RU2578618C1 - Manufacturing method of strips of low-alloyed weld steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to low alloyed steel used for making welded oil and gas pipelines, metal structures, suitable for use in far North conditions, for construction of marine structures and structures operated in aggressive media. Production of steel containing, WT%: carbon 0.05-0.13, Silicon 0.10-0.40, manganese 0.30-0.70, vanadium 0.04-0.12, niobium ≤0.01, aluminium 0.02-0.06, titanium ≤ 0.01, molybdenum ≤ 0.25, nitrogen ≤ 0.008, chrome 0.30-1.0, nickel ≤ 0.30, copper ≤ 0.30, sulphur ≤ 0.004, phosphorus ≤ 0.018, calcium 0.001-0.006, iron and impurities - the rest. Produced steel is subject to off-furnace treatment, vacuum, continuous casting into slabs and hot rolling. Hot rolling of strips is carried out in roughing stands to roll thickness not less than 4.3 units from nominal thickness of finished strip at rolling end temperature determined by expression RFT black = Ar₃(calc) + C×2350, [°C], and finishing stands at rolling end temperature determined by formula RFT = (960-806×C)±15, [°C], where C is the mass fraction of carbon in WT%. Strip is cooled at collecting roller table at a rate of not less than 10 °C/s and wound at temperature in range 530÷600 °C to obtain in the band banding of not more than 2 points and structure with the number of not more than 9 points without inclusions based on niobium and titanium carbonitrides particles.

EFFECT: higher corrosion resistance, cold resistance and yield ratio of hot-rolled strip.

3 cl, 3 tbl, 1 ex

Description

The invention relates to metallurgy, in particular to low alloy steels used for the manufacture of welded structures, oil and gas pipelines, suitable for operation in the Far North.

Hot-rolled low-alloy welded steel for main oil and gas pipelines operating in the Far North should combine high strength, ductility, corrosion resistance and cold resistance (table 1).

A known method of hot rolling strips of steels with carbonitride hardening, including heating the slabs to a temperature of 1100-1250 ° C and holding at this temperature for 3-5 hours, rough rolling with a total compression of at least 80% and an end temperature of 1020-1090 ° C , finish rolling with a total compression of at least 70% and an end temperature of 820-870 ° C, cooling of the strips with water to a temperature of 550-620 ° C and winding into rolls (RF Patent 2195505, IPC C21D 8/04, C22C 38/12, 27.12 .2002).

The disadvantage of the above production method is that in the structure of metal, mainly in the axial zone, large inclusions are formed based on niobium and titanium carbonitrides, which create local areas that differ in microstructure and properties from areas that are not adjacent to the axial area, which negatively affects the corrosion properties of rolled products, reducing them.

The closest in technical essence and the achieved results to the proposed invention is a method for the production of strips for oil and gas pipes, including steel smelting, continuous casting into slabs, heating slabs to a temperature of 1190-1250 ° C, hot rolling with an end temperature of 820-870 ° C, water cooling to a temperature of 500-580 ° C and strip winding into rolls, characterized in that the coiled coils are cooled at a speed of 5-20 ° C / h to a temperature not exceeding 100 ° C. Steel is melted containing 0.08-0.13% C, 0.50-0.70% Mn, 0.40-0.65% Si, 0.05-0.09% V, 0.015-0.040% Nb, 0.01-0.03% Ti, 0.02-0.05% Al, N <0.008%, Cr <0.3%, Ni <0.3%, Cu≤0.2%, S≤0.005% , P≤0.015%, Fe - the rest, under the following conditions: C _e = C + Mn / 6 + (Cr + V + Ti) / 5 + (Cu + Ni) / 15≤0.39%, while P _cm = C + (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 15 + V / 10≤0.24% (RF Patent 2348703, IPC C21D 8/04, C22C 38/42, C22C 38/46, 10/10/2008).

The disadvantage of this production method is the reduction of the corrosion properties of the rolled products due to the formation of large inclusions based on niobium and titanium carbonitrides mainly in the axial zone of the rolled products, while the yield on corrosion properties does not exceed 70%.

The technical result of the invention is to increase the corrosion resistance, cold resistance and yield of hot-rolled strip products.

The technical result is achieved by the fact that in the method for the production of strips of low alloy welded steel, including steelmaking, continuous casting into slabs, heating and hot rolling, according to the invention, steel of the following chemical composition is melted, wt. %: carbon 0.05-0.13, silicon 0.10-0.40, manganese 0.30-0.70, vanadium 0.04-0.12, niobium not more than 0.01, aluminum 0.02- 0.06, titanium no more than 0.01, molybdenum no more than 0.25, nitrogen no more than 0.008, chromium 0.3-1.0, nickel no more than 0.30, copper no more than 0.30, sulfur no more than 0.004 , phosphorus no more than 0.018, calcium 0.001-0.006, iron and impurities the rest, the steel is subjected to out-of-furnace treatment and evacuation to ensure the mass fraction of hydrogen and oxygen impurities is not more than 2 and 25 ppm, respectively, the score of non-metallic inclusions is not more than 2.5 in average value and not more than 3.0 at the maximum value and total content of impurities of arsenic, lead, zinc, tin, antimony, bismuth is not more than 0.020%, hot rolling of the strips is carried out in the rough group of stands to a thickness of rolling not less than 4.3 times the nominal thickness of the finished strip at the temperature of the end of rolling, determined by the expression Tkp black = Ar ₃ (calc) + C × 2350, [° C] and the finishing group of the stands at the temperature of the end of rolling, determined by the expression Tk.p. = (960-806 × C) ± 15, [° C], where C - mass fraction of carbon in%, cool the strip on the discharge roller table at a speed of at least 10 ° C / s and wind it at a temperature in the range of 530–600 ° C with obtaining in the banding band no more than 2 points and structures with a grain point no more than 9 points without inclusions based on particles of niobium and titanium carbonitrides. In this case, the following relations should be fulfilled:

Ce = C + Mn / 6 + (Cr + V + Nb + Ti) / 5 + (Ni + Cu) / 15≤0.43%,

Pcm = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + V / 10 + Mo / 15≤0.26%,

where Ce is the carbon equivalent,%;

Pcm - crack resistance coefficient,%;

Ca / S≥1,

in addition, during smelting, steel is treated with silicocalcium and / or ferrocerium.

The essence of the invention consists in the following: a high complex of corrosion characteristics and a yield of more than 95% is ensured by increasing the purity of steel by harmful impurities and non-metallic inclusions, optimizing the chemical composition, temperature conditions of rolling and structure.

In the proposed production method, the purity of harmful impurities, non-metallic inclusions and gases is provided by the method of smelting, after-treatment, vacuumization and casting of steel.

The removal of hydrogen from the metal is necessary to eliminate this type of steel failure, such as hydrogen-initiated cracking of steel (HIC). Hydrogen cracking HIC is manifested in the form of internal cracking, the formation of blisters in the absence of an external load, the presence of which during pipe production or pipeline operation is an integral part of its operation. In the volume of the metal, hydrogen fills the "traps" - areas in which hydrogen atoms have reduced free energy - these are substitution atoms, interstitial atoms, dislocation clusters, interphase and grain boundaries, structural imperfections, pore volume and micro-discontinuities. Traps that have high binding energy with hydrogen atoms capture them in large quantities; on them, hydrogen goes into molecular form, thereby creating internal gas pressure. As a result, a crack nucleus appears, surrounded by strong stress fields, hydrogen diffuses from these solid solutions or from weak traps. A region of high-pressure molecular hydrogen is formed, which leads to further development of the crack; therefore, its residual amount after the evacuation operation is limited to a value of not more than 2 ppm.

The limitation of the mass fraction of oxygen not more than 25 ppm is due to the minimization of its negative effect on the final properties of steel due to the formation of an extensive group of non-metallic inclusions, the points of which are also subject to normalization (not more than 3.0 points for the maximum value and not more than 2.5 points for the average) . It was experimentally established that accumulations of non-metallic inclusions larger than 3 points in the form of phosphides, sulfides, nitrides, hydrides lead to the destruction of samples during corrosion tests, which is unacceptable. In addition, non-metallic inclusions larger than 3 points reduce the cold resistance of steel, reduce the yield of hot-rolled strips.

The greatest danger is accumulations of flat and elongated manganese sulfides and oxides. To modify non-metallic inclusions, steel is treated with rare-earth metals, for example, cerium and / or silicocalcium.

In addition to contamination by non-metallic inclusions, the contamination of steel by impurity elements of non-ferrous metals has a negative effect on the resistance to hydrogen cracking and hydrogen sulfide corrosion. It was established that the majority of failures due to stress corrosion occurred on pipes made of a sheet of increased contamination with non-ferrous metals (Sb, Sn, Pb, Zn, etc.). The effect of impurity elements on the resistance of pipe steels is realized in two ways. First, some impurities form segregations in ferrite, which are most often located at grain boundaries. Secondly, intragranular concentration inhomogeneities may occur. Such concentration inhomogeneities cause local changes in the corrosion rate. Impurity elements can have an indirect effect on the resistance of steel to stress corrosion cracking by changing the characteristics of toughness and ductility of ferrite. The total content of arsenic, lead, zinc, tin, antimony and bismuth is not more than 0.020% is achieved through the use of pure charge in steelmaking.

The minimum degree of deformation in the finishing group of stands should be at least 4.3 times the nominal thickness of the finished product. With a decrease in the strain rate, the study of the structure of the roll significantly worsens, the uniformity of the microstructure along the strip thickness changes, which negatively affects the whole complex of mechanical and corrosion characteristics.

The optimum temperatures of the end of rough and finish rolling are assigned on the basis of empirical formulas, taking into account the influence of the chemical composition:

where C, Mn, Si, Cr, Ni, V, Ti, Al, Nb, W, S, P, N, B, Mo - mass fraction of carbon, manganese, silicon, chromium, nickel, vanadium, titanium, aluminum, niobium , tungsten, sulfur, phosphorus, nitrogen, boron, molybdenum,%.

The temperature thus calculated for the end of the finish rolling, depending on the mass fraction of carbon, provides the complete recrystallization process, which is necessary for the formation of a uniform equiaxed austenite grain.

The temperature of the completion of rolling in the finishing group of stands also depends on the chemical composition of the steel. This allows you to get more stable indicators of the microstructure in the rental and reduce the variation in the mechanical characteristics of metal. Then the strip is cooled on the discharge roller table at a speed of at least 10 ° C / s, obtaining a winding temperature in the range of 530 ÷ 600 ° C, which, in combination with the calculated finish temperature of finishing rolling, allows you to start accelerated cooling from the same structural state of the strip with minimized the influence of chemical composition. The use of accelerated post-deformation cooling at the indicated speed also prevents the appearance of coarse banding of a ferrite-pearlite structure (no more than 2 points) and provides a structure with grain no larger than 9 points.

When the structure of the steel contains a fine-grained structurally free ferrite with a grain number of at least 9 points with a structural banding of not more than 2 points, there is an additional increase in the resistance of steel against local corrosion and cold resistance. A decrease in the number of ferrite grains of less than 9 points, as well as an increase in structural banding of more than 2 points, worsen the corrosion resistance and cold resistance of steel.

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 0.13% impairs the ductility and toughness of the steel.

Silicon deoxidizes and strengthens the steel, increases its elastic properties. When the silicon content is less than 0.10%, the strength of the steel is insufficient. An increase in the silicon content of more than 0.40% leads to an increase in the number of silicate non-metallic inclusions, embrittlement of steel, and worsens its ductility.

Manganese is introduced to increase the strength of steel, the binding of impurity sulfur to sulfides. When the manganese content is less than 0.30%, the strength of steel and viscosity at low temperatures are reduced, which leads to an increase in rejection. An increase in the concentration of manganese in excess of 0.70% affects the ductility of steel, reduces cold resistance and increases the ratio of σ _t / σ _in more than 0.87.

Vanadium forms carbides (VC) with carbon, and nitrides (VN) with nitrogen. Small vanadium nitrides and carbonitrides are located along grain and subgrain boundaries, inhibit the movement of dislocations, and thereby strengthen steel. With a vanadium content of less than 0.04%, the properties of the steel are below an acceptable level. An increase in the concentration of vanadium over 0.12% causes the precipitation hardening of the rolled products and leads to its precipitation at the grain boundaries in the form of intermetallic compounds. This affects the properties and reduces the yield of hot rolled strips.

The niobium content is limited to no more than 0.01%, since when doped with niobium and its content is more than 0.01%, inclusions based on niobium carbides (NbC) and niobium nitrides (NbN) are formed in the axial zone, which creates local areas with increased stress and negative affects the corrosion resistance of metal.

Aluminum within the range of 0.02-0.06% deoxidizes steel, grinds grain, binds nitrogen at high temperatures, i.e. prevents the reduction of toughness. When the aluminum content is less than 0.02%, its effect is weak, steel has low mechanical properties. An increase in the aluminum content of more than 0.06% leads to an increased content of non-metallic inclusions, which leads to the formation of defects during welding.

карбонитрида титана в 2,0-2,5 раза отличаются от соответствующих характеристик низколегированной стали, и изменение температуры или внешних нагрузок приводит к возникновению значительных микродеформаций, раскалывающих включение или отслаивающих его от матрицы, при коррозионном растрескивании под напряжением (КРН) зарождение трещин происходит в первую очередь на неметаллических включениях Ti(CN).The titanium content in steel is limited to 0.015% in order to avoid the formation of accumulations of titanium carbonitrides Ti (CN) in the axial rolling zone, having the form of flat plates (films) with sharp edges. The modulus of normal elasticity E and the coefficient of thermal expansion

titanium carbonitride differs by a factor of 2.0-2.5 from the corresponding characteristics of low alloy steel, and a change in temperature or external loads leads to the occurrence of significant microdeformations, splitting the inclusion or delaminating it from the matrix, during stress corrosion cracking (SCC) cracking occurs in primarily on non-metallic inclusions Ti (CN).

Molybdenum is a highly effective modifier that increases the strength and toughness of steel, increases corrosion resistance and increases local corrosion resistance (local corrosion exists in any pipeline in which there are components for oxygen, carbon dioxide or hydrogen sulfide corrosion). An increase in its content of more than 0.25% impairs plasticity and leads to an overuse of alloying elements.

Nitrogen is a carbonitride forming element that strengthens steel. However, an increase in nitrogen concentration in excess of 0.008% leads to a decrease in the viscosity properties at low temperatures, which is unacceptable.

Chromium reduces the corrosion rate, increases strength due to the release of the secondary phase during tempering of steel, as a hardener it is less effective than vanadium. The chromium content is limited to not more than 1.0%.

Nickel and copper increase the strength and resistance to pitting corrosion, but when the content of Ni and Cu is more than 0.30%, there is a decrease in the cold resistance of steel at low temperatures.

Sulfur is a harmful impurity that reduces the plastic and viscous properties. At a sulfur concentration of not more than 0.005%, its harmful effect is weak and does not lead to a noticeable decrease in the mechanical properties of steel. At the same time, deeper sulfur removal makes steel more expensive and reduces the economic performance of production.

-железе, что ведет к упрочнению металлической матрицы. Однако увеличение содержания фосфора более 0,018% вызывает охрупчивание стали и снижение хладостойкости.Phosphorus in an amount of not more than 0.018% is completely dissolved in

- iron, which leads to hardening of the metal matrix. However, an increase in the phosphorus content of more than 0.018% causes embrittlement of steel and a decrease in cold resistance.

Calcium provides refinement of grain boundaries of the microstructure of steel. Acting as a surfactant, it cleans grain boundaries from unwanted impurities, thereby achieving a simultaneous increase in toughness at low temperatures and the corrosion resistance of steel. With a decrease in calcium content of less than 0.001%, its positive effect is weak. An increase in calcium content in excess of 0.006% leads to an increase in the number of non-metallic inclusions, which negatively affects the mechanical properties of hot-rolled products.

Additionally, a carbon equivalent limit is introduced - not more than 0.43%.

Ce = C + Mn / 6 + (Cr + Mo + V + Nb + Ti) / 5 + (Ni + Cu) / 15, where

Ce is the carbon equivalent,%;

C is the mass fraction of carbon,%;

Mn — mass fraction of manganese,%;

Cr is the mass fraction of chromium,%;

Mo is the mass fraction of molybdenum,%;

V is the mass fraction of vanadium,%;

Ni — mass fraction of nickel,%;

Nb is the mass fraction of niobium,%;

Cu is the mass fraction of copper,%;

6, 5, 15 - empirical coefficients.

In this case, the sum of V + Nb + Ti should be no more than 0.15%.

Steel with a carbon equivalent of not more than 0.43% has good weldability. With a carbon equivalent of more than 0.43%, the ability of steel to weld is reduced, because the tendency of the weld metal to hardening when it is cooled increases and provokes the receipt of various properties in the heat-affected zone and the base metal. It should be noted that before welding metal with a carbon equivalent of more than 0.45%, heating is required to avoid cracking, which leads to an increase in cost and complexity of the process.

The limitation of the coefficient of crack resistance not more than 0.26% characterizes the resistance of steel to cracking under the action of mechanical stress caused by the pressure in the pipeline of the pumped products containing aggressive components. In cases where there is an unfavorable combination of concentrations of steel components, i.e. if Pcm = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + V / 10 + Mo / 15 + 5B≥0.26%, the steel has a low resistance to cracking, which reduces the operational resistance of the pipeline.

The Ca / S≥1 ratio is an indicator of the quality of steel processing; when the ratio Ca / S≤1, the modification of non-metallic inclusions was not carried out in full, while the indicators of the resistance of metal to hydrogen cracking are reduced.

Steel modification with cerium is aimed at reducing the harmful effects of manganese sulfides with their replacement with cerium and calcium sulfides. The introduction of cerium has a positive effect on increasing the level of toughness at low temperatures and increasing corrosion characteristics.

Implementation example

Steel was smelted in an oxygen converter, deoxidized with ferromanganese, ferrosilicon, alloyed with ferrovanadium, metallic aluminum and silico-calcium were introduced. Desulfurization and dephosphorization of the melt and purging with argon were carried out.

The chemical composition of steels with different contents of alloying elements and impurities is shown in table 2.

The steel was subjected to continuous casting into slabs and hot rolling on a continuous broadband mill 2000 into strips 9.0 mm thick with a temperature of the end of rolling T _kn = 880 ° C, after which it was cooled with water to a temperature of T _cm = 590 ° C and rolled up.

Table 3 shows the various modes of production of hot rolled strips, mechanical properties and yield.

As follows from tables 2 and 3, when implementing the proposed method, hot-rolled strips (compositions No. 2-4) have increased corrosion resistance, cold resistance (impact strength at low temperatures). Due to this, the maximum yield of hot rolled strip is achieved.

In cases of transcendental values of the declared parameters (compositions No. 1 and 6), as well as when using the prototype method (composition No. 7), the corrosion resistance and cold resistance in the hot rolled steel deteriorate.

The described production technology provides for obtaining a fine-grained uniform microstructure having a ferrite grain score of no larger than 9.

Using the proposed method for the production of strips of low alloy welded steel provides a strength class of metal rolling of at least K52, the level of cold resistance is ensured up to -50 ° C, tests are carried out on KCV samples, requirements for corrosion resistance: CLR≤6%, CTR≤3%.

The prototype steel was selected as the base object in assessing the technical and economic efficiency of the proposed steel.

Using the claimed method will increase the life of pipelines up to 30%, makes it possible to use metal for the manufacture of metal structures operating in aggressive environments.

Literature used in the preparation of the description of the invention

1. RF patent 2195505, IPC C21D 8/04, C22C 38/12, 12/27/2002

2. RF patent 2348703, IPC C21D 8/04, C22C 38/42, C22C 38/46, 03/10/2009.

Claims (3)

1. A method for the production of strips of low alloy welded steel, including steelmaking, continuous casting into slabs, heating and hot rolling, characterized in that the steel is melted with the following chemical composition, wt.%: Carbon 0.05-0.13, silicon 0, 10-0.40, manganese 0.30-0.70, vanadium 0.04-0.12, niobium not more than 0.01, aluminum 0.02-0.06, titanium not more than 0.01, molybdenum not more than 0.25, nitrogen not more than 0.008, chromium 0.3-1.0, nickel not more than 0.30, copper not more than 0.30, sulfur not more than 0.004, phosphorus not more than 0.018, calcium 0.001-0.006, iron and impurities the rest is subjected to melted steel out of furnace processing and evacuation to ensure the mass fraction of impurities of hydrogen and oxygen is not more than 2 and 25 ppm, respectively, the point of non-metallic inclusions is not more than 2.5 in average and not more than 3.0 in maximum value and total content of impurities of arsenic, lead, zinc tin, antimony, bismuth is not more than 0.020%, hot rolling of the strips is carried out in the roughing group of stands to a thickness of rolling not less than 4.3 times the nominal thickness of the finished strip at the temperature of the end of rolling, determined by the expression Tkp black. = Ar ₃ (calculation) + C × 2350, [° C] and number group of stands at the end of rolling temperature, determined by the expression Tk. ° C / sec and wound at a temperature in the range of 530 ÷ 600 ° C with obtaining in the strip band not more than 2 points and structures with a grain point of not more than 9 points without inclusions based on particles of niobium and titanium carbonitrides. 2. The method according to p. 1, characterized in that the steel is melted, in which the contents of carbon, manganese, chromium, vanadium, titanium, copper, nickel, silicon, molybdenum, calcium and sulfur satisfy the following ratios:
Ce = C + Mn / 6 + (Cr + V + Nb + Ti) / 5 + (Ni + Cu) / 15≤0.43%,
Pcm = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + V / 10 + Mo / 15≤0.26%,
where Ce is the carbon equivalent,%;
Pcm - crack resistance coefficient,%;
Ca / S≥1. 3. The method according to p. 1, characterized in that during the smelting of steel is treated with silicocalcium and / or ferrocerium.