JP5308726B2 - Austenitic stainless steel sheet for press forming having a fine grain structure and method for producing the same - Google Patents

Description

  The present invention relates to an austenitic stainless steel sheet for press forming having a fine grain structure and an improvement of the manufacturing method thereof, and in particular, an austenitic stainless steel sheet for press forming having a fine grain structure having an average crystal grain size of 10 μm or less, and It relates to the manufacturing method.

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

  Furthermore, as utilizing the grain refinement effect in austenitic stainless steel, Patent Document 1 discloses a metal gasket and its material and manufacturing method thereof, and Patent Document 2 discloses a high-strength metastable austenitic stainless steel. Each is disclosed. Specifically, in Patent Document 1, a fine grain structure having a crystal grain size of 5 μm or less is utilized by utilizing the phase transformation from processing-induced martensite to austenite and precipitation of chromium nitride as described above in SUS301L defined in JIS G4305. Furthermore, high strength of Hv500 or higher is achieved by combination with temper rolling. On the other hand, in patent document 2, the crystal grain size of SUS304 is 19-50 micrometers using the fine dispersion of an oxide, and it aims at prevention of the crack by wire drawing or cold work.

  As described above, the formation of a fine grain structure in SUS304 or SUS301L is aimed at increasing the strength. That is, the fine grain structure of conventional austenitic stainless steel is aimed at increasing strength. However, conventionally, it has not been clarified at all about the utilization of the miniaturization effect for press molding applications.

  By the way, in the austenitic stainless steel sheet, a problem that cracks when left after deep drawing, that is, so-called time cracks, has been known for a long time. Therefore, Non-Patent Document 3 describes that time cracking is improved in order of increasing Ni content in SUS301, SUS304, and SUS305, and is improved as the C and N content is lower.

Furthermore, with respect to the improvement of press formability related to the deep drawability as described above, for example, as shown in Patent Documents 3 to 7, many studies on components have been made. Specifically, Patent Documents 3 and 4 disclose austenitic stainless steel for press forming that has improved time cracking by the combined addition of Mo, Cu, and Al. On the other hand, in Patent Document 5, C: 0.035% or less, Si: 0.6% or less, Mn: 0.8 to 1.8%, Ni: 7.5 to 8.3%, Cr: 16. 3 to 17.1%, Cu: 1.7 to 2.3%, Mo: 0.7 to 1.1%, N: 0.025% or less, and the value of the constitutive formula comprising these components is An austenitic stainless steel sheet excellent in prescribed press formability and corrosion resistance is disclosed. On the other hand, in Patent Document 6, C: 0.04% or less, Si: 1.0% or less, Mn: 1.7 to 2.1%, Ni: 5 to 9%, Cr: 15 to 20%, Cu : Soft austenitic stainless steel having 1.0 to 5.0%, N: 0.035% or less, and having improved workability by defining values of constitutive formulas composed of these components is disclosed. On the other hand, Patent Document 7 discloses an austenitic stainless steel excellent in secondary workability after deep drawing, in which the component range is further strictly defined.
WO / 2002/088410 JP 2001-262281 A Japanese Patent No. 3398250 Japanese Patent No. 3422592 Japanese Patent No. 3734428 JP 2005-68560 A JP 2006-52457 A Iron and steel, 78 (1992), p141-148 Iron and steel, 80 (1994), p249-253 Stainless Steel Handbook, 3rd edition, p560 Japan Steel Pipe Technical Report, No. 87 (1980), p51-60 Mater. Trans. , 45 (2004), p2272

  As described above, the austenitic stainless steels disclosed in Patent Documents 3 to 7 have a lower content of C and N, adjust the content of Ni, Cu, and Mn, which are austenite forming elements, and further range of components. The press formability associated with deep drawability is improved by strictly defining the amount of addition of other elements. Further, it is specified that a cold-rolled steel sheet is annealed at 1050 to 1100 ° C. as a method for producing these steels.

  However, in Non-Patent Document 4, even if the components of austenitic stainless steel are adjusted, if the annealing is performed at 1000 ° C. or higher, the limit of refining is the grain size No. It has been shown that less than 10, ie the crystal grain size is greater than 10 μm. Therefore, it is easily guessed that the crystal grain size of the austenitic stainless steel disclosed in Patent Documents 3 to 7 is larger than 10 μm.

  As described above, in the austenitic stainless steel, there has been no investigation in which a fine grain structure having an average crystal grain size of 10 μm or less is formed and tried to be applied to press forming applications. That is, until now, there has been no technical idea and such disclosure of utilizing the miniaturization effect to overcome the time crack, which is a drawback of austenitic stainless steel, and to improve the press formability.

  The present invention has been made in view of the above circumstances, and overcoming time cracking, which is a defect of austenitic stainless steel due to grain refinement effect, regardless of strict component adjustment and addition of alloy elements, An object of the present invention is to provide an austenitic stainless steel sheet for press forming having a fine grain structure with improved press formability and a method for producing the same.

In order to solve the above-mentioned problems, the present inventors have studied earnestly on the improvement effect of time cracking due to the optimal component balance and refinement effect for the formation of fine grain structure, targeting austenitic stainless steel softer than SUS304. To complete the present invention.
That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.085 % 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 is a steel component composed of Fe and inevitable impurities, Md value shown in the following formula (1) is in the range of -20 to 40, and the average crystal grain size is 10 μm or less. Ri, cylindrical deep drawing was allowed to austenitic stainless steel sheet for press forming having a fine grained structure drawing ratio for generating a timing cracking, characterized in der Rukoto than 2.2 after molding.
Md = 551-462 × ([C] + [N]) − 9.2 × [Si] −8.1 × [Mn] −13.7 × [Cr] −29 × ([Ni] + [Cu] ) -18.2 × [Mo] (1)
(Where [] is mass%)
(Here, the drawing ratio represents a value obtained by dividing the blank diameter (mm) by the punch diameter (mm).)
(2) The steel is further 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 Element: 0.5% or less, Al: 0.5% or less, Mg: 0.005% or less, Ca: 0.005% or less An austenitic stainless steel sheet for press forming having a fine grain structure as described in 1).
(3) above (1) or a method for producing a press-molding austenitic stainless steel sheet having a fine grain structure according to (2), hot having steel components described in the above (1) or (2) after annealing the rolled steel sheet, subjected to cold rolling, 650 to 9 50 ° C. in a final annealing method for producing a press-molding austenitic stainless steel sheet having a fine grain structure, characterized by.
( 4 ) The cold-rolled austenitic stainless steel sheet for press forming having a fine grain structure according to ( 3 ) above, wherein the rolling reduction is 70% or more and the rolling temperature is 50 ° C. or less. Production method.
( 5 ) In the final annealing, the press having a fine grain structure according to the above ( 3 ) or ( 4 ), wherein the annealing temperature is set to a temperature range of 650 to 900 ° C. and the annealing time is set to 10 minutes or more. A method for producing an austenitic stainless steel sheet for forming.

As described above, according to the austenitic stainless steel sheet for press forming having a fine grain structure according to the present invention, regardless of strict component adjustment and addition of alloy elements, the effect of austenitic stainless steel is achieved by the effect of grain refinement. It is possible to overcome the time crack that is a defect and to improve the press formability.
Moreover, according to the manufacturing method of the austenitic stainless steel sheet for press forming which has the fine grain structure of this invention, the austenitic stainless steel sheet for press molding which has the said fine grain structure can be manufactured.

  In order to solve the above-mentioned problems, the present inventors have made an optimal balance of components for the formation of a fine-grained structure and an improvement effect of the time cracking due to the refinement effect for austenitic stainless steel softer than SUS304. Various studies were conducted and the following new findings were obtained.

Below, the typical result of the said examination is demonstrated.
The austenitic stainless steel which shows a component in the following Table 1 was melted, and a hot-rolled sheet having a thickness of 3.0 mm was manufactured. Here, hot-rolled sheet annealing was performed at 1150 ° C. and pickled to prepare 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. Furthermore, 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 ranges from 1 minute to 24 hours in the temperature range of 600 to 1050 ° C. It was changed with. Furthermore, after pickling the cold-rolled annealed plate, the average crystal grain size was measured and a multistage deep drawing test was performed to determine the limit drawing ratio at which time cracking occurred.

  The average crystal grain size was determined by a steel-crystal grain size microscopic test method specified in JIS G0551 after embedding a plate cross-section in a resin, polishing it and electrolytically etching nitric acid. On the other hand, in the multi-stage deep drawing test, cylindrical deep drawing was performed under the conditions of a blank diameter of 72 mmφ, a punch diameter of 40 mmφ, a die diameter of 42 mmφ, and a wrinkle holding pressure of 1 ton. Carried out. In the second and subsequent cylindrical deep drawing, the punch diameter is set to the second stage: 35 mmφ, the third stage: 30 mmφ, the fourth stage: 25 mmφ, the fifth stage: 22 mmφ, and the sixth stage: 20 mmφ. The transition was carried out when no cracking occurred after standing for 48 hours.

(A) FIG. 1 is a graph showing the relationship between the average crystal grain size and the component balance (Md) when cold-rolled sheet annealing is held at 800 ° C. for 1 hour. Here, the Md value is defined as the following equation (1). As shown in FIG. 1, the average crystal grain size decreases as the Md value increases. That is, as the Md value increases, the amount of work-induced martensite generated by cold rolling increases. Therefore, it is considered that the increase in the Md value promoted 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 above. . Therefore, from this study, it is effective to set the Md value to -20 or more in order to make the target average crystal grain size 10 μm or less. In addition, even if the Md value is almost equivalent to SUS304, it was confirmed that the component (steel B in Table 1) in which Cr and Ni are reduced and Cu is added is more effective for miniaturization. .
Md = 551-462 × ([C] + [N]) − 9.2 × [Si] −8.1 × [Mn] −13.7 × [Cr] −29 × ([Ni] + [Cu] ) -18.2 × [Mo] (1)
(Where [] is mass%)

(B) FIG. 2 is a view showing an appearance after a multistage deep drawing test in steel B (Md = 29.5), and FIG. 2 (a) shows an appearance after a six-stage cylindrical deep drawing. ) Shows the external appearance after four-stage cylindrical deep drawing. Moreover, FIG. 3 is a figure which shows the observation result of the microstructure of the data which used for the multistage deep drawing test in Steel B, FIG.3 (a) shows the observation result of the microstructure of steel shown to Fig.2 (a). 3 (b) shows the observation results of the microstructure of the steel shown in FIG. 2 (b).
As shown in FIG. 2 (a), the steel (see FIG. 3 (a)) whose average crystal grain size has been refined to 3 μm has cracked even after the cylindrical deep drawing test up to the sixth stage. Absent. On the other hand, as shown in FIG. 2 (b), a steel (see FIG. 3 (b)) manufactured under normal annealing conditions (1050 ° C., held for 60 seconds) and having an average crystal grain size of 28 μm is Time cracking occurred after the fourth-stage cylindrical deep drawing test.
From this result, a novel finding was found that the periodical crack limit drawing ratio is drastically improved by the effect of crystal grain refinement.

(C) FIG. 4 is a graph showing the relationship between the time crack limit drawing ratio, the average crystal grain size, and the Md value. As shown in FIG. 4, when Md = 29.5, it is confirmed that the time crack limit drawing ratio is increased due to the effect of crystal grain refinement. Although this reason is not necessarily clear, it is estimated as follows. That is, in steel materials, increasing toughness by refining crystal grains is well known. For example, Non-Patent Document 5 described above reports the effect of miniaturization against brittle fracture of BCC metal that occurs at low temperatures. This time crack generated by deep drawing of austenitic stainless steel is a brittle crack (brittle fracture) occurring in a mixed state of austenite (FCC) of the parent phase and work-induced martensite (BCC). It is presumed that the inhibitory effect by miniaturization was developed for such brittle fracture as well as BCC metal.

(D) The expression of the above-described miniaturization effect is affected by the component balance (Md). In FIG. 3, in the case of Md = 43 (steel A shown in Table 1), the time crack limit drawing ratio does not increase even if it is miniaturized and slightly decreases. The reason for this is presumed that the residual stress on the side wall of the cup easily reached the limit of time cracking due to the hardening of the material itself by miniaturization and the generation of a large amount of work-induced martensite in deep drawing. From the results of this study, it is effective for the Md value to be 40 or less for the manifestation of the refinement effect that suppresses the time cracking.

(E) On the other hand, in the case of Md = −25 (steel G shown in Table 1) having a low Md value, it is difficult to form a fine grain structure as described in (a) above, and the material itself is processed. Since the induction martensite formation is suppressed, it is difficult to obtain a refinement effect that suppresses time cracking.

(F) Conventionally, in SUS304, which has been reported on many cases for studying miniaturization, it is difficult to obtain the effect of suppressing the time crack from FIG. 3 as in the case of (d). This reason is presumed to be because the material itself is hardened by miniaturization as previously reported.

(G) The refinement of crystal grains is affected by manufacturing conditions in addition to the component conditions described in (a) above. Therefore, 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, it is preferable to hold the final annealing performed after the cold rolling at a low temperature for as long as possible.

  The present inventions (1) to (6) 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%".

The austenitic stainless steel sheet for press forming having a fine grain structure of the present invention is, in mass%, C: 0.1% or less, Cr: 14 to 19%, Si: 2% or less, Mn: 4% or less, Ni : 5 to 8%, Cu: 4% or less, N: 0.1% or less, the balance is a steel component composed of Fe and inevitable impurities, and the value of Md shown in the following formula (2) is -20 to 40 The average crystal grain size is 10 μm or less.
Md = 551-462 × ([C] + [N]) − 9.2 × [Si] −8.1 × [Mn] −13.7 × [Cr] −29 × ([Ni] + [Cu] ) -18.2 × [Mo] (2)
(Where [] is mass%)
Further, the austenitic stainless steel sheet for press forming having a fine grain structure of the present invention, the above steel is further 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 element: 0.5% or less, Al: 0.5% or less, Mg: 0.005% or less, Ca: 0.005% or less It is preferable to contain two or more species.
Furthermore, it is preferable that the austenitic stainless steel sheet for press forming having a fine grain structure according to the present invention has a drawing ratio of 2.2 or more, which is left after cylindrical deep drawing to generate a time crack. Here, the drawing ratio represents a value obtained by dividing the blank diameter (mm) by the punch diameter (mm).

(A) The reason for limitation regarding the component will be described below.
The austenitic stainless steel sheet for press forming according to the present invention has a fine grain structure with an average crystal grain size of 10 μm or less, and the components and the component balance are defined in order to improve the time crack limit drawing ratio by the refinement effect. It is.

  C is an austenite-forming element and is added for the purpose of securing austenite stability. However, if added in a large amount, it becomes hard and processability is lowered, so the upper limit is made 0.1%. Preferably it is 0.05% or less. The lower limit is preferably 0.005% from the viewpoint of manufacturability. More preferably, the content is 0.01% or more.

  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% or more, more preferably 16% or more. On the other hand, when a large amount of Cr is added, 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% or less.

  Si is effective as a strong deoxidizing agent, but when added in a large amount, it becomes hard and inhibits manufacturability, so the upper limit is made 2% or less. Preferably it is 1% or less. The lower limit is preferably 0.1% or more from the viewpoint of manufacturability. More preferably, it is 0.2% or more.

  Mn is an austenite-forming element and is added for the purpose of ensuring austenite stability and improving deep drawability. However, if added in a large amount, MnS is formed and the corrosion resistance is lowered, so the upper limit is made 4% or less. Preferably, it is 3% or less. The lower limit is preferably 0.5 or more for the above purpose. More preferably, it is 1% or more.

  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% or more. 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 and hardens, 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, but Cu is a preferable element for saving Ni 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% or less. On the other hand, the lower limit is preferably 1% in order to obtain the above effects, and more preferably 1.5% or more.

  N is an austenite-forming element like C, and is added for the purpose of ensuring austenite stability. However, if added in a large amount, it becomes hard and the workability deteriorates, so the upper limit is made 0.1%. Preferably it is 0.06% or less. On the other hand, the lower limit is preferably 0.005% from the relationship with manufacturability, and more preferably 0.01% or more.

  Mo is not an essential element in the present invention, but is an element effective for improving corrosion resistance. However, since Mo is a very expensive and rare element, the upper limit is made 1%. Preferably it is 0.5% or less. On the other hand, the lower limit is preferably 0.05% in order to obtain the effect of improving the corrosion resistance, and more preferably the lower limit is 0.1%.

  V is not an essential element in the present invention, but is an element effective for improving corrosion resistance even if it does not reach Mo. However, V is an expensive element and is a solid solution strengthening element, which impairs workability. Therefore, the upper limit is 1%. Preferably it is 0.5% or less. On the other hand, the lower limit is preferably 0.1% in order to obtain the effect of improving the corrosion resistance.

  B and rare earth elements may be added in a timely manner in order to improve hot workability. However, if B exceeds 0.01%, the productivity and corrosion resistance may be significantly impaired. Therefore, the upper limit of B is preferably 0.01%, more preferably 0.005% or less. In addition, when adding B, it is preferable to make the minimum of B into 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 0.2% or less. In addition, when adding rare earth elements, it is preferable that the minimum of rare earth elements shall be 0.005%.

  Nb and Ti form carbonitrides and contribute to the refinement of crystal grains. Therefore, Nb and Ti may be added as appropriate. 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% or less. In addition, when adding Nb and Ti, it is preferable that a minimum is 0.005%. More preferably, it is 0.01% or more.

  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% or less, More preferably, it is 0.1% or less. In addition, when adding Al, it is preferable to make a minimum into 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 leads to deterioration of corrosion resistance and weldability, so the upper limit is made 0.005%. Preferably, it is 0.002% or less. In addition, when adding Mg and Ca, it is preferable to make a minimum into 0.0001%. More preferably, it is 0.0003% or more.

  Furthermore, the austenitic stainless steel of this invention may contain P and S in the following range as a part of inevitable impurities other than said component. P and S are elements harmful to hot workability and corrosion resistance. Therefore, P is preferably 0.1% or less, and more preferably 0.05% or less. On the other hand, S is preferably 0.01% or less, and more preferably 0.005% or less.

In the present invention, in addition to the component range described above, the optimum component balance for the formation of a fine grain structure is defined by the Md value shown in the following equation (3).
Md = 551-462 × ([C] + [N]) − 9.2 × [Si] −8.1 × [Mn] −13.7 × [Cr] −29 × ([Ni] + [Cu] ) -18.2 × [Mo] (3)
(Where [] is mass%)

  Metastable austenitic stainless steel causes martensitic transformation by plastic working even at a temperature equal to or higher than the Ms point (martensitic transformation start temperature). Thus, the upper limit temperature that causes transformation by processing is called the Md value. That is, the Md value is an index indicating the stability of austenite. Therefore, by designing the Md value in the range of -20 to 40, the formation of the fine grain structure targeted by the present invention and the effect of improving the time crack limit drawing ratio by this refinement effect can be obtained. However, when the Md value is less than −20, as described in the above (e), it becomes difficult to form a fine grain structure and to suppress the time cracking. On the other hand, when the Md value exceeds 40, as described in the above (d), it is effective for the formation of a fine grain structure, but it inhibits the effect of suppressing the time cracking. A more preferable Md value is in the range of −5 to 35.

(B) The reason for limitation regarding the manufacturing method will be described below.
The manufacturing method of the austenitic stainless steel sheet for press forming which has the fine grain structure of this invention performs cold rolling after annealing the hot-rolled steel sheet which has the steel component described in the said (A) term, 650-1050 It is characterized in that it is finally annealed at ℃.
In cold rolling, the rolling reduction is preferably 70% or more, and the rolling temperature is preferably 50 ° C. or less. In the final annealing, the annealing temperature is set to a temperature range of 650 to 900 ° C., and the annealing time is 10 minutes or more. It is preferable to do.

  That is, the method for producing an austenitic stainless steel sheet for press forming having a fine grain structure according to the present invention has the components described in the above item (A) and is specifically produced so as to have an average crystal grain size of 10 μm or less. It is preferable to control the conditions as follows.

  A known method can be applied to the production method up to hot rolling, and it is not particularly limited. In order to form a fine grain structure in the final annealing after cold rolling, it is effective to promote the work-induced martensitic transformation by cold rolling as described in (g) above. On the other hand, in order to obtain an average crystal grain size of 10 μm or less as an object of the present invention, it is effective to set the work-induced martensite volume ratio to 50% or more after cold rolling. Here, the final annealing conditions after cold rolling are specified to satisfy the above-described requirements. Furthermore, it is preferable to also define cold rolling conditions and consider hot-rolled sheet annealing.

  Hot rolled sheet (hot rolled steel sheet) annealing is performed at a temperature range of 1050 to 1200 ° C. 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 do. Here, when the temperature is lower than 1050 ° C., the austenite particle size may be less than 20 μm, which is not preferable. On the other hand, when the temperature exceeds 1200 ° C., surface quality such as pickling after annealing may be hindered. Moreover, annealing over 1200 ° C. has a heavy load on the equipment. Therefore, it is preferable to set it as the range of 1050-1200 degreeC as mentioned above, More preferably, it is the range of 1080-1180 degreeC.

  The final annealing is set to a temperature range of 650 to 1050 ° C. in order to make the average crystal grain size 10 μm or less. Here, a lower limit temperature of less than 650 ° C. is not preferable because strain in cold rolling is accumulated and recrystallization of austenite grains becomes insufficient and workability is significantly reduced. Accordingly, the lower limit temperature is 650 ° C., preferably 700 ° C., more preferably 750 ° C. On the other hand, when the upper limit temperature exceeds 1050 ° C., austenite crystal grain growth proceeds and the average crystal grain size exceeds 10 μm, which is not preferable. Accordingly, the upper limit temperature is 1050 ° C., preferably 1000 ° C., more preferably 900 ° C.

Here, when the temperature range of the final annealing is 650 to 900 ° C., the annealing time is preferably 10 minutes or more in order to promote recrystallization of austenite and remove the strain in cold rolling, and 30 minutes. More preferably. The upper limit of the holding time (annealing time) is not particularly limited, but it is preferably 24 hours or less assuming box annealing that is industrially known for chromium-based stainless steel. However, when manufacturing with a small-scale annealing facility, it is not limited to the above and may exceed 24 hours.
On the other hand, when the temperature range of final annealing is 900 to 1050 ° C., the annealing time is preferably held for a short time of 10 minutes or less, more preferably 1 minute or less in consideration of crystal grain growth.

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. Here, when the rolling reduction is less than 70%, the work-induced martensite volume fraction is less than 50%, and it is difficult to form a fine grain structure as described above for P and S which are inevitable impurities. It becomes. More preferably, the rolling reduction is 80% or more. In addition, although the upper limit of this rolling reduction is not specifically prescribed | regulated, 90% is preferable in consideration of hot-rolled sheet manufacture and cold-rolling equipment capability.
On the other hand, when the rolling temperature is higher than 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. Further, the lower limit of the rolling temperature is not particularly specified, but industrially, a temperature of 10 ° C. reached by water cooling is preferable. However, when manufacturing with a small-scale rolling facility, it is not limited to the above, and may be a low temperature (for example, −200 ° C.) reached by cooling liquid nitrogen or the like.

  The crystal grain size of the austenitic stainless steel for press molding of the present invention has the component of the above item (A), and the production condition of the above item (B) is carried out so that the average crystal particle size is 10 μm or less. Here, when the average crystal grain meter is more than 10 μm, it becomes difficult to suppress the time cracking due to the refinement effect aimed at by the present invention. Therefore, in order to take advantage of these refinement effects, the average crystal grain size is preferably 5 μm or less. The lower limit of the average crystal grain size is not particularly specified, but it is difficult to make the average crystal grain size less than 1 μm also from the description of the non-patent literature. Therefore, in consideration of practical use, the average crystal grain size is more preferably in the range of 1 to 5 μm.

  Below, the manufacturing method of this invention is implemented and the Example of the austenitic stainless steel plate made into the component and average grain size of this invention is described.

  First, an austenitic stainless steel slab having components shown in Table 2 was melted and hot-rolled to obtain a hot-rolled sheet having a thickness of 3.5 mm. Here, steel Nos. 1-23 satisfy | fills the component and Md value which are prescribed | regulated by this invention. On the other hand, Steel No. Nos. 24-28 are those in which one or both of the component defined in the present invention and the Md value deviate.

Next, each hot-rolled sheet was annealed and subjected to cold rolling and final annealing to obtain a product. Cold rolling and final annealing were performed under the conditions specified in the present invention and other conditions. Table 3 shows the manufacturing conditions for each step. As shown in Table 3, cold rolling is performed at a room temperature while cooling with water at a temperature lower than 30 ° C. (described as “<30 ° C.” in Table 3). The test was performed under the two conditions of over 50 ° C. (described as “> 50 ° C.” in Table 3).
After the final annealed plate was pickled, the average grain size was measured, the hardness was measured, and the multistage deep drawing test was performed to determine the limit drawing ratio at which time cracking occurred.

The average crystal grain size was determined by the method specified in JISG0551 as described above.
The hardness was measured by a Vickers hardness test specified in JISZ2244 using an embedded sample whose average crystal grain size was measured.
The multistage deep drawing test was performed by the method described in the above embodiment, and the limit drawing ratio at which time cracking occurred was measured.
The upper limit of the limit drawing ratio measured in this test is the blank diameter 72 mm / punch diameter 20 mm = 3.6. Accordingly, when the limit drawing ratio exceeds 3.6, it is set to 3.6 or more (the same applies to Table 3).

Table 3 shows the various evaluation results described above.
Test No. 6 has the component of this invention and Md value, and is manufactured at a well-known annealing temperature. This test No. The average crystal grain size of No. 6 is 30 μm, and the time crack limit drawing ratio is 2.2. In contrast, test no. 1-4, 7-28 have the component of this invention and Md value, and implemented the manufacturing conditions prescribed | regulated by this invention (it describes as an invention example in the remarks column of Table 3). The steel sheets of these inventive examples had an average crystal grain size of 10 μm or less, and the limit drawing ratio at which time cracking occurred was significantly higher than 2.2 due to the effect of crystal grain refinement. Thereby, the austenitic stainless steel sheet having the components of the present invention and the Md value and carrying out the production conditions defined in the present invention has extremely excellent time cracking resistance due to the refinement effect in deep drawing. It was confirmed that

  Test No. Although 5 has the component and Md value of this invention, it is outside the manufacturing method prescribed | regulated by this invention. Test No. 5 is a non-recrystallized structure having a low average annealing temperature and a low final annealing temperature. Therefore, Hv250 becomes hard due to the remaining strain during cold rolling, and the improvement of the time crack limit drawing ratio due to refinement was not observed.

  Test No. Although 29, 31, 33, 34, and 36 deviate from the components and Md values of the present invention, the production methods defined in the present invention were carried out. Here, test no. Nos. 29, 31, and 36 have Hv 220 or more and a time crack limit drawing ratio of less than 2.2 due to the refinement of crystal grains, and the improvement of the time crack limit draw ratio due to the refinement was not observed. On the other hand, test no. Nos. 33 and 34 did not reach the average grain size of 10 μm or less.

  Test No. Nos. 30, 32, and 35 are components and Md values of the present invention, and production methods are all out of the specified range of the present invention. These have an average crystal grain size of 28 and 30 μm, and exceed 2.2 due to the effects of conventionally known components (low C, low N, small Md value, and suppresses the formation of processing-induced martensite). The time crack drawing ratio was obtained.

  According to the present invention, time cracking, which is a defect of austenitic stainless steel, can be overcome by the refinement effect of crystal grains, regardless of strict component adjustment and addition of alloy elements. Thereby, an austenitic stainless steel sheet for press forming having a fine grain structure with improved press formability can be obtained.

FIG. 1 is a graph showing the relationship between the average crystal grain size and the component balance (Md) when cold-rolled sheet annealing is held at 800 ° C. for 1 hour. FIG. 2 is a view showing an appearance after the multistage deep drawing test. FIG. 3 is a diagram showing the observation result of the microstructure of the material subjected to the multistage deep drawing test. FIG. 4 is a graph showing the relationship between the time crack limit drawing ratio, the average crystal grain size, and the Md value.

Claims (5)

In mass%, C: 0.085 % 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, has a steel component and the balance being Fe and unavoidable impurities, and a range of values of Md is -20 to 40 shown below (1), Ri average crystal grain size der less 10 [mu] m, a cylindrical deep drawing left drawing ratio for generating a timing cracking in austenitic stainless steel sheet for press forming having a fine grain structure, characterized in der Rukoto than 2.2 after molding.
Md = 551-462 × ([C] + [N]) − 9.2 × [Si] −8.1 × [Mn] −13.7 × [Cr] −29 × ([Ni] + [Cu] ) -18.2 × [Mo] (1)
(Where [] is mass%)
(Here, the drawing ratio represents a value obtained by dividing the blank diameter (mm) by the punch diameter (mm).)   The steel 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, rare earth element: 0 2 or less, Al: 0.5% or less, Mg: 0.005% or less, Ca: 0.005% or less. An austenitic stainless steel sheet for press forming having a fine grain structure. A method for producing an austenitic stainless steel sheet for press forming having the fine grain structure according to claim 1 or 2 ,
After annealing the hot-rolled steel sheet having a steel composition according to claim 1 or 2, carried out cold rolling, austenite for press molding having a fine grain structure, characterized in that the final annealing at 650 to 9 50 ° C. Of manufacturing stainless steel sheet. 4. The method for producing an austenitic stainless steel sheet for press forming having a fine grain structure according to claim 3 , wherein the rolling reduction is 70% or more and the rolling temperature is 50 [deg.] C. or less. The austenitic stainless steel sheet for press forming having a fine grain structure according to claim 3 or 4 , wherein in the final annealing, an annealing temperature is set to a temperature range of 650 to 900 ° C, and an annealing time is set to 10 minutes or more. Manufacturing method.

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