U.S. patent application number 14/781440 was filed with the patent office on 2016-02-04 for steel sheet for nitriding and production method therefor.
The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Yoshimasa Funakawa, Takashi Kobayashi, Hayato Saito.
Application Number | 20160032431 14/781440 |
Document ID | / |
Family ID | 51657992 |
Filed Date | 2016-02-04 |
United States Patent
Application |
20160032431 |
Kind Code |
A1 |
Kobayashi; Takashi ; et
al. |
February 4, 2016 |
STEEL SHEET FOR NITRIDING AND PRODUCTION METHOD THEREFOR
Abstract
A steel sheet for nitriding has excellent formability and
punchability. The steel sheet has a composition including, in
percent by mass, 0.02% to 0.08% of C, 0.1% or less of Si, 0.2% to
1.8% of Mn, 0.05% or less of P, 0.02% or less of S, 0.01% to 0.06%
of Al, 0.5% to 1.5% of Cr, 0.01% or less of N, and the balance
being Fe and incidental impurities; and has a microstructure
including ferrite as a main phase and pearlite and/or bainite as a
secondary phase. The ferrite has a fraction of 70% or more in the
entire microstructure and an average grain diameter of 5 to 25 gm.
An average length of the major axis of cementite present in the
secondary phase in a cross section in the rolling direction of the
steel sheet is 3.0 .mu.m or less.
Inventors: |
Kobayashi; Takashi; (Tokyo,
JP) ; Saito; Hayato; (Tokyo, JP) ; Funakawa;
Yoshimasa; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
51657992 |
Appl. No.: |
14/781440 |
Filed: |
March 20, 2014 |
PCT Filed: |
March 20, 2014 |
PCT NO: |
PCT/JP2014/001603 |
371 Date: |
September 30, 2015 |
Current U.S.
Class: |
148/602 ;
148/333 |
Current CPC
Class: |
C22C 38/00 20130101;
C22C 38/38 20130101; C22C 38/02 20130101; C23C 8/26 20130101; C21D
2211/002 20130101; C22C 38/26 20130101; C21D 8/0263 20130101; C21D
8/0226 20130101; C22C 38/04 20130101; C22C 38/06 20130101; C22C
38/24 20130101; C21D 2211/005 20130101; C23C 8/32 20130101; C22C
38/28 20130101; C21D 6/005 20130101; C21D 2211/009 20130101; C22C
38/36 20130101; C21D 6/008 20130101; C21D 9/46 20130101; C22C
38/001 20130101; C21D 6/002 20130101 |
International
Class: |
C22C 38/38 20060101
C22C038/38; C21D 9/46 20060101 C21D009/46; C21D 6/00 20060101
C21D006/00; C22C 38/28 20060101 C22C038/28; C22C 38/00 20060101
C22C038/00; C22C 38/24 20060101 C22C038/24; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C21D 8/02 20060101 C21D008/02; C22C 38/26 20060101
C22C038/26 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 2, 2013 |
JP |
2013-076824 |
Claims
1.-4. (canceled)
5. A steel sheet for nitriding having a composition and a
microstructure: the composition comprising in percent by mass,
0.02% to 0.08% of C, 0.1% or less of Si; 0.2% to 1.8% of Mn, 0.05%
or less of P; 0.02% or less of S, 0.01% to 0.06% of Al; 0.5% to
1.5% of Cr, 0.01% or less of N; and the balance being Fe and
incidental impurities; and a microstructure including ferrite as a
main phase and pearlite and/or bainite as a secondary phase,
wherein the ferrite has a fraction of 70% or more in the entire
microstructure and an average grain diameter of 5 to 25 .mu.m, and
cementite present in the secondary phase has an average length of a
major axis of 3.0 .mu.m or less in a cross section in a rolling
direction of the steel sheet.
6. The steel sheet for nitriding according to claim 5, wherein the
composition further comprises, in percent by mass, at least one
selected from the group consisting of 0.005% to 0.075% of V, 0.005%
to 0.025% of Nb, and 0.005% to 0.025% of Ti.
7. A method of producing a steel sheet for nitriding, comprising:
heating a steel slab having a composition including, in percent by
mass, 0.02% to 0.08% of C, 0.1% or less of Si, 0.2% to 1.8% of Mn,
0.05% or less of P, 0.02% or less of S, 0.01% to 0.06% of Al, 0.5%
to 1.5% of Cr, 0.01% or less of N, and the balance being Fe and
incidental impurities, to 1,050.degree. C. to 1,250.degree. C.; hot
rolling the heated steel slab at a finishing temperature ranging
from the Ar.sub.3 transformation temperature to (the Ar.sub.3
transformation temperature+100.degree. C.); cooling the hot rolled
steel sheet at a cooling rate of 40.degree. C./s to 80.degree. C./s
in the temperature range from the finishing temperature to
750.degree. C. at a cooling rate of 15.degree. C./s to 35.degree.
C./s in the temperature range from 750.degree. C. to a cooling stop
temperature of 500.degree. C. to 650.degree. C.; and coiling the
cooled steel sheet at a coiling temperature of 500.degree. C. to
650.degree. C.
8. The method according to claim 7, wherein the composition of the
steel slab further includes, in percent by mass, at least one
selected from the group consisting of 0.005% to 0.075% of V, 0.005%
to 0.025% of Nb, and 0.005% to 0.025% of Ti.
9. The method according to claim 7, wherein the steel sheet has a
microstructure including ferrite as a main phase and pearlite
and/or bainite as a secondary phase, wherein the ferrite has a
fraction of 70% or more in the entire microstructure and an average
grain diameter of 5 to 25 .mu.m, and cementite present in the
secondary phase has an average length of a major axis of 3.0 .mu.m
or less in a cross section in a rolling direction of the steel
sheet.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a steel sheet for nitriding which
is used by being subjected to nitriding treatment to improve
durability and which is suitable as a material for machine parts,
in particular, a steel sheet for nitriding having excellent
formability and punchability before nitriding treatment, and a
method of producing the same. Background
[0002] In machine parts used for automotive transmissions and the
like, to improve fatigue strength and abrasion resistance, steel as
a material is formed into a desired part shape and then often
subjected to a surface-hardening treatment before use. Typical
examples of such a surface-hardening treatment are carburizing
treatment and nitriding treatment.
[0003] Carburizing treatment is the most common surface-hardening
treatment. However, in carburizing treatment, usually, carbon is
diffused and infiltrated (carburized) into the surface portion of
steel at a temperature of the A.sub.3 transformation point or more,
and then the steel is subjected to quenching. Consequently, under
the influence of distortion caused by quenching from the high
temperature, a decrease in shape accuracy of parts is unavoidable.
Furthermore, in the as-quenched state after carburizing, the
toughness of the steel is markedly decreased. Accordingly, after
quenching, it is necessary to perform tempering to recover
toughness and perform correction of the shape of parts. Therefore,
when a carburizing treatment is employed, the number of processes
required to manufacture parts increases, resulting in an increase
in manufacturing costs, which is disadvantageous.
[0004] In contrast, in nitriding treatment, usually, steel is
heated to a temperature of about 500.degree. C. to 600.degree. C.
that is lower than the A.sub.1 transformation temperature so that
nitrogen is diffused and infiltrated (nitrided) into the surface
portion of steel, and the surface of steel is hardened without
quenching, unlike the carburizing treatment. That is, in nitriding
treatment, the treatment temperature is relatively low, and
treatment is not accompanied by phase transformation of steel
during cooling. Therefore, a decrease in shape accuracy of parts
due to transformation strain does not occur, which is advantageous.
Furthermore, the volume variation of the surface portion of steel
due to nitriding is small, and it is easy to maintain good shape
accuracy of parts, which is also advantageous.
[0005] In nitriding using ammonia gas, according to existing
techniques, since the time required for nitriding is significantly
long, the nitriding treatment is not suitable for automotive parts
and the like that are supposed to be mass-produced. However, in
recent years, a nitriding treatment referred to as nitrocarburizing
in which a nitriding reaction is allowed to proceed rapidly by
utilizing a carburizing atmosphere has become common. Thus, the
significantly long treatment time, which is the problem in the
existing techniques, is in the process of being solved.
[0006] In nitrocarburizing treatment, by holding an object to be
treated in a treatment atmosphere at 550.degree. C. to 600.degree.
C. for several hours, nitrogen is diffused and introduced into
steel from the surface of steel by an iron carbide generating
reaction. In accordance with the nitrocarburizing treatment,
although the surface hardness obtained after the treatment is lower
than that in the existing nitriding treatment, the time required
for nitriding can be markedly reduced. For the reason described
above, in recent years, the number of cases in which
nitrocarburizing treatment is employed as the surface-hardening
treatment, as an alternative to carburizing treatment, has been
increasing.
[0007] On the other hand, according to existing techniques, machine
parts used for automotive transmissions and the like are generally
manufactured by subjecting intermediate products obtained by
casting or forging to machining However, in recent years, thin
steel sheets have been used as a material for machine parts, and
machine parts have been manufactured by subjecting thin steel
sheets to press working or the like to have a desired shape. The
reason for this is that, by replacing parts manufactured by
subjecting intermediate products obtained by casting or forging to
machining in the existing techniques with sheet-metal working
products of steel sheets, shortening of manufacturing processes and
reduction in manufacturing costs can be achieved. Under such a
background, there has been an increased demand for steel sheets for
nitriding having excellent formability as a steel material for the
machine parts.
[0008] Regarding a steel sheet for nitriding having excellent
formability, various techniques have been proposed. For example,
Japanese Unexamined Patent Application Publication No. 9-25513 and
Japanese Unexamined Patent Application Publication No. 9-25543
propose techniques for producing a steel sheet for nitriding in
which a steel having a composition including, in weight ratio,
0.01% to less than 0.08% of C, 0.005% to 1.00% of Si, 0.010% to
3.00% of Mn, 0.001% to 0.150% of P, 0.0002% to 0.0100% of N, more
than 0.15% to 5.00% of Cr, more than 0.060% to 2.00% of Al, and one
or two of Ti and V is hot-rolled, followed by coiling at
500.degree. C. or higher, and then optionally subjected to cold
rolling at a rolling reduction of 50% or more and subjected to
perform recrystallization annealing. In accordance with those
techniques, by producing a low-carbon steel sheet in which the
content of C that adversely affects formability is suppressed to
less than 0.08% and, at the same time, which contains Al, Cr, Ti
and/or V which are nitriding accelerating elements, it is possible
to obtain a steel sheet for nitriding having excellent formability
and nitriding property.
[0009] Furthermore, Japanese Unexamined Patent Application
Publication No. 2005-171331 proposes a technique on a steel sheet
for nitrocarburizing in which the steel sheet has a composition
including 0.01% to 0.10% by mass of C, 0.1% by mass or less of Si,
0.1% to 1.0% by mass of Mn, 0.05% by mass or less of P, 0.01% by
mass or less of S, 0.01% to 0.06% by mass of Al, 0.05% to 0.50% by
mass of Cr, 0.01% to 0.30% by mass of V, 0.01% by mass or less of
N, and the balance being Fe and incidental impurities. In
accordance with the technique proposed in Japanese Unexamined
Patent Application Publication No. 2005-171331, it is possible to
obtain a steel sheet for nitrocarburizing in which the production
cost is low because of reduced amounts of alloy elements, which has
excellent formability, and which can be used to produce a steel
sheet having an excellent surface hardening property by
nitrocarburizing treatment because of simultaneous addition of Cr
and V which are nitriding accelerating elements.
[0010] When machine parts used for automotive transmissions and the
like are manufactured by subjecting a thin steel sheet to forming,
in many cases, the thin steel sheet is blanked to a predetermined
size prior to forming, and holes with various shapes are pierced
after forming. Accordingly, the steel sheet as the material for the
parts is required to have excellent formability and excellent
punchability. When the punchability of the steel sheet is degraded,
occurrence of sags, burrs, and the like in punched surfaces during
punching increases markedly, which impairs the dimensional accuracy
of the machine parts. There may be cases where microcracks are
likely to occur in punched surfaces, thus adversely affecting the
strength property of the machine parts.
[0011] However, in the existing techniques described above, no
consideration is given to punchability of the steel sheets.
Furthermore, the problems described below remain.
[0012] In the techniques proposed in Japanese Unexamined Patent
Application Publication No. 9-25513 and Japanese Unexamined Patent
Application Publication No. 9-25543, the composition includes a
large amount of Al as a nitriding accelerating element. Therefore,
there is a concern that inner defects and surface defects resulting
from Al-containing inclusions may occur, and generation of a large
amount of Al-containing slag increases refining costs during
smelting.
[0013] In accordance with the technique proposed in Japanese
Unexamined Patent Application Publication No. 2005-171331, even if
the amounts of alloy elements for nitriding acceleration are
decreased, a sufficient hardening property can be imparted to a
steel sheet for nitrocarburizing. However, the resulting steel
sheet has insufficient strength, and it is difficult to apply the
technique to heavily-loaded parts.
[0014] It could therefore be helpful to provide a steel sheet for
nitriding which can be widely used as a material for parts such as
automotive transmissions, and which has excellent formability and
excellent punchability before nitriding treatment, and a method of
producing the same.
SUMMARY
[0015] We found that by adjusting the chemical composition and
microstructure of a steel sheet to certain ranges, a good hardening
property can be provided by nitriding treatment, and it is possible
to impart sufficient formability and punchability to a steel sheet
before nitriding treatment.
[0016] We thus provide:
[0017] [1] A steel sheet for nitriding having a composition and
microstructure; the composition including, in percent by mass,
0.02% to 0.08% of C, 0.1% or less of Si, 0.2% to 1.8% of Mn, 0.05%
or less of P, 0.02% or less of S, 0.01% to 0.06% of Al, 0.5% to
1.5% of Cr, 0.01% or less of N, and the balance being Fe and
incidental impurities, and the microstructure including ferrite as
a main phase and pearlite and/or bainite as a secondary phase,
wherein the ferrite has an area fraction of 70% or more in the
entire microstructure and an average grain diameter of 5 to 25
.mu.m, and cementite present in the secondary phase has an average
length of the major axis of 3.0 .mu.m or less in a cross section in
the rolling direction of the steel sheet.
[0018] [2] The steel sheet for nitriding according to [1], wherein
the composition further includes, in percent by mass, at least one
selected from the group consisting of 0.005% to 0.075% of V, 0.005%
to 0.025% of Nb, and 0.005% to 0.025% of Ti.
[0019] [3] A method of producing a steel sheet for nitriding
including:
[0020] heating a steel slab having a composition including, in
percent by mass, 0.02% to 0.08% of C, 0.1% or less of Si, 0.2% to
1.8% of Mn, 0.05% or less of P, 0.02% or less of S, 0.01% to 0.06%
of Al, 0.5% to 1.5% of Cr, 0.01% or less of N, and the balance
being Fe and incidental impurities, to 1,050.degree. C. to
1,250.degree. C.;
[0021] hot rolling the heated steel slab at a finishing temperature
ranging from the Ar.sub.3 transformation temperature to (the
Ar.sub.3 transformation temperature+100.degree. C.);
[0022] cooling the hot rolled steel sheet at a cooling rate of
40.degree. C./s to 80.degree. C./s in the temperature range from
the finishing temperature to 750.degree. C. and at a cooling rate
of 15.degree. C./s to 35.degree. C./s in the temperature range from
750.degree. C. to a cooling stop temperature of 500.degree. C. to
650.degree. C.; and coiling the cooled steel sheet at a coiling
temperature of 500.degree. C. to 650.degree. C.
[0023] [4] The method of producing a steel sheet for nitriding
according to [3], wherein the composition of the steel slab further
includes, in percent by mass, at least one selected from the group
consisting of 0.005% to 0.075% of V, 0.005% to 0.025% of Nb, and
0.005% to 0.025% of Ti.
[0024] It is possible to obtain a steel sheet having excellent
formability and punchability and a good hardening property by
nitriding treatment. The steel sheet is very suitable as a material
for formed parts to be subjected to nitriding treatment such as
automotive transmission parts, thus exhibiting industrially marked
effects. Furthermore, the steel sheet is not limited to being used
for gas nitrocarburizing treatment and salt bath nitrocarburizing
treatment, but can also be suitably used as any of various steel
sheets for nitriding such as plasma nitriding, gas nitriding,
carbonitriding, and nitrosulphurizing.
DETAILED DESCRIPTION
[0025] First, the microstructure of a steel sheet for nitriding
will be described. The steel sheet has a microstructure including
ferrite (which may also be referred to as "polygonal ferrite"),
which is a main phase, and a secondary phase. The secondary phase
includes pearlite and/or bainite. Furthermore, the fraction of the
ferrite in the entire microstructure is 70% or more, the average
grain diameter of the ferrite is 5 to 25 .mu.m, and the average
length of the major axis of cementite present in the secondary
phase in a cross section in the rolling direction of the steel
sheet is 3.0 .mu.m or less.
Main Phase: Ferrite
[0026] In the steel sheet, by using soft ferrite as a main phase,
formability of the steel sheet can be secured. When a material
other than ferrite is used as the main phase, it is not possible to
impart good formability to the steel sheet. However, in a steel
sheet having a ferrite single-phase microstructure, it is not
possible to secure sufficient strength as a steel sheet material
that can be widely used for automotive transmission parts and the
like. Therefore, the steel sheet has a microstructure including
ferrite as the main phase, and the secondary phase described
below.
Secondary Phase: Pearlite and/or Bainite
[0027] The secondary phase, which is the remainder other than
ferrite, includes one or two selected from the group consisting of
pearlite and bainite. The secondary phase in the steel sheet
microstructure has a role in reinforcing the strength of the steel
sheet having soft ferrite as the main phase. When reinforcement of
a microstructure having martensite as the secondary phase is
employed, martensite is softened by an increase in temperature
during nitriding treatment, resulting in an increased variation in
the strength of the steel sheet. Therefore, to stably maintain the
strength of the steel sheet even after being subjected to nitriding
treatment in which the steel sheet is held at about 500.degree. C.
to 600.degree. C., the secondary phase in the steel sheet
microstructure is required to be composed of pearlite and/or
bainite.
Area Fraction of Ferrite in the Entire Microstructure: 70% or
More
[0028] It is necessary to set the area fraction of the ferrite,
which is the main phase, to be 70% or more to impart good
formability to the steel sheet. When the area fraction of the
ferrite is less than 70%, formability of the steel sheet is likely
to be at an insufficient level. Furthermore, punchability of the
steel sheet decreases. For example, during punching of the steel
sheet, the sheared surface ratio of punched surfaces decreases. On
the other hand, when the area fraction of the ferrite is
excessively high, the strength of the steel sheet may not reach the
necessary level. Therefore, the area fraction of the ferrite is
preferably 97% or less, and more preferably 95% or less.
Average Grain Diameter of Ferrite: 5 to 25 .mu.m
[0029] When the average grain diameter of the ferrite is more than
25 .mu.m, the surface properties of the steel sheet may be degraded
during forming, and the smoothness of punched surfaces may be
degraded, resulting in degradation of punchability of the steel
sheet. Furthermore, when the grain diameter of the ferrite becomes
coarse, the number of grain boundaries decreases. Consequently,
grain boundary diffusion of N during nitriding treatment is
suppressed, and there is a concern that the time required for the
nitriding treatment may be elongated. On the other hand, when the
average grain diameter of the ferrite is less than 5 .mu.m, the
steel sheet hardens, and formability is likely to be degraded.
Therefore, the average grain diameter of the ferrite is 5 to 25
.mu.m, and preferably 5 to 15 .mu.m.
Average Length of the Major Axis of Cementite Present in the
Secondary Phase in a Cross Section in the Rolling Direction of the
Steel Sheet: 3.0 .mu.m or Less
[0030] When the average length of the major axis of cementite
present in the secondary phase in a cross section in the rolling
direction of the steel sheet exceeds 3.0 .mu.m, the stress
concentration ratio increases at the interface between the
cementite and the ferrite during punching of the steel sheet, which
makes it easy to cause microcracks, and the fracture surface ratio
at punched surfaces increases. Thus, punchability of the steel
sheet is degraded. Therefore, the average length of the major axis
is 3.0 .mu.m or less. However, when the cementite becomes extremely
small, microcracks are likely to occur in punched surfaces of the
steel sheet. Therefore, the average length of the major axis is
preferably 1.0 .mu.m or more.
[0031] Reasons for limiting the chemical composition of the steel
sheet will be described below. Hereinafter, "%", which is the unit
of measure for the constituent element content, means "percent by
mass" unless otherwise indicated.
C: 0.02% to 0.08%
[0032] C is an element having an effect of increasing the strength
of steel through solid solution strengthening and formation of the
secondary phase. When the C content is less than 0.02%, it is not
possible to secure a sufficient strength of the steel sheet as a
material for parts. On the other hand, when the C content is
exceeded 0.08%, the strength of the steel sheet increases
excessively, resulting in a decrease in formability. Furthermore,
the fraction of the secondary phase increases, and it is difficult
to obtain cementite having a desired form. Therefore, the C content
is 0.02% to 0.08%, and preferably 0.04% to 0.06%.
Si: 0.1% or Less
[0033] Si is an element effective in deoxidizing steel, and also
has an effect of strengthening steel by solid solution
strengthening. The Si content is preferably 0.01% or more to obtain
these effects. However, when the Si content exceeds 0.1%, hardly
eliminable scale is generated during hot rolling, and surface
properties of the steel sheet degrade markedly. Therefore, the Si
content is 0.1% or less, and preferably 0.05% or less.
Mn: 0.2% to 1.8%
[0034] Mn is an element that strengthens steel by solid solution
strengthening. Furthermore, Mn has an effect of fixing, as
precipitates, S which is present as an impurity in the steel, thus
reducing adverse effects caused by S. When the Mn content is less
than 0.2%, the effects cannot be obtained sufficiently, and it is
not possible to secure the required strength of the steel sheet. On
the other hand, when the Mn content is more than 1.8%, the strength
of the steel sheet increases excessively, and a band-like
microstructure due to microsegregation is likely to be formed,
resulting in degradation in formability and punchability of the
steel sheet. Therefore, the Mn content is 0.2% to 1.8%, and
preferably 0.2% to 1.2%.
P: 0.05% or Less
[0035] P is an element present as an impurity in the steel and,
when the P content is large, formability and toughness of the steel
sheet degrade. Therefore, the P content is 0.05% or less, and
preferably 0.03% or less.
S: 0.02% or Less
[0036] S is also an element present as an impurity in the steel
and, when the S content is large, formability and toughness of the
steel sheet degrade. Therefore, the S content is 0.02% or less, and
preferably 0.01% or less.
Al: 0.01% to 0.06%
[0037] Al is an element added for the purpose of deoxidizing steel.
When the Al content in the steel is less than 0.01%, it is not
possible to obtain a sufficient deoxidizing effect. On the other
hand, when the Al content in the steel is more than 0.06%, the
deoxidizing effect is saturated, and there is a possibility that
inner defects and surface defects will increase due to an increase
in inclusions in the steel. Therefore, the Al content is 0.01% to
0.06%, and preferably 0.02% to 0.05%.
Cr: 0.5% to 1.5%
[0038] Cr has an effect of increasing the hardness of the surface
portion of the steel sheet by forming nitrides in the steel by
nitriding treatment, and is an important alloy element. Cr also has
an effect of refining cementite in the steel. In terms of
sufficient exhibition of such effects, it is necessary to set the
Cr content at 0.5% or more. However, when the Cr content exceeds
1.5%, the hardened portion of the outermost layer is significantly
embrittled by nitriding treatment, and the depth of the hardened
portion may be decreased in some cases. Therefore, the Cr content
is 0.5% to 1.5%, and preferably 0.5% to 1.0%.
N: 0.01% or Less
[0039] N is an element present as an impurity in the steel. A large
amount of N degrades formability of the steel sheet, and there is a
possibility that N will combine with nitriding accelerating
elements such as Cr, before nitriding treatment, thus degrading the
hardening property due to nitriding. Therefore, the N content is
0.01% or less, and preferably 0.005% or less.
[0040] The steel sheet may contain, in addition to the composition
described above, one or two or more selected from the group
consisting of 0.005% to 0.075% of V, 0.005% to 0.025% of Nb, and
0.005% to 0.025% of Ti.
V: 0.005% to 0.075%
[0041] V is an element having an effect of increasing the hardness
of the surface portion of the steel sheet by forming nitrides in
the steel by nitriding treatment. Furthermore, since V is a
carbide/nitride forming element, V also has an effect of increasing
the strength of steel by particle dispersion strengthening
(precipitation strengthening). Accordingly, V can be incorporated
for the purpose of controlling the hardening property due to
nitriding treatment and adjusting the level of strength of the
steel sheet. In terms of sufficient exhibition of such effects, the
V content is preferably 0.005% or more. On the other hand, when the
V content is excessively large, formability is decreased by an
excessive increase in the strength of the steel sheet, the hardened
portion is embrittled by nitriding treatment, and also an
economical disadvantage is caused. Therefore, the V content is
preferably 0.005% to 0.075%, and more preferably 0.025% to
0.075%.
Nb: 0.005% to 0.025%
[0042] Nb is a carbide/nitride forming element having an effect of
increasing the strength of steel by particle dispersion
strengthening (precipitation strengthening). When the Nb content is
less than 0.005%, such an effect cannot be sufficiently obtained.
On the other hand, when the Nb content is more than 0.025%, there
is a concern that the strength of the steel sheet will increase
excessively, resulting in a decrease in formability. Therefore, the
Nb content is preferably 0.005% to 0.025%, and more preferably
0.005% to 0.015%. Ti: 0.005% to 0.025%
[0043] Ti is also a carbide/nitride forming element having an
effect of increasing the strength of steel by particle dispersion
strengthening (precipitation strengthening). When the Ti content is
less than 0.005%, such an effect cannot be sufficiently obtained.
On the other hand, when the Ti content is more than 0.025%, there
is a concern that the strength of the steel sheet may increase
excessively, resulting in a decrease in formability. Therefore, the
Ti content is preferably 0.005% to 0.025%, and more preferably
0.005% to 0.015%.
[0044] The balance other than the components described above
includes Fe and incidental impurities. As the incidental
impurities, 0.03% or less of Cu, 0.03% or less of Ni, 0.03% or less
of Mo, 0.003% or less of Sn, 0.003% or less of Sb, 0.005% or less
of 0, and the like are permissible.
[0045] A method of producing a steel sheet for nitriding will be
described below.
[0046] A steel sheet can be obtained by heating, hot rolling,
cooling, and coiling steel having the chemical composition
described above.
[0047] The steel can be refined by any of known refining processes
such as a converter process or an electric furnace process. The
refined steel is formed into a steel (slab) by continuous casting
or ingot casting and bloom rolling or the like. As necessary,
preliminary treatments, secondary smelting, cleaning of the steel
surface can be performed.
Heating Temperature of Steel: 1,050.degree. C. to 1,250.degree.
C.
[0048] When the heating temperature of the steel is lower than
1,050.degree. C., it is difficult to secure a desired finishing
temperature during hot rolling. On the other hand, when the heating
temperature of the steel exceeds 1,250.degree. C., energy required
for heating increases and defects in the surface properties of the
steel sheet are likely to occur. Therefore, the heating temperature
of the steel before hot rolling is 1,050.degree. C. to
1,250.degree. C., and preferably 1,100.degree. C. to 1,200.degree.
C.
[0049] When the steel is heated, the steel cooled to normal
temperature may be reheated, or the steel being cooled after
casting may be subjected to additional heating or
heat-retained.
[0050] After the steel is heated to the temperature range described
above, rough rolling and finish rolling (hot rolling) are
performed. The rough rolling may be performed under ordinary
conditions, and rough rolling conditions are not particularly
limited.
Finishing Temperature: Ar.sub.3 Transformation Temperature to (the
Ar.sub.3 Transformation Temperature+100.degree. C.)
[0051] When the finishing temperature in the hot rolling step is
lower than the Ar.sub.3 transformation temperature, an
un-recrystallized ferrite microstructure which is elongated in the
rolling direction and a pancake-shaped, coarse ferrite
microstructure are formed, and it is not possible to obtain ferrite
with a desired grain diameter. Furthermore, formability and
punchability of the steel sheet are degraded. Moreover, the
in-plane anisotropy of mechanical properties of the steel sheet is
increased. On the other hand, when the finishing temperature
exceeds (the Ar.sub.3 transformation temperature+100.degree. C.),
surface properties of the steel sheet are likely to be degraded,
the ferrite microstructure is likely to be coarsened, and it is
difficult to obtain ferrite with a desired grain diameter.
Therefore, the finishing temperature is the Ar.sub.3 transformation
temperature to (the Ar.sub.3 transformation temperature+100.degree.
C.), and preferably (the Ar.sub.3 transformation
temperature+20.degree. C.) to (the Ar.sub.3 transformation
temperature+100.degree. C.). The steel sheet under rolling may be
subjected to additional heating using a heating device such as a
sheet bar heater or an edge heater to secure a necessary finishing
temperature.
Cooling Rate from Finishing Temperature to 750.degree. C.:
40.degree. C./s to 80.degree. C./s
[0052] The steel sheet which has been subjected to hot rolling is
cooled (forced cooled) in the temperature range from the finishing
temperature to 750.degree. C. at a cooling rate of 40.degree. C./s
to 80.degree. C./s, and preferably 45.degree. C./s to 75.degree.
C./s. When the cooling rate in the temperature range is less than
40.degree. C./s, the microstructure of the hot-rolled steel sheet
is likely to be coarsened, and it is not possible to obtain ferrite
or cementite having a desired shape. On the other hand, when the
cooling rate in the temperature range is more than 80.degree. C./s,
martensite or excessively large amounts of bainite and pearlite are
likely to be generated in the hot-rolled steel sheet, and it is
difficult to obtain ferrite with a desired fraction or a desired
secondary phase.
Cooling Rate from 750.degree. C. to Cooling Stop Temperature:
15.degree. C./s to 35.degree. C./s
Cooling Stop Temperature: 500.degree. C. to 650.degree. C.
[0053] In the temperature range from 750.degree. C. to a cooling
stop temperature, cooling (forced cooling) is performed at a
cooling rate of 15.degree. C./s to 35.degree. C./s, and preferably
15.degree. C./s to 25.degree. C./s. When the cooling rate in the
temperature range is less than 15.degree. C./s, the microstructure
of the hot-rolled steel sheet is likely to be coarsened, and it is
difficult to obtain ferrite or cementite having a desired shape. On
the other hand, when the cooling rate in the temperature range is
more than 35.degree. C./s, progression of ferrite transformation is
suppressed and it is not possible to obtain ferrite with a desired
fraction.
[0054] When the cooling stop temperature is lower than 500.degree.
C., the steel sheet is hardened because martensite and an
excessively large amount of bainite are generated. As a result,
formability of the steel sheet is degraded and the strength of the
steel sheet after nitriding treatment becomes unstable. On the
other hand, when the cooling stop temperature is higher than
650.degree. C., pearlite is coarsened, and it is not possible to
obtain cementite having a desired shape. Therefore, the cooling
stop temperature is 500.degree. C. to 650.degree. C., and
preferably 500.degree. C. to 600.degree. C.
[0055] The steel sheet cooled to the cooling stop temperature may
be directly coiled, or may be allowed to cool for a short period of
time until it is coiled with a coiler. The term "being allowed to
cool" refers to air cooling in the air in which forced cooling by
pouring water is not performed. However, to remove cooling water
remaining on the steel sheet, it is permissible to spray
high-pressure water or compressed air for a very short period of
time onto the steel sheet which is being allowed to cool because a
decrease in the temperature of the steel sheet due to this is very
small.
Coiling Temperature: 500.degree. C. to 650.degree. C.
[0056] When the coiling temperature is lower than 500.degree. C.,
the steel sheet is hardened because martensite and an excessively
large amount of bainite are generated. As a result, formability of
the steel sheet is degraded and the strength of the steel sheet
after nitriding treatment becomes unstable. On the other hand, when
the coiling temperature is higher than 650.degree. C., pearlite is
coarsened, and it is not possible to obtain cementite having a
desired shape. Therefore, the coiling temperature is 500.degree. C.
to 650.degree. C., and preferably 500.degree. C. to 600.degree.
C.
[0057] The coiled steel sheet is used after scale is removed by
pickling or shot peening. Furthermore, the steel sheet may be
subjected to temper rolling for the purpose of shape straightening
and adjusting surface roughness. Performing such descaling or
temper rolling does not impair the advantages.
EXAMPLES
[0058] Steels A to L containing constituent elements shown in Table
1 and the balance being Fe and incidental impurities were prepared
in molten state. The resulting slabs were subjected to hot rolling
under the conditions shown in Table 2, and thus hot-rolled steel
sheets with a thickness of 2.3 mm were formed. Subsequently, the
resulting hot-rolled steel sheets were descaled by pickling, and
then subjected to temper rolling with an elongation of 0.5%.
Specimens were taken from the hot-rolled steel sheets after temper
rolling, and microstructure observation, a tensile test, and a
punching test were performed. Furthermore, the hot-rolled steel
sheets after temper rolling was subjected to nitriding treatment,
and a hardness test was performed on the hot-rolled steel sheets
after the nitriding treatment.
(1) Microstructure Observation
[0059] A specimen of a cross section in the thickness direction
parallel to the rolling direction at the 1/4 width position was
taken from each of the steel sheets before the nitriding treatment,
and the specimen was subjected to mirror polishing and etched with
nital. Using an image obtained by photographing the 1/4 thickness
position at an appropriate magnification of 500 to 5,000 times with
an optical microscope or a scanning electron microscope, the
microstructure was confirmed.
[0060] Regarding the fraction of ferrite in the microstructure,
using the image described above, the area ratio of ferrite was
obtained by image analysis, which was defined as the fraction of
ferrite.
[0061] Regarding the average grain diameter of ferrite, using the
image described above, grain diameters were determined in
accordance with the method stipulated in Japan Industrial Standard
JIS G 0551-2005, and the average grain diameter was calculated from
the grain size number.
[0062] Regarding the average length of the major axis of cementite
present in the secondary phase (pearlite and/or bainite), using the
image described above, the length of the major axis of each of
grains of cementite in the observation range was obtained, and the
average was calculated by arithmetic average. These results are
also shown in Table 2.
(2) Tensile Test (Evaluation of Formability)
[0063] Formability of the steel sheet was evaluated on the basis of
ductility determined by a tensile test. The tensile test was
carried out in accordance with JIS Z 2241-2011 using a No. 5 test
specimen stipulated in JIS Z 2241-2011 which was taken at the 1/4
width position of a steel sheet such that the testing direction
corresponded to the rolling direction. Tensile strength (TS) and
elongation after fracture (EL) were measured, and a
strength-elongation balance (TS.times.EL) was calculated. The steel
sheet having a strength-elongation balance value of 16 GPa% or more
was evaluated as having good formability.
(3) Punching Test (Evaluation of Punchability)
[0064] A disk-shaped test specimen with a diameter of 50 mm was
punched out from a steel sheet before nitriding treatment
(clearance: 5% of steel sheet thickness), the sheared surface ratio
at the punched surface of the test specimen was measured, and
presence or absence of microcracks in the fracture surface region
was confirmed. When the sheared surface ratio was 60% or more, and
no cracks were observed in the fracture surface region, the steel
sheet was evaluated as having good punchability.
(4) Hardness Test (Evaluation of Surface Hardening Property Due to
Nitriding Treatment)
[0065] A hot-rolled steel sheet after temper rolling was subjected
to gas nitrocarburizing treatment, and the cross section hardness
of the steel sheet after the gas nitrocarburizing treatment
(nitrided layer cross section hardness) was measured. Gas obtained
by mixing ammonia (NH.sub.3) and endothermic converted gas at the
equal volume ratio was used as nitriding gas. The gas
nitrocarburizing treatment temperature was 570.degree. C., and the
holding time at the gas nitrocarburizing treatment temperature was
150 minutes. After holding, oil cooling was performed. Regarding
the cross section hardness of the steel sheet, a specimen of a
cross section in the thickness direction parallel to the rolling
direction was taken from the steel sheet after the gas
nitrocarburizing treatment, and the Vickers hardness (HV0.1) at a
depth of 0.2 mm from the surface of the steel sheet was measured in
accordance with JIS Z 2244-2009. When the measured Vickers hardness
was 250 or more, the surface hardening property of the steel sheet
by nitriding treatment was evaluated to be good.
[0066] The results thereof are shown in Table 3.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) Steel C Si Mn
P S Al Cr N V Nb Ti A 0.02 0.08 1.8 0.02 0.007 0.02 1.5 0.003 -- --
-- B 0.04 0.05 0.3 0.02 0.010 0.05 0.9 0.002 -- -- -- C 0.04 0.03
0.2 0.02 0.005 0.04 0.8 0.002 0.005 -- -- D 0.05 0.03 0.6 0.03
0.005 0.06 1.0 0.002 -- -- -- E 0.05 0.03 0.8 0.05 0.005 0.03 1.0
0.002 -- 0.005 0.008 F 0.06 0.04 1.2 0.02 0.005 0.04 0.7 0.002 --
-- -- G 0.06 0.02 1.0 0.01 0.005 0.05 0.8 0.001 0.065 -- -- H 0.06
0.05 1.2 0.03 0.005 0.05 0.6 0.001 0.050 0.008 -- I 0.08 0.06 1.4
0.01 0.020 0.03 1.2 0.005 -- -- -- J 0.05 0.03 0.7 0.04 0.005 0.04
0.4 0.002 -- -- -- K 0.05 0.03 0.7 0.04 0.005 0.05 1.6 0.002 -- --
-- L 0.12 0.03 1.2 0.02 0.005 0.03 0.6 0.002 -- -- --
TABLE-US-00002 TABLE 2 Hot rolling conditions Steel sheet
microstructure *3 Ar.sub.3 Cooling Cooling F average C average
Steel trans- Heating Finishing rate rate Cooling stop Coiling F
grain length sheet formation temperature temperature 1 *1 2 *2
temperature temperature fraction diameter of major No. Steel point
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) (.degree.
C.) (.degree. C.) (.degree. C.) Type (%) (.mu.m) axis (.mu.m) 1 A
761 1100 860 70 15 650 650 F, P 95 18 3.0 2 A 761 1250 920 70 15
650 650 F, P 95 28 3.5 3 A 761 1100 860 70 10 650 650 F, P 96 27
3.2 4 A 761 1100 860 60 15 690 680 F, P 96 29 3.8 5 B 812 1150 900
75 20 600 600 F, P 92 12 2.6 6 B 812 1000 760 40 20 610 600 F, P 94
26 2.8 7 C 811 1150 900 65 25 600 600 F, P 94 11 2.5 8 C 811 1150
900 35 20 610 600 F, P 93 16 3.1 9 D 807 1250 840 40 30 560 550 F,
P 90 9 2.0 10 D 807 1250 840 80 45 550 540 F, P 68 8 1.6 11 E 806
1250 840 45 30 550 550 F, P 89 7 1.9 12 F 773 1200 860 60 25 580
580 F, P 82 7 1.7 13 F 773 1200 860 100 35 550 540 F, P, B 66 5 1.3
14 G 781 1200 860 60 25 590 580 F, P 78 7 1.5 15 G 781 1200 860 60
35 470 460 F, B, M 62 7 1.2 16 H 791 1200 860 55 25 550 540 F, P 74
6 1.4 17 I 744 1050 820 70 35 500 500 F, B 70 5 1.0 18 I 744 1050
840 65 25 520 520 F, P, B 72 6 1.2 19 J 810 1250 840 45 30 550 550
F, P 89 10 2.4 20 K 800 1250 840 45 30 550 550 F, P 91 8 1.6 21 L
749 1050 840 45 30 560 550 F, P 80 6 3.3 *1) Average cooling rate
in the temperature range from the finishing temperature to
750.degree. C. *2) Average cooling rate in the temperature range
from 750.degree. C. to the cooling stop temperature *3) F: ferrite,
P: pearlite, B: bainite, M: martensite, C: cementite present in
pearlite and/or bainite
TABLE-US-00003 TABLE 3 Formability Strength- Hardening Elongation
elongation Punchability property Tensile after balance Sheared
Nitrided layer Steel strength fracture TS .times. EL surface Cracks
in cross section sheet TS EL (GPa Eval- ratio fracture Eval-
hardness Eval- No. Steel (MPa) (%) %) uation *4 (%) surface uation
*4 HV0.1 (HV0.1) uation *4 Remarks 1 A 384 43.3 16.6 .largecircle.
68 Absent .largecircle. 262 .largecircle. Example of invention 2 A
371 41.8 15.5 X 63 Present X 249 X Comparative example 3 A 377 42.9
16.2 .largecircle. 64 Present X 256 .largecircle. Comparative
example 4 A 369 43.1 15.9 X 65 Present X 243 X Comparative example
5 B 358 46.2 16.5 .largecircle. 65 Absent .largecircle. 285
.largecircle. Example of invention 6 B 345 39.7 13.7 X 48 Present X
278 .largecircle. Comparative example 7 C 362 45.6 16.5
.largecircle. 63 Absent .largecircle. 281 .largecircle. Example of
invention 8 C 356 46.3 16.5 .largecircle. 61 Present X 273
.largecircle. Comparative example 9 D 379 43.9 16.6 .largecircle.
64 Absent .largecircle. 275 .largecircle. Example of invention 10 D
394 41.6 16.4 .largecircle. 58 Absent X 277 .largecircle.
Comparative example 11 E 411 40.7 16.7 .largecircle. 66 Absent
.largecircle. 287 .largecircle. Example of invention 12 F 409 42.8
17.5 .largecircle. 61 Absent .largecircle. 276 .largecircle.
Example of invention 13 F 440 40.6 17.9 .largecircle. 56 Absent X
280 .largecircle. Comparative example 14 G 458 40.2 18.4
.largecircle. 62 Absent .largecircle. 319 .largecircle. Example of
invention 15 G 522 34.7 18.1 .largecircle. 55 Present X 328
.largecircle. Comparative example 16 H 531 34.5 18.3 .largecircle.
60 Absent .largecircle. 337 .largecircle. Example of invention 17 I
603 27.6 16.6 .largecircle. 62 Absent .largecircle. 274
.largecircle. Example of invention 18 I 598 27.9 16.7 .largecircle.
63 Absent .largecircle. 271 .largecircle. Example of invention 19 J
371 44.4 16.5 .largecircle. 59 Absent X 171 X Comparative example
20 K 395 43.0 17.0 .largecircle. 65 Absent .largecircle. 222 X
Comparative example 21 L 587 27.5 16.1 .largecircle. 60 Present X
269 .largecircle. Comparative example *4) .largecircle.: Good, X:
Poor
[0067] Our steel sheets (examples) have good formability, excellent
punchability of the steel sheet, and an excellent surface hardening
property due to nitriding treatment. On the other hand, in other
steel sheets outside of our ranges in terms of the chemical
composition and microstructure (comparative examples), any of or
all of the formability, punchability, and surface hardening
property due to nitriding treatment are at insufficient levels.
[0068] This application claims the benefit of Japanese Patent
Application No. 2013-076824, filed Apr. 2, 2013, which is hereby
incorporated by reference herein in its entirety.
* * * * *