KR20190094717A - Super duplex stainless steel with improved corrosion resistance and manufacturing method thereof - Google Patents

Abstract

The present invention relates to super duplex stainless steel with improved corrosion resistance and a manufacturing method thereof. A homogenization thermal treatment step is performed before a solid solution thermal treatment step is performed for a shape of an austenite structure to be spherically controlled and uniformly distributed. Moreover, a method for manufacturing a super duplex stainless steel with improved corrosion resistance comprises: the homogenization thermal treatment step of thermal treating a steel plate which comprises 25.0 wt% of Cr, 6.8 wt% of Ni, 3.75-3.85 wt% of Mo, 0.79 wt% of Mn, 0.27 wt% of N, 0.18 wt% of Cu, 0.014 wt% of C, 0.01-0.02 wt% of W, and the remainder consisting of Fe and unavoidable impurities at a temperature of 1275-1325°C to homogenize an austenite structure; and the solid solution thermal treatment step of enabling the homogenized steel plate to be thermal treated and then rapidly cooled to be solidified.

Description

Super duplex stainless steel with improved corrosion resistance and manufacturing method

The present invention relates to a super duplex stainless steel with improved corrosion resistance and a method of manufacturing the same, and more particularly, to a homogeneous heat treatment step before performing the solid solution heat treatment step, thereby controlling the shape of the austenitic structure to be spherical and uniformly distributing it. The present invention relates to a super duplex stainless steel and a method of manufacturing the same having improved corrosion resistance.

In general, super duplex stainless steel is used in chemical plants, chemical tanks, desalination plants, and seawater pumps because of its excellent mechanical properties and corrosion resistance. In particular, when used in parts that are in primary contact with seawater, high yield strength and high corrosion resistance are required.

The super duplex stainless steel has a structure composed of the ferrite structure and the austenitic structure in the same ratio, the strength is higher than that of the austenitic stainless steel, pitting corrosion and resistance to stress corrosion cracking This has an excellent advantage.

In general, in the case of super duplex stainless steel, it is also an important task to control the balance between the austenitic phase and the ferrite phase to secure excellent corrosion resistance and to improve hot workability, which can suppress defects that may occur during sheet production.

The conventional super duplex stainless steel is manufactured through a total of four steps including a casting process, a forging process, a drawing process, and a solution annealing process. However, the austenite structure of the super duplex stainless steel manufactured by the conventional method starts to be formed unevenly after the casting process, becomes more unevenly formed through the forging process and the drawing process, and is uneven even when the solid solution heat treatment process proceeds. There was a problem that the state was maintained (see FIG. 1).

The super duplex stainless steel produced by the above conventional method includes a non-uniform austenite structure until the point at which processing is completed. Looking at this in detail with reference to Figure 2, the structure of the conventional super duplex stainless steel will generate a deficient zone (a) of austenite phase and a dense zone (b) of austenite phase, the surface area of this non-uniform austenite phase is a super duplex stainless steel It acts as a factor in lowering the strength, ductility and corrosion resistance of steels, and in particular promotes the development of formulas. As described above, the conventional super duplex stainless steel has a problem in that the austenite structure is uneven in shape and thus corrosion resistance is lowered.

Therefore, there is a need for a technology for improving the corrosion resistance by uniformly controlling the structure of the super duplex stainless steel.

KR 10-1735003 B1

Disclosure of Invention The present invention has been made to solve the above problems, and an object thereof is to provide a super duplex stainless steel and a method of manufacturing the same, which can improve the corrosion resistance by uniformly controlling the austenite structure of the super duplex stainless steel.

On the other hand, the object of the present invention is not limited to the above-mentioned object, other objects that are not mentioned will be clearly understood from the following description.

The present invention for solving the above technical problem is by weight, Cr 25.0%, Ni 6.8%, Mo 3.75% to 3.85%, Mn 0.79%, N 0.27%, Cu 0.18%, C 0.014%, W 0.01% to 0.02 A homogenization heat treatment step of homogenizing the austenite structure by heat-treating a steel sheet containing%, balance Fe and unavoidable impurities at 1275 ° C to 1325 ° C; And a solid solution heat treatment step of heat-treating the homogenized steel sheet and then quenching and solubilizing the steel sheet.

In addition, the shape of the austenite structure in the homogenization heat treatment step is characterized in that the sphere is controlled.

The present invention for solving the other technical problem is by weight, Cr 25.0%, Ni 6.8%, Mo 3.75% to 3.85%, Mn 0.79%, N 0.27%, Cu 0.18%, C 0.014%, W 0.01% to It includes 0.02%, residual Fe and unavoidable impurities, and the critical pitting termperature (CPT) is 80 ℃ to 90 ℃ relates to a super-duplex stainless steel with improved corrosion resistance.

In addition, the pitting resistance equivalent number (PREN) according to Equation 1 below is characterized in that 40 or more.

Equation 1 PREN = Cr + 3.3 (Mo + 0.5W) + 16N

Cr, Mo, W, N in the formula 1 is the weight percent content of each element.

In addition, by weight, the (Mo + W) content is characterized in that it is included in the range 3.81 to 3.82.

By means of solving the above problems, the super-duplex stainless steel and the method of manufacturing the improved corrosion resistance of the present invention by heat-treating the steel sheet at 1275 ℃ to 1325 ℃, by continuously solidifying, reducing the proportion of austenite structure Of course, the shape of the austenitic structure is controlled to be spherical and the distribution is uniform, thereby improving the corrosion resistance.

1 is a view showing a conventional manufacturing method and the structure of the super duplex stainless steel produced according to the method.
2 is a view for explaining the non-uniform state of the austenite structure of the super duplex stainless steel manufactured according to the conventional manufacturing method.
Figure 3a is a view for explaining the structure of the super duplex stainless steel according to the heat treatment heat treatment conditions of the conventional manufacturing method.
Figure 3b is a graph showing the phase ratio and PRE of the super duplex stainless steel according to the solid solution heat treatment conditions of the conventional manufacturing method.
4 is a view showing the structure of the super duplex stainless steel according to the homogenization heat treatment conditions of the present invention.
5 is a view showing the structure of the homogenized heat treatment and solid solution heat treatment super duplex stainless steel according to an embodiment of the present invention.
Figure 6 is a graph showing the phase percentage of the super duplex stainless steel manufactured according to an embodiment of the present invention.
7 is a graph showing the amount of secondary phase precipitation of the super duplex stainless steel manufactured according to an embodiment of the present invention.
8 is a graph showing the critical pitting temperature (CPT) of the super duplex stainless steel according to the heat treatment conditions of the present invention.
9 is a graph showing the electrochemical characteristics of the super duplex stainless steel prepared according to an embodiment of the present invention.
10 is a graph showing the physical properties of the super duplex stainless steel manufactured according to an embodiment of the present invention.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various forms, and only the embodiments are to make the disclosure of the present invention complete, and those skilled in the art to which the present invention pertains. It is provided to inform the full scope of the invention. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. In this specification, the singular forms also include the plural unless specifically stated otherwise in the phrases.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. On the other hand, illustrated and detailed description of the configuration and its operation and effects can be easily understood from those skilled in the art will be briefly or omitted and will be described in detail with respect to the parts related to the present invention.

Corrosion resistance improved super-duplex stainless steel manufacturing method according to the invention by weight, Cr 25.0%, Ni 6.8%, Mo 3.75% to 3.85%, Mn 0.79%, N 0.27%, Cu 0.18%, C 0.014%, W A homogenization heat treatment step of homogenizing the austenite structure by heat-treating a steel sheet containing 0.01% to 0.02%, balance Fe and inevitable impurities at 1275 ° C to 1325 ° C; And a solid solution heat treatment step of quenching and solidifying the heat-treated homogenized steel sheet.

Looking at each component contained in the steel sheet of the present invention is as follows.

Chromium (Cr) is a very effective element for increasing the corrosion resistance to most corrosion. In order to improve the corrosion resistance, it is preferable to add 25% by weight of chromium, and if it exceeds 25% by weight, there is a problem that the tendency of intermetallic precipitation increases.

Nickel (Ni) is used as an austenite stabilizing element and must be added in an appropriate amount to obtain the desired ferrite content. It is preferred to add 6.8% by weight for an appropriate ratio of austenite and ferrite phases.

Molybdenum (Mo) is an element that improves the corrosion resistance in the chloride environment, in particular reducing acids. If more molybdenum and chromium are added than the above-mentioned contents, the amount of intermetallic precipitation may increase, so it is preferable to add 3.75 to 3.85 wt% of molybdenum.

Manganese (Mn) is used as the austenite stabilizing element and must be added in an appropriate amount to obtain the desired ferrite content. It is preferable to add 0.79% by weight for an appropriate ratio of austenite and ferrite phases.

Nitrogen (N) is a very effective element that increases the corrosion resistance, tissue stability and strength of the material. If the content of nitrogen is increased, the regeneration of austenite after welding can be obtained to obtain a good weld joint with good characteristics, but if the content of nitrogen and chromium are high at the same time, chromium nitride can be precipitated, thus obtaining an excellent effect of nitrogen In order to add 0.27% by weight, it is preferable.

Copper (Cu) is an element that improves general corrosion resistance in an acid environment such as sulfuric acid, and also affects tissue stability. If the content of copper is high, the problem of excessively increasing the solid solution occurs, so it is preferable to add 0.18% by weight.

Tungsten (W) is an element that increases the corrosion resistance to point corrosion and crevice corrosion. If the content of chromium, molybdenum and tungsten is too high, the amount of precipitation between metals may increase, it is preferable to add 0.01 to 0.02% by weight.

In addition, the (Mo + W) content by weight may be included in the range from 3.81 to 3.82. When a large amount of tungsten (W) and molybdenum (Mo) are added in order to improve corrosion resistance and hot workability, secondary phases such as chi (χ) and sigma (σ) phases are precipitated at high temperatures, thereby impact properties and corrosion resistance. This is because it causes a problem of deterioration.

The homogenization heat treatment step is to solve the problem that the austenitic structure of the conventional super duplex stainless steel is non-uniformly formed to lower the corrosion resistance by heat-treating the steel sheet at 1275 ℃ to 1325 ℃ for 30 to 60 minutes, austenitic structure Can be made uniform. Preferably, the heat treatment temperature of the homogenization heat treatment step may be 1285 ° C to 1320 ° C, more preferably 1295 ° C to 1310 ° C.

By performing the homogenization heat treatment under the conditions of the temperature and time as described above, the austenitic structure is controlled to be spherical, thereby reducing the surface energy has the advantage of obtaining a more stable super duplex stainless steel.

In other words, the target temperature of 1275 ℃ to 1325 ℃ to prevent the secondary phase from being deposited during the heat treatment, by controlling the non-uniform shape and distribution of the austenitic structure was determined that the temperature can be uniformly distributed in the sphere In addition, the optimal time was derived.

In addition, in the solid solution heat treatment step, the heat-treated homogenized steel sheet at 1000 to 1100 ℃, it is preferable to quench the super duplex stainless steel in water below 5 ℃.

On the other hand, as another embodiment of the present invention, the present invention by weight, Cr 25.0%, Ni 6.8%, Mo 3.75% to 3.85%, Mn 0.79%, N 0.27%, Cu 0.18%, C 0.014%, W 0.01 It relates to a super-duplex stainless steel with improved corrosion resistance, characterized in that% to 0.02%, the balance Fe and unavoidable impurities, the critical pitting termperature (CPT) is 80 ℃ to 90 ℃.

And in weight percent, the (Mo + W) content may be included in the range from 3.81 to 3.82.

At this time, the description of the components and (Mo + W) content of the super duplex stainless steel is the same as described in the manufacturing method of the super duplex stainless steel.

In addition, the super duplex stainless steel is characterized in that the pitting resistance equivalent number (PREN) according to Equation 1 below 40 or more, Cr, Mo, W, N in the following Equation 1 is the weight percent of each element Means content.

Equation 1 PREN = Cr + 3.3 (Mo + 0.5W) + 16N

In the embodiment of the present invention to use the steel sheet having the components of Table 1 to achieve the above object, the physical properties before heat treatment of the steel sheet is shown in Table 2.

ingredient C N Mn Ni Cr Mo Cu W Fe weight% 0.014 0.27 0.79 6.8 25.0 3.8 0.18 0.02 Bal

0.2% Proof Stress (N / mm ² ) [ksi] minimum 550 [7.98] Ultimate Tensile Strength (N / mm ² ) [ksi] minimum 760-800 [116] Elongation (%) minimum 20 to 25 Hardness (HBN) 270 max Reduction of Cross Section Area (%) 45 Charpy V-notch Impact at ambient Temp (J) [ft.lb] 80min [59min] Charpy V-notch Impact at -46 ° C (J) [ft.lb] 45av, 35min [33av, 25.8min]

1. (Comparative example) Super duplex stainless steel according to the conventional manufacturing method

For comparison with the super duplex stainless steel manufactured according to the present invention, a super duplex stainless steel heat-treated according to a conventional manufacturing method will be described first.

1 is a view showing a conventional manufacturing method and the structure of the super duplex stainless steel manufactured according to the method, Figure 2 is a view for explaining the non-uniform state of the austenite structure of the super duplex stainless steel produced according to the conventional manufacturing method Drawing.

Figure 3a is a view for explaining the structure of the super duplex stainless steel according to the solid solution heat treatment conditions of the conventional manufacturing method, Figure 3b shows the phase percentage and PRE of the super duplex stainless steel according to the solid solution heat treatment conditions of the conventional manufacturing method As a graph, (a) is a graph showing the percentage of phase, and (b) is a graph showing the fitting resistance equivalent value (PRE).

Referring to FIG. 1, a conventional super duplex stainless steel is manufactured through a total of four steps including a casting process, a forging process, a drawing process, and a solid solution heat treatment process. Specifically, the austenite structure of the steel sheet begins to form unevenly after the casting process, and is more uniformly formed through the forging process and the drawing process, and continues to be uneven even after the solid solution heat treatment process proceeds. Appeared to be maintained.

The super duplex stainless steel produced by the above conventional method includes a non-uniform austenite structure until the point at which processing is completed.

The area indicated by (a) of FIG. 2 is a deficiency zone of austenite tissue, and the area indicated by (b) is a dense zone of austenite tissue. This non-uniform austenite structure acts as a deterioration in the corrosion resistance of super duplex stainless steel, and in particular promotes the formation of a formula, which causes the destruction of the material.

In order to solve this problem, it was confirmed that even if the temperature was increased in the conventional heat treatment heat treatment process, only the fraction of the tissue was affected, and the shape and distribution of the austenitic structure were kept in an uneven state (FIGS. 3A and FIG. 3). 3b).

2. (Example) Super duplex stainless steel according to one embodiment of the present invention

(1) Selection of homogenization heat treatment conditions

Figure 4 is a view showing the structure of the super duplex stainless steel according to the homogenization heat treatment conditions of the present invention, Figure 5 is a view showing the structure of the homogenization heat treatment and solid solution heat treatment super duplex stainless steel according to an embodiment of the present invention.

Referring to FIG. 4, as a result of homogenizing the austenite structure by heat-treating the steel sheet at 1200 ° C., a fine austenite structure was formed than before the heat treatment. Narrow austenitic and coarse austenitic structures of 10 μm or less remained on the outside, and segregation partially occurred, as indicated in yellow.

In addition, as a result of homogenizing the austenite structure by heat-treating the steel sheet at 1300 ° C., the homogenization of the austenite structure was generally obtained, resulting in a uniform distribution. It was confirmed that a spherical austenite structure having a diameter of 10 to 15 μm on the outside and a diameter of 10 to 30 μm on the inside was formed.

That is, in order to reduce the surface energy of the steel sheet to produce a more stable super duplex stainless steel, it is preferable to homogeneously heat-treat the steel sheet at 1300 ° C to control the shape of the austenite structure into a spherical shape.

Referring to Figure 5, the structure (a) of the steel sheet subjected to the homogenization heat treatment step at 900 ℃, 1000 ℃, 1100 ℃, 1200 ℃ and 1300 ℃ (a) and the structure of the super duplex stainless steel undergoing a solid solution heat treatment step at 1100 ℃ after homogenization (b) can be compared, through which suitable homogenization heat treatment conditions and solid solution heat treatment conditions can be selected.

As a result, after the homogenization heat treatment step at 1200 ℃ and 1300 ℃, the austenite structure of the super duplex stainless steel, which is solid solution heat treated at 1100 ℃ is spherical form and began to exhibit a uniform distribution, in particular homogenized heat treatment at 1300 ℃ The austenitic structure of the super duplex stainless steel showed a more dense and uniform distribution.

Accordingly, in order to control the shape and distribution of the austenitic structure, it is preferable to perform a homogenization heat treatment at 1200 ° C to 1300 ° C. In particular, it is preferable that the homogenization heat treatment is performed at 1300 ° C and then the solid solution heat treatment at 1100 ° C. Judging.

(2) Effect of Homogenization Heat Treatment on the Phase Ratio of Super Duplex Stainless Steel

Figure 6 is a graph showing the percentage by temperature homogenization heat treatment temperature of the super duplex stainless steel manufactured according to an embodiment of the present invention, (a) is a graph of the austenitic structure, (b) is a graph showing the ratio of the ferrite structure. . 7 is a graph showing the amount of secondary phase precipitation of the super duplex stainless steel manufactured according to an embodiment of the present invention.

At this time, the percentage of austenite structure and ferrite structure was measured according to ASTM E 1245.

Referring to FIG. 6, the ratio of the austenite structure was high when the super duplex stainless steel was homogenized at a temperature of less than 1100 ° C., but the ratio of the austenite structure and the ferrite structure was the same at 1100 ° C. and exceeded 1100 ° C. The proportion of ferrite tissue was higher than that of austenite tissue.

In addition, referring to FIG. 7, the secondary phase of 18.8% was precipitated in the steel sheet which was not heat-treated, but in the case of super duplex stainless steel homogenized and heat-treated at 1300 ° C., the secondary phase was suppressed and the precipitation amount was significantly reduced to 0.6%. In other words, it is safe to assume that the secondary phase hardly precipitates.

That is, by homogenizing heat treatment at 1300 ° C, the fraction of the ferrite structure of the super duplex stainless steel can be increased, the formation of the secondary phase can be suppressed, and the corrosion resistance can be improved.

(3) Critical formula temperature of super duplex stainless steel according to heat treatment conditions

8 is a graph showing the critical pitting temperature (CPT) of the super duplex stainless steel according to the heat treatment conditions of the present invention.

The critical formula temperature means the temperature at which the formula is stable in seawater. The higher the value, the higher the temperature at which the formula grows. That is, the critical formula temperature is used as an index indicating corrosion resistance.

In order to measure the critical formula temperature of the super duplex stainless steel of the present invention, a super duplex stainless steel specimen having a size of 18 mm 10 mm 10 mm was used, and corrosion resistance was evaluated according to ASTM G 5-150. Specific experimental conditions are shown in Table 3 below.

Test on the surface 1 cm ² Parameters temperature Starting temperature 0 ~ 3 ℃

CPT Cell Configuration

Pt mesh (CE) SCE (RE) Specimen (WE) Thermocouple Heating pad Potential 700 mV _SCE Temperature increasing 1 ℃ / min Electrolyte 1M NaCl

Referring to FIG. 8, the CPT of the steel sheet which was not heat-treated was measured at 59.6 ° C., and the CPT of the solid solution heat-treated specimen at 1100 ° C. was measured at 71.6 ° C. as in the conventional manufacturing method, as in the manufacturing method of the present invention. The CPT of the specimen homogenized at 1300 ° C. and then solid solution heat treated at 1100 ° C. was measured at 87.3 ° C.

That is, since the CPT of the specimen was increased through the homogenization heat treatment step before the solid solution heat treatment, it can be seen that the corrosion resistance is improved.

(4) Characterization of Super Duplex Stainless Steel According to Heat Treatment Conditions

9 is a graph showing the electrochemical characteristics of the super duplex stainless steel prepared according to an embodiment of the present invention, Figure 10 is a graph showing the physical characteristics of the super duplex stainless steel prepared according to an embodiment of the present invention.

Referring to FIG. 9, when the electrical characteristics of the specimen were examined in region (A), the potential of the specimen homogenized and heat treated at 1300 ° C. was the highest and the current density was the lowest. The electrical properties of the specimens in zone (B) showed that the homogeneous heat treatment at 1300 ° C and the solid solution heat-treated at 1100 ° C had the highest potential and lowest density.

Looking at the tensile shear strength of the specimen according to the heat treatment conditions with reference to Figure 10, the specimen not subjected to the heat treatment is about 780MPa, homogenized heat treatment at 1300 ℃ homogenized heat treatment at about 800MPa, 1300 ℃ and heat-treated heat treatment at 1100 ℃ The specimen was measured at about 830 MPa. That is, by sequentially performing a homogenization heat treatment and a solid solution heat treatment according to an embodiment of the present invention, it is possible to produce a super duplex stainless steel having high physical properties of about 830 MPa tensile shear strength.

In addition, the elongation of the specimen according to the heat treatment condition will be described with reference to FIG. 10. Elongation is the rate at which the material is stretched during the tensile test, the amount of elongation in percent and can be measured by standards such as ASTM M420. As a result of examining the elongation of the specimen according to the heat treatment condition, the specimen which was not heat treated was 30%, the homogenized heat treated specimen at 1300 ° C was 26%, the homogenized heat treated at 1300 ° C, and the solid solution heat treated at 1100 ° C was 28%. Was measured. That is, the specimens that were not heat treated had the highest austenite fraction and the highest elongation was measured. According to an embodiment of the present invention, the specimens subjected to homogenization heat treatment at 1300 ° C. and the solid solution heat treatment at 1100 ° C. were made of ferrite tissue. The elongation seems to have decreased due to the increased fraction and the reduced fraction of austenite tissue.

As described above, the present invention undergoes a homogenization heat treatment step at 1275 ° C to 1325 ° C, and then performs a solid solution heat treatment step continuously to control the shape of the austenite structure to be spherical and to make the distribution uniform. Excellent tensile resistance as well as tensile shear strength and 28% elongation can be obtained.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the claims that follow may be better understood. Those skilled in the art will appreciate that the present invention can be embodied in other specific forms without changing the technical spirit or essential features of the present invention. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive. The scope of the invention is indicated by the following claims rather than the above description, and all changes or modifications derived from the claims and their equivalents should be construed as being included in the scope of the invention.

Claims (5)

By weight percent Cr 25.0%, Ni 6.8%, Mo 3.75% to 3.85%, Mn 0.79%, N 0.27%, Cu 0.18%, C 0.014%, W 0.01% to 0.02%, balance Fe and inevitable impurities A homogenization heat treatment step of homogenizing the austenite structure by heat treating the steel sheet at 1275 ° C to 1325 ° C; And
And a solid solution heat treatment step of quenching and solidifying the heat-treated homogenized steel sheet.
The method of claim 1,
In the homogenization heat treatment step, the shape of the austenitic structure is a spherical shape, characterized in that the control of the super-duplex stainless steel with improved corrosion resistance.
By weight percent Cr 25.0%, Ni 6.8%, Mo 3.75% to 3.85%, Mn 0.79%, N 0.27%, Cu 0.18%, C 0.014%, W 0.01% to 0.02%, balance Fe and inevitable impurities ,
Critical pitting termperature (CPT) is a super duplex stainless steel with improved corrosion resistance, characterized in that 80 ℃ to 90 ℃. The method of claim 3,
Corrosion resistance improved super duplex stainless steel, characterized in that PREN (Pitting Resistance equivalent number) according to Equation 1 below 40.
Equation 1 PREN = Cr + 3.3 (Mo + 0.5W) + 16N
Cr, Mo, W, N in the formula 1 is the weight percent content of each element.
The method of claim 3,
By weight, (Mo + W) content of the super-duplex stainless steel with improved corrosion resistance, characterized in that included in the range 3.81 to 3.82.

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