DESCRIPTION
Title of Invention: Excellent Workability Steel Wire Rod
and Method for Production of Same
Technical Field
[0001] The present invention is an invention which
improves the workability of steel wire rod by the effect
such as delaying formation of internal microvoids, which
10 is the elementary step of causing fracturing or cracking
on working process such as forging that is a typical
process of wire drawing or bolt formation and that is
said to be essential in production processes using wire
rod to produce products. This invention is characterized
15 by being applicable to the general fields of working of
steel wire rods.
Background Art
[0002] The most generally used technology in the prior
art for improving the workability of steel wire rod is
20 the method of performing spheroidizing annealing. The
prior art utilizing spheroidizing annealing, as shown in
PLT 1, includes making the grain size of the austenite
crystal 100 ~ or more and making the volume fraction of
ferrite 20% or less. In particular, as a method fo~
25 promoting spheroidizing of cementite after annealing, Cr
is added.
[0003] In this prior art, to secure forgeability, the
grain size of the austenite crystal has to be made 100 ~
or more, so when performing a forging operation in which
30 a free surface is exposed and worked instead of
performing an upset operation, the skin of the free
surface part is caused to be uneven in shape. If the
extent of this is severe, the result may become to be
relatively noticeable unevenness like an orange peel.
35 Depending on use for products, the unevenness may become
a problem. Further, since a lot of Cr is added for
improving the formation of cementite, the cost of the
- 2 -
alloy steel also becomes somewhat higher and other
problems are incurred.
[0004] PLT 2 controls the structure of a steel
material so as to give degenerate pearlite: 10 area% or
5 more, bainite: 75 area% or less, and ferrite: 60 area% or
10
15
.
less , and achieves both shortening of the spheroidizing
annealing time of the steel material and improvement of
the workability and reduction of the deformation
resistance after spheroidizing.
[0005] Further, PLT 2 restricts the area% of
degenerate pearlite, bainite, and ferrite to desirable
ranges to thereby achieve a balance of workability and
deformation resistance and obtain a steel wire rod
exhibiting excellent cold formability.
[0006] Further, PLT 3 describes a method for producing
a rolled steel wire made of steel such as eutectoid
steel. The method is characterized by producing a high
tensile strength steel wire having excellent wire
drawability by performing heat treatment for isothermal
20 transformation immediately after completing the rolling
without allowing the steel material to be transformed
from the austenite phase in the integrated process from
casting to wire rod rolling.
[0007] However, in PLTs 1 to 4, causes by which steel
25 wire tends to be easily broken at the time of severely
working steel wire rod to produce steel wire have not
been researched. Further, the effects of the behavior of
microvoids formed at the time of shaping steel wire rod
into steel wire on the breakage of steel wire have not
30 been researched.
Citation List
Patent Literature
[0008] PLT 1: Japanese Patent Publication No. 2004-
68064A
35 PLT 2: Japanese Patent Publication No. 2006-225701A
PLT 3: Japanese Patent Publication No. 2009-275250A
PLT 4: Japanese Patent Publication No. 7-258734A
Summary of Invention
Technical Problem
- 3 -
[0009] The present invention was made in consideration
of such a situation and has as its object t~he provision
5 of steel wire rod having stable workability, which is
characterized by having a microstructural morphology of
cementite designed for delay of formation of microvoids
at the inside during a working operation so as to realize
stable wire drawability and forgeability.
10 Solution to Problem
[0010] To achieve the above object, the gist of the
present invention is as follows:
(1) An excellent workability steel wire rod comprising
steel components including, by mass%, C: 0.20 to 0.60%,
15 Si: 0.15 to 0.30%, Mn: 0.25 to 0.60%, P: ~0.020%, S:
~0.010%, and a balance of Fe and unavoidable impurities,
and an inside microstructure including cementite, wherein
by number ratio, 80% or more of the cementite in a crosssection
vertical to a longitudinal direction of the wire
20 rod has a short axis of 0.1 ~or less and a ratio of a
long axis to the short axis, defined as an aspect ratio,
of 2.0 or less.
[0011] (2) The excellent workability steel wire rod
according to (1) further containing, in addition to the
25 steel components, by mass%, one or more of Al: 0.06% or
less, Cr: 1.5% or less, Mo: 0.50% or less, Ni: 1.00% or
less, V: 0.50% or less, B: 0.005% or less, and Ti: 0.05%
or less.
[0012] (3) A method for production of an excellent
30 workability steel wire rod excellent in drawability and
forgeability comprising heating a billet of a chemical
composition according to (1) or (2) to 950°C to 1080°C,
supplying the billet to a wire rod rolling process to
obtain a wire rod, coiling the wire rod in a temperature
35 region of 750°C to 900°C, then subjecting the wire rod to
in-line heat treatment by a molten salt of 400°C to 430°C,
- 4 -
and ejecting the molten salt to the wire rod being dipped
in the molten salt at a stirring flow rate of 0.5 m/s to
2.0 m/s in range.
Advantageous Effects of Invention
5 [0013] The present invention suppresses wire breakage
and cracking during working operations in the fields of
typical processes of manufacture of steel wire rod such
as wire drawing or cold forging, enables the provision of
a wire rod having excellent workability, and contributes
10 to the stabilization of production activities in the
above-mentioned fields.
Brief Description of Drawings
[0014] FIG. 1 is a view showing an outline of a method
for measuring electrical resistance.
15 FIG. 2 is a comparative view showing a difference in
electrical resistances of steel wire rods of the present
invention and the prior art.
FIG. 3 is a graph showing a relationship between a void
shape and cementite short axis.
20 FIG. 4A is a schematic top view explaining an in-line
heat treatment process of a steel wire rod, while FIG. 4B
is a schematic side cross-sectional view explaining an
in-line heat treatment process of a steel wire rod.
FIG. SA is a schematic front cross-sectional view of an
25 apparatus 10 for performing an in-line heat treatment
process comprising a cooling tank in which piping 2 is
laid for discharging molten .salt A, while FIG. 5B is a
schematic side cross-sectional view of the apparatus 10.
Description of Embodiments
30 [0015] Below, the steel components according to the
present invention, the aspect ratio (long axis)/(short
axis) relating to the microstructural morphology of
cementite, the abundance ratios of different aspect
ratios in the total amount of cementite in a cross-
35 section, the short axis sizes, and details relating to
the method for production, particularly reasons for
defining the lower limits and upper limits of the
- 5 -
suitable ranges, will be specifically explained. The
relating to the steel components all show mass%.
[0016] C: 0.20 to 0.60%
"!:<-" 0
C, as is well known, is an element required for securing
5 strength. If less than 0.20%, a suitable strength in the
application can no longer be held. If over 0.60%, at the
time of cold forging, the load stress becomes higher, so
the lifetime of the forging punch etc. come to be
affected.
10 [0017] Si: 0.15 to 0.30%
Si is used as a deoxidizing material. If the amount of Si
is less than 0.15%, the deoxidation becomes insufficient
and surface defects due to pinhole defects which were
formed at the casting stage are caused at the surface
15 part of the billet. Further, if the amount of Si being
over 0.30%, selective oxidation at the stage of heating
the billet causes Si to concentrate at the interface
between scale and base iron. In view of concern for
having a detrimental effect on the descaling ability, the
20 upper limit was made 0.30%.
[0018] Mn: 0.25 to 0.60%
Mn, like Si, is an element required for deoxidation.
Further, it is an element important for securing the
ductility during hot rolling. The lower limit was made
25 0.25% to avoid insufficient deoxidation. Further, the
upper limit was made 0.60% because addition over this
amount would result in an increase of solid solution
strengthening amount, raise the deformation resistance at
the time of forging, and thereby invite deterioration of
30 tool life.
[0019] P: S0.020%
P is an element having the feature of causing
deterioration of the ductility of the steel material.
Further, the segregation ratio of P is also high, so
35 concentration of P easily occurs at the segregation
portions caused in the production stage. For this reason,
the upper limit of P was made 0.020%.
- 6 -
[0020] S: :0::0.010%
S bonds to Mn in the steel to produce MnS. Further, S
segregates at the center part in the processes between
refining process of the st-eel and solidification process
5 of the steel, so MnS becomes denser at the center part.
If S exceeds 0.010%, at the time of wire drawing etc.,
internal cracks may occur and the wire may break.
Therefore, S is made 0.010% or less.
[0021] The basic composition of chemical components in
10 the steel wire rod of the present invention is as
mentioned above. If further including, in addition to the
15
above composition, one or more elements selected from the
group comprised of Al: 0. 06% or less, Cr: 1. 50% or less,
Mo: 0.50% or less, Ni: 1.00% or less, V: 0.50% or less,
B: 0.005% or less, and Ti: 0.05% or less, the advantages
are obtained of improvement of the hardenability and
improvement of the strength in cold forging.
[0022] Al: 0.06% or less
Al has the effect of fixing N to suppress dynamic strain
20 aging during cold forging and reduce the deformation
resistance. To obtain this effect, it is preferable to
include at least 0.01%. However, if Al is included in
excess, it makes the toughness fall, so the upper limit
of Al is made 0.06%.
25 [0023] Cr: 1.50% or less, Mo: 0.50% or less, and Ni:
1.00% or less
Cr, Mo, and Ni are elements effective for improving the
hardenability. However, if included in excess, they cause
deterioration of the ductility, so the contents are kept
30 to within the above ranges.
V: 0.50% or less
V may be added for the purpose of precipitation
strengthening. However, if V is added in a large amount,
deterioration of the ductility is caused, so the content
35 is kept to within the above range.
[0024] B: 0.0050% or less and Ti: 0.05% or less
B is an element for improving the hardenability and may
- 7 -
be added as necessary. However, if included in excess, it
causes deterioration of the toughness, so the upper limit
is made 0.005%. Ti is an element effective for the
reduction of the def-ormation resistance at the time of
5 cold forging by the effect of suppression of dynamic
aging owing to fixing of solid solution N, so may be
added as necessary. However, if Ti is included in excess,
coarse TiN precipitates, the coarse TiN acts as
initiation points, and cracking is likely to occur, so
10 the upper limit is made 0.05%.
[0025] Next, the reasons for limitation of the aspect
ratio of cementite will be explained. As a method for
obtaining a grasp of the effect of the cementite shape on
the workability, the inventors used a die with a larger
15 approach angle than a usually used wire drawing die so as
to intentionally severely process a material and engaged
in various studies on the occurrence of microvoids formed
at the inside. As a result, they found that the shapes of
microvoids generated at the interface part between the
20 cementite and base iron have the following features.
[0026] The inventors performed drawing processes using
various types of steel wire rods in which the aspect
ratios are different from each other by highly angled
dies (approach angle 30°) in single passes (25% dra_wing
25 reduction of area), observed the microvoids in the crosssections
of the drawn steel wires, and measured the
shapes of the generated voids and the ratios of the
shapes. Specific examples of the observations are shown
in Table 1. The observation was performed by taking
30 lOOOOX SEM photographs of 265 ~m2 area region at the three
locations of the surface layer part, 1/4D part (D:
diameters of wire rods), and center part, respectively.
When the aspect ratio of the cementite shape was 2 or
less, the ratio at which microvoids exist individually
35 became extremely high. On the other hand, with regard to
cementite formed in a lamellar shape (aspect ratio: 10 or
more), the ratio at which microvoids are connected to
- 8 -
each other in adjoining cementites was high. Further,
with an aspect ratio of 2 to 10 in range, there was a
mixture of both independent and connected types. However,
observation bythis method is limited to a local visual
5 field in a cross-section.
[0027] Therefore, in order to increase the volume of
observation and stably get a grasp of formation of
internal microvoids, the inventors produced steel wires
by using the Steel Wire Rod Nos. 1 to 6 of the present
10 invention and the Steel Wire Rod Nos. 11 to 16 of the
comparative examples shown in Table 3 and attempted to
measure the electrical resistances of the steel wires by
the four-probe method shown in FIG. 1.
[0028] Table 1. State of Generation of Voids by Aspect
15 Ratio and Ratios of Voids of Those Types (%)
of
Aspect ratio of cementite
Features
void shapes 2 0 to 2
Over to
10
less than 10
or more
Independent 93.1 5.7 1.1
Connected 1.2 20.3 78.6
[0029] The results are shown in FIG. 2. As imagined
from the shapes of the voids actually observed, it was
confirmed that steel wires made of the steel wire rods of
20 the present invention are suppressed more in formation of
internal microvoids and are lower in electrical
resistance values since the numbers of generation of
microvoids are smaller. Based on the results of these
measurements, in the course of getting a grasp of the
25 state of generation of internal microvoids and observing
in detail the microstructural morphology, the inventors
discovered that there is a close relationship between the
formation of microvoids and the form of cementite by
initially applying wire drawing conditions severer than
30 usual to artificially cause the formation of microvoids.
When focusing on the shape of the cementite, it was found
that if the ratio of the long axis to the short axis
(below, called the ''aspect ratio'') is 2 or less, cracks
- 9 -
independently occur from the interface of the base iron
around the cementite.
[0030] On the other hand, in Table 1, if the aspect
ratio is over 2 to 10, while the trends differ depending
5 on the distance between adjoining cementite crystals,
both the independent and connected forms appear.
Furthermore, if the aspect ratio exceeds 10, the
connected form increases. This trend is also shown in
Table 1. Based on these findings, the inventors obtained
10 the findings that by suppressing the aspect ratio of the
cementite to 2 or less, the formation of internal
microvoids is suppressed and that controlling microvoids
to independent ones which are hard to connect to each
other is effective for providing wire rod excellent in
15 the wire drawability and forgeability.
[0031] Based on the above results of study, the
reasons for limitation of the microstructural morphology
will be explained below.

20 The aspect ratio is made 2 or less because of the
following: As shown in Table 1, after artificially severe
wire drawing was performed to inflict damage on the
cementite, microvoids were formed. The inventors
researched the formation of the microvoids in detail, and
25 they acquired insights into the formation of the
microvoids. From their insights into the formation of the
microvoids, the ratio of microvoids whereby independent
microvoids are formed and do not easily connect to each
other becomes highest when an aspect ratio is 2 or less.
30 The aspect ratio was determined based on the result of
this observation. Further, if the ratio, that is,
abundance ratio, of cementite with an aspect ratio of 1
to 2 is 80% or more in a cross-section, the desired
workability is obtained. Therefore, the lower limit of
35 the abundance ratio is made 80%. If the abundance ratio
is less than 80%, the ratio of the independent microvoids
connecting together rises and the workability is
affected.
[0032]
Cementite>
- 10 -

The microstructure varies depending on the
difference of the cooling speed at the different portions
20 in a cross-section arising at the stage of production of
the wire rods, so there is an inherent limit to how
uniform a microstructure in the overall cross-section can
be made. It is difficult to make the ratio of the
lamellar type structures 0. Various tests were performed.
25 As a result, it could be confirmed that if the ratio of
lamellar type structures is less than 5%, there was
little effect on the workability. Therefore, the upper
limit of the ratio of lamellar type structures is defined
30
as 5%.
[0034] Next, the method for production of the
excellent workability steel wire rod of the present
invention will be explained.

The billet is heated to 950°C to l080°C in range. After
35 heating, the billet is rolled to a wire rod. If less than
950°C, within the usual holding time, the internal
- 11 -
imbalance of heat inside the billet becomes greater and
warp of the steel material at the time of rolling or
problems accompanying the increase in the reaction force
arise. Further, the upper limit temperature is made 1080°C
5 because if the heating temperature is more than that, the
y (austenite) grain size will easily increase etc. Such an
increase in y grain size more than necessary would affect
the skin quality of the free surface of the final
10
product, so the upper limit is made 1080°C.
[0035]
After the heating process, the steel piece is coiled up
at a temperature of 750°C to 900°C in range. The lower
limit temperature varies somewhat due to the size of the
rolled wire rod, but is made 750°C to stably perform the
15 heat treatment after coiling. Further, if less than 750°C,
pearlite transformation occurs before the heat treatment
and the targeted metal microstructure can no longer be
obtained. On the other hand, coiling at a temperature
over 900°C would invite an increase in surface oxidation
20 etc. so is not desirable.
[0036]
In-line heat treatment is performed by dipping the wire
rod after the coiling process in a cooling tank
containing a molten salt of at least one of potassium
25 nitrate and sodium nitrate and of 400°C to 430°C while
stirring at a predetermined flow rate. The lower limit
temperature of the in-line heat treatment temperature is
made 400°C because with a temperature less than that, a
lower bainite structure is formed and the hardness of the
30 material rapidly ends up increasing, so the lifetime of a
tool used in a forging process etc. deteriorates. The
upper limit temperature of the heat treatment is made
430°C because if a temperature over this, there would be
regions where degenerate pearlite structures are mixed
35 into the upper bainite, so control of the aspect ratio of
5
- 12 -
the cementite would become difficult and the effect of
delaying formation of microvoids, which is the most
important in the present invention, would no longer be
able to be exhibited.
[0037] The condition which plays an important role in
the present invention is not only the above in-line heat
treatment temperature, but also the stirring flow rate
creating the jet flow explained here.
In the above-mentioned in-line heat treatment, the steel
10 wire rod is dipped in the cooling tank in the form of a
loose coil or other coil. In this case, even if the flow
of the molten salt in the cooling tank is maintained in a
constant direction, since the steel wire rod being heattreated
is a coil in shape, the direction in which the
15 molten salt strikes the steel wire rod will differ
depending on the location. It is considered de facto
difficult to make the direction of impact constant.
[0038] Therefore, it is thought that not only the flow
rate, but also the effect of the direction in which the
20 molten salt strikes the steel wire rod is an important
technical issue in realizing the present invention. With
that in mind, the effect was investigated. The
relationship between typical directions of the flow of
molten salt such as directions parallel to the conveyance
25 direction (F) of the steel wire rod (011 and 012 of FIGS.
4A and 48), directions vertical to the coil surface of
the steel wire rod (directions 031 and 032 of FIG. 48),
and directions horizontal to coil surface of steel wire
rod and vertical to conveyance direction (F) (directions
30 021 and 022 of FIG. 4A) and the abundance ratio of
cementite with an aspect ratio of 2 or less with respect
to the total amount of cementite in the cross-section was
investigated.
[0039] As shown in FIG. 4A and FIG. 48, the directions
35 012, 022 and 032 were made positive directions and the
directions 011, 021, and 031 were made negative
directions. The maximum flow rates and the minimum flow
- 13 -
rates of the molten salt A in each of three directions
vertical to each other were measured near the coil
surfaces llA and llB of the steel wire rod 1,
respectively. The average flow rates in each of the three
5 directions vertical to each other, calculated on the
basis of the maximum flow rates and the minimum flow
rates, were defined as the "stirring flow rate vectors''
and the magnitudes of the stirring flow rate vectors were
defined as the ''stirring flow rates". The relationship
10 between the stirring flow rate of the molten salt and the
abundance ratio of the cementite was investigated. As a
result, it was found that if the steel wire rod is a coil
shape, if the stirring flow rate of the molten salt is
0.5 m/s or more with respect to the coil surfaces of the
15 steel wire rod, the uniformity of the material quality in
the cross-section can be improved to a level not
substantially posing any problems.
[0040] Further, if the stirring flow rate is less than
0.5 m/s with respect to the coil surfaces, the cooling of
20 the wire rod by the molten salt becomes insufficient and
control to make the aspect ratio of the cementite 2 or
less can no longer be stably performed. On the other
hand, if making the stirring speed over 2 m/s with
respect to the coil surfaces, a rise in pressure of the
25 stirring flow in the molten salt is invited, the material
being heat treated, that is, the wire rod coil, starts to
shake and, therefore conveyance becomes unstable etc. The
upper limit of the stirring flow rate is limited from the
viewpoint of operational stability.
30 [0041] The positions for measurement of the stirring
flow rate may be the gap between adjoining rollers of the
conveyor rollers 6, for example. Further, the stirring
flow rate is particularly preferably measured at a
position where the flow rates up to reaching the coil
35 surfaces llA and llB are maintained to be substantially
constant ..
[0042] Further, with regard to the method of using a
- 14 -
gas as a medium for driving the stirring, the cooling of
the wire rod by the molten salt becomes insufficient, so
the aspect ratio of the cementite may be unable to be
controlled to 2 or less. Therefore, the wire rod may be
5 cooled either by using a stirring machine to directly
stir the molten salt in the cooling tank or by
discharging the molten salt itself into the molten salt
in the cooling tank.
Examples
10 [0043] Below, examples will be used to show the
advantageous effects of the present invention. Table 2-1
shows the chemical components of the test steels used for
the tests.
[0044] Each steel of Table 2-1 was smelted, then
1S continuously cast into a 300 mm x SOO mm casting size,
and then was bloomed to a 122 mm square billet. The
billet was reheated, and then rolled to obtain a wire
rod. The Wire Rod Nos. 1 to 10 of the invention examples
and Wire Rod Nos. 18 to 21 were coiled, then dipped in
20 molten salt in the in-line heat treatment apparatus 10
shown in FIGS. SA and SB for direct heat treatment to
obtain S.S mm~ wire rods. The Wire Rod No. 11 was directly
cooled in the molten salt without stirring the molten
salt after rolling the wire rod. Further, Wire Rod,Nos.
2S 12 to 17 are cases of comparative examples, which were
obtained by continuous casting to obtain cast billets of
the same sizes, then blooming them to obtain billets of
the same sizes, and then rolling them to obtain S.S mm~
wire rods with air blast cooling for heat treatment after
30 the wire rod rolling.
[0045] The in-line heat treatment of the wire rod
after coiling, as shown in FIGS. SA and SB, was performed
by conveying the steel wire rod 1 using the conveyor
rollers 6 in the in-line heat treatment apparatus 10 in
3S the F direction so that the entire coil shaped steel wire
rod 1 was dipped below the surface S of the molten salt
- 15 -
A. The in-line heat treatment apparatus 10 is structured
so as to have a cooling tank 3 in which piping 2 is laid
for discharging molten salt A. The piping 2 discharges
molten salt A toward the wire rod 1 from the lower side
5 to the upper side so as to create a flow 4 of molten salt
vertical to the coil surfaces 11 of the wire rod 1.
[0046] The stirring flow rate was calculated as the
average speed of the maximum speed and the minimum speed
of the flow 4 of the molten salt near the coil surfaces
10 11 of the steel wire rod 1.
[0047] As will be understood from Table 2-2, the
method for production of a wire rod according to the
present invention is characterized by dipping a wire rod
in a molten salt of 400 to 430°C which is relatively lower
15 in temperature as a direct heat treatment after wire rod
rolling and making the molten salt accompanied with a
stirring flow contact the heat treated material to
thereby strengthen the dipped wire rod by removal of
heat.
20 [0048] For this reason, unlike the wire rods of the
comparative examples, the microstructures of the steel
wire rods according to the present invention present F
(ferrite)+B (bainite). On the other hand, it is
understood that the microstructural morphologies of the
25 steel wire rods of the comparative examples present F+P
(pearlite) structure since the wire rod cooling speed
becomes slower than that in the method for production
according to the present invention. Next, as will be
understood from Table 3, the difference in the types of
30 microstructural morphologies appears in a factor of form
of cementite, that is, the aspect ratio.
[0049] That is, in the case of the steel wire rod of
the present invention, the temperature of the heat
treatment medium enables the aspect ratio to be made
35 smaller compared with the case of production by the usual
air blast cooling and easily enables the aspect ratio of
2 or less to be achieved. On the other hand, the Wire Rod
- 16 -
Nos. 12 to 17 of the comparative examples have lamellar
structures, so it is understood that the abundance ratios
of cementite with aspect ratios of 2 or less become
extremely small. Further, in each of the Wire Rod Nos. 18
5 to 21 of the comparative examples, amount of cementite
with an aspect ratio of 2 or less is less than 80% in the
cross-section. This is due to the fact that during the
in-line heat treatment, the stirring flow rate of the
molten salt. was less than 0.5 m/s, so the wire rods were
10 not sufficiently cooled by the molten salt.
[0050] The Wire Rod Nos. 1 to 21 were measured for
abundance ratios of cementite with short axes of 0.1 ~
or less and with aspect ratios of 2 or less among
cementites in the cross-sections vertical to a direction
15 of the wire rod. Further, the Wire Rod Nos. 1 to 21 were
drawn and measured for wire drawability, forgeability,
and electrical resistance and measured for numbers of
microvoids. The results are shown in Table 3.
[0051] First, as shown in Table 3, when wire drawing
20 the steel wire rods of the invention examples and the
steel wire rods of the comparative examples using dies
which have die half angles of 5°, no large difference is
observed between the workabilities of the two. Therefore,
the inventors provided intentionally severe wire d:J;awing
25 conditions by using a die having a die half angle of 15°,
and performed wire drawing. As a result, as shown in
Table 3, it was found that the features of the steel of
the present invention appeared and no generation of
microvoids could be observed inside at the time of
30 performing one die drawing of a 5.5 mm to 5 mm, while in
the case of steel wire rods of the comparative examples,
microvoids were generated in the inside.
- 17
[0052] Table 2-1
Steel Chemical components (mass%)
type c Si Mn p s Al Cr Mo Ni v Ti B
A 0.20 0.15 0.25 0.010 0.005
B 0.25 0.16 0.30 0.012 0.007
c 0.30 0.28 0.35 0.014 0. 009 - - - - - - -
D 0.35 0.20 0.40 0. 015 0.006 - - - - - - -
E 0.45 0.25 0.35 0. 019 0.005 - - - -
F 0.60 0.30 0.60 0.020 0.010
G 0.20 0.15 0.60 0.010 0.010 0.039 0.02 0.01 0.02
H 0.23 0.16 0.60 0.015 0.010 0.031 0.15 - 0.02 0.01 0.03 0.0015
I 0.42 0.20 0.60 0.020 0.010 0.026 1.09 - 1. 00 - - -
J 0.24 0.22 0.51 0.020 0.009 0.026 1. 50 - 0.26 -
K 0.45 0,30 0.50 0.020 0.010
L 0.20 0.15 0.25 0.010 0.005
M 0.25 0.16 0.30 0.012 0.007
N 0.30 0.28 0.35 0.014 0.009 - - - - - - -
0 0.35 0.20 0.40 0.015 0.006 - - - - -
p 0.45 0.25 0.35 0.019 0.005
Q 0.60 0.30 0.60 0.020 0.010
(ln the table " " lndlcates the amount of addltlon of the correspondlng
element to the steel material is 0 wt%.)
5 [0053] Table 2-2
Wire Heating Coiling
Coolant Stirring Micro-
Class rod
Steel Cooling temp. flow rate structural
type medium temp. "C temp. "C
No. "C m/s morphology
Inv. 1 A Molten salt 1080 750 400 0.5 F+B
ex. 2 B Molten salt 950 770 410 1 F+B
3 c Molten salt 1000 BOO 420 1.1 F+B
4 D Molten salt 980 850 417 1.5 F+B
5 E Molten salt 1020 87 5 415 1.7 F+B
6 F Molten salt 1050 900 430 1.9 F+B
7 G Molten salt lOBO 750 400 0.5 F+B
8 H Molten salt 1080 750 400 0.5 F+B
9 I Molten salt 1080 750 400 0.5 F+B
10 J Molten salt 1080 750 400 0.5 F+B
Comp. 11 K Molten salt 1050 900 425 None F+B
ex. Air blast Room
12 L cooling
1100 850 None F+P
temp.
13 M
Air blast
1080 850
Room
None F+P
cooling temp.
14
Air blast
1100 850
Room
N None F+P cooling temp.
15 0
Air blast
1120 850
Room
None F+P
cooling temp.
16
Air blast
1080 850
Room
p None F+P
cooling temp.
17 Q Air blast
1090 850
Room
None F+P
cooling temp.
18 G Molten salt 1050 900 425 0.3 F+P
19 H Molten salt 1050 900 425 0.3 F+P
20 I Molten salt 1050 900 425 0.3 F+P
21 J Molten salt 1050 900 425 0.3 F+P
[0054] Table 3
Wire Amount of cementite with Wire Electrical
Class rod Steel Type of aspect ratio of 2 or less
draw- High angle wire Forge- resistance Number of
No. type structures
(%) (*1) ability drawing (*3) (%) ability
(x10.30)
microvoids
1*2)
Inv. 1 A F+B 96 0 0 0 0.230 0
ex. 2 B F+B 93 0 0 0 0.234 0
3 c F+B 88 0 0 0 0.239 0
4 D F+B 94 0 0 0 0.241 0
5 E F+B 92 0 0 0 0.247 0
6 F F+B 81 0 0 0 0.250 0
7 G F+B 85 0 0 0 0.242 0
8 H F+B 87 0 0 0 0.240 0
9 I F+B 88 0 0 0 0.241 0
10 J F+B 87 0 0 0 0.240 0
Camp. 11 K F+B 75 0 Ll 50 0.280 18
ex. 12 L F+P 5 0 Ll 60 0. 298 16
13 M F+P 0 0 Ll 60 0. 295 15
14 N F+P 3 0 X 80 0.302 40
15 0 F+P 6 0 X 80 0.315 43
16 p F+P 2 0 X 100 0.365 38
17 Q F+P 1 0 X 100 0. 380 . 49
18 G F+P 78 0 Ll 45 0. 27 5 12
19 H F+P 77 0 Ll 45 0.273 10
20 I F+P 76 0 Ll 47 0. 271 9
21 J F+P 78 - 0 Ll 46__ 0.272 10
-·--
{*1): Abundance ratio of cementite with short axis of 0.1 ~or less and with aspect ratio of 2 or less among cementite in
cross-section vertical to longitudinal direction of wire rod.
( * 2) Die half angle: wire drawing by 5°.
(*3) Die half angle: wiring drawing by 15°
f-'
CD
- 19 -
[0055] The amounts of cementite with aspect ratios of
2 or less in Wire Rod Nos. 1 to 10 corresponding to the
invention examples were 80% or more. Further, in the
Steel Wire Rod Nos. 12 to 17 of Table 3, the majority of
5 the cementite was a lamellar type, and the abundance
ratio of the area of cementite with a short axis of 0.1
J.lm and an aspect ratio of 2 or less (Table 3, "Amount of
cementite with aspect ratio of 2 or less (%)") was only
6% or less.
10 On the other hand, if comparing the test results of the
wire drawability using dies with die half angles of 15°
between the Steel Wire Rod Nos. 1 to 10 (invention
examples) and the Steel Wire Rod Nos. 11 to 21
(comparative examples), the steel wire rods of the
15 invention examples have higher ductility due to the
delayed generation of microvoids.
From this result, it is understood that the high
ductility due to the delayed generation of microvoids
appears in a region where the average value of the aspect
20 ratio is 2 or less and the abundance ratio is 80% or
more.
[0056] Further, from the results of Table 3, it could
be confirmed that if the number of microvoids actually
generated becomes greater, the drawn steel wire also
25 increases in electrical resistance.
[0057] That is, as shown in Table 3, it was confirmed
that the steel wires of the present invention have
electrical conductivities of 0.23 to 0.25 xl0-3 Q in
range, while the steel wires of the comparative examples
30 have higher electrical conductivities of 0.28 to 0.38
xl0-3 Q in range. Compared with the steel wires of the
present invention, it could be confirmed that steel wires
of the comparative examples had clearly greater numbers
of generated microvoids.
35 [0058] The electrical resistivity was measured using
the four-probe method shown in FIG. 1. Further, the
- 20 -
number of microvoids was measured by drawing by a high
angle die (approach angle: 30") by one pass (25% drawing
reduction of area), observing the microvoids present in a
2.4 mm x 3.2 mm area at SOOX, and counting the number of
5 visually discernable microvoids.
[0059] The above-mentioned differences in the number
of internal microvoids generated appear in the
forgeability as an effect on actual workability.
[0060] Test pieces with L/D ratios (L: length, D:
10 diameter) of 1.5 were given V-notches along the
longitudinal direction at one location in the
circumferential direction. Using these test pieces,
forging tests were conducted five times with rolling
reduction rates of up to 90% and the rate of occurrence
15 of cracking at the bottoms of the notches (%) was
determined. The results are shown in the forgeability
column of Table 3.
[0061] As will be understood from the above results,
in the case of the steel wire rods according to the
20 present invention, no cracking can be observed and the
workability is good. On the other hand, in the steel wire
rods of the comparative examples, cracking occurred in
the range of 50 to 100%. These results are obtained as a
result of it being possible to delay the generatior; of
25 internal microvoids during shape processing in steel wire
rod according to the present invention where the shape of
the cementite is controlled to make the aspect ratio 2 or
less. The reason is that, as shown by the results of
observation shown in FIG. 3, the steel wire rod according
30 to the present invention is high in ratio of formation of
independent microvoids.
Industrial Applicability
[0062] The present invention suppresses the occurrence
of wire breakage or fracture during a working operation
35 in typical processes of manufacture using steel wire rod
as a material such as wire drawing or cold forging and
enables the provision of wire rod having excellent
- 21 -
workability. It is a significant invention able to
contribute to stabilization of production activities in
that field.

CLAIMS
Claim 1. An excellent workability steel wire rod
comprising s·teel components including, by mass%, C: 0. 20
to 0.60~, Si: 0.15 to 0.30%, Mn: 0.25 to 0.60%, P:
5 ~0.020%, S: ~0.010%, and a balance of Fe and unavoidable
impurities, and having an inside microstructure including
cementite,
wherein by number ratio, 80% or more of the
cementite in a cross-section vertical to a longitudinal
10 direction of the wire rod has a short axis of 0.1 J.Lm or
less and a ratio of a long axis to the short axis,
defined as an aspect ratio, of 2.0 or less.
Claim 2 . The excellent workability steel wire rod
according to claim 1 further containing, in addition to
15 said steel components, by mass%, one or more of Al: 0.06%
or less, Cr: 1.5% or less, Mo: 0.50% or less, Ni: 1.00%
or less, V: 0.50% or less, B: 0.005% or less, and Ti:
0.05% or less.
Claim 3. A method for production of an excellent
20 workability steel wire rod excellent in drawability and
forgeability comprising heating a billet of a chemical
composition according to claim 1 or 2 to 950°C to 1080°C,
supplying the billet to q wire rod rolling process to
obtain a wire rod, coiling the wire rod in a temperature
25 region of 750°C to 900°C, then subjecting the wire rod to
in-line heat treatment by a molten salt of 400°C to 430°C,
and ejecting the molten salt to the wire rod being dipped
in the molten salt at a stirring flow rate of 0.5 m/s to
2.0 m/s in range.