Low-alloyed steel

FIELD: metallurgy; making of low-alloy steels for manufacture of articles operating in aggressive media containing hydrogen sulfide and carbon dioxide.

SUBSTANCE: proposed low-alloyed steel contains the following components, mass-%: carbon, 0.05-0.15; manganese, 0.40-0.65; chromium, 0.50-0.80; silicon, 0.30-0.80; vanadium, 0.04-0.09; aluminum, 0.02-0.05; the remainder being iron and admixtures. Content of admixtures is limited as follows, mass-%: nitrogen, no more than 0.008; nickel, no more than 0.30; copper, no more than 0.25; hydrogen, no more than 0.0002; total calcium/sulfur ratio shall be no less than 1. In particular case, proposed steel may contain titanium and niobium; total amount of vanadium, titanium and niobium shall not exceed 0.15 mass-%. Proposed steel may be used for manufacture of pipes and joints for handling aggressive media, watery oil and formation water in particular characterized by high degree of mineralization and containing H2S and CO2.

EFFECT: enhanced strength characteristics and corrosion resistance.

2 cl, 4 tbl

 

The invention relates to the field of metallurgy, in particular to low-alloy steels intended for the manufacture of products used in highly aggressive environments containing hydrogen sulfide and carbon dioxide.

Known low-alloy steel, grade HF (Marochnik steels and alloys. Edited A.s.zubchenko. M., "engineering", 2001, page 172) the following chemical composition, wt.%:

Carbon0,12-0,18
Silicon0,17-0,37
Manganese0,40-0,70
Chrome0,80-1,10
Vanadium0,06-0,12
Coppernot exceed 0.30
Sulfurnot exceed 0.035
Phosphorusnot exceed 0.035
Ironrest

This steel has high strength characteristics, but low corrosion resistance, poor weldability and can not be used for fabrication of structural steel type pipe and fittings.

Known steel (patent RF №2196845, IPC With 22 38/46), used in chemical machinery parts and elements of welded structures, working under pressure in a wide range of temperatures, after having the respective chemical composition, wt.%:

Carbon0,18-0,22
Silicon0,20-0,40
Manganese0,50-0,80
Nickel0,05-0,40
Chromenot more than 0.4
Vanadium0,02-0,05
Aluminum0,01-0,04
Calcium0,01-0,02
Ironrest

This steel also has low corrosion resistance and cannot be used in products that are intended for use in aggressive environments containing H2S and CO2.

The closest essential features of the present invention is a low alloy steel by the RF patent №2200768, IPC With 22 38/46, having the following chemical composition, wt.%:

Carbon0,12-0,18
Manganese1,0-1,8
Silicon0,4-0,7
Chrome0,4-0,8
Aluminum0,01-0,05
Vanadium0,04-0,08
Nitrogen0,009-0,015
Copperof 0.1-0.4
Nickel0,1-0,34
Calcium0,001-0,05
Ironrest

This steel has higher strength characteristics, which allows its use in products for transportation environments under high pressure, but it has low corrosion resistance, resulting in unsuitable for the manufacture of products used in harsh saline environments.

The task, which is aimed by the invention, is the creation of low-alloy steel having sufficient strength characteristics and has high corrosion resistance that can be used for the manufacture of pipes and fittings designed for the transport of aggressive media, in particular, water-cut oil and water, characterized by a high degree of mineralization and contain N2S and CO2.

To achieve the objectives of the proposed low-alloy steel containing carbon, manganese, chromium, silicon, vanadium, aluminum, iron and impurities, which unlike the prototype has the following ratio of components, wt.%:

Carbon0,05-0,15
Manganese0,40-0,65
Chrome 0,50-0,80
Silicon0,30-0,80
Vanadium0,04-0,09
Aluminum0,02-0,05
Iron and impuritiesrest

This is further limited to the quantitative content of impurities in the following ratio, wt.%:

Nitrogenno more than 0,008
Nickelnot exceed 0.30
Coppernot more than 0,25
Hydrogenno more is 0.0002

and the total ratio of calcium/sulfur must be at least 1.

In the particular case of the proposed steel may further contain titanium and niobium, but the total content of vanadium, titanium and niobium should not exceed 0.15 wt.%.

The technical result provided by the claimed invention is as follows. Comparison of the chemical compositions of the prototype - steel RF patent No. 2200768 and proposed steel shows that the content of carbon, silicon, chromium, vanadium and aluminum fully or partially overlap. System alloying prototype - iron, manganese, silicon provides improved strength characteristics: tensile strength 895-960 MPa. In most cases, such a high strength is not needed for n is fahazavana pipes, used in the petroleum industry. In this industry often uses the pipes with regulatory tensile strength up to 588 MPa. Unlike the prototype of the proposed steel significantly reduced the content of manganese (system alloying iron-chrome), and also limited the content of impurities, particularly nitrogen, in addition, a limit on the amount of hydrogen and the ratio of calcium and sulfur. The specified ratio of the components allowed to obtain ferritic-pearlitic steel structure with a grain size not more than 10 μm (in the prototype, the grain size 15 μm), ferrite-pearlite banding does not exceed 1 point (prototype - 3 points), bainitic structures are not allowed (in the prototype Pets up to 40% bainite). In this case, as shown by experimental studies, the content of manganese is less than 0.40 wt.% cannot provide the required mechanical characteristics, and the increase in manganese content over to 0.65 wt.% leads to increased levels of ferrito-pearlitic poloschatosti, increases the probability of formation of bainitic structures, increases liquation heterogeneity that results in a decrease in the resistance of the claimed steel to hydrogen and sulfide stress corrosion cracking. Thus, the proposed interval manganese content provides the optimum combination of strength is pokazateli and high corrosion resistance, that allows you to use steel for the manufacture of products intended for the transport of aggressive media. When out-of-furnace treatment offered steel due to the specified ratio of calcium and sulfur is provided by the formation of uniformly distributed homogeneous oxysulfides rounded. In the prototype, the calcium content is 0.001 to 0.05 wt.% regardless of the sulfur content, which does not allow to get in out-of-furnace processing of non-metallic inclusions of the desired chemical composition. All the above leads to the resistance offered steel to General, pitting and drain corrosion and crack resistance, initiated by hydrogen and sulfide corrosion cracking under tension. Limiting the hydrogen content also improves corrosion resistance and cold resistance, since it provides no metal microcracks formed during malinali metallurgical hydrogen and leading to cracking hydrogen cracking in the process of sulfide corrosion. In addition, the selection of quantitative ratios of known components led to the unexpected achievement of the technical result, which is expressed in the fact that the proposed structure is characterized by low values of carbon equivalent (Ceand couples who tra resistance of steel against cracking during welding (P cm)that provides good weldability and the absence of hot and cold cracks. Limiting the hydrogen content at this prevents embrittlement of the weld and the formation of cracks. Known steel prototype is characterized by high values of carbon equivalent and setting the resistance of steel against rastreskivanija when welding up to 0.71 and to 0.38, respectively. Thus, this steel has poor weldability, which would entail the necessity of application of welding prior and concomitant heating and heat treatment of welded joints, which significantly increases the cost of welding. The proposed steel has, moreover, increased gladstonos and can be used for products operating in Arctic conditions at temperatures up to -60°C.

The invention is illustrated in the comparative examples and experimental results: table 1 presents the variations of the chemical composition of low alloy steels; table 2 presents the results of the strength tests specified variants; table 3 - results of the corrosion tests; table 4 - indicators weldability. Analysis of experimental data shows that the strength characteristics of the steel are reduced below an acceptable level, when the content of manganese, carbon, PU glue, the I is less than the minimum values (option 1). In the cases of output for the proposed maximum values included in the composition of the steel components along with increasing strength, the decrease of the corrosion resistance and weldability of steels (options 7, 8, 9). The mechanical characteristics of the claimed composition of steel (range 2-6) correspond to the strength class C that ensures reliable operation of pipelines for the transportation of water-cut oil and formation water.

If the composition of low-alloy steel, niobium and titanium in a greater degree increase its corrosion resistance and gladstonos due to the fixation of nitrogen in the carbonitrides. The limit of the total content of vanadium, titanium and niobium to 0.15 wt.% provides good weldability.

Thus, the obtained experimental data confirm the possibility of using the proposed steel flowlines transporting medium with high corrosion activity and operating in conditions of acid treatment reservoirs for enhanced oil recovery.

td align="center"> V
Table 1

The chemical composition of low alloy steels
number ofThe content of chemical elements, wt.%
MnCrSiAlNNiCuNTiNbFeCa/S
10,040,350,480,250,030,0160,0080,020,02is 0.00020,000,00OST0,9
20,050,400,600,400,090,0200,0080,200,25is 0.00020,000,00OST1
30,110,500,700,550,060,0350,0080,300,12is 0.00020,000,00OST1,2
40,120,500,500,800,090,0450,0080,250,15is 0.00020,000,00OST1,5
50,120,500,500,650,08being 0.0360,0080,25 0,14is 0.00020,010,05OST1,5
60,150,650,800,300,080,0350,0080,100,09is 0.00020,000,00OST1,6
70,120,500,600,940,090,0440,0080,250,280,00040,000,00OST1,5
80,160,701,000,400,120,0380,0100,350,17is 0.00020,000,00OST1,9
9 (prototype)0,181,800,800,700,080,0470,0150,340,20-0,000,00OST-

td align="center"> 19
Table 2

Mechanical properties of low alloy steels
number ofσin, The PA σt, MPaδ, %σintKCU-40J/cm2KCV-40J/cm2KCU-60J/cm2
1480390320,81-250230
2506430300.85-240210
3550470280,85-220180
4600517230,86-157140
5605512260,85-200180
6610540220,89-110130
7578530200,92-9381
8630570210,90-95105
9 (prototype)9308350,90127--

Table 3

Corrosion characteristics of low alloy steels
number ofThe corrosion rate in the environment NACE Km, mm/yearThe rate of corrosion in the environment of H2S+HCl, Km, mm/yearHydrogen crackingThreshold voltage SCRN σth,% σtThe threshold stress intensity factor KisscMPa·m1/2
CLR %CTR %
10,535,1417543
20,303,0007545
30,323,00about8046
40,50the 3.80about7037
50,352,90about8043
60,608,30 about7032
70,514,310,56523
80,6419,2316524
9 (prototype)1,7048,81755517

Table 4

Performance welding of low-alloy steel
Number ofSePCM
10,200,10
20,280,15
30,370,22
40,350,23
50,360,22
60,450,25
70,380,25
80,540,30
9 (prototype)0,690,37

1. Low alloy steel containing carbon, manganese, chromium, silicon, vanadium, aluminum, iron and impurities, characterized by the following ratio, wt.%:

Carbon0,05-0,15
Manganese0,40-0,65
Chrome0,50-0,80
Silicon0,30-0,80
Vanadium0,04-0,09
Aluminum0,02-0,05
Iron and impuritiesRest

while the quantitative content of impurities is further limited by the following ratio, wt.%:

NitrogenNo more than 0,008
NickelNot exceed 0.30
CopperNot more than 0,25
HydrogenNo more is 0.0002

and the total ratio of calcium/sulfur must be at least 1.

2. Low-alloy steel according to claim 1, characterized in that it additionally contains titanium and niobium, the total content of vanadium, titanium and niobium does not exceed 0.15 wt.%.



 

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