Technical Field
[0001] The present disclosure relates to a high-strength steel member.
Background Art
[0002] Among steel members used for machines, automobiles, bridges, or buildings, steel
members that are required to have particularly high strength are, for example, those
in which chromium steels or chromium-molybdenum steels defined in JIS G 4104 or JIS
G 4105 have been subjected to a quenching/tempering treatment. Some steel members,
such as gears, are carburized and then quenched to have high strength.
[0003] Quenching is made after heating steel members to high temperatures at which austenite
phases are formed. However, absorption of hydrogen into steel members from atmosphere
during heating may cause quenching crack after quenching. Further, for example, a
low tempering temperature of from 150 to 200°C as in the case of high-strength steel
members may cause deterioration in ductility or toughness after tempering, since hydrogen
absorbed into the steel members during quenching is not sufficiently released by the
tempering.
[0004] With regard to hydrogen embrittlement resistance of high-strength steel members (steel
members having tensile strengths of 1,000 MPa or higher), for example, Patent Literature
1 describes that adding V, Nb and Ti to steel to refine a prior austenite grain is
effective for improvement in delayed fracture resistance.
[0005] Patent Literatures 2 to 4 each describe a technique of dispersing, in steel, a fine
precipitate that exhibits a hydrogen trapping ability by high-temperature tempering
after quenching, to improve delayed fracture resistance.
SUMMARY OF INVENTION
Technical Problem
[0007] However, the conventional techniques described in Patent Literatures 1 to 4 and the
like have a limitation to fundamental improvement in delayed fracture resistance of,
for example, high-strength steel members that are subjected to low-temperature tempering
at from 150 to 200°C after quenching.
[0008] An object in one aspect of the present disclosure is to provide a high-strength steel
member excellent in delayed fracture resistance, which is one kind of hydrogen embrittlement
resistance.
Solution to Problem
[0009] Solutions for achieving the object in one aspect of the present disclosure include
the following aspects.
- <1> A high-strength steel member having a chemical composition of, in terms of percent
(%) by mass:
C: from 0.10 to 0.50%,
Si: from 0.02 to 2.00%,
Mn: from 0.05 to 2.00%,
Cr: from 0.10 to 2.00%,
Ti: from 0.20 to 1.00%,
N: from 0.0020 to 0.0250%,
Al: from 0 to 0.100%,
V: from 0 to 0.50%,
Nb: from 0 to 0.50%,
Mo: from 0 to 1.00%,
B: from 0 to 0.0100%,
Cu: from 0 to 2.00%,
Ni: from 0 to 3.00%, and
a balance consisting of Fe and impurities,
having a tensile strength of 1,000 MPa or higher,
containing 0.10% or more of, in terms of percent (%) by area, at least one Ti precipitate
that has an average size of from 30 to 200 nm in terms of an average equivalent circle
diameter and is selected from the group consisting of a Ti carbide, a Ti nitride,
and a composite compound thereof, at a location of 1 mm in depth from a surface of
the steel member, and
containing 0.5 ppm by mass or more of non-diffusible hydrogen that is released in
a temperature range of from 400 to 800°C in a thermal desorption hydrogen analysis.
- <2> The high-strength steel member according to <1>, having a chemical composition
including, in terms of percent (%) by mass, one or more of
Al: from 0.005 to 0.100%,
V: from 0.01 to 0.50%,
Nb: from 0.01 to 0.50%, or
Mo: from 0.01 to 1.00%.
- <3> The high-strength steel member according to <1> or <2>, having a chemical composition
including, in terms of percent (%) by mass,
B: from 0.0003 to 0.0100%.
- <4> The high-strength steel member according to any one of <1> to <3>, having a chemical
composition including, in terms of percent (%) by mass,
one or both of Cu: from 0.05 to 2.00% or Ni: from 0.05 to 3.00%.
- <5> The high-strength steel member according to any one of <1> to <4>, wherein the
Ti precipitate has an average aspect ratio of from 1.0 to 3.0.
Advantageous Effect of Invention
[0010] According to one aspect of the present disclosure, a high-strength steel member excellent
in delayed fracture resistance, which is one kind of hydrogen embrittlement resistance,
can be provided.
BRIEF DESCRIPTION OF DRAWING
[0011] Fig. 1 shows a schematic view for describing, in a case in which a measurement object
is larger than a "round rod steel having a size of 10 mm in diameter φ × 50 mm in
length L", an extraction location of a test piece when a test piece for measurement
of a content of non-diffusible hydrogen is extracted from the measurement object.
DESCRIPTION OF EMBODIMENTS
[0012] Hereinafter, an embodiment that is an example of the present disclosure will be specifically
described.
[0013] In the present description, a numerical value range represented by "(from)...to..."
means a range that encompasses respective numerical values indicated before and after
"to" as a lower limit value and an upper limit value.
[0014] A content of each element in a chemical composition is expressed as an element amount
(for example, a C amount, a Si amount).
[0015] The indication "%" with respect to the content of each element in a chemical composition
means "percent (%) by mass".
[0016] A high-strength steel member according to the present embodiment (hereinafter, also
simply referred to as "steel member") is a steel member having a predetermined chemical
composition and having a tensile strength of 1,000 MPa or higher. The tensile strength
of the steel member is a value obtained by measurement according to JIS-Z 2241 (2015).
[0017] The steel member according to the present embodiment contains 0.10% or more of, in
terms of percent (%) by area, at least one Ti precipitate that has an average size
of from 30 to 200 nm in terms of an average equivalent circle diameter and is selected
from the group consisting of a Ti carbide, a Ti nitride, and a composite compound
thereof, at a location of 1 mm in depth from a surface of the steel member, and contains
0.5 ppm by mass or more of non-diffusible hydrogen that is released in a range of
from 400 to 800°C in a thermal desorption hydrogen analysis.
[0018] The steel member according to the present embodiment has the configuration described
above and thus is a high-strength steel member excellent in delayed fracture resistance,
which is one kind of hydrogen embrittlement resistance. The steel member according
to the present embodiment has been found as follows.
[0019] The inventors have specifically analyzed a delayed fracture behavior, which is one
kind of hydrogen embrittlement phenomenon, using various strength of steel members
that are produced by quenching/tempering treatments.
[0020] It has been already found that, of hydrogen that has been absorbed from an external
environment into a steel member, diffusible hydrogen that diffuses in the steel member
at room temperature particularly causes delayed fracture. The diffusible hydrogen
can be measured from a curve having a peak at a temperature of about 100°C in a "curve
representing the relationship between the temperature and the rate of hydrogen release
from a steel member" obtained by heating the steel member at a rate of 100°C/hour.
[0021] Accordingly, if hydrogen that has been absorbed from an external environment is trapped
at some portion in a steel member so as not to be diffused, it is possible to render
hydrogen harmless and delayed fracture due to absorbed hydrogen is suppressed.
[0022] The presence of a trapping site of hydrogen (hereinafter, also referred to as "hydrogen-trapping
site") can be confirmed by comparing peak temperatures and peak heights of hydrogen
release curves obtained by heating, at 100°C/hour, a steel member before charging
with hydrogen and a steel member after charging with hydrogen, respectively. The amount
of hydrogen trapped in a certain hydrogen-trapping site (hereinafter, also referred
to as "hydrogen trapping capacity") can be determined by an area integral value of
the peak.
[0023] The inventors have performed the following evaluation with respect to delayed fracture
resistance of a steel member that has been subjected to low-temperature tempering
at from 150 to 200°C after quenching. A test piece of a circularly notched rod steel
having a diameter of 10 mm is heated for 20 minutes in an atmosphere of 1 atm containing
from 30 to 100% of hydrogen, quenched by cooling with water, and thereafter tempered
at 150°C for 30 minutes. A constant load (90% relative to tensile strength) is applied
to the test piece in the air and a time is measured until rupture is caused, thereby
evaluating delayed fracture resistance. A longer rupture time means that the steel
member is more favorable in delayed fracture resistance.
[0024] As a result, the inventors have found that a steel member having a steel structure
containing 0.10% or more of, in terms of percent (%) by area, at least one Ti precipitate
that has an average grain size of from 30 to 200 nm and is selected from the group
consisting of a Ti carbide, a Ti nitride, and a composite compound thereof, at a location
of 1 mm in depth from a surface of the steel member is excellent in delayed fracture
resistance.
[0025] The steel member that has such a steel structure and is excellent in delayed fracture
resistance exhibits, in a temperature range of from 400 to 800°C, a hydrogen release
peak which indicates that hydrogen stably trapped in a hydrogen-trapping site consisting
of the Ti precipitate described above has been released, in the case of performing
a thermal desorption hydrogen analysis at a rate of 100°C/hour after the heat treatment
under the conditions described above. The amount of hydrogen that is released (hydrogen
trapping capacity) is 0.5 ppm by mass or more.
[0026] The inventors have made investigation by comparison with the "technique of dispersing,
in steel, a fine precipitate that exhibits a hydrogen trapping ability by high-temperature
tempering after quenching, to improve delayed fracture resistance" described in Patent
Literature 2 (
JP-A No. 2000-26934) and, as a result, have obtained the following finding. A fine Ti precipitate (at
least one Ti precipitate selected from the group consisting of a Ti carbide, a Ti
nitride, and a composite compound thereof) is precipitated at a higher temperature
in the case of containing a large amount of 0.20% or more of Ti. Therefore, it is
possible to cause precipitation during heating in quenching without tempering, and
hydrogen that has been trapped is released at a higher temperature. As can be seen
therefrom, hydrogen is stably trapped, and thus hydrogen that has been absorbed from
a heating atmosphere during quenching can be trapped during cooling in quenching,
thereby allowing for rendering hydrogen harmless even in subsequent low-temperature
tempering. It has been thus found that delayed fracture resistance is excellent as
compared with the technique of Patent Literature 2.
[0027] It is noted that inclusion of an excess C causes deterioration in delayed fracture
resistance. It is also noted that no inclusion of a predetermined amount of N causes
generation of a coarse grain during quenching and deterioration in delayed fracture
resistance. Therefore, a C amount is set as from 0.10 to 0.50% and an N amount is
set as from 0.0020 to 0.0250%, as described below.
[0028] It has been found, based on the finding above, that the steel member according to
the present embodiment has the configuration described above and thus is a high-strength
steel member excellent in delayed fracture resistance, which is one kind of hydrogen
embrittlement resistance.
[0029] Further, a technique has been established which involves forming, in a steel member,
a steel structure where "at least one Ti precipitate selected from the group consisting
of a Ti carbide, a Ti nitride, and a composite compound thereof" serving as a hydrogen-trapping
site is finely precipitated.
[0030] The reason why the steel structure at a location of 1 mm in depth from a surface
of the steel member has been focused on is that delayed fracture due to hydrogen embrittlement
occurs at an inner portion that is at a depth of several hundred micrometers or more
from the surface of the steel member, originating at a site that is high in stress
triaxiality.
[0031] Hereinafter, the steel member according to the present embodiment will be specifically
described.
(Hydrogen Trapping Capacity)
[0032] First, the reason for limiting the hydrogen trapping capacity (namely, the content
of non-diffusible hydrogen) that is the most important for improvement in delayed
fracture characteristics of a high-strength steel member is described.
[0033] Diffusible hydrogen that causes delayed fracture of a steel member obtained by low-temperature
tempering after quenching is absorbed into the steel member from a heating atmosphere
during quenching. For example, several ppm by mass of hydrogen is absorbed during
heating to an austenite region, in a case of carburization quenching or quenching
with heat by RX gas (endothermic converted gas) firing. Hydrogen in a martensite structure
obtained by quenching is low in diffusion coefficient, and thus hydrogen may be difficult
to be completely released by low-temperature tempering after quenching, possibly resulting
in hydrogen embrittlement.
[0034] When hydrogen is stably trapped in a hydrogen-trapping site during heating in such
an atmosphere and quenching, the content of non-diffusible hydrogen after quenching
is increased, thereby allowing for suppression of hydrogen embrittlement. In other
words, hydrogen that is released during re-heating to a temperature range of from
400 to 800°C after quenching is in a form of hydrogen stably trapped in a hydrogen-trapping
site, and is rendered harmless and does not contribute to hydrogen embrittlement.
[0035] Therefore, the steel member according to the present embodiment is a steel member
containing 0.5 ppm by mass or more of non-diffusible hydrogen that is released in
a temperature range of from 400 to 800°C in a thermal desorption hydrogen analysis.
In other words, the hydrogen trapping capacity (content of non-diffusible hydrogen)
is 0.5 ppm by mass or more.
[0036] The hydrogen trapping capacity is preferably 0.8 ppm by mass or more, and more preferably
1.0 ppm by mass or more, from the viewpoint of improvement in delayed fracture resistance.
The content of non-diffusible hydrogen is preferably 3.0 ppm by mass or less, from
the viewpoint of suppression of deterioration in forgeability due to increase of a
precipitate.
[0037] The steel member is controlled so as to have a steel structure in which the amount
of non-diffusible hydrogen that is released in a temperature range of from 400 to
800°C in a thermal desorption hydrogen analysis is 0.5 ppm by mass or more, thereby
allowing delayed fracture characteristics to be improved.
[0038] The thermal desorption hydrogen analysis is performed as follows. First, a test piece
of a round rod steel having a size of 10 mm in diameter φ × 50 mm in length L is taken
from a steel member as a measurement object. Next, the test piece is heated at 100°C/hour
using a "gas chromatography type temperature rising hydrogen analysis apparatus" and
the amount (mass) of hydrogen that is released at each temperature is analyzed.
[0039] A hydrogen release curve is thus obtained which exhibits the relationship between
the temperature and the amount of hydrogen that is released. The amount of non-diffusible
hydrogen that is released in a temperature range of from 400 to 800°C, namely, the
hydrogen trapping capacity (the content of non-diffusible hydrogen) is determined
by an area integral value of the peak in the hydrogen release curve.
[0040] In a case in which the measurement object is larger than the "round rod steel having
a size of 10 mm in diameter φ × 50 mm in length L", a test piece obtained by scraping,
from the measurement object, a "round rod steel having a size of 10 mm in diameter
φ × 50 mm in length L", whose outer peripheral surface corresponds to a location at
a depth of 1 mm from a surface of the measurement object, is used as the test piece
(see Figure 1). In Figure 1, OM represents the steel member as the measurement object
and SP represents the test piece.
[0041] In contrast, in a case in which the measurement object is smaller than the "round
rod steel having a size of 10 mm in diameter φ × 50 mm in length L", the measurement
object is directly used as the test piece. This is because the content value of non-diffusible
hydrogen that is measured does not vary even in a case in which the test piece is
smaller than the "round rod steel having a size of 10 mm in diameter φ × 50 mm in
length L".
[0042] It is noted that, even in a case in which the test piece is provided with a circular
notch, the content of non-diffusible hydrogen that is measured does not vary depending
on presence or absence of the circular notch.
(Steel Structure)
[0043] The steel member according to the present embodiment contains 0.10% or more of, in
terms of percent (%) by area, at least one Ti precipitate that has an average size
of from 30 to 200 nm in terms of an average equivalent circle diameter and is selected
from the group consisting of a Ti carbide, a Ti nitride, and a composite compound
thereof, at a location of 1 mm in depth from a surface of the steel member. In other
words, a volume (area) fraction of the Ti precipitate is 0.10% or more in terms of
percent (%) by area.
[0044] The Ti precipitate has hydrogen trapping ability and serves as a hydrogen-trapping
site that releases hydrogen at a relatively high temperature of from 400 to 800°C.
The presence of the Ti precipitate having hydrogen trapping ability enables non-diffusible
hydrogen to be stably trapped during quenching of the steel member. In other words,
the hydrogen trapping capacity (the content of non-diffusible hydrogen) of 0.5 ppm
by mass or more can be achieved. Thus, the steel member can be improved in delayed
fracture characteristics.
[0045] Ti oxides also have hydrogen trapping ability. However, it is preferable that Ti
oxides are not included in the steel member from the viewpoint of securement of forgeability.
[0046] The Ti carbide, the Ti nitride, or the composite compound thereof (namely, Ti carbonitride)
in the Ti precipitate is a compound that mainly contains Ti as a metal component (Ti
occupies 50% by atom or more of a metal site) and has an FCC (face-centered cubic)
structure.
[0047] From the viewpoint of increase in hydrogen trapping capacity and improvement in delayed
fracture characteristics, the volume (area) fraction of the Ti precipitate is preferably
0.10% or more, and more preferably 0.20% or more, in terms of percent (%) by area.
From the viewpoint of securement of toughness, the volume (area) fraction of the Ti
precipitate is preferably 1.00% or less, and more preferably 0.50% or less, in terms
of percent (%) by area.
[0048] The volume (area) fraction of the Ti precipitate means a volume (area) fraction of
the entire Ti precipitate contained in the steel member.
[0049] From the viewpoint of increase in hydrogen trapping capacity and improvement in delayed
fracture characteristics with securement of tensile strength, the average size of
the Ti precipitate is preferably 100 nm or less, and more preferably 80 nm or less,
in terms of the average equivalent circle diameter. From the same viewpoint, the average
size of the Ti precipitate is preferably 60 nm or more.
[0050] From the viewpoint of increase in hydrogen trapping capacity and improvement in delayed
fracture characteristics with securement of tensile strength, an average aspect ratio
of the Ti precipitate is preferably from 1.0 to 3.0. An upper limit of the average
aspect ratio of the Ti precipitate is more preferably 2.0, and still more preferably
1.5.
[0051] The volume (area) fraction of the Ti precipitate, the average size of the Ti precipitate
(average equivalent circle diameter), and the average aspect ratio of the Ti precipitate
are each measured using a test piece prepared by an extraction replica method, by
means of a transmission electron microscope (TEM) equipped with an energy dispersive
X-ray analyzer (EDS). Specifically, the measurement is performed as follows.
[0052] A portion that is located 1 mm in depth from a surface of the steel member (hereinafter,
also referred to as a "measurement surface") is taken from an arbitrary site of the
steel member serving as the measurement object, and a test piece is prepared by an
extraction replica method.
[0053] Next, an arbitrary region of the measurement surface of the test piece (region having
a size of 5 µm × 5 µm) is observed using TEM-EDS at a magnification of 30,000.
[0054] Next, a component of the precipitate present in the field of view for observation
is subjected to analysis of an electron beam diffraction pattern with TEM and analysis
with EDS, thereby identifying the Ti precipitate.
[0055] Next, an area ratio of the entire Ti precipitate present in the field of view for
observation is calculated.
[0056] The foregoing operation is performed five times, and the average value of the resulting
area ratio of the Ti precipitate is defined as the volume (area) fraction of the Ti
precipitate.
[0057] An equivalent circle diameter of the entire Ti precipitate present in the field of
view for observation is determined.
[0058] The foregoing operation is performed five times, and the average value of the resulting
"equivalent circle diameter" is defined as the average size (average equivalent circle
diameter) of the Ti precipitate.
[0059] A long axis length and a short axis length of the entire Ti precipitate present in
the field of view for observation are determined. The long axis length of the Ti precipitate
is defined as the maximum diameter of the Ti precipitate. The short axis length of
the Ti precipitate is defined as the maximum length of the Ti precipitate along with
a direction perpendicular to the long axis.
[0060] The foregoing operation is performed five times, and the average value of the resulting
"aspect ratio (= ratio of long axis length and short axis length (long axis length/short
axis length))" is defined as the average aspect ratio of the Ti precipitate.
[0061] The steel member according to the present embodiment preferably includes a refined
prior austenite grain, from the viewpoint of improvement in delayed fracture characteristics.
[0062] A grain size of the prior austenite grain (hereinafter, also referred to as "prior
γ grain size") is preferably from 5 to 50 µm, more preferably from 10 to 40 µm, and
still more preferably from 15 to 30 µm, in terms of an equivalent circle diameter
at a location of 1 mm in depth from a surface of the steel member.
[0063] The prior γ grain size is measured by the following method.
[0064] A portion that is located 1 mm in depth from a surface of the steel member (hereinafter,
also referred to as a "measurement surface") is taken from an arbitrary site of the
steel member serving as the measurement object, and the measurement surface of the
sample that has been taken is subjected to embedded polishing, and thereafter etched
with a picral solution (mixed solution of hydrochloric acid, picric acid, and alcohol)
as a corrosive liquid. The measurement surface of the sample is imaged using an optical
microscope (at a magnification of 250), a γ grain boundary that has been imaged is
subjected to digitization and binarization, a grain size of the prior γ grain is measured,
and the average value thereof is determined.
(Chemical Composition)
[0065] The steel member according to the present embodiment preferably has a chemical composition
of, in terms of percent (%) by mass: C: from 0.10 to 0.50%, Si: from 0.02 to 2.00%,
Mn: from 0.05 to 2.00%, Cr: from 0.10 to 2.00%, Ti: from 0.20 to 1.00%, N: from 0.0020
to 0.0250%, Al: from 0 to 0.100%, V: from 0 to 0.50%, Nb: from 0 to 0.50%, Mo: from
0 to 1.00%, B: from 0 to 0.0100%, Cu: from 0 to 2.00%, Ni: from 0 to 3.00%, and a
balance consisting of Fe and impurities, from the viewpoint of improvement in delayed
fracture characteristics.
[0066] Al, V, Nb, Mo, B, Cu, and Ni in the chemical composition of the steel member according
to the present embodiment are optional components, namely, components that do not
have to be included in the steel member. In cases in which these components are contained,
each component is preferably contained in an amount equal to or greater than the lower
limit of the respective content amount of the component, which will be described below.
• C: from 0.10 to 0.50%
[0067] C is an element essential to secure the tensile strength of the steel member (hereinafter,
also referred to as "strength"). When the amount of C is less than 0.10%, required
strength cannot be obtained. In this regard, when the amount of C is more than 0.50%,
deterioration in delayed fracture resistance is caused as well as deterioration in
toughness. Thus, the amount of C is from 0.10 to 0.50%. The amount of C is preferably
from 0.20 to 0.40% from the viewpoint of strength and toughness.
• Si: from 0.02 to 2.00%
[0068] Si has an effect of increasing the strength of the steel member by a solid-solution
hardening action. When the amount of Si is less than 0.02%, the action cannot be exerted.
In this regard, when the amount of Si is more than 2.00%, the action is saturated
and it is not possible to expect any effect commensurate with the amount. Thus, the
amount of Si is 0.02 to 2.00%. The amount of Si is preferably from 0.20 to 2.00% from
the viewpoint of exertion of the solid-solution hardening action.
• Mn: from 0.05 to 2.00%
[0069] Mn is an element not only necessary for deoxidation and desulfuration, but also effective
for improvement in hardenability for providing a martensite structure. When the amount
of Mn is less than 0.05%, the effect cannot be obtained. In this regard, when the
amount of Mn is more than 2.00%, a Mn precipitate is segregated in a grain boundary
during heating to an austenite region, thereby resulting in not only embrittlement
of the grain boundary but also deterioration in delayed fracture resistance. Thus,
the amount of Mn is from 0.05 to 2.00%. The amount of Mn is preferably from 0.50 to
1.50% from the viewpoint of improvement in hardenability and delayed fracture resistance.
• Cr: from 0.10 to 2.00%
[0070] Cr is an element effective for improvement in hardenability and increase in softening
resistance during a tempering treatment. When the amount of Cr is less than 0.10%,
the effect cannot be sufficiently exerted. In this regard, when the amount of Cr is
more than 2.00%, deterioration in toughness and deterioration in cold workability
are caused. Thus, the amount of Cr is from 0.10 to 2.00%. The amount of Cr is preferably
from 0.50 to 1.50% from the viewpoint of improvement in hardenability, and suppression
of deterioration in toughness and deterioration in cold workability.
• Ti: from 0.20 to 1.00%
[0071] Ti is an element that forms a fine Ti precipitate (at least one Ti precipitate selected
from the group consisting of a Ti carbide, a Ti nitride, and a composite compound
thereof) having hydrogen trapping ability at a relatively high temperature of from
400 to 800°C, and that contributes to improvement in delayed fracture resistance.
Further, Ti has not only an effect of forming TiN during deoxidation and heat treatment,
thereby preventing coarsening of an austenite grain, but also an effect of fixing
N. When the amount of Ti is less than 0.20%, these effects cannot be exerted. In this
regard, when the amount of Ti is more than 1.00%, Ti cannot be melted even with heat
during rolling and a coarse Ti precipitate remains, thereby resulting in adverse effect
on machinability or toughness. Thus, the amount of Ti is from 0.20 to 1.00%. The amount
of Ti is preferably from 0.30 to 0.80%, and more preferably from 0.40 to 0.60%, from
the viewpoint of formation of a fine Ti precipitate, machinability, toughness, or
the like.
• N: from 0.0020 to 0.0250%
[0072] N is an element that forms a Ti nitride and contributes to improvement in delayed
fracture resistance. N has an effect of forming nitrides of Al, V, and Nb, thereby
allowing refinement of a prior austenite grain and increase in yield strength. When
the amount of N is less than 0.0020%, these effects are less exerted. In this regard,
when the amount of N is more than 0.0250%, these effect are saturated. Thus, the amount
of N is from 0.0020 to 0.0250%. The amount of N is preferably from 0.0030 to 0.0150%,
from the viewpoint of improvement in delayed fracture resistance, refinement of a
prior austenite grain, and increase in yield strength.
[0073] The chemical composition of the steel member according to the present embodiment
may include, in terms of percent (%) by mass, one or more of Al: from 0 to 0.100%,
V: from 0 to 0.50%, Nb: from 0 to 0.50%, or Mo: from 0 to 1.00%, and preferably includes,
in terms of percent (%) by mass, one or more of Al: from 0.005 to 0.100%, V: from
0.01 to 0.50%, Nb: from 0.01 to 0.50%, or Mo: from 0.01 to 1.00%.
• Al: from 0.005 to 0.100%
[0074] Al is an element that has not only an effect of forming AlN during deoxidation and
heat treatment, thereby preventing coarsening of an austenite grain, but also an effect
of fixing N. When the amount of Al is less than 0.005%, these effects are difficult
to be exerted. In this regard, when the amount of Al is more than 0.100%, these effects
are easily saturated. Thus, Al is preferably from 0.005 to 0.100%.
• V: from 0.01 to 0.50%
[0075] V is an element that is precipitated in combination with TiC and that contributes
to fine dispersion of a precipitate. Further, V is an element effective for forming
a carbonitride, thereby refining an austenite grain. It is noted that the effect is
less exerted when the amount of V is not 0.01% or more, and is easily saturated when
the amount of V is more than 0.50%. When the amount of V is more than 0.50%, workability
is easily impaired due to increase in deformation resistance. Thus, the amount of
V is preferably from 0.01 to 0.50%.
• Nb: from 0.01 to 0.50%
[0076] Nb is an element that is precipitated in combination with TiC and that contributes
to fine dispersion of a precipitate, as is the case with V. Further, Nb is an element
effective for forming a carbonitride, thereby refining an austenite grain. It is noted
that the effect is insufficient when the amount of Nb is less than 0.01%, and is easily
saturated when the amount of Nb is more than 0.50%. Thus, the amount of Nb is preferably
from 0.01 to 0.50%.
• Mo: from 0.01 to 1.00%
[0077] Mo is an element that is precipitated in combination with TiC and that contributes
to fine dispersion of a precipitate, as is the case with V. It is noted that the effect
is insufficient when the amount of Mo is less than 0.01%, and is easily saturated
when the amount of Mo is more than 1.00%. Furthermore, when the amount of Mo is more
than 1.00%, workability is easily impaired due to increase in deformation resistance.
Thus, the amount of Mo is preferably from 0.01 to 1.00%.
[0078] The chemical composition of the steel member according to the present embodiment
may include B: from 0 to 0.0100% in terms of percent (%) by mass, and preferably includes
B: from 0.0003 to 0.0100% in terms of percent (%) by mass.
• B: from 0.0003 to 0.0100%
[0079] B is an element that suppresses grain boundary breakage and that improves delayed
fracture resistance. Further, B is an element that is segregated in an austenite grain
boundary, thereby resulting in a remarkable improvement in hardenability. It is noted
that the effects are difficult to be exerted when the amount of B is less than 0.0003%,
and are easily saturated when the amount of B is more than 0.0100%. Thus, the amount
of B is preferably from 0.0003 to 0.0100%. The amount of B is more preferably from
0.0003 to 0.0050%, from the viewpoint of improvement in hardenability and delayed
fracture resistance.
[0080] The chemical composition of the steel member according to the present embodiment
may include one or both of Cu: from 0 to 2.00% or Ni: from 0 to 3.00% in terms of
percent (%) by mass, and preferably includes one or both of Cu: from 0.05 to 2.00%
or Ni: from 0.05 to 3.00% in terms of percent (%) by mass.
• Cu: from 0.05 to 2.00%
[0081] Cu is an element effective for improvement in softening resistance during a tempering
treatment. When the amount of Cu is less than 0.05%, the effect is difficult to be
exerted. Further, when the amount of Cu is more than 2.00%, hot workability is easily
deteriorated. Thus, the amount of Cu is preferably from 0.05 to 2.00%. The amount
of Cu is more preferably from 0.05 to 1.00% from the viewpoint of suppression of deterioration
in hot workability.
• Ni: from 0.05 to 3.00%
[0082] Ni is an element for improvement in ductility that is deteriorated with increase
in strength. Further, Ni is an element for improvement in hardenability during a heat
treatment to increase in tensile strength. When the amount of Ni is less than 0.05%,
these effects are less exerted. Further, when the amount of Ni is more than 3.00%,
these effects are saturated and effects commensurate with the amount are difficult
to be exerted. Thus, the amount of Ni is preferably from 0.05 to 3.00%.
[0083] The balance of the chemical composition of the steel member according to the present
embodiment consists of Fe and impurities.
[0084] The impurities refer to any components that are contained in a raw material, or any
components that are incorporated in the course of production and are not intentionally
contained. Further, the impurities also encompass any components that are contained
in amounts in ranges not having any effect on properties of the steel member, even
if the components are intentionally contained.
[0085] Examples of the impurities include P and S. An amount of P and an amount of S are
each preferably from 0 to 0.015%, for example, from the viewpoint of no effect on
delayed fracture resistance. It is noted that the respective lower limits of the amount
of P and the amount of S may be more than 0% from the viewpoint of reduction in cost
of removal of P and cost of removal of S.
(Method of Producing Steel Member)
[0086] It is important for a method of producing the steel member according to the present
embodiment to include previously precipitating a Ti precipitate that exhibits trapping
ability in a rolling step during production of a rolling steel member serving as a
material of the steel member, in order to address a variety of heat treatment conditions
during production of the steel member.
[0087] For example, in a case in which a rolled bar steel member is used as the steel member,
a steel piece (billet) having the chemical composition described above is heated to
a temperature of 1,250°C or higher during rod steel rolling to cause the Ti compound
to become a solid solution, thereafter hot-rolled at a finish rolling temperature
of from 900 to 1,000°C, followed by being cooled to from 700 to 750°C at an average
cooling rate of 40°C/s or less. Thus, an objective Ti precipitate can be precipitated.
In this case, the Ti precipitate is isotropically precipitated.
[0088] The heating temperature of the steel piece (billet) refers to a surface temperature
of the steel piece. The finish rolling temperature refers to a surface temperature
of the rolled bar steel member immediately after finish rolling. The average cooling
rate after finish rolling refers to a surface-cooling rate of the rolled bar steel
member after finish rolling.
[0089] The rolled bar steel member in which the objective Ti precipitate has been precipitated
is heated to an austenite region (for example, 850 to 1,050°C), cooled to from 20
to 100°C at a cooling rate of 40°C/s or less to be quenched, and subjected to low-temperature
tempering at a temperature of from 150 to 200°C for a duration of from 15 to 60 minutes,
thereby obtaining the steel member according to the present embodiment.
[0090] Even in a case of employing a method of producing the steel member without any rolling,
such an objective Ti precipitate can be formed in the steel member by appropriately
controlling solid-solution and precipitation of compounds.
EXAMPLES
[0091] Hereinafter, the present disclosure will be more specifically described with reference
to Examples. It is noted that the respective Examples are not intended to limit the
scope of the present disclosure.
[0092] A test material having a chemical composition shown in Table 1 was heated to a temperature
shown in Table 2, thereafter hot-rolled at a finish rolling temperature shown in Table
2, and cooled to 700°C at an average cooling rate shown in Table 2 for rolling to
a diameter φ of 20 mm, thereby producing a circularly notched test piece (notch depth
of 2 mm, notch bottom radius of 0.25 mm, and notch angle of 60 degrees) made of a
round rod steel having a size of 10 mm in diameter φ × 50 mm in length L.
[0093] The test piece was heated in a carburizing heating atmosphere or in a condition simulating
RX gas heating (1 atm, mixed atmosphere of 50% of hydrogen and Ar, heating temperature
of 1,000°C, and heating duration of 30 minutes), cooled to 20°C with water at a cooling
rate of 40°C/s or less to be quenched, and thereafter tempered at 150°C for 20 minutes.
[0094] Note that No. 28 as a comparative steel was tempered at 520°C for 30 minutes and
No. 29 as a comparative steel was tempered at 400°C for 40 minutes.
[0095] The resulting test piece was subjected to measurement of the rupture time with a
constant load test at 3,000 kgf up to 100 hours. The tensile strength was also measured.
[0096] Separately, the test piece immediately after tempered was subjected to thermal desorption
hydrogen analysis and the hydrogen trapping capacity released at a temperature range
of from 400 to 800°C was measured, in accordance with the method described above.
The prior γ grain size, the volume (area) fraction of the Ti precipitate, the average
size of the Ti precipitate (average equivalent circle diameter), and the average aspect
ratio of the Ti precipitate were also measured in accordance with the methods described
above.
[Table 1]
No |
Chemical composition (% by mass), balance = Fe + impurities |
Note |
C |
Si |
Mn |
P |
S |
Cr |
Ti |
N |
Al |
B |
Others |
1 |
0.41 |
0.20 |
0.80 |
0.010 |
0.005 |
1.21 |
0.55 |
0.0051 |
0.005 |
- |
1.00Mo |
Example steel |
2 |
0.20 |
0.19 |
0.76 |
0.008 |
0.006 |
1.20 |
0.61 |
0.0152 |
- |
- |
- |
Example steel |
3 |
0.18 |
0.15 |
0.75 |
0.006 |
0.006 |
1.50 |
0.21 |
0.0121 |
0.007 |
0.0012 |
- |
Example steel |
4 |
0.10 |
0.02 |
1.05 |
0.006 |
0.005 |
1.90 |
1.00 |
0.0123 |
0.005 |
- |
- |
Example steel |
5 |
0.20 |
2.00 |
2.00 |
0.008 |
0.004 |
1.17 |
0.62 |
0.0103 |
- |
- |
- |
Example steel |
6 |
0.48 |
0.20 |
0.80 |
0.008 |
0.006 |
0.10 |
0.51 |
0.0084 |
- |
- |
- |
Example steel |
7 |
0.22 |
0.15 |
0.77 |
0.007 |
0.004 |
1.00 |
0.62 |
0.0233 |
- |
- |
0.3V |
Example steel |
8 |
0.20 |
0.18 |
0.80 |
0.007 |
0.006 |
1.04 |
0.59 |
0.0152 |
- |
- |
0.01Nb |
Example steel |
9 |
0.18 |
0.20 |
0.76 |
0.007 |
0.004 |
1.20 |
0.20 |
0.0124 |
0.005 |
0.0003 |
0.10Cu |
Example steel |
10 |
0.22 |
0.20 |
0.05 |
0.008 |
0.004 |
1.38 |
0.51 |
0.0105 |
0.100 |
- |
- |
Example steel |
11 |
0.20 |
0.19 |
0.75 |
0.008 |
0.007 |
1.20 |
0.55 |
0.0151 |
- |
- |
3.00Ni |
Example steel |
12 |
0.10 |
0.10 |
1.00 |
0.006 |
0.005 |
2.00 |
1.00 |
0.0103 |
0.005 |
- |
- |
Example steel |
13 |
0.22 |
0.15 |
0.77 |
0.007 |
0.004 |
1.01 |
0.62 |
0.0020 |
- |
- |
0.01V |
Example steel |
14 |
0.20 |
0.20 |
0.75 |
0.008 |
0.004 |
1.19 |
0.61 |
0.0148 |
- |
0.0100 |
- |
Example steel |
15 |
0.11 |
0.19 |
0.72 |
0.007 |
0.008 |
1.20 |
0.21 |
0.0119 |
0.006 |
0.0051 |
0.05Cu |
Example steel |
16 |
0.40 |
0.20 |
0.80 |
0.010 |
0.005 |
1.20 |
0.50 |
0.0050 |
0.007 |
- |
0.01Mo, 0.50V |
Example steel |
17 |
0.19 |
0.22 |
0.75 |
0.008 |
0.004 |
1.18 |
0.55 |
0.0150 |
- |
- |
2.0Cu |
Example steel |
18 |
0.20 |
0.20 |
0.73 |
0.008 |
0.004 |
1.20 |
0.61 |
0.0155 |
- |
- |
0.05Ni |
Example steel |
19 |
0.20 |
0.18 |
0.80 |
0.007 |
0.004 |
1.05 |
0.60 |
0.0150 |
- |
- |
0.5Nb |
Example steel |
20 |
0.20 |
0.25 |
0.75 |
0.006 |
0.004 |
1.20 |
0.05 |
0.0150 |
- |
- |
- |
Comparative steel |
21 |
0.41 |
0.20 |
0.82 |
0.010 |
0.005 |
1.17 |
0.00 |
0.0050 |
0.005 |
- |
0.20Mo |
Comparative steel |
22 |
0.40 |
0.18 |
0.81 |
0.010 |
0.005 |
1.19 |
0.05 |
0.0052 |
0.008 |
- |
0.20Mo |
Comparative steel |
23 |
0.18 |
0.22 |
0.80 |
0.007 |
0.007 |
1.21 |
1.09 |
0.0050 |
- |
- |
- |
Comparative steel |
24 |
0.60 |
0.21 |
0.83 |
0.008 |
0.004 |
0.03 |
0.50 |
0.0120 |
- |
- |
- |
Comparative steel |
25 |
0.11 |
0.05 |
0.10 |
0.008 |
0.008 |
0.05 |
0.57 |
0.0152 |
- |
- |
- |
Comparative steel |
26 |
0.20 |
0.20 |
0.75 |
0.008 |
0.004 |
1.20 |
0.55 |
0.0015 |
- |
- |
- |
Comparative steel |
27 |
0.20 |
0.19 |
0.76 |
0.008 |
0.006 |
1.20 |
0.61 |
0.0152 |
- |
- |
- |
Comparative steel |
28 |
0.36 |
0.16 |
0.97 |
0.008 |
0.008 |
- |
0.29 |
0.0049 |
0.071 |
- |
0.05V, 2.93Mo |
Comparative steel |
29 |
0.10 |
0.10 |
1.00 |
0.006 |
0.005 |
2.00 |
1.00 |
0.0103 |
0.005 |
- |
- |
Comparative steel |
[Table 2]
No |
Production conditions |
Hydrogen trapping capacity released at from 400 to 800°C (ppm by mass) |
Prior γ grain size |
Ti precipitate |
Characteristics |
Note |
Heating temperature (°C) |
Finish rolling temperature (°C) |
Average cooling rate (°C/s) |
Volume (area) fraction (%) |
Average size (average equivalent circle diameter) |
Aspect ratio |
Rupture time in constant load test (hr) |
Tensile strength (Mpa) |
1 |
1280 |
950 |
20 |
0.83 |
20 µm |
0.32 |
60 nm |
1.6 |
No rupture |
1560 |
Example steel |
2 |
1300 |
900 |
15 |
0.90 |
28 µm |
0.25 |
160 nm |
1.3 |
No rupture |
1350 |
Example steel |
3 |
1265 |
925 |
20 |
0.53 |
31 µm |
0.18 |
90 nm |
1.1 |
No rupture |
1340 |
Example steel |
4 |
1310 |
930 |
15 |
2.10 |
21 µm |
0.36 |
190 nm |
1.4 |
No rupture |
1235 |
Example steel |
5 |
1290 |
900 |
25 |
1.05 |
33 µm |
0.28 |
100 nm |
1.5 |
No rupture |
1360 |
Example steel |
6 |
1280 |
1000 |
40 |
0.88 |
30 µm |
0.31 |
90 nm |
1.3 |
No rupture |
1570 |
Example steel |
7 |
1280 |
950 |
20 |
0.82 |
22 µm |
0.21 |
60 nm |
1.6 |
No rupture |
1370 |
Example steel |
8 |
1280 |
950 |
30 |
0.95 |
18 µm |
0.28 |
70 nm |
1.4 |
No rupture |
1360 |
Example steel |
9 |
1250 |
950 |
25 |
0.51 |
32 µm |
0.12 |
60 nm |
1.2 |
No rupture |
1330 |
Example steel |
10 |
1270 |
950 |
25 |
0.95 |
22 µm |
0.28 |
100 nm |
1.1 |
No rupture |
1340 |
Example steel |
11 |
1260 |
950 |
25 |
0.87 |
28 µm |
0.30 |
90 nm |
1.6 |
No rupture |
1350 |
Example steel |
12 |
1290 |
950 |
25 |
2.05 |
22 µm |
0.26 |
190 nm |
1.8 |
No rupture |
1110 |
Example steel |
13 |
1270 |
950 |
30 |
0.82 |
25 µm |
0.25 |
60 nm |
1.5 |
No rupture |
1320 |
Example steel |
14 |
1270 |
950 |
12 |
0.90 |
33 µm |
0.26 |
160 nm |
1.7 |
No rupture |
1330 |
Example steel |
15 |
1250 |
900 |
15 |
0.51 |
20 µm |
0.18 |
60 nm |
1.3 |
No rupture |
1310 |
Example steel |
16 |
1265 |
920 |
30 |
0.80 |
18 µm |
0.41 |
60 nm |
1.8 |
No rupture |
1520 |
Example steel |
17 |
1270 |
980 |
25 |
0.87 |
35 µm |
0.32 |
90 nm |
1.3 |
No rupture |
1360 |
Example steel |
18 |
1280 |
950 |
30 |
0.90 |
36 µm |
0.33 |
160 nm |
1.4 |
No rupture |
1340 |
Example steel |
19 |
1275 |
900 |
30 |
0.95 |
16 µm |
0.31 |
70 nm |
1.3 |
No rupture |
1330 |
Example steel |
20 |
1260 |
930 |
25 |
0.03 |
35 µm |
0.05 |
10 nm |
1.2 |
25 |
1330 |
Comparative steel |
21 |
1260 |
920 |
25 |
0.00 |
22 µm |
0.00 |
- |
1.3 |
4 |
1510 |
Comparative steel |
22 |
1260 |
950 |
25 |
0.05 |
21 µm |
0.04 |
20 nm |
1.2 |
6 |
1520 |
Comparative steel |
23 |
1250 |
970 |
25 |
0.18 |
35 µm |
0.68 |
310 nm |
1.1 |
76 |
1350 |
Comparative steel |
24 |
1260 |
920 |
30 |
0.81 |
37 µm |
0.35 |
- |
1.6 |
32 |
1620 |
Comparative steel |
25 |
1260 |
900 |
35 |
0.91 |
38 µm |
0.38 |
100 nm |
1.3 |
0 |
980 |
Comparative steel |
26 |
1260 |
900 |
20 |
0.86 |
65 µm |
0.25 |
120 nm |
1.2 |
52 |
1350 |
Comparative steel |
27 |
1115 |
920 |
16 |
0.21 |
26 µm |
0.24 |
350 nm |
1.6 |
83 |
1340 |
Comparative steel |
28 |
1260 |
920 |
25 |
0.13 |
36 µm |
1.40 |
10 nm |
5.4 |
32 |
1737 |
Comparative steel |
29 |
1250 |
945 |
24 |
1.84 |
21 µm |
0.25 |
180 nm |
1.9 |
0 |
960 |
Comparative steel |
[0097] In Tables 1 and 2, Nos. 1 to 19 correspond to Example steels and the others correspond
to Comparative steels. All of the Example steels exhibited 0.5 ppm by mass or more
of hydrogen trapping ability as shown in the Tables, and thus were found to be excellent
in delayed fracture resistance.
[0098] On the contrary, Nos. 20, 21 and 22 corresponding to Comparative steels were each
an example where the Ti content was so low that the size of the Ti precipitate was
small or no Ti precipitate was present, thereby resulting in a small amount of hydrogen
trapping.
[0099] No. 23 corresponding to Comparative steel was an example where the amount of Ti was
excess, by which a solid solution of TiC was not sufficiently formed and a coarse
carbide was generated during heating in rolling, thereby resulting in a small amount
of hydrogen trapping.
[0100] No. 24 corresponding to Comparative steel was an example where the amount of C was
excess, thereby resulting in deterioration in delayed fracture resistance.
[0101] No. 25 corresponding to Comparative steel was an example where the amount of Cr was
small and hardenability was insufficient, as a result of which the steel was low in
tensile strength after quenching and thus could not withstand any load in the constant
load test.
[0102] No. 26 corresponding to Comparative steel was an example where the steel was small
in amount of N, thereby resulting in generation of a coarse grain during heating in
quenching and deterioration in delayed fracture resistance.
[0103] No. 27 corresponding to Comparative steel was an example where the heating temperature
during rolling was low, by which a solid-solution of the Ti compound was not sufficiently
formed and a coarse Ti precipitate was generated, thereby resulting in deterioration
in delayed fracture resistance.
[0104] No. 28 corresponding to Comparative steel was an example where the tempering temperature
was high, by which most of the Ti precipitate was precipitated during tempering and
thus the size of the Ti precipitate was small, thereby resulting in deterioration
in delayed fracture resistance.
[0105] No. 29 corresponding to Comparative steel was an example where the tempering temperature
was high, as a result of which the tensile strength after tempering was low and thus
the steel could not withstand any load in the constant load test.
[0106] It was thus found that the Comparative steels were low in delayed fracture resistance.
[0107] The disclosure of Japanese Patent Application No.
2017-123347 is incorporated herein by reference in its entirety.
[0108] All documents, patent applications, and technical standards described herein are
incorporated herein by reference to the same extent as if each individual document,
patent application, or technical standard was specifically and individually indicated
to be incorporated herein by reference.