Leggett & Platt v. Simmons Bedding et al.
`Ex. 1004 / Page 1 of 12
`
`

`
`Patent Application Publication Dec. 6, 2007 Sheet 1 of 2
`
`US 2007/0277913 A1
`
`L: LENGH OF AILINZE
`
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`LAMELLAR SPACING A = L/N
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`
`Ex. 1004 / Page 2 of 12
`
`Ex. 1004 / Page 2 of 12
`
`

`
`Patent Application Publication Dec. 6, 2007 Sheet 2 of 2
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`Ex. 1004 / Page 3 of 12
`
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`

`
`US 2007/0277913 A1
`
`Dec. 6, 2007
`
`WIRE ROD EXCELLENT IN WIRE-DRAWING
`WORKABILITY AND METHOD FOR PRODUCING
`SAME
`
`BACKGROUND OF THE INVENTION
`
`[0001] The invention relates to a wire rod excellent in
`wire-drawing workability, out of which a drawn wire prod-
`uct, such as a steel cord, beading wire, PC steel wire, spring
`steel, can be efi‘iciently produced with high productivity, and
`a method for producing the same.
`
`In most cases of producing a drawn wire product,
`[0002]
`such as a steel cord, a drawing process is applied to a wire
`rod serving as material for the wire product in order to make
`adjustment in size and quality (physical properties), and
`therefore,
`it
`is extremely useful from the viewpoint of
`enhancement in productivity, and so forth to improve wire-
`drawing workability of the wire rod. In this connection, if
`improvement on wire drawing workability is implemented,
`this will not only improve productivity, due to an increase in
`drawing rate and a decrease in the number of drawing
`passes, but also provide many benefits such as reduction in
`wear and tear of draw dies.
`
`field,
`technical
`a pertinent
`in
`[0003] Accordingly,
`researches on enhancement in the wire drawing workability
`of the wire rod have been under way. For example,
`in
`Japanese Unexamined Patent Application Publication (JP-
`A) No. 91912/2004, there has been disclosed a technology
`for improving the wire-drawing workability by focusing
`attention on size of a pearlite block, a quantity of pro-
`eutectoid cementite formed, an average thickness of cement-
`ite, Cr concentration in cementite, and so forth, and by
`optimizing them.
`
`[0004] Further, in JP-A-295930/1996, there has been dis-
`closed that the wire-drawing workability is improved by
`controlling an area ratio of upper bainite formation, and
`growth size of intergranular bainite. In JP-A-130258/ 1987,
`there has been disclosed a technology for improving resis-
`tance to wire break, and a die life by controlling an amount
`of total oxygen, and nonviscous inclusion composition, in
`steel.
`
`[0005] However, a rise in wire-drawing rate, and an
`increase in reduction of area per one pass cause degradation
`in ductility of a drawn wire product, and deterioration in die
`life. Accordingly, in order to further enhance productivity in
`the pertinent technical field, there is still a demand for a wire
`rod excellent in the wire-drawing workability, capable of
`achieving excellent resistance to wire-break, and enhance-
`ment of the die life even in harsh wire-drawing conditions of
`high wire-drawing rate, and large reduction of area.
`
`SUMMARY OF THE INVENTION
`
`the
`[0006] Under circumstances described as above,
`invention has been developed, and it is therefore an object
`of the invention to provide a wire rod excellent in wire-
`drawing workability, insusceptible to wire break in spite of
`an increase in wire-drawing rate, and reduction of area, and
`capable of extending a die life by suppressing die wear, and
`a method for producing the same.
`
`[0007] According to one aspect of the invention, a wire
`rod that has succeeded in achieving the object is made of
`steel containing C: 0.6 to 1.1% (mass %, applicable to all
`
`components referred to hereunder), Si: 0.1 to 2.0%, Mn: 0.1
`to 1%, P: not more than 0.020% (0% exclusive), S: not more
`than 0.020% (0% exclusive), N: not more than 0.006% (0%
`exclusive), Al: not more than 0.03% (0% exclusive), and O:
`not more than 0.003% (0% exclusive), the balance including
`Fe, and unavoidable impurities, and further, the wire rod
`comprises a pearlite structure wherein an area ratio of a
`second-phase ferrite is not more than 11.0%, and a pearlite
`lamellar spacing is not less than 120 pm.
`
`[0008] The wire rod according to the aspect of the inven-
`tion may contain not more than 1.5% Cr for higher strength,
`and may further contain not more than 1% Cu, and/or not
`more than 1% Ni, for suppression of decarburization.
`
`[0009] The wire rod according to the aspect of the inven-
`tion preferably further contains at least one element selected
`from the group consisting of not more than 0.30% V, not
`more than 0.1% Ti, not more than 0.10% Nb, not more than
`0.5% Mo, and not more than 0.1% Zr from the viewpoint of
`refinement of the metal microstructure, and suppression of
`transformation into ferrite.
`
`[0010] The wire rod according to the aspect of the inven-
`tion may further contain at least one element selected from
`the group consisting of not more than 5 ppm Mg, not more
`than 5 ppm Ca, and not more than 1.5 ppm REM in order to
`soften oxides and enhance the wire drawing workability.
`Still further, the wire rod according to the invention may
`contain not more than 15 ppm B in order to enhance
`hardenability.
`
`In accordance with another aspect of the invention,
`[0011]
`there is provided a method for producing a wire rod,
`comprising the steps of heating a steel product meeting
`requirements for chemical components, as described here-
`inbefore, to a temperature in a range of 900 to 1250° C., hot
`rolling the steel product at a temperature not lower than 780°
`C., and finish-rolling the same at a temperature not higher
`than 1100° C. to be thereby formed into a wire rod, water-
`cooling the wire rod to a temperature range of 750 to 950°
`C. before coiling the same up to be placed on conveying
`equipment, cooling the wire rod at an average cooling rate
`of not less than 20° C./sec within 20 sec from the coiling of
`the wire rod to thereby drop temperature of the wire rod to
`a minimum value point (T1) in a temperature range of 550
`to 630° C., and subsequently heating the wire rod to thereby
`raise the temperature of the wire rod up to a maximum value
`point (T2) in a temperature range of 580 to 720° C., higher
`in temperature than T1, within 50 sec from the coiling of the
`wire rod.
`
`[0012] The inventor, et al. have found out to their surprise
`that a wire rod excellent in wire-drawing workability, insus-
`ceptible to wire break, and capable of extending a die life by
`suppressing die wear, can be obtained by specifying the
`respective contents of C, Si, Mn, P, S, N, Al, and O while
`controlling the area ratio of the second-phase ferrite and the
`pearlite lamellar spacing. With the use of the wire rod
`described, it will become possible to increase a wire-draw-
`ing rate, and reduction of area, thereby enabling productivity
`to be further enhanced.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`invention will be
`[0013] Embodiments of the present
`described in detail based on the following figures, wherein:
`
`Ex. 1004 / Page 4 of 12
`
`Ex. 1004 / Page 4 of 12
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`

`
`US 2007/0277913 A1
`
`Dec. 6, 2007
`
`[0014] FIG. 1 is a SEM photograph ofa location at D/4 on
`the cross-sectional
`face of a wire rod according to an
`embodiment of the invention (D: diameter of the wire rod);
`(the SEM photograph used for explaining about the structure
`of a second-phase ferrite),
`
`[0015] FIG. 2 is another SEM photograph ofa location at
`D/4on the cross-sectional face of the wire rod (D: diameter
`of the wire rod); (the SEM photograph used for explaining
`about a method of finding a pearlite lamellar spacing), and
`
`[0016] FIG. 3 is a schematic representation showing a
`treatment pattern adopted in a method for producing the wire
`rod according to the invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`[0017] A wire rod according to the invention has features
`lying in requirements for chemical components thereof, and
`requirements for a metal microstructure thereof (an area
`ratio of a second-phase ferrite, and a pearlite lamellar
`spacing). Accordingly, the chemical components of the wire
`rod (a steel product) are first described hereinafter.
`
`C: 0.6 to 1.1% (mass %, applicable to all components
`referred to hereunder)
`
`[0018] Carbon is an element intensely affecting strength of
`the wire rod, and in order to secure strength required of a
`steel cord, beading wire, PC steel wire, and so forth, as
`targets for which the invention has been developed, addition
`of not less than 0.6% C is required. On the other hand, if
`carbon content is excessive,
`there occurs degradation in
`ductility, so that an upper limit of the carbon content is set
`to 1.1%. The carbon content is preferably in a range of 0.8
`to 1.0%.
`
`Si: 0.1 to 2.0%
`
`[0019] For the purpose of deoxidation, in particular, Si is
`added to a wire rod to be subjected to intense drawing, and
`addition of not
`less than 0.1% Si
`is required. Further,
`because Si also contributes to enhancement in strength of the
`wire rod due to solid solution hardening, an addition amount
`thereof is increased as necessary. However, an excessive
`increase in the strength due to excessive addition of Si will
`cause deterioration in wire-drawing workability. Further-
`more, the excessive addition of Si will cause promotion of
`decarburization,
`to which attention should be given. For
`those reasons, with the invention, an upper limit of silicon
`content
`is set
`to 2.0% to prevent deterioration in wire-
`drawing workability, and promotion of decarburization. The
`silicon content is preferably in a range of 0.15 to 1.8%.
`Mn: 0.1 to 1%
`
`[0020] Addition of not less than 0.1% Mn is required for
`the purpose of deoxidization, and locking a deleterious
`element S in the form of MnS to thereby render S harmless.
`Further, Mn also acts so as to stabilize carbide in steel.
`However, because excessive Mn content will cause occur-
`rence of segregation, and supercooled structures, thereby
`causing deterioration in wire drawing workability, an upper
`limit of Mn content is set to 1%. The Mn content is more
`
`preferably in a range of 0.15 to 0.9%.
`
`P: not more than 0.020% (0% exclusive)
`
`P is an element deleterious to wire drawing work-
`[0021]
`ability, in particular, and because excessive P content causes
`degradation in tenacity and ductility of a wire rod, an upper
`limit of P content is set to 0.020%. The P content is more
`
`preferably not more than 0.015%, and further preferably, not
`more than 0.010%.
`
`S: not more than 0.020% (0% exclusive)
`
`S as well is an element deleterious to wire drawing
`[0022]
`workability,
`in particular. If Mn is contained, S can be
`locked in the form of MnS, as described above, however,
`excessive S content causes an increase in amount as well as
`
`size of MnS, thereby resulting in degradation of ductility, an
`upper limit of S content is set to 0.020%. The S content is
`more preferably not more than 0.015%, further preferably,
`not more than 0.010%.
`
`N: not more than 0.006% (0% exclusive)
`
`[0023] N is an element contributing to an increase in
`strength due to age hardening. A preferable lower limit of N
`content is 0.001%. However, because the N content causes
`degradation in ductility, an upper limit thereof is set to
`0.006%. The upper limit is preferably not more than 0.004%,
`more preferably, not more than 0.003%.
`
`A1: not more than 0.03% (0% exclusive)
`
`is an element effective as a deoxidizer, and
`[0024] Al
`further, is combined with N to form AlN, which contributes
`to refinement of a metal microstructure. A preferable lower
`limit of Al content is 0.0003%. However, if the Al content
`is excessive, this will cause coarse oxides to be formed,
`thereby resulting in deterioration of wire drawing workabil-
`ity, and an upper limit thereof is therefore set to 0.03%. The
`upper limit thereof is preferably not more than 0.01%, more
`preferably not more than 0.005%.
`
`O: not more than 0.003% (0% exclusive)
`
`If oxygen content in steel is high, the coarse oxides
`[0025]
`are prone to be easily formed, thereby resulting in deterio-
`ration of wire drawing workability, and an upper limit of the
`oxygen content is therefore set to 0.003%. The upper limit
`thereof is preferably not more than 0.002%, more preferably
`not more than 0.0015%.
`
`[0026] With the wire rod according to the invention, the
`chemical components described as above represent basic
`components, and the balance includes in effect Fe, and
`unavoidable impurities, however, the wire rod may contain
`the following elements if needs be.
`Cr: not more than 1.5%
`
`[0027] Cr is an element effective for rendering the wire rod
`higher in strength, and a preferable lower limit of Cr content
`is 0.01%. However, since excessive addition of Cr causes
`supercooled structures prone to be formed to thereby cause
`deterioration in wire drawing workability, an upper limit of
`the Cr content is set to 1.5%. The upper limit thereof is
`preferably not more than 1.0%.
`Cu: not more than 1%
`
`[0028] Cu is an element acting so as to enhance corrosion
`resistance besides acting so as to suppress decarburization in
`a surface layer, and therefore, Cu can be added as necessary.
`
`Ex. 1004 / Page 5 of 12
`
`Ex. 1004 / Page 5 of 12
`
`

`
`US 2007/0277913 A1
`
`Dec. 6, 2007
`
`A preferable lower limit of Cu content is 0.01%. However,
`because excessive addition of Cu will not only render the
`wire rod susceptible to cracking upon hot working, but also
`adversely affect wire drawing workability due to formation
`of supercooled structures, an upper limit of the Cu content
`is set to 1%. The upper limit thereof is preferably not more
`than 0.8%.
`
`Ni: not more than 1%
`
`[0029] Ni is an element effective for suppressing decar-
`burization in the surface layer, and enhancement in corro-
`sion resistance, as with the case of Cu, and therefore, Ni can
`be added as necessary. A preferable lower limit of Ni content
`is 0.01%. However, because excessive addition of Ni will
`cause deterioration in the wire drawing workability due to
`formation of supercooled structures, an upper limit of the Ni
`content is set to 1%. The upper limit thereof is preferably not
`more than 0.8%.
`
`V: not more than 0.30%
`
`[0030] V is an element contributing to refinement of the
`metal microstructure by forming carbide in carbon steel.
`Further, because V in solid solution state will enhance
`hardenability, and suppress transformation into ferrite, V can
`be added as necessary. A preferable lower limit of V content
`is 0.0010%. However, because excessive addition of V will
`cause deterioration in the wire drawing workability due to
`formation of supercooled structures, an upper limit of the V
`content is set to 0.3%. The upper limit thereof is preferably
`not more than 0.25%.
`
`Ti: not more than 0.1%
`
`[0031] Ti is an element contributing to the refinement of
`the metal microstructure, and the suppression of transfor-
`mation into ferrite as with the case of V, and Ti can therefore
`be added as necessary. A preferable lower limit of Ti content
`is 0.0010%. However, because excessive addition of Ti will
`cause deterioration in wire drawing workability, an upper
`limit of the Ti content is set to 0.1%. The upper limit thereof
`is preferably not more than 0.08%.
`Nb: not more than 0.10%
`
`[0032] Nb is an element contributing to the refinement of
`the metal microstructure, and the suppression of transfor-
`mation into ferrite as with the case of V, and Nb can
`therefore be added as necessary. A preferable lower limit of
`Nb content is 0.020%. However, because excessive addition
`of Nb will cause deterioration in wire drawing workability,
`an upper limit of the Nb content is set to 0.10%. The upper
`limit thereof is preferably not more than 0.08%.
`Mo: not more than 0.5%
`
`[0033] Mo is an element contributing to the refinement of
`the metal microstructure, and the suppression of transfor-
`mation into ferrite as with the case of V, and Mo can
`therefore be added as necessary. A preferable lower limit of
`Mo content is 0.05%. However, because excessive addition
`of Mo will cause deterioration in wire drawing workability,
`an upper limit of the Mo content is set to 0.5%. The upper
`limit thereof is preferably not more than 0.3%.
`Zr: not more than 0.1%
`
`mation into ferrite as with the case of V, and Zr can therefore
`be added as necessary. A preferable lower limit of Zr content
`is 0.010%. However, because excessive addition of Zr will
`cause deterioration in wire drawing workability, an upper
`limit of the Zr content is set to 0.1%. The upper limit thereof
`is preferably not more than 0.05%.
`
`Mg: not more than 5 ppm
`
`[0035] Mg is an element acting so as to soften oxides to
`thereby enhance wire drawing workability, and Mg can
`therefore be added as necessary. A preferable lower limit of
`Mg content is 0.1 ppm. However, because excessive addi-
`tion of Mg will cause oxides thereof to undergo a change in
`quality to thereby rather deteriorate wire drawing workabil-
`ity, an upper limit of the Mg content is set to 5 ppm. The
`upper limit thereof is preferably not more than 2 ppm.
`
`Ca: not more than 5 ppm
`
`[0036] Ca is an element acting so as to soften oxides as
`with the case of Mg, and Ca can therefore be added as
`necessary. Apreferable lower limit of Ca content is 0.3 ppm.
`However, because excessive addition of Ca will cause
`deterioration in wire drawing workability, an upper limit of
`the Ca content is set to 5 ppm. The upper limit thereof is
`preferably not more than 2 ppm.
`
`REM: not more than 1.5 ppm
`
`[0037] REM acts so as to soften oxides as with the case of
`Mg, and REM can therefore be added as necessary. A
`preferable lower limit of REM content is 0.1 ppm. However,
`because excessive addition of REM will cause deterioration
`
`in wire drawing workability, an upper limit of the REM
`content is set to 1.5 ppm. The upper limit thereof is prefer-
`ably not more than 0.5 ppm.
`
`B: not more than 15 ppm
`
`[0038] B is an element capable of enhancing hardenability,
`and addition of B enables the transformation into ferrite to
`
`be suppressed. A preferable lower limit of B content is 3
`ppm. However, because excessive addition of B causes
`supercooled structures prone to be easily formed, thereby
`adversely affecting wire drawing workability, an upper limit
`of the B content is set to 15 ppm. The upper limit thereof is
`preferably not more than 12 ppm.
`
`the metal microstructure of the wire rod
`[0039] Next,
`according to the invention is described hereinafter. The wire
`rod according to the invention has a feature in that the area
`ratio of the second-phase ferrite is not more than 11.0%.
`Herein, “the second-phase ferrite” according to the inven-
`tion refers to ferrite in a region without pearlite (lamellar
`structure of ferrite and cementite) formed therein, as indi-
`cated by respective arrows in FIG. 1 showing an SEM
`photograph of a cross-sectional face of the wire rod. Further,
`because there are times when it is difiicult to distinguish the
`second-phase ferrite from pearlite, “the second-phase fer-
`rite” according to the invention is more specifically defined
`as “BCC—Fe crystal grains in a region surrounded by a
`boundary differing in misorientation angle by not less than
`10 degrees from the periphery of the region, an area ratio of
`cementite present in the respective BCC—Fe crystal grains
`being not more than 6%”.
`
`[0034] Zr is an element contributing to the refinement of
`the metal microstructure, and the suppression of transfor-
`
`“The area ratio of the second-phase ferrite”,
`[0040]
`according to the invention, refers to an area ratio (%) of the
`
`Ex. 1004 / Page 6 of 12
`
`Ex. 1004 / Page 6 of 12
`
`

`
`US 2007/0277913 A1
`
`Dec. 6, 2007
`
`field of the
`second-phase ferrite to an observed visual
`cross-sectional face of the wire rod, magnified 500 to 1500
`times by the scanning electron microscope (SEM), that is,
`(an area of the second-phase ferrite within the observed
`visual field/an area of the observed visual field in whole)><
`100. In this case, the area of the second-phase ferrite can be
`found by use of image analysis software, for example,
`{Image-Pro (Ver 4.0)} developed by Media Cybernetics.
`Further, since there occurs variation in the area ratio of the
`second-phase ferrite by the visual field as observed, a mean
`value of several values thereof, found by observing not less
`than eight visual fields selected at random, is adopted as a
`value of the area ratio of the second-phase ferrite, according
`to the invention.
`
`[0041] The inventor, et al. have found out that a wire rod
`excellent in resistance to wire-break can be obtained by
`controlling the area ratio of the second-phase ferrite of the
`wire rod to not more than 11%, preferably 10.0%, and more
`preferably not more than 9%. A mechanism causing the
`above is not clearly known, however, the mechanism can be
`presumed to be as follows. The invention, however, is not to
`be limited in scope to the mechanism presumed as described
`hereunder.
`
`[0042] With a carbon steel wire rod to be subjected to
`drawing, such as the wire rod according to the invention, the
`primary constituent of the metal microstructure thereof is
`pearlite, however, in general, there also exists the region of
`the second-phase ferrite, without pearlite formed therein. It
`is presumed that strain concentration occurs to the second-
`phase ferrite lower in strength than pearlite when drawing is
`applied, so that voids are prone to occur thereto. The voids
`each can become a starting point of wire break. Accordingly,
`it is reasoned that resistance to wire break can be enhanced
`
`by decreasing the second-phase ferrite that is low in strength
`and is susceptible to the strain concentration.
`
`[0043] Further, the wire rod according to the invention has
`a feature in that the same comprises a pearlite structure
`wherein the pearlite lamellar spacing is not less than 120 um,
`preferably not less than 140 um, and more preferably not
`less than 170 pm. The wire rod according to the invention
`can at times include baimte, and/or martensite besides the
`second-phase ferrite, however, pearlite is the primary con-
`stituent of the metal microstructure thereof. In the case of
`
`bainite, and/or martensite being in existence, a ratio of a total
`area of those microconstituents is preferably not more than
`5%, more preferably not more than 2%, and still more
`preferably, bainite, and martensite do not in effect exist.
`
`[0044] With the invention, “the pearlite lamellar spacing”
`refers to a thickness of a lamellar layer in pearlite, composed
`of a pair of a ferrite layer and a cementite layer, in pearlite.
`However, since there occurs variation in the pearlite lamellar
`spacing by the position for observation of the metal micro-
`structure, what has been found in the following mar1ner is
`defined as a value of “the pearlite lamellar spacing” accord-
`ing to the invention.
`
`[0045] First, not less than six photographs of the cross-
`sectional face of the wire rod, as observed and magnified
`3000 to 10,000 times by the SEM, are taken. As shown in
`FIG. 2, in a colony (a region where the ferrite layers and the
`cementite layers, in pearlite, are aligned in same direction)
`in the respective photographs taken by the SEM, a line
`segment orthogonal to the ferrite layers and the cementite
`
`layers is drawn, and the pearlite lamellar spacing in the
`colony as “a length of the line segment/the number of
`lamellar layers within the line segment” is found on the basis
`of the length of the line segment, and the number of the
`lamellar layers within the line segment. Then, by finding the
`pearlite lamellar spacings within not more than five pieces of
`the colonies, respectively,
`in the respective photographs
`taken by the SEM, the respective pearlite lamellar spacings
`within not less than thirty pieces of the colonies altogether
`are worked out, and a mean value thereof is defined as a
`value of “the pearlite lamellar spacing” according to the
`invention.
`
`[0046] A mechanism whereby the resistance to the wire-
`break of the wire rod is enhanced if the pearlite lamellar
`spacing is not
`less than 120 pm is not clearly known,
`however, the mechanism can be presumed to be as follows.
`The invention, however, is not to be limited in scope to the
`mechanism presumed as described hereunder. Even if the
`second-phase ferrite exists in the wire rod, the strain con-
`centration occurring to the second-phase ferrite will be
`mitigated in the case that a difference in strength between the
`second-phase ferrite and microstructure on the periphery
`thereof is small, so that occurrence of voids likely to cause
`wire-break is presumed to be checked. Further, it is reasoned
`that if the pearlite lamellar spacing becomes wider, strength
`of pearlite becomes lower, and a difference in strength
`between pearlite and the second-phase ferrite is rendered
`relatively small, so that this will probably contribute to
`enhancement in the resistance to the wire-break of the wire
`rod.
`
`[0047] However, if the pearlite lamellar spacing becomes
`excessively wide, it is deemed that a likelihood of occur-
`rence of the voids will become greater on the contrary. An
`upper limit of the pearlite lamellar spacing is therefore
`preferably not more than 350 pm, more preferably not more
`than 300 um, and still more preferably, not more than 280
`pm.
`
`[0048] With the invention, a location on the cross-sec-
`tional face, adopted for observation by the SEM, in order to
`find “the area ratio of the second-phase ferrite”, and “the
`pearlite lamellar spacing”, is specified as a location at D/4 on
`the cross-sectional face of the wire rod (D: diameter of the
`wire rod). The reason for this is to extract average data on
`the metal microstructure of the wire rod. Parts in the surface
`
`layer are subjected to effects of decarburization and central
`parts are subjected to effects of segregation and so forth, so
`that variations in the data, at
`those locations,
`tend to
`increase.
`
`[0049] The wire rod according to the invention can be
`produced by, for example, a method described hereinafter
`(refer to FIG. 3). The wire rod according to the invention,
`however,
`is not limited to that produced by the method
`described hereinafter. First, a steel product meeting the
`requirements for the chemical components is heated up to
`900 to 1250° C. to be subsequently hot rolled at a tempera-
`ture not lower than 780° C., and a fimsh-rolling temperature
`is controlled to not higher than 1100° C. This is because
`heating is insufficient with a heating temperature lower than
`900° C., and conversely, if the heating temperature exceeds
`1250° C., decarburization in the surface layer spreads, so
`that scales capable of adversely affecting the wire-drawing
`workability tend to become harder to peel off. Further, if a
`
`Ex. 1004 / Page 7 of 12
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`Ex. 1004 / Page 7 of 12
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`US 2007/0277913 Al
`
`Dec. 6, 2007
`
`rolling temperature is lowered, decarburization in the sur-
`face layer is similarly promoted, and a lower limit tempera-
`ture for hot rolling is therefore set to 780° C. Conversely, if
`the finish-rolling temperature exceeds ll00° C., this will
`render it difficult to control
`transformation of the metal
`
`microstructure by cooling and reheating, to be executed in a
`subsequent process step, so that an upper limit of the
`finish-rolling temperature is set to ll00° C.
`
`[0050] A wire rod formed after the finish-rolling is water-
`cooled to a temperature range of 750 to 950° C., and is
`coiled up on conveying equipment, such as a Stelmor
`conveyer, to be then placed thereon. Temperature control
`executed after water-cooling is important for control of the
`transformation of the metal microstructure,
`taking place
`thereafter, and control of scales. If an ultimate temperature
`at the time of water-cooling is below 750° C., this will at
`times cause the supercooled structures to be formed in the
`surface layer, thereby adversely affecting the wire-drawing
`workability, and on the other hand, if the ultimate tempera-
`ture exceeds 950° C., this will cause loss in deformability of
`scales, so that scales will peel off in the course of transpor-
`tation, thereby creating a cause for rusting.
`
`It is of particular importance from the viewpoint of
`[0051]
`obtaining the wire rod meeting the requirements for the
`metal microstructure, excellent in the wire-drawing work-
`ability, to cool the wire rod at an average cooling rate of not
`less than 20° C./sec within 20 sec from the coiling of the
`wire rod to thereby drop temperature of the wire rod to a
`minimum value point (T1) in a temperature range of 550 to
`630° C. before raising the temperature of the wire rod up to
`a maximum value point (T2) in a temperature range of 580
`to 720° C., higher in value than T1, within 50 sec from the
`coiling of the wire rod. A reference time for “within 20 sec
`from the coiling” is a point in time when a rolled wire rod
`is coiled up in ring-like fashion to be placed on the convey-
`ing equipment, such as the conveyer. Further, since the wire
`rod is continuously coiled up, and is continuously cooled,
`there occurs time lag between the top part of the wire rod,
`coiled up, and the bottom part thereof, to be coiled up, with
`respect to a time when the wire rod is placed, and a time
`when the wire rod is cooled, respectively, however, respec-
`tive measurements on time from the coiling up to the cooling
`are started upon the coiling of the respective part of the wire
`rod.
`
`Since it is presumed that the second-phase ferrite
`[0052]
`prone to undergo strain concentration is formed at a rela-
`tively high temperature before pearlite transformation, for-
`mation of the second-phase ferrite can be suppressed by
`rapidly cooling the wire rod at the average cooling rate of
`not less than 20° C./sec down to a temperature region where
`ferrite is hard to be formed within 20 sec from the coiling of
`the wire rod. Further, because pearlite transformation nuclei
`in massive amounts are formed due to such rapid cooling,
`advantageous effect of the metal microstructure being
`micronized can be gained. However, if a cooling rate is
`excessively high, this will raise the risk of an increase in
`strength differential within the wire rod, due to localized
`formation of supercooled structures, and so forth, thereby
`causing deterioration in the wire drawing workability.
`Accordingly, the average cooling rate is preferably set to not
`more than 50° C./sec. Herein, “the average cooling rate”,
`according to the invention, refers to a cooling rate found on
`the basis of a temperature difference between the wire rod
`
`temperature upon the coiling thereof (that is, the wire rod
`temperature after water-cooling) and T1, and a cooling time
`length required for the wire rod temperature at the time of
`the coiling to drop down to T1.
`
`[0053] Further, if the wire rod is cooled down only to the
`minimum value point (T1) in excess of 630° C. in such a
`cooling process step as described above, it is not possible to
`sufficiently suppress the formation of the second-phase
`ferrite, so that coarse grains having adverse effects on the
`wire-drawing workability become prone to be easily formed.
`On the contrary, if the wire rod is excessively cooled down
`to the minimum value point (T1) below 550° C., this will
`lead to an increase in strength differential within the wire
`rod, due to the formation of the supercooled structures, and
`so forth.
`
`[0054] After the wire rod is cooled down to T1 in the
`temperature range during the cooling process step, the wire
`rod is reheated to thereby cause the pearlite transformation
`to occur. On this occasion, by increasing the wire rod
`temperature to a high temperature at 580° C. or higher, the
`pearlite lamellar spacing can be rendered wider. Further, it is
`presumed that the higher a transformation temperature, the
`wider the pearlite lamellar spacing can become, however,
`ductility becomes excessively low at
`the transformation
`temperature in excess of 720° C., raising the risk of the wire
`drawing workability undergoing deterioration contrary to
`expectation.
`
`is deemed possible that the pearlite lamellar
`It
`[0055]
`spacing can be rendered wider by slowly cooling the wire
`rod as usual, or holding the wire rod at a constant tempera-
`ture without rapidly cooling the same after the coiling
`thereof on the conveying equipment. However, there is a
`likelihood that the metal microstructure becomes coarser
`
`because a rate at which the pearlite transformation nuclei are
`formed is low in a high temperature region, thereby causing
`adverse effects on the wire drawing workability. Hence, the
`wire rod whose metal microstructure is fine, and has a wide
`pearlite lamellar spacing can be provided by a production
`method according to the invention, comprisin