DESCRIPTION
Title of Invention: High Carbon Steel Wire Rod Excellent
in Drawability and Fatigue Characteristics After Wire
Drawing
Technical Field
[OOOl] The present invention relates to wire for
drawing purposes, in particular high carbon steel wire
10 and wire for valve springs which are excellent in
drawability and fatigue characteristics after drawing.
[0002] The wire of the present invention is used,
after drawing, for example, for steel cord for automobile
tires, saw wire for cutting silicon for solar cells or
semiconductors, valve springs for automobile engines,
long rubber belts, aircraft tires, bridge ropes, etc.
Background Art
[0003] In general, high carbon steel wire which is
used for drawing purposes has to be able to be drawn at a
20 high speed and has to be excellent in fatigue resistance
after drawing. As one of the factors having a detrimental
effect on these characteristics, hard oxide-based
nonmetallic inclusions may be mentioned.
[0004] Among these oxide-based inclusions, inclusions
25 of A1203, Si02, CaO, Ti02, MgO, and other singlecomposition
or binary systems of Mg0 .A1203 or 2Mg0 -Si02
are high in hardness and nonviscous. Therefore, to
produce high carbon steel wire excellent in drawability,
it is necessary to raise the cleanliness of the molten
steel and soften the oxide-based inclusions.
[0005] As the method of raising the cleanliness of
steel and softening nonmetallic inclusions in this way,
PLT 1 shows a method of production of steel for high
carbon steel excellent in drawability. PLT 2 discloses a
35 method of production of extremely fine wire. The basic
idea of these arts is limited to three-component system
oxide-based nonmetallic inclusions of A1203-Si02-Mn0.
[0006] PLT 3 proposes making nonmetallic inclusions
the Spessartite region in a A1203, Si02, MnO threecomponent
phase diagram so as to improve the drawability
of the product. PLT 4 discloses a method of restricting
the amount of A1 which is added to the molten steel so as
to reduce harmful inclusions and improve the drawability.
[0007] PLT 5 relates to production of steel cords of a
nonviscous inclusion index of 20 or less and proposes to
blow CaO-containing flux together with a carrier gas
(inert gas) into the ladle molten steel while completely
restricting A1 so as to perform preliminary deoxidation,
then blow in an alloy which contains one or more of Ca,
Mg, and REM and thereby soften the inclusions.
[ 0 0 0 8 ] Among the above methods, with the method of
reforming ternary nonmetallic inclusions, stable control
of the composition is difficult. On the other hand, with
the method of control of multi-component nonmetallic
inclusions, it is difficult to reduce the size and number
of inclusions and secure the ductility. Therefore, it is
difficult to improve the drawability and fatigue
resistance after drawing.
[OOOS] Therefore, PLT 6 restricts the range of the
total oxygen content to a certain range to control the
amount and composition of the nonviscous inclusions,
reduces the size and number of nonviscous inclusions and
secures ductility to thereby make the amount and
distribution of size of nonviscous inclusions the
preferable state, further, adds the composition of
inclusions to Si02 and MnO, and reforms them to multicomponent
oxide-based inclusions which selectively
contain A1203, MgO, CaO, and Ti02 so as to soften the
inclusions and realize high carbon steel wire extremely
excellent in drawability and fatigue resistance after
drawing.
[OOlO] Furthermore, PLT 7 discloses the method of
limiting the sizes of hard high Si02 inclusions and
reducing the amount of use of expensive deoxidizing
alloys.
[ 0 0 11 ] Further, for control for facilitating
stretching of nonmetallic inclusions at a lower melting
point, several methods of utilizing alkali metal
5 compounds have been proposed. PLT 8 proposes to use a
mixture of an Si-based deoxidizing agent and an alkali
metal compound so as to control the amount of alkali
metal compound in the nonmetallic inclusions to 4 to 24%
and improve the stretchability.
[0012] Furthermore, PLT 9 proposes Si-killed steel
which is excellent in fatigue strength characterized by
including 0.5 to 10% of an oxide of an alkali metal in
A1203-Ca0-Si02-Mg0-Mn0-based low melting point inclusions.
[ 0 0 13 ] Furthermore, PLTs 10 and 11 disclose steel wire
for high cleanliness springs excellent in fatigue
characteristics characterized by including one or more of
Li02, Na20, and K20 in a total of Li02, Na20, and K20 of
0.5 to 20% in low melting point inclusions. These
describe that Li02, Na20, and K20 are not equal and that
20 by positively adding Li with its particularly strong
deoxidizing power as the source of formation of the
oxide-based inclusions, the effect is raised if including
a suitable amount of Li02 in the oxide-based inclusions.
Citations List
Patent Literature
[0014] PLT 1: Japanese Patent Publication No. 57-
22969B
PLT 2: Japanese Patent Publication No. 55-24961A
PLT 3: Japanese Patent Publication No. 54-7252B
PLT 4: Japanese Patent Publication No. 50-81907A
PLT 5: Japanese Patent Publication No. 57-35243B
PLT 6: Japanese Patent Publication No. 4-8499B
PLT 7: Japanese Patent No. 3294245A
PLT 8: Japanese Patent No. 2654099A
PLT 9: Japanese Patent No. 3719131A
PLT 10: Japanese Patent Publication No. 2005-29888A
PLT 11: Japanese Patent No. 4315825B
Non-Patent Literature
[ 0 0 15 ] NPLT 1: Iron and Steel Institute of Japan ed.
Third Edition Iron and Steel Handbook, II. Iron-making
and Steel-making, page 690
Summary of Invention
Technical Problem
[0016] As explained above, by securing the cleanliness
of the molten steel, softening the nonmetallic
inclusions, preventing contamination by Al, etc., high
carbon steel wire which has a high drawability could be
supplied.
[0017] In this regard, in recent years, an improvement
in productivity has been sought by elimination of the
patenting after primary drawing in the process of drawing
wire. Also, the market for saw wire finer than the steel
cord used for automobile tires has grown. Therefore, the
size of inclusions in wire causing breakage has become
much smaller than the past (20 pm or more). With only
conventional inclusion softening art, the inclusions
cannot be sufficiently stretched and handling becomes
difficult.
[0018] The present invention was made in consideration
of the above situation and has as its topic the supply of
high performance high carbon wire which is able to handle
even tough applications, which is extremely low in wire
breakage rate at the time of drawing, and which is
excellent in fatigue characteristics after wire drawing
by utilizing the basic technique of multi-component
control of oxide-based nonmetallic inclusions and
utilizing compounds other than oxides so as to cause a
distinctive drop in the melting point and viscosity of
nonmetallic inclusions and refine the inclusions after
wire rolling.
Solution to Problem
[0019] The inventors investigated in detail the
relationship between the composition of nonmetallic
inclusions and the melting point or viscosity. As a
result, they discovered that by establishing the
copresence of Na or another alkali metal and a fine
amount of fluorine in multi-component inclusions, it is
possible to further lower the melting point and viscosity
of inclusions, possible to control the formation of the
crystal phases, and as a result possible to refine the
inclusions after wire rolling.
[0020] Furthermore, the inventors discovered that
there is also the effect of delaying the formation of
crystal phases of nonmetallic inclusions in the NaF
molecules.
[0021] The present invention was made based on the
above discovery and has as its gist the following.
COO221 (1) High carbon steel wire excellent in
drawability and the fatigue characteristics after wire
drawing which contains, by mass%, C: 0.5 to 1-28, Si:
0.15 to 2.5%, Mn: 0.20 to 0.9%, P10.025%, S: 0.004 to
0.025%, Al: 0.000005 to 0.002%, Ca: 0.00001 to 0.002%,
Mg: 0.00001 to 0.001%, Na: 0.000005 to 0.001%, and F:
0.000003 to 0.001%, has a total oxygen amount of 16 to 30
ppm, and has a balance of Fe and unavoidable impurities,
the high carbon steel wire rod characterized in that the
number ratio of inclusions satisfying (%Si02)=40 to 95%,
(%Ca0)=0.5 to 30%, (%A1203)=0.5 to 30%, (%Mg0)=0.5 to 20%,
and (%Mn0)=0.5 to 10% and further satisfying (%Na)=0.2 to
7% and (%F)=0.17 to 8% (below, referred to as "inclusions
covered due to composition") in the oxide-based
nonmetallic inclusions of a short axis of 0.5 pm or more,
a long axis of 1.0 pm or more, and a circle equivalent
diameter (area converted to diameter) of 1 pm or more
which are seen in the L direction cross-section of the
wire (below, referred to as "inclusions covered due to
size"), that is, the number of inclusions covered due to
composition/number of inclusions covered due to sizex100,
is 80% or more:
[0023] where (%Si02), (%CaO), (%A1203), (%MgO), (%MnO),
(%Na) , and (%F) respectively are the contents of Si02,
CaO, A1203, MgO, MnO, Na, and F in the inclusions (mass%).
(Same below.)
[0024] (2) The high carbon steel wire excellent in
5 drawability and the fatigue characteristics after wire
drawing of (1) characterized by further including REM:
0.000005 to 0.001%, the inclusions covered due to
composition satisfying, by average concentration,
furthermore (%T. REM) =O. 3 to 1.0% and ( % S ) =O. 05 to 0.2%,
10 [0025] where, (%T. REM) and (%S) respectively include
the total of rare earth elements in the inclusions and
the content of S (mass%). (Same below.)
[0026] (3) The high carbon steel wire excellent in
drawability and the fatigue characteristics after wire
15 drawing of (1) or (2) characterized by further including
B: 0.0005 to 0.002%.
[0027] (4) The high carbon steel wire excellent in
drawability and the fatigue characteristics after wire
drawing of any one of (1) to (3) characterized by further
20 including one or more of Cr: 0.05 to 1.0%, Ni: 0.05 to
1.0%, Cu: 0.05 to 1.0%, Ti: 0.001 to 0.25%, Nb: 0.001 to
0.25%, V: 0.001 to 0.25%, Mo: 0.05 to 1.0%, and Co: 0.1
to 2%.
Advantageous Effects of Invention
25 100281 According to the present invention, it is
possible to obtain high performance high carbon wire
which can refine the inclusions after wire rolling, which
can handle even tough applications such as saw wire,
which is extremely low in wire breakage rate at the time
30 of drawing, and which is excellent in fatigue
characteristics after wire drawing as well.
Description of Embodiments
[0029] Below, the present invention will be explained
in detail. First, details of the mechanism of the present
35 invention will be explained. Note that, unless otherwise
indicated, below, "%" will be deemed to mean "mass%".
[0030] The conventional technique for multi-component
control of nonmetallic inclusions was a technique
lowering the melting point and viscosity of silicate
inclusions. In such silicate inclusions, Na and F are
extremely strong in affinity. Seen from a micro
5 viewpoint, Na ions and F ions are positioned close by and
influence the melting point and viscosity of silicate
inclusions as NaF molecules.
[0031] Oxides containing NaF start to melt at a
temperature of 1200°C or less, while oxides to which only
10 Na20 is added and oxides to which only F (for example,
CaF2) is added do not start melting until a high
temperature of over 1200°C. That is, by establishing the
copresence of Na and F, an extremely low melting point
can be realized.
15 [0032] This melting point of 1200°C or less is a
temperature close to not only the blooming in the
breakdown process of a continuously cast steel slab (1150
to 1300°C), but also to the wire rolling temperature (1000
to 1200°C). In the past, it was believed that the
20 stretching of inclusions at the time of rolling occurred
mainly in the blooming process. However, when Na and F
are copresent in inclusions, the inclusions are stretched
not only in the blooming process, but also in the wire
rolling process. Therefore, by establishing the
25 copresence of Na and F, it is possible to refine
inclusions much more.
[0033] In nonmetallic inclusions, there is the
potential for various crystal phases to be formed in
accordance with the composition, but in actuality crystal
30 phases are formed and become starting points of wire
breakage etc. when growing large. As opposed to this, if
adding NaF molecules, in addition to the effect of a drop
in the melting point and a drop in the viscosity, there
is the effect of a remarkable delay in the formation of
35 crystal phases. As a result, the starting points of wire
breakage are decreased, so the wire breakage rate at the
time of drawing becomes extremely low.
[0034] Note that, the effect of Na and F on the
stretchability of inclusions depends on the calculated
amount of NaF in the inclusions. If the calculated amount
of NaF increases, the stretchability is improved. Here,
the "calculated amount of NaF" means the mass% of NaF in
the inclusions when Na and F are bonded 1:l in molar
ratio.
[0035] When the balance of Na and F is poor and
10 excessive Na or F is present, there is almost no effect
on the stretchability of inclusions. For this reason,
these are preferably added so that (%Na) and (%F) become,
in terms of molar ratio, 1:1, that is, in terms of mass
ratio, 1:0.83.
15 [0036] PLTs 8 to 11 disclose methods using oxides of
alkali metals such as Na. However, none of these
literature allude to the need to basically use silicatebased
multi-component inclusions and establish copresence
of Na and F. That is, the inventions of these literature
20 and the present invention differ in technical idea.
[0037] Next, the reasons for setting the contents of
the oxides forming the oxide-based inclusions in the
present invention will be explained.
100381 First, the reasons for limitation of the total
25 oxygen amount in the steel will be explained. In wire
with a total oxygen amount of over 30 ppm, the amount of
nonmetallic inclusions becomes greater and wire breakage
cannot be sufficiently avoided in worked materials used
for tough applications, so the upper limit is made 30
30 ppm. On the other hand, if using a powerful deoxidizing
material such as A1 or Mg in a large amount, it is easy
to make the total oxygen amount less than 16 ppm, but to
control the composition of nonmetallic inclusions in the
wire of the present invention, a total oxygen content of
35 16 ppm or more is necessary. If the total oxygen amount
is less than 16 pprn or more than 30 ppm, the die life
becomes extremely poor. The more preferable range of the
total oxygen amount is 17 to 25 ppm.
[0039] Next, the compositions and types of the
nonmetallic inclusions in the present invention will be
explained.
[ 0 0 4 0 1 The steel wire of the present invention is
characterized in that inclusions satisfying (%Si02)=40 to
95%, (%Ca0)=0.5 to 30%, (%A1203)=05. to 30%, (%MgO)= O. 5 to
20%, and (%Mn0)=0.5 to 10% and further satisfying
(%Na)=0.2 to 7% and (%F)=0.17 to 8% (inclusions covered
10 due to composition) in oxide-based nonmetallic inclusions
of a short axis of 0.5 pm or more, a long axis of 1.0 pm
or more, and a circle equivalent diameter (area converted
to diameter) of 1 pm or more which are seen in the L
direction cross-section of the wire (inclusions covered
15 due to size) are 80% or more in terms of the number ratio
(number of inclusions covered due to composition/number
of inclusions covered due to sizex100).
[ 0 0 4 1 ] Inclusions with a short axis of less than 0.5
pm in the wire L cross-section are inclusions which are
20 inherently small in size or which easily deform during
rolling. Inclusions with a long axis of less than 1.0 pm
and a circle equivalent diameter of less than 1.0 pm are
inclusions which are inherently small in size. These
inclusions tend to not become causes of deterioration of
25 the drawability and fatigue characteristics.
[0042] Therefore, in the present invention, oxidebased
nonmetallic inclusions with a short axis of 0.5 pm
or more, a long axis of 1.0 pm or more, and a circle
equivalent diameter of 1 pm or more, seen in the cross-
30 section of a wire, were defined as inclusions for
evaluation and are called "inclusions covered due to
size".
[0043] Next, the reasons for limitation of the ranges
of composition will be explained for the inclusions
35 covered due to composition.
[ 0 0 4 4 ] To soften and refine the nonmetallic inclusions
aimed at in the present invention, first, a multicomponent
oxide composition has to be put together. The
oxide composition is based on a five-component system of
Si02-Ca0-A1203-Mg0-MnO. Na and F are simultaneously
included whereby for the first time the effects of
softening and refining the nonmetallic inclusions are
exhibited.
[0045] Si02 is an important oxide forming the basis of
the silicate inclusions. If (%Si02) is less than 40%, the
10 base multi-component inclusions themselves will not
become silicate inclusions and the effects of the present
invention cannot be exhibited. If (%SiO2) exceeds 95%, the
result no longer becomes multi-component inclusions and
large sized Si02 causes deterioration of quality.
[0046] (%CaO) has to be made 0.5% or more to obtain
the effect of reduction of the melting point and
viscosity by formation of multi-component inclusions. If
(%CaO) exceeds 30%, CaO-rich hard inclusions are formed
and a deterioration of quality occurs.
100471 A1203 contributes to softening of inclusions if
in a suitable quantity, but if exceeding 30% as (%A1203),
hard A1203 inclusions are formed and the quality greatly
deteriorates. If (%A1203) is less than 0.5%, the effect of
the multi-component inclusions is not obtained.
[ 0 0 4 8 ] (%MgO) has to be made 0.5% or more to obtain
the effect of reduction of the melting point and
viscosity by formation of multi-component inclusions. If
(%MgO) exceeds 20%, olivine or foresterite (2MgO.Si02) or
other harmful inclusions are formed.
[0049] (%MnO) has to be made 0.5% or more to obtain
the effect of reduction of the melting point and
viscosity by formation of multi-component inclusions. If
(%MnO) exceeds lo%, the inclusions become not silicate
inclusions, but Spessartite (Si02-Mn0-A1203)i nclusions
35 and the effects of addition of Na and F are no longer
exhibited.
[0050] Na and F are extremely important ingredients in
the present invention. If (%Na) is less than 0.2%, there
is no effect of improvement of stretchability of
inclusions. On the other hand, if (%Na) is over 7%, the
problems arise that the effect becomes saturated and the
5 amount of dust generated when adding Na rapidly rises.
The content is preferably less than 4%.
[0051] Further, if (%F) is less than 0.17%, there is
no effect of improvement of stretchability of inclusions.
If (%F) is over 8%, the effect becomes saturated and
10 problems such as a rapid increase in the refractory melt
loss become greater. Note that, as explained above, Na
and F exhibit their effects after forming NaF molecules
in the inclusions, so they are preferably added so that
the molar ratio of Na and F in the nonmetallic inclusions
15 becomes 1: 1, that is, by mass ratio, (%Na) : (%F) become
close to 1:0.83.
[0052] Note that, when counting the oxide-based
nonmetallic inclusions (inclusions covered due to size)
with a short axis of 0.5 pm or more, a long axis of 1.0
20 pm or more, and a circle equivalent diameter of 1 pm or
more, seen in the cross-section of a wire, it is
necessary that the number ratio of inclusions which
satisfy the above composition among the inclusions
covered due to size (inclusions covered due to
25 composition) (number of inclusions covered due to
composition/number of inclusions covered due to sizex100)
be 80% or more.
[0053] The number ratio falling under 80% means that
the effect of stretching of inclusions by Na+F cannot be
30 enjoyed. Further, falling under 80% means, for example,
the presence of a certain amount of inclusions of
compositions not belonging to multi-component inclusions
such as MgO-based or A1203-based hard inclusions. As a
result, the drawability and the fatigue characteristics
35 after wire drawing are impaired.
COO541 The reason for defining the size of the
inclusions is to count only inclusions of a size causing
deterioration of the drawability and fatigue
characteristics.
[OOSS] In the present invention, it is possible to add
both Na and F into the steel, include both Na and F in
the silicate-based multi-component oxide-based
inclusions, and control the composition of inclusions so
as to secure excellent drawability and fatigue
characteristics after wire drawing. Recently,
applications are increasing in which the steel wires are
drawn more finely. In such applications, the high carbon
steel wires of the present invention exhibit particularly
excellent performance.
100561 The method of addition of Na and F may be to
add them as an NaF compound. It is also possible to add
Na and F separately (for example, Nag03 and CaF2 etc. )
[0057] Note that, when adding F, if simultaneously
adding it with metal Si, SiF4 is formed and gasifies and
the yield of F deteriorates, so this should be avoided.
[0058] By controlling the (%T.REM) (total content of
La, Ce, Nd, and other rare earths) and (%S) in the
nonmetallic inclusions, the drawability can be further
improved. The reason is as follows:
COO591 REM (La, Ce, Nd, etc.) is strong in affinity
with S, fixes S in the form of REM oxysulfide (REM~OZS),
and is taken into the multi-component inclusions. Due to
this, it is possible to reduce the amount solid solution
S in the steel and precipitation of MnS is suppressed.
The MnS which is precipitated in the steel sometimes
30 becomes the starting point of breakage during drawing. By
suppressing this precipitation, the drawability and the
fatigue characteristics after wire drawing are improved.
[0060] The (%T.REM) of the inclusions covered due to
composition should be controlled to 0.3 to 1.0%, while
(%S) should be controlled to 0.05 to 0.2% in range. If
(%T.REM) is less than 0.3%, the S fixing ability is
insufficient, while if over 1.0%, the concentration of
REM oxides in the nonmetallic inclusions increases and
the stretchability is sometimes not sufficiently
improved. Further, if (%S) is less than 0.05%, the fixed
amount of S is too small and there is no effect, while if
5 over 0.2%, CaS etc. is formed in the nonmetallic
inclusions and the stretchability is sometimes not
sufficiently improved.
[0061] Note that, wire breakage starting from MnS is
less frequent compared with wire breakage starting from
10 oxide-based nonmetallic inclusions. Therefore, first, it
is necessary to suitably control the composition of
oxide-based nonmetallic inclusions in the steel.
[0062] Next, the provisions on the composition of
ingredients of the steel of the present invention will be
15 explained. So-called killed steel is used for wire which
is used as high carbon steel wire such as the piano wire -
of JISG3502, hard steel wire of JISG3506, and oil
tempered wire for valve springs of JISG3561. Considering
these JIS standards, ease of manufacture, and practical
20 aspects, the ranges of ingredients are defined as follows
in the present invention.
100631 That is, the steel is steel which contains, by
mass%, C: 0.5 to 1.2%, Si: 0.15 to 2.5%, Mn: 0.20 to
0.9%, P10.025%, S: 0.004 to 0.0258, Al: 0.000005 to
25 0.002%, Ca: 0.00001 to 0.002%, Mg: 0.00001 to 0.001%, Na:
0.000005 to 0.001%, and F: 0.000003 to 0.001% and which
contains, in accordance with need, one or more types of
Cr: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ti:
0.001 to 0.25%, V: 0.001 to 0.25%, Nb: 0.001 to 0.25%,
30 Mo: 0.05 to 1.0%, and Co: 0.1 to 2%.
[0064] Further, if including REM: 0.000005 to 0.001%,
the effect of the present invention becomes larger.
Furthermore, if adding B: 0.0005 to 0.002%, steel which
is much more excellent in drawability and the fatigue
35 characteristics after wire drawing is obtained.
I00651 C is an element which is economic and effective
in strengthening the steel. To obtain the strength
required as a hard steel wire, 0.5% or more is necessary.
However, if over 1.2%, the steel falls in ductility and
becomes brittle and secondary working becomes difficult,
so the content is made 1.2% or less. The more preferable
concentration of C is 0.51 to 1.1%.
[0066] Si and Mn are required for deoxidation and
control of the composition of inclusions. If Si is less
than 0.15% and Mn is less than 0.20%, there is no effect.
Further, while also effective as a steel strengthening
element, if Si is over 2.5% and Mn is over 0.9%, the
steel becomes brittle. The more preferable ranges of Si
and Mn are respectively 0.16 to 2.3% and 0.25 to 0.85%.
[0067] P causes degradation of the wire drawability in
high carbon steel and further causes degradation of the
ductility after wire drawing. Accordingly, the content of
P has to be made 0.025% or less, more preferably is
0.020% or less.
[0068] S also causes degradation of the wire
drawability and further causes degradation of the
ductility after wire drawing. On the other hand, to
secure descaling of the steel material, it is necessary
to secure a certain extent of concentration of S or more.
Accordingly, the content of S is 0.004 to 0.025%,
preferably 0.005 to 0.020%.
[0069] A1 is an element which influences the
composition of inclusion of the present invention. If too
great or too small, a predetermined composition of
inclusions cannot be obtained. Accordingly, the
concentration of A1 is 0.000005 to 0.002%, preferably
0.0002 to 0.001%.
[0070] Ca is also an element which influences the
composition of inclusions of the present invention. If
too great or too small, a predetermined composition of
inclusions cannot be obtained. Accordingly, the
concentration of Ca is made 0.00001 to 0.002%, preferably
0.000013 to 0.0015%.
[ 0 0 7 11 Mg also is an element which influences the
composition of inclusions of the present invention, If
too great or too small, a predetermined composition of
inclusions cannot be obtained. Accordingly, the
concentration of Mg is made 0.00001 to 0.001%, preferably
0.000011 to 00008%.
[0072] Further, Na and F are extremely important
ingredients in the composition of inclusions of the
present invention. The concentrations of Na and F in the
steel influences the composition of inclusions.
100731 Na is an element which influences the
composition of inclusions of the present invention. If
too great or too small, a predetermined composition of
inclusions cannot be obtained. Accordingly, the
concentration of Na is made 0.000005 to 0.001%,
15 preferably 0.000007 to 0.0005%.
[ 0 0 7 4 ] F is also an element which influences the
composition of inclusions of the present invention. If
too great or too small, a predetermined composition of
inclusions cannot be obtained. Accordingly, the
20 concentration of F is made 0.000003 to 0.001%, preferably
0.000005 to 0.0005%.
[0075] The steel of the present invention preferably
further contains the following ingredients.
[0076] Cr has the effect of refining the pearlite and
25 raising the strength of the steel. The amount required
for obtaining this effect is 0.05%. Addition of this or
more is preferable. However, if adding over 1.0%, the
ductility is impaired, so the upper limit is made 1.0%.
[ 0 0 7 7 1 Ni strengthens the steel by effects similar to
30 Cr. To obtain these effects, addition of 0.05% or more is
preferable. If adding over 1.0%, the ductility falls, so
the upper limit is made 1.0% or less.
[0078] Cu has the effect of improve the scale
characteristics and corrosive fatigue characteristics of
35 the wire. To obtain this effect, addition of 0.05% or
more is preferable. If adding over 1.0%, the ductility
falls, so the upper limit is made 1.0% or less.
[0079] Ti, Nb, and V have the effect of raising the
strength of wire by precipitation strengthening. In each
case, there is no effect if less than 0.001%, while if
over 0.25%, precipitation embrittlement is caused.
5 Accordingly, the content is made 0.001 to 0.25%. Further,
these elements are effective if added to reduce the y
grain size at the time of patenting.
[ 0 0 8 0 1 Mo is an element which improves the
hardenability of steel. In the case of the present
10 invention, it is possible to raise the strength of the
steel by adding this, but addition of an excessive amount
causes the steel to excessively harden and makes working
it difficult. Accordingly, the range of addition of Mo is
made 0.05 to 1.0%.
15 [0081] Co, by inclusion in 0.1 to 2%, has the effect
of suppressing the formation of pro-eutectoid cementite
of the hyper-eutectoid steel whereby the ductility is
improved.
[0082] B improves the hardenability of steel and, when
20 present in the austenite in the solid solution state,
concentrates at the grain boundaries to suppress the
formation of ferrite, degenerate pearlite, bainite, and
other nonpearlite precipitates and thereby improve the
drawability. If the amount of addition is too small, this
25 effect cannot be obtained, so the lower limit is made
0.0005%. On the other hand, if adding too much, the
precipitation of coarse Fe3(CB)6 carbides in the austenite
is promoted and the drawability is adversely influenced.
Therefore, the upper limit is made 0.002%.
30 [0083] An REM is an element which influences the
composition of inclusions of the present invention. If
the REM is too great or too small, the predetermined
composition of inclusions for improving the drawability
cannot be obtained, so the content is made 0.000005 to
35 0.001%.
[ 0 0 8 4 ] Next, the method of production of the high
carbon steel wire of the present invention will be
explained.
[0085] The steel of the present invention can be
produced by tapping molten steel which has finished being
refined in a converter or electric furnace into a ladle,
then refining it by simplified ladle refining. As the
simplified ladle refining, CAB (capped argon bubbling),
SAB (shield argon bubbling), CAS (adjustment of
ingredients by SAB) which are described in NPLT 1 may be
used.
[0086] To make the total oxygen amount in the steel 30
ppm or less, it is effective to suppress as much as
possible the contamination by converter slag which flows
out from the converter to the ladle at the time of
tapping and, further, to secure a sedation time of
simplified ladle refining (time from end of ladle
refining to start of continuous casting) of about 20 to
40 minutes, and to promote flotation and separation of
oxides. Further, it is also effective to prevent
oxidation by air of the molten steel between the ladle
and the tundish and between the tundish and the
continuous casting mold.
[0087] On the other hand, to make the total oxygen
amount in the steel 16 ppm or more, it is possible to not
add the strong deoxidizing element A1 or Mg into the
25 steel as much as possible, to keep the addition of Ti to
the minimum necessary extent as well, and to not perform
treatment of simplified ladle refining for a long period
of time.
[ 0 0 8 8 1 Specifically, the melting of the synthetic slag
and agitation of the molten steel, the secondary
deoxidation and fine adjustment of ingredients and
adjustment of molten steel temperature, and argon
bubbling in the ladle are assumed to take 25 to 40
minutes. Further, argon bubbling in the ladle is used to
35 uniformly mix the ingredients and cooling materials and
promote the flotation and separation of inclusions.
[0089] If performing vacuum degassing and other fullscale
ladle refining, the possibility of the total amount
of oxygen in the steel becoming less than 16 ppm becomes
higher, so this is not preferable.
[0090] Making the inclusions covered due to size in
the steel with an (%A1203) of 30% or less ones of a number
ratio of 80% or more requires effort to prevent
contamination of the steel by Al. It is preferable to not
use A1 as a deoxidizing material of course and to use an
ferroalloy with a low A1 content for the Fe-Si and Si-Mn
of the ferroalloy which is added at the time of tapping.
[009l] For example, usual Fe-Si contains about 1.5% of
Al, but low Al-Fe-Si with an A1 content of about 0.01 to
0.10% can be preferably used. Further, even if using
refractories with a small alumina content as ladle
refractories, it is effective to introduce inclusions
with an (%A1203) of 30% or less in a number ratio of 80%
or more.
[0092] Note that, there is always some contamination
by A1 derived from the A1 in the ferroalloy which is
added to the steel or the alumina in the ladle or tundish
refractories, so it is possible to make the inclusions
with an (%A1203) of 0.5% or more a number ratio of 80% or
more.
[0093] The (%CaO) and (%Si02) in the inclusions can be
25 made ones in the ranges of the present invention by
adjusting the contents of CaO and Si02 in the slag on the
ladle in simplified ladle refining and by employing
production conditions for making the total amount of
oxygen in the above steel 30 ppm or less.
COO941 Specifically, it is possible to adjust the
ingredients and amounts of the Si02-CaO-based synthetic
slag which is added to the ladle so as to adjust the
basicity of the ladle slag (CaO/Si02 mass ratio). The
basicity of the ladle slag is preferably 0.9 to 1.3.
35 Further, by adopting production conditions giving a total
oxygen amount in the steel of 30 ppm or less, it is
possible to prevent an increase in the (%Si02) of
inclusions due to oxidation of the Si ingredient in the
steel.
[0095] Note that, regarding the point of making the
inclusions ones with (%MgO) of 0.5 to 20% and (%MnO) of
0.5 to lo%, it is possible to make the inclusions ones in
the range of the present invention by smelting of
ordinary steel based on contamination from the sources of
MgO in the refractories and oxidation of Mn in the steel.
[0096] Regarding the point of making the inclusions
ones with (%Na) =0.2 to 7% and (%F) =O. 17 to 88, as
explained above, by adding Na and F into the steel, it is
possible to include both Na and F in the inclusions
within the ranges of the present invention.
[0097] At this time, the method of addition of Na and
F may be to add them as an NaF compound or to add Na and
F separately (for example, Na2C03, CaF2, etc.) Note that,
when adding F, if simultaneously adding it with metal Si,
SiF4 is formed and gasifies and the yield of F
deteriorates, so this should be avoided.
[0098] To make the (%T.REM) in the inclusions 0.3 to
1.0% and make the (%S) 0.05 to 0.2%, it is sufficient to
add REM into the steel in an amount corresponding to
several ppm. The REM which is added to the steel reacts
with the S in the steel to form REM oxysulfides and join
with the silicate-based inclusions. As a result, in the
inclusions covered due to composition, it is possible to
include (%T.REM)=0.3 to 1.0%, (%S)=0.05 to 0.2% in
average concentration.
Examples
[0099] The smelting of the present examples was
performed by an LD converter. When tapping steel from an
LD converter to a ladle, a so-called "dart" type
converter slag sealing tool is used to keep the outflow
of LD slag at a small amount (50 rnrn thickness or less).
[OlOO] Further, at the time of tapping, a
carburization material for adjusting the ingredients of
C, Si, and Mn and Fe-Si, Fe-Mn, Si-Mn, or other
deoxidizing ferroalloy were added. For the deoxidizing
ferroalloy, one containing as little Al, Mg, or other
powerful deoxidizing element as possible was used.
Further, during the tapping or after the tapping, argon
5 gas was blown in from the ladle bottom.
[OlOl] The molten steel in the ladle after receiving
the steel was deoxidized by the Si, Mn, etc., that is,
was so-called "killed steel". This ladle was moved to the
position for molten steel refining, then Si02-CaO-based
10 synthetic slag was added to the ladle, then argon was
blown from the ladle bottom to agitate the molten steel
in the ladle and perform CAB simplified ladle refining.
[0102] Next, the secondary deoxidizing material was
added to the molten steel as ferroalloy. The secondary
15 deoxidizing material included metal Ca, Al, Mg, Si, etc.
In accordance with need, Na, F, and REM were added to the
steel in the ladle. When adding both Na and F, NaF was
added, when adding Na alone, Na2C03 was added, and when
adding F alone, CaF2 was added. When adding F, it is added
20 at a timing separate from the addition of the alloy
containing Si or secondary deoxidizing material.
[0103] After adding the secondary deoxidizing
material, the ingredients were further finally adjusted
and the refining of molten steel in the ladle was ended.
25 After finishing refining the ladle molten steel, a
suitable sedation time (20 to 40 minutes or so) so that
the total oxygen amount in the steel becomes 16 to 30 ppm
was secured, then continuous casting was performed. The
molten steel was continuously cast from the ladle through
30 a tundish, but at that time, to suppress as much as
possible the oxidation by air between the ladle and
tundish and inside the tundish, inert gas was used to
seal the system. The obtained steel bloom was passed
through a bloom heating furnace, then bloomed, the steel
35 bloom was rolled to billet, and the steel billet was
straightened, then it was passed through a heating
furnace and rolled to produce 5.5 mrn$ wire.
[0104] The number and composition of nonmetallic
inclusions were investigated by cutting out samples of
0.5 meter length from one coil of 5.5 mm4 wire, cutting
out small samples of 11 mm length from any 10 locations
in the L direction (length direction), and investigating
the entire surfaces at the longitudinal cross-sections
passing through the center axes in the length direction.
The number and composition of nonmetallic inclusions were
determined by obtaining oxide-based nonmetallic
inclusions of a short axis of 0.5 pm or more, a long axis
of 1.0 pm or more, and a circle equivalent diameter of 1
pm or more as "inclusions covered due to size" and
analyzing the compositions of the individual inclusions
by X-ray spectroscopy.
[0105] Among the inclusions covered due to size, the
inclusions in the range of composition of the present
invention are referred to as the "inclusions covered due
to composition". These were evaluated for the number
ratio (number of inclusions covered due to
composition/nurnber of inclusions covered due to
sizex100). Further, the average composition of all of the
inclusions covered due to size was also calculated.
However, for REM and Sf the average composition of the
inclusions covered due to composition was calculated.
25 [0106] After that, 5.5 mrn4 wires were drawn to 0.175
mm4 or less and investigated for drawing characteristics
and die life. The drawing characteristics were evaluated
by the frequency of breakage for a certain amount of
drawing as the "wire breakage index". A wire breakage
30 index of 5 or less was considered good. The die life was
evaluated indexed to the minimum lifetime allowable for
materials in current processes as 100. The longer the
lifetime, the greater the index. A die life index of 100
or more was good.
35 [0107] Furthermore, to evaluate the fatigue
characteristics, wires drawn to 0.175 mm$ were subjected
to rotating fatigue tests. In the rotating fatigue tests,
the stress was changed in various ways and the number of
repetitions until breakage was investigated. The stress
at which wire breakage occurs with 100000 repetitions was
corrected by the coefficient of tension of a mechanical
test and evaluated as the stress index. A stress index of
15 or more was judged as good.
[0108] Tables 1 to 4 show the results of invention
examples and comparative examples. Numerical values which
10 are outside the ranges of the present invention are
underlined.

. .
d 2 i
Inv.
0 .. LO
u ex. P 3
I I ~ompe.x .
[0113] In the Invention Example Nos. 1 to 24, good
results could be obtained in each case. Nos. 8 to 18 are
examples of addition of REMs in addition to Na and F. In
this case, the die life and fatigue characteristics are
5 improved. Furthermore, Nos. 19 to 24 are examples of
addition of B to the steel. A further improvement in the
die life and fatigue characteristics was confirmed.
[ 0 11 4 ] Next, the results of the comparative examples
will be explained. No. 25 is a case of no addition of Na
10 and F, No. 26 is a case of addition of Na alone, and No.
27 is a case of addition of F alone. In each case, the
number ratio of inclusions (number of inclusions covered
due to composition/number of inclusions covered due to
sizex100, below referred to as the "number ratio of
15 inclusions") was zero. The wire breakage index, die life,
and fatigue characteristics all deteriorated compared
with the invention examples.
[OllS] No. 28 is the case where the seal in the
tundish is insufficient, so the total oxygen amount
20 becomes higher than the range of the present invention.
The number of inclusions was large and the die life and
fatigue characteristics deteriorated.
[0116] Nos. 29 to 32 are examples where the number
ratio of inclusions fell below 80%. No. 29 is an example
25 which uses refractories high in A1203 and MgO content, so
a large number of A1203-based and MgO-based inclusions
believed to be due to the refractories are present in the
inclusions. As a result, the number ratio of inclusions
fell and the wire breakage index, die life, and fatigue
30 characteristics all deteriorated.
[ 0 11 7 ] No. 30 is an example where the change in
composition of the Si02-CaO-based synthetic slag caused
the (%Si02) in the nonmetallic inclusions to drop, so the
number ratio of inclusions fell, some hard inclusions
35 appeared in the inclusions, and the wire breakage index,
die life, and fatigue characteristics all deteriorated
somewhat.
[0118] No. 31 is an example where the amount of
outflow of LD slag was somewhat large, coarse inclusions
of Si02 alone appeared in the deoxidizing process, and the
(%Si02) in the nonmetallic inclusions increased. As a
result, the number ratio of inclusions fell and the wire
breakage index and fatigue characteristics deteriorated.
[0119] No. 32 is an example which used as a
deoxidizing alloy not a low A1 ferroalloy, but a high A1
concentration usual ferroalloy. The (%A1203) in the
nonmetallic inclusions increased. As a result, the number
ratio of inclusions fell, a large number of hard A1203-
based inclusions were formed, and the wire breakage
index, die life, and fatigue characteristics were all
extremely poor.
[0120] No. 33 is an example where the concentration of
S in the steel is high, the (%S) in the nonmetallic
inclusions is a higher value than the range of the
present invention, and the wire breakage index, die life,
and fatigue characteristics deteriorate.
[0121] No. 34 is an example where has too much REM
added, so the (%T.REM) in the nonmetallic inclusions
becomes a value higher than the range of the present
invention and the wire breakage index, die life, and
fatigue characteristics deteriorate.

- 28 -
CLAIMS
Claim 1. High carbon steel wire rod excellent in
drawability and the fatigue characteristics after wire
drawing which contains, by mass%,
C : 0.5 to 1.2%,
Si: 0.15 to 2.5%,
Mn: 0.20 to 0.9%,
S: 0.004 to 0.025%,
Al: 0.000005 to 0.002%,
Ca: 0.00001 to 0.002%,
Mg: 0.00001 to 0.001%,
Na: 0.000005 to 0.0018, and
F: 0.000003 to 0.001%,
has a total oxygen amount of 16 to 30 ppm, and which has
a balance of Fe and unavoidable impurities,
the high carbon steel wire characterized in that the
number ratio of inclusions satisfying (%Si02)=40 to 95%,
(%Ca0)=0.5 to 30%, (%Al2o3)=0.5 to 30%, (%Mg0)=0.5 to 20%,
and (%Mn0)=0.5 to 10% and further satisfying (%Na)=0.2 to
7% and (%F)=0.17 to 8% (below, referred to as "inclusions
covered due to composition") in the oxide-based
nonmetallic inclusions of a short axis of 0.5 p or more,
a long axis of 1.0 p or more, and a circle equivalent
25 diameter (area converted to diameter) of 1 p or more
which are seen in the L direction cross-section of the
wire (below, referred to as "inclusions covered due to
size"), that is, the number of inclusions covered due to
composition/number of inclusions covered due to sizex100,
30 is 80% or more:
where (%Si02), (%CaO) , (%A1203), (%MgO) , (%MnO) , (%Na) ,
and (%F) respectively are the contents of Si02, CaO,
A1203, MgO, MnO, Na, and F in the inclusions (mass%) .
Claim 2. The high carbon steel wire excellent in
35 drawability and the fatigue characteristics after wire
drawing as set forth in claim 1 characterized by further
including REM: 0.000005 to 0.001%, said inclusions
covered due to composition satisfying, by average
concentration, furthermore (%T.REM)=0.3 to 1.0% and
(%S)=0.05 to 0.2%,
5 where, (%T.REM) and (%S) respectively include the total
of rare earth elements in the inclusions and the content
of S (mass%).
Claim 3. The high carbon steel wire excellent in
drawability and the fatigue characteristics after wire
10 drawing as set forth in claim 1 or 2 characterized by
-. . further including B: 0.0005 to 0.002%.
Claim 4. The high carbon steel wire excellent in
drawability and the fatigue characteristics after wire
drawing as set forth in claim 1 or 2 characterized by
15 further including one or more of:
Cr: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cu: 0.05 to 1.0%,
Ti: 0.001 to 0.25%,
20 Nb: 0.001 to 0.25%,
V: 0.001 to 0.25%,
Mo: 0.05 to 1.0%, and
Co: 0.1 to 2%.
Claim 5. The high carbon steel wire excellent in
25 drawability and the fatigue characteristics after wire
drawing as set forth in claim 3 characterized by further
containing one or more of
Cr: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
30 Cu: 0.05 to 1.0%,
Ti: 0.001 to 0.25%,
Nb: 0.001 to 0.25%,
V: 0.001 to 0.25%,
Mo: 0.05 to 1.0%, and
35 Co: 0.1 to 2%.
" _ " . ." --- " - - .- _.>-_L -
- - -
Dated this 1 1.07.2013
[NEHA SRIVASTAVA]
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]