U.S. patent application number 13/468255 was filed with the patent office on 2012-09-20 for austenitic stainless steel sheet and a method for its manufacture.
This patent application is currently assigned to SUMITOMO METAL INDUSTRIES, LTD.. Invention is credited to Masayoshi SAWADA.
Application Number | 20120237388 13/468255 |
Document ID | / |
Family ID | 44059630 |
Filed Date | 2012-09-20 |
United States Patent
Application |
20120237388 |
Kind Code |
A1 |
SAWADA; Masayoshi |
September 20, 2012 |
AUSTENITIC STAINLESS STEEL SHEET AND A METHOD FOR ITS
MANUFACTURE
Abstract
An austenitic stainless steel sheet for springs having both a
high strength and excellent formability has a chemical composition
comprising C: 0.01-0.15%, Si: at most 3.0%, Mn: at most 3.0%, Cr:
10.0-30.0%, Ni: 4.0-20.0%, N: at most 0.40%, and a remainder of Fe
and impurities, and it has a metallurgical structure such that the
austenite content .gamma.s (%) in the surface region of the steel
sheet and the austenite content .gamma.c (%) in the center region
of the sheet thickness satisfy (.gamma.s+.gamma.c)/2.ltoreq.55 and
.gamma.s/.gamma.c.gtoreq.0.10, with the remaining structure being
primarily strain-induced martensite.
Inventors: |
SAWADA; Masayoshi; (Osaka,
JP) |
Assignee: |
SUMITOMO METAL INDUSTRIES,
LTD.
Osaka
JP
|
Family ID: |
44059630 |
Appl. No.: |
13/468255 |
Filed: |
May 10, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2010/070350 |
Nov 16, 2010 |
|
|
|
13468255 |
|
|
|
|
Current U.S.
Class: |
420/49 ; 420/57;
420/61; 420/67; 72/200 |
Current CPC
Class: |
C22C 38/02 20130101;
C21D 6/004 20130101; C22C 38/42 20130101; C22C 38/44 20130101; C21D
9/46 20130101; C21D 2211/008 20130101; C22C 38/04 20130101; C22C
38/40 20130101; C21D 2211/001 20130101; C22C 38/001 20130101 |
Class at
Publication: |
420/49 ; 420/57;
420/61; 420/67; 72/200 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/44 20060101 C22C038/44; B21B 1/00 20060101
B21B001/00; C22C 38/42 20060101 C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 18, 2009 |
JP |
2009-263066 |
Claims
1. An austenitic stainless steel sheet characterized by having a
chemical composition comprising, in mass %, C: 0.01-0.15%, Si: at
most 3.0%, Mn: at most 3.0%, Cr: 10.0-30.0%, Ni: 4.0-20.0%, N: at
most 0.40%, and a remainder of Fe and impurities, and by having a
metallurgical structure in which the austenite content .gamma.s (%)
in a surface region of the steel sheet and the austenite content
.gamma.c (%) in a center region of the sheet thickness satisfy
(.gamma.s+.gamma.c)/2.ltoreq.55 and .gamma.s/.gamma.c.gtoreq.0.10,
with the remaining structure being primarily strain-induced
martensite.
2. An austenitic stainless steel sheet as set forth in claim 1
wherein the chemical composition contains, in mass %, at least one
of Mo: at most 3.0% and Cu: at most 3.0% in place of a portion of
Fe.
3. An austenitic stainless steel sheet as set forth in claim 1
wherein the chemical composition contains, in mass %, at least one
of Ti: at most 0.50%, Nb: at most 0.50%, and V: at most 1.0% in
place of a portion of Fe.
4. A method for manufacturing an austenitic stainless steel sheet
as set forth in claim 1 characterized by performing hot rolling of
a steel having the above-described chemical composition, then
performing cold rolling and annealing of the resulting hot rolled
steel sheet to obtain an annealed cold rolled steel sheet, and
subjecting the annealed cold rolled steel sheet to temper rolling
with the number of passes being at least the reduction (%)/10.
5. A method as set forth in claim 4 wherein the average grain
diameter of the austenite grains of the annealed cold rolled steel
sheet before temper rolling is at most 5 .mu.m.
6. An austenitic stainless steel sheet as set forth in claim 2
wherein the chemical composition contains, in mass %, at least one
of Ti: at most 0.50%, Nb: at most 0.50%, and V: at most 1.0% in
place of a portion of Fe.
7. A method for manufacturing an austenitic stainless steel sheet
as set forth in claim 2 characterized by performing hot rolling of
a steel having the above-described chemical composition, then
performing cold rolling and annealing of the resulting hot rolled
steel sheet to obtain an annealed cold rolled steel sheet, and
subjecting the annealed cold rolled steel sheet to temper rolling
with the number of passes being at least the reduction (%)/10.
8. A method for manufacturing an austenitic stainless steel sheet
as set forth in claim 3 characterized by performing hot rolling of
a steel having the above-described chemical composition, then
performing cold rolling and annealing of the resulting hot rolled
steel sheet to obtain an annealed cold rolled steel sheet, and
subjecting the annealed cold rolled steel sheet to temper rolling
with the number of passes being at least the reduction (%)/10.
9. A method for manufacturing an austenitic stainless steel sheet
as set forth in claim 6 characterized by performing hot rolling of
a steel having the above-described chemical composition, then
performing cold rolling and annealing of the resulting hot rolled
steel sheet to obtain an annealed cold rolled steel sheet, and
subjecting the annealed cold rolled steel sheet to temper rolling
with the number of passes being at least the reduction (%)/10.
Description
TECHNICAL FIELD
[0001] This invention relates to an austenitic stainless steel
sheet and a method for its manufacture. More specifically, it
relates to an austenitic stainless steel sheet for use as a spring
material having both a high strength and excellent formability and
to a method for its manufacture.
BACKGROUND ART
[0002] Spring materials which are used in flat springs, spiral
springs, frames, Belleville (coned disk) springs, dome switches,
and the like for electronic equipment, nuclear power facilities,
automobile parts and the like require having a high strength in
order to reduce the thickness of the material and having excellent
formability in order to fabricate it into a desired product
shape.
Materials which have thus far been used for these applications have
been SUS 301 (AISI 301) stainless steels which are classified as
metastable austenitic stainless steel. When a SUS 301 stainless
steel undergoes cold working, deformed portions transform into hard
strain-induced martensite, thereby making it possible to obtain a
high strength relatively easily. In addition, due to suppression of
local deformation by the TRIP (transformation induced plasticity)
effect, excellent formability can also be obtained. Such a spring
material is disclosed in below-identified Patent Documents 1-3, for
example.
[0003] Patent Document 1 discloses a stainless steel which has
excellent formability and which contains C: at most 0.03% (in this
description, unless otherwise specified, percent with respect to
chemical composition means mass percent), Si: at most 1.0%, Mn: at
most 2.5%, Ni: 4.0-10.0%, Cr: 13.0-20.0%, N: 0.06-0.30%, S: at most
0.01%, and O: at most 0.007%, with the value of
M=330-(480.times.C)-(2.times.Si)-(10.times.Mn)-(14.times.Ni)-(5.7.times.C-
r)-(320.times.N) being at least 30.
[0004] Patent Document 2 discloses a stainless steel which has
excellent spring properties and excellent fatigue properties in
deformed portions and which contains C: at most 0.08%, Si: at most
3.0%, Mn: at most 4.0%, Ni: 4.0-10.0%, Cr: 13.0-20.0%, N:
0.06-0.30%, and O: at most 0.007%, with the value of
above-described M being at least 40.
[0005] Patent Document 3 discloses a stainless steel which has
excellent formability and fatigue properties and which contains C:
at most 0.03%, Si: greater than 1.0% to at most 3.0%, Mn: at most
4.0%, Ni: 4.0-10.0%, Cr: 13.0-20.0%, N: at most 0.30%, S: at most
0.01%, and O: at most 0.007%, with the value of above-described M
being in the range of 30-100.
[0006] All of the stainless steels disclosed in Patent Documents
1-3 are manufactured by carrying out cold rolling with a rolling
reduction of at least 50% after hot rolling, then carrying out
finish annealing at a relatively low temperature for a relatively
short time so as to produce refined, uniform recrystallized grains
with an average grain diameter of at most 10 .mu.m, and finally
performing temper rolling. Namely, in each of these stainless
steels, it is attempted to obtain desired strength properties by
utilizing grain refinement, which is a strengthening mechanism
accompanied by little deterioration in formability. However, in
recent years, the strength demanded of spring materials is
increasing, and the stainless is steels disclosed in Patent
Documents 1-3 sometimes do not have sufficient strength demanded of
products.
[0007] Below-identified Patent Document 4 discloses a high strength
spring material based on a low-C, high-N SUS 301L steel and
specifically a stainless steel having a mixed phase structure
comprising at least 40% by area of martensite and a remainder of
austenite or a single-phase martensitic structure which is obtained
by temper rolling with a reduction of at least 30% of a stainless
steel having a chemical composition containing C: at most 0.03%,
Si: at most 1.0%, Mn: at most 2.0%, Cr: 16.0-18.0%, Ni: 6.0-8.0%,
N: at most 0.25%, and Nb: 0-0.30% and a structure in which the
percent area of recrystallized grains having an average grain
diameter of at most 5 .mu.m is at least 50% and less than 100% and
the percent area of unrecrystallized portions is greater than 0%
and at most 50%.
[0008] With the stainless steel disclosed in Patent Document 4,
after a metallurgical structure including a strain-induced
martensite is formed by temper rolling, the steel is formed into a
predetermined shape and then subjected to aging treatment, thereby
causing fine chromium nitrides to precipitate in martensite. By
utilizing the precipitation strengthening at this time, it is
possible to achieve a high strength without adding an additional
step.
PRIOR ART DOCUMENTS
Patent Documents
[0009] Patent Document 1: JP 04-214841 A [0010] Patent Document 2:
JP 05-279802 A [0011] Patent Document 3: JP 05-117813 A [0012]
Patent Document 4: JP 4,321,066 B
SUMMARY OF THE INVENTION
[0013] In order to decrease the size and improve the performance of
products, there is today a demand for spring materials having a
higher strength and better formability. Therefore, even the
stainless steel disclosed in Patent Document 4 is sometimes unable
to fully satisfy the performance demanded of the newest
products.
[0014] According to one aspect, the present invention is an
austenitic stainless steel sheet characterized by having a chemical
composition comprising C: 0.01-0.15%, Si: at most 3.0%, Mn: at most
3.0%, Cr: 10.0-30.0%, Ni: 4.0-20.0%, N: at most 0.40%, and a
remainder of Fe and impurities, and by having a metallurgical
structure in which the austenite content .gamma.s (%) in a surface
region of the steel sheet and the austenite content .gamma.c (%) in
a center region of the thickness of the steel sheet satisfy
(.gamma.s+.gamma.c)/2.ltoreq.55 and .gamma.s/.gamma.c.gtoreq.0.10,
with the remaining structure being primarily strain-induced
martensite.
[0015] The austenite content .gamma.s (%) in a surface region of
the steel sheet means the volume percent of austenite contained in
a region from the outermost surface of the steel sheet to a depth
of 10 .mu.m in the sheet thickness direction (this region being
referred to as the surface region of the steel sheet). The
austenite content .gamma.c (%) in a center region of the sheet
thickness means the volume percent of austenite contained in a
region at a depth of 10 .mu.m in the sheet thickness direction from
a surface which is formed by removing one-half of the original
sheet thickness by mechanical grinding and chemical polishing of
one side of the steel sheet (this region being referred to as the
center region of the steel sheet).
[0016] The chemical composition of an austenitic stainless steel
shv eet according to the present invention may further contain, in
place of a portion of Fe, [0017] (1) at least one of Mo: at most
3.0% and Cu: at most 3.0%, and/or [0018] (2) at least one of Ti: at
most 0.50%, Nb: at most 0.50%, and V: at most 1.0%.
[0019] From another aspect, the present invention is a method of
manufacturing an austenitic stainless steel sheet characterized by
hot rolling a steel having the above-described chemical
composition, then carrying out cold rolling and annealing on the
resulting hot rolled steel sheet to obtain an annealed cold rolled
steel sheet, and then subjecting the annealed cold rolled steel
sheet to temper rolling with the number of passes being at least
the rolling reduction (%)/10.
[0020] In the above-described method, the average grain diameter of
austenite grains in the annealed cold rolled steel sheet before
temper rolling is preferably at most 5 .mu.m.
[0021] The present invention provides an austenitic stainless steel
sheet having both a high strength and excellent formability and a
method for its manufacture.
BRIEF EXPLANATION OF THE DRAWINGS
[0022] FIG. 1 is an explanatory view showing an example of
processing steps of an austenitic stainless steel sheet according
to the present invention after temper rolling;
[0023] FIG. 2 is an explanatory view showing an example of the
relationship between the distribution in the sheet thickness
direction of the austenite content after temper rolling and
formability.
[0024] FIG. 3 is an explanatory view showing a method of evaluating
formability.
MODES FOR CARRYING OUT THE INVENTION
[0025] The present invention will be explained in greater detail
while referring to the attached drawings.
[0026] An austenitic stainless steel sheet according to the present
invention is a cold rolled steel sheet which has undergone temper
rolling. As shown in FIG. 1, the austenitic stainless steel sheet
is subjected, after temper rolling, to forming in order to impart a
desired shape and then, if necessary, to aging treatment, when
products (such as various types of springs) are fabricated
therefrom.
[0027] The present inventors came up with the idea that a
strain-induced martensitic structure formed by temper rolling could
be strengthened by increasing the content of C, which is a solid
solution strengthening element, thereby making it possible to
achieve an increase in strength of steel. The above-described
problem of insufficient strength can be solved by the combination
of strengthening of martensite phases by increasing the C content
and precipitation strengthening utilizing Cr.sub.2N.
[0028] To this end, if the amount of strain-induced martensite is
low and a large amount of austenite remains, a desired strength
cannot be obtained. Therefore, the average of the austenite content
.gamma.s (%) in the surface region of the steel sheet and the
austenite content .gamma.c (%) in the center region of the sheet
thickness, namely, (.gamma.s+.gamma.c)/2 (below, this value will be
referred to as the average austenite content) is made at most 55.
".gamma.s" and ".gamma.c" are defined as set forth above.
[0029] The present inventors also came up with the idea that
controlling the distribution of the austenite content in the sheet
thickness direction is effective at improving the formability of a
steel sheet, which decreases as strength increases. FIG. 2 is an
explanatory view showing an example of the relationship between the
distribution in the sheet thickness direction of the austenite
content after temper rolling and formability.
[0030] As shown in FIG. 2, even when the average austenite content
after temper rolling is the same, it is possible to greatly improve
the formability of the temper-rolled steel sheet by varying the
distribution in the sheet thickness direction of the austenite
content after temper rolling. Specifically, by increasing the
austenite content remaining in the surface region of a steel sheet
after temper rolling, strain-induced martensitic transformation
takes place sufficiently to adequately induce the TRIP effect in
the surface region of the steel sheet, which is most deformed in
subsequent forming operations. As a result, excellent formability
is obtained.
[0031] In addition, if this austenitic stainless steel sheet is
then treated by aging, fine Cr.sub.2N precipitates mainly in the
martensite phases having a low solubility for N during aging
treatment, thereby making it possible to also utilize the effect of
age strengthening. In this manner, an austenitic stainless steel
sheet according to the present invention can exhibit both a high
strength and excellent formability.
[0032] The heat generated by working within a steel sheet during
temper rolling increases as the amount of reduction per pass in
temper rolling increases. Therefore, the temperature of the surface
of a steel sheet, which is cooled with a rolling oil, becomes
markedly lower than the temperature of the center region of the
thickness of the sheet, and during the next pass of rolling, the
amount of martensite which forms in the surface region of the steel
sheet markedly increases, leading to a significant decrease in the
austenite content in the surface region of the steel sheet.
[0033] Namely, if temper rolling is carried out with a small number
of passes as in a conventional manner, the austenite content
remaining in the surface region of the steel sheet becomes markedly
smaller than the austenite content remaining in the center region
of the thickness of the sheet. During subsequent forming, the TRIP
effect resulting from strain-induced martensitic transformation of
austenite is not adequately obtained, and formability
decreases.
[0034] In contrast, if the number of passes in temper rolling is
increased and the rolling reduction per pass is decreased to
suppress the generation of heat during working, the amount of
austenite remaining in the surface region of the steel sheet after
temper rolling can be increased. As a result, a distribution of the
austenite content in the sheet thickness direction which is
desirable for subsequent forming can be achieved.
[0035] Specifically, if the distribution of the austenite content
in the sheet thickness direction of a steel sheet after temper
rolling is such that the austenite content .gamma.s (%) in the
surface region of the steel sheet and the austenite content
.gamma.c (%) in the center region of the sheet thickness satisfy
the condition .gamma.s/.gamma.c.gtoreq.0.10, during the subsequent
forming operations, the TRIP effect is sufficiently exhibited in
the surface region of the steel sheet, which is most greatly
deformed in the forming operations.
[0036] Even if the number of passes in temper rolling is increased
so as to decrease the rolling reduction in each pass, the
temperature of the surface region of the steel sheet becomes lower
than that of the center region of the sheet thickness. As a result,
it cannot be avoided that a larger amount of martensite is formed
in the surface region of the steel sheet than in the center region
of the sheet thickness, thereby making the austenite content in the
surface region lower than that in the center region. However, it
was found that sufficient formability for actual use is guaranteed
if the austenite content in the surface region of the steel sheet
is at least 1/10 of the austenite content in the center region of
the sheet thickness.
[0037] As explained above, the present invention is based on the
technical concept of "achieving a great increase in strength by the
combination of strengthening of martensite phases by an increased C
content and precipitation strengthening with Cr.sub.2N, and at the
same time achieving excellent formability by optimizing the
distribution of the austenite content in the sheet thickness
direction, whereby an austenitic stainless steel sheet can be
obtained which satisfies the demand for a material which can form
spring parts having decreased size and weight."
[0038] Next, the chemical composition, the metallurgical structure,
and a manufacturing method for an austenitic stainless steel sheet
according to the present invention will be explained.
(1) Chemical Composition
[0039] C: 0.01-0.15%
[0040] C is a solid solution strengthening element, and it is
extremely effective at strengthening a martensite phase which is
formed during cold working. Therefore, is the C content is made at
least 0.01%. However, if the C content is excessive, coarse
carbides are formed during manufacturing process of steel sheets,
and formability and corrosion resistance deteriorate, so the C
content is made at most 0.15%. The C content is preferably at least
0.03%.
[0041] Si: at most 3.0%
[0042] Si is a solid solution strengthening element, thereby
imparting a high strength to steel, and it is also used as a
deoxidizing agent at the time of melt refining. However, if the Si
content is excessive, coarse Si compounds are formed during
manufacturing process of steel sheets, and these coarse Si
compounds lead to a deterioration in hot workability and cold
workability. Therefore, the Si content is at most 3.0% and
preferably at most 2.8%.
[0043] Mn: at most 3.0%
[0044] Mn is used as a deoxidizing agent at the time of melt
refining. In addition, Mn is an austenite stabilizing element, and
it is contained in a suitable amount taking into consideration the
balance with other elements. However, if the Mn content is
excessive, coarse Mn compounds are formed during manufacturing
process of steel sheets, and the coarse Mn compounds become the
starting points of fractures, causing formability to deteriorate.
Therefore, the Mn content is at most 3.0% and preferably at most
2.8%.
[0045] Cr: 10.0-30.0%
[0046] Cr is a basic element in stainless steel. When its content
is at least 10.0%, it forms a passive film on the surface of steel
and provides the effect of increasing corrosion resistance.
Furthermore, when steel is subjected to aging treatment, it
contributes to an increase in the strength of steel by
precipitating as fine Cr.sub.2N. However, Cr is a ferrite-forming
element, so if the Cr content is excessive, delta (.delta.) ferrite
forms at high temperatures and the hot workability of steel is
markedly decreased. Therefore, the Cr content is at least 10.0% and
at most 30.0%, and preferably at least 12.0% and at most 25.0%.
[0047] Ni: 4.0-20.0%
[0048] Ni is a basic element in an austenitic stainless steel. At
least 4.0% of Ni is contained in order to stably obtain an
austenite phase having an excellent balance between strength and
ductility at room temperature. However, if the Ni content is
excessive, an austenite phase becomes too stable and the occurrence
of strain-induced martensitic transformation is suppressed, and a
high strength cannot be obtained. Therefore, the Ni content is at
least 4.0% and at most 20.0%, and preferably at least 4.5% and at
most 18.0%.
[0049] N: at most 0.40%
[0050] Like C, N is a solid solution strengthening element which
contributes to increasing the strength of steel. Furthermore, when
steel undergoes aging treatment, it contributes to increasing the
strength of steel by precipitating as fine Cr.sub.2N. However, if
the N content is too high, the occurrence of edge cracks is easily
induced at the time of hot working. Therefore, the N content is at
most 0.40% and is preferably at least 0.05% and at most 0.30%.
[0051] An austenitic stainless steel sheet according to the present
invention may further contain the following optional elements as
necessary.
[0052] One or both of Mo: at most 3.0% and Cu: at most 3.0%
[0053] Mo and Cu are both elements which precipitate as fine
intermetallic compounds during aging treatment and thereby
contribute to increasing the strength of a steel sheet, so they may
be added if necessary. However, if the Mo content or the Cu content
is excessive, delta (.delta.) ferrite may easily form at high
temperatures and precipitate at grain boundaries, resulting in a
marked deterioration in hot workability. Therefore, the Mo content
and the Cu content are each at most 3.0% and preferably each at
most 2.8%.
[0054] At least one of Ti: at most 0.5%, Nb: at most 0.5%, and V:
at most 1.0%
[0055] Ti, Nb, and V each precipitate as fine carbides or nitrides
during manufacturing process of steel sheets, thereby suppressing
the growth of crystal grains by the pinning effect and contributing
to increasing the strength of a steel sheet by precipitation
strengthening. Therefore, they may be contained if necessary.
However, if the content of these elements is excessive, they form
coarse carbides or nitrides which become starting points of
fracture at the time of deformation and markedly worsen
formability. Therefore, the Ti content and the Nb content are made
at most 0.5%, and the V content is made at most 1.0%. Preferably,
the Ti content and the Nb content are at most 0.4%, and the V
content is at most 0.8%.
[0056] The remainder other than the above-described elements is Fe
and impurities. Examples of typical impurities are P: at most 0.05%
and S: at most 0.03%.
(2) Metallurgical Structure
[0057] [Austenite Distribution in the Sheet Thickness
Direction]
[0058] As a result of carrying out a variety of tests, the present
inventors found that when the austenite content .gamma.s (%) in the
surface region of a steel sheet and the austenite content .gamma.c
(%) in the center region of the sheet thickness of the steel sheet
satisfy the following Equations (1) and (2) and the remainder of
the structure is constituted primarily by a strain-induced
martensitic structure, an austenitic stainless steel sheet having
both a high strength and formability is obtained:
(.gamma.s+.gamma.c)/2.ltoreq.55 Equation (1)
.gamma.s/.gamma.c.gtoreq.0.10. Equation (2)
[0059] By making the average austenite content, which is the
average of the austenite content .gamma.s in the surface region of
the steel sheet and the austenite content .gamma.c in the center
region of the sheet thickness, at most 55% as shown by Equation (1)
and making the remainder primarily high strength strain-induced
martensite, a high strength steel is obtained. The average
austenite content is preferably at most 50%, more preferably at
most 45%, still more preferably at most 40%, and most preferably at
most 35%. There is no particular lower limit on the average
austenite content, but if the austenite content is extremely small,
a sufficient TRIP effect may not be obtained in the surface of the
steel sheet at the time of forming, so it is preferably at least 5%
and more preferably at least 7.5%.
[0060] By making the ratio (.gamma.s/.gamma.c) of the austenite
content .gamma.s in the surface region of the steel sheet to the
austenite content .gamma.c in the center region of the sheet
thickness at least 0.10 as shown by Equation (2), the TRIP effect
resulting from the strain-induced martensitic transformation of
austenite is adequately exhibited even in the surface of a steel
sheet, which undergoes the greatest deformation at the time of
forming of a sheet, and excellent formability is obtained. The
ratio .gamma.s/.gamma.c is preferably at least 0.2, more preferably
at least 0.3, still more preferably at least 0.5, and most
preferably at least 0.6.
[0061] In the present invention, a high strength and excellent
formability can both be achieved by having the austenite content in
the surface region of the steel sheet and the austenite content in
the center region of the sheet thickness satisfy Equation (1) and
Equation (2).
[0062] The remainder of the metallurgical structure other than
austenite is primarily a strain-induced martensite phase. This
strain-induced martensite is formed by temper rolling of a steel
sheet which has been annealed after cold rolling. Therefore, an
austenitic stainless steel sheet according to the present invention
is a temper rolled material.
[0063] "Primarily a strain-induced martensite phase" means that
strain-induced martensite is at least 50 volume % of the remainder
of the structure other than austenite. In an austenitic stainless
steel sheet which is manufactured in accordance with the
below-described method according to the present invention, the
metallurgical structure consists substantially of austenite and
strain-induced martensite. Examples of other phases are fine
precipitates (carbides, nitrides, and carbonitrides), but the
amount of these phases is extremely small. A single-phase
martensitic structure in which .gamma.s=.gamma.c=100% is outside
the scope of the present invention.
[0064] As stated above, the austenite content is larger in the
center region of the sheet thickness than in the surface region of
the steel sheet. Therefore, even in an austenitic stainless steel
sheet according to the present invention having an increased
austenite content in the surface region of the steel sheet, the
relationship .gamma.s<.gamma.c (namely .gamma.s/.gamma.c<1)
is satisfied.
[0065] [Grain diameter of austenite grains before temper rolling:
at most 5 .mu.m]
[0066] Refinement of crystal grains is known as a method of
strengthening steel with little deterioration in ductility. This is
also an effective strengthening method in the stainless steel which
is the object of the present invention. In addition, by decreasing
the grain diameter and increasing the density of grain boundaries,
strains which concentrate at grain boundaries at the time of
forming are dispersed, thereby achieving the effect of suppressing
the occurrence of cracks. In the present invention, the grain
diameter of austenite grains in a steel sheet before temper rolling
(an annealed cold rolled material) is preferably at most 5
.mu.m.
(3) Manufacturing Method
[0067] In accordance with the present invention, the
above-described austenitic stainless steel sheet according to the
present invention can be manufactured by carrying out hot rolling
of a steel material having the above-described chemical is
composition, then subjecting the resulting hot rolled steel sheet
to cold rolling and annealing to obtain an annealed cold rolled
steel sheet, and subjecting the annealed cold rolled steel sheet to
temper rolling with the number of passes being at least the rolling
reduction (%)/10.
[0068] Hot rolling, cold rolling, and annealing may each be carried
out in a conventional manner. Cold rolling is preferably performed
one to three times so that the overall rolling reduction is around
30-90%, and after a predetermined overall reduction has been
obtained, annealing is then carried out. It is also possible to
repeat multiple passes of cold rolling and annealing. There is no
particular limit on the number of passes of cold rolling which are
carried out prior to temper rolling.
[0069] It is preferable to make the overall reduction in cold
rolling large enough to obtain a refined metallurgical structure in
which the average grain diameter of austenite grains in the
annealed cold rolled steel sheet which is subjected to the
subsequent temper rolling is at most 5 .mu.m, because particularly
formability is improved thereby.
[Temper Rolling Conditions]
[0070] In the present invention, temper rolling is carried out in a
strong manner in order to make the most use of strengthening
achieved by strain-induced martensitic transformation. The overall
reduction of temper rolling is preferably at least 40%, more
preferably at least 50%, and most preferably at least 60%. There is
no particular upper limit on the overall reduction of temper
rolling, but normally it is less than 100% and preferably at most
90%.
[0071] If such strong temper rolling is carried out with a small
number of passes, as stated above, strain-induced martensitic
transformation is promoted in the surface region of the steel
sheet, and the austenite content in this region is so decreased
that it is no longer possible to satisfy the condition that the
ratio of the austenite content .gamma.s in the surface region of
the steel sheet to the austenite content .gamma.c in the center
region of the sheet thickness (.gamma.c/.gamma.s) is at least 0.1,
leading to a deterioration in formability.
[0072] As a result of investigating the relationship between the
number of passes in temper rolling and the austenite distribution
in the thickness direction of a steel sheet, the present inventors
confirmed that by carrying out temper rolling such that is the
number of passes is at least the overall reduction (%)/10 as shown
by Equation (3), the ratio .gamma.c/.gamma.s becomes at least 0.10.
Therefore, temper rolling is carried out such that the number of
passes is at least the overall reduction in temper rolling (%)
divided by ten (10). For example, when the overall reduction in
temper rolling is 65%, the number of passes is made at least 7.
Number of passes in temper rolling.gtoreq.Overall reduction in
temper rolling(%)divided by 10. Equation (3)
[0073] Preferably, the reduction in each pass of temper rolling is
nearly the same. Accordingly, the reduction in each pass of temper
rolling is preferably made at most 10%. Since excessively
increasing the number of passes worsens productivity, the number of
passes is preferably in the range from the smallest number of
passes satisfying the total reduction (%)/10 to 2 passes larger
than the smallest number.
Example 1
[0074] Table 1 shows the chemical compositions of stainless steels
used in this example. Steels A-F are inventive steels which satisfy
the composition defined by the present invention, and steels G-M
are comparative steels which do not satisfy the composition defined
by the present invention.
[0075] Table 2 shows the manufacturing conditions and the test
results for steel sheets manufactured using steels A-M. Steel
sheets 1-8 are inventive steel sheets which satisfy the
requirements of the present invention, while steel sheets 9-18 are
comparative steel sheets which do not satisfy the requirements of
the present invention.
[0076] Steels having the chemical compositions shown in Table 1
were melted in a conventional atmospheric melting furnace to obtain
17-kg ingots. Each of these ingots was subjected to hot rolling and
annealing to obtain a hot rolled steel sheet with a thickness of
6.0 mm, and then this hot rolled steel sheet underwent cold to
rolling and annealing one to three times to obtain an annealed cold
rolled steel sheet having a thickness of 0.8-4.0 mm. This annealed
cold rolled steel sheet was subjected to a plurality of passes of
temper rolling to finally obtain a thin sheet with a thickness of
0.4 mm. Temper rolling was carried out under conditions such that
the reduction in each pass was the same.
[0077] Using test pieces which were taken from the steel sheets
before and after the temper rolling, the grain size, the austenite
content, formability, and tensile strength were investigated by the
following methods. Some of the steel sheets were subjected to aging
treatment at 300.degree. C. for one minute after temper rolling.
The tensile strength of these steel sheets was the value after
aging treatment.
[0078] (Average Grain Diameter After Annealing)
[0079] The grain diameter of austenite grains was determined as the
nominal grain diameter of austenite grains in a scanning electron
photomicrograph of an etched cross section of a test piece taken
from the annealed cold rolled steel sheet before temper
rolling.
[0080] (Austenite Content)
[0081] The austenite content was determined for the surface region
of a test piece taken from a steel sheet after temper rolling and
for the surface of the center region of the sheet thickness which
was exposed by mechanical grinding and chemical polishing. The
austenite content was determined using the integrated intensity
ratio obtained by X-ray diffraction measurement and a scanning
electron photomicrograph after etching. In Table 2, the austenite
content in the surface region of a steel sheet is indicated by
.gamma.s, and the austenite content in the surface of the center
region of the sheet thickness is indicated by .gamma.c.
[0082] (Formability)
[0083] FIG. 3 is an explanatory view showing a method for
evaluating formability. A 100 mm-square test piece taken from a
steel sheet after temper rolling was subjected to shallow draw
forming as shown in FIG. 3. Thereafter, the edge of the hole was
examined for cracks under an optical microscope. A case in which no
cracks were ascertained at all was evaluated as DOUBLE CIRCLE
(excellent), a case in which continuous cracks were not ascertained
was evaluated as CIRCLE (o, acceptable), and a case in which
continuous cracks were ascertained or in which fracture occurred
was evaluated as X (unacceptable).
[0084] (Tensile Strength)
[0085] Tensile strength was measured in accordance with JIS Z 2241
using a JIS No. 13B tensile test piece taken from a steel sheet
after temper rolling or after aging treatment. The measured value
is shown together with a CIRCLE (acceptable) for specimens having a
tensile strength exceeding 1500 N/mm.sup.2 and with an X
(unacceptable) for specimens which did not reach this level.
TABLE-US-00001 TABLE 1 Mark C N Cr Ni Si Mn Mo Cu Nb Ti V Inven- A
0.049 0.12 17.1 6.9 0.48 2.48 0.5 0.5 0 0 0 tive B 0.050 0.12 17.2
6.9 2.51 0.51 0.5 0.5 0.05 0.05 0.05 C 0.121 0.08 19.1 5.1 0.55
2.01 0.5 0.5 0 0 0 D 0.050 0.15 17.0 6.9 0.54 0.51 0 0 0 0 0 E
0.052 0.12 12.9 10.0 0.51 0.45 0.5 0.5 0 0 0 F 0.082 0.1 13.8 4.9
0.64 0.78 0.01 0.05 0 0 0 Compar- G 0.170 0.41 12.5 5.5 0.48 0.48
0.5 0.5 0 0 0 ative H 0.008 0.03 14.9 7.2 0.55 2.1 0.5 0.5 0 0 0 I
0.031 0.05 31.3 21.1 1.02 1.03 0.2 0.2 0 0 0 J 0.030 0.05 7.8 3.9
1.03 0.99 0.2 0.2 0 0 0 K 0.049 0.1 18.0 8.1 3.51 3.39 0.5 0.5 0 0
0 L 0.052 0.12 17.1 7.1 0.55 2.51 3.5 3.2 0 0 0 M 0.032 0.05 17.0
6.9 0.52 0.48 0.5 0.5 0 1.0 0 Note) The underlined values are
outside the range defined by the present invention.
TABLE-US-00002 TABLE 2 Manufacturing conditions Anneal- % number
Grain dia- Test results ing reduction of passes % Aging meter after
Tensile Steel Steel temp. in temper in temper reduction/ temp.
annealing .gamma.s .gamma.c (.gamma.s + .gamma.c)/2 Form- strength
sheet mark (.degree. C.) rolling rolling 10 (.degree. C.) (.mu.m)
(%) (%) (%) .gamma.s/.gamma.c ability (N/mm.sup.2) Inven- 1 A 950
70 8 7 300 10.1 21.5 27.5 24.5 0.78 .largecircle. .largecircle.1792
tive 2 A 950 70 7 7 10.0 20.0 28.0 24.0 0.71 .largecircle.
.largecircle.1712 3 A 850 70 7 7 300 4.0 25.0 34.5 29.8 0.72
.circleincircle. .largecircle.1827 4 B 900 60 6 6 300 4.8 24.0 45.0
34.5 0.53 .circleincircle. .largecircle.1856 5 C 950 50 5 5 300 9.8
33.0 54.0 43.5 0.61 .largecircle. .largecircle.1658 6 D 950 70 7 7
300 8.5 14.5 48.5 31.5 0.30 .largecircle. .largecircle.1712 7 E 950
90 10 9 300 9.8 9.5 11.5 10.5 0.83 .largecircle. .largecircle.1922
8 F 950 80 10 8 9.1 5.5 9.5 7.5 0.58 .largecircle.
.largecircle.1903 Compar- 9 A 950 90 5 9 300 10.1 2.0 27.5 14.8
0.07 X .largecircle.1908 ative 10 E 950 90 3 9 300 9.8 1.0 11.0 6.0
0.09 X .largecircle.1945 11 F 950 80 3 8 9.0 0.5 8.5 4.5 0.06 X
.largecircle.1906 12 G 950 70 7 7 300 11.2 54.5 62.0 58.3 0.88 X
.largecircle.1552 13 H 950 50 2 5 300 9.5 5.5 64.5 35.0 0.09 X
X1458 14 I 950 60 6 6 300 11.3 89.0 97.0 93.0 0.92 .largecircle.
X1022 15 J 950 50 2 5 300 9.3 2.5 29.5 16.0 0.08 X
.largecircle.1786 16 K 950 50 5 5 300 10.8 68.0 92.0 80.0 0.74 X
X1150 17 L 950 70 7 7 300 11.0 54.8 81.5 68.2 0.67 X X1332 18 M 950
70 3 5 300 9.5 3.5 38.0 20.8 0.09 X .largecircle.1703 Note)
Underlinesd values or marks are outside the range defined by the
present invention.
[0086] Steel sheets 1-8 in Table 2, which were steel sheets
according to the present invention, had excellent formability and a
high strength. Comparing steel sheets 1 and 2, it was ascertained
that a particularly high strength was obtained by precipitation of
fine Cr.sub.2N by aging treatment. In addition, it was ascertained
that steel sheets 3 and 4, which had a grain diameter of at most 5
.mu.m after annealing, had a particularly high strength and
excellent formability.
[0087] Steel sheets 9-18 were comparative examples for which the
chemical composition or the manufacturing conditions were outside
the range defined by the present invention.
[0088] Steel sheets 9-11 had a .gamma.s/.gamma.c ratio of less than
0.1, so a high strength was obtained but formability was poor.
Comparing steel sheet 7 with steel sheet 10 or steel sheet 8 with
steel sheet 11, although steel sheets 7 and 8 had both a high
strength and formability, steel sheets 10 and 11 had a high
strength but poor formability. Therefore, it was ascertained that
even if steels with the same chemical composition are manufactured
with the same reduction in temper rolling, the distribution of the
austenite content varies and properties greatly vary with the
number of rolling passes in temper rolling.
[0089] For steel sheet 12, the C content and the N content were
above the range for the present invention. As a result, coarse
carbonitrides were formed, and formability markedly worsened.
[0090] Steel sheet 13 had a Cr content below the range for the
present invention, so it had a low strength after aging treatment.
In addition, .gamma.s/.gamma.c was less than 0.1, so formability
was poor.
[0091] For steel sheet 14, the Cr content and the Ni content were
above the range for the present invention, and the average values
of .gamma.s and .gamma.c exceeded 55. Therefore, its strength was
low even after aging treatment.
[0092] For steel sheet 15, the Cr content and the Ni content were
below the range for the present invention and .gamma.s/.gamma.c was
less than 0.1, so formability was poor.
[0093] For steel sheet 16, the Si content and the Mn content were
above the range for the present invention, and the average values
of .gamma.s and .gamma.c exceeded 55. Therefore, the strength was
low even after aging treatment. In addition, coarse Si compounds
and Mn compounds formed, so formability was also poor.
[0094] For steel sheet 17, the Mo content and the Cu content were
above the range for the present invention, and the average values
of .gamma.s and .gamma.c exceeded 55, so the strength was low even
after aging treatment. In addition, coarse intermetallic compounds
were formed, and formability was also poor.
[0095] For steel sheet 18, the Ti content was above the range for
the present invention, so coarse TiN formed and formability was
poor.
* * * * *