DESCRIPTION
Title of Invention: Steel Sheet Provided With Hot Dip
Galvanized Layer Excellent in Plating Wettability and
Plating Adhesion and Method of Production of Same
Technical Field
The present invention relates to a hot dip
galvanized steel sheet and a method of production of the
same, more particularly relates to a steel sheet which is
provided with a hot dip galvanized layer excellent in
plating wettability and plating adhesion and which can be
applied as a member in the automotive field, household
appliance field, and construction material field and a
method of production of the same.
Background Art
[0002] In members in the automotive field, household
appliance field, and construction material field, surface
treated steel sheet which imparts corrosion prevention is
being used. In particular, hot dip galvanized steel sheet
which can be inexpensively produced and which is
excellent in corrosion prevention is being used.
[0003] In general, hot dip galvanized steel sheet is
produced by the following method using a continuous hot
dip galvanization facility. First/ a slab is hot rolled,
cold rolled, and heat treated to obtain a thin-gauge
steel sheet. This is degreased and/or pickled by a
pretreatment step for the purpose of cleaning the surface
of the base material steel sheet or, omitting the
pretreatment step, is heated in a preheating furnace to
burn off the oil on the surface of the base material
steel sheet surface, then is heated to recrystallize and
anneal it. The atmosphere at the time of
recrystallization and annealing is an Fe reducing
atmosphere since at the time of the later plating
treatment, Fe oxides would obstruct the wettability of
- 2 -
the plating layer and the base material steel sheet or
the adhesion of the plating layer and base material steel
sheet. After the recrystallization and annealing, without
contacting the air, the steel sheet is continuously
5 cooled to a temperature suitable for plating in an Fe
reducing atmosphere and dipped in a hot dip galvanization
bath for hot dip galvanization.
[0004] In a continuous hot dip galvanization facility,
the types of heating furnaces which perform the
10 recrystallization and annealing include DFF {direct flame
furnaces), NOF (nonoxidizing furnaces), all radiant tube
type {all reducing) types or combinations of the same
etc., but for ease of operation, less roll pickup in the
heating furnace, the ability to produce high quality
15 plated steel sheet at a lower cost', and other reasons,
the mainstream practice has been to make the entire
inside of the furnace an Fe reducing atmosphere and make
the heating furnace an all radiant tube type. The "roll
pickup" referred to here means the deposition of oxides
20 or foreign matter from the surface of the steel sheet on
the rolls in the furnace at the time of running through
the furnace. After deposition, defects in appearance
occur at the steel sheet, so this has a detrimental
effect on quality and productivity.
25 [0005] In recent years, in particular in the
automotive field, to achieve both the function of
protecting the passengers at the time of collision and
lighter weight aimed at improvement of the fuel
efficiency, use of hot dip galvanized steel sheet which
30 is made higher in strength of the base material steel
sheet by inclusion of elements such as Si and Mn has been
increasing.
[0006] However, Si and Mn are elements which are more
easily oxidizable compared with Fe, so at the time of
35 heating in recrystallization and annealing in the all
radiant tube type of furnace, even in a reducing
atmosphere of Fe, Si and Mn end up oxidizing. For this
_ 3 -
reason, in a steel sheet which contains Si and Mn, in the
process of recrystallization and annealing, the Si and Mn
present in the steel sheet surface oxidize. Further, the
Si and Mn which thermally diffuse from the inside of the
5 steel sheet oxidize at the steel sheet surface whereby
gradually the Si and Mn oxides become concentrated. If
the Si and Mn oxides concentrate at the steel sheet
surface, in the process of dipping the steel sheet in the
hot dip galvanization bath, contact between the molten
10 zinc and steel sheet would be obstructed, which would
cause a drop in the wettability of the plating layer and
adhesion of the plating layer. If the plating layer falls
in wettability, nonplating defects occur and result in
defects in appearance and/or defects in corrosion
15 prevention. If the plating adhesion falls, when press
forming this plated steel sheet, peeling of the plating
occurs and results in defects in appearance and/or
defects in corrosion prevention after forming, so becomes
a major problem.
20 [0007] As the art for suppressing concentration of
oxides of Si and Mn, as art focusing on the
recrystallization and annealing process, PLT 1 shows
oxidizing the steel sheet surface so that the thickness
of the oxide film becomes 400 to 10000A, then reducing
25 the Fe in the furnace atmosphere containing hydrogen and
then plating. Further, PLT 2 shows the method of
oxidizing the steel sheet surface and controlling the
oxygen potential in the reducing furnace to thereby
reduce the Fe and internally oxidize the Si so as to
30 suppress the concentration of Si oxides at the surface,
then plating. However, in these art, if the reduction
time is too long, Si concentrates at the surface, while
if too short, an Fe oxide film remains on the steel sheet
surface. In the actual case where the oxide film on the
35 steel sheet surface becomes uneven in thickness, there is
the problem that adjustment of the reducing time is
extremely difficult and issues in the plating layer
- 4 -
wettability and plating layer adhesion are insufficiently
resolved. Furthermore, if the Fe oxide film of the
surface at the time of oxidation becomes too thick, there
is the problem that roll pickup is caused.
5 [0008] PLT 3 solves the above problem which was due to
causing Fe to oxidize once, has as its object to suppress
the concentration of the Si and Mn oxides, and shows a
method comprising lowering the oxygen potential
(log (PH20/PH2) ) of the atmosphere in the recrystallization
10 and annealing in an all radiant tube type of furnace to a
value at which Fe and Si and Mn will not be oxidized {be
reduced), then plating. However, in this art, to reduce
Si and Mn, it is necessary to greatly lower the steam
concentration of the atmosphere or greatly raise the
15 hydrogen concentration, but there is the problem that
this is poor in industrial practicality and also the
problem that the Si and Mn which remain at the steel
sheet surface without being oxidized obstruct the
reaction between the plating and base material steel
20 sheet and, further, react with the oxides floating on the
surface of the bath to form Si and Mn oxides at the time
of dipping in the plating bath, so the plating
wettability and plating adhesion fall.
[0009] PLT 4 shows a method of raising the oxygen
25 potential in the atmosphere in the recrystallization and
annealing in an all radiant tube type of furnace until Si
and Mn internally oxidize, then plating. Further, PLTs 5
and 6 show methods of carefully controlling the means and
conditions for raising the oxygen potential to suppress
30 the surface concentration of both Fe oxides and Si and Mn
oxides, then plating. However, if raising the oxygen
potential, Si and Mn internally oxidize, but Fe oxidizes.
On the other hand, with an increase of oxygen potential
of an extent where Fe does not oxidize, the internal
35 oxidation of Si and Mn becomes insufficient and Si and Mn
oxides concentrate at the surface. In the arts of
adjusting the oxygen potential of the atmosphere which
- 5 -
are described in PLTs 4 to 6, there is the problem that
the issues in plating layer wettability and the plating
layer adhesion are not sufficiently resolved.
[0010] Furthermore, as art for suppressing the
5 concentration of Si and Mn oxides, as the above-mentioned
means increasing the steps of production of the general
continuous type hot dip galvanization, PLT 7 shows the
method of performing annealing two times, pickling and
removing the surface concentrates of Si which are formed
10 on the surface after the first annealing so as to
suppress the formation of surface concentrates at the
time of the second annealing, then plating. However, when
the Si concentration is high, pickling is not enough to
completely remove the surface concentrates, so the
15 plating wettability and plating adhesion are
insufficiently improved. Further, facilities for two
annealing operations and pickling facilities are newly
required for removing the surface concentrates of Si, so
there is the problem of an increase in the capital costs
20 and production costs.
[0011] PLTs 8 and 9 show methods of preplating the
steel sheet surface by Cr, Ni, Fe, etc. before or after
recrystallization and annealing, then plating. However,
in these art, there are the problem that when preplating
25 before recrystallization and annealing, the heating at
the time of annealing causes the preplated elements to
diffuse in the steel sheet and the steel sheet to fall in
strength and elongation and the problem that the Fe or Si
and Mn which diffuse at the steel sheet surface oxidizes.
30 Further, when preplating after recrystallization and
annealing, oxides are formed on the steel sheet surface,
so there is the problem that preplating unevenly deposits
on the steel sheet and has difficulty covering the
concentrated oxides. Further, this method has the problem
35 that no matter whether performing the preplating before
or after the recrystallization and annealing, cost is
incurred in the materials of the preplating or costs are
- 6 -
incurred in the preplating facilities, so the increase in
steps leads to an increase in the production costs.
[0012] Furthermore, in art which suppresses the
concentration of the Si and Mn, as art which focuses on
5 causing internal oxidation in advance in the hot rolling
step, PLT 10 shows the art of controlling the oxygen
potential in the hot rolling step so as to cause internal
oxidation of Si and using the resultant thin-gauge steel
sheet to produce hot dip galvanized steel sheet by a
10 continuous hot dip galvanization facility. However, in
this art, at the time of the cold rolling step and other
rolling, the layer of internal oxidation also ends up
being rolled together, so the internal oxidation layer
becomes smaller in thickness and Si oxides end up
15 concentrating at the surface in the recrystallization and
annealing process, so there is the problem that the
plating wettability and plating adhesion are
insufficiently improved. Further, there is the problem
that if causing internal oxidation in the hot rolling
20 step, the simultaneously formed Fe oxides cause roll
pickup.
[0013] PLT 11 shows the method of controlling the
oxygen potential in the atmosphere in the heating furnace
and the oxygen potential in the atmosphere at the topmost
25 part of the soaking furnace high in the same way and
controlling the oxygen potential of the top part of the
soaking furnace to be higher than the oxygen potential at
the bottom part of the furnace by a certain degree to
plate the high-Si containing steel sheet. However, by
30 this method as well, the plating adhesion is
insufficient.
Citations List
Patent Literature
35 [0014] PLT 1. Japanese Patent Publication No. 55-
122865A
PLT 2. Japanese Patent Publication No. 2001-323355A
- 7 -
10
PLT
PLT
PLT
PLT
PLT
PLT
PLT
PLT
PLT
3.
4.
5.
6.
7.
8.
9.
10
11
Japanese Patent Publication No. 2010-126757A
Japanese Patent Publication No. 2008-7842A
Japanese Patent Publication No. 2001-279412A
Japanese Patent Publication No. 2009-209397A
Japanese Patent Publication No. 2010-196083A
Japanese Patent Publication No. 56-33463A
Japanese Patent Publication No. 57-79160A
Japanese Patent Publication No. 2000-309847A
Japanese Patent Publication No. 2009-068041A
Summary of Invention
Technical Problem
[0015] The present invention has as its problem to
provide a hot dip galvanized steel sheet which uses a
15 steel sheet which contains the easily oxidizable elements
of Si and Mn as a base material and is provided with a
hot dip galvanized layer which is excellent in plating
wettability and plating adhesion and to provide a method
of production of the same.
20
Solution to Problem
[0016] To solve the above problem, the inventors took
note of the effect of the contents of components of the
hot dip galvanized layer and base material steel sheet in
25 a hot dip galvanized steel sheet, in particular the base
material steel sheet right under the plating layer, on
the plating wettability and plating adhesion and further
took note of, in the method of production of the same,
achieving both causing internal oxidation of Si and Mn
30 when raising the oxygen potential of the atmosphere and
reducing the Fe in a radiant tube type of heating furnace
by controlling the recrystallization and annealing to the
oxygen potential in the heating step and soaking step.
They engaged in various studies in depth and, as a
35 result, discovered that it is possible to produce hot dip
galvanized steel sheet which is excellent in plating
wettability and plating adhesion without increasing the
steps in a continuous hot dip galvanization facility
which is provided with an all radiant tube type of
heating furnace and thereby completed the present
invention.
5 [0017] That is, the present invention has as its gist
the following:
(I) A hot dip galvanized steel sheet comprising a steel
sheet which contains, by mass%,
C: 0.05% to 0.50%,
10 Si: 0.1% to 3.0%,
Mn: 0.5% to 5.0%,
P: 0.001% to 0.5%,
S: 0.001% to 0.03%,
Al: 0.005% to 1.0%, and
15 a balance of Fe and unavoidable impurities, having a hot
dip galvanized layer A on the surface of the steel sheet,
characterized by having the following B layer right below
the steel sheet surface and inside the steel sheet:
B layer: Layer which has thickness of 0.001 um to 0.5 Jim,
20 which contains, based on mass of the B layer, one or more
of Fe, Si, Mn, P, S, and Al oxides in a total of less
than 50 mass%, which contains C, Si, Mn, P, S, and Al not
in oxides in:
C: less than 0.05 mass%,
25 Si: less than 0.1 mass%,
Mn: less than 0.5 mass%,
P: less than 0.001 mass%,
S: less than 0.001 mass%, and
Al: less than 0.005 mass%, and
30 which contains Fe not in oxides in 50 mass% or more.
[0018] (2) A hot dip galvanized steel sheet comprising
a steel sheet which contains, by mass%,
C: 0.05% to 0.50%,
Si: 0.1% to 3.0%,
35 Mn: 0.5% to 5.0%,
P: 0.001% to 0.5%,
S: 0.001% to 0.03%,
- 9 -
Al: 0.005% to 1.0%,
one or more elements of Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W,
B, Ca, and REM in respectively 0.0001% to 1%, and
a balance of Fe and unavoidable impurities, having a hot
5 dip galvanized layer A on the surface of the steel sheet,
characterized by having the following B layer right below
the steel sheet surface and inside the steel sheet:
B layer: Layer which has thickness of 0.001 urn to 0.5 urn,
which contains, based on mass of the B layer, one or more
10 of Fe, Si, Mn, P, S, Al, Ti, Nb, Cr, Mo, Ni, Cu, Zr, V,
W, B, Ca, and REM oxides in a total of less than 50
mass%, which contains C, Si, Mn, P, S, Al, Ti, Nb, Cr,
Mo, Ni, Cu, Zr, V, W, B, Ca, and REM not in oxides in:
C: less than 0.05 mass%,
15 Si: less than 0.1 mass%,
Mn: less than 0.5 mass%,
P: less than 0.001 mass%,
S: less than 0.001 mass%,
Al: less than 0.005 mass%,
20 one or more of Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca,
and REM in respectively less than 0.0001 mass%,
and
which contains Fe not in oxides in 50 mass% or more.
[0019] (3) The hot dip galvanized steel sheet
25 according to (1) or (2), wherein the hot dip galvanized
layer A has a thickness of 2 urn to 100 urn.
[0020] (4) A method of production of a hot dip
galvanized steel sheet comprising casting, hot rolling,
pickling, and cold rolling a steel containing the
30 components described in (1) or (2) to obtain a cold
rolled steel sheet, and annealing the cold rolled steel
sheet and hot dip galvanizing the annealed steel sheet in
a continuous hot dip galvanization facility which is
provided with a heating furnace and a soaking furnace,
35 wherein,
in the heating furnace and the soaking furnace which
perform the annealing treatment, the temperature of the
- 10 -
cold rolled steel sheet in the furnaces being 500°C to
950°C in temperature range and running the cold rolled
steel sheet under the following conditions:
Heating furnace conditions: Using an all radiant tube
5 type of heating furnace, heating the cold rolled steel
sheet in the above temperature range for 10 seconds to
1000 seconds, wherein the log (PH2O/PH2) of the value of
the steam partial pressure (PH2O) in the heating furnace
divided by the hydrogen partial pressure (PH2) is -2 to 2,
10 and wherein the heating furnace has an atmosphere
comprised of hydrogen in a hydrogen concentration of 1
vol% to 30 vol%, steam, and nitrogen;
Soaking furnace conditions: After the heating furnace, in
the soaking furnace, soaking the cold rolled steel sheet
15 in the above temperature range for 10 seconds to 1000
seconds, wherein the log (PH20/PH2) of the value of the
steam partial pressure (PH20) in the soaking furnace
divided by the hydrogen partial pressure (PH2) is -5 to -
2, and wherein the soaking furnace has an atmosphere
20 comprised of hydrogen in a hydrogen concentration of 1
vol% to 30 vol%, steam, and nitrogen.
Advantageous Effects of Invention
[0021] According to the method of production of the
25 present invention, a hot dip galvanized steel sheet which
is excellent in plating wettability and plating adhesion
is obtained using a steel sheet which contains the easily
oxidizable elements Si and Mn as a base material.
30 Brief Description of Drawings
[0022] FIG. 1 shows the results of the plating
wettability/ plating adhesion determined by the
relationship of the thickness of the A layer and
thickness of the B layer obtained from the results of the
35 later explained Examples Al to A72, Bl to B72, Cl to C72,
Dl to D72, El to E72, Fl to F72, and Gl to G72 and
Comparative Examples Hi to H12 and H29 to H34.
- 11 -
FIG. 2 shows the relationship of the content of oxides of
the B layer and the plating wettability/adhesion obtained
from the results of the later explained Examples Al to
A72, Bl to B72, CI to C72, Dl to D72, El to E72, Fl to
5 F72, and Gl to G72 and Comparative Examples Hi to H12.
FIG. 3 shows the relationship of the Fe content of the B
layer and the plating wettability/adhesion obtained from
the results of the later explained Examples Al to A72, Bl
to B72, CI to C72, Dl to D72, El to E72, Fl to F72, and
10 Gl to G72 and Comparative Examples Hi to H12.
FIG. 4 shows the results of the plating
wettability/plating adhesion which are determined by the
relationship between the oxygen potential log{PH20/PH2) of
the heating furnace and the oxygen potential log (PH2O/PH2)
15 of the soaking furnace obtained from the results of the
later explained Examples Al to A72, Bl to B72, CI to C72,
Dl to D72, El to E72, Fl to F72, and Gl to G72 and
Comparative Examples HI to H12.
FIG. 5 shows the relationship between the hydrogen
20 concentration of the heating furnace and plating
wettability/plating adhesion obtained from the results of
the later explained Examples Al to A72, Bl to B72, CI to
C72, Dl to D72, El to E72, Fl to F72, and Gl to G72 and
Comparative Examples H25 to H28.
25 FIG. 6 shows the relationship between the hydrogen
concentration of the soaking furnace and the plating
wettability/plating adhesion as understood from the
results of the later explained Examples Al to A72, Bl to
B72, CI to C72, Dl to D72, El to E72, Fl to F72, and Gl
30 to G72 and Comparative Examples H25 to H28.
FIG. 7 shows the results of the plating
wettability/plating adhesion which is determined by the
relationship of the peak temperature of the cold rolled
steel sheet in the heating furnace and the time in the
35 temperature range of 500°C to 950°C which is obtained from
the results of the later explained Examples Al to A72, Bl
to B72, CI to C72, Dl to D72, El to E72, Fl to F72, and
/
- 12 -
Gl to G72 and Comparative Examples H13 to H18 and H22 to
H24.
FIG. 8 shows the results of the plating
wettability/plating adhesion which is determined by the
5 relationship between the minimum and maximum sheet
temperatures (sheet temperature range) at the soaking
furnace and the time in the temperature range of 500°C to
950°C which is obtained from the results of the later
explained Examples Al to A72, Bl to B72, CI to C72, Dl to
10 D72, El to E72, Fl to F72, and Gl to G72 and Comparative
Examples H13 to H24.
Description of Embodiments
[0023] Below, the present invention will be explained
15 in detail. First, the assumed components of the steel
sheet which is provided with the hot dip galvanized layer
of the present invention which are as follows. Further,
below, the % which is explained in the Description shall
be mass% unless otherwise indicated.
20 [0024] C: 0.05% to 0.50%
C is an element which stabilizes the austenite phase and
is an element which is necessary for raising the strength
of the steel sheet. If the amount of C is less than
0.05%, the steel sheet becomes insufficient in strength,
25 while if over 0.50%, the workability falls. For this
reason, the amount of C is 0.05% to 0.5%, preferably
0.10% to 0.40%.
[0025] Si: 0.1% to 3.0%
Si causes the solid solution C in the ferrite phase to
30 concentrate in the austenite phase and raises the temper
softening resistance of the steel sheet to thereby
improve the strength of the steel sheet. If the amount of
Si is less than 0.1%, the steel sheet becomes
insufficient in strength, while if over 3.0%, it falls in
35 workability. Further, the plating wettability and plating
adhesion are not sufficiently improved. For this reason,
the amount of Si is 0.1% to 3.0%, preferably 0.5% to
- 13 -
2.0%.
[0026] Mn: 0.5% to 5.0%
Mn is an element which is useful for raising the
hardenability and raising the strength of the steel
5 sheet. If the amount of Mn is less than 0.5%, the steel
sheet becomes insufficient in strength, while if over
5.0%, it falls in workability. Further, the plating
wettability and plating adhesion are not sufficiently
improved. For this reason, the amount of Mn is 0.5% to
10 5.0%, preferably 1.0% to less than 3.0%.
[0027] P: 0.001% to 0.5%
P contributes to improvement of the strength, so can
include P in accordance with the required strength level.
However, if the content of P is contained over 0.5%,
15 grain boundary segregation causes the material quality to
deteriorate, so the upper limit is made 0.5%. On the
other hand, to make the content of P less than 0.001%, a
tremendous increase in cost is required at the
steelmaking stage, so 0.001% is made the lower limit.
20 [0028] S: 0.001% to 0.03%
S is an unavoidably included impurity element. After cold
rolling, sheet shaped inclusions MnS are formed whereby
the workability drops, so the amount of S is preferably
as small as possible, but excessive reduction is
25 accompanied with an increase in the desulfurization costs
of the steelmaking process. Therefore, the amount of S is
0.001% to 0.03%.
[0029] Al: 0.005% to 1.0%
Al has a high affinity with the N in the steel sheet and
30 has the effect of fixing the solid solution N as
precipitates to thereby improve the workability. However,
excessive addition of Al conversely causes the
workability to deteriorate. For this reason, the amount
of Al is 0.005% to 1.0%.
35 [0030] The balance other than the above composition of
components is Fe and unavoidable impurities. In the
present invention, for the purpose of securing the
- 14 -
strength, improving the workability, etc., in accordance
with need, one or more elements which are selected from
Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM may be
suitably included in the steel sheet in respectively
5 0.0001% to 1%.
[0031] The method of production of steel sheet is not
particularly limited from casting to cold rolling. The
steel is processed by general casting, hot rolling,
pickling, and cold rolling to obtain cold rolled steel
10 sheet. The steel sheet has a thickness of preferably 0.1
mm to 3 mm.
[0032] Next, factors which are important in the
present invention, that is, the hot dip galvanized layer
of the steel sheet (A layer) and the layer which is
15 formed in the steel sheet (B layer), will be explained.
[0033] The hot dip galvanized steel sheet of the
present invention is has the A layer on the steel sheet
surface and the B layer right under the steel sheet
surface. The A layer is a hot dip galvanized layer which
20 is formed on the steel sheet surface to secure corrosion
prevention. The B layer is a layer mainly comprised of Fe
which is suppressed in contents of oxides and elements of
C, Si, Mn, etc. It is formed in the steel sheet right
under the base material steel sheet surface to thereby
25 improve the plating wettability and plating adhesion.
[0034] The A layer constituted by the hot dip
galvanized layer may have elements other than zinc added
in the layer so long as 50% or more of the constituent
components is zinc. Further, a hot dip galvannealed layer
30 which becomes an Fe-Zn alloy by heating after hot dip
galvanization treatment is also possible. In the case of
a hot dip galvannealed layer, if the content of Fe in the
Fe-Zn alloy is over 20 mass%, the plating adhesion falls,
so the content is preferably 20 mass% or less.
35 [0035] The Fe content in the Fe-Zn alloy of the hot
dip galvannealed layer referred to here is found by
cutting out a piece of a predetermined area from the hot
- 15 -
dip galvanized steel sheet, dipping it in hydrochloric
acid to dissolve only the plating layer, then analyzing
this solution by an ICP (emission spectrophotometry
analyzer) to measure the amount of Fe and amount of Zn
5 and thereby calculate the ratio of Fe.
[0036] The thickness of the A layer, as shown in FIG.
1, is preferably 2 urn to 100 urn. If less than 2 urn, the
corrosion prevention ability is insufficient. In
addition, it becomes difficult to make the plating
10 uniformly deposit on the base material steel sheet and
nonplating defects are formed, that is, a problem arises
relating to plating wettability. If over 100 (im, the
effect of improvement of the corrosion resistance by the
plating layer becomes saturated, so this is not
15 economical. Further, the residual stress inside the
plating layer increases, so the plating adhesion falls.
For this reason, the thickness of the A layer is
preferably 2 {im to 100 urn. Regarding the method of
measurement of the thickness of the A layer which is
20 mentioned here, there are various methods, but for
example the microscopic cross-section test method
described in JIS H 8501 may be mentioned. This is a
method of burying a cross-section of a sample in a resin,
polishing it, then in accordance with need etching it by
25 a corrosive solution and analyzing the polished surface
by an optical microscope or scan type electron microscope
(SEM), electron probe microanalyzer (EPMA), etc. and
finding the thickness. In the present invention, the
sample is buried in Technovit 4002 (made by Maruto
30 Instrument Co., Ltd.) and polished in order by #240,
#320, #400, #600, #800, and #1000 polishing paper (JIS R
6001), then he polished surface is analyzed by EPMA from
the surface of the plated steel sheet by line analysis,
the thickness at which Zn is no longer detected is found
35 at positions of any 10 locations separated from each
other by 1 mm or more, the found values are averaged, and
- 16 -
the obtained value is deemed the thickness of the A
layer.
[0037] In the case of a hot dip galvannealed layer,
due to the B layer inside the steel sheet right under the
5 steel sheet surface of the base material, the content of
the oxides of the steel sheet of the base material is
reduced whereby the reactivity between the Fe and plating
is promoted and the plating wettability and the plating
adhesion are further improved, so this is preferable.
10 [0038] The B layer which characterizes the present
invention is a layer which is formed by raising the
oxygen potential of the atmosphere in a radiant tube type
of heating furnace and lowering the oxygen potential of
the atmosphere to reduce the Fe in a soaking furnace. In
15 the heating furnace, Si and Mn internally oxidize and C
oxidizes and disassociates at the steel sheet surface as
a gas, so at a certain thickness under the steel sheet
surface, the concentration of Si, Mn, and C not in oxides
right under the steel sheet surface of the base material
20 is reduced, but the thickness follows the heat dispersion
of Si and Mn and C, so becomes greater than the thickness
of the internal oxidation layer. If just raising the
oxygen potential of the atmosphere, Fe oxides would be
formed at the steel sheet surface of the base material
25 and increase. Also, for example, in internal oxidation of
Si, internal oxides of the composite oxide with Fe called
"fayalite" (Fe2Si04) would be formed and increase.
However, with the method of the present invention, the Fe
is reduced in the soaking furnace, so Fe oxides can be
30 suppressed right under the steel sheet surface of the
base material. Therefore, the B layer according to the
present invention is a different layer from the "internal
oxidation layer" which is described in the prior art
literature etc.
35 [0039] The B layer in the steel sheet right under the
base material steel sheet surface, as shown in FIG. 1,
having a thickness of 0.001 (am to 0.5 urn is important for
- 17 -
improving the plating wettability and plating layer
adhesion. If less than 0.001 fim, the amount of the B
layer falls, so the plating wettability and adhesion are
not sufficiently improved, while if over 0.5 \xmf the
5 strength in the B layer is not secured and cohesive
failure occurs, so the plating adhesion falls. More
preferably, the B layer has a thickness of 0.01 urn to 0.4
ytm. The thickness of the B layer referred to here was
found as follows: the surface of the hot dip galvanized
10 steel sheet was sputtered while using an X-ray
photoelectron spectroscope (XPS) for analyzing the
composition in the depth direction. The depth at which Zn
could no longer be detected was designated as Dl. The
amounts of C, Si, Mn, P, S, and Al in the B layer were
15 respectively C: less than 0.05 mass%, Si: less than 0.1
mass%, Mn: less than 0.5 mass%, P: less than 0.001 mass%,
S: less than 0.001 mass%, and Al: less than 0.005 mass%,
so the depth at which C is detected in 0.05% or more or
the depth at which Si is detected in 0.1% or more, the
20 depth at which Mn is detected in 0.5% or more, the depth
at which P is detected in 0.001% or more, the depth at
which S is detected in 0.001% or more, and the depth at
which Al is detected in 0.005% or more are found and the
depth of the smallest value among these values is
25 designated as D2. The thickness of the B layer is made
the average value obtained by finding (D2-D1) for N=3.
However, the percentage which is shown here is based on
the display of the XPS system. The measurement method is
not limited. In addition to X-ray photoelectron
30 spectroscopy (XPS), glow discharge spectrometry (GDS),
secondary ion mass spectrometry (SIMS), time-of-flight
type secondary ion mass spectrometry (TOF-SIMS), TEM, or
another analysis means may be used.
[0040] By recrystallizing and annealing the steel
35 sheet which is hot dip galvanized of the present
invention, one or more types of oxides of E*e, Si, Mn, P,
- 18 -
S, and Al are formed right under the steel sheet surface.
As shown in FIG. 2, if the total of the contents of these
in the B layer of the hot dip galvanized steel sheet of
the present invention becomes 50% or more, the plating
5 wettability and the plating adhesion fall. Therefore, the
total of these oxides in the B layer is less than 50%,
preferably less than 25%.
[0041] The one or more types of oxides of Fe, Si, Mn,
P, S, and Al referred to here is not particularly limited
10 to the following, but, as specific examples, FeO, Fe203,
Fe304, MnO, Mn02/ Mn203, Mn304, Si02, P2O5, Al203, and S02 as
single oxides and respective nonstoichiometric
compositions of single oxides or FeSi03, Fe2Si04, MnSi03,
Mn2Si04, AlMn03/ Fe2P03, and Mn2P03 as composite oxides and
15 respective nonstoichiometric compositions of composite
oxides may be mentioned. The total of the rates of
content is found, in the same way as the above-mentioned
measurement of thickness of the B layer, by sputtering
the surface of the hot dip galvanized steel sheet while
20 analyzing the composition by an X-ray photoelectron
spectroscope (XPS) in the depth direction, totaling the
average values of the concentrations by mass of cations
of Fe, Si, Mn, P, S, and Al which are measured from the
depth at which Zn is no longer detected (Dl) to the depth
25 of the smallest value (D2) among the depth at which C is
detected in 0.05% or more or the depth at which Si is
detected in 0.1% or more, the depth at which Mn is
detected in 0.5% or more, the depth at which P is
detected in 0.001% or more, the depth at which S is
30 detected in 0.001% or more, and the depth at which Al is
detected in 0.005% or more, further adding the average
value of the concentration by mass of anions of 0, and
averaging the result for N=3 measurement results. The
measurement method is not particularly limited. In
35 accordance with need, glow discharge spectrometry (GDS),
secondary ion mass spectrometry (SIMS), time-of-flight
type secondary ion mass spectrometry (TOF-SIMS), TEM, or
- 19 -
another analysis means may be used.
[0042] Furthermore, in the B layer, the contents of C,
Si, Mn, P, S, and Al not.in oxides being suppressed is
also important for improving the plating wettability and
5 plating adhesion. This is because if decreasing the C,
Si, Mn, P, S, and Al which are added to the steel sheet
of the base material right under the steel sheet surface
to raise the ratio of Fe, the reactivity of the steel
sheet and plating rises, the plating becomes easily
10 wetted, and the adhesion between plating and base
material rises. In addition, this is because, regarding
the C, Si, Mn, P, S, and Al not in oxides present at the
steel sheet surface of the base material, if the oxides
which are present on the surface of the bath contact the
15 steel sheet of the base material when dipping the steel
sheet in the plating bath and treating the plating layer,
the Si, Mn, P, S, and Al oxidize and reduce the
reactivity of the steel sheet and plating, so decreasing
the C, Si, Mn, P, S, and Al not in oxides is effective
20 for improvement of the plating wettability and the
plating adhesion. A drop in the plating wettability and
adhesion is recognized with a content of C of the B layer
of 0.05% or more or a content of Si of 0.1% or more, a
content of Mn of 0.5% or more, a content of P of 0.001%
25 or more, a content of S of 0.001% or more, and a content
of Al of 0.005% or more, so preferably the content of C
of the B layer is made less than 0.05%, the content of Si
is made less than 0.1%, the content of Mn is made less
than 0.5%, the content of P is made less than 0.001%, the
30 content of S is made less than 0.001%, and the content of
Al is made less than 0.005%. The contents of C, Si, Mn,
P, S, and Al referred to here are found by sputtering the
surface of the hot dip galvanized steel sheet while
analyzing the composition by XPS in the depth direction
35 and averaging, by the N=3 results of measurement, the
average values of the concentrations by mass of C, Si,
Mn, P, S, and Al which are measured from the depth at
- 20 -
which Zn is no longer detected (Dl) to the depth of the
smallest value (D2) among the depth at which C is
detected in 0.05% or more or the depth at which Si is
detected in 0.1% or more, the depth at which Mn is
5 detected in 0.5% or more, the depth at which P is
detected in 0.001% or more, the depth at which S is
detected in 0.001% or more, and the depth at which Al is
detected in 0.005% or more. The measurement method is not
particularly limited. In accordance with need, glow
10 discharge spectrometry (GDS) , secondary ion mass
spectrometry (SIMS), time-of-flight type secondary ion
mass spectrometry (TOF-SIMS), TEM, or other analysis
means may be used.
[0043] If the content of Fe not in oxides in the B
15 layer, as shown in FIG. 3, is less than 50%, the
wettability and adhesion with the A layer and the
adhesion with the base material steel sheet fall. As a
result, the plating wettability and the plating adhesion
are made to fall, so the content of Fe not in oxides is
20 50% or more, preferably 70% or more. The content of Fe
referred to here is found by sputtering the surface of
the hot dip galvanized steel sheet while analyzing the
composition by XPS in the depth direction and averaging,
for N=3 measurement results, the average value of the
25 concentration by mass of Fe which is measured from the
depth at which Zn is no longer detected (Dl) to the depth
of the smallest value (D2) among the depth at which C is
detected in 0.05% or more or the depth at which Si is
detected in 0.1% or more, the depth at which Mn is
30 detected in 0.5% or more, the depth at which P is
detected in 0.001% or more, the depth at which S is
detected in 0.001% or more, and the depth at which Al is
detected in 0.005% or more. The measurement method is not
particularly limited. In accordance with need, glow
35 discharge spectrometry (GDS), secondary ion mass
spectrometry (SIMS), time-of-flight type secondary ion
mass spectrometry (TOF-SIMS), TEM, or other analysis
- 21 -
means may be used.
[0044] A more preferable embodiment of the B layer
will be explained next. This embodiment has as its object
to secure the strength, improve the workability, etc. and
5 is the case where one or more elements which are selected
from Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM are
contained in the steel sheet in respective amounts of
0.0001% to 1% as additional components.
[0045] The B layer between the hot dip galvanized
10 layer constituted by the A layer and the base material
steel sheet, as shown in FIG. 1, preferably has a
thickness of 0.001 urn to 0.5 Jim as explained above. More
preferably, the B layer similarly has a thickness of 0.01
Jim to 0.4 urn. The preferable thickness of the B layer
15 referred to here is found as follows: The surface of the
hot dip galvanized steel sheet is sputtered while XPS is
used to analyze the composition in the depth direction,
the depth at which Zn is no longer detected is designated
as Dl, the depth where C is detected in 0.05% or more or
20 the depth at which Si is detected in 0.1% or more, the
depth at which Mn is detected in 0.5% or more, the depth
at which P is detected in 0.001% or more, the depth at
which S is detected in 0.001% or more, the depth at which
Al is detected in 0.005% or more, and the depth at which
25 Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM are
detected in 0.0001% or more are found, and the depth of
the smallest value among these values is designated as
D2. The thickness of the B layer is made the average
value of (D2-D1) found for N=3. The measurement method is
30 not particularly limited. In accordance with need, glow
discharge spectrometry (GDS), secondary ion mass
spectrometry (SIMS), time-of-flight type secondary ion
mass spectrometry (TOF-SIMS), TEM, or other analysis
means may be used.
35 [0046] Furthermore, by recrystallization and annealing
of the steel sheet which is hot dip galvanized in the
present invention, oxides of one or more of Fe, Si, Mn,
- 22 -
Al, P, S, Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and
REM are formed right under the steel sheet surface. As
shown in FIG. 2, if the total of the rates of content in
the By layer after hot dip galvanization of the present
5 invention becomes 50% or more, the plating wettability
and the plating adhesion fall, so the total is less than
50%, preferably less than 25%.
[0047] The oxides of the one or more elements which
are selected from the Fe, Si, Mn, P, S, Al, Ti, Nb, Cr,
10 Mo, Ni, Cu, Zr, V, W, B, Ca, and REM referred to here are
not particularly limited to the following, but as
specific examples, FeO, Fe203, Fe304, MnO, Mn02, Mn203,
Mn304/ Si02, P205, Al203, S02, Ti02, NbO, Cr203, Mo02, NiO,
CuO, Zr02, V2O5, W02, B2O5, and CaO as single oxides and
15 respective nonstoichiometric compositions of single
oxides or FeSx03, Fe2Si04, MnSi03, Mn2Si04, AlMn03, Fe2P03,
and Mn2P03 as composite oxides and respective
nonstoichiometric compositions of composite oxides may be
mentioned. The total of the rates of content is found, in
20 the same way as the above-mentioned measurement of
thickness of the B layer, by sputtering the surface of
the hot dip galvanized steel sheet while using an X-ray
photoelectron spectroscope (XPS) to analyze the
composition in the depth direction, totaling the average
25 values of the respective concentrations of mass of
cations of Fe, Si, Mn, Al, P, S, Ti, Nb, Cr, Mo, Nx, Cu,
Zr, V, W, B, Ca, and REM which are measured from the
depth at which Zn is no longer detected {Dl) to the depth
of the smallest value (D2) among the depth at which C is
30 detected in 0.05% or more or the depth at which Si is
detected in 0.1% or more, the depth at which Mn is
detected in 0.5% or more, the depth at which P is
detected in 0.001% or more, the depth at which S is
detected in 0.001% or more, the depth at which Al is
35 detected in 0.005% or more, and the depth at which Ti,
Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, REM are detected in
0.0001% or more, further adding the average value of the
- 23 -
concentration of mass of 0 ions, and averaging the result
for N=3 measurement results. The method of measurement is
not particularly limited, but in accordance with need,
glow discharge spectrometry (GDS), secondary ion mass
5 spectrometry (SIMS), time-of-flight type secondary ion
mass spectrometry (TOF-SIMS), TEM, or other analysis
means may be used.
[0048] Furthermore, in the B layer, suppressing the
contents of C, Si, Mn, P, S, Al, Ti, Nb, Cr, Mo, Ni, Cu,
10 Zr, V, W, B, Ca, and REM not in oxides is also important
for improving the plating wettability and the plating
adhesion. This is because if reducing the C, Si, Mn, P,
S, Al, Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM
which are added to the steel sheet of the base material
15 right under the steel sheet surface and raising the ratio
of Fe, the reactivity of the steel sheet and the plating
rises and the plating becomes easily wetted and the
adhesion between the plating and the base material rises.
In addition, this is because, regarding the C, Si, Mn, P,
20 S, Al, Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM
not in oxides present at the steel sheet surface of the
base material, if the oxides which are present on the
surface of the bath contact the steel sheet of the base
material when dipping the steel sheet in the plating bath
25 and treating the plating layer, the Si, Mn, P, S, Al, Ti,
Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM oxidize and
reduce the reactivity of the steel sheet and plating, so
decreasing the C, Si, Mn, P, S, Al, Ti, Nb, Cr, Mo, Ni,
Cu, Zr, V, W, B, Ca, and REM not in oxides is effective
30 for improvement of the plating wettability and the
plating adhesion. A drop in the plating wettability and
adhesion is recognized when the B layer has a C content
of 0.05% or more or Si content of 0.1% or more, Mn
content of 0.5% or more, P content of 0.001% or more, S
35 content of 0.001% or more, Al content of 0.005% or more,
Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM content
of 0.0001% or more, so the B layer preferably has a C
- 24 -
content of less than 0.05%, Si content of less than 0.1%,
Mn content of less than 0.5%, P content of less than
0.001%, S content of less than 0.001%, Al content of less
than 0.005%, Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and
5 REM content of less than 0.0001%. The contents of the C,
Si, Mn, P, S, Al, Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B,
Ca, and REM referred to here are found by sputtering the
surface of the hot dip galvanized steel sheet while
analyzing the composition by XPS in the depth direction
10 and averaging, for N=3 measurement results, the average
values of the respective concentrations of mass of C, Si,
Mn, P, S, Al, Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca,
and REM which are measured from the depth at which Zn is
no longer detected to the depth of the smallest value
15 (D2) among the depth at which C is detected in 0.05% or
more or the depth at which Si is detected in 0.1% or
more, the depth at which Mn is detected in 0.5% or more,
the depth at which P is detected in 0.001% or more, the
depth at which S is detected in 0.001% or more, the depth
20 at which Al is detected in 0.005% or more, and the depth
at which Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM
are detected in 0.0001% or more. The method of
measurement is not particularly limited. In accordance
with need, glow discharge spectrometry (GDS), secondary
25 ion mass spectrometry (SIMS), time-of-flight type
secondary ion mass spectrometry (TOF-SIMS), TEM, or other
analysis means may be used.
[0049] If the content of Fe not in oxides at the B
layer, in the same way as above, is less than 50%, the
30 wettability and adhesion with the A layer and the
adhesion with the base material steel sheet falls and as
a result the plating wettability and plating adhesion are
decreased, so the content of Fe not in oxides is 50% or
more, preferably 70% or more. The content of Fe referred
35 to here is found by sputtering the surface of the hot dip
galvanized steel sheet while using XPS to analyze the
composition in the depth direction and averaging, for N=3
- 25 -
measurement results, the average value of the
concentration by mass of Fe which is measured from the
depth at which Zn is no longer detected (Dl) to the depth
of the smallest value (D2) among the depth at which C is
5 detected in 0.05% or more or the depth at which Si is
detected in 0.1% or more, the depth at which Mn is
detected in 0.5% or more, the depth at which P is
detected in 0.001% or more, the depth at which S is
detected in 0.001% or more, the depth at which Al is
10 detected in 0.005% or more, and the depth at which Ti,
Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM are detected
in 0.0001% or more. In particular, the method of
measurement is not limited. In accordance with need, glow
discharge spectrometry (GDS), secondary ion mass
15 spectrometry (SIMS), time-of-flight type secondary ion
mass spectrometry (TOF-SIMS), TEM, or other analysis
means may be used.
[0050] Next, a method of production of hot dip
galvanized steel sheet which is excellent in plating
20 wettability and plating adhesion of the present invention
will be explained.
[0051] As the method of production, one which works
steel of predetermined components into cold rolled steel
sheet using an ordinarily used method, then treats it to
25 anneal and hot dip galvanize it in a continuous hot dip
galvanization facility which Is provided with a heating
furnace and soaking furnace and is characterized by, in
the heating furnace and soaking furnace which perform the
annealing treatment, the temperature of the cold rolled
30 steel sheet in the furnaces being 500°C to 950°C in
temperature range and running the cold rolled steel sheet
under the following conditions is important for producing
the hot dip galvanized steel sheet of the present
invention:
35 Heating furnace conditions: Using an all radiant
tube type of heating furnace, heating the cold rolled
steel sheet in the above temperature range for 10 seconds
- 26 -
to 1000 seconds during which making the log(PH20/PH2) of
the value of the steam partial pressure 3, Fe304, MnO, Mn02, Mn203,
Mn304, Si02, P205/ AI2O3, S02, Ti02/ NbO, Cr203, Mo02, NiO,
CuO, Zr02, V205, WO2, B205, and CaO as single oxides and
15 respective nonstoichiometric compositions of single
oxides or FeSi03, Fe2Si04, MnSi03, Mn2Si04, AlMn03, Fe2P03,
and Mn2P03 as composite oxides and respective
nonstoichiometric compositions of composite oxides which
are internally oxidized may be mentioned.
20 [0055] With respect to the atmosphere in the heating
furnace in the sheet temperature range, as shown in FIG.
4, a log (PH20/PH2) in a nitrogen atmosphere which
contains water and hydrogen is preferably -2 to 2 in the
production of hot dip galvanized steel sheet of the
25 present invention. If the log (PH20/PH2) is less than -2,
the oxidation reaction of C does not sufficiently proceed
and, further, external oxides of one or more elements
which are selected from Si, Mn, P, S, Al, Ti, Nb, Cr, Mo,
Ni, Cu, Zr, V, W, B, Ca, and REM are formed at the steel
30 sheet surface, so the plating wettability and adhesion
fall. If log(PH20/PH2) is over 2, Fe oxides are
excessively formed at the steel sheet surface, so the
plating wettability and adhesion fall. In addition, the
internal oxidation of Si, Mn, P, S, Al, Ti, Nb, Cr, Mo,
35 Ni, Cu, Zr, V, W, B, Ca, and REM is performed excessively
right under the steel sheet surface whereby the internal
stress of the steel sheet due to the internal oxides
- 28 -
increases and the plating adhesion falls. More
preferably, the log is -2 to 0.5. The external oxides of
the one or more elements which are selected from Si, Mn,
P, S, Al, Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and
5 REM referred to here are not particularly limited to the
following, but, as specific examples, FeO, Fe203, Fe3<04,
MnO, Mn02/ Mn203, Mn304/ Si02/ P205, Al203, S02, Ti02, NbO,
Cr203, Mo02, NiO, CuO, Zr02/ V205, W02, B205, and CaO as
single oxides and respective nonstoichiometric
10 compositions of single oxides or FeSi03, Fe2Si04, MnSi03/
Mn2Si04, AlMn03, Fe2P03, and Mn2P03 as composite oxides and
respective nonstoichiometric compositions of composite
oxides which are externally oxidized may be mentioned.
[0056] Further, in the atmosphere in the heating
15 furnace in the sheet temperature range, the hydrogen
concentration, as shown in FIG. 5, is 1 vol% to 30 vol%.
If the hydrogen concentration is less than 1 vol%, the
ratio of nitrogen increases and a nitridation reaction
occurs at the steel sheet surface, so the plating
20 wettability or plating adhesion falls, while if over 30
vol%, the annealing treatment becomes inferior
economically and, in addition, hydrogen forms a solid
solution inside of the steel sheet whereby hydrogen
embrittlement occurs and the plating adhesion falls.
25 [0057] - Further, the heating time in the heating
furnace in the sheet temperature range is preferably 10
seconds to 1000 seconds from the viewpoint of production
of the hot dip galvanized steel sheet of the present
invention. If less than 10 seconds, the amounts of
30 oxidation of Si, Mn, and C are small, so the plating
wettability and adhesion fall, while if over 1000
seconds, the productivity of the annealing treatment fall
and the internal oxidation proceeds excessively right
under the steel sheet surface, so internal stress occurs
35 due to internal oxides and the plating adhesion falls.
The time in the heating furnace referred to here is the
time by which the cold rolled steel sheet runs in the
- 29 -
temperature range of a sheet temperature of 500°C to
950°C.
[0058] The speed of temperature rise in the heating
furnace is not particularly limited, but if too slow, the
5 productivity deteriorates. If too fast, the cost of the
heating facility increases, so 0.5°C/s to 20°C/s is
preferable.
[0059] The initial sheet temperature at the time of
entry into the heating furnace is not particularly
10 limited, but if too high, the steel sheet oxidizes, so
the plating wettability and plating adhesion fall, while
if too low, cost is incurred for cooling, so 0°C to 200°C
is preferable.
[0060] After the heating furnace, next in the
15 temperature range of the soaking furnace, by lowering the
oxygen potential log(PH2O/PH2) / the Fe-based oxides of the
steel sheet surface, specifically, FeO, Fe203, or Fes04 or
the composite oxides of Fe and Si and Fe and Cr of
Fe2Si04, FeSi03, and FeCr204 are reduced. That is, before
20 recrystallization and annealing, the steel sheet surface
is formed with compounds which naturally oxidize in the
atmosphere such as the Fe oxides of FeO, Fe203, and Fe304.
Further, in the heating step, FeO, Fe203/ and Fe304
increase and, in addition, the easily oxidizable elements
25 Si and Cr are oxidized, so Fe2Si04, FeSi03, and FeCr204 are
formed. Therefore, before the soaking step, the steel
sheet surface has compounds which obstruct the plating
wettability and plating adhesion such as FeO, Fe203, Fe304,
FeSi03, Fe2Si04, and FeCr204. By reducing these oxides in
30 the soaking step, the plating wettability and plating
adhesion are improved.
[0061] The atmosphere in the soaking furnace in the
sheet temperature range, as shown in FIG. 4, being a
nitrogen atmosphere containing water and hydrogen in
35 which the log(PH20/PH2) is -5 to less than -2 is
preferable in manufacture of the hot dip galvanized steel
- 30 -
sheet of the present invention. If the log(PH20/PH2) is
less than -5, not only does the annealing treatment
become poor in economy, but also the Si, Mn, P, S, Al,
Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM right
5 under the steel sheet which were internally oxidized in
the heating step end up being reduced whereby the plating
wettability and adhesion fall. If the log (PH20/PH2) is -2
or more, the Fe-based oxides are not sufficiently
reduced, so the plating wettability or adhesion falls.
10 More preferably, the value is -4 to less than -2.
[0062] Further, in the atmosphere of the soaking
furnace inside the above sheet temperature range, the
hydrogen concentration, as shown in FIG. 6, is 1 vol% to
30 vol%. If the hydrogen concentration is less than 1
15 vol%, the ratio of the nitrogen increases and a
nitridation reaction occurs at the steel sheet surface,
so the plating wettability and the plating adhesion fall,
while if over 30 vol%, the annealing treatment is
inferior economically. Further, at the inside of the
20 steel sheet, hydrogen forms a solid solution so hydrogen
embrittlement occurs and the plating adhesion falls.
[0063] Further, the heating time in the above sheet
temperature range of the soaking furnace is 10 seconds to
1000 seconds, but this is not preferable in production of
25 hot dip galvanized steel sheet of the present invention.
If less than 10 seconds, the Fe-based oxides are not
sufficiently reduced. Further, if over 1000 seconds, the
productivity of the annealing treatment falls and
external oxides of Si and Mn are formed, so the plating
30 wettability and adhesion fall. Further, in the soaking
furnace, even if the sheet temperature is a constant
temperature, the temperature may change in 500°C to 950°C
in temperature range.
[0064] Individual control of the atmospheric
35 conditions in the heating furnace and the soaking furnace
of the continuous hot dip galvanization facility is a
characteristic feature of the method of production of the
- 31 -
hot dip galvanized steel sheet of the present invention.
For individual control, it is necessary to charge the
furnaces with nitrogen, steam, and hydrogen while
controlling their concentrations. Further, the log
5 {PH2O/PH2) of the oxygen potential in the heating furnace
has to be higher than the log {PH20/PH2) of the oxygen
potential in the soaking furnace. For this reason, when
gas flows from the heating furnace toward the soaking
furnace, it is sufficient to introduce an additional
10 atmosphere of a higher hydrogen concentration or lower
steam concentration than the inside of the heating
furnace from between the heating furnace and the soaking
furnace toward the soaking furnace. When gas flows from
the soaking furnace toward the heating furnace, it is
15 sufficient to introduce an additional atmosphere of a
lower hydrogen concentration or higher steam
concentration than the inside of the soaking furnace from
between the heating furnace and soaking furnace toward
the heating furnace.
20 [0065] After the steel sheet leaves the heating
furnace and the soaking furnace, it can be run through
the general ordinary steps until being dipped in the hot
dip galvanization bath. For example, it can be run
through a slow cooling step, rapid cooling step,
25 overaging step, secondary cooling step, water guench
step, reheating step, etc. alone or in any combination.
It is also possible to similarly run it through general
ordinary steps after dipping in a hot dip galvanization
bath.
30 [0066] The steel sheet is run through the heating
furnace and soaking furnace, then is cooled and, in
accordance with need, held in temperature, is dipped in a
hot dip galvanization bath where it is hot dip
galvanized, then is treated for alloying in accordance
35 with need.
[0067] With hot dip galvanization treatment, it is
possible to use a hot dip galvanization bath which has a
- 32 -
bath temperature of 440°C to less than 550°C, a total of
concentration of Al in the bath and concentration of
cations of Al of 0.08% to 0.24%, and unavoidable
impurities.
5 [0068] If the bath temperature is less than 440°C, the
molten zinc in the bath may solidify, so this is
unsuitable. If the bath temperature exceeds 550°C, the
evaporation of the molten zinc at the bath surface
becomes severe, the operating cost rises, and vaporized
10 zinc sticks to the inside of the furnace, so there are
problems in operation.
[0069] When plating hot dip galvanized steel sheet, if
the total of the concentration of Al in the bath and the
concentration of cations of Al becomes less than 0.08%, a
15 large amount of £ layers is formed and the plating
adhesion falls, while if the total exceeds 0.24%, the Al
which oxidizes in the bath or on the bath increases and
the plating wettability falls.
[0070] When performing hot dip galvanization
20 treatment, then alloying treatment, the alloying
treatment is optimally performed at 440°C to 600°C. If
less than 440°C, the alloying proceeds slow. If over
600°C, due to the alloying, a hard, brittle Zn-Fe alloy
layer is overly formed at the interface with the steel
25 sheet, and the plating adhesion deteriorates. Further, if
over 600°C, the residual austenite phase of the steel
sheet breaks down, so the balance of strength and
ductility of the steel sheet also deteriorates.
30 Examples
[0071] Below, examples will be used to specifically
explain the present invention.
[0072] After the usual casting, hot rolling, pickling,
and cold rolling, Test Materials (TM) 1 to 72 of 1 mm
35 thickness cold rolled sheets which are shown in Table 1
were treated for annealing and treated to give plating
layers by a continuous hot dip galvanization facility
provided with an all radiant tube type heating furnace of
a relatively high productivity heating method with little
roll pickup as explained above. By using an all radiant
tube type of furnace, as explained above, there is little
roll pickup and the productivity is also good.
Table 1. Composition and Thickness of Cold Rolled Steel
Sheet
- 58 -
[0099] After the soaking furnace, the steel sheet is
treated by general slow cooling, rapid cooling,
overaging, and secondary cooling steps and then dipped in
a hot dip galvanization bath. The hot dip galvanization
5 bath had a plating bath temperature of 460°C and contained
0.13 mass% of Al. After the steel sheet was dipped in the
hot dip galvanization bath, it was wiped by nitrogen gas
to adjust the plating thickness to 8 urn per surface.
After that, in several examples, an alloying furnace was
10 used to heat the steel sheet to a temperature of 500°C for
30 seconds for alloying treatment. The obtained hot dip
galvanized steel sheet was evaluated for plating
wettability and plating adhesion. The results are shown
in Tables 2 to 7, while comparative examples are shown in
15 Table 8. In Tables 2 to 7, the performance of alloying
treatment is described by indicating the case where
alloying treatment is performed as "Yes" and the case
where it is not as "No".
[0100] The plating wettability was evaluated by
20 mapping Zn and Fe on any 200 urn x 200 urn area on the
surface of the plated steel sheet of each test material
by EPMA and judging the case where there is no Zn and
there are locations where Fe is exposed as poor in
wettability (Poor) and the case where Zn covers the
25 entire surface and there are no locations where Fe is
exposed as good in wettability (Good).
[0101] The plating adhesion was measured by a
powdering test. The case of a peeling length of over 2 mm
was evaluated as poor in adhesion (Poor), 2 mm to over 1
30 mm as good in adhesion (Good), and 1 mm or less as
extremely good in adhesion (Very good). A "powdering
test" is a method of examination of adhesion which
adheres Cellotape® to a hot dip galvanized steel sheet,
bends the tape surface by 90° (R=l), unbends it, then
35 peels off the tape and measures the peeled length of the
plating layer.
Further, the thickness of the B layer and the
total of the contents of the individual oxides or
composite oxides in the B layer, the content of Fe not in
oxides in the B layer, and the contents of Si, Mn, P, S,
Al, Ti, Cr, Mo, Nx, Cu, Zr, V, B, and Ca not in oxides in
the B layer were found by the methods of measurement by
the above-mentioned XPS (PHI5800, made by Ulvac Phi).
As a result of tests of the plating wettability
and plating adhesion of the examples (invention examples)
and comparative examples of the present invention, it was
learned that the examples of the present invention of
Tables 2 to 9 of Al to A72, Bl to B72, CI to C72, Dl to
D72, El to E72, Fl to F72, and Gl to G72 were better in
plating wettability and plating adhesion compared with
the comparative examples of Table 9 of the levels HI to
H34.
Industrial Applicability
[0104] The hot dip galvanized steel sheet which is
produced by the method of the present invention is
excellent in plating wettability and plating adhesion, so
application mainly as members in the automotive field and
the household appliance field and construction machine
field may be expected.
CLAIMS
Claim 1. A hot dip galvanized steel sheet comprising
a steel sheet which contains, by mass%,
C: 0.05% to 0.50%,
Si: 0.1% to 3.0%,
Mn: 0.5% to 5.0%,
P: 0.001% to 0.5%,
S: 0.001% to 0.03%,
Al: 0.005% to 1.0%, and
a balance of Fe and unavoidable impurities, having
a hot dip galvanized layer A on the surface of said steel
sheet, characterized by having the following B layer
right below said steel sheet surface and inside said
steel sheet:
B layer: Layer which has thickness of 0.001 fim to
0.5 (jm, which contains, based on mass of said B layer,
one or more of Fe, Si, Mn, P, S, and Al oxides in a total
of less than 50 mass%, which contains C, Si, Mn, P, S,
and Al not in oxides in:
C: less than 0.05 mass%,
Si: less than 0.1 mass%,
Mn: less than 0.5 mass%,
P: less than 0.001 mass%,
S: less than 0.001 mass%, and
Al: less than 0.005 mass%, and
which contains Fe not in oxides in 50 mass% or more.
Claim 2. A hot dip galvanized steel sheet comprising
a steel sheet which contains, by mass%,
C: 0.05% to 0.50%,
Si: 0.1% to 3.0%,
Mn: 0.5% to 5.0%,
P: 0.001% to 0.5%,
S: 0.001% to 0.03%,
Al: 0.005% to 1.0%,
one or more elements of Ti, Nb, Cr, Mo, Ni, Cu, Zr,
V, W, B, Ca, and REM in respectively 0.0001% to 1%, and
a balance of Fe and unavoidable impurities, having a
hot dip galvanized layer A on the surface of said steel
sheet, characterized by having the following B layer
right below said steel sheet surface and inside said
steel sheet:
B layer: Layer which has thickness of 0.001 urn to
0.5 \na, which contains, based on mass of said B layer,
one or more of Fe, Si, Mn, P, S, Al, Ti, Nb, Cr, Mo, Ni,
Cu, Zr, V, W, B, Ca, and REM oxides in a total of less
than 50 mass%, which contains C, Si, Mn, P, S, Al, Ti,
Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM not in
oxides in:
C: less than 0.05
Si: less than 0.1 mass%,
Mn: less than 0.5 mass%,
P: less than 0.001 ma s s %,
S: less than 0.001 mass%,
Al: less than 0.005 mass%,
one or more of Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B,
Ca, and REM in respectively less than 0.0001 mass%, and
which contains Fe not in oxides in 50 mass% or more.
Claim 3. The hot dip galvanized steel sheet
according to claim 1 or 2, wherein said hot dip
galvanized layer A has a thickness of 2 \xm to 100 urn.
Claim 4. A method of production of a hot dip
galvanized steel sheet comprising casting, hot rolling,
pickling, and cold rolling a steel containing the
components described in claim 1 or 2 to obtain a cold
rolled steel sheet, and annealing said cold rolled steel
sheet and hot dip galvanizing the annealed steel sheet in
a continuous hot dip galvanization facilities which are
provided with a heating furnace and a soaking furnace,
wherein, in said heating furnace and said soaking
furnace which perform said annealing treatment, the
temperature of said cold rolled steel sheet in the
furnaces being 500°C to 950°C in temperature range and
running said cold rolled steel sheet under the following