RU2630721C1 - Thick sheet of structural steel for manufacturing details of welded structures and method for its production in normalized condition - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel sheet thickness up to 50 mm contains, wt %: C 0.10-0.14, Si 0.16-0.30, Mn 1.35-1.60, Al 0.02-0.05, S not more than 0.005, P not more than 0.018, Ti 0.010 -0.025, Nb 0.025-0.040, V+Nb+Ti not more than 0.07, Cr+Ni+Cu not more than 0.3, N not more than 0.007, Fe and impurities the rest. At that Seq≤0.43% has ferrite and perlite microstructure, yield strength is at least 335 MPa, temporary resistance is at least 470 MPa, elongation is at least 22%, KV impact work at minus 50°C at least 34 J. When the sheet is produced, the continuously poured blank is heated to 1190-1210°C, conduct rough rolling at a starting temperature not lower than 950°C thickness of at least two thicknesses of the finished sheet, with relative reduction per pass of at least 10% for not less than 80% of the number of crimps in roughing, finishing rolling at a starting temperature based on Tst=(-1.05×h+860)±10°C, where h is the thickness of the sheet, mm, 1.05 is the empirical coefficient determined experimentally, °C/mm, and completed at a temperature of 820±10°C, after which the sheet is cooled in air.

EFFECT: ensuring weldability and increased impact performance at low temperatures.

2 cl, 3 tbl

Description

The invention relates to metallurgy, in particular to structural steels used in the production of thick hot-rolled sheets for the manufacture of welded structures operating under high loads at ambient temperature and at low temperatures, for example, structures of bridges, locks, tanks, etc.

Known structural steel used in the manufacture of hot-rolled sheets for the manufacture of a welded supporting body of a telescopic boom of an automobile crane (RF Patent No. 2075534, IPC C22C 38/50, publ. 20.03.1997). Steel contains carbon, manganese, silicon, chromium, nickel, copper, molybdenum, vanadium, aluminum, phosphorus, sulfur, calcium, iron, titanium in the following ratio, wt. %:

Carbon 0.12-0.18 Manganese 1.2-1.5 Silicon 0.5-0.8 Copper 0.03-0.3 Aluminum 0.02-0.05 Chromium 0.5-1.0 Nickel 0.5-0.8 Molybdenum 0.2-0.6 Vanadium 0.1-0.2 Sulfur 0.003-0.015 Calcium 0.006-0.03 Phosphorus 0.003-0.02 Titanium 0.01-0.03 Iron Rest

The finished steel was poured into slabs, which were calcined in a chamber furnace at a temperature of 700 ° C. Then the slabs were heated to a temperature of 1230 ° C and rolled into strips to a thickness of 6 mm. The temperature of the end of rolling was 860 ° C. The rolled strips were cooled with water at a rate of 15 ° C / s to a temperature of 550 ° C, after which they were rolled up.

The disadvantage of this steel is that it is designed for the production of rolled steel in the state after accelerated cooling with water, namely, the thickness of the rolled metal is limited to 25 mm, it is possible to reduce the strength after heating during the manufacturing of parts of welded structures at the consumer.

Known closest to the proposed steel containing carbon, silicon, manganese, chromium, copper, vanadium, aluminum, nickel, nitrogen, calcium and iron, niobium, titanium, sulfur and phosphorus in the following ratio of element content, wt. %:

Carbon 0.08-0.15 Silicon 0.1-0.6 Manganese 1.0-1.8 Chromium 0.3-0.9 Copper 0.1-0.5 Vanadium 0.02-0.10 Aluminum 0.01-0.06 Nickel 0.7-1.5 Nitrogen 0.002-0.015 Calcium 0.002-0.030 Niobium 0.01-0.05 Titanium 0.004-0.035 Sulfur No more than 0,010 Phosphorus No more than 0,020 Iron Rest

Steel was poured into slabs and subjected to homogenizing annealing at a temperature of 680 ° C. Then the slabs were heated to a temperature of 1230 ° C and rolled on a plate mill into sheets 10 mm thick. The sheets were subjected to thermal improvement by heating to a temperature of 920 ° C, quenching with water, tempering at a temperature of 660 ° C (RF Patent No. 225999, IPC C22C 38/50, C22C 38/58, publ. 10.07.2005).

The disadvantage of this steel is the need for an additional operation - thermal improvement of rolled products, as well as a possible decrease in strength after heating during the manufacturing of parts of welded structures at the consumer.

The purpose of the invention is to obtain rolled products with a thickness of up to 50 mm with a guarantee of standard properties after heating during the manufacturing process of parts of welded structures at the consumer while maintaining satisfactory weldability and increased impact work at low temperatures.

This goal is achieved in that a thick sheet of structural steel for the manufacture of parts of welded structures has the following chemical composition, wt. %:

Carbon 0.10-0.14 Silicon 0.16-0.30 Manganese 1.35-1.60 Aluminum 0.02-0.05 Sulfur No more than 0,005 Phosphorus No more than 0,018 Titanium 0.010-0.025 Niobium 0,025-0,040 total content of elements vanadium, niobium, titanium No more than 0,07 total content of elements chrome, nickel, copper No more than 0.3 Nitrogen No more than 0,007 Iron and impurities Rest,

moreover, the carbon equivalent SEC ≤ 0.43%, the microstructure is represented by ferrite and perlite with a ferrite grain score of not less than 6, and in which the yield strength is at least 335 MPa, the tensile strength is at least 470 MPa, the elongation is at least 22 %, work impact KV at minus 50 ° C at least 34 J in a normalized state. The goal is also achieved by the fact that in the method of obtaining a thick sheet of the above chemical composition in a normalized state, including austenization of a continuously cast billet from steel, rough rolling with regulated compression for a passage, reinforcing of the roll, finishing rolling, austenization of a continuously cast billet is carried out to a temperature of 1190-1210 ° C, rough rolling is started at a temperature of at least 950 ° C and is carried out at a thickness of at least two thicknesses of the finished sheet, with relative reductions per passage d not less than 10% for not less than 80% of the number of reductions during rough rolling, finish rolling begins at a temperature determined depending on the thickness of the sheet from the ratio

Tnchp = (- 1.05 × h + 860) ± 10 ° С,

where h is the thickness of the sheet, mm

1,05 - empirical coefficient determined empirically, ° C / mm,

and complete at a temperature of 820 ± 10 ° C, after which the sheet is cooled in air.

The invention consists in the following.

Carbon in this steel is one of the main reinforcing elements. When the carbon content is less than 0.10%, the required strength of steel is not provided, and an increase in its content of more than 0.14% reduces the ductility and toughness of steel at lower operating temperatures, and reduces weldability.

Silicon deoxidizes steel, increases its strength. When the silicon content is less than 0.16%, the strength of the steel is below the permissible level, and when the content is more than 0.30%, the ductility of the steel decreases.

Manganese deoxidizes and strengthens steel, improves desulfurization. When the manganese content is less than 1.35%, the strength decreases, and when its content is more than 1.60%, the austenite grain increases when heated before deformation and the required properties are not achieved without additional heat treatment.

Aluminum deoxidizes steel, binds nitrogen, grinding grain, increases its strength. When the aluminum content is less than 0.02%, a satisfactory degree of deoxidation is not provided, the strength of the steel decreases. An increase in the content of this element by more than 0.05% leads to a decrease in the plastic and viscous properties of steel.

Sulfur and phosphorus in this steel are harmful impurities, their concentration should be minimal, however, at a sulfur concentration of not more than 0.005% and phosphorus not more than 0.018%, their negative effect on the properties of steel is negligible. Moreover, a further reduction in impurities is possible only due to deeper desulfurization and dephosphorization of steel, which will significantly increase the cost of its production and is not practical.

The titanium content is limited to 0.010-0.025% to prevent the formation of crystallization of large TiN particles and / or complex globular particles based on them containing Nb, Ca, Mg, S, O, as well as excessive growth of austenite grain upon heating, which leads to coarsening of the microstructure sheets and reduce the level of mechanical properties.

The niobium content is limited to the level of 0.040% to reduce segregation heterogeneity, prevent the formation of large conglomerates of complex particles of Ti, Nb (C, N), in an amount of not less than 0.025% niobium is necessary to inhibit grain growth during rolling.

The total content of vanadium, niobium and titanium is limited to 0.07%, it is determined based on the maximum efficiency of these elements when the dispersion hardening mechanism is activated: the formation of carbide of each element occurs in different temperature ranges, an increase in their total content above 0.07% leads to inhibition carbide formation process and their inefficient use in the alloying system.

The total content of chromium, nickel and copper is limited to 0.3%, it is determined based on their ability to harden the solid solution, reducing the plastic properties of steel.

Nitrogen is necessary for the isolation of finely dispersed nitrides and to contain the coarsening of austenitic grains. When the nitrogen content exceeds 0.007%, its concentration in the solid solution increases, which affects the toughness at low temperatures.

The carbon equivalent of SEC is limited to 0.43% to produce well weldable steel.

When a continuously cast billet is heated to a temperature of not less than 1190 ° С, microalloying additives are dissolved in the steel matrix, then they are released in the form of dispersed phases during rolling. When heated above 1210 ° C, coarsening of austenite grain is observed.

The deformation temperature at the rough rolling stage was adopted at least 950 ° C, based on the need to grind austenite grain due to multiple recrystallization. To ensure a satisfactory study of the structure of the sheets by thickness, taking into account the high temperature of the end of the rolling, it is necessary to provide a thickness of intermediate reinforcing of at least 2 thicknesses of the finished sheet, with relative compressions per pass of at least 10% for at least 80% of the number of compressions during rough rolling. The remaining 20% of the number of reductions are allowed with a lesser degree per pass in order to adjust the geometric dimensions of the intermediate roll. During compression during a pass at the rough rolling stage, less than 10% with an amount of less than 80% of the number of compression during rough rolling due to uneven deformation along the sheet thickness, an inhomogeneous grain structure is formed and poor development of the central layers of the roll is observed.

The temperature interval of the beginning and end of deformation at the finishing stage of rolling is selected based on the need to prepare austenite for subsequent transformation, by creating deformed austenite grains containing deformation bands and having a high dislocation density, this allows the maximum grinding of ferrite grain, resulting in the required set of properties: the finish rolling start temperature, which is determined by the formula Tnchp = (- 1.05 × h + 860) ± 10 ° С, the end temperature is set equal to 820 ± 10 ° С. To increase the elongation and yield, the sheets are subjected to delayed cooling in a stack in the air. The target sheet structure is ferrite and perlite with a ferrite grain score of at least 6, in which the yield strength is at least 335 MPa, the tensile strength is at least 470 MPa, the elongation is at least 22%, and the impact work is KV at minus 50 ° C at least 34 J.

The implementation of the proposed technical solution allows to obtain the required quality of hot-rolled sheets used for the manufacture of welded structures operating under high loads at ambient temperature and at low temperatures, for example, bridge structures, locks, tanks, etc., which is achieved by choosing rational temperature-deformation modes for a specific chemical composition of steel. When the variable parameters go beyond the specified boundaries, there are cases of non-receipt of stably satisfactory results of mechanical tests. As a result, the data obtained confirm the correctness of the selected values of the technological parameters in the framework of the proposed method for the production of hot rolled sheets of structural steel for the manufacture of welded structures operating under high loads at ambient temperature and at low temperatures.

The application of the method is illustrated by an example of its implementation in the production of sheets of steel grade S355NL according to EN 10025-3 up to 50 mm thick on a plate mill 5000 of PAO Severstal.

Steel was smelted in an oxygen converter with a capacity of 370 tons with a magnesium desulfurization process in a pouring ladle. Primary alloying, preliminary deoxidation and metal treatment with solid slag mixtures with metal purging with argon in a steel pouring ladle were carried out at the outlet. Final alloying, microalloying, metal processing with calcium and metal overheating for evacuation were carried out on the complex steel finishing unit. The metal was degassed by evacuation. Casting was carried out at a continuous casting machine with metal protection with argon from secondary oxidation.

The chemical composition of steel is given in table 1.

Steel obtained with the following composition of chemical elements, wt. %: C = 0.12; Si = 0.29; Mn = 1.6; Cr = 0.03; Ni = 0.02; Cu = 0.03; Ti = 0.015; V = 0.005; Nb = 0.033; N = 0.005; Al = 0.03; S = 0.002; P = 0.016; iron and impurities - the rest. The carbon equivalent was SEC = 0.40%.

Continuously cast billets were heated to a temperature of 1196 ° C and rolled at the rough stage to a thickness of undercooling of 106.1 mm, cooled in air to a temperature of 829 ° C, and rolled at the finishing stage to a final thickness of 30.0 mm with the end of the deformation process at 814 ° C. Next, the sheets were cooled in air. Preliminary deformation at the rough rolling stage was started at a temperature of 1016 ° C and was carried out with regulated reductions of 10.3-10.8-10.7-13.3-13.4-12.8-15.8-12.0% ( all reductions are carried out with a degree of more than 10%).

Static tensile tests were carried out on five-fold flat samples according to BS EN ISO 6892-1, made from samples taken in the transverse direction relative to the rolling direction. Dynamic tests were carried out on samples with a V-shaped notch at a temperature of minus 50 ° C according to ISO 148-1. Grain size - EN ISO 643. Implementation options of the proposed method and test results are shown in tables 2 and 3, respectively.

The test results showed that the proposed method for the production of steel of the selected chemical composition (options No. 1 and 2) provides a satisfactory level of mechanical properties, determined by static tensile testing of the samples, as well as increased resistance to brittle fracture at low temperatures. With the prohibitive values of the proposed modes (options No. 3-9) and the prototype method (option No. 10), it is not possible to achieve the required level of strength and viscosity properties, including after heating during the manufacturing of welded structures at the consumer.

Thus, the application of the described rolling method ensures the achievement of the required results, namely, the production of rolled products up to 50 mm thick with a yield strength of at least 335 MPa, a temporary resistance of at least 470 MPa, an elongation of at least 22%, and impact work KV at minus 50 ° C at least 34 J in a normalized state.

The technical and economic advantages of the invention are that the use of the proposed method provides the production of hot-rolled sheets of structural steel with a thickness of up to 50 mm with a guarantee of standard properties after heating in the manufacturing process of parts of welded structures at the consumer while maintaining satisfactory weldability and increased impact work at low temperatures.

Claims (8)

1. A thick sheet of structural steel for parts of welded structures, having a composition, wt. %: carbon 0.10-0.14 silicon 0.16-0.30 manganese 1.35-1.60 aluminum 0.02-0.05 sulfur no more than 0,005 phosphorus no more than 0,018 titanium 0.010-0.025 niobium 0,025-0,040 total content vanadium + niobium + titanium no more than 0,07 total content chrome + nickel + copper no more than 0.3 nitrogen no more than 0,007 iron and impurities rest, moreover, the carbon equivalent SECv ≤0.43%, while the steel sheet has a microstructure consisting of ferrite and perlite with a ferrite grain score of at least 6 and a yield strength of at least 335 MPa, a temporary resistance of at least 470 MPa, elongation at least 22%, KV impact work at minus 50 ° C - at least 34 J. 2. A method of manufacturing a thick sheet of structural steel for parts of welded structures according to claim 1, including austenization of continuously cast billets of steel, rough rolling with regulated compression for the passage, reinforcing the roll, finishing rolling and cooling, while austenization of continuously cast billets is carried out to a temperature of 1190 -1210 ° C, rough rolling is started at a temperature of at least 950 ° C and is carried out at a thickness of at least two thicknesses of the finished sheet, with relative reductions per pass of at least 10% for not less than 80% of the number of reductions during rough rolling, finishing rolling starts at a temperature determined depending on the thickness of the sheet from the ratio Tnchp = (- 1,05 × h + 860) ± 10 ° C, where h is the thickness of the sheet, mm 1.05 - empirical coefficient determined empirically, ° C / mm, and complete at a temperature of 820 ± 10 ° C, after which the sheet is cooled in air.