U.S. patent application number 12/602764 was filed with the patent office on 2010-07-08 for cold rolledsteel sheet and method for manufacturing the same.
This patent application is currently assigned to HYUNDAI STEEL COMPANY. Invention is credited to Yongbin Im, Seongju Kim, Jeongsu Lee.
Application Number | 20100172788 12/602764 |
Document ID | / |
Family ID | 40854271 |
Filed Date | 2010-07-08 |
United States Patent
Application |
20100172788 |
Kind Code |
A1 |
Kim; Seongju ; et
al. |
July 8, 2010 |
COLD ROLLEDSTEEL SHEET AND METHOD FOR MANUFACTURING THE SAME
Abstract
A method of manufacturing a cold-rolled steel sheet includes:
adding, by weight %, carbon (C) 0.005% or less, nitrogen (N) 0.002
to 0.005%, manganese (Mn) 0.1 to 1.0%, phosphorous (P) 0.005 to
0.1%, niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.30 or less,
sulfur (S) 0.02% or less, aluminum 0.001 to 0.03%; adjusting the
atomic ratio of Nb/C to 1 or more and the atomic ratio of Al/N to
0.5 to 1.5, homogenizing a steel containing iron (Fe) and elements
inevitably contained in manufacturing the steel as the remainder at
temperature of 1150 to 1300.degree. C., setting the final
hot-rolling temperature to 890 to 950.degree. C. that is over an
Ar3 critical point; and hot-winding the hot-rolled steel sheet and
cold-rolling the hot-rolled steel sheet at 40 to 80% cold reduction
ratio.
Inventors: |
Kim; Seongju; (Gyeonggi-do,
KR) ; Lee; Jeongsu; (Chungcheongnam-do, KR) ;
Im; Yongbin; (Seoul, KR) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Assignee: |
HYUNDAI STEEL COMPANY
Incheon
KR
|
Family ID: |
40854271 |
Appl. No.: |
12/602764 |
Filed: |
October 23, 2008 |
PCT Filed: |
October 23, 2008 |
PCT NO: |
PCT/KR2008/006260 |
371 Date: |
December 2, 2009 |
Current U.S.
Class: |
420/87 ; 148/320;
148/603; 420/127; 72/364 |
Current CPC
Class: |
C21D 8/0236 20130101;
C22C 38/12 20130101; C22C 38/001 20130101; C22C 38/04 20130101;
C21D 9/46 20130101; C21D 8/0205 20130101; C22C 38/06 20130101; C21D
2211/004 20130101 |
Class at
Publication: |
420/87 ; 148/603;
148/320; 420/127; 72/364 |
International
Class: |
C22C 38/00 20060101
C22C038/00; C21D 8/02 20060101 C21D008/02; C22C 38/12 20060101
C22C038/12; B21D 31/00 20060101 B21D031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 2007 |
KR |
10-2007-0109220 |
Oct 22, 2008 |
KR |
10-2008-0103409 |
Claims
1. A method of manufacturing a cold-rolled steel sheet, comprising:
adding to iron (Fe), by weight %, carbon (C) 0.005% or less,
nitrogen (N) 0.002 to 0.005%, manganese (Mn) 0.1 to 1.0%,
phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%, silicon
(Si) 0.3% or less, sulfur (S) 0.02% or less, aluminum 0.001 to
0.03% to provide a steel; adjusting the atomic ratio of Nb/C to 1
or more and the atomic ratio of Al/N to 0.5 to 1.5, homogenizing
the steel at a temperature of 1150 to 1300.degree. C., hot-rolling
the steel at a final hot-rolling temperature to 890 to 950.degree.
C. that is over an Ar3 critical point; and hot-winding the
hot-rolled steel sheet and cold-rolling the hot-rolled steel sheet
at 40 to 80% cold reduction ratio.
2. The method of manufacturing a cold-rolled steel sheet according
to claim 1, wherein annealing is performed in a range of 750 to
880.degree. C. after the cold-rolling.
3. The method of manufacturing a cold-rolled steel sheet according
to claim 1, wherein the hot-winding is performed in a temperature
range of 450 to 650.degree. C.
4. A cold-rolled steel sheet formed of a steel comprising: carbon
0.005% or less, nitrogen (N) 0.002 to 0.005%, manganese (Mn) 0.1 to
1%, phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%,
silicon (Si) 0.3% or less, sulfur (S) 0.02% or less, aluminum 0.001
to 0.03%, by weight %, and the remainder of iron (Fe) and elements
inevitably contained in manufacturing the steel, wherein an atomic
ratio of Nb/C is adjusted to 1 or more and an atomic ratio of Al/N
is adjusted to 0.5 to 1.5.
5. A method of manufacturing a steel sheet, the method comprising:
providing a steel alloy comprising iron (Fe), carbon (C), nitrogen
(N), manganese (Mn), phosphorous (P), niobium (Nb), silicon (Si),
sulfur (S), aluminum (Al), wherein the atomic ratio of Nb/C is 1 or
greater, and the atomic ratio of Al/N is from 0.5 to 1.5 in the
steel alloy, hot-rolling the steel alloy to provide a hot-rolled
steel sheet; and cold-rolling the hot-rolled steel sheet.
6. The method of claim 5, wherein providing the steel alloy
comprises: providing a raw steel material; and adding to the raw
steel material one or more elements selected from the group
consisting of carbon (C), nitrogen (N), manganese (Mn), phosphorous
(P), niobium (Nb), silicon (Si), sulfur (S), aluminum (Al).
7. The method of claim 6, wherein Nb is added in such an amount to
make the atomic ratio of Nb/C1 or greater, wherein C is not
added.
8. The method of claim 6, wherein Nb and C are added in such
amounts to make the atomic ratio of Nb/C1 or greater.
9. The method of claim 6, wherein Al and N are added in such
amounts to make the atomic ratio of Al/N fall in a range from 0.5
to 1.5.
10. The method of claim 6, wherein C is added in an amount of 0.005
wt. % or less with reference to the total weight of the steel
alloy, wherein N is added in an amount of 0.002 to 0.005 wt. % with
reference to the total weight, wherein Mn is added in an amount of
0.1 to 1.0 wt. % with reference to the total weight, wherein P is
added in an amount of 0.005 to 0.1 wt. % with reference to the
total weight, wherein Nb is added in an amount of 0.015 to 0.04 wt.
% with reference to the total weight, wherein Si added in an amount
of 0.3 wt. % or less with reference to the total weight, wherein S
is added in an amount of 0.02 wt. % or less with reference to the
total weight, wherein Al is added in an amount of 0.001 to 0.03 wt.
% with reference to the total weight.
11. The method of claim 5, wherein providing the steel alloy
further comprises: homogenizing the element-added steel material at
a temperature between 1150 and 1300.degree. C.
12. The method of claim 5, wherein a final temperature of the
hot-rolling is in a range from 890 to 950.degree. C. over an Ar3
critical point.
13. The method of claim 5, wherein a cold reduction ratio of the
cold rolling is in a range from 40 to 80%.
14. A cold-rolled steel sheet manufactured from the method of claim
6.
15. A cold-rolled steel sheet manufactured from the method of claim
10.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for manufacturing
a cold-rolled steel sheet for an exterior plate of an automobile,
such as a door, hood, and trunk lid, more particularly a method for
manufacturing a cold-rolled steel sheet that is provided with
excellent strain aging resistance at room temperature and bake
hardenability by adjusting the amounts of niobium (Nb) and aluminum
(Al) for fixing carbon and nitrogen of a solid-solution element to
a low-carbon steel and appropriately adjusting the amounts of
manganese (Mn) and phosphorus (P) to adjust the strength of the
steel, while maintaining high yield strength required for an
exterior plate and a finished product after painting-heat
treatment.
BACKGROUND ART
[0002] Recently, cold-rolled steel sheets for vehicles require high
strength for improving fuel efficiency by decreasing weight of the
vehicles and for decreasing weight of the car body, and also
require sufficient yield strength and tensile strength, good
property press-forming property, spot weldability, fatigue
property, and painting-corrosion proof, etc.
[0003] In general, steel sheets have opposite characteristics in
terms of strength and formability, but dual phase steel sheets and
bake hardenable steel sheets have been known as steel sheets that
satisfy both properties.
[0004] In addition to having a low press-forming property as
compared with high tensile strength above 40 kgf/mm2 grade,
manufacturing of dual phase steel sheets are costly due to
significant addition of alloy elements, such as manganese and
chrome.
[0005] On the other hand, bake hardenable steel sheets show yield
strength close to soft steel sheets, when the tensile strength is
40 kgf/mm2 grade or less, so that they have excellent ductility and
their yield strength increases when bake-hardening after
press-forming.
[0006] The bake hardening is a process using a kind of strain aging
that is generated by fixing electric charges that are generated in
deformation of carbon or nitrogen that is interstitial elements
dissolved in the steel. As the amount of dissolved carbon and
nitrogen increases in the solid solution, the level of bake
hardening increases; however, natural aging can be accompanied by
excessively dissolved elements and press-forming property may
deteriorates. Therefore, it is important to control the amount of
dissolved elements.
[0007] Steel sheets for exterior plates of vehicles have been
manufactured to ensure bake hardenability by appropriately
adjusting the amount of titanium (Ti) or niobium (Nb) added in
ultra low carbon aluminum killed steel to adjust the amount of
dissolved elements in the steel, and to ensure yield strength by
adding phosphorous (P), manganese (Mn), and silicon (Si) etc.,
which are solution strengthening elements.
[0008] A method of controlling the amount of remaining carbon by
adding titanium to manufacture a bake hardenable steel may result
in varying qualities of the material because the amount of carbon
may significantly change. This is because titanium can bond with a
variety of elements, such as nitrogen (N), sulfur (S), carbon (C)
in the steel.
[0009] Further, according to other examples of manufacturing bake
hardenable steel, a method of controlling the amount of remaining
carbon by adding niobium (Nb) requires high temperature annealing,
and therefore the resulting material's qualify varies depending
upon the conditions of the annealing, and the quality of plating
may deteriorate in a hot dip plating process that may follow. In
addition, a method of ensuring bake hardenability using dissolved
carbon has difficulty in securing a long aging-warranty period
because the carbon has a relatively high diffusion speed in the
steel.
[0010] That is, bake hardened steels produced by maintaining carbon
atoms dissolved in the steel are of disadvantageous, while bake
hardenability is high, as strain aging resistance at room
temperature decreases as carbon atoms has a high diffusion speed at
room temperature.
[0011] Further, nitrogen is not used for the bake hardenability,
because most of is educed in a winding process into AlN in the case
of aluminum-deoxidized steel=and is educed into Tin at a high
temperature in the case of titanium added steel.,
[0012] Further, there is an additional technology for ensuring bake
hardenability, which removes dissolved carbon by applying high
temperature annealing and removes dissolved nitrogen by adding
aluminum, for low carbon steel having carbon content of 0.01% or
more. However, high temperature annealing has disadvantages in that
quality of the material may vary depending on the control
conditions and the dissolved carbon may not be sufficiently removed
after the annealing.
[0013] In this case, even if the dissolved carbon is removed by
adding titanium and niobium, formability is deteriorated and the
strain aging resistance at room temperature is not sufficiently
ensured by the remaining carbon unless management of NbC and TiC
are controlled.
Technical Problem
[0014] In order to overcome the problems, it is an object of the
invention to provide a cold-rolled steel sheet having excellent
dent resistance by adjusting the amount of niobium (Nb) and
aluminum (Al) for fixing carbon and nitrogen, appropriately
adjusting the amount of manganese (Mn) and phosphorous (P) to
adjust strength of the steel, and using low-temperature annealing
and low-temperature winding to maintain necessary yield strength
for an exterior plate and high yield strength in the final product
after painting-heat treatment.
Technical Solution
[0015] A method of manufacturing a cold-rolled steel sheet
according to the present invention includes: providing an alloy
steel comprising, by weight % with reference to the total weight of
the alloy steel, carbon (C) 0.0050 or less, nitrogen (N) 0.002 to
0.005%, manganese (Mn) 0.1 to 1%, phosphorous (P) 0.005 to 0.1%,
niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.3% or less, sulfur (S)
0.02% or less, aluminum 0.001 to 0.03%, and iron (Fe); adjusting
the atomic ratio of Nb/C to 1 or more and the atomic ratio of Al/N
to 0.5 to 1.5, homogenizing the alloy steel at a temperature of
1150 to 1300.degree. C., hot-rolling the steel alloy to provide a
hot-rolled steel sheet at a final hot-rolling temperature to 890 to
950.degree. C. that is at right over an Ar3 critical point; and
hot-winding the hot-rolled steel sheet and cold-rolling the
hot-rolled steel sheet at a cold reduction ratio from 40 to
80%.
[0016] Further, annealing is performed in a temperature range of
750 to 880.degree. C. after the cold rolling.
[0017] Further, it is preferable that the hot-winding is performed
in a range of 450 to 650.degree. C. temperature.
[0018] A cold-rolled steel sheet formed of a steel according to an
embodiment of the present invention comprises: carbon (C) 0.005% or
less, nitrogen (N) 0.002 to 0.005%, manganese (Mn) 0.1 to 1%,
phosphorous (P) 0.005 to 0.1%, niobium (Nb) 0.015 to 0.04%, silicon
(Si) 0.3% or less, sulfur (S) 0.02% or less, aluminum 0.001 to
0.03%, by weight %, and the remainder of iron (Fe) and elements
inevitably contained in manufacturing the steel, wherein an atomic
ratio of Nb/C is adjusted to 1 or more and an atomic ratio of Al/N
is adjusted to 0.5 to 1.5.
Advantageous Effects
[0019] The present invention is designed to manufacture a
cold-rolled steel sheet having excellent strain aging resistance at
room temperature and bake hardenability using carbon and nitrogen
as solid-solution elements. Accordingly, the present invention has
an advantage of manufacturing a cold-rolled steel sheet having
excellent strain aging resistance at room temperature and bake
hardenability and using low-temperature annealing and
low-temperature winding.
[0020] Further, the present invention has an advantage of
preventing non-uniform machining and ensuring strain aging
resistance at room temperature and a long aging-warranty period, by
maximally preventing solid-solution carbon to prevent effect of
carbon in bake hardening.
[0021] Further, according to embodiments of the present invention,
since the manganese content is decreased, workability and spot
weldability are improved. Reduction of strength of the steel sheet
due to reduction of the manganese content is compensated by the
bake hardening by controlling precipitates and solid-solution
nitrogen. Therefore, the present invention can be stably used for
exterior plates of vehicles.
DESCRIPTION OF DRAWINGS
[0022] FIG. 1 is a graph illustrating a comparison of an embodiment
according to a method of manufacturing a cold-rolled steel sheet
with a comparative example having different elements, and showing
change of bake hardening value to hot winding temperature.
[0023] FIG. 2 is a graph showing changes in bake hardening values
over annealing temperature of the embodiment of the present
invention and a comparative example.
[0024] Preferred embodiments of the invention are described
hereafter in detail with reference to the accompanying
drawings.
[0025] A cold-rolled steel sheet and a method for manufacturing the
same according an embodiment of the present invention includes:
providing a steel alloy comprising: by weight %, nitrogen (N) 0.02
to 0.05%, manganese (Mn) 0.1 to 1%, phosphorous (P) 0.005 to 0.1%,
niobium (Nb) 0.015 to 0.04%, silicon (Si) 0.30 or less, sulfur (S)
0.02% or less, aluminum 0.001 to 0.03%, iron (Fe) and inevitable
impurities; adjusting the atomic ratio of Nb/C to 1 or less and the
atomic ratio of Al/N to 0.5 to 1.5; homogenizing the steel alloy at
a temperature ranging from 1150 to 1300.degree. C. that is an
austenite region, forming a hot-rolled steel sheet by rolling the
steel at a temperature ranging from 890 to 950.degree. C. that is
over an Ar3 critical point in the final hot rolling; hot-winding
the hot-rolled steel sheet at a temperature range of 450 to
650.degree. C., rolling the hot-rolled steel sheet at a cold
reduction ratio ranging from 40 to 80% cold reduction ratio, and
performing annealing at a temperature ranging from 750 to
880.degree. C.
[0026] Thereafter, a hot dip plated steel sheet can be manufactured
by performing hot dip plating at a temperature of 460.degree. C. in
a process of galvalume or zinc plating on an alloying hot dip
plating line and performing alloying at a temperature ranging from
460 to 560.degree. C.
[0027] It is preferable to perform overaging at a temperature of
400.degree. C. after the annealing, but may be skipped when
annealing is performed at a low temperature.
[0028] The plating temperature of 460.degree. C. is a temperature
in a melter known in the art although it is not limited
thereto.
[0029] The temperature of hot-winding is performed at a temperature
lower than 450.degree. C., nitrogen can be bonded into AlN in a
slab re-heating process, such that bake hardenability may not be
ensured by the nitrogen.
[0030] On the contrary, when the winding temperature is higher than
650.degree. C., the bake hardenability can rapidly decrease, and
therefore the temperature of the hot-winding may be limited to a
range from 450 to 650.degree..
[0031] Further, according to the alloy composition of the present
invention, an ultra low carbon steel containing carbon of 0.005 wt
% or less can be used as raw steel to minimize the solid-solution
carbon, which controls bake hardening of the steel, with the
solid-solution nitrogen rather than carbon.
[0032] Control using the nitrogen is more advantageous to achieve
bake hardening than using the carbon. This is because the nitrogen
has a lower diffusion speed in steel than the carbon, such that it
is advantageous in strain aging resistance at room temperature.
Here, the term "strain aging resistance at room temperature"
regards the changes in the quality of steel as time passes, and
bake-hardened steel should be ensured in the strain aging
resistance at room temperature because it will be used in the
automotive manufacturing while after the steel manufacturing.
[0033] A very small amount of remaining solid-solution carbon can
be maximally removed by adjusting the atom ratio of Nb/C. The atom
ratio of Nb/C is adjusted to be 1 or greater, which educes all of
the solid-solution carbon in the steel into a precipitate of NbC
such that only solid-solution nitrogen is in present in the steel.
Accordingly, the solid-solution carbon plays little role in bake
hardening.
[0034] The solid-solution nitrogen is controlled by aluminum that
forms a precipitate with nitrogen. When the solid-solution nitrogen
is not appropriately controlled, the strain aging resistance at
room temperature and the formability may be deteriorated. The
atomic ratio of Al/N for controlling the solid-solution nitrogen
can be adjusted to a range from 0.5 to 1.5. This is because when
the atomic ratio of Al/N is below 0.5, the strain aging resistance
at room temperature may not be stably ensured, whereas when it is
over 1.5, appropriate amount of solid-solution nitrogen may not be
ensured, such that the bake hardenability can deteriorate.
[0035] Further, the alloy composition of the invention improves
workability and spot weldability by reducing the content amount of
manganese that can deteriorate the workability and spot
weldability. Reduction of strength of the cold-rolled steel sheet
that is caused by reducing the manganese content can be compensated
by homogenizing and making the structure minute by NbC and AlN
precipitation hardening.
[0036] Components contained in the cold-rolled steel sheet of
embodiments of the present invention are as follows, in reference
to weight % (referred to as % hereafter).
[0037] 1. Carbon (C) 0 005% or less
[0038] When the amount of carbon is 0.005% or higher, the amount of
niobium Nb for fixing the carbon can increase, such that not only
manufacturing cost of the steel may increase, but workability of
the steel may decrease.
[0039] Further, when fixing the carbon using the niobium is
insufficient, aging may rapidly progress due to the carbon, such
that the strain aging resistance at room temperature of the steel
may be reduced. Therefore, the amount of the carbon is limited to
0.005% or less.
[0040] 2. Silicon (Si): 0.3% or less
[0041] The silicon (Si) can increase activity of the carbon present
in a solid solution state in the steel, such that the strain aging
resistance at room temperature can deteriorate and the quality of
plating can be significantly reduced. Further, although as the
amount increases, the strength may be increased by solid solution
hardening, but which decreases ductility, such that the maximum
added amount of silicon is limited to 0.3%.
[0042] 3. Manganese (Mn): 0.1 to 1.0%
[0043] The manganese (Mn) is in present in the solid-solution state
in the steel and has a function of increasing the strength of the
steel. However, the amount of 1.0% or more can largely decrease the
ductility, such that the maximum added amount of manganese may be
limited to 1.0%. On the other hand, when no manganese is added to
the steel, hot shortness may be caused by sulfur present in the
steel, such that the minimum added amount of manganese is
preferably limited to 0.1%.
[0044] 4. Phosphorous (P): 0.005 to 0.1%
[0045] The phosphorous is present in the solid-solution state in
the steel has a function of increasing the strength of the steel.
The amount of 0.1% or more may considerably decrease the ductility
and weldability of the steel, such that the maximum added amount of
the phosphorous may be limited to 0.1%. However, when no manganese
is added to the steel, it may be difficult to ensure sufficient
strength of the steel, such that the minimum added amount of the
phosphorous is preferably limited to 0.005%.
[0046] 5. Niobium (Nb): 0.015 to 0.04%
[0047] The niobium is added to fix the carbon present in the
solid-solution state in the steel. The solid-solution carbon
present in the steel prevents a cold-rolling collective structure
from being formed, such that the workability of the steel
deteriorates. Further, when carbon in the solid-solution state
exists, the strain aging resistance at room temperature is
deteriorated by rapid diffusion of the carbon, such that a
sufficient amount of niobium is needed to fix the solid-solution
carbon. The necessary amount of niobium is set such that the atom
ratio of Nb/C is 1 or more; therefore, the minimum amount is
limited to 0.015% and the maximum is limited to 0.04% in
consideration of the amount of carbon.
[0048] 6. Nitrogen (N): 0.002 to 0.005%
[0049] In general, the nitrogen (N) is an element that is
inevitably added to the steel; however, it is needed to adjust the
added amount of nitrogen in the present invention because the
present invention controls the bake hardenability using the
nitrogen. When the added amount is too small, it is difficult to
ensure the bake hardenability and when the added amount is too
large, it may be possible to ensure sufficient bake hardenability
by the nitrogen, but may cause aging due to the solid-solution
nitrogen and deteriorate the workability. Therefore, the added
amount of nitrogen is in the range of 0.02 to 0.005%.
[0050] 7. Aluminum (Al): 0.001 to 0.03%
[0051] The aluminum is also added to deoxidize the steel, but is
used to control the bake hardenability by bonding with the nitrogen
in the present invention. When the amount of aluminum is 0.001% or
less, deoxidization is decreased and oxygen is in present in the
steel. Accordingly, when elements that form oxidized substances,
such as manganese and silicon, are added during manufacturing of
the steel, manganese oxide and silicon oxide are formed, such that
element control of the silicon etc. is difficult. However, when the
amount of aluminum is 0.03% or more, unnecessarily excessive amount
is added, such that it reacts with nitrogen being in present in the
steel and forms an aluminum nitride precipitate. Therefore, the
bake hardenability by the nitrogen cannot be achieved. Accordingly,
the maximum added amount of the nitrogen is limited to 0.03%
[0052] In addition, the sulfur (S) is an element that is generally
inevitably contained during manufacturing of the steel, such that
the addition range is limited to 0.02% or less.
[0053] The following Table 1 shows an embodiment of the invention
and a comparative example each having different components.
TABLE-US-00001 TABLE 1 Steel Chemical element No. C Nb Mn P S Al N
Al/N Reference 1 0.0023 0.028 0.2 0.011 0.007 0.005 0.0024 1.11
Embodiment 2 0.0030 0.030 0.4 0.040 0.005 0.005 0.0030 0.86
Embodiment 3 0.0021 0.025 0.6 0.030 0.005 0.006 0.0035 0.89
Embodiment 4 0.0031 0.030 0.3 0.060 0.005 0.010 0.0044 1.18
Embodiment 5 0.0022 0.020 0.2 0.020 0.005 0.040 0.0025 8.30
Comparative example 6 0.0025 0.050 0.2 0.011 0.006 0.02 0.0024 4.30
Comparative example 7 0.0023 0 0.2 0.011 0.006 0.01 0.0048 1.08
Comparative example 8 0.0025 0.018 0.3 0.06 0.005 0.045 0.0034 6.8
Comparative example
[0054] In Table 1, the Embodiments and Comparative examples are
achieved by maintaining the ingot of the solid-solution steel for
two hours in a heating furnace of 1250.degree. C. and then
hot-rolling it, in which the final temperature of the hot rolling
is 900.degree. C., the temperature of hot-winding is 560.degree.
C., and cold rolling is performed at 70% cold reduction ratio.
[0055] The cold-rolled sample is cooled at a cooling speed of
-3.degree. C./sec and continuously annealed at a temperature of
800.degree. C., and the sample after the continuous annealing has
undergone a tensile test in a universal testing machine.
[0056] The following Table 2 shows changes in the mechanical
properties according to heat treatment conditions and manufacturing
conditions of the embodiment and comparative example of Table
1.
TABLE-US-00002 TABLE 2 Mechanical property Yield Tensile Elongation
Steel strength strength ratio BH AI No. (MPa) (MPa) (%) (MPa) (MPa)
Reference 1 182 283 46 35 23 Embodiment 2 230 355 39 37 24
Embodiment 3 233 357 40 33 22 Embodiment 4 240 360 38 40 20
Embodiment 5 170 280 45 20 10 Comparative example 6 160 280 47 0 0
Comparative example 7 210 270 45 48 38 Comparative example 8 230
350 38 25 22 Comparative example
[0057] As shown in Table 2, sample Nos. 1 to 4 correspond to
embodiments of the present invention and have tensile strength of
270 to 360 MPa, elongation ratio of 38 to 47%, bake hardening
strength of 33 to 40 MPa, and aging index of 30 or less, such that
they achieve high-strength steel, maintain excellent ductility,
have high bake hardenability and excellent strain aging resistance
at room temperature.
[0058] On the other hand, in comparative examples Nos. 5, 6, and 8,
the addition amount of Al is high, such that even though the
winding process is performed at a low winding temperature,
sufficient bake hardenability cannot be ensured by the aluminum
fixing the nitrogen.
[0059] Further, in comparative No. 7, niobium is not added, such
that a large amount of carbon is in present in the solid-solution
state in the steel, and accordingly, the bake hardenability is
high, but the strain aging resistance at room temperature is
low.
[0060] FIG. 1 is a graph showing changes in bake hardening values
according to hot-winding temperature in one example of each of the
comparative example and the embodiment (Embodiment No. 1 and
Comparative Example No. 5) and FIG. 2 is a graph showing changes in
bake hardening values according to annealing temperature.
[0061] It can be seen from FIG. 1 that as the winding temperature
of Embodiment No. 1 decreases, the bake hardenability increases,
and particularly, the bake hardenability rapidly increases under
600.degree. C.
[0062] This is because precipitation of AlN is delayed when the
hot-rolling winding temperature decreases, such that nitrogen in a
large amount of solid-solution state can exist.
[0063] In Embodiment Nos. 1, 2, 3, and 4 having sufficient
solid-solution nitrogen is ensured in the hot-rolling winding
process, as shown in FIG. 2, it is possible to ensure sufficient
bake hardenability even at a low annealing temperature, such that
low-temperature annealing is possible. The lower the annealing
temperature, the more the energy is saved and the alloying dip
plating property is improved.
* * * * *