RU2358023C1 - Method of production strips out of low alloyed steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: at production of strips on continuous broad-strip mill for spiral seam electric welded tubes for transporting oil pipelines there is performed heating of slabs out of steel possessing following ratio of components, wt %: 0.11-0.14 C; 1.30-1.65 Mn; 0.4-0.6 Si; 0.02-0.05 Al; 0.01- 0.04 Nb; 0.01-0.03 Ti; 0.001-0.005 Ca; not more 0.01 V; not more 0.2 Cr; not more 0.2 Ni; not more 0.2 Cu; not more 0.015 P; not more 0.006 S; not more 0.010 N, Fe - the rest, its rough rolling to intermediate thickness, continuous finish rolling with temperature of beginning of rolling not more, than 970°C and temperature of end of rolling of 800-855°C, and cooling with water to the temperature of 540-580°C. In addition, total contents of C, Mn, Cr, Cu, Si, Ni and V in steel meet the ratio: C+(Mn+Cr+Cu)/20+Si/30+Ni/60+V/10≤0.24%, while finishing rolling of strips is carried out with total relative thickness reducing not less, than 64%.

EFFECT: upgraded impact resistance at negative temperature and fracture resistance of strips.

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 continuous broadband mills for spiral welded electric-welded pipes for main oil pipelines.

For the production of spiral-seam electric-welded pipes of main oil pipelines operating in seismic zones at negative temperatures, hot-rolled strips (strips) of 12-16 mm thick, 900-1800 mm wide of low alloy steel with the following set of mechanical and operational properties are required (Table 1):

Table 1. Strip Properties for Spiral Oil Pipes σ _in , N / mm ² σ _t , N / mm ² δ ₅ ,% σ _t / σ _in KCV ^-10 , J / cm ² KV ^-20 , J / cm ² KCU ^-40 J / cm ² 510-620 380-490 not less than 23 no more than 0.90 not less than 98 not less than 27 not less than 68 Note: Strips must withstand 180 ° cold bending.

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 a low toughness and crack resistance at low temperatures, poor weldability. This makes it impossible to use strips for the manufacture of spiral seam pipes of the northern design, 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 impact strength at low temperatures, have insufficient weldability and are unsuitable for the manufacture of spiral seam pipes of the northern version for operation in seismically hazardous areas.

The closest in its technical essence 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.

Slabs of low alloy steel are heated to a temperature of 1220–1280 ° С, subjected to rough rolling to an intermediate thickness, finish continuous rolling with a regulated rolling end temperature of 820–880 ° С, and water cooling to a winding temperature of 580–660 ° С [3].

The disadvantages of this method are that the strips have low impact strength at low temperatures and crack resistance.

The technical problem solved by the invention is to increase the impact strength at low temperatures and crack resistance of strips.

To solve the technical problem in the known method for the production of strips of low alloy steel, including heating slabs, rough rolling to an intermediate thickness, continuous finishing rolling with a regulated temperature of the rolling end, water cooling, according to the proposal, the steel has the following ratio of components, wt.%:

Carbon 0.11-0.14 Manganese 1.3-1.65 Silicon 0.40-0.60 Aluminum 0.02-0.05 Niobium 0.01-0.04 Titanium 0.01-0.03 Calcium 0.001-0.005 Vanadium no more than 0,01 Chromium no more than 0.2 Nickel no more than 0.2 Copper no more than 0.2 Phosphorus no more than 0.015 Sulfur no more than 0,006 Nitrogen no more than 0,010 Iron Rest,

the temperature of the beginning of the finish rolling is maintained no higher than 970 ° C, the end of the rolling is 800-855 ° C, and water cooling is carried out to a temperature of 540-580 ° C. In addition, the total content in carbon steel C, manganese Mn, chromium Cr, copper Cu, silicon Si, nickel Ni, vanadium V must satisfy the ratio: C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + V / 10≤0.24%, and finishing rolling of strips is carried out with a total relative compression in thickness of at least 64%.

The invention consists in the following. The use of low-alloy steel of the proposed composition under the condition C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + V / 10≤0.24% provides after hot rolling according to the proposed modes the obtaining of a given set of mechanical properties of strips, increasing viscous properties and crack resistance at low temperatures, high weldability of casing pipes. An increase in the complex of mechanical and operational properties of strips is achieved both by optimizing the chemical composition of steel and by the modes of hot rolling.

Carbon in low alloy steel of the proposed composition determines the strength properties of strips. A decrease in carbon content of less than 0.11% leads to a drop in their strength below the permissible level. An increase in carbon content of more than 0.14% affects the viscosity properties of strips and crack resistance at low temperatures.

A decrease in manganese content of less than 1.30% increases the oxidation of steel, impairs the strength and viscosity properties 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 over 0.9, which is unacceptable.

When the silicon content is less than 0.40%, the deoxidation of steel deteriorates, and the strength properties of strips decrease. An increase in the silicon content of more than 0.60% leads to an increase in the number of silicate inclusions, reduces the impact strength of strips, and worsens the performance of KV ^-20 KCU ^-40 .

Aluminum deoxidizes and modifies steel. By binding excess 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 at low temperatures.

Niobium grinds the grain of the microstructure, increases the strength and viscosity of the strips rolled according to the proposed modes. When the niobium content is less than 0.01%, the strips have insufficient viscosity at low temperatures. An increase in the niobium content in excess of 0.04% proved to be inappropriate, since it did not improve the mechanical and operational properties of the strips.

Titanium, being a strong carbide-forming element, helps to increase the strength properties of strips while increasing toughness and crack resistance at low temperatures. A decrease in the titanium content of less than 0.01% leads to a decrease in the strength and viscosity properties of strips, and a deterioration in crack resistance. An increase in titanium content of more than 0.03% leads to an increase in the ratio of σ _t / σ _in , which is impractical.

Calcium contributes to steel modification and grinding of microstructure grains during hot rolling of slabs. With a decrease in calcium content of less than 0.001%, it does not have a modifying effect, which leads to a deterioration in the complex of mechanical properties of strips. With an increase in calcium content of more than 0.005%, the number of non-metallic inclusions increases, and the viscosity and plastic properties of strips decrease.

Chrome, nickel and copper are impurity elements. When the concentration of each of them is not more than 0.2%, they do not adversely affect the toughness and crack resistance at low temperatures. When the concentration of each of these elements is more than 0.2%, the toughness, crack resistance and weldability of strips 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 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.

It was experimentally established that to ensure high crack resistance of strips in the hot-rolled state, it is necessary that C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + V / 10≤0.24%. In this case, strips rolled according to the proposed modes have maximum crack resistance. If C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + V / 10> 0.24%, this leads to a decrease in crack resistance and also weldability of steel.

Heating for rolling slabs of low alloy steel of the proposed composition ensures its austenitization, dissolution in the austenitic matrix of sulfides, phosphides, nitrides, alloying and impurity compounds, carbonitride reinforcing particles.

Continuous finishing rolling of steel strips with the adjusted chemical composition in the temperature range from the temperature of rolling start T _np ≤970 ° C to the temperature of the end of rolling T _kp = 800-855 ° C with a total relative compression ε _Σ ≥64% (controlled rolling mode) ensures due to the implementation of deformation-thermal cycling, dispersion of austenitic grains of the microstructure and a decrease in their growth rate during recrystallization. Also, during deformation-thermal cycling, complete precipitation of carbonitride reinforcing particles from the solid solution, deformation hardening of the metal matrix occurs. Subsequent water cooling of a strip with a dispersed microstructure to a winding temperature T _cm = 540-580 ° C ensures fixation of the fine-grained structure of perlite with granular morphology. As a result, strips in a hot-rolled state acquire increased toughness and crack resistance at low temperatures with high strength and plastic properties.

If T _np is higher than 970 ° C, then the heterogeneity of the microstructure of rolled strips increases, which leads to a decrease in strength and viscosity properties.

At T _cp above 855 ° C, the processes of dynamic and static recrystallization of the deformed microstructure are accelerated, the required degree of hardening of the strip and grinding of its microstructure to the optimum level are not achieved, which leads to an increase in the size of austenite grains, a decrease in the viscosity and strength properties of the strips, and deterioration of crack resistance. The decrease in T _CP less than 800 ° C leads to the formation of a variety of microstructure, decrease in toughness and crack resistance at low temperatures.

When the strips are cooled with water to T _cm above 580 ° C, the strip softens, and the viscosity properties decrease at low temperatures. The decrease in T _cm less than 540 ° C does not lead to an improvement in the properties of strips, but only increases the consumption of cooling water and energy consumption.

It was experimentally established that during finish rolling with a total compression of less than 64%, the necessary degree of grinding of austenitic grains of the microstructure of low alloy steel is not provided. As a result, the viscosity properties of strips and crack resistance at low temperatures are reduced.

Method implementation examples

In an oxygen converter, low alloy steels of various compositions are smelted (Table 2), which are continuously cast into 250 mm thick slabs.

Slabs are loaded into methodological furnaces and heated to austenitizing temperature T _a = 1250 ° C. A heated slab of steel with composition No. 3 is fed onto the furnace roll of a continuous broadband mill 2000 and subjected to rolling in a roughing stand group (rough rolling) into a roll with an intermediate thickness of H ₀ = 50 mm. Then, the roll at a temperature of 950 ° C is set into a continuous 7-stand finishing group of stands, where they are crimped into a strip of final thickness H ₁ = 14 mm. Thus, the total relative compression during finish rolling is:

The temperature of the end of the rolling is maintained equal to T _kn = 825 ° C by changing the rolling speed and interstand cooling of the strip.

The rolled strip during transportation along the discharge roller table is subjected to cooling by laminar streams of water to a temperature of T _cm = 560 ° C, and then wound into 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. 2-5), an increase in the viscosity properties and crack resistance of strips is achieved at negative temperatures. In the case of transcendental values of the declared parameters (options No. 1 and No. 6), the viscosity properties and crack resistance of the strips deteriorate. Also, lower viscous properties and crack resistance have strips produced according to the prototype method (option No. 7).

Technical and economic advantages of the proposed method are that the heating of slabs of low alloy steel of the proposed composition with a ratio of the chemical elements of steel C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + V / 10≤0.24 %, their subsequent rough hot rolling to an intermediate thickness, continuous finishing rolling from a temperature of no higher than 970 ° to a temperature of 800-855 ° C with a total relative compression of at least 64% in thickness, cooling with water to a temperature of 540-580 ° C ensures deformation thermal cycling and forming the optimal fine-grained pearlitic microstructure of steel. Due to this, an increase in toughness and crack resistance at negative temperatures is achieved.

Using the proposed method will provide an increase in the profitability of the production of strips for welded spiral seam pipes for northern oil pipelines by 10-15%

Information sources

1. Japanese application No. 61-163210, IPC C21D 8/00, 1986

2. Japanese application No. 61-223125, IPC C21D 8/02, C22C 38/54, 1986

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

table 2 The chemical composition of low alloy steels Composition number The content of chemical elements, wt.% FROM Mn Si Al Nb Ti Ca V Cr Ni Cu R S N

Fe one.
2.
3.
four.
5.
6.
7. 0,100
0,110
0,120
0.113
0.140
0.150
0.160 1.20
1.30
1.48
1.65
1.30
1.70
1,60 0.3
0.4
0.5
0.6
0.4
0.7
0.6 0.01
0.02
0,03
0.05
0,03
0.06
0.04 0.009
0.010
0,025
0,040
0,030
0,050
0,040 0.009
0.010
0,020
0,030
0,020
0,040
- 0,0009
0.0010
0.0030
0.0050
0.0020
0.0060
0.012 0.004
0.005
0.007
0.010
0.008
0.011
0.010 0.12
0.14
0.17
0.20
0.10
0.23
0.25 0.11
0.12
0.14
0.20
0.15
0.25
0.27 0.10
0.13
0.15
0.20
0.10
0.30
0.30 0.011
0.012
0.013
0.015
0.014
0.016
0.014 0.001
0.002
0.004
0.006
0.005
0.007
0.005 0.005
0.006
0.008
0.010
0.007
0.011
0.009 0.183
0.204
0.230
0.240
0.232
0.290
0.293 Rest
-: -
-: -
-: -
-: -
-: -
-: -

Table 3 Modes of production of strips of low alloy steel and their effectiveness Option No. Composition number Rolling modes Mechanical properties Crack resistance T _np ° s ε _Σ, ° C T _kp ° s T _eat ° C σ _in , N / mm ² σ _t N / mm ² δ ₅ ,% _in σ / σ _m KCV ^-10 , J / cm ² KV ^-20 J / cm ² KCU ^-40 J / cm ² one.
2.
3.
four.
5.
6.
7. 6.
2.
3.
four.
5.
one.
7. 930
940
950
960
970
980
970 62
64
72
75
80
68
80 790
800
825
840
855
860
880 530
540
560
570
580
590
660 650
520
550
600
620
500
510 500
405
413
462
465
455
400 19
32
33
33
32
26
25 0.77
0.78
0.75
0.77
0.75
0.91
0.78 96
108
110
103
109
92
78 24
32
35
thirty
thirty
25
22 47
72
78
71
75
49
51 unsatisfied.
sat.
sat.
sat.
sat.
unsatisfied.
unsatisfied.

Claims (2)

1. A method of manufacturing strips of low alloy steel, including heating slabs, rough rolling to an intermediate thickness, continuous finishing rolling with a regulated temperature of the end of rolling, water cooling, characterized in that the steel has the following ratio of components, wt.%:
carbon 0.11-0.14 manganese 1.3-1.65 silicon 0.40-0.60 aluminum 0.02-0.05 niobium 0.01-0.04 titanium 0.01-0.03 calcium 0.001-0.005 vanadium no more than 0,01 chromium no more than 0.2 nickel no more than 0.2 copper no more than 0.2 phosphorus no more than 0.015 sulfur no more than 0,006 nitrogen no more than 0,010 iron rest,
the temperature of the beginning of the finish rolling is maintained no higher than 970 ° C, the end of the rolling is 800-855 ° C, and water cooling is carried out to a temperature of 540-580 ° C. 2. The method according to claim 1, characterized in that the total content of carbon, manganese, chromium, copper, silicon, nickel, vanadium in the steel must satisfy the ratio: [C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + V / 10] ≤0.24%, and finishing strips are rolled with a total relative compression of at least 64% in thickness.