KR970009523B1 - High strength & high corrosion resistance of martensite stainless steel - Google Patents

Abstract

A martensitic stainless steel is provided which consists, by weight, of 12-16% of Cr, up to 5% of Ni, 1.5-5% of Mo, 0.1-0.6% of C, 0.05-0.3% of N, up to 0.5% of V, up to 2% of Si, up to 2% of Mn and the balance being Fe. The stainless steel has high strength and is excellent in corrosion resistance under corrosion atmosphere which containing Cl ion.

Description

High Strength High Corrosion Resistance Martensitic Stainless Steel

The present invention relates to martensitic stainless steel having high strength and excellent corrosion resistance and abrasion resistance in a corrosive atmosphere in which chlorine ions are present.

Generally, stainless steels are classified into austenitic stainless steels, ferritic stainless steels, martensitic stainless steels, and precipitation hardening stainless steels. In addition, stainless steels are roughly classified into fine structural steel duplex stainless steels.

Such stainless steels have the best corrosion resistance of austenitic stainless steels in a corrosive atmosphere in which chlorine ions are present, followed by ferritic, precipitation hardening, and martensitic stainless steels.

Therefore, even in the usage amount of stainless steel, the most austenitic 304 stainless steel having excellent corrosion resistance is the most, and the use amount of 316 stainless steel is steadily increasing.

The reason why the martensitic stainless steel is poor in corrosion resistance compared to other stainless steels is that the composition must be present in the austenite stabilization zone γloop at high temperature in order to obtain the martensite structure. This is because it is added as low as 14%. In general, chromium is known to improve the corrosion resistance by stabilizing the passivation film, but when chromium is added 12-14% to improve the corrosion resistance, ferrite is formed during heat treatment at high temperature, so martensite transformation cannot occur due to cooling. And a decrease in hardness, which can cause deterioration of corrosion resistance.

However, martensitic stainless steels have higher mechanical properties than other stainless steels in spite of the defects of corrosion resistance as described above, so that the petroleum and chemical plants, gas turbine engines, turbine blades of power plants, steam and oil As it is widely used in oil refinery pumps, cutting tools, gears, bearings, etc., if the corrosion resistance and mechanical properties are improved, the life of the machine can be extended, resulting in economical effects due to reduced replacement costs, as well as advanced medical care. It can be used as structural material for seawater such as equipment material, oil field pipe, and material for environment-related equipment, etc. to manufacture products with high added value.

Accordingly, it is an object of the present invention to provide martensitic stainless steel of high value and high corrosion resistance and high strength economically by improving the corrosion resistance and mechanical properties of martensitic stainless steel.

1 and 2 are graphs showing the results of the official test (anode polarization test) of the alloy specimens of the present invention and AISI 304, AISI 316, 420, J1, 420, J2 stainless steel.

3 is a graph showing a comparison of the change in hardness (HRC) value according to the (C + N)% content in the alloy specimen composition of the present invention.

4 is a graph showing a comparison of the formula potential (Ep: Fitting Equivalent) according to the PRE (Pitting Resistance Equivalent) value of the alloy specimens of the present invention and AISI 304, AISI 316, 420, J1, 420, J2 stainless steel.

The specific composition of the martensitic stainless steel according to the present invention is 12% by weight, 16% by chromium, 5% or less nickel, 1.5-5% molybdenum, 0.1-0.6% carbon, 0.05-0.3% nitrogen, 0.5% or less vanadium, It is composed of less than 2.0% of silicon, less than 2.0% of manganese and residual iron.

When explaining the role of the components in the alloy of each composition element constituting the martensitic stainless steel according to the present invention as described above in detail as follows.

12-16% chrome

The main purpose of the addition of chromium is to improve the corrosion resistance, and is an element necessary for obtaining the intrinsic corrosion resistance of stainless steel, and improves corrosion resistance and oxidation resistance by forming a thin oxide film to delay or prevent oxidation or corrosion.

However, if too much chromium is added, the steel cannot be hardened because the martensite transformation cannot be obtained by entering the ferrite zone out of the austenitic single phase zone and by heat treatment, and also forming delta ferrite because it is a ferrite forming element. Since the workability and corrosion resistance may be reduced, the content of chromium is limited to 12% or more and 16% or less.

Nickel 5% or less

The addition of nickel stabilizes the austenitic phase, inhibits the formation of the ferritic phase, and increases the cavitation corrosion resistance, erosion corrosion resistance, and bending fatigue strength in terms of corrosion.

However, when too much nickel is added, the austenite is stabilized so that martensite transformation does not occur due to air cooling or oil cooling. In the low nickel content range, the toughness increases with increasing nickel content, but when a certain threshold value is added. Austenitic is stabilized, and martensite and austenite coexist to reduce strength and toughness. Therefore, the content range of nickel is limited to 5% or less.

Molybdenum 1.5 ~ 5%

The purpose of adding molybdenum to stainless steel is to 1) improve corrosion resistance, 2) improve high temperature mechanical properties, and 3) improve martensitic stainless steel's resistance to tempering and strength. That is, the addition of molybdenum increases the resistance to tempering, and can cause the secondary curing reaction to obtain the optimum ductility, toughness, stress corrosion cracking resistance.

When molybdenum is present together with nitrogen, the corrosion resistance is greatly improved by synergism with nitrogen. Therefore, when molybdenum is added, when molybdenum is added too much, it forms delta ferrite because it is a forming element of ferrite. Because it has, it is limited to 5% or less.

0.1 ~ 0.6% carbon

Carbon contributes to stabilizing austenite and improves its strength. The addition of carbon expands the austenite zone, allowing the addition of chromium and molybdenum for improved corrosion resistance and inhibiting the formation of delta ferrite. .

As the carbon content increases, the hardness value is increased up to 0.6%, but the carbon content is high, so that the carbides are precipitated and cause intergranular corrosion and ductility deterioration.

Nitrogen 0.05 ~ 0.3%

The addition of nitrogen is aimed at improving the corrosion resistance and mechanical properties, and when such nitrogen is present with molybdenum, it is known to significantly increase the corrosion resistance by synergistic effect and also increase the grain boundary corrosion resistance.

Nitrogen, like carbon, is a very effective alloying element that expands the austenite zone, and is also employed as an invasive atom in solid solution to achieve a solid solution strengthening of steel.

However, if it is added above the solid solution limit, bubbles are generated during casting, so the content is limited from 0.05 to 0.3%.

Vanadium 0.5% or less

Vanadium is an effective element that suppresses the generation of chromium deficient layer, increases the high capacity of nitrogen, and suppresses grain growth of inversely modified austenite, but when added a lot, the ductility of steel is reduced, so the amount of addition is limited to 0.5% or less.

Less than 2.0% of silicon

Silicon is added for the purpose of deoxidation in the form of Ca-Si, which increases the aging hardening effect by interacting with copper in the aging treatment, and also strengthens the martensite phase and increases the fluidity in casting production.

However, it promotes the formation of delta ferrite and lowers the hot workability to within 2.0%.

Manganese 2.0% or less

Manganese is an austenite stabilizing element, which expands the austenite zone, reduces ductility, creates an oxidative scale during the annealing process, and damages the surface of the steel sheet.

In order to maintain the high strength and high corrosion resistance, the present invention made of the alloying elements described above should be subjected to solution temperature and time of martensitic heat treatment. If such a proper heat treatment is not performed, the general characteristics of the steel of the present invention may be deteriorated.

Description of the Preferred Embodiments

Next, the specimen manufacturing method according to the present invention and the mechanical properties and corrosion resistance evaluation test of the specimen will be described.

Psalm Fabrication

Specimen of the present invention is a high-frequency induction furnace using electrolytic iron (purity 99.9%), chromium (purity 99.2%), molybdenum (purity 99.9%), Fe-Si, Fe-Cr-N as the main materials Soluble in an alumina crucible in air. In this case, the charging order of the materials starts with pure iron, which is a high melting point metal, and is charged in the order of Mo, Cr, Ni, and Fe-Cr-N.

The tapping temperature of the molten metal was about 1600 ° C., and the molten metal was injected into a preheated Y-type mold to prepare an ingot. The manufactured ingot is processed by grinding or machining to a suitable size, soaked at 1100 ° C. for 1 hour, hot rolled, and then water cooled.

The hot rolled specimen is subjected to annealing heat treatment at 1150 ° C. for 1 hour, and then subjected to a pickling treatment by heating a mixed solution of 10% nitric acid and 3% hydrofluoric acid to 50 ° C. in order to remove an oxide scale formed on the surface.

The specimen thus prepared is subjected to austenitic hardening heat treatment and tempering, and evaluated for physical properties and corrosion resistance through the following test. Table 1 below shows the chemical components of the inventive alloy, comparative alloy and conventional steel.

Corrosion Resistance Test Results

(1) Anode Polarization Test Results

The test solution was Hank's solution (NaCl 8.0g / l, CaCl0.14g / l, KCl 0.4g / l, NaHCO0.35g / l, Glncose 1.0g / l, NaHPO · 2HO 0.1g / l, MgCl · 6HO 0.1g / 1, NaHPO.12HO 0.06g / l, MgSO.7HO 0.06g / l) were used, and the potential-current curve was measured while scanning the potential from the corrosion potential to the anode using a potentiostat.

Corrosion resistance in the bipolar polarization test of this method is evaluated by critical current density, passive current density, and formula generation potential. That is, the smaller the threshold current density or the passive current density, the greater the resistance, and the higher the formula generation potential, the greater the resistance.

1 and 2 are the results of the anode polarization test of the alloy steel, the comparative steel and the conventional steel according to the present invention. According to the test results, the formula generation potential of all the inventive steel is higher than AISI 304, and the passive current density is higher than that of AISI 304. Low, 1,9,10 of the invention steel had almost the same value as AISI 316.

Taken together, it can be seen that the corrosion resistance of the present invention is better than that of AISI 304, and the corrosion resistance is nearly 316.

Table 2 below shows the corrosion resistance of the present invention steel, comparative steel and conventional steel (Pitting Resistan ce Equivalent) , Eq (Pitting Potential), ip (Passive Current density) value, the mechanical properties are represented by HRC (Rockwell Hardness) value.

Figure 4 shows the relationship between the PRE value and the formula potential (Ep) of the alloy steel of the present invention, it can be seen that the Ep value generally increases with the increase of PRE.

(Note: PRE: stainless steels have pitting corrosion rather than normal corrosion in solutions containing C1-ions in seawater, so resistance to seawater is expressed as resistance to the formula and quantified in PRE)

PRE =% Cr + 3.3 ×% Mo + 30 ×% N

Ep (Pitting pontential): official potential

ip (passive current density): Passive current density

(2) weight loss test results

The test solution is immersed in a 6wt% FeCl, 6HO solution at room temperature (26 ℃) for 24 hours and then evaluated by the weight loss corrosion rate is determined that the smaller the weight loss is more resistant.

Table 3 below shows the inventive alloys 1, 2, 6, 8 and conventional steel 420J1, AISI 304, AISI 316 comparative steel Corrosion test results for. As can be seen from the test results, the alloy of the present invention has superior corrosion resistance than the conventional martensitic 420J, is superior to the austenitic AISI 304 and slightly lower than the AISI 316.

Hardness test results

Hardness value was measured by C Scale with 150kg load using Rockwell hardness tester and mechanical properties were examined through these hardness values. The results are as shown in Table 2 above.

All the present invention hardness hardness (HRC) is more than 50 showed similar or superior hardness value than 420J of the conventional steel, and showed significantly higher hardness values than AISI 304, 316.

Figure 3 shows the hardness value according to the amount of [C + N]% of the alloy components of the present invention steel, it can be seen that the hardness is slightly increased with HRC as the [C + N]% increase.

Based on the above experimental results, the alloy steel of the present invention has superior strength and corrosion resistance than the martensitic stainless steels SUS 420J1, J2 and austenitic stainless steel AISI 304, which are currently commercially available, and compared with AISI 316. Of course, it can be seen that it is excellent and shows almost the same level of corrosion resistance.

The manufacturing process and general characteristics of the alloy of the present invention will be more clearly understood by the following comparative examples and examples.

Comparative Example 1

Ingots of Comparative Steels 1 and 2 shown in Table 1 were prepared by using a high frequency induction furnace in the atmosphere using electrolytic iron, chromium, molybdenum, vanadium Fe-Si, and Fe-Cr-N as commercial materials. .

After ingoting this ingot for 1100 degreeC for 1 hour, hot rolling was performed to the thickness of 3 mm. After hot rolling, the annealing heat treatment was performed at 750 ° C. for 1 hour, and then pickling treatment was performed on a nitric acid-fluoric acid mixed solution maintained at 50 ° C. in order to remove an oxide scale formed on the surface. Thereafter, after cold rolling to a thickness of 1 mm, the austenitic hardening heat treatment was heat-treated at 1020 ° C, 1050 ° C, and 1080 ° C for 30 minutes in an Ar atmosphere to measure hardness.

It can be seen that Comparative Steel 1 has the highest hardness value at 1050 ° C and Comparative Steel 2 at 1080 ° C.

Comparative Example 2

10 kg of ingots of Comparative Steels 3, 4, and 5 in Table 1 were prepared using high-frequency induction furnaces in the atmosphere, mainly using commercially pure electrolytic iron, chromium, molybdenum, vanadium Fe-Si, and Fe-Cr-N. Prepared. From the hot rolling to the austenitic curing heat treatment, the manufacturing process was the same as in Comparative Example 1.

When the austenitic hardening heat treatment was performed at 950 ° C, 980 ° C, 1020 ° C, 1050 ° C and 1080 ° C for 30 minutes in an Ar atmosphere, the corrosion resistance and hardness were measured and comprehensively evaluated. It can be seen that the heat treatment at 1020 ° C. and Comparative Steel 5 is the most appropriate heat treatment condition.

Example 1

Ingots of the alloys 1 and 2 of the present invention were prepared by using a high frequency induction furnace in the air, using electrolytic iron, chromium, molybdenum, Fe-Si, and Fe-Cr-N as commercial materials having commercially pure grades, and austenitic from hot rolling. The manufacturing process was the same as that of Comparative Example 1 before the hardening heat treatment.

As shown in Table 1, the chemical components of the alloys 1 and 2 of the present invention were only the carbon component of the two alloys, and the other elements were almost the same. As shown in Table 2, in the corrosion resistance, the inventive alloy 1 was the inventive alloy. It was superior to 2, and hardness of invention alloy 2 was excellent.

As a result of performing austenitic curing heat treatment of the alloys 1 and 2 of the present invention at 980 ° C, 1020 ° C, 1050 ° C and 1080 ° C for 30 minutes, the invention alloy 1 has the best corrosion resistance at 1050 ° C and the invention alloy 2 at 980 ° C. And it can be seen that it has a hardness value.

Example 2

Ingots of the alloys 3, 4, 5, and 6 of the present invention were manufactured by using a high frequency induction furnace in the air, using electrolytic iron, chromium, molybdenum, Fe-Si, and Fe-Cr-N as commercial materials. From rolling to the austenitic hardening heat treatment, the manufacturing process was the same as in Comparative Example 1.

Chemical compositions of the alloys 3, 4, 5, and 6 of the present invention are shown in Table 1, and these alloys were examined for the effects of nitrogen by varying nitrogen in the alloying elements.

Among these alloys, 4, 5 has better corrosion resistance than 3, 6, because the invention alloy 3 is added to the nitrogen below the appropriate level, the invention alloy 6 is nitrogen is added enough, but the molybdenum is added less corrosion resistance I could see it fell.

Hardness Measurement The alloys 3, 4, 5, and 6 of the present invention exhibited hardness values higher than 420. The higher the amount of [carbon + nitrogen], the higher the hardness value. The alloy 6 of the present invention was subjected to austenitic hardening heat treatment at 980 ° C and 1020 ° C. The hardness and anode polarization tests were conducted for 30 minutes at 1050 ° C. and 1080 ° C., and it was found that 1020 ° C. to 1050 ° C. were an appropriate heat treatment temperature.

Example 3

Ingots of the alloys 7 and 8 of the present invention were prepared by using a high frequency induction furnace in the air with electrolytic iron, chromium, molybdenum, Fe-Si, and Fe-Cr-N as commercial materials having commercially pure grades, and austenitic from hot rolling. Preparation process before the curing heat treatment was the same as in Comparative Example 1.

Chemical compositions of the alloys 7 and 8 of the present invention are shown in Table 1, and these alloys were examined for the effect of this element by varying Mo in the alloying elements. Comparing the characteristics of these alloys and the alloy 3 of the present invention through Table 2, it can be seen that the corrosion resistance is excellent when Mo is added at least 2% in a state containing 0.10 or more nitrogen.

In order to find out the proper austenite curing heat treatment temperature of the alloy 8 of the present invention, the heat treatment temperature was different from 980 ° C., 1020 ° C., 1050 ° C., and 1080 ° C., and the best corrosion resistance and hardness were 1050 ° C.

Example 4

Ingots of the alloys 9, 10, and 11 of the present invention were prepared by using a high frequency induction furnace in the air, using electrolytic iron, chromium, molybdenum, Fe-Si, Fe-Cr-N as commercial materials having commercially pure grades. The manufacturing process was the same as that of Comparative Example 1 before the austenitic curing heat treatment.

Chemical compositions of the alloys 9, 10, and 11 of the present invention are shown in Table 1, and these alloys are different from Ni in raw water of alloys to examine the effect of elements. Table 2 shows the characteristics of these alloys, and the corrosion resistance is almost the same, and in terms of hardness, the alloy 9 of the present invention is slightly inferior to the alloys 10 and 11 of the present invention. Therefore, Ni should be added at least about 2.0% to obtain the optimal material properties.

In order to determine the proper austenite curing heat treatment temperature of the alloy 10 of the present invention, when the heat treatment temperature was changed to 980 ° C., 1020 ° C. and 1080 ° C., the temperature at which the excellent corrosion resistance and the hardness was excellent was 1050 ° C.

As can be seen from the above, the high strength, high corrosion resistance martensitic stainless steel according to the present invention has superior corrosion resistance and strength than conventional SUS 420 J1, 420 J2 martensite stainless steel, as well as austenitic stainless steel AISI 304. In addition to the use of martensitic stainless steels, it can be widely used in various industrial fields requiring better corrosion resistance and strength, and can be manufactured at a lower cost than general-purpose stainless steel AISI 304 in terms of economics. It is an invention.

Claims (1)

It is composed of 12 ~ 16% chromium, 5% or less nickel, molybdenum 1.5 ~ 5%, carbon 0.1 ~ 0.6%, nitrogen 0.05 ~ 0.3%, vanadium 0.5%, silicon 2.0%, manganese 2.0% and residual iron High strength, high corrosion resistance martensitic stainless steel.