RU2746599C1 - Sparingly alloyed cold-resistant high-strength steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of metallurgy, namely to high-strength cold-resistant steels, and can be used in the production of pressure vessels used for storage and transportation of compressed gases in a wide range of temperatures, including those operated at ambient temperatures from minus 50°C to plus 60°C. Steel contains components in the following ratio, wt%: carbon - 0.27-0.30, silicon - 0.90-1.20, manganese - 1.0-1.30, chromium - 0.90-1.20, nickel – 1.45-2.20, copper - 0.40-0.70, molybdenum - 0.25-0.40, titanium carbonitride - 0.03-0.10, zirconium carbonitride - 0.03-0.10, cerium - 0.001-0.02 , vanadium - 0.05-0.08, aluminum - 0.005-0.02, calcium - 0.005-0.01, titanium - 0.005-0.035, niobium - 0.005-0.035, barium - 0.005-0.025, if necessary, at least one of: gadolinium – 0.0008-0.0015, nitrogen - 0.005-0.012 and an element from the group containing lanthanum, yttrium, neodymium or mixtures thereof - 0.001-0.02, the rest is iron and impurities. The total content of low-melting impurities of lead, bismuth, tin, antimony and arsenic does not exceed 0.02, wt%, and the content of inevitable impurities of sulfur, phosphorus, oxygen and hydrogen does not exceed, wt%: sulfur ≤0.008, phosphorus ≤0.008, oxygen ≤0.005 and hydrogen ≤0.0005. The ratio of σ_0,2/σ_h is less than 0.90.

EFFECT: high level of mechanical properties and stability of characteristics, including strength, ductility and toughness.

3 cl

Description

The invention relates to metallurgy, in particular, to high-strength cold-resistant steels and can be used in the production of pressure vessels used for storage and transportation of compressed gases in a wide range of temperatures, including those operated at ambient temperatures from minus 50 ° C to plus 60 ° C.

In accordance with the requirements of GOST 12247-80 for pressure vessels, in particular, for cylinders with a volume of 1000 liters, with an outer diameter of 600 mm, a body length at an operating pressure of 31.4 MPa - 4850 mm, at an operating pressure of 39.2 MPa - 5050 mm and with a wall thickness of 25.4 mm and 31.1 mm, respectively, the mechanical properties of steel must correspond to the following data: σw = 883 MPa (90 kgf / mm ² ), σ _0.2 = 687 MPa (70 kgf / mm ² ), δ≥ 12%, KCU ₊₂₀ ≥49 J / cm ² , KCU _-50 ≥29.4 J / cm ² , HB = 269-341.

Known economically alloyed high-strength cold-resistant steel, which contains carbon, silicon manganese, chromium, copper, nickel., Molybdenum, vanadium, sulfur, phosphorus and iron and inevitable impurities, characterized in that it additionally contains cerium and boron, with the following ratio of components, wt ... %: carbon 0.23-0.27, silicon ≤0.30, manganese 0.30-0.60, chromium 0.90-1.15, nickel 2.40-2.80, molybdenum 0.40-0 , 50, vanadium 0.12-0.16, cerium 0.001-0.005, boron 0.0001-0.0010, sulfur ≤0.010, phosphorus ≤0.012, copper ≤0.10, iron and inevitable impurities - the rest (RU 2680557, C22 C 38/54, 02.22.2019).

Steel after heat treatment has _a tensile strength σ in the range 1128-1275 MPa, a yield stress σ _0,2 981-1128 MPa with an elongation of at least 13% and toughness at minus 50 ° C is not less than 39 J / cm ² at testing of specimens with a sharp notch.

The main disadvantage of steel with the specified alloying after tempering to a given level of strength, the steel has an increased tendency to brittle fractures during operation, which is assessed by the criterion of the ratio of the yield strength to the ultimate strength. According to A.G. Gumerov. the ratio of the yield strength to the tensile strength for high-strength alloy steels should not exceed 0.90, and for the prototype steel it is higher than 0.90.

(Gumerov A.G. Overhaul of underground oil pipelines. PDF format, 1999).

The specified alloying complex for steel does not provide sufficient resistance to brittle fracture at low temperatures down to minus 50 ° C, especially during long-term operation.

Known steel grade 30XN2MFA according to GOST 4543, containing, wt%: carbon 0.27-0.34, manganese 0.30-0.60, silicon 0.17-0.37, sulfur <0.025, phosphorus <0.025, chromium 0.60-0.90, nickel 2.0-2.4, molybdenum 0.20-0.30, vanadium 0.10-0.18, copper <0.30, iron - the rest. This composition of alloying elements after quenching and tempering at a temperature of 600 ° C provides good plasticity - relative elongation and relative contraction of more than 19% and 62%, respectively, but has at σ _0.2 = 830 MPa; σ _in = 1040 MPa low viscosity at a low temperature (-50 ° C), even on samples with a round notch: KCU ^-40 = 41 J / cm ² , KCU ^-60 = 35 J / cm ² , which does not guarantee sufficient resistance material of pressure vessels to brittle destruction during operation in northern latitudes.

Known steel XH654 (Germany), containing components in the following ratio, wt. %: carbon 0.25-0.35%; silicon 0.10-0.40%; manganese 0.40-0.70%; chromium 1.25-1.65%; nickel 1.45-1.75%; molybdenum 0.35-0.50%; vanadium 0.05-0.15%; sulfur up to 0.015%; phosphorus up to 0.020%; iron is the rest. After improvement, the material of the sheet provides an excessively high level of strength - up to σ _in = 1380 MPa. With a limited nickel content and a specified level of strength, this steel has an increased tendency to brittle fracture at negative operating temperatures.

The closest in technical essence and the achieved result is economically alloyed high-strength steel 30HGSN2A according to GOST 4543, containing, by weight. %: carbon 0.27-0.34; manganese 1.00-1.30; silicon 0.90-1.20; sulfur <0.025; phosphorus <0.025; chromium 0.90-1.20; nickel 1.40-1.80; copper <0.30 iron-balance.

The composition of alloying elements after hardening 900 ° C and tempering at a temperature of 600 ° C provides good ductility - elongation and reduction of area greater than 20% and 55%, respectively, has a strength (σ _0,2 = 900 MPa; σ _in = 1000 MPa) , which meets the requirements of GOST 12247-80, but has low viscosity at a low temperature (-50 ° C) even on samples with a round notch, which does not guarantee sufficient resistance of the pressure vessel material to brittle fracture during operation in northern latitudes.

The specified alloying complex does not have sufficient resistance to brittle fracture at low temperatures down to minus 50 ° C. Another disadvantage of steel 30HGSN2A is its tendency to temper brittleness during tempering.

The objective and technical result of the invention is the development of an economically alloyed cold-resistant steel of high strength and high cold resistance at temperatures up to minus 50 ° C, which allows using the developed steel for the manufacture of high-pressure vessels operated in the temperature range from plus 60 to minus 50 ° C.

The technical result is achieved in that an economically alloyed cold-resistant high-strength steel containing carbon, silicon, manganese, chromium, nickel, copper, molybdenum, cerium, vanadium, iron and impurities, characterized in that it additionally contains titanium carbonitride, zirconium carbonitride with a particle size of 30 -65 nm, aluminum, calcium, titanium, niobium, barium and, if necessary, at least one element selected from the group containing gadolinium, nitrogen and an element from the group containing lanthanum, yttrium, neodymium or mixtures thereof, in the following ratio components, wt%:

carbon 0.27-0.30 silicon 0.90-1.20 manganese 1.0-1.30 chromium 0.90-1.20 nickel 1.45-2.20 copper 0.40-0.70 molybdenum 0.25-0.40 titanium carbonitride 0.03-0.10 zirconium carbonitride 0.03-0.10 cerium 0.001-0.02 vanadium 0.05-0.08 aluminum 0.005-0.02 calcium 0.005-0.01 titanium 0.005-0.035 niobium 0.005-0.035 barium 0.005-0.025 if necessary, at least one of gadolinium 0.0008-0.0015 nitrogen 0.005-0.012 and element from the group containing lanthanum, yttrium, neodymium, or mixtures thereof 0.001-0.02 iron and impurities rest.

The technical result is also achieved in that the total content of impurities of low-melting metals - lead, bismuth, tin, antimony and arsenic, does not exceed 0.02 wt.%, And the content of inevitable impurities of sulfur, phosphorus, oxygen and hydrogen does not exceed, wt. %: sulfur ≤0.008; phosphorus ≤0.008, oxygen ≤0.005 and hydrogen ≤0.0005.

The technical result is also achieved in that the criterion ratio σ _0,2 / σ _in ≤0,90.

The carbon content in the selected range (0.27-0.30 wt.%) Provides the required level of strength, while an increase in weldability and cold resistance is achieved. An increase in the carbon content above 0.30 wt.% Causes a significant increase in strength, which will negatively affect a decrease in toughness and ductility.

Silicon, along with manganese and aluminum, is the main deoxidizer of steel, and is also present as an alloying element. It removes oxygen from the metal, and also slightly increases its strength and corrosion resistance. Its addition helps to remove oxygen dissolved in them from molten metals, which is a harmful impurity that degrades the mechanical properties of the metal. Silicon significantly increases the yield strength and strength of steel. Silicon content 0.90-1.20 wt. % is optimal. Content above 1.20 wt% negatively affects the toughness properties of the cold-resistant steel. Due to the low cost of silicon, its use as an alloying element is expedient.

Manganese strengthens cold-resistant steel, increases hardenability and can help reduce nickel content. When the manganese content is more than 0.60 wt%, the complex of viscoplastic properties of steel decreases. For this steel, the manganese content of 0.30-0.60 wt% is optimal and does not impair the cold resistance characteristics.

Chromium in the accepted range of 0.90-1.20 wt.%, Required to ensure the hardenability of steel in sections up to 70 mm and some hardening of the steel due to solid solution hardening. At the same time, the characteristics of cold resistance are not deteriorated.

Additives of chromium 0.90-1.20 wt.% In cold-resistant steel containing nickel during heat treatment from the intercritical interval stabilize the reverse transformation austenite to low temperatures, which improves plasticity and toughness at low temperatures.

Nickel is one of the few elements that simultaneously improves both the strength and toughness properties of cold-resistant steel. The minimum nickel content of 1.45 wt% is established based on the reliable operation of parts made of cold-resistant steel at an operating temperature of -50 ° C, and the maximum nickel content of 2.20 wt% reliably ensures the operating temperature of steel for all ranges of thicknesses of pipe billets. Increasing the nickel content above 2.20 wt% is not economically feasible.

Copper at a content of 0.40-0.70 wt% effectively improves the strength properties and hardenability of steel. When the copper content is within the selected limits, there is no negative effect on the toughness and ductility, and copper effectively improves the weather resistance of steel. The combined content of nickel and copper ensures high cold resistance. Steel containing copper is hardened by precipitation of e-copper. An increase in the copper content over 0.70 wt% is unreasonable due to contamination of the lining of furnaces, aggregates and casting ladles, which may require flushing of the used equipment or limit the smelting process to copper-containing steel grades.

Joint alloying with molybdenum, vanadium and niobium within the stated limits most effectively promotes steel hardening due to solid solution and precipitation hardening, as well as improving hardenability. With an increase in the content of molybdenum to 0.40 wt. % increase in yield strength, tensile strength and viscoplastic properties of steel. Molybdenum prevents the development of temper brittleness of steel. A further increase in the molybdenum content for cold-resistant steels is not economically feasible.

Vanadium in cold-resistant steels containing nickel is an effective dispersion hardener, but this is realized only with full heat treatment.

Niobium is an active carbide former and is almost entirely present in steel in the form of MC carbides, which provide grain boundary control and, in the presence of nickel, also effective precipitation hardening. In addition, niobium provides a favorable steel structure after hot deformation.

The dissolution temperature of niobium carbides in austenite is 50-70 ° C higher than that of vanadium carbides, as a result of which niobium carbides restrict the growth of austenite grains, and vanadium carbides released during tempering promote steel hardening. Thus, solid solution, grain boundary and dispersion hardening and grain refinement are simultaneously provided due to the introduction of niobium. All of this is an effective way to simultaneously increase the strength, low-temperature toughness and ductility of steel. The lower limit of the niobium content in steel is 0.005 wt%, since in such amounts it is practically inevitably present in charge materials, and it is advisable to limit the maximum niobium content to 0.035 wt%, since at a higher niobium content, eutectic NbC carbides are formed, which reduces viscoplastic properties of steel.

Titanium, even in small amounts, forms carbonitrides that are practically insoluble at temperatures of hot deformation and heat treatment, effectively controlling the grain boundaries. In addition, these elements, together with rare earth metals, are part of complex oxysulfides, which are small in size and have favorable morphology. At a titanium content of more than 0.035 wt%, the size of the Me (C, N) carbonitrides can noticeably increase, which can lead to a deterioration in the properties of the steel, especially the impact toughness and ductility. The minimum titanium content is 0.005 wt%.

Rare earth metals (REM), on the one hand, actively interact with oxygen, nitrogen, sulfur and other elements, forming non-metallic inclusions of favorable morphology, and on the other hand, they accumulate at grain boundaries, improving intergranular cohesion. Lanthanum, cerium, neodymium, praseodymium, also scandium, yttrium and their mixtures can be used as rare earth metals. Rare earth metals have a positive effect at a minimum content of 0.0005 wt.%, And an increase in their content of more than 0.04 wt.% Does not lead to a noticeable improvement in the properties of steel. Therefore, the optimal content of rare-earth metals or their total mixture is 0.005-0.04 wt.%.

Aluminum is used in steel as a deoxidizer and, with its content of 0.005-0.02 wt%, ensures complete deoxidation of steel with an insignificant content of aluminum oxides and nitrides in steel.

Aluminum in the composition of steel in combination with the reactive elements calcium and cerium favorably changes the shape of non-metallic inclusions, reduces the oxygen and sulfur content in the steel, reduces the amount of sulfide inclusions, cleans and strengthens grain boundaries and refines the steel structure, which leads to an increase in strength, ductility and impact strength. Calcium and cerium also favorably affect the nature of nitride inclusions, promote the transition of film inclusions of aluminum nitrides into globular complexes of oxysulfonitride formations.

Additional introduction of gadolinium in an amount of 0.0008-0.0015 wt.% Provides reactivity to oxygen, nitrogen and hydrogen, sulfur and other harmful impurities in the alloy. As a powerful deoxidizer, degasser and desulfurizer, gadolinium increases alloy density and lowers sulfur content. Strengthens grain boundaries, increases ductility, toughness and corrosion resistance of the alloy. The selected gadolinium content is effective both in terms of affecting the properties of steel and in terms of economic viability.

The combined action of aluminum, calcium, barium and cerium opens up additional possibilities in controlling the structure and properties of steel.

Calcium additives in an amount of 0.005-0.01 wt.% Makes it difficult to isolate excess phases along grain boundaries and contributes to an increase in plasticity and toughness. The joint introduction of calcium and barium into steel significantly improves the kinetics of the process of interaction of calcium with impurities. Barium in an amount of 0.005-0.01 wt% globularizes inclusions to a greater extent than calcium. A significant part of the inclusions becomes rounded. Barium additives contribute (compared to calcium and cerium) to the formation of smaller globules. Modification with calcium and barium refines sulfides and leads to a redistribution of inclusions in the dendritic structure as a result of an increase in sulfide inclusions in the axes.

Calcium, barium are included in the composition of slags and have a desulfurizing and modifying effect. In steel, these components are included in the composition of non-metallic inclusions and, at excessively high contents, can lead to an increase in the contamination of steel with non-metallic inclusions, including corrosive ones. Taking into account the minimum limits for the content of these elements in steel, their total residual content should be limited to 0.015 wt%.

Sulfur and phosphorus are harmful elements that reduce the complex of properties of steel, so their content should be as low as possible. However, deep desulfurization and dephosphorization are quite complex activities. Therefore, it is advisable to set the sulfur content not more than 0.008 wt.% And phosphorus not more than 0.008 wt. %, which are reliably provided by modern methods of steel production and allow maintaining the strength and viscoplastic properties of steel at the required level.

Nitrogen is an inevitable impurity in steel, which is present both in solid solution and in the form of nitrides and carbonitrides, which, with a nitrogen content of more than 0.012 wt. % can have an adverse effect on the complex of properties of steel. Since the minimum attainable nitrogen content in industrial steel is 0.002 wt%, the optimal nitrogen content is 0.002-0.012 wt%.

Oxygen is also inevitably present in the composition of steel in the form of non-metallic inclusions. When its content exceeds 0.005 wt%, the content of nonmetallic inclusions in steel increases, which worsens the properties of the steel and causes their heterogeneity. From the point of view of economic feasibility, the optimal oxygen content is not more than 0.005 wt. %.

Hydrogen is present in steel in a dissolved form and, under certain conditions, can cause the appearance of flakes. For thicknesses and production technology typical for pipe billets, it is advisable to limit the hydrogen content in steel to 2-5 ppm (0.0002-0.0005 wt.%). the optimum content is 0.00001-0.0005 wt%.

Arsenic, tin, lead, antimony and bismuth are impurities that negatively affect the toughness of steel. The effect of arsenic in steel on the properties of steel is similar to the effect of phosphorus, and with a mass fraction of not more than 0.008, arsenic does not have a negative effect on the properties of steel. Tin, lead, antimony and bismuth have a negative effect on the hot and cold ductility of steel during rolling and forging. It is advisable to limit their total content to 0.020 wt%, taking into account the minimum possible content of these elements in charge materials and extremely limited possibilities for their removal. When the total content of these elements is more than 0.02 wt%, there is a decrease in the values of impact toughness and a shift in the temperature of the ductile-brittle transition to the region of higher temperatures.

The introduction into the steel composition of the sum of finely dispersed titanium carbonitrides and zirconium carbonitrides with nanoscale dispersion (with a size of 30-65 nm) allows the formation of a large number of crystallization centers uniformly distributed in the volume of the metal.

Titanium carbonitrides are more rounded and smaller than titanium nitrides. Titanium carbonitrides are distributed relatively evenly in the cast metal; some of these inclusions tend to concentrate in the interbranches of dendrites and in the interdendritic space. In addition, the introduction of nanoparticles of titanium carbonitride and zirconium carbonitride with a size of 30-65 nm into the composition of steel makes it possible to form a large number of crystallization centers evenly distributed in the volume of the metal during solidification of the steel melt.

During the solidification of steel, chemically resistant particles of zirconium carbonitride and titanium carbonitrides being in the melt have increased resistance to dissociation and will be the centers of crystallization of austenite grains, which will significantly grind the primary austenite grain, increase the area of the boundaries of austenite grains, and significantly increase the dispersion of carbides and nitrides of vanadium and niobium falling out along the boundaries of austenite grains, which will provide an increase in strength properties and simultaneously indicators of plasticity and toughness

Experiments have shown that the introduction of modifiers into the melt in an amount of 0.06-0.20% of the melt mass leads to a significant change in both the resulting structure and the morphology and topography of the carbide phase. At the same time, there is a sharp grinding of the grain, the columnarity of the grains and the uneven graininess are eliminated. The dendritic structure of the cast metal is thin and uniform over the section of the ingot.

With a decrease in the amount of carbonitrides less than 0.06 wt.%, The modifying effect is sharply reduced. The boundary concentration of the grain boundary hardening elements decreases and, as a result, the cold resistance of the pipe billet may decrease. With a decrease in the amount of carbonitrides less than 0.06 wt%, an increase in strength properties is not ensured, since sufficient grain refinement and stabilization of grain boundaries are not ensured.

When the content of nanoparticles of titanium carbonitride and zirconium carbonitride in an amount of more than 0.10 wt.% Each, a decrease in the characteristics of plasticity and viscosity occurs, since zirconium carbonitride and titanium carbonitride begin to precipitate in excess amounts.

The presence of titanium and zirconium carbonitrides in an amount of 0.06-0.20 wt% has a barrier effect on the migrating grain boundary. The effectiveness of the action of carbonitrides is determined by the difference in the melting temperatures of carbonitrides and the temperature of the melt, i.e. the higher the difference, the more effective it is. The melting point of titanium carbonitride is 2930 ° C, zirconium carbonitride is 3530 ° C, and the steel melt temperature is about 1590-1630 ° C.

Carbonitride nanopowders 35-65 nm in size easily stick together, and they are also poorly wetted by liquid metal. To ensure wettability and uniform distribution of carbonitrides, their activation is carried out by introducing fine powders of iron, nickel and copper into the iron-nickel-copper matrix in a ratio of 97-5-2 wt.%. The composition of the matrix was selected for reasons of economic feasibility, since the use of a matrix entirely of nickel, copper and other ductile metals significantly increases the cost of steel.

Since, despite the high density of nanopowders, they easily form a dusty suspension in the air, which, under certain conditions, ignites and explodes spontaneously, to prevent this, either the mixture is compacted into tablets, or the mixture is placed in a metal shell (tube) of any section, with a wall thickness, depending on the volume of the processed melt. The casing is closed on one side by flattening, after filling with the mixture, the casing is closed on the other side (the casing diameter can be from 5 to 35 mm, and the thickness from 0.10 to 1.4 mm).

Due to the use of titanium and zirconium carbonitrides, the cost of manufacturing products is significantly reduced by reducing casting rejects and the possibility of using less alloyed steels with high strength and ductility characteristics.

To confirm the achievement of the technical result, centrifugally cast billets of pipes of 3 steel compositions according to the invention were smelted, they were hot deformed by forging, as well as heat treatment according to the following regime: 1. Quenching in oil from a temperature of 890-900 ° C, tempering at a temperature of 590-600 ° C ...

2. Quenching in oil from a temperature of 890-900 ° C, the second quenching from a temperature of 790-800 ° C, tempering at a temperature of 570-580 ° C. More than 6 variants were investigated in total

It has been found that the steel according to the invention, after appropriate heat treatment, provides the required level and stability of performance, including an increase in ductility (elongation) up to 20-30%, compared to the known steel.

So, depending on the composition and heat treatment in 1 mode, the yield point at room temperature is not lower than 950 MPa, the ultimate strength is not lower than 1250 MPa, and the elongation at 20 ° C is not lower than 25%. Impact strength (KCV) not less than 95 J / cm ² at a temperature of minus 50 ° С, according to the 2nd heat treatment mode, the yield strength at room temperature is not less than 1050 MPa, the ultimate strength is not less than 1350 MPa, elongation at 20 ° C is not less than 28 % respectively. Impact strength (KCV) not less than 105 J / cm ² at a temperature of minus 50 ° С,

Thus, the steel according to the invention at a low manufacturing cost (in comparison with the steels used for these purposes) reliably provides the required set of properties for tubular blanks of vessels for storing and transporting compressed gases operated at low temperatures down to -50 ° C.

Claims (4)

1. Economically alloyed cold-resistant high-strength steel containing carbon, silicon, manganese, chromium, nickel, copper, molybdenum, cerium, vanadium, iron and impurities, characterized in that it additionally contains titanium carbonitride, zirconium carbonitride with a particle size of 30-65 nm, aluminum, calcium, titanium, niobium, barium and, if necessary, at least one element selected from the group containing gadolinium, nitrogen and an element from the group containing lanthanum, yttrium, neodymium or mixtures thereof, in the following ratio of components, wt%: carbon 0.27-0.30 silicon 0.90-1.20 manganese 1.0-1.30 chromium 0.90-1.20 nickel 1.45-2.20 copper 0.40-0.70 molybdenum 0.25-0.40 titanium carbonitride 0.03-0.10 zirconium carbonitride 0.03-0.10 cerium 0.001-0.02 vanadium 0.05-0.08 aluminum 0.005-0.02 calcium 0.005-0.01 titanium 0.005-0.035 niobium 0.005-0.035 barium 0.005-0.025 if necessary, at least one of gadolinium 0.0008-0.0015 nitrogen 0.005-0.012 and an element from the group containing lanthanum, yttrium, neodymium, or mixtures thereof 0.001-0.02 iron and impurities rest 2. Steel under item 1, characterized in that the total content of low-melting impurities of lead, bismuth, tin, antimony and arsenic does not exceed 0.02 wt.%, And the content of inevitable impurities of sulfur, phosphorus, oxygen and hydrogen does not exceed, wt. %: sulfur ≤0.008, phosphorus ≤0.008, oxygen ≤0.005 and hydrogen ≤0.0005. 3. Steel according to claim. 1, characterized in that the criterion of the ratio σ _0.2 / σ _in is ≤0.90.