hot-rolling, and thus, induction l~ardcnabilit)o~f the steel discloscd in the patent
documents 5 to 7 is insufficient.
[Prior Art Docunnents]
[Patent Docun~ents]
5 [0004]
[Patent Document I] Jal>anese unexanlined patent application, First Poblication
No. S60-141832
[Patent Docun~er2~1t Japanese ~unexaminedp atent applicatiot~,F irst Publication
No. S62-103323
10 [Patent Doc~unen3t 1 Japanese unexamined patent application, First Poblication
No. S62-013523
[Patent Document 41 Japanese unexamined patent application, First Publication
No. HI-039324
[Patent Document 51 Japanese unexa~ninedp atent applicatiot~F, irst Publication
15 No. S6l-048521
[Patent Document 61 Japanese ut~examinedp atent application, First Publication
No. H2-213415
[Patent Doc~unent7 1 Japanese unexarnined patent application, First Poblication
No. 2010-168624
20 [Sununary of Invention]
[Technical P~.oble~n]
[OOOS]
In vie\\, of the above, tlie object of tlie present invention is to provide a
hot-rolled and directly-quenched steel bar for induction hardening, and to provide a
steel bar which is a t~~cdiuctuar bon steel; has esccllcr~ct rack propagatiou stopping
propcrtics and excelleut low tcmperature toughness; has excellent induction
hardenability and excellent machinability; has unifornl hardening depth; is
tl~anufacturedb y a t~~ethowdh ich does not include a thernial refining process; and has
5 high plodnctivity.
[Method for Solviug the Problem]
[0006]
The itlventors have conducted research to solve the above-described problems.
As a result, the inventors found that it is necessary to control the corllpositiotl of the
10 steel bar as well as optimize the method for manufach~ritlgth ereof to enhance crack
propagation stopping prope~liesl,o w tenlperahlre toughness, productivity, and induction
-
hardeiiabilit)~o f the hot-rolled and directly-q~iencheds teel bar for induction hardenitig,
which is a medium carbon steel. 111 particular, the it~vetitorsfo und that adequately
cotltrolling the heating tenlperah~rea nd heating time before hot-rolling; controlling the
15 hot-rolling tetnperature (especially, finish rolling temperature); controlling the flow
velocity of cooliug water to obtain a structure in which the bcc phase is fiue and the
total decarburization is low; adequately controlling water film thickness of the cooling
water and the reheating temperature to suppress unevenness of the structure of the steel
bar in the circumferetltial and longitudinal directions in order to provide adequate
20 hardness to tlie steel bar are nseful. In the present iuvet~tion",a steel bar of \vhich
induction bardenability is enhanced" indicates a steel bar in which tlie structure has a
predetermined hardness corresponding to the amount of carbon and unevenness of
hardness, and the structure of the steel bar is small after induction hardening.
[0007]
The preseut invcntion was achieved bascd on the abovc-dcscribed novcl
findings, and a summary of the present invention is as follows.
(1) A steel bar according to one e~nbodituento f the present invention includes,
as a cbernical co~npositioi~nl terms of mass%: C: 0.30 to 0.80%; Si: 0.01 to 1.50%; Mn:
5 0.05 to 2.50%; Ai: 0.010 to 0.30%; N: 0.0040 to 0.030%; P: 0.035% or less; S: 0.10%
or less; Cr: 0 to 3.0%; Mo: 0 to 1.5%; Cu: 0 to 2.0%; Ni: 0 to 5.0%; B: 0 to 0.0035%;
Ca: 0 to 0.0050%; ZI.: 0 to 0.0050%; Mg: 0 to 0.0050%; Rem: 0 to 0.0150%; Ti: 0 to
0.150%; Nb: 0 to 0.150%; V: 0 to 1.0%; W: 0 to 1.0%; Sb: 0 to 0.0150%; SII: o to
2.0%; Zn: 0 to 0.50%; Te: 0 to 0.20%; Bi: 0 to 0.50%; Pb: 0 to 0.50%, and a re~nai~lder
10 including Fe and impurities, wl~ereitla region which is along a liue extending between a
center of a cross section of the steel bar and a periphery of the cross section of the steel
-
bar and which has a hardness higher than the average hardness in the line by Hv20 or
Inore is a hardening region in the line, the minitnutn value of depth of the hardening
regions in the 8 lines of which the angle is 45" is the minimum hardening depth in the
15 cross section, and the maximum value of the depth of the hardening regions in the 8
lines is the tnaxilnnm hardening depth in the cross section, whcreitl the difference
between the ~naximu~hnar dening depth in the cross section and the mit~imutnh ardening
depth in the cross section is 1.5 mm or less, wherein the difference between the
maxinlurn value of the ~naxitnumh ardening depth and the mininntin value of the
20 maximum hardening depth it1 the cross sections at 3 points which are separated fiom
each other by 1650 nlm palallel to a longitudinal direction of the steel bar is 1.5 U I I ~o r
less, wherein the difference between the maximun~v alue of the minimun~h ardening
depth and the minimum value of the minimum hardening depth in the cross sections at
the 3 points which are separated &om each other by 1650 nun parallel to the
longitudinal direction of the steel bar is 1.5 rnm or less, \vherein a structure in an area
from a surface of the steel bar to a dcpth of 25% of a radius of the steel bar includcs 10
area% or less of a ferrite and a retnainder including one or more selected &om a group
consisting of a bainite and a mallensite, wherein a boundary between grains which are
5 adjacent to each other and of which an orientation difference is 15 degree or tnore is a
grain boundary, and an equivalent circle diameter of an area surrounded by the grain
boundary is a grain size, wherein the average value of the grain size of a bcc phase in
the area from the surface of the steel bar to the depth of 25% of the radius of the steel
bar is 1.0 to 10.0 ~LIIwI, herein the average value of the grain size of the bcc phase in an
10 area from the depth of 50% of the radius of the steel bar to the center of the steel bar is
1.0 to 15.0 pnl, wherein a hardness of a region of which a depth from the surface is 50
~ - -
ltm is Hv200 to Hv500, and wherein a total decarburized layer thickness DM-T is 0.20
mm or less.
[0009]
15 (2) The steel bar according to (1) tnay include, as the chenlical composition in
ternls of mass%: one or Inore selected from thc group consisting of Cr: 0.1 to 3.0%;
Mo: 0.10 to 1.5%; Cu: 0.10 to 2.0%; Ni: 0.1 to 5.0%; and B: 0.0010 to 0.0035%.
[OO 101
(3) 'l'he'stzel.bar according to (1) or (2) may include, as the chen~ical
20 composition in tenus of mass%: one or more selected from the group consisting of Ca:
0.0001 to 0.0050%; Zr: 0.0003 to 0.0050%; Mg: 0.0003 to 0.0050%; and Rem: 0.0001
to 0.0150%.
(4) The steel bar according to any one of (I) to (3) nlay include, as the
cl~cmicacl onlposition in tcrtns of mass%: one or tnore selectcd fro111 the group
cotlsisting of Ti: 0.0030 to 0.0150%; Nb: 0.004 to 0.150%; V: 0.03 to I .0%; and W:
0.01 to 1.0%.
100 121
5 (5) The steel bar accordit~gto any one of (1) to (4) may itlclude, as the
chemical con~positionin terms of mass%: one or more selected from the group
consisting of Sb: 0.0005 to 010150%; Sn: 0.005 to 2.0%; Zn: 0.0005 to 0.50%; Te:
0.0003 to 0.20%; Bi: 0.005 to 0.50%; and Pb: 0.005 to 0.50%.
[Advantageous Effects of It~vet~tion]
10 [0013]
Hot-rolled and directly-quenched steel bar for induction hardening according to
-
the above-described embodiments has high crack propagation stopping properties, and
the base material has low temperature tougluless. Further, the unevenness of the
hardening depth after hot-rolling of the steel bar is small, even if thermal refining is not
15 performed. Therefore, the present inventiotl can obtain a steel bar which is excellent in
productivity and induction hardenability.
[Brief Description of the Drawing]
[0014]
[Figure 11.. ." Figure showing distribution of hardening depth in cross
20 section of steel bar according to one embodiment of the present invention.
[Figure 21 Figure showing positions in longitudinal directiol~a, t which
the cross sectiotls of tl~est eel bar according to the one embodiment of the present
invention are observed.
[Figurc 31 Figure showing constructio~of~ t he steel bar according to the
one enlbodinle~lot f thc prcsent in\lention.
[Figure 41 Figure showing positions at which a grain size of a bcc phase
in the cross section of the steel bar according to the one embodiment of the present
5 invention is measured.
[Figure 51 Figure showing an exanlple of outline of rolling line and
water cooling apparahis const~uctingm anufacturing apparatus for the steel bar
according to the one enlbodinlent of the present invention.
[Figure 61 Figure sl~oo\\inagn example of outline of the water cooling
10 apparatus constructing mat~ufacturinga pparatus for the steel bar according to the one
e~libodinlenot f the present invention.
[Figure 71 Figure showing an example of o~itlineo f the water cooling
apparatus constructing tnanufachlring apparatus for the steel bar according to the one
embodiment of the present invention.
15 [Figure 81 Figure showing an example of outline of rapid-cooling just after
rolling and rchcating during method for nlanufactuting the steel bar according to the one
crnboditnent of the present invention.
[Description of Etnbodimet~ts]
[OOlS] -"
20 Hereinafter, details of an embodiment of the present invention (hereinafter,
called the present embodiment) \\ill be described.
[OOI 61
First, the reason for li~nitingt l~cl ~cniicalc onlposition of the steel bar
according to the presetit etiibodinicnt \will be described. Hereinafter, the anlounts of
alloy co~npositionsin mass% will sitnply be described as "%.
[0017]
5 (C: 0.30 to 0.80%)
C is an element having a great effect on strength of the steel bar. If an amount
of C is less than 0.30%, sufficient hardness cannot be obtained after induction hardening.
On the other hand, if the atnount of C is tnore thau 0.80%, a large atnount of residual
austenite fornis during the itlductiot~h ardenit~ga nd prevents the hardness iucreasing.
10 Tllerefore, the atnount of C of the steel bar according to the present embodiment is 0.30
to 0.80%. 111 order to advantageously obtain the above-described effects, the lower
litnit of the amount of C is preferably 0.40%, atid more preferably 0.50%.
roo1 81
(Si: 0.01 to 1.50%)
15 Si is an eletnent effective for deoxidizing the steel, as well as effective for
strengthening fcrrite and increasing resistance to temper softening. If an amount of Si
is less than 0.01%, the effect is insufficient. If the amount of Si is more than 1.50%,
material property is deteriorated due to etnbrittletueut of the steel bar, and
carburizability &deteriorated. Therefore, it is necessary that the amount of Si is
20 within a range of 0.01 to 1.50%. In order to ad\~antageouslyo btain the
above-described effects, the lo\wrer limit of the amount of Si is preferably 0.03%, and
more preferably 0.05%. The upper limit of the amount of Si is preferably 0.50%, and
more preferably 0.40%.
[0019]
(Mn: 0.05 to 2.50%)
Mn fixes S in the steel as MnS. MnS disperses in the steel. In addition, MII
is an elemeut necessary for increasing hardenability of the steel and for securing
stre~lgtho f the steel after quet~chingb y forming solid-solution of Mn with matrix.
5 However, ifthe amount of Mn is less than 0.05%, S in the steel co~nbinesw ith Fe to
for111 FeS which ernbrittles the steel. On the other hand, if the amount of Mn is more
than 2.50%, the above-described effects of Mn on the strength and the hardenability is
satu~.ated. Therefol-e, the amount of Mn is 0.05 to 2.50%. In order to obtain the
above-described effects more efiie~ltlyt,h e preferable lower litnit of the a~nounot f MII
10 is 0.20% and a more preferable lower limit of the amount of Mn is 0.30%. The
preferable apper litilit of the amount of Mn is 1.80% or less and a more preferable upper
limit of the amount of Mn is 1.60%.
[0020]
(Al: 0.010 to 0.30%)
15 Al has a deoxidizing effect. In addition, Al forms Al nitride (AIN), and
suppresses coarscuing of grain. Further~l~orAe,l fixes solid-solution N it1 the stccl as
AlN. If B is included it1 the steel, the solid-solutionN cotnbir~esw ith B in the steel to
fonu BN, and decreases the amount of solid-solution B. If B is i~lciudedin the steel,
A1 is effective for securing the amount of the solid-solution B which increases
20 hardenability. In order to obtain the above-described effects, it is necessary that
0..010% or more of A1 is included. On the other hand, if the amount of Al is excess,
,41203 fomms, and deteriorates fatigue strength as well as causes cold-forging crack.
Therefore, it is necessary that the apper limit of the amount of A1 is 0.30%. In order to
obtain the above-described effects more eEciet~tlyp, referable lo\ver litilit of the amount
of A1 is 0.015%, and a more preferable lowcr limit of the amount of A1 is 0.020%.
Thc i~rcfcrablcu pper lilnit of the amount of A1 is 0.25% or less and a more preferable
upper litnit of the amount of A1 is 0.15%.
[0021]
5 (N: 0.0040 to 0.030%)
N combines with AI, Ti, Nb, atid V in the steel to form fine nitrides or fine
carbonitridcs. The fine nitrides or the fine carbonitrides have an effect for suppressing
coarselling of the grain. If the alnoutlt ofN is less that1 0.0040%, the effect is
insufticient. If the atnount of N is more thaat10.030%, the effect is saturated. In
10 addition, if the atnount of N is more than 0.030%, carbonitrides which does not fonn
solid-solution during heating at hot-rolling or during heating at hot-forging retilain in
the steel bar, and the amount of the fine carbonitrides which is effective for suppressing
coarsening of the grain decreases. Therefore, it is necessary that the amount of N is
within a range of 0.0040 to 0.030%. In order to obtain the above-described effects
15 more efficiently, preferable lower litnit of the atnount of N is 0.0045% and a tnore
preferable lowcr limit of the amoutit of N is 0.0050%. Thc preferable upper litnit of
the amount of N is 0.015% or less and a more preferable upper limit of the atnount ofN
is 0.010%.
[0022] . . . . ..
20 (P: 0.035% or less)
P is an impurity element. If the amount of P is tnore than 0.035%, casting
property and hot wwrorkability deteriorate. In addition, if the amount of P is more than
0.035%, the hard~iesso f the steel bar before qoenchiog increases, and the niachinability
of the steel bar deteriorates. Therefore, the atnount of P is 0.035% or less. In order
to futrthcr supprcss deterioration of the macl~inability,t he hot workability, and the
casting property due to P, the prcfcrable upper litnit of the alnount of P is 0.025% and a
Inore preferable upper limit of the anlout~ot f P is 0.01 5%. It is preferable that the
anlount of P is as small as possible, and thus, it is not necessary to provide the lower
5 lin~iot f the a ~ n o uo~f ~P.t l'he lower limit of the amount of P may be 0%.
[0023]
(S: 0.1 0% or less)
S is an impurity clemeilt. In addition, S combines with Mn in the steel to
form MnS. Alth~ougMl~n is effective for increasitlg the tl~achinabilityo f the steel bar,
10 if the amount of S is more that^ 0.10%, MnS coarsens. The coarse MnS acts as a crack
origin during hot-rolling, and thus, the coarse MnS deteriorates hot workabilitp.
-
Therefore, it is necessary that the amount of S is 0.1 0% or less. In order to further
suppress deterioration of the hot workability, the preferable upper limit of the amount of
S is 0.05% and a more preferable upper litnit of the amount of S is 0.02%. It is not
15 necessary to provide the lower limit of the amount of S. The lower litnit of the amount
of S may bc 0%. On the other hand, in order to stably obtain the effect for etltlancing
the machinability, the lou~elri mit of the amount of S rnay be 0.02%.
[0024]
In order to enhance the hardenability and the strength, the steel bar may include
20 Cr: 0 to 3.0%, Mo:O to 1.5%, Cu:O to 2.0%, Ni:O to 5.0%, and B:O to 0.0035% as
optional elements.
[0025]
(Cr: 0 to 3.0%)
Cr is an optional elcment, and it is not nccessary that the steel bar itlcludes Cr
as chemical composition. Therefore, thc lower lin~iot f tlic alllount of Cr is 0%. 011
the other hand, Cr is at1 element which enhances the hardenability of the steel bar and
provides resistance to temper softenillg to the steel bar, and thus, the steel which needs
5 high strength may include Cr. If a large amount of Cr is included, Cr carbides form
and clllhrittle the steel bar. Therefore, the amount of Cr of the steel bar acconlir~gto
the present embodiment is 0 to 3.0%. In a case in which Cr is included for obtaining
the above-described effects, the preferable lower liniit of the amount of Cr is 0.1% and a
more preferable lower limit of the amount of Cr is 0.4%. The preferable upper limit of
10 the amount of Cr is 2.5% and a more preferable upper limit of the amount of Cr is 2.0%.
[0026]
(Ma: 0 to 1.5%)
Mo is an optional element, and it is not necessary that the steel bar includes Mo
as chemical con~position. Therefore, the lower litilit of the amount of Mo is 0%. On
15 the other hand, Mo provides the resistance to temper softening to the steel bar and
enhatlces the hardenability of the steel bar, and thus, the steel wvliich tlecds high strength
may include Mo. If the amount of Mo is more than IS%, the effect of Mo is saturated.
Therefore, in a case in which Mo is included, the upper limit of the amount of Mo is
1..5%. In a case-inlivhich Mo is included for obtaining the above-described effects,
20 preferable lower li~i~oift t he anloutlt of MO is 0.10% and a more preferable lower limit
of the amount of Mo is 0.15%. The preferable upper limit of the amount of Mo is
1 .l% and a more preferable npper limit of the amount of Mo is 0.70%.
[0027]
(Cu: 0 to 2.0%)
Cu is an optional clcment, and it is not necessary that the steel bar includes Cu
as cheulical composition. 'Sherefore, the lowvcr limit of the atnouut of Cu is 0%. On
the other hand, Cn is an element which is effeciive for strengthening ferrite, enhancing
the hardenabilitp, ant1 enhancing corrosion resistance. If the aniount of Cu is more
5 than 2.0%, the effects regarding mechanical prope~tya re saturated. Ant1 thus, in a case
in which Cu is included, the upper limit of the atnount oSCu is 2.0%. Particularly, Cu
(nay deteriorate hot ductility of the steel bar and may cause a flaw which fonns during
hot-rolling, and thus, it is preferable that Cu be included together with Ni. In order to
obtain the above-described effects tnore efficiently, the preferable lower limit of the
10 amount of Cu is 0.05% and a more preferable lower litnit of the amount of Cu is 0.10%.
The preferable upper limit of the amount of Co is 0.40% and a nlore preferable upper
--- -
linlit of the amount of Co is 0.30%.
[0028]
(Ni: 0 to 5.0%)
15 Ni is an optional element, and it is not necessary that the steel bar includes Ni
as chemical composition. Therefore, the lower litnit of the amount of Ni is 0%. On
the other hat~dN, i is an element which is effcctivc for enhancing ductility of the ferrite,
enhancing the hardenability, and enhancing the corrosion resistance. If the aatnount of
Ni is Inore than 5.0%; the effects regarding ~nccl~anicparlo perty are saturated and the
20 machinability of the steel bar deteriorates. h t l thus, in a case in which Ni is included,
the upper litnit of the amoutit ofNi is 5.0%. In order to obtain the above-described
effects more efficiently, the preferable lower limit of the arnount of Ni is 0.1% and a
more preferable lower lirnit of the aunount of Ni is 0.40%. 'The preferable upper limit
of the amount of Ni is 4.5% and a more preferable upper limit of the amount of Ni is
3.5%.
LO0291
(B: 0 to 0.0035%)
5 B is an optional elemeut, and it is riot necessary that the steel bar includes B as
chemical con~position. Therefore, the lower limit of the amount of B is 0%. On the
other hand, R segregates at grain boundary as solid-solution B to enhance the
hardetlability of the steel bar and the streilgth of the grain bou~idary,a nd thus, B
e~lhancestl ie fatigue strength and impact strength \\>11icha re required to machine
10 con~ponent. On the other hand, if the amount of B is more that^ 0.0035%, the
above-described effects are saturated and the hot ductility of the steel bar deteriorates
p~
significat~tly. And thus, in a case it1 which B is it~cludedt,h e upper limit of the amount
of B is 0.0035%. In order to obtain the above-described effects more efficiently, the
pxferable lower limit of the amount of B is 0.0010% and a more preferable lower limit
15 of the amount of B is 0.0015%. The preferable upper limit of the amount of B is
111 addition, in order to control the configuration of oxides and sulfides, tlie
steel bar accordingto the present embodiment may itlclude one or more selected from
20 the group consisting of Ca, Zr, Mg, and Rem as optional elements.
[003 11
(Ca: 0 to 0.0050%)
Ca is an optional element, and it is not necessary that the steel bar includes Ca
as chemical composition. Therefore, the lower limit of the amount of Ca is 0%. On
the other hand, Ca is a dcosidizitlg elelllent and for~nso xides in the steel bar. in steel
including Al, such as the steel bar accorditlg to the present e~nbodimcntC, a forills
calciu~na lutllitlate (CaOAI2O3). CaOA1203 is oxide of which the ~neltingp oint is
lower than that of Alz03, and for~ntso ol protection film during high speed cutting to
5 enhance the machii~abilityo f the steel bar. On the other hand, if the amount of Ca is
Inore than 0.0050%, CaS forms in the steel and deteriorates the ~nachinability.
Therefore, in a case in which Ca is included, the upper limit of the amount of Ca is
0.0050%. In order to obtain the above-described effects Illore efficiently, the
preferable lower limit of the alnount of Ca is 0.0001% and a tnore preferable lower limit
10 of the atnount of Ca is 0.0002%. The preferable upper limit of the amount of Ca is
0.0035% and a more preferable upper li~i~oift t he amount of Ca is 0.0030%.
-
[0032]
(Zr: 0 to 0.0050%)
Zr is an optional elenlent, and it is not necessary that the steel bar include Zr in
15 the chemical cotnposition. Therefore, the lower li~niot f the amount of Zr is 0%. On
the other hand, Zr is a dcoxidizing element and forms oxides in the steel bar. It is
assumed that the oxides are ZrO2. Since ZrOz acts as precipitation nuclei of MnS,
ZrOz increases the number of locations at which MnS precipitates to utufonnly disperse
MnS in the steel bar, and tl~usZ, rO2 has an effect for enhancing the machinability. In
20 addition, since Zr inco~~oratienst o MriS in a solid-solution state to form complex
snlfides axid decreases deforn~abilityo f MnS, Zr has an effect for suppressing
elongation of MnS during hot-rolling and hot forging. On the other hand, if the
amount of Zr is more than 0.0050%, yield of the steel bar significantly deteriorates, and
a huge alnount of hard compou~~sdusc h as ZrO2, ZrS, and the like form to deteriorate
thc mechanical properties of the stccl bar such as the nlachinability, impact value,
fatigue property, atld thc like. Therefore, in a case in which Zr is included, the upper
linlit of the alnoutlt of Zr is 0.0050%. In order to obtain the above-described effects
tnore efficiently, the preferable lower litnit of the amount of Zr is 0.0003%. ?'he
5 preferable upper li~niot f the amount of Zr is 0.0035%.
[0033]
(Mg: 0 to 0.0050%)
Mg is an optio~lael lement, and it is not necessary that the steel bar i~lcludesM g
as cl~emicacl onipositiotl. Therefore, the lower limit of the anloutlt of Mg is 0%. On
10 the other hand, Mg is a deoxidizing element and fortns oxides in the steel bar. In a
case in which deoxidizing with A1 is performed, Mg reform at least a part of AlzO,,
-
wl~ichd eteriorates the machinability, into MgO. Since MgO is relatively soft and
finely disperses, MgO does not deterio~ateth e machinability of the steel bar.
Tl~ereforeM, g has an effect for suppressing deterioration of the machinability due to the
15 deoxidization with Al. In addition, Mg oxides act as t~ucleoi f MnS, and thus, have an
effect for finely dispersing MnS. Furtherti~oreM, g fornls con~plexs ulfides with MnS,
and thus, Mg has an effect for spheroidizing MnS. On the other hand, if the amount of
Mg is more that1 0.0050%, Mg forms MgS to deteriorate the n~achinabilityo f the steel
bar. Therefort; in a case in which Mg is it~cludedt,h e upper limit of the amount of Mg
20 is 0.0050%. In order to obtain the above-described effects more eff~cientlyt,h e
preferable lower litnit of the amount of Mg is 0.0003%. The preferable upper litnit of
the amount of Mg is 0.0040%.
Re~n(r are-eat-th clcment) is an optional elenlent, and it is not necessary that the
steel bar includcs Rem as chemical compositiotl. Tlicrcfore, the lowvcr li~niot f the
amount of Re111 is 0%. On the other hand, Reni is a deoxidizing element, and has an
effect for forming low-melting oxides to suppress nozzle clogging during casting. In
5 addition, Ken1 incorporates into MnS in a solid-solution state or combines with MnS to
decrease defonnability of Ahis, and thus, Rem suppresses the elongation of MnS during
the hot-rolling and the hot forging. As described above, Rem is an element effective
for reducing anisotrop)' of the steel bar. If the anlount of Re~nis Illore than 0.0150%, a
huge amount of Rem sulfides form and deteriorate the inachinability. Therefore, in a
10 case in which Rem is included, the upper limit of the anloutit of Rem is 0.0150%. In
order to obtain the above-described effects Inore efficiently, the preferable lower linlit
~-~
of the amount of Rem is 0.0001%. The preferable upper limit of the amount of Rem is
15 In addition, in order to increase strength by forming carbonitrides and to size
austenite grains by the carbonitrides, one or more selected from thc group consisting of
Ti, Nb, V, and W may be included as optional elements.
[0036]
(Ti: 0 to 0.159%)
20 Ti is at1 optional element, and it is not necessary that the steel bar includes Ti as
chemical composition, Therefore, the lower limit of the amount of Ti is 0%. On the
other hand, Ti is an element contributing to suppressing growth of the austenite grains
and increasing strength of the austenite grains by forming the carbonitrides. A steel
bar wluch slioould have high strength and a steel bar in which strain thereof should be
reduced may includc Ti as a sizing eleinent for preventing the austenite grain coarsening.
I11 addition, Ti is a deoxidizing elei~le~aintd has an effect for e~lhancingth e
machinability of the steel bar by fortuiug soft oxides. Ou the other hand, if the anxount
of Ti is excessive, l'i-type sulfides forin and decrease the amount of MnS \\41ich
5 increases the machinability, and thus, the niachit~abilityo f the steel is deteriorated.
Therefore, the lrpper li~i~oift t he amount of Ti of the steel bar according to the present
embodiment is 0.150%. In order to obtain the above-described effects more efficiently,
the preferablc lower limit of the arnoutlt of Ti is 0.003%. The preferable npper limit of
the ainoutlt of Ti is 0.100%.
10 [0037]
(Nb: 0 to 0.150%)
- -
Nb is au optional element, and it is not necessary that the steel bar include Nb
as chc~nicacl ompositiot~. Therefore, the lower limit of the atl~ounot f Nb is 0%. On
the other hand, Nb is an element which forms carbonitrides, and contributes to
15 increasing the strength of the steel by secondary precipitation hardening and
suppressing the growth of the austetlite grains. A stcel bar which should have high
strength and a steel bar in which strain thereof shoold be reduced may it~cludcN b as a
sizing element for preventing the austcnite grain coarsening. If the amount of Nb is
more than 0. l'5w6:.coarse carbonitrides wliich do not for111 solid-solution and which
20 cause hot crack, and thus, il~echa~~ipcraolp erties are deteriorated. Therefore, in a case
in which Nb is included, the upper lin~iot fthe arnouilt of Nb is 0.150%. In order to
obtain the above-described effects more efficiently, the preferable lower limit of the
amount of Nb is 0.004%. The preferable upper limit of the aiuount of Nb is O.l00%.
[0038]
(V: 0 to 1.0%)
V is an optional element, and it is not necessary that the stcel bar includes V as
chemical composition. Therefore, the lower liniit of the amount of V is 0%. On the
other hand, V is an elenlent whic11 forms carbonitrides, and contributes to increasing the
5 strength of the steel by secondary precipitation hardening, suppressing the growth of the
austenite grains, and increasing the strength of the austenite grains. A steel bar which
should have high strength and a steel bar in which strain thereof should be reduced may
include V as a sizing element for preveuting the ai~steniteg rain coarsening. If the
a~nounot f V is inore than 1.0%, coarse carbonitrides which do not form solid-solution
10 and which cause hot crack, and thus, mechanical properties are deteriorated. Therefore,
in a case in which V is included, the upper limit of the amount of V is 1.0%. In order
to obtain the above-described effects tnore efficiently, the preferable lower limit of the
atnount of V is 0.03%.
[0039]
15 (W: 0 to 1.0%)
W is an optional element, and it is not necessary that the steel bar includes W
as chemical con~position. Therefore, the lower limit of the amount of W is 0%. On
the other hand, W is an element which forms carbonitrides, and contributes to
increasing th'strengtl~ of the steel by secondary precipitation hardening. If the amount
20 of W is more than 1.0%, coarse carbonitrides which do not form solid-solution and
which cause hot crack, and thus, mechanical propelties are deteriorated. Therefore, in
a case in which W is included, the upper li~niot f the arnount of W is 1.0%. In order to
obtain the above-described effects more efficiently, the preferable lower limit of the
amount of W is 0.0 I %.
[0040]
In addition, in order to cnhance the machinabilitp, one or more selected fio~om
the group consisting of Sb, Sn, Zn, Te, Bi, and Pb may be included as optional
elements.
5 [0041]
(Sb: 0 to 0.0150%)
Sb is an optional element, and it is not necessary that the steel bar includes Sh
as chemical composition. Therefore, the lower linlit of the amount of Sb is 0%. On
the other hand, Sn moderately en~brittlesfe rrite and enhances the n~achinabilityo f the
10 steel bar. In a case in wl~ictlh~e amount of solid-solution Al is large, the effect is
significantly exhibited. 011 the other hand, if the amount of Sb is more than 0.0150%,
the amount of macro segregation of Sb become excess, and thus, the impact value of the
steel bar significal~tlyd eteriorates. And thus, in a case in which Sb is included, the
upper limit of the amount of Sb is 0.0150%. It1 order to obtain the above-described
15 effects more eff~cientlyt,h e preferable lower lin~iot f the amount of Sb is 0.0005%.
[0042]
(Sn: 0 to 2.0%)
SII is an optional elenlent, and it is not necessary that the steel bar includes Sn
as chenlical co~nposition. Therefore, the lower limit of the amount of Sn is 0%. On
20 the other hand, Sn has an effect for embrittling the ferrite to extend the sewice life of
the tool and an effect for improving surface roughness of the steel bar. However, if the
amount of Sn is more than 2.0%, the effects are saturated. Therefore, in a case in
which Sn is included, the upper limit of the amount of Sn is 2.0%. In order to obtain
thc abovc-described effects inorc efficiently, the preferable lower li~niot f the amount of
Sn is 0.005%.
[0044]
(Zn: 0 to 0.50%)
5 ZII is an optional element, and it is not uecessary that the steel bar includes Zn
as chemical composition. Therefore, the lower limit of the a ~ ~ o uonf tZ n is 0%. On
the other hand, Zn has all effect for embrittling the ferrite to extend the sewice life of
the tool and an effect for improving the surface rouglu~esso f tlie steel bar. However, if
the atnount of ZII is more than 0.50%, the effects are satu~ated. Therefore, in a case in
10 which Zn is included, the upper limit of the amount of Zn is 0.50%. In order to obtain
the above-described effects more efficiently, the preferable lower limit of the allloutlt of
Zn is 0.0005%.
[0045]
(Te: 0 to 0.20%)
15 Te is an optional element, and it is not necessary that the steel bar includes Te
as chemical con~position. Therefore, the lower linlit of the amoutlt of Te is 0%. On
the other hand, Te is an elcn~enet nhancing the machinability. In addition, Te forms
MnTe which coexists with MnS and decreases defornlability of MnS, aud thus, Te has
an effect for suppressing the elongation of MnS. Accordingly, Te is an element
20 effective for reducing anisotropy of the steel bar. However, if the allloullt of Te is
more than 0.20%, the effects are saturated, and Te may cause flaw due to a decrease in
hot ductility. Therefore, in a case in which Te is included, the upper li~niot f the
anlount of Te is 0.20%. In order to obtain the above-described effects more efficiently,
the preferable lower limit of the amount of Te is 0.0003%.
100451
(Bi: 0 to 0.50%)
Bi is an optional element, and it is not necessary that the steel bar iucludes Bi
as cl~emicacl otnpositio~~.T herefore, the lower Ii~niot f tlte alnount of Bi is 0%. On
5 the other hand, Bi is an element etlhanciilg the machinability. However, if the amount
of Bi is more than 0.50%, the effect for enhatlculg the machinability is saturated, and Bi
map cause flaws doe to a decrease in hot ductility. heref fore, in a case in which Ri is
included, the upper limit of the arnount of Bi is 0.50%. In order to obtain the
above-described effects more efficiently, tlle preferable lower li~niot f the amount of Bi
10 is 0.005%.
[0046]
(Pb: 0 to 0.50%)
Pb is an optional element, and it is not necessary that the steel bar includes Pb
as chemical composition. Therefore, the lower limit of the amount of Pb is 0%. Pb is
15 an element enhancing the machinability. However, if the amount of Pb is more than
0.50%, the effect for enhancing the machinability is saturated, and Pb may cause fla\vs
due to a decrease in hot ductility. Therefore, in a case in which Pb is included, the
upper limit of the amount of Pb is 0.50%. In order to obtain the above-described
effects more efficiently, tlle preferable lower litnit of the amount of Pb is 0.005%.
20 [0047]
The cl~emicacl omposition of the steel bar according to the present e~nbodiment
is described above. Remainder of the cl~emicalc on~positiono f the steel bar according
to the present embodiment is Fe and impurity. The impurity is a component which is
itlcorporated from raw materials such as mineral or set-ap or by various factors in a
manufacturing process ~vhcnth e steel bar is industrially matmfactured, and is accepted
within a range that does not adversely affect the property of the stccl bar according to
the present e~nbodilnent. Although the preferable lo\ver li~nitso f the optional elelne~lts
are described, the properties of the steel bar according to the present etnbodiment are
5 not deteriorated even if the amounts of the optio~~ealle ments are lower than the
above-described the preferable lower limits. Therefore, the atnounts of the optional
elements included in the steel bar according to the present e~nbodi~nemnta y be lower
that1 the above-described the preferable lower limits.
[0048]
10 Nest, a reason for limitations regarding a structure and a hardness of the steel
bar according to the present etnbodiinent will be described wit11 reference to Figures 1
to 4 showving construction of the steel bar, Figures 5 to 7 showing cot~structiono f a
tnat~ufach~rinegq uipment of the steel bar, and Figure 8 sho\\ring method for
inanufacturing the steel bar.
Intensive studics have been carried out by inventors on a metliod which can
obtain the steel bar 1 having high crack propagation stopping properties, escellent basc
material low temperature tonghness, and high it~ductioha~r~de nability, and which call
niat~nfactureth e steel bar 1 with high efficiency and ~vithouth ermal refining. As a
20 result, the itlventors found that it is effective for obtaining the steel bar 1 having high
crack propagation stopping properties, excellent base material low temperature
toughness, and high induction hardenability that a structure of a surface layer area 13 of
the steel bar 1 is a tempered tnartensite, a bainite, or a mixed structure having the
tempered rnattensite and the bainite, that the structure of the surface layer area 13 of the
steel bar 1 is refined, and that formation of a ferrite is suppressed. In t11c present
invention, the surfacc layer area 13 is an area from a surface 15 of tlle steel bar 1 to a
depth of 25% of a radius r of the steel bar 1. 11%a ddition, in the present invention, the
tempered martensite may be simply referred as "martensite". Ivloreover, the inventors
5 found that it is effective for obtaining the steel bar 1 having above-described features
that steel bar 1 is rapidly cooled just after hot-rolling, aud then reheated.
[OOSO]
Typical ther~narle fining includes quenching and ten~pering. In rapid-cooling
during the quenching, the steel bar 1 is sufficietltly cooled so that a center portion
10 thereof is cooled, and then, the steel bar 1 is heated during the tempering. The thermal
refilling cat1 bring the steel bar 1 having predetemlined surface hardness, high crack
-~ -
propagatiotl stopping properties, and low temperature toughness. In an entire cross
section 10 of the steel bar 1 (a cross section perpendicular to a longitudinal direction of
the steel bar I), the structure is mainly the tempered martensite and the amount of the
15 ferrite is small, and the structure is refined. On the other hand, during nlanufach~ring
the steel bar 1 according to the present embodilllent, the steel bar 1 is rapidly cooled just
after hot-rolling, and tlicn tl~csu rface of the steel bar is heated by self-reheating due to
sensible heat of inner portion of the steel bar. In this case, although a surface part of
the steel bar 1 is heat-treated similar to the typical thermal refining, the center of tlie
20 steel bar 1 is not cooled and heated. In the case in which the steel bar 1 is sufficiently
cooled so that a center portion thereof is cooled, the reheating is not occur aud the
surface part of the steel bar 1 is not sufficiently heated. Therefore, surface hardtless of
the steel bar 1 after the rel~eatingin creases excessi~~ealynd the machit~abilityo f the steel
bar 1 deteriorates. The itlventors found that in order to suppress the increase of the
surface hardness of the steel bar 1 after the reheating, the structure of the surface layer
area 13 of the cross section 10 can be co~ltrolledto bc fiue tempered martetlsitc, fine
bainite, or fine mix stn~ctureo f the tempered martensite and the bainite by adequately
co~ltrollitlgc o~lditioot~f t he rapid-cooling to the steel bar 1 just after the hot-rolling so
5 that only the surface of the steel bar 1 is rapidly cooled and reheated. Further~~lorteh,e
inventors found that it is effective for increasing productivity if unevenness of
hardening depth after the reheating is suppressed.
I005 11
That is, the steel bar 1 according to the present embodimetlt is the steel bar 1
10 ~vhichis rapidly cooled just after hot-rolling and then reheated, in which a region which
is along a line (line segment) extending behveen a center 12 of a cross section 10 of the
-- -
steel bar 1 and a periphery 11 of the cross section 10 of the steel bar 1 and which has a
hardness higher than the average hardness in the lirie by IIv20 or more is a hardening
region 101 in the line, a minimum value of depth of the hardening regions 101 in the 8
15 lines of which the angle is 45" is a minirnum hardening depth 103 in the cross section
10, and the maximum value of the depth of the hardening regions 101 in the 8 lines is
thc maxitnutil hardening depth 102 in the cross section 10, in which a difference
between the niaxin~unhl ardening depth 102 in the cross section 10 and the minimum
hardening depths103 in the cross section 10 is 1.5 nun or less, in xvl~cha difference
20 bet\vcen the tuaximurn value of the maxin~umh ardening depth 102 and a minimum
value of the rnaxitllum hardening depth 102 in the cross sections 10 at 3 points C1, C2,
and C3 which are separated fro111 each other by 1650 mlix parallel to a Iongiti~dinal
direction of the steel bar I is 1.5 mni or less, in \vhieh a difference between the
maximum value of the tniuitnurn hardening depth 103 atid a mitlituutn value of the
I I ~ ~ I I ~ I ~hIaLrId~eIn ing dcpth 103 in the cross scctiotls 10 at thc 3 points CI,C z, and Cj
which arc scparated fro111 each other by 1650 IIUII parallel to the longituditlal direction
of the steel bar 1 is 1.5 mnl or less, it1 wvhich a structure iin an area fro111 a surface 15 of
the steel bar 1 to a depth of 25% of a radius r of the steel bar 1 includes 10 area% or less
5 of a fel-rite aud a remainder including one or more selected from a group consisting of a
bainite and a inartensite, in which a boundary between grains which are adjacent to each
other and of which an orientation differeuce is 15 degree or more is a grain boundary,
and an equivalent circle diameter of an area surrounded by the graitl boundary is a grain
size, in \vhich the average value of the grain size of a bcc phase in the area from the
10 surface 15 of the steel bar 1 to the depth of 20% of the radius r of the steel bar 1 is 1.0 to
10.0 1tn1,iti which the average value of the grain size of the bcc phase it1 at1 area from
the depth of 50% of the radius r of the steel bar 1 to the center 12 of the steel bar 1 is 1.0
to 15.0 ltm, in which a hardness of a region 105 of which a depth from the surface 15 is
50 pnl is Hv200 to Hv500, and in which a total decarburizcd layer thickness DM-T is
15 0.20 iilnl or less.
[0052]
(Diffcrcncc between maximum hardening dcpth in cross section and minimum
hardening depth it1 cross section: 1.5 Inn1 or less)
(Difference between li~axin~uvlall~ue of rnaxinlutn hardening depth and
20 tnininlum value of rnaxitlluln hardenitlg depth in cross sections at 3 points \vIiich are
sepa~atedfi om each other by 1650 mm parallel to loiigitudinal direction of steel bar: 1.5
mm or less)
(Difference between maximum value of ininimuni hardening depth and
rni~lirllu~vlal lue of minimum hardening depth ill cross sections at 3 poitlts which are
scparatcd fro111 each other by 1650 ti1111 parallcl to longitudinal direction of steel bar: 1.5
tntn or less)
In the steel bar 1 according to the present embodiment, a region wluch is along
a line extending between a center 12 of a cross section 10 of the steel bar 1 and a
5 periphery 11 of the cross section 10 of the steel bar 1 and which has a hardness higher
than the average hardness in the line by Hv20 or more is a hardening region 10 1, the
nlininiun~v alue of depth of the liardening regions 101 in the 8 lines of \vhich the angle
is 45" is the ~nini~nnhnxid eniug depth 103 in the cross section 10, and the lnaxinlunl
value of the depth of the hardening regions 101 in tlie 8 lines is the tnaxinnun hardening
10 depth 102 in the cross section 10.
[0053]
Definitions of the terms will be described in detail with Figure 1. The figure
1 shows an arbitra~yc ross section 10 (i.e. a section perpendicular to the longitudinal
direction of the steel bar 1) of the steel bar 1. In a case in \vIuch hardness is
15 continuously measured at any intervals, for example, at 200 p n ~in tervals along an
arbitrary line extending between a center 12 of the cross section 10 of the steel bar 1 and
a periphery 11 of the cross section 10 of the steel bar 1, the average hardness along the
arbitrary line can be obtained. In the steel bar 1 according to the present en~boditnent,
only the surface part thereof is quenched and tempered, and thus, hardness of the
20 surface part is higher than hardness of a center part. In the arbitrary line, a region
having hardness lugher than the average hardness in the arbitrary line by Hv20 or Inore
is assumed as a region in \vIiich quench hardening occurs. Therefore, the
above-described region of the steel bar 1 according to the present embodi~nenti,n which
the quench hardening occurs, is defined as a liardening region 101 in the line. Depth
of thc hardening region 101 regarding any liue is assumed as hardening depth it1 the linc.
In addition, in the stccl bar 1 according to the present embodirlle~ltt, he mit~iiuumv alue
of depth of the hardelling regions 101 in the 8 lines of wliich the angle is 45" is defined
as the mitlitnum hardening depth 103 in the cross section 10, a nlaxi~nuv~all~uc of the
5 depth of the hardening regions 101 in the 8 lines is defined as a n~axi~lllht~arnd ening
depth 102 in the cross section 10, and a differetlcc between the ~nini~nuh~arldle ning
depth 103 in the cross section 10 and the maxitnum hardening depth 102 in the cross
section 10 is defined as a quenching deflection 104 it1 the cross section. The
quet~clitlgd eflection 104 in the cross section is a value indicating unevenness in the
10 cross scctiotl 10, and it is assumed that a cross section 10 of whch the qucuching
dcflectiotl 104 in the cross section is small is quenched unifornlly along circumferential
direction of the cross section 10.
[0054]
The steel bar 1 according to the present erl~bodimcnits manufactured by
15 rapid-cooling a hot-rolled steel 20 after hot-rolling. During the rapid-cooling, along
the entire of the hot-rolled steel 20 it1 circumferet~tiadl irection and in lot~gituditlal
direction, the cooling is as unifornl as possible. The reason is that uneven cooling
makes the hardening depth uneven, which makes the structure and the hardness of the
hot-rolledsteel 20.and'the steel bat I uneven in the circu~nferentiald ircctiorl and in the
20 lo~~gituditldailr ection. The unevenness of the structure and the unevenness of the
hardness cause a warpage in the hot-rolled steel 20 after rapid-cooling to the hot-rolled
steel 20, or cause the warpage in the steel bar 1 after induction hardening to the steel bar
1. If a marked warpage occurs, it is necessary to correct the warpage and yield
decreases due to shape failure, and thus, the marked warpage decreases production
efllciency of the steel bar 1. In order to keep the protloction cfiicicncy of the steel bar
1 at a lcvcl prcfcrablc for industrial use, it is uecessary that an amount of the walpage of
the steel bar 1 is suppressed to less than 3 1n11111n.
[0055]
5 The it~ventorsfo und that it is necessary for keepiug the production efficiency
of tlie steel bar I at a preferable level by suppressing the amount of the warpage of the
steel bar 1 that tlie steel bar I is manufactured so that the quenching deflection 104 in
the cross section in arbitrary cross sections 10 of the steel bar 1 is 1.5 nim or less
Thereby, the steel bar 1 havitig uniform hardening depth in the circut~lferenced irection
10 can be obtained. In addition, the inventors found that it is tlecessaly that the steel bar 1
is manufactured so that a difference between a maximum value of the maxiti~~rni
hardening depth 102 and the minimum value of the n~axit~luhtnar dening depth 102 in
the cross sections 10 at 3 points CI, Cz, and C3 which are separated from each other by
1650 Inn1 palallel to, the longitudinal direction of the steel bar 1 (hereinaftel; referred as
15 "Arnax") is 1.5 mm or less and a difference between a maxinmm value of the minimum
hardening depth 103 and the minimum value of the minimum hardening depth 103 in
the cross sections 10 at the 3 points CI, Cz, and Cj which are separated from each other
by 1650 inn1 parallel to the longitudinal direction of the steel bar 1 (hereinafter, referred
as "Amitin) is 1.5 mm or less. Thereby, the steel bar 1 having uniform hardening depth
20 in the longitudinal direction can be obtained. If one or more of the quenching
deflection 104 in the cross section, the Amax, and the Amin is more than 1.5 mm, the
amount of the warpage of the steel bar 1 increases to be more than 3 1nnl1111. The
preferable upper limits of the quenching deflection 104 in the cross section, the Arnax,
and the Amin are 1.4 lnm, 1.3 mm, or 1.2 mm. Since the smaller the quenching
deflection 104 in thc cross section, thc Amax, and the Amin are, ll~en lore preferable it is,
the lower liniits of the quenching deflection 104 in the cross section, the Alllax, and thc
Anlin are 0 mm. Howeve!; it is difficult to completely relicve the itneventiess of the hat-dening
depth, and thus, sobstantial lo\\'el. limits of the quenching deflection 104 in the cross section,
5 the Amax, and the Amin [nay bc about 0.7 mto.
[0056]
Method for measuring the maxirnuo~h ardening depth 102 in the arbitrary cross
section 10 of the steel bar 1 and the minimu urn hardening depth 103 in the arbitrary cross
section 10 of the steel bar 1 will be described below. At first, along a first line
10 extending between a center 12 of a cross section 10 of the steel bar 1 and a periphery 11
of the cross section 10 of the steel bar 1, hardness is continuously measured at arbitraqr
intervals from the center 12 to the periphery 11. Next, the average hardness of the first
line is calculated based on the obtained hardness values. Then, a region having
hardness higher than the average lutrdness in the first line by 11~20or more is assutned
15 as a hardening region 101, and depth of the hardening region 10 1 (hardening depth) is
mcasurcd. And then, along nu, line ("n" is 2 to 8 of counting number) in which angle
bet\veen the rill, line and the I st line is 45" x (n-1) and which extends between a center
12 of a cross section 10 of the steel bar 1 and a periphe~y 11 of the cross section 10 of
the steel bar 1, hardness is cot~tinuouslym easured similar to the first line. 'rhe largest
20 of the 8 kinds of hardening depth obtained thereby is the maxinlu~nh ardening depth 102
in the arbitrary cross section 10 and the minimum of that is the minilnun1 hardening
depth 103 in the arbitrary ccmss section 10. Typically, the hardening region 101
obtained by the above-described measuring method is a continuous line of which the
origin is the periphery 11 ofthe cross section 10. If the hardening region 101 is not the
contitlt~ousli ne of \t,hich the origin is the periphcry I1 of the cross section 10, the
llardncss values used for defining the hardening region 101 may not be concct.
Cotlditions for measuring the hardness and the intewals during measuring the hardness
are not limited. In view of the diameter and the hardness of the steel bar according to
5 the present en~bodimentf,o r example, load during measuritlg the hardt~essm ay be 200g
and the intervals during measuring the hardness n~aybe 100 pm.
lo0571
(Average value of grain size of bcc phase in area from surface of steel bar to
depth of 25% of radius of steel bar: 1.0 to 10.0 ptn)
10 (Average value of grain size of bcc phase in area from depth of 50% of radius
of steel bar to center of steel bar: 1.0 to 15.0 pm)
111 view of safety, in a case in which the steel bar 1 is used for stmctore material
of the machine component and the like (for example, a shaft, a pin, a cylinder rod, a
steering rack bar, and a rebar, etc.), it is necessary that fracture morphology of the steel
15 bar 1 is bending when the steel bar 1 is broken by some kind of impact or load beyond
an expected level. Fracture lnorphology of typical structure ~natcriali s rupture, i.c. a
tnorphology by \vhich the structure tnatcrial is divided. On the other hand, it is
in~portanfto r safety of the stl~lch~mrea terial that the fiacture morphology of the
struch~rem aterial is a fracture n~orphologys uch as bendiug by which only deformation
20 occurs (i.e. breaking does not occur). The inventors made test pieces for supposiug a
circumstance in which the steel bar 1 is used for structure material by induction
hardening the surface part of the steel bar 1, and then machining the steel bar 1 so as to
be a shape having U notch of u~hichd epth is 1 mm. Nest, the inventors performed
three-point bend test on the test pieces in ethyl alcohol cooled to -40°C, aud studied the
effect of the grain sizc of bcc phase for the fracture morphology of each test pieces.
As a result, during the three-point bend tcst on tcst pieces of wvhich thc bcc phase were
suficie~ltlyre fined, i.e. test pieces in which average values of grain size of the bcc
phase in areas (surface layer areas 13) from the surfaces 15 of steel bars 1 to depth of
5 25% of radius r of the steel bars 1 were 10.0 11111 or less and in which average values of
the grain size of the bcc phase in areas (center areas 14) fro111 depth of 50% of radius r
of the steel bars 1 to the centers 12 of the steel bars 1 were 15.0 pln or less, although
cracks occurred from the bottonts of the U notches thereof, crack prol~agationw as
stopped. Therefore, the fracture t~lorphologyo f the test pieces ofw hich the bcc phase
10 were sufftciently refined were determined as bending. In addition, charpy impact test
pieces were co~.~ectefdro m the center portion oft he steel bars 1 of which the bcc phase
- -
were sufficiently refined and charpy i~ilpacte st at -40°C was perfornled on the charpy
impact test pieces, and it was found that cha~pyim pact values of the center portions of
the steel bars I of which the bcc phase wvere sufficiently refined were high. That is, the
15 center portions of the steel bars 1 of which the bcc phase were sufficie~ltlyr efined had
excellent toughness. On the other hand, the tl~ee-poinbt end test and the charpy
itnpact test were performed on test pieces of which the bcc phase were not sufficiently
refined, i.e. test pieces in which average values of grain size of the bcc phase in surface
layer areas 13 wvere Inore that~1 0.0 ptn and/or in which average values of the grain size
20 of the bcc phase in center areas 14 were more than 15.0 pm, and during the thee-point
bend test, the test pieces were 11ot bended and divided into two pieces. That is, the
fiach~rem orphology of the test pieces ofw hich the bcc phase were not sufficiently
refined were determined as ruph~re. In addition, based on the charpy impact test, it
was found that charm' itnpact values of the ceuter portions of the steel bars 1 of wvl~icl~
thc bcc phase were uot sufficiently refined were lo~v. In thc prcscnt invention, a
boundary betwccn grains which arc adjaccnt to each other and of which at1 orieutation
diffcrcnce is 15 degree or Illore is defined as a grain boundary, and an equivalent circle
diameter of an area surrounded by the grain boundary is tlefined as a grain size.
5 [0058]
In view of the above-described founding, in the steel bar 1 according to the
present embodiment, the average value of the grain size of the bcc phase in the surface
layer area 13 is defined as 1.0 to 10.0 11111 and the average value of the grain size of the
bcc phase in the center area 14 is defined as 1.0 to 15.0 pm. Since it is difiicult in an
10 industrially practicable wap to decrease the average value of the grain size of the bcc
phase to be 1.0 ptn or lowel; both of the lower li~iliot f the average grain size of the bcc
phase in the surface layer area 13 and that of the center area 14 is 1.0 pm. ,411
intermediate area from depth of 25% of radius r of the cross section to depth of 50% of
radius r of the cross section is a transitional area fsom the structure in the surface layer
15 area 13 to the stl~~ctuorfe t he center area 14. In order to obtain the demanded average
valuc of the grain size of the bcc phase, it is effective that finish rolling temperature 3 1
of hot-rolling is adequately controlled and rapid-cooling is performed just after the
hot-rolling with a sufficient anlount of water.
[0059] ... ., ..
20 Method for measuriug the average value of the grain size of the bcc phase in
the surface layer area 13 of the steel bar 1 and that of the center wea 14 of the steel bar
1 is not limited. For example, the values lnay be obtained by measuring the average
value of the grain size of the bcc phase at positions showtl in Figure 4 with an
Electron-Back-Scattering-Diffraction (EBSD) apparatus attached in a scanning electroll
micmscopc. An csa~npleo f method for mcasuritlg the average valuc of the grain size
of the bcc phase it1 the surface laycr area 13 of the steel bar 1 is as follows. At first,
crystal orietltatio~m~a ps of the bcc phase regarding areas of 400 ~111x 400 pm ill each of
eight measuring positior~s( black circle marks showvr~i n Figure 4) consisting of four
5 measuring positious in portion 16 of which the depth is 200 11111 from the surface 15 of
the steel bar 1 atid four measuring positious in portion 17 of which the depth is 25% of
the radius r from the surface 15 of the steel bar 1. Then, boundary in the crystal
orientatiotl maps of the bcc phase, at wvl~ich an orientation differetice is 15 degree or
more, is assumed as grain boulidaty of the bcc phase, atid the average values ofthe
10 grain size of the bcc phase in each of the eight measuring positions are measured using
method of Johnson-Saltykov (see "QUANTITATIVE MICROSCOPY", Uchida
-
Rokakuho, published at July 30, 1972, R. T. DeHoff and F. N. Rhines, p189). Thetl,
the average value of the grait~s ize of the bcc phase in the surface layer area 13 of the
steel bar 1 can be obtained by further averaging the average values of the grain size of
15 the bcc phase it1 each of the eight measuring positions. An example of method for
measuring the average value of the grain size of the bcc phase in the center area 14 of
the steel bar 1 is as follo\vs. At first, average values of the grain size of the bcc phase
it1 each of 9 measuring positiolls (white circle marks shown in Figure 4) consisting of
four n~easuritlgpositioIIs'in portion 18 of which the depth is 50% of the radius r from
20 the surface 15 of the steel bar 1, four ~lleasurit~pgo sitions in poltion 19 of which the
depth is 75% of the radius r fro111 the surface 15 of the steel bar 1, and one measuring
position in center 12 of the cross section I0 of the steel bar 1 are measured using
above-described method. Then, the average value of the grain size of the bcc phase in
the center area 14 of the steel bar 1 can be obtained by further averaging the average
valucs of the grain size of tlle bcc phasc in each of the 9 nleasuring positions. The four
tneasuring positions are selected so that the angles betwccn adjacent lines \vhich are
between the four nleasuring positious and the center 12 of the cross section 10 of the
steel bar 1 are about 90 degree. The four measuring positions in the portion 17 of
5 which the depth is 25% ofthe I-adius r from the surface 10 of the steel bar 1, the four
measuring positions in the portion 18 of \\~hichth e depth is 50% of the radius r fiom the
surface 10 of the steel bar 1, and the four measuring positions in the portion I9 of which
the depth is 75% of the radius r from the surface 10 of the steel bar I are selected
similarly.
10 [0060]
(Structure in area from surface of steel bar to depth of 25% of radius of steel
bar: 10 area% or less of ferrite and remainder including one or more selected fio~na
group consisting of bainite and mattensite)
(Total decarburized layer thickness DM-T: 0.20 tnm or less)
15 In a case in which the steel bar 1 is used for structure material of the machine
component and the like (for exatnple, a shaft, a pin, a cylinder rod, and a steering rack
bar, etc.), in order to provide a surface portion thereof with rcquired strength and wear
resistance, induction hardening is perfor~nedth ereon. Therefore, induction
hardenability is required for the steel bar 1 used as the struchrre material. If carbon
20 content in the steel bar 1 decreases, the itlductio~h~ar denability deteriorates, and thus,
the predeterniined hardness cannot be obtained. And thus, it is necessary that
decarburization of the surface of the steel bar 1 is suppressed. In addition, if the
amount of ferrite in the surface layer area 13 of the steel bar 1 increases, since the
induction hardening is a short period (few seconds) of heating, the carbon does not
sufficiently diffi~scin thc ferrite even if the induction hardening is perfor~licd. In this
case, the carbon content it1 a portion which was the ferrite dccrcascs and the hardness
thereof after the induction hardenitlg decreases, and thus, i~lductiothl ardenability
deteriorates.
5 LO06 1 J
The inventors found that it is necessary that a total decarburized layer thickness
DM-T defined in JIS G 0558 "STEELS-DETERMINATION OF DEPTH OF
DECARBURIZATION" is 0.20 IIUII or less for the good induction hardenability. If
the total decarburized layer tluckness DM-T is more than 0.20 mm, deficiencies such as
10 lack of sttrface hardness after the induction hardening, and the like occurs.
[0062]
In addition, the inventors determined that a structure in the surface layer area
13 of the steel bar 1 includes 10 area% or less of a ferrite and a remainder includitlg one
or more selected from the group consisting of a bainite and a martensite. If the
15 structure is oat of the determined range, deficiencies such as lack of surface hardness
after the induction hardening, unevenness of the hardness, and the like occurs. In
order to suppress the total decarburization, it is effective that billct heating tetnperaturc
and billet heating time at hot-rolling is adequately contro1led and rapid-cooling is
performed on the hot-rolled steel 20 just after the hot-rolling. In order to suppress
20 precipitation of the ferrite, it is effective that the hot-rolled steel 20 is quenched by the
rapid-cooling on the hot-rolled steel 20 just after the hot-rolling so illat the structure of
the steel bar 1 includes one or Inore of the martensite and the baitlite. In addition to
the martensite and/or the bainite, the remainder of the struch~reo f the surface layer arca
13 of the steel bar 1 may iuclude 5 area% or less of a pearlite and other structure of
which the amouot is snlall enough so that the properties of thc steel bar according to the
present enlbodinler~tis not affected thereby. Howcw~cl;t he pearlite and the other
structure are not essetltial. The structure of a portion other than the surface layer area
13 of the steel bar 1 according to the present embodiment may have various
5 configuration and does not seriously affect the properties of the steel bar 1, and thus, the
structure thereof does not limited. For example, the structure of the portion other than
tlie surface layer area 13 of the steel bar 1 according to tlie present embodiment may be
mainly ferrite-pearlite structure and may include other stroctures such as the bainite, the
martensite, and the like.
10 [0063]
(Hardness of region of which depth frotn surface is 50 pn~H: v200 to Hv500)
In a case in which the steel bar 1 is used for structure material of the machine
cotnpotlent and the like (for example, a shaft, a pin, a cylinder rod, and a steering rack
bar, etc.), typically, the steel bar is worked to be a desired shape by machine wwrork such
15 as cutting. In a case in which the hot-rolled steel 20 after the hot-rolling is
rapid-cooled in order to refine the structure, the hardriess of the steel bar 1 increases.
How\~everi, f the hardness of the steel bar 1 is excess, the machinability of the steel bar 1
deteriorates, and thus, yield rate deteriorates and cost for cutting increases. Therefore,
it is necessary to control the hardness of the steel bar 1. The inventors studied the
20 machitlability with plunge cutting, and found that the machinability of a steel bar 1 of
which surface hardness (region 105 of which a depth fioni the sitsface is 50 I I I ) after
reheating wwras more than Hv500 was significantly poor. Therefore, the surface
hardness of the steel bar 1 according to the present embodiment is determined to be
1-Iv500 or less (preferably Hv450 or less, and Inore preferably Hw1400 or less). On the
other hand, if the surface hardncss of the steel bar 1 is lower than Hv200, strength
requircd for parts cannot be obtained, and thus, the lower linlit of thc surface hardness
after reheating is Hv200. The hardness at the region 105 of which the depth fro111 the
surface 15 of the steel bar 1 is 50 pm can be obtained by nleasuring I~ardnesso f tlic
5 region 105 in the cross section 10 of the steel bar 1, the region being 50 kum inside from
the periphery 1 I of the cross section 10.
[0064]
The diameter of the steel bar 1 according to the present embodiment is not
limited. However, in view of capacity of the manufacturing equipment, the diameter
10 ofthe steel bar 1 is substantially 19 to 120 mm.
[0065]
Next, a method for manufacturing the steel bar 1 according to the present
en~bodin~ewntil l be described. For example, the steel bar 1 according to the present
e~nbodirnenti s tnanufactured by a method having heating a steel (billet) having a
15 chemical composition of the steel bar 1 according to the present en~bodimento 1000 to
120O0C, keeping the steel thcrein during 100 to 130 sccond, hot-rolling the steel with a
finish rolling temperature 31 being 850 to 950°C to obtain a hot-rolled steel 20, cooling
the hot-rolled steel 20 just after finishing of the hot-rolling under a condition in mhich a
water film thickness 283 /diameter of the hot-rolled steel 20 is 0.1 to 0.5, and in which
20 le~lgtho f a water cooling zone (an area it1 a water cooling apparatus 24 fiom a water
cooling starting point to a water cooling stopping point), passing speed of the hot-rolled
steel 20 through the water cooling zone, and flow velocity of a cooling water 29 in the
water cooling zone is adequately set, reheating a surface of the hot-rolled steel 20 to 500
to 600°C, and cooling the hot-rolled steel 20 to room temperature. It is necessary that
the length of thc watcr cooling zonc, thc passing spccd of tllc hot-rolled steel 20 tlwo~ougll
the water cooling zone, and the flow velocity of the coolitlg water 29 in the watcr
cooling zone are set so that surface te~ilperatureo f tlie hot-rolled steel 20 after the
cooling rises to 500 to 600°C.
5 [0066]
In order to manofactute the structure as described above, a rolling line and a
cooling apparatus illustrated in Figores 5 to 7 can be used. The hot-rolled steel 20 can
be obtained by hot-rolling tlie steel, which is heated in the heating furnace 21, with the
hot-rollit~gt nill22. The hot-rolled steel 20 which is hot-rolled is rapid-cooled just
10 after the hot-roiling in the water cooli~lga pparatus 24. The water cooli~iga pparatus 24
is cotifigured by a plurality of water cooling pipes 28 filled with cooling water 29,
though which the cooling water 29 flows. When the hot-rolled steel 20 passes
through the water coolit~gp ipes 28, the coolitlg water 29 has a predetel~nit~ewda ter filrn
thickness 283. The water fi1111 thickness 283 is the average distance between the ituier
15 surface of the cooling pipes 28 and the outer surface of the hot-rolled steel 20. That is,
the water film thickness 283 is a value of a radius of the inner surface of tlie cooling
pipcs 28 minus a radius of the lot-rolled steel 20. A diameter of the hot-rolled steel 20
is substantially equal to the diameter of the steel bar 1. The hot-rolled steel 20 passes
through a plurality of thewater cooling pipes 28 under adequate conditions so that only
20 surface part of the hot-rolled steel 20 can be quenched. The surface part of the
hot-rolled steel 20 leave fro111 the water cooling apparatus 24 is reheated and
self-tempered by sensible heat of inner portion of the hot-rolled steel 20. Temperature
of the hot-rolled steel 20 just after the hot-rolling (wvl~icli is substalltially equal to the
finish rollit~gte mperature 3 1) call be measured by an infmred thermometer 23 for
measuring the finish rolling tctnperaturc installed at an exit of tl~cho t-rolling 111il1 22,
and tlic wwatcr cooling temperature 32 can be nleasurcd by an infrared tl~er~~~om2c5 tcr
for measuring the water cooling tcmperahlre installed at an exit of the water cooling
apparatus 24. The reheating temperature 33 can be measured by an infrared
5 therniometer 26 for ~neasnringr eheating temperature installed at a place in which the
reheating is perfo~lned. As shown in Figure 8, the reheating temperature 33 is the
maximurn tenmperah~reo f tlie surface of the hot-rolled steel 20 after finish of the water
cooling.
[0067]
10 If the heating tetnperah~reb efore the hot-rolling is less than 1000°C,
defommation resistance during rolling increase, and thus, rolling force increases. In this
case, deficiencies such as inipossibility of the rolling, formation of a lot of rolling flaws
even ifthe rolling can be perfor~l~eadn,d the like may occur. In addition, if the heating
temperature before the hot-rolling is more than 120OoC, deficiencies such as increasing
15 the decarburized layer thickness of tlie steel bar 1, in \w41ich the hardness after the
induction hardening lacks, and the like nmap occur.
[0068]
If the keeping time of the heating before the hot-rolling is less than 100 second,
unevenness of the'teniperature distribution in the billet increases, and tllus, cracks occur
20 during the hot-rolling. On the other hand, if the keeping time of the l~eatit~bgef ore the
hot-rolling is more than 130 second, excess decarburization occurs.
100691
If the finish temperature of tlie hot-rolling is less than 850°C, deficiencies such
as occurring the rolling flaw, and increasing deforn~ationre sistance occur. On the
other hand, if the finish tenlperature of the hot-rolling is more that1 950°C, deficicncics
such as coarsening the grain sizc of thc bcc phase after rolling tnay occur, it1 which the
structure after the induction hardening coarsens and crack propagation stoppit~g
properties of the steel bar 1 deteriorates.
5 [0070]
Thc hardening depth and the reheating temperature 33 required for the steel bar
1 according to the present embodiment can be achieved by adequately controlling a
t~utnbero f the water cooling pipe 28 (i.e. total length of tllc water cooling pipe 28),
transit speed of the hot-rolled steel 20, and flow velocity of the cooling water 29 it1 the
10 water cooling pipe 28. Passing direction 281 of cooling water is opposite to passing
direction 282 of the hot-rolled steel 20. The larger the number of the water coolit~g
pipes 28, the slower the transit speed of the hot-rolled steel 20, andlor the faster the flow
velocity of the coolit~gw ater 29, tl~ed eeper the hardenit~gd epth and the lower the
reheating temperature. On the other hand, the smaller the number of the water coolit~g
15 pipes 28, the faster the transit speed of the hot-rolled steel 20, and/or the slower the flow
velocity of the cooli~~wgat er 29, the shallowver thc hardening depth and the higher the
reheating temperature. Howcvcr, controlling cooling condition with changit~gth e total
length of the water cooli~~pigp es 28 causes enlargement and co~nplicationo f the cooling
. apparatus. In addition, controlling cooling condition with controlling the transit speed
20 of the hot-rolled steel 20 makes the productivity of the steel bar 1 unstable. Therefore,
in vie\\( of the industrial applicability, a method in which the number of the water
cooli~~pigp e 28 (i.e. the total length of tl~ew ater cooling pipe 28) and the transit speed
of the hot-rolled steel 20 arc constant and the flow velocity of the cooling water 29 is
cot~trolledis an easiest way for co~~trollinthge cooling condition.
It is liecessary that the cooliilg condition is controlled so that thc reheating
temperature (a niaxi~uumv alue of thc surface temperature of the hot-rolled steel 20
rise11 by the reheating) after the coolitlg is 500 to 600°C. For example, in a case it1
which the total length of the water cooling pipe 28 is 20111 and the transit speed of the
5 hot-rolled steel 20 is 4111/s, the lower liulit of the flow velocity of the cooling water 29
may be 0.4111/s, preferably O.6n1/s, and Illore preferably O.Xm/s. In a case in which the
total length of the water cooling pipe 28 is 20m and the transit speed of the hot-rolled
steel 20 is 4111/s, the upper limit of the flow velocity of the cooling water 29 is 21n/s. I11
a case such as the flow velocity of the cooling water 29 is escessive, the reheating
10 teliiperature after the cooling is lower than 500°C.
[0071]
-
In a process in which the hot-rolled steel 20 is in-line rapid-cooled just after the
hot-rolling, it is impoitant to evenly cool tlie hot-rolled steel 20. Utleverl cooling
causes unevenness of the hardetling depth, and thus, the uneven cooli~lgc auses
15 uneveilness of tlie structure of the hot-rolled steel 20 and tlie steel bar 1 it1
cireunifercntial direction and longituditial direction. As described above, uneven
structure (uneveil hardening depth) causes warpage of the hot-rolled steel 20 aftcr the
rapid-cooling and warpage of the steel bar 1 after the i~tductionh ardening. If a marked
warpage occurs, it is necessary to correct the warpage and yield decreases due to shape
20 failure, and tlms, the marked warpage decreases productiotl efficiency of the steel bar 1.
In order to suppress the decrease in the production efficiency of the steel bar 1, the
unevenness of the hardening depth after the rapid-cooling just after the rolling and the
relieating may be suppressed.
[0072]
44
111 order to supprcss the quencl~itigd eflection 104 in cross section, the Amax,
and thc Amin to be 1.5 111111 or less, a ratio R of the thickuess of the water tilt11 covering
the hot-rolled steel 20 and the diameter of the hot-rolled steel 20 (i.e. R = "water filtll
thickness 283" /"diameter of hot-rolled steel 20") and the flow velocity of the coolillg
5 water 29 are adequately controlled nrhile the hot-rolled steel is cooled by passing the
hot-rolled steel 20 t111.ougli the water cooling pipes 28. It is effective that R is
controlled to be a predetermined value or more and tlie flo\v velocity is controlled
within an adequate range for unifor~lllyc ooli~~thge hot-rolled steel 20. The inventors
found that in a case in which R was 0.1 or more, the qqael~chingd eflection 104 in the
10 cross section, the Amax, and the Amin of the steel bar were 1.5 Inn1 or less. Therefore,
the lower litnit of R is 0.1, preferably 0.15 and more preferably 0.2. On the other hand,
- -
if R is excess, resistance during conveyance of the hot-rolled steel 20 increases, and thus,
failure of the conveyance of the hot-rolled steel 20 occurs and productivity deteriorates.
And thus, the upper limit of R is 0.5.
15 [0073]
It is necessaly that the other cooling co~iditionsa re controlled so that the
reheating temperature 33 (the rnaximurn value of the surface ternpcrature of the
hot-rolled steel 20 risen by tlie reheating) after the cooling is 500 to 60OoC. For
example, in a case in which the total length of the water cooling pipe 28 is 20111 and the
20 transsit speed of the hot-rolled steel 20 is 4nds, the lower lunit of the flow velocity of the
cooling water 29 may be 0.4nds, preferably 0.6mls, and more preferably 0.8nds. If the
flow velocity of the cooling water 29 is excess, the reheating temperature 33 cantlot be
secured and the surface hardtless after the reheating increases, and thus, it1 a case in
n~hicltih e total length of the water cooling pipe 28 is 20111 and the transit speed of the
hot-rollcd steel 20 is 4m/s, the upper limit of the flow velocity of the cooling water 29 is
If the reheating temperature is less than 500°C, the tempering is not sufliciently
performed, and thus, the surface hardness of the steel bar increases and the
5 machinability of the steel bar deteriorates. If the reheating temperature is more than
600°C, the hardeuit~gd epth is insufficient.
[Examples]
[0074]
I.Iereinafter, the present invention will be described with exatnples. Tl~e
10 examples are merely for describing the present invention, and do not limit tlle scope of
the invention.
~-
[0075]
Hot-rolled steels having 935 ti~mw ere obtained by hot-rolling billets having
chet~~iccaolm position sliown in Figure 1, having a height of 162 nlnl and a width of 162
15 mm and having a weight of 2 tons under conditions shown in Figure 2 with a hot-rolling
mill. Just after the hot-rolling, the hot-rolled steels having 935 lnm were rapid-cooled
with a water cooling apparatus, and thcn reheated. Steel bars were obtained by
air-cooling the hot-rolled steels after the reheating to room temperature. The finish
temperature of the hot-roiling, the cooling temperature, and the reheating temperature
20 were measured with infrared thern~ometers. Positional relation between each of the
infixed thennometers, the hot-rolling mill, the water cooling apparatus, and a cooling
bed is sliown in Figures 5 to 7, and progression of the temperature of the steel bars is
shown in Figure 8.
[0076]
Hereinafter, the above-described nlethod for manufactusing will be described
with reference to thc Figures 5 to 7 showing an exanlple of sum~naryo f thc hot-rolling
line according to the present invention. The hot-rolled steels 20 were obtained by
hot-rolling the billets (steels), which were heated in a heating furllace 21, with the
5 hot-rolling ulill22. The finish rolling teulperature 3 1 was measured with an infrared
thern~otneter2 3 for measuring the finish rolling lemperatnre. Just after the hot-mlling,
the hot-rolled steels 20 were rapid-cooled with tlie water cooling apparatus 24. Then,
the hot-rolled steels 20 were reheated, the reheating temperature 33 thereof was
measured with an infrared tliern~ometer2 6 for measuring reheating temperature, atid the
10 hot-rolled steels 20 were air-cooled with the cooling bed 27. In Tables 2-1 to 2-3, the
"HEATING TEMP." was the heating ten~peratureo f tlie hot-rolled steels 20 before the
-~ -
hot-rolling, the "HEA'SING TIME" was the time during keeping the hot-rolled steels 20
before the hot-rolling within the above-described heating temperature, the "FINISH
ROLLING TEMP." was the finish temperature of the hot-rolling, "WATER FILM
15 THICKNESS / DIA. OF STEEL" was the ratio R of the thickness of the water filnl and
the diarneter of the hot-rolled steel 20 (i.c. R = "water film thickuess 283" 1 "diameter of
hot-rolled steel 20"), the "LENGTH OF WATER COOLING ZONE" was thc total
length of water cooling pipes 28, "SPEED PASSING WATER COOLING ZONE" was
the speed of the hot-rolled steels 20 passing though the water cooling zone, and
20 "FLOW VELOCITY" was the flow velocity of cooling water 29.
[0077]
Hereinafter, tl~esu rface temperature histoty of surfaces of the steel bars during
the above-described method for manufacturing will be described with reference the
Figure 8 showing example of summary of the rapid-cooling just after the hot-rolling
according to tlic present imre~~tion.C ooli~~wgva tcr 29 was poured on the surfaccs of
the hot-rolled steels 20 just after the finish rolling at the fitlish rolli~lgtc mperatore 3 1.
By the pouring, temperature of the surface parts of the hot-mlled steels 20 \wrere cooled
to water coolitlg telnperature 32. Then, the surfaces of the hot-rolled steels 20 were
5 reheated to the reheating temperature 33 by seiisible heat of inner portions of the
hot-rolled steels 20. And then, the hot-rolled steels 20 were air-cooled in the cooling
bed 27.
(Atllout~ot f warpage)
10 The steel bars I were obtained by cooling the hot-rolled steels 20 to room
temperature, and then, tlie steel bars 1 were cut to a length of 5111. Then, a string was
extended behveen the both sides of the steel bars 1 having a length of 5m, and a
distance between the string and tlie surfaces 15 of the steel bars 1 was nieasured at the
center in the lotigitudinal disection of tlie steel bars 1 having a length of 5111. The
15 measured values of the distance divided by the length of the steel bars 1 (i.e. 5111) were
assumed as the amount of warpage of the steel bars 1.
[0079]
(Decarburized layer thickness)
Decarburized layer thickness was obtained by measuring a total decarburized
20 laper thickness DM-T with a method defined in JIS G 0558 "STEELS
DETERMINATION OF DEPTH OF DECARBURIZATION.
[0080]
(Hardness of cross section and Ifardening depth)
As shonrn in Figurc 2 sho\vitlg positions CI, C2, and C3 (cross section
observatio~pi ositions) in lo~lgitudi~ldailr ection in which the cross sections 10 of the
steel bar 1 are observed, the steel bars 1 were ve~ticallyc ut in the longitudinal direction
at the thee cross section observation positions consisting of C1 and C3, which were
5 positions separated from the ends of the steel bars 1 having a length of 3500 mm, and
C2, which were in the center in the longitudinal direction of the steel bars 1. C1, C2,
and C1 were arranged at 1650 mm interval. The cut planes (cross sections 10) were
polished and the hardness thereof was nleasured based on a procedure described
hereinafter. At first, along a first line extending behveen a center 12 of a cross section
10 10 of the steel bar I and a periphery 11 of the cross section 10 of the steel bar 1,
hardness \\.as contitluously measured at arbitra~yi~ ltervalsfr om the center 12 to the
periphery 11. Next, the average hardness of the first line was calculated based on the
obtained hardness values. Then, a region having a hardness higher than the average
hardness in the first line by I-Iv20 or more was assun~eda s a hardening region 101, and
15 the depth of the hardening region 101 (hardening depth) was measured. And then,
along thc n,h line ("11" is 2 to 8 of countit~gn umber) in \vhich angle behvccn the nth line
and the 1st line was 45O x (n-I) and which extended between a center 12 of a cross
section 10 of the steel bar 1 and a periphery 11 of the cross section 10 of the steel bar 1,
tlie hardness was contitluously measured similarly to the first line. The largest of the 8
20 kinds of hardening depth obtained thereby was the maxinlu~nh ardening depth 102 in
the arbitrary cross section 10, the minimum of that was the minimum hardening depth
103 in the arbitrary cross section 10 of the steel bar 1, and difference of the maximmn
hardening depth 102 and the nlininluln hardening depth 103 was quenching deflection
104 in the cross section.
[OD8 11
Maxi~uum valne of the quenching deflection 104 ill the cross section was a
n~asitnu~vnal ue of the quenching deflection 104 it1 the cross sections at C1, Cz, and C,.
The maximum value of the quenching deflection 104 in the cross section indicated
5 uuevetuiess of hardening depth iu the cross section.
[0082]
Amin was a difference between a maximum value of the minimum hardening
depth 103 and the ininimum value of tlie ~nininlu~hnar denitig depth 103 in tlie cross
sections at C,, C2, aud C3. Alnin indicated unevenness of the l~ardet~indgep th in the
10 lotlgitudinal direction.
[0083]
Amax was a difference between a maxinlum value of the maxitnum hardening
depth 102 and the n~ininiutnv alue of the lnasimum hardening depth 102 it1 the cross
sections at CI, Cz, and C3. Anlax indicated unevenness of the hardening depth it1 the
15 lo~~gitudindailr ection.
[0084]
(Amouut of ferrite in surface 1a)~ear rea of steel bar)
The cross sections of the steel bars were polished, and etched with nital, and
photographs of structure therein at positions of 25% depth of radius of the steel bars
20 from the surfaces of the steel bas were take11 with an optical microscope and with a
magnification ratio of 500. Then, the photographs were printed out, regions \vliich
were not ferrite were painted in black, and regions which were ferrite a11d white in color
were not painted. Thereafter, the papers were bit~arizedw ith an image analyzing
device, and ratios of area of the \white regions in area of the papers (i.e, measured views)
were calculated. Thc ratios of the area of the \vllitc regions in the area of the measured
views were assu~~t~oe bde the amount of the ferrite.
[0085]
(Average value of grain size of bcc phase)
5 The average values of the grain size of the bcc phase were measured with an
Electron Back Scatteriug Diffraction (EBSD) apparatus attached to a scatuiing electron
microscope in C-cross sections of the steel ba~(si .e. cross sections perpendicular to
rolling direction of the steel bars, or cross sections of the steel bars). Hereinafter,
method for measuring will be described with reference to Figure 4.
10 The average values of the grain size of the bcc phase in the surface layer areas
13 of the steel bars 1 were obtained as follows. At f ~ s tc,r ystal orientation tnaps of the
bee phase regarding areas of 400 pm x 400 p111 in each of eight ll~easuritlgp ositions
consisting of four measuring positions in po~tions 16 of \vhich the depth were 200 prn
from the surfaces 15 of the steel bars 1 and four measuring positions in portions 17 of
15 which the depth were 25% of the radius r from the surfaces 15 of the steel bars 1.
Then, boundary in the crystal orieutation maps of the bcc phase, at which an orientation
difference was 15 degree or more, was assumed to be the grain boundary of the bcc
phase, and the average values of the grain size of the bcc phase it1 each of the eight
measuring positions were measured using method of Johnson-Saltykov (see
20 "QUANTITATIVE MICROSCOPY", Uchida Rokakuho, published at July 30, 1972, R.
T. DeHoff aud F. N. Rhines, p189). Then, the average values of the grain size of the
bcc phase in the surface layer areas 13 were obtained by further averaging the average
values of the grain size of the bcc phase in each of the eight measuring positions.
The average values of the grain size of the bcc phasc in the center areas 14 of
the steel bars 1 wwrcre n~casurcd as follows. At first, average values of thc grain sizc of
the bcc phase in each of 9 measuring positions consisting of four tneasuring positious in
portions 18 of which the depth were 50% of the radius r fro111 the surfaces 15 of the
5 steel bars 1, four measuring positious in portions 19 of which the depth were 75% of the
radius r from the surfaces 15 of the steel bars 1, and one measuring position in the
center 12 of the cross sections 10 of the steel bars I were measured using
above-described method. Theleil, the average values of the grain size of the bcc phase it1
the center area 14 wvere obtained by further averaging the average values of the grain
10 size of the bcc phase in each of the 9 tneasuritlg positions. four measuring positions
were selected so that the angles between adjacent lines which were bet~veenth e four
measuring positions and the centers 12 of the cross sections 10 of the steel bars 1 were
about 90 degrees. The four tneasuring positions in the portions 17 of which the depth
were 25% of the radius r fro111 the surfaces 10 of the steel bars 1, the four tneasuring
15 positions in the portions 18 of which the depth were 50% of the radius r from the
surfaces 10 of the steel bars 1, and the four tneasuring positions in the portions 19 of
which the depth were 75% of the radius r from the surfaces 10 of the steel bars 1 wvere
selected similarly.
[OOSS]
20 (Induction hardening)
Induction hardening was perfomled under a condition in which frequenc)~w vas
300 kHz and heating tin~ew as 1.8 sec, and tempering wvas perfonlied under a condition
in which heating temperahwe was 170°C and heating time was 1 hour. The hardness
of surfaces of the steel bars after the induction hardening were niinimom values of 8
mcasurcd values obtained by ~ueasuri~aitg 8 positio~isin thc cut sections (cross sectio~ls
10) perpendicular to the longitudinal direction of the steel bars 1, of which dcpth were
50 ptn fro111 the surfaces of the steel bars, with a micro-Vickers hardness tester of wwliicli
load was 200g. Above-described 8 positio~~wse re unifor~nlpd istribnted along
5 peripheries of the steel bars. That is, tlie angles between adjacent lines which wvere
between the 8 positions and tlie centers ofthe steel bars 1 were about 45 degree.
Samples having a hardness of less than Hv700 after the induction liardenitig were
determined as "fail" regarding it~di~ctiloiat~rd enability. "HARDNESS AFTER
INDUCTION HARDENING" showvti in Tables 2-4 to 2-6 indicates the hardness of the
10 surfaces of the steel bars after the inductior~h ardening.
(Tlwee-point bend)
Three-point bend test pieces wwrere manufactured by it~ductionh ardening the
steel bars 1 having 1935 mrn under the above-described condition, grinding the surfaces
15 15 to depth of 0.5 lnm from the surfaces 15, and walking U-notch having depth of 1
~utnat surfaces after the grinding. Tlicn, a three-point bend test was performed on tlic
tlxee-point bend test pieces in ethyl alcohol cooled to -40°C under JIS Z 2248
"METALLIC MATERIALS - BEND TEST". The type of the test pieces was No.2
test piece. Bending was perfonned by loweritlg a puticl~w ith velocity of 10 nun/niin.
20 111 addition, tlie bending was perfornied until bend angle of the test pieces is 150 degree.
The test pieces in which breaking occurred during the tlxee-point bend test were
detemiined as "fail".
[0088]
(Impact value)
Test piece ~naterialsh aving height of 10 mm, width of 10 nlln, and a length of
55 mm were cut off from centcrs of the cross sections 10 of the stcel bars 1. U-notches
having a depth of 2 Inn1 were fornled in the test piece materials to lnanufacturc U-notch
charpy inlpact test pieces. Charpy impact test at -40°C was perfonned on the U-notch
6 charpy impact test pieces in accordance with JIS Z 2242 "METHOD FOR CHARPY
PENDUI,UM IMPACT TEST OF METALLIC AIATERIALS", and test pieces of
which absorbed energy in the Charpy impact test were less than 90 .I/cn~w~er e
deter~nineda s "fail"
[0089]
10 As shown in Table 3, inventive examples \\,ere excellent in unevenness of
hardening depth, fiacture morphology, which indicates crack propagation stopping
properties, in the thee-point bend test, and impact value in cotnparison with
comparative exarnples of wl~ichth e amount of C was same thereto, as well as there was
no probletn in haxdness after the induction hardening.
15 [0090]
In conlparative example No.21, amount of C was lower that1 the defined range,
and thus, the surface layer hardness after reheating was low, the hardness after induction
hardeniug was low, and inductiou hardenability was poor.
[0091]
20 111 comparative examples 22 to 30, the finish rolling temperature was higher
than the defined range, and thus, the average values of the grain size of the hcc phase in
the surface layer areas and the center areas exceeded the defined range. In addition, in
comparative exa~nples2 2 to 30, the crack propagation formed at the bottom of the notch
did not stop, and breaking occu~sedd uring the three-point bend test. Furtlierlliore, t11c
i~npacvt alues of the conlparative exa~llplcsN o. 22 to 30 were low.
[0092]
111 comparative examples 31 to 39, the flow velocity of cooling water was high,
5 the comparative examples 3 1 to 39 were excessively cooled, and reheating te~liperature
was low. Thus, the surface hardness after reheating of the comparative examples 3 1 to
39 was higher than the defined range, and workability was poor.
[0093]
111 con~parativee xanlples 40 to 48, heating temperature before hot-rolling was
10 high, heating time before the hot-rolling was long, and the finish rolling telnperature
was low. In the cotnparative exan~ples4 0 to 48, total decarburized layer thickness
exceeded the defined range, tlle hardness after the induction hardening mas low, and the
induction hardenability was poor.
[0094]
15 In comparative examples No. 49 to 57, the finish rolling temperature was lower
than the defined range and the flow velocity of tlle cooling water after the hot-rolling
was slow, and tllus, the reheating temperature exceeded the defuied range. 111 the
comparative examples 49 to 57, area ratio of ferrite excessed the defined range, and thus,
quenching was it~completelyp erfornied. Therefore, tlle grain size of bcc phase in
20 surface layer areas and center areas thereof coarsened, crack propagation formed at the
bottom of the notch did not stop and breaking occutsed, impact values thereof were low,
and base material toughness thereof were lo\\'. In addition, tnaximu~uq uenching
deflection in the cross section, Amax, and Amin therein, \vI~ich indicated unevenness of
hardening depth, escccdcd thc defined ranges, and thus, the amount of ~varpagcw as
large and productivity was deterioratcd.
[0095]
I11 comparative esatnples No. 58 to 66, water film thickness with respect to the
5 diameter of the steel bars were thin, and thus, Amas, and Aniin therein, which indicated
an llllevemless of hardetiing depth, esceeded the defined ranges, the amount of warpage
was large, and productivity was deteriorated.
[0096]
[Table 1-11
[0097]
[Table 1-21
[0098]
[Table 1-31
[0099]
[Table 2-11
[O 1001
[Table 2-21
[OlOl~
[rable 2-31
[O 1021
[Table 2-41
[0103]
[Table 2-51
[0 1041
[Table 2-61
[Reference Signs I,ist]
[0105]
1: Steel bar
10: Cross section
I I: Periphery
12: Center
13: Surface layer area
14: Center area
15: Smface
16: Poltion of which the depth is 200 pm
Portiot~o f which the depth is 25% of the radius
Portion of which the depth is 50% of the radius
Portio~oi f which the depth is 75% of the radius
Hardening region
Maximum hardening depth of cross section
Mininiuo~h ardening depth of cross section
Quenching deflection in cross section
-,Reyiulrof which a depth from the surface is 50 }un~
C,, CZ, and C3: Cross section observation positions
20: Not-rolled steel
21: Heating ft~rnace
22: Hot-rolling Inill
23: Infrared thern~otneterf or llleasuring finish rolling ternperatuse
Watcr coolitlg apparatus
Infrared tllennomctel. for measuring water cooling temperature
Infrared thernlometer for tneasuring reheating tetnperature
Coolitlg bed
Water cooling pipe
Cooling water
Passing direction of coolitlg water
Passing direction of hot-rolled steel
Water film thickness
Finish tetnperature
Water cooling temperature
Reheating temperature
TABLE 1-1

TABLE 1-3

1 PROPERTIES OF STEEL
AREA RATIO SURFACE LAYER AVERAGE DR.
TYPE AVERAGE
CURVE DM-T
OF FERRITE IN HARDNESS
SURFACE AFTER OF bcc IN
LAYER AREA REHEATING
SURFACE
CENTER AREA
IMPACT
VALUE
(-40°C)
RESULT OF
THREE-POIN?
BEND TEST
(-40°C)
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
UNBLOKEN
HARDNESS
AFTER
INDUCTION
HARDENING

[Doclrmcnt type] Clainis
1. A steel bar,
wherein the steel bar comprises, as a chcmical composition in terms of mass%:
C: 0.30 to 0.80%;
Si: 0.01 to 1.50%;
Mn: 0.05 to 2.50%;
AI: 0.010 to 0.30%;
N: 0.0040 to 0.030%;
10 P: 0.035% or less;
S: 0.10% or less;
Cr: 0 to 3.0%;
Mo: 0 to 1.5%;
Cu: 0 to 2.0%;
Ni: 0 to 5.0%;
B: 0 to 0.0035%;
Ca: 0 to 0.0050%;
Zr: 0 to 0.0050%,
Mg: 0 to 0.0050%;.
Kern: 0 to 0.0150%;
Ti: 0 to 0.150%;
Nb: 0 to 0.150%;
V: 0 to 1 .O%;
M': 0 to 1 .ow.
Sb: 0 to 0.0150%;
211: 0 to 0.50%;
Te: 0 to 0.20%;
Bi: 0 to 0.50%;
Pb: 0 to 0.50%, and
a re~naindeirn cluding Fe and impurities,
wvliereitl a region which is along a line extending between a center of a cross
section of the steel bar and a periphery of the cross section of the steel bar and which
10 has a hardness higher than an average hardness in the line by Hv20 or more is a
hardening region it1 the line, a minimum value of depth of the hardening regions in the 8
lines of which the angle is 45" is a tniturnun~h ardening depth in the cross section, and a
maximum value of the depth of the hardening regions it1 the 8 lines is a maximutn
hardening depth in the cross section,
15 wherein a difference between the maxinlutn hardening depth in the cross
section and the minimum hardening depth in the cross section is 1.5 lmn or less,
wherein a difference bctwwen a maximum value of the maxitnum hardening
depth and a minimum value of the ~tllaximu~hna rdening depth in the cross sections at 3
points which are separated from each other by 1650 mn1 parallel to a lot~gitudinal
20 direction of the steel bar is 1.5 lmn or less,
wlierein a differelice between a nlaximum value of the niini~nulnh ardelling
depth and a minimum value of the minimum hardening depth in the cross sections at the
3 points which are separated from each other by I650 mm parallel to the longitudiual
direction of the steel bar is 1.5 luln or less,
wherein a structure in an area from a surface of the steel bar to a depth of 25%
of ZI radius of the stccl bar includes 10 arm% or less of a fcrritc and a remainder
including one or Inore selected from a group consisting of a bainite and a martensite,
wherein a boundary between grains \\~hicha re adjacent to each other and of
5 which an orientatioti difference is 15 degree or more is a graiii boundary, am1 an
equivalent circle diaineter of an area surrounded by the grain boundary is a grain size,
wl~ereitla n average value of the grain size of a bcc pllase in tlli area from the
surface of the steel bar to tlie depth of 25% of the radius of the steel bar is 1.0 to 10.0
10 wherein an average value of the grain size of the bcc phase in an area from the
depth of 50% of the radius of the steel bar to the center of the steel bar is 1.0 to 15,O pm,
wherein a hardness of a region of which a depth from the surface is 50 ptn is
Iiv200 to Hv500, and
\\therein a total decarburized layer tliick~lessD M-T is 0.20 nun or less.
15
2. The steel bar according to claitn 1, comprising, as the cl~cmicacl oinposition in
one or Inore selected from tile group consisting of
Cr: 0.1 to 3.0%;
Mo: 0.10 to 1.5%;
Cu: 0.10 to 2.0%;
Ni: 0.1 to 5.0%; and
B: 0.0010 to 0.0035%
3. The steel bar according to c1ai111 1 or 2, comprisitlg, as the chemical
composition in tcr~nso f mass%:
one or more selected fro~tlh~e group consisting of
Ca: 0.0001 to 0.0050%;
Zr: 0.0003 to 0.0050%;
Mg: 0.0003 to 0.0050%; and
Rem: 0.0001 to 0.01 50%.
4. The steel bar according to ally one of clai~l~1 sto 3, colllprising, as the chelllical
10 colnpositiotl in terms of mass%:
one or more selected fro111 the group consisting of
'Ii0:. 0030 to 0.0150%;
Nb: 0.004 to 0.150%;
V: 0.03 to 1.0%; and
W: 0.01 to 1.0%.
5. The steel bar accordii~gto any one of claims 1 to 4, comprising, as the chen~ical
cornpositiotl in terms of mass%:
one or more selected fro111 the groitp cootlsistitlg of
Sb: 0.0005 to 0.0150%;
Sn: 0.005 to 2.0%;
Zn: 0.0005 to 0.50%;
Te: 0.0003 to 0.20%;
Bi: 0.005 to 0.50%; .