U.S. patent application number 09/953788 was filed with the patent office on 2002-07-11 for hot-dip galvanized steel sheet and method for producing the same.
This patent application is currently assigned to NKK CORPORATION. Invention is credited to Harada, Kozo, Inazumi, Toru, Kitano, Fusato, Kobayashi, Akio, Nagataki, Yasunobu, Node, Shunsaku, Sato, Shogo, Tomita, Kunikazu, Urabe, Toshiaki.
Application Number | 20020088510 09/953788 |
Document ID | / |
Family ID | 26584048 |
Filed Date | 2002-07-11 |
United States Patent
Application |
20020088510 |
Kind Code |
A1 |
Nagataki, Yasunobu ; et
al. |
July 11, 2002 |
Hot-dip galvanized steel sheet and method for producing the
same
Abstract
A hot-dip galvanized steel sheet is produced by rough rolling a
steel, finish rolling the rough rolled steel at a temperature of
Ar3 point or more, coiling the finish rolled steel at a temperature
of 700.degree. C. or less, and hot-dip galvanizing the coiled steel
at a pre-plating heating temperature of Ac1 to Ac3. A continuous
hot-dip galvanizing operation is performed by soaking a pickled
strip at a temperature of 750 to 850.degree. C., cooling the soaked
strip to a temperature range of 600.degree. C. or less at a cooling
rate of 1 to 50.degree. C. per second, hot-dip galvanizing the
cooled strip, and cooling the galvanized strip so that the
residence time at 400 to 600.degree. C. is within 200 seconds. The
steel sheet has a structure consisting essentially of ferrite and
martensite.
Inventors: |
Nagataki, Yasunobu;
(Fukuyama, JP) ; Urabe, Toshiaki; (Fukuyama,
JP) ; Kitano, Fusato; (Fukuyama, JP) ;
Kobayashi, Akio; (Kawasaki, JP) ; Tomita,
Kunikazu; (Kawasaki, JP) ; Node, Shunsaku;
(Kasaoka, JP) ; Harada, Kozo; (Fukuyama, JP)
; Sato, Shogo; (Yokohama, JP) ; Inazumi, Toru;
(Ann Arbor, MI) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN,
LANGER & CHICK, P.C.
767 THIRD AVENUE
NEW YORK
NY
10017-2023
US
|
Assignee: |
NKK CORPORATION
Tokyo
JP
|
Family ID: |
26584048 |
Appl. No.: |
09/953788 |
Filed: |
September 17, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09953788 |
Sep 17, 2001 |
|
|
|
PCT/JP01/00403 |
Jan 23, 2001 |
|
|
|
Current U.S.
Class: |
148/533 ;
428/659 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/04 20130101; C21D 8/0263 20130101; C22C 38/24 20130101;
C22C 38/38 20130101; Y10T 428/12799 20150115; C21D 8/0278 20130101;
C22C 38/12 20130101; C23C 2/02 20130101; C23C 2/40 20130101; C21D
2211/005 20130101; C21D 2211/008 20130101; Y10S 428/939 20130101;
C21D 9/56 20130101 |
Class at
Publication: |
148/533 ;
428/659 |
International
Class: |
B32B 015/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2000 |
JP |
2000-014921 |
Jan 28, 2000 |
JP |
2000-019616 |
Claims
What is claimed is:
1. A hot-dip galvanized steel sheet comprising: a steel sheet
containing 0.04 to 0.12% of C, 0.5% or less of Si, 1.0 to 2.0% of
Mn, 0.05% or less of P, 0.005% or less of S, 0.05 to 1.0% of Cr,
0.005 to 0.2% of V, 0.1% or less of sol. Al, and 0.01% or less of n
by weight %; said steel sheet having a structure consisting
essentially of ferrite and martensite; and a hot-dip galvanizing
layer formed on the steel sheet:
2. The hot-dip galvanized steel sheet according to claim 1, wherein
said steel sheet is a hot rolled steel sheet.
3. The hot-dip galvanized steel sheet according to claim 1, wherein
said steel sheet is a cold rolled steel sheet.
4. The hot-dip galvanized steel sheet according to claim 1, wherein
said steel sheet has a martensite volume percentage of at least
7%.
5. The hot-dip galvanized steel sheet according to claim 1, wherein
the content of Si is 0.1% or less.
6. The hot-dip galvanized steel sheet according to claim 1, wherein
the content of Cr is 0.05 to 0.2%.
7. The hot-dip galvanized steel sheet according to claim 1, wherein
the content of V is 0.02 to 0.1%.
8. A method for producing a hot-dip galvanized steel sheet,
comprising the steps of: rough rolling a steel containing 0.04 to
0.12% of C, 0.5% or less of Si, 1.0 to 2.0% of Mn, 0.05% or less of
P, 0.005% or less of S, 0.05 to 1.0% of Cr, 0.005 to 0.2% of V,
0.1% or less of sol. Al, and 0.01% or less of N by weight %; finish
rolling the rough rolled steel at a temperature of the Ar3 point or
more; coiling the finish rolled steel at a temperature of
700.degree. C. or less; and hot-dip galvanizing the coiled steel at
a pre-plating heating temperature of Ac1 to Ac3.
9. The method according to claim 8, further comprising the step of
alloying the hot-dip galvanized steel.
10. The method according to claim 8, wherein the content of Si is
0.1% or less.
11. A hot-dip galvanized steel sheet comprising: a steel sheet
containing 0.04 to 0.13% of C, 0.5% or less of Si, 1.0 to 2.0% of
Mn, 0.05% or less of P, 0.01% or less of S, 0.05% or less of sol.
Al, 0.007% or less of N, 0.05 to 0.5% of Mo, and 0.2% or less of Cr
by weight %; said steel sheet having a structure consisting
essentially of ferrite having an average grain size of 20 .mu.m or
less and martensite with a volume percentage of 5 to 40%; and a
hot-dip galvanizing layer formed on the steel sheet.
12. The hot-dip galvanized steel sheet according to claim 11,
wherein said steel sheet further contains 0.02 to 0.2% of V.
13. The hot-dip galvanized steel sheet according to claim 11,
wherein said steel sheet is a hot rolled steel sheet.
14. The hot-dip galvanized steel sheet according to claim 11,
wherein said steel sheet is a cold rolled steel sheet.
15. A method for producing a hot-dip galvanized steel sheet,
comprising the steps of: rolling a steel containing 0.04 to 0.13%
of C, 0.5% or less of Si, 1.0 to 2.0% of Mn, 0.05% or less of P,
0.01% or less of S, 0.05% or less of sol. Al, 0.007% or less of N,
0.05 to 0.5% of Mo, and 0.2% or less of Cr by weight % to produce a
strip; pickling said strip; and performing a continuous hot-dip
galvanizing, said continuous hot-dip galvanizing comprising the
steps of: soaking the pickled strip at a temperature of 750 to
850.degree. C.; cooling the soaked strip to a temperature range of
600.degree. C. or lower at a cooling rate of 1 to 50.degree. C. per
second; hot-dip galvanizing the cooled strip; and cooling the
galvanized strip so that the residence time at 400 to 600.degree.
C. seconds.
16. The method according to claim 15, wherein said strip is a hot
rolled strip.
17. The method according to claim 15, wherein said strip is a cold
rolled strip obtained by cold rolling the hot rolled strip with a
cold rolled reduction of 40% or more.
18. The method according to claim 15, further comprising the step
of alloying said galvanized strip after the step of hot-dip
galvanizing.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a hot-dip galvanized steel
sheet used for automotive structural members, mechanical structural
parts, and the like, and a method for producing the same.
DESCRIPTION OF THE RELATED ARTS
[0002] In order to improve fuel economy and safety on collision, a
high-tensile strength steel sheet has been demanded for vehicle
body structural members and suspension members, and a high strength
has been required since a long time ago. In addition, in recent
years, a hot rolled steel sheet used for vehicle body structural
members and suspension members is required to have excellent press
formability, especially high ductility, because it is subjected to
severe forming consisting mainly of bulging. In this situation,
dual-phase structure type hot rolled steel sheets, basically having
a microstructure consisting of ferrite and martensite, have been
developed.
[0003] Furthermore, a steel sheet obtained by hot-dip galvanizing
the dual-phase structure type hot rolled steel sheet having both
high ductility and corrosion resistance has been demanded, and has
been disclosed in Unexamined Japanese Patent Publication No.
56-142821. The steel sheet disclosed in this Publication is
characterized in that a steel sheet containing 0.15% or less of C
and 1.0 to 2.5% of Mn+Cr by weight % as basic components and the
balance of Fe and unavoidable impurities is caused to have a
dual-phase structure by a continuous hot-dip galvanizing line
(hereinafter, referred to as CGL) on which a pre-plating heating
temperature, cooling rate before plating bath, alloying
temperature, and cooling rate after alloying are specified in
detail.
[0004] Specifically, after dual-phases of ferrite phase and
austenite phase are formed in the process of pre-plating heating,
the austenite phase is changed to a martensite phase by hardening
on the CGL.
[0005] However, in order to secure hardenability on the CGL line,
an alloy element must be added as a steel component, or the line
speed of CGL must be increased. The addition of an alloy element
increases the cost of steel. Also, for many CGLs, hardenability
cannot be secured at a line speed determined from the security of
stability of zinc deposition control and the restriction of
reaction rate for alloying.
[0006] On the other hand, a high-strength hot-dip galvanized steel
sheet having a tensile strength exceeding 440 MPa, which has
advantages of excellent rust preventing property and high proof
stress, has been used widely for construction members, mechanical
structural parts, automotive structural parts, and the like.
Therefore, a great number of inventions relating to the
high-strength hot-dip galvanized steel sheet have been disclosed.
In particular, since a need for workability has increased as the
application range extends, many inventions relating to a
high-strength hot-dip galvanized steel sheet having high
workability have been disclosed, for example, in Unexamined
Japanese Patent Publication Nos. 5-311244 and 7-54051.
[0007] In recent years, while a need for workability of a steel
sheet as is manufactured has increased, attention has been paid to
the properties of weld portion as a need for a product. This is
because as the technology to which the steel sheet is applied
expands, a steel sheet is fabricated in a state of including a weld
portion as in the case of tailored blank material, or a requirement
for high-speed deformation behavior of a structural member
including a weld portion becomes stringent.
[0008] However, the above-described conventional high-strength
hot-dip galvanized steel sheet has a serious drawback in that a
weld heat-affected zone (hereinafter, referred to as HAZ) softens
at the time of welding because the main strengthening mechanism
generally uses a low-temperature transformation phase such as
martensite and bainite obtained by quenching of austenite phase.
Such softening phenomenon occurring at the time of welding leads to
decreased formability for, for example, a tailored blank material,
and also causes a decrease in properties for structural member such
as deformation strength, rupture strength, and high-speed
deformation strength even when the steel sheet is used for other
applications.
SUMMARY OF THE INVENTION
[0009] It is an object of the present invention to provide a method
for manufacturing a hot-dip galvanized steel sheet with high
workability without the use of an expensive alloy element and
without being subject to any restriction of CGL facility, and a
steel sheet manufactured by the manufacturing method.
[0010] To achieve the object, the present invention provides a
hot-dip galvanized steel sheet comprising:
[0011] a steel sheet containing 0.04 to 0.12% of C, 0.5% or less of
Si, 1.0 to 2.0% of Mn, 0.05% or less of P, 0.005% or less of S,
0.05 to 1.0% of Cr, 0.005 to 0.2% of V, 0.1% or less of sol. Al,
and 0.01% or less of N by weight %;
[0012] the steel sheet having a structure consisting essentially of
ferrite and martensite; and
[0013] a hot-dip galvanizing layer formed on the steel sheet.
[0014] The steel sheet may be a hot rolled steel sheet or a cold
rolled steel sheet.
[0015] Further, the present invention provides a method for
producing for a hot-dip galvanized steel sheet, comprising the
steps of:
[0016] rough rolling a steel containing 0.04 to 0.12% of C, 0.5% or
less of Si, 1.0 to 2.0% of Mn, 0.05% or less of P, 0.005% or less
of S, 0.05 to 1.0% of Cr, 0.005 to 0.2% of V, 0.1% or less of sol.
Al, and 0.01% or less of N by weight %;
[0017] finish rolling the rough rolled steel at a temperature not
lower than the Ar3 point;
[0018] coiling the finish rolled steel at a temperature of
700.degree. C. or less; and
[0019] hot-dip galvanizing the coiled steel at a pre-plating
heating temperature of Ac1 to Ac3.
[0020] It is another object of the present invention to provide a
new high-strength hot-dip galvanized steel plate having a property
such that a change in hardness of HAZ is very small in welding such
as laser welding, mush-seam welding, or arc welding, and a method
for producing the same.
[0021] To achieve the object, the present invention provides a
hot-dip galvanized steel sheet comprising:
[0022] a steel sheet containing 0.04 to 0.13% of C, 0.5% or less of
Si, 1.0 to 2.0% of Mn, 0.05% or less of P, 0.01% or less of S,
0.05% or less of sol. Al, 0.007% or less of N, 0.05 to 0.5% of Mo,
and 0.2% or less of Cr by weight %;
[0023] the steel sheet having a structure consisting essentially of
ferrite having an average grain size of 20 .mu.m or less and
martensite with a volume percentage of 5 to 40%; and
[0024] a hot-dip galvanizing layer formed on the steel sheet.
[0025] The steel sheet may be a hot rolled steel sheet or a cold
rolled steel sheet.
[0026] Further, the present invention provides a method for
producing a hot-dip galvanized steel sheet, comprising the steps
of:
[0027] rolling a steel containing 0.04 to 0.13% of C, 0.5% or less
of Si, 1.0 to 2.0% of Mn, 0.05% or less of P, 0.01% or less of S,
0.05% or less of sol. Al, 0.007% or less of N, 0.05 to 0.5% of Mo,
and 0.2% or less of Cr by weight % to manufacture a strip;
[0028] pickling the strip; and
[0029] performing a continuous hot-dip galvanizing, said hot-dip
galvanizing comprising the steps of:
[0030] soaking the pickled strip at a temperature of 750 to
850.degree. C.;
[0031] cooling the soaked strip to a temperature range of
600.degree. C. or less at a cooling rate of 1 to 50.degree. C. per
second;
[0032] hot-dip galvanizing the cooled strip; and
[0033] cooling the galvanized strip so that the residence time at
400 to 600.degree. C. is within 200 seconds.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a diagram showing an influence of the content of
Cr+V in accordance with the present invention on a martensite
volume percentage;
[0035] FIG. 2 is a diagram showing a relationship between the
content of Mo and V in accordance with the present invention and
.DELTA.Hv; and
[0036] FIGS. 3(a), 3(b) and 3(c) are diagrams schematically showing
a change in hardness of HAZ caused by an excessive and insufficient
content of Mo, V and Cr.
EMBODIMENT FOR CARRYING OUT THE INVENTION
[0037] Embodiment 1
[0038] The inventors conducted a study on a composition for
obtaining a dual-phase structure consisting mainly of ferrite and
martensite that provides high hardenability even when the line
speed of CGL is relatively low. As the result, we found that proper
contents of C, Si, Mn, etc. and combined addition of Cr and V relax
the restriction of line speed significantly. The present invention
has been made by adding further studies to the above knowledge. The
gist of the present invention is as follows:
[0039] 1. A hot-dip galvanized high tensile strength steel sheet
having high workability, characterized by containing 0.04 to 0.12%
of C, 0.5% or less of Si, 1.0 to 2.0% of Mn, 0.05% or less of P,
0.005% or less of S, 0.05 to 1.0% of Cr, 0.005 to 0.2% of V, 0.1%
or less of sol. Al, and 0.01% or less of N by weight % and further
having a structure consisting essentially of ferrite and
martensite.
[0040] 2. A manufacturing method for a hot-dip galvanized high
tensile strength steel sheet having high workability, characterized
in that a steel containing 0.04 to 0.12% of C, 0.5% or less of Si,
1.0 to 2.0% of Mn, 0.05% or less of P, 0.005% or less of S, 0.05 to
1.0% of Cr, 0.005 to 0.2% of V, 0.1% or less of sol. Al, and 0.01%
or less of N by weight % is rough rolled; the rough rolled steel is
finish rolled at a temperature higher than the Ar3 point; the
finish rolled steel is coiled at a temperature of 700.degree. C. or
lower; and the coiled steel is hot-dip galvanized at a pre-plating
temperature of Acl to Ac3.
[0041] 3. A manufacturing method for a hot-dip galvanized high
tensile strength steel sheet having high workability, characterized
in that a steel containing 0.04 to 0.12% of C, 0.5% or less of Si,
1.0 to 2.0% of Mn, 0.05% or less of P, 0.005% or less of S, 0.05 to
1.0% of Cr, 0.005 to 0.2% of V, 0.1% or less of sol. Al, and 0.01%
or less of N by weight % is rough rolled; the rough rolled steel is
finish rolled at a temperature higher than the Ar3 point; the
finish rolled steel is coiled at a temperature of 700.degree. C. or
lower; the coiled steel is hot-dip galvanized at a pre-plating
temperature of Acl to Ac3; and further the galvanized steel is
alloyed.
[0042] The following is a description of the reason for restricting
the components, the reason for restricting the microstructure, the
hot rolling conditions, and the hot dip galvanizing conditions of
the present invention.
[0043] Chemical composition
[0044] C: 0.04% or more and 0.12% or less
[0045] C is essential to producing martensite and securing a target
strength, and the content thereof of 0.04% or more is needed. On
the other hand, if the content of C exceeds 0.12%, the workability
decreases. Therefore, the content of C should be 0.04% or more and
0.12% or less.
[0046] Si: 0.5% or less
[0047] When the content of Si is high, it is difficult to galvanize
a steel sheet in hot-dip galvanizing, and the content exceeding
0.5% reduces the adhesion property of plating layer. Therefore, the
content of Si should be 0.5% or less. The content of Si should
preferably 0.1% or less.
[0048] Mn: 1.0% or more and 2.0% or less
[0049] Mn acts advantageously in forming the structure, and is
added to improve strength by solid strengthening. To secure
necessary strength, 1.0% or more of Mn is added. The content of Mn
exceeding 2.0% decreases the workability such as press formability.
Therefore, the content of Mn should be 1.0% or more and 2.0% or
less.
[0050] P: 0.05% or less
[0051] P is an impurity element that decreases the weldability and
press formability, so that the content is restricted to 0.05% or
less. However, the content should preferably be reduced to the
utmost in the range allowed in terms of economy.
[0052] S: 0.005% or less
[0053] S is an impurity element that produces A-series inclusion
together with Mn and decreases the press formability, so that the
content is restricted to 0.005% or less. However, the content
should preferably be reduced to the utmost in the range allowed in
terms of economy.
[0054] Cr: 0.05% or more and 1.0% or less
[0055] V: 0.005% or more and 0.2% or less
[0056] The present invention is characterized by improving the
hardenability of steel by the combined addition of Cr and V. In
order to significantly relax the restriction of line speed of CGL
at which a dual-phase structure type steel sheet can be hardened,
0.05% or more of Cr and 0.005% or more of V are added combinedly.
On the other hand, even if these elements are added in large
amounts, the effect saturates, and the manufacturing cost
increases. Therefore, the contents of Cr and V should be 1.0% or
less and 0.2% or less, respectively. When only either Cr or V is
added singly, the hardenability cannot be secured sufficiently. The
content of Cr should preferably be 0.05 to 0.2%, and the content of
V should preferably be 0.002 to 0.1%.
[0057] Sol. Al: 0.01% or less
[0058] Sol. Al is an essential element for deoxidization. However,
if the content exceeds 0.01%, the effect saturates, and Al-series
inclusion increases, so that the press formability decreases.
Therefore, the content of sol. Al should be 0.10% or less.
[0059] N: 0.01% or less
[0060] A high content of N decreases the ductility. Therefore, the
content of N should be 0.01% or less.
[0061] Microstructure
[0062] In the present invention, in order to secure necessary
strength and satisfactory ductility, the microstructure of steel
consists essentially of ferrite and martensite. This structure can
contain bainite in the range such that the operation and effects
are not ruined.
[0063] Hot rolling conditions
[0064] Next, the hot rolling conditions will be described. In the
present invention, dual-phases of ferrite and austenite are
separated in the hot-dip galvanizing process after hot rolling, and
hardening is performed. In the hot rolling process, the finishing
temperature in finish rolling and coiling temperature are specified
so that a desirable structure can be obtained in the hot-dip
galvanizing process.
[0065] Finishing temperature: Ar3 transformation temperature or
higher
[0066] If the finishing temperature is lower than the Ar3
transformation temperature, the rolling of an .alpha.+.gamma.
dual-phase region produces a mixed grain structure, and this
problem is not solved after a steel sheet has passed through the
CGL, so that the ductility decreases. Therefore, the finishing
temperature should be the Ar3 transformation temperature or higher.
Coiling temperature: 700.degree. C. or lower
[0067] If the coiling temperature exceeds 700.degree. C., carbides
precipitated in the cooling process are coarsened, so that it takes
much time to dissolve carbides necessary before plating. Therefore,
the line speed of CGL must be decreased, which is disadvantageous
in hardening the steel sheet and decreases the production
efficiency. For this reason, the coiling temperature should be
700.degree. C. or lower. This tendency is strengthened when a steel
sheet is charged in the CGL without being cold rolled.
[0068] The hot rolling operation may be performed by a method using
a slab manufactured by the ordinary ingot making process or
continuous casting process, or may be performed by a method using
direct hot rolling process without operation in a heating furnace.
The method for hot rolling is not subject to any special
restriction. The slab heating temperature may be any temperature
such that a weight loss due to scale formation is proper, rough
rolling and finish rolling can be performed, and a finish rolling
temperature not lower than the Ar3 transformation temperature can
be secured. The slab heating temperature is not subject to any
special restriction. Also, a semi-finished product may be heated
before finish rolling in an atmosphere furnace or by high-frequency
heating.
[0069] Hot-dip galvanizing conditions
[0070] As described above, in the present invention, the structure
of steel sheet is controlled so as to be a dual-phase structure
having necessary strength and workability in the hot-dip
galvanizing process. For this purpose, the pre-plating heating
condition is specified.
[0071] Pre-plating heating condition: The heating temperature
should be Acl point or higher and Ac3 point or lower, and the
holding time should be 5 seconds to 10 minutes.
[0072] At the stage of pre-plating heating, the steel sheet is
heated to a temperature of Acl point or higher and Ac3 point or
lower to effect tow-phase separation. After plating, or during
cooling to a temperature lower than the alloying temperature in the
case where alloying is performed after plating, hardening is
performed, by which the structure consisting essentially of ferrite
and martensite is formed. In order to sufficiently effect
dual-phase separation, the holding time may be 5 seconds at the
minimum. If the holding time is longer than 5 seconds, there is no
problem from the viewpoint of structure control, but if the holding
time is too long, the production efficiency decreases. Therefore,
the holding time should be within 10 minutes.
[0073] On the CGL, precise control of heat cycle is difficult to
carry out, and therefore it is usually difficult to control the
microstructure so that desired properties can be obtained. In the
present invention, however, the combined addition of Cr and V
eliminates the need for specially restricting the manufacturing
conditions on the CGL, except the specification of pre-plating
heating temperature. Even if the cooling rate after plating or
during cooling to a temperature lower than the alloying temperature
in the case where alloying is performed after plating is as low as
3.5 to 9.3.degree. C. per second, the structure consisting
essentially of ferrite and martensite can be obtained.
[0074] In the case where the quality of hot-dip galvanization is
further stabilized, it is preferable to perform pickling after hot
rolling and before hot-dip galvanizing. Also, after hot-dip
galvanizing, alloying can be carried out.
[EXAMPLE 1]
[0075] A steel having a chemical composition given in Table 1 was
made by a converter, and a slab was formed by continuous casting.
The balance not given in Table 1 were Fe and unavoidable
impurities. Steel types A and B are steels to which Cr and V are
combinedly added, and have a composition in the range of the
present invention. Steel type C is a steel to which neither Cr nor
V is added, and steel types D to F are steels to which either Cr or
V is added, these steel types having a composition outside the
range of the present invention.
[0076] Then, the slab was finish rolled to a sheet thickness of 2.0
mm at a temperature of 860.degree. C., which is higher than the Ar3
point, and the rolled sheet was coiled at 500.degree. C. After
being pickled, the steel sheet was heated to 800.degree. C. and
held at that temperature for two minutes on the CGL. Thereafter,
the steel sheet was hot-dip galvanized on both surfaces with a
coating weight of 45 g/m.sup.2, and then was alloyed under the
condition of 550.degree. C..times.10 sec. At this time, the line
speed was increased from the coil head to the coil end for each
coil.
[0077] From the coil that has passed through the CGL, samples were
taken from portions corresponding to line speeds 30, 80 and 165
mpm. Using a JIS No. 5 tensile test piece, the yield strength (YS),
tensile strength (TS), yield ratio (YR), and elongation (El) were
determined, and also the microstructure was observed. Table 2 gives
the results. The cooling rate from the alloying temperature
(550.degree. C.) to the Ms point is determined according to the
line speed, and is shown in Table 2 as cooling rate.
[0078] For examples A1 to B3 of the present invention, which are
examples corresponding to the steel type A to which Cr and V are
added, a dual-phase structure consisting essentially of ferrite and
martensite can be obtained regardless of the line speed of CGL, and
satisfactory ductility is provided while necessary strength is
secured. On the other hand, comparative examples C1 to F3 are
examples corresponding to steel types to which both Cr and V are
not combinedly added, having a composition outside the range of the
present invention. For the steel types C, D and E, the
hardenability is insufficient, and a dual-phase structure
consisting essentially of ferrite and martensite cannot be
obtained, so that the strength and ductility are insufficient,
except for examples D3 and E3 in which the line speed of CGL is 165
mm.
[0079] For the steel type F, a structure corresponding to a
dual-phase structure is formed at any line speed, and a strength
not lower than 590 MPa is secured. However, because this steel type
is a type to which Cr is singly added and therefore a large amount
of Cr is added, the manufacturing cost is high. The line speed of
165 mpm is close to the upper limit in operation, so that this
speed is undesirable because of high percent defective of
alloying.
[0080] FIG. 1 shows an influence of the content of Cr+V in a steel
on a martensite volume percentage of a steel sheet manufactured
under the conditions given in Table 2. In the case where Cr and V
are combinedly added, a martensite volume percentage of 7% or
higher can be obtained regardless of the line speed. On the other
hand, in the case where Cr or V is singly added, a martensite
volume percentage of 3% or higher can be obtained only at a line
speed of 165 mpm. This fact reveals that the combined addition of
Cr and V is effective.
1 TABLE 1 Steel type Chemical composition (wt %) Classification
symbol C Si Mn P S Sol.Al N Cr V Present A 0.10 0.05 1.65 0.019
0.001 0.046 0.0038 0.05 0.006 invention B 0.07 0.04 1.57 0.009
0.004 0.033 0.0040 0.93 0.189 Comparative C 0.09 0.08 1.62 0.025
0.002 0.039 0.0045 0.03 0.003 example D 0.09 0.05 1.66 0.023 0.002
0.025 0.0048 0.06 0.003 E 0.10 0.06 1.63 0.017 0.001 0.028 0.0039
0.02 0.007 F 0.08 0.06 1.58 0.011 0.001 0.026 0.0044 1.14 0.002
[0081]
2 TABLE 2 Steel CGL line Cooling Tensile property Reference type
speed rate YS TS YR El character symbol (mpm) (.degree. C./s) (MPa)
(MPa) (%) (%) Microstructure Classification A1 A 30 3.5 419 592 71
27 Ferrite + martensite + Present bainite invention A2 80 9.3 402
597 67 28 Ferrite + martensite + Present bainite invention A3 165
19.1 391 605 65 27 Ferrite + martensite Present invention B1 B 30
3.5 499 690 72 24 Ferrite + martensite + Present bainite invention
B2 80 9.3 504 744 68 22 Ferrite + martensite Present invention B3
165 19.1 509 769 66 21 Ferrite + martensite Present invention C1 C
30 3.5 425 521 82 30 Ferrite + fine pearlite Comparative example C2
80 9.3 420 528 80 29 Ferrite + fine pearlite Comparative example C3
165 19.1 418 543 77 29 Ferrite + bainite + Comparative fine
pearlite example D1 D 30 3.5 420 519 81 30 Ferrite + fine pearlite
Comparative example D2 80 9.3 407 541 75 29 Ferrite + bainite +
Comparative fine pearlite example D3 165 19.1 388 590 66 28 Ferrite
+ martensite + Comparative bainite example E1 E 30 3.5 445 565 79
27 Ferrite + bainite Comparative example E2 80 9.3 438 574 76 27
Ferrite +bainite Comparative example E3 165 19.1 409 591 69 27
Ferrite + martensite + Comparative bainite example F1 F 30 3.5 499
620 80 25 Ferrite + bainite + Comparative fine martensite example
F2 80 9.3 500 651 77 24 Ferrite + bainite + Comparative fine
martensite example F3 165 19.1 493 699 71 22 Ferrite + martensite +
Comparative bainite example
[Example 2]
[0082] A steel type G to which Cr and V were combinedly added,
having a chemical composition in the range of the present invention
as given in Table 3 (the balance not given in Table 3 were Fe and
unavoidable impurities), was made by a converter, and a slab was
formed by continuous casting. Subsequently, the slab was hot rolled
under the conditions of a finish temperature of 860.degree. C.
higher than the Ar3 point and a coiling temperature (CT) of 400 to
750.degree. C. to produce a strip with a thickness of 2.0 mm. After
being pickled, the strip was heated to 800.degree. C. and held at
that temperature for two minutes on the CGL. Thereafter, the strip
was hot-dip galvanized on both surfaces with a coating weight of 45
g/m.sup.2, and then was alloyed under the condition of 550.degree.
C..times.10 sec.
[0083] At this time, the line speed was increased from the coil
head to the coil end for each coil. From the coil that has passed
through the CGL, samples were taken from portions corresponding to
line speeds 30, 80 and 160 mpm. Using a JIS No. 5 tensile test
piece, yield strength (YS), tensile strength (TS), yield ratio
(YR), and elongation (El) were determined, and also the
microstructure was observed. Table 4 gives the results. The cooling
rate from the alloying temperature (550.degree. C.) to the Ms point
at each portion is determined according to the line speed, and is
shown in Table 4 as cooling rate.
[0084] For examples 1 to 5 of the present invention, since the
coiling temperature is 700.degree. C. or lower, a dual-phase
structure consisting of ferrite and martensite can be obtained at
all line speeds, so that proper strength and satisfactory ductility
are provided. For comparative examples 6 to 8, since the coiling
temperature is as high as 750.degree. C., being outside the range
of the present invention. When the coiling temperature is as high
as 750.degree. C., carbides precipitate as coarse carbides after
hot rolling and coiling, and are not dissolved sufficiently even by
heating before plating on the CGL. In the case of the comparative
examples 7 and 8, carbides partially consisting essentially of
cementite in addition to ferrite and martensite are contained, so
that a strength-ductility balance is insufficient although the
strength is proper. For the comparative example 6, since the line
speed is as low as 30 mpm, the dissolution of carbides is
sufficient, but the production efficiency is low. Therefore, this
comparative example is undesirable.
3TABLE 3 Steel type Chemical composition (wt %) symbol C Si Mn P S
Sol.Al N Cr V G 0.08 0.04 1.52 0.008 0.003 0.036 0.0046 0.45
0.08
[0085]
4 TABLE 4 CGL line Cooling Tensile property Reference CT speed rate
YS TS YR El character (.degree. C.) (mpm) (.degree. C./s) (MPa)
(MPa) (%) (%) Microstructure Classification 1 400 80 9.3 435 648 67
25 Ferrite + martensite Present invention 2 600 80 9.3 413 641 64
26 Ferrite + martensite Present invention 3 700 30 3.5 416 614 68
28 Ferrite + martensite Present invention 4 700 80 9.3 422 628 67
27 Ferrite + martensite Present invention 5 700 160 18.5 437 637 69
26 Ferrite + martensite Present invention 6 750 30 3.5 509 769 66
21 Ferrite + martensite + Comparative bainite example 7 750 80 9.3
445 602 74 26 Ferrite + martensite + Comparative carbide example 8
750 160 18.5 438 596 73 26 Ferrite + martensite + Comparative
carbide example
[0086] Embodiment 2
[0087] Embodiment 2-1 is a hot-dip galvanized steel sheet
characterized by containing 0.04 to 0.13% of C, 0.5% or less of Si,
1.0 to 2.0% of Mn, 0.05% or less of P, 0.01% or less (including 0%)
of S, 0.05% or less of sol. Al, 0.007% or less (including 0%) of N,
0.05 to 0.5% of Mo, and 0.2% or less (including 0%) of Cr by weight
%, the balance consisting essentially of Fe and unavoidable
impurities, and having a structure consisting essentially of
ferrite having an average grain size of 20 .mu.m or smaller and
martensite with a volume percentage of 5 to 40%.
[0088] Embodiment 2-2 is a hot-dip galvanized steel sheet
characterized by further containing 0.02 to 0.2% of V in addition
of the components of the embodiment 2-1, and having a structure
consisting essentially of ferrite having an average grain size of
20 .mu.m or smaller and martensite with a volume percentage of 5 to
40%.
[0089] Embodiment 2-3 for solving the before-mentioned problems is
a manufacturing method for a hot-dip galvanized steel sheet
described in Embodiment 2-1 or 2-2. This manufacturing method is
characterized in that a steel having the components described in
Embodiment 2-1 or 2-2 is cast and then hot rolled into a strip;
after being pickled, the strip is cold rolled as necessary with a
cold rolled reduction of 40% or more; on the succeeding continuous
hot-dip galvanizing line, after the strip is soaked at a
temperature of 750 to 850.degree. C., it is cooled to a temperature
range of 600.degree. C. or lower at a cooling rate of 1 to
50.degree. C. per second, and then is galvanized; as necessary, the
strip is further alloyed; and thereafter, the strip is cooled in a
state in which the residence time at 400 to 600.degree. C. is
within 200 seconds.
[0090] The expression of "the balance consisting essentially of Fe
and unavoidable impurities" means that a steel sheet containing
minute amounts of other elements including unavoidable impurities
is embraced in the scope of the present invention unless the
effects of the present invention are eliminated. In this
description and the accompanying drawings, the percentage %
indicating the content of component of steel means weight % unless
otherwise specified. Also, "structure consisting essentially of
ferrite and martensite with a volume percentage of 5 to 40%" means
that a steel sheet containing a structure such as small amounts of
cementite, bainite, or retained austenite is embraced in the scope
of the present invention.
[0091] (Progress in making invention and reason for restricting Mo,
V, Cr and structure)
[0092] In order to solve the before-mentioned problems, the
inventors studied the influence of steel component and structure on
a change in strength of weld portion. As the result, we found that
by containing a proper amount of Mo in a steel containing basic
components of C, Si, Mn, etc. in restricted amounts and providing a
structure consisting essentially of ferrite having an average grain
size of 20 .mu.m or smaller and martensite with a volume percentage
restricted to 5 to 40%, a high-strength galvanized steel sheet that
scarcely decreases the hardness of HAZ can be manufactured. Also,
we found that this effect is enhanced by containing a proper amount
of V.
[0093] It is generally known that if a high temperature of 400 to
800.degree. C. is kept, a low-temperature transformation phase
obtained by quenching austenite phase such as martensite and
bainite is tempered easily in a short period of time, or carbides
are coarsened, by which the strength is decreased suddenly. The
inventors fully studied the influence of steel component and
microscopic structure. As the result, we found that the following
control is effective in preventing a decrease in strength.
[0094] (1) By making martensite having high dislocation density a
hard phase and utilizing secondary precipitation strengthening, a
decrease in strength of hard phase can be reduced when the
temperature rises in a short period of time. For this purpose, it
is effective to contain Mo or V. However, if the contents of these
elements are high, the hardness of HAZ partially increases as
compared with the base metal, which is undesirable in preventing
the strength from decreasing. Also, Cr, which is known as a
secondary precipitation strengthening element like Mo and V,
deposits rapidly when the temperature rises in a short period of
time, so that a change in hardness of HAZ increases, so that a high
content of Cr is undesirable.
[0095] (2) The volume percentage of martensite phase in which a
change in hardness is large at the time of welding is restricted to
40% or less, and the balance is made ferrite, by which a change in
hardness as a whole can be decreased. However, if the volume
percentage of martensite is too low, inversely the secondary
precipitation strengthening of martensite phase cannot be utilized
effectively for resistance to softening HAZ. Therefore, the lower
limit of volume percentage is specified at 5%.
[0096] (3) Furthermore, the control of ferrite grain size is also
important. The average grain size is specified at 20 .mu.m or
smaller to increase the grain boundary area, by which the
deposition of austenite at the grain boundary is promoted when the
temperature rises in a short period of time. Thereby, a rise in the
Ac3 transformation temperature, at which the hardness of martensite
phase decreases most greatly, can be avoided, so that the decrease
in hardness of martensite phase can be restrained.
[0097] The following is a description of the reason for restricting
the content of Mo, V and Cr.
[0098] Mo: 0.05% to 0.5%
[0099] Mo is an essential element in obtaining the effect of the
present invention. As described above, the reason for this is that
softening due to tempering of martensite phase caused by a
temperature rise at HAZ at the time of welding is restrained by the
precipitation of carbides of Mo. Therefore, the content of 0.05%,
which achieves the effect, is set as the lower limit. If Mo is
contained excessively, the hardness of HAZ increases greatly, and a
change in hardness of HAZ increases. For this reason, the upper
limit is specified at 0.5%. The content of Mo should preferably
0.15 to 0.4%.
[0100] Cr: 0.2% or less (including 0%)
[0101] In making the present invention, a study was conducted on an
element that seems to be effective for resistance to softening due
to tempering of other martensite phases containing Mo as a base,
specifically V and Cr. As the result, it was revealed that when the
temperature rises in a short period of time as in the case of HAZ
at the time of welding, the influence of the kind of element
differs, and even a minute amount of Cr contained greatly increases
the hardness of HAZ, and thus a content of Cr exceeding 0.2%
increases the change in hardness of HAZ. In the present invention,
therefore, the content of Cr is restricted to 0.2% or less
(including 0%).
[0102] V: preferably 0.02 to 0.2%
[0103] An element to which attention was paid in this study was V.
The combined addition of Mo and V greatly decreased the change in
hardness of HAZ. It was thought that the reason for this is that
the precipitation strengthening due to V carbide at the time when
the temperature of martensite phase rises in a short period of time
is not so great, and moreover the temperature at which V carbide
precipitates is different from the temperature at which Mo carbide
precipitates, so that in a wide heat history region of HAZ, uniform
resistance to softening due to tempering can be provided. The lower
limit of V content for achieving such an effect is 0.02%. If V is
contained excessively, the hardness of HAZ increases greatly as in
the case of Cr, so that the upper limit is specified at 0.2%. The
reason for restricting the lower limit of V in the embodiment 2-2
is as described above. Therefore, in the embodiment 2-1, a steel
sheet containing 0.02% or less of V is not precluded.
[0104] FIGS. 3(a) to 3(c) schematically show a change in hardness
of HAZ caused by an excessive and insufficient content of Mo, V and
Cr. FIG. 3(a) shows a case where the contents of Mo and V are lower
than the proper values, showing that a difference in hardness
.DELTA.Hv between the most softened portion of HAZ and the base
metal is large. FIG. 3(b) shows a case where the contents of Mo, V
and Cr exceed the proper values, showing that although the
softening degree of HAZ is small, the base metal is also softened,
so that the .DELTA.Hv increases eventually. FIG. 3(c) shows a case
where the contents of Mo, V and Cr are within the range of the
present invention, showing that the .DELTA.Hv is small.
[0105] (Reason for restricting other components) C: 0.04 to
0.13%
[0106] C is an essential element in securing a desired strength.
However, if the content of C increases, the martensite volume
percentage becomes too high, so that the hardness of HAZ increases
greatly. Therefore, the lower limit is specified at the minimum
value for securing the strength, and the upper limit is specified
as described above in order for the martensite volume percentage
that greatly decreases the hardness of HAZ not to exceed 40%. Si:
0.5% or less
[0107] Si is an essential element in stably obtaining a dual-phase
structure of ferrite and martensite. However, if the content of Si
increases, the adhesion property of galvanizing layer and the
appearance of surface deteriorate remarkably. Therefore, the upper
limit is specified at 0.5%. Mn: 1.0 to 2.0%
[0108] Mn, like C, is an essential element in securing a desired
strength. Although a content of 1.0% is necessary to obtain a
desired strength as the lower limit, if Mn is contained
excessively, the martensite volume percentage increases, and thus
the hardness of HAZ decreases greatly. Therefore, the upper limit
is specified at 2.0%.
[0109] P: 0.05% or less
[0110] P, like Si, is an essential element in stably obtaining a
dual-phase structure of ferrite and martensite. However, if the
content of P increases, the toughness of weld portion decreases.
Therefore, the upper limit is specified at 0.05%.
[0111] S: 0.01% or less
[0112] S is an impurity, so that a high content thereof decreases
the toughness of weld portion as in the case of P. Therefore, the
upper limit is specified at 0.01%.
[0113] Sol. Al: 0.05% or less
[0114] The content of Sol. Al contained in the ordinary steel does
not ruin the effects of the present invention, and 0.05% or less of
sol. Al has no problem. Therefore, the upper limit is specified at
0.05%.
[0115] N: 0.007% or less (including 0%)
[0116] The content of N contained in the ordinary steel does not
ruin the effects of the present invention, and 0.007% or less of N
has no problem. Therefore, the upper limit is specified at
0.007%.
[0117] For other elements that have not been described above,
unless the content thereof is extremely high, the effects of the
present invention are not especially ruined. For example, when Nb
or Ti is added to provide a higher strength or finer structure of
steel, the content thereof within 0.05% has no problem.
[0118] (Manufacturing method)
[0119] The following is a description of a manufacturing method for
the hot-dip galvanized steel sheet in accordance with the present
invention.
[0120] In order to obtain the steel in accordance with the present
invention, the composition of each component must be restricted as
described above, and also the structure must be controlled so as to
be a structure consisting essentially of ferrite having an average
grain size of 20 .mu.m or smaller and martensite with a volume
percentage of 5 to 40%.
[0121] First, a steel having a predetermined composition is cast,
and then is hot rolled into a strip. After being pickled, the strip
is further cold rolled with a cold rolled reduction of 40% or more
as necessary to prepare a substrate for plating. The conditions for
hot rolling are not specified. Unless the hot rolling method is
such that the grain size of hot rolled sheet becomes remarkably
large, for example, due to a finish rolling temperature lower than
the Ar3 transformation point or a low cooling rate of 10.degree.
C./sec or lower after the finish of hot rolling, there does not
especially arise any problem. Inversely, a method which decreases
the grain size of hot rolled sheet, for example, due to rapid
cooling with a high cooling rate of 100 to 300.degree. C./sec
performed within one second after the finish of hot rolling or a
combination of finish hot rolling with a high reduction with the
rapid cooling does not ruin the effects of the present invention.
The reason for specifying the reduction at the time of cold rolling
at 40% or more is that a reduction lower than 40% is liable to
increase the grain size in annealing.
[0122] On the succeeding continuous hot-dip galvanizing line, after
the strip is soaked at a temperature of 750 to 850.degree. C., it
is cooled to a temperature range of 600.degree. C. or lower at a
cooling rate of 1 to 50.degree. C. per second, and then is
galvanized so that the residence time at 400 to 600.degree. C. is
within 200 seconds. As necessary, the strip is further alloyed. A
soaking temperature not lower than 750.degree. C. is necessary for
stably obtaining the austenite phase. However, if the soaking
temperature exceeds 850.degree. C., the grain size increases, so
that desired properties cannot be obtained. Therefore, the upper
limit is specified at 850.degree. C. Thereafter, the strip is
cooled to a temperature range of 600.degree. C. or lower at a
cooling rate of 1 to 50.degree. C. per second. The purpose for this
is that pearlite is not produced and fine ferrite is precipitated
with a desired volume percentage. The lower limit of cooling rate
is specified because a cooling rate lower than this value produces
pearlite and increases the grain size of ferrite. The upper limit
of cooling rate is specified because if a cooling rate is higher
than this value, not only ferrite does not precipitate sufficiently
but also the martensite volume percentage increases to 40% or
more.
[0123] The pickled sheet or a cold rolled sheet is cooled to a
temperature range of 600.degree. C. or lower and then is
galvanized, and further is alloyed as necessary. Finally, the sheet
is cooled to room temperature. According to the study conducted by
the inventors, it was revealed that in the process of cooling to
room temperature, the residence time at 400 to 600.degree. C. has a
large influence on the formation of structure. Specifically, if the
residence time is long, the precipitation of cementite from
austenite is remarkable, and thus not only the volume percentage of
martensite phase decreases so that the strength decreases but also
the effect of resistance to softening of HAZ due to the
precipitation of Mo and V carvide is not achieved. Based on the
result of study conducted by the inventors, the upper limit of
residence time is specified at 200 seconds.
[0124] In the present invention, the structure is specified as a
structure consisting essentially of ferrite and martensite with a
volume percentage of 5 to 40%. However, even if the structure
contains cementite, bainite, or retained austenite with a volume
percentage within 5%, the effects of the present invention are not
ruined.
[0125] Although not mentioned specially, other means such as a slab
manufacturing method such as ingot making or continuous casting,
continuous hot rolling by means of rough hot rolled bar joint in
hot rolling, and temperature rise within 200.degree. C. using an
induction heater in the process of hot rolling have no influence on
the effects of the present invention.
[0126] [Example]
[0127] The following is a description of examples of the present
invention and comparative examples.
[0128] Steels A to X having a chemical composition in the range of
the present invention as given in Table 5 and steels a to m of
comparative examples having a chemical composition outside the
range of the present invention were manufactured by a converter,
and slabs were formed by continuous casting. These slabs were hot
rolled to form strips at the heating temperature and coiling
temperature given in Table 6. After being pickled, some of strips
were cold rolled with a draft of 65% to prepare a substrate for
plating. Succeedingly, on a continuous hot-dip galvanizing line, a
hot-dip galvanized steel sheet or an alloyed hot-dip galvanized
steel sheet was manufactured under the conditions given in Table 7.
The heat cycle on the continuous hot-dip galvanizing line was set
in the preferable range shown in the embodiment 2-3.
[0129] Table 7 gives evaluation results for structure, tensile
strength, and change in hardness .DELTA.Hv of HAZ caused by laser
welding of each of these steels. The steel number in Table 7
corresponds to that in Table 6. The laser welding conditions were
an output of 5 kw and a welding speed of 2 m/min. The welding speed
was especially decreased so that the HAZ is easily softened.
[0130] FIG. 2 is a diagram in which .DELTA.Hv of HAZ of the steel
given in Table 7 is summarized by the contents of Mo and V. In this
figure, .DELTA.Hv is evaluated by three grades of .largecircle.
(.DELTA.Hv.ltoreq.10), (10<.DELTA.Hv.ltoreq.20), and
(.DELTA.Hv>20). As seen from FIG. 2, by setting the contents of
Mo and other elements in the range specified by the present
invention, high resistance to softening of HAZ of
.DELTA.Hv.ltoreq.20 can be obtained. Further, by setting the
content of V in the range described in the embodiment 2-2, the
resistance of .DELTA.Hv.ltoreq.10 can be obtained. (In FIG. 2,
steels in which the content of C is outside the range of the
present invention, like steel Nos. 26 and 27 in Table 7, and steels
in which the content of Cr is outside the range of the present
invention, like steel Nos. 36 to 38 are excluded.)
5 TABLE 5 TS Composition (wt %) Calculated Steel C Si Mn P S sol.Al
N Mo V Cr Other value Remark A 0.048 0.25 1.71 0.03 0.001 0.02
0.0025 0.3 0.002 - 626 P B 0.05 0.2 1.4 0.025 0.0006 0.031 0.0014
0.13 0.05 - 542 P C 0.049 0.36 1.9 0.014 0.001 0.014 0.0023 0.07
0.07 - 692 P D 0.051 0.1 1.82 0.045 0.003 0.019 0.0025 0.43 0.002 -
668 P E 0.05 0.02 1.8 0.01 0.007 0.02 0.0036 0.5 0.17 - 805 P F
0.06 0.01 1.65 0.026 0.003 0.021 0.0044 0.4 0.02 - P G 0.063 0.1
1.6 0.03 0.002 0.032 0.0036 0.07 0.03 - P H 0.065 0.25 1.62 0.015
0.004 0.012 0.0021 0.13 0.035 - P I 0.064 0.23 1.35 0.032 0.002
0.024 0.002 0.15 0.11 - P J 0.065 0.25 1.6 0.025 0.0002 0.022
0.0028 0.31 0.11 - P K 0.063 0.15 1.58 0.026 0.002 0.023 0.0011
0.35 0.05 - Nb:0.01 P L 0.068 0.25 1.66 0.032 0.002 0.018 0.0048
0.23 0.15 - P M 0.067 0.1 1.6 0.019 0.001 0.031 0.0032 0.48 0.01 -
P N 0.064 0.48 1.63 0.011 0.002 0.026 0.0033 0.06 0.002 - P O 0.068
0.1 1.6 0.011 0.002 0.022 0.0015 0.25 0.03 - P P 0.07 0.01 1.22
0.016 0.001 0.038 0.0019 0.08 0.16 - P Q 0.072 0.05 1.2 0.029 0.006
0.031 0.0022 0.39 0.05 - P R 0.071 0.11 1.65 0.022 0.001 0.025
0.0019 0.45 0.13 - P S 0.07 0.01 1.2 0.016 0.001 0.024 0.0029 0.38
0.19 - P T 0.074 0.3 1.52 0.015 0.003 0.022 0.0021 0.27 0.19 - P U
0.075 0.3 1.6 0.015 0.0005 0.035 0.0036 0.15 0.2 - P V 0.079 0.01
1.2 0.016 0.001 0.021 0.0021 0.38 0.07 0.1 P W 0.088 0.25 1.1 0.03
0.002 0.023 0.0024 0.37 0.13 - P X 0.096 0.29 1.6 0.032 0.001 0.024
0.0044 0.15 0.07 0.18 P Y 0.128 0.25 1.55 0.012 0.004 0.025 0.0031
0.18 0.018 - P a 1 0.15 1.5 0.021 0.003 0.03 0.0016 0.2 0.08 - C b
2 0.13 1.53 0.02 0.0006 0.036 0.0021 0.35 0.1 - C c 0.082 0.25 1.41
0.03 0.001 0.024 0.0022 3 0.18 - C d 0.068 0.36 1.6 0.012 0.002
0.028 0.003 4 0.002 - C e 0.065 0.1 1.63 0.03 0.002 0.021 0.0019 5
0.12 - C f 0.074 0.01 1.23 0.016 0.001 0.023 0.0026 6 0.1 - C g
0.075 0.3 1.6 0.026 0.005 0.026 0.0022 7 0.062 - C h 0.072 0.01 1.2
0.016 0.001 0.019 0.0026 0.2 8 - C i 0.07 0.02 1.18 0.015 0.001
0.04 0.0041 0.07 9 - C j 0.075 0.3 1.6 0.015 0.007 0.025 0.0031
0.45 10 - C k 0.093 0.25 1.62 0.033 0.001 0.026 0.0029 0.23 0.07 11
C l 0.081 0.25 1.42 0.018 0.001 0.021 0.0021 0.18 0.05 12 C m 0.053
0.45 1.8 0.045 0.003 0.028 0.003 0.28 0.07 13 C Thick frame
indicates that the value is outside the range of present invention.
Minus mark indicates that the content is less than 0.05%. P:
Present invention C: Comparative example
[0131]
6 TABLE 6 Hot-dip galvanizing condition Hot rolling condition Sheet
Soaking Cooling Residence Steel Steel Heating Coiling Reduc-
thickness temperature rate time at 400 No. type temperature
(.degree. C.) temperature (.degree. C.) tion (%) Substrate (mm)
(.degree. C.) (.degree. C./sec) to 600.degree. C. Alloying 1 A 1220
580 -- Pickled sheet 2.3 800 7 120 .largecircle. 2 B 1260 630 --
Pickled sheet 2.3 800 7 100 X 3 C 1230 600 -- Pickled sheet 2.3 780
12 120 .largecircle. 4 D 1170 530 -- Pickled sheet 2.3 830 15 180
.largecircle. 5 E 1220 620 65 Cold rolled sheet 1.2 800 3 70
.largecircle. 6 F 1200 600 -- Pickled sheet 2.3 800 8 180
.largecircle. 7 G 1200 580 -- Pickled sheet 2.3 850 20 140
.largecircle. 8 H 1200 580 -- Pickled sheet 2.3 850 15 100 x 9 I
1200 580 -- Pickled sheet 2.3 820 10 120 .largecircle. 10 J 1200
580 65 Cold rolled sheet 1.2 820 10 120 .largecircle. 11 K 1200 580
-- Pickled sheet 2.3 800 2 100 .largecircle. 12 L 1270 580 --
Pickled sheet 2.3 800 7 100 .largecircle. 13 M 1230 580 -- Pickled
sheet 2.3 800 25 140 .largecircle. 14 N 1200 580 -- Pickled sheet
2.3 800 20 140 .largecircle. 15 O 1200 550 -- Pickled sheet 2.3 820
10 45 X 16 P 1200 550 -- Pickled sheet 2.3 780 10 120 X 17 Q 1200
620 -- Pickled sheet 2.3 840 5 140 .largecircle. 18 R 1200 620 --
Pickled sheet 2.3 800 7 120 .largecircle. 19 S 1200 620 -- Pickled
sheet 2.3 800 5 120 .largecircle. 20 T 1200 580 -- Pickled sheet
2.3 800 28 120 .largecircle. 21 U 1200 580 65 Cold rolled sheet 1.2
800 10 30 X 22 V 1200 580 -- Pickled sheet 2.3 800 13 120
.largecircle. 23 W 1200 580 -- Pickled sheet 2.3 750 9 120
.largecircle. 24 X 1280 600 65 Cold rolled sheet 1.2 780 5 120
.largecircle. 25 Y 1200 600 -- Pickled sheet 2.3 800 27 120
.largecircle. 26 a 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 27 b 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 28 c 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 29 d 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 30 e 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 31 f 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 32 g 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 33 h 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 34 i 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 35 j 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 36 k 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 37 l 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle. 38 m 1200 600 -- Pickled sheet 2.3 800 10 120
.largecircle.
[0132]
7 TABLE 7 Structure Property Steel Ferrite grain size Martensite
volume Change in hardness No. (.mu.m) percentage (%) TS (MPa) of
HAZ (.DELTA.Hv) Symbol Classification 1 13 10 626 17 .DELTA.
Present invention 2 12 8 542 8 .smallcircle. Present invention 3 8
12 692 10 .smallcircle. Present invention 4 10 13 668 15 .DELTA.
Present invention 5 18 12 690 12 .DELTA. Present invention 6 9 9
638 12 .DELTA. Present invention 7 7 15 572 9 .smallcircle. Present
invention 8 10 14 624 5 .smallcircle. Present invention 9 11 13 619
8 .smallcircle. Present invention 10 12 12 726 8 .smallcircle.
Present invention 11 10 12 661 6 .smallcircle. Present invention 12
9 15 761 9 .smallcircle. Present invention 13 8 17 666 13 .DELTA.
Present invention 14 11 16 616 19 .DELTA. Present invention 15 9 17
627 9 .smallcircle. Present invention 16 13 19 587 10 .smallcircle.
Present invention 17 19 20 579 7 .smallcircle. Present invention 18
9 18 781 9 .smallcircle. Present invention 19 11 18 682 10
.smallcircle. Present invention 20 6 21 790 12 .DELTA. Present
invention 21 9 20 790 11 .DELTA. Present invention 22 8 20 602 6
.smallcircle. Present invention 23 10 25 677 8 .smallcircle.
Present invention 24 11 28 725 8 .smallcircle. Present invention 25
5 37 796 17 .DELTA. Present invention 26 10 14 782 28 x Comparative
example 27 9 15 810 36 x Comparative example 28 11 22 698 23 x
Comparative example 29 9 14 591 31 x Comparative example 30 8 13
648 25 x Comparative example 31 10 19 659 29 x Comparative example
32 8 16 750 33 x Comparative example 33 11 17 666 26 x Comparative
example 34 13 18 651 31 x Comparative example 35 7 22 730 33 x
Comparative example 36 8 25 737 38 x Comparative example 37 10 20
633 37 x Comparative example 38 8 12 570 38 x Comparative example
Thick frame indicates that the value is outside the range of
present invention. P: Present invention C: Comparative example
[0133] Table 8 gives the results of studies on a change in
property, which were conducted by changing the heat cycle
especially on a continuous hot-dip galvanizing line for steel H of
an example of the present invention. Since the soaking temperature
is improper for steel Nos. 1 and 5, the cooling rate is improper
for steel Nos. 6 and 11, and the residence time at 400 to
600.degree. C. is too long for steel No. 16, the structure
specified in the present invention is not obtained, and desired
resistance to softening of HAZ is not obtained. Contrarily, for the
steel of the present invention manufactured under the manufacturing
conditions described in Embodiment 2-3, the structure described in
Embodiment 2-1 is obtained, and high resistance to softening of HAZ
of .DELTA.Hv.ltoreq.20 is obtained.
8 TABLE 8 Hot rolling condition Hot-dip galvanizing condition
Heating Coiling Sheet Soaking Cooling Steel Steel temperature
temperature Reduc- thickness temperature rate No. type (.degree.
C.) (.degree. C.) tion (%) Substrate (mm) (.degree. C.) (.degree.
C. /sec) 1 H 1220 580 -- Pickled sheet 2.3 16 10 2 H 1220 580 --
Pickled aheet 2.3 750 10 3 H 1220 580 -- Pickled aheet 2.3 800 10 4
H 1220 580 -- Pickled sheet 2.3 850 10 5 H 1220 580 -- Pickled
sheet 2.3 17 10 6 H 1220 580 -- Pickled sheet 2.3 800 18 7 H 1220
580 -- Pickled sheet 2.3 800 2 8 H 1220 580 -- Pickled sheet 2.3
800 5 9 H 1220 580 -- Pickled sheet 2.3 800 20 10 H 1220 580 --
Pickled sheet 2.3 800 50 11 H 1220 580 -- Pickled sheet 2.3 800 19
12 H 1220 580 -- Pickled sheet 2.3 800 10 13 H 1220 580 -- Pickled
sheet 2.3 800 10 14 H 1220 580 -- Pickled sheet 2.3 800 10 15 H
1220 580 -- Pickled sheet 2.3 800 10 16 H 1220 580 -- Pickled sheet
2.3 800 10 Structure Property Hot-dip galvanizing condition
Martensite Change in Resisdence Ferrite volume hardness Steel time
at grain size percentage TS of HAZ No. 400 to 600.degree. C.
Alloying (.mu.m) (%) (MPa) (.DELTA.Hv) Classification 1 120
.smallcircle. 12 20 571 28 C 2 120 .smallcircle. 10 18 615 13 P 3
120 .smallcircle. 10 17 610 10 P 4 120 .smallcircle. 18 18 600 8 P
5 120 .smallcircle. 21 20 590 23 P 6 120 .smallcircle. 22 10 570 30
C 7 120 .smallcircle. 13 18 605 10 C 8 120 .smallcircle. 10 16 607
9 P 9 120 .smallcircle. 8 17 612 8 P 10 120 .smallcircle. 6 37 625
16 P 11 120 x 7 23 670 22 C 12 40 .smallcircle. 8 22 605 15 P 13 90
.smallcircle. 9 18 612 9 P 14 160 .smallcircle. 10 18 608 7 P 15
190 .smallcircle. 12 17 590 13 P 16 24 .smallcircle. 15 25 563 31 C
*mark indicates that much cementite deposits. P: Present invention
C: Comparative example
* * * * *