KR100413822B1 - ferritic stainless steel with improved dissimilar materials crevice corrosion resistance - Google Patents

Abstract

The present invention relates to a ferritic stainless steel used for seawater heat exchanger, and to ferritic stainless steel for seawater heat exchanger, which has improved corrosion resistance and toughness by adding tungsten and Ni to existing component systems (UNS S44800, UNS S44700, UNS S44735). It aims to provide.

In particular, the present invention is characterized in that Nb + Ti is added as a stabilizing element and the composition is satisfied to satisfy the condition of adding Al so that the oxygen content is 0.005% or less. It also provides a high chromium ferritic stainless steel which improves the corrosion resistance by the addition of W and improves the toughness without changing the high temperature phase precipitation rate by the addition of Ni and W.

According to the present invention, it is possible to obtain a high chromium ferrite stainless steel which improves the corrosion resistance by the addition of W and improves the toughness without changing the high temperature phase precipitation rate by the addition of Ni and W.

Description

Ferritic stainless steel with improved dissimilar materials crevice corrosion resistance}

The present invention relates to a ferritic stainless steel used for seawater heat exchanger, more specifically, by adding tungsten (W) and nickel (Ni) compared to conventional component system (UNS S44800, UNS S44700, UNS S44735) corrosion resistance and toughness It relates to a ferri-based stainless steel for seawater heat exchanger with improved.

Conventionally, copper alloy stainless steel, Ti alloy and the like have been used as heat exchanger tube materials, and stainless steel having low corrosion resistance and excellent corrosion resistance has been used as a seawater heat exchanger material. In particular, high chromium ferritic steel, which is cheaper, has higher modulus of elasticity, and has higher thermal conductivity than austenitic, has been used, but recently, 30Cr- is used as a material for tubes under mechanical bonding environments such as expansion with 316 steel. Corrosion of 4Mo steels (S44700, S44735, S44800) has occurred and has been a problem. R.G.Kelly explains this phenomenon and notes that it is because of the lower potential and higher current when activation is observed compared to the 300 series (vol. 51, CORROSION p. 826).

In order to suppress corrosion of high chromium ferrite steel under galvanic couple with 316 steel, it is required to add alloying elements to lower the current in the active region. In general, W has been reported to improve the passivation properties of alloys, and steel grades having improved corrosion resistance using these properties have been previously filed (application number: 195-17466). However, previously applied steel species are limited to an ideal system having a high content of Ni in the steel and are not reported for ferritic stainless steels.

In general, ferritic steels are very weak in toughness, so improving the toughness becomes a very important property and it is necessary to minimize the addition of alloying elements (Cr, Mo, W, C, N, O, P). In particular, Ni may increase the deposition rate of high temperature precipitates to reduce toughness. However, when the formation of high temperature precipitates is suppressed (Mo <2%), Ni has been reported to increase toughness by increasing C and N solid solubility in steel. . In the case of 30Cr-4Mo steel, since the Mo content is more than 4%, the toughness may be reduced due to the formation of high temperature precipitates. However, if rapid cooling is possible during the performance, the toughness improvement by Ni addition is not impossible. In addition, there is a need for a method of increasing corrosion resistance and reducing toughness through the addition of microalloys. According to the patent application of high-alloy ferritic stainless steel (Application No. 98-26854), the toughness and corrosion resistance of ferritic stainless steel added with% Nb <12 × (C + N) and Al (0.05% or less) It is reported to be excellent.

Therefore, the present invention is to improve the corrosion resistance by adding W, without changing the pitting index, compared to the existing ferritic steel, and the purpose of the development of ferritic stainless steel with improved toughness by adding Ni and fine alloying elements (Ti, Al) have.

In order to achieve the above object, the present invention, in the ferritic stainless steel, by weight%, C: 0.015% or less, N: 0.02% or less, Si: 0.5% or less, Mn: 0.5% or less, P: 0.03% or less , S: 0.004% or less, O: 0.005% or less, Nb: 0.36% or less, Ti: 0.15% or less, Cu: 0.5% or less, Al: 0.15% or less, Mo: 3-5%, Ni: 2% or less, W: 3.5% or less, Cr: 28-32%, balance Fe and unavoidable impurities, Nb + Ti is added as a stabilizing element, and is formed by satisfying the condition of adding Al so that the oxygen content is 0.005% or less. It is an object of the present invention to provide a ferritic stainless steel for seawater heat exchanger.

1 is a schematic diagram showing a configuration of a general heat exchanger.

Figure 2 (a), (b), (c), (d) is a graph showing the high temperature precipitation rate according to the alloy component.

Hereinafter, the present invention will be described with reference to the drawings.

In the present invention, similar to the chemical composition of the high-chromium ferritic steel (application number: 98-26854) patented by the addition of the fine alloy (Nb + Ti) as a stabilizing element of the composite addition and deoxidized with Al, respectively The amount of element added is determined in the following limits, unlike conventional ferritic steels. In order to improve toughness, Ti is added as long as coarse precipitation of TiN (2 μm or less) is suppressed by Ti addition, and the stabilization element content required for inhibition of sensitization is Nb added instead of Ti. In addition, Al is added as a deoxidizer, so the maximum amount of Al added (0.15% or less) is determined by the maximum size of Al-oxide, and the maximum size is 1 μm or less.

As such, it provides a ferritic stainless steel for seawater heat exchanger which minimizes Ti and Al contents and adds Ni and W to improve toughness and corrosion resistance.

In the present invention, by weight%, C: 0.015% or less, N: 0.02% or less, Si: 0.5% or less, Mn: 0.5% or less, P: 0.03% or less, S: 0.004% or less, O: 0.005% or less, Nb : 0.36% or less, Ti: 0.15% or less, Cu: 0.5% or less, Al: 0.15% or less, Mo: 3-5%, Ni: 2% or less, W: 3.5% or less, Cr: 28-32%, balance It is composed of Fe and unavoidable impurities, Nb + Ti is added as a stabilizing element, and the composition is satisfied to satisfy the condition of adding Al so that the oxygen content is 0.005% or less. Further, the corrosion resistance by the addition of W is improved. At the same time, the toughness is improved without changing the high temperature phase precipitation rate by the addition of Ni and W.

Hereinafter, the reason for limiting the composition range of the steel of the present invention will be described in detail.

C and N: In order to minimize the reduction in toughness, the maximum of each of the C and N element contents is 0.015% and 0.02%. However, the smaller the content, the better the properties of the material, e.g. elongation, impact toughness, corrosion resistance, etc., the minimum is not determined.

Si: Silicon is an element that increases deoxidation and oxidation resistance. In order to suppress the decrease in toughness, the Si content is limited to within 0.5%.

Mn: Manganese is an element that increases deoxidation. However, the inclusion MnS reduces the corrosion resistance, limiting the Mn content to within 0.5%.

P: Since phosphorus not only reduces corrosion resistance but also toughness, it limits the P content to within 0.03%.

S: Sulfur forms MnS to reduce corrosion resistance, thus limiting the S content to within 0.004%.

O: Oxygen increases inclusion content, reducing toughness and corrosion resistance. Therefore, it is good to suppress the oxygen content as much as possible and limit it to within 0.005%.

Cu: Copper not only increases corrosion resistance in a reducing atmosphere, but also increases pitting resistance. However, excessive addition reduces stress corrosion resistance and hot workability, thus limiting the addition amount to within 0.5%.

Al: Aluminum is an element added for deoxidation. The addition of Al increases the toughness, but the Al-oxide is formed when excessively added to reduce the corrosion resistance, so Al in steel should be suppressed as much as possible and the upper limit should be controlled within 0.15%.

Ti: Titanium is not used for welding because the element added to inhibit MnS formation that not only inhibits sensitization but also suppresses the corrosion resistance of steel or coarse TiN formation during solidification decreases toughness. If no Ti is added, it is advantageous in terms of toughness of the product. However, when welding is inevitable due to the use of the material, the welded toughness of the Ti-added steel is similar to the welded toughness of the Ti-free steel, but Ti is advantageous in terms of weld ductility. In addition, corrosion resistance after pickling steels added with Ti is superior to those without Ti. Therefore, it is necessary to minimize the formation of coarse TiN by adding Ti to 0.15% or less in consideration of toughness.

Nb: Niobium is an element added to prevent sensitization. The addition of 0.36% or more leads to a decrease in toughness due to the precipitated phase, so the maximum content is 0.36%. The appropriate stabilization ratio to prevent sensitization is (Ti + Nb) / (C + N)> 10. Therefore, the content of titration Nb is 10x% (C + N)-% Ti (-0.03%). Although the content of (C + N) of the base material is limited to 0.02%, the concentration of nitrogen may increase due to contamination of nitrogen in the air during welding, so the appropriate Nb content is determined by the degree of contamination. Determined by the equation.

% Nb = 10 ×% (C + N)-% Ti (˜0.03%)

% (C + N) = weight content of welded part (C + N)

Cr: Chromium increases the corrosion resistance, but as the Cr content increases, the increase in the precipitation rate of Cr (C, N) and Cr-oxide causes a decrease in toughness, so the appropriate range of Cr is limited to 28 to 32%. If the Cr content is 28% or less, the effect of improving corrosion resistance cannot be expected. If the content is 32% or more, the strength is too high, the elongation is low, the moldability is worsened, and the toughness is sharply lowered due to sigma precipitation during cooling.

Mo: Molybdenum causes a decrease in toughness due to high-temperature phase (σ, x) precipitation but increases corrosion resistance, so a minimum amount of corrosion resistance is required and an appropriate range is 3 to 5%.

W: Tungsten moves the precipitation region of the high temperature phase (σ, x) to a high temperature and at the same time increases the precipitation rate but improves the passivation characteristics, so the appropriate range is within 3.5%.

Ni: Nickel decreases toughness by increasing high temperature (σ, x) precipitation rate, but suppression of precipitation through rapid quenching in the precipitation zone increases the toughness of carbon and nitrogen in the steel, thus improving toughness. Is the range that can suppress the precipitation phase by rapid cooling and is less than 2%.

Hereinafter, the present invention will be described in detail through examples.

(Example 1)

The chemical composition of 25 Kg ingots is shown in Tables 2 and 3. Each ingot was melted in a vacuum induction furnace and subjected to homogenization by heat treatment at 1270 ° C. for 2 hours in an Ar atmosphere. After the heat treatment was rolled to a thickness of 6mm and water cooled. These plates were heat treated at 1100 ° C. for 1 minute before cold rolling to a thickness of 3 mm. Finally, cold annealing was performed at 1050 to 1100 ° C. for 30 seconds. Each specimen was ground to SIC # 200 prior to testing.

Table 5 shows the crevice corrosion resistance of steel. The crack corrosion resistance was expressed as the magnitude of the current in the active region after polarization test in chlorine solution (0.8M HCl + 3.2M LiCl, room temperature). That is, the lower the current value, the higher the crack corrosion resistance. Compared to the existing steels (No 5. CO1, CO2), the austenitic stainless steels (CO3, CO4) containing a large amount of Ni have excellent crack corrosion resistance. However, W-added ferritic stainless steel (No 7 9) shows very good corrosion resistance (see Table 2).

Table 6 is a table evaluating the resistance of the material in the galvanic couple environment with 316 steel. The figures mentioned in the table indicate the current generated during electrical contact between the material of interest and 316 steel in a brine solution (0.5M HCl + 3.2M LiCl, room temperature) .The higher the absolute value of the current value, the lower the resistance of the material. it means. In addition, "-" sign means corrosion of the material connected to the 316-based steel, "+" sign means corrosion of the 316-based steel. From an engineering point of view, corrosion should not occur in either material, so the magnitude of the current value that represents the rate of corrosion is more important than the sign. Ferrite steel without W showed higher current value than austenitic steel, but ferrite steel with W was similar to that of austenitic steel. Therefore, austenitic steel containing expensive Ni is the best, but considering the material cost, ferritic steel with W is considered to be most suitable.

Table 7 shows the passivation current density according to the alloying elements in the saline solution (0.8M HCl + 3.2M LiCl, room temperature) to evaluate the stability of the passivation film. The lower the current density of the passivation film, the more stable the passivation film is, which means higher pitting resistance. No change of passive current density was observed with addition of Ni, but the addition of W improved the pitting resistance regardless of the co-addition of Ni (reduced the passive current density by about 10 times), while the same pitting formula compared to ferritic steels. The passivation current density of steels with index was about 10 times higher, which shows the good pitting resistance in ferritic steels.

Figure 2 is a surface perpendicular to the rolling surface and parallel to the rolling direction by polishing the following 3mm thick annealing material to 0.3μm after heat treatment to the following 3mm thick annealing material in order to investigate the effect of high temperature phase precipitation rate The precipitated phase was observed using an SEM at 500 magnification, and the fraction of the precipitated phase calculated using an image analyzer is shown.

No increase in precipitation rate was observed in the 30Cr-2Mo-2W steel with 2% W added based on the 30Cr-4Mo steel. Since the temperature in the region with the highest precipitation rate tends to increase, the precipitation fraction is expected to decrease in the actual process, so it is predicted that there will be little change in toughness due to the addition of W. However, the addition of Ni rapidly increases the high temperature precipitation rate, so it is necessary to limit the addition of Ni. However, this is related to the cooling process equipment, so if the speed of cooling can be higher than the rate of precipitation phase, it is necessary to add Ni in terms of improving room temperature toughness.

Steel grade Heat treatment time (minutes) Heat treatment temperature (℃) 5-9 10, 60, 180, 600 725, 775, 825, 875, 925, 975

no Cr Mo C N Ti Nb Al Ni W P Cu Si Mn S O 5 30.1 4.0 .008 .01 .05 .26 .05 NA NA 0.03 0.5 or less 0.33 0.3 .002 .007 6 29.8 3.5 .01 .011 .06 .31 .05 NA 1.0 0.03 0.27 0.3 .002 .007 7 29.9 3.2 .011 .01 .06 .26 .05 NA 2.15 0.03 0.3 0.3 .002 .005 8 29.8 3.2 .013 .011 .06 .26 .05 1.05 2.07 0.03 0.27 0.3 .002 .004 9 29.7 3.2 .015 .01 .06 .26 .06 2.0 2.18 0.03 0.29 0.3 .002 .004

Alloy Cr Mo Ni C N Ti Nb Cu Mn P S Si AI129-4C 28.8 3.78 0.26 0.02 .031 0.36 0.29 - 0.26 0.02 .002 0.28 AI29-4-2 28.9 3.83 2.42 .002 .008 - - ≤0.1 <1 0.02 0.01 0.11 AI6XN 20.4 6.23 23.9 .023 0.21 - - 0.17 0.29 0.02 .0004 0.41 254SMO 20 6.1 17.9 .012 0.20 - - 0.68 0.49 .019 .001 0.35

River bell Activation Current Density (mA / cm ² ) Steel grade number Chemical composition NO 5 30Cr-4Mo 15 NO 7 30Cr-3Mo-2W Not detected NO 9 30Cr-3Mo-2W-2Ni Not detected CO 1 29Cr-4Mo 11 CO 2 29Cr-4Mo-2Ni 11 CO 3 20Cr-6Mo-18Ni 1.5 CO 4 20Cr-6Mo-24Ni 0.8

River bell Corrosion rate after galvanic couple (mA / cm ² ) Steel grade number Chemical composition NO 5 30Cr-4Mo -160 NO 7 30Cr-3Mo-2W -78 NO 9 30Cr-3Mo-2W-2Ni -80 CO 1 29Cr-4Mo -163 CO 2 29Cr-4Mo-2Ni -165 CO 3 20Cr-6Mo-18Ni +58 CO 4 20Cr-6Mo-24Ni +56

River bell Passive current density (㎂ / cm ² ) Steel grade number Chemical composition NO 5 30Cr-4Mo 2 × 10 ^-2 No 6 30Cr-3.5Mo-1W 4 × 10 ^-4 NO 7 30Cr-3Mo-2W 10 ^-4 No 8 30Cr-3Mo-2W-1Ni 10 ^-4 NO 9 30Cr-3Mo-2W-2Ni 10 ^-4 CO 1 29Cr-4Mo 2 × 10 ^-3 CO 2 29Cr-4Mo-2Ni 2 × 10 ^-3 CO 3 20Cr-6Mo-18Ni 10 ^-2 CO 4 20Cr-6Mo-24Ni 10 ^-2

As described above, ferritic steel with W added to seawater heat exchanger as compared to the conventional ferritic steel not only has excellent pitting resistance and crack corrosion resistance, but also has a high degree of cracking during mechanical bonding with 315 steel, which is a plate material. In the environment, the corrosion resistance was about 2 times better than the existing ferritic steel, and the corrosion resistance was similar to that of the austenitic steel to which expensive Ni was added. In addition, since there is no increase in high temperature precipitation rate due to the addition of W, there is no need for additional equipment for rapid cooling other than the production facilities of existing steel, and there is no effect of deterioration of toughness due to high temperature precipitation.

Claims (1)

In ferritic stainless steel, By weight%, C: 0.015% or less, N: 0.02% or less, Si: 0.5% or less, Mn: 0.5% or less, P: 0.03% or less, S: 0.004% or less, O: 0.005% or less, Nb: 0.36% Ti: 0.15% or less, Cu: 0.5% or less, Al: 0.15% or less, Mo: 3-5%, Ni: 2% or less, W: 3.5% or less, Cr: 28-32%, balance Fe and inevitable A ferritic stainless steel for seawater heat exchanger, comprising: an impurity; and Nb + Ti is added as a stabilizing element; and an Al content is added so that an oxygen content is 0.005% or less.