[Name of Document] DESCRIPTION
[Title of the Invention] WIRE MATERIAL FOR NON-HEAT TREATED
COMPONENT, STEEL WlRE FOR NON-HEAT TREATED COMPONENT,
AND NON-HEAT TREATED COMPONENT AND MANUFACTURING
5 METHOD THEREOF
[Technical Field]
[0001] The present invention relates to a non-heat treated component
manufactured from a wire inaterial, used for automotive parts and various
industrial machineries having an axial shape such as a bolt, a torsion bar, a
10 stabilizer, and whose tensile strength is 900 MPa to 1300 MPa, a steel wire to
inanufacture the above, further, the wire material to manufacture the steel
wire, and a ~nanufacturingm ethod thereof. Note that, architectural bolts and
so on are included in machine components being objects of the present
invention. This application is based upon and claims the benefit of priority
15 of the prior Japanese Patent Application No. 201 1-184737, filed on August 26,
201 1, the entire contents of which are incorporated herein by reference.
[Background Art]
[0002] A high-strength niachine component having a tensile-strength of
900 MPa or Inore is used for a vehicle and various industrial machineries to
20 reduce weight and size thereof. Conventionally, this kind of high-strength
machine conlponent is nlanufactured by using steel nlaterials of an alloy steel
and a special steel in which alloying elelnents such as Mn, Cr, Mo, or B are
added to a carbon steel for machine structural use, performing spheroidizing
annealing after hot-rolling to soften the material, fo~liling into a
25 predetermined shape by perfonning cold forging and form rolling, and
thereafter, supplying strength by perforlning a quench-hardening and
tempering process.
[0003] However, a steel cost of these steel materials is high because the
alloying elements are contained, and a manufacturing cost thereof increases
because soften annealing before it is formed into a component shape and the
5 quench-hardening and tempering process after the forming are required.
[0004] An ait is known in which wire drawing is performed for a wire
material whose strength is increased by quick cooling, precipitation
strengthening, and so on without performing the soften annealing and the
quench-hardening and tempering process to supply a predetermined strength.
10 This art is used for a bolt and so on, and the bolt manufactured by using this
art is called as a non-heat treated bolt.
[0005] In Patent Docunlent 1, a inanufacturing method of the non-heat
treated bolt is disclosed in which a wire material containing C: 0.15% to
0.30%, Si: 0.03% to 0.55%, Mn: 1.1% to 2.0% is cooled in a boiling water
15 bath, and a drawing process is performed with a reduction of area of 20% to
50%. In this nlanufacturing method, it is possible to omit the spheroidizing
annealing and the quench-hardening and tempering process, but a nlaximum
strength of the bolt described in the example is 88 kgf/mm2, and it cannot be
said that this bolt has enough strength, and there is a limit in
20 high-strengthening.
[0006] In Patent Document 2, a cold forging steel containing C: 0.4% to
1.0%, whose chemical composition satisfies a specific conditional expression,
and whose structure is made up of pearlite and pseudo pearlite is disclosed.
A C amount of this steel is large, and cold forgeability thereof deteriorates
25 compared to a carbon steel for machine structural use and an alloy steel for
~nachine structural use which are conventionally used for machine
components such as a bolt.
[0007] As stated above, a machine co~nponent having good cold
forgeability and a strength of 900 MPa or more and a steel wire and a wire
material to manufacture the above cannot be obtained by non-heat treated
5 wire lnaterials according to the conventional at-ts.
[Prior Art Document]
[Patent Document]
[0008] Patent Document 1: Japanese Laid-open Patent Publication No.
H02-2748 10
10 Patent Document 2: Japanese Laid-open Patent Publication No.
2000-144306
[Disclosure of the Invention]
[Problen~tso Be Solved by the Invention]
[0009] The present invention is made in consideration of the above-stated
15 problenls in the conventional arts, and an object thereof is to provide (a) a
high-strength machine component capable of being ~nanufactured at low-cost,
and having a tensile strength of 900 MPa to 1300 MPa, (b) a steel wire used
for tnanufacturing the machine conlponent, and capable of onlitting heat
treatments such as soften annealing and quench-hardening and tempering
20 process, (c) a wire material to manufacture the steel wire, and (d) a
manufacturing method lnanufacturing the above.
[Means for Solving the Problen~s]
[00 101 The present inventors investigated a relationship between a
chemical conlposition and a structure of a steel material to obtain a
25 high-strength machine component having a tensile strength of 900 MPa or
more capable of perfornling cold forging even if softening heat treattnent is
not performed and without perfolming a thermal refining process such as
quench-hardening and tempering to attain the above-stated object. The
present invention is made based on a metallurgical knowledge obtained by the
investigation, and an outline thereof is as descried below.
5 [OOll] [I]
A wire material for a non-heat treated co~nponent used for
manufacturing the non-heat treated component whose tensile strength is 900
MPa to 1300 MPa, contains, in mass%:
C: 0.20% to 0.50%, Si: 0.05% to 2.0%, Mn: 0.20% to 1.0%, being
10 limited to contain P: 0.030% or less, S: 0.030% or less, N: 0.005% or less, Fl
defined by the following expression (I) is less than 0.60, with the balance
made up of Fe and inevitable impurities,
wherein a metal structure contains a pearlite structure of 64 x (C%) +
52% or Inore in a volume fraction with the balance made up of one kind or
15 two kinds of a pro-eutectoid ferrite structure and a bainite structure,
an average block grain diameter of the pearlite structure at a region
from a surface layer to 0.1 D is 15 kun or less when a dialneter of the wire
material is set to be D, and (the average block grain diameter of the pearlite
structure at the region from the surface layer to 0.1 D) / (an average block
20 grain diameter of the pearlite structure at a range from 0.25 D to a center) is
less than 1.0.
F1 =C(%)+Si(%)/24+Mn(%)/6 ... (1)
[00121 [21
The wire material for the non-heat treated conlponent according to [I],
25 further contains, in mass%:
one kind or two or more kinds fi.0111 among Al: 0.003% to 0.050%, Ca:
0.001% to 0.010%, Mg: 0.001% to 0.010%, Zr: 0.001% to 0.010%.
[00131 [31
A manufacturing method of a wire material for a non-heat treated
component used for manufacturing the non-heat treated co~nponent whose
5 tensile strength is 900 MPa to 1300 MPa, includes:
heating a steel billet containing, in mass%, C: 0.20% to 0.50%, Si:
0.05% to 2.0%, Mn: 0.20% to 1.0%, being limited to contain P: 0.030% or
less, S: 0.030% or less, N: 0.005% or less, F1 defined by the following
expression (1) is less than 0.60, with the balance made up of Fe and inevitable
10 impurities;
hot-rolling into a wire nlaterial shape;
coiling at a coiling temperature of 800°C to 900°C;
cooling at a cooling rate of 20°C/s to 100°C/s from a coiling finish
temperature to 600°C, filrther cooling at the cooling rate of 20°C/s or less
15 fronl 600°C to 550°C;
thereafter, isothermally holding in a molten salt tank 1 at 400°C to
600°C and a successive molten salt tank 2 at 500°C to 600°C for 5 seconds to
150 seconds each; and
subsequently cooling.
Fl =C(%)+Si(%)/24+Mn(%)/6 ... (1)
[00141 [41
A steel wire for a non-heat treated component used for manufacturing
the non-heat treated conlponent whose tensile strength is 900 MPa to 1300
MPa, contains, in mass%:
25 C: 0.20% to 0.50%, Si: 0.05% to 2.0%, Mn: 0.20% to 1.0%, being
limited to contain P: 0.030% or less, S: 0.030% or less, N: 0.005% or less, F1
defined by the following expression (1) is less than 0.60, with the balance
made up of Fe and inevitable impurities,
wherein a metal structure contains a pearlite structure of 64 x (C%) +
52% or more in a volume fsaction with the balance made up of one kind or
5 two kinds of a pro-eutectoid ferrite structure and a bainite structure,
an average block grain diameter of the pearlite stsucture at a region
from a surface layer to 0.1 D is 15 Lim or less when a diameter of the steel
wire is set to be D, and (the average block grain diameter of the pearlite
structure at the region from the surface layer to 0.1 D) I (an average block
10 grain diameter of the pearlite structure at a range fsom 0.25 D to a center) is
less than 1 .O,
an area ratio of a structure made up of a pearlite block whose aspect
ratio is 2.0 or more is 70% or more relative to a whole pearlite structure at a
region from a surface layer to 1.0 Inn1 at a cross section in parallel to an axial
15 direction of the steel wire.
F1 =C(%)+Si(%)/24+Mn(%)/G ... (1)
[OO15l [51
The steel wire for the non-heat treated component according to [4],
further contains, in mass%:
20 one kind or two or more kinds fsom among Al: 0.003% to 0.050%, Ca:
0.001% to 0.010%, Mg: 0.001% to 0.010%, Zr: 0.001% to 0.010%.
[OO 161 C61
A manufacturing method of a steel wire for a non-heat treated
colnponent used for manufacturing the non-heat treated coinponent wllose
25 tensile strength is 900 MPa to 1300 MPa, includes:
heating a steel billet containing, in inass%, C: 0.20% to 0.50%, Si:
0.05% to 2.0%, Mn: 0.20% to 1.0%, being limited to contain P: 0.030% or
less, S: 0.030% or less, N: 0.005% or less, F1 defined by the following
expression (I) is less than 0.60, with the balance made up of Fe and inevitable
impurities;
hot-rolling into a wire inaterial shape;
coiling at a coiling temperature of 800°C to 900°C;
cooling at a cooling rate of 20"Cls to 100°C/s fsom a coiling finish
temperature to 600°C, fusther cooling at the cooling rate of 20°C/s or less
from 600°C to 550°C;
10 thereafter, isothern~ally holding in a molten salt tank 1 at 400°C to
600°C and a successive molten salt tank 2 at 500°C to 600°C for 5 seconds to
150 seconds each;
subsequently cooling; and
thereafter, perfornling wire drawing at a total red~ictiono f area of 15%
15 to 80%.
F1 =C (%)+Si (%)/24+Mn(%)/G ... (1)
[00171 [71
A non-heat treated component whose tensile strength is 900 m a to
1300 MPa, manufactured by cold-\vorking a steel wire containing, in inass%:
20 C: 0.20% to 0.50%, Si: 0.05% to 2.094 Mn: 0.20% to 1.0%, being limited to
contain P: 0.030% or less, S: 0.030% or less, N: 0.005% or less, F1 defined
by the following expression (1) is less than 0.60, with the balance inade up of
Fe and inevitable impurities,
wherein a metal stmcture contains a pearlite structure of 64 x (C%) +
25 52% or more in a volume fraction, with the balance tuade up of one kind or
two kinds of a pro-eutectoicl ferrite structure and a bainite structure,
an average block grain diameter of the pearlite structure at a region
from a surface layer to 0.1 D is 15 pin or less when a diameter of the steel
wire is set to be D, and (the average block grain diameter of the pearlite
structure at the region from the surface layer to 0.1 D) / (an average block
5 grain diameter of the pearlite structure at a range from 0.25 D to a center) is
less than 1.0, and
an area ratio of a structure made up of a pearlite block whose aspect
ratio is 2.0 or inore is 70% or Inore relative to a whole pearlite st~~~ctaut rae
region fsom a surface layer to 1.0 lnin at a cross section in parallel to an axial
10 direction of the steel wire.
F1 =C (%)+Si (%)/24+Mn(%)/6 ... (1)
[00181 P I
The non-heat treated component according to [7], further contains in
mass%:
15 one kind or two or more kinds fsom among Al: 0.003% to 0.050%, Ca:
0.001% to 0.010%, Mg: 0.001% to 0.010%, Zr: 0.001% to 0.010%.
[00191 [91
A manufacturing method of a non-heat treated comnponent whose
tensile strength is 900 MPa to 1300 MPa, includes:
20 heating a steel billet containing in mass%, C: 0.20% to 0.50%, Si:
0.05% to 2.0%, Mn: 0.20% to 1.0%, being limited to contain P: 0.030% or
less, S: 0.030% or less, N: 0.005% or less, F1 defined by the following
expression (I) is less than 0.60, with the balance mnade up of Fe and inevitable
impurities;
25 hot-rolling into a wire inaterial shape;
coiling at a coiling tenlperature of 800°C to 900°C;
cooling at a cooling rate of 20°C/s to 100°C/s from a coiling finish
temperature to 600°C, further cooling at the cooling rate of 20°C/s or less
from 600°C to 550°C;
thereafter, isothermally holding in a molten salt tank 1 at 400°C to
5 GOO°C and a successive molten salt tank 2 at 500°C to 600°C for 5 seconds to
150 seconds each;
subsequently cooling;
thereafter, performing wire drawing at a total reduction of area of 15%
to 80%; and
10 further, performing cold-working.
Fl=C(%)+Si(%)/24+Mn(%)/6 ... (1)
[0020] [20]
The illanufacturing nlethod of the non-heat treated conlponent
according to [9],
15 wherein after the wire drawing is performed, cold-working is
performed without perfor~ninga softening heat treatment.
[0021] [21]
The manufacturing inethod of the non-heat treated component
according to [9], further includes:
20 holding at 200°C to 600°C for 10 nlinutes or Inore after the
cold-working is perfonned.
[Effect of the Invention]
[0022] According to the present invelltion, it is possible to provide a
high-strength nlachine component having a tensile-strength of 900 MPa to
25 1300 MPa contributing to reduction in weight and size of a vehicle, various
kinds of industl-ial machineries, and architectural ~nelnbersa t low-cost.
[Brief Description of the Drawings]
[0023] [FIG. 11 FIG. 1 is a view illustrating a relationship between a
tensile strength (TS) and a ratio between an average block grain diameter of a
pearlite structure within a range from a surface layer to 0.1 D and an internal
5 average block grain diameter.
[Mode for Carrying out the Invention]
[0024] The present inventors investigated in detail about the relationship
between the chemical co~nposition and the structure of the steel material to
obtain the high-strength machine co~nponent having the tensile strength of
10 900 MPa or Inore capable of perfornling cold forging even if the softening
heat treatment is not perfor~ned as stated above and without perfor~ning a
thermal refining process sucli as the quench-hardening and te~npering. Then,
the present inve~ltorsc ontinued a total study as for a series of inanufacturing
lnethod relating to an inline heat treatment using a retained heat at a
15 hot-rolling time of a wire ~naterial,a nd up to a subsequent steel wire, machine
co~nponent based on the n~etallurgical knowledge obtained by the
investigation to nlanufacture the high-strength machine component at low
cost and come to the following conclusions.
[0025] (x) To supply the high-strength to a wire material by wire drawing
20 and cold forging, it is effective to change a steel structi~re into a pearlite
structure excellent in a work hardening ability, but workability is low, a
defor~nation resistance is high, and working cracks are easy to occur in the
pearlite structure.
[0026] (y) To improve the workability of the pearlite structure, it is
25 effective (yl) to reduce an alnount of alloying elements, and (y2) to make a
block grain dia~netero f the pearlite str~lcturea t a surface layer minute.
[0027] (2) Namely, it is possible to extremely improve cold workability
of the pearlite structure if C (%) + Si (%) / 24 + Mn (%) / 6 is set to be less
than 0.60, and the grain diameter of the pearlite block at a region from the
surface layer to 0.1 D (D: a diameter of the wire material) is set to be 15 ym
5 or less, and a ratio with a grain diameter of a pearlite block at inside of the
wire material is set to be less than 1 .O.
[0028] As stated above, it becomes possible to secure the excellent work
hardening ability, maintain the high-strength even if the quench-hardening
and tempering process is not perfonned, and improve the cold forgeability by
10 iinproving the chemical co~npositiona nd the stlx~ctureo f the steel material.
[0029] A steel wire to be a material to obtain the inachine component
capable of perfon~lingt he cold forging even if the softening heat treatment is
not perfonned, and being high-strength without perforining the thernlal
refining process such as the quench-hardening and tempering is one already
15 having a microstructure with the above-stated characteristics at a stage of the
steel wire, and it is effective to work into a component for machine structural
use without perfor~ningth e heat treatment before the working.
[0030] In this case, the cold workability is deteriorated but a soften
annealing cost and a quench-hardening and tempering cost after the work can
20 be reduced, and therefore, the present invention is advantageous in cost
colnpared to a conventional ~nanufacturingm ethod in which the spheroidizing
annealing is perfornled for softening.
[003 11 Further, as for a manufacturing method of the wire material to be a
nlaterial of the steel wire, it is possible to obtain the steel material in almost
25 perfect pearlite structure without adding any expensive alloying elements if it
is itnmersed in a molten salt bath made irp of two tanks just after rolling while
using remaining heat at the hot-rolling time. This manufacturing method is
the best manufacturing method capable of obtaining excellent material
characteristics at low cost.
[0032] Namely, the present invention is a series of manufacturing method
5 in which the steel material whose chemical composition is adjusted to be the
pearlite structure is immersed in the molten salt bath by using the remaining
heat at the hot-rolling time to obtain the wire material having the allnost
perfect pearlite structure, then wire drawing is perfor~ned at a room
temperature under a specific condition, an adjustment is performed to be
10 high-strength pearlite structure, it is formed into a machine component, and
thereafter, heat treatnlent at a relatively low tetnperature is perfor~ned to
recover ductility thereof,
[0033] Therefore, according to the present invention, it is possible to
nlanufactuse the nlachine component whose tensile strength is 900 MPa to
15 1300 MPa at low cost though it is extremely difficult to tna~lufacture it
according to the conventional manufacturing method and knowledge. . ~~
[0034] At first, reasons for limiting the che~nicacl omposition of the steel
material (wire material, steel wire, non-heat treated component) of the present
inventio~al re described. Hereinafter, a symbol "%" relating to the chemical
20 co~npositionm eans mass%.
[0035] C is added to secure a predete~lninedt ensile strength. When it is
less than 0.20%, it is difficult to secure the tensile strength of 900 MPa or
more, on the other hand, when it exceeds 0.50%, cold forgeability deteriorates,
and therefore, C is set to be 0.20% to 0.50%. A preferable range to enable
25 both the strength and the cold forgeability is 0.35% to 0.48%.
. [0036]. Si f~~nctioanss a deoxidizing element and has an effect enhancing
the tensile strength by solid-solution strengthening. When it is less than
0.05%, an addition effect is insufficient. When it exceeds 2.0%, the addition
effect is saturated, hot ductility deteriorates, flaws are easy to occur, and
manufacturability is lowered. Accordingly, Si is set to be 0.05% to 2.0%.
5 A preferable range in consideration of the manufacturability is 0.18% to
0.5%.
[0037] Mn has an effect enhancing the tensile strength of the steel after a
pearlite transformation. When it is less than 0.20%, an addition effect is
insufficient, and when it exceeds 1.0%, the addition effect is saturated, and
10 therefore, a range of Mn is set to be 0.20% to 1.0%. A more preferable
range is 0.50% to 0.8%.
[0038] P and S are inevitable impurities. These eleinents segregate at a
grain boiundary to deteriorate hydrogen embrittletne~irt esistant characteristics,
and therefore, the less it is, the better. Accordingly, an upper limit is each set
15 at 0.030%. It is preferably 0.015% or less. A lower limit includes "0"
(zero)%, but both P and S are inevitably mixed for at least approxiinately
0.0005%.
COO391 N deteriorates the cold workability by dynamic strain aging, and
therefore, the less it is, the better, so an upper limit is set at 0.005%. It is
20 preferably 0.004% or less. A lower limit illcludes "0" (zero)%, but it is
inevitably mixed for at least approximately 0.0005%.
[0040] When a relational expression (1) of contents of C, Si, and Mn: F1
= C (%) -I- Si (%) / 24 + Mn (%) / 6 becomes 0.60 or more, a deforlnation
resistance increases and the cold workability deteriorates, and therefore, F1 is
25 set to be less than 0.60.
[0041] C, Si, and Mn are elements ilnproving the strength. F1 is an
expression restricting a total amount of C, Si, and Mn in consideration of a
degree of contribution to the strength improvement.
[0042] In the present invention, A1 may be contained for 0.003% to
0.050%. Al filnctions as the deoxidizing element, and in addition, forms
5 AIN to reduce solid-solution N, and suppresses the dynamic strain aging.
A1N functions as a pinning particle to refine crystal grains and improves the
cold workability.
[0043] When it is less than 0.003%, there is no addition effect, and when
it exceeds 0.050%, the addition effect is saturated, and the manufacturability
10 deteriorates, and therefore, A1 is set to be 0.003% to 0.050%. It is preferably
0.008% to 0.045%.
[0044] In the present invention, one kind or two more Inore kinds may be
contained from anlong Ca: 0.001% to 0.010%, Mg: 0.001% to 0.010%, Zr:
0.001% to 0.010% as the deoxidizing elements. These elements function as
15 the deoxidizing elements, and fosm sulfide such as CaS, MgS to fix
solid-solution S and has an effect improving hydrogen e~nbrittle~nernets istant
characteristics.
[0045] Cr, Mo, Ni, Ti, Nb and V enhance the strength, deteriorate the
cold workability, and therefore, they are necessary to be reduced. Note that
20 when an atnount contained as the inlpurities is less than 0.60 in a value of C
(%)+Si(%)/24+Mn(%)/6+(Cr(%)+Mo(%))/S+Ni(%)/40+(Ti
(%) + Nb (%) + V (%)) / 5, an effect on the cold workability is small, and
therefore, Cr, Mo, Ni, Ti, Nb and V are allowed within a range of less than
0.60 in the above-stated value. The above-stated value is preferably 0.58 or
25 less.
[0046] Note that 0 inevitably exists in a nlode of an oxide of A1, Ca
andlor Mg in the steel. When an 0 amount is large, coarse oxide may be
generated, and it may cause a fatigue fracture, and therefore, it is preferably
0.01% or less. Note that 0 is inevitably mixed for at least approximately
0.001%.
5 [0047] In the present invention, a steel billet having the above-stated
chemical composition is necessary to be hot-rolled to change it into a steel
material (wire material, steel wire, non-heat treated component) having a
specific microstructure. Next, limitation reasons of the microstructure of the
steel material (wire material, steel wire, non-heat treated component) are
10 described.
100481 The pearlite structure is a structure having excellent work
hardening characteristics. When a volume fsaction is less than "64 x (C%) +
52%", work hardening at the wire drawing time and the cold forging time
beconles s~nallt,h e strength is lowered, and working cracks are easy to occur
15 at the cold forging time because a non-pearlite structure part becomes a
starting point of the fsacture. Accordingly, a lower limit of the volume
fraction of the pearlite structure is set to be "64 x (C%) + 52%".
[0049] It is possible to contain a pro-eutectoid ferrite structure and a
bainite structure as a remaining structure other than the pearlite structure. A
20 martensite stri~cturei s not contained because the cracks at the wire drawing
time and the cold forging time are easy to occur and the hydrogen
enlbrittlement resistant cllaracteristics are deteriorated.
[0050] The volume fiaction of the pearlite stn~ctureis found, for example,
by photographing a C-cross section of the wire ~naterial (a cross section
25 perpendicular to a longitudinal direction of the wire material) at a
magnification of 1000 times by using a scanning electron microscope, and by
performing image analysis. For example, at the C-cross section of the wire
material, a region of 125 ym x 95 Fun is photographed at each of a region in a
vicinity of a surface layer (surface) of the wire material, a 114 D part (a part
kept off for 1/4 of a diameter D of the wire material from the surface of the
5 wire material in a center direction of the wire material), and a 112 D part (a
center part of the wire material). An area ratio of a structure contained in a
microscopic observation surface (C-cross section) is equal to the volu~ne
fraction of the structure, and therefore, the area ratio obtained by the image
analysis is the voluine fraction of the structure. Note that the voluine
10 fractions of the pearlite struct~~reosf the steel wire and the non-heat treated
co~nponenat re si~nilarlyd efined.
[005 11 When an average block grain dia~netero f the pearlite structure at a
range from the surface layer to 0.1 D exceeds 15 ~tmt,h e working cracks are
easy to occur at the cold forging time, and therefore, an upper liinit of the
15 average block grain diaineter is set to be 15 pm.
[0052] When (the average block grain diameter of the pearlite structure at
the region fsom the surface layer to 0.1 D) 1 (an average block grain diaineter
of the pearlite structure at a range from 0.25 D to the center) becomes 1.0 or
Inore, the working cracks are easy to occur, and therefore, a ratio of the
20 average block grain dia~netersi s set to be less than 1.0. A preferable upper
limit thereof is 0.90.
[0053] Next, in the present invention, an area ratio of a structure made up
of a pearlite block whose aspect ratio is 2.0 or nlore at a region from a surface
layer to 1.0 lnin at a cross section which is in parallel to an axial direction of
25 the steel wire is 70% or Inore relative to a whole pearlite structure at the steel
wire obtained by wire drawing the wire material. A relationship between a
tensile strength (TS) and a ratio of the average block grain dialneter of the
pearlite structure at the range fiom the surface layer to 0.1 D and an internal
average block grain diameter is illustrated in Fig. 1. In the drawing, a black
square represents a case of a steel material whose chemical composition is out
5 of a range of the present invention, and F1 is 0.6 or more.
[0054] 111 the drawing, a black triangle represents a case of a steel wire
whose chemical colnposition is within the range of the present invention, but
whose volume fraction of the structure made up of the pearlite block whose
aspect ratio is 2.0 or more is less than 70% relative to the wllole pearlite
10 structure to be out of the range of the present invention, and O represents a
case of a steel wire whose chenlical colnposition is within the range of the
present invention, and whose volurne fraction of the struct~~mrea de up of the
pearlite block wliose aspect ratio is 2.0 or more is 70% or more relative to the
whole pearlite structure.
15 [0055] The average block grain diameter can be measured by using, for
example, an EBSP (Electron Back Scattering Pattern) device. Specifically, a
region of 275 pn x 165 pnl is lneasured at each of the range fiom the surface
layer to 0.1 D and a range from the 1/4 D part (a part kept off for 1/4 of the
dialneter D of a steel wire fsom the surface of the steel wire in a center
20 direction of the steel wire) to the 1/2 D part (the center palt of the steel wire)
at the wire material cross section perpendicular to the longitudinal direction of
the wire material.
[0056] A boundary where a nlisorientation beconles 10" or inore from a.
clystal orientation map of a bcc structure lneasured by the EBSP device is set
25 to be a block grain boundary. A circle-equivalent grain dialneter of one
block grain is defined as a block grain diameter, and a volu~nea verage thereof
is defined as an average grain diameter.
100571 The non-heat treated component is a machine co~nponenitn which
the heat treat~nentss uch as the soften annealing and the quench-hardening and
tempering process are not performed, and the strength is supplied by working
5 effects such as the wire drawing and the forging. Here, it is the machine
component whose reduction of area from an initial cross section is 10% or
more,
[0058] Next, a manufacturing method of the steel material (the wire
material, the steel wire, the non-heat treated conlponent) is described. A
10 steel billet made up of a predeter~nined chemical coinposition is heated, then
hot-rolled into a wire state, and thereaftel; it is coiled LIP in a ring state. A
coiling temperature is set at 800°C to 900°C, and it is cooled at a cooling rate
of 20°C/sec to 100"C/sec from a coiling finish teniperature to 600°C, further
it is cooled at a cooling rate of 20°C/sec or less from 600°C to 550°C.
15 [0059] The coiling temperature affects on the pearlite block grain after
transformation. When the coiling temperature exceeds 900°C, the pearlite
block grain diameter of the wire ~naterial after the hot-rolling beconies a
coarse grain to exceed 15 pin at a surface layer part, and the cold forgeability
is deteriorated. When the coiling temperature is less than 800°C, the volume
20 fraction of the pro-eutectoid fesrite of the structure after transfolmation
increases, and the volume fraction of the pearlite structure becomes less than
"64 x (C%) + 52%". Accordingly, the coiling temperature is set at 800°C to
900°C. -
[0060] When the cooling rate after the coiling is less than 20°C/sec, the
25 volume fiaction of the pro-eutectoid ferrite structure of the wire ~naterial
increases and the volunle fraction of the pearlite structure becomes less than -
"64 x (C%) + 52%". An excessive cooling equipment is required to enable
the cooling rate of over 1OO0C/sec. Accordingly, the cooling rate after the
coiling to 600°C is set at 20°C/sec to 100°C/sec.
[0061] When the cooling rate f?om 600°C to 550°C exceeds 20°C/sec,
5 the bainite structure is generated in the structure to deteriorate the cold
forgeability, and therefore, an upper limit of the cooling rate f?om 600°C to
550°C is set at 20°C/sec. A lower limit is preferably 1°C/sec from a point of
view of productivity.
[0062] Next, the wire material is immersed in the nlolten salt tank by
10 using the remaining heat at the hot-rolling time to generate an isotherlnal
pearlite transforlnation.
[0063] After it is cooled to 550°C, the wire inaterial is i~n~nerseindt o a
nlolten salt tank 1 at 400°C to 600°C and a successive inolten salt tank 2 at
500°C to 600°C, and it is isother~nally held for 5 seconds to 150 seconds
15 respectively, and thereafter, it is cooled to man~~facturthee wire material
having the above-stated ~nicrostruct~ure.
[0064] When the temperature of the molten salt tank 1 is less than 400°C,
bainite is generated to deteriorate the cold forgeability. When the
tenlperature of the nlolten salt tank 1 exceeds 600°C, a time required for the
20 pearlite transforlnation becollles long. Accordingly, the temperature of the
nlolten salt tank 1 is set at 400°C to 600°C.
[0065] At the nlolten salt tank 2 subsequent to the molten salt tank 1, the
temperature is set at 500°C to 600°C to finish the pearlite transfor~nation
within a mini~nulnti me. An ini~nersionti me to the inolten salt tank is set to
25 be 5 seconds to 150 seconds at each tank from points of view of enough
temperature keeping and the productivity of the steel material. The cooling
after it is held in the molten salt tank for a predetermined time may be a water
cooling or a standing-to-cool.
[0066] Note that the similar effect can be obtained by using equipments
such as a lead tank and a fluidized bed as the immersion tank instead of the
5 molten salt tank, but the present invention is superior in points of environment
and manufacturing cost.
[0067] To change the wire material manufact~~reads stated above into a
steel wire having the desired strength and cold forgeability by performing the
wire drawing, a mode of the pearlite structure at a region fkoln the surface
10 layer to 1.0 nun is important.
[0068] When the volume fraction of the str~icturem ade up of the pearlite
block whose aspect ratio is 2.0 or Inore is less than 70% relative to the whole
pearlite structure at the region from the surface layer to 1.0 lnln of the steel
wire, the ilnprovement effect of the cold forgeability is not obtained.
15 Accordingly, a lower lilnit of the volume fraction of the structure made up of
the pearlite block whose aspect ratio is 2.0 or--more is set at 70%. A
preferable lower limit of the volurne fraction of the structure is 80% because
the less the volume fraction of the block whose aspect ratio is less than 2.0 is,
the better.
20 [0069] When the aspect ratio of the pearlite block is less than 2.0, the
improvement effect of the cold forgeability is small, and therefore, a lower
limit of the aspect ratio is set at 2.0. Note that the aspect ratio is a ratio
between a nlajol. axis and a n~inor axis of a block grain, and it is equal to a
ratio between a diameter in an axial direction and a diameter in a
25 perpendicular direction to the axis after the wire drawing (the diameter in the
axial directionhhe diameter in the perpendicular direction to the axik).
[0070] In the wire drawing, the reduction of area is set at 15% to 80%.
When the reduction of area of the wire drawing is less than 15%, the work
hardening is insufficient and the strength is in short, and therefore, a lower
litnit of the reduction of area is set at 15%. When the reduction of area
5 exceeds SO%, the working cracks are easy to occur at the cold forging time,
and therefore, an upper limit of the reduction of area is set at 80%. A
preferable reduction of area is 20% to 65%. Note that the wire drawing may
be performed once or plural times.
[0071] The steel wire obtained as stated above is shaped into a final
10 machine coniponent, but a heat treatnient is not necessarily performed before
the shaping to ~iiaintainth e above-stated characteristics of the microstructure.
The steel wire obtained as stated above is cold forged (cold working), and
thereby, a non-heat treated component \vhose tensile strength is 900 MPa to
1300 MPa is obtained. A foundation of the present invention is to obtain the
15 non-heat treated colnponent whose tensile strength is 900 MPa or more.
When the strength as a conlponent is less than 900 MPa in the tensile strength,
it is not necessary to apply the present invention. On the other hand, a
coniponent exceeding 1300 MPa is difficult to manufacture by the cold
forging, and the manufactwing cost increases. Accordingly, the tensile
20 strength is set to be 900 MPa to 1300 MPa as the colnponent strength.
[0072] The tensile strength is preferably 900 MPa to 1250 MPa, more
preferably 900 MPa to less than 1200 MPa. The niachine conlponent nlay
be held at 200°C to 600°C for 10 nlinutes to 5 hours after it is cold forged into
the component shape, and thereafter, cooled so as to improve other material
25 characteristics required as the machine component such as a yield strength, a
yield ratio, or ductility though it is high-strength as it is as the machine
component.
Examples
[0073] Next, examples of the present invention are described.
Conditions in the examples are an conditional example applied to verify
5 feasibility and effects of the present invention, and the present invention is not
limited to the conditional example. The present invention is able to apply
various conditions within a range not departing &om the spirit of the
invention and attaining an object of the invention.
[0074] Chemical compositions of the steel materials supplied for the
10 example and values of the expression Fl = (C%) + (Si%) / 24 + (Mn%) / 6 are
represented in Table 1. Steel types L, M, N and 0 are conlparative examples
out of the range of the present invention.
[0075] [Table 11
15 [0076] Steel billets made up of these steel types are hot-rolled into wire
nlaterials each having the wire diameter of 8.0 lnnl to 15.0 mm. -After the
hot-rolling, coiling, cooling are performed, and the isothermal transfor~nation
process is perfornled at the molten salt tanks 1, 2 on a rolling line, and then
cooled.
20 [0077] A wire diameter of each of the hot-rolled wire nlaterials, a coiling
temperature after the hot-rolling, a cooling rate from the coiling temperature
to 600°C, a cooling rate fronl 600°C to 550°C, an isother~nal holding
te~nperaturea nd an isothern~ahl olding time at each of the~~noltesnal t tanks 1,
2 are represented in Table 2. The wire drawing is perfor~ned for each of the
25 hot-rolled wire materials after the cooling with the reduction of areas
-. ..- _.-. .. . . represented in Table 2, and a heat treatment-is p.erformed,. Respective heat
treatment temperatures and holding times of the heat treatment are
represented in Table 2.
[0078] [Table 21
5 [0079] A metal structure, a volume fraction of a pearlite structure, an
average block grain diameter of the pearlite structure at a region from a
surface layer to 0.1 D, an average block grain diameter of an internal pearlite
structure (an average block grain diameter of the pearlite structure at a range
from 0.25 D to a center), and a ratio of the average block grain diaineters
10 between the surface layer and the internal of each of the wire materials
obtained by performing the isothernlal transfor~nationp rocess at the molten
salt tanks 1, 2 and then cooled are represented in Table 3. Note that in the
metal structure, F represents a pro-eutectoid ferrite, P represents pearlite, B
represents bainite, and M represents martensite.
15 [0080] [Table 31
[0081] Structures of the steel wires after the wire drawing are the same as
the structures represented in Table 3. In Table 3, each ratio of a structure
made up of a pearlite block whose aspect ratio is 2.0 or nlore relative to a
20 whole pearlite structure at a region from a surface layer to 1.0 nun at a cross
section in parallel to an axial direction of the steel wire is represented.
Besides, each lower linlit of the volun~e fiaction of the pearlite structure
calculated by "64 x (C%) + 52%" is represented in Table. 3.
[0082] Each tensile strength at a final nlachine component obtained by
25 performing the cold-forging (cold working) of the steel wire, and each cold
forgeability of the steel wire before the heat treatment are represented in Table
4.
[0083] [Table 41
[0084] The tensile strength is evaluated by using a 9A test piece of JIS Z
5 2201 and performing a tensile test based on a test method of JIS Z 2241.
The cold forgeability is evaluated by a maximum stress (deformation
resistance) and a lnaximu~n cornpression ratio (limit co~npression ratio)
without any cracks by using a sample of $5.0 mm x 7.5 lnln prepared by
nlachining the steel wire after the wire drawing, when an end face of the
10 sample is constrained and conlpressed with a metal Inold having a groove in a
concentric state, and lnachined at a conlpression ratio of 57.3% corresponding
to a distoltion of 1 .O.
[0085] When the inaxilnunl stress when it is machined at the co~npression
ratio of 57.3% is 1200 MPa or less, it is judged that the deformation
15 resistance is excellent, and when the lnaxirnum co~npressionr atio without any
cracks is 65% or inore, it is judged that the limit co~npression ratio is
excellellt.
100861 A level 10 is a conventional ~nanufacturing lnetllod in which the
isothermal transfor~nationp rocess is not perfor~neda fter the coiling, and it is
20 cooled on Stelnlor as represented in Table 2, and the volume fraction of the
pearlite structure is out of the range of the present invention.
[0087] A level I1 is a conlparative example in which tlie wire material of
the level 10 manufactured by cooling on the Stellnor is heated at 950°C for 10
minutes, and held in a lead bath at 580°C for 100 seconds. The average
25 block grain dianleter of the pearlite structure at the range from the surface
layer to 0.1 D, and the ratio of the average block grain dianleters between the
surface layer and the internal are out of the range of the present invention.
A level 13 is an example in which the coiling telnperature exceeds the
upper limit of the present invention. The average block grain diameter of
the pearlite stlucture at the range fiom the surface layer to 0.1 D, and the ratio
5 of the average block grain diameters of the surface layer and the internal are
out of the range of the present invention.
[0088] A level 15 is an example in which the wire drawing reduction of
area is s~nallerth an the lower limit of the range of the present invention, and
the volume fiaction of the pearlite structure whose aspect ratio is 2.0 or more
10 does not reach the lower linlit of the range of the present invention.
[0089] A level 16 is an exanlple in which the tenlperat~~oref the nlolten
salt bath is lower than the lower limit of the range of the present invention,
and the nlartensite structure is nlixed in the metal structure to be out of the
structure of the present invention, in addition, the volume fraction of the
15 pearlite structi~re and the volunle fraction of the pearlite structure whose
aspect ratio is 2.0 or Inore do not reach the lower limit of the range of the
present invention. In the level 16 in which the martensite structure is mixed,
wire drawability deteriorates, and wire breakage occurred during the wire
drawing.
20 A level 22 is an example in which the coiling temperature is less than
the lower limit of the range of the present invention. The pro-eutectoid
ferrite is generated, and the volunle fraction of the pearlite structure is less
than the lower limit of the range of the present invention.
A level 23 is an exanlple in which the temperature of the molten salt
25 bath 1 exceeds the upper limit of the range of the present invention. The
martensite strilcture is mixed in the metal structure to be out of the structure
of the present invention, in addition, the volume fraction of the pearlite
structure is less than the lower limit of the range of the present invention.
A level 24 is an example in which the temperature of the molten salt
bath 2 exceeds the upper limit of the range of the present invention. The
5 martensite structure is inixed in the metal structure to be out of the structure
of the present invention, in addition, the volume fraction of the pearlite
structure and the volume fraction of the pearlite structure whose aspect ratio is
2.0 or more do not reach the lower limit of the range of the present invention.
A level 25 is an example in which the holding times of the inolten salt
10 tank 1 and the molten salt tank 2 are less than the lower linlit of the range of
the present invention. The inartensite structure is niixed in the metal
structure to be out of the structure of the present invention, in addition, tlie
volume fraction of the pearlite structure and the volume fraction of the
pearlite structure whose aspect ratio is 2.0 or more do not reach the lower
15 limit of the range of the present invention. In the level 25 in which the
martensite structure is mixed, the wire drawability deteriorates, and wire
breakage occurred during the wire drawing.
[0090] Mechanical properties of respective levels are represented in Table
4.
20 [0091] All of the limit compression ratios are less than 65% and bad in
the level 10 in which tlie volume fraction of the pearlite structure and the ratio
of the average block grain diameters between the surface layer and tlie
internal are out of the range of the present invention, the level 11 in \vllich the
average block grain diameter of the pearlite struct~tre at the range frosii the
25 surface layer to 0.1 D and the ratio the ratio of the average block grain
diameters between thesurface layer andthe internal are out of the range of-the.. . . ... .; .
present invention, the level 13 in which the average block grain diameter of
the pearlite structure at the range fsom the surface layer to 0.1 D is out of the
range of the present invention, the level 15 in which the ratio of the average
block grain diameters between the surface layer and the internal is out of the
5 range of the present invention, each of the level 16 and level 24 in which the
martensite structure is mixed in the metal structure to be out of the structure
of the present invention and the volu~nef raction of the pearlite structure and
the the volu~ne fsaction of the pearlite structure whose aspect ratio is 2.0 or
Inore are out of the range of the present invention, the level 18 in which the
lo voiu~ne fraction of the pearlite structure and the the volume fi-action of the
pearlite structure whose aspect ratio is 2.0 or nlore are out of the range of the
present invention, the level 22 in cvhich the volume fsaction of the pearlite
structure is out, and the level 23 in which the mastensite structure is mixed in
the metal structure to be out of the strilcture of the present invention, and the
15 volume fsaction of the pearlite structure is out of the range of the present
invention.
[0092] In each of a level 19 using the steel type M in which Cr and Mo
are out of the range of the present invention, a level 20 using the steel type N
in which C and F 1 are out of the range of the present invention, and a level 21
20 using the steel type 0 in which C and N are out of the range of the present
invention, the stress at the colnpression ratio of 57.3% exceeds 1200 MPa,
and the defor~nationr esistance is bad.
[0093] It can be seen fi.o~nt he above-stated description that the machine
component according to the present invention has workability in which the
25 cold forging is possible even if the soften annealing is not perfor~neda, nd has
the strength of 900 MPa to 1300 MPa even if the quench-hardening and
tempering process is not performed.
[Industrial Applicability]
[0094] As stated above, according to the present invention, it is possible
to provide the high-strength lnachine component contributing to reduction in
5 weight and size of a vehicle, various kinds of industrial machineries, and
architectural members at low-cost. Accordingly, the present invention is
applicable for mechanical industries.
Table 1
Table 2
vg RE
DUrXmEmEjR
COOLING RATE FROM
mfmacl P_C O'L'NGi 0riE\O o'C
COOUNG RA1E FROM
600I-? r-iTri 0mbc
'.EMWXTfJKE HOLDING 1M TEMPERATURE
OF MOLTEN OF MOLTEN OF MIXTEN
SALTJANK 1 1 SALTCNK 1 SALTZNK 2
HOLDING
TIME OF
MQ\~€N~S$T
HOLDING TIME REMARKS
(h)
HEAT
TREATMENT
TEMPL@TURE
WIRE DRAWING
CRACK
Table 3
18
19
20
21
22
23
24
25
3
M
N - 0
I
K
K
K
F.P.8
F.P.E
F.?.E
F.P.8
F.P.9
F.P.8.M
F.P.0.M
F.P.6.M
62.9
82.7
372
88.5
81.4
83.4
83.4
83.4
5 4 11.7
90 1 122
94 1 13.5
95 1 14.2
10.6
13.9
14.8
15.3
12.9
15.2
13.2
13.8
71
72
69
39
10.3
14.3
10.9
12.3
1.10
0.88
0.91
0.93
62 xaMp,
COpPAMVVE
74
76
75
0.80 I 70
0.97 1 72
0.83 67
0.89 1 54
co PPR TYE
cogpsp.p~
cOk%wjECO~
PAE$T~E
Table 4
[Name of Document] What is claimed is
[Claim 11 A wire material for a non-heat treated cotnponent used for
manufacturing the non-heat treated component whose tensile strength is 900
MPa to 1300 MPa, containing, in mass%,
5 C: 0.20% to 0.50%, Si: 0.05% to 2.0%, Mn: 0.20% to 1.0%, being
limited to contain P: 0.030% or less, S: 0.030% or less, N: 0.005% or less, F1
defined by the following expression (1) is less than 0.60, with the balance
made up of Fe and inevitable impurities,
wherein a metal structure contains a pearlite stsucture of 64 x (C%) +
10 52% or Inore in a volume fraction, with the balance made up of one kind or
two kinds of a pro-eutectoid ferrite structure and a bainite structure,
an average block grain diameter of the pearlite structure at a region
from a surface layer to 0.1 D is 15 pm or less when a diameter of the wire
nlaterial is set to be D, and (the average block grain diameter of the pearlite
15 structure at the region fsom the surface layer to 0.1 D) / (an average block
grain diameter of the pearlite structure at a range fsom 0.25 D to a center) is
less than 1 .O.
F1 =C(%)+Si(%)/24+Mn(%)/6 ... (1)
[Claim 21 The wire ~naterial for the non-heat treated component
20 according to claiin 1, further containing, in mass%,
one kind or two or more kinds fsom alllong Al: 0.003% to 0.050%, Ca:
0.001% to 0.010%, Mg: 0.001% to 0.010%, Zr: 0.001% to 0.010%.
[Claim 31 A manufacturing method of a \vise material for a non-heat
treated colnponent used for ~nanufacturing the non-heat treated component
25 whose tensile strength is 900 MPa to 1300 MPa, comprising:
lleating a steel billet containing, in mass%, C: 0.20% to 0.50%, Si:
0.05% to 2.0%, Mn: 0.20% to 1.0%, being limited to contain P: 0.030% or
less, S: 0.030% or less, N: 0.005% or less, F1 defined by the following
expression (1) is less than 0.60, with the balance made up of Fe and inevitable
impurities; hot-rolling into a wire material shape; coiling at a coiling
5 ternperature of 800°C to 900°C;
cooling at a cooling rate of 20°C/s to 100°C/s from a coiling finish
ternperature to 600°C, further cooling at the cooling rate of 20°C/s or less
from 600°C to 550°C;
thereaftel; isothernlally holding in a molten salt tank 1 at 400°C to
10 600°C and a successive nlolten salt tank 2 at 500°C to 600°C for 5 seconds to
150 seconds each; and
subsequently cooling.
Fl=C(%)+Si(%)/24+Mn(%)/G ... (1)
[Clainl 41 A steel wire for a non-heat treated colnponent used for
15 manufacturing the non-heat treated component whose tensile strength is 900
MPa to 1300 MPa, containing, in inass%,
C: 0.20% to 0,50%, Si: 0.05% to 2.0%, Mn: 0.20% to 1.096, being
linlited to contain P: 0.030% or less, S: 0.030% or less, N: 0.005% or less, F1
defined by the following expression (1) is less than 0.60, with the balance
20 inade LIP of Fe and inevitable impurities,
wherein a metal structure contains a pearlite structure of 64 x (C%) +
52% or nlore in a volume fraction, with the balance made up of one kind or
two kinds of a pro-eutectoid ferrite structure and a bainite structure,
an average block grain diameter of the pearlite structure at a region
25 from a surface layer to 0.1 D is 15 11111 or less when a diameter of the steel
wire is set to be D, and (the average block grain diameter of the pearlite
structure at the region from the surface layer to 0.1 D) / (an average block
grain diameter of the pearlite structure at a range from 0.25 D to a center) is
- less than 1 .O, and
an area ratio of a structure made up of a pearlite block whose aspect
5 ratio is 2.0 or more is 70% or more relative to a whole pearlite structure at a
region from a surface layer to 1.0 inm at a cross section in parallel to an axial
direction of the steel wire.
F1 =C(%)+Si(%)/24+Mn(%)/6 ... (1)
[Claim 51 The steel wire for the non-heat treated component according to
10 claim 4, further containing, in mass%,
one kind or two or more kinds fro111 among Al: 0.003% to 0.050%, Ca:
0.001% to 0.010%, Mg: 0.001% to 0.010%, Zr: 0.001% to 0.010%.
[Claim 61 A manufacturing method of a steel wire for a non-heat treated
component used for manufacturing the non-heat treated component whose
15 tensile strength is 900 MPa to 1300 MPa, comprising:
heating a steel billet containing, in mass%, C: 0.20% to 0.50%, Si:
0.05% to 2.0%, Mn: 0.20% to 1.0%, being limited to contain P: 0.030% or
less, S: 0.030% or less, N: 0.005% or less, F1 defined by the following
expression (1) is less than 0.60, with the balance made up of Fe and inevitable
20 inlpurities; hot-rolling into a wire material shape; coiling at a coiling
temperature of 800°C to 900°C;
cooling at a cooling rate of 20°C/s to 100°C/s fkoin a coiling finish
temperature to 600°C, further cooling at the cooling rate of 20°C/s or less
from 600°C to 550°C;
25 thereafter, isother~nally holding in a molten salt tank 1 at 400°C to
600°C and a successive molten salt tank 2 at 500°C to 600°C for 5 seconds to
150 seconds each;
subsequently cooling; and
.- thereafter, performing wire drawing at a total reduction of area of 15%
to 80%.
5 F1 =C(%)+Si(%)/24+Mn(%)/6 ... (1)
[Claim 71 A non-heat treated component whose tensile strength is 900
MPa to 1300 MPa, manufactured by cold-working a steel wire containing, in
mass%, C: 0.20% to 0.50%, Si: 0.05% to 2.0%, Mn: 0.20% to 1.0%, being
limited to contain P: 0.030% or less, S: 0.030% or less, N: 0.005% or less, F1
10 defined by the following expression (1) is less than 0.60, with the balance
made up of Fe and inevitable inlpurities,
wherein a metal structure contains a pearlite structure of 64 x ((2%) +
52% or more in a volunle fraction, with the balance made up of one kind or
two kinds of a pro-eutectoid ferrite structure and a bainite structure,
15 an average block grain diameter of the pearlite structure at a region
from a surface layer to 0.1 D is 15 pm or less when a diameter of the steel
wire is set to be D, and (the average block grain dianleter of the pearlite
structure at the region from the surface layer to 0.1 D) / (an average block
grain diameter of the pearlite structure at a range from 0.25 D to a center) is
20 less than 1 .O,
an area ratio of a structure made up of a pearlite block whose aspect
ratio is 2.0 or Inore is 70% or inore relative to a whole pearlite structure at a
region from a surface layer to 1.0 lum at a cross section in parallel to an axial
direction of the steel wire.
25 Fl=C(%)+Si(%)/24+Mn(%)/b ... (1)
. . [Claim 8.1 . . . .The non-heattreated component according to clairn 7,.further ~.
containing, in mass%,
one kind or two or Inore kinds from among Al: 0.003% to 0.050%, Ca:
0.001% to 0.010%, Mg: 0.001% to 0.010%, Zr: 0.001% to 0.010%.
[Claim 91 A manufacturing method of a non-heat treated component
5 whose tensile strength is 900 MPa to 1300 MPa, comprising:
heating a steel billet containing, in mass%, C: 0.20% to 0.50%, Si:
0.05% to 2.0%, Mn: 0.20% to 1.0%, being limited to contain P: 0.030% or
less, S: 0.030% or less, N: 0.005% or less, F1 defined by the following
expression (1) is less than 0.60, with the balance made up of Fe and inevitable
10 i~npurities; hot-rolling into a wire nlaterial shape; coiling at a coiling
temperature of 800°C to 900°C;
cooling at a cooling rate of 20°C/s to 100°C/s fsom a coiling finish
telnperature to 600°C, f~n-therc ooling at the cooling rate of 20°Cls or less
fron16OO~Cto 550°C;
15 thereafter, isothermally holding in a nlolten salt tank 1 at 400°C to
600°C and a successive nlolten salt tank 2 at 500°C to 600°C for 5 seconds to
150 seconds each;
subsequently cooling;
thereafter, perforn~ingw ire drawing at a total reduction of area of 15%
20 to 80%; and
further, perfor~ningc old-working.
F1 =C(%)+Si(%)/24+Mi1(%)/6 ... (1)
[Claim 101 The manufacturing illethod of the non-heat treated component
according to claim 9,
25 wherein after the wire drawing is perfor~ned, cold-working is
perforlned without perfor~ninga softening heat treatment.
[Claim 1 I] The manufacturing method of the non-heat treated component
according to claim 9, further comprising:
holding at 200°C to 600°C for 10 lninutes or more after the
cold-working is performed.