US20120237388A1

US20120237388A1 - Austenitic stainless steel sheet and a method for its manufacture

Info

Publication number: US20120237388A1
Application number: US13/468,255
Authority: US
Inventors: Masayoshi SAWADA
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2009-11-18
Filing date: 2012-05-10
Publication date: 2012-09-20
Also published as: CN102639742B; WO2011062152A1; JPWO2011062152A1; KR101289518B1; JP5056985B2; CN102639742A; KR20120083930A

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 γs (%) in the surface region of the steel sheet and the austenite content γc (%) in the center region of the sheet thickness satisfy (γs+γc)/2≦55 and γs/γc≧0.10, with the remaining structure being primarily strain-induced martensite.

Description

TECHNICAL FIELD

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

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.
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×C)−(2×Si)−(10×Mn)−(14×Ni)−(5.7×Cr)−(320×N) being at least 30.
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.
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.
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 μm, 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.
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 μm is at least 50% and less than 100% and the percent area of unrecrystallized portions is greater than 0% and at most 50%.
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

Patent Document 1: JP 04-214841 A
Patent Document 2: JP 05-279802 A
Patent Document 3: JP 05-117813 A
Patent Document 4: JP 4,321,066 B

SUMMARY OF THE INVENTION

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.
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 γs (%) in a surface region of the steel sheet and the austenite content γc (%) in a center region of the thickness of the steel sheet satisfy (γs+γc)/2≦55 and γs/γc≧0.10, with the remaining structure being primarily strain-induced martensite.
The austenite content γs (%) 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 μm in the sheet thickness direction (this region being referred to as the surface region of the steel sheet). The austenite content γc (%) in a center region of the sheet thickness means the volume percent of austenite contained in a region at a depth of 10 μm 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).
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,

- (1) at least one of Mo: at most 3.0% and Cu: at most 3.0%, and/or
- (2) at least one of Ti: at most 0.50%, Nb: at most 0.50%, and V: at most 1.0%.

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.
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 μm.
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

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;

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.

FIG. 3 is an explanatory view showing a method of evaluating formability.

MODES FOR CARRYING OUT THE INVENTION

The present invention will be explained in greater detail while referring to the attached drawings.
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.
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 Cr₂N.
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 γs (%) in the surface region of the steel sheet and the austenite content γc (%) in the center region of the sheet thickness, namely, (γs+γc)/2 (below, this value will be referred to as the average austenite content) is made at most 55. “γs” and “γc” are defined as set forth above.
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.
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.
In addition, if this austenitic stainless steel sheet is then treated by aging, fine Cr₂N 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.
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.
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.
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.
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 γs (%) in the surface region of the steel sheet and the austenite content γc (%) in the center region of the sheet thickness satisfy the condition γs/γc≧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.
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.
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 Cr₂N, 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.”
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

C: 0.01-0.15%
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%.
Si: at most 3.0%
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%.
Mn: at most 3.0%
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%.
Cr: 10.0-30.0%
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 Cr₂N. However, Cr is a ferrite-forming element, so if the Cr content is excessive, delta (δ) 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%.
Ni: 4.0-20.0%
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%.
N: at most 0.40%
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 Cr₂N. 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%.
An austenitic stainless steel sheet according to the present invention may further contain the following optional elements as necessary.
One or both of Mo: at most 3.0% and Cu: at most 3.0%
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 (δ) 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%.
At least one of Ti: at most 0.5%, Nb: at most 0.5%, and V: at most 1.0%
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%.
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

[Austenite Distribution in the Sheet Thickness Direction]
As a result of carrying out a variety of tests, the present inventors found that when the austenite content γs (%) in the surface region of a steel sheet and the austenite content γc (%) 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:
(γs+γc)/2≦55 Equation (1)
γs/γc≧0.10. Equation (2)
By making the average austenite content, which is the average of the austenite content γs in the surface region of the steel sheet and the austenite content γc 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%.
By making the ratio (γs/γc) of the austenite content γs in the surface region of the steel sheet to the austenite content γc 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 γs/γc 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.
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).
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.
“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 γs=γc=100% is outside the scope of the present invention.
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 γs<γc (namely γs/γc<1) is satisfied.
[Grain diameter of austenite grains before temper rolling: at most 5 μm]
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 μm.

(3) Manufacturing Method

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.
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.
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 μm, because particularly formability is improved thereby.

[Temper Rolling Conditions]

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%.
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 γs in the surface region of the steel sheet to the austenite content γc in the center region of the sheet thickness (γc/γs) is at least 0.1, leading to a deterioration in formability.
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 γc/γs 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≧Overall reduction in temper rolling(%)divided by 10. Equation (3)
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

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.
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.
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.
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° C. for one minute after temper rolling. The tensile strength of these steel sheets was the value after aging treatment.
(Average Grain Diameter After Annealing)
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.
(Austenite Content)
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 γs, and the austenite content in the surface of the center region of the sheet thickness is indicated by γc.
(Formability)
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).
(Tensile Strength)
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/mm²and with an X (unacceptable) for specimens which did not reach this level.

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 2

	Manufacturing conditions

Anneal-

%

number

Grain dia-

Test results

ing

reduction

of passes

%

Aging

meter after

Tensile

Steel

temp.

in temper

reduction/

temp.

annealing

γs

γc

(γs + γc)/2

Form-

strength

sheet

mark

(° C.)

rolling

10

(° C.)

(μm)

(%)

γs/γc

ability

(N/mm²)

Inven-	1	A	950	70	8	7	300	10.1	21.5	27.5	24.5	0.78	◯	◯1792
tive	2	A	950	70	7	7		10.0	20.0	28.0	24.0	0.71	◯	◯1712
	3	A	850	70	7	7	300	4.0	25.0	34.5	29.8	0.72	⊚	◯1827
	4	B	900	60	6	6	300	4.8	24.0	45.0	34.5	0.53	⊚	◯1856
	5	C	950	50	5	5	300	9.8	33.0	54.0	43.5	0.61	◯	◯1658
	6	D	950	70	7	7	300	8.5	14.5	48.5	31.5	0.30	◯	◯1712
	7	E	950	90	10	9	300	9.8	9.5	11.5	10.5	0.83	◯	◯1922
	8	F	950	80	10	8		9.1	5.5	9.5	7.5	0.58	◯	◯1903
Compar-	9	A	950	90	5	9	300	10.1	2.0	27.5	14.8	0.07	X	◯1908
ative	10	E	950	90	3	9	300	9.8	1.0	11.0	6.0	0.09	X	◯1945
	11	F	950	80	3	8		9.0	0.5	8.5	4.5	0.06	X	◯1906
	12	G	950	70	7	7	300	11.2	54.5	62.0	58.3	0.88	X	◯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	◯	X1022
	15	J	950	50	2	5	300	9.3	2.5	29.5	16.0	0.08	X	◯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	◯1703

Note)
Underlinesd values or marks are outside the range defined by the present invention.

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 Cr₂N by aging treatment. In addition, it was ascertained that steel sheets 3 and 4, which had a grain diameter of at most 5 μm after annealing, had a particularly high strength and excellent formability.
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.
Steel sheets 9-11 had a γs/γc 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.
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.
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, γs/γc was less than 0.1, so formability was poor.
For steel sheet 14, the Cr content and the Ni content were above the range for the present invention, and the average values of γs and γc exceeded 55. Therefore, its strength was low even after aging treatment.
For steel sheet 15, the Cr content and the Ni content were below the range for the present invention and γs/γc was less than 0.1, so formability was poor.
For steel sheet 16, the Si content and the Mn content were above the range for the present invention, and the average values of γs and γc 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.
For steel sheet 17, the Mo content and the Cu content were above the range for the present invention, and the average values of γs and γc exceeded 55, so the strength was low even after aging treatment. In addition, coarse intermetallic compounds were formed, and formability was also poor.
For steel sheet 18, the Ti content was above the range for the present invention, so coarse TiN formed and formability was poor.

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 γs (%) in a surface region of the steel sheet and the austenite content γc (%) in a center region of the sheet thickness satisfy (γs+γc)/2≦55 and γs/γc≧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 μm.

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.