Non-heat treated wire rod with excellent wire drawability and impact toughness and manufacturing method therefor
Provided are a non-quenched and tempered wire rod having excellent drawability and impact toughness suitable for materials for automobiles or mechanical parts and a method of manufacturing the same. According to an embodiment of the present disclosure, the non-quenched and tempered wire rod includes, in percent by weight (wt %), 0.05 to 0.35% of carbon (C), 0.05 to 0.5% of silicon (Si), 0.5 to 2.0% of manganese (Mn), 1.0% or less of chromium (Cr), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), 0.01 to 0.07% of soluble aluminum (sol.Al), 0.01% or less of nitrogen (N), at least one of 0.1% or less of niobium (Nb), 0.5% or less of vanadium (V), and 0.1% or less of titanium (Ti), and the remainder of iron (Fe) and inevitable impurities, and includes a ferrite-pearlite layered structure, as a microstructure, in a rolling direction.
1 . A non-quenched and tempered wire rod comprising: in percent by weight (wt %), 0.05 to 0.35% of carbon (C), 0.05 to 0.5% of silicon (Si), 0.5 to 2.0% of manganese (Mn), 1.0% or less of chromium (Cr), 0.03% or less of phosphorus (P), 0.03% or less of sulfur(S), 0.01 to 0.07% of soluble aluminum (sol.Al), 0.01% or less of nitrogen (N), at least one of 0.1% or less of niobium (Nb), 0.5% or less of vanadium (V), and 0.1% or less of titanium (Ti), and the remainder of iron (Fe) and inevitable impurities; and
a ferrite-pearlite layered structure, as a microstructure, in a rolling direction,
wherein an average thickness of a ferrite band in an L cross-section, as a cross-section parallel to the rolling direction, is from 10 μm to 23 μm,
wherein an average room temperature impact toughness is from 160 J to 253 J at 35% drawing, from 153 J to 221 J at 45% drawing, and from 162 J to 224 J at 55% drawing,
wherein a difference between a maximum hardness and a minimum hardness in a C cross-section perpendicular to the rolling direction, is 30 Hv or less,
wherein an average particle diameter of the ferrite in the C cross-section is from 3 μm to 20 μm, and
wherein an average lamellar space of the pearlite is from 0.03 μm to 0.3 μm,
wherein the wire rod satisfies I max −I min <40 J in 30% to 60% drawing,
wherein I max is a maximum value of average room temperature impact toughness after drawing and I min is a minimum value of average room temperature impact toughness after drawing.
2 . The non-quenched and tempered wire rod of claim 1 , wherein a fraction of the ferrite is from 30% to 90%.
3 . The non-quenched and tempered wire rod of claim 1 , wherein a carbon equivalent C eq represented by the following formula is from 0.4 to 0.6:
Ceq
=
[
C
]
+
[
Si
]
/
9
+
[
Mn
]
/
5
+
[
Cr
]
/
12
(wherein [C], [Si], [Mn], and [Cr] are contents (wt %) of corresponding elements, respectively).
4 . A method of manufacturing the non-quenched and tempered wire rod of claim 1 , the method comprising: preparing a billet comprising, in percent by weight (wt %), 0.05 to 0.35% of carbon (C), 0.05 to 0.5% of silicon (Si), 0.5 to 2.0% of manganese (Mn), 1.0% or less of chromium (Cr), 0.03% or less of phosphorus (P), 0.03% or less of sulfur(S), 0.01 to 0.07% of soluble aluminum (sol.Al), 0.01% or less of nitrogen (N), at least one of 0.1% or less of niobium (Nb), 0.5% or less of vanadium (V), and 0.1% or less of titanium (Ti), and the remainder of iron (Fe) and inevitable impurities;
reheating the billet at a reheating temperature Tr satisfying Equation (2) below;
rolling the reheated billet into a wire rod; and
coiling the rolled wire rod followed by cooling, and thereby producing the non-quenched and tempered wire rod of claim 1 ;
wherein the rolling of the reheated billet into the wire rod comprises rolling the reheated billet at a final rolling temperature Tf satisfying Equation (3) below,
T
1
≤
Tr
≤
1200
°
C
.
(
2
)
T
2
≤
Tf
≤
T
3
(
3
)
(
wherein
T
1
=
757
+
606
[
C
]
+
80
[
Nb
]
/
[
C
]
+
1023
√
[
Nb
]
+
330
[
V
]
,
T
2
=
955
-
396
[
C
]
+
24.6
[
Si
]
-
68.1
[
Mn
]
-
24.8
[
Cr
]
-
36.1
[
Nb
]
-
20.7
[
V
]
,
T
3
=
734
+
465
[
C
]
-
355
[
Si
]
+
360
[
Al
]
+
891
[
Ti
]
+
6800
[
Nb
]
-
650
f
[
Nb
]
+
730
[
V
]
-
2321
[
V
]
,
and
[C], [Si], [Mn], [Cr], [Al], [Ti], [Nb], and [V] are contents (wt %) of corresponding elements, respectively).
5 . The method of claim 4 , wherein the cooling comprises cooling the wire rod at an average rate of 0.1° C./s to 2° C./s.