U.S. patent application number 11/885805 was filed with the patent office on 2008-07-24 for alloyed hot-dip galvanized steel sheet and method of producing the same.
This patent application is currently assigned to JFR Steel Corporation, a corporation of Japan. Invention is credited to Yutaka Awajiya, Takayuki Futatsuka, Hiroshi Matsuda, Yasunobu Nagataki, Tatsuya Nakagaito.
Application Number | 20080175743 11/885805 |
Document ID | / |
Family ID | 39641405 |
Filed Date | 2008-07-24 |
United States Patent
Application |
20080175743 |
Kind Code |
A1 |
Futatsuka; Takayuki ; et
al. |
July 24, 2008 |
Alloyed Hot-Dip Galvanized Steel Sheet and Method of Producing the
Same
Abstract
An alloyed hot-dip galvanized steel sheet has a composition
containing about 0.05 to about 0.25 wt % of C, about 0.5 wt % or
less of Si, about 1 to about 3 wt % of Mn, about 0.1 wt % or less
of P, about 0.01 wt % or less of S, about 0.1 to about 2 wt % of
Al, and less than about 0.005 wt % of N, and satisfying the
relations, Si+Al.gtoreq.0.6 wt %, (0.0006.times.Al) wt
%.ltoreq.N.ltoreq.(0.0058-0.0026.times.Al) wt %, and
Al.ltoreq.(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn) wt %,
the balance including Fe and inevitable impurities. The alloyed
hot-dip galvanized steel sheet can be obtained without passing
through a complicated process, has excellent surface appearance and
anti-secondary work embrittlement and high strength, and is thus
suitable for application to an automobile steel sheet.
Inventors: |
Futatsuka; Takayuki;
(Hiroshima, JP) ; Nakagaito; Tatsuya; (Hiroshima,
JP) ; Matsuda; Hiroshi; (Chiba, JP) ;
Nagataki; Yasunobu; (Chiba, JP) ; Awajiya;
Yutaka; (Hiroshima, JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER US LLP
ONE LIBERTY PLACE, 1650 MARKET ST, SUITE 4900
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFR Steel Corporation, a
corporation of Japan
Chiyoda-ku, Tokyo
JP
|
Family ID: |
39641405 |
Appl. No.: |
11/885805 |
Filed: |
March 31, 2006 |
PCT Filed: |
March 31, 2006 |
PCT NO: |
PCT/JP06/07396 |
371 Date: |
September 6, 2007 |
Current U.S.
Class: |
420/120 ;
420/128 |
Current CPC
Class: |
C21D 2211/001 20130101;
C23C 28/023 20130101; C21D 9/48 20130101; C22C 38/06 20130101; C21D
8/0405 20130101; C21D 2211/002 20130101; C23C 2/28 20130101; C23C
2/02 20130101; C21D 2211/009 20130101; C21D 2211/008 20130101; C23C
28/02 20130101; C22C 38/04 20130101; C21D 6/005 20130101; C21D
2211/005 20130101 |
Class at
Publication: |
420/120 ;
420/128 |
International
Class: |
C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2005 |
JP |
2005-103833 |
Mar 3, 2006 |
JP |
2006-058460 |
Claims
1. An alloyed hot-dip galvanized steel sheet comprising a
composition containing about 0.05 to about 0.25 wt % of C, about
0.5 wt % or less of Si, about 1 to about 3 wt % of Mn, about 0.1 wt
% or less of P, about 0.01 wt % or less of S, about 0.1 to about 2
wt % of Al, and less than about 0.005 wt % of N, and satisfying the
relations, Si+Al.gtoreq.0.6 wt %, (0.0006.times.Al) wt
%.ltoreq.N.ltoreq.(0.0058-0.0026.times.Al) wt %, and
Al.ltoreq.(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn) wt %,
the balance including Fe and inevitable impurities.
2. The alloyed hot-dip galvanized steel sheet according to claim 1,
wherein the composition further contains at least one element
selected from the group consisting of about 1 wt % or less of Cr,
about 1 wt % or less of V, and about 1 wt % or less of Mo.
3. The alloyed hot-dip galvanized steel sheet according to claim 1,
wherein the composition further contains at least one element
selected from the group consisting of about 0.1 wt % or less of Ti,
about 0.1 wt % or less of Nb, about 0.005 wt % or less of B, and
about 1 wt % or less of Ni.
4. The alloyed hot-dip galvanized steel sheet according to claim 2,
wherein the composition further contains at least one element
selected from the group consisting of about 0.1 wt % or less of Ti,
about 0.1 wt % or less of Nb, about 0.005 wt % or less of B, and
about 1 wt % or less of Ni.
5. The alloyed hot-dip galvanized steel sheet according to claim 1,
wherein the composition further contains at least one element
selected from the group consisting of Ca and REM in a total of
about 0.01 wt % or less.
6. The alloyed hot-dip galvanized steel sheet according to claim 2,
wherein the composition further contains at least one element
selected from the group consisting of Ca and REM in a total of
about 0.01 wt % or less.
7. The alloyed hot-dip galvanized steel sheet according to claim 3,
wherein the composition further contains at least one element
selected from the group consisting of Ca and REM in a total of
about 0.01 wt % or less.
8. The alloyed hot-dip galvanized steel sheet according to claim 4,
wherein the composition further contains at least one element
selected from the group consisting of Ca and REM in a total of
about 0.01 wt % or less.
9. The alloyed hot-dip galvanized steel sheet according to claim 1,
wherein the steel sheet has a metal structure containing a residual
austenite phase at a volume ratio of about 3 to about 20%.
10. A method of producing an alloyed hot-dip galvanized steel
sheet, the method comprising casting, hot-rolling, and cold-rolling
steel for a steel sheet having the composition according to claim
1, holding the steel at about 730.degree. C. to about 900.degree.
C. for about 60 to about 300 seconds, cooling the steel at about 3
to about 100.degree. C./s, further holding the steel at about
350.degree. C. to about 600.degree. C. for about 30 to about 250
seconds, hot-dip galvanizing the steel, and then alloying the steel
at about 470.degree. C. to about 600.degree. C.
Description
RELATED APPLICATION
[0001] This is a .sctn.371 of International Application No.
PCT/JP2006/307396, with an international filing date of Mar. 31,
2006 (WO 2006/104275 A1, published Oct. 5, 2006), which is based on
Japanese Patent Application Nos. 2005-103833, filed Mar. 31, 2005,
and 2006-058460, filed Mar. 3, 2006.
TECHNICAL FIELD
[0002] This disclosure relates to an alloyed hot-dip galvanized
steel sheet having high strength, excellent surface appearance, and
excellent anti-secondary work embrittlement and a method of
producing the same. The alloyed hot-dip galvanized steel sheet is
suitably applied to automobile steel sheets.
BACKGROUND
[0003] In recent years, from the viewpoint of conservation of the
global environment, there have been advanced thinning of steel
sheets by weight reduction of car bodies for improving mileages and
increases in high strength steel sheets for improving safety.
However, increases in strength of steel sheets decrease ductility
and toughness, and thus there have been desired steel sheets having
both high strength and high formability and excellent toughness
after forming (anti-secondary work embrittlement).
[0004] For such a requirement, there have been developed various
steel sheets such as ferrite-martensite dual phase steel (so-called
Dual-Phase steel) and steel using transformation-induced plasticity
of residual austenite (so-called TRIP steel).
[0005] In some cases, the surfaces of these steel sheets are
galvanized for improving rust prevention in practical use. As such
galvanized steel sheets, alloyed hot-dip galvanized steel sheets
subjected to heat treatment for diffusing Fe of the steel sheets
into plating layers after hot-dip galvanization are widely used
from the viewpoint of securing press property, spot weldability,
coating adhesion.
[0006] For example, in Japanese Unexamined Patent Application
Publication No. 11-279691, there has been proposed an alloyed
hot-dip galvanized steel sheet in which residual austenite (also
referred to as "residual .gamma.") is secured by adding a large
amount of Si to the steel sheet, thereby achieving high ductility.
However, Si decreases the adhesion of zinc plating, and thus a
complicated process of Ni pre-plating, applying a special chemical,
reducing an oxide layer on the surface of a steel sheet, and/or
appropriately controlling the thickness of an oxide layer is
required for adhesion of zinc plating to such high-Si steel.
[0007] In Japanese Unexamined Patent Application Publication No.
2002-030403, there has been proposed an alloyed hot-dip galvanized
steel sheet in which instead of Si, Al with a small adverse effect
on zinc plating adhesion is added to the steel sheet, thereby
securing excellent ductility and improving wettability and
anti-powdering qualities of zinc plating. However, in high-Al
steel, N and Al in the steel forms AlN which precipitates in large
amounts at austenite grain boundaries during continuous casting,
thereby embrittling the grain boundaries. Since bending correction
from the vertical direction to the horizontal direction is
performed in usual continuous casting, the embrittlement of the
grain boundaries easily causes cracking in a slab at a corrected
portion. When a slab having a crack is rolled, the crack remains in
a final product to significantly deteriorate the surface
appearance. In this case, a process of removing the crack of the
slab with a grinder is required, thereby significantly increasing
cost.
[0008] In U.S. Pat. No. 3,596,316, there has been proposed a method
of avoiding slab cracking in which Ti is added to a steel sheet to
fix N as TiN in order to avoid the slab cracking. However,
precipitation of AlN actually starts at a temperature higher than
the temperature where N is completely fixed as TiN, and thus it is
difficult to completely avoid the slab cracking. Further, Al is a
strong ferrite stabilizing element and increases the A.sub.3
transformation point, and thus ferrite is easily produced, due to
an increase in the transformation point, at a slab corner where a
temperature drop easily occurs until the slab width is reduced
after the slab is discharged from a heating furnace during hot
rolling. As a result, local distortion is concentrated at a corner
during width reduction, thereby easily causing surface defects such
as scabs.
[0009] Any one of the proposed techniques has the problem of high
surface sliding resistance and low anti-secondary work
embrittlement in comparison to cold-rolled steel sheets because the
high-tension steel sheets are hot-dip galvanized in a subsequent
process.
[0010] On the other hand, in Japanese Unexamined Patent Application
Publication No. 2004-211140, there has been proposed a hot-dip
galvanized steel sheet in which in order to improve the
anti-secondary work embrittlement of a galvanized steel sheet, the
amount of B added to a Dual-Phase steel sheet used as a steel sheet
is controlled to decrease the grain size of ferrite grains,
improving grain boundary strength. However, a steel sheet utilizing
a martensite phase cannot utilize an improvement in ductility (TRIP
effect) due to strain-induced transformation of residual austenite
and thus does not have sufficient ductility.
[0011] It could therefore be advantageous to provide an alloyed
hot-dip galvanized steel sheet having high strength and excellent
surface appearance and excellent anti-secondary work embrittlement
and a method of producing the same.
SUMMARY
[0012] We provide an alloyed hot-dip galvanized steel sheet having
a composition containing about 0.05 to about 0.25 wt % of C, about
0.5 wt % or less of Si, about 1 to about 3 wt % of Mn, about 0.1 wt
% or less of P, about 0.01 wt % or less of S, about 0.1 to about 2
wt % of Al, and less than about 0.005 wt % of N, and satisfying the
relations, Si+Al.gtoreq.0.6 wt %, (0.0006.times.Al) wt
%.ltoreq.N.ltoreq.(0.0058-0.0026.times.Al) wt %, and
Al.ltoreq.(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn) wt %,
the balance including Fe and inevitable impurities.
[0013] The alloyed hot-dip galvanized steel sheet has the
composition which may further contain at least one element selected
from the group consisting of about 1 wt % or less of Cr, about 1 wt
% or less of V, and about 1 wt % or less of Mo.
[0014] Each of the alloyed hot-dip galvanized steel sheet described
above has the composition which may further contain at least one
element selected from the group consisting of about 0.1 wt % or
less of Ti, about 0.1 wt % or less of Nb, about 0.005 wt % or less
of B, and about 1 wt % or less of Ni.
[0015] Each of the alloyed hot-dip galvanized steel sheet described
above has the composition which may further contain at least one
element selected from the group consisting of Ca and REM in a total
of about 0.01 wt % or less.
[0016] Each of the alloyed hot-dip galvanized steel sheet described
above has a metal structure preferably containing a residual
austenite phase at a volume ratio of about 3 to about 20%.
[0017] Further, we provide a method of producing an alloyed hot-dip
galvanized steel sheet, the method including casting, hot-rolling,
and cold-rolling steel for a steel sheet having any one of the
above-described compositions, holding the steel at about
730.degree. C. to about 900.degree. C. for about 60 to about 300
seconds, cooling the steel at about 3 to about 100.degree. C./s,
further holding the steel at about 350.degree. C. to about
600.degree. C. for about 30 to about 250 seconds, hot-dip
galvanizing the steel, and then alloying the steel at about
470.degree. C. to about 600.degree. C.
DETAILED DESCRIPTION
[0018] We found that precipitation and coarsening of AlN influence
not only the surface quality of a final product due to slab
cracking, but also the anti-secondary work embrittlement of a final
product and the anti-secondary work embrittlement is improved when
an N amount is N.ltoreq.(0.0058-0.0026.times.Al) wt %. Although
details of reasons are not entirely understood, a conceivable
reason is that breakage starts from coarse AlN due to embrittlement
at a low temperature and, when the Al amount is increased,
precipitation of AlN starts from a high temperature to easily
coarsen the precipitate, thereby deteriorating the anti-secondary
work embrittlement.
[0019] On the other hand, fine AlN precipitating at a high
temperature suppresses coarsening of austenite grains, and thus
ferrite grains are made fine, thereby improving the anti-secondary
work embrittlement. Therefore, we found that there is a required
minimum N amount, and a suitable N amount satisfies
(0.0006.times.Al) wt %.ltoreq.N for improving the anti-secondary
work embrittlement.
[0020] We also found that the occurrence of scabs in hot rolling is
suppressed by controlling the Al amount to
Al.ltoreq.(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn) wt %.
This is adapted for balancing an increase in the Ar.sub.3
transformation point due to addition of Al and Si and a decrease in
the transformation point due to addition of C and Mn. The reason
for this is that when the components are controlled in the above
range, ferrite formation at a corner of a slab before width
reduction is suppressed.
[0021] Therefore, slab cracking in continuous casting, scabs in hot
rolling, and non-plating after hot-dip galvanization are suppressed
to improve the surface appearance of products. In addition, the
anti-secondary work embrittlement is improved. Further, an alloyed
hot-dip galvanized steel sheet with high strength can be obtained
without passing through a complicated process.
[0022] Our steel sheets will be described in detail below.
[0023] First, the reasons for specifying the composition of the
alloyed hot-dip galvanized steel sheet will be described.
Hereinafter, "%" represents "% by mass."
C: about 0.05 to about 0.25%
[0024] C is an element for stabilizing austenite and a necessary
element for securing a martensite amount and causing austenite to
remain at room temperature. When the C amount is less than about
0.05%, it is difficult to simultaneously secure the strength of the
steel sheet and the amount of residual austenite to achieve high
ductility. On the other hand, when the C amount exceeds about
0.25%, welds and heat-affected zone are significantly hardened,
thereby deteriorating weldability. Therefore, the C amount is in
the range of about 0.05 to about 0.25%.
Si: about 0.5% or Less
[0025] Si is an element effective in strengthening steel. Si is
also a ferrite forming element which promotes the concentration of
C in austenite and suppresses the formation of carbides and thus
has the function of promoting the formation of residual austenite.
However, when the Si amount exceeds about 0.5%, galvanizing is
deteriorated, and plating is difficult in a usual hot-dip
galvanization process. Therefore, the Si amount is about 0.5% or
less. Usually, the Si amount is preferably about 0.01 to about 0.5%
and more preferably about 0.01 to about 0.4%.
Mn: about 1 to about 3%
[0026] Mn is an element effective in strengthening steel. Mn is
also an element for stabilizing austenite and an element necessary
for increasing residual austenite. However, when the Mn amount is
less than about 1%, these effects cannot be easily obtained. On the
other hand, when the Mn amount exceeds about 3%, a second phase
fraction is excessively increased, and the amount of solid-solution
hardening is increased, thereby significantly increasing strength
and greatly decreasing ductility. Therefore, such a Mn amount is
unsuitable for application to automobile steel sheets. Thus, the Mn
amount is in the range of about 1 to about 3%.
P: about 0.1% or Less
[0027] P is an element effective in strengthening steel. However,
when the P amount exceeds about 0.1%, plating defects or
non-plating occurs, and P segregates at grain boundaries to
deteriorate the anti-secondary work embrittlement. Therefore, the P
amount is about 0.1% or less. From the viewpoint of anti-secondary
work embrittlement, the P amount is preferably about 0.05% or less.
Usually, the P amount is preferably about 0.001 to about 0.05%.
S: about 0.01% or Less
[0028] S forms inclusions such as MnS and causes deterioration in
impact resistance and cracking along metal flow of welds.
Therefore, the S amount is preferably as small as possible.
However, from the viewpoint of production cost, the S amount is
about 0.01% or less. Usually, the S amount is preferably about
0.0001 to about 0.01%.
Al: about 0.1 to about 2%
Si+Al.ltoreq.0.6%
[0029] Al and Si are ferrite forming elements which promote the
concentration of C in austenite and suppress the formation of a
carbide and thus have the function of promoting the formation of
residual austenite. When a total of Al and Si added is less than
about 0.6%, sufficient ferrite or residual .gamma. cannot be
obtained, thereby significantly decreasing ductility. Even when Al
is added in an amount of less than about 0.1% and Si is added up to
an upper limit, the total of Al+Si is less than about 0.6%. On the
other hand, when the Al amount exceeds about 2%, the amount of the
inclusion in the steel sheet is increased, deteriorating ductility.
Therefore, the Al amount is in the range of about 0.1 to about 2%,
and Si+Al.gtoreq.0.6% is satisfied.
Al.ltoreq.(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn) %
[0030] When the Al content exceeds
(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn) %, scabs easily
occur in hot rolling. Therefore, the Al amount satisfies
Al.ltoreq.(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn)
%.
N: Less than about 0.005%
(0.0006.times.Al) %.ltoreq.N.ltoreq.(0.0058-0.0026.times.Al) %
[0031] N is an important element and causes deterioration in the
anti-secondary work embrittlement when the amount of AlN
precipitate is increased with an increase in the N amount. To avoid
such deterioration in the anti-secondary work embrittlement, the N
amount is limited to less than about 0.005%, and the relational
expression,
(0.0006.times.Al).ltoreq.N.ltoreq.(0.0058-0.0026.times.Al) %, is
satisfied. When the N amount is about 0.005% or more, AlN
excessively precipitates, causing cracking in a slab. Therefore,
the N amount is less than about 0.005%. Usually, the N amount is
preferably about 0.001 to less than about 0.005%. Cr, V, Mo: each
about 1% or less
[0032] Cr, V, and Mo have the function of suppressing the formation
of pearlite in cooling from an annealing temperature and thus can
be added according to demand. When the amount of each of these
elements is about 1% or less, appropriate steel sheet strength is
obtained, and ductility and adhesion of zinc plating are little
adversely affected. Therefore, when Cr, V, and Mo are added, the
amount of each of the elements is preferably about 1% or less. In
general, the amount is preferably about 0.01 to 1% and more
preferably about 0.01 to about 0.5%.
Ti, Nb: Each about 0.1% or Less
[0033] Ti and Nb are effective in precipitation strengthening of
steel and can thus be added according to demand. However, when the
amount of each of Ti and Nb is about 0.1% or less, formability and
shape fixability can be easily maintained. Therefore, when Ti and
Nb are added, the amount of each of the elements is preferably
about 0.1% or less. In general, the amount is preferably about 0.01
to about 0.1% and more preferably about 0.01 to about 0.05%.
B: about 0.005% or Less
[0034] B is effective in strengthening steel and can thus be added
according to demand. When the B amount is about 0.005% or less, an
increase in strength can be easily controlled, thereby achieving
excellent formability. Therefore, when B is added, the amount is
preferably about 0.005% or less. In general, the amount is
preferably about 0.0001 to about 0.005% and more preferably about
0.0001 to about 0.003%.
Ni: about 1% or Less
[0035] Ni is an austenite stabilizing element which causes
austenite to remain and is effective in increasing strength, and
thus can be added according to demand. When the Ni amount is about
1% or less, ductility of a steel sheet can be easily increased.
Therefore, when Ni is added, the amount is preferably about 1% or
less. In general, the amount is preferably about 0.01 to about
1%.
Ca and REM: at Least One in Total of about 0.01% or Less
[0036] "REM" represents at least one of the rare earth elements. Ca
and REM have the function of controlling the form of sulfide
inclusions and thus have the effect of improving elongation and
stretch flange formability of a steel sheet, and thus can be added
according to demand. When the total of these elements exceeds about
0.01%, these effects are saturated. Therefore, when Ca and REM are
added, the total of at least one of the elements is preferably
about 0.01% or less. In general, the total is preferably about
0.001 to about 0.01% and more preferably about 0.001 to about
0.005%.
[0037] Besides the above-described elements and Fe in the balance,
various impurities in the production process and trace amounts of
essential elements in the production process are inevitably mixed.
However, these inevitable impurities are permissible as long as
they have no particular influence on the disclosed advantage.
[0038] From the viewpoint of anti-secondary work embrittlement, it
is preferable to satisfy the relational expression,
(-10.times.C+5.times.Si-Mn+6.times.Al) %.gtoreq.-0.5. Although
details of reasons for this are not completely understood, a
conceivable reason is that to suppress coarsening of austenite
grains at a high temperature due to a decrease in the Ac.sub.3
point, a decrease in Ac.sub.3 point due to the addition of C and Mn
and an increase in Ac.sub.3 point due to the addition of Si and Al
are balanced by satisfying the above range, thereby improving the
anti-secondary work embrittlement.
[0039] Next, the metal structure of the steel sheet will be
described.
Residual Austenite Phase: Volume Ratio of about 3 to about 20%
[0040] The strain-induced transformation of a residual austenite
phase is effectively utilized so that excellent surface appearance
and anti-secondary work embrittlement and not only high strength
but also high ductility can be imparted to the alloyed hot-dip
galvanized steel sheet as a final product. Therefore, it is very
important to control the volume ratio of the residual austenite.
From the viewpoint of securing high ductility, the ratio of the
residual austenite phase is preferably about 3% or more. On the
other hand, when the ratio of the residual austenite phase is about
20% or less, the formation of martensite after forming is
suppressed, and thus such a ratio is suitable in view of
brittleness. Therefore, the ratio of the residual austenite phase
is preferably about 20% or less. The metal structure of a steel
sheet used as the alloyed hot-dip galvanized steel sheet includes a
ferrite main phase and a residual austenite phase as a second
phase. However, the volume ratio of the ferrite phase is preferably
about 40 to about 90% from the viewpoint of securing high
ductility. Examples of a metal structure other than the residual
austenite phase in the second phase include a bainite phase, a
martensite phase and/or a pearlite phase. The total volume ratio of
these phases is preferably about 7 to about 50%. The average grain
size of the ferrite main phase is preferably about 15 .mu.m or less
because the anti-secondary work embrittlement can be easily
improved.
[0041] Next, a preferred method of producing the alloyed hot-dip
galvanized steel sheet will be described.
[0042] Steel satisfying the conditions of the above composition is
continuously cast to form a cast slab, and then the cast slab is
hot-rolled and cold-rolled to prepare a steel sheet. However, the
conditions for these processes are not particularly limited. Then,
in a continuous galvanizing line, the steel sheet is annealed by
holding in a temperature range of about 730.degree. C. to about
900.degree. C. for about 60 to about 300 seconds, cooled at about 3
to about 100.degree. C./s, held in a temperature range of about
350.degree. C. to about 600.degree. C. for about 30 to about 250
seconds, hot-dip galvanized, and then alloyed at about 470.degree.
C. to about 600.degree. C.
[0043] In the production method, excellent surface appearance and
anti-secondary work embrittlement and not only high strength but
also high ductility can be imparted to the alloyed hot-dip
galvanized steel sheet as the final product.
[0044] Each of the production conditions will be described in
further detail below.
Annealing Temperature: about 730 to about 900.degree. C. Holding
Time: about 60 to about 300 Seconds
[0045] Annealing is performed in the austenite region or
intercritical region including an austenite phase and a ferrite
phase. When the annealing temperature is about 730.degree. C. or
more and the holding time is about 60 seconds or more, it is easy
to dissolve a carbide in the steel sheet, completely recrystallize
ferrite, and impart ductility. On the other hand, when the
annealing temperature is about 900.degree. C. or less, coarsening
of austenite grains is suppressed, and the number of ferrite
nucleation sites formed from the second phase by subsequent cooling
is easily increased. Further, when the holding time is about 300
seconds or less, coarsening of AlN is suppressed, and the
anti-secondary work embrittlement is easily improved. Therefore,
the annealing temperature is about 730 to about 900.degree. C., and
the holding time is about 60 to about 300 seconds.
Cooling Rate: about 3 to about 100.degree. C./s
[0046] When the cooling rate is about 3.degree. C./s or more,
precipitation of pearlite is suppressed, dissolved Carbon content
in untransformed austenite tends to increase, and thus the intended
metal structure (residual austenite phase) can be easily obtained.
When the cooling rate is about 100.degree. C./s or less, growth of
ferrite is promoted to increase the volume ratio of ferrite, and
thus sufficient ductility can be easily secured. Therefore, the
cooling rate is preferably about 3 to about 100.degree. C./s.
Although the scope of our disclosure includes a case in which the
cooling rate changes during cooling, the average cooling rate is
preferably about 10.degree. C./s or more and more preferably over
about 20.degree. C./s from the viewpoint of productivity.
Holding Temperature Range: about 350.degree. C. to about
600.degree. C.
[0047] When the holding temperature is about 600.degree. C. or
less, precipitation of a carbide in untransformed austenite is
suppressed. When the holding temperature is about 350.degree. C. or
more, precipitation of a carbide in bainitic ferrite due to lower
bainite transformation is suppressed to easily form stable residual
austenite. Therefore, the holding temperature is preferably about
350.degree. C. to about 600.degree. C. In order to stably produce
residual austenite, the holding temperature is preferably about
500.degree. C. or less.
Holding Time: about 30 to about 250 Seconds
[0048] The holding time pays a very important role for controlling
residual austenite. Namely, when the holding time is about 30
seconds or more, stabilization of untransformed austenite proceeds,
and thus the amount of residual austenite is easily secured, easily
obtaining desired properties. On the other hand, when the holding
time is about 250 seconds or less, line speed need not be extremely
decreased even in a CGL line where austempering cannot be performed
for a long time, thereby causing an advantage of productivity.
Therefore, the holding time is about 30 to about 250 seconds. The
holding time is preferably about 70 seconds or more in order to
stably secure residual austenite and preferably about 200 seconds
or less from the viewpoint of productivity.
Alloying Temperature: about 470.degree. C. to about 600.degree.
C.
[0049] The alloying temperature after hot-dip galvanization must be
higher than the plating bath temperature, and the lower limit is
about 470.degree. C. When the alloying temperature is about
600.degree. C. or less, like in the case where the holding
temperature is about 600.degree. C. or less, precipitation of a
carbide in untransformed austenite is suppressed, and thus stable
residual austenite can be easily obtained. Therefore, the alloying
temperature is about 470.degree. C. to about 600.degree. C.
[0050] In the production method, the specified annealing
temperature, holding temperature, and alloying temperature need not
be constant as long as they are in the above-respective ranges. The
plating conditions may be in a usual operation range, i.e., coating
weight may be about 20 to about 70 g/m.sup.2, and the amount of Fe
in a plating layer may be about 6 to about 15%.
EXAMPLES
Example 1
Effect of Composition of Steel Sheet
[0051] Molten steel having each of the compositions shown in Table
1 was prepared by a converter and continuously cast to form a cast
slab. The resulting slab was heated to 1250.degree. C. and then
hot-rolled at a finish rolling temperature of 900.degree. C. to
prepare a hot-rolled steel sheet having a thickness of 3.0 mm. The
hot-rolled steel sheet produced as described above was visually
observed for the occurrence of scabs. The presence of scabs is
shown in Table 2.
[0052] After hot-rolling, the hot-rolled steel sheet was pickled
and further cold-rolled to prepare a cold-rolled steel sheet having
a thickness of 1.2 mm. Then, in a continuous galvanizing line, each
cold-rolled steel sheet was annealed at 820.degree. C., cooled at a
rate of 10.degree. C./s, galvanized with coating weight of 50/50
g/m.sup.2 by a zinc plating bath at 460.degree. C., and then
alloyed at 520.degree. C. to prepare an alloyed hot-dip galvanized
steel sheet.
[0053] Since the surface appearance of an alloyed hot-dip
galvanized steel sheet are significantly inhibited by scabs,
cracking in a slab, and plating defects, the surface appearance of
the alloyed hot-dip galvanized steel sheets are also shown in Table
2. Good surface appearance represent a state in which a final
product has a uniform surface with beauty without defects, and poor
surface appearance represent a state in which surface defects such
as scabs or non-plating occur.
[0054] Further, each of the resulting alloyed hot-dip galvanized
steel sheets was temper-rolled of 0.5%, and mechanical properties
were examined. As the mechanical properties, tensile strength (TS)
and elongation (El) were measured using a JIS No. 5 tensile
specimen obtained from each steel sheet in a direction
perpendicular to the rolling direction. The measured values and
values of TS.times.El are also shown in Table 2.
[0055] The anti-secondary work embrittlement was evaluated by the
following method: [0056] A cylindrical sheet having a diameter of
95 mm was obtained from each of the resulting alloyed hot-dip
galvanized steel sheets and then formed into a cylindrical cup
having a diameter of 50 mm by deep drawing at a drawing ratio of
1.9. The edge of each of the cylindrical cups was trimmed to
prepare a sample having a height of 30 mm. Then, the sample was
placed on a truncated cone-shaped mold with a tip of 60.degree. so
that the bottom faced upward, and the whole of a tester was cooled
to a predetermined temperature. After holding for a predetermined
time, a load was applied to the sample from above, and a critical
temperature (longitudinal crack transition temperature) where a
brittle crack occurred in the side wall of the cylindrical cup was
determined. The critical temperature was used as an index for
anti-secondary work embrittlement, for evaluating the steel sheets
obtained from steel Nos. 1-A to 1-Z. The results are also shown in
Table 2. [0057] The alloyed hot-dip galvanized steel sheets
obtained from steel Nos. 1-A to 1-N satisfying our compositions
were excellent in surface appearance and anti-secondary work
embrittlement. However, the alloyed hot-dip galvanized steel sheets
obtained from steel Nos. 1-O to 1-V, 1-Y, and 1-Z were inferior in
one or both of the surface appearance and the anti-secondary work
embrittlement. Steel No. 1-W having a low C content had
insufficient strength, and the alloyed hot-dip galvanized steel
sheet obtained from steel No. 1-X having a small total of Al+Si
showed insufficient elongation (El).
[0058] The above-mentioned results indicate that an alloyed hot-dip
galvanized steel sheet obtained from steel satisfying our
conditions has excellent surface appearance, high strength, and
excellent anti-secondary work embrittlement.
Example 2
Effect of Composition of Steel Sheet and Residual Austenite
[0059] Molten steel having each of the compositions shown in Table
3 was prepared by a converter and a hot-rolled steel sheet having a
thickness of 3.0 mm was prepared by the same method as in Example
1. The hot-rolled steel sheet produced as described above was
visually observed for the occurrence of scabs. The presence of
scabs is shown in Tables 4-1 and 4-2.
[0060] After hot-rolling, the hot-rolled steel sheet was pickled
and further cold-rolled to prepare a cold-rolled steel sheet having
a thickness of 1.2 mm. Then, in a continuous galvanizing line, each
cold-rolled steel sheet was heat-treated under the conditions shown
in Tables 4-1 and 4-2, plated at 50/50 g/m.sup.2, and then alloyed
to prepare an alloyed hot-dip galvanized steel sheet.
[0061] Like in Example 1, the surface appearances of the resulting
alloyed hot-dip galvanized steel sheets are also shown in Tables
4-1 and 4-2.
[0062] Further, each of the resulting alloyed hot-dip galvanized
steel sheets was measured with respect to tensile strength (TS) and
elongation (El) by the same method as in Example 1. The measured
values and values of TS.times.El are also shown in Tables 4-1 and
4-2.
[0063] With respect to the anti-secondary work embrittlement, a
critical temperature (longitudinal crack transition temperature)
where a brittle crack occurred in the side wall of a cylindrical
cup was determined by the same method as in Example 1. The critical
temperature was used as an index for the anti-secondary work
embrittlement, for evaluating steel sheets of Invention Examples
2-1 to 2-28 and Comparative Examples 2-29 to 2-38. The results are
also shown in Tables 4-1 and 4-2.
[0064] Tables 3, 4-1, and 4-2 indicate that in the alloyed hot-dip
galvanized steel sheets obtained from steel Nos. 2-S, 2-U to 2-W,
and 2-Y not satisfying any one of the N amount of less than 0.005%,
Al.ltoreq.(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn) %,
and the Si amount of 0.5% or less, the surface appearance were
deteriorated. In addition, in the alloyed hot-dip galvanized steel
sheets obtained from steel Nos. 2-Q to 2-U not satisfying
(0.0006.times.Al) %.ltoreq.N.ltoreq.(0.0058-0.0026.times.Al) %, the
anti-secondary work embrittlement was deteriorated.
[0065] Further, Tables 4-1 and 4-2 indicate that in the alloyed
hot-dip galvanized steel sheets of Invention Examples 2-2, 2-5,
2-8, 2-12, 2-14, 2-19, 2-20, 2-25, and 2-28 containing small
amounts of residual austenite, both the elongation and the values
of TS.times.El are low, but these steel sheets have excellent
surface appearance and anti-secondary work embrittlement and high
tensile strength (TS). On the other hand, in the alloyed hot-dip
galvanized steel sheets of Comparative Examples 2-29 to 2-38, any
one of the surface appearance, anti-secondary work embrittlement,
and tensile strength (TS) is deteriorated.
[0066] Since the alloyed hot-dip galvanized steel sheets of
Invention Examples 2-1, 2-3, 2-4, 2-6, 2-7, 2-9 to 2-11, 2-13, 2-15
to 2-18, 2-21 to 2-24, 2-26, and 2-27 were produced under the
preferred production conditions and satisfied the condition of the
amount of residual austenite, these steel sheets have not only
excellent surface appearance and anti-secondary work embrittlement
and high tensile strength but also high elongation (El) and high
values of TS.times.El.
Example 3
Effects of Composition of Steel Sheet and Residual Austenite, and
Cracking in Slab
[0067] Molten steel having each of the compositions shown in Table
5 was prepared by a converter and continuously cast to form a cast
slab. The occurrence of cracking in the slab is shown in Tables 6-1
and 6-2. The occurrence of cracking was determined by visual
observation as well as color check after the slab was cooled to
room temperature.
[0068] The resulting slab was heated to 1250.degree. C. and then
hot-rolled at a finish rolling temperature of 900.degree. C. to
prepare a hot-rolled steel sheet having a thickness of 3.0 mm. The
hot-rolled steel sheet produced as described above was visually
observed for the occurrence of scabs. The presence of scabs is
shown in Tables 6-1 and 6-2.
[0069] After hot-rolling, the hot-rolled steel sheet was pickled
and further cold-rolled to prepare a cold-rolled steel sheet having
a thickness of 1.2 mm. Then, in a continuous galvanizing line, each
cold-rolled steel sheet was heat-treated under the conditions shown
in Tables 6-1 and 6-2, plated at 50/50 g/m.sup.2, and then alloyed
so that the Fe amount in the plating layer was 9%.
[0070] Further, each of the resulting alloyed hot-dip galvanized
steel sheets was measured with respect to tensile strength (TS) and
elongation (El) by the same method as in Example 1. The measured
values and values of TS.times.El are also shown in Tables 6-1 and
6-2.
[0071] Table 6-1 indicates that the alloyed hot-dip galvanized
steel sheets of Invention Examples 3-5 and 3-8 to 3-19 satisfying
our steel sheet compositions cause no cracking in the slabs and no
scabs in the hot-rolled steel sheets and have excellent surface
appearance. Also, the anti-secondary work embrittlement and tensile
strength are excellent.
[0072] On the other hand, in the alloyed hot-dip galvanized steel
sheets Comparative Examples 3-1 to 3-4, 3-6, 3-7, and 3-20 to 3-30
not satisfying any one of the steel sheet components, i.e., the N
amount, (0.0006.times.Al)
%.ltoreq.N.ltoreq.(0.0058-0.0026.times.Al) %, and
Al.ltoreq.(1.25.times.C.sup.0.5-0.57.times.Si+0.625.times.Mn) %, a
problem occurred in at least one of deterioration in the surface
appearance due to cracking in the slabs, scabs in the hot-rolled
steel sheets, or non-plating, and the anti-secondary work
embrittlement. Comparative Example 3-30 also had low tensile
strength (TS).
[0073] In particular, as shown in Comparative Example 3-31 in Table
6-2, steel 3-W containing a large amount of Mn exhibits a
significant increase in strength but very low elongation. Further,
as shown in Comparative Example 3-32 in Table 6-2, steel 3-X having
a small total of Al+Si exhibits very low elongation for strength
and a low value of TS.times.El.
[0074] Since the alloyed hot-dip galvanized steel sheets of
Invention Examples 3-5, 3-8, 3-11, 3-12, and 3-15 to 3-18 satisfy
the preferred production conditions, these steel sheets have
appropriate amounts of residual austenite and have not only
excellent surface appearance and anti-secondary work embrittlement
and high strength but also high elongation (El) and high values of
TS.times.El.
TABLE-US-00001 TABLE 1 1.25C.sup.0.5 - Component of steel (% by
mass) 0.0058 - 0.57Si + 0.625 Ti, Nb, B, Al + Si 0.0006Al 0.0026Al
Mn Steel C Si Mn P S Al N Mo, V, Cr, Ni Ca, REM (mass %) (mass %)
(mass %) (mass %) 1-A 0.15 0.21 2.1 0.016 0.0018 0.9 0.0025 -- B:
0.0005 -- 1.1 0.0005 0.0035 1.68 1-B 0.09 0.01 2.4 0.029 0.0010 1.5
0.0012 Mo: 0.1, -- Ca: 0.001 1.5 0.0009 0.0019 1.87 V: 0.005 1-C
0.11 0.41 1.8 0.008 0.0025 1.0 0.0023 -- Nb: 0.02 -- 1.4 0.0006
0.0032 1.36 1-D 0.17 0.28 1.7 0.009 0.0003 0.7 0.0030 Cr: 0.3 -- --
1.0 0.0004 0.0040 1.42 1-E 0.16 0.45 1.6 0.031 0.0017 0.5 0.0036
Cr: 0.3 -- -- 1.0 0.0003 0.0045 1.24 1-F 0.12 0.16 1.5 0.012 0.0036
1.2 0.0018 Cr: 0.1, Ni: 0.1 -- 1.4 0.0007 0.0027 1.28 V: 0.01 1-G
0.11 0.30 1.8 0.022 0.0008 1.0 0.0028 Cr: 0.3 -- -- 1.3 0.0006
0.0032 1.37 1-H 0.23 0.37 1.9 0.008 0.0014 1.3 0.0021 -- Ti: 0.02
REM: 0.002 1.7 0.0008 0.0024 1.58 1-I 0.12 0.05 1.8 0.022 0.0019
0.8 0.0032 Cr: 0.3 -- -- 0.9 0.0005 0.0037 1.53 1-J 0.13 0.18 1.5
0.007 0.0024 1.1 0.0022 -- -- Ca: 0.002 1.3 0.0007 0.0029 1.29 1-K
0.12 0.27 1.6 0.011 0.0009 0.7 0.0018 Cr: 0.3 -- -- 1.0 0.0004
0.0040 1.28 1-L 0.10 0.36 1.8 0.015 0.0028 0.4 0.0035 -- Nb: 0.03
REM: 0.003 0.8 0.0002 0.0048 1.32 1-M 0.19 0.24 2.4 0.008 0.0011
0.8 0.0013 Cr: 0.2 Ni: 0.05 -- 1.0 0.0005 0.0037 1.91 1-N 0.12 0.40
2.3 0.016 0.0003 0.5 0.0027 V: 0.005 B: 0.002 -- 0.9 0.0003 0.0045
1.64 1-O 0.20 0.02 2.0 0.009 0.0041 1.2 0.0030 Mo: 0.45 -- Ca:
0.003 1.2 0.0007 0.0027 1.80 1-P 0.10 0.01 1.2 0.008 0.0028 0.8
0.0044 V: 0.1 Ti: 0.01 REM: 0.005 0.8 0.0005 0.0037 1.14 1-Q 0.15
0.01 2.0 0.023 0.0014 1.7 0.0009 Mo: 0.2 -- -- 1.7 0.0010 0.0014
1.73 1-R 0.12 0.12 2.0 0.013 0.0009 0.8 0.0045 -- Ti: 0.02, -- 0.9
0.0005 0.0037 1.61 B: 0.0005 1-S 0.08 0.35 2.1 0.009 0.0029 1.7
0.0012 Cr: 0.2 -- -- 2.1 0.0010 0.0014 1.47 1-T 0.17 0.01 2.2 0.015
0.0016 1.0 0.0054 -- Ti: 0.4 -- 1.0 0.0006 0.0032 1.88 1-U 0.11
0.21 1.4 0.011 0.0020 0.9 0.0056 -- Nb: 0.01, -- 1.1 0.0005 0.0035
1.17 B: 0.003 1-V 0.12 0.43 1.6 0.012 0.0033 1.3 0.0051 -- -- --
1.7 0.0008 0.0024 1.19 1-W 0.03 0.42 2.0 0.002 0.0008 0.8 0.0032 V:
0.05 -- Ca: 0.003 1.2 0.0005 0.0037 1.23 1-X 0.11 0.13 1.7 0.010
0.0032 0.3 0.0041 -- Ni: 0.2 -- 0.4 0.0002 0.0050 1.40 1-Y 0.15
0.80 1.9 0.008 0.0002 1.1 0.0031 Cr: 0.2, -- -- 1.9 0.0007 0.0029
1.22 Mo: 0.1 1-Z 0.13 0.24 0.5 0.012 0.0041 1.2 0.0011 -- -- -- 1.4
0.0007 0.0027 0.63 Underline: out of the range of the present
invention.
TABLE-US-00002 TABLE 2 Alloyed hot-dip galvanized steel sheet Scabs
of Mechanical properties Anti-secondary hot-rolled Surface TS EI TS
.times. EI (MPa work Steel steel sheet appearance*.sup.1 (MPa) (%)
%) embrittlement*.sup.2 Remarks 1-A No .largecircle. 794 26.3 20874
.circle-w/dot. Invention Example 1-B No .largecircle. 849 25.9
22022 .circle-w/dot. Invention Example 1-C No .largecircle. 740
29.9 22134 .circle-w/dot. Invention Example 1-D No .largecircle.
817 28.2 23039 .circle-w/dot. Invention Example 1-E No
.largecircle. 793 27.0 21411 .circle-w/dot. Invention Example 1-F
No .largecircle. 650 32.7 21224 .circle-w/dot. Invention Example
1-G No .largecircle. 652 32.8 21386 .circle-w/dot. Invention
Example 1-H No .largecircle. 921 24.9 22946 .circle-w/dot.
Invention Example 1-I No .largecircle. 621 34.1 21176
.circle-w/dot. Invention Example 1-J No .largecircle. 567 37.0
20979 .circle-w/dot. Invention Example 1-K No .largecircle. 670
31.9 21373 .circle-w/dot. Invention Example 1-L No .largecircle.
659 29.5 19434 .largecircle. Invention Example 1-M No .largecircle.
996 20.4 20334 .largecircle. Invention Example 1-N No .largecircle.
906 22.0 19903 .largecircle. Invention Example 1-O No .largecircle.
1002 20.8 20875 X Comparative Example 1-P No .largecircle. 390 50.3
19617 X Comparative Example 1-Q No .largecircle. 868 26.3 22819 X
Comparative Example 1-R No .largecircle. 727 27.2 19760 X
Comparative Example 1-S Present X 875 27.9 24444 .circle-w/dot.
Comparative Example 1-T No X 1450 13.8 20031 X Comparative Example
1-U No X 678 30.3 20588 X Comparative Example 1-V Prsent X 684 33.9
23233 X Comparative Example 1-W No .largecircle. 479 31.7 15177
.circle-w/dot. Comparative Example 1-X No .largecircle. 618 20.8
12854 .largecircle. Comparative Example 1-Y No X 1006 24.1 24205 X
Comparative Example 1-Z Present X 424 36.8 15603 .circle-w/dot.
Comparative Example Underline: out of the range of the present
invention. *.sup.1.largecircle. indicates a galvanized steel sheet
with good surface appearance, and X indicates a galvanized steel
sheet with poor surface appearance. *.sup.2.circle-w/dot. incidates
a longitudinal crack transition temperature of less than
-70.degree. C., .largecircle. incidates a longitudinal crack
transition temperature of -70 to -40.degree. C., and X incidates a
longitudinal crack transition temperature of over -40.degree.
C.
TABLE-US-00003 TABLE 3 1.25C.sup.0.5 - Component of steel (% by
mass) 0.0058 - 0.57Si + 0.625 Mo, V, Ti, Nb, B, Al + Si 0.0006Al
0.0026Al Mn Steel C Si Mn P S Al N Cr, Ni Ca, REM (mass %) (mass %)
(mass %) (mass %) 2-A 0.10 0.13 1.9 0.012 0.0010 0.8 0.0035 Mo: 0.1
-- -- 0.9 0.0005 0.0037 1.51 2-B 0.12 0.01 1.7 0.014 0.0003 0.8
0.0025 Cr: 0.3 -- -- 0.8 0.0005 0.0037 1.49 2-C 0.20 0.35 2.0 0.009
0.0017 1.5 0.0012 -- -- REM: 0.002 1.9 0.0009 0.0019 1.61 2-D 0.10
0.33 1.6 0.029 0.0009 1.0 0.0020 -- Nb: 0.01 Ca: 0.003 1.3 0.0006
0.0032 1.21 2-E 0.17 0.01 1.7 0.031 0.0008 1.0 0.0017 Cr: 0.3 -- --
1.0 0.0006 0.0032 1.57 2-F 0.12 0.24 1.7 0.019 0.0020 0.7 0.0031 --
Nb: 0.02 -- 0.9 0.0004 0.0040 1.36 2-G 0.16 0.15 1.8 0.023 0.0036
1.2 0.0018 Cr: 0.3 Ti: 0.01 REM: 0.001 1.4 0.0007 0.0027 1.54 2-H
0.08 0.01 2.3 0.008 0.0014 1.7 0.0012 V: 0.1 -- Ca: 0.002 1.7
0.0010 0.0014 1.79 2-I 0.12 0.05 1.7 0.028 0.0023 1.0 0.0016 Cr:
0.3 -- -- 1.1 0.0006 0.0032 1.47 2-J 0.17 0.32 1.7 0.009 0.0008 0.7
0.0032 Cr: 0.3 -- -- 1.0 0.0004 0.0040 1.40 2-K 0.12 0.26 1.0 0.016
0.0011 0.7 0.0036 -- Ti: 0.2, REM: 0.003 1.0 0.0004 0.0040 0.91 B:
0.002 2-L 0.11 0.28 1.6 0.005 0.0024 0.7 0.0030 Cr: 0.3 -- -- 1.0
0.0004 0.0040 1.25 2-M 0.16 0.17 2.3 0.012 0.0015 1.3 0.0023 Mo:
0.2 Ni: 0.1 -- 1.5 0.0008 0.0024 1.84 2-N 0.09 0.31 1.8 0.025
0.0031 0.4 0.0044 Cr: 0.1 Nb: 0.01 -- 0.7 0.0002 0.0048 1.32 2-O
0.15 0.07 2.1 0.008 0.0011 0.8 0.0032 -- -- -- 0.9 0.0005 0.0037
1.76 2-P 0.12 0.18 2.5 0.014 0.0008 0.7 0.0027 V: 0.1 Ti: 0.01 --
0.9 0.0004 0.0040 1.89 2-Q 0.17 0.01 2.3 0.026 0.0013 1.8 0.0008
Cr: 0.2 -- -- 1.8 0.0011 0.0011 1.95 Mo: 0.1 2-R 0.08 0.28 1.9
0.011 0.0002 0.7 0.0042 Cr: 0.3 B: 0.002 -- 1.0 0.0004 0.0040 1.38
2-S 0.10 0.03 1.7 0.023 0.0003 1.3 0.0057 -- -- -- 1.3 0.0008
0.0024 1.44 2-T 0.14 0.01 2.0 0.010 0.0016 1.2 0.0031 -- Ni: 0.05
Ca: 0.002 1.2 0.0007 0.0027 1.71 2-U 0.12 0.33 1.7 0.009 0.0026 1.5
0.0041 Mo: 0.1 B: 0.001 -- 1.8 0.0009 0.0019 1.31 2-V 0.10 0.42 1.6
0.018 0.0011 1.3 0.0015 -- Ti: 0.01, -- 1.7 0.0008 0.0024 1.16 Nb:
0.01 2-W 0.14 0.90 1.8 0.036 0.0034 0.2 0.0047 Cr: 0.2, -- -- 1.1
0.0001 0.0053 1.08 V: 0.005 2-X 0.02 0.13 2.2 0.014 0.0010 1.3
0.0021 -- Ti: 0.01 -- 1.4 0.0008 0.0024 1.48 2-Y 0.11 0.28 0.4
0.008 0.0160 1.0 0.0026 Mo: 0.3 -- Ca: 0.001 1.3 0.0006 0.0032 0.50
2-Z 0.15 0.15 1.5 0.023 0.0003 0.2 0.0045 Cr: 0.2 -- REM: 0.001 0.4
0.0001 0.0053 1.34 Underline: out of the range of the present
invention.
TABLE-US-00004 TABLE 4-1 Alloyed hot-dip galvanized steel sheet
Production condition Cool- Scabs of Annealing ing Holding Holding
Alloying hot-rolled Temp. Holding rate temp. time temp. Steel steel
sheet (.degree. C.) time (s) (.degree. C./s) (.degree. C.) (s)
(.degree. C.) Invention example 2-1 2-A No 840 100 38 510 130 540
Invention example 2-2 2-A No 830 80 30 470 20 550 Invention example
2-3 2-B No 820 140 9 440 90 540 Invention example 2-4 2-B No 830
140 11 440 50 540 Invention example 2-5 2-B No 800 110 8 440 110
620 Invention example 2-6 2-C No 790 220 19 500 100 540 Invention
example 2-7 2-C No 810 180 73 460 120 530 Invention example 2-8 2-C
No 800 200 25 650 130 540 Invention example 2-9 2-D No 830 170 17
440 160 540 Invention example 2-10 2-E No 800 110 18 420 90 540
Invention example 2-11 2-F No 820 100 12 490 90 560 Invention
example 2-12 2-F No 800 30 11 510 100 540 Invention example 2-13
2-G No 830 90 23 450 80 530 Invention example 2-14 2-G No 700 100
20 430 100 530 Invention example 2-15 2-H No 810 180 32 460 110 530
Invention example 2-16 2-H No 830 120 44 410 120 540 Invention
example 2-17 2-I No 840 90 26 450 110 540 Invention example 2-18
2-J No 820 150 11 420 90 550 Invention example 2-19 2-J No 950 170
17 460 120 540 Invention example 2-20 2-J No 800 120 2 400 100 530
Invention example 2-21 2-K No 800 80 17 450 80 520 Invention
example 2-22 2-L No 840 110 36 470 110 530 Invention example 2-23
2-M No 850 100 23 440 100 540 Invention example 2-24 2-N No 830 200
10 480 170 530 Invention example 2-25 2-N No 840 110 17 310 90 530
Invention example 2-26 2-O No 780 180 32 420 140 530 Invention
example 2-27 2-P No 820 120 9 500 200 530 Invention example 2-28
2-P No 850 100 180 520 160 540 Alloyed hot-dip galvanized steel
sheet Mechanical properties Residual .gamma. Surface TS EI TS
.times. EI Anti-secondary work (%) appearance*.sup.1 (MPa) (%) (MPa
%) embrittlement*.sup.2 Invention example 2-1 6.6 .largecircle. 668
29.7 19808 .circle-w/dot. Invention example 2-2 1.5 .largecircle.
691 27.3 18864 .circle-w/dot. Invention example 2-3 6.1
.largecircle. 630 33.9 21357 .circle-w/dot. Invention example 2-4
4.2 .largecircle. 657 31.1 20433 .circle-w/dot. Invention example
2-5 0.4 .largecircle. 621 28.7 17823 .largecircle. Invention
example 2-6 12.7 .largecircle. 898 26.3 23647 .circle-w/dot.
Invention example 2-7 12.5 .largecircle. 903 26.4 23839
.circle-w/dot. Invention example 2-8 1.6 .largecircle. 752 24.2
18198 .circle-w/dot. Invention example 2-9 4.9 .largecircle. 524
38.9 20384 .circle-w/dot. Invention example 2-10 8.1 .largecircle.
786 27.8 21851 .circle-w/dot. Invention example 2-11 7.8
.largecircle. 634 31.4 19935 .circle-w/dot. Invention example 2-12
1.3 .largecircle. 490 31.3 15337 .circle-w/dot. Invention example
2-13 10.2 .largecircle. 705 30.5 21497 .circle-w/dot. Invention
example 2-14 0.7 .largecircle. 553 30.3 16756 .circle-w/dot.
Invention example 2-15 11.5 .largecircle. 826 27.6 22819
.circle-w/dot. Invention example 2-16 11.3 .largecircle. 820 28.5
23370 .circle-w/dot. Invention example 2-17 7.7 .largecircle. 612
34.8 21198 .circle-w/dot. Invention example 2-18 7.6 .largecircle.
788 30.0 23640 .circle-w/dot. Invention example 2-19 2.3
.largecircle. 729 27.2 19829 .circle-w/dot. Invention example 2-20
2.2 .largecircle. 515 31.7 16326 .largecircle. Invention example
2-21 6.9 .largecircle. 832 24.1 20031 .circle-w/dot. Invention
example 2-22 8.5 .largecircle. 651 33.4 21743 .circle-w/dot.
Invention example 2-23 10.8 .largecircle. 1021 21.5 21990
.circle-w/dot. Invention example 2-24 7.4 .largecircle. 634 30.1
19075 .largecircle. Invention example 2-25 1.6 .largecircle. 506
31.8 16091 .circle-w/dot. Invention example 2-26 8.1 .largecircle.
721 27.1 19521 .largecircle. Invention example 2-27 7.0
.largecircle. 891 22.0 19648 .largecircle. Invention example 2-28
2.1 .largecircle. 1024 10.8 11059 .largecircle.
*.sup.1.largecircle. indicates a galvanized steel sheet with good
surface appearance, and X indicates a galvanized steel sheet with
poor surface appearance. *.sup.2.circle-w/dot.incidates a
longitudinal crack transition temperature of less than -70.degree.
C., .largecircle. incidates a longitudinal crack transition
temperature of -70 to -40.degree. C., and X incidates a
longitudinal crack transition temperature of over -40.degree.
C.
TABLE-US-00005 TABLE 4-2 Alloyed hot-dip galvanized steel sheet
Production condition Cool- Scab of hot- Annealing ing Holding
Holding Alloying rolled steel Temp. Holding rate temp. time temp.
Steel sheet (.degree. C.) time (s) (.degree. C./s) (.degree. C.)
(s) (.degree. C.) Comparative exmaple 2-29 2-Q No 840 230 12 390
100 510 Comparative exmaple 2-30 2-R No 820 110 29 460 80 540
Comparative exmaple 2-31 2-S No 810 150 26 410 110 530 Comparative
exmaple 2-32 2-T No 780 210 16 530 120 530 Comparative exmaple 2-33
2-U Present 830 120 40 460 100 530 Comparative exmaple 2-34 2-V
Present 850 90 15 400 180 540 Comparative exmaple 2-35 2-W No 780
90 10 420 90 540 Comparative exmaple 2-36 2-X No 780 120 12 510 120
540 Comparative exmaple 2-37 2-Y Present 820 150 7 450 120 530
Comparative exmaple 2-38 2-Z No 800 130 21 490 160 540 Alloyed
hot-dip galvanized steel sheet Mechanical properties Anti-secondary
Residual .gamma. Surface TS EI TS .times. EI work (%)
appearance*.sup.1 (MPa) (%) (MPa %) embrittlement*.sup.2
Comparative exmaple 2-29 11.3 .largecircle. 1009 23.0 23217 X
Comparative exmaple 2-30 8.9 .largecircle. 826 24.4 20126 X
Comparative exmaple 2-31 9.7 X 583 36.6 21321 X Comparative exmaple
2-32 9.1 .largecircle. 729 28.6 20827 X Comparative exmaple 2-33
13.4 X 808 29.1 23552 X Comparative exmaple 2-34 11.2 X 692 33.5
23185 .circle-w/dot. Comparative exmaple 2-35 9.3 X 850 24.8 21098
.circle-w/dot. Comparative exmaple 2-36 1.9 .largecircle. 383 40.9
15665 .circle-w/dot. Comparative exmaple 2-37 2.3 X 510 33.5 17085
.circle-w/dot. Comparative exmaple 2-38 1.2 .largecircle. 567 21.3
12077 .largecircle. Underline: out of the range of the present
invention. *.sup.1.largecircle. indicates a galvanized steel sheet
with good surface appearance, and X indicates a galvanized steel
sheet with poor surface appearance. *.sup.2.circle-w/dot.incidates
a longitudinal crack transition temperature of less than
-70.degree. C., .largecircle. incidates a longitudinal crack
transition temperature of -70 to -40.degree. C., and X incidates a
longitudinal crack transition temperature of over -40.degree.
C.
TABLE-US-00006 TABLE 5 1.25C.sup.0.5 - Component of steel (% by
mass) 0.0058 - 0.57Si + Mo, V, Ti, Nb, Al + Si 0.0006Al 0.0026Al
0.625Mn Steel C Si Mn P S Al N Cr B, Ni Ca, REM (mass %) (mass %)
(mass %) (mass %) 3-A 0.15 0.01 1.6 0.011 0.0020 1.4 0.0025 -- --
-- 1.4 0.0008 0.0022 1.48 3-B 0.07 0.01 2.6 0.010 0.0025 1.7 0.0011
Cr: 0.3 -- -- 1.7 0.0010 0.0014 1.95 3-C 0.20 0.02 2.0 0.009 0.0041
1.2 0.0030 Mo: 0.45 -- Ca: 0.003 1.2 0.0007 0.0027 1.80 3-D 0.10
0.01 1.2 0.008 0.0028 0.8 0.0044 V: 0.1 Ti: 0.01 REM: 0.005 0.8
0.0005 0.0037 1.14 3-E 0.12 0.20 2.2 0.012 0.0014 0.8 0.0025 -- Ti:
0.04 -- 1.0 0.0005 0.0037 1.69 3-F 0.14 0.30 1.8 0.009 0.0008 1.0
0.0009 -- Nb: 0.05 -- 1.3 0.0006 0.0032 1.42 3-G 0.16 0.45 1.8
0.007 0.0032 1.2 0.0022 -- B: 0.0020 -- 1.7 0.0007 0.0027 1.37 3-H
0.08 0.10 2.2 0.012 0.0035 1.3 0.0020 Mo: 0.1, -- -- 1.4 0.0008
0.0024 1.67 Cr: 0.2 3-I 0.23 0.50 1.8 0.010 0.0020 0.3 0.0043 --
Ni: 0.2 -- 0.8 0.0002 0.0050 1.44 3-J 0.12 0.25 1.6 0.012 0.0018
0.7 0.0033 -- Nb: 0.02, Ca: 0.003, 1.0 0.0004 0.0040 1.29 B: 0.0020
REM: 0.003 3-K 0.16 0.01 1.9 0.014 0.0013 1.5 0.0015 Mo: 0.5 Ti:
0.01 Ca: 0.002 1.5 0.0009 0.0019 1.68 3-L 0.12 0.45 1.9 0.001
0.0015 0.25 0.0020 Cr: 0.2 -- REM: 0.002 0.7 0.0002 0.0052 1.36 3-M
0.18 0.01 2.1 0.010 0.0030 1.8 0.0035 -- -- -- 1.8 0.0011 0.0011
1.84 3-N 0.08 0.30 2.0 0.008 0.0028 0.7 0.0055 Mo: 0.3 -- Ca: 0.002
1.0 0.0004 0.0040 1.43 3-O 0.13 0.60 1.9 0.023 0.0013 0.2 0.0053 --
Nb: 0.03 -- 0.8 0.0001 0.0053 1.30 3-P 0.17 0.15 1.6 0.010 0.0016
1.2 0.0044 -- Ti: 0.02 -- 1.4 0.0007 0.0027 1.43 3-Q 0.13 0.00 1.9
0.010 0.0036 1.8 0.0022 V: 0.1 Ti: 0.01, REM: 0.003 1.8 0.0011
0.0011 1.64 Nb: 0.02 3-R 0.12 0.01 1.4 0.021 0.0020 1.6 0.0040 --
-- -- 1.6 0.0010 0.0016 1.30 3-S 0.07 0.30 1.2 0.010 0.0034 1.1
0.0035 Cr: 0.1 Nb: 0.01 -- 1.4 0.0007 0.0029 0.91 3-T 0.13 0.01 1.8
0.011 0.0009 1.5 0.0030 -- Ti: 0.03 -- 1.5 0.0009 0.0019 1.57 3-U
0.04 0.20 1.80 0.012 0.0042 1.4 0.0037 -- Ti: 0.02, -- 1.6 0.0008
0.0022 1.26 Nb: 0.03 3-V 0.09 0.02 0.70 0.010 0.0025 1.0 0.0029 Mo:
0.3 Ti: 0.01 Ca: 0.002 1.0 0.0006 0.0032 0.80 3-W 0.12 0.10 3.50
0.011 0.0023 1.2 0.0018 Cr: 0.3 -- -- 1.3 0.0007 0.0027 2.56 3-X
0.10 0.10 1.80 0.010 0.002 0.4 0.0032 -- -- -- 0.5 0.0002 0.0048
1.46 Underline: out of the range of the present invention.
TABLE-US-00007 TABLE 6-1 Alloyed hot-dip galvanized steel sheet
Production condition Scrab of hot- Annealing Cooling Holding
Holding Alloying Cracking rolled steel temp. rate temp. time temp.
Steel in slab sheet (.degree. C.) (.degree. C./s) (.degree. C.) (s)
(.degree. C.) Invention example 3-5 3-B No No 800 33 400 110 510
Invention example 3-8 3-E No No 810 27 420 90 480 Invention example
3-9 3-E No No 700 22 460 100 530 Invention example 3-10 3-E No No
820 28 430 20 540 Invention example 3-11 3-F No No 810 43 370 220
510 Invention example 3-12 3-G No No 810 50 510 115 520 Invention
example 3-13 3-G No No 800 2 410 105 520 Invention example 3-14 3-G
No No 790 32 280 120 530 Invention example 3-15 3-H No No 850 16
420 80 520 Invention example 3-16 3-I No No 820 27 460 90 540
Invention example 3-17 3-J No No 810 44 390 150 500 Invention
example 3-18 3-K No No 790 31 500 100 490 Invention example 3-19
3-L No No 820 22 640 90 520 Alloyed hot-dip galvanized steel sheet
Mechanical properties Anti-secondary Residual .gamma. Surface TS EI
TS .times. EI work (%) appearance*.sup.1 (MPa) (%) (MPa %)
embrittlement*.sup.2 Invention example 3-5 10.9 .largecircle. 955
23.9 22823 .circle-w/dot. Invention example 3-8 9.2 .largecircle.
817 24.6 20110 .circle-w/dot. Invention example 3-9 0.6
.largecircle. 643 25.1 16139 .circle-w/dot. Invention example 3-10
1.9 .largecircle. 824 21.3 17551 .circle-w/dot. Invention example
3-11 9.1 .largecircle. 786 27.3 21447 .circle-w/dot. Invention
example 3-12 11.8 .largecircle. 886 25.9 22959 .circle-w/dot.
Invention example 3-13 2.6 .largecircle. 625 25.9 16188
.circle-w/dot. Invention example 3-14 2 .largecircle. 837 18.8
15738 .circle-w/dot. Invention example 3-15 10.4 .largecircle. 846
25.6 21649 .circle-w/dot. Invention example 3-16 7.3 .largecircle.
879 22.8 20050 .circle-w/dot. Invention example 3-17 8.7
.largecircle. 707 28.3 20008 .circle-w/dot. Invention example 3-18
10.9 .largecircle. 996 22.1 22004 .circle-w/dot. Invention example
3-19 1.4 .largecircle. 903 19.4 17518 .circle-w/dot.
*.sup.1.largecircle. indicates a galvanized steel sheet with good
surface appearance, and X indicates a galvanized steel sheet with
poor surface appearance. *.sup.2.circle-w/dot.incidates a
longitudinal crack transition temperature of less than -70.degree.
C., .largecircle. incidates a longitudinal crack transition
temperature of -70 to -40.degree. C., and X incidates a
longitudinal crack transition temperature of over -40.degree.
C.
TABLE-US-00008 TABLE 6-2 Alloyed hot-dip galvanized steel sheet
Production condition Scrab of Annealing Cooling Holding Holding
Alloying Cracking hot-rolled temp. rate temp. time temp. Steel in
slab steel sheet (.degree. C.) (.degree. C./s) (.degree. C.) (s)
(.degree. C.) Comparative exmaple 3-1 3-A No No 820 25 430 85 520
Comparative exmaple 3-2 3-A No No 840 31 470 90 610 Comparative
exmaple 3-3 3-A No No 920 18 440 90 530 Comparative exmaple 3-4 3-A
No No 830 150 460 105 520 Comparative exmaple 3-6 3-C No No 840 26
480 160 510 Comparative exmaple 3-7 3-D No No 820 8 520 75 550
Comparative exmaple 3-20 3-M Present No 800 35 410 180 520
Comparative exmaple 3-21 3-M Present No 820 47 410 130 520
Comparative exmaple 3-22 3-N Present No 780 28 460 125 530
Comparative exmaple 3-23 3-O Present No 820 41 370 110 540
Comparative exmaple 3-24 3-P Present No 820 9 530 80 510
Comparative exmaple 3-25 3-Q Present Preesnt 840 36 420 130 500
Comparative exmaple 3-26 3-R Present Preesnt 800 12 400 85 530
Comparative exmaple 3-27 3-S No Preesnt 780 24 490 100 490
Comparative exmaple 3-28 3-T Present No 830 39 430 90 510
Comparative exmaple 3-29 3-U Present Preesnt 810 21 510 75 520
Comparative exmaple 3-30 3-V No Preesnt 830 27 410 75 550
Comparative exmaple 3-31 3-W No No 820 15 430 90 510 Comparative
exmaple 3-32 3-X No No 820 48 420 120 520 Alloyed hot-dip
galvanized steel sheet Mechanical properties Anti-secondary
Residual .gamma. Surface TS EI TS .times. EI work (%)
appearance*.sup.1 (MPa) (%) (MPa %) embrittlement*.sup.2
Comparative exmaple 3-1 10.2 .largecircle. 617 35.1 21646 X
Comparative exmaple 3-2 2.4 .largecircle. 599 27.9 15712 X
Comparative exmaple 3-3 8.1 .largecircle. 572 30.3 17332 X
Comparative exmaple 3-4 7.8 .largecircle. 654 28.2 18443 X
Comparative exmaple 3-6 12.3 .largecircle. 1002 20.8 20850 X
Comparative exmaple 3-7 3.2 .largecircle. 390 50.3 19632 X
Comparative exmaple 3-20 6.2 X 738 26.6 19631 X Comparative exmaple
3-21 15.3 X 844 27.5 23198 X Comparative exmaple 3-22 9.6 X 810
25.0 20247 X Comparative exmaple 3-23 7.8 X 768 25.6 19662
.circle-w/dot. Comparative exmaple 3-24 10.3 X 691 31.1 21484 X
Comparative exmaple 3-25 12.0 X 822 28.2 23193 X Comparative
exmaple 3-26 9.7 X 542 41.3 22394 X Comparative exmaple 3-27 8.3 X
500 43.6 21809 X Comparative exmaple 3-28 10.6 X 714 30.8 21990 X
Comparative exmaple 3-29 0.5 X 510 26.7 13617 X Comparative exmaple
3-30 1.6 X 404 39.7 16039 .circle-w/dot. Comparative exmaple 3-31
13.1 .largecircle. 1323 8.1 10716 .circle-w/dot. Comparative
exmaple 3-32 2.3 .largecircle. 538 23.6 12697 .circle-w/dot.
Underline: out of the range of the present invention.
*.sup.1.largecircle. indicates a galvanized steel sheet with good
surface appearance, and X indicates a galvanized steel sheet with
poor surface appearance. *.sup.2.circle-w/dot.incidates a
longitudinal crack transition temperature of less than -70.degree.
C., .largecircle. incidates a longitudinal crack transition
temperature of -70 to -40.degree. C., and X incidates a
longitudinal crack transition temperature of over -40.degree.
C.
INDUSTRIAL APPLICABILITY
[0075] An alloyed hot-dip galvanized steel sheet causing no
cracking in a slab in continuous casting, no scab in hot rolling,
and no plating defect after hot-dip galvanizing and thus having
excellent surface appearance can be obtained without passing
through a complicated process. In addition, the alloyed hot-dip
galvanized steel sheet has both excellent anti-secondary work
embrittlement and high strength, and is thus suitable as an
automobile steel sheet and can widely contribute to the industrial
field.
* * * * *