Medium carbon steel

Abstract

FIELD: metallurgy.SUBSTANCE: continuously cast hot-rolled medium-carbon steel used for the manufacture of machine parts operating under conditions of high load and corrosive activity. The steel contains the following components, wt.%: carbon 0.36-0.44, manganese 0.25-0.50, silicon 0.17-0.37, chromium 2.0-2.4, vanadium 0.09-0.15, aluminum 0.02-0.04, nitrogen 0.02-0.04, sulfur 0.03-0.05, phosphorus 0.01-0.03, copper no more than 0.3, nickel 0.01-0.3, calcium 0.003-0.009, iron and inevitable impurities - the rest. The ratio of aluminum to nitrogen content ranges from 0.5 to 2, and the total amount of inevitable impurities is no more than 0.1 wt.%. The manganese sulfides and aluminum oxides contained in steel have a globular shape and are uniformly distributed throughout the volume of steel.EFFECT: provides increased strength, wear and corrosion resistance.1 cl, 2 tbl, 1 ex

Description

[0001] The invention relates to metallurgy, to steel compositions and can be used for the production of structural alloyed steel grades used for the manufacture of machine parts operating under conditions of high load and corrosive activity.

[0002] A medium-carbon free-cutting steel is known, described in patent CN 106544594 A dated 28.10.2016, IPC C21C 7 / 06, C21C 7 / 10; C21D 8 / 06; C22C 33 / 06; C22C 38 / 02; C22C 38 / 04; C22C 38 / 06; C22C 38 / 22; C22C 38 / 24. The known steel contains, by weight %: carbon 0.33-0.38; silicon 0.17-0.37; manganese 0.50-0.70; sulfur no more than 0.02; phosphorus no more than 0.025; chromium 1.0-1.2; Vanadium 0.10-0.15; Aluminum 0.015-0.060; Molybdenum 0.2-0.3; Oxygen no more than 0.0015. The disadvantages of this steel include low corrosion resistance in aggressive environments, despite the addition of chromium (1.0-1.2%). Another disadvantage of this steel is its highly alloyed chemical composition, the use of expensive alloying elements (Mo, V, Cr), the main purpose of such alloying is to improve the mechanical properties of steel due to dispersion hardening, which makes their combined use impractical and increases the cost of steel.

[0003] The closest steel for the same purpose to the claimed invention in terms of a combination of features is described in GOST 4543. The well-known medium-carbon steel grade 40KhFA, taken as the prototype, contains, in wt. %: carbon 0.37-0.44, silicon 0.17-0.37, manganese 0.5-0.8, chromium 0.8-1.1, vanadium 0.1-0.18, nickel and copper up to 0.3, sulfur and phosphorus up to 0.025, the rest is iron and unavoidable impurities. This steel is characterized as chromium-vanadium structural. Good strength properties are achieved by alloying with chromium, vanadium and manganese. The disadvantages of the prototype are a tendency to grain growth during heating, low corrosion resistance and machinability. For modern industries with high demands on strength and wear resistance, the steel chosen as a prototype can only be used in a heat-treated condition, which complicates the production process of the parts.

[0004] The objective of the invention is to create steel with improved strength characteristics, wear resistance properties, and corrosion resistance relative to the prototype.

[0005] The technical result of the present invention is to achieve the stated objective.

[0006] The said technical result of the invention is achieved due to the fact that the proposed medium-carbon steel, containing carbon, manganese, silicon, chromium, vanadium, sulfur, copper, phosphorus, nickel, iron and inevitable impurities, according to the invention, additionally contains aluminum, nitrogen and calcium, in the following ratio of components, mass %:

[0007] carbon - 0.36-0.44;

[0008] manganese - 0.25-0.50;

[0009] silicon - 0.17-0.37;

[0010] chrome - 2.0-2.4;

[0011] vanadium - 0.09-0.15;

[0012] aluminum - 0.02-0.04;

[0013] nitrogen - 0.02-0.04;

[0014] sulfur - 0.03-0.05;

[0015] phosphorus - 0.01-0.03;

[0016] copper - no more than 0.3;

[0017] nickel - 0.01-0.3;

[0018] calcium - 0.003-0.009;

[0019] iron and unavoidable impurities are the rest, while the ratio of aluminum to nitrogen is from 0.5 to 2, and the total amount of unavoidable impurities is not more than 0.1%.

[0020] The differences of the proposed medium-carbon structural steel are the increased content of chromium and sulfur, the reduced content of manganese, the addition of calcium, nitrogen and aluminum to the steel, which allows for the necessary strength characteristics and technological properties to be achieved.

[0021] The advantages of the claimed invention over the prototype are the higher strength characteristics of the steel, in particular its mechanical properties, due to the presence of additional components in the alloy that prevent the development of microcracks in the interdendritic space and enhance dispersion hardening, thereby increasing the strength, hardness and wear resistance of the steel, as well as corrosion properties.

[0022] The invention provides high strength properties to steel through the formation of chromium and vanadium carbides and carbonitrides, as well as vanadium nitrides, which are uniformly distributed throughout the steel due to the microalloying of calcium and aluminum. The formation of dispersed, globular refractory particles improves the wear resistance of the material by fine-tuning the grain size, and the increased chromium content improves the corrosion resistance of the steel compared to the prior art.

[0023] Manganese improves the strength properties and also promotes steel deoxidation and improved machinability through the formation of manganese sulfides. However, manganese content above 0.5% has a negative impact on grain growth at high temperatures and temper brittleness. Higher sulfur content (0.03-0.05%) allows for the production of higher amounts of manganese sulfide, which significantly improves steel machinability and simplifies production. If the sulfur concentration in the steel composition is below the specified range, the smaller amounts of manganese sulfides formed will not improve the technological properties of the steel; if the sulfur content exceeds the specified range, it leads to red brittleness.

[0024] Chromium strengthens steel and increases its resistance to corrosion and wear. The chromium concentration in steel ranges from 2.0 to 2.4%. This level ensures the formation of stable and refractory chromium carbides, increasing the strength and wear resistance of the steel. A higher chromium content (2.0 to 2.4%) forms a denser Cr2O3 passivation layer, significantly increasing corrosion resistance compared to the reference material. Chromium concentrations below this range lead to weakening and a significant reduction in corrosion resistance. Excess chromium reduces the ductility and impact toughness of the steel.

[0025] The primary increase in steel strength and hardness is achieved through the dispersion strengthening effect of vanadium addition. Vanadium concentration in steel ranges from 0.09% to 0.15%. If the vanadium concentration in the steel is below this range, no increase in strength will occur. The upper limit of the range is determined by achieving optimal strength and impact toughness, as exceeding the vanadium concentration significantly reduces the toughness of the steel.

[0026] Aluminum is present in steel as a deoxidizer. The aluminum concentration in steel is 0.02-0.04%. A minimal amount of aluminum is necessary for the formation of aluminum oxide and the reduction of oxygen in the steel. Aluminum content within the specified limits effectively improves the steel structure, creating a fine-grained structure and improving its mechanical properties and resistance to overheating. Excess aluminum also interacts with nitrogen in the steel, forming aluminum nitrides during crystallization. However, if the aluminum concentration in steel exceeds the permissible limit, this will lead to an increase in the size of solid non-metallic inclusions in the steel, reducing machinability.

[0027] Nitrogen increases the strength of steel while maintaining ductility. The combination of C, V, N, and Cr provides precipitation hardening through the formation of carbides, carbonitrides, and nitrides. The nitrogen concentration in steel ranges from 0.02 to 0.04%. This concentration allows for the production of the required amount of vanadium nitrides. Nitrogen alloying also increases the stability of carbonitrides and reduces the steel's susceptibility to overheating. If the nitrogen concentration in the steel is below this range, it is insufficient to form hardening nitrides.

[0028] Excess nitrogen (concentrations greater than 0.03%) can form nitride clusters at grain boundaries, reducing impact toughness. High nitrogen content also makes the steel structure unstable, which can lead to uncontrolled aging during service due to the precipitation of nitrides from solid solution. However, the addition of aluminum binds nitrogen into refractory nitrides, AlN, during crystallization, preventing its harmful effects. AlN forms earlier than VN or Cr2N and is distributed evenly throughout the grain. Furthermore, without the addition of aluminum, vanadium and chromium nitrides tend to grow at grain boundaries, creating brittle zones. AlN, however, serves as crystallization centers for VN, reducing their size, increasing strength without sacrificing ductility. Therefore, it is necessary to control the aluminum to nitrogen ratio in the range of 0.5-2.0, which ensures the binding of nitrogen into dispersed aluminum nitrides and prevents embrittlement.At ratios below 0.5, there is insufficient aluminum to fully bind nitrogen, leading to a negative residual nitrogen effect and deterioration of mechanical properties. At ratios greater than 2.0, excess aluminum leads to the formation of large aluminum oxide inclusions and a decrease in the effectiveness of vanadium alloying, as some of the vanadium is bound into aluminum-containing phases instead of strengthening VN.

[0029] Calcium promotes the modification of non-metallic inclusions, which serve as crystallization centers and can influence grain growth and phase distribution. Calcium concentrations in steel range from 0.003 to 0.009%. This amount globulates manganese sulfides and aluminum oxides, ensuring their uniform distribution within the steel. This reduces the anisotropy of the alloy's mechanical properties, as modified non-metallic inclusions serve as crystallization centers for strengthening dispersoids. This calcium content also improves the steel's machinability by globulating manganese sulfides, a necessary criterion for further processing. If the calcium concentration in the steel exceeds the upper limit of this range, this will lead to the formation of a brittle eutectic at the grain boundaries, reducing ductility and impact toughness.If the concentration of calcium in the steel is below the specified range, the required effect of alloying will not be achieved.

[0030] The total amount of unavoidable impurities must be less than 0.1%. This amount of impurities does not affect the properties of the steel, but is inevitably present during smelting. Unavoidable impurities include elements not specified in the composition of the invention and not intentionally added, but inevitably present in the smelted steel. The composition of unavoidable impurities depends on the steelmaking method, as well as the chemical composition of the charge used in production.

[0031] Implementation example. The claimed steel and the prototype steel were produced using similar technologies: continuous casting and hot rolling. The composition of the claimed steel and the prototype steel is shown in Table 1. The mechanical properties of the hot-rolled bar are listed in Table 2. Corrosion resistance tests were conducted under identical conditions for each sample, according to NACE Standard TMO177-2005, Method A, testing metal for resistance to stress corrosion cracking in an H2S-containing environment.

[0032]

[0033]

[0034] Conclusion: The patented steel exhibits higher yield strength, tensile strength, elongation, and hardness compared to the prior art. The invention demonstrates advantages in properties in the unheated state, simplifying the production of parts. The patented steel has a nearly 15% higher σ в and σ0,2 , thanks to its increased chromium content and dispersion strengthening with vanadium nitrides and chromium carbides. It also has superior corrosion resistance compared to the prototype. The proposed steel boasts greater hardness, which translates into superior wear resistance compared to the prototype.

Claims

Medium-carbon steel containing carbon, manganese, silicon, chromium, vanadium, sulfur, phosphorus, nickel, iron and inevitable impurities, characterized in that it additionally contains aluminum, nitrogen and calcium in the following ratio of components, wt.%: carbon 0,36-0,44 manganese 0,25-0,50 silicon 0,17-0,37 chromium 2,0-2,4 vanadium 0,09-0,15 aluminum 0,02-0,04 nitrogen 0,02-0,04 sulfur 0,03-0,05 phosphorus 0,01-0,03 copper no more than 0.3 nickel 0,01-0,3 calcium 0,003-0,009 iron and inevitable impurities rest, The ratio of aluminum to nitrogen is from 0.5 to 2, and the total amount of unavoidable impurities is no more than 0.1%, while the steel is obtained by continuous casting and subsequent hot rolling, and the manganese sulfides and aluminum oxides contained in the steel have a globular shape and are uniformly distributed throughout the volume of the steel.

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