[Name of Document] DESCRIPTION
[Title of the Invention] STEEL WIRE ROD OR STEEL BAR HAVING EXCELLENT
COLD FORGEABILITY
[Technical Field]
5 [OOOI]
The present invention relates to a steel wire rod or steel bar (including bar-in-coil;
the same shall apply hereinafter) as hot-rolled having excellent cold forgeability afier
spheroidizing annealing. This application claims the priority right of Japanese Patent
Application No. 2012-86844, filed in Japan on April 5, 2012, and the content of which is
10 incorporated herein.
[Background Art]
[0002]
Recently, there is a growing need for cold forging that can reduce or abbreviate
machining such as cutting, for improvement in productivity. As compared to hot forging,
15 cold forging has a problem that deformation resistance is high, and deformability
(ductility) is poor, thus there are problems that mold crack and steel crack are likely to be
caused.
[0003]
Therefore, the steel material to be subjected to cold forging is generally subjected
20 to spheroidizing annealing aiming at reducing deformation resistance and improving
deformability. Patent Literature 1 discloses a wire rod or steel bar having excellent cold
workability, that is softened by specifying the ferrite fraction to have low deformation
resistance even as hot rolled.
25 In addition, it is lcnown that deformability after spheroidizing annealing is
strongly affected by a structure before spheroidizing annealing, i.e., pre-structure. For
example, Patent Literature 2 discloses a method for improving deformability by using a
pre-structure having a pro-eutectoid ferrite fraction of 5 to 30% by area, with the balance
comprising a structure mainly consisting of bainite, and in which, also, the average value
of the lath interval of cementite in the bainite is set to 0.3 pm or more. Also, Patent
5 Literature 3 discloses "steel wire rod or bar steel for case hardening having excellent cold
forgeability after spheroidizing" in which refinement of carbide is possible when
performing spheroidizing annealing and having high deformability by having a mixed
structure comprising ferrite, bainite and pearlite and specifying the area fraction of the
bainite to 30% or more. In addition, Patent Literature 4 discloses an invention in
10 consideration of preventing crack during cold working for the structure after spheroidizing
annealing by specifying the ferrite fraction of the surface layer structure to 10% or less.
[Prior Art Literatures]
[Patent Literatures]
[0005]
15 [Patent Literature 11 JP 2002-146480A
[Patent Literature 21 JP 2001-89830A
[Patent Literature 31 JP 2005-220377A
[Patent Literature 41 JP 2001-18 1791A
[Summary ofthe Invention]
20 [Problems to Be Solved by the Invention]
[0006]
Patent Literature 1 is originally a technique that can omit annealing, and, different
from a technique of preventing crack of steel material that is an essential problem in cold
working with high working degree, is not a technique to improve the crack of steel
25 material.
[0007]
The methods disclosed in Patent Literature 2, Patent Literature 3 and Patent
Literature 4 relate to a technique of preventing crack of steel material that is an essential
problem in cold working with high working degree. However, also regarding these
methods, there has been still a room for further improvement for preventing crack. The
5 present invention has been made in consideration of the problems described above, and an
object of the present invention is to provide a steel wire rod or steel bar for cold forging as
hot-rolled having excellent ductility after spheroidizing annealing, that can prevent crack
of steel materia1,that is afi inhibiting factor of cold forging in working with further higher
working degree.
10 [Means for Solving the Problems]
[OOOS]
The present inventors have intensively studied, and consequently found that it is
useful for improving deformability to prevent the crack of steel material during cold
forging to appropriately control the surface roughness of the steel basis material, in
15 addition to the steel material component and pre-structure before spheroidizing annealing
[0009]
The present invention has been made based on the above novel knowledge, and
the gist of the presel~ti nvention is as described below.
[OO lo]
20 [I1
A steel wire rod or steel bar as hot-rolled, having excellent cold forgeability,
including,
by mass %, as a chemical composition,
C: 0.1 to 0.6%,
Si: 0.01 to IS%,
Mn: 0.05 to 2.5%,
Al: 0.015 to 0.3%,
N: 0.0040 to 0.0150%, and
P: limited to 0.035% or less,
S: limited to 0.025% or less, and the balance substantially consisting of iron and
5 unavoidable impurities, wherein a depth of d (mm) from the surface of the surface layer
region with 20 HV 0.2 or more higher, relative to HV 0.2 that is the average hardness in the
region where the depth from the surface is from sectional radius R x 0.5 (mm) to the center
satisfies the following formula (I); the steel structure of the surface layer region has a
ferrite fraction of 10% or less by area ratio, with the balance being one or two or more of
10 martensite, bainite and pearlite; the steel structure where the depth from the surface is from
the sectional radius R x 0.5 (mm) to the center is ferrite-pearlite or ferrite-bainite; and the
surface roughness Ra in the circumferential direction when scales adhering to the surface
have been removed is 4 pm or less.
0.5 2 d/R 2 0.03 .. . (1)
15 [0011]
[21
The steel wire rod or steel bar according to [I], further including one or two or
more of,
by mass %, as the chemical composition of the steel,
20 Cr: 3.0% or less,
Mo: 1.5% or less,
Cu: 2.0% or less,
Ni: 5.0% or less, and
B: 0.0035% or less.
25 [0012]
[3 I
The steel wire rod or steel bar according to [I] or [2], further including one or two
or more of,
by mass %, as the chemical composition of the steel,
Ca: 0.005% or less,
Zr: 0.005% or less,
Mg: 0.005% or less, and
Rem: 0.015% or less.
[00 131
141
10 The steel wire rod or steel bar according to any of [I] to [3], further including one
or two or more of,
by mass %, as the cllemical composition of the steel,
Ti: 0.20% or less,
Nb: 0.1% or less,
15 V: 1 .O% or less, and
W: 1.0% or less.
[0014]
r51
The steel wire rod or steel bar according to any of [I] to [4], further including one
20 or two or more of,
by mass %, as a chemical composition of the steel,
Sb: 0.0150% or less,
Sn: 2.0% or less,
Zn: 0.5% or less,
Te: 0.2% or less,
Bi: 0.5% or less, and
Pb: 0.5% or less.
[00 151
[61
The steel wire rod or steel bar according to any of [I] to [5], further satisfying the
5 following formula (2), by mass %, as the chemical composition of the steel.
31Si+15Mn+23Cr+26Mo+lOOV> 55 ... Formula(2)
[00 161
[71
The steel wire rod or steel bar according to any of [I] to [6], further including,
10 by mass %, as the chemical composition of the steel,
Ti: 0.02 to 0.20% and
B: 0.0005 to 0.0035%.
[Effects of the Invention]
[OO 171
15 The steel wire rod or steel bar of the present invention can prevent crack of steel
material that occurs during cold forging. The present invention can realize cold forging
with high working degree that is conventionally impossible, or abbreviate intermediate
annealing of the stcp in which cold forging is conventionally inlpossible without
intermediate annealing.
20 [Brief Description of the Drawing]
[0018]
[FIG. 11 FIG. 1 is a graph showing a relationship between the value of formula (2)
and tempered hardness at 300°C.
[Modes for Carrying out the Invention]
25 [0019]
Hereinafier, embodiments for carrying out the present invention will be described
in detail. First, the reason for limiting the chemical composition of the present invention
will be described. Hereinafter, % by mass in the composition is simply denoted by %.
[0020]
C: 0.1 to 0.6%
5 C is an element having a major effect on the basic strength of the steel material.
However, in a case where the C content is less than 0.1%, a sufficient strength cannot be
obtained, and other alloy elements must be further added in large amounts. On the other
hand, with a C content exceeding 0.6%, the material hardness increases, and deformation
resistance markedly increases, resulting in significant degradation in machinability.
10 Accordingly, in the present invention, the C content is set to 0.1 to 0.6%. The preferred
range is from 0.4 to 0.6%.
[0021]
Si: 0.01 to 1.5%
Si is an element effective for deoxidization of steel, and is also an element
15 effective for strengthening ferrite and improving temper softening resistance. With Si
less than 0.01%, the effects are insufficient. On the other hand, with Si exceeding 1.5%,
the steel becomes brittle, material characteristics degrade, also, machinability significantly
deteriorates, and fuillier, carburizing properties are inhibited. Accordingly, the Si content
needs to be set in the range of 0.01 to 1.5%. The preferred range is from 0.05 to 040%.
20 [0022]
Mn: 0.05 to 2.5%
Mn fixes and disperses S in steel as MnS. Also, Mn is an element necessary to
improve hardenability and secure strength after quenching by foiming a solid solution in
the matrix. However, with an Mn content of less than 0.05%, S in steel bonds with Fe so
25 as to form FeS, and the steel becomes brittle. On the other hand, when the Mn content
increases, specifically, the Mn content exceeds 2.5%, the hardness of the basis material
increases, cold workability degrades, and also the effects on strength and hardenability are
also saturated. Accordingly, the Mn content is set to 0.05% to 2.5%. The preferred
range is from 0.30 to 1.25%.
[0023]
5 Al: 0.015 to 0.3%
A1 is effective for, besides deoxidization of steel, fixation of solid solution N
present in steel as AlN, and crystal grain refinement. Also, when B is contained, it is
useful for securing solid s~lutionB . In order to obtain the above effects, 0.015% or more
of A1 is required. However, with a content exceeding 0.3%, A1203 is excessively
10 produced, and degradation of fatigue strength and cold forging crack are caused, thus the
A1 content is set to 0.015% to 0.3%.
[0024]
N: 0.0040 to 0.0150%
N bonds with Al, Ti, Nb and V in steel to produce nitride or carbonitride, and
15 suppresses coarsening of crystal grain. In addition, with a content less than 0.0040%, the
effect is insufficient. However, with a content exceeding 0.0150%, the effect is saturated,
and also non-solid solution carbonitride does not form a solid solution and remains during
heating before hot lolling or hot forging, thus it is difficult to increase the amount of fine
carbonitride effective to suppress coarsening of crystal grain. Accordingly, the content
20 thereof needs to be set in the range of 0.0040 to 0.0150%.
[0025]
P: 0.035% or less
When the P content increases, specifically, with a P content exceeding 0.035%,
the hardness of the basis material increases in steel, and cold workability, hot worlcability
25 and casting characteristics also degrade. Accordingly, the P content is set to 0.035% or
less. The preferred range is 0.02% or less.
S: 0.035% or less
With an S content exceeding 0.035%, MnS is coarsened, and becomes a starting
point of crack during cold working. For the above reason, the S content needs to be set to
5 0.035% or less. The preferred range is 0.01% or less.
[0027]
Furthermore, as optionally contained elements, for improving hardenability and
imparting strength, one or two or more of Cr: 3.0% or less, Mo: 1.5% or less, Cu: 2.0% or
less, Ni: 5.0% or less and B: 0.0035% or less may be contained.
10 [0028]
Cr: 3.0% or less
Cr is an element for improving hardenability and also imparting temper softening
resistance, and is added to steel in which a high strength is required. In order to stably
improve hardenability, the Cr content is desirably 0.1% or more. Also, when Cr is
15 contained in an amount exceeding 3.0%, Cr carbide is produced, and steel becomes brittle.
Accordingly, in the present invention, when Cr is contained, the content thereof is set to
3.0% or less. The preferred range is from 0.1 to 2.00%.
[0029]
Mo: 1.5% or less
20 Mo is an element for imparting temper softening resistance and also improving
hardenability, and is added to steel in which a high strength is required. In order to stably
improve hardenability, the Mo content is desirably 0.01% or more. Also, even when Mo
is contained in an amount exceeding 1.5%, the effects are saturated. Accordingly, when
Mo is contained, the content thereof is set to 1.5% or less. The preferred range is from
25 0.05 to 0.25%.
[0030]
Cu: 2.0% or less
Cu is an element effective for strengthening ferrite and also improving
hardenability and improving corrosion resistance. In order to stably improve
hardenability and corrosion resistance, the Cu content is desirably 0.1% or more. Also,
Ij 5 even when Cu is contained in an amount exceeding 2.0%, the effects are saturated in terms
!
I
I of mechanical properties. Accordingly, when Cu is contained, the content thereof is set to
:!
2.0% or less. Meanwhile, Cu particularly degrades hot ductility, and causes defect during
rolling, and thus is preferzbly added together with Ni.
[003 11
10 Ni: 5.0% or less
Ni is an element effective for strengthening ferrite, improving ductility and also
improving hardenability and improving corrosion resistance. In order to stably improve
hardenability and corrosion resistance, the Ni content is desirably 0.1% or more. Also,
even when Ni is contained in an amount exceeding 5.0%, the effects are saturated in terms
15 of mechanical properties, and machinability degrades. Accordingly, when Ni is contained,
the content thereof is set to 5.0% or less.
[0032]
B: 0.0035% or less
Solid solution B improves hardenability and also improves grain boundary
20 strength, and improves fatigue strength and impact strength as machine parts. In order to
stably improve hardenability and cold workability, the B content is desirably 0.0005% or
more. Also, even when B is contained an amount exceeding 0.0035%, the effects are
saturated in terms of mechanical properties, and further, hot ductility markedly degrades.
Accordingly, when B is contained, the content thereof is set to 0.0035% or less.
25 [0033]
Furthermore, as optionally contained elements, one or two or more of Ca, Zr, Mg
and Rem may be contained.
[0034]
Ca: 0.005% or less
Ca is a deoxidizing element, and produces an oxide. In steel containing 0.015%
5 or more as total A1 (T-AI) as in the steel of the present invention, calcium aluminate
(CaOA1203) is formed when Ca is contained. CaOA1203 is an oxide having a lower
melting point as compared to A1203, thus serves as a tool protective film during
high-speed cutting, and improves machinability. In order to stably improve machinability,
the Ca content is desirably 0.0002% or more. Also, with a Ca content exceeding 0.005%,
10 CaS is produced in steel, and conversely, machinability degrades. Accordingly, when Ca
is contained, the content thereof is set to 0.005% or less.
LO0351
Zr: 0.005% or less
Zr is a deoxidizing element, and produces an oxide in steel. The oxide is
15 considered to be Zr02, and this Zr02 becomes a precipitation nucleus of MnS, thus has
effects of increasing the precipitation sites of MnS and uniformly dispersing MnS. In
addition, Zr also has an action of forming a solid solution in MnS so as to produce a
complex sulfide, lowcr deformability, and suppress stretching of MnS during rolling and
hot forging. As such, Zr is an element effective for reducing the anisotropy. In order to
20 stably obtain these effects, the Zr content is desirably 0.0003% or more. On the other
hand, even when Zr is contained in an amount exceeding 0.005%, the yield becomes
extremely poor so as to produce large amounts of hard compounds such as Zr02 and ZrS,
and conversely, mechanical properties such as machinability, impact values and fatigue
characteristics degrade. Accordingly, when Zr is contained, the content thereof is set to
25 0.005% or less.
COO361
Mg: 0.005% or less
Mg is a deoxidizing element, and produces an oxide in steel. Moreover, hard
A1203 is modified into MgO or A1203-Mg0, which is relatively soft and finely dispersed
to improve machinability. In addition, an oxide thereof is liable to become a nucleus of
5 MnS, and also has an effect of finely dispersing MnS. In order to stably obtain these
effects, the Mg content is desirably 0.0003% or more. Also, Mg produces a complex
sulfide with MnS and spheroidize MnS; however, when Mg is excessively contained,
specifically, with an Mg content exceeding 0.005%, the production of sole MgS is
accelerated and conversely deteriorates machinability. Accordingly, when Mg is
10 contained, the content thereof is set to 0.005% or less.
[0037]
Rem: 0.015% or less
Rem (rare earth element) is a deoxidizing element, produces an oxide having a
low melting point, and suppresses nozzle clogging during casting, and also has an action of
15 forming a solid solution in MnS or bonds with MnS, lower the deformability thereof, and
suppress stretching of the MnS shape during rolling and hot forging. As such, Rem is an
element effective for reducing the anisohopy. In order to stably obtain these effects, the
Rem content is desirably 0.0001% or more. Also, with Rem is contaiucd in an amount
exceeding 0.015%, a large amount of a sulfide of Rem is produced, and machinability
20 deteriorates. Accordingly, when Rem is contained, the content thereof is set to 0.015% or
less.
[0038]
Furthermore, as optionally contained elements, one or two or more of Ti, Nb, V
and W may be contained.
25 [0039]
Ti: 0.20% or less
Ti is an element that forms carbonitride, contributes to suppression of the growth
or strengthening of austenite grains, and is used as a granulating element for preventing
coarsening of grains in steel in which a high strength is required and steel in which a low
strain is required. In addition, Ti is also a deoxidizing element, and has an effect of
5 forming a soft oxide so as to improve machinability. In order to stably obtain the above
effects, the content is preferably 0.001% or more. In addition, with a Ti content
exceeding 0.1%, a non-solid solution coarse carbonitride which causes hot cracking is
precipitated, and converscly, mechanical properties are impaired. Accordingly, when Ti
is contained in the present invention, the content thereof is set to 0.20% or less. The
10 preferred range is from 0.001 to 0.20%.
[0040]
Nb: 0.1% or less
Nb is also an element that forms carbonitride, contributes to strengthening of steel
through secondary precipitation hardening, and suppression of the growth and
15 strengthening of austenite grains, and is used as a granulating element for preventing
coarsening of grains in steel in which a high strength is required and steel in which a low
strain is required. In order to stably obtain the effect of increasing the strength, the Nb
content is desirably 0.01% or more. In addition, when Nb is contained in an amount
exceeding 0.1%, a non-solid solution coarse carbonitride which causes hot cracking is
20 precipitated, and conversely, mechanical properties are impaired. Accordingly, when Nb
is contained, the content thereof is set to 0.1% or less.
[0041]
V: 1 .O% or less
V is also an element that forms carbonitride and can strengthen steel through
25 secondary precipitation hardening, and is contained in steel in which a high strength is
required. However, in order to stably obtain the effect of increasing the strength, the V
content is desirably 0.03% or more. In addition, when V is contained in an amount
exceeding 1.0%, a non-solid solution coarse carbonitride which causes hot cracking is
precipitated, and conversely, mechanical properties are impaired. Accordingly, when V is
contained, the content thereof is set to 1 .O% or less.
5 [0042]
W: 1.0% or less
W is also an element that forms carbonitride and can strengthen steel through
secondary precipitation hardening. In order to stably obtain the effect of increasing the
strength, the W content is desirably 0.01% or more. In addition, when W is contained in
10 an amount exceeding 1.0%, a non-solid solution coarse carbonitride which causes hot
cracking is precipitated, and conversely, mechanical properties are impaired. Accordingly,
when W is contained, the content thereof is set to 1 .O% or less.
[0043]
Furthermore, as optionally contained elements, one or two or more of Sb, Sn, Zn,
15 Te, Bi and Pb may be contained.
[0044]
Sb: 0.0150% or less
Sb makes fcrrite brittle to an appropriate extent, and improves machinability. In
order to stably obtain the effect of improving machinability, the Sb content is desirably
20 0.0005% or more. In addition, when the Sb content increases, specifically, exceeds
0.0150%, the macro segregation of Sb becomes excessive, and the impact value
significantly decreases. Accordingly, the Sb content is set to 0.0150% or less.
[0045]
Sn: 2.0% or less
25 Sn has effects of making ferrite brittle so as to extend the service life of a tool and
improving the surface roughness. In order to stably obtain these effects, the Sn content is
desirably 0.005% or more. Also, even when Sn is contained in an amount exceeding
2.0%, the effects are saturated. Accordingly, when Sn is contained, the content thereof is
set to 2.0% or less.
[0046]
5 Zn: 0.5% or less
Zn has effects of making ferrite brittle so as to extend the service life of a tool and
improving the surface roughness. In order to stably obtain these effects, the Zn content is
desirably 0.0005% or mcre. Also, even when Zn is contained in an amount exceeding
0.5%, the effects are saturated. Accordingly, when Zn is contained, the content thereof is
10 set to 0.5% or less.
[0047]
Te: 0.2% or less
Te is a machinability-improving element. In addition, Te has an action of
producing MnTe, and coexisting with MnS so that the deformability of MnS degrades and
15 stretching of the MnS shape is suppressed. As such, Te is an effective element for
reducing anisotropy. In order to stably obtain these effects, the Te content is desirably
0.0003% or more. In addition, with a Te content exceeding 0.2%, not only is the effect
saturated, but hot duclility also degrades such that it is highly likely that tlcfects are caused.
Accordingly, when Te is contained, the content thereof is set to 0.2% or less.
20 [0048]
Bi: 0.5% or less
Bi is a machinability-improving element. In order to stably obtain the effect of
improving machinability, the Bi content is desirably 0.005% or more. In addition, even
when Bi is contained in an amount exceeding 0.596, not only is the
25 machinability-improving effect saturated, but hot ductility also degrades such that it is
highly likely that defects are caused. Accordingly, when Bi is contained, the content
thereof is set to 0.5% or less.
[0049]
Pb: 0.5% or less
Pb is a machinability-improving element. In order to stably obtain the effect of
5 improving machinability, the Pb content is desirably 0.005% or more. In addition, even
when Pb is contained in an amount exceeding 0.5%, not only is the
machinability-improving effect saturated, but hot ductility also degrades such that it is
highly likely that defects are caused. Accordingly, when Pb is contained, the content
thereof is set to 0.5% or less.
10 [0050]
In addition to the above composition range, Si, Mn, or further one or two or more
of Cr, Mo and V are contained so as to satisfy the following formula (2), whereby the steel
wire rod or steel bar of the present invention can be molded to, for example, a gear, by cold
forging, and then when carburized, quenched and tempered and used, softening resistance
15 after carburizing quenching and tempering is increased, and high temperature hardness can
be kept high, and it is possible to improve the surface fatigue strength. The gear
instantaneously reaches about 300°C by the friction when meshing, thus softening at
tempering of 300°C is suppressed and the hardness is secured, whereby it is possible to
manufacture gear parts having further excellent surface fatigue strength.
20 [0051]
Si, Mn, Cr, Mo and V are conventionally efficient for temper softening resistance.
In the level of steel 30 with a component composition of C: 0.11 to 0.60% (% by mass, the
same shall apply hereinafter.), Si: 0.10 to 1.5%, IMn: 0.05 to 2.46%, P: 0.01 to 0.03 %, S:
0.007 to 0.01%, Al: 0.02 to 0.025%, Cr: 0 to 3.0%, Mo: 0 to 1.5%, V: 0 to 0.4% and N:
25 0.0040 to 0.0140%, as a result 01 investigating tempered hardness at 300°C of the steel
material by performing carburizing, quenching and tempering (quenching was performed
after gas carburizing in the conditions of 950°C x 300 minutes and a carbon potential of
0.8, then tempering at 150°C x 90 minutes was performed.) and then retaining the steel at
30OoC x 90 minutes, it has been found that there is a certain relationship between the value
of formula (2) and tempered hardness at 30OoC, as shown in FIG. 1. Based on FIG. 1,
5 the value of the formula (2) is set to 55 or more, whereby it is possible to obtain tempered
hardness of JIS SCM 420 or more at 30OoC, commonly used as a gear.
31% + 15Mn + 23Cr + 26Mo + 100V > 55 ... Formula (2)
[0052]
When B: 0.0005 to 0.0035% and Ti: 0.02 to 0.20% are contained, B improves
10 hardenability, and Ti fixes N as TiN to suppress production of BN and increase the amount
of solid solution B, whereby hardenability can be further increased. Furthermore, the
steel wire rod or steel bar of the present invention can be molded to, for example, a gear,
by cold forging, and then when carburized, quenched and tempered and used, solid
solution B is segregated in particle boundary after carburizing, quenching and tempering,
15 thereby increasing the grain boundary strength, and it is possible to manufacture parts
excellent in low-cycle fatigue strength.
[0053]
Next, the reasons for specifying the structure and hardness appltcd to the present
invention will be described.
20 [0054]
The present inventors have intensively studied for a means of improving ductility
of a steel wire rod for cold forging, and revealed that, in order to prevent forging crack, it is
important that the structure after spheroidizing annealing is uniform and fine. Moreover,
in order to achieve that, it was found to be effective that the ferrite haction was suppressed
25 to the specific amount or less, for the structure before spheroidizing annealing of the steel
wire rod, and the balance was a mixed structure of one or two or more of fine martensite,
bainite and pearlite.
[0055]
The present invention is a steel wire rod or steel bar as hot-rolled, wherein a depth
of d (mm) from the surface of the surface layer region with 20 HV 0.2 or more higher,
5 relative to HV 0.2 that is the average hardness in the region where the depth from the
surface is from sectional radius R x 0.5 (mm) to the center satisfies the following formula
(1). Also, the steel structure of the surface layer region comprises a ferrite fraction of
10% or less, with the balance being one or two or more of martensite, bainite and pearlite.
Moreover, the steel structure where the depth from the surface is from the sectional radius
10 R x 0.5 (mm) to the center is ferrite-pearlite or ferrite-bainite.
0.5 2 c!iR 2 0.03 ... (1)
[0056]
Here, d is a depth (rnm) from the surface of the surface layer region with 20 HV
0.2 or more higher, relative to HV 0.2 that is the average hardness in the region where the
15 depth from the surface is from sectional radius R x 0.5 (mm) to the center. R is a
sectional radius of a steel wire rod or steel bar.
[0057]
The reasons h r specifying the hardness distribution and structuic distribution will
be described.
20 [0058]
In a case where a cylindrical member is upset, it is dynamically prone to cracking
more on the surface, but the present inventors have experimentally investigated at what
depth from the surface should be uniform and fine structure that is hardly cracked. As a
result, when a depth of d from the surface of the surface layer region with 20 HV 0.2 or
25 more higher, relative to HV 0.2 that is the average hardness in the region where the depth
from the surface is from sectional radius R x 0.5 (mm) to the center is less than 0.03R,
cracking occurs from the vicinity of depth d, and critical cracking characteristics
deteriorate, thus it was set as d 2 0.03 R. With d exceeding 0.5 R, deformation resistance
markedly increases, causing a reduction in mold life, thus it was set as d I 0.5 R.
I00591
5 The reason why the ferrite fraction of the surface layer region is set to 10% or less
by area ratio is as follows. When the ferrite fraction of the structure (pre-structure) before
spheroidizing annealing is high, dispersion of cementite after spheroidizing annealing
concentrates on the portion other than ferrite portion in the pre-structure. As a result,
distribution of cementite after spheroidizing annealing becomes nonuniform, and critical
10 cracking characteristics deteriorate. This phenomenon becomes remarkable with a ferrite
fraction exceeding 10% by area ratio, thus the fraction is limited to 10% or less, and is
preferably 5% or less and more preferably 3% or less. A structure of the balance other
than the ferrite is one or two or more of the martensite, bainite, and pearlite.
[0060]
15 In the steel structure where the depth from the surface is from sectional radius R x
0.5 (mm) to the center, ferrite-pearlite or ferrite-bainite are used, and as long as satisfying
the above hardness distribution, the structure fraction is not particularly limited.
[0061]
In order to have the hardness distribution and structure distribution described
20 above, by pouring water to the surface of the steel material immediately after the finish
rolling, the water pouring is stopped after once cooling the surface temperature of the steel
material to 100 to 60OoC, and the surface temperature of the steel material is recuperated to
200 to 700°C with internal potential heat. Thus, it is possible to suppress ferrite
transformation of the surface layer, and set the ferrite fraction to 10% or less, with the
25 balance as a mixed structure of one or two or more of marlensite, bainite and pearlite. In
the present invention, a steel wire rod or steel bar that is hot rolled and then cooled by
pouring water to the surface of the steel material is referred to as a "steel wire rod or bar as
hot-rolled".
[0062]
On the other hand, as the steel structure where the depth from the surface is from
5 the sectional radius R x 0.5 (mrn) to the center, an effect of pouring water to the surface of
the steel material is small, thus ferrite is produced and forms ferrite-pearlite or
ferrite-bainite.
[0063]
Next, the reason for specifying the surface roughness will be described.
10 [0064]
After subjecting a steel wire rod or steel bar as hot-rolled to spheroidizing
annealing, critical cracking characteristics in a case where upsetting is performed by a test
piece cut in the longitudinal direction are affected by the surface roughness of the basis
material. Here, in the steel wire rod or steel bar as hot-rolled, the surface of the basis
15 material is in a state of being covered by scales. In a case where the surface roughness is
simply measured, the surface roughness of the scales that cover the basis material is
measured, and the surface roughness of the basis material affecting the critical cracking
characteristics cannot be known. Therefore, the scales adhered to the surface are
removed, and the surface roughness in the circumferential direction is measured, whereby
20 it is possible to measure the surface roughness of the basis material affecting the critical
cracking characteristics. As a result of investigating the surface roughness and critical
cracking characteristics after removing scales from a rolled material rolled in various
conditions to greatly change the surface roughness, the critical cracking characteristics
degrade as the surface roughness is high, but when the surface roughness is reduced to Ra
25 5 4 pm, the critical cracking characteristics do not degrade, thus it was specified as Ra 5 4
pm. Ra was calculated according to the Ra defined in JIS B0601: '82.
[0065]
Here, scales can be removed by pickling, shot blasting and the like. Pickling is
carried out, for example, in the treatment conditions in a hydrochloric acid solution with a
concentration of 10% by mass at 60°C for an immersion time of 3 to 14 minutes
5 (preferably 4 to 12 minutes, more preferably 5 to 10 minutes). Other than the
hydrochloric acid, sulfuric acid may be used. Shot blasting is carried out, for example, by
projecting a steel ball with a diameter of 0.5 mm and a hardness of 47.3 HRC at a
projection density of 90 Kglm3 and a projection velocity of 70 mls.
[0066]
10 In order to have a surface roughness Ra in the circumferential direction when
pickling the steel wire rod or steel bar of 4 pm or less, it is necessary to appropriately carry
out descaling before rough rolling, after extracting the billet from the heating furnace, and
also to keep the surface temperature of the steel material during passing the rolled material
from rough rolling to finish rolling high at a constant temperature or more. It is achieved
15 by having a minimum temperature of the surface temperature of the steel material during
passing the rolled material of 860°C or more, preferably 900°C or more, and further
preferably 910°C or more. When the surface temperature of the steel material during
passing the rolled material is low, deformability deteriorates to form fine wrinkle-like
deformation, thus the surface roughness increases. After extracting the billet from the
20 heating furnace, the descaling before hot rolling or during rolling is usually carried out by
high water pressure, and in order to appropriately carry out descaling, it is necessary to set
the descaling water pressure high. However, at a high descaling water pressure, the
surlace temperature of the steel material during passing the rolled material is lowered, thus,
in order to secure the minimum temperature, billet heating temperature and descaling water
25 pressure need to be appropriately properly set.
[Examples]
[0067]
Hereinafter, the present invention will be specifically described in detail based on
examples. These examples are provided to describe the present invention, and do not
limit the scope of the present invention.
5 [0068]
162 mm square billets having the chemical compositions shown in Table 1 and
Table 2 were rolled in the conditions of Table 3 and Table 4. As for all examples except
for test No. 17, test pizces were collected from steel bars after beiug rolled, and
microstructure and hardness distribution, and surface roughness after pickling were
10 investigated. As for test No. 17, after being rolled, the outer periphery was lathe turned
by one side of 0.5 mm to form a 4144 steel bar, further a test piece was collected from the
steel bar, and microstructure and hardness distribution, and surface roughness were
investigated.
[0069]
15 Next, the steel bars once cooled to room temperature after being rolled (for test
No. 17, after being cut) were heated and retained in the range of Acl + 5°C to Ac3 - 5'C for
20 minutes, and subjected to spheroidizing annealing heat treatment of cooling the steel
bars to Acl - 70°C :1t a cooling rate of 5.5"Cihr or less. Then, an upsetting test was
performed with a compression test piece cut perpendicular to the rolling direction of the
20 steel bar so as to be a height of 1.5 times of the rolling diameter in the longitudinal
direction to investigate the critical compression ratio. The results are collectively shown
in Tables 3 and 4.
[0070]
[Hardness distribution, microstructure]
25 For a steel bar in which section (C section) cut perpendicular to the rolling
direction of the steel bar was embedded with resin, the hardness distribution was examined
in 100 pm pitch using micro Vickers in the condition of a test force of 1.961 N, and the
region with 20 HV 0.2 or more higher, relative to HV 0.2 that is the average hardness in the
region where the depth from the surface is from sectional radius R x 0.5 (mm) to the center
was defined as a depth of d mm from the surface.
5 [0071]
Next, under an optical microscope, the surface layer part was observed at a total
of eight points at a 200 pm depth from the surface layer and a d mm depth from the surface
layer in the four directio~sd ifferent by 90 degrees on the C section of the wire rod, at a
magnification of 1000 times, and the ferrite fraction was measured. In the range from the
10 surface layer to d mm, the balance of the ferrite was one or two or more of the martensite,
bainite and pearlite.
[0072]
[Surface rouglmess]
In a case of pickling, the steel bar was pickled by being immersed in a
15 hydrochloric acid solution with a concentration of 10% by mass at a temperature of 60°C
for 5 to 10 minutes, and after visually confirming that scale was removed from the entire
circumference, roughness in the circumferential direction was measured, and Ra as defined
in JIS B0601: '82 was calculated.
[0073]
20 [Critical compression test]
The compression ratio (%) to have a failure probability of 50% from the upsetting
test in the conditions to have a strain rate of 10s - I was invesiigaied. Craclting was
defined as cracking with a crack length of 0.5 mm or more, observed visually, or under an
optical microscopy as necessary. Due to pressure on the mold surface, the upper limit of
25 the compression ratio was set to 80%. When cracking did not occur at 80%, the critical
compression ratio was defined to be 80%.
[0074]
As is apparent from Table 3 and Table 4, it can be seen that the critical
compression ratios of inventive examples (test Nos. 1 to 27, 37 to 78) are remarkably
excellent as compared to the critical compression ratios of comparative examples (test Nos.
5 28 to 36).
[0075]
In test Nos. 28, 31 and 32 of comparative examples, since the range of d was
outside of the specification, and the surface layer structure before spheroidizing annealing
was not good, the cementite after spheroidizing annealing was not sufficiently uniformly
10 dispersed, and thus the critical compression ratio was reduced. It was caused by
insufficient cooling due to laclc of water amount during cooling in Nos. 28 and 31, and
rapid material passing rate in water-cooling band in No. 32.
[0076]
In comparative examples Nos. 29 and 30, since the rolling temperature was low,
15 deformability during rolling deteriorated, thus the surface roughness deteriorated, and the
critical limit compression ratio was reduced.
[0077]
In comparalive examples Nos. 33 and 34, the chemical composit~ono f P or S that
lowers the cold workability exceeded the specification of the present application, and
20 working limit was consequently lowered.
[0078]
In comparative example No. 35, after extracting the billet from the heating
furnace, the descaling water pressure before hot rolling was too low, thus descaling was not
sufficiently performed. Therefore, the surface roughness exceeded the speciication of
25 the present application, and the working limit was consequently lowered.
[0079]
In comparative example No. 36, after extracting the billet from the heating
furnace, the descaling water pressure before hot rolling was too high, thus the minimum
temperature on the surface of the steel material during passing of the rolled material was
low, and the billet was outside of the specification of the present application. Therefore,
5 deformability during rolling deteriorated, thus the surface roughness deteriorated, and the
working limit was lowered.
[0080]
Furthermore, for Examples 37 to 78, carburizing, quenching and tempering
(quenching was performed after gas carburizing in the conditions of 950°C x 300 minutes
10 and a carbon potential of 0.8, then tempering at 15O0C x 90 minutes was performed.) were
performed after spheroidizing annealing.
[0081]
[Surface fatigue strength]
A small roller (with a cy!indrical surface with a diameter of 26 mm x width of 18
15 rnm) for a roller pitting test was prepared, and a roller pitting fatigue test was conducted in
the conditions of a Hertz stress of 3000 MPa, a slip ratio of -40%, and an ATF oil
temperature of 80°C. The number of repetitions until pitting occurred was listed in Table
4. In a case where pitting did not occur, the roller pitting fatigue test was repeated until
10,000,000 times.
20 [0082]
[Low-cycle fatigue strength]
A four-point bending fatigue test piece (13 mm x 80 mm L, 3 mm V notch in the
central part) was prepared, and a four-point bending low-cycle fatigue test was performed
at a frequency of 1 Hz with a sine wave at a stress ratio of 0.1. In Table 4, 500 times
25 strength was listed.
[00831
The surface fatigue strength is high in Examples 37 to 76 satisfying the formula
(2), as compared to Examples 77 and 78.
[0084]
It can be seen that Examples 57 to 78 containing Ti: 0.02 to 0.20% and B: 0.0005
5 to 0.0035% are excellent in low cycle fatigue as compared to Examples 37 to 56 not
containing Ti and B.
[0085] [Table 11
[0086] [Table 21
[Table 31
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[0088] [Table 41
pame of Document] CLAIMS
[Claim 1]
A steel wire rod or steel bar as hot-rolled, having excellent cold forgeability,
comprising:
5 by mass %, as a chemical composition,
C: 0.1 to 0.6%,
Si: 0.01 to IS%,
Mn: 0.05 to 2.5%,
Al: 0.015 to 0.3%,
10 N: 0.0040 to 0.0150%, and
P: limited to 0.035% or less,
S: limited to 0.025% or less, and the balance substantially consisting of iron and
unavoidable impurities, wherein a depth of d (rnm) from the surface of the surface layer
region with 20 HV 0.2 or more higher, relative to IN 0.2 that is the average hardness in the
15 region where the depth from the surface is from sectional radius R x 0.5 (mm) to the center
satisfies the following formula (1); the steel structure of the surface layer region has a
ferrite fraction of 10% or less by area ratio, with the balance being one or two or more of
martensite, bainite and pearlite; the steel structure where the depth from the surface is from
the sectional radius R x 0.5 (mm) to the center is ferrite-pearlite or ferrite-bainite; and the
20 surface roughness Ra in the circumferential direction when scales adhering to the surface
have been removed is 4 pm or less.
0.5 > d/R > 0.03 . . . (1)
[Claim 2]
The steel wire rod or steel bar according to claim 1, further comprising one or two
25 or more of,
by mass %, as the chemical composition of the steel:
Cr: 3.0% or less,
Mo: 1.5% or less,
Cu: 2.0% or less,
Ni: 5.0% or less, and
5 B: 0.0035% or less.
[Claim 3]
The steel wire rod or steel bar according to claim 1 or 2, further comprising one or
two or more of,
by mass %, as the chemical composition of the steel,
10 Ca: 0.005% or less,
Zr: 0.005% or less,
Mg: 0.005% or less, and
Rem: 0.015% or less.
[Claim 4]
15 The steel wire rod or steel bar according to any of claims 1 to 3, further
comprising one or two or more of,
by mass %, as the chemical composition of the steel,
Ti: 0.20% 01, less,
Nb: 0.1% or less,
V: 1 .O% or less, and
W: 1.0% or less.
[Claim 5]
The steel wire rod or steel bar according to any of claims 1 to 4, further
comprising one or two or more of,
by mass %, as a chemical composition of the steel,
Sb: 0.0150% or less,
Sn: 2.0% or less,
Zn: 0.5% or less,
Te: 0.2% or less,
Bi: 0.5% or less, and
5 Pb: 0.5% or less.
[Claim 6]
The steel wire rod or steel bar according to any of claims 1 to 5, furtl~ers atisfying
the followii1g formula (2); by mass %, as the chemical composition of the steel.
31% + 15IVhl+ 23Cr i- 26Mo + 10OV 2 55 .. . Formula (2)
10 [Claim 7]
The steel wire rod or steel bar according to any of claims 1 to 6, hrther
conlprising:
by mass %, as the chemical composition of the steel.