RU2225793C2 - Clad corrosion resistant steel and an item made out of it - Google Patents

Abstract

FIELD: metallurgy; production of a clad corrosion-resistant steel used in petroleum and chemical engineering industry. SUBSTANCE: the invention is dealt with metallurgy, in particular with production of a clad corrosion- resistant steel, which may be used for manufacture of the steel sheets and pipes for petroleum and chemical engineering industry, for construction of the oil pipelines, heat supply networks, etc. The offered clad corrosion-resistant steel consists of a primary layer made out of carbon or a low-alloy steel and, at least, of one clad layer from a corrosion-resistant chromium steel with a adhesive strength of layers of no less than the strength of steel of the primary layer. At that the corrosion-resistant steel of the clad layer contains components in the following ratio, (in mass %): carbon - 0.01-0.08; manganese 0.20 - 0.80; silicon 0.5- 2.5; chromium - 11-15; sulfur - no more than 0.007; phosphorus no more than 0. 035; iron and the imminent impurities - the rest. The contents of chromium, silicon and carbon in the indicated layer corresponds to the condition: [Cr] +4 [Si]-20 [C] = 16-20 %. The steel of the clad layer may additionally contain molybdenum - in amount of 0.5 - 2.5 mass %; niobium - in amount defined from a condition: 8 [C] ≤ [Nb] ≤ 1.5 mass %, and titanium in amount defined from a condition: 4 [C] ≤ [Ti] ≤ 1.0 mass %. Technical result of the invention is an increase of the clad steel rust resistance against over-all and local corrosion at preservation of mechanic and technical characteristics. EFFECT: increased the steel over-all and local corrosion resistance at preservation of mechanic and technical characteristics. 5 cl, 1 tbl, 1 ex

Description

The invention relates to the field of metallurgy, and in particular to the production of corrosion-resistant bimetal and products from it, in particular sheets and pipes used for equipment of oil and chemical engineering, for pipelines, heating networks and other purposes in any sectors of the economy in environments of various aggressiveness, in including causing the appearance of local types of corrosion. In such conditions, the main criterion for the selection of material for specific products is the need to ensure a sufficient level of resistance against general, intergranular, pitting and other types of corrosion with sufficient strength, ductility, viscosity and weldability. Bimetals are a unique type of metal products, in which it is possible, with the right choice of layers, to provide the required combination of corrosion resistance, mechanical and technological characteristics, which, in turn, leads to increased durability and reliability of the equipment.

Known two-layer corrosion-resistant sheet steel according to GOST 10885, traditionally used for equipment in the oil, chemical and pulp and paper industry. Carbon or low alloy steels are used as the main layer, corrosion-resistant ferritic or austenitic steels are used as the clad layer, and clad layers of high alloy steels and alloys of the type 08Kh17N13M2T, 08Kh17N13M3T, 06KhN28MDT and others with high chromium content are used to provide high resistance to pitting corrosion. nickel, molybdenum and other elements. The disadvantages of such two-layer steels in addition to high cost include low technological characteristics. In particular, the increased deformation resistance of the steel of the cladding layer at high temperatures is accompanied by uneven deformation of the layers and an increase in the load on the processing equipment during hot processing. In addition, during the welding of such steels, there is a high probability of the formation of crystallization cracks in welded joints. This dictates the advisability of creating a bimetal with a cladding layer of economically alloyed corrosion-resistant steels, for example, chromium - ferritic or ferritic-martensitic, with higher technological characteristics and at the same time resistant to pitting corrosion.

Known two-layer high-strength corrosion-resistant steel and products made from it, for example, sheets, welded storage tanks for transporting and storing aggressive liquids. The steel of the base layer contains the following components, wt.%:

Carbon 0.08-0.10

Silicon 0.17-0.37

Manganese 0.3-0.6

Chrome 0.6-0.9

Nickel 2.0-3.0

Copper 0.4-0.7

Molybdenum 0.35-0.45

Aluminum 0.02-0.06

Niobium 0.02-0.05

Sulfur 0.001-0.010

Phosphorus 0.001-0.015

Iron Else

The protective, cladding layer is made of steel, containing, wt.%:

Carbon 0.01-0.12

Silicon 0.2-0.8

Manganese 1.3-2.5

Chrome 17.0-20.5

Nickel 8.0-11.5

Niobium 0.7-1.2

Iron Else

The thickness of the cladding layer is 5.7-16.7% of the total thickness of the bimetal (RF patent No.2016912, IPC B 32 B 15/14, publ. 30.07.1994). Such two-layer steel is resistant to general and intergranular corrosion in many environments. However, due to the high content of alloying elements, its cost is high. In addition, the resistance of such steel to pitting corrosion is insufficient.

Known composite stainless steel material with a layered structure, including the main, transitional and surface structural layers made of chromium-silicon or chromium-silicon alloy steel of regulated composition, and the surface layer contains the following components, wt.%:

Chrome 6-25

Silicon Up to 6.5

when the ratio Si / Cr <0.4, and may also optionally contain one or more components selected from the group, wt.%:

Carbon Up to 1.0

Nickel Up to 34.0

Manganese Up to 6.0

Aluminum Up to 4.0

Molybdenum Up to 3.0

Tungsten Up to 4.0

Vanadium Up to 0.5

Copper Up to 5.0

Titanium Up to 1.0

Niobium Up to 1.0

Zirconium Up to 1.0

Yttrium Up to 0.5

REM Up to 0.5

Nitrogen Up to 0.8

Iron and impurities Else

(RF patent No. 2155134, IPC B 32 V 15/18, publ. 08.27.2000). The material has an increased strength of the connection of the layers and, in this regard, high adaptability. However, its corrosion resistance in many environments may be insufficient.

Known products - pipelines for transporting oil and natural gas with high corrosion resistance and good weldability. The pipes are made of clad steel, the outer layer of which contains the following components, wt.%:

Carbon 0.02-0.25

Silicon 0.1-1.0

Manganese 0.05-2.0

Aluminum 0.005-0.10

Phosphorus Not more than 0.015

Sulfur Not more than 0.005

Iron and inevitable impurities

The inner layer in direct contact with the transported medium contains the following components, wt.%:

Carbon Not more than 0.03

Chrome 7.5-16.0

Silicon 0.01-1.0

Manganese 0.01-3.0

Nitrogen Not more than 0.02

Iron and inevitable impurities

The inner layer may also contain one or more elements from the following group, wt.%:

Nickel Up to 4.0

Copper Up to 4.5

Molybdenum Up to 2.5

Tungsten Up to 5.0

and also may additionally contain one or more elements from the following group, wt.%:

Niobium 0.01-1.0

Vanadium 0.01-1.0

Titanium 0.01-1.0

Zirconium 0.01-1.0

(US patent, No. 5275893, NKI 428/683, IPC B 32 V 15/18, publ. 4.01.1994) Pipelines have high corrosion resistance during transportation of pure oil and natural gas. However, when an aqueous fraction appears in the transported medium, as well as in some other environments, the resistance of such clad steel and pipes from it against general and pitting corrosion is insufficient.

The closest analogue of the claimed invention is clad corrosion resistant steel, consisting of a base layer of carbon or low alloy steel and at least one clad layer of corrosion resistant steel, while the carbon content in the steel of the base layer is 0.02-0.20%, adhesion strength layers is not lower than the strength of the base layer, and the ratio of the thickness of the cladding layer to the total thickness of the layers is determined depending on the thermal coefficients of linear expansion of the cladding steels and the base basal layers according

0.5 × [1- (α _o.s. -α _ps ) / α _o.s. ] ≤h _ps / h _total ≤ 0.20,

where α _o.s. and α _ps - thermal coefficients of linear expansion of the steels of the main and cladding layers, respectively,

h _total and h _P.c. - the total thickness of the layers and the thickness of the cladding layer, respectively.

The steel of the base layer contains the following components, wt.%:

Carbon 0.02-0.2

Silicon 0.05-0.8

Manganese 0.3-2.1

one or more elements from the group including

Niobium 0.005-0.1

Vanadium 0.005-0.12

Titanium 0.005-0.08

Boron 0.001-0.006

Iron Else

the alloying elements are connected by a number of dependencies, and the corrosion-resistant steel of the cladding layer contains 5-20% chromium and not more than 0.01% sulfur, for example, 08Kh18FB steel or 08Kh18N10 steel. The above clad stainless steel products are made in the form of strips 2.5 mm thick, or sheets 5 mm thick, or welded pipes, having high resistance to corrosion and mechanical damage, in particular against stress corrosion cracking, high viscosity, cold resistance, strength , plasticity, weldability (RF patent No. 21115559, IPC B 32 V 15/18, publ. 07/20/1998, description - prototype of the invention “steel” and “product”). Such clad steel and its product have low resistance to general and pitting corrosion in many operating environments, in particular in aqueous environments with a high content of chlorine ions.

The problem solved by this invention is to provide high corrosion resistance, mechanical properties and manufacturability of clad steel and products made from it, at a low cost.

The technical result of this invention is to increase corrosion resistance, in particular resistance to general and local corrosion of clad steel and products made from it, while maintaining mechanical and technological characteristics, as well as cost.

The technical result is achieved by the fact that in the known clad corrosion-resistant steel, consisting of a base layer of carbon or low alloy steel and at least one cladding layer of chromium corrosion-resistant steel, with adhesion layers not lower than the strength of the steel of the base layer, according to the invention, corrosion-resistant clad layer steel contains carbon, manganese, silicon, chromium, sulfur, phosphorus, iron and inevitable impurities, in the following ratio, wt.%:

Carbon 0.01-0.08

Manganese 0.20-0.80

Silicon 0.5-2.5

Chrome 11-15

Sulfur Not more than 0.007

Phosphorus Not more than 0,035

Iron and inevitable impurities

while the content of chromium, silicon and carbon meets the condition

where [Cr], [Si] and [C] are the contents of chromium, silicon and carbon, respectively, wt.%, also the fact that the steel of the cladding layer additionally contains molybdenum in an amount of 0.5-2.5 wt.%,

also the fact that the steel of the cladding layer additionally contains niobium in an amount determined from the condition

where [C] and [Nb] are the carbon and niobium contents, respectively, wt.%, or titanium in an amount determined from the condition

where [C] and [Ti] are the carbon and titanium contents, respectively, wt.%,

as well as the fact that the product is made of the specified clad stainless steel.

Obtaining high resistance of clad steel and products made from it against various types of general and local corrosion is achieved by observing a certain chemical composition of the clad layer steel. The carbon content in the proposed range allows you to provide the required combination of corrosion resistance and mechanical properties that determine the technological characteristics of steel. At a higher carbon content, the resistance of steel against intergranular and pitting corrosion decreases. With a lower carbon content, there is a sharp increase in the tendency to grain growth, which negatively affects the characteristics of ductility and viscosity.

The manganese content not lower than the proposed one provides the required degree of deoxidation of steel, which positively affects its corrosion resistance. A further increase in the manganese content decreases the resistance to pitting corrosion due to the appearance of a martensitic component in the steel structure and the release of sulfide non-metallic inclusions.

The proposed chromium content provides high resistance of steel against general corrosion in many environments. With a lower chromium content, corrosion resistance will be insufficient. A further increase in its content above the proposed level leads to an increase in the cost of steel and products from it without a significant increase in corrosion resistance.

The silicon content in the proposed range allows for a given level of chromium to further increase the corrosion resistance of steel. A further increase in the silicon content leads to the appearance of segregation zones in steel with an increased content thereof, while structural heterogeneity is enhanced and resistance to local corrosion is reduced. A lower silicon content than proposed does not increase the corrosion resistance of steel.

Limiting the content of impurities - sulfur and phosphorus - positively affects the resistance of steel and its products against pitting corrosion.

An additional limitation of the content of chromium, silicon and carbon in accordance with expression (1) ensures the formation of a uniform ferritic structure in steel, which makes it possible to increase resistance to pitting corrosion. At lower values of this expression, the presence of other phases in the structure that negatively affect the corrosion resistance of steel is possible. At higher values of expression (1), the formation of the σ phase is possible, which negatively affects not only the corrosion resistance of steel, but also leads to its embrittlement and a decrease in manufacturability in the manufacture of products.

An additional introduction of molybdenum in the proposed range allows you to further increase the resistance against pitting corrosion, as well as against general corrosion in a number of redox environments.

The proposed content of niobium and titanium, as well as their regulation depending on the carbon content in accordance with expressions (2) and (3), can ensure the resistance of steel against intergranular corrosion, regardless of the carbon content in the steel of the clad layer.

Examples of concrete implementation of the proposal

Double-layer sheet steel with a base layer of steel 20sp according to GOST 1050 and five variants of cladding steel was obtained at Severstal using the electroslag surfacing method. The chemical composition of the slabs of the main layer of steel 20sp, wt.%:

Carbon 0.18

Silicon 0.31

Manganese 0.48

Sulfur 0.007

Phosphorus 0.015

Chrome 0.10

Nickel 0.08

Copper 0.05

Aluminum 0.04

Iron Else

A layer of corrosion-resistant steel was deposited with consumable electrodes onto the surface of a slab with dimensions of 250 × 1300 × 5000 mm, while the thickness of the deposited layer was 40-50 mm. After heating in the methodical furnaces, two-layer slabs were rolled on a continuous broad-band mill “2000” into strips 10 mm thick. The strips were wound into rolls at a temperature of 650-670 ° C. Sheet blanks were cut from strips of all variants, from which welded tubes with a diameter of 426 mm and a length of 150 mm were made with an inner cladding layer of stainless steel. Billets were cut from strips and from nozzles and samples were prepared for mechanical, electrochemical, and corrosion tests, including resistance to general, pitting, and intergranular corrosion.

The following clad layer steels were tested:

option 1 - ferritic corrosion-resistant steel containing 0.02% carbon, 0.58% manganese, 1.3% silicon, 12.9% chromium, 0.003% sulfur, 0.012% phosphorus, the rest is iron and inevitable impurities, with the value [ Cr] +4 [Si] -20 [C] amounted to 17.7%, which corresponds to the expression (1) (option corresponds to claim 1 of the claims);

option 2 - ferritic corrosion-resistant steel containing 0.02% carbon, 0.51% manganese, 1.6% silicon, 13.0% chromium, 0.003% sulfur, 0.014% phosphorus, 1.3 molybdenum, the rest is iron and unavoidable impurities while the value of [Cr] +4 [Si] -20 [C] was 19.0%, which corresponds to expression (1) (the variant corresponds to claim 2);

option 3 - ferritic corrosion-resistant steel containing 0.04% carbon, 0.55% manganese, 0.5% silicon, 10.2% chromium, 0.003% sulfur, 0.012% phosphorus, the rest is iron and inevitable impurities, with the value [ Cr] +4 [Si] -20 [C] was 11.4%. which does not correspond to expression (1) (the variant does not correspond to the claims);

option 4 - ferritic stainless steel containing 0.06% carbon, 0.60% manganese, 1.7% silicon, 12.5% chromium, 0.003% sulfur, 0.014% phosphorus, 0.8% niobium, the rest is iron and unavoidable impurities, while the value of [Cr] +4 [Si] -20 [C] was 18.1%, which corresponds to expression (1) (the variant corresponds to claim 3 of the claims);

option 5 - ferritic stainless steel containing 0.06% carbon, 0.56% manganese, 1.2% silicon, 13.1% chromium, 0.004% sulfur, 0.009% phosphorus, 0.5% titanium, the rest is iron and unavoidable impurities, while the value of [Cr] +4 [Si] -20 [C] was 16.7%, which corresponds to expression (1) (the variant corresponds to claim 4).

When rolling sheets for all obtained options, a satisfactory technological plasticity and the required level of deformation resistance were noted. Loads on equipment during rolling did not exceed permissible, non-uniformity of deformation was insignificant. At the same time, a high surface quality of the sheets was obtained.

The billets selected from all the obtained options were welded using two types of welding — automatic arc and manual — in the manufacture of nozzles. As an additive material used austenitic wire Sv-07X25H13. In the case of manual arc welding, this wire was used in electrodes of the EA-2 type. The resulting welded joints were tested for bending at an angle of 120 degrees. Test results are satisfactory. In addition, a metallographic study of welded joints was carried out, which showed a complete absence of welding cracks and other defects both in the weld metal and in the heat-affected zone. The results obtained indicate the high welding and technological characteristics of clad steel of all the options obtained. The mechanical properties of two-layer samples — yield strength, tensile strength, and elongation were determined during tensile tests in accordance with GOST 1497. For all five variants, a high level of mechanical characteristics was obtained: yield strength 340-370 N / mm2, temporary strength 490-530 N / mm2, elongation δ5-25-28%.

Corrosion tests were carried out according to the following methods:

Method 1 - the rate of general corrosion of steels in non-oxidizing acidic media was determined (1.0 N H ₂ SO ₄ ; 0.1 N H ₂ SO ₄ ; 0.1 N H ₂ SO ₄ + 0.9 N Na ₂ SO ₄ ; pH 0.5-2.5) at room temperature and a test duration of 90 days. The gravimetric method was used to determine the weight loss of the samples during the test and the rate of general corrosion was calculated. In this case, the condition for ensuring the required corrosion resistance was considered to be the obtaining of corrosion rate values of not more than 1 mm / year.

Method 2 — resistance to pitting corrosion in an aqueous medium containing 0.17 mol / L NaCl + 0.13 mol / L KCl + 0.008 mol / L Na ₂ SO ₄ + 0.008 mol / L NaHCO _{3 was} determined at a temperature of 10-30 ° C and exposure time of 90 days. After testing using optical microscopy, the presence, size, and number of pits formed on the surface of the steels were determined. The main criterion for resistance to pitting corrosion was considered the absence of pitting on the surface of steel after testing.

Method 3 - determination of resistance to pitting corrosion by electrochemical methods according to GOST 9.912-89 in solutions: No. 1 - model of recycled water; No. 2 is a model of sea water. A condition for ensuring the required resistance to pitting corrosion is the positive value of the difference between the critical potential of pitting corrosion and the potential of free corrosion when tested in each of these environments.

Method 4 - resistance to intergranular corrosion was determined according to GOST 6032.

The results of corrosion tests of samples of the investigated options are presented in table 1.

It can be seen that options 1, 2, 4 and 5, corresponding to the claims, provide high resistance against all considered types of corrosion. Especially high resistance to general and pitting corrosion is the steel obtained in option 2 - with molybdenum. The introduction of niobium and titanium into steel makes it possible to obtain steel that is resistant to intergranular corrosion, even with a relatively high carbon content of 0.06% (options 4 and 5).

With reduced values of [Cr] +4 [Si] -20 [C] for option 3, the corrosion resistance sharply decreases, first of all, the resistance to pitting corrosion; while for steel option 3, not containing titanium or niobium, unsatisfactory test results were obtained for resistance to intergranular corrosion.

Thus, the use of this proposal significantly increases the corrosion resistance of ferritic stainless steels while maintaining their technological characteristics and cost.

Claims (5)

1. Clad corrosion-resistant steel, consisting of a base layer of carbon or low alloy steel and at least one cladding layer of corrosion-resistant chrome steel, with a bond strength of layers not lower than the strength of the steel of the base layer, characterized in that resistant steel cladding layer contains carbon, manganese, silicon, chromium, sulfur, phosphorus, iron and inevitable impurities in the following ratio, wt.%: Carbon 0.01-0.08 Manganese 0.20-0.80 Silicon 0.5-2.5 Chrome 11-15 Sulfur Not more than 0.007 Phosphorus Not more than 0,035 Iron and inevitable impurities while the content of chromium, silicon and carbon meets the condition [Cr] +4 [Si] -20 [C] = 16-20%, where [Cr], [Si] and [C] are the contents of chromium, silicon and carbon, respectively, wt.%. 2. Clad corrosion-resistant steel according to claim 1, characterized in that the clad layer steel further comprises molybdenum in an amount of 0.5-2.5 wt.%. 3. Clad corrosion-resistant steel according to claim 1 or 2, characterized in that the steel of the cladding layer further comprises niobium in an amount determined from the condition 8 [C] ≤ [Nb] ≤1.5 wt.%, where [C] and [Nb] are the carbon and niobium contents, respectively, wt.%. 4. Clad corrosion-resistant steel according to claim 1 or 2, characterized in that the clad layer steel further comprises titanium in an amount determined from the condition 4 [C] ≤ [Ti] ≤1.0 wt.%, where [C] and [Ti] are the carbon and titanium contents, respectively, wt.%. 5. The product is made of clad corrosion-resistant steel, characterized in that it is made of clad corrosion-resistant steel according to one of claims 1 to 4.