RU2262537C1 - Method of production of strips from low-alloyed steel - Google Patents

Abstract

FIELD: metallurgy; rolling process; manufacture of strips used for production of electrically welded casing pipes for construction of oil and gas wells in continuous wide-strips mills.

SUBSTANCE: proposed method includes heating the slabs, roughing these slabs to intermediate thickness and finish rolling at specified temperature of end of rolling, cooling with water to reeling temperature; slabs are heated to temperature of 1220-1280°C; temperature of end of reeling and reeling temperature are maintained within 820-880°C and 580-660°C, respectively. Strips are rolled from steel containing the following constituents, mass-%: 0.12-0.17 C; 1.3-1.6 Mn; 0.3-0.6 Si;0.02-0.06 Al; 0.01-0.05 V and/or Nb; no more than 0.3 Cr; no more than 0.3 Ni; no more than 0.3 Cu; no more than 0.015 P; no more than 0.006 S; no more than 0.010 N; no more than 0.02 Ca; the remainder being Fe.

EFFECT: improved mechanical properties of strips; enhanced weldability.

3 tbl, 6 ex

Description

The invention relates to the field of metallurgy, and more specifically to rolling production, and can be used in the manufacture of continuous wideband mills strips for electric-welded casing, designed to equip oil and gas wells in northern seismic zones.

For the production of casing pipes operating in seismic zones at low temperatures, hot-rolled strips (strips) of 6-10 mm thick, 1200-1800 mm wide of low alloy steel, with the following set of mechanical properties are required (Table 1):

In addition to the indicated mechanical properties, casing 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 strips of this steel have low viscous properties at low temperatures, poor weldability. This makes it impossible to use it for the manufacture of northern casing pipes operating in seismically hazardous areas. In addition, the need for thermal improvement (hardening and tempering) of strips after rolling complicates and increases the cost of production.

There is also known a method of manufacturing a sheet of 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 production method, the sheets have insufficient strength and ductility with a ratio of σ _t / σ _in exceeding 0.92. Such sheets do not meet the requirements for viscosity at low temperatures, have insufficient weldability and are unsuitable for the manufacture of casing pipes of the northern version for operation in seismically hazardous areas.

The closest in technical essence and the achieved results to the proposed invention is a method for the production of strips from 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, finishing 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 mechanical properties at low temperatures, which reduces the yield of strips. In addition, strips are characterized by insufficient weldability: when testing samples for breaking, their destruction occurs along the weld.

The technical problem solved by the invention is to increase the mechanical properties of strips and ensure high weldability.

To solve the technical problem in the known method for the production of strips of low alloy steel, including heating slabs, their rough rolling to an intermediate thickness and finish rolling with a regulated temperature of the end of rolling, water cooling to a winding temperature, according to the proposal, the slabs are heated to a temperature of 1220-1280 ° C, and the temperature of the end of rolling and winding is maintained in the ranges of 820-880 ° C and 580-660 ° C, respectively.

In addition, for the production of strips using low-alloy steel of the following chemical composition, wt.%:

Carbon 0.12-0.17 Manganese 1.3-1.6 Silicon 0.3-0.6 Aluminum 0.02-0.06 Vanadium and / or Niobium 0.01-0.05 Chromium no more than 0.3 Nickel no more than 0.3 Copper no more than 0.3 Phosphorus no more than 0.015 Sulfur no more than 0,006 Nitrogen no more than 0,010 Calcium no more than 0,02 Iron Rest

The invention consists in the following. Heating slabs of low alloy steel of the proposed composition to a temperature of 1220-1280 ° C provides its austenization, 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 to an intermediate thickness 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 the rolling.

Subsequent finishing rolling of the strip with a temperature of rolling end of 820-880 ° C provides the necessary degree of grinding 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 580-660 ° C is a ferrite-pearlite mixture with uniform grains of the 11th point, and the mechanical properties of the strip in the hot-rolled state fully comply with the requirements (Table 1) without additional heat treatment. Casing pipes from such strips well resist seismic displacements of soil sections at negative temperatures without fracture. In addition, due to the limitation of the concentration of carbon and other alloys in steel, low-alloy steel, 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, but on the base metal.

The use of low-alloy 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 regimes, the stable obtaining of the specified mechanical properties of strips, increasing the viscosity properties at low temperatures, and high weldability of casing 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. A decrease in this temperature below 1220 ° C leads to incomplete dissolution of austenite carbonitride hardening particles, a decrease in technological plasticity, steel hardening (σ _in > 610 N / mm ² ), and a decrease in 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 820 ° C leads to excessive grinding of the microstructure, the deterioration of the mechanical properties of the strips. Improving the mechanical properties by increasing the alloyed ™ steel increases the cost of production and affects the weldability of steel.

Increasing the temperature of the winding T _cm above 660 ° C contributes to the formation of variability of the microstructure, reducing the strength properties below acceptable values. The decrease in T _cm less than 580 ° C leads to the failure of the specified ratio σ _t / σ _in , which is unacceptable.

Carbon in low alloy steel of the proposed composition determines the strength properties of strips. A decrease in carbon content of less than 0.12% leads to a drop in their strength below the permissible level. An increase in carbon content of more than 0.17% affects the viscosity properties of strips and their weldability.

A decrease in manganese content of less than 1.3% increases the oxidation of steel, degrades the strength and weldability of strips. An increase in the manganese content of more than 1.6% increases the ratio of yield strength to temporary tensile strength σ _t / σ _in excess of 0.8, which is unacceptable,

When the silicon content is less than 0.3%, the deoxidation of steel deteriorates, and the strength properties of strips decrease. An increase in the silicon content of more than 0.6% leads to an increase in the number of silicate inclusions, reduces the toughness of strips, and worsens the KCU ^-40 index and the weldability of steel.

Aluminum deoxidizes and modifies steel. By binding nitrogen to nitrides, it inhibits its negative effect on the properties of strips. When the aluminum content is less than 0.02%, the complex of mechanical properties of strips decreases. An increase in its concentration of more than 0.06% leads to a deterioration in the viscosity properties of strips.

Vanadium and niobium, both individually and together, grind the grain of the microstructure, increase the strength and viscosity of the strips rolled according to the proposed modes. Steel may contain either only vanadium or only niobium, or vanadium and niobium together. When the content of vanadium and / or niobium (V + Nb) is less than 0.01%, the strips have insufficient viscosity at low temperatures. An increase in the content of vanadium and / or niobium (V + Nb) in excess of 0.05% was impractical, since it did not improve the properties of strips.

Chrome, nickel and copper are impurity elements. When the concentration of each of them is not more than 0.3%, 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 0.3%, the viscous, plastic properties and strip weldability deteriorate.

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 steel of the proposed composition do not have a noticeable negative effect on the quality of the strips, while their removal from the steel melt significantly increases production costs and complicates the process. An increase in the concentration of these harmful impurities over the suggested values worsens the whole complex of mechanical properties of strips.

Calcium contributes to the modification of steel and grinding of microstructure grains during rough hot rolling of slabs in the temperature range from 1220-1280 ° C to 820-880 ° C. Calcium enters steel when it is smelted from limestone and slag. However, an increase in calcium content of more than 0.02% leads to an increase in the number of non-metallic inclusions and a deterioration in the viscosity and plastic properties of strips, which is unacceptable.

Method implementation examples

In converter production, low-alloy steels of various compositions are smelted and cast (Table 2).

Slabs with a thickness of 250 mm are loaded into methodological furnaces and heated to austenization temperature T _a = 1250 ° C. The heated slabs are discharged onto the furnace rolling table of a continuous broadband 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 970 ° C is set into a continuous 7-stand finishing group of stands, where it is crimped to a final thickness of 8 mm. The regulated temperature of the end of rolling T _KP = 850 ° C is supported by a change in the rolling speed and intercell cooling of the strip.

Laminated strip is issued to the discharge roller table, where it is cooled with water to a winding temperature T _cm = 620 ° 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), an increase in mechanical properties and weldability is achieved. In the case of transcendental values of the declared parameters (options No. 1 and No. 5), the mechanical properties and weldability of the strips deteriorate. Also, lower properties and weldability are strips produced according to the prototype method (option No. 6).

The technical and economic advantages of the proposed method are that heating slabs of low alloy steel of the proposed composition to a temperature of 1220-1280 ° C, their subsequent hot rolling into strips of a given thickness with a temperature of rolling end of 820-880 ° C and cooling with water to a winding temperature of 580 -200 ° C provides the formation of the optimal fine-grained ferritic-pearlite microstructure of steel. Due to this, an increase in mechanical properties, weldability of strips with minimal alloying is achieved. This helps to reduce production costs.

Using the proposed method will increase the profitability of steel production and strips for casing pipes of the northern version by 8-10%.

Sources of information:

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 pipelines. M., Metallurgy, 1989, p.262-266 - prototype.

table 2
The chemical composition of low alloy steels Composition number The content of chemical elements, wt.% FROM Mn Si Al V and / or Nb Cr Ni Cu R S N Sa Fe 1 0.11 1.20 0.20 0.01 0.009 0.10 0.11 0.12 0.011 0.003 0.006 0.005 Rest 2 0.12 1.30 0.30 0.02 0.010 0.12 0.14 0.11 0.012 0.004 0.007 0.009 -: - 3 0.14 1.45 0.35 0.04 0,030 0.25 0.22 0.19 0.013 0.005 0.008 0.012 -: - 4 0.17 1,60 0.60 0.06 0,050 0.30 0.30 0.30 0.015 0.006 0.010 0,020 -: - 5 0.18 1.70 0.70 0,07 0,060 0.35 0.32 0.33 0.016 0.007 0.011 0,022 -: - 6 (prototype) 0.19 1.20 0.50 - - 0.28 0.25 0.27 0,032 0,036 0.007 - -: - Note: composition 6 additionally contains 0.07% As.

Table 3
Modes of production of strips of low alloy steel and their effectiveness Option No. Composition number Temperature conditions, ° С Mechanical properties Weldability T _a , ° C T _CP , ° C T _cm , ° C σ _in , N / mm ² σ _t , N / mm ² σ _t / σ _in δ ₄ ,% KCU ^-40 , J / cm ² KCV ^-20 , J / cm ² 1 5 1210 810 570 650 590 0.90 thirteen 37 34 unsatisfied. 2 2 1220 820 580 610 480 0.79 28 54 53 sat. 3 3 1250 850 620 560 420 0.75 thirty 58 57 sat. 4 4 1280 880 660 510 395 0.77 25 51 52 sat. 5 1 1290 890 670 450 395 0.87 22 44 43 unsatisfied. 6 6 1250 870 690 500 445 0.89 17 42 39 unsatisfied.

Claims (1)

1. Method for the production of strips of low alloy steel, including heating slabs, heating, 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 steel contains the following ratio of components, wt.%: Carbon 0.12-0.17 Manganese 1.3-1.6 Silicon 0.3-0.6 Aluminum 0.02-0.06 Vanadium and / or niobium 0.01-0.05 Chromium No more than 0.3 Nickel No more than 0.3 Copper No more than 0.3 Phosphorus No more than 0.015 Sulfur No more than 0,006 Nitrogen No more than 0,010 Calcium No more than 0,02 Iron Rest the slabs are heated to 1220-1280 ° C, the temperature of the end of rolling is maintained in the range of 820-880 ° C, and the temperature of the winding is in the range of 580-660 ° C.