U.S. patent application number 09/408124 was filed with the patent office on 2002-01-31 for steel plate for paint use and manufacturing method thereof.
Invention is credited to KAN, TOSHIAKI, OKANO, SHIGEO, SAKAI, MASAHIKO, TAKESHITA, SATORU.
Application Number | 20020011286 09/408124 |
Document ID | / |
Family ID | 26552530 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011286 |
Kind Code |
A1 |
KAN, TOSHIAKI ; et
al. |
January 31, 2002 |
STEEL PLATE FOR PAINT USE AND MANUFACTURING METHOD THEREOF
Abstract
A steel plate for paint use which contains C (0.12% or less), Si
(1.0% or less), Mn (2.5% or less), P (0.05% or less), S (0.02% or
less), and Cr (0.05% or less), Cu (0.05-3.0%), Ni (0.05-6.0%), Ti
(0.025-0.15%), and Cu+Ni (0.50% or more), with P.sub.CM being 0.23%
or less, in terms of mass %. Said steel plate may contain at least
one additional component selected from B (0.0005-0.0030%), Al
(0.05-0.50%), Ca (0.0001-0.05%), Ce (0.0001-0.05%), La
(0.0001-0.05%), Nb (0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%),
and Mo (0.05-0.5%), in terms of mass %. This steel plate provides
good weldability as well as good painting durability in a
salt-polluted environment.
Inventors: |
KAN, TOSHIAKI;
(KAKOGAWA-SHI, JP) ; OKANO, SHIGEO; (KAKOGAWA-SHI,
JP) ; TAKESHITA, SATORU; (KAKOGAWA-SHI, JP) ;
SAKAI, MASAHIKO; (KAKOGAWA-SHI, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
26552530 |
Appl. No.: |
09/408124 |
Filed: |
September 29, 1999 |
Current U.S.
Class: |
148/332 ;
148/333; 148/654; 420/91 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/005 20130101; C22C 38/16 20130101; C21D 8/0263 20130101;
C22C 38/08 20130101; C21D 1/18 20130101; C22C 38/42 20130101; C21D
8/0226 20130101; C22C 38/58 20130101 |
Class at
Publication: |
148/332 ;
148/654; 148/333; 420/91 |
International
Class: |
C22C 038/42; C22C
038/50; C21D 008/00; C22C 038/16 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 1998 |
JP |
10277716 |
Dec 25, 1998 |
JP |
10370422 |
Claims
What is claimed i,s:
1. A steel plate for paint use which contains C (0.12% or less,
excluding O;), Si (1.0% or less, excluding 0%), Mn (2.5% or less,
excluding O%), P (0.05% or less, excluding 0%), S (0.02% or less,
excluding 0%), Cr (0.05% or less, excluding 0%), Cu (0.05-3.0%), Ni
(0.05-6.0%), Ti (0.025-0.15%), Cu+Ni (0.50% or more), and P.sub.CM
(0.23% or less), in terms of mass %.
2. A steel plate for paint use as defined in claim 1, which further
contains at least one additional component selected from Al
(0.05-0,50%), Ca (0.0001-0.05%), Ce (0.0001-0.05%), and La
(0.0001-0.05%), in terms of mass %.
3. A steel plate for paint use as defined in claim 1, which further
contains B (0.0005-0.0030%), in terms of mass %.
4. A steel plate for paint use as defined in claim 2, which further
contains B (0.0005-0.0030%), in terms of mass %.
5. A steel plate for paint use as defined in claim 1, which further
contains at least one additional component selected from Nb
(0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo
(0.05-0.5%), in terms of mass %.
6. A steel plate for paint use as defined in claim 2, which further
contains at least one additional component selected from Nb
(9.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo
(0.05-0.5%), in terms of mass %.
7. A steel plate for paint use as defined in claim 3, which further
contains at least one additional component selected from Nb
(0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo
(0.05-0.5%), in terms of mass %.
8. A steel plate for paint use as defined in claim 4, which further
contains at least one additional component selected from Nb
(0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%), and Mo
(0.05-0.5%), in terms of mass %.
9. A manufacturing method of a steel plate for paint use, said
process comprising hot-rolling a steel plate which is defined in
any of claims 1 to 8 and contains Ti and C in such an amount that
the Ti/C ratio exceeds 4, in such a way that the heating
temperature is 850-1200.degree. C. and the temperature at the end
of rolling is 950.degree. C. or lower, said hot rolling being
followed by air cooling or water cooling (at a cooling rate
1.degree. C./s or higher).
10. A manufacturing method of a steel plate for paint use, said
process comprising hot-rolling a steel plate which is defined in
any of claims 1 to 8 and contains Ti and C in such an amount that
the Ti/C ratio exceeds 4, in such a way that the heating
temperature is 850-1200.degree. C. and the temperature at the end
of rolling is 950.degree. C. or lower, said hot rolling being
followed by direct quenching from a temperature of
Ar.sub.3.about.950.degree. C. or reheating-quenching from a
temperature of Ac.sub.3.about.950.degree. C., and tempering.
11. A manufacturing method of a steel plate for paint use, said
process comprising hot-rolling a steel plate which is defined in
any of claims 1 to 8 and contains Ti and C in such an amount that
the Ti/C ratio is 4 or less, in such a way that the heating
temperature is 850.about.(1200-50.times.Ti/C) .degree. C. and the
temperature at the end of rolling is
(Ar.sub.3+50.times.Ti/C+100.times.[Ni].sup.2) .degree. C. or lower,
which is followed by air cooling or water cooling (at a cooling
rate 1.degree. C./s or higher). (where [Ni] represents the content
of Ni.)
12. A manufacturing method of a steel plate for paint use, said
process comprising hot-rolling a steel plate which is defined in
any of claims 1 to 8 and contains Ti and C in such an amount that
the Ti/C ratio is 4 or less, in such a way that the heating
temperature is 850.about.(1200-50.times.Ti/C) .degree. C. and the
temperature at the end of rolling is
(Ar.sub.3+50.times.Ti/C+100.times.[Ni].sup.2) .degree. C. or lower,
which is followed by direct quenching from a temperature at the end
of rolling or reheating-quenching from a temperature of
(Ac.sub.3+50.times.Ti/C+100.times.[Ni].sup.2) .degree. C. or lower,
and tempering. (where [Ni] represents the content of Ni.)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a steel plate to be used
for steel structures (such as bridges and towers) which present
difficulties in routine maintenance work such as repainting and
also to a manufacturing method thereof. More particularly, the
present invention relates to a steel plate to be used with a
painted film in coastland or cold district where steel structures
are subjected to salt damage by airborne salt or deicing salt (as
an antifreezing agent scattered on the road) and also to a
manufacturing method thereof.
[0003] 2. Description on the Related Art
[0004] There are two kinds of corrosion-resistant steels specified
in the Japanese Industrial Standards (JIS). They are
corrosion-resistant hot-rolled steel for welded structures
(designated as SMA, JIS G-3114) and highly corrosion-resistant
rolled steel (designated as SPA, JIS G-3125). These steels contain
Cr, Cu, Ni, P, etc. in an adequate amount. Such corrosion-resistant
steels are also disclosed in Japanese patents mentioned later.
Corrosion-resistant steels form a dense and excellently adhesive
layer of stable rust thereon which protects them from corrosion.
They have been widely used in inland areas.
[0005] Unfortunately, corrosion-resistant steels need a long time
of 10 years or more until they form a layer of stable rust.
Practically, they pose a problem of initial corrosion and
rust-laden water. This is true particularly in Japan where the
climate is warm and humid. Rust stabilization is common practice to
prevent corrosion-resistant steels from posing landscape or
environment with rust-laden water until they form stable rust when
they are used without a painted film. This practice, however,
merely avoids rust-laden water and hinders the formation of compact
rust layer when steels are used in a salt-polluted environment.
[0006] Several means have been proposed to address the
above-mentioned problems involved in corrosion-resistant steels.
For example, resin painting on the surface of corrosion-resistant
steel, which is intended to promote the formation of stable rust
while isolating steel surface from its environment, is disclosed in
Japanese Patent Publication Nos. 22530/1978, 33991/1981,
39915/1983, 17833/1983, and 21273/1994, and Japanese Patent
Laid-open No. 133480/1990. A surface treating solution to promote
the formation of stable rust, which contains Fe.sub.3O.sub.4 of
scaly crystal structure, phosphoric acid, and butyral resin
dissolved in a solvent, is disclosed in Japanese Patent Laid-open
No. 133480/1990. A method of surface treatment for rust
stabilization, which consists of applying a painting solution
composed of more than one compound of P, Cu, Cr, N, Si, and Mo,
Fe.sub.2O.sub.3+Fe.sub.3O.sub.4, phosphoric acid, a bisphenol epoxy
resin, and auxiliaries dissolved in a solvent, is disclosed in
Japanese Patent Publication No. 21273/1994. The above-mentioned
means, however, neither improve the corrosion-resistant steels
themselves nor promote the formation of stable rust satisfactorily.
In other words, a resin painted film usually has minute defects at
which the film effect is not produced. Such defects cause corrosion
to take place in the interface between the painted film and base
metal, with the result that the painted film exfoliate before the
stable rust layer is formed. Therefore, the use of
corrosion-resistant steel is limited in the salt-polluted
environment.
[0007] In the meantime, an important subject in the world of bridge
is to save maintenance cost for repainting as well as construction
cost. The latter object is achieved by reducing the number of main
girders, adopting rationalized girders, reducing the frequencies of
site welding, and reducing maintenance management. This stimulates
a demand for steel with large thickness and high strength capable
of welding with a large amount of heat input which obviates
preheating to prevent cold cracking at the time of welding.
OBJECT AND SUMMARY OF THE INVENTION
[0008] The present invention was completed in order to address the
above-mentioned problems. Accordingly, it is an object of the
present invention to provide a steel plate for paint use and a
manufacturing method thereof, said steel plate imparting good
durability to the painted film thereon when used in a salt-polluted
environment and also being superior in weldability.
[0009] The gist of the present invention resides in a steel plate
for paint use which contains C (0.12% or less), Si (1.0% or less),
Mn (2.5% or less), P (0.05% or less), S (0.02% or less), Cr (0.05%
or less), Cu (0.05-3.0%), Ni (0.05-6.0%), Ti (0.025-0.15%), Cu+Ni
(0.50% or more), and Pa (0.23% or less), in terms of mass %.
[0010] The above-specified steel plate may contain at least one
additional component selected from B (0.0005-0.0030%), Al
(0.05-0.50%), Ca (0.0001-0.05%), Ce (0.0001-0.05%), La
(0.0001-0.05%), Nb (0.002-0.05%), V (0.01-0.10%), Zr (0.002-0.05%),
and Mo (0.05-0.5%), in terms of mass %.
[0011] The above-specified steel plate, with the Ti/C ratio higher
than 4, is produced by hot-rolling in such a way that the heating
temperature (T) is 850-1200.degree. C. and the temperature at the
end of rolling is 950.degree. C. or lower, which is followed by air
cooling or water cooling (at a cooling rate 1.degree. C./s or
higher), or by direct quenching from a temperature of
Ar.sub.3.about.950.degree. C. or reheating-quenching from a
temperature of Ac.sub.3.about.950.degree. C., and tempering.
[0012] The above-specified steel plate, with the Ti/C ratio 4 or
lower, is produced by hot-rolling in such a way that the heating
temperature (T) is 850.ltoreq.T.ltoreq.(1200-50.times.Ti/C)
.degree. C. and the temperature at the end of rolling is
(Ar.sub.3+50.times.Ti/C+100.times.Ni.sup.2) .degree. C. or lower,
which is followed by air cooling or water cooling (at a cooling
rate 1.degree. C./s or higher), or by direct quenching from a
temperature at the end of rolling or reheating-quenching from a
temperature of (Ac.sub.3+50.times.Ti/C+100.times.Ni.sup.2) .degree.
C. or lower, and tempering. P.sub.CM, Ar.sub.3, and AC.sub.3 used
above are defined as follows.
[0013] P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B
[0014] Ar.sub.3=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo+0.35(t-8) (where
t represents the plate thickness.)
[0015]
Ac.sub.3=908-223.7C+438.5P+30.49Si+37.92V-34.43Mn-23Ni+2(100C-54+6N-
i) (where the term 2(100C-54+6Ni) is applicable only when it is
positive.)
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a graph showing how toughness is affected by the
heating temperature and the Ti/C ratio.
[0017] FIG. 2 is a graph showing how toughness is affected by the
difference between FRT and Ar.sub.3 and the Ti/C ratio, in the case
where the amount of Ni is 1.0%.
[0018] FIG. 3 is a graph showing how toughness is affected by the
difference between FRT and Ar.sub.3 and the Ti/C ratio, in the case
where the amount of Ni is 0.5%.
[0019] FIG. 4 is a graph showing how toughness is affected by the
difference between the quenching temperature and Ac.sub.3 and the
Ti/C ratio, in the case where the amount of Ni is 1.0%.
[0020] FIG. 5 is a graph showing how toughness is affected by the
difference between the quenching temperature and Ac.sub.3 and the
Ti/C ratio, in the case where the amount of Ni is 0.5%.
[0021] FIG. 6 is a figure showing the shape of the specimen
subjected to the accelerated test and the atmospheric exposure
test.
[0022] FIG. 7 is a figure illustrating the cycle of accelerated
tests.
[0023] FIG. 8 is a graph showing the relation between the corrosion
resistance and the total amount of Cu+Ni.
[0024] FIG. 9 is a graph showing the relation between the corrosion
resistance and the amount of Ti added.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] It is known that if steel has a compact stable rust layer
thereon, its corrosion only proceeds at a negligibly low rate even
though it has no special anti-corrosion treatment, because the rust
layer physically or electrochemically prevents
corrosion-accelerating factors (such as moisture, oxygen, and
chlorine ions present in the environment) from reaching the base
metal (or steel). The corrosion-resistant steel effectively
utilizes the action of self-corrosion resistance by compact
rust.
[0026] To be concrete, it is possible to obtain the
corrosion-resistant steel by adding such elements as Cr, Cu, Ni,
and P, which promote the formation of compact rust, in very small
amounts. In other words, corrosion-resistance steel produces its
effect when it is used without a painted film. However, as
mentioned earlier, the corrosion-resistant steel does not fully
produce its effect of promoting the formation of stable rust when
it is used in an environment severely contaminated with salt.
Several means to cope with this situation have been devised. One of
them is to paint the steel surface with a thin resin film so as to
protect steel from salt until stable rust forms on the steel
surface. However, the resin painted film is not satisfactory
because of film defects as mentioned above.
[0027] The present inventors extensively studied the mechanism of
corrosion in the defective part of a painted film. It was found
that Cr as a steel component is a corrosion-accelerating element.
In other words, it was found that when steel corrosion starts at
the defective part of a painted film, Cr dissolves together with
iron atoms, giving rise to Cr ions which, in conjunction with Cl
ions, lower the pH in the defective part, thereby acidifying
condensed water therein. The resulting acid water causes corrosion
in the interface between the painted film and the base metal.
[0028] It is concluded from the mechanism of corrosion just
mentioned above that it is important to take into account the
following three points for improvement in durability of
resin-painted corrosion-resistant steel in salt-polluted areas.
[0029] (1) Reduce the amount of Cr as far as possible so as to
remove the corrosion-accelerating element in the defective part in
a painted film.
[0030] (2) Add an element, in place of Cr, which promotes the
formation of stable rust. (Since a painted steel has its base
protected from salt by the painted film, it will have a long life
even in a corrosive environment so long as it contains an element
which prevents the pH from decreasing in the defective part of the
painted film.)
[0031] (3) Add an element which moderates the decrease of pH in the
defective part of a painted film or which rather increases pH when
dissolved.
[0032] If the above-mentioned requirements are met, steel will form
stable rust in the defective part of the painted film. Painting a
common organic resin is recommended because of economy,
workability, and simplicity. Among various resins (such as
polyester, epoxy, and urethane), butyral resin is the best because
of its toughness, flexibility, impact strength, and good adhesion
to metal.
[0033] The steel for paint use will permit reduction in the number
of main girders and adoption of rationalized girders, which leads
to cost reduction in bridge construction, if it has good
weldability, good low-temperature toughness, sufficient thickness,
and high strength. For the steel plate to have good weldability, it
is necessary to control the C content and the weld cracking
parameter of material P.sub.CM. For the steel plate to have good
toughness, it is necessary to control the precipitation of TiC or
to specify the heating, rolling, and heat-treating conditions
according to the Ti/C ratio. For the steel plate to have sufficient
thickness and high strength, it is necessary to add B, Nb, V, Zr,
and Mo. For the steel plate to have good toughness at the part
affected by welding heat and to be capable of welding with a large
amount of heat, it is necessary to specify the upper limit of the
content of C and Ti and to effectively utilize B.
[0034] The present invention is based on the above-mentioned ideas.
Mention is made below of the effect of each composition and the
reason why the amount of each composition is limited.
[0035] Regarding Cu, Ni, and Ti as essential elements for the
corrosion-resistant steel.
[0036] Cu is an element which is electrochemically nobler than
iron. It forms compact rust and grows stable rust. It produces its
effect when it is contained in an amount of 0,05% or more. Its
effect levels off when its content exceeds 3.0%. With an excess
amount, it makes the steel brittle at the time of hot rolling.
Therefore, the adequate content of pu should be 0.05-3.0%.
[0037] Ni is an element which, like Cu, improves corrosion
resistance. It produces its effect when it is contained in an
amount of 0.05% or more. In addition, Ni prevents hot brittleness
which may occur if Cu is contained. Its effect levels off when its
content exceeds 6.0%. Therefore, the adequate content of Ni should
be 0.05-6.0%.
[0038] In addition, the present invention requires that the total
amount of Cu+Ni be 0.50% or more. FIG. 8 shows the relation between
the total amount of Cu+Ni and the corrosion resistance, which was
found by the present inventors' experiment with the sample
according to claim 1. The test method is shown in FIG. 7. The test
result was rated in terms of the width of blistering at the
defective part of painted film. In FIG. 8, the corrosion resistance
index is indicated by 1-a (where a is the average width (mm) of
blistering). The larger the index, the better the corrosion
resistance. It is apparent from FIG. 8 that the corrosion
resistance increases according as the total amount of Cu+Ni
increases. Good effects are produced when the total amount of Cu+Ni
is 0.50% or more.
[0039] Ti is an essential element to supersede Cr which was
selected under the idea mentioned in (2) above. Like Cr, Cu, and
Ni, this element forms compact rust and grows stable rust. It also
provides outstanding corrosion resistance and produces the effect
of purifying steel. These effects are remarkable when the content
is 0.025% or more. With its content exceeding 0.15%, Ti does not
produce any additional effect but rather aggravates the toughness
of the part affected by welding heat. Therefore, the adequate
content of Ti should be 0.025-0.15%.
[0040] FIG. 9 shows the relation between the content of Ti and the
corrosion resistance, which was found by the present inventors'
experiment with the sample according to claim 1. The test method
and the rating of the test result are the same as mentioned above.
It is apparent from FIG. 9 that the corrosion resistance increases
according as the content of Ti increases. Good effects are produced
when the content of Ti is 0.05% or more.
[0041] Mention is made below of the effect of P, Cr, C, Si, and Mn.
P and Cr are necessary for conventional steels to be used without
coating. Since they greatly aggravate weldability, their content is
limited to 0.05% in the steel plate of the present invention which
is used mainly for bridges and other structures that need site
welding frequently. The content of Cr should not exceed 0.05%,
because Cr decreases pH and acidifies condensed water in the
defective part of painted film, thereby causing corrosion in the
interface between the painted film and the base metal.
[0042] C is an essential element for the steel plate to have a
desired strength. With an increasing content of C, the steel plate
becomes poor in weldability and corrosion resistance. Therefore,
the content of C should be 0.12% or less. Incidentally, for the
steel plate to have satisfactory weldability and corrosion
resistance, the content of C should be 0.10% or less. For good
weldability, P.sub.CM should be 0.23% or less according to the
present invention.
[0043] Si promotes solid-solution hardening, accelerates the
formation of stable rust, and improves corrosion resistance.
However, Si in an excess amount aggravates weldability. Therefore,
the adequate content of Si should be 1.0% or less.
[0044] Mn provides strength, like C. A large amount of Mn in steel
has an adverse effect on workability, toughness, and corrosion
resistance (due to MnS formed from it). Therefore, the adequate
content of Mn should be 2.5% or less.
[0045] S combines with Mn or Fe to form MnS or FeS, respectively.
These compounds provides a starting point for corrosion. Therefore,
the adequate content of S should be 0.02% or less.
[0046] Al, as well as Ti, is an element to supersede Cr which was
selected under the idea mentioned in (2) above. Like Cr, Cu, and
Ni, this element forms compact rust and grows stable rust. It
produces its effect when its content is 0.05% or more. It produces
an enhanced effect when used in combination with Ti. With an amount
exceeding 0.50%, it produces no additional effect but rather
aggravates the toughness of the base metal. Therefore, the adequate
content of Al should be 0.05-0.50%.
[0047] Ca, Ce, and La are elements to moderate pH decrease in the
defective part of painted film, which were selected under the idea
mentioned above in (3). These elements slightly dissolve as the
corrosion of iron proceeds under the painted film. They are
alkaline and hence they moderate pH decrease, thereby preventing
corrosion in the defective part in painted film. They produce their
effect when they are present in an amount of 0.0001% or more. Their
effect levels off even though their amount is increased. Therefore,
their respective content should be 0.0001-0.05%.
[0048] B is an element which improves the hardenability and
strength of steel and forms fine ferrite in the part affected by
welding heat, thereby compensating for embrittlement due to TiC
precipitation, An amount of 0.0005% or more is necessary for B to
produce its effect. An excess amount more than 0.0030% aggravates
weldability rather than enhancing the effect. Therefore, the
content of B should be 0.0005-0.0030%.
[0049] Mention is made below of Mo, Nb, Zr, and V. These elements
are added to thick steel plates (50 mm and above) and high-strength
steel (590 N/mm.sup.2 and above), but they produce very little
effect on corrosion resistance.
[0050] Mo, as well as B, is an element which effectively increases
the strength of steel. An amount of 0.05% or more is necessary for
Mo to produce its effect. An excess amount more than 0.5%
aggravates weldability rather than enhancing the effect. Therefore,
the content of Mo should be 0.05-0.5%.
[0051] Nb and Zr are elements which form their carbo-nitrides to
increase strength. They produce this effect when they are present
in an amount of 0.002% or more. An excess amount more than 0.05%
aggravates toughness rather than enhancing the effect. Therefore,
the content of Nb and Zr should be 0.002-0.05% each.
[0052] V, as well as Nb and Zr, is an element which increases the
strength of steel. An amount of 0.01% or more is necessary for it
to produce its effect. An excess amount more than 0.10% aggravates
toughness rather than enhancing the effect. Therefore, the content
of V should be 0.01-0.10% each.
[0053] Mention is made below of the manufacturing method according
to the present invention. The method of the invention is
characterized in adding Ti in a large amount so that the steel
exhibits good corrosion resistance when it is given coating.
Unfortunately, Ti precipitates in the form of TiC, thereby greatly
aggravating the toughness of the base metal. In the production of
steel plates, it is important to suppress the deterioration of
toughness due to TiC. There are two ways to achieve this object,
(1) by preventing Ti from forming solid solution when steel is
heated for hot rolling and quenching, or (2) by making dissolved Ti
(in solid solution) harmless. The process of production was
investigated in two ways according to the Ti/C ratio which is
either greater than 4 or smaller than 4.
[0054] Incidentally, it is not necessary to investigate the
deterioration of toughness due to TiC particles which have
precipitated before heating for hot rolling or quenching, because
the TiC particles are too large to affect toughness. In other
words, those TiC particles which exist before heating for hot
rolling are formed during air cooling after casting, and those TiC
particles which exist before heating for quenching are formed
during air cooling after hot rolling. Air cooling after casting is
very slow because the slab is thick and hence the precipitated TiC
particles grow and become large. In the case of hot rolling which
ends at a high temperature and is followed by air cooling, TiC
particles grow and become large, Such grown TiC particles do not
affect toughness and hence they can be neglected.
[0055] Case 1 in which the Ti/C ratio is 4 or less and steel does
not undergo quenching and tempering.
[0056] The effect of heating temperature was investigated to find
the condition under which Ti does not form solid solution. Several
steel samples were prepared, with the Ti/C ratio varied for the
base composition of 0.05%C-0.55%Cu-0.50% Ni-0.05% Ti. In order to
make dissolved Ti harmless, hot rolling was carried out in such a
way that the finish rolling temperature (FRT) is 760.degree. C.
(which is close to Ar.sub.3), with the heating temperature varied.
Hot rolling, followed by air cooling, gave 25-mm thick steel
plates. These steel plates were tested for toughness. The results
are shown in FIG. 1. (The object of making dissolved Ti harmless is
achieved if hot rolling is carried out to such an extent the low
region of the temperature of .gamma. solid solution is reached. Hot
rolling at high temperature induces strain to precipitate TiC
particles, which become coarser during subsequent rolling to such
an extent that they do not match the matrix any longer. Thus it is
possible to suppress the deterioration of toughness.)
[0057] FIG. 1 is a graph showing how toughness varies depending on
the heating temperature and the Ti/C ratio. It is apparent from
FIG. 1 that if the heating temperature (T) is (1200-50.times.Ti/C)
.degree. C. or more (in the region under the oblique line), the
desired value of vE.sub.0>100J is achieved. The lower limit of
heating temperature is 850.degree. C. in view of the productivity
at the time of rolling, because steel is difficult to roll due to
increased deformation resistance when the heating temperature is
low.
[0058] Investigations were carried out into the finish rolling
temperature which is adequate to make the dissolved Ti harmless.
Several steel samples were prepared, with the Ti/C ratio varied for
the base composition of 0.05% C-0.55% Cu-0.05% Ti. According to the
present invention, the steel plate for coating is positively
incorporated with Ni for improvement in toughness. To see the
effect of Ni on toughness, two samples were tested, one containing
0.5% Ni and the other containing 1.0% Ni. Judging from the results
mentioned above, the heating temperature was kept low at
1050.degree. C., which is the lower limit available for the
continuous heating furnace. Several kinds of 25-mm thick steel
plates were prepared by hot rolling, followed by air cooling, with
the finish rolling temperature varied. These samples were tested
for toughness. The results are shown in FIGS. 2 and 3.
[0059] FIGS. 2 and 3 show how toughness is affected by the
difference between FRT and Ar.sub.3 and the Ti/C ratio, with the
amount of Ni kept at 1.0% or 0.5%. It is apparent from FIGS. 2 and
3 that if FRT is (Ar.sub.3+50.times.Ti/C+100.times.[Ni].sup.2)
.degree. C. or lower (in the region under the oblique line), the
desired value of vE.sub.0.gtoreq.100J is achieved. For high
toughness, FRT should preferably be 700-800.degree. C.
[0060] Case 2 in which the Ti/C ratio is 4 or less and steel
undergo quenching and tempering.
[0061] The effect of quenching and tempering temperature was
investigated to find the condition under which Ti does not form
solid solution. Several steel samples were prepared, with the Ti/C
ratio varied for the base composition of 0.05% C-0.55% Cu-0.50%
Ni-0.05% Ti. This steel was incorporated with 10 ppm of B. As in
the case mentioned above, the amount of Ni was kept at 0.5% and
1.0%. Hot rolling was carried out such that the heating temperature
is 1100.degree. C. (which is generally applied to steels for welded
structures) and the finish rolling temperature (FRT) is 850.degree.
C. Hot rolling, followed by air cooling, gave 25-mm thick steel
plates.
[0062] The thus obtained steel plates underwent quenching at varied
temperatures and tempering at 640.degree. C. (which is applied to
ordinary steels (570 N/mm.sup.2) for welded structures). Quenching
was carried out at a cooling rate of 20.degree. C./s. The resulting
samples were tested for toughness. The results are shown n FIGS. 4
and 5.
[0063] FIGS. 4 and 5 show how toughness is affected by the
difference between annealing temperature and Ac.sub.3 and the Ti/C
ratio, with the amount of Ni kept at 1.0% or 0.5%. It is apparent
from FIGS. 4 and 5 that if the quenching temperature is
(Ac.sub.3+50.times.Ti/C+100.times.[Ni].su- p.2) .degree. C. or
lower (in the region under the oblique line), the desired value of
vE.sub.0.gtoreq.100J is achieved. For high toughness, the annealing
temperature should preferably be 850-880.degree. C.
[0064] The above-mentioned explanation of the quenching temperature
is applicable to reheating-hardening. However, it is also
applicable to direct quenching if the heating temperature and FRT
are the same as those in the case where the Ti/C ratio is higher
than 4, and the desired value of vE.sub.0.gtoreq.100J is achieved
as a matter of course. Hot rolling is followed by water cooling at
a controlled cooling rate in view of the plate thickness in order
to obtain the desired strength. In the case where high toughness is
required, FRT should be 700-800.degree. C. and hot rolling should
be followed directly by quenching.
[0065] Case 3 in which the Ti/C ratio is higher than 4.
[0066] In the case where the Ti/C ratio is higher than 4, TiC
precipitates incoherently in austenite (without deteriorating
toughness), with very little coherent precipitation (which
deteriorates toughness) in ferrite. Therefore, it is basically
unnecessary to specify the heating temperature, FRT, and quenching
temperature. In the present invention, they are specified as
follows in consideration of production cost and productivity.
Heating temperature: 1200.degree. C. as the upper limit (in
consideration of fuel consumption) and 850.degree. C. as the lower
limit (in consideration of rolling productivity).
[0067] Finish rolling temperature (FRT): 950.degree. C. as the
upper limit (in consideration of strength). Improved strength needs
fine crystal particles. For high toughness, FRT should preferably
be 700-800.degree. C.
[0068] Quenching temperature: 950.degree. C. as the upper limit (in
consideration of fuel consumption), and Ac.sub.3 as the lower limit
(in consideration of strength). Hot rolling may be followed
directly by quenching. However, there may be an instance where it
is necessary to carry out quenching in the two-phase region in
order to achieve a low yield ratio.
EXAMPLE 1
[0069] The invention will be described with reference to the
following examples.
[0070] Steel sheets were prepared, each having the chemical
composition as shown in Table 1. They were painted with resin
paints as shown in Table 2. The painted film on the steel plate was
given a cross cut as shown in FIG. 6. The samples with a cross cut
(artificial coating defect) were examined for long-term durability
by means of accelerated test and atmospheric exposure test.
[0071] The painted film on the steel sheet was preceded by sand
blasting for surface preparation, and the painting was accomplished
by spraying so that a painted film thickness of 10 .mu.m was
attained. In Table 2 showing paints, B denotes butyral resin, P
denotes polyester resin, E denotes epoxy resin, U denotes urethane
resin, and F denotes fluorine resin.
[0072] The accelerated test consists of three steps of (1)
irradiation with a carbon arc lamp, (2) dipping in salt water
(0.1%, 0.5%, and 3.0%), and (3) keeping at constant temperature and
constant humidity, which are turned sequentially 60 cycles.
[0073] After the accelerated test, the samples were examined for
external appearance and corrosion spreading from the cross cut in
the painted film.
[0074] The atmospheric exposure test consists of exposing the
samples (directed southward and inclined 30.degree. to the
horizontal) to the atmosphere for one year. After the atmospheric
exposure test, the samples were examined for external appearance
and corrosion spreading from the crosscut in the painted film.
[0075] Corrosion was rated by measuring the width of corrosion
spread at eight points and expressed in terms of average.
[0076] The appearance was rated on a scale of one to ten, with one
indicating the severest damage (or corrosion on the entire surface)
and ten indicating the best appearance. The relative overall
judgment is indicated by .circleincircle., .largecircle., , and X.
The results are shown in Table 2.
[0077] It is apparent from Table 2 that the painted steel plates
according to the present invention are by far superior to the
comparative steel plates, Comparative Examples 1 to 3 are explained
below.
[0078] No. 1 represents plain steel. No. 2 represents so-called
corrosion-resistant steel. Since it contains Cr. it has widely
spread corrosion due to a lowered pH. No. 3 represents a steel
which does not contain any element (functioning like Cr) which
promotes the formation of stable rust and moderate the decrease in
pH. Hence it is poor in corrosion resistance. The results shown in
Table 2 prove the usefulness of the present invention.
1TABLE 1 Chemical composition (mass %) Steel C Si Mn P S Cu Ni Cr
Ti Al Ca Others Cu + Ni P.sub.CM Ti/C Remarks 1 0.09 0.21 1.15
0.010 0.003 0.01 0.01 0.03 -- 0.026 -- -- 0.02 0.16 -- Comparative
2 0.12 0.20 0.75 0.015 0.003 0.36 0.21 0.50 -- 0.024 -- -- 0.57
0.21 -- Comparative 3 0.11 0.22 0.66 0.021 0.004 0.34 0.23 0.02 --
0.023 -- -- 0.57 0.17 -- Comparative 4 0.11 0.22 0.66 0.021 0.024
3.50 0.80 0.01 0.080 0.024 -- -- 4.30 0.34 0.7 Comparative 5 0.15
0.25 1.40 0.010 0.007 0.35 0.22 0.02 0.050 0.030 -- -- 0.57 0.25
0.3 Comparative 6 0.05 0.35 1.46 0.007 0.002 0.54 0.31 0.03 0.030
-- -- -- 0.85 0.17 0.6 Example 7 0.04 0.35 1.46 0.007 0.002 0.54
0.31 0.03 0.070 -- -- -- 0.85 0.16 1.8 Example 8 0.02 0.35 1.65
0.010 0.007 0.55 0.30 0.03 0.110 -- -- -- 0.85 0.15 5.5 Example 9
0.01 0.20 0.52 0.010 0.007 2.23 2.50 0.03 0.050 -- -- -- 4.73 0.21
5.0 Example 10 0.01 0.25 1.60 0.010 0.007 0.35 5.53 0.03 0.051 --
-- -- 5.88 0.22 5.1 Example 11 0.02 0.35 1.65 0.010 0.007 0.55 0.30
0.03 0.070 2.05 -- -- 0.85 0.15 3.5 Example 12 0.05 0.25 1.45 0.010
0.007 0.35 0.20 0.03 0.050 0.082 0.0035 La:0.004 0.55 0.15 1.0
Example 13 0.05 0.25 1.45 0.010 0.007 0.40 0.20 0.03 0.080 --
0.0015 Ce:0.0050 0.60 0.16 1.6 Example 14 0.05 0.35 1.23 0.007
0.002 0.55 0.30 0.03 0.045 -- -- B:0.0007 0.85 0.16 0.9 Example 15
0.06 0.25 1.70 0.010 0.007 0.45 0.20 0.03 0.080 -- -- B:0.0025 0.65
0.19 1.3 Example 16 0.05 0.25 1.51 0.010 0.007 0.51 0.20 0.03 0.050
-- -- B:0.0016 0.71 0.17 1.0 Example Nb:0.012 17 0.08 0.25 1.45
0.010 0.007 0.55 0.20 0.03 0.050 -- -- V:0.053 0.75 0.20 0.6
Example Mo:0.20 18 0.11 0.25 1.45 0.010 0.007 0.35 0.22 0.03 0.050
-- 0.0025 B:0.0008 0.57 0.22 0.5 Example Mo:0.012 19 0.05 0.25 1.45
0.010 0.007 0.50 0.20 0.03 0.050 0.105 0.0035 Nb:0.03 0.70 0.16 1.0
Example V:0.035 Zr:0.013
[0079]
2 TABLE 2 Accelerated test Accelerated test Accelerated test
Atmospheric (0.1% salt water) (0.5% salt water) (3.0% salt water)
exposure test Appear- Corrosion Appear- Corrosion Appear- Corrosion
Appear- Corrosion Over- ance spread Rat- ance spread Rat- ance
spread Rat- ance spread Rat- all Steel Paint (RN) (mm) ing (RN)
(mm) ing (RN) (mm) ing (RN) (mm) ing rating Remarks 1 B 3 1.48 X 6
0.53 X X Comparative 2 B 4 1.64 X 7 0.44 X X Comparative 3 B 2 2.02
X 7 0.52 X X Comparative 4 B -- -- -- -- -- -- -- -- X Comparative
5 B 10 <0.50 .circleincircle. 8 0.70 .smallcircle. 7 0.67
.DELTA. 9 0.24 .circleincircle. .smallcircle. Comparative 6 B 9
<0.51 .circleincircle. 7 0.84 .DELTA. 9 0.26 .smallcircle.
.smallcircle. Example 7-1 B 10 <0.50 .circleincircle. 10 0.61
.circleincircle. 9 0.55 .circleincircle. 10 0.23 .circleincircle.
.circleincircle. Example 7-2 P 10 <0.50 .circleincircle. 8 0.66
.smallcircle. 9 0.22 .circleincircle. .circleincircle. Example 7-3
E 10 <0.50 .circleincircle. 8 0.68 .smallcircle. 9 0.23
.circleincircle. .circleincircle. Example 7-4 U 10 <0.50
.circleincircle. 8 0.64 .smallcircle. 9 0.20 .circleincircle.
.circleincircle. Example 7-5 F 10 <0.50 .circleincircle. 8 0.66
.smallcircle. 9 0.21 .circleincircle. .circleincircle. Example 8 B
10 <0.50 .circleincircle. 10 0.61 .circleincircle. 9 0.55
.circleincircle. 10 0.23 .circleincircle. .circleincircle. Example
9 B 10 <0.50 .circleincircle. 10 0.51 .circleincircle. 10 0.18
.circleincircle. .circleincircle. Example 10 B 10 <0.50
.circleincircle. 10 0.50 .circleincircle. 10 0.18 .circleincircle.
.circleincircle. Example 11 B 10 <0.50 .circleincircle. 8 0.64
.smallcircle. 8 0.60 .DELTA. 9 0.20 .smallcircle. .smallcircle.
Example 12 B 10 <0.50 .circleincircle. 8 0.68 .smallcircle. 7
0.62 .DELTA. 10 0.21 .smallcircle. .smallcircle. Example 13 B 10
<0.50 .circleincircle. 10 0.53 .circleincircle. 10 0.54
.circleincircle. 10 0.19 .circleincircle. .circleincircle. Example
14 B 10 <0.50 .circleincircle. 10 0.60 .circleincircle. 9 0.55
.circleincircle. 10 0.20 .circleincircle. .circleincircle. Example
15 B 10 <0.50 .circleincircle. 10 0.62 .circleincircle. 9 0.56
.circleincircle. 10 0.21 .circleincircle. .circleincircle. Example
16 B 10 <0.50 .circleincircle. 10 0.62 .circleincircle. 8 0.60
.smallcircle. 10 0.24 .circleincircle. .circleincircle. Example 17
B 10 <0.50 .circleincircle. 10 0.62 .circleincircle. 8 0.60
.smallcircle. 10 0.24 .circleincircle. .circleincircle. Example 18
B 11 <0.51 .circleincircle. 10 0.64 .circleincircle. 9 0.55
.smallcircle. 10 0.20 .circleincircle. .circleincircle. Example 19
B 12 <0.52 .circleincircle. 10 0.56 .circleincircle. 10 0.50
.circleincircle. 10 0.16 .circleincircle. .circleincircle.
Example
EXAMPLE 2
[0080] Steel billets were prepared, each having the chemical
composition as shown in Table 1. They were made into steel plates
(25-80 mm thick) under the conditions shown in Table 3. The
resulting steel plates were tested for tensile strength,
low-temperature toughness, preheating temperature to prevent weld
crack (according to JIS Z-3158), and toughness of the heat affected
zone. The results are shown in Table 3. For the last item mentioned
above, a weld joint was made by electro-gas arc welding (with heat
input of 120 kJ/cm). Toughness was measured at three points: one at
the bond (boundary between the welded metal and the base metal),
one 1 mm from the bond toward the base metal, and one 3 mm from the
bond toward the base metal. The lowest value of three measurements
was accepted.
[0081] Sample No. 5 (as comparative example) has a high value of
P.sub.CM and hence has a preheating temperature to prevent weld
cracking which is as high as 100.degree. C. In addition, it has a
low value of toughness (20 J) at the part affected by welding
heat.
[0082] Sample No. 7-6 (as comparative example) has a heating
temperature which is higher than that specified in the present
invention, Sample No. 7-7 (as comparative example) has a finish
rolling temperature which is higher than that specified in the
present invention. Therefore, they do not meet the requirement that
the base metal should have a value of toughness greater than 100 J
(their values are 60 J and 80 J, respectively). Samples Nos. 8-1 ad
8-2 (as comparative examples) have the Ti/C ratio exceeding 4. The
former has a heating temperature which is higher than that
specified in the present invention. The latter has a finish rolling
temperature which is higher than that specified in the present
invention. Therefore, they do not meet the requirement that the
base metal should have a value of toughness greater than 100 J
(their values are 85 J and 76 J, respectively).
[0083] Sample No. 15-1 (as comparative example) has the Ti/C ratio
exceeding 4. It has a quenching temperature which is higher than
that specified in the present invention. Therefore, its base metal
has a value of toughness lower than 80 J.
[0084] Examples according to the present invention are superior in
base metal characteristics, preheating temperature to prevent weld
crack, and toughness of the heat affected zone, regardless of
whether the Ti/C ratio is higher than 4 or lower than 4, as shown
in Table 2.
[0085] Samples Nos. 15-2 and 19 (as examples) were obtained by hot
rolling which was followed by direct quenching. They gave the same
results as obtained in the case where reheating quenching was
carried out according to the present invention,
3TABLE 3 Hardening Plate Ar.sub.3 + 50 .times. Ac.sub.3 + 50
.times. Heating Finish rolling temperature Annealing thickness 1200
- 50 .times. Ti/C + 100 .times. Ti/C + 100 .times. temperature
temperature Cooling (.degree. C.) (DQ: temperature Steel Ti/C (mm)
Ti/C (.degree. C.) Ni.sup.2 (.degree. C.) Ni.sup.2 (.degree. C.)
(.degree. C.) (.degree. C.) method direct quenching) (.degree. C.)
5 0.3 25 1185 758 853 1100 800 Air cooling 6 0.6 25 1170 795 893
1050 780 Air cooling 7 1.8 25 1110 858 955 1050 780 Water cooling
7-6 1.8 25 1110 858 955 1200 800 Water cooling 7-7 1.6 25 1110 858
955 1050 1000 Water cooling 7-8 1.8 50 1110 858 955 1050 780 Water
cooling 8 5.5 25 -- Ar.sub.3:750 Ac.sub.3:855 1100 900 Water
cooling 8-1 5.5 25 -- Ar.sub.3:750 Ac.sub.3:855 1250 910 Water
cooling 8-2 5.5 25 -- Ar.sub.3:750 Ac.sub.3:855 1100 1000 Water
cooling 9 5.0 25 -- Ar.sub.3:689 Ac.sub.3:841 1100 900 Air cooling
10 5.1 25 -- Ar.sub.3:473 Ac.sub.3:735 1100 880 Air cooling 11 3.5
25 1025 934 1039 1000 880 Water cooling 12 1.0 25 1150 820 908 1100
800 Water cooling 13 1.6 25 1120 849 938 1100 820 Water cooling 14
0.9 25 1155 828 861 1050 780 Air cooling 15 1.3 25 1135 811 913
1050 760 Air cooling 880 640 15-1 1.3 25 1135 811 913 1050 760 Air
cooling 930 640 15-2 1.3 80 1135 811 913 1050 760 Water cooling DQ
640 16 1.0 25 1150 813 907 1050 760 Air cooling 880 640 17 0.6 25
1170 789 866 1050 760 Air cooling 870 640 18 0.5 25 1175 776 870
1050 760 Air cooling 860 640 19 1.0 50 1150 817 910 950 760 Water
cooling DQ 640 Base metal characteristics Yield Yield Preheating
temperature to Charpy V-notch strength strength protect weld crack
(.degree. C.) impact Steel (N/mm.sup.2) (N/mm.sup.2) VE.sub.0(J)
(RT: room temperature) properties vEo(J) Remarks 5 453 555 100 20
Comparative 6 435 510 >300 <RT 110 Example 7 476 573 >300
<RT 100 Example 7-6 460 585 60 <RT Comparative 7-7 453 603 80
<RT Comparative 7-8 456 563 >300 <RT 110 Example 8 335 466
>300 <RT 110 Example 8-1 355 480 85 <RT Comparative 8-2
363 503 76 <RT Comparative 9 430 520 >300 <RT 110 Example
10 435 534 >300 <RT 115 Example 11 441 598 >300 <RT 110
Example 12 445 536 >300 <RT 120 Example 13 437 533 >300
<RT 120 Example 14 450 515 >300 <RT 185 Example 15 556 628
>300 <RT 170 Example 15-1 568 645 80 <RT Comparative 15-2
528 625 >300 <RT 150 Example 16 560 633 >300 <RT 150
Example 17 550 628 >300 <RT 115 Example 18 563 635 >300
<RT 155 Example 19 551 635 >300 <RT 120 Example
* * * * *