U.S. patent application number 12/810852 was filed with the patent office on 2011-01-27 for high strength thin steel sheet excelling in weldability and process for producing the same.
This patent application is currently assigned to POSCO. Invention is credited to Kwang Geun Chin, Hee Jae Kang, Jong Sang Kim, Jin Keun Oh.
Application Number | 20110017363 12/810852 |
Document ID | / |
Family ID | 40824486 |
Filed Date | 2011-01-27 |
United States Patent
Application |
20110017363 |
Kind Code |
A1 |
Kang; Hee Jae ; et
al. |
January 27, 2011 |
HIGH STRENGTH THIN STEEL SHEET EXCELLING IN WELDABILITY AND PROCESS
FOR PRODUCING THE SAME
Abstract
Provided are a high strength thin steel sheet having tensile
strength of about 800 MPa or more, and a manufacturing method
thereof. The thin steel sheet is mainly used for construction
materials, home appliances, and automobiles. The thin steel sheet
has excellent plating characteristic, welding characteristic,
bending workability, and hole expansion ratio. The thin steel sheet
includes, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn:
1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, SoLAl: 0.01-0.40%, N:
0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%,
and including at least one material selected from the group
consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo:
0.003-0.08%, and includes Fe and other inevitable impurities as a
remainder. Here, Si, Mn, B, Sb, P, and S meet conditions of
5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27. Also,
the manufacturing method can secure workability of the thin steel
sheet.
Inventors: |
Kang; Hee Jae; (Gwangyang,
KR) ; Oh; Jin Keun; (Gwangyang, KR) ; Chin;
Kwang Geun; (Gwangyang, KR) ; Kim; Jong Sang;
(Gwangyang, KR) |
Correspondence
Address: |
THE WEBB LAW FIRM, P.C.
700 KOPPERS BUILDING, 436 SEVENTH AVENUE
PITTSBURGH
PA
15219
US
|
Assignee: |
POSCO
Pohang
KR
|
Family ID: |
40824486 |
Appl. No.: |
12/810852 |
Filed: |
August 8, 2008 |
PCT Filed: |
August 8, 2008 |
PCT NO: |
PCT/KR2008/004627 |
371 Date: |
October 12, 2010 |
Current U.S.
Class: |
148/534 ;
148/330; 148/337; 148/653 |
Current CPC
Class: |
C22C 38/32 20130101;
C22C 38/06 20130101; C22C 38/38 20130101; C22C 38/60 20130101 |
Class at
Publication: |
148/534 ;
148/653; 148/330; 148/337 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/12 20060101 C22C038/12; C22C 38/14 20060101
C22C038/14 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2007 |
KR |
10-2007-0140446 |
Claims
1. A high strength thin steel sheet having an excellent welding
characteristic, the thin steel sheet comprising: in weight %, C:
0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S:
0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr:
0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at
least one material selected from the group consisting of Ti:
0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe
and other inevitable impurities as a remainder, wherein Si, Mn, B,
Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and
C+Mn/20+Si/30+2P+4S<0.27.
2. The thin steel sheet of claim 1, wherein the steel sheet has a
structure comprising at least one selected from the group
consisting of bainite and bainitic ferrite occupying about 40% or
more, and ferrite and martensite phases occupying the
remainder.
3. The thin steel sheet of claim 1, wherein the steel sheet
comprises a hot-dip galvanized (GI) layer or a galvannealed (GA)
layer on a surface of the steel sheet.
4. A method for manufacturing a high strength thin steel sheet
having an excellent welding characteristic, the method comprising:
reheating a slab of the steel sheet comprising, in weight %, C:
0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S:
0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr:
0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at
least one material selected from the group consisting of Ti:
0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe
and other inevitable impurities as a remainder, wherein Si, Mn, B,
Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and
C+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at a
temperature of a finish rolling exit side between the Ar.sub.3
transformation point and 950.degree. C.; pickling a wound hot
rolled steel sheet and performing cold rolling on the same at a
reduction ratio of 40-80%; and performing continuous annealing on
an obtained cold rolled steel sheet at a temperature range of
740-860.degree. C., cooling the cold rolled steel sheet down to
250-600.degree. C. at a cooling rate satisfying a condition of -5
Log CR+25C-17Si+40Cr+13,000B>30 in a cooling rate range of
3-150.degree. C./s, and cooling the same at a cooling rate of
5.degree. C./minute or more.
5. The method of claim 4, wherein the steel sheet has a structure
comprising at least one selected from the group consisting of
bainite and bainitic ferrite occupying 40% or more, and ferrite and
martensite phases occupying the remainder.
6. The method of claim 4, further comprising performing hot-dip
galvanizing (GI) or galvannealing (GA).
Description
TECHNICAL FIELD
[0001] The present invention relates to a high strength thin steel
sheet having a tensile strength of about 800 MPa or more, mainly
used for construction materials, home appliances, and automobiles,
and a manufacturing method thereof, and more particularly, to a
high strength thin steel sheet having excellent plating
characteristic, welding characteristic, bending workability, and
hole expansion ratio (HER) as well as high tensile strength, and a
manufacturing method thereof.
BACKGROUND ART
[0002] Recently, a steel sheet for automobiles has required even
higher strength to improve fuel economy or durability. A high
strength steel sheet having a high strength of about 800 MPa is
increasingly used for a car's body structures or a reinforcing
material in aspects of collision safety and passenger protection.
However, since the high strength of a steel sheet causes a
reduction in moldability and a welding characteristic, the
development of a material for complementing this problem is highly
required. In response to this requirement, steel sheets of various
composite structures such as ferrite-martensite dual phase steel or
transformation-induced plasticity (TRIP) steel sheet using
transformation-induced plasticity of retained austenite have been
developed up to now.
[0003] For example, Japanese Laid-Open Patent Publication No.
6-145892 proposes a method for manufacturing a steel sheet having
excellent moldability by controlling chemical components and an
amount of retained austenite. Japanese Patent No. 2660644 and
Japanese Patent No. 2704350 propose a method for manufacturing a
high strength steel sheet having press moldability by controlling
chemical components and fine structures of the steel sheet. Also,
Japanese Patent No. 3317303 proposes a steel sheet including
retained austenite of 5% or more and having an excellent
moldability, particularly, excellent local elongation. However,
most of the above-described related arts have been developed to
improve ductility. Sufficient considerations of bending
workability, a hole expansion ratio, welding characteristic, etc.,
which are important standards during actual part processing, have
not been made.
[0004] Among the required characteristics of a steel sheet, a most
crucial characteristic of a steel sheet used for a car's body
structure or a reinforcing material mainly requiring a steel sheet
of high strength of 800 MPa or more is a spot welding
characteristic. The steel used for a car's body structure or a
reinforcing material protects passengers by absorbing collision
energy during a collision. If the strength of a spot welded portion
is not sufficient, the portion will be destroyed and cut, so that a
sufficient level of collision energy absorption cannot be obtained.
For technology regarding high strength steel sheet with
consideration of a welding characteristic, there exists Japanese
Laid-Open Patent Publication No. 2003-193194, but it does not meet
a welding characteristic actually required by the market.
[0005] Also, Japanese Laid-Open Patent Publication No. 2005-105367
proposes technology of securing a welding characteristic and
ductility for steel of 780 MPa or more. In the case of
manufacturing a steel sheet having a high strength of 800 MPa or
more in a real process, a cold rolling characteristic is remarkably
reduced due to the high strength of a hot strip, which is an
intermediate material. Also, since a rapid cooling heat treatment
condition should be applied during an annealing process,
workability is also remarkably reduced. Japanese Laid-Open Patent
Publication No. 2005-105367 has no sufficient consideration of
these problems.
DISCLOSURE OF INVENTION
Technical Problem
[0006] The present invention has been made to solve the foregoing
problems with the prior art, and therefore an object of the present
invention is to provide a steel sheet having excellent plating
characteristic, welding characteristic, bending workability, and
hole expansion ratio in manufacturing a thin steel sheet having
high tensile strength of 800 MPa or more. Also, another object of
the present invention is to provide a method of securing
workability of a steel sheet.
Technical Solution
[0007] According to an aspect of the present invention, there is
provided a steel sheet including, in weight %, C: 0.02-0.20%, Si:
1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less,
Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B:
0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one
material selected from the group consisting of Ti: 0.003-0.08%, Nb:
0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other
inevitable impurities as a remainder, wherein Si, Mn, B, Sb, P, and
S meet conditions of 5<(Si/Mn+150B)/Sb<20 and
C+Mn/20+Si/30+2P+4S<0.27.
[0008] According to another aspect of the present invention, there
is provided a method for manufacturing a steel sheet, the method
including: reheating a slab of the steel sheet including, in weight
%, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%,
S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr:
0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at
least one material selected from the group consisting of Ti:
0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe
and other inevitable impurities as a remainder, wherein Si, Mn, B,
Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and
C+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at a
temperature of a finish rolling exit side between the Ar.sub.3
transformation point and 950.degree. C.; pickling a wound hot
rolled steel sheet and performing cold rolling on the same at a
reduction ratio of 40-80%; and performing continuous annealing on
an obtained cold rolled steel sheet at a temperature range of
740-860.degree. C., cooling the cold rolled steel sheet down to
250-600.degree. C. at a cooling rate satisfying a condition of -5
Log CR+25C-17Si+40Cr+13,000B>30 in a cooling rate range of
3-150.degree. C./s, and cooling the same at a cooling rate of
5.degree. C./minute or more.
[0009] The steel sheet may have a structure including at least one
selected from the group consisting of bainite and bainitic ferrite
occupying 40% or more, and ferrite and martensite phases occupying
the remainder.
ADVANTAGEOUS EFFECTS
[0010] The present invention can provide a steel sheet having
excellent plating characteristic, welding characteristic, bending
workability, and hole expansion ratio while having high a tensile
strength of about 800 MPa or more, and a manufacturing method
thereof that can secure the manufacturability of the steel
sheet.
BEST MODE FOR CARRYING OUT THE INVENTION
[0011] A steel sheet includes, in weight %, C: 0.02-0.20%, Si: 1.5%
or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al:
0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb:
0.001-0.10%, and including at least one material selected from the
group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo:
0.003-0.08%, and including Fe and other inevitable impurities as
the remainder, and Si, Mn, B, Sb, P, and S meet conditions of
5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27.
[0012] Also, a method for manufacturing the steel sheet includes:
reheating a slab of the steel sheet including, in weight %, C:
0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S:
0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr:
0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at
least one material selected from the group consisting of Ti:
0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe
and other inevitable impurities as the remainder, wherein Si, Mn,
B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and
C+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at a
temperature of a finish rolling exit side between Ar.sub.3
transformation point and 950.degree. C.; pickling a wound hot
rolled steel sheet and performing cold rolling on the same at a
reduction ratio of 40-80%; and performing continuous annealing on
an obtained cold rolled steel sheet at a temperature range of
740-860.degree. C., cooling the cold rolled steel sheet down to
250-600.degree. C. at a cooling rate satisfying a condition of -5
Log CR+25C-17Si+40Cr+13,000B>30 in a cooling rate range of
3-150.degree. C./s, and cooling the same at a cooling rate of
5.degree. C./minute or more.
[0013] The steel sheet includes at least one selected from the
group consisting of bainite and bainitic ferrite occupying 40% or
more, and ferrite and martensite occupying the remainder.
[0014] Hereinafter, the present invention will be described in
detail.
[0015] Carbon (C) is preferably in a weight % of 0.02-0.20 weight %
(hereinafter simply referred to as %).
[0016] Carbon in steel is an element added in order to strengthen a
transformation structure. However, when an amount of C exceeds
0.20%, a hole extension characteristic and a welding characteristic
is reduced. On the other hand, when an amount of C is less than
0.02%, it is difficult to secure strength.
[0017] Silicon (Si) is preferably in 1.5% or less.
[0018] Silicon in steel is an element that can be effectively used
in order to improve strength. However, since silicon not only
causes surface scale defects but also reduces the surface
characteristic of a plated steel sheet in connection with a surface
characteristic. Also, silicon reduces a chemical treatment
characteristic. Therefore, generally, silicon content is limited to
1.0% or less. Since recent progress in plating technology allows
silicon content in steel up to 1.5% without a great problem during
a manufacturing process, the content is limited to 1.5% or
less.
[0019] Mn is preferably in 1.5-3.0%.
[0020] Mn in steel is an element having a very high solid-solution
strengthening effect and simultaneously, facilitates the formation
of a composite structure including ferrite and martensite. When Mn
content in steel is less than 1.5%, it is difficult to secure the
strength required by the present invention. When Mn content exceeds
3.0%, there is a high possibility that problems in a welding
characteristic and a hot rolling characteristic will occur.
[0021] P is preferably in 0.001-0.10%.
[0022] P in steel is an element having an effect of strengthening
the steel. When P content is less than 0.001%, not only can the
strengthening effect can be secured but a problem in manufacturing
costs may also be generated. On the other hand, when P content is
excessively added, press moldability may reduce and brittleness of
steel may occur.
[0023] S is preferably in 0.010% or less.
[0024] S in steel is an impurity element, hindering the ductility
and welding characteristic of a steel sheet. When S content in
steel exceeds 0.01%, there is a high possibility of hindering the
ductility and welding characteristic of a steel sheet.
[0025] Sol.Al is preferably in 0.01-0.4%.
[0026] Sol.Al in steel is an effective element to combine with
oxygen in steel to perform a deoxidation operation, distribute
carbon inside ferrite to austenite to improve martensite hardening
ability. When Sol.Al content is less than 0.01%, such an effect
cannot be secured. On the other hand, when Sol.Al content exceeds
0.4%, such an effect is saturated and manufacturing costs may
increase.
[0027] N is preferably 0.020% or less.
[0028] N in steel is an element that effectively stabilizes
austenite. When N content in steel exceeds 0.020%, the stability of
austenite greatly increases to prevent the formation of bainite,
which is a fine structure intended by the present invention.
[0029] Cr is preferably in 0.3-1.5%.
[0030] Cr in steel is an element added to improve the hardening
ability of steel and to secure high strength. In the present
invention, Cr plays an important role of facilitating the formation
of bainite. When Cr content in steel is less than 0.3%, such an
effect is difficult to secure. When Cr content in steel exceeds
1.50%, such an effect is saturated and is disadvantageous
economically.
[0031] Boron (B) is preferably in 0.0010-0.0060%.
[0032] Boron in steel is an element used to delay the
transformation of austenite into pearlite during a cooling process
of an annealing process. B is added as an element of suppressing
forming of ferrite and facilitating forming of bainite. However,
when B content in steel is less than 0.0010%, such an effect is
difficult to obtain. When B content in steel exceeds 0.0060%,
excessive B is inspissated on a surface to cause deterioration in
plating adhesion.
[0033] Sb is preferably 0.001-0.1%.
[0034] Sb in steel is an indispensable element added in order to
secure an excellent plating characteristic in the present
invention. Sb has an outstanding effect in suppressing surface
inspissation of oxides such as MnO, SiO.sub.2, Al.sub.2O.sub.3,
etc. to reduce surface defects, and suppressing coarsening of
surface inspissation materials by temperature rise and a change in
a hot rolling process. When Sb content is less than 0.001%, such an
effect is difficult to secure, and even when an added amount
continuously increases, such an effect does not increase greatly
and problems of manufacturing costs and moldability reduction may
be generated. Therefore, Sb content is limited to 0.001-0.1%.
[0035] According to the present invention, one or two or more
materials selected from Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo:
0.003-0.08% are added to the steel formed of the above elements to
achieve a strength increase and miniaturization of grain
diameters.
[0036] When an added amount of Ti, Nb, and Mo is less than 0.003%
in its lower limit, an effect of achieving a strength increase and
miniaturization of grain diameters is difficult to secure. When an
added amount exceeds 0.08% in its upper limit, manufacturing costs
may be increased and ductility may be remarkably reduced due to
excessive eduction materials.
[0037] The steel of the present invention is formed with Fe and
other inevitable impurities as the remainder besides the
above-described elements.
[0038] According to the present invention, an alloy constituent
ratio of Si, Mn, B, Sb, P, and S may satisfy the following Math
Figures 1 and 2 in designing an alloy of a steel sheet having the
above-described component ranges.
MathFigure1
5<(Si/Mn+150B)/Sb<20 [Math.1]
MathFigure2
C+Mn/20+Si/30+2P+4S<0.27 [Math.2]
[0039] Math Figure 1 is a component relation that can secure
surface quality, obtained as an empirical numerical value. That is,
Mn, Si, and B in steel are elements having a characteristic of
forming inspissation materials on a surface during an annealing
process. As inspissation materials of these elements increase, a
plating characteristic is reduced. On the other hand, since Sb
hinders a grain boundary diffusion of the above surface
inspissation elements, Sb is very advantageous in an aspect of
surface quality. For example, when a value calculated by Math
Figure 1 is between 5 and 20, it means that a good surface quality
can be secured.
[0040] Meanwhile, Math Figure 2 is a component relation that can
secure a desirable welding characteristic, obtained as an empirical
numerical value. That is, C, Mn, Si, P, and S in steel raise a
carbon equivalent. As well known in the art, when a carbon
equivalent is high, a welding characteristic is reduced. Setting a
condition by which a welding defect is not generated during spot
welding, which is a welding method primarily performed when steel
of the present invention is used through repeated experiments
provides Math Figure 2. When a value calculated using Math Figure 2
exceeds 0.27, it means that there is a high possibility that a
welding defect may be generated.
[0041] A steel sheet of the present invention has a structure in
which one or more selected from bainite and bainitic ferrite occupy
40% or more, and ferrite and martensite phases occupy the
remainder. Ferrite and martensite may occupy 25% or less and 35% or
less, respectively.
[0042] Hereinafter, a method for manufacturing steel sheet formed
of the above components using a cold rolled steel sheet will be
described in detail.
[0043] A slab whose components have been formed using the
above-described alloy designing method is reheated and hot rolling
is performed. Finish rolling in the hot rolling may be performed at
a temperature of an exit side between the Ar.sub.3 transformation
point and 950.degree. C. That is, at a hot finish rolling
temperature below the Ar.sub.3 transformation point, there is a
high possibility that hot transformation resistance rapidly
increases, and a problem in manufacturing may be generated. At a
temperature exceeding 950.degree. C., not only may excessively
thick oxidation scales occur, but there is also high possibility
that a steel sheet may be coarsened.
[0044] A hot rolled steel sheet manufactured using the above
process is pickled and cold-rolled.
[0045] A reduction ratio of the cold rolling may be 40-80%. When a
reduction ratio is less than 40%, recrystallization driving force
is weakened, so that there is possibility that a problem may be
generated in obtaining good recrystalline grain. When a reduction
ratio exceeds 80%, a rolling load increases rapidly.
[0046] The above obtained cold rolled steel sheet is continuously
annealed at a temperature of preferably 740-860.degree. C. When
temperature is less than 740.degree. C. during continuous
annealing, a danger that non-recrystallization grain is formed
increases. When temperature exceeds 860.degree. C., a large grain
may be formed and simultaneously, a strip passing ability may be
defective due to a high temperature annealing operation.
[0047] After the continuous annealing, the cold rolled steel sheet
is continuously cooled down to a temperature of 250-600.degree. C.
at a cooling rate allowing a value calculated by following Math
Figure 3 to exceed 30 within the cooling rate (CR) of 3-150.degree.
C./s, and then is gradually cooled down at a cooling rate of
5.degree. C./min. or more. A high strength thin steel sheet having
tensile strength of 800 MPa and having good plating characteristic,
welding characteristic, and hole expansion ratio can be easily
manufactured by continuously annealing the thin steel sheet under
the above condition.
MathFigure3
5 Log CR+25C-17S+40Cr+13,000B>30 [Math.3]
[0048] where CR is a cooling rate.
[0049] When a cooling rate is lowered to less than 3.degree. C./s
after the continuous annealing, ferrite or pearlite is formed, so
that strength intended by the present invention is difficult to
secure. Also, if the cooling rate is too higher than 150.degree.
C./s, hard phase of martensite, etc. is excessively formed, so that
bending workability and a hole expansion ratio is greatly reduced,
and also reduction in a strip passing ability due to a shape defect
during a process is greatly worried. Therefore, cooling may be
performed in the cooling rate (CR) of 3-150.degree. C./s as
described above.
[0050] Also, to accomplish excellent bending workability and hole
expansion ratio, which are the characteristics of steel according
to the present invention, a cooling rate allowing a value
calculated by Math Figure 3 to exceed 30 should be applied. That
is, when a value calculated by Math Figure 3 is less than 30,
bainite or bainitic ferrite phase, in which the steel of the
present invention intends to obtain as its fine structure, is
difficult to obtain by as much as 40% or more. When the
bainite-based structure is obtained by as much as 40% or more, a
product having excellent bending workability and hole expansion
ratio while having high strength of about 800 MPa, which are the
characteristics of the steel according to the present invention,
can be manufactured.
[0051] Meanwhile, a cooling final temperature for a cooling
operation may be a temperature between 250 and 600.degree. C. When
a cooling final temperature is less than 250.degree. C., a danger
that a large amount of martensite will be formed increases. When a
cooling final temperature exceed 600.degree. C., a large amount of
soft phases of ferrite or pearlite, etc., are formed, so that an
intended material is difficult to accomplish.
[0052] The above-described manufacturing method can be likewise
applied to a plated product such as a hot-dip galvanized material
(GI) and a galvannealed material (GA) as well as a cold rolled
steel sheet.
MODE FOR THE INVENTION
[0053] Hereinafter, the present invention is described in more
detail using an embodiment thereof.
[0054] As illustrated in Table 1, a slab having the component
composition of the present invention is heated to a temperature of
1200.degree. C. and extracted, and then rolling is performed at a
cold reduction ratio of 55% using, as a material, a hot rolled
steel sheet manufactured by hot-rolling the slab under a condition
of a finish rolling temperature of 900.degree. C. Continuous
annealing heat treatment is performed (CR) at the annealing
temperature and cooling condition of Table 2. A plated product is
manufactured by performing hot-dip galvanizing (GI) and
galvannealing (GA) processes. Conditions and galvannealing process
time applied during continuous annealing are given below. [0055]
Annealing furnace atmosphere: N.sub.2-10% H.sub.2O (dew point
-32.degree. C.) [0056] Annealing furnace heating rate: 3.degree.
C./sec [0057] Annealing time: 90 sec [0058] Plating temperature:
460.degree. C. [0059] Galvannealing time: 24 sec (in case of GA
product)
[0060] As illustrated in Table 2, plating characteristics
(appearance and adhesion characteristic) and the quality of a
material (tensile strength, hole expansion ratio, and bending
workability) are measured and results thereof are shown together
with a comparison material.
[0061] In Table 2, a plated appearance is represented by
non-plating or .largecircle. for a case not including other plating
defects. A defect name is written for a case where a plating defect
is generated.
[0062] In Table 2, a plating adhesion appraisal has been made in
the following way, in which: a plated sheet is cut off by 20
mm.times.50 mm, a bending test is performed on the plated sheet,
the plated sheet is unfolded again, a tape is attached on the
folded portion of the plated sheet, and the width of a plated layer
detached from the plated sheet is appraised using the following
criteria.
[0063] .circleincircle.: No detached plating or width of detached
plating is within about 1 mm
[0064] .largecircle.: Width of detached plating is within about 1-3
mm
[0065] .DELTA.: Width of detached plating is within about 3-5
mm
[0066] X: Width of detached plating is about 5 mm or more
[0067] In Table 2, a hole expansion ratio (HER) is obtained by
making a hole having a diameter of 10 mm in a test piece having a
size of 120.times.120 mm, expanding the hole using a punch having a
forming portion angle of 60 degrees until a crack is generated, and
calculating a ratio of an expanded hole to the initial hole of 10
mm in diameter. Also, in Table 2, bending workability has been
appraised by performing a bending test on a test piece using a 90
degree V-shaped punch, and measuring a smallest punch radius (mm)
that does not cause breakage.
TABLE-US-00001 TABLE 1 Math Math FIG. 1 FIG. 2 Steel No. C Si Mn P
S Al N Ti Nb Mo Cr B Sb value value 1 0.06 0.1 2.5 0.01 0.004 0.035
0.005 0.02 0.05 0.03 0.9 0.0018 0.02 15.5 0.22 2 0.07 0.15 2.2
0.015 0.003 0.05 0.004 0.025 0.055 0.05 0.7 0.0023 0.03 13.8 0.23 3
0.05 0.05 2.1 0.007 0.003 0.22 0.003 0.015 0.045 0.01 0.5 0.0013
0.02 10.9 0.18 4 0.03 0.1 2.5 0.008 0.004 0.043 0.005 -- 0.06 --
0.7 0.0019 0.04 8.1 0.19 5 0.15 0.1 2.7 0.005 0.003 0.052 0.003
0.04 -- -- 1.0 0.0021 0.03 11.7 0.31 6 0.08 0.5 2.1 0.009 0.003
0.35 0.007 0.02 0.03 -- 0.8 0.0032 0.04 18.0 0.23 7 0.07 0.20 2.1
0.012 0.003 0.04 0.003 -- -- 0.04 0.9 0.0017 0.02 17.5 0.22 8 0.15
0.2 2.7 0.015 0.008 0.043 0.005 -- -- -- -- 0.0012 -- -- 0.35 9
0.11 0.3 2.4 0.015 0.008 0.043 0.005 -- -- 0.04 -- -- -- -- 0.30 10
0.18 0.2 1.8 0.011 0.005 0.038 0.004 -- 0.05 -- -- -- -- -- 0.32
Math FIG. 1 = (Si/Mn + 150B)/SbMath FIG. 2 = C + Mn/20 + Si/30 + 2P
+ 4S When a value calculated by Math FIG. 1 is betwwen 5-20, and a
value calculated by Math FIG. 2 is less than 0.27, an alloy design
condition of the present invention is satisfied.
TABLE-US-00002 TABLE 2 Continuous Hole annealing Cooling Math
Tensile expansion Bending Steel temperature rate FIG. 3 Plated
Plating strength ratio workability Bainite No. Product (.degree.
C.) (.degree. C./sec) value appearance adhesion (MPa) (%) (mm)
ratio Remark 1 CR 840 8 54.7 -- -- 1045 39 0 R 55 Steel of the 2 GA
830 20 50.6 .largecircle. .circleincircle. 995 45 0 R 60 present
invention 3 GA 855 20 30.8 .largecircle. .circleincircle. 830 68 0
R 45 4 GI 860 30 44.4 .largecircle. .circleincircle. 874 54 0 R 65
5 GA 810 20 62.8 .largecircle. .circleincircle. 1076 48 0 R 60 6 CR
820 10 62.1 -- -- 1012 60 0 R 55 7 GA 820 20 49.9 -- -- 982 54 0 R
45 8 GA 810 20 -- Non-plated X 1087 12 2 R 15 Steel of comparison 9
GA 810 20 -- .largecircle. .DELTA. 990 8 2 R 10 10 GA 810 20 --
Non-plated .DELTA. 1040 7 2 R 10 Math FIG. 3 = -5LogCR + 25C - 17Si
+ 40Cr + 13,000B*A value calculated by Math FIG. 3 is 30 or more,
the manufacturing condition of the present invention is
satisfied.
[0068] As shown in Table 2, when a steel sheet is manufactured
according to the method of the present invention, a high strength
thin steel sheet having tensile strength of about 800 MPa or more,
having excellent surface characteristic and mechanical
characteristic compared to an existing comparison material, and
having excellent plating characteristic, welding characteristic,
bending workability, and hole expansion ratio can be
manufactured.
[0069] According to the steel of the present invention, the steel
sheet has a structure in which one selected from bainite and
bainitic ferrite occupies 40% or more, and ferrite and martensite
occupy 25% or less and 35% or less, respectively.
* * * * *