U.S. patent application number 10/686357 was filed with the patent office on 2004-04-22 for high strength hot rolled steel sheet and method for manufacturing the same.
This patent application is currently assigned to NKK CORPORATION. Invention is credited to Funakawa, Yoshimasa, Maeda, Eiji, Murao, Yasuhiro, Nakata, Hiroshi, Saito, Takanobu, Sato, Kaoru, Shiozaki, Tsuyoshi, Suwa, Minoru, Tomita, Kunikazu, Yamamoto, Tetsuo.
Application Number | 20040074573 10/686357 |
Document ID | / |
Family ID | 27573726 |
Filed Date | 2004-04-22 |
United States Patent
Application |
20040074573 |
Kind Code |
A1 |
Funakawa, Yoshimasa ; et
al. |
April 22, 2004 |
High strength hot rolled steel sheet and method for manufacturing
the same
Abstract
The present invention relates to a high strength hot rolled
steel sheet containing 0.15% or less C, 0.02 to 0.35% Ti, and 0.05
to 0.7% Mo by weight percentage and consisting essentially of a
matrix of ferrite structure single phase and fine precipitates with
a grain size of smaller than 10 nm dispersed in the matrix, for
example, a high strength hot rolled steel sheet which consists
essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn,
0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less
N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo by weight percentage, and the
balance being Fe, wherein fine precipitates with a grain size of
smaller than 10 nm are dispersed in a matrix of ferrite structure
single phase at a number per unit volume of
5.times.10.sup.4/.mu.m.sup.3 or higher. This steel sheet, which has
tensile strength of not lower than 550 MPa, high elongation and
excellent stretch flangeability, is suitable for intricately shaped
automotive chassis parts such as a suspension arm.
Inventors: |
Funakawa, Yoshimasa;
(Yokohama, JP) ; Shiozaki, Tsuyoshi; (Yokohama,
JP) ; Tomita, Kunikazu; (Yokohama, JP) ;
Saito, Takanobu; (Kawasaki, JP) ; Nakata,
Hiroshi; (Fukuyama, JP) ; Sato, Kaoru;
(Yokohama, JP) ; Suwa, Minoru; (Fukuyama, JP)
; Yamamoto, Tetsuo; (Yokohama, JP) ; Murao,
Yasuhiro; (Kawasaki, JP) ; Maeda, Eiji;
(Yokohama, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
NKK CORPORATION
Tokyo
JP
|
Family ID: |
27573726 |
Appl. No.: |
10/686357 |
Filed: |
October 14, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10686357 |
Oct 14, 2003 |
|
|
|
10108691 |
Mar 28, 2002 |
|
|
|
6666932 |
|
|
|
|
10108691 |
Mar 28, 2002 |
|
|
|
PCT/JP01/09469 |
Oct 29, 2001 |
|
|
|
Current U.S.
Class: |
148/602 |
Current CPC
Class: |
C22C 38/12 20130101;
Y10T 428/12799 20150115; C22C 38/02 20130101; B32B 15/013 20130101;
C21D 2211/004 20130101; C22C 38/14 20130101; C22C 38/04 20130101;
C21D 8/0226 20130101; C23C 2/40 20130101; C21D 2211/005
20130101 |
Class at
Publication: |
148/602 |
International
Class: |
C21D 008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 31, 2000 |
JP |
2000-331681 |
Oct 31, 2000 |
JP |
2000-331682 |
Jan 31, 2001 |
JP |
2001-022726 |
Feb 20, 2001 |
JP |
2001-044351 |
Feb 20, 2001 |
JP |
2001-044352 |
Feb 20, 2001 |
JP |
2001-044354 |
Feb 20, 2001 |
JP |
2001-044355 |
Sep 18, 2001 |
JP |
2001-282774 |
Claims
What is claimed is:
1. A method for manufacturing a high strength hot rolled steel
sheet, comprising the steps of: producing a steel slab which
consists essentially of 0.06% or less C, 0.5.% or less Si, 0.5 to
2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006%
or less N, 0.05 to 0.6% Mo, 0.02 to 0.10% Ti by weight percentage,
and the balance being Fe, and satisfies the equation of
0.8.ltoreq.(C/12)/[(Ti/48)+(Mo/96)].ltoreq.- 1.3; producing a hot
rolled steel sheet by hot rolling said steel slab at a temperature
of Ar3 transformation point or higher; and coiling said hot rolled
steel sheet at a temperature of 550 to 700.degree. C.
2. A method for manufacturing a high strength hot rolled steel
sheet, comprising the steps of: producing a steel slab which
consists essentially of 0.06% or less C, 0.5% or less Si, 0.5 to
2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006%
or less N, 0.05 to 0.6% Mo, 0.02 to 0.10% Ti, at least one element
selected from 0.08% or less Nb and 0.15% or less V by weight
percentage, and the balance being Fe; producing a hot rolled steel
sheet by hot rolling said steel slab at a temperature of Ar3
transformation point or higher; and coiling said hot rolled steel
sheet at a temperature of 550 to 700.degree. C.
3. A method for manufacturing a high strength hot rolled steel
sheet, comprising the steps of: producing a steel slab which
consists essentially of more than 0.06% and 0.15% or less C, 0.5%
or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1%
or less Al, 0.006% or less N, more than 0.10% and 0.35-% or less
Ti, 0.3 to 0.7% Mo by weight percentage, and the balance being Fe,
and satisfies the equation of 0.8.ltoreq.(C/12)/[(Ti/48)+(Mo/96)] s
1.3; producing a hot rolled steel sheet by hot rolling said steel
slab at a temperature of Ar3 transformation point or higher; and
coiling said hot rolled steel sheet at a temperature of 550 to
700.degree. C.
Description
[0001] This application is a divisional application of Ser. No.
10/108,691 filed Mar. 28, 2002 (now allowed), which is a
continuation application of International Application
PCT/JP01/09469 filed on Oct. 29, 2001.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a high strength steel sheet
suitable for automotive chassis parts or the like and, more
particularly, to a high strength hot rolled steel sheet having
tensile strength of not lower than 550 MPa, and to a method for
manufacturing the same.
[0004] 2. Description of Related arts
[0005] From the viewpoint of increased fuel economy leading to
environmental protection, hot rolled steel sheets for automobiles,
-having higher tensile strength and smaller thickness, have been
developed. In particular, higher tensile strength and smaller
thickness have been demanded for hot rolled steel sheets used for
automotive chassis parts such as a suspension arm, to which a great
effect of higher tensile strength and smaller thickness is
obtained. On the other hand, since the automotive chassis parts
have an intricate shape, the hot rolled steel sheet used for them
is required to have high elongation and excellent stretch
flangeability as well as high tensile strength.
[0006] Conventionally, various types of high strength hot rolled
steel sheets used for automotive chassis parts have been proposed.
For example, JP-A-4-329848 has disclosed a dual phase steel sheet
having excellent fatigue property and stretch flangeability, which
consists of a ferrite structure and a second phase (pearlite,
bainite, martensite, retained austenite, etc.). However, sufficient
stretch flangeability cannot be obtained because of the existence
of the hard second phase.
[0007] JP-A-6-172924 has proposed a steel sheet having excellent
stretch flangeability, which has a bainitic ferrite structure with
high dislocation density. However, sufficient elongation cannot be
obtained because of the existence of the bainitic ferrite structure
with high dislocation density.
[0008] JP-A-6-200351 has proposed a steel sheet, consisting mainly
of a polygonal ferrite structure, which has excellent stretch
flangeability and is provided with high tensile strength by
utilizing precipitation strengthening due to TiC or solid solution
strengthening. However, high elongation and excellent stretch
flangeability cannot be obtained steadily because much Ti must be
added and thus coarse precipitates are yielded easily.
[0009] JP-A-7-11382 has proposed a steel sheet having excellent
stretch flangeability, which has an acicular ferrite structure in
which fine TiC or NbC is precipitated. However, sufficient
elongation cannot be obtained because of the existence of the
acicular ferrite structure with high dislocation density.
[0010] JP-A-11-152544 has proposed a steel sheet in which the grain
size of ferrite is controlled to be as fine as 2 .mu.m or smaller
by adding Ti, Nb, V and Mo. However, high elongation cannot be
obtained because of the fine grain size of ferrite of 2 .mu.m or
smaller.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide a high
strength hot rolled steel sheet having high tensile strength of not
lower than 550 MPa, high elongation, and excellent stretch
flangeability, which is suitable for intricately shaped automotive
chassis parts and a method for manufacturing the same.
[0012] The object of the present invention can be achieved by a
high strength hot rolled steel sheet containing 0.15% or less C,
0.02 to 0.35% Ti, and 0.05 to 0.7% Mo by weight percentage and
consisting essentially of a matrix of ferrite structure single
phase and fine precipitates with a grain size smaller than 10 nm
dispersed in the matrix. More concretely, the object thereof can be
achieved by the following high strength hot rolled steel
sheets.
[0013] (1) A high strength hot rolled steel sheet which consists
essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn,
0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less
N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo by weight percentage, and the
balance being Fe, wherein fine precipitates with a grain size
smaller than 10 nm are dispersed in a matrix of ferrite structure
single phase at a number per unit volume of
5.times.10.sup.4/.mu.m.sup.3 or higher.
[0014] (2) A high strength hot rolled steel sheet which consists
essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0 % Mn,
0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less
N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo, at least one element selected
from 0.08% or less Nb and 0.15% or less V by weight percentage, and
the balance being Fe, wherein fine precipitates with a grain size
smaller than 10 nm are dispersed in a matrix of ferrite structure
single phase at a number per unit volume of
5.times.10.sup.4/.mu.m.sup.3 or higher.
[0015] (3) A high strength hot rolled steel sheet which consists
essentially of more than 0.06% and 0.15% or less C, 0.5% or less
Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less
Al, 0.006% or less N, more than 0.10% and 0.35% or less Ti, 0.3 to
0.7% Mo by weight percentage, and the balance being Fe, wherein
fine precipitates with a grain size smaller than 10 nm are
dispersed in a matrix of ferrite structure single phase at a number
per unit volume of 5.times.10.sup.4/.mu.m.sup.3 or higher.
[0016] The above-described high strength hot rolled steel sheets
can be manufactured, for example, by a method comprising the steps
of: producing a hot rolled steel sheet by hot rolling a steel slab
having the above-described compositions at a temperature of Ar3
transmission point or higher; and coiling the hot rolled steel
sheet at a temperature of 550 to 700.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a graph showing the relationship between TS and
number per unit volume of fine carbides;
[0018] FIG. 2 is a graph showing the relationship between grain
size of Fe carbide and .lambda.;
[0019] FIG. 3 is a graph showing the relationship between volume
percentage of Fe carbide and .lambda.;
[0020] FIG. 4 is a graph showing the relationship between Si and Mo
contents and surface property;
[0021] FIG. 5 is a graph showing the relationship between
TS.times.EL/t.sup.0.2 and press formability; and
[0022] FIG. 6 is a graph showing the relationship between solid
solution C content and EL.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The inventors conducted studies on the elongation and the
stretch flangeability of a high strength hot rolled steel sheet,
and, as a result, obtained the knowledge described below.
[0024] (1) Precipitates of carbide, nitride, etc. are made fine by
the addition of Mo.
[0025] (2) If the precipitates of carbide, nitride, etc. are
dispersed finely in a matrix of ferrite structure single phase with
low dislocation density and high ductility by the addition of Mo,
high elongation and excellent stretch flangeability as well as high
tensile strength can be achieved at the same time.
[0026] Based on this knowledge, we found that 0.15% or less C, 0.02
to 0.35% Ti, and 0.05 to 0.7% Mo by weight percentage, the matrix
made essentially of a ferrite structure single phase, and fine
precipitates with a grain size smaller than 10 nm dispersed in the
matrix allow to produce a hot rolled steel sheet having high
tensile strength and excellent stretch flangeability.
[0027] The reasons for the limitation of contents of C, Ti and Mo
are as described below.
[0028] If the content of C is higher than 0.15%, the fine
precipitates are sometimes liable to become coarse, and thus the
strength decreases. Therefore, the content of C should be 0.15% or
lower.
[0029] If the content of Ti is smaller than 0.02%, the amount of
fine precipitates is small, and thus it is difficult to obtain
tensile strength of not lower than 550 MPa. If the Ti content is
higher than 0.35%, the ferrite structure becomes fine, and thus the
total elongation is liable to decrease. Therefore, the content of
Ti should be in the range of 0.02 to 0.35%.
[0030] If the content of Mo is 0.05% or higher, fine composite
precipitates combining with Ti are deposited while pearlite
transformation is restrained, and thus the steel can be
strengthened while maintaining high elongation and excellent
stretch flangeability. However, if the Mo content is higher than
0.7%, a hard phase is formed, and thus the stretch flangeability
degrades. Therefore, the content of Mo should be in the range of
0.05 to 0.7%.
[0031] In the present invention, the ferrite structure single phase
need not always consist of a 100% ferrite phase. If the ferrite
phase is 95% or more, the object of the present invention can be
achieved.
[0032] Also, if the grain size of fine precipitates deposited in
the matrix is 10 nm or larger, tensile strength of not lower than
550 MPa can hardly be obtained. Therefore, if an attempt is made to
strengthen the steel by the precipitates with a grain size of 10 nm
or larger, the volume percentage of precipitates must be increased.
Thereby, the ferrite structure is made fine, and thus the
elongation is decreased. Therefore, the grain size of fine
precipitates should be smaller than 10 nm, preferably 5 nm or
smaller. To achieve higher tensile strength, the grain size of fine
precipitates should preferably be 3 nm or smaller.
[0033] The fine precipitates are composite carbides containing Ti
and Mo. It is thought that since Mo, which has lower diffusion rate
than Ti, forms carbides together with Ti, the growth rate of the
carbides becomes low, so that fine carbides are deposited.
[0034] Even if W with a content in the range of 0.01 to 1.5% is
added in place of Mo, or together with Mo, fine precipitates with a
grain size smaller than 10 nm can be deposited, and thus a high
strength hot rolled steel sheet having high elongation, excellent
stretch flangeability and tensile strength of not lower than 550
MPa can be obtained.
[0035] Since the amount of fine precipitates exerts a great
influence on the strength, the relationship between amount of fine
precipitates and tensile strength TS was investigated using hot
rolled steel sheets with a thickness of 2.3 mm which contained 0.03
to 0.15% C, 0.03 to 0.25% Ti, and trace to 0.7% Mo, and were hot
rolled at a finishing temperature of 900.degree. C. and coiled at a
coiling temperature of 500 to 800.degree. C. As a result, it is
found that as shown in FIG. 1, if the number per unit volume of
fine precipitates with a grain size smaller than 10 nm is
5.times.10.sup.4/.mu.m.sup.3 or larger, TS of not lower than 550
MPa can surely be obtained, if the number thereof is
1.times.10.sup.5/.mu.m.sup.3 or larger, TS of not lower than 700
MPa can surely be obtained, and if the number thereof is
2.times.10'/.mu.m.sup.3 or larger, TS of not lower than 780 MPa can
surely be obtained.
[0036] The high strength hot rolled steel sheet in accordance with
the present invention sometimes contains Fe carbides with a
relatively large grain size in addition to the fine precipitates
with a grain size of smaller than 10 nm. Since the Fe carbides with
a large grain size is undesirable for stretch flangeability, the
relationship between grain size (average grain size) and volume
percentage of Fe carbides and stretch flangeability was
investigated using the above-described hot rolled steel sheets. In
this investigation, the grain size (average grain size) and volume
percentage of Fe carbides were determined by image analysis after
the observation under an electron microscope. Also, the stretch
flangeability was evaluated as follows. A specimen having a hole
was blanked with a clearance of 12.5% in the center of a steel
sheet of 130 mm square by using a punch of 10 mm in diameter, and
the blanked hole was pushed up from the burr-free side. Then, a
hole diameter was measured at the time when cracking passed through
the steel sheet, and evaluation was made by a hole expanding ratio
.lambda. expressed by the following equation.
.lambda.(%)=[(d-10)/10].times.100
[0037] FIG. 2 shows the relationship between grain size of Fe
carbides and hole expanding ratio. FIG. 3 shows the relationship
between volume percentage of Fe carbides and hole expanding
ratio.
[0038] It is found that if the grain size of Fe carbides exceeds 1
.mu.m or if the volume percentage of Fe carbides exceeds 1%,
.lambda. becomes lower than 80%, and thus the stretch flangeability
degrades. Therefore, it is preferable that the grain size of Fe
carbides be smaller than 1 .mu.m, and the volume percentage thereof
be 1% or lower.
[0039] If the contents of C, Ti and Mo expressed by weight
percentage satisfy the equation of
0.8.ltoreq.(C/12)/[(Ti/48)+(Mo/96)].ltoreq.1.3, fine precipitates
with a grain size of smaller than 10 nm are surely formed at a
number per unit volume of 5.times.10.sup.4/.mu.m.sup.3 or higher.
Also, If the contents of Ti and Mo expressed by atomic percentage
in the fine precipitates satisfy the equation of
0.1.ltoreq.Ti/Mo.ltoreq.- 3, fine precipitates with a grain size of
smaller than 10 nm are surely formed at a number per unit volume of
5.times.10.sup.4/.mu.m.sup.3 or higher, so that tensile strength of
not lower than 550 MPa can surely be obtained. Further, in order to
achieve higher tensile strength, the equation of 0.5.ltoreq.5
Ti/Mo.ltoreq.2 should preferably be satisfied.
[0040] Even if in addition to C, Ti and Mo, at least one element
selected from 0.08% or less Nb and 0.15% or less V is contained,
the same effect can be obtained. In this case, however, the
contents of Ti, Mo, Nb and V expressed by atomic percentage in the
fine precipitates must satisfy the equation of
0.25.ltoreq.Mo/(Ti+Nb+V+Mo).
[0041] Similarly, in the case where W is contained in place of Mo,
if the contents of Ti and W expressed by atomic percentage in the
fine precipitates satisfy the equation of 0.1.ltoreq.Ti/W, fine
precipitates with a grain size smaller than 10 nm are surely formed
at a number per unit volume of 5.times.10.sup.4/.mu.m.sup.3 or
higher.
[0042] Also, if both Mo and W are contained, if the equation of
0.1.gtoreq.Ti/(Mo+W) is satisfied, fine precipitates with a grain
size of smaller than 10 nm are surely formed at a number per unit
volume of 5.times.10.sup.4/.mu.m.sup.3 or higher.
[0043] If the ratio of element contents in the fine precipitates is
controlled in this manner, not only the number of fine precipitates
but also the dispersion state thereof is made homogeneous, so that
more homogeneous tensile characteristic values can be obtained in
the steel sheet, and also the sheet shape after rolling becomes
good.
[0044] The investigation on the structure and the fine precipitates
in the present invention was made by the method described
below.
[0045] A specimen was prepared by the electrolytic polishing
process using the twin jet method, and observation was made under a
transmission electron microscope at an acceleration voltage of 200
kV. At this time, the observation was made by the defocus method in
which the crystal orientation of parent phase was controlled so
that the fine precipitates had a measurable contrast with the fine
precipitates, and the focus was shifted from the normal focus to
decrease the miscount of precipitates to the minimum. Also, the
thickness of specimen in a zone in which the precipitate grain was
measured was evaluated by measuring an elastic scattering peak and
an inelastic scattering peak by using the electric energy loss
spectroscopy. By this method, the number of grains and the
thickness of specimen can be measured for the same zone. The number
of grains was measured at four locations in a zone of 0.5.times.0.5
.mu.m of the specimen, and was calculated as the number of grains
observed by being projected per 1 .mu.m.sup.2. From this value and
the thickness of specimen, the number of precipitate grains per 1
.mu.m.sup.2 was calculated. Also, chemical analysis of precipitates
was performed by the energy dispersive X-ray spectroscopy.
[0046] As examples of high strength hot rolled steel sheet in
accordance with the present invention, the following high strength
hot rolled steel sheets can be cited.
[0047] (1) A high strength hot rolled steel sheet which consists
essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn,
0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less
N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo by weight percentage, and the
balance being Fe, wherein fine precipitates with a grain size of
smaller than 10 nm are dispersed in a matrix of ferrite structure
single phase at a number per unit volume of
5.times.10.sup.4/m.sup.3 or higher.
[0048] (2) A high strength hot rolled steel sheet which consists
essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn,
0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less
N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo, at least one element selected
from 0.08% or less Nb and 0.15% or less V by weight percentage, and
the balance being Fe, wherein fine precipitates with a grain size
of smaller than 10 nm are dispersed in a matrix of ferrite
structure single phase at a number per volume of
5.times.10.sup.4/.mu.m.sup.3 or higher.
[0049] (3) A high strength hot rolled steel sheet which consists
essentially of more than 0.06% and 0.15% or less C, 0.5% or less
Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less
Al, 0.006% or less N, more than 0.10% and 0.35% or less Ti, 0.3 to
0.7% Mo by weight percentage, and the balance being Fe, wherein
fine precipitates with a grain size of 10 nm or smaller are
dispersed in a matrix of ferrite structure single phase at a number
per unit volume of 5.times.10.sup.4/.mu.m.sup.3 or higher.
[0050] The steel sheets of (1) and (2) are high strength hot rolled
steel sheets having tensile strength of not lower than 780 MPa, and
the steel sheet of (3) is a high strength hot rolled steel sheet
having tensile strength of not lower than 950 MPa.
[0051] The following is a description of the reasons for the
limitation of composition.
[0052] C: C forms carbides, and is effective in strengthening the
steel depending on the contents of Ti, Mo, Nb and V described
later. However, in the case of a steel sheet having tensile
strength of around 780 MPa, if the content of C is higher than
0.06%, pearlite is formed or the precipitate becomes coarse, so
that the elongation and the stretch flangeability deteriorate.
Therefore, the content of C should be 0.06% or lower. On the other
hand, in the case of a steel sheet having tensile strength of
around 980 MPa, the content of C should be 0.15% or lower for the
same reason, and the content should be higher than 0.06% to obtain
tensile strength of around 980 MPa. Therefore, the content of C
should be higher than 0.06% and 0.15% or lower. Also, it is
desirable that the relationship between the content of C and the
later-described contents of Ti and Mo should satisfy
0.8.ltoreq.(C/12)/[(Ti/48)+(Mo/96)].ltoreq.1.3. By satisfying the
above-described equation, fine composite carbides containing Ti and
Mo can be precipitated, and the deterioration in elongation and
stretch flangeability caused by the formation of pearlite or coarse
precipitates can be restrained.
[0053] Si: Si is an element effective for solid solution
strengthening. However, if the content of Si is higher than 0.5%,
the precipitation of C from ferrite is accelerated, and thus coarse
Fe carbides are liable to precipitate on grain boundaries, so that
the stretch flangeability degrades. Also, if the content of Si is
higher than 0.5%, the hot rolling of thin steel sheets with a
thickness of 2.5 mm or smaller becomes unstable. Therefore, the Si
content should be 0.5% or lower.
[0054] Mn: The content of Mn should be 0.5% or higher from the
viewpoint of solid solution strengthening. However, if the content
is higher than 2.0%, segregation occurs or a hard phase is formed,
so that the stretch flangeability degrades. Therefore, the content
of Mn should be in the range of 0.5 to 2.0%.
[0055] P: P is effective for solid solution strengthening. However,
if the content of P is higher than 0.06%, segregation occurs, so
that the stretch flangeability degrades. Therefore, the content of
P should be 0.06% or lower.
[0056] S: A lower content of S is more desirable. If the content of
S is higher than 0.005%, the stretch flangeability deteriorates.
Therefore, the content of S should be 0.005% or lower.
[0057] Al: Al is added as a deoxidizer. If the content of Al is
higher than 0.1%, both of the elongation and the stretch
flangeability deteriorate. Therefore, the content of Al should be
0.1% or lower.
[0058] N: A lower content of N is more desirable. If the content of
N is higher than 0.006%, coarse nitrides increase, so that the
stretch flangeability degrades. Therefore, the content of N should
be 0.006% or lower.
[0059] Ti: As described above, Ti forms fine composite carbides,
and thus strengthens the steel while the high elongation and the
excellent stretch flangeability are maintained. In the case of a
steel sheet having tensile strength of around 780 MPa, the content
of Ti should be in the range of 0.02 to 0.10%, and in the case of a
steel sheet having tensile strength of around 950 MPa, the content
of Ti should be higher than 0.10% and 0.35% or lower.
[0060] Mo: For the same reason as that in the case of Ti, in the
case of a steel sheet having tensile strength of around 780 MPa,
the content of Mo should be in the range of 0.05 to 0.6%, and in
the case of a steel sheet having tensile strength of around 950
MPa, the content of Mo should be in the range of 0.3 to 0.7%.
[0061] In the case of a steel sheet having tensile strength of
around 780 MPa, at least one element selected from 0.08% or less Nb
and 0.15% or less V can further be contained. Nb and V are
effective in making the structure fine, and also form composite
precipitates together with Ti and Mo, which contributes to the
increase in elongation and stretch flangeability. However, if the
content of Nb is higher than 0.08%, or if the content of V is
higher than 0.15%, the elongation decreases. Therefore, the content
of Nb should be 0.08% or lower, and the content of V should be
0.15% or lower. From the viewpoint of the formation of fine
structure by using Nb and V, it is preferable that the content of
Nb should be 0.005% or higher and the content of V should be 0.001%
or higher.
[0062] As concrete examples of high strength hot rolled steel sheet
in accordance with the present invention, the following hot rolled
steel sheets containing W can be cited in addition to the
above-described three types of high strength hot rolled steel
sheets.
[0063] (4) A high strength hot rolled steel sheet which consists
essentially of 0.1% or less C, 0.5% or less Si, 2% or less Mn,
0.06% or less P, 0.01% or less S, 0.1% or less Al, 0.006% or less
N, 0.5% or less Cr, 0.02 to 0.2% Ti, 0.08% or less Nb, 0.01 to 1.5%
W by weight percentage, and the balance being Fe, wherein fine
precipitates having a grain size of smaller than 10 nm are
dispersed in a matrix of ferrite structure single phase at a number
per volume of 5.times.10.sup.4/.mu.m.s- up.3 or higher and the
contents of Ti and W represented by atomic percentage in the
precipitates satisfy the equation of 0.1.ltoreq.Ti/W.
[0064] (5) A high strength hot rolled steel sheet which consists
essentially of 0.1 t or less C, 0.5% or less Si, 2% or less Mn,
0.06% or less P, 0.01% or less S, 0.1% or less Al, 0.006% or less
N, 0.5% or less Cr, 0.02 to 0.2% Ti, 0.08% or less Nb, 0.05 to 0.6%
Mo, 0.01 to 1.5% W by weight percentage, and the balance being Fe,
wherein fine precipitates having a grain size of smaller than 10 nm
are dispersed in a matrix of ferrite structure single phase at a
number per volume of 5.times.10.sup.4/.mu.m.sup.3 or higher and the
contents of Ti, Mo and W represented by atomic percentage in the
precipitates satisfy the equation of 0.1.ltoreq.Ti/(Mo+W).
[0065] In the above-described high strength hot rolled steel sheets
(1) to (5), as shown in the later-described example 7, if the
content of solid solution C is 0.0020% or lower, the elongation
increases further.
[0066] FIG. 4 shows the relationship between contents of Si and Mo
and surface property. In this figure, .circleincircle. indicates
very good degree of scale defect caused by Si, .largecircle.
indicates good degree, and X indicates poor degree.
[0067] In the above-described high strength hot rolled steel sheets
(1) to (5), if the contents of Si and Mo by weight percentage
satisfy the equation of (Si+Mo).ltoreq.0.5%, a very good surface
property can be obtained.
[0068] Even if at least one element selected from 0.15% or less Cr,
0.15% or less Cu, and 0.15% or less Ni is contained, the effects of
the present invention can be achieved.
[0069] The above-described high strength hot rolled steel sheet (1)
can be manufactured by a method for manufacturing a high strength
hot rolled steel sheet, which comprises the steps of: producing a
hot rolled steel sheet by hot rolling a steel slab, which consists
essentially of, for example, 0.06% or less C, 0.5% or less Si, 0.5
to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al,
0.006% or less N, 0.05 to 0.6% Mo, 0.02 to 0.10% Ti by weight
percentage, and the balance being Fe, and satisfies the equation of
0.8.ltoreq.(C/12)/[(Ti/48)+(Mo/96)].ltoreq.1.3, at a temperature of
Ar3 transformation point or higher; and coiling the hot rolled
steel sheet at a temperature of 550 to 700.degree. C.
[0070] The above-described high strength hot rolled steel sheet (2)
can be manufactured by producing a steel slab, which consists
essentially of, for example, 0.06% or less C, 0.5% or less Si, 0.5
to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al,
0.006% or less N, 0.05 to 0.6% Mo, 0.02 to 0.10% Ti, at least one
element selected from 0.08% or less Nb and 0.15% or less V by
weight percentage, and the balance being Fe, under the same hot
rolling conditions as those for the steel sheet (1).
[0071] The above-described high tensile hot rolled steel sheet (3)
can be manufactured by producing a steel slab, which consists
essentially of, for example, more than 0.06% and 0.15% or less C,
0.5% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S,
0.1% or less Al, 0.006% or less N, more than 0.10% and 0.35% or
less Ti, 0.3 to 0.7% Mo by weight percentage, and the balance being
Fe, and satisfies the equation of
0.8.ltoreq.(C/12)/[(Ti/48)+(Mo/96)] s 1.3, under the same hot
rolling conditions as those for the steel sheet (1).
[0072] If the hot rolling is performed at a temperature lower than
Ar3 transformation point, coarse grains are produced, and thus the
elongation decreases, and the precipitates are made coarse by a
strain inducing phenomenon. Therefore, the hot rolling must be
performed at a temperature of Ar3 transformation point or higher,
preferably, at a temperature of 880.degree. C. or higher. After the
hot rolling, in order to provide a ferrite structure single phase,
the steel sheet must be coiled at a temperature of 550.degree. C.
or higher, preferably at a temperature of 600.degree. C. or higher.
Also, in order to prevent the precipitates from becoming coarse,
the steel sheet must be coiled at a temperature of 700.degree. C.
or lower, preferably, at a temperature of 660.degree. C. or
lower.
[0073] The steel slab can be rolled by the direct rolling method in
which hot rolling is performed immediately after casting or after
heat retention. Alternatively, it can be rolled by the continuous
rolling method in which the rolled steel is heated or heat-retained
before or during finish rolling, or the rolled steels are joined
after rough rolling.
[0074] The above-described high tensile hot rolled steel sheet (1)
to (5) can be subjected to hot dip galvanizing treatment. For the
high strength hot rolled steel sheet in accordance with the present
invention, since stable fine precipitates are dispersed, even if
the hot dip galvanizing treatment is accomplished, the precipitates
do not change, and thus the steel sheet does not soften.
[0075] Since the automotive chassis parts such as a suspension arm,
a reinforcement, a side member, a seat frame, and a seat rail have
an intricate shape, it is difficult to fabricate these parts by
press forming the conventional high strength hot rolled steel
sheet. However, if the above-described high tensile hot rolled
steel sheets (1) to (5) are applied, these parts can be fabricated
with high quality and high yields.
[0076] In particular, a high strength hot rolled steel sheet which
has hole expanding ratio of 80% or higher and tensile strength TS
of 700 MPa or higher and in which TS, elongation EL (%), and sheet
thickness t (mm) satisfy the equation of
(TS.times.EL)/t.sup.0.2>12000 is suitable for the automotive
chassis parts for the reason described below.
[0077] The product of TS and EL, TS.times.EL, serves as an index of
absorbed energy of material, and is effective as an index
representing the difficulty in developing press cracking. The value
required for this differs according to TS of steel sheet. Also,
since EL is a value that depends on the sheet thickness t, a
desirable range of TS.times.EL represented as a function of t that
does not cause a problem of press cracking due to insufficient EL
exists for each TS. The EL of steel sheet with a different t can be
converted by using Oliver's equation, and is proportional to
t.sup.0.2.
[0078] A suspension arm was fabricated with a 1200-ton press by
using a hot rolled steel sheet that contains 0.04% C, 0.09% Ti, and
0.2% Mo and has a thickness of 3.5 to 2.7 mm and TS of 710 to 850
MPa. Then, the relationship between (TS.times.EL)/t.sup.0.2 and
press formability (no cracking: OK, cracking: NG) was
investigated.
[0079] As shown in FIG. 5, if (TS.times.EL)/t.sup.0.2 exceeds
12000, cracking does not occur at all.
EXAMPLE 1
[0080] Steels A to E having a chemical composition given in Table 1
were hot rolled under the conditions given in Table 1, by which
steel sheets 1 to 5 were manufactured. Thereafter, a tensile test
using a JIS No. 5 test piece was conducted, and also the
investigation of structure and precipitate under a transmission
electron microscope, the measurement of hole expanding ratio
(.lambda.), and the real press test of suspension arm were carried
out by the above-described method. Table 1 additionally gives the
structure of matrix, the grain size of precipitates, and the number
per unit volume of precipitates.
[0081] The results are given in Table 2.
[0082] According to the observation under a transmission electron
microscope, in the steel sheets 1 to 3 of examples of this
invention, fine precipitates with a grain size of smaller than 10
nm were dispersed homogeneously in a matrix of ferrite structure
single phase. Also, these fine precipitates were carbides
containing Ti and Mo. The steel sheets 1 to 3 had TS of not lower
than 800 MPa and high EL and .lambda., and were not cracked in the
press test.
[0083] On the other hand, the steel sheet 4 of comparative example,
having a bainite (B) structure, had low EL and was cracked in the
press test. Also, the steel sheet 5, having a ferrite+pearlite
(F+P) structure, had low .lambda. and was cracked.
1 TABLE 1 Hot rolling conditions Fine Finishing Coiling precipitate
temper- temper- Grain Number Chemical composition (wt %) ature
ature Struc- size (10.sup.4/ Steel C Si Mn P S Al N Ti Nb Mo Cr
(.degree. C.) (.degree. C.) ture (nm) .mu.m.sup.3) Remarks A 0.045
0.05 1.67 0.004 0.001 0.044 0.0020 0.085 0.007 0.20 0.056 890 600 F
1 21 Inventive example B 0.046 0.08 1.65 0.005 0.001 0.040 0.0025
0.084 0.008 0.19 0.056 900 625 F 2 21 Inventive example C 0.044
0.08 1.59 0.006 0.001 0.041 0.0022 0.087 0.005 0.21 0.057 890 650 F
2 22 Inventive example D 0.045 0.06 1.64 0.005 0.001 0.041 0.0020
0.088 0.006 0.21 0.058 900 480 B 1 4.0 Comparative example E 0.122
0.46 1.84 0.011 0.001 0.046 0.0041 0.091 0.041 -- 0.195 860 570 F +
P 30 4.4 Comparative example F: ferrite, B: bainite, P:
pearlite
[0084]
2TABLE 2 Steel Thickness TS EL .lambda. Press sheet Steel (mm)
(MPa) (%) TS .times. EL (%) formability Remarks 1 A 3.2 821 21.1
17323 105 OK Inventive example 2 B 3.2 810 23.1 18711 110 OK
Inventive example 3 C 3.2 815 22.3 18175 118 OK Inventive example 4
D 3.2 841 16.5 13877 95 NG Comparative example 5 E 3.2 856 20.7
17719 30 NG Comparative example
EXAMPLE 2
[0085] Steels A to P having a chemical composition given in Table 3
were heated to a temperature of 1250.degree. C. and then hot rolled
at a finishing temperature of 880 to 930.degree. C., by which steel
sheets 1 to 16 with a thickness of 3.2 mm were manufactured by
changing the cooling rate and the coiling temperature. Thereafter,
the same tests as those in example 1 were conducted.
[0086] The results are given in Table 4.
[0087] The steel sheets 1 to 10 of examples of this invention,
having a ferrite structure single phase, had precipitates with a
grain size of smaller than 10 nm, a value of Mo/(Ti+Nb+V+Mo) of
0.25 or higher by atomic percentage, and TS of not lower than 550
MPa and high EL and k. Also, according to the observation under an
electron microscope, fine precipitates were dispersed homogeneously
in the ferrite structure.
[0088] Contrarily, for the steel sheets 11 and 12 of comparative
examples to which Mo was not added, pearlite was yielded, and
precipitates were coarse, so that both EL and .lambda. were low.
Also, for the steel sheet 13, the grain size of precipitates was 10
nm or larger, and the C content was low and the amount of
precipitates was small, so that TS was lower than 550 MPa. For the
steel sheet 14, the Mn content was high and the segregation was
remarkable, and also martensite was formed, so that both EL and
.lambda. were low. For the steel sheet 15, the Ti content was low
and the amount of precipitates was small, so that TS was lower than
550 MPa. For the steel sheet 16, although the Ti content was high
and composite precipitates of Ti and Mo existed, the ratio of Mo in
the composite precipitates was low and the Si content was high, so
that the precipitates were coarse, and thus both EL and .lambda.
were low.
3 TABLE 3 Chemical composition (wt. %) Steel C Si Mn P S Al N Mo Ti
Nb V Others A 0.045 0.20 1.60 0.003 0.0007 0.046 0.0041 0.20 0.085
-- -- -- B 0.047 0.22 1.59 0.002 0.0006 0.048 0.0040 0.20 0.082
0.012 -- -- C 0.042 0.18 1.56 0.004 0.0006 0.052 0.0040 0.11 0.080
0.037 -- -- D 0.045 0.18 1.39 0.006 0.0006 0.049 0.0031 0.21 0.084
0.008 -- -- E 0.035 0.09 1.65 0.005 0.0009 0.050 0.0045 0.30 0.040
0.011 0.069 -- F 0.038 0.11 1.12 0.004 0.0008 0.046 0.0043 0.15
0.043 0.009 -- -- G 0.040 0.08 0.89 0.005 0.0007 0.045 0.0042 0.14
0.025 0.041 -- -- H 0.043 0.17 1.56 0.005 0.0008 0.046 0.0039 0.19
0.083 0.010 -- Cr: 0.12 I 0.042 0.16 1.57 0.004 0.0008 0.047 0.0041
0.20 0.082 0.007 -- Cu: 0.14 J 0.042 0.18 1.55 0.004 0.0006 0.049
0.0039 0.20 0.085 0.008 -- Ni: 0.11 K 0.063 0.20 1.72 0.008 0.0006
0.054 0.0028 -- 0.084 0.040 -- L 0.051 0.09 1.70 0.006 0.0009 0.048
0.0040 -- 0.101 0.038 -- M 0.006 0.16 1.61 0.005 0.0010 0.046
0.0042 0.13 0.070 0.011 -- N 0.045 0.15 2.20 0.004 0.0008 0.050
0.0045 0.15 0.061 0.022 -- O 0.047 0.19 1.54 0.005 0.0009 0.045
0.0038 0.12 0.010 0.012 -- P 0.050 0.48 1.03 0.012 0.0010 0.030
0.0025 0.21 0.150 0.050 --
[0089]
4 TABLE 4 Precipitate composition Steel TS EL .lambda. Precipitate
ratio sheet Steel (MPa) (%) (%) Structure grain size (nm) Mo/(Ti +
Nb + V + Mo) Remarks 1 A 806 24.6 109 F 3 0.47 Inventive example 2
B 807 24.4 111 F 3 0.46 Inventive example 3 C 795 23.3 83 F 4 0.39
Inventive example 4 D 793 24.9 108 F 4 0.48 Inventive example 5 E
789 24.7 101 F 3 0.28 Inventive example 6 F 603 33.9 133 F 4 0.67
Inventive example 7 G 598 34.1 138 F 4 0.75 Inventive example 8 H
803 24.0 108 F 3 0.46 Inventive example 9 I 801 24.7 100 F 3 0.47
Inventive example 10 J 805 24.5 107 F 3 0.47 Inventive example 11 K
811 20.5 39 F + P 16 0 Comparative example 12 L 786 20.7 46 F 15 0
Comparative example 13 M 495 36.7 121 F 11 0.51 Comparative example
14 N 802 19.3 43 F + M 5 0.49 Comparative example 15 O 508 37.1 125
F 6 0.83 Comparative example 16 P 801 20.4 78 F 12 0.01 Comparative
example
EXAMPLE 3
[0090] Steels having a chemical composition given in Table 5 were
heated to an austenite zone, then hot rolled at a finishing
temperature of 880.degree. C., and coiled at a coiling temperature
given in Table 5, by which steel sheets 17 to 29 with a thickness
given in Table 5 were manufactured. The steel sheets 17 to 23 are
ones manufactured with the aim of providing TS of not lower than
780 MPa, and the steel sheets 24 to 29 are ones manufactured with
the aim of providing TS of not lower than 590 MPa. A tensile test
was conducted by using a JIS No. 5 test piece. Also, the sheet
shape after rolling was judged visually. A flat sheet was indicated
by mark .largecircle., and a remarkably wavy sheet was indicated by
mark x. Further, precipitates were investigated under a
transmission electron microscope.
[0091] For the steel sheets 17 to 21 and 24 to 27 of examples of
this invention, the value of Mo/(Ti+Nb+Mo) was 0.25 or higher, high
TS and EL were obtained, and the sheet shape was good.
[0092] On the other hand, for the steel sheets 22, 23, 28 and 29 of
comparative examples, the value of Mo/(Ti+Nb+Mo) was lower than
0.25, EL was low, and the sheet shape was remarkably wavy. The
reason for this is that quenching was performed after hot rolling
to secure strength, so that a second phase was formed.
5 TABLE 5 Preci- pitate compo- sition Coil- ratio ing Mo/ Tem- (Ti
+ Thick- per- Steel Chemical composition (wt %) Nb + ness ature TS
EL sheet C Si Mn P S Al N Mo Ti Nb V + Mo) (mm) (.degree. C.) (MPa)
(%) Shape Remarks 17 0.055 0.12 1.81 0.005 0.001 0.045 0.0025 0.22
0.088 0.010 0.48 2.0 625 801 20.1 .largecircle. Inventive example
18 " " " " " " " " " " 0.48 1.6 625 810 19.2 .largecircle.
Inventive example 19 " " " " " " " " " " 0.48 1.4 625 805 18.6
.largecircle. Inventive example 20 " " " " " " " " " " 0.48 1.2 625
807 18.1 .largecircle. Inventive example 21 " " " " " " " " " "
0.45 2.0 600 795 18.9 .largecircle. Inventive example 22 " " " " "
" " " " " 0.20 2.0 540 821 16.3 x Comparative example 23 " " " " "
" " " " " 0.16 2.0 500 830 14.1 x Comparative example 24 0.020 0.01
1.31 0.001 0.001 0.050 0.0034 0.18 0.051 0.020 0.53 2.3 640 605
35.1 .largecircle. Inventive example 25 " " " " " " " " " " 0.53
1.8 640 610 33.4 .largecircle. Inventive example 26 " " " " " " " "
" " 0.53 1.4 640 608 31.8 .largecircle. Inventive example 27 " " "
" " " " " " " 0.53 1.2 640 611 30.8 .largecircle. Inventive example
28 " " " " " " " " " " 0.22 1.4 540 631 28.1 x Comparative example
29 " " " " " " " " " " 0.18 1.4 510 642 25.3 x Comparative
example
EXAMPLE 4
[0093] Steels having a chemical composition given in Table 6 were
heated to a temperature of 1250.degree. C., then hot rolled at a
finishing temperature of 890.degree. C., and coiled at a coiling
temperature of 650.degree. C., by which steel sheets with a
thickness of 3.2 mm were manufactured. A tensile test was conducted
by taking JIS No. 5 test pieces from a central position in the
width direction and a position 65 mm distant from the edge in the
central portion in the longitudinal direction of the steel sheet.
Also, precipitates were investigated under a transmission electron
microscope. Further, a scale defect caused by Si was evaluated by
the above-described method.
[0094] The results are given in Table 6.
[0095] For the steel sheets 2 to 4 of examples of this invention,
homogeneous properties were obtained in the width direction, and
the surface property was good. In particular, for the steel sheets
2 and 3 in which the value of (Si+Mo) is 0.5 or lower, a very good
surface property was obtained. For the steel sheet 3, according to
the investigation under a transmission electron microscope, fine
carbides containing Ti and Mo were dispersed homogeneously in the
ferrite structure.
[0096] Contrarily, for the steel sheet 1 of comparative example to
which Mo was not added, the structure was a ferrite+pearlite
structure, and the variation in properties in the width direction
ATS was 30 MPa or larger, and also AEL was as large as 2% or
larger. Also, for the steel sheet 5 of comparative example in which
much Mo was added and Ti/Mo is less than 0.1, the variation in
properties was small, but the elongation decreases greatly.
[0097] For the steel sheets 7 to 9 of examples of this invention,
the variation in properties in the width direction was small, and
the surface property was very good.
[0098] Contrarily, for the steel sheet 6 of comparative example to
which Mo was not added, the structure was a ferrite+pearlite
structure, and the variation in properties ATS was 30 MPa or
larger, and also AEL was as large as 2% or larger. For the steel
sheet 10 of comparative example to which much Cr was added, the
formation of second phase could not be restrained, and the
structure was an acicular ferrite structure, so that EL decreases
significantly. For all of these steel sheets, since the value of
(Si+Mo) was 0.5% or lower, the surface property was very good.
[0099] For the steel sheets 12 to 14 of examples of this invention,
the variation in properties in the width direction was small, and
the surface property was also good.
[0100] Contrarily, for the steel sheet 11 of comparative example to
which Ti was not added, pearlite and cementite were precipitated,
and although TS is low, EL is as low as that of the steel sheet 12,
and the variation in properties was also large.
[0101] For the steel sheets 16 to 18 of examples of this invention,
the variation in properties in the width direction was small, and
the surface property was good. In particular, for the steel sheet
16 in which the value of (Si+Mo) was 0.5% or lower, the surface
property was very good.
[0102] Contrarily, for the steel sheet 15 of comparative example to
which Mo was not added, the variation in properties was large, and,
for the steel sheet 19 of comparative example to which much Mo was
added, EL was low.
6TABLE 6 Steel Chemical composition (wt %) sheet C Si Mn P S Al N
Mo Ti Nb Cr 1 0.040 0.18 1.62 0.005 0.002 0.050 0.0030 -- 0.084
0.011 0.056 2 0.041 0.18 1.60 0.006 0.003 0.055 0.0033 0.11 0.080
0.010 0.051 3 0.043 0.19 1.61 0.006 0.003 0.054 0.0028 0.20 0.085
0.010 0.056 4 0.044 0.17 1.59 0.003 0.003 0.048 0.0025 0.36 0.084
0.012 0.050 5 0.044 0.17 1.63 0.006 0.003 0.057 0.0036 0.72 0.085
0.011 0.055 6 0.035 0.15 1.61 0.012 0.003 0.035 0.0016 -- 0.040
0.011 0.230 7 0.035 0.15 1.45 0.011 0.003 0.036 0.0018 0.23 0.040
0.013 0.081 8 0.036 0.14 1.43 0.013 0.002 0.037 0.0019 0.19 0.041
0.014 0.151 9 0.036 0.13 1.44 0.011 0.003 0.031 0.0018 0.20 0.043
0.013 0.425 10 0.035 0.14 1.46 0.014 0.004 0.035 0.0019 0.22 0.042
0.014 0.740 11 0.056 0.26 1.30 0.009 0.003 0.061 0.0039 0.34 --
0.034 0.240 12 0.057 0.27 1.40 0.008 0.002 0.060 0.0038 0.35 0.030
0.034 0.260 13 0.058 0.26 1.36 0.009 0.003 0.069 0.0041 0.46 0.057
0.035 0.260 14 0.054 0.26 1.38 0.007 0.002 0.068 0.0037 0.35 0.089
0.034 0.240 15 0.045 0.38 1.39 0.011 0.003 0.050 0.0031 -- 0.085
0.041 0.154 16 0.044 0.39 1.38 0.008 0.004 0.049 0.0030 0.08 0.084
0.043 0.162 17 0.046 0.37 1.39 0.010 0.003 0.045 0.0029 0.23 0.085
0.041 0.158 18 0.044 0.28 1.38 0.011 0.001 0.043 0.0033 0.31 0.086
0.045 0.187 19 0.044 0.37 1.37 0.009 0.002 0.048 0.0031 0.77 0.084
0.042 0.177 Tensile properties Variation in TS properties Surface
Structure Ti/Mo (MPa) EL (%) .DELTA. TS .DELTA. EL property Remarks
1 F + P -- 781 19.8 45 2.4 .circleincircle. Comparative example 2 F
1.53 816 21.4 15 0.5 .circleincircle. Inventive example 3 F 0.82
807 24.4 12 0.1 .circleincircle. Inventive example 4 F 0.61 812
24.2 11 0.2 .largecircle. Inventive example 5 AF 0.05 890 14.9 10
0.1 x Comparative example 6 F + P -- 572 36.8 43 2.3
.circleincircle. Comparative example 7 F 0.46 610 35.6 12 0.2
.circleincircle. Inventive example 8 F 0.44 615 34.6 12 0.3
.circleincircle. Inventive example 9 F 0.45 624 35.5 9 0.1
.circleincircle. Inventive example 10 AF 0.48 687 28.9 9 0.2
.circleincircle. Comparative example 11 F + P 0 512 34.1 48 4.3
.largecircle. Comparative example 12 F 0.23 623 34.3 8 0.9
.largecircle. Inventive example 13 F 0.25 781 24.9 9 1.1
.largecircle. Inventive example 14 F 0.51 812 24.1 11 1.3
.largecircle. Inventive example 15 F + P -- 735 22.4 53 4.2
.largecircle. Comparative example 16 F 2.10 782 24.1 12 1.3
.circleincircle. Inventive example 17 F 0.74 791 23.3 13 1.2
.largecircle. Inventive example 18 F 0.49 829 22.2 11 0.7
.largecircle. Inventive example 19 AF 0.22 898 17.4 10 1.2 x
Comparative example F: ferrite, AF: acicular ferrite, P:
pearlite
EXAMPLE 5
[0103] Steels having a chemical composition given in Table 7 were
hot rolled under the conditions given in Table 7, by which steel
sheets 21 to 38 with a thickness of 3.6 mm were manufactured. As in
the case of example 4, the tensile characteristic values were
investigated in the central and edge portions in the width
direction of steel sheet. Also, precipitates were investigated
under a transmission electron microscope.
[0104] The results are given in Table 7.
[0105] For the steel sheets 21 to 25 in which the finishing
temperature was changed, the variation in properties was very small
when the finishing temperature was 880.degree. C. or higher.
[0106] Also, for the steel sheets 26 to 30 in which the coiling
temperature was changed, the variation in properties of the steel
sheets 27 to 29 in which the coiling temperature was 550 to
700.degree. C. was very small, and the elongation thereof was also
high.
[0107] Contrarily, for the steel sheets 26 and 30 whose coiling
temperature is out of the range of 550 to 700.degree. C., the
variation in properties was large.
[0108] For the steel sheets 31 to 35 in which the finishing
temperature and coiling temperature were changed, the variation in
properties of the steel sheets 32 to 34 in which the finishing
temperature was 880.degree. C. or higher and the coiling
temperature was 550 to 700.degree. C. was very small.
[0109] For the steel sheets 36 and 37, the Mo content was high, the
ratio of Ti/Mo of precipitates was lower than 0.1, and the
variation in properties was large. Also, for the steel sheet 38,
the coiling temperature was as low as 500.degree. C., a bainite
structure was formed, and the variation in TS was especially
large.
7 TABLE 7 Chemical composition (wt %) Steel sheet C Si Mn P S Al N
Mo Ti Nb Cr 21 0.042 0.18 1.54 0.001 0.001 0.055 0.0029 0.21 0.085
0.008 0.060 22 " " " " " " " " " " " 23 " " " " " " " " " " " 24 "
" " " " " " " " " " 25 " " " " " " " " " " " 26 0.045 0.17 1.39
0.006 0.001 0.049 0.0031 0.22 0.088 0.009 0.067 27 " " " " " " " "
" " " 28 " " " " " " " " " " " 29 " " " " " " " " " " " 30 " " " "
" " " " " " " 31 0.035 0.25 1.55 0.005 0.001 0.034 0.0022 0.15
0.035 0.015 0.080 32 " " " " " " " " " " " 33 " " " " " " " " " " "
34 " " " " " " " " " " " 35 " " " " " " " " " " " 36 0.05 0.50 1.50
0.01 0.001 0.035 0.0025 0.99 0.150 0.050 -- 37 " " " " " " " " " "
" 38 0.05 0.48 1.03 0.012 0.001 0.030 0.0025 0.21 0.150 0.050 --
Hot rolling conditions Finishing Coiling Tensile temper- temper-
properties Variation in ature ature Struc- TS properties (.degree.
C.) (.degree. C.) ture Ti/Mo (Mpa) EL (%) .DELTA. TS .DELTA. EL 21
865 603 F 1.30 703 21.2 28 1.2 22 880 604 F 1.32 819 20.5 15 0.4 23
885 604 F 1.41 825 20.6 10 0.3 24 910 610 F 1.30 820 21.3 8 0.2 25
920 606 F 1.32 823 21.1 10 0.2 26 892 500 AF 0.03 764 20.6 31 1.7
27 891 580 F 2.30 821 23.8 10 0.2 28 890 530 F 1.20 831 24.3 5 0.2
29 893 650 F 1.10 824 24.4 3 0.1 30 894 720 F + P 1.10 703 20.3 25
1.9 31 843 592 F 0.61 544 31.1 24 1.6 32 892 623 F 0.65 620 35.0 5
0.3 33 899 632 F 0.67 613 35.4 6 0.2 34 891 600 F 0.61 611 34.4 4
0.2 35 901 490 BF 0.03 531 27.5 22 1.7 36 895 590 F 0.08 635 26.3
26 2.3 37 903 700 F + P 0.08 725 22.3 35 3.1 38 910 500 B 0.08 815
18.2 42 2.9 F: ferrite, AF: acicular ferrite, P: pearlite, B:
bainite, BF: bainitic ferrite
EXAMPLE 6
[0110] Steels A to T having a chemical composition given in Table 8
were hot rolled under the hot rolling conditions given in Table 9,
by which steel sheets 1 to 23 with a thickness of 3.2 mm were
manufactured. Thereafter, a tensile test and a hole expanding test
were conducted. Also, a metallographic structure was observed under
an optical microscope and a scanning electron microscope. Further,
precipitates were investigated under a transmission electron
microscope. In this example, a steel sheet having TS of 780 MPa or
higher, EL of 20% or higher, and .lambda. of 70% or higher was
judged to be good.
[0111] The results are given in Table 9.
[0112] For the steel sheets 1 to 3 and 5 to 15 of examples of this
invention, the chemical composition was within the range of the
present invention, and the structure was a ferrite structure in
which carbides containing Ti and Mo were dispersed homogeneously,
so that EL and .lambda. were high.
[0113] Contrarily, for the steel sheet 4 of comparative example, a
bainite structure with high dislocation density was formed because
of the low coiling temperature, so that EL was low. For the steel
sheets 16 to 18 of comparative examples, pearlite or martensite was
formed, so that both EL and .lambda. were low. Also, for the steel
sheet 19, carbides and nitrides of Ti and Nb were not fine because
a small amount of Mo was added, and few Mo carbides were
precipitated, so that A was low. For the steel sheet 21, a small
amount of Ti was added and the amount of Ti carbides was small, so
that TS and .lambda. were low. For the steel sheets 22 and 23,
since Mo was not added and much Ti was added, the precipitates were
coarse and the amount thereof was excessive, so that both EL and
.lambda. were low.
8 TABLE 8 Chemical composition (wt. %) Steel C Si Mn P S Al N Mo Ti
Nb Cr A 0.043 0.20 1.61 0.005 0.0010 0.054 0.0028 0.20 0.085 0.008
0.056 B 0.042 0.18 1.56 0.004 0.0006 0.052 0.0040 0.10 0.080 0.037
0.082 C 0.045 0.18 1.39 0.006 0.0006 0.049 0.0031 0.21 0.084 0.008
0.067 D 0.057 0.19 1.59 0.005 0.0015 0.050 0.0034 0.18 0.076 0.010
0.710 E 0.047 0.28 1.63 0.008 0.0011 0.048 0.0046 0.19 0.082 0.012
-- F 0.043 0.18 1.91 0.012 0.0015 0.046 0.0051 0.21 0.066 0.011
0.094 G 0.045 0.20 1.50 0.014 0.0016 0.044 0.0044 0.12 0.084 0.013
0.081 H 0.048 0.07 1.41 0.009 0.0009 0.050 0.0049 0.38 0.077 0.009
-- I 0.045 0.10 1.40 0.011 0.0018 0.049 0.0031 0.20 0.033 0.040 --
J 0.048 0.14 1.60 0.008 0.0011 0.048 0.0035 0.20 0.096 0.043 0.070
K 0.040 0.11 1.36 0.009 0.0014 0.039 0.0040 0.18 0.079 0.009 0.081
L 0.050 0.12 1.62 0.005 0.0008 0.041 0.0039 0.43 0.088 0.010 -- M
0.066 0.20 1.50 0.002 0.0006 0.051 0.0039 -- 0.166 0.057 -- N 0.065
0.18 1.72 0.006 0.0008 0.050 0.0034 0.20 0.080 0.010 0.061 O 0.041
0.21 2.20 0.008 0.0013 0.036 0.0035 0.15 0.081 0.008 0.070 P 0.043
0.19 1.60 0.008 0.0009 0.038 0.0028 0.04 0.078 0.009 0.053 Q 0.050
0.20 1.20 0.006 0.0011 0.041 0.0033 0.49 0.068 0.008 -- R 0.046
0.18 1.64 0.012 0.0013 0.032 0.0028 0.16 0.016 0.051 0.051 S 0.040
0.51 1.51 0.012 0.0012 0.030 0.0026 -- 0.250 -- -- T 0.030 0.51
1.48 0.010 0.0012 0.035 0.0026 -- 0.151 0.013 --
[0114]
9TABLE 9 Heating Finishing Coiling Steel temperature temperature
temperature TS EL .lambda. sheet Steel (.degree. C.) (.degree. C.)
(.degree. C.) (MPa) (%) (%) Structure Remarks 1 A 1250 910 560 782
20.2 85 F Inventive example 2 A 1200 880 600 820 21.4 86 F
Inventive example 3 A 1250 900 675 813 24.1 113 F Inventive example
4 A 1200 880 510 740 17.2 91 B Comparative example 5 B 1250 900 640
798 23.6 81 F Inventive example 6 C 1250 915 650 831 24.2 85 F
Inventive example 7 D 1200 880 610 799 22.1 73 F Inventive example
8 E 1250 900 640 822 22.6 80 F Inventive example 9 F 1250 890 630
836 20.6 82 F Inventive example 10 G 1200 900 600 783 24.9 89 F
Inventive example 11 H 1250 915 590 828 21.6 81 F Inventive example
12 I 1250 890 620 780 23.1 90 F Inventive example 13 J 1200 900 600
857 20.1 79 F Inventive example 14 K 1200 900 610 850 20.2 78 F
Inventive example 15 L 1250 900 450 811 23.1 100 F Inventive
example 16 M 1250 915 650 820 18.6 43 F + P Comparative example 17
N 1200 900 710 830 18.1 52 F + P Comparative example 18 O 1250 900
500 841 17.2 50 F + M Comparative example 19 P 1250 890 580 791
20.4 55 F Comparative example 20 Q 1250 900 510 820 17.9 46 F + M
Comparative example 21 R 1250 915 630 760 20.1 50 F Comparative
example 22 S 1250 900 600 830 17.4 39 F Comparative example 23 T
1200 890 610 814 17.8 48 F Comparative example F: ferrite. B:
bainite. M: martensite. P: pearlite
EXAMPLE 7
[0115] The steels A, F and M given in Table 8 were heated at a
temperature of 1250.degree. C., then hot rolled at a finishing
temperature of 890.degree. C., and coiled at a coiling temperature
of 630.degree. C., by which steel sheets with a thickness of 3.0 mm
were manufactured. Also, the content of solid solution C was
measured by the internal friction measurement.
[0116] FIG. 6 shows the relationship between solid solution C
content and EL.
[0117] It was verified that if the solid solution C content is
0.0020% or lower, EL is high.
EXAMPLE 8
[0118] Steels A to M having a chemical composition given in Table
10 were heated at a temperature of 1250.degree. C., then hot rolled
at a finishing temperature of 880 to 930.degree. C., by which steel
sheets 1 to 13 with a thickness of 2.6 mm were manufactured by
changing the cooling rate and coiling temperature. The coiling
temperature was changed in the temperature range exceeding
600.degree. C. Thereafter, a tensile test and a hole expanding test
were conducted. Also, precipitates were investigated under a
transmission electron microscope.
[0119] The results are given in Table 11.
[0120] For the steel sheets 1 to 9 of examples of this invention,
the structure consisted of a ferrite structure single phase, the
average grain size of precipitates was smaller than 10 nm, and the
composition ratio of precipitates satisfied the equation of
0.1.ltoreq.Ti/Mo.ltoreq.3- , so that TS was not lower than 950 MPa,
and EL and .lambda. were high.
[0121] Contrarily, for the steel sheet 10 of comparative example,
the C content was too high, pearlite was formed because of no
addition of Mo, and the precipitates were coarse, so that both EL
and .lambda. were low. Also, for the steel sheet 11, the
precipitates were coarse because of no addition of Mo, so that both
EL and .lambda. were low. For the steel sheet 12, the segregation
was remarkable because the content of Mn is too high, and
martensite was formed, so that both EL and .lambda. were low. For
the steel sheet 13, although composite precipitates of Ti and Mo
existed, the content of C was too high, pearlite was formed, and
the precipitates were coarse, so that both EL and .lambda. were
low.
10 TABLE 10 Chemical compositon (wt %) Steel C Si Mn P S Al N Mo Ti
Nb V Others A 0.075 0.19 1.59 0.002 0.0007 0.045 0.0040 0.34 0.17
-- -- -- B 0.085 0.21 1.60 0.003 0.0008 0.046 0.0039 0.37 0.19
0.011 -- -- C 0.090 0.16 1.57 0.004 0.0007 0.050 0.0041 0.50 0.19
0.033 -- -- D 0.119 0.17 1.59 0.005 0.0006 0.048 0.0032 0.51 0.24
0.009 -- -- E 0.130 0.08 1.65 0.004 0.0009 0.044 0.0036 0.60 0.17
0.012 0.068 -- F 0.089 0.09 0.84 0.005 0.0007 0.047 0.0041 0.52
0.18 0.039 -- -- G 0.094 0.16 1.54 0.006 0.0007 0.045 0.0039 0.41
0.19 0.011 -- Cr: 0.10 H 0.090 0.15 1.58 0.004 0.0007 0.044 0.0042
0.38 0.19 0.007 -- Cu: 0.15 I 0.094 0.16 1.56 0.003 0.0006 0.046
0.0039 0.40 0.19 0.007 -- Ni: 0.12 J 0.181 0.19 1.70 0.008 0.0007
0.051 0.0030 -- 0.21 0.037 -- -- K 0.090 0.07 1.71 0.007 0.0009
0.047 0.0037 -- 0.27 0.036 -- -- L 0.091 0.13 2.23 0.003 0.0009
0.048 0.0045 0.37 0.18 0.020 -- -- M 0.168 0.48 1.06 0.011 0.0010
0.032 0.0031 0.29 0.24 0.048 -- --
[0122]
11TABLE 11 Precipitate Steel TS EL .lambda. Precipitate grain
composition ratio sheet Steel (MPa) (%) (%) Structure size (nm)
Ti/Mo Remarks 1 A 961 20.9 83 F 5 0.96 Inventive example 2 B 990
19.6 81 F 5 0.89 Inventive example 3 C 1003 18.5 80 F 6 1.38
Inventive example 4 D 1052 16.9 97 F 6 1.04 Inventive example 5 E
1048 16.0 83 F 5 1.94 Inventive example 6 F 965 20.1 101 F 6 0.59
Inventive example 7 G 994 19.0 96 F 5 0.79 Inventive example 8 H
992 19.0 105 F 5 0.96 Inventive example 9 I 999 18.3 81 F 5 1.04
Inventive example 10 J 991 16.1 21 F + P 15 -- Comparative example
11 K 973 16.6 41 F 16 -- Comparative example 12 L 988 14.7 23 F + M
7 1.13 Comparative example 13 M 992 16.0 68 F + P 13 3.00
Comparative example F: ferrite, P: pearlite, M: martensite
EXAMPLE 9
[0123] Steels A to E having a chemical composition given in Table
12 were heated at a temperature of 1250.degree. C., then hot rolled
at a finishing temperature of 890.degree. C., and coiled at a
temperature of 5200.degree. C., by which steel sheets 1 to 5 with a
thickness of 3.2 mm were manufactured. Thereafter, as in the case
of example 4, the tensile properties and hole expanding ratio were
investigated in the central and edge portions in the width
direction of steel sheet. Also, precipitates were investigated
under a transmission electron microscope.
[0124] The results are given in Table 13.
[0125] For the steel sheets 1 to 4 of examples of this invention,
the variation in properties was very small.
[0126] On the other hand, for the steel sheet 5 of comparative
example, only Ti is contained as a carbide/nitride forming element,
so that EL and .lambda. were low, and the variation in properties
was large.
12 TABLE 12 Chemical composition (wt %) Ti/ Steel C Si Mn P S Al N
Ti Nb Mo Cr W (Mo + W) Structure A 0.048 0.01 1.48 0.008 0.001
0.045 0.0038 0.081 0.010 0.16 0.050 0.071 0.8 F B 0.048 0.03 1.49
0.008 0.001 0.043 0.0037 0.088 0.012 0.15 0.051 0.098 0.9 F C 0.049
0.02 1.50 0.007 0.001 0.044 0.0035 0.085 0.013 0.10 0.052 0.200 0.8
F D 0.047 0.01 1.50 0.008 0.001 0.045 0.0035 0.087 0.012 -- 0.055
0.400 0.8 F E 0.043 0.01 1.53 0.005 0.001 0.044 0.0034 0.186 0.009
-- 0.051 -- -- F F: ferrite
[0127]
13 TABLE 13 Tensile properties Variation in Steel TS EL .lambda.
properties sheet Steel (MPa) (%) (%) .DELTA.TS .DELTA.EL Remarks 1
A 813 22 100 8 1.0 Inventive example 2 B 822 20 105 13 1.2
Inventive example 3 C 801 23 110 5 1.1 Inventive example 4 D 808 21
108 3 0.9 Inventive example 5 E 762 17 60 35 3.8 Comparative
example
* * * * *