[0001]The present invention relates to a hot dip galvanized steel sheet and a method for producing
the same, mainly relates to a high strength hot dip galvanized steel sheet to be worked into
10 various shapes by press forming etc., as a steel sheet for automobile use and a method for
producing the same.
BACKGROUND
[0002]
15 In recent years, improvement of the fuel efficiency of automobiles has been sought from the
viewpoint of control of hot house gas emissions accompanying the campaign against global
warming. Application of high strength steel sheet for lightening the weight of car bodies and
securing collision safety has been increasingly expanding. In particular, recently, the need for
ultrahigh strength steel sheet with a tensile strength of 980 MPa or more has been increasingly
20 rising. Further, high strength hot dip galvanized steel sheet which is hot dip galvanized on its
surface is being sought for portions in car bodies where rust prevention is demanded.
[0003]
Hot dip galvanized steel sheet used for auto parts requires not only strength, but also press
formability, weldability, and various other types of workability necessary for forming parts.
25 Specifically, from the viewpoint of press formability, excellent elongation (total elongation in
tensile test: El) and stretch flangeability (hole expansion rate: ) are required from steel sheet.
[0004]
In general, press formability deteriorates along with the higher strength of steel sheet. As
means for achieving both higher strength and press formability of steel, TRIP (transformation
30 induced plasticity) steel sheet utilizing transformation induced plasticity of retained austenite is
known.
[0005]
PTLs 1 to 3 disclose art relating to high strength TRIP steel sheet controlled in fractions of
structural constituents to predetermined ranges and improved in elongation and hole expansion
35 rates. Further, PTL 4 describes high strength steel sheet having a predetermined chemical
composition, and including, in terms of volume fraction, 15% or less of ferrite having an average
2
crystal grain diameter of 2 m or less, 2 to 15% of retained austenite having an average crystal
grain diameter of 2 m or less, 10% or less of martensite having an average crystal grain
diameter of 3 m or less, and a balance of bainite and tempered martensite having an average
crystal grain diameter of 6 m or less, wherein the average number of cementite grains having a
5 grain diameter of 0.04 m or more existing in the bainite and tempered martensite grains is 10 or
more, and describes that this high strength steel sheet has a tensile strength of 1180 MPa or more
and has a high elongation and hole expandability and excellent bending workability
accompanying the same.
[0006]
10 Furthermore, TRIP type high strength hot dip galvanized steel sheet is disclosed in several
literature.
[0007]
Normally, to produce hot dip galvanized steel sheet in a continuous annealing furnace, it is
necessary to heat the steel sheet to the reverse transformation temperature region (>Ac1), then in
15 the middle of the process for cooling down to room temperature, dip the steel sheet in a 460C or
so hot dip galvanizing bath. Alternatively, after cooling down to room temperature, it is
necessary to again heat the steel sheet to the hot dip galvanizing bath temperature and dip the
steel sheet in the coating bath. Furthermore, to produce hot dip galvannealed steel sheet, it is
necessary to perform alloying treatment after dipping the steel sheet in the coating bath. For
20 example, PTL 5 describes that the steel sheet is heated to Ac1 or more, is then rapidly cooled
down to the martensite transformation start temperature (Ms) or less, is then reheated to the
bainite transformation temperature region and held at the temperature region to stabilize the
austenite (austemper it), and is then reheated to the coating bath temperature or alloying
treatment temperature for galvannealing. However, with such a production method, since the
25 martensite is excessively tempered in the coating and alloying step, there was the problem that
the material quality became poor.
[0008]
PTLs 6 to 10 disclose a method for producing hot dip galvanized steel sheet comprising
cooling the steel sheet after the coating and alloying and reheating it to temper the martensite.
30
[CITATIONS LIST]
[PATENT LITERATURE]
[0009]
[PTL 1] WO 2013/051238
35 [PTL 2] Japanese Unexamined Patent Publication No. 2006-104532
[PTL 3] Japanese Unexamined Patent Publication No. 2011-184757
3
[PTL 4] WO 2017/179372
[PTL 5] WO 2014/020640
[PTL 6] Japanese Unexamined Patent Publication No. 2013-144830
[PTL 7] WO 2016/113789
5 [PTL 8] WO 2016/113788
[PTL 9] WO 2016/171237
[PTL 10] Japanese Unexamined Patent Publication No. 2017-48412
SUMMARY
10 [TECHNICAL PROBLEM]
[0010]
On the other hand, hot dip galvanized steel sheet used for members for automobiles is
required to not only possess press formability, but also not fracture in a brittle manner at the time
of deformation upon collision. In particular, in steel sheet used for members for automobiles, not
15 the toughness before press forming, but the toughness after introduction of plastic strain due to
press forming has to be excellent. However, in the prior art, the improvement of the toughness
after introduction of plastic strain has not necessarily been sufficiently studied. For this reason,
there is still room for improvement of the properties of hot dip galvanized steel sheet, in
particular hot dip galvanized steel sheet used for members for automobiles.
20 [0011]
An object of the present invention is to provide hot dip galvanized steel sheet excellent in
press formability and toughness after press forming and a method for producing the same.
[SOLUTION TO PROBLEM]
25 [0012]
The inventors engaged in intensive studies for solving this problem and as a result obtained
the following findings:
(i) In the continuous hot dip galvanization heat treatment step, martensite is formed by
cooling down to the Ms or less after coating or coating and alloying. Further, after that, the steel
30 is reheated and held isothermally to suitably temper the martensite and stabilize the retained
austenite. By such heat treatment, the martensite is no longer excessively tempered by the
coating or coating and alloying, and therefore the balance of strength and ductility is improved.
(ii) Originally, tempered martensite is a structure excellent in balance of strength and
toughness, but if large in size acts as a factor degrading the toughness. The inventors studied
35 means effective for decreasing the number of coarse tempered martensite. As a result, they
discovered that it is effective to hold the steel isothermally in a suitable temperature region
4
before dipping in the coating bath and the alloying treatment following that and partially
advance bainite transformation. By holding it isothermally, the nontransformed austenite which
will later become martensite is split by the bainite. By splitting the austenite by the bainite, it is
possible to reduce the size of the martensite formed from such austenite and in relation to this
5 decrease the coarse tempered martensite in the final structure. As a result, the toughness is
greatly improved.
(iii) To improve the toughness after introduction of plastic strain, the metallic structure at
the time when plastic strain is introduced must not contain a large amount of hard brittle fresh
martensite (martensite which is not tempered, i.e., martensite not containing carbides). To reduce
10 such fresh martensite, in the production steps from the hot rolling to the continuous hot dip
galvanization, it was found that limiting the producing conditions so that dispersion of Mn
between the ferrite and austenite is suppressed, then performing continuous hot dip galvanization
heat treatment satisfying the above (i) and (ii) is effective. The details are not necessarily clear,
but it is guessed that the source of formation of fresh martensite seen in the metallic structure at
15 the time of introduction of plastic strain is not only the fresh martensite present from before the
introduction of plastic strain, but also the unstable retained austenite transforming to martensite
by deformation induced transformation due to introduction of slight plastic strain. Such unstable
retained austenite is believed to be easily formed at the Mn concentrated part where
austempering (stabilization of austenite by dispersion of carbon atoms) is difficult to proceed in
20 a continuous hot dip coating step. The Mn concentrated part may be considered to originally
have been a segregated region formed at the time of casting, but if steel further subsequently
dwells at the two phase temperature region of ferrite and austenite, an alloy is dispersed between
the two phases and the Mn concentrated part becomes more marked. In the two-phase
temperature region present in the steps from hot rolling to the continuous hot dip galvanization,
25 by controlling the producing conditions so that not much dispersion of Mn is allowed, the
formation of the Mn concentrated part is reduced. Along with this, the amount of unstable
retained austenite easily formed at the Mn concentrated part can be made to decrease. As a
result, since the amount of martensite resulting from deformation induced transformation is
decreased from such unstable retained austenite at the time of introduction of plastic strain, it is
30 guessed that the fresh martensite contained in the metallic structure at the time of introduction of
plastic strain is decreased.
[0013]
The present invention was realized based on the above findings and specifically is as
follows:
35 (1) A hot dip galvanized steel sheet comprising a base steel sheet and a hot dip galvanized
layer on at least one surface of the base steel sheet, wherein the base steel sheet has a chemical
5
composition comprising, by mass%,
C: 0.100% to 0.350%,
Si: 0.50% to 2.50%,
Mn: 1.00% to 3.50%,
5 P: 0.050% or less,
S: 0.0100% or less,
Al: 0.001% to 1.500%,
N: 0.0100% or less,
O: 0.0100% or less,
10 Ti: 0% to 0.200%,
V: 0% to 1.00%,
Nb: 0% to 0.100%,
Cr: 0% to 2.00%,
Ni: 0% to 1.00%,
15 Cu: 0% to 1.00%,
Co: 0% to 1.00%,
Mo: 0% to 1.00%,
W: 0% to 1.00%,
B: 0% to 0.0100%,
20 Sn: 0% to 1.00%,
Sb: 0% to 1.00%,
Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%,
Ce: 0% to 0.0100%,
25 Zr: 0% to 0.0100%,
La: 0% to 0.0100%,
Hf: 0% to 0.0100%,
Bi: 0% to 0.0100%,
REM other than Ce and La: 0% to 0.0100% and
30 a balance of Fe and impurities,
a steel microstructure at a range of 1/8 thickness to 3/8 thickness centered about a position
of 1/4 thickness from a surface of the base steel sheet contains, by volume fraction,
ferrite: 0% to 50%,
retained austenite: 6% to 30%,
35 bainite: 5% or more,
tempered martensite: 5% or more,
6
fresh martensite: 0% to 10%, and
pearlite and cementite in total: 0% to 5%,
a number density of tempered martensite with a circle equivalent diameter of 5.0 m or
more is 20/1000 m
2
or less, and
5 an area ratio of fresh martensite with a circle equivalent diameter of 2.0 m or more after
imparting 5% plastic strain is 10% or less.
(2) A method for producing the hot dip galvanized steel sheet according to (1),
comprising:
(A) a hot rolling step comprising heating a slab having the chemical composition according
10 to (1) and finish rolling the heated slab by a plurality of rolling stands then coiling it up, wherein
the hot rolling step satisfies the conditions of the following (A1) to (A3):
(A1) an average heating rate from Ac1 to Ac1+30C at the time of heating the slab is 2
to 50C/min,
(A2) in the finish rolling by the plurality of rolling stands, a rolling reduction per pass
15 is 37% or less, a first pass inlet side temperature is 1000C or more, a final pass exit side
temperature is 900C or more, an average time between stands is 0.20 second or more, and a
time from an end of finish rolling to a start of cooling is 1 second or more, and
(A3) a coiling temperature is 450 to 680C, and
(B) a hot dip galvanizing step comprising heating the obtained steel sheet to first soak it,
20 first cooling then second soaking the first soaked steel sheet, dipping the second soaked steel
sheet in a hot dip galvanizing bath, second cooling the coated steel sheet, and heating the second
cooled steel sheet then third soaking it, wherein the hot dip galvanizing step satisfies the
conditions of the following (B1) to (B6):
(B1) in the heating of the steel sheet before the first soaking, an average heating rate
25 from Ac1 to Ac1+30C is 0.5C/s or more,
(B2) the steel sheet is held at a maximum heating temperature of Ac1C+30C to
950C for 1 second to 1000 seconds (first soaking),
(B3) an average cooling rate in a temperature range of 700 to 600C at the first cooling
is 10 to 100C/s,
30 (B4) the first cooled steel sheet is held in a range of 250 to 480C for 80 seconds to
500 seconds (second soaking),
(B5) the second cooling is performed down to 150C or less, and
(B6) the second cooled steel sheet is heated to a temperature region of 300 to 420C,
then held in the temperature region for 100 to 1000 seconds (third soaking).
35
[ADVANTAGEOUS EFFECTS OF INVENTION]
7
[0014]
According to the present invention, it is possible to obtain high strength hot dip galvanized
steel sheet excellent in press formability, specifically high strength hot dip galvanized steel sheet
excellent in ductility and hole expandability and further toughness after introduction of plastic
5 strain.
BRIEF DESCRIPTION OF DRAWINGS
[0015]
FIG. 1 shows a reference view of an SEM secondary electron image.
10
DESCRIPTION OF EMBODIMENTS
[0016]

The hot dip galvanized steel sheet according to the embodiment of the present invention
15 comprises a base steel sheet and a hot dip galvanized layer on at least one surface of the base
steel sheet, wherein the base steel sheet has a chemical composition comprising, by mass%,
C: 0.100% to 0.350%,
Si: 0.50% to 2.50%,
Mn: 1.00% to 3.50%,
20 P: 0.050% or less,
S: 0.0100% or less,
Al: 0.001% to 1.500%,
N: 0.0100% or less,
O: 0.0100% or less,
25 Ti: 0% to 0.200%,
V: 0% to 1.00%,
Nb: 0% to 0.100%,
Cr: 0% to 2.00%,
Ni: 0% to 1.00%,
30 Cu: 0% to 1.00%,
Co: 0% to 1.00%,
Mo: 0% to 1.00%,
W: 0% to 1.00%,
B: 0% to 0.0100%,
35 Sn: 0% to 1.00%,
Sb: 0% to 1.00%,
8
Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%,
Ce: 0% to 0.0100%,
Zr: 0% to 0.0100%,
5 La: 0% to 0.0100%,
Hf: 0% to 0.0100%,
Bi: 0% to 0.0100%,
REM other than Ce and La: 0% to 0.0100% and
a balance of Fe and impurities,
10 a steel microstructure at a range of 1/8 thickness to 3/8 thickness centered about a position
of 1/4 thickness from a surface of the base steel sheet contains, by volume fraction,
ferrite: 0% to 50%,
retained austenite: 6% to 30%,
bainite: 5% or more,
15 tempered martensite: 5% or more,
fresh martensite: 0% to 10%, and
pearlite and cementite in total: 0% to 5%,
a number density of tempered martensite with a circle equivalent diameter of 5.0 m or
more is 20/1000 m
2
or less, and
20 an area ratio of fresh martensite with a circle equivalent diameter of 2.0 m or more after
imparting 5% plastic strain is 10% or less.
[0017]
[Chemical Composition]
First, the reasons for limitation of the chemical composition of the base steel sheet
25 according to the embodiment of the present invention (below, also simply referred to as the
“steel sheet”) as described above will be explained. In this Description, the “%” used in
prescribing the chemical composition are all “mass%” unless otherwise indicated. Further, in
this Description, “to” when showing the ranges of numerical values unless otherwise indicated
will be used in the sense including the lower limit values and upper limit values of the numerical
30 values described before and after it.
[0018]
[C: 0.100% to 0.350%]
C is an element essential for securing the steel sheet strength. If less than 0.100%, the
required high strength cannot be obtained, and therefore the content of C is 0.100% or more. The
35 content of C may be 0.120% or more or 0.150% or more as well. On the other hand, if more than
0.350%, the workability or weldability falls, and therefore the content of C is 0.350% or less.
9
The content of C may be 0.340% or less, 0.320% or less, or 0.300% or less as well.
[0019]
[Si: 0.50% to 2.50%]
Si is an element suppressing formation of iron carbides and contributing to improvement of
5 strength and shapeability, but excessive addition causes the weldability of the steel sheet to
deteriorate. Therefore, the content is 0.50 to 2.50%. The content of Si may be 0.60% or more or
0.80% or more as well and/or may be 2.40% or less, 2.20% or less, or 2.00% or less as well.
[0020]
[Mn: 1.00% to 3.50%]
10 Mn (manganese) is a powerful austenite stabilizing element and an element effective for
increasing the strength of the steel sheet. Excessive addition causes the weldability or low
temperature toughness to deteriorate. Therefore, the content is 1.00 to 3.50%. The content of Mn
may be 1.20% or more or 1.50% or more as well and/or may be 3.40% or less, 3.20% or less, or
3.00% or less as well.
15 [0021]
[P: 0.050% or less]
P (phosphorus) is a solution strengthening element and an element effective for increasing
the strength of the steel sheet, but excessive addition causes the weldability and toughness to
deteriorate. Therefore, the content of P is limited to 0.050% or less. Preferably it is 0.045% or
20 less, 0.035% or less, or 0.020% or less. However, since extreme reduction of the content of P
would result in high dephosphorizing costs, from the viewpoint of economics, a lower limit of
0.001% is preferable.
[0022]
[S: 0.0100% or less]
25 S (sulfur) is an element contained as an impurity and forms MnS in steel to cause the
toughness and hole expandability to deteriorate. Therefore, the content of S is restricted to
0.0100% or less as a range where the toughness and hole expandability do not remarkably
deteriorate. Preferably it is 0.0050% or less, 0.0040% or less, or 0.0030% or less. However,
since extreme reduction of the content of S would result in high desulfurizing costs, from the
30 viewpoint of economics, a lower limit of 0.001% is preferable.
[0023]
[Al: 0.001% to 1.500%]
Al (aluminum) is added in at least 0.001% for deoxidation of the steel. However, even if
excessively adding it, not only does the effect become saturated and is a rise in cost invited, but
35 also the transformation temperature of the steel is raised and the load at the time of hot rolling is
increased. Therefore, an amount of Al of 1.500% is the upper limit. Preferably it is 1.200% or
10
less, 1.000% or less, or 0.800% or less.
[0024]
[N: 0.0100% or less]
N (nitrogen) is an element contained as an impurity. If its content is more than 0.0100%, it
5 forms coarse nitrides in the steel and causes deterioration of the bendability and hole
expandability. Therefore, the content of N is limited to 0.0100% or less. Preferably it is 0.0080%
or less, 0.0060% or less, or 0.0050% or less. However, since extreme reduction of the content of
N would result in high denitriding costs, from the viewpoint of economics, a lower limit of
0.0001% is preferable.
10 [0025]
[O: 0.0100% or less]
O (oxygen) is an element contained as an impurity. If its content is more than 0.0100%, it
forms coarse oxides in the steel and causes deterioration of the bendability and hole
expandability. Therefore, the content of O is limited to 0.0100% or less. Preferably it is 0.0080%
15 or less, 0.0060% or less, or 0.0050% or less. However, from the viewpoint of the producing
costs, a lower limit of 0.0001% is preferable.
[0026]
The basic chemical composition of the base steel sheet according to the embodiment of the
present invention is as explained above. The base steel sheet may further contain the following
20 elements according to need.
[0027]
[Ti: 0% to 0.200%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to 1.00%, Cu:
0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1.00%, B: 0% to 0.0100%, Sn: 0%
to 1.00%, and Sb: 0% to 1.00%]
25 Ti (titanium), V (vanadium), Nb (niobium), Cr (chromium), Ni (nickel), Cu (copper), Co
(cobalt), Mo (molybdenum), W (tungsten), B (boron), Sn (tin), and Sb (antimony) are all
elements effective for raising the strength of steel sheet. For this reason, one or more of these
elements may be added in accordance with need. However, if excessively adding these elements,
the effect becomes saturated and in particular an increase in cost is invited. Therefore, the
30 contents are Ti: 0% to 0.200%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to
1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1.00%, B: 0% to
0.0100%, Sn: 0% to 1.00%, Sb: 0% to 1.00%. The elements may also be 0.005% or more or
0.010% or more. In particular, the content of B may be 0.0001% or more or 0.0005% or more.
[0028]
35 [Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to
0.0100%, Hf: 0% to 0.0100%, Bi: 0% to 0.0100%, and REM other than Ce and La: 0% to
11
0.0100%]
Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf
(hafnium), and REM (rare earth elements) other than Ce and La are elements contributing to
microdispersion of inclusions in the steel. Bi (bismuth) is an element lightening the
5 microsegregation of Mn, Si, and other substitution type alloying elements in the steel. Since
these respectively contribute to improvement of the workability of steel sheet, one or more of
these elements may be added in accordance with need. However, excessive addition causes
deterioration of the ductility. Therefore, a content of 0.0100% is the upper limit. Further, the
elements may be 0.0005% or more or 0.0010% or more as well.
10 [0029]
In the base steel sheet according to the embodiment of the present invention, the balance
other than the above elements is comprised of Fe and impurities. “Impurities” are constituents
entering due to various factors in the producing process, first and foremost the raw materials
such as the ore and scrap, when industrially producing the base steel sheet and encompass all
15 constituents not intentionally added to the base steel sheet according to the embodiment of the
present invention. Further, “impurities” encompass all elements other than the constituents
explained above contained in the base steel sheet in levels where the actions and effects
distinctive to those elements do not affect the properties of the hot dip galvanized steel sheet
according to the embodiment of the present invention.
20 [0030]
[Steel Structures Inside Steel Sheet]
Next, the reasons for limitation of the internal structure of the base steel sheet according to
the embodiment of the present invention will be explained.
[0031]
25 [Ferrite: 0 to 50%]
Ferrite is a soft structure excellent in ductility. It may be included to improve the elongation
of steel sheet in accordance with the demanded strength or ductility. However, if excessively
contained, it becomes difficult to secure the desired steel sheet strength. Therefore, the content is
a volume fraction of 50% as the upper limit and may be 45% or less, 40% or less, or 35% or less.
30 The content of ferrite may be a volume fraction of 0%. For example, it may be 3% or more, 5%
or more, or 10% or more.
[0032]
[Tempered martensite: 5% or more]
Tempered martensite is a high strength tough structure and is an essential metallic structure
35 in the present invention. To balance the strength, ductility, and hole expandability at a high level,
it is included in a volume fraction of at least 5% or more. Preferably, it is a volume fraction of
12
10% or more. It may be 15% or more or 20% or more as well. For example, the content of the
tempered martensite may be a volume fraction of 85% or less, 80% or less, or 70% or less.
[0033]
[Fresh martensite: 0 to 10%]
5 In the present invention, fresh martensite means martensite which is not tempered, i.e.,
martensite not containing carbides. This fresh martensite is a brittle structure, so becomes a
starting point of fracture at the time of plastic deformation and causes deterioration of the local
ductility of the steel sheet. Therefore, the content is a volume fraction of 0 to 10%. More
preferably it is 0 to 8% or 0 to 5%. The content of fresh martensite may be a volume fraction of
10 1% or more or 2% or more.
[0034]
[Retained austenite: 6% to 30%]
Retained austenite improves the ductility of steel sheet due to the TRIP effect of
transformation into martensite due to work induced transformation during deformation of steel
15 sheet. For this reason, it is contained in a volume fraction of 6% or more. It may be contained in
8% or more or 10% or more as well. The greater the retained austenite, the more the elongation
rises, but to obtain a large amount of retained austenite, C and other alloying elements must be
included in large amounts. For this reason, the upper limit value of the retained austenite is a
volume fraction of 30%. It may be 25% or less or 20% or less as well.
20 [0035]
[Pearlite and cementite in total: 0 to 5%]
Pearlite includes hard coarse cementite and forms a starting point of fracture at the time of
plastic deformation, so causes the local ductility of the steel sheet to deteriorate. Therefore, the
content, together with the cementite, is a volume fraction of 0 to 5%. It may also be 0 to 3% or 0
25 to 2%.
[0036]
[Bainite: 5% or more]
In the present invention, to suppress the formation of coarse tempered martensite, bainite
transformation is partially advanced before the martensite transformation. For this reason, to
30 obtain this effect, the content of bainite has to be a volume fraction of 5% or more. The content
of bainite may also be a volume fraction of 8% or more or 12% or more. The upper limit value
of the content of bainite is not particularly set, but, for example, may be a volume fraction of
50% or less, 40% or less, or 35% or less.
[0037]
35 [Total of number density of tempered martensite with circle equivalent diameter of 5.0 m or
more of 20/1000 m
2
or less]
13
To improve the toughness after introduction of plastic strain, the number density of coarse
tempered martensite with a circle equivalent diameter of 5.0 m or more is limited to 20/1000
m
2
or less. Preferably it is 15/1000 m
2
or less or 10/1000 m
2
or less. The number density
may also be 0/1000 m
2
or 1/1000 m
2
or more.
5 [0038]
[Area ratio of fresh martensite with circle equivalent diameter of 2.0 m or more after imparting
5% plastic strain: 10% or less]
For the toughness after introduction of plastic strain, it is important to decrease the fresh
martensite present after introduction of plastic strain. In particular, coarse fresh martensite with a
10 circle equivalent diameter of more than 2.0 m has a large detrimental effect. Therefore, in the
steel sheet according to the embodiment of the present invention, the area ratio of fresh
martensite with a circle equivalent diameter of 2.0 m or more after introduction of 5% plastic
strain is limited to 10% or less. For example, the area ratio of fresh martensite may be 8% or less
or 6% or less as well. Further, the area ratio of fresh martensite may be 0% or may be 1% or
15 more.
[0039]
The fractions of the steel structures of the hot dip galvanized steel sheet are evaluated by
the SEM-EBSD method (electron backscatter diffraction method) and SEM secondary electron
image observation.
20 [0040]
First, a sample is taken from the cross-section of thickness of the steel sheet parallel to the
rolling direction so that the cross-section of thickness at the center position in the width direction
becomes the observed surface. The observed surface is machine polished and finished to a
mirror surface, then electrolytically polished. Next, in one or more observation fields at a range
25 of 1/8 thickness to 3/8 thickness centered about 1/4 thickness from the surface of the base steel
sheet at the observed surface, a total area of 2.010- 9 m
2
or more is analyzed for crystal
structures and orientations by the SEM-EBSD method. The data obtained by the EBSD method
is analyzed using “OIM Analysis 6.0” made by TSL. Further, the distance between evaluation
points (steps) is 0.03 to 0.20 m. Regions judged to be FCC iron from the results of observation
30 are deemed retained austenite. Further, boundaries with differences in crystal orientation of 15
degrees or more are deemed grain boundaries to obtain a crystal grain boundary map.
[0041]
Next, the same sample as that observed by EBSD is corroded by Nital and observed by
secondary electron image for the same fields as observation by EBSD. Since observing the same
35 fields as the time of EBSD measurement, Vickers indents and other visual marks may be
provided in advance. From the obtained secondary electron image, the area ratios of the ferrite,
14
retained austenite, bainite, tempered martensite, fresh martensite, and pearlite are respectively
measured and the results deemed the volume fractions. Regions having lower structures in the
grains and having several variants of cementite, more specifically two or more variants,
precipitating are judged to be tempered martensite (for example, see reference drawing of FIG.
5 1). Regions where cementite precipitates in lamellar form are judged to be pearlite (or pearlite
and cementite in total). Regions which are small in brightness and in which no lower structures
are observed are judged to be ferrite (for example, see reference drawing of FIG. 1). Regions
which are large in brightness and in which lower structures are not revealed by etching are
judged to be fresh martensite and retained austenite (for example, see reference drawing of FIG.
10 1). Regions not corresponding to any of the above regions are judged to be bainite. The volume
ratios of the same are calculated by the point counting method and used as the volume ratios of
the structures. The volume ratio of the fresh martensite can be found by subtracting the volume
ratio of retained austenite found by X-ray diffraction.
[0042]
15 The volume ratio of retained austenite is measured by the X-ray diffraction method. At a
range of 1/8 thickness to 3/8 thickness centered about 1/4 thickness from the surface of the base
steel sheet, a surface parallel to the sheet surface is polished to a mirror finish and measured for
area ratio of FCC iron by the X-ray diffraction method. This is used as the volume fraction of the
retained austenite.
20 [0043]
The number density of tempered martensite with a circle equivalent diameter of 5.0 m or
more is determined by calculating the circle equivalent diameters of tempered martensite in the
observed fields by image processing for tempered martensite identified by the above EBSD
observation and SEM observation and determining the frequency of presence of tempered
25 martensite with a circle equivalent diameter of 5.0 m or more.
[0044]
The area ratio of fresh martensite with a circle equivalent diameter of 2.0 m or more after
introduction of plastic strain is evaluated by the following method. First, a tensile test piece is
taken using the width direction of the steel sheet as the long direction of the test piece and
30 prestraining it in advance using a tensile test machine so that the amount of plastic strain
becomes 5%. From the center of the parallel part of the prestrained test piece, a sample for
observation of the microstructure is taken so that the cross-section of thickness parallel to the
rolling direction of the steel sheet becomes the observed surface. The observed surface is
machine polished and finished to a mirror surface, then electrolytically polished. After that, the
35 above-mentioned method is used for EBSD observation and SEM observation to identify the
fresh martensite, then image processing is used to measure the area ratio of fresh martensite with
15
a circle equivalent diameter of 2.0 m or more.
[0045]
(Hot dip galvanized layer)
The base steel sheet according to the embodiment of the present invention has a hot dip
5 galvanized layer on at least one surface, preferably on both surfaces. This coating layer may be a
hot dip galvanized layer or hot dip galvannealed layer having any composition known to persons
skilled in the art and may include Al and other additive elements in addition to Zn. Further, the
amount of deposition of the coating layer is not particularly limited and may be a general amount
of deposition.
10 [0046]

Next, the method for producing the hot dip galvanized steel sheet according to the
embodiment of the present invention will be explained. The following explanation is meant to
illustrate the characteristic method for producing the hot dip galvanized steel sheet according to
15 the embodiment of the present invention and is not meant to limit the hot dip galvanized steel
sheet to one produced by the production method explained below.
[0047]
The method for producing the hot dip galvanized steel sheet comprises
(A) a hot rolling step comprising heating a slab having the same chemical composition as
20 the chemical composition explained above relating to the base steel sheet and finish rolling the
heated slab by a plurality of rolling stands then coiling it up, wherein the hot rolling step satisfies
the conditions of the following (A1) to (A3):
(A1) an average heating rate from Ac1 to Ac1+30C at the time of heating the slab is 2
to 50C/min,
25 (A2) in the finish rolling by the plurality of rolling stands, a rolling reduction per pass
is 37% or less, a first pass inlet side temperature is 1000C or more, a final pass exit side
temperature is 900C or more, an average time between stands is 0.20 second or more, and a
time from an end of finish rolling to a start of cooling is 1 second or more, and
(A3) a coiling temperature is 450 to 680C, and
30 (B) a hot dip galvanizing step comprising heating the obtained steel sheet to first soak it,
first cooling then second soaking the first soaked steel sheet, dipping the second soaked steel
sheet in a hot dip galvanizing bath, second cooling the coated steel sheet, and heating the second
cooled steel sheet then third soaking it, wherein the hot dip galvanizing step satisfies the
conditions of the following (B1) to (B6):
35 (B1) in the heating of the steel sheet before the first soaking, an average heating rate
from Ac1 to Ac1+30C is 0.5C/s or more,
16
(B2) the steel sheet is held at a maximum heating temperature of Ac1C+30C to
950C for 1 second to 1000 seconds (first soaking),
(B3) an average cooling rate in a temperature range of 700 to 600C at the first cooling
is 10 to 100C/s,
5 (B4) the first cooled steel sheet is held in a range of 250 to 480C for 80 seconds to
500 seconds (second soaking),
(B5) the second cooling is performed down to 150C or less, and
(B6) the second cooled steel sheet is heated to a temperature region of 300 to 420C,
then held in the temperature region for 100 to 1000 seconds (third soaking).
10 [0048]
Below, the method for producing the hot dip galvanized steel sheet will be explained in
detail.
[0049]
[(A) Hot Rolling Step]
15 First, in the hot rolling step, a slab having the same chemical composition as the chemical
composition explained above relating to the base steel sheet is heated before hot rolling. The
heating temperature of the slab is not particularly limited, but for sufficient dissolution of the
borides, carbides, etc., generally 1150C or more is preferable. The steel slab used is preferably
produced by the continuous casting method from the viewpoint of producing ability, but may
20 also be produced by the ingot making method or thin slab casting method.
[0050]
[Average heating rate from Ac1 to Ac1+30C: 2 to 50C/min]
In this method, the average heating rate from Ac1 to Ac1+30C at the time of slab heating
is controlled to 2 to 50C/min. At the two-phase (austenite and ferrite) temperature region right
25 above the Ac1, alloying elements particularly easily become dispersed between the austenite and
ferrite. For this reason, when reheating the slab, the slab is heated in this temperature region by a
2C/min or more relatively fast average speed. If the heating rate falls below 2C/min, in the
final structure after plastic strain, the amount of coarse fresh martensite increases. On the other
hand, if performing rapid heating with a heating rate of more than 50C/min, the temperature
30 distribution in the thickness direction of the slab becomes uneven and thermal stress occurs, so
sometimes heat deformation and other inconveniences occur in the slab. For example, the above
average heating rate may be 4C/min or more and/or may be 40C/min or less, 30C/min or less,
20C/min or less, or 10C/min or less. The Ac1 point is calculated by the following formula:
The mass% of the elements are entered for the element symbols in the following formula. For
35 elements not contained, 0 mass% is entered:
Ac1 (C)723-10.7Mn-16.9Ni+29.1Si+16.9Cr
17
Further, in the present invention, “the average heating rate from Ac1 to Ac1+30C at the
time of slab heating” means the value obtained by dividing the difference from Ac1 to
Ac1+30C, i.e., 30C, by the elapsed time from Ac1 until Ac1+30C.
[0051]
5 [Rough rolling]
In this method, for example, the heated slab may be rough rolled before the finish rolling so
as to adjust the sheet thickness etc. Such rough rolling is not particularly limited, it is preferable
to perform it to give a total rolling reduction at 1050C or more of 60% or more. If the total
rolling reduction is less than 60%, since the recrystallization during hot rolling becomes
10 insufficient, sometimes this leads to unevenness of the structure of the hot rolled sheet. The
above total rolling reduction may, for example, be 90% or less.
[0052]
[Finish rolling by plurality of rolling stands]
The finish rolling is performed in a range satisfying the following conditions: maximum
15 rolling reduction per pass of 37% or less, first pass inlet side temperature of 1000C or more,
final pass exit side temperature of 900C or more, average time between stands of 0.20 second
or more, and time from end of the finish rolling to start of cooling of 1 second or more. In the
finish rolling, the larger the strain energy accumulated at the austenite, the more easily ferrite
transformation occurs at a high temperature after the end of the finish rolling. The lower the
20 ferrite transformation temperature, the more the dispersion of alloying elements, particularly Mn,
occurring between the ferrite and austenite can be suppressed. Accordingly, to reduce the strain
energy accumulated at the austenite, finish rolling is performed in a range satisfying the above
requirements. For example, the maximum rolling reduction per pass may be 30% or less, 25% or
less, or 20% or less and/or may be 5% or more. The first pass inlet side temperature may be
25 1100C or less. The final pass exit side temperature may be 1000C or less or 990C or less. The
average time between stands may be 0.50 second or more and/or may be 10 seconds or less. The
time from the end of the finish rolling to the start of cooling may be 2 seconds or more or 3
seconds or more and/or may be 10 seconds or less.
[0053]
30 [Coiling temperature: 450 to 680C]
The coiling temperature is 450 to 680C. If the coiling temperature falls below 450C, the
strength of the hot rolled sheet becomes excessive and sometimes the cold rolling ductility is
impaired. On the other hand, if the coiling temperature is more than 680C, the ferrite
transformation occurs more easily at a high temperature, so dispersion of alloying elements, in
35 particular Mn, easily occurs between the ferrite and austenite. The coiling temperature may be
500C or more and/or may be 650C or less or 600C or less.
18
[0054]
In the present method, the obtained hot rolled steel sheet (hot rolled coil) may be pickled or
otherwise treated as required. The hot rolled coil may be pickled by any ordinary method.
Further, the hot rolled coil may be skin pass rolled to correct its shape and improve its pickling
5 ability.
[0055]
[Cold Rolling Step]
In this method, after the hot rolling and/or pickling, the steel sheet may be heat treated as is
by a continuous hot dip galvanization line or may be cold rolled, then heat treated on a
10 continuous hot dip galvanization line. If performing cold rolling, the cold rolling reduction is
preferably 25% or more or 30% or more. On the other hand, since excessive rolling reduction
results in an excessive rolling force and leads to increases in load of the cold rolling mill, the
upper limit is preferably 75% or 70%.
[0056]
15 [(B) Hot Dip Galvanizing Step]
In this method, after the hot rolling step, the obtained steel sheet is coated in a hot dip
galvanizing step. In the hot dip galvanizing step, first, the steel sheet is heated and is subjected to
first soaking. While not particularly limited to this, at the time of heating the steel sheet, the
average heating rate from 600C to Ac1 is preferably limited to for example 10.0C/s or less. If
20 the average heating rate is more than 10.0C/s, recrystallization of ferrite does not sufficiently
proceed and the steel sheet will sometimes deteriorate in elongation. The average heating rate
may also be 6.0C/s or less. The lower limit value of the average heating rate is not particularly
limited, but for example may be 1.0C/s or more. In the present invention, “the average heating
rate from 600C to Ac1” means a value obtained by dividing the difference between 600C and
25 Ac1 by the elapsed time from 600C to Ac1.
[0057]
[Average heating rate from Ac1 to Ac1+30C: 0.5C/s or more]
The average heating rate from Ac1 to Ac1+30C at the time of heating the steel sheet is
limited to 0.5C/s or more. If the average heating rate from Ac1 to Ac1+30C falls below
30 0.5C/s, the dispersion of Mn between the ferrite and austenite becomes remarkable, so in the
final structure after plastic strain, the amount of coarse fresh martensite increases. This average
heating rate may be 1.0C/s or more. The upper limit value of the average heating rate is not
particularly set, but for example may be 10.0C/s or less. In the present invention, “the average
heating rate from Ac1 to Ac1+30C at the time of heating the steel sheet” means a value
35 obtained by dividing the difference from Ac1 to Ac1+30C, i.e., 30C, by the elapsed time from
Ac1 to Ac1+30C.
19
[0058]
[First soaking treatment: Holding at maximum heating temperature of Ac1+30C to 950C for 1
second to 1000 seconds]
To cause sufficient austenite transformation to proceed, the steel sheet is heated to at least
5 Ac1+30C or more and held at that temperature (maximum heating temperature) as soaking
treatment. However, if excessively raising the heating temperature, not only is deterioration of
the toughness due to coarsening of the austenite grain size invited, but also damage to the
annealing facilities is led to. For this reason, the upper limit is 950C, preferably 900C. If the
soaking time is short, austenite transformation does not sufficiently proceed, so the time is at
10 least 1 second or more. Preferably it is 30 seconds or more or 60 seconds or more. On the other
hand, if the soaking time is too long, the productivity is damaged, so the upper limit is 1000
seconds, preferably 500 seconds. During soaking, the steel sheet does not necessarily have to be
held at a constant temperature. It may also fluctuate within a range satisfying the above
conditions. The “holding” in the first soaking treatment and the later explained second soaking
15 treatment and third soaking treatment means maintaining the temperature within a range of a
predetermined temperature20C, preferably 10C, in a range not exceeding the upper limit
value and lower limit value prescribed in the soaking treatments. Therefore, for example, a
heating or cooling operation which gradually heats or gradually cools whereby the temperature
fluctuates by more than 40C, preferably 20C, with the temperature ranges prescribed in the
20 soaking treatments are not included in the first, second, and third soaking treatments according
to the embodiment of the present invention.
[0059]
[First cooling: Average cooling rate in temperature range of 700 to 600C: 10 to 100C/s]
After holding at the maximum heating temperature, the steel sheet is cooled by the first
25 cooling. The cooling stop temperature is 480C to 600C which becomes the following second
soaking temperature. The average cooling rate in a temperature range of 700C to 600C is 10 to
100C/s. If the average cooling rate is less than 10C/s, sometimes the desired ferrite fraction
cannot be obtained. The average cooling rate may be 15C/s or more or 20C/s or more. Further,
the average cooling rate may also be 80C/s or less or 60C/s or less. Further, in the present
30 invention, “the average cooling rate in a temperature range of 700 to 600C” means the value
obtained by dividing the temperature difference between 700C and 600C, i.e., 100C, by the
elapsed time from 700C to 600C.
[0060]
[Second soaking treatment: Holding in range of 250C to 480C for 80 to 500 seconds]
35 Second soaking treatment is performed by holding the steel sheet in a range of 250C to
480C for 80 to 500 seconds to partially advance the bainite transformation. Due to the present
20
heat treatment, the nontransformed austenite which later becomes martensite is split by the
bainite, so in the final structure, the coarse tempered martensite is reduced and thereby the
toughness after introduction of plastic strain can be improved. The temperature of the second
soaking treatment may be 280C or more and may be 450C or less. Further, the holding time
5 may be 100 seconds or more and may be 400 seconds or less. In relation to this, even if simply
suitably performing the second soaking treatment, if the dispersion of Mn between the ferrite
and austenite is not sufficiently suppressed from the hot rolling step to the hot dip galvanizing
step, it will not be possible to reduce the amount of unstable retained austenite which is easily
formed at the Mn concentrated part and as a result in the structure after plastic strain, the amount
10 of coarse fresh martensite will increase and the toughness will fall. Therefore, in the method for
producing hot dip galvanized steel sheet according to the embodiment of the present invention,
to improve the toughness after introduction of plastic strain, it is important to satisfy the
conditions of (A1) to (A3) explained above in the hot rolling step while suitably performing the
second soaking treatment in the hot dip galvanizing step.
15 [0061]
After the second soaking treatment, the steel sheet is dipped in a hot dip galvanization bath.
The steel sheet temperature at this time has little effect on the performance of the steel sheet, but
if the difference between the steel sheet temperature and the coating bath temperature is too
large, since the coating bath temperature will change and sometimes hinder operation, provision
20 of a step for reheating the steel sheet to a range of the coating bath temperature-20C to the
coating bath temperature+20C is desirable. The hot dip galvanization may be performed by an
ordinary method. For example, the coating bath temperature may be 440 to 470C and the
dipping time may be 5 seconds or less. The coating bath is preferably a coating bath containing
Al in 0.08 to 0.2%, but as impurities, Fe, Si, Mg, Mn, Cr, Ti, and Pb may also be contained.
25 Further, controlling the basis weight of the coating by gas wiping or another known method is
preferable. The basis weight is preferably 25 to 75 g/m2
per side.
[0062]
[Alloying treatment]
For example, the hot dip galvanized steel sheet formed with the hot dip galvanized layer
30 may be treated to alloy it as required. In this case, if the alloying treatment temperature is less
than 460C, not only does the alloying rate becomes slower and is the productivity hindered, but
also uneven alloying treatment occurs, so the alloying treatment temperature is 460C or more.
On the other hand, if the alloying treatment temperature is more than 600C, sometimes the
alloying excessively proceeds and the coating adhesion of the steel sheet deteriorates. Further,
35 sometimes pearlite transformation proceeds and the desired metallic structure cannot be
obtained. Therefore, the alloying treatment temperature is 600C or less.
21
[0063]
[Second cooling: Cooling to 150C or less]
The steel sheet after the coating treatment or coating and alloying treatment is cooled by the
second cooling which cools it down to the martensite transformation start temperature (Ms) or
5 less so as to make part of the austenite transform to martensite. The martensite produced here is
tempered by the subsequent reheating and third soaking treatment to become tempered
martensite. If the cooling stop temperature is more than 150C, the tempered martensite is not
sufficiently formed, so the desired metallic structure is not obtained. Therefore, the cooling stop
temperature is 150C or less. It may also be 100C or less. The Ms point is calculated by the
10 following formula: The mass% of the elements are entered for the element symbols in the
following formula. For elements not contained, 0 mass% is entered:
Ms (C)550-361C-39Mn-35V-20Cr-17Ni-10Cu-5Mo+30Al
[0064]
[Third soaking treatment: Holding in range of 300C to 420C for 100 to 1000 seconds]
15 After the second cooling, the steel sheet is reheated to a range of 300C to 420C for the
third soaking treatment. In this step, to obtain the desired amount of retained austenite, carbon is
made to concentrate in the austenite to stabilize the austenite (austemper it). In addition, the
martensite produced at the time of the second cooling is tempered. If the holding temperature is
less than 300C or the holding time is less than 100 seconds, since the bainite transformation
20 does not sufficiently proceed, it becomes difficult to obtain the desired amount of retained
austenite or the nontransformed austenite which later becomes martensite is not sufficiently split
by the bainite and as a result sometimes a large amount of coarse fresh martensite is produced
after introduction of plastic strain. On the other hand, if the holding temperature is more than
420C or if the holding time is more than 1000 seconds, since the martensite is excessively
25 tempered and bainite transformation excessively proceeds, it becomes difficult to obtain the
desired strength and metallic structure. The temperature of the third soaking treatment may be
350C or more and may be 400C or less. Further, the holding time may be 150 seconds or more
and may be 600 seconds or less.
[0065]
30 After the third soaking treatment, the steel sheet is cooled down to room temperature to
obtain the final finished product. The steel sheet may also be skin pass rolled to correct the
flatness and adjust the surface roughness. In this case, to avoid deterioration of the ductility, the
elongation rate is preferably 2% or less.
35 EXAMPLES
[0066]
22
Next, examples of the present invention will be explained. The conditions in the examples
are illustrations of conditions employed for confirming the workability and effects of the present
invention. The present invention is not limited to these illustrations of conditions. The present
invention can employ various conditions so long as not deviating from the gist of the present
5 invention and achieving the object of the present invention.
[0067]
[Example A]
Steels having the chemical compositions shown in Table 1 were cast to prepare slabs. The
balance other than the constituents shown in Table 1 comprised Fe and impurities. These slabs
10 were hot rolled under the conditions shown in Table 2 to produce hot rolled steel sheets. After
that, the hot rolled steel sheets were pickled to remove the surface scale. After that, they were
cold rolled. Further, the obtained steel sheets were continuously hot dip galvanized under the
conditions shown in Table 2 and suitably treated for alloying. In the soaking treatments shown in
Table 2, the temperatures were held within a range of the temperatures shown in Table 2 10C.
15 The chemical compositions of the base steel sheets obtained by analyzing samples taken from
the produced hot dip galvanized steel sheets were equal with the chemical compositions of the
steels shown in Table 1.
[0068]
[Table 1-1]
23
Table 1-1
Steel type C Si Mn P S Al N O Cr Mo V Nb
A 0.163 1.02 2.66 0.011 0.0023 0.100 0.0037 0.0005 0.28
B 0.230 1.88 1.28 0.009 0.0020 0.015 0.0024 0.0017 1.08
C 0.116 0.63 3.32 0.023 0.0030 0.030 0.0024 0.0011
D 0.212 1.49 2.17 0.012 0.0015 0.024 0.0031 0.0025 0.012
E 0.298 1.66 1.51 0.016 0.0024 0.023 0.0013 0.0018
F 0.157 2.02 1.68 0.025 0.0020 0.076 0.0014 0.0028
G 0.185 0.97 2.20 0.007 0.0025 0.652 0.0049 0.0017 0.23
H 0.132 0.81 2.94 0.031 0.0005 1.120 0.0053 0.0010
I 0.206 1.60 2.27 0.010 0.0015 0.018 0.0014 0.0017
J 0.339 0.95 1.09 0.045 0.0007 0.151 0.0043 0.0007 0.21 0.40
K 0.135 2.40 1.39 0.004 0.0024 0.008 0.0050 0.0022 0.08
L 0.193 1.22 2.41 0.027 0.0017 0.056 0.0057 0.0012
M 0.205 1.29 2.73 0.009 0.0029 0.417 0.0040 0.0011
N 0.177 1.90 2.46 0.013 0.0020 0.037 0.0013 0.0015 0.13
O 0.224 0.95 2.52 0.006 0.0011 0.592 0.0035 0.0005 0.22
P 0.094 1.51 2.50 0.009 0.0026 0.042 0.0050 0.0007
Q 0.197 0.30 2.53 0.015 0.0026 0.014 0.0035 0.0020
R 0.180 1.41 0.92 0.018 0.0030 0.103 0.0034 0.0020
S 0.192 1.08 3.80 0.008 0.0027 0.032 0.0051 0.0026
T 0.370 1.71 1.77 0.014 0.0012 0.044 0.0015 0.0021
U 0.167 2.77 2.08 0.010 0.0021 0.005 0.0060 0.0028
V 0.194 1.01 2.60 0.013 0.0018 1.710 0.0015 0.0008
Bold underlines indicate outside scope of present invention.
Empty field in table indicates corresponding chemical constituent not intentionally added.
[0069]
5 [Table 1-2]
24
Table 1-2
Steel type Ti B Cu Ni Co W Sn Sb Others Ac1 Ms
A 724 381
B 782 396
C 0.27 0.25 702 373
D 743 390
E 0.025 0.0018 755 384
F 0.41 0.18 764 430
G REM: 0.0036 728 416
H 715 421
I 0.14 0.11 745 388
J Ca: 0.0029 743 383
K 0.59 Ce: 0.0050,Zr: 0.0064 768 437
L Hf: 0.0042 733 388
M 0.056 0.0021 Bi: 0.0068 731 382
N 0.019 0.0020 Mg: 0.0051 752 391
O 0.020 0.0035 724 388
P 740 420
Q 705 381
R 754 452
S 714 333
T 754 349
U 781 409
V 725 430
Bold underlines indicate outside scope of present invention.
Empty field in table indicates corresponding chemical constituent not intentionally added.
[0070]
5 [Table 2-1]
25
Table 2-1
No. Steel
type
Hot rolling step Cold rolling step
Slab heating
temp.
Slab heating rate
(Ac1~Ac1+30C)
Total rolling reduction of
rough rolling at 1050C or
more
Finish inlet
side temp.
Finish exit side
temp.
Max.
rolling
reduction
Average time
between stands
Time from end of finish
rolling to start of cooling
Coiling
temp. Cold rolling reduction
C C/min % C C % s s C %
1 A 1220 6 88 1070 960 20 0.5 3 600 56
2 A 1270 8 88 1010 920 20 0.5 3 600 56
3 A 1210 5 88 1090 980 20 0.5 3 540 56
4 A 1240 0.4 88 1040 930 20 0.5 3 550 56
5 A 1220 6 88 1070 960 20 0.5 3 600 56
6 B 1260 7 88 1010 900 20 0.5 3 500 56
7 B 1240 5 88 1030 940 20 0.5 3 500 56
8 B 1250 7 88 950 850 20 0.5 3 590 56
9 B 1260 7 88 1010 900 20 0.5 3 500 56
10 C 1210 9 88 1020 900 20 0.5 3 570 56
11 C 1230 9 88 1040 920 20 0.5 3 530 56
12 C 1240 6 88 1050 940 20 0.5 3 500 56
13 C 1210 9 88 1020 900 20 0.5 3 570 56
14 D 1210 6 88 1020 900 20 0.5 3 540 56
15 D 1260 7 88 1040 940 20 0.5 3 480 56
16 D 1230 5 88 1030 930 20 0.5 3 580 56
17 D 1270 5 88 1060 940 20 0.5 3 530 56
18 D 1270 5 88 1060 940 20 0.5 3 530 56
19 D 1210 6 88 1020 900 20 0.5 3 540 56
20 E 1220 4 88 1070 970 20 0.5 3 510 56
21 E 1280 6 88 1060 970 20 0.5 3 600 56
22 E 1260 5 88 1020 920 38 0.07 3 600 56
23 E 1220 4 88 1070 970 20 0.5 3 510 56
24 F 1220 8 88 1060 960 20 0.5 3 500 56
25 F 1230 7 88 1050 950 20 0.5 3 590 56
26 F 1240 7 88 1070 940 20 0.5 3 580 56
27 F 1220 8 88 1040 940 20 0.5 0.3 560 56
Bold underlines indicate outside scope of present invention.
[0071]
5 [Table 2-2]
26
Table 2-2
No. Steel
type
Hot rolling step Cold rolling step
Slab heating
temp.
Slab heating rate
(Ac1~Ac1+30C)
Total rolling reduction of
rough rolling at 1050C or
more
Finish inlet
side temp.
Finish exit side
temp.
Max.
rolling
reduction
Average time
between stands
Time from end of finish
rolling to start of cooling
Coiling
temp. Cold rolling reduction
C C/min % C C % s s C %
28 G 1280 8 88 1050 930 20 0.5 3 580 56
29 G 1250 5 88 1030 920 20 0.5 3 550 30
30 G 1230 2 65 1070 940 29 0.5 3 650 67
31 G 1260 9 88 1060 950 20 0.5 3 770 56
32 H 1280 8 88 1080 990 20 0.5 3 570 56
33 I 1250 5 88 1030 940 20 0.5 3 560 56
34 J 1210 4 88 1080 970 20 0.5 3 510 56
35 K 1210 4 88 1080 980 20 0.5 3 550 56
36 L 1220 4 88 1030 920 20 0.5 3 530 56
37 M 1250 7 88 1010 910 20 0.5 3 520 56
38 M 1230 6 88 1010 940 20 0.5 3 560 56
39 M 1250 7 88 1010 910 20 0.5 3 520 56
40 N 1240 4 88 1040 940 20 0.5 3 580 56
41 N 1250 6 88 1030 950 20 0.5 3 550 56
42 N 1250 4 88 1040 950 20 0.5 3 560 56
43 O 1270 5 88 1060 950 20 0.5 3 540 56
44 P 1220 9 88 1040 920 20 0.5 3 580 56
45 Q 1250 8 88 1030 920 20 0.5 3 510 56
46 R 1250 8 88 1070 950 20 0.5 3 590 56
47 S 1220 5 88 1040 920 20 0.5 3 520 56
48 T 1220 5 88 1060 940 20 0.5 3 490 56
49 U 1260 4 88 1090 1000 20 0.5 3 570 56
50 V 1280 6 88 1060 940 20 0.5 3 510 56
51 D 1240 7 88 1050 970 20 0.5 3 560 56
52 D 1220 7 88 1040 910 20 0.5 3 550 56
53 D 1240 5 88 1030 900 35 0.3 1 590 56
54 E 1230 6 88 1070 960 25 0.5 3 560 56
Bold underlines indicate outside scope of present invention.
[0072]
5 [Table 2-3]
27
Table 2-3
No.
Hot dip galvanizing step
Heating First soaking First cooling Second soaking Alloying Second cooling Third soaking
Heating rate between
600C-Ac1
Heating rate between Ac1-
Ac1+30C
Temp. Holding
time
Average cooling rate
between 700-600C
Temp. Holding
time Alloying temp. Cooling stop.
temp. Temp. Holding
time
C/s C/s C s C/s C s C C C s
1 4.6 1.4 815 90 27 320 105 485 60 390 180
2 5.1 1.8 815 90 27 320 105 495 190 390 180
3 5.3 1.7 815 90 27 320 105 495 50 400 1500
4 4.9 1.7 810 90 28 320 105 480 60 400 180
5 4.6 1.5 815 90 27 300 105 - 60 380 180
6 5.2 1.6 840 90 33 300 105 500 70 400 180
7 5.3 1.7 740 90 25 300 105 480 60 400 180
8 5.0 1.8 820 90 26 300 105 490 70 390 180
9 5.2 1.6 840 90 33 300 105 - 70 400 180
10 4.5 1.6 805 90 22 330 105 505 60 410 370
11 5.0 1.6 805 90 28 310 105 505 50 250 180
12 4.5 1.3 800 90 2 310 105 495 60 400 180
13 4.5 1.6 800 90 22 330 105 - 50 400 370
14 5.3 1.5 820 90 36 310 105 485 60 410 180
15 4.8 1.8 820 90 29 310 105 475 60 400 75
16 4.6 1.8 820 90 29 310 105 495 60 450 180
17 4.9 1.6 820 90 29 340 900 495 50 390 180
18 4.8 1.7 840 90 40 520 105 500 50 400 180
19 5.3 1.5 820 90 31 310 105 - 60 400 180
20 5.2 1.5 830 90 29 350 105 480 90 390 180
21 5.3 1.8 830 90 29 330 50 495 60 400 180
22 4.5 1.2 830 90 30 330 105 480 40 380 180
23 5.2 1.5 820 90 29 350 105 - 90 390 180
24 4.6 1.5 860 90 38 310 105 475 50 390 180
25 4.7 1.6 860 90 35 450 105 480 60 380 180
26 4.9 1.7 850 90 33 390 105 485 90 300 180
27 4.8 1.6 860 90 35 370 105 470 50 390 180
Bold underlines indicate outside scope of present invention.
[0073]
5 [Table 2-4]
28
Table 2-4
No.
Hot dip galvanizing step
Heating First soaking First cooling Second soaking Alloying Second cooling Third soaking
Heating rate between
600C-Ac1
Heating rate between Ac1-
Ac1+30C
Temp. Holding
time
Average cooling rate
between 700-600C
Temp. Holding
time Alloying temp. Cooling stop.
temp. Temp. Holding
time
C/s C/s C s C/s C s C C C s
28 5.2 1.4 890 90 38 290 105 490 50 370 180
29 5.0 1.6 890 90 27 410 105 550 70 370 105
30 4.2 1.6 880 90 30 320 105 500 30 380 180
31 4.6 2.0 880 90 36 360 105 490 80 380 180
32 5.3 1.7 900 90 40 300 105 480 90 410 180
33 5.0 1.7 830 90 30 260 105 485 80 380 180
34 4.8 1.4 820 90 29 290 105 500 90 390 180
35 1.9 0.6 880 270 13 300 315 480 50 360 540
36 5.0 1.7 860 90 36 310 105 495 40 400 180
37 4.9 1.7 865 90 52 360 105 530 90 400 180
38 5.5 2.2 900 90 60 370 105 530 100 400 180
39 4.9 1.7 865 90 52 360 105 - 80 380 180
40 4.6 1.3 865 90 52 350 105 530 70 380 180
41 4.6 1.3 860 90 52 400 105 540 80 300 180
42 4.8 1.4 870 90 52 380 105 - 80 370 180
43 5.8 2.1 870 90 41 370 105 - 80 380 370
44 4.9 1.7 850 90 46 300 105 480 60 400 180
45 4.5 1.5 815 90 39 340 105 490 60 380 180
46 4.5 1.5 900 90 37 350 105 470 80 380 180
47 5.2 1.5 810 90 46 340 105 490 50 390 180
48 4.7 1.4 840 90 42 340 105 495 70 410 180
49 4.8 1.7 900 90 44 300 105 475 70 400 180
50 5.2 1.6 900 90 44 320 105 500 50 400 180
51 4.2 0.2 830 90 35 350 105 490 80 400 180
52 4.7 1.5 820 90 38 380 105 480 50 - -
53 6.0 1.7 830 90 37 330 105 470 30 400 180
54 5.1 1.9 830 90 32 - - 480 50 400 180
Bold underlines indicate outside scope of present invention.
29
[0074]
A JIS No. 5 tensile test piece was taken from each of the thus obtained steel sheets in a
direction perpendicular to the rolling direction and was subjected to a tensile test based on JIS
Z2241: 2011 to measure the tensile strength (TS) and total elongation (El). Further, each test
5 piece was tested by the “JFS T 1001 Hole Expansion Test Method” of the Japan Iron and Steel
Federation Standards to measure the hole expansion rate (). A test piece with a TS of 980 MPa
or more and a TSEl
0 . 5 /1000 of 80 or more was judged good in mechanical properties and
as having press formability preferable for use as a member for automobiles.
[0075]
10 The toughness after the introduction of plastic strain (toughness after press forming) was
evaluated by the following technique. A JIS No. 5 tensile test piece was taken in a direction
perpendicular to the rolling direction and given 5% plastic strain by a tensile test. A 2 mm Vnotched Charpy test piece was taken from a parallel part of the tensile test piece after imparting
strain. After that, a strained material and nonstrained material were subjected to a Charpy test at
15 the test temperature and -20C. A test piece with a ratio of Charpy absorption energy after
imparting strain/Charpy absorption energy before imparting plastic strain of 0.7 or more was
judged as “very good”, one of 0.5 to 0.7 was judged as “good”, and one of 0.5 or less was judged
as “poor”. Ones evaluated as very good and good were considered as passing.
[0076]
20 The results are shown in Table 3. In Table 3, “GA” means hot dip galvannealing, while GI
means hot dip galvanizing without alloying treatment.
[0077]
[Table 3-1]
30
Table 3-1
No. Steel
type Coating
Microstructure Mechanical properties
Remarks Ferrite Retained
austenite
Tempered
martensite
Fresh
martensite Pearlite+cementite Bainite
Fresh
martensite
after prestrain
(2 m)
Tempered
martensiteﾄ
(5 m)
Press formability
Toughness
TS El  after prestrain TSEl
0.5
/1000
% % % % % % % /1000m2 MPa % %
1 A GA 25 9 27 3 0 36 5 2 1023 19.7 33 116 Very good Ex.
2 A GA 25 11 6 8 0 50 12 0 1050 16.8 19 77 Poor Comp. ex.
3 A GA 25 4 27 0 0 44 0 2 988 14.0 32 78 Very good Comp. ex.
4 A GA 28 12 20 8 0 32 11 5 1058 19.0 20 90 Poor Comp. ex.
5 A GI 25 10 24 2 0 39 4 3 1007 20.0 35 119 Very good Ex.
6 B GA 34 16 36 2 0 12 4 3 1087 22.5 26 125 Very good Ex.
7 B GA 77 2 0 5 6 10 0 0 689 29.7 20 92 Good Comp. ex.
8 B GA 40 14 22 7 0 17 13 5 1029 22.0 21 104 Poor Comp. ex.
9 B GI 4 16 32 2 0 46 5 3 1078 22.1 25 119 Very good Ex.
10 C GA 9 6 71 6 0 8 8 11 1036 17.0 21 81 Good Ex.
11 C GA 9 3 70 11 0 7 11 11 1289 11.8 20 68 Poor Comp. ex.
12 C GA 65 5 12 5 6 7 8 1 854 19.9 20 76 Good Comp. ex.
13 C GI 9 7 68 3 0 13 6 10 1022 17.0 24 85 Good Ex.
14 D GA 32 14 22 2 0 30 4 2 1008 24.3 35 145 Very good Ex.
15 D GA 35 12 24 9 0 20 12 3 1026 22.6 17 96 Poor Comp. ex.
16 D GA 35 9 24 13 0 19 15 3 1033 20.8 15 83 Poor Comp. ex.
17 D GA 35 11 2 5 0 47 8 0 900 21.1 17 78 Very good Comp. ex.
18 D GA 20 11 60 1 0 8 2 21 1119 17.7 45 133 Poor Comp. ex.
19 D GI 37 13 20 1 0 29 4 2 997 24.8 31 138 Very good Ex.
20 E GA 19 22 37 4 0 18 8 12 1140 25.4 22 136 Very good Ex.
21 E GA 19 16 60 2 0 3 9 21 1174 21.2 23 119 Poor Comp. ex.
22 E GA 32 22 21 9 0 16 14 5 1138 19.8 16 90 Poor Comp. ex.
23 E GI 19 22 33 4 0 22 7 11 1129 26.0 21 135 Very good Ex.
24 F GA 33 13 25 3 0 26 4 4 982 23.1 38 140 Very good Ex.
25 F GA 33 11 43 2 0 11 6 10 1068 21.0 34 131 Very good Ex.
26 F GA 38 10 18 9 0 25 10 3 1110 17.3 25 96 Good Ex.
27 F GA 44 13 29 4 0 10 12 5 999 22.5 26 115 Poor Comp. ex.
Bold underlines indicate outside scope of present invention.
[0078]
5 [Table 3-2]
31
Table 3-2
No. Steel
type Coating
Microstructure Mechanical properties
Remarks Ferrite Retained
austenite
Tempered
martensite
Fresh
martensite Pearlite+cementite Bainite
Fresh
martensite
after prestrain
(2 m)
Tempered
martensiteﾄ
(5 m)
Press formability
Toughness
TS El  after prestrain TSEl
0.5
/1000
% % % % % % % /1000m2 MPa % %
28 G GA 30 13 33 2 0 22 5 3 1005 21.6 37 132 Very good Ex.
29 G GA 38 13 10 5 0 34 5 1 982 23.0 28 120 Very good Ex.
30 G GA 48 12 18 4 0 18 9 5 1013 22.0 21 102 Very good Ex.
31 G GA 63 12 9 5 0 11 12 0 950 23.8 27 117 Poor Comp. ex.
32 H GA 47 9 18 1 0 25 4 1 995 23.2 26 118 Very good Ex.
33 I GA 31 12 25 2 0 30 4 3 1024 23.6 33 139 Very good Ex.
34 J GA 45 27 10 6 0 12 10 0 1045 30.1 16 126 Good Ex.
35 K GA 30 9 39 8 0 14 9 1 1078 20.6 15 86 Good Ex.
36 L GA 12 12 51 2 0 23 6 9 1003 25.0 28 133 Very good Ex.
37 M GA 16 7 55 1 0 21 2 12 1236 14.2 51 125 Very good Ex.
38 M GA 0 7 70 0 0 23 0 12 1259 12.1 60 118 Very good Ex.
39 M GI 17 9 55 1 0 18 2 12 1225 14.7 49 126 Very good Ex.
40 N GA 13 9 64 1 0 13 2 11 1198 15.0 41 115 Very good Ex.
41 N GA 15 10 58 5 0 12 8 9 1202 15.5 30 102 Very good Ex.
42 N GI 9 10 52 3 0 26 3 10 1184 14.8 41 112 Very good Ex.
43 O GI 20 8 60 0 0 12 1 11 1221 14.9 50 129 Very good Ex.
44 P GA 22 2 50 1 0 25 1 4 891 13.1 30 64 Very good Comp. ex.
45 Q GA 17 3 36 2 0 42 2 4 1097 12.0 31 73 Very good Comp. ex.
46 R GA 66 10 0 2 0 22 0 0 701 31.5 48 153 Very good Comp. ex.
47 S GA 9 6 66 12 0 7 13 23 1374 12.0 18 70 Poor Comp. ex.
48 T GA 8 25 38 6 0 23 14 11 1310 20.4 19 116 Poor Comp. ex.
49 U GA 18 7 62 12 0 1 15 21 1233 15.8 18 83 Poor Comp. ex.
50 V GA 70 9 0 12 0 9 13 0 901 22.9 16 83 Poor Comp. ex.
51 D GA 23 12 20 8 0 30 12 3 1095 18.3 19 87 Poor Comp. ex.
52 D GA 34 11 0 24 0 31 26 - 1121 15.7 18 75 Poor Comp. ex.
53 D GA 26 11 24 6 0 33 9 2 1046 20.5 23 103 Good Ex.
54 D GA 20 10 65 3 0 2 10 23 1191 19.8 21 108 Poor Comp. ex.
Bold underlines indicate outside scope of present invention.
32
[0079]
In Comparative Example 2, the second cooling stop temperature in the hot dip galvanizing
step was higher than 150C. As a result, the desired metallic structure could not be obtained and
the press formability and toughness after prestraining were poor. In Comparative Example 3, the
5 holding time of the third soaking treatment in the hot dip galvanizing step was more than 1000
seconds. As a result, the desired metallic structure could not be obtained and the press
formability was poor. In Comparative Example 4, the slab heating rate was less than 2C/min.
As a result, the desired metallic structure could not be obtained and the toughness after
prestraining was poor. In Comparative Example 7, the temperature of the first soaking treatment
10 in the hot dip galvanizing step was less than Ac1+30C (812C). As a result, the desired metallic
structure could not be obtained and the press formability was poor. In Comparative Example 8,
the first pass inlet side temperature of the finish rolling in the hot rolling step was less than
1000C and the final pass exit side temperature was less than 900C. As a result, the desired
metallic structure could not be obtained and the toughness after prestraining was poor. In
15 Comparative Example 11, the temperature of the third soaking treatment in the hot dip
galvanizing step was less than 300C. As a result, the desired metallic structure could not be
obtained and the press formability and toughness after prestraining were poor. In Comparative
Example 12, the average cooling rate of the first cooling in the hot dip galvanizing step was less
than 10C/s. As a result, the desired metallic structure could not be obtained and the press
20 formability was poor. In Comparative Example 15, the holding time of the third soaking
treatment was less than 100 seconds. As a result, the desired metallic structure could not be
obtained and the toughness after prestraining was poor.
[0080]
In Comparative Example 16, the temperature of the third soaking treatment in the hot dip
25 galvanizing step was higher than 420C. As a result, the desired metallic structure could not be
obtained and the toughness after prestraining was poor. In Comparative Example 17, the holding
time of the second soaking treatment in the hot dip galvanizing step was more than 500 seconds.
As a result, the desired metallic structure could not be obtained and the press formability was
poor. In Comparative Example 18, the temperature of the second soaking treatment was more
30 than 480C. As a result, the desired metallic structure could not be obtained and the toughness
after prestraining was poor. In Comparative Example 21, the holding time of the second soaking
treatment in the hot dip galvanizing step was less than 80 seconds. As a result, the desired
metallic structure could not be obtained and the toughness after prestraining was poor. In
Comparative Example 22, the maximum rolling reduction of the finish rolling in the hot rolling
35 step was more than 37% and the average time between stands was less than 0.20 second. As a
result, the desired metallic structure could not be obtained and the toughness after prestraining
33
was poor. In Comparative Example 27, the time from the end of the finish rolling to the start of
cooling was less than 1 second. As a result, the desired metallic structure could not be obtained
and the toughness after prestraining was poor. In Comparative Example 31, the coiling
temperature in the hot rolling step was more than 680C. As a result, the desired metallic
5 structure could not be obtained and the press formability and toughness after prestraining were
poor. In Comparative Examples 44 to 50, the chemical compositions were not controlled to
within the predetermined ranges, so the press formability and/or toughness after prestraining
were poor. In Comparative Example 51, the average heating rate from Ac1 to Ac1+30C in the
hot dip galvanizing step was less than 0.5 second. As a result, the desired metallic structure
10 could not be obtained and the toughness after prestraining was poor. In Comparative Example
52, the third soaking treatment was omitted, so the desired metallic structure could not be
obtained and the press formability and toughness after prestraining were poor. In Comparative
Example 54, the second soaking treatment was omitted, so the desired metallic structure could
not be obtained and the toughness after prestraining was poor.
15 [0081]
In contrast to this, the hot dip galvanized steel sheets of the examples have a tensile strength
of 980 MPa or more and TSEl
0 . 5 /1000 of 80 or more and, further have excellent toughness
after prestraining, so it is learned that they are excellent in press formability and toughness after
press forming.
20 [0082]
[Example B]
In this example, the inventors studied the presence or absence of a specific soaking
treatment. First, they prepared a slab having the chemical composition shown in Table 1, then, as
shown in Table 4, made the first cooling gradual cooling to eliminate the second soaking
25 treatment. Other than that, the same procedure was followed as the case of Example A to obtain
hot dip galvanized steel sheet. The steel structures and mechanical properties in the obtained hot
dip galvanized steel sheet were investigated by methods similar to the case of Example A. The
results are shown in Table 5. In the different soaking treatments shown in Table 4, the
temperature was maintained within a range of the temperature shown in Table 4 10C.
30 [0083]
34
[Table 4-1]
Table 4-1
No. Steel
type
Hot rolling step Cold rolling step
Slab heating
temp.
Slab heating rate
(Ac1~Ac1+30C)
Total rolling reduction of
rough rolling at 1050C or
more
Finish inlet side
temp.
Finish exit side
temp.
Maximum
rolling
reduction
Average
time
between
stands
Time from end of finish
rolling to start of cooling
Coiling
temp. Cold rolling reduction
C C/min % C C % s s C %
55 A 1230 5 88 1050 950 20 0.4 3 600 56
Bold underlines indicate outside scope of present invention.
5 [0084]
[Table 4-2]
Table 4-2
No.
Hot dip galvanizing step
Heating First soaking First cooling Alloying Third soaking
Heating rate between 600CAc1
Heating rate between Ac1-
Ac1+30C
Temp. Holding
time
Average cooling rate
between 700~cooling stop
temp.
Cooling stop
temp. Alloying temp. Temp. Holding
time
C/s C/s C s C/s C C C s
55 4.8 1.5 820 90 2 80 485 400 180
Bold underlines indicate outside scope of present invention.
10 [0085]
[Table 5]
Table 5
No. Steel
type Coating
Microstructure Mechanical properties
Remarks Ferrite Retained
austenite
Tempered
martensite
Fresh
martensite Pearlite+cementite Bainite
Fresh martensite after
prestrain
(2 m)
Tempered
martensiteﾄ
(5 m)
Press formability Toughness after
TS El  prestrain TSEl
0.5
% % % % % % % /1000 /1000m2 MPa % %
55 A GA 40 8 6 9 6 31 12 2 1010 19.6 20 89 Poor Comp. ex.
Bold underlines indicate outside scope of present invention.
35
[0086]
As clear from the results of Table 5, if making the first cooling gradual cooling to eliminate
second soaking treatment, the desired metallic structure could not be obtained and the toughness
after prestraining was poor.

WE CLAIMS

[Claim 1]A hot dip galvanized steel sheet comprising a base steel sheet and a hot dip galvanized layer
5 on at least one surface of the base steel sheet, wherein the base steel sheet has a chemical
composition comprising, by mass%,
C: 0.100% to 0.350%,
Si: 0.50% to 2.50%,
Mn: 1.00% to 3.50%,
10 P: 0.050% or less,
S: 0.0100% or less,
Al: 0.001% to 1.500%,
N: 0.0100% or less,
O: 0.0100% or less,
15 Ti: 0% to 0.200%,
V: 0% to 1.00%,
Nb: 0% to 0.100%,
Cr: 0% to 2.00%,
Ni: 0% to 1.00%,
20 Cu: 0% to 1.00%,
Co: 0% to 1.00%,
Mo: 0% to 1.00%,
W: 0% to 1.00%,
B: 0% to 0.0100%,
25 Sn: 0% to 1.00%,
Sb: 0% to 1.00%,
Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%,
Ce: 0% to 0.0100%,
30 Zr: 0% to 0.0100%,
La: 0% to 0.0100%,
Hf: 0% to 0.0100%,
Bi: 0% to 0.0100%,
REM other than Ce and La: 0% to 0.0100% and
35 a balance of Fe and impurities,
a steel microstructure at a range of 1/8 thickness to 3/8 thickness centered about a position
37
of 1/4 thickness from a surface of the base steel sheet contains, by volume fraction,
ferrite: 0% to 50%,
retained austenite: 6% to 30%,
bainite: 5% or more,
5 tempered martensite: 5% or more,
fresh martensite: 0% to 10%, and
pearlite and cementite in total: 0% to 5%,
a number density of tempered martensite with a circle equivalent diameter of 5.0 m or
more is 20/1000 m
2
or less, and
10 an area ratio of fresh martensite with a circle equivalent diameter of 2.0 m or more after
imparting 5% plastic strain is 10% or less.
[Claim 2]
A method for producing the hot dip galvanized steel sheet according to claim 1,
15 comprising:
(A) a hot rolling step comprising heating a slab having the chemical composition according
to claim 1 and finish rolling the heated slab by a plurality of rolling stands then coiling it up,
wherein the hot rolling step satisfies the conditions of the following (A1) to (A3):
(A1) an average heating rate from Ac1 to Ac1+30C at the time of heating the slab is 2
20 to 50C/min,
(A2) in the finish rolling by the plurality of rolling stands, a rolling reduction per pass
is 37% or less, a first pass inlet side temperature is 1000C or more, a final pass exit side
temperature is 900C or more, an average time between stands is 0.20 second or more, and a
time from an end of finish rolling to a start of cooling is 1 second or more, and
25 (A3) a coiling temperature is 450 to 680C, and
(B) a hot dip galvanizing step comprising heating the obtained steel sheet to first soak it,
first cooling then second soaking the first soaked steel sheet, dipping the second soaked steel
sheet in a hot dip galvanizing bath, second cooling the coated steel sheet, and heating the second
cooled steel sheet then third soaking it, wherein the hot dip galvanizing step satisfies the
30 conditions of the following (B1) to (B6):
(B1) in the heating of the steel sheet before the first soaking, an average heating rate
from Ac1 to Ac1+30C is 0.5C/s or more,
(B2) the steel sheet is held at a maximum heating temperature of Ac1C+30C to
950C for 1 second to 1000 seconds (first soaking),
35 (B3) an average cooling rate in a temperature range of 700 to 600C at the first cooling
is 10 to 100C/s,
38
(B4) the first cooled steel sheet is held in a range of 250 to 480C for 80 seconds to
500 seconds (second soaking),
(B5) the second cooling is performed down to 150C or less, and
(B6) the second cooled steel sheet is heated to a temperature region of 300 to 420C,
5 then held in the temperature region for 100 to 1000 seconds (third soaking).