HIGH STRENGTH STEEL MATERIAL AND HIGH
STRENGTH BOLT EXCELLENT IN DRLAYED FRACTURE RESISTANCE
AND METHODS OF PRODUCTION OF SAME
Technical Field
The present invention relates to a high strength
steel material which is used for wire rods, PC steel bars
(steel bars for prestressed concrete use), etc., more
particularly relates to a high strength steel material
and high strength bolts of a tensile strength of 1300 MPa
or more which are excellent in delayed fracture
resistance and methods for the production of the same.
Background Art
The high strength steel which is used in large
amounts for machines, automobiles, bridges, and building
structures is medium carbon steel with an amount of C of
0.20 to 0.35%, for example, SCr, SCM, etc: defined by JIS
G 4104 and JIS G 4105 which is quenched and tempered.
However, in all types of steels, if the tensile strength
exceeds 1300 MPa, the risk of delayed fracture occurring
becomes larger.
As methods for improving the delayed fracture
resistance of high strength steel, the method of making
the steel structure a bainite structure or the method of
refining the prior austenite grains is effective.
PLT 1 discloses steel which is refined in prior
austenite grains and improved in delayed fracture
resistance, while PLT's 2 and 3 disclose steels which
suppress segregation of steel ingredients to improve the
delayed fracture resistance. However, with refinement of
prior austenite grains or with suppression of segregation
of ingredients, it is difficult to greatly improve the
delayed fracture resistance.
A bainite structure contributes to improvement of
the delayed fracture resistance, but formation of a
bainite structure requires suitable additive elements or
heat treatment, so the cost of the steel rises.
PLT1s 4 to 6 disclose wire rods for high strength
5 bolts containing 0.5 to 1.0 mass% of C in which an area
ratio 80% or more of the pearlite structure is strongly
drawn to impart 1200~/mrn* or more strength and excelAent
delayed fracture resistance. However, the wire rods which
are described in PLT1s 4 to 6 are high in cost due to the
10 drawing process. Further, manufacture of thick wire rods
is difficult.
PLT 7 discloses a coil spring using oil-tempered
wire of a hardness of the inside cross-section of Hv550
or more to suppress the occurrence of delayed fracture
15 after cold coiling. However, the surface hardness of the
product after nitriding is Hv900 or more. Under high load
stress such as with bolts or PC steel bars, the delayed
fracture characteristics are low. If the corrosive
environment becomes severer, there is the problem of
20 delayed fracture occurring.
PLT 8 discloses a steel material which is excellent
in dela.yed fracture resistance composed of steel of a
required composition which is nitrided and mainly
consists of a tempered martensite structure. The high
25 strength steel material which is disclosed in PLT 8
exhibits delayed fracture resistance even under a
corrosive environment containing hydrogen.
However, in recent years, corrosive environments
have become harsher. A high strength steel material which
30 exhibits excellent delayed fracture resistance even under
severe corrosive environments has been sought.
Citations List
Patent Literature
PLT 1: JP-B2-64-4566
PLT' 2: JP-A-3-243744
PLT 3: JP-A-3-243745

(1) A steel material which is excellent in delayed
fracture resistance containing, by mass%, C: 0.10 to
0.55%, Si: 0.01 to 3%, and Mn: 0.1 to 2%, further
containing one or both of V: 1.5% or less and Mo: 3.0% or
5 less, the contents of V and Mo satisfying V+1/2Mo>0.4%,
further containing one or more of Cr: 0.05 to 1.5%, Nb:
0.001 to 0.05%, Cu: 0.01 to 4%, Ni: 0.01 to 4%, and B:
0.0001 to 0.005%, and having a balance of Fe and
unavoidable impurities, the structure being a mainly
10 tempered martensite structure,
the surface of the steel material being formed with
(a) a nitrided layer having a thickness from the
surface of the steel material of 200 p or more and a
nitrogen concentration of 12.0 mass% or less and higher
15 than the nitrogen concentration of the steel material by
0.02 mass% or more and
(b) a low carbon region having a depth from the
surface of the steel material of 100 pm or more to 1000
p or less and having a carbon concentration of 0.05
20 mass% or more and 0.9 time or less the carbon
concentration of the steel material.
(2) A steel material which is excellent in delayed
fracture resistance as set forth in (1) characterized in
that due to the presence of the nitrided layer and low
25 carbon region, the amount of absorption of hydrogen in
the steel material is 0.5 ppm or less and the critical
diffusible hydrogen content of the steel material is 2.00
ppm or more.
(3) A steel material which is excellent in delayed
30 fracture resistance as set forth in (1) or (2) further
characterized in that the steel material contains, by
mass%, one or more of Al: 0.003 to 0.1%, Ti: 0.003 to
0.058, Mg: 0.0003 to 0.01%, Ca: 0.0003 to 0.01%, and Zr:
0.0003 to 0.01%.
35 (4) A steel material which is excellent in delayed
fracture resistance as set forth in any one of (1) to (3)
characterized in that the nitrided layer has a thickness
of 1000 pm or less.
(5) A steel material which is excellent in delayed
fracture resistance as set forth in any one of (1) to (4)
5 characterized in that the tempered martensite has an area
ratio of 85% or more.
(6) A steel material which is excellent in delayed
fracture resistance as set forth in any one of (1) to (5)
characterized in that the steel material has a
10 compressive residual stress at the surface of 200 MPa or
more.
(7) A steel material which is excellent in delayed
fracture resistance as set forth in any one of (1) to (6)
characterized in that the steel material has a tensile
15 strength of 1300 MPa or more.
(8) A bolt which is excellent in delayed fracture
resistance obtained by working a steel material
containing, by mass%, C: 0.10 to 0.55%, Si: 0.01 to 3%,
and Mn: 0.1 to 2%, further containing one or both of V:
20 1.5% or less and Mo: 3.0% or less, the contents of V and
Mo satisfying V+1/2Mo>0.4%, further containing one or
more of Cr: 0.05 to 1.5%, Nb: 0.001 to 0.05%, Cu: 0.01 to
4%, Ni: 0.01 to 4%, and B: 0.0001 to 0.005%, and having a
balance of Fe and unavoidable impurities, the structure
25 being a mainly tempered martensite structure,
the surface of the bolt being formed with
(a) a nitrided layer having a thickness from the
surface of the bolt of 200 pm or more and a nitrogen
concentration of 12.0 mass% or less and higher than the
30 nitrogen concentration of the steel material by 0.02
mass% or more and
(b) a low carbon region having a depth from the
surface of the bolt of LOO pm or more to 1000 pm or less
and having a carbon concentration of 0.05 mass% or more
35 and 0.9 time or less the carbon concentration of the
steel material.
(9) A high strength bolt which is excellent in
delayed fracture resistance as set forth in (8)
characterized in that due to the presence of the nitrided
layer and low carbon region, the amount of absorption of
5 hydrogen in the bolt is 0.5 ppm or less and the critical
diffusible hydrogen content of the bolt is 2.00 ppm or
more.
(10) A high strength bolt which is excellent in
delayed fracture resistance as set forth in (8) or (9)
10 characterized in that the steel material further
contains, by mass%, one or more of Al: 0.003 to 0.1%, Ti:
0.003 to 0.05%, Mg: 0.0003 to 0.01%, Ca: 0.0003 to 0.01%,
and Zr: 0.0003 to 0.01%.
(11) A high strength bolt which is excellent in
15 delayed fracture resistance as set forth in any one of
(8) to (10) characterized in that the nitrided layer has
a thickness of 1000 pm or less.
(12) A high strength bolt which is excellent in
delayed fracture resistance as set forth in any one of
20 (8) to (11) characterized in that the tempered martensite
has an area ratio of 85% or more.
(13) A high strength bolt which is excellent in
delayed fracture resistance as set forth in any one of
(8) to (12) characterized in that the bolt has a
25 compressive residual stress at the surface of 200 MPa or
more.
(14) A high strength bolt which is excellent in
delayed fracture resistance as set forth in any one of
(8) to (13) characterized in that the bolt has a tensile
30 strength of 1300 MPa or more.
(15) A method of production of a high strength steel
material which is excellent in delayed fracture
resistance as set forth in any one of (1) to ( 7 ) ,
the method of production of a high strength steel
35 material which is excellent in delayed fracture
resistance characterized by
(1) heating a steel material having a composition
as set forth in (1) or (3) to form a low carbon region
having a depth from the surface of the steel material of
100 pm or more to 1000 pm or less and having a carbon
concentration of 0.05 mass% or more and 0.9 time or less
5 the carbon concentration of the steel material, then
cooling as it is to make the steel material structure a
mainly martensite structure, then
(2) nitriding the steel material at over 500°C to
650°C or less to form on the surface of the steel material
10 a nitrided layer having a nitrogen concentration of 12.0
mass% or less and higher than the nitrogen concentration
of the steel material by 0.02 mass% and having a
thickness from the surface of the steel material of 200
pm or more and to make the steel material structure a
15 mainly tempered martensite structure.
(16) A method of production of a high strength steel
material which is excellent in delayed fracture
resistance as set forth in (15) characterized in that the
nitrided layer has a thickness of 1000 pm or less.
20 (17) A method of production of a bolt which is
excellent in delayed fracture resistance as set forth in
any one of (8) to (14),
the method of production of a bolt which is
excellent in delayed fracture resistance characterized by
25 (1) heating a bolt obtained by working a steel
material having a composition as set forth in (8) or (10)
to form a low carbon region having a depth from the
surface of the bolt of 100 pm or more to 1000 pm or less
and having a carbon concentration of 0.05 mass% or more
30 and 0.9 time or less the carbon concentration of the
steel material, then cooling as it is to make the steel
material structure a mainly martensite structure, then
(2) nitriding the bolt at over 500°C to 650°C or
less to form on the surface of the bolt a nitrided layer
35 having a nitrogen concentration of 12.0 mass% or less and
higher than the nitrogen concentration of the steel
material by 0.02 mass% and having a thickness from the
surface of the bolt of 200 pm or more and to make the
steel material structure a mainly tempered martensite
structure.
5 (18) A method of production of a bolt which is
excellent in delayed fracture resistance as set forth in
(17), characterized in that the nitrided layer has a
thickness of 1000 p or less.
10 Advantageous Effect of Invention
According to the present invention, it is possible
to provide a high strength steel material (wire rod or PC
steel bar) and high strength bolt which exhibit excellent
delayed fracture resistance even in a severe corrosive
15 environment and methods for production able to produce
these inexpensively.
Brief Description of Drawings
FIG. l(a) is a view which schematically shows a
20 hydrogen evolution rate curve which is obtained by
hydrogen analysis by the thermal desorption analysis.
FIG. l(b) is a view which schematically shows the
relationship between a fracture time obtained by a
constant load delayed fracture test of a steel material
25 and an amount of diffusible hydrogen.
FIG. 2 is a view which shows a method of finding a
depth (thickness) of a low carbon region from a carbon
concentration curve which is obtained by an Energy
Dispersion type X-ray Spectrometer (EDX).
30 FIG. 3 is a view which shows a method of finding a
thickness (depth) of a nitrided region from a nitrogen
concentration curve which is obtained by an Energy
Dispersion type X-ray Spectrometer (EDX).
FIG. 4 is a view which shows a test piece which is
35 used for a delayed fracture test of a steel material.
FIG. 5 is a view which shows one mode of a delayed
fracture test machine.
FIG. 6 is a view which shows a relationship between
temperature and humidity in an accelerated corrosion test
and time.
5 Description of Embodiments
It is known that hydrogen in steel causes delayed
fracture. Further, absorption of hydrogen into the steel
occurs along with corrosion in actual environments. The
ingress of diffusible hydrogen into the steel
10 concentrates at the concentrated parts of tensile stress
and results in occurrence of delayed fracture.
FIG. l(a) schematically shows a hydrogen evolution
rate curve obtained by hydrogen analysis by the thermal
desorption analysis. As shown in FIG. l(a), the amount of
15 release of diffusible hydrogen reaches a peak near 100°C.
In the present invention, a sample is raised in
temperature by 100°C/h and the cumulative value of the
amount of hydrogen which is released from room
temperature to 400°C is defined as the amount of
20 diffusible hydrogen. Note that, the amount of released
hydrogen can be measured by a gas chromatograph.
In the present invention, the minimum amount of
diffusible hydrogen at which delayed fracture occurs is
referred to as the "critical diffusible hydrogen
25 content". The limit amount of diffusible hydrogen differs
according to the type of the steel.
FIG. l(b) schematically shows the relationship
between the fracture time obtained by a constant load
delayed fracture test of the steel material and the
30 amount of diffusible hydrogen. As shown in FIG. l(b), if
the amount of diffusible hydrogen is great, the fracture
time is short, while if the amount of diffusible hydrogen
is small, the fracture time is long.
That is, if the amount of diffusible hydrogen is
35 small, delayed fracture does not occur, while if the
amount of diffusible hydrogen is great, delayed fracture
occurs. In the present invention, a constant load delayed
fracture test of the steel material is run and, as shown
in FIG. l(b), the maximum value of the amount of
diffusible hydrogen at which no fracture occurs for 100
5 hours or more was made the critical diffusible hydrogen
content.
If comparing the amount of absorption of hydrogen
and the critical diffusible hydrogen content and if the
critical diffusible hydrogen content is greater than the
10 amount of absorption of hydrogen, delayed fracture does
not occur. Conversely if the critical diffusible hydrogen
content is smaller than the amount of absorption of
hydrogen, delayed fracture occurs. Therefore, the larger
the critical diffusible hydrogen content, the more the
15 occurrence of delayed fracture is suppressed.
However, if the amount of absorption of hydrogen in
the steel material from a corrosive environment exceeds
the critical diffusible hydrogen content, delayed
fracture occurs.
20 Therefore, to prevent the occurrence of delayed
fracture, it is effective to suppress absorption of
hydrogen into the steel material. For example, if forming
a nitrided layer at the surface of the steel material by
nitriding, the amount of absorption of hydrogen due to
25 corrosion is suppressed, so the delayed fracture
resistance is improved.
However, if forming a nitrided layer at the steel
material surface, due to hardening of the surface layer,
the critical diffusible hydrogen content decreases and
30 the delayed fracture resistance is not improved.
Therefore, the inventors studied the composition of the
steel material predicated on trapping and rendering
harmless the hydrogen due to ingress in the steel
material by fine precipitates so as to increase the
35 critical diffusible hydrogen content.
As a result, the inventors confirmed that if
including suitable amounts of one or both of V and Mo and
causing fine precipitates composed of carbides, nitrides,
and/or carbonitrides of V or Mo to form in the steel
material, it is possible to increase the critical
diffusible hydrogen content.
5 However, on the other hand, it was confirmed that
even if the steel material is made to contain one or both
of V and Mo, sometimes improvement of the delayed
fracture resistance cannot be sufficiently obtained.
Therefore, the inventors engaged in repeated
10 intensive studies focusing on the relationship between
the crystal structure of the fine precipitates and the
critical diffusible hydrogen content and amount of
absorption of hydrogen and as a result learned the
following.
15 The hexagonal carbides, nitrides, and carbonitrides
of Mo have a larger effect of increase of the critical
diffusible hydrogen content than the effect of increase
of the amount of absorption of hydrogen, so contribute to
the improvement of the delayed fracture resistance, but
20 compared with the NaC1-type structures of carbides,
nitrides, and carbonitrides of V, the effect of
improvement of strength by fine precipitation is small.
On the other hand, NaC1-type structures of carbides,
nitrides, and carbonitrides of V are excellent in the
2 5 effect of improvement of strength due to fine
precipitation. However, the NaC1-type structures of
carbides, nitrides, and carbonitrides of V increase the
critical diffusible hydrogen content, while also increase
the amount of absorption of hydrogen, so the effect of
30 improvement of the delayed fracture resistance is smaller
compared with the hexagonal carbides, nitrides, and
carbonitrides of Mo.
That is, in a steel material, to secure the required
strength and secure excellent delayed fracture
35 resistance, it is necessary to suitably set the contents
of Mo and V.
Therefore, the inventors engaged in in-depth studies
to suitably set the contents of Mo and V. As a result,
they learned that if including one or both of V: 1.5
mass% or less and Mo: 3.0 mass% or less and making the
contents of V and Mo (mass%) satisfy V+1/2Mo>0.4 mass%,
5 the critical diffusible hydrogen content is increased and
an excellent delayed fracture resistance is obtained.
Further, the inventors studied lowering the hardness
of the nitrided layer to improve the delayed fracture
resistance, carried out accelerated corrosion tests and
10 exposure tests on steel materials which were decarburized
on the surface and, furthermore, nitrided, and
investigated the hydrogen ingress characteristics and
delayed fracture resistance of the steel material.
As a result, the inventors learned that if forming a
15 nitrided layer of a predetermined nitrogen concentration
and thickness on the surface of a steel material which
has a predetermined composition and structure and,
furthermore, forming a low carbon region of a
predetermined carbon concentration and depth on the steel
20 material surface, the delayed fracture resistance is
remarkably improved compared with the case of forming
only a nitrided layer on the steel material surface.
This is believed to be due to the synergistic effect
of (1) suppression of the amount of absorption of
25 hydrogen compared with the case of a nitrided layer alone
due to the formation of a nitrided layer at the low
carbon region which is formed at the steel material
surface and (2) suppression of excessive hardening of the
surface and increase of the critical diffusible hydrogen
30 content due to the formation of the low carbon region at
the steel material surface.
Basically, they learned that if forming, on the
surface of a steel material of a predetermined
composition and structure, (a) a nitrided layer having a
3.5 thickness from the surface of the steel material of 200
pm or more and a nitrogen concentration of 12.0 mass% or
less and higher than the nitrogen concentration of the
steel material by 0.02 mass% or more and (b) a low carbon
region having a depth from the surface of the steel
material of 100 pm or more to 1000 pm or less and having
a carbon concentration of 0.05 mass% or more and 0.9 time
5 or less the carbon concentration of the steel material,
it is possible to increase the critical diffusible
hydrogen content of the steel material and reduce the
amount of absorption of hydrogen.
Further, the inventors discovered that by heating
10 and rapid cooling at the time of nitriding, compressive
residual stress occurs at the steel material surface and
the delayed fracture resistance is improved. In
particular, in the case of a high strength bolt in which
strain is introduced into the surface by working,
15 formation of a nitrided layer is promoted. Further, the
nitrogen concentration becomes higher, so the delayed
fracture resistance is remarkably improved.
Below, the present invention will be explained in
detail.
20 The high strength steel material and high strength
bolt of the present invention are composed of
predetermined compositions of ingredients and have a
nitrided layer and a low carbon region simultaneously
present on the surface.
25 That is, at the surface of the high strength steel
material and high strength bolt of the present invention,
there is a region with a nitrogen concentration of 12.0
mass% or less and higher than the nitrogen concentration
of the steel material by 0.02 mass% or more and with a
30 carbon concentration of 0.05 mass% or more and 0.9 time
or less the steel material (low carbon nitrided layer).
When the thickness of the nitrided layer is greater
than the thickness of the low carbon region, the carbon
concentration at the location deeper than the low carbon
35 region is equal to the carbon concentration of the steel
material and the nitrogen concentration is higher than
the nitrogen concentration of the steel material.
On the other hand, when the thickness of the low
carbon region is greater than the thickness of the
nitrided layer, the result is a low carbon region with a
carbon concentration of 0.05 mass% or more and 0.9 times
5 or less of the carbon concentration of the steel material
and with contents of other elements equal to the steel
material is present under the nitrided layer.
First, the low carbon region will be explained. In
the present invention, the low carbon region is a region
10 with a carbon concentration of 0.05 mass% or more and 0.9
time or less the carbon concentration of the high
strength steel material or high strength bolt.
In the high strength steel material and high
strength bolt of the present invention, a low carbon
15 region is formed at a depth of 100 pm or more to 1000 pn
from the steel material surface.
The depth and carbon concentration of the low carbon
region adjust the heating atmosphere, heating
temperature, and holding time at the time of heat
20 treatment which forms the low carbon region. For example,
if the carbon potential of the heating atmosphere is low,
the heating temperature is high, and the holding time is
long, the low carbon region becomes deeper and the carbon
concentration of the low carbon region falls.
25 If the carbon concentration of the low carbon region
is less than 0.05 mass%, this becomes less than half of
the lower limit 0.10 mass% of the carbon concentration of
the steel material, so it is not possible to secure a
predetermined strength and hardness by the low carbon
30 region. If the carbon concentration of the low carbon
region is over 0.9 time the carbon concentration of the
steel material, this is substantially equal to the carbon
concentration of the steel material and the effect of
presence of the low carbon region ends up becoming
35 weaker.
For this reason, in the present invention, the low
carbon region was defined as a region where the carbon
concentration is 0.05 mass% or more and 0.9 time or less
the carbon concentration of the steel material.
If the carbon concentration of the low carbon region
is 0.05 mass% or more and 0.9 time or less of the carbon
5 concentration of the steel material, it is possible to
reduce the amount of increase in the surface hardness due
to formation of the nitrided layer. As a result, the
hardness of the surface of the steel material becomes
equal to the hardness of the steel material or lower than
10 the hardness of the steel material and can prevent a
reduction of the critical diffusible hydrogen content.
The depth (thickness) of the low carbon region was
made a depth (thickness) of 100 p or more from the
surface of the steel material or bolt so that the effect
15 is obtained. The depth (thickness) of the low carbon
region is preferably greater in depth (thickness), but if
over 1000 p, the strength of the steel material as a
whole or the bolt as a whole falls, so the depth
(thickness) of the low carbon region is given an upper
20 limit of 1000 pm.
Next, a nitrided layer will be explained. In the
present invention, the nitrided layer is a region with a
nitrogen concentration of 12.0 mass% or less and higher
than the nitrogen concentration of the steel material or
25 bolt by 0.02 mass% or more. Further, the nitrided layer
is formed by a thickness of 200 pn or more from the
surface of the steel material or bolt.
The thickness and nitrogen concentration of the
nitrided layer can be adjusted by the heating atmosphere,
30 heating temperature, and holding time at the time of
nitriding. For example, if the concentration of ammonia
or nitrogen in the heating atmosphere is high, the
heating temperature is high, and the holding time is
long, the nitrided layer becomes thicker and the nitrogen
35 concentration of the nitrided layer becomes higher.
If the nitrogen concentration of the nitrided layer
is higher than the nitrogen concentration of the steel
material, it is possible to reduce the amount of
absorption of hydrogen in the steel material from a
corrosive environment, but if the difference of the
nitrogen concentration of the nitrided layer and the
nitrogen concentration of the steel material is less than
0.02 mass%, the effect of reduction of the amount of
absorption of hydrogen cannot be sufficiently obtained.
For this reason, the nitrogen concentration of the
nitrided layer was made a concentration higher than the
nitrogen concentration of the steel material by 0.02
mass% or more.
On the other hand, if the nitrogen concentration
exceeds 12.3 mass%, the nitrided layer excessively rises
in hardness and becomes brittle, so 12.0 mass% was made
the upper limit.
If the steel material surface is formed with a
nitrided layer which has a nitrogen concentration of 12.0
mass% or less and higher than the nitrogen concentration
of the steel material by 0.02 mass% or more and a depth
of 200 pm or more from the surface, the amount of
absorption of hydrogen in the steel material from the
corrosive environment is greatly reduced.
The nitrided layer was limited to a thickness
(depth) of 200 p or more from the surface of the steel
material or bolt so that the effect is obtained. The
upper limit of the thickness of the nitrided layer is not
particularly defined, but if the thickness is over 1000
pm, the productivity falls and a rise in cost is invited,
so 1000 p or less is preferable.
The depth (thickness) of the low carbon region which
is formed on the high strength steel material or high
strength bolt of the present invention can be found from
the curve of the carbon concentration from the surface of
the steel material or bolt.
A cross-section of a steel material or bolt which
has a low carbon region and nitrided layer on the surface
is polished and an Energy Dispersive X-ray
Spectroscopyter (below, sometimes referred to as "EDX")
or a Wavelength Dispersive X-ray Spectroscopy (below,
5 sometimes referred to as "WDS") is used for line analysis
to measure the carbon concentration in a depth direction
from the surface.
FIG. 2 shows the method of finding the depth
(thickness) of the low carbon region from the curve of
10 the carbon concentration which is obtained by EDX. That
is, FIG. 2 is a view which shows the relationship between
the distance from the steel material surface, obtained by
measuring the carbon concentration in the depth direction
from the surface using EDX, and the carbon concentration.
15 As shown in FIG. 2, the carbon concentration
increases along with the increased distance (depth) from
the steel material surface. This is because due to
decarburization, a low carbon region is formed on the
surface of the steel material.
20 In the region not affected by the decarburization,
the carbon concentration is substantially constant
(average carbon concentration "a"). The average carbon
concentration "a" is the carbon concentration of the
region not affected by the decarburization and is equal
25 to the amount of carbon of the steel material before
decarburization.
Therefore, in the present invention, the chemical
analysis value of the carbon concentration of the steel
material is made the reference value when finding the
30 depth of the low carbon region.
As shown in FIG. 2, it is possible to discriminate
the range where the carbon concentration from the steel
material surface to the required depth becomes lower than
10% or more of the average carbon concentration "a" (a x
35 0.1) (range of 0.9 time or less of the carbon
concentration of the steel material) and find the
distance (depth) from the steel material surface at the
boundary of that range in the depth direction so as to
evaluate the depth (thickness) of the low carbon region.
The thickness (depth) of the nitrided layer can be
found from the change of the nitrogen concentration from
5 the surface of the steel material or bolt in the same way
as the low carbon region. Specifically, a cross-section
of the steel material or bolt which has a low carbon
region and nitrided layer on the surface is polished and
an EDX or WDS is used for line analysis to measure the
10 nitrogen concentration in the depth direction from the
surface.
FIG. 3 shows the method of finding the thickness
(depth) of the nitrided layer from the nitrogen
concentration curve obtained by an Energy ~is~ersiotny pe
15 X-ray Spectrometer (EDX) . That is, FIG. 3 is a view
showing the relationship between the distance from the
steel material surface and the nitrogen concentration
which is obtained by measuring the nitrogen concentration
in the depth direction from the surface using EDX.
20 As the distance (depth) from the steel material
surface becomes longer, the nitrogen concentration
decreases, but in the region not affected by nitriding,
the carbon concentration is substantially constant
(average nitrogen concentration).
25 The average nitrogen concentration is a range of
nitrogen concentration not affected by nitriding and is
equal to the amount of nitrogen of the steel material
before nitriding. Therefore, in the present invention,
the chemical analysis value of the nitrogen concentration
30 of the steel material is made the reference value when
finding the thickness of the nitrided layer.
As shown in FIG. 3, it is possible to discriminate
the region in which the nitrogen concentration from the
steel material surface down to the required depth becomes
35 higher than the average nitrogen concentration by 0.02
mass% or more and finding the distance (depth) from the
steel material surface at the boundary of that region in
the depth direction so as to evaluate the thickness
(depth) of the nitrided layer.
The depth of the low carbon region and the thickness
of the nitrided layer are found by obtaining simple
5 averages of the values which were measured at any five
locations at the cross-section of the steel material or
bolt.
Note that, the carbon concentration and nitrogen
concentration of the steel material may be found by
10 measuring the carbon concentration and nitrogen
concentration at a position sufficiently deeper than the
depth of the low carbon region and nitrided layer, for
example, a position at a depth of 2000 pn or more from
the surface. Further, it is also possible to obtain an
15 analytical sample from a position at a depth of 2000 pm
or more from the surface of the steel material or bolt
and chemically analyze it to find them.
In the high strength steel Inaterial of the present
invention, as explained above, the delayed fracture is
20 remarkably improved by the synergistic effect of (1)
suppression of the amount of absorption of hydrogen due
to the formation of a nitrided layer at the low carbon
region which is formed at the steel material surface and
(2) increase of the critical diffusible hydrogen content
25 due to the formation of the low carbon region at the
steel material surface.
According to investigations by the inventors, the
surface of the steel material has a nitrided layer and a
low carbon region copresent on it, whereby the amount of
30 absorption of hydrogen in the steel material can be
suppressed to 0.5 ppm or less and the critical diffusible
hydrogen content of the steel material can be raised to
2.00 ppm or more.
Next, the reasons for limitation of the composition
35 of the steel material will be explained. Below, the %
according to the composition mean mass%.
C: C is an essential element in securing the
strength of a steel material. If less than 0.10%, the
required strength is not obtained, while if over 0.55%,
the ductility and toughness fall and the delayed fracture
resistance also falls, so the content of C was made 0.10
Si: Si is an element which improves strength by
solution strengthening. If less than 0.01%, the effect of
addition is insufficient, while if over 3%, the effect
becomes saturated, so the content of Si was made 0.01 to
10 3%.
Mn: Mn is an element which not only performs
deoxidation and desulfurization, but also gives a
martensite structure, so lowers the transformation
temperature of the pearlite structure or bainite
15 structure to raise the hardenability. If less than 0.1%,
the effect of addition is insufficient, while if over 2%,
it segregates at the grain boundary at the time of
heating of austenite to embrittle the grain boundary and
degrades the delayed fracture resistance, so the content
2 0 of Mn was made 0.1 to 2%.
The high strength steel material or high strength
bolt of the present invention contains one or both of V
and Mo to improve the delayed fracture resistance.
V: V is an element which causes the formation of
25 fine precipitates of carbides, nitrides, and/or
carbonitrides in the steel material. These fine
precipitates act to trap and render harmless the hydrogen
due to ingress in the steel material and increase the
critical diffusible hydrogen content. Further, the fine
30 precipitates contribute to the improvement of strength of
the steel material.
Further, V is an element which lowers the
transformation temperature of the pearlite structure or
bainite structure and increases the hardenability and
35 also raises the resistance to softening during tempering
at the time of tempering to contribute to improvement bf
the strength.
If the content of V exceeds 1.5%, the solution
temperature for precipitation strengthening becomes
higher and, further, the ability for trapping hydrogen
also becomes saturated, so the content was made 1.5% or
5 less. If the content of V is less than 0.05%, an effect
of improvement of strength due to the formation of fine
precipitates is not sufficiently obtained, so 0.05% or
more is preferable.
Mo: Mo is an element which causes the formation of
10 fine precipitates of carbides, nitrides, and/or
carbonitrides in the steel material. The fine
precipitates act to trap and render harmless the hydrogen
due to ingress in the steel material and increase the
critical diffusible hydrogen content. Further, fine
15 precipitates contribute to an improvement of strength of
the steel material and increases the critical diffusible
hydrogen content to contribute to the improvement of the
delayed fracture resistance.
Further, Mo is an element which lowers the
20 transformation temperature of the pearlite structure or
bainite structure to raise the hardenability and raises
the resistance to softening during tempering at the time
of tempering to contribute to the improvement of the
strength.
25 If the content of Mo exceeds 3.0%, the solution
temperature for precipitation strengthening becomes
higher and, further, the ability for trapping hydrogen
also becomes saturated, so the content was made 3.0% or
less. If the content of Mo is less than 0.4%, an effect
30 of increase of the critical diffusible hydrogen content
due to the formation of fine precipitates is not
sufficiently obtained, so 0.4% or more is preferable.
V+1/2Mo: In the present invention, the contents of
Mo and V have to satisfy V+l/ZMo>0.4%. If the contents of
35 V and Mo satisfy this formula, the amount of V whereby
not only the critical diffusible hydrogen content, but
also the amount of absorption of hydrogen increases
becomes relatively smaller than t6e amount of Mo, so the
critical diffusible hydrogen content increases and an
excellent delayed fracture resistance is obtained.
The high strength steel material or high strength
5 bolt of the present invention may further contain one or
more of Cr, Nb, Cu, Ni, and B in a range not impairing
the excellent delayed fracture resistance for the purpose
of improving the strength.
Cr: Cr is an element which lowers the transformation
10 temperature of the pearlite structure or bainite
structure to raise the hardenability and, further, raises
the resistance to softening during tempering at the time
of tempering to contribute to the improvement of the
strength. If less than 0.05%, the effect of addition is
15 not sufficiently obtained, while if over 1.5%,
deterioration of the toughness is invited, so the content
of Cr was made 0.05 to 1.5%.
Nb: Nb, like Cr, is an element which raises the
hardenability and the tempering softening resistance to
20 contribute to the improvement of the strength. If less
than 0.001%, the effect of addition is not sufficiently
obtained. If over 0.05%, the effect of addition becomes
saturated, so the content of Nb was made 0.001 to 0.05%.
Cu: Cu is an element which contributes to the
25 improvement of the hardenability, increase of the temper
softening resistance, and improvement of strength by the
precipitation effect. If less than 0.01%, the effect of
addition is not sufficiently obtained, while if over 4%,
grain boundary embrittlement occurs and the delayed
30 fracture resistance deteriorates, so the content of Cu
was made 0.01 to 4%.
Ni: Ni is an element which raises the hardenability
and is effective for improvement of the ductility and
toughness which fall along with increased strength. If
35 less than 0.01%, the effect of addition is not
sufficiently obtained, while if over 4%, the effect of
addition becomes saturated, so the content of Ni was made
B: B is an element which suppresses grain boundary
fracture and is effective for improvement of the delayed
fracture resistance. Furthermore, B is an element which
5 segregates at the austenite grain boundary and remarkably
raises the hardenability. If less than 0.0001%, the
effect of addition cannot be sufficiently obtained, while
if over 0.005%, B carbides and Fe borocarbides form at
the grain boundaries, grain boundary embrittlement
10 occurs, and delayed fracture resistance falls, so the
content of B is made 0.0001 to 0.005%.
The high strength steel material and high strength
bolt of the present invention may further contain, for
the purpose of refining the structure, one or more of Al,
15 Ti, Mg, Ca, and Zr in a range not detracting from the
excellent delayed fracture resistance.
Al: A1 is an element which forms oxides or nitrides
and prevents coarsening of austenite grains to suppress
deterioration of the delayed fracture resistance. If less
20 than 0.003%, the effect of addition is insufficient,
while if over 0.1%, the effect of addition becomes
saturated, so the content of A1 is preferably 0.003 to
0.1%.
Ti: Ti also, like Al, is an element which forms
25 oxides or nitrides to prevent coarsening of austenite
grains and suppress deterioration of the delayed fracture
resistance. If less than 0.003%, the effect of addition
is insufficient, while if over 0.05%, the Ti
carbonitrides coarsen at the time of rolling or working
30 or at the time of heating in heat treatment and the
toughness falls, so the content of Ti is preferably 0.003
to 0.05%.
Mg: Mg is an element which has a deoxidizing and
desulfurizing effect and, further, forms Mg oxides, Mg
35 sulfides, Mg-A1, Mg-Ti, and Mg-Al-Ti composite oxides or
composite sulfides, etc. to prevent coarsening of
austenite grains and suppress deterioration of delayed
fracture resistance. If less than 0.0003%, the effect of
addition is insufficient, while if over 0.01%, the effect
of addition becomes saturated, so the content of Mg is
preferably 0.0003 to 0.01%.
5 Ca: Ca is an element which has a deoxidizing and
desulfurizing effect and, further, forms Ca oxides, Ca
sulfides, Al, Ti, and Mg composite oxides or composite
sulfides, etc. to prevent coarsening of austenite grains
and suppress deterioration of delayed fracture
10 resistance. If less than 0.0003%, the effect of addition
is insufficient, while if over 0.01%, the effect of
addition becomes saturated, so the content of Ca is
preferably 0.0003 to 0.01%.
Zr: Zr is an element which forms Zr oxides, Zr
15 sulfides, Al, Ti, Mg, and Zr composite oxides or
composite sulfides, etc. to prevent coarsening of
austenite grains and suppress deterioration of delayed
fracture resistance. If less than 0.0003%, the effect of
addition is insufficient, while if over 0.01%, the effect
20 of addition becomes saturated, so the content of Zr is
preferably 0.0003 to 0.01%.
Steel Structure
Next, the structure of the high strength steel
material and high strength bolt of the present invention
25 (below, sometimes called "the steel structure of the
present invention") will be explained. The steel
structure of the present invention is mainly tempered
martensite, so the structure is excellent in ductility
and toughness even if the tensile strength is 1300 MPa or
30 more.
The steel structure of the present invention is
preferably a structure where the area ratio of the
tempered martensite in the region excluding the low
carbon region and nitrided layer is 85% or more and the
35 balance is composed of one or more of residual austenite,
bainite, pearlite, and ferrite.
The area ratio of the tempered martensite is
measured at a deeper position between the depth at which
the carbon concentration becomes constant in the carbon
concentration curve which is shown in FIG. 2 and the
depth where the nitrogen concentration becomes constant
in the nitrogen concentration curve which is shown in
FIG. 3.
For example, it is sufficient to measure the depth
of 2000 pn or more from the surface of the steel material
or bolt or the area ratio of the tempered martensite at
locations of 1/4 of the thickness or diameter of the
steel material.
Note that, the area ratio of martensite can be found
by observing the cross-section of the steel material
using an optical microscope and measuring the area of
martensite per unit area. Specifically, the cross-section
of the steel material is etched by a ~ital etching
solution, the areas of martensite in five fields in a
range of 0.04 mm2 are measured, and the average value is
calculated.
Further, in the steel material of the present
invention, compressive residual stress of the steel
material surface occurs due to the heating and rapid
cooling at the time of nitriding whereby the delayed
fracture resistance is improved. If the compressive
residual stress occurs by 200 MPa or more, the delayed
fracture resistance is improved, so the compressive
residual stress of the surface of the steel material of
the present invention is preferably 200 MPa or more.
The compressive residual stress can be measured by
X-ray diffraction. Specifically, the residual stress of
the steel material surface is measured, then the steel
material surface is etched 25 pn at a time by
electrolytic polishing and the residual stress in the
depth direction is measured. It is preferable to measure
any three locations and use the average value of the
same.
In a steel material in which no low carbon region
and nitrided layer are formed on the surface, if the
tensile strength becomes 1300 MPa or more, the frequency
of occurrence of delayed fracture remarkably increases.
Therefore, if the tensile strength is 1300 MPa or more,
5 the delayed fracture resistance of the steel material of
the present invention on which a low carbon region and
nitrided layer are formed on the surface is remarkably
excellent.
The upper limit of the tensile strength of the
10 present invention is not particularly limited, but over
2200 MPa is technically difficult at the present point of
time, so 2200 MPa is provisionally made the upper limit.
Note that the tensile strength may be measured based on
JIS Z 2241.
15 Method of Production
Next, a method of production of a steel material of
the present invention will be explained.
The method of production of a steel material of the
present invention is composed of a decarburization step
20 of heating a steel material of a required composition
(wire rod or PC steel bar or steel material worked to a
predetermined shape) to decarburize it, a hardening step
of cooling the decarburized steel material to make the
steel structure a mainly martensite structure, and a step
25 of nitriding the hardened steel material at over 500°C to
650°C or less.
Note that, due to the nitriding step, the structure
of the steel material of the present invention becomes a
structure of mainly tempered martensite.
30 In the decarburization step, the steel material of
the present invention is decarburized to make the carbon
concentration, down from the surface of the steel
material by a depth of 100 p or more to 1000 p or less,
0.05% or more and 0.9 time or less the carbon
35 concentration of the steel material. The atmosphere in
the heating furnace is, for example, adjusted to a
concentration of methane gas to make it weakly
decarburizing and form a low carbon region.
The heating temperature in the decarburization is
preferably Ac3 to 950°C. By heating to Ac3 or more, it is
possible to make the steel structure austenite, promote
decarburization from the surface layer, and easily form a
low carbon region.
The upper limit of the heating temperature is
preferably 950°C in the point that this suppresses
coarsening of the crystal grains and improves the delayed
fracture resistance. The holding time at the heating
temperature is preferably 30 to 90 minutes. By holding at
the heating temperature for 30 minutes or more, it is
possible to sufficiently secure the depth of the low
carbon region and possible to make the steel structure
uniform. If considering the productivity, the holding
time at the heating temperature is preferably 90 minutes
or less
At the hardening step, the heated steel material is
cooled to obtain a mainly martensite structure. The
heated steel material may be oil quenched as it is for
hardening.
In the steel structure of the present invention, the
area ratio of the tempered martensite is preferably 85%
or more, so the area ratio of the martensite after
hardening is preferably 85% or more.
At the hardening step, to secure an area ratio of
the martensite of 85% or more, at the time of hardening,
it is preferable to make the cooling rate in the range
from 700 to 300°C 5OC/s or more. if the cooling rate is
less than 5OC/s, sometimes the area ratio of the
martensite becomes less than 85%.
At the nitriding step, a steel material with a steel
structure of mainly martensite and formed with a low
carbon region at the surface layer is nitrided. Due to
the nitriding, a nitrided layer is formed with a
thickness from the steel material surface of 200 p or
more and a nitrogen concentration of 12.0% or less and
higher than the nitrogen concentration of the steel
material by 0.02% or more. At the same time, the steel
material is tempered to make the steel structure a mainly
tempered martensite structure and to form fine
precipitates which trap hydrogen.
In the method of production of a steel material of
the present invention, the steel material contains one or
both of V and Mo, so in the nitriding step, fine
carbides, nitrides, and/or carbonitrides (fine
precipitates) which act to trap the hydrogen are formed.
The nitriding is performed by, for example, heating
the steel material in an atmosphere containing ammonia or
nitrogen. The nitriding is preferably performed by
holding the sample at over 500°C to 650°C or less for 30
to 90 minutes. If the nitriding temperature exceeds 650°C,
the steel material falls in strength, so the nitriding
temperature is made 650°C or less.
20 ,If the nitriding temperature is 500°C or less, fine
precipitates do not sufficiently form, so the nitriding
temperature is made over 500°C. Further, if the nitriding
temperature is made over 500°C, the time required for
diffusion of nitrogen from the steel material surface
25 becomes shorter, the treatment time is shortened, and the
productivity rises.
If the nitriding time is less than 30 minutes, the
depth of the nitrided layer is liable not to reach a
depth of 200 pm or more from the surface, so the
30 nitriding time is preferably 30 minutes or more. The
upper limit of the nitriding time is not defined, but in
the present invention, the nitriding temperature is high,
so even at 90 minutes or less, a sufficient thickness of
a nitrided layer can be formed.
35 Note that, in the nitriding step, the gas nitriding
method, nitrocarburizing method, plasma nitriding method,
salt bath nitriding method, or other general nitriding
method may be used.
Next, the method of production of the high strength
bolt of the present invention (below, sometimes referred
5 to as "the present invention bolts") will be explained.
The method of production of the bolt of the present
invention is composed of a working step of working the
steel material of the present invention having the
required composition into a bolt, a decarburization step
10 of heating the bolt to decarburize it, a hardening step
of cooling the heated bolt to make the steel structure a
mainly martensite structure, and a nitriding step of
nitriding the hardened bolt at a temperature of over 500°C
to 650°C or less.
15 In the nitriding step, the steel structure of the
bolt becomes a mainly tempered martensite structure.
Note that, in the working step, for example, the
steel material wire rod is cold forged and rolled to form
a bolt.
20 The method of production of the bolt of the present
invention differs from the method of production of the
steel material of the present invention only in the
working step for working the steel material into a bolt
shape, so the explanation of the other steps will be
25 omitted.
The method of production of the steel material of
the present invention and the method of production of the
bolt of the present invention preferably performs rapid
cooling, after nitriding, in a range from 500 to 200°C by
30 a cooling rate of 10 to 100°C/s. By rapidly cooling after
nitriding, it is possible to make the compressive
residual stress of the surface of the steel material or
bolt 200 MPa or more. Due to the presence of this
compressive residual stress, the delayed fracture
35 resistance is improved more.
Examples
Next, examples of the present invention will be
explained, but the conditions of the examples are an
example of the conditions adopted for confirming the
5 workability and effect of the present invention. The
present invention is not limited to this example of the
conditions. In the present invention, various conditions
can be adopted so long as not departing from the gist of
the present invention and achieving the object of the
10 present invention.
(Examples)
Molten steels of the compositions of ingredients
which are shown in Table 1 were cast in accordance with
an ordinary method. The cast slabs were hot worked to
15 obtain steel materials (wire rods). The steel materials
were heated. to Ac3 to 950°C and cooled as is for
hardening.
Note that, at the time of heating, the atmosphere in
the heating furnace was controlled to be weakly
20 decarburizing. The hardening was performed by oil
quenching so that the cooling rate in the range of 700 to
300°C became 5OC/s or more. Further, the depth of the low
carbon region was investigated by the carbon potential of
the atmosphere of the heating furnace, heating
25 temperature, and holding time.
00 XI" 10 171 1
After that, the steel materials were nitrided by
nitrocarburizing to form nitrided layers. After
nitriding, the materials were rapidly cooled in the range
of 500 to 200°C by the cooling rates which are shown in
5 Table 2 (cooling rates after tempering) to obtain the
high strength steel materials of Manufacturing Nos. 1 to
25.
Note that, the nitriding was performed at the
temperatures which are shown in Table 2 while making the
10 ammonia volume ratios in the treatment gas atmosphere 30
to 50% and making the treatment times 30 to 90 minutes.
The nitrided layers were adjusted in thickness by
changing the heating temperature and the holding time.
The nitrided layers were adjusted in nitrogen
15 concentration by changing the ammonia volume ratio in the
treatment gas atmosphere.
Table 2
I I I I I 1 Nitriding 1 I 1 I Critical 1 I I
Man.
("C/s) (P) (PP~) fracture
Steel
type
Tempered
martensite
ratio ( % )
Strength
(MPa)
Low
carbon
region
depth
Temperature
("c)
Cooling
rate
Nitrided
layer
thickness
Compressive
residual
stress
(MPa)
Amount of
absorption
of
hydrogen
diffusible
hydrogen
content of
delayed
Delayed
fracture
presence
Remarks
The steel materials which are shown in Table 1 (wire
rods) were worked into bolts by the same process as the
high strength steel materials (wire rods) of
Manufacturing Nos. 1 to 25 to obtain the high strength
5 bolts of Manufacturing Nos. 26 to 40. The nitriding was
performed in the temperature ranges which are shown in
Table 3. After the nitriding, the materials were rapidly
cooled in the range of 500 to 200°C by the cooling rates
which are shown in Table 3 (cooling rates after
10 tempering).
Table 3
The tempered martensite ratios, tensile strengths,
low carbon region depths, nitrided layer thicknesses,
compressive residual stresses, amounts of absorption of
hydrogen, limit amounts of diffusible hydrogen, and
5 delayed fracture resistances of the high strength steel
materials of Manufacturing Nos. 1 to 25 (Table 2) and the
high strength bolts of Manufacturing Nos. 26 to 40 (Table
3) were measured by the methods which are shown below.
The results are shown in Table 2 and Table 3 together.
10 Tempered Martensite Ratio
The tempered martensite ratio was found by polishing
the cross-section of each of the high strength steel
materials of Manufacturing Nos. 1 to 25 and the high
strength bolts of Manufacturing Nos. 26 to 40, etching by
15 a Nital etching solution, using an optical microscope to
measure the areas of the martensite in five fields in a
0.04 mm2 range, and finding the average value.
Note that in each of the high strength steel
materials of Manufacturing Nos. 1 to 25 and the high
20 strength bolts of Manufacturing Nos. 27 to 41, the
structure of the remaining part of the tempered
martensite was a balance of one or more of austenite,
bainite, pearlite, and ferrite.
Tensile Strength
25 The tensile strength was measured based on JIS Z
2241.
Low Carbon Region Depth and Nitrided Layer Thickness
A cross-section of each of the high strength steel
materials of Manufacturing Nos. 1 to 25 and the high
30 strength bolts of Manufacturing Nos. 26 to 40 was
polished and measured for the carbon concentration and
nitrogen concentration in the depth direction from the
surface using an EDX at any five locations in the
longitudinal direction.
35 The depth (thickness) of the region where the carbon
concentration is 0.9 time or less of the carbon
concentration of the steel material ((carbon
concentration of low carbon region/carbon concentration
of steel material) 10.9) was defined as the "low carbon
region depth", while the depth (thickness) of the region
where the nitrogen concentration is higher than the
5 nitrogen concentration of the steel material by 0.02% or
more (nitrogen concentration of nitrided layer-nitrogen
concentration of steel material 20.02) was defined as the
"nitrided layer thickness".
Note that, the low carbon region depth and the
10 nitrided layer thickness were the averages of values
measured at any five locations in the longitudinal
direction.
Compressive Residual Stress
An X-ray residual stress measurement apparatus was
15 used to measure the compressive residual stress of the
surface. The residual stress of the surface of each of
the high strength steel materials of Manufacturing Nos. 1
to 25 and the high strength bolts of Manufacturing Nos.
26 to 40 was measured, then the surface was etched by 25
20 pm at a time by electrolytic polishing and the residual
stress in the depth direction was measured. Note that,
the compressive residual stress was made the average of
the values measured at any three locations.
Critical diffusible hydrogen content and Delayed
25 Fracture Resistance
From each of the high strength steel materials of
Manufacturing Nos. 1 to 25 and the high strength bolts of
Manufacturing Nos. 26 to 40, a delayed fracture test
piece of the shape which is shown in FIG. 4 was prepared
30 and subjected to absorption of hydrogen. For absorption
of hydrogen, the electrolytic hydrogen charge method was
used to change the charge current and change the amount
of absorption of hydrogen as shown by Table 2 and Table
3.
35 The surface of each delayed fracture test piece
which was subjected to absorption of hydrogen was plated
with Cd to prevent dissipation of the diffusible
hydrogen. The test piece was leaved at room temperature
for 3 hours to even the concentration of hydrogen at the
inside.
5 After that, a delayed fracture test machine which is
shown in FIG. 5 was used to run a constant load delayed
fracture test applying a tensile load of 90% of the
tensile strength to the test piece 1. Note that, in the
test machine which is shown in FIG. 5, when applying a
10 tensile load to the test piece 1, a balance weight 2 was
placed at one end of a lever having the fulcrum 3 as the
fulcrum and the test piece 1 was placed at the other end
to conduct the test.
Further, as shown in FIG. l(b), the maximum value of
15 the amount of diffusible hydrogen of a test piece 1 which
did not fracture even after performing the constant load
delayed fracture test for 100 hours or more was made the
critical diffusible hydrogen content. The amount of
diffusible hydrogen of the test piece 1 was measured by
20 raising the delayed fracture test piece in temperature at
100°C/h and measuring the cumulative value of the amounts
of hydrogen which were released between room temperature
to 400°C by a gas chromatograph.
When comparing the amount of absorption of hydrogen
25 and the critical diffusible hydrogen content of delayed
fracture and the critical diffusible hydrogen content is
greater than the amount of absorption of hydrogen,
delayed fracture does not occur. Conversely, if the
critical diffusible hydrogen content is smaller than the
30 amount of absorption of hydrogen, delayed fracture
occurs.
Therefore, the delayed fracture resistance was
evaluated as "without delayed fracture" when the amount
of absorption of hydrogen which is shown in Table 2 and
35 Table 3 was less than critical diffusible hydrogen
content and as "with delayed fracture" when the amount of
absorption of hydrogen was the critical diffusible
hydrogen content or more.
Amount of Absorption of hydrogen
The amount of absorption of hydrogen was determined
by preparing a test piece of each of the high strength
5 steel materials of Manufacturing Nos. 1 to 25 and the
high strength bolts of Manufacturing Nos. 26 to 40 and
running an accelerated corrosion test of the pattern of
temperature, humidity, and time which is shown in FIG. 6
for 30 cycles,.
10 The corroded layer at the surface of the test piece
was removed by sandblasting, then the hydrogen was
analyzed by the Thermal desorption analysis. The amount
of hydrogen which was released from room temperature to
400°C was measured to find the amount of absorption of
15 hydrogen.
As shown in Table 2, the high strength steel
materials of Manufacturing Nos. 1 to 15 of the invention
examples had a low carbon region depth of 100 pn or more
and a nitrided layer thickness of 200 pm or more. All had
20 a tensile strength of 1300 MPa or more, an amount of
absorption of hydrogen of 0.5 ppm or less, a critical
diffusible hydrogen content of 2.00 ppm or more, an
amount of absorption of hydrogen of less than the
critical diffusible hydrogen content, and no delayed
25 fracture.
Further, the high strength steel materials of
Manufacturing Nos. 1 to 15 all had a tempered martensite
rate of 50% or more and a structure of mainly tempered
martensite. Further, the high strength steel materials of
30 Manufacturing Nos. 1 to 15 all had a compressive residual
stress of 200 MPa or more.
As opposed to this, as shown in Table 2, the high
strength steel material of Manufacturing No. 16 of the
comparative example was an example where the amount of C,
35 the amount of Si, and the amount of Mn were small and the
strength was low.
Manufacturing No. 17 is an example where the amount
of C was large, Manufacturing No. 18 is an example where
the amount of Mn was large, Manufacturing No. 19 is an
example where the amount of Cr was large, Manufacturing
5 No. 21 is an example where the amount of Cu was large,
Manufacturing No. 22 is an example where the amount of B
was large, and Manufacturing No. 20 is an example where
V+1/2Mo was low. In each case, the critical diffusible
hydrogen content was low and the resistance was "with
10 delayed fracture".
Further, Manufacturing No. 23 is an example where
the heating time of the hardening was short, the low
carbon region depth was less than 100 p , the critical
diffusible hydrogen content was low, and the resistance
15 was "with delayed fracture". Manufacturing No. 24 is an
example where the nitriding time was short, the nitrided
layer thickness was less than 200 p , the amount of
absorption of hydrogen was large, and the resistance was
"with delayed fracture".
20 Manufacturing No. 25 is an example in which the
concentration of ammonia in the gas of the nitriding was
lowered, so at a location down to the depth of 200 pm
from the surface, the difference of the nitrogen
concentration from the steel material became 0.01 mass%,
25 the amount of absorption of hydrogen was larger, and the
resistance was "with delayed fracture".
As shown in Table 3, the high strength bolts of
Manufacturing Nos. 26 to 40 of the invention examples had
a low carbon region depth of 100 pm or more and a
30 nitrided layer thickness of 200 p or more. All had a
tensile strength of 1300 MPa or more, an amount of
absorption of hydrogen of 0.5 ppm or less, a critical
diffusible hydrogen content of 2.00 ppm or more, an
amount of absorption of hydrogen of less than the
35 critical diffusible hydrogen content, and a resistance
"without delayed fracture".
The high strength bolts of Manufacturing Nos. 26 to
40 all had a tempered martensite ratio of 50% or more, a
structure of mainly tempered martensite, and a
compressive residual stress of 200 MPa or more.
5 From Table 2 and Table 3, it will be understood that
the high strength bolts of Manufacturing Nos. 26 to 40,
which differ from the high strength steel materials of
Manufacturing Nos. 1 to 15 only in the point of working
the steel material (wire rod) to bolts (high strength
10 bolts of Manufacturing Nos. 26 to 40 correspond to high
strength steel materials of Manufacturing Nos. 1 to 15),
were further suppressed in the amount of absorption of
hydrogen compared with the high strength steel materials.
15 Industrial Applicability
As explained above, according to the present
invention, it is possible to provide a high strength
steel material (wire rod or PC steel bar) and high
strength bolt which exhibit excellent delayed fracture
20 resistance even in a severe corrosive environment and a
method of production enabling inexpensive production of
these. Accordingly, the present invention is extremely
high in applicability in industries manufacturing and
using steel materials.

Claims
A steel material which is excellent in delayed
5 fracture resistance containing,
by mass%,
C: 0.10 to 0.55%,
Si: 0.01 to 3%, and
Mn: 0.1 to 2%,
10 further c'ontaining one or both of
V: 1.5% or less and Mo: 3.0% or less, the contents
of V and Mo satisfying
V+1/2Mo>0.48,
further containing one or more of
15 Cr: 0.05 to 1.5%,
Nb: 0.001 to 0.05%,
Cu: 0.01 to 4%, I
Ni: 0.01 to 4%, and
B: O.CO01 to 0.005%, and
20 having a balance of Fe and unavoidable impurities, the
structure being a mainly tempered martensite structure,
the surface of the steel material being formed with
(a) a nitrided layer having a thickness from the
surface of said steel material of 200 pn or more and a
25 nitrogen concentration of 12.0 mass% or less and higher
than the nitrogen concentration of said steel material by
0.02 mass% or more and
(b) a low carbon region having a depth from the
surface of said steel material of 100 pm or more'to 1000
30 pm or less and having a carbon concentration of 0.05
mass% or more and 0.9 time or less the carbon
concentration of said steel material.
Claim 2
35 A steel material which is excellent in delayed
fracture resistance as set forth in claim 1 characterized
in that due to the presence of said nitrided layer and
low carbon region, the amount of absorption of hydrogen
in the steel material is 0.5 ppm or less and the critical
diffusible hydrogen content of the steel material is 2.00
5 ppm or more.
Claim 3
A steel material which is excellent in delayed
fracture resistance as set forth in claim 1 or 2 further
10 characterized in that said steel material contains, by
mass%, one or more of
Al: 0.003 to 0.1%,
Ti: 0.003 to 0.05%,
Mg: 0.0003 to 0.01%,
Ca: 0.0003 to 0.01%, and
Zr: 0.0003 to 0.01%.
Claim 4
A steel material which is excellent in delayed
20 fracture resistance as set forth in any one of claims 1
to 3 characterized in that said nitrided layer has a
thickness of 1000 pn or less.
Claim 5
25 A steel material which is excellent in delayed
fracture resistance as set forth in any one of claims 1
to 4 characterized in that said tempered martensite has
an area ratio of 85% or more.
30 Claim 6
A steel material which is excellent in delayed
fracture resistance as set forth in any one of claims 1
to 5 characterized in that said steel material has a
compressive residual stress at the surface of 200 MPa or
35 more.
Claim 7
A steel material which is excellent in delayed
fracture resistance as set forth in any one of claims 1
to 6 characterized in that said steel material has a
tensile strength of 1300 MPa or more.
5
Claim 8
A bolt which is excellent in delayed fracture
resistance obtained by working a steel material
containing,
10 by mass%,
C: 0.10 to 0.55%,
Si: 0.01 to 3%, and
Mn: 0.1 to 2%,
further containing one or both of
15 V: 1.5% or less and
Mo: 3.0% or less, the contents of V and Mo
satisfying
V+1/2Mo>0.4%,
further containing one or more of
20 Cr: 0.05 to 1.5%,
Nb: 0.001 to 0.05%,
Cu: 0.01 to 4%,
Ni: 0.01 to 4%, and
B: 0.0001 to 0.005%, and
25 having a balance of Fe and unavoidable impurities, the
structure being a mainly tempered martensite structure,
the surface of said bolt being formed with
(a) a nitrided layer having a thickness from the
surface of said bolt of 200 pm or more and a nitrogen
30 concentration of 12.0 mass% or less and higher than the
nitrogen concentration of said steel material by 0.02
mass% or more and
(b) a low carbon region having a depth from the
surface of said bolt of 100 pm or more to 1000 p or less
35 and having a carbon concentration of 0.05 mass% or more
and 0.9 time or less the carbon concentration of said
steel material.
Claim 9
A high strength bolt which is excellent in delayed
fracture resistance as set forth in claim 8 characterized
in that due to the presence of said nitrided layer and
low carbon region, the amount of absorption of hydrogen
in the bolt is 0.5 ppm or less and the critical
diffusible hydrogen content of the bolt is 2.00 ppm or
more.
Claim 10
A high strength bolt which is excellent in delayed
fracture resistance as set forth in claim 8 or 9
characterized in that said steel material further
contains, by mass%, one or more of
Al: 0.003 to 0.1%,
Ti: 0.003 to 0.05%,
Mg: 0.0003 to 0.01%,
Ca: 0.0003 to 0.01%, and
Zr: 0.0003 to 0.01%.
Claim 11
A high strength bolt which is excellent in delayed
fracture resistance as set forth in any one of claims 8
to 10 characterized in that said nitrided layer has a
thickness of 1000 pm or less.
Claim 12
A high strength bolt which is excellent in delayed
fracture resistance as set forth in any one of claims 8
to 11 characterized in that said tempered martensite has
an area ratio of 85% or more.
Claim 13
A high strength bolt which is excellent in delayed
fracture resistance as set forth in any one of claims 8
to 12 characterized in that said bolt has a compressive
residual stress at the surface of 200 MPa or more.
Claim 14
A high strength bolt which is excellent in delayed
5 fracture resistance as set forth in any one of claims 8
to 13 characterized in that said bolt has a tensile
strength of 1300 MPa or more.
Claim 15
10 A method of production of a high strength steel
material which is excellent in delayed fracture
resistance as set forth in any one of claims 1 to 7,
said method of production of a high strength steel
material which is excellent in delayed fracture
15 resistance characterized by
(1) heating a steel material having a composition
as set forth in claim 1 or 3 to form a low carbon region
having a depth from the surface of said steel material of
100 pm or more to 1000 pm or less and having a carbon
20 concentration of 0.05 mass% or more and 0.9 time or less
the carbon concentration of said steel material, then
cooling as it is to make the steel material structure a
mainly martensite structure, then
(2) nitriding said steel material at over 500°C to
25 650°C or less to form on the surface of the steel material
a nitrided layer having a nitrogen concentration of 12.0
mass% or less and higher than the nitrogen concentration
of said steel material by 0.02 mass% and having a
thickness from the surface of said steel material of 200
30 pm or more and to make the steel material structure a
mainly tempered martensite structure.
Claim 16
A method of production of a high strength steel
35 material which is excellent in delayed fracture
resistance as set forth in claim 15 characterized in that
said nitrided layer has a thickness of 1000 pm or less.
Claim 17
A method of production of a bolt which is excellent
in delayed fracture resistance as set forth in any one of
claims 8 to 14,
said method of production of a bolt which is
excellent in delayed fracture resistance characterized by
(1) heating a bolt obtained by working a steel
material having a composition as set forth in claim 8 or
10 to form a low carbon region having a depth from the
surface of said bolt of 100 pm or more to 1000 pm or less
and having a carbon concentration of 0.05 mass% or more
and 0.9 time or less the carbon concentration of said
steel material, then cooling as it is to make the steel
material structure a mainly martensite structure, then
(2) nitriding said bolt at over 500°C to 650°C or
less to form on the surface of the bolt a nitrided layer
having a nitrogen concentration of 12.0 mass% or less and
2 0 higher than the nitrogen concentration of said steel
material by 0.02 mass% and having a thickness from the
surface of said bolt of 200 pm or more and to make the
steel material structure a mainly tempered martensite
structure.
2 5
Claim 18
A method of production of a bolt which is excellent
in delayed fracture resistance as set forth in claim 17,
characterized in that said nitrided layer has a thickness
30 of 1000 prn or less.