D E C L A R A T I O N
I, Yuki Yamada , c/o Seiwa Patent & Law,
Toranomon 37 Mori Bldg., 5-1, Toranomon 3-chome,. Minatoku,
Tokyo, Japan, hereby verify that I am the translator
of the attached translation of International Application
No. PCT/JP2011/056482 and that I believe the attached
translation is a true and accurate translation of the
same.
This 20th day of April , 2012
- 1 -
DESCRIPTION
Y774
Title of Invention: HIGH STRENGTH STEEL AND HIGH STRENGTH
BOLT EXCELLENT IN DELAYED FRACTURE RESISTANCE AND METHODS
5 OF PRODUCTION OF SAME
Technical Field
The present invention relates to a high strength
steel which is used for wire rods, PC steel bars (steel
10 bars for prestressed concrete use), etc., more
particularly relates to a high strength steel 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.
15
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
20 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.
25 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
30 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
35 of ingredients, it is difficult to greatly improve the
delayed fracture resistance.
A bainite structure contributes to improvement of
- 2 -
the delayed fracture resistance, but formation of a
bainite structure requires suitable additive elements or
heat treatment, so the cost of the steel rises.
PLT's 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 1200N/mm 2 or more strength and excellent
delayed fracture resistance. However, the wire rods which
are described in PLT's 4 to 6 are high in cost due to the
10 drawing process. Further, manufacture of thick wire rods
is difficult.
PLT7 discloses a coil spring in which development of
a delayed fracture after cold-coiling is prevented, using
an oil tempered wire having a hardness in the inner part
15 of cross section of >_ Hv 550. However, the coil spring
has a surface layer hardness after nitriding of Hv 900 or
more, and a product, for example, in the form of bolt or
PC steel bar has a low delayer fracture under a high load
stress. Thus, developing a delayed fracture in a severe
20 corrosion environment is a problem.
PLTB discloses a high strength steel having
excellent delayed fracture resistance mainly comprised of
tempered martensite structure, which is obtained by
nitriding a steel having a certain composition. The high
25 strength steel disclosed in PLTB displays a delayed
fracture resistance even in a corrosion environment
containing hydrogen.
Nevertheless, corrosion environments have recently
become severe, and a high strength steel displaying
30 excellent delayed fracture resistance even in severe
corrosion environments is needed.
Citations List
Patent Literature
PLT 1: JP-B2-64-4566
35 PLT 2: JP-A-3-243744
PLT 3: JP-A-3-243745
PLT 4: JP-A-2000-337332
- 3
PLT 5: JP-A-2000-337333
PLT 6: JP-A-2000-337334
PLT 7: JP-A-10-251803
PLT 8: JP-A-2009-299180
5
Summary of Invention
Technical Problem
As explained above, in high strength steels, there
is a limit to improving the delayed fracture resistance
10 by conventional methods. As a method for improving the
delayed fracture resistance, there is the method of
causing fine precipitates to diffuse in the steel and
trapping hydrogen by the precipitates. However, even if
employing this method, it is difficult to effectively
15 suppress delayed fracture when the amount of hydrogen
which enters from the outside is large.
The present invention, in view of this current
situation, has as its object to provide a high strength
steel (wire rod or PC steel bar) and high strength bolt
20 which exhibit excellent delayed fracture resistance even
under a severe corrosive environment and methods of
production for producing these inexpensively.
Solution to Problem
25 The inventors engaged in intensive research on the
techniques for solving the above problem. As a result,
they learned that if (a) decarburizing and nitriding the
surface of the steel (al) to form a low carbon region to
suppress hardening and (a2) to form a nitrided layer to
30 obstruct absorption of hydrogen, the delayed fracture
resistance is remarkably improved.
The present invention was made based on the above
discovery and has as its gist the following:
(1) A steel which is excellent in delayed fracture
35 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
more of Cr: 0.05 to 1.5%, V: 0.05 to 0.2%, Mo: 0.05 to
- 4 -
0.4%, 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
tempered martensite structure,
5 the surface of the steel being formed with
(a) a nitrided layer having a thickness from the
surface of the steel of 200 pm or more and a nitrogen
concentration of 12.0 mass% or less and higher than the
nitrogen concentration of the steel by 0.02 mass% or more
10 and
(b) a low carbon region having a depth from the
surface of the steel 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 the
15 steel.
(2) A high strength steel which is excellent in
delayed fracture resistance as set forth in said (1)
characterized in that due to the presence of the nitrided
layer. and low carbon region, the absorbed hydrogen
20 content in the steel is 0.5 ppm or less and the critical
diffusible hydrogen content of the steel is 0.20 ppm
(2.00 ppm?) or more.
(3) A high strength steel which is excellent in
delayed fracture resistance as set forth in said (1) or
25 (2) characterized in that said steel 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%.
(4) A high strength steel which is excellent in
30 delayed fracture resistance as set forth in any of said
(1) to (3) characterized in that the nitrided layer has a
thickness of 1000 pm or less.
(5) A high strength steel which is excellent in
delayed fracture resistance as set forth in any of said
35 (1) to (4) characterized in that the tempered martensite
has an area ratio of 85% or more.
- 5 -
(6) A high strength steel which is excellent in
delayed fracture resistance as set forth in any of said
(1) to (5) characterized in that the steel has a
compressive residual stress at the surface of 200 MPa or
5 more.
(7) A high strength steel which is excellent in
delayed fracture resistance as set forth in any of said
(1) to (6) characterized in that the steel has a tensile
strength of 1300 MPa or more.
10 (8) A high strength bolt which is excellent in
delayed fracture resistance obtained by working a steel
containing, by mass%, C: 0.10 to 0.55%, Si: 0.01 to 3%,
and Mn: 0.1 to 2%, further containing one or more of Cr:
0.05 to 1.5%, V: 0.05 to 0.2%, Mo: 0.05 to 0.4%, Nb:
15 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
tempered martensite structure,
the surface of the bolt being formed with
20 (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
nitrogen concentration of the steel by 0.02 mass% or more
and
25 (b) 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
and 0.9 time or less the carbon concentration of the
steel.
30 (9) A high strength bolt which is excellent in
delayed fracture resistance as set forth in said (8)
characterized in that due to the presence of the nitrided
layer and low carbon region, the absorbed hydrogen
content in the bolt is 0.5 ppm or less and the critical
35 diffusible hydrogen content of the bolt is 0.20 (2.00?)
ppm or more.
(10) A high strength bolt which is excellent in
- 6 -
delayed fracture resistance as set forth in said (8) or
(9) characterized in that said steel 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:
5 0.0003 to 0.01%.
(11) A high strength bolt which is excellent in
delayed fracture resistance as set forth in any of said
(8) to (10), characterized in that the nitrided layer of
the bolt has a thickness of 1000 pm or less.
10 (12) A high strength bolt which is excellent in
delayed fracture resistance as set forth in any of said
(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
15 delayed fracture resistance as set forth in any of said
(8) to (12), characterized in that the bolt has a
compressive residual stress at the surface of 200 MPa or
more.
(14) A high strength bolt which is excellent in
20 delayed fracture resistance as set forth in any of said
(8) to (13), characterized in that the bolt has a tensile
strength of 1300 MPa or more.
(15) A method of production of a high strength steel
which is excellent in delayed fracture resistance as set
25 forth in any of said (1) to (7),
the method of production of a high strength steel
which is excellent in delayed fracture resistance
characterized by
(1) heating a steel having a composition as set
30 forth in said (1) or (3) to form a low carbon region
having a depth from the surface of the steel 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 the steel, then cooling as it is to make
35 the steel structure a mainly martensite structure, then
(2) nitriding the steel at 500°C or less to form on
- 7
the surface of the steel a nitrided layer having a
nitrogen concentration of 12.0 mass% or less and higher
than the nitrogen concentration of the steel by 0.02
mass% and having a thickness from the surface of the
5 steel of 200 pm or more and to make the steel structure a
mainly tempered martensite structure.
(16) A method of production of a high strength steel
which is excellent in delayed fracture resistance as set
forth in said (15) characterized in that the nitrided
10 layer has a thickness of 1000 pm or less.
(17) A method of production of a bolt which is
excellent in delayed fracture resistance as set forth in
any of said (8) to (14),
the method of production of a bolt which is
15 excellent in delayed fracture resistance characterized by
(1) heating a bolt obtained by working a steel
having a composition as set forth in said (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
20 having a carbon concentration of 0.05 mass% or more and
0.9 time or less the carbon concentration of the steel,
then cooling as it is to make the steel structure a
mainly martensite structure, then
(2) nitriding the bolt at 500°C or less to form on
25 the surface of the bolt a nitrided layer having a
nitrogen concentration of 12.0 mass% or less and higher
than the nitrogen concentration of the steel by 0.02
mass% and having a thickness from the surface of the bolt
of 200 pm or more and to make the steel structure a
30 mainly tempered martensite structure.
(18) A method of production of a bolt which is
excellent in delayed fracture resistance as set forth in
said (17), characterized in that the nitrided layer has a
thickness of 1000 pm or less.
35
Advantageous Effect of Invention
8
According to the present invention, it is possible
to provide a high strength steel (wire rod or PC steel
bar) and high strength bolt which exhibit excellent
delayed fracture resistance even in a severe corrosive
5 environment and methods for production able to produce
these inexpensively.
Brief Description of Drawings
FIG. 1(a) is a view which schematically shows a
10 hydrogen evolution curve which is obtained by hydrogen
analysis by the Thermal desorption analysis.
FIG. 1(b) is a view which schematically shows the
relationship between a fracture time obtained by a
constant load delayed fracture test of a steel and an
15 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
Dispersive x-ray Spectroscopy (EDX).
20 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
Dispersive x-ray Spectroscopy (EDX).
FIG. 4 is a view which shows a test piece which is
25 used for a delayed fracture test of a steel.
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
30 and time.
Description of Embodiments
It is known that hydrogen in steel causes delayed
fracture. Further, absorption of hydrogen into the steel
35 occurs along with corrosion in actual environments. The
absorption of diffusible hydrogen into the steel
concentrates at the concentrated parts of tensile stress
- 9 -
and results in occurrence of delayed fracture.
FIG. 1(a) schematically shows absorption of hydrogen
curve obtained by hydrogen analysis by the Thermal
desorption analysis. As shown in FIG. 1(a), the amount of
5 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 desorbed from room
temperature to 400°C is defined as the amount of
10 diffusible hydrogen. Note that, the amount of desorbed
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
15 content". The critical diffusible hydrogen content
differs according to the type of the steel.
FIG. 1(b) schematically shows the relationship
between the fracture time obtained by a constant load
delayed fracture test of the steel and the amount of
20 diffusible hydrogen. As shown in FIG. 1(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
25 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 is run and, as shown in FIG.
1(b), the maximum value of the amount of diffusible
30 hydrogen at which no fracture occurs for 100 hours or
more was made the critical diffusible hydrogen content.
If comparing the absorbed hydrogen content and the
critical diffusible hydrogen content and if the critical
diffusible hydrogen content is greater than the absorbed
35 hydrogen content, delayed fracture does not occur.
Conversely if the critical diffusible hydrogen content is
- 10 -
smaller than the absorbed hydrogen content, delayed
fracture occurs. Therefore, the larger the critical
diffusible hydrogen content, the more the occurrence of
delayed fracture is suppressed.
5 However, if the absorbed hydrogen content in the
steel from a corrosive environment exceeds the critical
diffusible hydrogen content, delayed fracture occurs.
Therefore, to prevent the occurrence of delayed
fracture, it is effective to suppress absorption of
10 hydrogen into the steel. For example, if forming a
nitrided layer at the surface of the steel by nitriding,
the absorbed hydrogen content due to corrosion is
suppressed, so the delayed fracture resistance is
improved.
15 However, if forming a nitrided layer at the steel
surface, due to hardening of the surface layer, the
critical diffusible hydrogen content decreases and the
delayed fracture resistance is not improved.
Therefore, the inventors studied lowering the
20 excessively high hardness of the nitrided layer to
improve the delayed fracture resistance. Specifically,
they decarburized and further nitrided the surfaces of
various steels, carried out accelerated corrosion tests
and exposure tests, and investigated the hydrogen
25 absorption characteristics and delayed fracture
resistance.
As a result, the inventors learned that if forming a
nitrided layer of a predetermined nitrogen concentration
and thickness on the surface of a steel which has a
30 predetermined composition and structure and, furthermore,
forming a low carbon region of a predetermined carbon
concentration and depth on the steel surface, the delayed
fracture resistance is remarkably improved compared with
the case of forming only a nitrided layer on the steel
35 surface.
This is believed to be due to the synergistic effect
of (1) suppression of the absorbed hydrogen content
- 11 -
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 surface and (2)
suppression of excessive hardening of the surface and
5 increase of the critical diffusible hydrogen content due
to the formation of the low carbon region at the steel
surface.
Basically, they learned that if forming, on the
surface of a steel of a predetermined composition and
10 structure, (a) a nitrided layer having a thickness from
the surface of the steel of 200 pm or more and a nitrogen
concentration of 12.0 mass% or less and higher than the
nitrogen concentration of the steel by 0.02 mass% or more
and (b) a low carbon region having a depth from the
15 surface of the steel 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 the
steel, it is possible to increase the critical diffusible
hydrogen content of the steel and reduce the absorbed
20 hydrogen content.
Further, the inventors discovered that by heating
and rapid cooling at the time of nitriding, compressive
residual stress occurs at the steel surface and the
delayed fracture resistance is improved. In particular,
25 in the case of a high strength bolt in which strain is
introduced into the surface by working, formation of a
nitrided layer is promoted. Further, the nitrogen
concentration becomes higher, so the delayed fracture
resistance is remarkably improved.
30 Below, the present invention will be explained in
detail.
The high strength steel and high strength bolt of
the present invention are composed of predetermined
compositions of ingredients and have a nitrided layer and
35 a low carbon region simultaneously present on the
surface. That is, at the surface of the high strength
steel and high strength bolt of the present invention,
- 12 -
there is a region with a nitrogen concentration of 12.0
mass% or less and higher than the nitrogen concentration
of the steel by 0.02 mass% or more and with a carbon
concentration of 0.05 mass% or more and 0.9 time or less
5 the steel (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
region is equal to the carbon concentration of the steel
10 and the nitrogen concentration is higher than the
nitrogen concentration of the steel. 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
15 mass% or more and 0.9 time or less of the carbon
concentration of the steel and with contents of other
elements equal to the steel is present under the nitrided
layer.
First, the low carbon region will be explained. In
20 the present invention, the low carbon region is a region
with a carbon concentration of 0.05 mass% or more and 0.9
time or less the carbon concentration of the high
strength steel or high strength bolt.
In the high strength steel and high strength bolt of
25 the present invention, a low carbon region is formed at a
depth of 100 pm or more to 1000 pm from the steel
surface. The depth and carbon concentration of the low
carbon region are adjusted by the heating atmosphere,
heating temperature, and holding time at the time of heat
30 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
35 region falls.
If the carbon concentration of the low carbon region
is less than 0.05 mass%, this becomes less than half of
- 13 -
the lower limit 0.10 mass% of the carbon concentration of
the steel, so it is not possible to secure a
predetermined strength and hardness by the low carbon
region. If the carbon concentration of the low carbon
5 region is over 0.9 time the carbon concentration of the
steel, this is substantially equal to the carbon
concentration of the steel and the effect of presence of
the low carbon region ends up becoming weaker.
For this reason, in the present invention, the low
10 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.
If the carbon concentration of the low carbon region
is 0.05 mass% or more and 0.9 time or less of the carbon
15 concentration of the steel, 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 becomes equal to the
hardness of the steel or lower than the hardness of the
20 steel 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 μm or more from the
surface of the steel or bolt so that the effect is
25 obtained. The depth (thickness) of the low carbon region
is preferably greater in depth (thickness), but if over
1000 μm, the strength of the steel as a whole or the bolt
as a whole falls, so the depth (thickness) of the low
carbon region is given an upper limit of 1000 μm.
30 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 or bolt by
0.02 mass% or more. Further, the nitrided layer is formed
35 by a thickness of 200 pm or more from the surface of the
steel or bolt.
- 14 -
The thickness and nitrogen concentration of the
nitrided layer can be adjusted by the heating atmosphere,
heating temperature, and holding time at the time of
nitriding. For example, if the concentration of ammonia
5 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
concentration of the nitrided layer becomes higher.
If the nitrogen concentration of the nitrided layer
10 is higher than the nitrogen concentration of the steel,
it is possible to reduce the absorbed hydrogen content in
the steel from a corrosive environment, but if the
difference of the nitrogen concentration of the nitrided
layer and the nitrogen concentration of the steel is less
15 than 0.02 mass%, the effect of reduction of the absorbed
hydrogen content 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 by 0.02 mass% or more.
20 On the other hand, if the nitrogen concentration
exceeds 12.0 mass%, the nitrided layer excessively rises
in hardness and becomes brittle, so 12.0 mass% was made
the upper limit.
If the steel surface is formed with a nitrided layer
25 which has a nitrogen concentration of 12.0 mass% or less
and higher than the nitrogen concentration of the steel
by 0.02 mass% or more and a depth of 200 pm or more from
the surface, the absorbed hydrogen content in the steel
from the corrosive environment is greatly reduced.
30 The nitrided layer was limited to a thickness
(depth) of 200 pm or more from the surface of the steel
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
35 pm, the productivity falls and a rise in cost is invited,
so 1000 pm or less is preferable.
- 15 -
The depth (thickness) of the low carbon region which
is formed on the high strength steel or high strength
bolt of the present invention can be found from the curve
of the carbon concentration from the surface of the steel
5 or bolt.
A cross-section of a steel or bolt which has a low
carbon region and nitrided layer on the surface is
polished and an Energy Dispersive x-ray Spectroscopy
(below, sometimes referred to as "EDX") or a Wavelength
10 Dispersive X-ray Spectroscopy (below, 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
15 (thickness) of the low carbon region from the curve of
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 surface, obtained by
measuring the carbon concentration in the depth direction
20 from the surface using EDX, and the carbon concentration.
As shown in FIG. 2, the carbon concentration
increases along with the increased distance (depth) from
the steel surface. This is because due to
decarburization, a low carbon region is formed on the
25 surface of the steel. 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
30 decarburization and is equal to the amount of carbon of
the steel before decarburization.
Therefore, in the present invention, the chemical
analysis value of the carbon concentration of the steel
is made the reference value when finding the depth of the
35 low carbon region.
As shown in FIG. 2, it is possible to discriminate
the range where the carbon concentration from the steel
- 16 -
surface to the required depth becomes lower than 10% or
more of the average carbon concentration "a" (a x 0.1)
(range of 0.9 time or less of the carbon concentration of
the steel) and find the distance (depth) from the steel
5 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
10 the surface of the steel or bolt in the same way as the
low carbon region. Specifically, a cross-section of the
steel 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 nitrogen
15 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 Dispersive xray
Spectroscopy (EDX). That is, FIG. 3 is a view showing
20 the relationship between the distance from the steel
surface and the nitrogen concentration which is obtained
by measuring the nitrogen concentration in the depth
direction from the surface using EDX.
As the distance (depth) from the steel surface
25 becomes longer, the nitrogen concentration decreases, but
in the region not affected by nitriding, the carbon
concentration is substantially constant (average nitrogen
concentration).
The average nitrogen concentration is a range of
30 nitrogen concentration not affected by nitriding and is
equal to the amount of nitrogen of the steel before
nitriding. Therefore, in the present invention, the
chemical analysis value of the nitrogen concentration of
the steel is made the reference value when finding the
35 thickness of the nitrided layer.
As shown in FIG. 3, it is possible to discriminate
the region in which the nitrogen concentration from the
- 17 -
steel surface down to the required depth becomes higher
than the average nitrogen concentration by 0.02 mass% or
more and finding the distance (depth) from the steel
surface at the boundary of that region in the depth
5 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
averages of the values which were measured at any five
10 locations at the cross-section of the steel or bolt.
Note that, the carbon concentration and nitrogen
concentration of the steel may be found by measuring the
carbon concentration and nitrogen concentration at a
position sufficiently deeper than the depth of the low
15 carbon region and nitrided layer, for example, a position
at a depth of 2000 pm or more from the surface. Further,
it is also possible to obtain an analytical sample from a
position at a depth of 2000 pm or more from the surface
of the steel or bolt and chemically analyze it to find
20 them.
In the high strength steel of the present invention,
as explained above, the delayed fracture is remarkably
improved by the synergistic effect of (1) suppression of
the absorbed hydrogen content due to the formation of a
25 nitrided layer at the low carbon region which is formed
at the steel surface and (2) increase of the critical
diffusible hydrogen content due to the formation of the
low carbon region at the steel surface.
According to investigations by the inventors, the
30 surface of the steel has a nitrided layer and a low
carbon region copresent on it, whereby the absorbed
hydrogen content in the steel can be suppressed to 0.10
ppm or less and the critical diffusible hydrogen content
of the steel can be raised to 0.20 ppm or more.
35 Next, the reasons for limitation of the composition
of the steel will be explained. Below, the % according to
the composition mean mass%.
- 18 -
C: C is an essential element in securing the
strength of a steel. 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
5 resistance also falls, so the content of C was made 0.10
to 0.55%.
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
10 becomes saturated, so the content of Si was made 0.01 to
3°o.
Mn: Mn is an element which not only performs
deoxidation and desulfurization, but also gives a
martensite structure, so lowers the transformation
15 temperature of the pearlite structure or bainite
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
20 degrades the delayed fracture resistance, so the content
of Mn was made 0.1 to 2%.
The high strength steel or high strength bolt of the
present invention may further contain one or more of Cr,
V, Mb, Nb, Cu, Ni, and B in a range not impairing the
25 excellent delayed fracture resistance for the purpose of
improving the strength.
Cr: Cr is an element which lowers the transformation
temperature of the pearlite structure or bainite
structure to raise the hardenability and, further, raises
30 the resistance to softening during tempering to
contribute to the improvement of the strength. If less
than 0.05%, the effect of addition is not sufficiently
obtained, while if over 1.5%, deterioration of the
toughness is invited, so the content of Cr was made 0.05
35 to 1.5%.
V: Like Cr, this is an element which lowers the
transformation temperature of the pearlite structure or
- 19 -
bainite structure to raise the hardenability and,
further, raises the resistance to softening during
tempering to contribute to the improvement of the
strength. If less than 0.05%, the effect of addition is
5 not sufficiently obtained, while if over 0.2%, the effect
of addition is saturated, so the content of V was made
0.05 to 0.2%.
Mo: Mo, like Cr and V, is an element which lowers
the transformation temperature of the pearlite structure
10 or bainite structure to raise the hardenability and,
further, raises the resistance to softening during
tempering to contribute to the improvement of the
strength. If less than 0.05%, the effect of addition is
not sufficiently obtained, while if over 0.4%, the effect
15 of addition is saturated, so the content of V was made
0.05 to 0.4%.
Nb: Nb, like Cr, V, and Mo, is an element which
raises the hardenability and the tempering softening
resistance to contribute to the improvement of the
20 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
- 20 -
0.01 to 4%.
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 and high strength bolt of
the present invention may further contain, for the
purpose of refining the structure, one or more of Al, Ti,
15 Mg, Ca, and Zr in a range not detracting from the
excellent delayed fracture resistance.
Al: Al 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 Al 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-Al, Mg-Ti, and Mg-Al-Ti composite oxides or
composite sulfides, etc. to prevent coarsening of
austenite grains and suppress deterioration of delayed
- 21 -
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 and
high strength bolt of the present invention (below,
25 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 more.
30 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
balance is composed of one or more of residual austenite,
35 bainite, pearlite, and ferrite.
The area ratio of the tempered martensite is
measured at a deeper position between the depth at which
- 22 -
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
5 FIG. 3.
For example, it is sufficient to measure the depth
of 2000 pm or more from the surface of the steel or bolt
or the area ratio of the tempered martensite at locations
of 1/4 of the thickness or diameter of the steel.
10 Note that, the area ratio of martensite can be found
by observing the cross-section of the steel using an
optical microscope and measuring the area of martensite
per unit area. Specifically, the cross-section of the
steel is etched by a Nital etching solution, the areas of
15 martensite in five fields in a range of 0.04 mm 2 are
measured, and the average value is calculated.
Further, in the steel of the present invention,
compressive residual stress of the steel surface occurs
due to the heating and rapid cooling at the time of
20 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 of the present invention is
25 preferably 200 MPa or more.
The compressive residual stress can be measured by
X-ray diffraction. Specifically, the residual stress of
the steel surface is measured, then the steel surface is
etched 25 pm at a time by electrolytic polishing and the
30 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 in which no low carbon region and
nitrided layer are formed on the surface, if the tensile
35 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,
- 23 -
the delayed fracture resistance of the steel of the
present invention on which a low carbon region and
nitrided layer are formed on the surface is remarkably
excellent.
5 The upper limit of the tensile strength of the
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
10 JIS Z 2241.
Method of Production
Next, a method of production of a steel of the
present invention will be explained.
15 The method of production of a steel of the present
invention is composed of a decarburization step of
heating a steel of a required composition (wire rod or PC
steel bar or steel worked to a predetermined shape) to
decarburize it, a hardening step of cooling the
20 decarburized steel to make the steel structure a mainly
martensite structure, and a step of nitriding the
hardened steel at over 500°C to 650°C or less.
Note that, due to the nitriding step, the structure
of the steel of the present invention becomes a structure
25 of mainly tempered martensite.
In the decarburization step, the steel of the
present invention is decarburized to make the carbon
concentration, down from the surface of the steel by a
depth of 100 μm or more to 1000 μm or less, 0.05% or more
30 and 0.9 time or less the carbon concentration of the
steel. 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.
35 The heating temperature in the decarburization is
preferably Ac3 to 950°C. By heating to Ac3 or more, it is
- 24 -
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
5 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
10 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
15 At the hardening step, the heated steel is cooled to
obtain a mainly martensite structure. The heated steel
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%
20 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 5°C/s or
25 more. if the cooling rate is less than 5°C/s, sometimes
the area ratio of the martensite becomes less than 85%.
At the nitriding step, a steel with a steel
structure of mainly martensite and formed with a low
carbon region at the surface layer is nitrided. Due to
30 the nitriding, a nitrided layer is formed with a
thickness from the steel surface of 200 pm or more and a
nitrogen concentration of 12.0% or less and higher than
the nitrogen concentration of the steel by 0.02% or more.
At the same time, the steel is tempered to make the steel
35 structure a mainly tempered martensite structure.
The nitriding is performed by, for example, heating
- 25 -
the steel in an atmosphere containing ammonia or
nitrogen. The nitriding is preferably performed by
holding the sample at 500°C or less, for example 400 to
500°C, for 1 to 12 hours. If the nitriding temperature
5 exceeds 500°C, the steel falls in strength, so the
nitriding temperature is made 500°C or less.
The lower limit of the nitriding temperature is not
particularly limited, but if the nitriding temperature is
less than 400°C, time is taken for diffusion of nitrogen
10 from the steel surface and the manufacturing cost rises.
If the nitriding time is less than 1 hours, the
depth of the nitrided layer is liable not to reach a
depth of 200 μm or more from the surface, so the
nitriding time is preferably 1 hour or more. The upper
15 limit of the nitriding time is not defined, but if over
12 hours, the manufacturing cost rises, so the nitriding
time is preferably 12 hours or less.
Note that, in the nitriding step, the gas nitriding
method, nitrocarburizing method, plasma nitriding method,
20 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
to as "the present invention bolts") will be explained.
25 The method of production of the bolt of the present
invention is composed of a working step of working the
steel of the present invention having the required
composition into a bolt, a decarburization step of
heating the bolt to decarburize it, a hardening step of
30 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. In the nitriding step, the steel
structure of the bolt becomes a mainly tempered
35 martensite structure.
Note that, in the working step, for example, the
- 26 -
steel wire rod is cold forged and rolled to form a bolt.
The method of production of the bolt of the present
invention differs from the method of production of the
steel of the present invention only in the working step
5 for working the steel into a bolt shape, so the
explanation of the other steps will be omitted.
The method of production of the steel of the present
invention and the method of production of the bolt of the
present invention preferably performs rapid cooling,
10 after nitriding, in a range from 500 to 200°C by 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 or bolt 200 MPa or more. Due to
the presence of this compressive residual stress, the
15 delayed fracture resistance is improved more.
Examples
Next, examples of the present invention will be
explained, but the conditions of the examples are an
20 example of the conditions adopted for confirming the
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
25 the present invention and achieving the object of the
present invention.
(Examples)
Molten steels of the compositions of ingredients
which are shown in Table 1 were cast in accordance with
30 an ordinary method. The cast slabs were hot worked to
obtain steels (wire rods). The steels 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 decarburizing. The hardening
35 was performed by oil quenching so that the cooling rate
in the range of 700 to 300°C became 5°C/s or more.
- 27 -
Further, the depth of the low carbon region was
investigated by the carbon potential of the atmosphere of
the heating furnace, heating temperature, and holding
time.
Table 1
Composition (masse)
Steel
type
C Si Mn Cr V Mo Nb Cu Ni B Al Ti Mg Ca Zr
Al 0.11 2.20 1.52 0.53 0.17 0.38 0-049 3.85 2.25 0.0045 0.032 - - 0.0003 -
A2 0.16 2.50 1.98 0.4 - 0.36 0.039 2.01 1.01 0.0031 0.089 0.005 0.0003 - 0.0032
A3 0.21 2.92 0.55 0.77 0.15 0.35 0.035 - 2.95 0.0025 - 0.012 0.0012 0.0056 0.0003
A4 0.26 0.98 0.98 1.48 0.18 0.39 - 1.52 0.78 0.0019 0.098 0.009 0.0022 0.0031 0.0044
AS 0.31 1.98 0.78 1.28 0-09 0.29 0.025 3.98 2.01 0.0002 - 0.031 0.0033 - -
A6 0.34 1.23 0.15 1.17 0.05 0.35 0.015 0.74 3.98 0.0004 0.021 0.049 0.0041 0.0021 0.0045
A7 0.34 0.95 0.34 0.07 0.18 0.35 0.006 0.49 0.28 0.0049 0.067 0.044 - - -
AS 0.41 0.61 0.45 0.64 - 0.05 0.002 - 0.01 0.0044 0.011 0.039 0.0099 0.0067 0.0036
A9 0.41 0.35 0.72 0.29 0.2 - 0.013 0.01 - 0.0028 0.052 0.003 0.0088 0.0099 0.0079
A10 0.45 0.20 0.12 - 0.14 0.09 0.021 0.31 0.17 0.0027 0.003 0.025 0.0052 0.0023 0.0035
All 0.51 0.02 0.22 0.24 0.08 0.12 0.007 0.11 0.06 - 0.033 0.021 0.0038 0.0011 0.0048
A12 0.55 0.11 0.34 0.19 0.06 0.21 0.002 0.05 0.03 0.0034 0.026 0.033 0.0055 0.0016 0.0088
A13 0.39 0.25 0.79 1.12 - - - - - - - - -
A14 0.39 0.25 0.76 1.06 - 0.25 - - - - - -
A15 0.39 0.25 0.76 1.06 - 0.25 - - - - 0.025 - - - -
31 0.09 0.009 0.08 0.53 0.17 0.38 0.044 3.85 - 0.0045 0.032 - - 0.0003 -
B2 0.60 0.12 0.22 0.24 0.08 0.12 0.007 0.11 0.06 - 0.033 0.021 0.0038 0.0011 0.0048
23 0.34 0.95 2.1 0.12 0.18 0.35 0.006 0.49 0.28 0.0045 0.067 0.044 0.0055 0.0033 0.0068
B4 0.21 2.54 0.55 1.6 0.15 0.35 - - 2.95 0.0025 - 0.012 0.0012 0.0056 0.0011
B5 0.31 1.98 0.78 1.28 0.09 0.29 0.025 4.1 2.01 0.0033 0.029 0.031 0.0033 - 0.0012
B6 0.41 0.51 0.45 0.64 - 0.12 0.002 0.03 0.0061 0.011 0.039 0.0076 0.0067 0.0036
(Note) - in table means not deliberately included
- 29 -
After that, the steel was nitrided by
nitrocarburizing to form a nitrided layer. After
nitriding, it was rapidly cooled in the range of 500 to
200°C by the cooling rate which is shown in Table 2
5 (cooling rate after tempering) to obtain the high
strength steels of Manufacturing Nos. 1 to 27.
Note that, the nitriding was performed at a
temperature which is shown in Table 2 while making the
ammonia volume ratio in the treatment gas atmosphere 30
10 to 50% and making the treatment time 1 to 12 hours.
The nitrided layer was adjusted in thickness by
changing the heating temperature and the holding time.
The nitrided layer was adjusted in nitrogen concentration
by changing the ammonia volume ratio in the treatment gas
15 atmosphere.
Table 2
Man. Steel Tempered Strength Low Nitriding Nitrided Compres- Pene- Critical Delayed Remarks
no. type martensite
ratio (%)
(MPa) carbon.
region
depth
(μm)
Temp.
('C)
Cooling
rate
(°C/s)
layer
thickness
(μm)
sive
residual
stress
(MPa)
trated
hydrogen
(ppm)
diffusible
hydrogen
content of
delayed
fracture
(ppm)
fracture
presence
1 Al 90 1312 120 400 32 236 306 0.05 0.35 No Inv. ex.
2 A2 87 1304 980 400 12 323 202 0.06 0.31 No
3 A3 86 1303 560 400 15 336 205 0.05 0.30 No
4 A4 94 1356 450 400 72 232 405 0.06 0.20 No
S AS 89 1334 268 400 38 245 332 0.07 0.24 No
6 AS 90 1320 976 400 36 246 331 0.07 0.34 No
7 A6 92 1427 966 400 82 201 432 0.07 0.31 No
8 A7 98 1463 643 450 25 212 258 0.06 0.22 No
9 AS 99 1579 109 450 46 256 384 0.07 0.25 No
10 A9 94 1532 153 450 19 321 274 0.07 0.26 No
11 AlO 95 1502 104 450 36 278 345 0.06 0.37 No
12 All 98 1503 889 450 28 261 274 0.07 0.33 No
13 A12 99 1587 346 460 86 244 456 0.08 0.34 No
14 A13 85 1526 256 490 98 223 501 0.08 0.29 No
15 A14 89 1478 348 460 76 243 435 0.07 0.28 No
16 A15 91 1508 412 460 65 234 421 0.08 0.37 No
17 A15 94 1535 256 460 75 233 411 0.06 0.34 No
18 A4 94 1351 210 450 8 205 193 0.06 0.15 No
19 31 92 1168 103 400 11 336 201 0.05 0.33 No Comp.
20 B2 93 1535 135 450 98 450 501 0.08 0.07 Yes ex.
21 B3 91 1546 146 450 35 289 333 0.09 0.08 Yes
22 34 98 1577 143 450 32 245 345 0.08 0.08 Yes
23 25 92 1563 169 450 82 338 432 0.08 0.08 Yes
24 26 92 1564 213 450 29 269 294 0.07 0.07 Yes
25 A3 99 1345 96 400 12 234 203 0.09 0.07 Yes
26 Ar 94 1356 211 450 15 195 203 0.19 0.11 Yes
27 A4 99 1354 209 440 18 204 0.18 0.11 Yes
(Note) Underlines in table are conditions outside scope of present invention
(Note) "-" in table means nitrided layer with nitrogen concentration 0.02% or more higher than base material is
not formed.
- 31 -
The steels which are shown in Table 1 (wire rods)
were worked into bolts by the same process as the high
strength steels (wire rods) of Manufacturing Nos. 1 to 27
to obtain the high strength bolts of Manufacturing Nos.
5 28 to 44. 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 tempering).
Table 3
Man. Steel Tempered Strength Low Nitriding Nitrided Compres- Pene- Critical Delayed
no. type martensite
ratio (°s)
(MPa) carbon
region
depth
(pm)
Temp.
(°C)
Cooling
rate
(°C/s)
layer
thickness
(μm)
sive
residual
stress
(MPa)
trated
hydrogen
(ppm)
diffusible
hydrogen
content of
delayed
fracture
(ppm)
fracture
presence
28 Al 90 1311 121 400 33 236 315 0.01 0.35 No
29 A2 87 1303 981 400 11 323 203 0.02 0.31 No
30 A3 86 1302 559 400 12 336 204 0.01 0.30 No
31 A4 94 1355 448 400 75 232 412 0.02 0.20 No
32 AS 89 1333 467 400 38 245 333 0.03 0.24 No
33 AS 90 1319 978 400 37 246 328 0.03 0.34 No
34 A6 92 1426 970 400 76 201 433 0.02 0.31 No
35 A7 98 1462 645 450 25 212 256 0.02 0.22 No
36 AS 99 1578 110 450 39 256 378 0.03 0.25 No
37 A9 94 1531 148 450 19 321 277 0.03 0.26 No
38 A10 95 1501 103 450 36 278 342 0.02 0.37 No
39 All 98 1502 888 450 19 261 278 0.02 0.33 No
40 A12 99 1586 346 460 99 244 755 0.03 0.34 No
41 A13 85 1525 257 490 85 223 499 0.03 0.29 No
42 A14 89 1477 346 460 85 243 441 0.02 0.28 No
43 A15 91 1507 411 460 82 234 432 0.03 0.37 No
44 A15 94 1534 258 460 76 233 412 0.01 0.34 No
- 33 -
The tempered martensite ratios, tensile strengths,
low carbon region depths, nitrided layer thicknesses,
compressive residual stresses, absorbed hydrogen content,
critical diffusible hydrogen contents, and delayed
5 fracture resistances of the high strength steels of
Manufacturing Nos. 1 to 27 (Table 2) and the high
strength bolts of Manufacturing Nos. 28 to 44 (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 steels of
Manufacturing Nos. 1 to 27 and the high strength bolts of
Manufacturing Nos. 28 to 44, etching by a Nital etching
15 solution, using an optical microscope to measure the
areas of the martensite in five fields in a 0.04 mm 2
range, and finding the average value.
Note that in each of the high strength steels of
Manufacturing Nos. 1 to 27 and the high strength bolts of
20 Manufacturing Nos. 28 to 44, 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 steels
of Manufacturing Nos. 1 to 27 and the high strength bolts
30 of Manufacturing Nos. 28 to 44 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.
The depth (thickness) of the region where the carbon
35 concentration is 0.9 time or less of the carbon
concentration of the steel ((carbon concentration of low
carbon region/carbon concentration of steel) _<0.9) was
- 34 -
defined as the "low carbon region depth", while the depth
(thickness) of the region where the nitrogen
concentration is higher than the nitrogen concentration
of the steel by 0.02% or more (nitrogen concentration of
5 nitrided layer-nitrogen concentration of steel >_0.02) was
defined as the "nitrided layer thickness".
Note that, the low carbon region depth and the
nitrided layer thickness were the averages of values
measured at any five locations in the longitudinal
10 direction.
Compressive Residual Stress
An X-ray residual stress measurement apparatus was
used to measure the compressive residual stress of the
surface. The residual stress of the surface of each of
15 the high strength steels of Manufacturing Nos. 1 to 27
and the high strength bolts of Manufacturing Nos. 28 to
44 was measured, then the surface was etched by 25 μm at
a time by electrolytic polishing and the residual stress
in the depth direction was measured. Note that, the
20 compressive residual stress was made the average of the
values measured at any three locations.
Critical Diffusible Hydrogen Content and Delayed
Fracture Resistance
From each of the high strength steels of
25 Manufacturing Nos. 1 to 27 and the high strength bolts of
Manufacturing Nos. 28 to 44, a delayed fracture test
piece of the shape which is shown in FIG. 4 was prepared
and subjected to absorption of hydrogen. For absorption
of hydrogen, the electrolytic hydrogen charge method was
30 used to change the charge current and change the absorbed
hydrogen content as shown by Table 2 and Table 3. 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
35 piece was left at room temperature for 3 hours to even
the concentration of hydrogen at the inside.
After that, a delayed fracture test machine which is
- 35 -
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
5 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. 1(b), the maximum value of
10 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
15 raising the delayed fracture test piece in temperature at
100°C/h and measuring the cumulative value of the amounts
of hydrogen which were desorbed between room temperature
to 400°C by a gas chromatograph.
When comparing the absorbed hydrogen content and the
20 critical diffusible hydrogen content of delayed fracture
and the critical diffusible hydrogen content is greater
than the absorbed hydrogen content, delayed fracture does
not occur. Conversely, if the critical diffusible
hydrogen content is smaller than the absorbed hydrogen
25 content, delayed fracture occurs.
Therefore, the delayed fracture resistance was
evaluated as "without delayed fracture" when the absorbed
hydrogen content which is shown in Table 2 and Table 3
was less than critical diffusible hydrogen content and as
30 "with delayed fracture" when the absorbed hydrogen
content was the critical diffusible hydrogen content or
more.
Absorbed hydrogen content
The absorbed hydrogen content was determined by
35 preparing a test piece of each of the high strength
steels of Manufacturing Nos. 1 to 27 and the high
strength bolts of Manufacturing Nos. 28 to 44 and running
- 36 -
an accelerated corrosion test of the pattern of
temperature, humidity, and time which is shown in FIG. 6
for 30 cycles. The corroded layer at the surface of the
test piece was removed by sandblasting, then the hydrogen
5 was analyzed by the Thermal desorption analysis. The
amount of hydrogen which was desorbed from room
temperature to 400°C was measured to find the absorbed
hydrogen content.
As shown in Table 2, the high strength steels of
10 Manufacturing Nos. 1 to 18 of the invention examples had
a low carbon region depth of 100 μm or more and a
nitrided layer thickness of 200 pm or more. Further, the
high strength steels of Manufacturing Nos. 1 to 18 all
had a tempered martensite rate of 50% or more and a
15 structure of mainly tempered martensite.
Further, regarding the compressive residual stress,
the high strength steels of Manufacturing Nos. 1 to 17
all had a compressive residual stress of 200 MPa or more,
but Manufacturing No. 18 has a stress of less than 200
20 MPa.
The high strength steels of Manufacturing Nos. 1 to
17 of the invention examples all had a tensile strength
of 1300 MPa or more, an absorbed hydrogen content of 0.1
ppm or less, a critical diffusible hydrogen content of
25 0.20 ppm or more, an absorbed hydrogen content of less
than the critical diffusible hydrogen content, and a
resistance of "without delayed fracture".
The high strength steel of Manufacturing No. 18 is
an invention example, but the cooling rate after
30 tempering was slow, so the compressive residual stress
was lower than the high strength steels of Manufacturing
Nos. 1 to 17 and the critical diffusible hydrogen fell
content, but the tensile strength was 1300 MPa or more,
the absorbed hydrogen content was 0.1 ppm or less, the
35 critical diffusible hydrogen content, and the resistance
was "without delayed fracture".
- 37 -
As opposed to this, as shown in Table 2, the high
strength steel of Manufacturing No. 19 of the comparative
example was an example where the amount of C, the amount
of Si, and the amount of Mn were small and the strength
5 was low. Manufacturing No. 20 is an example where the
amount of C was large, Manufacturing No. 21 is an example
where the amount of Mn was large, Manufacturing No. 22 is
an example where the amount of Cr was large,
Manufacturing No. 23 is an example where the amount of Cu
10 was large, and Manufacturing No. 24 is an example where
the amount of B was large, so the critical diffusible
hydrogen content was low and the resistance was "with
delayed fracture".
Further, Manufacturing No. 25 is an example where
15 the heating time of the hardening was short, the low
carbon region depth was less than 100 pm, the critical
diffusible hydrogen content was low, and the resistance
was "with delayed fracture". Manufacturing No. 26 is an
example where the nitriding time was short, the nitrided
20 layer thickness was less than 200 pm, the absorbed
hydrogen content was large, and the resistance was "with
delayed fracture".
Manufacturing No. 27 is an example in which the
concentration of ammonia in the gas of the nitriding was
25 lowered, so at a location down to the depth of 200 pm
from the surface, the difference of the nitrogen
concentration from the steel became 0.01 mass%, the
absorbed hydrogen content was larger, and the resistance
was "with delayed fracture".
30 As shown in Table 3, the high strength bolts of
Manufacturing Nos. 28 to 44 of the invention examples had
a low carbon region depth of 100 pm or more and a
nitrided layer thickness of 200 pm or more. All had a
tensile strength of 1300 MPa or more, an absorbed
35 hydrogen content of 0.1 ppm or less, a critical
diffusible hydrogen content of 0.20 ppm or more, an
- 38 -
absorbed hydrogen content of less than the critical
diffusible hydrogen content, and a resistance "without
delayed fracture".
The high strength bolts of Manufacturing Nos. 28 to
5 44 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.
From Table 2 and Table 3, it will be understood that
the high strength bolts of Manufacturing Nos. 28 to 44,
10 which differ from the high strength steels of
Manufacturing Nos. 1 to 17 only in the point of working
the steel (wire rod) to bolts (high strength bolts of
Manufacturing Nos. 28 to 44 correspond to high strength
steels of Manufacturing Nos. 1 to 17), were further
15 suppressed in the absorbed hydrogen content compared with
the high strength steels.
Industrial Applicability
As explained above, according to the present
20 invention, it is possible to provide a high strength
steel (wire rod or PC steel bar) and high strength bolt
which exhibit excellent delayed fracture resistance even
in a severe corrosive environment and a method of
production enabling inexpensive production of these.
25 Accordingly, the present invention is extremely high in
applicability in industries manufacturing and using
steels.
Reference Signs List
30 1 test piece
2 balance weight
3 fulcrum
- 39 -

CLAIMS
Claim 1
A high strength steel which is excellent in delayed
fracture resistance containing,
5 by mass%,
C: 0.10 to 0.55%,
Si: 0.01 to 3%, and
Mn: 0.1 to 2%,
further containing one or more of
10 Cr: 0.05 to 1.5%,
V: 0.05 to 0.2%,
Mo: 0.05 to 0.4%,
Nb: 0.001 to 0.05%,
Cu: 0.01 to 4%,
15 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 tempered martensite structure,
the surface of the steel being formed with
20 (a) a nitrided layer having a thickness from the
surface of the steel of 200 pm or more and a nitrogen
concentration of 12.0 mass% or less and higher than the
nitrogen concentration of the steel by 0.02 mass% or more
and
25 (b) a low carbon region having a depth from the
surface of the steel 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 the
steel.
30
Claim 2
A high strength steel which is excellent in delayed
fracture resistance as set forth in claim 1 characterized
in that due to the presence of the nitrided layer and low
35 carbon region, the absorbed hydrogen content in the steel
is 0.10 ppm or less and the critical diffusible hydrogen
content of the steel is 0.20 ppm or more.
- 40 -
Claim 3
A high strength steel which is excellent in delayed
fracture resistance as set forth in claim 1 or 2
5 characterized in that said steel 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%,
10 Ca: 0.0003 to 0.01%, and
Zr: 0.0003 to 0.01%.
Claim 4
A high strength steel which is excellent in delayed
15 fracture resistance as set forth in any one of claims 1
to 3 characterized in that the nitrided layer has a
thickness of 1000 pm or less.
Claim 5
20 A high strength steel which is excellent in delayed
fracture resistance as set forth in any one of claims 1
to 4 characterized in that the tempered martensite has an
area ratio of 85% or more.
25 Claim 6
A high strength steel which is excellent in delayed
fracture resistance as set forth in any one of claims 1
to 5 characterized in that the steel has a compressive
residual stress at the surface of 200 MPa or more.
30
Claim 7
A high strength steel which is excellent in delayed
fracture resistance as set forth in any one of claims 1
to 6 characterized in that the steel has a tensile
35 strength of 1300 MPa or more.
Claim 8
- 41 -
A high strength bolt which is excellent in delayed
fracture resistance obtained by working a steel
containing,
by mass%,
5 C: 0.10 to 0.55%,
Si: 0.01 to 3%, and
Mn: 0.1 to 2%,
further containing one or more of
Cr: 0.05 to 1.5%,
10 V: 0.05 to 0.2%,
Mo: 0.05 to 0.4%,
Nb: 0.001 to 0.05%,'
Cu: 0.01 to 4%,
Ni: 0.01 to 4%, and
15 B: 0.0001 to 0.005%, and
having a balance of Fe and unavoidable impurities, the
structure being a mainly tempered martensite structure,
the surface of the bolt being formed with
(a) a nitrided layer having a thickness from the
20 surface of the bolt of 200 pm or more and a nitrogen
concentration of 12.0 mass% or less and higher than the
nitrogen concentration of the steel by 0.02 mass% or more
and
(b) a low carbon region having a depth from the
25 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
and 0.9 time or less the carbon concentration of the
steel.
30 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 the nitrided layer and low
carbon region, the absorbed hydrogen content in the bolt
35 is 0.10 ppm or less and the critical diffusible hydrogen
content of the bolt is 0.20 ppm or more.
- 42 -
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 further contains, by
5 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
10 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
15 to 10, characterized in that the nitrided layer has a
thickness of 1000 pm or less.
Claim 12
A high strength bolt which is excellent in delayed
20 fracture resistance as set forth in any one of claims 8
to 11, characterized in that the tempered martensite has
an area ratio of 85% or more.
Claim 13
25 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 the bolt has a compressive
residual stress at the surface of 200 MPa or more.
30 Claim 14
A high strength bolt which is excellent in delayed
fracture resistance as set forth in any one of claims 8
to 13, characterized in that the bolt has a tensile
strength of 1300 MPa or more.
35
Claim 15
A method of production of a high strength steel
- 43 -
which is excellent in delayed fracture resistance as set
forth in any one of claims 1 to 7,
the method of production of a high strength steel
which is excellent in delayed fracture resistance
5 characterized by
(1) heating a steel having a composition as set
forth in claim 1 or 3 to form a low carbon region having
a depth from the surface of the steel of 100 pm or more
to 1000 pm or less and having a carbon concentration of
10 0.05 mass% or more and 0.9 time or less the carbon
concentration of the steel, then cooling as it is to make
the steel structure a mainly martensite structure, then
(2) nitriding the steel at 500°C or less to form on
the surface of the steel a nitrided layer having a
15 nitrogen concentration of 12.0 mass% or less and higher
than the nitrogen concentration of the steel by 0.02
mass% and having a thickness from the surface of the
steel of 200 pm or more and to make the steel structure a
mainly tempered martensite structure.
20
Claim 16
A method of production of a high strength steel
which is excellent in delayed fracture resistance as set
forth in claim 15 characterized in that the nitrided
25 layer has a thickness of 1000 pm or less.
Claim 17
A method of production of a high strength bolt which
is excellent in delayed fracture resistance as set forth
30 in any one of claims 8 to 14,
the method of production of a bolt which is
excellent in delayed fracture resistance characterized by
(1) heating a bolt obtained by working a steel
having a composition as set forth in claim 8 or 10 to
35 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
- 44 -
having a carbon concentration of 0.05 mass% or more and
0.9 time or less the carbon concentration of the steel,
then cooling as it is to make the steel structure a
mainly martensite structure, then
5 (2) nitriding the bolt at 500°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 higher
than the nitrogen concentration of the steel by 0.02
mass% and having a thickness from the surface of the bolt
10 of 200 pm or more and to make the steel structure a
mainly tempered martensite structure.
Claim 18
A method of production of a high strength bolt which
15 is excellent in delayed fracture resistance as set forth
in claim 17, characterized in that the nitrided layer has
a thickness of 1000 pm or less.