ROLLED STEEL BAR OR WIRE ROD FOR HOT FORGING
TECHNICAL FIELD
[0001]
The present invention relates to a rolled steel bar or wire rod for hot
forging. More particularly, the present invention relates to a rolled steel bar
or wire rod for hot forging, which is able to be welded and excellent in strength
and toughness (especially toughness at low temperatures).
BACKGROUND ART
[0002]
With regard to suspension parts for an automobile, such as a spindle, a
high strength and an excellent toughness are required.
[0003]
As the starting material for such a part, in general, the "Carbon and
Alloy Steel for Machine Structural Use" containing medium carbon level, such
as S48C specified in JIS G 4051 (2009) or SCM435H specified in JIS G 4051
(2009), has been used.
[0004]
Usually, the above-described steel is subjected to a quench-and-temper
heat treatment after being hot forged, and thereafter is worked into the
predetermined shape by machining. Further, in order to join to other parts,
the said machined steel is subjected to working such as bolt hole forming and
spline shaft grinding, and thereby is finished into the desired part.
[0005]
However, the cost for working a high strength part, which is the said
machined steel, into the desired part is very high. Therefore, the said
machined part has been desired to join to other parts by welding.
[0006]
If the content of C is set to low level, weld cracking can be prevented, so
that the parts can be welded.
[0007]
However, if the content of C is set to low level, it is difficult to provide the
part with a high strength.
[0008]
Furthermore, in the case of welding, grains coarsen in the heat affected
zone (hereinafter, referred to as "HAZ"), so that it is difficult to attain an
excellent toughness especially at low temperatures. Moreover, with regard to
the suspension part for an automobile such as a spindle to which a great impact
is assumed to be applied, not only the decrease in toughness of the HAZ must
be restrained, but also the toughness of the base metal itself of part must be
enhanced.
[0009]
Therefore, assuming that the part is used in a cold district, the starting
material for the above-described part is required to be provided with a HAZ and
base metal that can have an excellent toughness especially at low
temperatures.
[0010]
Accordingly, the Patent Literature 1 discloses the "WELDABLE STEEL
HAVING HIGH STRENGTH AND HIGH TOUGHNESS, AND METHOD FOR
PRODUCING MEMBER USING THE SAME".
LIST OF PRIOR ART DOCUMENT
PATENT LITERATURE
[0011]
Patent Literature 1: JP 2007-84909 A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0012]
In the technique disclosed in the Patent Literature 1, due consideration
has not been given to the toughness of HAZ and base metal, and the weld
cracking. Furthermore, in order to enhance the toughness of base metal, it is
necessary that the heating temperature at the time of hot forging be lowered to
a temperature of 1100°0 or less, and a controlled forging be carried out.
However, if the heating temperature at the time of hot forging is lowered, the
deformation resistance of a steel material, which is a starting material,
increases. Therefore, the technique disclosed in the Patent Literature 1 has
not necessarily been desirable in terms of producing parts.
[0013]
With regard to a starting material which is required to have weldability,
there is a thick plate. Unlike the thick plate, a steel bar and a wire rod, which
are starting materials used for parts such as a spindle, are formed into a part
shape by hot forging after heating a rolled material to a high temperature of
about 1200°0; and thereafter they are further subjected to a
quench-and-temper heat treatment. Therefore, in the case where the whole of
the starting material is heated again to a high temperature of about 1200°0
after rolling, the steel bar and the wire rod are also required to have stable
toughness as the base metal.
[0014]
The present invention has been made in view of the above-described
situation, and accordingly an objective thereof is to provide a rolled steel bar or
wire rod for hot forging, which is able to be welded, moreover is excellent in
strength and toughness of a base metal and in toughness of a HAZ, and is
suitable as a starting material for suspension parts of an automobile, such as a
spindle.
MEANS FOR SOLVING THE PROBLEMS
[0015]
The present inventors first carried out various examinations and studies
concerning the prevention of weld cracking and the attainment of excellent
strength of base metal among the above-described problems. As a result, the
following findings were obtained.
[0016]
(a) Concerning the weld cracking:
(a-I) The occurrence of weld cracking depends on the maximum hardness
of HAZ (hereinafter, referred to as "Hmax"). If Hmax in Vickers hardness
(hereinafter, referred to as "HV hardness") exceeds 400, weld cracking is liable
to occur.
[0017]
(a-2) With regard to an index for predicting Hmax, a carbon equivalent
(Ceq) has often been used. For example, JIS G 3106 (2008) describes the
following formula;
Ceq = C + (Sil24) + (Mnl6) + (Nil40) + (Cr/5) + (Mo/4) + (V/14);
wherein each symbol C, Si, Mn, Ni, Cr, Mo and V represents the content by
mass percent of the element concerned.
[0018]
Many relational formulas between Ceq and Hmax have been proposed.
However, the present inventors predicted Hmax by using the following formula
based on the data given in EXAMPLES described after;
Hmax = 583 x Ceq + 65.
[0019]
(a-3) From the above items (a-I) and (a-2), it can be seen that in order to
prevent weld cracking, it is effective to regulate not only the contents of C, Si,
Mn and Cr as elements to be contained intentionally in the base metal but also
the contents of Ni, Mo and V in impurities.
[0020]
(b) Concerning the strength of base metal
(b-I) The content of C has the greatest influence on Hmax correlating
with weld cracking, in other words, on Ceq. Therefore, it may be
advantageous that weld cracking should be prevented by decreasing the
content of C, and the strength should be attained by containing elements for
enhancing the hardenability in place of C.
[0021]
(b-2) In order to attain a sufficient strength (800 MPa or more in tensile
strength) after a quench-and-temper heat treatment has been carried out,
especially after tempering at 400 to 500°C (particularly, tempering at 475°C)
has been carried out, the chemical composition of base metal may
advantageously be controlled so that the ideal critical diameter (Dr) expressed
by the following formula is 70 or more;
DI = 25.4 x 0.311 x CO.5 x (1 + 0.64 x si) x (1 + 4.10 x Mn) x (1 + 2.83 x p)
x (1 - 0.62 x S) x (1 + 2.33 x Cr) x (1 + 0.52 x Ni) x (1 + 3.14 x Mo) x (1 + 0.27 x
Cu) x {I + 1.50 x (0.90 - C)};
wherein each symbol C, Si, Mn, P, S, Cr, Ni, Mo and Cu in the above formula
represents the content by mass percent of the element concerned.
[0022]
Even if weld cracking can be prevented, and in addition, a high strength
can be attained in the base metal, by using the above-described method, the
HAZ and base metal are not necessarily excellent in toughness.
[0023]
Accordingly, the present inventors next carried out various examinations
and studies concerning the attainment of excellent toughness of HAZ and base
metal, which were left ones of the above-described problems. As a result, the
following findings were obtained.
[0024]
(c) Concerning the toughness of HAZ
(c-1) When welding is performed, the HAZ IS heated to a high
temperature exceeding 1200°C. With regard to a "pinning particle" for
restraining grain coarsening in the HAZ at such a high temperature, TiN
(titanium nitride) having a high dissolving temperature may advantageously
be used because carbides orland carbo"nitrides dissolve in matrix at such a high
temperature.
[0025]
(c-2) TiN does not dissolve in matrix even at the time of heating for hot
forging and at the time of heating for quenching. Therefore, if TiN is present
in a proper size at the stage of steel bar or wire rod after hot rolling, the said
grain coarsening in the HAZ can be restrained when welding is performed.
That is to say, by controlling the precipitation mode of TiN, which precipitates
at the stage of steel bar or wire rod after hot rolling, the toughness of HAZ can
be enhanced.
[0026]
(d) Concerning the toughness of base metal
(d-1) Even if the chemical composition of base metal is controlled so that
the DI is 70 or more, in the case where the contents of C, Si, Mn and Cr become
too high, the hardness of base metal also becomes too high; and thus the
toughness of base metal decreases.
[0027]
(d-2) The toughness of base metal is affected by not only the contents of C,
Si, Mn and Cr but also the contents of P, S, Cu, Ni and Mo in impurities.
[0028]
(d-3) In order to attain an excellent toughness of base metal, the chemical
composition of base metal may advantageously be controlled so that the DI does
not exceed 170.
[0029]
(d-4) In order to enhance the toughness of base metal, in addition to the
control of the DI, the coarsening of austenite grains, which occur at the time of
heating for quenching, must be prevented.
[0030]
(d-5) The aforementioned TiN has a large size as a pinning particle at the
time of heating for quenching, so that the pinning effect is little. In contrast,
TiC and TiC in which N is dissolved, that is to say Ti(C, N), are finer than TiN,
so that TiC and Ti(C, N) act as a "pinning particle" when heating for quenching
to a temperature of about 900°C is performed, and have an effect for
restraining the coarsening of austenite grains.
[0031]
(d-6) In the case where TiC and Ti(C, N) have a too small size, TiC and
Ti(C, N) dissolve in matrix on account of the heating at the time of hot forging
to a temperature of about 1200°C. On the other hand, in the case where TiC
and Ti(C, N) have a large size, the number of TiC and Ti(C, N) decreases.
[0032]
(d-7) In order for TiC and Ti(C, N) to act sufficiently as a "pinning
particle" when the heating for quenching is performed, it is important that a
large number of fine TiC and Ti(C, N) be distributed before the heating for
quenching, that is to say, in the state after hot forging.
[0033]
(d-8) In the state in which TiC and Ti(C, N) are present in a proper size at
the stage of steel bar or wire rod after hot rolling, the above condition (d-7) can
be satisfied; and therefore, the coarsening of austenite grains can be restrained
when the heating for quenching is performed. That is to say, by controlling
the precipitation mode of TiC and Ti(C, N), which precipitate at the stage of
steel bar or wire rod after hot rolling, the toughness of base metal can be
enhanced. For this reason, the parts can be used under low temperature
environments.
[0034]
(d-9) Ti (titanium) combines with N in preference to C. Therefore, in
order to use both of [TiN] and [TiC and Ti(C, N)] as "pinning particles", it is
necessary that Ti be contained in an amount sufficient to combine with both of
N and C.
[0035]
The present invention has been accomplished on the basis of the
above-mentioned findings. The main points of the present invention are the
rolled steel bar or wire rod for hot forging described in the following (1) and the
method for producing a rolled steel bar or wire rod for hot forging described in
the following (2).
[0036]
(1) A rolled steel bar or wire rod for hot forging, having a chemical
composition comprising, by mass percent;
C: 0.10 to 0.20%,
Si: 0.01 to 0.30%,
Mn: 1.00 to 2.30%,
S: 0.040% or less,
Cr: 0.10 to 0.80%,
AI: 0.010 to 0.080%,
B: 0.0002 to 0.0050%,
Ti: 0.010 to 0.080%, and
N: 0.0020 to 0.0080%,
with the balance being Fe and impurities,
in which the contents of P, Cu, Ni, Mo and V among the impurities are
restricted so that
p: 0.040% or less,
Cu: less than 0.10%,
Ni: less than 0.10%,
Mo: less than 0.05%, and
V: 0.01% or less;
further, the fnl expressed by the following formula (1) is 0.001 or more, the Ceq
expressed by the formula (2) is 0.57 or less, and the DI expressed by the
formula (3) is 70 to 170;
and further, in an area of 100 Jlm2, ten or more Ti precipitates of 0.07 to 1.0 Jlm
in equivalent circle diameter and ten or more Ti precipitates of 0.01 to 0.05 Jlm
in equivalent circle diameter are present;
fnl = Ti - 3.4N ... (1),
Ceq = C + (Si/24) + (Mnl6) + (Nil40) + (Cr/5) + (Mo/4) + (V/14) ... (2),
DI = 25.4 x 0.311 x CO.5 x (1 + 0.64 x Si) x (1 + 4.10 x Mn) x (l + 2.83 x P)
x (1 - 0.62 x S) x (1 + 2.33 x Cr) x (1 + 0.52 x Ni) x (1 + 3.14 x Mo) x (1 + 0.27 x
Cu) x {I + 1.50 x (0.90 - C)} ... (3);
wherein each symbol Ti, N, C, Si, Mn, Ni, Cr, Mo, V, P, Sand Cu in the above
formulas represents the content by mass percent of the element concerned.
[0037]
The aforementioned Ti precipitates indicate TiN, TiC and THc, N). In
the following, TiC in which N is dissolved, that is to say THc, N), is sometimes
referred to as TiC.
[0038]
In addition, the term "impurities" so referred to in the phrase "the
balance being Fe and impurities" indicates those elements which come from the
raw materials such as ore and scrap, and/or the production environment when
the steel material is produced on an industrial scale.
[0039]
(2) A method for producing a rolled steel bar or wire rod for hot forging, in
which,
a molten steel having the chemical composition according to the above (1)
is continuously cast under the condition that the cooling rate is 2.0°C/min or
more; and
a bloom obtained by the casting process is heated to a temperature range
of 1100°C or more and is held for 30 minutes or more, being subjected to
blooming and steel bar rolling or wire rod rolling under a condition satisfying
the following formula (4), and moreover, after being subjected to steel bar
rolling or wire rod rolling, the rolled steel bar or wire rod is cooled at a cooling
rate of 5 to 100°C/min in a temperature range of 800 to 600°C;
Y = (Tl + 273) x log (tl + t2T(2» ::: 7.5 x 103 ••• (4);
in the formula (4), T is a heating temperature in the unit of "oC", t is a hoiding
time at heating temperature T in the unit of "s", subscript 1 representing
blooming process and subscript 2 representing steel bar rolling process or wire
rod rolling process, and T(2) equals (T2 + 273)/(TI + 273).
[0040]
That is to say, Tl is a heating temperature (oC) in the blooming process,
T2 is a heating temperature (oC) in the steel bar rolling process or wire rod
rolling process, tl is a holding time (s) at T10C in the blooming process, and t2 is
a holding time (s) at T2°C in the steel bar rolling process or wire rod rolling
process.
[0041]
The temperature and cooling rate in the each process mentioned above
refer to the temperature and cooling rate such that the surface is a reference.
..J{t
/1
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0042]
The rolled steel bar or wire rod for hot forging of the present invention is
able to be welded, moreover is excellent in strength and toughness of base
metal and in toughness of HAZ, and therefore can be suitably used as a starting
material for suspension parts of an automobile, such as a spindle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043]
[Fig. 1] Figure 1 is a schematic VIew for explaining the positional
relationship between a test piece having a width of 25 mm for lap joint welding
used in the EXAMPLES and an R/2 portion ("R" represents a radius) in a round
bar having a diameter of 36 mm from which the said test piece is cut off. In
this figure, the direction of a greater dimension of test piece is referred to as a
"width direction".
[Fig. 2] Figure 2 shows the dimensions and shape of a test piece for lap
joint welding used in the EXAMPLES, in which the units of the dimensions are
"mm".
[Fig. 3] Figure 3 is a schematic view for explaining a state in which two
test pieces for lap joint welding are used to perform lap joint welding.
[Fig. 4] Figure 4 is a schematic view for explaining a position at which
the grain size in a HAZ was measured by using test pieces subjected to lap joint
welding that were cut longitudinally into two equal parts at the central position
in the width direction, and further, were cut to 30 mm so that, in the
longitudinal cross section, the weld metal was positioned in the central portion
in the longitudinal direction in the EXAMPLES, in which the units of the
dimensions are "mm".
[Fig. 5] Figure 5 is a schematic view for explaining a state in which an
HV hardness was measured by using test pieces subjected to lap joint welding
that were cut longitudinally into two equal parts at the central position in the
width direction, and further, were cut to 30 mm so that, in the longitudinal
cross section, the weld metal was positioned in the central portion in the
longitudinal direction in the EXAMPLES, in which the units of the dimensions
are "mm".
MODE FOR CARRYING OUT THE INVENTION
[0044]
In the following, all of the requirements of the present invention are
described in detail. In the following description, the symbol "%" for the
content of each element means "% by mass".
[0045]
(A) Chemical composition of the steel bar or wire rod:
C: 0.10 to 0.20%
C (carbon) has an effect for increasing the hardness of martensite after
quenching of steel. In addition, C has an action for restraining grain
coarsening at the time of heating for quenching by forming TiC together with Ti.
In order to sufficiently achieve these effects, it is necessary that 0.10% or more
of C be contained. On the other hand, C increases the hardness of HAZ, and
resultantly causes weld cracking. Therefore, the upper limit of the C content
is set; and the content of C is set to 0.10 to 0.20%. The content of C is
preferably set to 0.12% or more, and 0.18% or less.
[0046]
Si: 0.01 to 0.30%
Si (silicon) is an element effective for deoxidizing a steel, and also
contributes to the improvement in hardenability. In order to achieve these
effects, it is necessary that 0.01% or more of Si be contained. On the other
hand, if the content of Si exceeds 0.30%, not only the above-described effects are
.,..l2'
13
saturated but also the hot ductility decreases. Therefore, the content of Si is
set to 0.01 to 0.30%. The content of Si is preferably set to 0.25% or less.
[0047]
Mn: 1.00 to 2.30%
Mn (manganese) has an action for enhancing the tensile strength due to
the improvement of hardenability. In order to achieve this effect, it is
necessary that the content of Mn be set to 1.00% or more. On the other hand,
if the content of Mn becomes excessive, the hardness of HAZ increases, and
resultantly weld cracking does occur. Therefore, the upper limit of the Mn
content is set; and the content ofMn is set to 1.00 to 2.30%. The content ofMn
is preferably set to 1.50% or more, and 2.00% or less.
[0048]
S: 0.040% or less
S (sulfur) is an element contained as an impurity in the steel. In
addition, if being contained positively, S combines with Mn to form MnS, so
that S has an effect for improving the machinability. However, if the content
of S increases and especially exceeds 0.040%, S segregates at the grain
boundaries, and decreases the toughness of base metal. Therefore, the content
of S is set to 0.040% or less. In the case where importance is attached to the
toughness of base metal, the content ofS is preferably set to 0.020% or less, and
the lower the content of S is, the more desirable. On the other hand, in the
case where importance is attached to the machinability, it is desirable to
positively contain S exceeding 0.020%.
[0049]
Cr: 0.10 to 0.80%
Cr (chromium) is an element effective in enhancing the hardenability.
In order to achieve this effect, it is necessary that 0.10% or more of Cr be
contained. On the other hand, if the content of Cr becomes excessive, the
hardness of HAZ increases, and resultantly weld cracking does occur.
Therefore, the upper limit of the Cr content is set; and the content of Cr is set to
0.10 to 0.80%. The content of Cr is preferably set to 0.20% or more, and 0.60%
or less.
[0050]
AI: 0.010 to 0.080%
AI (aluminum) is added as a deoxidizer. In order to achieve this effect, it
is necessary that 0.010% or more of AI be contained. However, even if AI
exceeding 0.080% is contained, the above-described effect is saturated, and the
alloying cost alone increases. Therefore, the content of AI is set to 0.010 to
0.080%.
[0051]
B: 0.0002 to 0.0050%
B (boron) is a very important element for enhancing the hardenability.
In order to achieve this effect, it is necessary that 0.0002% or more of B be
contained. On the other hand, if the content of B exceeds 0.0050%, not only
the hardenability improving effect is saturated but also the cost increases.
Therefore, the upper limit of the B content is set; and the content of B is set to
0.0002 to 0.0050%. The content of B is preferably set to 0.0030% or less.
[0052]
Ti: 0.010 to 0.080%
Ti (titanium) is an important element in the present invention. That is
to say, Ti combines with free N to preferentially form TiN, and therefore has an
effect of preventing B, which is effective for hardenability, from combining with
N. The said TiN acts as a pinning particle, and has an effect for restraining
grain coarsening in the HAZ at the welding time and enhancing the toughness.
Further, Ti combines with C to form TiC. The said TiC also acts as a pinning
particle, and has an effect for restraining the coarsening of austenite grains in
the base metal at the time of heating for quenching to a temperature of about
900°C and enhancing the toughness. In order to achieve these effects, it is
necessary that 0.010% or more of Ti be contained. However, if the content of
Ti increases and exceeds 0.080%, coarse TiN is formed; and thus the toughness
of base metal is deteriorated. Therefore, the content of Ti is set to 0.010 to
0.080%. The content of Ti is preferably set to 0.020% or more, and 0.060% or
less.
[0053]
N: 0.0020 to 0.0080%
N (nitrogen) is an important element in the present invention. That is
to say, as described above, TiN formed by the combination of N with Ti acts as a
pinning particle, restrains the grain coarsening in the HAZ at the welding time,
and enhances the toughness. In order to achieve these effects, it is necessary
that 0.0020% or more of N be contained. On the other hand, when the content
of N increases BN, which decreases the hardenability improving effect of B, is
formed. Further, the excessive amount ofN forms coarse TiN which decreases
the toughness of base metal. Therefore, the upper limit of the N content is set;
and the content of N is set to 0.0020 to 0.0080%.
[0054]
The rolled steel bar or wire rod for hot forging of the present invention
has a chemical composition comprising elements described above with the
balance being Fe and impurities, in which the contents of P, Cu, Ni, Mo and V
among the impurities are restricted to the ranges described below, and further,
the fnl expressed by the aforementioned formula (1) is 0.001 or more, the Ceq
expressed by the formula (2) is 0.57 or less, and the DI expressed by the
formula (3) is 70 to 170.
[0055]
p: 0.040% or less
P (phosphorus) is an element contained as an impurity in the steel. If
the content of P increases and especially exceeds 0.040%, P segregates at the
grain boundaries, and decreases the toughness of base metal. Therefore, the
content of P among the impurities is set to 0.040% or less. The content of P
among the impurities is preferably set to 0.030% or less.
[0056]
Cu: less than 0.10%
Cu (copper) is sometimes contained in the steel as an impurity though in
small amounts. Like C, Si, Mn and Cr, Cu has an influence on the
hardenability, and enhances the hardenability. If the hardenability becomes
excessive, the toughness of base metal decreases, so that the content of Cu
must be set to as low as possible. Therefore, the content of Cu among the
impurities is set to less than 0.10%.
[0057]
Ni: less than 0.10%
Ni (nickel) is sometimes contained in the steel as an impurity though in
small amounts. Like C, Si, Mn and Cr, Ni has an influence on the hardness of
HAZ and the hardenability, and enhances the hardness of HAZ and the
hardenability. If the hardness of HAZ increases, the danger of the occurrence
of weld cracking increases, and if the hardenability becomes excessive, the
toughness of base metal decreases. Therefore, the content of Ni among the
impurities must be set to as low as possible, and is set to less than 0.10%.
[0058]
Mo: less than 0.05%
Mo (molybdenum) is sometimes contained in the steel as an impurity
though in small amounts. Like C, Si, Mn and Cr, Mo has an influence on the
hardness of HAZ and the hardenability, and enhances the hardness of HAZ and
the hardenability. If the hardness of HAZ increases, the danger of the
occurrence of weld cracking increases, and if the hardenability becomes
excessive, the toughness of base metal decreases. Therefore, the content ofMo
among the impurities must be set to as low as possible, and is set to less than
0.05%.
[0059]
v: 0.01% or less
V (vanadium) is sometimes contained in the steel as an impurity though
in small amounts. Like C, Si, Mn and Cr, V has an influence on the hardness
of HAZ, and enhances the hardness of HAZ. If the hardness of HAZ increases,
the danger of the occurrence of weld cracking increases. Therefore, the
content ofV among the impurities must be set to as low as possible, and is set
to 0.01% or less.
[0060]
fnl: 0.001 or more
Ti combines with N in preference to C. Therefore, in the present
invention in which both of TiN and TiC are used as pinning particles, it is
necessary that Ti be contained in an amount sufficient to combine with both of
Nand C.
[0061]
In the case where the fnl, that is to say, the value expressed by the
following formula is less than 0.001, Ti of a sufficient amount to combine with C
cannot be obtained, so that the pinning effect of TiC is insufficient;
fnl = Ti - 3AN ... (1).
Therefore, the fnl expressed by the said formula (1) is set to 0.001 or more.
The value of the fnl can be 0.073 in the case where the content of Ti is 0.80%,
which is the upper limit regulated by the present invention, and the content of
N is 0.0020%, which is the lower limit regulated by the present invention.
[0062]
Ceq: 0.57 or less
The occurrence of weld cracking depends on Hmax (the maxImum.
hardness of HAZ), and if Hmax exceeds 400 in HV hardness, weld cracking is
liable to occur. The said Hmax is correlated with the Ceq expressed by the
following formula (2);
Hmax =583 x Ceq + 65,
Ceq = C + (Sil24) + (Mnl6) + (Nil40) + (Cr/5) + (Mo/4) + (V/14) ... (2).
[0063]
Therefore, in order to restrain weld cracking from occurring, the Hmax
should be as follows;
Hmax = 583 x Ceq + 65 ~ 400.
[0064]
Consequently, from the following calculation, the Ceq expressed by the
aforementioned formUla (2) is set to 0.57 or less;
Ceq ~ 335/583 = 0.57.
The said Ceq can be 0.29 in the case where the contents of respective elements
take their lower limit values regulated by the present invention.
[0065]
DI: 70 to 170
If, after a quench-and-temper heat treatment has been performed,
especially after a tempering at 400 to 500°C (particularly, after a tempering at
475°C) has been performed, the DI, that is to say, the value expressed by the
following formula is less than 70, a sufficient strength (800 MPa or more in
tensile strength) cannot be attained;
DI = 25.4 x 0.311 x CO.5 x (1 + 0.64 x si) x (1 + 4.10 x Mn) x (1 + 2.83 x P)
x (1 - 0.62 x S) x (1 + 2.33 x Cr) x (1 + 0.52 x Ni) x (1 + 3.14 x Mo) x (1 + 0.27 x
Cu) x {I + 1.50 x (0.90 - C)} ... (3).
• On the other hand, if the said DI increases and exceeds 170, the hardness of
base metal becomes too high, and the toughness of base metal decreases.
Therefore, the DI expressed by the aforementioned formula (3) is set to 70 to
170.
[0066]
(B) Size and precipitation density of the Ti precipitates:
The Ti precipitates in the present invention are TiN, TiC and Ti(C, N).
TiN is a precipitate which precipitates at high temperatures. On the other
hand, TiC and Ti(C, N) are precipitates which precipitate at low temperatures.
Because TiN which precipitates at high temperatures and TiC and Ti(C, N)
which precipitate at low temperatures have the similar crystal structure or for
other reasons, it is difficult to determine the respective precipitation densities
thereof. However, among these Ti precipitates, it has been known that TiN
takes a hexahedral mode. Consequently, the present inventors examined the
mode and size of Ti precipitates which are present in the steel bar after hot
rolling by the observation using a transmission electron microscope. As a
result, it was revealed that in the case where the equivalent circle diameter is
0.07 /lm or more, most of the Ti precipitates are angular, and in the case where
the equivalent circle diameter is 0.05 /lm or less, angular Ti precipitates are
scarcely seen. Therefore, the present inventors regulated the precipitation
density of the Ti precipitates by supposing that a Ti precipitate of 0.07 /lm or
more in equivalent circle diameter is TiN which precipitates at high
temperatures, and a Ti precipitate of 0.05 /lm or less in equivalent circle
diameter is TiC or Ti(C, N) which precipitates at low temperatures.
[0067]
(B-1) Size and precipitation density of the Ti precipitate of 0.07 to 1.0 /lm
in equivalent circle diameter:
TiN does not dissolve in matrix even at the time of heating for hot forging
and at the time of heating for quenching. Therefore, at the stage of steel bar
or wire rod after hot rolling, the size and precipitation density of TiN are
controlled, whereby, when welding is performed, by the pinning action of TiN,
the grain coarsening in the HAZ are restrained.
[0068]
If at the stage of steel bar or wire rod after hot rolling, in an area of 100
Ilm2, ten or more Ti precipitates of 0.07 to 1.0 Ilm in equivalent circle diameter
are present, the grain coarsening in the HAZ are restrained; and thus the HAZ
is easily provided with excellent toughness.
[0069]
That is to say, when welding is performed, the HAZ is heated to a high
temperature exceeding 1200°C. However, even at such a high temperature,
the Ti precipitates of 0.07 Ilm or more in equivalent circle diameter do not
dissolve in matrix. Therefore, these Ti precipitates act as pinning particles.
.On the other hand, the Ti precipitates exceeding 1.0 Ilm in equivalent circle
diameter do not have the pinning action because of being too large.
[0070]
However, at the stage of steel bar or wire rod after hot rolling, even if the
size of Ti precipitate is 0.07 to 1.0 Ilm in equivalent circle diameter, in the case
where ten or more Ti precipitates are nQt ~;re~ent in an area of 100 Ilm2, when
welding is performed, the grain coarsening in the HAZ is unavoidable.
[0071]
Therefore, the rolled steel bar or wire rod for hot forging of the present
invention is regulated such that ten or more Ti precipitates of 0.07 to 1.0 Ilm in
equivalent circle diameter are present in an area of 100 Ilm2. With regard to
the Ti precipitates of 0.07 to 1.0 Ilm in equivalent circle diameter, even if the
precipitation density is too high, the pinning effect is saturated. Therefore,
~
21
the number of the said Ti precipitates may advantageously be one thousand or
less.
[0072]
(B-2) Size and precipitation density of the Ti precipitate of 0.01 to 0.05
/lm in equivalent circle diameter:
TiC acts as a pinning particle at the time of heating for quenching to a
temperature of about 900°C, and restrains the coarsening of austenite grains.
Therefore, TiC contributes to the improvement in toughness of base metal.
[0073]
In order to achieve the above-described effects sufficiently, it is important
that a large number of fine TiC be distributed before the heating for quenching,
namely in the state after hot forging.
[0074]
That is to say, it may be advantageous that a large number of fine TiC
are present after hot forging, which is a process before quenching. For this
purpose, at the stage of starting material before hot forging, that is to say, at
the stage of steel bar or wire rod after hot rolling, ten or more Ti precipitates of
0.01 to 0.05 /lm in equivalent circle diameter must be present in an area of 100
/lm2•
[0075]
With regard to the Ti precipitates at the stage of steel bar or wire rod
after hot rolling, in the case where the size thereof is too small, that is to say,
the size is less than 0.01 /lm in equivalent circle diameter, the said Ti
precipitates dissolve in matrix on account of the heating at the time of hot
forging to a temperature of about 1200°C. Therefore, the pinning effect cannot
be ensured at the time of heating for quenching. On the other hand, if the size
of Ti precipitate exceeds 0.05 /lm in equivalent circle diameter at the
above-mentioned stage, coarse Ti precipitates remain when heating for
quenching is performed, and the number of fine Ti precipitates decreases, so
that the pinning effect runs short. Consequently, in order for Ti precipitates
to achieve an effect as a pinning particle when heating for quenching is
performed, it is necessary that the size of Ti precipitate be 0.01 to 0.05 !lm in
equiv~entcircle diameter at the stage of steel bar or wire rod after hot rolling.
[0076]
However, even if the size ofTi precipitate is 0.01 to 0.05!lm in equivalent
circle diameter at the stage of steel bar or wire rod after hot rolling, unless ten
or more Ti precipitates are present in an area of 100 !lm2, the coarsening of
austenite grains cannot be restrained at the time of heating for quenching.
[0077]
Therefore, the rolled steel bar or wire rod for hot forging of the present
invention is regulated such that ten or more Ti precipitates of 0.01 to 0.05 !lm
in equivalent circle diameter are present in an area of 100 !lm2• With regard
to the Ti precipitates of 0.01 to 0.05 !lm in equivalent circle diameter, even if
the precipitation density is too high, the pinning effect is saturated. Therefore,
the number of the said Ti precipitates may advantageously be one thousand or
less.
[0078]
(C) Method for producing the rolled steel bar or wire rod for hot forging:
The rolled steel bar or wire rod for hot forging in accordance with the
present invention described in the above (1) can be produced, for example, by
the method for producing a rolled steel bar or wire rod for hot forging described
in the above (2).
[0079]
In the concrete, a molten steel having the chemical composition described
in the above item (A) is continuously cast under the condition that the cooling
rate is 2.0°C/min or more; and
a bloom obtained by the casting process is heated to a temperature range
of 1100°C or more and is held for 30 minutes or more, being subjected to
blooming and steel bar rolling or wire rod rolling under a condition satisfying
the following formula (4), and moreover, after being subjected to steel bar
rolling or wire rod rolling, the rolled steel bar or wire rod is cooled at a cooling
rate of 5 to 100°C/min in a temperature range of 800 to 600°C;
whereby, the rolled steel bar or wire rod for hot forging can be produced;
Y = (Tl + 273) x log (tl + t2T(2» ~ 7.5 x 103 ••• (4).
[0080]
In the above formula (4), T is a heating temperature (oC), t is a holding
time (s) at TOC, subscript 1 representing blooming process and subscript 2
representing steel bar rolling process or wire rod rolling process, and T(2)
equals (T2 + 273)/(Tl + 273).
[0081]
As described already, the temperature and cooling rate in the each
process mentioned above refer to the temperature and cooling rate such that
the surface is a reference.
[0082]
(C-1) Concerning the cooling rate at the time of continuous casting of
molten steel:
TiN precipitates at the time of cooling in continuous casting. In order to
obtain the precipitation density ofTi precipitates of 0.07 to 1.0 f..I.m in equivalent
circle diameter having the pinning action described in the above item (B-1) at
the stage of steel bar or wire rod after hot rolling, it is preferable that the
molten steel be continuously cast under the condition that the cooling rate is
2.0°C/min or more. Since the increase in cooling rate has a limitation, the
range of cooling rate of 50°C/min or less is preferable.
[0083]
If the cooling rate is low, the TiN, which had precipitated at the time of
cooling in continuous casting, grows in the cooling process thereof, and it
becomes difficult for ten or more Ti precipitates of 0.07 to 1.0 /-lm in equivalent
circle diameter to precipitate in an area of 100 /-lm2•
[0084]
The cooling rate at the time of continuous casting refers to a cooling rate
in a temperature range in which TiN precipitates, that is to say, a cooling rate
at which the steel is cooled to IOOO°C after the steel has solidified.
[0085]
(C-2) Concerning the blooming, hot steel bar rolling and wire rod rolling:
In order to reduce the segregation in a bloom obtained by the casting
process, it is said to be usually desirable that the heating temperature at the
time when the said bloom is rolled be increased, and in addition, the holding
time at that heating temperature be prolonged.
[0086]
However, in the case where the said holding time is prolonged, TiN
coagulates and coarsens, and therefore it becomes difficult for ten or more Ti
precipitates of 0.07 to 1.0 /-lm in equivalent circle diameter to precipitate in an
area of 100 /-lm2• Therefore, when welding is performed, the effect of
restraining the grain coarsening in the HAZ, which is achieved by the pinning
action of TiN, is lost.
[0087]
If blooming and steel bar rolling or wire rod rolling are performed, by
making the heating temperature of the said bloom 1100°C or more and by
making the holding time at that heating temperature for soaking up to the
central portion 30 minutes or more, and in addition, under the condition
satisfying the aforementioned formula (4), TiN can be restrained from
coagulating and coarsening; and thereby, the grain coarsening in the HAZ can
be restrained easily at the time of welding.
[0088]
In the following, the formula (4) is explained.
[0089]
The degree of Ostwald growth of TiN is affected by the heating
temperature T(oC) and the heating time t(s). Accordingly, the present
inventors thought that the degree of Ostwald growth of TiN is arranged by
using the tempering parameter of "(T + 273) x log (t)".
[0090]
In general, the steel bar or wire rod is produced by subjecting a bloom
obtained by the casting process to two stages of rolling processes: blooming and
steel bar rolling or wire rod rolling.
[0091]
In the following, the production of steel bar or wire rod is explained in
detail by taking the production of steel bar as an example.
[0092]
Taking the heating temperatures in a blooming process and a steel bar
rolling process as T1(oC) and T2(oC), respectively, and taking the holding times
at the said heating temperatures as tl(S) and t2(S), respectively; and thus, the
tempering parameters in the each rolling process are "(T1 + 273) x log t{ and
"(T2 + 273) x log t2".
[0093]
Therefore, the time x(s) required for producing the Ostwald growth of
TiN, which growth is equal to that produced at the heating time in the steel bar
rolling process, at the heating temperature T1(oC) in the blooming process, is
determined.
[0094]
First, the tempering parameter of "(T2 + 273) x log t2" for the Ostwald
growth of TiN in the steel bar rolling process can be expressed as the following
formula (a) by using the heating temperature Tl(ac) in the blooming process;
(T2 + 273) log t2 = (Tl + 273) log x ... (a).
[0095]
Next, if T(2) equals (T2 + 273)/(Tl + 273), the said time xes) required for
producing the Ostwald growth of TiN, which growth is equal to that produced
at the heating time in the steel bar rolling process, at the heating temperature
Tl(ac) in the blooming process, can be expressed as the following formula (b);
x = t2T(2) '" (b).
[0096]
Finally, by expressmg the degree of Ostwald growth of TiN which
occurs in the steel bar rolling process of the heating temperature T2(ac) and the
holding time t2(S) by using the heating temperature Tl(ac) in the blooming
process and the holding time xes) at the said temperature Tl(ac), the degree Y
of Ostwald growth of TiN in two rolling processes of the blooming process and
the steel bar rolling process can be expressed by the following formula (c) as the
parameter of one process of the blooming process; and in addition, by
substituting the said formula (b) into the formula (c), the following formula (d)
can be obtained;
Y = (Tl + 273) x log (tl + x) ... (c),
Y = (Tl + 273) x log (tI + t2T(2» '" (d).
[0097]
As the result of detailed examination of the relationship between the
thus determined parameter Y in the formula (d) and the size and precipitation
number of TiN, it was revealed that if the Yvalue is 7.5 x 103 or less, the rolled
steel bar for hot forging in accordance with the present invention described in
the above (1) can be obtained.
[0098]
From the above description, as the parameter which represents the
degree of Ostwald growth of TiN, the following formula (4) was regulated;
Y = (Tl + 273) x log (tl + t2T(2» ::; 7.5 x 103 ••• (4).
[0099]
Even if the said Y value is within the regulated range, the heating
temperatures in the blooming process and the steel bar rolling process are
preferably set to 1300°C or less, and further preferably set to 1270°C or less
from the viewpoint of energy saving. In addition, even if the said Y value is
within the regulated range, the holding times at the heating temperatures in
the blooming process and the steel bar rolling process are preferably set to
18,000s (300 min) or less, and further preferably set to 14,400s (240 min) or less
from the viewpoint of energy saving. From the viewpoint of productivity, the
said Y value is preferably 4.0 x 103 or more.
[0100]
The parameter Y representing the degree of Ostwald growth of TiN
shown in the aforementioned formula (d) is a parameter in a general case in
which the steel bar is produced at two stages of rolling processes of the
blooming and the steel bar rolling.
[0101]
A parameter Y' which represents the degree of Ostwald growth of TiN in
the case where rolling processes are performed at "i stages" (i: a positive integer
of 3 or more) can be expressed as the formula (e). In this case, if the Y' value is
7.5 x 103 or less, the rolled steel bar for hot forging in accordance with the
present invention described in the above (1) can be obtained.
[0102]
Y' = (Tl + 273) x log {L(tiT(i)} ... (e);
in which T(i) equals (Ti + 273)/(Tl + 273).
[0103]
In the case where a bloom obtained by the casting process is hot worked
at two stages of rolling processes of the blooming and the wire rod rolling, and
even in the case where the said bloom is hot worked at "i stages" (i: a positive
integer of 3 or more) of rolling processes, the same as the above description is
true.
[0104]
(C-3) Concerning the cooling after steel bar rolling or wire rod rolling:
In order to obtain the precipitation density of Ti precipitates of 0.01 to
0.05 /lm in equivalent circle diameter having the pinning action described in
the above item (B-2) at the stage of steel bar or wire rod after hot rolling, it is
preferable that the rolled steel bar or wire rod be cooled at a cooling rate of 5 to
100°C/min in the temperature range of 800 to 600°C.
[0105]
If the said cooling rate is too high, TiC does not precipitate sufficiently.
On the other hand, if the said cooling rate is too low, the precipitated TiC
coagulates and grows. Therefore, in both the cases, it is difficult for ten or
more Ti precipitates of 0.01 to 0.05 /lm in equivalent circle diameter to
precipitate in an area of 100 /lm2•
[0106]
The following examples illustrate the present invention more specifically.
These examples are, however, by no means limited to the scope of the present
invention.
EXAMPLES
[0107]
The steels 1 to 14 shown in Table 1 were melted by use of a 70-tons
converter, and were continuously cast into blooms respectively, under the
conditions shown in Table 2. Thereafter, the said each bloom was bloomed
into billets of 180 mm x 180 mm; and further the said each billet was steel bar
rolled to produce steel bars having a diameter of 54 mm.
[0108]
The steels 1 to 7 and 12 to 14 shown in Table 1 are steels of examples of
the present invention with chemical compositions being within the range
regulated by the present invention. On the other hand, the steels 8 to 11 are
steels of comparative examples with chemical compositions being out of the
range regulated by the present invention.
[0109]
[Table 1]
~
Table 1
tP~
Stool Chemical comoosition (% bv mass) Balance: Fe and im urities
C : Si Mn'l P S eu Ni Cr Al Mo V Ti B N fnl Ceq DI
1 0.10 0.02 1.61 0.014 0.015 0.02 0.03 0.38 0.039 0.01 0.001 0.022 0.0022 0.0043 0.007 0.45 95
2 0.11 0.01 1.72 0.017 , 0.003 0.01 0.01 0.39 0.033 0.01 0.001 0.036 I 0.0020 0.0021 0.029 0.48 106
3 0.14 0.07 I 1.81 0.015 i 0.015 0,02 0.03 0.42 0.040 0.01 0.001 0.054 I 0.0022 0.0045 0.039 0.53 133
<1 0.16 0.021 1.66 0.014 0.014 0.02 0.03 0.28 0.040 0.01 0.001 0.022 ! 0.0019 0.0038 0.009 0.50 101
5 0.17 0.01 1.72 0.016 0.003 0.01 0.01 0.30 0.031 0.01 0.001 0.037 0.0020 0.0040 0.023 0.52 110
6 0.18 0.07 . 1.75 0.014 0.014 0.02 0.03 0.32 0.041 0.01 0.001 0.053 0.0019 0.0042 0.039 0.54 122
7 0.16 0.07 I 1.71 0.014 0.013 0.06 0.07 0.34 0.041 0.01 0.001 0.044 0.0020 0.0040 0.030 0.52 120
8 o 14 0.02 1.78 0.014 0.015 0.02 0.01 0.31 0.038 0.01 0.001 0.012 0.0019 0.0078 *-0.015 0.50 lOG
9 0.18 0.15 1.98 0.010 0.015 0.02 0.02 0.34 0.041 0.01 0.001 0.039 0.0030 0.0056 0.020 ... 0.59 145
!
10 0.19 0.17 1.72 0.038 0.008 , 0.09 *0.10 I 0.36 0.041 1*0.05 0.001 0.036 0.0021 0.0042 0.022 0.57 * 172
0.12 i 0.02 ! 1.32
!
11 0.011 0.017 . 0.03 0.02 I 0.22 0.039 0.01 0.001 0.048 0.0015 0.0064 0.026 0.39 * 66
12 0.19 I 00211.68 0.016 0.013 0.02 0.02 0.40 0.040 I 0.01 0.001 0.029 0.0021 0.0043 0.014 0.55 131
13 0.16 I 0.01 1.87 0.015 0.014 0.02 0.01 0.37 0.038 0.01 0.001 0.062 0.0018 0.0067 0.039 0.55 128
14 0.15 0.08 1.72 0.014 0.018 0.02 0.01 0.38 0.041 0.01 0.001 0.033 0.0023 0.0051 0.016 0.52 122
fn1 = "Pi .- 3AN
Ce:>q = C + (Sif24) + (Mu/G) + (Ni/40) T (Cr/5) + (:"10/4) + (V/14)
DI = 25.'1 x 0.311 x CO.5 x (l + 0.64 x Si) x (1 + 4.10 x Mn) x (1 + 2.83 x P) x (1 - 0.62 x S) x (1 + 2.33 x Cr) x (1 + 0.52 x ND x (1
-i- 3.14 x Mo) x (1 + 0.27 x Cu) x {I + 1.50 x (0.90 - C)}
The mark " llldicates falling outside the conditions of chemical compositions regulated by the present invention.
g~.LL3!AJ&£u£au,&£aU$ UJuam; WJ£OE hi •.aUk. ,.WW UM iiiii UZS,iCt& _wazI2,!ea JiiU!i&4i4¥4UtZU £ 4Zk Q ZS@i1(;;:::ZS:;;;;W dFj~'
O-J~
~
Table 2
Cooling rate at the time Heating condition in the Heating condition in
Test ISteel Iof continuous cast.ing bloomin~ process the steel bar rolling I y
No. of molten swel 1'emperaturc ,Holding time Temperature Holding time
(OCimin) (oe) (min) (oC) (min)
1 1 2.:3 129:i 103 1213 22 6.0x 10J
2 2 2.1 1288 un 1221 22 6.lxI03
oj :~ 2.6 1308 132 1220 20 G.2x10 3
'J
·1 4 2.5 1292 125 1204 24 G.lx10 3
5 5 :n 1291 121 1211 25
I
6.1x 103
(i (j .')......, 1298 112 1209 21 6.1X10 J
'i 7 ., ., .1279 135 1224 21 6.1XI0 3
.:..o.l 8 I I . 8 2.2 1288 135 1209 28 6.2x 10·
9 . 9 2.2 129G 131 1212 I 27 6.2x 10·
10 ~ 10 2.3 1:120 133 1241 21 6.3xlO·
II * 11 oJ ., 130;j 120 1222 I 24 G.2x L0 3
_.4) I
12 12 i!- L7 1302 115 1201)
I
'}" 6.1x 103 _.
13 13 2.3 1347 720 1302 132 ~ 7.6xlO· ..
1-1 1<1 2.3 1296 103 1213 28 6.1x10 3
y = ('I'; + 273) x log (tl + bTllll.
In the alHlVl~ tin·mula. T is a heating temperature (eel. t is a holding time (s) at temperature T(oCl, subscript 1
subscnpt 2 representing steel bar rolling process. and T(:,l) equals ('1': + 273)i(T, + 27a).
The mark * indicates falling outside the conditions of l:hemical compositions regulated by the present invenrion.
The mark # indicates falling outside the conditions of producing method regulated by the present invention.
Cooling rate in the tempel'Uture
range of 800 to 600°C after
steel bar rolling
(oC/min)
18
19·
19
18
18
18
18
18
18
18
18
18
18
# 4
repredollting blooming process und
~
q'
~
ttl
t-.:>
'--'
5'
i-£
i-£ o
'--'
[0111]
With regard to the said each steel bar having the diameter of 54 mm
which was prepared as described above, the precipitation density of TiN and
TiC was examined.
[0112]
That is to say, from the longitudinal cross section of an R/2 portion ("R"
represents a radius) of the steel bar having t4e diameter of 54 mm, a specimen
was sampled by the extraction replica method, and the precipitation density of
Ti precipitates was examined by the observation using a transmission electron
microscope provided with an energy dispersive X-ray spectroscope (EDX).
[0113]
In the concrete, 20 fields were observed at a magnification of 40,000
times, the area of Ti precipitates was calculated by image analysis, and then
the calculated each area was converted into the area of circle; and thus the
equivalent circle diameter was determined.
[0114]
Next, the number of Ti precipitates having an equivalent circle diameter
of 0.07 to 1.0 ~m was counted, and the counted number was converted into the
number per an area of 100 ~m2. Similarly, the number of Ti precipitates
having an equivalent circle diameter of 0.01 to 0.05 ~m was counted, and the
counted number was converted into the number per an area of 100 ~m2.
[0115]
Furthermore, the said each steel bar having the diameter of 54 mm was
heated at 1200°C for 30 minutes and was hot forged; and thereby a round bar
having a diameter of 36 mm was prepared. Next, this round bar having the
diameter of 36 mm was heated at 900°C for one hour and was water quenched,
and thereafter was tempered at 475°C for one hour.
[0116]
By using the said quench-and-temper heat treated round bar having the
diameter of 36 mm, tensile tests, impact tests, and HRC hardness
measurements were carried out.
[0117]
With regard to the tensile test, a No. 14A test piece (the diameter of
parallel part: 5 mm) specified in JIS Z 2201 (1998) was cut off from the R/2
portion of the said each round bar having the diameter of 36 mm, and by the
ordinary method, the tensile test was carried out at 25°C to measure the tensile
strength (TS). The target TS was set to 800 MPa or more.
[0118]
With regard to the impact tests, U-notch standard impact test pieces
(notch depth: 2 mm, notch bottom radius: 1 mm) specified in JIS Z 2242 (2005)
were cut off from the R/2 portions of the said each round bar having the
diameter of 36 mm, and the Charpy impact tests were carried out at 25°C and
-40°C to measure the respective impact values. The each target impact value
was set to 160 J/cm2 or more at 25°C, and 135 J/cm2 or more at -40°C.
[0119]
With regard to the HRC hardness measurement, the said each round bar
having the diameter of 36 mm was cut transversely, and the cut plane was
polished so as to be used as the test plane. The measurement of HRC
hardness was carried out at four points in the R/2 portion and at one point in
the central portion, and the arithmetical mean value of the five points was set
to the HRC hardness.
[0120]
Furthermore, from the R/2 portion of the said quench-and-temper heat
treated round bar having the diameter of 36 mm, test pieces for lap joint
welding shown in Figure 2 were cut off in the positional relationship shown in
Figure 1, and lap joint welding was performed as shown in Figure 3.
[0121]
The said lap joint welding was performed usmg an 800 MPa class
welding wire (the ER110S-G specified in AWS A5.28 (2005» by the MAG
welding method under the following welding conditions; voltage: 20V, current:
180A, and welding speed: 40 em/min. First, the occurrence of weld cracking
was examined, and next, the grain size in the HAZ was examined. In addition,
the HV hardness of HAZ was measured, and the Hmax (the maximum
hardness of HAZ) was evaluated.
[0122]
The occurrence of weld cracking was judged by the liquid penetrant
testing.
[0123]
The method for evaluating the grain size in the HAZ was as described
below. First, the test pieces having been subjected to lap joint welding were
cut longitudinally into two equal parts to 12.5 mm at the central position in the
width direction, and further, were cut to 30 mm so that, in the longitudinal
cross section, the weld metal was positioned in the central portion in the
longitudinal direction, being embedded in a resin, and were mirror-like
polished. Next, the polished test pieces were etched with a picric acid
saturated aqueous solution to which a surface active agent was added, and as
shown in Figure 4, for the upper test piece of the two lapped test pieces, the
prior-austenite grain size was measured at a position 1.5 mm distant from the
joint surface in the thickness direction, which was the center of the thickness of
3 mm, and 0.3 mm distant from the weld metal. The target grain size was set
to 3.0 or more in the grain size number.
[0124]
The method for evaluating the HV hardness of HAZ was as described
below. The test pieces embedded in the resin, which have been used for the
grain size measurement, were polished again. As shown in Figure 5, with
regard to the upper test piece of the two lapped test pieces, the HV hardness at
a position 1.5 mm distant from the top surface thereof in the thickness direction,
which was the center of the thickness of 3 mm, was measured, and with regard
to the lower test piece, the HV hardness at a position 1.0 mm distant from the
top surface thereof in the thickness direction was measured. The said each
HV hardness was measured at a pitch of 0.5 mm from end to end of the test
pieces cut to 30 mm. The target Hmax was set to 400 or less in HV hardness.
[0125]
The above"mentioned respective examination results are collectively
shown in Table 3.
[0126]
[Table 3]
e
Table 3
l}>~
~
Properties of the quench-Ilnd,tilmper Properties of the test piece having been
Number of '1'1 pr<''Clpitares Number of Ti P10ilcipitates heat treated base metal subiected to lao ioint weldin!!
Classification Test Steel having an equivalent having an equivalent Tensile !Impact Impact HRC Hmax Weld 1Prior'austenite
No. crrde diameter circle diameter strength value value hardness (HV) cracking ! gram size numb('1'
of 0.07 to 1.0 lJ,m of O.oI to 0.05 lJ,m (MPa) at 25"C at -45°C i in the H.~
(J/cm 2 ) (J/cm 2 ) ! I
1 1 18 13 8:~O 281 262 24 310 non'existence 3.6
2 2 23 30 855 357 308 27 3·14 non'existence 3,7
Inventive 3 v" a2 34 9:~0 270 229 28 367 non'existence 4,0
eXllmples :j. 4 17 16 899 273 242 27 358 non-existence 3.6
5 ;j 35 24 912 317 277 29 368 non'existence 4.0 (, G ·12 35 fJ46 242 226 29 397 non 'existence 4.1
7 7 40 31 922 236 201 28 371 non-cxistence 4.0
8 . 8 14 * 2 910 i 5: 147 S 102 28 361 non-existence 3.2
f) • 9 28 30 96:1 I 222 196 31 $ 419 existence 3.8
Comparative 10 " 10 :18 24 989 I $ 155 $ [26 33 379 non-exit;tence 4.0
11 " 11 40 29 $ 776 331 297 21 aOl non'existence 4.0
exampl"s 12 12 . 7 19 928 268 235 30 388 non'existence $ 2.3
n 1:3 .. (; '12 879 287 250 28 392 non-existence $ 2.2
II 14 2-1 ... (; 864 S 158 $ 128 27 35(; non'existence 3.8
"Numb"r of Ti precipitates" indicar.cs the numbers thereof per an area of 100 l-Im2•
The lIlark " indicates falling outside the conditions of I:hemical compositions and number of Ti precipitates l"\~gulated by [he present invention,
The mark S indicates falling ShOl·t of the tar!!l,t \'allll1 in the present invention.
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[0127]
From Table 3, it is apparent that, in the case of test Nos. 1 to 7 which use
steels 1 to 7 with chemical compositions being within the range regulated by
the present invention, and in which ten or more of each of Ti precipitates
having two kinds of equivalent circle diameters regulated by the present
invention were present, the each Hmax was less than 400 in HV hardness, so
that weld cracking did not occur, and moreover, the strength and toughness of
each base metal were excellent. Further, in the case of the above-described
test numbers, the prior-austenite grain size numbers in the HAZ were as large
as 3.6 to 4.1 (in other words, the prior-austenite grain sizes were smaII), and
therefore, it is presumed that the toughness ofHAZ was also excellent.
[0128]
In contrast, in the case of test Nos. 8 to 14 of the "comparative examples"
in which all of the conditions regulated by the present invention were not met
at the same time, at least one of weld cracking, strength of base metal,
toughness of base metal, and toughness of HAZ has a problem.
[0129]
That is to say, in the case of test No.8, the fn1 of the used steel 8 was
"-0.015", and deviated from the condition regulated by the present invention.
Therefore, the precipitation density of Ti precipitates of 0.01 to 0.05 J-lm in
equivalent circle diameter deviated from the condition regulated by the present
invention, and thus the toughness of base metal was poor.
[0130]
In the case of test No.9, the Ceq of the used steel 9 was as large as 0.59,
and deviated from the condition regulated by the present invention. For this
reason, the Hmax was as high as 419 in HV hardness, and exceeded 400.
Therefore, weld cracking did occur.
[0131]
In the case of test No. 10, the DI of the used steel 10 was as large as 172,
and deviated from the condition regulated by the present invention. Therefore,
the toughness of base metal was poor.
[0132]
In the case of test No. 11, the DI of the used steel 11 was as small as 66,
and deviated from the condition regulated by the present invention. Therefore,
the tensile strength of base metal was low.
[0133]
In the case of test Nos. 12 and 13, although the chemical compositions of
the used steels 12 and 13 were within the range regulated by the present
invention, the precipitation density of Ti precipitates of 0.07 to 1.0 J.lm in
equivalent circle diameter deviated from the condition regulated by the present
invention. Therefore, the prior-austenite grain size numbers in the HAZ were
as low as 2.3 and 2.2, respectively, (in other words, the prior-austenite grain
sizes were large), and therefore, it is presumed that the toughness of HAZ was
poor.
[0134]
Similarly, in the case of test No. 14, although the chemical composition of
the used steel 12 was within the range defined in the present invention, the
precipitation density of Ti precipitates of 0.01 to 0.05 J.lm in equivalent circle
diameter deviated from the condition regulated by the present invention.
Therefore, the toughness of base metal was poor.
INDUSTRIAL APPLICABILITY
[0135]
The rolled steel bar or wire rod for hot forging of the present invention is
able to be welded, moreover is excellent in strength and toughness of base
metal and in toughness of HAZ, and therefore can be suitably used as a starting
material for suspension parts of an automobile, such as a spindle.

We claim:
1. A rolled steel bar or wire rod for hot forging, having a chemical
composition comprising, by mass percent;
C: 0.10 to 0.20%,
Si: 0.01 to 0.30%,
Mn: 1.00 to 2.30%,
S: 0.040% or less,
Cr: 0.10 to 0.80%,
AI: 0.010 to 0.080%,
B: 0.0002 to 0.0050%,
Ti: 0.010 to 0.080%, and
N: 0.0020 to 0.0080%,
with the balance being Fe and impurities,
in which the contents of P, Cu, Ni, Mo and V among the impurities are
restricted so that
p: 0.040% or less,
Cu: less than 0.10%,
Ni: less than 0.10%,
Mo: less than 0.05%, and
V: 0.01% or less;
further, the fnl expressed by the following formula (1) is 0.001 or more, the Ceq
expressed by the formula (2) is 0.57 or less, and the DI expressed by the
formula (3) is 70 to 170;
and further, in an area of 100 j.lm2, ten or more Ti precipitates of 0.07 to 1.0 IJm
in equivalent circle diameter and ten or more Ti precipitates of 0.01 to 0.05 IJm
in equivalent circle diameter are present;
fnl = Ti - 3.4N ... (1),
Ceq = C + (Sil24) + (Mnl6) + (Nil40) + (Cr/5) + (Mo/4) + (VI14) ... (2),
Dr = 25.4 x 0.311 x CO.5 x (1 + 0.64 x si) x (1 + 4.10 x Mn) x (1 + 2.83 x p)
x (1 - 0.62 x S) x (1 + 2.33 x cd x (1 + 0.52 x Ni) x (1 + 3.14 x Mo) x (1 + 0.27 x
Cu) x {I + 1.50 x (0.90 - C)} ... (3);
wherein each symbol Ti, N, C, Si, Mn, Ni, Cr, Mo, V, P, Sand Cu in the above
formulas represents the content by mass percent of the element concerned, and
further, the above Ti precipitates indicate TiN, TiC and Ti(C,N).
2. A method for producing a rolled steel bar or wire rod for hot forging,
in which,
a molten steel having the chemical composition according to claim 1 is
continuously cast under the condition that the cooling rate is 2.0°C/min or
more; and
a bloom obtained by the casting process is heated to a temperature range
of 1100°C or more and is held for 30 minutes or more, being subjected to
blooming and steel bar rolling or wire rod rolling under a condition satisfying
the following formula (4), and moreover, after being subjected to steel bar
rolling or wire rod rolling, the rolled steel bar or wire rod is cooled at a cooling
rate of 5 to 100°C/min in a temperature range of 800 to 600°C;
Y= (Tl + 273) x log (tl + t2T(2» ~ 7.5 x 103 .. , (4);
in the formula (4), T is a heating temperature in the unit of "oC", t is a holding
time at heating temperature T in the unit of "s", subscript 1 representing
blooming process and subscript 2 representing steel bar rolling process or wire
rod rolling process, and T(2) equals (T2 + 273)/(Tl + 273).