RU2798238C1 - High strength low alloy steel for agricultural machinery - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: high-strength steel for the manufacture of critical elements of agricultural earthmovers. The steel contains elements in the following ratio, wt.%: carbon 0.32-0.35, silicon 1.6-1.8, chromium 0.5-0.7, manganese 1.0-1.5, molybdenum 0, 2-0.3, niobium 0.03-0.05, titanium 0.02-0.04, boron no more than 0.01, aluminum no more than 0.01, sulfur no more than 0.01, phosphorus no more than 0, 01, iron and inevitable impurities - the rest. After hardening from a temperature of 900°C into the water and drawing at a temperature of 200 to 500°C for 60 minutes followed by air cooling, it has a uniform tempering martensite structure.

EFFECT: high strength, ductility and impact strength of steel.

1 cl, 1 tbl, 3 ex

Description

The invention relates to the field of metallurgy, namely to high-strength steels and can be used for the manufacture of critical elements of agricultural earth-moving machinery.

To date, steel for agricultural and earthmoving equipment is subject to certain requirements in terms of hardness, yield strength, tensile strength, resistance to abrasive wear with sufficient ductility and toughness. To improve steels intended for products of earth-moving and agricultural machines, it is necessary to ensure a high level of performance.

Known hot-rolled steel described in patent RU 2605037 C1 dated 12/20/2016. According to this patent, high-strength steel contains, wt. %: C 0.16-0.45, Si 0.05-0.70, Mn 0.50-1.50, S 0.002-0.008, P not more than 0.015, Cr not more than 0.15, Ni not more than 0 ,15, Cu not more than 0.15, Nb from 0.005 to less than 0.01, acid-soluble Al 0.02-0.05, Fe and inevitable impurities - the rest, while the ratio between Mn and S is connected by the dependence [Mn]⋅[ S]<0.005. The method consists in heating the billet to a temperature in the range from 1250°C to 1300°C and subsequent hot rolling. The disadvantage of this method are low strength characteristics: yield strength less than 440 MPa and tensile strength less than 685 MPa.

Known high-strength structural steel used in aircraft structures, disclosed in the patent SU 1316285 A1 dated 10/15/1994. High-strength steel has the following chemical composition, wt. %: C 0.30-0.38, Mn 0.05-0.5, Si 2.1-2.8, Cr 0.8-1.2, Ni 2.6-3.0, Mo 0, 3-1.2, V 0.05-0.15, one element from the group of rare earth metals, including La and Ce 0.001-0.05, at least one element from the group of alkaline earth metals, including Mg and Ca 0.001-0.05 , Fe - rest. The method consists in heating the billet to 900°C, followed by cooling in oil and subsequent tempering at temperatures of 200-250°C for 2-3 hours. The use of rare earth metals in steel smelting leads to the formation of a pyroelectric effect with the release of a large amount of smoke containing harmful substances, which worsens the sanitary, hygienic and environmental conditions for the production of the alloy; the use of very expensive elements such as rare earth and alkali metals significantly increases the cost of castings.

High-strength sheet steel is known, see patent RU 2677888 C2 dated January 22, 2019 (or European patent WO 2016/001887 dated January 7, 2016). According to this patent, high-strength steel contains, wt. %: C 0.1-0.4, Mn 4.5-5.5, Si 1-3, Mo 0.2-0.5, the rest includes Fe and inevitable impurities. The proposed method includes the following steps: annealing a rolled sheet obtained from said steel by holding it at an annealing temperature AT greater than the Ac ₃ transformation temperature of the steel, quenching the sheet by cooling it to a quenching temperature QT in the range between the transformation temperatures Ms and Mf of the steel , heating the sheet to the overaging temperature RT in the range of 300° C. to 500° C. and holding it at the overaging temperature RT for over aging time Pt greater than 10 sec, and cooling the sheet down to ambient temperature. Sheet steel according to this method has a yield strength of not more than 1400 MPa, a tensile strength of 1360-1560 MPa, a uniform relative elongation of not more than 12%.

The disadvantage of this steel is that the highest mechanical characteristics are achieved only through additional technological operations. At the same time, the strength level also remains relatively low.

The closest in technical essence to the proposed invention and taken as a prototype is high-strength and wear-resistant steel disclosed in patent RU 2606825 C1 dated 01/10/2017. According to this patent, high-strength steel is used in the manufacture of highly loaded parts of the working bodies of tillage, sowing, forage harvesting, vegetable harvesting and other agricultural machines. Steel contains in wt. %: 0.30-0.50 C, 0.30-0.50 C, 0.25-0.40 Ni, 0.20-0.40 Cu, 0.05-0.15 Mo, 0.10 -0.30 Si, 0.80-1.00 Mn, 0.01-0.03 V, 0.01-0.04 Nb, 0.001-0.005 V, 0.01-0.03 Ti, 0.01 -0.05 Al, not more than 0.015 N, 0.001-0.02 Ca, not more than 0.02 S, not more than 0.020 P, Fe and impurities - the rest, while the total content of P and S is not more than 0.025. The method of obtaining steel includes the following steps: smelting, forging, rolling at a temperature of 1150-870°C after heating in a chamber furnace to 1200°C; heat treatment was carried out according to the regime: quenching in water from a temperature of 870-930°C, followed by tempering in the temperature range of 200-350°C for 4-6 hours. The resulting steel with 0.30-0.35% C with a sheet thickness of 3 to 20 mm has a yield strength of 1240-1435 MPa, a relative elongation of at least 7-8%, impact strength at +20°C of at least 20-30 J /cm ² , relative uniform elongation of at least 2.0-2.5% and high wear resistance.

The main disadvantage of the known high-strength steel:

- the chemical composition is characterized by a large number of alloying elements, which is less profitable from an economic point of view. The steel is characterized by low deformation ability due to the high content of harmful impurities (sulphur, phosphorus) and copper, in addition, additional hot rolling is required to obtain steel, which complicates the technological process to obtain the required level of characteristics. And also too long duration of heat treatment (more than 4 hours), which is impractical in industrial conditions.

From the analysis of known similar technical solutions, it was revealed that the technical problem in this area is the need to create new compositions of high-strength low-alloy steel for the manufacture of critical technical elements of agricultural earth-moving equipment.

The technical result, to which the invention is directed, is to obtain a composition of high-strength low-alloy steel, providing high strength, ductility and toughness.

To solve the technical problem and achieve the claimed result, high-strength low-alloy steel for agricultural machinery contains carbon, silicon, chromium, manganese, molybdenum, niobium, titanium, boron, aluminum, sulfur, phosphorus and iron in the following ratio of chemical elements, wt. %:

carbon 0.32-0.35 silicon 1.6-1.8 chromium 0.5-0.7 manganese 1.0-1.5 molybdenum 0.2-0.3 niobium 0.03-0.05 titanium 0.02-0.04 boron no more than 0.01 aluminum no more than 0.01 sulfur no more than 0.01 phosphorus no more than 0.01 iron and inevitable impurities rest,

while the steel is subjected to quenching from a temperature of 900°C in water and additional tempering at a temperature of 200°C to 500°C for 60 minutes, followed by cooling in air.

The carbon included in the steel provides high strength and hardness of the alloy. Reducing the carbon content below the stated level leads to a decrease in strength, and a higher content compared to the declared limits adversely affects ductility. Carbon also has a positive effect on the hardenability of said steel. In this regard, the carbon content is limited to a range of 0.32 to 0.35 mass. %.

Silicon has a positive effect on the hardenability and increases the elasticity of the steel. To ensure high hardness and strength, steel composition includes from 1.6 to 1.8 wt. % silicon. Too high a silicon content has a negative effect on the hardness, strength and malleability of the alloy. To eliminate these negative effects, the silicon content in the proposed steel is limited to 1.6 - 1.8 wt. %.

Alloying steel with manganese leads to deoxidation and hardening, and also binds sulfur, forming manganese sulfides. The content of manganese in the range of 1.0 - 1.5 mass. % leads to an improvement in toughness and hardness.

Chromium increases the strength of steel. Manganese and chromium increase the hardenability of steel, allowing a significant increase in the thickness of hardened parts while reducing the cooling rate during hardening.

Alloying of steel with molybdenum in the range of 0.2-0.3 wt. % leads to an increase in corrosion resistance, hardness, and also improves its hardenability. Molybdenum also prevents temper brittleness during heat treatment. Alloying steel molybdenum more than 0.3 wt. % is not economically feasible.

Alloying of steel with niobium in the range of 0.03 - 0.05 wt. % leads to steel hardening, as well as to the formation of fine grains and contributes to the appearance of a subgrain structure, fixed and stabilized by dispersed particles of niobium carbides and carbonitrides. The increase in the content of niobium more than 0.10 wt. % leads to the formation of large niobium carbonitrides, leading to a decrease in viscosity and is not economically feasible due to the increase in alloying costs.

Titanium in the amount of 0.02-0.04 wt. % is a necessary technological additive for nitrogen fixation, as well as to prevent the formation of boron nitrides. The isolation of fine MX particles containing titanium is aimed at increasing the strength of the steel.

Aluminum is needed to bind nitrogen to aluminum nitrides and to effectively control grain growth, as well as to prevent boron from binding to nitrides. Therefore, the aluminum content in steel should not exceed 0.01 wt. %.

When the boron content is more than 0.01 wt. %. iron borides are formed, which impair the manufacturability of steel, in which embrittlement after heat treatment is manifested.

Phosphorus and sulfur are harmful impurities that worsen the technological ductility of steel, so the total content should not exceed 0.02 wt. %.

To optimize the properties of the composition is subjected to additional holiday at temperatures of 200°C, 280°C, 400°C and 500°C for 60 minutes, followed by cooling in air.

As a rule, heat treatment consists of quenching or normalizing followed by tempering. As a result, a uniform tempering martensite structure is observed throughout the volume, which leads to an increase in plasticity.

The heating temperature of more than 940°C during hardening leads to a deterioration in the properties of the steel due to excessive growth of microstructure grains, which worsens the toughness properties of the steel sheet at low temperatures. A decrease in the hardening temperature below 900°C can lead to incomplete dissolution of carbide particles in the matrix, as a result of which the maximum hardening of the steel is not achieved, its toughness deteriorates, and the properties of the steel change with a change in the content of alloying elements and impurities.

To relieve internal stresses after quenching, tempering is carried out at a temperature of 200-500°C. Tempering hardened steel at temperatures above 500°C reduces the strength properties below an acceptable level. Reducing the tempering temperature below 200°C leads to the loss of plastic and toughness properties of high-strength steel. Complex alloying with niobium, chromium and molybdenum contributes to an increase in the dispersion of the steel structure and an increase in strength during tempering.

Implementation examples.

Example 1. Received high-strength steel with the following chemical composition of the masses. %: 0.32 C, 1.6 Si, 0.5 Ccr, 1.0 Mn, 0.2 Mo, 0.03 Nb, 0.02 Ti, B<0.01, A1<0.01 rest Fe and impurities (the total content of S and P does not exceed 0.02%).

Example 2. High-strength steel was obtained with the following chemical composition of the masses. %: 0.35 C, 1.8 Si, 0.7 Cr, 1.5 Mn, 0.3 Mo, 0.05 Nb, 0.04 Ti, B<0.01, A1<0.01 rest Fe and impurities (the total content of S and P does not exceed 0.02%).

Example 3. High-strength steel was obtained with the following chemical composition of the masses. %: 0.34 C, 1.7 Si, 0.56 Cr, 1.35 Mn, 0.2 Mo, 0.04 Nb, 0.031 Ti, B<0.01, A1<0.01 rest Fe and impurities (the total content of S and P does not exceed 0.012%).

Steel production is carried out according to the following technological operations:

1) Casting of an ingot by electroslag submerged arc remelting and its cooling in air to room temperature;

2) Heating in a muffle furnace to a deformation temperature of 1150°C for 2 hours;

3) Forging from a temperature of 1150°C to 950°C into a billet with a size of 60x150x450 mm and cooling to room temperature;

4) Quenching consisting in austenization at a temperature of 900°C for 40 minutes, followed by cooling into water;

5) is additionally subjected to tempering at temperatures of 200°C, 280°C, 400°C and 500°C for 60 minutes, followed by cooling in air.

The method provides a set of high performance characteristics, namely high strength, ductility, hardness and impact strength (table 1).

The method ensures the simultaneous achievement of a high level of strength, ductility, hardness and impact strength.

The results of tensile tests at room temperature, determination of impact strength by the Charpy method on samples with a V-shaped notch and hardness by the Rockwell method, performed in accordance with GOST, are shown in Table 1.

Compared with the prototype, the proposed composition of high-strength low-alloy steel makes it possible to provide high strength, ductility and toughness.

Claims (3)

High-strength steel for the manufacture of elements of agricultural earth-moving equipment, characterized in that it contains carbon, silicon, chromium, manganese, molybdenum, niobium, titanium, boron, aluminum, sulfur, phosphorus, iron and inevitable impurities in the following ratio of elements, wt.%: carbon 0.32-0.35 silicon 1.6-1.8 chromium 0.5-0.7 manganese 1.0-1.5 molybdenum 0.2-0.3 niobium 0.03-0.05 titanium 0.02-0.04 boron no more than 0.01 aluminum no more than 0.01 sulfur no more than 0.01 phosphorus no more than 0.01 iron and inevitable impurities rest, however, after quenching from a temperature of 900°C into water and tempering at a temperature of 200°C to 500°C for 60 minutes, followed by cooling in air, it has a uniform tempering martensite structure.