RU2333970C1 - Tubing stock made from low-alloyed steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention concerns metallurgy field. Particularly it concerns manufacturing of tubing stock of diameter from 80 till 180 mm. For providing of increased level of consumer properties, favourable correlation between strength, flexibility and viscosity, reduced tendency to temper brittleness stock is made from steel containing mass %: carbon 0.34-0.40, manganese 1.40-1.65, silicon 0.17-0.37, vanadium 0.06-0.12, chrome 0.005-0.30, nitrogen 0.005-0.015, arsenic 0.0001-0.03, tin 0.0001-0.02, lead 0.0001-0.01, zinc 0.0001-0.005, iron and unavoidable admixtures -the rest, at fulfillment of following ratio; (As+Sn+Pb+5×Zn) ≤0.07, [C+Mn/6+(Cr+V)/5]≤0.72. In the capacity of unavoidable admixtures steel contains, mass%: niobium not more than 0.02, phosphorus not more than 0.030, sulphur not more than 0.030, nickel not more than 0.30, copper not more than 0.30, molybdenum not more than 0.10. Hot-rolled tubing stock is of lamellar ferrite-pearlitic structure. Dimension of valid grain is 6-9 points, macrostructure: center porosity, spot heterogeneity, segregation square, subshrink segregation is not more than 3 points for each kind, segregation streaks are not more than 2 points, nonmetallic inclusions: sulphides, spot oxides, stitch oxides, fragile silicates, flexible silicates, non-strained silicates are not more than 4.5 points for each kind of inclusions, tensile strength is not less than 689 N/mm², yield strength is not less than 552 N/mm², percentage elongation is not less than 13%, contraction ratio is not less 48%, impact resistance KCU+20°C is not less 60 J/mm².

EFFECT: receiving of steel tubing stock with high mechanical properties.

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 180 mm from low alloy steel.

Known tube billet of low alloy steel containing carbon, silicon, manganese, niobium, molybdenum, sulfur, phosphorus, chromium, copper, nickel, aluminum, titanium and iron - the rest, subjected to hot rolling (RU 2252972 C1, C21D 9/08, 27.05 .2005).

Known pipe billet of low alloy steel containing carbon, manganese, silicon, chromium, nickel, vanadium, niobium, titanium, aluminum, calcium, sulfur, phosphorus, nitrogen, copper, antimony, tin, arsenic, iron - the rest, made from hot rolled sheet (RU 2180691 C1, C21D 9/08, 03/20/2002).

The objective 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, a homogeneous macro- and microstructure of rolled products, as well as a reduced tendency to temper brittleness.

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

carbon 0.34-0.40 manganese 1.40-1.65 silicon 0.17-0.37 vanadium 0.06-0.12 chromium 0.005-0.30 nitrogen 0.005-0.015 arsenic 0.0001-0.03 tin 0.0001-0.02 lead 0.0001-0.01 zinc 0.0001-0.005 iron and inevitable impurities rest,

when performing the following ratios:

amount (arsenic + tin + lead + 5 × zinc) ≤0.07;

amount: [carbon + manganese / 6 + (chromium + vanadium) / 5] ≤0.72,

continuously cast, hot-rolled, has a lamellar ferritoperlite structure, the actual grain size is 6–9 points, the macrostructure is central porosity, point heterogeneity, segregation square, shrink segregation is not more than 3 points for each type, segregation strips are not more than 2 points, non-metallic inclusions: sulfides, point oxides, line oxides, brittle silicates, plastic silicates, undeformed silicates - not more than 4.5 points for each type of inclusions, mechanical properties after normalization are temporary tear resistance not less than 689 N / mm ² , yield strength not less than 552 N / mm ² , elongation not less than 13%, relative narrowing not less than 48%, impact strength KCU + 20 ° C not less than 60 J / mm ² .

As impurities, the steel additionally contains niobium, phosphorus, sulfur, nickel, copper and molybdenum in the following proportions, wt.%:

niobium - not more than 0.02

phosphorus - not more than 0,030

sulfur - not more than 0,030

nickel - no more than 0.30

copper - no more than 0.30

molybdenum - not more than 0.10

The above combinations of alloying elements make it possible to obtain a ferritoperlite finely dispersed structure in the finished product with a favorable combination of strength and ductility, weldability and a reduced tendency to reversible temper brittleness.

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 (0.40%) is due to the need to ensure the required level of ductility of steel, and the lower - 0.34%, respectively - to ensure the required level of strength and hardenability of this steel.

Vanadium is introduced into the composition of this steel in order to provide a finely dispersed, uniform grain structure. Moreover, it controls 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 temperature of recrystallization and, as a result, affects the nature of the γ-α transformation). The upper limit of the vanadium content is 0.12% due to the need to ensure the required level of ductility of steel, and the lower limit, respectively 0.06%, to ensure the required level of strength 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. The upper level of manganese - 1.65% and chromium - 0.30% is determined by the need to ensure the required level of ductility of steel, and the lower, manganese - 1.40% and chromium - 0.005%, respectively, by the need to provide the required level of strength and hardenability of this become.

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 0.37% adversely affects the ductility characteristics of steel.

Nitrogen promotes the formation of nitrides in steel. The upper limit of nitrogen content - 0.015% 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.

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% for each element, 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 the tendency of steel to reversible temper brittleness.

The ratio [C + Mn / 6 + (Cr + V) / 5] ≤0.72 determines the strength and toughness characteristics of the studied steel, while the ratio (As + Sn + Pb + 5 × Zn) ≤0.07 determines a lower the tendency of steel to manifest reversible temper embrittlement.

An example embodiment of the invention

Smelting of the test steel (chemical composition, wt.%: Carbon - 0.36, manganese - 1.54, silicon - 0.25, chromium - 0.18, vanadium - 0.10, arsenic - 0.009, tin - 0.004, lead - 0.002, zinc - 0.001, nitrogen - 0.009) is produced in 150-ton steel-arc furnaces using 100% metallized pellets in the charge, which ensures that the mass fraction of nitrogen before discharge 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 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. Casting is carried out on four-strand radial-type ONRSs with an ingot of 300 × 360 mm in size with a drawing speed of 0.6 ÷ 0.7 m / min, with metal protection from oxidation by using coating 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 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 specimens, GOST 1497-84 on an INSTRON-1185 test 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:

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 is α.

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

As a result of hot rolling, a заготов100 mm pipe billet is obtained, with a length of 11800 mm. The structure is ferritic perlite, the real grain score is 9. Macrostructure: central porosity - 1 point, point heterogeneity - 2 points, segregation square - 0.5 points, shrink segregation - 1 point, segregation strips - 1 point. Non-metallic inclusions: sulfides - 1 point, point oxides - 1 point, strox oxides - 1 point, brittle silicates - 0.5 points, plastic silicates - 0.5 points, non-deforming silicates - 1 point. Mechanical properties after normalization at 870 ° С, 1 hour, air: temporary tensile strength 710 N / mm ² , yield strength 565 N / mm ² , elongation 15%, relative narrowing 51%, impact strength KCU + 20 ° C - 75 J / mm ² ; (As + Sn + Pb + 5 × Zn) = 0.020, [C + Mn / 6 + (Cr + V) / 5] = 0.67.

Implementation of the proposed method for the production of tube billets from alloy steel provides an increase in the level of consumer properties while providing a favorable ratio of strength, ductility and toughness, a minimum level of anisotropy of mechanical properties, a reduced tendency to reversible temper brittleness, satisfactory weldability, low content of non-metallic inclusions, homogeneous macro- and microstructure rental.

Claims (2)

1. The pipe billet of low alloy steel, characterized in that it is obtained from steel containing the following ratio of components in wt.%: carbon 0.34-0.40 manganese 1.40-1.65 silicon 0.17-0.37 vanadium 0.06-0.12 chromium 0.005-0.30 nitrogen 0.005-0.015 arsenic 0.0001-0.03 tin 0.0001-0.02 lead 0.0001-0.01 zinc 0.0001-0.005 iron and inevitable impurities rest, with the following ratios of components: (As + Sn + Pb + 5 Zn) ≤0.07; [C + Mn / 6 + (Cr + V) / 5] ≤0.72, Moreover, it is made of continuously cast, hot-rolled and has a lamellar ferritic perlite structure, the actual grain size is 6–9 points, the macrostructure of the species: central porosity, point heterogeneity, segregation square, shrink segregation no more than 3 points for each type, segregation strips no more than 2 points, non-metallic inclusions of the type: sulfides, point oxides, line oxides, brittle silicates, plastic silicates, undeformed silicates not more than 4.5 points for each type of inclusions, mechanical properties VA after normalization - temporary tensile strength not less than 689 N / mm ² , yield strength not less than 552 N / mm ² , elongation not less than 13%, relative narrowing not less than 48%, impact strength KCU + 20 ° C not less than 60 J / mm ² . 2. The pipe billet according to claim 1, characterized in that as inevitable impurities the steel contains niobium, phosphorus, sulfur, nickel, copper and molybdenum in the following proportions, wt.%: Niobium not more than 0.02, phosphorus not more than 0.030, sulfur is not more than 0.030, nickel is not more than 0.30, copper is not more than 0.30, molybdenum is not more than 0.10.