KR20120108785A - Ferritic stainless steel having excellent high temperature strength - Google Patents

Abstract

PURPOSE: Ferritic stainless steel with excellent high-temperature strength is provided to ensure high physical properties and improve processing efficiency by controlling the content of Mo below 0.8wt.%. CONSTITUTION: Ferritic stainless steel with excellent high-temperature strength comprises C of 0-0.01wt.%, Si of 0-0.5wt.%, Mn of 0-2.0wt.%, P of 0-0.02wt.%, S of 0-0.02wt.%, Cr of 12-19wt.%, Mo of 0-1.0wt.%, W of 2-7wt.%, Ti of 0-0.3wt.%, Nb of 0-0.6wt.%, N of 0-0.01wt.%, Al of 0-0.01wt.%, and Fe and inevitable impurities of the remaining amount. [Reference numerals] (AA) Sigma phase

Description

Ferritic Stainless Steel having Excellent High Temperature Strength}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to ferritic stainless steel having excellent high temperature characteristics and formability for use in automotive exhaust manifolds. More particularly, the present invention relates to high temperature oxidation resistance, high temperature salt corrosion resistance, and high temperature strength by controlling the amount of alloying elements. The present invention relates to a ferritic stainless steel having excellent thermal fatigue characteristics and formability.

In recent years, in response to the seriousness of environmental problems caused by automobile exhaust gas, a number of countries, such as Europe, the United States, and Japan, are implementing legal regulations on the emission of harmful substances. Accordingly, materials used in exhaust systems such as automobiles have also been studied in various directions to cope with high environmental regulations in each country. Specifically, the weight reduction of automobile materials, the rise of exhaust gas temperature by turbo engine, and the improvement of fuel efficiency of automobiles have been studied. For this purpose, the ferrite for high temperature, which has superior high temperature strength, heat resistance and corrosion resistance, is higher than the existing ferritic stainless steel. The demand for stainless steels is increasing.

Ferritic stainless steel is widely used as a vehicle exhaust system material because of its low thermal expansion coefficient, excellent thermal fatigue characteristics, excellent oxidation resistance and oxide film adhesion at high temperatures, and low cost. Recently, as regulations on environmental pollution are strengthened, regulations on automobile exhaust gas are also being tightened. In other words, the combustion temperature of the engine and the exhaust gas temperature gradually increases, fuel efficiency is improved and the exhaust gas purification efficiency is increasing. Therefore, as a material for an exhaust manifold having a function of inducing exhaust gas from an engine to a catalyst device, ferritic stainless steel having excellent high temperature strength, thermal fatigue characteristics, high temperature oxidation resistance, high temperature salt corrosion resistance, and moldability is required.

Exhaust manifold material has been used in the past, nodular graphite cast iron or Si nodular cast iron having excellent heat resistance and high temperature characteristics, and in recent years, a lot of stainless steels have been used in accordance with the demand for exhaust gas temperature rise and weight reduction. The stainless steel is initially used 409L steel, 430J1L steel, and recently, 429EM steel, 441 steel and 444 steel are used a lot as the exhaust gas temperature rise, workability improvement and low-cost material requirements. On the other hand, such a material limits the maximum use temperature of the exhaust manifold to 870 ° C, and there is a problem in that the materials are not applicable when the exhaust manifold is used at a temperature higher than the temperature.

On the other hand, vehicle exhaust system components such as exhaust manifolds are required to be mounted in a limited space, so various configurations are required, and therefore they are often designed in a complicated shape. In order to be suitable for this, the exhaust manifold material is required to have a variety of properties, such as the required properties to ensure sufficient processability, there should be no shape deformation of the thickness reduction after molding. In recent years, in order to improve the exhaust gas purification function, research is being conducted to quickly reach a converter of a high temperature exhaust gas. As a result, the shape of the molded article is further complicated by shortening the distance between the exhaust manifold and the converter by directly connecting the exhaust manifold and the converter. Therefore, the material for exhaust manifold must improve heat resistance and workability simultaneously, and at the same time, low cost is required.

An object of the present invention devised to solve the above problems is to provide a ferritic stainless steel with improved durability at high temperatures by suppressing the generation of sigma phase.

In addition, another object of the present invention is to provide a ferritic stainless steel having a predetermined high tensile strength and excellent formability.

According to a feature of the present invention for achieving the object as described above, the present invention is by weight, C: more than 0 wt% to 0.01 wt%, Si: more than 0 wt% to 0.5 wt%, Mn: 0 greater than or equal to wt% and less than or equal to 2.0 wt%, P: greater than or equal to 0 wt% and less than or equal to 0.02 wt%, S: greater than or equal to 0 wt% and less than or equal to 0.02 wt%, Cr: greater than or equal to 12 wt% and less than or equal to 19 wt%, and Mo: 0 wt% Or more to 1.0 wt% or less, W: 2 wt% or more to 7 wt% or less, Ti: 0 wt% or more to 0.3 wt% or less, Nb: more than 0 wt% to 0.6 wt% or less, N: more than 0 wt% to Ferrite-based stainless steel having excellent high temperature strength, characterized by containing 0.01 wt% or less, Al: 0 wt% or more and 0.01 wt% or less, balance Fe and other unavoidable impurities.

The Mo may be characterized in that less than 0.8 wt% by weight.

In addition, the hot-rolled annealing structure of the ferritic stainless steel may include a sigma phase fraction of 5% or less.

W may be 3 wt% or more and 6 wt% or less by weight.

Mo + 0.83W may be at least 3.5 wt% and up to 5.0 wt% by weight.

In addition, in the present invention, [(Ti + 1 / 2Nb) / (C + N)] may be 19.5 or more and 32 or less.

According to the present invention as described above, the generation of sigma phase can be suppressed to provide a ferritic stainless steel having improved durability at high temperatures.

In addition, the present invention can provide a ferritic stainless steel having excellent moldability while maintaining high moldability.

1 is a graph showing the evaluation results for the effect of Mo and W on high temperature strength.
Figure 2a is an optical micrograph of the hot-annealed structure in Mo-added steel.
2b is an optical micrograph of a hot rolled annealing structure in Mo + W composite additive steel.
3 is a graph evaluating the thermal fatigue characteristics of ferritic stainless steel according to the addition amount of Mo and W.
Figure 4 is a graph evaluating the high temperature oxidation resistance of ferritic stainless steel according to the addition amount of Mo and W.
5 is a graph evaluating the high temperature salt corrosion resistance of ferritic stainless steel according to the addition amount of Mo and W.

Specific details of other embodiments are included in the detailed description and the drawings.

Advantages and features of the present invention, and methods of achieving the same will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments described below, but may be embodied in various forms. In the following description, it is assumed that a part is connected to another part, But also includes a case in which other elements are electrically connected to each other in the middle thereof. In the drawings, parts not relating to the present invention are omitted for clarity of description, and like parts are denoted by the same reference numerals throughout the specification.

The present invention relates to a ferritic stainless steel that can be used for exhaust manifolds for automobiles, and the ferritic stainless steel according to the present invention has excellent oxidation resistance, corrosion resistance, strength, and thermal fatigue characteristics at high temperatures. It has moldability. Specifically, the ferritic stainless steel can reduce the cost by eliminating unnecessary losses by managing the amount of the element that can improve the high temperature characteristics such as oxidation resistance, corrosion resistance, etc. at high temperatures. In addition, the ferritic stainless steel according to the present invention may be characterized in that it basically ensures excellent oxidation resistance, salt corrosion resistance and high temperature characteristics to be suitable for the exhaust manifold for automobiles.

Ferritic stainless steel according to a preferred embodiment of the present invention has excellent high temperature strength, by weight% C: more than 0 wt% to 0.01 wt%, Si: more than 0 wt% to 0.5 wt%, Mn: 0 greater than or equal to wt% and less than or equal to 2.0 wt%, P: greater than or equal to 0 wt% and less than or equal to 0.02 wt%, S: greater than or equal to 0 wt% and less than or equal to 0.02 wt%, Cr: greater than or equal to 12 wt% and less than or equal to 19 wt%, and Mo: 0 wt% Or more to 1.0 wt% or less, W: 2 wt% or more to 7 wt% or less, Ti: 0 wt% or more to 0.3 wt% or less, Nb: more than 0 wt% to 0.6 wt% or less, N: more than 0 wt% to 0.01 wt% or less, Al: 0 wt% or more and 0.01 wt% or less, balance Fe and other unavoidable impurities.

The ferritic stainless steel is excellent in high temperature oxidation resistance, high temperature corrosion resistance, high temperature strength, thermal fatigue characteristics and formability. This may be affected by the respective alloying elements included in the ferritic stainless steel and the amount thereof added. Hereinafter, each component system which comprises the ferritic stainless steel of this invention is demonstrated in detail. The following component systems were based on the weight percentage.

In the ferritic stainless steel, C may be more than 0 wt% to 0.01 wt% or less. Since the C may increase the room temperature strength of the ferritic stainless steel, the C may be added in excess of 0 wt%. On the other hand, when the amount of C added exceeds 0.01 wt%, the room temperature strength of the ferritic stainless steel is increased, while the ductility, workability, toughness, etc. at relatively high temperature strength and room temperature may decrease. Therefore, C is preferably added at more than 0 wt% to 0.01 wt% or less. More preferably C may be added at 0.005 wt% or less.

Si may be included at more than 0 wt% and up to 0.5 wt%. Si is an element that acts as a deoxidizer in the molten steel state of ferritic stainless steel and is an element necessary in the steelmaking process. In addition, the Si may advantageously work to improve the oxidation resistance of ferritic stainless steel. On the other hand, when the Si is added in excess of 0.5 wt%, since the hardness of the ferritic stainless steel can be increased by the Si solid-solution strengthening phenomenon, the elongation and workability of the ferritic stainless steel may be lowered. Therefore, Si according to the present invention is preferably added at more than 0 wt% to 0.5 wt% or less.

Mn may be included between 0 wt% and 2.0 wt% or less. When ferritic stainless steel is used as a material for automobile exhaust manifolds, scale and the like may be produced at high temperatures. At this time, the generated scale can be easily dropped, and the dropped scale can flow into the catalytic device (converter) to block the passage of the catalytic device. Therefore, the ferritic stainless steel should have peeling resistance with respect to scale, and may include Mn for this purpose. On the other hand, when the amount of Mn added exceeds 2.0 wt%, Mn and S may react with each other to form MnS. Since MnS may adversely affect the corrosion resistance of ferritic stainless steel, the MnS is preferably included in more than 0 wt% to 2.0 wt% or less.

P may be included at 0 wt% or more and 0.02 wt% or less. The P may increase the strength of the ferritic stainless steel, but may decrease workability. In the steelmaking process of ferritic stainless steel, P is often treated with impurities, so it is desirable to reduce as much as possible. On the other hand, the extremely low P in the process step is inefficient in terms of refining cost and productivity, it is preferable that P is included in less than 0.02 wt%.

S may be included at 0 wt% or more and 0.02 wt% or less. S may exist as an inclusion in the ferritic stainless steel and may also serve as an impurity that lowers the corrosion resistance. Therefore, in order to improve the corrosion resistance of the ferritic stainless steel, it is preferable to lower the amount of S added as much as possible, but extremely lowering S in the process step may be inefficient in terms of cost or time. Therefore, it may be desirable to control the amount of S added to 0.02 wt% or less, and it may be particularly desirable to control the amount of S added to 0.003 wt% or less.

Cr may be included at 12 wt% or more and 19 wt% or less. Cr is an alloying element that must be added to improve corrosion resistance and oxidation resistance of ferritic stainless steel. That is, ferritic stainless steel is difficult to obtain sufficient corrosion resistance when the addition amount of Cr is low, the Cr may be included in more than 12 wt%. On the other hand, when the addition amount of Cr exceeds 19 wt%, while the corrosion resistance of the ferritic stainless steel is improved, the strength is too high and thus the elongation and impact characteristics may be drastically lowered. Therefore, in ferritic stainless steel, Cr is preferably contained at 12 wt% or more and 19 wt% or less.

Ti may be included at 0 wt% or more and 0.3 wt% or less. Ti is an alloying element added to improve high temperature strength and intergranular corrosion resistance of ferritic stainless steel. If the amount of Ti added in the ferritic stainless steel exceeds 0.3 wt%, steelmaking inclusions may increase and surface defects such as scab may frequently occur, and nozzle clogging may occur during performance, thereby lowering process efficiency. have. In addition, the increase in the solid solution Ti can reduce the elongation and low temperature impact of the ferritic stainless steel. In addition, when Ti and Nb are added together in the ferritic stainless steel, when the ferritic stainless steel is used for a long time at a high temperature, Fe 3 Nb 3 C carbide may be precipitated and coarsening may occur, causing high temperature deterioration. Therefore, in the present invention, the amount of Ti added may be 0.3 wt% or less.

The amount of N added may be greater than 0 wt% and up to 0.01 wt%. N acts to increase the strength of ferritic stainless steel like C, but may reduce ductility and workability. In particular, in order to ensure sufficient weld toughness and workability of the ferritic stainless steel, the N is preferably contained in more than 0 wt% to 0.01 wt% or less. In this case, it may be particularly preferable that N is included in an amount of 0.007 wt% or less.

The amount of Mo added in the ferritic stainless steel may be 0 wt% or more and 1.0 wt% or less. In addition, Mo may be particularly preferably 0.8 wt% or less, wherein the hot-rolled annealing structure of the ferritic stainless steel may include a sigma phase fraction of 5% or less. Preferably, W may be 2 wt% or more and 7 wt% or less, and more preferably, W may be included in 3 wt% or more and 6 wt% or less.

Ferritic stainless steel has been researched and made various efforts such as adding Mo to improve high temperature strength. If Mo is added to the ferritic stainless steel is more than 3 wt% in the method of adding Mo, there is a disadvantage in generating a sigma phase of the ferritic stainless steel. The sigma phase may not only cause defects in manufacturing the ferritic stainless steel, but also may be a problem in durability when used for automobile exhaust manifolds. Ferritic stainless steel according to the present invention can reduce the Mo to suppress the generation of sigma phase. In addition, the ferritic stainless steel according to the present invention may be added in an amount of 1 wt% or less in order to secure high temperature strength.

It may be particularly preferable that the amount of Mo added is 0.8 wt% or less. In the ferritic stainless steel making process, since the steel fixing is made in a large amount, it is not easy to control the amount of the added substance in a small amount, and thus an effort to control the steel may be inefficient. On the other hand, since elements such as Mo are expensive raw materials, fine adjustment of the amount of Mo may advantageously reduce production costs. Therefore, by controlling the amount of Mo added to 0.8 wt% or less, there is an advantage that the process efficiency can be improved while maintaining the physical properties of the predetermined ferritic stainless steel.

When the amount of W added is less than 2 wt%, the amount of nano-precipitated precipitates, such as Fe 2 W, and the amount of W dissolved in the matrix are lowered, and ferritic stainless steel hardly obtains sufficient high temperature strength and thermal fatigue characteristics. In addition, when the amount of the W added is more than 7 wt%, the raw material cost of the ferritic stainless steel may be increased, and a large amount of Fe 2 W is generated in the ferritic stainless steel, which adversely affects the line flowability, thereby lowering the production efficiency. , Weldability, moldability and the like can be reduced. The ferritic stainless steel further includes W, so that the tensile strength is 40 MPa or more in the high temperature tensile test tested at 900 ° C, and is applicable to an exhaust manifold for automobiles requiring high strength at high temperatures.

When the amount of Mo is 0.8 wt% or less, the ferritic stainless steel may further include W to secure high temperature oxidation resistance, high temperature salt corrosion resistance, high temperature strength and thermal fatigue characteristics. In this case, the added Mo and W may be represented by Mo + 0.84W = 3.5 wt% or more to 5.0 wt% or less when considering the effect on the high temperature strength of the ferritic stainless steel. At this time, when the Mo + 0.84W is less than 3.5 wt%, the high temperature strength, thermal fatigue life, high temperature oxidation resistance, high temperature salt corrosion resistance characteristics of the ferritic stainless steel may be lowered, and when higher than 5.0 wt%, the high temperature characteristics are excellent. However, the elongation, which is a room temperature workability factor, may be lowered, and the toughness of the welded part and the base material may also be reduced.

The ferritic stainless steel, Mo + 0.83W may comprise 3.5 wt% or more to 5.0 wt% or less. With respect to Mo and W contained in ferritic stainless steel, when the value of Mo + 0.83W is less than 3.5 wt%, the ferritic stainless steel has sufficient high temperature strength and thermal fatigue characteristics for use in automobile exhaust manifolds. Difficult to have That is, when the Mo + 0.83W value is less than 3.5wt% in ferritic stainless steel, the maximum use temperature of the automotive exhaust manifold using the same is not more than 900 ° C, which cannot be used at higher temperatures. There is. In addition, when the value of Mo + 0.83W exceeds 5.0 wt%, it may cause a problem in the line flowability of the ferritic stainless steel, resulting in a decrease in productivity, a decrease in moldability and weldability, and the like.

In the ferride stainless steel, Ti is 0 wt% or more and 0.3 wt% or less, Nb is more than 0 wt% and 0.6 wt% or less, N is more than 0 wt% and 0.01 wt% or less, and Al is 0.01. It may be less than or equal to wt%. In this case, the relationship between the elements [(Ti + 1 / 2Nb) / (C + N)] may be 19.5 or more to 32 or less.

In order to secure high temperature strength and thermal fatigue characteristics of ferritic stainless steel, predetermined Ti and Nb should be added. At this time, when the addition amount of Ti and Nb is less than a predetermined amount, grain boundary corrosion may occur in the weld heat affected zone of ferritic stainless steel, or high temperature strength and thermal fatigue characteristics may be reduced. Therefore, the addition amount of Ti and Nb can be adjusted so that (Ti + 1 / 2Nb) / (C + N) is added 19.5 or more. On the other hand, when (Ti + 1 / 2Nb) / (C + N) exceeds 32, it may be advantageous for the high temperature characteristics of the ferritic stainless steel, but the amount of solid solution Nb is excessively high, resulting in low room temperature elongation, toughness and workability. Can be. Therefore, (Ti + 1 / 2Nb) / (C + N) may be included in 19.5 or more and 32 or less.

Hereinafter, the present invention will be described through Examples and Comparative Examples, which are intended to facilitate the present invention, and the present invention is not limited by the following Examples and Comparative Examples.

1. Specimen Fabrication

Table 1 shows the chemical composition table of the specimen used in each Example and Comparative Example. Referring to Table 1, each of Examples and Comparative Examples is based on Fe-15 wt% Cr, by changing the addition amount of Mo, W, Nb and (Ti + 1 / 2Nb) / (C + N) Ferritic stainless steels were prepared, respectively. Some of the hot rolled parts were 20mmt and 5mmt, respectively, and a 2.0mm thick coil and 20mmt bar specimens were produced through a hot-rolled annealing step and a cold-rolled annealing step at a temperature of 1050 ° C. Specimens prepared as described above are Examples 1 to 7 and Comparative Examples 1 to 4 as shown in Table 1 below.

(Table 1)

2. Evaluation of characteristics of ferritic stainless steel at room temperature and high temperature

For each of Examples 1 to 7 and Comparative Examples 1 to 4 produced as described in Table 1, the room temperature tensile test, the high temperature tensile test at 900 ° C., and the high temperature characteristics according to the respective components were examined. Thermal fatigue life, oxidation resistance and salt corrosion resistance were evaluated.

First, thermal fatigue specimens were manufactured by processing the specimens according to Table 1, and thermal fatigue life was evaluated using a thermal fatigue specimen prepared as described above at a temperature of 200 to 900 ° C. with a restraint rate of 0.3. In addition, in order to evaluate oxidation resistance, each Example 1-7 and Comparative Examples 1-4 were heated at 1000 degreeC for 200 hours. Thus, the oxidation scale produced by heating was pickled and the weight change after removal was measured to confirm the oxidation resistance at high temperature. A 26% NaCl solution was prepared to evaluate the salt corrosion resistance. After keeping the specimens according to Examples 1 to 7 and Comparative Examples 1 to 4 at isothermal temperature for 2 hours at 500 ° C. for 10 minutes, the sample was immersed in a 26% NaCl solution for 5 minutes, and then the weight loss was measured. Corrosion was evaluated.

Table 2 below shows the evaluation results for each Example and Comparative Example according to Table 1, for room temperature tensile strength, r-bar value, high temperature tensile strength, thermal fatigue life, high temperature oxidation resistance and high temperature salt corrosion resistance A table showing values.

(Table 2)

Referring to Table 1 and Table 2, it was confirmed that the room temperature tensile strength and the r-bar value, which are the items for evaluating the ease of molding, satisfy the predetermined target values in Examples 1 to 7 and Comparative Examples 1 to 4, respectively. On the other hand, in the high temperature tensile strength, thermal fatigue life, high temperature oxidation resistance, and high temperature salt corrosion resistance, which are the items for evaluating the characteristics at high temperatures, Examples 1 to 7 satisfy the predetermined target values required for automobile exhaust manifold materials. On the other hand, in Comparative Examples 1 to 4, it was confirmed that the evaluation items were not satisfied.

Examples 1 to 7 according to the present invention are all contained in an amount of 0.8 wt% or less of Mo added. In addition, Examples 1 to 7 according to the present invention all contain W in an amount of 3 wt% or more and 6 wt% or less. It is confirmed that this product satisfies the high temperature tensile strength required for the exhaust manifold material by exhibiting a high tensile strength of 41 MPa or higher at 900 ° C., despite being a steel added less than the amount of Mo commonly used. Could. In addition, the embodiments according to the present invention was confirmed that the room temperature elongation, which is an item capable of evaluating the formability of the ferritic stainless steel, is superior to 31% and r-bar value of 0.9 or more. In addition, the thermal fatigue life of Examples 1 to 3 and 5 according to the present invention was 2250, 2260, 1860, and 2520, respectively, and it was confirmed that all satisfy 1800 cycles or more.

An embodiment the maximum value of each of the Examples the weight changing amount in the evaluation of high-temperature oxidation resistance of 1-7 is 10.56 mg / cm ² (Example 6), the minimum value is found to be 7.8mg / cm ² (Example 5) . On the other hand, in Comparative Examples 1 to 4, it was confirmed that scale dropout occurred in Comparative Examples 2 and 3.

In the case of high temperature salt corrosion resistance, in Examples 1 to 7, the maximum value is 2.996 g / mm ² and the weight loss is 3 g / mm ² or less, whereas in Comparative Examples 1 to 4, the maximum value is 20.2 g / mm ² and the minimum value is 14.378. g / mm ² (Comparative Example 4) was confirmed that the weight loss is significantly increased compared to the Example.

In other words, when the above experimental results are examined, in the Examples and Comparative Examples, the room temperature elongation and the r-bar value, which are the characteristics of formability at room temperature, show roughly comparable values, while the thermal fatigue properties and the high temperature oxidation resistance which are high temperature characteristics And in the high temperature salt corrosion resistance it was confirmed that the comparative example is deteriorated in performance compared to the example. Specifically, in each of Examples 1 to 7, the high temperature oxidation resistance satisfies the high temperature oxidation resistance of all specimens with a weight change of 11 mg / cm ² or less. On the other hand, in the case of the comparative example, Comparative Examples 1 and 4 did not satisfy the conditions aimed at high temperature oxidation resistance, and in Comparative Examples 2 and 3 it could be confirmed that even the scale drop out. In addition, in the case of repeated high temperature corrosion resistance, Examples 1 to 7 have weight loss of 3 g / mm ² or less, whereas in Comparative Example, the maximum value of weight loss is 20.2 g / mm ^2, and all four specimens have high temperature resistance. It was confirmed that the corrosion was not satisfied.

Examples 1 to 7 according to the present invention can be confirmed that not only the moldability at room temperature, but also satisfies all the properties at high temperature. Therefore, the ferritic stainless steel according to the present invention can be used at a high temperature, and the moldability can also satisfy predetermined conditions. Therefore, the above embodiments were found to be applicable to the exhaust manifold for automobiles that require precise formability.

The ferritic stainless steel according to the present invention has the advantage that the manufacturing cost can be lowered since it has a suitable physical property for the exhaust manifold while reducing the addition amount of Mo which is an expensive raw material. On the other hand, in the case of the comparative example that does not satisfy the content of Mo and W, it was confirmed that the thermal fatigue life, high temperature oxidation resistance and high temperature salt corrosion resistance, which are characteristics at high temperature, are not all satisfied.

In addition, referring to Table 1 and Table 2, Mo + 0.83W value is 3.5% or more and 5% or less, and the value of [(Ti + 1 / 2Nb) / (C + N)] is 19.5 or more and 32 or less. Controlled specimens were also found to meet all the required properties, with both high temperature and room temperature formation.

1 is a graph showing the evaluation results of the effect of Mo and W on high temperature strength.

Referring to FIG. 1, the weight percent of C: 0.005 wt%, N: 0.0006 wt%, Cr: 15 wt%, Nb: 0.4 wt%, and Ti: 0.1 wt% are the same, but the addition amount of Mo and W is equal to The effects of Mo and W on the high temperature strength of ferritic stainless steels were evaluated.

First, to evaluate the effect of Mo and W on the high temperature strength, 900 ℃ high temperature tensile strength was evaluated by changing the addition amount of Mo alone and Mo + W composite additive steel. The X axis represents the amount of Mo added (Mo added steel) or the amount of Mo + W added (Mo + W composite added steel), and the Y axis shows the high temperature tensile strength according to the graph and derives a relational expression. The regression equation is used to show the relationship with the Mo or Mo + W addition on high temperature strength. As a result, the relation of high temperature tensile strength (MPa) = 22.4 + 4.67Mo + 3.91W at 900 ° C can be derived. Therefore, the contribution of W element to high temperature strength is 84% (W / Mo = 3.91 / 4.67 = 0.84 ) Level.

FIG. 2A is an optical micrograph of a hot-annealed structure in Mo-added steel, and FIG. 2B is an optical micrograph of a hot-annealed structure in Mo + W composite steel.

2A and 2B are optical micrographs comparing sigma phase fractions in hot-annealed structures of Mo-added steels and Mo + W composite-added steels, respectively. Figure 2a is a hot-rolled annealing structure of the ferritic stainless steel containing Mo 3 wt%, the hot-rolled annealing structure of the ferritic stainless steel can be confirmed that the sigma phase fraction is up to 20%. In addition, Figure 2b is a ferritic stainless steel containing 0.5 wt% Mo, 4.5 wt% W, it can be seen that the sigma phase fraction of the thermally annealed structure of the ferritic stainless steel is less than 5% sigma phase. Referring to Figures 2a and 2b, it can be seen that the Mo + W composite additive steel can control the sigma phase of the hot-rolled annealing structure at a lower fraction than Mo alone steel.

3 is a graph evaluating the thermal fatigue characteristics of ferritic stainless steel according to the addition amount of Mo and W.

Referring to Figure 3, the Example was confirmed that the thermal fatigue life at 900 ℃ superior to the comparative example. That is, when the X axis is a high tensile strength at 900 ° C. and the Y axis is a thermal fatigue life cycle, the values for the examples are all located on the upper right side of the graph, whereas the comparative example is on the left side of the graph. It could be confirmed that it is located at the bottom. That is, while the Example is excellent in high temperature tensile strength and thermal fatigue life at 900 ℃, it was confirmed that the comparative example is relatively weak.

Figure 4 is a graph evaluating the high temperature oxidation resistance of ferritic stainless steel according to the addition amount of Mo and W.

Referring to FIG. 4, two items in which the change in weight is relatively small in the comparative example indicate a case in which scale occurs on the surface, and the remaining comparative example except this is more in weight change in the high temperature oxidation resistance result than the example. It can be seen that the larger. That is, the Example was confirmed to be excellent in high temperature oxidation resistance compared to the comparative example.

5 is a graph evaluating high temperature salt corrosion resistance of ferritic stainless steel according to the addition amount of Mo and W.

5 is a graph showing the weight change according to the results of the high temperature salt corrosion resistance test according to the addition amount of Mo or Mo + W. As the amount of Mo or Mo + W added increased, the change in weight was less than 7.5 g / cm ^{2 at} about 3 wt% or more, whereas at 2 wt% or less of Mo or Mo + W, the change in weight was 12.5 g / cm. ^It could be confirmed that it appeared as ² or more. That is, the addition amount of Mo + W may affect the high temperature salt corrosion resistance of ferritic stainless steel, and the addition amount of Mo + W is better in the weight change for the high temperature salt corrosion resistance than 3 wt% or more than 2 wt% or less. I could confirm it.

It will be understood by those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the foregoing detailed description, and all changes or modifications derived from the meaning and scope of the claims and the equivalents thereof are included in the scope of the present invention Should be interpreted.

Claims (6)

C: greater than 0 wt% to 0.01 wt%, Si: greater than 0 wt% to 0.5 wt% or less, Mn: greater than 0 wt% to 2.0 wt% or less, P: greater than 0 wt% to 0.02 wt% or less , S: 0 wt% or more and 0.02 wt% or less, Cr: 12 wt% or more and 19 wt% or less, Mo: 0 wt% or more and 1.0 wt% or less, W: 2 wt% or more and 7 wt% or less, Ti : 0 wt% or more and 0.3 wt% or less, Nb: more than 0 wt% and 0.6 wt% or less, N: more than 0 wt% and 0.01 wt% or less, Al: 0 wt% or more and 0.01 wt% or less, balance Fe and Ferritic stainless steel with excellent high temperature strength, characterized by containing other unavoidable impurities. The method of claim 1,
The Mo is ferritic stainless steel, characterized in that less than 0.8 wt% by weight. The method of claim 2,
The hot-rolled annealing structure of the ferritic stainless steel is a ferritic stainless steel, characterized in that it comprises a sigma phase fraction of 5% or less. The method of claim 1,
W is a ferritic stainless steel, characterized in that 3% by weight or more and 6% by weight or less by weight. The method of claim 1,
Mo + 0.83W is a ferritic stainless steel, characterized in that the weight percentage of 3.5 wt% or more to 5.0 wt% or less. 6. The method according to any one of claims 1 to 5,
[(Ti + 1 / 2Nb) / (C + N)] is ferritic stainless steel, characterized in that 19.5 or more and 32 or less.

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