RU2336321C1 - Tube stock out of low carbon steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention refers to metallurgy, particularly to production of tube stock with diameter from 100 to 180 mm. To facilitate upgraded level of consumer characteristics at a minimal level of anisotropy of mechanical properties the tube stock is fabricated out of steel, containing, wt %: carbon 0.06-0.14, manganese 0.35-0.65, silicon 0.17-0.37, titanium 0.01-0.03, aluminium 0.020-0.060, vanadium 0.005-0.02, molybdenum 0.005-0.15, niobium 0.005-0.02, nitrogen 0.005-0.005, iron and unavoidable impurities - the rest, as impurities steel contains wt. %: chromium not more, than 0.25, nickel not more, than 0.30, copper not more, than 0.25, sulphur not more, than 0.013, phosphorus not more, than 0.18, at that steel has ferrite-pearlite structure, the dimension of the actual grain is 5-8 points, its macrostructure for central porosity, pointed non-uniformity, liquation square, subshrinking liquation - not more, than 2 points for each kind, liquation strips - not more, than 1 point, non-metallic inclusions - sulphides, pointed oxides, stripped oxides, fragile silicates, plastic silicates, non-deformed silicates - not more, than 4.5 points for each kind of inclusions, tensile strength is 350-480 N/mm², yield point is 235 N/mm², specific elongation is not less, than 28%, contraction ratio is not less, than 55%.

EFFECT: production of tube stock with upgraded level of consumer characteristics.

1 cl, 1 ex

Description

The invention relates to the field of metallurgy, in particular to the production of pipe billets with a diameter of 100 to 180 mm from low carbon steel.

The closest in technical essence and the achieved effect to the proposed invention is the known tubular billet of low carbon steel containing (wt.%): Carbon 0.05-0.30, manganese 0.35-1.50, silicon 0.15-1 , 0, chromium 0.005-0.5, nickel 0.005-0.50, copper 0.005-0.50, sulfur not more than 0.015, phosphorus not more than 0.020, aluminum 0.01-0.05, niobium 0.01-0, 06, iron and inevitable impurities - the rest, the tube billet has predetermined parameters for non-metallic inclusions, mechanical properties and a certain microstructure (RU 2221875 C2, C21C 5/52, 01.20.2004).

The most important requirement for a low-carbon steel billet is, on the one hand, ensuring the steel structure, improving metallurgical quality parameters: uniformity of micro- and macrostructures with a low content of non-metallic inclusions, and, on the other hand, providing an increased range of consumer properties.

The technical result 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.

To achieve a technical result in a pipe billet of low-carbon steel, continuously cast, hot-rolled, having predetermined parameters of non-metallic inclusions, structure and mechanical properties, the steel contains the following ratio of components, wt.%:

carbon 0.06-0.14 manganese 0.35-0.65 silicon 0.17-0.37 titanium 0.01-0.03 aluminum 0,020-0,060 vanadium 0.005-0.02 molybdenum 0.005-0.15 niobium 0.005-0.020 nitrogen 0.005-0.008 iron and inevitable impurities  rest

The pipe billet has a ferrite-pearlite structure, the actual grain size is 5–8 points for the macrostructure — central porosity, point heterogeneity, segregation square, shrink segregation no more than 2 points for each species, segregation strips no more than 1 point, for non-metallic inclusions sulfides, oxides point, line oxides, brittle silicates, plastic silicates, non-deforming silicates - not more than 4.5 points for each type of inclusions.

As unavoidable impurities, steel additionally contains the following components, wt.%: Chromium no more than 0.25, nickel no more than 0.30, copper no more than 0.25, sulfur no more than 0.013, phosphorus no more than 0.018. Mechanical properties after normalization: temporary tensile strength 350-480 N / mm ² , yield strength not less than 235 N / mm ² , elongation not less than 28%, relative narrowing not less than 55%.

The given combinations of alloying elements make it possible to obtain a ferrite-pearlite finely dispersed structure, a low content of non-metallic inclusions, a homogeneous macrostructure, and a favorable combination of strength and ductility characteristics in the finished product.

Carbon is introduced into the composition of steel in order to ensure a given level of its strength and hardenability. The upper limit of the carbon content of 0.14 is due to the need to ensure the required level of ductility of steel, and the lower, respectively 0.06, to ensure the required level of strength and hardenability of this steel.

Vanadium and niobium are introduced into the composition of this steel in order to provide a finely dispersed, uniform grain structure. Moreover, they control the processes in the lower part of the austenitic region (they determine the tendency for austenite grain to grow, stabilize the structure during thermomechanical treatment, increase the recrystallization temperature and, as a result, affect the nature of the γ-α transformation). The upper limit of the vanadium and niobium content of 0.02% is due to the need to ensure the required level of ductility of steel, and the lower - respectively 0.005% - to ensure the required level of strength of this steel.

Manganese and molybdenum are used, on the one hand, as solid solution hardeners, and on the other hand, as elements that increase the stability of supercooled austenite of steel. In this case, the upper level of the content of manganese 0.65% and molybdenum 0.15% is determined by the need to ensure the required level of ductility of steel, and the lower 0.35% and 0.005%, respectively, by the need for the required level of strength and hardenability of this steel.

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

Aluminum and titanium are used as deoxidizers and nitride former. So, the lower level of the content of these elements (0.02 and 0.01, respectively) is determined by the requirement to ensure a given level of uniformity and dispersion of the steel structure, and the upper level (0.06 and 0.03) is determined by the requirement to ensure a given level of ductility of steel.

Nitrogen promotes the formation of nitrides in steel. The upper limit of nitrogen content of 0.008% is due to the need to obtain a given level of ductility and toughness of steel, and the lower limit of 0.005% is due to issues of manufacturability.

An example of manufacturing a pipe billet.

Steel smelting, chemical composition, wt.%: Carbon 0.11, manganese 0.5, silicon 0.22, titanium 0.024, aluminum 0.033, vanadium 0.015, molybdenum 0.03, niobium 0.01, nitrogen 0.007 is produced in 150 -tonic arc steel-smelting furnaces using 100% metallized pellets in the charge, which ensures that the mass fraction of nitrogen before leaving the particleboard is not more than 0.003%, as well as a low content of impurities, wt.%: chromium 0.11, nickel 0.08, copper 0.06, sulfur 0.009, phosphorus 0.012.

The preliminary alloying of the metal with manganese and silicon is carried out in a ladle upon discharge from particleboard. After release, the metal is purged with argon through the bottom purge unit, during which the steel is 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 adjusted according to the manganese content, and it is heated to a temperature that ensures further processing. After processing on AKOS, the metal is subjected to vacuum treatment on a batch vacuum. During evacuation, a final adjustment is made in chemical composition. After evacuation, the metal is treated with silicocalcium and transferred to casting. The casting is carried out on a four-strand radial-type ONRS with an ingot of 300 × 360 mm in size with a drawing speed of 0.6 ÷ 0.7 m / min with protection of the metal from oxidation by using coating slag mixtures in the intermediate ladle and mold, protective tubes, immersion glasses and feeding argon. This provides a low content of nitrogen and oxygen, and the purity of the metal on non-metallic inclusions. After casting and cutting to a measured length, the continuously cast billets obtained are cooled in controlled cooling furnaces. Hot rolling of long products starts at a temperature of 1180-1150 ° C and ends at a temperature of 840-950 ° C. To determine the mechanical properties of the tubular billet is subjected to normalization at 920 ° C with an exposure of 1 hour.

The mechanical characteristics at room temperature were determined on type I samples, GOST 1497-84 on an INSTRON-1185 testing machine with strain-strain registration. The loading speed of the sample is 5 mm / min. The strength characteristics σ _b and σ _0.2 and ductility δ and φ were determined.

The average values of the characteristics were calculated according to the test results of at least three samples per point. The significance of the differences in the average values of the analyzed values was evaluated using the student criterion, calculated as follows:

where M ₁ and M ₂ are the average values of the compared values; S ₁ ² and S ₂ ² - variance of the average; t _kr ^0.05 (α) is the critical value of the Student criterion at a significance level of 0.95 and the number of degrees of freedom α.

The macrostructure was controlled in accordance with TU 14-1-5212-93 and GOST 10243-75.

As a result of hot rolling, a billet with a diameter of 122 mm and a length of 11800 mm is obtained. The structure is ferrite-pearlite, the actual grain score is 7. Macrostructure: central porosity 1 point, point heterogeneity 0.5 points, segregation square 1 point, shrink segregation 1 point, segregation strips 0.5 points. Non-metallic inclusions: sulfides - 1 point, point oxides 0.5 points, line oxides 1 point, brittle silicates 1 point, plastic silicates 1 point, non-deforming silicates 1.5 points. Mechanical properties after normalization 920 ° C, 1 hour: temporary tensile strength 390 N / mm ² , yield strength 245 N / mm ² , elongation 32%, relative narrowing 65%.

The introduction of the production of tube billets from low alloy steel 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. Continuous cast billet of low carbon steel, hot rolled with specified parameters for metal inclusions, mechanical properties and macrostructure, characterized in that it is made of steel containing the following ratio of components, wt.%: carbon 0.06-0.14 manganese 0.35-0.65 silicon 0.17-0.37 titanium 0.01-0.03 aluminum 0,020-0,060 vanadium 0.005-0.02 molybdenum 0.005-0.15 niobium 0.005-0.02 nitrogen 0.005-0.008 iron and inevitable impurities  rest, it has a ferrite-pearlite structure, the actual grain size is 5-8 points, the macrostructure according to central porosity, point heterogeneity, segregation square, shrink segregation is not more than 2 points for each species, segregation strips are not more than 1 point, non-metallic inclusions for sulfides, point oxides, oxides stitch, brittle silicates, silicates plastic, silicates undeformed not more than 4.5 points for each mean tensile strength 350-480 N / mm ^2, yield stress of not less than 235 N / mm ^2, classifies Flax elongation of not less than 28%, relative narrowing of at least 55%. 2. The pipe billet according to claim 1, characterized in that the steel contains the following components as inevitable impurities, wt.%: Chromium not more than 0.25, nickel not more than 0.30, copper not more than 0.25, sulfur not more than 0.013, phosphorus not more than 0.018.