RU2203343C2 - Two-phase stainless steel at high corrosion resistance in aggressive media - Google Patents

Abstract

FIELD: metallurgy; making corrosion-resistant steels of austenitic ferrite class; shipbuilding; construction of chemicals tankers; oil producing and gas industries; chemical and power engineering. SUBSTANCE: steel may be used in structures working in aggressive media, in aqueous media in particular which contain chloride ions, sulfides and products of oil producing process, in acids of various concentration and in sea water. Temperature interval is from +50 to +350 C. Steel contains components at the following relationship, mas. %: carbon, 0.01-0.03; silicon, 0.3-0.8; manganese, 0.8-1.2; chromium, 24-26; nickel, 6.5-7.5; molybdenum, 3.5-4.5; nitrogen, 0.15-0.30; sulfur, 0.001-0.015; phosphorus, 0.001- 0.020; copper, 0.1-0.5; calcium, 0.01-0.023; cerium, 0.001-0.02; magnesium, 0.01-0.04; iron, the balance, provided calcium/sulfur ratio ≥1,5. is satisfied. Said steel possesses high resistance to pitting corrosion. EFFECT: enhanced strength and ductility of steel at hot deformation. 2 tbl

Description

The invention relates to metallurgy of corrosion-resistant steels of the austenitic-ferritic class, containing iron with a different content of alloying elements as a base, and can be used in petrochemical, power engineering, shipbuilding, metallurgy, and other industries, in structures that work for a long time in aggressive environments, in particular in aqueous media containing chlorine ions, sulfur compounds, oil and gas products, in acids of various concentrations, in marine ode. The temperature range of use from -50 to +350 ^o C.

 The chemical compositions of the most famous corrosion-resistant austenitic-ferritic steels were analyzed [1]. Of the foreign steels, the closest in composition is SAF 2507 steel [2], of domestic - steel O3X24N6AM4 (ZI-130). These steels have a high resistance to pitting in chloride solutions. However, they are not technologically advanced during hot deformation due to the suboptimal combination of alloying elements, or insufficiently use the capabilities of complex alloying to increase the resistance of pitting and crevice corrosion in aqueous media with sulfur compounds and chlorides.

Closest to the claimed composition in composition is steel (according to patent GB 2160221 A, C 22 C 38/06 of 12/18/1985 [3]) with the following chemical composition, wt.%:
Carbon - ≤0.03
Silicon - 0.1-1.0
Manganese - 0.1-2.0
Phosphorus - ≤0.03
Sulfur - ≤0.01
Nickel - 3.0-8.0
Chrome - 21.0-28.0
Molybdenum - 1.0-4.0
Nitrogen - 0.08-0.25
Aluminum - ≤0.02
Iron - Else
This steel does not possess the required resistance against pitting corrosion in a wide range of aggressive environments and does not provide the necessary processability in the production of pipes at the stage of metallurgical redistribution. The deformation on rolling mills, as well as pressing at 1100-1200 ^o With requires from the metal the value of the relative elongation of at least 60%. With less elongation, cracks appear during rolling. In addition, for tanks and pipelines with sea water, it is necessary to increase the resistance to local corrosion.

There is evidence that the resistance of austenitic-ferritic steels to pitting in water with chlorides depends on the contamination of the metal with non-metallic inclusions, which are the centers of the onset of corrosion damage. The deoxidation of steel by sulfur, phosphorus, and oxygen is facilitated by the deoxidation of steel by calcium, magnesium, the introduction of elements such as Y, Ce, which can bind S, P, O ₂ , clean grain boundaries from oxides and form finely dispersed compounds with them.

 The aim of the invention is the creation of steel with increased strength, higher ductility during hot deformation and high resistance to pitting corrosion, which improves operational reliability and resource.

This goal is achieved in that the steel additionally contains copper, calcium, magnesium, cerium in the following ratio of components, wt. %:
Carbon - 0.01-0.03
Silicon - 0.3-0.8
Manganese - 0.8-1.2
Chrome - 24-26
Nickel - 6.5-7.5
Molybdenum - 3.5-4.5
Nitrogen - 0.15-0.30
Sulfur - 0.001-0.015
Phosphorus - 0.1-0.5
Copper - 0.1-0.5
Calcium - 0.01-0.023
Cerium - 0.001-0.02
Magnesium - 0.001-0.02
Iron - Else
in this case, the condition Ca / S ≥ 1.5 must be observed.

 The selected content of the components provides an austenitic-ferritic structure.

The introduction of copper, calcium, cerium and magnesium into steel according to patent GB 2160221 A increases the strength properties, plastic properties at hot deformation temperatures (1100-1200 ^o C) and corrosion properties (resistance to pitting).

 It is known that the introduction of copper [4] in stainless steel increases the strength and corrosion resistance in sea water, firstly, due to the grinding of austenitic grains and secondly, due to the compaction of grain boundaries during the formation of small austenitic grains. Steels used in contact with seawater are usually alloyed with copper.

 Magnesium was introduced into steel in order to reduce the oxygen content in liquid steel and convert it to light pop-up oxides, which leads to a decrease in the amount of oxides in steel. This increases the resistance against pitting.

 Calcium interacts with sulfur and thus neutralizes the harmful effect of sulfur [5]. The absence of sulfur at the grain boundaries leads to an increase in high-temperature ductility during hot deformation. The fulfillment of the ratio Ca / S ≥ 1.5 [6] provides high-temperature ductility of more than 60%, which guarantees the absence of cracks during forging and rolling.

 Cerium was introduced in order to regulate the shape, reduce the number and size of the formed excess phases, in particular for spheroidization of oxides and sulfides [7]. With spheroidization, the amount of oxides and sulfides is significantly reduced. This phenomenon significantly increases manufacturability at the stage of hot metallurgical redistribution. Resistance against pitting also increases with a decrease in the number and size of non-metallic inclusions, therefore, the temperature of the onset of pitting formation of steel with the introduction of cerium increases.

 The obtained higher level of mechanical, technological and corrosion properties of the claimed steel is ensured by complex alloying of the composition in the above ratio.

 With the introduction of copper, manganese, calcium, cerium outside the specified limits and with values of the Ca / S ratio <1.5, the effectiveness of their positive effect on the level of strength, hot ductility, and pitting resistance decreases.

 In TsNII KM "Prometey" in accordance with the plan of scientific research carried out a series of laboratory works for the smelting, plastic and heat treatment of the declared steel and steel prototype. The metal was smelted in 60 kg electric furnaces with a special selection of the charge and casting ingots weighing 17 kg. The resulting metal was subjected to pressure treatment on rolling equipment and was heat treated. The chemical composition of the investigated materials is presented in table. 1.

The strength properties, corrosion resistance in aggressive aqueous media were investigated (the critical temperature of the onset of pitting formation in a solution of 10% FeCl ₃ • 6H ₂ O was determined), as well as the process plasticity at high temperatures (δ,% at 1100 and 1200 ^o С). The test results are given in table. 2, confirm the advantages of the proposed steel in terms of strength at room temperature, elongation at forging temperatures and critical temperature of the onset of pitting in 10% FeCl ₃ • 6H ₂ O.

 The technical and economic effect of the use of the invention is expressed in increasing operational reliability and safety of work, as well as in increasing the resource of products.

Sources of information
1. Voronenko B. I. Modern corrosion-resistant austenitic-ferritic steels. MITOM, 10, 1997

 2. Sandvik SAF 2507. A high performance duplex stainless steel. S - 1875 - ENG, February, 1997.

 3. UK Patent Application GB 2160221A. 12/18/1985.

 4. Patent 4985091, USA 01/15/1997

 5. A.s. 1694685, Central Research Institute of the World Cup, USSR, 1989.

 6. Patent 4664725, USA, 1987.

 7. A.S. 1710594, USSR, 1992

Claims (1)

Two-phase stainless steel with high corrosion resistance in aggressive environments, containing carbon, silicon, manganese, chromium, nickel, molybdenum, nitrogen, sulfur, phosphorus and iron, characterized in that it additionally contains copper, calcium, magnesium, cerium in the following ratio of components , wt.%:
Carbon - 0.1 - 0.03
Silicon - 0.3 - 0.8
Manganese - 0.8 - 1.2
Chrome - 24 - 26
Nickel - 6.5 - 7.5
Molybdenum - 3.5 - 4.5
Nitrogen - 0.15 - 0.30
Sulfur - 0.001 - 0.015
Phosphorus - 0.001 - 0.020
Copper - 0.1 - 0.5
Calcium - 0.01 - 0.023
Cerium - 0.001 - 0.02
Magnesium - 0.01 - 0.04
Iron - Else
subject to a calcium / sulfur ratio of ≥ 1.5.

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