[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.
[0006]
2
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 5 transformation temperature region (>Ac1) and
soak it, then in the middle of the process for cooling down to room temperature, dip it in a 460C
or so hot dip galvanizing bath. Alternatively, after heating and soaking, then cooling down to
room temperature, it is necessary to reheat the steel sheet to the hot dip galvanizing bath
temperature and dip it in the bath. Furthermore, usually, to produce hot dip galvannealed steel
10 sheet, it is necessary to perform alloying treatment after dipping the steel sheet in the coating
bath, then reheat the steel sheet to a 460C or more temperature region. For example, PTL 4
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
15 (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 martensite
and bainite is excessively tempered in the coating and alloying step, there was the problem that
the material quality became poor.
[0008]
20 PTLs 5 to 9 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.
[0009]
On the other hand, if applying an ultra high strength steel sheet having a tensile strength of
more than 980 MPa as a member for an automobile, it is necessary to not only secure press
25 formability of course and also solve the problem of hydrogen embrittlement cracking of the steel
sheet (also called “delayed fracture”). “Hydrogen embrittlement fracture” is the phenomenon
where a steel member on which a high stress acts in the situation of use suddenly fractures due to
the hydrogen penetrating the steel from the environment. In general, hydrogen embrittlement
cracking occurs more easily the more the strength of the steel sheet rises. This is believed to be
30 because the residual stress after shaping increases the higher the strength of the steel sheet. This
susceptibility to hydrogen embrittlement cracking is called the hydrogen embrittlement
resistance.
[0010]
Up to now as well, several attempts to improve the hydrogen embrittlement resistance of
35 steel sheet have been made. Examples of such studies are shown below.
[0011]
3
PTLs 10 and 11 disclose a method for producing ultra high strength steel sheet making the
steel structure mainly a martensite structure by heating cold rolled steel sheet having a
predetermined chemical composition to the Ac3 point or more, quenching it, and tempering it,
and describe that these ultra high strength steel sheets have excellent delayed fracture resistance.
5 [0012]
PTL 12 discloses high strength cold rolled steel sheet having a chemical composition
containing Cu, Cr, Nb, Ni, etc., in trace amounts, having a steel structure rendered a mainly
bainite structure, and having a tensile strength of 120 kgf/mm2 or more, and describes that such
high strength cold rolled steel sheet is excellent in delayed fracture resistance.
10 [0013]
PTL 13 discloses a method for producing cold rolled steel sheet having a structure inside
the steel rendered a mainly tempered martensite structure while softened at the surface layer and
having a 1270 MPa or more tensile strength by decarburizing annealing steel sheet having a
predetermined chemical composition, then heating it to the Ar3 point or more, quenching, and
15 tempering it, and describes that such cold rolled steel sheet is excellent in bendability and
delayed fracture resistance.
[0014]
PTL 14 discloses high strength thin-gage steel sheet utilizing the hydrogen trap action of
retained austenite by controlling the amount and dispersion mode of retained austenite contained
20 in the steel structure, and describes that this high strength thin-gage steel sheet is excellent in
hydrogen embrittlement resistance.
[0015]
PTL 15 describes that adjusting the volume fractions of the soft phases (ferrite) and the
hard phases (martensite and retained austenite) acting as the sources of formation of voids to
25 cause the formation of the hard intermediate phases of tempered martensite or bainite
(hardnesses between the soft phases of ferrite and the hard phases of martensite) and further
making the crystal grains finer makes it possible to secure strength and hole expandability while
containing soft ferrite to a certain degree, and containing cementite in the structure of the steel
sheet to cause the formation of hydrogen trap sites makes it possible to secure strength, and
30 obtain elongation, delayed fracture resistance (hydrogen embrittlement resistance) and hole
expandability.
[CITATIONS LIST]
[PATENT LITERATURE]
35 [0016]
[PTL 1] WO 2013/051238
4
[PTL 2] Japanese Unexamined Patent Publication No. 2006-104532
[PTL 3] Japanese Unexamined Patent Publication No. 2011-184757
[PTL 4] WO 2014/020640
[PTL 5] Japanese Unexamined Patent Publication No. 2013-144830
5 [PTL 6] WO 2016/113789
[PTL 7] WO 2016/113788
[PTL 8] WO 2016/171237
[PTL 9] Japanese Unexamined Patent Publication No. 2017-48412
[PTL 10] Japanese Unexamined Patent Publication No. 10-001740
10 [PTL 11] Japanese Unexamined Patent Publication No. 9-111398
[PTL 12] Japanese Unexamined Patent Publication No. 6-145891
[PTL 13] WO 2011/105385
[PTL 14] Japanese Unexamined Patent Publication No. 2007-197819
[PTL 15] WO 2017/179372
15
SUMMARY
[TECHNICAL PROBLEM]
[0017]
In steel sheet for automobiles, if considering the usage, the hydrogen embrittlement
20 resistance after introduction of plastic strain due to press forming has to be excellent. However,
in the prior art, the improvement of the hydrogen embrittlement resistance after introduction of
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.
25 [0018]
Therefore, an object of the present invention is to provide hot dip galvanized steel sheet
excellent in press formability and hydrogen embrittlement resistance after plastic working and
having a tensile strength of 980 MPa or more and a method for producing the same.
30 [SOLUTION TO PROBLEM]
[0019]
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
35 cooling down to the Ms or less after coating or coating and alloying. Further, after that, the steel
may be reheated and held isothermally to suitably temper the martensite and, in the case of steel
5
sheet containing retained austenite, further 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) Hydrogen embrittlement cracking proceeds by cracks advancing along the crystal grain
boundaries. Therefore, stabilization of 5 the crystal grain boundaries is effective for improvement
of the hydrogen embrittlement resistance. Therefore, the inventors looked at boron (B) which
has the effect of stabilization of austenite grain boundaries and thought of increasing the
segregation and concentration of boron at the austenite grain boundaries. Specifically, they
discovered that by holding the steel sheet isothermally in the 480 to 600C or so temperature
10 region where boron easily segregates at the austenite grain boundaries after the soaking step and
cooling step in continuous hot dip galvanization heat treatment, the segregation and
concentration of boron at the grain boundaries rise. However, if performing such treatment after
coating or after martensite transformation, the powdering property of the coating layer
deteriorates and the balance of strength and ductility deteriorates due to the martensite being
15 excessively tempered. Therefore, the isothermal holding operation has to be performed before
the coating treatment.
(iii) To improve the effect of grain boundary segregation of boron more, in the process
leading up to the continuous hot dip galvanization heat treatment, the precipitation and
coarsening of borides has to be suppressed and the boron has to be present in a solid solution
20 state. Specifically, the hot rolling and cooling conditions after hot rolling are limited to suppress
the precipitation and coarsening of borides in the hot rolling step. In addition, the hydrogen
embrittlement resistance is improved more by performing continuous hot dip galvanization heat
treatment satisfying the above (i) and (ii).
[0020]
25 The present invention was realized based on the above findings and specifically is as
follows:
(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
composition comprising, by mass%,
30 C: 0.050% to 0.350%,
Si: 0.10% to 2.50%,
Mn: 1.00% to 3.50%,
P: 0.050% or less,
S: 0.0100% or less,
35 Al: 0.001% to 1.500%,
N: 0.0100% or less,
6
O: 0.0100% or less,
Ti: 0.005% to 0.200%,
B: 0.0005% to 0.0100%,
V: 0% to 1.00%,
5 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%,
10 Mo: 0% to 1.00%,
W: 0% to 1.00%,
Sn: 0% to 1.00%,
Sb: 0% to 1.00%,
Ca: 0% to 0.0100%,
15 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%,
20 Bi: 0% to 0.0100%,
REM other than Ce and La: 0% to 0.0100% and
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,
25 ferrite: 0% to 50%,
retained austenite: 0% to 30%,
tempered martensite: 5% or more,
fresh martensite: 0% to 10%, and
pearlite and cementite in total: 0% to 5%,
30 when there are remaining structures, the remaining structures consist of bainite,
a concentration of B atoms at prior austenite grain boundaries is 2.0 atm% or more, and
an average effective crystal grain size is 7.0 m or less.
(2) The hot dip galvanized steel sheet according to (1) wherein the steel microstructure
further contains, by volume fraction, retained austenite: 6% to 30%.
35 (3) A method for producing the hot dip galvanized steel sheet according to (1) or (2),
comprising
7
(A) a hot rolling step comprising finish rolling a slab having the chemical composition
according to the above (1), then coiling it up, wherein the hot rolling step satisfies the conditions
of the following (A1) to (A4):
(A1) a time during which the slab dwells at the temperature TB or less from extracting
5 the slab to the finish rolling inlet side is 300 seconds or less,
[Mathematical 1]
(where [B] and [N] respectively represent mass% of boron (B) and nitrogen (N)),
(A2) in the finish rolling, the finish rolling inlet side temperature is 900 to 1050C, the
10 finish rolling exit temperature is 850C to 1000C and a total rolling reduction is 70 to 95%,
(A3) in cooling of the steel sheet after the finish rolling, the average cooling rate from
the finish rolling exit temperature to 800C is VC/s or more,
[Mathematical 2]
15 (where [B] and [N] respectively represent mass% of boron (B) and nitrogen (N)), and
(A4) 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
20 cooled steel sheet then third soaking it, wherein the hot dip galvanizing step satisfies the
conditions of the following (B1) to (B7):
(B1) in the heating of the steel sheet before the first soaking, an average heating rate
from 650C to a maximum heating temperature of Ac1+30C or more and 950C or less is
0.5C/s to 10.0C/s,
25 (B2) the steel sheet is held at the maximum heating temperature 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,
(B4) the first cooled steel sheet is held in a range of 480 to 600C for 80 seconds to
30 500 seconds (second soaking),
8
(B5) when dipping the second soaked steel sheet in a hot dip galvanizing bath, then
alloying it, the alloying treatment is performed in a range of 460 to 600C,
(B6) the second cooling is performed down to Ms-50C or less, and
(B7) the second cooled steel sheet is heated to a temperature region of 200 to 420C,
then held 5 in the temperature region for 5 to 1000 seconds (third soaking).
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0021]
According to the present invention, it is possible to obtain hot dip galvanized steel sheet
10 excellent in press formability, specifically ductility and hole expandability and further hydrogen
embrittlement resistance after introduction of plastic strain.
BRIEF DESCRIPTION OF DRAWINGS
[0022]
15 FIG. 1 shows a reference view of an SEM secondary electron image.
FIG. 2 is a temperature-thermal expansion curve when simulating a heat cycle
corresponding to hot dip galvanization treatment according to the embodiment of the present
invention by a thermal expansion measurement apparatus.
FIG. 3 is a view schematically showing a test method for evaluating hydrogen
20 embrittlement resistance after plastic deformation.
DESCRIPTION OF EMBODIMENTS
[0023]

25 The hot dip galvanized steel sheet according to the embodiment of the present invention
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: 0.050% to 0.350%,
Si: 0.10% to 2.50%,
30 Mn: 1.00% to 3.50%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.001% to 1.500%,
N: 0.0100% or less,
35 O: 0.0100% or less,
Ti: 0.005% to 0.200%,
9
B: 0.0005% to 0.0100%,
V: 0% to 1.00%,
Nb: 0% to 0.100%,
Cr: 0% to 2.00%,
5 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%,
10 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%,
15 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
20 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: 0% to 30%,
25 tempered martensite: 5% or more,
fresh martensite: 0% to 10%, and
pearlite and cementite in total: 0% to 5%,
when there are remaining structures, the remaining structures consist of bainite,
a concentration of B atoms at prior austenite grain boundaries is 2.0 atm% or more, and
30 an average effective crystal grain size is 7.0 m or less.
[0024]
[Chemical Composition]
First, the reasons for limitation of the chemical composition of the base steel sheet
according to the embodiment of the present invention (below, also simply referred to as the
35 “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
10
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
values described before and after it.
[0025]
5 [C: 0.050% to 0.350%]
C is an element essential for securing the steel sheet strength. If less than 0.050%, the
required high strength cannot be obtained, and therefore the content of C is 0.050% or more. The
content of C may be 0.070% or more, 0.085% or more, or 0.100% 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
10 0.350% or less. The content of C may be 0.340% or less, 0.320% or less, or 0.300% or less as
well.
[0026]
[Si: 0.10% to 2.50%]
Si is an element suppressing formation of iron carbides and contributing to improvement of
15 strength and shapeability, but excessive addition causes the weldability of the steel sheet to
deteriorate. Therefore, the content is 0.10 to 2.50%. The content of Si may be 0.20% or more,
0.30% or more, 0.40% or more, or 0.50% or more as well and/or may be 2.20% or less, 2.00% or
less, or 1.90% or less as well.
[0027]
20 [Mn: 1.00% to 3.50%]
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 1.10% or more, 1.30% or more, or 1.50% or more as well and/or may be 3.30% or less,
25 3.10% or less, or 3.00% or less as well.
[0028]
[P: 0.050% or less]
P (phosphorus) is a solution strengthening element and an element effective for increasing
the strength of the steel sheet. Excessive addition causes the weldability and toughness to
30 deteriorate. Therefore, the content of P is limited to 0.050% or less. Preferably it is 0.045% or
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.
[0029]
35 [S: 0.0100% or less]
S (sulfur) is an element contained as an impurity and forms MnS in steel to cause the
11
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
viewpoint 5 of economics, a lower limit of 0.0001% is preferable.
[0030]
[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
10 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
less, 1.000% or less, or 0.800% or less.
[0031]
[N: 0.0100% or less]
15 N (nitrogen) is an element contained as an impurity. If its content is more than 0.0100%, it
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
20 0.0001% is preferable.
[0032]
[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
25 expandability. Therefore, the content of O is limited to 0.0100% or less. Preferably it is 0.0080%
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.
[0033]
[Ti: 0.005% to 0.200%]
30 Ti (titanium) fixes the N (nitrogen) present as an impurity in the steel as TiN and is added
to keep the B (boron) from precipitating as nitrides. To obtain the above effect, at least 0.005%
must be added. On the other hand, if excessively added, not only does the effect become
saturated, but also coarse titanium carbide (TiC) is formed and the ductility or toughness of the
steel sheet deteriorates. For this reason, the upper limit of the addition amount is 0.200%. The
35 content of Ti may be 0.008% or more, 0.010% or more, or 0.013% or more and/or may be
0.150% or less, 0.120% or less, or 0.100% or less.
12
[0034]
[B: 0.0005% to 0.0100%]
B (boron) segregates at the prior austenite grain boundaries and causes a drop in the energy
of the prior austenite grain boundaries thereby improving the hardenability of the steel sheet.
Furthermore, in the present invention, B atoms segregate 5 at the prior austenite grain boundaries
and raise the peel strength of the prior austenite grain boundaries, and therefore improve the
hydrogen embrittlement resistance. To obtain the above effect, at least 0.0005% or more must be
added. On the other hand, if excessively added, not only does the effect become saturated, but
also borides are formed in the steel and the hardenability of the steel sheet is lowered. For this
10 reason, the upper limit of the addition amount is 0.0100%. The content of B may be 0.0006% or
more, 0.0008% or more, or 0.0010% or more and/or may be 0.0060% or less, 0.0040% or less,
or 0.0035% or less.
[0035]
The basic chemical composition of the base steel sheet according to the embodiment of the
15 present invention is as explained above. The base steel sheet may further contain the following
elements according to need.
[0036]
[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%, Sn: 0% to 1.00%, and Sb: 0% to 1.00%]
20 V (vanadium), Nb (niobium), Cr (chromium), Ni (nickel), Cu (copper), Co (cobalt), Mo
(molybdenum), W (tungsten), 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 contents are V: 0% to
25 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%, Sn: 0% to 1.00%, Sb: 0% to 1.00%. The elements
may also be 0.005% or more or 0.010% or more.
[0037]
[Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to
30 0.0100%, Hf: 0% to 0.0100%, Bi: 0% to 0.0100%, and REM other than Ce and La: 0% to
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
35 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
13
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.
[0038]
In the base steel sheet according 5 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
constituents not intentionally added to the base steel sheet according to the embodiment of the
10 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.
[0039]
15 [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.
[0040]
[Ferrite: 0 to 50%]
20 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.
The content of ferrite may be a volume fraction of 0%. For example, it may be 3% or more, 5%
25 or more, or 10% or more.
[0041]
[Tempered martensite: 5% or more]
Tempered martensite is a high strength tough structure and is an essential metallic structure
in the present invention. To balance the strength, ductility, and hole expandability at a high level,
30 it is included in a volume fraction of at least 5% or more. Preferably, it is a volume fraction of
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 96% or less, 85% or less, 80% or less, or 70%
or less.
[0042]
35 [Fresh martensite: 0 to 10%]
In the present invention, fresh martensite means martensite which is not tempered, i.e.,
14
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
5 1% or more or 2% or more.
[0043]
[Retained austenite: 0% 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
10 sheet. On the other hand, 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. However, if
desiring to improve the ductility of the steel sheet, the content is preferably a volume fraction of
6% or more, 8% or more, or 10% or more. Further, if the content of the retained austenite is 6%
15 or more, the content of Si in the base steel sheet is preferably, by mass%, 0.50% or more.
[0044]
[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
20 content, together with the cementite, is a volume fraction of 0 to 5%. It may also be 0 to 3% or 0
to 2%.
[0045]
The remaining structures besides the above structures may be 0%, but if there are any
present, they are bainite. The remaining bainite structures may be upper bainite or lower bainite
25 or may be mixed structures of the same.
[0046]
[Concentration of B atoms at prior austenite grain boundaries: 2.0 atm% or more]
The base steel sheet according to an embodiment of the present invention has a
concentration of B atoms at the prior austenite grain boundaries of 2.0 atm% or more. In the
30 present invention, the B atoms segregate at the prior austenite grain boundaries to thereby raise
the peel strength of the prior austenite grain boundaries and improve the hydrogen embrittlement
resistance. If less than 2.0 atm%, the effect of improvement of the hydrogen embrittlement
resistance is not sufficiently obtained. The concentration of B atoms at the prior austenite grain
boundaries may be 2.5 atm% or more or 3.0 atm% or more.
35 [0047]
[Average effective crystal grain size: 7.0 m or less]
15
The base steel sheet according to an embodiment of the present invention has an average
effective crystal grain size of 7.0 m or less. In the present invention, the “average effective
crystal grain size” means the value calculated when defining a grain having a difference in
orientation from adjoining grains of 15 degrees or more of one crystal grain. In addition to the
improvement of the peel strength of the 5 prior austenite grain boundaries by the B atoms, it is
possible to improve the hydrogen embrittlement resistance by reducing the average effective
crystal grain size. If the average effective crystal grain size is more than 7.0 m, the effect of
improvement of the hydrogen embrittlement resistance is not sufficiently obtained. The average
effective crystal grain size may be 6.0 m or less, 5.5 m or less, or 5.0 m or less as well.
10 [0048]
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.
[0049]
15 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
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- 9m2 20 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
are deemed retained austenite. Further, boundaries with differences in crystal orientation of 15
25 degrees or more are deemed grain boundaries to obtain a crystal grain boundary map.
[0050]
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
fields as the time of EBSD measurement, Vickers indentations and other visual marks may be
30 provided in advance. From the obtained secondary electron image, the area ratios of the ferrite,
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.
35 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
16
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.
1). Regions not corresponding to any of the above regions are judged to be bainite. The volume
ratios of the same are 5 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.
[0051]
The volume ratio of retained austenite is measured by the X-ray diffraction method. At a
10 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.
[0052]
15 In the present invention, the concentration of B atoms at the prior austenite grain
boundaries is found by the STEM-EELS method. Specifically, for example, it is found by the
method disclosed in METALLURGICAL AND MATERIALS TRANSACTIONS A: vol. 45A,
p.1877 to 1888. 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
20 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 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- 9m2 or more is analyzed
for crystal structures and orientations by the SEM-EBSD method to identify the prior austenite
25 grain boundaries. Next, the region including the prior austenite grain boundaries is extracted by
FIB in the SEM. After that, Ar ion milling etc., is used to reduce the thickness down to about 70
nm. The electron energy loss spectrum (EELS) is obtained along a line traversing the prior
austenite grain boundaries by aberration spectrum STEM from the test piece reduced in
thickness. The scan steps at the time of line analysis is preferably 0.1 nm or so.
30 [0053]
The “average effective crystal grain size” in the present invention is determined using the
value found by the above EBSD analysis. Specifically, the value calculated by the following
formula using a boundary of a difference in orientation of 15 degrees or more as a grain
boundary is the average effective crystal grain size. In the formula, N is the number of crystal
35 grains included in the region for evaluation of the average effective crystal grain size, Ai is the
area of the i-th (i1, 2,, N) grain, and di is the circle equivalent diameter of the i-th crystal
17
grain. These data is easily found by EBSD analysis.
[Mathematical 3]
[0054]
5 (Hot dip galvanized layer)
The base steel sheet according to the embodiment of the present invention has a hot dip
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
10 amount of deposition of the coating layer is not particularly limited and may be a general amount
of deposition.
[0055]

Next, the method for producing the hot dip galvanized steel sheet according to the
15 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
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.
[0056]
20 The method for producing the hot dip galvanized steel sheet comprises
(A) a hot rolling step comprising finish rolling a slab having the same chemical
composition as the chemical composition explained above relating to the base steel sheet, then
coiling it up, wherein the hot rolling step satisfies the conditions of the following (A1) to (A4):
(A1) a time during which the slab dwells at the temperature TB or less from extracting
25 the slab to the finish rolling inlet side is 300 seconds or less,
18
[Mathematical 4]
(where [B] and [N] respectively represent mass% of boron (B) and nitrogen (N)),
(A2) in the finish rolling, the finish rolling inlet side temperature is 900 to 1050C, the
finish rolling exit temperature is 8505 C to 1000C and a total rolling reduction is 70 to 95%,
(A3) in cooling of the steel sheet after the finish rolling, the average cooling rate from
the finish rolling exit temperature to 800C is VC/s or more,
[Mathematical 5]
10 (where [B] and [N] respectively represent mass% of boron (B) and nitrogen (N)),
(A4) 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
15 cooled steel sheet then third soaking it, wherein the hot dip galvanizing step satisfies the
conditions of the following (B1) to (B7):
(B1) in the heating of the steel sheet before the first soaking, an average heating rate
from 650C to a maximum heating temperature of Ac1+30C or more and 950C or less is
0.5C/s to 10.0C/s,
20 (B2) the steel sheet is held at the maximum heating temperature 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,
(B4) the first cooled steel sheet is held in a range of 480 to 600C for 80 seconds to
25 500 seconds (second soaking),
(B5) when dipping the second soaked steel sheet in a hot dip galvanizing bath, then
alloying it, the alloying treatment is performed in a range of 460 to 600C,
(B6) the second cooling is performed down to Ms-50C or less, and
(B7) the second cooled steel sheet is heated to a temperature region of 200 to 420C,
30 then held in the temperature region for 5 to 1000 seconds (third soaking).
19
[0057]
Below, the method for producing hot dip galvanized steel sheet will be explained in detail.
[0058]
[(A) Hot Rolling Step]
First, in the hot rolling step, a slab having 5 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
10 also be produced by the ingot making method or thin slab casting method.
[0059]
[Time where slab dwells at temperature TB or less from extraction of slab to finish rolling inlet
side: 300 seconds or less]
In the present method, the time where the slab extracted from the casting facility dwells at
15 the temperature TB or less expressed by the following formula (1) until the finish rolling inlet
side is controlled to 300 seconds or less. TB is the temperature where a thermodynamic drive
force at which boron nitride (BN) is precipitated is generated. When steel dwells at a TB or less
temperature for a long time, the BN starts to precipitate, sufficient solid solution B can no longer
be obtained, and the amount of segregation of B at the austenite grain boundaries in the final
20 product decreases. Therefore, the time dwelling at TB or less is limited to 300 seconds or less.
For example, it may be 200 seconds or less or 150 seconds or less.
[Mathematical 6]
(where [B] and [N] respectively represent mass% of boron (B) and nitrogen (N)).
25 [0060]
[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
30 rolling reduction is less than 60%, since the recrystallization during hot rolling becomes
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.
[0061]
20
[Finish rolling inlet side temperature: 900 to 1050C, finish rolling exit side temperature: 850C
to 1000C, and total rolling reduction: 70 to 95%]
The finish rolling is performed in a range satisfying the conditions of a finish rolling inlet
side temperature of 900 to 1050C, a finish rolling exit side temperature of 850C to 1000C,
and a total rolling reduction of 5 70 to 95%. If the finish rolling inlet side temperature falls below
900C, the finish rolling exit side temperature falls below 850C, or the total rolling reduction
exceeds 95%, the hot rolled steel sheet develops texture, so sometimes anisotropy appears in the
final finished product sheet. On the other hand, if the finish rolling inlet side temperature rises
above 1050C, the finish rolling exit side temperature rises above 1000C, or the total rolling
10 reduction falls below 70%, the hot rolled steel sheet becomes coarser in crystal grain size
sometimes leading to coarsening of the final finished product sheet structure and resulting in the
average effective crystal grain size not satisfying a predetermined range. For example, the finish
rolling inlet side temperature may be 950C or more. The finish rolling exit side temperature
may be 900C or more. The total rolling reduction may be 75% or more or 80% or more.
15 [0062]
[Average cooling rate from finish rolling exit side temperature to 800C: VC/s or more]
To keep boron from precipitating as nitrides after the finish rolling, the obtained steel sheet
is cooled by an average cooling rate of VC/s or more in the range from the finish rolling exit
side temperature to 800C. If the cooling rate falls below VC/s, the boron present in a solid
20 solution state due to precipitation of BN decreases, so the amount of segregation of boron at the
austenite grain boundaries in the final product decreases. Here, V is expressed by the following
formula (2).
[Mathematical 7]
25 In the formula, [B] and [N] respectively represent mass% of boron (B) and nitrogen (N).
Further, in the present invention, the “average cooling rate from finish rolling exit side
temperature to 800C” means a value obtained by dividing the difference of the finish rolling
exit side temperature and 800C by the elapsed time from the finish rolling exit side temperature
to 800C.
30 [0063]
[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
21
impaired. On the other hand, if the coiling temperature exceeds 680C, the cementite coarsens
and undissolved cementite remains, so sometimes the workability is impaired. Further, coarse
borides precipitate in the hot rolled steel sheet and the boron present in a solid solution state
decreases, so the amount of segregation of boron at the austenite grain boundaries at the final
product decreases. The coiling temperature may 5 be 470C or more and/or may be 650C or less.
[0064]
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
10 ability.
[0065]
[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
15 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%.
[0066]
20 [(B) Hot Dip Galvanization Step]
[Average heating rate from 650C to maximum heating temperature of Ac1+30C or more and
950C or less: 0.5 to 10.0C/s]
In this method, after the hot rolling step, the obtained steel sheet is coated in a hot dip
galvanization step. In the hot dip galvanization step, first, the steel sheet is heated and subjected
25 to first soaking treatment. At the time of heating the steel sheet, the average heating rate from
650C to the maximum heating temperature of Ac1+30C or more and 950C or less is limited
to 0.5 to 10.0C/s. If the heating rate is more than 10.0C/s, the recrystallization of ferrite does
not sufficiently proceed and sometimes the elongation of the steel sheet becomes poor. On the
other hand, if the average heating rate falls below 0.5C/s, the austenite becomes coarse, so
30 sometimes the finally obtained steel structures become coarse. This average heating rate may be
1.0C/ or more and/or may be 8.0C/s or less or 5.0C/s or less. In the present invention, the
“average heating rate” means the value obtained by dividing the difference between 650C and
the maximum heating temperature by the elapsed time from 650C to the maximum heating
temperature.
35 [0067]
[First soaking treatment: Holding at maximum heating temperature of Ac1+30C or more and
22
950C or less for 1 second to 1000 seconds]
To cause sufficient austenite transformation to proceed, the steel sheet is heated to at least
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 5 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
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
10 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
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
15 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
soaking treatments are not included in the first, second, and third soaking treatments according
to the embodiment of the present invention.
20 [0068]
[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
cooling. The cooling stop temperature is 480C to 600C of the following second soaking
treatment temperature. The average cooling rate in a temperature range of 700C to 600C is 10
25 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
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
30 elapsed time from 700C to 600C.
[0069]
[Second soaking treatment: Holding in range of 480C to 600C for 80 to 500 seconds]
Second soaking treatment holding the steel sheet in a range of 480C to 600C for 80 to 500
seconds is performed for making the concentration of segregation of the B atoms in the austenite
35 grain boundaries rise more. If the temperature of the second soaking treatment falls below 480C
or becomes higher than 600 or if the holding time falls below 80 seconds, the segregation of the
23
B atoms to the austenite grain boundaries does not sufficiently proceed. On the other hand, if the
holding time becomes more than 500 seconds, since bainite transformation will excessively
proceed, the metal structures according to the embodiment of the present invention will not be
able to be obtained. The temperature of the second soaking treatment may be 500C or more
and/or may be 570C or less. Further, the 5 holding time may be 100 seconds or more and/or may
be 400 seconds or less. In relation to this, even if simply suitably performing the second soaking
treatment, if not suitably securing sufficient solid solution B in the hot rolling step, the amount
of segregation of B at the austenite grain boundaries in the final product decreases. Therefore, in
the method for producing the hot dip galvanized steel sheet according to an embodiment of the
10 present invention, to make the amount of segregation of B to the austenite grain boundaries in
the final product increase, it is important to satisfy the conditions of (A1), (A3), and (A4)
explained above in the hot rolling step while suitably performing the second soaking treatment in
the hot dip galvanization step.
In the present method, to produce the hot dip galvanized steel sheet according to an
15 embodiment of the present invention, after the second soaking treatment, predetermined coating
treatment has to be performed, but if the second soaking treatment were performed after dipping
in the coating bath, sometimes the powdering resistance of the coated layer would remarkably
deteriorate. This is because if performing heat treatment after dipping in a coating bath at 480C
or more for 80 seconds or more, the alloying reaction between the coating and steel sheet
20 excessively proceeds and the structure inside the coating film changes from the  phases
excellent in ductility to the Γ phases poor in ductility.
[0070]
After the second soaking treatment, the steel sheet is dipped in a hot dip galvanizing bath.
The steel sheet temperature at this time has little effect on the performance of the steel sheet, but
25 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
of a step for cooling 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 460C and the
30 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.
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.
[0071]
35 [Alloying treatment: 460 to 600C]
The hot dip galvanized steel sheet formed with the hot dip galvanized layer may be treated
24
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 5 adhesion of the steel sheet deteriorates. Further, sometimes
pearlite transformation proceeds and the desired metallic structure cannot be obtained.
Therefore, the alloying treatment temperature is 600C or less. The alloying treatment
temperature may be 500C or more or may be 580C or less.
[0072]
10 [Second cooling: Cooling to Ms-50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)-
50C or less so as to make part or the majority of the austenite transform to martensite. The
martensite produced here is tempered by the subsequent reheating and third soaking treatment to
15 become tempered martensite. If the cooling stop temperature is more than Ms-50C, since the
tempered martensite is not sufficiently formed, the desired metallic structure is not obtained. If
desiring to utilize the retained austenite for improving the ductility of the steel sheet, it is
desirable to provide a lower limit to the cooling stop temperature. Specifically, the cooling stop
temperature is desirably controlled to a range of Ms-50C to Ms-180C.
20 [0073]
The martensite transformation in the present invention occurs after the ferrite
transformation and bainite transformation. Along with the ferrite transformation and bainite
transformation, C is diffused in the austenite. For this reason, this does not match the Ms when
heating to the austenite single phase and rapidly cooling. The Ms in the present invention is
25 found by measuring the thermal expansion temperature in the second cooling. For example, the
Ms in the present invention can be found by using a Formastor tester or other apparatus able to
measure the amount of thermal expansion during continuous heat treatment, reproducing the
heat cycle of the hot dip galvanization line from the start of hot dip galvanization heat treatment
(corresponding to room temperature) to the above second cooling, and measuring the thermal
30 expansion temperature at that second cooling. However, in actual hot dip galvanization heat
treatment, sometimes cooling is stopped between Ms to room temperature, but at the time of
measurement of thermal expansion, cooling is performed down to room temperature. FIG. 2 is a
temperature-thermal expansion curve simulating by a thermal expansion measurement device a
heat cycle corresponding to the hot dip galvanization treatment according to an embodiment of
35 the present invention. Steel sheet linearly thermally contracts in the second cooling step, but
departs from a linear relationship at a certain temperature. The temperature at this time is the Ms
25
in the present invention.
[0074]
[Third soaking treatment: Holding in temperature region of 200C to 420C for 5 to 1000
seconds]
After the second cooling, the steel sheet is reheated to 5 a range of 200C to 420C for the
third soaking treatment. In this step, the martensite produced at the time of the second cooling is
tempered. If the holding temperature is less than 200C or the holding time is less than 5
seconds, the tempering does not sufficiently proceed. On the other hand, since bainite
transformation does not sufficiently proceed, it becomes difficult to obtain the desired amount of
10 retained austenite. 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 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 240C or more and
may be 400C or less. Further, the holding time may be 15 seconds or more or may be 100
15 seconds or more and may be 600 seconds or less.
[0075]
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
20 elongation rate is preferably 2% or less.
EXAMPLES
[0076]
Next, examples of the present invention will be explained. The conditions in the examples
25 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
invention and achieving the object of the present invention.
[0077]
30 [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
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
35 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
26
Table 2, the temperatures were held within a range of the temperatures shown in Table 2 10C.
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.
5 [0078]
[Table 1-1]
Table 1-1
Steel
type
C Si Mn P S Al N O Cr Mo V Nb
A 0.187 1.77 2.54 0.007 0.0011 0.025 0.0027 0.0006
B 0.201 1.52 1.50 0.010 0.0016 0.022 0.0035 0.0018
C 0.212 0.95 2.27 0.012 0.0012 0.015 0.0046 0.0018 0.23 0.022
D 0.140 1.29 1.12 0.007 0.0018 0.037 0.0014 0.0014 1.06 0.15
E 0.236 1.90 2.23 0.009 0.0019 0.019 0.0025 0.0015
F 0.313 1.84 3.06 0.009 0.0007 0.029 0.0018 0.0020 0.25 0.08
G 0.165 1.10 2.72 0.012 0.0025 0.021 0.0043 0.0014 0.27
H 0.203 1.59 1.51 0.005 0.0026 0.041 0.0025 0.0019
I 0.132 0.34 2.33 0.007 0.0024 0.014 0.0017 0.0022 0.19 0.033
J 0.071 0.46 2.55 0.009 0.0007 0.123 0.0016 0.0021 0.49 0.10
K 0.113 0.78 2.32 0.019 0.0005 0.027 0.0047 0.0010 0.80 0.009
L 0.085 0.13 2.86 0.018 0.0013 0.029 0.0021 0.0018
M 0.295 1.88 2.04 0.009 0.0014 0.049 0.0010 0.0023
N 0.199 0.92 2.60 0.010 0.0028 0.652 0.0032 0.0005
O 0.166 1.04 2.47 0.015 0.0017 1.188 0.0027 0.0024 0.23
P 0.222 0.48 2.60 0.008 0.0011 0.033 0.0029 0.0004 0.15 0.010
Q 0.045 1.31 2.55 0.015 0.0017 0.046 0.0039 0.0006
R 0.193 1.69 2.64 0.010 0.0016 0.017 0.0029 0.0006
S 0.145 1.18 3.96 0.011 0.0020 0.031 0.0018 0.0011
T 0.210 1.36 0.77 0.015 0.0017 0.044 0.0040 0.0009
U 0.166 2.88 1.52 0.010 0.0030 0.044 0.0028 0.0013
V 0.390 1.22 2.09 0.013 0.0011 0.035 0.0025 0.0013
W 0.194 1.40 2.25 0.016 0.0022 0.020 0.0019 0.0023
Bold underlines show outside ranges of present invention.
Empty fields in the table show corresponding constituents not intentionally added.
5 [0079]
[Table 1-2]
28
Table 1-2
Steel
type
Ti B Cu Ni Co W Sn Sb Others Ac1
A 0.015 0.0023 Mg: 0.0045 747
B 0.101 0.0006 751
C 0.010 0.0018 726
D 0.013 0.0029 766
E 0.022 0.0008 0.25 0.16 752
F 0.016 0.0021 748
G 0.064 0.0012 Ce: 0.0050 726
H 0.011 0.0022 0.23 0.37 753
I 0.029 0.0018 708
J 0.018 0.0035 Ca: 0.0044 717
K 0.023 0.0010 0.11 0.08 734
L 0.020 0.0038 La: 0.0013,Hf: 0.0056 696
M 0.009 0.0029 Bi: 0.0067 756
N 0.018 0.0020 REM: 0.0072 722
O 0.034 0.0014 Zr: 0.0053 727
P 0.015 0.0018 0.12 709
Q 0.018 0.0021 734
R 0.019 0.0003 744
S 0.025 0.0028 715
T 0.023 0.0015 754
U 0.019 0.0020 791
V 0.020 0.0025 736
W 0.013 0.0108 740
Bold underlines show outside ranges of present invention.
Empty fields in the table show corresponding constituents not intentionally added.
5 [0080]
[Table 2-1]
29
Table 2-1
No.
Steel
type
Hot rolling step Cold rolling step
Slab
heating
temp.
TB
Dwell time at
TB or less
Total rolling
reduction of rough
rolling at 1050C or
more
Finish inlet side
temp.
Finish exit side
temp.
Finish rolling total
rolling reduction
V
Cooling rate
from finish exit
side to 800C
Coiling temp. Cold rolling reduction
C C s % C C % C/s C/s C %
1 A 1200 1064 23 84 1050 900 92 3.9 5.5 580 53
2 A 1240 1064 70 84 1020 910 92 3.9 6.8 490 53
3 A 1270 1064 55 84 1020 920 92 3.9 6.3 500 53
4 A 1200 1064 39 84 1040 930 92 3.9 5.9 600 53
5 A 1270 1064 33 84 1030 920 92 3.9 7.0 550 53
6 A 1250 1064 20 84 1050 920 92 3.9 5.6 510 53
7 B 1240 1006 0 84 1050 910 92 1.6 5.2 470 53
8 B 1270 1006 11 84 990 900 92 1.6 4.7 530 53
9 B 1220 1006 0 84 1040 920 92 1.6 4.9 470 53
10 C 1250 1080 101 84 1030 940 92 6.2 7.1 520 53
11 C 1190 1080 459 84 980 920 92 6.2 6.8 490 53
12 C 1220 1080 59 84 1050 950 92 6.2 1.8 610 53
13 C 1260 1080 73 84 1030 950 92 6.2 7.9 600 53
14 D 1240 1041 14 84 1030 920 92 2.4 6.0 490 53
15 D 1250 1041 40 84 1000 930 92 2.4 6.2 620 53
16 D 1250 1041 46 84 1000 950 92 2.4 5.9 470 53
17 D 1230 1041 0 84 1050 940 92 2.4 6.1 570 53
18 E 1230 1004 0 84 1050 900 92 1.6 5.3 620 53
19 E 1280 1004 3 84 1000 920 92 1.6 5.4 520 53
20 E 1270 1004 0 84 1010 950 92 1.6 5.2 570 53
Bold underlines show outside ranges of present invention.
[0081]
5 [Table 2-2]
30
Table 2-2
No.
Steel
type
Hot rolling step Cold rolling step
Slab
heating
temp.
TB
Dwell time at
TB or less
Total rolling
reduction of rough
rolling at 1050C or
more
Finish inlet side
temp.
Finish exit side
temp.
Finish rolling total
rolling reduction
V
Cooling rate
from finish exit
side to 800C
Coiling temp. Cold rolling reduction
C C s % C C % C/s C/s C %
21 F 1220 1037 19 86 1020 950 88 2.3 5.8 610 40
22 F 1280 1037 16 86 1020 930 92 2.3 6.0 480 40
23 F 1270 1037 6 86 1030 950 92 2.3 6.1 590 40
24 G 1270 1054 72 84 1000 950 92 3.1 5.0 590 53
25 G 1230 1054 16 84 1040 920 92 3.1 6.5 580 53
26 G 1280 1054 75 84 1000 900 92 3.1 6.3 540 53
27 G 1230 1054 53 84 1010 910 92 3.1 6.8 480 53
28 H 1250 1057 21 84 1040 950 92 3.4 5.9 490 53
29 H 1280 1057 44 84 1010 930 92 3.4 6.2 480 53
30 H 1270 1057 31 84 1030 930 92 3.4 5.7 500 53
31 H 1250 1057 19 84 1040 950 92 3.4 5.9 540 53
32 I 1260 1026 0 84 1030 920 92 2.0 5.2 520 53
33 I 1250 1026 30 84 1000 920 92 2.0 5.7 510 53
34 I 1280 1026 0 84 1130 1040 92 2.0 5.9 530 53
35 J 1260 1058 62 84 1010 910 92 3.4 6.0 540 53
36 J 1220 1058 70 84 1020 910 92 3.4 6.0 550 53
37 J 1210 1058 40 84 1030 960 92 3.4 5.5 600 53
38 K 1240 1049 43 84 1010 920 92 2.8 6.1 500 53
39 K 1240 1049 0 84 1050 950 92 2.8 5.7 600 53
40 K 1250 1049 17 84 1030 960 92 2.8 5.8 550 53
Bold underlines show outside ranges of present invention.
[0082]
5 [Table 2-3]
31
Table 2-3
No.
Steel
type
Hot rolling step Cold rolling step
Slab
heating
temp.
TB
Dwell time at
TB or less
Total rolling
reduction of rough
rolling at 1050C or
more
Finish inlet side
temp.
Finish exit side
temp.
Finish rolling total
rolling reduction
V
Cooling rate
from finish exit
side to 800C
Coiling temp. Cold rolling reduction
C C s % C C % C/s C/s C %
41 L 1220 1078 66 84 1040 960 92 5.8 6.2 580 75
42 L 1260 1078 59 84 1040 910 92 5.8 6.3 590 75
43 M 1210 1023 3 84 1020 950 92 1.9 5.3 580 53
44 M 1230 1023 0 84 1040 950 92 1.9 6.6 560 53
45 M 1230 1023 3 84 1020 920 92 1.9 6.6 570 53
46 N 1260 1066 89 84 1010 930 92 4.1 6.6 600 53
47 N 1210 1066 20 84 1050 950 50 4.1 5.4 600 53
48 N 1240 1066 28 84 1040 970 92 4.1 6.7 570 53
49 O 1210 1037 0 84 1050 950 92 2.3 6.3 540 53
50 O 1240 1037 0 84 1040 960 92 2.3 6.5 530 53
51 P 1220 1055 84 84 1010 930 92 3.2 6.7 600 53
52 P 1210 1055 24 84 1040 940 92 3.2 5.6 540 53
53 P 1220 1055 56 84 1020 910 92 3.2 5.3 550 53
54 Q 1240 1080 54 84 1040 920 92 6.1 8.1 590 53
55 R 1260 963 0 84 1010 900 92 1.2 7.1 560 53
56 S 1270 1053 55 84 1010 890 92 3.0 7.5 500 53
57 T 1250 1062 63 84 1020 880 92 3.7 6.9 540 53
58 U 1230 1058 34 84 1030 940 92 3.4 7.6 520 53
59 V 1210 1064 35 84 1040 920 92 4.0 8.2 600 53
60 W 1270 1134 259 84 1050 960 92 91.3 8.0 580 53
61 A 1220 1064 16 84 1050 940 92 3.9 6.2 730 53
Bold underlines show outside ranges of present invention.
[0083]
5 [Table 2-4]
32
Table 2-4
No.
Hot dip galvanization step
Ms at hot dip galvanization
step
Heating First soaking treatment First cooling Second soaking treatment Alloying treatment Second cooling Third soaking treatment
Heating rate 650C to
maximum heating temp.
Temp. Holding time Cooling rate Temp. Holding time Alloying temp. Cooling stop temp. Temp. Holding time
C/s C s C/s C s C C C s C
1 1.4 850 90 28 550 100 530 250 380 330 366
2 2.1 890 90 30 560 100 540 250 400 330 382
3 1.7 850 90 39 420 100 540 250 390 330 359
4 2.0 860 90 22 560 60 530 230 390 330 369
5 1.9 850 90 23 540 100 540 260 500 330 367
6 1.6 850 90 21 550 100 - 240 390 330 363
7 1.8 840 90 25 520 100 510 250 400 330 353
8 1.6 840 90 28 530 100 500 40 390 330 364
9 2.0 850 90 29 520 590 520 150 400 330 217
10 1.5 820 90 24 550 100 530 230 390 330 362
11 1.4 830 90 22 540 100 530 270 390 330 364
12 1.5 830 90 23 550 100 520 250 390 330 364
13 1.6 830 90 22 550 100 - 240 380 330 364
14 1.6 870 90 29 550 100 550 200 300 330 427
15 1.8 850 90 24 550 100 540 290 320 330 419
16 1.5 850 90 25 570 390 540 280 320 330 417
17 1.8 860 90 21 650 100 550 280 320 330 357
18 1.5 840 90 19 560 100 530 250 400 330 361
19 1.6 870 90 21 560 100 530 250 340 330 372
20 0.9 760 90 21 550 100 530 250 380 330 91
Bold underlines show outside ranges of present invention.
[0084]
5 [Table 2-5]
33
Table 2-5
No.
Hot dip galvanization step
Ms at hot dip galvanization
step
Heating First soaking treatment First cooling Second soaking treatment Alloying treatment Second cooling Third soaking treatment
Heating rate 650C to
maximum heating temp.
Temp. Holding time Cooling rate Temp. Holding time Alloying temp. Cooling stop temp. Temp. Holding time
C/s C s C/s C s C C C s C
21 1.7 860 90 31 540 100 560 210 380 330 313
22 1.0 860 135 14 540 150 550 210 380 495 312
23 1.5 860 90 27 550 100 - 180 390 330 313
24 1.3 840 90 23 550 100 580 150 300 110 365
25 1.6 840 90 19 560 100 570 160 180 330 366
26 1.3 840 90 25 550 100 570 160 300 3 365
27 1.4 840 90 28 550 100 570 160 240 330 366
28 1.6 850 90 27 490 100 540 280 390 330 373
29 1.9 850 90 29 490 100 - 280 380 330 371
30 1.4 850 90 3 500 100 530 220 390 330 278
31 1.5 840 90 29 490 100 630 260 390 330 351
32 1.7 820 90 23 560 100 550 80 280 15 402
33 1.7 820 90 24 550 470 540 60 280 15 401
34 1.8 830 90 25 550 100 550 80 280 15 405
35 1.3 810 90 21 550 100 560 70 290 15 401
36 0.1 800 90 19 560 100 540 80 280 15 398
37 1.1 810 90 18 570 100 - 50 290 15 402
38 1.6 810 90 24 550 100 580 50 280 15 395
39 1.5 810 90 23 560 80 590 70 300 15 395
40 1.4 820 90 21 560 100 - 90 300 15 398
Bold underlines show outside ranges of present invention.
[0085]
5 [Table 2-6]
34
Table 2-6
No.
Hot dip galvanization step
Ms at hot dip galvanization
step
Heating First soaking treatment First cooling Second soaking treatment Alloying treatment Second cooling Third soaking treatment
Heating rate 650C to
maximum heating temp.
Temp. Holding time Cooling rate Temp. Holding time Alloying temp. Cooling stop temp. Temp. Holding time
C/s C s C/s C s C C C s C
41 1.2 810 90 23 540 100 560 60 240 15 389
42 1.3 810 90 17 550 100 - 50 240 15 391
43 2.1 870 90 22 550 100 530 250 400 330 355
44 2.2 870 450 30 550 100 530 230 400 330 363
45 1.7 870 90 26 560 100 - 250 400 330 354
46 2.1 870 90 26 530 100 560 260 410 330 367
47 1.7 880 90 29 540 100 550 250 400 330 377
48 1.7 870 90 33 530 100 - 230 400 330 369
49 1.8 920 90 26 560 100 550 260 400 330 360
50 2.1 920 90 25 560 100 - 270 400 330 357
51 1.6 850 90 23 550 100 510 60 280 15 367
52 1.9 850 90 23 550 100 520 100 250 330 367
53 1.8 860 90 21 550 100 - 50 280 15 367
54 1.6 870 90 25 550 100 510 290 380 330 420
55 2.0 850 90 26 550 100 500 280 390 330 349
56 1.2 820 90 24 550 100 530 220 390 330 340
57 1.6 880 90 23 550 100 490 200 380 330 160
58 1.7 890 90 31 550 100 590 240 400 330 386
59 1.2 820 90 23 550 100 540 200 370 330 307
60 1.9 870 90 26 540 100 580 210 380 330 280
61 1.8 850 90 36 550 100 510 250 400 330 348
Bold underlines show outside ranges of present invention.
35
[0086]
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
piece was tested by the “JFS T 1001 Hole Expansion Te 5 st 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.
[0087]
10 The hydrogen embrittlement resistance after the plastic deformation was evaluated by a Ubending
test. First, a 30 mm x 120 mm strip-shaped test piece was taken from the steel sheet so
that the longitudinal direction of the test piece and the rolling direction of the steel sheet became
vertical and holes were drilled for fastening the bolts at the two ends of the test piece. Next, as
shown in FIG. 3, a radius 5 mm punch was used bending by 180. After that, the sprung back U15
bending test piece was stressed by fastening it using bolts and nuts. At that time, a GL5 mm
strain gauge was attached to the top part of the U-bending test piece and stress was applied by
control of the amount of strain. The applied stress corresponded to 1000 MPa. At this time, the
strain was converted to stress from the stress-strain curve obtained in advance by a tensile test.
After that, the test piece was dipped in a pH 1.0 hydrochloric acid for 24 hours. The end faces of
20 the U-bent test piece was left as sheared. A test piece with cracks recognized at the bent tip after
the end of the test was judged as “poor”, while one with no cracks recognized was judged as
“very good”. An evaluation of very good was deemed passing.
[0088]
The results are shown in Table 3. In Table 3, “GA” means hot dip galvannealing, while GI
25 means hot dip galvanizing without alloying treatment.
[0089]
[Table 3-1]
36
Table 3-1
No.
Steel
type
Coating
Microstructure Mechanical properties
Ferrite Remarks
Retained
austenite
Tempered
martensite
Fresh
martensite
Pearlite+
cementite
Bainite
Former 
grain
boundary
B conc.
Average
effective grain
size
Press formability
Hydrogen
embrittlement
resistance
TS El  TSEl0.5
/1000
% % % % % % atm% m MPa % %
1 A GA 18 10 54 2 0 16 3.8 4.8 1216 17.1 43 136 Very good Ex.
2 A GA 0 9 73 1 0 17 4.1 5.3 1257 14.5 57 138 Very good Ex.
3 A GA 20 11 32 3 0 34 1.7 4.5 1148 17.7 30 111 Poor Comp. ex.
4 A GA 15 9 60 0 0 16 1.8 5.0 1221 16.4 41 128 Poor Comp. ex.
5 A GA 18 3 55 5 6 13 4.0 4.7 1075 14.2 22 72 Very good Comp. ex.
6 A GI 19 11 46 1 0 23 4.0 4.9 1209 17.5 44 140 Very good Ex.
7 B GA 34 12 17 2 0 35 2.3 3.0 1025 23.6 30 132 Very good Ex.
8 B GA 34 4 50 0 0 12 2.9 3.4 1103 14.4 34 93 Very good Ex.
9 B GA 28 10 3 2 0 57 2.5 3.7 931 21.2 26 101 Very good Comp. ex.
10 C GA 20 9 58 1 0 12 3.4 4.0 1196 16.6 51 142 Very good Ex.
11 C GA 16 9 49 1 0 25 1.6 3.9 1183 15.2 38 111 Poor Comp. ex.
12 C GA 17 9 55 1 0 18 1.5 4.2 1190 15.5 40 117 Poor Comp. ex.
13 C GI 17 10 52 1 0 20 3.6 4.0 1188 16.5 54 144 Very good Ex.
14 D GA 13 6 77 2 0 2 5.1 4.1 1230 11.3 48 96 Very good Ex.
15 D GA 21 7 56 2 0 14 4.7 4.8 1201 14.6 40 111 Very good Ex.
16 D GA 22 7 50 1 0 20 5.5 4.8 1183 15.0 35 105 Very good Ex.
17 D GA 57 6 15 3 0 19 1.4 6.3 960 19.1 27 95 Poor Comp. ex.
18 E GA 10 16 58 2 0 14 2.8 4.5 1277 18.2 31 129 Very good Ex.
19 E GA 0 14 77 1 0 8 2.7 4.4 1369 14.4 27 102 Very good Ex.
20 E GA 75 6 0 4 6 9 2.0 6.1 944 15.5 18 62 Very good Comp. ex.
Bold underlines show outside ranges of present invention.
[0090]
5 [Table 3-2]
37
Table 3-2
No.
Steel
type
Coating
Microstructure Mechanical properties
Ferrite Remarks
Retained
austenite
Tempered
martensite
Fresh
martensite
Pearlite+
cementite
Bainite
Former 
grain
boundary
B conc.
Average
effective grain
size
Press formability
Hydrogen
embrittlement
resistance
TS El  TSEl0.5
/1000
% % % % % % atm% m MPa % %
21 F GA 0 20 78 2 0 0 5.1 4.9 1506 19.1 30 158 Very good Ex.
22 F GA 0 19 70 1 0 10 5.0 5.2 1477 19.5 26 147 Very good Ex.
23 F GI 0 17 83 0 0 0 4.7 4.8 1491 19.4 27 150 Very good Ex.
24 G GA 12 7 64 4 0 13 3.0 3.9 1195 12.0 40 91 Very good Ex.
25 G GA 12 3 62 11 0 12 3.0 4.0 1262 10.7 24 66 Poor Comp. ex.
26 G GA 13 2 62 12 0 11 3.2 4.4 1230 10.9 28 71 Poor Comp. ex.
27 G GA 12 6 63 7 0 12 3.1 4.3 1221 11.5 35 83 Very good Ex.
28 H GA 26 12 29 1 0 32 4.6 5.9 1114 19.5 33 125 Very good Ex.
29 H GI 26 13 25 2 0 34 4.5 5.5 1112 19.8 30 121 Very good Ex.
30 H GA 61 13 10 4 0 12 4.2 6.5 969 23.6 18 97 Very good Comp. ex.
31 H GA 33 7 12 4 6 38 4.6 5.9 991 18.1 19 78 Very good Comp. ex.
32 I GA 13 2 70 2 0 13 4.3 4.1 1234 10.8 47 91 Very good Ex.
33 I GA 13 2 59 3 0 23 5.0 4.4 1196 11.2 40 85 Very good Ex.
34 I GA 8 1 74 3 0 14 6.1 9.3 1208 10.1 45 82 Poor Comp. ex.
35 J GA 37 0 45 4 0 14 4.8 4.2 1016 13.1 53 97 Very good Ex.
36 J GA 41 0 43 3 0 13 5.2 8.9 1007 13.8 46 94 Poor Comp. ex.
37 J GI 37 0 55 2 0 6 4.9 4.3 1009 13.3 53 98 Very good Ex.
38 K GA 16 1 75 3 0 5 5.6 4.7 1195 10.6 60 98 Very good Ex.
39 K GA 15 1 72 3 0 9 2.4 4.5 1189 10.9 54 95 Very good Ex.
40 K GI 8 1 70 3 0 18 5.0 4.8 1200 10.8 56 97 Very good Ex.
Bold underlines show outside ranges of present invention.
[0091]
5 [Table 3-3]
38
Table 3-3
No.
Steel
type
Coating
Microstructure Mechanical properties
Ferrite Remarks
Retained
austenite
Tempered
martensite
Fresh
martensite
Pearlite+
cementite
Bainite
Former 
grain
boundary
B conc.
Average
effective grain
size
Press formability
Hydrogen
embrittlement
resistance
TS El  TSEl0.5
/1000
% % % % % % atm% m MPa % %
41 L GA 34 0 40 2 0 24 4.9 5.0 1033 11.8 52 88 Very good Ex.
42 L GI 33 0 45 2 0 20 5.1 4.9 1024 12.0 50 87 Very good Ex.
43 M GA 4 20 51 2 0 23 5.3 4.7 1237 21.5 32 150 Very good Ex.
44 M GA 0 19 66 2 0 13 5.3 5.2 1260 20.1 41 162 Very good Ex.
45 M GI 5 19 50 3 0 23 5.5 4.9 1206 21.2 33 147 Very good Ex.
46 N GA 25 11 43 2 0 19 4.0 6.4 1148 18.6 33 123 Very good Ex.
47 N GA 17 10 49 2 0 22 4.2 7.5 1134 16.9 25 96 Poor Comp. ex.
48 N GI 25 12 50 1 0 12 4.2 6.1 1172 17.0 40 126 Very good Ex.
49 O GA 48 9 15 1 0 27 4.4 6.5 1011 21.0 21 97 Very good Ex.
50 O GI 48 10 9 2 0 31 4.5 6.6 987 21.2 22 98 Very good Ex.
51 P GA 0 2 96 2 0 0 5.0 4.5 1512 8.1 51 87 Very good Ex.
52 P GA 0 4 92 4 0 0 4.9 4.7 1530 9.0 48 95 Very good Ex.
53 P GI 0 2 96 2 0 0 5.2 4.5 1514 8.3 50 89 Very good Ex.
54 Q GA 35 1 37 0 0 27 4.7 6.8 886 19.8 54 129 Very good Comp. ex.
55 R GA 19 12 41 2 0 26 0.8 6.5 1114 19.3 32 122 Poor Comp. ex.
56 S GA 7 4 69 20 0 0 4.9 4.0 1331 10.2 11 45 Poor Comp. ex.
57 T GA 51 9 0 0 0 40 3.0 6.2 748 27.9 29 112 Very good Comp. ex.
58 U GA 39 5 15 18 0 23 4.2 6.0 1229 11.9 9 44 Poor Comp. ex.
59 V GA 8 24 29 12 0 27 5.1 6.6 1278 21.1 8 76 Poor Comp. ex.
60 W GA 58 9 12 3 0 18 1.0 6.1 951 18.4 23 84 Poor Comp. ex.
61 A GA 29 9 31 2 0 29 1.2 6.0 1123 16.7 35 111 Poor Comp. ex.
Bold underlines show outside ranges of present invention.
39
[0092]
Comparative Examples 3 and 4 had temperatures of the second soaking treatment in the hot
dip galvanization step lower than 480C or holding times of the second soaking treatment of less
than 80 seconds. As a result, the concentrations of the solid solution B at the prior austenite grain
boundaries became less than 2.0 atm% and h 5 ydrogen embrittlement resistances were poor.
Comparative Example 5 had a temperature of the third soaking treatment at the hot dip
galvanization step was higher than 420C. As a result, the desired metallic structure was not
obtained and press formability was poor. Comparative Example 9 had a holding time of the
second soaking treatment in the hot dip galvanization step of more than 500 seconds. As a result,
10 the desired metallic structure was not obtained and press formability was poor. Comparative
Example 11 had a dwell time of TB or less in the hot rolling step of more than 300 seconds. As a
result, the concentration of the solid solution B at the prior austenite grain boundaries becomes
less than 2.0 atm% and the hydrogen embrittlement resistance was poor. Comparative Example
12 had an average cooling rate from the finish rolling exit side temperature to 800C in the hot
15 rolling step of less than VC/s. As a result, the concentration of the solid solution B of the prior
austenite grain boundaries became less than 2.0atm% and the hydrogen embrittlement resistance
was poor. Comparative Example 17 had a temperature of the second soaking treatment at the hot
dip galvanization step of more than 600C. As a result, the desired metallic structure was not
obtained, the concentration of solid solution B at the prior austenite grain boundaries became
20 less than 2.0 atm%, and the press formability and hydrogen embrittlement resistance were poor.
Comparative Example 20 had a temperature of the first soaking temperature in the hot dip
galvanization step which was less than the lower limit prescribed in the present invention, so was
less than Ac1+30C. As a result, the desired metallic structure was not obtained and press
formability was poor.
25 [0093]
Comparative Examples 25 and 26 had temperatures of the third soaking treatment at the hot
dip galvanization step of less than 200C or the holding times of the third soaking treatment of
less than 5 seconds. As a result, the desired metallic structures were not obtained and the press
formabilities and hydrogen embrittlement resistances were poor. Comparative Example 30 had
30 an average cooling rate of the first cooling in the hot dip galvanization step of less than 10C/s.
As a result, the desired metallic structure was not obtained and press formability was poor.
Comparative Example 31 had a temperature in the alloying treatment in the hot dip galvanization
step of more than 600C. As a result, the desired metallic structure was not obtained and press
formability was poor. Comparative Example 34 had a finish rolling inlet side temperature and
35 finish rolling exit side temperature at the hot rolling step of more than 1050C and more than
1000C. As a result, the average effective crystal grain size of the metallic structure was more
40
than 7.0 m and the hydrogen embrittlement resistance was poor. Comparative Example 36 had
an average heating rate in the hot dip galvanization step of less than 0.5C/s, had an average
effective crystal grain size of the metallic structure becoming more than 7.0 m, and had a
hydrogen embrittlement resistance which was poor. Comparative Example 47 had a total rolling
reduction of the finish rolling in the hot rolling 5 step of less than 70%. As a result, the average
effective crystal grain size of the metallic structure became more than 7.0 m and the hydrogen
embrittlement resistance was poor. Comparative Examples 54 to 60 had chemical compositions
not controlled to within the predetermined ranges, so the press formabilities and/or hydrogen
embrittlement resistances were poor. Comparative Example 61 had a coiling temperature at the
10 hot rolling step was more than 680C. As a result, the concentration of the solid solution B at the
prior austenite grain boundaries became less than 2.0 atm% and the hydrogen embrittlement
resistance was poor.
[0094]
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 15 /1000 of 80 or more and further have excellent hydrogen
embrittlement resistance after plastic deformation, so it is learned that they are excellent in press
formability and hydrogen embrittlement resistance after plastic forming.
[0095]
[Example B]
20 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
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
25 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.
[0096]
[Table 4]
41
Table 4
No.
Steel
type
Hot rolling step
Cold
rolling
step
Hot dip galvanization step
Ms at hot dip
galvanization
step
Slab
heating
temp.
TB
Dwell
time at
TB or
less
Total rolling
reduction
of rough
rolling at
1050C
or more
Finish
inlet side
temp.
Finish
exit side
temp.
Finish
rolling
total
rolling
reduction
V
Cooling rate
from
finish exit
side to
800C
Coiling
temp.
Cold
rolling
reduction
Heating
First soaking
treatment
First cooling
Alloying
treatment
Third soaking
treatment
Heating rate
from 650C to
maximum
heating temp.
Temp.
Holding
time
Cooling
rate
Cooling
step
temp.
Alloying
temp.
Temp.
Holding
time
C C s % C C % C/s C/s C % C/s C s C/s C C C s C
62 A 1220 1064 23 84 1050 900 92 3.9 5.5 580 53 1.4 850 90 1 250 520 380 330 348
Bold underlines show outside ranges of present invention.
[0097]
5 [Table 5]
Table 5
No.
Steel
type
Coating
Microstructure Mechanical properties
Ferrite Remarks
Retained
austenite
Tempered
martensite
Fresh
martensite
Pearlite+
cementite
Bainite
Former 
grain
boundary
B conc.
Average
effective grain
size
Press formability
Hydrogen
embrittlement
resistance
TS El  TSEl0.5
/1000
% % % % % % atm% m MPa % %
62 A GA 30 7 28 7 3 25 1.5 4.6 1128 15.7 32 100 Poor Comp. ex.
Bold underlines show outside ranges of present invention.
42
[0098]
As clear from the results of Table 5, if making the first cooling gradual cooling to eliminate
second soaking treatment, the concentration of the solid solution B at the prior austenite grain
boundaries became less than 2.0 atm% and the hydrogen embrittlement resistance was poor.
5 [0099]
[Example C]
In the example, the relationship between the soaking treatment and the coating treatment
was studied in the same way. First, a slab having the chemical composition shown in Table 1
was prepared. Next, as shown in Table 6, except for performing the coating and alloying
10 treatment not after the second soaking treatment but after the third soaking treatment, the same
procedure was followed to obtain hot dip galvanized steel sheets. The steel structures and
mechanical properaties 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 7. In the soaking
treatments shown in Table 6, the temperature was maintained in the range of the temperature
15 shown in Table 6 10C.
[0100]
[Table 6]
43
Table 6
No.
Steel
type
Hot rolling step
Cold
rolling
step
Hot dip galvanization step
Ms at hot dip
galvanization
step
Slab
heating
temp.
TB
Dwell
time at
TB or
less
Total
rolling
reduction
of rough
rolling at
1050C
or more
Finish
inlet
side
temp.
Finish
exit side
temp.
Finish
rolling
total
rolling
reduction
V
Cooling rate
from
finish exit
side to
800C
Coiling
temp.
Cold
rolling
reduction
Heating
First soaking
treatment
First
cooling
Second
soaking
treatment
Second
cooling
Third soaking
treatment
Alloying
treatment
Heating
rate from
650C to
maximum
heating
temp.
Temp.
Holding
time
Cooling
rate
Temp.
Holding
time
Cooling
stop
temp.
Temp.
Holding
time
Alloying
temp.
C C s % C C % C/s C/s C % C/s C s C/s C s C C s C C
64 B 1250 1006 0 84 1040 960 92 1.6 5.0 500 53 1.8 840 90 25 520 100 250 400 330 560 341
Bold underlines show outside ranges of present invention.
[0101]
5 [Table 7]
Table 7
No.
Steel
type
Coating
Microstructure Mechanical properties
Ferrite Remarks
Retained
austenite
Tempered
martensite
Fresh
martensite
Pearlite+
cementite
Bainite
Former 
grain
boundary
B
concentration
Average
effective grain
size
Press formability
Hydrogen
embrittlement
resistance
TS El  TSEl0.5
/1000
% % % % % % atm% m MPa % %
64 B GA 33 4 20 8 6 29 2.2 3.4 922 18.8 20 78 Very good Comp. ex.
Bold underlines show outside ranges of present invention.
44
[0102]
As clear from the results of Table 7, if performing coating alloying treatment after the third
soaking treatment, the desired metallic structure could not be obtained and the press formability
was poor.

WE CLAIMS

[Claim 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: 0.050% to 0.350%,
Si: 0.10% to 2.50%,
Mn: 1.00% to 3.50%,
P: 0.050% or less,
10 S: 0.0100% or less,
Al: 0.001% to 1.500%,
N: 0.0100% or less,
O: 0.0100% or less,
Ti: 0.005% to 0.200%,
15 B: 0.0005% to 0.0100%,
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%,
Sn: 0% to 1.00%,
25 Sb: 0% to 1.00%,
Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%,
Ce: 0% to 0.0100%,
Zr: 0% to 0.0100%,
30 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,
35 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,
46
ferrite: 0% to 50%,
retained austenite: 0% to 30%,
tempered martensite: 5% or more,
fresh martensite: 0% to 10%, and
5 pearlite and cementite in total: 0% to 5%,
when there are remaining structures, the remaining structures consist of bainite,
a concentration of B atoms at prior austenite grain boundaries is 2.0 atm% or more, and
an average effective crystal grain size is 7.0 m or less.
[Claim 2]
10 The hot dip galvanized steel sheet according to claim 1, wherein the steel microstructure
further contains, by volume fraction, retained austenite: 6% to 30%.
[Claim 3]
A method for producing the hot dip galvanized steel sheet according to claim 1 or 2,
comprising
15 (A) a hot rolling step comprising finish rolling a slab having the chemical composition
according to claim 1, then coiling it up, wherein the hot rolling step satisfies the conditions of
the following (A1) to (A4):
(A1) a time during which the slab dwells at the temperature TB or less from extracting
the slab to the finish rolling inlet side is 300 seconds or less,
20 [Mathematical 1]
(where [B] and [N] respectively represent mass% of boron (B) and nitrogen (N)),
(A2) in the finish rolling, the finish rolling inlet side temperature is 900 to 1050C, the
finish rolling exit temperature is 850C to 1000C and a total rolling reduction is 70 to 95%,
25 (A3) in cooling of the steel sheet after the finish rolling, the average cooling rate from
the finish rolling exit temperature to 800C is VC/s or more,
[Mathematical 2]
(where [B] and [N] respectively represent mass% of boron (B) and nitrogen (N)), and
47
(A4) 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 5 the hot dip galvanizing step satisfies the
conditions of the following (B1) to (B7):
(B1) in the heating of the steel sheet before the first soaking, an average heating rate
from 650C to a maximum heating temperature of Ac1+30C or more and 950C or less is
0.5C/s to 10.0C/s,
10 (B2) the steel sheet is held at the maximum heating temperature 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,
(B4) the first cooled steel sheet is held in a range of 480 to 600C for 80 seconds to
15 500 seconds (second soaking),
(B5) when dipping the second soaked steel sheet in a hot dip galvanizing bath, then
alloying it, the alloying treatment is performed in a range of 460 to 600C,
(B6) the second cooling is performed down to Ms-50C or less, and
(B7) the second cooled steel sheet is heated to a temperature region of 200 to 420C,
20 then held in the temperature region for 5 to 1000 seconds (third soaking).