U.S. patent application number 15/031016 was filed with the patent office on 2016-09-15 for hot-rolled steel sheet having excellent surface hardness after carburizing heat treatment and excellent drawability.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steet, 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 | 20160265078 15/031016 |
Document ID | / |
Family ID | 52992905 |
Filed Date | 2016-09-15 |
United States Patent
Application |
20160265078 |
Kind Code |
A1 |
KAJIHARA; Katsura ; et
al. |
September 15, 2016 |
HOT-ROLLED STEEL SHEET HAVING EXCELLENT SURFACE HARDNESS AFTER
CARBURIZING HEAT TREATMENT AND EXCELLENT DRAWABILITY
Abstract
A hot-rolled steel sheet having a sheet thickness of 2-10 mm and
containing specific amounts of C, Mn, Al, and N with iron and
unavoidable impurities. With regard to all grains existing at a
position of t/4 in depth, t being sheet thickness, an area ratio of
grains having a sheet-plane orientation within 10.degree. from
(123) plane is 20% or more, a total area ratio of grains having a
crystal direction within 10.degree. from <001> direction and
grains having a crystal direction within 10.degree. from
<110> direction, in a rolling direction, is 25% or less, and
an average grain size of all grains is 3-50 .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 Steet, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
52992905 |
Appl. No.: |
15/031016 |
Filed: |
October 21, 2014 |
PCT Filed: |
October 21, 2014 |
PCT NO: |
PCT/JP2014/077983 |
371 Date: |
April 21, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0278 20130101;
C22C 38/52 20130101; C22C 38/00 20130101; C22C 38/38 20130101; C23C
8/22 20130101; C22C 38/002 20130101; C21D 8/0273 20130101; C21D
2201/05 20130101; C22C 38/12 20130101; C22C 38/58 20130101; C22C
38/08 20130101; C21D 8/0226 20130101; C22C 38/04 20130101; C21D
8/0473 20130101; C21D 2211/005 20130101; C22C 38/005 20130101; C21D
2211/009 20130101; C21D 9/46 20130101; C22C 38/06 20130101; C22C
38/60 20130101; C22C 38/02 20130101; C21D 1/06 20130101; C22C 38/14
20130101; C21D 8/0463 20130101; C22C 38/50 20130101; C22C 38/001
20130101; C22C 38/42 20130101; C21D 8/0205 20130101; C21D 8/0263
20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/60 20060101 C22C038/60; C22C 38/58 20060101
C22C038/58; C22C 38/52 20060101 C22C038/52; C22C 38/50 20060101
C22C038/50; C22C 38/42 20060101 C22C038/42; C22C 38/38 20060101
C22C038/38; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/08 20060101 C22C038/08; 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; C23C 8/22 20060101 C23C008/22 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 2013 |
JP |
2013-219468 |
Claims
1. A hot-rolled steel sheet excellent in drawability and surface
hardness after a carburizing heat treatment, wherein: the
hot-rolled steel sheet has a sheet thickness of from 2 to 10 mm;
and the hot-rolled steel sheet comprises 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%, N: from 0.003 to 0.30%,
iron, and unavoidable impurities; a microstructure of the
hot-rolled steel sheet mainly comprises ferrite and pearlite; and,
with respect to all grains including the ferrite and the pearlite,
existing at a position of t/4 in depth (t: sheet thickness), an
area ratio of grains having a sheet-plane orientation within
10.degree. from (123) plane is 20% or more, a total area ratio of
grains having a crystal direction within 10.degree. from
<001> direction and grains having a crystal direction within
10.degree. from <110> direction, in a rolling direction, is
25% or less, 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
hot-rolled steel sheet further comprises at least one member 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 (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
hot-rolled steel sheet further comprises at least one member 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 (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 be enhanced in formability by virtue of a configuration
where the average r value is 1.2 or more, the r value (rL) in the
rolling direction is 1.3 or more, the r value (rD) in the
45.degree. direction with respect to the rolling direction is 0.9
or more, the r value (rC) in a direction perpendicular to the
rolling direction is 1.2 or more, and the X-ray reflection surface
random intensity ratios of {111}, {100} and {110} in the sheet
plane at 1/2 sheet thickness of the steel sheet are 2.0 or more,
1.0 or more, and 0.2 or more, respectively.
[0008] The hot-rolled steel sheet disclosed in Patent Document 3 is
supposed to be enhanced in the ductility and burring formability by
virtue of a configuration where the steel sheet has a
microstructure containing from 40 to 95% by volume of bainite
phase, with the remainder being a ferrite phase, the average grain
diameter of the ferrite is 1.2 .mu.m or more and less than 4 .mu.m,
and at least either one property of the following (a) and (b) is
satisfied:
[0009] (a) the ratio (ds/dc) between the average grain diameter
(ds) of ferrite at a position of 1/8 thickness in the sheet
thickness direction from the surface of the steel sheet and the
average grain diameter (dc) of ferrite at the center of sheet
thickness is from 0.3 to 0.7; and
[0010] (b) the sum of pole densities of {110}<111>,
{110}<001> and {211}<111> at a position of 1/8
thickness in the sheet thickness direction from the surface of the
steel sheet is 5 times or more of one not having a texture, and
each pole density is 1.5 times or more.
[0011] The hot-rolled steel sheet disclosed in Patent Document 4 is
supposed to be enhanced in rigidity by virtue of a configuration
where the average grain size of a ferrite phase is 5 .mu.m or less,
the steel sheet has a microstructure allowing a ferrite phase to be
present at an area ratio of 50% or more, and the average ODF
analysis intensity fin (113)[1-10] to (223)[1-10] orientations in
the sheet plane at 1/4 sheet thickness of the steel sheet is 4 or
more.
[0012] The hot-rolled steel sheet disclosed in Patent Document 5 is
supposed to have enhanced draw formability by virtue of a
configuration where .DELTA.r1 specified by the following formula
(1) is -0.20 or more and 0.20 or less and .DELTA.r2 specified by
the following formula (2) is 0.42 or less.
.DELTA.r1=(r0-2r45+r90)/2 (1)
.DELTA.r2=rmax-rmin (2)
[0013] Each symbol in the formulae represents the following
value:
[0014] r0: an r value measured on a specimen sampled in parallel
with the rolling direction of sheet plane,
[0015] r45: an r value measured on a specimen sampled in the
45.degree. direction with respect to the rolling direction of sheet
plane,
[0016] r90: an r value measured on a specimen sampled in the
90.degree. direction with respect to the rolling direction of sheet
plane,
[0017] rmax: a maximum value among r0, r45 and r90, and
[0018] rmin: a minimum value among r0, r45 and r90.
[0019] The hot-rolled steel sheet disclosed in Patent Document 6 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.
[0020] Although the hot-rolled steel sheets disclosed in Patent
Documents 2 to 6 are excellent in the cold formability such as
drawability, the surface hardness after a carburizing heat
treatment is not referred to at all, and the improvement effect
thereon is unknown.
[0021] On the other hand, the hot-rolled steel sheet (carburized
steel strip) disclosed in Patent Document 7 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.
[0022] Although the hot-rolled steel sheet (carburized steel strip)
disclosed in Patent Document 7 is excellent in the surface hardness
after a carburizing heat treatment, cold formability as well as
drawability are not referred to at all, and the improvement effect
thereon is unknown.
[0023] As described above, almost no studies have been heretofore
made on a hot-rolled steel sheet having both drawability and
surface hardness after a carburizing heat treatment.
PRIOR ART LITERATURE
Patent Documents
[0024] Patent Document 1: Japanese Patent No. 3,094,856
[0025] Patent Document 2: Japanese Patent No. 3,742,559
[0026] Patent Document 3: Japanese Patent No. 4,161,935
[0027] Patent Document 4: Japanese Patent No. 4,867,257
[0028] Patent Document 5: JP-A-2013-119635
[0029] Patent Document 6: Japanese Patent No. 4,276,504
[0030] Patent Document 7: JP-A-2010-222663
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0031] The object of the present invention is to provide a
hot-rolled steel sheet having both drawability 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
[0032] The invention according to claim 1 is a hot-rolled steel
sheet excellent in drawability and surface hardness after a
carburizing heat treatment, having:
[0033] a sheet thickness of from 2 to 10 mm;
[0034] a component composition containing, in mass % (hereinafter,
the same applies to chemical components),
[0035] C: from 0.05 to 0.30%,
[0036] Mn: from 0.3 to 3.0%,
[0037] Al: from 0.015 to 0.1%, and
[0038] N: from 0.003 to 0.30%,
[0039] with the remainder being iron and unavoidable impurities;
and
[0040] a microstructure mainly containing ferrite and pearlite, in
which
[0041] with respect to all of grains including the ferrite and
pearlite (hereinafter, referred to as "all grains"), existing at a
position of t/4 in depth (t: sheet thickness),
[0042] an area ratio of grains having a sheet-plane orientation
within 10.degree. from (123) plane is 20% or more,
[0043] a total area ratio of grains having a crystal direction
within 10.degree. from <001> direction and grains having a
crystal direction within 10.degree. from <110> direction, in
a rolling direction, is 25% or less, and
[0044] an average grain size of the all grains is from 3 to 50
.mu.m.
[0045] 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.
[0046] 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 one member of the following (a) to
(f):
[0047] (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%);
[0048] (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%);
[0049] (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%);
[0050] (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%);
[0051] (e) Sb: 0.02% or less (exclusive of 0%); and
[0052] (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
[0053] According to the present invention, in a microstructure
mainly containing ferrite+pearlite, by controlling the texture of a
hot-rolled steel sheet to a predetermined microstructure
configuration and thereby increasing the deformability during
processing even in parts requiring drawability, the life of a metal
mold could be extended, and a hot-rolled steel sheet hardly allows
generation of cracking in the steel sheet and can ensure a
predetermined surface hardness for parts obtained after a
carburizing heat treatment can be provided.
MODE FOR CARRYING OUT THE INVENTION
[0054] 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 not only
the texture of the hot-rolled steel sheet is controlled to a
predetermined microstructure configuration but also the grains are
refined.
Sheet Thickness of Steel Sheet of the Present Invention: from 2 to
10 mm
[0055] 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.
[0056] 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
[0057] <C: from 0.05 to 0.30%>
[0058] 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%.
[0059] <Mn: from 0.3 to 3.0%>
[0060] 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%.
[0061] <Al: from 0.015 to 0.1%>
[0062] 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 AlN and thereby preventing grain growth.
[0063] 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.
[0064] <N: from 0.003 to 0.30%>
[0065] 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.
[0066] 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.
[0067] <Si: 0.5% or less>
[0068] 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%.
[0069] <P: 0.030% or less>
[0070] 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.
[0071] <S: 0.035% or less>
[0072] 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.
[0073] 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.
[0074] <At least one member selected from the group consisting
of
[0075] Cr: 3.0% or less (exclusive of 0%),
[0076] Mo: 1.0% or less (exclusive of 0%) and
[0077] Ni: 3.0% or less (exclusive of 0%)>
[0078] 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.
[0079] <Cu: 2.0% or less (exclusive of 0%) and/or
[0080] Co: 5% or less (exclusive of 0%)>
[0081] 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.
[0082] 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.
[0083] <At least one member selected from the group consisting
of
[0084] V: 0.5% or less (exclusive of 0%),
[0085] Ti: 0.1% or less (exclusive of 0%) and
[0086] Nb: 0.1% or less (exclusive of 0%)>
[0087] 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%.
[0088] <Ca: 0.08% or less (exclusive of 0%) and/or
[0089] Zr: 0.08% or less (exclusive of 0%)>
[0090] 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%.
[0091] <Sb: 0.02% or less (exclusive of 0%)>
[0092] 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%.
[0093] <At least one member selected from the group consisting
of
[0094] REM: 0.05% or less (exclusive of 0%),
[0095] Mg: 0.02% or less (exclusive of 0%),
[0096] Li: 0.02% or less (exclusive of 0%),
[0097] Pb: 0.5% or less (exclusive of 0%), and
[0098] Bi: 0.5% or less (exclusive of 0%)>
[0099] 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.
[0100] 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.
[0101] 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.
[0102] 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.
[0103] 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.
[0104] 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.
[0105] The microstructure characterizing the steel sheet of the
present invention is described below.
Microstructure of Steel Sheet of the Present Invention
[0106] As described above, the steel sheet of the present invention
has a microstructure mainly containing ferrite and pearlite and in
particular, is characterized in that the texture configuration in
steel is more strictly controlled. In the present invention, the
microstructure mainly containing ferrite and pearlite means that
the total amount of ferrite and pearlite is 90% or more in terms of
the area ratio. As long as the total amount of ferrite and pearlite
is 90% or more in terms of the area ratio, other microstructures
(e.g., bainite, martensite) may be produced in a small amount, but
the amount of other microstructures is preferably as small as
possible.
[0107] In general, with respect to the texture control for the
enhancement of formability of a steel sheet, both experiments and
theory have heretofore clarified that, as for the deep drawability
of a thin steel sheet for use in a body outer panel of an
automobile, as the plastic anisotropy (r value (Lankford value):
the ratio of a sheet-width strain to a sheet-thickness strain in a
tensile test) of a material is larger, the drawability is higher
and furthermore, it is essential for the enhancement of deep
drawability to strongly develop the {111} plane parallel with a
sheet-plane orientation and weaken the {100} plane orientation in
the recrystallization texture (see, Iron and Steel Institute of
Japan: "Recrystallization, Texture and Their Application to
Structural Control", March, 1999, p. 208).
[0108] Therefore, in a steel sheet for the body outer panel,
various approaches to enhancing the formability by the texture
control are taken as disclosed in Patent Documents 2 to 4, but such
approaches have not been attempted in a steel sheet for a
carburizing heat treatment.
Texture of Steel Sheet of the Present Invention
[0109] The steel sheet of the present invention is characterized in
that, with respect to grains of all phases including ferrite and
pearlite, the orientation and size of the grains are controlled to
specific ranges.
[0110] With respect to all of grains including ferrite and pearlite
(hereinafter, referred to as "all grains"), existing at a position
of t/4 in depth (t: sheet thickness),
[0111] an area ratio of grains having a sheet-plane orientation
within 10.degree. from (123) plane is 20% or more>
[0112] How the texture is formed differs depending on the
processing method, despite the same crystal system, and in the case
of a rolled material, it is expressed by a rolling surface and a
rolling direction. As described below, the rolling surface is
represented by {ooo}, and the rolling direction is represented by
<.DELTA..DELTA..DELTA.>. Here, o or .DELTA. indicates an
integer. The expression of each orientation is described, for
example, in Shinichi Nagashima (compiler), "Texture" (published by
Maruzen Co. Ltd.).
[0113] In the present invention, with respect to all grains
existing at a position of t/4 in depth, the area ratio of those
having a sheet-plane orientation within 10.degree. from (123) plane
is controlled to be 20% or more, whereby the cold formability and
drawability of a steel sheet for a carburizing heat treatment can
be enhanced.
[0114] With respect to the sheet-plane orientation of grains, it
has been conventionally known to be effective in enhancing the deep
drawability to strongly develop the (111) plane orientation
parallel with the sheet plane and on the other hand, weaken the
(001) plane orientation. Such a control of the sheet-plane
orientation may have been possible in the process involving
applying a cold rolling step and an annealing step, but this
sheet-plane orientation control has been difficult in the steel
sheet of the present invention.
[0115] Therefore, in the present invention, a grain having a
sheet-plane orientation of (123) plane is newly introduced, whereby
a texture control can be achieved in the steel sheet of the present
invention and enhancement of the cold formability in a softened
state can be realized.
[0116] As described above, a grain having (123) plane as the
sheet-plane orientation has an action of enhancing the cold
formability in a softened state, and in order to effectively bring
out such an action, 20% or more in terms of area ratio is needed.
It is preferably 22% or more, more preferably 24% or more and still
more preferably 26% or more.
[0117] Here, since a sheet material has a microstructure
distribution in the sheet thickness direction, the microstructure
configuration has been specified by employing, as the
representative position, a position of 1/4 sheet thickness in
depth. In addition, since grains having a sheet-plane orientation
within 10.degree. from the above-described ideal plane orientation
((123) plane) are considered to have a substantially equivalent
action, the microstructure is specified by the area ratio of grains
having a sheet-plane orientation in this range.
[0118] <A total area ratio of grains having a crystal direction
within 10.degree. from <001> direction and grains having a
crystal direction within 10.degree. from <110> direction, in
a rolling direction, is 25% or less>
[0119] As this total area ratio is larger, the in-plane anisotropy
in drawability increases, and for this reason, the ratio is limited
to 25% or less, preferably 23% or less and more preferably 20% or
less.
[0120] In addition, due to the above-described effect of reducing
the in-plane anisotropy, martensite transformation proceeds in
quenching after a carburizing heat treatment while maintaining the
original crystal orientation relationship of grains, as a result,
an effect of reducing transformation strain and enhancing the
dimensional accuracy of parts can also be obtained. In another
interpretation, it may also be thought that occurrence of in-plane
anisotropy during the forming process indicates generation of
unevenness in the strain in the parts and since this causes
deterioration of the dimensional accuracy of parts after
carburizing heat treatment and quenching, the effect of reducing
the in-plane anisotropy by the above-described texture control
brings about an effect of enhancing the dimensional accuracy of
parts.
[0121] <An average grain size of the all grains is from 3 to 50
.mu.m>
[0122] The average grain size of all grains needs to be in the
range of from 3 to 50 .mu.m so as to enhance the formability
(drawability, bendability, press formability) of steel sheet and to
satisfy the surface quality after processing. If the grains are
excessively refined, the deformation resistance is too much
increased, and therefore, the average grain size is set to be 3
.mu.m or more, preferably 4 .mu.m or more, and more preferably 5
.mu.m or more. On the other hand, if the grains grow excessively,
not only the toughness, fatigue properties, etc. are deteriorated
but also the bendability or press moldability such as overhanging
is significantly reduced even when the crystal orientation is
controlled, and cracking during forming process or a defect such as
surface roughening is likely to be generated. Therefore, the
average grain size is set to be 50 .mu.m or less, preferably 45
.mu.m or less and more preferably 40 .mu.m or less. Similarly to
the above, since a size distribution of grains is present in the
sheet thickness direction, the average grain size of all grains has
been specified by employing, as the representative position, a
position of 1/4 sheet thickness in depth.
Method for Measuring Sheet-Plane Orientation of Grain
[0123] The sheet-plane orientation of the grain is
measured/analyzed by means of SEM-EB SP (Electron Back Scattering
Pattern) and EBSD (Electron Back Scattering Diffraction). For
example, SEM (JEOLJSM5410) manufactured by JEOL Ltd. is used as the
SEM apparatus and, for example, EB SP (OM) manufactured by TSL
Corp. is used as the EBSP measurement/analysis system. Although
these may vary depending on the size of the grain, the sample
measurement region is set to 300 to 1,000 .mu.m.times.300 to 1,000
.mu.m and the measurement step interval is set, for example, to 1
to 3 .mu.m. Out of the crystal orientations of respective grains
thus identified, those having an orientation within 10.degree. from
each ideal plane orientation are totalized to determine a total
area, followed by dividing by the area of the measurement region,
whereby an area ratio is determined for every ideal plane
orientation.
Method for Measuring Crystal Direction in Rolling Direction of
Grain
[0124] As for the crystal direction in the rolling direction of the
grain, the cross-section (side surface) in the rolling direction of
the steel sheet is measured with EBSP, and grains having a crystal
direction within 10.degree. from <001> direction and grains
having a crystal direction within 10.degree. from <110>
direction, in the rolling direction, are identified by analysis.
With respect to the measuring method, although these may vary
depending on the size of the grain, the sample measurement region
is set to 300 to 1,000 .mu.m.times.300 to 1,000 .mu.m at a 1/4 part
in the sheet thickness direction and the measurement step interval
is set, for example, to 1 to 3 .mu.m. Out of the crystal directions
of respective grains thus identified, those having an orientation
within 10.degree. from each ideal plane direction are totalized to
determine a total area, followed by dividing by the area of the
measurement region, whereby an area ratio is determined for every
ideal crystal direction.
Method for Measuring Average Grain Size of the All Grains
[0125] As for the average grain size of the above-described all
grains, maximum diameters of each grain observed in a predetermined
measurement region were measured by employing SEM-EBSP above and
the measurement conditions thereof, and an average value of
measured values was determined as the average grain size.
[0126] 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
[0127] 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
[0128] 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.
[0129] 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.
[0130] 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.
[0131] 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.
[0132] 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.
[0133] 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.
[0134] 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.
[0135] 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.
[0136] 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.
[0137] 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.
[0138] 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.
[0139] 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.
[0140] 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.
[0141] 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.
[0142] 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
[0143] 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
[0144] In the rough rolling, the microstructure control of
recrystallized austenite is performed so as to ensure the
percentage of a grain with a predetermined crystal orientation
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 percentage of the grain with a
predetermined crystal orientation can be controlled. The rough
rolling temperature is more preferably from 900 to 1,100.degree.
C.
Hot Finish Rolling
[0145] 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.
Rolling Reduction in Final Pass of Hot Finish Rolling
[0146] If the rolling reduction in the final pass of the hot finish
rolling is too high, the texture configuration of the present
invention cannot be obtained, and the anisotropy increases. On the
contrary, if the rolling reduction is too low, the texture cannot
be developed. Therefore, the rolling reduction in the final pass of
hot finish rolling is set to be from 10 to 18%, preferably from 11
to 17% and particularly preferably from 12 to 16%.
Rapid Cooling After Hot Rolling
[0147] 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
[0148] 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. 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,
hard phases (bainite and martensite) are more produced, and the
cold formability is thereby reduced.
Rapid Cooling and Coiling After Slow Cooling
[0149] 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.
[0150] 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
[0151] 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.
[0152] 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.
[0153] 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.
[0154] 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 cooling rate after stop of rapid cooling
is not shown, but in each production example, a condition of
10.degree. C./s is employed.
[0155] With respect to each of the thus-obtained as-hot rolled
sheets, the sheet-plane orientation of grain, the crystal direction
in the rolling direction of grain, and the average grain size of
all grains were examined by the measuring methods described in the
item of "MODE FOR CARRYING OUT THE INVENTION" above. Here, it has
been confirmed that in all of Steel Nos. 1 to 27 shown in Table 3,
the total amount of ferrite and pearlite was 90% or more in terms
of area ratio (the microstructure mainly contained ferrite and
pearlite).
[0156] In addition, in order to evaluate the drawability on each of
the as-hot rolled sheets above, a JIS No. 5 piece was sampled to be
angled 0.degree. (parallel with rolling direction), 45.degree. or
90.degree. (perpendicular to rolling direction) with respect to the
rolling direction and subjected to a tensile test, and the r value
(r0, r45, r90) at each angle was determined. The average r value
and the .DELTA.r value were calculated according to the following
formula. Here, the .DELTA.r value is an indicator for evaluating
the in-plane anisotropy of the r value.
Average r value=(r0+2.times.r45+r90)/4
.DELTA.r value=(r0+r90)/2-r45
[0157] Those where all of r0, r45, r90, and the average r value are
0.85 or more and the .DELTA.r value is within .+-.0.1 were judged
as passed.
[0158] 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
[0159] 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
[0160] 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.
[0161] 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 Rolling Reduction
Heating Rough Rolling Finish Rolling in Final Pass of Rapid Cooling
Coiling Thickness of Production Steel Temperature Temperature
Temperature Finish Rolling Stop Temperature Temperature Hot-Rolled
No. Species (.degree. C.) (.degree. C.) (.degree. C.) (%) (.degree.
C.) (.degree. C.) Sheet (mm) 1 a 1250 1119 879 13 600 569 3 2 a
1250 1054 906 11 609 598 8 3* a 1000* 891* 771* 13 528* 453* 5 4* a
1250 1081 871 10 651 585 12* 5* a 1250 1184 826 6* 625 560 5 6 b
1250 1098 876 14 601 552 5 7 c 1250 1188 872 14 678 575 5 8 d 1250
958 858 10 640 578 5 9 e 1250 1148 888 14 652 598 5 10 f 1250 1111
853 15 630 565 5 11 g 1250 1199 838 16 638 578 5 12 h 1250 1178 855
14 637 574 5 13 i 1250 1020 874 17 639 547 5 14 j 1250 995 862 10
599 554 5 15 k 1250 962 878 15 638 607 5 16 l 1250 1020 890 16 655
561 5 17 m 1250 1160 863 13 623 621 5 18 n 1250 985 899 13 654 592
5 19 o 1250 1032 862 15 614 595 5 20 p 1250 1098 845 16 613 561 5
21 q 1250 1016 842 13 617 571 5 22 r 1250 1164 894 10 598 598 5 23
s 1250 1118 871 12 616 556 5 24 t 1250 1144 842 12 605 577 5 25 u
1250 1017 897 14 617 578 5 26 v 1250 1047 892 16 599 557 5 27 w
1250 1036 846 12 646 609 5 (underlined = outside the scope of the
present invention, *= outside the recommended range)
TABLE-US-00003 TABLE 3 Surface Grain Hardness after Area Ratio Area
Ratio of Carburizing of (123) <001> Direction + Average
Drawability Hardness Steel Steel Production Plane <110>
Direction Grain Size Average .DELTA.r at Depth of No. Species No.
(%) (%) (.mu.m) r0 r45 r90 r value value 0.8 mm (Hv) Remarks 1 a 1
27 10 25 0.89 0.90 0.87 0.89 -0.021 387 Steel of Invention 2 a 2 42
11 37 0.89 0.90 0.92 0.90 0.002 358 Steel of Invention 3 a 3* 26 32
17 0.80 0.92 0.81 0.86 -0.115 393 Comp. steel 4 a 4* 15 12 56 0.81
0.85 0.82 0.83 -0.035 382 Comp. steel 5 a 5* 16 29 15 0.81 0.91
0.80 0.86 -0.110 394 Comp. steel 6 b 6 31 16 24 0.87 0.88 0.91 0.89
0.008 398 Steel of Invention 7 c 7 30 20 23 0.88 0.89 0.90 0.89
-0.004 401 Steel of Invention 8 d 8 37 15 17 0.86 0.87 0.91 0.88
0.015 405 Steel of Invention 9 e 9 44 17 19 0.86 0.87 0.93 0.88
0.022 402 Steel of Invention 10 f 10 29 18 20 0.87 0.88 0.91 0.88
0.013 402 Steel of Invention 11 g 11 35 17 13 0.88 0.91 0.90 0.90
-0.020 417 Steel of Invention 12 h 12 29 16 12 0.88 0.89 0.89 0.88
-0.002 409 Steel of Invention 13 i 13 29 14 17 0.90 0.91 0.88 0.90
-0.017 405 Steel of Invention 14 j 14 25 14 22 0.88 0.89 0.88 0.88
-0.011 396 Steel of Invention 15 k 15 42 12 22 0.90 0.91 0.89 0.90
-0.012 403 Steel of Invention 16 l 16 39 11 17 0.88 0.89 0.88 0.88
-0.009 412 Steel of Invention 17 m 17 27 16 20 0.87 0.88 0.87 0.88
-0.010 400 Steel of Invention 18 n 18 38 17 18 0.91 0.92 0.90 0.91
-0.017 398 Steel of Invention 19 o 19 27 15 18 0.87 0.88 0.92 0.89
0.018 400 Steel of Invention 20 p 20 39 10 22 0.87 0.88 0.90 0.88
0.007 402 Steel of Invention 21 q 21 36 12 43 0.90 0.91 0.90 0.91
-0.011 246 Comp. steel 22 r 22 26 17 21 0.82 0.83 0.82 0.83 -0.010
458 Comp. steel 23 s 23 32 13 22 0.87 0.88 0.87 0.88 -0.010 292
Comp. steel 24 t 24 37 16 21 0.81 0.85 0.81 0.83 -0.040 410 Comp.
steel 25 u 25 28 12 19 0.82 0.87 0.81 0.84 -0.058 405 Comp. steel
26 v 26 33 19 18 0.83 0.90 0.82 0.86 -0.077 405 Comp. steel 27 w 27
30 18 24 0.82 0.87 0.82 0.85 -0.054 402 Comp. steel (underlined =
outside the scope of the present invention, *= outside the
recommended range)
[0162] 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, both the indicator of drawability 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 drawability can be obtained.
[0163] 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 indicator of drawability and the surface hardness
after a carburizing heat treatment does not meet the acceptance
criteria.
[0164] For example, Steel No. 3 satisfies the requirements for the
component composition, but since all of the heating temperature
before hot rolling, the rough rolling temperature, the finish
rolling temperature, the rapid cooling stop temperature, and the
coiling temperature are outside the recommended range and too low,
grains having <001> direction and <110> direction as
the crystal direction in the rolling direction are excessively
formed, resulting in poor drawability, among others, the anisotropy
of r value.
[0165] 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, a grain having a
sheet-plane orientation of (123) plane is lacking and the grain
grows, resulting in poor drawability.
[0166] Steel No. 5 satisfies the requirements for the component
composition, but since the rolling reduction in the final pass of
finish rolling is outside the recommended range and too small, a
grain having a sheet-plane orientation of (123) plane is lacking
and grains having <001> direction and <110> direction
as the crystal direction in the rolling direction are excessively
formed, resulting in poor drawability, among others, the anisotropy
of r value.
[0167] In Steel No. 21 (steel species q), the hot rolling
conditions are in the recommended range but the C content is too
low, resulting in poor surface hardness after a carburizing heat
treatment.
[0168] In Steel No. 22 (steel species r), the hot rolling
conditions are in the recommended range but the C content is too
high, resulting in poor drawability.
[0169] In Steel No. 23 (steel species s), the hot rolling
conditions are in the recommended range but the Mn content is too
low, resulting in poor surface hardness after a carburizing heat
treatment.
[0170] In Steel No. 24 (steel species t), the hot rolling
conditions are in the recommended range but the Mn content is too
high, resulting in poor drawability.
[0171] In Steel No. 25 (steel species u), the hot rolling
conditions are in the recommended range but the Al content is too
low, resulting in poor drawability.
[0172] 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, resulting in poor drawability as well.
[0173] In Steel No. 27 (steel species w), the hot rolling
conditions are in the recommended range but the N content is too
high, resulting in poor drawability.
[0174] From the above, the applicability of the present invention
could be confirmed.
[0175] 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.
[0176] This application is based on Japanese Patent Application
(Patent Application No. 2013-219468) filed on Oct. 22, 2013, the
contents of which are incorporated herein by way of reference.
INDUSTRIAL APPLICABILITY
[0177] The hot-rolled steel sheet of the present invention exhibits
good cold formability during processing, and, after a carburizing
heat treatment, displays a hardness on a predetermined surface as
well as at a predetermined deep portion from the surface and is
excellent in the wear resistance and anti-fatigue properties, and
therefore, is useful for, for example, clutches, dampers, gears,
etc. used in automobiles.
* * * * *