KR20020042151A - Method of manufacturing austenitic stainless steel with improved high temperature creep resistance - Google Patents

Abstract

PURPOSE: A method of manufacturing austenitic stainless steels with improved high temperature creep resistance by heat treatment in nitrogen atmosphere is provided. CONSTITUTION: In a method of manufacturing austenitic stainless steel, the method is characterized in that after rolling, fine TiN precipitates are deposited by heat treatment at 1000-1100°C in bright annealing furnace in 10-40 vol.% of nitrogen gas. In this method, the feeding speed of austenitic stainless steel into bright annealing furnace is controlled to 5-10 mpm to secure enough time to diffuse nitrogen into steel.

Description

A method of manufacturing austenitic stainless steel with improved high temperature creep resistance

The present invention relates to a method for producing an austenitic stainless steel sheet having excellent creep properties, and more particularly, to a method for producing an austenitic stainless steel sheet having excellent creep strength using heat treatment using a nitrogen atmosphere.

In general, in order to be used in an apparatus for generating high temperature steam, the creep strength at high temperature and high pressure must be excellent as well as good corrosion resistance at high temperature.

As a method of improving the high temperature creep characteristics of stainless steel, 1) adding alloying elements to increase the strength at high temperatures (British Patent Publication No. 2 204 060 A), mainly using elements such as Ti, Mo, Nb, V, and Cu. 2) Method of increasing high temperature creep strength by adjusting hot rolling conditions and cooling rate after adding alloying elements (Japanese Patent 7268454), 3) Carbon to suppress carbide formation at grain boundaries that degrade creep characteristics. Method to keep the content of 0.03% or less and maintain high solubility nitrogen below 0.15% (Japanese Patent Laid-Open Publication No. 10-298719), r, li 4) By setting the ratio of B and Mo to prevent sensitization of grain boundary and There is a method of improving the creep characteristics by miniaturizing (Japanese Patent Laid-Open No. 63-69947).

However, these methods have the disadvantage of adding an additional alloy or complicated process. In particular, Ti, which is added to increase the high temperature strength by forming a fine precipitate, is an element that causes nozzle clogging during the play, resulting in a sharp decrease in productivity. In addition, in the case of Ti-added steel, creep resistance decreases due to coarse TiN formed during solidification at high temperature.

Thus, the present inventors conducted research and experiments to solve the problems of the conventional methods described above, and proposed the present invention based on the results, the present invention is 304 steel species produced without the addition of additional Ti steel The purpose of the present invention is to provide an austenitic stainless steel sheet having excellent high temperature creep resistance by forming a fine TiN extract by heat treatment in an atmosphere gas including nitrogen in a bright annealing furnace after rolling.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention provides a method for producing an austenitic stainless steel sheet having excellent creep strength using heat treatment using a nitrogen atmosphere, unlike a method of adding Ti such as 321 steel to improve high temperature creep properties.

The present invention relates to a method for improving the high temperature creep characteristics of the austenitic stainless steel plate 304 without additional addition of TI. A method of depositing fine TiN on a steel sheet by heat treatment in a bright annealing furnace composed of hydrogen and nitrogen after rolling. It is about. Bright annealing heat treatment does not supply nitrogen to the surface of the steel sheet when the sole hydrogen is used, so TiN is not formed. On the other hand, when the nitrogen content is high, the discoloration of the surface occurs. Therefore, nitrogen is 10% to 40% by volume. The working speed is passed through the strip at a speed of 5 to 10mpm to provide a sufficient supply of nitrogen to the steel sheet. If the working speed is high, the infiltration of nitrogen is insufficient and the desired TiN precipitate cannot be obtained. Heat treatment temperature is set at 1000 ^o C ~ 1100 ^o C. This is because when the heat treatment temperature is low, the decomposition of carbides is insufficient, resulting in deterioration of corrosion resistance. In addition, if the heat treatment temperature is too high, it is difficult to expect the improvement of the creep characteristics because the TiN precipitates. The present invention relates to a method of finally manufacturing an austenitic stainless steel sheet having excellent high temperature creep strength.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

Slabs are prepared by melting steel having a chemical composition as shown in Table 1, followed by ordinary rolling. As shown in Table 2, the breaking time was measured under a stress of 700 ^° C. and 15 kg / mm ² depending on the volume of nitrogen in the atmosphere gas. When nitrogen was not included, the break time was measured as 5 hours, and as the nitrogen content increased, the break time tended to increase in proportion to the increase of the nitrogen content from 85 hours to 98 hours. However, when the nitrogen content exceeds 40%, discoloration on the surface appears. Therefore, the nitrogen content is limited to 10% to 40%. In Comparative Steel 3, in which 0.25% of Ti, which has excellent creep properties, was observed, discoloration of the surface was severely observed, and coarse TiN precipitates formed during solidification were included. The rupture time when heat-treated in a nitrogen atmosphere shows superior creep resistance compared to the gingham added Ti. Table 3 shows the results of measuring breakage time under stress at 700 ^o C and 15 kg / mm ² according to the working speed (mpm: strip running speed per minute) at 1000 ^o C after maintaining the nitrogen content at 20%. Beyond this, the break time is decreasing. In other words, as the working speed increases, the penetration of nitrogen into the steel sheet surface is insufficient, thereby delaying the formation of heavy TiN. Therefore, work speed is limited to 5 ~ 10mpm. When the working speed is 3mpm and 5mpm, there is no difference in breaking time. That is, TiN is sufficiently formed at a working speed of 5mpm. Therefore, work speed is limited to 5 ~ 10mpm. After the nitrogen content of the atmosphere gas was maintained at 20%, the annealing temperature was changed from 950 ^° C to 1150 ^° C. Since the break time was measured under the stress of 700 ^° C, 15kg / mm ² (Table 4) When the annealing temperature is 950 ^° C because the solid solution of carbide is not enough, the precipitation of many carbides at the grain boundary was observed. In the case of 1150 ^o C, creep resistance was decreased by coarsening of grains.

Table 2 shows the rupture time under 700 ^o C, 15kg / mm ² stress due to nitrogen volume, Table 3 shows the rupture time under 700 ^o C, 15kg / mm ² stress due to working speed, and Table 4 shows the annealing The breaking time is shown under a stress of 700 ^° C. and 15 kg / mm ² depending on the temperature.

Remarks Cr Mo Nb Ti Al C + N Si Mn Fe Annealing Heat Treatment Method Invention steel 18.2 0.15 - 0.017 - 0.070 0.6 1.2 Balance Bright Annealing Comparative Steel 1 18.2 0.15 - 0.017 - 0.070 0.6 1.2 Balance General Annealing, Pickling Comparative Steel 2 18.2 0.15 - 0.1 - 0.070 0.6 1.2 Balance General Annealing, Pickling Comparative Steel 3 18.2 0.15 - 0.25 - 0.070 0.6 1.2 Balance General Annealing, Pickling

Nitrogen volume ＼ time Break time 700 ℃, 15Kg / mm2 Remarks 0 5 No surface discoloration 10 85 No surface discoloration 20 90 No surface discoloration 30 90 No surface discoloration 40 93 No surface discoloration 50 95 Surface discoloration 100 98 Surface discoloration Comparative Steel 1 5 Comparative Steel 2 5.5 Comparative Steel 3 80 TiN Coarsening

Working speed ＼ time Break time 700 ℃, 15Kg / mm2 Remarks 3 92 No surface discoloration 5 90 No surface discoloration 10 90 No surface discoloration 15 45 No surface discoloration 20 35 No surface discoloration

Annealing temperature (℃) Break time 700 ℃, 15Kg / mm2 Remarks 950 65 Grain Carbide Precipitation 1000 90 - 1050 90 - 1100 90 - 1150 35 Grain coarsening

As described above, the present invention avoids the use of alloys to improve the existing creep resistance and uses bright annealing, but by controlling the working speed, annealing temperature and nitrogen content, it is possible to produce austenitic stainless steel sheet having excellent creep resistance. As a method, the creep strength is very excellent compared to the steel sheet produced by the conventional method.

Claims (3)

A method for producing austenitic stainless steels, the austenitic stainless steels having excellent high temperature creep strength, characterized in that the heat treatment is performed in a bright annealing furnace under an atmosphere gas containing nitrogen content of 10 to 40% by volume to precipitate fine TiN. Method of manufacturing knight-based stainless steel. The method for producing austenitic stainless steel having excellent high temperature creep strength according to claim 1, wherein the working speed is 5 to 10mpm to sufficiently infiltrate nitrogen. The method of claim 1, wherein the heat treatment temperature is performed at 1000 ^° C to 1100 ^° C.