Method for the production of sheets of low-alloy steel

 

(57) Abstract:

The invention relates to metallurgy, and more particularly to rolling production, and can be used in the manufacture of the sheet reversing mills low alloy steel for the construction of offshore platforms. The method includes heating the slabs, multi-pass roughing and finishing rolling with compression rolls in thickness in a regulated temperature range. The technical result - improving complex of mechanical properties and yield of sheet metal. For this purpose, the slab is heated to a temperature 1150-1190°C, and multi-pass finishing rolling is produced in the temperature interval from 880-900 to 730-740°With the total compression of not less than 70%. In addition, before the rough rolling the slab additionally rolled in a temperature range from 1200-1280 to 1000-1100°With compression in the thickness of 30-45%. Rolling is subjected to a slab of low-alloy steel of the following chemical composition, wt.%: 0,06-0,09 With, and 0.40-0,55 Si; 1,45-1,65 Mn; 0,005-0,015 V; 0,035-0,050 Nb; 0,005-0,02 Ti; 0,020-0,055 Al; not more than 0.15 Cr, not more than 0.15 Ni, not more than 0.15 Cu; not more than 0.01 Mo; no more than 0,006 CA; no more than 0,0008 In; no more than 0,010 N; no more than 0,006 S; less than 0.015 R; the rest of the iron. 2 C.p. f-crystals. 3 table.

The invention amnesia reversing mills low alloy steel sheet for construction of offshore platforms.

In the construction of steel structures, platforms for oil and gas offshore use sheets of welded low-alloy steels, which must meet the following complex of mechanical properties (table.1).

In addition to these mechanical properties, the sheets must meet the requirements for weldability: when test gap destruction of the sample should not occur along the seam and base metal.

A method of producing steel sheets, including smelting and continuous casting into slabs of low-alloy steel, containing by mass %:

Carbon 0,04-0,10

Silicon 0,01-0,50

Manganese 0,4-1,5

Chrome 0,05-1,0

Molybdenum 0,05-1,0

Vanadium 0,01-0,1

Bor 0,0005-0,005

Aluminum 0,001-0,1

Iron and impurities Else

Cast slabs are heated to a temperature of 1250°C and rolled with a total compression of not less than 75%. Laminated sheets subjected to quenching from the austenitic region and high temperature tempering [1].

The disadvantages of this method are that the steel sheet has a low plastic and viscous properties at negative temperain structures of offshore platforms. In addition, the need for thermal improvement (quenching and tempering) of the sheet after rolling complicates and increases the cost of production.

There is also known a method of production of steel plate, low alloy steel, including the casting of slabs the following chemical composition, wt.%:

Carbon 0,02-0,3

Manganese 0,5-2,5

Aluminum 0,005-0,1

Silicon 0,05-1,0

Niobium 0.003 To 0.01 To

Iron Rest

The slabs are heated to a temperature of 950-1050°C and rolled at a temperature above the point ANDr3with a total compression of 50-70%. Laminated sheets are cooled in air [2].

With this method of production sheets have insufficient strength and ductility ratio t/inin excess of 0.94. These leaves do not meet the requirements for weldability and are not suitable for the manufacture of structures of offshore platforms.

The closest in technical essence and the achieved results of the present invention is a method [3] production of sheets of low-alloy steel grade 17GS (according to GOST 19281-89) the following chemical composition, wt.%:

Carbon 0,14-0,20

Manganese 1,0-1,4

Silicon 0.4 to exceed 0.035

Sulfur is Not more 0,040

Arsenic Not more than 0.08

Nitrogen Not more than 0,008

Iron Rest

Slabs are heated in furnaces to a temperature 1220-1280°C, is subjected to rough rolling in the temperature range 1050-1180°C to an intermediate thickness of 30-40 mm and finish rolling at a regulated temperature range 900-1050°C. To improve the mechanical properties of hot-rolled sheet is subjected to heat treatment (normalizing, thermal improvement).

The disadvantage of this method is that even after the additional heat treatment it does not obtain high complex mechanical properties of the sheets. This reduces the yield of sheet metal for construction of offshore platforms.

The technical problem solved by the invention is to improve the range of mechanical properties and yield of sheet metal.

The problem is solved in that in the known method of producing sheets of low-alloy steel comprising heating slabs, multi-pass roughing and finishing rolling with compression rolls in thickness in a regulated temperature range, according to imperatorem interval from 880-900 to 730-740°With the total compression of not less than 70%. Before the rough rolling the slab additionally rolled in a temperature range from 1200-1280 to 1000-1100°With compression in the thickness of 30-45%, and low-alloy steel has the following chemical composition, wt.%:

Carbon 0,06-0,09

Silicon 0,40-0,55

Manganese 1,45-1,65

Vanadium 0,005-0,015

Niobium 0,035-0,050

Titanium 0,005-0,02

Aluminum 0,020-0,055

Chromium Not more than 0.15

Nickel is Not more than 0.15

Copper Not over 0.15

Molybdenum is Not more than 0.01

Calcium Not more than 0,006

Boron is Not more 0,0008

Nitrogen Not more than 0,010

Sulfur is Not more than 0,006

Phosphorus less than 0.015

Iron Rest

The invention consists in the following. Heating of slabs of low-alloy steel to a temperature 1150-1190°To provide austenitization, complete dissolution in austenite matrix sulphides, phosphides, nitrides, alloying and impurity compounds, density of strengthening particles. This increases process flexibility and deformability of the slab during rolling to an intermediate thickness. Furthermore, since the rolling process is a continuous decrease of the temperature of the metal at the specified level, necessary for carrying out low-temperature finish rolling.

Subsequent finish rolling of leaves in the temperature interval from 880-900 to 730-740°With the total relative compression of not less than 70% provides the necessary degree of crushing of the microstructure, complete loss of solid solution density of strengthening particles, strain hardening of the metal matrix. In the microstructure of the sheets after cooling consists of a ferritic-pearlitic mixture with uniform grains 11th grade and mechanical properties of hot rolled sheets in the position to fully meet the requirements sheets for welded structures of offshore platforms (PL.1) without additional heat treatment. In addition, the leaves of low-alloy steel with such a microstructure characterized by high weldability: when test gap destruction of samples is not on the weld and base metal.

The use of low-alloyed steel of the proposed structure while carrying out the stated ratios in it alloying elements and impurities provides after hot rolling on the said stable modes poluester interval from 1200-1280 to 1000-1100°With compression thickness 35-40% improves the performance of subsequent roughing and finishing pochatok for controlled modes, to increase the thickness of slabs without deterioration of the mechanical properties of the finished sheet.

It was established experimentally that the heating of slabs of low-alloy steel above 1190°C does not improve the complex mechanical properties of the sheets, and only increases the heating time and requires additional podstugivaniya piece before the finishing rolling, which reduces the efficiency of the process. Reduction of the temperature below 1150°With leads to incomplete dissolution in austenite density of strengthening particles, reduction of technological plasticity, as well as plastic and viscous properties of the leaves.

When the temperature of the beginning of the finish rolling above 900°C is not achieved the necessary degree of grinding grain microstructure, which leads to deterioration of mechanical properties of the sheets. Reduction of the temperature below 880°C leads to the formation of unstable microstructure and properties of the finished sheet.

When the temperature of end finishing above 740°C, and when the total compression than 70% is not achieved the required degree of thermomechanical hardening sheets. Lowering the temperature of the end of the finish rolling below 730°C leads to the formation of non-uniform microstructure became the rink slab at a temperature started rolling above 1280°With increasing time of heating for rolling and leads to oxidation of the grain boundaries of the microstructure at the surface. Reduction of the temperature below 1200°With leads to deterioration of ductility nicolegraves steel and the growth of energy consumption for rolling.

Lowering the temperature of the completion of the preliminary rolling below 1000°or an increase in compression over 45% worsen the geometry of the slab and the quality of the finished sheets. The increase in this temperature in excess of 1100°With increases okalinoobrazovanie, and decrease the compression of less than 30% requires increasing the number of passes in the subsequent rolling of the leaves, which reduces the productivity of the rolling mill.

Carbon low alloy steel of the proposed composition determines the strength and weldability of the leaves. The lower carbon content of less than 0.06% leads to a drop in their strength below acceptable levels. Increasing the carbon content of more than 0,09% affects the rheological properties of the leaves and their weldability.

When the silicon content less than 0.40% worse recyclenet steel, reduced strength properties of the sheets. The increase in silicon concentration greater than 0.55% leads to an increase in the number of silicate inclusions reduces the impact strength of the sheets, affects the rate KV-40and the weldability of steel.

The decrease in the content of manganese is less 1,45% increases arise yield stress to the temporary tear resistance t/inin excess of 0.93, which is unacceptable.

Vanadium grinds grain microstructure, increases the strength and toughness of the sheets, laminated on the proposed modes. When the vanadium content of less than 0,005% sheets have insufficient viscosity at low temperatures. The increase in the content of vanadium in excess of 0.015% was found to be inappropriate, as it does not improve the properties of the sheets.

Niobium in steel at temperatures of finish rolling, when the total relative compression of not less than 70% contributes to obtaining a cellular dislocation microstructure of steel provides a combination of strength and plastic properties of the sheets without additional heat treatment. When the concentration of niobium, less than 0.035% of mechanical properties of hot rolled sheets in the state is not high enough. The increased concentration of more than 0.05% does not lead to further enhancement of the mechanical properties of the sheets, so it is impractical.

Titanium is a strong carbidopa element, reinforcing steel. When the titanium content of less than 0,005% of its hardening is insufficient, the leaves have low strength and toughness. Increasing the concentration of titanium in excess of 0.02% does not provide further improve the properties of l nitrides, reducing its harmful effects on the rheological properties. When the aluminum content less than 0.02% of its influence is small, viscous properties become worse. The increase in the content of this element is more to 0.055% causes graphitization of carbon and reduce the strength characteristics of steel sheet products.

Chromium, Nickel and copper are impurity elements and increase the strength and corrosion resistance of leaves. When the concentration of each of them to 0.15% they do not render harmful influence on the weldability of leaves, but can extend scrap metal in the smelting, which reduces the production of leaves.

Molybdenum provides obtaining of hot-rolled sheets with the desired properties when the content is less than 0.01 percent. Otherwise, the strength properties above an acceptable level.

Calcium has a modifying effect, which improves the impact strength of steel at -40°C. However, the increase in the calcium content of more than 0,006% leads to increase of non-metallic inclusions, reduce ductility and toughness of low alloy steel.

Boron contributes to the grinding of the grains of the microstructure when the roughing hot rolling of slabs. However, increasing with the properties leaves, what is unacceptable.

The proposed steel composition may contain as impurities not more than 0,010% nitrogen, not more than 0,006% sulfur, not more than 0.015% phosphorus. When these limiting concentrations of these elements in the steel of the proposed structure does not adversely impact on the quality of the sheets, while their removal from the melt of steel significantly increases production costs and complicates the process.

An example implementation of the method

In BOF production of produce production and casting low alloy steels of different composition (table.2) in a slab thickness of 250 mm Cast slabs are heated in heater wells to a temperature TNK=1240°C and rolled on the blooming mill at a thickness of 150 mm with a total relative compression Eb=40%. The temperature of the finish rolling on the blooming is TKB=1050°C.

The slab 150 mm thick loads in furnaces and heated to a temperature of austenitization Tand=1170°C. the Heated slabs after compression in the vertical mill rolled in a reversing roughing stand Duo 2800 to an intermediate thickness 36 mm as rolling in the roughing stand the temperature of the rolls is reduced to values of TTM=890° what I see in the leaves of finite thickness 9.0 mm with a relative total compression E=75%. Rolling sheets completed at the temperature TCP=735°C.

Laminated sheets are cooled in air, carry out tests of mechanical properties and weldability. Leaves, tested, used for the manufacture of welded structures sea platforms.

From table.3 shows that when implementing the proposed method (options No. 2-4) is achieved by increasing the range of mechanical properties and weldability. This has the effect of increasing the yield of leaves. In the case of over-range values of the declared options (options # 1 and # 5) the complex of mechanical properties and weldability of the leaves are falling. The sheets are not suitable for the manufacture of welded structures of offshore platforms. Also lower complex of mechanical properties and weldability are achieved even after thermal improvement of the sheets obtained according to the method prototype (option # 6).

Technical appraisal and economic benefits of the proposed method lies in the fact that its use produces optimal microstructure, high complex of mechanical properties and weldability sheets for welded structures of offshore platforms.

These advantages are achieved by the project in the calculation of technical and economic advantages of the proposed method is adopted prototype method. The use of the proposed method will improve profitability sheets category 4A 25-30%.

Literature

1. Japan's bid No. 61-163210, IPC 21 D 8/00, 1986

2. Japan's bid No. 61-223125, IPC 21 D 8/02, With 22 38/54, 1986

3. Sailors Y. I. and other Steel for main gas pipelines. M.: metallurgy, 1989, S. 242-244, 268 prototype.

1. Method for the production of sheets of low-alloy steel comprising heating slabs, multi-pass roughing and finishing rolling with compression rolls in thickness in a regulated temperature range, characterized in that the slab is heated to a temperature 1150-1190°C, and multi-pass finishing rolling is produced in the temperature interval from 880-900 to 730-740°With the total compression of not less than 70%.

2. The method according to p. 1, characterized in that before the rough rolling the slab additionally rolled in a temperature range from 1200-1280° up to 1000-1100°With compression in the thickness of 30-45%.

3. The method according to p. 1 or 2, characterized in that the low-alloy steel has the following chemical composition, wt.%:

Carbon 0,06-0,09

Silicon 0,40-0,55

Manganese 1,45-1,65

Vanadium 0,005-0,015

Neither the e more than 0.15

Copper Not over 0.15

Molybdenum is Not more than 0.01

Calcium Not more than 0,006

Boron is Not more 0,0008

Nitrogen Not more than 0,010

Sulfur is Not more than 0,006

Phosphorus less than 0.015

Iron Rest



 

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