KR20210028382A - High corrosion resistant austenitic stainless steel with excellent impact toughness and hot workability - Google Patents

Abstract

Disclosed in the present specification is a highly corrosion-resistant austenitic stainless steel having excellent impact toughness and hot workability. According to one embodiment of the disclosed highly corrosion-resistant austenitic stainless steel, the highly corrosion-resistant austenitic stainless steel comprises, in wt%, 0.03% or less (excluding 0) of C, 1.0% or less of Si, 1.0% or less of Mn, 18 to 24% of Cr, 16 to 24% of Ni, 5.0 to 7.0% of Mo, 0.1 to 2.0% of Cu, 1.0% or less of W, 0.18 to 0.3% of N, 0.02 to 0.1% of Al, 0.01% or less of O, 0.002 to 0.01% of Ca, less than 0.001% of S, and a balance of Fe and inevitable impurities, and satisfies an O/Al of 0.01 to 0.12, and a S/Ca of 0.01 to 0.4.

Description

{High corrosion resistant austenitic stainless steel with excellent impact toughness and hot workability}

The present invention relates to a high corrosion resistance austenitic stainless steel excellent in impact toughness and hot workability. The austenitic stainless steel according to the present invention can be applied as a material for industrial facilities such as desulfurization facilities, heat exchangers, desalination facilities, food and beverage facilities.

Austenitic stainless steel is used in a wide range because it has excellent corrosion resistance, workability and weldability. STS 316 stainless steel with improved corrosion resistance by adding 2% of Mo to STS 304 stainless steel represented by the composition of 18Cr-8Ni is applied to various fields such as kitchens, home appliances, and industrial facilities.

Corrosion resistance of an austenitic stainless steel can be ensured by adding elements such as Cr, Mo, and N. However, when the content of elements such as Cr, Mo, and N to be added is increased, intermetallic compounds such as σ phase are precipitated in the matrix structure, thereby reducing corrosion resistance and impact toughness, and remarkably reducing hot workability.

In order to solve this problem, Patent Documents 1 and 2 disclose a technique of suppressing formation of a sigma phase by adding tungsten (W) instead of Mo. However, since the high-alloy austenitic stainless steel should generally have a component within the standard range, it is not preferable to add W instead of Mo. In addition, when a large amount of W is contained, there is a concern that another intermetallic compound such as a chi (χ) phase may be precipitated.

In Patent Document 3, the σ phase is controlled by adjusting the component so that the value of the sigma (σ) equivalent (SGR) represented by the following equation is 18 or less. However, Patent Document 3 considers only Cr, Mo, N, and Mn Cu as alloy elements that affect the control of the σ phase, and there is a problem that intermetallic compounds such as the σ phase still precipitate in the matrix structure.

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

0001) Korean Patent Application Publication No. 10-2001-0038199 (Publication date: April 0, 2001) 0002) Korean Patent Application Publication No. 10-1999-0005962 (Publication date: September 15, 2000) US Patent Publication No. 2015-0050180 (published date: February 19, 2015)

In order to solve the above-described problems, the present invention is to provide a highly corrosion-resistant austenitic stainless steel having excellent corrosion resistance and impact toughness while also having excellent hot workability.

As a means for achieving the above object, the highly corrosion-resistant austenitic stainless steel having excellent impact toughness and hot workability according to an example of the present invention is weight %, C: 0.03% or less (excluding 0), Si: 1.0% or less , Mn: 1.0% or less, Cr: 18 to 24%, Ni: 16 to 24%, Mo: 5.0 to 7.0%, Cu: 0.1 to 2.0%, W: 1.0% or less, 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%, containing the remaining Fe and unavoidable impurities, O/Al: 0.01 to 0.12, S/Ca: 0.01 to 0.4 Is satisfied.

In each of the high corrosion resistance austenitic stainless steels having excellent impact toughness and hot workability of the present invention, the impact toughness (CNV _TH ) value expressed by the following formula (1) may be 80 or more.

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

In the above formula (1), C, Si, Mn, Cr, Ni, Mo, Cu, W, and N mean the weight percent of each alloy element, and T _σ is the temperature at which the sigma (σ) phase is completely decomposed thermodynamically. Means, and T means the actual solution heat treatment temperature.

In each of the high corrosion resistance austenitic stainless steels having excellent 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 alloy element.

In each of the high corrosion resistance austenitic stainless steels having excellent impact toughness and hot workability of the present invention, the σ phase measured in an area ^{of 26 mm 2 at a magnification of 50 times from the surface to the depth of 1/4 to 3/4 of the thickness} The area ratio may be 1.0% or less.

In each of the high corrosion resistance austenitic stainless steels having excellent impact toughness and hot workability of the present invention, the critical pitting temperature may be 80°C or higher.

According to the present invention, it is possible to provide a highly corrosion-resistant austenitic stainless steel that can be applied as a material for industrial facilities such as desulfurization facilities, heat exchangers, desalination facilities, food and beverage facilities, and has excellent corrosion resistance and impact toughness and excellent hot workability. .

In the alloy component defined by the present invention, the PREW-Mn value is controlled to be 40 or more and 50 or less, and the formation of intermetallic compounds is suppressed to ensure high corrosion resistance, _{and the alloy component and the impact toughness (CNV TH} ) value are 80 or more. Excellent impact toughness is secured by controlling the heat treatment conditions, and excellent hot workability can be secured by controlling trace elements to satisfy O/Al: 0.01 to 0.12 and S/Ca: 0.01 to 0.4.

1 is a graph showing the critical formula temperature (CPT) according to the change of PREW-Mn in each example.
2 is a graph showing the S/Ca and O/Al values of each example.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, embodiments of the present invention are provided in order to more completely explain the present invention to those with average knowledge in the art.

The terms used in the present application are only used to describe specific examples. So, for example, a singular expression includes a plural expression unless the context clearly has to be singular. In addition, terms such as "include" or "include" used in the present application are used to clearly refer to the existence of features, steps, functions, components or combinations thereof described in the specification, but other features It should be noted that it is not used to preliminarily exclude the presence of elements, steps, functions, components, or combinations thereof.

Meanwhile, unless otherwise defined, all terms used in the present specification should be viewed as having the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Therefore, unless clearly defined in the specification, a specific term should not be interpreted as an excessively ideal or formal meaning. For example, in the present specification, expressions in the singular include plural expressions unless the context clearly has exceptions.

In addition, "about", "substantially" and the like in the present specification are used at or close to the numerical value when manufacturing and material tolerances specific to the stated meaning are presented, and are accurate to aid understanding of the present invention. Or absolute figures are used to prevent unreasonable use of the stated disclosure by unconscionable infringers.

The high corrosion resistance austenitic stainless steel having excellent impact toughness and hot workability according to an example of the present invention is weight %, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr: 18 to 24% , Ni: 16 to 24%, Mo: 5 to 7%, Cu: 0.1 to 2.0%, W: 1.0% or less, 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%, may contain the remaining Fe and unavoidable impurities.

Hereinafter, the reasons for limiting the alloy composition will be described in detail. All of the following component compositions refer to% by weight unless otherwise specified.

Carbon (C): 0.03% by weight or less (excluding 0)

C is a strong austenite phase stabilizing element, and its strength is increased by solid solution strengthening. However, when the C content is excessive, it is easily combined with a carbide-forming element such as Cr, which is effective in corrosion resistance at the austenite phase boundary, to form a carbide, and the formed carbide lowers the Cr content around the grain boundary to lower the 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 a ferrite phase stabilizing element, and is an element that improves corrosion resistance and serves as a deoxidizing agent. However, excessive Si content promotes precipitation of intermetallic compounds such as σ phase, thereby lowering mechanical properties and corrosion resistance related to impact toughness, and may cause 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 safety element and improves the solubility of N. However, when the Mn content is excessive, inclusions such as MnS may be formed, thereby reducing corrosion resistance. Accordingly, it is preferable to limit the upper limit of the Mn content to 1.0% by weight or less.

Chrome (Cr): 18 to 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 secure high corrosion resistance in which the PREW-Mn value is 40 or more. However, Cr is a ferrite phase stabilizing element, and when the Cr content is excessive, the ferrite fraction increases to lower the hot workability, and the formation of the sigma phase is promoted, thereby reducing mechanical properties and corrosion resistance. In consideration of this, it is preferable to limit the upper limit of the Cr content to 24% by weight or less.

Nickel (Ni): 16 to 24% by weight

Ni is the most powerful austenite phase stabilizing element, and in order to maintain the austenite phase, Ni may be added in an amount of 16% by weight or more. However, as the Ni content increases, the raw material price increases, 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 the present invention, in order to secure high corrosion resistance in which the value of PREW-Mn is 40 or more, Mo may be added in an amount of 5.0% by weight or more. Mo is an element useful in terms of mechanical properties and corrosion resistance in an annealed state, but is a representative element that generates a σ phase during processes such as aging heat treatment, hot rolling, or welding. Accordingly, when the Mo content is excessive, sigma-phase formation is promoted and mechanical properties and corrosion resistance may be deteriorated. Therefore, it is preferable to limit the upper limit of the Mo content to 7.0% by weight or less.

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. For this, Cu may be added in an amount of 0.1% by weight or more. However, if the Cu content is excessive, pitting resistance in a chlorine atmosphere decreases and hot workability decreases. 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. In addition, since W has a large atomic radius, it is known to be an effective element in inhibiting the formation of a σ phase by interfering with the diffusion of Cr and Mo at high temperatures. However, it is preferable that the high alloy austenitic stainless steel has a component within the standard range, and when a large amount of W is added, it promotes the precipitation of intermetallic compounds such as chi (χ) phase, thereby reducing corrosion resistance and impact toughness, and hot workability. There is a risk of hindering. 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, for the purpose of improving corrosion resistance, N may be added in an amount of 0.18% by weight or more. However, if the N content is excessive, 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, it can remove oxygen from molten steel and improve the hot workability of steel. In consideration of this, Al may be added in an amount of 0.02% by weight or more. However, if the Al content is excessive, non-metallic inclusions may be formed, thereby lowering the cleanliness of the steel and causing deterioration of the material such as lower impact toughness due to the formation of AlN. Accordingly, 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 lowers the hot workability of steel. Accordingly, it is preferable that the O content be as low as possible, and the upper limit of the O content can be controlled to 0.01% by weight or less. In order to secure more excellent 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, and it combines with S in molten steel to form a stable CaS compound, thereby suppressing the tendency of S to segregate at grain boundaries, thereby improving the hot workability of steel. In consideration of this, Ca may be added in an amount of 0.002% by weight or more. However, if the Ca content is excessive, there is a concern that non-metallic inclusions are formed and the cleanliness of the steel is reduced. Accordingly, it is preferable to limit the upper limit of the Ca content to 0.01% by weight or less. In order to increase the cleanliness of the steel, more preferably, the upper limit of the Ca content may be limited to 0.0045% by weight or less.

Sulfur (S): less than 0.001% by weight

S is an element that segregates at grain boundaries and lowers 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, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a typical manufacturing process, this cannot be excluded. Since the impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.

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

In general, the corrosion resistance of austenitic stainless steel is indirectly expressed by the Pitting Resistance Equivalent Number (PREN). The pitting resistance equivalent 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 alloy element means a weight percent of the corresponding element.

PREN = Cr + 3.3*Mo + 16*N

However, W is also an element that improves the corrosion resistance of austenitic stainless steel, and Mn is an element that adversely affects corrosion resistance by forming water-soluble inclusions, so there is a limit to expressing the corrosion resistance by the above PREN equation. Accordingly, in the present invention, in consideration of both the effects of W and Mn, the above PREN equation is modified as PREW-Mn expressed by the following equation. In the following formula, each alloy element means a weight percent of the corresponding element.

PREW-Mn = Cr + 3.3*(Mo + 0.5*W) + 16*N-0.5*Mn

In order to secure sufficient corrosion resistance of steel in an environment containing a large amount of salt such as seawater or an extremely corrosive environment including an acidic substance, the value of the PREW-Mn may be 40 or more and 50 or less. If the PREW-Mn value is less than 40, sufficient corrosion resistance cannot be secured, so it cannot withstand a long time in a corrosive environment. If it exceeds 50, σ phase, an intermetallic compound due to a large amount of Cr, Mo, W content, is precipitated in the matrix structure. As a result, there is a concern that the corrosion resistance may be lowered. As a result of controlling the PREW-Mn value to 40 or more and 50 or less, the critical formula temperature of the austenitic stainless steel according to an example of the present invention may be 80°C or more.

In addition, the austenitic stainless steel according to the present invention has excellent impact toughness. Hereinafter, the technical means for securing the impact toughness of the steel in the present invention will be described in detail.

The impact toughness of steel can be determined by intermetallic compounds. The intermetallic compound is mainly a σ phase containing Cr, Mo, etc., and the σ phase precipitates in the matrix structure, reducing corrosion resistance, impact toughness, and hot workability. The higher the content of the alloy components such as Cr and Mo, the more the formation of the σ phase is promoted. Therefore, it is necessary to control the appropriate alloy components so as to suppress the formation of the σ phase.

In addition, when the steel is subjected to solution heat treatment at a high temperature, the σ phase is decomposed as elements such as Cr and Mo in the σ phase diffuse into the matrix structure. Typically, the solution heat treatment temperature of the high corrosion resistance austenitic stainless steel containing 316-based Mo is 1,100°C or higher, and thus the solution heat treatment temperature in the present invention may be 1,100°C or higher in order to decompose the σ phase in consideration of this. However, since excessive high temperature and long-term solution heat treatment affects heat treatment facilities, the solution heat treatment temperature is limited to 1,200°C or less.

Since the formation and decomposition of the σ phase is affected by the alloy component and the solution heat treatment temperature, the solution heat treatment condition and the alloy component must be properly controlled in order to suppress the σ phase that lowers the impact toughness. In the present invention, the impact toughness can be secured by setting the value of _{the impact toughness (CNV TH} ) expressed by the following equation as a function of the alloy component and the solution heat treatment temperature to be 80 or more. The CNV _TH value corresponds to the theoretical value of impact toughness according to the present invention. In the following CNV _TH , T _σ is the temperature at which the sigma (σ) phase is completely decomposed thermodynamically, and T is the actual solution heat treatment temperature. In the following formula CNV _TH , each alloying element refers to the weight% of the corresponding element, and T has a value between 1,100 and 1,200°C.

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

According to the present invention, _{as a result of controlling the value of the formula CNV TH} to be 80 or more, the sigma phase can be suppressed. For example, the austenitic stainless steel according to the present invention may have a σ phase area ratio of 1.0% or less, measured in an area ^{of 26 mm 2} at a magnification of 50 times in an area from the surface of the specimen to a depth of 1/4 to 3/4 of the thickness. have.

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

In order to secure corrosion resistance of austenitic stainless steel, inevitably a large amount of alloying elements such as Cr, Mo, and N must be added. When the content of elements such as Cr, Mo, and N increases, the grain boundaries become brittle during hot working due to impurities segregating at the grain boundaries, and hot workability deteriorates. Therefore, in order to secure corrosion resistance and at the same time secure hot workability, it is important to add alloying elements such as Cr, Mo, and N while minimizing impurities segregating at grain boundaries so that grain boundaries are not embrittled during hot working.

The impurities segregating at the grain boundaries of austenitic stainless steel are typically oxygen (O) and sulfur (S). In the present invention, excellent hot workability is secured by controlling trace elements to minimize impurities such as oxygen and sulfur segregated at grain boundaries.

In order to lower the oxygen content in the steel, the deoxidation process is important, and Al can be used as the main deoxidizer. Al combines with oxygen to form slag, thereby removing oxygen from molten steel and improving the hot workability of steel. However, if the Al content is excessive, non-metallic inclusions are formed to reduce the cleanliness of the steel, and there is a concern that the impact toughness of the steel may decrease due to the formation of AlN. In consideration of this, in the present invention, the change in oxygen content due to the addition of Al is indexed to O/Al, and the value of O/Al can be controlled to be 0.01 or more and 0.12 or less.

In addition, in the present invention, Ca, which combines with sulfur in molten steel to form a stable CaS compound, is added to lower the sulfur content in steel. Ca can improve the hot workability of steel by forming a CaS compound to suppress the tendency of sulfur to segregate at grain boundaries. However, if the Ca content is excessive, there is a concern that non-metallic inclusions are formed and the cleanliness of the steel is reduced. In consideration of this, in the present invention, the change in sulfur content due to the addition of Ca is indexed into S/Ca, and the value of S/Ca can be controlled to be 0.01 or more and 0.4 or less.

The present invention is controlled to satisfy O/Al: 0.01 to 0.12 and S/Ca: 0.01 to 0.4 so that cracks do not occur on the surface or edge of the steel during hot working.

According to the present invention, high corrosion resistance is secured by controlling the PREW-Mn value to be 40 or more and 50 or less, and the alloy components and heat treatment conditions are controlled so that the _{impact toughness (CNV TH) value is 80 or more to secure excellent impact toughness.} , O/Al: 0.01 to 0.12, S/Ca: By controlling the trace elements to satisfy 0.01 to 0.4, excellent hot workability can be secured.

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

{Example}

After melting the steel having the chemical composition shown in Table 1 below in a vacuum induction melting furnace, hot rolling was performed, and then solution heat treatment was performed in a range of 1,100 to 1,200°C to prepare a hot rolled plate having a thickness of 5 mm.

(* is outside the range defined by the present invention.)

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

The PREW-Mn value in Table 2 is derived by substituting the content (% by weight) of each alloy element in Table 1 in the following equation.

PREW-Mn = Cr + 3.3*(Mo + 0.5*W) + 16*N-0.5*Mn

The critical formula temperature (CPT) in Table 2 was measured for the surface part CPT according to the ASTM G150 method, and the higher the temperature, the better the corrosion resistance. Based on the fact that when the critical pitting temperature is measured by the above method for the super austenitic stainless steel having the best corrosion resistance among austenitic stainless steels, a value of 80°C or higher is obtained. It was determined to have secured.

_{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 in each example.

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

The surface cracks in Table 2 are indicated as'Good' when cracks of 5 mm or more in length are observed with a frequency of less than 5 on a surface of 150 mm x 250 mm, and'Bad' when observed with a frequency of 5 or more.

For the σ phase area ratio in Table 2, after the final annealing heat treatment, the cross section of the steel was mirror-polished with a diamond paste of 1 μm size, and then a specimen was prepared so that the σ phase and the matrix structure were separated by etching with NaOH solution, and then the specimen prepared as above. Was calculated by continuously measuring 10 fields of view in an area ^{of 26 mm 2} at a magnification of 50 times in the area from the surface to the depth of 1/4 to 3/4 of the thickness.

_{The CNV TH} value in Table 2 is the theoretical value of the impact toughness according to the present invention, and _{the CNV TH} value was derived by substituting the _{weight% values and T σ and T values of each alloy component in the following equation.} The derived CNV _TH value was expressed up to 2 significant figures.

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

_{The CNV EX} values in Table 2 are experimental values of the Charpy notch impact toughness results, and after processing the specimen to have a thickness of 4 mm, the notched impact toughness was measured at room temperature (25°C).

_{Comparing the CNV TH} value and CNV _EX value in Table 2, the experimental value and the theoretical value of impact toughness are almost similar without any deviation, so that the actual impact toughness value can be accurately derived without a large error using the equation of _{CNV TH proposed in the present invention.} You can see that you can.

(* is outside the range defined by the present invention.)

Hereinafter, with reference to Tables 1 and 2, each invention example and a comparative example will be compared and evaluated.

Inventive Examples 1 to 8 satisfied the range of alloy components defined by the present invention. In addition, Inventive Examples 1 to 8 had a PREW-Mn value of 40 or more and 50 or less, and a critical formula temperature exceeding 80° C. to ensure high corrosion resistance. Inventive Examples 1 to 8 were able to secure excellent impact toughness with _{an impact toughness (CNV EX} ) of 80J or more by controlling the alloy components and heat treatment conditions so that the σ area ratio is 1.0% or less and the CNV _{TH value is 80 or more.} Inventive Examples 1 to 8 were able to secure excellent hot workability without surface cracking during hot working by controlling trace elements to satisfy O/Al: 0.01 to 0.12 and S/Ca: 0.01 to 0.4.

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, precipitation of intermetallic compounds such as the σ phase was promoted, so that the σ area ratio exceeded 1.0%, and the impact toughness value was about 32 J, which was inferior to the invention example.

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

In Comparative Example 4, the Cr and Mo content exceeded the upper limit of the Cr and Mo content defined in the present invention, and the PREW-Mn value exceeded 50. It precipitated, and the corrosion resistance was rather lowered. Referring to Table 2, the σ area ratio exceeded 1.0%, and as a result, the corrosion resistance was lowered and the impact toughness value was 35J, which was inferior to the invention example.

In Comparative Examples 5 and 6, the content of Al and Ca was less than the lower limit of the Al and Ca content defined in the present invention, and thus the content of oxygen and sulfur was relatively high, and exceeded the upper limit of the O/Al and S/Ca values defined in the present invention. As a result, surface cracks occurred during hot working, and the hot workability was inferior to the invention example.

In Comparative Example 7, the content of Al and Ca is within the range of the content of Al and Ca defined in the present invention. However, Comparative Example 7 exceeded the upper limit of the O/Al and S/Ca values defined in the present invention, and as a result, surface cracks occurred during hot working, and the hot workability was inferior to the invention example.

In addition, the above results can be visually confirmed from FIGS. 1 and 2 of the present invention. 1 is a graph showing the critical formula temperature (CPT) according to the change of PREW-Mn in each example. 2 is a graph showing S/Ca and O/Al values of each example. A shaded area in each drawing corresponds to a range area targeted by the present invention.

Referring to FIG. 1, when the PREW-Mn value does not correspond to 40 or more and 50 or less for the purpose of the present invention, the critical formula temperature (CPT) does not reach 80°C, or the critical formula temperature (CPT) exceeds 100°C. Even if, due to excessive Cr and Mo content, the intermetallic compound σ phase, etc., precipitated in the matrix structure, resulting in lower corrosion resistance.

Referring to FIG. 2, when the S/Ca and O/Al values are out of the target range of the present invention, it can be seen that surface cracks have occurred during hot working. In particular, Comparative Example 7 is within the range of Al and Ca content defined by the present invention, but as shown in FIG. 2, the O/Al and S/Ca values of Comparative Example 7 are outside the present invention, and the surface during hot working Cracks occurred.

From the above-described example results, the PREW-Mn value is controlled to be 40 or more and 50 or less in the alloy component defined by the present invention to ensure high corrosion resistance, _{and the alloy component and the impact toughness (CNV TH} ) value are 80 or more. It can be seen that excellent impact toughness was secured by controlling the heat treatment conditions, and excellent hot workability was secured by controlling trace elements to satisfy O/Al: 0.01 to 0.12 and S/Ca: 0.01 to 0.4.

As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and those of ordinary skill in the art are within the scope of not departing from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (5)

In% by weight, C: 0.03% or less (excluding 0), Si: 1.0% or less, Mn: 1.0% or less, Cr: 18 to 24%, Ni: 16 to 24%, Mo: 5.0 to 7.0%, Cu: 0.1 To 2.0%, W: 1.0% or less, 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%, remaining Fe and unavoidable impurities Including, O/Al: 0.01 to 0.12, S/Ca: high corrosion resistance austenitic stainless steel having excellent impact toughness and hot workability satisfying 0.01 to 0.4. The method of claim 1,
High corrosion-resistant austenitic stainless steel with excellent impact toughness and hot workability with _{an impact toughness (CNV TH} ) of 80 or more represented by the following formula (1):
(1) CNV _TH = 336-1432*C-22.1*Si + 64.1*Mn + 8.5*Cr + 0.11*Ni -10.1*Mo-3.3*Cu + 22.1*W-392*N-293*(T _σ / T)
(In the above formula (1), C, Si, Mn, Cr, Ni, Mo, Cu, W, and N mean the weight percent of each alloy element, and T _σ is the temperature at which the sigma (σ) phase is completely decomposed thermodynamically. Means, and T means the actual solution heat treatment temperature). The method of claim 1,
High corrosion resistance austenitic stainless steel having excellent impact toughness and hot workability with a PREW-Mn value of 40 or more and 50 or less represented by the following formula (2):
(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 alloy element). The method of claim 1,
High corrosion resistance austenitic stainless steel with excellent impact toughness and hot workability with an area ratio of σ of less than 1.0%, measured in an area ^{of 26 mm 2} at a magnification of 50 times in the area from the surface to the depth of 1/4 to 3/4 of the thickness. The method of claim 1,
High corrosion resistance austenitic stainless steel with excellent impact toughness and hot workability with a critical pitting temperature of 80℃ or higher.

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