KR20100069178A

KR20100069178A - Chrome containing steel having high resistance of grain boundary to corrosion

Info

Publication number: KR20100069178A
Application number: KR1020080127790A
Authority: KR
Inventors: 김상윤; 김현철; 엄상호
Original assignee: 주식회사 포스코
Priority date: 2008-12-16
Filing date: 2008-12-16
Publication date: 2010-06-24

Abstract

PURPOSE: A containing chromium steel with a superior grain boundary corrosion is provided to prevent the decrease of the lecture corrosion resistance. CONSTITUTION: A containing chromium steel with a superior grain boundary corrosion comprises less than C 0.01 weight% less than N 0.015 weight% less than the Si 1.0 weight% less than the Mn 1.0 weight% less than P 0.04 weight% less than S 0.01 weight% less than the Ni 0.5 weight% less than the Cu 0.5 weight% less than the Mo 0.5 weight% less than the Al 0.15 weight% Cr 10.5~13 weight%, Ti 0.05~0.4 weight%, Nb 0.1~0.6 weight%, and the rest Fe and other inevitable impurities. The content of P is limited less than 0.01 weight%.

Description

Chromium-containing steel with excellent intergranular corrosion resistance {CHROME CONTAINING STEEL HAVING HIGH RESISTANCE OF GRAIN BOUNDARY TO CORROSION}

The present invention relates to chromium-containing steels having improved intergranular corrosion resistance for vehicle exhaust system components, in particular for piping pipes which are placed in a corrosive environment by condensate.

Vehicle exhaust system components, especially the muffler located at the end of the exhaust system, are not only excellent in workability and weldability because they are made of pipes, bends and welds, but also corrosion resistance to withstand the harsh corrosive environment caused by condensate generated by exhaust gases. This needs to be excellent.

Typically, the condensed water is a weak alkali having a pH of 7 to 9, and includes ammonia ion, chlorine ion, sulfate ion, sulfite ion, carbonate ion, nitrate ion, organic acid ion, etc., and the electrical conductivity and chlorine ion increased by these ions. Corrosiveness is strong due to. When the temperature of the exhaust gas rises and the muffler temperature rises, the condensate evaporates and disappears. However, the corrosive environment becomes more severe due to the pH decrease and the concentration of corrosive chlorine ions. In fact, the corrosion inside the muffler is reported to occur more frequently in a vehicle with a lot of short-distance operation and the repetition of condensate generation and drying processes.

On the other hand, in a gasoline vehicle equipped with a three-way catalyst, the muffler is heated to 400 ° C. or higher by exhaust gas when driving long distances. Will appear. The grain boundary sensitization is a phenomenon in which Cr, Fe, etc. combine with C and precipitate at grain boundaries in the form of M ₂₃ C ₆ when stainless steel is exposed for a long time in the region of 450 to 550 ° C. It is significantly reduced compared to the concentration of Cr in the mouth, and when the Cr concentration around the grain boundary becomes 10.5% or less, the minimum Cr concentration at which the Cr passivation film layer can be formed, grain boundary corrosion occurs.

In recent years, many car makers have extended the muffler warranty period of new cars (for example, from 1 year or 20,000 km to 3 years or 60,000 km), and are now using 11-12 Cr steel, a steel material used in mufflers. Is expected to lack corrosion resistance. Therefore, development of steel materials excellent in corrosion resistance is calculated | required more conventionally.

An object of the present invention is to provide a chromium-containing steel excellent in intergranular corrosion resistance that can withstand longer than conventional in the harsh corrosive environment due to condensate by adjusting the composition of the chromium-containing steel used in conventional vehicle exhaust system components.

The chromium-containing steel excellent in intergranular corrosion resistance according to the present invention for achieving the above object is, in weight%, C: 0.01% or less, N: 0.015% or less, Si: 1.0% or less, Mn: 1.0% or less, P : 0.04% or less, S: 0.01% or less, Ni: 0.5% or less, Cu: 0.5% or less, Mo: 0.5% or less, Al: 0.15% or less, Cr: 10.5 to 13%, Ti: 0.05 to 0.4%, Nb : 0.1-0.6%, remaining Fe and other unavoidable impurities, satisfying the relationship of (2 x Ti + Nb) / (C + N) ≥ 25.

Preferably, the chromium-containing steel according to the present invention is C: 0.002 to 0.008%, N: 0.002 to 0.010% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% Ni: 0.5% or less, Cu: 0.5% or less, Mo: 0.5% or less, Al: 0.15% or less, Cr: 11.0 to 12.0%, Ti: 0.05 to 0.3%, and Nb: 0.20 to 0.4%.

Also, preferably, the content of P is controlled to 0.01% or less.

As described above, the chromium-containing steel has improved intergranular corrosion resistance through appropriate control of alloying elements such as Ti and Nb, so that it can withstand longer than in the harsh corrosive environment due to condensate.

Hereinafter, a chromium-containing steel having excellent intergranular corrosion resistance according to the present invention will be described in detail with reference to the accompanying drawings.

One of the first measures to prevent grain boundary sensitization in stainless steels discussed in the background is to lower the C concentration in stainless steels extremely low. However, it is practically difficult to lower the concentration of C to such an extent that grain boundary sensitization can be prevented.

In the second method, a C stabilizing element such as Ti is added to inhibit the binding of Cr and C. However, when a large amount of Ti is added, there is a problem that Ti is re-oxidized during steelmaking refining process, resulting in greatly deterioration of cleanliness of the steel, resulting in surface defects. The present invention has been obtained through various studies and experiments aimed at improving intergranular corrosion resistance according to the second method. In particular, by adding Ti-Nb in an optimal form, the intergranular corrosion resistance of chromium-containing steel is greatly improved.

Hereinafter, the reason for the composition design of the chromium-containing steel according to the present invention will be described.

Since carbon (C) deteriorates moldability and corrosion resistance and causes grain boundary corrosion, the smaller the content is, the better the content is limited to 0.01% by weight or less. However, since excessive C reduction is directly connected to an increase in refining cost, it is preferably managed at 0.002 to 0.008% by weight.

Nitrogen (N), like C, deteriorates moldability and corrosion resistance, so the smaller the content, the better the content is. However, since excessive reduction leads to an increase in refining cost, it is preferably managed at 0.002 to 0.010% by weight.

Silicon (Si) contributes to deoxidation in the steel refining process. However, if excessively contained, the workability is deteriorated, so it is controlled to 1.0 wt% or less.

Manganese (Mn), like Si, contributes to deoxidation in the steel refining process. However, the content of more than 1.0% by weight not only lowers the ductility, but also forms MnS, which lowers the corrosion resistance. Therefore, it is managed to 1.0 weight% or less.

Phosphorus (P) is a solid solution strengthening element like Mn and Si, and considering the effect on the properties of steel, the smaller the content, the better the content is limited to 0.04% by weight or less. On the other hand, P is generally known only to reduce the workability and toughness of steel, but in the present invention, P is an element that must be managed to be important for reducing grain boundary segregation. In other words, P is a solid solution in steel at high temperature, but exceeds the solid solution limit in low temperature region to cause grain boundary segregation or surface segregation. Decrease. Preferably, the content of P needs to be controlled to 0.01% by weight or less.

Sulfur (S), like P, is an element segregated at the grain boundary in the low temperature region. The sulfur (S) is combined with Ti or C to reduce the amount of solid solution Ti, coarsen the precipitates, and lower the corrosion resistance. Therefore, the content of S needs to be managed to 0.01% by weight or less.

Nickel (Ni) is effective in improving toughness and improving salt corrosion, but when contained in an amount of more than 0.5% by weight, the corrosion resistance of the steel is reduced.

Copper (Cu) is lowered in ductility, and when contained in more than 0.5% by weight is reduced to pitting resistance, stress corrosion resistance is controlled to 0.5% by weight or less.

Molybdenum (Mo) improves the corrosion resistance, but not only causes a decrease in toughness due to high-temperature phase precipitation, but also manages to 0.5% by weight or less.

Aluminum (Al) is an element added as a deoxidation element. In addition, Al increases toughness, but when excessively added, aluminum oxide is formed to reduce corrosion resistance, so that the content is limited to 0.15% by weight or less.

Chromium (Cr) is a basic component that imparts corrosion resistance to stainless steel, and the more it is added, the higher the corrosion resistance of the steel is. However, as the Cr content increases, the precipitation rates of Cr (C, N) and Cr oxides also increase, leading to a decrease in toughness and an increase in manufacturing cost. In view of such circumstances and to be applied for parts of the exhaust system, the Cr content is preferably 10.5 to 13% by weight, more preferably, considering the production cost and corrosion resistance, the content needs to be controlled to 11.0 to 12.0% by weight. There is.

Titanium (Ti) combines with C, N and S to suppress grain boundary sensitization and to suppress the formation of MnS, which lowers the corrosion resistance of steel, thereby improving corrosion resistance and intergranular corrosion resistance. In addition, according to the present invention, Ti improves workability (deep drawing) and increases the amount of solid solution Nb by adding Nb and complex. However, when excessively added, the surface quality is lowered and the toughness is lowered. Therefore, it is necessary to be controlled at 0.05 to 0.4% by weight, preferably 0.05 to 0.3% by weight.

Niobium (Nb) is an element added to prevent grain boundary sensitization of steel, and it is a function that contributes to the development of recrystallized texture that affects the corrosion resistance and plastic anisotropy (r) value of products by fixing C and N as carbonitrides. Also do. However, when excessively added, it precipitates in a Lavas phase and causes a deterioration of workability, so the content thereof needs to be controlled to 0.1 to 0.6% by weight, more preferably, intergranular corrosion resistance, moldability and manufacture. Considering the cost, it needs to be controlled at 0.20 to 0.4% by weight.

On the other hand, according to the present invention, the content control between C, N, Ti, Nb, which is a major component of the steel, depends on the relationship (2 × Ti + Nb) / (C + N) ≧ 25. The chromium-containing steel according to the present invention is mainly a ferritic stainless steel, and this chromium-containing ferritic steel generates a significant change in its corrosion amount depending on the Ti / (C + N) ratio. That is, when the Ti / (C + N) ratio is about 11.8, the corrosion rate hardly changes even at high temperatures, and when lower, the corrosion resistance at high temperatures is considerably reduced. Therefore, the Ti / (C + N) ratio is preferably 12 or more. According to the present invention, Ti and Nb are added in combination, and the atomic weight of Ti is 47.9 and the atomic weight of Nb is 92.9, where Nb is about twice the weight of Ti. Therefore, the content control formula between C, N, Ti, Nb is finally expressed as shown in the above relation.

Experiment example

As an experimental example for checking the properties of the chromium-containing steel according to the present invention, look at the results of the corrosion test for Examples A1, A2 having a composition according to the present invention and Comparative Example B not. The compositions of Examples A1, A2 and Comparative Example B are as shown in Table 1 below, and Comparative Example B is a SUH409L steel generally used in vehicle exhaust system systems.

division C Si Mn P S Cu Al Ni Cr Mo Ti Nb N (2 × Ti + Nb)
/ (C + N) A1 0.007 0.46 0.34 0.025 0.0008 0.05 0.045 0.12 11.16 0.02 0.21 0.311 0.008 49 A2 0.005 0.46 0.34 0.009 0.0008 0.05 0.045 0.12 11.16 0.02 0.17 0.242 0.007 48 B 0.006 0.52 0.24 0.026 0.0009 0.06 0.036 0.17 11.21 0.02 0.22 0.001 0.008 31

Specimens used in the experiment were prepared as follows. That is, the slab having a composition corresponding to Table 1 is hot-rolled after casting to make a hot rolled coil of 3.5mm thickness, and then cold-rolled to 1.2mm hot-rolled coil is manufactured by annealing and pickling treatment. The annealing temperature of the cold rolled sheet was set to 850 to 900 ° C in order to set the crystal grain size number to about 6-9. Each specimen was cut into pieces of 30 mm x 80 mm obtained as described above.

Looking at the intergranular corrosion experiment process for the specimen, first, considering that the exhaust system parts are manufactured by welding, as shown in Figure 1, two specimens were superimposed on each other and then subjected to GMA welding (Gas Metal Arc Welding). As welding conditions, the current (AC) was 71 A, the voltage was 16.7 V, the welding speed was 0.72 m / min, and Ar + 2% O ₂ was used as the protective gas, and Y308 (1.2 mmφ) was used as the welding rod. . In order to satisfy the grain boundary sensitization conditions, the welded specimens were placed in an electric furnace and heated at 500 ° C. for 10 hours, and then air-cooled.

Next, after preparing 10 specimens heat-treated and welded as described above for each of Examples A1, A2, and Comparative Example B, each of them was a test solution filled with 750 ml in a 1 L beaker as shown in FIG. % CuSO ₄ , 0.5% H ₂ SO ₄ ) was immersed in a boiling (about 105 ℃) for 20 hours. Then, after removing the scale of the specimens soaked, as shown in Figure 3, it was divided into three to observe each cross-sectional structure. The summary of the observation is shown in Table 2.

division Cross-sectional observation Psalm 1 Psalm 2 Psalm 3 Psalm 4 Psalm 5 Psalm 6 Psalm 7 Psalm 8 Psalm 9 Psalm 10
A1 Section 1 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Section 2 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Section 3 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
A2 Section 1 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Section 2 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Section 3 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
B Section 1 × ○ × × × ○ × × × × Section 2 × × × × × × × × ○ × Section 3 × × × × × × × × × ×

○: Good condition (no grain boundary corrosion), ×: Bad state (grain boundary corrosion occurred)

As shown in Table 2, in the case of Examples A1 and A2, no intergranular corrosion occurred, but in the case of Comparative Example B, the intergranular corrosion occurred. An example of such a cross-sectional observation result photograph is as shown in FIG. 4.

While specific embodiments of the present invention have been illustrated and described, those of ordinary skill in the art may vary the present invention without departing from the spirit of the invention as set forth in the following claims. It is to be understood that modifications and variations are possible.

1 to 3 is a view for explaining an experimental method according to an embodiment of the present invention, Figure 1 is a welding process, Figure 2 shows a corrosion resistance measurement process, Figure 3 shows a specimen cutting process for tissue observation,

Figure 4 is a photograph of the grain boundary corrosion of the specimen according to the position shown in FIG.

Claims

By weight%, C: 0.01% or less, N: 0.015% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Ni: 0.5% or less, Cu: 0.5% Or less, Mo: 0.5% or less, Al: 0.15% or less, Cr: 10.5-13%, Ti: 0.05-0.4%, Nb: 0.1-0.6%, remaining Fe and other unavoidable impurities, and (2 × Ti + A chromium-containing steel excellent in intergranular corrosion resistance, characterized by satisfying a relationship of Nb) / (C + N) ≧ 25.

The method according to claim 1,

C: 0.002-0.008%, N: 0.002-0.010% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Ni: 0.5% or less, Cu: 0.5% or less , Mo: 0.5% or less, Al: 0.15% or less, Cr: 11.0 to 12.0%, Ti: 0.05 to 0.3%, Nb: 0.20 to 0.4%.

The chromium-containing steel having excellent intergranular corrosion resistance according to claim 1 or 2, wherein the content of P is 0.01% or less.