RU2397255C1 - Procedure for production of sheets out of alloyed steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention refers to metallurgy, particularly to production of structure steel of high strength and improved weldability for usage in ship building, heating power complex, transport, heavy duty machine building, bridge engineering, etc. The procedure for production of plate iron consists in production of a continuous cast work-piece of specified chemical composition, in austenisation performed at temperature 1200-1250°C, and in rolling. At the first stage the work-piece is rolled to thickness of not less, than 60 mm and equal to 2-3 of final thickness of a sheet. At the second stage the work-piece is cooled with water to temperature 970±10°C, further it is finish rolled within the range of temperature 950-980°C and deformation per every pass not less 10%, whereupon the work piece is cooled at an average rate 20-80°/sec to temperature 150-250°C. Sheets are heated at an average rate 1-1.5°/min to temperature 580-630°C, conditioned at 10-16 min/mm and cooled in air.

EFFECT: production of heavy duty plate iron with upgraded indices of strength at simultaneous increase of cold resistance and low temperature viscosity in thickness up to 35 mm.

2 cl, 3 tbl, 1 ex

Description

The invention relates to metallurgy, and more particularly to the production of structural steels of high strength and improved weldability for use in shipbuilding, fuel and energy complex, transport and heavy engineering, bridge building, etc.

A known method of producing sheet metal from steel of the following chemical composition, wt.%:

Carbon 0.05-0.15 Manganese 1.2-2.0 Silicon 0.2-0.6 Niobium 0.01-0.10 Titanium 0.005-0.03 Aluminum 0.01-0.10 Chromium 0.03-0.50 Nickel 0.03-0.50 Copper 0.03-0.50 Nitrogen 0.005-0.020 Iron the rest [1] is a prototype.

Using the method of thermomechanical processing, which consists in obtaining a preform, its austenitization, deformation with a total degree of compression of 50-80% to a thickness of 14 mm, cooling from a temperature of the end of deformation of 760-900 ° C at a speed of 10-60 ° C / s to a temperature of 300 ± 20 ° C, reheated to a temperature of 590-740 ° C with a shutter speed of 0.2-3.0 min / mm and final cooling in air.

The main disadvantages of this method of production are insufficient strength, cold resistance, low temperature viscosity of the obtained steel, as well as providing properties in thicknesses of only up to 14 mm.

The technical result of this invention is the production of critical rolled products with increased strength indicators, while increasing cold resistance and low temperature viscosity in thicknesses up to 35 mm.

The specified technical result is achieved by the fact that in the production method of plate products, including steelmaking, continuous casting into billets, preliminary rolling, intermediate undermining of the rolled products, finishing rolling, cooling of the rolled products and tempering, steel of the following composition is melted, wt.%:

Carbon 0.08-0.10 Silicon 0.20-0.30 Manganese 0.40-0.60 Chromium 0.40-0.60 Copper 0.50-0.70 Nickel 1.90-2.20 Molybdenum 0.25-0.31 Aluminum 0.005-0.040 Vanadium 0.01-0.03 Calcium 0.030 (calculated) Sulfur 0.001-0.003 Phosphorus 0.001-0.008 Arsenic 0.001-0.020 Iron rest

The carbon content in steel largely determines its strength. At the same time, an increase in the carbon fraction over 0.10% is impractical due to a significant decrease in ductility, viscosity, cold resistance, as well as an increase in the tendency of steel to crack during welding.

Silicon is used as a deoxidizing agent and helps to ensure strength. The upper limit of the silicon content is determined by the fact that a significant amount of it in steel adversely affects the viscosity of the metal in the heat affected zone of the welded joint.

Manganese effectively strengthens steel while being a deoxidizer. In addition, when the content in steel is up to 0.6%, manganese increases the viscosity of steel, including at low temperatures.

Chrome with a content in the range of 0.3-0.7% helps to ensure the required strength, while at a higher content it negatively affects the level of steel viscosity.

Within the indicated limits, copper and nickel provide the necessary strength of steel and its viscosity at negative temperatures by means of solid solution hardening, as well as by increasing the stability of austenite in the ferrite region during γ-transformation and the formation of predominantly bainitic-martensitic structures.

Molybdenum helps to obtain the required strength, and also prevents the development of temper brittleness of steel. Above 0.4%, molybdenum lowers the viscosity of steel.

Aluminum is introduced into steel as a deoxidizer, as well as for the purpose of grinding grain. When the aluminum content in steel exceeds 0.05%, the steel purity decreases with respect to non-metallic inclusions of the aluminum oxide system, which adversely affects the mechanical properties of the base metal and welded joints.

Vanadium contributes to the achievement of the required level of strength and viscosity due to grinding of the microstructure due to the release of dispersed particles of carbides and carbonitrides during the rolling process. The introduction of vanadium in an amount of more than 0.04% is impractical due to the deterioration of weldability.

Calcium in an amount of up to 0.03% is introduced into steel in order to modify non-metallic inclusions, which favorably affects the isotropy of the mechanical properties and viscosity of steel, including at low temperatures.

Sulfur, phosphorus and arsenic are impurity elements that adversely affect the isotropy of the mechanical properties of steel, ductility and toughness at low temperatures, and therefore, the content of these elements in steel must be minimized.

Sheet rolling after austenitization is carried out in two stages. At the first stage, the workpiece is deformed to an intermediate thickness equal to 2-3 sheet thicknesses, but not less than 60 mm. The workpiece is cooled between stages of deformation by water.

Accelerated cooling between stages is necessary to prevent the occurrence of collective recrystallization processes in the middle of the rolled thickness. The second stage of deformation is performed in the temperature range 950–980 ° C with compression in each pass of at least 10%. The specified conditions for the deformation at the second stage of rolling will eliminate the heterogeneity of the thickness of the rolled product due to incomplete occurrence of recrystallization processes and thus ensure the isotropy of the mechanical properties of the sheet. After rolling, the billet is cooled at a mass-average rate of 20-80 ° / s to a temperature of 150-250 ° C. Then the sheets are heated at a mass-average speed of 1-1.5 ° / min to a temperature of 580-630 ° C with a holding time of 10-16 min / mm and cooled in air.

The main factors of hardening (increasing yield strength) of bainitic-martensitic steels are solid-solution (20-35%), dislocation (15-20%), grain-boundary (20-30%), small-angle boundaries (20-30%) and hardening by dispersed particles ( 2-10%).

An increase in the yield strength of steel usually leads to an increase in the tendency to brittle fracture. The only mechanism that simultaneously with the increase in the yield strength causes an increase in cold resistance is grain grinding. Grain grinding is achieved by adding aluminum, vanadium and nitrogen, which, forming fine carbides, inhibit the growth of austenite grain when heated. The use of thermomechanical processing provides grinding of austenitic grain at the initial stage of deformation. At the final stage of deformation in the γ phase, a developed substructure is formed with a large number of uniformly distributed defects in the crystal structure. During accelerated cooling after deformation, additional hardening of the steel occurs due to phase hardening and substructural hardening during the γ → α transformation. The final structure and properties of the rental are formed after additional heating and exposure at tempering temperature.

An example of the method

Steel was smelted in an electric furnace and, after out-of-furnace refining and evacuation, was cast into continuously cast slabs with a cross section of 200 × 1550 mm. Rolling per sheet was performed on a single-shaft reversing mill "5000"

The chemical composition is shown in table 1, technological conditions in table 2.

Mechanical properties were determined on transverse samples. Tensile tests were carried out on type III specimens No. 4 of GOST 1497, and shock bending on specimens with a V-shaped notch (type 11, GOST 9454). Table 3 shows the average values of three results of tests for static tensile and six results of tests for impact bending.

The critical temperature of the viscous-brittle transition NDT was determined according to ASTM E208.

Literary sources used in the preparation of the description of the invention:

1. Patent of the Russian Federation No. 2062795, IPC C21D 9/46, C21D 8/02, 1996

Claims (2)

1. Method for the production of plate products, including steelmaking, casting onto billets, austenitization, rolling in a given temperature range and cooling to a regulated temperature, reheating and holding, characterized in that austenitization is performed at a temperature of 1200-1250 ° C, rolling at the first stages are carried out until a workpiece thickness of at least 60 mm is reached and equal to 2-3 final sheet thicknesses, then the workpiece is cooled with water to a temperature of 970 ± 10 ° C, final rolling in the temperature range of 950-980 ° C with def by irrigation in each pass at least 10%, after which it is cooled at a mass-average speed of 20-80 ° / s to a temperature of 150-250 ° C, sheets are heated at a mass-average rate of 1-1.5 ° / min to a temperature of 580-630 ° with a shutter speed of 10-16 min / mm and cooled in air. 2. The method according to claim 1, characterized in that the steel is melted containing the following ratio of elements, wt.%:
carbon 0.08-0.10 silicon 0.20-0.30 manganese 0.40-0.60 chromium 0.40-0.60 copper 0.50-0.70 nickel 1.90-2.20 molybdenum 0.25-0.31 aluminum 0.005-0.040 vanadium 0.01-0.03 calcium 0.030 (calculated) sulfur 0.001-0.003 phosphorus 0.001-0.008 arsenic 0.001-0.020 iron rest