RU2336320C1 - Tube stock out of micro alloyed steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention refers to metallurgy, particularly to production of tube stock with diameter from 80 to 180 mm designed for fabricating of seamless tubes of various applications. The tube stock is made out of steel, containing, wt %: carbon 0.16-0.27, manganese 0.34-1.00, silicon 0.17-0.37, chromium 0.005-0.20, molybdenum 0.001-0.10, copper 0.005-0.20, nickel 0.005-0.20; nitrogen 0.005 -0.008, niobium 0.005-0.010, vanadium 0.005-0.020, titanium 0.005-0.040, iron and unavoidable impurities - the rest, at maintaining the following conditions: V+Nb≤0.02; C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15≤0.43. As impurities steel contains (wt. %): aluminium not more, than 0.50, sulphur not more, than 0.025, phosphorus not more, than 0.025. The stock is made continuous cast, hot rolled, it has ferrite-pearlite structure, the dimension of the actual grain is 5-8 points, macrostructure for central porosity, pointed non-uniformity, liquation square, subshrinking liquation is not more, than 2 points for each kind, liquation strips not more, than 1 point, non-metallic inclusions for sulphides, pointed oxides, stripped oxides, fragile silicates, plastic silicates, non-deformed silicates not more, than 2.5 for each kind. Tensile strength is not less, than 415 N/mm², yield point is not less, than 241 N/mm², specific elongation is not less, than 24%.

EFFECT: facilitating optimal combination of hardness, plasticity and viscosity of tube stock.

2 cl, 1 ex

Description

The invention relates to the field of metallurgy, in particular to the production of pipe billets with a diameter of 80 to 180 mm from microalloy steel, intended for the production of seamless pipes for various purposes.

Known pipe billet from low alloy 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 no more than 0.015, phosphorus no more than 0.020, aluminum 0.01-0.05, niobium 0.01-0.06, iron and the rest unavoidable impurities. The hot-rolled tube billet has a high level of steel purity for non-metallic inclusions and a certain microstructure (RU 2221875 C2, 01.20.2004, C21C 5/52).

The closest analogue is the known seamless pipe production method in which a hot-rolled tubular billet of microalloyed steel is disclosed having predetermined parameters for non-metallic inclusions, mechanical properties, real grain size and a certain microstructure (RU 2210604 C2, C21D 8/10, 08/20/2003 )

The most important requirement for a pipe billet made of microalloyed steel is, on the one hand, to ensure a given structure, to improve metallurgical quality parameters: uniformity of micro- and macrostructure with a low content of non-metallic inclusions, and on the other hand, to provide an increased range of consumer properties.

The technical result of the invention is to provide an increased 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 technical result is achieved by the fact that in a pipe billet of microalloyed steel, continuously cast, hot rolled, having predetermined parameters of metallurgical quality for non-metallic inclusions and structure, mechanical properties, steel contains the following component ratios in wt.%:

carbon 0.16-0.27 manganese 0.34-1.00 silicon 0.17-0.37 chromium 0.005-0.20 molybdenum 0.001-0.10 copper 0.005-0.20 nickel 0.005-0.20 nitrogen 0.005-0.008 niobium 0.005-0.010 vanadium 0.005-0.020 titanium 0.005-0.040 iron and inevitable impurities  rest

when performing the following element ratios:

V + Nb≤0.02,

C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15≤0.43,

the pipe billet has a ferrite-pearlite structure, the actual grain size is 5-8 points, according to the macrostructure is central porosity, point heterogeneity, light contour, shrink segregation is not more than 2 points for each type, segregation strips are not more than 1 point, for non-metallic inclusions sulfides, point oxides, line oxides, brittle silicates, plastic silicates, non-deforming silicates - not more than 2.5 points for each type of inclusions. The mechanical properties after normalization are the temporary tensile strength of at least 415 N / mm ² , the yield strength of at least 241 N / mm ² , the elongation of at least 24%. As inevitable impurities, steel additionally contains in wt.%: Aluminum no more than 0.050, sulfur no more than 0.025, phosphorus no more than 0.025.

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 this steel in order to ensure a given level of its strength and hardenability. The upper limit of the carbon content of 0.27% is due to the need to ensure the required level of ductility of steel, and the lower - respectively 0.16% - to ensure the required level of strength and hardenability of this steel.

Manganese, molybdenum and chromium 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. The upper level of manganese 1.00%, molybdenum 0.10% and chromium 0.20% is determined by the need to ensure the required level of ductility of steel, and the lower - 0.34%, 0.001% and 0.005%, respectively - by the need to provide the required level of strength , hardenability and heat resistance 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% will adversely affect the ductility characteristics of steel.

Vanadium, niobium and titanium 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 (determines the tendency to growth of austenite grain, stabilizes the structure during thermomechanical processing, increases the recrystallization temperature and, as a result, affects the nature of the γ-α transformation. The upper limit of the vanadium content is 0.02%, niobium 0.01% and titanium 0.040% due to the need to ensure the required level of ductility of steel, and the lower - respectively 0.005%, 0.005%, 0.005% - to ensure the required level of strength of this 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.

Nickel within the specified limits affects the characteristics of hardenability and toughness of steel. The lower level of nickel content of 0.005% is due to the need to ensure a given level of viscosity of steel, and the upper level of 0.20% due to the need to obtain a martensitic structure during hardening of steel (since nickel is an austenitizer).

Copper determines the characteristics of hot ductility of steel. Moreover, the lower level of its content of 0.005% is determined by the requirements to ensure a given level of ductility of steel. The upper level of 0.20% is due to the need to provide a given level of hardenability of steel.

To ensure a given level of ductility of steel, the following ratio of elements is required:

V + Nb≤0.02.

The ratio C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15≤0.43 determines the weldability conditions of the workpiece.

An example of manufacturing a pipe billet. Steel smelting, chemical composition in wt.%: Carbon 0.19, manganese 0.95, silicon 0.26, chromium 0.12, nickel 0.10, molybdenum 0.06, copper 0.09, niobium 0.01, vanadium 0.01, titanium 0.02, nitrogen 0.007, at

V + Nb = 0.02,

C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 = 0.40,

they are produced in 150-ton electric arc furnace of particleboard using 100% metallized pellets in the charge, which ensures that the mass fraction of nitrogen before being released from the particleboard is not 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 a ladle upon discharge from 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 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 of the chemical composition is made. 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 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. 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 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. After rolling, normalization is carried out to determine the mechanical properties.

The mechanical characteristics at room temperature are 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 are determined.

The average values of the characteristics 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 is 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 is controlled in accordance with TU 14-1-5212-93 and GOST 10243-75.

As a result of hot rolling, a заготов110 mm pipe billet is obtained, 11800 mm long, ferrite-pearlite structure, real grain score is 7. Macrostructure: 1 point central porosity, 1 point point heterogeneity, 1 point light contour, 1 point shrink segregation, liquation strips of 0.5 points. Non-metallic inclusions: sulfides 1.0 point, point oxides 0.5 points, line oxides 1 point, brittle silicates 0.5 points, plastic silicates 0.5 points, non-deforming silicates 1.5 points. Mechanical properties after normalization 920 ° C, 1 hour: temporary tensile strength 452 N / mm ² , yield strength 323 N / mm ² , elongation 26%,

V + Nb = 0.02,

C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 = 0.40.

The introduction of a tubular billet of microalloyed steel made it possible to provide 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 microalloyed steel, hot rolled, having predetermined parameters of non-metallic inclusions, actual grain size, mechanical properties, macrostructure, characterized in that it is made of steel containing the following ratio of components, wt.%: carbon 0.16-0.27 manganese 0.34-1.00 silicon 0.17-0.37 chromium 0.005-0.20 molybdenum 0.001-0.10 copper 0.005-0.20 nickel 0.005-0.20 nitrogen 0.005-0.008 niobium 0.005-0.010 vanadium 0.005-0.020 titanium 0.005-0.040 iron and inevitable impurities  rest, subject to the conditions: V + Nb≤0.02; C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15≤0.43, at the same time, 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 by sulfides, point oxides, line oxides, brittle silicates, plastic silicates, undeformed silicates not more than 2.5 points for each type of inclusions, temporary tensile strength not less than 415 N / mm ² , yield strength not less its 241 N / mm ² , elongation of at least 24%. 2. The pipe billet according to claim 1, characterized in that, as unavoidable impurities, the steel contains, wt.%: Aluminum, not more than 0.050, sulfur, not more than 0.025, phosphorus, not more than 0.025.