RU2432403C1 - Procedure for manufacture of cold resistant flat - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: for manufacture of flat of 1070 mm thickness with upgraded indices for resistance to brittle fracture at low temperature there is melted steel containing wt %: carbon - 0.06-0.12, manganese - 0.60-1.20, silicon -0.15-0.35, nickel - 0.05-0.40, aluminium - 0.02-0.05, molybdenum - 0.003-0.08, titanium 0.002-0.02, niobium - 0.02-0.06, vanadium - 0.02-0.05, nitrogen - 0.001-0.008, sulphur - 0.001-0.008, phosphorus - 0.003-0.012, calcium 0.005-0.03, copper 0.05-0.30, iron - the rest. Steel is cast in blanks which are subjected to austenisation at temperature 1180-1210C. Further, they are preliminary deformed with regulated reductions not less, than 12% at temperature 1000-1050C, cooled in air to temperature of beginning of finish deformation and finally deformed at temperature 880-770C. Also, each successive reduction at 1-4% exceeds the previous one. Temperature of end of rolling of flats is calculated by formula: Tre=Ar3+(100-130)-37.7ln(t), where t - thickness of flat. Accelerated cooling is carried out in interval of temperatures 620-510C, further, flat is slowly cooled in pile to temperature of environment. ^ EFFECT: high strength and improved weldability of steel. ^ 1 ex, 2 tbl

Description

The invention relates to metallurgy, and more particularly to the production of structural steels of normal and increased strength, improved weldability for use in shipbuilding, construction, bridge building and other industries.

For structures of northern design for various purposes (shipping berths, bridge supports, power lines, shaped structures, hoisting cranes, elements of oil drilling platforms, etc.), cold-resistant sheet steel with a thickness of up to 70 mm with high resistance to crack propagation under operational loads is required.

A known method of producing steel, in which steel of the specified chemical composition is smelted in a converter, the metal is cast into continuously cast billets, after heating the slabs for rolling, preliminary deformation is carried out with a total degree of reduction of 35-60% at a temperature of 900-800 ° C, then undermining, the final deformation is carried out at a temperature of 830-750 ° C with a total degree of compression of 65-75%, accelerated cooling to temperatures of 500-260 ° C, then delayed cooling in a caisson to a temperature not exceeding 150 ° C (RF patent 2265067 [1]).

A disadvantage of the analogue is the ability to provide mechanical properties and cold resistance in rolled products up to 50 mm thick.

A known method of producing sheet metal with a thickness of up to 70 mm, adopted as a prototype, from steel of the following chemical composition, wt.%:

Carbon 0.04-0.10 Manganese 1.00-1.40 Silicon 0.15-0.35 Niobium 0.02-0.06 Vanadium 0.02-0.10 Aluminum 0.02-0.06 Nickel 0.10-0.80 Molybdenum 0.01-0.08 Phosphorus 0.003-0.012 Sulfur 0.001-0.008 Iron rest

A method for the production of cold-resistant steel includes steelmaking. the specified chemical composition in the converter, casting metal into continuously cast billets, heating slabs for rolling, preliminary deformation with a total degree of compression of 58-65% with regulated minimum reductions in the first four passes: (12-15%) - (13-17%) - (14-18%) - (14-20%), at a temperature of 940-990 ° С, cooling of the obtained workpiece by 70-100 ° С, final deformation at a temperature of 830-750 ° С with a total reduction ratio of 35-42%, accelerated cooling to temperatures of 550-400 ° C, then delayed cooling in a caisson to a temperature not exceeding 150 ° C (patent RF №2345149, IPC C21D 8/02, C22C 38/12, C21D 9/46 [2]).

The main disadvantage of this method is the lack of stability of the performance characteristics of sheet metal with a thickness of 40-70 mm, primarily the unstable characteristics that evaluate the tendency of steel to brittle fracture according to the results of testing large-sized technological samples for static bending at low temperatures (temperature Tkb), and unstable results when assessing the characteristics of crack resistance (critical opening at the crack tip CTOD) at low temperatures, which explains the absence recrystallization to roughing rolling step carried out at temperatures of 940-990 ° C, and forming thereby separate from unrecrystallized austenite grain morphology bainite lath, which causes the unstable characteristics of resistance to brittle resolution [3].

The technical result of this invention is to obtain rolled products with a thickness of 10-70 mm for critical purposes with stable indices of resistance to brittle fractures at low temperatures — cold resistance and crack resistance.

The technical result is achieved by the fact that in a method for the production of cold-resistant rolled sheets with a thickness of 10 ... 70 mm, including smelting, casting to billets, austenitization, deformation in a given temperature range and cooling to a regulated temperature, unlike the closest analogue, steel containing wt.% Is smelted :

carbon 0.06-0.12 manganese 0.60-1.20 silicon 0.15-0.35 nickel 0.05-0.40 aluminum 0.02-0.05 molybdenum 0.003-0.08 titanium 0.002-0.02 niobium 0.02-0.06 vanadium 0.02-0.05 nitrogen 0.001-0.008 sulfur 0.001-0.008 phosphorus 0.003-0.012 calcium 0.005-0.03 copper 0.05-0.30 iron rest while SECV% not more than 0.36,

austenitization is carried out at a temperature of 1180-1210 ° C, preliminary deformation with regulated compressions of at least 12% is carried out at a temperature of 1000-1050 ° C, then the resulting workpiece is cooled in air to the temperature at which the final deformation begins, the final deformation is carried out at a temperature of 880-770 ° C, and each subsequent compression is 1-4% higher than the previous one, and the temperature of the end of rolling of the sheets is calculated by the formula: Tkp = Ar ₃ + (100-130) -37,7ln (t), where t is the thickness of the sheet, accelerated cooling is carried out in the range of pace temperature 620-510 ° C, then sheet metal is slowly cooled in a stack to ambient temperature.

An increase in resistance to brittle fractures (cold resistance and crack resistance) in low alloy steels is achieved by ensuring high metallurgical quality in relation to harmful impurities, gases and non-metallic inclusions, grain refinement and the formation of a structure of a given morphology.

Modification of liquid steel with calcium reduces the overall level of metal contamination with non-metallic inclusions, allows for a low mass fraction of sulfur and prevents the formation of inclusions of unfavorable morphology (acute-angled, film), leading to a decrease in cold resistance. Oxide and sulfide inclusions during the modification of steel with calcium are small inclusions of a globular shape that do not affect the level of cold resistance [4-7].

Regulation of the content of impurity elements, especially sulfur and phosphorus, provides high resistance of steel to brittle and layered fractures in the direction of the thickness of the rolled metal in the composition of welded joints. With an increase in sulfur content, the number of sulfide inclusions causing layered fracture increases, the work of crack propagation and impact strength decrease [8]. Sulfur increases the tendency of metal to form cracks during welding due to the formation of dispersed film precipitates of sulfides in the weld zone. The harmful effect of phosphorus is based on its influence on the expansion of the liquidus-solidus region, leading to the development of primary segregation processes, as well as a significant narrowing of the γ-region, which facilitates the development of segregation in the solid state [9]. The resulting segregation is poorly resolved due to the relatively low diffusion rate of phosphorus in α and γ solutions. Phosphorus causes an increased tendency to brittle fracture with lower test temperatures due to enrichment of grain boundaries.

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.

The most effective mechanism for improving the cold resistance is the grinding of real grain. Grinding the structure is achieved by alloying with titanium, nitrogen, vanadium, and niobium, which, forming fine carbonitrides, inhibit the growth of austenite grains during heating and have an inhibitory effect on collective recrystallization during the high-temperature rolling stage.

Titanium is a strong carbonitride-forming element, contributing to the grinding of grain at a selected concentration due to the formation of dispersed precipitates with nitrogen. Dispersed nitrides modify the cast structure, providing a fine austenitic grain that is not subject to significant growth at selected heating temperatures for rolling.

Doping with nitrogen, titanium, vanadium and niobium within the claimed limits most effectively contributes to the creation of ultrafine-grained ferrite-pearlite or ferrite-bainitic structure with fine particles of vanadium and niobium carbonitrides during rolling and accelerated cooling, which effectively stabilize the created structure under operational influences - static and cyclic loads .

Niobium forms finely dispersed Nb (C, N) particles over a wide temperature range, which, by choosing the appropriate regime, are used to limit the growth of austenite grain and, during deformation, to regulate the recrystallization process [10].

Vanadium is a highly effective element for the dispersion hardening of steel, which is carried out due to the release of finely dispersed particles of carbonitride V (C, N) in the ferrite region upon cooling of the rolled metal [11] or during tempering.

The limitation of the carbon equivalent value guarantees high processability in welding without preheating. The requirements for limiting the maximum values of carbon equivalent are provided by varying the content of chemical elements.

The main distinguishing features of the production method are:

- restriction of grain growth due to fine precipitates of titanium carbonitrides during heating for rolling in the temperature range of 1180-1210 ° C, allowing for the most complete dissolution of vanadium and niobium carbonitrides for subsequent hardening of steel;

- increasing the temperature range of the rough rolling stage to 1000-1050 ° C for grinding austenitic grain due to recrystallization processes while limiting the niobium content within specified limits as an element that most significantly suppresses recrystallization processes;

- ensuring the temperature of the end of rolling of sheet metal with a thickness of 10-70 mm in the temperature range of 880-770 ° C, which for sheets of various thicknesses is calculated by the formula: Tkp = Ar ₃ + (100-130) -37,7ln (t), where t - the thickness of the sheet, to avoid hardening of the σ-phase in the formed steel structure;

- regulation of the temperature range of accelerated cooling at temperatures of 510-620 ° C, allowing to form a finely dispersed ferrite-pearlite or ferrite-bainitic structure with a fraction of perlite not more than 10%, acicular bainite from 10 to 20%, the rest is quasi-polygonal ferrite.

Tests of sheet metal manufactured by this technology showed that the proposed modes for steel of the selected chemical composition provide stable resistance to brittle fracture at temperatures up to minus 40 ° C after static tests on large-sized technological samples and stable crack resistance (CTOD) in thick sheet 10-70 mm.

Example

Steel was smelted in a 370 t oxygen converter with a magnesium desulfurization process in a casting ladle. Initial alloying, preliminary deoxidation, and metal processing with solid slag mixtures with metal purging with argon in a steel pouring ladle were carried out at the outlet. The final alloying, microalloying, metal treatment with calcium and metal overheating for evacuation were carried out on the on-off furnace "Ladle-Bucket". The metal was degassed by evacuation. The 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.

According to the specified method, the preform was subjected to austenitization at a temperature of 1180-1210 ° C for 4-6 hours. Rolling on sheets with a thickness of 10, 25, 35, 40, 50 and 70 mm was carried out on a single-strand mill with a maximum force of up to 11 thousand tons in reverse mode. Preliminary deformation was carried out with strictly regulated compressions (at least 12%) in the temperature range of 1000-1050 ° C. The tackle was blown up in air to the temperature of the beginning of the second stage of deformation. The final deformation was carried out at a temperature of 880-770 ° C, and the temperature of the end of deformation for sheets of different thicknesses was determined by the formula: Tkp = Ar ₃ + (100-130) -37.7ln (t), where t is the thickness of the sheet. After deformation, the sheets were cooled in an accelerated cooling unit to a temperature of 510-620 ° C in one pass. Slow cooling was carried out in a stack in air to ambient temperature.

The mechanical properties (table 2) of sheet metal were determined on transverse samples. Static tensile tests were carried out on type III samples No. 6 or type III No. 4 according to GOST 1497, and on impact bending - on samples with a V-shaped notch (type 11, GOST 9454), including after mechanical aging according to GOST P52927- 2008. Resistance to layered fractures was evaluated by the relative narrowing of samples cut according to GOST 28870-90 in the direction of sheet thickness.

To test technological samples for fracture by static bending, one sample was cut from a sheet across the direction of rolling from the middle third of the sheet width in accordance with GOST R52927-2008. The test was carried out by static bending in accordance with GOST R52927-2008 at room temperature (assessment of the type of fracture) and low temperature (determination of the temperature Tkb corresponding to the minimum temperature at which 70% of the fibrous component is observed in the fracture of a process sample of full thickness tested for static bending). After the test, a visual assessment of the fracture surface was carried out for compliance with the requirements of GOST R52927-2008. During the test, the percentage of the viscous component in the fracture was evaluated as a percentage.

Weldability was evaluated by calculating the fracture toughness parameter when welding SECV according to the above formula.

The CTOD test procedure for cold-resistant steels, equipment and measuring requirements were in accordance with Part I of the British Standard BS 7448 [12]. Requirements for CTOD values in accordance with [13]. For testing, we used samples for rectangular rectangular bending with a single-sided notch (SENB type according to BS 7448) and smooth side surfaces. Fatigue crack growth was carried out at a frequency of 5-8 Hz. The total number of loading cycles for the sample was not less than 55,000. During the tests, a deformation diagram was recorded in the coordinates “load - opening of crack faces”. The determination of displacements (opening of the crack faces) was carried out by the DSR 10/50 sensor.

The test results show that the proposed production method for steel of the selected chemical composition provides a more stable at low temperatures level of brittle fracture resistance characteristics (cold resistance when testing large samples and crack resistance), satisfying the requirements of the "Rules ..." of the Russian Maritime Register of Shipping [13] than the well-known way.

Information sources

1. Patent of the Russian Federation No. 2265067.

2. Patent of the Russian Federation No. 2345149, IPC C21D 8/02, C22C 38/12, C21D 9/46, 2009

3. Gorynin I.V., Rybin V.V., Malyshevsky V.A., Khlusova E.I. The principles of alloying, phase transformations, structure and properties of cold-resistant welded shipbuilding steels. // Metallurgy and heat treatment of metals, 2007, No. 1, pp. 9-15.

4. Berezhnitsky L.T., Gromyak R.S., Trush I.I. // FKHMM. 1975, No. 5, p.40.

5. Brodetsky I. L., Kharchevnikov V. P., Trotsan A. I. et al. On the effect of calcium on grain boundary embrittlement of structural steel with carbonitride hardening. MITOM. 1995, No. 5, pp. 24-26.

6. Kovalenko B.C., Kuchkin V.I., Pilguk V.E., Zayats E.L. On the effect of calcium on the structure and properties of steel. Metals, 1983, No. 6, p. 92-96.

7. Volchok I.P., Fedkov V.A., Lugov M.V. Non-metallic inclusions and steel failure at low temperatures. FKHMM, 1977, No. 2, pp. 10-12.

8. Odessa P.D., Smirnov L.A., Kulik D.V. Micro-alloyed steels for northern and unique metal structures. M .: Intermet Engineering, 2006, 176 p.

9. Goodremont E. Special steel. 2nd ed. M .: Metallurgy, 1966, v. 1-2.

10. G. Akben, I. Weiss and J. J. Jonas: Acta Metall. Mater., 1981, 29, 111-121.

11. Jonas J.J., Weiss J. // Metal Science - 1979 - No. 3-4 - P.238-245.

12. BS 7448. Fracture Mechanics Toughness Test. Part 1. Method for determination of critical CTOD and critical J values of welds in metallic materials, 1997. Part 2. Method for determination of K1c, critical ÑTOD and critical J-values of metallic materials.

13. Rules for the classification, construction and equipment of floating drilling rigs and offshore stationary platforms. Russian Maritime Register of Shipping, 2006

Claims (1)

Method for the production of cold-resistant rolled sheets with a thickness of 10 ÷ 70 mm, including steel smelting, casting to billets, austenitization, deformation in a given temperature range and cooling to a regulated temperature, characterized in that the composition is smelted, wt.%:
carbon 0.06-0.12 manganese 0.60-1.20 silicon 0.15-0.35 nickel 0.05-0.40 aluminum 0.02-0.05 molybdenum 0.003-0.08 titanium 0.002-0.02 niobium 0.02-0.06 vanadium 0.02-0.05 nitrogen 0.001-0.008 sulfur 0.001-0.008 phosphorus 0.003-0.012 calcium 0.005-0.03 copper 0.05-0.30 iron rest
wherein С _{equiv is} not more than 0.36%, austenitization is carried out at a temperature of 1180-1210 ° C, preliminary deformation with regulated compressions of at least 12% is carried out at a temperature of 1000-1050 ° C, then the resulting billet is cooled in air to the initial final temperature deformation, the final deformation is carried out at a temperature of 880-770 ° C, and each subsequent compression is 1-4% higher than the previous one, and the temperature of the end of rolling of the sheets is calculated by the formula Tkp = Ar ₃ + (100-130) -37,7ln (t) where t is the thickness of the sheet, accelerated cooling of the wasp estvlyayut in the temperature interval 620-510 ° C, more sustained rolled sheet in the stack is cooled to ambient temperature.