U.S. patent application number 13/918403 was filed with the patent office on 2014-01-09 for manufacturing method of high-strength and high-toughness thin steel and heat treatment apparatus.
This patent application is currently assigned to Delta Tooling Co., Ltd.. The applicant listed for this patent is DELTA TOOLING CO., LTD.. Invention is credited to Etsunori FUJITA, Seiji KAWASAKI, Shigeyuki KOJIMA, Shigeru MAEDA, Soichi MAKITA, Yumi OGURA, Hideyuki YAMANE, Seiya YOSHIDA.
Application Number | 20140008847 13/918403 |
Document ID | / |
Family ID | 42665499 |
Filed Date | 2014-01-09 |
United States Patent
Application |
20140008847 |
Kind Code |
A1 |
FUJITA; Etsunori ; et
al. |
January 9, 2014 |
MANUFACTURING METHOD OF HIGH-STRENGTH AND HIGH-TOUGHNESS THIN STEEL
AND HEAT TREATMENT APPARATUS
Abstract
To provide a technique suitable for elevating strength and
toughness of a thin low-carbon steel. By performing rapid heating
and rapid cooling to a thin low-carbon steel which is an ordinary
steel with a thickness of 1.2 mm or less, a steel where a
microstructure becomes a duplex grain size structure mixed with
crystal grains having different grain diameters, which is not
homogeneous, preferably, hard phase structures are contained in
addition to the duplex grain size structure is obtained, and a
high-strength and high-toughness thin low-carbon steel is obtained.
Further, by performing a heat treatment process involving rapid
heating and rapid cooling multiple times, a duplex grain size
structure of crystal grains with smaller grain diameters or a hard
phase structure contained therein is obtained, so that a thin
low-carbon steel with higher strength and higher toughness is
obtained.
Inventors: |
FUJITA; Etsunori;
(Hiroshima-shi, JP) ; OGURA; Yumi; (Hiroshima-shi,
JP) ; KAWASAKI; Seiji; (Hiroshima-shi, JP) ;
KOJIMA; Shigeyuki; (Hiroshima-shi, JP) ; MAKITA;
Soichi; (Hiroshima-shi, JP) ; MAEDA; Shigeru;
(Aki-gun, JP) ; YAMANE; Hideyuki; (Aki-gun,
JP) ; YOSHIDA; Seiya; (Aki-gun, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DELTA TOOLING CO., LTD. |
Hiroshima-shi |
|
JP |
|
|
Assignee: |
Delta Tooling Co., Ltd.
Hiroshima-shi
JP
|
Family ID: |
42665499 |
Appl. No.: |
13/918403 |
Filed: |
June 14, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13202991 |
Oct 28, 2011 |
|
|
|
PCT/JP10/52651 |
Feb 22, 2010 |
|
|
|
13918403 |
|
|
|
|
Current U.S.
Class: |
266/121 ;
266/259 |
Current CPC
Class: |
C21D 1/785 20130101;
Y02P 10/25 20151101; C21D 1/42 20130101; C22C 38/02 20130101; C22C
38/04 20130101; C21D 1/18 20130101; C21D 2211/008 20130101; C22C
38/08 20130101; C21D 1/26 20130101; Y02P 10/253 20151101; C22C
38/16 20130101 |
Class at
Publication: |
266/121 ;
266/259 |
International
Class: |
C21D 1/78 20060101
C21D001/78 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 24, 2009 |
JP |
2009-041571 |
Claims
1-19. (canceled)
20. A heat treatment apparatus which uses a thin low-carbon steel
which is an ordinary steel which has been worked to have a
thickness of 1.2 mm or less as the steel raw material which is a
reception material of heat treatment and which is used to
manufacture a high-strength and high-toughness thin steel by a
first process of rapidly cooling the thin low-carbon steel after
rapid heating thereof to obtain a martensite structure and a second
process of rapidly cooling the thin low-carbon steel which have
been subjected to the first process after rapidly reheating the
same to a temperature lower than a temperature at a rapid heating
time in the first process, wherein a work supporting section which
supports the thin low-carbon steel which is a target to be treated,
a first heating section which performs the rapid heating treatment
in the first process, a first cooling section which performs the
rapid cooling treatment in the first process, a second heating
section which performs the rapid heating treatment in the second
process, and a second cooling section which performs the rapid
cooling treatment in the second process are sequentially arranged;
and the first heating section, the first cooling section, the
second heating section, and the second cooling section are provided
so as to be movable relative to the work supporting section;
wherein one heating section having a predetermined length extending
in a moving direction, the first cooling section arranged on the
side opposite to the heating section via the work, and the second
cooling section arranged on the same side as the heating section or
on the same side as the first cooling section via the work to be
separated from the heating section or the first cooling section by
a predetermined distance rearward in the moving direction are
provided, and the heating section has such a length that a vicinity
of a front portion thereof corresponds to the first cooling section
and a vicinity of a rear portion thereof extends rearward in the
moving direction beyond the first cooling section; and the heating
section is configured to have two functions such that the vicinity
of the front portion of the heating section has a function of the
first heating section which performs the rapid heating in the first
process and the vicinity of the rear portion of the heating section
has a function of the second heating section which performs the
rapid heating in the second process.
21. The heat treatment apparatus according to claim 20, wherein the
second cooling section is disposed on the same side as the heating
section.
22. The heat treatment apparatus according to claim 20, wherein the
work supporting section is rotatably provided in a supporting state
of the thin low-carbon steel.
23. The heat treatment apparatus according to claim 20, wherein
each of the heating sections includes a coil performing
high-frequency induction heating.
24. The heat treatment apparatus according to claim 20, wherein
each of the heating sections is provided with a laser performing
laser heating.
25. The heat treatment apparatus according to claim 20, wherein
each of the first cooling section and the second cooling section is
a cooling water supplying section supplying cooling water.
26. The heat treatment apparatus according to claim 20, wherein the
first process includes a step of rapidly heating the thin
low-carbon steel up to a temperature of 1000.degree. C. or higher
at a rate of 300.degree. C./second or more and a step of rapidly
cooling the thin low-carbon steel at a rate of 300.degree.
C./second or more after the thin low-carbon steel is held at a
temperature of 900.degree. C. or higher within ten seconds; and the
second process includes a step of rapidly heating the thin
low-carbon steel up to a temperature of 700.degree. C. or higher at
a rate of 300.degree. C./second or more after the cooling in the
first process and a step of rapidly cooling the thin low-carbon
steel at a rate of 300.degree. C./second or more after the thin
low-carbon steel is held at a temperature of 600.degree. C. or
higher within ten seconds.
27. The heat treatment apparatus according to claim 26, wherein
rapid heating is performed up to a temperature in a range of
1000.degree. C. to 1250.degree. C. at the rapid heating step in the
first process and rapid heating is performed up to a temperature in
a range of 750.degree. C. to 1050.degree. C. at the rapid heating
step in the second process.
28. The heat treatment apparatus according to claim 26, wherein a
holding time before the rapid cooling after the rapid heating in
the first process is set within five seconds, and the holding time
before the rapid cooling after the rapid heating in the second
process is set within five seconds.
29. The heat treatment apparatus according to claim 26, wherein C
content of the thin low-carbon steel is in a range of 0.01 to 0.12%
by mass % and the rest thereof is composed of iron and inevitable
impurities.
30. The heat treatment apparatus according to claim 26, wherein the
steel raw material which is a reception material of heat treatment
is a thin low-carbon steel with a thickness of 1.0 mm or less.
31. The heat treatment apparatus according to claim 26, wherein the
steel raw material which is a reception material of heat treatment
is a thin low-carbon steel with a thickness of 0.8 mm or less
32. The heat treatment apparatus according to claim 26, wherein the
steel raw material which is a reception material of heat treatment
is a thin low-carbon steel with a thickness of 0.5 mm or less.
Description
CROSS REFERENCE
[0001] This application is a division of and is based upon and
claims the benefit of priority under 35 U.S.C. .sctn.120 for U.S.
Ser. No. 13/202,991, filed Oct. 28, 2011, the entire contents of
which is incorporated herein by reference. U.S. Ser. No. 13/202,991
is a National Stage of PCT JP10/052,651, filed Feb. 22, 2010, and
claims the benefit of priority under 35 U.S.C. .sctn.119 from
Japanese Patent Application No. 2009-041571, filed Feb. 24,
2009.
TECHNICAL FIELD
[0002] The present invention relates to a manufacturing method of a
high-strength and high-toughness thin steel which performs heat
treatment to a thin low-carbon steel as a reception material to
manufacture a high-strength and high-toughness thin steel, and a
heat treatment apparatus.
BACKGROUND ART
[0003] For example, a seat frame for transportation equipment such
as an automobile or an airplane is strongly required to be reduced
in weight in view of fuel consumption improvement, carbon dioxide
emission control or the like, and thus, high strength of a steal
material used for forming a seat frame is in demand. On the other
hand, the seat frame is also required to have not only high
strength but also high toughness (also including ductility) in view
of impact absorbing properties owing to deformation or the like. As
techniques satisfying these demands, for example, high-strength
steel plates disclosed in Patent Literatures 1 to 3 are known.
[0004] Each of the high-strength steel plates disclosed in these
Literatures assumes control on an addition amount of an alloy
element other than carbon, and it is made to contain, for example,
Mn, Mo, Cr, or the like in a predetermined amount or more to secure
a predetermined hardness or ductility. Then, for use as a steel
material for an automobile or the like, the high-strength steel
plate is finally cold-rolled down to a thickness of 1.2 mm, but
heat treatment performed at a step before the cold rolling is to
heat-roll a steel slab to a thickness of 3.2 mm. That is, since the
techniques disclosed in these Literature are techniques for
obtaining a steel plate with a thickness of several mm or thicker,
it is necessary to achieve evenness of a microstructure including a
plate-thickness direction in the steel plate in the heat treatment,
and thus, control on an addition amount of an alloy element is an
important factor.
[0005] On the other hand, in Patent Literatures 4 to 5, techniques
of achieving high strength of an ordinary low-carbon steel have
been disclosed. Patent Literature 4 discloses a technique proposed
in order to solve such a problem that, since tempering property of
an ordinary low-carbon steel was poor in the previous technique,
when a martensitic state was utilized as an originating structure,
an uneven duplex grain structure was produced during an annealing
time so that a predetermined high-strength and high-ductility steel
material could not be obtained. Therefore, in Patent Literature 4,
after an ordinary low-carbon steel is tempered to achieve
martensitic phase of 90% or more, an ultra-fine crystal grain
ferrite structure with grain diameters of 1.0 .mu.m or less is
obtained by performing cold-rolling with a total reduction ranging
from 20% to less than 80% and performing annealing. Patent
Literature 5 is the technique which has been proposed by the
present applicant, where high strength is achieved by performing
working process for elevating internal stress, such as press
forming, and achieving refinement and duplex grain sizing of a
metal structure of a low-carbon steel by heat treatment.
CITATION LIST
Patent Literature
[0006] Patent Literature 1: Japanese Patent No. 4005517 [0007]
Patent Literature 2: JP-A-2005-213640 [0008] Patent Literature 3:
JP-A-2008-297609 [0009] Patent Literature 4: Japanese Patent No.
4189133 [0010] Patent Literature 5: JP-A-2008-13835
SUMMARY OF INVENTION
Technical Problem
[0011] A demand for reduction of cost or recycling efficiency of
resources of a seat frame for an automobile or the like
increasingly becomes high from now due to energy saving,
accommodation to environmental problems, or the like. Therefore,
rather than high strength or high toughness achieved by alloying
like the techniques described in Patent Literatures 1 to 3,
achievement using an ordinary low-carbon steel which elevates the
recycling efficiency is desired. Further, these techniques are
techniques mainly implemented by iron and steel material
manufacturers for producing a predetermined high-strength and
high-toughness steel from a steel slab and they are not techniques
which can be utilized by processing manufacturer which manufactures
a seat frame or the like using a commercially-available steel. If
the processing manufacturer purchases an ordinary steel (common
steel) which is inexpensive and easy to work from an iron and steel
material manufacturer rather than purchasing a material sold as a
high-strength and high-toughness steel by the iron and steel
material manufacturer, and can achieve high-strength and
high-toughness at a required portion of the ordinary steel if
necessary, cost reduction of a seat frame can be achieved.
[0012] The technique disclosed in Patent Literature 4 is a
technique for obtaining desired strength and ductility using an
ordinary low-carbon steel as a reception material of heat
treatment, but it requires a process where after the whole steel
material is martensitized, it is cold-rolled to achieve refinement
homogeneously. Therefore, an installation provided with a rolling
function is required, which includes a problem regarding
installation cost and manufacturing cost. As apparent from such a
fact that an ordinary low-carbon steel material with a thickness of
2 mm is exemplified in an Example in patent Literature 4, in order
to achieve high strength and high ductility of a steel with a
certain thickness, it is necessary to achieve homogeneous
refinement in a plate-thickness direction so that a cold-rolling
step under predetermined conditions is essential after the
martensitization.
[0013] In the case of the technique disclosed in Patent Literature
5, refinement and high strength are achieved by heat-treating a
thin cold-rolled steel plate and a thin hot-rolled steel plate
having a thickness of 1.2 mm and a thickness of 1.0 mm in Examples,
but there is room for further improvement of toughness.
[0014] The present invention has been made in view of the above
circumstances, and a problem to be solved thereof is to provide a
technique excellent in cost reduction and recyclability as well as
suitable for elevating strength and toughness of a thin low-carbon
steel which is an ordinary steel on the side of a processing
manufacturer.
Solution to Problem
[0015] As a result of the present inventors' keen study for solving
the above problem, since a thin low-carbon steel which is an
ordinary steel (common steel) with a thickness of 1.2 mm or less is
thin, it has a high heat capacity and it tends to be rapidly heated
and rapidly cooled easily. The present inventors have focused on
such a fact that, owing to a duplex grain size structure where
crystal grains different in grain diameter and formed by rapid
heating and rapid cooling have been mixed, preferably, owing to a
structure where hard phase structure higher in hardness than such a
duplex grain size structure has been contained in addition to the
duplex grain size structure, a steel where strength and toughness
have been balanced in high level can be obtained even if refined
crystal grains with a size of 1 .mu.m or less are not contained at
a high percentage as in a thick steel or even unless homogenization
is achieved in a plate-thickness direction. The present inventors
have also focused on such a fact that it is effective to perform a
heat treatment process involving rapid heating and rapid cooling
multiple times without requiring a cold-rolling step after heat
treatment or the like in order to obtain a duplex grain size
structure where grain diameters of crystal grains are different in
this manner in a thin low-carbon steel.
[0016] That is, a manufacturing method of a high-strength and
high-toughness thin steel according to the present invention is a
method for manufacturing a high-strength and high-toughness thin
steel by heat-treating a steel raw material, comprising: using a
thin low-carbon steel which is an ordinary steel worked to have a
thickness of 1.2 mm or less as the steel raw material which is a
reception material of heat treatment; a first process of rapidly
cooling the thin low-carbon steel after rapid heating thereof to
obtain a martensite structure; and a second process of rapidly
cooling the thin low-carbon steel which have been subjected to the
first process after rapidly reheating the same to a temperature
lower than a temperature at a rapid heating time in the first
process, wherein the first process and the second process are
implemented while the thin low-carbon steel is being relatively
moved to each heating section and each cooling section which
perform rapid heating and rapid cooling treatments in the first
process and the second process; the first process includes a step
of rapidly heating the thin low-carbon steel up to a temperature of
1000.degree. C. or higher at a rate of 300.degree. C./second or
more and a step of rapidly cooling the thin low-carbon steel at a
rate of 300.degree. C./second or more after the thin low-carbon
steel is held at a temperature of 900.degree. C. or higher within
ten seconds; and the second process includes a step of rapidly
heating the thin low-carbon steel up to a temperature of
700.degree. C. or higher at a rate of 300.degree. C./second or more
after the cooling in the first process and a step of rapidly
cooling the thin low-carbon steel at a rate of 300.degree.
C./second or more after the thin low-carbon steel is held at a
temperature of 600.degree. C. or higher within ten seconds.
[0017] It is preferred that the rapid heating in the first process
and the rapid heating in the second process are implemented by
high-frequency induction heating. Further, the rapid heating in the
first process and the rapid heating in the second process can be
implemented by laser heating. The treatments in the first process
and the second process can also be performed multiple times.
[0018] It is preferred that C content of the thin low-carbon steel
is in a range of 0.01 to 0.12% by mass % and the rest thereof is
composed of iron and inevitable impurities. It is preferred that
rapid heating is performed up to a temperature in a range of
1000.degree. C. to 1250.degree. C. at the rapid heating step in the
first process and rapid heating is performed up to a temperature in
a range of 750.degree. C. to 1050.degree. C. at the rapid heating
step in the second process. It is preferred that a holding time
before the rapid cooling after the rapid heating in the first
process is set within five seconds and the holding time before the
rapid cooling after the rapid heating in the second process is set
within five seconds.
[0019] It is preferred that a heat treatment apparatus which treats
the thin low-carbon steel is provided with a first heating section
which performs the rapid heating treatment in the first process, a
first cooling section which performs the rapid cooling treatment in
the first process, a second heating section which performs the
rapid heating treatment in the second process, and a second cooling
section which performs the rapid cooling treatment in the second
process, and the thin low-carbon steel is sequentially treated in
the first heating section, the first cooling section, the second
heating section, and the second cooling section. Further, such a
configuration can be adopted that the first heating section and the
second heating section are composed of one heating section having a
predetermined length extending in a moving direction, and the
cooling treatment in the first process and the cooling treatment in
the second process can be performed to the thin low-carbon steel
which is a target to be treated from opposite faces thereof.
[0020] It is preferred that, when the thin low-carbon steel is
pipe-shaped, treatment is performed while the thin low-carbon steel
is being rotated. Further, the steel raw material which is a
reception material of heat treatment is preferably a thin
low-carbon steel with a thickness of 1.0 mm or less, the steel raw
material which is a reception material of heat treatment is more
preferably a thin low-carbon steel with a thickness of 0.8 mm or
less, and the steel raw material which is a reception material of
heat treatment is further preferably a thin low-carbon steel with a
thickness of 0.5 mm or less.
[0021] A heat treatment apparatus according to the present
invention is a heat treatment apparatus which uses a thin
low-carbon steel which is an ordinary steel which has been worked
to have a thickness of 1.2 mm or less as the steel raw material
which is a reception material of heat treatment and which is used
to manufacture a high-strength and high-toughness thin steel by a
first process of rapidly cooling the thin low-carbon steel after
rapid heating thereof to obtain a martensite structure and a second
process of rapidly cooling the thin low-carbon steel which have
been subjected to the first process after rapidly reheating the
same down to a temperature lower than a temperature at a rapid
heating time in the first process, wherein a work supporting
section which supports the thin low-carbon steel which is a target
to be treated, a first heating section which performs the rapid
heating treatment in the first process, a first cooling section
which performs the rapid cooling treatment in the first process, a
second heating section which performs the rapid heating treatment
in the second process, and a second cooling section which performs
the rapid cooling treatment in the second process are sequentially
arranged; and the first heating section, the first cooling section,
the second heating section, and the second cooling section are
provided so as to be movable relative to the work supporting
section.
[0022] Further, the heat treatment apparatus according to the
present invention is configured such that one heating section
having a predetermined length extending in a moving direction, the
first cooling section arranged on the side opposite to the heating
section via the work, and the second cooling section arranged on
the same side as the heating section or on the same side as the
first cooling section via the work to be separated from the heating
section or the first cooling section by a predetermined distance
rearward in the moving direction are provided, and the heating
section has such a length that a vicinity of a front portion
thereof corresponds to the first cooling section and a vicinity of
a rear portion thereof extends rearward in the moving direction
beyond the first cooling section; and the heating section is
configured to have two functions such that the vicinity of the
front portion of the heating section has a function of the first
heating section which performs the rapid heating in the first
process and the vicinity of the rear portion of the heating section
has a function of the second heating section which performs the
rapid heating in the second process. In this case, it is preferred
that the second cooling section is disposed on the same side as the
heating section.
[0023] Further, such a configuration can be adopted that the work
supporting section is rotatably provided in a supporting state of
the thin low-carbon steel. Furthermore, it is preferred that such a
configuration is adopted that each of the heating sections includes
a coil performing high-frequency induction heating, and such a
configuration can be adopted that each of the heating sections is
provided with a laser performing laser heating.
Advantageous Effect of the Invention
[0024] According to the manufacturing method of a high-strength and
high-toughness thin steel and the heat treatment apparatus, by
performing rapid heating and rapid cooling to a thin low-carbon
steel which is an ordinary steel with a thickness of 1.2 mm or
less, a duplex grain size structure where a microstructure is
inhomogeneous and crystal grains different in grain diameter have
been mixed is obtained, preferably, a structure where a hard phase
structure is contained in addition to the duplex grain size
structure is obtained, so that a thin low-carbon steel with high
strength and high toughness is obtained. Further, by performing a
heat treatment process involving rapid heating and rapid cooling
multiple times, a duplex grain size structure of crystal grains
having smaller grain diameters or a hard phase structure contained
therein is obtained, so that a thin low-carbon steel with higher
strength and higher toughness is obtained. In addition, by using a
heat treatment apparatus provided with two heating sections and two
cooling sections arranged in a predetermined order, the plurality
of times of the rapid heating and the rapid cooling can be
efficiently performed. Furthermore, by using one heating section
having a predetermined length and arranging the first cooling
section on the side opposite to the heating section via a work, an
apparatus can be made simpler, which results in contribution to
manufacturing cost reduction of a high-strength and high-toughness
thin steel.
BRIEF DESCRIPTION OF DRAWINGS
[0025] FIG. 1A is a diagram showing one example of a schematic
configuration of a high-frequency induction heating apparatus, FIG.
1B is a diagram showing a schematic configuration of a preferred
example of the high-frequency induction heating apparatus, and FIG.
1C is a diagram showing a schematic configuration of a
high-frequency induction heating apparatus where one heating
section which performs rapid heating in a first process and in a
second process is provided and rapid cooling treatment is performed
from both sides of a work;
[0026] FIG. 2 is a graph showing temperature conditions of
treatment conditions (A) and (B) in Test Example 1;
[0027] FIGS. 3A to 3C are electron microscope photographs of
microstructures of Samples 1 to 3 which were treated under the
treatment conditions (A) and (B) of Test Example 1;
[0028] FIG. 4 is a graph showing a temperature condition of a
treatment condition (C) in Test Example 2;
[0029] FIG. 5 is an electron microscope photograph of a
microstructure of Sample 1 which was treated under the treatment
condition (C) of Test Example 2;
[0030] FIG. 6A is electron microscope photographs of
microstructures of raw material states of Sample 1 and Sample 2,
and FIGS. 6B and 6C are electron microscope photographs of
respective microstructures of Sample 1 (Comparative Sample 1) and
Sample 2(Comparative Sample 2) which were treated in Comparative
Example 1;
[0031] FIG. 7 is a graph showing a relationship between hardness
(Hv) and fractal dimension of Sample 1 to Sample 3 which were
treated by Test Example 1, Test Example 2, and Comparative Example
1;
[0032] FIG. 8 is a graph showing a relationship between breaking
elongation and fractal dimension of Sample 1 to Sample 2 which were
treated by Test Example 1, Test Example 2, and Comparative Example
1;
[0033] FIGS. 9A and 9B are diagrams for explaining a measuring
method of a bending test of Test Example 3;
[0034] FIG. 10 is a graph showing a measurement result of the
bending test of Test Example 3;
[0035] FIG. 11 is a graph showing a measurement result of a tensile
test of Test Example 4; and
[0036] FIG. 12 is a graph showing a measurement result of a tensile
test of a pipe-shaped steel of Test Example 5.
DESCRIPTION OF EMBODIMENTS
[0037] In a method for manufacturing a high-strength and
high-toughness thin steel according to the present invention, a
steel raw material which is a reception material of heat treatment
is a commercially-available ordinary steel (common steel), which is
a thin and low-carbon one (hereinafter, called "thin low-carbon
steel"). As the thin low-carbon steel, a rolled steel plate which
is inexpensive and excellent in workability and which is used in a
seat frame of an automobile or the like is suitable, the rolled
steel plate including both a cold-rolled steel plate and a
hot-rolled steel plate. The thickness of the rolled steel plate is
1.2 mm or less. When the thickness of the rolled steel is thicker
than 1.2 mm, a large heat source and a large-scaled cooling
installation are required for performing rapid heating and rapid
cooling in order to achieve high strength and high toughness, and
because homogeneity of crystal grains is required in a
plate-thickness direction, it is difficult to perform control, so
that such a thick rolled steel plate is unfit for a steel raw
material which is a target to be treated of the present invention.
A steel raw material to be treated of the present invention, which
does not involve a rolling process and whose high strength and high
toughness should be achieved by only a heat treatment process of
rapid heating and rapid cooling is preferably a thin low-carbon
steel with a thickness of 1.0 mm or less, more preferably a thin
low-carbon steel with a thickness of 0.8 mm or less, and further
preferably a thin low-carbon steel with a thickness of 0.5 mm or
less.
[0038] As the above thin low-carbon steel, a low-carbon steel whose
carbon content is in a range of 0.01 to 0.3% and whose rest is
composed of iron and inevitable impurities can be used, but an
extremely-low-carbon steel whose carbon content is in a range of
0.01 to 0.12% and whose rest is composed of iron and inevitable
impurities is preferably used. By using a material whose carbon
content is lower and is further inexpensive, reduction of a
manufacturing cost of a seat frame or the like can be achieved.
Further, in the present invention, by performing limitation to a
thin material, even if carbon content is low, strength can be
elevated and balance with toughness can be achieved, so that it is
unnecessary to perform addition of an alloy element other than
carbon and recycling efficiency is excellent. On the other hand,
since there is no limitation about a component except for the above
carbon content, for example, a recycle steel material mixed with a
material which was used as an ordinary steel, where various
components other than carbon were mixed, is also usable.
Incidentally, the thin low-carbon steel which is a target to be
treated includes both a plate-shaped one and a pipe-shaped one.
[0039] It is preferred that a process of heat-treating the above
thin low-carbon steel is performed according to the following two
steps. That is, the process of performing heating treatment
includes a first process of rapidly cooling a thin low-carbon steel
after rapid heating thereof to obtain a martensite structure and a
second process of rapidly cooling the thin low-carbon steel which
has been subjected to the first process after rapidly reheating the
thin low-carbon steel up to a temperature lower than a temperature
at the rapid heating time in the first process. Incidentally, it is
possible to perform the treatments of the thin low-carbon steel in
the first process and the second process multiple times in a
repeating manner.
[0040] The first process includes a step of rapidly heating the
thin low-carbon steel up to a temperature of 1000.degree. C. or
higher, preferably, up to a temperature in a range of 1000.degree.
C. to 1250.degree. C., at a rate of 300.degree. C./second or more
and a step of, after the rapid heating, maintaining the thin
low-carbon steel within ten seconds, preferably, within five
seconds, until the temperature of the thin low-carbon steel drops
to a predetermined temperature of 900.degree. C. or higher,
preferably, drops to a temperature in a range of 1000.degree. C. to
1100.degree. C., and thereafter rapidly cooling the thin low-carbon
steel at a rate of 300.degree. C./second or more. By rapidly
heating the thin low-carbon steel up to the above temperature, a
metal structure of the thin low-carbon steel is austenitized and a
martensite structure is formed by the rapid cooling, but since the
thickness of the thin low-carbon steel which is a target to be
treated of the present invention is 1.2 mm or less, a homogeneous
martensite structure which have escaped relatively coarsening can
be formed by, so to speak, ultra-rapid heating and ultra-rapid
cooling such as 300.degree. C./second or more. Incidentally, the
rapid heating rate and the rapid cooling rate are more preferably
set to 500.degree. C./second or more.
[0041] The second process includes a step of rapidly heating the
thin low-carbon steel up to 700.degree. C. or higher, preferably,
up to a temperature in a range of 750.degree. C. to 1050.degree.
C., at a rate of 300.degree. C./second or more, after the cooling
in the first process, and a step of, after the rapid heating,
maintaining the thin low-carbon steel within ten seconds,
preferably, within five seconds, until the temperature of the thin
low-carbon steel drops to a predetermined temperature of
600.degree. C. or higher, preferably, to a temperature in a range
of 700.degree. C. to 950.degree. C. and thereafter rapidly cooling
the thin low-carbon steel at a rate of 300.degree. C./second or
more. When the thin low-carbon steel which has been subjected to
the first process is heat-treated in the second process again, it
is preferred that the heat treatment is performed after the
temperature of the thin low-carbon steel drops to 200.degree. C. or
lower by the rapid cooling in the first process. The heat treatment
in the second process may be performed in another line at a lower
temperature, for example, after the temperature drops to room
temperature. Incidentally, the rapid heating rate and the rapid
cooling rate in the second process are more preferably set to
500.degree. C./second or more similarly to the first process.
[0042] By performing the above ultra-rapid heating and ultra-rapid
cooling in the second process, the martensite structure is changed,
and a duplex grain size structure which includes a duplex grain
size structure of crystal grains different in grain diameter in a
range from 1 .mu.m to 30 .mu.m (the term "grain diameter" in this
text indicates "grain diameter such as a circular phase"), where
crystal grains different in grain diameter having an average grain
diameter smaller than an average grain diameter of a martensite
obtained when heat treatment has been performed in order to form
the martensite, namely, an average grain diameter of a martensite
obtained when the heat treatment in the first process has been
performed have aggregated, can be finally obtained.
[0043] The duplex grain size structure is preferably a structure
having a configuration where crystal grains with grain diameters of
1 .mu.m to less than 5 .mu.m and crystal grains with grain
diameters of 5 .mu.m to 30 .mu.m have been mixed, and it is further
preferably a structure having a configuration where crystal grains
with grain diameters of 1 .mu.m to less than 5 .mu.m and crystal
grains with grain diameters of 5 .mu.m to 20 .mu.m have been mixed.
Since the heat-treated steel has the duplex grain size structure
different in grain diameter in this manner instead of a homogeneous
grain diameter, a partial elongation occurs in the case of the thin
low-carbon steel, so that a steel having higher toughness can be
obtained. In order to obtain higher strength, it is preferred that
hard phase structures higher in hardness than the duplex grain size
structure are dispersed in the duplex grain size structure. For
example, when the duplex grain size structure is a ferrite
structure different in grain diameter, it is preferred that
island-shaped martensites having a grain diameter of 30 .mu.m or
less, preferably, 20 .mu.m or less are dispersed in the duplex
grain size structure. Thereby, a thin low-carbon steel with high
strength and high toughness where a reaction force due to
deflection of a beam due to a bending moment at a transition point
from an elastic region to a plastic region is at least 1.5 times
that before heat treatment in a bending property, a yield point in
a tensile property has a strength of at least 1.5 times that before
heat treatment, and a breaking elongation is at least 1.5 times
that in a state where heat treatment forming martensite has been
performed in a thin low-carbon steel, namely, when the heat
treatment in the first process has been performed can be
obtained.
[0044] In the high-strength and high-toughness thin steel obtained
by the present invention, a microstructure is the duplex grain size
structure of crystal grains different in grain diameter, as
described the above, preferably, a structure where hard phase
structures such as martensite have been dispersed in the duplex
grain size structure. In the present invention, the thin low-carbon
steel provided with high strength and high toughness is obtained by
such a structure control, but the present inventors have found that
the microstructure can be regulated from the view point of fractal
dimension of a grain diameter. Though described in detail later,
the microstructure of the thin low-carbon steel which has been
controlled by the heat treatment like the present invention has a
fractal dimension of a grain diameter higher than that of a grain
diameter in martensite obtained when heat treatment for forming
martensite has been performed, namely, by only the heat treatment
in the first process.
[0045] Incidentally, the term "fractal dimension" is a measure
representing the degree of complexity, where in a figure having a
self-similarity, when the figure is composed of m similar figures
obtained by reducing the figure into a size of 1/n thereof, fractal
dimension (similarity dimension) D is expressed by D=log (m)/log
(n)=log (the number of similar figures to an original figure)/log
(the number of equal divisions). Accordingly, the "fractal
dimension of a grain diameter" in this text becomes higher
according to advance to further refinement of crystal grains.
[0046] It is preferred that, as a heat treatment apparatus which
performs each heat treatment in the first process and the second
process, a high-frequency induction heating apparatus is used.
Further, it is preferred that a heating section (a coil configuring
an induction heating section in the case of an induction heating
apparatus) and a cooling section (a cooling water supplying section
supplying cooling water) of the high-frequency induction heating
apparatus move at a predetermined speed relative to the thin
low-carbon steel which is a target to be heat-treated and the work
supporting section. Thereby, the rapid heating and the rapid
cooling treatment in the above-described extremely short time can
be realized even by a small-scaled installation. A moving speed of
the heating section (a coil configuring an induction heating
section in the case of an induction heating apparatus) and the
cooling section of the high-frequency induction heating apparatus
is preferably set in a range within 30 mm/second, more preferably
set in a range within 18 mm/second. Incidentally, a work (thin
low-carbon steel) is supported by the work supporting section, and
when the work is a plate-shaped one, the work supporting section
may be configured with a flat plate-shaped table on which the
plate-shaped work can be placed or a grasping section (see FIG. 1A
to 1C) which grasps an end portion of the work. Further, when the
work is pipe-shaped, it is preferred that treatment is performed
while the work is being rotated, so that it is preferred that such
a configuration is adopted that the work supporting section has a
grasping section which can grasp the pipe-shape one and the
grasping section is rotatable.
[0047] As shown in FIG. 1A, as the high-frequency induction heating
apparatus, one provided with a heating section and a cooling water
supplying section in this order can be used. The heating section
and the cooling water supplying section are provided by only one
set thereof, where when the treatment in the first process is
performed, the heating section is controlled to a predetermined
temperature so that the heating section is made to function as a
first heating section (coil) to perform treatment, and the cooling
water supplying section is similarly made to function as a first
cooling section (a cooling water supplying section). After the
treatment of the first process is performed, the treatment in the
second process is performed again by the high-frequency induction
heating apparatus shown in FIG. 1A. In this case, the heating
section is controlled to a temperature lower than that in the
treatment performed in the first process to be made to function as
a second heating section (coil), while the cooling section is made
to function as a second cooling section, so that the treatment is
performed. Incidentally, the high-frequency induction heating
apparatus is not limited to one provided with only one set of a
heating section and a cooling water supplying section, but such a
configuration is preferably adopted that a first heating section
(coil) and a first cooling section (first cooling water supplying
section) which perform the treatment in the first process and a
second heating section (coil) and a second cooling section (second
cooling water supplying section) which perform the treatment in the
second section are provided in this order, as shown in FIG. 1B.
According to the apparatus shown in FIG. 1B, the first process and
the second process can be performed continuously, so that a
treatment rate of a work is improved.
[0048] Further, as shown in FIG. 1C, such a configuration can be
adopted that both functions of the first heating section in the
first process and the second heating section in the second process
are satisfied by using a heating section (coil) having a
predetermined length or longer in a moving direction, for example,
a lengthy one having a length of about 5 to 10 cm. That is, the
heating section is disposed on the side of one face of a work (thin
low-carbon steel), and a cooling section (first cooling water
supplying section) is provided on the side opposite to the work so
as to correspond to a vicinity of a front portion of the heating
section in the moving direction. Thereby, the vicinity of the front
portion of the heating section in the moving direction performs the
rapid heating treatment in the first process and the first cooling
water supplying section corresponding thereto performs the rapid
cooling treatment in the first process. The heating section and the
first cooling water supplying section move as a set thereof. Then,
a site on the work which has been subjected to the rapid heating
and rapid cooling treatments in the first process is rapidly
reheated by the vicinity of a rear portion of the heating section.
Thereby, the rapid heating treatment in the second process is
performed. Thereafter, a cooling section (second cooling water
supplying section) disposed to be separated from the heating
section by a predetermined distance rearward in the moving
direction rapidly cools the site on the work which has been rapidly
heated by the vicinity of the rear portion of the heating section
to apply the rapid cooling treatment in the second process to the
site. Accordingly, when the lengthy heating section (coil) shown in
FIG. 1C is used, the rapid heating in the first process and the
second process can be performed by one heating section (coil), so
that a high-frequency induction heating apparatus which has a
simple and inexpensive structure can be realized. Incidentally, as
shown in FIG. 1C, the second cooling water supplying section is
disposed on the same side as the heating section via the work, but
it may be disposed on the same side as the first cooling water
supplying section. However, in order to perform efficient rapid
cooling, it is preferred that the second cooling water supplying
section is disposed on the same side as the heating section, as
shown in FIG. 1C.
[0049] Incidentally, as the first heating section and the second
heating section in the first process and the second process, a
laser is provided, so that each rapid heating treatment may be
performed by laser heating.
Test Example 1
[0050] Heat treatment was applied to following respective
Samples.
(1) Sample 1: a cold-rolled steel plate of an ordinary steel (SPCC)
[0051] Chemical Components (%): C=0.04, Si=0.02, Mn=0.26, P=0.011,
and S=0.006 [0052] Thickness: 0.5 mm, Width: 100 mm, and Length:
200 mm (2) Sample 2: a cold-rolled steel plate of an ordinary steel
(SPCC) [0053] Chemical Components (%): C=0.037, Si=0.004, Mn=0.19,
P=0.013, S=0.012, sol Al=0.015, Cu=0.02, Ni=0.02, and B=14 (PPM)
[0054] Thickness: 0.5 mm, Width: 100 mm, and Length: 200 mm (3)
Sample 3: a cold-rolled steel plate of an ordinary steel (JSC440)
[0055] Chemical Components (%): C=0.12, Si=0.06, Mn=1.06, P=0.022,
and S=0.005 [0056] Thickness: 0.6 mm, Width: 100 mm, and Length:
200 mm
[0057] As the heat treatment apparatus, a high-frequency induction
heating apparatus provided with one set of the heating section and
the cooling section shown in FIG. 1A was used, where after the
treatment in the first process was performed by the heating section
and the cooling water supplying section, each Sample was left down
to room temperature, and the treatment in the second process was
then performed by the same high-frequency induction heating
apparatus. As the treatment condition, the following two treatment
conditions (A) and (B) were adopted.
[0058] Treatment Condition (A)
[0059] First Process
[0060] (1) Moving speed of the heating section and the cooling
water supplying section: 800 mm/min.
[0061] (2) The coil of the heating section was adjusted to 120 A. A
sample was pre-heated according to gradual temperature rising as
the heating section came relatively close to the sample, but the
sample was rapidly heated from 400.degree. C. to 1200.degree. C. in
about one second. Thereafter, the sample was held for about 2.5
seconds until the temperature thereof dropped to 1050.degree. C.,
and it was then rapidly cooled to 200.degree. C. or lower in about
0.5 seconds by supplying cooling water from the cooling water
supplying section (a solid line in the first process shown in FIG.
2).
[0062] Second Process
[0063] (1) Moving speed of the heating section and the cooling
water supplying section: 800 mm/min.
[0064] (2) After the sample dropped to room temperature, it was set
in the high-frequency induction heating apparatus again. A current
to be made to flow in the coil of the heating section was adjusted
to 100 A, and after the sample was pre-heated to 400.degree. C., it
was rapidly heated up to 900.degree. C. in about 0.5 seconds. The
sample was held for about 2.5 seconds until the temperature dropped
to 800.degree. C., it was then rapidly cooled down to about
200.degree. C. or lower in about 0.5 seconds by supplying cooling
water from the cooling water supplying section, and it was
thereafter left until the temperature reached room temperature (a
solid line in the first process shown in FIG. 2).
[0065] Treatment Condition (B)
[0066] First Process
[0067] (1) Moving speed of the heating section and the cooling
water supplying section: 800 mm/min.
[0068] (2) The coil of the heating section was adjusted to 120 A. A
sample was pre-heated according to gradual temperature rising as
the heating section came relatively close to the sample, but the
sample was rapidly heated from 400.degree. C. to 1200.degree. C. in
about one second. Thereafter, the sample was held for about 2.5
seconds until the temperature thereof dropped to 1050.degree. C.,
and it was then rapidly cooled to 200.degree. C. or lower in about
0.5 seconds by supplying cooling water from the cooling water
supplying section (a solid line in the first process shown in FIG.
2).
[0069] Second Process
[0070] (1) Moving speed of the heating section and the cooling
water supplying section: 1000 mm/min.
[0071] (2) After the sample dropped to room temperature, it was set
in the high-frequency induction heating apparatus again. A current
to be made to flow in the coil of the heating section was adjusted
to 100 A, and after the sample was pre-heated to 400.degree. C., it
was rapidly heated up to 800.degree. C. in about 0.5 seconds. The
sample was held for about 2.5 seconds until the temperature dropped
to 700.degree. C., it was then rapidly cooled down to about
200.degree. C. or lower in about 0.5 seconds by supplying cooling
water from the cooling water supplying section, and it was
thereafter left until the temperature reached room temperature (a
broken line in the first process shown in FIG. 2).
[0072] FIG. 3A are electron microscope photographs of
microstructures of Samples 1 which were treated according to the
treatment conditions (A) or (B) and which were observed by cutting
near their central portions in their longitudinal directions, and
FIG. 3B are electron microscope photographs of microstructures of
the Samples 2 which were treated according to the treatment
conditions (A) or (B) and which were observed by cutting near their
central portions in their longitudinal directions (incidentally,
regarding the microstructures of the raw material states of Samples
1 and Samples 2, see the column "Raw Material" in FIG. 6A). FIG. 3C
are electron microscope photographs of microstructures of Samples 3
which were treated according to the treatment conditions (A) or (B)
and which were observed by cutting near their central portions in
their longitudinal directions.
[0073] From FIG. 3A, Sample 1 which was treated according to the
treatment condition (A) was composed of a duplex grain size
structure of a ferrite structure of fine grains having grain
diameters of 1 .mu.m to less than 5 .mu.m and a ferrite structure
of grains having grain diameters of 5 to 30 .mu.m, where
island-shaped martensites having a grain diameter of 30 .mu.m or
less were contained in the duplex grain size structure in an amount
of less than 5%. On the other hand, in the case of the treatment
condition (B) where the moving speed was faster than that of the
treatment condition (A) and the heating temperature in the second
process was lower than that of the treatment condition (A), Sample
1 was composed of a duplex grain size structure of a ferrite
structure of fine grains having grain diameters of 1 .mu.m to less
than 5 .mu.m and a ferrite structure of grains having grain
diameters of 5 to 20 .mu.m, so that crystal grains of Sample 1
according to the treatment condition (A) were slightly larger than
those of Sample 1 according to the treatment condition (B).
[0074] In the case of FIG. 3B, Sample 2 which was treated according
to the treatment condition (A) contained island-shaped martensites
having grain diameters of 30.mu. in an amount of about 20% in
addition to a duplex grain size structure of a ferrite structure of
fine grains having grain diameters of 1 .mu.m to less than 5 .mu.m
and a ferrite structure of grains having grain diameters of 5 to 30
.mu.m. In the case of the treatment condition (B), Sample 2 was
composed of a duplex grain size structure of a ferrite structure of
fine grains having grain diameters of 1 .mu.m to less than 5 .mu.m
and a ferrite structure of grains having grain diameters of 5 to 20
.mu.m.
[0075] Samples 3 contained C in an amount of 0.12% which was more
than those of Samples 1 and Samples 2. Accordingly, as shown in
FIG. 3C, both Samples which were treated according to the treatment
condition (A) or (B) contained island-shaped martensites having
grain diameters of 30 .mu.m or less in addition to a duplex grain
size structure of a ferrite structure of fine grains having grain
diameters of 1 .mu.m to less than 5 .mu.m and a ferrite structure
of fine grains having grain diameters of 5 to 30 .mu.m, where the
island-shaped martensites were contained in an amount of about 50
to 60%.
Test Example 2
[0076] The above Samples 1 was heat-treated by a high-frequency
induction heating apparatus provided with a heating section
comprising a lengthy coil with a length of 6 cm shown in FIG. 1C
and first and second cooling water supplying sections. A treatment
condition was as the following (C).
[0077] Treatment Condition (C)
[0078] First Process
[0079] (1) Moving speed of the heating section and the first and
second cooling water supplying sections: 800 mm/min.
[0080] (2) The coil of the heating section was adjusted to 120 A. A
sample was pre-heated according to gradual temperature rising as
the heating section came relatively close to the sample, but the
sample was rapidly heated from 400.degree. C. to 1200.degree. C. in
about one second. Thereafter, the sample was held for about 2.5
seconds until the temperature thereof dropped to 1050.degree. C.,
and it was then rapidly cooled to 200.degree. C. or lower in about
0.5 seconds by supplying cooling water from the cooling water
supplying section (a solid line in the first process shown in FIG.
4).
[0081] Second Process
[0082] (1) Moving speed of the heating section and the first and
second cooling water supplying sections: 1000 mm/min.
[0083] (2) A current to be made to flow in the coil of the heating
section was adjusted to 90 A and Sample 1 whose temperature dropped
to about 200.degree. C. was rapidly heated up to 800.degree. C. in
about 0.5 seconds by the rear portion of the heating section.
Sample 1 was held for about 2.5 seconds until its temperature
dropped to 700.degree. C., it was then rapidly cooled to
200.degree. C. or less in about 0.5 seconds by supplying cooling
water from the second cooling water supplying section, and
thereafter it was left until its temperature reached room
temperature (a solid line in the second process in FIG. 4).
[0084] FIG. 5 is an electron microscope photograph of a
microstructure of Sample 1 which was treated according to the
treatment condition (C) and which was observed by cutting near its
central portion in its longitudinal direction. From FIG. 5, Sample
1 which was treated according to the treatment condition (C)
included island-shaped martensites having grain diameters of about
5 to 10 .mu.m formed in an amount of about 20% in addition to a
duplex grain size structure of a ferrite structure of fine grains
having grain diameters of 1 .mu.m to less than 5 .mu.m and a
ferrite structure of grains having grain diameters of 5 to 20
.mu.m.
Comparative Example 1
[0085] As the heat treatment apparatus, the high-frequency
induction heating apparatus provided with one set of the heating
section and the cooling water supplying section shown in FIG. 1A
was used, and heat treatment where rapid heating and rapid cooling
were only once performed was performed to Sample 1 (Comparative
Sample 1) and Sample 2 (Comparative Sample 2).
[0086] Regarding the treatment condition, a case where after rapid
heating was performed up to 1200.degree. C. by the heating section
(coil), rapid cooling was performed by the cooling water supplying
section (Heat Treatment 1) and a case where after rapid heating was
performed up to 900.degree. C. by the heating section (coil), rapid
cooling was performed by the cooling water supplying section (Heat
Treatment 2) were tested. The condition of Heat Treatment 1 was
aimed to produce a martensite structure while the condition of Heat
Treatment 2 was aimed to produce a duplex grain size structure or a
duplex grain size structure including island-shaped martensites.
Electron microscope photographs of microstructures of respective
Samples whose were observed by cutting at their central portions in
their longitudinal directions are shown in FIG. 6A to 6C.
Incidentally, in these figures, "Raw Material" indicates
microstructures of Sample 1 and Sample 2 before heat treatment is
performed thereto.
[0087] From FIG. 6A, both Sample 1 and Sample 2 in their raw
material states have approximately-even ferrite structures of
grains with grain diameters of 10 .mu.m or less. Both Comparative
Sample 1 and Comparative Sample 2 in their states of "Heat
Treatment 1" shown in FIG. 6B have coarse martensite structures of
grains with grain diameter of 20 to 100 .mu.m. Comparative Sample 2
in its state of "Heat Treatment 2" shown in FIG. 6C has a duplex
grain size structure of a ferrite structure of fine grains having
grain diameters of 1 .mu.m to less than 5 .mu.m and a ferrite
structure of grains having grain diameters of 5 to 30 .mu.m. In the
case of Comparative Sample 1, island-shaped martensites having a
grain diameter of about 5 to 10 .mu.m are formed in addition to a
duplex grain size structure of a ferrite structure of fine grains
having grain diameters of 1 .mu.m to less than 5 .mu.m and a
ferrite structure of grains having grain diameters of 5 to 30
.mu.m.
[0088] FIG. 7 is a graph where an average hardness (Hv) is
represented on a horizontal axis while fractal dimension of a grain
diameter is represented on a vertical axis, and respective values
of respective Samples 1, 2 and 3 of Test Example 1 and Test Example
2, and Comparative Samples 1 and 2 of Comparative Example 1 are
plotted. As apparent from this figure, in the case of Sample 1 and
Sample 2, ones where the duplex grain size structure was formed or
island-shaped martensites were formed in the duplex grain size
structure in both Test Examples 1 and 2 were higher in fractal
dimension than Comparative Samples 1 and 2 (Heat treatment 1) where
martensite structures were formed in Comparative Example 1. It was
found that, regarding inclinations obtained by least-square method,
shown in FIG. 7, Test Examples 1 tended to be higher in fractal
dimension than Comparative Examples 1 as a whole, and by performing
rapid heating treatment and rapid cooling treatment multiple times
like Test Example 1, even in ones with same duplex grain size
structure formed or with same island-shaped martensites formed in a
duplex grain size structure, Samples 1 and 2--A treatment
(treatment according to the treatment condition (A)) and Samples 1
and 2--B treatment (treatment according to the treatment condition
(B)) could be made finer in grain diameter and higher in toughness
than Comparative Samples 1 and 2 (Heat Treatment 2). Further, even
in the case of Sample 1--C treatment (treatment according to the
treatment condition (C)) of Test Example 2, though hardness became
high, fractal dimension was approximately equal to the case of
Comparative Sample 1 (Heat Treatment 2) formed with the duplex
grain size structure.
[0089] Further, very high hardness was obtained in Sample 3 of Test
Example 1. This is because a dispersion percentage of island-shaped
martensites is high, and Sample 3 of Test Example 1 is inferior to
Samples 1 and 2--A treatment and Samples 1 and 2--B treatment in
toughness. However, when compared with Comparative Samples 1 and 2
(Heat Treatment 1), it is found that Sample 3 of Test Example 1
maintained hardness which was not inferior to that of the case
having the martensite structure and shown in FIG. 6B, while it
became high in fractal dimension, so that it could be increased in
toughness while it maintained hardness higher than those of
Comparative Samples 1 and 2 (Heat Treatment 1). However, when C
content is more than that of Sample 3 of Test Example 1, there is a
possibility that the toughness is further inferior, so that it is
more desirable that the C content is set to 0.12% or less.
[0090] Incidentally, when high strength is achieved by refining
crystal grains in a metal structure, the fractal dimension is
largely raised according to Law of Hall-Petch as shown by arrow X
as compared with the state of raw materials shown in FIG. 7, but
the fact that high strength does not depend on refinement of
crystal grains in the case of the technique of the present
invention utilizing ultra-rapid heating and ultra-rapid cooling is
also understood from such a fact that a rising percentage of a
value of a fractal dimension showed a small tendency.
[0091] FIG. 8 is a graph where a breaking elongation (%) is
represented on a horizontal axis while fractal dimension of a grain
diameter is represented on a vertical axis, and respective values
of respective Samples 1 and 2 of Test Example 1 and Test Example 2,
and Comparative Samples 1 and 2 of Comparative Example 1. For
example, "Sample 1--A treatment" indicates one where island-shaped
martensites are contained in the above-described duplex grain size
structure in an amount of less than 5% and whose breaking
elongation is 18.16%, while "Sample 2--A treatment" indicates one
which is composed of the above-described duplex grain size
structure and whose breaking elongation is 20.44%. The breaking
elongation which is one of indexes of the toughness tends to become
large according to the increase of the fractal dimension, so that a
correlation between the above-described fractal dimension and
toughness became apparent. Then, it was also found from FIG. 8 that
Samples 1 and 2 of the present invention which were subjected to
rapid heating treatment and rapid cooling treatment multiple times
could elevate their toughness as compared with Comparative Samples
1 and 2 which were subjected to rapid heating treatment and rapid
cooling treatment only once.
Text Example 3
Bending Test
[0092] Three kinds of samples having the same chemical components
as those of the cold-rolled steel plate of the ordinary steel of
Sample 1 and whose thicknesses were 0.5 mm, 0.8 mm, and 1.0 mm,
respectively, were heat-treated such that the heat treatment covers
their ranges of a width of 30 mm and a length of 100 mm (see FIG.
9A). In the heat treatment, each treatment included in the first
process and the second process was performed according to the above
"Treatment Condition (A)".
[0093] As shown in FIG. 9B, each of the above-described Samples was
set on the supporting stand supporting the vicinities of both ends
thereof, and a load was imparted on a central portion of the
heat-treated range of each Sample in a longitudinal direction
thereof at a loading rate of 10 mm/min by a crosshead. Respective
ones of non-heated ones (represented as "Raw Material" in FIG. 10)
and heat-treated ones ("Heat Treatment" in FIG. 10) of all Samples
were tested. The test result is shown in FIG. 10.
[0094] As apparent from FIG. 10, regarding a reaction force due to
deflection of a beam caused by a bending moment at a transition
point from an elastic region to a plastic region in a bending
property, Sample with a thickness of 0.5 mm which was heat-treated
is about twice the Sample before heat-treated, and Samples with a
thickness of 0.8 mm and with a thickness of 1.0 mm which were
heat-treated are about 2.5 times those before heat-treated.
Accordingly, by using Sample with a thickness of 0.5 mm which was
heat-treated instead of a raw material with a thickness of 0.8 mm
or using Sample with a thickness of 0.8 mm which was heat-treated
instead of a raw material with a thickness of 1.0 mm, contribution
to weight reduction of a seat frame or the like can be
achieved.
Test Example 4
Tensile Test
[0095] Tests were performed by grasping end portions of samples
with a length of 150 mm and a width of 30 mm in their longitudinal
directions by a chuck. The samples were Samples 1 with a thickness
of 0.5 mm and with a thickness of 0.8 mm which were used in the
above-described bending tests and Sample 2 with a thickness of 0.5
mm. The result is shown in FIG. 11. In FIG. 11, "Heat treatment--A
(Sample 1)" and "Heat Treatment--A (Sample 2)" were heat-treated
according to the above-described treatment condition (A) of Test
Example 1, where the microstructure was the duplex grain size
structure or it was the duplex grain size structure formed therein
with island-shaped martensites. "Heat treatment 1" is a sample
which was heat-treated according to the above-described "Heat
Treatment 1" of Comparative Example 1 and which resulted in
martensite structure.
[0096] As a result, the yield point (proof stress) of the sample
formed with the martensite structure of Heat Treatment 1 in
Comparative Example 1 is high but the breaking elongation thereof
is low. On the other hand, the yield points (proof stresses) of
"Heat treatment--A (Sample 1)" and "Heat Treatment--A (Sample 2)",
when having a thickness of 0.5 mm, were about twice that of a raw
material before heat-treated and they were lower than that of one
formed with a martensite structure, but the breaking elongations
thereof were at least three times that of the one formed with a
martensite structure. The yield points (proof stresses) of "Heat
treatment--A (Sample 1)" and "Heat Treatment--A (Sample 2)", when
having a thickness of 0.8 mm, were about 2.5 times that of a raw
material before heat-treated, but the breaking elongation thereof
were about twice that of one formed with a martensite
structure.
Test Example 5
[0097] A steel pipe made of carbon steel for machine or structure
(STKM-13C) with a diameter of 12 mm, a thickness of 1.0 mm, and C
content of 0.08% was heat-treated while being rotated at a rotation
speed of 400 rpm. Regarding the case where the heat treatments in
the first process and the second process were performed by the
high-frequency induction heating apparatus shown in FIG. 1A
(represented as "two-stage heat treatment" in FIG. 12) and the case
where only the heat treatment in the first process was performed
(represented as "one-stage heat treatment" in FIG. 12), tensile
tests were performed and compared with each other. The result is
shown in FIG. 12.
[0098] As apparent from FIG. 12, samples which were subjected to
the two-stage heat treatment of the present invention became about
at least twice that of a raw material regarding the yield point
(proof stress) and they had breaking elongation about twice that of
a sample which was subjected to the one-stage heat treatment.
Incidentally, since the "Raw Material" in FIG. 12 was not attached
with an elongation meter, a rising of a graph thereof was different
from those of the other samples which were heat-treated, but the
breaking elongation thereof was corrected from actual measured
values. Further, in graphs of samples which were heat-treated, the
reason why loads applied to the samples dropped halfway was because
a measuring machine was stopped and the elongation meter was
detached from the samples halfway since while the sample was being
attached with a measuring tool, it could not be measured until
breaking took place.
[0099] From the above, it was found that all of the hardness, the
yield point (proof stress), the tensile strength, the reaction
force due to deflection of a beam caused by a bending moment, and
the breaking elongation of a steel where the microstructure which
was subjected to the heat treatment of the present invention was a
duplex grain size structure or a duplex grain size structure formed
with island-shaped martensites, namely, a steel which was subjected
to the rapid heating and rapid cooling treatments in the first
process and the second process were maintained in high level, and a
steel having high strength and high toughness (high ductility)
could be obtained while it was obtained by heat-treating a
commercially-available ordinary steel.
* * * * *