[Document Type] SPECIFICATION
[Title of the Invention] NON-IIEAT TREATED STEEL
[Tecluiical Field]
[OOOI]
The present invention relates to a non-lieat treated steel which is capable of
on~ittinga heat treatment including qnencliing and tenipering and which is performed
immediately after liot forging for fornling the steel into a steel component, and more
particc~larlyto a ~liaterialf or the steel component which is used with fracture splitting.
[Background Art]
[0002]
Recently, a non-heat treated steel to be hot-forged (hereinafter, referred to as a
non-heat treated steel) in which heat tleatliients can be onlitted is applied to a forged
component for a veliiclc engine and a forged colnponent for a \~cliiclcs uspension.
Tlte cliemical compositioti of tlie non-heat treated steel is designed to realize excellent
oieclianical ploperties even \\,lien tlie non-lieat treated steel IS as a~r-cooledo r as
forced-air-cooled after liot forging, that is, even \vI~ent hc lieat treatments including
qiic~ichi~aignd tempering according to the related att are omitted.
[0003]
As one of the coniponents to which the non-heat treated steel is widely
applied, there is a connecting rod (liereinaftel; called a conrod) for an engine. The
conrod is a component \vh~cIic onverts a reciprocating motion of a piston in the engine
into a lotating motion of a crankshaft to transmit power and is composed ofhvo parts,
i.e. a cap and a rod. The conrotl is mounted to tlie crankshaft by interposing the
crankshaft beheen tlie cap and the rod and fastening tlieoi with bolts. Hitherto, tlie
conrod is manofactured by separately forging tlie cap and the rod or mechanically
cutting a product forged to have a sliape in which tlie cap and the rod are integrated,
and thereafter processing joint surfaces of the cap aid the rod by n~acliiningw ith high
accuracy. I11 addition, in liiany cases, pin-cutting is performed to prevent tlie joint
surfaces fro111 misalignment. Therefore, the processing becomes more coniplex, and
tli11s there is a problem in that tlie mant~facturingc osts are increased.
[0004]
Therefore, in recent years, a method including hot forging a steel to form the
steel into a shape in which a cap and a rod are integrated, notching the inside of a large
end portion of tlie fornied pro(lnct, cold fracture splitting the fortncd product into the
cap and the rod by applying an impact tensile stress to the foi-nied procluct, and
moonting the cap and the rod to a crankshafi using the fractured surfaces thereof as
joint surfaces is employed. I11 this method, machining of the joint surfaces is
unnecessary. In addition, pin-cutting to prcvcnt ti~isalignmenct an be oniitted as
necessary using irregularities of tlie fractured surfaces. Therefore, the processing cost
of components can be reduced. Moreover, since the area of the joint surfaces can be
reduced by oniitting pins, it is possible to achieve reductions in tlie size and weight of
the conrod itself.
[0005]
In Europe and USA \\here such fiacture split conrod is widely supplied,
C70S6 in the DIN standards is supplied as asteel for the fracture split conrods. This
is a high carbon non-heat treated steel containing 0.7 weight% of carbon, and in oraer
to suppress changes in dimensions during frachlre splitting, almost the entire strncturc
thereof is a pearlite structure having low ductility and lo\\' toughness. An anlount of
plastic deformation of C70S6 in the vicinity of a fractured surface at the time of
fracture is sniall and thus C70S6 has excellent fracture separability. On the other
haid, C70S6 has a coarse structure compared to a ferrite-pearlite strocture of a medium
carbon non-heat treated steel \\~liichis a current steel for a conrod and tltus has a low
yield ratio (i.e. yield strengtl~tensilest rength). Tlicrcfore, tltere is a problem in that
C70S6 canllot be applied to a high strength conrod \vliicli requires high bockling
strength.
[0006]
In order to increase the yield ratio, it is necessary to control an amount of
carbon lo\\, and to increase a ferrite fraction. Ho\~revcrw, hen tlte ferrite fraction is
increascd, ductility and toughicss are enhanced, and thus the amount of plastic
deformation in the vicinity of tlie fractured surface during fracture splitting is increased,
resulting in an increase in the amount of deforniation of tlie inside diameter of the large
elid portion of tlte conrod. Therefore, tltere is a probleni in that the fracture
scparabiiitp is degraded.
[0007]
In order to solve the problems, a medic1111 carbon non-heat treated steels
having excellent fracture separability arc proposed. For example, in Patent
Docunients 1 and 2, a technique of adding a large aaliiou~tot f an e~nbrittlinge lenie~it
such as Si or P in order to degrade ductility and touglit~esso f a material itself so as to
improve fracture separability is described. In Patent Doc~~~nc3n atnsd 4, a tecliaiqoe
of degrading the ductility and tougllness of ferrite using precipitation strengthening of
second phase particles so as to improve fracture separability is described. In Patent
Docu~ne~i5t st o 8, a technique of controlling the form of Mn sulfides so as to improve
fracture separability is described. In Patent Document 9, a technique of heating a
steel to an ultra-11igl1 temperature close to a solidus line or a liquidas line in order to
. . significantly coarsen the structure oftlic steel so as to impmve fracture separability is
described. Ho\vever, in the techniqoes, while the amount of defotn~ationo fthe
fsactured surface obtained by the fsactore splitting is snlall, the material becomes
brittle, and thos chipping occurs during fracture splitting or during engaging the
fsach~~lresudr faces with each other. Chips of the fractnred surfaces cause
n~isaligtunenitn a position during the engagement between the fractured surfaces, and
thus there tilay be a problem in \\41ich the engagenlent cannot be performed with highaccuracy.
[Prior Art Docua~ent]
[Patcnt Document]
[0008]
[Patent Document 11 Japanese Patent (Granted) Publication No. 3637375
[Patent Document 21 Japanese Patent (Granted) Publication No. 3756307
[Patent Docun~en3t 1 Japanese Patent (Granted) Publication No. 3355 132
[Patent Document 41 Japanese Patent (Granted) Pt~blication No. 3988661
[Patent Docunient 51 Japanese Patent (Granted) Publication No. 431485 1
[Patent Document 61 Japanese Patent (Granted) Publication No. 3671688
[Patcnt Docu~nen7t 1 Japanese Patent (Granted) Publication No. 4268194
[Patcnt Docu~nen8t 1 PCT Lntemational Pitblicatio~N~o . W020091107282
[Patcnt Document 91 Japanese Patent (Granted) Publication No. 4086734
[Patent Document 101 Japanese Patent (Granted) Publication No. 4705740
[Disclosure of the lm~entio~i]
[Problems to be solved by the Invention]
[0009]
In consideration of the foregoing circumstances, an object of the present
invention is to provide a oon-heat treated steel in which an an~ounot f plastic
defornlation in the vicinity of a fractured surface at the time of frach~reis small,
chipping of the fractured surface is suppressed, and fracture separability is excellent.
[Means for Solving the Proble~n]
[OOl 01
The inventors have found that an anlount of deformation during fracture
splitting is reduced by including a larger amount of V than that of the related art and
chips of a fractured surface after the frach~resp litting can be reduced by reducing
segregation of V in a steel, and have completed the present invcl~tion. The subject
nlatter of the present invention is as described belo\v.
[OOll]
(a) A non-heat treated steel according to an aspect of the present in\lention
includes, as steel composition, by mass%, 0.20 to 0.60% of C, 0.50 to 2.0% of Si, 0.20
to 2.0% ofMn, 0.010 to 0.15% ofP, 0.010 to 0.15% of S, 0.10 to 0.50% ofV, 0.002 to
0.02% of N, and a balance consisting of Fe and itnpurities, in which when a ratio of a
~naximum value of a V content in the steel to an average value of the V content in the
steel in a cross section of the steel is defined as a segregation ratio of V, the segregatiol~
ratio of V is 1.0 or more and less than 3.0.
[0012]
(b) The non-heat treated steel described in (a) may filrther include at least one
of, by Inass %, 0.005% or less of Ca, 0.005% or less of Mg, and 0.005% or less of Zr.
[00 131
(c) The non-heat treated steel described in (a) or (b) may further include at
least one of, by Illass %, 0.25% or less of Cr, 0.10% or less of Ti, and 0.05% or less of
Nb.
[Effects of thc Invention]
[0014]
The non-heat treated steel according to the aspect of the present invention has
excellerlt fracture separability in wl~icha n atnount of plastic defonuatioa in the vicinity
of a fractured surface is small and chips of the fractured surface are stnall \\,hen
fracture splitting is performed after perfomling air-cooling or forced-air-cooling after
hot forging. Due to the characteristics in which the amount of plastic deformation of
the fractured surface is small and fi~rthermorec hips are small, the frach~reds nrfaces
can be engaged with each other with high-accuracy without n~isaligntnenti n position
during the engagenlent between the frachlred surfaces, thereby increasing the yield in
component prodt~ction. I11 addition, due to the characteristics, a process of
eliminating tile chips can be omitted, which resc~ltsin a reduction in the manufacturitlg
cost. This is extremely effective in the industty.
[Brief Description of the Dra\\~ing]
[OO 1 51
FIG. 1 is a diagram illustrating a specimen having a shalte cor~espondingto a
large end portion of a conrod used in a fracture separability evaluation test, in which
(a) is a plan view and (b) is a side vie\\:
FIG 2 is a diagram illustrating the relationship between a segregation ratio of
V and an amount of chip generation in a fractured surface.
[Embodiments of the invention]
[00 161
The inventors have intensively researched various factors which influence an
amount of plastic deformation in the vicinity of a fractured surface and \vhich
influence chipping of the frach~reds urface after fracture splitting and have obtained the
follo\\ing knowledge.
(I) By inclocling a large a~nounot f V, an amo~uiot f plastic defor~nationi n the
vicinity of the fractured surface after the fracture splitting can be reduced. During
cooling after hot forging, V carbides and V carbonitrides precipitate in a ferrite
structure and strengthen the ferrite with precipitation strengthening. Ductility and
toughness are redoced by strengthening the ferrite. Due to sufficient reductions in
ductility and tougluless, the anioulit of deformation after the fracture splitting is
reduced. However, generally, the fractured surface becomes brittle due to the
reductions in ductility and toughness, and accordingly, there may be cases where
chipping of the fractured surface occurs.
[0017]
(2) By reducing a segregation of V in a steel, such chips of the fractored
surface are reduced. The microstructore of the steel beconles inhomogeneous by
iticluditlg a large amount of V, since V is significantly segregated, the a~nounot f V
becomes irregular, and thus a ferrite transformation start temperature in the steel
becolnes irregular. When the steel is subjected to the fsacture splitting, the
inholllogeneous structlire significantly changes the crack propagation direction and
branchcs the crack to generate sub-cracking, which result in a large alnount of chips.
[OO 1 81
111 the present invention, "segregation of V" is defined as "segrcgation ratio of
V The "segregation ratio of V" refers to a ratio of a maximum value of V co~lte~init
the steel to an average value of V content in the steel (i.e. maximum valuelaverage
value) in a cross-section of the steel product.
[0019]
Hereinafter, the reason for limiting an amount of each of elements contained
in a stccl according to this e~nbotli~tle\n\rti ll be described. Here, " % regarding a
clie~iiicalc o~llpositionsll iealts "~iiass%.
[0020]
C: 0.20 to 0.60%
C has an effect of ensuring tensile strength of a component and an effect of
realizing good fracture separability by i~icreasi~alg v olume fraction of pearlite (i.e.
pearlite fraction), the pearlite having low ductility and lo\\, toughness, to reduce an
amount of plastic deformation ict the vicinity of the fractured surface at the time of
fracture. In order to obtain the effects, the lo\ver liiltit of the antount of C needs to be
0.20%. The lower limit of tlie arltou~lot f C is preferably 0.25%, and is more
preferably 0.30%. In ternis of i~nprovingf racture separability, tlie upper lilliit of the
amount of C does not need to be specified. Ilo\\~everw, hen C is excessively
contained, the pearlite fractioii becomes excessive, and the structule is coarsened,
resulting in a reduction in yield ratio, \vhich is itot preferable ia a case where the steel
is applied to a high stlength coltrod that requires buckling strength. Therefore, the
upper limit of the amount of C is 0.60%. The upper limit of the amount of C is
preferably 0.50%, atid is lliore preferably 0.48%.
[0021]
Si: 0.50 to 2.0%
Si strengthens the ferrite through solid solutioti strengthening, aiid reduces
ductility and tougllaess. The reduction in ductility and toughness reduces the amount
of plastic tleforlltation in the vicinity of the fractured surface at the time of the fracture
and tltus has an effect of realizing good fracture separability, In order to obtain this
effect, the lower limit of the amount of Si needs to be 0.50%. When Si is excessively
contained, the ferrite fraction may become excessive, and thus there may be cases
\\41ere the fracture separability of the steel is degraded. Thercforc, the ttppcr linlit of
the a~nounot f Si is 2.0%. The upper litnit of the anlount of Si is preferablp 1.5%, and
is niore preferably 1.25%.
roo221
Mn: 0.20 to 2.0%
Mn strengthens the ferrite through solid solution strengthening, and reduces
ductility and toughness. The reduction in ductility and toughness reduces the amount
of plastic defortnation in the vicinity of the fractured surface at the time of the fracture
and thus has all cffect of realizing good fracture separability. 111 addition, Mn
conibi~~teos S and fonus MI sulfide. Wlleen the steel is subjected to the fracture
splitting, the crack propagates along the MI sulfide that extends along a rolling
tlirection. Therefore, including Mu has an effect of enlarging irregularities of tlte
fractured surface and thus preventing misalignment in position during engagement
between the fractured st~rfaccs. III order to obtain the effects, the lolver limit ofthe
amount of Mn needs to be 0.20%. The lower limit of the amount of Mn is preferably
0.30%, and more preferably 0.45%. When MII is escessively contained, a lamellar
spacing of the pearlite is reduced, and the ductility and toughness of the pearlite is
increased. Therefore, the a~~~oouf nplta stic deformation in the vicinity of the
fractured surface at the time of the fracture is increased, and thos fracture separability
is degraded. Moreover, when Mn is excessively contained, a bainite strt~ctureis
likely to be generated, and thus there may be cases where fracture sepat.ability is
significantly degraded. Accordingly, the upper limit of the amount of Mn is 2.0%.
The upper limit of the amount of Mn is preferably 1.5%, is more preferably 1.2%, and
is even Inore preferably 1.0%.
[0023]
P: 0.010 to 0.15%
P reduces the ductility and touglll~esso f the ferrite and the pearlite. The
reductio~in~ d uctility and tougl~nessr educes the amoulit of plastic defortl~atioti~n the
vicinity of the fractured surface at the tillte of the fracture and thos has an effect of
realizing good fracture separability. 111 order to obtain this effect, the lower litnit of
the allloutlt of P needs to be 0.010%. The lower liniit of the amount ofP is preferably
0.030%. When P is excessively cot~tainedt,h ere may be cases \\,here hot ductility is
degraded and thus cracking and defects are likely to occur during hot working.
Therefore, the upper litl~iot f tlie amount of P is 0.15%. The upper li~niot f the
amount of P is preferably 0.10%, and is more preferably 0.070%.
[0024]
S: 0.010 to 0.15%
S is bonded to MI and forlns Mn sulfides. When the steel is subjected to the
fracture splitting, the crack propagates along the MI) sulfide that extends along tlie
rolling direction. Therefore, including S enlarges irregularities of the fractured
surface and thus has an etl'ect of preventing misalignment in position during
engagement bctwce~th~e fractured surfaccs. In order to obtain this effect, the lower
limit of the atliount of S needs to be 0.010%. When S is excessively coatained, the
amotlllt of plastic defor~nationin the vicinity of the fractored surface during the
fracture splitting is increased, and thus there may be cases ~vl~efrrea cture separability
is degraded. In addition, w11e1l S is excessively contailled, there may be cases \\there
hot ductility is degraded and thus cracking or defects are likely to occur during hot
o r k i g . Therefore, the upper limit of tile amount of S is 0.15%. The upper limit of
the amount of S is preferably 0.12%, and is more preferably 0.10%.
[0025]
V: 0.10 to 0.50%
V is an important elelllent in tlie steel accorditig to this embodiment. V
reduces dttctility and tougluiess by forniing nlainly carbides or carbonitrides during
cooling after tlie hot forging to strengthen tlie ferrite. The reduction in ductility and
tooghtiess reduces tlie amount of plastic deihr~nationin the vicitiity of the fractured
surface at tlie time ofthe fiacture and enables the steel to have good fiacture
separability. In addition, V has an effect of increasing the yield ratio of the steel by
the precipitation strei~gtkeningo f carbides or carbonitrides. In order to obtain this
effect, the lower liniit of the amount of V needs to be 0.10%. The lower litnit of the
an~ounot f V is prcfcrably 0.15%, and is illore preferably 0.20%. 011 the otllcr lia~~d,
even \vhen V is excessively contained, the effect is saturated, and thus the upper liniit
of the amount of V is 0.50%. Tlie upper limit of tlie amoant of V is preferably 0.35%.
[0026]
N: 0.002 to 0.02%
N acts as a trat~sfor~nationno cleus of the ferrite by forming inainly V nitrides
or V carbonitrides during cooling after hot forging to accelerate ferrite transfor~nation. ..
Accordingly, N has an effect of suppressing generation of the bainite structure that
sig~iifica~ithlya r~nsth e fracture separability of the steel. In order to obtain this effect,
the lower liniit of tlie amount of N ~~eettols b e 0.002%. Whea N is cscessively
contai~icdt,h ere may be cases where hot ductility is degraded and thus cracking and
defects are likely to occur during hot working. Therefore, tihe upper linlit ofthe
amount ofN is 0.02%. The upper li~iiiot fthe amount of N is preferably 0.01%.
[0027]
At least one of Ca: 0.005% or less, Mg: 0.005% or less, and Zr: 0.005%
All of Ca, Mg, and Zr form oxides to become crystallization nuclei or
. ~ . precipitation nuclei of Mn sulfides, atid thus utiifor~iilya nd finely disperse tlie~Mi
sulfide. The Mn sulfide lias an effect of becoming propagatio~pl aths of cracks during
tlie fracture splitti~lgto reduce tlie aliioulit of plastic defor~natioi~ni t lie vicinity of the
frach~reds urface aid to enhance fsacture separability. Wlieti Ca, Mg, and Zr are
excessively contained, the effect is satorated, and thus thc upper liniit of each of the
amounts of Ca, Mg, and Zr is 0.005%. hi order to sl~fficietltlye xhibit the effect, tlie
lower limit of each of the aalllol~ntso f Ca, Mg, and Zr is preferably 0.0005%.
[0028]
The stecl according to this c~iibodi~nem~aiyt furtl~crc o~ltaia~ti l east one of
0.25% or less of Cr, 0.10% or less of Ti, aid 0.05% or less of Nb as necessary.
[0029]
Cr: 0.25% or less
Si~iiilarlyto Mn, Cr strengthens the ferrite through solid solntion
strengthening, atid reduces ductility and toughness. The reduction ill dt~ctilitya nd
toughness reduces tlie amount of plastic deformation in the vicinity of tlie fractured
surface at the time of the fracture and tlius has all efl'ect of obtaining good fsach~re
separability. Ho\\,ever, when Cr is excessively contained, a latilellar spacing of the
pearlite decreases, and tlie ductility and toughness of the pearlite are increased.
Therefore, the arnotrnt of plastic deformation in tlie vicinity of the frachlretl surfacc at
the ti~ilco f the fsacture is increased, and tlius fracture separability is degraded.. In
additioo, when Cr is excessively contained, a bai~iites tructure is likely to be generated,
atid tlius there may be cases \\lliere frachire separability is significantly degraded.
Accordingly, in a case \\4iere Cr is contained ill order to obtain the above-described
effects, the amoutlt of Cr is 0.25% or less. The upper litiiit of tlie a~iiounto f Cr is
preferably 0.15%. In order to sufficiently exhibit the effect of Cr, the lower litiiit of
the amount of Cr is preferably 0.01%. ~ ~ ~ . .
[0030]
Ti: 0.10% or less
Ti fornis niaillly carbides or carbotiitrides during coolilig after hot forging,
strengthens the fcrrite through precipitation strengthening, and reduces ductility and
toughness. The reduction in ductility and toughness has an effect of redocing the
atlioont of plastic defomiation in tlie vicinity of the fractured sutface at the titlie of tlie
fsach~reto obtain good fracture separability. However, whet1 Ti is excessively
contained, the effcct is saturated. Therefore, in a case wllere Ti is colltaitied in order
to obtain the above-described cffcct, the upper litnit of thc alnount of Ti is 0.10%. In
order to sofftcielttly exbibit tlie effect of Ti, the lower limit of tlie amount of Ti is
preferably 0.005%. An appropriate range of the amount of Ti is 0.010 to 0.030%.
[0031]
Nb: 0.05% or less
Nb forms mainly carbides or carbonitrides during cooling after hot forging
and strengthens the ferrite through precipitation strengthening to reduce ductility and
touglincss. The reduction in ductility and tough~lessh as an effect of reducing the
amount of plastic defor~ilatiotiin the vicinity of tlie fractured sl~rfacea t the titlie of the
fracture to obtain good fracture separability. Ho\vever, \\?lien Nb is escessivcly
contained, tlie erect is satorated. Therefore, in a case where Nb is contained in order
to obtain the above-described erect, the upper limit of the amount of Nb is 0.05%. In
order to sufficiently exhibit the effect of Nb, the lower limit of tlie amount of Nb is
preferably 0.005%. An appropriate range of the atnount of Nb is 0.01 0 to 0.030%.
[0032]
The balance of the steel according to this e~llbodi~ilein~ciltu des iron and
impurities; The impurities are referred to as those incorporated from raw materials
such as ore or scraps and fro111 a matiufacturing environmetlt. Moreover, the steel
accordi~lgto this e~iibodimentm ay contain Te, Zn, Sti, and the like in addition to the
above-described elements in a range that does not hart11 the effects of the steel
accordi~lgto this embodiment.
[0033]
Next, the reason for the segregation ratio of V of the steel being 1.0 or Inore
and less than 3.0 will be described.
[0034]
W11e1i a large amount of V is contained, the steel has low ductility and low
toughness, arid the amount of plastic defor~i~atioinn tlie vicinity of the fractured
st~rfaccd uring the fiacture splitting is reduced. On the other band, when a large
a~uoutito f V is contained, the fractured surface beco~ilesb rittle and the chipping is
likely to occur. When a large amoutit of V is contained, segregation of V significantly
occurs and thus tlie strtlcture after the hot forging becomes inhomogeneous. This
significantly cliatiges the propagation directio~io f cracks and blanches tlie cracks to
generate sub-cracking during the fracture splitting to the stccl. This results in a large
amount of chips. The it~vctitorsh ave researched the relationship between the
segregation ratio of V and thc chipping of the fractured surface.
[0035]
A steel havi~iga composition including 0.38% (mass%, and the same applies
to the follo\ving) of C, 0.88% of Si, 0.69% of MII, 0.054% of P, 0.073% of S, 0.30% of
V, 0.0104% of N, and tlie balance consisting of Fe and tlie i~iipt~ritiwesa s melted in a
converter to be manufactured by continuous casting, and was subjected to hot rolling to
have a steel bar shape having a diameter of 56 cnm. At this time, a plurality of steel
products \\it11 varying segregation ratios of Vwere brcpared byadjusting whether or . ~ .. ..
not to perfonit electroniagnetic stirring in a die during the continuous casting, a degree
of superheating molten steel in a tondish (13 to 52OC), aid a rolling reduction gradient
(0.0 to 3.0 1im1/m) during light rolling reduction of a final solidification portion.
[0036]
The segregation ratio of V is,au index which represents a degree of the
segregation of V. Here, by using an electron probe ~nicroanalyzer( EPMA), line
analysis was performed on a cross-section perpendicular to a hot rolling direction of
the steel bar having a diameter of 56 111111 in a radial direction from tlie surface to the
center and from the center to the surface, tlie niaxinlum vaiuc and the average value of
the V content \\,ere measured, aid the ratio (i.e. tlie niaxiniun~v alue/thc average value
of tlie V co~icentratio~thi)e reof was calculated. Therefore, the value of the
segregation ratio is high when the segregation is significant, and the value of the
segregation ratio is 1.0 when no segregation occurrs.
[0037]
In order to evaluate tlie chipping of tlie fiactt~reds urface, a specimen
corresponding to a forged conrod was manufactured by hot forging. Specifically, a
steel bar having a dianieter of 56 nun and a length of 100 nilu was heated at 1250°C,
was thereafter forged in a direction perpendicillar to the lengthwise direction of the
steel bar so as to have a thickness of 20 mm, and was filrtlier cooled to room
temperature by air-cooling (being left in the air). Thereafier, the steel bar is subjected
to cutting work to be used as a sltecitiien having a shape corresponding to a large end
portion of tlie conrod. In tlie specimen, as illustrated in FIG. I, a hole having a
dianieter of 50 111111 mas bored in the center portion having a plate shape having
dimensions of 80 mmx80 nun and a thickness of 18 om, and 45-degree V notches
having a dcptli of 1 mm and a tip curvature of 0.5 num were forn~edo n the inner
surface of tlie hole having a diameter of 50 mtn at hvo points positioned at 590 degrees
with respect to tlie lengtl~wised irection of the steel bar which is a n~ateriabl efore
fo~ging. Fu~tlier~norteh,r ough-holes having a diameter of 8 nnn as bolt holes were
nade to be positioned at points where the center lines thereof were positioned at 8 mln
fioni the side surfaces where tthe notch was forn~ed.
[0038]
A fiacture splitting apparatus is constrt~ctedf rom a separated die and a dropweight
tester. The separated die has a shape in which a colunui having a diameter of
46.5 tnrn and fortiled on a rectangular steel is divided into two parts along tlie ccriter
line thereof One divided part of the column is fixed, and the other divided part
tnoves on a rail. Wedge holes are fornied in joint surfaces of the two selnicirc~~lar
col~tmns. 111 order to fracture tthe specimen, the column having a diameter of 46.5 nit11
of the separated die is fitted into the hole having a diameter of 50 mm of the specimen,
a \\,edge is inserted therebet\veen, and installed on a drop-weight. The drop-\!,eight
has a mass of200 kg, and has a niechanism of falling along a guide. When tlie dropweight
falls, tlie \vedgc is stuck, and tlie speci~nenis tensile fractured into hvo parts.
111 addition, so as not to cause the specnilen to be separated fro111 the sepatated die at
the time of the fiactore, the periphery of tlie specimen is fixed to be pressed by the
separated die.
100391
In this test, after the specimen is fractured under a drop-weight height of 100
nun, an opelation of facing and fastening the fractured surfaces with a bolt under a
torque of 20 N.m to be assembled with each other and an operation of nc~fasteriiiigth e
bolt to release the frach~reds urfaces are repeated ten times, and the total weight of
fraglnents getie~atetal t the operations is defined as an amount of chip generation of ttic
fractured surface.
[0040]
FIG. 2 illustrates the relationship between the segregation ratio of V and the
anlol~nto f chip generation of the fractured surface. The a~nounot f chip generation of
the fract~lreds urface is reduced due to the reduction in the segregation ratio of V. In
order to suppress the amount of chip generation to be 1.0 nlg or less for the purpose of
omitting a process of eliminating the chips, the segregation ratio of V needs to be less
than 3.0. Therefore, the upper limit of the segregation ratio of V was set to be less
than 3.0. In order to fnrther suppress tlie amount of chip generation, the segregation
ratio of V is preferably 2.5 or less, atld more preferably 2.0 or less.
[0041]
As described above, control of the segregation ratio ofV can be realized by
adjusting \\,l~etl~oer not to perfonn the electromagnetic stirring in thc die during
continuous casting, tlie degree of superheating molten steel in the tundish, and the
rolling reduction gradient during light rolling reduction of the final solidification
portion. M~licnth e electro~~~ag~st~ireritnigc is performed, the degree of sopcrl~cating
molten steel in the tondish is 13°C or higher and 40°C or less, and the rolling reduction
gradient during the light rolling reduction of the final solidification portion is 0.5
mmlm or higl~cra ncl2.0 nlndm or less, the segregation ratio of V can be 1.0 or higher
and less than 3.0.
[0042]
Examples of the present invention fill be described below in detail. In
addition, the examples are for explaining the technical meaning and effects of the
present invention and do not limit the scope of the present invention.
[Examples] .~~ ~ ... . ..
[0043]
A bloom was produced by continuous casting of a steel having a contposition
sho\\811 in Table 1 and melting in a converter. The bloom was subjected to bloo~nit~g
to be for~iiedin to a billet of 162 nun square, and tlien was subjected to hot rolli~igto be
formed into a steel bar shape having a diameter of 56 mln. The symbol "-" hi the
table represents that the amount of the element associated with the positio~w~it h the
symbol is equal to or less than its detection limit vale. Furthenuore, steel'products
\\?erep repared in which the segregatiot~r atios of V thereof are varied by adjusting
whether or not to perfonn electro~~~agnesttiircri ng in a die during continuous casting, a
degree of superheating ~noltens teel in a tundish, and a rolling reductiot~g radient
during light rolli~~regd uction of a final solidification portion as shown in Table 2.
LVlien the electro~liag~ietsitci rring was perfor~~ietdli,e stirring was perfofom~edu nder a
flo\v ratc of 65 cm/scc. hi addition, the steel \\,as poured into the die in a rangc of the
degree of superheating molten steel in a tundish of 13 to 52OC, and the rolling
r e d ~ ~ c t\\i'aos~ p~e rfo~n~ethde reon in a range of the rolling reduction gradient duriog
light rolli~~regd uction of a final soiidificatio~pi ortion of 0 to 1.4 mmnh. A heating
temperature and a heating time for the bloo111 before the blooming \\,ere respectively
1270°C and 140 min, and a heating temperature and a iieatulg time for the billet before
thc hot rolli~~\vge rc respectively 1240°C and 90 inin. The i~~~derlip~oirteiodn in the
comparative steels of Table 1 represent that they are not in tlie range of the present
invention.
[0044]
['rable 11
[0045]
[Table 21
[0046]
Next, in order to exatiiine the degree of segregatiot~o f V, by using tlie electron
probe microanalyzer (EPMA), line analysis was perforti~edo n a cross-section
perpendicular to a hot rolling direction of the steel bar having a diameter of 56 mm in a
radial direction fronl the surface to the center atid fro111 tlie center to tlle surface, the V
content distribution was measured, and tlie segregation ratio which is the ratio of tlie
maximum value to tlie average value of the V content was calcolated.
[0047]
Subsequently, in order to exallline fracture separability and mechanical
properties (tensile property), the speciti~enc orresponding to the forged conrod was
produced by hot Corging. specific all^: an element steel bar having a diameter of 56
mm and a length of 100 IIIIII \\'as lieated to 1150 to 128OoC, was tl~ereaflerf orged in a
direction perpendicular to thc lengthwise direction oftlie steel bar so as to have a
thickness of 20 mm, and were ft~rtlierc ooled to room temperature by air-cooling (being
left in the air). AJlS No. 4 tensile specinlen and a specimen for fracture separability
evaluation having a shape corresponding to tlie large cnd portion of the conrod were
cut fro111 the forged niaterial after the coolinig. Tbe JIS No. 4 tensile specin~en\b ras
collected along tlie longitl~dinadl irection at a position of 30 mm froni tlie side surface
of the forged material. In tlie specimen used in a fracture separability evaluation, as
illustrated in FIG. 1, a hole having a dia~neteor f 50 ~ii~\viais bored in the center portion
having a plate shape having dimensions of SO mmx80 n11n and a tliicliness of I8 mm,
and 45-degree V notches having a depth of lmm and a tip cut~atureo f 0.5 mm were
fomied on the inner surface of tlie hole having a dian~eteor f 50 ninl at hwco points
positioned at +90 degrees with respect to the lengthwise direction of the steel bar
. . . ~vl~icish a material before tlie forging. Furtherti~ore,t hroogli-holcs having a dianieter
of 8 nun as the bolt holes were made to be positioned at points of lie re the center lines
thereof were positioned at 8 ninl from the side surfaces where the notch was fomled.
[0048]
Atesting machine for the fracture separability evaluation is constituted by a
separated die and a drop-weight tester. The separated die has a shape in which a
colu~iinh aving a dianleter of 46.5 mm and formed on a rectangular steel is divided into
two parts along the center line thereof. One divided part is fixed, and the other
dividetl part iiloves on a rail. Wedge holes are fornled i~tjoinstu rfaces of the two
senlicircular colu~nns. During tlie fracture test, the column having a dianteter of 46.5
mnt of the separated (lie is fitted into the hole having a dianlctcr of 50 tutn of the
specimen, a wedge is inserted therebetween and installed on a tlrop-weight. The
drop-weight has a Illass of 200 kg, and has a mechanism of falling along a guide.
When the drop-weight falls, the wedge is stuck, and the specimen is tensile fractnred
into hvo parts. In addition, so as not to cause the specimen to be separated from the
separated die at the time of tlie fracture, the peripheiy of the specimen is fixed to be
pressed by the separated die.
[0049j
In this test, the fractore was perfortiled under a drop-weight hcight of 100 nint,
the specimens after the fracture were allowed to face each other and fastened by a bolt,
and the difference between the inner dianteter in a fracture direction and the inner
diameter in a direction perpendicular to the fracture direction was measured and \\>as
defined as the amount of deformation by the fracture splitting. Thereafter, an
operation of facing and fastening the fractured surfaces with a bolt at a torque of 20
N.m to be assembled with each other and an operation of unfastening the bolt to
release the fract~~resudr faces are,repeated ten times, and the total wcight offragments . . .
generated at the operations is defined as a11 amount of chip generation of the fractured
surface. Regarding tile fracture separability, the specimen having a11 aniount of
deformation by tlie fracture splitting of higher than 100 pm or having an aniou~lot f
chip generatioti of the fracturcd surface of higher than 1.0 mg was regarded as not
reaching the target.
In addition, regarding a yield ratio, the specimen having a yield ratio of less
than 0.70 was regarded as not reaching the target. Regarding an elongation, the
specimen having an elotigation of higher than 18% was regarded as not reaching the
target.
[OOSO]
It was seen that all of Invention Examples of test Nos. 1 to 22 achieve the
targets and have excellent fsactore separability. On the other hand, the amounts of C,
Si, Mn, P, and V in test Nos. 23 to 26,28, and 30 were not in the rangc of the present
invention. Therefore, in test Nos. 23 to 26,28, and 30, the ferrite fractions were high
or the ductility of tlie ferrite and pearlite str~~cturwesa s not sufficiently reduced and
resulted in high ductility, and thus the amount of defomiatioti during the fracture
splitting was high and fractt~res eparability \\,as poor. The amounts of Mn and Cr in
test Nos. 27 and 3 1 were not in tlie range of the present invention. Therefore, in test
Nos. 27 and 31, a bainite structure is generated or the doctility of tlie pearlite structure
was not sufticiently reduced, and thas the amount of deformation during the fracture
splitting was high and the fracture separability mas poor. The amount of S in test No.
29 \!,as not in the range of the present invention. Therefore, in test No. 29, the
amount of Mn sulfides having a high aspect ratio was increased such that separation
had occurred, which results in cracks parallel to the estension direction of tlie Mn
sulfides. Accordingl): intest No. 29, the amount of deforniatio~di uring the fiactorc . . . . . .. . . ... .
splitting was high and frachlre separability was poor. In test Nos. 32 to 38, althougl~
the steel compositions of the steel were in the range of the present invention,
electronlagnetic stirring in the die during the continuous casting was not performed,
the degree of superlieating molten steel in a tundish was above 4OoC, or the condition
during light rolling reduction of a final solidification portion was not in the specified
range. Therefore, in test Nos. 32 to 38, the segregation ratio of V was 3.0 or higher,
and the aniount of chip generation of the fractured surface bad not reached the target.
[Industrial Applicability]
[0051]
The non-heat treated steel of the present ilivelltion bas excellelit fracture
separability in which an amount of plastic deformation in the vicinity of a fractured
surface is snlall and chips of the fractured st~rfacea re small when the non-heat treated
steel is hot forged, air-cooled or forced-air-cooled, and then fracture split. Due to the
characteristics in \\lhich the atnotint of plastic deformation of the fractured surface is
snlall and furthermore chips are small, the fsactured surfaces can be engaged with each
other with good accoracy without n~isalignn~einnt position during the engagement
bet\veeo the fractured surfaces, thereby increasing yield in prodoction of parts. In
addition, due to the characteristics, a process of eliminating the chips can be omitted,
which results in a reduction in manufacturing cost. This is extremely effective in the
industry.
[Description of Reference Ninuerals and Signs]
[0052]
1: SPECIMEN
2: HOLE
3:. VNOTCH ., . ~
4: THROUGH-HOLE

TABLE. 2 DEGREE OF
ROLLING FRACTURE SEPARABlLI TY TENSILE PROPERTIES
PERFORMING OF SUPERHEATING SEGREGATION AMOUNT OF AMOUNT OF TNEoS. T CLASSIFICATION ELECSTTRIORMRAINGGN ETIC MIONLT TEUNN SDTISEHE L ROLLINLIGG HT OF DEFORMATION STYRIEELNDG TH STTERNESNILGET H
E L O ~ ~ ~ T 1 O N
("C) (r-dn) (me> (MPa) (MPa)
-%
* THE UNDERLINED PARTS ARE UNDER CONDITIONS OUT OF THE RANGE OF THE PRESENT INVENTION.

[Document Type] CLALMS
[Claim 11
Anon-heat treated steel comprising, as steel conlposition, by mass%,
C: 0.20 to 0.60%,
Si: 0.50 to 2.0%,
Mn: 0.20 to 2.0%,
P: 0.010 to 0.15%,
S: 0.010 to 0.15%,
Y 0.10 to 0.50%,
N: 0.002 to 0.02%, and
a balance colisisting of Fe and impurities, \vI~erein
nrlien a ratio of a maxini~tmv alue of a V content in the steel to an average
value of tlie V content in tlie steel it1 a cross section of the steel is defined as a
segregation ratio of V, the segregation ratio of V is 1.0 or more and less than 3.0.
[Claim 21
The non-heat treated steel according to claim 1, fiirtlier comprising at least
one of, by niass %,
Ca: 0.005% or less,
Mg: 0.005% or less, and
Zr: 0.005% or less.
[Claini 31
'She non-heat treated steel accordilig to claim 1 or 2, fi~rtlierc oniprisiag at
least one of, by mass %,
Cr: 0.25% or less,
Ti: 0.10% or less, and
Nb: 0.05% or less.