RU2243288C1 - Steel - Google Patents

Abstract

FIELD: metallurgy, in particular low-alloy steel.

SUBSTANCE: invention relates to low-alloy, thin-sheet, weldable, structural steel for manufacturing of heavy hauler platforms. Claimed steel contains (mass %): carbon 0.12-0.20; manganese 1.0-1.8; silicium 0.4-0.7; chromium 0.4-0.8; aluminium 0.02-0.05; vanadium 0.04-0.08; nitrogen <=0.015; copper 0.1-0.4; nickel <=2.5; calcium 0.002-0.050; niobium <=0.06; titanium <=0.03; boron 0.001-0.005; phosphorus <=0.02; sulfur <=0.015; and balance: iron.

EFFECT: steel with improved tensile strength and reduced shrinkage of hot-rolled thin-sheet product.

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Description

The invention relates to metallurgy, in particular to low alloy plate welded structural steels intended for the manufacture of platforms for heavy vehicles operating in the Far North.

For the manufacture of platforms for heavy dump trucks operating at temperatures up to -50 ° C, hot-rolled sheets with a thickness of 9-50 mm in heat-treated condition from welded cold-resistant low alloy steel are used. Hot rolled steel sheets for this purpose must combine high strength and toughness at low temperatures. The required mechanical properties of hot rolled sheets in the delivery state are given in table. 1.

Known low alloy steel having the following chemical composition, wt.%:

Carbon 0.11-0.16

Manganese 1.0-1.4

Silicon 0.15-0.35

Titanium 0.08-0.14

Copper 0.02-0.30

Aluminum 0.02-0.06

Chrome 0.02-0.15

Nickel 0.02-0.15

Molybdenum 0.005-0.015

Vanadium 0.005-0.015

Iron Else [1].

The disadvantages of steel of known composition are that the hot rolled sheets after heat treatment have low strength and viscosity properties at -40 ° C.

Also known low alloy steel containing, wt.%:

Carbon 0.05-0.2

Manganese 0.15-1.6

Phosphorus 0.015

Silicon Not more than 0.5

Sulfur 0.002-0.008

Copper 0.2-0.5

Aluminum Less than 0.1

Niobium and / or Less than 0.05

Vanadium 0.1

Molybdenum 0.5

Chrome Less than 0.5

Nickel Less than 0.3

Calcium 0.0001-0.005

Iron Else [2].

The disadvantages of steel of the specified composition is the low strength and toughness of thick hot-rolled sheets even after thermal improvement.

The closest in composition and properties to the proposed steel is a low alloy welded steel of the following composition, wt.%:

Carbon 0.12-0.18

Manganese 1.0-1.8

Silicon 0.4-0.7

Chrome 0.4-0.8

Aluminum 0.01-0.05

Vanadium 0.04-0.08

Nitrogen 0.009-0.015

Copper 0.1-0.4

Nickel 0.01-0.34

Calcium 0.001-0.05

Iron Else [3].

The disadvantages of the known steel are as follows. To increase the complex of mechanical properties, thick hot-rolled sheets of this steel must be subjected to thermal improvement (hardening + tempering). But even after thermal improvement, the viscosity properties of thick sheets at a temperature of -40 ° C (KCU ^-40 and KCV ^-40 ) remain below the acceptable level. In addition, thermal stresses arising in the process of hardening of thick sheets lead to a violation of their flatness (warping).

The technical problem solved by the invention is to increase the viscous properties and flatness of plate in heat-treated condition.

The stated technical problem is solved in that the steel containing carbon, manganese, silicon, chromium, aluminum, vanadium, nitrogen, copper, nickel, calcium and iron, additionally contains niobium, titanium, boron, phosphorus and sulfur in the following ratio of elements, wt .%:

Carbon 0.12-0.20

Manganese 1.0-1.8

Silicon 0.4-0.7

Chrome 0.4-0.8

Aluminum 0.02-0.05

Vanadium 0.04-0.08

Nitrogen Not more than 0.015

Copper 0.1-0.4

Nickel No more than 2.5

Calcium 0.002-0.050

Niobium Not more than 0.06

Titanium Not more than 0.03

Boron 0.001-0.005

Phosphorus Not more than 0,020

Sulfur Not more than 0.015

Iron Else

 The essence of the invention lies in the fact that the proposed chemical composition of steel with boron, niobium and titanium in a heat-treated state acquires a cellular structure that increases the proportion of the viscous component in the fracture of the sample. Due to this, an increase in the viscosity properties of thick sheets at negative temperatures is achieved even when using normalization instead of thermal improvement. Due to the fact that hardening is not used, plate in heat-treated state has a high flatness. Sulfur and phosphorus, as inevitable impurities, in the indicated concentrations do not affect the properties of steel.

Carbon reinforces steel. With a carbon content of less than 0.12%, the required strength of the steel is not achieved, and with its content of more than 0.20%, the weldability of the steel deteriorates.

Manganese deoxidizes and strengthens steel, binds sulfur. When the manganese content is less than 1.0%, the strength and wear resistance of the steel are insufficient. An increase in manganese content of more than 1.8% leads to a decrease in viscosity at a temperature of -40 ° C.

Silicon deoxidizes steel, increases its strength. At a silicon concentration of less than 0.4%, the strength of the steel is lower than permissible, and at a concentration of more than 0.7%, ductility decreases, the steel does not stand the test for cold bending.

Chrome increases the strength and toughness of steel. When its concentration is less than 0.4%, the strength is below acceptable values. An increase in chromium content of more than 0.8% leads to a loss of ductility due to the growth of chromium carbides.

Aluminum deoxidizes steel and grinds grain. It binds nitrogen to nitrides, reducing its harmful effect on viscosity properties. When the aluminum content is less than 0.02%, its effect is small, the viscosity properties of steel deteriorate. An increase in the content of this element over 0.05% leads to graphitization of carbon and a decrease in strength characteristics.

Vanadium in combination with aluminum are strong deoxidizing and carbide forming elements. When the vanadium content is less than 0.04%, the strength and ductility of the steel in the heat-treated state decreases. An increase in the content of vanadium over 0.08% is impractical, because does not lead to further improvement of properties, but only increases the consumption of alloying.

Nitrogen in steel is an impurity element. It does not lead to a significant deterioration in properties when the concentration in steel is not more than 0.015%. Deeper deazotation significantly increases the cost of steel production.

Copper is introduced to increase the stability of austenite, which is especially important when normalizing thick sheets. An increase in copper content of more than 0.4% leads to graphitization of low alloy steel, which reduces the complex of mechanical properties. A decrease in copper content of less than 0.1% impairs the toughness and strength properties of low alloy steel after normalization.

Nickel helps to increase the strength of steel, and when its content is not more than 2.5%, the complex of mechanical properties of thick sheets does not deteriorate, but the possibilities of using nickel-containing scrap metal for smelting are expanded.

Calcium has a modifying effect, which allows to increase the properties of thick sheets in the Z-direction, to increase the impact strength at -40 ° C. When the calcium content is less than 0.002%, its positive effect is weak, thick sheets have low mechanical properties. An increase in calcium content of more than 0.050% leads to an increase in non-metallic inclusions, a decrease in ductility and toughness of low alloy steel.

Niobium and titanium help stabilize the microstructure of low alloy steel over sheet thickness. However, if the niobium content is more than 0.06% or titanium more than 0.03%, then the improvement of the properties of plate steel will not occur, but only the cost of alloying materials will increase.

Boron strengthens the solid solution by the introduction mechanism, increases the strength and toughness of steel, and grinds the microstructure. With a boron content of less than 0.001%, its effect is negligible. An increase in boron content of more than 0.005% leads to the appearance of excess phases (borides) at the grain boundaries, which reduces the toughness of steel at low temperatures.

Phosphorus and sulfur in this steel are harmful impurities, their concentration should be as low as possible. However, when the concentration of phosphorus is not more than 0.020% and sulfur is not more than 0.015%, their negative effect is negligible. At the same time, deeper dephosphorization and desulfurization of steel will significantly increase the cost of its production, which is impractical.

Low alloy steels of various chemical composition were smelted in an electric arc furnace. In the ladle, steel was deoxidized with ferromanganese, ferrosilicon, alloyed with ferrochrome, ferrovanadium, ferroboron, ferrotitanium, metallic aluminum, nickel, copper, niobium, and also silicocalcium were introduced. Using synthetic slags, the excess sulfur and phosphorus were removed, the excess nitrogen was eliminated by evacuating the steel during its out-of-furnace treatment. The chemical composition of smelted steels is given in table.2.

Steel was poured into slabs, which were subjected to homogenizing annealing at a temperature of 700 ° C. Then the slabs were heated to a temperature of 1250 ° C and rolled on a plate mill 2800 in sheets with a thickness of 25 mm Laminated sheets were subjected to normalization by heating to 900 ° C and cooling in air. After cooling, samples were taken from the sheets and mechanical properties tested. Table 3 shows the test results of the properties of hot rolled sheets.

From table 2 and 3 it follows that the proposed steel (compositions 2-4) has a higher impact strength at a temperature of -40 ° C. Due to the fact that hardening is excluded during heat treatment (normalization), the non-flatness of thick sheets does not exceed 6 mm / m.

At transcendental concentrations of elements (compositions 1, 5), the viscosity properties of steel at low temperatures deteriorate, the non-flatness of thick sheets increases. Also, the lower viscosity properties have a steel prototype (option 6). In addition, thick sheets of this steel are characterized by a flatness of 30-55 mm / m after quenching. The use of normalization for steel sheets of the prototype in order to reduce the non-flatness of thick sheets did not allow to obtain the specified strength and viscosity characteristics.

The technical and economic advantages of the proposed steel consist in the fact that the introduction of 0.001-0.005% boron and not more than 0.03% titanium into it, while limiting the impurity contents of sulfur and phosphorus to the optimal concentration of the remaining elements, increased the toughness at low temperatures and replaced thermal improvement to normalize. The exception of hardening made it possible to increase the flatness of plate steel.

The prototype steel was adopted as the base object. Using the proposed steel will increase the profitability of the production of thick sheets for BelAZ platforms of northern design by 10-15%.

Sources of information

1. Auth. testimonial. USSR No. 1652373, IPC C 22 C 38/50, 1991

2. Japanese application No. 5247521, IPC C 22 C 38/42, 1977

3. Patent of the Russian Federation No. 2200768, IPC C 22 C 38/46, 38/58, 1997 - prototype.

Claims (1)

Steel containing carbon, manganese, silicon, chromium, aluminum, vanadium, nitrogen, copper, nickel, calcium and iron, characterized in that it additionally contains niobium, titanium, boron, phosphorus and sulfur in the following ratio of element content, wt.% : Carbon 0.12-0.20 Manganese 1.0-1.8 Silicon 0.40-0.70 Chrome 0.4-0.8 Aluminum 0.02-0.05 Vanadium 0.04-0.08 Nitrogen Not more than 0.015 Copper 0.1-0.4 Nickel No more than 2.5 Calcium 0.002-0.050 Niobium Not more than 0.06 Titanium Not more than 0.03 Boron 0.001-0.005 Phosphorus Not more than 0,020 Sulfur Not more than 0.015 Iron Else