JP2022003160A - Austenitic stainless steel - Google Patents

Abstract

To achieve a stainless steel that has improved workability during press molding, and also has small dimensional change due to heat treatment after press molding.SOLUTION: In the inventive austenitic stainless steel, a value of Md30 calculated by the formula (1) is -60 or more and -20 or less, and a volumetric change index value calculated by the formula (2) is 14.5 or less. Md30=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo (1). Index value=Cr+6(Si+Al)+2Mo-0.5 Ni-Cu (2).SELECTED DRAWING: None

Description

The present invention relates to austenitic stainless steel.

Conventionally, various press-molded products have been manufactured by press-molding austenitic stainless steel (hereinafter referred to as stainless steel). Here, various problems arise when the stainless steel is press-formed. For example, in SUS304, which is a typical stainless steel, at least a part of the austenite phase in the stainless steel is transformed into a martensitic phase (hereinafter referred to as martensitic transformation) due to strain during press forming. If work hardening of stainless steel progresses due to martensitic transformation, there is a risk that work defects, such as cracks, will occur in the press-molded product. In order to solve this problem, for example, Patent Document 1 discloses stainless steel that can reduce processing defects of a press-molded product.

Japanese Unexamined Patent Publication No. 2006-291296

Depending on the purpose of use of the press-molded product, after press-molding the stainless steel, heat treatment may be performed for the purpose of strain removal, brazing, diffusion bonding, or the like. By this heat treatment, at least a part of the martensite phase in the stainless steel is transformed into the austenite phase (hereinafter referred to as austenite transformation), and the volume of the stainless steel is reduced.

Here, with respect to the stainless steel disclosed in Patent Document 1, measures against the decrease in volume when the above heat treatment is performed are not always sufficient. Therefore, in the stainless steel disclosed in Patent Document 1, when a press-molded product is manufactured by heat treatment after press molding, the dimensions of the press-molded product become smaller than the desired dimensions beyond an allowable range, and the press-molding is performed. There is a risk that the dimensional accuracy of the product will decrease.

One aspect of the present invention has been made in view of the above problems, and an object thereof is stainless steel, which has a small dimensional change when heat treatment is applied after press molding while improving processability at the time of press molding. It is to realize steel etc.

In order to solve the above problems, the austenitic stainless steel according to one aspect of the present invention has C: 0.025% by mass or less, Si: 1.00% by mass or less, Mn: 0.30% by mass or more and 3 .50% by mass or less, P: 0.04% by mass or less, S: 0.015% by mass or less, Ni: 7.00% by mass or more and 12.00% by mass or less, Cr: 16.00% by mass or more and 20 Austenitic stainless steel with .00% by mass or less, Cu: 3.50% by mass or less, N: 0.025% by mass or less, Al: 0.1% by mass or less, and the balance is Fe except for unavoidable impurities. , (C + N): 0.035% by mass or less, _{the value of Md 30} calculated by the following formula (1) is -60 or more and -20 or less, and the martensite phase is calculated by the following formula (2). The volume change index value, which is an index for predicting the degree of volume change during transformation from to austenitic phase, is 14.5 or less.

Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo ... (1)
Volume change index value = Cr + 6 (Si + Al) + 2Mo-0.5Ni-Cu ... (2)
According to the above configuration, since _{the value of Md 30} is −20 or less, the amount of martensite phase transformed from the austenite phase during press forming of stainless steel can be reduced. Therefore, when the stainless steel after press forming is heat-treated, the amount of the austenite phase transformed from the martensite phase can be reduced.

Here, the crystal structure of the martensite phase is a body-centered cubic lattice (bcc), and the crystal structure of the austenite phase is a face-centered cubic lattice (fcc). In general, fcc has a higher atomic packing factor than bcc, so the transformation from the martensite phase to the austenite phase causes a volume reduction in the stainless steel. Therefore, by _{setting the value of Md 30} to −20 or less and reducing the amount of the austenite phase that transforms from the martensite phase during the heat treatment, the dimensional change of the press-molded article can be reduced. As a result, the dimensional accuracy of the heat-treated press-molded product can be improved.

Further, since _{the value of Md 30} is -60 or more, martensitic transformation can be caused during press forming of stainless steel. In general, martensitic transformation improves the workability of stainless steel during press forming. Therefore, by _{setting the value of Md 30} to -60 or more, the workability at the time of press forming of stainless steel can be improved.

Further, the volume change index value calculated by the equation (2) is 14.5 or less. Here, through diligent studies by the present inventors, it is found that the volume change index value calculated by the equation (2) is an index for predicting the degree of volume change during the transformation from the martensite phase to the austenite phase. It was issued. The lower the volume change index value, the lower the volume change rate during austenite transformation. Therefore, even when a part of the stainless steel is transformed from the martensite phase to the austenite phase by heat treatment, the alloy component of the stainless steel is adjusted so that the volume change index value is 14.5 or less. It is possible to reduce the change in the volume of steel. Therefore, it is possible to reduce the dimensional change of the stainless steel that occurs when the stainless steel after the heat treatment is heat-treated, and to more reliably improve the dimensional accuracy of the press-molded product that has been heat-treated.

Further, the value of (C + N) is 0.035% by mass or less. Here, both C and N increase the strength of the stainless steel to lower the press formability, and also have a strong solid solution strengthening action, so that time cracking after press forming is likely to occur. Therefore, by setting not only the individual values of C and N but also their total values to the predetermined values or less, stainless steel is compared with the case where only the individual values of the alloy components are set to the predetermined values or less. The workability during press forming of steel can be further improved.

The austenitic stainless steel according to one aspect of the present invention has Mo: 1.0% by mass or less, V: 0.50% by mass or less, B: 0.0001% by mass or more and 0.01% by mass or less. .. According to the above configuration, since Mo: 1.0% by mass or less, the dimensional change due to the heat treatment of the press-molded stainless steel product can be further reduced, and the dimensional accuracy of the press-molded product can be further improved. Further, since V: 0.50% by mass or less, the formation of a ferrite phase can be suppressed, the austenite phase can be stabilized, and the raw material cost of stainless steel can be reduced. Further, since B: 0.0001% by mass or more, it is possible to improve the hot workability of stainless steel and reduce cracks during hot rolling. Then, B: Since 0.01 wt% or less, to reduce the precipitation of Cr ₂ B, it is possible to maintain the corrosion resistance of stainless steel.

Further, from the austenitic stainless steel according to one aspect of the present invention, a press-formed product in which the volume ratio of the martensite phase to the austenitic stainless steel is 50% or less can be produced by press molding. According to the above-mentioned press-molded product, since the volume ratio of the martensite phase is 50% or less, the dimensional change due to the heat treatment of the press-molded product can be more reliably reduced, and the dimensional accuracy of the press-molded product is more reliably improved. Can be made to.

According to one aspect of the present invention, it is possible to realize stainless steel or the like in which the dimensional change that occurs when heat treatment is applied after press molding is small while improving the workability during press molding.

It is a schematic diagram which shows the method of manufacturing the press-molded article by press-molding from the austenitic stainless steel which concerns on one Embodiment of this invention.

Hereinafter, one embodiment of the present invention will be described in detail. In the austenitic stainless steel (hereinafter referred to as stainless steel) according to the embodiment of the present invention, the content of each alloy element is specified, (C + N): 0.035% by mass or less, and the value of _{Md 30 is} It is -60 or more and -20 or less, and the volume change index value is 14.5 or less.

[Value of _{Md 30]
Md 30} calculated by the following equation (1) is defined as the temperature at which 50% of the austenite phase undergoes martensitic transformation in a tensile deformation with a true strain of 0.3. The _{higher the value of Md 30,} the more unstable the austenite phase.

Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo ... (1)
Here, the crystal structure of the martensite phase is a body-centered cubic lattice (bcc), and the crystal structure of the austenite phase is a face-centered cubic lattice (fcc). Generally, in the case of a crystal structure composed of a single element, it is known that the filling rate of an atom of bcc is 68% and the filling rate of an atom of fcc is 74%. That is, in the case of a crystal structure composed of a single element, fcc has a higher atomic packing factor than bcc.

The above general theory of atomic packing factor also applies to crystal structures composed of a plurality of elements. Even in the case of a crystal structure composed of a plurality of elements, fcc has a higher atomic packing factor than bcc. Therefore, even for stainless steel, which is an alloy containing multiple elements, the transformation from the martensite phase to the austenite phase increases the atomic filling rate and increases the density of the stainless steel, so the volume of the stainless steel per unit mass. Decreases. Therefore, in the heat treatment after press molding, when the amount of the martensite phase transformed into the austenite phase is large, the dimensional change in the heat treatment becomes large.

When _{the value of Md 30} is higher than -20, the proportion of martensite phase produced by press forming is high. When the ratio of the martensite phase after press molding is high, the amount of the martensite phase transformed into the austenite phase increases due to the subsequent heat treatment. Therefore, the dimensional change due to the heat treatment of the press-molded product becomes large, and the dimensional accuracy of the press-molded product deteriorates. Therefore, _{the value of Md 30} is set to -20 or less. The press-molded body is a structure produced by press-molding stainless steel and in a state before being heat-treated. Therefore, if heat treatment is not applied, the press-molded product becomes a press-molded product as a final product.

On the other hand, when _{the value of Md 30} is lower than -60, the austenite phase is stabilized more than necessary, so that martensitic transformation due to press molding is unlikely to occur. Therefore, the TRIP (Transformation Induced Plasticity) phenomenon is unlikely to occur, and the workability during press forming (hereinafter referred to as press formability), particularly the processability during overhanging, is lowered. Therefore, _{the value of Md 30} is set to -60 or more.

[Volume change index value]
According to the diligent studies of the present inventors, there were cases where the dimensional change due to the heat treatment was different even if the ratio of the martensite phase was the same. The present inventors speculate that the influence of each element of the alloy component on the lattice constants of the austenite phase and the martensite phase may affect the dimensional change due to the heat treatment.

Then, through diligent studies by the present inventors, it has been found that the volume change index value calculated by the following equation (2) is an index for predicting the degree of volume change during the transformation from the martensite phase to the austenite phase. It was issued.

Volume change index value = Cr + 6 (Si + Al) + 2Mo-0.5Ni-Cu ... (2)
Here, the volume change index value is defined based on the influence of each element on the lattice constants of the martensite phase and the austenite phase, based on the lattice constants of the martensite phase and the austenite phase of Fe.

Specifically, first, the base composition is stainless steel in which Cr: 18% by mass, Ni: 8% by mass, C: 0.06% by mass, and the balance is Fe excluding unavoidable impurities, and Cr, Ni, Si, Prepare a plurality of types of stainless steels having different compositions of Al, Mo and Cu. Next, for these stainless steels, the volume V100 per gram in the state where the austenite phase is 100% and the volume V50 per gram in the state where the volume ratio between the austenite phase and the martensite phase is 50%: 50%. demand. The stainless steel in a state where the volume ratio of the austenite phase and the martensite phase is 50%: 50% can be produced by processing by adjusting the temperature and strain. Then, the above formula (2) can be calculated based on the influence of the content of each element on the ratio between V100 and V50.

The higher the volume change index value, the larger the dimensional change due to the heat treatment of the press-molded stainless steel, and as a result, the dimensional accuracy of the press-molded product decreases. Therefore, the volume change index value is set to 14.5 or less.

[Alloy component]
The present invention is applied to, for example, stainless steel containing the following components, but the present invention is not limited thereto. The stainless steel according to the embodiment of the present invention may contain various alloy components in addition to the following alloy components. In this specification, the content of each alloying element in stainless steel is simply referred to as the content, and is expressed in units:% by mass.

<C and N>
Both C and N are austenite-forming elements and are required to obtain an austenite phase. However, C and N increase the strength of the stainless steel and lower the press formability, and also have a strong solid solution strengthening action, so that time cracking after press forming is likely to occur. Therefore, the contents of C and N are 0.025% by mass or less, respectively, and the total content of C and N (C + N) is 0.035% by mass or less.

<Cr>
Cr is necessary to enhance corrosion resistance, and the Cr content is set to 16% by mass or more. However, if Cr is excessively added, the volume change during austenite transformation due to heat treatment becomes large. Therefore, the Cr content is 20.0% by mass or less, preferably 18.5% by mass or less.

<Si>
Si is effective for use as a deoxidizing material. However, when Si is excessively added, in addition to increasing work hardening and lowering press formability, the volume change during austenite transformation due to heat treatment becomes large as in Cr. Therefore, the Si content is 1.00% by mass or less, preferably 0.70% by mass or less.

<Mn>
Mn is an austenite stabilizing element and is effective for obtaining an austenite phase. Is. However, if Mn is excessively added, damage to the refractory is promoted during steelmaking, and Mn-based inclusions that are the starting points of machining cracks are increased. Therefore, the Mn content is 0.30% by mass or more and 3.50% by mass or less.

<Al>
Al is necessary for use as a deoxidizing material. However, when Al is excessively added, in addition to increasing work hardening and lowering press formability, the volume change during austenite transformation due to heat treatment becomes large as in Cr. Further, when Al exists as alumina (Al ₂ O ₃ ), the alumina serves as a cracking starting point during press molding. Therefore, the Al content is 0.1% by mass or less.

<Ni>
Ni is an austenite-forming element and is required to obtain the austenite phase. Further, Ni is also effective for reducing the volume change at the time of austenite transformation due to heat treatment. Therefore, the Ni content is set to 7.00% by mass or more. However, if Ni is added in excess, the austenite phase is stabilized more than necessary, resulting in deterioration of moldability. Therefore, the Ni content is 12.00% by mass or less, preferably 9.00% by mass or less in order to reduce the cost of stainless steel, for example.

<Cu>
Cu is an austenite-forming element and is effective for obtaining an austenite phase. In addition, Cu reduces work hardening of the austenite phase, and thus has an effect of improving moldability. Further, Cu is also effective for reducing the volume change during austenite transformation due to heat treatment. Therefore, the Cu content is preferably set to 1.00% by mass or more. However, if Cu is excessively added, a CuMn phase having a low melting point is formed in the center of the slab, which deteriorates hot workability. Therefore, the Cu content is 3.50% by mass or less.

<Mo>
Mo is effective for obtaining corrosion resistance and can be added as needed. However, if Mo is added in excess, the volume change during austenite transformation due to heat treatment becomes large. Therefore, the Mo content is 1.00% by mass or less. When the Mo content is 1.00% by mass or less, the dimensional change due to the heat treatment of the press-molded stainless steel product can be further reduced, and the dimensional accuracy of the press-molded product can be further improved.

<P and S>
P reduces corrosion resistance. Therefore, the content of P is preferably 0.04% by mass or less. S also reduces corrosion resistance. Therefore, the content of S is 0.015% by mass or less, more preferably 0.003% by mass or less.

<V>
V is effective for increasing corrosion resistance. However, V is a ferrite-forming element and is an expensive raw material. Therefore, in order to suppress the formation of the ferrite phase, stabilize the austenite phase, and reduce the raw material cost of stainless steel, the V content is preferably 0.50% by mass or less.

<B>
B may be added as necessary in order to improve hot workability and reduce cracks during hot rolling. The content of B is preferably 0.0001% by mass or more, for example. However, when B is excessively added, precipitates as Cr 2 _B, reducing the corrosion resistance. Therefore, the content of B is preferably 0.01% by mass or less.

[Manufacturing method of stainless steel strip]
The stainless steel according to the embodiment of the present invention can be produced by a general method for producing stainless steel, for example, melting. Further, also when the stainless steel strip is manufactured from the stainless steel according to the embodiment of the present invention, a general method for manufacturing the stainless steel strip can be used.

Hereinafter, an example of a method for manufacturing a stainless steel strip from stainless steel according to an embodiment of the present invention will be described. First, the slab produced by continuous casting is heated at 1100 ° C. or higher and 1300 ° C. or lower, and then hot-rolled to obtain a hot-rolled steel strip. Next, the hot-rolled steel strip is annealed if necessary and then pickled. When annealing, it is preferable to heat the hot-rolled steel strip at about 900 ° C. or higher and 1150 ° C. or lower, but the annealing temperature is not limited to this range. Next, the hot-rolled steel strip after pickling is cold-rolled to a predetermined plate thickness, and then finish annealing is performed. The finish annealing temperature may be, for example, 700 ° C. or higher and 1150 ° C. or lower, but is not limited to this range. Further, for example, when the final plate thickness required after finish annealing is thin, intermediate rolling and intermediate annealing may be performed before cold rolling to obtain the final plate thickness, if necessary. For example, in the case of manufacturing a product with a final plate thickness of 0.3 mm, a hot-rolled steel strip with a plate thickness of 3.5 mm is cold-rolled to a plate thickness of 1.0 mm by intermediate rolling, and after intermediate annealing, it is 0.3 mm. It may be further cold-rolled to finish annealing. After the finish annealing, the finish pickling may be performed.

[Manufacturing method of press-molded products]
A press-molded product can be produced from a stainless steel strip according to an embodiment of the present invention by general press molding. Examples of the types of press forming include drawing, overhanging, stretch flange processing, and bending processing. As the material of the press-formed product, a stainless steel plate manufactured by using the stainless steel according to the embodiment of the present invention may be used instead of the stainless steel strip.

The press molding may be carried out at room temperature or may be carried out by heating. In the press-molded product manufactured from the stainless steel strip according to the embodiment of the present invention, the stainless steel strip is press-molded at room temperature, for example, 10 ° C. or lower, and even when heat treatment is not performed after press molding, for example, martensite described later will be described. The volume ratio of the site phase is 50% or less. In order to further reduce the volume ratio of the martensite phase, it is preferable to press-mold while keeping the temperature of the stainless steel high. In order to keep the temperature of the stainless steel high, for example, the room where the press molding is performed may be air-conditioned and kept at 10 ° C. or higher. Alternatively, the temperature of the die may be maintained at 10 ° C. or higher by using a heating die or by utilizing the processing heat generated by the continuous press.

[Eriksen height]
The formability at the time of press forming can be evaluated by, for example, the Eriksen height defined in JIS Z2247. The Eriksen height is the moving distance of the punch until a through crack occurs by pushing the punch with the spherical end into the test piece tightened between the wrinkle holder and the die, that is, the depth of the dent. Defined. Therefore, it is interpreted that the higher the Eriksen height, the better the formability at the time of press molding. The Eriksen height is preferably 13.0 mm or more when measuring, for example, a test piece having a thickness of 1.0 mm.

[Time break]
In the following description, for convenience, the press-molded product in the state after press-molding and before heat treatment is also referred to as "press-molded product". If the processing conditions at the time of press molding are harsh, cracks may occur in the press molded product over time even if there is no abnormality in the press molded product immediately after press molding. This crack is commonly referred to as a timed crack. It is preferable that time cracking does not occur.

[Dimensional change]
The dimensional change in the heat treatment of the press-molded product can be evaluated by, for example, the shrinkage rate SF (%) calculated by the following formula (3) for the cylindrical press-molded product.

SF (%) = 100 × (D1-D2) / D1 ... (3)
In the formula (3), D1 is the outer diameter of the press-molded product before heat treatment, and D2 is the outer diameter of the press-molded product after heat treatment at 1050 ° C.

The shrinkage rate SF is preferably 0.3% or less.

[Martensite ratio]
The volume ratio of the martensite phase of the press-molded product (hereinafter referred to as the martensite ratio) can be used as one of the indexes of the dimensional change during the heat treatment of the press-molded product. The lower the martensite ratio, the less the martensite phase that can be transformed into the austenite phase during the heat treatment, so it is predicted that the dimensional change during the heat treatment will be small.

The martensite ratio can be measured using, for example, X-ray diffraction, electron backscatter diffraction, magnetic induction, or the like. In the present specification, considering that the measurement is relatively easy, the martensite ratio was measured using FERITSCOPE FMP30 manufactured by Fischer using a magnetic induction method.

The martensite ratio of the press-molded product becomes the maximum value at the place where the strain at the time of press-molding is maximum, for example, at the place where the flange is formed at the time of drawing. The martensite ratio in the press-molded product is preferably 50% or less at any point of the press-molded product.

The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention.

An embodiment of the present invention will be described below. First, stainless steels A1 to A4 and B1 to B4 having the compositions shown in Table 1 are melted, and then hot rolling, annealing, pickling, cold rolling, finish annealing and finish pickling are performed in this order as a 2D material. , 1.0 mm thick stainless steel plates A1 to A4 and B1 to B4, respectively. Finish annealing was performed by setting the target temperature of stainless steel to 1050 ° C. and adjusting the furnace temperature. The oxidation scale generated after finish annealing was finally removed by finish pickling with an aqueous solution mixed with nitric acid and hydrofluoric acid.

_{Table 2 shows (C + N) Md 30} and volume change index values calculated based on the composition of stainless steels A1 to A4 and B1 to B4. As shown in Table 2, the stainless steels A1 to A4 have (C + N): 0.035% by mass or less, _{the value of Md 30} is -60 or more and -20 or less, and the volume change index value is 14. It is 5 or less.

On the other hand, the stainless steel B1 has (C + N): 0.035% by mass or less and the volume change index value is 14.5 or less, but _{the value of Md 30} is lower than -60. The stainless steel B2 has _{a value of Md 30} of -60 or more and -20 or less, but a (C + N) value of more than 0.035% by mass and a volume change index value of more than 14.5. The stainless steel B3 has (C + N): 0.035% by mass or less and the volume change index value is 14.5 or less, but _{the value of Md 30} is higher than -20. The stainless steel B4 has (C + N): 0.035% by mass or less, _{the value of Md 30} is -60 or more and -20 or less, but the volume change index value exceeds 14.5.

[Eriksen height]
The Eriksen height was measured for stainless steel plates A1 to A4 and B1 to B4 having a thickness of 1.0 mm. As shown in Table 3, the Eriksen heights of the stainless steel plates A1 to A4 and B2 to B4 were 13.0 mm or more, indicating that the moldability was good. On the other hand, the Eriksen height of the stainless steel plate B1 was as low as 12.5 mm. It is presumed that the reason for this is that since _{the value of Md 30} of stainless steel B1 is as low as -124.0, the austenite phase is stabilized more than necessary and the TRIP phenomenon due to the martensite phase is unlikely to occur.

〔Press molding〕
FIG. 1 is a schematic view showing a method of producing a press-molded product by press-molding from stainless steel according to an embodiment of the present invention. By the method shown in FIG. 1, stainless steel sheets A1 to A4 and B2 to B4 having an Eriksen height of 13.0 mm or more were press-formed by multi-stage drawing at room temperature.

Specifically, first, a disk-shaped blank 10 having a diameter of 96 mm was prepared from stainless steel plates A1 to A4 and B2 to B4, respectively. The blank 10 was first drawn using a punch having a punch diameter of 48 mm to prepare a first drawn product 20. At this time, the die used was a die having a clearance of 50% with respect to the initial plate thickness, and the radius of curvature of the punch and the die was set to 8.0 mm. The first drawn product 20 includes a substantially cylindrical cylindrical portion 21 and a bottom portion 22 provided at one end of the cylindrical portion in the axial direction, and an opening is provided at the other end of the cylindrical portion 21 in the axial direction. 23 is formed. The diameter of the cylindrical portion 21 was 48 mm, and the axial length was about 36 mm.

Similarly, the first drawn product 20 was subjected to multi-stage drawing using punches having punch diameters of 44 mm, 40 mm, 35 mm, 30 mm and 25 mm in order to produce a press-molded product 30. At this time, the die used was a die having a clearance of 50% with respect to the initial plate thickness, and the radius of curvature of the punch and the die was set to 8.0 mm.

The press-molded product 30 includes a substantially cylindrical cylindrical portion 31 and a bottom portion 32 provided at one end of the cylindrical portion in the axial direction, and an opening 33 is provided at the other end of the cylindrical portion 31 in the axial direction. Is formed. The diameter of the cylindrical portion 31 was 25 mm, and the length in the axial direction was about 90 mm. In the following description, the press-molded products 30 made from the stainless steel plates A1 to A4 and B2 to B4 will be referred to as press-molded products A1 to A4 and B2 to B4, respectively.

[Time break]
With respect to the press-molded products A1 to A4 and B2 to B4, the presence or absence of time cracking was visually observed one week after press molding using a 25 mm punch. As shown in Table 3, no time cracking was observed in the press-molded products A1 to A4 and B3 and B4. On the other hand, in the press-molded product B2, a time crack occurred. It is presumed that the reason for this is that the (C + N) of stainless steel B2 is as high as 0.070% by mass.

[Martensite ratio]
The martensite ratios of the press-molded products A1 to A4 and B3 and B4 in which no time crack was observed were measured by FERITSCOPE FMP30 manufactured by Fisher. As shown in FIG. 1, the measurement point 34 is a point in the cylindrical portion 31 where the distance d4 from the opening 33 is 5 mm.

As shown in Table 3, the martensite ratio of the press-molded products A1 to A4 and B4 was as low as 50% or less. On the other hand, the martensite ratio of the press-molded product B3 was higher than 50%. It is presumed that the reason why the martensite ratio of the press-formed product B3 is high is that _{the value of Md 30} of the stainless steel B3 is as high as 24.3, and the ratio of transformation from the austenite phase to the martensite phase during press forming is high. Will be done.

〔Shrinkage factor〕
The press-molded products A1 to A4 and B3 and B4 in which no time crack was observed were heated at 1050 ° C. for 1 hour in a vacuum atmosphere with a pressure of 0.001 Pa or less and then cooled. For each press-molded product, the outer diameter before heat treatment was D1 and the outer diameter after heat treatment was D2, and the shrinkage rate SF (%) was calculated by the following formula (3). In addition, D1 and D2 were measured at the edge of the opening 33 in the cylindrical portion 31 of the press-molded product 30 shown in FIG.

SF (%) = 100 × (D1-D2) / D1 ... (3)
As shown in Table 3, the shrinkage rate of the press-molded products A1 to A4 was 0.3% or less, and the dimensional accuracy of the press-molded products A1 to A4 was good. On the other hand, it was shown that the shrinkage rate of the press-molded products B3 and B4 was larger than 0.3%, and the dimensional accuracy of the press-molded products B3 and B4 was lower than that of the press-molded products A1 to A4.

It is presumed that the reason why the dimensional accuracy of the press-molded product B3 is low is that since the martensite ratio of the press-molded product B3 is high, there are many stainless steels that are transformed from the martensite phase to the austenite phase by heat treatment. On the other hand, the press-molded product B4 had low dimensional accuracy even though the martensite ratio was as low as 41%. The reason for this is that since the volume change index value of stainless steel B3 is as high as 16.1, each element of the alloy component of stainless steel B3 increases the volume change during the transformation from the austenite phase to the martensite phase. It is presumed.

The present invention can be used for austenitic stainless steel and press-formed products of austenitic stainless steel.

10 Blank 30 Press-molded product 31 Cylindrical part 32 Bottom part 33 Opening part 34 Measurement point

Claims (2)

C: 0.025% by mass or less, Si: 1.00% by mass or less, Mn: 0.30% by mass or more and 3.50% by mass or less, P: 0.04% by mass or less, S: 0.015% by mass Hereinafter, Ni: 7.00% by mass or more and 12.00% by mass or less, Cr: 16.00% by mass or more and 20.00% by mass or less, Cu: 3.50% by mass or less, N: 0.025% by mass. Hereinafter, it is an austenite-based stainless steel in which Al: 0.1% by mass or less and the balance is Fe except for unavoidable impurities.
(C + N): 0.035% by mass or less,
_{The value of Md 30} calculated by the following formula (1) is -60 or more and -20 or less.
An austenitic stainless steel having a volume change index value of 14.5 or less, which is calculated by the following formula (2) and is an index for predicting the degree of volume change during transformation from the martensite phase to the austenitic phase.
Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo ... (1)
Volume change index value = Cr + 6 (Si + Al) + 2Mo-0.5Ni-Cu ... (2) The austenitic stainless steel according to claim 1, wherein Mo: 1.0% by mass or less, V: 0.50% by mass or less, B: 0.0001% by mass or more and 0.01% by mass or less.

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