RU2341567C2 - Tubing stock made of low-alloy steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: tubing stock of diameter from 80 till 180 mm is implemented of steel containing wt %: carbon 0.18-0.23, manganese 0.45-0.65, silicon 0.17-0.37, chrome 0.005-0.70, vanadium 0.050-0.090, aluminium 0.020-0.050, calcium 0.001-0.05, niobium 0.005-0.030, copper is not more than 0.25, nickel is not more than 0.25, iron and unavoidable admixtures - the rest, at fulfillment of conditions: C+Mn/6+(Cr+V)/5+(Ni+Cu)/15≤0.43, at that it has ferrite - pearlite structure, effective grain size 5-8 points, macrostructure by center porosity, dotted nonuniformity, undershrinked gathering, segregate square is not more than 2 points for each kind, segregate strips is not more than 1 point, nonmetallics by sulfides, dotted oxides, stitch oxides, fragile silicates, plastic silicates, unstrained silicates not more than 4.0 points by each kind. In the capacity of unavoidable admixtures steel contains in wt %: arsenic is not more than 0.03, tin is not more than 0.02, lead is not more than 0.01, zinc is not more than 0.005, nitrogen is not more than 0.008, sulphur is not more than 0.010, phosphorus is not more than 0.015. Mechanical properties after normalising: tensile strength is not less than 500 N/mm², yield strength is not less than 390 N/mm², tensile strain is not less than 16%, contraction ratio is not less than 45%, and impact viscosity at room temperature KCU is not less than 20 J/cm².

EFFECT: favorable ratio of strength, flexibility and viscosity.

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 80 to 160 mm from low alloy steel.

Closest to the proposed invention is a known tubular billet made of 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 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 pipe 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 most important requirements for a low-alloy steel billet are, on the one hand, ensuring the uniformity of micro- and macrostructures, low content of non-metallic inclusions, and, on the other hand, ensuring an increased range of consumer properties.

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

To achieve a technical result, a tubular billet of low alloy steel, continuously cast, hot rolled, having predetermined parameters of metallurgical quality for non-metallic inclusions and structure, mechanical properties, hardenability, contains the following component ratios in wt.%:

carbon 0.18-0.23 manganese 0.45-0.65 silicon 0.17-0.37 chromium 0.005-0.70 vanadium 0,050-0,090 aluminum 0,020-0,050 calcium 0.001-0.05 niobium 0.005-0.030 copper no more 0.25 nickel no more 0.25 iron and inevitable impurities rest,

when performing the following element ratios:

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

has a ferrite-pearlite structure, the actual grain size is 5-8 points, for the macrostructure - central porosity, point heterogeneity, shrinkable segregation, segregation square of not more than 2 points for each species, segregation strips - not more than 1 point, for non-metallic inclusions: sulfides , point oxides, line oxides, brittle silicates, plastic silicates, undeformed silicates - not more than 4.0 points for each type of inclusions. As inevitable impurities, steel additionally contains in wt.%: Arsenic no more than 0.03, tin no more than 0.02, lead no more than 0.01, zinc no more than 0.005, nitrogen no more than 0.008, sulfur no more than 0.010, no phosphorus more than 0.015. Mechanical properties after normalization - temporary tensile strength not less than 500 N / mm ² , yield strength not less than 390 N / mm ² , elongation not less than 16%, relative narrowing not less than 45%, impact strength at room temperature KCU not less than 20 J / cm ² .

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 (0.23%) is due to the need to ensure the required level of ductility of steel, and the lower - respectively 0.18% - to ensure the required level of strength and hardenability of this steel.

Manganese 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. In this case, the upper level of manganese — 0.65% and chromium — 0.70% —is determined by the need to ensure the required level of ductility of steel, and the lower — 0.45% and 0.005%, respectively, by the need to provide the required level of strength and hardenability, and heat resistance of this become.

Silicon refers to ferrite-forming elements. The lower limit on silicon - 0.17% - is due to the technology of deoxidation of steel. A silicon content above 0.37% adversely affects the ductility characteristics of 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 content is 0.09%, niobium is 0.03% due to the need to ensure the required level of ductility of steel, and the lower limit is 0.05% and 0.005%, respectively, to ensure the required level of strength of this steel.

Aluminum and titanium are used as deoxidizers. So the lower level of aluminum content - 0.02 - is determined by the requirement to ensure uniformity of the structure, and the upper level of 0.05 - by the requirement to ensure a given level of ductility of steel.

Calcium is a modifier of non-metallic inclusions. The lower level - 0.001% - is determined by the need to provide the required morphology of non-metallic inclusions, and the upper level - 0.05% - by the need to provide a given level of ductility of steel.

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

An example of obtaining a tube billet.

Smelting of the test steel, chemical composition in wt.%: Carbon - 0.19, manganese - 0.51, silicon - 0.28, chromium - 0.20, niobium - 0.01, vanadium - 0.06, aluminum - 0.035 , calcium - 0.08, nitrogen - 0.007, copper - 0.025, nickel - 0.02, provided C + Mn / 6 + (Cr + V) / 5 + (Ni + Cu) / 15 = 0.33 is produced in 150-ton electric arc furnace of chipboard with the use of 100% metallized pellets in the charge, which ensures a mass fraction of nitrogen before discharge from the chipboard of 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 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. 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.

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 are determined. The characteristics of impact strength at room temperature are determined on samples of type I, GOST 9454-78, on a mechanical head MK-30. The viscous component in the fractures of shock samples is determined visually.

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:

и

- дисперсии среднего;

- критическое значение критерия Стьюдента при уровне значимости 0.95 и числе степеней свободы - α.where M ₁ and M ₂ are the average values of the compared values;

and

- average variance;

- the critical value of the Student criterion with 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, structure: ferrite-perlite, real grain score is 7. Macrostructure: central porosity - 1 point, point heterogeneity - 1 point, light contour - 1 point, shrinkage segregation - 1 point, liquidation strips - 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, undeformed silicates - 1.5 points.

The mechanical properties after normalization are the tensile strength 562 N / mm ² , yield strength 424 N / mm ² , elongation 17%, relative narrowing 52%, impact strength at room temperature KCU 54 J / cm ² .

The introduction of the proposed low-alloy steel billet 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 low alloy steel, hot rolled, having predetermined parameters of non-metallic inclusions, structure, actual grain size and mechanical properties, characterized in that it is made of steel containing the following ratio of components, wt.%: carbon 0.18-0.23 manganese 0.45-0.65 silicon 0.17-0.37 chromium 0.005-0.70 vanadium 0,050-0,090 aluminum 0,020-0,050 calcium 0.001-0.05 niobium 0.005-0.030 copper no more than 0.25 nickel no more than 0.25 iron and inevitable impurities rest subject to the ratio C + Mn / 6 + (Cr + V) / 5 + (Ni + Cu) / 15≤0.43, Moreover, it has a ferrite-pearlite structure, the actual grain size is 5-8 points, the macrostructure according to central porosity, point heterogeneity, shrink segregation, the segregation square is not more than 2 points for each species, the 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 4.0 points for each type of inclusions, temporary tensile strength not less than 500 N / mm ² , yield strength not less its 390 N / mm ² , elongation of not less than 16%, relative narrowing of not less than 45%, impact strength at room temperature KCU not less than 20 J / cm ² . 2. The pipe billet according to claim 1, characterized in that the steel contains inevitable impurities, wt.%: Arsenic not more than 0.03, tin not more than 0.02, lead not more than 0.01, zinc not more than 0.005, nitrogen is not more than 0.008, sulfur is not more than 0.010, phosphorus is not more than 0.015.