U.S. patent application number 13/583295 was filed with the patent office on 2013-05-30 for method for manufacturing high strength steel sheet.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is Yoshimasa Funakawa, Hiroshi Matsuda, Yasushi Tanaka. Invention is credited to Yoshimasa Funakawa, Hiroshi Matsuda, Yasushi Tanaka.
Application Number | 20130133786 13/583295 |
Document ID | / |
Family ID | 44563168 |
Filed Date | 2013-05-30 |
United States Patent
Application |
20130133786 |
Kind Code |
A1 |
Matsuda; Hiroshi ; et
al. |
May 30, 2013 |
METHOD FOR MANUFACTURING HIGH STRENGTH STEEL SHEET
Abstract
A method for manufacturing a high strength steel sheet includes
heating a steel sheet containing at least 0.10 mass % of carbon to
either a temperature in an austenite single phase region or a
temperature in an (austenite+ferrite) two-phase region; cooling the
steel sheet to a cooling stop temperature as a target temperature
set within a cooling temperature region ranging from Ms to
(Ms-150.degree. C.) to allow a portion of non-transformed austenite
to proceed to martensitic transformation; retaining a coldest part
in a sheet widthwise direction of the steel sheet at a temperature
in a temperature range from the cooling stop temperature as the
target temperature to (the cooling stop temperature+15.degree. C.)
for 15 seconds to 100 seconds; and heating the sheet to a
temperature to temper said martensite, wherein "Ms" represents
martensitic transformation start temperature and said cooling
temperature region is exclusive of Ms and inclusive of
(Ms-150.degree. C.).
Inventors: |
Matsuda; Hiroshi; (Tokyo,
JP) ; Funakawa; Yoshimasa; (Tokyo, JP) ;
Tanaka; Yasushi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Matsuda; Hiroshi
Funakawa; Yoshimasa
Tanaka; Yasushi |
Tokyo
Tokyo
Tokyo |
|
JP
JP
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
44563168 |
Appl. No.: |
13/583295 |
Filed: |
February 28, 2011 |
PCT Filed: |
February 28, 2011 |
PCT NO: |
PCT/JP2011/001163 |
371 Date: |
November 5, 2012 |
Current U.S.
Class: |
148/533 ;
148/661 |
Current CPC
Class: |
C21D 8/02 20130101; C22C
38/001 20130101; C21D 6/001 20130101; C21D 6/002 20130101; C21D
8/0205 20130101; C21D 9/46 20130101; C21D 6/008 20130101; C22C
38/04 20130101; C22C 38/06 20130101; C23C 2/28 20130101; C21D 1/22
20130101; C21D 2211/008 20130101; C22C 38/02 20130101; C21D 8/00
20130101; C21D 2211/002 20130101; C21D 6/005 20130101; C23C 2/02
20130101 |
Class at
Publication: |
148/533 ;
148/661 |
International
Class: |
C21D 6/00 20060101
C21D006/00; C23C 2/02 20060101 C23C002/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 9, 2010 |
JP |
2010-052323 |
Claims
1. A method for manufacturing a high strength steel sheet
comprising: heating a steel sheet containing at least 0.10 mass %
of carbon to either a temperature in an austenite single phase
region or a temperature in an (austenite+ferrite) two-phase region;
cooling the steel sheet to a cooling stop temperature as a target
temperature set within a cooling temperature region ranging from Ms
to (Ms-150.degree. C.) to allow a portion of non-transformed
austenite to proceed to martensitic transformation; retaining a
coldest part in a sheet widthwise direction of the steel sheet at a
temperature in a temperature range from the cooling stop
temperature as the target temperature to (the cooling stop
temperature+15.degree. C.) for 15 seconds to 100 seconds; and
heating the sheet to a temperature to temper said martensite,
wherein "Ms" represents martensitic transformation start
temperature and said cooling temperature region is exclusive of Ms
and inclusive of (Ms-150.degree. C.).
2. The method of claim 1, further comprising subjecting the steel
sheet to a hot dip galvanizing process or a galvannealing process
either: between completion of the heating process to a temperature
in either the austenite single phase region or the
(austenite+ferrite) two-phase region and completion of the cooling
process; or during the tempering process; or during a process after
the tempering process.
3. The method of claim 1, wherein the steel sheet has a composition
including by mass %, C: 0.10% to 0.73%, Si: 3.0% or less, Mn 0.5%
to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, N:
0.010% or less, and remainder as Fe and incidental impurities.
4. The method of claim 3, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Cr: 0.05% to 5.0%, V: 0.005% to 1.0%
and Mo: 0.005% to 0.5%.
5. The method of claim 3, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ti: 0.01% to 0.1% and Nb: 0.01% to
0.1%.
6. The method of claim 3, wherein the composition of the steel
sheet further comprises, by mass %, B: 0.0003% to 0.0050%.
7. The method of claim 3, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ni: 0.05% to 2.0% and Cu: 0.05% to
2.0%.
8. The method of claim 3, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001%
to 0.005%.
9. The method of claim 2, wherein the steel sheet has a composition
including by mass %, C: 0.10% to 0.73%, Si: 3.0% or less, Mn 0.5%
to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, N:
0.010% or less, and remainder as Fe and incidental impurities.
10. The method of claim 4, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ti: 0.01% to 0.1% and Nb: 0.01% to
0.1%.
11. The method of claim 4, wherein the composition of the steel
sheet further comprises, by mass %, B: 0.0003% to 0.0050%.
12. The method of claim 5, wherein the composition of the steel
sheet further comprises, by mass %, B: 0.0003% to 0.0050%.
13. The method of claim 4, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ni: 0.05% to 2.0% and Cu: 0.05% to
2.0%.
14. The method of claim 5, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ni: 0.05% to 2.0% and Cu: 0.05% to
2.0%.
15. The method of claim 6, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ni: 0.05% to 2.0% and Cu: 0.05% to
2.0%.
16. The method of claim 4, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001%
to 0.005%.
17. The method of claim 5, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001%
to 0.005%.
18. The method of claim 6, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001%
to 0.005%.
19. The method of claim 7, wherein the composition of the steel
sheet further comprises by mass % at least one element selected
from the group consisting of Ca: 0.001% to 0.005% and REM: 0.001%
to 0.005%.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2011/001163, with an inter-national filing date of Feb. 28,
2011 (WO 2011/111332 A1, published Sep. 15, 2011), which is based
on Japanese Patent Application No. 2010-052323, filed Mar. 9, 2010,
the subject matter of which is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to a method for manufacturing a high
strength steel sheet being excellent in formability in particular
ductility and stretch-flangeability and having tensile strength of
at least 980 MPa for use in the industrial fields of automobiles,
electric appliances and the like.
BACKGROUND
[0003] Improving fuel efficiency of automobiles has been an
important task in recent years from the viewpoint of global
environment protection. Due to this, there has been vigorous trend
toward making vehicle body parts thin by increasing the strength of
vehicle body materials to reduce weight of vehicles.
[0004] In general, the proportion of a hard phase such as
martensite and bainite with respect to the entire microstructure of
a steel sheet must be increased to increase the strength of the
steel sheet. However, enhancing the strength of a steel sheet by
increasing the proportion of a hard phase thereof tends to
deteriorate formability of the steel sheet. Therefore, there has
been a demand for a steel sheet having both high strength and good
formability in a compatible manner. There have been developed up to
now various types of multi-phase steel sheets such as
ferrite-martensite dual phase steel (DP steel), TRIP steel
utilizing transformation-induced plasticity of retained austenite,
and the like.
[0005] In a case where the proportion of a hard phase is increased
in a multi-phase steel sheet, formability of the steel sheet is
strongly influenced by formability of the hard phase because not
only deformability of polygonal ferrite, but deformability of a
hard phase itself directly affects formability of the steel sheet.
As a result, formability of a resulting steel sheet significantly
deteriorates if formability of the hard phase is insufficient as
described above where the proportion of a hard phase is increased.
In contrast, deformability of polygonal ferrite dominates
formability of a steel sheet to ensure good formability, e.g., good
ductility, in spite of poor formability of a hard phase where the
steel sheet contains soft polygonal ferrite at a relatively high
content and the hard phase at a relatively low content.
[0006] In view of this, there have conventionally been attempts to:
subject a cold rolled steel sheet to a thermal treatment to adjust
the content of polygonal ferrite therein generated by our annealing
process and cooling process thereafter; allow martensite to be
formed by water-quenching the steel sheet thus treated; and temper
martensite by heating the steel sheet to relatively high
temperature and retaining the steel sheet in that state to allow
carbides to form in martensite as a hard phase, thereby improving
formability of martensite.
[0007] In such a case of employing such conventional facilities for
continuous annealing and water-quenching as described above,
however, the temperature of a steel sheet after quenching naturally
drops to a temperature around the water temperature and most of
non-transformed austenite experiences martensitic transformation,
whereby it is difficult to utilize low-temperature transformed
microstructure such as retained austenite and the like. In other
words, improvement of formability of a hard microstructure totally
depends on an effect caused by martensite tempering. Improvement of
formability of a steel sheet is thus significantly limited in the
case of employing facilities for continuous annealing and
water-quenching.
[0008] Alternatively, there has been proposed as a steel sheet
having a hard phase other than martensite a steel sheet including
polygonal ferrite as a main phase and bainite and pearlite as hard
phases with carbides formed in bainite and pearlite as the hard
phases. This steel sheet aims to improve formability thereof not
only by use of polygonal ferrite as the main phase, but also by
formation of carbides in the hard phases to improve formability in
particular stretch-flangeability of the hard phases themselves.
[0009] JP-A 04-235253, example, proposes a high tensile strength
steel sheet having excellent bendability and impact properties,
manufactured by specifying alloy components and obtaining steel
microstructure constituted of fine and uniform bainite having
retained austenite.
[0010] JP-A 2004-076114 proposes a multi-phase steel sheet having
excellent bake hardenability, manufactured by specifying types and
contents of alloy components, obtaining steel microstructure mainly
constituted of bainite having retained austenite and controlling
the content of the retained austenite in bainite.
[0011] Further, JP-A 11-256273 proposes a multi-phase steel sheet
having excellent impact resistance, manufactured by specifying
types and contents of alloy components, obtaining steel
microstructure including at least 90% (by area ratio) bainite
having retained austenite and 1%-15% retained austenite in bainite
and setting hardness (HV) of bainite in a specific range.
[0012] The aforementioned steel sheet, however, has problems
described below.
[0013] The component composition described in JP-A 04-235253 cannot
ensure a sufficient content of stable retained austenite to express
a TRIP effect in a high strain region of a resulting steel sheet
when the steel sheet is imparted with strains, whereby the steel
sheet exhibits poor ductility prior to reaching plastic instability
and poor stretchability, although bendability thereof is relatively
good.
[0014] The steel sheet of JP-A 2004-076114, although it has good
bake hardenability, experiences difficulties not only in achieving
high tensile strength (TS) equal to or higher than 980 MPa or 1050
MPa, but also in ensuring satisfactory formability such as
ductility and stretch-flangeability when strength thereof is
ensured or increased due to its microstructure primarily containing
bainite or ferrite with martensite reduced as best as possible.
[0015] The steel sheet of JP-A 11-256273 primarily aims to improve
impact resistance and the microstructure thereof includes as a main
phase bainite having hardness (HV) of 250 or less at a content
exceeding 90%, whereby it is very difficult to achieve tensile
strength (TS) of at least 980 MPa in that steel sheet.
[0016] It is reasonably assumed that, among automobile parts to be
formed by press-forming, automobile structural members having
relatively complicated shapes such as a center pillar inner
generally require a tensile strength of at least 980 MPa and, in
the future, possibly at least 1180 MPa class.
[0017] Further, a steel sheet for use as a material of vehicle
parts requiring high strength in particular such as a door impact
beam, a bumper reinforcement to suppress deformation during a car
collision generally necessitates a tensile strength of at least
1180 MPa class and, in the future, possibly at least 1470 MPa
class.
[0018] Various types of steel sheets have been developed as
described above as the demand for a steel sheet having higher
strength increases. It is very important to ensure good stability
in mechanical properties of a high strength steel sheet in terms of
reliably obtaining good formability of the steel sheet in a stable
manner. In view of this, there has been developed, for example, a
multi-phase high strength steel sheet including various types of
hard microstructures manufactured by utilizing various types of
hard microstructures, transformed from non-transformed austenite in
a relatively low temperature range to avoid having an overall
microstructure constituted of a single phase such as martensite. It
is very important in such a multi-phase microstructure as described
above to control fractions of respective hard phases or
microstructures with good precision in terms of stabilizing
mechanical properties of the resulting multi-phase high strength
steel sheet. However, precision in fraction control is not yet
sufficiently high in such a case as described above.
[0019] Specifically, variation in sheet temperature within a steel
sheet tends to occur when the steel sheet is subjected to thermal
treatment such as finish annealing. Accordingly, when such a steel
sheet having a variation in sheet temperature as described above is
rapidly cooled to a target temperature to allow martensite to be
formed by a predetermined content, martensite is not formed at a
uniform content, but the formation ratio of martensite rather
varies across the steel sheet due to the aforementioned variation
in sheet temperature. As a result, there arises variations in
mechanical properties of the resulting steel sheet.
[0020] It could therefore be helpful to provide a method for
manufacturing a high strength steel sheet having tensile strength
(TS) of at least 980 MPa, being excellent in formability in
particular ductility and stretch-flangeability and exhibiting good
stability in mechanical properties.
[0021] Specifically, it could be helpful to provide a high strength
steel sheet having high strength and good formability in a
compatible manner by transforming a portion of non-transformed
austenite into tempered martensite and the rest of the
non-transformed austenite into microstructures such as bainite and
retained austenite. The high strength steel sheet should also
include a steel sheet of which surface has been further treated by
hot dip galvanizing or galvannealing.
SUMMARY
[0022] Hereinafter, "Being excellent in formability" represents a
condition that a product of tensile strength and total elongation,
i.e., (TS.times.T. EL), is equal to or higher than 20000 MPa% and a
condition that a product of tensile strength and critical hole
expansion ration, i.e., (TS.times..lamda.), is equal to or higher
than 25000 MPa% are both satisfied. Further, "Being excellent in
stability of mechanical properties" represents that the standard
deviation .sigma. of TS in the sheet widthwise direction and the
standard deviation .sigma. of T. EL are not larger than 10 MPa and
not larger than 2.0%, respectively.
[0023] Wherein a desired microstructure, e.g., a predetermined
ratio of martensite, is to be formed in a steel sheet, the steel
sheet is cooled to particular target temperature which is set
accordingly. However, the steel sheet to be thus cooled tends to
have variation in sheet temperature due to the preceding thermal
treatment as described above. Accordingly, in a case where a such a
steel sheet having variation in sheet temperature thereof as
described above is cooled and when the temperature of a part of the
steel sheet where temperature is lowest (the coldest part) reaches
the target temperature as shown in FIG. 1(a), martensite has not
been so sufficiently formed in a part of the steel sheet where
temperature is highest (the hottest part) as in the coldest part,
whereby variation arises in microstructure of the steel sheet.
Meanwhile, when the temperature of the hottest part of the steel
sheet reaches the target temperature as shown in FIG. 1(b),
martensitic transformation has proceeded too far in the coldest
part of the steel sheet, thereby worsening variation in
microstructure of the steel sheet.
[0024] In short, variation in sheet temperature within a steel
sheet results in non-uniform microstructure of steel and thus
inevitably in variation in mechanical properties of the steel
sheet.
[0025] We discovered that the microstructure of a steel sheet is
made uniform and thus variations in mechanical properties such as
strength of the steel sheet can be reduced by setting thermal
treatment conditions around a target temperature to select the
coldest part of a steel sheet as the reference region, cool the
coldest part to the target temperature as shown in FIG. 1(c), and
retain the steel sheet in a temperature range slightly above target
temperature for a predetermined time.
[0026] We thus provide: [0027] (1) A method for manufacturing a
high strength steel sheet, comprising the steps of: heating a steel
sheet containing at least 0.10 mass % of carbon to either
temperature in the austenite single phase region or temperature in
the (austenite+ferrite) two-phase region; cooling the steel sheet
to cooling stop temperature as target temperature set within a
cooling temperature region ranging from Ms to (Ms-150.degree. C.)
to allow a portion of non-transformed austenite to proceed to
martensitic transformation; and heating the sheet temperature to
temper the martensite, characterized in that the method further
comprises retaining the coldest part in the sheet widthwise
direction of the steel sheet at temperature in a temperature range
from the cooling stop temperature as the target temperature to (the
cooling stop temperature+15.degree. C.) for a period ranging from
15 seconds to 100 seconds (inclusive of 15 seconds and 100
seconds), [0028] wherein "Ms" represents martensitic transformation
start temperature and the cooling temperature region is exclusive
of Ms and inclusive of (Ms-150.degree. C.). [0029] (2) The method
for manufacturing a high strength steel sheet of (1) above, further
comprising subjecting the steel sheet to hot dip galvanizing
process or galvannealing process either: between completion of the
heating process to temperature in either the austenite single phase
region or the (austenite+ferrite) two-phase region and completion
of the cooling process; or during the tempering process; or during
a process after the tempering process. [0030] (3) The method for
manufacturing a high strength steel sheet of (1) or (2) above,
wherein the steel sheet has a composition including by mass %,
[0031] C: 0.10% to 0.73%, [0032] Si: 3.0% or less, [0033] Mn 0.5%
to 3.0%, [0034] P: 0.1% or less, [0035] S: 0.07% or less, [0036]
Al: 3.0% or less, [0037] N: 0.010% or less, and [0038] remainder as
Fe and incidental impurities. [0039] (4) The method for
manufacturing a high strength steel sheet of (3) above, wherein the
composition of the steel sheet further includes by mass % at least
one type of elements selected from [0040] Cr: 0.05% to 5.0%, [0041]
V: 0.005% to 1.0%, and [0042] Mo: 0.005% to 0.5%. [0043] (5) The
method for manufacturing a high strength steel sheet of (3) or (4)
above, wherein the composition of the steel sheet further includes
by mass % at least one type of elements selected from [0044] Ti:
0.01% to 0.1%, and [0045] Nb: 0.01% to 0.1%. [0046] (6) The method
for manufacturing a high strength steel sheet of any of (3) to (5)
above, wherein the composition of the steel sheet further includes,
by mass %, B: 0.0003% to 0.0050%. [0047] (7) The method for
manufacturing a high strength steel sheet of any of (3) to (6)
above, wherein the composition of the steel sheet further includes
by mass % at least one type of elements selected from [0048] Ni:
0.05% to 2.0%, and [0049] Cu: 0.05% to 2.0%. [0050] (8) The method
for manufacturing a high strength steel sheet of any of (3) to (7)
above, wherein the composition of the steel sheet further includes
by mass % at least one type of elements selected from [0051] Ca:
0.001% to 0.005%, and [0052] REM: 0.001% to 0.005%.
[0053] It is thus possible to provide a high strength steel sheet
being excellent in formability and exhibiting excellent stability
in mechanical properties thereof. As a result, it is possible to
reduce thickness of a steel sheet and weight thereof, thereby
effectively reducing weight of an automobile body.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] FIGS. 1(a)-1(c) are diagrams each showing a temperature
pattern in a thermal treatment for forming martensite by a
predetermined ratio by heating and rapidly cooling a steel
sheet.
[0055] FIG. 2 is a diagram showing a temperature pattern in a
thermal treatment in our method for manufacturing a high strength
steel sheet.
DETAILED DESCRIPTION
[0056] Our steel sheets and methods will be described in detail
hereinafter.
[0057] First, a steel sheet material as a starting steel material
for manufacturing a high strength steel sheet is prepared by
subjecting a steel sheet having a component composition adjusted to
contain at least 0.10 mass % of carbon ("mass %" for a steel sheet
component will be abbreviated to "%" hereinafter) to hot rolling
process and, optionally, cold rolling process. These hot rolling
and cold rolling processes are not particularly restricted and may
be carried out according to the conventional methods.
[0058] The high strength steel sheet needs to contain at least
0.10% of carbon therein because carbon is an essential element in
terms of increasing strength of the steel sheet, ensuring
necessitated content of martensite and making austenite be retained
at the room temperature.
[0059] Typical manufacturing conditions of a cold rolled steel
sheet as a steel sheet material are as follows.
[0060] Manufacturing conditions of a cold rolled steel sheet
include, for example: heating a steel material to temperature in
the range of 1000.degree. C. to 1300.degree. C.; finishing hot
rolling at temperature in the range of 870.degree. C. to
950.degree. C.; and subjecting a hot rolled steel sheet thus
obtained to coiling at temperature in the range of 350.degree. C.
to 720.degree. C., pickling, and cold rolling at rolling reduction
rate in the range of 40% to 90% to obtain a cold rolled steel sheet
(a steel sheet material).
[0061] It is acceptable in preparing a steel sheet material to skip
at least a part of the hot rolling process by employing thin slab
casting, strip casting or the like.
[0062] A high strength steel sheet is then manufactured from the
(cold rolled) steel sheet material thus obtained according to our
method including the following processes.
[0063] FIG. 2 shows one example of temperature pattern in thermal
treatment of the method for manufacturing a high strength steel
sheet.
[0064] A steel sheet material is heated for annealing to either
temperature in the austenite single phase region or temperature in
the (austenite+ferrite) two-phase region as shown in FIG. 2. The
annealing temperature is not particularly restricted as long as it
is equal to or higher than the temperature within the
(austenite+ferrite) two-phase region. However, if the annealing
temperature exceeding 1000.degree. C. causes austenite grains to
grow excessively, thereby coarsening gains of respective
microstructures generated by cooling thereafter, which
microstructures constitute a resulting steel sheet, to eventually
deteriorate toughness and the like of the steel sheet. Accordingly,
the annealing temperature is preferably 1000.degree. C. or
lower.
[0065] When the annealing time is shorter than 15 seconds, carbides
already existing in a steel sheet prior to the annealing may not be
dissolved sufficiently and/or reverse transformation of the
microstructures of the steel sheet into austenite may not proceed
sufficiently. When the annealing time exceeds 600 seconds, the
processing cost increases due to too much energy consumption.
Accordingly, the annealing time is to be in the range of 15 seconds
to 600 seconds.
[0066] The steel sheet thus annealed is cooled to a first
temperature region ranging from (martensite start temperature
Ms-150.degree. C.) to Ms (inclusive of (Ms-150.degree. C.) and
exclusive of Ms) as shown in FIG. 2. Cooling stop temperature: T1
(which will be referred to as "T1" hereinafter) as the target
temperature is set within the first temperature region.
[0067] The purpose of this cooling process is to cool the steel
sheet below the Ms point such that a portion of austenite proceeds
to martensitic transformation. In a case where the lower limit of
the first temperature region is set to be below (Ms-150.degree.
C.), most of non-transformed austenite proceeds to martensitic
transformation by the cooling process and thus it is not possible
to utilize microstructures like retained austenite which are
effective in terms of improving formability of a steel sheet.
[0068] In a case where the upper limit of the first temperature
region is set to exceed the Ms point, martensite may not have been
formed in a sufficient content in the steel sheet when the cooling
process is stopped, whereby tempered martensite cannot be reliably
obtained in a sufficient content in the heating or tempering
process thereafter. Accordingly, the first temperature region,
within which T1 is set, is to range from (Ms-150.degree. C.) to Ms,
wherein (Ms-150.degree. C.) is inclusive and Ms is exclusive.
[0069] The average cooling rate of a steel sheet until the
temperature of the steel sheet drops to the first temperature
region is not particularly restricted. However, the cooling rate
lower than 3.degree. C./s (".degree. C./s" represents ".degree.
C./second") results in excess formation and growth of polygonal
ferrite and precipitation of pearlite and the like, which makes it
impossible to obtain the desired microstructure of a steel sheet.
Accordingly, the average cooling rate from the annealing
temperature to the first temperature region is to be at least
3.degree. C./s.
[0070] It is particularly important that, when a portion of
non-transformed austenite is made to proceed to martensitic
transformation by cooling, the temperature of the coldest part in
the sheet widthwise direction of a steel sheet is retained within
the first temperature region (indicated as a hatched area in FIG.
2) and also in a temperature range or sub-region ranging from the
target cooling stop temperature T1 to (T1+15.degree. C.). In a case
where the temperature of the coldest part of the steel sheet is
below T1.degree. C., non-transformed austenite proceeds to
martensitic transformation excessively in some parts of the steel
sheet to form too much martensite, thereby exceeding the target
content thereof expected at the target temperature T1. As a result,
variations in martensite cannot be eliminated and the desired
properties cannot be stably obtained in these parts of the steel
sheet even after retaining the steel sheet for a predetermined
period. In a case where the temperature of the coldest part of the
steel sheet exceeds (T1+15.degree. C.), martensite is not formed
sufficiently and fails to meet the target content thereof expected
at the target temperature T1 in some part of the steel sheet. As a
result, there arise variations in contents of bainite, retained
austenite and tempered martensite formed during the heating or
tempering process thereafter, whereby the desired properties cannot
be stably obtained in the resulting steel sheet.
[0071] It is necessary to retain the coldest part of a steel sheet
at a temperature in the range of T1 to (T1+15.degree. C.) for a
period ranging from 15 seconds to 100 seconds. Sheet temperature of
parts other than the coldest part of a steel sheet may not
sufficiently drop and these parts may fail to have the desired
steel sheet microstructure, thereby generating variations in
formability within the steel sheet, in a case where the retention
time of the coldest part of the steel sheet at a temperature in the
range of T1 to (T1+15.degree. C.) is shorter than 15 seconds. A
retention time exceeding 100 seconds would simply meaninglessly
prolong the processing time because then an effect of making sheet
temperature of parts other than the coldest part of a steel sheet
follow the temperature of the coldest part, caused by the retention
time, reaches a plateau.
[0072] "The coldest part" of a steel sheet represents the part at
which sheet temperature is coldest in the sheet widthwise direction
of the steel sheet. The coldest part of a steel sheet is normally
an edge portion of the steel sheet, but may be another portion
depending on the characteristics of a production line. In a case
where there is a possibility that a portion other than an edge
portion of a steel sheet will be the coldest part of the steel
sheet, it is preferable that the steel sheet is in advance tested
to investigate the coldest part thereof so that sheet temperature
of the coldest part can be reliably controlled during the actual
manufacturing process.
[0073] Manufacturing facilities are preferably equipped with a
thermometer capable of confirming sheet temperature distribution
across the entire sheet width of a steel sheet in terms of
achieving reliable measurement of actual temperature of the coldest
part of the steel sheet. If manufacturing facilities lack such a
thermometer as described above, these facilities can still control
thermal processing conditions by finding out the coldest part of a
steel sheet by an experiment in advance as described above and
measuring and controlling the temperature of the coldest part of
the steel sheet thus determined.
[0074] Further, sectioning a steel sheet in the sheet widthwise
direction into several blocks and carrying out feedback control of
respective sheet temperatures in the respective blocks are
effective in terms of reliably keeping sheet temperature of the
steel sheet which is being retained within the temperature range of
T1 to (T1+15.degree. C.).
[0075] As described above, it is possible to remarkably decrease
variations in mechanical properties such as tensile strength within
a high strength steel sheet by retaining the coldest part of the
steel sheet at a predetermined temperature for a predetermined
period.
[0076] The mechanism of such a decrease in variations as described
above is not clear. We believe that: if the concentration of
martensite formed within a steel sheet has varied because
temperatures of some parts of the steel sheet dropped too low from
the Ms point due to variations in sheet temperature in the sheet
thickness direction and the widthwise direction with respect to the
sheet-feeding direction, the magnitude of martensite formation
within the steel sheet can be made stable by carrying out the
aforementioned unique thermal processing; and as a result the
magnitude of martensitic transformation across the entire steel
sheet is made uniform and mechanical properties of the steel sheet
are rendered stable across the entire steel sheet.
[0077] Next, the steel sheet thus retained at temperature in the
first temperature region is heated by a conventional method and
subjected to martensite-tempering process as shown in FIG. 2.
[0078] Although a temperature range for this tempering process is
not particularly restricted, the tempering temperature is
preferably equal to or higher than 200.degree. C. in view of
tempering efficiency of martensite. In a case where the cooling
stop temperature is equal to or higher than 200.degree. C., the
heating process for tempering can be omitted by simply retaining a
steel sheet at a temperature in the temperature range equal to or
higher than 200.degree. C. The tempering temperature is preferably
equal to or lower than 570.degree. C. because carbides are
precipitated from non-transformed austenite and desired
microstructures may not be obtained when the upper limit of the
tempering temperature exceeds 570.degree. C.
[0079] Retention time after raising the temperature of a steel
sheet to the tempering temperature is not particularly restricted.
However, a retention time shorter than 5 seconds may result in
insufficient tempering of martensite, which makes it impossible to
obtain the desired microstructures in a resulting steel sheet and
possibly deteriorates formability of the steel sheet. A retention
time exceeding 1000 seconds, for example, causes carbides to be
precipitated from non-trans-formed austenite and stable retained
austenite having relatively high carbon concentration cannot be
obtained as the final microstructure of a resulting steel sheet,
whereby the resulting steel sheet may not have at least one of
desired strength and ductility. Accordingly, the retention time of
retaining a steel sheet for tempering is preferably in the range of
5 seconds to 1000 seconds.
[0080] The retention temperature in the aforementioned thermal and
tempering processes need not be constant and may vary within such a
predetermined temperature range as described above. In other words,
a variation in the retention temperature within the predetermined
temperature range does not have an adverse effect. Similar
tolerance is applied to the cooling rate and the cooling rate may
vary to some extent. Further, the steel sheet may be subjected to
the relevant thermal treatments in any facilities as long as the
required thermal history is satisfied. Yet further, subjecting a
surface of the steel sheet to temper-rolling for shape correction
and/or a surface treatment such as electrolytic plating after the
thermal treatment is included.
[0081] The method for manufacturing a high strength steel sheet may
further include subjecting the steel sheet to hot dip galvanizing
process or galvannealing process (galvannealing process is
combination of hot dip galvanizing and alloying process
thereafter). In the case of carrying out hot dip galvanizing
process or galvannealing process during the martensite-tempering
process in the tempering temperature range, the total retention
time at temperature in the tempering temperature region, including
processing time for the hot dip galvanizing process or the
galvannealing process, is still within the range of 5 seconds to
1000 seconds.
[0082] The hot dip galvanizing process and the galvannealing
process are preferably carried out in a continuous galvanizing
line.
[0083] In the method for manufacturing a high strength steel sheet,
it is acceptable to complete the method to the final thermal
treatment to obtain a high strength steel sheet and then subject
the high strength steel sheet to hot dip galvanizing process and
galvannealing process later.
[0084] A method for subjecting the steel sheet to hot dip
galvanizing process and a method for subjecting the steel sheet to
galvannealing process are typically carried out as follows.
[0085] A steel sheet is immersed in a plating bath and then coating
weight is adjusted by gas wiping or the like. Aluminum content
dissolved in the plating bath is preferably in the range of 0.12
mass % and 0.22 mass % in hot dip galvanizing and in the range of
0.08 mass % and 0.18 mass % in galvannealing, respectively.
Temperature of a plating bath may be in the range of 450.degree. C.
to 500.degree. C. in hot dip galvanizing. In a case where
galvannealing is further carried out, temperature during the
alloying process is preferably 570.degree. C. or lower.
[0086] An alloying temperature exceeding 570.degree. C. results in
precipitation of carbides from non-transformed austenite and
possibly formation of pearlite, which may lead to failure in
obtaining at least one of good strength and good formability, as
well as deterioration of anti-powdering property of a coating layer
in a resulting coated steel sheet. However, the galvannealing
process may not proceed smoothly when the alloying temperature is
below 450.degree. C. Accordingly, the alloying temperature is
preferably equal to or higher than 450.degree. C.
[0087] The coating weight per one surface of a steel sheet is
preferably in the range of 20 g/m.sup.2 to 150 g/m.sup.2 in a case
where the steel sheet is subjected to coating such as galvanizing.
A coating weight less than 20 g/m.sup.2 results in poor corrosion
resistance, while a corrosion resisting effect reaches a plateau
and production cost meaninglessly increases when the coating weight
exceeds 150 g/m.sup.2.
[0088] The alloy degree of a coating layer (i.e., Fe % or Fe
content in a coating layer) is preferably in the range of 7% to
15%. An alloy degree of a coating layer less than 7% results in
uneven alloying to deteriorate appearance quality of a resulting
coated steel sheet and/or formation of what is called .zeta. phase
in the coating layer to deteriorate sliding properties of a
resulting coated steel sheet. An alloy degree of a coating layer
exceeding 15 mass % results in excess formation of hard and brittle
.GAMMA. phase to deteriorate coating adhesion properties of a
resulting coated steel sheet.
[0089] In addition to the foregoing descriptions of the primary
features regarding conditions in manufacturing a high strength
steel sheet, a component composition of a steel sheet preferable as
a steel sheet material for the manufacturing method will be
described next.
C: 0.10% to 0.73%
[0090] At least 0.10% of carbon is required in the steel sheet as
described above.
[0091] However, a carbon content exceeding 0.73% significantly
hardens a welded portion and surrounding portions affected by
welding heat, thereby deteriorating weldability of a resulting
steel sheet. Accordingly, the upper limit of carbon content in
steel is preferably 0.73%. Carbon content in steel is more
preferably in the range of 0.15% to 0.48% (exclusive of 0.15% and
inclusive of 0.48%).
Si: 3.0% or less
[0092] Silicon is a useful element which contributes to increasing
strength of a steel sheet through solute strengthening. However,
silicon content in steel exceeding 3.0% deteriorates: formability
and toughness due to increase in content of solute Si in polygonal
ferrite and bainitic ferrite; and coatability and coating adhesion
of plating when the steel sheet is subjected to hot dip
galvanizing. Accordingly, Si content in steel is 3.0% or less,
preferably 2.6% or less, and more preferably 2.2% or less.
[0093] The silicon content in steel is preferably at least 0.5%
because silicon is a useful element in terms of suppressing
formation of carbide and facilitating formation of retained
austenite. However, silicon need not be added and thus Si content
may be zero % in a case where formation of carbide is suppressed by
only aluminum.
Mn: 0.5% to 3.0%
[0094] Manganese is an element which effectively increases steel
strength. A manganese content less than 0.5% in steel causes
carbide to be precipitated at temperature higher than the
temperature at which bainite and martensite are formed when a steel
sheet is cooled after annealing, thereby making it impossible to
reliably obtain a sufficient content of hard phase contributing to
steel strengthening. An Mn content exceeding 3.0% may deteriorate
forgeability of the steel. Accordingly, the Mn content in the steel
is preferably in the range of 0.5% to 3.0% and more preferably in
the range of 1.5% to 2.5%.
P: 0.1% or less
[0095] Phosphorus is a useful element in terms of increasing steel
strength. However, a phosphorus content in steel exceeding 0.1%;
makes steel brittle due to grain boundary segregation of phosphorus
to deteriorate impact resistance of a resulting steel sheet; and
significantly slows galvannealing (alloying) rate down in a case
the steel sheet is subjected to galvannealing. Accordingly, the
phosphorus content in the steel is 0.1% or less and preferably
0.05% or less.
[0096] The lower limit of the phosphorus content in the steel is
preferably around 0.005% because an attempt to reduce the
phosphorus content below 0.005% significantly increases production
costs, although the phosphorus content in the steel is to be
decreased as best as possible.
S: 0.07% or less
[0097] Sulfur forms inclusions such as MnS and may be a cause of
deterioration of impact resistance and generation of cracks along
metal flow at a welded portion of a steel sheet. It is thus
preferable that sulfur content in the steel is reduced as best as
possible. However, decreasing the sulfur content in the steel to an
exorbitantly low level increases production costs. Accordingly,
presence of sulfur in the steel is tolerated unless the sulfur
content in the steel exceeds 0.07% or so. The sulfur content in the
steel is preferably 0.05% or less, and more preferably 0.01% or
less. The lower limit of the sulfur content in the steel is around
0.0005% in view of production costs because decreasing the sulfur
content in the steel below 0.0005% significantly increases
production costs.
Al: 3.0% or less
[0098] Aluminum is a useful element added as a deoxidizing agent in
a steel manufacturing process. However, an aluminum content
exceeding 3.0% may deteriorate ductility of a steel sheet due to
too much inclusion in the steel sheet. Accordingly, the aluminum
content in steel is 3.0% or less and preferably 2.0% or less.
[0099] Further, aluminum is a useful element in terms of
suppressing formation of carbides and facilitating formation of
retained austenite. The aluminum content in steel is preferably at
least 0.001% and preferably at least 0.005% to sufficiently obtain
this good effect of aluminum.
[0100] The aluminum content represents content of aluminum
contained in a steel sheet after deoxidization.
N: 0.010% or less
[0101] Nitrogen is an element which most significantly deteriorates
anti-aging property of steel and thus the content thereof in the
steel is preferably decreased as best as possible. However, the
presence of nitrogen in the steel is tolerated unless the nitrogen
content in the steel exceeds 0.010% or so. The lower limit of the
nitrogen content in the steel is around 0.001% in view of
production costs because decreasing the nitrogen content in the
steel below 0.001% significantly increases production costs.
[0102] The composition of the steel sheet may further include, in
addition to the aforementioned optional components other than
carbon, the following components in an appropriate manner.
At least one type of element selected from Cr: 0.05% to 5.0%, V:
0.005% to 1.0%, and Mo: 0.005% to 0.5%
[0103] Chromium, vanadium and molybdenum are elements which each
suppress formation of pearlite when a steel sheet is cooled from
the annealing temperature. These good effects of Cr, V and Mo are
obtained when the contents of Cr, V and Mo in steel are at least
0.05%, at least 0.005% and at least 0.005%, respectively. However,
the contents of Cr, V and Mo in steel exceeding 5.0%, 1.0% and
0.5%, respectively, results in too much formation of hard
martensite, which strengthens a resulting steel sheet too much to
make the steel sheet brittle. Accordingly, in a case where the
composition of the steel sheet includes at least one of Cr, V and
Mo, the contents thereof are Cr: 0.05% to 5.0%, V: 0.005% to 1.0%,
and Mo: 0.005% to 0.5%.
At least one type of element selected from Ti: 0.01% to 0.1%, and
Nb: 0.01% to 0.1%
[0104] Titanium and niobium are useful elements in terms of
precipitate strengthening/hardening of steel. Titanium and niobium
can each cause this effect when the contents thereof in steel are
at least 0.01%, respectively. In a case where at least one of the
Ti content and Nb content in the steel exceeds 0.1%, formability
and shape fixability of a resulting steel sheet deteriorate.
Accordingly, in a case where the steel sheet composition includes
Ti and Nb, the contents thereof are Ti: 0.01% to 0.1%, and Nb:
0.01% to 0.1%, respectively.
B: 0.0003% to 0.0050%
[0105] Boron is a useful element in terms of suppressing formation
and growth of ferrite from austenite grain boundary. This good
effect of boron can be obtained when the boron content in the steel
is at least 0.0003%. However, a boron content in the steel
exceeding 0.0050% deteriorates formability of a resulting steel
sheet. Accordingly, when the steel sheet composition includes
boron, the boron content in steel is B: 0.0003% to 0.0050%.
At least one type of elements selected from Ni: 0.05% to 2.0%, and
Cu: 0.05% to 2.0%
[0106] Nickel and copper are elements which each effectively
increase strength of steel. Further, these elements each cause an
effect of facilitating internal oxidation of a surface layer
portion of a steel sheet to improve coating adhesion property in a
case the steel sheet is subjected to galvanizing or galvannealing.
These good effects of Ni and Cu are obtained when the contents
thereof in the steel are at least 0.05%, respectively. In a case
where at least one of Ni content and Cu content in the steel
exceeds 2.0%, formability of a resulting steel sheet deteriorates.
Accordingly, in a case where the steel sheet composition includes
Ni and Cu, the contents thereof are Ni: 0.05% to 2.0%, and Cu:
0.05% to 2.0%, respectively.
At least one element selected from Ca: 0.001% to 0.005% and REM:
0.001% to 0.005%
[0107] Calcium and REM are useful elements in terms of making
sulfides spherical to lessen the adverse effects of the sulfides on
stretch flangeability of a steel sheet. Calcium and REM can each
cause this effect when the contents thereof in steel are at least
0.001%, respectively. In a case where at least one of the Ca
content and REM content in the steel exceeds 0.005%, inclusions
increase to cause surface defects, internal defects and the like of
a resulting steel sheet. Accordingly, in a case where the steel
sheet composition includes Ca and REM, the contents thereof are Ca:
0.001% to 0.005% and REM: 0.001% to 0.005%, respectively.
[0108] Components other than those described above are Fe and
incidental impurities in the steel sheet. However, our steel sheets
do not exclude the possibility that the steel composition thereof
includes a component other than those described above unless
inclusion of the component has an adverse effect.
EXAMPLES
Example 1
[0109] Our steel sheets and methods will be described further in
detail by Examples hereinafter. These Examples, however, do not
restrict this disclosure by any means. Needless to say, any changes
in structure can be made without having an adverse effect.
[0110] A steel material, obtained from steel having a component
composition as shown in Table 1 by using ingot techniques, was
heated to 1200.degree. C. and subjected to finish hot rolling at
870.degree. C. to obtain a hot rolled steel sheet. The hot rolled
steel sheet was subjected to coiling at 650.degree. C., pickling,
and cold rolling at rolling reduction rate of 65% to obtain a cold
rolled steel sheet having sheet thickness: 1.2 mm. The cold rolled
steel sheet thus obtained was subjected to thermal treatment under
the conditions shown in Table 2.
[0111] The thermal treatment temperatures (the annealing
temperatures) shown in Table 2 were all within either the austenite
single phase region or the (austenite+ferrite) two-phase region,
except for that of sample No. 4.
[0112] Some of the cold rolled steel sheet samples were each
subjected to hot dip galvanizing or galvannealing either during the
tempering process or after the tempering process. The hot dip
galvanizing process was carried out such that respective surfaces
of a cold rolled steel sheet sample were coated at coating weight
(per one surface): 50 g/m.sup.2 at plating path temperature:
463.degree. C. The galvannealing process was carried out such that
respective surfaces of a cold rolled steel sheet sample were first
subjected to coating at coating weight (per one surface): 50
g/m.sup.2 at plating path temperature: 463.degree. C. and then
alloying, under alloying conditions adjusted as required, at
temperature equal to or lower than 550.degree. C. to achieve alloy
degree (i.e., Fe % or Fe content in a coating layer) of 9 mass
%.
[0113] Each of the steel sheet samples thus obtained was subjected
to temper-rolling at rolling reduction rate (elongation rate): 0.3%
either directly after the thermal treatment in a case where the
sample was not subjected to any coating process or after the hot
dip galvanizing process or the galvannealing process in a case
where the sample was subjected to a coating process.
TABLE-US-00001 TABLE 1 Steel Steel sheet component (mass %) type C
Si Mn Al P S N Cr V Mo Ti Nb B Ni Cu Ca REM Note A 0.400 1.99 1.98
0.036 0.012 0.0040 0.0023 -- -- -- -- -- -- -- -- -- -- Steel B
0.310 2.02 1.52 0.040 0.010 0.0030 0.0041 -- -- -- -- -- -- -- --
-- -- Steel C 0.090 0.80 2.50 0.042 0.015 0.0050 0.0040 -- -- -- --
-- -- -- -- -- -- Comparative steel D 0.302 2.01 2.03 0.039 0.009
0.0040 0.0037 -- -- -- -- -- -- -- -- -- -- Steel E 0.402 1.80 0.50
0.041 0.010 0.0040 0.0037 0.9 -- -- -- -- -- -- -- -- -- Steel F
0.498 2.05 1.50 0.039 0.013 0.0040 0.0032 -- 0.05 -- -- -- -- -- --
-- -- Steel G 0.604 1.98 1.49 0.041 0.010 0.0030 0.0039 -- -- -- --
-- -- -- -- -- -- Steel H 0.298 2.00 1.81 0.037 0.029 0.0030 0.0041
-- -- 0.03 -- -- -- -- -- -- -- Steel I 0.301 2.41 1.92 0.037 0.029
0.0030 0.0041 -- -- -- -- 0.03 -- -- -- -- -- Steel J 0.412 1.10
1.52 1.02 0.013 0.0030 0.0037 -- -- -- -- -- -- 0.20 0.20 -- --
Steel K 0.480 1.70 1.30 0.038 0.012 0.0030 0.0041 -- -- -- 0.020 --
0.0015 -- -- -- -- Steel L 0.185 1.52 2.33 0.041 0.011 0.0040
0.0029 -- -- -- -- -- -- -- -- 0.002 -- Steel M 0.145 1.51 2.09
0.039 0.013 0.0030 0.0040 -- -- -- -- -- -- -- -- -- 0.003
Steel
TABLE-US-00002 TABLE 2 Average Cooling rate down Target to first
cooling stop Retention Temperature Annealing process temperature
temperature: time of the range of the Ms - Tempering process Sample
Steel Temperature Time region T1 coldest part coldest part Ms
150.degree. C. Temperature Time No. type (.degree. C.) (s)
(.degree. C./s) (.degree. C.) (s) (.degree. C.) (.degree. C.)
(.degree. C.) (.degree. C.) (s) Note 1 A 900 300 18 250 18 250~260
325 175 400 120 Example 2 B 900 200 20 400 30 393~400 378 228 420
100 Comparative Example 3 B 890 150 50 100 20 102~110 378 228 400
90 Comparative Example 4 B 670 200 15 250 20 252~260 378 228 400
120 Comparative Example 5 B 900 180 20 270 20 270~273 378 228 400
90 Example 6 B 900 180 15 240 25 210~245 378 228 400 90 Comparative
Example 7 C 900 180 20 290 20 290~294 443 293 360 90 Comparative
Example 8 D 890 200 15 250 18 252~258 366 216 390 90 Example 9 E
880 300 20 250 20 251~259 358 208 410 120 Example 10 F 870 400 15
190 20 191~201 297 147 400 300 Example 11 G 870 500 18 170 30
171~182 252 102 420 400 Example 12 H 900 200 20 225 20 226~232 364
214 400 180 Example 13 I 900 200 20 220 20 220~233 348 198 410 350
Example 14 J 900 200 20 220 20 221~227 318 168 400 300 Example 15 K
880 300 15 165 18 167~174 299 149 400 400 Example 16 L 900 200 35
280 16 282~285 407 257 390 90 Example 17 M 900 200 20 290 20
290~299 412 262 400 100 Example
[0114] Various properties of each of the steel sheet samples and
the coated steel sheet samples thus obtained were evaluated by the
following methods.
[0115] A tensile test was carried out according to JIS Z 2241 by
using a JIS No. 5 test piece collected from the steel sheet sample
in a direction orthogonal to the rolling direction thereof. TS
(tensile strength) and T.EL (total elongation) of the test piece
were measured and the product of the tensile strength and the total
elongation (TS.times.T. EL) was calculated to evaluate balance
between strength and formability (ductility) of the steel sheet
sample. TS.times.T. EL.gtoreq.20000 (MPa%) is evaluated to be good
balance between strength and elongation.
[0116] Stretch flangeability of each of the steel sheet samples and
the coated steel sheet samples thus obtained was evaluated
according to The Japan Iron and Steel Federation Standard (JFS)
T1001 by: cutting the steel sheet sample into a test piece (100
mm.times.100 mm); forming a hole (diameter: 10 mm) by punching in
the test piece with clearance corresponding to 12% of the sheet
thickness between a steel sheet edge and the hole; pushing a
60.degree. cone punch into the hole in a state where the test piece
was set on a die (inner diameter: 75 mm) with fold pressure: 88.2
kN exerted thereon; measuring a critical hole diameter at crack
initiation; and calculating a critical hole expansion ratio .lamda.
(%) according to Formula (1) below:
Critical hole expansion ratio .lamda.
(%)={(D.sub.f-D.sub.0)/D.sub.0}.times.100 (1).
In Formula (1), D.sub.f represents critical hole diameter at crack
initiation (mm) and D.sub.o represents the initial hole diameter
(mm).
[0117] Further, balance between strength and stretch-flangeability
of the steel sheet sample was evaluated by calculating the product
of strength and critical hole expansion ratio (TS.times..lamda.) by
using .lamda. thus determined through measurement.
[0118] Stretch-flangeability is evaluated to be good when
TS.times..lamda..gtoreq.25000 (MPa%).
[0119] The results obtained by the measurements described above are
shown in Table 3.
TABLE-US-00003 TABLE 3 TS .times. TS .times. T.EL .lamda. Sample
Steel TS T.EL .lamda. (MPa (MPa No. type (MPa) (%) (%) %) %) Note 1
A 1477 22 18 32494 26586 Example 2 B 1212 20 17 24240 20604
Compara- tive Example 3 B 1520 11 46 16720 69920 Compara- tive
Example 4 B 836 22 40 18392 33440 Compara- tive Example 5 B 1382 16
44 22112 60808 Example 6 B 1451 13 44 18863 63844 Compara- tive
Example 7 C 1119 8 50 8952 55950 Compara- tive Example 8 D 1370 16
37 21920 50690 Example 9 E 1471 19 30 27949 44130 Example 10 F 1563
18 17 28134 26571 Example 11 G 1678 19 15 31882 25170 Example 12 H
1482 14 35 20748 51870 Example 13 I 1474 18 41 26532 60434 Example
14 J 1498 16 35 23968 52430 Example 15 K 1750 12 18 21000 31500
Example 16 L 1198 20 29 23960 34742 Example 17 M 992 25 40 24800
39680 Example
[0120] As is obvious from Table 3, the steel sheet samples
manufactured according to our method all satisfied tensile strength
of at least 980 MPa, (TS.times.T. EL).gtoreq.20000 (MPa%) and
(TS.times..lamda.).gtoreq.25000 (MPa%). That is, it is confirmed
from Table 3 that each of the steel sheet samples has
satisfactorily high strength and excellent formability in
particular excellent stretch-flangeability.
[0121] In contrast, sample No. 4, in which the annealing
temperature failed to reach the (austenite+ferrite) two-phase
region, did not obtain the desired microstructures of a steel sheet
and had tensile strength (TS) below 980 MPa and (TS.times.T. EL)
below 20000 (MPa%), although (TS.times..lamda.).gtoreq.25000 (MPa%)
and stretch-flangeability was relatively good therein.
[0122] Each of sample No. 2 and sample No. 3, in which T1 was
beyond the first temperature region, did not obtain the desired
microstructures of a steel sheet and failed to satisfy at least one
of (TS.times.T. EL).gtoreq.20000 (MPa%) and
(TS.times..lamda.).gtoreq.25000 (MPa%), although it met tensile
strength (TS).gtoreq.980 MPa.
[0123] Sample No. 6, in which temperature of the coldest part of
the steel sheet dropped below the target temperature during the
retention time, i.e., was outside our range, did not obtain the
desired microstructures of a steel sheet and failed to satisfy
(TS.times.T. EL).gtoreq.20000 (MPa%), although it met tensile
strength (TS).gtoreq.980 MPa.
[0124] Sample No. 7, of which carbon content was outside our range,
did not obtain the desired microstructures of a steel sheet and
failed to have the desired properties of the steel sheet.
Example 2
[0125] Further, samples Nos. 18-22 prepared by using steel type A
shown in Table 1 were subjected to thermal treatment conditions
show in Table 4, respectively. Table 5 shows results of
investigating mechanical properties and variations therein for each
of these samples. Variations in mechanical properties of each steel
sheet sample were determined by: cutting 20 sheets of test
materials (length in the rolling direction: 40 mm.times.width: 250
mm) from a portion (length of the rolling direction: 1000 mm) of
the steel sheet sample, wherein these test materials to be
evaluated were originally evenly distributed (located) across the
entire width of the steel sheet (i.e., from one edge via the center
portion to the other edge of the steel sheet) and then cut and
collected, respectively; obtaining JIS No. 5 test pieces from these
20 test materials, respectively; subjecting each of the respective
JIS No. 5 test pieces to tensile test; and calculating standard
deviations of tensile strength and T. EL. for each of the test
pieces. Standard deviation a of tensile strength .ltoreq.10 MPa and
standard deviation .sigma. of T. EL..ltoreq.2.0% are evaluated to
be good, respectively.
TABLE-US-00004 TABLE 4 Average cooling Target Retention rate down
cooling time of to first stop the Temperature Sam- Annealing
process temperature temperature: coldest range of the Tempering
process ple Steel Temperature Time region T1 part coldest part Ms
Ms - 150.degree. C. Temperature Time No. type (.degree. C.) (s)
(.degree. C./s) (.degree. C.) (s) (.degree. C.) (.degree. C.)
(.degree. C.) (.degree. C.) (s) Note 18 A 900 250 20 250 18 251~253
325 175 400 120 Example 19 A 900 250 20 250 20 245~263 325 175 400
100 Comparative Example 20 A 900 300 20 300 2 300~302 325 175 400
100 Comparative Example 21 A 900 300 20 300 7 300~305 325 175 400
120 Comparative Example 22 A 900 300 20 300 30 300~308 325 175 400
120 Example
TABLE-US-00005 TABLE 5 Standard Standard Sample deviation .sigma.
deviation .sigma. No. of TS (MPa) of T. EL. (%) Note 18 5 0.9
Example 19 12 1.9 Comparative Example 20 22 3.1 Comparative Example
21 15 2.5 Comparative Example 22 6 1.3 Example
[0126] As shown in Table 5, sample No. 18 and sample No. 22
subjected to our thermal treatment each satisfy standard deviation
.sigma. of tensile strength .ltoreq.10 MPa and standard deviation a
of T. EL..ltoreq.2.0%, i.e., good stability in mechanical
properties. In contrast, sample No. 19 having temperature of the
coldest part of the steel sheet beyond the range of T1 to
(T1+15.degree. C.) and samples Nos. 20 and 21 each having retention
time of the coldest part of the steel sheet beyond the range of 15
s to 1000 s all exhibit large variations, i.e., at least one of
standard deviation .sigma. of tensile strength >10 MPa and
standard deviation .sigma. of T. EL.>2.0%.
[0127] Further, the mechanical properties and variations therein
were analyzed for each of our steel sheet samples shown in Table 3
in the same manner as described above in connection with samples
Nos. 18-22. It was confirmed that our steel sheet samples each
satisfied both standard deviation .sigma. of tensile strength 10
MPa and standard deviation .sigma. of T. EL..ltoreq.2.0%, i.e.,
good mechanical stability.
INDUSTRIAL APPLICABILITY
[0128] Our high strength steel sheet, being excellent in
formability and tensile strength (TS) and exhibiting good stability
in mechanical properties, is very useful in the industrial fields
of automobile, electric appliances and the like and in particular
contributes to reducing weight of automobile body.
* * * * *