RU2241769C1 - Method for production of candies from low-alloy steel - Google Patents

Abstract

FIELD: metallurgy, in particular, rolling-mill industry, electric-welded pipes for oil pipelines in seismic regions.

SUBSTANCE: claimed method includes slab heating, breaking-down to intermediate thickness and finish rolling in regulated temperature region. Slab is heated to 1170-1240⁰C, finish rolling is carried out at 910-710⁰C with total relative cogging of 60-80 %. In one embodiment of invention low-alloy steel for candies contains (mass %): 0.06-0.12 C; 1.4-1.7 Mn; 0.20-0.45 Si; 0.06-0.10 V; 0.04-0.08 Nb; 0.005-0.035 Ti; 0.01-0.3 Cr; 0.01-0.3 Ni; 0.01-0.3 Ni; 0.01-0.3 Cu; 0.02-0.05 Al; 0.01-0.50 Mo; ≤0.006 S; ≤0.015 P; ≤0.006 B; ≤0.010 N; and balance: Fe, provided that C +Mn/6 + (Cr + Mo + V)/5 + (Ni +Cu)/15 = 0.35 - 0.40 % and C + V/10 + Mo/15 + (Cr +Mn + Cu)/20 +Si/30 +Ni/60 + 5B ≤ 0.22 %.

EFFECT: candy with improved mechanical properties and weldability.

2 cl, 3 tbl, 1 ex

Description

The invention relates to the field of metallurgy, and more particularly to rolling production, and can be used in the manufacture of electric-welded pipes for the construction of oil pipelines in seismic zones.

For the production of oil pipeline pipes under the Sakhalin-2 project, hot-rolled sheets (strips) of thickness 8-20 mm, strength category X65, from low alloy steel, with the following set of mechanical properties are required (Table 1).

In addition to the indicated mechanical properties, the strips must satisfy the weldability requirements, and the grain size of the microstructure N should not be larger than the 9th point according to ASTM E112. Due to this, increased stability of the pipeline against destruction in zones of seismic activity and the formation of main cracks is achieved.

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 plastic and viscous properties at low temperatures, poor weldability. This makes it impossible to use it for the manufacture of oil pipes 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 the longitudinal direction in excess of 0.92. Such sheets do not meet the weldability requirements and are unsuitable for the manufacture of oil pipe 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 17GS (according to GOST 19281-89) of the following chemical composition, wt.%:

Carbon 0.14-0.20

Manganese 1.0-1.4

Silicon 0.4-0.6

Chrome 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 not more than 0.08

Nitrogen no more than 0,008

Iron Else

The slabs are heated in a methodical furnace to a temperature of 1220-1280 ° C, subjected to rough rolling in the temperature range of 1050-1180 ° C to an intermediate thickness of 30-40 mm and finishing rolling in a regulated temperature range of 900-1050 ° C. To improve the mechanical properties, hot-rolled strips are subjected to heat treatment (normalization, thermal improvement).

The disadvantage of this method is that it does not provide directly after hot rolling a given set of mechanical properties and weldability of strips. This leads to the need for additional thermal improvement (quenching + tempering).

The technical problem solved by the invention is to improve the complex of mechanical properties, weldability and eliminate the need for heat treatment of strips.

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 finishing rolling in a regulated temperature range, according to the proposal, the slabs are heated to a temperature of 1170-1240 ° C, and finishing rolling is carried out in the temperature range of 910-710 ° C with a total relative compression of 60-80%.

A possible embodiment of the method in which the strips are rolled from low alloy steel of the following chemical composition, wt.%:

Carbon 0.06-0.12

Manganese 1.4-1.7

Silicon 0.20-0.45

Vanadium 0.06-0.10

Niobium 0.04-0.08

Titanium 0.005-0.035

Chrome 0.01-0.30

Nickel 0.01-0.30

Copper 0.01-0.30

Aluminum 0.02-0.05

Molybdenum 0.01-0.50

Sulfur no more than 0,006

Phosphorus no more than 0.015

Boron no more than 0,006

Nitrogen no more than 0.010

Iron Else

wherein

and

The invention consists in the following. Heating slabs of low alloy steel to a temperature of 1170-1240 ° 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, by the time the rough rolling is finished, the temperature of the roll decreases to the optimum level necessary for low-temperature finishing rolling.

Subsequent finishing rolling of the strip in the temperature range of 910-710 ° C with a total relative compression of 60-80% 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 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 without destruction, main cracks and hydrogen cracking. In addition, strips made of low alloy steel with such a microstructure are characterized by high weldability: during tensile testing, fracture of the samples occurs not 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 modes stable production of the specified mechanical properties of strips, high weldability and resistance to hydrogen embrittlement of oil pipelines.

It was experimentally established that increasing the heating temperature of slabs of low alloy steel above 1240 ° C does not improve the complex of mechanical properties of strips, but only increases the heating time and requires additional reinforcement of the roll before finishing rolling, which reduces the productivity of the process. A decrease in this temperature below 1170 ° C leads to incomplete dissolution of the carbonitride hardening particles in austenite, a decrease in technological plasticity, steel hardening (σ _t prod.> 500), and a decrease in the plastic and viscosity properties of strips.

At a finish rolling temperature above 910 ° С or a total reduction of less than 60%, the required degree of strip hardening (σ _at <540 MPa, σ _t cross-section <485 MPa) and grinding of its microstructure (N less than 9 points) are not achieved. As a result, to ensure the required complex of mechanical properties, it is necessary to conduct thermal improvement of strips after hot rolling. A decrease in the temperature of finish rolling below 710 ° C or an increase in the total degree of deformation of more than 80% leads to excessive grinding of the microstructure, a fall of δ ₂ , δ _{equal to} KV- ²² and KCV- ²² below acceptable values, the formation of anisotropy and the set ratios σ _t / σ _in in the longitudinal and transverse directions.

The carbon in the low alloy steel of the proposed composition determines the strength of the strips. A decrease in carbon content of less than 0.06% leads to a drop in their strength below the permissible level. An increase in carbon content of more than 0.12% affects the plastic and viscous properties of strips and their weldability.

A decrease in the manganese content of less than 1.4% increases the oxidation of steel, worsens the weldability of strips. An increase in the manganese content of more than 1.7% increases the ratio of the yield strength to the tensile strength σ _t / σ _in excess of 0.93, which is unacceptable.

When the silicon content is less than 0.20%, the deoxidation of steel deteriorates, and the strength properties of strips decrease. An increase in the silicon content of more than 0.45% leads to an increase in the number of silicate inclusions, reduces the impact strength of the strips, worsens the KV- ²² 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 0.06%, the strips have insufficient viscosity at low temperatures. An increase in the content of vanadium in excess of 0.10% was impractical, since it did not improve the properties of strips.

Niobium in steel at a rolling temperature in the finishing stand from 910-720 ° C to less than 820 ° C with a total relative compression of 60-80% contributes to the production of a cellular dislocation microstructure of steel, providing a combination of strength and plastic properties of strips without additional heat treatment. At a niobium concentration of less than 0.04%, the mechanical properties of strips in a hot-rolled state are not high enough. Increasing the concentration of more than 0.08% does not lead to a further increase in the mechanical properties of the strips, therefore, it is impractical.

Titanium is a strong carbide forming element that strengthens steel. When the titanium content is less than 0.005%, its strengthening effect is not enough manifested, strips have low strength and viscosity. An increase in the concentration of titanium in excess of 0.035% does not provide a further increase in the properties of strips; therefore, it is impractical.

Chrome, nickel and copper increase the strength and corrosion resistance of strips. At a concentration of each of them up to 0.3%, 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 of strips. When the concentration of each of these elements is less than 0.01%, their effect on the properties of strips is practically not manifested, therefore, such a decrease is impractical.

Aluminum deoxidizes and modifies steel. At a concentration of less than 0.02%, its effect is weak, which affects the mechanical properties of strips. An increase in its content of more than 0.05% graphitizes carbon, which also degrades the quality of strips.

Molybdenum provides hot rolled strips of a given strength category X65, when its content is 0.01-0.10%. An increase in the content of molybdenum in excess of 0.50% does not lead to a further increase in the quality of strips, but only increases the consumption of alloys, which is impractical. When the concentration of molybdenum is less than 0.01%, its beneficial effect is practically not manifested, the mechanical properties of strips (σ _in , σ _t , KV- ²² , KCV- ²² ) are below permissible.

The steel of the proposed composition may contain in the form of impurities not more than 0.008% 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.

Boron contributes to the grinding of microstructure grains during rough hot rolling of slabs in the temperature range 1170-1240 ° C. However, an increase in boron content of more than 0.006% leads to an increase in the number of non-metallic inclusions and a deterioration in the viscosity properties of strips, which is unacceptable.

то необходимый комплекс механических свойств штрипсов достигается при всех возможных комбинациях температурно-деформационных режимов горячей прокатки листов из заявленных диапазонов как для непрервно литых, так и катаных слябов. Снижение рассматриваемой суммы менее 0,35% приводит к тому, что прочностные свойства штрипсов снижаются. Увеличение этой суммы более 0,40% приводит к некоторому снижению вязкостных свойств и доли волокнистой составляющей в изломе при отрицательных температурах. Это снижает качество штрипсов.It was experimentally established that if the content of chemical elements satisfies the sum

then the necessary complex of mechanical properties of strips is achieved with all possible combinations of temperature-deformation modes of hot rolling of sheets from the declared ranges for continuously cast and rolled slabs. A decrease in the considered amount of less than 0.35% leads to the fact that the strength properties of strips are reduced. An increase in this amount of more than 0.40% leads to some decrease in the viscosity properties and the proportion of the fibrous component in the fracture at low temperatures. This reduces the quality of the strips.

как показали экспериментальные исследования, свариваемость штрипсов наилучшая, разрыв образцов со сваркой происходит по основному металлу. Если же

то свариваемость штрипсов ухудшается из-за общей "перелегированности" стали, хотя механические свойства остаются в заданных пределах.At

As shown by experimental studies, the weldability of the strips is the best, the rupture of the samples with welding occurs on the base metal. If

then the weldability of strips is deteriorated due to the general “alloying” of the steel, although the mechanical properties remain within specified limits.

Method implementation examples

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

Прокатку штрипсов завершают при температуре Т_кп=740°С. Прокатанные штрипсы охлаждают на воздухе, обрезают на ширину 1880 мм, проводят испытания механических свойств и свариваемости. Штрипсы, прошедшие испытания, используют для изготовления электросварных труб для нефтепровода.Slabs with a thickness of 150 mm are loaded into methodological furnaces and heated to austenitization temperature T _n = 1200 ° C. The heated slabs after crimping in a vertical stand are rolled in a reversible roughing stand duo 2800 to an intermediate thickness of 32 mm As rolling in the roughing stand, the temperature of the rolls is reduced to a value of T _nn = 880 ° C and then they are transferred to a finishing reversible stand of quarto 2800. When finishing rolling, the rolls are pressed into strips of a final thickness of 9.5 mm with a relative total compression:

The rolling of the strips is completed at a temperature T _kn = 740 ° C. Laminated strips are cooled in air, cut to a width of 1880 mm, mechanical properties and weldability are tested. The tested strips are used for the manufacture of electric-welded pipes for an oil pipeline.

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. 2-5), an improvement in the complex of mechanical properties and weldability is achieved while eliminating the need for heat treatment of strips. In the case of transcendental values of the declared parameters (options No. 1 and No. 6), the complex of mechanical properties and weldability of the strips deteriorate. Also, lower properties and weldability are achieved even after thermal improvement of the strips obtained according to the prototype method (option No. 7).

The technical and economic advantages of the proposed method are that heating slabs of low alloy steel to a temperature of 1170-1240 ° C, rough rolling to an intermediate thickness and subsequent finishing rolling with a total relative compression of 60-80% in the temperature range of 910-720 ° C provides the formation of an optimal microstructure, a high complex of mechanical properties and weldability of strips for oil pipes operating in seismically hazardous regions. This eliminates the need for additional heat treatment of hot rolled strips. These advantages are achieved when using low alloy steel of the proposed chemical composition.

As a basic object in the calculation of the technical and economic advantages of the proposed method adopted the prototype method. Using the proposed method will increase the profitability of the production of strips of strength category X65 by 25-30%.

Literature

1. Japanese application No. 611-163210, 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.242-244, 268 - prototype.

Claims (2)

1. A method for the production of strips of low alloy steel, including heating slabs, their rough rolling to an intermediate thickness and finishing rolling in a regulated temperature range, characterized in that the slabs are heated to a temperature of 1170-1240 ° C, and finishing rolling is carried out in a temperature range of 910 -710 ° C with a total relative compression of 60-80%. 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 0.06-0.12 Manganese 1.4-1.7 Silicon 0.20-0.45 Vanadium 0.06-0.10 Niobium 0.04-0.08 Titanium 0.005-0.035 Chrome 0.01-0.30 Nickel 0.01-0.30 Copper 0.01-0.30 Aluminum 0.02-0.05 Molybdenum 0.01-0.50 Sulfur Not more than 0.006 Phosphorus Not more than 0.015 Boron Not more than 0,006 Nitrogen Not more than 0.010 Iron Else wherein

and