Title of invention: austenitic stainless steel with improved strength
Technical field
[One]
The present invention relates to an austenitic stainless steel, and in particular, to an austenitic stainless steel having improved strength while securing elongation and corrosion resistance.
Background
[2]
Stainless steel refers to steel that has strong corrosion resistance by suppressing corrosion, which is the weak point of carbon steel. In general, stainless steel is classified according to its chemical composition or metal structure. According to the metal structure, stainless steel can be classified into austenite-based, ferrite-based, martensite-based and dual phase-based.
[3]
Among them, austenitic stainless steel is a steel containing a large amount of chromium (Cr) and nickel (Ni), and is most commonly used. For example, in the case of 316L stainless steel, it is applied to various industrial fields by securing corrosion resistance and molding characteristics with a component system based on 16 to 18% Cr, 10 to 14% Ni, and 2 to 3% molybdenum (Mo). have.
[4]
However, in the case of Ni and Mo, there is a problem in terms of price competitiveness due to high material prices, and raw material supply and demand are unstable due to extreme fluctuations in material prices, and it is difficult to secure supply price stability.
[5]
Therefore, research has been conducted to reduce the content of Ni and Mo while securing corrosion resistance and formability comparable to that of conventional 316L stainless steel. As a substitute for such 316L stainless steel, 200 series stainless steel, for example, 216 steel, which reduced Ni and increased the content of Mn was developed.
[6]
216 stainless steel basically reduces the Ni content to a certain amount, lowering the material price, and at the same time adding a large amount of Mn by 7% or more to secure the stability of the austenite phase according to the Ni reduction, and is 17.5~22% by weight. It contains Cr, 5~7% Ni, 7.5~9% Mn and 2~3% Mo.
[7]
By this component-based design, 216 stainless steel can secure a level of corrosion resistance similar to that of 316L stainless steel, but due to the generation of a large amount of Mn fume during the steelmaking process by the addition of a large amount of Mn, improvement is not only required in terms of environment, but also When generating rigid inclusions (MnS), it causes a decrease in productivity in the manufacturing process and a decrease in the surface quality of the final material.
[8]
Meanwhile. Duplex stainless steel is a substitute for 316L stainless steel.
[9]
Duplex stainless steel is a stainless steel having a microstructure in which an austenite phase and a ferrite phase are mixed. Specifically, austenite phase and a ferrite phase exist in about 35 to 65% by volume, respectively, and are characteristic of austenitic stainless steel and ferritic stainless steel. Represent all.
[10]
Duplex stainless steel is in the spotlight as a steel for industrial facilities such as desalination facilities, pulp, paper, and chemical facilities that require corrosion resistance because it is economical and easy to secure high strength due to its low Ni content while securing corrosion resistance equivalent to 316L stainless steel.
[11]
In particular, among duplex stainless steels, expensive alloying elements such as Ni and Mo are reduced, limiting 19 to 23% of Cr, 1.8 to 3.5% of Ni, 0 to 2% of Mn, and 0.5 to 1.0% of Mo. Research on lean duplex stainless steel, which further highlights the advantages of low alloy cost through the addition of 0.3% high nitrogen, is actively being conducted.
[12]
However, in the case of lean duplex stainless steel, there is a problem that formability and elongation are inferior due to the formation of an upper interface between austenite and ferrite. Accordingly, there is a need to develop an austenitic stainless steel with improved strength while reducing Ni, Mo, etc. while securing elongation and corrosion resistance.
Detailed description of the invention
Technical challenge
[13]
Embodiments of the present invention are to provide an austenitic stainless steel with improved strength while securing the elongation and corrosion resistance of the existing 316L stainless steel.
Means
[14]
Austenitic stainless steel with improved strength according to an embodiment of the present invention, by weight, C: 0.02 to 0.14%, Si: 0.2 to 0.6%, S: less than 0.01%, Mn: 2.0 to 4.5%, Ni : 2.5 to 5.0%, Cr: 19.0 to 22.0%, Cu: 1.0 to 3.0%, Mo: less than 1.0%, N: 0.25 to 0.40%, the remainder contains Fe and inevitable impurities, expressed by the following formula (1) The SNL (Solubility of Nitrogen in Liquid) value is greater than or equal to the content of N.
[15]
Equation (1): SNL= -0.188- 0.0423×C -0.0517×Si+ 0.012×Mn +0.0048×Ni + 0.0252×Cr -0.00906×Cu +0.00021×Mo
[16]
(Here, C, Si, Mn, Ni, Cr, Cu, and Mo mean the content (% by weight) of each element.)
[17]
In addition, according to an embodiment of the present invention, C+N: may be 0.5% or less (excluding 0).
[18]
In addition, according to an embodiment of the present invention, it may further include one or more of B: 0.001 to 0.005% and Ca: 0.001 to 0.003%.
[19]
In addition, according to an embodiment of the present invention, the Md 30 value expressed by the following equation (2) may satisfy -50 or less.
[20]
Equation (2): Md 30 = 551 -462 × (C +N) -9.2 × Si -8.1 × Mn -13.7 × Cr -29 × (Ni +Cu)-8.5 × Mo
[21]
(Here, C, N, Si, Mn, Cr, Ni, Cu, and Mo mean the content (% by weight) of each element.)
[22]
In addition, according to an embodiment of the present invention, the following equation (3) may be satisfied.
[23]
Equation (3): Creq/Nieq ≤ 1.8
[24]
(Where, Creq = Cr +Mo +1.5 × Si, Nieq = Ni +0.5 × Mn +30 × (C +N) +0.5 × Cu.)
[25]
In addition, according to an embodiment of the present invention, a value of the pitting expression index expressed by the following equation (4) may satisfy 22 or more.
[26]
Equation (4): Phrase resistance index (PREN) = 16 +3.3Mo +16N -0.5Mn
[27]
(Here, Mo, N, and Mn mean the content (% by weight) of each element.)
[28]
Further, according to an embodiment of the present invention, the austenitic stainless steel may have a yield strength (0.2 off-set) of 400 to 450 MPa, and a tensile strength of 700 to 850 MPa.
[29]
In addition, according to an embodiment of the present invention, the elongation of the austenitic stainless steel may be 35% or more.
Effects of the Invention
[30]
According to an embodiment of the present invention, it is possible to provide an austenitic stainless steel with improved strength while securing elongation and corrosion resistance comparable to that of the existing 316L stainless steel.
Brief description of the drawing
[31]
1 is a thermocalc for each component for deriving a SNL (Solubility of Nitrogen in Liquid) value of an austenitic stainless steel according to an embodiment of the present invention. This is a graph to explain the correlation between the calculation result and the applied value of the regression equation.
Best mode for carrying out the invention
[32]
Austenitic stainless steel with improved strength according to an embodiment of the present invention, by weight, C: 0.02 to 0.14%, Si: 0.2 to 0.6%, S: less than 0.01%, Mn: 2.0 to 4.5%, Ni : 2.5 to 5.0%, Cr: 19.0 to 22.0%, Cu: 1.0 to 3.0%, Mo: less than 1.0%, N: 0.25 to 0.40%, the remainder contains Fe and inevitable impurities, expressed by the following formula (1) The SNL (Solubility of Nitrogen in Liquid) value is greater than or equal to the content of N.
[33]
Equation (1): SNL= -0.188- 0.0423×C -0.0517×Si+ 0.012×Mn +0.0048×Ni + 0.0252×Cr -0.00906×Cu +0.00021×Mo
[34]
(Here, C, Si, Mn, Ni, Cr, Cu, and Mo mean the content (% by weight) of each element.)
Mode for carrying out the invention
[35]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented in order to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited to the exemplary embodiments presented here, but may be embodied in other forms. In the drawings, in order to clarify the present invention, portions not related to the description may be omitted, and the size of components may be slightly exaggerated to aid understanding.
[36]
Throughout the specification, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless specifically stated to the contrary.
[37]
Expressions in the singular number include expressions in the plural unless the context clearly has exceptions.
[38]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[39]
The austenitic stainless steel having improved strength according to an aspect of the present invention is, by weight, C: 0.02 to 0.14%, Si: 0.2 to 0.6%, S: less than 0.01%, Mn: 2.0 to 4.5%, Ni: 2.5 to 5.0%, Cr: 19.0 to 22.0%, Cu: 1.0 to 3.0%, Mo: less than 1.0%, N: 0.25 to 0.40%, the remainder contains Fe and unavoidable impurities.
[40]
Hereinafter, the reason for limiting the numerical value of the content of the alloying component in the examples of the present invention will be described. Hereinafter, the unit is% by weight unless otherwise specified.
[41]
The content of C is 0.02 to 0.14%.
[42]
Carbon (C) is an element that is effective for stabilizing the austenite phase, but when the content is low, 0.02% or more may be added as additional austenite stabilizing elements are required. However, if the content is excessive, it not only degrades workability due to the solid solution strengthening effect, but also induces grain boundary precipitation of Cr carbide due to latent heat after hot-rolled coiling and heat-affected zone of the weld, which may adversely affect ductility, toughness, and corrosion resistance. Therefore, the upper limit can be limited to 0.14%.
[43]
The content of Si is 0.2 to 0.6%.
[44]
Silicon (Si) serves as a deoxidizer during the steelmaking process and is an element effective in improving corrosion resistance, and can be added by 0.2% or more. However, Si is an effective element for stabilizing the ferrite phase, and when excessively added, it promotes the formation of delta ferrite in the casting slab, thereby lowering the hot workability and lowering the ductility/toughness of the steel due to the solid solution strengthening effect. It can be limited.
[45]
The content of Mn is 2.0 to 4.5%.
[46]
Manganese (Mn) is an austenitic stabilizing element that is added instead of nickel (Ni) in the present invention, and is effective in improving cold rolling properties by suppressing the generation of processing organic martensite, and the solubility of nitrogen (N) during the steelmaking process to be described later. As an element to increase the can be added 2.0% or more. However, if the content is excessive, the upper limit may be limited to 4.5% because it may reduce the ductility, toughness, and corrosion resistance of the steel as it causes an increase in S-based inclusions (MnS).
[47]
The content of Ni is 2.5 to 5.0%.
[48]
Nickel (Ni) is a strong austenite-phase stabilizing element and is essential to secure good hot workability and cold workability. In particular, even if a certain amount of Mn or more is added, it is essential to add 2.5% or more. However, as Ni is an expensive element, it causes an increase in raw material cost when a large amount is added. Accordingly, the upper limit can be limited to 5.0% in consideration of both the cost and efficiency of the steel.
[49]
The content of Cr is 19 to 22%.
[50]
Although chromium (Cr) is a ferrite stabilizing element, it is effective in suppressing the formation of martensite phase, and is a basic element that secures the corrosion resistance required for stainless steel. In addition, 19% or more may be added as an element that increases the solubility of nitrogen (N) during a steelmaking process to be described later. However, if the content is excessive, the manufacturing cost increases, and the formation of delta (δ) ferrite in the slab leads to a decrease in hot workability.Therefore, there is a problem that additional addition of austenite stabilizing elements such as Ni and Mn is required. That upper limit can be limited to 22%.
[51]
The content of P is less than 0.1%.
[52]
The upper limit of phosphorus (P) may be limited to 0.1% as it lowers corrosion resistance or hot workability.
[53]
The content of S is less than 0.01%.
[54]
As sulfur (S) lowers corrosion resistance or hot workability, its upper limit may be limited to 0.01%.
[55]
The content of Cu is 1.0 to 3.0%.
[56]
Copper (Cu) is an austenite-phase stabilizing element added instead of nickel (Ni) in the present invention, and improves formability by improving corrosion resistance in a reducing environment and reducing Stacking Fault Energy (SFE). In order to sufficiently express this effect, it may be added at least 1.0%. However, if the content is excessive, the upper limit may be limited to 3.0% because it may increase the material cost as well as lower the hot workability.
[57]
The content of Mo is less than 1.0%.
[58]
Molybdenum (Mo) is an effective element in improving the corrosion resistance of stainless steel by modifying a passive film. However, since Mo is an expensive element, when a large amount of Mo is added, it causes an increase in raw material cost and has a problem of deteriorating hot workability. Accordingly, in consideration of the cost-efficiency and hot workability of the steel, the upper limit can be limited to 1.0%.
[59]
The content of N is 0.25 to 0.40%.
[60]
Nitrogen (N) is an element effective in improving corrosion resistance and is a strong austenite stabilizing element. Therefore, nitrogen alloying can reduce material cost by enabling lower use of Ni, Cu, and Mn. 0.25% or more may be added to sufficiently express this effect. However, if the content is excessive, the upper limit may be limited to 0.40%, since workability and formability may be deteriorated due to the solid solution strengthening effect.
[61]
The content of C+N is 0.5% or less.
[62]
C and N are elements effective in improving strength, but when the content is excessive, there is a problem of lowering the workability, and the upper limit of the total may be limited to 0.5%.
[63]
In addition, the austenitic stainless steel having improved strength according to an embodiment of the present invention may further include one or more of B: 0.001 to 0.005 and Ca: 0.001 to 0.003%.
[64]
The content of B is 0.001 to 0.005%.
[65]
Boron (B) is an element effective in securing good surface quality by suppressing the occurrence of cracks during casting, and can be added by 0.001% or more. However, if the content is excessive, nitride (BN) may be formed on the product surface during the annealing/pickling process to reduce the surface quality, and the upper limit may be limited to 0.005%.
[66]
The content of Ca is 0.001 to 0.003%.
[67]
Calcium (Ca) is an element that improves product cleanliness by suppressing the formation of MnS steel-making inclusions generated at grain boundaries when high Mn is contained, and can be added by 0.001% or more. However, if the content is excessive, it may cause a decrease in hot workability and a decrease in product surface quality due to formation of Ca-based inclusions, and the upper limit may be limited to 0.003%.
[68]
The remaining component of the present invention is iron (Fe). However, since unintended impurities from the raw material or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.
[69]
In order to secure price competitiveness of austenitic stainless steel, it is necessary to reduce the content of expensive austenite stabilizing elements such as Ni and Mn, and it is required to predict the amount of N addition that can compensate for this. To this end, it is necessary to set the optimum N content through calculation of the solubility limit of N in consideration of each alloy component.
[70]
Accordingly, the content of N that can be dissolved in the molten metal temperature at 1150℃ is derived according to the addition amount of each alloy element (C, Si, Mn, Ni, Cr, Cu, Mo) using the state diagram prediction program Thermocalc. I did.
[71]
1 is a thermocalc for each component for deriving a SNL (Solubility of Nitrogen in Liquid) value of an austenitic stainless steel according to an embodiment of the present invention. This is a graph to explain the correlation between the calculation result and the applied value of the regression equation.
[72]
Referring to FIG. 1, the limit value at which nitrogen is dissolved in the molten metal is calculated and expressed as "N solubility (The.)".
[73]
Based on the calculated value of Thermocalc. according to this component change, the SNL (Solubility of Nitrogen in Liquid) regression equation of Equation (1) was derived.
[74]
Equation (1): SNL= -0.188- 0.0423×C -0.0517×Si+ 0.012×Mn +0.0048×Ni + 0.0252×Cr -0.00906×Cu +0.00021×Mo
[75]
When the derived regression equation was applied, it was confirmed that the R(sq) value corresponds to a high correlation with 100%, and the thermocalc for each component for deriving the SNL (Solubility of Nitrogen in Liquid) which is the N melting limit value. It was confirmed that it is possible to secure suitability in the relationship between the calculation result and the regression equation.
[76]
In the austenitic stainless steel having improved strength according to an embodiment of the present invention, the SNL value is greater than or equal to N content. In this way, when the SNL value was set higher than the N content to increase the nitrogen solubility limit, it was confirmed that the steelmaking operation of the target alloy component was performed satisfactorily.
[77]
In the case of austenitic stainless steel, it is applied to products that require a beautiful surface. In the case of products that require a beautiful surface, it is common to perform bright annealing of cold-rolled materials. Such bright annealing is performed by performing heat treatment in a reducing atmosphere (Dew point -40 ~ -60℃) using nitrogen (N 2 ), hydrogen (H 2 ), etc. of the stainless steel cold-rolled material. It is a heat treatment technology that keeps the surface bright and beautiful without changing the color and properties of the surface by preventing reoxidation. Bright annealing using hydrogen as the atmospheric gas used for such bright annealing is the most common, because it is most widely used for suppressing discoloration of the surface as well as high heat capacity.
[78]
Compared to general austenitic stainless steel, in stainless steel having reduced austenite stabilizing elements such as Ni and Mn as in the present invention, there is a point to be considered when applying bright annealing in a hydrogen atmosphere.
[79]
This is a point that there is a high possibility that a problem of poor workability due to hydrogen embrittlement defects occurs in the final material due to the penetration of hydrogen during bright annealing. In the case of stainless steel with reduced austenite stabilizing elements such as Ni and Mn, stress-induced or processing-induced martensite is formed around the surface layer during cold rolling before final bright annealing, and the martensite phase formed on such surface layer is heat treated during bright annealing. As a result, it comes into contact with hydrogen atoms, which are inert gases, before being transformed into an austenite phase, and some of these hydrogen atoms penetrate into the martensite phase. As the existing stress-organic or process-organic martensite is phase-transformed into an austenite phase by bright annealing, hydrogen atoms that have penetrated inside cannot be discharged to the outside and are trapped in an atomic state at the surface.
[80]
The hydrogen atoms penetrating into the surface layer are naturally bake-out after a certain period of time at room temperature for ferrite or martensite, which are general BCC and BCT structures, and do not significantly affect physical properties.
[81]
On the other hand, when the martensitic phase of the surface layer is transformed into an austenite phase by bright annealing, that is, when hydrogen atoms are present in the lattice structure of FCC, the natural bakeout of hydrogen atoms is not smooth even after a considerable period of time at room temperature and is a long-term material. It will exist within.
[82]
These hydrogen atoms are known to cause hydrogen embrittlement, and hydrogen atoms trapped in the material due to some processing or deformation change to the state of hydrogen molecules (gas), and when a certain pressure is reached, cracks are formed under a certain load. It acts as a starting point and causes a decrease in elongation.
[83]
Therefore, in the case of an austenitic stainless steel having relatively low Ni and Mn, it is possible to secure beautiful surface quality and workability through bright annealing only by controlling the amount of martensite phase formed on the surface by additional work hardening together with alloy components.
[84]
Accordingly, the austenitic stainless steel having improved strength according to an embodiment of the present invention satisfies a range of -50° C. or less in the Md 30 value expressed by the following formula (2) .
[85]
Equation (2): Md 30 = 551 -462 × (C +N) -9.2 × Si -8.1 × Mn -13.7 × Cr -29 × (Ni +Cu)-8.5 × Mo
[86]
In the austenitic stainless steel, martensitic transformation occurs by plastic working at a temperature equal to or higher than the martensitic transformation start temperature (Ms). The upper limit temperature that causes phase transformation by such processing is represented by the Md value, and is a measure of the degree to which phase transformation occurs due to processing.
[87]
In particular, the temperature (°C) at which 50% phase transformation to martensite occurs when 30% strain is applied is defined as Md 30 . When the Md 30 value is high, it is easy to generate the process organic martensite phase, whereas when the Md 30 value is low, it can be judged as a steel type that is relatively difficult to form the process organic martensite phase. In general, the Md 30 value is used as an index to determine the austenite stabilization degree of a conventional austenitic stainless steel, and can be calculated through the Nohara regression equation expressed by Equation (2).
[88]
The reason why various kinds of phases are formed by the difference in alloy component content is that the effect of each added alloy component on the phase balance is different.
[89]
The degree to which each alloy component affects the phase balance can be calculated through Creq and Nieq, and the phase generated at room temperature can be predicted through the Creq/Nieq ratio expressed as in Equation (3) below.
[90]
Equation (3): Creq/Nieq
[91]
Here, Creq = Cr +Mo +1.5 × Si, Nieq = Ni +0.5 × Mn +30 × (C +N) +0.5 × Cu.
[92]
In other words, when the Creq/Nieq ratio is low, the austenite single phase can be formed at room temperature due to relatively high austenite stabilization, whereas when the Creq/Nieq ratio is high, the austenite stability is low and the ferrite phase is likely to be formed locally. .
[93]
The present inventors examined by applying the Creq/Nieq ratio to various alloy components, and as a result, it was confirmed that the formation of a single-phase austenite matrix structure was possible when the Creq/Nieq ratio was 1.8 or less.
[94]
Various methods are used as a criterion for evaluating the corrosion resistance of stainless steel, but the use of PREN is a simple method of examining the discrimination power of alloy components.
[95]
PREN is generally used as having an influence of Cr, Mo, and N, but in the case of a steel type having a relatively high content of Mn, it is necessary to consider the effect of Mn as well, so the following formula (4) component relation was derived in the present invention.
[96]
When the alloy component system of generally used high corrosion resistance 316L stainless steel is applied to the following formula, it shows a value of about 22. Accordingly, in the present invention, in order to secure corrosion resistance equal to or higher than that of 316L stainless steel, the PREN value was set to 22 or higher.
[97]
Equation (4): Phrase resistance index (PREN) = 16 +3.3Mo +16N -0.5Mn
[98]
[99]
Hereinafter, the present invention will be described in more detail through examples.
[100]
For various alloy component ranges shown in Table 1 below, a slab having a thickness of 200 mm was prepared by melting an ingot, heated at 1,240° C. for 2 hours, and then hot-rolled to prepare a hot-rolled steel sheet having a thickness of 3 mm.
[101]
[Table 1]
C Si Mn S Ni Cr Cu Mo N C+N
Example 1 0.104 0.48 2.91 0.005 3.53 20.8 2.1 0.52 0.3 0.404
Example 2 0.103 0.49 3.4 0.005 3.35 19.6 1.16 0.39 0.27 0.373
Example 3 0.088 0.31 3.41 0.004 3.7 21.7 2.51 0.10 0.34 0.428
Example 4 0.035 0.31 3.8 0.006 4.2 21 2.48 0.20 0.33 0.365
Comparative Example 1 0.02 0.52 1.4 0.004 10.4 16.6 0.39 2.00 0.018 0.038
Comparative Example 2 0.014 0.55 2.4 0.006 2.4 20.3 0.1 1.30 0.2 0.166
Comparative Example 3 0.1 0.38 3.8 0.006 3.4 17.2 1.45 0.10 0.21 0.310
Comparative Example 4 0.15 0.46 3.8 0.004 3.6 21.6 2.04 0.32 0.35 0.500
[102]
After performing a solution treatment at 1,150° C. for 1 minute, microstructure observation and evaluation of various mechanical properties were performed.
[103]
Mechanical properties were measured using a No. 5 test piece specified in Japanese Industrial Standard JIS Z 2201. Specifically, a tensile test was conducted using JIS Z 2201, and the measured yield strength (Yield Strength, MPa), tensile strength (Tensile Strength, MPa), and elongation (Elongation, %) were shown in Table 2 below. .
[104]
In addition, SNL calculation results, Md30 calculation results, Creq/Nieq ratio calculation results, and PREN calculation results for the 4 Examples and 4 Comparative Examples in Table 1 are shown in Table 2 below.
[105]
[Table 2]
Steel grade N dissolved (The.) N dissolved (Reg.) Md 30 (℃) Creq/Nieq PREN Phase analysis Mechanical properties
YS(MPa) TS(MPa) El(%)
Example 1 0.3238 0.3244 -121 1.2140 25.861 Austenite 490 780 44%
Example 2 0.3067 0.3080 -60 1.2322 23.507 Austenite 460 760 50%
Example 3 0.3582 0.3590 -170 1.0914 25.765 Austenite 510 800 44%
Example 4 0.3472 0.3488 -136 1.1845 25.04 Austenite 470 750 42%
Comparative Example 1 0.2205 0.2204 -60 1.5585 22.788 Austenite 220 540 58%
Comparative Example 2 0.3230 0.3233 76 2.6076 25.822 Duplex 480 700 45%
Comparative Example 3 0.2552 0.2556 -5 1.1661 18.99 Austenite 380 720 54%
Comparative Example 4 0.3544 0.3550 -180 1.0507 26.356 Austenite 530 830 32%
[106]
In the case of Comparative Example 1, which corresponds to the component system of general 316L stainless steel, it can be seen that a structure composed of an austenite phase is shown, and a PREN value of 22 or more is shown. However, less than 0.25% of nitrogen is added, and the mechanical properties evaluation results show a yield strength of 220 MPa and a tensile strength of 540 MPa, which corresponds to the physical properties of a generally widely used soft austenitic stainless steel and is applied to materials requiring high strength. There is a problem that is difficult to do.
[107]
In the case of Comparative Example 2 in which the Creq/Nieq ratio exceeds 1.8, as Mo is added at a certain level or more, the PREN value is about 26 levels, indicating excellent pitting resistance. In addition, it can be seen that the mechanical property evaluation results show a yield strength of 480 MPa, a tensile strength of 700 MPa, and an elongation of 45%.
[108]
However, it was confirmed that the austenite phase and the ferrite phase formed a duplex structure with about 5:5 when the microstructure was observed at room temperature as both Ni and N were relatively low level. This is because the stabilization of ferrite in the phase balance is relatively higher than that of 316L stainless steel. In the duplex structure, there is a possibility that cracks may occur at the interface between the austenite phase and the ferrite phase, and thus it is difficult to apply to a material requiring molding such as bending.
[109]
In the case of Comparative Example 3 in which the content of Ni and Mn was slightly increased compared to Comparative Example 2 and the Creq/Nieq ratio was set to 1.8 or less, a structure composed of an austenite phase was formed when microstructure was observed, and in the case of mechanical properties, Comparative Example 1 It can be seen that it is hard compared to the 316L of Comparative Example 2, and exhibits soft physical properties compared to the Duplex stainless steel of Comparative Example 2.
[110]
However, the Md 30 value is -5℃, and hydrogen embrittlement is likely to occur when producing bright annealing materials with beautiful surfaces in the future. In addition, since the dissolution limit of N, which is greatly affected by the Cr content, is low, the amount of N added is 0.21%, and the nitrogen factor of the PREN value cannot be maximized, making it difficult to secure pitting resistance at the level of 316L.
[111]
In the case of Comparative Example 4 in which the content of N, C, and Cr was increased compared to Comparative Example 3, it was suitable for manufacturing a bright annealed material as it showed the Md 30 value at the level of -180°C , and the Creq/Nieq ratio was set to 1.8 or less It can be seen that it is possible to secure a single-phase austenite structure.
[112]
However, it can be seen that the C+N content is 0.5%, exceeding 0.5%, which is the upper limit of the present invention, and exhibits hard mechanical properties, and the elongation is less than 35%.
[113]
Referring to Table 2, in the case of Examples 1 to 4 of the present invention, it is possible to secure an Md 30 value of -50°C or less , so that the possibility of occurrence of hydrogen embrittlement during bright annealing is low, as well as nickel equivalent (Nieq) and The ratio of the chromium equivalent (Creq) (Creq/Nieq) satisfies the range of 1.8 or less, so that a single-phase austenite structure can be formed at room temperature.
[114]
In addition, it was confirmed that the content of Ni and Mo was relatively low, while securing price competitiveness, and exhibiting a PREN value of 22 or more.As a result of mechanical property evaluation, it was confirmed that high strength characteristics compared to 316L can be realized and a good elongation of 35% or more can be secured. I did.
[115]
From the above results in weight%, C: 0.02 to 0.14%, Si: 0.2 to 0.6%, P: less than 0.1%, S: less than 0.01%, Mn: 2.0 to 4.5%, Ni: 2.5 to 5.0%, Cr: 19.0 To 22.0%, Cu: 1.0 to 3.0%, Mo: less than 1.0%, N: 0.25 to 0.40%, and the remainder of the austenitic stainless steel containing Fe and inevitable impurities newly proposed by the present invention SNL (Solubility of Nitrogen in Liquid) value control for securing price competitiveness and ease of steel making, Md 30 value control for securing austenite phase stability , Creq/Nieq ratio control for austenite phase formation in microstructure, and securing corrosion resistance. It can be seen that it is possible to manufacture stainless steel that can improve price competitiveness and strength while securing processability and corrosion resistance comparable to that of existing 316L stainless steel through the control of the PREN for the purpose.
[116]
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 not departing from the concept and scope of the following claims. It will be appreciated that various changes and modifications are possible in.
Industrial applicability
[117]
The austenitic stainless steel according to the present invention has improved strength while securing elongation and corrosion resistance, and thus can be used as a substitute material for existing 316L stainless steel.
Claims
[Claim 1]
In% by weight, C: 0.02 to 0.14%, Si: 0.2 to 0.6%, S: less than 0.01%, Mn: 2.0 to 4.5%, Ni: 2.5 to 5.0%, Cr: 19.0 to 22.0%, Cu: 1.0 to 3.0 %, Mo: less than 1.0%, N: 0.25 to 0.40%, the remainder contains Fe and inevitable impurities, and the strength of SNL (Solubility of Nitrogen in Liquid) expressed by the following formula (1) is greater than or equal to the content of N. Austenitic stainless steel. Equation (1): SNL= -0.188- 0.0423×C -0.0517×Si+ 0.012×Mn +0.0048×Ni + 0.0252×Cr -0.00906×Cu +0.00021×Mo (here, C, Si, Mn, Ni, Cr, Cu , Mo means the content (% by weight) of each element.)
[Claim 2]
The austenitic stainless steel according to claim 1, wherein C+N: 0.5% or less (excluding 0) is improved.
[Claim 3]
The austenitic stainless steel according to claim 1, further comprising at least one of B: 0.001 to 0.005% and Ca: 0.001 to 0.003%.
[Claim 4]
The austenitic stainless steel with improved strength according to claim 1, wherein the Md 30 value represented by the following formula (2) satisfies -50 or less. Equation (2): Md 30 = 551 -462 × (C +N) -9.2 × Si -8.1 × Mn -13.7 × Cr -29 × (Ni +Cu)-8.5 × Mo (here, C, N, Si, Mn, Cr, Ni, Cu, and Mo mean the content (% by weight) of each element.)
[Claim 5]
The austenitic stainless steel according to claim 1, wherein the strength satisfying the following formula (3) is improved. Equation (3): Creq/Nieq ≤ 1.8 (where Creq = Cr +Mo +1.5 × Si, Nieq = Ni +0.5 × Mn +30 × (C +N) +0.5 × Cu.)
[Claim 6]
The austenitic stainless steel according to claim 1, wherein the austenitic stainless steel with improved strength satisfies a pitting resistance index value of 22 or more represented by the following formula (4). Equation (4): Plenty resistance index (PREN) = 16 +3.3Mo +16N -0.5Mn (here, Mo, N, Mn means the content (% by weight) of each element.)
[Claim 7]
The austenitic stainless steel with improved strength according to claim 1, wherein the yield strength (0.2 off-set) is 400 to 450 MPa, and the tensile strength is 700 to 850 MPa.
[Claim 8]
The austenitic stainless steel with improved strength according to claim 1, wherein the elongation is 35% or more.