JP2008019479A - Rolled austenitic stainless steel plate with excellent strength and ductility, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolled product excellent in mechanical properties over all the cross sections, which is an extra thick stainless steel plate of ≥100 mm thickness for use at very low temperatures, such as a structural material for a superconducting coil of a nuclear fusion reactor and a structural material for LNG, and also to provide its manufacturing method. <P>SOLUTION: The objective rolled product can be manufactured by subjecting a steel ingot, which contains, by mass, ≤0.08% C, 0.10 to 0.22% N, ≥0.12% of (C+S), 0.01 to 2.0% Si, 0.1 to 2.0% Mn, 15 to 27% Cr, 8 to 20% Ni, ≤4% Mo, ≤0.1% Co, 0.1 to 3% Cu, 0.001 to 0.10% Al and 0.0005 to 0.01% Ca and has a calculated δ-ferrite quantity value (δcal) of -7 to 4% and a thickness of ≥650 mm, to forging at ≥0.5 reduction of area, to hot rolling at ≥1.5 reduction ratio and then to solution heat-treatment. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a cryogenic austenitic stainless rolled steel sheet particularly useful for a cryogenic temperature of 150 K or less, which is used as a structural material for a superconducting coil of a nuclear fusion reactor or a structural material for LNG (liquefied natural gas). In particular, the present invention relates to a rolled steel sheet having a thickness of 100 mm or more that could not be manufactured conventionally.

  A structural material for a superconducting coil of a nuclear fusion reactor, which is expected as a next-generation energy source, is non-magnetic and requires high strength characteristics in a superconducting temperature cryogenic region. Moreover, since the superconducting coil is expected to be a large device, a thick plate having a large thickness is indispensable for the structural material.

  Conventionally, for this application, as described in Patent Document 1, Patent Document 2, and Patent Document 3, an austenitic stainless steel in which C is reduced as much as possible and a large amount of N is added to ensure low temperature strength and the γ phase is stabilized. Steel has been used.

Non-Patent Document 1 defines hot-rolled stainless steel plates and steel strips. For example, SUS316LN specifies a proof stress of 245 N / mm ² or more, a tensile strength of 550 N / mm ² , and an elongation of 40 or more.

  However, it has been difficult in the manufacturing process to obtain a rolled steel sheet that satisfies this standard over the entire cross section in a steel sheet having a thickness of 100 mm or more. The reason is that it is difficult to obtain a fine recrystallized structure over the entire cross section of a rolled product.

  In order to secure strength and elongation, it is necessary to crush the cast structure of the steel ingot over the entire cross section and to introduce a processing strain throughout the entire cross section in order to obtain a fine recrystallized structure after heat treatment. It is effective to make the ratio as large as possible. However, for thick-walled products, the thickness of the material before rolling is limited due to the limitations of the rolling mill, and the reduction ratio is also limited. Therefore, there is a cross-sectional area where machining strain cannot be introduced, and in particular, machining strain does not easily enter from the ¼ area of the plate thickness to the central area, so that even after heat treatment, the cast structure remains and becomes coarse, resulting in strength and elongation. Low sites will result.

  Patent Document 1 discloses a cryogenic structural austenitic stainless steel having a high N and Mn. A thickness of 100 mm or more is not disclosed.

  With respect to Patent Document 2, a structural austenitic stainless steel having a very high N, Mn, and Al cryogenic resistance and toughness is disclosed. A thickness of 100 mm or more is not disclosed.

  Patent Document 3 discloses a cryogenic austenitic stainless steel having excellent reheat resistance with Nb content and high Mn. A thickness of 100 mm or more is not disclosed.

  Patent Document 4 discloses a method for producing a stainless steel plate having a thickness of 100 mm or more that has excellent cryogenic properties. The addition of Cu or Ti is not disclosed. Further, a heat treatment method for obtaining a uniform sized structure over the entire cross section is disclosed, but homogenization by refining the solidified structure is not disclosed.

  Patent Document 5 discloses a hot rolling method for 50 mm and 100 mm thick austenitic stainless steel thick plates for grain refinement. The component range is not disclosed.

  Patent Document 6 discloses a method for producing a high-strength austenitic stainless steel sheet (containing Ti). Ti is added to refine the solidification structure and prevent surface flaws during rolling. The solution treatment is omitted, and the provision of a thickness of 100 mm or more is not disclosed.

  Patent Document 7 discloses a method for producing a 97 mm thick stainless steel extra-thick steel sheet that eliminates cracks on the surface of a continuously cast slab by forging without causing cracks. The thickness of 100 mm or more and the component range are not disclosed.

  From the above, stainless steel with excellent mechanical properties such as proof stress and toughness at cryogenic temperatures has been disclosed as the prior art. However, regarding rolled products with an extremely large thickness of 100 mm or more, the mechanical properties were excellent over the entire cross section. A rolled product and a manufacturing method with an extremely large thickness of 100 mm or more are not disclosed.

  Forging compared to rolling, the material thickness in the forging machine is smaller than that of the rolling mill, and the reduction ratio can be increased, and processing can be performed in the direction of increasing plate thickness. As a result, it is easy to achieve a fine recrystallized structure over the entire cross section. However, if everything is finished only by forging, cost increases and productivity declines.

JP-A-60-13063 JP-A-61-52351 Japanese Patent Laid-Open No. 2-57668 JP 7-316653 A JP 7-310120 A JP-A-8-104920 Japanese Patent Laid-Open No. 11-131138 JIS G4304 (2005)

  The present invention relates to a material used at a very low temperature, such as a structural material of a superconducting coil of a nuclear fusion reactor or a structural material of LNG (liquefied natural gas). An object is to provide an excellent rolled product and a production method for obtaining the same.

  High strength austenitic stainless steel with a large amount of N added at extremely low temperatures, especially thick rolled products with a large thickness, are processed by forging or rolling at a thickness of 1/4 to 3/4 of the surface layer. It is a site where distortion is difficult to enter, and it is particularly difficult to ensure the strength and ductility of the site.

  Therefore, the present inventors have obtained a finding that, by examining various alloys in detail, first, by specifying the components, the minimum required strength and ductility can be secured even at extremely low temperatures. In other words, in order to ensure the strength and ductility of the rolled steel sheet over its entire thickness, it is necessary to adjust the elements effective for high strength to an appropriate range and refine the solidification structure by adjusting the components in order to obtain a stable and high effect. It is valid.

  Next, by properly combining the forging and rolling processes, the cast structure of the steel ingot can be crushed over the entire cross section, and hot crystallization distortion can be introduced into the entire cross section to promote recrystallization and obtain a uniform recrystallized structure. Was revealed. That is, by rolling a steel ingot after forging, it is possible to realize a rolled product having excellent mechanical properties over the entire cross section and a manufacturing method for obtaining the same regarding a stainless steel plate having an extremely large thickness of 100 mm or more.

The present invention is configured based on this finding, and the gist thereof is as follows.
(1) By mass%, C: 0.08% or less, N: 0.10% or more and 0.22% or less, C + N: 0.12% or more, Si: 0.01% or more and 2.0% or less, Mn : 0.1% to 2.0%, Cr: 15% to 27%, Ni: 8% to 20%, Mo: 4% or less, Co: 0.1% or less, Cu: 0.1% 3% or less, Al: 0.001% or more and 0.10% or less, Ca: 0.0005% or more and 0.01% or less, consisting of remaining iron and inevitable impurities, defined by the following formula (I) The calculated amount of δ ferrite (δcal; volume%) is -7% or more and 4% or less, and the elongation in the width and length directions is 30% or more at any part in the thickness direction. The above austenitic stainless steel sheet.
δcal (volume%)
= 2.9 × ([Cr] +0.3 [Si] + [Mo]) − 2.6 × ([Ni] + 0.3 × [Mn] + 0.25 × [Cu] + 35 × [C] + 20 × [N])-18 (I)
However, [element symbol] means the content (% by mass) of each element.
The elongation is a value measured at 4K.
(2) The stainless rolled steel sheet according to (1), further containing Ti: 0.010% to 0.030% by mass%.
(3) A steel ingot having a thickness of 650 mm or more is forged at an area reduction rate of 0.5 or more, hot rolled at a reduction ratio of 1.5 or more, and then subjected to a solution heat treatment, The stainless rolled steel sheet according to 1) or (2) and a method for producing the same.
Here, the area reduction rate (A) of forging is defined as follows.
Steel ingot cross-sectional area before forging (thickness x width): A ₀
Steel ingot cross-sectional area after forging (thickness x width): A ₁
A = (A ₀ −A ₁ ) / A ₀
The rolling reduction ratio (R) is defined as follows.
Slab thickness before rolling: B ₀
Slab thickness after rolling: B ₁
R = B ₀ / B ₁
(4) For a steel ingot with a thickness of 500 mm or more, forging with an area reduction rate of 0.3 or more in the direction of decreasing thickness and forging with an area reduction rate of 0.15 or more in the direction of increasing thickness alternately. The stainless rolled steel sheet according to (1) or (2), which is performed at least once, and then subjected to solution heat treatment after hot rolling at a reduction ratio of 1.5 or more, and a method for producing the same.
Here, the area reduction rate (C) of forging in the direction in which the thickness decreases or increases is defined as follows.
Steel ingot cross-sectional area after n-th forging (thickness x width): C _n
Steel ingot cross-sectional area after n-1th forging (thickness × width): C _n-1
C = (C _n-1 −C _n ) / C _n-1
The rolling reduction ratio (R) is defined as follows.
Slab thickness before rolling: B ₀
Slab thickness after rolling: B ₁
R = B ₀ / B ₁

  Forging here is free forging using a press, but the press may be divided into several times in the same direction of the same surface of the steel ingot until the entire steel ingot reaches a predetermined cross-sectional shape. In this case, that is, a series of pressing steps by pressing in the same direction of the same surface of the steel ingot until the entire steel ingot reaches a predetermined cross-sectional shape is regarded as one forging step.

  According to the present invention, a thick plate having high strength and ductility at a very low temperature and a thickness of 100 mm or more can be obtained. The steel of the present invention can be applied as a structural material for a coil for a thermonuclear fusion reactor (ITER), which is expected as a next-generation energy source.

  In addition, the steel of the present invention can be applied to the enlargement of superconducting equipment, LNG (liquefied natural gas) structures, etc., so it is expected to greatly contribute to various industrial fields including the future energy industry. Industrial and social effects are great.

  Conventionally, forged products having a thickness of 100 mm or more existed, but rolled steel sheets could not be obtained. The reason is that in order to obtain a uniform crystal structure in the cross section, it is necessary to add a strain to the entire cross section and perform solution heat treatment. For this purpose, it is necessary to increase the reduction ratio by increasing the slab thickness as much as possible. However, this is because the rolling mill has a limited slab thickness and cannot be manufactured.

  Although it is possible to increase the slab thickness in the forging process, finishing all with only forging results in increased costs and reduced productivity.

  In the present invention, an austenitic stainless rolled steel sheet with a thickness of 100 mm or more, which is excellent in strength and ductility, is manufactured by forging a steel ingot defining a component range effective for strength improvement in the first half of the process and rolling in the second half of the process. Clarified that it is possible.

  However, a coarse solidified structure may remain from the surface to 1/4 to 3/4 of the thickness, and the elongation at the site is low, which becomes a neck and limits the strength-ductility balance of the entire cross section. Sometimes.

  Therefore, in order to secure further ductility, the solidification structure is refined by adding Ti, and it is manufactured in combination with the forging-rolling process, and the crystal structure of the product is refined in the entire cross section. -The ductility balance can be improved.

  Next, the limiting conditions of this invention are shown.

  When C is added in a large amount, the strength at cryogenic temperature is improved, but Cr carbonitride is precipitated in a large amount in the production process, and the toughness at cryogenic temperature is deteriorated, so 0.08% is made the upper limit. did.

  N is an extremely effective element for stabilizing the austenite phase and ensuring strength at an extremely low temperature. However, if the content is less than 0.10%, the effect is small. If the content exceeds 0.22%, the weldability is remarkably deteriorated, and welding cracks and blowholes are frequently generated. Therefore, the N content is 0.1 to 0.22%. And limited.

  For C + N, it is said that the cryogenic strength is correlated with the amount of C + N in the steel, and the strength increases as the amount of C + N increases. Since C and N each define a range, C + N is set to 0.12% or more.

  If Si is less than 0.01%, the cleanliness of the steel becomes poor and the toughness deteriorates. If it exceeds 2.0%, the hot workability deteriorates and it becomes difficult to produce a thick plate. It was limited to 0.01 to 2.0%.

  If Mn is less than 0.1%, the cleanliness of the steel becomes poor. If it exceeds 2.0%, the hot workability deteriorates, so the Mn content is limited to 0.1 to 2%.

  Cr needs to be 15% or more in order to ensure corrosion resistance at the time of processing the member. However, if it exceeds 27%, a brittle σ phase is generated and the toughness is deteriorated, so the Cr content is limited to 15 to 27%. .

  Ni is an element that stabilizes the austenite phase and improves strength toughness and ductility at cryogenic temperatures. However, if less than 8%, the effect of stabilizing the austenite phase is insufficient, so 8% was made the lower limit. However, since it is an extremely expensive element, the upper limit is set to 20% at which the austenite stabilizing effect is saturated from the viewpoint of cost.

  Mo added to ensure strength exceeds 4%, and intermetallic compounds such as sigma phase are produced to deteriorate toughness at extremely low temperatures. Limited to 4% or less. In addition, since the strength improvement effect will become small if it is less than 0.5%, addition of 0.5% or more is desirable.

  When Co is mixed as an impurity element, it is harmful when activated. In order to reduce activation, it was made 0.1% or less.

  Since Cu is an element that increases corrosion resistance, it is positively added. If it is less than 0.1%, there is no effect, and if it exceeds 3%, the hot workability is hindered.

  Al is an element that improves the cleanliness of steel as a deoxidizer. However, when the content is less than 0.001%, this effect is not obtained. When the content exceeds 0.10%, the hot workability deteriorates, so the Al content is limited to 0.001 to 0.10%.

  Ca added for improving hot rolling processability is less than 0.0005%, and there is no effect of improving hot workability. If it exceeds 0.01%, the cleanliness is poor, so the amount added is 0.0005. Limited to -0.01%.

  Ti is desirably added to refine the solidified structure and improve the strength and elongation more stably. If it is less than 0.010%, there is no effect, and if it exceeds 0.030%, coarse nitrides precipitate and deteriorate toughness. Therefore, it was made into 0.010% or more and less than 0.030%.

  S contained as an inevitable impurity is an element that decreases hot workability and toughness, and is desirably reduced to 0.003% or less.

  P contained as an inevitable impurity is an element harmful to corrosion resistance, and is desirably reduced to 0.040% or less.

  Next, the point that the calculated amount of δ ferrite (δcal; volume%) defined by the following formula (I) is −7% or more and 4% or less will be described.

δcal (volume%)
= 2.9 × ([Cr] +0.3 [Si] + [Mo]) − 2.6 × ([Ni] + 0.3 × [Mn] + 0.25 × [Cu] + 35 × [C] + 20 × [N])-18 (I)
However, [element symbol] means the content (% by mass) of each element.

  δcal is the recommended formula of DJKOTECKI & TASIEWERT (Weld.J., 71 (1992), 171s) and the recommended formula of TASIEWERT, CNMcCOWAN & DLOLSON (Weld.J., 67 (1988), 289s) This is a combined calculation method and represents the ratio of δ ferrite content in the solidified structure. Since the actual slab is very large, it is difficult to measure the amount of δ-ferrite. Therefore, the amount of δ-ferrite precipitation was estimated using the formula obtained by experiments with small ingots. It was confirmed by some sampling surveys that the amount of δ ferrite of the large-section steel ingot targeted by the present invention shows a value of about −0, + 8% than the amount of δ ferrite precipitated predicted by this calculation formula. If δ ferrite appears during solidification, it is effective in refining the austenite solidification structure, and if δ ferrite is finely dispersed in the steel sheet, it suppresses crystal grain coarsening during heating. If δcal is less than −7%, the above effects do not become apparent, and if it exceeds 4%, these effects are saturated, and mechanical properties such as strength and elongation start to deteriorate. From the above, δcal was set to -7% or more and 4% or less.

  There are three directions of elongation of the steel plate: width, length, and thickness. However, considering the practicality, the elongation in the width and length directions was specified. The elongation is set to 30% or more because there is no practical problem if it is 30% or more at an arbitrary portion in the thickness direction at a temperature of room temperature or lower and up to 4K. Higher ductility is required for application to parts that require higher processing, and the elongation is preferably 40% or more.

  Regarding the manufacturing method of the present invention, the reason why the steel ingot having a thickness of 650 mm or more is forged at an area reduction rate of 0.5 or more, hot-rolled at a reduction ratio of 1.5 or more, and then subjected to solution heat treatment. explain.

  In order to make the cross-sectional structure of the extremely thick material uniform, it is desirable to increase the reduction ratio by using a steel ingot that is as thick as possible. Considering the thickness of the product, it was limited to steel ingots with a thickness of 650 mm or more. In addition, it is desirable to reduce as much as possible the area reduction rate of forging, which can apply large strain locally, to crush coarse solidified structure. The area reduction rate was made 0.5 or more by introducing.

  Hot rolling is applied to the product thickness in the post-forging process, but the reduction ratio of hot rolling is increased in order to obtain a uniform recrystallized structure after solution treatment by increasing the reduction ratio as much as possible and introducing strain throughout the entire section. It was set to 1.5 or more.

  The solution heat treatment is performed for the purpose of obtaining sufficient characteristic values for mechanical properties such as strength and elongation and corrosion resistance by solidifying the constituent elements and making the metal structure and crystal grain size uniform. The solution heat treatment conditions can be rapidly cooled from 920 ° C. to 1200 ° C. depending on the alloy components and the manufacturing process.

  By using a steel ingot having a thickness of 650 mm or more, forging at an area reduction rate of 0.5 or more, hot rolling at a reduction ratio of 1.5 or more, and performing a solution heat treatment An austenitic stainless rolled steel sheet having an elongation in the width and length directions of 30% or more at an arbitrary portion in the thickness direction and a thickness of 100 mm or more can be produced.

  The reason why the manufacturing method according to claim 4 of the present invention is limited to adding a process of forging at an area reduction rate of 0.15 or more in the direction of increasing the thickness of the steel ingot will be described.

  In order to make the cross-sectional structure of the very thick material uniform, it is important to introduce a processing strain over the entire cross-section and obtain a recrystallized structure by solution heat treatment. For that purpose, it is effective not only to forge in the direction of decreasing thickness, but also to forge in the direction of increasing thickness of the steel ingot, and then rolling it using the thick steel ingot described above. I found out.

Here, the area reduction rate (C) of forging in the direction in which the thickness decreases or increases is defined as follows.
Steel ingot cross-sectional area after n-th forging (thickness x width): C _n
Steel ingot cross-sectional area after n-1th forging (thickness × width): C _n-1
C = (C _n-1 −C _n ) / C _n-1
The rolling reduction ratio (R) is defined as follows.
Slab thickness before rolling: B ₀
Slab thickness after rolling: B ₁
R = B ₀ / B ₁
When the area reduction rate per time in the forging in the direction of increasing the thickness is less than 0.15, the effect is small, and since a uniform structure was obtained at 0.15 or more, the area reduction rate was limited to 0.15 or more. Moreover, the area reduction rate per time in the forging in the direction in which the thickness decreases is also 0.3 or more. The forging in the direction of decreasing the thickness and the forging in the direction of increasing the thickness may be started from either direction. And it is sufficient to forge one or more times alternately. For example, a forging pattern such as “increase direction−decrease direction−increase direction”, “decrease direction−increase direction−decrease direction”, or the like can be employed.

  By forging from the direction perpendicular to the thickness direction, strain can be applied from two directions to increase the slip surface, and machining strain can be introduced in all directions from all directions, increasing the thickness of the steel ingot. It is considered that the forging process had a remarkable effect in forming a uniform structure. In this case, the initial steel ingot thickness is reduced to 500 mm or more. Moreover, it is the same as that of the above-mentioned method about implementing hot rolling by the reduction ratio 1.5 or more after forging, and performing solution heat treatment.

  The steel shown in Table 1-1 and Table 2 is cast ingot, and forging is performed with a reduction ratio of 1.5, 2.0, 2.5, and then hot with a reduction ratio of 1.4 to 3.7. Rolling was performed to produce a thick plate having a thickness of 100 mm to 250 mm. Moreover, the manufacturing conditions and manufacturing results of the steels in Table 1-1 are shown in Tables 1-2 and 1-3. Further, in Table 2, Example No. 1-13, Comparative Example No. The production conditions and production results of Nos. 1 to 11 (Claims 1 to 3) are shown in Table 3; 14-18, Comparative Example No. Table 4 shows the production conditions and production results of 12-13 (Claim 4).

  First, examples shown in Tables 1-1 to 1-3 will be described.

  Solid solution heat treatment was performed by a method in which a thick plate forged and rolled under the production conditions shown in Table 1-2 was heated to 1100 ° C. and cooled with water.

  Thereafter, the strength was evaluated at 4K, and the results are shown in Tables 1-2 and 1-3. As the test piece, a JISZ2201 14A test piece (diameter 6 mm, gauge distance 30 mm, total length 110 mm) was used. The test piece is the product surface layer (surface 10 mm part), a quarter thickness part from the surface, a half thickness part from the surface, a 3/4 thickness part, and a surface layer on the back. The strength was evaluated by a tensile test.

  Tables 1-2 and 1-3 show the values of 0.2% proof stress, tensile strength, and elongation in the width and length direction of the thick plate from the site in the same cross section for representative invention steels. In all the inventive steels, the 0.2% proof stress, tensile strength, and elongation were the lowest in the region of 1/4 to 3/4 thickness, and the surface layer did not become the lowest. The cast structure in the rolled steel sheet tends to remain, and the structure becomes nonuniform, corresponding to the 1/4 to 3/4 thickness portions. Therefore, these minimum values in the same cross section are in the range of ¼ thickness to ¾ thickness.

  Based on the above results, the steels shown in Table 2 showed the lowest values of the 0.2% proof stress, tensile strength, and elongation of the ¼ thickness to ¾ thickness portions in the same cross section for all examples.

  Among the steels shown in Table 2, the present invention steel No. 1-13, comparative steel No.1. About 1-11, it manufactured according to the manufacturing conditions shown in Table 2.

  Invention Steel No. No. 14 is a forging process before rolling, forging at an area reduction rate of 0.25 in the width direction of the steel ingot, increasing the thickness from 730 mm to 900 mm, forging at an area reduction rate of 0.6 in the thickness direction, and then pressing The product was rolled at a reduction ratio of 1.8 to give a product with a thickness of 200 mm.

  Invention Steel No. No. 15 is a forging process before rolling, forging at an area reduction rate of 0.15 in the longitudinal direction of the steel ingot, increasing the thickness from 730 mm to 900 mm, forging at an area reduction rate of 0.6 in the thickness direction, and then pressing The product was rolled at a reduction ratio of 1.8 to give a product with a thickness of 200 mm.

  Invention Steel No. No. 16 is a forging process before rolling, forging at an area reduction rate of 0.25 in the width direction of the steel ingot and increasing the thickness from 500 mm to 614 mm, then forging at an area reduction rate of 0.6 in the thickness direction, and then pressing. The product was rolled at a reduction ratio of 1.8 to obtain a product with a thickness of 137 mm.

  Invention Steel No. No. 17 is a forging process before rolling, forging at an area reduction rate of 0.30 in the thickness direction of the steel ingot, reducing the thickness from 730 mm to 510 mm, and then forging at an area reduction rate of 0.15 in the width direction. The thickness was increased to 600 mm, and the thickness was reduced to 358 mm by forging at an area reduction rate of 0.30 in the thickness direction, and then rolled at a reduction ratio of 1.8 to obtain a product having a thickness of 200 mm.

  Invention Steel No. No. 18 is forged at an area reduction rate of 0.25 in the width direction of the steel ingot in the forging process before rolling and increased in thickness from 500 mm to 614 mm, and then forged at an area reduction rate of 0.6 in the thickness direction. The thickness is reduced to 430 mm, then forged at an area reduction rate of 0.25 in the width direction to increase the thickness to 471 mm, and then forged at an area reduction rate of 0.6 in the thickness direction to reduce the thickness to 330 mm. The product was rolled at a reduction ratio of 1.8 to a thickness of 184 mm.

  Comparative steel No. No. 12 shows the difference in strength due to the difference in forging area reduction rate. In order to compare with No. 14, the forging process in the forging process before rolling was forged at an area reduction rate of 0.10 in the width direction of the steel ingot and increased in thickness from 730 mm to 832 mm, and then forged at an area reduction rate of 0.57 in the thickness direction. Then, the thickness was reduced to 358 mm, and then rolled at a reduction ratio of 1.8 to obtain a product having a thickness of 200 mm.

  Comparative steel No. No. 13 shows the difference in strength due to the difference in the forging area reduction rate. In order to compare with No. 18, the forging process before rolling was forged at an area reduction rate of 0.23 in the width direction of the steel ingot to increase the thickness from 500 mm to 631 mm, and then forged at an area reduction rate of 0.29 in the thickness direction. The thickness is reduced to 448 mm, and then forged with an area reduction rate of 0.13 in the width direction to increase the thickness to 502 mm and then forged with an area reduction rate of 0.29 in the thickness direction to reduce the thickness to 356 mm. Then, it was rolled at a reduction ratio of 1.8 to obtain a product having a thickness of 199 mm.

  The solution heat treatment was performed by heating the thick plate forged and rolled under the above production conditions to 1100 ° C. and cooling it with water.

  The production results are shown in Table 3 and Table 4, respectively. The evaluation method was the same as that used in Tables 1-2 and 1-3.

  The steel of the present invention was confirmed to have high strength at 4K and large elongation because the crystal grains were uniformly refined. On the other hand, in the steel out of the component range, the effect of making the crystal grains uniform and fine was poor and a partially coarse structure remained. Therefore, even though high strength was obtained, high elongation was not obtained. In addition, even when the steel ingot thickness, the area reduction rate in forging, and the hot rolling reduction ratio were outside the claimed range, some coarse structures remained, so even if high strength was obtained, high elongation was not obtained. .

Claims (4)

% By mass
C: 0.08% or less,
N: 0.10% or more and 0.22% or less,
C + N: 0.12% or more,
Si: 0.01% or more and 2.0% or less,
Mn: 0.1% or more and 2.0% or less,
Cr: 15% or more and 27% or less,
Ni: 8% or more and 20% or less,
Mo: 4% or less,
Co: 0.1% or less,
Cu: 0.1% to 3%,
Al: 0.001% or more and 0.10% or less,
Ca: 0.0005% or more and 0.01% or less, comprising the balance iron and inevitable impurities,
The calculated amount of δ ferrite (δcal; volume%) defined by the following formula (I) is −7% or more and 4% or less,
It is characterized by an elongation of 30% or more in the width and length directions at any part in the thickness direction
An austenitic stainless rolled steel sheet having a thickness of 100 mm or more.
δcal (volume%)
= 2.9 × ([Cr] +0.3 [Si] + [Mo]) − 2.6 × ([Ni] + 0.3 × [Mn] + 0.25 × [Cu] + 35 × [C] + 20 × [N])-18 (I)
However, [element symbol] means the content (% by mass) of each element.
The elongation is a value measured at 4K. The stainless rolled steel sheet according to claim 1, further comprising Ti: 0.010% or more and 0.030% or less by mass%. 3. A steel ingot having a thickness of 650 mm or more is forged at an area reduction rate of 0.5 or more, and is subjected to a solution heat treatment after hot rolling at a reduction ratio of 1.5 or more. Manufacturing method for stainless steel sheet.
Here, the area reduction rate (A) of forging is defined as follows.
Steel ingot cross-sectional area before forging (thickness x width): A ₀
Steel ingot cross-sectional area after forging (thickness x width): A ₁
A = (A ₀ −A ₁ ) / A ₀
The rolling reduction ratio (R) is defined as follows.
Slab thickness before rolling: B ₀
Slab thickness after rolling: B ₁
R = B ₀ / B ₁ For steel ingots with a thickness of 500 mm or more, forging with an area reduction rate of 0.3 or more in the direction of decreasing thickness and forging with an area reduction rate of 0.15 or more in the direction of increasing thickness alternately one or more times each. The method for producing a stainless rolled steel sheet according to claim 1 or 2, characterized in that, after that, hot rolling is performed at a reduction ratio of 1.5 or more, and solution heat treatment is performed.
Here, the area reduction rate (C) of forging in the direction in which the thickness decreases or increases is defined as follows.
Steel ingot cross-sectional area after n-th forging (thickness x width): C _n
Steel ingot cross-sectional area after n-1th forging (thickness × width): C _n-1
C = (C _n-1 −C _n ) / C _n-1
The rolling reduction ratio (R) is defined as follows.
Slab thickness before rolling: B ₀
Slab thickness after rolling: B ₁
R = B ₀ / B ₁