RU2276695C1 - Stainless steel for production of pipes and method of production of stainless steel pipes - Google Patents

Abstract

FIELD: ferrous metallurgy, stainless steels in particular; production of pipes of enhanced corrosion resistance for erection of pipes lines carrying corrosive media.

SUBSTANCE: proposed stainless steel contains carbon, silicon, manganese, chromium, nickel, molybdenum, iron and unavoidable admixtures. Method of production of pipes from stainless steel includes making steel, teeming it into ingots, deformation of ingots into tubular billets, hot extrusion followed by heat treatment. After teeming of steel and deformation of ingots, pipe billets are molded in pipes and pipes are subjected to fast cooling in water; heat treatment is performed by austenization at temperature of 1030-1050°C at holding pipe wall thickness within 1 min/mm followed by cooling in air. This method may be used for production of pipes of large diameters (up to 159-219 mm); proposed steel has balanced chemical and phase composition.

EFFECT: improved mechanical properties.

3 cl, 2 dwg, 2 tbl, 2 ex

Description

The invention relates to ferrous metallurgy, in particular to stainless steels, and can be used in the manufacture of pipes of increased corrosion resistance for various purposes in engineering industries, for example, for the construction of pipelines transporting corrosive corrosive media (aqueous media containing chlorine ions, hydrogen sulfide, carbon dioxide, etc.). Conventional steels in such conditions can be subject to general and local corrosion, stress corrosion cracking, corrosion erosion, which in turn will lead to damage to the pipeline.

It should be noted that the chemical composition, the microstructure of the steel, the amount, composition and properties of non-metallic inclusions affect the corrosion resistance of steel and its strength properties, and the method of steel production should include minimal alloying during the smelting process, providing a high degree of purity for non-metallic inclusions.

Martensitic stainless steel for pipe production is known in the art (see EP 0178338, C 22 C 38/18, 04/23/1986). Steel contains components in the following ratio, wt.%:

Carbon no more than 0.3 Silicon no more than 1,0 Manganese no more than 2.0 Chromium 11.0-14.0 Aluminum 0.005-0.10 Nitrogen no more than 0.10 Phosphorus no more than 0.20 Sulfur no more than 0,003 Iron rest

one or more elements from the group:

Nickel not less than 3,5 Copper not less than 2.0 Molybdenum not less than 2.5 Niobium not less than 0.10 Vanadium 3-10

Also known from the prior art is a method of manufacturing steel for pipes with increased corrosion resistance, including steel smelting, casting, accelerated cooling of the resulting billet in water and its heat treatment (see RU 2184155 C2, C 21 D 8/10, 06/27/2002).

The closest analogue to the claimed group of inventions for the combination of essential features and purpose is SU 1686026 A1, C 22 C 38/48, 10.23.1991, of which structural stainless steel is known, used for the manufacture of welded pipes of high pressure pipelines, and a method of manufacturing pipes from of stainless steel. The considered steel of this composition and obtained by this method has increased strength, ductility and resistance to intergranular corrosion of welded joints. Steel contains components in the following ratio, wt.%:

Carbon 0.05-0.08 Chromium 16.5-17.5 Nickel 4,5-5,2 Manganese 1.0-1.5 Silicon 1.5-1.8 Molybdenum 1.0-3.0 Niobium 0.04-0.16 Nitrogen 0.15-0.19 Iron rest

The method includes steel smelting, its casting into ingots, deformation and subsequent heat treatment.

The disadvantage of the steel disclosed in the closest analogue is reduced mechanical properties, in particular yield strength at a temperature of 350 ° C. At the same time, the surface quality of the pipes was low (Rz = 80 μm and coarser), therefore, an additional operation was required to “ovulate” the pipes — cold deformation in the correct machine — followed by grinding the surface in order to remove traces of ovalization, which, of course, leads to additional labor costs.

The technical result of the invention is the creation of a new grade of stainless steel for the production of pipes, including pipes of "large" diameter (⌀ 159-219 mm), having a balanced chemical and phase composition and having an increased level of mechanical properties, in particular, an increased level of limit values yield strength (σ _0.2 ) at a temperature of 350 ° C, as well as reducing labor costs in the production of pipes from the declared steel due to the exclusion of additional pipe processing operations.

The technical result is achieved by the fact that the proposed stainless steel for the production of pipes containing carbon, silicon, manganese, chromium, nickel, molybdenum, iron and inevitable impurities, while it additionally contains aluminum, titanium and boron, in the following ratio of components, wt.% :

carbon 0.03-0.1 silicon no more than 0.8 manganese 1.0-2.0 chromium 17.0-19.0 nickel 9.0-11.0 molybdenum no more than 0.5 aluminum no more than 0,05 titanium 0.015-0.80 boron 0.002-0.005 Fe and inevitable impurities rest,

the ratio of titanium to carbon in steel is 7-10,

as well as a method of manufacturing stainless steel pipes, including steel smelting, casting it into ingots, deformation of ingots into a pipe billet, hot pressing, subsequent heat treatment, stainless steel is smelted, and after steel casting and deformation of the ingot, the pipe billet is pressed into pipes and subsequent accelerated cooling of pressed pipes in water, and heat treatment is carried out by austenization at a temperature of 1030-1050 ° C with holding for 1 min / mm of the pipe wall thickness and cooling in air.

At the same time, sulfur, phosphorus and copper are inevitable and undesirable impurities in steel.

The microalloying of steel with boron (0.002-0.005 wt.%) Contributes to the grinding of austenitic grain due to the displacement of excess phases, including carbide, from the boundaries into the grain body, which affected the increase in the strength properties of steel by 25-30% at 350 ° C. This made it possible to obtain the required set of physicomechanical properties of pipes, including σ _0.2 at 350 ° C, which eliminated the need for additional ovalization operations and subsequent grinding of pipes.

A decrease in the silicon content in the steel and the introduction of aluminum within the proposed range will increase the strength properties and weldability without reducing corrosion resistance due to inhibition of the growth of austenitic grain.

The molybdenum content is reduced due to additional alloying with titanium, which, like molybdenum, is a carbide-forming element and has a greater affinity for carbon compared to molybdenum.

The ratio of titanium to carbon in steel is selected in the range of 7-10, since it is with this ratio that titanium allows efficient bonding of carbon and nitrogen, which is an impurity. This increases the absorption of titanium by molten steel. A change in this ratio will lead to the appearance of titanium oxides and an increase in the contamination of steel with non-metallic inclusions.

The selected type of heat treatment, namely, the temperature range of heating for austenization, is associated with the prevention of the possibility of overheating of the hereditarily fine-grained structure, which will lead to a decrease in all mechanical properties of steel. Exposure for 1 min per mm of the wall thickness of the pipe makes it possible to fully go through the process of redistribution of alloying components and the process of inheritance of the original structure.

Example. A series of laboratory swimming trunks and studies of the structure of steel and its mechanical properties were carried out: steel melting with boron content of 0.002, 0.003 and 0.005 wt.%, Subsequent casting into ingots of 15 kg, their deformation by pressing into pipes and heat treatment according to specified conditions. After pressing, accelerated cooling of the pipes into water was carried out to fix the structure and prevent the growth of austenitic grain.

The chemical analysis showed the content of alloying components and impurities, which are presented in Table 1. The mechanical properties are presented in Table 2. The microstructure of the obtained steel after heat treatment is presented in figures 1 and 2.

Table 1
The chemical composition of stainless steel for the production of pipes, wt.% No. FROM Si Mn Cr Ni Mo Al Ti AT Impurities: P, S, Cu one 0,037 1,5 1.37 16.9 5.5 1,0 - - - 0.020; 0.005; 0.28 2 0,034 0.52 1.36 16.8 9,4 0.50 0.05 0.30 0.002 0.021; 0.005; 0.28 3 0,036 0.52 1.36 16.95 9,4 0.50 0.05 0.25 0.004 0.021; 0.005; 0.28 four 0,033 0.51 1.35 16.85 9.36 0.50 0.05 0.27 0.005 0.020; 0.005; 0.27

Where smelting No. 1 corresponds to steel from the closest analogue, melting No. 2, 3, 4 correspond to the composition of the inventive steel with a boron content of 0.002, 0.003 and 0.005 wt.%, Respectively.

table 2
Mechanical properties at 20 and 350 ° С, heat treatment 1050 ° С No. σ _in , MPa σ _0.2 , MPa δ ₅ ,% ψ,% Austenite grain number twenty 350 twenty 350 twenty 350 twenty 350 one 660 430 390 267 56 31 - 73 6-7 2 673 448 437 327 fifty 28 - 71 8-9 3 685 438 420 330 53 36 - 73 9-10 four 691 467 440 356 58 27 - 70 8-9

The measurements were carried out and calculated as the average of 5 obtained values.

The size of the austenitic grain, characterized by the number (score), was determined according to the standard scale of microstructures (GOST 5639-65). The data obtained showed that the austenitic grain is fine-grained. Therefore, when using the proposed steel grade and during the above-described heat treatment, the grain size decreases, which positively affects the mechanical properties of the steel.

Example. Steel is smelted in an electric furnace with casting at a temperature of 1560 ° C, then it is cast into ingots, the ingots are deformed into a pipe billet with a diameter of 150 mm at a temperature of 1120 ° C. Peeling of the tube billet is carried out to a roughness of RZ - 80 μm; then hot pressing with a force of 2200 tons of tube billet on a press into a pipe, heat treatment of the pipe by quenching from 1030 ° C with cooling in water, followed by finishing.

Thus, the use of the proposed steel will significantly increase the strength characteristics, including yield strength at 350 ° C, and reduce labor costs in the production of pipes.

Claims (2)

1. Stainless steel for the production of pipes containing carbon, silicon, manganese, chromium, nickel, molybdenum, iron and inevitable impurities, characterized in that it additionally contains aluminum, titanium and boron, in the following ratio, wt.%: Carbon 0.03-0.1 Silicon No more than 0.8 Manganese 1.0-2.0 Chromium 17.0-19.0 Nickel 9.0-11.0 Molybdenum No more than 0.5 Aluminum No more than 0,05 Titanium 0.015-0.80 Boron 0.002-0.005 Iron and inevitable impurities Rest the ratio of titanium to carbon in steel is 7-10. 2. A method of manufacturing stainless steel pipes, including steel smelting, casting it into ingots, deformation of ingots into pipe billets, hot pressing, subsequent heat treatment, and stainless steel is melted according to claim 1, after steel casting and ingot deformation, the pipe billets are pressed into pipes and subsequent accelerated cooling of pressed pipes in water, and heat treatment is carried out by austenization at a temperature of 1030-1050 ° C with holding for 1 min / mm of the pipe wall thickness and cooling in air.

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