U.S. patent application number 12/866382 was filed with the patent office on 2010-12-23 for high-strength hot-rolled steel sheet and method for manufacturing same.
Invention is credited to Shinjiro Kaneko, Noriaki Moriyasu, Kaneharu Okuda, Tetsuo Shimizu, Masahide Watabe.
Application Number | 20100319819 12/866382 |
Document ID | / |
Family ID | 40952299 |
Filed Date | 2010-12-23 |
United States Patent
Application |
20100319819 |
Kind Code |
A1 |
Kaneko; Shinjiro ; et
al. |
December 23, 2010 |
HIGH-STRENGTH HOT-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING
SAME
Abstract
A high-strength hot-rolled steel sheet has a tensile strength
(TS) of 540 to 780 MPa, only small variations in strength, and
excellent uniformity in strength using a general-purpose
Ti-containing steel sheet, which is inexpensive. The high-strength
hot-rolled steel sheet includes, on a mass percent basis,
0.05%-0.12% C, 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less P,
0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti,
and the balance being Fe and incidental impurities. The
microstructure have a volume fraction of polygonal ferrite of 70%
or more, and the amount of Ti present in a precipitate having a
size of less than 20 nm is 50% or more of the value of Ti*
calculated using formula (1): Ti*=[Ti]-48/14.times.[N] (1) where
[Ti] and [N] represent a Ti content (percent by mass) and a N
content (percent by mass), respectively, of the steel sheet.
Inventors: |
Kaneko; Shinjiro; (Tokyo,
JP) ; Okuda; Kaneharu; (Tokyo, JP) ; Shimizu;
Tetsuo; (Tokyo, JP) ; Moriyasu; Noriaki;
(Tokyo, JP) ; Watabe; Masahide; (Tokyo,
JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER LLP (US)
ONE LIBERTY PLACE, 1650 MARKET ST, SUITE 4900
PHILADELPHIA
PA
19103
US
|
Family ID: |
40952299 |
Appl. No.: |
12/866382 |
Filed: |
February 4, 2009 |
PCT Filed: |
February 4, 2009 |
PCT NO: |
PCT/JP2009/052244 |
371 Date: |
August 5, 2010 |
Current U.S.
Class: |
148/602 ;
148/320 |
Current CPC
Class: |
C21D 9/46 20130101; C21D
2211/005 20130101; C22C 38/04 20130101; C22C 38/14 20130101; C21D
8/0226 20130101; C21D 8/0263 20130101; C22C 38/06 20130101; C21D
2211/004 20130101 |
Class at
Publication: |
148/602 ;
148/320 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06; C22C 38/14 20060101
C22C038/14 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 8, 2008 |
JP |
2008-028455 |
Claims
1. A high-strength hot-rolled steel sheet comprising, on a mass
percent basis, 0.05%-0.12% C, 0.5% or less Si, 0.8%-1.8% Mn, 0.030%
or less P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N,
0.030%-0.080% Ti, the balance being Fe and incidental impurities,
and microstructures whose volume fraction of polygonal ferrite is
70% or more, wherein the amount of Ti present in a precipitate
having a size of less than 20 nm is 50% or more of the value of Ti*
calculated using formula (1): Ti*=[Ti]-48/14.times.[N] (1) where
[Ti] and [N] represent a Ti content (percent by mass) and a N
content (percent by mass), respectively, of the steel sheet.
2. A method for manufacturing a high-strength hot-rolled steel
sheet comprising: heating a steel slab to 1150.degree. C. to
1300.degree. C., the steel slab containing, on a mass percent
basis, 0.05%-0.12% C; 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less
P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080%
Ti, and the balance being Fe and incidental impurities; subjecting
the slab to finish hot rolling at a finishing temperature of
800.degree. C. to 950.degree. C.; starting cooling at a cooling
rate of 20.degree. C./s or more within 2 seconds after completion
of the finish hot rolling; stopping cooling at 650.degree. C. to
750.degree. C.; performing natural cooling for 2 seconds to 15
seconds; cooling the steel sheet at a cooling rate of less than
100.degree. C./s; and winding the steel sheet into a coil in a
temperature range of 550.degree. C. to 650.degree. C.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2009/052244, with an international filing date of Feb. 4,
2009 (WO 2009/099237 A1, published Aug. 13, 2009), which is based
on Japanese Patent Application No. 2008-028455, filed Feb. 8, 2008,
the subject matter of which is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to a high-intensity hot-rolled steel
sheet having a tensile strength (TS) of 540 to 780 MPa, only small
variations in strength, and excellent uniformity in strength
between coils and within a coil, and being useful as a steel sheet
for automobiles and so forth, and to a method for manufacturing the
same.
BACKGROUND
[0003] From the viewpoint of global environmental protection,
improvement in the fuel economy of automobiles has recently been
required to regulate the amount of CO.sub.2 emission. In addition,
it is also required to improve safety by focusing on collision
characteristics of automobile bodies to ensure the safety of
passengers in case of accident. Thus, both weight reduction and
strengthening of automobile bodies are being actively promoted. To
simultaneously achieve the weight reduction and strengthening of
automobile bodies, an increase in the strength of material for
members and a reduction in weight by reducing the thickness of
sheets to the extent that rigidity is not impaired are said to be
effective. Nowadays, high-strength steel sheets are positively used
for automotive parts. The use of high-strength steel sheets results
in a significant weight reduction effect. Thus, in the motor
vehicle industry, for example, there is a trend toward the use of
steel sheets as a structural member with a tensile strength (TS) of
540 MPa or more.
[0004] Many automotive parts made from steel sheets are
manufactured by press forming. Regarding the formability of
high-strength steel sheets, dimensional accuracy is important in
addition to prevention of cracking and wrinkling. In particular,
the control of springback is an important problem. Nowadays, new
automobiles are developed very efficiently by computer assisted
engineering (CAE) and can predict the amount of springback more
accurately by the input of the characteristics of the steel sheet.
So, it is not necessary to make as many dies. Variations in the
amount of springback cause a problem when parts are connected to
each other and thus should be reduced. If the steel sheets used for
automobile parts have wide variations in strength, CAE can not
predict the amount of springback. So, in particular, a
high-strength steel sheet having only small variations in strength
and excellent uniformity in strength is required.
[0005] As a method for reducing variations in strength in a coil,
Japanese Unexamined Patent Application Publication No. 4-289125
discloses the following method: In the case of hot-rolling
Nb-containing low-manganese steel (Mn: 0.5% or less), a
rough-rolled sheet bar is temporarily wound into a coil. Next, the
sheet bar is joined to the preceding sheet bar while being unwound,
and then continuously finish-rolled to achieve uniformity in the
strength of the high-strength hot-rolled steel sheet in a coil.
Japanese Unexamined Patent Application Publication No. 2002-322541
discloses a high-strength hot-rolled steel sheet with excellent
uniformity in strength, i.e., only small variations in strength,
produced by the addition of both Ti and Mo to form very fine
precipitates uniformly dispersed therein.
[0006] However, the method described in JP 4-289125 has a problem
in which when the sheet is wound into a coil, the sheet is divided.
Furthermore, the addition of Nb causes an increase in cost, which
is economically disadvantageous. In the steel sheet described in JP
2002-322541, which is a Ti system, it is necessary to add Mo, which
is expensive, thus causing an increase in cost. Moreover, in both
of those publications, two-dimensional uniformity in strength in
the in-plane directions including both of the width direction and
the longitudinal direction of the coil is not taken into
consideration. Disadvantageously, even if the coiling temperature
is uniformly controlled, the variations in the in-plane strength of
the coil are inevitably caused by different cooling histories for
each position in the wound coil.
[0007] In consideration of the above-described situation, it could
be helpful to provide a high-strength hot-rolled steel sheet having
a tensile strength (TS) of 540 to 780 MPa, only small variations in
strength, and excellent uniformity in strength using a
general-purpose Ti-containing steel sheet, which is
inexpensive.
SUMMARY
[0008] We conducted intensive studies and successfully provided a
high-strength hot-rolled steel sheet having excellent uniformity in
strength and only small variations in strength over the entire area
of the hot-rolled steel sheet by controlling the chemical
composition and the metal microstructure of the steel sheet and the
precipitation state of Ti that contributes to precipitation
strengthening.
[0009] Our high-strength hot-rolled steel sheets and methods for
manufacturing the high-strength hot-rolled steel sheet are
described below, the steel sheet having only small variations in
in-plane strength and excellent uniformity in strength.
[0010] We thus provide: [0011] A high-strength hot-rolled steel
sheet includes, on a mass percent basis, 0.05%-0.12% C, 0.5% or
less Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01% or less S,
0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti, the balance
being Fe and incidental impurities, and microstructures whose
volume fraction of polygonal ferrite is 70% or more, in which the
amount of Ti present in a precipitate having a size of less than 20
nm is 50% or more of the value of Ti* calculated using formula
(1):
[0011] Ti*=[Ti]-48/14.times.[N] (1) where [Ti] and [N] represent a
Ti content (percent by mass) and a N content (percent by mass),
respectively, of the steel sheet.
[0012] We also provide: [0013] A method for manufacturing a
high-strength hot-rolled steel sheet includes the steps of heating
a steel slab to 1150.degree. C. to 1300.degree. C., the steel slab
containing, on a mass percent basis, 0.05%-0.12% C; 0.5% or less
Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01% or less S, 0.005%-0.1%
Al, 0.01% or less N, 0.030%-0.080% Ti, and the balance being Fe and
incidental impurities, subjecting the slab to finish hot rolling at
a finishing temperature of 800.degree. C. to 950.degree. C.,
starting cooling at a cooling rate of 20.degree. C./s or more
within 2 seconds after the completion of the finish hot rolling,
stopping cooling at 650.degree. C. to 750.degree. C., subsequently
performing natural cooling for 2 seconds to 15 seconds, cooling the
steel sheet at a cooling rate of less than 100.degree. C./s, and
winding the steel sheet into a coil in the temperature range of
550.degree. C. to 650.degree. C.
[0014] It is thus possible to reduce variations in strength in a
coil of a high-strength hot-rolled steel sheet having a tensile
strength (TS) of 540 to 780 MPa, thereby achieving the
stabilization of the shape fixability of the steel sheet at the
time of press forming and the strength and durability of a part.
This leads to improvement in reliability at the time of the
production and use of an automotive part. Furthermore, the
above-mentioned effect is provided without adding an expensive raw
material such as Nb, thus reducing the cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows the investigation results of the relationship
between the volume fraction of polygonal ferrite (%) and the
tensile strength TS (MPa).
[0016] FIG. 2 shows the investigation results of the relationship
between the proportion of the amount of Ti contained in a
precipitate having a size of less than 20 nm with respect to Ti*
and the tensile strength TS (Mpa).
DETAILED DESCRIPTION
[0017] Our steel sheets and methods will be described in detail
below.
1) A method for evaluating small variations in strength, i.e.,
uniformity in strength, will be described.
[0018] An example of a target steel sheet is a coiled steel sheet
having a weight of five tons or more and a steel sheet width of 500
mm or more. In this case, in an as-hot-rolled state, the innermost
turn including the front end in the longitudinal direction, the
outermost turn including the rear end in the longitudinal
direction, and regions extending from both sides to 10 mm from both
sides in the width direction are not evaluated. Variations in the
strength of the steel sheet are evaluated on the basis of
tensile-strength distribution obtained from two-dimensional
measurement at least 10 points in the longitudinal direction and at
least 5 points in the width direction. Furthermore, we provide a
steel sheet having a tensile strength (TS) of 540 MPa to 780
MPa.
2) The reason for the limitation of the chemical components
(composition) of steel will be described below.
[0019] The units of the content of each component in the steel
composition are "percent by mass" and are simply indicated by "%"
unless otherwise specified.
C: 0.05% to 0.12%
[0020] C is an important element as well as Ti described below in
our steel sheets. C forms a carbide with Ti and is effective in
increasing the strength of a steel sheet by precipitation
strengthening. The C content is preferably 0.05% or more and more
preferably 0.06% or more from the viewpoint of precipitation
strengthening. A C content exceeding 0.12% liable to adversely
affect satisfactory elongation and flangeability. Thus, the upper
limit of the C content is set to 0.12% and preferably 0.10% or
less.
Si: 0.5% or less
[0021] Si is effective in enhancing solid-solution strengthening
and improving ductility. To provide the effect described above, the
Si content is effectively 0.01% or more. A Si content exceeding
0.5% is liable to cause the occurrence of a surface defect called
red scale during hot rolling, which can reduce the quality of
surface appearance when a steel sheet is produced. Thus, the Si
content is preferably 0.5% or less and more preferably 0.3% or
less.
Mn: 0.8% to 1.8%
[0022] Mn is effective in achieving higher strength and has the
effect of reducing the transformation point and the ferrite grain
size. The Mn content needs to be 0.8% or more. More preferably, the
Mn content is set to 1.0% or more. A Mn content exceeding 1.8%
causes the formation of a low-temperature transformation phase
after hot rolling to reduce the ductility and is liable to make TiC
precipitation unstable. Thus, the upper limit of the Mn content is
set to 1.8%.
P: 0.030% or less
[0023] P is an element effective for solid-solution strengthening.
P also has the effect of reducing scale defects due to Si. An
excessive P content more than 0.030%, however, is liable to cause
the segregation of P into grain boundaries and reduce toughness and
weldability. Thus, the upper limit of the P content is set to
0.030%.
S: 0.01% or less
[0024] S is an impurity and causes hot tearing. Furthermore, S is
present in the form of an inclusion in steel, deteriorating the
various characteristics of a steel sheet. Thus, the S content needs
to be minimized. Specifically, the S content is set to 0.01%
because the S content is allowable to 0.01%.
Al: 0.005% to 0.1%
[0025] Al is useful as a deoxidizing element for steel. Al also has
the effect of fixing dissolved N present as an impurity, thereby
improving resistance to room-temperature aging. To provide the
effect, the Al content needs to be 0.005% or more. An Al content
exceeding 0.5% leads to an increase in alloy cost and is liable to
cause surface defects. Thus, the upper limit of the Al content is
set to 0.1%.
N: 0.01% or less
[0026] N is an element which degrades the resistance to
room-temperature aging and in which the N content is preferably
minimized. A higher N content causes a reduction in resistance to
room-temperature aging. To fix dissolved N, it is necessary to
perform the addition of large amounts of Al and Ti. Thus, the N
content is preferably minimized. The upper limit of the N content
is set to 0.01%.
Ti: 0.030% to 0.080%
[0027] Ti is an important element to strengthen steel by
precipitation strengthening. Ti contributes to precipitation
strengthening by forming a carbide with C.
[0028] That is, to produce a high-strength steel sheet having a
tensile strength TS of 540 MPa to 780 MPa, it is preferred to form
fine precipitates each having a size of less than 20 nm.
Furthermore, it is important to increase the proportion of the fine
precipitates (each having a size of less than 20 nm). One of the
reasons for this may be that precipitates having a size of 20 nm or
more are less likely to provide the effect of suppressing
dislocation migration and fail to sufficiently harden polygonal
ferrite, which can reduce strength. It is thus preferred that the
precipitates have a size of less than 20 nm. The fine precipitates
containing Ti and each having a size of less than 20 nm are formed
by the addition of Ti and C within the above ranges. In this
specification, the precipitates containing Ti and C are generically
referred to as a "Ti-containing carbide." Examples of the
Ti-containing carbide include TiC and Ti.sub.4C.sub.2S.sub.2. The
carbide may further contain N as a component and may be
precipitated in combination with, for example, MnS.
[0029] In the high-strength steel sheet, it is observed that the
Ti-containing carbide is mainly precipitated in polygonal ferrite.
This is probably because supersaturated C is easily precipitated as
a carbide in polygonal ferrite because of a low solid-solubility
limit of C in polygonal ferrite. The precipitates allow soft
polygonal ferrite to harden, thereby achieving a tensile strength
(TS) of 540 MPa to 780 MPa. Furthermore, Ti is readily bonded to
dissolved N and thus an element suitable for fixation of dissolved
N. From that standpoint, the Ti content is set to 0.030% or more.
However, an excessive addition of Ti only results in the formation
of coarse undissolved TiC or the like, which is a carbide of Ti but
does not contribute to strength, and is thus uneconomical, which is
not preferred. The upper limit of the Ti content is set to 0.080%
from this viewpoint.
[0030] It is preferred that the composition of the balance other
than the components described above be substantially iron and
incidental impurities.
3) The reason for the limitation of the steel microstructure of the
steel sheet will be described below.
[0031] The steel sheet has a microstructure whose volume fraction
of polygonal ferrite is 70% or more, and the amount of Ti in a
precipitate having a size of less than 20 nm is 50% or more of the
value of Ti* calculated using formula (I).
[0032] The strength of the high-strength hot-rolled steel sheet is
determined by the superposition of the amounts of strengthening
based on three strengthening mechanisms, i.e., solid-solution
strengthening, microstructural strengthening, and precipitation
strengthening, on the base strength of the steel itself. The base
strength is inherent strength of iron. The amount of solid-solution
strengthening is almost uniquely determined by a chemical
composition. Thus, these two strengthening mechanisms are
negligibly involved in the variations in strength in a coil. The
strengthening mechanism that is the most closely related to the
variations in strength is precipitation strengthening, followed by
microstructural strengthening.
[0033] The amount of strengthening by precipitation strengthening
is determined by the size and dispersion of precipitates
(specifically, precipitate spacing). The dispersion of precipitates
can be expressed by the amount and size of the precipitates. Thus,
if the size and amount of the precipitates are determined, the
amount of strengthening by precipitation strengthening will be
determined. Microstructural strengthening is determined by the type
of steel microstructure. The type of steel microstructure is
determined by a transformation-temperature range from austenite. If
a chemical composition and a steel microstructure are determined,
the amount of strengthening will be determined.
4) Experimental facts will be described below.
[0034] Steel A in which the amount of Ti added was 0.04% and steel
B in which the amount of Ti added was 0.06%, each of steel A and
steel B having a basic chemical composition of
0.08C-0.1Si-1.5Mn-0.011P-0.002S-0.017Al-0.005N, were ingoted in a
laboratory into cast strands. These cast strands were formed into
sheet bars each having a thickness of 25 mm by slabbing. Each of
the sheet bars was heated to 1230.degree. C., hot-rolled in five
passes to have a finishing temperature of 880.degree. C., and
water-cooled at a cooling rate of 25.degree. C./s 1.7 seconds after
finish rolling. At that time, the cooling stop temperature was
changed between 720.degree. C. and 520.degree. C. After water
cooling, each sheet bar was subjected to natural cooling for 10
seconds. Each sheet bar was inserted into an electric furnace
having a temperature of 500.degree. C. to 700.degree. C. and wound.
The holding time in the furnace was changed between 1 and 300
minutes. Then, in the case where the difference between the cooling
stop temperature and the furnace temperature was 30.degree. C. or
higher, after the natural cooling, water cooling was performed at a
cooling rate of 25.degree. C./s in such a manner that the sheet bar
had a temperature 30.degree. C. higher than the furnace
temperature. Hot-rolled steel sheets having different precipitation
states of Ti and different steel microstructures were manufactured
by the method described above. The hot-rolled steel strips were
subjected to pickling and then temper rolling at an elongation of
0.5%. Test pieces for a tensile test and analytical samples of
precipitates were taken.
[0035] Steel sheets in which the amount of Ti contained in
precipitates having a size of less than 20 nm was 50% or more of
the amount of Ti* expressed as formula (1) described below were
selected from the hot-rolled steel sheets manufactured as described
above. FIG. 1 shows the investigation results of the relationship
between the volume fraction of polygonal ferrite (%) and the
tensile strength TS (MPa). As shown in FIG. 1, the tensile strength
TS tends to decrease as the volume fraction of polygonal ferrite
increases. At a volume fraction of polygonal ferrite of 70% or
more, a change in TS is small, and TS is stabilized.
[0036] For example, the volume fraction of polygonal ferrite can be
determined as follows. A portion of an L section (a section
parallel to a rolling direction) of a steel sheet, the portion
excluding surface layers each having a thickness equal to 10% of
the thickness of the sheet, is etched with 5% nital. The
microstructures of the etched portion are photographed with a
scanning electron microscope (SEM) at a magnification of
1000.times.. Smooth ferrite crystal grains in which grain
boundaries have a small step height of less than 0.1 .mu.m and
corrosion marks are not left in the grains are defined as polygonal
ferrite. Polygonal ferrite is distinguished from other ferrite
phases and different transformation phases such as pearlite and
bainite. These are color-coded with image-analysis software. The
area ratio is defined as the volume fraction of polygonal
ferrite.
[0037] Similarly, steel sheets each having a volume fraction of
polygonal ferrite of 70% or more were selected from the hot-rolled
steel sheets manufactured as described above. FIG. 2 shows the
investigation results of the relationship between the proportion of
the amount of Ti contained in a precipitate having a size of less
than 20 nm with respect to Ti* expressed as formula (1) described
below and the tensile strength TS (MPa). As described above, the
precipitates each having a size of less than 20 nm and contributing
to precipitation strengthening are composed of added Ti. Thus,
whether Ti is efficiently precipitated as fine precipitates or not
can be determined by the grasp of the amount of Ti in the
precipitate having a size of less than 20 nm. As shown in FIG. 2,
TS tends to increase as the amount of Ti contained in the
precipitate having a size of less than 20 nm increases. In the case
where the amount of Ti contained in the precipitate is 50% or more
of Ti*, a change in TS is small, and TS is stabilized.
[0038] From the above result, it is conceivable that in the case
where the steel microstructures are controlled to have a volume
fraction of polygonal ferrite of 70% or more and where the amount
of Ti contained in the precipitate having a size of less than 20 nm
is controlled in the range of 50% or more of Ti* expressed as
formula (1) described below, the resulting variations in strength
are significantly small and practically satisfactory even if
inevitable variations in strength occur because the cooling
histories of the coil after winding are different for each
position,
Ti*=[Ti]-48/14.times.[N] (1)
where [Ti] and [N] represent a Ti content (percent by mass) and a N
content (percent by mass), respectively, of the steel sheet.
[0039] Thus, in the case where a steel sheet has microstructures
whose volume fraction of polygonal ferrite is 70% or more and that
the amount of Ti contained in a precipitate having a size of less
than 20 nm is 50% or more of Ti* expressed as formula (1) described
above are met, at any position of a steel sheet, even if the
cooling histories of a coil are different for each position,
substantially the same amount of strengthening is obtained at any
position of the steel sheet. Thus, the steel sheet has only small
variation in strength and excellent uniformity in strength.
5) The amount of Ti contained in a precipitate having a size of
less than 20 nm can be measured by a method described below.
[0040] After a sample is electrolyzed in an electrolytic solution
by a predetermined amount, the test piece is taken out of the
electrolytic solution and immersed in a solution having
dispersibility. Then precipitates contained in this solution are
filtered with a filter having the pore size of 20 nm. Precipitates
passing through the filter having a pore size of 20 nm together
with the filtrate each have a size of less than 20 nm. After the
filtration, the filtrate is appropriately analyzed by
inductively-coupled-plasma (ICP) emission spectroscopy, ICP mass
spectrometry, atomic absorption spectrometry, or the like to
determine the amount of Ti in the precipitates having a size of
less than 20 nm.
6) An example of a preferred method for manufacturing a
high-strength hot-rolled steel sheet will be described below.
[0041] The composition of a steel slab used in the manufacturing
method is the same as the composition of the steel sheet described
above. Furthermore, the reason for the limitation of the
composition is the same as above. The high-strength hot-rolled
steel sheet is manufactured through a hot-rolling step of
subjecting a raw material to rough hot rolling to form a hot-rolled
steel sheet, the raw material being the steel slab having a
composition within the range described above.
i) Heating Temperature: 1150.degree. C. to 1300.degree. C.
[0042] With respect to the heating temperature of a slab, the
hot-rolled steel sheet is preferably heated to 1150.degree. C. or
higher such that an undissolved Ti-containing carbide, such as TiC,
may not be present in a heating stage. This is because the presence
of the undissolved Ti-containing carbide adversely affects the
tensile strength of a hot-rolled steel sheet. Hence, absence of the
undissolved Ti-containing carbide is preferred. However, heating at
excessively high temperatures causes problems, for example, an
increase in scale loss due to an increase in oxidation weight.
Thus, the upper limit of the heating temperature of the slab is
preferably set to 1300.degree. C.
[0043] The steel slab heated under the foregoing conditions is
subjected to hot rolling in which rough rolling and finish rolling
are performed. The steel slab is formed into a sheet bar by the
rough rolling. The conditions of the rough rolling need not be
particularly specified. The rough rolling may be performed
according to a common method. It is preferred to use what is called
a "sheet-bar heater" from the viewpoint of reducing the heating
temperature of the slab and preventing problems during the hot
rolling.
[0044] Then, the sheet bar is subjected to finish rolling to form a
hot-rolled steel sheet.
ii) Finishing Temperature (FDT): 800.degree. C. to 950.degree.
C.
[0045] A high finishing temperature results in coarse grains to
reduce formability and is liable to cause scale defects. Hence, the
finishing temperature is set to 950.degree. C. or lower. A
finishing temperature of less than 800.degree. C. results in an
increase in rolling force to increase the rolling load and an
increase in rolling reduction to develop an abnormal texture in
austenite non-recrystallization, which is not preferred from the
viewpoint of achieving uniform strength. Accordingly, the finishing
temperature is set in the range of 800.degree. C. to 950.degree. C.
and preferably 840.degree. C. to 920.degree. C.
[0046] To reduce the rolling force during the hot rolling, some or
all passes of the finish rolling may be replaced with lubrication
rolling. Lubrication rolling is effective from the viewpoint of
improving uniformity in the shape of a steel sheet and uniformity
in strength. The coefficient of friction during the lubrication
rolling is preferably in the range of 0.10 to 0.25. Furthermore, a
continuous rolling process is preferred in which a preceding sheet
bar and a succeeding sheet bar are joined to each other and then
the joined sheet bars are continuously finish-rolled. The use of
the continuous rolling process is desirable from the viewpoint of
achieving the stable operation of the hot rolling.
iii) Cooling (primary cooling) at a cooling rate of 20.degree. C./s
or more within 2 seconds after finish hot rolling
[0047] When a time exceeding 2 seconds elapses between the start of
cooling and the completion of the finish rolling, a strain
accumulated during the finish rolling is relieved. Thus, even if
cooling control described below is performed, ferrite is not
effectively formed, failing to stably precipitate TiC. Furthermore,
the same phenomenon is liable to occur when the cooling rate is
less than 20.degree. C./s.
iv) Stop of cooling in a temperature range of 650.degree. C. to
750.degree. C. and natural cooling step for 2 seconds to 15
seconds
[0048] With respect to a temperature during natural cooling, to
effectively precipitate the Ti-containing carbide such as TiC in a
short time required for the passage of a steel sheet through a
run-out table, it is necessary to hold the steel sheet for a
predetermined period of time in a temperature range where ferrite
transformation is maximized. At a natural cooling (holding)
temperature of less than 650.degree. C., it is impossible to ensure
the amount of the Ti-containing carbide required for the desired
amount of strengthening because of a low growth rate of
precipitates of the Ti-containing carbide. At a natural cooling
temperature exceeding 750.degree. C., coarse Ti-containing carbide
grains are sparsely distributed because of the insufficient
nucleation of precipitates and a high growth rate, thereby reducing
the strengthening ability. Accordingly, the natural cooling
temperature is set in the range of 650.degree. C. to 750.degree.
C.
[0049] A natural cooling time of less than 2 seconds results in an
insufficient amount of the Ti-containing carbide precipitating. It
is thus difficult to ensure the amount of strengthening required. A
natural cooling time exceeding 15 seconds causes a reduction in
strengthening ability because coarse Ti-containing carbide grains
are sparsely distributed. Therefore, the natural cooling time is
set in the range of 2 seconds to 15 seconds.
v) Cooling (secondary cooling) at a cooling rate of 100.degree.
C./s
[0050] In the case where the cooling rate subsequent to the natural
cooling treatment is 100.degree. C. or more, the controllability of
a coiling temperature is reduced, causing difficulty in achieving
stable strength. Thus, the cooling rate is set to less than
100.degree. C./s. The lower limit of the cooling rate is not
particularly limited but is 5.degree. C./s or more from the
viewpoint of inhibiting the coarsening of precipitates.
vi) Winding the steel sheet into a coil in a temperature range of
550.degree. C. to 650.degree. C.
[0051] In the case where the coiling temperature is less than
550.degree. C., a portion that is not transformed on the run-out
table is formed as a low-temperature transformed phase to cause
variations in strength and a reduction in ductility. In the case
where the coiling temperature exceeds 650.degree. C., the growth of
the Ti-containing carbide such as TiC proceeds after the completion
of the winding so that coarse Ti-containing carbide grains are
sparsely distributed, thereby reducing the strengthening ability.
This is also liable to cause variations in strength corresponding
to cooling histories after winding. Thus, the coiling temperature
is set in the range of 550.degree. C. to 650.degree. C.
[0052] In the case where the variations in strength are taken into
consideration in the coil, the precipitation of the Ti-containing
carbide such as TiC proceeds mainly in a cooling stage after
completion of the winding. Hence, it is desirable to take into
consideration of the cooling histories of the steel sheet after the
winding. In particular, the front end and the rear end of the coil
are rapidly cooled so that precipitation of the Ti-containing
carbide does not sufficiently proceed in some cases. Thus, the
temperatures of the front and rear ends of the coil are increased
with respect to the temperature of the inner portion of the coil
other than the front and rear ends, thereby further improving the
variations in strength.
Example 1
[0053] An example will be described below.
[0054] Molten steels having compositions shown in Table 1 were made
with a converter and formed into slabs by a continuous casting
process. These steel slabs were heated to 1250.degree. C. and
rough-rolled into sheet bars. Then the resulting sheet bars were
subjected to a hot-rolling step in which finish rolling was
performed under conditions shown in Table 2, thereby forming
hot-rolled steel sheets.
[0055] These hot-rolled steel sheets were subjected to pickling and
temper rolling at an elongation of 0.5%. Regions extending from
both sides to 10 mm from both sides in the width direction were
removed by trimming. Various properties were evaluated. Steel
sheets were taken at positions at which the innermost turn
including the front end and the outermost turn including the rear
end of the coil in the longitudinal direction were cut.
Furthermore, steel sheets were taken at 20 equally divided points
of the inner portion in the longitudinal direction. Test pieces for
a tensile test and analytical samples of precipitates were taken
from both sides of each of the steel sheets in the width direction
and 8 equally divided points of each steel sheet in the width
direction.
[0056] The test pieces for a tensile test were taken in a direction
(L direction) parallel to a rolling direction and processed into
JIS No. 5 test pieces. The tensile test was performed according to
the regulation of JIS Z 2241 at a crosshead speed of 10 mm/min to
determine tensile strength (TS). Table 2 shows the investigation
results of tensile properties of the resulting hot-rolled steel
sheets.
[0057] With respect to microstructures, a portion of an L section
(a section parallel to a rolling direction) of each of the steel
sheets, the portion excluding surface layers each having a
thickness equal to 10% of the thickness of the sheet, was etched
with nital. The microstructures of the etched portion were
identified with a scanning electron microscope (SEM) at a
magnification of 1000.times.. The volume fraction of polygonal
ferrite was measured by the method described above with image
processing software.
[0058] The quantification of Ti in a precipitate having a size of
less than 20 nm was performed by a quantitative procedure described
below.
[0059] The resulting hot-rolled steel sheets described above were
cut into test pieces each having an appropriate size. Each of the
test pieces was subjected to constant-current electrolysis in a 10%
AA-containing electrolytic solution (10 vol % acetylacetone-1 mass
% tetramethylammonium chloride-methanol) at a current density of 20
mA/cm.sup.2 so as to be reduced in weight by about 0.2 g.
[0060] After electrolysis, each of the test pieces having surfaces
to which precipitates adhered was taken from the electrolytic
solution and immersed in an aqueous solution of sodium
hexametaphosphate (500 mg/l) (hereinafter, referred to as an "SHMP
aqueous solution"). Ultrasonic vibration was applied thereto to
separate the precipitates from the test piece. The separated
precipitates were collected in the SHMP aqueous solution. The SHMP
aqueous solution containing the precipitates was filtered with a
filter having a pore size of 20 nm. After the filtration, the
resulting filtrate was analyzed with an ICP emission spectrometer
to measure the absolute quantity of Ti in the filtrate. Then the
absolute quantity of Ti was divided by an electrolysis weight to
obtain the amount of Ti (percent by mass) contained in the
precipitates each having a size of less than 20 nm. Note that the
electrolysis weight was determined by measuring the weight of the
test piece after the separation of the precipitates and subtracting
the resulting weight from the weight of the test piece before
electrolysis. Next, the resulting amount of Ti (percent by mass)
contained in the precipitates each having a size of less than 20 nm
was divided by Ti* calculated by substituting the Ti content and
the N content shown in Table 1 in formula (1), thereby determining
the proportion (%) of the amount of Ti contained in the
precipitates each having a size of less than 20 nm.
TABLE-US-00001 TABLE 1 Chemical component (% by mass) Symbol C Si
Mn P S Al N Ti Ti* Remarks A 0.071 0.01 1.35 0.008 0.005 0.034
0.0035 0.035 0.023 Adaptation example B 0.075 0.01 1.30 0.008 0.003
0.032 0.0032 0.045 0.034 Adaptation example C 0.082 0.01 1.25 0.008
0.004 0.040 0.0030 0.058 0.048 Adaptation example D 0.090 0.01 1.35
0.010 0.005 0.034 0.0015 0.05 0.045 Adaptation example E 0.085 0.01
1.40 0.008 0.005 0.034 0.0020 0.032 0.025 Adaptation example F
0.078 0.01 1.65 0.008 0.003 0.035 0.0015 0.042 0.037 Adaptation
example G 0.079 0.25 1.35 0.008 0.005 0.035 0.0030 0.034 0.024
Adaptation example H 0.081 0.01 0.50 0.008 0.003 0.036 0.0032 0.042
0.031 Comparative example I 0.040 0.01 1.35 0.009 0.002 0.034
0.0032 0.045 0.034 Comparative example J 0.095 0.01 1.35 0.008
0.005 0.034 0.0032 0.025 0.014 Comparative example K 0.082 0.01
1.35 0.008 0.005 0.036 0.0033 0.1 0.089 Comparative example
TABLE-US-00002 TABLE 2 Heating Finishing Cooling Primary Natural
cooling Natural Secondary Steel Steel Thickness temperature
temperature start time cooling rate temperature cooling time
cooling rate sheet No. No. mm .degree. C. (FDT) .degree. C. s
.degree. C./s .degree. C. s .degree. C./s 1 A 6.0 1220 880 1.7 25
700 10 25 2 2.6 1220 880 0.8 55 700 7 55 3 6.0 1100 880 1.7 25 700
10 25 4 6.0 1220 1000 1.7 25 700 10 25 5 6.0 1220 880 3.4 25 700 10
25 6 6.0 1210 880 1.7 15 700 10 15 7 6.0 1210 880 1.7 25 760 10 25
8 6.0 1210 880 1.7 25 700 1 25 9 6.0 1220 880 1.7 25 700 10 25 10
6.0 1220 880 1.7 25 700 10 25 11 B 4.5 1220 880 1.4 35 700 10 35 12
1.6 1220 880 0.6 100 700 5 80 13 4.5 1220 880 1.4 35 640 10 35 14
4.5 1220 880 1.4 35 700 30 35 15 C 3.2 1230 880 0.9 50 700 6 50 16
D 6.0 1220 880 1.7 25 700 10 25 17 E 6.0 1210 880 1.7 25 700 10 25
18 F 6.0 1230 870 1.7 25 700 10 25 19 G 4.5 1230 880 1.4 35 700 10
35 20 H 6.0 1230 890 1.7 25 700 10 25 21 I 6.0 1230 890 1.7 25 700
10 25 22 J 6.0 1230 870 1.7 25 700 10 25 23 K 6.0 1220 870 1.7 25
700 10 25 Amount of Ti Proportion of Coiling present in amount of
Ti temperature Volume precipitate contained in Proportion of (CT)
after fraction of with size of precipitate with compliant steel
Proportion Steel finish hot polygonal less than 20 nm size of less
than TS microstructure of compliant .DELTA.TS sheet No. rolling
.degree. C. ferrite % % by mass 20 nm % MPa % TS % MPa Remarks 1
600 91 0.019 82 623 100 100 44 Inventive example 2 600 92 0.018 79
603 100 100 33 Inventive example 3 600 87 0.007 32 582 56 92 64
Comparative example 4 600 63 0.012 53 604 6 100 68 Comparative
example 5 600 97 0.009 38 597 5 97 51 Comparative example 6 600 86
0.010 45 605 7 95 61 Comparative example 7 600 95 0.009 39 602 5 85
73 Comparative example 8 600 53 0.011 49 619 0 89 53 Comparative
example 9 600 90 0.007 31 579 3 57 55 Comparative example 10 600 42
0.009 40 624 0 72 51 Comparative example 11 600 84 0.025 73 632 100
100 40 Inventive example 12 600 84 0.026 75 621 100 100 39
Inventive example 13 600 56 0.015 44 653 0 93 75 Comparative
example 14 600 85 0.010 30 602 3 83 68 Comparative example 15 600
95 0.036 76 665 100 100 33 Inventive example 16 600 97 0.030 67 658
100 100 49 Inventive example 17 600 86 0.016 65 600 100 100 36
Inventive example 18 600 97 0.027 72 617 100 100 43 Inventive
example 19 600 83 0.017 72 645 100 100 40 Inventive example 20 600
90 0.013 43 526 4 0 46 Comparative example 21 600 94 0.013 37 504 3
0 62 Comparative example 22 600 91 0.0005 37 536 5 0 33 Comparative
example 23 600 59 0.059 67 792 6 100 64 Comparative example
[0061] In the results shown in Table 2, values of the proportion of
the volume fraction of polygonal ferrite, the amount of Ti
contained in precipitates each having a size of less than 20 nm
with respect to Ti* expressed as formula (1), and the tensile
strength TS are defined as representative values at a middle
portion in the longitudinal and transverse directions. The
proportion of compliant steel microstructures is defined as the
proportion of points where both requirements of the volume fraction
of polygonal ferrite and the proportion of the amount of Ti in the
precipitates each having a size of less than 20 nm is satisfied to
189 measurement points. The proportion of compliant TS is defined
as the proportion of points where TS is 540 MPa or more to 189
measurement points. .DELTA.TS is a value obtained by determining
the standard deviation .sigma. of TS values at 189 measurement
points and multiplying the standard deviation .sigma. by 4.
[0062] As is clear from the investigation results shown in Table 2,
in our example, a steel sheet having satisfactory uniformity in
strength is manufactured, in which the steel sheet has a TS of 540
MPa or more, which is high strength, and the variations in strength
(.DELTA.TS) in the coil in the in-plane direction are 50 MPa or
less.
INDUSTRIAL APPLICABILITY
[0063] It is possible to stably manufacture a hot-rolled steel
sheet having a tensile strength (TS) of 540 MPa or more and only
small variations in strength at low cost, which provides a marked,
industrially beneficial effect. For example, the use of a
high-strength hot-rolled steel sheet for automotive parts reduces
variations in the amount of springback after formation using the
high-tensile steel sheet and variations in collision
characteristics, thus making it possible to design automobile
bodies with higher accuracy and to contribute sufficiently to the
collision safety and weight reduction of automobile bodies.
* * * * *