Technical Field
[0001] The present disclosure relates to a steel material having high strength and excellent
impact toughness, usable for a land-based wind power generator and the like; and a
method for manufacturing the same.
Background Art
[0002] Recently, as a tower height of a land-based wind power generator has gradually become
more advanced, demand for a thick, high strength steel material having excellent load
resistance capacity has increased, and at the same time, a guarantee of impact toughness
has also been required.
[0003] In order to realize high strength and excellent impact toughness of a steel material,
refinement of a grain thereof is essential, and a rolling process may be one of various
representative methods for the refinement of the grain. When rolling is performed
at a temperature at which recrystallization may occur, a new fine grain of austenite
may be generated using internal stress generated by a roll separation force, as a
driving force. Meanwhile, a grain may receive stress by rolling in a temperature within
a range in which recrystallization does not occur, to form a band structure in a rolling
direction, and when many dislocations occur therein to undergo phase transformation
of austenite, more nucleation sites may be provided to cause a grain refinement effect.
[0004] However, as a thickness of the steel material increases, a roll separation force
that may be applied by rolling becomes limited, and as an internal structure, especially
a central portion of the steel material is closer thereto, it may be less easy to
form a fine grain by the rolling. This is because the grain of austenite tends to
grow at temperatures equal to or higher than Ae3, as a temperature increases and a
heating time period increases.
[0005] Meanwhile, it is often difficult to secure a grain having a sufficiently small size
only by reheating and rolling of a slab, which may be processes in which refinement
of a grain of austenite mainly occurs. In particular, as a temperature of the steel
material to be rolled increases, deformation resistance during the rolling may decrease.
For easy rolling, the reheating of the slab may be usually performed at a temperature
much higher, as compared to the Ae3 temperature, and at that time, the grain of austenite
may grow significantly. When a grain refinement effect by rolling is not sufficient,
an additional grain refining effect of austenite may be expected by a re-heating treatment
after the rolling process, which generally may include a normalizing heat treatment.
[0006] As a material for a wind tower, a steel material that has been subjected to a normalizing
heat treatment has traditionally been applied. However, when a heat treatment, as
above, is applied during a manufacturing process, manufacturing costs may increase
significantly. Accordingly, the material for a wind tower may not be easy to produce
commercially, as compared to an as-rolled steel material or a thermo-mechanical controlled
process (TMCP) steel material. Accordingly, there may be demand for manufacturing
a steel material having properties, similar to those of a normalized heat-treated
steel material without undergoing a normalizing heat treatment.
[0007] Patent Document 1 proposes a method of manufacturing a steel material having excellent
impact toughness without a normalizing heat treatment. However, Patent Document 1
has a low carbon content, which is advantageous in securing sufficient low-temperature
impact toughness, but it may be difficult to secure sufficient strength, and furthermore,
there may be a limitation in that strength decreases significantly as thickness increases.
Summary of Invention
Technical Problem
[0009] An aspect of the present disclosure is to provide a steel material with excellent
strength and impact toughness, even when a heat treatment process is omitted, and
a method for manufacturing the same.
[0010] An object of the present disclosure is not limited to those mentioned above. The
additional problems of the present disclosure may be described throughout the specification,
and those skilled in the art will have no difficulty in understanding the additional
problems of the present disclosure from those described in the specification of the
present disclosure.
Solution to Problem
[0011] According to an aspect of the present disclosure, a steel material having high strength
and excellent impact toughness, comprises, by weight, C: 0.12 to 0.18%, Si: 0.2 to
0.5%. Mn: 1.0 to 1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015 to 0.045%,
Nb: 0.02 to 0.05%, V: 0.01 to 0.08%, Ti: 0.005 to 0.017%, N: 0.002 to 0.01%, and a
remainder of Fe and inevitable impurities;
[0012] wherein a carbon equivalent (Ceq) of the following Relationship 1 is 0.48 or less,
[0013] a microstructure comprises 60 to 85% of ferrite by area fraction and remaining pearlite,
and the microstructure includes at least one precipitate of NbC or VC, and a size
of the precipitates is 50 nm or less:

[0014] Where C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.
[0015] According to another aspect of the present disclosure, a method of manufacturing
a steel material having high strength and excellent impact toughness, comprises heating
a steel slab comprising, by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%. Mn: 1.0 to
1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.02 to 0.05%,
V: 0.01 to 0.08%, Ti: 0.005 to 0.017%, N: 0.002 to 0.01%, and a remainder of Fe and
inevitable impurities, and in which a carbon equivalent (Ceq) of the following Relationship
1 is 0.48 or less, under conditions of the following Relationship 2; and
[0016] rough-rolling the heated steel slab at a temperature within a range of 900 to 1100°C,
and then finishing hot-rolling to Ar3 or higher after the rough-rolling:

[0017] Where C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.
Slab Extraction Temperature (°C) > 10300 / {4.09 - log([Nb][C]0.24[N]0.65)} - 273
[0018] Where [Nb], [C], and [N] mean an amount (% by weight) in an alloy composition, respectively.
Advantageous Effects of Invention
[0019] According to the present disclosure, a steel material securing excellent strength
and impact toughness without performing normalizing heat treatment after rolling may
be provided, and may be widely used for a wind power structure or the like. In addition,
a commercially useful steel material may be provided by reducing manufacturing costs
by omitting a heat treatment.
[0020] Various advantages and effects of the present disclosure are not limited to those
described above, and can be more easily understood through description of specific
embodiments of the present disclosure.
Brief Description of Drawings
[0021] FIG. 1 is a graph illustrating a relationship between slab extraction temperature
and yield strength, illustrated in an inventive example of the present disclosure.
Best Mode for Invention
[0022] Hereinafter, terms used in the present specification are for describing the present
disclosure, and are not intended to limit the present disclosure. Additionally, as
used herein, singular forms include plural forms unless relevant definitions clearly
indicates the contrary.
[0023] The meaning of "including" or "comprising" used in the specification specifies a
configuration, and does not exclude the presence or addition of another configuration.
[0024] Unless otherwise defined, all terms, including technical and scientific terms, used
in the present specification have the same meaning as commonly understood by a person
of ordinary skill in the technical field to which the present disclosure pertains.
Terms defined in the dictionary may be interpreted to have meanings consistent with
related technical literature and current disclosure.
[0025] The present inventors recognized that a normalized rolling (NR) method, as a manufacturing
method in which a steel material is hot-rolled and then air-cooled in a temperature
within a range having properties, equal to or better than those of a normalized heat-treated
steel material, without performing normalizing heat treatment after rolling, may secure
properties, equal to or better than a normalizing heat treatment material, by establishment
of an optimal component design and manufacturing conditions.
[0026] In particular, in structural steel used in a land-based wind tower or the like, as
a large size and economic efficiency are required, a method is needed to secure properties
required for a material thereof and manufacture the same economically. Accordingly,
in an alloy design, it was confirmed that a steel material having target properties
could be provided by identifying and optimizing an alloy composition and a relationship
between some components and optimizing manufacturing conditions, leading to completion
of the present disclosure.
[0027] Hereinafter, an embodiment of a steel material of the present disclosure will be
described in detail.
[0028] First, an alloy composition of the steel material will be described in detail. The
steel material of the present disclosure may comprise, by weight, C: 0.12 to 0.18%,
Si: 0.2 to 0.5%. Mn: 1.0 to 1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015
to 0.045%, Nb: 0.02 to 0.05%, V: 0.01 to 0.08%, Ti: 0.005 to 0.017%, and N: 0.002
to 0.01%; and
[0029] may further include one or more of Cu: 0.5% or less and Ni: 0.5% or less.
[0030] Carbon (C): 0.12 to 0.18% by weight (hereinafter, referred to as %, unless specifically
mentioned in the present disclosure, an amount of each element is based on weight%.)
[0031] C may be an element effective in improving strength of steel. For this purpose, C
may be included in an amount of 0.12% or more. When an amount thereof exceeds 0.18%,
a degree of segregation in a central portion of the steel may increase, and a martensite-austenite
(MA) structure may be formed, which may significantly reduce low-temperature impact
toughness. More advantageously, 0.17% or less may be included.
Silicon (Si): 0.2 to 0.5%
[0032] Si may not be only used as a deoxidizing agent, but may be also an element advantageous
for improving strength and toughness of steel. To sufficiently obtain this effect,
Si may be included in an amount of 0.2% or more. When an amount thereof exceeds 0.5%,
there may be a risk of excessive formation of MA and poor low-temperature impact toughness.
Therefore, Si may be in an amount of 0.2 to 0.5%.
Manganese (Mn): 1.0 to 1.7%
[0033] Mn may be an element advantageous for improving strength of steel by a solid solution
strengthening effect. To fully obtain the effect, Mn may be included in an amount
of 1.0% or more. When an amount thereof exceeds 1.7%, it combines with sulfur (S)
in the steel to form MnS, which may greatly impair low-temperature impact toughness.
Therefore, Mn may be included in an amount of 1.0 to 1.7%, and more advantageously,
may be included in an amount of 1.35 to 1.65%.
Phosphorus (P): 0.012% or less
[0034] P may be an element advantageous in improving strength of steel and securing corrosion
resistance thereof, but may greatly impair impact toughness of the steel. Therefore,
it is desirable to limit an amount thereof as low as possible. In the present disclosure,
even when P is included at a maximum of 0.012%, there may be no difficulty in securing
target properties. Therefore, an amount thereof is limited to be 0.012% or less. Considering
a level to be unavoidably added, 0% may be excluded.
Sulfur (S): 0.003% or less
[0035] S may be an element that greatly inhibits the hydrogen-induced cracking resistance
and impact toughness of steel by combining with Mn in the steel to form MnS and the
like. Therefore, it is advantageous to manage S in a low amount as possible. In the
present disclosure, even when S is included at a maximum of 0.003%, there may be no
difficulty in securing target properties. Therefore, an amount thereof is limited
to be 0.003% or less. Considering a level to be unavoidably added, 0% may be excluded.
Aluminum (Al): 0.015 to 0.045%
[0036] Al may be an element that may inexpensively deoxidize molten steel. To sufficiently
obtain the above-mentioned effect, Al may be included in an amount of 0.015% or more.
When an amount thereof exceeds 0.045%, nozzle clogging may occur during continuous
casting. This may be undesirable because not only does it cause damage, but impact
toughness may be significantly reduced due to formation of Al-based oxidizing inclusions.
Therefore, Al may be included in 0.015 to 0.045%.
Niobium (Nb): 0.02 to 0.05%
[0037] Nb may precipitate to form NbC or Nb(C,N), greatly improving strength of a base material,
and when reheated at high temperature, dissolved Nb may suppress recrystallization
of austenite and transformation of ferrite or bainite to obtain a structure refinement
effect. For this purpose, Nb may be included in an amount of 0.02% or more. When an
amount thereof is excessive, undissolved Nb may form TiNb(C,N), which causes UT defects
and impedes low-temperature impact toughness. Therefore, an upper limit of Nb may
be 0.05%. More advantageously, Nb may contain 0.035 to 0.045%.
Vanadium (V): 0.01 to 0.08%
[0038] V may have a low solid solution temperature, as compared to other alloy elements,
and may form VC during an air cooling process after hot-rolling, to contribute significantly
to increasing strength. A steel material such as those of the present disclosure may
not have sufficient strength after post-welding heat treatment (PWHT). Therefore,
a strength improvement effect may be obtained by including 0.01% or more of V. When
an amount thereof exceeds 0.08%, a fraction of a hard phase such as MA may increase,
causing a problem in that low-temperature impact toughness is significantly reduced.
Therefore, an amount of V may be 0.01 to 0.08%.
Titanium (Ti): 0.005 to 0.017%
[0039] Ti may be included together with N to form TiN, thereby reducing occurrence of surface
cracks due to formation of AlN precipitates, and may be included in an amount of 0.005%
or more. When an amount thereof exceeds 0.017%, coarse TiN may be formed during reheating
of a steel slab, which acts as a factor in impeding low-temperature impact toughness.
Therefore, Ti may be in an amount of 0.005 to 0.017%, and more preferably 0.01 to
0.015%.
Nitrogen (N): 0.002 to 0.01%
[0040] It may be advantageous that N may be included together with Ti to form TiN and suppress
grain growth due to thermal effects during welding. To sufficiently obtain the above-described
effects when adding Ti, N may be included in an amount of 0.002% or more. When an
amount thereof exceeds 0.01%, it is undesirable because coarse TiN is formed and low-temperature
impact toughness is impaired. Therefore, N may be in an amount of 0.002 to 0.01%.
[0041] Additionally, in addition to the above composition, one or more of copper (Cu): 0.5%
or less and nickel (Ni) : 0.5% or less may be further included.
Copper (Cu): 0.5% or less
[0042] Cu may be an element that may greatly improve strength by solid solution strengthening.
When an amount of Cu is excessive, it may not only impair weldability due to an increase
in carbon equivalent, but also significantly deteriorate surface quality of a product.
Therefore, when adding Cu, it may be included at a maximum of 0.5%. In the present
disclosure, there may be no difficulty in securing target properties even when Cu
is not added. Therefore, it is noted that Cu is not essential.
Nickel (Ni): 0.5% or less
[0043] NI may be an element that may simultaneously improve strength of a base material
and low-temperature impact toughness thereof, but may be an expensive element. When
an amount thereof exceeds 0.5%, economic feasibility may be greatly reduced. Therefore,
Ni may be included in an amount of 0.5% or less. In the present disclosure, there
may be no difficulty in securing target properties even when Ni is not added. Therefore,
it is noted that Ni is not essential.
[0044] The remainder may include iron (Fe) and inevitable impurities. Inevitable impurities
may be unintentionally mixed in the normal steel manufacturing process, and, thus,
may not be completely excluded, and any engineer in the normal steel manufacturing
field can easily understand meaning thereof. In addition, the present disclosure does
not completely exclude addition of compositions, other than the steel compositions
mentioned above.
[0045] To secure impact toughness as well as a target level of strength in a steel material
of the present disclosure, it is desirable to appropriately adjust amounts of elements
advantageous for improving properties by adding a certain amount thereof. Therefore,
a carbon equivalent (Ceq) of the following Relationship 1 may be 0.48 or less. When
the carbon equivalent (Ceq) exceeds 0.48, it is advantageous in securing strength,
but there may be a risk that properties after welding may be greatly impaired. In
addition, when large amounts of alloy elements are included, the costs will increase
and economic feasibility will be impaired. Therefore, the carbon equivalent (Ceq)
may be 0.48 or less.

[0046] Where C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.
[0047] A microstructure of the steel material of the present disclosure may include 60 to
85% of ferrite by area fraction and remaining pearlite. When the ferrite fraction
is less than 60% or the remaining pearlite fraction exceeds 40%, it is advantageous
to secure strength, but impact toughness may decrease significantly. In addition,
when the ferrite fraction exceeds 85%, it is advantageous to secure impact toughness,
but it is difficult to secure sufficient strength. Therefore, the steel material of
the present disclosure may include 60 to 85 area % of ferrite and remaining pearlite.
[0048] A grain size of the ferrite may be 30um or less. When an average grain size of the
ferrite exceeds 30um, it is difficult to secure yield strength, and impact toughness
may be greatly reduced. Therefore, the average grain size may be 30um or less.
[0049] The microstructure of the steel material may include a precipitate of NbC and/or
VC. A size of the precipitate may be 50 nm or less. When the size of the precipitate
exceeds 50 nm, it is undesirable because impact toughness may be greatly reduced.
The precipitates may exist within a grain of ferrite.
[0050] When the steel material is evaluated perpendicular to a rolling direction at a t/4
point in a thickness direction (where t means a thickness (mm) of the steel material),
a yield strength may be 370 MPa or more, a tensile strength may be 520 MPa or more,
and an average Charpy impact absorption energy (CVN, -20°C) value at a temperature
of - 20°C may be 40J or more, to have excellent strength and low-temperature impact
toughness.
[0051] Next, an embodiment of a method for manufacturing a steel material of the present
disclosure will be described in detail. The above method may be manufactured by heating
and hot-rolling a steel slab having an alloy composition, as described above, and
a carbon equivalent (Ceq) of 0.48 or less in Relationship 1. Hereinafter, each process
will be described in detail.
Heating of Steel Slab
[0052] Homogenization treatment may be performed by heating a steel slab satisfying an alloy
composition, as described above. In this case, it is desirable to perform heating
to satisfy temperature conditions defined by the following Relationship 2. A slab
extraction temperature in the following Relationship 2 may not exceed 1200°C.
[0053] When a heating temperature of the steel slab does not satisfy the conditions of the
following Relationship 2, a precipitate (carbide, nitride) formed in the slab may
not be sufficiently re-dissolved, thereby reducing formation of precipitate in a process
after hot-rolling, and it is ultimately difficult to satisfy specified yield strength
and tensile strength presented in the present disclosure. When the slab extraction
temperature exceeds 1200°C, a grain of austenite may coarsen and properties of the
steel may deteriorate. Therefore, the slab extraction temperature may not exceed 1200°C.
Slab Extraction Temperature (°C) > 10300 / {4.09 - log([Nb][C]0.24[N]0.65)} - 273
[0054] Where [Nb], [C], and [N] mean an amount (% by weight) in an alloy composition, respectively.
Hot-Rolling
[0055] The heated steel slab may be hot-rolled. The heated steel slab may be rough-rolled
at a temperature within a range of 900 to 1100°C, and may be then finish hot-rolled
to Ar3 or higher. When a temperature during the rough-rolling may be less than 950°C,
there may be a problem in that the temperature becomes too low during the subsequent
finishing hot-rolling. When a temperature during the finishing hot-rolling is lower
than Ar3, a rolling load may increase and there may be a risk of quality defects such
as surface cracks or the like.
Ar3 = 910-310C-80Mn-20Cu-55Ni-80Mo+119V+124Ti-18Nb+179Al
[0056] Where each element means an amount (% by weight))
[0057] After the hot-rolling, air cooling may be performed.
[0058] The steel material of the present disclosure manufactured by the above method may
secure high strength and excellent impact toughness without performing subsequent
heat treatment, such as normalizing heat treatment or the like.
Mode for Invention
[0059] Next, examples of the present disclosure will be described.
[0060] Various modifications to the following examples may be made by those skilled in the
art without departing from the scope of the present disclosure. The following examples
are for understanding of the present disclosure, and the scope of the present disclosure
should not be limited to the following examples, but should be determined by the claims
described below as well as their equivalents.
(Example 1)
[0061] A slab was manufactured by continuously casting molten steel having an alloy composition
(% by weight, the remainder being Fe and inevitable impurities) illustrated in Table
1 below. In this case, the slab was manufactured to have a thickness of 300 mm. In
Table 1, Inventive Examples 1 to 4 were cases in which both the alloy composition
and Relationship 1 presented in the present disclosure were satisfied, Comparative
Example 1 was a case in which an amount of C and Relationship 1 were outside the values
presented in the present disclosure, and Comparative Example 3 indicates that an amount
of Nb was outside the value presented in the present disclosure.
[Table 1]
|
C |
Si |
Mn |
P |
S |
Al |
Nb |
Cu |
Ni |
V |
Ti |
N |
Relationship 1 |
Inventive Example 1 |
0.1 50 |
0.4 50 |
1.5 30 |
0.0 08 |
0.0 02 |
0.0 30 |
0.0 45 |
0.0 00 |
0.0 00 |
0.0 40 |
0.0 12 |
0.0 035 |
0.413 |
Inventive Example 2 |
0.1 60 |
0.4 50 |
1.6 50 |
0.0 08 |
0.0 02 |
0.0 30 |
0.0 45 |
0.0 00 |
0.0 00 |
0.0 40 |
0.0 12 |
0.0 036 |
0.443 |
Inventive Example 3 |
0.1 55 |
0.4 50 |
1.6 00 |
0.0 09 |
0.0 02 |
0.0 30 |
0.0 35 |
0.0 00 |
0.0 00 |
0.0 55 |
0.0 12 |
0.0 035 |
0.433 |
Inventive Example 4 |
0.1 60 |
0.4 00 |
1.6 50 |
0.0 08 |
0.0 02 |
0.0 30 |
0.0 45 |
0.1 00 |
0.2 00 |
0.0 45 |
0.0 12 |
0.0 035 |
0.464 |
Comparative Example 1 |
0.1 85 |
0.4 00 |
1.6 50 |
0.0 09 |
0.0 02 |
0.0 30 |
0.0 40 |
0.1 00 |
0.1 50 |
0.0 45 |
0.0 12 |
0.0 036 |
0.486 |
Comparative Example 2 |
0.1 50 |
0.4 50 |
1.6 00 |
0.0 08 |
0.0 02 |
0.0 30 |
0.0 45 |
0.0 00 |
0.0 00 |
0.0 30 |
0.0 12 |
0.0 035 |
0.423 |
Comparative Example 3 |
0.1 60 |
0.4 50 |
1.5 50 |
0.0 08 |
0.0 02 |
0.0 30 |
0.0 15 |
0.0 00 |
0.0 00 |
0.0 35 |
0.0 12 |
0.0 036 |
0.425 |
[0062] In Table 1 above, Relationship 1 may be calculated as follows:

[0063] Where C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.
[0064] The slab was heated and rough-rolled under conditions in Table 2, finish hot-rolled
at a temperature within a range of 880 to 900°C to produce a hot-rolled steel sheet
having a thickness of 5 mm, and air-cooled to room temperature. Inventive Examples
1 to 4 and Comparative Examples 1 and 3 satisfied the process conditions presented
in the present disclosure, but Comparative Example 2 did not satisfy the conditions
of Relationship 2 below.
[Table 2]
|
Extractio n Temp.(°C) |
Furnace Time (min.) |
Remaining Reduction Ratio (%) |
Roll Start Temp. (°C) |
Roll Finish Temp. (°C) |
Relati onship 2 |
Satisfaction of Relationship 2 |
Inventive Example 1 |
1161 |
362 |
50 |
930 |
900 |
1151 |
○ |
Inventive Example 2 |
1165 |
360 |
50 |
928 |
898 |
1154 |
○ |
Inventive Example 3 |
1160 |
360 |
50 |
915 |
880 |
1131 |
○ |
Inventive Example 4 |
1162 |
362 |
50 |
920 |
890 |
1153 |
○ |
Comparati ve Example 1 |
1160 |
362 |
50 |
920 |
890 |
1147 |
○ |
Comparati ve Example 2 |
1125 |
360 |
50 |
925 |
895 |
1151 |
X |
Comparati ve Example 3 |
1164 |
361 |
50 |
930 |
900 |
1066 |
○ |
[0065] In Table 2 above, Relationship 2 may be as follows:
Slab Extraction Temperature (°C) > 10300 / {4.09 - log([Nb][C]0.24[N]0.65)} - 273
[0066] Where [Nb], [C], and [N] mean an amount (% by weight) in an alloy composition, respectively.
[0067] A microstructure and mechanical properties of a steel material manufactured as above
were evaluated. The microstructure was observed using an optical microscope, and then
a fraction and diameter of ferrite grains were measured using an analysis program,
and an average diameter of a precipitate was measured using a transmission electron
microscope. In this case, the microstructure was measured at a point t/4 (t may be
the steel thickness, mm) in the thickness direction of each steel material, and the
results may be illustrated in Table 3 below.
[0068] In addition, the mechanical properties were evaluated at a 1/4t point in a thickness
direction of each steel material. In this case, tensile specimens were collected from
each thickness direction point in a direction, perpendicular to a rolling direction,
to measure tensile strength (TS), yield strength (YS), and elongation (El) was measured,
and the impact specimen was taken from a JIS No. 4 standard test specimen at a 1/4t
point in the thickness direction in the rolling direction, and the average impact
toughness (CVN) at -20°C was measured. The results may be illustrated in Table 4 below.
[Table 3]
|
Microstructure |
Polygonal Ferrite (area %) |
Pearlite (area %) |
Grain Size of Ferrite (µm) |
Size of Precipitate (nm) |
Inventive Example 1 |
80 |
20 |
26 |
35 |
Inventive Example 2 |
76 |
24 |
22 |
34 |
Inventive Example 3 |
77 |
23 |
27 |
42 |
Inventive Example 4 |
76 |
24 |
20 |
41 |
Comparative Example 1 |
68 |
32 |
36 |
67 |
Comparative Example 2 |
81 |
19 |
28 |
36 |
Comparative Example 3 |
76 |
24 |
34 |
30 |
[0069] As illustrated in Table 3, Inventive Steels 1 to 4 manufactured according to the
alloy composition, component relationship, and manufacturing conditions, proposed
in the present disclosure, satisfied a fraction, a grain size, and a precipitate size
of polygonal ferrite proposed in the present disclosure. Comparative Examples 1 and
3 were satisfied with the polygonal ferrite fraction, but a grain size of ferrite
was outside the value presented in the present disclosure. Additionally, Comparative
Example 1 deviated from a size of the precipitate presented in the present disclosure.
[Table 4]
|
As-rolled |
Normalized |
Tensile Properties |
Impact Toughness (J) |
Tensile Properties |
Impact Toughness (J) |
YP (MPa) |
TS (MPa) |
El.(%) |
YP (MPa) |
TS (MPa) |
El.(%) |
Inventive Example 1 |
383 |
536 |
27 |
210 |
377 |
524 |
28 |
223 |
Inventive Example 2 |
402 |
552 |
26 |
196 |
396 |
530 |
27 |
200 |
Inventive Example 3 |
410 |
556 |
25 |
174 |
400 |
542 |
26 |
186 |
Inventive Example 4 |
401 |
548 |
26 |
195 |
393 |
529 |
27 |
203 |
Comparative Example 1 |
410 |
544 |
24 |
35 |
374 |
524 |
24 |
42 |
Comparative Example 2 |
336 |
497 |
29 |
168 |
316 |
488 |
30 |
173 |
Comparative Example 3 |
351 |
520 |
26 |
75 |
330 |
509 |
28 |
94 |
[0070] Table 4 above illustrates tensile properties and low-temperature impact toughness
before and after normalizing. In this case, normalizing treatment was held at 870°C
for 128 minutes, and was then air-cooled.
[0071] In Inventive Examples 1 to 4, a component range, Relationships 1 and 2, and microstructure
properties presented in the present disclosure were satisfied, and both tensile properties
and low-temperature impact toughness were satisfied. Specifically, in Inventive Examples
1 to 4, when comparing results after as-rolled and normalizing heat treatment, yield
strength and tensile strength slightly decreased after heat treatment, but still satisfy
the strength presented in the present disclosure. Impact toughness increased slightly,
as compared to those manufactured by an NR method after heat treatment, and it can
be confirmed that impact toughness presented by the present disclosure was satisfied.
[0072] In Comparative Example 1, as component systems in which an amount of C and Relationship
1 were outside the range presented in the present disclosure, but it can be confirmed
that yield/tensile strength satisfied the values presented in the present disclosure
due to excessive addition of C, and impact toughness did not satisfy the values. Comparative
Example 2 satisfied all of the component ranges presented in the present disclosure,
but did not satisfy Relationship 2 to have a very low slab extraction temperature.
It can be confirmed that, after both as-rolled and normalizing heat treatments, yield
strength and tensile strength presented in the present disclosure were lowered. In
addition, it can be confirmed that a decrease in yield strength was very large, as
compared to Inventive Examples 1 to 4. This was be believed to be due to a significant
decrease in strength as Nb was not sufficiently dissolved in the slab and NbC was
not sufficiently precipitated during rolling. Comparative Example 3 was a case in
which an amount of Nb deviated from the value presented in the present disclosure.
Even though Nb was heated at a temperature at which Nb was sufficiently dissolved
in a slab, an amount itself was very low and an NbC precipitate was not sufficiently
precipitated. Therefore, it can be confirmed that the NbC precipitate was not sufficiently
precipitated, and yield strength and tensile strength were not satisfied, and in Comparative
Example 3, it can also be confirmed that yield strength decreased significantly after
normalizing heat treatment.
(Example 2)
[0073] As a separate example, a steel material having a thickness of 75 mmt was manufactured
by rolling a slab having the components of Inventive Example 1 of Example 1. In this
case, to confirm yield strength according to a slab extraction temperature, when results
of Relationship 2 of the extraction temperature were varied, results of relationship
between an extraction temperature and yield strength were illustrated in FIG. 1. It
can be confirmed that, when the extraction temperature did not satisfy Relationship
2, yield strength presented in the present disclosure did not satisfied, whereas when
the extraction temperature satisfied Relationship 2, all exhibited excellent yield
strength.
1. A steel material having high strength and excellent impact toughness, comprising:
by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%. Mn: 1.0 to 1.7%, P: 0.012% or less,
S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.02 to 0.05%, V: 0.01 to 0.08%, Ti: 0.005
to 0.017%, N: 0.002 to 0.01%, and a remainder of Fe and inevitable impurities;
wherein a carbon equivalent (Ceq) of the following [Relationship 1] is 0.48 or less,
a microstructure comprises 60 to 85% of ferrite by area fraction and remaining pearlite,
and the microstructure includes at least one precipitate of NbC or VC, and a size
of the precipitates is 50 nm or less:

Where C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.
2. The steel material of claim 1, further comprising at least one of Cu: 0.5% or less
or Ni: 0.5% or less.
3. The steel material of claim 1, wherein a grain size of the ferrite is 30um or less.
4. The steel material of claim 1, wherein the at least one precipitate of NbC or VC is
provided in a grain of the ferrite.
5. The steel material of claim 1, wherein, when the steel material is evaluated perpendicular
to a rolling direction at a t/4 point in a thickness direction (where t means a thickness
(mm) of the steel material), a yield strength is 370 MPa or more, a tensile strength
is 520 MPa or more, and an average Charpy impact absorption energy (CVN, -20°C) value
at a temperature of -20°C is 40J or more.
6. A method of manufacturing a steel material having high strength and excellent impact
toughness, comprising:
heating a steel slab comprising, by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%. Mn:
1.0 to 1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.02 to
0.05%, V: 0.01 to 0.08%, Ti: 0.005 to 0.017%, N: 0.002 to 0.01%, and a remainder of
Fe and inevitable impurities, and in which a carbon equivalent (Ceq) of the following
[Relationship 1] is 0.48 or less, under conditions of the following [Relationship
2]; and
rough-rolling the heated steel slab at a temperature within a range of 900 to 1100°C,
and then finishing hot-rolling to Ar3 or higher after the rough-rolling:

Where C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.
Slab Extraction Temperature (°C) > 10300 / {4.09 - log([Nb][C]0.24[N]0.65)} - 273
Where [Nb], [C], and [N] mean an amount (% by weight) in an alloy composition, respectively.
7. The method of claim 6, wherein the steel slab further comprises at least one of Cu:
0.5% or less or Ni: 0.5% or less.
8. The method of claim 6, wherein the slab extraction temperature is 1200°C or lower.