RU2806645C1 - Method for production of high-strength cold-resistant sheet metal - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the production of rolled sheets from cold-resistant steel with a strength class of 800-950 MPa, used for the metallurgical industry, transport and heavy engineering, production of lifting mechanisms and means of transporting goods operating in extreme conditions of the Far North. Steel is smelted with the following composition, wt.%: carbon 0.18-0.28, silicon 0.40-0.65, manganese 1.10-1.60, molybdenum 0.01-0.30, aluminium 0.02- 0.05, chromium 0.16-0.30, nickel 0.16-0.30, copper no more than 0.10, titanium up to less than 0.01, vanadium no more than 0.01, niobium up to less than 0.01, boron 0.001-0.005, sulphur not more than 0.005, phosphorus not more than 0.013, nitrogen 0.001-0.008, iron - the rest. Continuous casting of steel into slabs, heating them in the temperature range 1180-1250°C, multi-pass hot rolling of sheets and water quenching at a temperature of 850-950°C followed by tempering at a temperature of 450-500°C is carried out.

EFFECT: ensures the production of rolled sheets with the required level of mechanical properties and cold resistance down to minus 70°C.

1 cl, 4 tbl

Description

The invention relates to the field of metallurgy, in particular to the production of high-strength rolled sheets from steel with a strength class of 800 MPa or more with cold resistance down to minus 70 ° C, and can be used in shipbuilding, bridge building, mechanical engineering, for example, in the production of lifting mechanisms, means of transporting goods working in extreme conditions of the Far North.

A known method for producing sheet steel with high wear resistance from steel of the following chemical composition, wt. %:

carbon 0.17-0.28 silicon 0.10-0.30 manganese 0.75-1.50 chromium 0.60-1.20 nickel 0.60-1.20 molybdenum 0.20-0.40 vanadium 0.04-0.10 aluminum 0.02-0.08 nitrogen 0.001-0.010 copper 0.01-0.10 niobium 0.001-0.020 titanium 0.002-0.040 boron 0.001-0.005 sulfur no more than 0.010 phosphorus no more than 0.015 iron rest.

The known production method involves continuous casting of steel into slabs, heating, multi-pass hot rolling of sheets and subsequent water quenching. In this case, the slabs are heated in the temperature range of 1180-1250°C, the finish rolling end temperature is set to no higher than 960°C, and hardening, including rolling heating, is carried out at a temperature of 920-970°C. Additionally, it is possible to carry out tempering after hardening at a temperature of 150-250°C (RF patent No. 2625861, C21D 8/02).

The disadvantage of this known method is that it obtains low strength properties of rolled sheets, and therefore does not provide a given set of mechanical properties. In addition, the insufficient stability of the performance characteristics of rolled sheets at temperatures below minus 40°C does not allow the use of this rolled material in low temperature conditions.

The closest analogue to the claimed invention is a method for the production of high-strength cold-resistant rolled sheets from low-alloy steel, including continuous casting of steel into slabs, their heating in the temperature range of 1180-1250°C, multi-pass hot rolling of sheets, water quenching at a temperature of 920-970°C with subsequent tempering, while continuous casting of steel is carried out from steel of the following chemical composition, wt. %:

carbon 0.16-0.27 silicon 0.33-0.62 manganese 1.30-1.90 molybdenum 0.01-0.30 aluminum 0.02-0.07 chromium no more than 0.15 nickel no more than 0.15 copper no more than 0.10 titanium 0.001-0.015 vanadium 0.001-0.01 niobium 0.001-0.008 boron 0.001-0.005 nitrogen 0.001-0.008 sulfur no more than 0.005 phosphorus no more than 0.012 iron rest,

in this case, the temperature of the end of finishing rolling is set from 860 to 980°C, and tempering is carried out at a temperature of 500-650°C (RF patent No. 2674797, C21D8/02, C22C38/00).

The disadvantage of this known method is the low level of impact strength of steel at subzero temperatures, as well as the formation of low strength and plastic properties of steel at temperatures down to minus 70°C.

The technical problem solved by the claimed invention is the production of high-strength cold-resistant sheet metal for the manufacture of critical products used in the Far North.

The technical result provided by the invention is to obtain the required set of mechanical properties of rolled steel (yield strength of at least 800 MPa, tensile strength of 950-1200 MPa) by selecting the optimal chemical composition of steel and a rational mode of its heat treatment.

This result is achieved by the fact that in the method of producing high-strength cold-resistant sheet metal, including casting steel into slabs, heating in the temperature range of 1180-1250°C, multi-pass hot rolling of sheets, water quenching followed by tempering, according to the change, continuous casting of the following steel is carried out chemical composition, wt. %:

carbon 0.18-0.28 silicon 0.40-0.65 manganese 1.10-1.60 molybdenum 0.01-0.30 aluminum 0.02-0.05 chromium 0.16-0.30 nickel 0.16-0.30 copper no more than 0.10 titanium to less than 0.01 vanadium no more than 0.01 niobium no more than 0.01 boron 0.001-0.005 sulfur no more than 0.005 phosphorus no more than 0.013 nitrogen 0.001-0.008 iron rest

in this case, hardening is carried out at a temperature of 850-950°C, and tempering at a temperature of 450-550°C.

After rolling, a fine microstructure is formed in the steel of the proposed composition, and subsequent heat treatment makes it possible to obtain specified and uniform properties. The claimed chemical composition of the steel was selected taking into account the following features.

The carbon in steel in the claimed range is selected to ensure ductility, reduce brittleness and eliminate the likelihood of cold cracks. Its content in steel should not exceed 0.28%. At the same time, with a carbon concentration of less than 0.18%, the required strength and hardness of steel is not achieved.

Silicon deoxidizes steel, strengthens the ferrite matrix, and reduces the cold resistance of steel. When the silicon content is less than 0.40%, the strength of steel is lower than permissible, and at a concentration of more than 0.65%, the impact strength and ductility of steel decreases, which leads to its embrittlement.

Manganese in steel with a content of 1.10-1.60% ensures deoxidation of steel. In the claimed quantity, it dissolves in ferrite, enters carbides as an alloying element and thereby ensures high strength of steel. When the manganese content is less than 1.10%, its strengthening effect is insufficient. And when the manganese content is over 1.60%, it leads to an increase in the proportion of the pearlite component without a significant change in the structure and, consequently, to strengthening of the steel with a deterioration in toughness and cold resistance.

As is known, the main factors of strengthening (increasing the yield strength) are solid solution, dislocation, substructural and dispersion strengthening. Increasing the yield strength of steel usually results in an increased susceptibility to brittle failure. The only mechanism that, simultaneously with an increase in the yield strength, causes an increase in cold resistance, is the grinding of the actual grain. Refinement of the structure is achieved by using alloying with vanadium, niobium, and boron, which, forming finely dispersed carbides, prevent the growth of austenite grains during heating and have an inhibitory effect on collective recrystallization during the high-temperature rolling stage.

Alloying with molybdenum is used to increase the strength and toughness of steel by refining the grain microstructure. When the molybdenum content is less than 0.01%, the strength of steel is below the required level, and an increase in its content by more than 0.30% impairs ductility and leads to excessive consumption of alloying elements.

Vanadium increases hardness and strength, refines grain, and increases the density of steel, as it is a good deoxidizer. Vanadium content of more than 0.01% can lead to a decrease in cold resistance.

The introduction of niobium into the steel composition is used for dispersion strengthening of steel, as well as for effectively increasing its toughness due to grain refinement. The niobium content of no more than 0.01% is due to the saving of an expensive alloying element.

Alloying with boron increases hardening during hardening, increases the hardenability, strength and wear resistance of steel, and refines the microstructure. When the boron content is less than 0.001%, its effect is negligible. However, an increase in boron content of more than 0.005% leads to the appearance of excess phases (borides) along the grain boundaries, which reduces the impact strength of steel at subzero temperatures.

Aluminum is one of the elements that increases the viscosity properties and corrosion resistance of steel. The content of aluminum in the claimed range contributes to obtaining a fine-grained structure. When the aluminum concentration is less than 0.02%, its positive effect does not appear, and limiting its content to no more than 0.05% is associated with preventing the formation of non-metallic inclusions.

Chromium increases the ability of steels to be thermally hardened and their resistance to corrosion and oxidation. If the chromium content exceeds 0.30%, it will lead to a deterioration in the ductility of the metal due to the growth of the carbide phase.

Nickel reduces the cold brittleness of steel to the greatest extent and is completely soluble in iron, since it has a similar crystalline lattice structure. Nickel is not a carbide-forming element; it is found in a solid solution of ferrite or austenite, strengthening the ferrite, increasing its viscosity and helping to reduce the cold brittleness threshold. The nickel content in steel should not exceed 0.30%, which is due to its scarcity.

Adding copper up to 0.10% increases the strength and corrosion resistance of steel. Increasing the copper content is not advisable due to additional economic costs, as well as the likelihood of red brittleness.

Titanium is a strong carbonitride-forming element, providing a cellular dislocation microstructure of steel, with a combination of high impact toughness and high strength properties of the metal at low temperatures. When the titanium content is above 0.01%, the excess amount of carbonitrides formed strengthens the steel and reduces ductility, which leads to a decrease in the viscosity properties of the metal.

Nitrogen leads to the formation of finely dispersed nitrides along the grain boundaries, preventing their growth, and allows increasing the yield strength and impact strength of the metal. An increased amount of nitrogen causes strain aging. Also, the limitation of nitrogen content is due to the need to obtain a given level of ductility and toughness of steel.

Sulfur, practically not dissolving in ferrite, accumulates in the form of sulfides located along the grain boundaries, being stress concentrators around which cracks arise and develop. Such sulfides are harmful from the point of view of cold-resistant steel, as they lead to weakening of grain boundaries and make plastic deformation more difficult. When the sulfur concentration is no more than 0.005%, its negative effect on the properties of steel is insignificant.

An increase in phosphorus content leads to a decrease in impact strength at subzero temperatures, having a sharply negative effect on the cold resistance of steel. When the phosphorus concentration is no more than 0.013%, its negative effect on the properties of steel is insignificant.

Thus, the claimed chemical composition of steel provides the most stable level of mechanical properties and cold resistance at temperatures down to minus 70°C.

The claimed heat treatment modes are due to the following features. Heating slabs of steel of the stated chemical composition to a temperature not lower than 1180°C ensures its austenitization, complete dissolution of sulfides, phosphides, nitrides, alloying and impurity compounds, and carbonitride reinforcing particles in the austenitic matrix. Thanks to this, the technological plasticity and deformability of steel during rolling increases. Heating slabs above 1250°C is impractical due to excessive growth of austenite grains and energy costs.

Water hardening of hot-rolled sheets is carried out at a temperature of 850-950°C. Temperatures below 850°C do not ensure stable achievement of the specified strength properties, and temperatures above 950°C lead to an unacceptable reduction in the impact strength of sheet steel at low temperatures.

To reduce or completely eliminate internal stresses, reduce brittleness and obtain the required structure and mechanical properties of hardened steel, it is tempered in the temperature range 450-550°C. The specified temperature range ensures the best combination of strength and toughness. As a result, there is an almost complete removal of internal stresses and the formation of structures in the form of sorbitol and troostite, tempered depending on the temperature.

Tempering of hardened sheets at temperatures above 550°C is impractical, since it reduces the strength properties of rolled sheets below the permissible level. Tempering temperatures below 450°C do not reach the level of plastic and viscosity properties of high-strength sheets, which reduces their yield.

Thus, the claimed temperature conditions for the production of rolled sheets make it possible to form an optimal phase composition with a high range of operational and mechanical properties of steel.

The required set of properties of hot-rolled sheets in the delivered state is given in Table. 1.

An example of the method

Steel smelting of the selected alloying systems was carried out using a ZG-0.06L vacuum induction furnace. Commercially pure iron (Armco iron) was used as the initial metal charge. To ensure the required chemical composition, alloying additives in the form of ferroalloys or pure metals were introduced into the melt (Table 2).

The billets were heated for rolling in an electric chamber furnace with a PVP-300 bogie hearth. The heating temperature of the metal for rolling was 1180-1250°C. The blanks were loaded into a heated oven, the holding time was determined at the rate of 2.5 minutes per 1 mm of thickness.

The compression of the ingots was carried out using a hydraulic press (roughing stage) and a single-stand reversible hot rolling mill 500 DUO (finishing stage). The temperature at the end of the finishing stage of rolling varied in the range from 860 to 980°C.

Heat treatment (additional heating for hardening, tempering) of rolled products was carried out in an electric chamber furnace according to hardening modes from a temperature of 850-950°C and subsequent tempering at a temperature of 450-550°C (Table 3).

The results of the analysis of the obtained microstructures of the samples indicate that after the quenching operation, all samples have predominantly the structure of fine lath martensite. After tempering at a temperature of 450°C, the lath structure of the α-phase is generally preserved, but in some areas it is destroyed. This structure is usually identified as tempered troostomartensite. With an increase in the tempering temperature from 450 to 550°C, the microstructure of the steel is characterized by an increase in the areas in which destruction of the lath structure is observed, and the appearance of a larger number of round-shaped carbide particles, which indicates the beginning of their spheroidization at this tempering temperature. Such a structure can be identified as tempered troosto-sorbitol.

Next, samples were made from the obtained rolls for mechanical tests for tensile, hardness and impact bending (Table 4).

Mechanical properties were determined using standard methods:

- tensile tests were carried out according to GOST 1497-84;

- impact bending tests were carried out in accordance with GOST 9454-78 on samples with a V-shaped notch at a temperature of -70°C;

- Brinell hardness testing was carried out in accordance with GOST 9012-59.

The test results presented in Table 3 showed that in sheet steel obtained using the proposed method (experiments No. 2-6), a combination of the required strength, plastic and toughness properties is achieved. In cases of deviations from the declared parameters (experiments No. 1 and 7), as well as when using the prototype method, the declared set of mechanical properties is not ensured.

Thus, the claimed invention ensures the achievement of a high level of mechanical characteristics: temporary tensile strength 950-1200 MPa; relative elongation of at least 14%; Brinell hardness 280-350 units, as well as increased cold resistance down to minus 70°C (impact work KV ^-70 no less than 40 J).

Claims (3)

A method for producing high-strength cold-resistant sheet metal, including continuous casting of steel into slabs, heating them to a temperature of 1180-1250°C, multi-pass hot rolling of sheets, water quenching followed by tempering, characterized in that continuous casting of steel is carried out containing, wt.%: carbon 0.18-0.28 silicon 0.40-0.65 manganese 1.10-1.60 molybdenum 0.01-0.30 aluminum 0.02-0.05 chromium 0.16-0.30 nickel 0.16-0.30 copper no more than 0.10 titanium to less than 0.01 vanadium no more than 0.01 niobium no more than 0.01 boron 0.001-0.005 sulfur no more than 0.005 phosphorus no more than 0.013 nitrogen 0.001-0.008 iron rest, in this case, hardening is carried out in the temperature range of 850-950°C, and tempering at a temperature of 450-550°C.