CN102002646A - Economic biphase stainless steel with high mechanical property and superior corrosion resistance - Google Patents

Abstract

The invention provides an economical duplex stainless steel with high mechanical properties and excellent corrosion resistance, its composition and weight percentage are: C: ≤0.03%, Cr: 21.5-23.0%, Ni: 1.50-2.50%, N: 0.20-0.40%, Mo: 0.20-0.40%, Cu: 0.20-0.50%, Mn: 0.5-2.0%, S: <0.005%, P: <0.005%, and the rest are Fe and unavoidable impurities. After the steel is heat-treated in the temperature range of 980-1080°C, the ferrite and austenite contents in the obtained dual-phase structure are roughly the same, the local corrosion resistance and uniform corrosion resistance are better than 304 austenitic stainless steel, and the mechanical properties are similar to 316L , can be used as a substitute material for 304, 316L austenitic stainless steel widely used at present.

Description

Economical duplex stainless steel with high mechanical properties and excellent corrosion resistance

technical field

The invention relates to an economical duplex stainless steel with high mechanical properties and excellent corrosion resistance, which can be used as a substitute material for typical austenitic stainless steel 304 and 316L used in structural applications, drinking water systems, pulp and paper industries.

Background technique

Austenitic stainless steel is currently the most widely used, accounting for more than 60% of the total amount of stainless steel. It is characterized by good corrosion resistance and easy processing and manufacturing, but its disadvantages are low strength and performance, and it is affected by the fluctuation of alloy cost and price. larger. Although ferritic stainless steel is used more and more in thin-gauge applications, due to its own grain coarsening tendency (especially in the heat-affected zone of the weld), it is necessary to replace austenitic stainless steel in specifications above 3mm Difficult to achieve. The 200 series manganese nitrogen replaced nickel austenitic stainless steel is limited to use in less corrosive environments due to its low chromium content. The duplex stainless steel based on the Fe-Cr-Ni alloy series is generally composed of ferrite (30% to 70%) and austenite, with high mechanical properties and good corrosion resistance. Seawater desalination and chemical industry are widely used. Its typical representative grade 2205 duplex stainless steel has successfully replaced 316L austenitic stainless steel in the fields of air pollution control equipment and chemical tankers. Although 2304 duplex stainless steel, which has been developed for more than 20 years, is not as good as 2205 duplex stainless steel in performance, when the price of nickel and molybdenum fluctuates greatly, and its raw material price exceeds the price difference between austenitic stainless steel and duplex stainless steel, 2304 Duplex stainless steel has become an ideal material to replace 316L stainless steel with excellent cost. In recent years, a new type of duplex stainless steel has been developed based on the idea of replacing nickel with nitrogen and manganese, such as the publication number CN 101613839A. The nickel content can be controlled below 1.0%, which can effectively reduce production costs; in addition, the duplex stainless steel can be reduced by adding trace rare earth elements. Duplex stainless steel with excellent comprehensive properties can also be obtained from medium-rare precious metals, such as publication numbers CN 101215674A and CN 101768705A. While ensuring the unique comprehensive excellent performance, in order to better improve the economy and reduce nickel, molybdenum and other resources, further research and development of resource-saving duplex stainless steel has become an inevitable development trend.

Contents of the invention

The purpose of the present invention is to provide an economical duplex stainless steel that can replace 304 and 316L austenitic stainless steels, which is resource-saving, has excellent mechanical properties and corrosion resistance, has good mechanical stability and toughness in the welding heat-affected zone, and is cost-effective.

In order to achieve the above object, the composition design principle of the duplex stainless steel of the present invention is: to resist various forms of corrosion, the chromium content must be maintained at a relatively high level; in order not to increase the cost, to obtain good crevice corrosion resistance and toughness, the nickel content Optimize and adjust the content of carbon and nitrogen so that it can obtain approximately the same content of ferrite and austenite after high-temperature heat treatment; lower molybdenum content to ensure high structural stability of dual-phase steel and reduce costs; Strictly control manganese content and harmful elements of sulfur and phosphorus to reduce the adverse effects of various manganese compounds on pitting corrosion resistance and uniform corrosion resistance; adding a small amount of copper element is beneficial to the formation of austenite and improves corrosion resistance.

According to the above principles, the present invention uses thermo-calc thermodynamic software to calculate and refer to the influence of a certain element content change in the multi-element alloy system on the stability of different phases in the phase diagram, optimize the design of the components, and propose the following technical solutions:

An economical duplex stainless steel with high mechanical properties and excellent corrosion resistance is characterized in that its composition and weight percentage are: C: ≤0.03%, Cr: 21.5-23.0%, Ni: 1.5-2.5%, N: 0.20-0.40%, Mo: ≤0.40%, Cu: 0.20-0.50%, Mn: 0.5-2.0%, S: <0.005%, P: <0.005%, and the rest are Fe and unavoidable impurities.

Smelt duplex stainless steel according to the above composition range, cast it into a slab of less than 200mm, cool it with water to a temperature of 520°C, then cool it at a cooling rate above 0.6°C/s, and then heat the slab at a temperature of 1250°C for 3 Hot rolling is carried out for more than 1 hour, and the final rolling temperature is controlled above 830°C. After hot rolling, forced water cooling is carried out at a cooling rate above 10°C/s, and annealing is carried out within a temperature range of 960°C to 1140°C. After annealing, perform forced water cooling at a cooling rate above 10°C/s. In this way, approximately the same content of ferrite and austenite duplex structure can be finally obtained. During the annealing treatment or welding cooling process, it is not easy to form hard and brittle σ phase, and precipitated phases such as Ф, R and carbide. Therefore, the structure of the duplex stainless steel of the present invention is stable, especially the mechanical stability of the welding heat-affected zone is high, and it has high toughness, and plates, pipes, wires, etc. can be manufactured according to different production processes.

Compared with the prior art, the economical duplex stainless steel of the invention has low carbon content, moderate chromium content, relatively low nickel and molybdenum content, is insensitive to raw material price fluctuations, and has better and stable economic efficiency. After annealing at 980-1080°C, the yield strength and ultimate tensile strength at room temperature are higher than 500MPa and 710MPa respectively, and the elongation is higher than 36%. Under the same corrosive environment, its crevice corrosion resistance and uniform corrosion resistance are better than austenitic stainless steel 304, which is equivalent to 316L; the pitting corrosion resistance equivalent value PREN≥25, and the pitting corrosion resistance is better than 304L and 316L austenitic stainless steel Stainless steel; the critical pitting temperature is also higher than 304L and 316L, close to the level of 316LN.

Invention effect

The economical duplex stainless steel of the present invention can produce hot-rolled medium plate, cold-rolled thin plate, hot-rolled wire rod, cold-drawn wire rod, and can be used to replace the standard austenitic stainless steel (304, 304, 316L).

Description of drawings

Fig. 1. Calculation range of melting point of economical duplex stainless steel of the present invention

Fig. 2. The microstructure of the economical duplex stainless steel hot-rolled plate of the present invention at room temperature

Figure 3. Proportional distribution of the two-phase structure of the economical duplex stainless steel in the high temperature region of the present invention

Figure 4. Variations of various compositional phases and contents of the economical duplex stainless steel of the present invention at 600-1000°C

Fig. 5. Microstructure of economical duplex stainless steel of the present invention after aging treatment

Fig. 6. The pitting corrosion potential of steel and comparison steel 316L according to the present invention

Figure 7. The critical pitting temperature (CPT) of steel and comparative steel 316L according to the present invention

Detailed ways

Example

1. Implement the composition and process of steel

Proportioning is carried out according to the composition range of the economical duplex stainless steel of the present invention, and the vacuum induction furnace method is used for smelting to obtain the chemical composition (weight percentage) of the steel shown in Table 1. Casting was carried out according to the conditions in Table 1. After casting, water spray cooling was carried out in the temperature range from 1000°C to 500°C. There was no σ phase precipitation in the microstructure, and no cracks appeared on the slab surface, and high-quality cast slabs could be obtained.

Chemical composition and pouring temperature (weight percent, %) of table 1 invention stainless steel

The slab was subjected to soaking treatment at 1250°C for 3 hours, and hot-rolled into hot-rolled sheets with a thickness of 6mm, 10mm and 16mm through multiple passes. The final rolling temperature was controlled at 1000°C, and there were no cracks and could be rolled smoothly. Cooling with water after rolling prevents the precipitation of harmful phases such as σ.

The hot-rolled sheet was subjected to solution treatment at 1050° C. for 30 minutes, followed by rapid water cooling to obtain a finished product.

2. Changes in microstructure and phase content

The melting point range of the implemented steel was determined by using thermo-calc software, which is about 1435-1478 °C, as shown in Figure 1. Figure 2 is a photo of the microstructure of a hot-rolled sheet after solution treatment at 1050°C for 30 minutes (the white part is austenite, and the dark part is ferrite), and the proportions of ferrite and austenite are roughly equal and evenly distributed. The ratio distribution of the two-phase structure of the steel in the high temperature region (950-1250°C) is shown in Figure 3. Figure 4 shows the variation law of phase composition and phase content with temperature calculated for the steel in the range of 600-1000°C. In the temperature range of 600 ~ 700 ° C, there is a σ phase, and it also shows that a very small amount of carbides are precipitated. Figure 5 shows the microstructure of the steel in this implementation after aging treatment at 700°C for 15 minutes. No secondary phase precipitation was found on the grain boundaries and inside the grains.

3. Mechanical performance test

Table 2 shows the test results of mechanical properties at room temperature of hot-rolled steel plates with different thicknesses implemented in the present invention. It can be seen that the comprehensive mechanical properties of the economical duplex stainless steel are excellent.

Table 2 Mechanical properties of duplex stainless steel in this embodiment after heat treatment

4. Corrosion resistance test (compared with 316L)

According to the ASTM B117 standard, the corrosion resistance test was carried out on the implementation steel of the present invention and the comparison steel 316L, and the comparison results are shown in Figure 6. According to the ASTM G150 standard, the critical pitting temperature (CPT) was measured for the implementation steel of the present invention and the comparison steel 316L, and the comparison results are shown in Figure 7. It can be seen that the pitting corrosion resistance of this economical duplex stainless steel is significantly better than that of 316L austenitic stainless steel. In addition, the crevice corrosion resistance, uniform corrosion resistance and atmospheric corrosion resistance are all better than 304, which is equivalent to 316L.

It should be emphasized that the embodiments of the present invention are only used to illustrate the present invention rather than limit the present invention, and the present invention is not limited to the details described herein, as long as within the essential core scope of the present invention, changes to the above-described embodiments , modifications will fall within the scope of the claims of the present invention.

Claims (1)

1. An economical duplex stainless steel with high mechanical properties and excellent corrosion resistance, characterized in that its composition and weight percentage are: C: ≤0.03%, Cr: 21.5-23.0%, Ni: 1.5-2.5%, N: 0.20-0.40%, Mo: 0.20-0.40%, Cu: 0.20-0.50%, Mn: 0.5-2.0%, S: <0.005%, P: <0.005%, and the rest are Fe and unavoidable impurities.

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