JP2011117024A - Fine grained austenitic stainless steel sheet exhibiting excellent stress corrosion cracking resistance and processability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fine granular austenitic stainless steel sheet in which a stress-corrosion cracking as a defect is eliminated, and good workability is also obtained by regulating average crystal grain diameter to ≤10 μm in Ni content of ≤8% and the characteristics of the crystal grain boundary. <P>SOLUTION: The austenitic stainless steel sheet has the steel composition composed of, by mass%, ≤0.05% C, 14-19% Cr, ≤2% Si, ≤4% Mn, 5-8% Ni, ≤4% Cu, ≤0.1% N, and the balance Fe with inevitable impurities, wherein Md=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.2Mo is in the range of -20 to 40; the average crystal diameter is ≤10 μm; and the ratio of a large bevel grain boundary of ≥15° is more than 80%. In the manufacturing method, a cold-rolling is performed under rolling reduction ratio of ≥70% and rolling temperature of ≤50°C; a last annealing is performed at 700-900°C; and holding time is 4-24 hr. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an austenitic stainless steel sheet having a fine grain structure with an average crystal grain size of 10 μm or less and excellent in stress corrosion cracking resistance and workability.

  In recent years, it has been known that refining crystal grains in steel materials is the most effective method for increasing strength and toughness regardless of the addition of alloy elements. Also in austenitic stainless steel, Non-Patent Documents 1 and 2 disclose the refinement of crystal grains using the phase transformation from work-induced martensite to austenite in SUS304 defined in JIS G4305. By such a method, a fine grain structure having a crystal grain size of 1 to 5 μm is formed. As a refinement effect, non-patent document 1 shows an increase in yield strength (0.2% proof stress) and non-patent document 2 shows 650. The development of superplasticity at ˜750 ° C. has been reported.

  In austenitic stainless steel, Patent Document 1 discloses a metal gasket, a material thereof, and a method for manufacturing the same as one utilizing a grain refinement effect. In this publication, a fine grain structure having a crystal grain size of 5 μm or less is obtained by utilizing the phase transformation from processing-induced martensite to austenite and precipitation of chromium nitride described in SUS301L defined in JIS G4305. High strength of Hv500 or more is achieved by combination.

  Conventionally, grain refinement of austenitic stainless steel is aimed at increasing 0.2% proof stress and increasing strength in SUS304 and SUS301L as described above with a crystal grain size of 1 to 5 μm.

  For a long time, it has been well known that austenitic stainless steel sheets cause stress corrosion cracking in a corrosive environment containing chloride ions. In Non-Patent Document 3, as a countermeasure, Ni-containing ferritic stainless steel is surely changed, and when it is difficult to use ferritic stainless steel from the viewpoint of workability and weldability, high Ni is used. It is described that SUSXM15J1 austenitic stainless steel (11.5 to 15%) and having an increased amount of Si and Cu are effective.

  The alloy addition described above works effectively for the improvement of stress corrosion cracking starting from pitting corrosion and crevice corrosion. Patent Document 2 discloses an austenitic stainless steel with excellent resistance to stress corrosion cracking and pitting corrosion, in which Ni is added in the order of 9%, Cu is added in an amount exceeding 1.5% and less than 2.5%, and Mo and N are contained in a small amount. Steel is disclosed. In Patent Document 3, Cr: 18 to 35%, Ni: 25 to 50%, Mo: 8% or less, Mn: 6% or less, N: 0.5% or less, C: 0.03% or less An austenitic alloy excellent in stress corrosion cracking resistance characterized by Cr and high Ni is disclosed. In Patent Document 4, C: 0.08% or less, Si: 0.1 to 3%, Cr: 18 to 23%, Ni: 8.5 to 12%, Mo: 0.2 to 2%, Cu: 0 .2 to 3.5%, N: 0.03 to 0.25%, adjusting the contents of Mn and S, adding Cu and N in combination, and further small amounts of Co, W, V , Nb has been disclosed as an austenitic stainless steel excellent in weather resistance, crevice corrosion resistance and stress corrosion cracking resistance.

  In addition, since intergranular cracking also occurs in stress corrosion cracking, Patent Documents 5 to 7 disclose improvement of intergranular stress corrosion cracking. Patent Document 5 discloses an austenitic stainless steel having excellent intergranular corrosion resistance and intergranular stress corrosion cracking resistance, characterized by containing either one or both of Mo and Nb. In Patent Documents 6 and 7, C is limited to 0.03% or less, N is contained at 0.15% or less, and carbide precipitation is reduced by adjusting the heating temperature and time of the steel slab, An austenitic stainless steel excellent in intergranular stress corrosion cracking resistance, characterized by reducing the Cr deficiency in the vicinity, and a method for producing the same are disclosed.

All of the austenitic stainless steels disclosed in Non-Patent Document 3 and Patent Documents 2 to 7 described above contain more than 8% Ni, Cu, Mo, Si, and Nb, Co, W, and trace elements as trace elements. Stress corrosion cracking resistance is improved by adding V or the like.
The annealing temperature in industrial production is known in Non-Patent Documents 3 and 4, and the crystal grain size is known in Non-Patent Document 5. Usually, austenitic stainless steel is annealed at 1000 to 1100 ° C., and even if the components are adjusted, the limit of refining is the grain size no. It is described that it is less than 10, that is, the crystal grain size is larger than 10 μm.

International Publication No. 02/088410 Pamphlet Japanese Patent Laid-Open No. 61-9557 JP 62-180037 A JP-A-62-247048 JP 62-287051 A JP-A-8-269550 JP-A-10-317104 Japanese Patent Application No. 2008-157717

Iron and steel, 78 (1992), 141-148 Iron and steel, 80 (1994), 249-253 Stainless Steel Handbook, 3rd edition, 560 Nishiyama Memorial Technology Course "Recent Advances in Stainless Steel Manufacturing Technology" 115 Japan Iron and Steel Institute Japan Steel Pipe Technical Report, No. 87 (1980), 51-60 OIM ACADEMY, TSL Solutions Inc.

In the conventional grain refinement of austenitic stainless steel, the grain refinement effect on the stress corrosion cracking resistance has not been clarified at all.
Further, as described above, it is usually described that austenitic stainless steel is annealed at 1000 to 1100 ° C., and the crystal grain size becomes larger than 10 μm even if the components are adjusted. Patent Documents 2 to 7 do not specifically describe the production method (annealing temperature) and the crystal grain size. Therefore, the steel disclosed in Patent Documents 2 to 7 is also easily estimated that the crystal grain size is larger than 10 μm as in Non-Patent Document 3, unless a special manufacturing method different from usual is disclosed. it can.

  As described above, in the austenitic stainless steel, there has been no study that attempted to improve the stress corrosion cracking resistance with an Ni amount of 8% or less. Furthermore, technology that reduces stress corrosion cracking, which is a drawback of austenitic stainless steels, due to the refinement effect of crystal grains, and achieves compatibility with workability, regardless of the amount of Ni of 8% or less and expensive addition of Mo. There is no thought or such disclosure at all.

  The object of the present invention is to overcome the stress corrosion cracking, which is a defect of austenitic stainless steel, by the effect of refining crystal grains, regardless of the addition of Ni with an amount of Ni of 8% or less and expensive workability. The object is to provide an austenitic stainless steel sheet having a fine grain structure with an average grain size of 10 μm or less.

(1) In mass%, C: 0.05% or less, Cr: 14-19%, Si: 2% or less, Mn: 4% or less, Ni: 5-8%, Cu: 4% or less, N: 0.1% or less, balance Fe and unavoidable impurities, and having a steel component with the following Md in the range of -20 to 40, an average crystal grain size of 10 μm or less, and a large tilt angle of 15 ° or more A fine-grained austenitic stainless steel sheet excellent in stress corrosion cracking resistance and workability, characterized in that the proportion occupied by grain boundaries exceeds 80%.
Md = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr
-29 (Ni + Cu) -18.2Mo

(2) The steel component is further in mass%, Mo: 1% or less, V: 1% or less, B: 0.010% or less, Nb: 0.5% or less, Ti: 0.5% or less, One or more rare earth elements: 0.5% or less, Al: 0.5% or less, Mg: 0.005% or less, Ca: 0.005% or less (1) ) Fine-grained austenitic stainless steel sheet with excellent stress corrosion cracking resistance and workability as described in (3) Boiling a molded product obtained by cylindrical deep drawing in a drawing ratio of 1.5 to 2.0. The fine-grained austenitic stainless steel sheet excellent in stress corrosion cracking resistance and workability as described in (1) or (2), wherein cracks do not occur when immersed in a 42% magnesium chloride aqueous solution for 4 hours. The aperture ratio is a value obtained by dividing the blank diameter by the punch diameter.
(4) The stress corrosion cracking resistance and processing according to any one of (1) to (3), wherein the 0.2% proof stress obtained by a tensile test is less than 400 MPa and the uniform elongation is more than 30%. Fine-grained austenitic stainless steel sheet with excellent properties.

  Hereinafter, the inventions related to the steel sheets (1) to (4) are referred to as the present invention. The inventions (1) to (4) may be collectively referred to as the present invention.

  As described above, according to the austenitic stainless steel sheet having a fine grain structure of the present invention, stress corrosion cracking, which is a defect of austenitic stainless steel, regardless of the amount of Ni of 8% or less and expensive addition of Mo. It is possible to overcome the problem and achieve compatibility with workability.

It is the graph which showed the relationship between an average crystal grain size and Md. It is the photograph which showed the external appearance of the molded article after 4 hours immersion in boiling 42% magnesium chloride aqueous solution. It is a photograph of the microstructure of steel of (i)-(ii) of Drawing 2. It is the graph which showed the relationship between the crack generation time in the boiling 42% magnesium chloride aqueous solution, an average crystal grain size, and Md. It is the graph which showed the relationship between the crack generation time in the boiling 42% magnesium chloride aqueous solution of a fine grain material, and the ratio of a large inclination grain boundary.

In order to solve the above-mentioned problems, the present inventors have aimed at an austenitic stainless steel having an Ni content of 8% or less, and have an optimum component balance for the formation of a fine grain structure and an action of improving stress corrosion cracking by a refinement effect. The present invention was completed by earnestly studying the compatibility between the process and workability. The typical experimental results will be described below.
In the present invention, the fine grain structure means that the average crystal grain size is 10 μm or less.

  The austenitic stainless steel which shows a steel component in Table 1 was melted, and a 3.0 mm-thick hot rolled sheet was manufactured. Hot-rolled sheet annealing was performed at 1150 ° C. and pickled to produce a cold-rolled sheet having a thickness of 0.5 mm. In cold rolling, the plate temperature was kept at 10 ° C. while cooling with water, and the generation of processing-induced martensite was promoted by suppressing processing heat generation. In cold-rolled sheet annealing, in order to form a fine grain structure by utilizing the phase transformation from work-induced martensite to austenite, the holding time was changed in the range of 600 to 1050 ° C. and 1 minute to 24 hours. After cold rolling, the final annealed steel sheet was pickled, and then subjected to measurement of the average crystal grain size, measurement of the ratio of the large-angle grain boundary, and measurement of crack generation time.

The average crystal grain size was determined by a steel-crystal grain size microscopic test method specified in JIS G 0551 after embedding and polishing a plate cross-section in a resin and electrolytic etching with nitric acid.
The ratio of the large tilt grain boundaries was measured by the grain boundary map display of the EBSP method. In the EBSP method, a low-angle grain boundary of less than 15 ° and a large-angle grain boundary of 15 ° or more can be identified by the grain boundary map display, and the ratio of the large-angle grain boundary in all the crystal grain boundaries can be calculated. Here, the measurement magnification was adjusted to include 3000 or more crystal grains, which are reported in Non-Patent Document 6 to statistically reflect bulk properties.

  Crack generation time is 1.9 cylinder deep drawing with a drawing ratio (blank diameter divided by punch diameter) of 1.9 under the conditions of blank diameter 67.5 mmφ, punch diameter 35 mmφ, die diameter 37 mmφ, and wrinkle holding pressure 1 ton. The obtained molded product was allowed to stand for 48 hours to confirm that no aging cracks occurred, and then immersed in a boiling 42% magnesium chloride aqueous solution specified in JIS G 0576, and the crack generation time was measured.

(A) FIG. 1 shows the relationship between the average crystal grain size and the component balance (Md) when the cold-rolled sheet annealing is held at 800 ° C. for 4 hours. Md is defined as in the following formula (1). From FIG. 1, the average crystal grain size decreases with increasing Md. As the Md increases, the amount of work-induced martensite generated by cold rolling increases. Therefore, it is considered that the increase in Md is promoted by the refinement utilizing the phase transformation from work-induced martensite to austenite in the annealing after cold rolling as described in Non-Patent Documents 1 and 2 described above. . From this study, it is effective to set Md to -20 or more in order to refine the target to an average crystal grain size of 10 μm or less. Moreover, even if Md is substantially equivalent to SUS304, it was confirmed that the steel component (steel D) in which Cr and Ni are reduced and Cu is added is more effective for miniaturization.

Md = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr
-29 (Ni + Cu) -18.2Mo (1)

(B) In FIG. 2, steel B (i) to (ii) and SUS316L (iii) having different average crystal grain diameters (d) are subjected to cylindrical deep drawing in the same manner as the measurement of the crack occurrence time. The appearance of the obtained molded product (cylindrical deep-drawing material) after immersion for 4 hours in a boiling 42% magnesium chloride aqueous solution is shown. Furthermore, in FIG. 3, the photograph of the microstructure of steel B (i)-(ii) used for the test of FIG. 2 is shown.

  As shown in FIG. 2, when the average crystal grain size is refined to 3 μm (FIG. 2 (i)), the average crystal grain size produced by normal annealing (1050 ° C., 1 minute hold) is 28 μm (FIG. 2 (ii) Compared with), no cracking occurs when the cylindrical deep-drawn material is immersed in a boiling 42% magnesium chloride aqueous solution. In this test, expensive austenitic stainless steel SUS316L (17Cr-12Ni-2Mo) which contains high Ni and Mo and exhibits resistance to stress corrosion cracking compared to general-purpose SUS304 (18Cr-8Ni) (FIG. 2). (Iii)). From these results, the inventors have found a novel finding that the stress corrosion cracking resistance (crack generation time) is remarkably improved by the effect of crystal grain refinement.

(C) FIG. 4 shows the relationship between crack generation time, average crystal grain size and Md in the boiling 42% magnesium chloride aqueous solution. 4 indicates that the crack occurrence time is longer than the plotted point.
In the case of Md = 29.5 (steel B), it can be seen that the crack generation time has increased dramatically due to the effect of refining crystal grains (average crystal grain size of 10 μm or less). The reason is not necessarily clear, but is estimated as follows. Stress corrosion cracking is basically intragranular cracking, and the grain area ratio, which is the starting point of cracking, is greatly reduced by the refinement of crystal grains. In addition, it is known that fracture toughness in steel materials is remarkably improved by crystal grain refinement. These factors seem to have exerted a considerable effect on the stress corrosion cracking resistance. In SUS316L as a comparison, cracks occurred when immersed for 2 to 3 hours under the same test conditions. The target of the present invention was that crack generation time clearly exceeded SUS316L under the same test conditions, and cracking did not occur when immersed for 4 hours (hr).

  In addition, the steel plate (steel B) improved in stress corrosion cracking resistance by grain refinement shown in FIG. 4 was subjected to final annealing of the cold-rolled sheet after cold rolling at 800 ° C. for 4 hours and 24 hours. It is a thing. In addition, the steel sheet having a crack generation time of less than 4 hours and an average crystal grain size of more than 10 μm shown in FIG. 4 was obtained by performing final annealing of the cold-rolled sheet after cold rolling at 900 ° C. to 1050 ° C. for 1 minute to 4 hours. The steel sheet having a crack generation time of less than 4 hours and an average crystal grain size of 10 μm or less is obtained by subjecting the cold-rolled sheet after cold rolling to final annealing at 800 ° C. for 4 hours.

(D) The expression of the refinement effect on the stress corrosion cracking resistance is affected by the component balance (Md). In order to develop a refinement effect that suppresses stress corrosion cracking, Md needs to be in the range of -20 to 40.
In FIG. 4, in the case of Md = 43 (steel A), the crack generation time does not increase greatly even if it is miniaturized. The reason for this is that the material itself has been hardened by miniaturization, a large amount of work-induced martensite has been generated in the deep drawing of the cylinder, and the effect of inhibiting stress corrosion cracking has not been manifested due to an increase in residual stress on the cup side wall. To do. From this examination, Md: 40 or less is effective for the expression of the refinement effect which suppresses stress corrosion cracking.

(E) In the case of Md = −25 (steel G) having a low Md value, it becomes difficult to form a fine grain structure as described in (a). Therefore, in FIG. 4, it is difficult to obtain a refinement effect that suppresses stress corrosion cracking by setting the average grain size to 10 μm or less. From this result, Md: −20 or more is effective for the expression of the refinement effect for suppressing stress corrosion cracking.

(F) In addition to Md, the ratio of the large-angle grain boundary in the crystal grain boundary affects the stress corrosion cracking resistance of the fine grain material. FIG. 5 is a graph showing the relationship between the crack occurrence time in steel B and the ratio of large-angle boundaries of 15 ° or more. Note that the up arrow (↑) in FIG. 5 indicates that the crack occurrence time is longer than the plotted point. As shown in FIG. 5, the fine grain material of Steel B has the stress corrosion cracking resistance described in (b) and (c) above when the ratio of the large-angle grain boundaries of 15 ° or more exceeds 80%. Improvement effect is obtained. The reason is considered as follows. The fine-grained material is produced by generating as much work-induced martensite as possible by cold rolling and utilizing reverse transformation from work-induced martensite to austenite by annealing at a lower temperature than usual. Due to large strain accumulation during cold rolling and low temperature annealing, residual strain after annealing tends to increase. In such a case, recrystallization of austenite grains is in progress, and there are many small-angle grain boundaries of less than 15 ° that are not recognized as large-angle grain boundaries. Therefore, a decrease in the ratio of the large-angle grain boundaries means that the residual strain of the steel is large, and it is assumed that the residual strain of the steel hinders the stress corrosion cracking resistance.

  Steel B shown in FIG. 5 is obtained by changing the final annealing of the cold-rolled sheet after cold rolling at 800 ° C. in the range of 10 minutes to 24 hours. In the case where the ratio of the large-angle grain boundaries shown in FIG. 5 is more than 80%, the final annealing of the cold-rolled sheet after cold rolling is performed at 800 ° C. for more than 1 hour.

(G) Refinement of crystal grains is affected by manufacturing conditions in addition to steel components. In order to utilize the phase transformation from work-induced martensite to austenite, it is effective to promote the work-induced martensite transformation in cold rolling. Therefore, it is preferable that the rolling reduction is increased by cold rolling and the processing heat generation is suppressed. Furthermore, in order to increase the ratio of the large-angle grain boundaries and develop the stress corrosion cracking resistance with the fine grain material, it is preferable that the final annealing performed after the cold rolling is held at a low temperature for as long as possible. Specifically, it is effective to set the final annealing condition at 700 to 900 ° C. for more than 1 hour. Further, increasing the ratio of the large tilt grain boundaries of 15 ° or more is effective in reducing the 0.2% proof stress and increasing the elongation, and contributes to the improvement of workability.

In the patent document 8, the present inventors have already proposed an austenitic stainless steel sheet for press forming having a fine grain structure with an average crystal grain size of 10 μm or less and a method for producing the same. Patent Document 8 aims to improve “aging cracking” after deep-drawing processing = delayed fracture of material, and is different from the phenomenon involving “stress corrosion cracking” = material corrosion and dissolution improved in the present invention. is there. In Patent Document 8, the ratio of the large tilt grain boundary of 15 ° or more that affects the stress corrosion cracking described above is not studied at all. The final annealing time is substantially 1 hour or less.
In order to improve the stress corrosion cracking resistance of the fine-grained steel proposed in Patent Document 8, the present invention finds a ratio of a large-angle grain boundary of 15 ° or more, which is an influential factor, and sets the final annealing time to 1 hour. It was found that super-control is extremely effective.

The present inventions (1) to (7) have been completed based on the findings (a) to (g).
Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(A) The reason for limitation regarding the steel component will be described below.
The austenitic stainless steel sheet according to the present invention has a component and a component balance (Md) in order to form a fine grain structure with an average crystal grain size of 10 μm or less and improve stress corrosion cracking resistance by a refinement effect. It is.

  C is an austenite-forming element and is added for the purpose of ensuring austenite stability. When added in a large amount, it becomes hard and the workability is lowered, and the precipitation of carbide is promoted to inhibit the stress corrosion cracking resistance aimed at by the present invention. Therefore, the upper limit is made 0.05%. Preferably it is 0.03%. The lower limit is preferably 0.005% from the viewpoint of manufacturability.

  Since Cr needs to be 14% or more in order to obtain sufficient corrosion resistance, the lower limit is made 14%. Preferably it is 15%, more preferably 16%. On the other hand, if added in a large amount, the workability is lowered due to hardening or formation of δ ferrite. Furthermore, the refinement | miniaturization of the crystal grain made into the objective of this invention is inhibited. Therefore, the upper limit is 19%. Preferably it is 18%.

  Si is effective as a strong deoxidizing agent, but when added in a large amount, it hardens and inhibits manufacturability, so the upper limit is made 2%. Preferably it is 1.5%. On the other hand, it has the effect of improving the stress corrosion cracking resistance which is the object of the present invention. In order to obtain these effects, 0.5% or more is preferably added. The lower limit is preferably 0.1% in terms of manufacturability.

  Mn is an austenite-forming element and is added for the purpose of ensuring austenite stability and improving workability. When added in a large amount, MnS is formed, and the stress corrosion cracking resistance aimed at by the present invention is hindered due to a decrease in corrosion resistance. Therefore, the upper limit is 4%. Preferably, it is 3%. The lower limit is preferably 0.5% for the above purpose.

  Ni is an indispensable element for austenitic stainless steel, and the lower limit is made 5% from the viewpoint of ensuring austenite stability and workability. Preferably it is 6%. On the other hand, Ni is an expensive and rare element, and also has an element that hinders the refinement of crystal grains targeted by the present invention, so the upper limit is made 8%. Preferably it is 7.5% or less.

  Like Ni, Cu is added for the purpose of austenite stability and softening. However, Cu is a preferable element for saving Ni and improving stress corrosion cracking resistance and promoting the refinement of crystal grains. However, the addition of a large amount adversely affects the quality of molten steel, the quality of discharged slag and its effective use, which deteriorates hot workability and eliminates the need for Cu metal components. Therefore, the upper limit is 4%. Preferably it is 3%. The lower limit is preferably 1% in order to obtain the above effect. More preferably, it is 1.5%.

  N is an austenite-forming element like C, and is added for the purpose of securing austenite stability. However, if added in a large amount, it becomes hard and the workability is lowered. Therefore, the upper limit is made 0.1%. Preferably it is 0.06% or less. The lower limit is preferably 0.005% from the viewpoint of manufacturability. More preferably, the content is 0.01%.

  Mo is not an essential element in the present invention, but may be added in a timely manner in order to improve the corrosion resistance and the stress corrosion cracking resistance targeted by the present invention. However, since Mo is a very expensive and rare element, even when it is added, the upper limit is made 1%. Preferably it is 0.5%. In order to obtain the above effect, the lower limit is preferably 0.1%.

  V is not an essential element in the present invention, but may be added as appropriate in order to improve the corrosion resistance and the stress corrosion cracking resistance aimed at by the present invention even if it does not reach Mo. However, V is an expensive element and is a solid solution strengthening element, which impairs workability. Therefore, even when adding, the upper limit is made 1%. Preferably it is 0.5%. The lower limit for obtaining the above effect is preferably 0.1%.

B and rare earth elements (REM) may be added in a timely manner in order to improve hot workability. However, if B exceeds 0.010%, the productivity and corrosion resistance may be significantly impaired. Therefore, the upper limit when adding is 0.010%. Preferably it is 0.005%. When added, the lower limit of B is preferably 0.0005%.
On the other hand, if the rare earth element exceeds 0.5%, the manufacturability and economy may be impaired. Therefore, the upper limit of the rare earth element is preferably 0.5%. More preferably, it is 0.2%. When added, the lower limit of the rare earth element is preferably 0.005%.

  Nb and Ti form carbonitrides and suppress the formation of Cr carbides, and thus contribute to the improvement of stress corrosion cracking resistance. However, if added in a large amount, the workability and manufacturability deteriorate, so the upper limit is made 0.5%. Preferably it is 0.3%. When added, the lower limit is preferably 0.005%. More preferably, it is 0.01%.

  Since Al is an effective element as a deoxidizing element, it may be added as appropriate. However, excessive addition leads to deterioration of workability and weldability, so the upper limit is made 0.5%. Preferably it is 0.3%, more preferably 0.1%. When added, the lower limit is preferably 0.01%.

  Mg and Ca form an oxide together with Al in the molten steel and act as a deoxidizer, so may be added as appropriate. Ca fixes S and has an effect of improving hot workability. However, excessive addition of Mg and Ca leads to deterioration of corrosion resistance and weldability, so the upper limit is made 0.005%. Preferably, the content is 0.002%. When added, the lower limit is made 0.0001%. More preferably, the content is 0.0003%.

  Furthermore, the austenitic stainless steel of the present invention may contain P and S in the following ranges as a part of inevitable impurities in addition to the above components. P and S are elements harmful to hot workability and corrosion resistance. P is preferably 0.1% or less. More preferably, it is 0.05% or less. S is preferably 0.01% or less. More preferably, it is 0.005% or less.

In the present invention, in addition to the component ranges described above, the optimum component balance for the formation of a fine grain structure is defined by Md shown in equation (1).
Md = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr
-29 (Ni + Cu) -18.2Mo (1)

Metastable austenitic stainless steel undergoes martensitic transformation by plastic working even at temperatures above the Ms point. The upper limit temperature that causes transformation by processing is called the Md point. That is, the Md point is an index indicating the stability of austenite.
By designing the Md shown in the formula (1) in the range of -20 to 40, it is possible to obtain the effect of improving the stress corrosion cracking resistance due to the formation of the fine grain structure and the refinement effect as the object of the present invention. When Md is less than −20, it is difficult to form a fine grain structure and develop stress corrosion cracking resistance as described in the above (d) and (e). On the other hand, when Md exceeds 40, as described in the above (d) and (e), it is effective in forming a fine grain structure, but it inhibits the development of stress corrosion cracking resistance. A preferred Md range is -5 to 35.

(B) A manufacturing method is demonstrated below.
When producing the fine-grained austenitic stainless steel sheet of the present invention, it has the steel components described in the section (A), the average crystal grain size is 10 μm or less, and the ratio of the large-angle grain boundaries of 15 ° or more is In order to make it more than 80% and to express stress corrosion cracking resistance effectively, it is preferable to set it as the following manufacturing conditions.

The production method up to hot rolling is not particularly limited and may be known.
In order to form a fine grain structure by final annealing after cold rolling, it is effective to promote work-induced martensitic transformation by cold rolling as described in (g) above. In order to obtain an average crystal grain size of 10 μm or less, which is the object of the present invention, it is effective to set the work-induced martensite volume ratio to 50% or more after cold rolling. Preferably, the processing-induced martensite volume fraction is more than 60%. The final annealing conditions after cold rolling are specified in order to refine crystal grains and increase the ratio of large-angle grain boundaries of 15 ° or more. Preferably, cold rolling conditions are also defined, more preferably hot-rolled sheet annealing should be considered.

  In the hot-rolled sheet annealing, in order to coarsen austenite grains used for cold rolling to 20 μm or more and promote work-induced martensitic transformation by cold rolling, it is preferable to set the temperature within a range from 1050 to 1200 ° C. Below 1050 ° C., the austenite grain size may be less than 20 μm. If it exceeds 1200 ° C., surface quality such as pickling after annealing may be impaired. Moreover, annealing over 1200 ° C. has a heavy load on the equipment. More preferably, it is set as the range of 1080-1180 degreeC.

  The final annealing after the cold rolling is in the range of 700 to 1050 ° C. in order to make the average crystal grain size 10 μm or less and the ratio of the large-angle grain boundary more than 80%. When the temperature is lower than 700 ° C., the strain in cold rolling is accumulated, the austenite grains are insufficiently recrystallized, the workability is remarkably lowered, and the ratio of the large-angle grain boundaries of 15 ° or more is small. Inhibits the intended stress corrosion cracking resistance. Preferably, the lower limit temperature is 750 ° C. or higher. Above 1050 ° C., austenite crystal grain growth proceeds and the average crystal grain size exceeds 10 μm. Preferably it is 900 degrees C or less. In order to realize a fine grain structure having a large tilt grain boundary ratio of 80% or more as a target of the present invention, a more preferable final annealing temperature is in a range of 750 to 850 ° C.

When the final annealing temperature is 700 to 900 ° C., the annealing time is preferably more than 1 hour in order to promote the recrystallization of austenite and increase the ratio of the large tilt grain boundaries of 15 ° or more. More preferably, it is 2 hours or more. The upper limit of the holding time is not limited, but it is preferably 24 hours or shorter in consideration of box annealing that is industrially known for chromium-based stainless steel. In order to realize a fine grain structure in which the ratio of the large tilt grain boundary targeted by the present invention is more than 80%, a more preferable final annealing time is set in a range of 4 to 24 hours. When it manufactures with a small-scale annealing equipment, it may not exceed that and may exceed 24 hours.
When the final annealing temperature is 900 to 1050 ° C., the annealing time is preferably maintained for a short time of 10 minutes or less in consideration of crystal grain growth. More preferably, the holding time may be 1 minute or less.

  In cold rolling, in order to promote work-induced martensitic transformation, it is preferable that the rolling reduction is 70% or more and the rolling temperature is 50 ° C. or less. When the rolling reduction is less than 70%, the processing-induced martensite volume fraction is less than 50%, and it becomes difficult to form a fine grain structure as described above. More preferably, the rolling reduction is 80% or more. The upper limit is not particularly specified, but 90% or less is preferable in consideration of hot-rolled sheet production and cold-rolling equipment capacity. When the rolling temperature exceeds 50 ° C., the work-induced martensite volume ratio is less than 50%, and it becomes difficult to form a fine grain structure as described above. The lower limit of the rolling temperature is not particularly defined, but industrially, the temperature reached by water cooling is preferably 10 ° C. or higher. When manufacturing with a small-scale rolling facility, it is not limited thereto, and may be a low temperature (for example, −200 ° C.) reached by cooling liquid nitrogen or the like.

(C) The reasons for limiting the metal structure will be described below.
In the fine-grained austenitic stainless steel sheet of the present invention, the slab having the steel component of the item (A) is used, and the preferable production conditions of the item (B) are carried out, and the average crystal grain size is 10 μm or less and 15 ° or more. The ratio of the large tilt grain boundaries is over 80%.

  When the average crystal grain size exceeds 10 μm, it becomes difficult to develop stress corrosion cracking resistance due to the refinement effect aimed at by the present invention. In addition, even if the average crystal grain size is 10 μm or less, if the ratio of the large angle grain boundaries of 15 ° or more is less than 80%, the stress corrosion cracking resistance due to the refinement effect as described in (f) above. Impairs sex improvement. In order to effectively develop the stress corrosion cracking resistance aimed at by the present invention, the ratio of the large tilt grain boundaries having an average crystal grain size of 5 μm or less and 15 ° or more is preferably 85% or more. The lower limit of the average crystal grain size is not particularly specified, but it is difficult to make it less than 1 μm from Non-Patent Documents 1 and 2 and Patent Document 1. Therefore, in consideration of practical use, the average crystal grain size is preferably in the range of 1 to 5 μm.

  As described above, the ratio of the large tilt grain boundaries of 15 ° or more is more than 80%, preferably more than 85%. Increasing the ratio of the large-angle grain boundaries is effective in reducing the 0.2% proof stress and increasing the elongation of the fine-grained material, and contributes to the improvement of workability. From the background described above, the workability targeted by the present invention is preferably superior to that of ferritic stainless steels and close to that of austenitic stainless steels such as SUS304. Therefore, it is preferable that the 0.2% proof stress is less than 400 MPa and the uniform elongation is more than 30%. In order to satisfy both the stress corrosion cracking resistance and the workability, the ratio of the large tilt grain boundaries of 15 ° or more is preferably more than 85%, more preferably more than 90%. In the present invention, the mechanical properties of 0.2% proof stress and uniform elongation are evaluated by JIS No. 13 B tensile test.

Examples of the present invention will be described below.
Austenitic stainless steel slabs having steel components shown in Table 2 were melted and hot-rolled to form hot rolled sheets having a thickness of 4 mm. Steel No. 1-23 satisfy | fills the steel component prescribed | regulated by this invention. Steel No. Nos. 24-28 are for removing the steel components defined in the present invention.

  The hot-rolled sheet was annealed, and cold rolling and final annealing were performed under other conditions in addition to the preferable conditions of the present invention. Cold rolling was performed under two conditions that were less than 30 ° C. while cooling with water at room temperature (<30 ° C.), and did not carry out water cooling and exceeded 50 ° C. (> 50 ° C.) in the middle of cold rolling due to processing heat generation. . After cold rolling, pickling the final annealed steel sheet, measuring the average crystal grain size, measuring the large angle grain boundary ratio of 15 ° or more by the EBSP method, measuring the stress corrosion cracking resistance (cracking time), The mechanical properties (0.2% proof stress, uniform elongation) were measured.

  Various evaluation methods were performed as described above. Specifically, the crack generation time is the same as in the measurement method described above, with a blank diameter of 67.5 mmφ, a punch diameter of 35 mmφ, a die diameter of 37 mmφ, and a draw ratio of 1 ton (the blank diameter divided by the punch diameter). 1.9) After performing cylindrical deep drawing of 1.9 and leaving the resulting molded product to stand for 48 hours and confirming that aging cracks do not occur, in a boiling 42% magnesium chloride aqueous solution specified in JIS G 0576 This was carried out by measuring the time when cracking (stress corrosion cracking) occurred after immersion. The presence or absence of cracks was determined visually. The mechanical properties were evaluated by JIS No. 13 B tensile test. Table 3 shows the relationship between manufacturing conditions and characteristics.

  Test No. 7 has the steel component of the present invention and is manufactured at a known annealing temperature, the final annealing temperature is high, the average crystal grain size is 30 μm, the large-angle grain boundary ratio is 98%, and the stress corrosion cracking occurrence time is 3 hours. The improvement of the stress corrosion cracking resistance due to the refinement of the crystal grains was not observed.

  Test No. 1-4 and 8-29 have the steel component of this invention, and implement the preferable manufacturing conditions of this invention. These steel sheets had an average crystal grain size of 10 μm or less and a ratio of large-angle grain boundaries of 15 ° or more exceeding 80%, and the results of stress corrosion cracking generation time significantly exceeded the above-mentioned target of 4 hours or more. Furthermore, these steel sheets have mechanical properties with a 0.2% proof stress of less than 400 MPa and a uniform elongation of more than 30%, and have reached the preferred workability described above in addition to the stress corrosion cracking resistance. As a result, the austenitic stainless steel sheet having the steel components of the present invention and carrying out the preferred production conditions of the present invention exhibits excellent stress corrosion cracking resistance due to the refinement effect, and is compatible with workability. It was.

Test No. No. 5 has the steel component of the present invention, and the average crystal grain size is as small as 6 μm, but the ratio of large-angle grain boundaries of 15 ° or more is 75%, which is less than 80%. In addition, Test No. 5 is a stress corrosion crack occurrence time of 3 hours.
Test No. Although 6 has the steel component of this invention, it remove | deviates from the preferable manufacturing method of this invention. Test No. No. 6 has an average crystal grain size as small as 1 μm, but the final annealing temperature is low and the ratio of the large-angle grain boundaries of 15 ° or more is less than 80%. For this reason, the stress corrosion cracking resistance was not improved due to the residual strain during cold rolling, and the workability was also lowered due to the hardening of the material accompanying the increase in 0.2% proof stress.

    Test No. Although 30, 32, 34, 35, and 37 deviate from the steel components of the present invention, the preferred production method of the present invention was carried out. Test No. In Nos. 30, 32, and 37, even when the crystal grains were refined, the stress corrosion cracking occurrence time was less than 4 hr, and the improvement of the stress corrosion cracking resistance targeted by the present invention was not observed. Test No. Nos. 34 and 35 did not reach the target average grain size of 10 μm or less.

  Test No. 31, 33, and 36 deviate from the steel components and the preferred manufacturing method. These have an average crystal grain size of 28 μm or 30 μm and did not reach the target stress corrosion cracking resistance of the present invention as expected from the conventionally known components.

 According to the present invention, the stress corrosion cracking, which is a defect of the austenitic stainless steel, is overcome by the grain refining effect regardless of the amount of Ni of 8% or less and expensive Mo, and the compatibility with workability is achieved. The intended fine-grained austenitic stainless steel sheet can be obtained.

Claims (4)

In mass%, C: 0.05% or less, Cr: 14-19%, Si: 2% or less, Mn: 4% or less, Ni: 5-8%, Cu: 4% or less, N: 0.1 % Or less, the balance Fe and unavoidable impurities, and the following Md is in the range of −20 to 40, the average grain size is 10 μm or less, and the grain angle boundary is 15 ° or more. A fine-grained austenitic stainless steel sheet excellent in stress corrosion cracking resistance and workability, characterized in that its proportion is more than 80%.
Md = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr
-29 (Ni + Cu) -18.2Mo   Further, the steel components are in mass%, Mo: 1% or less, V: 1% or less, B: 0.010% or less, Nb: 0.5% or less, Ti: 0.5% or less, rare earth elements: 2. One or more of 0.5% or less, Al: 0.5% or less, Mg: 0.005% or less, Ca: 0.005% or less are contained. Fine grain austenitic stainless steel plate with excellent stress corrosion cracking resistance and workability.   2. A molded article obtained by subjecting a cylindrical deep drawing to a drawing ratio of 1.5 to 2.0 is immersed in a boiling 42% magnesium chloride aqueous solution for 4 hours so that no cracks are generated. Or the fine grain austenitic stainless steel plate excellent in the stress corrosion cracking resistance and workability of Claim 2.   The fineness excellent in stress corrosion cracking resistance and workability according to any one of claims 1 to 3, wherein the 0.2% proof stress obtained by a tensile test is less than 400 MPa and the uniform elongation is more than 30%. Grain austenitic stainless steel sheet.