RU2242525C1 - Method for producing of strips from low-alloy steel - Google Patents

Abstract

FIELD: metallurgy, in particular, manufacture of rolled products.

SUBSTANCE: method involves heating slabs; performing rough rolling to provide for intermediate thickness and finish rolling at regulated rolling finishing temperature; cooling to winding temperature. Slabs are heated to temperature of 1,230-1,270⁰C. Rolling finishing temperature and winding temperature are maintained within the range of 805-855⁰C and 520-580⁰C, respectively. Strips are produced from low-alloy steel composition containing, wt%: C 0.05-0.08; Mn 1.55-1.65; Si 0.15-0.25; V 0.03-0.04; Nb 0.05-0.06; Ti 0.01-0.02; Al 0.02-0.05; Cr not in the excess of 0.1; Ni not in the excess of 0.1; Cu not in the excess of 0.1; S not in the excess of 0.005; P not in the excess of 0.015; B not in the excess of 0.005; N not in the excess of 0.010, with following ratio: Al/N≥2.0 being satisfied. Method is used for manufacture of electrically welded pipes employed in construction of oil pipelines in seismic zones.

EFFECT: increased viscosity characteristics at negative temperatures and improved weldability of steel strips.

2 cl, 3 tbl, 1 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 broadband mills for electric welded pipes intended for the construction of northern oil pipelines in seismic zones.

For the production of pipes of a northern oil trunk pipeline with a pipe diameter of 508 mm, operating in seismic zones at low temperatures, hot rolled sheets (strips) of 7–9 mm thickness from low alloy steel are required, which have the following set of mechanical properties (Table 1):

In addition to the indicated mechanical properties, 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 by weight,%:

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

Boron 0.0005-0.005

Aluminum 0.001-0.1

Iron and impurities Else

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 plate steel has low viscous properties at low temperatures, poor weldability. This makes it impossible to use it for the manufacture of pipes of northern pipelines operating in seismically hazardous areas. 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 Else

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 viscosity requirements at low temperatures, have insufficient weldability and are not suitable for the manufacture of pipes of northern pipelines 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

Chrome Not more than 0.30

Nickel Not more than 0.30

Copper Not more than 0.30

Phosphorus Not more than 0,035

Sulfur Not more than 0,040

Arsenic Not more than 0.08

Nitrogen Not more than 0.008

Iron Else

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 to a winding temperature T _cm = 620-700 ° C [3].

The disadvantages of this method are that the strips have low viscous properties at low temperatures. In addition, strips are characterized by insufficient weldability: during testing of a specimen for rupture, its destruction occurs along the weld.

The technical problem solved by the invention is to increase the viscosity properties at low temperatures and the weldability of strips.

The stated technical problem is solved by the fact that in the known method for the production of strips of low alloy steel, including heating slabs, their rough rolling to an intermediate thickness and finishing rolling with a regulated temperature of the end of rolling, cooling to a winding temperature, according to the proposal, the slabs are heated to a temperature of 1230-1270 ° C, and the temperature of the end of rolling and winding is maintained in the ranges of 805-855 ° C and 520-580 ° C, respectively.

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

Carbon 0.05-0.08

Manganese 1.55-1.65

Silicon 0.15-0.25

Vanadium 0.03-0.04

Niobium 0.05-0.06

Titanium 0.01-0.02

Aluminum 0.02-0.05

Chrome Not more than 0.1

Nickel Not more than 0.1

Copper Not more than 0.1

Sulfur Not more than 0.005

Phosphorus Not more than 0.015

Boron Not more than 0.005

Nitrogen Not more than 0.010

Iron Else

In this case, the following ratio of the content of chemical elements in steel should be satisfied: A1 / N≥2.0.

The invention consists in the following. Heating slabs of low alloy steel of the proposed composition to a temperature of 1230-1270 ° C ensures 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 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 the end of rolling of 805-855 ° 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 520-580 ° 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. Oil pipelines from such strips well resist seismic displacements of soil sections at negative temperatures without destruction. In addition, due to the limitation of the concentration of carbon and other alloying steels, low-alloy steel, having a given strength and high viscosity at low temperatures, is characterized by high weldability: during tensile testing, fracture The samples do not occur along the weld, but along 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 in 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 pipes and oil pipelines.

It was experimentally established that increasing the heating temperature of slabs of low alloy steel above 1270 ° 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. Reduction of temperature below 1230 ° C results in incomplete dissolution in austenite carbonitride strengthening particles, reduction of technological plasticity, pereuprochneniyu Steel (σ _T> 590 N / mm ^2), strips reduce viscosity properties at low temperatures.

When the temperature of the end of rolling T _CP above 855 ° C, the required degree of hardening of the strip and grinding of its microstructure to the optimum level are not achieved. The decrease in temperature T _CP below 805 ° C leads to excessive grinding of the microstructure, deterioration of the viscosity properties of the strips.

Increasing the temperature of the winding T _cm above 580 ° C contributes to the formation of variability of the microstructure, reducing the strength properties below acceptable values. The decrease in T _cm less than 520 ° 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 0.05% leads to a drop in their strength below the permissible level. An increase in carbon content of more than 0.08% affects the viscosity properties of strips and their weldability.

A decrease in manganese content of less than 1.55% increases the oxidation of steel, impairs the strength and weldability of strips. An increase in the manganese content of more than 1.65% increases the ratio of yield strength to temporary tensile strength σ _t / σ _in excess of 0.88, which is unacceptable.

When the silicon content is less than 0.15%, the deoxidation of steel deteriorates, and the strength properties of strips decrease. An increase in the silicon content of more than 0.25% leads to an increase in the number of silicate inclusions, reduces the impact strength of the strips, worsens the KV- ⁴⁰ index and the weldability of steel.

Vanadium grinds the grain of the microstructure, increases the strength and viscosity of the strips rolled according to the proposed modes. When the vanadium content is less than 0.03%, the strips have insufficient viscosity at low temperatures. An increase in the content of vanadium in excess of 0.04% was impractical, since it did not improve the properties of strips.

Niobium in steel at a temperature of rolling end Т _кп = 805-855 ° С promotes the production of a cellular dislocation microstructure of steel, which provides a combination of high strength and viscosity properties of strips. At a niobium concentration of less than 0.05%, the mechanical properties of the strips in the hot-rolled state are not high enough. An increase in its concentration of more than 0.06% does not lead to a further increase in the mechanical properties of strips; therefore, it is impractical.

Titanium is a strong carbide forming element that strengthens steel. When the titanium content is less than 0.01%, its strengthening effect is not enough manifested, strips have low strength and viscosity. An increase in titanium concentration in excess of 0.02% does not provide a further increase in the properties of strips; therefore, it is impractical.

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 0.05% leads to a deterioration in the viscosity properties of strips.

Chrome, nickel and copper are impurity elements. When the concentration of each of them is not more than 0.1%, they do not adversely affect the weldability of strips in the production of 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 0.1%, the viscosity properties and strip weldability deteriorate.

The steel of the proposed composition may contain in the form of impurities not more than 0.005% sulfur, not more than 0.015% phosphorus 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.

Boron contributes to the grinding of microstructure grains during rough hot rolling of slabs in the temperature range from 1230-1270 ° C and below. However, an increase in boron content of more than 0.005% leads to an increase in the number of non-metallic inclusions and a deterioration in the viscosity properties of strips, which is unacceptable.

It has been experimentally established that, with the ratio A1 / N≥2, the necessary set of mechanical properties of strips is achieved at all the proposed temperature conditions for hot rolling of strips. A decrease in the ratio under consideration less than 2 leads to a decrease in the viscosity properties and the proportion of the fibrous component in the fracture at low temperatures, and to a deterioration in the weldability of strips.

An example implementation of the method

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 austenitization temperature T _a = 1250 ° C. The heated slabs are discharged onto the furnace roll of a continuous broadband mill 2000 and subjected to rolling in a roughing group of stands (rough rolling) to an intermediate thickness of 45 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 _kn = 830 ° 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 = 550 ° 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 the table. 3 it follows that when implementing the proposed method (options No. 2-4) is achieved by increasing the viscosity properties at low temperatures and weldability. In the case of transcendental values of the declared parameters (options No. 1 and No. 5), the viscosity properties at low temperatures and the 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 the heating of slabs of low alloy steel of the proposed composition to a temperature of 1230-1270 ° C, their subsequent hot rolling into strips of a given thickness with a temperature of rolling end of 805-855 ° C and cooling with water to a winding temperature of 520 -580 ° C provides the formation of the optimal fine-grained ferritic-pearlitic microstructure of steel. Due to this, an increase in the viscosity properties at negative temperatures and weldability of the strips is achieved.

Using the proposed method will increase the profitability of the production of strips for pipelines of the northern version by 8-10%.

Literature

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, with. 262-266 is a prototype.

Claims (2)

1. A method of manufacturing strips of low alloy steel, including heating slabs, rough rolling to an intermediate thickness and finishing rolling with a regulated temperature of the end of rolling, cooling to a winding temperature, characterized in that the slabs are heated to a temperature of 1230-1270 ° C, and the temperature of the end rolling and winding support in the ranges of 805-855 and 520-580 ° C, respectively. 2. The method according to claim 1, characterized in that the strips are rolled from low alloy steel of the following chemical composition, wt.%: Carbon 0.05-0.08 Manganese 1.55-1.65 Silicon 0.15-0.25 Vanadium 0.03-0.04 Niobium 0.05-0.06 Titanium 0.01-0.02 Aluminum 0.02-0.05 Chrome Not more than 0.1 Nickel Not more than 0.1 Copper Not more than 0.1 Sulfur Not more than 0.005 Phosphorus Not more than 0.015 Boron Not more than 0.005 Nitrogen Not more than 0.010 Iron Else while the following ratio should be satisfied: A1 / N≥2.0, where A1 and N are the contents of aluminum and nitrogen, respectively.