DESCRIPTION
[Title of the Invention] GALVANIZED STEEL SHEET FOR HOT FORMING
[Technical Field]
[OOO 11
The present disclosure relates to a galvanized steel sheet for hot forming,
particularly a galvanized steel sheet for hot forming that is suitable for use in the
manufacture. of automobile under-carriage members and reinforcing components.
[Background Art]
[00021
In recent years, for weight reduction of automobiles, an effort has been made to
reduce the thickness of steel sheets to be used, through an increase in strength of the
steel sheets. As a technique for subjecting difficult-to-form material such as a high
strength steel sheet to press forming, a hot forming technique such as a hot press has
become more and more adopted in which a material that is to be subjected to forming is
preheated before forming.
[0003]
The hot forming method is advantageous in that forming is carried out at an
elevated temperature that renders deformation resistance low and hardening can be
carried out simultaneously with the forming. Accordingly, the hot forming method is
an excellent forming method that can simultaneously ensure strengthening of the
member and the formability. The hot forming method, however, requires heating of a
steel sheet to a high temperature of 700°C or above before forming and thus poses a
problem of oxidation of the steel sheet surface. Scale formed of iron oxide produced as
a result of the oxidation of the steel sheet surface disadvantageously comes off during
pressing, is adhered to a mold, and results in lowered productivity, or the scale stays in
products after pressing and leads to poor appearance. Further, when such scale stays,
the adhesion between the steel sheet and the coating film is so low that the corrosion
resistance is lowered in coating in the next step. Accordingly, after pressing, descaling
such as shot blasting is necessary.
[0004]
In order to solve the above problems, the use of galvanized steel sheets coated
with a zinc-based plating or an aluminum-based plating has been proposed as a material
for hot forming with a view to inhibiting the oxidation of the base-material steel surface
and/or improving the corrosion resistance of press formed products. For example,
Patent Literatures 1 and 2 use galvanized steel sheets in hot forming.
[0005]
Further, Patent Literature 3 proposes a steel sheet that can simplify or eliminate
the step of separating the oxide of the formed product surface through an improved
adhesion of an oxide film formed in hot forming by specifying the concentration of C, Si,
P, and/or Ti in steel and specifying the coverage of Zn on a steel sheet surface and the
concentration of A1 in a film.
[Prior Art Literature(s)]
[Patent Literature(s)]
[0006]
[Patent Literature 11 JP 2003-73774A
[Patent Literature 21 JP 2001-353548A
[Patent Literature 31 JP 2005-48254A
[Summary of the Invention]
[Problem(s) to Be Solved by the Invention]
[0007]
In galvanized steel sheets for hot forming manufactured by conventional
techniques, however, it has been found that, when the amount of a zinc oxide layer
formed in the hot forming is excessively large, deposition or spark sometimes occurs in
spot welding after the hot forming.
[0008]
As well known, panels for automobile bodies are assembled by joining the
panels pressed into various shapes to each other by resistance welding (particularly spot
welding). In particular, when spot welding is carried out, since welding is continuously
carried out at a number of spots, the number of times of continuous dotting by an
identical electrode tip should be maximized for ,productivity improvement purposes.
[0009]
In order to increase the number of times of continuous dotting in the spot
welding, the suppression of wear of the electrode tip is important. When the deposition
of the electrode tip or the spark occurs during the welding, the wear of the electrode is
accelerated and the electrode tip comes off. As a result, subsequent welding cannot be
continued. For this reason, the suppression of the occurrence of deposition or spark
during the spot welding is important from the viewpoint of improving the productivity.
[OO lo]
An object of the present disclosure is to solve the problem of spot weldability
after the hot forming of the galvanized steel sheet for hot forming.
[Means for Solving the Problem(s)]
[OO 111
The inventors of the present disclosure have made investigations on the
formation of zinc oxide in hot forming of a galvanized steel sheet for hot forming and
have made extensive and intensive studies on an improvement in spot weldability after
hot forming. As a result, the inventors of the present disclosure have found that
excessive formation of the zinc oxide layer can be suppressed during the hot forming,
and it becomes possible to improve spot weldability after the hot forming, by bringing
the chemical composition of a steel sheet as a substrate to be plated, the coverage of
plating and the amount and concentration of A1 in a galvanized layer to respective proper
ranges.
[00.12]
The present disclosure based on such finding is as follows.
(1)
A galvanized steel sheet for hot forming, the galvanized steel sheet including a
galvanized film provided on a surface of a steel sheet,
wherein the steel sheet has a chemical composition consisting of, in mass%,
C: 0.10% to 0.45%,
Si: less than 0.3%,
Mn: 0.5% to 3.0%,
P: less than or equal to 0.02%,
S: less than or equal to 0.004%,
sol. Al: 0.005% to 1.0%,
N: less than or equal to 0.01%, and
the balance: Fe and impurities,
wherein the galvanized film has aplating coverage of 40 g/m2 to 1 1oglm2, an A1
content of more than or equal to 150 mg/m2 within the galvanized film, and an A1
concentration of less than or equal to 0.5 mass %, and
wherein the galvanized steel sheet is used for an application in which the
galvanized steel sheet is heated to 700°C or above and is then subjected to hot forming.
[00 1 31
(2)
The galvanized steel sheet for hot forming according to (I), wherein the
chemical composition includes, in mass%,
Ti: less than or equal to 0.01% instead of a part of Fe.
[00 141
(3)
The galvanized steel sheet for hot forming according to (1) or (2), wherein the
chemical composition includes, instead of a part of Fe, one or more selected from the
group consisting of, in mass%,
Cr: less than or equal to 0.3%, and
B: less than or equal to 0.0050%.
[00 1 51
(4)
The galvanized steel sheet for hot forming according to any one of (1) to (3),
wherein the chemical composition includes, instead of a part of Fe, one or more selected
from the group consisting of, in mass%,
Mg: less than or equal to 0.05%,
Ca: less than or equal to 0.05%, and
REM: less than or equal to 0.05%.
[0016]
(5)
The galvanized steel sheet for hot forming according to any one of (1) to (4),
wherein the chemical composition includes, in mass%,
Bi: less than or equal to 0.05% instead of a part of Fe.
[0017]
In the present disclosure, "galvanization" as used herein means both zinc plating
and zinc alloy plating.
[Effect(s) of the Invention]
[OO 1 81
In the galvanized steel sheet for hot forming according to the present disclosure,
excessive formation of zinc oxide in hot forming is suppressed, and, thus, the occurrence
of deposition and spark in spot welding after the hot forming can be suppressed. As a
result, in an automobile body assembling process, the number of times of continuous
dotting in welding can be improved, and, thus, the necessary frequency of repair of the
electrode tip can be reduced. Further, the frequency of the occurrence of a spark
phenomenon can be reduced, and the necessity of repair of the surface of automobile
bodies can be eliminated. Thus, troubles involved in welding can be avoided,
advantageously leading to dramatically improved productivity of an automobile body
assembling process.
[Modes for Carrying out the Invention]
[00 191
The chemical composition of a base steel sheet as a substrate, a plating film, a
hot forming method, and a manufacturing method for the galvanized steel sheet for hot
forming according to the present disclosure will be described in more detail. In the
present specification, in all cases, "%" in chemical composition represents "mass%."
[0020]
1. Chemical composition of steel sheet as substrate
[C: 0.10% to 0.45%]
C is a very important element that enhances the hardenability of the steel sheet
and mainly determines strength after hardening. Further, C is an element that lowers an
AC3 point and promotes a lowering in a hardening treatment temperature. When the
content of C is less than 0.10%, the contemplated effect is not satisfactory. Therefore,
the C content is more than or equal to 0.10%. On the other hand, when the C content is
more than 0.45%, the toughness of a hardened portion is significantly deteriorated.
Thus, the C content is less than or equal to 0.45%. The C content is preferably less
than or equal to 0.35%.
[002 11
[Si: less than 0.3%]
Si is an element that is contained as an impurity, suppresses mutual diffusion of
Fe and Zn in the galvanized layer and the base steel sheet, and lowers the alloying rate
of the galvanized layer. Further, in heating before hot forming, Si is enriched in the
interface of a zinc oxide layer formed by the heating and the steel sheet and lowers the
adhesion of the zinc oxide layer. The Si content is less than 0.3% from the viewpoint
of ensuring the adhesion of the zinc oxide layer high enough to withstand a difference in
thermal expansion caused in hot forming or rapid cooling. The Si content is more
preferably less than or equal to 0.10%.
[0022]
[Mn: 0.5% to 3.0%]
Mn is an element that is very effective in enhancing the hardenability of the
steel sheet and stably ensuring the strength after hardening. When the content of Mn is
less than 0.5%, the contemplated effect is not satisfactory. Therefore, the Mn content is
more than or equal to 0.5%.' The Mn content is preferably more than or equal to 1.0%.
On the other hand, when the Mn content is more than 3.0%, the effect is saturated.
Further, in this case, difficulties are sometimes encountered in ensuring stable strength
after hardening. Thus, the Mn content is less than or equal to 3.0%. The Mn content
is preferably less than or equal to 2.4%.
[0023]
[P: less than or equal to 0.02%]
P is an element that is contained as an impurity, suppresses mutual diffusion of
Fe and Zn'.in the galvanized layer and the base steel sheet, and lowers the alloying rate
of the galvanized layer. In order to avoid an excessive increase in the zinc oxide layer
in heating before hot forming, a method is useful in which Zn that is an original plating
ingredient is incorporated as an Fe-Zn solid solution phase in the surface layer of the
steel sheet to suppress oxidation. When the P content is more than 0.02%, the
suppression of oxidation is difficult. Accordingly, the P content is less than or equal to
0.02%. The P content is preferably less than or equal to 0.01%.
[0024]
[S: less than or equal to 0.004%]
S is an element that is contained as an impurity, and functions to render the steel
brittle. Accordingly, the S content is less than or equal to 0.004%. The S content is
more preferably less than or equal to 0.003%.
[0025]
[Sol. Al: 0.005% to 0.1%]
A1 functions to deoxidize steel and thus to render a steel product sound. When
the content of sol. A1 is less than 0.005%, the functional effect cannot be attained
without difficulties. Therefore, the sol. A1 content is more than or equal to 0.005%.
On the other hand, when the sol. A1 content is more than 0.1%, the functional effect is
saturated and is not cost-effective. Thus, the sol. A1 content is less than or equal to
0.1%
[0026]
[N: Less than or equal to 0.01%]
N is an element that is contained as an impurity, forms an inclusion in the steel,
and deteriorates the toughness after hot forming. Thus, the N content is less than or
equal to 0.01%, preferably less than or equal to 0.008%, and more preferably less than or
equal to 0.005%.
[0027]
The steel sheet as a substrate may contain, in addition to the above-described
elements, the following elements.
[Ti: less than or equal to 0.10%]
Ti is an element that promote mutual diffusion of Fe and Zn in the galvanized
layer and the base steel sheet, enhance an alloying rate of the galvanized layer, and
suppress the formation of a molten Zn alloy layer, for example, in hot forming.
Accordingly, Ti may be contained. When the content of Ti is more than 0.10%, the
functional effect is saturated and, thus, this is not cost-effective. Therefore, the content
of Ti is less than or equal to 0.10%. The content of Ti is preferably less than or equal
to 0.03%. In order to more reliably attain the functional effect, preferably, the content
of Ti is more than or equal to 0.01%.
[0028]
[One or two selected from the group consisting of Cr: less than or equal to 0.3%, and B:
less than or equal to 0.0050%]
Cr and B are elements that are effective in enhancing the hardenability of the
steel sheet and stably ensuring strength after hardening. Therefore, one or two of these
elements may be contained. When the content of Cr is more than 0.3% or when the
content of B is more than 0.0050%, the contemplated effect is saturated and, thus, this is
not cost-effective. Thus, the content of Cr is less than or equal to 0.3%, and the content
of B is less than or equal to 0.0050%. The content of B is preferably less than or equal
to 0.0030%. In order to more reliably attain the contemplated effect, preferably, any
one of Cr: more than or equal to O.l%, and B: more than or equal to 0.0010% is satisfied.
[0029]
[One or more selected from the group consisting of Ca: less than or equal to 0.05%, Mg:
less than or equal to 0.05%, and REM: less than or equal to 0.05%]
Ca, Mg, and REM function to refine the form of inclusions in the steel and thus
to prevent the occurrence of inclusion-derived cracking in hot forming. Therefore, one
or more of these elements may be contained. When these elements are added in an
excessive amount, the effect of refining the form of inclusions in the steel is saturated,
disadvantageously leading to an increased cost. Thus, the Ca content, the Mg content,
and the REM content are less than or equal to 0.05%, less than or equal to 0.05%, and
less than or equal to 0.05%, respectively. In order to more reliably attain the functional
effect, preferably, any one of Ca: more than or equal to 0.0005%, Mg: more than or
equal to 0.0005%, and REM: more than or equal to 0.0005% is satisfied.
[0030]
Here REM refers to 17 elements in total of Sc, Y, and lanthanoids, and the
content of REM refers to the total content of these elements. Lanthanoids are
industrially added as misch metal.
[003 11
[Bi: less than or equal to 0.05%]
Bi is an element that becomes a solidification nucleus in a solidification process
of a molten steel and reduces a secondary arm spacing of dendrite, thereby suppressing
the segregation of Mn and the like that segregate within the secondary arm spacing of
the dendrite. Therefore, Bi may be contained. In particular, for steel sheets in which
a large amount of Mn is contained, such as steel sheets for hot pressing, Bi is effective
in suppressing a deterioration in toughness derived from the segregation of Mn.
Accordingly, preferably, Bi is contained in such steel grade. When Bi is contained in
an amount of more than 0.05%, the functional effect is saturated, disadvantageously
leading to an increased cost. Thus, the Bi content is less than or equal to 0.05%. The
Bi content is preferably less than or equal to 0.02%. In order to more reliably attain the
functional effect, the Bi content is preferably more than or equal to 0.0002%. The Bi
content is more preferably more than or equal to 0.0005%.
[0032]
2. Galvanized film
[Coverage of galvanizing]
The steel sheet for hot forming according to the present disclosure is a
galvanized steel sheet including a galvanized layer provided on a surface of a steel sheet.
The coverage of galvanizing is 40 g/m2 to 110 g/m2 per one surface (the same shall
apply hereinafter). When the coverage of the galvanizing is excessively large, Zn in
the galvanized film cannot be satisfactorily incorporated in the base-material steel sheet
as solid solution phase during heating before hot forming and the zinc oxide layer is
disadvantageously excessively formed, resulting in lowered adhesion. The coverage of
the galvanizing is excessively small, difficulties are encountered in forming the zinc
oxide layer in an amount large enough to suppress the oxidation of the steel sheet in
heating before hot forming.
[003 31
[Composition of galvanizing]
The composition of the galvanized film is not particularly limited, and the film
may be a pure zinc plated film or alternatively may be a zinc alloy plated film in which
one or more alloying elements selected, for example, from Al, Mn, Ni, Cr, Co, Mg, Sn,
and Pb have been incorporated in a proper amount according to contemplated purposes
(for Al, the concentration of A1 is limited to less than or equal to 0.5 mass%, as
described below). In some cases, one or more selected, for example, from Fe, Be, By Si,
P, S, Ti, V, W, Mo, Sb, Cd, Nb, Cu, and Sr that are sometimes unavoidably included, for
example, from raw materials are contained in the plated film. Further, the film may be
a Zn-Fe alloy plated film formed by heat treating a galvanized film or an alloyed hot dip
galvanized film, that is, a hot dip galvanized film.
[0034]
The method for glavannealing is also not particularly limited. However, hot
dip galvanizing is advantageous from the viewpoint of providing a galvanizing coverage
of more than or equal to 40 g/m2. The galvanized film is preferably a hot dip
galvanized film and an alloyed hot dip galvanized film.
[003 51
The amount of A1 in the galvanized film is more than or equal to 150 mg/m2.
When the amount of A1 in the film is less than 150 mg/m2, the amount of A1 oxide
produced in the film surface layer in heating before hot forming is so small that the
oxidation of zinc is not inhibited and zinc oxide is produced in an excessive amount,
disadvantageously leading to occurrence of spark or deposition in spot welding. From
the viewpoint of promoting the diffusion of zinc in the base-material steel in hot forming,
the concentration of A1 in the galvanized film is less than or equal to 0.5 mass%,
preferably less than or equal to 0.4 mass%.
[0036]
The amount of A1 in the film of the hot dip galvanized steel sheet produced in a
continuous hot dip galvanizing line is influenced, for example, by an atmosphere, a bath
temperature, an intrusion material temperature, a dipping time, and a concentration of A1
in bath in heating before annealing. The amount of A1 in the film can be brought to
more than or equal to 150 mg/m2 by experimentally determining the relationship
between these production conditions and the amount of A1 in the film. In order to bring
the amount of A1 in the film to more than or equal to 150 mg/m2, the concentration of A1
in the bath is preferably in the range of about 0.12 to 0.18 mass%, and more preferably
in the range of 0.14 to 0.16 mass%.
LO03 71
Among hot dip galvanized steel sheets, alloyed hot dip galvanized steel sheets
are particularly preferred for hot forming applications, because the separation of the
galvanized film after hot forming is significantly small. In the alloyed hot dip
galvanized steel sheet, the melting point of the galvanizing is high, and an Fe-Al-based
alloy layer is absent in the interface of the base-material steel and the galvanized film.
Accordingly, the alloyed hot .dip galvanized steel sheet is advantageous in that zinc is
diffused in the base-material steel in heating before hot forming to form a solid solution
phase. In pure zinc galvanizing, for example, in hot dip galvanized steel sheets, the
melting point of the galvanizing is low and about 420°C. Accordingly, zinc is likely to
be evaporated, and the Fe-A1 layer present at the interface inhibits the diffusion of Zn.
Thus, a thick oxide film composed mainly of zinc is likely to be formed.
[003 81
The concentration of Fe in the galvanized film in the alloyed hot dip galvanized
steel sheet is preferably in the range of 8 to 15%. When the concentration of Fe is
below the lower limit of a defined range, a pure zinc phase having a low melting point is
likely to stay on the surface, and a thick oxide film composed mainly of zinc is also
likely to be formed. On the other hand, when the concentration of Fe in the galvanized
film is above the upper limit of the defined range, a powdering phenomenon in which
the galvanized layer is separated is disadvantageously likely to occur.
[003 91
3. Hot forming
The galvanized steel sheet according to the present disclosure is usually heated
to a temperature of about 700 to 1000°C, and, subsequently, hot forming such as press
forming is carried out.
[0040]
Examples of heating methods include heating, for example, by electric furnaces
and gas furnaces, and flame heating, electric heating, high-frequency heating, induction
heating and other heating. When achieving hardening of the material by heating is also
contemplated, a method is adopted that includes, after heating of a material to a
hardening temperature (usually about 700 to 1000°C) that provides a target hardness,
keeping the temperature for a given period of time, subjecting the material to pressing in
the high-temperature state with a mold through which, for example, a cooling pipe is
passed, and, in this case, rapidly cooling the material through contact with the mold. A
method may be of course adopted in which properties of pro'ducts after hot pressing are
regulated by varying the hardening temperature or the cooling rate in a preheated
pressing mold.
[004 11
4. Manufacturing method
As described above, in the hot forming of the steel sheet, the steel sheet is
heated in hot forming to a temperature of or near an austenite region, and forming is
carried out in the temperature region. Therefore, mechanical properties of the base
steel sheet at room temperature before heating are not important. Thus, the
metallographic structure of the base steel sheet before heating is not particular limited.
That is, the base steel sheet before galvanizing may be any one of a hot-rolled steel sheet
and a cold-rolled steel sheet and may be manufactured by any method without particular
limitation. However, manufacturing methods suitable from the viewpoint of
productivity will be described.
[0042]
[Hot rolling]
From the viewpoint of stability of the rolling, hot rolling is preferably
performed in the austenite region. When the coiling temperature is low, a martensite
structure occurs, strength increases, and it is difficult to perform continuous hot dip
galvanizing line passing or perform cold rolling. On the other hand, when the coiling
temperature is high, oxidized scale becomes thick and efficiency of subsequent pickling
is reduced or, in a case where the pickling is not performed and plating is directly
performed, plating adhesion is reduced. Thus, the coiling temperature is preferably 500
to 650°C.
[0043]
[Cold rolling]
Cold rolling is carried out by an ordinary method. In the steel sheet according
to the present disclosure, the amount of carbon is so 'large that cold rolling at an
excessively high rolling reduction leads to an increased burden on a mill. An
excessively enhanced strength after cold rolling by work hardening poses a problem of
weld strength in coil connection or a line passing property in a galvanizing line. Thus,
the rolling reduction is preferably less than or equal to 80%, more preferably less than or
equal to 70%.
[0044]
Note that, since cost is increased by the cold rolling, the cold rolling is
preferably omitted and a steel sheet that has not been cold-rolled after hot rolling is
preferably used in a case of the steel sheet having a sheet thickness and sheet width
capable of being produced by the hot rolling.
[0045]
[Galvanizing]
The formation of the galvanized layer may be performed by hot dip galvanizing,
electroplating, thermal spraying, deposition, or the like. The method for forming the
galvanized layer is not limited. The galvanizing (plating) may be continuously
performed on a steel strip, or may be performed on a single cutlength sheet. In general,
the. use of a continuous hot dip galvanizing line having an excellent production
efficiency is preferred. When the substrate is a hot-rolled steel sheet, coil is rewound
followed by galvanizing. On the other hand, when the substrate is a cold-rolled steel
sheet, annealing is generally followed by hot dip galvanizing.
[0046]
A galvanizing method will be described by taking as an example hot dip
galvanizing or alloyed hot dip galvanizing in a continuous hot dip galvanizing line.
LOO471
In the normal continuous hot dip galvanizing line, a heating furnace, a cooling
zone, a hot dip galvanizing bath, an alloying furnace are continuously installed. In the
present invention, the metallographic structure of the base steel sheet is not particular
limited. Accordingly, heat patterns in the heating furnace and in the cooling zone are
not particular limited.
[0048]
However, the base steel sheet has a chemical composition in which an amount of
carbon is large and that causes easy quenching. Accordingly, in the line, there is a risk
that the base steel sheet becomes a steel sheet strengthened too much. From the
viewpoint of ease of passing, a producible range (sheet thickness and sheet width), it is
preferable to set a heat pattern by which the steel sheet is not strengthened too much.
Next, this point is explained.
[0049]
In a case where the highest heating temperature of the steel product in the
heating furnace is a temperature of less than Acl point, recovering and recrystallizing of
a steel sheet occurs during the hearing, and strength is lowered than that before gearing.
Accordingly, there is no problem with regard to passability. In addition, a cooling rate
after heating does not affect the metallographic structure. Accordingly the cooling can
be performed at any speed. From the viewpoint of saving heating energy in the furnace,
heating at a low temperature that does not disturb a galvanizing property is preferable.
[0050]
On the other hand, in a case where the highest heating temperature of the steel
product is a temperature of more than or equal' to Acl point, recovering and
recrystallizing of a steel sheet occurs during the hearing, and an austenite phase appears.
In addition, in accordance with a subsequent cooling condition, a high-strength
transformation phase is formed. In a case where heating is performed at a temperature
of more than or equal to Acl point and austenite is formed, when a cooling rate is too
fast, the austenite transforms into a structure mainly consisting of bainite or martensite,
and a steel sheet has a high strength. Accordingly, such cooling rate is not preferable.
Specifically, an average cooling rate from the highest heating temperature to 500°C is
preferably set to be less than or equal to a critical cooling rate. Note that, the critical
cooling rate is used as an indicator of the hardenability of the steel sheet. Even if the
steel sheet cooled under the above-described condition includes a small amount of
bainite or martensite, the effect of a production method described here is not denied.
However, from a viewpoint that a lowest possible strength increases passability, it is
preferable to slow the cooling rate as possible and not to form the martensite.
[005 11
The hot dip galvanizing may be carried out by dipping the steel sheet in a hot
dip galvanizing bath and pulling up the steel sheet according to an ordinary method.
The coverage of the galvanizing is regulated by a pulling-up rate and by regulating the
flow rate of a wiping gas blown out through a nozzle. The concentration of A1 in the
galvanized film can be regulated by regulating the composition of the galvanizing bath,
the temperature of the galvanizing bath, and the time of dipping in the galvanizing bath.
The content of A1 in the galvanized film can also be achieved by regulating the coverage
of the galvanizing.
[0052]
Alloying treatment is carried out by, after hot dip galvanizing treatment,
reheating the material, for example, in a gas furnace or an induction heating furnace.
Metal diffusion is carried out between the galvanizing layer and the base steel sheet, and
alloying (Zn-Fe alloy formation) of the galvanized film proceeds. In order to enhance
the content (%) of Fe in the galvanized layer, the alloying temperature is preferably
brought to 480°C or above. When the temperature is below 480°C, the alloying rate is
so low that the line speed is lowered, and productivity is lowered, or it is necessary to
take a measure in equipment such as an increase in length of the alloying furnace. The
higher the alloying temperature, the higher the alloying rate. At an Acl point or above,
the steel sheet disadvantageously increases in strength for the same reason as the highest
heating temperature. The alloying temperature is preferably in the range of 500 to
650°C.
[0053]
When temper rolling is carried out after hot dip galvanizing, flatness correction
and surface roughness of the steel sheet can be regulated. Accordingly, temper rolling
may be carried out in some applications.
[0054]
In the galvanized steel sheet, as well known in the art, a chemical conversion
film may be formed on the surface of the galvanized film from the viewpoint of
enhancing the corrosion resistance or the coatability. The chemical conversion
treatment is preferably carried out with a non-chromium-based chemical conversion
treatment solution.
[Examples]
[0055]
Slabs having chemical compositions specified in Table 1 were hot-rolled.
Subsequently, the steel sheets were cooled and coiled at a sheet temperature of 600°C to
obtain a hot-rolled steel sheet having a thickness of 2.8 mm. The hot-rolled steel sheet
was pickled and was then cold-rolled to prepare a 1.2 mm-thick cold-rolled steel sheet.
Some hot-rolled steel sheets were not cold-rolled after the pickling.
[0056]
[Table 11
[0057]
The hot-rolled steel sheets and the cold-rolled steel sheets were galvanized in a
hot dip galvanizing equipment. Plating was carried out under varied conditions of bath
temperature: 450 to 460°C, A1 concentration in bath: 0.10 to 0.15%, and coverage of
galvanizing per one surface: 40 to 8og/m2. After hot dip galvanizing, alloying was
carried out at a sheet temperature of 500 to 600°C to prepare alloyed hot dip galvanized
steel sheets. Some hot dip galvanized steel sheets were not subjected to the alloying
treatment.
C
D
E
F
G
H
I
J
0.21
0.20
0.21
0.21
0.21
0.13
0.10
0.30
0.04
0.05
0.21
0.07
0.07
0.07
0.06
0.20
1.2
1.3
1.2
2.1
2.1
2.0
2.3
1.7
0.01
0.02
0.01
0.01
0.01
0.01
0.02
0.01
0.001
0.002
0.002
0.001
0.001
0.001
0.001
0.001
0.004
0.004
0.003
0.004
0.004
0.005
0.004
0.003
0.04
0.03
0.04
0.04
0.04
0.04
0.04
0.04
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.02
0.02
0.02
0.03
0.03
0.03
0.02
0.03
0.0016
0.0014
0.0015
0.0020
0.0020
0.0023
0.0022
0.0013
Bi: 0.002
REM: 0.001
[0058]
Specimens for hot pressing (specimen size: 250 mm in width x 200 mm in
length) were extracted from the manufactured hot dip galvanized steel sheets and alloyed
hot dip galvanized steel sheets. The temperature of the steel sheet specimen within the
heating furnace was allowed to reach 900°C. The steel sheet specimen was held at the
temperature for 3 minutes and was then taken out, and, immediately after that, hot
pressing was carried out with a sheet steel mold, followed by rapid cooling. .
[0059]
In such a state that two hot pressed steel sheet specimens were superimposed on
top of each other, spot welding was carried out under the following conditions, followed
by a 1000-point continuous dotting test for the evaluation of weldability. A circle is
put in a cell of a case where spark or deposition was not occurred, and a cross is put in a
cell of a case where spark or deposition was occurred
[0060]
- Applied pressure: 400 kgf
Weld time: 15 cycles
- Holding time: 9 cycles
- Welding current: Current immediately before dust
- Shape of electrode tip: DR type, end of tip is 6 mm$-40R
The results of the spot weldability test, together with manufacturing conditions
(cold rolling and alloying treatment done or not done) and the results of analysis of the
galvanized film, are summarized in Table 2.
1006 11
[Table 21
Spot weldability afte
present disclosure
Comparative Example
[0062]
In Table 2, test Nos. 1 to 13 are examples of the present disclosure, and test Nos.
14 to 34 are examples where the amount of A1 in the galvanized film is excessively small.
In all of test Nos. 1 to 13, the spot weldability was good. On the other hand, in all of
test Nos. 14 to 24, the spot weldability was poor. From the above results, it was found
that, when the content of A1 in the galvanized film is larger than 1 5 0 ~ / ma~ d,e terioration
in spot weldability of the hot formed galvanized steel sheet can be prevented.
[Name of Document] CLAIMS
[Claim 11
A galvanized steel sheet for hot forming, the galvanized steel sheet comprising a
galvanized film provided on a surface of a steel sheet,
wherein the steel sheet has a chemical composition consisting of, in mass%,
C: 0.10% to 0.45%,
Si: less than 0.3%,
Mn: 0.5% to 3.0%,
P: less than or equal to 0.02%,
S: less than or equal to 0.004%,
sol. Al: 0.005% to 1.0%,
N: less than or equal to 0.01%, and
the balance: Fe and impurities,
wherein the galvanized film has a plating coverage of 40 g/m2 to 1 1og/m2, an A1
content of more than or equal to 150 mg/m2 within the galvanized film, and an A1
concentration of less than or equal to 0.5 mass %, and
wherein the galvanized steel sheet is used for an application in which the
galvanized steel sheet is heated to 700°C or above and is then subjected to hot forming.
[Claim 21
The galvanized steel sheet for hot forming according to claim 1, wherein the
chemical composition includes, in mass%,
Ti: less than or equal to 0.01% instead of a part of Fe.
[Claim 31
The galvanized steel sheet for hot forming according to claims 1 or 2, wherein
the chemical composition includes, instead of a part of Fe, one or more selected from the
group consisting of, in mass%,
Cr: less than or equal to 0.3%, and
B: less than or equal to 0.0050%.
[Claim 41
Z:'PG f-JEk"+XH~' ~ - Q17~:Qz- ~ Q ~ ~
The galvanized steel sheet for hot forming according to any one of claims 1 to 3,
wherein the chemical composition includes, instead of a part of Fe, one or more selected
from the group consisting of, in mass%,
Mg: less than or equal to 0.05%,
Ca: less than or equal to 0.05%, and
REM: less than or equal to 0.05%.
[Claim 51
The galvanized steel sheet for hot forming according to any one of claims 1 to 4,
wherein the chemical composition includes, in mass%,
Bi: less than or equal to 0.05% instead of a part of Fe.
[Claim 61
The galvanized steel sheet for hot forming according to any one of claims 1 to 5,
wherein the galvanized steel sheet is an alloyed hot dip galvanized steel sheet.