius (mm), t: sheet thickness (mm)).
[0006] Patent Document 4 describes, as a cold-rolled steel sheet having
good ductility and bendability, a cold-rolled steel sheet which has a chemical
composition containing, in mass %, C: 0.08 to 0.20%, Si: 1.0% or less, Mn:
1.8 to 3.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.005 to 0.5%, N:
0.01% or less and Ti: 0.02 to 0.2% with a balance being constituted of Fe and
impurities, has a steel structure constituted of, in vol%, ferrite: 10% or more,
bainite: 20 to 70%, retained austenite: 3 to 20% and martensite: 0 to 20% in
which an average grain diameter of the ferrite is 10 pm or less, an average
grain diameter of the bainite is 10 pm or less, an average grain diameter of the
retained austenite is 3 pm or less, and an average grain diameter of the
martensite is 3 pm or less, has mechanical characteristics such that a tensile
strength (TS) is 780 MPa or more, a product (TS x El value) of tensile
strength (TS) and total elongation (El) is 14000 MPa.% or more, and a
minimum bend radius in a bending test is 1.5t or less (t: sheet thickness), and
has a sheet thickness of 2.0 mm or more, and describes that plating is
provided on a surface of the cold-rolled steel sheet.
[0007] Patent Document 5 describes, as an alloyed hot-dip galvanized
steel sheet excellent in bendability, an alloyed hot-dip galvanized steel sheet
which has a chemical composition containing, in mass%, C: 0.03 to 0.12%,
-
Si: 0.02 to 0.50%, Mn: 2.0 to 4.0%, P: 0.1% or less, S: 0.01% or less, sol. Al:
0.01 to 1 .O% and N: 0.01% or less and further containing one or both of Ti:
0.50% or less and Nb: 0.50% or less in the range satisfling Ti + Nbl2 2 0.03
with a balance being constituted of Fe and impurities, and has a steel structure
such that an area ratio of ferrite is 60% or more and an average grain diameter
of ferrite is 1.0 to 6.0 pm, in which an alloyed hot-dip galvanized layer
contains, in mass%, Fe: 8 to 15% and Al: 0.08 to 0.50% with a balance being
constituted of Zn and impurities, and the alloyed hot-dip galvanized steel
sheet has a tensile strength of 540 MPa or more and has excellent bendability.
[OOOS] Patent Document 6 describes as a high-strength hot-dip
galvanized steel sheet excellent in workability, one having a hot-dip
galvanized layer on a base steel sheet containing, in mass%, C: 0.03 to 0.17%,
Si: 0.01 to 0.75%, Mn: 1.5 to 2.5%, P: 0.080% or less, S: 0.010% or less, sol.
Al: 0.01 to 1.20%, Cr: 0.3 to 1.3% with a balance being constituted of Feand
inevitable impurities, and having a steel structure constituted of, in volume
fraction, 30 to 70% ferrite, less than 3% retained austenite, and martensite of
the balance, in which 20% or more of the martensite is tempered martensite.
[0009] Patent Document 7 describes, as an ultra-high-strength cold-rolled
steel sheet excellent in bending workability, a steel containing, by wt%, C:
0.12 to 0.30%, Si: 1.2% or less, Mn: 1 to 3%, P: 0.020% or less, S: 0.010% or
less, sol. Al: 0.01 to 0.06% with a balance being constituted of Fe and
inevitable impurities, the steel having a soft layer of C: 0.1 wt% or less in a
surface layer part on both surfaces by 3 to 15 vol% per surface with a balance
being constituted of a complex structure of retained austenite of less than 10
vol% and a low-temperature transformation phase or further ferrite.
[Prior Art Document]
[Patent Document]
[OO 101 Patent Document 1 : Japanese Laid-open Patent Publication No.
Patent Document 2: Japanese Laid-open Patent Publication No. 2009-2 15616
Patent Document 3 : Japanese Laid-open Patent Publication No. 2009-270 126
Patent Document 4: Japanese Laid-open Patent Publication No. 2010-59452
Patent Document 5: Japanese Laid-open Patent Publication No. 20 10-65269
10 Patent Document 6: Japanese Laid-open Patent Publication No. 20 10-70843
Patent Document 7: Japanese Laid-open Patent Publication No. H5-195 149
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0011] However, the conventional technologies are not able to obtain
15 sufficient bendability when bending is performed on a high-strength
galvanized steel sheet, and thus further improvement of bendability has been -
required.
In view of the above situations, the present invention provides a
high-strength galvanized steel sheet having excellent bendability and a
20 manufacturing method thereof.
[Means for Solving the Problems]
[0012] The present inventors have conducted intensive studies in order to
obtain a high-strength galvanized steel sheet with maximum tensile strength
of 900 MPa or more by which excellent bendability can be obtained by
25 preventing all of crack inside a steel sheet which is a base material, necking in
an interface between a steel sheet surface and a plating layer, and destruction
and peeling of the plating layer, which occur in a deformation part by
performing bending. As a result, the present inventors found that it may be a
high-strength galvanized steel sheet having an alloyed galvanized layer with
an iron content of 8 to 12% formed on a surface of a base steel sheet having
predetermined chemical components, in which in a base steel sheet structure,
retained austenite is limited to 8% or less in volume fraction, kurtosis K* of
hardness distribution, which will be described later, is -0.30 or less, and a
ratio between Vickers hardness of surface layer and Vickers hardness of 114
thickness "(Vickers hardness of surface layer)/(Vickers hardness of 1/4
thickness)" is 0.35 to 0.70.
[0013] Specifically, although such a high-strength galvanized steel sheet
has maximum tensile strength of 900 MPa or more, the Vickers hardness of
surface layer of the base steel sheet is low compared to the Vickers hardness
of 114 thickness, the surface layer of the base steel sheet easily deforms when
bending is performed, and moreover the retained austenite, which becomes a
starting point of-destruction, is limited to 8% or less in volume fraction in the
base steel sheet structure. Thus, a crack does not easily occur in the inside
of the base steel sheet.
[0014] Moreover, in such a high-strength galvanized steel sheet, since the
kurtosis K* of hardness distribution is -0.30 or less and dispersion in
distribution of hardness in the base steel sheet is small, there are less
boundaries where regions which largely differ in hardness are in contact with
each other, and a crack does not easily occur in the inside of the base steel
sheet when bending is performed.
Further, in such a high-strength galvanized steel sheet, since the
Vickers hardness of surface layer of the base steel sheet is low compared to
the Vickers hardness of 114 thickness and ductility of the surface layer of the
base steel sheet is excellent, necking is prevented on the base steel sheet side
in the interface between the surface of the base steel sheet and the alloyed
galvanized layer when bending is performed, and thus necking does not easily
occur in the interface between the surface of the base steel sheet and the
alloyed galvanized layer.
COO1 51 Further, in such a high-strength galvanized steel sheet, the content
of iron of the alloyed galvanized layer is 8 to 12%, and adhesion in the
interface between the surface of the base steel sheet and the alloyed
galvanized layer is excellent. Thus, destruction and peeling of the alloyed
galvanized layer do not easily occur when bending is performed.
The present invention was completed based on such knowledge, and
the gist thereof is as follows.
[00161 (1)
A high-strength galvanized steel sheet excellent in bendability with
maximum tensile strength of 900 MPa or more, including an alloyed
galvanized layer formed on a surface of a base steel sheet containing, in
mass%, C: 0.075 to 0.300%, Si: 0.30 to 2.50%, Mn: 1.30 to 3.50%, P: 0.001
to 0.050%, S: 0.0001 to 0.0100%, Al: 0.005-to 1.^500%, N: 0.0001 to 0.0100%,
and 0: 0.0001 .to 0.0100% with a balance being constituted of iron and
inevitable impurities, wherein: retained austenite is limited to 8% or less in
volume fraction in a range of 118 thickness to 318 thickness of the base steel
sheet: when plural measurement regions with a diameter of 1 pm or less are
set in the range of 118 thickness to 318 thickness of the base steel sheet,
measurement values of hardness in the plural measurement regions are
arranged in an ascending order to obtain a hardness distribution, an integer
N0.02 is obtained, which is a number obtained by multiplying a total number
of measurement values of hardness by 0.02 and rounding up this number
when this number includes a fraction, hardness of a measurement value which
is N0.02-th largest from a measurement value of minimum hardness is taken
5 as 2% hardness, an integer N0.98 is obtained, which is a number obtained by
multiplying a total number of measurement values of hardness by 0.98 and
rounding down this number when this number includes a fraction, and
hardness of a measurement value which is N0.98-th largest fi-om a
measurement value of minimum hardness is taken as 98% hardness, kurtosis
10 K* of the hardness distribution between the 2% hardness and the 98%
hardness is -0.30 or less; a ratio between Vickers hardness of surface layer of
the base steel sheet and Vickers hardness of 114 thickness of the base steel
sheet is 0.35 to 0.70; and a content of iron in the alloyed galvanized layer is 8
to 12% in mass%.
15 [0017] (2)
The high-strength galvanized steel sheet excellent in bendability
according to (I), wherein the structure of the base steel sheet contains, in
volume fraction, 10 to 75% ferrite, 10 to 50% in total of either or both of
bainitic ferrite and bainite, 10 to 50% tempered martensite in the range of 118
20 thickness to 318 thickness of the base steel sheet, the fresh martensite is
limited to 15% or less in volume fraction, and perlite is limited to 5% or less
in volume fraction.
[00181 (3)
The high-strength galvanized steel sheet excellent in bendability
25 according to (I), wherein the base steel sheet further contains, in mass%, one
or both of Ti: 0.005 to 0.150%, and Nb: 0.005 to 0.150%.
-
[00191 (4)
The high-strength galvanized steel sheet excellent in bendability
according to (I), wherein the base steel sheet further contains, in mass%, one
or more of B: 0.0001 to 0.0100%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu:
5 0.01to2.00%,Mo:0.01to1.00%,andW:0.01to1.00%.
[00201 (5)
The high-strength galvanized steel sheet excellent in bendability
according to (I), wherein the base steel sheet further contains, in mass%, V
0.005 to 0.150%.
10 [0021] (6)
The high-strength galvanized steel sheet excellent in bendability
according to (I), wherein the base steel sheet further contains, 0.0001 to
0.5000 mass% in total of one or more of Ca, Ce, Mg, Zr, Hf, and REM.
[0022] The high-strength galvanized steel sheet excellent in bendability
15 according to (I), wherein either or both of a coating film, constituted of a
phosphorus oxide and a coating film constituted of a composite oxide
containing phosphorus is or are formed on a surface of the alloyed galvanized
layer.
[0023] A manufacturing method of a high-strength galvanized steel sheet
20 excellent in bendability, the method including: a hot-rolling step of heating to
1050°C or more a slab containing, in mass%, C: 0.075 to 0.300%, Si: 0.30 to
2.50%, Mn: 1.30 to 3.50%, P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Al:
0.005 to 1.500%, N: 0.0001 to 0.0100%, and 0: 0.0001 to 0.0100% with a
balance being constituted of iron and inevitable impurities, completing hot
25 rolling at a finish hot-rolling temperature of 880°C or more, and coiling in a
temperature region of 750°C or less; a continuous annealing step of heating - - --
the steel sheet in a temperature range between 600°C and Acl transformation
point at an average heating rate of 1°C or more, retaining the steel sheet for
20 seconds to 600 seconds at an annealing temperature between (Acl
transformation point + 40)"C and Ac3 transformation point and in an
5 atmosphere in which log(water partial pressurehydrogen partial pressure) is
-3.0 to 0.0, performing bending-unbending deformation processing two or
more times using a roll with a radius of 800 mm or less so as to make a
difference in accumulated strain amount between a fkont and rear surface be
0.0050 or less, thereafter cooling the steel sheet in the temperature range of
10 740°C to 650°C at an average cooling rate of 1.0 to 5.0°C/second, and cooling
the steel sheet in the temperature range of 650°C to 500°C at an average
cooling rate of 5 to 200°C/second; and a plating alloying step of performing
an alloying treatment including dipping the steel sheet after the continuous
annealing step in a galvanizing bath, and then retaining the steel sheet at a
15 temperature of 470 to 650°C for 10 to 120 seconds.
LOO241 (9) - -
The manufacturing method of the high-strength galvanized steel sheet
excellent in bendability according to (8), wherein after the hot-rolling step
and before the continuous annealing step, a cold-rolling step of cold rolling
20 with a reduction ratio of 30 to 75% is performed.
[0025] (10)
The manufacturing method of the high-strength galvanized steel sheet
excellent in bendability according to (8), wherein after the alloying treatment
step, the steel sheet is retained at a temperature of 200 to 350°C for 30 to
25 1000 seconds.
The manufacturing method of the high-strength galvanized steel sheet
excellent in impact resistance characteristic according to (8), wherein after the
alloying treatment step, a step of adding a coating film constituted of a
phosphorus oxide andlor a composite oxide containing phosphorus is
performed.
[Effect of the Invention]
[0027] According to the present invention, a high-strength galvanized
steel sheet excellent in bendability with maximum tensile strength of 900
MPa or more and a manufacturing method thereof can be provided.
[Best Mode for Carrying out the Invention]
LO0281 A high-strength galvanized steel sheet of the present invention is a
high-strength galvanized steel sheet with tensile strength of 900 MPa or more,
including an alloyed galvanized layer formed on a surface of a base steel
sheet containing, in mass%, C: 0.075 to 0.300%, Si: 0.30 to 2.50%, Mn: 1.30
to 3.50%, P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Al: 0.005 to 1.500%, N:
0 to 0.01 00%, 0: 0 to 0.01 00% with a balance being constituted of iron and
inevitable impurities.
[0029] (Chemical components of the base steel sheet)
First, chemical components (composition) of the base steel sheet
constituting the high-strength galvanized steel sheet of the present invention
will be described. Note that [%I in the following description is [mass%].
[0030] "C: 0.075 to 0.300%"
C is contained for increasing strength of the base steel sheet.
However, when the content of C exceeds 0.300%, weldability becomes
insufficient. In view of weldability, the content of C is preferably 0.250% or
less, more preferably 0.220% or less. On the other hand, when the content
of C is less than 0.075%, the strength decreases and it is not possible to ensure
the maximum tensile strength of 900 MPa or more. In order to increase the
strength, the content of C is preferably 0.090% or more, more preferably
0.1 00% or more.
5 [003 11 "Si: 0.30 to 2.50%"
I Si is an element which suppresses generation of iron-based carbide in 1 I
the base steel sheet, and is necessary for increasing strength and formability.
A.
Further, it is also an element which improves stretch flangeability because it .I I
increases hardness of surface layer of the base steel sheet as a solid-solution I
10 strengthening element. However, when the content of Si exceeds 2.50%, the i
base steel sheet becomes brittle and ductility deteriorates. In view of I
I
I ductility, the content of Si is preferably 2.20% or less, more preferably 2.00% I
or less. On the other hand, when the content of Si is less than 0.30%, a large
I
amount of coarse iron-based carbides is generated during an alloying
15 treatment of the alloyed galvanized layer, deteriorating strength and
I
formability. - In view of this, the lower limit value of Si is preferably 0.50% - -
or more, more preferably 0.70% or more.
[0032] "Mn: 1.30 to 3.50%"
Mn is contained for increasing strength of the base steel' sheet:
20 However, when the content of Mn exceeds 3.50%, a coarse Mn concentrated
portion occurs in a sheet thickness center portion of the base steel sheet,
embrittlement occurs easily, and a trouble such as breaking of a cast slab
occurs easily. Further: when the content of Mn exceeds 3.50%, weldability
also deteriorates. Therefore, the content of Mn needs to be 3.50% or less.
25 In view of weldability, the content of Mn is preferably 3.20% or less, more
preferably 3.00% or less. On the other hand, when the content of Mn is less
..
than 1.30%, a large amount of soR structures is formed during cooling after
annealing, and thus it becomes difficult to ensure the maximum tensile
strength of 900 MPa or more. Thus, the content of Mn needs to be 1.30% or
more. The content of Mn is, for further increasing the strength, preferably
5 1.50% or more, more preferably 1.70% or more.
[0033] "P: 0.001 to 0.050%"
P tends to segregate in the sheet thickness center portion of the base
steel sheet, and embrittles a weld zone. When the content of P exceeds
0.050%, the weld zone becomes quite brittle, and thus the content of P is
10 limited to 0.50% or less. Although effects of the present invention are
exhibited without particularly setting the lower limit of the content of P,
setting the content of P to less than 0.001% accompanies large increase in
manufacturing costs, and thus 0.001% is set as the lower limit value.
[0034] "S: 0.0001 to 0.0100%"
15 S adversely affects weldability and manufacturability during casting
and hot rolling. Thus, the upper limit value of the content of S is set to
0.0100% or less. Further, S couples with Mn to form coarse MnS and
decreases ductility and stretch flangeability. Thus, it is preferably 0.0050%
or less, more preferably 0.0025% or less. Effects of the present invention
20 are exhibited without particularly setting the lower limit of the content of S.
However, setting the content of S to less than 0.0001% accompanies large
increase in manufacturing costs, and thus 0.0001% is set as the lower limit
value.
[0035] "Al: 0.005 to 1.500%''
2 5 A1 suppresses generation of iron-based carbide to increase strength
and formability of the base steel sheet. However, when the content of A1
exceeds 1.500%, weldability worsens, and thus the upper limit of A1 content
is set to 1.500%. In view of this, the content of A1 is preferably 1.200% or
less, more preferably 0.900% or less. Further, although A1 is an effective
element as a deoxidizing material, when the content of A1 is less than 0.005%,
5 the effect as the deoxidizing material cannot be obtained sufficiently, and thus
the lower limit of the content of A1 is 0.005% or more. To obtain the
deoxidizing effect sufficiently, the content of A1 is preferably 0.010% or
more.
[0036] "N: 0.0001 to 0.0100%"
10 N forms a coarse nitride and deteriorates ductility and stretch
flangeability, and thus its added amount should be suppressed. When the
content of N exceeds 0.0100%, this tendency becomes significant, and thus
the range of N content is set to 0.0100% or less. Further, N causes
generation of blow hole during welding, and thus a smaller amount is better.
15 Although effects of the present invention are exhibited without particularly
setting the lower limit of the content of N, setting the content of N to less than
0.0001% accompanies large increase in manufacturing costs, and thus
0.0001% is set as the lower limit value.
[0037] "0: 0.0001 to 0.0100%"
20 0 forms an oxide and deteriorates ductility and stretch flangeability,
and thus its content needs to be suppressed. When the content of 0 exceeds
0.0 loo%, deterioration of stretch flangeability becomes significant, and thus
the upper limit of 0 content is set to 0.0100% or less. The content of 0 is
preferably 0.0080% or less, more preferably 0.0060% or less. Although
25 effects of the present invention are exhibited without particularly setting the
I
I lower limit of the content of 0, setting the content of 0 to less than 0.0001% -
I
I
I
I
accompanies large increase in manufacturing costs, and thus 0.0001% is set as
the lower limit.
[0038] The base steel sheet forming the high-strength galvanized steel
sheet of the present invention may further contain the following elements as
5 necessary.
"Ti: 0.005 to 0.150%"
Ti is an element which contributes to strength increase of the base
steel sheet by precipitate strengthening, fine grain strengthening by growth
suppression of ferrite crystal grains, and dislocation strengthening through
10 suppression of recrystallization. However, when the content of Ti exceeds
0.150%, precipitation of the carbonitride increases and formability
deteriorates, and thus the content of Ti is preferably 0.150% or less. In view
of formability, the content of Ti is more preferably 0.100% or less,
hrthermore preferably 0.070% or less. Although effects of the present
15 invention are exhibited without particularly setting the lower limit of the
content of Ti, the content of Ti is preferably 0.005% or more so as to
sufficiently obtain the strength increasing effect of Ti. To increase strength
of the base steel sheet, the content of Ti is more preferably 0.010% or more,
furthermore preferably 0.0 1 5% or more.
20 [0039] "Nb: 0.005 to 0.150%" !
Nb is an element which contributes to strength increase of the base ~
I
steel sheet by precipitate strengthening, fine grain strengthening by growth
suppression of ferrite crystal grains, and dislocation strengthening through
suppression of recrystallization. However, when the content of Nb exceeds
25 0.150%, precipitation of the carbonitride increases and formability
deteriorates, and thus the content of Nb is preferably 0.150% or less. In
view of formability, the content of Nb is more preferably 0.100% or less,
hrthermore preferably 0.060% or less. Although effects of the present
invention are exhibited without particularly setting the lower limit of the
content of Nb, the content of Nb is preferably 0.005% or more so as to
5 sufficiently obtain the strength increasing effect of Nb. To increase strength
of the base steel sheet, the content of Nb is preferably 0.010% or more,
furthermore preferably 0.0 15% or more.
[0040] "B: 0.0001 to 0.0100%"
B suppresses phase transformation at high temperature and is an
10 element effective for increasing strength, and may be added in place of part of
C andlor Mn. When the content of B exceeds 0.0100%, workability during
hot working is impaired and productivity decreases. Thus, the content of B
is preferably 0.0100% or less. In view of productivity, the content of B is
more preferably 0.0050% or less, furthermore preferably 0.0030% or less.
15 Although effects of the present invention are exhibited without particularly
setting the lower limit of the content 0-f B, the content of B is preferably
0.0001% or more so as to sufficiently obtain the effect of strength increase by
B. To increase strength, the content of B is preferably 0.0003% or more,
hrthermore preferably 0.0005% or more.
20 [0041] "Cr: 0.01 to 2.00%"
Cr suppresses phase transformation at high temperature and is an
element effective for increasing strength, and may be added in place of part of
C andlor Mn. When the content of Cr exceeds 2.00%, workability during
hot working is impaired and productivity decreases, and thus the content of Cr
25 is preferably 2.00% or less. Although effects of the present invention are
exhibited without particularly setting the lower limit of the content of Cr, the
content of Cr is preferably 0.01% or more so as to sufficiently obtain the
effect of strength increase by Cr.
100421 "Ni: 0.01 to 2.00%"
Ni suppresses phase transformation at high temperature and is an
5 element effective for increasing strength, and may be added in place of part of
C and/or Mn. When the content of Ni exceeds 2.00%, weldability is
impaired, and thus the content of Ni is preferably 2.00% or less. Although
effects of the present invention are exhibited without particularly setting the
lower limit of the content of Ni, the content of Ni is preferably 0.01% or more
10 so as to sufficiently obtain the effect of strength increase by Ni.
[0043] "Cu: 0.01 to 2.00%"
Cu is an element which increases strength by existing as fine particles
in steel, and can be added in place of part of C and/or Mn. When the content
of Cu exceeds 2.00%, weldability is impaired, and thus the content of Cu is
15 preferably 2.00% or less. Although effects of the present invention are
.... exhibited without particularly setting the lower limit of the content of Cu, the
content of Cu is preferably 0.01% or more so as to sufficiently obtain the
effect of strength increase by Cu.
- - '[0044] "Mo: 0.01 to 1.00%"
2 o Mo suppresses phase transformation at high temperature and is an
element effective for increasing strength, and may be added in place of part of
C andlor Mn. When the content of Mo exceeds 1.00%, workability during
hot working is impaired and productivity decreases. Thus, the content of
Mo is preferably 1.00% or less. Although effects of the present invention
25 are exhibited without particularly setting the lower limit of the content of Mo,
the content of Mo is preferably 0.01% or more so as to sufficiently obtain the
. -
effect of strength increase by Mo.
[0045] "W: 0.01 to 1.00%"
W suppresses phase transformation at high temperature and is an
element effective for increasing strength, and may be added in place of part of
C andfor Mn. When the content of W exceeds 1.00%, workability during
hot working is impaired and productivity decreases, and thus the content of W
is preferably 1.00% or less. Although effects of the present invention are
exhibited without particularly setting the lower limit of the content of W, the
content of W is preferably 0.01% or more so as to sufficiently obtain the
effect of strength increase by W.
LO0461 "V 0.005 to 0.150%"
V is an element which contributes to strength increase of the base steel
sheet by precipitate strengthening, fine grain strengthening by growth
suppression of ferrite crystal grains, and dislocation strengthening through
suppression of recrystallization. However, when the content of V exceeds
- - 0.150%, precipitation of the carbonitride increases and :formability
deteriorates, and thus the content of V is preferably 0.150% or less.
Although effects of the present invention are exhibited without particularly
setting the lower limit of the content of V, the content of V is preferably- -
0.005% or more so as to sufficiently obtain the strength increasing effect of V.
[0047] "0.0001 to 0.5000% in total of one or more of Ca, Ce, Mg, Zr, Hf,
REM"
Ca, Ce, Mg, Zr, Hf, REM are elements effective for improving .
formability, and one or more of them may be added. However, when a total
content of one or more of Ca, Ce, Mg, Zr, Hf, REM exceeds 0.5000%, it is
possible that ductility is impaired on the contrary. Accordingly, the total
content of the elements is preferably 0.5000% or less. Although effects of
the present invention are exhibited without particularly setting the lower limit
of the total content of one or more of Ca, Ce, Mg, Zr, Hf, REM, the total
content of these elements is preferably 0.0001% or more so as to sufficiently
5 obtain the effect of improving formability of the base steel sheet. In view of
formability, the total content of one or more of Ca, Ce, Mg, Zr, Hf, REM is
more preferably 0.0005% or more, furthermore preferably 0.0010% or more.
[0048] Note that REM stands for Rare Earth Metal, and refers to an
element belonging to the lanthanoid series. In the present invention, REM
10 or Ce is often added in misch metal, and may contain elements of the
lanthanoid series other than La and Ce in a complex form. Effects of the
present invention are exhibited even when elements of the lanthanoid series
other than La and Ce are contained as inevitable impurities. Further, effects
of the present invention are exhibited even when metals La and Ce are added.
15 COO491 (Structure of the base steel sheet)
The reasons for defining the structure of the base steel sheet of the
high-strength galvanized steel sheet of the present invention are as follows.
"Retained austenite: 8% or less"
In the structure of the base steel sheet, retained austenite is limited to
20 8% or less in volume fraction in the range of 118 thickness to 318 thickness of
the base steel sheet.
Retained austenite largely improves strength and ductility, but on the
other hand, it becomes a starting point of destruction and largely deteriorates
bendability. Accordingly, in the high-strength galvanized steel sheet of the
25 present invention, retained austenite contained in the structure of the base
steel sheet is limited to 8% or less in volume fraction. To further improve
bendability of the high-strength galvanized base steel sheet, the volume
fraction of retained austenite is preferably 5% or less.
Note that in the entire structure of the base steel sheet, the retained
I
austenite is desirably limited to 8% or less in volume fraction. However, the
5 metal structure in the range of 118 thickness to 318 thickness with 114 of the
sheet thickness of the base steel sheet being the center represents the -structure
of the entire base steel sheet. Therefore, when the retained austenite is
limited to 8% or less in volume fraction in the range of 118 thickness to 318
thickness of the base steel sheet, it can be assumed that the retained austenite
10 is substantially limited to 8% or less in volume fraction in the entire structure
of the base steel sheet. Accordingly, in the present invention, the range of
volume fraction of retained austenite in the range of 118 thickness to 318
thickness of the base steel sheet is defined.
[0050] Besides that the above-described retained austenite is limited to
15 8% or less in volume fraction, the structure of the base steel sheet of the
high-strength galvanized steel sheet of the present invention preferably , -
contains, in volume fraction, 10 to 75% ferrite, 10 to 50% in total of either or
both of bainitic ferrite and bainite, and 10 to 50% tempered martensite in the
range of 118 thickness to 318 thickness with 114 of the sheet thickness being
20 the center. Further, preferably, the fresh martensite is limited to 15% or less
in volume fraction, and perlite is limited to 5% or less in volume fraction.
When the base steel sheet of the high-strength galvanized steel sheet of the
present invention has such structure, kurtosis K* of a hardness distribution
which will be described later becomes -0.30 or less, making it be a
25 high-strength galvanized steel sheet having more excellent bendability.
Note that similarly the metal structure of these ferrite and so on is
desirably in a predetermined range in the entire structure of the base steel
sheet. However, the metal structure in the range of 118 thickness to 318
thickness with 114 of the sheet thickness of the base steel sheet being the
center represents the entire structure of the base steel sheet. Therefore, when
10 to 75% ferrite, 10 to 50% in total of either or both of bainitic ferrite and
bainite, and 10 to 50% tempered martensite in volume fraction are contained
in the range of 118 thickness to 318 thickness of the base steel sheet, fresh
martensite is limited to 15% or less in volume fraction, and perlite is limited
to 5% or less in volume fraction, it can be assumed that the metal structure of
these ferrite and so on is substantially in a predetermined range in the entire
structure of the base steel sheet. Accordingly, in the present invention, the
range of volume fraction of the metal structure of these ferrite and so on is
defined in the range of 118 thickness to 318 thickness of the base steel sheet.
[0051] "Ferrite: 10 to 75%"
Ferrite is a structure effective for improving ductility, and is contained
preferably by 10 to 75% 3n volume fraction in the structure of the base steel
sheet. When the volume fraction of ferrite is less than lo%, it is possible
that sufficient ductility is not obtained. The volume fraction of ferrite
contained in the structure of the base steel sheet is more preferably 15% or I
more, hrthermore preferably 20% or more in view of ductility. Further, since ferrite has a soft structure, when its volume fraction exceeds 75%, it is
possible that sufficient strength cannot be obtained. To sufficiently increase
tensile strength of the base steel sheet, the volume fraction of ferrite contained
in the structure of the base steel sheet is preferably 65% or less, more
preferably 50% or less.
[0052] "Perlite: 5% or less"
When there is a large amount of perlite, ductility deteriorates. Thus,
the volume fraction of perlite contained in the structure of the base steel sheet
is preferably limited to 5% or less, more preferably 2% or less.
[0053] " 10 to 50% in total of either or both of bainitic ferrite and bainite"
5 Bainitic ferrite and bainite have a structure excellent in balance
between strength and ductility, and preferably 10 to 50% bainitic ferrite and
bainite in total in volume fraction are contained in the structure of-the base
steel sheet. Further, bainitic ferrite and bainite have a microstructure having
strength which is in the middle between soft ferrite and hard martensite, and
10 tempered martensite and retained austenite, and they are preferably contained
by 15% or more in total, more preferably 20% or more in total in view of
bendability. On the other hand, when the volume fraction of bainitic ferrite
and bainite exceeds 50% in total, yield stress increases excessively and shape
fixability deteriorates, which is hence not preferable. Note that only one of
I5 bainitic ferrite and bainite may be contained, or both of them may be
contained. ..---
[0054] "Fresh martensite: 15% or less"
The fresh martensite largely improves tensile strength, but on the other
hand, it becomes a starting point of destruction and largely deteriorates I
20 bendability. Accordingly, they are preferably limited to 15% or less in I
volume fraction in the structure of the base steel sheet. To increase
bendability, the volume fraction of fresh martensite is more preferably 10% or
less, furthermore preferably 5% or less.
[0055] "Tempered maitensite: 10 to 50%"
25 Tempered martensite has a structure which largely improves tensile
strength, and may be contained by 50% or less in volume fraction in the
structure of the base steel sheet. In view of tensile strength, the volume
fraction of tempered martensite is preferably 10% or more. On the other
hand, when the volume fraction of tempered martensite contained in the
structure of the base steel sheet exceeds 50%, yield stress increases
5 excessively and there is a concern of deteriorating shape fixability, which is
hence not preferable.
[0056] "Other structure"
The structure of the base steel sheet of the high-strength galvanized
steel sheet of the present invention may contain a structure such as coarse
10 cementite other than the above-described structures. However, when there is
a large amount of coarse cementite in the structure of the base steel sheet,
bendability deteriorates. Thus, the volume fraction of coarse cementite
contained in the structure of the base steel sheet is preferably 10% or less,
more preferably 5% or less.
15 [0057] The volume fractions of respective structures contained in the
structure of the base steel sheet of the high-strength galvanized steel sheet of
the present invention can be measured by, for example, the following method.
Regarding the volume fraction of retained austenite, X-ray diffraction
is performed on an observation surface which Is a surface in parallel to a sheet
20 surface of the base steel sheet and at 114 thickness, and an area fraction is
calculated, which can then be assumed as the volume fraction.
[0058] Regarding the volume fractions of ferrite, perlite, bainitic ferrite,
bainite, tempered martensite, and fresh martensite contained in the structure
of the base steel sheet of the high-strength galvanized steel sheet of the
25 present invention, a sample is collected from an observation surface which is
a thicknesswise cross section perpendicular to a sheet surface of the base steel
sheet and in parallel with a rolling direction (rolling reduction direction), the
observation surface is polished and nital etched, the range of 118 thickness to
318 thickness with 114 of the sheet thickness being the center is observed with
a field emission scanning electron microscope (FE-SEM), and area fractions
5 of respective structures are measured, which can be assumed as the volume
fractions of respective structures.
As described above, since the metal structure in the range of 118
thickness to 318 thickness with 114 of the sheet thickness of the base steel
sheet being the center represents the structure of the entire base steel sheet,
10 the metal structure of the entire structure of the base steel sheet can be
recognized by using the volume fraction of retained austenite at 114 thickness
of the base steel sheet and the volume fractions of metal structures, such as a
metal structure of ferrite, and so on, in the range of 118 thickness to 318
thickness of the base steel sheet.
[0059] Ferrite is a mass of crystal grains and is a region where there is no
iron-based carbide with a major axis of 100 nm or more in its inside. Note
that the volume fraction of ferrite is the sum of the volume fractions of ferrite
remaining at the maximum heating temperature and ferrite newly generated in
a ferrite transforrhation temperature region.
Bainitic ferrite is an aggregation of lath-shaped crystal grains which
contains no iron-based carbide with a major axis of 20 nm or more in the
inside of the lath.
Bainite is an aggregation of lath-shaped crystal grains which has
plural iron-based carbides with a major axisof 20 nm or more in the inside of
the lath, and these carbides further belong to a single variant, that is,
iron-based carbide group extending in the same direction. Here, the
iron-based carbide group extending in the same direction means ones having a
difference of 5" or less in stretch direction of the iron-based carbide group.
Tempered martensite is an aggregation of lath-shaped crystal grains
which has plural iron-based carbides with a major axis of 20 nrn or more in
the inside of the lath, and these carbides further belong to plural variants, that
is, plural iron-based carbide groups extending in different directions.
Note that bainite and tempered martensite can be distinguished easily
by observing iron-based carbides in lath-shaped crystal grains by using the
FE-SEM and checking stretch directions thereof.
[0060] Further, fresh martensite and retained austenite are not corroded
sufficiently by nital etching. Therefore, they are distinguished clearly from
the above-described structures (ferrite, bainitic ferrite, bainite, tempered
martensite) in observation with the FE-SEM.
Therefore, the volume fraction of fresh martensite is obtained as a
difference between the area fraction of a non-corroded region observed with
the FE-SEM and the area fraction of retained austenite measured with X-rays.
[006 11 (Kurtosis K* of hardness distribution)
In the high-strength galvanized steel sheet of the present invention,
kurtosis K* in the hardness distribution of a predetennined range of the baseL
steel sheet is -0.30 or less. Here, the hardness distribution in the
high-strength galvanized steel sheet of the present invention is defined as
follows. Specifically, plural measurement regions with a diameter of 1 pm
or less are set in the range of 118 thickness to 318 thickness of the base steel
sheet, and hardness in the plural measurement regions is measured. Then,
measurement values of the respective measurement regions are arranged in an
ascending order to obtain the hardness distribution. Then, an integer N0.02
is obtained, which is a number obtained by multiplying a total number of
measurement values of hardness by 0.02 and rounding up this number when it
I
includes a fraction. Then, hardness of a measurement value which is the
~0.02-thla rgest from a measurement value of minimum hardness is taken as
I 5 2% hardness. Further, an integer N0.98 is obtained, which is a number,
I obtained by multiplying a total number of measurement values of hardness by
I 0.98 and.rounding down this number when it includes a fraction. - Then,
I hardness of a measurement value which is the N0.98-th largest from a
measurement value of minimum hardness is taken as 98% hardness. Then,
10 in the high-strength galvanized steel sheet of the present invention, the
kurtosis K* in the hardness distribution between the 2% hardness and the
98% hardness is set in the range below -0.30 or less.
I Specifically, for example, when measurement regions with a diameter
of 1 pm or less are set at 1000 positions in the range of 118 thickness to 318
15 thickness of the base steel sheet, and hardness is measured in measurement
I . regions at these 1000 points, "the total number of measurement values of
I hardness" is 1000. Then, the hardness distribution can be obtained by
I arranging the measurement values of hardness measured in the respective
I measurement regions at these 1000 points in an ascending order.
In this case, a number resulting from multiplying the total number of
measurement values of hardness (that is, 1000) by 0.02 (= 20) is the "integer
N0.02". Then, in the obtained hardness distribution, the hardness of the
N0.02-th (that is, 20th) largest measurement value from the measurement
value of minimum hardness is 2% hardness.
Further, similarly, a number resulting from multiplying the total
number of measurement values of hardness (that is, 1000) by 0.98 (= 980) is
the "integer N0.98". Then, in the obtained hardness distribution, the
hardness of the N0.98-th (that is, 980th) largest measurement value from the
measurement value of minimum hardness is 98% hardness.
Note that although the case where the total number of measurement
5 values of hardness is 1000 has been described, when the total number of
measurement values of hardness is 2000 (that is, when hardness is measured
at 2000 points), the "integer N0.02" is 40 and the "integer N0.98" is 1960.
Then, the hardness of the 40-th largest measurement value from the
measurement value of minimum hardness is 2% hardness, and the hardness of
the 1960-th largest measurement value is 98% hardness.
Further, when the "integer N0.02" is obtained by the above-described
procedure, if the number obtained by multiplying by 0.02 includes a fraction,
a number obtained by rounding up after the decimal point is the "integer
N0.02". Similarly, when the "integer N0.98" is obtained, if the number
obtained by multiplying by 0.98 includes a fraction, a number obtained by
rounding up after the decimal point is the "integer N0.98". - - -
[0062] Here, the "hardness" used for hardness distribution in the present
invention means a measurement value measured by the following method.
Specifically, a dynamic micro hardness tester having a Berkovich type
20 triangular pyramid indenter is used to measure hardness by push-in depth
measurement method with a push-in load of 1 g weight. Note that the
measurement position of hardness is in the range of 118 thickness to 318
thickness of the base steel sheet with 114 of the sheet thickness being the
center in the thicknesswise cross section in parallel with the rolling direction
25 of the base steel sheet. Further, the total number of measurement values of
hardness is in the range of 100 to 10000, preferably 1000 or more.
invention, the above-described kurtosis K* of hardness distribution between
2% hardness and 98% hardness is -0.30 or less, and there is a small dispersion
in distribution of hardness in the base steel sheet. Therefore, a boundary
5 where regions which differ largely in hardness are in contact with each other
decreases, and excellent bendability can be obtained. To obtain more I
excellent bendability, the kurtosis K* is preferably -0.40 or less, more
preferably -0.50 or less. Although effects of the present invention are
exhibited without particularly setting the lower limit of the kurtosis K*, it is
10 difficult from experiences to make K* be -1.20 or less, which is hence set as I
the lower limit. However, in the high-strength galvanized steel sheet of the I
present invention, this kurtosis K* may be more than -0.40, and for example,
may be about -0.35 to -0.38.
[0064] Note that the kurtosis K* is a number obtained with the following
15 equation from data of measurement values of hardness in plural measurement
regions, and is. a value evaluated by comparing a frequency distribution of . I
data with a normal distribution. When the kurtosis becomes a negative I
value, it represents that a frequency distribution curve of data is relatively flat I
and means that thelarger the absolute value thereof, the more it deviates fi-om
20 the normal distribution.
[0065] [Equation 11
I COO661 Note that in the above equation, Hi indicates hardness of the i-th
largest measurement point from the measurement value of minimum
25 hardness, H* indicates average hardness from the N0.02-th largest
. -
measurement point to the N0.98-th largest measurement point from the
minimum hardness, and s* indicates a standard deviation from the N0.02-th
largest measurement point to the N0.98-th largest measurement point from the
minimum hardness.
5 [0067] (Ratio of Vickers hardness between surface layer and 114
thickness of the base steel sheet)
Further, in the high-strength galvanized steel sheet of the present
invention, a ratio between Vickers hardness of surface layer of the base steel
sheet and Vickers hardness of 114 thickness of the base steel sheet "(Vickers
10 hardness of surface layer)/(Vickers hardness of 114 thickness)" is 0.35 to 0.70.
Note that in the present invention, the "Vickers hardness of surface layer of
the base steel sheet" means the Vickers hardness at the point entering the base
steel sheet side by 10 ym from the interface between a surface of the base
steel sheet and the alloyed galvanized layer.
15 [0068] The Vickers hardness of surface layer of the base steel sheet and
Vickers hardness of 114 thickness of the base steel sheet can be measured by a
method which will be described below. Specifically, Vickers hardness is
measured at five points separated by 1 mm or more fiom each other in the
rolling direction of the base steel sheet at each of the point entering the base
20 steel sheet side by 10 ym from the interface between the surface of the base
steel sheet and the alloyed galvanized layer and the point of 114 thickness of
the base steel sheet, the maximum value and the minimum value are
discarded, and the average value of remaining three positions is employed.
In the measurement of Vickers hardness, the load is 100 gf.
25 [0069] In the high-strength galvanized steel sheet of the present
invention, since the ratio between the Vickers hardness of surface layer of the
base steel sheet and Vickers hardness of 114 thickness of the base steel sheet is
in the above-described range, the Vickers hardness of surface layer of the base
steel sheet is sufficiently low as compared to the Vickers hardness of 114
thickness, and the surface layer of the base steel sheet has a microstructure
5 excellent in ductility. Accordingly, necking on the base steel sheet side in
the interface between the surface of the base steel sheet and the alloyed
galvanized layer in the case where bending of the high-strength galvanized
steel sheet is performed is prevented, and necking in the interface between the
surface of the base steel sheet and the alloyed galvanized layer does not easily
10 occur.
[0070] When the ratio between the Vickers hardness of surface layer of
the base steel sheet and the Vickers hardness of 114 thickness of the base steel
sheet exceeds 0.70, the surface layer of the base steel sheet is hard and
necking in the surface of the base steel sheet cannot be prevented sufficiently,
15 which hence results in insufficient bendability. To obtain more excellent
bendability, the ratio between the,Vickers hardness of surface layer of the base
steel sheet and the Vickers hardness of 114 thickness of the base steel sheet is
preferably 0.60 or less. Further, when the ratio between the Vickers
hardness of surface layer of the base steel sheet and the Vickers hardness of
20 114 thickness of the base steel sheet is less than 0.35, stretch flangeability
deteriorates. To obtain good stretch flangeability, the ratio between the
Vickers hardness of surface layer of the base steel sheet and the Vickers
hardness of 114 thickness of the base steel sheet is preferably 0.38 or more.
LO07 11 (Alloyed galvanized layer)
25 On the high-strength galvanized steel sheet of the present invention,
an alloyed galvanized layer is formed on the surface of the base steel sheet.
A main body of the alloyed galvanized layer is an Fe-Zn alloy formed by
diffusion of Fe in steel in the zinc plating by alloying reaction, and the content
of iron in the alloyed galvanized layer is 8 to 12% in mass%. In the present
invention, since the content of iron in the alloyed galvanized layer is 8 to
12%, destruction and peeling of the alloyed galvanized layer can be prevented
sufficiently when bending is performed on the high-strength galvanized steel
sheet. The content of iron in the alloyed galvanized layer is 8.0% or more
for ensuring good flaking resistance, and is preferably 9.0% or more.
Further, the content of iron in the alloyed galvanized layer is 12.0% or less for
ensuring good powdering resistance, and is preferably 11.0% or less.
Further, the alloyed galvanized layer may contain A1 as impurity.
[0072] The alloyed galvanized layer may contain one or more of Pb, Sb,
Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I, Cs, REM, or they
may be mixed therein. When the alloyed galvanized layer contains one or
more of the above elements or have them mixed in, effects of the present
invention are not impaired, and there may even be cases where they improve
corrosion resistance, workability, and/or the like depending on their contents,
and hence are preferable.
[0073] The coating weight of the alloyed galvanized layer is not
particularly limited, but it is desirably 20 g/m2 or more in view of corrosion
resistance and 150 g/m2 or less in view of economy. Further, an average
thickness of the alloyed galvanized layer is 1.0 pm or more and 50 pm or less.
When it is less than 1.0 pm, sufficient corrosion resistance cannot be
obtained. Preferably, it is 2.0 pm or more. On the other hand, when it is
more than 50.0 pm, strength of the steel sheet is impaired, which is hence not
preferable. In view of raw material costs, the thickness of the alloyed
galvanized layer is thinner the better, preferably 30.0 pm or less.
[0074] Moreover, either or both of a coating film constituted of a
. .
phosphorus oxide and a coating film constituted of a composite oxide
containing phosphorus may be formed on the surface of the alloyed
5 galvanized layer.
[0075] (Manufacturing method)
Next, a method of manufacturing the high-strength galvanized steel
sheet of the present invention will be described in detail.
To manufacture the high-strength galvanized steel sheet of the present
10 invention, first, a steel sheet to be the base steel sheet is manufactured. To
manufacture the steel sheet, first, a slab having the above-described chemical
components (composition) is cast. Then, a hot-rolling step is performed,
including heating to 1050°C or more, completing hot rolling at a finish
hot-rolling temperature of 880°C or more, and coiling in a temperature region
15 of 750°C or less.
- - [0076] (Hot-rolling step) --
As the slab to be subjected to the hot-rolling step, a continuously cast
slab or a slab produced by a thin slab caster or the like can be used. The
manufacturing method of the high-strength galvanized steel sheet of the
20 present invention is compatible with a process like continuous casting-direct
rolling (CC-DR) in which hot rolling is performed immediately after casting.
[0077] In the hot-rolling step, the slab heating temperature needs to be
1050°C or more. When the slab heating temperature is excessively low, the
finish rolling temperature becomes lower than an AT3 transformation point.
25 and rolling with a two-phase region of ferrite and austenite is performed.
Thus, a duplex grain structure with a heterogeneous hot-rolling structure is
generated, and a heterogeneous structure will not be resolved even after
undergoing cold-rolling step and continuous annealing step, resulting in a
base steel sheet with poor ductility and bendability. Further, decrease in slab
heating temperature leads to excessive increase in rolling load, and there are
5 concerns of dificulty in rolling, causing a defective shape of the base steel
sheet after rolling, and the like. Although effects of the present invention are
exhibited without particularly setting the upper limit of the slab heating
temperature, setting an excessively high heating temperature is not preferable
in view of economy, and thus the upper limit of the slab heating temperature
10 is desirably 1350°C or less.
[0078] Note that the Ar3 transformation point is calculated with the
following formula.
AT3 =901 -325 x C + 33 x Si - 92 x (Mn +Nil2 + Cr12 + Cu12 +
Mo12) + 52 x A1
15 In this formula, C, Si, Mn, Ni, Cr, Cu, Mo, A1 represent the contents
of respective elements [mass%]. When an element is not contained, it is
calculated as 0.
[0079] Further, the finish hot-rolling temperature needs to be 880°C or
more. When the finish hot-rolling temperature is less than 880°C, the rolling
20 load during the ,finishing rolling becomes high, and there are concerns of
making the hot rolling difficult, causing a defective shape of the hot-rolled
steel sheet to be obtained after hot rolling, and the like. Further, the finish
hot-rolling temperature of hot rolling is preferably equal to or more than the
Ar; transformation point. When the finish hot-rolling temperature is less
25 than the Ar3 transformation point, the hot rolling becomes two-phase rolling
of ferrite and austenite, and the structure of the hot-rolled steel sheet may
I
become a heterogeneous duplex grain structure.
On the other hand, although effects of the present invention are
exhibited without particularly setting the upper limit of the finish hot-rolling
temperature, when an excessively high finish hot-rolling temperature is set,
5 the slab heating temperature must be set excessively high in order to ensure
this temperature. Thus, the upper limit of the finish hot-rolling temperature
is desirably 1000°C or less.
[OOSO] To prevent excessive increase in thickness of the oxide formed on
the surface of the hot-rolled steel sheet and deterioration of picklability
10 property, a coiling temperature of 750°C or less is set. To further increase
picklability, the coiling temperature is preferably 720" or less, more
preferably 700°C or less.
On the other hand, when the coiling temperature is less than 500°C,
I strength of the hot-rolled steel sheet increases excessively and makes cold
15 rolling difficult, and thus the coiling temperature is 500°C or more. To
reduce a cold-rolling load, the coiling temperature is preferably 530°C ormore,
more preferably 600°C or more.
[OOS 11 Next, preferably, pickling of the hot-rolled steel sheet
I manufactured thus is 'performed. The pickling- is to remove oxides- on
I 20 surfaces of the hot-rolled steel sheet, and hence is important for improving
platability of the base steel sheet. Further, the pickling may be once or may
be performed plural times separately.
[0082] (Cold-rolling step)
Although it is also possible to subject the hot-rolled steel sheet after
25 pickling as is to a continuous annealing step, a cold-rolling step may be
performed on the pickled hot-rolled steel sheet for the purpose of sheet
thickness adjustment and/or shape correction. When the cold-rolling step is
performed, a reduction ratio is preferably set in the range of 30 to 75% so as
to obtain a base steel sheet having an excellent shape with high sheet
thickness precision. When the reduction ratio is less than 30%, it is difficult
5 to keep its shape flat, possibly resulting in bad ductility of the final product.
The reduction ratio in the cold-rolling step is preferably 40% or more, more
preferably 45% or more. On the other hand, in cold rolling with a reduction
ratio of more than 75%, a cold-rolling load becomes too large and makes the
cold rolling difficult. Thus, the reduction ratio is preferably 75% or less.
10 In view of cold-rolling load, the reduction ratio is more preferably 70% or
less.
[0083] Note that in the cold-rolling step, effects of the present invention
are exhibited without particularly defining the number of times of cold-rolling
pass and a reduction ratio of each rolling pass.
15 [0084] (Continuous annealing step)
Next, a continuous annealing step is performed in which the hot-rolled
steel sheet obtained afier the hot-rolling step or the cold-rolled steel sheet
obtained afier the cold-rolling step is passed through a continuous annealing '
line. In the continuous annealing step of the present invention, the steel
20 sheet is heated in a temperature range between 600°C and Acl transformation
point at an average heating rate of 1°C/second or more. Then, the steel sheet
is retained for 20 seconds to 600 seconds at an annealing temperature between
(Aci transformation point + 40)"C and Ac3 transformation point and in an
atmosphere in which log(water partial pressurehydrogen partial pressure) is
25 -3.0 to 0.0, and bending-unbending deformation processing is applied two or
more times to the steel sheet by using a roll with a radius of 800 mm or less,
thereby performing a treatment so as to make a difference in accumulated
strain amount between a front and rear surface be 0.0050 or less. Thereafter,
the steel sheet is cooled in the temperature range of 740°C to 650°C at an
average cooling rate of 1.0 to 5.0°Clsecond.
5 [0085] In the present invention, by performing the continuous annealing
step, a distribution of C amount inside the hot-rolled steel sheet or the
cold-rolled steel sheet is controlled, hardness inside the cold-rolled steel sheet
is ensured, and meanwhile hardness of a surface layer is made moderately
low.
10 In the continuous annealing step, first, the hot-rolled steel sheet
obtained after the hot-rolling step or the cold-rolled steel sheet obtained after
the cold-rolling step is heated at the average heating rate of 1°C/second or
more in the temperature range between 600°C and Acl transformation point.
When the temperature of the steel sheet becomes 600" or more,
15 decarburization from the steel sheet begins. In the temperature range
between 600°C and Ac, transformation point, iron contained in the steel sheet
is the same bcc iron in both inside and surface. In the present invention, the
bcc iron is a generic name of ferrite, bainite, bainitic ferrite, and martensite
having a body-centered cubic lattice.
20 [0086] In the temperature range between 600°C and Ac, transformation
point, since all the iron contained in the steel sheet is bcc iron, not only
carbon in a surface layer of the steel sheet but also carbon in a center portion
of the steel sheet can escape easily from an outermost layer. When the
average heating rate in the temperature range between 600°C and Acl
25 transformation point is less than 1°C/second, it takes a long time for the steel
sheet to reach the Acl transformation point from 600°C, and thus there is a
possibility that the C amount escaping from the steel sheet in the temperature
range between 600°C and Acl transformation point becomes too large,
resulting in insufficient strength of the galvanized steel sheet. To ensure
strength of the galvanized steel sheet, the average heating rate in the
temperature range between 600°C and Acl transformation point is preferably
2"CIsecond or more. Although it would be no problem when the upper limit
of the average heating rate between 600°C and Acl transformation point is not
particularly defined, it is preferably 100°C/second or less in view of cost.
[0087] Thereafter, the steel sheet which have reached the Acl
transformation point is hrther heated, and the steel sheet is retained at an
annealing temperature between (Acl transformation point + 40)"C and Ac3
transformation point and in an atmosphere in which log(water partial
pressurehydrogen partial pressure) is -3.0 to 0.0 for 20 seconds to 600
seconds, and bending-unbending deformation processing is applied two or
more times to the steel sheet by using a roll with a radius of 800 mm or less,
thereby performing annealing so as to make a difference in accumulated strain
amount between a front and rear surface be 0.0050 or less.
[0088] In the temperature region (annealing temperature) between (Acl
transformation point + 40)"C and Ac3 transformation point, the steel sheet is
in a state that a microstructure in the surface layer of the steel sheet is bcc iron
and a microstructure in the center portion of the steel sheet is austenite. As
compared to bcc iron, more carbon can solid-dissolve in austenite.
Accordingly, carbon does not easily diffuse from austenite to bcc iron, but
easily defuses from bcc iron to the outside or to austenite. Therefore, at the
annealing temperature, the carbon in the center portion of the steel sheet
remains in the center portion, part of the carbon in the surface layer of the
steel sheet difises to the center portion, and the rest escapes from the
outermost layer. Thus, the steel sheet has, as a result, a distribution such that
the carbon amount in the center portion is larger than in the surface layer.
[0089] When the annealing temperature is less than (Acl transformation
point + 40)"C, carbon does not easily difise from bcc iron to the outside or
austenite, and the distribution of C amount in the steel sheet does not become
larger in the center portion than in the surface layer. Thus, the annealing
temperature is preferably (Acl transformation point + 50)"C or more, more
preferably (Acl transformation point + 40)"C or more. Further, when the
annealing temperature exceeds the Ac3 transformation point, the bcc iron
cannot exist, hardness of the surface layer is difficult to control, and the
volume fraction of retained austenite increases, thereby deteriorating
bendability. Therefore, the annealing temperature is preferably (Ac3 -
10)"C or less, more preferably (Ac3 - 19°C or less.
[0090] In the present invention, the atmosphere for performing annealing
is set so that log(water partial pressurelhydrogen partial pressure) is -3.0 to
0.0. By making the logarithm of the ratio between water partial pressure and
hydrogen partial pressure be -3.0 to 0.0, decarburization from the steel sheet
surface layer by performing annealing is facilitated moderately. When the
logarithm of the ratio between water partial pressure and hydrogen partial
pressure is less than -3.0, decarburization from the steel sheet surface layer by
performing annealing becomes insufficient. To facilitate decarburization, I
the logarithm of the ratio between water partial pressure and hydrogen partial
pressure is preferably -2.5 or more. - When the logarithm of the ratio between
water partial pressure and hydrogen partial pressure is more than 0.0,
decarburization fiom the steel sheet surface layer by perfonning annealing is
facilitated excessively, and it is possible that strength of the base steel sheet of
the galvanized steel sheet becomes insufficient. To ensure strength of the
base steel sheet, the logarithm of the ratio between water partial pressure and
t
hydrogen partial pressure is preferably -0.5 or less. Further, preferably, the
5 atmosphere for performing annealing includes nitrogen, water vapor, and
hydrogen and is mainly constituted of nitrogen, and oxygen may be contained
besides nitrogen, water vapor, and hydrogen.
[0091] In the present invention, retention time in the annealing
temperature and the atmosphere described above is 20 seconds to 600
10 seconds. When the retention time is less than 20 seconds, the amount of
carbon difising from bcc iron to the outside or austenite becomes
insufficient. To ensure the amount of carbon difising from bcc iron, the
retention time is preferably 35 seconds or more, more preferably 50 seconds
or more. Further, when the retention time exceeds 600 seconds, the amount
15 of carbon escaping from the outermost layer becomes large, and hardness of
the surface layer decreases excessively. To ensure hardness of the surface
layer, the retention time is preferably 450 seconds or less, more preferably
300 seconds or less.
[0092] When performing annealing, bending-unbending deformation
20 processing is performed two or more times by using a roll with a radius of
800 mm or less at the annealing temperature and in the above atmosphere, so
as to make a difference in accumulated strain amount between a front and rear
surface be 0.0050 or less. Through this bending-unbending deformation
processing, strain is introduced into the surface layer of a steel sheet to be the
25 base steel sheet, and the outermost layer is transformed into bcc iron
efficiently. In the present invention, since the difference in accumulated
strain amount between the front and rear surface is made to be 0.0050 or less,
bias in bendability between the front and- rear surface in the base steel sheet of
the finally obtained galvanized steel sheet becomes sufficiently small. On
the other hand, when the amount of strain introduced into the surface layer of
5 the steel sheet is biased to one of the front and rear surface and the difference
in accumulated strain amount between the front and rear surface exceeds
0.0050, a hardness distribution in the front and rear surface becomes
imbalanced, resulting in different bendability in the front and rear surface in
the base steel sheet of the finally obtained galvanized steel sheet, which is not
10 preferable. The difference in accumulated strain amount between the front
and rear surface of the steel sheet is smaller the better, preferably 0.0030 or
less.
LO0931 Further, although there is no particular upper limit of the number
of times of bending-unbending deformation processing, the shape of the steel
sheet cannot be maintained when the accumulated strain amount between the
front and rear surface of the steel sheet exceeds 0.100, and thus the
accumulated strain amount between the front and rear surface is preferably in
the range of 0.100 or less.
The roll used for the bending-unbending deformation processing has a
radius of 800 mm or less. By having the radius of the roll of 800 mm or
less, strain can be introduced easily into the surface layer of the steel sheet.
When the radius of the roll is larger than 800 mm, strain cannot be introduced
sufficiently into the surface of the steel sheet, the surface layer is not
transformed into bcc iron, and thus hardness of the surface layer does not
become sufficiently low.
[0094] In the bending-unbending deformation processing, bending is
performed plural times in which the amount of strain provided by one time of
bending on an outside of bending is limited in the range of 0.0007 or more to
0.091 or less by tensile strain. To allow sufficient phase transformation, the
amount of strain provided by one time of bending is preferably 0.0010 or
5 more on the outside of bending. When the amount of strain provided on the
outside of bending by one time of bending exceeds 0.091, the shape of the
steel sheet cannot be maintained. In view of this, the amount of strain
provided on the outside of bending by one time of bending is preferably 0.050
or iess, more preferably 0.025 or less.
10 [0095] Further, while ferrite transformation in the surface layer of the
steel sheet proceeds by the bending-unbending deformation processing in the
vicinity of the highest temperature of annealing, in the inside of the steel sheet
where strain is small, ferrite transformation is delayed and the ratio of
austenite increases, and there occurs a difference in hardness between the
15 surface layer and the inside (114 thickness). To make an effective difference
in hardness occur between the surface layer and the inside (114 thickness), the
sheet thickness of the steel sheet is desirably 0.6 mm or more and 5.0 mm or
less. When it is less than 0.6 mm, it is difficult to maintain the shape of the
steel sheet. When it is more than 5.0 mm, it is difficult to control
20 temperature of the steel sheet, and target characteristics cannot be obtained.
Further, when the roll diameter is more than 800 mm, sufficient strain cannot
be introduced into the surface layer of the steel sheet. Although the lower
limit of the roll diameter is not particularly set, 50 mm or more is preferable
because maintenance costs of equipment increase when a roll less than 50 mm
25 is used.
[0096] Next, the steel sheet after the bending-unbending deformation
42
processing is performed is cooled at the average cooling rate of 1.0 to
5.0°C/second in the temperature range of 740°C to 650°C. Thus, ferrite
which is bcc iron is generated in the microstructure in the center portion of
the steel sheet, and accompanying this, part of C is difised from the steel
5 sheet center portion to the surface layer portion. Thus, a concentration
difference in C amount between the center portion and the surface layer of the
steel sheet becomes small, and the distribution of C amount in the steel sheet
corresponds to the range of the ratio between Vickers hardness of-surface
layer and Vickers hardness of 114 thickness "(Vickers hardness of surface
I 10 layer)/(Vickers hardness of 114 thickness)" in the base steel sheet of the
I
I high-strength galvanized steel sheet of the present invention.
I
[0097] When the average cooling rate in the temperature range of 740°C
to 650°C is less than l.O°C/second, the retention time in the temperature
range of 740°C to 650°C becomes long and a large amount of ferrite is
15 generated. Thus, diffusion of C from the center portion of the steel sheet to
the surface layer portion is facilitated, and the-difference between hardness of
the center portion and hardness of the surface layer of the steel sheet becomes
insufiicient. Further, when the average cooling rate in the temperature range
of 740°C to 650°C exceeds 5.0°C/second, the amount of-ferrite generated in
20 the microstructure of the center portion of the steel sheet is insufficient, and
the concentration difference of the C amount between the center portion and
the surface layer of the steel sheet is too large.
[0098] Note that when the steel sheet is cooled at the average cooling rate
of 1.0 to 5.0°C/second in the temperature range of 740°C to 650°C after the
25 bending-unbending deformation processing is performed, preferably, it is in
an atmosphere in which log(water partial pressurehydrogen partial pressure)
is -3.0 or less. Thus, difision of C from the surface layer portion of the
steel sheet to the outside in the temperature range of 740°C to 650°C can be
stopped, C in the surface layer portion can be increased more efficiently, and
strength of the base steel sheet of the high-strength galvanized steel sheet can
be ensured.
[0099] Next, in this embodiment, in the temperature range of 650°C to
500°C, the steel sheet can be cooled at an average cooling rate of 5 to
200°C/second. By the steel sheet being cooled to a temperature range of
5 0 0 " ~or less, growth of ferrite in the microstructure of the center portion of
the steel sheet is stopped, and diffusion of C across a long distance between
the center portion and the surface layer portion of the steel sheet is stopped.
When the average cooling rate in the temperature range of 650°C to
500°C is less than 5"C/second, a large amount of perlite andlor iron-based
carbide is generated, and thus the retained austenite becomes insufficient. In
view of this, the average cooling rate is preferably 7.0"CIsecond or more,
more preferably 8.0°C/second or more. On the other- hand, although effects
of the present invention are exhibited without particularly setting the upper
limit of the average cooling rate in the temperature range of 650°C to 500°C,
special equipment is needed for making the average cooling rate' exceed
200°C, and thus the upper limit of the cooling rate is set to 200°C/second in
view of costs.
[0100] Next, in this embodiment, the steel sheet is preferably retained for
15 to 1000 seconds in the temperature range of 500°C to 400°C. Thus, the
steel sheet to be the base steel sheet obtains preferable amounts of retained
austenite, bainite, andlor bainitic ferrite. At 400°C or less, bainite
transformation proceeds excessively, C concentration to retained austenite
proceeds, and thus a large amount of retained austenite remains. Thus, it
becomes difficult to make the volume fraction of retained austenite to be 8%
or less. Further, when the retention time in the temperature range of 500°C
to 400°C exceeds 1000 seconds, coarse iron-based carbide, which works as a
5 starting point of destruction, is generated and grows, and thus bendability
deteriorates largely.
[O 10 11 (Plating alloying step)
Next, an alloying treatment is performed, including dipping the steel
sheet after the continuous annealing step in a galvanizing bath, and then
10 retaining at a temperature of 470 to 650°C for 10 to 120 seconds. Thus, the
high-strength galvanized steel sheet of the present invention is formed, which
contains Zn-Fe alloy in the surface of the base steel sheet and in which an
alloyed galvanized layer with an iron content of 8 to 12% is formed.
Note that normally, the larger the carbon content of the base steel
15 sheet, the lower the content of iron contained in the alloyed galvanized layer
-- and the lower the adhesion between the base steel sheet and the alloyed
galvanized layer. Further, in the present invention, in order to make a
high-strength galvanized steel sheet with maximum tensile strength of 900
MPa or more, a large amount of carbon which is an element that improves
20 strength is contained. However, in the present invention, since the carbon
concentration in the surface layer of the cold-rolled steel sheet to be the base
steel sheet obtained after the continuous annealing step is low, the alloyed
galvanized layer excellent in adhesion with an iron content of 8 to 12% is
formed in the plating alloying step.
25 [O102] The galvanizing bath is not particularly limited, effects of the
present invention are not impaired when one or more of Pb, Sb, Si, Sn, Mg,
Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I, Cs, REM is mixed in the
galvanizing bath, and there may even be cases where they improve corrosion
resistance, workability, and/or the like depending on their contents, and hence
are preferable. Further, A1 may be contained in the galvanizing bath. In
5 this case, the A1 concentration in the bath is preferably 0.05% or more and
0.15% or less.
Further, the temperature of the alloying treatment is preferably 480 to
560°C, and the retention time of the alloying treatment is preferably 15 to 60
seconds.
10 [0103] In this embodiment, after the alloying treatment, retaining at a
I temperature of 200 to 350°C for 30 to 1000 seconds is preferable. This
I
I makes the base steel sheet structure of the high-strength galvanized steel sheet
contain tempered martensite. As a result, the base steel sheet structure of the
high-strength galvanized steel sheet has retained austenite, ferrite, bainite
15 andlor bainitic ferrite. and tempered martensite, and by having such base steel
sheet-structure, it becomes a steel sheet in which the above-described kurtosis
K* of hardness distribution is -0.30 or less.
[I01041 Note that instead of retaining at a temperature of 200 to 350°C for
30 to 10@0 seconds after the alloying treatment, the steel sheet after the-
20 alloying treatment may be cooled to 350°C or less to generate martensite, and
thereafter, it may be reheated to the temperature range of 350" or more and
550°C or less and retained for two seconds or more to generate tempered
martensite. Further, tempered martensite is generated in the base steel sheet
structure also by further cooling the steel sheet, which has been cooled to the
25 temperature region of 500°C or less in the continuous annealing step, to
350°C or less to generate martensite and then reheating it, and retaining at
400 to 500°C.
[0105] Moreover, in this embodiment, it is also possible to perform cold
rolling with a reduction ratio of 0.05 to 3.00% for shape correction on the
high-strength galvanized steel sheet cooled to room temperature.
[0 1061 Note that the present invention is not limited to the
above-described examples.
For example, in the present invention, it is also possible to add a
coating film constituted of a P oxide and/or a composite oxide containing P on
the surface of the alloyed gal;anized layer of the galvanized steel sheet
obtained by the above-described method.
A coating film constituted of a phosphorus oxide andlor a composite
oxide containing phosphorous can function as a lubricant when a steel sheet is
processed, and can protect the alloyed galvanized layer formed on the surface
of the base steel sheet.
15 EXAMPLES
[0107] The present invention will- be described in further detail using - - -
examples. I
Slabs having chemical components (composition) of A to Z, AA to AC
illustrated in Table 1 and Table 2, and BA to BF illustrated in Table 3 were
20 cast, hot rolled under the conditions (slab heating temperature, finish
hot-rolling temperature) illustrated in Table 4 to Table 7 just after casting,
cooled, coiled at temperatures illustrated in Table 4 to Table 7, and subjected
to pickling. Experimental examples 4, 10, 16, 22,49, 54, 102, 106 were just
hot rolled (no cold rolling), and other experimental examples were cold rolled
25 under the conditions (reduction ratios) illustrated in Table 3 to Table 5.
Then, a continuous annealing step and a plating alloying step were performed
under the conditions illustrated in Table 8 to Table 11 on respective steel
sheets of experimental examples 1 to 109,201 to 2 18.
[0108] [Tablel]

[Olll] [Table41
36
37
38
39
G 1 1270 753
753
753
753
G
G
G
929 1 672 1 41 EXAMPLE
938 f 591 1 47 EXAMPLE
B42 1 654
936 [ 658 f
1235
1245
1250
[O 1 121 [Table 51

[O 1 1 51 [Table 81
[O 1 1 61 [Table 91

[O 1 191 The Acl transformation point and the Ac3 transformation point in
Table 8 to Table 11 were obtained by cutting out a small piece from the steel
sheets treated under the conditions of Tables 4 to 7 before performing
annealing processing, and measuring a cubical expansion curve thereof when
heated by 10°C/second.
In annealing, a decarburization treatment was performed including
passing in the temperature range between 600°C and Acl - A*. cl transformation point
at an average heating rate described in Table 8 to Table 11, heating to a
maximum heating temperature (annealing temperature) described in Table 8
to Table 118, and retaining for a retention time (retention time in the
continuous annealing step) described in Table 8 to Table 11 in an atmosphere
mainly constituted of nitrogen in which water partial pressure and hydrogen
partial pressure (log(PH20/PH2) is controlled under the conditions described
in Table 8 to Table 11.
[0120] In the decarburization treatment (in the continuous annealing
step), in experimental examples 1 to 12 and experimental examples 16 to 29,
a roll with a radius of 450 mm was used and bending-unbending deformation
processing was performed 6 times in total. In experimental examples 13 to
15, a roll with a radius of 450 mm was used and bending-unbending
deformation processing was performed 7 times in total. In experimental
examples 30 to 44, a roll with a radius of 730 mm was used and
bending-unbending deformation processing was performed 4 times in total.
In experimental examples 45 to 48, experimental examples 55 to 69, and
experimental examples 73 to 109, a roll with a radius of 600 mm was used
and bending-unbending deformation processing was performed 6 times in
total. In experimental examples 49 to 54 and experimental examples 70 to
72, a roll with a radius of 780 mm was used and bending-unbending
deformation processing was performed 6 times in total.
On the other hand, in experimental examples 201 to 218,
bending-unbending deformation processing was performed plural times (2 to
12 times) by the number of times of bending-unbending deformation
processing illustrated in Table 1 1. Further, in experimental examples 20 1 to
21 8, the radius of the roll for performing the bending-unbending deformation
processing was varied. Minimum roll radii (rnrn) and average roll radii
(mm) of the rolls used for the respective bending-unbending deformation
processing performed in experimental examples 20 1 to 2 18 are illustrated in
Table 11. Further, in the bending-unbending deformation processing, among
total strain amounts introduced respectively into a front surface and a rear
surface of the steel sheet, a larger strain amount is illustrated as a maximum
total strain. Further, in experimental examples 201 to 218, the sheet
thickness of the steel sheet was varied from 0.70 to 8.00 mm.
A& described in Table 8 to Table 11 indicates the absolute value of a
difference in strain amounts introduced by performing the bending-unbending
deformation processing, which are calculated for each of the front and rear
surface of the steel sheet.
[0121] Thereafter, cooling at an average cooling rate illustrated in Table 8
to Table 11 in the temperature range of 740°C to 650°C was performed, and
cooling at an average cooling rate illustrated in Table 8 to Table 11 in the
temperature range of 650°C to 500°C was performed. Note that in
experimental examples 47 and 52, when the steel sheet was cooled in the
temperature range of 740°C to 650°C, the atmosphere in a cooling bath was
set so that log(water partial pressure/hydrogen partial pressure) = -4.0.
[0122] Next, the steel sheet after cooling was retained for a retention time
(retention time between the continuous annealing step and an alloying
treatment) described in Table 8 to Table 11 in the temperature range of 500 to
400°C. Thereafter, an alloying treatment was performed including dipping
5 the steel sheet in a galvanizing bath and retaining for a retention time
described in Table 8 to Table 11 at the temperature described in Table 8 to
Table .. 1- 1 .
After the alloying treatment, the steel sheet was retained in the
temperature range of 200 to 350°C for a retention time described in Table 8 to
10 Table 11 (retention time of alloying treatment).
[0123] After cooling to room temperature, cold rolling with a reduction
ratio of 0.15% was performed in experimental examples 7 to 34, cold rolling
with a reduction ratio of 1.50% was performed in experimental example 53,
cold rolling with a reduction ratio of 1.00% was performed in experimental
15 example 54, and cold rolling with a reduction ratio of 0.25% was performed
in conditions 6 1 .to 100.
Thereafter, in experimental examples 9 and 49, a coating film
constituted of composite oxide containing P in the surface layer of the
galvanized steel sheet was added.
20 lo1241 Experimental examples 9 and 49 are examples in which a coating
film constituted of composite oxide containing P in the surface layer of the
alloyed hot-dip galvanized steel sheet was added, and a high-strength alloyed
hot-dip galvanized steel sheet excellent in formability can be obtained.
[0125] -Microstructures in the range of 118 thickness to 318 thickness in
25 the steel sheets of experimental examples 1 to 109 and 201 to 218 were
observed and volume fractions were measured. Results thereof are
illustrated in Table 12 to Table 15. In Table 12 to Table 15, "F" means
ferrite, "B" means bainite, "BF" means bainitic ferrite, "TM" means tempered
martensite, "M" means fresh martensite, and "retained y" means retained
austenite.
[0126] Among the microstructure fractions, the amount of retained
austenite was measured by X-ray diffraction, and others were obtained by
nital etching a cross section obtained by cutting out and mirror polishing a --
thicknesswise cross section in parallel with the rolling direction of the steel
sheet, and observing the cross section using a field emission scanning electron
microscope (FE-SEM).
Further, the content of iron in 112 thickness of the alloyed galvanized
layer was measured using EDX. Results thereof are illustrated in Table 12 to
Table 1 5.

[0128] [Table 131
I] [Table 141
-90 1 V 55 0 118120 0 f 5 2 8.4 EXAMPLE
91 1 w 46 17)Il 12 0 1 10.7 EXAMPLE
96 I X 1 7 3 1 2 I 3 11 0 0 1 . 10.8 W ~ I P L E ~
97 1 X 53 2510 17 3 2 0 10.8 EXAMPLE
1 98 1 X 61 23 1113 1 0 1 10,8 -
99 Y 25,,t6 331251 1 0 0 9.6 EXAFilPLE
1 I 100 Y . 3 8 [ 1 4 2 2 ( T 9 / 2 j 5 0 g. 5 ~XAMPLE
[O 13 11 Hardness of experimental examples 1 to 109 and 20 1 to 2 18 was
measured by a method described below. Results thereof are illustrated in
Table 16 to Table 19.
Regarding hardness of the surface layer and 114 thickness of the base
steel sheet, Vickers hardness was measured at five points, which are separated
by 1 mm or more from each other in the rolling direction, the maximum value
and the minimum value were discarded, and the average value of remaining
three positions was employed. In the measurement of Vickers hardness, the
load was 100 gf. Note that the Vickers hardness of the surface layer was
measured on a line entering the base steel sheet side by 40 pm from the
interface between the alloyed galvanized layer and the base steel sheet.
Kurtosis K* of hardness distribution was calculated using results of
measuring hardness by push-in depth measurement method with a push-in
load of 1 g weight by using a dynamic micro hardness tester having a
Berkovich type triangular pyramid indenter. Note that the measurement
position of hardness was in the range of 1/8 thickness to 318 thickness with
114 of the sheet thickness being the center in the thicknesswise cross section
perpendicular to the sheet surface of the steel sheet and in parallel with the -
rolling direction (rolling reduction direction). Further, the total number of
measurement values of hardness was set to 1000.
5
[O 1321 [Table 161

[O 1 341 [Table 1 81
[O 1361 Table 20 to Table 23 illustrate results of evaluating characteristics
of the steel sheets of experimental examples 1 to 109 and 201 to 21 8 by a
method described below.
5 Tensile test pieces according to JIS Z 2201 were sampled from the
steel sheets of experimental examples 1 to 109 and 201 to 2 18, a tensile test
was performed according to JIS Z 2241, and yield stress "YS", tensile
strength "TS", and total elongation "EL" were measured.
Further, a hole expansion test (JFST100 1) for evaluating flangeability
L -
10 was performed, and a hole expansion limit value "h" as an index of stretch
flangeability was calculated.
Further, a 90-degree V bending test was performed. A test piece of
35 mm x 100 mm was cut out from the steel sheets of experimental examples
1 to 109, a shear cut surface was mechanically polished, and a bend radius
. -
was set to double the sheet thickness, to thereby perform evaluation. Then,
one that became a predetermined shape was evaluated as passed (O), and one
5 that did not become the predetermined shape was evaluated as failed (X).
Further, at the time of the bending test, presence of crack, necking, and
plating peeling was evaluated separately by visual observation, and one . - * t - . .
having none of them was evaluated as passed (O), and one having any of
them was evaluated as (X).
10

[O 13 81 [Table 2 11

[O 1401 [Table 231
I
[0141] As illustrated in Table 20 to Table 23, the tensile strength was 900
MPa or more and the result of bending test was 0 in all the experimental
examples which are examples of the present invention among experimental
5 examples 1 to 109 and 201 to 2 18.
On the other hand, in the experimental examples which are
comparative examples among experimental examples 1 to 109 and 201 to
218, the tensile strength was less than 900 MPa or X was included in results
of bending test, and they did not satisfy the excellence in both high strength
t 0 and bendability.
[0142] In experimental example 107, the added amount of C is small and
a hard stl-ucture cannot be obtained, and thus strength is inferior.
In experimental example 108, the added amount of Si is small,
solid-solution strengthening of soft structure is insufficient, surface hardness
of the steel sheet softens largely compared to its inside, and thus stretch
flangeability and strength are inferior.
In experimental example 109, the added amount of Mn is small, the
volume fraction of retained austenite which becomes a starting point of
5 destruction is large, and thus stretch flangeability and bendability are inferior.
[0143] Experimental example 94 is an example in which completion
temperature of hot rolling is low, the microstructure extends in one direction
and is heterogeneous, and thus ductility, stretch flangeability, and bendability
are inferior.
10 Experimental example 98 is an example in which temperature for
coiling on a coil is high after hot rolling, the microstructure becomes quite
coarse, and thus ductility, stretch flangeability, and bendability are inferior.
Experimental example 6 is an example in which the heating rate in the
annealing step is slow, decarburization in the steel sheet proceeds, hardness of
15 the surface layer decreases largely, and thus stretch flangeability and
bendability are inferior.
[O144] Experimental example 11 is an example in which the maximum
heating temperature in the annealing step is low, many coarse iron-based
carbides which become a starting point of destruction are contained, and thus
20 ductility, stretch flangeability, and bendability are inferior.
On the other hand, experimental example 12 is an example in which
the maximum heating temperature in the annealing step is high, the volume
fraction of retained austenite which becomes a starting point of destruction is
large, and thus stretch flangeability and bendability are inferior.
25 [0145] Experimental example 17 is an example in which retention time in
the decarburization treatment temperature region is short, hardness of the
surface layer is excessively high, and thus bendability is inferior.
On the other hand, experimental example 18 is an example in which
retention time in the decarburization treatment temperature region is long,
hardness of the surface layer decreased excessively, and thus stretch
5 flangeability and bendability are inferior.
[0146] Experimental example 23 is an example in which water vapor
. . partial pressure in the atmosphere in the . -- decarburization treatment .?..
temperature region is high, hardness of the surface layer decreases
excessively, and thus bendability is inferior.
10 On the other hand, experimental example 24 is an example in which
water vapor partial pressure in the atmosphere in the decarburization
treatment temperature region is low, hardness of the surface layer is
excessively high, and thus bendability is inferior.
[0147] Experimental examples 28, 29 are examples in which there is a
15 large difference AE in total strain amounts which are introduced respectively
into the front surface and the rear. surface in the decarburization treatment
temperature region, and bendability is inferior.
Experimental example 33 is an example in which the average cooling
rate of 740°C to 650°C is low, the kurtosis in hardness distribution inside the
20 steel sheet is large, and thus stretch flangeability and bendability are inferior.
On the other hand, experimental example 34 is an example in which
the average cooling rate of 740°C to 650°C is high, the kurtosis in hardness
distribution inside the steel sheet is large, and thus ductility and bendability
are inferior.
25 [0148] Experimental example 5 is an example in which the average
cooling rate of 650°C to 500°C is low, a hardness difference between the steel
sheet surface layer and the inside is small, many iron-based carbides are also
generated, and bendability is inferior.
Experimental example 38 is an example in which alloying treatment
temperature of the plating layer is high, Fe% in the plating layer is excessive,
5 coarse iron-based carbides which become a starting point of destruction are
also generated inside the steel sheet, and thus ductility, stretch flangeability,
and bendability are inferior.
< - . ,-
On the other hand, experimental example 39 is an example in which
alloying treatment temperature of the plating layer is low, Fe% in the plating
10 layer is insuficient, and thus bendability is inferior.
[0 1491 Experimental example 43 is an example in which alloying
treatment time of the plating layer is short, Fe% in the plating layer is
insufficient, and bendability is inferior.
On the other hand, experimental example 44 is an example in which
15 alloying treatment time of the plating layer is long, coarse iron-based carbides
which become a starting point of destruction are generated inside the steel
sheet, and thus ductility, stretch flangeability, and bendability are inferior.
Experimental example 203 is an example in which the sheet thickness
of the steel sheet is significantly thin, flatness of the steel sheet cannot be
20 maintained, and it was not possible to perform the predetermined
characteristic evaluation test.
Experimental example 206 is an example in which there is a large
difference AE in total strain amounts which are introduced respectively into
the front surface and the rear surface, and bendability is inferior.
2 5 In experimental examples 209 and 218, the amount of strain
introduced in one bending is small, hardness of the surface layer is
excessively hard, and hence bendability is inferior.
In experimental examples 212 and 215, the amount of strain
introduced in one bending is large, the shape of the steel sheet is impaired,
sufficient flatness is not obtained, and it was not possible to perform the
5 predetermined characteristic evaluation test.

Fame of Document] What is claimed is
[Claim 11 A high-strength galvanized steel sheet excellent in bendability
with maximum tensile strength of 900 MPa or more, comprising an alloyed
galvanized layer formed on a surface of a base steel sheet containing, in
mass%,
Si: 0.30 to 2.50%,
.< .
Mn: 1.30 to 3.50%,
P: 0.001 to 0.050%,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.500%,
N: 0.0001 to 0.0100%, and
0: 0.0001 to 0.0100% with a balance being constituted of iron and
inevitable impurities, wherein:
retained austenite is limited to 8% or less in volume fraction in a range
of 118 thickness to 318 thickness of the base steel sheet;
when plural measurement regions with a diameter of 1 pm or less are
set in the range of 118 thickness to 318 thickness of the basc steel sheet,
measurement values of hardness in the plural measurement regions are
arranged in an ascending order to obtain a hardness distribution, an integer
N0.02 is obtained, which is a number obtained by multiplying a total number
of measurement values of hardness by 0.02 and rounding up this number
when this number includes a fraction, hardness of a measurement value which
is N0.02-th largest from a measurement value of minimum hardness is taken
as 2% hardness, an integer N0.98 is obtained, which is a number obtained by
multiplying a total number of measurement values of hardness by 0.98 and
rounding down this number when this number includes a fraction, and
hardness of a measurement value which is N0.98-th largest from a
measurement value of minimum hardness is taken as 98% hardness, kurtosis
K* of the hardness distribution between the 2% hardness and the 98%
5 hardness is -0.30 or less;
a ratio between Vickers hardness of surface layer of the base steel
sheet and Vickers hardness of 114 thickness of the base steel sheet is . .- 0.35 to
0.70; and
a content of iron in the alloyed galvanized layer is 8 to 12% in mass%.
lo [Claim 21 The high-strength galvanized steel sheet excellent in I
bendability according to claim 1, wherein the structure of the base steel sheet
contains, in volume fraction, 10 to 75% ferrite, 10 to 50% in total of either or
both of bainitic ferrite and bainite, 10 to 50% tempered martensite in the
range of 118 thickness to 318 thickness of the base steel sheet, the fresh
15 martensite is limited to 15% or less in volume fraction, and perlite is limited I
to 5% or less in volume fraction.
[Claim 31 The high-strength galvanized steel sheet excellent in
bendability according to claim 1, wherein the base steel sheet fkrther contains,
in mass%, one or both of
20 Ti: 0.005. to 0.150%, and
Nb: 0.005 to 0.150%.
[Claim 41 The high-strength galvanized steel sheet excellent in
bendability according to claim 1, wherein the base steel sheet fkrther contains,
in mass%, one or more of
25 B: 0.000 1 to 0.0 loo%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
- .
Mo: 0.01 to 1.00%, and
W: 0.01 to 1.00%.
[Claim 51 The high-strength galvanized steel sheet excellent in
bendability according to claim 1, wherein the base steel sheet further contains,
in mass%,
V: 0.005 to 0.150%.
[Claim 61 The high-strength galvanized steel sheet excellent in
bendability according to claim 1, wherein the base steel sheet further contains,
0.0001 to 0.5000 mass% in total of one or more of
Ca, Ce, Mg, Zr, Hf, and REM.
[Claim 71 The high-strength galvanized steel sheet excellent in
bendability according to claim 1, wherein either or both of a coating film
constituted of a phosphorus oxide and a coating film constituted of a
composite oxide containing phosphorus is or are formed on a surface of the
alloyed galvanized layer.
[Claim 81 A manufacturing method of a high-strength galvanized steel
sheet excellent in bendability, the method comprising:
a hot-rolling step of heating to 1050°C or more a slab containing, in
mass%,
C: 0.075 to 0.300%,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.050%,
S: 0.000~to 0.0100%,
Al: 0.005 to 1.500%,
N: 0.0001 to 0.0100%, and
0: 0.0001 to 0.0100% with a balance being constituted of iron and
inevitable impurities, completing hot rolling at a finish hot-rolling
temperature of 880°C or more, and coiling in a temperature region of 750°C
or less;
a continuous annealing step of heating the steel sheet in a temperature
% .
range between 600°C and Acl transformation point at an average heating rate
of 1°C or more, retaining the steel sheet for 20 seconds to 600 seconds at an
annealing temperature between (Acl transformation point + 40)"C and Ac3
transformation point and in an atmosphere in which log(water partial
pressurehydrogen partial pressure) is -3.0 to 0.0, performing
bending-unbending deformation processing two or more times using a roll
with a radius of 800 mm or less so as to make a difference in accumulated
strain amount between a front and rear surface be 0.0050 or less, thereafter
cooling the steel sheet in the temperature range of 740°C to 650°C at an
average cooling rate of 1.0 to 5.0°C/second, and cooling the steel sheet in the
temperature range of 650°C to 500°C at an average cooling rate of 5 to
200°C/second; and
a plating alloying step of performing an alloying treatment including
dipping the steel sheet after the continuous annealing step in a galvanizing
bath, and then retaining the steel sheet at a temperature of 470 to 650°C for 10
to 120 seconds.
[Claim 91 - The manufacturing method of the high-strength galvanized
steel sheet excellent in bendability according to claim 8, wherein after the
hot-rolling step and before the continuous annealing step, a cold-rolling step
*
of cold rolling with a reduction ratio of 30 to 75%
[Claim 101 The manufacturing method of the high-strength galvanized
steel sheet excellent in bendability according to claim 8, wherein after the
alloying treatment step, the steel sheet is retained at a temperature of 200 to
5 350°C for 30 to 1000 seconds.
[Claim 111 The manufacturing method of the high-strength galvanized
steel sheet excellent in impact resistance characteristic according to claim 8,
wherein after the alloying treatment step, a step of adding a coating film
constituted of a phosphorus oxide and/or a composite oxide containing
10 phosphorus is performed.