RU2249628C1 - Round-profiled rolled iron from low-carbon steel for cold die forging of high-strength especially high-profiled fastening members - Google Patents

Abstract

FIELD: ferrous metallurgy.

SUBSTANCE: invention provides round-profiled iron smelted from low-carbon steel composed of, wt %: carbon 0.17-0.25, manganese 0.30-0.65, silicon 0.01-0.17, sulfur 0.005-0.020, vanadium 0.005-0.07, niobium 0.005-0.02, calcium 0.001-0.010, iron and unavoidable impurities - the balance. When following relationships are fulfilled: 12/C-Mn/0.02 ≥ 27; 0.46 ≥ 6V+8Nb ≥ 0.22; Ca/S ≥ 0.065, rolled iron has following characteristics: maximum degree of pollution with nonmetal inclusions, in particular sulfides, oxides, silicates, and nitrides, does not exceed 3 points for each type of inclusions; longitudinally uniform spheroidized structure composed of at least 80% grainy perlite; effective grain size 5-10 points; diameter 10-25 mm; carbon-free layer not exceeding 1.5% of diameter; cold setting value at least 1/3 height; point of maximum load not higher than 550 MPa; relative elongation at least 20%; and relative contraction at least 60%.

EFFECT: ensured optimal conditions for cold die forging of high-strength geometrically complex fastening members and simultaneously ensured improved characteristics of in-process plasticity and low level of stray hardening.

Description

The invention relates to the field of metallurgy, in particular the production of long products, round, from low-carbon high-ductility steel for cold forging of fasteners of particularly complex shapes.

Known long sections made of microalloyed steel containing carbon and alloying elements having a given structure, for example, cold-deformed martensite, tensile strength of at least 1800 MPa and wire diameter of 0.1-0.5 mm [1].

Known long products, round of low-carbon steel, containing, wt.%: Carbon 0.17-0.20%, silicon 0.17-0.37%, manganese 0.65-1.00%, chromium 0.55- 0.70%, vanadium 0.05-0.08%, niobium 0.02-0.04%, iron - the rest, when the ratio (chromium / carbon) (vanadium / niobium) ² = 16.4-65, 9 [2]

The closest in technical essence and the achieved effect to the proposed invention is long products, round, made of steel containing, wt.%: Carbon 0.18-0.22%, manganese 0.27-0.67%, silicon 0.17-0.37%, vanadium 0 , 05-0.10%, niobium 0.01-0.04%, the rest is iron in the following ratio of components, wt.%: 6V + 8Nb≥ 0.56 and 100000VNb ² ≤ 8.0 [3]. The disadvantages are that with a relatively high level of variation in the carbon content, the factor of boron protection from binding to nitrides is not taken into account, which does not allow to obtain increased hardenability characteristics.

The objective of the invention is to provide rational conditions for the cold forging of complex-profile fasteners while ensuring uniform mechanical properties along the rolled section and increased characteristics of technological ductility.

The most important requirement for long products, round, high-ductility low-carbon steel for cold forming for fasteners of especially complex shape, is, on the one hand, high technological ductility and low coefficient of strain hardening in the delivery state and, on the other hand, the ability to provide a given level consumer properties.

The problem is solved in that the well-known long products, round, from high-ductility low-carbon steel, having a given structure, temporary tensile strength and hardness, according to the invention is made of steel containing the following ratio of components, wt.%:

carbon 0.17-0.25

Manganese 0.30-0.65

silicon 0.01-0.17

sulfur 0.005-0.020

vanadium 0.005-0.07

niobium 0.005-0.02

calcium, 0.001-0.010

iron and

unavoidable impurities rest

Moreover:

the maximum contamination score for steel with non-metallic inclusions for sulfides, oxides, silicates and nitrides does not exceed 3 points for each type of inclusions, the rolled product has a uniform spheroidized structure in length, consisting of at least 80% granular perlite, the actual grain size is 5-10 points, diameter the wire is from 10 to 25 mm, has a decarburized layer of not more than 1.5% of the diameter, cold draft of not less than 1/3 of the height, temporary tensile strength of not more than 550 MPa, elongation of at least 20%, relative constriction of at least 60%.

The given combinations of alloying elements (item 1) make it possible to obtain a homogeneous finely dispersed tempering martensite structure with a favorable combination of strength and ductility characteristics in a finished product (bolt, nut, stud with a diameter of up to 23 mm).

Carbon and carbonitride-forming elements (vanadium, niobium) are introduced into the composition of this steel in order to provide a fine grain structure, which will increase both its strength level and provide a given level of ductility. At the same time, niobium and vanadium control processes in the austenitic region (they determine the tendency for austenite grain to grow, stabilize the structure during thermomechanical treatment, increase the temperature of recrystallization and, as a result, affect the nature of the γ -α transformation. Niobium and vanadium also contribute to the hardening of steel upon thermal improvement The upper limit of carbon content (0.25%), niobium (0.02%) and vanadium (0.07%) is due to the need to ensure the required level of ductility of steel, and the lower - 0.17%, 0.005% and 0.005%, respectively - the required level of strength of this steel.

Manganese is used, on the one hand, as a hardener of a solid solution, and on the other hand, as an element that significantly increases the stability of supercooled austenite. In this case, the upper level of manganese content is 0.65%, determined by the need to ensure the required level of ductility of steel, and the lower - 0.30%, by the need to provide the required level of strength of steel.

Silicon refers to ferrite-forming elements. The lower limit on silicon is 0.01%, due to the technology of deoxidation of steel. A silicon content above 0.17% will adversely affect the ductility characteristics of steel.

Sulfur determines the level of ductility of steel. The upper limit (0.020%) is due to the need to obtain a given level of ductility and toughness of steel, and the lower limit (0.005%) is due to issues of manufacturability.

Calcium is an element that modifies 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 due to issues of manufacturability.

определяют условия обеспечения заданных характеристик пластичности и упрочняемости стали при холодной объемной штамповке сложнопрофильных крепежных деталейThe ratio

determine the conditions for ensuring the specified characteristics of ductility and hardenability of steel during cold forging of complex profile fasteners

Therefore, the claimed combination of features meets the criterion of ″ significant differences ″.

The following is an example implementation of the invention, not excluding others in the scope of the claims.

High-ductility low-carbon steels are smelted in the Fuchs electric shaft furnace. For a guaranteed low nitrogen content, a special technology has been developed, which includes batch melting with molten iron up to 40% of the total volume of the charge. The oxidation period provides for high rates of carbon oxidation in the range of 0.05-0.07% / min. The electric mode provides for turning off the furnace when the carbon content is 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 outlet from the furnace is 1640-1680 ° С. Ferroalloys are introduced, steel is treated to remove non-metallic inclusions 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 is cast into a mold widened to the top. The weight of the ingot is 7.85 tons. To ensure a low nitrogen content during casting, the metal stream is protected by argon through a special ring device. The ingots are heated in the crimping workshop in recuperative wells to a rolling start temperature of 1250-1270 ° C. The ingots are rolled in blooming (1300 mill) and then on a continuous billet mill for a billet with a cross section of 100 × 100 mm. To remove the decarburized layer formed during heating of the ingots, the workpieces are subjected to abrasive cleaning. Then, the obtained workpiece was hot rolled at a wire mill 150 or a small-grade mill 250 in diameters from 5.5 to 23 mm in coils. To ensure the size of the decarburized layer is not more than 1% of the diameter, the rate of delivery of billets from the furnace is limited to at least 100 t / h for mill 150 and at least 56 t / h for mill 250. The temperature at which billets are rolled starts at 1220-1240 ° C for mill 250 and 1270-1290 ° C for mill 150. Hot rolling of long products is completed at a temperature of 1000-1050 ° C, then accelerated cooling to 880-900 ° C, followed by cooling in air to 300 ° C and subsequent winding into riots.

As a result of hot rolling, we get long products with a diameter of 21 mm with a granular perlite structure (99%), a decarbonized layer with a depth of 0.12 mm, a real grain score of 9, a cold draft of wire with a diameter of 21 mm by 90%, a temporary tensile strength of 500 MPa, an elongation of 22 %, narrowing 70% ..

Ratio

С=0,20%, Mn=0,50%

C = 0.20%, Mn = 0.50%

V=0,01%, Nb=0,01%

V = 0.01%, Nb = 0.01%

Ca=0,0011%, S=0,008%

Ca = 0.0011%, S = 0.008%

The introduction of the proposed product - long products, round, from mild steel with increased ductility provides directly in the stream of the mill (without additional spheroidizing annealing) the structure of long products, guaranteeing rational conditions for the cold forging of complex profile fasteners.

SOURCES OF INFORMATION

1. RU 2177510 C2, C 21 D 8/06, 12/27/2001.

2. SU 1703709, C 22 C 38/54, 01/07/1992.

3. SU 1772208, C 22 C 38/12, Bull. No. 40, 10.30.1992 (prototype).

Claims (1)

Sections, round, smelted from low-carbon steel, containing carbon and alloying elements, having specified steel quality parameters for non-metallic inclusions, structure, mechanical properties, tensile strength and technological ductility, characterized in that the steel contains the following ratio of components in wt.% : Carbon 0.17-0.25 Manganese 0.30-0.65 Silicon 0.01-0.17 Sulfur 0.005-0.020 Vanadium 0.005-0.07 Niobium 0.005-0.02 Calcium 0.001-0.010 Iron and inevitable impurities when the relations

the maximum contamination score of steel with non-metallic inclusions for sulfides, oxides, silicates and nitrides does not exceed 3 points for each type of inclusions, the rolled product has a uniform spheroidized structure in length, consisting of at least 80% granular perlite, actual grain size is 5-10 points, diameter from 10 to 25 mm, the decarburized layer is not more than 1.5% of the diameter, the cold precipitation is not less than 1/3 of the height, the tensile strength is not more than 550 MPa, the elongation is not less than 20%, the relative narrowing is not less than 60%.