U.S. patent application number 12/148876 was filed with the patent office on 2008-08-28 for multi-phase steel sheet excellent in hole expandability and method of producing the same.
This patent application is currently assigned to Nippon Steel Corporation. Invention is credited to Takehiro Hoshino, Osamu Kawano.
Application Number | 20080202645 12/148876 |
Document ID | / |
Family ID | 34678202 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080202645 |
Kind Code |
A1 |
Kawano; Osamu ; et
al. |
August 28, 2008 |
Multi-phase steel sheet excellent in hole expandability and method
of producing the same
Abstract
The present invention provides a steel sheet excellent in both a
balance between strength and elongation and a balance between
strength and hole expandability, in other words, a multi-phase
steel sheet having an excellent balance between strength and hole
expandability. The present invention is a multi-phase steel sheet
excellent in hole expandability characterized in that: the steel
sheet contains, as chemical components in mass, C: 0.03 to 0.15%,
P: not more than 0.010%, S: not more than 0.003%, and either one or
both of Si and Al in a total amount of 0.5 to 4%, and one or more
of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%, with the
balance consisting of Fe and unavoidable impurities; the
microstructure at a section of the steel sheet is composed of
either one or both of retained austenite and martensite which
account(s) for 3 to 30% in total in area percentage and the balance
consisting of either one or both of ferrite and bainite; the
maximum length of the crystal grains in the microstructure is not
more than 10 microns; and the number of inclusions 20 microns or
larger in size at a section of the steel sheet is not more than 0.3
piece per square millimeter.
Inventors: |
Kawano; Osamu; (Oita-shi,
JP) ; Hoshino; Takehiro; (Oita-shi, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Assignee: |
Nippon Steel Corporation
Tokyo
JP
|
Family ID: |
34678202 |
Appl. No.: |
12/148876 |
Filed: |
April 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10741607 |
Dec 18, 2003 |
|
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12148876 |
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Current U.S.
Class: |
148/546 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/04 20130101 |
Class at
Publication: |
148/546 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Claims
1-6. (canceled)
7: A method of producing a multi-phase steel sheet excellent in
hole expandability, which steel sheet contains, as chemical
components in mass, C: 0.03 to 0.15%, P: not more than 0.010%, S:
not more than 0.003%, and either one or both of Si and Al in a
total amount of 0.5 to 4%, and one or more of Mn, Ni, Cr, Mo and Cu
in a total amount of 0.5 to 4%, with the balance consisting of Fe
and unavoidable impurities, characterized by: when molten steel
with said components is refined, circulating the molten steel not
less than 1.5 times after flux for desulfurization is added at the
time of the desulfurization of the molten steel; further, when a
steel sheet is produced by hot-rolling a slab obtained by casting
said molten steel, conducting the finish rolling by controlling the
finish-rolling entry temperature to 950.degree. C. or higher and
the finish-rolling exit temperature within the range from 780 to
920.degree. C.; and coiling the steel sheet thus obtained at a
temperature of 500.degree. C. or lower.
8: A method of producing a multi-phase steel sheet excellent in
hole expandability according to claim 7, characterized in that the
steel sheet further contains, as chemical components in mass, one
or more of Nb, V and Ti in a total amount of 0.3% or less.
9. A method of producing a multi-phase steel sheet excellent in
hole expandability according to claim 7, characterized in that the
steel sheet further contains, as a chemical component in mass, B of
0.01% or less.
10: A method of producing a multi-phase steel sheet excellent in
hole expandability according to claim 7, characterized in that the
steel sheet further contains, as chemical components in mass,
either one or both of Ca of 0.01% or less and REM of 0.05% or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a multi-phase steel sheet
excellent in hole expandability, aiming at the application for
automobiles, such as passenger cars and trucks, etc., for
industrial machines, or the like, and a method of producing the
same.
BACKGROUND ART
[0002] In recent years, demands for high strength steel sheets have
been growing with the increasing needs mainly for the weight
reduction of automobile bodies and the assurance of the safety of
passengers in a collision. In particular, the application of steels
of TS 590 MPa class (60 kgf/mm.sup.2 class) in tensile strength has
rapidly expanded.
[0003] As a steel sheet used for such application, a multi-phase
steel sheet comprising retained austenite and/or martensite is
widely known. For example, as Japanese Unexamined Patent
Publication No. H9-104947 discloses, a steel sheet having an
excellent balance between strength and elongation (a total
elongation is 33.8 to 40.5% when a tensile strength is 60 to 69
kgf/mm.sup.2) is obtained by containing retained austenite in an
appropriate quantity therein. In this technology, however, a
technology regarding the balance between strength and hole
expandability has not been sufficiently considered and, in
particular, technological requirements for ultra-low P, the control
of the maximum length of a microstructure and inclusions and the
control of the hardness of a microstructure are not, in the least,
taken into consideration. Therefore, the properties of the steel
sheet have been inferior (a hole expansion ratio d/d0 is 1.46 to
1.68, namely 46 to 68% in terms of a net hole expansion rate, when
a tensile strength is 60 to 69 kgf/mm.sup.2) and the application
has been limited.
[0004] In the meantime, Japanese Unexamined Patent Publication No.
H3-180426 discloses a bainite sheet steel excellent in the balance
between strength and hole expandability (a hole expansion ratio
d/d0 is 1.72 to 2.02, namely 72 to 102% in terms of a net hole
expansion rate, when a tensile strength is 60 to 67 kgf/mm.sup.2).
However, since this technology provides not a multi-phase structure
but the equalization of a structure (a bainite single phase
structure), as a means of improving the net hole expansion rate,
the balance between strength and elongation is rather insufficient
(a total elongation is 27 to 30% when a tensile strength is 60 to
67 kgf/mm.sup.2) and the application is again limited.
[0005] That is, though, in the press forming of auto parts, punch
stretch formability represented by the balance between strength and
elongation and stretch flange formability represented by the
balance between strength and hole expandability are two major
components of forming, such a technology, satisfying both the
components simultaneously, has not been available and the
excellence in both has been the key to the expansion of the
application.
[0006] In recent years while the shift to high strength steel
sheets is progressing at an increasing rate due to global
environmental issues, as their application to components with high
degree of forming difficulty has been taken into consideration, a
steel sheet excellent in both the balance between strength and
elongation and the balance between strength and hole expandability,
in other words, a multi-phase steel sheet excellent in the balance
between strength and hole expandability, has been demanded.
DISCLOSURE OF THE INVENTION
[0007] The object of the present invention is, by solving the
problems of the conventional steel sheets, to provide a steel sheet
having both the excellent balance between strength and hole
expandability (not less than 35,000 MPa %, preferably not less than
46,000 MPa %, in terms of the value obtained by multiplying a
tensile strength by a net hole expansion rate) and the excellent
balance between strength and elongation (not less than 18,500 MPa
%, preferably not less than 20,000 MPa %, in terms of the value
obtained by multiplying a tensile strength by a total elongation),
that is, a multi-phase steel sheet excellent in hole expandability,
and a method of producing the same.
[0008] Both of the balance between strength and hole expandability
(MPa%), and the balance between strength and elongation (MPa%) are
indexes of press-formability. If these values are large, the
resultant products exhibit excellent properties. The balance
between strength and hole expandability is represented by the
product of the value of strength (MPa) obtained by tensile test and
the value of hole expansion ratio (%) obtained by hole expansion
test. Further, the balance between strength and elongation is
represented by the product of the value strength (MPa) obtained by
tensile test and the value of total elongation obtained by tensile
test. In the steel sheet which is generally used, if tensile
strength increases, both of hole expansion ratio and elongation
decrease and, as a result, both of the balance between strength and
hole expandability (MPa%), and the balance between strength and
elongation (MPa%) exhibit low values. On the other hand, according
to the present invention, lowering the value both of hole expansion
ratio and elongation can be restrained and it is possible to obtain
the high values of the balance between strength and hole
expandability (MPa%), and the balance between strength and
elongation (MPa%).
[0009] The present inventors have earnestly studied, from the
viewpoint of integrated manufacturing from steelmaking to hot
rolling, and have finally invented a multi-phase steel sheet
excellent in hole expandability and a method of producing the
same.
[0010] The gist of the present inventions is as follows:
[0011] (1) A multi-phase steel sheet excellent in hole
expandability characterized in that:
[0012] the steel sheet contains, as chemical components in
mass,
[0013] C: 0.03 to 0.15%,
[0014] P: not more than 0.010%,
[0015] S: not more than 0.003%, and
either one or both of Si and Al in a total amount of 0.5 to 4%, and
one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to
4%, with the balance consisting of Fe and unavoidable
impurities;
[0016] the microstructure at a section of the steel sheet is
composed of either one or both of retained austenite and martensite
which account(s) for 3 to 30% in total in area percentage and the
balance consisting of either one or both of ferrite and
bainite;
[0017] the maximum length of the crystal grains in the
microstructure is not more than 10 microns; and
[0018] the number of inclusions 20 microns or larger in size at a
section of the steel sheet is not more than 0.3 pieces per square
millimeter.
[0019] (2) A multi-phase steel sheet excellent in hole
expandability characterized in that:
[0020] the steel sheet contains, as chemical components in
mass,
[0021] C: 0.03 to 0.15%,
[0022] P: not more than 0.010%,
[0023] S: not more than 0.003%, and
either one or both of Si and Al in a total amount of 0.5 to 4%, and
one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to
4%, with the balance consisting of Fe and unavoidable
impurities;
[0024] the microstructure at a section of the steel sheet is
composed of either one or both of retained austenite and martensite
which account(s) for 3 to 30% in total in area percentage, pearlite
which accounts for more than 0% to not more than 3% in area
percentage, and the balance consisting of either one or both of
ferrite and bainite;
[0025] the maximum length of the crystal grains in the
microstructure is not more than 10 microns; and
[0026] the number of inclusions 20 microns or larger in size at a
section of the steel sheet is not more than 0.3 pieces per square
millimeter.
[0027] (3) A multi-phase steel sheet excellent in hole
expandability according to the item (1) or (2), characterized in
that the micro Vickers hardness of bainite is less than 240.
[0028] (4) A multi-phase steel sheet excellent in hole
expandability according to any one of the items (1) to (3),
characterized by further containing, as chemical components in
mass, one or more of Nb, V and Ti in a total amount of 0.3% or
less.
[0029] (5) A multi-phase steel sheet excellent in hole
expandability according to any one of the items (1) to (4),
characterized by further containing, as a chemical component in
mass, B of 0.01% or less.
[0030] (6) A multi-phase steel sheet excellent in hole
expandability according to any one of the items (1) to (5),
characterized by further containing, as chemical components in
mass, either one or both of Ca of 0.01% or less and REM of 0.05% or
less.
[0031] (7) A method of producing a multi-phase steel sheet
excellent in hole expandability, which steel sheet contains, as
chemical components in mass,
[0032] C: 0.03 to 0.15%,
[0033] P: not more than 0.010%,
[0034] S: not more than 0.003%, and
either one or both of Si and Al in a total amount of 0.5 to 4%, and
one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to
4%, with the balance consisting of Fe and unavoidable impurities,
characterized by:
[0035] when molten steel with said components is refined,
circulating the molten steel not less than 1.5 times after flux for
desulfurization is added at the time of the desulfurization of the
molten steel;
[0036] further, when a steel sheet is produced by hot-rolling a
slab obtained by casting said molten steel, conducting the finish
rolling by controlling the finish-rolling entry temperature to
950.degree. C. or higher and the finish-rolling exit temperature
within the range from 780 to 920.degree. C.; and
[0037] coiling the steel sheet thus obtained at a temperature of
500.degree. C. or lower.
[0038] (8) A method of producing a multi-phase steel sheet
excellent in hole expandability according to the item (7),
characterized in that the steel sheet further contains, as chemical
components in mass, one or more of Nb, V and Ti in a total amount
of 0.3% or less.
[0039] (9) A method of producing a multi-phase steel sheet
excellent in hole expandability according to the item (7) or (8),
characterized in that the steel sheet further contains, as a
chemical component in mass, B of 0.01% or less.
[0040] (10) A method of producing a multi-phase steel sheet
excellent in hole expandability according to any one of the items
(7) to (9), characterized in that the steel sheet further contains,
as chemical components in mass, either one or both of Ca of 0.01%
or less and REM of 0.05% or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a graph showing the effect of the chemical
component P on a net hole expansion rate.
[0042] FIG. 2 is a graph showing the effect of the maximum length
of a microstructure on a net hole expansion rate.
[0043] FIG. 3 is a graph showing the effect of the number of
inclusions on a net hole expansion rate.
[0044] FIG. 4 is a schematic drawing showing the refining of molten
steel when an RH is used.
[0045] FIG. 5 is a graph showing the effect of the frequency of the
reflux of molten steel after flux addition for desulfurization on
the number of inclusions.
[0046] FIG. 6 is a graph showing the effect of finish-rolling entry
and exit temperatures at the finishing mill in hot rolling on the
maximum length of a microstructure.
BEST MODE FOR CARRYING OUT THE INVENTION
[0047] The present invention is explained in detail hereunder.
[0048] First, the chemical components are explained.
[0049] C is an important element for stabilizing austenite and
obtaining a multi-phase structure, and C is added at not less than
0.03 mass % in order to stabilize austenite and to obtain either
one or both of retained austenite and martensite in the total
amount of not less than 3% in area percentage. However, the upper
limit of C content is set at not more than 0.15 mass %, preferably
not more than 0.11 mass %, in order to avoid the deterioration of
weldability and an adverse influence on a net hole expansion
rate.
[0050] P is a key element among the addition elements of the
present invention. The effect of P is demonstrated in FIG. 1. FIG.
1 shows the result of the investigation on the relationship between
the P content and the net hole expansion rate of a steel sheet,
using the steel sheets having the chemical components of Steel No.
1 in Table 1.
TABLE-US-00001 TABLE 1 Steel Chemical components (mass %) No. C Si
Mn P S Al N Al + Si 1 0.11 1.88 1.40 0.006-0.034 0.001 0.03 0.003
1.41 2 0.10 1.40 1.40 0.008 0.001 0.04 0.002 1.44
[0051] A net hole expansion rate is calculated based on the Japan
Iron and Steel Federation Standard JFS T 1001-1996. From FIG. 1,
the net hole expansion rate improves remarkably and exponentially
by controlling the P content to not more than 0.010 mass % and its
effect on the net hole expansion rate, which has not yet been
assumed within the range of conventional concepts, is recognized.
By so doing, press cracking can be avoided. Although the reason is
not completely clear, it is supposed that the reduction of P
content improves the properties of the edge of a punched hole (for
instance: the minimization of facet size, the reduction of
roughness and the reduction of microcracks on a fractured plane;
the suppression of the deterioration of workability in a
microstructure on a sheared plane; and the like), and leads to the
improvement of a net hole expansion rate.
[0052] S content is set at not more than 0.003 mass %, preferably
not more than 0.001 mass %, from the viewpoint of preventing the
deterioration of a net hole expansion rate and weldability caused
by sulfide-system inclusions.
[0053] Si and Al are elements useful for obtaining a multi-phase
structure. They make either one or both of retained austenite and
martensite account for not less than 3% in total in area percentage
and have the function of improving a net hole expansion rate, by
promoting the formation of ferrite and suppressing the formation of
carbide, and further by strengthening ferrite, thus reducing the
hardness difference between ferrite and hard phases (such as
bainite and martensite) and contributing to the uniformity of a
structure. Moreover, they act also as deoxidizing elements. From
the above-mentioned viewpoint, the lower limit of the total
addition amount of either one or both of Si and Al should be not
less than 0.5 mass %. Considering the balance between the cost and
the effect, the upper limit of the total addition amount is set at
not more than 4 mass %.
[0054] With regard to the addition amount of each of Si and Al, the
following may be taken into consideration.
[0055] When excellent surface quality is required in particular,
either one of the means of avoiding Si scale by controlling the Si
content to less than 0.1 mass %, preferably not more than 0.01 mass
%, and the means of making Si scale harmless (making scale less
conspicuous by forming the scale all over the surface) by
controlling the Si content rather to more than 1.0 mass %,
preferably more than 1.2 mass %, may be adopted.
[0056] It is also possible to increase the addition amount of Al
and reduce the addition amount of Si to meet the requirement of
material properties, for example, in a case where it is desired to
lower a tensile strength by making use of the difference between Si
and Al in the function of strengthening ferrite.
[0057] Al may be limited to not more than 0.2 mass %, preferably
not more than 0.1 mass %, considering the drawbacks in steelmaking,
such as the erosion of refractory materials, nozzle clogging and
the like, and the material properties.
[0058] Mn, Ni, Cr, Mo, and Cu are elements useful for obtaining a
multi-phase structure, and also are elements which strengthen
ferrite. From the above-mentioned viewpoint, the lower limit of the
total addition amount of one or more of them should be not less
than 0.5 mass %. However, considering the balance between the cost
and the effect, the upper limit of the total addition amount is set
at not more than 4 mass %.
[0059] Furthermore, one or more of Nb, V, Ti, B, Ca and REM may be
added as selective elements.
[0060] Nb, V and Ti are elements effective for a higher strength.
However, considering the balance between the cost and the effect,
the total addition amount of one or more of those elements is set
at not more than 0.3 mass %.
[0061] B has a function as a strengthening element, and may be
added by not more than 0.01 mass %. In addition, B also has the
effect of mitigating the adverse effect of P.
[0062] Ca may be added by not more than 0.01 mass % since Ca
further improves a net hole expansion rate by controlling the shape
of sulfide-system inclusions (spheroidizing).
[0063] Moreover, REM may also be added by not more than 0.05 mass %
for the same reason.
[0064] In addition, N may be added by not more than 0.02 mass %, if
required, aiming at the stabilization of austenite and the
strengthening of a steel sheet.
[0065] Next, a microstructure is explained hereunder.
[0066] In order to obtain an excellent net hole expansion rate,
from the viewpoint of not deteriorating the uniformity of a
fractured surface size, one of the properties of the edge of a
punched hole, and the like, which uniformity has been improved by
the ultimate reduction of P, the control of the maximum length of
crystal grains in a microstructure and the control of the amount
and size of inclusions are especially important. Therefore, that is
explained first.
[0067] As the crystal grain size of a microstructure affects the
fractured surface size at the edge of a punched hole, it affects a
net hole expansion rate remarkably. Even in the case where the
average size of crystal grains in a microstructure is fine, if the
maximum grain size is large, it adversely affects a net hole
expansion rate. As a microstructure is composed of many crystal
grains, a net hole expansion rate cannot be governed by the average
grain size: when a big crystal grain exists among many crystal
grains, it adversely affects the net hole expansion rate even if
the average grain size is fine. Here, with regard to the size of a
crystal grain, not a circle-reduced diameter but the maximum length
thereof affects a net hole expansion rate.
[0068] FIG. 2 shows the result of the investigation on the
relationship between the maximum length of a microstructure in a
steel sheet and the net hole expansion rate of the steel sheet,
using the steel sheets having the chemical components of Steel No.
2 in Table 1. As shown in FIG. 2, the net hole expansion rate
improves remarkably and exponentially when the maximum length of a
microstructure is not larger than 10 microns, and its effect on the
net hole expansion rate, which has not yet been assumed within the
range of the conventional concept, is recognized. By so doing,
press cracking can be avoided.
[0069] Here, the maximum length of a microstructure was calculated
from an optical micrograph under the magnification of 400 taken at
a section perpendicular to the rolling direction of a steel sheet
after the section was etched with a nitral reagent and the reagent
disclosed in Japanese Unexamined Patent Publication No. S59-219473,
averaging all over the section along the thickness direction.
[0070] Moreover, with regard to inclusion control, a net hole
expansion rate can be improved by reducing the number of coarse
inclusions. The number of coarse inclusions was obtained by
observing a polish-finished section along the rolling direction of
a steel sheet with a microscope (400 magnifications) and
integrating the number of coarse inclusions 20 microns or larger in
maximum length. FIG. 3 shows the result of the investigation on the
relationship between the number of coarse inclusions (20 microns or
larger in maximum length) in a steel sheet and the net hole
expansion rate, using the steel sheets having the chemical
components of Steel No. 2 in Table 1. It is understood that, when
the number of coarse inclusions (20 microns or larger in maximum
length) is not more than a specified number (not more than 0.3
piece per square millimeter), the net hole expansion rate can be
improved remarkably and press cracking can be avoided.
[0071] In addition, controlling the micro Vickers hardness of
bainite to less than 240 acts preferably on the improvement of hole
expandability. The reduction of the hardness of bainite lowers the
hardness difference between ferrite and bainite and thus
contributes to the improvement of the uniformity of a structure.
However, if the micro Vickers hardness of bainite exceeds 240, the
hardness difference between ferrite and bainite deviates from the
range desirable for hole expandability and further the
deterioration of hole expandability is caused by the deterioration
of workability of the bainite itself. The reduction of P (not more
than 0.01%) largely contributes to enhancing the effect, but
details are not known.
[0072] Here, the micro Vickers hardness of bainite is obtained by
identifying bainite by etching a section perpendicular to the
rolling direction of a steel sheet with the reagent disclosed in
Japanese Unexamined Patent Publication No. S59-219473, and by
averaging the values measured at five points (averaging the values
excluding the maximum and minimum values from among the values
measured at seven points) under a load of 1 to 10 gr.
[0073] Furthermore, in order to obtain an excellent balance between
strength and elongation as well as an excellent balance between
strength and hole expandability, it is essential to control the
kind and the area percentage of a multi-phase structure.
[0074] An excellent balance between strength and elongation (not
less than 18,500 MPa % in terms of the value obtained by
multiplying a tensile strength by a total elongation) and an
excellent balance between strength and hole expandability (not less
than 35,000 MPa % in terms of the value obtained by multiplying a
tensile strength by a net hole expansion rate) are obtained by
controlling the total area percentage of either one or both of
retained austenite and martensite to 3 to 30%.
[0075] When the total area percentage of either one or both of
retained austenite and martensite is less than 3%, it becomes
impossible to obtain the stable effect of improving the balance
between strength and elongation, which is to be obtained by the
retained austenite and martensite. Therefore, its lower limit is
set at 3%.
[0076] When the total area percentage of either one or both of
retained austenite and martensite is more than 30%, the effect of
improving the balance between strength and elongation is saturated
and the deterioration of a net hole expansion rate and the like are
caused. Therefore, from the viewpoint of press formability, the
upper limit of the total area percentage is set at 30%.
[0077] Here, it is preferable that pearlite is not contained in a
steel sheet since it hinders a balance between strength and
elongation and a balance between strength and hole expandability.
Therefore, the area percentage of pearlite is determined to be not
more than 3% at most, preferably not more than 1%.
[0078] It is more desirable to add the following restrictions in
addition to the above restrictions.
[0079] When a particularly excellent balance between strength and
elongation (not less than 20,000 MPa %) is required, it is
desirable that the area percentage of retained austenite is set at
not less than 3%.
[0080] Moreover, when a particularly excellent balance between
strength and hole expandability (not less than 46,000 MPa % in
terms of the value obtained by multiplying a tensile strength by a
net hole expansion rate) is required, it is desirable that the area
percentage of martensite is set at not more than 3%.
[0081] On the other hand, when a low yield ratio (not more than 70%
in terms of yield ratio YR which is a value obtained by dividing a
yield stress by a tensile strength and then multiplying the divided
value by 100) is required from the viewpoint of the shape
fixability, the area percentage of martensite is set at not less
than 3%.
[0082] Preferably, by controlling the maximum length of the
microstructure of retained austenite and/or martensite to not more
than 2 microns, the effect increases yet further.
[0083] The remainder structure of a microstructure consists of
either one or both of ferrite and bainite, and by controlling the
total area percentage of ferrite and bainite to not less than 80%,
the deterioration of press. formability, which is caused by hard
structures other than ferrite and bainite combining with each other
in the form of a network, can be suppressed.
[0084] Due to the effect described above, both an excellent balance
between strength and hole expandability (not less than 35,000 MPa
%, preferably not less than 46,000 MPa %, in terms of the value
obtained by multiplying a tensile strength by a net hole expansion
rate) and an excellent balance between strength and elongation (not
less than 18,500 MPa %, preferably not less than 20,000 MPa %, in
terms of the value obtained by multiplying a tensile strength by a
total elongation) can be obtained simultaneously, and press
formability improves markedly.
[0085] Here, the identification of the constitution of a
microstructure, the measurement of an area percentage, and the
measurement of the maximum length of retained austenite and/or
martensite were carried out with an optical micrograph under the
magnification of 1,000 taken at a section perpendicular to the
rolling direction of a steel sheet after the section was etched
with a nitral reagent and the reagent disclosed in Japanese
Unexamined Patent Publication No. S59-219473, and by X-ray
analysis.
[0086] Next, the production method is explained hereunder.
[0087] Firstly, when molten steel is refined in a steelmaking
process, it is important to let the molten steel reflux not less
than 1.5 times after the addition of flux for desulfurization at
the time when the molten steel is desulfurized using a secondary
refining apparatus such as an RH. Here, the reflux of molten steel
is represented by the amount of molten steel that circulates the
inside of a secondary refining apparatus, such as an RH, per unit
time, and there are various formulas for the computation. For
example, as disclosed in "The Refining Limitation of Impurity
Elements in a Mass Production Scale" (Iron and Steel Institute of
Japan, the Forum of Elevated Temperature Refining Process Section,
and Japan Society for the Promotion of Science, the 19th
Steelmaking Committee, Reaction Process Workshop, March 1996, P.
184-187), the amount of refluxed molten steel Q expressed by the
following Equation 1 is defined as the refluxed amount of one
time:
Refluxed amount
Q=11.4.times.V.sup.1/3.times.D.sup.4/3.times.{ln(P1/P0)}.sup.1/3.times.k
Eq 1,
where Q: Amount of refluxed molten steel (t/min.), V: Flow rate of
refluxed gas (Nl/min.), D: Inner diameter of snorkel (m), P0:
Pressure in vacuum chamber (Pa), P1: Pressure at injection port of
refluxed gas (Pa), and k: Constant (a constant determined based on
secondary refinement apparatus, 4 in this case).
[0088] The schematic drawing of the refining of molten steel using
an RH is shown in FIG. 4. Two snorkels 3 of the degassing chamber 2
are dipped into the molten steel ladle 1, gas is blown from
underneath one of these snorkels (in this case, Ar is blown from
underneath one of the snorkels through the injection lance 4),
then, the molten steel in the molten steel ladle 1 rises and enters
the degassing chamber 2, and after the degassing process, the
molten steel descends and returns from the other snorkel 3 to the
molten-steel ladle. Here, though the example wherein a secondary
refining apparatus employing an RH is used is shown, it is needless
to say that other apparatus (for example, a DH) may be used.
[0089] FIG. 5 shows the result of investigating the relationship
between the frequency of the reflux of molten steel after flux for
desulfurization is added when molten steel having the components of
Steel No. 2 in Table 1 is refined and the number of inclusions 20
microns or larger in size per square mm at a section of a steel
sheet obtained by hot-rolling a slab cast from the molten steel. As
shown in FIG. 5, by increasing the frequency of the reflux of
molten steel, the surfacing of the desulfurization flux system
inclusions is notably promoted, the number of coarse inclusions (20
microns or larger) can be reduced to not more than a prescribed
number (not more than 0.3 per square mm), the net hole expansion
rate is improved, and thus press cracking is avoided.
[0090] Next, the condition of the temperature at finish rolling in
a hot-rolling process when a hot-rolled steel sheet according to
the present invention is produced is examined. FIG. 6 shows the
result of summarizing the relation among finish-rolling entry and
exit temperatures when a slab having the components of Steel No. 2
in Table 1 is hot-rolled, and the maximum length of crystal grains
in the microstructure at a section of the steel sheet obtained.
[0091] As shown in FIG. 6, by regulating the finish-rolling entry
temperature at not lower than 960.degree. C. and the finish-rolling
exit temperature at not lower than 780.degree. C., the maximum
length of the microstructure is certainly controlled to not larger
than 10 microns and, therefore, a net hole expansion rate can be
improved and press cracking can be avoided. Preferably, it is
desirable to regulate the finish-rolling entry temperature in
accordance with chemical components, finish-rolling speed and
finish-rolling exit temperature.
[0092] Here, if a finish-rolling exit temperature exceeds
920.degree. C., the whole microstructure coarsens, the drawbacks
such as the deterioration of press formability and the generation
of scale defects remarkably appear, and therefore the temperature
is determined to be the upper limit.
[0093] Though conditions on a cooling table after finish rolling
are not particularly specified, the multi-step control of a cooling
rate (the combination of quenching, slow cooling and isothermal
retention) or immediate quenching at the finish-rolling exit, which
are generally known, may be employed, aiming at the control of the
area percentage of a microstructure and the promotion of the fining
of a microstructure and the formation of a multi-phase
structure.
[0094] The upper limit of a coiling temperature is set at
500.degree. C. in order for either one or both of retained
austenite and martensite to account for 3% or more in total in area
percentage. If a coiling temperature exceeds 500.degree. C., the
total area percentage of 3% or more cannot be secured and thus an
excellent balance between strength and elongation (tensile strength
multiplied by total elongation) is not obtained.
[0095] Here, either air cooling or forced cooling may be employed
for the cooling of a steel sheet after it is coiled.
[0096] In addition, a slab may be subjected to rolling after once
being cooled and then reheated, or rolling by HCR or HDR. Further,
a slab may be produced by so-called thin slab continuous
casting.
[0097] Furthermore, a steel sheet according to the present
invention may be plated with Zn or the like for improving corrosion
resistance, or may be coated with a lubricant or the like for
further improving press formability.
EXAMPLE
[0098] The chemical compositions other than Fe of the steels
subjected to the test are shown in Table 2.
[0099] The production conditions in the steelmaking and hot rolling
of the steels subjected to the test are shown in Table 3. The
microstructures and material properties of hot-rolled steel sheets
obtained are shown in Tables 4 and 5.
TABLE-US-00002 TABLE 2 Steel Chemical components (mass %) No. C Si
Mn P S Al N Ni Cr Cu Mo Others *1 *2 Remarks 1 0.11 1.38 1.40 0.009
0.001 0.03 0.003 -- -- -- -- 1.41 1.40 2 0.10 1.40 1.40 0.008 0.001
0.04 0.002 -- -- -- -- Ca: 0.0035 1.44 1.40 3 0.09 1.35 1.36 0.010
0.001 0.04 0.002 -- -- -- -- B: 0.001 1.39 1.36 4 0.11 1.42 1.39
0.009 0.002 0.02 0.003 -- -- -- -- V: 0.0011 1.44 1.39 5 0.06 1.55
1.50 0.008 0.001 0.10 0.002 -- -- -- -- Ti: 0.014 1.65 1.50 6 0.06
1.51 1.53 0.006 0.001 0.03 0.002 -- -- -- -- Nb: 0.012, 1.54 1.63
REM: 0.0015 7 0.08 1.50 1.45 0.006 0.001 0.04 0.004 -- -- -- 0.10
1.54 1.55 8 0.08 1.49 1.54 0.015 0.002 0.03 0.004 -- -- 0.15 --
1.52 1.69 P being outside range of present invention 9 0.14 1.39
1.26 0.009 0.001 0.03 0.003 -- 0.16 -- -- 1.42 1.42 10 0.13 1.40
1.20 0.008 0.001 0.03 0.004 0.14 -- -- -- 1.43 1.43 11 0.06 1.25
1.29 0.008 0.001 0.03 0.003 -- -- -- -- 1.28 1.39 12 0.06 1.21 1.35
0.015 0.001 0.02 0.003 -- -- -- -- 1.23 1.35 P being outside range
of present invention 13 0.07 1.22 1.32 0.010 0.002 0.03 0.003 -- --
0.12 -- 1.25 1.44 14 0.09 1.10 1.30 0.009 0.001 0.03 0.004 -- 0.08
-- -- 1.13 1.38 15 0.08 1.05 1.30 0.006 0.002 0.02 0.002 0.12 -- --
-- 1.07 1.42 16 0.11 1.98 1.95 0.009 0.001 0.03 0.003 -- -- -- --
2.01 1.95 17 0.14 1.45 1.05 0.005 0.001 0.02 0.003 -- -- -- -- 1.47
1.05 *1 Al + Si (not including unavoidable impurities) *2 Mn + Ni +
Cr + Cu + Mo (not including unavoidable impurities)
TABLE-US-00003 TABLE 3 Steelmaking conditions Hot-rolling
conditions Frequency of Slab size, Finish-rolling Finish-rolling
Size after finish Steel circulation of molten thickness .times.
entry temperature, exit temperature, rolling, thickness .times. No.
No. steel, t (time) *1 width (mm) FT0 (.degree. C.) FT7 (.degree.
C.) width (mm) 1 1 1.7 250 .times. 950 960 855 3.2 .times. 850 2 1
3.5 250 .times. 950 1010 865 3.2 .times. 850 3 1 2.5 250 .times.
950 935 825 3.2 .times. 850 4 1 0.9 250 .times. 950 990 850 3.2
.times. 850 5 1 0.6 250 .times. 950 1040 895 3.2 .times. 850 6 2
1.5 250 .times. 1100 1035 875 3.2 .times. 850 7 3 1.6 250 .times.
1400 915 810 3.2 .times. 850 8 4 2.0 250 .times. 850 1020 860 3.2
.times. 850 9 5 1.7 245 .times. 1100 1025 865 2.9 .times. 920 10 6
1.6 245 .times. 1100 1000 855 2.9 .times. 920 11 7 0.7 250 .times.
1000 980 850 2.6 .times. 1000 12 8 2.4 250 .times. 1000 1020 870
2.6 .times. 1000 13 9 2.3 250 .times. 1000 945 835 3.0 .times. 990
14 10 1.7 250 .times. 950 985 856 3.0 .times. 990 15 11 2.6 250
.times. 950 990 860 9.2 .times. 1250 16 12 1.5 245 .times. 900 1010
890 3.2 .times. 1350 17 13 0.8 245 .times. 850 995 860 2.9 .times.
1215 18 14 1.8 235 .times. 1000 920 820 3.5 .times. 1350 19 15 1.2
235 .times. 950 1025 880 3.5 .times. 900 20 16 1.7 260 .times. 950
1000 855 9.2 .times. 850 21 17 1.7 260 .times. 950 960 855 9.2
.times. 850 22 17 1.7 250 .times. 950 960 930 3.2 .times. 850 Steel
Cooling conditions Coiling conditions No. No. on cooling table
Coiling temperature, CT (.degree. C.) Remarks 1 1 50.degree.
C./sec. 415 2 1 50.degree. C./sec. 475 3 1 50.degree. C./sec. 400
FT0 being outside range of present invention 4 1 50-15-50.degree.
C./sec. 405 t being outside range of present invention 5 1
50.degree. C./sec. 400 t being outside range of present invention 6
2 50.degree. C./sec. 870 7 3 50.degree. C./sec. 410 FT0 being
outside range of present invention 8 4 50.degree. C./sec. 405 9 5
55.degree./sec. 505 CT being outside range of present invention 10
6 55.degree. C./sec. 440 11 7 60.degree. C./sec. 415 t being
outside range of present invention 12 8 60.degree. C./sec. 360 13 9
55.degree. C./sec. 395 FT0 being outside range of present invention
14 10 55.degree. C./sec. 410 15 11 50.degree. C./sec. after air
cooling <100 for 5 sec. 16 12 50-15-50.degree. C./sec. <100
17 13 50.degree. C./sec. after air cooling <100 t being outside
range of for 5 sec. present invention 18 14 50.degree. C./sec.
after air cooling <100 FT0 being outside range for 5 sec. of
present invention 19 15 50.degree. C./sec. after air cooling
<100 t being outside range of for 5 sec. present invention 20 16
50.degree. C./sec. 400 21 17 50.degree. C./sec. 600 CT being
outside range of present invention 22 17 50-15-50.degree. C./sec.
<100 FT7 being outside range of present invention *1 The
frequency of the reflux of molten steel can be calculated by, for
example, the following equation. The amount of refluxed molten
steel Q expressed by the following equation is defined as the
refluxed amount of one time: Refluxed amount Q = [11.4 .times.
V.sup.1/3 .times. D.sup.4/3 .times. {1n(P1/P0)}.sup.1/3] .times. 4,
where V: Flow rate of refluxed gas (N1/min.), D: Sectional area of
snorkel (m.sup.2), P0: Pressure in vacuum chamber (Pa), and P1:
Pressure at injection port of refluxed gas (Pa).
TABLE-US-00004 TABLE 4 F B F + B Retained .gamma. Retained .gamma.
+ M Area Area Area Area M Area Average percentage percentage
percentage percentage Area percentage grain size Remainder No. (%)
(%) (%) (%) percentage (%) (micron) microstructure 1 84 11 95 5 0 5
2 2 85 11 96 3 0 3 2 1% P 3 83 9 92 8 0 8 2 4 85 10 95 5 0 5 2 5 86
11 97 3 0 3 3 6 84 10 94 6 0 6 2 7 83 11 94 6 0 6 2 8 85 11 96 4 0
4 2 9 82 16 98 1 0 1 2 1% P 10 83 13 96 4 0 4 3 11 82 13 95 5 0 5 2
12 60 13 93 3 4 7 2 13 82 10 92 8 0 8 2 14 82 11 93 7 0 7 2 15 80 6
86 2 12 14 3 16 80 4 84 3 13 16 3 17 80 5 85 3 12 15 3 18 82 6 88 0
12 12 3 19 83 5 88 1 11 12 3 20 65 30 95 5 0 5 2 21 77 15 92 0 0 0
-- 8% P 22 69 0 69 0 31 31 >10 Maximum length of Number of
inclusions microstructure being 20 microns or larger in Micro
Vickers hardness No. 10 microns or smaller size being 0.3 or less
of B being 240 or less Remarks 1 .smallcircle. .smallcircle.
.smallcircle. 2 .smallcircle. .smallcircle. .smallcircle. 3 x
.smallcircle. x Maximum length of microstructure being outside
range of present invention 4 .smallcircle. x .smallcircle. Number
of inclusions being outside range of present invention 5
.smallcircle. x .smallcircle. Number of inclusions being outside
range of present invention 6 .smallcircle. .smallcircle.
.smallcircle. 7 x .smallcircle. x Maximum length of microstructure
being outside range of present invention 8 .smallcircle.
.smallcircle. .smallcircle. 9 .smallcircle. .smallcircle.
.smallcircle. .gamma. + M being outside range of present invention
10 .smallcircle. .smallcircle. .smallcircle. 11 .smallcircle. x
.smallcircle. Number of inclusions being outside range of present
invention 12 .smallcircle. .smallcircle. x 13 x .smallcircle. x
Maximum length of microstructure being outside range of present
invention 14 .smallcircle. .smallcircle. .smallcircle. 15
.smallcircle. .smallcircle. .smallcircle. 16 .smallcircle.
.smallcircle. x 17 .smallcircle. x .smallcircle. Number of
inclusions being outside range of present invention 18 x
.smallcircle. x Maximum length of microstructure being outside
range of present invention 19 .smallcircle. x .smallcircle. Number
of inclusions being outside range of present invention 20
.smallcircle. .smallcircle. .smallcircle. 21 .smallcircle.
.smallcircle. .smallcircle. .gamma. + M and P being outside range
of present invention 22 x .smallcircle. -- .gamma. + M and maximum
length being outside range of present invention Microstructure: F;
ferrite, B; bainite, retained .gamma.; retained austenite, M;
martensite, and P; pearlite
TABLE-US-00005 TABLE 5 Punching hole expandability Static tensile
properties Net hole expansion TS .times. .lamda. TS YS T.El YR TS
.times. TEl No. rate .lamda. (%) MPa % MpA MpA % % MPa % Remarks 1
80 48320 604 476 36 0.79 21744 Invented example 2 94 56870 605 495
34 0.82 20570 Invented example 3 57 34884 612 494 34 0.81 20808
Comparative example 4 56 34832 622 465 35 0.75 21770 Comparative
example 5 53 32595 615 491 32 0.80 19680 Comparative example 6 98
60760 620 496 33 0.80 20460 Invented example 7 56 34664 619 477 35
0.77 21665 Comparative example 8 85 51340 604 480 34 0.79 20536
Invented example 9 112 67312 601 485 28 0.81 16828 Comparative
example 10 104 62400 600 466 35 0.78 21000 Invented example 11 55
33385 607 471 34 0.78 20638 Comparative example 12 50 30950 619 490
32 0.79 19808 Comparative example 13 55 34375 625 468 35 0.75 21875
Comparative example 14 105 64890 618 472 34 0.76 21012 Invented
example 15 75 46575 621 410 33 0.66 20493 Invented example 16 45
27810 618 399 32 0.65 19776 Comparative example 17 40 25160 629 411
31 0.65 19499 Comparative example 18 46 29440 640 421 29 0.66 18560
Comparative example 19 45 28530 634 409 31 0.65 19654 Comparative
example 20 81 63261 781 560 29 0.72 22649 Invented example 21 70
42350 605 540 26 0.89 15780 Comparative example 22 30 23400 780 535
23 0.69 17940 Comparative example
[0100] Here, the evaluations of properties and microstructures were
carried out by the following methods.
[0101] Tensile test was carried out with JIS No. 5 test pieces, and
tensile strength (TS), yield strength (YS), yield ratio
(YR=YS/TS.times.100), total elongation (T.EL), and the balance
between strength and elongation (TS.times.T.EL) were obtained.
[0102] A net hole expansion rate was calculated based on the Japan
Iron and Steel Federation Standard JFS T1001-1996.
[0103] The maximum length of crystal grains in a microstructure was
calculated from an optical micrograph under the magnification of
400 taken at a section perpendicular to the rolling direction of a
steel sheet after the section was etched with a nitral reagent and
the reagent disclosed in Japanese Unexamined Patent Publication No.
S59-219473.
[0104] The number of coarse inclusions in a steel sheet was
obtained by observing a polish-finished section perpendicular to
the rolling direction of a steel sheet with a microscope (400
magnifications) and integrating the number of coarse inclusions 20
microns or larger in maximum length.
[0105] The identification of the constitution of a microstructure,
the measurement of an area percentage, and the measurement of the
maximum length of retained austenite and/or martensite were carried
out with an optical micrograph under a magnification of
1,000.times. taken at a section perpendicular to the rolling
direction of a steel sheet after the section was etched with a
nitral reagent, the reagent disclosed in Japanese Unexamined Patent
Publication No. S59-219473 and the reagent disclosed in Japanese
Unexamined Patent Publication No. H5-163590, and with X-ray
analysis.
[0106] An area percentage of retained austenite (F.gamma.: in %)
was calculated according to the following equation based on
Mo-K.alpha. rays in X-ray analysis:
F.gamma.(%)=(2/3){100/(0.7.times..alpha.(211)/.gamma.(220)+1)}+(1/3){100-
/(0.78.times..alpha.(211)/.gamma.(311)+1)},
where, .alpha.(211), .gamma.(220), .alpha.(211), and .gamma.(311)
represent the intensity on the respective planes.
[0107] In the examples of the present invention (Nos. 1, 2, 6, 8,
10, 14, 15 and 20), as shown in Table 5, high strength hot-rolled
steel sheets excellent in press formability, having both an
excellent balance between strength and hole expandability (not less
than 35,000 MPa % in terms of the value obtained by multiplying a
tensile strength by a net hole expansion rate) and an excellent
balance between strength and elongation (not less than 18,500 MPa %
in terms of the value obtained by multiplying a tensile strength by
a total elongation), are obtained.
[0108] On the other hand, in the comparative examples (Nos. 3 to 5,
7, 9, 11 to 13 and 16 to 19), since some conditions are outside the
range of the present invention as explained at the remarks in
Tables 1 to 3, steel sheets having poor mechanical properties (poor
properties in a balance between strength and hole expandability and
a balance between strength and elongation) are obtained by all
means.
[0109] The present invention has made it possible to provide,
stably and at a low cost, a multi-phase steel sheet excellent in
press formability, having both an excellent balance between
strength and hole expandability and an excellent balance between
strength and elongation, and a method of producing the steel sheet,
and, consequently, the ranges of the application and the service
conditions have markedly been expanded and the industrial and
economical effects of the present invention are remarkable.
* * * * *