U.S. patent application number 16/960922 was filed with the patent office on 2020-11-12 for abrasion resistant steel and method for producing same.
This patent application is currently assigned to NIPPON STEEL CORPORATION. The applicant listed for this patent is NIPPON STEEL CORPORATION. Invention is credited to Motomichi HARA, Takahiro KAMO, Takumi MIYAKE, Masaki MIZOGUCHI, Yasunori TAKAHASHI.
Application Number | 20200354808 16/960922 |
Document ID | / |
Family ID | 1000004977929 |
Filed Date | 2020-11-12 |
United States Patent
Application |
20200354808 |
Kind Code |
A1 |
MIZOGUCHI; Masaki ; et
al. |
November 12, 2020 |
ABRASION RESISTANT STEEL AND METHOD FOR PRODUCING SAME
Abstract
Abrasion resistant steel having a chemical composition
comprising, by mass %, C: 0.10 to 0.20%, Si: 0.01 to 1.20%, Mn:
0.01 to 2.00%, P: less than 0.017%, S: 0.010% or less, Cu: 0.01 to
0.70%, Ni: 0.01 to 1.00%, Cr: 0 to 1.50%, Mo: 0 to 0.80%, W: 0 to
0.50%, Nb: 0 to 0.050%, V: 0 to 0.20%, Ti: 0 to 0.030%, B: 0 to
0.0030%, N: 0.0001 to 0.0070%, Al: 0.001 to 0.10%, Ca: 0 to
0.0050%, Zr: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, and
a balance of Fe and impurities, having metal structures at a
position of 1/4 of the thickness from the surface in a thickness
direction, which have a total of the area ratios of martensite and
lower bainite of 50 to 100% and a prior austenite average grain
size of 5 to 23 .mu.m, and having a Brinell hardness at a position
of 1 mm from the surface in the thickness direction of 360 to 440,
and a method for producing the same.
Inventors: |
MIZOGUCHI; Masaki; (Tokyo,
JP) ; KAMO; Takahiro; (Tokyo, JP) ; HARA;
Motomichi; (Tokyo, JP) ; MIYAKE; Takumi;
(Tokyo, JP) ; TAKAHASHI; Yasunori; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL CORPORATION
Tokyo
JP
|
Family ID: |
1000004977929 |
Appl. No.: |
16/960922 |
Filed: |
December 25, 2018 |
PCT Filed: |
December 25, 2018 |
PCT NO: |
PCT/JP2018/047681 |
371 Date: |
July 9, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 2211/001 20130101;
C22C 38/58 20130101; C22C 38/44 20130101; C22C 38/42 20130101; C21D
8/0263 20130101; C21D 6/005 20130101; C22C 38/46 20130101; C22C
38/48 20130101; C22C 38/001 20130101; C22C 38/06 20130101; C21D
6/008 20130101; C22C 38/005 20130101; C21D 8/0205 20130101; C21D
9/46 20130101; C22C 38/54 20130101; C22C 38/50 20130101; C21D 6/004
20130101; C22C 38/002 20130101; C21D 2211/002 20130101; C22C 38/02
20130101; C21D 2211/008 20130101; C21D 8/0226 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/58 20060101 C22C038/58; C22C 38/54 20060101
C22C038/54; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/42 20060101 C22C038/42; C22C 38/06 20060101
C22C038/06; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2018 |
JP |
2018-053878 |
Claims
1-4. (canceled)
5. Abrasion resistant steel having a chemical composition
comprising, by mass %, C: 0.10 to 0.20%, Si: 0.01 to 1.20%, Mn:
0.01 to 2.00%, P: less than 0.017%, S: 0.010% or less, Cu: 0.01 to
0.70%, Ni: 0.01 to 1.00%, Cr: 0 to 1.50%, Mo: 0 to 0.80%, W: 0 to
0.50%, Nb: 0 to 0.050%, V: 0 to 0.20%, Ti: 0 to 0.030%, B: 0 to
0.0030%, N: 0.0001 to 0.0070%, Al: 0.001 to 0.10%, Ca: 0 to
0.0050%, Zr: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, and
a balance of Fe and impurities, having metal structures having a
total area ratios of martensite and lower bainite of 50 to 100% and
a prior austenite average grain size of 5 to 20 .mu.m at a position
of 1/4 of a thickness from a surface in a thickness direction, and
having a Brinell hardness of 360 to 440 at a position of 1 mm from
the surface in the thickness direction.
6. The abrasion resistant steel according to claim 5, wherein a
thickness is 15 mm or more.
7. A method for producing abrasion resistant steel, the method
comprising: heating a slab having a chemical composition according
to claim 5 to 1000 to 1350.degree. C., hot rolling the heated slab
at 1000 to over 825.degree. C. by a 20% or more rolling reduction,
then at 825 to 730.degree. C. by a 10% or more rolling reduction,
and ending the hot rolling at a temperature of 730.degree. C. or
more, cooling the hot rolled steel plate, and reheating the cooled
steel plate to 860.degree. C. or more, then quenching the reheated
steel plate.
8. The abrasion resistant steel according to claim 5, wherein a
prior austenite average grain size is 18 .mu.m or less.
9. A method for producing abrasion resistant steel, the method
comprising: heating a slab having a chemical composition according
to claim 6 to 1000 to 1350.degree. C., hot rolling the heated slab
at 1000 to over 825.degree. C. by a 20% or more rolling reduction,
then at 825 to 730.degree. C. by a 10% or more rolling reduction,
and ending the hot rolling at a temperature of 730.degree. C. or
more, cooling the hot rolled steel plate, and reheating the cooled
steel plate to 860.degree. C. or more, then quenching the reheated
steel plate.
10. The abrasion resistant steel according to claim 6, wherein a
prior austenite average grain size is 18 .mu.m or less.
Description
FIELD
[0001] The present invention relates to abrasion resistant steel
having a high toughness suitable for use as a component part of a
construction machine, industrial machine, or other machine in which
abrasion resistance is demanded and to a method for producing the
same.
BACKGROUND
[0002] The abrasion resistance of a component part of a machine is
strongly governed by its surface hardness, so high hardness steel
has been used for component parts of machines such as construction
machines for civil engineering or mining use or industrial machines
in which abrasion resistance is demanded. For this high hardness
steel, the property of having stable abrasion resistance making it
able to withstand long term use has been demanded. Further, in
recent years, demand for construction machines or industrial
machines used in cold regions has been increasing. A steel material
having a low temperature toughness suitable for use in such cold
regions has been demanded.
[0003] PTL 1 proposes a method for producing abrasion resistant
steel plate comprising controlling the chemical constituents,
heating then hot rolling, then reheating and accelerated
cooling.
[0004] PTL 2 proposes a method for producing abrasion resistant
thick gauge steel plate having low temperature toughness comprising
controlling the chemical constituents and using microprecipitates
having a diameter of 50 nm or less to inhibit the growth of
austenite grains during production.
[0005] PTL 3 proposes a method for producing low alloy abrasion
resistant steel comprising controlling the chemical constituents
and heating, then hot rolling and applying accelerated cooling
right after hot rolling.
CITATIONS LIST
Patent Literature
[0006] [PTL 1] Japanese Unexamined Patent Publication No.
2012-214890
[0007] [PTL 2] Japanese Unexamined Patent Publication No.
2014-194042
[0008] [PTL 3] Japanese Unexamined Patent Publication No.
2016-509631
SUMMARY
Technical Problem
[0009] The steel plate produced by the method described in PTL 1
has a large C content, so is difficult to make higher in toughness.
Further, in the method described in PTL 1, the rolling conditions
at the time of hot rolling were not sufficiently studied.
Therefore, there was still room for improvement from the viewpoint
of enhancing the toughness. Furthermore, the examples of PTL 1 had
many with low reheating temperatures, therefore there were
technical issues from the viewpoint of securing a high
hardness.
[0010] PTL 2 taught to make microprecipitates disperse in the steel
to inhibit the growth of austenite during reheating by the pinning
effect and making the austenite grains finer. However, with the
method of making such microprecipitates disperse in the steel,
slight differences in the chemical constituents or differences in
the reheating temperature cause large fluctuations in the state of
dispersion of the precipitates, so it is difficult to stably refine
the austenite grains and high toughness can not necessarily be
realized. Further, the P content is not always kept sufficiently
low. Furthermore, sometimes a drop in toughness is caused.
[0011] The steel plate produced by the method described in PTL 3 is
difficult to make higher in toughness by a high C content. Further,
the fact that by performing the cooling (quenching) right after hot
rolling at a low temperature, anisotropy occurs in the steel
material structures became clear by studies of the inventors.
Therefore, there is the problem that the toughness when causing
fracture in the rolling direction becomes lower.
[0012] The present invention was made in consideration of such a
situation and has as its object to provide abrasion resistant steel
having excellent low temperature toughness able to be used even in
cold regions and a method for producing the same by a novel
constitution. Specifically, it has as its object the provision of
abrasion resistant steel having an absorption energy of 27 J or
more in a Charpy impact test at -40.degree. C. at a position of 1/4
of the thickness from the surface in a thickness direction and
having a Brinell hardness (Brinell hardness at a position of 1 mm
from the surface in the thickness direction) of 360 to 440 and a
method for producing the same.
Solution to Problem
[0013] The gist of the present invention is as follows:
[0014] (1) Abrasion resistant steel having a chemical composition
comprising, by mass %,
[0015] C: 0.10 to 0.20%,
[0016] Si: 0.01 to 1.20%,
[0017] Mn: 0.01 to 2.00%,
[0018] P: less than 0.017%,
[0019] S: 0.010% or less,
[0020] Cu: 0.01 to 0.70%,
[0021] Ni: 0.01 to 1.00%,
[0022] Cr: 0 to 1.50%,
[0023] Mo: 0 to 0.80%,
[0024] W: 0 to 0.50%,
[0025] Nb: 0 to 0.050%,
[0026] V: 0 to 0.20%,
[0027] Ti: 0 to 0.030%,
[0028] B: 0 to 0.0030%,
[0029] N: 0.0001 to 0.0070%,
[0030] Al: 0.001 to 0.10%,
[0031] Ca: 0 to 0.0050%,
[0032] Zr: 0 to 0.0050%,
[0033] Mg: 0 to 0.0050%,
[0034] REM: 0 to 0.0050%, and
[0035] a balance of Fe and impurities,
[0036] having metal structures having a total area ratios of
martensite and lower bainite of 50 to 100% and a prior austenite
average grain size of 5 to 23 .mu.m at a position of 1/4 of a
thickness from a surface in a thickness direction, and
[0037] having a Brinell hardness of 360 to 440 at a position of 1
mm from the surface in the thickness direction.
[0038] (2) The abrasion resistant steel according to claim 1,
wherein a thickness is 15 mm or more.
[0039] (3) A method for producing abrasion resistant steel, the
method comprising:
[0040] heating a slab having a chemical composition according to
(1) or (2) to 1000 to 1350.degree. C.,
[0041] hot rolling the heated slab at 1000 to over 825.degree. C.
by a 20% or more rolling reduction, then at 825 to 730.degree. C.
by a 10% or more rolling reduction, and ending the hot rolling at a
temperature of 730.degree. C. or more,
[0042] cooling the hot rolled steel plate, and
[0043] reheating the cooled steel plate to 860.degree. C. or more,
then quenching the reheated steel plate.
Advantageous Effects of Invention
[0044] According to the present invention, abrasion resistant steel
having excellent low temperature toughness enabling use in cold
regions is obtained. In particular, even if the plate thickness is
large, abrasion resistant steel having excellent low temperature
toughness can be obtained.
DESCRIPTION OF EMBODIMENTS
[0045] Abrasion Resistant Steel
[0046] In general, if raising the hardness of a steel material, the
toughness tends to fall. It is not easy to secure low temperature
toughness by a high hardness steel material such as abrasion
resistant steel. The inventors engaged in repeated studies to
obtain abrasion resistant steel having high toughness at a low
temperature and as a result learned to make the prior austenite
average grain size 5 to 23 .mu.m at a position of 1/4 of the
thickness from the surface of the steel plate in the thickness
direction.
[0047] The inventors engaged in various studies on the production
conditions for refining prior austenite grains and as a result
learned that at the time of reheating at the time of quenching, it
is important to increase the nucleation sites when transforming
bainite or martensite back to austenite. This is because by
remarkably increasing the nucleation sites for back transformation
to austenite, it is possible to refine the austenite grains when
the structures finish being transformed back to austenite.
[0048] Further, they learned that to make the nucleation sites of
the austenite back transformation increase, it is important to
control the temperature and rolling reduction at the time of hot
rolling.
[0049] Further, they learned that the nucleation sites of austenite
at the time of reheating when quenching are the large grain
boundaries such as the prior austenite grain boundaries of the
bainite and martensite. Further, as explained above, by controlling
the temperature and rolling reduction at the time of hot rolling,
it is possible to refine and flatten the austenite grains at the
time of hot rolling. Due to this, it becomes possible to increase
the area of the large grain boundaries per unit volume at the time
of reheating when quenching, that is, the nucleation sites for
austenite back transformation. Furthermore, by such control at the
time of hot rolling, it is believed there is also the effect that
it is possible to give rolling strain to the steel and increase the
energy stored at the crystal grain boundaries and thereby promote
back transformation
[0050] Furthermore, controlling the temperature at the time of
ending the hot rolling is also important. This is because if the
temperature at the time of ending the hot rolling is made too low,
the prior austenite grains after the reheating and quenching will
become excessively refined and thereby the steel will not be
sufficiently quenched and sometimes the hardness will drop.
[0051] Further, to obtain abrasion resistant steel having low
temperature toughness, with just control of the average grain size
of the prior austenite, the toughness is not sufficiently improved.
The metal structures should be made ones mainly comprised of
martensite and lower bainite. In addition, for improvement of
toughness, suitable combination of various types of alloys is
important.
[0052] In particular, when obtaining fine austenite grains, in
general the hardenability tends to fall. Even if quenching,
sometimes sufficient hardness cannot be obtained. Therefore, Cu and
Ni may be added to raise the hardenability.
[0053] Chemical Composition of Abrasion Resistant Steel
[0054] Below, the constituent requirements of the abrasion
resistant steel according to the present invention will be
explained. First, the reasons for limiting the chemical composition
of the steel will be explained. In this Description, the "%" used
for the chemical contents mean mass %.
[0055] C: 0.10 to 0.20%
[0056] C (carbon) is an element effective for increasing the
hardness of the steel. In the present invention, to secure the
hardness, the lower limit of the C content is made 0.10%. The
preferable lower limit of the C content is 0.11%. The more
preferable lower limit of the C content is 0.12%. On the other
hand, if the C content is more than 0.20%, sometimes the Brinell
hardness of 440 or less in range targeted by the present invention
will no longer be satisfied and, therefore, the toughness will
fall, so the upper limit of the C content is made 0.20%. To better
improve the toughness, the upper limit of the C content is
preferably made 0.16%, more preferably is made 0.15%.
[0057] Si: 0.01 to 1.20%
[0058] Si (silicon) is a deoxidizing element. It also contributes
to improvement of the hardness by solution strengthening.
Therefore, in the present invention, the lower limit of the Si
content is made 0.01%. The lower limit of the Si content is
preferably 0.10%, more preferably 0.20%. However, if the Si content
is too high, the toughness and weldability deteriorate, so the
upper limit of the Si content is made 1.20%. The preferable upper
limit of the Si content is made 0.80%. The more preferable upper
limit of the Si content is made 0.70% or 0.50%.
[0059] Mn: 0.01 to 2.00%
[0060] Mn (manganese) contributes to raising the hardness through
enhancement of the hardenability, so in the present invention, the
lower limit of the Mn content is made 0.01%. To raise the strength
further, the lower limit of the Mn content is preferably made
0.50%, more preferably is made 1.00%. On the other hand, if the Mn
content is more than 2.00%, the toughness and weldability
deteriorate, so the upper limit of the Mn content is made 2.00%.
The preferable upper limit of the Mn content is 1.70% or 1.50%,
while the more preferable upper limit is 1.40% or 1.30%.
[0061] P: less than 0.017%
[0062] P (phosphorus) is an impurity. It segregates at the grain
boundaries etc. and aggravates the occurrence of brittle fracture,
so in the present invention, the P content is made less than
0.017%. Preferably, the P content is 0.013% or less. More
preferably, the P content is 0.010% or less. If becoming 0.017% or
more, the toughness remarkably falls. The P content is preferably
as small as possible. The lower limit is 0%, but if making the
content less than 0.001%, the production costs remarkably increase,
so for example the lower limit of the P content may also be 0.001%,
0.002%, 0.003%, or 0.005%.
[0063] S: 0.010% or less
[0064] S (sulfur) is an impurity. It forms MnS and other sulfides
to lower the toughness, so in the present invention, the S content
is made 0.010% or less. Preferably, the S content is 0.007% or
less. More preferably, the S content is 0.005% or less. If more
than 0.010%, the toughness sometimes falls. The S content is
preferably as small as possible. The lower limit is 0%, but if
making the content less than 0.001%, the production costs
remarkably increase, so for example the lower limit of the S
content may also be 0.001%, 0.002%, or 0.003%.
[0065] Cu: 0.01 to 0.70%
[0066] Cu (copper) contributes to a rise in hardness through
enhancement of the hardenability, so the Cu content is made 0.01%
or more. The lower limit of the Cu content is preferably 0.10%,
more preferably 0.20%. However, excessive addition of Cu causes a
drop in toughness or fracture of the slab after casting or a drop
in the weldability, so the upper limit of the Cu content is made
0.70%. Preferably, the upper limit of the Cu content is made 0.60%,
more preferably it is made 0.50%.
[0067] Ni: 0.01 to 1.00%
[0068] Ni (nickel) contributes to a rise in hardness through
enhancement of the hardenability and, further, contributes to
enhance of the toughness, so the lower limit of the Ni content is
made 0.01%. The preferable Ni content is 0.10% or more, while the
more preferable Ni content is 0.30% or more. Excessive addition of
Ni invites a rise in the costs, so the upper limit of the Ni
content is made 1.00%. Preferably, the upper limit of the Ni
content is made 0.90%, more preferably is made 0.80%.
[0069] Cr: 0 to 1.50%
[0070] Cr (chromium) is an element contributing to a rise in
hardness through enhancement of the hardenability. The lower limit
of the Cr content is 0%, but to reliably obtain this effect, the
lower limit of the Cr content is preferably made 0.01%. Making it
0.05% is more preferable. However, if the Cr content is more than
1.50%, the toughness and weldability are made to fall. Therefore,
the upper limit of the Cr content is made 1.50%. Preferably, the
upper limit of the Cr content is made 1.00%, more preferably it is
made 0.95%.
[0071] Mo: 0 to 0.80%
[0072] Mo (molybdenum) is an element contributing to a rise in
hardness through enhancement of the hardenability. The lower limit
of the Mo content is 0%, but to reliably obtain this effect, the
lower limit of the Mo content is preferably made 0.01%. Making it
0.05% is more preferable. However, if the Mo content is more than
0.80%, the toughness and weldability are made to fall. Therefore,
the upper limit of the Mo content is made 0.80%. Preferably, the
upper limit of the Mo content is made 0.60%, more preferably it is
made 0.55%.
[0073] W: 0 to 0.50%
[0074] W (tungsten) is an element contributing to a rise in
hardness through enhancement of the hardenability. The lower limit
of the W content is 0%, but to reliably obtain this effect, the
lower limit of the W content is preferably made 0.001%. Making it
0.01% is more preferable while making it 0.05% is further more
preferable. However, excessive addition of W causes the toughness
and weldability to fall. Therefore, the upper limit of the content
is made 0.50%. Preferably, the upper limit of the content is made
0.08%, more preferably the lower limit of the content is made 0.07%
or 0.06%.
[0075] Nb: 0 to 0.050%
[0076] Nb (niobium) is an element contributing to a rise in
hardness through enhancement of the hardenability. The lower limit
of the Nb content is 0%, but to reliably obtain this effect, the
lower limit of the Nb content is preferably made 0.001% or more.
Making it 0.005% is more preferable. On the other hand, if
excessively adding Nb, the toughness and weldability are made to
fall. Therefore, the upper limit of the Nb content is made 0.050%.
Preferably, the upper limit of the Nb content is made 0.040%, more
preferably it is made 0.030%.
[0077] V: 0 to 0.20%
[0078] V (vanadium) is an element contributing to a rise in
hardness through enhancement of the hardenability and precipitation
strengthening. The lower limit of the V content is 0%, but to
reliably obtain this effect, the lower limit of the V content is
preferably made 0.001%. Making it 0.010% is more preferable. On the
other hand, excessive addition of V causes the toughness and
weldability to fall. Therefore, the upper limit of the V content is
made 0.20%. Preferably, the upper limit of the V content is made
0.15%, more preferably it is made 0.10%.
[0079] Ti: 0 to 0.030%
[0080] Ti (titanium) is an element which forms TiN to fix the N in
the steel. The lower limit of the Ti content is 0%, but to reliably
obtain this effect, the lower limit of the Ti content is preferably
made 0.001%. Further, TiN has the effect of refining the austenite
grains before hot rolling by the pinning effect, so the lower limit
of the Ti content is more preferably made 0.005%. On the other
hand, if the Ti content is more than 0.030%, coarse TiN is formed
and the toughness is impaired, so the upper limit of the Ti content
is made 0.030%. Preferably, the upper limit of the Ti content is
made 0.020%, more preferably the upper limit of the Ti content is
made 0.015%.
[0081] B: 0 to 0.0030%
[0082] B (boron) is an element contributing to a rise in hardness
through enhancement of the hardenability. Further, it is an element
segregating at the grain boundaries and strengthening the grain
boundaries to improve the toughness. The lower limit of the B
content is 0%, but to reliably obtain this effect, the lower limit
of the B content is preferably made 0.0001%. Making it 0.0005% is
more preferable. On the other hand, excessive addition of B causes
the toughness and weldability to fall. Therefore, the upper limit
of the B content is made 0.0030%. Preferably, the upper limit of
the B content is made 0.0015%, more preferably it is made
0.0010%.
[0083] N: 0.0001 to 0.0070%
[0084] N (nitrogen) is an element which forms TiN and contributes
to refining the metal structure and precipitation strengthening.
Therefore, the lower limit of the N content is made 0.0001%.
Preferably, the lower limit of the N content is made 0.0010%, more
preferably it is made 0.0020%. However, if the N content becomes
excessive, the toughness falls and becomes a cause of surface
cracking at the time of casting or strain aging of the steel
material produced and the resultant poor quality, so the upper
limit of the N content is made 0.0070%. Preferably, the upper limit
of the N content is made 0.0050%, more preferably it is made
0.0040%.
[0085] Al: 0.001 to 0.10%
[0086] Al (aluminum) is necessary as a deoxidizing element in the
present invention. The lower limit of the Al content for obtaining
the effect of deoxidizing is made 0.001%. The lower limit of the Al
content is preferably made 0.010%, while making it 0.030% is more
preferable. On the other hand, if excessively adding Al, the Al
oxides coarsen and become base points for brittle fracture and the
toughness falls, so the upper limit of the Al content is made
0.10%. Preferably, the upper limit of the Al content is made
0.080%, more preferably it is made 0.070%.
[0087] Ca: 0 to 0.0050%
[0088] Ca (calcium) is an element effective for control of the
morphology of sulfides. It suppresses the formation of coarse MnS
and contributes to improvement of the toughness. The lower limit of
the Ca content is 0%, but to reliably obtain this effect, the lower
limit of the Ca content is preferably made 0.0001%. Making it
0.0010% is more preferable. On the other hand, if the Ca content
becomes more than 0.0050%, sometimes the toughness falls, so the
upper limit of the Ca content is made 0.0050%. The preferable upper
limit of the Ca content is 0.0030%, while the more preferable upper
limit of the Ca content is 0.0025%.
[0089] Zr: 0 to 0.0050%
[0090] Zr (zirconium) precipitates as carbides and nitrides and
contributes to precipitation strengthening of the steel. The lower
limit of the Zr content is 0%, but to reliably obtain this effect,
the lower limit of the Zr content is preferably made 0.0001%.
Making it 0.0010% is more preferable. On the other hand, if the Zr
content is more than 0.0050%, coarsening of the carbides and
nitrides of Zr is invited and the toughness sometimes falls, so the
upper limit of the Zr content is made 0.0050%. The preferable upper
limit of the Zr content is 0.0030%, while the more preferable upper
limit of the Zr content is 0.0020%.
[0091] Mg: 0 to 0.0050%
[0092] Mg (magnesium) contributes to improvement of the toughness
of the base material and toughness of the weld HAZ. The lower limit
of the Mg content is 0%, but to reliably obtain this effect, the
lower limit of the Mg content is preferably made 0.0001%. Making it
0.0005% is more preferable. On the other hand, even if adding more
than 0.0050% of Mg, the above effect is saturated, so the upper
limit of the Mg content is made 0.0050%. The preferable upper limit
of the content of Mg is 0.0040%, while the more preferable upper
limit is 0.0030%.
[0093] REM: 0 to 0.0050%
[0094] An REM (rare earth metal) contributes to improvement of the
toughness of the base material and toughness of the weld HAZ. The
lower limit of the REM content is 0%, but to reliably obtain this
effect, the lower limit of the REM content is preferably made
0.0001%. Making it 0.0005% is more preferable. On the other hand,
even if adding more than 0.0050% of an REM, the above effect is
saturated, so the upper limit of the REM content is made 0.0050%.
The preferable upper limit of the content of an REM is 0.0040%,
while the more preferable upper limit is 0.0030%.
[0095] In the abrasion resistant steel of the present invention,
the balance besides the above elements is comprised of Fe and
impurities. Here, "impurities" are elements which enter due to
various factors in the production process from ore, scraps, and
other such raw materials when industrially producing abrasion
resistant steel.
[0096] Physical Properties of Abrasion Resistant Steel
[0097] Next, the reasons for limiting the area ratios of the metal
structures and the prior austenite average grain size will be
explained. In the present invention, the area ratios of the metal
structures and the prior austenite average grain size are measured
at a position of 1/4 of the thickness from the surface of the steel
plate in the thickness direction.
[0098] Area Ratios of Metal Structures
[0099] The area ratios of the metal structures of the abrasion
resistant steel according to the present invention are measured by
examining a sample taken from a position of 1/4 of the thickness
from the surface of the steel plate in the thickness direction by a
scan electron microscope (SEM) after corrosion by a Nital solution.
Specifically, on an image of the corroded sample captured by the
SEM, 20.times.20 straight lines are drawn at 10 .mu.m intervals
vertically and horizontally. Whether the metal structures at the
positions of the lattice points are martensite, lower bainite, or
upper bainite is judged. Next, from the results of judgment, the
total of the area ratios of martensite and lower bainite (area %)
at a position of 1/4 of the thickness from the surface in a
thickness direction is calculated. Here, in this Description,
"upper bainite" means the bainite with cementite present at the
interfaces of the laths (between the laths) while "lower bainite"
means the bainite with cementite present inside the laths. "Laths"
are metal structures formed inside the prior austenite grain
boundaries by martensite transformation. According to the present
invention, to obtain abrasion resistant steel having low
temperature toughness, the total of the area ratios of the
martensite and lower bainite of the metal structures at a position
of 1/4 of the thickness from the surface of the steel material in a
thickness direction has to be 50 to 100%. If the total of the area
ratios of the martensite and lower bainite is less than 50%, the
toughness falls. Further, the upper limit of the total of the area
ratios of the martensite and lower bainite is 100%, but may also be
99% or 98%. The lower limit of the total of the area ratios of
martensite and lower bainite is preferably 60%, more preferably
70%, 80%, 90%, or 95%. The lower limit of the area ratio of the
martensite may also be made 50%. If necessary, the lower limit of
the area ratio of the martensite may also be made 70%, 80%, or 90%.
The upper limit of the area ratio of the martensite may also be
made 100% or 95%.
[0100] Prior Austenite Average Grain Size
[0101] To determine the prior austenite average grain size in metal
structures of the abrasion resistant steel according to the present
invention, the cutting method (JIS G0551: 2013) is employed.
Specifically, first, a sample taken from a position of 1/4 of the
thickness from the surface in a thickness direction is corroded in
a picric acid solution to expose the prior austenite grain
boundaries. Next, the sample is photographed by an optical
microscope and a length 2 mm to 10 mm straight test line (may also
be divided into a plurality of sections) is drawn on the
photographed image. The number of crystal grain boundaries which
the test line crosses is counted. Next, the length of the test line
is divided by the number of crystal grain boundaries which the test
line crosses so as to find the average line segment length (that
is, average line segment length=test line length/number of crystal
grain boundaries which test line crosses) to thereby calculate the
prior austenite average grain size at a position of 1/4 of the
thickness from the surface in a thickness direction. According to
the present invention, to obtain abrasion resistant steel having
low temperature toughness, the prior austenite average grain size
at a position of 1/4 of the thickness from the surface of the steel
plate in the thickness direction has to be 23 .mu.m or less. If the
prior austenite average grain size is more than 23 .mu.m, the
toughness falls. Preferably, the prior austenite average grain size
is 20 .mu.m or less, more preferably 18 .mu.m or less. Further, to
prevent the hardenability from falling, the lower limit value of
the prior austenite average grain size is made 5 .mu.m. Preferably,
the prior austenite average grain size is 7 .mu.m or more, more
preferably 9 .mu.m or more or 11 .mu.m.
[0102] In the abrasion resistant steel obtained by the method for
production explained later, direct quenching by water cooling right
after hot rolling is not employed, so there are no stretched prior
austenite grains compared with case of direct quenching. For this
reason, the average aspect ratio of the prior austenite grains at a
position of 1/4 of the thickness from the surface of the steel
plate in the thickness direction may be made 2.0 or less. This
average aspect ratio is more preferably 1.5 or less, still more
preferably 1.2 or less. In the present invention, the "average
aspect ratio of the prior austenite grains" is the average value of
the aspect ratio of the prior austenite grains at a position of 1/4
of the thickness from the surface of the steel plate in the
thickness direction. However, the number of prior austenite grains
measured was made 50 grains. Here, the aspect ratio of a certain
single prior austenite grain is found by dividing a length of the
prior austenite grain in the rolling direction by the length of the
prior austenite grain in the plate thickness direction. The lengths
of the prior austenite grains in the rolling direction and plate
thickness direction can be measured by using an optical microscope
to examine a surface of the steel plate including the thickness
direction and rolling direction (surface vertical to width
direction of steel plate), that is, an L-cross-section.
[0103] Brinell Hardness
[0104] The hardness of the steel is shown by the Brinell hardness.
The abrasion resistant steel according to the present invention has
a Brinell hardness of 360 to 440. The measurement position of the
Brinell hardness is a position of 1 mm from the steel material
surface in the thickness direction. However, the measured surface
is a surface parallel to the steel material surface. On this
surface, the Brinell hardness is measured at three points. The
average value is made the Brinell hardness of the present
invention. The Brinell hardness is measured based on JIS Z2243:2008
using a diameter 10 mm cemented carbide sphere of an indenter and a
3000 kgf test force (HBW10/3000). The abrasion resistant steel
according to the present invention has a Brinell hardness of
preferably 370 or more, more preferably 380 or more, still more
preferably 390 or more.
[0105] Absorption Energy of Charpy Impact Test at -40.degree.
C.
[0106] The toughness of steel can be shown by the absorption energy
of a Charpy impact test. For example, if evaluated by a Charpy
impact test at -40.degree. C., the abrasion resistant steel
according to the present invention has an absorption energy of 27 J
or more. The Charpy impact test is performed at -40.degree. C.
based on JIS Z2242: 2005 using a Charpy test piece taken from a
position of 1/4 of the thickness from the surface in a thickness
direction for evaluating the low temperature toughness. The
absorption energy of the Charpy impact test at -40.degree. C. of
the abrasion resistant steel according to the present invention is
preferably 40 J or more, more preferably 50 J or more, still more
preferably 60 J or more, most preferably 70 J or more. The upper
limit does not have to be particularly set, but may be 400 J or 300
J.
[0107] The abrasion resistant steel according to the present
invention, that is, abrasion resistant steel having the
above-mentioned chemical composition and having the metal
structures at a position of 1/4 of the thickness in a thickness
direction from the surface which have a total of the area ratios of
the martensite and lower bainite of 50 to 100% and a prior
austenite average grain size of 5 to 23 .mu.m, has a 27 J or more
absorption energy of the Charpy impact test at -40.degree. C.
Further, the total of the area ratios of the martensite and lower
bainite at the position of 1/4 of the thickness is 50 to 100%. The
steel has a Brinell hardness of 360 to 440 at a position of 1 mm
from the surface in the thickness direction.
[0108] Thickness of Abrasion Resistant Steel
[0109] The thickness (plate thickness) of the abrasion resistant
steel is not particularly restricted. For example, it may be 15 mm
or more, 20 mm or more, 30 mm or more, or 40 mm or more and 100 mm
or less, 90 mm or less, 80 mm or less, or 70 mm or less. According
to the present invention, by controlling the temperature at the
time of hot rolling, the rolling reduction, and the temperature at
the time of end of hot rolling and furthermore controlling the
temperature of the reheating and quenching, regardless of the plate
thickness, it is possible to suitably refine the prior austenite
grains and secure sufficient hardenability. More specifically, by
hot rolling while controlling the temperature and rolling
reduction, cooling, then reheating at a suitable temperature, it is
possible to increase the nucleation sites of austenite back
transformation. As a result, after back transformation to austenite
is completed, it becomes possible to suitably refine the prior
austenite grains inside of the steel material. This effect can be
obtained regardless of the thickness of the steel plate. For
example, the invention can also be applied to the case, difficult
in the past, of a large plate thickness (for example, 15 mm or
more, in particular, 40 mm or more). The shape of the abrasion
resistant steel does not have to be particularly limited, but may
be steel plate.
[0110] Method for Producing Abrasion Resistant Steel
[0111] Next, one example of the method for producing the abrasion
resistant steel according to the present invention will be
explained.
[0112] The method for producing the slab used for producing the
abrasion resistant steel according to the present invention is not
particularly limited. For example, the slab can be obtained by
casting molten steel after adjusting its chemical composition. The
thickness of the slab is preferably made 200 mm or more from the
viewpoint of productivity. Further, if considering reduction of
segregation, homogenization of the heating temperature before hot
rolling, etc., the thickness of the slab is preferably 350 mm or
less. Such a slab can be used in the method for producing abrasion
resistant steel according to the present invention explained
below.
[0113] Heating Step
[0114] Next, before performing the hot rolling, the slab is heated
to 1000 to 1350.degree. C. If the heating temperature of the slab
is less than 1000.degree. C., sometimes the alloy elements can no
longer form solid solutions, so the lower limit is made
1000.degree. C. On the other hand, if the heating temperature of
the slab becomes higher than 1350.degree. C., the scale on the
surface of the slab used as the material liquefies and can obstruct
production, so the upper limit is made 1350.degree. C.
[0115] Note that, before this heating, it is also possible apply
heating at 1100 to 1350.degree. C. for the purpose of forming solid
solutions of the alloy elements or reducing segregation.
[0116] Hot Rolling Step
[0117] In the present invention, to raise the density of austenite
nucleation at the time of reheating by refinement and flattening of
the prior austenite grains after hot rolling, the heated slab is
hot rolled at 1000 to over 825.degree. C. by a 20% or more rolling
reduction. If this rolling reduction falls under 20%, sometimes the
prior austenite grains become insufficiently refined after hot
rolling and the toughness falls. The rolling reduction at 1000 to
over 825.degree. C. is preferably 25% or more, more preferably 30%
or more. Note that to prevent a drop in the hardenability due to
excessive refinement of the austenite grains at the time of
reheating and quenching, the upper limit of the rolling reduction
at 1000 to over 825.degree. C. is preferably made 75% or less.
Further, to leave rolling strain after hot rolling and after
cooling and raise the density of austenite nucleation at the time
of reheating, furthermore, the hot rolling is performed at 825 to
730.degree. C. by a 10% or more rolling reduction. If this rolling
reduction falls under 10%, sometimes the prior austenite grains are
insufficiently refined after hot rolling and the toughness falls.
The rolling reduction at 825 to 730.degree. C. is preferably 15% or
more, more preferably 20% or more. Note that to prevent a drop in
the hardenability due to excessive refinement of the austenite
grains at the time of reheating and quenching, the upper limit of
the rolling reduction at 825 to 730.degree. C. is preferably made
80%. Furthermore, in the present invention, the temperature at the
time of the end of the hot rolling is 730.degree. C. or more. If
the temperature at the time of the end of the hot rolling becomes
less than 730.degree. C., sometimes the prior austenite grains are
excessively refined after reheating and quenching, the
hardenability falls, and the hardness becomes insufficient. The
temperature at the time of the end of the hot rolling is preferably
740.degree. C. or more, 750.degree. C. or more, or 760.degree. C.
or more. Further, the temperature at the time of the end of the hot
rolling is preferably 820.degree. C. or less, 810.degree. C. or
less, 800.degree. C. or less, 790.degree. C. or less, or
780.degree. C. or less.
[0118] Due to the hot rolling step according to the present
invention, it is possible to refine and flatten the austenite
grains at the time of hot rolling. Due to this, it becomes possible
to increase the nucleation sites of austenite back transformation
at the time of reheating when quenching after hot rolling.
Accordingly, even if the thickness of the steel plate is great, it
is possible to suitably refine the prior austenite grains inside
the steel plate and thereby becomes possible to secure high
hardness and low temperature toughness.
[0119] Cooling Step
[0120] Next, the hot rolled steel plate is cooled in the
atmosphere. By not using water cooling, it is possible to greatly
keep down shape defects in the steel plate. If water cooling,
sometimes hydrogen embrittlement becomes a problem. Cooling need
only be performed until for example 400.degree. C.
[0121] Reheating and Quenching Step
[0122] Next, the steel plate cooled after hot rolling is reheated
to an 860.degree. C. or more temperature then acceleratedly cooled
(water cooled) to quench it. That is, the steel plate obtained by
performing this process is a reheated quenched material (RQ
material). If the reheating temperature becomes less than
860.degree. C., the alloy elements may insufficiently form solid
solutions and the austenite back transformation may not be 100%
completed so the hardenability may fall, therefore the lower limit
of the reheating temperature is made 860.degree. C. If the
reheating temperature is too high, coarsening of the austenite
grains may cause a drop in the toughness after quenching, so the
upper limit of the reheating temperature is preferably 930.degree.
C. The quenching being performed by a cooling speed of 5.degree.
C./sec or more is preferable in securing the hardness and
toughness. The steel plate obtained by performing this step (RQ
material) can be made finer in prior austenite grains compared with
steel plate obtained by direct quenching (DQ material) not
including reheating and quenching. Further, compared with a DQ
material, it is sometimes possible to reduce the average aspect
ratio of the prior austenite grains.
[0123] The abrasion resistant steel produced by hot rolling and
quenching under the above conditions has excellent hardness and low
temperature toughness. Specifically, such abrasion resistant steel
has a Brinell hardness of 360 to 440 and has an absorption energy
of 27 J or more of the Charpy impact test at -40.degree. C. at a
position of 1/4 of the thickness from the surface in a thickness
direction. Further, the method for producing the abrasion resistant
steel according to the present invention does not require advanced
steelmaking art and enables reduction of the production load and
shortening of the work period. Therefore, it is possible to improve
the reliability of construction machines etc. without detracting
from their economicalness. The contribution to industry is
extremely remarkable.
EXAMPLES
[0124] Steels having the chemical compositions shown in Table 1
were smelted and continuously cast to produce thickness 240 to 300
mm slabs. The steels were smelted in a converter then primary
deoxidized. Alloy elements were added to adjust the chemical
compositions and vacuum degassing was performed in accordance with
need. The thus obtained slabs were heated and hot rolled, cooled,
then quenched to produce steel plates. Note that Production Nos. 57
and 58 (comparative examples) were DQ materials which were
acceleratedly cooled (water cooled) right after hot rolling (were
not reheated). The contents of the elements shown in Table 1 were
found by chemically analyzing samples taken from the produced
steels.
TABLE-US-00001 TABLE 1 Chemical Composition of Steel Sample
Chemical Content (mass %) composition no C Si Mn P S Cu Ni Cr Mo W
Nb V Ti B 1 0.200 0.03 1.94 0.012 0.008 0.10 0.05 0.024 2 0.195
0.05 1.89 0.010 0.007 0.20 0.20 0.81 0.72 0.048 0.191 0.020 3 0.180
0.15 1.70 0.009 0.007 0.51 0.81 1.39 0.69 0.39 0.030 0.128 0.017
0.0023 4 0.180 0.20 1.59 0.007 0.005 0.61 0.94 0.009 5 0.161 1.14
1.40 0.004 0.007 0.39 0.64 0.60 0.31 0.012 0.0012 6 0.160 0.87 1.32
0.003 0.009 0.30 0.55 0.010 7 0.142 0.11 0.49 0.015 0.002 0.49 0.68
1.20 0.79 0.035 0.149 0.003 0.0011 8 0.139 0.30 1.20 0.014 0.002
0.69 0.80 0.015 9 0.130 0.25 1.05 0.013 0.002 0.25 0.40 1.00 0.60
0.015 0.035 0.012 0.0010 10 0.129 0.41 0.80 0.013 0.008 0.30 0.89
0.80 0.50 0.20 0.010 0.038 0.011 11 0.110 0.45 1.50 0.016 0.008
0.25 0.50 0.009 12 0.111 0.50 1.15 0.004 0.007 0.22 0.30 0.99 0.040
0.060 0.007 0.0012 13 0.110 0.39 0.71 0.005 0.007 0.20 0.15 1.10
0.60 0.030 0.010 0.0010 14 0.101 0.60 1.70 0.004 0.004 0.40 0.69
0.010 15 0.100 0.32 1.30 0.005 0.003 0.25 0.45 0.79 0.038 0.030
0.015 0.0012 16 0.125 0.24 1.05 0.012 0.002 0.25 0.42 0.97 0.59
0.015 0.034 0.011 0.0011 17 0.140 0.14 0.48 0.016 0.003 0.50 0.70
1.11 0.80 0.034 0.143 0.009 0.0010 18 0.160 0.87 1.32 0.003 0.009
0.30 0.55 0.010 19 0.181 0.21 1.61 0.007 0.006 0.59 0.96 0.010 20
0.179 0.20 1.60 0.008 0.004 0.55 0.99 0.009 21 0.091 0.05 0.80
0.010 0.008 0.20 0.20 0.009 22 0.210 0.34 1.32 0.011 0.008 0.30
0.11 1.01 0.62 0.030 0.120 0.012 0.0012 23 0.145 1.25 1.39 0.009
0.009 0.20 0.09 1.00 0.55 0.040 0.011 0.0012 24 0.155 0.70 2.21
0.011 0.010 0.30 0.10 0.99 0.45 0.20 0.030 0.041 0.014 25 0.154
0.71 1.56 0.018 0.009 0.20 0.10 0.011 26 0.149 0.68 1.60 0.012
0.014 0.22 0.18 0.50 0.015 27 0.160 0.68 1.50 0.014 0.008 0.78 0.19
0.51 0.20 0.040 0.010 28 0.130 0.25 1.10 0.009 0.007 0.25 0.40 1.00
0.59 0.013 0.033 0.009 0.0010 29 0.129 0.24 1.12 0.008 0.007 0.25
0.38 0.99 0.58 0.014 0.033 0.033 0.0009 30 0.125 0.25 1.08 0.012
0.006 0.23 0.39 0.88 0.46 0.020 0.032 0.012 0.0008 31 0.131 0.24
1.01 0.010 0.003 0.40 0.38 Chemical Content (mass %) composition no
N Al Ca Zr Mg REM Remarks 1 0.0010 0.072 Inv. ex. 2 0.0059 0.060 3
0.0070 0.054 0.0039 0.0025 4 0.0022 0.043 5 0.0017 0.088 0.0030 6
0.0029 0.023 7 0.0045 0.033 0.0020 8 0.0041 0.030 9 0.0030 0.050 10
0.0032 0.031 0.0025 11 0.0020 0.019 12 0.0033 0.021 13 0.0038 0.029
14 0.0029 0.033 15 0.0030 0.028 16 0.0032 0.049 0.0020 17 0.0051
0.030 0.0020 Ce: 0.0025 18 0.0029 0.023 0.0031 19 0.0029 0.040
0.0015 Y: 0.0030 20 0.0032 0.038 0.0032 La: 0.0019 21 0.0030 0.050
Comp. ex. 22 0.0032 0.040 23 0.0028 0.032 24 0.0019 0.028 25 0.0020
0.023 26 0.0030 0.013 27 0.0044 0.029 28 0.0035 0.121 29 0.0044
0.039 30 0.0082 0.042 0.0021 31 0.0032 0.044 Inv. ex. Empty fields
mean elements not intentionally added. Underlines indicate outside
scope of present invention.
[0125] The heating temperature, hot rolling, and other production
conditions of a slab at the time of production, the Brinell
hardness of the produced sample, the total of the area ratios of
the martensite and lower bainite at a position of 1/4 of the
thickness from the surface in a thickness direction, the prior
austenite average grain size at a position of 1/4 of the thickness
from the surface in a thickness direction, and the value of the
absorption energy of the Charpy impact test at -40.degree. C. at a
position of 1/4 of the thickness from the surface in a thickness
direction are shown in Table 2 and Table 3.
[0126] The total of the area ratios of the martensite and lower
bainite of the metal structures at a position of 1/4 of the
thickness from the surface in a thickness direction, as explained
above, can be judged by observation of the steel slab using a SEM
after corrosion by a Nital solution. Specifically, on an image
captured by the SEM, 20.times.20 straight lines were drawn at 10
.mu.m intervals vertically and horizontally. Whether the metal
structures at the positions of the lattice points were martensite,
lower bainite, or upper bainite was judged and the total of the
area ratios of the martensite and lower bainite was calculated by
the area %.
[0127] The prior austenite average grain size at a position of 1/4
of the thickness from the surface in a thickness direction, as
explained above, was found by corroding a steel slab by a picric
acid solution to expose the prior austenite grain boundaries,
drawing a length 2 mm to 10 mm straight test line (may also be
divided into a plurality of sections) on the image photographed by
the optical microscope, and counting the number of crystal grain
boundaries which the test line crossed. Next, the length of the
test line was divided by the number of crystal grain boundaries
which the test line crossed so as to find the average line segment
length and thereby calculate the prior austenite average grain
size. Further, in all of the examples according to the present
invention, the average aspect ratio of the prior austenite grain
sizes was 2.0 or less.
[0128] The Charpy impact test was performed based on JIS Z2242:
2005 at -40.degree. C. The Brinell hardness was measured at a
position of 1 mm from the surface in the thickness direction based
on JIS Z2243: 2008 using a cemented carbide sphere of a diameter of
10 mm of an indenter by a test force of 3000 kgf (HBW10/3000).
[0129] The target values of the hardness and toughness of the
abrasion resistant steel according to the present invention were a
Brinell hardness of 360 to 440 and an absorption energy of the
Charpy impact test at -40.degree. C. of 27 J or more.
TABLE-US-00002 TABLE 2 Production Conditions and Physical
Properties of Abrasion Resistant Steel (1) 1000.degree. C. to
825.degree. C. Produced over 825.degree. C. or less Chem. plate
Heating rolling rolling Rolling end Reheating Prod. comp. thickness
temp. reduction reduction temp. temp. no. no. (mm) (.degree. C.)
(%) (%) (.degree. C.) (.degree. C.) 1 1 12 1200 50 32 801 900 2 2
12 1200 50 32 805 900 3 3 20 1150 40 35 789 900 4 4 20 1150 40 35
788 900 5 5 30 1150 42 29 735 900 6 6 30 1150 42 29 738 900 7 7 50
1250 48 25 759 910 8 8 50 1250 48 25 762 910 9 9 70 1150 30 20 799
900 10 9 70 1150 17 13 810 900 11 9 70 1150 24 5 805 900 12 10 70
1150 30 20 800 900 13 11 40 1100 45 19 810 890 14 12 40 1100 45 19
810 890 15 12 60 1150 29 18 802 840 16 13 60 1150 29 18 798 890 17
14 50 1150 48 25 767 920 18 15 50 1150 48 25 777 920 19 16 70 1150
30 20 803 900 20 17 30 1250 48 25 765 910 21 18 70 1150 42 29 744
900 22 19 20 1150 40 35 790 900 23 20 20 1150 40 35 786 900 24 21
50 1150 48 25 790 900 25 22 50 1150 48 25 785 900 26 23 40 1150 45
19 810 900 27 24 40 1150 45 19 820 900 28 25 40 1150 45 19 822 900
29 26 40 1150 45 19 815 900 30 27 40 1150 45 19 817 900 31 28 40
1150 45 19 808 900 32 29 40 1150 45 19 817 900 33 30 40 1150 45 19
778 900 Total of area ratios of martensite and lower bainite Prior
austenite grain at position of size at position of Charpy 1/4 in
1/4 in absorption thickness thickness energy Prod. Brinell
direction direction (vE-40.degree. C.) no. hardness (%) (.mu.m) (J)
Remarks 1 408 59 15 110 Inv. ex. 2 430 80 14 170 Inv. ex. 3 422 94
14 177 Inv. ex. 4 402 60 13 99 Inv. ex. 5 418 78 10 181 Inv. ex. 6
403 58 9 123 Inv. ex. 7 422 98 15 169 Inv. ex. 8 400 72 16 103 Inv.
ex. 9 418 99 19 210 Inv. ex. 10 419 99 27 24 Comp. ex. 11 424 99 30
18 Comp. ex. 12 435 89 18 167 Inv. ex. 13 389 67 17 95 Inv. ex. 14
415 82 15 155 Inv. ex. 15 354 42 13 15 Comp. ex. 16 428 100 21 201
Inv. ex. 17 387 67 18 89 Inv. ex. 18 411 78 18 178 Inv. ex. 19 422
99 17 234 Inv. ex. 20 418 97 14 189 Inv. ex. 21 399 60 8 145 Inv.
ex. 22 401 62 12 120 Inv. ex. 23 397 59 11 123 Inv. ex. 24 352 65
18 198 Comp. ex. 25 450 98 19 20 Comp. ex. 26 412 97 15 26 Comp.
ex. 27 418 99 15 18 Comp. ex. 28 392 88 17 25 Comp. ex. 29 423 84
18 20 Comp. ex. 30 410 91 17 17 Comp. ex. 31 417 86 14 26 Comp. ex.
32 428 93 16 14 Comp. ex. 33 412 94 13 9 Comp. ex. Underlines
indicate outside scope of present invention.
TABLE-US-00003 TABLE 3 Production Conditions and Physical
Properties of Abrasion Resistant Steel (2) 1000.degree. C.
825.degree. C. Produced to over 825.degree. C. or less Chem. plate
Heating rolling rolling Rolling end Reheating Prod. comp. thickness
temp. reduction reduction temp. temp. Brinell no. no. (mm)
(.degree. C.) (%) (%) (.degree. C.) (.degree. C.) hardness 34 31 70
1150 30 20 790 900 420 35 9 60 1150 29 18 722 900 355 36 9 60 1150
29 0 835 900 429 37 9 70 1150 40 32 769 900 415 38 9 50 1150 35 35
758 900 413 39 9 40 1150 48 25 759 900 410 40 9 20 1150 60 30 760
900 414 41 17 70 1150 40 32 770 890 408 42 17 50 1150 35 35 767 890
409 43 17 40 1150 48 25 767 890 402 44 17 20 1150 60 30 760 890 404
45 9 70 1150 40 32 813 900 420 46 9 50 1150 35 35 804 900 419 47 9
40 1150 48 25 804 900 418 48 9 20 1150 60 30 805 900 418 49 9 70
1150 52 0 850 900 421 50 9 50 1150 45 0 848 900 418 51 9 40 1150 56
0 852 900 421 52 9 20 1150 72 0 846 900 420 53 17 70 1150 40 0 844
890 417 54 17 50 1150 45 0 849 890 418 55 17 40 1150 56 0 852 890
419 56 17 20 1150 72 0 838 890 418 57 9 40 1150 55 0 890 No 401
reheating 58 9 40 1150 48 25 770 No 351 reheating Total of area
ratios of martensite and lower bainite Prior austenite grain at
position of size at position of Charpy 1/4 in 1/4 in absorption
thickness thickness energy Prod. direction direction (vE-40.degree.
C.) no. (%) (.mu.m) (J) Remarks 34 99 17 219 Inv. ex. 35 90 4 249
Comp. ex. 36 99 26 25 Comp. ex. 37 87 16 220 Inv. ex. 38 85 15 232
Inv. ex. 39 88 13 210 Inv. ex. 40 83 11 240 Inv. ex. 41 97 15 210
Inv. ex. 42 95 14 199 Inv. ex. 43 96 13 221 Inv. ex. 44 96 11 229
Inv. ex. 45 99 20 138 Inv. ex. 46 99 17 154 Inv. ex. 47 100 16 145
Inv. ex. 48 97 14 158 Inv. ex. 49 100 37 9 Comp. ex. 50 99 34 19
Comp. ex. 51 99 33 22 Comp. ex. 52 99 27 24 Comp. ex. 53 99 35 10
Comp. ex. 54 99 34 16 Comp. ex. 55 98 31 20 Comp. ex. 56 98 26 26
Comp. ex. 57 67 65 21 Comp. ex. 58 48 39 25 Comp. ex. Underlines
indicate outside scope of present invention.
[0130] As shown in Table 2 and Table 3, the invention examples of
Production Nos. 1 to 9, Nos. 12 to 14, and Nos. 16 to 23, 34, and
37 to 48 had chemical compositions, heating temperatures, rolling
reductions by hot rolling at 1000 to over 825.degree. C., rolling
reductions by hot rolling at 825 to 730.degree. C., temperatures at
the end of hot rolling, and reheating temperatures satisfying the
scopes of the present invention. As a result, the totals of the
area ratios of the martensite and lower bainite and the prior
austenite average grain sizes were within the scopes of the present
invention, the Brinell hardnesses were within the range of 360 to
440 targeted by the present invention, and the absorption energies
of the Charpy impact test at -40.degree. C. satisfied the 27 J or
more targeted by the present invention.
[0131] On the other hand, Production No. 10, No. 11, No. 15, and
Nos. 24 to 33 of Table 2 and Production No. 35, No. 36, and Nos. 49
to 58 of Table 3 failed to satisfy the above targets for one or
both of the Brinell hardness and absorption energy of the Charpy
impact test at -40.degree. C.
[0132] Production No. 10 is an example in which the rolling
reduction at 1000 to over 825.degree. C. was low, so the prior
austenite average grain size at a position of 1/4 of the thickness
from the surface in a thickness direction was more than 23 .mu.m
resulting in the absorption energy of the Charpy impact test at
-40.degree. C. failing to reach the target value.
[0133] Production No. 11 is an example in which the rolling
reduction at 825 to 730.degree. C. was low, so the prior austenite
average grain size at a position of 1/4 of the thickness from the
surface in a thickness direction was more than 23 .mu.m resulting
in the absorption energy of the Charpy impact test at -40.degree.
C. failing to reach the target value.
[0134] Production No. 15 is an example in which the reheating
temperature was less than 860.degree. C., so the hardenability fell
and in which the total of the area ratios of the martensite and
lower bainite became less than 50%, whereby the Brinell hardness
and absorption energy of the Charpy impact test at -40.degree. C.
failed to reach the target values.
[0135] Production No. 24 is an example in which the C content was
small and in which the Brinell hardness failed to reach the target
value. Further, Production No. 25 is an example in which the C
content was great and the Brinell hardness and absorption energy of
the Charpy impact test at -40.degree. C. failed to reach the target
values. Furthermore, Production No. 26 is an example where the Si
content was great, Production No. 27 is an example where the Mn
content was great, Production No. 28 is an example where the P
content was great, Production No. 29 is an example where the S
content was great, Production No. 30 is an example where the Cu
content was great, Production No. 31 is an example where the Al
content was great, Production No. 32 is an example where the Ti
content was great, and Production No. 33 is an example where the N
content was great, so in each of the steel samples, the absorption
energy of the Charpy impact test at -40.degree. C. failed to reach
the target value.
[0136] Further, Production No. 35 of Table 3 is an example in which
the temperature at the end of the hot rolling was less than
730.degree. C., so the prior austenite grains after reheating and
quenching were excessively refined, the hardenability fell, and the
Brinell hardness failed to reach the target value.
[0137] Production No. 36 and Nos. 49 to 56 are examples in which
the rolling reductions at 825 to 730.degree. C. were 0%, so the
prior austenite grain sizes after reheating and quenching exceeded
23 .mu.m and thereby the absorption energies of the Charpy impact
tests at 40.degree. C. failed to satisfy the target values.
[0138] Production No. 57 is an example in which the rolling
reduction at 825 to 730.degree. C. was 0% and reheating and
quenching was not performed (was a DQ material), so the prior
austenite grain size exceeded 23 .mu.m and thereby the absorption
energy of the Charpy impact test at -40.degree. C. failed to reach
the target value.
[0139] Production No. 58 is an example in which reheating and
quenching was not performed (was a DQ material), so the prior
austenite grain size exceeded 23 .mu.m and the total of the area
ratios of martensite and lower bainite became less than 50%. Due to
this, the Brinell hardness and absorption energy of the Charpy
impact test at -40.degree. C. failed to reach the target
values.
INDUSTRIAL APPLICABILITY
[0140] According to the present invention, it is possible to obtain
abrasion resistant steel having excellent low temperature toughness
able to be used even in cold regions.
* * * * *