Specification
DESCRIPTION
HOT STAMP MOLDED BODY, AND METHOD FOR PRODUCING HOT STAMP
MOLDED BODY
Technical Field
[0001] The present invention relates to a hot stamp molded body, which is a component
molded and quenched at the same time by hot press molding, and applied mainly to a skeletal
component, a reinforcing component, a chassis component, or the like of an automobile body,
and a method for producing the same.
Background Art
|"0002| In recent years, for the sake of weight reduction of an automobile leading to
improvement in fuel efficiency, weight reduction of a steel sheet to be used by increasing the
strength of a steel sheel has been endeavored. However, when the strength of a slcel sheet lo
be used is increased, there occurs a problem of occurrence of scoring or steel sheet fracture
during molding, or instability of the shape of a molded item due lo a spring-back
phenomenon.[0003J As a technology for producing a high strength component, there is a
method by which the strength is increased after press molding, instead of pressing a high
strength steel sheet. An example of the same is hot stamp molding. Hot stamp molding is
a method by which a slcel sheel to be molded is heated in advance for facilitating molding,
and subjected to press molding keeping the high temperature as also described in Patent
Literature 1, and 2. As a molding material therefor, a qucnchablc steel grade is selected, and
a higher strength is achieved by quenching on the occasion of cooling after pressing. By this
procedure, the strength of a steel sheel can be enhanced at the same lime as press molding
without conducting a separate heal treatment step for strength increase after press molding.
[0004] However, since hot stamp molding is a molding method by which a healed steel sheet
is processed, formation of a Fc scale by surface oxidation of the sleel sheet is unavoidable.
Even in a case in which a steel sheet is heated in a non-oxidizing atmosphere, when the sheet
is taken out from a healing furnace for press molding, a Fc scale is formed on a surface due lo
exposure to the air. Further, healing in such a non-oxidi/.ing atmosphere is cosily.
10005J In a case in which a Fc scale is formed on a sleel sheet surface during heating, the Fe
scale may be peeled oJVduring pressing to slick lo a mold, so as lo develop such a problem
thai the productivity of pressing may be impaired, or the Fc scale remains on a product after
pressing lo disfeature the appearance. Further, in a case in which such an oxide film remains,
since a Fe scale on a surface of a molded item is poor in adhesiveness, when a conversion
1
treatment and painting are performed on a molded item without removing the scale, a problem
in paint adhesiveness will be developed.
[0006] Therefore, ordinarily a Fe scale is removed by applying a sandblasting treatment or a
shotblasting treatment after hot stamping, and thereafter a conversion treatment or painting is
carried out as described in Patent Literature 3. However, such a blasting treatment is
troublesome, and impairs remarkably the productivity of hot stamping. Further, a strain may
be generated in a molded item.
[0007] Meanwhile, a technology, by which hot stamping is conducted on a zinc-based
coated steel sheet or an aluminum coated steel sheet, while suppressing Fe scale generation,
has been disclosure in Patent Literature 4 to 6. Further, a technology for preforming a hot
press on a coated steel sheet is also disclosed in Patent Literature 7 to 9.
[0008] Further, a method for producing a zinc-based coated steel sheet is disclosed in Patent
Literature 10 and 11.
[0009]
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 1107-116900
Patent Literature 2: JP-A No. 2002-102980
Patent 1 ,iteraluie 3: JP-A No. 2003-2058
Patent Literature 4: JP-A No. 20.00r3£64.0
Patent Literature 5: JP-A No. 2001-353548
Patent Literature 6: JP-A No. 2003-126921
Patent Literature 7: JP-A No. 2011-202205
Patent Literature 8: JP-A No. 2012-233249
Patent Literature 9: J J'-A No. 2005-74464
Patent Literature 9: JP-A No. 2003-126921
Patent Literature 10: JP-A No. 1104-191354
Patent Literature II: JP-A No. 2012-17495
SUMMARY OF INVENTION
Technical Problem
j0010J However, in a case in which an aluminum coated steel sheet, especially a hot-dip
aluminum coaled sleel sheet is hot-stamped, counter diffusion of a plated layer and a steel
matrix material lakes place during sleel sheet healing and an intcrmelallic compound, such as
Fe-AI and Fe-Al-Si, is formed al a plating interface. Further, an oxide film of aluminum is
formed on a surface of a plated layer. The aluminum oxide film compromises paint
adhesiveness, although not so seriously as an iron oxide film, and cannot necessarily satisfy
2
such severe paint adhesiveness as required for an automobile outer plate, a chassis component,
etc. Further, it is difficult to form a conversion coating used broadly as a painting surface
treatment.
[0011] Meanwhile, in a case in which a zinc-based coated steel sheet, especially a hot-dip
zinc coated steel sheet is hot-stamped, a Zn-Fe intermetallic compound or a Fe-Zn solid
solution phase is formed by counter diffusion of a plated layer and a steel matrix material
during steel sheet heating, and a Zn-based oxide film is formed on the outermost surface.
The compound, phase, or oxide film does not impair paint adhesiveness or conversion
treatability, unlike the aluminum-based oxide film.
[0012] In recent years, as a producing process for a steel sheet for hot stamping, a technique
by which a steel sheet can be rapidly heated by Joule heating or induction healing has been
acquiring popularity. In this case, the total of the temperature elevation time and the
retention time at hot stamping is frequently less than 1 min. When a zinc-based eoatcd steel
sheet is hot-stamped under such conditions, a soft plated layer sticks lo a mold, which requires
frequent maintenance works of a mold, and therefore there has been a drawback in that the
productivity is impaired.
fOOl 3J An object of the invention is to overcome the above problems and lo provide a hot
stamp molded body that can be.produced highly efficiently without causing slicking of plating
to a mold, when an cleelrogalvanizcd steel sheet with a light plating weight is hot-stamped
using a rapidly heating method such as Joule healing and induction healing, and can secure
favorable paint adhesiveness without a posit realmenl such as sholblasling alter hot stamping,
as well as a method for producing the same.
Solution to Problem
10014] The essentials of the invention arc as follows.
11] A hot stamp molded body produced by hot-stamping an elcctrogalvanized steel sheet
comprising as components of a sled sheet, by mass %:
C:
Si:
Ai:
Mn:
P:
S:
N:
Ti:
Nb:
from 0.10 lo 0.35%,
from 0.01 to 3.00%,
from 0.01 lo3.00%,
from 1.0 to 3.5%,
from 0.001 to 0.100%,
from 0.001 lo 0.010%,
from 0.0005 to 0.0100%,
from 0.000 lo 0.200%,
from 0.000 to 0.200%,
Mo: from 0.00 to 1.00%,
Cr: from 0.00 to 1.00%,
V: from 0.000 to 1.000%,
Ni: from 0.00 to 3.00%,
B: from 0.0000 to 0.0050%,
Ca: from 0.0000 to 0.0050%, and
Mg: from 0.0000 to 0.0050%,
a balance being Fe and impurities,
wherein the steel sheet is electrogalvanized on each face with a plating weight not
less than 5 g/m2 and less than 40 g/m2;
wherein a galvanized layer of the hot stamp molded body is configured with 0 g/m
to 15 g/m2 of a Zn-Fe intermetallic compound and a Fe-Zn solid solution phase as a balance,
and
wherein in the galvanized layer of the hot stamp molded body, 1x10 pes to IxlO4 pes
of particulate matter with an average diameter of from 10 nm to 1 um are present per 1 mm
length of the galvanized layer.
[0015]
[2] The hot stamp jnolded body-according to [1] above, wherein the steel sheet
comprises, by mass %, one, or two or more kinds of:
Ti: from 0.001 to 0.200%,
Nb: from 0.001 to 0.200%,
Mo: from 0.01 to 1.00%,
Cr: from 0.01 to 1.00%,
V: from 0.001 to 1.000%,
Ni: from 0.01 to 3.00%,
B: from 0.0002 to 0.0050%,
Ca: from 0.0002 to 0.0050%, or
Mg: from 0.0002 to 0.0050%.
[0016]
[3] The hot stamp molded body according to [1] or [2] above, wherein the particulate
matter is one, or two or more kinds of oxides containing one, or two or more kinds out of Si,
Mn, Cr or Al.
[0017]
[4] The hot stamp molded body according to any one of claims [1] to [3] above, wherein
the electrogalvanized steel sheet is an electrolytic zinc alloy-coated steel sheet.
4
[0018]
[5] A method for producing a hot stamp molded body, in which a steel comprising as
components, by mass %:
C:
Si:
Al:
Mn:
P:
S:
N:
Ti:
Nb:
Mo:
Or:
V:
Ni:
B:
Ca:
Mg:
from 0.10 to 0.35%,
from 0.01 to 3.00%,
from 0.01 to 3.00%,
from 1.0 to 3.5%,
from 0.001 to 0.100%,
from 0.001 to 0.010%,
from 0.0005 to 0.0100%,
from 0.000 lo 0.200%,
from 0.000 to 0.200%,
from 0.00 lo 1.00%,
from 0.00 lo 1.00%,
from 0.000 to 1.000%,
from 0.00 lo 3.00%,
from 0.0000 lo 0.0050%,
from 0.0000.to 0.0050%.and
from 0.0000 lo 0.0050%,
a balance being he and impurities, is subjected to a hot rolling step, a pickling step, a cold
rolling step, a continuous annealing step, a temper rolling step, and an eleetrogalvani/.ing step
lo yield an eleetrogalvani/.ed steel sheet, and the electrogalvani/ed steel sheet is subjeeled to a
hot stamp molding *stcp lo produce a hot slamp molded body;
wherein in the continuous annealing step, the steel sheet is subjected lo repealed
bending at a bending angle of from 90° to 220° (bur or more times during heating of the steel
sheet in an atmosphere gas containing hydrogen al from 0.1 volume % lo 30 volume %, and
IlaO corresponding lo a dew point of from -70°C to -20"C as well as nitrogen and impurities
as a balance at a sheet temperature within a range of from 350°C to 700°C,
wherein in the elcclrogalvanizing step, each face of the steel sheet is
eleclrogalvanized with a plating weight of not less than 5 g/m and less than 40 g/m', and
wherein in the hoi slump molding step, the electrogalvani/ed steel sheet is healed
with an average tcnijieiaturc elevation rate of 50°C/see or more to a temperature range of from
700°C lo 1100°C, hol-slampcd within 1 min from the initiation of the temperature elevation,
and thereafter cooled to normal temperature.
[0019]
5
[6] The method for producing r a hot stamp molded body according to [5] above,
wherein the steel comprises, by mass %, one, or two or more kinds of:
Ti: from 0.001 to 0.200%,
Nb: from 0.001 to 0.200%,
Mo: from 0.01 to 1.00%,
Cr: from 0.01 to 1.00%,
V: from 0.001 to 1.000%,
Ni: from 0.01 to 3.00%,
B: from 0.0002 to 0.0050%,
Ca: from 0.0002 to 0.0050%, and
Mg: from 0.0002 to 0.0050%.
Advantageous Effects of Invention
[0020J According lo the invention, a hot stamp molded body that can be produced highly
efficiently without causing slicking of plating lo a mold, when an zinc coated steel sheet with
a light plating weight is hot-stamped using a rapidly healing method such as Joule heating and
induction healing, and can secure favorable paint adhesiveness without a posllrealmenl such
as shotblasting after hot stamping, as well as a method for producing the same can be
provided.
BRIEF DESCRIPTION OF DRAWINGS
[0021]
Fig. I is a diagram showing a heal history during heating for hot stamping, increase
in a Fc concentration in a plated layer, and a phase change of a tissue.
Fig. 2 is a graph showing a relationship between the remaining amount of a Zn-Fc
intennetallic compound after heating for hot stamping and the degree of sticking ofplaling to
a mold.
Fig. 3A is a schematic diagram showing a relationship between the remaining amount
of a Zn-Fc intennetallic compound after heating for hot stamping and the structure of a plated
layer in a case in which a residual Zn-Fc inlermetallie compound is not present.
big. 313 is a schematic diagram showing a relationship between the remaining amount
of a Zn-Fc inlermetallie compound after heating for hot stamping and the structure of a plated
layer in a case in which the remaining amount of a Zn-Fc intennetallic compound is 15 g/m'
or less.
Fig. 3C is a schematic diagram showing a relationship between the remaining amount
of a Zn-Fc inlermetallie compound after heating for hot stamping and the structure of a plated
layer in a ease in which the remaining amount of a Zn-Fe intennetallic compound is beyond
6
15g/m2.
Fig. 4 is a graph showing a relationship between a Zn plating weight before hot
stamping and the amount of a Zn-Fe intermetallic compound after plating.
Fig. 5 is a graph showing a relationship between the formation amount of an oxide
inside a steel sheet and the paint adhesiveness.
Fig. 6Ais a graph showing a relationship between the number of 90° bending during
heating and the formation amount of an oxide inside a steel sheet, with respect to the number
of bending of 0, 1, 2, and 3 times.
Fig. 6B is a graph showing a relationship between the number of 90° bending during
heating and the formation amount of an oxide inside a steel sheet, with respect to the number
of bending of 4, 5, and 7 times.
Fig. 6C is a graph showing a relationship between the number of 90° bending during
heating and the formation amount of an oxide inside a steel sheet, with respect to the number
of bending of 9, and 10 times.
Fig. 7 is a graph showing a relationship between the bending angle inflicted on a
sample during healing and the formation amount of an oxide inside a steel sheet.
DESCRIPTION OF EMBODIMENTS
[0022] The invention will be described in detail below. A numerical range expressed
herein by "x to y" includes, unless otherwise specified, the values of x and y in the range as
the minimum and maximum values respectively.
[0023J The inventor conducted hot stamp molding using cleclrogalvanizcd steel sheets with
a plurality of plating weights under various heating conditions. As the results, it has been
made clear that slicking of plating to a mold can be suppressed with a structure, in which the
amount of a Zn-Fe intcrmetallie compound in a plated layer after heating for hot stamping is
controlled within 0 g/m to 15 g/m , and a balance is a Fe-Zn solid solution phase, wherein a
particulate matter with a predetermined size is present in the plated layer in an appropriate
amount. The details will be described below.
10024] Since a Zn-Fe intcrmetallie compound is soft in a high temperature condition in
which a hot stamp molding is conducted, the Zn-Fe intermetallic compound may slick to a
mold, when the Zn-Fe intcrmetallie compound receives a sliding action during pressing.
Therefore, as shown in Fig. 1, the Fe conccnlialion in a plalcd layer is increased by promoting
a Zn-Fc alloying reaction by healing. When a structure, in which a Zn-Fe intermetallic
compound composed of a 1' phase (FC3Z1110) is not present in a steel sheet surface and only a
Fe-Zn solid solution phase composed of an u-Fc phase is present (the solid line arrow in Ihe
7
Figure), is formed by the above means, sticking of plating to a mold can be suppressed.
Further, it has been known that, even when a Zn-Fe intermetallic compound remains, insofar
as the remaining amount is 15 g/m or less, such severe sticking of plating to a mold as
disturbs production does not occur.
[0025] Next, a relationship between the remaining amount of a Zn-Fe intermetallic
compound after heating for hot stamping and the degree of sticking of plating to a mold is
shown in Fig. 2. When an electrogalvanized steel sheet with a plating weig ht of 30 g/m2 was
heated to 850°C, then cooled to 680°C, and hot-stamped, the remaining amount of a Zn-Fe
intermetallic compound was regulated by adjusting the retention time at 850°C. Then, the
relationship between the remaining amount of a Zn-Fe intermetallic compound and the
sticking to a mold after heating for hot stamping was determined. Based on the remaining
amount of a Zn-Fe intermetallic compound after hot stamping, evaluation of the remaining
amount of a Zn-Fc intermetallic compound was graded in; a double circle: (here is no need for
mold maintenance work (slicking of plating to a mold is extremely insignificant), a circle:
adhered substances can be simply wiped off with rags, or the like (sticking of plating to a
mold is insignificant), and a cross mark: polishing of a mold is necessary (slicking of plating
lo a mold is significant), wherein a double circle and a circle were deemed as acceptable as
on-spccillcation. As obvious from Fig. 2, when the remaining amount of a Zn-Fe
intermetallic compound exceeds 15 g/m', the degree of sticking of plating to a mold becomes
severer.
[0026] The reasons, although based on a presumption, are described referring to Fig. 3A lo
Fig. 3G. Fig. 3 lo Fig. 3C are schematic diagrams showing a relationship between the
remaining amount of a Zn-Fc intermetallic compound after heating for hot stamping and the
structure of a plated layer. When the remaining amount of a Zn-Fc intermetallic compound
is 15 g/m' or less, a Zn-Fe intermetallic compound does not cover any surface of a steel sheet,
or remains in a stale where the compound is present in small pieces as shown in Fig. 3A and
Fig. 313, and therefore sticking of plating to a mold presumably occurs hardly. Meanwhile,
when the remaining amount of a Zn-Fc intermetallic compound exceeds 15 g/m , a Zn-Fc
inlcrmctallie compound covers the entire surface of a steel sheet as shown in Fig. 3C, and
therefore slicking of plating lo a mold presumably occurs easily.
[0027J In this regard, after heating for hoi stamping, there is only a slight or almost no
change in the amount of a Zn-Fc intermetallic compound before and after hot stamping
(pressing). Consequently, the amount of a Zn-Fe intermetallic compound after heating for
hot stamping may be examined after cooling before hot stamping (pressing), or may be
examined on a formed body after hoi stamping (pressing). In other words, when the amount
8
ofa Zn-Fe intermetallic compound remaining in a plated layer ofa hot-pressed body is from 0
g/m2 to 15 g/m2, sticking of plating to a mold can be suppressed.
[0028] Further, in recent years in need of rapid heating for productivity improvement, a
technique for heating rapidly a steel sheet, such as Joule heating and induction heating, has
been introduced in a producing process for a hot stamp molded body. In this case, the
temperature elevation rate can be 50°C/s or more on the occasion of hot stamping, and in most
cases the total of temperature elevation time and retention time is 1 min or less. In order to
reduce the remaining amount ofa Zn-Fe mtermetallic compound to 15 g/m or less near the
outer surface layer of a steel sheet after hot stamping, it is required to adjust the plating
weight according to the heating time or the heating temperature.
[0029] In order to mitigate sticking of plating to a mold, the amount ofa Zn-Fe intermetallic
compound in a plated layer after heating is preferably 0 g/m . However, when the remaining
amount ofa Zn-Fe intermetallic compound is 15 g/m or less, a Zn-Fe intermetallic compound
is in a formation stale, in which the compound does not cover the entire surface ofa steel
sheet, lather remains in small pieces, and sticking of plating to a mold as severe as obstructive
to production does not occur. The remaining amount ofa Zn-Fe intermetallic compound is
preferably 10 g/m or less.
[0030] An amount of a Zn-Fc inicrmctallie compound in a plated layer allcr healing is
determined by constant current electrolysis of the sample at 4 mA/cm in a 150 g/L aqueous
solution of NH4CI using a saturated calomel electrode as a reference electrode. Namely, a
weight ofa Zn-Fe intermetallic compound per unit area can be determined by measuring a
time period, when the electric potential is -800 mV vs. SCli or less during execution of the
constant current electrolysis, and deriving a quantity of electricity flown per unit area during
the time period. Meanwhile, although not quantitatively, existence or nonexistence ofa
Zn-Fe intermetallic compound can be roughly estimated by observation ofa backscattered
electron image.
[0031] In a production process ofa hot stamp molded body, a steel sheet is ordinarily healed
to approx. from 700°C to 1100°C. It has come to be known, in a case in which a sheet is
heated to the steel sheet temperature by the rapid heating, that the remaining amount ofa
Zn-Fe intermetallic compound disadvantageous])' exceeds 15 g/m . This is because the total
duration of heating is short to follow the dolled line pattern in Fig. 1 so that a Fe-Zn solid
solution phase cannot be secured sufficiently, and rather a Zn-Fe inlermetallic compound
lends lo be formed. Additionally, in the ease of conventional radiant heat transfer healing,
there appears a temperature gradient for heat transfer from Ihe surface ofa steel sheet to Ihe
inside so dial there appears a gradient in Ihe thickness direction ofa plated layer with respect
9
to formation of a Zn-Fe intermetallic compound, however in the case of rapid heating by
Joule heating, induction heating, or the like, since a heating current flows along the steel sheet
surface, the steel sheet surface, namely the entire plated layer is rapidly and actively heated,
so that a Zn-Fe intermetallic compound is presumably formed uniformly in the thickness
direction of the plated layer.
[0032] Consequently, in order to avoid generation of a Zn-Fe intermetallic compound,
subject to conditions, such as a heating temperature and a retention time, a strategy for
avoidance of increase in a generation amount of a Zn-Fe intermetallic compound was decided
such that the plating weight of an original plated layer was tried to be reduced and its
preferable range was narrowed.
[00331 Fig. 4 shows a relationship between a plating weight before heating for hot stamping
and the amount of a Zn-Fe intermetallic compound after heating for hot skimping. The
above is a result with respect to a steel sheet, which was heated in the air at a rate of 50°C/s to
a temperature of 950°C, maintained there for 2 s, then cooled at a rale of 20°C/s to 680°C, and
pressed.
[0034J When a plating weight is 40 g/m' or more, a Zn-Fe intermetallic compound in a
plated layer can be hardly decreased to 15 g/m or less. Therefore, in the present process, a
plating weight is required to be Jess, than 40 g/m .
Since a plating weight is required to be 5 g/m or more from a viewpoint of
suppression of scaling during healing for hot stamping, this value is deemed as the lower
limit.
The plating weight is preferably from 10 g/m to 30 g/m .
Meanwhile, in a case in which clectrogalvanizcd coating is electric zinc alloy plating,
the amount of Zn in a plated layer is from the same viewpoints from 5 g/m to 40 g/m , and
preferably from 10 g/m7' to 30 g/m2.
[00351 In this regard, for measuring a plating weight and a Zn amount, a broadly prevailing
analytical method for a plating weight and a Zn amount can he applied without a hitch, for
example, a measurement of a plating weight and a Zn amount can be performed by dipping a
plated steel sheet in a hydrochloric acid solution containing hydrochloric acid at a
concentration of 5% and a corrosion inhibitor for pickling at a temperature of 25°C until the
plating is dissolved, and analyzing the obtained solution by a TCP emission analyzer.
[0036J Although an clectrogalvanizcd coating may be either of electric zinc plating, and
electric zinc alloy plating, electric zinc alloy plating is preferable. Namely, a steel sheet for
hot slam]) molding is preferably an electrolytic zinc alloy-coaled slecl sheet.
[00371 However, in the ease of clectrogalvanizcd coating with a light plating weight, when
10
an electrogalvanized steel sheet with a small plating weight was heated by a rapidly heating
method as described above and subjected to hot stamp molding, there arose a new problem
that the paint adhesiveness of a formed body after hot stamping became inferior.
[0038] The reasons behind the above are presumed as follows. When a heating time is
short and the plating weight is small, a Zn-based oxide film to be formed during heating on
the outermost surface of a plated layer becomes also thin, and a Zn-Fe alloying reaction
advances rapidly before a Zn-based oxide film grows sufficiently so that most part of Zn in
the plated layer is consumed in a Fe-Zn solid solution phase. Presumably, a Zn-based oxide
film can grow when a plated layer is in a form of Zn-Fe intermetailic compound, in which the
Zn activity is relatively high, but when a plated layer comes to take a form of Fe-Zn solid
solution phase, the growth is not any more possible due to increase in the Fe activity and
decrease in the Zn activity. In (he case of a thin Zn-bascd oxide film, when a steel sheet
receives a sliding action during pressing, a Fe-Zn solid solution phase is exposed easily where
Fe scales are formed presumably, and the paint adhesiveness becomes inferior.
10039] In order to improve the paint adhesiveness of a formed body, the inventors carried
out hot slumping tests using electrogalvanized sleel sheets produced under various conditions.
As the result, it was found, through observation of a sleel sheet cross-section tissue of a
formed body having favorable, paint adhesiveness, that a Zn-bascd oxide film was not peeled
oifand could remain mostly on a steel sheet surface, when there were a certain atnount of fine
particulate matters with an average diameter of 1 pm or less.
Further, it was confirmed that the paint adhesiveness ol such a hot stamp molded
body was superior to a case where a particulate mailer is not present.
J 0040] The particulate matters were analyzed lo find that they were mostly an oxide
containing an easily oxidi/ablc element contained in steel, such as Si, Mn, Cr, and Al.
To study the phenomenon thai the adhesiveness of a Zn-based oxide film is superior,
when there are a certain amount of fine particulate matters (mainly an oxide as described
below) in a plated layer, the tissue of a steel sheet which was heated at the same condition as
for hot stamp molding but not pressed and directly cooled was investigated. As the result, it
has been known thai when there are a certain amount of fine particulate matters in a plated
layer, moderate ruggedness appears at an interface between a Zn-bascd oxide film and a
plated layer. Since it was known that when an interface had a complex morphology, a
keying effect at the interface developed generally lo improve the paint adhesiveness, it was
presumed that the adhesiveness of a Zn-based oxide film was enhanced similarly by a keying
effeel, and exposure of a Fe-Zn solid solution phase was suppressed during pressing and
therefore generation of the Fe scale was avoided lo enhance the paint adhesiveness.
II
[0041] A particulate matter causing formation of moderate ruggedness at the interface is
considered as follows.
It is presumed from the component and the generation amount that a particulate
matter is an oxide of not an impurity element in a plated layer, but mainly an element
contained in steel, which has been conceivably present before heating for hot stamping at an
interface between a plated layer and a steel matrix, or inside a steel matrix. Further, it is
believed that the oxide has been formed in a steel sheet production process during annealing
of a steel sheet after cold rolling.
It is believed that, when an oxide is present at an interface between a plated layer and
a steel matrix, the oxide exhibits generally a barrier effect so as to suppress locally a Zn-Fe
alloying reaction during healing for hot stamping. It is, however, further believed that in the
case of a line particulate oxide with an average diameter of 1 um or less, the suppression
effect on a Zn-Fe alloying reaction is weak, and therefore influence of an oxide at an interface
on a Zn-Fe alloying reaction is small.
[0042] Meanwhile, when an oxide is formed inside a steel matrix, by pinning a crystal grain
boundary near a steel sheet surface during annealing, growth of a crystal grain is suppressed.
When a crystal grain near a steel sheet surface is small, and the number of crystal grain
boundaries is large, the Zn-Fe.alloying reaction rale becomes high. In olher words, where an
inside oxide is present, a Zn-Fe alloying reaction is conceivably becomes high locally.
[0043] Examples of the oxide mentioned here include, but are not particularly limited to,
oxides containing one, or two or more kinds out of Si, Mn, Cror Al. Specific examples
include single oxides, such as MnO, M11O2, Mn?.O.j, M113O4, SiC>2, AI2O3, and O2O3, and
single oxides with a non-stoichiomctric composition corresponding to each of these; complex
oxides, such as FcSi(>t, I't^SKX), MnSiCh, MnzSiO^ AIM11O3, FcCi^O/i, Fe?.CrO-i, MnCr?.05
O
y
CM
( 1
U-,
en
en
Example
<
0
0
y
00
en
O
MExample
<
<
0
y
0
0
0
X
CO
Example
<
0
y
0
y
en
0
d
W
00
en
Example
<
0
0
y
V3
en
0
d
K-J
Example
<
<<
0
y
0
en
r-i
'/1
0
0
S
0
[0104]
[Table 5]
Test
Number
41;
42
43
44'
45'
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
61
Steel
grade
N
0
P
Q
R
S
T
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
Plated layer of hot press formed body
Amount of
Zn:Fe intermetallic
compound
(g/m2)
3.5
0.0
0.0
0.0
0.0
0.0
6.9
0.0
0.0
1.4
5.1
0.0
3.4
0.0
0.0
0.0
0.0
0.0
0.0
2.3
4.3
0.0
4.7
0.0
0.0
0.0
0.0
Average diameter of
particulate matter
(nm)
25
27
26
20
16
18
22
18
18
23
22
24
28
22
19
13
26
16
15
21
18
19
22
19
26
11
21
Number of
particulate matter
log (pcs/mm)
3.2
2.3
3.8
1.7
1.6
2.2
3.1
2
2.2
2.8
2.4
3.2
3.7
3.8
1.7
1.6
3.8
1.5
2.4
2.6
2.4
3.1
3.7
3.6
1.2
1.1
3.4
Evaluation
Plating stuck to
mold
Existent or not
No
No
V- No
;•. NO
v: No
;•• No
'•- No
No
No
No
No
No
No
No
No
No
NO'
No
No
No
No
No
No
No
No
No
No
Formation of Fe
scale
Existent or not
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Paintin
adhesiven
AA
AA
A
AA
AA
AA
A
AA
AA
AA
AA
AA
A
A
AA
AA
A
AA
AA
AA
AA
AA
AA
AA
AA
A
AA
32
68
69
70
71
72
73
74
A
A
A
A
A
A
A
17.4 18 1.7
Unevaluable due to formation of Fe scales over the entire surface
0.0
0.0
0.0
0.0
22.0
8
4
12
16
20
0.4
0.2
0.4
0.3
1.8
Yes
Fe scale sticking
No
No
No
No
Yes
No
Yes
Yes
Yes
Yes
Yes
No
AA
C
C
c
c
c
AA
33
[0105] Although the invention has been described in terms of the preferred Embodiments
and Examples according to the invention, such Embodiments and Examples are just an
example within the range of the essentials of the invention, and addition, omission,
replacement, and other alternations of the constitution without departing from the spirit of the
invention are possible. Namely, the foregoing description is not intended to limit the scope
of the invention, and various alterations are no doubt possible within the scope of the
invention.
[0106] The entire contents of the disclosures by Japanese Patent Application No.
2013-122351 are incorporated herein by reference.
All the literature, patent application, and technical standards cited herein are also
herein incoiporated to the same extent as provided for specifically and severally with respect
to an individual literature, patent application, and technical standard to the effect that the same
should be so incorporated by reference.
CLAIMS
[Claim 1] Ahot stamp molded body produced by hot-stamping an electrogalvanized
steel sheet comprising as components of a steel sheet, by mass %:
C:
Si:
Al:
Mn:
P:
S:
N:
'11:
Nb:
Mo:
Cr:
V:
Ni:
B:
Ca:
Mg:
from 0.10 to 0.35%,
horn 0.01 to 3.00%,
from 0.01 to 3.00%,
from 1.0 to 3.5%,
horn 0.001 to 0.100%,
fiom 0.001 to 0.010%,
from 0.0005 to 0.0100%,
from 0.000 to 0.200%,
from 0.000 to 0.200%,
from 0.00 to 1.00%,
from 0.00 to 1.00%,
from 0.000 to 1.000%,
from 0.00 to 3.00%,
from 0.0000 to 0.Q050%,
from 0.0000 to 0.0050%, and
from 0.0000 to 0.0050%,
a balance being l;e and impurities,
wherein the steel sheet is electrogalvanized on each face with a plating weight not
less than 5 g/m' and less than 40 g/ni ;
wherein a galvanized layer of the hot stamp molded body is configured with 0 g/m2
to 15 g/m2 of a Zn-Ve inlermetallic compound and a Vc-Zn solid solution phase as a balance,
and
wherein, in the galvanized layer of the hot stamp molded body, 1x10 pes to IxIO4 pes
of particulate matter with an average diameter of from 10 nni to 1 pin arc present per 1 mm
length of the galvanized layer.
[Claim 2j The hot stamp molded body according to claim 1, wherein the steel sheet
comprises, by mass %, one, or two or more kinds of:
Ti: from 0.001 to 0.200%,
Nb: from 0.001 to 0.200%,
Mo: from 0.01 to 1.00%,
Cr: from 0.01 to 1.00%,
35
V: from 0.001 to 1.000%,
Ni: from 0.01 to 3.00%,
B: from 0.0002 to 0.0050%,
Ca: from 0.0002 to 0.0050%, or
Mg: from 0.0002 to 0.0050%.
[Claim 3] The hot stamp molded body according to claim 1 or 2, wherein the
particulate matter is one, or two or more kinds of oxides containing one, or two or more kinds
out of Si, Mn, Cr or Al.
[Claim 4] The hot stamp molded body according to any one of claims 1 to 3, wherein
the electrogalvanized steel sheet is an electrolytic zinc alloy-coated steel sheet.
[Claim 5] A method for producing a hot stamp molded body, in which a steel
comprising as components, by mass %:
C:
Si:
Al:
Mn:
P:
S:
N:
Ti:
Nb:
Mo:
Cr:
V:
Ni:
B:
Ca:
Mg:
from 0.10 to 0.35%,
from 0.01 to 3.00%,
from 0.01 to 3.00%,
from 1.0 to 3.5%,
from 0.001 to 0.100%,
from 0.001 to .O.OJO.%, ..._-•
from 0.0005 to 0.0100%,
from 0.000 to 0.200%,
from 0.000 to 0.200%,
from 0.00 to 1.00%,
from 0.00 to 1.00%,
from 0.000 to 1.000%,
from 0.00 to 3.00%,
from 0.0000 to 0.0050%,
from 0.0000 to 0.0050%, and
from 0.0000 to 0.0050%,
a balance being Fe and impurities, is subjected to a hot rolling step, a pickling step, a cold
rolling step, a continuous annealing step, a temper rolling step, and an electrogalvanizing step
to yield an electrogalvanized steel sheet, and the electrogalvanized steel sheet is subjected to a
hot stamp molding step to produce a hot stamp molded body;
wherein in the continuous annealing step, the steel sheet is subjected to repeated
bending at a bending angle of from 90° to 220° four or more times during heating of the steel
sheet in an atmosphere gas containing hydrogen at from 0.1 volume % to 30 volume %, and
36
H2O corresponding to a dew point of from -70°C to -20°C as well as nitrogen and impurities
as a balance at a sheet temperature within a range of from 350°C to 700°C,
wherein in the electrogalvanizing step, each face of the steel sheet is
electrogalvanized with a plating weight of not less than 5 g/m and less than 40 g/m , and
wherein in the hot stamp molding step, the electrogalvanized steel sheet is heated
with an average temperature elevation rate of 50°C/sec or more to a temperature range of from
700°C to 1100°C, hot-stamped within 1 min from the initiation of the temperature elevation,
and thereafter cooled to normal temperature.
[Claim 6] The method for producing a hot stamp molded body according to claim 5,
wherein the steel comprises, by mass %, one, or two or more kinds of:
Ti: from 0.001 to 0.200%,
Nb: from 0.001 to 0.200%,
Mo: from 0.01 to 1.00%,
Cr: from 0.01 to 1.00%,
V: from 0.001 to 1.000%,
Ni: from 0.01 to 3.00%,
B: from 0.0002 to 0.0050%,
Ca: from 0.0002.1O &Q05fi%,_Qr
Mg: from 0.0002 to