JP2013227669A - Duplex phase stainless steel with good acid resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a general-purpose duplex phase stainless steel that achieves cost reduction of alloy elements by reducing Ni content and suppresses deterioration of acid resistance without promoting deposition of nitride.SOLUTION: A duplex phase stainless steel with good acid resistance includes, in mass%, 0.06% or less C, 0.1-1.5% Si, 4.0-10.0% Mn, 0.05% or less P, 0.005% or less S, 20.0-24.0% Cr, 0.5-4.0% Ni, 2.0-4.0% Mo, 0.1-2.5% Cu, 0.003-0.050% Al, 0.007% or less O, 0.10-0.30% N, and the balance being Fe and unavoidable impurities. In the duplex phase stainless steel, an area ratio of the ferrite phase is 40-70%, (1) GI represented in an expression 1 is 50 or more but 200 or less, (2) GIα is 55 or more, (3) GIdif represented in an expression 2 is 0 or more. The expression 1: GI=-Cr+18Ni+30Cu-10Mn+35Mo, the expression 2: GIdif=GIα-GIγ. In the expression 2, GIα and GIγ are the value calculated by the expression 1 using element contents (mass%) in the ferrite phase and austenitic phase respectively.

Description

  The present invention relates to a general-purpose duplex stainless steel, which is inexpensive and has good acid resistance and manufacturability, among the duplex stainless steels having two phases of an austenite phase and a ferrite phase, The amount of replacement from expensive Ni to cheap N can be increased, and while lowering the cost, acid degradation due to Mn is suppressed by utilizing the preferential dissolution of the austenite phase, resulting in low cost, acid resistance and manufacturability It relates to a duplex stainless steel with good acid resistance that does not impede.

  Duplex stainless steel has both an austenite phase and a ferrite phase in the steel structure, and has been used for petrochemical equipment materials, pump materials, chemical tank materials, etc. as a material having high strength and high corrosion resistance. Furthermore, since duplex stainless steel is generally a low Ni component system, it has been attracting attention as a material with lower alloy costs and less fluctuation than the austenitic stainless steel, which is the mainstream of stainless steel, due to the recent surge in metal raw materials. I'm bathing.

  The latest topics of duplex stainless steel are cost reduction and increased usage. Patents 1, 2 and the like correspond to steel types that further increase the merit of low alloy costs. Both are standardized by ASTM-A240, Patent Document 1 corresponds to S32101 (representative component 22Cr-1.5Ni-5Mn-0.22N), and Patent Document 2 is S82441 (representative component 24Cr-3.6Ni-). 3Mn-0.27N). In either case, in general-purpose type duplex stainless steels such as JIS standard SUS329J3L and ASTM standards S31803 and S32205, expensive Ni is replaced with inexpensive Mn, and expensive Mo is replaced with inexpensive Cu and N. By substituting it, cost reduction was achieved. Further, there is Patent Document 3 that defines the components between the phases. According to the document, by specifying that the difference in pitting corrosion resistance between the ferrite phase and the austenite phase is reduced, the corrosion resistance of the duplex stainless steel is disclosed. Can be improved.

WO2002 / 27056 WO 2010/070202 JP 2003-293090 A

Luhua Zhang et al. : Electrochimica Acta, 54 (2009), p5387-5392

  Among these, when reducing the cost of S31803, which is a general-purpose duplex stainless steel that is used in an environment where an acid is used and requires corrosion resistance, reduction of acid resistance and deterioration of characteristics become problems. When designing a component to reduce expensive Ni for cost reduction, the acid resistance improvement effect is lost, and the phase balance is lost, resulting in precipitation of nitrides, leading to deterioration of properties. It was.

  In the present invention, an economical general-purpose duplex stainless steel that suppresses a decrease in acid resistance even when an expensive Ni component is reduced, suppresses precipitation of nitrides, and reduces problems when used in an acid environment. The purpose is to provide.

  The present inventors have investigated in detail a method for suppressing a decrease in acid resistance due to cost reduction based on a component system in which Cr is 24% or less, Mo is 4.0% or less, and Ni is 4.0% or less. As a result, the following knowledge about the improvement measures was obtained.

In the case of single phase stainless steel, it is said that acid resistance and pitting corrosion resistance correspond to the amount of additive elements. Therefore, test steel slabs were prepared by changing various addition amounts in the above-mentioned base component system, and the magnitude of the suppression effect and acceleration effect of each element on acid resistance and pitting corrosion resistance was specifically quantified by a corrosion test. . As a result, it was found that the GI value represented by the formula (1) needs to be 50 or more in order to use in the above environment.
GI = -Cr + 18Ni + 30Cu-10Mn + 35Mo (1)
However, each element name in the formula represents its content (mass%)
In addition, when an element in the formula is not contained or its content is unknown, the element is calculated as 0.

  Furthermore, the inventors have found that the dissolved phase of ferrite and austenite changes by observing the cross section. In particular, it was found that when the austenite phase distributed individually is preferentially dissolved and the ferrite phase as a parent phase serves as a barrier to prevent the progress of corrosion, the acid resistance is good. That is, it has been found that it is effective to control the GI value of the ferrite phase because the acid resistance of ferrite, not austenite, greatly affects the acid resistance of the duplex stainless steel. As a result of analyzing the components, it was found that when the GI value GIα in the ferrite is 55 or more, the acid resistance is good.

Further, it has been found that the acid resistance of the ferrite phase is not simply improved but the acid resistance difference between the phases is also important. That is, it has been found that it is not necessary to reduce the difference between the ferrite phase and the austenite phase as in Patent Document 3, and conversely, increasing the acid resistance difference between the phases is more effective for acid resistance. The acid resistance difference between the phases is expressed by the formula (2). In order to obtain the effect of the present invention, GIdif is required to be 0 or more, and particularly in steel types of 10 or more, the acid resistance effect tends to be further enhanced.
GIdif = GIα−GIγ (2)
However, GIα in the formula means a GI value calculated by the element content in the ferrite phase, and GIγ means a GI value calculated by the element content in the austenite phase.

  The present inventors designed the component composition based on the above knowledge. As can be seen from the above equation (1), the GI value takes a larger value by increasing the amount of addition of Ni, Cu, and Mo and decreasing the amount of Cr and Mn. Among them, it concentrates in the ferrite phase and does not decrease Mo, which has a large effect of increasing the acid resistance of the ferrite phase, but concentrates in the austenite phase, reducing the effect on the acid resistance of the ferrite phase and lowering expensive Ni In this way, the cost was improved. However, since the austenite phase also decreases as Ni decreases, it is necessary to adjust the phase balance. N and Cu can be expected to increase the austenite phase at a low cost. Since Cu has a smaller effect on increasing the austenite phase than N, and when added in a large amount, it may cause hot brittleness. Therefore, improvement was attempted by adding N.

  However, although the effect of increasing the austenite phase is high by adding N, nitrides are precipitated during production, which causes a decrease in corrosion resistance and a deterioration in toughness due to the formation of a Cr-deficient layer. Therefore, the precipitation of nitride was suppressed by adding Mn. As described above, Mn is an element that deteriorates acid resistance, but the deterioration of acid resistance was smaller than expected. The detailed mechanism of this effect is unknown, but by controlling the acid resistance difference between the phases described above, unlike the conventional duplex stainless steel, the austenite phase in which Mn is concentrated dissolves first. This is presumably because the Mn concentration decreases and the adverse effect on acid resistance is reduced. Thus, by adding Mn, it succeeded in making the suppression of nitride precipitation at the time of manufacture and ensuring acid resistance compatible.

  As described above, the present inventors have studied in detail the conditions under which both low cost and good acid resistance can be achieved by prescribing the components based on the newly obtained knowledge, and the completion of the present invention. It came to.

From the above findings, the gist of the present invention is as follows.
That is,
(1) In mass%,
C: 0.06% or less, Si: 0.1-1.5%, Mn: 4.0-10.0%, P: 0.05% or less, S: 0.005% or less, Cr: 20. 0-24.0%,
Ni: 0.5-4.0%, Mo: 2.0-4.0%, Cu: 0.1-2.5%,
Al: 0.003 to 0.050%, O: 0.007% or less, N: 0.10 to 0.30%, the balance is made of Fe and inevitable impurities, and the following (1) to A duplex stainless steel with good acid resistance characterized by satisfying (3) and having a ferrite phase area ratio of 40 to 70%.
(1) GI represented by formula (1) is 50 or more and 200 or less (2) GIα is 55 or more (3) GIdif represented by formula (2) is 0 or more GI = −Cr + 18Ni + 30Cu-10Mn + 35Mo 1)
GIdif = GIα−GIγ (2)
However, each element name in said formula represents the content (mass%).
GIα means the GI value calculated by the element content in the ferrite phase,
GIγ means a GI value calculated from the element content in the austenite phase.
(2) Further, by mass%, Ti: 0.003 to 0.050%, Nb: 0.01 to 0.20% 1 type or 2 types are contained, The above-mentioned (1) Duplex stainless steel with good acid resistance.
(3) Further, by mass%, one or more of Ca: 0.0050 or less, Mg: 0.0030 or less, B: 0.0005 to 0.0050%, REM: 0.10% or less. The duplex stainless steel with good acid resistance according to (1) or (2), characterized in that it is contained.
(4) Further, it contains one or more of Sn: 0.03 to 0.2%, V: 0.05 to 0.5%, and Co: 2.0% or less in mass%. The duplex stainless steel having good acid resistance according to any one of (1) to (3).
(5) Furthermore, the average of the major axis Lγ of the austenite phase of the L cross section is 50 μm or more and satisfies the formula (4). The two-phase excellent acid resistance according to any one of the above (1) to (4) Stainless steel.
1000 ≦ Lγ × t ≦ 10000 (4)
t represents the product plate thickness (mm).
(6) A method for producing a duplex stainless steel including at least a hot rolling step and a final heat treatment step, in the hot rolling step, in a temperature region Tγ40 in which an austenite phase is present at 40% or more, with respect to the original slab thickness. In any one of the above (1) to (5), the cumulative sheet thickness reduction rate R value is 5% to 50% hot-rolled, and heat-treated at 900 to 1100 ° C in the final heat treatment step. A method for producing a general-purpose duplex stainless steel having good acid resistance as described.

  ADVANTAGE OF THE INVENTION According to this invention, suppression of the acid-resistant fall which is one of the big problems in the general purpose type duplex stainless steel which reduced the alloy cost can be solved, without inhibiting manufacturability. As a result, general-purpose duplex stainless steel with reduced alloy costs can be expanded to applications where acid resistance has been an issue, and the industrial contribution is extremely large.

The present invention is described in detail below.
First, the reason for limitation according to claim 1 of the present invention will be described. In addition,% about a component means the mass%.
C limits the content to 0.06% or less in order to ensure the corrosion resistance of the stainless steel. If the content exceeds 0.06%, Cr carbide is generated and the corrosion resistance is deteriorated. Preferably, it is 0.04% or less. On the other hand, since extremely reducing the content significantly increases the cost, the lower limit is preferably made 0.001%.

  Si is added in an amount of 0.1% or more for deoxidation. However, if added over 1.5%, the toughness deteriorates. Therefore, the upper limit is limited to 1.5%. A preferable range is 0.2 to 1.0%.

  Mn is added in an amount of 4.0% or more because it suppresses precipitation of nitrides in the duplex stainless steel. However, if it exceeds 10.0%, the acid resistance deteriorates. Therefore, the upper limit is limited to 10.0%. A preferable range is from more than 4.0 to 8.0%, and more preferably from 5.0 to 6.5%.

  P is an element inevitably contained in the steel and is limited to 0.05% or less in order to deteriorate the hot workability. Preferably it is 0.03% or less. On the other hand, since reducing the content extremely increases the cost, the preferable lower limit is 0.005%.

  S is an element inevitably contained in steel like P, and is also limited to 0.005% or less in order to deteriorate hot workability and corrosion resistance. Preferably, it is 0.002% or less. On the other hand, reducing the content extremely increases the cost significantly, so the lower limit is preferably made 0.0001%.

  Cr is an element that is basically necessary to ensure corrosion resistance, and is a relatively inexpensive alloy. Therefore, in the present invention, Cr is contained in an amount of 20.0% or more. On the other hand, it is an element that increases the ferrite phase. If the content exceeds 24.0%, the amount of ferrite becomes excessive and the corrosion resistance is impaired. Preferably, the content is 21.0% or more and 23.0% or less.

  Ni is an element effective for increasing the austenite phase in the duplex stainless steel and improving the corrosion resistance against various acids, and is added in an amount of 0.5% or more. Suppress it as much as possible and make it 4.0% or less. A preferable range is 1.0 to 3.0%.

  Mo is a very effective element that additionally increases the corrosion resistance of stainless steel, and is added in an amount of 2.0% or more. However, since it is a very expensive element, it is suppressed as much as possible in the present invention, and the upper limit is defined as 4.0% or less. A preferable range is 2.5 to 3.5%.

  Cu is an element effective for increasing the austenite phase in the duplex stainless steel and improving the corrosion resistance against various acids like Ni, and is an inexpensive alloy compared with Ni. However, if the content exceeds 2.5%, the hot workability is impaired, so the upper limit was made 2.5%. A preferred range is 0.5% to 2.0%.

  Al is an important element for deoxidation of steel, and it is necessary to contain 0.003% or more in order to reduce oxygen in the steel. On the other hand, Al is an element having a relatively large affinity with N, and if added excessively, AlN is generated and the toughness of the base material is inhibited. The degree depends on the N content, but when Al exceeds 0.050%, the toughness deteriorates remarkably, so the upper limit of the content is set to 0.050%. Preferably it is 0.030% or less.

  O is a harmful element constituting an oxide that is representative of nonmetallic inclusions, and excessive inclusion inhibits toughness. In addition, the formation of coarse clustered oxides causes surface defects. For this reason, the upper limit of the content was set to 0.007%. Preferably it is 0.005% or less. On the other hand, since excessive reduction leads to an increase in cost, the preferable lower limit is made 0.0005%.

  N is an effective element that dissolves in the austenite phase to increase strength and corrosion resistance and increase the austenite phase of the base material and the heat-affected zone. For this purpose, 0.10% or more is contained. On the other hand, if the content exceeds 0.30%, Cr nitride precipitates at the time of manufacture and causes a decrease in corrosion resistance. Therefore, the upper limit of the content was set to 0.30%. A preferable content is 0.15 to 0.25%.

Ti and Nb have an effect of forming a nitride in a very small amount and suppressing the precipitation of Cr nitride, and one or two of them can be added as necessary.
Regarding Ti, 0.003% or more of addition is necessary to exhibit the above effect.
On the other hand, if it exceeds 0.050% and is contained in the duplex stainless steel, coarse TiN is generated and the toughness of the steel is inhibited. For this reason, the content was defined as 0.003 to 0.050%. Preferably, it is 0.003 to 0.020%.

  Nb has the effect of further improving corrosion resistance in addition to the effect of suppressing the precipitation of Cr nitride. Further, nitrides and carbides formed by Nb are generated in the course of hot working and heat treatment, and have the action of suppressing crystal grain growth and strengthening the steel material. For this purpose, 0.01% or more is contained as necessary. On the other hand, excessive addition causes precipitation as an undissolved precipitate during heating before hot rolling and impairs toughness, so the upper limit of its content was set to 0.20%. Preferably, it is 0.01% to 0.10%.

  Ca, Mg, B, and REM are elements that improve hot workability, and can be added as necessary. In order to obtain this effect, one or more of Ca: 0.0050% or less, Mg: 0.0030% or less, B: 0.0005 to 0.0050%, REM: 0.10% or less are used. It is necessary to add. Moreover, since the toughness will fall when all exceed an upper limit, when adding, it is set to each upper limit. Preferably, they are Ca: 0.0010-0.0050%, Mg: 0.0001-0.0015%, B: 0.0005-0.0030%, REM: 0.005-0.05%. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

  Sn, V, and Co are elements that improve corrosion resistance, and can be added as necessary. In order to obtain this effect, it is necessary to add one or more of Sn: 0.03-0.2%, V: 0.05-0.5%, Co: 2.0% or less It is. On the other hand, if the upper limit is exceeded, Sn and V impair hot workability, and Co increases the cost. A preferable range of Co is 0.03 to 1.0%.

  In addition, in order to obtain a favorable characteristic in the said duplex stainless steel, it is necessary to make a ferrite phase area ratio into the range of 40 to 70%. If it is less than 40%, poor toughness occurs, and if it exceeds 70%, problems of hot workability and stress corrosion cracking occur. In either case, the corrosion resistance is poor.

Furthermore, in the present invention, GI is set as an acid resistance index. The larger the GI, the lower the corrosion rate of the metal base, and thus the more difficult the plate thickness decreases. The smaller the GI, the higher the corrosion rate of the metal base and the thickness tends to decrease. As described above, the acid resistance is determined by the amount of alloy element added and can be expressed by GI. Specifically, the GI value represented by the following formula (1) may be 50 or more and 200 or less.
GI = −Cr + 18Ni + 30Cu-10Mn + 35Mo (1)
When the GI value is less than 50, the acid resistance of the metal matrix is inferior, so that it cannot be applied to uses that require acid resistance. Moreover, since an effect will be saturated and only cost will increase when it exceeds 200, an upper limit is limited to 200. Preferably, it is 75-175.

  Moreover, in this invention, it is necessary to prescribe | regulate the acid resistance of a ferrite phase based on the knowledge mentioned above. GIα needs to be 55 or more when GIα is calculated by calculating the GI value based on the amount of elements contained in the ferrite phase. When GIα is less than 55, the ferrite phase has insufficient acid resistance and cannot be applied to uses that require acid resistance.

GIdif indicates the difference between the GI values of the ferrite phase and the austenite phase. The larger the GIdif, the more the dissolution of the austenite phase is promoted and the acid resistance of the ferrite phase is improved. Specifically, the GIdif value expressed by the following formula (2) may be 0 or more. Preferably it is 10 or more.
GIdif = GIα−GIγ (2)
Note that GIα and GIγ can be calculated by measuring the element content in each phase by the method described later in the examples and substituting the contents into equation (1) for obtaining GI. . Further, a known method can be used to control GIdif, that is, to control the amounts of elements contained in the ferrite phase and the austenite phase, respectively. For example, the method described in Non-Patent Document 1 can be used.

  Furthermore, it became clear that the further acid-proof improvement effect can be achieved by controlling the shape of an austenite phase. As a result of the examination, it was found that when the austenite phase was extended for a longer time, an effect of improving acid resistance was observed, and the length Lγ was 50 μm or more. When heat treatment was performed at a general temperature of 850 to 1150 ° C., Lγ tended to increase as the heat treatment temperature decreased. In addition, there is a correlation between the length Lγ of the austenite phase and the product sheet thickness t. As t decreases, Lγ increases and the product Lγ × t can be used as an index of manufacturability. As a result of the examination, when Lγ × t was 1000 or more and 10,000 or less, the productivity was good. It is assumed that 1000 ≦ Lγ × t ≦ 10000.

  The method for producing the duplex stainless steel of the present invention includes at least a hot rolling step and a final heat treatment step. In order to extend the austenite phase and increase Lγ, in the hot rolling process, it is necessary to hot-roll in a temperature region where a large amount of austenite phase exists and at a high sheet thickness reduction rate. If the temperature region Tγ40 where at least 40% of the austenite phase exists is not present, the effect of increasing Lγ is small.

  Furthermore, the cumulative sheet thickness reduction rate R value relative to the original slab thickness at Tγ40 during hot rolling is important. When the R value is high, the austenite phase is extended, and when it is low, the extension effect is reduced. When the R value is less than 5%, the acid resistance improving effect is small. Furthermore, manufacturability was good in the region of 50% or less. For this reason, in order to further improve acid resistance, hot rolling is performed at an R value of 5% to 50% at a temperature Tγ40 where the austenite phase is present at 40% or more. In the hot rolling process, it is important to control the R value at Tγ40, and the rolling rate in a temperature region other than Tγ40 may be appropriately set according to the product plate thickness.

  About a final heat treatment process, it is preferable to set it as 900-1100 degreeC. This is because, as described above, Lγ tends to increase as the heat treatment temperature decreases, but manufacturability is improved when Lγ × t is controlled in the range of 1000 to 10,000.

  Examples are described below. Table 1 shows the chemical composition of the test steel. In addition, content described in this Table 1 is the amount contained in steel materials, and is different from the element amount contained in the ferrite phase mentioned later and the element amount contained in the austenite phase. Other than the components are Fe and inevitable impurity elements. Note that the blank indicates that measurement was not performed because the element was not intentionally added.

  These component steels were melted in an MgO crucible by a laboratory 50 kg vacuum induction furnace and cast into a steel ingot of about 120 mm square. After forging to a size of 80tx110wxl from the main body of the steel ingot, the hot rolling material is processed, heated to a temperature of 1180 ° C for 1 to 2 hours, and rolled at a finish temperature of 900 ° C for 12mm. A hot-rolled steel sheet having a thickness of about 700 mm was obtained. Spray cooling was performed from a state where the steel material temperature immediately after rolling was 800 ° C. or higher to 200 ° C. or lower. The final solution heat treatment was carried out under conditions of water cooling after soaking at 1050 ° C. for 20 minutes.

The characteristic evaluation was performed as follows about the thick steel plate obtained by the above.
In the evaluation of hot workability, the longest ear crack length of about 700 mm of the rolled material was defined as the ear crack length, and when there was an ear crack of 5 mm or more, it was determined that the hot workability was poor. Moreover, even if the ear crack length was within the range, when surface flaws were observed, it was similarly determined that the hot workability was poor. As for the evaluation results of the ear crack length, Table 2 shows the ear crack length. In addition, if surface flaws are seen, they are listed in the remarks column.

  For acid resistance, a 25 × 25 × 3 mm test piece was taken from the surface layer, and a test piece whose surface was polished by # 600 was used. First, as a basic corrosion resistance evaluation, the sample was immersed in sulfuric acid at 60 ° C. and 10% for 30 minutes. Thereafter, the corrosion rate was calculated from the weight change before and after the test, and the acid resistance between the steel types was compared. A value of 0.1 mm / y or less required for acid-resistant applications was considered good. The evaluation results are shown in Table 2 with the corrosion rate. In addition, as a more severe test for further improvement of properties, the sample was immersed in 60% 30% sulfuric acid for 30 minutes. Thereafter, similarly, the corrosion rate was calculated from the weight change before and after the test, and the acid resistance between the steel types was compared. A value of 0.1 mm / y or less required for acid-resistant applications was considered good. The evaluation results are shown in Table 3 with the corrosion rate.

Moreover, about pitting corrosion resistance, the pitting potential measurement based on JISG0577 was performed. Specifically, a 25 × 10 × 2 mm test piece was collected from the surface layer, and a test piece coated with a silicon coating material was prepared leaving a 1 cm ² test surface. The test piece that had been polished # 600 immediately before the start of the test to remove the passive film was immersed in a 1.0 mol / L NaCl aqueous solution degassed at 50 ° C. for 10 minutes, and then the potential was applied at a speed of 20 mV / min. The potential when the current density was 100 μA / cm ² after sweeping was defined as the pitting potential. 0.40V or more was determined to be good. As a result of the evaluation, the pitting corrosion potential is shown in Table 2.

  Analysis of the elements in the phase was performed by EPMA. The beam system was narrowed down to 1 μm, and the central portion of the ferrite phase and austenite phase was point analyzed to measure the alloy element concentration of each phase. GIα and GIγ were calculated using the amounts of alloy elements contained in the ferrite phase and the austenite phase obtained by this method. As described above, the calculation method was calculated by substituting the element content in each phase into the equation (1) for obtaining GI. Moreover, GIdif was calculated by substituting GIα and GIγ into the equation (2). Among these, the calculated values of GIα and GIdif are shown in Table 1.

The evaluation results are shown in Table 2.
In the steel according to the present invention, both the edge crack and acid resistance of the rolled material showed good values.

  Regarding hot workability, each of Si, P, S, Cu, Al, Sn, V, Ca, Mg, REM, B, Ti, and Nb addition amount excessive No. D, G, H, P, Q, U, V, X, Y, Z, AA, AB, and AC resulted in a result that the ear crack of the hot-rolled sheet exceeded 5 mm.

  For acid resistance, No. GI is less than 50. No. A, F, I, M, O and GIdif are less than 0. In B, the corrosion rate exceeded 0.1 mm / y, and the acid resistance was inferior. Regarding pitting corrosion resistance, No. with a large amount of C. No. with less C and Mn No. with less E and Cr No. with many K and N No. with less S and Mo No. in which corrosion resistance is reduced due to excessive amounts of M and α. The pitting corrosion potential was less than 0.40 V for L and R, and the pitting corrosion resistance was inferior. In addition, No. T was inferior. No. from the viewpoint of cost. J, N, and W were excluded.

  The relationship between the R value and Lγ × t is as follows: In the laboratory, a test piece with an R value of 2 to 70% and t of 2 to 50 mm was prepared, and the final solution heat treatment was performed at 1050 ° C. for 20 minutes. The condition was post-water cooling. For Lγ, a micro test piece with a sample L cross section was processed, grain boundaries were revealed by oxalic acid electrolytic etching, and the length was measured with an optical microscope.

The evaluation results are shown in Table 3.
An R value of less than 5% For a, f, and k, Lγ was 50 μm or less, and the corrosion rate was more than 0.1 mm / y. The heat treatment temperature is 900-1100 ° C., 1000 ≦ Lγ × t ≦ 10000 and R ≦ 50%. a, b, c, d, f, h, i, k, m, and n had good manufacturability.

  As can be seen from the above examples, it has been clarified that a general-purpose duplex stainless steel excellent in acid resistance can be obtained by the present invention.

  According to the present invention, in general-purpose duplex stainless steel, it is possible to reduce the cost of the alloy by reducing Ni, and to suppress the deterioration of acid resistance and manufacturability, which are one of the major problems, and as a result, the cost of the alloy can be reduced. Of the intended use of the general-purpose duplex stainless steel, the cost can be reduced by expanding the application range to the use for which acid resistance has been an issue, and the industrial contribution is extremely large.

Claims (6)

In mass%
C: 0.06% or less,
Si: 0.1 to 1.5%,
Mn: 4.0 to 10.0%,
P: 0.05% or less,
S: 0.005% or less,
Cr: 20.0 to 24.0%,
Ni: 0.5 to 4.0%,
Mo: 2.0 to 4.0%,
Cu: 0.1 to 2.5%,
Al: 0.003 to 0.050%,
O: 0.007% or less,
N: 0.10 to 0.30%
In which the balance is Fe and inevitable impurities, satisfies the following (1) to (3), and has a ferrite phase area ratio of 40 to 70%. steel.
(1) GI represented by formula (1) is 50 or more and 200 or less (2) GIα is 55 or more (3) GIdif represented by formula (2) is 0 or more GI = −Cr + 18Ni + 30Cu-10Mn + 35Mo・ (1)
GIdif = GIα−GIγ (2)
However, each element name in said formula represents the content (mass%).
GIα means the GI value calculated by the element content in the ferrite phase,
GIγ means a GI value calculated from the element content in the austenite phase. Furthermore, Ti: 0.003 to 0.050% by mass%,
Nb: 0.01-0.20%
The duplex stainless steel with good acid resistance according to claim 1, comprising one or two of the following: Furthermore, by mass% Ca: 0.0050% or less,
Mg: 0.0030% or less,
B: 0.0005 to 0.0050%,
The duplex stainless steel with good acid resistance according to claim 1 or 2, characterized by containing one or more of REM: 0.10% or less. Furthermore, Sn: 0.03-0.2% by mass%
V: 0.05-0.5%
Co: 2.0% or less of one type or two or more types, Duplex stainless steel with good acid resistance according to any one of claims 1 to 3. Furthermore, the average of the long diameter L (gamma) of the austenite phase of a L cross section is 50 micrometers or more, and satisfy | fills (4) Formula, The duplex stainless steel with favorable acid resistance in any one of Claims 1-4 characterized by the above-mentioned.
1000 ≦ Lγ × t ≦ 10000 (4)
t represents the product plate thickness (mm). A method for producing a duplex stainless steel including at least a hot rolling step and a final heat treatment step,
In the hot rolling step, in the temperature region Tγ40 where the austenite phase is present at 40% or more, the cumulative sheet thickness reduction rate R value with respect to the original slab thickness is hot rolled 5% to 50%,
The method for producing a general-purpose duplex stainless steel with good acid resistance according to any one of claims 1 to 5, wherein heat treatment is performed at 900 to 1100 ° C in the final heat treatment step.