RU2228385C1 - Steel for welded structures - Google Patents

Abstract

FIELD: ferrous metallurgy. SUBSTANCE: invention provides steel appropriate for welded structures operated under normal temperature conditions and in water-based corrosive media. Steel contains following components, wt %: carbon 0.06-0.12, manganese 0.65-0.85, silicon 0.05-0.11, nickel 0.32-0.40, vanadium 0.04- 0.08, copper 0.32-0.40, aluminum 0.02-0.04, barium 0.020-0.050, nitrogen 0.002-0.004, sulfur up to 0.015, phosphorus up to 0.015, and iron - the balance, wherein summary content Mn+Si+Al+Ba is 0.80-1.05 wt %. EFFECT: improved combination of mechanical properties and corrosion resistance in nearly neutral aqueous corrosive media. 2 cl, 2 tbl

Description

The invention relates to the field of ferrous metallurgy, in particular to alloy steels for metal structures, and can be used in the production of steels of ordinary quality for welded structures operating at normal temperatures and in water-based corrosive environments similar in pH to neutral pH values.

Known low-alloy structural steel for welded structures of the brand 09G2, containing, wt.%; carbon <0.12; manganese 1.40-1.80; silicon 0.17-0.37; chrome <0.30; nickel <0.30; sulfur <0.040; phosphorus <0.035; nitrogen <0.008; copper <0.30; arsenic <0.08; iron - the rest (Marochnik of steels and alloys / V.G. Sorokin, A.V. Volosnikova, S.A. Vyatkin, etc. / Under the general editorship of V.G. Sorokin. - M.: Engineering, 1989, p. . 97-99).

A disadvantage of the known steel is a reduced level of strength and toughness properties, Thus, for sheets and rolled thickness of about 20 mm tensile strength σ _B = 440 MPa, yield strength σ _0,2 = 305 MPa, elongation δ = 21%, the shock viscosity KCV ^-40 = 29 J / cm ² . In addition, manganese contributes to the growth of austenite grain during heating, which leads to a decrease in properties; 09G2 steel is also characterized by low corrosion resistance in aqueous media.

The closest analogue of the claimed invention is steel for pipes and other welded structures containing components in the following ratio, wt.%: Carbon <0.3; silicon 0.01-0.8; manganese 0.5-2; copper <0.5; nickel <2; aluminum <0.1; barium 0.003-0.015; phosphorus <0.04; sulfur <0.03; vanadium <0.1; iron - the rest (Japanese application JP 57-023047, publ. 06.02.1982).

Signs of the closest analogue, coinciding with the essential features of the claimed invention: the presence of carbon, manganese, silicon, nickel, vanadium, copper, aluminum, barium, nitrogen, sulfur, phosphorus and iron in steel.

Known steel does not provide a high complex of mechanical properties and corrosion resistance in corrosive environments based on water, close to neutral for the following reasons.

The limited amount of barium in the known steel in the range of 0.003-0.015 wt.% Is not enough to provide a significant reduction in the oxidation of steel to prevent oxidation of the carbonitride-forming element, vanadium. This reduces the level of control effect on the quality of steel and leads to an increase in the dispersion of indicators of mechanical properties, reducing the grade of steel. A small amount of barium reduces its modifying effect, and also contributes to an increase in the size of globular inclusions, which leads to a decrease in the complex of mechanical properties.

The permissible sulfur and phosphorus content in the known steel, equal to 0.03 and 0.04 wt.%, Respectively, prevents the production of steel with high corrosion resistance, which, together with a low barium content, also leads to a decrease in the ductility and toughness of steel, reducing complex of mechanical properties.

In the known steel, the nitrogen content is not limited, therefore, it can vary widely depending on the method of steelmaking, for example, from 0.002-0.005 wt.% In converter production to 0.007-0.010 wt.% In electric steel production (Goldstein M.I. and other Special steels. Textbook for high schools. - M .: Metallurgy, 1985, p. 29). In this case, the vanadium content is limited only by the upper limit (≤0.1 wt.%), Which, together with an unlimited nitrogen content, leads to unpredictable indicators of carbonitride hardening and a decrease in the set of mechanical properties of steel.

The basis of the invention is the task of improving the steel for welded structures by optimizing the quantitative composition.

The expected technical result is to obtain a high complex of mechanical properties and corrosion resistance in corrosive environments based on water, close to neutral.

The problem is solved in that the steel for welded structures containing carbon, manganese, silicon, nickel, vanadium, copper, aluminum, barium, nitrogen, sulfur, phosphorus and iron, according to the invention it contains components in the following ratio, wt.%:

Carbon 0.06-0.12

Manganese 0.65-0.85

Silicon 0.05-0.11

Nickel 0.32-0.40

Vanadium 0.04-0.08

Copper 0.32-0.40

Aluminum 0.02-0.04

Barium 0.020-0.050

Nitrogen 0.002-0.004

Sulfur ≤0.015

Phosphorus ≤0.015

Iron rest

In addition, the combined content (Mn + Si + Al + Ba) is 0.80 - 1.05 wt.%.

The steel of the proposed composition provides a high complex of mechanical properties, namely: temporary tensile strength σ _at ≥ 550 MPa, conditional yield strength σ _0.2 ≥ 500 MPa, elongation δ = 22%, impact strength KCV ⁺²⁰ ≥ 50 J / cm ² .

In addition, steel is well welded and has high corrosion resistance in water-based media, close in pH to neutral; In this regard, steel can be used for oil and gas pipes and metal structures of general purpose and ordinary quality.

The carbon content in the inventive steel is limited to 0.06-0.12 wt.%: This ensures good weldability and ductility of the steel. To obtain carbon in an amount of less than 0.06 wt.%, Complication of melting is required and the level of strength properties is reduced, and therefore such a carbon content is impractical. When the carbon content is more than 0.12 wt.%, It becomes possible to reduce ductility and the deterioration of corrosion resistance due to an increase in microgalvanic pairs.

Manganese is contained in an amount of 0.65-0.85 wt.%. The overall decrease in manganese relative to steel according to the closest analogue is due to the fact that during casting of large ingots due to segregation of manganese microvolumes enriched by it can be formed, so that with accelerated cooling of rolled products, nonequilibrium structures with low ductility are obtained. When the content is less than 0.65 wt.% Manganese, it does not sufficiently solidify the solid solution. With a content of more than 0.85 wt.% Manganese in rolled thin sections there are areas with reduced ductility.

Silicon in the inventive steel is limited to 0.05-0.11 wt.%. With this silicon content, sufficient deoxidation is achieved by the combined action (Mn + Si + Al + Ba), which is regulated in the inventive steel. Obtaining very low silicon values (less than 0.05 wt.%) Complicates the smelting technology and is impractical; when the silicon content is more than 0.11 wt.%, the probability of obtaining a noticeable amount of silicate inclusions increases, which reduces the resistance to brittle fracture, especially with cyclic loads.

The optimum nickel content is 0.32-0.40 wt.%. In these quantities, its positive effect is manifested on impact strength. If the content is less than 0.32 wt.%, The solid solution will not be sufficiently alloyed with nickel, more than 0.40 wt.% Of nickel will significantly increase the cost of steel, and from the point of view of the conditions and field of use of the inventive steel is impractical.

Vanadium in an amount of 0.04-0.08 wt.% Provides, together with aluminum in an amount of 0.02-0.04 wt.%, A fine-grained structure, which leads to an increase in the viscosity and ductility of steel. Vanadium in the declared range with a limited nitrogen content of 0.002-0.004 wt.%, Forms carbonitrides, almost completely eliminating the presence of free nitrogen in steel, and aluminum - oxides, which inhibit the growth of austenite grain during heating. In addition, they affect the quality of steel as deoxidizing agents.

Barium in the amount of 0.020-0.050 wt.% Has a positive effect on the globularization of inclusions in steel and, as a result, increases the viscosity of steel, has a positive effect on machinability, and reduces the corrosion current in the globular inclusion - solid solution microvoltaic pairs. The barium content in the claimed amounts helps to reduce the oxidation of steel, providing a more complete use of vanadium, which is part of the inventive steel as a carbonitride binder, which virtually eliminates the presence of free nitrogen in the steel. In this case, vanadium that did not react with nitrogen and carbon increases the viscosity and ductility of the steel. In addition, barium in the claimed amounts modifies the sulfide and phosphide components of non-metallic inclusions, forming strong complexes, which leads to an increase in the complex of mechanical properties of steel and provides high corrosion resistance in water-based media, preventing the formation of blistering - surface hydrogen cracking - one of the common defects pipe steel. With a content of less than 0.020 wt.% Barium, the effect is not stable, and with more than 0.050 wt.% Barium, the ductility of the steel decreases below permissible.

Copper is contained in an amount of 0.30-0.40 wt.%. This amount is necessary to give the steel corrosion resistance in water-based media that are close in pH to neutral, and the positive effect of copper on corrosion resistance is enhanced in the presence of phosphorus and barium. In addition, copper strengthens the solid solution, providing strength properties in combination with nickel.

In the inventive steel, the sulfur and phosphorus content is limited to an upper limit of 0.015 wt.%. A higher sulfur content increases the amount of sulfides, increases the tendency of steel to corrosion cracking and hydrogen saturation, which leads to a decrease in ductility. A greater than 0.015 wt.% Phosphorus content leads to the tendency of brittle fracture of steel at the lowest operating temperatures.

Nitrogen in the proposed steel is contained in an amount of 0.002-0.004 wt.%. This amount is necessary for the formation of carbonitrides, hardening steel. When the nitrogen content is less than 0.002 wt.%, The carbonitrides content decreases while the carbide fraction increases, which leads to a decrease in the complex of mechanical properties of steel. With a nitrogen content of more than 0.004 wt.% And a limited vanadium content of 0.04-0.08 wt.%, The aging effect may occur, accompanied by a decrease in ductility and toughness.

An embodiment of the inventive steel, not excluding other options in the scope of the claims.

The experimental steels were smelted in a semi-industrial crucible furnace with a nominal crucible capacity of 30 kg. For the charge used low-carbon steel scrap. The chemical composition of the experimental steels is given in table 1 (No. of smelts 1-3 - proposed steel, No. 4 - steel according to the closest analogue).

Under the same conditions, a comparative ingot with steel composition was smelted according to the closest analogue.

The obtained ingots weighing 25 kg were forged onto billets with a cross section of 30 × 120 mm, then they were rolled into strips with a thickness of 15 mm. Samples for heat treatment, mechanical, corrosion testing and weldability were made from strips. Corrosion tests were carried out in tap water with a pH of 7.32. The rate of general corrosion was evaluated by weight loss, weldability was assessed visually by inspection of the weld and the heat-affected zone.

The test results of the experimental steels are shown in table 2, from which it can be seen that the claimed steel according to the set of mechanical properties and corrosion resistance in water-based corrosive environments, close to neutral, preferably known.

Claims (2)

1. Steel for welded structures containing carbon, manganese, silicon, nickel, vanadium, copper, aluminum, barium, nitrogen, sulfur, phosphorus and iron, characterized in that it contains components in the following ratio, wt.%: Carbon 0.06-0.12 Manganese 0.65-0.85 Silicon 0.05-0.11 Nickel 0.32-0.40 Vanadium 0.04-0.08 Copper 0.32-0.40 Aluminum 0.02-0.04 Barium 0.020-0.050 Nitrogen 0.002-0.004 Sulfur ≤0.015 Phosphorus ≤0.015 Iron Else 2. Steel according to claim 1, characterized in that the combined content of Mn + Si + Al + Ba is 0.80-1.05 wt.%.