CN116391055A

CN116391055A - High corrosion-resistant austenitic stainless steel and method for producing same

Info

Publication number: CN116391055A
Application number: CN202180050113.0A
Authority: CN
Inventors: 渡边隆之; 平田茂
Original assignee: Nippon Yakin Kogyo Co Ltd
Current assignee: Nippon Yakin Kogyo Co Ltd
Priority date: 2020-07-31
Filing date: 2021-07-27
Publication date: 2023-07-04
Also published as: WO2022025078A1; JP6823221B1; JP2022026828A; US20230357879A1; DE112021003995T5

Abstract

The high corrosion resistant austenitic stainless steel of the present invention comprises, in mass%, C: 0.005-0.030%, si:0.05 to 0.30 percent of Mn:0.05 to 0.40 percent of P: 0.005-0.050%, S: 0.0001-0.0010%, ni:22.0 to 32.0 percent, cr:19.0 to 28.0 percent of Mo:5.0 to 7.0 percent, N:0.18 to 0.25 percent of Al: 0.005-0.100%, cu:0.05 to 0.50 percent of W:0.05% or less, sn:0.0005 to 0.0150 percent, co:0.030 to 0.300 percent, B:0.0005 to 0.0050% and the balance of Fe and unavoidable impurities, satisfying the following formula (1), wherein the area ratio of sigma phase is 1% or less, and CPT by ASTM G48Method C is 60 ℃ or more as corrosion resistance, and the reduction of corrosion resistance can be suppressed even in a precipitation temperature region exposed to sigma phase. 0.05.ltoreq.10 [%B ] +2[%P ] +6[%Sn ] +0.03[%Si ]. Ltoreq.0.20 … (1).

Description

High corrosion-resistant austenitic stainless steel and method for producing same

Technical Field

The present invention relates to a high corrosion resistant austenitic stainless steel used in an environment such as a chemical plant where extremely excellent corrosion resistance is required, and more particularly, to a high corrosion resistant austenitic stainless steel which is harmless by delaying the decrease in corrosion resistance caused by precipitation of sigma phase which is a detrimental intermetallic compound.

Background

Because of its good corrosion resistance, high corrosion resistant austenitic stainless steel is used in various fields and is applied in environments containing corrosive substances, such as various industrial fields of seawater environments, flue gas desulfurization apparatuses, oil wells, food factories, chemical factories, or atomic energy factories. In such an environment, when SUS430, SUS304, or the like, which is a general-purpose stainless steel, is applied, there is a case where the corrosion resistance is insufficient, and thus, there is a case where localized corrosion such as pitting corrosion, crevice corrosion, stress corrosion cracking, or the like occurs, and the use thereof is greatly restricted. Accordingly, attempts have been made to improve corrosion resistance by adding a large amount of elements effective for corrosion resistance, such as Cr, mo, and N, to austenitic stainless steel.

For example, patent document 1 proposes austenitic stainless steel having a Cr content of at most 35%. Patent document 2 proposes austenitic stainless steel having a Mo content of at most 8.0%. In addition, patent document 3 proposes austenitic stainless steel having an N content increased to 0.50% at the maximum, which is suitable for a severe corrosion environment.

In high corrosion resistant austenitic steels, cr and Mo are contained in large amounts for the purpose of improving corrosion resistance, but these are also elements that promote precipitation of sigma phase, which is a detrimental intermetallic compound, and therefore precipitation of sigma phase is extremely rapid when exposed to a temperature range of about 700 to 1000 ℃ as compared with general austenitic stainless steels. If the sigma phase precipitates in the steel, cr and Mo are deficient around the sigma phase, resulting in a decrease in corrosion resistance.

In the production process of producing a plate, a strip or a bar from austenitic stainless steel, hot forging or hot rolling is performed, and further, if necessary, cold rolling or the like is performed. Then, although so-called solution heat treatment is performed for softening and homogenizing the structure, after heat treatment, rapid cooling such as water cooling is rapidly performed to prevent precipitation of sigma phase.

In contrast, when a high corrosion resistance austenitic stainless steel sheet is used as a raw material and a carbon steel or the like is used as a base material to form a coating, or after a structure such as a tank or a reactor is manufactured by welding, a heat treatment is performed to remove softening or residual stress. In the former, if the thickness of the coating is increased, cooling of the inside of the plate becomes slow, and in the latter, if the coating is a large-sized structure, a portion where cooling becomes slow occurs due to the structure, and in either case, it is difficult to avoid precipitation of the sigma phase. In addition, in the production process of the product, a brazing process using a BA furnace may be performed. This is to keep a temperature region around 900 ℃ and to melt the solder to join, and then to cool it in an atmosphere. In this case, the cooling is slow even when the alloy is exposed to the temperature of around 900 ℃ at which the sigma phase is precipitated for several minutes to several tens of minutes. When a highly corrosion-resistant austenitic stainless steel is used in such a process, it is difficult to obtain a predetermined corrosion resistance.

As described above, in austenitic stainless steel with high corrosion resistance, it is desired to suppress precipitation of sigma phase as much as possible, and various composition and heat treatment conditions have been proposed for this purpose. For example, in patent document 4, alloys having a σ phase area ratio of 0.1% or less and excellent nitric acid corrosion resistance have been developed by suppressing the upper limits of Cr and Mo contents to 27.00% and 3.20% respectively, and further performing solution heat treatment at 1050 to 1150 ℃ and then performing quenching. However, the method of avoiding the precipitation of the sigma phase is performed in a temperature range where the sigma phase is not precipitated at a solid solution temperature of 1050 to 1100 ℃, and is achieved by rapid cooling thereafter. Therefore, the steel in this document is subjected to solution heat treatment, and nothing is considered about precipitation of sigma phase in the heat treatment thereafter. In the examples, even if the chemical components within the scope of the invention of patent document 4 were subjected to heat treatment at 1000 ℃.

In patent literatureIn document 5, there is proposed a high corrosion resistance austenitic stainless steel sheet obtained by determining M by the relation of Cr, ni, mo, mn, cu, si, al, fe, N, C _d Value sum M _dc The value is regulated to be equal to or smaller than a predetermined value, so that the precipitation of sigma phase in the segregation part of the whole steel and the center of the plate thickness is inhibited, and the area ratio of sigma phase is lower than 1.0%, thereby ensuring manufacturability and realizing thinning. However, in the present invention, the inhibition of the precipitation of the sigma phase is also achieved by annealing after cold rolling, and there is no study on the inhibition of the precipitation of the sigma phase and the influence on the corrosion resistance when the heat treatment is exposed to the temperature region of the precipitation of the sigma phase. In addition, patent document 5 also describes that the corrosion resistance can be improved by selectively containing the compound from the group of Ti, nb, ta, zr, V, W, sn, sb, ga. Further, ti, nb, ta, zr is fixed to C, N to produce carbonitride, whereby grain boundary corrosion resistance is improved, and addition of V, W improves crevice corrosion resistance in particular. However, sn, sb, and Ga are described as being added solely for the purpose of improving corrosion resistance, but the relationship with the σ phase is not mentioned.

In the case of the patent document 6, a control unit, it is described that by the method of the following steps in O: less than 50ppm, al: less than 50ppm, si: the impurity content is reduced within an industrially possible range of 400ppm or less, and precipitation of sigma phase is delayed to a usable extent even in aging heat treatment of 650 ℃ x 5000hr, but an alloy as a matrix is close to SUS310S, and it is difficult to obtain corrosion resistance useful in factories. In addition, in this alloy (20 Ni-28 Cr), the precipitation of sigma phase was significantly slower than that of high corrosion resistant stainless steel containing 5% or more of Mo.

Patent document 7 proposes austenitic stainless steel which exhibits excellent corrosion resistance even after a final heat treatment in a temperature range of 850 to 980 ℃ by reducing the Mn content in the steel, and which is excellent in use as a clad partner material unavoidable for the heat treatment in the temperature range in the production step. However, the basic composition is Fe-0.02C-0.5Mn-14Ni-18Cr-3.2Mo-0.06N, which is in conformity with SUS317, and the alloy content is small, and it is still difficult to obtain corrosion resistance that can be used in factories that are required to withstand severe corrosive environments.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 5-247597,

patent document 2: japanese patent laid-open No. 10-060603,

patent document 3: japanese patent application laid-open No. 2010-31313,

patent document 4: japanese patent publication No. WO/2012/1768802,

patent document 5: japanese patent publication No. WO/2016/076254,

patent document 6: japanese patent laid-open No. 10-140291,

patent document 7: japanese patent laid-open No. 61-223167.

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made in view of the above-described problems in the prior art, and an object of the present invention is to provide a highly corrosion-resistant austenitic stainless steel excellent in corrosion resistance even when exposed to a temperature range of sigma phase precipitation, specifically, to a temperature range of 700 to 1000 ℃, particularly, to around 850 ℃ in which the corrosion resistance is drastically changed.

The inventors have intensively studied to solve the above problems. As a result, in order to improve corrosion resistance, it is necessary to increase the Ni, cr, mo, N concentration in steel, but among them, the addition amount of Mo and W, which have a large effect of precipitation of the sigma phase, is minimized as much as possible, and similarly, mn and Si, which are elements for precipitating the sigma phase, are reduced within a range that does not excessively increase the production load, does not affect deoxidation or the like, and Ni, N, and Co, which are austenite phase stabilizing elements, are contained in an allowable range in terms of cost and weldability. However, it is found that the suppression of the σ phase is insufficient by only these measures.

Therefore, the inventors have sought an effect for further suppressing the precipitation of the sigma phase in addition to the above. Focusing on the grain boundary triple point, which is the position where the sigma phase preferentially precipitates, various methods of delaying the movement of Cr, mo, and the like, which are constituent elements of the sigma phase, to this point have been studied. As a result, it was found that by properly controlling the contents of Sn, B, P, and Si, which are elements segregated in the grain boundaries, precipitation of the sigma phase can be delayed, and good corrosion resistance can be ensured even when exposed to the temperature region in which the sigma phase is precipitated.

Next, the inventors have repeatedly studied the condition that the delay effect of the σ phase precipitation is sufficiently exhibited by controlling the amounts of Sn, B, and P. As a result, it was found that control of the crystal grain size is an important factor. In order to form a solid solution in the sigma phase, it is necessary to perform solution heat treatment at a sufficiently high temperature, and the crystal grain size is necessarily coarsened. In this case, the points of the grain boundary triple points, which are the preferential precipitation sites of the σ phase, are extremely small, and when the subsequent heat treatment is performed, the grain boundary diffusion of Cr and Mo is concentrated in such small sites. In this case, even if the effect of the above elements is exhibited, the precipitation of the sigma phase cannot be suppressed, and further, since coarse particles are used, sensitization due to carbide is liable to occur, and it is judged that the corrosion resistance is lowered. On the other hand, when the crystal grain size is excessively fine, the total area of the grain boundaries becomes large, the distribution of the Sn, B, and P amounts in the grain boundaries becomes sparse, and it is determined that the effect of delaying the σ phase precipitation cannot be sufficiently obtained. Thus, it was found that the precipitation of sigma phase can be delayed by controlling the crystal grain size in the range of 3.0 to 7.0 based on JIS G0511 and allowing Sn, P, B, si to exist properly in the grain boundaries.

In addition, as a method of controlling the crystal grain size, the inventors focused attention on carbonitride. It was found that the crystal grain size according to JIS G0551 can be controlled within the range of 3.0 to 7.0 by adding 1 or 2 or more of V, nb, B having a high affinity for C, N within an appropriate range. In addition, the effect of difficult precipitation of Cr carbide that causes sensitization is also obtained. Further, mn enhances the solubility of N and suppresses precipitation of these carbonitrides, and thus, in the control of the crystal grain size in the present invention, it was found that adjustment of the Mn amount is also an important factor.

The present invention has been completed by the above-described procedures. That is, the high corrosion resistant austenitic stainless steel of the present invention is characterized by comprising, in mass%, C: 0.005-0.030%, si:0.05 to 0.30 percent of Mn:0.05 to 0.40 percent of P: 0.005-0.050%, S: 0.0001-0.0010%, ni:22.0 to 32.0 percent, cr:19.0 to 28.0 percent of Mo:5.0 to 7.0 percent, N:0.18 to 0.25 percent of Al: 0.005-0.100%, cu:0.05 to 0.50 percent of W:0.05% or less, sn:0.0005 to 0.0150 percent, co:0.030 to 0.300 percent, B:0.0005 to 0.0050 percent, the balance being Fe and unavoidable impurities,

and satisfies the following formula (1), the area ratio of sigma phase is 1% or less, and CPT by ASTM G48Method C is 60 ℃ or more as corrosion resistance.

0.05≤10[％B]+2[％P]+6[％Sn]+0.03[％Si]≤0.20...(1)

In the high corrosion resistant austenitic stainless steel of the present invention, it is preferable that Nb: 0.005-0.250%, V: 1 or 2 of 0.005 to 0.250% and satisfies the following formula (2), and the crystal grain size of the substrate based on JIS G0511 is in the range of 3.0 to 7.0.

1.2≤100{2([％V]+[％Nb])+6[％B]}×([％N]+[％C]-0.1[％Mn])≤5.0...(2)

The austenitic stainless steel with high corrosion resistance according to the present invention is produced by a method characterized in that the temperature range of 700 to 1000 ℃ is allowed to pass through 10 to 60 minutes by isothermal holding, or cooling or heating steps as a thermal history after solution heat treatment.

Effects of the invention

According to the present invention, even when exposed to a temperature range such as σ phase precipitation, the decrease in corrosion resistance can be suppressed, and therefore, the present invention can be suitably used as a joining material for clad steel joined to thick-walled carbon steel or the like, a highly corrosion-resistant material used in a process of passing through a line furnace for brazing, or the like.

Drawings

FIG. 1 (a) is an electron micrograph of a metal sample according to the present invention, and (b) is an electron micrograph of an EBSD method.

FIG. 2 is a graph showing the relationship between the sigma phase area ratio and CPT at each aging heat treatment temperature in the present invention.

FIG. 3 is a graph showing the relationship between corrosion resistance and formula (1) and the relationship between the number of cracks generated and formula (1) in the present invention.

FIG. 4 is a graph showing the relationship between corrosion resistance and crystal grain size in the present invention.

FIG. 5 is a graph showing the relationship between the crystal grain size and the formula (2) in the present invention.

FIG. 6 is a graph showing the relationship between corrosion resistance and crystal grain size in the present invention.

Detailed Description

The present inventors have conducted the following < experiment 1> to < experiment 3> to conduct studies, and completed the present invention. The study is described below.

Conventionally, a method of quantitatively evaluating sigma phase causing deterioration of corrosion resistance has been mainly performed by a point algorithm represented by ASTM E562. This is a method of evaluating the ratio of intersection points and σ of lattice-like graticules attached to a microscope with respect to a metal structure subjected to etching. Therefore, the evaluation result is affected by the quality of etching at the time of observation, and there is a possibility that the actual σ phase deposition amount contains about a few percent of errors. Accordingly, the inventors evaluated the area ratio of the σ phase by measurement based on a field emission scanning electron microscope and a back scattering electron diffraction method (hereinafter, referred to as EBSD method), which are methods that can perform high-definition measurement and obtain high reliability by crystal structure determination.

In the conventional patent related to sigma phase suppression, the effect of suppressing sigma phase was also compared with the above-mentioned "area ratio", but the relationship between the amount of sigma phase deposition and the change in corrosion resistance when the annealing temperature was varied variously was not clear. Therefore, the inventors examined the relationship between the amount of sigma phase deposition and the corrosion resistance when the heat treatment temperature and the holding time were varied variously.

< experiment 1>

The melting of steel having Fe-0.01% C-25% Cr-23% Ni-6% Mo-0.20% N-0.4% Cu as a basic component was performed using a high frequency induction furnace. After a steel ingot was produced with a melting amount of 20kg, a plate having a thickness of 8mm and a width of 70mm was produced by hot forging at a heating temperature of 1200 ℃. Then, the forged sheet was annealed and pickled, and further cold-rolled to a thickness of 2mm, to prepare a cold-rolled sheet. The cold-rolled sheet was subjected to solution heat treatment at 1150 ℃ for 1min, and cooled by forced air cooling. Further, the cold-rolled sheet is subjected to aging heat treatment in which the temperature and the holding time are variously changed in a range of 700 to 1100 ℃ for 1 to 60 minutes. For this aging heat treated material, the σ phase area ratio and the corrosion resistance by the EBSD method were measured.

The sigma phase area ratio was evaluated by electropolishing a small piece cut from a heat-treated cold-rolled sheet at right angles to the rolling direction by using "Tenupol-5" manufactured by Streers, inc., and then measuring a measurement region of 80 μm by 240 μm at a position of 1/4 in the thickness direction of the cold-rolled sheet under a step size of 0.2 μm using a back-scattering electron diffraction device (OIM Analysis7.3 "manufactured by TSL SOLUTIONS, inc., JSM-7001F), attached to a field emission scanning electron microscope (manufactured by Japan electronics, inc.).

Corrosion resistance was evaluated by measuring the critical pitting corrosion occurrence temperature CPT by performing an aqueous solution immersion test of an aqueous solution of a chloride specified in ASTM G48Method C. The test piece was obtained by collecting a 25mm×50mm test piece from a cold-rolled sheet subjected to aging heat treatment, polishing the whole surface with a water-resistant polishing paper of No. 120 SiC, degreasing with acetone, and then subjecting to the test. The test solution was 600ml per 1 sample, and after immersing for 72 hours, the minimum temperature CPT (critical pitting temperature) at which pitting with a depth of 25 μm or more was generated was measured.

Fig. 1 and 2 show the measurement results of the σ -phase area ratio and the corrosion resistance, respectively. Fig. 1 is an example of a sigma phase area rate evaluation based on the EBSD method. An extremely fine sigma phase (white spot) having a grain size of about 0.3 μm precipitated along a grain boundary in the secondary electron image of fig. 1 (a) can be detected as a white spot in the EBSD image of fig. 1 (b). Fig. 2 shows the results of the relationship between the area ratio of the fine and minute amount of sigma phase and the corrosion resistance in the EBSD method. The relationship between the sigma phase area ratio and the CPT when the alloy was maintained at each annealing temperature for 600sec is shown.

In the steel of the present invention, although the decrease in corrosion resistance was observed in the range of 700 to 1000 ℃ at the annealing temperature, as shown in fig. 2, the minimum corrosion resistance temperature was 850 ℃ although the sigma phase area ratio was the same or slightly larger. Therefore, as a reproduction of the case where the present alloy is supplied to a temperature region where the decrease in corrosion resistance is remarkable particularly due to precipitation of sigma phase, the corrosion resistance was evaluated after various changes were made in soaking time at an aging heat treatment temperature of 850 ℃.

< experiment 2>

In order to obtain an effect of delaying deterioration of corrosion resistance due to sigma phase precipitation, a delay effect of grain boundary diffusion due to Sn, B, P, and Si, which are elements segregated at grain boundaries, is thought to be obtained. 20kg of steel having various contents of Sn, B, P and Si as a basic component, which was Fe-25% Cr-23% Ni-6.0% Mo-0.20% N-0.4% Cu, was melted using a high frequency induction furnace. Then, a forged sheet and a cold-rolled sheet were obtained by the same method as in experiment 1. At this time, the hot workability was evaluated by cracking generated in the side face of the forged plate. The cold-rolled sheet was subjected to solution heat treatment at 1150 ℃ for 1min, and cooled by forced air cooling. Further, the cold-rolled sheet was subjected to aging heat treatment at 850 ℃. In this experiment, the holding time was varied within 1.5 hr. The aging heat treatment material was evaluated for corrosion resistance and the crystal grain size was measured.

Regarding hot workability, when cracking occurring in the side surface of the forged plate was visually observed and cracking of 20mm or more did not occur, since workability was particularly excellent, it was evaluated as excellent (excellent), as good (o) when each 100mm was less than 3 in the longitudinal direction, as acceptable (Δ) when 3 or more to 6 or less, and as poor (x) when 6 or more occurred.

The corrosion resistance was evaluated by measuring the critical pitting corrosion occurrence temperature CPT in the same manner as in experiment 1. In the case where the soaking time exceeds 1.5hr and the CPT still exceeds 60 ℃, the deterioration of pitting resistance under aging is particularly excellent, and therefore, it is evaluated as excellent (excellent), the soaking time before the CPT reaches 60 ℃ is 1.2hr or more and less than 1.5hr is evaluated as good (o), the soaking time is evaluated as acceptable (Δ) when the soaking time is 1hr or more and less than 1.2hr, and the CPT is judged as inferior (x) when the soaking time is less than 1hr and the CPT is reduced to 60 ℃.

The grain size of the steel was measured on a heat-treated cold-rolled sheet at 1150 ℃ for 1min based on JIS G0551.

TABLE 1

The test results are shown in table 1. Fig. 3 is a graph showing the test results of table 1, and shows that when heat treatment at 850 ℃ is performed in the corrosion resistance test, the relationship between B, P, sn of formula (1) and the Si content (horizontal axis) indicates that the soaking time (left vertical axis) required until CPT reaches 60 ℃ needs 1hr or more.

0.05≤10[％B]+2[％P]+6[％Sn]+0.03[％Si]≤0.20...(1)

Referring to FIG. 3, nos. 1 to 18 having formula (1) of 0.05 or more show good retardation effect of corrosion resistance degradation, 1hr or more before lowering to 60 ℃. In addition, it was found that B was more effective than P, sn even when added in a small amount. In addition, regarding Nos. 1 to 18, in which the soaking time before CPT was lowered to 60℃was 1hr or more, it was found that the area ratio of the sigma phase when aging heat treatment was performed at 850℃for 60 minutes was 1.0% or less, and the grain size of the sigma phase was 2 μm or less. It was confirmed that as the area ratio of the σ phase increases, the particle size of the σ phase also increases.

However, although the range of B, P, sn, si is appropriate, nos. 11 to 14 and 17 are shorter than Nos. 1 to 10 before being lowered to 60℃and are barely satisfactory to the extent of 1hr (the symbol Δ of FIG. 3). The crystal grain size of the steel was measured and found to be 7.0 to 7.5, and the crystal grains were all fine, suggesting that the corrosion resistance deterioration suppression effect could not be sufficiently exhibited even if the addition amounts of Sn, B, and P were controlled. In addition, in nos. 17 and 18 when the formula (1) exceeds 0.20, it is determined that the cracking occurring in the side face is 6 or more and cannot be applied to high temperature processing. No.19, 20 having formula (1) below 0.05 is below 1hr before CPT is reduced to 60 ℃. Therefore, the formula (1) needs to be controlled within a range of 0.05 to 0.20.

In table 1 and fig. 3, since nos. 11 to 14 and 17 each have a low effect of suppressing deterioration of corrosion resistance as fine particles, it is thought that the crystal grain size is related to the effect exerted by Sn, B, P, si. Therefore, the steel 6 of table 1 was examined for the correlation between the crystal grain size and the effect of delaying deterioration of corrosion resistance by varying the temperature and time of the solution heat treatment, and the results are shown in table 2 and fig. 4.

TABLE 2

As shown in table 2 and fig. 4, the soaking time before CPT was reduced to 60 ℃ varied depending on the grain size, suggesting that there was an appropriate range. No.6-c, which was solution heat treated at 1080℃for 1min, had a grain size as fine as 7.5. On the other hand, no.6-b is as high as 1150℃and soaking is as long as 30min, so the particle size is 2.5, and is coarse.

Therefore, in order to prevent the sigma phase from remaining, it is necessary to perform solution heat treatment at a high temperature, but crystal grains are inevitably coarsened, precipitation of the sigma phase is accelerated, and corrosion resistance is lowered due to carbide precipitation. However, the crystal grain size becomes fine in the low-temperature heat treatment, and the effect of delaying the precipitation of the sigma phase due to the inclusion of P, B, sn, si cannot be obtained sufficiently. In addition, the sigma phase may not be completely eliminated. Therefore, even in a high-temperature heat treatment, a technique for controlling the crystal grain size within an appropriate range is necessary.

< experiment 3>

According to the results of experiment 2, by properly adjusting the contents of B, P, sn and Si, the corrosion resistance deterioration when exposed to the temperature region where the sigma phase is precipitated can be delayed. However, in order to fully exert this effect, it is known that further control of the crystal grain size is required. Therefore, the control method thereof is repeatedly studied.

20kg of steel containing Fe-0.2% Si-25% Cr-23% Ni-6.0% Mo-0.4% Cu-0.003% Sn-0.020% P as a basic component was melted by a high-frequency induction furnace. In order to control the crystal grain size by the pinning effect during the melting, it is conceivable to precipitate carbonitrides and to variously change the component contents of V, nb, B and C, N, mn. The value of formula (1) in this case is in the range of 0.05 to 0.10. A cold-rolled sheet having a thickness of 2mm was obtained in the same manner as in experiment 1 except that the solution heat treatment was performed at 1150℃for 1min, the aging heat treatment was performed at 850℃for a period of time varying within 1.5hr, and the obtained material was used as a test material, and the corrosion resistance evaluation and the crystal grain size measurement were performed by the same methods as described above. The test results are shown in table 3. Fig. 5 is a graph showing the relationship between the crystal grain size and the following formula (2).

1.2≤100{2([％V]+[％Nb])+6[％B]}×([％N]+[％C]-0.1[％Mn])≤5.0...(2)

TABLE 3

In fig. 5, it was found that the larger the value of formula (2), the finer the crystal grains, and the crystal grain size can be controlled. When the value of the formula is in the range of 1.2 to 5.0, the crystal grain size is in the range of 3.0 to 7.0.

Fig. 6 is a graph showing a relationship between a range in which the soaking time is 1hr or more before the CPT reaches 60 ℃ and the grain size based on JISG 0511. According to FIG. 6, if the crystal grain size is smaller than 3.0, i.e., coarse particles, CPT is lower than 1hr before reaching 60 ℃. When the crystal grain size is in the range of 3.0 to 7.0, the effect of suppressing deterioration of corrosion resistance is excellent by exceeding 1hr before 60 ℃. In particular, particle sizes in the range of 4 to 6 are shown to be optimal. However, if the crystal grain size is larger than 7, i.e., fine particles, the time is again lower than 1hr. From this, it is found that in order to sufficiently retard the deterioration of corrosion resistance, the crystal grain size is preferably adjusted within an appropriate range, and it is necessary to control the grain size within a range of 3.0 to 7.0.

Next, the reasons for limitation of the composition of each element, the relational expression, and the like in the present invention will be described. Hereinafter,% represents mass%.

C：0.005～0.030％

C is an element effective for stabilizing the austenite phase, and suppresses precipitation of the sigma phase. Is also an important element for forming carbonitrides for controlling the grain size. Therefore, it is necessary to add at least 0.005%. However, if the content is excessive, the crystal grain size becomes fine due to the pinning effect of carbonitrides, the effect of delaying the precipitation of sigma phase cannot be obtained, and precipitation of Cr carbide becomes easy in welding or the like, and corrosion resistance is deteriorated. Therefore, the upper limit is set to 0.030%. The preferable lower limit of the content is 0.007%, the more preferable lower limit is 0.009%, the preferable upper limit is 0.025%, and the more preferable upper limit is 0.020%.

Si：0.05～0.30％

Si is an important element constituting the invention having deoxidizing action, and is present in grain boundaries together with Sn, B and P, and is an indispensable element for delaying the precipitation of sigma phase. However, if Si is excessively contained, precipitation of sigma phase is promoted, and oxide scale is easily formed, which deteriorates wettability during brazing. Therefore, the Si content is set to 0.05 to 0.30%. The preferable lower limit of the content is 0.07%, the more preferable lower limit is 0.09%, the preferable upper limit is 0.25%, and the more preferable upper limit is 0.23%.

Mn：0.05～0.40％

Mn is an element added as a deoxidizer, and has an effect of stabilizing an austenite phase and improving the solubility of N, and is therefore an element necessary for controlling the particle size by carbonitride. Therefore, it is necessary to contain Mn at 0.05% or more. However, excessive addition promotes precipitation of sigma phase, and reduces corrosion resistance. MnS is formed and becomes a starting point of pitting, and corrosion resistance is deteriorated. Therefore, the Mn content is set to 0.05 to 0.40%. The preferable lower limit of the content is 0.06%, the more preferable lower limit is 0.07%, the preferable upper limit is 0.30%, and the more preferable upper limit is 0.25%.

P：0.005～0.050％

P is an element that is inevitably mixed into steel as an impurity, but in the present invention, is an element that exists in grain boundaries and is necessary to delay precipitation of sigma phase. In order to obtain this effect, it is necessary to add at least 0.005% or more. However, when the content exceeds 0.050%, corrosion resistance and hot workability are significantly deteriorated. Therefore, the content of P is set to 0.005 to 0.050%. The preferable lower limit of the content is 0.010%, the more preferable lower limit is 0.012%, the preferable upper limit is 0.040%, and the more preferable upper limit is 0.035%.

S：0.0001～0.0010％

S is an impurity element inevitably mixed into steel, and deteriorates hot workability, and forms sulfide as a starting point of pitting, thereby adversely affecting corrosion resistance. In this experiment, no retardation effect of deterioration in corrosion resistance due to precipitation of sigma phase was found as in P. Therefore, the S content is preferably extremely small, and the upper limit is preferably 0.0010%. However, S improves the fluidity of the melt at the time of melting, and is also an element that improves weldability. From the viewpoint of obtaining good weldability, it is preferable to contain 0.0001% or more. The preferable lower limit of the content is 0.0002%, the more preferable lower limit is 0.0003%, the preferable upper limit is 0.0008%, and the more preferable upper limit is 0.0007%.

Ni：22.0～32.0％

Ni is an element for stabilizing the austenite phase, and is an important element for suppressing precipitation of intermetallic compounds such as σ and improving pitting corrosion resistance and entire surface corrosion resistance. However, if the Ni content exceeds 32.0%, the thermal deformation resistance increases and the cost increases. Therefore, the Ni content is set to 22.0 to 32.0%. The preferable lower limit of the content is 23.0%, the more preferable lower limit is 23.5%, the preferable upper limit is 31.5%, and the more preferable upper limit is 30.0%.

Cr：19.0～28.0％

Cr is an element that is essential for improving crevice corrosion resistance and grain boundary corrosion resistance, including pitting corrosion resistance. However, excessive Cr content promotes precipitation of sigma phase, which in turn deteriorates corrosion resistance. Therefore, the Cr content is set to 19.0 to 28.0%. The preferable lower limit of the content is 21.0%, the more preferable lower limit is 22.0%, the preferable upper limit is 27.0%, and the more preferable upper limit is 25.0%.

Mo：5.0～7.0％

Mo is an element that improves pitting corrosion resistance and crevice corrosion resistance, similarly to Cr, N, and the like. However, when Mo is excessively contained, precipitation of the sigma phase is greatly promoted, and corrosion resistance is deteriorated. Therefore, the Mo content is set to be in the range of 5.0 to 7.0%. The preferable lower limit of the content is 5.1%, the more preferable lower limit is 5.2%, the preferable upper limit is 6.7%, and the more preferable upper limit is 6.5%.

N：0.18～0.25％

N is an element for stabilizing the austenite phase, and is an element effective for suppressing precipitation of the sigma phase. Further, the pitting resistance and crevice corrosion resistance are greatly improved as in Cr and Mo, and the element forming carbonitride for controlling the grain size is the same as in C. Therefore, it is necessary to add at least 0.18%. However, if the N content is excessive, a large amount of carbonitride precipitates, the crystal grain size becomes fine, and the effect of delaying the precipitation of the σ phase cannot be obtained. Therefore, it may not exceed 0.25%. The preferable lower limit of the content is 0.19%, the more preferable lower limit is 0.20%, the preferable upper limit is 0.24%, and the more preferable upper limit is 0.23%.

Al：0.005～0.100％

Al is a component added as a deoxidizer. In addition, in CaO-SiO ₂ -Al ₂ O ₃ The desulfurization is promoted by deoxidization in the coexistence of MgO-based slag, and is an important element for stabilizing the yield of B in refining. However, when the metal is excessively contained, scale is easily formed, and wettability of brazing is deteriorated. Therefore, the content of Al is set to 0.005 to 0.100%. The preferable lower limit of the content is 0.008%, the more preferable lower limit is 0.010%, the preferable upper limit is 0.080%, and the more preferable upper limit is 0.070%.

Cu：0.05～0.50％

Cu is an element that stabilizes the austenite phase and contributes to improvement of acid resistance. In order to obtain this effect, it is necessary to contain 0.05% or more. However, since excessive addition increases the cost and deteriorates the hot workability, the upper limit is set to 0.50% or less. Therefore, the content is set to 0.05 to 0.50%. The preferable lower limit of the content is 0.07%, the more preferable lower limit is 0.08%, the preferable upper limit is 0.45%, and the more preferable upper limit is 0.40%.

Sn：0.0005～0.0150％

Sn exists in the grain boundary together with B, P in the present invention, and becomes an important element for retarding precipitation of sigma phase. In order to obtain this effect, it is necessary to add at least 0.0005% or more. However, when the content exceeds 0.0150%, sn itself has an effect of promoting precipitation of sigma phase. Therefore, the Sn content is set to 0.0005 to 0.0150%. The preferable lower limit of the content is 0.0010%, the more preferable lower limit is 0.0012%, the preferable upper limit is 0.0100%, and the more preferable upper limit is 0.0090%.

Co：0.030～0.300％

Co has the effect of stabilizing the austenite phase and also suppressing the precipitation of the sigma phase, similarly to Ni. Further, the alloy is a useful element having a higher sigma phase suppressing effect per unit weight than Ni. In order to obtain this effect, it is necessary to contain at least 0.030% or more. However, co is an element more expensive than Ni, and excessive addition leads to higher costs. Therefore, the upper limit is set to 0.300%. The preferable lower limit of the content is 0.040%, the more preferable lower limit is 0.050%, the preferable upper limit is 0.295%, and the more preferable upper limit is 0.290%.

B：0.0005～0.0050％

B is an important element constituting the present invention, and is present in the grain boundary together with P, sn, and exhibits an effect of delaying the precipitation of sigma phase. In addition, the proper control of the grain size of steel together with V, nb plays an important role in retarding the precipitation of sigma phase. Therefore, it is necessary to add at least 0.0005% or more. However, if B is excessively contained, a large amount of carbonitride is precipitated, and the crystal grain size becomes fine due to excessive pinning effect, and the effect of delaying the σ phase precipitation cannot be obtained. In addition, hot workability is significantly deteriorated. Therefore, the upper limit is set to 0.0050%. The preferable lower limit of the content is 0.0007%, the more preferable lower limit is 0.0008%, the preferable upper limit is 0.0035%, and the more preferable upper limit is 0.0032%.

0.05≤10[％B]+2[％P]+6[％Sn]+0.03[％Si]≤0.20...(1)

By containing B, P, sn, si, which is an element constituting the above-described composition, in a predetermined range and satisfying the relationship of the above-described formula, sn, B, and P segregate in the grain boundaries, and the effect of further delaying the deterioration of corrosion resistance due to the precipitation of the sigma phase can be obtained. The preferable lower limit is 0.06, the more preferable lower limit is 0.08, the preferable upper limit is 0.18, and the more preferable upper limit is 0.16.

The sigma phase area ratio is less than 1.0%

When heat treatment at 850 ℃ is performed, in which the decrease in corrosion resistance is particularly remarkable, the area ratio of the sigma phase is 1.0% or less when the time before the CPT is reduced to 60 ℃ is 1hr or more, which is clear from the precise quantification of the area ratio of the sigma phase by EBSD and the corrosion test thereof. Therefore, the σ area ratio needs to be 1.0% or less. It is required to be preferably 0.8% or less, more preferably 0.7% or less. In addition, the large amount of sigma phase precipitate, which means that the degree of the Cr and Mo deficiency layer formed around the sigma phase is worse. Therefore, in order to reduce the delayed corrosion resistance, the particle size of the sigma phase is preferably small. In the present invention, the upper limit of the size is 2.0 μm or less. Preferably 1.8 μm, more preferably 1.6 μm or less.

Nb、V：0.005～0.250％

Nb and V are important elements constituting the present invention. Nb combines with C, N together with V, B to form carbide, nitride or carbonitride to control the crystal grain size, thereby having an effect of retarding the precipitation of sigma phase. In order to obtain this effect, it is necessary to contain 1 or more of 0.005% or more. However, even if Nb or V is contained in an amount exceeding 0.250%, precipitation of intermetallic compounds is promoted, and corrosion resistance is lowered. Therefore, it is set as an upper limit. The preferable lower limit of the content is 0.006%, the more preferable lower limit is 0.007%, the preferable upper limit is 0.230%, and the more preferable upper limit is 0.210%.

The effect of controlling the particle size of Nb and V is obtained by either 1 kind of Nb or 2 kinds of Nb or V, respectively, and thus the effect can be exerted by selectively containing 1 or more kinds of Nb or V.

1.2≤100{2([％V]+[％Nb])+6[％B]}×([％N]+[％C]-0.1[％Mn])≤5.0...(2)

By adding 1 or 2 of the constituent elements C, N and B and V, nb having the above-described constitution in an appropriate range and satisfying the relationship concerning carbonitride precipitation shown above, an appropriate pinning effect can be obtained, and the rate of precipitation of the sigma phase can be retarded by controlling the crystal grain size based on JIS G0551 to be in the range of 3.0 to 7.0. The preferred lower limit is 1.3, the more preferred lower limit is 1.4, the preferred upper limit is 4.5, and the more preferred upper limit is 4.2.

The substrate has a crystal grain size of 3.0 to 7.0 based on JIS G0511

Since the precipitation rate of the sigma phase is affected by the crystal grain size, it is necessary to control the same. When the grain size exceeds 3.0 and is coarse, that is, the grain size number becomes smaller, the points of the grain boundary triple points, which are the preferential sites of sigma phase precipitation, become smaller, and the grain boundary diffusion of Cr and Mo becomes concentrated, thereby accelerating the growth of sigma phase. On the other hand, when the crystal grain size is finer than 7.0, that is, the grain size number is larger, the total area of the grain boundaries becomes larger, the distribution of the Sn, B, and P amounts in the grain boundaries becomes sparse, and the effect of delaying the sigma phase precipitation cannot be sufficiently obtained. Therefore, the crystal grain size is set to 3.0 to 7.0. The preferable lower limit is 3.5, the more preferable lower limit is 4.0, the preferable upper limit is 6.5, and the more preferable upper limit is 6.0.

The balance of the high corrosion resistance austenitic stainless steel of the present invention, excluding the above-mentioned components, is composed of Fe and unavoidable impurities. The unavoidable impurities herein are components which are inevitably mixed in for various reasons in the industrial production of stainless steel, and are components which are allowed to be contained within a range which does not adversely affect the effect of the present invention.

Next, a method for producing the high corrosion resistance austenitic stainless steel according to the present invention will be described.

The method for producing the stainless steel of the present invention is not particularly limited, and is preferably produced by the following method. Firstly, raw materials such as scrap iron or stainless steel scraps, ferrochrome, ferronickel, pure nickel, metallic chromium and the like are melted by an electric furnace. Then, in an AOD furnace or VOD furnace, oxygen and argon are blown to perform decarburization refining, and quicklime, fluorite, al, si, and the like are charged to perform desulfurization and deoxidation treatment. The slag composition in this treatment is preferably adjusted to CaO-Al ₂ O ₃ -SiO ₂ MgO-F system. In addition, the slag preferably satisfies CaO/Al in order to effectively perform desulfurization at the same time ₂ O ₃ ≥2、CaO/SiO ₂ And is more than or equal to 3. Further, the refractory of the AOD furnace or VOD furnace is preferably a magnesium-chromium alloyOr dolomite. After refining in the AOD furnace or the like, the composition and temperature are adjusted in the LF process, and then continuous casting is performed to produce a slab, which is then hot rolled and cold rolled as necessary to produce a thin sheet such as a thick sheet, a hot-rolled steel sheet, or a cold-rolled steel sheet.

Examples

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples as long as the gist thereof is not exceeded. First, raw materials such as scrap iron, stainless steel scrap, ferrochrome, etc. are melted by an electric furnace of 60 tons. Then, in the AOD process, oxygen and argon are blown, and decarburization refining is performed. Then, quicklime, fluorite, al, si are charged to perform desulfurization and deoxidation. Then, ingots were cast by a continuous casting machine to obtain slabs (samples 1 to 45) having chemical compositions shown in table 4.

TABLE 4

* The designation of ()'s indicates a deviation from the scope of the invention

The chemical components other than C, S, N were analyzed by fluorescent X-ray analysis. N was analyzed by an inert gas-pulse heat fusion method, and C, S was analyzed by a combustion-infrared absorption method in an oxygen stream.

The slab was then hot-rolled according to a conventional method to obtain a hot-rolled steel sheet having a sheet thickness of 8.0 mm. At this time, the hot workability was evaluated by cracking that had occurred in the side surfaces of the hot-rolled steel sheet. Then, the hot-rolled steel sheet was subjected to solution heat treatment, cold rolling was performed, and a product annealing and pickling step was performed to obtain a cold-rolled strip having a sheet thickness of 2.0 mm. The annealing of the article was performed under water-cooling after maintaining at 1150 ℃ for 1 min. Further, the cold-rolled strip is subjected to aging heat treatment at 850 ℃ for a holding time varying variously within a range of not more than 1.5 hr. The aging heat treatment material was evaluated for corrosion resistance, which will be described below, and the crystal grain size was measured based on JIS G0551. Further, quantitative evaluation of the σ phase area ratio and the σ phase crystal grain size was performed by the EBSD method similar to < experiment 1 >.

< test for evaluating Hot workability >

The cracks that have occurred in the side surfaces of the hot-rolled steel sheet were visually observed, and if 40mm or more cracks were not occurred, they were evaluated as excellent (excellent) because of particularly excellent workability, were evaluated as good (o) if each 100mm was less than 3 in the longitudinal direction, were evaluated as ok (Δ) if 3 or more to 6 or less, were judged as not being available for working if 6 or more occurred, and were evaluated as bad (x).

< test for evaluating Corrosion resistance >

For the cold rolled strip subjected to the aging heat treatment, an iron chloride solution immersion test conforming to ASTM G48 (Method C) was performed under the following conditions, and the critical pitting corrosion occurrence temperature (CPT) was measured to evaluate the corrosion resistance.

Test piece: width 25mm x length 50mm x thickness 2mm;

test solution: 6 mass% FeCl ₃ +1 mass% aqueous HCl;

test liquid amount: 600ml per 1 test piece;

surface grinding: carrying out whole-surface wet grinding by using #120 SiC grinding paper;

test temperature: 55-100 ℃;

dipping time: 100hr;

number of test pieces (n number): 2 conditions;

evaluation criteria: the pitting depth of the test piece was measured, and the Critical Pitting Temperature (CPT) was obtained and evaluated at a pitting depth of 25 μm or more. In the aging heat treatment, since the deterioration of pitting resistance under aging is particularly excellent in suppressing deterioration even when the soaking time is 1.5hr and the CPT exceeds 60 ℃, the aging heat treatment is evaluated as excellent (excellent), the soaking time before the CPT reaches 60 ℃ is 1.2hr or more and less than 1.5hr, the soaking time is evaluated as good (good), the soaking time is evaluated as acceptable (delta) when the soaking time is 1hr or more and less than 1.2hr, and the soaking time is evaluated as poor (x) when the CPT is reduced to 60 ℃ when the soaking time is less than 1hr.

< measurement of sigma phase area Rate >

The sigma phase area ratio of a cold rolled strip subjected to an aging heat treatment at 850 ℃ for 60min was measured by the EBSD method similar to that of < experiment 1 >.

Test piece collection direction: collecting from a direction at right angles to the rolling process;

sample grinding: electrolytic polishing was performed by using "Tenupol-5" manufactured by Strauss corporation;

EBSD assay: a field emission scanning electron microscope (manufactured by japan electronics corporation, "JSM7001F", attached to a back scattering electron diffraction device (manufactured by TSL SOLUTIONS, manufactured by EBSD analysis software OIM analysis7.3 ");

measurement region: 80 μm by 240 μm;

step size: 0.2 μm.

< measurement of Crystal System of sigma phase >

The same sample as the sample for which the above sigma phase area ratio was obtained was subjected to a scanning electron microscope to obtain a sigma phase crystal grain size from a 5000-fold composition image.

The evaluation results are shown in table 5 below. Table 5 shows the following relational expressions for suppressing corrosion resistance degradation,

0.05≤10[％B]+2[％P]+6[％Sn]+0.03[％Si]≤0.20...(1)

and a relation for controlling the grain size of the crystals,

1.2≤100{2([％V]+[％Nb])+6[％B]}×([％N]+[％C]-0.1[％Mn])≤5.0...(2)

the case where the relationship is satisfied is indicated by an o symbol, the case where the relationship is not satisfied is indicated by an x symbol, the case where both are satisfied at the same time is indicated by an o, and the case where either one is not satisfied is indicated by an x.

TABLE 5

* The designation of ()'s indicates a deviation from the scope of the invention

As shown in Table 5, test numbers 1 to 18, in which the respective components satisfy the ranges of the present invention, show good retardation effect of corrosion resistance degradation, with a time period of 1hr or more before CPT reaches 60 ℃. The crystal grain size is in the range of 3.0 to 7.0. Although the ranges of the components of the present invention are satisfied, the test numbers 19 to 30 of the formula (2) below 1.2 or above 5.0 deviate from the ranges of 3.0 to 7.0 in all the crystal grain sizes, and the CPT stays slightly above 1hr before reaching 60 ℃.

In contrast, although the ranges of the components of the present invention are satisfied, the test numbers 31 to 33 in which the formula (1) is less than 0.05 are less than 1hr before the CPT reaches 60 ℃. At this time, the σ phase area ratio was more than 1% and precipitated, and the particle diameter was more than 2. Mu.m.

In addition, in the test No. 31, in the formula (2), the crystal grain size was 9.0, and the CPT was excessively fine, and was only 0.3hr before reaching 60 ℃.

Test No. 32 was less than 1.2 in the formula (2), the crystal grain size was 2.0, and it was excessively coarse, and it was only 0.4hr before CPT reached 60 ℃.

Further, the test numbers 34 to 37 in which the formula (1) exceeded 0.20, and the CPT was 1hr or more before 60℃was reached, and a satisfactory delay effect of deterioration in corrosion resistance was exhibited, but it was judged that the hot-rolled steel sheet had 6 or more cracks on the side surface and could not be subjected to high-temperature processing.

Although the formulas (1) and (2) were satisfied, any one of the test numbers Sn, B, P, si was lower than the test numbers 38 to 41 within the scope of the invention, and the retardation effect of the sigma phase precipitation by these could not be sufficiently obtained, and the CPT was lower than 1hr before reaching 60 ℃. In this case, the sigma phase area ratio is more than 1%, and the particle diameter thereof is more than 2. Mu.m.

Further, any of Sn, B and P exceeds the test numbers 42 to 44 within the scope of the invention, and CPT was 1hr or more before 60 ℃ was reached, and a satisfactory delay effect of deterioration in corrosion resistance was exhibited, but it was judged that cracking that had occurred in the side surfaces of the hot-rolled steel sheet was 6 or more and was not available for high-temperature working.

Si exceeds test number 45 of the scope of the invention, and is less than 1hr before CPT reaches 60 ℃. In this case, the sigma phase area ratio exceeds 1%, and the particle diameter exceeds 2. Mu.m.

Industrial applicability

According to the present invention, even when exposed to a temperature range such as σ phase precipitation, the decrease in corrosion resistance can be suppressed, and therefore, the present invention can be suitably used as a joining material for clad steel to be roll-joined to thick-walled carbon steel or the like, a highly corrosion-resistant material used in a process of passing through a line furnace for brazing, or the like.

Claims

1. A highly corrosion-resistant austenitic stainless steel characterized by comprising, in mass%,

contains C: 0.005-0.030%, si:0.05 to 0.30 percent of Mn:0.05 to 0.40 percent of P: 0.005-0.050%, S: 0.0001-0.0010%, ni:22.0 to 32.0 percent, cr:19.0 to 28.0 percent of Mo:5.0 to 7.0 percent, N:0.18 to 0.25 percent of Al: 0.005-0.100%, cu:0.05 to 0.50 percent of W:0.05% or less, sn:0.0005 to 0.0150 percent, co:0.030 to 0.300 percent, B:0.0005 to 0.0050 percent, the balance being Fe and unavoidable impurities,

and satisfies the following formula (1), the area ratio of sigma phase is 1% or less, CPT based on ASTM G48Method C is 60 ℃ or more as corrosion resistance,

0.05≤10[％B]+2[％P]+6[％Sn]+0.03[％Si]≤0.20...(1)。

2. the high corrosion resistant austenitic stainless steel according to claim 1, comprising Nb: 0.005-0.250%, V:0.005 to 0.250% and satisfies the following formula (2), the crystal grain size of the substrate based on JIS G0511 is in the range of 3.0 to 7.0,

1.2≤100{2([％V]+[％Nb])+6[％B]}×([％N]+[％C]-0.1[％Mn])≤5.0...(2)。

3. a method for producing a highly corrosion-resistant austenitic stainless steel according to claim 1 or 2, characterized in that,

as a thermal history after the solution heat treatment, a temperature range of 700 to 1000 ℃ is allowed to pass through for 10 to 60 minutes by isothermal holding, or cooling or heating steps.