JP2010222695A - Alloy-saving two-phase stainless steel material having excellent corrosion resistance, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive Ni-saving type two-phase stainless steel material having corrosion resistance and superior impact toughness. <P>SOLUTION: This stainless steel material includes, by mass%, 0.06% or less C, 0.1-1.5% Si, 0.1-6.0% Mn, 0.05% or less P, 0.005% or less S, 0.25-4.0% Ni, 19.0-23.0% Cr, 0.05-1.0% Mo, 3.0% or less Cu, 0.15-0.25% N, 0.003-0.050% Al and 0.007% or less O, while controlling Ni-bal. represented by expression <1>: Ni-bal.=(Ni+0.5Mn+0.5Cu+30C+30N)-1.1(Cr+1.5Si+Mo+W)+8.2, to -8 to -4, wherein each element name in the expression represents the content (%) and includes 40-70% by an area rate of austenite phases. In the austenite phase, 50 pieces or more of crystal grains, each of which has the major axis of 1-20 μm, exist in the observed visual field of 0.1 mm<SP>2</SP>in a plane parallel to the surface of the steel material. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention reduces the content of expensive alloys such as Ni and Mo among duplex stainless steels having two phases of an austenite phase and a ferrite phase used in an atmospheric environment, an aqueous environment, and a chloride environment. Related to alloy duplex stainless steel. Specifically, it is a duplex stainless steel rolled with solution heat treatment. For example, conventional austenite as piping and heat exchangers in plants such as dams, sluices, vacuum equipment materials, seawater desalination materials, petroleum refining and chemical industries. The steel of the present invention can be used in place of a part of the field where the stainless steel has been used.

Duplex stainless steel has both an austenite phase and a ferrite phase in the steel structure, and has been used as a high-strength, high-corrosion-resistant material for petrochemical equipment materials, pump materials, chemical tank materials, and the like. Furthermore, since duplex stainless steel is generally a low Ni component system, it has a lower alloy cost and less fluctuation than the austenitic stainless steel, which is the mainstream of stainless steel, due to the recent surge in metal raw materials. Has attracted attention as.
The latest topic of duplex stainless steel is the development of an alloy-saving type and an increase in its usage. The alloy-saving type is a steel type that suppresses the content of an alloy that is more expensive than conventional duplex stainless steel and further increases the merit of low alloy costs, and is disclosed in Patent Documents 1 to 3 and the like. These are standardized by ASTM-A240 and correspond to S32001, S32101, and S32304, respectively. The main types of conventional duplex stainless steels are JIS SUS329J3L and SUS329J4L, but these are more corrosion resistant than austenitic high corrosion resistant steel SUS316L, and expensive Ni and Mo are about 6-7% and about 3 respectively. Add ~ 4%. In contrast, alloy-saving duplex stainless steel uses Ni or Mo for N or Mn instead of SUS316L or general-purpose steel SUS304L for corrosion resistance, and Ni or Mo is about 1-4% for S32304. In S32101, it is greatly reduced to about 0 to 1%.
In particular, N is an effective element that stabilizes the austenite phase and improves the strength and corrosion resistance by forming a solid solution in the austenite phase. Particularly in the case of duplex stainless steel, Cr and Mo are concentrated in the ferrite phase, so In order to ensure corrosion resistance, addition of N is important. Furthermore, in the case of steel materials to be welded, the ratio of ferrite phase increases due to heating in the heat affected zone of duplex stainless steel, but the cooling rate is high during cooling, so diffusion cannot catch up, and high ferrite content does not completely transform into austenite. However, the corrosion resistance may be greatly reduced. However, the addition of N having a high diffusion rate has an effect of securing an austenite phase even in such a case, so it is actively added.

US / 4828630 JP-A 61-56267 WO2002 / 27056

These alloy-saving duplex stainless steels are originally designed to have corrosion resistance comparable to that of SUS304 or SUS316L, but the austenitic steel grades with which corrosion resistance originally corresponds, for example, S32101 is SUS304, S32304. The phenomenon of lower than SUS316L often occurred especially in alloy-saving duplex stainless steels with high N by substituting Ni for N.
An object of the present invention is to provide an alloy-saving duplex stainless steel that does not cause a decrease in corrosion resistance as described above, while keeping the cost of the alloy as low as possible without changing the component design of the alloy-saving duplex stainless steel. And

The present inventors have conducted hot rolling / solution heat treatment experiments using Ni-saving duplex stainless steel containing 21Cr-1.5Ni-5Mn-0.21% N and various laboratory molten steel pieces. As a result of proceeding with corrosion resistance evaluation and structure observation, it is possible to distinguish steel materials with good corrosion resistance by metal structure observation, and by specifying the conditions of solution heat treatment after hot rolling, without adding corrosion resistance improving components Furthermore, it has been found that the corrosion resistance of the alloy-saving duplex stainless steel material is improved.
First, as a result of evaluating the corrosion resistance of the test material and observing the structure of the steel material by various methods, it is the sensitization caused by precipitation of nitride at the ferrite phase grain boundary that brings about a decrease in corrosion resistance in the steel material. I found. Next, as a result of various changes in the heat treatment conditions for the method of suppressing the precipitation of nitrides and extensive studies, the ferrite phase was crystallized by crystallizing fine austenite in the coarse ferrite phase scattered in the duplex stainless steel. It has been found that the precipitation of nitrides at the grain boundaries can be suppressed. Furthermore, as a result of examining the conditions under which the fine austenite can be crystallized, the temperature of the solution heat treatment is shown in relation to the reheating temperature of the slab or steel slab during hot rolling at a relatively low temperature for a short time. It was found that it can be realized by making it within a certain range. Details will be described below.

  In the alloy-saving duplex stainless steel with a high nitrogen content, the mechanism causing sensitization is as follows. Nitrogen is dissolved in the ferrite phase by solution heat treatment, but originally the nitrogen solid solubility limit of the ferrite phase is relatively small, and the amount of nitrogen exceeding the solid solubility limit during cooling precipitates at the grain boundaries as nitrides. Causes sensitization. Since the precipitation rate corresponds to the amount of supersaturated nitrogen, the precipitation of nitride is suppressed by reducing the amount of nitrogen dissolved in the ferrite phase when the temperature of nitride precipitation is 800 ° C or lower. it can. In order to reduce the amount of nitrogen solid solution in the ferrite phase, nitrogen in the ferrite phase may be moved into the austenite phase while the ferrite phase is higher than the nitride precipitation temperature range. In this case, a problem is that when the ferrite phase is coarse crystal grains, nitrogen may not reach the adjacent austenite phase. Although the diffusion rate of nitrogen in the ferrite phase is overwhelmingly faster than that of the austenite phase, in the case of duplex steel, water cooling is performed after heat treatment to prevent sensitization due to precipitation of chromium carbide in the austenite phase. In such a rapid cooling, there may be a case where sufficient time for reducing the solid solution N cannot be secured.

When a method for preventing this was studied, if fine austenite crystal grains were crystallized in the coarse ferrite phase observed in the steel, the fine austenite crystal grains could be spent even if the time available for nitrogen diffusion was short. It can be absorbed by being in the vicinity and the amount of nitrogen in the ferrite can be reduced. Specifically, when the microscopic structure is observed in an observation field of at least 0.5 mm ² with a plane parallel to the steel surface as the spectroscopic surface, the austenite phase grains having a major axis of 1 μm or more and 20 μm or less are observed in an observation field of 0.1 mm. 50 or more per ² need only exist.

Furthermore, it was clarified as a method for realizing the organization as follows. First, in order to reliably crystallize the austenite phase at a low temperature, it is necessary to use a component system in which austenite is relatively stable among the duplex stainless steels. Therefore, Ni-bal., Which is widely used as a formula for estimating the amount of austenite. Stipulated.
This Ni-bal. Assuming a component system that defines the following, the following method has been found for solution heat treatment conditions. In general, in the case of duplex steel, the lower the heat treatment temperature, the lower the amount of equilibrium ferrite and the more austenite. Accordingly, when heat treatment is performed at a low temperature where the amount of equilibrium ferrite at the heat treatment temperature is less than the amount of ferrite before heat treatment after hot rolling, an austenite phase is generated.
Specifically, it has been found that the solution heat treatment temperature may be 930 ° C. or higher and 150 ° C. or lower than the reheating temperature of the slab or steel slab during hot rolling. In this case, since dislocations caused by hot rolling and precipitates precipitated during cooling are scattered in the ferrite grains, a fine austenite phase is generated using this as a nucleus. However, if the heat treatment is performed for a long time, fine austenite grains are combined and become coarse, and as a result, the nitrogen moving distance from the ferrite phase to the austenite phase becomes long, so it is necessary to set an upper limit for the heat treatment conditions. Specifically, it has been found that the heat treatment time T (hr) may be set so as to satisfy the following formula <2>.
LMP = (t + 273) × (20 + log ₁₀ T) ≦ 25200... <2>
Where t is the heat treatment temperature (° C.) and T is the heat treatment time (hr).
As a result, the present invention has been clarified with respect to the chemical composition, structure and manufacturing method of the Ni-saving duplex stainless steel material.

That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.06% or less, Si: 0.1 to 1.5%, Mn: 0.1 to 6.0%, P: 0.05% or less, S: 0.005 %: Ni: 0.25-4.0%, Cr: 19.0-23.0%, Mo: 1.0% or less, Cu: 3.0% or less, N: 0.15-0.25 %, Al: 0.003 to 0.050%, O: 0.007% or less, with the balance being Fe and unavoidable impurities, Ni-bal. Is -8 or more and -4 or less, the austenite phase area ratio is 40 to 70%, the plane parallel to the steel material surface is used as the spectroscopic surface, and the microscopic structure is observed in an observation field of at least 0.5 mm ² or more. The austenite phase crystal grains having a major axis of 1 μm or more and 20 μm or less are present in an amount of 50 or more per observation field of 0.1 mm ^{2. The} alloy-saving duplex stainless steel material Ni-bal. = (Ni + 0.5Mn + 0.5Cu + 30C + 30N)
-1.1 (Cr + 1.5Si + Mo + W) +8.2... <1>
In the above formula, each element name represents its content (%).
(2) Further, by mass%, Ti: 0.003 to 0.05%, Nb: 0.02 to 0.15%, V: 0.05 to 0.5%, or one or more of them The alloy-saving duplex stainless steel material having good corrosion resistance as described in (1).
(3) Further, it contains one or more of W: 0.03 to 1.0% and Co: 0.02 to 1.0% by mass% (1) or The alloy-saving duplex stainless steel material having good corrosion resistance as described in (2).
(4) Further, by mass%, B: 0.0005 to 0.0040%, Ca: 0.0005 to 0.0050%, Mg: 0.0001 to 0.0030%, REM: 0.005 to 0.00. The alloy-saving duplex stainless steel material with good corrosion resistance according to any one of (1) to (3), which contains one or more of 050%.

(5) In the step of re-heating and then hot-rolling the duplex stainless steel slab or steel slab having the composition described in any one of (1) to (4), followed by solution heat treatment The temperature of 930 ° C. or more and 150 ° C. or less lower than the reheating temperature of the slab or steel slab, and the heat treatment time T (hr) is in the range represented by the following formula <2> (1) ) To (4), the method for producing an alloy-saving duplex stainless steel material having good corrosion resistance.
LMP = (t + 273) × (20 + log ₁₀ T) ≦ 25200... <2>
Where t is the heat treatment temperature (° C.) and T is the heat treatment time (hr).

  According to the present invention, deterioration in corrosion resistance due to sensitization of ferrite grain boundaries in Ni-saving duplex stainless steel could be avoided. This makes it possible to provide an inexpensive Ni-saving duplex stainless steel hot rolled steel with corrosion resistance and excellent impact toughness that is used in atmospheric, water, and chloride environments. The steel material of the present invention is used in place of some of the fields where austenitic stainless steel has been conventionally used as piping and heat exchangers in plants for vacuum equipment, seawater desalination, petroleum refining, chemical industry, etc. There are significant industrial contributions such as being able to do so.

It is an example of the structure | tissue photograph of the duplex stainless steel material of this invention, and the fine austenite grain was shown with the white arrow. It is a figure which shows the appropriate range of the temperature and time of solution heat treatment in this invention.

Below, the reason for limitation of the steel composition as described in (1) of this invention is demonstrated first.
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 and toughness deteriorate.

  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 0.1% or more for deoxidation. Furthermore, addition of 1% or more has the effect of increasing the austenite phase and improving toughness, increasing the solid solubility of N, making it difficult to precipitate nitrides, and improving corrosion resistance. However, if it is added over 6.0%, the above effect is saturated and the passive film is weakened to deteriorate the corrosion resistance. Therefore, the upper limit is limited to 6%. The preferable content is 2.0 to 6.0%, and the most preferable range from the point of corrosion resistance is more than 2.0 to less than 4.0%.

  P is limited to 0.05% or less in order to deteriorate hot workability and toughness. Preferably, it is 0.03% or less.

  S degrades hot workability, toughness, and corrosion resistance, so it is limited to 0.005% or less. Preferably, it is 0.0020% or less.

  Ni is an element effective for increasing the austenite phase in duplex stainless steel, suppressing the formation of work-induced martensite and improving toughness, and further improving the corrosion resistance against various acids. 25% or more is essential, but since it is an expensive alloy, it is suppressed as much as possible in the present invention to 4.0% or less. A preferable range is 1.0 to less than 3.0%.

  Cr is contained in an amount of 19.0% or more to ensure basic corrosion resistance. On the other hand, if the content exceeds 23.0%, the ferrite phase fraction increases and the toughness and the corrosion resistance of the weld zone are impaired. Therefore, the Cr content is set to 19% or more and 23% or less. A preferable content is 19.0 to 22.0%.

  Mo is a very effective element that additionally increases the corrosion resistance of stainless steel. In the steel of the present invention, the upper limit is 1.0% or less in terms of cost, but it is an extremely expensive element, and more preferably 0.5% or less.

  Cu is an element that additionally enhances the corrosion resistance of stainless steel to acids, and has the effect of stabilizing the austenite phase and improving toughness. If the content exceeds 3.0%, εCu precipitates exceeding the solid solubility and embrittlement occurs, so the upper limit was made 3.0%. A preferable content is 0.5 to 2.0%.

As described above, N is an effective element that stabilizes the austenite phase and dissolves it in the austenite phase to increase the strength and corrosion resistance. In particular, in the case of a steel material to be welded, N is an effective element that increases the corrosion resistance of the weld heat affected zone. Added.
On the other hand, in the case of an alloy-saving duplex stainless steel with a small amount of Cr and Mo that increases the solid solution limit of N, if it is high N, the problem of characteristic deterioration due to nitride precipitation at the grain boundary as described above occurs, which is shown in the present invention. It will be necessary to optimize the manufacturing conditions. The amount of N that causes the problem is the case of exceeding 0.15% in the steel of the present invention, and this is the lower limit of the N amount of the present invention. On the other hand, if the content exceeds 0.25%, Cr nitride is precipitated regardless of the heat treatment conditions and the toughness and corrosion resistance are impaired, so the upper limit of the content was made 0.25%.

  Al is an important element for deoxidation of steel, and is contained together with Si in order to reduce oxygen in the steel. When the Si content exceeds 0.3%, it may not be necessary to add, but the reduction of the oxygen content is essential for securing toughness, and for this reason, a content of 0.003% or more is necessary. . On the other hand, Al is an element having a relatively large affinity with N, and if added excessively, AlN is generated and inhibits the toughness of stainless steel. 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 (oxygen) is an important element constituting an oxide that is representative of non-metallic 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.

Next, in order to obtain good characteristics in the duplex stainless steel of the present invention, the austenite phase area ratio needs to be in 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. In particular, in the steel of the present invention, it is better to increase the austenite phase as much as possible. The austenite amount can be secured by adjusting the content ratio of the austenite increasing element and the ferrite phase increasing element within the specified range of the present invention. Specifically, Ni-bal. The expression is in the range of −8 to −4. Preferably it is -7.1--4.
Ni-bal. = (Ni + 0.5Cu + 0.5Mn + 30C + 30N)
-1.1 (Cr + 1.5Si + Mo + W) +8.2 ... <1>

The distribution of austenite crystal grains is an important factor for determining the corrosion resistance of the rolled steel material of the present invention. The present inventors have conducted hot rolling / solution heat treatment experiments using Ni-saving duplex stainless steel containing 21Cr-1.5Ni-5Mn-0.21% N and various laboratory molten steel pieces. When the microstructure is observed in an observation field of at least 0.1 mm ² with the plane parallel to the steel surface as the specular surface, the crystal grain length is 1 μm or more and 20 μm or less. It was found that the corrosion resistance was good when 50 or more austenite phase grains were present per observation field of 0.1 mm ² .

  The grounds for setting are as follows. The reason why the specular surface is a plane parallel to the steel surface is because the following points are superior to the case where the cross section is perpendicular to the processing direction. When duplex is rolled, the structure is compressed in the thickness direction, and very thin α and γ phases are arranged alternately. On the other hand, the structure parallel to the steel surface is not compressed, a relatively coarse structure is observed, and the following measurement is easy. Furthermore, since the surface is a surface that is actually exposed to a corrosive environment, it may be directly evaluated, and the surface was selected.

The specular surface is buffed, and after performing etching that exposes austenite grain boundaries by, for example, electrolytic etching in a 10% oxalic acid solution, observation with a microscope is performed. The observation visual field is at least 0.5 mm ² or more. This is because the surface of the duplex stainless steel has a coarse ferrite phase and austenite phase of several tens of μm depending on the location, and fine austenite grains are observed in the coarse ferrite. This is because it is necessary to observe a certain area.

  The fine austenite crystal grains to be counted here are limited to those having a major axis of 1 μm or more and 20 μm or less. The upper limit value was derived by conducting an evaluation experiment and measuring the diameter of fine austenite grains found in a material having an effect of improving corrosion resistance. The lower limit was defined as the limit that can be clearly observed with the austenite phase with an optical microscope.

FIG. 1 shows an example of a tissue photograph. The ones indicated by white arrows are fine austenite grains. As the number of fine austenite crystal grains, as shown in the examples, it was found that the corrosion resistance was good if 50 or more particles were present per observation field of 0.1 mm ² . As a specific evaluation procedure, for example, when a tissue photograph of 12 fields of view is taken at 500 times the same as the JIS standard, it becomes 0.5425 mm ^{2 for} 12 fields of view of an L size 89 mm x 127 mm. (0.5425 ÷ 0.1) = 272 or more fine austenite grains may be confirmed.

Next, the reason for limiting the steel composition of the alloy-saving duplex stainless steel material described in (2) of the present invention will be described.
Ti has an effect of forming a nitride in a very small amount and suppressing the precipitation of Cr nitride, and is added as necessary. Addition of 0.003% or more is necessary to exert the above effect.
On the other hand, if it exceeds 0.05% 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 0.05%. A suitable content of Ti is 0.003 to 0.020%.

  Nb also has the effect of suppressing the precipitation of Cr nitride and increasing the corrosion resistance. 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 reason, it is contained by 0.02% or more. On the other hand, excessive addition causes precipitation as an undissolved precipitate during heating before hot rolling, which impairs toughness, so the upper limit of its content was set to 0.15%. The preferable content range in the case of adding is 0.03% to 0.10%.

  V is also contained in an amount of 0.05% or more for the purpose of improving the corrosion resistance. However, if V is contained in an amount exceeding 0.5%, coarse V-based carbonitrides are produced and the toughness deteriorates. Therefore, the upper limit is limited to 0.5%. The preferable content when added is in the range of 0.06 to 0.30%.

Next, the reason for limiting the steel composition of the alloy-saving duplex stainless steel material described in (3) of the present invention will be described.
W, like Mo, is an element that additionally improves the corrosion resistance of stainless steel, and has a higher solid solubility than V. For the purpose of enhancing the corrosion resistance in the steel of the present invention, 0.03 to 1.0% is contained.

  Co is an element effective for enhancing the toughness and corrosion resistance of steel, and is selectively added. If the content is less than 0.02%, the effect is small. If the content exceeds 1.0%, the element is an expensive element, so that the effect corresponding to the cost is not exhibited. Therefore, the content when added is determined to be 0.02 to 1.0%.

Furthermore, the reason for limiting the steel composition of the alloy-saving duplex stainless steel material described in (4) of the present invention will be described.
B, Ca, Mg, and REM are all elements that improve the hot workability of steel, and one or more of them are added for that purpose. In any case, excessive addition of B, Ca, Mg, and REM lowers the hot workability and toughness, so the upper and lower limits of the content were determined as follows. B is 0.0005 to 0.0040%, Ca is 0.0005 to 0.0050%, Mg is 0.0001 to 0.0030%, and REM is 0.005 to 0.050%. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

Furthermore, the reason for limitation of the manufacturing method of the alloy-saving duplex stainless steel material as described in (5) of this invention is demonstrated.
As a manufacturing method of the steel material of the present invention, a slab or a slab of duplex stainless steel having the composition described in any one of the above (1) to (4) is reheated and hot-rolled, and then solutionized. Heat treatment is performed.
The solution heat treatment is carried out in order to dissolve Cr precipitates precipitated during hot rolling, recrystallize the processed crystal grains, and further crystallize the fine austenite necessary for the present invention. For this purpose, a temperature of 930 ° C. or higher is necessary. In order to crystallize the fine austenite phase, the heat treatment temperature is controlled so that the equilibrium ferrite content is lower than the ferrite content of the hot-rolled material. It is considered that the amount of ferrite in the hot-rolled material is roughly affected by the amount of equilibrium ferrite during reheating of the cast slab and steel slab during hot rolling. It has been found that fine austenite grains can be crystallized. The solid solution and recrystallization of the precipitate proceed as the heat treatment temperature is increased, but if it is excessively high, the fine austenite grains targeted by the present invention become coarse. In order to define the conditions, the temperature and the time may be specified under such conditions that the LMP represented by the formula <2> is 25200 or less. The LMP equation is known as the Larson-Miller parameter, and is an index of diffusion expressed by temperature and time as disclosed in, for example, Japanese Patent Application Laid-Open No. 7-316744. As this value increases, diffusion proceeds, and therefore grain growth proceeds and the grain size of fine austenite grains increases. Therefore, by setting an upper limit to this value, coarsening of the fine austenite grains is suppressed. As a result of experiments, it was found that when the value is 25200 or less, fine austenite grains can be suppressed to 20 μm or less. Specific heat treatment conditions are as shown in FIG. 2 shows the steel No. in Table 2 of the examples described below. The experimental results of 1 and 2 are plotted.

  Examples will be described below. Table 1 shows the chemical composition of the test steel. A blank column indicates that no analysis was performed because no additive was added. The components other than those listed in Table 1 are Fe and unavoidable impurity elements. REM in the table means lanthanoid rare earth elements, and the content indicates the total of these elements. These steels were melted in a MgO crucible by a laboratory 50 kg vacuum induction furnace and cast into a flat steel ingot having a thickness of about 100 mm. The material for hot rolling was processed from the main body of the steel ingot. This material was heated for 1 to 2 hours at the temperature indicated by “reheating temperature” in Table 2, and then rolled under a finishing temperature of about 950 ° C. to obtain a hot-rolled steel sheet 12 mm thick × about 700 mm long. In addition, spray cooling was implemented from 200 degreeC or less to the steel material temperature immediately after rolling from 800 degreeC or more. The solution heat treatment was soaked under the conditions shown in “Heat treatment temperature” and “Heat treatment time” in Table 2 and then cooled with water.

Next, the characteristics were evaluated as follows. In the evaluation of hot workability, the length of the longest ear crack of about 700 mm of the rolled material was regarded as the ear crack length, and the sizes were compared. As for the austenite area ratio, the cross section parallel to the rolling direction is embedded and mirror polished, electrolytic etching is performed in an aqueous KOH solution, and then the ferrite area ratio is measured by image analysis through observation with an optical microscope. The austenite area ratio was used. Further, in order to evaluate the corrosion resistance, the surface of each test piece collected from each sample from the surface layer was polished by # 600, and CPT (critical pitting corrosion temperature) measurement defined by ASTM G48 method E was performed.
Furthermore, the surface of the test piece collected from the surface layer was subjected to electrolytic etching in a 10% oxalic acid solution to remove the irregularities and then embedded in a resin and mirror polished to expose the structure. The material was observed under a light microscope at a magnification of 500 times, 12 L-size photographs were taken, and the number of austenite grains having a major axis of 1 μm or more and 20 μm or less was counted with an image analyzer.

The evaluation results are shown in Table 2. Steel No. 1 satisfying the steel composition and solution heat treatment conditions of the present invention. Inventive examples 1 to 13 all showed good corrosion resistance and toughness. In addition, about toughness, the V notch Charpy value at −20 ° C. of the base material was determined to be 170 J / cm ² or more as good.
Steel No. For 1, 2, and A, the solution heat treatment conditions were variously changed and evaluated. As a result, the number of austenite grains was different. It was found that when the number of austenite grains exceeds 50 / 0.1 mm ² , the CPT exceeds 10 ° C. and the corrosion resistance of the steel can be maintained well. Further, by performing heat treatment in the range shown in FIG. 1, the number of fine austenite grains could be increased to 50 / 0.1 mm ² or more. The heat resistance at 900 ° C. was good in corrosion resistance but poor in recrystallization. All the other steels of the inventive examples were able to secure the number of fine austenite grains by 1180 ° C. reheating-1000 ° C. × 20 minutes heat treatment and good CPT. In addition, steel composition No. which steel composition does not correspond to this invention and whose N is low. In A, corrosion resistance was maintained even at high temperature heat treatment.

About hot workability, when P, S, Cu was excessive, it fell, and the ear crack became 25 mm or more (steel No. F, G, J). Moreover, it improved by addition of B, Ca, Mg, and REM (steel No. 2, 5, 6, 7, 9 of Table 2), and the ear crack became very few.
Regarding toughness, steels No. D and L with high Si and Al, and steel No. with high O due to poor deoxidation due to too little. C and K had low toughness due to inclusions. Steel No. I is too high in Cr. N is Ni-bal. Is too low, ferrite stability is high, and only a small amount of fine austenite grains can be obtained.
Regarding corrosion resistance, steel No. 1 containing excess C, Mn and S was used. Steel No. with too little B, E, G and Cr H was bad. High N steel No. Even when M was heat-treated at 1180 ° C.-1000 ° C. × 20 minutes, the corrosion resistance decreased due to nitride precipitation.
As can be seen from the above examples, it has become clear that the present invention can provide an alloy-saving duplex stainless steel with good corrosion resistance.

  According to the present invention, some of the fields where austenitic stainless steel has been conventionally used as piping and heat exchangers in plants such as dams, sluices, vacuum equipment materials, seawater desalination materials, petroleum refining, chemical industries, etc. The industrial contribution such as the use of inexpensive Ni-saving type duplex stainless steel material instead is very great.

Claims (5)

In mass%, C: 0.06% or less, Si: 0.1 to 1.5%, Mn: 0.1 to 6.0%, P: 0.05% or less, S: 0.005% or less, Ni: 0.25 to 4.0%, Cr: 19.0 to 23.0%, Mo: 1.0% or less, Cu: 3.0% or less, N: 0.15 to 0.25%, Al : 0.003 to 0.050%, O: 0.007% or less, with the balance being Fe and unavoidable impurities, Ni-bal. Is -8 or more and -4 or less, the austenite phase area ratio is 40 to 70%, the plane parallel to the steel material surface is used as the spectroscopic surface, and the microscopic structure is observed in an observation field of at least 0.5 mm ² or more. An alloy-saving duplex stainless steel material with good corrosion resistance, characterized in that 50 or more austenite phase grains having a major axis of 1 μm or more and 20 μm or less are present per observation field of 0.1 mm ² .
Ni-bal. = (Ni + 0.5Mn + 0.5Cu + 30C + 30N)
-1.1 (Cr + 1.5Si + Mo + W) +8.2... <1>
In the above formula, each element name represents its content (%).   Furthermore, it contains one or more of Ti: 0.003-0.05%, Nb: 0.02-0.15%, and V: 0.05-0.5% by mass%. The alloy-saving duplex stainless steel material having good corrosion resistance according to claim 1.   Furthermore, it contains one or more of W: 0.03 to 1.0% and Co: 0.02 to 1.0% by mass%. The alloy-saving duplex stainless steel material with good corrosion resistance as described.   Furthermore, by mass%, B: 0.0005 to 0.0040%, Ca: 0.0005 to 0.0050%, Mg: 0.0001 to 0.0030%, REM: 0.005 to 0.050% The alloy-saving duplex stainless steel material with good corrosion resistance according to any one of claims 1 to 3, characterized by containing one or more of them. A solution heat treatment temperature t (° C) in the step of re-heating and casting the duplex stainless steel slab or steel slab having the composition according to any one of claims 1 to 4, followed by solution heat treatment. Is not less than 930 ° C and not more than 150 ° C lower than the reheating temperature of the slab or steel slab, and the solution heat treatment time T (hr) is in the range represented by the following formula <2>. 5. The method for producing an alloy-saving duplex stainless steel material having good corrosion resistance according to any one of items 4 to 4.
LMP = (t + 273) × (20 + log ₁₀ T) ≦ 25200... <2>
Where t is the heat treatment temperature (° C.) and T is the heat treatment time (hr).