U.S. patent application number 15/763151 was filed with the patent office on 2018-09-27 for high strength stainless steel sheet excellent in fatigue characteristics, and production method therefor.
The applicant listed for this patent is NISSAN MOTOR CO., LTD.. Invention is credited to Yasuhiro EHARA, Hiroyasu MATSUBAYASHI, Shun SAITO.
Application Number | 20180272397 15/763151 |
Document ID | / |
Family ID | 57123196 |
Filed Date | 2018-09-27 |
United States Patent
Application |
20180272397 |
Kind Code |
A1 |
EHARA; Yasuhiro ; et
al. |
September 27, 2018 |
HIGH STRENGTH STAINLESS STEEL SHEET EXCELLENT IN FATIGUE
CHARACTERISTICS, AND PRODUCTION METHOD THEREFOR
Abstract
A thin steel sheet contains, in terms of percentage by mass,
from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less of Si,
from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00% of Ni,
from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from 0 to
3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti, from 0
to 0.008% of Al, and the balance of Fe, with unavoidable
impurities; and having a number density of a non-metallic inclusion
lining up with an interparticle distance in the rolling direction
of 20 mm or less and an interparticle distance in the sheet
thickness direction of 10 mm or less that has a length in the
rolling direction of 40 mm or more of 3.0 or less per square
millimeter on the L cross section.
Inventors: |
EHARA; Yasuhiro; (Tokyo,
JP) ; SAITO; Shun; (Tokyo, JP) ; MATSUBAYASHI;
Hiroyasu; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NISSAN MOTOR CO., LTD. |
Tokyo |
|
JP |
|
|
Family ID: |
57123196 |
Appl. No.: |
15/763151 |
Filed: |
July 5, 2016 |
PCT Filed: |
July 5, 2016 |
PCT NO: |
PCT/JP2016/069952 |
371 Date: |
March 26, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21C 7/0075 20130101;
C22C 38/58 20130101; C21C 7/068 20130101; C21C 7/076 20130101; C21C
7/0685 20130101; C21D 2211/004 20130101; C21C 7/0087 20130101; C21D
9/46 20130101; C22C 38/04 20130101; C21D 2211/008 20130101; C22C
38/42 20130101; C22C 38/001 20130101; C22C 38/40 20130101; C22C
38/02 20130101; B21B 3/02 20130101; C21D 2211/001 20130101; C22C
38/44 20130101 |
International
Class: |
B21B 3/02 20060101
B21B003/02; C21C 7/068 20060101 C21C007/068; C21C 7/076 20060101
C21C007/076; C21D 9/46 20060101 C21D009/46; C22C 38/40 20060101
C22C038/40 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 29, 2015 |
JP |
2015-191221 |
Claims
1. A stainless steel sheet: having a sheet thickness of from 20 to
500 .mu.m; having a chemical composition containing, in terms of
percentage by mass, from 0.010 to 0.200% of C, more than 2.00% and
4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00% or more and
less than 10.00% of Ni, from 11.00 to 20.00% of Cr, from 0.010 to
0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0
to 0.008% of Ti, from 0 to 0.008% of Al, and the balance of Fe,
with unavoidable impurities; and having, with an assumption that in
a cross section (L cross section) in parallel to a rolling
direction and a sheet thickness direction, a group of non-metallic
inclusion particles that line up with an interparticle distance in
the rolling direction of 20 .mu.m or less and an interparticle
distance in the sheet thickness direction of 10 .mu.m or less is
one non-metallic inclusion, a number density of the non-metallic
inclusion having a length in the rolling direction of 40 .mu.m or
more of 3.0 or less per square millimeter on the L cross
section.
2. The stainless steel sheet according to claim 1, wherein the
non-metallic inclusion having a length in the rolling direction of
40 .mu.m or more contains one or two of (i) and (ii): (i) a TiN
based inclusion particle and (ii) a spinel based inclusion particle
containing one or more of Al and Mg.
3. The stainless steel sheet according to claim 1, wherein the
stainless steel sheet has a tensile strength in the rolling
direction of 2,000 N/mm.sup.2 or more.
4. The stainless steel sheet according to claim 1, wherein the
stainless steel sheet has a mixed structure of a deformation
induced martensite phase and an austenite phase as a matrix (metal
basis material).
5. A production method of a stainless steel sheet comprising: a
step of providing a molten steel having a chemical composition (A)
below, in such a manner that in component control by adding an
auxiliary raw material and a slag forming flux to a molten steel
having a C content of 0.20% or less having a Cr oxide-containing
slag on a molten steel surface after subjecting to a
decarburization process by blowing oxygen into a Cr-containing
molten iron, a container housing the molten steel, the auxiliary
raw material, and the slag forming flux used are selected to make a
Ti content in the molten steel of 0.008% by mass or less and an Al
content therein of 0.008% by mass or less, at least an Fe--Si alloy
as the auxiliary raw material is dissolved in the molten steel to
perform deoxidation, reduction and recovery of Cr in the slag into
the molten steel, and control of a Si content in the steel, and a
Ca-containing slag forming flux is added to control a slag basicity
(i.e., a mass ratio of CaO/SiO.sub.2) to from 1.3 to 1.5; a step of
providing a cast piece by casting the molten steel obtained in the
preceding step; a step of providing a hot rolled steel sheet by
subjecting the cast piece to hot working including at least hot
rolling; and a step of providing a cold rolled steel sheet having a
sheet thickness of from 20 to 500 .mu.m by subjecting the hot
rolled steel sheet to annealing and cold rolling one or more times:
(A) in terms of percentage by mass, from 0.010 to 0.200% of C, more
than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00%
or more and less than 10.00% of Ni, from 11.00 to 20.00% of Cr,
from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00%
of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and the
balance of Fe, with unavoidable impurities.
6. The production method of a stainless steel sheet according to
claim 5, wherein the container housing the molten steel used is a
container having a refractory constituting the inner surface of the
container that is not used for housing a molten steel (i.e., a new
ladle).
7. The production method of a stainless steel sheet according to
claim 5, wherein the Fe--Si alloy used has an Al content of 0.05%
by mass or less, and a Ti content of 0.05% by mass or less.
8. The production method of a stainless steel sheet according to
claim 5, wherein the production method further comprises: a step of
subjecting the cold rolled steel sheet to an aging treatment.
9. The production method of a stainless steel sheet according to
claim 8, wherein the steel sheet obtained by the production method
has a mixed structure of a deformation induced martensite phase and
an austenite phase for a matrix (metal basis material), and has a
tensile strength in the rolling direction of 2,000 N/mm.sup.2 or
more.
Description
TECHNICAL FIELD
[0001] The present invention relates to, in a stainless steel
species capable of providing an extremely high strength by
utilizing formation of a deformation induced martensite phase,
solid-solution strengthening by a large Si content, and age
hardening, a steel sheet that is significantly suppressed in
formation of a coarse hard non-metallic inclusion. The invention
also relates to a production method therefor.
BACKGROUND ART
[0002] As a high strength stainless steel, a metastable austenite
stainless steel represented by SUS301 has been widely used.
However, for providing a high strength with SUS301, the cold
rolling reduction ratio is necessarily increased, and accordingly
the reduction in toughness is associated therewith. As a technique
for avoiding the problem to achieve both a high strength and a high
toughness at high levels simultaneously, a measure of achieving a
high strength by utilizing formation of a deformation induced
martensite phase, solid-solution strengthening by a large Si
content, and age hardening has been known, and has been used in
such purposes as an ID saw blade substrate and the like (PTL
1).
[0003] The stainless steel of the type described in PTL 1 has a
high strength and a high toughness through formation of a dual
phase structure of a deformation induced martensite and austenite
through cold rolling, and has good fatigue resistance
characteristics as a rotary member having a sheet thickness of 0.1
mm or more, such as an ID saw blade. However, a further enhancement
of the fatigue resistance characteristics is demanded in the case
where the stainless steel is worked into a thin sheet material
having a sheet thickness of less than 0.1 mm, particularly from 20
to 70 .mu.m, and is applied to the purpose of a spring material,
which repeatedly receives elastic deformation. Examples of the
factors deteriorating the fatigue resistance characteristics of a
steel material include the presence of a non-metallic inclusion.
Even assuming inclusions having the same size, when the sheet
thickness is decreased, the length proportion in the sheet
thickness direction of the inclusion occupied in the sheet
thickness is increased, and stress is concentrated to the
circumference of the inclusion particles, which function as a
starting point or a propagation path of cracks. It is more
difficult in a thinner sheet material to avoid the decrease of the
fatigue characteristics caused by the non-metallic inclusion.
[0004] As a method for decreasing the amount of the non-metallic
inclusion in a steel material (i.e., increasing the cleanliness
degree thereof), measures for optimizing the slag composition in
refining have been variously investigated. However, from the
standpoint of preventing working cracks and fatigue breaking, it is
not necessarily sufficient that only the cleanliness degree is
simply enhanced, and the control of the composition of the
non-metallic inclusion is said to be effective. For example, PTL 2
describes a measure, in which in manufacture of general purpose
austenite steel species, such as SUS304, the composition of the
non-metallic inclusion is controlled by adjusting the slag basicity
to from 1.4 to 2.4 with a refining furnace having a lining of a
dolomite refractory, and thereby an austenite stainless steel
having no working crack is provided. However, according to the
researches by the present inventors, it has been found that even
though the measure described in PTL 2 is tried for a steel species
having a large Si content, it is difficult to improve significantly
the fatigue characteristics of the thin sheet material.
[0005] PTL 3 describes a technique, in which in a large Si content
austenite stainless steel, the total amount of the B.sub.1 type
inclusions having high melting points mainly formed of alumina is
decreased, and thereby the corrosion resistance in high-temperature
and high-concentration nitric acid is improved. It is described
that for suppressing formation of the B.sub.1 type inclusions, the
reduction recovery of Cr with Al is not performed, and an Fe--Si
alloy having a small Al content of approximately 0.1% is added
(paragraphs 0052 and 0053). However, the target steel in PTL 3 is
an austenite single phase steel having a Ni content of 10% or more
(paragraph 0033), which is not a steel species that intends the
enhancement of the strength through the formation of a deformation
induced martensite phase. The literature does not teach a measure
for improving the fatigue resistance characteristics in a thin
sheet material for a purpose of a spring. As described later, it is
important that the formation of the TiN based inclusion is
suppressed for improving the fatigue resistance characteristics of
the steel species targeted in the invention, but the manufacturing
method described in the literature cannot stably decrease the TiN
based inclusion.
CITATION LIST
Patent Literatures
[0006] PTL 1: Japanese Patent No. 3,219,117
[0007] PTL 2: Japanese Patent No. 3,865,853
[0008] PTL 3: Japanese Patent No. 5,212,581
SUMMARY OF INVENTION
Technical Problem
[0009] Among non-metallic inclusions contained in the steel, an
inclusion of a kind having a high melting point and a large
hardness remains as a granular matter after hot rolling, and after
cold rolling, the hard grains having been crushed to a certain
extent remain and line up in the rolling direction. Therefore, it
is considered that the fatigue resistance characteristics of the
thin sheet material can be enhanced by significantly suppressing
the formation of the hard non-metallic inclusion of this type.
However, for providing a considerably high strength level for the
steel species of the deformation induced martensite forming type,
it is necessary to contain Si in a large amount exceeding 2% by
mass. When the Si content in the steel is increased in this manner,
it is considerably difficult to suppress the formation of the hard
non-metallic inclusion. As described in PTL 3, even such a measure
is applied that Al is not added, and an Fe--Si alloy having a small
Al content is added, the fatigue resistance characteristics of the
thin sheet material cannot be stably improved.
[0010] The invention is to achieve, in mass production sites, a
thin sheet material having a sheet thickness of from 20 to 500
.mu.m having a distribution mode of a non-metallic inclusion that
is effective for improving the fatigue resistance characteristics,
in a stainless steel of the deformation induced martensite forming
type having a large Si content.
Solution to Problem
[0011] As a result of detailed investigations by the inventors, it
has been found that in a large Si content stainless steel species
of the deformation induced martensite forming type, for improving
the fatigue resistance characteristics in the use as a thin sheet
spring material, it is considerably effective to decrease the
amount of a TiN based inclusion and a spinel based inclusion
containing one or more of Al and Mg each having a circle equivalent
diameter of 6.0 .mu.m or more present in the hot rolled steel
sheet. It has also been found that the decrease of the coarse
inclusions can be achieved in mass production operation by strictly
managing the incorporation of Ti and Al from the attachment of the
container housing the molten steel, the auxiliary raw material, and
the slag forming flux, and controlling the basicity of the final
slag formed after the addition of Si to a range that is lower than
the ordinary range. When the hot rolled steel sheet is formed into
a thin sheet through a cold rolling process, the inclusions in the
thin sheet are in such an existence form that is considerably
advantageous for the improvement of the fatigue resistance
characteristics. The invention has been made based on the
knowledge.
[0012] For achieving the object, the invention provides a stainless
steel sheet: having a sheet thickness of from 20 to 500 .mu.m;
having a chemical composition containing, in terms of percentage by
mass, from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less
of Si, from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00%
of Ni, from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from
0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti,
from 0 to 0.008% of Al, and the balance of Fe, with unavoidable
impurities; and having, with an assumption that in a cross section
(L cross section) in parallel to a rolling direction and a sheet
thickness direction, a group of non-metallic inclusion particles
that line up with an interparticle distance in the rolling
direction of 20 .mu.m or less (i.e., from 0 to 20 .mu.m) and an
interparticle distance in the sheet thickness direction of 10 .mu.m
or less (i.e., from 0 to 10 .mu.m) is one non-metallic inclusion, a
number density of the non-metallic inclusion having a length in the
rolling direction of 40 .mu.m or more of 3.0 or less per square
millimeter on the L cross section.
[0013] Herein, the interparticle distance (.mu.m) in the rolling
direction of two particles appearing on the L cross section is the
distance (.mu.m) in the rolling direction between the areas in the
rolling direction where the particles are present respectively in
the case where the areas in the rolling direction do not overlap
each other, or is 0 .mu.m in the case where the areas overlap each
other. Similarly, the interparticle distance (.mu.m) in the sheet
thickness direction of two particles appearing on the L cross
section is the distance (.mu.m) in the sheet thickness direction
between the areas in the sheet thickness direction where the
particles are present respectively in the case where the areas in
the sheet thickness direction do not overlap each other, or is 0
.mu.m in the case where the areas overlap each other. The two
particles that have an interparticle distance in the rolling
direction of 20 .mu.m or less and an interparticle distance in the
sheet thickness direction of 10 .mu.m or less belong to the same
"group".
[0014] In the non-metallic inclusion having a length in the rolling
direction of 40 .mu.m or more, examples thereof that exert a
particularly large influence on the fatigue resistance
characteristics include a material containing one or two of (i) and
(ii): (i) a TiN based inclusion particle and (ii) a spinel based
inclusion particle containing one or more of Al and Mg.
[0015] The Ti content is the total Ti content in the steel
including Ti that is present as an inclusion. Similarly, the Al
content is the total Al content in the steel including Al that is
present as an inclusion.
[0016] The number density of the inclusion can be measured by
observing an observation plane obtained by mirror-polishing the L
cross section with an SEM (scanning electron microscope). The
discrimination in terms of kind of the inclusion among the TiN
based inclusion and the spinel based inclusion containing one or
more of Al and Mg can be made, for example, by elemental analysis
with an EDX (energy dispersive X-ray spectrometry) attached to the
SEM.
[0017] FIG. 1 shows an example of the SEM micrograph of the
inclusion appearing on the L cross section of the conventional cold
rolled steel sheet having a sheet thickness of 120 .mu.m
(Conventional Example No. 1 described later). The horizontal
direction in the figure agrees with the rolling direction, and the
vertical direction in the figure agrees with the sheet thickness
direction. Groups of non-metallic inclusion particles lining up
substantially along the rolling direction appear at two positions
(A) and (B). The interparticle distance in the sheet thickness
direction between the proximate particles of the groups (A) and (B)
is shown in the symbol S in FIG. 1. The interparticle distance in
the sheet thickness direction S exceeds 10 .mu.m, and therefore
taking all the particles of the groups (A) and (B) as the target,
the particles do not correspond to the "group of non-metallic
inclusion particles that line up with an interparticle distance in
the rolling direction of 20 .mu.m or less and an interparticle
distance in the sheet thickness direction of 10 .mu.m or less".
Taking only the particles of the group (A) as the target, all the
constitutional particles each have an interparticle distance in the
rolling direction of 20 .mu.m or less and an interparticle distance
in the sheet thickness direction of 10 .mu.m or less with respect
to at least one of the other particles, and therefore the particles
constituting the group (A) correspond to the "group of non-metallic
inclusion particles that line up with an interparticle distance in
the rolling direction of 20 .mu.m or less and an interparticle
distance in the sheet thickness direction of 10 .mu.m or less".
Accordingly, the particles constituting the group (A) are assumed
to be one non-metallic inclusion. Similarly, the particles
constituting the group (B) are also assumed to be one non-metallic
inclusion. Consequently, two non-metallic inclusions are present in
FIG. 1, and the lengths in the rolling direction thereof are shown
by L.sub.A and L.sub.B respectively in FIG. 1. Therein, L.sub.A is
40 .mu.m or more, and therefore in the two non-metallic inclusions,
the non-metallic inclusion having a length in the rolling direction
L.sub.A corresponds to the "non-metallic inclusion having a length
in the rolling direction of 40 .mu.m or more".
[0018] As a result of an EDX analysis, these non-metallic
inclusions each are a TiN based inclusion.
[0019] FIG. 2 shows an example of the SEM micrograph of the
inclusion appearing on the L cross section of the cold rolled steel
sheet having a sheet thickness of 120 .mu.m according to the
invention (Invention Example No. 5 described later) in a different
view field from FIG. 1. The horizontal direction in the figure
agrees with the rolling direction, and the vertical direction in
the figure agrees with the sheet thickness direction. The
non-metallic inclusion particles present in the form of lining up
in FIG. 2 correspond to the "group of non-metallic inclusion
particles that line up with an interparticle distance in the
rolling direction of 20 .mu.m or less and an interparticle distance
in the sheet thickness direction of 10 .mu.m or less", and
therefore the non-metallic inclusion particles are assumed to be
one non-metallic inclusion. The non-metallic inclusion has a length
in the rolling direction that slightly exceeds 40 .mu.m, and
therefore corresponds to the "non-metallic inclusion having a
length in the rolling direction of 40 .mu.m or more".
[0020] As a result of an EDX analysis, these non-metallic
inclusions each are a TiN based inclusion.
[0021] The number density of the non-metallic inclusion having a
length in the rolling direction of 40 .mu.m or more on the L cross
section of the steel sheet can be obtained in the following
manner.
Measurement Method for Number Density of Non-Metallic Inclusion
Having Length in Rolling Direction of 40 .mu.m or More
[0022] The observation plane obtained by mirror-polishing the cross
section (L cross section) in parallel to the rolling direction and
the sheet thickness direction of the steel sheet is observed with
an SEM. A measurement field having a length in the rolling
direction of 100 .mu.m or more and a length in the sheet thickness
direction of the overall length in the sheet thickness direction is
arbitrarily determined, and in all the "non-metallic inclusions
having a length in the rolling direction of 40 .mu.m or more" that
are present entirely or partially in the measurement field, the
number of the inclusion that is present entirely in the measurement
field and the inclusion that partially protrudes outside the
measurement field but is present in the measurement field with a
half or more part thereof in the rolling direction present in the
measurement field is counted. This operation is performed for one
measurement field or two or more measurement fields that do not
overlap each other until the total area of the measurement fields
reaches 10 mm.sup.2 or more, and the value obtained by dividing the
sum of the counted numbers for the measurement fields by the total
area of the measurement fields is designated as the "number density
(per square millimeter) of the non-metallic inclusion having a
length in the rolling direction of 40 .mu.m or more".
[0023] In the stage of the hot rolled steel sheet, the total number
density of the TiN based inclusion and the spinel based inclusion
containing one or more of Al and Mg each having a circle equivalent
diameter of 6.0 .mu.m or more on the L cross section is preferably
0.05 or less per square millimeter.
[0024] The circle equivalent diameter is the particle diameter
converted to the diameter of the circle that has the same area as
the projected area of the inclusion particle appearing on the
observation surface. The circle equivalent diameters of each of the
inclusion particles can be calculated, for example, by computer
image processing of an SEM micrograph of the inclusion. The number
density of the inclusion in the hot rolled steel sheet can be
obtained in the following manner.
Measurement Method for Number Density of Inclusion in Hot Rolled
Steel Sheet
[0025] The observation plane obtained by mirror-polishing the cross
section (L cross section) in parallel to the rolling direction and
the sheet thickness direction of the steel sheet is observed with
an SEM. A rectangular measurement field is determined in a randomly
selected view field, and in all the inclusion particles observed in
the view field that are a TiN based inclusion or a spinel based
inclusion containing one or more of Al and Mg and have a circle
equivalent diameter of 6.0 .mu.m or more, the number of the
particle that is present entirely in the measurement field and the
particle that partially protrudes outside the measurement field but
is present in the measurement field with a half or more part of the
particle area present in the measurement field is counted. This
operation is performed for plural view fields that do not overlap
each other until the total area of the measurement fields reaches
200 mm.sup.2 or more, and the value obtained by dividing the sum of
the counted numbers for the view fields by the total area of the
measurement fields is designated as the "total number density (per
square millimeter) of the TiN based inclusion and the spinel based
inclusion containing one or more of Al and Mg each having a circle
equivalent diameter of 6.0 .mu.m or more".
[0026] With the hot rolled steel sheet exhibiting the
aforementioned inclusion distribution, a thin sheet material
obtained therefrom through subsequent cold rolling provides the
aforementioned prescribed inclusion distribution, providing a
significant improvement effect of the fatigue resistance
characteristics. As the thin sheet material, examples thereof that
are particularly preferred include a thin sheet material having a
tensile strength in the rolling direction of 2,000 N/mm.sup.2 or
more. The steel sheet obtained by cold rolling the hot rolled steel
sheet having the aforementioned composition has a mixed structure
of a deformation induced martensite phase and an austenite phase
for the matrix (metal basis material).
[0027] As a production method of the aforementioned steel sheet,
there is provided a production method of a stainless steels sheet
containing:
[0028] a step of providing a molten steel having the aforementioned
chemical composition, in such a manner that in component control by
adding an auxiliary raw material and a slag forming flux to a
molten steel having a C content of 0.20% or less having a Cr
oxide-containing slag on a molten steel surface after subjecting to
a decarburization process by blowing oxygen into a Cr-containing
molten iron, a container housing the molten steel, the auxiliary
raw material, and the slag forming flux used are selected to make a
Ti content in the molten steel of 0.008% by mass or less and an Al
content therein of 0.008% by mass or less, at least an Fe--Si alloy
as the auxiliary raw material is dissolved in the molten steel to
perform deoxidation, reduction and recovery of Cr in the slag into
the molten steel, and control of a Si content in the steel, and a
Ca-containing slag forming flux is added to control a slag basicity
(i.e., a mass ratio of CaO/SiO.sub.2) to from 1.3 to 1.5;
[0029] a step of providing a cast piece by casting the molten steel
obtained in the preceding step;
[0030] a step of providing a hot rolled steel sheet by subjecting
the cast piece to hot working including at least hot rolling;
and
[0031] a step of providing a cold rolled steel sheet having a sheet
thickness of from 20 to 500 .mu.m by subjecting the hot rolled
steel sheet to annealing and cold rolling one or more times.
[0032] Herein, the operation "selecting the container housing the
molten steel, the auxiliary raw material, and the slag forming flux
used to make a Ti content in the molten steel of 0.008% by mass or
less and an Al content therein of 0.008% by mass or less" means
that the container housing the molten steel having a small amount
of the attachment or no attachment is used, and the auxiliary raw
material and the slag forming flux having an impurity content
managed to a low level are used, so as to prevent the Ti content in
the molten steel from exceeding 0.008% by mass and to prevent the
Al content therein from exceeding 0.008% by mass, due to Ti and Al
incorporated from the attachment of the container housing the
molten steel, the auxiliary raw material, and the slag forming flux
into the molten steel. The molten steel in the stage after
completing the decarburization process by blowing oxygen into the
Cr-containing molten iron can be assumed to have the Ti and Al
contents of substantially zero. Therefore, a steel having a Ti
content of from 0 to 0.008% and an Al content of from 0 to 0.008%
can be obtained by preventing or decreasing as much as possible the
incorporation thereof from the outside.
[0033] The container housing the molten steel, the auxiliary raw
material, and the slag forming flux used are more preferably
selected to make a Ti content in the molten steel of 0.006% by mass
or less and an Al content therein of 0.006% by mass or less.
[0034] Specific examples of the container housing the molten steel
include a refining vessel and a ladle that are lined with a
refractory. The ladle may be used directly as the refining vessel.
The container housing the molten steel used is preferably a
container having a refractory constituting the inner surface of the
container that is not used for housing a molten steel (i.e., a new
ladle).
[0035] The Fe--Si alloy used preferably has an Al content of 0.05%
by mass or less, and a Ti content of 0.05% by mass or less.
[0036] While the content of Mg, which is an element forming the
spinel based inclusion, in the steel is not particularly defined,
it has been confirmed that the Mg content can be decreased to a
level causing no problem through the aforementioned selection of
the container housing the molten steel, the auxiliary raw material,
and the slag forming flux effective for decreasing the Ti and Al
contents. In this case, the total Mg content in the steel may be
0.002% by mass or less.
[0037] The cold rolled steel sheet may be subjected to an aging
treatment, and thereby providing a steel sheet having a mixed
structure of a deformation induced martensite phase and an
austenite phase for the matrix (metal basis material) and a tensile
strength in the rolling direction, for example, of 2,000 N/mm.sup.2
or more.
Advantageous Effects of Invention
[0038] According to the invention, a thin sheet material having a
significantly decreased number of a hard non-metallic inclusion
having a large length in the rolling direction in a large Si
content stainless steel species of the deformation induced
martensite forming type can be achieved in a mass production
operation. The large Si content stainless steel species of this
type can exhibit a maximum level of strength among the stainless
steel species, and has been used mainly such purposes as an ID saw
blade. The thin sheet material can be improved in fatigue
resistance characteristics by the inclusion control according to
the invention, and thus the thin sheet material can be applied to
the purpose of a thin sheet spring material. Accordingly, the
invention can contribute to the further size reduction of the thin
sheet spring component used in electronic devices and the like
through the utilization of the high strength characteristics
inherent to the steel species.
BRIEF DESCRIPTION OF DRAWINGS
[0039] FIG. 1 is the SEM micrograph of the non-metallic inclusion
appearing on the L cross section of the cold rolled steel sheet of
Conventional Example No. 1.
[0040] FIG. 2 is the SEM micrograph of the non-metallic inclusion
appearing on the L cross section of the cold rolled steel sheet of
Invention Example No. 5.
[0041] FIG. 3 is the SEM micrograph of the typical TiN based
inclusion observed on the L cross section of the hot rolled steel
sheet of Conventional Example No. 4.
[0042] FIG. 4 is the SEM micrograph of the typical TiN based
inclusion observed on the L cross section of the hot rolled steel
sheet of Invention Example No. 5.
DESCRIPTION OF EMBODIMENTS
Chemical Composition
[0043] The percentage for the chemical composition is percentage by
mass unless otherwise indicated.
[0044] The invention targets a steel having the following chemical
composition (A).
[0045] (A) In terms of percentage by mass, from 0.010 to 0.200% of
C, more than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of
Mn, 3.00% or more and less than 10.00% of Ni, from 11.00 to 20.00%
of Cr, from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to
1.00% of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and
the balance of Fe, with unavoidable impurities.
[0046] The steel species having the composition forms deformation
induced martensite and is increased in strength in cold rolling.
Furthermore, in the subsequent aging treatment, the solute atoms,
such as C and N, form a Cottrell atmosphere mainly in the
martensite phase to fix the dislocation, which causes a function
enhancing the strength (strain aging). Moreover, Si present in a
large amount in the steel provides solid-solution strengthening of
the martensite phase and the residual austenite phase, which
contributes to the enhancement of the strength.
[0047] In the invention, for sufficiently providing the benefit of
the aforementioned strength enhancing function of Si, a steel
having a Si content exceeding 2.00% is targeted. However, with an
excessively large Si content, such problems that hot working cracks
tend to occur, and the like may be conspicuous. Herein, the Si
content is restricted to 4.00% or less.
[0048] C is an element forming an austenite phase and is necessary
for strengthening the steel. However, an excessive amount of C may
cause deterioration of the corrosion resistance and the toughness.
In the invention, a steel having a C content of from 0.010 to
0.200% is targeted, and particularly in the case where a high
strength is targeted, the C content is advantageously in a range of
from 0.050 to 0.100%.
[0049] N is an element forming an austenite phase and is necessary
for strengthening the steel. However, an excessive amount of N may
be a factor promoting the formation of a TiN based inclusion. In
the invention, a steel having a N content of from 0.010 to 0.200%
is targeted. With the range, the particle diameter distribution of
the TiN based inclusion can be optimized to the range defined in
the invention by the production method of suppressing the
incorporation of Ti described later. The preferred range of the N
content is from 0.050 to 0.085%.
[0050] Mn is an element, with which the austenite stability can be
easily controlled by controlling the content thereof, and the
content thereof is controlled in a range of from 0.01 to 3.00%. A
large amount of Mn contained may prevent the deformation induced
martensite phase from being induced. The Mn content is more
preferably controlled in a range of 1.00% or less, and may be
managed to a range of 0.50% or less.
[0051] Ni is an element forming an austenite phase, and the content
thereof of 3.00% or more is ensured for providing a metastable
austenite phase at ordinary temperature. A too large Ni content may
prevent the deformation induced martensite phase from being
induced, and therefore the Ni content is less than 10.00%. The Ni
content is more preferably from 7.00 to 9.50%.
[0052] Cr is an element necessary for ensuring the corrosion
resistance. In the invention, a steel having a Cr content of from
11.00 to 20.00% is targeted. Cr is an element forming a ferrite
phase, and with a larger amount thereof exceeding the above range,
an austenite single phase structure may not be obtained at a high
temperature in some cases. The more preferred range of the Cr
content is from 12.00 to 15.00%.
[0053] Mo may be contained depending on necessity since Mo has a
function enhancing the corrosion resistance, and also has a
function contributing the strengthening through the formation of a
Mo precipitate in the aging treatment and preventing the structure
work-hardened in the cold rolling from being softened in the aging
treatment. For sufficiently providing the benefit of the functions,
it is preferred to ensure a Mo content of 1.0% or more. In the case
where a strength level with a tensile strength in the rolling
direction of 2,000 N/mm.sup.2 or more is intended, it is extremely
effective that Mo is contained in an amount of 2.00% or more.
However, a too large amount of Mo contained may cause formation of
a .delta. ferrite phase at a high temperature, and therefore in the
case where Mo is contained, the content thereof is in a range of
3.00% or less, and may be managed to a range of 2.50% or less.
[0054] Cu may be contained depending on necessity since Cu has a
function increasing the strength through the mutual action with Si
in the aging treatment. In this case, the Cu content is more
preferably 0.01% or more. A too large content of Cu may be a factor
decreasing the hot workability. In the case where Cu is contained,
the content thereof is in a range of 1.00% or less.
[0055] Ti is an element forming a TiN based inclusion, and the Ti
content is necessarily suppressed to a low level particularly in a
large Si content steel, in which TiN is easily formed. As a result
of the various investigations, the Ti content is necessarily 0.008%
or less, and is preferably 0.006% by mass or less. The Ti content
is preferably as small as possible, but in view of the cost in a
mass production operation, it is rational that the Ti content is in
a range of 0.001% or more.
[0056] Al is a factor generating a spinel based inclusion through
the formation of Al.sub.2O.sub.3, and the Al content is necessarily
suppressed to a low level particularly in a molten steel of a large
Si content steel, in which Al.sub.2O.sub.3 is easily formed. In the
invention, the Al content is preferably as small as possible. As a
result of the various investigations, the Al content is necessarily
0.008% or less, and is preferably 0.006% by mass or less. The Al
content is preferably as small as possible, but in view of the cost
in a mass production operation, it is rational that the Al content
is in a range of 0.001% or more. However, even with an Al content
in the aforementioned range, it may be difficult to make stably the
particle diameter distribution of the spinel based inclusion within
the range defined in the invention unless the slag basicity after
the addition of Si is optimized as described later.
[0057] As the unavoidable impurities, it is preferred that the P
content is 0.040% or less, and the S content is 0.002% or less, and
it is also preferred that the Mg content is from 0 to 0.002%.
[0058] For controlling the easiness of the formation of the
deformation induced martensite phase in the cold rolling, the value
Md.sub.30 defined by the following expression (1) is preferably in
a range of from -50 to 0.
Md.sub.30=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo (1)
[0059] Herein, the element symbols in the expression (1) each
represent the content of the corresponding element in terms of
percentage by mass.
Non-Metallic Inclusion
[0060] The non-metallic inclusion present in the steel is roughly
classified into a soft type having a low melting point and a hard
type having a high melting point. In the steel targeted by the
invention, the former soft type is mainly a CaO--SiO.sub.2 based
material. The soft type inclusion is extended in the rolling
direction in the hot rolling since it is in a liquid state at the
hot rolling temperature, and is then collapsed and finely dispersed
in the subsequent cold rolling. The soft inclusion of this type
exerts substantially no adverse effect on the fatigue resistance
characteristics of the thin sheet material.
[0061] What becomes a problem is the latter hard inclusion. The
inclusion of this type remains as a granular material after the hot
rolling, and after the cold rolling, hard particles having been
crushed to a certain extent remain in the form of lining up in the
rolling direction. The proportion of the length in the sheet
thickness direction of the inclusion occupied in the sheet
thickness is increased with the smaller sheet thickness, and the
inclusion particles tend to function as a starting point and a
propagation path of cracks due to the stress concentration around
the inclusion particles. It has been found that the hard type
inclusion that becomes a problem in the steel targeted by the
invention is a TiN based inclusion and a spinel based inclusion
containing one or more of Al and Mg. In particular, the TiN based
inclusion has a tendency of growing associated with the decrease of
the solubility of Ti in the process where the temperature of the
molten steel is decreased in casting, and thus tends to become a
problem.
[0062] According to the investigations by the inventors, when the
number ratio of the TiN based inclusion and the spinel based
inclusion described above that have a circle equivalent diameter of
6.0 .mu.m or more is decreased in the stage of the hot rolled steel
sheet, the inclusion distribution mode that is advantageous for
improving the fatigue resistance characteristics on repeatedly
receiving elastic deformation can be obtained in the form of a thin
sheet material having a sheet thickness, for example, of from 20 to
500 .mu.m. Specifically, it is extremely advantageous to provide a
structure state in the stage of the hot rolled steel sheet, in
which the total number density of the TiN based inclusion and the
spinel based inclusion containing one or more of Al and Mg each
having a circle equivalent diameter of 6.0 .mu.m or more on the
cross section in parallel to the rolling direction and the sheet
thickness direction (L cross section) is 0.05 or less per square
millimeter.
[0063] Examples of the more preferred structure state of the hot
rolled steel sheet include a metal structure, in which in addition
to the aforementioned provision of the number density, the maximum
particle diameter of the circle equivalent diameter of the TiN
based inclusion and the spinel based inclusion containing one or
more of Al and Mg on the L cross section is 10.0 .mu.m or less. In
this case, the measurement area of the L cross section for
determining the maximum particle diameter suffices to be 200
mm.sup.2 or more.
[0064] It has been found that in a thin sheet material having a
sheet thickness of from 20 to 500 .mu.m, the number density of the
non-metallic inclusion having a length in the rolling direction of
40 .mu.m or more is particularly effectively 3.0 or less per square
millimeter on the L cross section for improving the fatigue
resistance characteristics on repeatedly receiving elastic
deformation. Herein, a group of the non-metallic inclusion
particles that line up with an interparticle distance in the
rolling direction of 20 .mu.m or less and an interparticle distance
in the sheet thickness direction of 10 .mu.m or less is assumed to
be one non-metallic inclusion, as described in the foregoing.
[0065] As for the fatigue resistance characteristics, adjacent
inclusion particles that line up closely to each other to a certain
extent function as a starting point of occurrence of cracks, as
similar to the case where the particles are present as a continuous
one group of particles. As a result of the various investigations,
a group of particles formed of plural non-metallic inclusion
particles that line up while maintaining an interparticle distance
in the rolling direction of 20 .mu.m or less and an interparticle
distance in the sheet thickness direction of 10 .mu.m or less
(which is assumed to be one non-metallic inclusion), particularly
having a length in the rolling direction of 40 .mu.m or more tends
to be a starting point of cracks on repeated application of elastic
deformation in the high strength steel as the target of the
invention. However, even with the non-metallic inclusion of this
type, the fatigue resistance characteristics can be improved by
decreasing the number density thereof to 3.0 or less per square
millimeter on the L cross section. It is estimated that the
mechanism therefor is that in the case where the density of the
non-metallic inclusion having a length in the rolling direction of
40 .mu.m or more is sufficiently decreased, the non-metallic
inclusion is prevented from exerting a function as a propagation
path of cracks.
[0066] The number density of the non-metallic inclusion having a
length in the rolling direction of 40 .mu.m or more on the L cross
section that is as low as possible is advantageous for the
enhancement of the fatigue resistance characteristics of the thin
sheet material. In the case where a steel is manufactured by using
raw materials having high purities without the use of scraps, for
example, with an experimental melting furnace, it is considered
that a thin sheet material containing an extremely small amount of
a non-metallic inclusion can be produced. However, in the case
where a steel sheet having a thickness of from 20 to 500 .mu.m is
manufactured in mass production sites, the complete prevention of
the formation of the non-metallic inclusion having a length in the
rolling direction of 40 .mu.m or more increases the load in the
steel manufacturing, which leads to increase of the cost.
Therefore, it is rational that in the thin sheet material having a
sheet thickness of from 20 to 500 .mu.m, the number density of the
non-metallic inclusion having a length in the rolling direction of
40 .mu.m or more is in a range of from 0.1 to 3.0 per square
millimeter on the L cross section.
Production Method
[0067] The stainless steel sheet having the optimized particle size
distribution of the hard non-metallic inclusion described above can
be produced by utilizing an ordinary manufacturing equipment for a
stainless steel. Representative examples thereof include a VOD
process and an AOD process. In any process, a molten steel having a
C content of 0.20% or less having a Cr oxide-containing slag on a
molten steel surface after subjecting to a decarburization process
by blowing oxygen into a Cr-containing molten iron is produced
firstly. The steel making process up to this stage may be performed
according to the ordinary method except that such a container is
selected as the container housing the molten steel that Ti and Al
are substantially or completely not incorporated from an attachment
and the like of the container.
[0068] The molten steel in this stage is the molten steel having
been subjected to decarburization by blowing oxygen, and therefore
easily oxidizable elements, Ti, Al, Mg, and Si, are substantially
oxidized and removed. That is, Ti, Al, Mg, Si are substantially not
present in the molten steel. Furthermore, a part of Cr, which is
contained in a large amount in the molten steel, is oxidized to
form a slag as a Cr oxide on the molten steel surface. The slag
formed mainly of a Cr oxide contains oxides of Ti, Al, Mg, and Si
having been removed from the molten steel. The molten steel also
contains a large amount of oxygen dissolved therein having been
blown for the decarburization. Accordingly, deoxidation is
necessarily performed before casting. In the invention, Si is
necessarily contained in the steel for producing a large Si content
steel having a Si content exceeding 2.00%. Furthermore, it is
desirable to perform a process of returning Cr escaping from the
molten steel in the decarburization from the slag to the steel
(reduction and recovery of Cr). In the invention, accordingly, the
"deoxidation", the "control of the Si content", and the "reduction
and recovery of Cr" are performed at one time by adding an Fe--Si
alloy to the molten steel. Furthermore, the component control is
performed by adding other auxiliary raw materials depending on
necessity.
[0069] By controlling the Si content to more than 2.00% by adding
the Fe--Si alloy to the molten steel, the deoxidation in the steel
proceeds with the Si source added in a large amount. The oxygen
concentration in the steel by the deoxidation is determined through
the chemical equilibrium of the chemical reaction of the expression
(2) below.
Si(in metal)+2O(in metal)=SiO.sub.2(in slag) (2)
[0070] The equilibrium constant K is shown by the expression (3)
below.
K=A(SiO.sub.2)/A(Si)/A(O).sup.2 (3)
[0071] In the expression, A(X) represents the activity of the
component X. As understood from the expression (3), when the Si
activity (i.e., the Si concentration) in the molten steel is
larger, the oxygen activity (i.e., the oxygen concentration) in the
molten steel shows an equilibrium at a lower level. Therefore, in
the molten steel having a large amount of the Si source added
targeted by the invention, the oxygen concentration in the molten
steel becomes smaller than a small Si content steel (for example,
an ordinary steel, such as SUS 304).
[0072] The chemical equilibrium based on the following expression
(4) is established between the Al oxide in the slag and oxygen in
the molten steel.
2Al(in metal)+3O(in metal)=Al.sub.2O.sub.3(in slag) (4)
[0073] According to the chemical equilibrium, the equilibrium is
retained by increasing the Al concentration in the molten steel
when the oxygen concentration in the molten steel is small. The
relationship is also applied to Ti and Mg. Consequently, when the
oxygen concentration in the molten steel is small, the Al
concentration, the Ti concentration, and the Mg concentration in
the molten steel are increased.
[0074] When the Al concentration and the Mg concentration in the
molten steel are larger, the spinel based inclusion tends to be
formed and grown. When the Ti concentration in the molten steel is
larger, the TiN based inclusion tends to be formed and grown.
Therefore, for suppressing the formation and growth of the
inclusions, the decrease of the oxygen concentration in the molten
steel associated with the increase of the Si concentration in the
molten steel is necessarily suppressed as much as possible. For
suppressing the decrease of the oxygen concentration in the molten
steel, it is advantageous that the SiO.sub.2 concentration in the
slag is larger. Accordingly, the invention uses a measure for
controlling the slag basicity (i.e., the mass ratio of
CaO/SiO.sub.2) to a low level. Specifically, the amount of the
Ca-containing substance added as a slag forming flux is controlled.
The slag forming flux added may be quick lime CaO. Ca is also
supplied to the slag from CaF.sub.2, which is added as a flux
component depending on necessity. Ca that is supplied from
CaF.sub.2 is converted to the CaO amount and added to the CaO value
used for calculating the basicity. As a result of the various
investigations, it has been found that it is effective to make the
slag basicity in a range of 1.3 or more and 1.5 or less in the slag
present on the molten steel surface after completing the addition
of the Fe--Si alloy. The slag basicity is more preferably 1.3 or
more and 1.45 or less, and further preferably 1.3 or more and 1.4
or less. With a smaller slag basicity, the decrease of the oxygen
concentration in the molten steel is suppressed, and Ti, Al, and Mg
are prevented from being incorporated into the molten steel from
the slag. However, with a too small slag basicity, other
inclusions, such as Cr.sub.2O.sub.3, are formed in a large amount.
The desulfurization capability is also lowered. Therefore, it is
extremely effective to control the final slag basicity to a range
that is not lower than 1.3.
[0075] As described above, the molten steel having been subjected
to the decarburization by blowing oxygen contains substantially no
Ti, Al, and Mg, and these elements are present as oxides in the
slag formed mainly of Cr oxide. Ti, Al, and Mg in the slag include
those incorporated from the raw materials and the refractory and
those incorporated from a slag, a metal, and the like of the
previous batch attached to the facilities, such as an electric
furnace and a converter furnace. For optimizing the particle
diameter distribution of the hard inclusion in the steel sheet by
controlling the slag basicity as described above, it is necessary
that Ti, Al, and Mg are prevented from being newly incorporated
after the time when the decarburization by blowing oxygen is
completed, i.e., after the time when the Fe--Si alloy is added. In
particular, it has been confirmed that when a slag attached in the
previous charge remains in the container housing the molten steel,
a coarse hard inclusion tends to be formed due to small amounts of
Ti and Al incorporated from the attachment. For preventing the
incorporation thereof from the container housing the molten steel,
it is most preferred to use a container having a refractory
constituting the inner surface of the container that is not used
for housing a molten steel (i.e., a new ladle). It has also been
confirmed that an Fe--Si alloy that is generally used in the
production site of stainless steels contains impurities, such as Al
and Ti, and Al and Ti incorporated therefrom become a factor
forming a coarse hard inclusion. Therefore, in the invention, it is
necessary to use an Fe--Si alloy having a high purity.
Specifically, it is preferred to use an Fe--Si alloy having an Al
content of 0.05% by mass or less and a Ti content of 0.05% by mass
or less. It is also desirable that Ti and Al are prevented as much
as possible from being incorporated from the other auxiliary raw
materials and slag forming flux.
[0076] It is important that the container housing the molten steel,
the auxiliary raw material, and the slag forming flux used are
selected to make a Ti content in the molten steel of 0.008% by mass
or less and an Al content therein of 0.008% by mass or less
finally. In the case where the final Ti content and the final Al
content in the steel exceed the aforementioned values, it is
difficult to achieve stably the aforementioned intended particle
diameter distribution of the hard inclusion even though the slag
basicity is controlled in the aforementioned manner. While the
content of Mg is desirably controlled to 0.002% by mass or less in
the molten steel finally, it has been confirmed that when the
container housing the molten steel, the auxiliary raw material, and
the slag forming flux used are selected to make the Ti and Al
contents within the aforementioned range, the particle diameter
distribution of the spinel based inclusion is in the aforementioned
intended state even though the Mg content in the steel is not
particularly restricted, and no problem occurs.
[0077] It is more preferred that the Ti content in the molten steel
is 0.006% by mass or less, and the Al content therein is 0.006% by
mass or less.
[0078] The casting may be performed according to an ordinary
method. In general, a cast piece is obtained by a continuous
casting method. In the description herein, a steel material
obtained by casting (i.e., a material having a solidification
structure) is referred to as a cast piece. Accordingly, a steel
ingot obtained by an ingot making method is encompassed in the cast
piece referred herein for the sake of convenience.
[0079] The resulting cast piece is subjected to hot working
including at least hot rolling, so as to provide a hot rolled steel
sheet. In the case where the ingot making method is employed, after
performing blooming and hot forging, hot rolling is performed. The
heating temperature in the hot rolling may be from 1,100 to
1,250.degree. C., and the sheet thickness of the hot rolled steel
sheet may be, for example, from 2.5 to 6.0 mm. According to the
manners, the stainless steel hot rolled sheet, in which the total
number density of the TiN based inclusion and the spinel based
inclusion containing one or more of Al and Mg each having a circle
equivalent diameter of 6.0 .mu.m or more on the cross section in
parallel to the rolling direction and the sheet thickness direction
(L cross section) is 0.05 or less per square millimeter, can be
obtained.
[0080] Subsequently, the hot rolled steel sheet is subjected to
annealing, cold rolling, and an aging treatment, and thereby a thin
sheet material of the high strength stainless steel sheet can be
obtained. The step of cold rolling may be performed plural times
including an intermediate annealing step. After each of the heat
treatments, acid pickling may be performed depending on necessity.
The condition for the annealing applied to the hot rolled steel
sheet (i.e., the hot rolled sheet annealing) may be, for example,
from 1,000 to 1,100.degree. C. for from 40 to 120 seconds, the
final cold rolling reduction ratio (which is the cold rolling
reduction ratio after the final intermediate annealing in the case
where the intermediate annealing is performed) may be, for example,
from 40 to 70%, and the condition for the aging treatment may be,
for example, from 400 to 600.degree. C. for from 10 to 60 minutes.
For the purpose of a thin sheet spring material, the final sheet
thickness is, for example, preferably 150 .mu.m or less, and more
preferably less than 100 .mu.m. For example, a thin sheet material
having a thickness of from 20 to 70 .mu.m may be provided.
According to the manners, the thin sheet material of the high
strength stainless steel having a mixed structure of a deformation
induced martensite phase and an austenite phase as the matrix
(metal basis material) can be obtained. The ratio of an area ratio
M of the deformation induced martensite phase and an area ratio A
of the austenite phase in terms of M/A is generally in a range of
from 30/70 to 50/50. In the material containing Mo in an amount,
for example, of 2.00% or more, a high strength with a tensile
strength in the rolling direction of 2,000 N/mm.sup.2 can be
obtained. In the thin sheet material, assuming that a group of the
non-metallic inclusion particles that line up with an interparticle
distance in the rolling direction of 20 .mu.m or less and an
interparticle distance in the sheet thickness direction of 10 .mu.m
or less is one non-metallic inclusion, a structure state, in which
the number density of the non-metallic inclusion having a length in
the rolling direction of 40 .mu.m or more is 3.0 or less per square
millimeter on the L cross section can be obtained, and the thin
sheet material exhibits good fatigue resistance characteristics in
the purpose of a spring material, which repeatedly receives elastic
stress.
Examples
[0081] The steels shown in Table 1 were manufactured by a VOD
process. In each of the steels, the final decarburization process
by blowing oxygen into the Cr-containing molten iron was completed
in the VOD equipment, and a molten steel having a C content of
0.10% or less having a Cr oxide-containing slag on the molten steel
surface was obtained. In this stage, the C content is substantially
equal to the final C content shown in Table 1.
TABLE-US-00001 TABLE 1 Chemical composition (% by mass) Class No. C
Si Mn P S Ni Cr Mo Cu N Ti Al Md.sub.30 Conventional 1 0.080 2.64
0.25 0.020 0.001 8.32 13.42 2.22 0.10 0.068 0.023 0.010 -12.8
Example 2 0.086 2.74 0.23 0.020 0.001 8.25 13.43 2.22 0.10 0.066
0.018 0.010 -13.5 3 0.086 2.65 0.42 0.029 0.001 8.34 13.57 2.23
0.18 0.066 0.010 0.009 -21.3 4 0.072 2.60 0.28 0.030 0.001 8.28
13.71 2.14 0.07 0.073 0.010 0.006 -11.7 Invention 5 0.086 2.73 0.22
0.028 0.001 8.32 13.73 2.23 0.28 0.069 0.004 0.004 -26.3
Example
[0082] In the final decarburization in the VOD equipment, the
container housing the molten steel used was a ladle, and the same
ladle was used in the process up to casting. As the ladle, a ladle
that had been used for manufacturing a Ti-containing stainless
steel as the previous charge was used for Conventional Examples
Nos. 1 and 2, a ladle that had been used for manufacturing a
Ti-free stainless steel as the previous charge was used for
Conventional Examples Nos. 3 and 4, and a ladle having a refractory
constituting the inner surface of the ladle that was not used for
housing a molten steel (i.e., a new ladle) was used for Invention
Example No. 5.
[0083] An Fe--Si alloy was added to the molten steel having been
completed for the final decarburization, so as to control the Si
content in the molten steel to the target value, and to perform
deoxidation, and reduction and recovery of Cr in the slag. In this
stage, the Si content in the molten steel is substantially equal to
the final Si content shown in Table 1. As the Fe--Si alloy, a
material corresponding to the ferrosilicon No. 2 defined in JIS
G2302:1998 was used for Conventional Examples Nos. 1 to 4. As a
result of the analysis, the material corresponding to the
ferrosilicon No. 2 contained approximately 1.0% by mass of Al,
approximately 0.07% by mass of Mg, and approximately 0.08% by mass
of Ti, while there was a certain fluctuation depending on the
product lot. On the other hand, a high quality Fe--Si alloy having
an Al content having been extremely decreased was used for
Invention Example No. 5. As a result of the analysis, the Al, Mg,
and Ti contents of the high quality Fe--Si alloy were 0.009% by
mass for Al, less than 0.001% by mass for Mg, and 0.012% by mass
for Ti.
[0084] Subsequent to the addition of the Fe--Si alloy, industrial
quick lime (CaO) was added as a slag forming flux to the slag.
Thereafter, the slag was collected and subjected to compositional
analysis. As a result, the slag basicity was from 1.60 to 1.65 for
Conventional Examples Nos. 1 to 4, and 1.33 for Invention Example
No. 5.
[0085] In each of the examples, the molten steel thus obtained
above was continuously cast and subjected to hot rolling, so as to
provide a hot rolled steel sheet having a sheet thickness of 3.8
mm. The heating temperature in the hot rolling was 1,230.degree. C.
For the resulting hot rolled steel sheet, the L cross section was
observed with an SEM, and according to the aforementioned section
"Measurement Method for Number Density of Inclusion in Hot Rolled
Steel Sheet", the total number density of the TiN based inclusion
and the spinel based inclusion containing one or more of Al and Mg
each having a circle equivalent diameter of 6.0 .mu.m or more was
measured. As a result, the total number density was from 0.20 to
0.45 per square millimeter for Conventional Examples Nos. 1 to 4,
and 0.02 per square millimeter for Invention Example No. 5. It was
understood that the formation of a coarse hard inclusion was
significantly suppressed by the manufacturing method according to
the invention.
[0086] FIG. 3 exemplifies the SEM micrograph of the typical TiN
based inclusion observed on the L cross section of the hot rolled
steel sheet of Conventional Example No. 4, and FIG. 4 exemplifies
the SEM micrograph of the typical TiN based inclusion observed on
the L cross section of the hot rolled steel sheet of Invention
Example No. 5. In all the micrographs, the horizontal direction
agrees with the rolling direction. The cross-hair cursor appearing
in the micrographs shows the beam irradiation position in the EDX
analysis.
[0087] Subsequently, a specimen collected from the hot rolled steel
sheet was subjected to hot rolled sheet annealing at 1,050.degree.
C. for 60 seconds, cold rolling, intermediate annealing at
1,050.degree. C. for 60 seconds, cold rolling, and an aging
treatment at 500.degree. C. for 30 minutes, so as to produce a thin
sheet material having a sheet thickness of 120 .mu.m having a mixed
structure of a deformation induced martensite phase and an
austenite phase as the matrix (metal basis material). In all the
resulting thin sheet materials, the tensile strength in the rolling
direction exceeded 2,000 N/mm.sup.2.
[0088] The L cross section of the thin sheet material was measured
for the number density of the non-metallic inclusion having a
length in the rolling direction of 40 .mu.m or more according to
the aforementioned section "Measurement Method for Number Density
of Non-metallic Inclusion having Length in Rolling Direction of 40
.mu.m or more". As described above, a group of the non-metallic
inclusion particles that lined up with an interparticle distance in
the rolling direction of 20 .mu.m or less and an interparticle
distance in the sheet thickness direction of 10 .mu.m was assumed
to be one non-metallic inclusion. As a result of the measurement,
the number density of the non-metallic inclusion having a length in
the rolling direction of 40 .mu.m or more on the L cross section
was from 8.2 to 33.2 per square millimeter for Conventional
Examples Nos. 1 to 4, and 2.4 per square millimeter for Invention
Example No. 5. As a result of the EDX analysis, all the
non-metallic inclusions counted were constituted by TiN based
inclusion particles or spinel based inclusion particles containing
one or more of Al and Mg.
[0089] In Invention Example, the number of the hard non-metallic
inclusion having a length in the rolling direction of 40 .mu.m or
more, which was a factor deteriorating the fatigue resistance
characteristics, was largely decreased as compared to Conventional
Examples.
[0090] For reference, a production example for the case where SUS
304 (having a Si content of 0.55%) is subjected to deoxidation with
an Fe--Si alloy is shown. A ladle that had been used for
manufacturing a Ti-containing stainless steel as the previous batch
was used as the container housing the molten steel, and the final
decarburization process by blowing oxygen into the Cr-containing
molten iron was completed in the VOD equipment, thereby providing a
molten steel having a C content of approximately 0.05% having a Cr
oxide-containing slag on the molten steel surface. The Fe--Si alloy
corresponding to the ferrosilicon No. 2 was added to the molten
steel, so as to control the Si content. Industrial quick lime (CaO)
was added as a slag forming flux. Thereafter, chromium nitride was
added to complete the component control. As a result of the
analysis of the final slag collected, the slag basicity was 1.65.
The molten steel was continuously cast and subjected to hot rolling
in the ordinary method, so as to provide a hot rolled steel sheet
having a sheet thickness of 3.5 mm. The hot rolled steel sheet was
investigated for the state of the presence of the non-metallic
inclusion in the same manner as in Nos. 1 to 5. As a result, a TiN
based inclusion and a spinel based inclusion containing one or more
of Al and Mg each having a circle equivalent diameter of 6.0 .mu.m
or more were not found. It is understood from the comparison
between the example of SUS 304 and Conventional Examples Nos. 1 to
4 that it is considerably difficult to suppress the formation of a
hard inclusion in a stainless steel species having a large Si
content.
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