U.S. patent application number 16/063985 was filed with the patent office on 2018-12-27 for low-yield-ratio type high-strength steel, and manufacturing method therefor.
This patent application is currently assigned to POSCO. The applicant listed for this patent is POSCO. Invention is credited to Mun-Young JUNG, Seng-Ho YU.
Application Number | 20180371590 16/063985 |
Document ID | / |
Family ID | 59090793 |
Filed Date | 2018-12-27 |
United States Patent
Application |
20180371590 |
Kind Code |
A1 |
YU; Seng-Ho ; et
al. |
December 27, 2018 |
LOW-YIELD-RATIO TYPE HIGH-STRENGTH STEEL, AND MANUFACTURING METHOD
THEREFOR
Abstract
A low-yield-ratio type high-strength steel includes 0.02-0.11 wt
% of carbon (C), 0.1-0.5 wt % of silicon (Si), 1.5-2.5 wt % of
manganese (Mn), 0.01-0.06 wt % of aluminum (Al), 0.1-0.6 wt % of
nickel (Ni), 0.01-0.03 wt % of titanium (Ti), 0.005-0.08 wt % of
niobium (Nb), 0.1-0.5 wt % of chromium (Cr), 0.01 wt % or less of
phosphorus (P) (excluding 0 wt %), 0.01 wt % or less of sulfur (S)
(excluding 0 wt %), 5-30 weight ppm of boron (B), 20-70 weight ppm
of nitrogen (N), 50 weight ppm or less of calcium (Ca) (excluding 0
weight ppm), 5-50 weight ppm or less of tin (Sn) (excluding 0
weight ppm), and a remainder thereof, being iron (Fe), and other
inevitable impurities.
Inventors: |
YU; Seng-Ho; (Pohang-si,
Gyeongsangbuk-do, KR) ; JUNG; Mun-Young; (Seoul,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si, Gyseongsangbuk -do |
|
KR |
|
|
Assignee: |
POSCO
Pohang-si, Gyeongsangbuk-do
KR
|
Family ID: |
59090793 |
Appl. No.: |
16/063985 |
Filed: |
December 2, 2016 |
PCT Filed: |
December 2, 2016 |
PCT NO: |
PCT/KR2016/014135 |
371 Date: |
June 19, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/06 20130101;
C21D 2211/002 20130101; C22C 38/001 20130101; C21D 8/0226 20130101;
C22C 38/48 20130101; C22C 38/54 20130101; C22C 38/46 20130101; C22C
38/008 20130101; C22C 38/58 20130101; C22C 38/44 20130101; C21D
2211/008 20130101; C22C 38/50 20130101; C22C 38/02 20130101; C22C
38/42 20130101; C22C 38/00 20130101; C21D 8/02 20130101; C21D
2211/005 20130101; C21D 8/0205 20130101; C22C 38/002 20130101; C22C
38/04 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/50 20060101 C22C038/50; C22C 38/54 20060101
C22C038/54; C22C 38/48 20060101 C22C038/48; C22C 38/02 20060101
C22C038/02; C22C 38/06 20060101 C22C038/06; C22C 38/00 20060101
C22C038/00; C21D 8/02 20060101 C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 2015 |
KR |
10-2015-0186522 |
Claims
1. Low-yield-ratio type, high-strength steel comprising: 0.02 to
0.11 wt % of carbon (C); 0.1 to 0.5 wt % of silicon (Si); 1.5 to
2.5 wt % of manganese (Mn); 0.01 to 0.06 wt % of aluminum (Al); 0.1
to 0.6 wt % of nickel (Ni); 0.01 to 0.03 wt % of titanium (Ti);
0.005 to 0.08 wt % of niobium (Nb); 0.1 to 0.5 wt % of chromium
(Cr); 0.01 wt % or less of phosphorus (P); 0.01 wt % or less of
sulfur (S); 5 to 30 wt ppm of boron (B); 20 to 70 wt ppm of
nitrogen (N); 50 wt ppm or less of calcium (Ca) (excluding 0); 5 to
50 wt ppm or less of tin (Sn); iron (Fe) as a remainder thereof;
and other inevitable impurities.
2. The low-yield-ratio type, high-strength steel of claim 1,
wherein the steel further includes one or more of 0.1 to 0.5 wt %
of copper (Cu), 0.15 to 0.3 wt % of molybdenum (Mo), and 0.005 to
0.3 wt % of vanadium (V).
3. The low-yield-ratio type, high-strength steel of claim 1,
wherein a refined structure of the steel includes bainitic ferrite
and granular bainite in a primary phase and includes M-A in a
secondary phase.
4. The low-yield-ratio type, high-strength steel of claim 3,
wherein the bainitic ferrite has an area fraction of 80 to 95%, the
granular bainite has an area fraction of 5 to 20%, and the M-A has
an area fraction of 3% or less (including 0%).
5. The low-yield-ratio type, high-strength steel of claim 1,
wherein PImax. (111)/PImax. (100) as a ratio of pole intensity
(PImax.) of (100) and (111) crystalline surfaces of the steel is
1.0 or more or 1.8 or less (where the PImax. (111) is pole
intensity of the (111) crystalline surface and the PImax. (100) is
pole intensity of the (100) crystalline surface).
6. The low-yield-ratio type, high-strength steel of claim 1,
wherein the steel has a yield ratio of 0.85 or less and has tensile
strength of 800 MPa or more.
7. The low-yield-ratio type, high-strength steel of claim 1,
wherein the steel has a thickness of 60 mm or less.
8. A method of manufacturing low-yield-ratio type, high-strength
steel, the method comprising: heating a slab including 0.02 to 0.11
wt % of carbon (C), 0.1 to 0.5 wt % of silicon (Si), 1.5 to 2.5 wt
% of manganese (Mn), 0.01 to 0.06 wt % of aluminum (Al), 0.1 to 0.6
wt % of nickel (Ni), 0.01 to 0.03 wt % of titanium (Ti), 0.005 to
0.08 wt % of niobium (Nb), 0.1 to 0.5 wt % of chromium (Cr), 0.01
wt % or less of phosphorus (P), 0.01 wt % or less of sulfur (S), 5
to 30 wt ppm of boron (B), 20 to 70 wt ppm of nitrogen (N), 50 wt
ppm or less of calcium (Ca) (excluding 0), 5 to 50 wt ppm or less
of tin (Sn), iron (Fe) as a remainder thereof, and other inevitable
impurities, at 1050 to 1250.degree. C.; rough-rolling the heated
slab at 950 to 1050.degree. C. to acquire a bar; hot-rolling the
bar at final rolling temperature of 700 to 950.degree. C. to
acquire hot rolled steel; and cooling the hot rolled steel at
cooling speed of 25 to 50.degree. C./s up to cooling termination
temperature of Bs or less.
9. The method of claim 8, wherein the slab further includes one or
more of 0.1 to 0.5 wt % of copper (Cu), 0.15 to 0.3 wt % of
molybdenum (Mo), and 0.005 to 0.3 wt % of vanadium (V).
10. The method of claim 8, wherein the hot-rolling is performed at
a reduction ratio of 50 to 80%.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to low-yield-ratio type,
high-strength steel and a manufacturing method therefor, and more
particularly, to low-yield-ratio type, high-strength steel that is
appropriate for use as steel for construction due to a low yield
ratio and high tensile strength, and a manufacturing method
therefor.
BACKGROUND ART
[0002] Recently, according to the trend for extremely tall and long
span structures, such as domestic and foreign buildings and
bridges, there has been a need to develop extra thick high strength
steel. When high strength steel is used, the structures of
buildings and bridges are rationalized and lightened due to high
allowable stress of the high strength steel and, thus, economical
construction is possible and a plate thickness is reduced, thereby
allowing machining such as cutting or perforation and welding
operations to be easily performed.
[0003] When the strength of steel is increased, a yield ratio
(yield strength/tensile strength) that is a ratio of tensile
strength to yield strength is frequently increased. In this regard,
when a yield ratio is increased, a stress difference up to a point
in time at which deconstruction occurs from a point in time (yield
point) at which plastic deformation occurs is not high and, thus,
buildings have a low construction margin for preventing buildings
from disintegrating through absorbing energy via deformation and,
accordingly, when a large external force, such as that of an
earthquake is applied, there is a problem in that it is difficult
to ensure safety. Accordingly, steel for construction needs to
satisfy both high strength and low yield ratio.
[0004] In general, it is well known that a yield ratio of steel is
lowered by realizing a structure in which a metal structure of
steel including a soft phase material such as ferrite as a major
structure and a hard phase material such as bainite or martensite
is appropriately distributed.
[0005] To acquire a structure in which a hard phase material is
appropriately distributed in the above soft phase-based refined
structure, Patent Document 1 discloses a method of lowering a yield
ratio via appropriate quenching and tempering in a dual phase
region of ferrite and austenite. However, the method requires an
additional number of thermal treatment processes in addition to a
rolling manufacturing process and, thus, there is a problem in that
it is inevitable to reduce productivity and to increase
manufacturing costs.
[0006] Accordingly, there has been a need to develop
low-yield-ratio type, high-strength steel and a manufacturing
method therefor, for ensuring super high strength and low yield
ratio as well as overcoming the problem in terms of reduced
productivity, increased manufacturing costs, etc.
CITED REFERENCE
[0007] (Patent Document 1) Japanese Patent Laid-Open Publication
No. sho 55-97425
DISCLOSURE
Technical Problem
[0008] An aspect of the present disclosure is to provide
low-yield-ratio type, high-strength steel and a manufacturing
method therefor, and in detail, low-yield-ratio type, high-strength
steel and a manufacturing method therefor, for ensuring super high
strength and a low yield ratio without reduction in productivity
and increase in manufacturing costs.
[0009] The objective of the present disclosure is not limited to
the above description and the following detailed description of the
present disclosure are exemplary and explanatory and are intended
to provide further explanation of the feature as claimed by one of
ordinary skill in the art without undue difficulty.
Technical Solution
[0010] According to an aspect of the present disclosure,
low-yield-ratio type, high-strength steel includes
[0011] 0.02 to 0.11 wt % of carbon (C), 0.1 to 0.5 wt % of silicon
(Si), 1.5 to 2.5 wt % of manganese (Mn), 0.01 to 0.06 wt % of
aluminum (Al), 0.1 to 0.6 wt % of nickel (Ni), 0.01 to 0.03 wt % of
titanium (Ti), 0.005 to 0.08 wt % of niobium (Nb), 0.1 to 0.5 wt %
of chromium (Cr), 0.01 wt % or less of phosphorus (P) (excluding 0
wt %), 0.01 wt % or less of sulfur (S) (excluding 0 wt %), 5 to 30
wt ppm of boron (B), 20 to 70 wt ppm of nitrogen (N), 50 wt ppm or
less of calcium (Ca) (excluding 0 wt ppm), 5 to 50 wt ppm or less
of tin (Sn) (excluding 0 wt ppm), iron (Fe) as a remainder thereof,
and other inevitable impurities.
[0012] According to another aspect of the present disclosure, a
method of manufacturing low-yield-ratio type, high-strength steel
includes heating a slab including 0.02 to 0.11 wt % of carbon (C),
0.1 to 0.5 wt % of silicon (Si), 1.5 to 2.5 wt % of manganese (Mn),
0.01 to 0.06 wt % of aluminum (Al), 0.1 to 0.6 wt % of nickel (Ni),
0.01 to 0.03 wt % of titanium (Ti), 0.005 to 0.08 wt % of niobium
(Nb), 0.1 to 0.5 wt % of chromium (Cr), 0.01 wt % or less of
phosphorus (P), 0.01 wt % or less of sulfur (S), 5 to 30 wt ppm of
boron (B), 20 to 70 wt ppm of nitrogen (N), 50 wt ppm or less of
calcium (Ca) (excluding 0), 5 to 50 wt ppm or less of tin (Sn), a
balance thereof, being iron (Fe), and other inevitable impurities,
at 1050 to 1250, rough-rolling the heated slab at 950 to
1150.degree. C. to acquire a bar, hot-rolling the bar at final
rolling temperature of 700 to 950.degree. C. to acquire hot rolled
steel, and cooling the hot rolled steel at cooling speed of 25 to
50.degree. C./s up to cooling termination temperature of Bs or
less.
[0013] The objective of the present disclosure is not limited to
the above description and various features and advantages thereof
are understood will be more clearly understood from the following
detailed description.
Advantageous Effects
[0014] As set forth above, according to an exemplary embodiment in
the present disclosure, low-yield-ratio type, high-strength steel
and a manufacturing method therefor may ensure extra high strength
and a low yield ratio without reduction in productivity and
increase in manufacturing costs.
BEST MODE FOR INVENTION
[0015] Exemplary embodiments of the present disclosure will now be
described more fully. The exemplary embodiments of present
disclosure may, however, be embodied in many different forms and
should not be construed as being limited to the embodiments set
forth herein; rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the
concept of the feature to those skilled in the art.
[0016] Hereinafter, low-yield-ratio type, high-strength steel
according to an aspect of the present disclosure is described in
detail.
[0017] The low-yield-ratio type, high-strength steel according to
an aspect of the present disclosure may include 0.02 to 0.11 wt %
of carbon (C), 0.1 to 0.5 wt % of silicon (Si), 1.5 to 2.5 wt % of
manganese (Mn), 0.01 to 0.06 wt % of aluminum (Al), 0.1 to 0.6 wt %
of nickel (Ni), 0.01 to 0.03 wt % of titanium (Ti), 0.005 to 0.08
wt % of niobium (Nb), 0.1 to 0.5 wt % of chromium (Cr), 0.01 wt %
or less of phosphorus (P) (excluding 0 wt %), 0.01 wt % or less of
sulfur (S) (excluding 0 wt %), 5 to 30 wt ppm of boron (B), 20 to
70 wt ppm of nitrogen (N), 50 wt ppm or less of calcium (Ca)
(excluding 0 wt ppm), 5 to 50 wt ppm or less of tin (Sn) (excluding
0 wt ppm), a balance thereof, being iron (Fe), and other inevitable
impurities.
[0018] Carbon (C): 0.02 to 0.11 wt %
[0019] C is an important element for forming bainite or martensite
and determining a size and fraction of the formed bainite or
martensite.
[0020] When a content of C is greater than 0.11 wt %, low
temperature toughness may be lowered and, when a content of C is
less than 0.02 wt %, formation of bainite or martensite may be
obstructed to degrade strength. Accordingly, content of C may be
0.02 to 0.11 wt %.
[0021] For high weldability of a plate used as a steel structure
for welding, an upper limit of content of C may be 0.08 wt %.
[0022] Silicon (Si): 0.1 to 0.5 wt %
[0023] Si is an element used as a deoxidizer for enhancing strength
and toughness.
[0024] When a content of Si is greater than 0.5 wt %, low
temperature toughness and weldability may be lowered and thick
scale is formed on a plate surface to cause gas cuttability
defects, other surface cracks, and so on. On the other hand, when a
content of Si is less than 0.1 wt %, a deoxidation effect may be
insufficient. Accordingly, a content of Si may be 0.1 to 0.5 wt %
and, more particularly, 0.15 to 0.35 wt %.
[0025] Manganese (Mn): 1.5 to 2.5 wt %
[0026] Mn may be an element useful to enhance strength via
solid-solution strengthening and, thus, it is necessary to add 1.5
wt % or more of Mn. However, when a content of Mn is greater than
2.5 wt %, toughness of a welding portion may be largely lowered due
to excessive increase in hardenability. Accordingly, a content of
Mn may be 1.5 to 2.5 wt %.
[0027] Aluminum (Al): 0.01 to 0.06 wt %
[0028] Al may be an element that inexpensively deoxidizes molten
steel and stabilizes ferrite. When a content of Al is less than
0.01 wt %, the aforementioned effect may be insufficient. On the
other hand, when a content of Al is greater than 0.06 wt %, a
nozzle may clog in the case of continuous casting. Accordingly, a
content of Al may be 0.01 to 0.06 wt %.
[0029] Nickel (Ni): 0.1 to 0.6 wt %
[0030] Ni may be an element that simultaneously enhances strength
and toughness of a parent material. According to the present
disclosure, to acquire the aforementioned effect, 0.1 wt % or more
of Ni may be added. However, Ni is an expensive element and, thus,
when Ni is added in an amount greater than 0.6 wt %, economization
may be reduced and weldability may be lowered. Accordingly, content
of Ni may be 0.1 to 0.6%.
[0031] Titanium (Ti): 0.01 to 0.03 wt %
[0032] Ti prevents growth of crystal grain while being reheated to
largely enhance low temperature toughness and, thus, 0.01 wt % or
more of Ti may be added. However, when a content of Ti is greater
than 0.03 wt %, there may be a problem in terms of a reduction in
low temperature toughness due to clogging of a nozzle for
continuous casting or crystallization of a central portion.
Accordingly, content of Ti may be 0.01 to 0.03 wt %.
[0033] Niobium (Nb): 0.005 to 0.08 wt %
[0034] Nb may be an important element for manufacturing TMCP steel
and is precipitated in the form of NbC or NbCN to enhance strength
of a parent material and a welding portion. Nb that is solid-solved
while being reheated at high temperature prevents
re-crystallization of austenite and modification of ferrite or
bainite to refine a structure. Furthermore, according to the
present disclosure, bainite may be formed even at low cooling speed
when a slab is cooled after rough rolling is performed, and
stability of austenite may also be enhanced when a slab is cooled
after last rolling is performed and, accordingly, generation of
martensite may also be facilitated in the case of cooling at low
speed.
[0035] To sufficiently acquire the aforementioned effect, content
of Nb may be 0.005 wt % or more. However, when a content of Nb is
greater than 0.08 wt %, brittleness crack may occur at an edge of
steel. Accordingly, content of Nb may be 0.005 to 0.08 wt %.
[0036] Chromium (Cr): 0.1 to 0.5 wt %
[0037] To acquire strength, Cr may be an element added to ensure
strength and may enhance quenching property. To sufficient the
aforementioned effect, it may be required to add 0.1% or more of
Cr. However, when a content of Cr is greater than 0.5%, rigidity of
a welding portion may be excessively increased and toughness may be
degraded. Accordingly, content of Cr may be 0.1 to 0.5%.
[0038] Phosphorus (P): 0.01 wt % or less
[0039] P may be an element that is advantageous for enhancing
strength and anti-corrosion but largely obstructs impact toughness
and, thus, content of P may be advantageously lowered and, thus, an
upper content limit of P may be 0.01 wt %.
[0040] Sulfur (S): 0.01 wt % or less
[0041] S may be an element that forms MnS or the like and largely
obstructs impact toughness and, thus, content of S may be
advantageously lowered and an upper content limit of S may be 0.01
wt %.
[0042] Boron (B): 5 to 30 wt ppm
[0043] B may be a much inexpensive element indicating high
hardenability and may be an element that is largely useful to form
bainite at low speed when being cooled after rough rolling is
performed.
[0044] By simply adding a small amount of B, strength may be
largely enhanced and, thus, 5 wt ppm or more of B may be added.
However, when a content of B is greater than 30 wt ppm, Fe.sub.23
(CB).sub.6 may be formed to rather reduce hardenability and to
largely reduce low temperature toughness. Accordingly, content of B
may be 5 to 30 wt ppm.
[0045] Nitrogen (N): 20 to 70 wt ppm
[0046] N enhances strength but largely reduces toughness and, thus,
content of N may be controlled to be 70 wt ppm or less. However,
when a content of N is controlled to be less than 20 wt ppm, steel
manufacturing load is increased and, thus, a lower content limit of
N may be 20 wt ppm.
[0047] Calcium (Ca): 60 wt ppm or less (excluding 0)
[0048] Ca may be used as an element that mainly prevents
non-metallic inclusion of MnS and enhances low temperature
toughness. However, when an excessive amount of Ca is added, Ca
reacts with oxygen included in steel to generate CaO that is
non-metallic inclusion and, thus, an upper content limit of Ca may
be 60 wt ppm.
[0049] Tin (Sn): 5 to 50 wt ppm
[0050] Sn may be an important element for ensuring
anti-corrosion.
[0051] In terms of ensuring of anti-corrosion, 5 ppm or more of Sn
may be added. However, when a content of Sn is greater than 50 ppm
wt %, there may be a problem in terms of a lot of amount of detects
in which a scale inflates or bursts like a blister on a steel
surface rather than an effect of enhancing anti-corrosion. In
addition, Sn enhances strength of steel but degrades elongation and
low-temperature impact toughness and, thus, an upper content limit
of Sn may be 50 wt ppm.
[0052] The remaining element in the present disclosure may be iron
(Fe). However, unintended impurities from a material or a
surrounding environment are inevitably introduced and, thus, the
impurities are not capable of being disregarded. These impurities
are known to one of ordinary skill in the art in a common
manufacturing process and, thus, information on the impurities is
not described throughout this specification.
[0053] Steel with the aforementioned advantageous steel composition
may have a sufficient effect by simply including an alloy element
with the aforementioned content range but properties such as
strength and toughness of steel, and toughness and weldability of a
welding heat affected zone may be further enhanced by further
including one or more of 0.1 to 0.5 wt % of copper (Cu), 0.15 to
0.3 wt % of molybdenum (Mo), and 0.005 to 0.3 wt % of vanadium
(V).
[0054] Copper (Cu): 0.1 to 0.5 wt %
[0055] Cu may be an element that minimizes reduction in toughness
of a parent material and, simultaneously, enhances strength. To
acquire the aforementioned effect, Cu of 0.1 wt % or more may be
added. However, when a content of Cu is greater than 0.5 wt %,
product surface properties may be largely degraded. Accordingly,
content of Cu may be 0.1 to 0.5 wt %.
[0056] Molybdenum (Mo): 0.15 to 0.3 wt %
[0057] Even if a small amount of Mo is added, hardenability may be
largely enhanced to enhance strength and, thus, it may be required
to add 0.15 wt % or more of Mo. However, when Mo is added in an
amount greater than 0.3 wt %, rigidity of a welding portion may be
excessively increased and toughness may be degraded. Accordingly, a
content of Mo may be 0.15 to 0.3 wt %.
[0058] Vanadium (V): 0.005 to 0.3 wt %
[0059] V is solid-solved at lower temperature than other refined
alloy and is precipitated in a welding heat affected zone and
prevent reduction in strength. To sufficiently acquire the
aforementioned effect, 0.005 wt % or more of V may be added.
However, when a content of V is greater than 0.3 wt %, toughness
may be rather reduced. Accordingly, content of V may be 0.005 to
0.3 wt %.
[0060] The refined structure of the steel according to the present
disclosure may include bainitic ferrite and granular bainite in a
primary phase and include M-A (martensiteaustenite constituent) in
a secondary phase.
[0061] Bainitic ferrite includes many high tilt boundaries in a
grain while maintaining an initial austenite crystal grain system
and, thus, may be useful to enhance strength and impact toughness
by virtue of a crystal grain refining effect.
[0062] Granular bainite maintains an initial austenite crystal
grain like bainitic ferrite but a secondary phase such as M-A is
present in a grain or a grain boundary. A high tilt boundary is not
present in a grain boundary and, thus, it is disadvantageous for
impact toughness but a large amount of low tilt boundaries are
present like inter-grain electric potential and, thus, strength is
slightly increased.
[0063] Bainitic ferrite and granular bainite are included in a
primary phase and, thus, a low yield ratio and high strength may be
ensured.
[0064] In this case, in terms of an area fraction, the bainitic
ferrite may be 80 to 95%, the granular bainite may be 5 to 20%, and
the M-A may be 3% or less (including 0%).
[0065] When an area fraction of bainitic ferrite is less than 80%,
it may be difficult to ensure high tensile strength and, when the
area fraction is greater than 95%, there is a problem in terms of
an increased yield ratio.
[0066] When an area fraction of granular bainite is less than 5%,
yield strength as well as tensile strength may be increased and,
thus, it is not possible to ensure a low yield ratio and, when the
area fraction is greater than 20%, it may not be possible to
effectively refine an initial coarsened austenite crystal grain
and, thus, tensile strength may be degraded.
[0067] A secondary phase such as M-A may be a refined structure
useful to achieve a low yield ratio and, thus, may have an area
fraction of 3% or less. When the area fraction of M-A is greater
than 3%, a yield ratio may be reduced but this may function as an
initiation point of cracking with respect to external stress and,
thus, it may adversely affect ensuring of tensile strength.
[0068] PImax. (111)/PImax. (100) of steel according to the present
disclosure may be 1.0 or more or 1.8 or less. The PImax. (111) may
be pole intensity (PImax.) of (111) crystalline surface acquired
using a method such as X-ray diffraction or electron backscatter
diffraction and the PImax. (100) may be pole intensity (PImax.) of
a (100) crystalline surface.
[0069] The pole intensity of the crystalline surface may be
determined depending on a last refined structure of steel according
to an aspect of the present disclosure. When bainitic ferrite and
granular bainite are in a primary phase, as a fraction of bainitic
ferrite is increased, a value of the PImax. (111) is increased and,
as a fraction of granular bainite is increased, a value of the
PImax. (100) may be increased. According to an aspect of the
present disclosure, the last refined structure of the steel has
bainitic ferrite with a higher area fraction than granular bainite
and, when PImax. (111)/PImax. (100) is equal to or less than 1.8,
it may be possible to manufacture low-yield-ratio type,
high-strength steel and, when PImax. (111)/PImax. (100) is greater
than 1.8, a low yield ratio may not be satisfied and, thus, an
upper limit may be 1.8 or less. In detail, PImax. (111)/PImax.
(100) may be 1.6 or less.
[0070] When PImax. (111)/PImax. (100) is less than 1.0, a fraction
of granular bainite is increased to be greater than 20% and, thus,
there is a problem in that it is difficult to ensure high strength.
Accordingly, a lower limit of PImax. (111)/PImax. (100) may be
equal to or greater than 1.0 and, in more detail, may be 1.2 or
more.
[0071] The steel according to the present disclosure may have a
yield ratio of 0.85 or less and may ensure tensile strength of 800
MPa or more and, thus, the steel may be used as steel for
construction, or the like.
[0072] The steel according to the present disclosure may have a
thickness of 60 mm or less.
[0073] Since the steel according to the present disclosure may
ensure high strength and a low yield ratio, the steel may have a
plate thickness of 60 mm or less and, thus, it may be easy to
perform machining such as cutting or perforation and a welding
operation. Accordingly, the steel may have a thickness of 60 mm or
less. In more detail, the thickness may be 40 mm or less and, more
particularly, 30 mm or less.
[0074] A lower thickness limit of the steel may not be particularly
limited but may be 15 mm or more to use the steel as steel for a
construction structure.
[0075] Hereinafter, a manufacturing method of low-yield-ratio type,
high-strength steel according to another aspect of the present
disclosure is described in detail.
[0076] The manufacturing method of low-yield-ratio type,
high-strength steel according to another aspect of the present
disclosure may include heating a slab with the aforementioned alloy
composition at 1050 to 1250.degree. C., rough-rolling the heated
slab at 950 to 1150.degree. C. to obtain a bar, hot-rolling the bar
at final rolling temperature of 700 to 950.degree. C. to acquire
hot rolled steel, and cooling the hot rolled steel at cooling speed
of 25 to 50.degree. C./s up to cooling termination temperature of
Bs or less.
[0077] Heating Slab
[0078] A slab having the aforementioned alloy composition may be
heated to a temperature of 1050 to 1250.degree. C.
[0079] Rough Rolling The heated slab may be rough-rolled at 950 to
1050.degree. C. to acquire a bar.
[0080] When the rough rolling temperature is less than 950.degree.
C., as austenite is deformed in a state in which re-crystallization
does not occur, there is a concern of coarsening of particles and,
when the rough rolling temperature is greater than 1050.degree. C.,
re-crystallization occurs and, simultaneously, particles are grown
and, thus, there is also a concern of coarsening of austenite
particles.
[0081] Hot-rolling
[0082] The bar is hot-rolled at final rolling temperature of 700 to
950.degree. C. to acquire hot rolled steel.
[0083] When the final rolling temperature is less than 700.degree.
C., a temperature of a plate is low and, thus, a load is generated
in a rolling mill and there is a concern in that the plate is not
capable of being rolled to a final thickness and, when the final
rolling temperature is greater than 950.degree. C., there is a
concern of re-crystallization during rolling.
[0084] In this case, a reduction ratio of the hot rolling may be 50
to 80%.
[0085] When the final rolling reduction ratio is less than 50%,
load applied to a material is increased during rolling and there is
danger of facility accidents and, when the final rolling reduction
ratio is greater than 80%, the number of rolling paths is increased
and, thus, there is a concern of not ensuring a final thickness up
to rolling termination temperature.
[0086] Cooling
[0087] The hot rolled steel may be cooled at cooling speed of 25 to
50.degree. C./s up to cooling termination temperature of Bs or
less.
[0088] When cooling of the hot rolled steel is terminated at
temperature greater than Bs, a phase of bainitic ferrite and
granular bainite is not sufficiently transitioned and, thus,
strength is not capable of being ensured. A cooling speed has a
physical limit depending on a plate thickness but, as soft ferrites
are generated at a cooling speed less than 25.degree. C./s, it may
be difficult to satisfy tensile strength of 800 MPa or more. As the
possibility that martensite which is low temperature transformed
structure is generated at cooling speed greater than 50.degree.
C./s is high, yield strength as well as tensile strength is also
increased and, thus, it may be difficult to satisfy a yield ratio
of 0.85 or less.
MODE FOR INVENTION
[0089] Hereinafter, the present disclosure will be described in
detail by explaining exemplary embodiments. However, the features
of the present disclosure will be more clearly understood and
should not be limited by the exemplary embodiments of the present
disclosure. The scope of the present disclosure should be
determined by the appended claims and their equivalents coming
within the meaning of the appended claims are intended to be
embraced therein.
[0090] A slab that satisfies a component system shown in Table 1
below was heated to 1160.degree. C., was rough-rolled at
1000.degree. C. and, then, was hot-rolled and cooled to satisfy a
manufacturing condition shown in Table 2 below to acquire steel.
The yield strength, tensile strength, yield ratio, and refined
structure of the steel were measured and shown in Table 3
below.
[0091] In addition, pole strength of (100) and (110) crystalline
surfaces of the steel was measured to obtain a value of PImax.
(111)/PImax. (100) and the value was shown in Table 3 below.
[0092] The yield strength and the tensile strength were measured
using a universal tensile tester.
[0093] A refined structure was obtained by mirror-like grinding the
steel and, then, performing chemical corrosion and, then, was
observed by an optical microscope.
[0094] Pole strength and entire structure strength were measured
via an X-ray diffractometer and an electron backscatter
diffractometer.
[0095] A unit of content of each element is wt % in Table 1
below.
TABLE-US-00001 TABLE 1 Steel Type C Si Mn P S Al Cr Ni Ti Nb B N Ca
Sn Invention 0.045 0.17 2.12 0.007 0.002 0.029 0.32 0.40 0.018 0.04
0.0016 0.0037 0.0010 0.0008 steel A Invention 0.052 0.15 2.48 0.008
0.001 0.026 0.30 0.15 0.016 0.04 0.0015 0.0035 0.0007 0.0042 steel
B Invention 0.065 0.16 1.75 0.011 0.001 0.030 0.29 0.29 0.019 0.04
0.0014 0.0029 0.0012 0.0021 steel C Invention 0.054 0.25 2.29 0.007
0.002 0.030 0.31 0.50 0.011 0.03 0.0013 0.0042 0.0005 0.0034 steel
D Comparison 0.045 0.11 1.91 0.005 0.003 0.006 0.04 1.52 0.008 0.01
0.0001 0.0040 0.0011 0.0004 Steel E Comparison 0.049 0.15 2.85
0.009 0.002 0.029 0.28 0.41 0.018 0.03 0.0015 0.0040 0.0014 0.0003
Steel F
TABLE-US-00002 TABLE 2 Hot final rolling Cooling Temper- Reduction
Cooling Termination Bs Steel ature Ratio Speed Temperature
Temperature type Division (.degree. C.) (%) (C/s) (.degree. C.)
(.degree. C.) Invention Invention 844 75 46.6 523 589 steel A
Example 1 Invention 860 70 41.1 537 Example 2 Invention 892 60 40.6
492 Example 3 Invention Invention 873 70 41.2 536 565 steel B
Example 4 Invention 890 60 37.7 506 Example 5 Invention 901 60 26.2
441 Example 6 Invention Invention 899 60 25.8 451 623 steel C
Example 7 Invention 890 60 26.3 447 Example 8 Invention 859 70 41.4
528 Example 9 Invention Comparison 852 75 51.4 534 568 steel D
Example 1 Comparison 863 75 57.7 507 Example 2 Comparison 904 45
6.4 182 Example 3 Comparison Comparison 870 72 34.1 350 574 Steel E
Example 4 Comparison 871 66 24.1 356 Example 5 Comparison 869 52
20.2 357 Example 6 Comparison Comparison 864 78 48.5 505 526 Steel
F Example 7 Comparison 877 65 31.4 502 Example 8 Comparison 835 55
20.4 496 Example 9
TABLE-US-00003 TABLE 3 Central portion Refined Yield Tensile Steel
structure strength Strength Yield PImax.(111)/ Type Division BF GB
M.A (MPa) (MPa) Ratio PImax.(100) Invention Invention 86 12 2 677
843 0.80 1.14 Steel A Example 1 Invention 89 10 1 703 872 0.81 1.25
Example 2 Invention 91 8 1 717 909 0.79 1.50 Example 3 Invention
Invention 87 10 3 697 866 0.80 1.16 Steel B Example 4 Invention 92
6 2 736 898 0.82 1.64 Example 5 Invention 88 11 1 707 871 0.81 1.27
Example 6 Invention Invention 92 7 1 761 919 0.83 1.52 Steel C
Example 7 Invention 93 7 0 786 926 0.85 1.71 Example 8 Invention 83
15 2 686 860 0.80 1.10 Example 9 Invention Comparison 97 3 0 797
931 0.86 1.98 Steel D Example 1 Comparison 98 2 0 893 981 0.91 1.96
Example 2 Comparison 71 24 5 613 780 0.79 0.87 Example 3 Comparison
Comparison AF: 72, B: 28 562 694 0.81 1.08 Steel E Example 4
Comparison AF: 79, B: 21 530 643 0.82 1.05 Example 5 Comparison AF:
74, B: 26 504 612 0.82 1.07 Example 6 Comparison Comparison BF: 97,
GB: 3, 876 984 0.89 1.97 Steel F Example 7 MA: 0 Comparison BF: 72,
GB: 24, 725 841 0.86 0.85 Example 8 MA: 4 Comparison BF: 66, GB:
31, 660 776 0.85 0.82 Example 9 MA: 3
[0096] In Table 3 above, BF is bainitic ferrite, GB is granular
bainite, MA is martensite austenite constituent, AF is accicular
ferrite, and B is bainite and their unit is area %.
[0097] It may be seen that Invention Examples 1 to 9 that satisfy
the alloy composition and manufacturing condition according to the
present disclosure are capable of ensuring a low yield ratio of
0.85 or less and tensile strength of 800 MPa or more.
[0098] On the other hand, Comparison Examples 1 to 3 satisfy the
alloy composition according to the present disclosure but do not
satisfy the manufacturing condition and, thus, it may be seen that
a low yield ratio is not capable of being ensured or tensile
strength is degraded.
[0099] Comparison Examples 4, 7, and 8 satisfy the manufacturing
condition according to the present disclosure but do not satisfy
the alloy composition and, thus, it may be seen that a low yield
ratio is not capable of being ensured.
[0100] While the present disclosure has been described referring to
the exemplary embodiments of the present disclosure, those skilled
in the art will appreciate that many modifications and changes can
be made to the present disclosure without departing from the spirit
and essential characteristics of the present disclosure.
* * * * *