RU2330896C2 - Pipe shell made of low-carbon low-alloyed steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention concerns the metallurgy field. Particularly it concerns manufacturing of pipe shell of diameter from 100 till 180 mm, intended for producing seamless pipe for various purposes. Pipe shell is made of low-carbon low-alloyed steel, which consists in mass % of: C - (0.23÷0.29), Mn - (1.35÷1.65), Si - (0.17÷0.43), Al - (0.02÷0.04), N - (0.005÷0.010), As -(0.0001÷0.03), Sn - (0.0001÷0.02), Pb - (0.0001÷0.01), Zn - (0.0001÷0.005), iron and inevitable admixtures, in ratio: (As+Sn+Pb+5×Zn)≤0.07; 0.46≤(C+Mn/6)≤0.55. Admixtures: phosphorus is not more than 0.025%, sulphur is not more than 0,030%, chrome is not more than 0.30%, nickel is not more than 0.25%, copper is not more than 0.30%. Pipe blank has ferrite-pearlite structure, effective grain size is 5-10 points, macrostructure is: center porosity, spot inhomogeneity, liquating square, undershrinked sweat is not more than 2 points; liquating strip is not more than 1 point, nonmetallic: spot sulphides, spot oxide, stitch oxide, fragile silicate, plastic silicate, unstrained silicate - not more than 2.5 points (average point) for every kind. mechanical properties after normalisation - tensile strength is not less than 517 N/mm², yield strength is 379-552 N/mm², percentage elongation is not less than 18%.

EFFECT: heightened level of consumer behavior for pipe shell.

2 cl, 1 ex

Description

The invention relates to the field of metallurgy, in particular to the production of tube billets with a diameter of 100 to 180 mm from low-carbon low-alloy steel, intended for the production of seamless pipes for various purposes.

Known tubular billet of low carbon low alloy steel having a predetermined structure, mechanical properties, the composition of the steel includes carbon, silicon, chromium, manganese, nickel, molybdenum, niobium, aluminum, nitrogen (RU 2070585 C1, C21D 9/14, 12.20.1996) .

Known tubular billet of low-carbon low-alloy steel containing carbon, silicon, manganese, molybdenum, sulfur, phosphorus, chromium, copper, nickel, aluminum, titanium, antimony, tin, arsenic and iron, the rest, made of hot-rolled sheet, having specified mechanical properties and a predetermined structure (RU 2252972 C1, C21D 9/08, 05.27.2005).

The most important requirement for a low-carbon low-alloy steel billet is on the one hand to ensure uniformity of the micro- and macrostructure, low content of non-metallic inclusions, and on the other hand, to provide an increased complex of consumer properties and a given morphology of non-metallic inclusions.

The objective of the invention is to provide a high level of consumer properties while providing a favorable ratio of strength, ductility and viscosity, a minimum level of anisotropy of mechanical properties, low content of non-metallic inclusions, homogeneous macro- and microstructure of rolled products.

The problem is solved in that the tube billet of low-carbon low-alloy steel is made of steel containing the following ratio of components in wt.%:

Carbon [C] 0.23-0.29 Manganese [Mn] 1.35-1.65 Silicon [Si] 0.17-0.43 Aluminum [Al] 0.02-0.04 Nitrogen [N] 0.005-0.010 Arsenic [As] 0.0001-0.03 Tin [Sn] 0.0001-0.02 Lead [Pb] 0.0001-0.01 Zinc [Zn] 0.0001-0.005 Iron and inevitable impurities rest,

when fulfilling the ratio: (As + Sn + Pb + 5 × Zn) ≤0.07; 0.46≤ (C + Mn / 6) ≤0.55, hot-rolled, improved, has a ferrite-pearlite structure, the actual grain size is 5-10 points, the macrostructure is central porosity, point heterogeneity, segregation square, non-shrink segregation is not more than 2.0 points for each type; segregation strips - not more than 1 point, non-metallic inclusions: point sulphides, point oxides, line oxides, brittle silicates, plastic silicates, undeformed silicates not more than 2.5 points, average for each type of inclusions, mechanical properties in a normalized state: temporary tensile strength not less than 517 N / mm ² , yield strength 379-552 N / mm ² , elongation of at least 18%. As impurities, steel additionally contains in wt.%: Phosphorus no more than 0.025, sulfur no more than 0.030, chromium no more than 0.30, nickel no more than 0.25, copper no more than 0.30.

The given combinations of alloying elements (item 1) make it possible to obtain a finely dispersed ferrite-pearlite structure, optimal content and morphology of nonmetallic inclusions, a homogeneous macrostructure, and a favorable combination of strength and ductility characteristics in the finished product.

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 (0.29%) is due to the need to ensure the required level of ductility of steel, and the lower - respectively 0.23% - to ensure the required level of strength and hardenability of this steel.

Manganese is used on the one hand as a hardener of solid solution, on the other hand as an element that increases the stability of supercooled austenite of steel. Moreover, the upper limit of the manganese content - 1.65% is determined by the need to ensure the required level of ductility of steel, and the lower - 1.35%, the need to provide the required level of strength and hardenability and heat resistance of this steel.

Silicon refers to ferrite-forming elements. The lower limit for silicon - 0.17% due to the technology of deoxidation of steel. A silicon content above 0.43% will adversely affect the ductility characteristics of steel.

Nitrogen promotes the formation of nitrides in steel. The upper limit of nitrogen content - 0.010% is due to the need to obtain a given level of ductility and toughness of steel, and the lower limit - 0.005% by issues of manufacturability.

Aluminum is a strong carbonitride former and steel deoxidizer. The upper limit of the aluminum content is 0.04% due to the need to obtain a given level of ductility and toughness of steel, and the lower limit is 0.02% by issues of manufacturability.

Arsenic, tin, lead and zinc are colored impurities that determine the overall level of ductility of steel and its tendency to manifest reversible temper brittleness during subsequent heat treatment of finished products from the pipe billet under consideration. The lower limit for arsenic, tin, lead, and zinc (0.0001%, respectively, is determined by the technology of steel production, and the upper limit (0.03%, 0.02%, 0.01%, and 0.005%, respectively) determines the increased tendency of steel to be reversible temper fragility.

The ratio (As + Sn + Pb + 5 × Zn) ≤0.07 determines the reduced tendency of steel to manifest reversible temper brittleness. The ratio of 0.46≤ (C + Mn / 6) ≤0.55 determines the parameters of the viscosity and hardenability of steel.

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 - providing an increased level of consumer properties while ensuring a favorable ratio of strength, ductility and viscosity, a minimum level of anisotropy of mechanical properties, low content of non-metallic inclusions, homogeneous macro - and microstructure rolled. Therefore, the claimed combination of features corresponds to the novelty and inventive step.

An example embodiment of the invention, not excluding others in the scope of the claims. Smelting of the test steel, chemical composition in wt.%: Carbon - 0.25%, manganese - 1.53, silicon - 0.26, aluminum - 0.032, nitrogen - 0.009, arsenic - 0.010, tin - 0.007, lead - 0.006, zinc - 0.002, is produced in 150-ton arc steel-smelting furnaces (DSP) using 100% metallized pellets in the charge, which ensures that the mass fraction of nitrogen before discharge from the DSP is no more than 0.003%, as well as a low content of color impurities. The preliminary alloying of the metal with manganese and silicon is carried out in the ladle upon discharge from the particleboard. After the release, the metal was purged with argon through the bottom purge unit, during which the steel was deoxidized by aluminum. After that, the metal enters the integrated steel processing unit (AKOS), where it is possible to heat the metal to the required temperature, purge it with argon through the bottom blowing unit, dosed the necessary ferroalloys and treat the steel with flux-cored wire with various fillers. At AKOS, refining slag is imposed with an additive of lime and fluorspar, slag is deoxidized with granulated aluminum, the metal is alloyed with aluminum to a content of 0.050%, the metal is refined according to the manganese content, and it is heated to a temperature that ensures further processing. After processing at AKOS, the metal is subjected to vacuum treatment at a batch vacuum. During evacuation, a final adjustment of the chemical composition is made. After evacuation, the metal is treated with silicocalcium and transferred to casting. The casting is carried out on radial four-strand UNRS in an ingot 300 × 360 mm in size with a drawing speed of 0.6 ÷ 0.7 m / min, with metal protection from oxidation by using cover slag mixtures in the intermediate ladle and mold, protective tubes, immersion glasses and argon feed. It also provides a low nitrogen and oxygen content and metal purity from non-metallic inclusions. After casting and cutting to a measured length, the continuously cast billets obtained were cooled in controlled cooling furnaces. Hot rolling of long products starts at a temperature of 900-950 ° C and ends at a temperature of 740-850 ° C, with a deformation in the last passes of at least 20%. Heat treatment of rolled products - normalization from 910-930 ° С.

As a result of hot rolling we get a pipe billet ⌀110 mm, length - 4800 mm. Structure - lamellar ferrite-perlite, real grain score - 9, macrostructure: central porosity - 1 point, point heterogeneity - 1 point, segregation square - 1 point, shrink segregation - 1 point, segregation strips - 1 point, non-metallic inclusions: point sulphides - 1 point, point oxides - 1 point, line oxides - 1 point, brittle silicates - 1 point, plastic silicates - 0.5 points, undeformed silicates - 1.5 points.

The mechanical properties in the normalized state are the temporary tensile strength of 542 N / mm ² , yield strength 412 N / mm ² , elongation of 20%. (As + Sn + Pb + 5 × Zn) = 0.033; (C + Mn / 6) = 0.505.

A low-carbon low-alloy steel billet provides an increased level of consumer properties of rolled products while ensuring a favorable ratio of strength, ductility and toughness, a minimum level of anisotropy of mechanical properties, a low content of non-metallic inclusions, and a homogeneous macro- and microstructure of rolled products.

Claims (2)

1. A pipe billet of low carbon low alloy steel having predetermined parameters of the structure and mechanical properties, characterized in that the billet is made of steel containing, wt.%: carbon [C] 0.23-0.29 Manganese [Mn] 1.35-1.65 silicon [Si] 0.17-0.43 aluminum [Al] 0.02-0.04 nitrogen [N] 0.005-0.010 arsenic [As] 0.0001-0.03 tin [Sn] 0.0001-0.02 lead [Pb] 0.0001-0.01 zinc [Zn] 0.0001-0.005 iron and inevitable impurities rest, when the relations are satisfied: (As + Sn + Pb + 5 · Zn) ≤0.07; 0.46≤ (C + Mn / 6) ≤0.55, hot-rolled, normalized and has a ferritoperlite structure with a real grain size of 5-10 points, a macrostructure with central porosity, point heterogeneity, segregation square, shrinkable segregation of not more than 2.0 points, segregation strips no more than 1 point, non-metallic inclusions of the type: point sulfides, point oxides, line oxides, brittle silicates, plastic silicates and non-deformed silicates with an average size for each type of inclusions no more than 2.5 points, temporary resistance a tensile strength of not less than 517 N / mm ² , yield strength 379-552 N / mm ² , elongation of at least 18%. 2. The pipe billet according to claim 1, characterized in that the steel contains inevitable impurities, wt.%: Phosphorus not more than 0.025, sulfur not more than 0.030, chromium not more than 0.30, nickel not more than 0.25, copper not more than 0.30.