RU2477323C1 - Manufacturing method of rolled low-alloy plate - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method involves heating of continuously cast billets, roughing till intermediate thickness, cooling of intermediate billet, finish rolling of the semifinished rolled product to the specified thickness of 10-40 mm; accelerated cooling of the obtained rolled plate; at that, finish rolling is performed at restricted interval of the rolling end temperature: Ar₃+K_E+20≤T_{roll end}≤Ar₃+K_E+70, where T_{roll end} - rolling end temperature, °C; Ar₃ -physical temperature of polymorphous ferritic steel conversion, which is determined based on content in low-alloy steel of 14 chemical elements, °C; K_e - additive correction for deformation, which is set depending on relative crimping in the last finishing pass, °C.

EFFECT: achieving the required set of mechanical properties of the strip and avoiding the necessity of carrying out heat treatment of finished plates.

4 tbl, 1 ex

Description

The invention relates to the field of metallurgy, and more specifically to the rolling production of low alloy steels of various strength classes, and can be used for the production of finished sheets used as the initial blank for longitudinal welded pipes of large diameter.

For modern pipeline projects, such as East Siberia - Pacific Ocean-2 (ESPO-2), Sakhalin-Khabarovsk-Vladivostok, etc., hot-rolled sheets (strips) are required that combine high strength and cold resistance, 10-40 thick mm strength category K56-K60 of low alloy steel, having the following set of mechanical properties (table 1).

In addition to the indicated mechanical properties, the strips must have good plastic properties (relative elongation δ ₅ ≥22%, uniform elongation δ _{equal to} ≥7%, relative narrowing Ψ≥64%,), the value of the actual grain of the metal must not be lower than the 9th number according to GOST 5639 (scale 1), hardness HV10≤280 units.

A known method of rolling low-alloy strip for main pipes on a plate reversing mill (patent RU 2403105 C1 dated 10.11.2010, IPC B21B 1/34), which includes rough rolling of a continuously cast billet, intermediate curing of the rolled material to a predetermined temperature and subsequent finishing rolling to a predetermined size . Rough rolling of the initial billet is carried out in two stages, with a total degree of compression of 45-65%, first in the longitudinal direction until the length of the roll is 0.70-0.98 of the length of the work rolls, then a 90 ° planing is performed and planed transverse direction to obtain a tack width of 0.6-0.85 of the width of the finished strip with a thickness of 3.5-6 thicknesses of the finished strip, finishing rolling is also carried out in two stages with a total degree of compression of the specified tack height 70-83% and total number of passes 9-25, in the pope echnom direction rolled at a reduction ratio of 25-40% in height to give them the width corresponding to the width of the finished strip, followed by reverse tilting tackle 90 ° and in terms of rolling in the longitudinal direction to obtain a predetermined thickness and length of the finished strip.

The disadvantages of this method are that only the parameters of the deformation process are described. The absence of temperature conditions for controlled strip rolling with a high degree of probability will lead to unsatisfactory results of mechanical testing of the rolled product, in part the lack of the required levels of strength, ductility and toughness. Which, in turn, will lead to the need for thermal improvement (hardening and tempering) of the sheets after rolling, and this complicates and increases the cost of production.

The closest in technical essence is the method of production of low-alloy plate (patent RU 2414515 C1 dated 03/20/2011, IPC C21D 8/02), which includes smelting, casting of steel into continuously cast billets, heating the billet, rough rolling, subsequent cooling of the intermediate billet, fine rolling, accelerated cooling of the obtained sheet metal to a predetermined temperature and its subsequent delayed cooling. In this case, the continuously cast billet is heated at a temperature of 1170-1210 ° C for at least 7 hours, rough rolling with a transition from longitudinal to transverse rolling with a breakdown of the width is started at a temperature of at least 950 ° C and is carried out at a thickness of 4.0 -5.5 thickness of the finished strip with relative compressions per pass of at least 10%, and the subsequent cooling in air of the intermediate billet is carried out to 770-800 ° C, and the breakdown of the width is completed at the finish rolling stage in no more than two passes with a total compression of 8 -15%, p after that, longitudinal rolling is carried out with a compression of at least 8% per pass, with the exception of the last two passes, in which the compression ratio is at least 1%, and finish rolling is completed at a temperature of at least 740 ° C, and accelerated cooling of the obtained strip is carried out to a temperature, determined depending on the carbon equivalent _Сequ from the relation: _Тco = (500 · _Сequiv + 385 ° С) ± 15 ° С, where 500 is an empirical coefficient, ° С.

The disadvantage of the prototype is that the described method for the production of plate low-alloy rolled products does not guarantee a high (90% or more) share of the viscous component in the fracture of the samples when tested with a falling load at a temperature of -20 ° C (IPG ^-20 ). A high proportion of the viscous component in the fracture of samples is an integral requirement of manufacturers of longitudinally-welded large-diameter electric welded pipes - consumers of plate steel from low alloy steels (see Table 1).

The technical result of the invention is the achievement of the required set of mechanical properties of the strip and eliminating the need for heat treatment of finished sheets.

The specified technical result is achieved by the fact that in the known method for the production of low-alloy plate, including heating a continuously cast billet, its rough rolling to an intermediate thickness, cooling the intermediate billet, finishing rolling to a thickness of the finished strip 10-40 mm, accelerated cooling of the obtained sheet according to the invention finishing rolling is carried out in the temperature range of the end of rolling, determined from the expression:

Ar ₃ + K _E + 20≤T _cn ≤Ar ₃ + K _E +70,

where T _kn - the temperature of the end of rolling, ° C;

Ar ₃ - the physical temperature of the polymorphic ferritic transformation of steel, determined by the following dependence, ° C:

Ar ₃ = 735 + 180 ([C] + [Cr]) + 1207 ([S] + [P]) - 11 ([Si] + [Mn] + [Ni] + [Cu] + [Mo]) + 755 ([Al] + [N]) - 329 ([V] + [Nb] + [Ti]),

where [C], [Si], [S], [P], [Mn], [Cr], [Ni], [Cu], [Mo], [Al], [N], [V], [ Nb], [Ti] - the content in the steel, respectively, of carbon, silicon, sulfur, phosphorus, manganese, chromium, nickel, copper, molybdenum, aluminum, nitrogen, vanadium, niobium and titanium, wt.%; 735, 180, 1207, 11, 755, 329 - empirical coefficients obtained by processing the data of dilatometric studies;

K _E - additive correction for deformation, set depending on the relative compression in the last finishing pass, K _E = 30 ° C at εrel≥20%; To _E = 20 ° C at 10% ≤εrel <20%; To _E = 10 ° C at 5% ≤εrel <10%; To _E = 5 ° C at εrel <5%.

From the point of view of structure formation, the thermomechanical rolling process is aimed at achieving the main goal - grinding ferrite grains by combining plastic deformation and controlling the processes of microstructure formation.

The rough rolling process is carried out at such temperatures when austenite is recrystallized. The control of this process leads to effective grain refinement due to repeated recrystallization after each pass. Deformed austenite has the following features: austenite grains have an elongated “muffin” shape (increased specific surface area of the boundaries), deformation bands, twin boundaries and a dislocation cellular structure are observed inside the grains.

During thermomechanical rolling, the size of the ferrite grain is determined by the total deformation at temperatures when recrystallization cannot occur. Therefore, an important parameter is the thickness of the rolled product for the finishing stage of rolling, which should be 4-5.5 times higher than the final thickness of the rolled product.

The finishing stage of rolling is completed in the austenitic region 20 ... 70 ° C above the point Ar ₃ . Such a temperature interval of the end of rolling allows the metal to be deformed in the region of unrecrystallized austenite, as a result of which deformation bands appear in the deformed austenite. During the conversion of austenite to ferrite, the deformation bands, as well as the boundaries of austenitic grains, serve as sites for the formation of nuclei of the ferritic phase. Accelerated water cooling in combination with thermomechanical processing of rolled products in the indicated temperature range additionally increases the dispersion of the structure, which makes it possible to grind grain to 5-6 microns or less. This method of production of plate low-alloy rolled products economically feasible allows you to perform the IPG test and prevent the formation of extensive destruction of oil and gas pipelines.

Of decisive importance is the temperature of the phase transformation of ferrite from austenite directly during the rolling process. It is known that the most reliable and simple way to determine the temperatures of phase transformations of materials is the dilatometric method of research (see Romanov P.V., Radchenko V.P. Transformation of austenite during continuous cooling of steel: Atlas of thermokinetic diagrams. Part 1. Novosibirsk, 1960, 51 s). The dependence allowing to calculate the temperature of the polymorphic phase austenite-ferrite transformation Ar ₃ low alloy steel was obtained by processing

results of dilatometric studies conducted at OJSC MMK. The chemical elements in the presented dependence are arranged according to the groups of influence on the structural-phase austenite-ferrite transformation of low alloy pipe steel.

[C], [Cr] - reinforcing elements, directly affect the interval of existence of δ-ferrite, which allows homogenizing a solid solution, increasing the uniformity of the distribution of chemical elements due to the fact that the diffusion mobility of carbon and chromium atoms in δ-ferrite is several orders of magnitude higher than the speed their diffusion in austenite.

[S], [P] are harmful impurities, due to their reduced solubility in ferrite, they diffuse to the grain boundaries, affecting the quantity and quality of the “nuclei” —the sites of formation of the ferrite phase.

[Si], [Mn], [Ni], [Cu], [Mo] - alloying elements that make up the austenite solid solution. Lower the temperature of the onset of decomposition of austenite.

[Al], [N] are technological impurities, the influence of these elements is due to the fact that their solubility in α-iron is extremely small compared with γ-iron and a supersaturated solid solution is formed during polymorphic γ → α-transformation.

[V], [Nb], [Ti] - carbonitride-forming elements, form a solid substitution solution with iron. The mismatch of the atomic radii of these elements and iron leads to a distortion of the crystal lattice of the solid solution and, as a result, to a slowdown of all processes controlled by diffusion, including recrystallization and phase transformations.

However, the temperature Ar ₃ determined by the dilatometric test method without taking into account the influence of deformation modes of metal processing will differ from the actual temperature that occurs directly in the rolling process (see Khlestov V.M., Dorozhko G.K. Transformation of deformed austenite in steel. Monograph / / Mariupol: Publishing house of the Perm State Technical University, 2002 .-- 407 p.). Therefore, an additive temperature correction for deformation K _{E is} required, which is set depending on the relative compression in the last finishing pass (table 2).

table 2 Additive temperature correction for deformation K _E εrel in the last finishing pass,% K _E , ° C εrel≥20 thirty 10≤εrel <20 twenty 5≤εrel <10 10 εrel <5 5

The given numerical values of K _E are empirical and were obtained by processing experimental data on the production of thick sheets at the hot rolling mill “5000” of OJSC “Magnitogorsk Iron and Steel Works”.

The low temperature of the end of rolling (Tcp <Ar ₃ + K _E +20) causes premature austenite-ferritic γ + α-transformation of the surface layers of the roll before it reaches the accelerated cooling unit, which leads to an incorrect cooling process and the formation of a rolled subgrain microstructure steel and the formation of unfavorable crystallographic texture of ferrite. All this, ultimately, leads to heterogeneity, anisotropy of the mechanical properties of the rolled product in general, and an unsatisfactory IPG test in particular.

The high temperature of the end of rolling (T _kp > Ar ₃ + K _E +70) due to the temperature gradient along the roll section will lead to postdynamic recrystallization of the central (hotter) layers of the roll, and consequently to spontaneous growth of austenitic grain, and a decrease in the specific surface area grain boundaries, twins and a dislocation cellular structure. All this during subsequent austenitic-ferritic transformation during cooling will lead to the formation of the actual metal grain at the level of 10-11th number according to GOST 5639 (scale 1), unsatisfactory IPG test and plastic characteristics of the rolled product, which does not meet the requirements of the consumer.

The combination of thermal deformation parameters of the production of plate low-alloy rolled according to the invention allows achieving the required level of mechanical properties of the finished sheets (see table 1) and is not found in the known technical solutions.

Based on the foregoing, we can conclude that for a specialist, the inventive method does not follow explicitly from the prior art, and therefore meets the patentability condition "inventive step".

An example of the method

At the reversible plate mill “5000” of hot rolling mill of OJSC MMK, sheets 16.0 × 3133 × 11650 of low alloy steel of strength class K60 are rolled according to TU 14-101-782-2010 with a wt.%: 0.07 C; 1.66 Mn; 0.32 Si; 0.046 Nb; 0.05 V; 0.022 Ti; 0.03 Cr; 0.26 Ni; 0.16 Cu; 0.033 Al; 0.005 Mo; 0.003 S; 0.009 P; 0.005 N.

A slab 300 mm thick, heated to a temperature of 1220 ° C, enters the hot rolling mill, which includes heating furnaces, hydroscale, a reversing rolling mill, a preliminary dressing machine (MPP), a discharge roller table with cooling sections, a hot dressing machine (MGP ), a section of delayed cooling of peals.

The roughing stage of rolling is carried out after hydroscale of furnace scale in the temperature range of 970-1110 ° C to an intermediate tack thickness equal to 5.5 thicknesses of the finished strip (90.0 / 16.0≈5.5). Further, the rolling undercoating (cooling of the intermediate billet) in a natural (in air) way is produced by reciprocating movements of the rolling directly on the roller table in the rolling line. The temperature of the end of rolling T _KP is determined as follows:

Ar ₃ = 735.6 + 180.1 ([C] + [Cr]) + 1206.9 ([S] + [P]) - 10.9 ([Si] + [Mn] + [Ni] + [ Cu] + [Mo]) + 755.3 ([Al] + [N]) - 328.8 ([V] + [Nb] + [Ti]) = 735.6-180.1 * (0.07 +0.03) + 1206.9 * (0.003 + 0.009) -10.9 * (0.32 + 1.66 + 0.26 + 0.16 + 0.005) + 755.3 * (0.033 + 0.005) - 328.8 * (0.05 + 0.046 + 0.022) ≈732 ° C.

The parameters of the final rolling stage are presented in table 3.

Tab. 3 Finishing Stage Settings Passage number Thickness mm Relative compression,% Width mm Length mm Surface temperature, ° C whipping up 90.00 - 3331 7235 891 one 73.14 18.21 3320 8765 872 2 59.26 18.97 3322 10802 839 3 48.21 18.83 3325 13308 858 four 38.92 19.09 3327 16417 853 5 32.14 17.43 3328 19879 878 6 26.75 16.77 3329 23841 861 7 22.51 15.85 3329 28337 870 8 19.06 15.33 3329 33413 841 9 16.40 13.97 3332 38892 805

Compression ate. the final pass is 13.97%, K _E = 20 ° C (see table 2).

Ar ₃ + K _E + 20≤T _kp ≤Ar ₃ + K _E +70

732 + 20 + 20≤T _cn ≤732 + 20 + 70

Tkp = 772 ÷ 822 ° C.

Accelerated cooling of peals is carried out on the installation of accelerated combined cooling. Variants of technological parameters, according to which, according to the claimed method and prototypes, rolling was carried out at mill 5000 of OJSC MMK, are presented in table 4.

Tab. four Technological parameters of hot rolling of sheets 16.0 × 3133 × 11650 from steel of strength class K60 according to TU 14-101-782-2010 at mill 5000 of OJSC MMK Mode of production Thickness, mm The temperature range of the finishing stage of rolling, ° C Temporary resistance, σ _in , N / mm ² Yield strength
σ _t , N / mm ² Impact strength KCV, J / cm ² Elongation δ ₅ ,% IPG ^-20 ,
% proposed 16,0 772 ... 822 648 573 330 24.0 95 by analogue method 16,0 not regulated 516 398 98 34.0 fifty according to the prototype method 16,0 not less than 740 611 543 263 23.0 75 TU norms 590-700 510- not not 90-100 14-101-782-2010: 610 less than 110 less than 22.5

Compliance with the technology of rolling and cooling in accordance with the proposed method for the production of low-plate thick-rolled plates provides the desired mechanical properties of strips (see table 1) and, therefore, eliminates the need for post-deformation heat treatment of finished sheets.

Based on the foregoing, we can conclude that the claimed method is workable and eliminates the disadvantages that occur in the prototype.

The present method can be widely used on hot-rolled plate mills in the production of thick sheets of low-alloy steels of high strength classes with the required regulated physical and mechanical properties of hot-rolled steel. Therefore, the claimed method meets the condition of patentability "industrial applicability".

Claims (1)

A method of manufacturing a low-alloy plate, including heating a continuously cast billet, rough rolling to an intermediate thickness, cooling the intermediate billet, finishing rolling to a finished strip thickness of 10-40 mm, accelerated cooling of the obtained sheet metal, characterized in that the finish rolling is carried out in the temperature range of the end of rolling defined from the expression
Ar ₃ + K _E + 20≤T _cn ≤Ar ₃ + K _E +70,
where T _kn - the temperature of the end of rolling, ° C,
Ar ₃ - the physical temperature of the polymorphic ferritic transformation of steel, determined by the following dependence, ° C:
Ar ₃ = 735 + 180 ([C] + [Cr]) + 1207 ([S] + [P]) - 11 ([Si] + [Mn] + [Ni] + [Cu] + [Mo]) + 755 ([Al] + [N]) - 329 ([V] + [Nb] + [Ti]),
where [C], [Si], [S], [P], [Mn], [Cr], [Ni], [Cu], [Mo], [Al], [N], [V], [ Nb], [Ti] - the content in the steel, respectively, of carbon, silicon, sulfur, phosphorus, manganese, chromium, nickel, copper, molybdenum, aluminum, nitrogen, vanadium, niobium and titanium, wt.%,
735, 180, 1207, 11, 755, 329 - empirical coefficients obtained by processing the data of dilatometric studies,
K _E - additive correction for deformation, set depending on the relative compression in the last finishing pass, K _E = 30 ° C at ε _rel. ≥20%; K _E = 20 ° С at 10% ≤ε _rel. <20%; K _E = 10 ° С at 5% ≤ε _rel. <10%; K _E = 5 ° C at ε _rel. <5%.