JP2017145487A - Ferrite austenite stainless steel sheet excellent in moldability and manufacturing method therefor - Google Patents

Abstract

Provided is a ferritic / austenitic stainless steel sheet which does not contain a large amount of expensive and rare element Ni and does not exhibit TRIP and has a uniform elongation and a large local elongation, and a method for producing the same. SOLUTION: N: 0.080 to 0.300% in mass%, PI value less than 30 Md value calculated from chemical composition in austenite phase is less than −10 ° C., perpendicular to the rolling width direction of the steel sheet The average projected area equivalent circle diameter of the austenite grains in a simple cross section is 1.0 + 5.0 × the nitrogen content (mass%) μm or more of the whole steel sheet, and the maximum relative to the total area of the austenite grains in the cross section perpendicular to the rolling width direction Rolling in the cross section perpendicular to the area ratio of austenite grains with a shape aspect ratio of less than 0.5, expressed by the reciprocal of the value obtained by dividing the length of the major axis by the maximum length in the direction orthogonal thereto, and the thickness direction of the steel sheet A ferritic-austenitic stainless steel sheet having an F value of 120 or more calculated from the number of ferrite / austenite phase boundaries in a direction 1000 μm perpendicular to the direction. [Selection] Figure 3

Description

  The present invention relates to a ferritic / austenitic stainless steel sheet and a method for producing the same.

  Stainless steel is roughly classified into austenitic stainless steel, ferritic stainless steel, and ferritic / austenitic stainless steel. Austenitic stainless steel contains 7% or more of expensive Ni and many steel types have excellent formability. Ferritic stainless steel contains almost no Ni and generally has a much lower formability than austenitic stainless steel.

  On the other hand, ferrite and austenitic stainless steels have a relatively low Ni content, and many steel types have intermediate characteristics between austenitic stainless steels and ferritic stainless steels in terms of formability and corrosion resistance.

  In recent years, ferritic / austenitic stainless steels with low Ni content and excellent economic efficiency are sometimes used as substitutes for austenitic stainless steels. Therefore, further improvement in the strength and ductility of the ferritic / austenitic stainless steel is required. Therefore, a technique for improving the strength and ductility has been developed by controlling the crystal grain size, crystal grain shape, and the like of ferrite-austenitic stainless steel.

  One such technique is an improvement in elongation due to the development of transformation-induced plasticity (TRIP) due to work-induced martensitic transformation. The processing-induced martensitic transformation is a phenomenon in which a relatively soft austenite phase is transformed into a relatively hard martensite phase by processing.

  When the material is subjected to a tensile test, constriction occurs after exhibiting a certain degree of uniform elongation, and deformation concentrates on the constricted portion, leading to fracture. When processing-induced martensitic transformation occurs by processing, the martensite phase in the steel is hard, so that deformation does not progress afterwards at the constricted portion. As a result, the ductility of the material is improved.

Patent Document 1 discloses a ferrite-austenite stainless steel in which the Md value, the crystal grain size and aspect ratio of the austenite phase are specified, and TRIP is easily developed to increase uniform elongation.
Patent Document 2 discloses a ferrite-austenitic stainless steel in which the texture of ferrite is controlled by multi-pass rolling to improve ridging properties, and TRIP is expressed by regulating the austenite phase ratio to increase ductility. It is disclosed.
Patent Document 3 discloses a ferrite-austenitic stainless steel in which the austenite phase is metastable by adjusting the amount of elements and the workability and impact resistance are improved by developing TRIP during deformation and impact. Has been.

  In Patent Document 1, there is a technology that regulates the stability of austenite grains by the Md value, reduces the austenite phase grain size, and makes the aspect ratio close to a sphere, thereby facilitating the expression of TRIP and increasing the uniform elongation. Have been described. However, Patent Document 1 does not describe a technique for increasing elongation in steel that does not exhibit TRIP.

  Patent Document 2 describes a technique for increasing the uniform elongation by expressing TRIP by defining the austenite phase ratio. However, there is no description of a technique for increasing elongation in steel that does not exhibit TRIP.

  Patent Document 3 describes a technique for improving workability and impact absorption by adjusting the amount of elements to make the austenite phase metastable and exhibiting TRIP. However, Patent Document 3 does not describe a technique for increasing elongation in steel that does not exhibit TRIP.

  As described above, there is knowledge for improving the ductility of ferritic / austenitic stainless steels by controlling the steel components and the metal structure. However, all of the conventional findings are techniques aimed at the expression of TRIP. In steel in which TRIP is manifested, stress remains due to processing, and problems such as aging cracking occur, and thus a process such as heat treatment may be required. Therefore, improvement in formability is required for stainless steel that does not exhibit TRIP and does not require a process such as heat treatment. As described above, Patent Documents 1 to 3 do not describe any technique for improving formability in steel that does not exhibit TRIP.

JP 2009-52115 A JP 2009-209448 A JP 2009-197326 A

  In view of the above technical background, the present invention has an object to provide a ferritic / austenitic stainless steel sheet having a component that does not exhibit TRIP due to processing-induced martensitic transformation and having good formability, and a method for producing the same. And

In order to investigate the metal structure factor governing elongation in the steel that does not express TRIP, the present inventors conducted the following experiments.
First, 13 types of steel containing N: 0.051 to 0.310% by mass% and having a PI value of less than 30 were cast and hot-rolled, and then annealed and cold-rolled to obtain a steel plate. In manufacturing, manufacturing conditions such as hot rolling conditions and annealing temperature were changed. The obtained steel plate was a ferrite-austenitic stainless steel plate having an Md value of -3 to -106. Table 1 shows the components of the steel. Moreover, Md value of the obtained steel plate is shown to Tables 2-4. Ingredient number 16 shown in Table 1 and Table 4 is an example in which the N content is excessive. Component number 17 is an example in which the Md value is −10 or more.

  Uniform elongation and local elongation were selected as the evaluation items serving as moldability indicators. The elongation at break, which is a general index of elongation, is expressed as the sum of uniform elongation until local thickness reduction (necking) occurs and local elongation after the thickness reduction occurs. That is, increasing the uniform elongation is effective for improving the stretch formability, and increasing the local elongation is effective for improving the bendability and hole expandability. In order to improve the overall formability, it is necessary to increase both uniform elongation and local elongation.

  With ferrite-austenitic stainless steel, it is difficult to improve local elongation. However, since the ferrite-austenitic stainless steel has a higher breaking strength than the ferritic stainless steel, the fracture resistance is large even with the same elongation, and good formability is obtained. The higher the absolute value of uniform elongation and local elongation, the higher the formability. In a ferritic / austenitic stainless steel sheet with a thickness of 1 mm, if the uniform elongation is 20% or more and the local elongation is 10% or more, the formability is relatively high. However, it will exhibit the same formability as ferritic stainless steel. Further, by devising the processing method, even if it is a ferrite-austenitic stainless steel, it is possible to perform processing with substantially the same shape as that of the austenitic stainless steel. More preferably, the uniform elongation is 21% or more and the local elongation is 10.5% or more. Measurement of elongation at break and uniform elongation was carried out by a method in accordance with JIS Z 2241 by collecting a JIS No. 13 B tensile test piece in parallel with the rolling direction from the steel plate having a thickness of 1 mm obtained above. The local elongation was determined as the difference between the breaking elongation and the uniform elongation.

  Moreover, the chemical composition in the austenite grain in structure | tissue was investigated using the electron beam microanalyzer (EPMA), and Md value which is a parameter | index of the stability of an austenite phase was calculated from the obtained chemical composition. Furthermore, in order to investigate the structure of the steel sheet, electron backscatter analysis images of the metal structure of the cross section perpendicular to the rolling width direction (L cross section) and the cross section perpendicular to the sheet thickness direction (Z cross section) at the center position in the rolling width direction are used. Investigated by law (EBSP). In this EBSP survey, phase identification, crystal grain size measurement, and aspect ratio measurement were performed in the L section, phase identification was performed in the Z section, and the number of ferrite / austenite grain boundaries in the direction perpendicular to the rolling direction was measured. It was. Data obtained from EBSP was classified into ferrite grains (BCC phase) and austenite grains (FCC phase) for each crystal grain, and the boundary was defined as a grain boundary. In addition, when grains having the same crystal structure are adjacent to each other, a portion having a crystal orientation difference of 15 ° or more at adjacent measurement points is regarded as a crystal grain boundary. A measurement example in which the crystal grain boundaries are indicated by black lines is shown in FIG. FIG. 1 shows EBSP measurement results of the L cross section, FIG. 1 (a) shows the BCC phase, and FIG. 1 (b) shows the FCC phase in white. The aspect ratio was measured as the reciprocal of the value obtained by dividing the projected area equivalent diameter and the longest diameter of each austenite grain (FCC phase) by the maximum diameter perpendicular to the projected area. FIG. 2 shows the EBSP measurement results of the Z cross section, FIG. 2 (a) shows the result of sample 1-1 with a cold rolling reduction ratio of 50%, and FIG. 2 (b) shows the result of sample 2-1 with a cold rolling reduction ratio of 80%. FIG. In FIG. 2, the BCC phase is shown in black and the FCC phase is shown in white, and the crystal grain boundaries are not shown.

Regarding the above experimental results, focusing on the grain size and aspect ratio of the austenite phase and investigating the relationship between the uniform elongation and local elongation of the tensile test, in the steel sheet that does not express TRIP, in the cross section perpendicular to the rolling width direction ,
(I) Uniform elongation is large when the crystal grain size of the austenite phase is large.
(Ii) The local elongation is large when the shape of the austenite phase extends in the rolling direction.
Furthermore, in a cross section perpendicular to the thickness direction,
(Iii) Local elongation is large when the number of ferrite / austenite grain boundaries in the direction perpendicular to the rolling direction is large.
I got the knowledge. The present invention has been completed based on the above findings, and the gist thereof is as follows.

<1>% by mass, N: 0.080 to 0.300%, a ferrite austenitic stainless steel sheet having a PI value represented by the formula (1) of less than 30,
The Md value calculated by the formula (2) from the chemical composition in the austenite phase is less than −10 ° C .;
In the cross section perpendicular to the rolling width direction of the steel sheet, the average of the projected area circle equivalent diameter of the austenite grains is 1.0 + 5.0 × the nitrogen content (mass%) μm or more in the whole steel sheet,
The shape aspect ratio expressed by the reciprocal of the value obtained by dividing the length of the longest diameter by the maximum length in the direction perpendicular to the total area of the austenite grains in the cross section perpendicular to the rolling width direction of the steel sheet is less than 0.5. The F value obtained by the formula (3) is 120 or more from the area ratio of the austenite grains and the number of ferrite / austenite phase boundaries in a direction perpendicular to the rolling direction of 1000 μm in the cross section perpendicular to the thickness direction of the steel sheet. A ferritic / austenitic stainless steel sheet.
PI value = {Cr} +3.3 {Mo} +16 {N} (1)
Md value = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18.5 [Mo] (2)
F value = 100 × area ratio of austenite grains having an aspect ratio of less than 0.5 + 0.28 × number of ferrite / austenite grain boundaries (3)
[However, the element symbol surrounded by {} in the formulas (1) to (3) is the content (mass%) of the element in the whole steel plate, and the element symbol surrounded by [] is This is the content (mass%) of the element in the austenite phase, and 0 is substituted when the element is not contained in the formulas (1) and (2). ]
<2> Further, by mass%, C: 0.050% or less, Si: 2.00% or less, Mn: 1.00 to 8.00%, P: 0.050% or less, S: 0.050% or less , Cr: 17.0 to 25.0%, Ni: 0.10 to 6.00%, and the balance consisting of iron and unavoidable impurities <1> Stainless steel sheet.
<3> Further, by mass%, Cu: 0.10 to 3.00%, Mo: 0.10 to 2.50%, Sn: 1.00% or less, W: 0.01 to 1.00%, V : The ferrite-austenitic stainless steel sheet according to <2>, containing one or more of 0.01 to 1.00%.
<4> Further, by mass%, Nb: 0.50% or less, Ti: 0.50% or less, B: 0.0050% or less, or one or more kinds <2> or The ferritic / austenitic stainless steel sheet according to <3>.
<5> Further, by mass%, Ca: 0.0050% or less, Mg: 0.0050% or less, Al: 0.50% or less, Ta: 0.0050% or less, Ga: 0.0050% or less, rare earth element : The ferrite austenitic stainless steel sheet according to any one of <2> to <4>, characterized by containing one or more of 0.50% or less.
<6><1> to <5> The steel of the component according to any one of <5> is continuously cast, and the obtained steel slab is heated to 1150 ° C. to 1250 ° C., and the rolling reduction ratio in all passes is 35. %, And the hot rolling is performed at 1050 ° C. to 1150 ° C., and the cold rolling reduction is 50% to 80%. The following cold rolling, or two or more cold rollings with intermediate annealing in between, is performed, and the final annealing is performed in 920 + r + 300 × total steel sheet nitrogen content (mass%) (° C.) to 940 + r + 300 × whole steel sheet. A method for producing a ferritic / austenitic stainless steel sheet, wherein the nitrogen content (mass%) (° C.) and the cooling rate from the end of holding to 400 ° C. are performed at a rate of more than 30 ° C./s and less than 70 ° C./s. Here, the cold rolling reduction ratio (%) is a value calculated by (sheet thickness before cold rolling−sheet thickness after cold rolling) / plate thickness before cold rolling × 100, and r is the cold rolling reduction ratio. It is the temperature (° C) of the same value. However, when cold rolling is performed a plurality of times and intermediate annealing is performed between the cold rollings, the rolling reduction rate of the final cold rolling is used for the calculation of r.

  According to the present invention, it is possible to obtain a ferritic / austenitic stainless steel sheet having excellent formability with a component system that does not exhibit TRIP. Therefore, it does not require additional steps such as heat treatment and trimming to prevent aging cracking, and can be applied to parts that have conventionally used austenitic stainless steel sheets containing a large amount of Ni, greatly contributing to the improvement of product processing efficiency. Is.

It is the figure which classified the measurement result of the electron backscattering analysis image method (EBSP) of the L section into the BCC phase and the FCC phase, (a) is a figure which shows a BCC phase by white display, and (b) is a FCC phase. Is a diagram showing in white display. FIG. 3 is a diagram in which measurement results of electron backscattering analysis image method (EBSP) of the Z cross section are classified into a BCC phase and an FCC phase, and (a) is a diagram showing the results of Sample 1-1 with a cold rolling reduction ratio of 50%. And (b) is a diagram showing the results of Sample 2-1 having a cold rolling reduction rate of 80%. Is a graph showing the relationship between X _{1 ([mu]} m) and uniform elongation is calculated by the average and -5.0 nitrogen content in the entire × steel projected equivalent circle diameter of austenite grains (wt%) (%). The shape aspect ratio expressed by the reciprocal of the value obtained by dividing the length of the longest diameter by the maximum length in the direction perpendicular to the total area of the austenite grains in the cross section perpendicular to the rolling width direction of the steel sheet is less than 0.5. The F value and local elongation (%) obtained by the formula (3) from the area ratio of austenite grains and the number of ferrite / austenite phase boundaries in a direction perpendicular to the rolling direction of 1000 μm in the cross section perpendicular to the thickness direction of the steel sheet. It is a graph which shows the relationship.

  The present invention is described in detail below. Unless otherwise noted,% of element content in this specification means mass%.

The ferritic / austenitic stainless steel sheet of the present invention contains 0.080 to 0.300% of N. N is an element that greatly affects the stability of the austenite phase. Further, N has an effect of improving the corrosion resistance when present in a solid solution. Therefore, N is added in an amount of 0.080% or more. However, when N exceeds 0.300%, uniform elongation and local elongation may be reduced. On the other hand, when N is added in an amount exceeding 0.300%, Cr nitride is liable to precipitate, resulting in a decrease in corrosion resistance. Therefore, N is preferably 0.300% or less. The preferable addition amount of N is in the range of 0.150 to 0.250%.
N is an element that has a large effect on uniform elongation and local elongation. Therefore, if only the production conditions according to the N component range and the N content are defined, the effect of improving the moldability of the present invention can be obtained. For other elements, the element type and the content thereof are appropriately selected. good.

(PI value: less than 30)
The PI value is a general index indicating the pitting corrosion resistance of a stainless steel plate, and is calculated from the average composition of the steel plate according to the following formula (1). When the PI value is 30 or more, the steel sheet is hardened and the workability is impaired, and the alloy cost is increased. Therefore, the upper limit is less than 30. The lower limit is not particularly required, but 20 or more is desirable for obtaining corrosion resistance equivalent to SUS304. More desirably, it is 23 or more.

PI value = {Cr} +3.3 {Mo} +16 {N} (1)
However, the element symbol enclosed in {} in (1) Formula means the average content (mass%) in the whole steel plate.

(Md value calculated from chemical composition in austenite phase: less than −10 ° C.)
The Md value is an index of the stability of austenite grains and is calculated by the following formula (2). When the Md value is small, TRIP is difficult to express. Steel sheets with Md values of -10 or more have a significantly larger uniform elongation than steel sheets with Md values of less than -10 ° C, and within 48 hours when the final product is subjected to cylindrical drawing with a drawing ratio of 2.2. Aging cracks occur. This is thought to be due to the expression of TRIP. Therefore, the Md value is preferably less than −10 ° C. More preferably, the Md value is less than −20 ° C. Although the lower limit is not particularly required, addition of an alloy element is required to lower the Md value, and therefore, the lower limit of the Md value is desirably more than −200 ° C. from the viewpoint of manufacturing cost.

Md value = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18.5 [Mo] (2)
However, [] in the formula (2) is the composition (mass%) in the austenite phase of the steel sheet, and {} is the average composition (mass%) of the entire steel sheet.

  The chemical composition in the austenite phase on which the Md value is calculated is measured by, for example, EPMA. [] In the above formula for calculating the Md value may be the content (mass%) of each element in the austenite phase measured by EPMA. However, since C is difficult to measure with EPMA, it is preferable to use the content (% by weight) of the entire steel sheet, not the composition in the austenite phase.

(Average crystal grain size of austenite grains: 1.0 + 5.0 × nitrogen content (mass%) μm or more in the whole steel sheet)
In the present invention, the average crystal grain size of the austenite grains is determined from the measurement result of the projected circle equivalent diameter of the austenite grains by EBSP measurement of the L cross section (a plane observed in the plate width direction). In the present invention, it has been found that good uniform elongation can be obtained by increasing the average crystal grain size of austenite grains. On the other hand, nitrogen is an element that increases the strength of steel and decreases the elongation. Therefore, in order to obtain good uniform elongation, it is necessary to control the average crystal grain size of austenite grains according to the nitrogen content. Figure 3 shows the results of examining the relationship of the average of the projected equivalent circle diameter of austenite grains nitrogen content of the overall from ([mu] m) 5.0 × steel uniform elongation value X ₁ obtained by subtracting the (mass%) (%) . The black symbols in FIG. 3 are samples of component numbers 1 to 15 in Table 1, the semi-black symbols are component numbers 16, and the white symbols are component numbers 17. When X ₁ is 1.0 or more, the uniform elongation is 20% or more, the nitrogen content in the overall average grain size 1.0 + 5.0 × steel sheet austenite grain (mass%) [mu] m or more. More preferably, it is 1.2 + 5.0 × the nitrogen content (mass%) of the whole steel plate or more. The upper limit of the average crystal grain size does not need to be specified in particular. However, when the average crystal grain size is increased, the strength is lowered, so that it is preferably less than 20 μm.

(Area ratio of austenite grains whose shape aspect ratio in the cross section perpendicular to the rolling width direction of the steel sheet is less than 0.5, and ferrite contained in a direction perpendicular to the rolling direction of 1000 μm in the cross section perpendicular to the thickness direction of the steel sheet / F value calculated by the formula (3) from the number of austenite grain boundaries is 120 or more)
In the present invention, rolling is performed by EBSP measurement of the aspect ratio of each austenite grain and EBSP measurement of the Z cross section (plane perpendicular to the thickness direction) by EBSP measurement of the L cross section (surface observed in the plate width direction). As a result of measuring the number of ferrite / austenite phase boundaries included in the direction 1000 μm perpendicular to the direction, the ratio of the austenite grains having a shape aspect ratio of less than 0.5 to the total austenite, the number of phase boundaries, and nitrogen content in the entire steel sheet Using the amount (mass%), it has been found that the F value calculated by the following formula (3) is 120 or more and good local elongation can be obtained.

F value = 100 × area ratio of austenite grains whose aspect ratio is less than 0.5 + 0.28 × ferrite / austenite grain boundary number (3)

  In formula (3), the shape aspect ratio is the reciprocal of the value obtained by dividing the length of the longest diameter of each grain by the length of the maximum diameter orthogonal thereto. Therefore, the upper limit of the shape aspect ratio is 1. The number of ferrite / austenite phase boundaries is the number of phase boundaries included in the direction 1000 μm perpendicular to the rolling direction. The result of investigating the relationship between F value and local elongation (%) is shown in FIG. The black symbols in FIG. 4 are samples of component numbers 1 to 15 in Table 1, the semi-black symbols are component numbers 16, and the white symbols are component numbers 17. From FIG. 3, it can be seen that a local elongation of 10% or more is obtained when the F value is 120 or more. Moreover, the tendency for local elongation to become large is recognized, so that F value is large. The F value is more preferably 130 or more.

  The number of crystal grains for measuring the average crystal grain size and the shape aspect ratio is 100 or more. The number of ferrite / austenite phase boundary measurements is 10 or more. In the present invention, the uniform elongation is improved by increasing the grain size of austenite grains, and the local elongation is improved when the area ratio of grains having a shape aspect ratio of less than 0.5 and the number of ferrite / austenite phase boundaries are increased. However, the reason for this is unknown at this stage. However, the reason for the improvement in uniform elongation is considered to be a decrease in strength due to the coarsening of the grain size, and the reason for the improvement in local elongation is that the grains having a shape aspect ratio of less than 0.5 and the ferrite / austenite phase boundary are voids. It is conceivable that the connection is suppressed and resistance to breakage occurs.

  As described above, in the present invention, the steel component composition other than the N content can be set as appropriate. Therefore, elements other than N and their contents are not limited at all. Hereinafter, the preferable component composition of the ferritic / austenitic stainless steel sheet according to the present invention will be described.

  As a preferable example of the ferritic / austenitic stainless steel sheet according to the present invention, C: 0.050% or less, Si: 2.00% or less, Mn: 1.00 to 8.00%, P: 0.050% or less, S: 0.050% or less, Cr: 17.0 to 25.0%, Ni: 0.10 to 6.00%, with the balance being iron and inevitable impurities.

(C: 0.050% or less)
C, like N, is an element that greatly affects the stability of the austenite phase. If more than 0.050% of C is added, the uniform elongation and local elongation of the steel sheet may decrease. In addition, when more than 0.050% of C is added, intergranular corrosion is caused to promote precipitation of Cr carbide. Therefore, the addition amount of C is preferably 0.050% or less, and more preferably 0.040% or less. Moreover, although it is preferable to make C low from the point of corrosion resistance, in existing steelmaking equipment, in order to reduce C amount to 0.002% or less, a big cost increase is caused, so C may be 0.002% or more. preferable.

(Si: 2.00% or less)
Si may be used as a deoxidizing element or may be added to improve oxidation resistance. However, the addition of more than 2.00% Si results in hardening of the steel sheet and lowers the uniform elongation. Accordingly, Si is preferably 2.00% or less, more preferably 1.50% or less, and even more preferably 1.00% or less. Further, in order to reduce the amount of Si to an extremely small amount, the cost for refining steel is increased, so Si is preferably 0.03% or more.

(Mn: 1.00 to 8.00%)
Mn concentrates in the austenite phase and plays an important role in changing the Md value. However, the addition of a large amount of Mn decreases the uniform elongation, and also reduces the corrosion resistance and hot workability. Therefore, the Mn content is preferably 8.00% or less, and more preferably 6.00% or less. Further, if the amount of Mn is less than 1.00%, an increase in cost and an increase in Md value in the steel refining process are caused, so the amount of Mn is preferably 1.00% or more, and 3.00 % Or more is more preferable.

(P: 0.050% or less)
P is an element that is inevitably mixed in, and is also contained in raw materials such as Cr, so it is difficult to reduce it. However, if P is contained in a large amount, formability is lowered. The smaller the amount of P, the better. The amount is preferably 0.050% or less, and more preferably 0.040% or less.

(S: 0.050% or less)
S is an element inevitably mixed in, and may combine with Mn to form inclusions, which may serve as a starting point for igniting. Therefore, S is preferably 0.050% or less. Since corrosion resistance improves so that the amount of S is low, it is more preferable to set it as 0.0030% or less.

(Cr: 17.0 to 25.0%)
Cr is an element necessary for ensuring corrosion resistance, and addition of 17.0% or more is preferable. However, the addition of a large amount of Cr causes hot working cracks or increases the cost of the refining process, so the upper limit is preferably made 25.0%. The amount of Cr is more preferably 20.0 to 23.0%, and further preferably 21.0 to 22.0%.

(Ni: 0.10 to 6.00% or less)
Ni is an austenite stabilizing element and is an important element for adjusting the stability of the austenite phase. Ni also has the effect of suppressing the precipitation of nitrides and improving the corrosion resistance. Therefore, Ni is preferably added in an amount of 0.10% or more, and more preferably 1.00% or more. On the other hand, addition of Ni exceeding 6.00% leads to an increase in raw material cost, and a problem such as stress corrosion cracking may occur due to an increase in the austenite phase ratio, so Ni is 6.00% or less. Preferably, the content is 3.00% or less, more preferably 2.00% or less.

  Furthermore, the ferrite-austenitic stainless steel sheet of the present invention can be optionally added with one or more of Cu, Mo, Sn, W, and V in order to improve the corrosion resistance.

(Cu: 0.10 to 3.00%)
Cu is an austenite stabilizing element and is an important element for adjusting the Md value. Further, Cu has the effect of suppressing the precipitation of nitride and improving the corrosion resistance. Therefore, Cu may be added by 0.10% or more. However, the addition of Cu exceeding 3.00% brings about an increase in weight reduction cost and deteriorates hot workability. Therefore, Cu is preferably 3.00% or less, and more preferably 1.5% or less.

(Mo: 0.10 to 2.50%)
Since Mo is an element that improves the corrosion resistance, 0.10% or more may be added. However, if the addition amount of Mo exceeds 2.50%, the raw material cost greatly increases. Therefore, Mo is preferably 2.50% or less, and more preferably 1.00% or less.

(Sn: 1.00% or less)
Sn is an element that improves the corrosion resistance. Since the effect is exhibited by the addition of 0.030% or more of Sn, it is preferable to set this as the lower limit. However, when Sn exceeds 1.00%, the hot workability is deteriorated, so this is preferably set as the upper limit.

(V: 0.01-1.00%)
Since V is an element that improves the corrosion resistance, 0.01% may be added as a lower limit. However, if V exceeds 1.00%, the load during rolling is increased and it becomes easy to produce a manufacturing iron, so this is the upper limit. A more preferable upper limit of V is 0.50% or less.

(W: 0.01-1.00%)
Since W is an element that improves the corrosion resistance, 0.01% may be added as a lower limit. However, if W exceeds 1.00%, the load at the time of rolling is increased, and it becomes easy to produce a production iron, so this is the upper limit. A more preferable upper limit of W is 0.50% or less.

  Furthermore, in order to further improve the hot workability and formability, one or more of the ferrite and austenitic stainless steel sheets of the present invention can be optionally added from Nb, Ti and B.

(Nb: 0.50% or less)
Nb may be added because it has an effect of preventing the weld heat affected zone from becoming coarse. In order to exhibit the effect of Nb, addition of 0.005% or more is necessary, so this may be added as the lower limit. However, addition of Nb in excess of 0.50% lowers uniform elongation and local elongation, so Nb is preferably 0.50% or less.

(Ti: 0.50% or less)
Ti, like Nb, has an effect of preventing the weld heat-affected zone from becoming coarse, and further has the effect of finely equiaxing the solidified structure. In order to exert such an effect, it is preferable to add Ti by 0.005% or more. However, addition of Ti exceeding 0.50% lowers uniform elongation and local elongation, so Ti is preferably 0.50% or less.

(B: 0.0050% or less)
B has an effect of improving hot workability and may be added. In order to exert these effects, B is preferably added in an amount of 0.0003% or more. However, if B exceeds 0.0050%, the corrosion resistance is remarkably deteriorated. Therefore, B is preferably 0.0050% or less, more preferably 0.0030% or less.

  Furthermore, the ferritic / austenitic stainless steel sheet of the present invention may be optionally added with one or more of Ca, Mg, Al, Ta, Ga and rare earth elements for deoxidation and desulfurization during refining. Can do.

(Ca: 0.0050% or less)
Ca may be slightly added for desulfurization and deoxidation. Since the effect is exhibited by the addition of 0.0001% or more of Ca, this may be added as the lower limit. However, hot work cracking is likely to occur due to the addition of over 0.0050% Ca, and the corrosion resistance is lowered, so the Ca content is preferably 0.0050% or less.

(Mg: 0.0050% or less)
Mg has an effect of refining the solidified structure as well as deoxidation, and may be added. In order to exert these effects, 0.0001% or more of Mg needs to be added, and this may be added as the lower limit. Moreover, since addition of 0.0050% Mg brings about an increase in cost in the steelmaking process, Mg is preferably 0.0050% or less.

(Al: 0.50% or less)
Al may be slightly added for desulfurization and deoxidation. Since the effect is exhibited by the addition of 0.0030% or more of Al, this may be added as the lower limit. However, addition of more than 0.50% leads to an increase in manufacturing costs and an increase in raw material costs, so 0.50% or less is preferable.

(Ta: 0.050% or less)
Ta is an element that improves corrosion resistance by modification of inclusions, and may be added as necessary. Since the effect is exhibited by the addition of 0.001% or more of Ta, this may be added as the lower limit. However, addition of more than 0.050% of Ta causes lowering of normal temperature ductility and toughness, so 0.050% or less is preferable, and more preferably 0.020% or less.

(Ga: 0.050% or less)
Ga is an element that suppresses corrosion resistance improvement and hydrogen embrittlement, and may be added as necessary. Since the effect is exhibited by the addition of 0.001% or more of Ga, this may be added as the lower limit. However, addition of more than 0.050% of Ta causes a decrease in workability, so 0.050% or less is preferable, and more preferably 0.020% or less.

(Rare earth elements: 0.50% or less)
Rare earth elements are a generic term for 15 elements (lanthanoids) from Sc, Y, and La to Lu, and may be added to improve hot workability. Since the effect is exhibited by the addition of a total amount of rare earth elements of 0.005% or more, this may be added as the lower limit. However, if the total amount of rare earth elements exceeds 0.50%, the productivity is impaired and the cost is increased, so the upper limit is made 0.50%. A more preferable rare earth element addition range is 0.020 to 0.20%.

  Next, the manufacturing method of the ferrite austenitic stainless steel sheet of the present invention will be described. As described above, in order to obtain a good uniform elongation, it is necessary to control the metal structure. Such a metal structure can be realized by combining the chemical composition of steel and appropriate manufacturing conditions.

  As a hot rolled material, a steel piece obtained by continuous casting is used. The steel slab composition may be steel having the component composition described above. The heating temperature before hot rolling is 1150 ° C to 1250 ° C. When the heating temperature is less than 1150 ° C., ear cracks occur during hot rolling, so this is the lower limit. Further, when the heating temperature exceeds 1250 ° C., the steel slab is deformed in the heating furnace or flaws are likely to occur during hot rolling, so the heating temperature is set to 1250 ° C. or less. A more preferable range of the heating temperature is 1180 ° C to 1220 ° C.

  In hot rolling, the rolling reduction is 35% or less in all passes, and the final finishing temperature is controlled to 950 ° C. or more. In order to obtain good uniform elongation in the present invention, it is necessary to increase the austenite grain size. Therefore, it is necessary to increase the grain size of the austenite grains in the hot rolling process. When there is one or more passes with a rolling reduction of over 35%, or when the finishing temperature is less than 950 ° C., the austenite grain size is 1.0 + 5.0 × nitrogen content in the entire steel sheet in the metal structure after cold rolling annealing The amount (mass%) is less than μm, and the uniform elongation during the tensile test becomes insufficient. More preferably, the reduction ratio is 34% or less and the final finishing temperature is 980 ° C. or more in all passes. As long as the rolling reduction of 35% is satisfied, the rolling reduction may be different in each pass, or the rolling reductions of all the passes may be the same.

  The annealing temperature of the hot-rolled sheet after hot rolling is set to 1050 ° C to 1150 ° C. When the annealing temperature is less than 1050 ° C., the austenite grain size after cold rolling and annealing is less than 1.0 + 5.0 × the total nitrogen content (mass%) μm of the steel sheet, and the uniform elongation during the tensile test is reduced. To do. When the annealing temperature is higher than 1150 ° C., the area ratio of austenite grains having an aspect ratio of less than 0.5 after cold rolling and annealing decreases, so the F value calculated by formula (3) is less than 120, The local elongation during the tensile test decreases. A more preferable range of the annealing temperature is 1050 ° C to 1100 ° C.

  After the annealing, cold rolling with a cold rolling reduction of 50% or more and 80% or less is performed. Cold rolling may be performed only once or twice. When cold rolling is performed twice, intermediate annealing may be performed between cold rolling. When performing the intermediate annealing, it is necessary to set the reduction ratio to 50% or more and 80% or less in all cold rolling. When there is at least one cold rolling with a cold rolling reduction of less than 50%, the number of ferrite / austenite phase boundaries decreases, so the F value calculated by equation (3) is less than 120, and the local elongation during the tensile test Decreases. When there is at least one cold rolling with a cold rolling reduction of more than 80%, the particle size becomes small, and the uniform elongation during the tensile test is lowered. A more preferable range of the cold rolling reduction is 60% or more and 75% or less. Here, the cold rolling reduction ratio (%) is a value calculated by ((plate thickness before cold rolling−plate thickness after cold rolling) / plate thickness before cold rolling × 100). However, when intermediate annealing is performed, the thickness after intermediate annealing is used as the thickness before cold rolling. The intermediate annealing temperature needs to be 1050 ° C. to 1150 ° C., similarly to the annealing for the hot-rolled sheet after hot rolling. When the intermediate annealing temperature is less than 1050 ° C., the grain size of the austenite grains after cold rolling and annealing is 1.0 + 5.0 × the nitrogen content (mass%) of the entire steel sheet is less than μm, and uniform during the tensile test Elongation decreases. Further, when the intermediate annealing temperature is higher than 1150 ° C., the proportion of grains having an aspect ratio of austenite grains after cold rolling and annealing of less than 0.5 decreases, so the F value calculated by the formula (3) is less than 120. Thus, the local elongation during the tensile test decreases. A more preferable range of the intermediate annealing temperature is 1050 ° C to 1100 ° C.

  Final annealing is performed on the cold-rolled steel sheet after cold rolling. The final annealing temperature is 920 + r + 300 × nitrogen content (% by mass) (° C.) to 940 + r + 300 × total nitrogen content (% by mass) (° C.) depending on the cold rolling reduction ratio and nitrogen content. carry out. Here, r is a temperature (° C.) equivalent to the cold rolling reduction ratio, and the cold rolling reduction ratio is calculated by (sheet thickness before cold rolling−thickness after cold rolling) / plate thickness before cold rolling × 100. Value. However, when cold rolling is performed a plurality of times and intermediate annealing is performed between the cold rollings, the rolling reduction of the final cold rolling is used.

  When the cold rolling reduction ratio is high, the austenite grain diameter tends to be small because more dislocations are introduced, while the number of ferrite / austenite grain boundaries and the area ratio of austenite grains with an aspect ratio of less than 0.5 increase. That is, the F value tends to increase. Also, when the cold rolling reduction is low, the austenite grain diameter tends to increase because less dislocations are introduced. On the other hand, the number of ferrite / austenite grain boundaries and the area ratio of austenite grains with an aspect ratio of less than 0.5 Tends to decrease, that is, the F value tends to decrease. When the annealing temperature is high, the particle size is larger than when the annealing temperature is low, and the area ratio of austenite grains having an aspect ratio of less than 0.5 is small. Furthermore, when the nitrogen content is high, the strength of the steel sheet increases, and the uniform elongation decreases. Therefore, it is necessary to increase the annealing temperature and increase the austenite grain size. On the other hand, when the nitrogen content is high, a lot of fine austenite phases are precipitated, and the number of ferrite / austenite grain boundaries increases. Therefore, even if the annealing temperature is increased, the F value does not decrease so much. The converse is true when the nitrogen content is low. Therefore, in order to obtain good uniform elongation and local elongation at the same time, it is necessary to adjust the annealing temperature after cold rolling in accordance with the rolling reduction of cold rolling and the nitrogen content of the steel sheet.

  When the final annealing temperature is within the specified range, good uniform elongation and local elongation are obtained. When the final annealing temperature is lower than the specified range, the austenite phase grain size is less than 1.0 + 5.0 × the total nitrogen content (mass%) μm of the steel sheet, so that the uniform elongation is lowered. When the final annealing temperature is higher than the specified range, the proportion of grains having an aspect ratio of less than 0.5 is lowered, so that the F value calculated by the formula (3) is less than 120, and the local elongation is lowered. A more preferable range of the final annealing temperature is 925 + r + 300 × nitrogen content (% by mass) (° C.) in the whole steel plate to 935 + r + 300 × nitrogen content (% by mass) (° C.) in the whole steel plate.

  The soaking time for annealing, intermediate annealing and final annealing on the hot-rolled sheet can be set as appropriate. Typically, it is 5 to 300 seconds.

  The cooling rate from the end of soaking in the final annealing to 400 ° C. is more than 30 ° C./s and less than 70 ° C./s. If the cooling rate after the holding is 70 ° C./s or more, the elements cannot be sufficiently diffused during the cooling process, so that the austenite grain components vary. As a result, TRIP appears in austenite grains having an Md value exceeding −10 ° C., and aging cracking occurs within 48 hours when the final product is subjected to cylindrical drawing with a drawing ratio of 2.2. When the cooling rate after the holding is less than 30 ° C./s, corrosion resistance and toughness are reduced due to the precipitation of inclusions. In consideration of productivity and pickling properties, the cooling rate is preferably 40 to 60 ° C./sec, and the cooling method may be appropriately selected such as air-water cooling or water cooling.

According to the ferrite-austenitic stainless steel plate of the present embodiment, a ferrite-austenitic stainless steel plate excellent in formability can be obtained with a component system that does not exhibit TRIP.
In addition, the ferrite-austenitic stainless steel sheet of the present embodiment has a Ni content of 0.10 to 6.00% and a relatively small amount so that TRIP is not expressed without containing a large amount of Ni. it can.
Therefore, it does not require additional steps such as heat treatment and trimming to prevent aging cracking, and can be applied to parts that have conventionally used austenitic stainless steel sheets containing a large amount of Ni, greatly contributing to the improvement of product processing efficiency. Is.

  Steel having a chemical composition shown in Table 1 was melted to form a steel piece, and a steel plate having a thickness of 1.0 mm was produced through steps of hot rolling, annealing of hot-rolled sheet, cold rolling, and final annealing. In manufacturing the steel sheet, in order to change the metal structure, the heating temperature before hot rolling, the hot rolling pass schedule, the final finishing temperature of the hot rolling, the annealing temperature of the hot rolled sheet, the intermediate annealing temperature, the cold rolling reduction ratio The final annealing temperature was changed.

And the tensile test was implemented with respect to the obtained product board, and uniform elongation and local elongation were measured. Further, EPMA measurement was performed on the metal structure of the obtained product plate, the composition of the austenite phase was identified, and the Md value of the austenite phase was determined. Furthermore, the phase identification by EBSP and the austenite grain size and aspect ratio were investigated from the steel sheet / L cross-section metal structure, the phase identification by EBSP from the steel sheet Z / cross-section metal structure, and the ferrite / The number of austenite phase boundaries was investigated. Each condition is as described above.
Tables 2 to 4 show the manufacturing conditions and the measurement results.

In addition, the code | symbol in Table 1 and Table 2 is as showing below.
PI value: Value calculated from the average composition of steel by the following formula.
PI value = {Cr} +3.3 {Mo} +16 {N}
Here, {} is an average composition (mass%).
T ₁ : Heating temperature before hot rolling (° C.)
N: Number of rolling reductions having a rolling reduction of more than 35% in the hot rolling process.
T ₂ : Hot rolling final finishing temperature (° C.)
HA: Hot-rolled sheet annealing temperature (° C), or intermediate annealing temperature (° C) when intermediate annealing is performed
FA: Final annealing temperature (° C)
Cold rolling rate: Cold rolling reduction rate (%)
V: Cooling rate (° C./s) from the end of holding in the final annealing to 400 ° C.
Md value: A value calculated from the composition in the austenite phase (average composition of C only) by the following formula.
Md value = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 Cu-18.5 [Mo]
In the above formula, [] is the content (mass%) of the element in the austenite phase, and {} is the average content (mass%) of the element in the entire steel.
X ₁ : austenite phase grain size -5.0 × nitrogen content (% by mass) (μm) in the whole steel plate
F value: the ratio of the area of austenite grains having an aspect ratio of less than 0.5% in the L cross section to the total austenite grain area and the number of ferrite / austenite phase boundaries in the direction 1000 μm perpendicular to the rolling direction in the Z cross section The value calculated by.
F value = 100 × area ratio of austenite grains having an aspect ratio of less than 0.5 + 0.28 × ferrite / austenite phase boundary manufacturability: a sample in which ear cracks and wrinkles occurred during hot rolling, or a final product Cylindrical drawing was performed at a drawing ratio of 2.2, and a sample in which aging cracking occurred after 48 hours was indicated by x.

Sample 1-1 is an example of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 1-2 had a high final annealing temperature (FA), the F value deviated from the present invention. Since Sample 1-3 had a low hot rolling final finishing temperature (T ₂ ), the particle size (X ₁ ) of the austenite phase deviated from the present invention.

Sample 2-1 is an example of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 2-2 had a low final annealing temperature (FA), the particle size (X ₁ ) of the austenite phase deviated from the present invention. Since Sample 2-3 has a high final annealing temperature (FA), the F value deviated from the present invention. Since Sample 2-4 has one rolling reduction (N) having a rolling reduction of more than 35%, the particle size (X ₁ ) of the austenite phase deviated from the present invention.

  Samples 3-1 and 3-2 are examples of the present invention, and good uniform elongation and local elongation are obtained. Sample 3-3 had a problem in manufacturability because the cooling rate (V) from the end of holding in the final annealing to 400 ° C. was high. Since Sample 3-4 had a high final annealing temperature (FA), the F value deviated from the present invention.

  Samples 4-1 and 4-2 are examples of the present invention, and good uniform elongation and local elongation are obtained. Since Samples 4-3 and 4-4 had a low cold rolling rate, the F value deviated from the present invention.

Samples 5-1 and 5-2 are examples of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 5-3 had a high heating temperature (T ₁ ) before hot rolling, there was a problem in manufacturability. Since Sample 5-4 had a low hot rolling final finishing temperature (T ₂ ), the particle size (X ₁ ) of the austenite phase deviated from the present invention.

Sample 6-1 is an example of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 6-2 has one rolling reduction (N) having a rolling reduction of more than 35%, the particle size (X ₁ ) of the austenite phase deviated from the present invention. Since Sample 6-3 had a low cold rolling rate, the F value deviated from the present invention. Since Sample 6-4 has a high cold rolling rate, the particle size (X ₁ ) of the austenite phase deviated from the present invention.

Samples 7-1 and 7-2 are examples of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 7-3 had a low annealing temperature (HA) of the hot-rolled sheet, the particle size (X ₁ ) of the austenite phase deviated from the present invention. Since sample 7-4 had a low heating temperature (T ₁ ) before hot rolling, there was a problem in manufacturability.

Sample 8-1 is an example of the present invention, and good uniform elongation and local elongation are obtained. Sample 8-2 had a problem in manufacturability because the cooling rate (V) from the end of holding in the final annealing to 400 ° C. was high. Since Sample 8-3 had a high annealing temperature (HA) of the hot-rolled sheet, the F value deviated from the present invention. Since Sample 8-4 had a low final annealing temperature (FA), the particle size (X ₁ ) of the austenite phase deviated from the present invention.

Sample 9-1 is an example of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 9-2 had a low annealing temperature (HA) of the hot-rolled sheet, the particle size (X ₁ ) of the austenite phase was out of the present invention. Since Sample 9-3 had a low heating temperature (T ₁ ) before hot rolling, there was a problem in manufacturability.

Samples 10-1 and 10-2 are examples of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 10-3 had a low heating temperature (T ₁ ) before hot rolling, a problem occurred in manufacturability. Since Sample 10-4 had a high annealing temperature (HA) of the hot-rolled sheet, the F value deviated from the present invention.

Sample 11-1 is an example of the present invention, and good uniform elongation and local elongation are obtained. Since sample 11-2 had a high annealing temperature (HA) of the hot-rolled sheet, the F value deviated from the present invention. Since Sample 11-3 had one rolling reduction (N) having a rolling reduction of more than 35%, the particle size (X ₁ ) of the austenite phase deviated from the present invention. Since Sample 11-4 had a high final annealing temperature (FA), the F value deviated from the present invention.

Samples 12-1 and 12-2 are examples of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 12-3 had a low hot rolling final finishing temperature (T ₂ ), the particle size (X ₁ ) of the austenite phase deviated from the present invention. Since Sample 5-4 had a low final annealing temperature (FA), the particle size (X ₁ ) of the austenite phase deviated from the present invention.

Samples 13-1 and 13-2 are examples of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 13-3 had a high heating temperature (T ₁ ) before hot rolling, there was a problem in manufacturability. Sample 13-4 had a problem in manufacturability because the cooling rate (V) from the end of the holding in the final annealing to 400 ° C. was high.

Sample 14-1 is an example of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 14-2 had a low final annealing temperature (FA), the particle size (X ₁ ) of the austenite phase deviated from the present invention. Since sample 14-3 had a low heating temperature (T ₁ ) before hot rolling, there was a problem in manufacturability.

Samples 15-1 and 15-2 are examples of the present invention, and good uniform elongation and local elongation are obtained. Since Sample 15-3 had one rolling reduction (N) having a rolling reduction of more than 35%, the particle size (X ₁ ) of the austenite phase deviated from the present invention. Since Sample 15-4 had a high cold rolling rate, the particle size (X ₁ ) of the austenite phase was out of the present invention.

  In Samples 16-1, 16-2, and 16-3, the component system did not satisfy the scope of the present invention, so that good uniform elongation and local elongation were not obtained.

  Samples 17-1, 17-2, and 17-3 all had problems in manufacturability because the Md values did not satisfy the scope of the present invention.

As described above, good manufacturability, uniform elongation, and local elongation were obtained in the examples of the present invention. On the other hand, in the comparative example, the T ₁ or Md value or the cooling rate in the final annealing does not satisfy the conditions, causing a problem in manufacturability, or the aspect ratio is T ₂ , the grain size X _{1 of the} austenite phase, or the L section. The F value calculated from the ratio of the austenite grains less than 0.5% to the total austenite grains and the number of ferrite / austenite phase boundaries in the direction of 1000 μm perpendicular to the rolling direction in the Z cross section does not satisfy the condition, and is uniform Elongation or local elongation is low.

<1>% by mass, N: 0.080 to 0.300%, a ferrite austenitic stainless steel sheet having a PI value represented by the formula (1) of less than 30,
The Md value calculated by the formula (2) from the chemical composition in the austenite phase is less than −10 ° C .;
In the cross section perpendicular to the rolling width direction of the steel sheet, the average of the projected area circle equivalent diameter of the austenite grains is 1.0 + 5.0 × the nitrogen content (mass%) μm or more in the whole steel sheet,
The shape aspect ratio expressed by the reciprocal of the value obtained by dividing the length of the longest diameter by the maximum length in the direction perpendicular to the total area of the austenite grains in the cross section perpendicular to the rolling width direction of the steel sheet is less than 0.5. The F value calculated by the formula (3) is 120 or more from the area ratio of the austenite grains and the number of ferrite / austenite phase boundaries in a direction perpendicular to the rolling direction of 1000 μm in the cross section perpendicular to the thickness direction of the steel sheet. The
In addition,
C: 0.050% or less,
Si: 2.00% or less,
Mn: 1.00 to 8.00%,
P: 0.050% or less,
S: 0.050% or less,
Cr: 17.0-25.0%,
Ni: 0.10 to 6.00%,
Containing, ferritic-austenitic stainless steel sheet balance being Rukoto such iron and unavoidable impurities.
PI value = {Cr} +3.3 {Mo} +16 {N} (1)
Md value = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18.5 [Mo] (2)
F value = 100 × area ratio of austenite grains having an aspect ratio of less than 0.5 + 0.28 × number of ferrite / austenite grain boundaries (3)
[However, the element symbol surrounded by {} in the formulas (1) to (3) is the content (mass%) of the element in the whole steel plate, and the element symbol surrounded by [] is This is the content (mass%) of the element in the austenite phase, and 0 is substituted when the element is not contained in the formulas (1) and (2). ]
< 2 > Further, by mass%, Cu: 0.10 to 3.00%, Mo: 0.10 to 2.50%, Sn: 1.00% or less, W: 0.01 to 1.00%, V : The ferrite-austenitic stainless steel sheet according to < 1 >, which contains one or more of 0.01 to 1.00%.
< 3 > Further, by mass%, Nb: 0.50% or less, Ti: 0.50% or less, B: 0.0050% or less, or one or more kinds < 1 > or < 2 > The ferritic / austenitic stainless steel sheet according to < 2 >.
< 4 > Further, by mass%, Ca: 0.0050% or less, Mg: 0.0050% or less, Al: 0.50% or less, Ta: 0.0050% or less, Ga: 0.0050% or less, rare earth element : The ferritic / austenitic stainless steel sheet according to any one of < 1 > to < 3 >, comprising one or more of 0.50% or less.
< 5 > A method for producing a ferritic / austenitic stainless steel sheet according to any one of <1> to < 4 >, wherein the steel of the component according to any one of <1> to < 4 > is used. Continuously cast, and the obtained steel slab is heated to 1150 ° C to 1250 ° C, hot rolling is performed with a rolling reduction of 35% or less in all passes and a finishing temperature of 950 ° C or more. The hot-rolled sheet obtained by rolling is annealed at 1050 ° C. to 1150 ° C., and the cold-rolling reduction ratio is 50% or more and 80% or less once, or two or more cold rolling sandwiching the intermediate annealing. 290 + r + 300 × total nitrogen content (mass%) (° C.) to 940 + r + 300 × total nitrogen content (mass%) (° C.) and the cooling rate from the end of holding to 400 ° C. Over 30 ° C / s and less than 70 ° C / s A method for producing a ferritic / austenitic stainless steel sheet.
Here, the cold rolling reduction ratio (%) is a value calculated by (sheet thickness before cold rolling−sheet thickness after cold rolling) / plate thickness before cold rolling × 100, and r is the cold rolling reduction ratio. The temperature is the same value (° C.). However, when cold rolling is performed a plurality of times and intermediate annealing is performed between the cold rollings, the rolling reduction rate of the final cold rolling is used for the calculation of r.

Claims (6)

A ferrite-austenitic stainless steel sheet containing N: 0.080 to 0.300% in mass% and having a PI value represented by the formula (1) of less than 30;
The Md value calculated by the formula (2) from the chemical composition in the austenite phase is less than −10 ° C .;
In the cross section perpendicular to the rolling width direction of the steel sheet, the average of the projected area circle equivalent diameter of the austenite grains is 1.0 + 5.0 × the nitrogen content (mass%) μm or more in the whole steel sheet,
The shape aspect ratio expressed by the reciprocal of the value obtained by dividing the length of the longest diameter by the maximum length in the direction perpendicular to the total area of the austenite grains in the cross section perpendicular to the rolling width direction of the steel sheet is less than 0.5. The F value obtained by the formula (3) is 120 or more from the area ratio of the austenite grains and the number of ferrite / austenite phase boundaries in a direction perpendicular to the rolling direction of 1000 μm in the cross section perpendicular to the thickness direction of the steel sheet. A ferritic / austenitic stainless steel sheet.
PI value = {Cr} +3.3 {Mo} +16 {N} (1)
Md value = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18.5 [Mo] (2)
F value = 100 × area ratio of austenite grains having an aspect ratio of less than 0.5 + 0.28 × number of ferrite / austenite grain boundaries (3)
[However, the element symbol surrounded by {} in the formulas (1) to (3) is the content (mass%) of the element in the whole steel plate, and the element symbol surrounded by [] is This is the content (mass%) of the element in the austenite phase, and 0 is substituted when the element is not contained in the formulas (1) and (2). ] In addition,
C: 0.050% or less,
Si: 2.00% or less,
Mn: 1.00 to 8.00%,
P: 0.050% or less,
S: 0.050% or less,
Cr: 17.0-25.0%,
Ni: 0.10 to 6.00%,
The ferritic / austenitic stainless steel sheet according to claim 1, wherein the balance is made of iron and inevitable impurities. In addition,
Cu: 0.10 to 3.00%,
Mo: 0.10 to 2.50%,
Sn: 1.00% or less,
W: 0.01-1.00%,
V: 0.01-1.00%
The ferritic / austenitic stainless steel sheet according to claim 2, comprising one or more of the following. In addition,
Nb: 0.50% or less,
Ti: 0.50% or less,
B: One type or two or more types of 0.0050% or less are contained, The ferrite austenitic stainless steel sheet according to claim 2 or 3 characterized by things. In addition,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
Al: 0.50% or less,
Ta: 0.0050% or less,
Ga: 0.0050% or less,
Rare earth element: 0.50% or less of 1 type or 2 types or more are contained, The ferrite austenitic stainless steel plate as described in any one of Claims 2-4 characterized by the above-mentioned. Continuously casting the steel of the component according to any one of claims 1 to 5,
The obtained steel slab is heated to 1150 ° C. to 1250 ° C., the rolling reduction in all passes is 35% or less, and the finishing temperature is 950 ° C. or more,
Annealing the hot-rolled sheet obtained by hot rolling at 1050 ° C. to 1150 ° C.,
Perform one cold rolling with a cold rolling reduction of 50% or more and 80% or less, or two or more cold rolling sandwiching intermediate annealing,
Final annealing is 920 + r + 300 × nitrogen content (mass%) (° C.) in the whole steel sheet to 940 + r + 300 × nitrogen content (mass%) (° C.) in the whole steel sheet, and the cooling rate from the end of holding to 400 ° C. is 30 ° C. / A method for producing a ferritic / austenitic stainless steel sheet, characterized by being carried out at a temperature exceeding 70 ° C./s.
Here, the cold rolling reduction ratio (%) is a value calculated by (sheet thickness before cold rolling−sheet thickness after cold rolling) / plate thickness before cold rolling × 100, and r is the cold rolling reduction ratio. It is the temperature (° C) of the same value. However, when cold rolling is performed a plurality of times and intermediate annealing is performed between the cold rollings, the rolling reduction rate of the final cold rolling is used for the calculation of r.

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