KR20080061834A - Ferritic stainless steel with excellent corrosion resistance properties for pickling treatment easily - Google Patents

Abstract

Ferritic stainless steel with excellent corrosion resistance properties for facilitating a pickling treatment is provided to restrict the formation of chrome carbide causing intergranular corrosion. Ferritic stainless steel comprises 0.01 mass percent or less of C, 0.01 mass percent or less of N, 0.4 mass percent or less of Si, 0.3 mass percent or less of Mn, 20 to 23 mass percent or less of Cr, 0.2 to 0.4 mass percent or less of Ni, 0.1 to 0.25 mass percent or less of Ti, 0.1 to 0.2 mass percent or less of Nb, 0.30 to 0.50 mass percent or less of Cu, 0.02 to 0.06 mass percent or less of Zr, 0.02 to 0.06 mass percent or less of Al, 0.002 mass percent or less of S, Fe, and inevitable impurities. The content of Cr is increased to improve the corrosion resistant properties. A few amount of Cu AND Ni is added to lower the corrosion rate.

Description

Ferritic stainless steel with excellent corrosion resistance properties for pickling treatment easily}

The present invention relates to a ferritic stainless steel that is easy to pickle and has excellent corrosion resistance in order to replace the austenitic stainless steel containing expensive Ni element, and more specifically, it is used as a material for building materials and has a corrosion resistance of 304. Development of highly corrosion-resistant ferritic stainless steel with controlled Ti / C + N, Nb / C + N, Cu, Ni, Si, and Zr components to improve to equivalent level, especially Ti + Nb composite addition and Si content of 0.4% It relates to a highly corrosion-resistant ferritic stainless steel that can facilitate pickling by examining the pickling properties appearing when raised.

Existing general-purpose austenitic stainless steel, 304 steel has 18% Cr-8% Ni as the main component of the alloy, which has high Ni content and ferritic 436L steel, which is equivalent to corrosion resistance, contains expensive Mo element. There is a problem that the cost of production is inferior to the high corrosion-resistant stainless steel material due to the increase in manufacturing cost by raw materials. Therefore, the present invention has developed a steel that can secure corrosion resistance equal to or higher than the existing Mo-added steel 436L without adding Mo. Since such steel is difficult to pickle, it is easy to pick up the alloy component system while ensuring corrosion resistance. It is an object of the present invention.

The present invention has been made in accordance with the above requirements to replace the expensive Ni and Mo in the existing 304 (18Cr-8Ni) and 436L (18Cr-1.2Mo-0.2Ti) steel, a new alloy Cr, Cu, Si, Ni, Zr In order to stabilize the C and N components by adding elements such as Ti and Nb components, Ti / C + N and Nb / C + N fractions are 12 or more and Ti, Nb is TiC or TiN, NbC at high temperature. Another object of the present invention is to provide a ferritic stainless steel which improves corrosion resistance by inhibiting formation of chromium carbide (Cr ₂₃ C ₆ ), which is a cause of grain boundary corrosion by forming precipitates such as NbN.

In the present invention, the stabilizing elements such as Ti and Nb have a side to improve corrosion resistance by combining with carbon. However, in the present invention, the Ti + Nb / C + N fraction is 12 or more because it reacts with oxygen to form a poorly soluble oxide. It provides high Cr ferritic stainless steel that satisfies the stabilization effect and is excellent in pickling.

The present invention, in order to achieve the above object, by mass% C; 0.01% or less, N; 0.01% or less Si: 0.4% or less, Mn: 0.3% or less, Cr: 20-23%, Ni: 0.2-0.4%, Ti: 0.10 to 0.25%, Nb: 0.1 to 0.20%, Cu: 0.30 to 0.50%, Zr: 0.02 to 0.06%, Al: 0.02 to 0.06%, S: 0.002% or less It is characterized by providing a pickling method of Mo-free high chromium ferritic stainless steel made of impurities.

In addition, the present invention was to increase the Cr content instead of expensive Mo in order to improve the corrosion resistance in the composition of the ferritic stainless steel, and to add a small amount of Cu and Ni to reduce the corrosion rate and very low C, N to improve workability By adding 12 or more Ti / C + N ratios or 12 or more Nb / C + N ratios to the content, the corrosion resistance and processability by Ti and Nb precipitates are improved during hot and cold annealing, and Si addition amount is increased to 0.4%. It is characterized by providing a method for pickling ferritic stainless steel further improved corrosion resistance by the improvement of the surface passivation film.

According to the present invention, the Ti / C + N fraction is 12 or more, or the Nb / C + N fraction is 12 or more, as a result of the test of 21% Cr-Base ferritic stainless steel, in which an appropriate amount is replaced by Cr instead of the expensive Mo element. It prevented the chromium carbide precipitation by the formation of Ti, Nb carbide in the hot and cold rolling annealing heat treatment, and improved the corrosion resistance to above 304 steel level by adding Cr, Si, Cu, and Ni elements, and added a small amount of Zr element. As a result of improving the impact toughness of the welded part, there is a big difference in pickling properties depending on the Ti, Nb / C + N ratio. Therefore, an object of the present invention is to set an alloy element range capable of improving pickling properties while maintaining corrosion resistance. Looking at the characteristics of the main alloying elements added as follows.

Ti: C, N elements are stabilized with Ti (C, N) precipitates to inhibit chromium carbide precipitation at high temperatures, improving corrosion resistance, and improving reproducibility by controlling the recrystallized texture during annealing heat treatment. Since there is a problem that a defect occurs, limit to Ti / C + N = 12 or more and 0.25% or less

Nb: C and N elements are stabilized with Nb (C, N) precipitates to inhibit chromium carbide precipitation at high temperatures, improving corrosion resistance. Although there is an effect to prevent the occurrence of excess, there is a problem that the workability decreases and Ridging surface defect occurs when excessive addition, Nb / C + N = 12 or more and less than 0.20%

Cr: It is an element that promotes the formation of oxide film of stainless steel, which increases corrosion resistance to the equivalent level of 304 steel, and requires more than 21% Cr to replace Mo element of 436L steel. Since there is a problem that the defect is increased, the upper limit is 23%.

Ni: As an element stabilizing an austenite phase such as C and N, it is an element that improves corrosion resistance by slowing the corrosion rate and is expensive, so it is limited to 0.4% or less in consideration of economic efficiency.

Si: It improves the high temperature oxidation resistance and strengthens the passivation film in stainless steel to improve the corrosion resistance, so it is 0.2% or more.

Looking at the effect of the alloying elements on the pickling properties in the steel grades used in the present invention, Ti and Nb precipitates Ti (C, N) and Nb (C, N) as described in the role of the alloying elements to form Cr carbide In addition, it has the effect of controlling the recrystallized texture during the annealing heat treatment and miniaturizing the crystal grains to improve the corrosion resistance. However, the oxidation rate tends to be reduced due to the formation of Ti / Nb oxide. Therefore, in the present invention, the Ti, Nb / C + N ratio, which is easy to pickle, is important while Ti, Nb serves as a stabilizing steel in high chromium steel.

Hereinafter, embodiments of the present invention will be described.

(Example)

Table 1 shows the components of the alloying elements used in the experiment.

The pickling results in Table 1 show the final results after hot and cold rolling in Tables 2 and 3. The steel used in the examples is a laboratory dissolved steel

After the hot blasting, shot blasting was performed after shot blasting. For complete pickling, pre-treatment was performed at a concentration of 150 g / l of sulfuric acid bath and a temperature of 55 ° C. It should be prepared with l and pickled at a speed of 30 ~ 40mpm. Here, when the hydrofluoric acid concentration is less than 30 g / l, the acid pickling occurs, and when the hydrofluoric acid concentration exceeds 35 g / l, peracid tax occurs and is limited to this range.

Hot-rolled specimens were cold rolled to 1.2 mmt and pickled under cold rolling and pickling conditions in Table 3. Cold-rolled specimens were subjected to annealing heat treatment at 970 ~ 1000 ℃ where recrystallization was sufficiently performed, and then experimented in the order of Table 3. Annealed heat treated specimens were pretreated in salt baths and sulfuric acid baths to facilitate pickling, and then nitric acid pickling was performed. Nitric acid electrolysis was performed at normal concentration (6%) and current density (0.1-0.2 A / cm ² ). The object of the present invention is to determine the Ti, Nb / C + N ratio, which is easy to pickle, and thus the cold rolling pickling conditions are carried out under ordinary operating conditions. The results after pickling are summarized in Table 1. As can be seen in the examples of Table 1, it can be seen that the Ti and Nb to C + N ratios are closely related to pickling.

As can be seen from Examples 1 to 6, the Ti-added steel of 19-21Cr steel has a good pickling property when hot-rolled and cold-rolled in the conditions of Tables 2 and 3 under a Ti / C + N ratio of 20 or less. On the other hand, in Examples 7 to 8, even after Ti / C + N was 20, the scale remained after pickling, and in Example 16, the pickling occurred even though the Ti / C + N ratio was very low as 10. This shows that pickling property is inferior when Ti addition amount exceeds 0.25. Therefore, the pickling properties of the Ti-added steels were found to be related to both the Ti / C + N ratio and the absolute Ti addition amount. It can be seen from Example 10 that even if the Ti addition amount is 0.25 or less, pickling is not performed when the Ti / C + N ratio is 20 or more. In conclusion, in order to completely descale the Ti-added steel, both Ti / C + N ratio of 20 or less and absolute Ti addition of 0.25 or less must be satisfied. As in the Nb-added steels, as in Examples 14 and 17, even if Nb / C + N is less than 20, it can be seen that pickling is not completely performed when the amount of Nb added is 0.2% or more. As in Example 15, the Nb addition amount should be limited to 0.2% or less to completely descale under the cold electrolytic pickling conditions of Table 3.

Alloy composition unit of steel used in the experiment:% Steel grade C (ppm) Cr Nb Ti N Al Mn Si Cu Ti, Nb / C + N Pickling Example 1 40 19 0.1 40 0.04 0.2 0.1 12 O Example 2 40 21 0.1 40 0.04 0.2 0.2 12 O Example 3 80 19 0.2 80 0.04 0.2 0.2 12 O Example 4 80 21 0.2 80 0.04 0.2 0.1 12 O Example 5 40 19 0.16 40 0.04 0.2 0.2 20 O Example 6 40 21 0.16 40 0.04 0.2 0.1 20 O Example 7 80 19 0.32 80 0.04 0.2 0.1 20 X Example 8 80 21 0.32 80 0.04 0.2 0.2 20 X Example 9 60 20 0.2 60 0.04 0.2 0.15 16 O Example 10 50 21 0.23 50 0.04 0.2 0.2 23 X Example 11 60 19 0.336 60 0.04 0.2 0.15 0.4 28 X Example 12 60 20 0.336 60 0.04 0.2 0.15 0.4 28 X Example 13 60 20 0.24 60 0.04 0.2 0.15 0.4 20 X Example 14 60 20 0.20 60 0.04 0.2 0.15 0.4 17 O Example 15 60 21 0.15 60 0.04 0.2 0.15 0.4 13 O Example 16 150 18.4 0.25 100 0.04 0.4 0.4 10 O Example 17 150 19.3 0.4 100 0.4 0.4 16 X Example 18 50 21 0.2 50 0.04 0.2 0.15 0.4 20 O

* O: complete descale X: scale remaining

 Hot Rolled Pickling Conditions Annealing Condition Sulfuric acid bath conditions Mixed acid conditions Whiteness Surface condition Temperature (° C) L / S (mpm) Sulfuric acid concentration (g / l) Temperature (° C) Nitrate Concentration (g / l) Folic acid concentration (g / l) Temperature (° C) 970 38 150 80 100 25 55 72.8 D 970 38 150 80 100 30 55 76.9 0 970 38 150 80 100 35 55 79.2 O 1000 38 150 80 100 25 55 72.4 D 1000 38 150 80 100 30 55 77.2 O 1000 38 150 80 100 35 55 76.5 O 970 30 150 80 100 25 55 73.6 D 970 30 150 80 100 30 55 78.1 O 970 30 150 80 100 35 55 78.1 O 1000 30 150 80 100 25 55 72.4 D 1000 30 150 80 100 30 55 76.5 O 1000 30 150 80 100 35 55 79.5 O

* O: complete descale D: scale remaining

Cold Rolling Pickling Condition division Annealing Furnace Air cooling Salt bath wash Sulfuric acid tank Nitric acid Condition 970 ~ 1000 ℃ (87 seconds) ~ 500 ℃ (60 seconds) 500 ℃ (11 seconds) ~ 55 ℃ 55 ℃ (11 seconds) 50 ℃ (51 seconds)

As can be seen in Table 3, the pickling conditions in the cold rolling step is subjected to the process of annealing furnace, air cooling, salt bath, washing, sulfuric acid bath, nitrate electrolyte. Nitric acid electrolysis was performed in 6-8% nitric acid solution.

According to the present invention, the corrosion resistance of 19-21Cr steel is low-cost chromium (Cr), copper (Cu), titanium (Ti) or niobium (Nb), silicon (Si) to replace expensive Ni in austenitic 304 steel. ), The addition of zirconium (Zr) can improve the corrosion resistance to the equivalent level of 304. Among the elements added to achieve such high corrosion resistance, Ti and Nb elements form C and N elements and Ti (C + N) or Nb (C + N) at high temperature, thereby affecting the corrosion resistance and pickling resistance. _It is effective to suppress the precipitation of ₂₃ C ₆ ). However, when an excessive amount of these elements is added, the corrosion resistance is improved, but pickling resistance is lowered. This improvement results in the formation of a relatively small amount of chromium depleted layer around the chromium carbide when the chromium carbide is formed, which facilitates the pickling reaction. Therefore, in the present invention, the absolute amount of Ti, Nb and the Ti, Nb / C + N ratio which can improve corrosion resistance and pickling are derived.

As described above, according to the present invention, a new alloy Cr, Cu, Si, Ni, Zr to replace expensive Ni and Mo in the existing 304 (18Cr-8Ni) and 436L (18Cr-1.2Mo-0.2Ti) steel In order to stabilize the C and N components by adding elements such as Ti and Nb components, Ti / C + N and Nb / C + N fractions are 12 or more and Ti, Nb is TiC or TiN, NbC at high temperature. Or by forming a precipitate, such as NbN, there is an effect of improving the corrosion resistance by inhibiting the formation of chromium carbide (Cr ₂₃ C ₆ ) which causes grain boundary corrosion. In addition, although the stabilizing element such as Ti and Nb in the present invention has a side to improve corrosion resistance by combining with carbon, the Ti + Nb / C + N fraction is 12 in the present invention because it hardly pickles by reacting with oxygen to form a poorly soluble oxide. As described above, a high Cr ferritic stainless steel that satisfies the stabilization effect and has excellent pickling properties can be obtained.

Claims (1)

C by mass; 0.01% or less, N: 0.01% or less, Ti: 0.25% or less, Nb: 0.05-0.20%, Si: 0.2-0.4%, Mn: 0.3% or less, Cr: 20-23%, Ni: 0.2-0.4%, High corrosion resistance ferritic stainless steel with easy pickling of Cu: 0.3 ~ 0.5%, Al: 0.03 ~ 0.10%, S: 0.002% or less, Ti / C + N: 12-20, Nb / C + N: 12-20 .