U.S. patent application number 14/410264 was filed with the patent office on 2015-10-01 for high-carbon steel tube having superior cold workability, machinability, and hardenability and method for manufacturing the same.
This patent application is currently assigned to JFE Steel Corporation. The applicant listed for this patent is Masatoshi Aratani, Kenichi Iwazaki, Yoshikazu Kawabata, Takatoshi Okabe, Shunsuke Toyoda. Invention is credited to Masatoshi Aratani, Kenichi Iwazaki, Yoshikazu Kawabata, Takatoshi Okabe, Shunsuke Toyoda.
Application Number | 20150275339 14/410264 |
Document ID | / |
Family ID | 49782512 |
Filed Date | 2015-10-01 |
United States Patent
Application |
20150275339 |
Kind Code |
A1 |
Aratani; Masatoshi ; et
al. |
October 1, 2015 |
HIGH-CARBON STEEL TUBE HAVING SUPERIOR COLD WORKABILITY,
MACHINABILITY, AND HARDENABILITY AND METHOD FOR MANUFACTURING THE
SAME
Abstract
A raw steel tube has a composition containing, by mass, 0.25% to
0.60% carbon, 0.01% to 2.0% silicon, 0.2% to 3.0% manganese, 0.001%
to 0.1% aluminum, 0.001% to 0.05% phosphorus, 0.02% or less sulfur,
0.0010% to 0.0100% nitrogen, 0.0003% to 0.0050% boron, and 0.0001%
to 0.0050% calcium, the balance being iron and incidental
impurities. The raw steel tube is heated to and soaked at an
Ac.sub.3 transformation point or higher and is then subjected to
stretch-reducing rolling at a finishing temperature of rolling of
900.degree. C. to (Ac.sub.1 transformation point) with a cumulative
reduction in diameter of 30% to 70% within the temperature range of
900.degree. C. or lower.
Inventors: |
Aratani; Masatoshi; (Handa,
JP) ; Toyoda; Shunsuke; (Kawasaki, JP) ;
Okabe; Takatoshi; (Handa, JP) ; Kawabata;
Yoshikazu; (Tokyo, JP) ; Iwazaki; Kenichi;
(Handa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Aratani; Masatoshi
Toyoda; Shunsuke
Okabe; Takatoshi
Kawabata; Yoshikazu
Iwazaki; Kenichi |
Handa
Kawasaki
Handa
Tokyo
Handa |
|
JP
JP
JP
JP
JP |
|
|
Assignee: |
JFE Steel Corporation
Tokyo
JP
|
Family ID: |
49782512 |
Appl. No.: |
14/410264 |
Filed: |
June 28, 2012 |
PCT Filed: |
June 28, 2012 |
PCT NO: |
PCT/JP2012/067138 |
371 Date: |
June 9, 2015 |
Current U.S.
Class: |
148/593 ;
148/330 |
Current CPC
Class: |
B23K 2101/06 20180801;
C21D 6/008 20130101; B21B 17/14 20130101; B23K 13/025 20130101;
C21D 2211/005 20130101; C21D 9/08 20130101; C22C 38/12 20130101;
C21D 8/105 20130101; B23K 2103/04 20180801; C22C 38/02 20130101;
B23K 11/08 20130101; C22C 38/001 20130101; B23K 11/16 20130101;
C22C 38/16 20130101; C21D 6/005 20130101; C22C 38/14 20130101; C22C
38/002 20130101; C22C 38/04 20130101; C21D 2211/003 20130101; C22C
38/06 20130101 |
International
Class: |
C22C 38/16 20060101
C22C038/16; C21D 9/08 20060101 C21D009/08; C21D 6/00 20060101
C21D006/00; C22C 38/00 20060101 C22C038/00; C22C 38/12 20060101
C22C038/12; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 8/10 20060101
C21D008/10; C22C 38/14 20060101 C22C038/14 |
Claims
1-7. (canceled)
8. A high-carbon steel tube having a composition comprising, by
mass: 0.25% to 0.60% carbon, 0.01% to 2.0% silicon, 0.2% to 3.0%
manganese, 0.001% to 0.1% aluminum, 0.001% to 0.05% phosphorus,
0.0001% to 0.02% sulfur, 0.0010% to 0.0100% nitrogen, 0.0003% to
0.0050% boron, and 0.0001% to 0.0050% calcium, the balance being
iron and incidental impurities, the high-carbon steel tube having a
microstructure comprising a ferrite base phase and cementite grains
finely dispersed in the base phase, wherein an average grain size d
of the cementite grains is 0.1 to less than 0.5 .quadrature.m, and
an average distance L between surfaces of adjacent cementite grains
is 0.5 to 10 .quadrature.m.
9. The high-carbon steel tube according to claim 8, wherein the
composition further comprises, by mass, at least one selected from
2.0% or less copper, 2.0% or less chromium, 2.0% or less
molybdenum, 2.0% or less tungsten, 1.0% or less vanadium, and 0.1%
or less niobium.
10. The high-carbon steel tube according to claim 8, wherein the
composition further comprises, by mass, 0.1% or less titanium.
11. A method of manufacturing a high-carbon steel tube comprising:
providing a high-carbon steel tube as a raw steel tube; heating and
soaking the raw steel tube; and subjecting the raw steel tube to
stretch-reducing rolling into a product steel tube, the raw steel
tube being a steel tube having a composition comprising, by mass:
0.25% to 0.60% carbon, 0.01% to 2.0% silicon, 0.2% to 3.0%
manganese, 0.001% to 0.1% aluminum, 0.001% to 0.05% phosphorus,
0.0001% to 0.02% sulfur, 0.0010% to 0.0100% nitrogen, 0.0003% to
0.0050% boron, and 0.0001% to 0.0050% calcium, the balance being
iron and incidental impurities, wherein the raw steel tube is
heated to and soaked at an Ac3 transformation point or higher and
is then subjected to stretch-reducing rolling at a finishing
temperature of rolling of 900.degree. C. to (Ac1 transformation
point) with a cumulative reduction in diameter of 30% to 70% within
a temperature range of 900.degree. C. or lower.
12. The method according to claim 11, wherein the high-carbon steel
tube is a high-carbon electric-resistance-welded steel tube formed
by a tube-making process including continuously roll-forming a
high-carbon steel strip having the composition into a substantially
cylindrical open pipe and joining together ends of the open pipe by
electric resistance welding.
13. The method according to claim 11, wherein the composition
further comprises, by mass, at least one selected from 2.0% or less
copper, 2.0% or less chromium, 2.0% or less molybdenum, 2.0% or
less tungsten, 1.0% or less vanadium, and 0.1% or less niobium.
14. The method according to claim 11, wherein the composition
further comprises, by mass, 0.1% or less titanium.
15. The high-carbon steel tube according to claim 9, wherein the
composition further comprises, by mass, 0.1% or less titanium.
16. The method according to claim 12, wherein the composition
further comprises, by mass, at least one selected from 2.0% or less
copper, 2.0% or less chromium, 2.0% or less molybdenum, 2.0% or
less tungsten, 1.0% or less vanadium, and 0.1% or less niobium.
17. The method according to claim 12, wherein the composition
further comprises, by mass, 0.1% or less titanium.
18. The method according to claim 13, wherein the composition
further comprises, by mass, 0.1% or less titanium.
19. The method according to claim 16, wherein the composition
further comprises, by mass, 0.1% or less titanium.
Description
TECHNICAL FIELD
[0001] The present invention relates to high-carbon steel tubes
suitable for automotive parts such as steering rack bars mounted in
automotive rack-and-pinion type steering devices or the like,
steering shafts and drive shafts, and methods for manufacturing
such high-carbon steel tubes, and particularly to improvements in
cold workability, machinability, and hardenability.
BACKGROUND ART
[0002] There has recently been a strong need for improved
automobile mileage for the conservation of the global environment.
Accordingly, efforts have been made to reduce the weight of
automotive bodies. The need for a reduction in the weight of
automotive bodies has prompted the replacement of solid-core parts
made of steel bars with hollow parts made of steel tubes in the
field of steering-associated parts such as steering rack bars and
steering shafts for mounting in steering devices, which transfer
the motion of an automotive steering wheel to wheels, and
drive-shaft-associated parts such as drive shafts, which transfer
the power of an engine to wheels.
[0003] Raw steel tubes used for making parts such as steering rack
bars, steering shafts, and drive shafts are subjected to cold
working processes such as cold drawing working, cold pressing, and
cold forging and are then subjected to cutting work to be cut into
the final part shape, often followed by quenching treatment to
achieve a predetermined strength required of the parts.
[0004] For example, a hollow steering rack bar disclosed in Non
Patent Literature 1 is manufactured by providing a high-carbon
steel tube as a raw steel tube, forming a flat part on the upper
side of the tube by a cold process, and cutting gear teeth in the
flat part into the final shape, followed by high-frequency
hardening to achieve a predetermined strength. Thus, a raw steel
tube used for making a hollow steering rack bar is subjected to
cold working, cutting, and quenching treatment. This requires the
steel tube to have superior cold workability, machinability, and
hardenability.
[0005] To achieve a predetermined strength of parts after quenching
treatment, a raw steel tube having a high carbon content needs to
be used. A high carbon content, however, deteriorates the cold
workability. Accordingly, in the related art, a high-carbon steel
is subjected to spheroidizing annealing to achieve decreased
strength. Spheroidizing annealing, however, involves heat treatment
at about 700.degree. C. for an extended period of time, i.e.,
several hours. This causes the problem of decreased productivity
and increased production costs.
[0006] To address this problem, for example, Patent literature 1
discloses a method for manufacturing a steel bar having a
spheroidized microstructure. This method includes heating a steel
containing 2 mass % or less carbon to the Ac.sub.1 point or higher,
cooling the steel to the temperature range of (Ar.sub.1-50.degree.
C.) to (Ar.sub.1-200.degree. C.) before finish rolling during hot
rolling, finish-rolling the steel to cause a plastic deformation of
10% or more so that the resulting heat of deformation heats the
steel again to the temperature range of the Ac.sub.3 point to
Ac.sub.1 -- 100.degree. C., and maintaining the steel in the
temperature range of the Ac.sub.1 point to 500.degree. C. for 7
minutes or more. The technique disclosed in Patent literature 1 is
intended to form spheroidized carbide by forming carbide before
finish rolling, deforming and crushing the carbide by finish
rolling while dividing the carbide by heating with the heat of
deformation, and subsequently cooling and maintaining the carbide
at constant temperature. Thus, the technique disclosed in Patent
literature 1 allows an as-rolled steel to have a spheroidized
microstructure. This considerably reduces the time for subsequent
spheroidizing annealing and may eliminate the need for
spheroidizing annealing, depending on the application.
[0007] Patent literature 2 discloses a method for manufacturing an
electric-resistance-welded steel tube with superior cold
workability and hardenability. This method includes heating a steel
tube containing, by mass, 0.25% to 0.50% carbon, 0.35% or less
silicon, 0.60% to 1.59% manganese, 0.0025% or less sulfur, and
0.010% or less phosphorus to (Ac.sub.1 transformation
temperature-20.degree. C.) to (Ac.sub.1 transformation
temperature), maintaining the steel at that temperature for a
predetermined period of time, and cooling the steel in air, or
heating the steel to (Ac.sub.1 transformation temperature) to
(Ac.sub.1 transformation temperature+30.degree. C.), maintaining
the steel at that temperature for a predetermined period of time,
cooling the steel to (Ar.sub.1 transformation point-20.degree. C.)
to (Ar.sub.1 transformation point) at 0.01.degree. C./s to
1.0.degree. C./s, and cooling the steel in air, or maintaining the
steel at that temperature for a predetermined period of time and
cooling the steel in air. According to Patent literature 2, this
technique provides an electric-resistance-welded steel tube having
good cold workability and good hardenability that allows for a
sufficient increase in strength after quenching.
[0008] Patent literature 3 discloses a method for manufacturing a
steel tube with improved cold workability and high-frequency
hardenability. This method includes heating or soaking a raw steel
tube having a composition containing, by mass, 0.3% to 0.8% carbon,
2% or less silicon, and 3% or less manganese and then drawing the
steel tube to a cumulative reduction in diameter of 30% or more
within the temperature range of (Ac.sub.1 transformation
point-50.degree. C.) to the Ac.sub.1 transformation point to form a
microstructure containing cementite grains having grain sizes of
1.0 .mu.m or less.
CITATION LIST
Patent Literature
[0009] PTL 1: Japanese Examined Patent Application Publication No.
05-76524 [0010] PTL 2: Japanese Unexamined Patent Application
Publication No. 2006-9141 [0011] PTL 3: Japanese Unexamined Patent
Application Publication No. 2001-355047
Non Patent Literature
[0011] [0012] NPL 1: Inoue, Sumitomo Metal, vol. 48, No. 4(1996),
p. 29
SUMMARY OF INVENTION
Technical Problem
[0013] However, the technique disclosed in Patent literature 1
leaves the problems to be solved, including deteriorated
high-frequency hardenability due to spheroidized carbide and a
shortened cutting tool life and poor finished surface due to
increased cutting resistance. Although the technique disclosed in
Patent literature 2 provides improved cold workability and
hardenability, it leaves the problems due to deteriorated
machinability to be solved, including a shortened cutting tool life
and poor finished surface due to increased cutting resistance.
[0014] In Patent literature 3, there is no discussion of the
machinability in the technique disclosed in this patent. In the
present application, to achieve improved machinability, it is
important that the grain size of and the dispersion distance
between cementite grains fall within their respective appropriate
ranges. Accordingly, stretch-reducing rolling is performed at the
Ac.sub.1 transformation point or higher, preferably at a
temperature higher than the Ac.sub.1 transformation point but not
higher than 900.degree. C. In contrast, Patent literature 3
discloses that stretch-reducing rolling is performed in the range
of (Ac.sub.1 transformation point-50.degree. C.) to the Ac.sub.1
transformation point. This leaves the problem of insufficient
machinability to be solved.
[0015] An object of the present invention is to advantageously
solve the foregoing problems in the related art and provide a
high-carbon electric-resistance-welded steel tube that has all of
superior cold workability, machinability, and hardenability and a
method for manufacturing such a high-carbon
electric-resistance-welded steel tube. The term "superior cold
workability" as used herein refers to an elongation El before
quenching of 40% or more.
Solution to Problem
[0016] To achieve the foregoing object, the inventors have
conducted extensive research on the influence of various factors on
the cold workability, machinability, and hardenability of
high-carbon electric-resistance-welded steel tubes. Through this
research, the inventors have discovered that, among various
microstructure factors, the grain size of and dispersion spacing
between cementite grains precipitated and dispersed in a ferrite
base are important factors that affect the cold workability,
machinability, and hardenability, particularly the machinability,
of a high-carbon steel tube. The term "ferrite" as used herein
refers to granular ferrite (also referred to as "polygonal
ferrite").
[0017] In a mixed microstructure of ferrite and cementite, voids
tend to form in ferrite-cementite interfaces after plastic
deformation, particularly during cutting, because they differ in
plastic deformability. These voids may cause cementite to peel or
may coalesce into a crack. Such peeling of cementite or coalescence
of voids would result in a poor finished surface and thus
deteriorate the machinability. After further research, the
inventors have found that there were the grain size of and
dispersion spacing between cementite grains that cause no peeling
of cementite or coalescence of voids during cutting. The inventors
have discovered that controlling the grain size of and dispersion
spacing between cementite grains within their respective
appropriate ranges provides an excellent finished surface and thus
significantly improves the machinability without peeling of
cementite or coalescence of voids during cutting.
[0018] The inventors have also discovered that good machinability
is achieved if the average grain size of cementite grains is 0.1 to
less than 0.5 .mu.m and the dispersion spacing between the surfaces
of adjacent cementite grains is 0.5 to 10 .mu.m. The inventors have
discovered that a high-carbon steel tube having a microstructure in
which such cementite grains are dispersed has all of superior cold
workability, superior machinability and superior hardenability.
[0019] After further research, the inventors have discovered that,
to provide a high-carbon electric-resistance-welded steel tube
having a microstructure containing cementite grains having a grain
size and a dispersion spacing within their respective appropriate
ranges described above, it is important to heat a high-carbon steel
tube to the Ac.sub.3 transformation point or higher and then
subject it to stretch-reducing rolling at a finishing temperature
of rolling of 900.degree. C. to (Ac.sub.1 transformation point)
with a cumulative rolling reduction of 30% to 70% within the
temperature range of 900.degree. C. or lower.
[0020] The present invention has been made based on the foregoing
discoveries and additional research. Specifically, a summary of the
present invention is as follows.
[0021] (1) A high-carbon steel tube with superior cold workability,
machinability, and hardenability has a composition containing, by
mass, 0.25% to 0.60% carbon, 0.01% to 2.0% silicon, 0.2% to 3.0%
manganese, 0.001% to 0.1% aluminum, 0.001% to 0.05% phosphorus,
0.02% or less sulfur, 0.0010% to 0.0100% nitrogen, 0.0003% to
0.0050% boron, and 0.0001% to 0.0050% calcium, the balance being
iron and incidental impurities. The high-carbon steel tube has a
microstructure containing a ferrite base phase and cementite grains
dispersed in the base phase. The average grain size d of the
cementite grains is 0.1 to less than 0.5 .mu.m, and the average
distance L between surfaces of adjacent cementite grains is 0.5 to
10 .mu.m.
[0022] (2) In the high-carbon steel tube according to Item (1), the
composition further contains, by mass, at least one selected from
2.0% or less copper, 2.0% or less chromium, 2.0% or less
molybdenum, 2.0% or less tungsten, 1.0% or less vanadium, and 0.1%
or less niobium.
[0023] (3) In the high-carbon steel tube according to Item (1) or
(2), the composition further contains, by mass, 0.1% or less
titanium.
[0024] (4) A method for manufacturing a high-carbon steel tube with
superior cold workability, machinability, and hardenability
includes providing a high-carbon steel tube as a raw steel tube,
heating and soaking the raw steel tube, and subjecting the raw
steel tube to stretch-reducing rolling into a product steel tube.
The raw steel tube is a high-carbon steel tube having a composition
containing, by mass, 0.25% to 0.60% carbon, 0.01% to 2.0% silicon,
0.2% to 3.0% manganese, 0.001% to 0.1% aluminum, 0.001% to 0.05%
phosphorus, 0.02% or less sulfur, 0.0010% to 0.0100% nitrogen,
0.0003% to 0.0050% boron, and 0.0001% to 0.0050% calcium, the
balance being iron and incidental impurities. The raw steel tube is
heated to and soaked at an Ac.sub.3 transformation point or higher
and is then subjected to stretch-reducing rolling at a finishing
temperature of rolling of 900.degree. C. to (Ac.sub.1
transformation point) with a cumulative reduction in diameter of
30% to 70% within the temperature range of 900.degree. C. or
lower.
[0025] (5) In the method for manufacturing a high-carbon steel tube
according to Item (4), the high-carbon steel tube is a high-carbon
electric-resistance-welded steel tube formed by a tube-making
process including continuously roll-forming a high-carbon steel
strip having the composition into a substantially cylindrical open
pipe and joining together ends of the open pipe by electric
resistance welding.
[0026] (6) In the method for manufacturing a high-carbon steel tube
according to Item (4) or (5), the composition further contains, by
mass, at least one selected from 2.0% or less copper, 2.0% or less
chromium, 2.0% or less molybdenum, 2.0% or less tungsten, 1.0% or
less vanadium, and 0.1% or less niobium.
[0027] (7) In the method for manufacturing a high-carbon steel tube
according to any one of Items (4) to (6), the composition further
contains, by mass, 0.1% or less titanium.
Advantageous Effects of Invention
[0028] The present invention has an industrially significant
advantage in that a high-carbon steel tube that has superior cold
workability comparable to or higher than that of a steel tube
subjected to spheroidizing annealing as well as superior
machinability and high-frequency hardenability and that is suitable
for automotive parts such as steering rack bars, steering shafts,
and drive shafts can be easily manufactured at low cost without
spheroidizing annealing. The present invention also has an
advantage in that it contributes to a reduction in the weight of
automotive bodies and thus contributes to the conservation of the
global environment.
DESCRIPTION OF EMBODIMENTS
[0029] The reasons for the limitations on the composition of a
high-carbon steel tube according to the present invention will be
described first. In the following description, percentages are by
mass unless otherwise stated.
0.25% to 0.60% Carbon
[0030] Carbon is an element that functions to increase the quench
hardness and is therefore important for providing the desired
strength parts. To achieve this effect, a carbon content of 0.25%
or more is necessary. A carbon content of more than 0.60%, however,
would significantly deteriorate the cold workability and would also
deteriorate the weldability, which, if electric resistance welding
is performed, would result in a poor quality of electric resistance
weld. Thus, the carbon content is limited to the range of 0.25% to
0.60%. A carbon content of 0.30% to 0.50% is preferred.
0.01% to 2.0% Silicon
[0031] Silicon is an element that functions as a deoxidizing agent
and also contributes to increased strength by forming a solid
solution. To achieve these effects, a silicon content of 0.01% or
more is necessary. A silicon content of more than 2.0%, however,
would deteriorate the cold workability and, if electric resistance
welding is performed, would also result in a poor quality of
electric resistance weld because its oxide would form during
electric resistance welding and remain after upsetting. Thus, the
silicon content is limited to the range of 0.01% to 2.0%. A silicon
content of 0.1% to 0.5% is preferred.
0.2% to 3.0% Manganese
[0032] Manganese is an element that improves the hardenability and
also contributes to increased strength by forming a solid solution.
To achieve these effects, a manganese content of 0.2% or more is
necessary. A manganese content of more than 3.0%, however, would
deteriorate the cold workability and, if electric resistance
welding is performed, would also result in a poor quality of
electric resistance weld because manganese oxide tends to remain in
the electric resistance weld. Thus, the manganese content is
limited to the range of 0.2% to 3.0%. A manganese content of 0.5%
to 2.0% is preferred.
0.001% to 0.1% Aluminum
[0033] Aluminum is an element that functions effectively as a
deoxidizing agent. To achieve this effect, an aluminum content of
0.001% or more is necessary. An aluminum content of more than 0.1%,
however, would result in the formation of more alumina-based
inclusions, which would degrade the surface properties. Thus, the
aluminum content is limited to the range of 0.001% to 0.1%. An
aluminum content of 0.01% to 0.05% is preferred.
0.001% to 0.05% Phosphorus
[0034] Phosphorus is an element that contributes to increased
strength, and this effect is particularly remarkable when
phosphorus is present in an amount of 0.001% or more. Phosphorus,
however, tends to segregate, and a phosphorus content of more than
0.05% would result in noticeable grain boundary segregation and
center segregation. This would deteriorate the ductility and would
also significantly deteriorate the weldability. Thus, the
phosphorus content is limited to the range of 0.001% to 0.05%. A
phosphorus content of 0.001% to 0.02% is preferred.
0.02% or Less Sulfur
[0035] Whereas it is desirable to minimize the sulfur content
because sulfur is present in the steel as sulfide-based inclusions
that are likely to form origins of cracking during forming, a
sulfur content of 0.02% or less is acceptable. Thus, the sulfur
content is limited to 0.02% or less. A sulfur content of 0.01% or
less is preferred. Also, a sulfur content of 0.0001% or more is
preferred because excessively reducing the sulfur content would
involve high refining costs.
0.0010% to 0.0100% Nitrogen
[0036] Nitrogen is an element that contributes to increased
strength by forming a solid solution. To achieve this effect, a
nitrogen content of 0.0010% or more is necessary. A nitrogen
content of more than 0.0100%, however, would deteriorate the
workability. Thus, the nitrogen content is limited to the range of
0.0010% to 0.0100%. A nitrogen content of 0.0050% or less is
preferred.
0.0003% to 0.0050% Boron
[0037] Boron is an element that significantly improves the
hardenability of the steel by segregating at grain boundaries, even
when it is contained in small amounts. To achieve this effect, a
boron content of 0.0003% or more is necessary. A boron content of
more than 0.0050%, however, is economically disadvantageous since
it would not have a greater effect commensurate with the content
thereof and would also promote intergranular fractures by
segregating at grain boundaries in large amounts. Thus, the boron
content is limited to the range of 0.0003% to 0.0050%. A boron
content of 0.0005% to 0.0030% is preferred.
0.0001% to 0.0050% Calcium
[0038] Calcium is an element that contributes effectively to
morphology control of inclusions by forming spherical nonmetallic
inclusions (sulfide-based inclusions). The formation of spherical
nonmetallic inclusions alleviates stress concentration around the
nonmetallic inclusions and thus reduces origins of cracking during
forming and origins of cracking at fatigue failure. To achieve this
effect, a calcium content of 0.0001% or more is necessary. A
calcium content of more than 0.0050%, however, would result in the
formation of more nonmetallic inclusions, which would deteriorate
the cleanliness of the steel. Thus, the calcium content is limited
to the range of 0.0001% to 0.0050%. A calcium content of 0.0001% to
0.0030% is preferred.
[0039] The composition described above is the basic composition. In
addition to the basic composition, the high-carbon steel tube
according to the present invention may further contain, as optional
constituents, at least one selected from 2.0% or less copper, 2.0%
or less chromium, 2.0% or less molybdenum, 2.0% or less tungsten,
1.0% or less vanadium, and 0.1% or less niobium, and/or 0.1% or
less titanium.
[0040] At Least One Selected from 2.0% or Less Copper, 2.0% or Less
Chromium, 2.0% or Less Molybdenum, 2.0% or Less Tungsten, 1.0% or
Less Vanadium, and 0.1% or Less Niobium
[0041] The high-carbon steel tube may optionally contain at least
one selected from copper, chromium, molybdenum, tungsten, vanadium,
and niobium, all of which are elements that contribute to increased
steel strength.
[0042] Copper is an element that contributes to increased strength
by improving the hardenability and is therefore effective in
improving the fatigue resistance. To achieve this effect, a copper
content of 0.01% or more is preferred. A copper content of more
than 2.0%, however, would significantly deteriorate the cold
workability. Thus, if the high-carbon steel tube contains copper,
the copper content is preferably limited to 2.0% or less. A copper
content of 0.1% to 1.0% is more preferred.
[0043] Chromium is an element that contributes to increased
strength by improving the hardenability. To achieve this effect, a
chromium content of 0.01% or more is preferred. Chromium, however,
tends to form oxide, and in the case that electric resistance
welding is performed, a chromium content of more than 2.0% would
result in a poor quality of electric resistance weld because
chromium oxide tends to remain in the electric resistance weld.
Thus, if the high-carbon steel tube contains chromium, the chromium
content is preferably limited to the range of 2.0% or less. A
chromium content of 0.1% to 1.0% is more preferred.
[0044] Molybdenum is an element that contributes to increased
strength by improving the hardenability and promoting precipitation
strengthening with carbide and is therefore effective in improving
the fatigue resistance. To achieve this effect, a molybdenum
content of 0.01% or more is preferred. A molybdenum content of more
than 2.0%, however, would significantly deteriorate the cold
workability. A high molybdenum content would also involve high
material costs. Thus, if the high-carbon steel tube contains
molybdenum, the molybdenum content is preferably limited to 2.0% or
less. A molybdenum content of 0.1% to 0.5% is more preferred.
[0045] Tungsten is an element that contributes to increased
strength by promoting precipitation strengthening with carbide. To
achieve this effect, a tungsten content of 0.01% or more is
preferred. A tungsten content of more than 2.0%, however, would
result in the precipitation of excess carbide, which would
deteriorate the cold workability. A high tungsten content would
also involve high material costs. Thus, if the high-carbon steel
tube contains tungsten, the tungsten content is preferably limited
to 2.0% or less. A tungsten content of 0.1% to 0.5% is more
preferred.
[0046] Vanadium is an element that contributes to increased
strength by promoting precipitation strengthening with carbide and
also improves the temper softening resistance. To achieve these
effects, a vanadium content of 0.01% or more is preferred. A
vanadium content of more than 1.0%, however, is economically
disadvantageous since it would not have any greater effect. A high
vanadium content would also deteriorate the cold workability. Thus,
if the high-carbon steel tube contains vanadium, the vanadium
content is preferably limited to 1.0% or less. A vanadium content
of 0.1% to 0.5% is more preferred.
[0047] Niobium is an element that contributes to increased strength
by improving the hardenability and promoting precipitation
strengthening with carbide. To achieve this effect, a niobium
content of 0.0010% or more is preferred. A niobium content of more
than 0.1%, however, is economically disadvantageous since it would
not have any greater effect. A high niobium content would also
deteriorate the cold workability. Thus, if the high-carbon steel
tube contains niobium, the niobium content is preferably limited to
0.1% or less. A niobium content of 0.0010% to 0.05% is more
preferred.
0.1% or Less Titanium
[0048] The high-carbon steel tube may optionally contain titanium,
which is an element that functions to inhibit coarsening of crystal
grains during heat treatment by forming carbide and nitride. To
achieve this effect, a titanium content of 0.001% or more is
preferred. A titanium content of more than 0.1%, however, would
deteriorate the cold workability. Thus, if the high-carbon steel
tube contains titanium, the titanium content is preferably limited
to 0.1% or less. A titanium content of 0.0010% to 0.05% is more
preferred.
[0049] The balance is iron and incidental impurities. The
high-carbon steel tube may contain 0.01% or less oxygen as an
incidental impurity.
[0050] The reasons for the limitations on the microstructure of the
high-carbon steel tube according to the present invention will then
be described.
[0051] The high-carbon steel tube according to the present
invention has a microstructure containing a ferrite base phase in
which cementite grains having an average grain size within a
predetermined range are dispersed at a spacing within a
predetermined range. Spheroidizing cementite grains tends to
improve the cold workability.
[0052] The term "ferrite base phase" means that the area fraction
of ferrite phase is larger than the area of any other phase as
determined by corroding a cross-section (L cross-section) parallel
to the longitudinal direction of the tube or a cross-section (C
cross-section) parallel to the circumferential direction of the
tube with a nital corrosion solution and analyzing an image of the
metal microstructure captured under a light microscope or a
scanning microscope.
[0053] Specifically, the area fraction of ferrite is 50% or more,
preferably 60% or more. The total area fraction of phases other
than ferrite, including pearlite, bainite, and cementite, is 40% or
less, preferably 30% or less. The term "ferrite" as used herein
refers to granular ferrite (also referred to as "polygonal
ferrite") and differs from "bainitic ferrite".
[0054] Further, the above microstructure in which the cementite
grains are dispersed is a microstructure in which the average grain
size d of the cementite grains is 0.1 to less than 0.5 .mu.m and
the average distance L between the surfaces of the adjacent
cementite grains is 0.5 .mu.m to 10 .mu.m. Average Grain Size d of
Cementite Grains: 0.1 .mu.m to Less Than 0.5
[0055] Excessively fine cementite grains, i.e., cementite grains
having an average grain size d of less than 0.1 .mu.m, would not
allow the cold workability to be sufficiently improved. Large
cementite grains, i.e., cementite grains having an average grain
size d of not less than 0.5 .mu.m, would not dissolve sufficiently
during quench heating (high-frequency heating) and thus deteriorate
the hardenability. As a result, the desired quenching hardness
(product hardness) would not be provided. Excessively large
cementite grains would also increase the resistance during cutting
(cutting resistance) and thus shorten the life of a cutting tool.
Thus, the average grain size d of the cementite grains is limited
to the range of 0.1 to less than 0.5 .mu.m. An average grain size d
of 0.3 to less than 0.5 .mu.m is preferred. The average grain size
d of the cementite grains is measured as described in the
Examples.
Average Distance L Between Surfaces of Adjacent Cementite Grains:
0.5 to 10 .mu.m
[0056] The average distance L between the surfaces of the adjacent
cementite grains affects the properties of a finished surface after
cutting. An average distance L of less than 0.5 .mu.m would be
likely to result in coalescence of voids at ferrite-cementite
interfaces into a crack during cutting and thus degrade the surface
properties of the finished surface. An average distance L of more
than 10 .mu.m would inevitably result in the formation of large
cementite grains, which would deteriorate the hardenability and
would also increase the cutting resistance and thus deteriorate the
machinability. Thus, in the present invention, the average distance
L between the surfaces of the adjacent cementite grains is limited
to the range of 0.5 to 10 .mu.m. An average distance L of 5 .mu.m
or less is preferred. The average distance L between the surfaces
of the adjacent cementite grains is measured as described in the
Examples.
[0057] The adjustment of the average grain size d of the cementite
grains and the average distance L between the surfaces of the
adjacent grains within the above respective ranges provides a
high-carbon steel tube that has all of cold workability,
hardenability, and machinability.
[0058] A preferred method for manufacturing a high-carbon steel
tube according to the present invention will then be described.
[0059] A high-carbon steel tube having the above composition is
used as a raw steel tube. The raw steel tube may be any type of
steel tube having the above composition, such as a seamless steel
tube, an electric-resistance-welded steel tube, or a forge-welded
pipe, and may be manufactured by any process.
[0060] For example, an electric-resistance-welded steel tube is
typically manufactured by a tube-making process including
continuously roll-forming a steel strip into a substantially
cylindrical open pipe and joining together ends of the open pipe by
electric resistance welding. Although a hot-rolled steel strip
having the above composition is preferred for reduced manufacturing
costs, a cold-rolled steel strip can also be used without any
problem.
[0061] The high-carbon steel tube used as the raw steel tube is
heated to and soaked at a heating temperature higher than or equal
to the Ac.sub.3 transformation point, preferably 1,100.degree. C.
or lower. A heating temperature lower than the Ac.sub.3
transformation point would not allow carbon to diffuse sufficiently
into the electric resistance weld and could therefore result in low
hardness locally during quenching. A high heating temperature above
1,100.degree. C. would degrade the surface properties of the steel
tube. For improved surface properties and homogeneity, the
retention time (soaking time) at the heating temperature is
preferably about 0.1 to 10 minutes.
[0062] After heating, the raw steel tube is subjected to
stretch-reducing rolling.
[0063] Stretch-reducing rolling is preferably performed at a
finishing temperature of rolling of 900.degree. C. to (Ac.sub.1
transformation point) with a cumulative reduction in diameter of
30% to 70% within the temperature range of 900.degree. C. or lower.
The cumulative reduction in diameter from the start to the end of
rolling is preferably controlled within the range of 35% to 70%,
depending on the size of the raw steel tube and the size of the
product steel tube, to divide pearlite and thereby form fine
cementite grains.
[0064] A high finishing temperature of rolling above 900.degree. C.
at the surface of the steel tube would not allow spheroidizing of
cementite grains because no carbide would remain after rolling.
Such a high finishing temperature of rolling would also degrade the
surface properties of the product steel tube. A finish temperature
of rolling lower than the Ac.sub.1 transformation point would
result in the formation of excessively fine cementite grains at a
narrow dispersion spacing and thus degrade the finished surface.
Thus, the finishing temperature of rolling is limited to the range
of 900.degree. C. to (Ac.sub.1 transformation point), preferably
the range higher than the Ac.sub.1 transformation point. A
finishing temperature of rolling of 850.degree. C. to 750.degree.
C. is preferred.
[0065] A cumulative reduction in diameter of less than 30% within
the temperature range of 900.degree. C. or lower would not allow
spheroidizing of cementite grains because pearlite would not be
sufficiently divided during stretch-reducing rolling. A cumulative
reduction in diameter of more than 70% within the temperature range
of 900.degree. C. or lower would result in the formation of
excessively fine cementite grains and excessive work hardening and
thus deteriorate the cold workability. Such a large cumulative
reduction in diameter would also lead to low productivity during
the manufacture of parts.
[0066] By applying the method of manufacture described above to a
raw steel tube, a high-carbon steel tube can be easily manufactured
that has a microstructure containing cementite grains having an
average grain size and an average distance between the surfaces of
adjacent cementite grains within their respective appropriate
ranges.
[0067] The present invention is further illustrated by the
following examples.
EXAMPLES
[0068] Raw steel tubes were formed from hot-rolled steel strips
(thickness: 7.0 mm) having the compositions shown in Table 1 by a
tube-making process including continuously roll-forming the steel
strips into substantially cylindrical open pipes and joining
together the ends of the open pipes by electric resistance welding
to form electric-resistance-welded steel tubes (outer diameter:
89.1 mm).
[0069] These raw steel tubes were subjected to stretch-reducing
rolling under the conditions shown in Table 2 to form product steel
tubes. As related-art examples, some of the steel tubes were
subjected to annealing treatment at 700.degree. C. for 10 hours or
to normalizing treatment at 925.degree. C. for 15 minutes. The raw
steel tubes of the related-art examples were worked to a diameter
of 40 mm before the above treatment.
[0070] The microstructure observation of the resulting steel tubes
was performed, and they were also examined for their cold
workability, hardenability, and machinability. These examinations
were performed as follows.
(1) Microstructure Observation
[0071] Each of the resulting product steel tubes was cut into a
test specimen for microstructure observation. A cross-section (C
cross-section) of the test specimen perpendicular to the
longitudinal direction of the tube was polished and corroded with a
nital corrosion solution and was observed under an electron
scanning microscope (at a magnification ratio of 2,000 times).
Images containing 100 or more cementite grains were captured in 10
or more fields of view. The resulting images were analyzed to
determine the area of each cementite grain, and the circle
equivalent diameter thereof was calculated to determine the grain
size of each grain. The arithmetic average grain size of the
cementite grains was calculated to determine the average grain size
d of the cementite grains in the steel tube. The resulting images
were also analyzed to determine the distances between the surfaces
of the adjacent cementite grains, and the arithmetic average
distance was calculated to determine the average distance L between
the surfaces of the adjacent cementite grains in the steel
tube.
(2) Cold Workability
[0072] Each of the resulting product steel tubes was cut into a JIS
No. 11 A tensile test specimen (GL: 50 mm). A tensile test was
performed in accordance with JIS 22241 to determine the tensile
strength TS and the elongation El. Test specimens having an
elongation El of 40% or more were rated as good, indicating
"superior cold workability", and other test specimens were rated as
poor.
(3) Hardenability
[0073] Each of the resulting product steel tubes was cut into a
test piece (length: 300 mm). The test piece was heated to a surface
temperature of 1,000.degree. C. with a high-frequency dielectric
heater in accordance with JIS G0559 and was quenched by spraying
water on the outer surface of the tube. The heating conditions were
as follows: the frequency was 10 kHz, and the feed rate of the
induction heating coil was 20 mm/s. The quenched test piece was cut
into a test specimen. The cross-sectional hardness distribution
across the thickness of the test specimen was measured with a
Vickers hardness tester (load: 4.9 N). The effective hardened layer
depth was determined as the depth of a region having a hardness of
95% or more of the maximum hardness depending on the carbon
content.sup.*1. For *1), the quenching hardness depending on the
carbon content was determined from the following reference and
conversion formula: [0074] Reference: William C. Leslie, The
Physical Metallurgy of Steels, Maruzen Co., Ltd., p. 235, Table
VII. 2
[0075] Hardness Conversion Table (HRC-to-HV Hardness conversion
table): SAE J417
[0076] Test specimens having a hardness of 95% or more of the
maximum hardness over a region extending from the outer surface of
the tube to a depth of 95% or more of the wall thickness were rated
as good, indicating "superior hardenability", and other test
specimens were rated as poor.
(4) Machinability
[0077] The resulting product steel tubes were grooved (V-grooved)
in the inner surface thereof. The working conditions were as
follows:
[0078] Rotational speed: 100 rpm
[0079] Feed rate: 0.3 mm/rev
[0080] V-groove cut depth: 1 mm
[0081] Tip: cemented carbide tip
[0082] The cemented carbide tip was a common tungsten cemented
carbide tip for cutting.
[0083] After 100 product steel tubes were grooved, the turning tool
(tool) was removed and inspected for its condition.
[0084] The case where the turning tool did not fractured or chipped
at the edge thereof and the finished surfaces had no defect was
rated as good, and the case where at least one of them occurred was
rated as poor.
[0085] The results are shown in Table 3.
[0086] The high-carbon steel tubes of the inventive examples all
had a larger elongation than those of the related-art examples
subjected to annealing treatment, demonstrating that they had
superior cold workability. In addition, the high-carbon steel tubes
of the inventive examples all had a higher hardenability than those
of the related-art examples subjected to normalizing treatment,
demonstrating that they had superior high-frequency hardenability.
Furthermore, the high-carbon steel tubes of the inventive examples
all provided less tool wear and better finished surface properties
than those of the related-art examples subjected to normalizing
treatment, demonstrating that they had superior machinability. In
contrast, the comparative examples beyond the scope of the present
invention had low cold workability, low hardenability, low
machinability, or all of them.
TABLE-US-00001 TABLE 1 Steel Symbol C Si Mn P S Al N B Ca Cr A
0.300 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 B
0.350 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 C
0.400 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 D
0.450 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 E
0.500 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 F
0.350 0.20 0.50 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 G
0.350 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 H
0.350 0.20 1.30 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.20 I
0.350 0.20 0.70 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 J
0.350 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 K
0.350 0.20 0.80 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 L
0.350 0.20 1.60 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 M
0.200 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 N
0.650 0.20 1.35 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 O
0.300 0.20 1.35 0.010 0.0210 0.030 0.0025 0.0015 0.0015 0.00 P
0.350 0.20 1.60 0.010 0.0015 0.030 0.0025 0.0015 0.0015 0.00 Q
0.400 0.20 1.35 0.010 0.0050 0.030 0.0025 0.0005 0.0001 0.00 Steel
Ac.sub.3 Ac.sub.1 Symbol Mo Nb Ti V W Cu (.degree. C.) (.degree.
C.) Remarks A 0.00 0.000 0.000 0.000 0.0000 0.000 786 714
Conforming example B 0.00 0.000 0.001 0.000 0.0000 0.000 778 714
Conforming example C 0.00 0.000 0.000 0.000 0.0000 0.000 769 714
Conforming example D 0.00 0.000 0.000 0.000 0.0000 0.000 761 714
Conforming example E 0.00 0.000 0.001 0.000 0.0000 0.000 754 714
Conforming example F 0.00 0.000 0.010 0.000 0.0000 0.000 807 723
Conforming example G 0.00 0.000 0.001 0.000 0.0000 0.000 778 714
Conforming example H 0.00 0.000 0.001 0.000 0.0000 0.000 777 718
Conforming example I 0.15 0.000 0.001 0.000 0.0000 0.000 802 721
Conforming example J 0.00 0.010 0.010 0.000 0.0000 0.000 781 714
Conforming example K 0.00 0.000 0.001 0.200 0.0000 0.000 815 720
Conforming example L 0.00 0.000 0.001 0.000 0.1000 0.200 768 712
Conforming example M 0.00 0.000 0.001 0.000 0.0000 0.000 807 714
Comparative example N 0.00 0.000 0.001 0.000 0.0000 0.000 734 714
Comparative example O 0.00 0.000 0.001 0.000 0.0000 0.000 787 714
Comparative example P 0.00 0.000 0.000 0.000 0.0000 0.000 770 712
Conforming example Q 0.00 0.000 0.000 0.000 0.0000 0.000 769 714
Conforming example
TABLE-US-00002 TABLE 2 Stretch reducing conditions Cumulative
reduction in diameter (%) Product Heating Finish rolling
Temperature Product steel tube Steel Ac.sub.3 Ac.sub.1 temperature
temperature range of 900.degree. tube size Heat treatment symbol
symbol (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.)
Total C. or lower (mm) conditions Remarks 1 A 786 714 900 750 55 50
.phi. 40 .times. t7.0 -- Inventive example 2 A 786 714 -- -- --
.phi. 40 .times. t7.0 Normalizing Comparative at 920.degree. C. for
example 15 minutes 3 A 786 714 -- -- -- -- .phi. 40 .times. t7.0
Spheroidizing Comparative annealing example at 700.degree. C. for
10 hours 4 B 778 714 950 820 55 40 .phi. 40 .times. t7.0 --
Inventive example 5 B 778 714 -- -- -- -- .phi. 40 .times. t7.0
Normalizing Comparative at 920.degree. C. for example 15 minutes 6
B 778 714 -- -- -- -- .phi. 40 .times. t7.0 Spheroidizing
Comparative annealing example at 700.degree. C. for 10 hours 7 C
769 714 980 840 55 38 .phi. 40 .times. t7.0 -- Inventive example 8
C 769 714 -- -- -- -- .phi. 40 .times. t7.0 Normalizing Comparative
at 920.degree. C. for example 15 minutes 9 C 769 714 -- -- -- --
.phi. 40 .times. t7.0 Spheroidizing Comparative annealing example
at 700.degree. C. for 10 hours 10 D 761 714 900 750 55 55 .phi. 40
.times. t7.0 -- Inventive example 11 D 761 714 -- -- -- -- .phi. 40
.times. t7.0 Normalizing Comparative at 920.degree. C. for example
15 minutes 12 D 761 714 -- -- -- -- .phi. 40 .times. t7.0
Spheroidizing Comparative annealing example at 700.degree. C. for
10 hours 13 E 754 714 950 750 55 55 .phi. 40 .times. t7.0 --
Inventive example 14 E 754 714 -- -- -- -- .phi. 40 .times. t7.0
Normalizing Comparative at 920.degree. C. for example 15 minutes 15
E 754 714 -- -- -- -- .phi. 40 .times. t7.0 Spheroidizing
Comparative annealing example at 700.degree. C. for 10 hours 16 F
807 723 1000 850 55 50 .phi. 40 .times. t7.0 -- Inventive example
17 G 778 714 1010 850 55 35 .phi. 40 .times. t7.0 -- Inventive
example 18 H 777 718 950 820 55 38 .phi. 40 .times. t7.0 --
Inventive example 19 I 802 721 900 750 55 50 .phi. 40 .times. t7.0
-- Inventive example 20 J 781 714 900 670 55 55 .phi. 40 .times.
t7.0 -- Inventive example 21 K 815 720 950 800 55 50 .phi. 40
.times. t7.0 -- Inventive example 22 L 768 712 900 780 55 50 .phi.
40 .times. t7.0 -- Inventive example 23 M 807 714 900 750 55 50
.phi. 40 .times. t7.0 -- Comparative example 24 N 734 714 950 800
55 42 .phi. 40 .times. t7.0 -- Comparative example 25 O 787 714 960
810 55 40 .phi. 40 .times. t7.0 -- Comparative example 26 B 778 714
1100 950 55 0 .phi. 40 .times. t7.0 -- Comparative example 27 B 778
714 880 600 55 50 .phi. 40 .times. t7.0 -- Comparative example 28 B
778 714 950 850 55 15 .phi. 40 .times. t7.0 -- Comparative example
29 P 770 712 950 820 55 40 .phi. 40 .times. t7.0 -- Inventive
example 30 Q 769 714 980 840 55 38 .phi. 40 .times. t7.0 --
Inventive example
TABLE-US-00003 TABLE 3 Metal microstructure before cold working
Phase Cementite Area frac- Average Distance between Tensile Product
tion of fer- grain surfaces of ad- properties tube Steel rite phase
size d jacent grains TS EL symbol symbol Type*) (%) (.mu.m) (.mu.m)
(MPa) (%) Hardenability Machinability Remarks 1 A F, C 90 0.4 1.1
560 51 Good Good Inventive example 2 A F, C, P 10 Not spheroidized
620 34 Poor Poor Comparative example 3 A P, F, C 90 1.6 12 575 38
Poor Poor Comparative example 4 B F, C, P 85 0.4 1.2 610 48 Good
Good Inventive example 5 B P, F, C 7 Not spheroidized 695 26 Poor
Poor Comparative example 6 B F, C, P 85 0.8 11 593 32 Poor Poor
Comparative example 7 C F, C, P 80 0.3 1.9 635 46 Good Good
Inventive example 8 C P, F, C 6 Not spheroidized 719 20 Poor Poor
Comparative example 9 C F, C, P 80 1.8 15 635 33 Poor Poor
Comparative example 10 D F, C, P 80 0.3 0.8 667 44 Good Good
Inventive example 11 D P, F, C 5 Not spheroidized 765 17 Poor Poor
Comparative example 12 D F, C, P 80 1.9 12 671 25 Poor Poor
Comparative example 13 E F, C, P, B 75 0.2 0.6 695 43 Good Good
Inventive example 14 E P, F, C 3 Not spheroidized 789 16 Poor Poor
Comparative example 15 E F, C, P, B 75 1.9 12 676 19 Poor Poor
Comparative example 16 F F, C 90 0.4 1.3 615 48 Good Good Inventive
example 17 G F, C 88 0.3 0.8 620 47 Good Good Inventive example 18
H F, C 90 0.4 1 605 49 Good Good Inventive example 19 I F, C 82 0.3
2.1 615 48 Good Good Inventive example 20 J F, C 90 0.3 0.8 620 48
Good Good Inventive example 21 K F, C 87 0.4 1 615 50 Good Good
Inventive example 22 L F, C 90 0.4 1 610 51 Good Good Inventive
example 23 M F, C 95 1.5 11 420 55 Poor Poor Comparative example 24
N F, C, P, B 40 1.5 12 850 15 Good Poor Comparative example 25 O F,
C 90 1.9 16 540 20 Poor Poor Comparative example 26 B F, C, P 85
2.1 13 610 38 Poor Poor Comparative example 27 B F, C, P 85 1.2 11
610 38 Poor Poor Comparative example 28 B F, C, P 85 1.3 15 610 25
Poor Poor Comparative example 29 P F, C, P 84 0.4 1 610 38 Good
Good Inventive example 30 Q F, C, P 81 0.3 1.9 635 36 Good Good
Inventive example *)F: ferrite, C: cementite, P: pearlite, B:
bainite
* * * * *