RU2363754C1 - Steel of increased hardenability and treatability - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to ferrous metallurgy, particularly to manufacturing of high-strength case-hardened steel for manufacturing of heavy loaded components in automobile industry, particularly toothed wheels of gearboxes, components of undercarriage of tractors and automobile of heavy payload. Steel is made with usage of metallised pellet and contains components in a following ratio wt %: carbon 0.15-0.25, manganese 0.80-1.10, silicon 0.15-0.40, sulphur 0.020-0.065, chrome 0.45-0.80, nickel 0.45-0.75, molybdenum 0.15-0.30, aluminium 0.02-0.06, nitrogen 0.004-0.015, calcium 0.0005-0.0050, boron ≤0.0008, iron and unavoidable admixtures are the rest. Overall content of manganese, chrome and molybdenum is not less than 1.7, and ratio of aluminium to the nitrogen is not less than 1.8. In the capacity of unavoidable admixtures steel contains phosphorus not more than 0.025 wt % and copper not more than 0.2 wt %.

EFFECT: there are increased hardenability properties of steel, improved treatability and reduced distortion at thermal treatment, that provides guaranteed level of consumer properties of manufactured from steel products.

2 cl, 1 ex

Description

The invention relates to ferrous metallurgy, in particular to the production of steels of high hardenability and machinability used for the production of critical automobile parts.

Known steel 20HGNM for the manufacture of products hardened by cementation, containing, wt.%:

Carbon 0.18-0.23

Silicon 0.17-0.37

Manganese 0.7-1.1

Chrome 0.4-0.7

Nickel 0.4-0.7

Molybdenum 0.15-0.25

Iron - the rest

(GOST 4543-71 "Rolled products from alloy structural steel").

The disadvantage of this steel is the low level of hardenability - steel

does not achieve the required hardness of the parts.

The closest analogue is steel for cementation (RF patent No. 227172, p. 1), containing, wt.%:

Carbon 0.12-0.26

Manganese 0.7-1.5

Silicon ≤0.4

Sulfur 0.01-0.05

Chrome 0.4-1.2

Nickel 0.4-0.7

Molybdenum ≤0.3

Iron and inevitable impurities - the rest, with the total content of manganese and chromium not less than 1.6 wt.%.

The disadvantages of the prototype are:

- wide concentration limits of the content of elements (carbon, manganese, chromium) do not ensure the stability of the properties of steel;

- the lack of regulations on the nitrogen content, lower molybdenum content does not allow to obtain a stable level of hardenability;

- factors of the influence of aluminum, boron and nitrogen on hardenability and warpage of products during chemical-thermal treatment of products are not taken into account, which in industrial conditions is often critical.

The technical result of the invention is to increase the hardenability characteristics of steel, improve workability and reduce warpage during heat treatment, which will ensure a guaranteed level of consumer properties of products.

To achieve the result, a steel containing, wt.%:

Carbon 0.15-0.25

Manganese 0.80-1.10

Silicon 0.15-0.40

Sulfur 0.020-0.065

Chrome 0.45-0.80

Nickel 0.45-0.75

Molybdenum 0.15-0.30

Aluminum 0.02-0.06

Nitrogen 0.004-0.015

Calcium 0.0005-0.0050

Boron ≤0,0008

Iron and impurities - the rest, with the total content of manganese, chromium, molybdenum not less than 1.7 ((Mn + Cr + Mo) ≥1.7), and the ratio of aluminum to nitrogen not less than 1.8 (Al / N≥1, 8). As impurities, steel contains regulated phosphorus ≤0.025 wt.% And copper ≤0.20 wt.%.

The given combinations of alloying elements make it possible to obtain a homogeneous fine-grained structure with a favorable combination of hardenability characteristics in the proposed steel after chemical-thermal treatment.

Carbon is introduced into the composition of this steel in order to ensure the level of its strength and hardenability. The upper limit of carbon content - 0.25% is due to the need to ensure the required level of ductility of steel, and the lower - 0.15% - to ensure the required level of strength.

Manganese, chromium, molybdenum and nickel are used, on the one hand, as solid solution hardeners, and on the other hand, as elements that significantly increase the stability of supercooled austenite and increase the hardenability of steel. Moreover, the upper level of the content of these elements, respectively, Mn - 1.10%; Cr - 0.80%; Mo - 0.30%; Ni - 0.75% is determined by the need to ensure the required level of ductility of steel, and the lower, respectively, Mn - 0.80%; Cr - 0.45%; Mo - 0.15%; Ni - 045% by the need to provide the required level of strength of finished products and hardenability of steel.

Silicon refers to ferrite-forming elements. The lower limit on silicon - 0.15% is due to the technology of deoxidation of steel. A silicon content above 0.40% adversely affects the ductility characteristics of steel.

Calcium and sulfur determine the level of workability of steel. The lower limit for calcium - 0.0005% and sulfur - 0.020% is due to the need to obtain the optimum machinability of steel, and the upper limit - 0.0050% and 0.065%, respectively - to issues of manufacturability and to eliminate increased metal contamination by non-metallic inclusions.

Aluminum is used as a deoxidant, which provides protection against grain growth during heat treatment. The lower level of aluminum content - 0.02% is determined by the requirement to ensure fine-grained steel, and the upper level - 0.06% - by issues of manufacturability.

Nitrogen is an element involved in the formation of carbonitrides, while the lower level of its content - 0.004% is determined by the requirement to ensure a given level of strength, and the upper level - 0.015% - by the requirement to ensure a given level of ductility and hardenability.

Boron contributes to a sharp increase in hardenability of steel, but leads to increased warpage of parts. The regulated upper limit of boron content - 0.0008% is determined for reasons of obtaining minimum warpage of products while ensuring the required level of hardenability.

To ensure complete binding of nitrogen to nitrides of the AlN type as a result of reactions:

[Al] + [N] = AlN

the following ratio is required: Al / N of at least 1.8. Otherwise, boron is not protected from binding to nitrides and the probability of warping of parts sharply increases.

The ratio (Mn + Cr + Mo)> 1.7 makes it possible to determine the strength and toughness characteristics of the studied steel.

The proposed ratio of elements in steel was found experimentally and is optimal, since it allows you to get a comprehensive technical effect. If the ratio of elements is violated, the properties of steel deteriorate, their instability is observed and the effect is not achieved.

To obtain the proposed steel for residual elements and impurities, a high-tech method was used to use metallized pellets obtained by direct reduction of iron from ore in steel smelting, thereby minimizing the effect of residual elements and impurities on the operational properties of steel.

An example of the implementation of the invention is given.

Smelting of steel containing carbon 0.22%, silicon 0.25%, manganese 0.93%, chromium 0.60%, nickel 0.55, molybdenum 0.21%, sulfur 0.028%, aluminum 0.032%, calcium 0, 0015%, nitrogen 0.008%, is produced in 150-ton steel arc furnaces (EAF) using metallized pellets in a charge. The preliminary alloying of the metal with manganese, silicon, chromium, molybdenum and nickel is carried out in the ladle upon discharge from the particleboard. After release, argon is purged through the bottom purge unit, during which the steel is deoxidized with aluminum. Next, the steel is evacuated on a circulating type vacuum. The metal enters the integrated steel processing unit (AKOS), on which the metal is heated to the required temperature, purged with argon through the bottom blowing unit, the dosed additives of the necessary ferroalloys are made, and the steel is treated with flux-cored wire with various fillers. At AKOS, refining slag is induced with an additive of lime and fluorspar, slag is deoxidized with granulated aluminum in an amount of 30 kilograms for every 100 millimeters of slag. Before transferring the smelting to casting, after fine-tuning all the elements, the aluminum content in the metal is adjusted by calculation to 0.020%, after which the smelting is processed with silicocalcium and the final alloying with aluminum and sulfur. Casting is carried out on four-strand continuous casting caster of radial type into an ingot 300 × 360 in size with a drawing speed of 0.5-0.6 m / min. The protection of the metal from secondary oxidation is carried out by using cover slag mixtures in the intermediate ladle and mold, protective tubes and immersion glasses. After casting and cutting to a measured length, the continuously cast billets obtained are cooled in controlled cooling furnaces.

Thus, the proposed steel composition provides stability of properties during heat treatment and allows to obtain products with a given level of operational characteristics.

Claims (2)

1. Steel of high hardenability and machinability, smelted using metallized pellets, containing carbon, manganese, silicon, chromium, nickel, molybdenum, sulfur, iron and inevitable impurities, characterized in that it additionally contains aluminum, nitrogen, calcium and boron in the next regulated ratio of elements, wt.%:
carbon 0.15-0.25 manganese 0.80-1.10 silicon 0.15-0.40 sulfur 0,020-0,065 chromium 0.45-0.80 nickel 0.45-0.75 molybdenum 0.15-0.30 aluminum 0.02-0.06 nitrogen 0.004-0.015 calcium 0.0005-0.0050 boron ≤0,0008 iron and inevitable impurities rest
the total content of manganese, chromium and molybdenum is not less than 1.7, and the ratio of aluminum to nitrogen is not less than 1.8. 2. Steel according to claim 1, characterized in that as inevitable impurities it contains phosphorus not more than 0.025 wt.% And copper not more than 0.20 wt.%.