U.S. patent application number 16/079722 was filed with the patent office on 2019-03-07 for high-strength cold rolled steel sheet with excellent yield strength and ductility, coated steel plate, and method for manufacturing same.
The applicant listed for this patent is POSCO. Invention is credited to Kyoo-Young LEE, Sea-Woong LEE, Won-Hwi LEE, Joo-Hyun RYU.
Application Number | 20190071745 16/079722 |
Document ID | / |
Family ID | 59966126 |
Filed Date | 2019-03-07 |
United States Patent
Application |
20190071745 |
Kind Code |
A1 |
LEE; Kyoo-Young ; et
al. |
March 7, 2019 |
HIGH-STRENGTH COLD ROLLED STEEL SHEET WITH EXCELLENT YIELD STRENGTH
AND DUCTILITY, COATED STEEL PLATE, AND METHOD FOR MANUFACTURING
SAME
Abstract
Provided is a high-strength steel sheet for use in lightening
materials for electronic products and materials for vehicles
including automobiles, trains and ships and, more particularly, to
a high-strength cold rolled steel sheet, a coated steel plate, and
a method for manufacturing the same, wherein yield strength and
ductility are improved by controlling the internal oxidation depth
and the amount of residual austenite after first annealing
operation, and the high-strength cold rolled steel sheet and the
coated steel plate is stably manufactured and provided without any
dent defects during the manufacturing thereof.
Inventors: |
LEE; Kyoo-Young;
(Gwangyang-si, KR) ; LEE; Won-Hwi; (Gwangyang-si,
KR) ; LEE; Sea-Woong; (Gwangyang-si, KR) ;
RYU; Joo-Hyun; (Gwangyang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
59966126 |
Appl. No.: |
16/079722 |
Filed: |
March 28, 2017 |
PCT Filed: |
March 28, 2017 |
PCT NO: |
PCT/KR2017/003351 |
371 Date: |
August 24, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 2211/001 20130101;
C21D 8/02 20130101; C22C 38/00 20130101; C22C 38/06 20130101; C22C
38/14 20130101; C21D 8/0236 20130101; C22C 38/02 20130101; C21D
9/46 20130101; Y02P 10/20 20151101; C21D 8/0226 20130101; C22C
38/12 20130101; C21D 2211/005 20130101; C22C 38/60 20130101; C21D
2211/008 20130101; C22C 38/04 20130101; C22C 38/001 20130101; C21D
8/0247 20130101 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/12 20060101 C22C038/12; C22C 38/14 20060101
C22C038/14; C22C 38/60 20060101 C22C038/60; C22C 38/00 20060101
C22C038/00; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/06 20060101 C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2016 |
KR |
10-2016-0036872 |
Claims
1. A high-strength cold rolled steel sheet, excellent in terms of
yield strength and ductility, comprising, by weight, 0.1% to 0.3%
of carbon (C), 0.1% to 2.0% of silicon (Si), 0.005% to 1.5% of
aluminum (Al), 1.5% to 3.0% of manganese (Mn), 0.04% or less
(excluding 0%) of phosphorus (P), 0.015% or less (excluding 0%) of
sulfur (S), 0.02% or less (excluding 0%) of nitrogen (N), 0.01% to
0.1% of antimony (Sb), a remainder of iron (Fe) and unavoidable
impurities, and the sum of Si and Al (Si+Al, wt %) satisfies 1% to
3.5%, wherein a microstructure comprises, by an area fraction, 5%
or less of polygonal ferrite having a ratio of a short axis to a
long axis of more than 0.4, 70% or less of acicular ferrite having
a ratio of a short axis to a long axis of 0.4 or less, 0.6% to 25%
of retained austenite, and a remainder of martensite, an internal
oxidation depth is 1 .mu.m or less from a surface.
2. The high-strength cold rolled steel sheet according to claim 1,
wherein, in the cold rolled steel sheet, the microstructure before
the second annealing operation comprises 0.6% or more of acicular
retained austenite, and the remainder is composed of one or more of
bainite, martensite and tempered martensite.
3. The high-strength cold rolled steel sheet according to claim 1,
wherein the cold rolled steel sheet further comprises one or more
selected from the group consisting of 0.005% to 0.1% of titanium
(Ti), 0.005% to 0.1% of niobium (Nb), 0.005% to 0.1% of vanadium
(V), 0.005% to 0.1% of zirconium (Zr), and 0.005% to 0.5% of
tungsten (W).
4. The high-strength cold rolled steel sheet according to claim 1,
wherein the cold rolled steel sheet further comprises one or more
selected from the group consisting of 1% or less (excluding 0%) of
molybdenum (Mo), 1% or less (excluding 0%) of nickel (Ni), 0.5% or
less (excluding 0%) of copper (Cu), and 1% or less (excluding 0%)
of chromium (Cr).
5. The high-strength cold rolled steel sheet according to claim 1,
wherein the cold rolled steel sheet further comprises one or more
selected from the group consisting of 0.01% or less (excluding 0%)
of calcium (Ca), 0.1% or less (excluding 0%) of bismuth (Bi), and
0.01% or less (excluding 0%) of boron (B).
6. The high-strength cold rolled steel sheet according to claim 1,
wherein tensile strength of the cold rolled steel sheet is 780 MPa
or greater.
7. The high-strength cold rolled steel sheet according to claim 1,
wherein one of a hot-dip galvanized layer, a hot-dip galvannealed
layer, an aluminum-silicon plated layer and a
zinc-magnesium-aluminum plated layer is formed on a surface of the
cold rolled steel sheet.
8. A method of manufacturing a high-strength cold rolled steel
sheet, excellent in terms of yield strength and ductility,
comprising: heating a steel slab comprising, by weight, 0.1% to
0.3% of carbon (C), 0.1% to 2.0% of silicon (Si), 0.005% to 1.5% of
aluminum (Al), 1.5% to 3.0% of manganese (Mn), 0.04% or less
(excluding 0%) of phosphorus (P), 0.015% or less (excluding 0%) of
sulfur (S), 0.02% or less (excluding 0%) of nitrogen (N), 0.01% to
0.1% of antimony (Sb), a remainder of iron (Fe) and unavoidable
impurities, and the sum of Si and Al (Si+Al, wt %) satisfies 1% to
3.5%, at a temperature of 1,000.degree. C. to 1,300.degree. C.; hot
rolling the heated steel slab at a temperature of 800.degree. C. to
950.degree. C. to produce a hot rolled steel sheet; coiling the hot
rolled steel sheet at a temperature of 750.degree. C. or lower;
cold rolling the coiled hot rolled steel sheet to produce a cold
rolled steel sheet; performing a first annealing operation in which
the cold rolled steel sheet is annealed at a temperature of Ac3 or
higher and cooled at an average cooling rate of 25.degree. C./sec
or lower; and performing a second annealing operation in which,
after the first annealing operation, the first annealed cold rolled
steel sheet is heated and maintained at a temperature of Ac1 to
Ac3, cooled in an average cooling rate of lower than 20.degree.
C./sec to a temperature of 500.degree. C. or lower, maintained at
the temperature of 500.degree. C. or lower 1 second or more, and
then cooled.
9. The method according to claim 8, wherein the steel slab further
comprises one or more selected from the group consisting of 0.005%
to 0.1% of titanium (Ti), 0.005% to 0.1% of niobium (Nb), 0.005% to
0.1% of vanadium (V), 0.005% to 0.1% of zirconium (Zr), and 0.005%
to 0.5% of tungsten (W).
10. The method according to claim 9, wherein the steel slab further
comprises one or more selected from the group consisting of 1% or
less (excluding 0%) of molybdenum (Mo), 1% or less (excluding 0%)
of nickel (Ni), 0.5% or less (excluding 0%) of copper (Cu), and 1%
or less (excluding 0%) of chromium (Cr).
11. The method according to claim 9, wherein the steel slab further
comprises one or more selected from the group consisting of 0.01%
or less (excluding 0%) of calcium (Ca), 0.1% or less (excluding 0%)
of bismuth (Bi), and 0.01% or less (excluding 0%) of boron (B).
12. The method according to claim 8, further comprising forming one
of a hot-dip galvanized layer, a hot-dip galvannealed layer, an
aluminum-silicon plated layer and a zinc-magnesium-aluminum plated
layer, on a surface of the cold rolled steel sheet after the second
annealing operation.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a high-strength cold
rolled steel sheet with excellent yield strength and ductility, a
coated steel sheet, and a method of manufacturing the same, which
is preferably used as a material for electronic products, and as a
material for vehicles, including automobiles, trains and ships.
BACKGROUND ART
[0002] In order to reduce a thickness of a steel sheet to reduce a
weight of a material for electronic products, and a material for
vehicles, including automobiles, trains and ships, it is necessary
to increase the strength of a steel material. Generally, there is a
disadvantage, in that ductility may be lowered as strength is
increased. In order to overcome this problem, many studies have
been conducted into improving the relationship between strength and
ductility. As a result, transformed structure steels utilizing a
retained austenite phase, together with martensite and bainite,
have been developed and applied.
[0003] Transformed structure steels may be classified as so-called
two phase (DP) steel, transformation induced plasticity (TRIP)
steel, and complex phase (CP) steel. Each type of steel may have
different mechanical properties, i.e., tensile strength and
elongation, depending on a type and fraction of a matrix and a
second phase. In particular, when the TRIP steel includes retained
austenite, a balance (TS.times.El) between tensile strength and
elongation may have the highest value.
[0004] Among such transformed structure steels, CP steel may have
elongation lower than other types of steel, and may only be used
for simple processing operations, such as roll forming operations.
The DP steel and the TRIP steel having high ductility may be
applied to cold rolled press forming operations or the like.
[0005] Patent Document 1 proposes a method of producing a
high-strength steel sheet having excellent formability, using an
annealing operation twice. In a first annealing operation, a single
phase of austenite is heated, and is cooled to a temperature of an
Ms point or higher and a Bs point or lower at an average cooling
rate of 20.degree. C./sec or higher; and, in a second annealing
operation, 50% or more of tempered bainite is included and about 3%
to 30% of retained austenite is secured in the final structure by
performing a two phase region annealing operation.
[0006] In Patent Document 2, the same process as in Patent Document
1 is carried out, but in the first annealing operation, a single
phase of austenite is heated, and is cooled to a temperature of an
Ms point or lower at an average cooling rate of 20.degree. C./sec
or higher; and, in the second annealing operation, 50% or more of
tempered martensite is included and about 3% to 20% of retained
austenite is secured in the final structure by performing a two
phase region annealing operation.
[0007] Patent Documents 1 and 2 may have an advantage of improving
stretch flange properties and ductility simultaneously. However,
there are disadvantages, in that process costs may be increased by
performing an annealing operation twice, and, when steel including
a large amount of Si and Mn is subject to a high-temperature
annealing operation in the first annealing operation, dents in an
annealing furnace may be induced, such that homogeneous materials
may not be continuously treated. In addition, when the first
annealing operation is performed at a rapid average cooling rate of
20.degree. C./sec or higher after the austenite heat treatment,
there is a disadvantage in that the important shape of the material
for press forming may be poor.
[0008] In addition to the above transformed structure steels, there
may be twinning induced plasticity (TWIP) steel in which a large
amount of C and Mn are added to the steel to obtain a
microstructure of steel as a single phase of austenite.
[0009] In the case of the TWIP steel disclosed in Patent Document
3, a balance of tensile strength and elongation (TS.times.El) may
be 50,000 MPa % or more, and may exhibit excellent material
properties. However, in order to produce such TWIP steel, when the
content of C is 0.4 wt %, the content of Mn may be required to be
about 25 wt % or more. When the content of C is 0.6 wt %, the
content of Mn may be about 20 wt % or more. However, when not
satisfied, an austenite phase causing a twinning phenomenon in the
matrix may not be stably secured, and epsilon martensite
(.epsilon.), having an HCP structure as well as a martensite
(.alpha.'), having a BCT structure, may be extremely harmful to
processability. Therefore, a large amount of elements for
stabilizing austenite should be added, such that austenite may be
stably present at room temperature. As described above, TWIP steels
having a large amount of alloying components added thereto may have
problems in that the processes such as casting and rolling may be
very difficult due to problems caused by the alloying components,
and manufacturing costs may be increased in terms of economy.
[0010] Recently, so-called 3.sup.rd generation steel or extra
advanced high-strength steel (X-AHSS), having higher ductility than
that of the DP steel and the TRIP steel, and lower ductility than
TWIP steel, has been developed, but there has been no great
achievement to date.
[0011] For example, Patent Document 4 discloses a quenching and
partitioning process (Q&P) for forming retained austenite and
martensite as a main structure. As illustrated in a report
utilizing the process (Non-Patent Document 1), when the content of
carbon is relatively low, such as about 0.2%, the yield strength
may be as low as 400 MPa. Further, the elongation obtained from the
final product may merely be comparable to that of conventional TRIP
steel. In addition, a method of increasing yield strength by
increasing amounts of the alloy of carbon and manganese has been
derived. However, in this case, there may be a disadvantage, in
that weldability may be deteriorated due to the addition of the
alloying component in an excessive amount.
[0012] In order to solve the problem of the product by the Q&P
heat treatment, Patent Document 5 proposes a method of improving
final properties by controlling the microstructure before the
annealing Q&P heat treatment. However, there has been a problem
in that, when steel including a large amount of Si and Mn is
subject to the high-temperature annealing operation in the first
annealing operation, a dent may be induced in the annealing
furnace.
[0013] Patent Document 1: Japanese Patent Laid-Open Publication No.
2002-309334
[0014] Patent Document 2: Japanese Patent Laid-Open Publication No.
2002-302734
[0015] Patent Document 3: Korean Patent Laid-Open Publication No.
1994-0002370
[0016] Patent Document 4: US Patent Publication No.
2006-0011274
[0017] Patent Document 5: Korean Patent Laid-Open Publication No.
2015-0130612
[0018] Non-Patent Document 1: ISIJ International, Vol. 51, 2011, p.
137-144
DISCLOSURE
Technical Problem
[0019] An aspect of the present disclosure is to provide a
high-strength cold rolled steel sheet having relatively lower alloy
costs, as compared to TWIP steel, in which high yield strength and
ductility required for materials for automobile structural members
is secured, and which has good shape quality without dents being
induced in an annealing furnace during operations; a coated steel
sheet; and a method of manufacturing the same.
Technical Solution
[0020] According to an aspect of the present disclosure, a
high-strength cold rolled steel sheet, excellent in terms of yield
strength and ductility, includes: by weight, 0.1% to 0.3% of carbon
(C), 0.1% to 2.0% of silicon (Si), 0.005% to 1.5% of aluminum (Al),
1.5% to 3.0% of manganese (Mn), 0.04% or less (excluding 0%) of
phosphorus (P), 0.015% or less (excluding 0%) of sulfur (S), 0.02%
or less (excluding 0%) of nitrogen (N), 0.01% to 0.1% of antimony
(Sb), a remainder of iron (Fe) and unavoidable impurities, and the
sum of Si and Al (Si+Al, wt %) satisfies 1% to 3.5%, wherein a
microstructure includes, by an area fraction, 5% or less of
polygonal ferrite having a ratio of a short axis to a long axis of
more than 0.4, 70% or less of acicular ferrite having a ratio of a
short axis to a long axis of 0.4 or less, 0.6% to 25% of acicular
retained austenite, and a remainder of martensite, an internal
oxidation depth is 1 .mu.m or less from a surface.
[0021] Another aspect of the present disclosure relates to a
high-strength cold rolled steel sheet, excellent in terms of yield
strength and ductility, wherein one of a hot-dip galvanized layer,
a hot-dip galvannealed layer, an aluminum-silicon plated layer and
a zinc-magnesium-aluminum plated layer is formed on a surface of
the cold rolled steel sheet.
[0022] According to another aspect of the present disclosure, a
method of manufacturing a high-strength cold rolled steel sheet,
excellent in terms of yield strength and ductility, includes:
heating a steel slab satisfying the alloy composition described
above to a temperature of 1,000.degree. C. to 1,300.degree. C.; hot
rolling the heated steel slab at a temperature of 800.degree. C. to
950.degree. C. to produce a hot rolled steel sheet; coiling the hot
rolled steel sheet at a temperature of 750.degree. C. or lower;
cold rolling the coiled hot rolled steel sheet to produce a cold
rolled steel sheet; performing a first annealing operation in which
the cold rolled steel sheet is annealed at a temperature of Ac3 or
higher and cooled at an average cooling rate of 25.degree. C./sec
or lower; and performing a second annealing operation in which,
after the first annealing operation, the first annealed cold rolled
steel sheet is heated and maintained at a temperature of Ac1 to
Ac3, cooled in an average cooling rate of lower than 20.degree.
C./second to a temperature of 500.degree. C. or lower, maintained
at the temperature of 500.degree. C. or lower 1 second or more, and
then cooled.
[0023] Another aspect of the present disclosure relates to a method
of manufacturing a high-strength cold rolled steel sheet, excellent
in terms of yield strength and ductility, further comprising
forming one of a hot-dip galvanized layer, a hot-dip galvannealed
layer, an aluminum-silicon plated layer and a
zinc-magnesium-aluminum plated layer, on a surface of the cold
rolled steel sheet after the second annealing operation.
Advantageous Effects
[0024] According to an aspect of the present disclosure, a
high-strength cold rolled steel sheet, a coated steel sheet, and a
method of manufacturing the same may be provided, which is
excellent in terms of yield strength and ductility and has a
tensile strength of 780 MPa or more, as compared to high ductility
transformed structure steel such as the conventional DP steel or
TRIP steel, and Q&P steel subjected to Q&P heat treatment;
and may be stably produced and provided without dent defects being
generated in production.
[0025] In addition, the ultra-high-strength steel sheet of the
present disclosure may be highly used for weight reduction of a
material for electronic products and a material for vehicles
including automobiles, trains and ships.
DESCRIPTION OF DRAWINGS
[0026] FIG. 1 illustrates an example of an annealing operation
according to the present disclosure, and illustrates (a) a first
annealing operation and (b) a second annealing operation;
[0027] FIG. 2 illustrates internal oxidation depth and formation
after (a) Inventive Example 2 and (b) Comparative Example 5 after a
hot rolling operation; and
[0028] FIG. 3 is a graph illustrating yield strength improvement of
Comparative Examples 1 to 4 and Inventive Examples 1 and 2
according to amounts of retained austenite after a first annealing
operation.
BEST MODE FOR INVENTION
[0029] The present inventors have conducted intensive research into
the effects of configuration of a phase obtained by a first
annealing operation performed before a second annealing operation
in two annealing processes on final physical properties. As a
result, the present inventors have found that yield strength and
ductility of a final product may be improved by forming
appropriately retained austenite in the first annealing operation,
in a different manner to a conventional method.
[0030] In addition, the present inventors have found that, in order
to suppress indent defects in an annealing furnace, mainly
occurring in a case of annealing a material to which large amounts
of Si, Mn and Al are added, Sb may be added to significantly reduce
an internal oxidation depth, and may suppress dent defects caused
by surface-enriched layers of Si, Mn and Al due to a
high-temperature heat treatment of an austenite single phase
annealing operation, corresponding to the first annealing
operation, and the present inventors have then accomplished the
present disclosure, based on the above.
[0031] Hereinafter, a high-strength cold rolled steel sheet
excellent in terms of yield strength and ductility, which is one
aspect of the present disclosure, will be described in detail.
[0032] The high-strength cold rolled steel sheet excellent in terms
of yield strength and ductility, which is one aspect of the present
disclosure, includes, by weight, 0.1% to 0.3% of carbon (C), 0.1%
to 2.0% of silicon (Si), 0.005% to 1.5% of aluminum (Al), 1.5% to
3.0% of manganese (Mn), 0.04% or less (excluding 0%) of phosphorus
(P), 0.015% or less (excluding 0%) of sulfur (S), 0.02% or less
(excluding 0%) of nitrogen (N), 0.01% to 0.1% of antimony (Sb), a
remainder of iron (Fe) and unavoidable impurities, and the sum of
Si and Al (Si+Al, wt %) satisfies 1% to 3.5%, wherein a
microstructure comprises, by an area fraction, 5% or less of
polygonal ferrite having a ratio of a short axis to a long axis of
more than 0.4, 70% or less of acicular ferrite having a ratio of a
short axis to a long axis of 0.4 or less, 0.6% to 25% of retained
austenite, and a remainder of martensite, an internal oxidation
depth is 1 .mu.m or less from a surface.
[0033] First, the reasons for restricting the composition of the
alloying component as described above will be described in detail.
Hereinafter, the content of each component is based on weight %
unless otherwise specified.
[0034] C: 0.1% to 0.3%
[0035] Carbon (C) may be an effective element for strengthening
steel, and, in the present disclosure, may be an important element
added for stabilizing retained austenite and securing the strength
thereof. The content is preferably 0.1% or more to obtain the
above-mentioned effect. When the content thereof exceeds 0.3%, the
risk of slab defects occurring may increase, and weldability may
also be significantly reduced. Therefore, the content of C in the
present disclosure is preferably limited to 0.1% to 0.3%.
[0036] Si: 0.1% to 2.0%
[0037] Silicon (Si) may be an element which inhibits precipitation
of carbides in ferrite and promotes diffusion of carbon in ferrite
into austenite, and consequently contributes to the stabilization
of retained austenite. It is preferable to add 0.1% or more to
obtain the above-mentioned effect. When the content thereof exceeds
2.0%, the hot and cold rolling properties may be significantly
deteriorated, and oxides may be formed on a surface of the steel to
reduce plating properties. Therefore, in the present disclosure,
the content of Si is preferably limited to 0.1% to 2.0%.
[0038] Al: 0.005% to 1.5%
[0039] Aluminum (Al) may be an element which is combined with
oxygen in the steel to perform a deoxidizing action, and, to this
end, the content thereof is preferably maintained at 0.005% or
more. Also, Al may contribute to stabilization of retained
austenite by suppressing a formation of carbide in ferrite, similar
to Si. When the content of Al exceeds 1.5%, it may be difficult to
produce a good slab through a reaction with a mold plus during
casting, and also, surface oxides may be formed to deteriorate
plating properties. Therefore, the content of Al in the present
disclosure is preferably limited to 0.005% to 1.5%.
[0040] As mentioned above, both Si and Al may contribute to
stabilization of retained austenite. To effectively achieve this,
it is preferable that the sum of Si and Al (Si+Al, wt %) satisfies
1.0% to 3.5%.
[0041] Mn: 1.5% to 3.0%
[0042] Manganese (Mn) may be an element effective for forming and
stabilizing retained austenite while controlling transformation of
ferrite. When the content of Mn is less than 1.5%, ferrite
transformation occurs on a large scale, and it may be difficult to
obtain a desired degree of strength. On the other hand, when the
content thereof exceeds 3.0%, in a case of a second Q&P heat
treatment, there is a problem in which it may be difficult to
secure an intended degree of ductility, as phase transformation is
overly delayed to form large amounts of martensite. Therefore, the
content of Mn in the present disclosure is preferably limited to
1.5% to 3.0%.
[0043] Sb: 0.01 to 0.1%
[0044] Antimony (Sb) may have an effect of suppressing internal
oxidation after hot rolling by inhibiting surface enrichment of Si,
Al, and the like, and movement of oxidizing elements through grain
boundary segregation, and for the same reason, may have the effect
of improving the quality of a plated surface by inhibiting
oxidation by the surface enrichment of Si, Al, and the like. When
the content thereof is less than 0.01%, the effect of inhibiting
the internal oxidation may be not sufficient, such that the
internal oxidation depth of the final product exceeds 1 .mu.m from
the surface. Meanwhile, when the content thereof exceeds 0.1%,
there may be a problem in which galvannealing of the galvanized
layer is delayed.
[0045] P: 0.04% or Less (Excluding 0%)
[0046] Phosphorus (P) may be an element which obtains a solid
solution strengthening effect and stabilizes retained austenite.
When the content thereof exceeds 0.04%, weldability may be lowered,
and the risk of brittleness of the steel may increase. Therefore,
in the present disclosure, the content of P may be 0.04% or less,
more preferably 0.02% or less.
[0047] S: 0.015% or Less (Excluding 0%)
[0048] Sulfur (S) may be an impurity element inevitably included in
the steel, and its content is preferably suppressed to the maximum.
In theory, it is advantageous to limit the content of S to 0%, but
it may be important to manage the upper limit thereof because it is
inevitably included in the manufacturing process. When the content
thereof exceeds 0.015%, the ductility and weldability of the steel
sheet may be highly impaired. Therefore, in the present disclosure,
it is preferable to be limited to 0.015% or less.
[0049] N: 0.02% or Less (Excluding 0%)
[0050] Nitrogen (N) may be an element effective in stabilizing
austenite. When the content thereof exceeds 0.02%, the risk of
brittleness of steel may be increased, and AlN may be excessively
precipitated by reaction with Al, thereby causing a problem of
deteriorating quality in a continuous casting process. Therefore,
in the present disclosure, it is preferable to limit the N content
to 0.02% or less.
[0051] At this time, the cold rolled steel sheet of the present
disclosure may further include one or more of Ti, Nb, V, Zr and Win
addition to the above-mentioned components for the purpose of
strength improvement and the like.
[0052] One or More of Ti: 0.005% to 0.1%, Nb: 0.005% to 0.1%, V:
0.005% to 0.1%, Zr: 0.005% to 0.1%, and W: 0.005% to 0.5%
[0053] Titanium (Ti), niobium (Nb), vanadium (V), zirconium (Zr)
and tungsten (W) may be effective elements for precipitation
strengthening and crystal grain refinement of the steel sheet. When
the content thereof is less than 0.005%, respectively, there may be
a problem in which it is difficult to secure the above-mentioned
effects. On the other hand, when the content thereof exceeds 0.1%
in the case of Ti, Nb, V, and Zr, and exceeds 0.5% in the case of
W, there may be a problem in which the above-mentioned effects may
be saturated, manufacturing costs may be greatly increased, and
precipitates may be excessively formed to significantly lower
ductility.
[0054] The cold rolled steel sheet of the present disclosure may
further comprise at least one of Mo, Ni, Cu, and Cr.
[0055] One or More of Mo: 1% or Less (Excluding 0%), Ni: 1% or Less
(Excluding 0%), Cu: 0.5% or Less (Excluding 0%), and Cr: 1% or Less
(Excluding 0%).
[0056] Molybdenum (Mo), nickel (Ni), copper (Cu), and chromium (Cr)
may be elements that contribute to stabilization of retained
austenite. These elements may act together with C, Si, Mn, Al, and
the like, in combination, to contribute to stabilization of
austenite. When the content thereof exceeds 1.0% in cases of Mo,
Ni, and Cr, and exceeds 0.5% in a case of Cu, there may be a
problem in which production cost is excessively increased.
Therefore, it is preferable to control these contents to not exceed
the above content.
[0057] Further, when Cu is added, it may cause brittleness during a
hot rolling operation, and therefore it is more preferable that Ni
is added simultaneously.
[0058] In addition, the cold rolled steel sheet of the present
disclosure may further include one or more of Ca, Bi, and B.
[0059] Ca: 0.01% or Less (Excluding 0%), Bi: 0.1% or Less
(Excluding 0%), B: 0.01% or Less (Excluding 0%)
[0060] Calcium (Ca) may be an element which is advantageous for
improving processability by controlling a form of sulfide. When the
content thereof exceeds 0.01%, the above-mentioned effect may be
saturated. Therefore, it is preferable that the content thereof is
0.01% or less.
[0061] Bismuth (Bi) may be an element having an effect of improving
quality of a plated surface by inhibiting movement of surface
oxidation elements such as Si, Al, and the like, through grain
boundary segregation. When the content thereof exceeds 0.1%, the
above-mentioned effect may be saturated. Therefore, it is
preferable that the content thereof is 0.1% or less.
[0062] Boron (B) may have an effect of improving quenchability due
to the combined effect with Mn, Cr, and the like to suppress soft
ferrite transformation at a relatively high temperature. When the
content thereof exceeds 0.01%, excessive amounts of B may be
concentrated on the steel surface during plating operation to cause
to deterioration of plating adhesiveness. Therefore, it is
preferable that the content thereof is 0.01% or less.
[0063] The remainder of the present disclosure may be iron (Fe). In
the conventional steel manufacturing process, since impurities
which are not intended from raw materials or surrounding
environment may be inevitably incorporated, the impurities may not
be excluded. All of these impurities are not specifically mentioned
in this specification, as they are known to anyone skilled in the
art of steel making.
[0064] The cold rolled steel sheet according to the present
disclosure satisfying the above-mentioned component composition may
include, by an area fraction, 5% or less of polygonal ferrite
having a ratio of a short axis to a long axis of more than 0.4, 70%
or less of acicular ferrite having a ratio of a short axis to a
long axis of 0.4 or less, 0.6% to 25% of retained austenite, and a
remainder of martensite, as a microstructure.
[0065] Since the polygonal ferrite having a ratio of a short axis
to along axis of more than 0.4 may serve to lower the yield
strength of the steel of the present disclosure applied to
structural members and the like, the ratio may be limited to 5% or
less. The acicular ferrite having a ratio of a short axis to a long
axis of 0.4 or less, and the retained austenite may be main
structures of the present disclosure, and may be structures
advantageous in securing strength and ductility.
[0066] The retained austenite may be advantageously needed to
ensure a balance between strength and ductility. When a fraction
thereof exceeds 25% (upper limit value), there may be a problem in
which stabilization of the retained austenite is insufficient,
because a carbon may be dispersed and diffused. Therefore, it is
preferable that the fraction of the retained austenite in the
present disclosure satisfies 25% or less. Meanwhile, the lower
limit thereof may basically be 0.6% or more, which is a fraction of
the retained austenite which should preferably be secured after the
first annealing operation in the present disclosure.
[0067] In addition, the steel sheet of the present disclosure
preferably has an internal oxidation depth of 1 .mu.m or less from
a surface, together with the characteristics of the
microstructure.
[0068] In the present disclosure, Sb may be basically included in
an amount of 0.01 to 0.1%. The element may have an effect of
suppressing internal oxidation by binding surface-enriched elements
such as Mn, Si and Al, and oxygen to be diffused into the steel
during cooling and coiling operations after a hot rolling operation
through the surface enrichment of Sb (FIG. 2). An internal oxide
layer formed after the hot rolling operation may cause cracking of
the internal oxide layer through the subsequent pickling and cold
rolling operations, which may then cause dent defects in the steel
sheet due to detachment and attachment to the inner roll in the
annealing furnace during the subsequent annealing operation. These
dent defects may deteriorate a surface quality of annealed coils to
be subsequently operated, including the product coil, making normal
product production difficult. When an internal oxidation depth of
the final cold rolled steel sheet exceeds 1 .mu.m, the above
problems may arise.
[0069] Meanwhile, a lower limit of the internal oxidation depth is
not particularly limited, including 0, since it is advantageous to
suppress the dent defects, as the internal oxidation does not
occur.
[0070] The cold rolled steel sheet of the present disclosure
satisfying the above-described alloy composition and microstructure
may have a tensile strength of 780 MPa or more, and excellent yield
strength and ductility, and may suppress dent defects during
annealing operation, thereby ensuring excellent productivity.
[0071] Meanwhile, the cold rolled steel sheet according to the
present disclosure may be manufactured through a manufacturing
process described below. In this case, the microstructure after the
first annealing operation, that is, the microstructure before the
second annealing operation, should include 0.6% or more of the
acicular retained austenite. Further, the remainder may be composed
of one or more of bainite, martensite and tempered martensite.
[0072] This may be to ensure the excellent yield strength and
ductility of the cold rolled steel sheet produced in the final
annealing operation. When the content of the retained austenite is
less than 0.6% after the first annealing operation, there may be a
problem in which the yield strength is lowered and the elongation
is lowered. Therefore, it is preferable that the retained austenite
is 0.6% or more. When 1.5% or more of the retained austenite is
secured, excellent physical properties that YS.times.El (MPa %) of
the final annealed product is 16,000 or more may be provided.
Therefore, it is more desirable to secure retained austenite of
1.5% or more after the first annealing operation.
[0073] One of a hot-dip galvanized layer, a hot-dip galvannealed
layer, an aluminum-silicon plated layer and a
zinc-magnesium-aluminum plated layer may be formed on a surface of
a high-strength coated steel sheet excellent in terms of yield
strength and ductility, which is another aspect of the present
disclosure.
[0074] Hereinafter, a method of manufacturing a high-strength cold
rolled steel sheet having excellent yield strength and ductility
according to another aspect of the present disclosure will be
described in detail.
[0075] The method for manufacturing a high-strength cold rolled
steel sheet having excellent yield strength and ductility according
to another aspect of the present disclosure may be manufactured by
subjecting a steel slab satisfying the above alloy composition to
heating-hot rolling-coiling-cold rolling-annealing operations, and
the conditions of the respective operations will be described in
detail below.
[0076] Steel Slab Heating Operation
[0077] In the present disclosure, it is preferable to carry out an
operation of heating and homogenizing the steel slab before
performing hot rolling, and it is more preferably performed in a
temperature within a range of 1,000.degree. C. to 1,300.degree.
C.
[0078] When a temperature during the heating operation is lower
than 1,000.degree. C., there may be a problem in which the rolling
load increases sharply. On the other hand, when the temperature
exceeds 1,300.degree. C., energy cost may increase, and amounts of
the surface scale may be excessive. Therefore, in the present
disclosure, it is preferable to carry out the heating operation of
the steel slab at 1,000.degree. C. to 1,300.degree. C.
[0079] Hot Rolling Operation
[0080] The heated steel slab may be hot rolled into a hot rolled
steel sheet, and the hot rolling is preferably performed at a
temperature within a range of 800.degree. C. to 950.degree. C.
[0081] When the hot rolling temperature at the time of hot rolled
finishing operation is lower than 800.degree. C., the rolling load
may be greatly increased, and the rolling may be difficult to
operate. On the other hand, when the hot rolling temperature
exceeds 950.degree. C., thermal fatigue of the rolling roll may be
greatly increased to cause surface quality deterioration due to
surface oxide film formation. Therefore, in the present disclosure,
the hot rolling finishing temperature during the hot rolling
operation is preferably limited to a temperature within a range of
800.degree. C. to 950.degree. C.
[0082] Coiling Operation
[0083] The hot rolled steel sheet produced according to the above
operations may be coiled, and, at this time, the coiling
temperature is preferably 750.degree. C. or lower, and more
preferably 650.degree. C. or lower for suppressing the internal
oxide layer.
[0084] When the coiling temperature is overly high at the time of
performing a coiling operation, scale may be excessively generated
on the surface of the hot rolled steel sheet to cause surface
defects and deteriorate plating ability. Further, in the steel
including a large amount of Mn, Si, Al or the like as in the
present disclosure, internal oxidation may be promoted to cause
dent defects in the subsequent annealing operation. Therefore, the
coiling operation is preferably performed at 750.degree. C. or
lower, more preferably, 650.degree. C. or lower. At this time, a
lower limit of the coiling temperature is not particularly limited,
but it is more preferably carried out at Ms.about.750.degree. C. in
consideration of the difficulty of subsequent cold rolling
operation as the strength of the hot rolled steel sheet in the
formation of martensite may be overly high.
[0085] Production outside of the above hot rolling conditions may
not greatly change the physical properties of the final product,
but may affect the productivity thereof.
[0086] Cold Rolling Operation
[0087] The hot rolled steel sheet may be pickled to remove the
oxide layer, and then cold rolled to obtain a cold rolled steel
sheet for matching the shape and thickness of the steel sheet. In
general, when the general annealing operation is carried out, it is
common to set a lower limit of a cold rolling reduction rate to
prevent the formation of coarse crystal grains during
recrystallization. In a case of the first annealing operation
before the final annealing operation as in the present disclosure,
there may be no restriction on the reduction rate in the cold
rolling operation.
[0088] Annealing Operation (First Annealing Operation and Second
Annealing Operation)
[0089] The present disclosure is directed to manufacturing a cold
rolled steel sheet comprising, by an area fraction, 5% or less of
polygonal ferrite having a ratio of a short axis to a long axis of
more than 0.4, 70% or less of acicular ferrite having a ratio of a
short axis to a long axis of 0.4 or less, 0.6% to 25% of retained
austenite, and a remainder of martensite, wherein an internal
oxidation depth is 1 .mu.m or less from a surface. The subsequent
annealing operation may be important to be controlled to obtain
such a cold rolled steel sheet.
[0090] Particularly, in the present disclosure, to secure a desired
microstructure through redistribution of elements such as carbon
and manganese during an annealing operation, a conventional
annealing process including Austempering or Q&P heat treatment
after a cold rolling operation may be not carried out, to produce a
final product. As will be described later, the present disclosure
is characterized in that, after the first annealing operation, a
low-temperature structure including 0.6% or more of acicular
retained austenite may be secured, subsequently, in the second
annealing operation, the steel sheet may be heated and maintained
at a temperature in the range of Ac1 to Ac3, and then cooled at an
average cooling rate of less than 20.degree. C. per second at a
temperature of 500.degree. C. or lower, maintained for 1 second or
longer, and then cooled.
[0091] The more the amount of retained austenite secured after the
first annealing operation, the higher the yield strength and
ductility. When the retained austenite of 1.5% or more after the
first annealing operation is secured, the YS.times.El (MPa %) of
the final annealed product may be 16,000 or more, exhibiting very
good physical properties. Therefore, it is more preferable to
secure retained austenite of 1.5% or more after the first annealing
operation.
[0092] First Annealing Operation
[0093] First, it is preferable that the cold rolled steel sheet as
above is subjected to a first annealing operation in which
annealing operation is performed at a temperature equal to or
higher than Ac3, and a cooling operation is performed at an average
cooling rate equal to or lower than 25.degree. C./sec (see (a) of
FIG. 1).
[0094] The cooling rate may be limited to secure retained austenite
at 0.6% or more. When cooled to 25.degree. C./sec or lower, it may
be possible to more stably secure the retained austenite by dynamic
partitioning, and secure 0.6% or more of retained austenite.
Dynamic partitioning means that alloying elements are redistributed
between phases during cooling at high temperatures.
[0095] In (a) of FIG. 1, No. 5 of the heat treatment indicates a
normal austempering heat treatment, which indicates a condition in
which an average cooling rate is very slow.
[0096] The microstructure of the first annealed cold rolled steel
sheet should include 0.6% or more of retained austenite, and the
formation of soft polygonal ferrite is preferably suppressed to a
minimum because it inhibits obtaining a finely finished annealed
structure during the subsequent annealing heat treatment, and the
remainder may be secured in any one of bainite, martensite, and
tempered martensite, which are low-temperature microstructures.
[0097] In the present disclosure, it may be important that the
ferrite region illustrated in (a) of FIG. 1 barely penetrates
during cooling operation to secure the polygonal ferrite to 5% or
less after the final annealing operation.
[0098] Therefore, when any one of the austempering heat treatment
and the Q&P heat treatment is performed in a 1-step heat
treatment or a 2-step heat treatment, as illustrated No. 5 in (a)
of FIG. 1, the initial cooling rate may be controlled to be slow,
and the average cooling rate may be controlled to be 25.degree.
C./sec or lower, such that the ferrite region may be not passed and
the retained austenite may be secured simultaneously.
[0099] The above-mentioned heat treatment conditions are to ensure
excellent yield strength and ductility of the cold rolled steel
sheet manufactured through the austempering or Q&P process in
the final annealing operation. When after the first annealing
operation. When the acicular retained austenite after the first
annealing operation is included in less than 0.6%, there may be a
disadvantage in that the yield strength is lowered and the
elongation is lowered.
[0100] Second Annealing Operation
[0101] After the first annealing operation, the steel sheet may be
heated and maintained at a temperature in the range of Ac1 to Ac3,
and then cooled to an average cooling rate of lower than 20.degree.
C./sec to a temperature of 500.degree. C. or lower. After cooling
to a temperature of 500.degree. C. or lower, general austempering
or Q&P heat treatment may be performed ((b) of FIG. 1).
[0102] In the present disclosure, the heating operation in the
range of Ac1 to Ac3 may be intended to secure the stability of
austenite through distribution of alloying elements into the
austenite during annealing operation, and ensure retained austenite
in the final structure at room temperature. It may be easy to
secure the acicular structure by the acicular retained austenite
formed after the first annealing operation.
[0103] It may be easy to secure a fine structure even after the
second annealing operation in which an inverse transformation is
carried out by the presence of such an acicular retained
austenite.
[0104] The cooling temperature after the two phase region annealing
operation is preferably 500.degree. C. or lower. This may be
because the austenite phase is transformed into pearlite, when
maintained at a temperature higher than 500.degree. C. for a long
time, and the retained austenite is not ensured smoothly.
Therefore, it is preferable to heat the steel sheet to a
temperature of 500.degree. C. or lower for long-term maintenance,
and inevitably increase the temperature to 500.degree. C. or more
at the time of the galvannealing heat-treatment for fusion
alloying. The galvannealing heat treatment within 1 minute does not
significantly deteriorate the physical properties of the steel of
the present disclosure.
[0105] At this time, a slow cooling section immediately after the
annealing operation may be passed to suppress meandering of the
steel sheet during the cooling operation after the annealing
operation, but the transformation into the polygonal ferrite in the
slow cooling section should be suppressed as much as possible, to
secure the microstructure and physical properties of the steel of
the present disclosure.
[0106] As described above, in the present disclosure, a
low-temperature structure including 0.6% or more of acicular
retained austenite may be heated and maintained in a range of Ac1
to Ac3 to secure the acicular microstructure at the time of
performing the second annealing operation. Therefore, high yield
strength and ductility may be secured, in comparison with physical
properties obtained by the general annealing operations twice,
which does not secure the retained austenite in the conventional
Austempering, the Q&P process and the first annealing
operation.
[0107] Meanwhile, a method of manufacturing a high-strength cold
rolled steel sheet, excellent in terms of yield strength and
ductility, which is another aspect of the present disclosure,
further comprises an operation of forming a plated layer on the
surface of the cold rolled steel sheet after the second annealing
operation.
[0108] In the operation of forming the plated layer, a hot-dip
galvanized layer may be formed by a dip operation in a hot-dip
galvanizing bath, or a galvannealed hot-dip galvanized layer may be
formed by alloying of the formed hot-dip galvanized layer. Further,
the aluminum-silicon plated layer or the zinc-magnesium-aluminum
plated layer may be formed by immersion in an aluminum-silicon or
zinc-magnesium-aluminum melting port.
MODE FOR INVENTION
[0109] Hereinafter, the present disclosure will be described more
specifically by way of examples. It should be noted, however, that
the following examples are intended to illustrate the present
disclosure in more detail, and not to limit the scope of the
present disclosure. The scope of the present disclosure to be
protected may be determined by the matters described in the claims
and matters able to be reasonably deduced therefrom.
[0110] An ingot having a thickness of 90 mm and a width of 175 mm
having a composition illustrated in Table 1 below was prepared
through vacuum melting, homogenized by heating at 1,200.degree. C.
for 1 hour, subjected to hot rolling at 900.degree. C. or higher,
cooled to 630.degree. C. or higher, charged into a furnace already
heated at 630.degree. C. and maintained for 1 hour, and hot rolling
operation was simulated by performing a furnace cooling operation.
Thereafter, the hot rolled sheet material was cold rolled at a cold
reduction rate of 50% to 60%, and then subjected to annealing heat
treatment under the conditions provided in Table 2 below to prepare
a final cold rolled steel sheet. Strength and elongation were
measured, and the results are illustrated in Table 2 below.
[0111] In the following Table 1, the unit of each of element
content is % by weight.
[0112] In Table 1, the unit in Bs (bainite transformation start
temperature), Ms (martensitic transformation start temperature),
Ac1 (austenite appearance temperature at the time of increasing
temperature), Ac3 (temperature at which the ferrite is completely
disappeared and the austenite single phase heat treatment starts,
at the time of increasing temperature) is .degree. C., and may be
calculated by the following relationship. In the following
relationship, the symbol of each element represents the content of
each element in weight %.
Bs=830-270C-90Mn-37Ni-70Cr-83Mo
Ms=539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo
Ac1=723-10.7Mn-16.9Ni+29.1Si+16.9Cr+290As+6.38W
Ac3=910-203
C-15.2Ni+44.7Si+104V+31.5Mo+13.1W-30Mn-11Cr-20Cu+700P+400Al+120As+400Ti
TABLE-US-00001 TABLE 1 Steel C Mn Si P S Al Ti Nb Sb N Bs Ms Ac1
Ac3 *IS 1 0.22 2.17 1.47 0.006 0.002 0.03 -- -- 0.021 0.005 575 380
743 832 IS 2 0.15 2.2 1.49 0.012 0.004 0.03 -- -- 0.018 0.003 592
409 743 852 IS 3 0.18 2.56 1.44 0.011 0.004 0.42 0.02 0.023 0.051
0.004 551 385 738 995 **CS 1 0.22 2.21 1.51 0.01 0.004 0.03 -- --
-- 0.004 572 379 743 835 CS 2 0.08 1.74 1.42 0.011 0.004 0.03 -- --
0.019 0.003 652 452 746 884 CS 3 0.27 4.83 1.57 0.012 0.003 0.03 --
-- 0.02 0.003 322 278 717 750 *IS: Inventive Steel, **CS:
Comparative Steel,
TABLE-US-00002 TABLE 2 Internal 2.sup.nd Microstructure, Oxidation
annealing Ms or less Reheating Over-aging Yield Tensile after
1.sup.st Depth Tem. Cooling Tem. Tem. Strength Strength Elongation
Steel Example annealing (.mu.m) (.degree. C.) Tem. (.degree. C.)
(.degree. C.) (.quadrature.) (MPa) (MPa) (%) *IS 1 ****CE 1 Cold
rolled 0 810 None None 420 412 1012 23 structure + Retained .gamma.
0% CE 2 Martensite + 0 810 None None 420 521 1033 25 Retained
.gamma. 0% CE 3 Martensite + 0 810 None None 420 537 1017 25
Retained .gamma. 0.08% CE 4 Martensite + 0 810 None None 420 572
999 27 Retained .gamma. 0.54% ***IE 1 Martensite + 0 810 None None
420 584 1010 27 Retained .gamma. 1.02% IE 2 Bainite + 0 810 None
None 420 594 981 29 Martensite + Retained .gamma. 5.17% IS 2 IE 3
Martensite + 0 810 250 440 None 580 842 27 Retained .gamma. 0.6% IS
3 IE 4 Martensite + 0 810 250 440 None 598 1150 18 Retained .gamma.
1.3% **CS 1 CE 5 Bainite + 12.3 810 None None 420 588 983 28
Martensite + Retained .gamma. 4.51% CS 2 CE 6 Martensite + 0 810
350 440 None 455 640 33 Retained .gamma. 0.6% CS 3 CE 7 Martensite
+ 0 730 150 440 None 1050 1215 13 Retained .gamma. 0.64% *IS:
Inventive Steel, **CS: Comparative Steel, ***IE: Inventive Example,
****CE: Comparative Example
[0113] In Table 2, an example of Comparative Example 1, in which
the microstructure after a first annealing operation is a cold
rolled structure, was subjected to a final annealing operation (a
second annealing operation) without performing the first annealing
operation after a cold rolling operation. In other examples,
microstructure was obtained by performing the first annealing
operation and cooling operation in a single phase of austenite. In
Table 2, Ms or less cooling temperatures expressed in the side
column of the final annealing temperature represent cooling
temperatures in ranges of Ms to Mf at the time of Q&P heat
treatment, and reheating temperatures represent heat treatment
temperatures increased for second redistribution. Examples in which
the two temperatures are expressed as "None" are examples in which
over-aging treatments of the general annealing operation, instead
of Q&P, were applied, and illustrate the heat treatment
temperature in the heat indicated by the over-aging temperature.
The examples in which Q&P heat treatment was performed are
illustrated as being distinguished from each other by denoting
"None" in columns of over-aging temperature.
[0114] Effects of the amount of retained austenite secured after
the first annealing operation on the physical properties after the
final annealing operation were examined under different cooling
conditions.
[0115] Comparative Example 2 ({circle around (1)} in (a) of FIG. 1)
was cooled to room temperature through a water cooling operation
(average cooling rate: 1000.degree. C./sec or higher), and further
cooled with liquid nitrogen.
[0116] Comparative Example 3 ({circle around (2)} in (a) of FIG. 1)
was cooled to room temperature through a water cooling operation
(average cooling rate: 1000.degree. C./sec or higher).
[0117] Comparative Example 4 ({circle around (3)} in (a) of FIG. 1)
was cooled through a Mist cooling operation (average cooling rate
180.degree. C./sec).
[0118] Inventive Example 1 ({circle around (4)} in (a) of FIG. 1)
was cooled to room temperature at a cooling rate of 25.degree.
C./sec.
[0119] Inventive Example 2 ({circle around (5)} in (a) of FIG. 1)
was cooled by an austempering operation as in No. 5 in (a) of FIG.
1.
[0120] Inventive Examples 3 and 4 and Comparative Examples 6 and 7
were cooled to room temperature at a cooling rate of 25.degree.
C./sec. In Comparative Example 5, the same austempering as in No. 5
in (a) of FIG. 1.
[0121] To confirm yield strength improvement of Comparative Example
1 in which no annealing operation was performed twice for Inventive
Steel 1, and Comparative Examples 2 to 4 and Inventive Examples 1
and 2 in which cooling conditions after the first annealing
operation varied, and multiplication of yield strength and
elongation, multiplication of tensile strength and elongation,
ratio of yield strength to tensile strength, and yield strength
improvement based on Comparative Example 1 were measured and
described. FIG. 3 is a graph illustrating yield strength
improvement after the second annealing operation according to
amounts of acicular retained austenite after the first annealing
operation.
TABLE-US-00003 TABLE 3 YS .times. El TS .times. El YS improvement
YS/TS Example (MPa %) (MPa %) (%) (%) **CE 1 9476 23276 -- 41 CE 2
13030 25833 26.5 50 CE 3 13425 25425 30.3 53 CE 4 15444 26973 38.8
57 *IE 1 15768 27270 41.7 58 IE 2 17226 28449 44.2 61 *IE:
Inventive Example, **CE: Comparative Example
[0122] In Comparative Examples 2 to 4 and Inventive Examples 1 and
2 in which an annealing operation was applied twice, the yield
strength was improved and the multiplication of yield strength and
elongation were both improved, compared with Comparative Example 1
in which a conventional cold hot rolled-annealing treatment was
applied. In particular, it may be seen that the higher the fraction
of retained austenite, the better the yield strength and the better
the multiplication of yield strength and elongation.
[0123] It can be seen from Table 3 and FIG. 3 that inventive
examples 1 and 2, in which the fraction of the acicular retained
austenite is 0.6% or more, provide an improvement of 40% or more,
as compared to a case in which the acicular retained austenite is
less than 0.6%. Particularly, in the case of Inventive Example 2,
in which the amount of acicular retained austenite was very high,
excellent improvements such as YS.times.El, TS.times.El, and the
like, were achieved.
[0124] Invention Example 3 using Inventive Steel 2 had carbon
contents lower than Inventive Steel 1 and a relatively lower
tensile strength, but still exhibited a relatively higher yield
ratio.
[0125] Invention Example 4 using Inventive Steel 3 had a relatively
lower yield ratio due to the introduction of ferrite by addition of
a large amount of Al, but had an excellent properties, such as
TS.times.El of 20,000 MPa % or more by securing 1.3% of the
acicular retained austenite before the second annealing
operation.
[0126] In Comparative Example 6 using Comparative Steel 2, it was
difficult to obtain a tensile strength standard which was limited
in the present disclosure, because the amount of carbon added was
very low. In Comparative Example 7 using Comparative Steel 3, the
strength was excellent due to addition of a large amount of Mn, but
a decrease in elongation is great, and TS.times.El was less than
20,000 MPa %.
[0127] Comparative steel 1 has similar components as those of
Invention Steel 1, except that no Sb is added. In Comparative
Example 5 using comparative steel 1, there was almost no difference
in physical properties from Inventive Example 2 using Inventive
Steel 1, but as illustrated in FIG. 2, the internal oxidation depth
after hot rolling operation was 12.3 .mu.m, and surface cracks and
dents may occur in the subsequent operations, such as pickling
operation, cold rolling operation, and annealing operation.
[0128] While exemplary embodiments have been illustrated and
described above, it will be apparent to those skilled in the art
that modifications and variations could be made without departing
from the scope of the present disclosure as defined by the appended
claims.
* * * * *