JP2022546776A - Highly corrosion-resistant austenitic stainless steel with excellent impact toughness and hot workability - Google Patents

Abstract

A highly corrosion-resistant austenitic stainless steel having excellent corrosion resistance, impact toughness, and hot workability is provided. A highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability of the present invention comprises, in weight percent, C: 0.03% or less (excluding 0), Si: 1.0% or less, Mn: 1.0% or less, Cr: 18-24%, Ni: 16-24%, Mo: 5.0-7.0%, Cu: 0.1-2.0%, W: 1.0% Below, N: 0.18 to 0.3%, Al: 0.02 to 0.1%, O: 0.01% or less, Ca: 0.002 to 0.01%, S: less than 0.001% , Fe and unavoidable impurities, and are characterized by satisfying O/Al: 0.01 to 0.12 and S/Ca: 0.01 to 0.4. [Selection drawing] Fig. 1

Description

TECHNICAL FIELD The present invention relates to a highly corrosion-resistant austenitic stainless steel having excellent impact toughness and hot workability. The austenitic stainless steel according to the present invention is applied to materials for industrial equipment such as desulfurization equipment, heat exchangers, freshwater equipment, and food and beverage equipment.

Austenitic stainless steel is widely used due to its excellent corrosion resistance, workability and weldability. STS316 stainless steel, which has improved corrosion resistance by adding 2% Mo to STS304 stainless steel represented by 18Cr-8Ni, is being applied to various fields such as kitchens, home appliances, and industrial equipment.

Corrosion resistance of austenitic stainless steel can be ensured by adding elements such as Cr, Mo and N. However, when the content of added elements such as Cr, Mo, and N increases, intermetallic compounds such as the σ phase precipitate in the matrix structure, degrading corrosion resistance and impact toughness, and deteriorating hot workability. However, there is a problem that it significantly lowers.

In order to solve such problems, Patent Documents 1 and 2 disclose techniques for suppressing the formation of the σ phase by adding tungsten (W) instead of Mo. However, since high alloy austenitic stainless steels should generally have compositions within the specification range, adding W instead of Mo is not preferred. In addition, when a large amount of W is contained, other intermetallic compounds such as the chi (χ) phase may be precipitated.

Patent document 3 controls the σ phase by adjusting the components so that the value of the sigma (σ) equivalent (SGR) represented by the following formula is 18 or less. However, Patent Document 3 restrictively considers only Cr, Mo, N, and MnCu as alloying elements that affect the control of the σ phase, and there is still the problem that intermetallic compounds such as the σ phase precipitate in the matrix structure. be.

SGR=Cr+2Mo-40N+0.5Mn-2Cu

Korean Patent Publication No. 10-2001-0038199 (Published date: April 06, 2001) Korean Patent Publication No. 10-1999-0005962 (Published date: September 15, 2000) US Patent Application Publication No. 2015-0050180 (Published date: February 19, 2015)

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, an object of the present invention is to provide a highly corrosion-resistant austenitic stainless steel which is excellent in corrosion resistance, impact toughness, and hot workability.

As a means for achieving the above object, a highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability according to an example of the present invention has a C content of 0.03% or less (excluding 0) by weight %. , Si: 1.0% or less, Mn: 1.0% or less, Cr: 18-24%, Ni: 16-24%, Mo: 5.0-7.0%, Cu: 0.1-2. 0%, W: 1.0% or less, N: 0.18-0.3%, Al: 0.02-0.1%, O: 0.01% or less, Ca: 0.002-0.01 %, S: less than 0.001%, the remaining Fe and inevitable impurities, O / Al: 0.01 to 0.12, S / Ca: 0.01 to 0.4. .

In the highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability of the present invention, the impact toughness (CNV _TH ) value represented by the following formula (1) may be 80 or more.
(1) CNV _TH = 336-1432 x C-22.1 x Si + 64.1 x Mn + 8.5 x Cr + 0.11 x Ni-10.1 x Mo-3.3 x Cu + 22.1 x W-392 x N- 293×( _Tσ /T)
In the above formula (1), C, Si, Mn, Cr, Ni, Mo, Cu, W, and N mean the weight percent of each alloying element, and _Tσ is the thermodynamic sigma (σ) phase. means the complete decomposition temperature and T means the actual solution heat treatment temperature.

In each highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability of the present invention, the PREW-Mn value represented by the following formula (2) may be 40 or more and 50 or less.
(2) PREW−Mn=Cr+3.3×(Mo+0.5×W)+16×N−0.5×Mn
In the above formula (2), Cr, Mo, W, N, and Mn mean weight percent of each alloying element.

In the highly corrosion-resistant austenitic stainless steel of the present invention, which is excellent in impact toughness and hot workability, the area from the surface to the depth of 1/4 to 3/4 of the thickness is measured at a magnification of 50 times with an area of 26 mm ² . The area ratio of the σ phase may be 1.0% or less.

In the highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability of the present invention, the critical pitting corrosion temperature may be 80° C. or higher.

According to the present invention, it has excellent corrosion resistance and impact toughness that can be applied to materials for industrial equipment such as desulfurization equipment, heat exchangers, freshwater equipment, food and beverage equipment, and high corrosion resistance with excellent hot workability. Austenitic stainless steel can be provided.

The PREW-Mn value is controlled to be 40 or more and 50 or less within the alloy composition defined by the present invention, the formation of intermetallic compounds is suppressed to ensure high corrosion resistance, and the impact toughness (CNV _TH ) value is 80 or more. Ensure excellent impact toughness by controlling the alloy components and heat treatment conditions so that O / Al: 0.01 to 0.12, S / Ca: trace elements to satisfy 0.01 to 0.4 can be controlled to ensure excellent hot workability.

4 is a graph showing critical pitting temperature (CPT) according to changes in PREW-Mn in each example. It is a graph which shows S/Ca and O/Al value of each Example.

The high corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability according to an example of the present invention has, in weight percent, C: 0.03% or less (excluding 0), Si: 1.0% or less, Mn: 1.0% or less, Cr: 18-24%, Ni: 16-24%, Mo: 5.0-7.0%, Cu: 0.1-2.0%, W: 1.0% or less, N: 0.18-0.3%, Al: 0.02-0.1%, O: 0.01% or less, Ca: 0.002-0.01%, S: less than 0.001%, rest of Fe and inevitable impurities, satisfying O/Al: 0.01 to 0.12 and S/Ca: 0.01 to 0.4.

Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified into various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Moreover, the embodiments of the present invention are provided so that the invention will be more fully understood by those of average skill in the art.

The terminology used in this application is merely used to describe specific examples. Thus, for example, singular references include plural references unless the context clearly dictates otherwise. Furthermore, terms such as "including" or "comprising" as used in this application expressly refer to the presence of the features, steps, functions, components, or combinations thereof described in the specification. and does not preclude the presence of other features, steps, functions, components or combinations thereof.

On the other hand, unless defined otherwise, all terms used herein should be assumed to have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. be. Accordingly, unless explicitly defined herein, certain terms should not be construed in an overly idealistic and formal sense. For example, the singular references herein include plural references unless the context clearly dictates otherwise.

Also, as used herein, "about," "substantially," and the like are used at or near the numerical value when manufacturing and material tolerances inherent in the referenced meaning are presented. To aid understanding of the invention, precise and absolute numerical values are used to prevent unauthorized use of the disclosed material referred to.

The highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability according to an example of the present invention has C: 0.03% or less, Si: 1.0% or less, and Mn: 1.0% or less by weight. , Cr: 18-24%, Ni: 16-24%, Mo: 5-7%, Cu: 0.1-2.0%, W: 1.0% or less, N: 0.18-0.3 %, Al: 0.02 to 0.1%, O: 0.01% or less, Ca: 0.002 to 0.01%, S: less than 0.001%, the remaining Fe and unavoidable impurities good.

Hereinafter, the reasons for limiting the alloy composition will be specifically described. Unless otherwise specified, all of the component compositions below are expressed in weight percent.

Carbon (C): 0.03% by weight or less (excluding 0)
C is a strong austenite phase stabilizing element and increases strength through solid solution strengthening. However, if the C content is too high, it easily combines with carbide-forming elements such as Cr, which are effective for corrosion resistance, at the austenite phase boundaries to form carbides, and the formed carbides reduce the Cr content around the grain boundaries. reduce corrosion resistance. Accordingly, it is preferable to limit the upper limit of the C content to 0.03% by weight or less.

Silicon (Si): 1.0% by Weight or Less Si is an element that stabilizes the ferrite phase, improves corrosion resistance, and acts as a deoxidizing agent. However, if the Si content is too high, it may promote the precipitation of intermetallic compounds such as the σ phase, degrading mechanical properties related to impact toughness and corrosion resistance, and may induce cracks during hot rolling. Accordingly, it is preferable to limit the upper limit of the Si content to 1.0% by weight or less.

Manganese (Mn): 1.0% by Weight or Less Mn is an austenite phase stabilizing element and improves N solid solubility. However, if the Mn content is too high, inclusions such as MnS may be formed to reduce corrosion resistance. Accordingly, it is preferable to limit the upper limit of the Mn content to 1.0% by weight or less.

Chromium (Cr): 18-24% by weight
Cr is a typical element for improving corrosion resistance of stainless steel, and in the present invention, Cr may be added in an amount of 18% by weight or more in order to ensure high corrosion resistance with a PREW-Mn value of 40 or more. However, Cr is a ferrite phase stabilizing element. If the Cr content is too high, the ferrite fraction increases, deteriorating hot workability, and promoting the formation of σ phase, degrading mechanical properties and corrosion resistance. sell. Considering this, it is preferable to limit the upper limit of the Cr content to 24% by weight or less.

Nickel (Ni): 16-24% by weight
Ni is the strongest austenite phase stabilizing element, and to maintain the austenite phase, Ni may be added in an amount of 16% by weight or more. However, if the Ni content increases, the cost of raw materials rises, so it is preferable to limit the upper limit of the Ni content to 24% by weight or less.

Molybdenum (Mo): 5.0 to 7.0% by weight
Mo is a ferrite phase stabilizing element and improves corrosion resistance. In order to ensure high corrosion resistance with a PREW-Mn value of 40 or more in the present invention, Mo may be added in an amount of 5.0% by weight or more. Mo is a useful element in terms of mechanical properties and corrosion resistance in an annealed state, and is a representative element that generates a σ phase during processes such as aging heat treatment, hot rolling, and welding. Therefore, if the Mo content is too high, the formation of the σ phase may be promoted and the mechanical properties and corrosion resistance may be degraded.

Copper (Cu): 0.1 to 2.0% by weight
Cu is an austenite phase stabilizing element, suppresses phase transformation to martensite phase during cold deformation, and improves corrosion resistance in a sulfuric acid atmosphere. Therefore, Cu may be added in an amount of 0.1% by weight or more. However, if the Cu content is too high, it reduces the pitting corrosion resistance in a chlorine atmosphere and degrades the hot workability. Accordingly, it is preferable to limit the upper limit of the Cu content to 2.0% by weight or less.

Tungsten (W): 1.0% by weight or less W is a ferrite phase stabilizing element and improves corrosion resistance. Moreover, W is known as an element effective in suppressing the formation of the σ phase by preventing the diffusion of Cr and Mo at high temperatures because of its large atomic radius. However, high-alloy austenitic stainless steel preferably has a composition within the standard range, and when W is added in a large amount, it promotes the precipitation of intermetallic compounds such as the chi (χ) phase and lowers corrosion resistance and impact toughness. , there is a risk of impairing the hot workability. Accordingly, it is preferable to limit the upper limit of the W content to 1.0% by weight or less.

Nitrogen (N): 0.18 to 0.3% by weight
N is an austenite phase stabilizing element and improves corrosion resistance in a chlorine atmosphere. Therefore, 0.18% by weight or more of N may be added for the purpose of improving corrosion resistance. However, if the N content is too high, the hot workability is deteriorated, so it is preferable to limit the upper limit of the N content to 0.3% by weight or less.

Aluminum (Al): 0.02 to 0.1% by weight
Al is an element that acts as a strong deoxidizing agent, and by combining with oxygen to form slag, Al can remove oxygen in molten steel and improve the hot workability of steel. Considering this, Al may be added in an amount of 0.02 wt% or more. However, if the Al content is too high, non-metallic inclusions are formed to deteriorate the cleanliness of the steel, and the formation of AlN causes deterioration of the material such as reduction in impact toughness. Therefore, it is preferable to limit the upper limit of the Al content to 0.1% by weight or less.

Oxygen (O): 0.01% by weight or less O is an element that segregates at grain boundaries and reduces the hot workability of steel. Accordingly, it is preferable to reduce the O content as much as possible, and the upper limit of the O content is controlled to 0.01% by weight or less. In order to ensure better hot workability, the O content may be more preferably controlled to 0.0035% by weight or less.

Calcium (Ca): 0.002 to 0.01% by weight
Ca is an element that acts as a deoxidizing agent. Ca combines with S in molten steel to form a stable CaS compound, thereby suppressing the tendency of S to segregate in the grain system and improving the hot workability of the steel. can be improved. Taking this into account, Ca may be added in an amount of 0.002% by weight or more. However, if the Ca content is too high, it may form non-metallic inclusions and degrade the cleanliness of the steel. Accordingly, it is preferable to limit the upper limit of the Ca content to 0.01% by weight or less. More preferably, the upper limit of the Ca content may be limited to 0.0045% by weight or less in order to improve the cleanliness of the steel.

Yellow (S): less than 0.001% by weight S is an element that segregates at grain boundaries and reduces the hot workability of steel. Accordingly, it is preferable to control the upper limit of the S content to less than 0.001% by weight.

The remaining component of the present invention is iron (Fe). However, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during normal manufacturing processes, and this cannot be excluded. The full content of the impurities is not specifically mentioned herein because any person skilled in the normal manufacturing process can know the impurities.

The austenitic stainless steel according to the present invention has excellent corrosion resistance and can be applied as a material for industrial equipment such as desulfurization equipment, heat exchangers, freshwater equipment, and food and beverage equipment. Hereinafter, technical means for ensuring the corrosion resistance of steel in the present invention will be described in detail.

Corrosion resistance of austenitic stainless steel is generally indirectly expressed by a Pitting Resistance Equivalent Number (PREN). The pitting resistance index (PREN) is expressed by the following formula using the contents of Cr, Mo, and N components, which are elements that affect corrosion resistance. In the following formula, each alloying element means weight percent of the element.

PREN = Cr + 3.3 x Mo + 16 x N

However, W is also an element that improves the corrosion resistance of austenitic stainless steel, and Mn is an element that forms water-soluble inclusions and adversely affects corrosion resistance. exist. Accordingly, the present invention modifies the PREN formula to PREW-Mn, which is expressed by the following formula, considering all the effects of W and Mn. In the following formula, each alloying element means weight percent of the element.

PREW−Mn=Cr+3.3×(Mo+0.5×W)+16×N−0.5×Mn

In order to ensure sufficient corrosion resistance of steel in an environment containing a large amount of salt such as seawater or an extremely corrosive environment containing acidic substances, the value of PREW-Mn is 40 or more and 50 or less. good. If the PREW-Mn value is less than 40, sufficient corrosion resistance cannot be ensured, so it cannot withstand a corrosive environment for a long time. It may be precipitated in the matrix structure and rather deteriorate the corrosion resistance. As a result of controlling the PREW-Mn value to 40 or more and 50 or less, the critical pitting corrosion temperature of the austenitic stainless steel according to an example of the present invention may be 80° C. or more.

Also, the austenitic stainless steel according to the present invention is excellent in impact toughness. Hereinafter, technical means for ensuring the impact toughness of steel in the present invention will be described in detail.

The impact toughness of steel may be determined by intermetallic compounds. The intermetallic compound is a σ phase containing mainly Cr, Mo, etc., and the σ phase precipitates in the matrix structure, deteriorating corrosion resistance, impact toughness and hot workability. Since the formation of the σ phase is promoted as the content of alloying components such as Cr and Mo increases, it is necessary to appropriately control the alloying components so as to suppress the formation of the σ phase.

When steel is subjected to solution heat treatment at a high temperature, the σ phase is decomposed while elements such as Cr and Mo in the σ phase diffuse into the matrix structure. Normally, the solution heat treatment temperature of 316 series Mo-added highly corrosion-resistant austenitic stainless steel is 1,100° C. or higher. , 100° C. or higher. However, solution heat treatment at an excessively high temperature for a long time affects the heat treatment equipment, so the solution heat treatment temperature is limited to 1,200° C. or less.

Since the formation and decomposition of the σ phase are affected by the alloy composition and solution heat treatment temperature, in order to suppress the σ phase, which lowers the impact toughness, the solution heat treatment conditions and alloy composition must be appropriately controlled. not. In the present invention, impact toughness is ensured by setting the value of impact toughness (CNV _TH ) represented by the following formula, which is a function of alloy composition and solution heat treatment temperature, to 80 or more. The CNV _TH value corresponds to the theoretical value of impact toughness according to the invention. In CNV _TH below, Tσ is the temperature at which the sigma ( _σ ) phase is completely decomposed thermodynamically, and T is the actual solution heat treatment temperature. In the formula CNV _TH below, each alloying element means the weight percent of that element, and T has a value between 1,100 and 1,200.degree.

CNV _TH = 336-1432 x C-22.1 x Si + 64.1 x Mn + 8.5 x Cr + 0.11 x Ni-10.1 x Mo-3.3 x Cu + 22.1 x W-392 x N-293 x ( _Tσ /T)

According to the present invention, as a result of controlling the value of the formula CNV _TH to be 80 or more, the σ phase can be suppressed. For example, the austenitic stainless steel according to the present invention has a σ phase area ratio of 1.0% measured in an area of 26 mm ² at a magnification of 50 times in a region from the surface of the sample to a depth of 1/4 to 3/4 in thickness. It may be below.

Also, the austenitic stainless steel according to the present invention is excellent in hot workability. Hereinafter, technical means for ensuring the hot workability of steel in the present invention will be described in detail.

In order to secure the corrosion resistance of austenitic stainless steel, a large amount of alloying elements such as Cr, Mo and N must be added. When the contents of elements such as Cr, Mo, and N become high, the grain boundaries during hot working become embrittled due to segregation of impurities at grain boundaries, resulting in deterioration of hot workability. Therefore, in order to ensure corrosion resistance and hot workability, alloying elements such as Cr, Mo and N are added and impurities segregated at grain boundaries are minimized so that the grain boundaries during hot working are kept. It is important to avoid embrittlement.

Impurities that segregate at grain boundaries of austenitic stainless steel are typically oxygen (O) and sulfur (S). In the present invention, trace elements are controlled to minimize impurities such as oxygen and sulfur that segregate at grain boundaries, thereby ensuring excellent hot workability.

In order to reduce the oxygen content in steel, the deoxidizing process is important, and Al may be used as the main deoxidizing agent. Al combines with oxygen to form slag, thereby removing oxygen from molten steel and improving the hot workability of the steel. However, if the Al content is too high, non-metallic inclusions may be formed to lower the cleanliness of the steel, and the formation of AlN may reduce the impact toughness of the steel. Considering this, in the present invention, the change in oxygen content due to the addition of Al is indexed by O/Al, and the value of O/Al is controlled to 0.01 or more and 0.12 or less.

In addition, in the present invention, in order to reduce the sulfur content in the steel, Ca is added which combines with sulfur in the molten steel to form a stable CaS compound. Ca can form a CaS compound to suppress the tendency of sulfur to segregate at grain boundaries and improve the hot workability of steel. However, if the Ca content is too high, it may form non-metallic inclusions and degrade the cleanliness of the steel. Considering this, in the present invention, the change in sulfur content due to Ca addition may be indexed by S/Ca, and the value of S/Ca may be controlled to 0.01 or more and 0.4 or less.

In the present invention, O/Al: 0.01 to 0.12, S/Ca: 0.01 to 0.4 are controlled so that cracks do not occur on the surface or edge of the steel during hot working. prevent it from occurring.

According to the present invention, the PREW-Mn value is controlled to be 40 or more and 50 or less to ensure high corrosion resistance, and the alloy composition and heat treatment conditions are controlled so that the impact toughness (CNV _TH ) value is 80 or more. and ensure excellent impact toughness, and control trace elements to satisfy O/Al: 0.01 to 0.12 and S/Ca: 0.01 to 0.4 to ensure excellent hot workability. do.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are for the purpose of illustrating and describing the present invention in more detail, and are not intended to limit the scope of rights of the present invention. This is because the scope of rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

[Example]
Steel having the chemical composition shown in Table 1 below is melted in a vacuum induction melting furnace, hot rolled, and then solution heat treated in the range of 1,100 to 1,200 ° C. to obtain a heat of 5 mm in thickness. Inter-rolled plate material was produced.

Table 2 shows the PREW-Mn value and critical pitting temperature (CPT), T _σ , T, O/Al, S/Ca, surface cracks, σ phase area ratio, impact toughness (CNV _TH , CNV _EX ) values are shown, respectively.

The PREW-Mn values in Table 2 are derived by substituting the content (% by weight) of each alloying element in Table 1 into the following formula.

PREW−Mn=Cr+3.3×(Mo+0.5×W)+16×N−0.5×Mn

The critical pitting temperature (CPT) in Table 2 is measured by surface CPT according to the ASTM G150 method, and the higher the temperature, the better the corrosion resistance. Super austenitic stainless steel, which has the best corrosion resistance among austenitic stainless steels, has a critical pitting corrosion temperature of 80°C or higher when measured by the above method. If the critical pitting corrosion temperature was 80° C. or higher, it was determined that sufficient corrosion resistance was ensured.

T _σ in Table 2 is the temperature at which the sigma (σ) phase is completely decomposed thermodynamically, and T is the actual solution heat treatment temperature for each example.

O/Al and S/Ca in Table 2 are derived by substituting the content (% by weight) of each alloying element in Table 1.

The surface cracks in Table 2 are "Good" when cracks with a length of 5 mm or more are observed at a frequency of less than 5 on the surface of an area of 150 mm × 250 mm, and "Good" when they are observed at a frequency of 5 or more. Bad”.

The σ phase area ratio in Table 2 was obtained by mirror-polishing the cross section of the steel after the final annealing heat treatment with a diamond paste of 1 μm size and etching it with a NaOH solution so that the σ phase and the matrix structure could be separated. After preparing the specimen prepared as described above, continuously measure 10 fields of view with an area of 26 mm ² at 50 times magnification in the area from the surface to the depth of 1/4 to 3/4 of the thickness. calculated by

The CNV _TH value in Table 2 is a theoretical value of impact toughness according to the present invention, and the CNV _TH value was derived by substituting the weight % value of each alloy component, T _σ , and T value into the following formula. Derived CNV _TH values are shown to two significant figures.

CNV _TH = 336-1432 x C-22.1 x Si + 64.1 x Mn + 8.5 x Cr + 0.11 x Ni-10.1 x Mo-3.3 x Cu + 22.1 x W-392 x N-293 x ( _Tσ /T)

The CNV _EX values in Table 2 are experimental values of the Charpy notch impact toughness results, and the notch impact toughness was measured at room temperature of 25° C. after processing the test pieces to a thickness of 4 mm.

Comparing the CNV _TH value and the CNV _EX value in Table 2, the experimental value and the theoretical value of impact toughness are almost similar without deviation, and the actual value of impact toughness can be calculated by the formula of CNV _TH proposed in the present invention. It can be seen that it can be derived accurately without large errors.

Hereinafter, each invention example and comparative example are comparatively evaluated with reference to Tables 1 and 2.

Inventive Examples 1 to 8 satisfied the range of alloy composition defined by the present invention. In addition, invention examples 1 to 8 had a PREW-Mn value of 40 or more and 50 or less, and the critical pitting corrosion temperature exceeded 80°C, ensuring high corrosion resistance. In invention examples 1 to 8, the alloy composition and heat treatment conditions are controlled so that the σ area ratio is 1.0% or less and the CNV _TH value is 80 or more, and the impact toughness (CNV _EX ) value is 80 J or more. I was able to secure toughness. In invention examples 1 to 8, trace elements are controlled to satisfy O/Al: 0.01 to 0.12 and S/Ca: 0.01 to 0.4, and surface cracks do not occur during hot working. It was possible to ensure good hot workability.

On the other hand, in Comparative Examples 1 and 2, the Si content exceeded 1.0% by weight, which is the upper limit of the Si content defined in the present invention. As a result, the precipitation of intermetallic compounds such as the σ phase was promoted, the σ area ratio exceeded 1.0%, and the impact toughness value was about 32 J, which was inferior to the invention examples.

In Comparative Example 3, the Cr and Mo contents did not reach the lower limits of the Cr and Mo contents defined in the present invention, the PREW-Mn value was less than 40, and the critical pitting corrosion temperature did not reach 80°C, so sufficient corrosion resistance was obtained. could not be secured.

In Comparative Example 4, the Cr and Mo contents exceed the upper limits of the Cr and Mo contents defined in the present invention, and the PREW-Mn value exceeds 50, and the σ phase, which is an intermetallic compound due to the excessive Cr and Mo contents, is the base. Precipitated in the structure, rather the corrosion resistance decreased. Referring to Table 2, the σ area ratio exceeded 1.0%, and as a result, the corrosion resistance decreased and the impact toughness value was 35J, which was inferior to the invention examples.

In Comparative Examples 5 and 6, the Al and Ca contents did not reach the lower limits of the Al and Ca contents defined in the present invention, the oxygen and sulfur contents were relatively high, and the O/Al and S/Ca contents defined in the present invention The upper limit of the value was exceeded, and as a result, surface cracks occurred during hot working, and the hot workability was inferior to that of the invention examples.

In Comparative Example 7, the Al and Ca contents are within the limits of the Al and Ca contents defined in the present invention. However, Comparative Example 7 exceeds the upper limits of the O/Al and S/Ca values stipulated in the present invention, and as a result, surface cracks occur during hot working, and the hot workability is inferior to the invention examples. rice field.

Also, the above results can be visually confirmed from FIGS. 1 and 2 of the present invention. FIG. 1 is a graph showing critical pitting temperature (CPT) according to changes in PREW-Mn in each example. FIG. 2 is a graph showing the S/Ca and O/Al values of each example. The shaded area in each drawing corresponds to the target area of the present invention.

Referring to FIG. 1, when the PREW-Mn value does not correspond to 40 or more and 50 or less, which is the object of the present invention, when the critical pitting temperature (CPT) does not reach 80 ° C., or when the critical pitting temperature (CPT) is Even when the temperature exceeded 100° C. (Comparative Example 4), the σ phase, which is an intermetallic compound due to excessive Cr and Mo contents, was precipitated in the matrix structure, and rather the corrosion resistance was lowered.

Referring to FIG. 2, when the S/Ca and O/Al values were out of the ranges targeted by the present invention (Comparative Examples 5, 6, and 7), surface cracks occurred during hot working. . In particular, in the case of Comparative Example 7, the Al and Ca contents are within the ranges defined by the present invention, or as shown in FIG. , surface cracks occurred during hot working.

From the results of the above-described examples, the PREW-Mn value is controlled to be 40 or more and 50 or less within the alloy composition defined by the present invention to ensure high corrosion resistance, and the impact toughness (CNV _TH ) value is 80 or more. Ensure excellent impact toughness by controlling the alloy composition and heat treatment conditions so that the It can be seen that control was performed to ensure excellent hot workability.

While illustrative embodiments of the invention have been described above, the invention is not so limited and those of ordinary skill in the art will appreciate the concept and scope of the claims set forth below. Various modifications and variations are possible without departing from the scope of the invention.

The austenitic stainless steel according to the present invention is suitable as a material for various industrial facilities such as desulfurization facilities, heat exchangers, fresh water facilities, food and beverage facilities.

Claims (5)

% by weight, C: 0.03% or less (excluding 0), Si: 1.0% or less, Mn: 1.0% or less, Cr: 18-24%, Ni: 16-24%, Mo: 5 .0-7.0%, Cu: 0.1-2.0%, W: 1.0% or less, N: 0.18-0.3%, Al: 0.02-0.1%, O : 0.01% or less, Ca: 0.002 to 0.01%, S: less than 0.001%, the remaining Fe and inevitable impurities, O / Al: 0.01 to 0.12, S / A highly corrosion-resistant austenitic stainless steel having excellent impact toughness and hot workability, characterized by satisfying Ca: 0.01 to 0.4. The highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability according to claim 1, characterized in that the impact toughness (CNV _TH ) value represented by the following formula (1) is 80 or more.
(1) CNV _TH = 336-1432 x C-22.1 x Si + 64.1 x Mn + 8.5 x Cr + 0.11 x Ni-10.1 x Mo-3.3 x Cu + 22.1 x W-392 x N- 293×( _Tσ /T)
(In the above formula (1), C, Si, Mn, Cr, Ni, Mo, Cu, W, and N mean the weight percent of each alloying element, and T _σ is the thermodynamic sigma (σ) phase. means the temperature at which is completely decomposed and T means the actual solution heat treatment temperature). A highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability according to claim 1, characterized in that the PREW-Mn value represented by the following formula (2) is 40 or more and 50 or less.
(2) PREW−Mn=Cr+3.3×(Mo+0.5×W)+16×N−0.5×Mn
(In the formula (2), Cr, Mo, W, N, and Mn mean weight percent of each alloying element). Claim 1, characterized in that the area ratio of the σ phase measured in an area of 26 mm ² at a magnification of 50 times in a region from the surface to the depth of 1/4 to 3/4 of the thickness is 1.0% or less. 2. A highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability according to . A highly corrosion-resistant austenitic stainless steel excellent in impact toughness and hot workability according to claim 1, characterized by having a critical pitting corrosion temperature of 80°C or higher.

Images