RU2285056C2 - Bar made from medium-carbon steel - Google Patents

Abstract

FIELD: metallurgy; production of round merchant shapes made from medium-carbon steel of enhanced machinability for manufacture of automobile shock absorber rods.

SUBSTANCE: steel used for manufacture of round bars contains the following components, mass-%: carbon, 0.40-0.52; manganese, 0.40-0.95; silicon, 0.17-37; chromium, 0.01-0.25; sulfur,0.020-0.045; vanadium, 0.005-0.02; aluminum, 0.03-0.05; calcium, 0.001-0.010; nitrogen, 0.005-0.015; nickel, no more than 0.25%; copper, no more than 0.25; molybdenum, no more than 0.10; arsenic, no more than 0.08; phosphorus, no more than 0.030; the remainder being iron and unavoidable admixtures at calcium-to-sulfur ratio ≥0.065. Nonmetallic inclusions of sulfides have double layer structure: sulfide with envelope. Curvature of bars does not exceed 0.5 mm/m. Shapes have laminated ferrito-pearlite structure at actual size of grain of 5-8 points; diameter of bars ranges from 10 to 300 mm; it has no decarbonized layer; hardness of billet is 229-255; rupture strength is no less than 660 Mpa; relative elongation is no less than 8% and relative reduction of area is no less than 35%.

EFFECT: improved ductility characteristics; low level of strain hardening at further heat treatment.

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Description

The invention relates to the field of metallurgy, in particular to the production of long products, from medium-carbon steel with high machinability, used for the manufacture of automobile shock absorber rods.

Known long sections made of steel containing (wt.%): Carbon 0.38-0.47%, manganese 0.8-1.2%, silicon 0.17-0.37%, vanadium 0.08-0.18%, boron 0.001-0.005 %, nitrogen 0.005-0.025%, sulfur 0.036-0.080%, calcium 0.001-0.010%, the rest, provided that the ratio of manganese to calcium is 100-1100 (USSR author's certificate SU 1689424 A1, C 22 C 38/60 dated 07.11. 1989, Bull. No. 41). The disadvantage of this steel is the relatively high nitrogen content and the absence of elements in the composition that protect boron from binding to nitrides, which in some cases will not allow the authors claimed effect to increase the hardenability characteristics.

Known long products of round steel made of steel (wt.%): Carbon 0.42-0.50%, silicon 0.17-0.37%, manganese 0.50-0.80%, sulfur - not more than 0.040%, phosphorus - not more than 0.035%, the rest is iron. Impurities: chromium - not more than 0.25%, nickel - not more than 0.30%, arsenic - not more than 0.08%, nitrogen - not more than 0.008%, copper - not more than 0.30% (Marochnik of steels and alloys, edited by A.S. Zubchenko, M., Engineering, 2003, p. 102).

The closest in technical essence and the achieved effect to the proposed invention is long products, with special surface finish, round, in bars, from medium carbon steel containing (wt.%): Carbon 0.42-0.50%, silicon 0.17-0.37%, manganese 0.50-0.80%, sulfur - not more than 0.040%, phosphorus - not more than 0.035%, the rest - iron. Impurities: chromium - not more than 0.25%, nickel - not more than 0.30%, arsenic - not more than 0.08%, nitrogen - not more than 0.008%, copper - not more than 0.30% (Marochnik of steels and alloys, edited by A.S. Zubchenko, M., Engineering, 2003, p. 102).

The most important requirement for long products, round, from medium-carbon steel used for the manufacture of shock absorber rods by turning with a perpendicular feed of the cutter, is, on the one hand, providing improved machinability by cutting, while ensuring a favorable ratio of strength, ductility and toughness, a minimum level of anisotropy mechanical properties and at the same time increase hardenability characteristics while ensuring through hardenability of high-quality rolled metal ata with a diameter up to 30 mm.

The technical result of the invention is the provision of rational processing conditions by cutting the rods of automobile shock absorbers while ensuring uniform mechanical properties along the rolled section.

In order to achieve a technical result in round sections, in bars melted from medium-carbon steel containing carbon and alloying elements, having predetermined parameters of steel quality for non-metallic inclusions, structure, mechanical properties, hardenability and machinability by cutting, steel contains the following ratio of components in wt. %:

carbon 0.40-0.52 manganese 0.40-0.95 silicon 0.17-0.37 chromium 0.01-0.25 sulfur 0,020-0,045 vanadium 0.005-0.02 aluminum 0.03-0.05 calcium 0.001-0.010 nitrogen 0.005-0.015 nickel no more than 0.25 copper no more than 0.25 molybdenum no more than 0.10 arsenic  no more than 0.08 phosphorus no more than 0,030 iron and inevitable impurities rest

when the calcium / sulfur ratio is ≥0.065, non-metallic sulfide inclusions in rolled products have a two-layer structure of sulfide with an oxide shell, the rolled products have a lamellar ferrite-pearlite structure, grain size 5-8 points, diameter from 10 to 30 mm, decarburized layer no more than 1, 5% of rolled diameter, hardness 229-255НВ, temporary tensile strength not less than 660 MPa, elongation not less than 8%, relative narrowing not less than 35%, curvature not more than 0.5 mm / m.

The given combinations of alloying elements make it possible to obtain in a finished product (a shock absorber rod with a diameter of up to 3 mm) after turning, quenching with high-frequency currents and subsequent grinding, a ferrite-pearlite fine structure with a favorable combination of strength and ductility characteristics.

Carbon is introduced into the composition of this steel in order to ensure a given level of its strength and hardenability. The upper limit of the carbon content (0.52%) is due to the need to ensure the required level of ductility of steel, and the lower - 0.40%, respectively - to ensure the required level of strength and hardenability of this steel.

A carbonitride-forming element, vanadium, is introduced into the composition of this steel in order to provide a finely dispersed, uniform grain structure, which will increase both its strength level and provide a given level of ductility. In this case, vanadium controls the processes in the lower part of the austenitic region and in the intercritical temperature range (determines the tendency to growth of austenite grain, stabilizes the structure during thermomechanical treatment, increases the recrystallization temperature and, as a result, affects the nature of the γ-α transformation. Vanadium also contributes to hardening the upper limit of the vanadium content is 0.02%, due to the need to ensure the required level of ductility of steel, and the lower - respectively 0.005% - providing rebuemogo strength level of the steel.

Manganese and chromium are used, on the one hand, as solid solution hardeners, and on the other hand, as elements that significantly increase the stability of supercooled austenite of steel. In this case, the upper level of manganese — 0.95% and chromium — 0.25% is determined by the need to ensure the required level of ductility of steel, and the lower, manganese — 0.40% and 0.01% of chromium, respectively, by the need to provide the required level of strength and hardenability of this steel.

Silicon refers to ferrite-forming elements. The lower limit on silicon - 0.17% is due to steel deoxidation technology. A silicon content above 0.37% will adversely affect the ductility characteristics of steel.

Sulfur determines the level of ductility of steel. The upper limit (0.040%) is due to the need to obtain a given level of ductility and toughness of steel, and the lower limit (0.020%) due to issues of manufacturability, as well as providing a given level of machinability by cutting this steel.

Aluminum is used as a deoxidizing agent for steel and an element that provides the formation of a finely dispersed, uniform grain structure. The upper limit (0.050%) is due to the need to obtain a given level of ductility and toughness of steel, and the lower limit (0.030%) is due to technological issues of production, as well as ensuring a uniform grain structure of steel.

Calcium is an element modifying non-metallic inclusions. The upper limit (0.010%), as in the case of sulfur, is due to the need to obtain a given level of ductility and toughness of steel, and the lower (0.001%) limit is due to issues of manufacturability.

Nitrogen promotes the formation of nitrides in steel. The upper limit of nitrogen content - 0.015% is due to the need to obtain a given level of ductility and toughness of steel, and the lower limit - 0.005% - due to issues of manufacturability.

The ratio of calcium / sulfur ≥0.065% determines the conditions for the formation of globular non-metallic inclusions (sulfides). If this ratio is fulfilled, the sulfides are globular, otherwise elongated sulfides are present in the steel, which increases the anisotropy of the properties of the steel and worsens the strength-toughness ratio, especially strongly in the transverse direction of the rolled product.

The analysis of patent and scientific and technical information did not reveal solutions having a similar set of features that would achieve a similar effect - improving the machinability of cutting while maintaining a favorable ratio of strength-ductility and toughness of steel.

The following is an example implementation of the invention.

Smelting of the studied steel (chemical composition in wt.%: Carbon - 0.48%, manganese - 0.72%, silicon - 0.32%, chromium - 0.10%, vanadium - 0.01%, sulfur - 0.034%, aluminum - 0.037%, calcium - 0.0025% , nitrogen - 0.010%) was carried out in a Fuchs mine electric furnace. Used a batch of smelting with molten iron up to 40% of the total volume of the charge. The oxidation period provided for high rates of carbon oxidation in the range of 0.05-0.07% / min. The electric mode provided for turning off the furnace with a carbon content of 0.2-0.4% above the lower limit for a given one, carbon blowing is carried out without an electric arc. The temperature of the exhaust from the furnace 1640-1680 ° C. The introduction of ferroalloys, the processing of steel to remove non-metallic inclusions was carried out at the ladle furnace equipped with an electric heating or chemical heating system. The temperature of the steel before casting is 60 ° C higher than the liquidus temperature of the brand. Steel casting was carried out in molds broadened to the top. The weight of the ingot is 7.85 tons. To ensure a given nitrogen content during casting, the metal stream was protected by argon through a special ring device. The ingots were heated in a crimping workshop in recuperative wells to a rolling start temperature of 1250-1270 ° C. The ingots were rolled on blooming (mill 1300) and then on a continuous billet mill to a workpiece with a section of 100 × 100 mm. To remove the decarburized layer formed during heating of the ingots, the workpieces were subjected to abrasive cleaning. Then, the resulting billet was hot rolled at a wire mill 150 or a fine mill 250 in diameters from 5.5 to 25 mm in coils. To ensure the size of the decarburized layer is not more than 1.5% of the diameter, the rate of delivery of the billets from the furnace is not less than 100 t / h for the mill 150 and not less than 56 t / h for the mill 250. The temperature for the start of rolling of the billets is 1220-1240 ° C for Mill 250 and 1270-1290 ° C for Mill 150. Hot rolling of long products was completed at a temperature of 1000-1050 ° C, then accelerated cooling to 880-900 ° C. Further, the hire was subjected to special surface treatment. Finishing included the operations of dressing, control of surface defects and ultrasonic control of internal defects, selective abrasive cleaning, continuous abrasive grinding, turning of round bars. The accuracy of rolling after turning corresponds to the h11 quality. At the BUNT-PRUTOK installation, turned coils of bar up to 6 meters long with a cutting accuracy of ± 5 mm are obtained from coils of hot-rolled steel.

As a result of hot rolling and subsequent special surface treatment, we obtain long products with a diameter of 22.5 mm, a length of 5900 mm, and the curvature of the rods not exceeding 0.7 mm / m. The structure of lamellar perlite, a decarburized layer with a depth of 0.05 mm, a real grain score of 7, the hardness of the billet 229-241 HB, temporary tensile strength 680 MPa, elongation of 9%, relative narrowing of 42%.

Calcium / sulfur ratio = 0.074, calcium content - 0.0025%, sulfur - 0.034%.

Implementation of the proposed method for the production of long products from medium carbon steel with increased machinability provides two-layer sandwich-nonmetallic inclusions that guarantee, on the one hand, improved performance by cutting, and on the other hand, a favorable ratio of ductility and toughness of steel.

Claims (1)

Sections of steel, round, in bars, melted from medium-carbon steel, containing carbon and alloying elements, having predetermined parameters of steel quality for non-metallic inclusions, structure, mechanical properties, hardenability and machinability by cutting, characterized in that the steel contains the following ratio of components, wt.% : Carbon 0.40-0.52 Manganese 0.40-0.95 Silicon 0.17-0.37 Chromium 0.01-0.25 Sulfur 0,020-0,045 Vanadium 0.005-0.02 Aluminum 0.03-0.05 Calcium 0.001-0.010 Nitrogen 0.005-0.015 Nickel No more than 0.25 Copper No more than 0.25 Molybdenum No more than 0.10 Arsenic No more than 0,08 Phosphorus No more than 0,030 Iron and inevitable impurities Rest when fulfilling the ratio of calcium: sulfur≥0,065, nonmetallic sulfide inclusions in rolled products have a two-layer sulfide structure with an oxide shell, rolled products have a lamellar ferritoperlitic structure, grain size 5-8 points, diameter 10 ÷ 30 mm, decarburized layer not more than 1.5% of rolled diameter, hardness 229-255НВ, temporary tensile strength not less than 660 MPa, relative elongation of at least 8%, relative narrowing of at least 35%, curvature of not more than 0.5 mm / m.