RU2292404C1 - Strip making method for producing tubes - Google Patents

Abstract

FIELD: metallurgy, namely manufacture of strips of low alloy steel in continuous wide strip rolling mills for producing electrically welded tubes designed for building oil- and gas pipelines operating in Far North regions.

SUBSTANCE: method is realized by using steel of next chemical composition, mass %: carbon, 0.05 - 0.09; silicon, 0.15 - 0.40; manganese, 1.0 - 1.4; aluminum, 0.01 - 0.06; titanium, 0.01 - 0.04; vanadium, 0.01 - 0.04; niobium, 0.02 - 0.06; molybdenum, 0.01 or less; chrome, 0.10 or less; nickel, 0.10; copper, 0.10; phosphorus, 0.015 or less; sulfur, 0.006 or less; calcium, 0.005 or less; nitrogen, 0.010 or less; iron, the balance. Content of elements in steel satisfies relation: C +Mn/6 +(Cr + Mo + V + Ti + Nb)/5 + (Ni + Cu)/15 = 0.40 or less. Slabs are heated up to temperature 1200 -1280°C; temperature of rolling process termination is sustained in range 830 - 880°C and coiling process temperature is sustained in range 540 - 580°C.

EFFECT: enhanced viscous properties of steel strips at negative temperatures, high welding capability and ductility of steel strips.

3 ex, 3 tbl

Description

The invention relates to the field of metallurgy, and more specifically to rolling production, and can be used in the manufacture on continuous broadband mills of low-alloy steel strips for electric welded pipes intended for the construction of oil and gas pipelines operating in the Far North.

For the production of oil and gas pipes operating in the Far North, hot-rolled strips (strips) of 4-8 mm thick with a width of 1050 mm made of low alloy steel are required, which have the following set of mechanical properties (Table 1):

Table 1 The mechanical properties of strips (TS 105-430-2004) σ _in , N / mm ² σ _t N / mm ² δ ₅ ,% KCV ^-60 , J / cm ² B ^-60 ; % 180 ° cold bend 540-660 no less than 380 not less than 23.0 not less than 40 not less than 50 having stood. Notes: 1. All tests are carried out on samples whose axis is transverse to the strip rolling direction; 2. B ^-60 - the proportion of the viscous component in the fracture of the sample at -60 ° C.

In addition to the indicated mechanical properties, pipe strips must have high weldability.

A known method for the production of steel sheets, including smelting and continuous casting into slabs of low alloy steel, containing, wt.%:

Carbon 0.04-0.10 Silicon 0.01-0.50 Manganese 0.4-1.5 Chromium 0.05-1.0 Molybdenum 0.05-1.0 Vanadium 0.01-0.1 Boron 0.0005-0.005 Aluminum 0.001-0.1 Iron and impurities Rest.

The cast slabs are heated to a temperature of 1250 ° C and rolled with a total compression of at least 75%. Laminated sheets are subjected to quenching from the austenitic region and high-temperature tempering [1].

The disadvantages of this method are that the sheets of this steel have low viscous properties at low temperatures, poor weldability. This makes it impossible to use it for the manufacture of oil and gas pipes of northern design. In addition, the need for thermal improvement (hardening and tempering) of the sheets after rolling complicates and increases the cost of production.

There is also known a method of production of plate low alloy steel, including casting slabs of 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 0.003-0.01 Iron Rest.

The slabs are heated to a temperature of 950-1050 ° C and rolled at a temperature above point A _r3 with a total compression of 50-70%. Laminated sheets are cooled in air [2].

With this method of production, the sheets have insufficient strength and ductility. Sheets also do not meet the requirements for viscosity at low temperatures, have insufficient weldability and are unsuitable for the manufacture of pipes for oil and gas pipelines of the northern version.

The closest in its technical essence and the achieved results to the proposed invention is a method for the production of strips of low alloy steel grade 17G1S (according to GOST 19281) of the following chemical composition, wt.%:

Carbon 0.15-0.20 Manganese 1.15-1.6 Silicon 0.4-0.6 Chromium no more than 0.30 Nickel no more than 0.30 Copper no more than 0.30 Phosphorus no more than 0,035 Sulfur no more than 0,040 Arsenic no more than 0.08 Nitrogen no more than 0,008 Iron Rest.

Slabs of low-alloy steel 17G1S are heated to a temperature of 1250 ° C, subjected to rough rolling on a continuous broadband mill to an intermediate thickness of 20-40 mm, finish rolling with a regulated rolling end temperature T _kp = 830-880 ° C and cooled with water to a winding temperature T _cm = 620-700 ° C [3].

The disadvantages of this method are that the strips have low viscosity properties at low temperatures. In addition, the strips are characterized by insufficient weldability: when testing samples for breaking, their destruction occurs along the weld, and have insufficient ductility.

The technical problem solved by the invention is to increase the viscosity properties of steel strips at low temperatures, giving them high weldability and ductility.

To solve the technical problem in the known method of producing strips for the manufacture of pipes, including heating steel slabs, rough rolling to an intermediate thickness and finish rolling with a regulated temperature of the end of rolling, cooling with water to a winding temperature, according to the proposal, the steel contains the following ratio of components, wt.% :

Carbon 0.05-0.09 Silicon 0.15-0.40 Manganese 1.0-1.4 Aluminum 0.01-0.06 Titanium 0.01-0.04 Vanadium 0.01-0.04 Niobium 0.02-0.06 Molybdenum no more than 0,01 Chromium no more than 0.10 Nickel no more than 0.10 Copper no more than 0.10 Phosphorus no more than 0.015 Sulfur no more than 0,006 Calcium no more than 0,005 Nitrogen no more than 0,010 Iron rest

the content of elements in steel should satisfy the ratio: C + Mn / 6 + (Cr + Mo + V + Ti + Nb) / 5 + (Ni + Cu) / 15≤0.40, the slabs are heated to a temperature of 1200-1280 ° C, the temperature of the end of rolling is maintained in the range of 830-880 ° C, and the temperature of the winding is in the range of 540-580 ° C.

The invention consists in the following. The mechanical properties and weldability of hot rolled strips are determined by both the chemical composition of the steel and the temperature conditions of their hot rolling. Therefore, to give the hot-rolled strips the highest impact strength at negative temperatures, weldability and ductility, it is necessary to optimize the chemical composition of steel, the temperature range of heating and deformation of slabs into strips, as well as the temperature at which the cooling of the strips with water is completed.

Carbon in steel of the proposed composition determines the strength properties of the strips. A decrease in carbon content of less than 0.05% leads to a drop in their strength below the permissible level. An increase in carbon content of more than 0.09% affects the viscosity properties of sheets at low temperatures and their weldability.

When the silicon content is less than 0.15%, the deoxidation of steel deteriorates, and the strength properties of the strips decrease. An increase in the silicon content of more than 0.40% leads to an increase in the number of silicate inclusions, reduces the ductility of the strips, worsens the KCV ^-60 and steel weldability.

A decrease in manganese content of less than 1.0% increases the oxidation of steel, impairs the strength and weldability of the sheets. Increasing the manganese content of more than 1.40% increases the strength above the acceptable level, reduces the plastic properties of hot-rolled sheets.

Aluminum deoxidizes and modifies steel. By binding nitrogen to nitrides, it inhibits its negative effect on the properties of the sheets. When the aluminum content is less than 0.01%, the complex of mechanical properties of the sheets decreases. An increase in its concentration of more than 0.06% leads to a deterioration in the viscosity properties of strips.

Titanium is a strong carbide forming element that strengthens steel. The finely dispersed titanium carbides precipitated during hot rolling and cooling of the strips with water are highly resistant to overheating. During welding, their dissolution and softening of the weld zone do not occur. When the titanium content is less than 0.01%, the strength of the hot rolled sheets decreases. An increase in titanium content in excess of 0.04% leads to a decrease in viscosity properties at a temperature of -60 ° C, which is unacceptable.

Vanadium and niobium, both individually and together, grind the grain of the microstructure, increase the strength and viscosity of the hot-rolled strips rolled according to the proposed modes. With a vanadium content of less than 0.01% or niobium of less than 0.02%, the bands have insufficient viscosity at low temperatures. An increase in the vanadium content of more than 0.04% or niobium of more than 0.06% proved to be impractical, since it worsened welding without further improving the properties of the hot-rolled strips.

Molybdenum in this steel is an impurity element; it enters steel from scrap metal during smelting. However, with an increase in the molybdenum concentration of more than 0.01%, the weldability of the hot rolled strips deteriorates.

Chrome, nickel and copper are also impurity elements. At a concentration of each of them not more than 0.10%, they do not adversely affect the weldability of strips in the production of casing pipes, but expand the possibilities of using scrap metal for smelting, which reduces the cost of production. When the concentration of each of these elements is more than 0.10%, the viscosity properties deteriorate at a temperature of -60 ° C, and the plastic properties and weldability of hot-rolled strips decrease.

The steel of the proposed composition may contain in the form of impurities not more than 0.015% phosphorus, not more than 0.006% sulfur and not more than 0.010% nitrogen. At the indicated maximum concentrations, these elements in the hot-rolled strips of steel of the proposed composition do not have a noticeable negative effect on the mechanical properties of the strips, while their removal from the molten steel significantly increases production costs and complicates the process. An increase in the concentration of these harmful impurities more than the proposed values worsens the whole complex of mechanical properties of the strips and in particular impact strength at a temperature of -60 ° C.

Calcium contributes to steel modification and grinding of microstructure grains during hot rolling of slabs in the temperature range from 1200-1280 ° C to 830-880 ° C. Calcium enters steel when it is smelted from limestone and slag. However, an increase in calcium content of more than 0.005% leads to an increase in the number of nonmetallic inclusions and a deterioration in the viscous and plastic properties of hot-rolled sheets, which is unacceptable.

The weldability of steel is determined by its chemical composition, and various components have a different effect. In all cases, an increase in the degree of alloying causes a deterioration in weldability. Therefore, in the steel of the proposed composition, in order to impart high weldability to the hot-rolled strips, as shown by experiments, it is necessary to fulfill the following relationship:

C _e = C + Mn / 6 + (Cr + Mo + V + Ti + Nb) / 5 + (Ni + Cu) / 15≤0.40.

In cases where the specified ratio is not satisfied, i.e.

C _e = C + Mn / 6 + (Cr + Mo + V + Ti + Nb) / 5 + (Ni + Cu) / 15> 0.40,

weldability of hot rolled sheets is deteriorating.

Heating slabs of steel of the proposed composition to a temperature of 1200-1280 ° C provides its austenitization, complete dissolution in the austenitic matrix of sulfides, phosphides, nitrides, alloying and impurity compounds, carbonitride reinforcing particles. Due to this, the technological plasticity and deformability of the slabs during rolling are increased. In addition, since during the rolling process there is a continuous drop in the temperature of the metal, at the indicated heating temperature, at the time the rough rolling is finished, the temperature of the roll decreases to the optimum level necessary to ensure a given temperature at the end of rolling.

Subsequent finishing rolling of the strip with a temperature of rolling end of 830-880 ° C provides the necessary degree of refinement of the microstructure, complete precipitation of carbonitride reinforcing particles from the solid solution, and strain hardening of the metal matrix. As a result, the microstructure of the strip after cooling to a winding temperature of 540-580 ° C is a ferrite-pearlite mixture with uniform grains of the 11th point, and the mechanical properties of the strip fully comply with the requirements (Table 1). Pipes from such strips well resist the formation of main cracks at negative temperatures in the Far North.

In addition, due to the limitation of the concentration in the steel of a specific composition of carbon and other alloying elements:

C _e = C + Mn / 6 + (Cr + Mo + V + Ti + Nb) / 5 + (Ni + Cu) / 15≤0.40

Hot-rolled strips, having a given strength and high viscosity at low temperatures, are characterized by high weldability: during tensile testing, fracture of the samples occurs not along the weld or heat affected zone from the action of heat of electric welding, but along the base metal.

The use of steel of the proposed composition while simultaneously fulfilling the stated ratios of alloying elements and impurities in it ensures, after hot rolling according to the above-mentioned modes, stable obtaining of the specified mechanical properties of the strips, increasing the viscosity properties at low temperatures, and high weldability of the pipes. Improving the mechanical properties of strips due to the simultaneous optimization of the chemical composition of steel and hot rolling modes can minimize the cost of alloying materials. This is an additional advantage of the proposed method.

It was experimentally established that increasing the heating temperature of slabs of low alloy steel above 1280 ° C does not improve the complex of mechanical properties of strips, but only increases the heating time and requires a decrease in the rolling rate, which reduces the productivity of the process. Lowering this temperature below 1200 ° C leads to incomplete dissolution of austenite carbonitride hardening particles, reducing technological ductility, steel hardening (σ _in > 660 N / mm ² ), lowering the viscosity properties of strips at low temperatures.

When the temperature of the end of rolling T _kp above 880 ° C is not achieved the required degree of hardening of the strip and grinding its microstructure to the optimum level. The decrease in temperature T _CP below 830 ° C leads to excessive grinding of the microstructure, the deterioration of the mechanical properties of the strip.

Increasing the temperature of the winding T _cm above 580 ° C contributes to the formation of variability of the microstructure, lowering the strength properties below acceptable values. The decrease in T _cm less than 540 ° C leads to a deterioration in plastic properties: the strip does not withstand tests for cold bending.

Method implementation examples

In converter production, steel is smelted and casted of various compositions (Table 2).

Slabs with a thickness of 250 mm are loaded into methodological furnaces and heated to austenitization temperature T _a = 1240 ° C. The heated slabs are discharged onto the rolling table of a continuous wide-band mill 2000 and subjected to rolling in a roughing group of stands (rough rolling) to an intermediate thickness of 40 mm. Then, the roll at a temperature of 990 ° C is set into a continuous 7-stand finishing group of stands, where it is crimped to a final thickness of 6 mm. The regulated temperature of the end of rolling T _KP = 855 ° C is supported by a change in the rolling speed and intercell cooling of the strip.

The rolled strip is issued to the discharge roller table, where it is cooled with water to a winding temperature T _cm = 560 ° C. The cooled strip is wound onto a roll.

The options for rolling strips in various modes from steels of various compositions are given in Table 3.

From table 3 it follows that when implementing the proposed method (options No. 2-4) is achieved by increasing the viscosity properties of steel strips at low temperatures, the strips have high weldability and ductility. In the case of transcendental values of the declared parameters (options No. 1 and No. 5), the viscosity properties at low temperatures, ductility and weldability of strips are deteriorated. Also lower viscosity properties at a temperature of -60 ° C, ductility and weldability have strips produced according to the prototype method (option No. 6).

The technical and economic advantages of the proposed method are that heating the slabs of steel of the proposed composition to a temperature of 1200-1280 ° C, their subsequent hot rolling into strips of a given thickness with a temperature of the end of rolling of 830-880 ° C and cooling with water to a coil temperature of 540- 580 ° C provides the formation of the optimal fine-grained ferrite-pearlite microstructure of steel. Due to this, an increase in toughness at -60 ° C and the share of the viscous component in the fracture, weldability and ductility of hot-rolled strips is achieved.

Using the proposed method will increase the profitability of the production of strips for oil and gas pipes of the Northern version by 10-15%.

Information sources

1. Japanese application No. 61-163210, IPC C 21 D 8/00, 1986;

2. Japanese application No. 61-223125, IPC C 21 D 8/02, C 22 C 38/54, 1986;

3. Sailors Yu.I. and others. Steel for gas mains. M .: Metallurgy, 1989, p.262-266 - prototype.

table 2 The chemical composition of steels Composition number The content of chemical elements, wt.% C _e ,% FROM Si Mn Al Ti V Nb Mo Cr Ni Cu R S Sa N Fe one 0.04 0.14 0.9 0.009 0.009 0.009 0.01 0.001 0.08 0.05 0.06 0.010 0.003 0.002 0.005 rest 0.22 2 0.05 0.15 1,0 0.010 0.010 0.010 0.02 0.003 0,07 0.04 0.04 0.011 0.004 0.003 0.006 -: - 0.24 3 0,07 0.27 1,2 0,035 0,025 0,025 0.04 0.005 0.06 0,07 0,07 0.013 0.004 0.004 0.008 -: - 0.31 four 0.09 0.40 1.4 0,060 0,040 0,040 0.06 0.010 0.10 0.10 0.10 0.015 0.006 0.005 0.010 -: - 0.38 5 0.10 0.42 1,5 0,070 0,050 0,050 0,07 0.012 0.12 0.11 0.11 0.016 0.007 0.006 0.011 -: - 0.42 6 0.19 0.50 1,2 - - - - 0.28 0.25 0.27 0,032 0,036 - 0.007 -: - - Note: composition 6 steel additionally contains 0.07% As.

Table 3 Strip production modes and their effectiveness Option No. Composition number Temperature conditions, C Mechanical properties Weldability T _a , ° C; T _CP , ° C T _cm , ° C σ _in , N / mm ² σ _t N / mm ² δ ₅ ,% KCV ^-60 , J / cm ^{2 -60} % 180 ° cold bend one 5 1190 820 530 690 650 19 27 38 not withstanding. unsatisfied. 2 2 1200 830 540 660 480 31 44 54 having stood. sat. 3 3 1240 855 560 590 420 32 58 66 having stood. sat. four four 1280 880 580 540 390 31 52 58 having stood. sat. 5 one 1290 890 590 470 375 22 32 47 not withstanding. unsatisfied. 6 6 1250 870 690 500 445 eighteen 24 44 not withstanding. unsatisfied.

Claims (1)

A method of manufacturing strips for the manufacture of pipes, including heating steel slabs, rough rolling to an intermediate thickness and finishing rolling with a regulated temperature of the end of rolling, cooling with water to a winding temperature, characterized in that the slab is obtained from steel containing the following ratio of components, wt.%: Carbon 0.05-0.09 Silicon 0.15-0.40 Manganese 1.0-1.4 Aluminum 0.01-0.06 Titanium 0.01-0.04 Vanadium 0.01-0.04 Niobium 0.02-0.06 Molybdenum No more than 0,01 Chromium No more than 0.10 Nickel No more than 0.10 Copper No more than 0.10 Phosphorus No more than 0.015 Sulfur No more than 0,006 Calcium No more than 0,005 Nitrogen No more than 0,010 Iron Rest when fulfilling the relations: C + Mn: 6+ (Cr + Mo + V + Ti + Nb): 5+ (Ni + Cu): 15 <0.40, the slabs are heated to a temperature of 1200-1280 ° C, the temperature of the end of rolling is maintained in the range of 830 -880 ° C, and the winding temperature is in the range of 540-580 ° C.