U.S. patent application number 15/031030 was filed with the patent office on 2016-08-18 for hot-rolled steel sheet having excellent surface hardness after carburizing heat treatment and excellent cold workability.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Katsura Kajihara, Takehiro Tsuchida.
Application Number | 20160237515 15/031030 |
Document ID | / |
Family ID | 52992820 |
Filed Date | 2016-08-18 |
United States Patent
Application |
20160237515 |
Kind Code |
A1 |
Kajihara; Katsura ; et
al. |
August 18, 2016 |
HOT-ROLLED STEEL SHEET HAVING EXCELLENT SURFACE HARDNESS AFTER
CARBURIZING HEAT TREATMENT AND EXCELLENT COLD WORKABILITY
Abstract
A hot-rolled steel sheet that: has a sheet thickness of 2-10 mm,
contains specific amounts of C, Mn, Al, and N, the remainder being
iron and unavoidable impurities, and contains ferrite and pearlite
in a specified area ratio, the remainder being bainite. In the
hot-rolled steel sheet, a number of grains having an aspect ratio
of major axis to minor axis of 3 or less is 60% or more of a number
of all grains and an average grain size of the all grains is from 3
to 50 .mu.m.
Inventors: |
Kajihara; Katsura; (Hyogo,
JP) ; Tsuchida; Takehiro; (Hyogo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
52992820 |
Appl. No.: |
15/031030 |
Filed: |
October 17, 2014 |
PCT Filed: |
October 17, 2014 |
PCT NO: |
PCT/JP2014/077742 |
371 Date: |
April 21, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/002 20130101;
C21D 1/74 20130101; C22C 38/00 20130101; C22C 38/44 20130101; C21D
9/46 20130101; C21D 1/18 20130101; C22C 38/001 20130101; C21D 1/06
20130101; C22C 38/02 20130101; C22C 38/105 20130101; C21D 8/0247
20130101; C21D 2211/009 20130101; C21D 2211/005 20130101; C21D
2211/002 20130101; C22C 38/06 20130101; C22C 38/42 20130101; C22C
38/60 20130101; C22C 38/12 20130101; C22C 38/08 20130101; C21D
8/0226 20130101; C23C 8/22 20130101; C22C 38/14 20130101; C22C
38/04 20130101 |
International
Class: |
C21D 1/06 20060101
C21D001/06; C22C 38/42 20060101 C22C038/42; C22C 38/08 20060101
C22C038/08; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C23C 8/22 20060101 C23C008/22; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 8/02 20060101
C21D008/02; C21D 9/46 20060101 C21D009/46; C22C 38/44 20060101
C22C038/44; C22C 38/10 20060101 C22C038/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 2013 |
JP |
2013-219467 |
Claims
1. A hot-rolled steel sheet excellent in cold formability and
surface hardness after a carburizing heat treatment, having: a
sheet thickness of from 2 to 10 mm; a component composition
comprising, in mass % (hereinafter, the same applies to chemical
components), C: from 0.05 to 0.30%, Mn: from 0.3 to 3.0%, Al: from
0.015 to 0.1%, and N: from 0.003 to 0.30%, with the remainder being
iron and unavoidable impurities; and a microstructure comprising,
in area ratio, ferrite: from 10 to 50%, pearlite: from 15 to 50%
and remainder: bainite, wherein with respect to grains of all
phases including the ferrite and the pearlite (hereinafter,
referred to as "all grains"), a number of grains having an aspect
ratio (major axis/minor axis) of 3 or less is 60% or more of a
number of the all grains, and an average grain size of the all
grains is from 3 to 50 .mu.m.
2. The hot-rolled steel sheet according to claim 1, wherein, in the
unavoidable impurities, Si is 0.5% or less, P is 0.030% or less and
S is 0.035% or less.
3. The hot-rolled steel sheet according to claim 1, wherein the
component composition further comprises at least any one of the
following (a) to (f): (a) at least one member selected from the
group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or
less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%); (b)
at least one member selected from the group consisting of Cu: 2.0%
or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%); (c)
at least one member selected from the group consisting of V: 0.5%
or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and
Nb: 0.1% or less (exclusive of 0%); (d) at least one member
selected from the group consisting of Ca: 0.08% or less (exclusive
of 0%) and Zr: 0.08% or less (exclusive of 0%); (e) Sb: 0.02% or
less (exclusive of 0%); and/or (f) at least one member selected
from the group consisting of REM: 0.05% or less (exclusive of 0%),
Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive
of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less
(exclusive of 0%).
4. The hot-rolled steel sheet according to claim 2, wherein the
component composition further comprises at least any one of the
following (a) to (f): (a) at least one member selected from the
group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or
less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%); (b)
at least one member selected from the group consisting of Cu: 2.0%
or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%); (c)
at least one member selected from the group consisting of V: 0.5%
or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and
Nb: 0.1% or less (exclusive of 0%); (d) at least one member
selected from the group consisting of Ca: 0.08% or less (exclusive
of 0%) and Zr: 0.08% or less (exclusive of 0%); (e) Sb: 0.02% or
less (exclusive of 0%); and/or (f) at least one member selected
from the group consisting of REM: 0.05% or less (exclusive of 0%),
Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive
of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less
(exclusive of 0%).
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot-rolled steel sheet
exhibiting good cold formability during processing before a heat
treatment and exhibiting a predetermined surface hardness and a
desired hardness even in a deep portion from the surface after a
carburizing heat treatment. More specifically, it relates to, among
steel materials used as various structural parts, a hot-rolled
steel sheet useful as a material for the manufacture of parts, for
example, clutches, dampers, gears, and the like employed in each
portion of automobiles, etc, which are subjected to a surface
hardening treatment by a carburizing-quenching or
carbonitriding-quenching treatment so as to improve the wear
resistance and anti-fatigue properties. In the following
description, explanations are given by referring typically to a
case of application in clutches but the present invention is of
course not limited to the manufacture thereof and can be
effectively utilized as a material for the manufacture of parts
requiring high surface hardness and excellent impact properties by
taking advantage of excellent carburizing-quenching performance and
carbonitriding-quenching performance thereof to harden the surface
portion while maintaining high toughness in the core portion.
BACKGROUND ART
[0002] In recent years, from the standpoint of environmental
protection, a requirement for weight reduction, i.e., higher
strength, of steel materials for use in various parts for
automotive, for example, transmission parts such as gears, and
casings is more and more increasing with the purpose of enhancing
the fuel efficiency of automobiles. To meet this requirement for
weight reduction and higher strength, a steel material prepared by
hot-forging a steel bar (hot-forged material) has been used as a
commonly-employed steel material (for example, see Patent Document
1). In addition, in order to reduce CO.sub.2 emission amount in the
process of producing parts, a requirement for cold forging of parts
such as gears, which had been heretofore worked by hot forging, is
also increasing.
[0003] Cold working (cold forging) is advantageous in that the
productivity thereof is high compared with hot working and warm
working and moreover, both the dimensional accuracy and the steel
material yield are good. On the other hand, a problem occurring in
the case of producing parts by the cold working is that a steel
material having high strength, i.e., high deformation resistance,
must be necessarily used so as to ensure that the strength of
cold-worked parts is equal to or more than a predetermined value
expected. However, a steel material with a higher deformation
resistance to be used has a disadvantage of leading to a shortening
of the life of a metal mold for cold working.
[0004] Under the above-mentioned background, in the field of
transmission parts, studies has been carried out to switch from the
conventional forged products (e.g., hot-forged and cold-forged) of
steel bars to the manufacture of parts using steel sheets, with an
aim of achieving weight reduction or cost reduction of parts. Among
others, in the parts of which surface is exposed to a contact
pressure, such as gears, dampers and clutches, the surface hardness
is increased by applying a carburizing heat treatment after
machining of a steel sheet into parts, so as to impart wear
resistance and anti-fatigue properties. As the steel sheet for the
manufacture of these parts, a general soft steel (e.g., SPHC) has
been conventionally used, but further higher strength and higher
hardness are demanded.
[0005] High-strength parts assured of predetermined strength and
surface hardness are manufactured by performing a carburizing heat
treatment after cold forming (e.g., press forming) of a steel sheet
into a predetermined shape. In order to increase the hardness of
the carburized surface, it may be thought to increase the amount of
a principal component, mainly the C amount, or of an additive
element, but this causes a reduction in the cold formability before
the heat treatment. Accordingly, a solution capable of achieving
both of ensuring the cold formability and enhancing the surface
hardness after a carburizing heat treatment has been required.
[0006] As described above, the present invention targets a
hot-rolled steel sheet. Conventional techniques related to a
hot-rolled steel sheet include, for example, the following Patent
Documents 2 to 6.
[0007] The hot-rolled steel sheet disclosed in Patent Document 2 is
supposed to have enhanced balance between strength and stretch
flange formability by virtue of a configuration where 70% or more
of the metal microstructure in terms of area percentage is a
ferrite phase, the average grain size thereof is 50 .mu.m or less,
the aspect ratio thereof is 3 or less, 70% or more of the ferrite
grain boundary is composed of a large-angle grain boundary, the
maximum diameter of the ferrite phase formed of the large-angle
grain boundary is 30 .mu.m or less, the area percentage of a
precipitate having a minimum diameter of 5 nm or more is 2% or less
of the metal microstructure, the average grain size of a second
phase having a maximum area percentage among residual phases
excluding the ferrite phase and the precipitate is 50 .mu.m or
less, and the large-angle grain boundary of the ferrite phase is
present between nearest second phases.
[0008] The hot-rolled steel sheet disclosed in Patent Document 3 is
supposed to have enhanced stretch flange formability by virtue of a
configuration where the ferrite average grain size is from 1 to 10
.mu.m, the standard deviation of the ferrite grain size is 3.0
.mu.m or less and the shape ratio of an inclusion is 2.0 or
less.
[0009] The hot-rolled steel sheet disclosed in Patent Document 4 is
supposed to have enhanced stretch flange formability by virtue of a
configuration where the microstructure is a ferrite-bainite
microstructure having a ferrite phase fraction of 50% or more with
the remainder being bainite, and the Mn microsegregation in the
range of 1/8t to 3/8t of the sheet thickness t falls within the
range satisfying 0.10.gtoreq..sigma./Mn.
[0010] The hot-rolled steel sheet disclosed in Patent Document 5 is
supposed to have enhanced elongation and stretch flange formability
by virtue of a configuration where the microstructure contains a
ferrite phase with an area ratio of 20% or more, a tempered
martensite phase with an area ratio of 10 to 60%, a martensite
phase with an area ratio of 0 to 10%, and a retained austenite
phase with a volume percentage of 3 to 15%.
[0011] Although the hot-rolled steel sheets disclosed in Patent
Documents 2 to 5 are excellent in the cold formability, the surface
hardness after a carburizing heat treatment is not referred to at
all, and the improvement effect thereon is unknown.
[0012] On the other hand, the hot-rolled steel sheet (carburized
steel strip) disclosed in Patent Document 6 is supposed to enable
reducing the "shear droop" during stamping and omitting a
carburizing treatment after the stamping by virtue of a
configuration where the average hardness to a depth of 50 .mu.m in
the surface layer part in the sheet thickness direction is 170 HV
or more, the metal microstructure is ferrite+pearlite, and the
difference .DELTA.C=CS-CM between the surface carbon concentration
CS (mass %) and the in-steel average carbon concentration CM (mass
%) is 0.1 mass % or more.
[0013] Although the hot-rolled steel sheet (carburized steel strip)
disclosed in Patent Document 6 is excellent in the surface hardness
after a carburizing heat treatment, cold formability is not
referred to at all, and the improvement effect thereon is
unknown.
[0014] As described above, almost no studies have been heretofore
made on a hot-rolled steel sheet having both cold formability and
surface hardness after a carburizing heat treatment.
PRIOR ART LITERATURE
Patent Documents
[0015] Patent Document 1: Japanese Patent No. 3,094,856
[0016] Patent Document 2: Japanese Patent No. 3,821,036
[0017] Patent Document 3: Japanese Patent No. 4,276,504
[0018] Patent Document 4: Japanese Patent No. 4,644,075
[0019] Patent Document 5: JP-A-2011-168861
[0020] Paten Document 6: JP-A-2010-222663
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0021] The object of the present invention is to provide a
hot-rolled steel sheet having both cold formability and surface
hardness after a carburizing heat treatment. In the present
invention, the carburizing heat treatment encompasses also the case
of a heat treatment for carbonitridation, in addition to for normal
carburization.
Means for Solving the Problems
[0022] The invention according to claim 1 is a hot-rolled steel
sheet excellent in cold formability and surface hardness after a
carburizing heat treatment, having:
[0023] a sheet thickness of from 2 to 10 mm;
[0024] a component composition containing, in mass % (hereinafter,
the same applies to chemical components),
[0025] C: from 0.05 to 0.30%,
[0026] Mn: from 0.3 to 3.0%,
[0027] Al: from 0.015 to 0.1%, and
[0028] N: from 0.003 to 0.30%,
[0029] with the remainder being iron and unavoidable impurities;
and
[0030] a microstructure containing, in area ratio,
[0031] ferrite: from 10 to 50%,
[0032] pearlite: from 15 to 50% and
[0033] remainder: bainite,
in which
[0034] with respect to grains of all phases including the ferrite
and the pearlite (hereinafter, referred to as "all grains"),
[0035] a number of grains having an aspect ratio (major axis/minor
axis) of 3 or less is 60% or more of a number of the all grains,
and an average grain size of the all grains is from 3 to 50
.mu.m.
[0036] The invention according to claim 2 is the hot-rolled steel
sheet according to claim 1, in which, in the unavoidable
impurities, Si is 0.5% or less, P is 0.030% or less and S is 0.035%
or less.
[0037] The invention according to claim 3 is the hot-rolled steel
sheet according to claim 1 or 2, in which the component composition
further contains at least any one of the following (a) to (f):
[0038] (a) at least one member selected from the group consisting
of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive
of 0%) and Ni: 3.0% or less (exclusive of 0%);
[0039] (b) at least one member selected from the group consisting
of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive
of 0%);
[0040] (c) at least one member selected from the group consisting
of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive
of 0%) and Nb: 0.1% or less (exclusive of 0%);
[0041] (d) at least one member selected from the group consisting
of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less
(exclusive of 0%);
[0042] (e) Sb: 0.02% or less (exclusive of 0%); and
[0043] (f) at least one member selected from the group consisting
of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less
(exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or
less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%).
Advantage of the Invention
[0044] According to the present invention, in a microstructure
mainly containing ferrite+pearlite, grains thereof are equi-axed
and refined, whereby a hot-rolled steel sheet making it possible to
obtain a predetermined surface hardness after a carburizing heat
treatment while ensuring cold formability can be provided.
Mode for Carrying Out the Invention
[0045] The hot-rolled steel sheet according to the present
invention (hereinafter, sometimes referred to as "steel sheet of
the present invention" or simply as "steel sheet") is described in
more detail below. The steel sheet of the present invention is
common with the hot-forged material (high-strength high-toughness
steel for case-hardening) described in Patent Document 1 in terms
of the component composition but differs in that the microstructure
is a microstructure mainly containing ferrite+pearlite and grains
thereof are equi-axed and refined.
[Sheet Thickness of Steel Sheet of the Present Invention: from 2 to
10 mm]
[0046] The steel sheet of the present invention targets one having
a sheet thickness of 2 to 10 mm. If the sheet thickness is less
than 2 mm, the rigidity as a structure cannot be ensured. On the
other hand, if the sheet thickness exceeds 10 mm, the
microstructure configuration specified in the present invention can
be hardly achieved, and the desired effects cannot be obtained. The
lower limit of the sheet thickness is preferably 3 mm or more and
more preferably 4 mm or more. The upper limit thereof is preferably
9 mm or less and more preferably 7 mm or less.
[0047] Next, the component composition constituting the steel sheet
of the present invention is described. In the following, all of the
units of chemical components are mass %.
[Component Composition of Steel Sheet of the Present Invention]
[0048] <C: from 0.05 to 0.30%>
[0049] C is an element indispensable for ensuring the strength of
the core portion as a carburizing- (or carbonitriding-)quenched
part finally obtained, and if it is less than 0.05%, sufficient
strength cannot be obtained. However, if it is excessively
contained, not only the toughness is deteriorated but also the
machinability or cold forgeability is reduced to impair the
formability, and therefore, the upper limit thereof is 0.30%. The
preferred content of C is in the range of from 0.08 to 0.25%.
<Mn: from 0.3 to 3.0%>
[0050] Mn is an element effective for the deoxidation of molten
steel and in order to effectively bring out such an effect, it must
be contained in an amount of 0.3% or more. If it is excessively
contained, cold formability or machinability is adversely affected
and the segregation amount at grain boundary is increased to
decrease the grain boundary strength, resulting in an adverse
effect on the impact properties. Therefore, the content thereof
must be 3.0% or less. The preferred content of Mn is in the range
of from 0.5 to 2.0%.
<Al: from 0.015 to 0.1%>
[0051] Al is an element contained in the steel as a deoxidizer for
the steel material and has an action of binding to N in the steel
to form AN and thereby preventing grain growth. In order to
effectively bring out such an effect, it must be contained in an
amount of 0.015% or more. The effect is saturated at about 0.1%,
and if the content exceeds it, the element binds with oxygen to
form a nonmetallic inclusion and adversely affects impact
properties, etc. Therefore, the upper limit thereof has been
specified to be 0.1%, and is preferably 0.08% or less, more
preferably 0.06% or less and especially preferably 0.04% or
less.
<N: from 0.003 to 0.30%>
[0052] N has an action of binding to Al, V, Ti, Nb, etc. in the
steel to form a nitride and thereby suppressing grain growth, and
this effect is effectively exerted when it is contained in an
amount of 0.003% or more. It is preferably 0.005% or more. However,
such an effect is saturated at about 0.30%, and if contained by not
less than the above amount, the nitride works as an inclusion and
adversely affects the physical properties. Therefore, the upper
limit has been specified to be 0.30%, and is preferably 0.10% or
less, more preferably 0.05% or less and especially preferably 0.03%
or less.
[0053] The steel sheet of the present invention fundamentally
contains the above-described components, with the remainder being
iron and unavoidable impurities. The contents of Si, P and S
unavoidably getting mixed in are desirably kept as small as
possible for the following reasons.
<Si: 0.5% or Less>
[0054] Si effectively acts as a strengthening element or a
deoxidizing element but, on the other hand, promotes grain boundary
oxidation to deteriorate the bending fatigue properties and
adversely affects the cold forgeability. Accordingly, in order to
remove such the problems, the content thereof must be kept at 0.5%
or less and, among others, when high-level bending fatigue
properties are required, the content thereof is preferably kept at
0.1% or less. From such a viewpoint, the more preferred content of
Si is in the range of from 0.02 to 0.1%.
<P: 0.030% or Less>
[0055] P segregates at the grain boundary to reduce the toughness
and therefore, the upper limit thereof has been specified to be
0.030%. The more preferred content of P is 0.020% or less and
further preferably 0.010% or less.
<S: 0.035% or Less>
[0056] S produces MnS and contributes to enhancement of
machinability. In the case of applying the present invention to
gears, etc., not only vertical impact properties but also lateral
impact properties are important, and the anisotropy needs to be
reduced to enhance the lateral impact properties. For this purpose,
the S content must be kept at 0.035% or less. The more preferred
content of S is 0.025% or less and further preferably 0.020% or
less.
[0057] The steel sheet of the present invention may contain the
following tolerable components, in addition to the above-described
basic components, in the ranges not impairing the actions of the
present invention.
<At Least One Member Selected from the Group Consisting of Cr:
3.0% or Less (Exclusive of 0%), Mo: 1.0% or Less (Exclusive of 0%)
and Ni: 3.0% or Less (Exclusive of 0%)>
[0058] These elements are useful elements in terms of having an
action of improving the quenchability or refining the quenched
microstructure. In particular, Cr has an excellent effect of
enhancing the quenchability; Mo effectively acts to decrease an
incompletely quenched microstructure, enhance the quenchability and
furthermore, increase the grain boundary strength; and Ni refines
the microstructure after quenching and thereby contributes to
enhancement of impact resistance. These effects are effectively
exerted by preferably containing at least one member of Cr: 0.2% or
more, Mo: 0.08% or more and Ni: 0.2% or more. However, if the Cr
amount exceeds 3.0%, Cr produces a carbide and causes grain
boundary segregation to reduce the grain boundary strength and in
turn, adversely affect the toughness; the above-described effects
of Mo are saturated at about 1.0%; and the above-described effects
of Ni are also saturated at 3.0%. Therefore, addition by not less
than those amounts is utterly useless from an economical
viewpoint.
<Cu: 2.0% or Less (Exclusive of 0%) and/or Co: 5% or Less
(Exclusive of 0%)>
[0059] Cu is an element effectively acting to enhance the corrosion
resistance, and the effect is effectively exerted by being
contained in an amount of preferably 0.3% or more. However, the
effect is saturated at a content of 2.0% and therefore, containing
by not less than the above amount is useless. When Cu is contained
alone, the hot formability of the steel material tends to be
deteriorated and in order to avoid such an ill effect, Ni having an
effect of enhancing the hot formability is desirably used in
combination in the above-described content range.
[0060] In addition, both Cu and Co are elements having an action of
causing strain-aging and hardening of a steel material and
effective for enhancing the strength after processing. In order to
effectively bring out such the actions, these elements are
preferably contained each in an amount of 0.1% or more and
furthermore 0.3% or more. However, if the Co content is excessively
large, the effect of causing strain-aging and hardening of a steel
material and the effect of enhancing the strength after processing
may be saturated, or cracking may be encouraged. Therefore, it is
recommended that the Co content is 5% or less, furthermore 4% or
less and particularly 3% or less.
<At Least One Member Selected from the Group Consisting of V:
0.5% or Less (Exclusive of 0%), Ti: 0.1% or Less (Exclusive of 0%)
and Nb: 0.1% or Less (Exclusive of 0%)>
[0061] These elements contribute to enhancing the toughness (impact
resistance) by binding to C or N to produce a carbide or a nitride
and by thus refining the grains. However, since the effect is
saturated around each upper limit value and the machinability or
cold formability may be rather adversely affected, they must be
kept equal to or less than the respective upper limit values. The
preferable lower limit values for effectively bringing out the
addition effect of these elements are V: 0.03%, Ti: 0.005% and Nb:
0.005%.
<Ca: 0.08% or Less (Exclusive of 0%) and/or Zr: 0.08% or Less
(Exclusive of 0%)>
[0062] Ca envelops a hard inclusion in a flexible inclusion, and Zr
spheroidizes MnS, both thereby contributing to enhancing the
machinability. In addition, both elements have an effect of
increasing the lateral impact properties by virtue of the reduction
in the anisotropy by the spheroidization of MnS. However, these
effects are each saturated at 0.08% and therefore, it is
recommended that the content of each is 0.08% or less, furthermore
0.05% or less and particularly 0.01% or less. The preferable lower
limit values for effectively bringing out the above-described
effects of these elements are Ca: 0.0005% (furthermore 0.001%) and
Zr: 0.002%.
<Sb: 0.02% or Less (Exclusive of 0%)>
[0063] Sb is an effective element for suppressing the grain
boundary oxidation and thereby increasing the bending fatigue
strength. However, since the effect is saturated at 0.02%, addition
by not less than the amount is useless from an economical
viewpoint. The preferable lower limit value for effectively
bringing out the addition effect of Sb is 0.001%.
<At Least One Member Selected from the Group Consisting of REM:
0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of
0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less
(exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%)>
[0064] REM is, similarly to Zr and Ca, an element spheroidizing a
sulfide compound-based inclusion such as MnS to thereby enhance the
deformation performance of steel and contributing to enhancement of
the machinability. In order to effectively bring out such actions,
REM is preferably contained in an amount of 0.0005% or more and
furthermore 0.001% or more. However, even if contained too much,
the effect thereof is saturated and an effect consistent with the
content cannot be expected. Therefore, recommended are 0.05% or
less, furthermore 0.03% or less and particularly 0.01% or less.
[0065] The "REM" in the present invention means to include
lanthanoid elements (15 elements from La to Ln) as well as Sc
(scandium) and Y (yttrium). Among these elements, it is preferable
to contain at least one element selected from the group consisting
of La, Ce and Y, and it is more preferable to contain La and/or
Ce.
[0066] Mg is, similarly to Zr and Ca, an element spheroidizing a
sulfide compound-based inclusion such as MnS to thereby enhance the
deformation performance of steel and contributing to enhancement of
the machinability. In order to effectively bring out such actions,
Mg is preferably contained in an amount of 0.0002% or more and
furthermore 0.0005% or more. However, even if contained too much,
the effect thereof is saturated and an effect consistent with the
content cannot be expected. Therefore, recommended are 0.02% or
less, furthermore 0.015% or less and particularly 0.01% or
less.
[0067] Li is, similarly to Zr and Ca, an element spheroidizing a
sulfide compound-based inclusion such as MnS to allow for
enhancement of the deformation performance of steel and
contributing to improvement of the machinability by lowering the
melting point of an Al-based oxide and thereby making it harmless.
In order to effectively bring out such actions, Li is preferably
contained in an amount of 0.0002% or more and furthermore 0.0005%
or more. However, even if contained too much, the effect thereof is
saturated and an effect consistent with the content cannot be
expected. Therefore, recommended are 0.02% or less, furthermore
0.015% or less and particularly 0.01% or less.
[0068] Pb is an element effective for enhancing the machinability.
In order to effectively bring out such an action, Pb is preferably
contained in an amount of 0.005% or more and furthermore 0.01% or
more. However, if contained too much, there arises a problem with
production such as formation of a roll mark. Therefore, recommended
are 0.5% or less, furthermore 0.4% or less and particularly 0.3% or
less.
[0069] Bi is, similarly to Pb, an element effective for enhancing
the machinability. In order to effectively bring out such an
action, Bi is preferably contained in an amount of 0.005% or more
and furthermore 0.01% or more. However, even if contained too much,
the effect of enhancing the machinability is saturated. Therefore,
recommended are 0.5% or less, furthermore 0.4% or less and
particularly 0.3% or less.
[0070] The microstructure characterizing the steel sheet of the
present invention is described below.
[Microstructure of Steel Sheet of the Present Invention]
[0071] As described above, the steel sheet of the present invention
has ferrite+pearlite as a principal microstructure and in
particular, is characterized in that with respect to grains of all
phases including ferrite and pearlite, each of the equiaxing degree
and the size is controlled to a specific range.
<Microstructure Containing Ferrite: from 10 to 50%, Pearlite:
from 15 to 50% and Remainder: Bainite>
[0072] The percentage of a phase is an important factor in
determining the strength level of the steel sheet. In the present
invention, from the standpoint of ensuring cold formability and the
matrix strength in the central part of sheet thickness after a heat
treatment, it is necessary to be set to approximately from 350 to
700 MPa as a tensile strength. If the tensile strength is less than
350 MPa, although the surface hardness may be ensured even after a
carburizing heat treatment, the hardness in a deep portion from the
surface is insufficient, and the strength and hardness in the
central part of sheet thickness are insufficient. On the other
hand, if the tensile strength exceeds 700 MPa, the cold formability
before a heat treatment cannot be ensured. In correspondence with
the strength level, if the percentage of ferrite is too small
and/or the percentage of pearlite is too large, the tensile
strength is excessively increased to make the forming impossible.
On the other hand, if the percentage of ferrite is too large and/or
the percentage of pearlite is too small, the base strength lacks,
leading to insufficient strength in the central part of sheet
thickness and a decrease in the fatigue strength. Therefore, the
microstructure is specified to contain, in area ratio, ferrite:
from 10 to 50% and pearlite: from 15 to 50%. The remainder is
bainite.
<The Number of Grains Having an Aspect Ratio (Major Axis/Minor
Axis) of 3 or Less is 60% or More of the Number of All
Grains>
[0073] The shape of the grain is required to be equi-axed grain for
achieving both enhancement of the stretch flange formability (hole
expandability) and assurance of the hardness, after a heat
treatment, in the central part of sheet thickness, i.e., in a deep
portion from the surface. For this purpose, the number of grains
having an aspect ratio (major axis/minor axis) of 3 or less, which
are an equi-axed grain, is set to be 60% or more, preferably 70% or
more and more preferably 80% or more, of the number of all grains.
The "all grains" herein means grains of all phases including the
above-described ferrite and pearlite.
<The Average Grain Size of All Grains is in the Range of from 3
to 50 .mu.m>
[0074] If the grain is too large, the surface quality is degraded
to cause surface cracking and to deteriorate the hole
expandability. Therefore, the average grain size of all grains is
set to be 50 .mu.m or less, preferably 40 .mu.m or less and more
preferably 30 .mu.m or less. On the other hand, in regard to the
lower limit value, as the grain is finer, the properties thereof
are improved, but the rolling capacity or cooling capacity must be
raised, which reduces the productivity. Therefore, the average
grain size of all grains is set to be 3 .mu.m or more, preferably 5
.mu.m or more and more preferably 7 .mu.m or more.
[Method for Measuring Area Ratio of Each Phase]
[0075] As for the area ratio of each phase above, each test steel
sheet is ground to a depth of t/4 (t: sheet thickness), then
subjected to Nital etching, and photographed for five visual fields
by a scanning electron microscope (SEM, magnification: 1,000
times), thereby determining each percentage of ferrite and pearlite
by a point counting method. The remainder was regarded as
bainite.
[Method for Measuring Aspect Ratio of Grain]
[0076] Grains of all phases including the above-described ferrite
and pearlite are measured for the maximum Feret diameter and the
minimum Feret diameter, and the ratio thereof (major axis/minor
axis) was defined as the aspect ratio.
[Method for Measuring Average Grain Size]
[0077] Individual centroid diameters as for all grains are
determined by performing image analysis on the above images
photographed by the scanning electron microscope, and a value
obtained by arithmetically averaging these centroid diameters by
the number of all grains is defined as the average grain size of
all grains.
[0078] The preferable production method for obtaining the steel
sheet of the present invention is described below.
[Preferable Production Method of Steel Sheet of the Present
Invention]
[0079] The steel sheet of the present invention can be produced,
for example, as a hot-rolled coil obtained by melting a raw
material steel having the above-described component composition,
casting it to form a slab, and subjecting the slab as it is or the
slab after surface chamfering to respective steps of heating, hot
rough rolling and finish rolling. Pickling and skin pass rolling
may be thereafter further applied according to the required
conditions such as surface state and sheet thickness accuracy.
[Preparation of Molten Steel]
[0080] First, desired oxides can be produced by adding
predetermined alloy elements in a predetermined order to a molten
steel in which the dissolved oxygen amount and the total oxygen
amount are adjusted. Above all, in the present invention, it is
very important to adjust the dissolved oxygen amount and
thereafter, adjust the total oxygen amount so as to inhibit
production of a coarse oxide.
[0081] The "dissolved oxygen" means oxygen that is present in the
molten steel without forming an oxide and kept in a free state. The
"total oxygen" means the total of all oxygens contained in the
molten steel, i.e., free oxygen and oxygen forming an oxide.
[0082] The dissolved oxygen amount in the molten steel is first
adjusted to a range of 0.0010 to 0.0060%. If the dissolved oxygen
amount in the molten steel is less than 0.0010%, a predetermined
amount of Al--O-based oxide cannot be ensured due to shortage of
the dissolved oxygen amount in the molten steel, and a desired size
distribution cannot be obtained. In addition, if the dissolved
oxygen amount is insufficient, in the case of adding REM, the REM
forms a sulfide, and an inclusion is thereby coarsened, giving rise
to deterioration of the properties. Therefore, the dissolved oxygen
amount is set to be 0.0010% or more. The dissolved oxygen amount is
preferably 0.0013% or more and more preferably 0.0020% or more.
[0083] On the other hand, if the dissolved oxygen amount exceeds
0.0060%, not only the reaction of oxygen and the elements above in
the molten steel becomes vigorous due to an excessively large
oxygen amount in the molten steel, which is disadvantageous in view
of melting operation, but also a coarse oxide is produced to rather
deteriorate the properties. Therefore, the dissolved oxygen amount
should be kept at 0.0060% or less. The dissolved oxygen amount is
preferably 0.0055% or less and more preferably 0.0053% or less.
[0084] The dissolved oxygen amount in a molten steel having been
subjected to primary refining in a converter or an electric furnace
usually exceeds 0.010%. Therefore, in the production method of the
present invention, the dissolved oxygen amount in the molten steel
needs to be adjusted to the range above in some way.
[0085] The method for adjusting the dissolved oxygen amount in the
molten steel includes, for example, a method of performing vacuum C
deoxidation by using an RH-type degassing refining apparatus and a
method of adding a deoxidizing element such as Si, Mn and Al, and
the dissolved oxygen amount may also be adjusted by appropriately
combining these methods. In addition, the dissolved oxygen amount
may be adjusted by using a ladle heating-type refining apparatus, a
simple molten metal treatment system, etc., in place of the RH-type
degassing refining apparatus. In this case, since the dissolved
oxygen amount cannot be adjusted by vacuum C deoxidation, a method
of adding a deoxidizing element such as Si may be employed for the
adjustment of the dissolved oxygen amount. In the case of employing
the method of adding a deoxidizing element such as Si, the
deoxidizing element may be added when the steel is tapped from the
converter to the ladle.
[0086] After adjusting the dissolved oxygen amount in the molten
steel to the range of 0.0010 to 0.0060%, the molten steel is
stirred to float and separate an oxide in the molten steel, whereby
the total oxygen amount in the molten steel is adjusted to the
range of 0.0010 to 0.0070%. Thus, in the present invention, after
removing unnecessary oxides by stirring a molten steel in which the
molten oxygen amount is appropriately controlled, the production of
a coarse oxide, i.e., a coarse inclusion, can be prevented.
[0087] If the total oxygen amount is less than 0.0010%, the desired
amount of oxide lacks and therefore, the amount of oxide
contributing to a fine size distribution of inclusions cannot be
ensured. Therefore, the total oxygen amount is set to be 0.0010% or
more. The total oxygen amount is preferably 0.0015% or more and
more preferably 0.0018% or more.
[0088] On the other hand, if the total oxygen amount exceeds
0.0070%, the amount of oxide in the molten steel is excessively
large, as a result, a coarse oxide, i.e., a coarse inclusion, is
produced to deteriorate the properties. Therefore, the total oxygen
amount should be kept at 0.0070% or less. The total oxygen amount
is preferably 0.0060% or less and more preferably 0.0050% or
less.
[0089] The total oxygen amount in the molten steel generally varies
in a correlated manner in response to the stirring time of the
molten steel and therefore, can be controlled, for example, by
adjusting the stirring time. Specifically, the total oxygen amount
in the molten steel is appropriately controlled while stirring the
molten steel and measuring from time to time the total oxygen
amount in the molten steel after removing an oxide floated.
[0090] In the case of adding REM and Ca to the steel material,
after adjusting the total oxygen amount in the molten steel to the
above-described range, REM is added and casting is then performed.
A desired oxide can be obtained by adding the elements above to a
molten steel in which the total oxygen amount has been
adjusted.
[0091] The forms of REM and Ca to be added to the molten steel are
not particularly limited and, for example, pure La, pure Ce, pure
Y, etc. as REM, or pure Ca, and furthermore, Fe--Si--La alloy,
Fe--Si--Ce alloy, Fe--Si--Ca alloy, Fe--Si--La--Ce alloy, Fe--Ca
alloy, or Ni--Ca alloy may be added. A misch metal may also be
added to the molten metal. The misch metal is a mixture of cerium
group rare-earth elements and, specifically, contains approximately
from 40 to 50% of Ce and approximately from 20 to 40% of La.
However, the misch metal often contains Ca as an impurity and in
the case where the misch metal contains Ca, the preferable range
specified in the present invention must be satisfied.
[0092] In the case where REM is added, in the present invention,
the molten steel after the addition of REM is preferably stirred
for in the range of not more than 40 minutes so as to promote the
removal of a coarse oxide. If the stirring time exceeds 40 minutes,
an oxide is coarsened due to occurrence of aggregation/coalescence
of fine oxides in the molten steel, whereby the properties are
deteriorated. Therefore, the stirring time is preferably 40 minutes
or less. The stirring time is more preferably 35 minutes or less
and still more preferably 30 minutes or less. The lower limit value
of the stirring time of the molten steel is not particularly
limited, but if the stirring time is too short, the concentrations
of additive elements are non-uniform, and the desired effect as the
entire steel material cannot be obtained. Accordingly, a desired
stirring time in accordance with the container size is
required.
[0093] In this way, a molten steel having an adjusted component
composition can be obtained. By using the obtained molten steel,
casting is performed to obtain a billet.
[0094] Next, heating, hot rolling including finish rolling, rapid
cooling after hot rolling, slow cooling after stop of rapid
cooling, rapid cooling and coiling after slow cooling are performed
for the production.
[Heating]
[0095] The heating before hot rolling is performed at 1,150 to
1,300.degree. C. This heating provides for an austenite single
phase, whereby a solid solution element (including an additive
element such as V and Nb) is dissolved in solid in the austenite.
If the heating temperature is less than 1,150.degree. C., the
element cannot be dissolved in solid in the austenite, and a coarse
carbide is formed, as a result, an effect of improving the fatigue
properties cannot be obtained. On the other hand, a temperature
exceeding 1,300.degree. C. is difficult in view of operation. In
the case of containing Ti as an additive element, from the
standpoint of forming a solid solution of Ti, which has a highest
solution treatment temperature among carbides, a temperature equal
to or more than the solution treatment temperature of TiC and
1,300.degree. C. or less is necessary. The more preferred lower
limit of the heating temperature is 1,200.degree. C.
[Hot Rough Rolling]
[0096] In the rough rolling, the microstructure control of
recrystallized austenite is performed so as to ensure the presence
percentage of an equiaxial grain with a predetermined shape
specified in the present invention. Taking into account to ensure
the temperature in the subsequent finish rolling, the rough rolling
temperature is set to be from 900 to 1,200.degree. C., and the
austenite grain is refined and repeatedly recrystallized in the
rough rolling, whereby the presence percentage of the equiaxial
grain with a predetermined shape can be controlled. The rough
rolling temperature is more preferably from 900 to 1,100.degree.
C.
[Hot Finish Rolling]
[0097] Hot rolling is performed so that the finish rolling
temperature can be 800.degree. C. or more. If the finish rolling
temperature is set too low, ferrite transformation occurs at a high
temperature, and a precipitated carbide in ferrite is coarsened.
Therefore, the finish rolling temperature needs to be not less than
a given level. The finish rolling temperature is more preferably
set to be 850.degree. C. or more so that the austenite grain can
grow and the grain size of bainite can be increased.
[Difference Between Entry-Side Temperature and Exit-Side
Temperature of Hot Finish Rolling]
[0098] The difference between the entry-side temperature and the
exit-side temperature of hot finish rolling is set to be
150.degree. C. or less. In the case where this temperature
difference exceeds 150.degree. C., which is the case where the
temperature before finish rolling is high, not only the grain
(austenite grain) grows but also the recrystallized grain produced
during finish rolling readily grows large. In addition, when the
temperature difference between the entry side and the exit side is
large, the recrystallized microstructure produced during finish
rolling is likely to become non-uniform, and a grain having a large
aspect ratio tends to remain. For these reasons, the number of
grains having an aspect ratio of 3 or less becomes less than 60% of
the number of all grains. The temperature difference is more
preferably 100.degree. C. or less.
[Rapid Cooling After Hot Rolling]
[0099] Within 5 seconds after the completion of the finish rolling,
rapid cooling is performed at a cooling rate (rapid cooling rate)
of 20.degree. C./s or more, and the rapid cooling is stopped at a
temperature (rapid cooling stop temperature) of 580.degree. C. or
more and less than 680.degree. C. This is done for lowering the
ferrite transformation start temperature and thereby refining the
precipitated carbide formed in ferrite. If the cooling rate (rapid
cooling rate) is less than 20.degree. C./s, pearlite transformation
is promoted, or if the rapid cooling stop temperature is less than
580.degree. C., pearlite transformation or bainite transformation
is promoted and, as a result, cold formability is reduced. On the
other hand, if the rapid cooling stop temperature is 680.degree. C.
or more, the precipitated carbide in ferrite is coarsened, and the
anti-fatigue properties cannot be ensured. The rapid cooling stop
temperature is preferably from 600 to 650.degree. C. and more
preferably 610 to 640.degree. C.
[Slow Cooling After Stop of Rapid Cooling]
[0100] After the stop of the rapid cooling, slow cooling is
performed at a cooling rate (slow cooling rate) of 5.degree. C./s
or more and less than 20.degree. C./s. The slow cooling rate is set
to be 5.degree. C./s or more so as to suppress formation of
proeutectoid ferrite during hot rolling, appropriately refine the
precipitated carbide in ferrite, and control the grain
microstructure in the hot-rolled sheet, thereby controlling the
texture configuration in the final steel sheet.
[0101] If the slow cooling rate is less than 5.degree. C./s, not
only the amount of proeutectoid ferrite formed is increased,
allowing production of a coarse grain, but also a coarse grain is
formed in the final steel sheet to cause a non-uniform state of
carbide and deteriorate the cold formability. If the cooling rate
is 20.degree. C./s or more, martensite is produced, and the cold
formability is thereby reduced.
[Rapid Cooling and Coiling After Slow Cooling]
[0102] After the slow cooling, coiling is performed at more than
550.degree. C. and 650.degree. C. or less. If the coiling
temperature exceeds 650.degree. C., many surface oxide scales are
formed to deteriorate the surface quality, and on the other hand,
if it is less than 550.degree. C., many martensites are formed to
reduce the cold formability.
[0103] The present invention is described in greater detail below
by referring to Examples, but the following Examples are not
limiting in nature on the present invention, and the present
invention can be implemented by making appropriate changes within
the scope conformable to the gist described hereinbefore and
hereinafter, and all of them are included in the technical scope of
the present invention.
EXAMPLES
[0104] A steel having the component composition shown in Table 1
below was melted by a vacuum melting method and cast into an ingot
having a thickness of 120 mm, followed by performing hot rolling
under the conditions shown in Table 2 below to produce a hot-rolled
steel sheet. In all tests, the cooling after the stop of rapid
cooling was slow cooling under the conditions of a cooling rate of
10.degree. C./s or less for 5 to 20 seconds.
[0105] A test steel containing the chemical components shown in
Table 1 was melted by using a vacuum melting furnace (capacity: 150
kg) and cast into 150 kg of an ingot, followed by cooling. When the
test steel was melted in the vacuum melting furnace, component
adjustment was applied to the elements except for Al, REM and Ca,
and the dissolved oxygen amount in the molten steel was adjusted by
deoxidation by using at least one element selected from C, Si and
Mn. The molten steel in which the dissolved oxygen amount is
adjusted was stirred for approximately from 1 to 10 minutes to
float and separate oxides in the molten steel, and the total oxygen
amount in the molten steel was thereby adjusted.
[0106] In the case of adding REM and Ca, they were added to a
molten steel in which the total oxygen amount had been adjusted,
thereby obtaining a molten steel adjusted for component thereof.
Here, REM was added in the form of a misch metal containing about
25% of La and about 50% Ce, and Ca was added in the form of an
Ni--Ca alloy, a Ca--Si alloy, or an Fe--Ca green compact.
[0107] The obtained ingot was hot-rolled under the conditions shown
in Table 2 to produce a hot-rolled sheet having a predetermined
thickness. In Table 2, the rapid cooling rate after hot rolling and
the cooling rate after stop of rapid cooling are not shown, but in
each production example, the conditions employed are 40.degree.
C./s for the rapid cooling after hot rolling and 10.degree. C./s
for the cooling after stop of rapid cooling.
[0108] With respect to each of the thus-obtained as-hot rolled
sheets, the area ratio of each phase in the steel sheet, the aspect
ratio of the grain, the number thereof, etc. were examined by the
measuring methods described in the item of "MODE FOR CARRYING OUT
THE INVENTION" above.
[0109] In addition, each of the as-hot rolled sheets above was
measured for the tensile strength and the hole expansion ratio for
evaluating the cold formability, and those where the tensile
strength was in the range of from 350 to 700 MPa and the hole
expansion ratio was 20% or more were judged as passed.
[0110] Furthermore, each of the as-hot rolled sheets above was
subjected to a carburizing-quenching test under the following
conditions for evaluating the surface hardness after a carburizing
heat treatment.
[Carburizing-Quenching Conditions]
[0111] A carburizing treatment was applied by holding at
900.degree. C. for 2.5 hours and further at 850.degree. C. for 0.5
hours in a gas atmosphere with a carbon potential (CP value)=0.8%
and thereafter, performed were oil-quenching at 100.degree. C.,
holding at 160.degree. C. for 2 hours for subjecting to a tempering
treatment, and air-cooling.
<Surface Hardness After Carburizing Heat Treatment>
[0112] The Vickers hardness (Hv) was measured by using a Vickers
hardness tester under the conditions of load: 1,000 g, measurement
position: a position of 0.8 mm in depth from the steel sheet
surface, and number of measurements: 5 times, and those where the
hardness was 350 Hv or more were judged as passed. Here, the
measurement position was set to a position of 0.8 mm in depth from
the surface because it was specified as a necessary condition to
exhibit desired hardness (strength) even at a deep position from
the surface after a carburizing heat treatment.
[0113] These measurement results are shown in Table 3 below.
TABLE-US-00001 TABLE 1 Steel Spe- Components (mass %) [remainder:
Fe and unavoidable impurities] cies C Si Mn P S Al N Cr Mo Ni Cu Co
V Ti Nb Ca Zr Sb Others a 0.23 0.05 2.50 0.008 0.012 0.032 0.010 --
-- -- -- -- -- -- -- -- -- -- -- b 0.08 0.10 2.10 0.010 0.018 0.026
0.005 -- -- -- -- -- -- -- -- -- -- -- -- c 0.15 0.06 1.88 0.008
0.012 0.027 0.090 1.03 -- -- -- -- -- -- -- -- -- -- -- d 0.13 0.02
2.20 0.012 0.020 0.016 0.008 0.99 -- -- -- -- -- -- -- -- -- -- --
e 0.17 0.12 2.01 0.010 0.021 0.022 0.009 -- 0.44 -- -- -- -- -- --
-- -- -- -- f 0.21 0.41 1.32 0.010 0.008 0.033 0.010 -- -- 0.99 --
-- -- -- -- -- -- -- -- g 0.17 0.05 1.40 0.008 0.012 0.032 0.010
1.07 0.01 0.02 0.01 -- -- -- -- -- -- -- -- h 0.19 0.07 1.52 0.010
0.027 0.022 0.080 1.01 -- 0.28 -- 0.32 -- -- -- -- -- -- -- i 0.18
0.06 1.39 0.009 0.010 0.03 0.090 -- -- -- -- -- 0.33 -- -- -- -- --
-- j 0.19 0.06 1.41 0.010 0.011 0.031 0.070 -- -- -- -- -- -- 0.05
-- -- -- -- -- k 0.17 0.07 1.40 0.008 0.012 0.03 0.090 -- -- -- --
-- -- -- 0.03 -- -- -- -- l 0.18 0.06 1.38 0.007 0.010 0.029 0.010
1.03 0.02 0.02 0.01 -- -- -- -- 0.002 -- -- -- m 0.18 0.05 1.39
0.007 0.010 0.029 0.010 0.99 0.02 0.03 -- 0.01 -- -- -- -- 0.002 --
-- n 0.17 0.05 1.41 0.007 0.009 0.032 0.080 1.07 0.01 0.02 0.01 --
-- -- -- -- -- 0.008 -- o 0.17 0.05 1.41 0.007 0.009 0.032 0.080
1.07 0.01 0.02 0.01 -- -- -- -- -- -- -- REM: 0.003, Li: 0.001 p
0.19 0.06 1.36 0.007 0.009 0.029 0.070 1.01 0.01 0.02 0.01 -- -- --
-- -- -- -- Mg: 0.001, Pb: 0.001, Bi: 0.05 q 0.03 0.05 2.20 0.008
0.001 0.03 0.009 -- -- -- -- -- -- -- -- -- -- -- -- r 0.31 0.05
2.20 0.007 0.001 0.03 0.009 -- -- -- -- -- -- -- -- -- -- -- -- s
0.17 0.07 0.20 0.007 0.001 0.03 0.009 -- -- -- -- -- -- -- -- -- --
-- -- t 0.17 0.07 3.15 0.007 0.001 0.03 0.009 -- -- -- -- -- -- --
-- -- -- -- -- u 0.17 0.07 2.10 0.007 0.001 0.009 0.009 -- -- -- --
-- -- -- -- -- -- -- -- v 0.17 0.07 2.10 0.007 0.001 0.11 0.009 --
-- -- -- -- -- -- -- -- -- -- -- w 0.17 0.07 2.10 0.007 0.001 0.025
0.310 -- -- -- -- -- -- -- -- -- -- -- -- (--: not added,
underlined: outside the scope of the present invention)
TABLE-US-00002 TABLE 2 Hot Rolling Conditions Rough Finish Rolling
Finish Rolling Temperature Rapid Thickness of Steel Heating Rolling
Entry-Side Exit-Side Difference Between Cooling Stop Coiling
Hot-Rolled Production Spe- Temperature Temperature Temperature
Temperature Entry Side - Exit Side Temperature Temperature Sheet
No. cies (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.)
(.degree. C.) (.degree. C.) (.degree. C.) (mm) 1 a 1250 1121 987
880 107 601 569 3 2 a 1250 1056 991 912 79 612 598 8 3* a 1000* 920
864 775* 89 532* 457* 5 4* a 1250 1086 975 872 103 655 586 12* 5* a
1250 1189 997 831 166* 629 563 5 6 b 1250 1101 942 880 62 602 556 5
7 c 1250 1193 924 875 49 673 576 5 8 d 1250 964 935 864 71 642 579
5 9 e 1250 1151 952 890 62 657 598 5 10 f 1250 1113 923 855 68 635
568 5 11 g 1250 1199 970 843 127 638 581 5 12 h 1250 1183 925 856
69 643 576 5 13 i 1250 1025 943 880 63 644 552 5 14 j 1250 1000 891
867 24 603 558 5 15 k 1250 967 923 879 44 643 609 5 16 l 1250 1025
911 890 21 655 562 5 17 m 1250 1163 903 868 35 624 622 5 18 n 1250
986 966 901 65 657 596 5 19 o 1250 1038 913 867 46 615 601 5 20 p
1250 1102 911 850 61 615 567 5 21 q 1250 1021 931 844 87 622 571 5
22 r 1250 1169 947 897 50 603 601 5 23 s 1250 1120 962 876 86 620
558 5 24 t 1250 1146 876 843 33 607 579 5 25 u 1250 1021 953 901 52
620 582 5 26 v 1250 1048 924 893 31 604 559 5 27 w 1250 1039 923
851 72 648 612 5 (underlined = outside the scope of the present
invention, *= outside the recommended range)
TABLE-US-00003 TABLE 3 Ratio of Number Surface Hardness Area Ratio
of of Grains with Cold Formability After Carburizing Steel Each
Phase Aspect Ratio of Average Tensile Hole Expansion Hardness at
Steel Spe- Production Ferrite Pearlite Bainite 3 or Less Grain Size
Strength Ratio Depth of 0.8 mm No. cies No. (%) (%) (%) (%) (.mu.m)
(MPa) (%) (Hv) Remarks 1 a 1 42 45 13 73 28 583 34 390 Steel of
Invention 2 a 2 43 42 15 66 41 541 33 361 Steel of Invention 3 a 3*
20 70 10 42 19 723 11 394 Comp. steel 4 a 4* 55 39 6 68 58 523 17
386 Comp. steel 5 a 5* 47 45 8 53 18 584 18 398 Comp. steel 6 b 6
38 32 30 74 25 594 31 403 Steel of Invention 7 c 7 39 36 25 72 24
587 32 406 Steel of Invention 8 d 8 41 29 30 72 22 576 28 411 Steel
of Invention 9 e 9 40 30 30 70 22 583 29 405 Steel of Invention 10
f 10 37 29 34 71 21 591 28 403 Steel of Invention 11 g 11 42 45 13
79 15 601 28 422 Steel of Invention 12 h 12 44 37 19 89 16 595 29
413 Steel of Invention 13 i 13 42 38 20 84 18 600 31 411 Steel of
Invention 14 j 14 39 45 16 72 22 645 28 399 Steel of Invention 15 k
15 38 44 18 89 23 645 33 403 Steel of Invention 16 l 16 41 46 13 89
21 588 31 415 Steel of Invention 17 m 17 43 37 20 81 24 566 36 401
Steel of Invention 18 n 18 45 33 22 89 23 638 35 403 Steel of
Invention 19 o 19 36 35 29 82 21 566 33 404 Steel of Invention 20 p
20 37 39 24 79 26 605 31 406 Steel of Invention 21 q 21 78 11 11 86
46 320 51 246 Comp. steel 22 r 22 11 74 15 43 26 853 -- 460 Comp.
steel 23 s 23 61 13 26 44 24 420 35 294 Comp. steel 24 t 24 9 66 25
15 24 687 17 411 Comp. steel 25 u 25 42 44 14 13 23 590 15 407
Comp. steel 26 v 26 41 42 17 12 21 603 13 406 Comp. steel 27 w 27
46 47 7 14 26 602 17 402 Comp. steel (underlined = outside the
scope of the present invention, *= outside the recommended range,
--: not measured due to generation of cracking)
[0114] As shown in Table 3, all of Steel Nos. 1, 2 and 6 to 20 are
Steels of the Invention produced by using a steel species
satisfying the requirements specified for the component composition
of the present invention under the recommended hot rolling
conditions, as a result, allowed to satisfy the requirements
specified for the microstructure of the present invention, and in
the steels, all of the tensile strength, the hole expansion ratio
and the surface hardness after a carburizing heat treatment meet
the acceptance criteria. It could be confirmed that a hot-rolled
steel sheet exhibiting a predetermined hardness (strength) after a
carburizing heat treatment while ensuring good cold formability can
be obtained.
[0115] On the other hand, Steel Nos. 3 to 5 and 21 to 27 are
Comparative Steels failing in satisfying at least one of the
requirements specified for the component composition and the
microstructure in the present invention, and in the steels, at
least one of the tensile strength, the hole expansion ratio and the
surface hardness after a carburizing heat treatment does not meet
the acceptance criteria.
[0116] For example, Steel No. 3 satisfies the requirements for the
component composition, but since the heating temperature before hot
rolling is outside the recommended range and too low, pearlite is
excessively formed and the grain is flattened, resulting in too
high tensile strength and poor hole expandability.
[0117] Steel No. 4 satisfies the requirements for the component
composition, but since the sheet thickness after hot rolling is
outside the specified range and too large, ferrite is excessively
formed and the grain grows, resulting in poor hole
expandability.
[0118] Steel No. 5 satisfies the requirements for the component
composition, but since the difference between the entry-side
temperature and the exit-side temperature in finish rolling is
outside the recommended range and too large, the grain is
flattened, resulting in poor hole expandability.
[0119] In Steel No. 21 (steel species q), the hot rolling
conditions are in the recommended range but the C content is too
low, and therefore ferrite is excessively formed, and the tensile
strength is excessively decreased resulting in poor surface
hardness after a carburizing heat treatment.
[0120] In Steel No. 22 (steel species r), the hot rolling
conditions are in the recommended range but the C content is too
high, and therefore pearlite is excessively formed and the grain is
flattened, resulting in too high tensile strength and generation of
cracking at the time of a hole expansion test (poor hole
expandability).
[0121] In Steel No. 23 (steel species s), the hot rolling
conditions are in the recommended range but the Mn content is too
low, and therefore ferrite is excessively formed and the grain is
flattened, resulting in poor surface hardness after a carburizing
heat treatment.
[0122] In Steel No. 24 (steel species t), the hot rolling
conditions are in the recommended range but the Mn content is too
high, and therefore ferrite is not sufficiently formed while
pearlite is excessively formed, and in addition, the ratio of the
number of grains having an aspect ratio of 3 or less is low,
resulting in poor hole expandability.
[0123] In Steel No. 25 (steel species u), the hot rolling
conditions are in the recommended range but the Al content is too
low and the ratio of the number of grains having an aspect ratio of
3 or less is low, resulting in poor hole expandability.
[0124] On the other hand, in Steel No. 26 (steel species v), the
hot rolling conditions are in the recommended range but the Al
content is too high and the ratio of the number of grains having an
aspect ratio of 3 or less is low, resulting in poor hole
expandability as well.
[0125] In Steel No. 27 (steel species w), the hot rolling
conditions are in the recommended range but the N content is too
low and the ratio of the number of grains having an aspect ratio of
3 or less is low, resulting in poor hole expandability.
[0126] From the above, the applicability of the present invention
could be confirmed.
[0127] While the present invention has been described in detail and
with reference to specific embodiments thereof, it will be apparent
to one skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope of
the present invention.
[0128] This application is based on Japanese Patent Application
(Patent Application No. 2013-219467) filed on Oct. 22, 2013, the
contents of which are incorporated herein by way of reference.
INDUSTRIAL APPLICABILITY
[0129] The hot-rolled steel sheet of the present invention exhibits
good cold formability during processing, and, after a carburizing
heat treatment, is excellent in the hardness on the surface as well
as at a predetermined depth and excellent in the wear resistance,
anti-fatigue properties, etc., and therefore, is useful as
materials for the production of clutches, dampers, gears, etc. of
automobiles.
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