RU2358024C1 - Method of production of strips out of low alloyed steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention can be implemented for production of strips with hardness grade of X80; said strips are used at construction of oil and gas pipelines. Slabs are heated to temperature of austenisation, further rough and finish multi-pass rolling is performed and strips are cooled with water; also finish rolling is carried out with total relative reducing not less, than 50%; temperature of end of rolling is maintained equal to 700-880°C, while cooling strips with water is performed at rate not less, than 10°C/sec to temperature not exceeding 580°C. In addition, low alloyed steel has the following chemical composition, wt %: 0.05-0.10 C; 0.20-0.40 Si; 1.50-1.90 Mn; 0.15-0.40 Mo; 0.02-0.06 Nb; 0.01-0.03 Ti; not more 0.01 B; not more 0.05 Al; not more 0.30 Cr; not more 0.50 Ni; not more 0.30 Cu; not more 0.010 P; not more 0.004 S; Fe - the rest.

EFFECT: upgraded hardness, plasticity and impact resilience of strips.

2 cl, 3 tbl

Description

The invention relates to the field of metallurgy, and more specifically to rolling production, and can be used to obtain strips of strength category X80 used in the construction of oil and gas pipelines.

Electric pipes from strips of strength category X80 used for the construction of oil and gas pipelines, with the same structural strength, have a wall thickness of ~ 25% less than pipes from strips of strength category X60-X65. Due to this, a significant reduction in the metal consumption of the pipeline is achieved. Table 1 shows the requirements for the mechanical properties of strips of low alloy welded steel of strength category X80.

Table 1
Mechanical properties of strips of strength category X80 σ _in , MPa σ _t , MPa σ _t / σ _in δ ₅ ,% KCV ^-10 , J / cm ² IPG ^-20 ,% C _e ,% 640-710 not less than 570 no more than 0.90 not less than 20 not less than 180 not less than 90 no more than 0.43 Notes: 1. IPG - the proportion of the viscous component in the fracture of the sample when tested by a falling load;
2. C _e is the carbon equivalent characterizing the weldability of steel.

A known method for the production of strips of low alloy steel [1], including heating slabs to a temperature of 1160-1190 ° C, rough rolling, finish rolling with a total relative compression of at least 70% at a temperature of rolling end not higher than 820 ° C. After rolling, the strips are subjected to water quenching from a temperature of 900-950 ° C and tempering at a temperature of 600-730 ° C. In this case, low alloy steel has the following chemical composition, wt.%:

Carbon 0.07-0.12 Manganese 1.4-1.7 Silicon 0.15-0.50 Vanadium 0.06-0.12 Niobium 0.03-0.05 Titanium 0.01-0.03 Aluminum 0.02-0.05 Chromium no more than 0.3 Nickel no more than 0.3 Copper no more than 0.3 Sulfur no more than 0,005 Phosphorus no more than 0.015 Nitrogen no more than 0,010 Iron rest

The disadvantages of this method are that the strips have low mechanical properties that do not correspond to the strength category X80. In addition, additional thermal improvement of strips increases the cost of their production.

There is also a known method for the production of strips of strength category X65 from low alloy steel of the following 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 Chromium 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 rest,

wherein

and

The method includes heating the slabs to a temperature of 1170-1420 ° C, their rough rolling to an intermediate thickness and finishing rolling in the temperature range of 910-710 ° C with a total relative compression of 60-80% [2].

The disadvantages of this method are that it does not provide high strength, plastic and viscous properties of strips corresponding to strength category X80.

The closest analogue in terms of characteristics and the achieved results to the proposed invention is a method for the production of strips from 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 method includes heating the slabs to an austenitizing temperature of 1220-1280 ° C, multi-pass rough rolling to an intermediate thickness, multi-pass finishing rolling with a rolling end temperature of 820-880 ° C and accelerated cooling of the strips with water to a temperature of 580-660 ° C [3].

The disadvantages of this method are that it does not provide high strength, plastic and viscous properties of strips corresponding to category X80.

The technical problem solved by the invention is to increase the strength, plastic and viscous properties of strips.

To solve the technical problem in the known method for the production of strips of low alloy steel, including heating slabs to the temperature of austenitization, multi-pass roughing and finishing rolling with a controlled temperature of the end of rolling, and cooling of the strips with water, according to the invention, the finish rolling is carried out with a total relative compression of at least 50% , the temperature of the end of rolling is maintained equal to 700-880 ° C, and the strips are cooled with water at a speed of at least 10 ° C / s to a temperature of no higher than 580 ° C. In this case, low alloy steel has the following chemical composition, wt.%:

Carbon 0.05-0.10 Silicon 0.20-0.40 Manganese 1.50-1.90 Molybdenum 0.15-0.40 Niobium 0.02-0.06 Titanium 0.01-0.03 Boron no more than 0,01 Aluminum no more than 0,05 Chromium no more than 0.30 Nickel no more than 0.50 Copper no more than 0.30 Phosphorus no more than 0,010 Sulfur no more than 0,004 Iron rest

The essence of the proposed technical solution is as follows. The complex of mechanical properties of strips is determined by the microstructural-phase state of low alloy steel, which, in turn, depends on the chemical composition of the steel and the modes of its multi-cycle deformation-heat treatment.

With known methods for the production of strips of low alloy steel, the microstructure of the strip is perlite with inclusions of ferrite, which results in insufficient strength of the strips. One of the ways to increase the strength of hot-rolled steels with a ferrite-pearlite phase composition is rolling at a low temperature in the α-γ region. However, such rolling, in addition to increasing energy costs for its implementation, leads to a sharp deterioration in the viscosity and plastic properties of steel, which is why it is unacceptable for the production of strips of category X80.

The proposed deformation-thermal conditions for the production of strips and the chemical composition of low-alloy steel ensure the formation, after multi-pass rolling and water cooling, of a two-phase ferrite-martensitic microstructure consisting of a ferrite matrix hardened by dispersed martensite, as well as a small amount of bainite with residual austenite and inclusions of needle martensite. Due to the presence of a ferrite matrix, strips acquire high viscous and plastic properties, and the presence of dispersed martensite in it increases the strength properties. Moreover, the volume content of dispersed martensite in the ferrite matrix formed upon accelerated cooling of the strips with water is about 3%. This ensures that the specified strength σ _in = 650-710 MPa without reducing ductility and toughness.

It has been experimentally established that with multi-pass finishing rolling with a total compression of less than 50%, the optimum degree of grinding of the microstructural components of steel is not achieved, as a result of which the microstructure and phase composition of the strips are uneven, and the plastic and viscosity properties are reduced.

When the temperature of the end of rolling T _kp below 700 ° C, the deformation of the steel of the proposed composition occurs in the α-γ region. As a result, the strength of the strips increases with a decrease in the viscosity and plastic properties, which is unacceptable. At a temperature of the end of rolling T _kp above 880 ° C, due to intensive dynamic recrystallization, austenite grains grow, the required degree of hardening of the strip and grinding of its microstructure to the optimum level are not achieved.

The cooling of rolled strips with water at a rate of less than 10 ° C / s leads to a decrease in the volume content of dispersed martensite in the ferrite matrix, which hardens steel. Therefore, the strength properties of strips with a ferrite matrix are below the permissible value.

With accelerated cooling of the strips with water to a temperature above 560 ° C, tempering of dispersed martensite occurs, which leads to softening of the strips below the permissible level.

Carbon in steel of the proposed composition determines its strength properties. A decrease in carbon content of less than 0.05% leads to a drop in strength below an acceptable level. An increase in carbon content in excess of 0.10% affects the weldability, ductility and toughness of steel.

Silicon deoxidizes the steel and strengthens the ferrite phase. When the silicon content is less than 0.2%, the strength of the steel is insufficient. An increase in silicon content of more than 0.4% leads to an increase in the number of silicate non-metallic inclusions, worsens its ductility and toughness.

Manganese is introduced to deoxidize and increase the strength of steel, binding impurity sulfur to sulfides. When the manganese content is less than 1.50%, the strength of steel decreases, the proportion of the viscous component in the fracture and the viscosity at low temperatures. An increase in the concentration of manganese in excess of 1.90% leads to a decrease in ductility and an increase in the ratio σ _t / σ _c .

Molybdenum plays a decisive role in providing both high strength, toughness and ductility of strips. Due to the presence of molybdenum in steel during cooling after a multi-cycle deformation, a phase of finely dispersed martensite is formed in it, which provides hardening of the ferrite matrix and obtaining hot rolled strips of a given strength category X80. A decrease in the molybdenum content of less than 0.15% reduces the strength properties of strips below an acceptable level. An increase in the molybdenum content of more than 0.40% reinforces the steel, leads to an increase in alloying consumption, which is impractical.

Niobium form NbC carbides with carbon. Small niobium carbides are located along the boundaries of grains and subgrains, inhibit the movement of dislocations and thereby strengthen the steel. When the niobium content is less than 0.02%, its effect is not large enough, the strength properties have become lower than the permissible level. An increase in niobium concentration of more than 0.06% causes dispersion hardening and embrittlement of grain boundaries. This leads to an increase in the ratio of σ _t / σ _in and the deterioration of the properties of steel.

Titanium is a strong carbide forming element that strengthens steel. A decrease in titanium content of less than 0.01% impairs the strength and ductility of the steel. However, during welding, titanium completely burns out, so its amount in steel should not exceed 0.03%.

Boron contributes to the grinding of microstructure grains during strip rolling, the formation of the martensitic phase. It displaces sulfur and phosphorus from the grain boundaries, reducing their harmful effect on mechanical properties. However, an increase in boron content of more than 0.01% leads to an increase in the number of non-metallic inclusions in steel and a deterioration in the viscosity properties of strips, which is unacceptable.

Aluminum is a deoxidizing and modifying element. However, an increase in aluminum content of more than 0.05% leads to a deterioration in the weldability of strips and pipes.

Impurities of chromium, nickel and copper contribute to the increase of strength properties, but with a content of more than 0.3% chromium, more than 0.5% nickel or more than 0.3% copper, there is a decrease in impact strength and weldability of strips.

Sulfur and phosphorus are harmful impurities that reduce the plastic and viscous properties. When the concentration of phosphorus is not more than 0.010% and sulfur is not more than 0.004%, their harmful effect is weak and does not lead to a noticeable decrease in the mechanical properties of strips. At the same time, a deeper removal of sulfur and phosphorus makes steel more expensive, making its production unprofitable.

The carbon equivalent C _e characterizing the weldability of low alloy steels is determined by the ratio:

An example implementation of the method

In an oxygen converter, low alloy steels are smelted to produce X80 category strips. Smelting is carried out in an oxygen converter from pig iron with additives of selected scrap metal. The resulting melts are deoxidized with ferrosilicon, ferromanganese, alloyed with ferroniobium, ferrotitanium, ferroboron, ferromolybdenum, and metal aluminum is introduced. Desulfurization and dephosphorization of the melt, purging with argon. Smelted low alloy steel of all compositions is continuously cast into 250 mm thick slabs.

The chemical composition of the steels for strips is given in table 2.

table 2
The chemical composition of low alloy steels for the production of strips of strength category X80 Composition number The content of chemical elements, wt.% FROM Si Mn Mo Nb Ti AT A1 Cr Ni Si R S Se Fe one. 0.04 0.19 2.1 0.14 0.01 0.009 - 0.01 0.10 0.20 0.10 0.004 0.001 0.44 rest 2. 0.05 0.20 1.9 0.15 0.02 0.010 0.003 0.02 0.01 0.01 0.01 0.005 0.002 0.40 -: - 3. 0.06 0.30 1.7 0.30 0.04 0,020 0.007 0,03 0.02 0.20 0.20 0.006 0.003 0.43 -: - four. 0.05 0.35 1,5 0.15 0.05 0,025 - 0.04 0.01 0.50 0.30 0.008 0.004 0.39 -: - 5. 0.08 0.40 1,5 0.40 0.06 0,030 0.010 0.05 0.10 0.04 0.02 0.010 0.003 0.43 -: - 6. 0.10 0.32 1,5 0.20 0,03 0,023 0.005 0,03 0.01 0.02 0,03 0.007 0.001 0.40 -: - 7. 0.06 0.25 1,6 0.15 0.04 0.017 0.009 0.04 0.31 0,03 0.01 0.008 0.003 0.42 -: - 8. 0.11 0.41 1.4 0.45 0,07 0,040 0.011 0.06 0.40 0.60 0.31 0.011 0.005 0.57 -: - 9. 0.15 0.50 1,6 - 0.01 - - 0.04 0.25 0.25 0.30 0.014 0.005 0.51 Note: composition No. 8 additionally contains 0.04% V and 0.01% Ca.

Continuously cast slabs of steel with composition 3 (Table 2) are loaded into a methodical furnace and heated to austenitization temperature T _a = 1160 ° C. After the temperature is equalized over the cross section, the next slab is fed to the quarto 5000 thick-plate reversing mill. The slab is subjected to multi-pass rough rolling with a breakdown of the width into a roll with an intermediate thickness of H _p = 60 mm. Then the roll is baked to a temperature of 950 ° C and rolled in 7 passes (finishing rolling) into a strip of final thickness N _W = 24.0 mm with a total relative compression

The temperature of the end of the rolling support equal to T _kn = 790 ° C.

The rolled strip is immediately subjected to accelerated cooling with water in a controlled cooling installation at a speed of V _o = 15 ° C / s to a temperature of T _about = 570 ° C.

Table 3 shows the implementation options for the production of strips of strength category X80 from low alloy steels, as well as indicators of their mechanical and operational properties.

From the data given in tables 2 and 3 it follows that when implementing the proposed method (options No. 2-6), an increase in the strength, plastic and viscosity properties of strips made of low alloy steel is achieved. As a result, they fully comply with the requirements for strips of strength category X80.

In the case of transcendental values of the declared parameters (options No. 1, No. 7 and No. 8), the complex of mechanical properties of the strips decreases. In addition, there is an increase in the carbon equivalent C _e , which indicates a deterioration in weldability.

Also lower strength, ductility and toughness are strips produced by the prototype method (option No. 9). They do not meet the strength category X80. The carbon equivalent C _{e of} the prototype steel also exceeds the permissible value.

Table 3
Modes of production of strips of strength category X80 and their operational properties No. p / p Hot rolling and cooling modes Performance indicators strips Composition number ε _Σ ,% T _CP , ° C V _o , ° C / c T _o , ° C σ _in , MPa σ _t , MPa σ _t / σ _in δ ₅ ,% KCV ^-10 , J / cm ² IPG ^-20 ,% C ^e ,% one. one 49 890 9 620 620 565 0.91 19.3 98 47 0.44 2. 2 fifty 880 10 580 640 570 0.89 28 340 99 0.40 3. 3 60 790 fifteen 570 700 580 0.83 32 360 one hundred 0.43 four. four 64 820 19 560 710 610 0.86 thirty 350 99 0.39 5. 5 68 800 eighteen 550 660 570 0.86 31 355 97 0.43 6. 6 70 700 twenty 540 710 640 0.90 29th 350 99 0.40 7. 7 75 690 12 580 630 575 0.91 eighteen 360 98 0.42 8. 8 48 685 21 590 740 725 0.98 17 170 82 0.57 9. 9 80 850 not regl. 620 560 448 0.80 26 200 88 0.51

The technical and economic advantages of the proposed method are that the heating of slabs of low alloy steel of the proposed composition to the austenitizing temperature, their subsequent multi-pass rough rolling and multi-pass finish rolling with a total relative compression of at least 50%, with a temperature of rolling end of 700-880 ° C and immediate cooling of the strips with water at a rate of at least 10 ° C / s to a temperature of no higher than 580 ° C provides the formation of a specific two-phase ferrite-martensitic microstructure ipsov. Due to this, strips have increased strength, ductility and toughness and fully comply with the strength category X80.

Using the proposed method will increase the profitability of the production of strips of increased strength for pipes of oil and gas pipelines by 15-20%.

Sources used in the preparation of the description of the invention

1. Patent of the Russian Federation No. 225123, IPC C21D 8/02, C22C 38/58, 2005

2. Patent of the Russian Federation No. 2241769, IPC C21D 8/02, C22C 38/58, B21B 1/26, 2004

3. Patent of the Russian Federation No. 2262537, IPC C21D 8/02, C22C 38/46, 2005 - prototype.

Claims (2)

1. A method for the production of strips of low alloy steel, including heating slabs to an austenitizing temperature, multi-pass roughing and finishing rolling with a regulated temperature of the end of rolling and cooling of the strips with water, characterized in that the final rolling is carried out with a total relative compression of at least 50%, the temperature of the end of rolling maintain equal to 700-880 ° C, and the cooling strips with water is carried out at a speed of not less than 10 ° C / s to a temperature of not higher than 580 ° C. 2. The method according to claim 1, characterized in that the low alloy steel has the following chemical composition, wt.%:
carbon 0.05-0.10 silicon 0.20-0.40 manganese 1.50-1.90 molybdenum 0.15-0.40 niobium 0.02-0.06 titanium 0.01-0.03 boron no more than 0,01 aluminum no more than 0,05 chromium no more than 0.30 nickel no more than 0.50 copper no more than 0.30 phosphorus no more than 0,010 sulfur no more than 0,004 iron rest