U.S. patent application number 10/557416 was filed with the patent office on 2006-08-31 for warm rolling method.
Invention is credited to Tadanobu Inoue, Eijiro Muramatsu, Kotobu Nagai, Shiro Torizuka.
Application Number | 20060191613 10/557416 |
Document ID | / |
Family ID | 33475458 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060191613 |
Kind Code |
A1 |
Torizuka; Shiro ; et
al. |
August 31, 2006 |
Warm rolling method
Abstract
A multi-directional warm-rolling method for manufacturing an
ultrafine grain steel material with an ultrafine grain structure of
3 .mu.m or smaller in average grain size. When roiling of two
passes or more is performed for a steel material in the rolling
temperature range of 350 to 800.degree. C., a rolling by an oval
shape caliber and a rolling by the other shape caliber an performed
at leas one time so that a large amount of strain can be introduced
into the material by a simple means with less section reduction
ratio and less number of passes. Steel materials having the
ultrafine grain structure and excellent strength and ductility can
be manufactured by this method.
Inventors: |
Torizuka; Shiro; (Ibaraki,
JP) ; Muramatsu; Eijiro; (Tsukuba-shi, JP) ;
Inoue; Tadanobu; (Tsukuba-shi, JP) ; Nagai;
Kotobu; (Tsukuba-shi, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
33475458 |
Appl. No.: |
10/557416 |
Filed: |
May 20, 2004 |
PCT Filed: |
May 20, 2004 |
PCT NO: |
PCT/JP04/07277 |
371 Date: |
March 20, 2006 |
Current U.S.
Class: |
148/648 |
Current CPC
Class: |
C21D 8/0231 20130101;
B21B 1/18 20130101; C21D 8/06 20130101; C21D 8/005 20130101 |
Class at
Publication: |
148/648 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 20, 2003 |
JP |
2003-180290 |
Claims
1. A warm rolling method for manufacturing an ultrafine grain steel
material having an ultrafine grain structure of average grain size
of 3 .mu.m or less, which comprises rolling by a caliber of oval
shape and rolling by a caliber of other shape at least once or more
each when rolling two passes or more to a steel material in a
rolling temperature range of 350 to 800.degree. C.
2. The warm rolling method of claim 1, wherein rolling by a caliber
of oval shape is followed by rolling by a caliber of other
shape.
3. The warm rolling method of claim 1, wherein the caliber of other
shape is square or round shape.
4. The warm rolling method of claim 1, wherein the rolling by oval
shape caliber is executed by 2 times or more to maximum N/2 times
or less in the case of N>2, wherein N is the total number of
passes.
5. The warm rolling method of claim 1, wherein continuous two
passes rolling are executed.
6. The warm rolling method of claim 5, wherein the section
reduction ratio after square shape caliber rolling from material is
20% or more in two passes rolling by calibers of oval shape and
square shape.
7. The warm rolling method of claim 1, wherein the section
reduction ratio is, in combination rolling of two passes rolling by
calibers of oval shape and square shape, 40% or more in combination
rolling by two times, and the section reduction ratio is 60% or
more in combination rolling by three times.
8. The warm rolling method of claim 1, further including a rolling
step in which the maximum shorter axis length of material after
rolling by oval shape caliber is 75% or less of the material
diagonal length before oval rolling.
9. The warm rolling method of claim 1, wherein a plastic strain of
1.5 or more is introduced at least in a region of 50 vol. % inside
of the material.
10. The warm rolling method of claim 9, wherein a plastic strain of
2 or more is introduced in a region of 90 vol. % or more inside of
the material.
11. The warm rolling method of claim 1, wherein the rolling
condition parameter Z expressed in the following formula (1) is 11
or more in the case that crystal structure of Fe just before
rolling is bcc such as the structure of ferrite, bainite,
martensite, pearlite or other, or 20 or more in the case that
structure just before rolling is austenite and Fe crystal structure
is fcc; Z = log .function. [ t .times. exp .function. ( Q 8.31
.times. ( T + 273 ) ) ] ( 1 ) ##EQU3## .epsilon.: strain t: time
from start of rolling till end (s) T: rolling temperature (.degree.
C., average of rolling temperature of each pass in the case of
multi-pass rolling) Q: 254,000 if structure just before rolling is
a primary phase of ferrite, bainite, martensite or pearlite; or
300,00 if mother phase is austenite.
12. The warm rolling method of claim 1, wherein the section
reduction ratio of initial material to final rolling is 90% or
less.
13. The warm rolling method of claim 1, wherein an ultrafine grain
steel having an average grain size of C section or L section of 3
microns or less is manufactured.
14. The warm rolling method of claim 1, wherein an ultrafine grain
steel having an average grain size of C section or L section of 1
micron or less is manufactured.
15. The warm rolling method of claim 2, wherein the caliber of
other shape is square or round shape.
16. The warm rolling method of claim 2, wherein the rolling by oval
shape caliber is executed by 2 times or more to maximum N/2 times
or less in the case of N>2, wherein N is the total number of
passes.
17. The warm rolling method of claim 3, wherein the rolling by oval
shape caliber is executed by 2 times or more to maximum N/2 times
or less in the case of N>2, wherein N is the total number of
passes.
18. The warm rolling method of claim 15, wherein the rolling by
oval shape caliber is executed by 2 times or more to maximum N/2
times or less in the case of N>2, wherein N is the total number
of passes.
19. The warm rolling method of claim 2, wherein continuous two
passes rolling are executed.
20. The warm rolling method of claim 3, wherein continuous two
passes rolling are executed.
Description
TECHNICAL FIELD
[0001] The present invention relates to a new warm rolling method
for manufacturing an ultrafine grain steel material with an
ultrafine grain structure of 3 .mu.m or smaller in grain size and
excellent in strength and ductility.
BACKGROUND ART
[0002] An ultrafine grain steel material can be extremely enhanced
in strength without adding alloying elements, and is considered to
be decreased extremely in the ductile to brittle transition
temperature at the same time, and hence the present inventors have
been promoting researches in order to realize the ultrafine grain
steel industrially, and have so far invented a method of warm
multi-pass rolling (document 1) and a method of multi-directional
working (document 2).
[0003] If warm multidirectional rolling can be realized easily, it
may lead to wider use of ultrafine grain steel, but it was found
not easy in the process of studies by the inventors.
[0004] As one of the technical difficulties, not less than a
certain amount of strain must be introduced into the material. For
example, the critical strain is 1.5 to 2.3, preferably about 3, and
a strain of 3 corresponds to a section reduction ratio of 95%, and
a large deformation processing is needed. To obtain a round bar of
10 mm in diameter as final product, warm rolling process must be
started from diameter of 45 mm, and to introduce such large strain
in warm rolling temperature region of high deformation resistance,
a large material is needed. As a result, the number of rolling
passes increases.
[0005] If a larger strain can be Introduced into the material by a
smaller section reduction ratio and a smaller number of passes, an
ultrafine grain structure can be obtained more easily, lots of
industrial benefits are expected such as enhancement of rolling
efficiency.
[0006] The inventors have proposed various methods about
multi-directional rolling, such as a method of compressing from
multiple directions by using anvil (document 2) and two-directional
screw-down rolling technology. Although the multi directional
working is a technology for introducing a large amount of strain
efficiently, but processing from at least two directions involved
very difficult technical problems.
[0007] Document 1: Japanese Patent Application Laid-Open No.
2001-309850
[0008] Document 2: Japanese Patent Application Laid Open No.
2001-240912
[0009] The present invention is devised in the light of the above
background, and is intended to present a new multi-directional warm
rolling method capable of introducing a large strain into the
material by a smaller section reduction ratio and a smaller number
of passes, by easier means, by further advancing from the findings
obtained so far by the inventors, and a manufacturing method of
steel material having ultrafine grain structure, and excellent in
strength and ductility.
DISCLOSURE OF INVENTION
[0010] To solve the problems, it is a first aspect of the invention
to present a warm rolling method for manufacturing an ultrafine
grain steel material having an ultrafine grain structure of average
grain size of 3 .mu.m or less, more particularly a warm rolling
method characterized by rolling by a caliber of oval shape and
rolling by a caliber of other shape at least once or more
respectively, when rolling two passes or more in a rolling
temperature range of steel material of 350 to 800.degree. C., and
it is a second aspect to present a warm rolling method, in which
rolling by a caliber of oval shape is followed by rolling by a
caliber of other shape.
[0011] It is a third aspect to present the above-mentioned warm
rolling method in which the caliber of other shape is square or
round shape.
[0012] It is a fourth aspect to present a warm rolling method in
which the rolling by oval shape caliber is executed by 2 times or
more to maximum N/2 times or less in the case of N>2, wherein N
is the total number of passes, it is a fifth aspect to present a
warm roiling method in which continuous two passes rolling are
executed, it is a sixth aspect to present a warm rolling method in
which the section reduction ratio after square shape caliber
rolling from material is 20% or more in two passes rolling by
calibers of oval shape and square shape, it is a seventh aspect to
present a warm roiling method in which the section reduction ratio
is 40% or more in combination rolling by two times, and the section
reduction ratio is 60% or more in combination rolling by three
times, in combination rolling of two passes rolling by calibers of
oval shape and square shape.
[0013] It is an eighth aspect of the invention to present any of
the above-mentioned warm rolling methods including a rolling step
in which the maximum shorter axis length of material after rolling
by oval shape caliber is 75% or less of the material diagonal
length before oval rolling, it is a ninth aspect to present a warm
rolling method in which a plastic strain of 1.5 or more is
introduced at least in a region of 50 vol. % inside of the
material, it is a tenth aspect to present a warm rolling method in
which a plastic strain of 2 or more is introduced in a region of 90
vol. % or more inside of the material, and it is an eleventh aspect
to present a warm rolling method in which the rolling condition
parameter Z expressed in the following formula (1) is 11 or more
(the structure just before rolling is ferrite, bainite, martensite,
pearlite or other Fe crystal structure of bcc) or 20 or more
(structure just before rolling is austenite and Fe crystal
structure of fcc). Z = log .function. [ t .times. exp .function. (
Q 8.31 .times. ( T + 273 ) ) ] ( 1 ) ##EQU1## [0014] .epsilon.:
strain [0015] .tau.: time from start of rolling till end (s) [0016]
T: rolling temperature (.degree. C., average of rolling temperature
of each pass in the case of multi-pass rolling) [0017] Q: 254,000
if structure just before rolling is a primary phase of ferrite,
bainite, martensite or pearlite; or 300,00 if mother phase is
austenite.
[0018] It is a twelfth aspect to present a warm rolling method in
which the section reduction ratio of initial material to after
final rolling is 90% or less it is a thirteenth aspect to present a
warm rolling method for manufacturing an ultrafine grain steel
having an average grain size of C section or L section of 3 microns
or less, and it is a fourteenth aspect to present a warm rolling
method for manufacturing an ultrafine grain steel having an average
grain size of C section or L section of 1 micron or less.
[0019] The invention having such features is devised on the basis
of new findings obtained by the investigations by the inventors.
That is, hitherto, it is known that caliber rolling which is
rolling by using a roil having a hole groove is common as the steel
bar manufacturing method, and shapes of caliber are roughly
classified into square shape (square shape, diamond shape), oval
shape and round shape. By performing caliber (groove roll) rolling
in warm rolling temperature region, a structure mainly composed of
ultrafine grain ferrite is obtained by multi-pass rolling (document
1). By using the oval shape caliber, it is found to be effective
for equiaxtalization of ferrite grain shape of L section of steel
bar (section parallel to longitudinal direction of bar).
[0020] As a result of intensive studies by the inventors, it has
been found that a large strain can be introduced into the material
even at a relatively small section reduction ratio, by performing
caliber rolling combining oval shape caliber and square, round or
other shape caliber, in an appropriate temperature region, so that
the technology is established
BRIEF DESCRIPTION OF DRAWINGS
[0021] FIG. 1 is a diagram of caliber in embodiment 1.
[0022] FIG. 2 is a photograph of C section of seed bar after
rolling.
[0023] FIG. 3 is a material mesh diagram.
[0024] FIG. 4 is a diagram showing plastic strain after 1 pass of
oval shape caliber rolling.
[0025] FIG. 5 is a diagram showing plastic strain after 2 passes of
square shape caliber rolling.
[0026] FIG. 6 is a diagram showing plastic strain after 3 passes of
oval shape caliber rolling.
[0027] FIG. 7 is a diagram showing plastic strain after 4 passes of
square shape caliber rolling.
[0028] FIG. 8 is a diagram showing plastic strain after 5 passes of
oval shape caliber rolling.
[0029] FIG. 9 is a diagram showing plastic strain after 6 passes of
round shape caliber rolling.
[0030] FIG. 10 is a SEM image of structure after 2 passes of square
shape caliber rolling.
[0031] FIG. 11 is a SEM image of structure after 4 passes of square
shape caliber rolling.
[0032] FIG. 12 is a SEM image of structure in embodiments 2 to
4.
[0033] FIG. 13 is a diagram of caliber.
[0034] FIG. 14 is a photograph of C section of steel bar after
rolling.
[0035] FIG. 15 is a SEM image of structure.
[0036] FIG. 16 is a SEM image of structure of comparative example
1.
[0037] FIG. 17 is a diagram showing the relation of parameter Z and
average grain size.
BEST MODE FOR CARRYING OUT THE INVENTION
[0038] The invention has the features as mentioned above, and the
individual embodiments are described specifically below.
[0039] The warm rolling method of the invention is capable of
manufacturing an ultrafine grain steel material having an ultrafine
grain structure of average gram size of 3 .mu.m or less, as
mentioned above, by combining rolling by a caliber of oval shape
and roiling by a caliber of other shape. In this case, the caliber
rolls used in rolling are oval shape caliber and other shape
caliber.
[0040] As for the caliber roll of oval shape caliber, the hole
shape formed by upper die and lower die is not circular (round),
hut the shape that circular (round) is flattened. The caliber of
other shape combined with the oval shape caliber includes square,
rhombus (diamond), round or similar shapes thereof.
[0041] In the invention, as the warm rolling method for
manufacturing an ultrafine grain steel material having an ultrafine
grain structure of average grain size of 3 .mu.m or less, both of
rolling by oval shape caliber and rolling by other shape caliber
are executed at least once or more in rolling of two passes or more
to steel material in rolling temperature range from 350 to
800.degree. C.
[0042] Actually, a preferable embodiment is that rolling by a
caliber of oval shape is followed by rolling by a caliber of other
shape, and that the rolling by oval shape caliber is executed by 2
times or more to maximum N/2 times or less in the case of N>2,
wherein N is the total number of passes.
[0043] For example, when combining oval shape caliber and square
caliber, the rolling condition is considered so as to include, in
all passes of rolling, two times or more of combination rolling
using oval shape caliber and square shape caliber (oval-square), to
include rolling by square shape caliber in the middle of
combination rolling of oval-square such as
oval-square-square-oval-square, or to roll of 4 passes of
oval-square-oval-square, or to roll of 6 passes of
oval-square-oval-square-oval-square. In this case, too, the square
shape caliber may be replaced by a caliber of round, rhombus or
other shape.
[0044] In the rolling method of the invention, a microscopic local
orientation difference caused by introduction of large strain by
warm rolling originates ultrafine grain, and in the recovery
process taking place during or after working, the dislocation
density in grain decreases and grain boundary is formed at the same
time, and thereby an ultrafine grain structure is formed. However,
the recovery is not sufficient if the temperature is low, then
deformation texture with high dislocation density is remained. On
the other hand, if the temperature is too high, the grain becomes
coarse due to discontinuous recrystallization or ordinary grain
growth, and ultrafine grain structure of 3 .mu.m or less is not
formed. Hence, the rolling temperature is limited within 350 to
800.degree. C.
[0045] In the present invention, ultrafine grains are generated
from the deformed grains flattened by warm rolling, and increase
along with increase of strain. And, a strain of at least 1.5 is
required in order to obtain a structure almost composed of
ultrafine grains.
[0046] More specifically, by introducing a plastic strain of 1.5 or
more, preferably 2 or more, in the region of at least 50 vol. % of
the material inside, ultrafine grains can be formed in this region.
Preferably, by introducing a plastic strain of 2 or more in the
region of 90% or more of material inside, an ultrafine grain region
can be formed in this region.
[0047] The greater the strain to be introduced is, the greater the
orientation differential angle among ultrafine grains is. That is,
large angular grain boundaries increase. If a strain of 3 can be
introduced, the rate of large angular grain boundaries is
sufficient in the grain boundary of ultrafine grass. Therefore, us
fur us the region of strain of 3 or more is 50% or more, or
preferably 80% or more of the entire section, a steel bar of
excellent dynamic properties is obtained.
[0048] In addition to processing in principal screw-down direction,
when combined with screw-down from other direction forming an angle
of about 90.degree., by applying processing strain from at least
two directions, the orientation of ultrafine grains is dispersed,
and the rate of large angular grain boundaries can be
increased.
[0049] According to the studies accumulated so far by the
inventors, it has been disclosed that the average grain size of
ultrafine grains formed by warm rolling depends on the processing
temperature and strain speed. The grain size becomes smaller along
with increase of rolling condition parameter Z of formula (1) as
the function of rolling temperature and strain speed. To obtain a
structure of average grain size or 1 .mu.m or less, the rolling
condition parameter Z must be set higher than a specific critical
value. As a result of experiment by one-pass large strain
compressive working using small samples, the critical value is
found to be about 11 in the case of bcc structure iron (ferrite,
bainite, martensite, pearlite, etc.), and about 20 in fcc structure
(austenite) (FIG. 17).
[0050] In formula (1), strain (.epsilon.) may be a true strain that
is industrially easy-to-se strain. For example, supposing the
initial area of steel bar to be So, and the area of C section after
rolling to be S, the section reduction ratio R is R=(So-S)/So (2)
Hence, the true strain .epsilon. is expressed as follows;
.epsilon.=-Ln(1-R)
[0051] Instead of true strain, the value calculated by finite
element method may be used (for example, Keizaburo Ham, et al.
"Introduction to finite element method," Kyoritsu Publishing, Mar.
15, 1990). More specifically, the plastic strain is calculated
according to the flow shown in Table 1 below. TABLE-US-00001 TABLE
1 Calculation flow of plastic strain 1. Obtain stress-strain curve
corresponding to the processing temperature of material. 2. Prepare
for finite element calculation method. (1) Create mesh in
workpiece. (2) Determine contact condition: coefficient of friction
= 0.3 coulomb condition. (3) Determine stress-strain curve,
material property values. 3. On the basis of condidons of (1) to
(3), calculate by universal finite element method, for example,
ABAQUS. The plastic strain a is calculated in the formula below,
and each strain increment is calculated by the universal finite
element method code. = 2 3 .times. [ 1 2 .times. { ( d .times.
.times. x - d .times. .times. y ) 2 + ( d .times. .times. y - d
.times. .times. z ) 2 + ( d .times. .times. z - d .times. .times. x
) 2 } + 3 4 .times. ( d .times. .times. .gamma. xy 2 + d .times.
.times. .gamma. yz 2 + d .times. .times. .gamma. zx 2 ) ] ##EQU2##
d.epsilon..sub.xd.epsilon..sub.yd.epsilon..sub.x: strain increment
of x, y, z d.gamma..sub.xyd.gamma..sub.yzd.gamma..sub.zx: shearing
stress increment
[0052] In the warm rolling method of the invention, hence, it is
preferred to set the rolling condition so that parameter Z may be
11 or more (bcc structure) or 20 or more (fcc structure).
[0053] Preferred embodiments of the invention include rolling
processes wherein section reduction ratio is 20% or more in two
passes rolling of oval shape caliber rolling and square shape
caliber rolling to material, section reduction ratio is 40% or more
in rolling of combined two times of two passes rolling by oval
shape caliber and square shape caliber, section reduction ratio is
60% or more in rolling of combined three times of two passes
rolling, and the maximum shorter axis length of material after
rolling by oval shape caliber is 70% or less of the diagonal length
of material before oval rolling.
[0054] Further, concerning the composition of the steel material to
which the warm rolling method of the invention can be applied, the
composition of steel is not limited at all because mechanism for
heightening the strength by phase transformation is not utilized at
all and addition of alloying element is not needed for enhancing
the strength, and therefore steel materials of wide composition
range can be used such as steel types free from phase
transformation, for example, ferrite single phase steel or
austenite single phase steel. More specifically, the following
composition, by wt. %, is preferred.
[0055] C: 0.001% or more to 1.2% or less,
[0056] Si: 0.1% or more to 2% or less,
[0057] Mn: 0.1% or more to 3% or less,
[0058] P: 0.2% or less,
[0059] S: 0.2% or less,
[0060] Al: 1.0% or less,
[0061] N: 0.02% or less,
[0062] Cr, Mo, Cu, and Ni in total: 30% or less,
[0063] Nb, Ti, V in total: 0.5% or less,
[0064] B: 0.01 or less, and
[0065] balance of Fe and inevitable impurities. Such composition
free from alloying elements may be presented as an example. The
alloy elements such as Cr, Mo, Cu, Ni, Nb, T, V, B, etc. may be
added more than the specified range as required, or may not be
added at all.
[0066] Presenting embodiments, the invention is more specifically
described below. But it must be noted that the invention is not
limited to the embodiments alone.
EMBODIMENTS
[0067] Table 2 shows chemical composition of sample steels used in
embodiments (the balance is Fe). TABLE-US-00002 TABLE 2 Chemical
composition of sample steels (mass %) C Si Mn P S Al a 0.15 0.3 1.5
0.01 0.001 0.03 b 0.11 0.3 0.5 0.02 0.005 0.03
Embodiment 1
[0068] A steel bar of 24 mm square having a ferrite+pearlite
structure of average ferrite grain size of 5 microns of the
composition shown in Table 2a was roiled in 6-pass caliber using
the caliber shown in FIG. 1 at rolling temperature of 520 to
450.degree. C. In FIG. 1, the outline of caliber dimension (mm) is
as shown in Table 3. TABLE-US-00003 TABLE 3 Radius of Longer axis
Shorter axis curvature 1st pass, oval 54 12 64 3rd pass, oval 41 9
49 5th pass, oval 19 10 12 6th pass, round Diameter 12
[0069] FIG. 2 shows the sectional shape changes and section
reduction ratio of each pass of rolling. The section reduction
ratio of rolling the 24.times.24 mm square bar by oval shape
caliber in the first pass is 37%, the section reduction ratio of
roiling material by square shape caliber in the second pass is 21%,
the section reduction ratio of rolling material by oval shape
caliber in the third pass is 15%, the section reduction ratio of
rolling material by square shape caliber in the fourth pass is 24%,
the section reduction ratio of rolling material by oval shape
caliber in the fifth pass is 13%, and the section reduction ratio
of rolling material by round shape caliber in the sixth pass is
12%. The section reduction ratio from the material to the square
bar of 17 mm in the second pass is 44%, the section reduction ratio
from the material to the square bar of 13 mm in the fourth pass is
71%, and the section reduction ratio from the material to the round
bar of 12.5 mm in the sixth pass is 80%.
[0070] FIG. 3 to FIG. 9 show distribution of plastic strain in the
material inside calculated by the finite element method. FIG. 5
suggests there is a region exceeding the plastic strain of 1.5 in
the material already in the second pass of oval-square shape
caliber. Its area rate is 75%. As shown in FIG. 6, after rolling of
three passes of oval-square-oval, a region over plastic strain of 2
occupies 92% of all area, and in FIG. 7, after rolling of four
passes of oval-square-oval-square, a region over plastic strain of
3 occupies 95%, and after oval-round rolling in FIG. 9, the plastic
strain is 3 or more in 100% region.
[0071] The section reduction ratio after two passes is about 44%
(when section reduction ratio R is converted merely into true
strain e, from e=-Ln(1-R/100), e=0.67), after four passes, 71%
(section reduction R merely converted into true strain of 1.23),
and after six passes, 80% (section reduction R merely converted
into true strain of 1.61), but it is found that a very large
plastic strain is formed inside the material. This is because the
oval shape caliber and square shape caliber are combined in
rolling, and the strain is far larger than the strain calculated
from a mere section reduction area.
[0072] FIG. 10 and FIG. 11 show SEM images of the structure. In
positions {circle around (1)} and {circle around (2)} in FIG. 10
corresponding to FIG. 5, ultrafine ferrite grains of 1 micron or
less are produced, while ultrafine grains are not found at position
of {circle around (3)}. In the microstructure of FIG. 11
corresponding to FIG. 7, almost entire region is covered with
ultrafine grain structure of ultrafine ferrite grains of 1 micron
or less.
[0073] Table 4 shows dynamic properties of material of 13 mm square
after four passes. Properties of 24 square bar before rolling are
shown for reference. Without causing brittle breakdown at double
yield strength and liquid nitrogen temperature, an absorption
energy of J was recognized. TABLE-US-00004 TABLE 4 Ductile-brittle
Central Ferrite Yield Tensile transition Absorption Vickers grain
strength strength temperature energy hardness size (.mu.m) (MPa)
(MPa) (.degree. C.) (J) -120.degree. C. (-) Embodiment 1 0.5 840
850 <-196 118 290 Embodiment 4 0.6 800 810 <-196 80 270-310
Comparative 5 460 580 -40 0 example 2
Embodiment 2 to 4
[0074] A steel bar of 24 nun square having a ferrite +pearlite
structure of average ferrite grain size of 5 microns of the
composition shown in Table 1a was rolled in 2-pass caliber using
the caliber shown in FIG. 1 (1), (2) at rolling temperature of
400.degree. C., 600.degree. C., and 700.degree. C. FIG. 12(a), (b),
(c) show SEM images of central part of steel bar (corresponding to
position {circle around (1)} in FIG. 10), in which fine ferrite
grains of average grain size of 0.5, 1, and 1.5 microns are
formed.
Embodiment 5
[0075] A steel bar of 15 mm square having a ferrite+pearlite
structure of average ferrite grain size of 20 microns of the
composition shown in Table 1b was rolled in 6-pass caliber until
diameter of 8 mm, using the caliber shown in FIG. 13 at rolling
temperature of 450 to 550.degree. C. Table 5 shows outline of
caliber dimension. FIG. 14 shows the sectional shape changes and
section reduction ratio in each pass of rolling. FIG. 15 shows SEM
images of structure after six passes, in which a fine ferrite grain
structure was formed in spite of section reduction ratio of about
74%. Concerning disc properties, the Vickers hardness is shown in
the bottom of the photograph in FIG. 15, and an excellent property
of over 800 MPa is obtained at tensile strength of 270 to 310.
TABLE-US-00005 TABLE 5 Radius of Longer axis Shorter axis curvature
1st pass, oval 31 6.8 38 3rd pass, oval 27 5.3 35.9 5th pass, oval
15 6.5 10.7 6th pass, round Diameter 8
COMPARATIVE EXAMPLE 1
[0076] A steel bar of 24 mm square having a ferrite+pearlite
structure of average ferrite grain size of 5 microns of the
composition shown in Table 1a was rolled in 7-pass caliber at
section reduction ratio of 70% (strain 1.2) until 13 mm square,
using the caliber shown in FIG. 1 at rolling temperature of
500.degree. C. It was not rolled by oval shape caliber. As shown in
SEM image in FIG. 16, fine grains were not formed in the center of
the steel bar.
COMPARATIVE EXAMPLE 2
[0077] A steel bar of 115 mm square of the composition shown in
Table 1a was heated to 900.degree. C., and rolled in caliber at
section reduction ratio of 94% (strain 3.1) until 24 mm square,
using the square shape caliber at rolling temperature of 870 to
850.degree. C. It was not rolled by oval shape caliber. The average
grain was 5 .mu.m, and fine grains were not formed. Dynamic
properties ware shown in Table 2, and the yield strength and
tensile strength were respectively 480 and 560 MPa.
INDUSTRIAL APPLICABILITY
[0078] As described herein, the invention presents a new warm
rolling method capable of introducing a greater strain into the
material by a smaller section reduction ratio and a smaller number
of passes by an easier means, and further presents a manufacturing
method of steel materials excellent in strength and ductility,
having ultrafine grain structure.
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