RU2469103C1 - Manufacturing method of plate from complex alloyed structural steel of increased strength - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: after making of complex alloyed structural steel, its continuous pouring, immediately prior to hot rolling there performed are dilatometric tests of specimens made from that steel and taken from continuously cast ingot. As a result of the above tests, actual temperature of phase change of Ar₃ of that steel is determined. Considering the obtained data, production temperature modes of end of rolling Tr.e.=Ar₃+(30-60)°C and beginning of water accelerated cooling of rolled product Ta.c.b≥Ar₃+10°C are determined.

EFFECT: achievement in finished rolled steel of strength properties as per yield strength of 510-610 MPa, as per ultimate tensile strength of 590-700 MPa.

3 cl, 4 tbl

Description

The present invention relates to rolling production and can be used in the manufacture of sheets and strips of highly alloyed structural steels of increased strength for use in shipbuilding, fuel and energy complex, transport and heavy engineering, bridge building and other sectors of the economy.

The complex of mechanical properties of hot-rolled strips and sheets of complex alloyed structural steels largely depends on the main parameters of rolling: the amount of compression in the last stand / passage of the mill, temperatures of the end of rolling (Tk.p) and accelerated cooling (Tu.o), etc. One of the decisive factors that determine the microstructure of the steel sheet and as a consequence, its strength and plastic properties, is the Ar ₃ temperature of the polymorphic transformation of steel (the beginning of the ferrite transformation from the austenite), which is also called a temperature of selfless process under cooling (see Quick Reference metallurgist / Ed V.P.Adrianovoy -... M .: Metallurgy, 1960, p.232). The value of this temperature depends primarily on the chemical composition of the steel and its exact knowledge for a specific steel grade is a guarantee of suitable metal rolling with a stable set of mechanical properties.

Ignorance or inaccurate determination of the temperature of Ar ₃ leads to erroneous forecasts of the phase state of the metal structure at the end of thermal deformation processing and, consequently, after accelerated cooling, which invariably leads to unsatisfactory results of testing the strength characteristics of finished products and to obtain inappropriate products.

где Т₀ - температура конца прокатки для листов минимальной толщины, °С, a С_э=C+Si/3+Mn/9, мас.%, С, Si и Mn - содержание в стали соответственно углерода, кремния и марганца.A known method of hot rolling of steel plate grade 10HSND (see patent RU 2334565 C2, class B21B 1/26, publ. 09/27/2008), in which the temperature of the end of the rolling set depending on the carbon equivalent

where T ₀ is the temperature of the end of rolling for sheets of minimum thickness, ° C, a C _e = C + Si / 3 + Mn / 9, wt.%, C, Si and Mn are the carbon, silicon and manganese contents in steel, respectively.

There is also known a method of hot rolling on a continuous broadband carbon steel mill (see patent RU 2200199 C2, class C21D 8/04, B21B 1/26, publ. 03/10/2003), where the temperature of the end of the rolling is set depending on the chemical composition of the steel and the thickness of the bands from the expression Tk.n = A / h + B, where A = -43-238 [C] -4.63 [Si] -198 [Mn]; B = 774-418 [C] -244 [Si] +248 [Mn]; [Si] and [Mn] are the silicon and manganese contents in steel, wt.%, H is the final rolled thickness in mm.

The disadvantages of the known methods is the lack of accuracy in the calculation of Tk.p for the production of modern structural steels having a complex composition of chemical composition consisting of 10 or more chemical elements, due to the fact that there is an uncertainty in the values of C _e and A to the combined effect of all alloying chemical elements on them, as well as the absence of influence on these values of the parameters of the deformation process, in particular the degree and speed of deformation.

Closest to the claimed object is a method of producing a strip for pipes of main pipelines, including steelmaking, casting into slabs, preliminary rolling of the slab across the longitudinal axis with a total degree of deformation of 60-80%, then the tackle is cooled in air to the temperature of the beginning of the finish rolling equal to ( Ar ₃ +150) ° С, and finish rolling is carried out in the direction of the longitudinal axis with a temperature of the end of rolling equal to (Ar ₃ + 20-40) ° С, then cooled to a temperature of 350-450 ° С at a speed of 15-50 ° С / s, and then with a speed of not more than 1 ° / s, the ratio of the total degrees of deformation of the preliminary rolling and final rolling is (1: 4) - (1: 8) (see patent RU 2397254 C1, C21D 8/02, C22C 38/12, published on 08/20/2010) .

The disadvantage of this method is that the minimum temperature is not determined for the onset of accelerated cooling of the roll and a specific technical tool for determining the temperature of the phase transformation Ar _{3 is} not given.

An object of the present invention is to provide a hot-rolled product from complex alloyed structural steels with a stable complex of mechanical properties in a high-performance hot rolling mill.

The technical result of the invention is the achievement in finished products of the level of strength characteristics in terms of yield strength of at least 510-610 MPa, in tensile strength of not less than 590-700 MPa.

The problem is solved in that in a method of manufacturing a sheet of highly alloyed structural steel of increased strength, including steel smelting, continuous casting into slabs, hot rolling with a temperature of rolling end above Ar ₃ , accelerated cooling, in contrast to the closest analogue, conduct dilatometric tests before hot rolling from said steel samples to determine the phase transformation temperature Ar _3, then hot rolling the slab is carried out with a rolling finishing temperature of from above Ar ₃ +40 C to Ar ₃ + 60 ° C, and a minimum start temperature of accelerated cooling Tn.u.o≥Ar ₃ + 10 ° C.

The invention consists in the following.

The mathematical dependences that govern the temperature of rolling completion (the final finishing stage of the process) and further accelerated water cooling of the rolled products are empirical and obtained by processing the experimental data during the rolling of this range at the 5000 hot rolling mill of OJSC Magnitogorsk Iron and Steel Works.

The temperature of the end of rolling during high-temperature thermomechanical processing usually lies above the upper critical point of the polymorphic transformation, and conditions of high-temperature plastic deformation and subsequent cooling are created under which the development of recrystallization processes is suppressed and a special structural state is created, characterized by an increased density of imperfections and their special distribution with the formation of a substructure polygonization. At the same time, a unique combination of properties is formed when, along with an increase in strength, ductility, toughness and resistance to brittle fracture simultaneously increase (see ML Bernstein. Thermomechanical Processing of Metals and Alloys, v. 1-2, M .: Metallurgy, 1968).

Polymorphic transformations (the phenomenon of polymorphism of solid solutions based on α- and γ-iron solid solutions) of complex composition during cooling occur in a certain interval limited by lower Ar ₁ and upper Ar ₃ critical points.

The first consequence of the polymorphism of solid solutions of steel is recrystallization in the solid state, which is a phenomenon of a change in the crystalline structure of steel that occurs upon cooling to a certain temperature — a critical point. The recrystallization process itself consists in the formation of new grains. The formation of these new grains in shape and size at low degrees of supercooling occurs diffusely by the appearance and subsequent growth of new phase nuclei - the so-called crystallization centers.

The number of crystallization centers and their growth rate depend on many factors, the main of which are:

- the temperature of heating or cooling of the steel relative to the temperature of the polymorphic transformation, i.e. critical points;

- the presence of insoluble impurities that facilitate the formation of crystallization centers;

- plastic deformation, causing a flow during recrystallization;

- recrystallization, accompanying in some cases of plastic deformation.

It is known that the most reliable and simplest method for determining the temperature of critical points 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 from.). First of all, this is explained by the fact that non-uniform changes in the linear dimensions of samples of low alloy steels accompany phase transformations that occur in steels with temperature. The results of dilatometric experiments, presented in the form of thermokinetic diagrams, supplemented by data on measuring the hardness of the initial and obtained microstructures, as well as their metallographic images, give an almost complete picture of the transformations of supercooled austenite.

However, the temperature of phase transformations (critical points) determined by the dilatometric test method without taking into account the influence of the main alloying elements (sparingly soluble impurities) and without deformation-thermal regimes of processing the material as close as possible to the production will significantly differ from the actual ones that occur directly during the rolling process. The specified set of features in the known technical solutions are not found.

Based on the above analysis of known sources of information, we can conclude that for a specialist the inventive method for hot rolling of sheets and strips of complex alloyed steels does not follow explicitly from the prior art, and therefore meets the patentability condition "inventive step".

An example of the method

In the oxygen-converter shop of OJSC MMK, 3 different grades of high-alloy structurally alloyed structural steel of high strength are smelted and poured into continuously cast ingots. The chemical composition of these steels is presented in table 1.

At the plate mill “5000” (5000 mm is the length of the barrel of the work roll) of hot rolling, MMK plans to roll large-format sheets of 17.8 * 3800 * 11500 mm for the production of large-diameter electric-welded pipes from the above steels.

According to the invention, before hot rolling, dilatometric tests of samples of the specified steel are carried out according to the regimes as close as possible to the planned modes of rolling the slab (table 2).

Samples for dilatometric studies are taken from the body of a continuously cast ingot after it has been cooled for 72 hours in the foot after casting. The parameters of heating, holding, deformation, and cooling of the samples are as close as possible to the planned modes of slab rolling (Table 2):

heating to a temperature of 1200-1220 ° C;

holding at this temperature for 3-10 minutes (taking into account the small size of the sample, sufficient time to equalize the temperature in the volume of the sample and dissolve the "excess phases" of the carbo-, nitride-forming elements);

cooling to a temperature of 1050 ° C at a rate of 0.5-3 ° C / s;

preliminary deformation in the temperature range 1000-1050 ° C, with a strain rate of 5-8 s ^-1 and a relative degree of deformation of 10-25%;

cooling to a temperature of 850 ° C at a rate of 0.5-3 ° C / s;

final deformation in the temperature range of 820-840 ° C, with a strain rate of 2-5 s ^-1 and a relative degree of deformation of 5-15%;

accelerated cooling to a temperature of 600-630 ° C at a speed of 16-20 ° C / s.

The actual temperatures corresponding to the polymorphic ferritic transformation upon cooling (Ar ₃ ), determined by dilatometric tests of samples of these steels, are presented in table 3.

Variants of technological parameters, according to which, according to the claimed method, rolling was carried out at the mill “5000” of OJSC “MMK”, are presented in table 4.

The inventive method of hot rolling of sheets and strips of highly alloyed structural steels of increased strength provides a stably high complex of mechanical properties under the conditions of the high-performance hot rolling mill “5000” of OJSC “MMK”.

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

The present method can be widely used in hot rolling mills in the production of sheets and strips from highly alloyed structural steels of increased strength with the required regulated strength properties of hot rolled sheets and strips at maximum mill productivity. Therefore, the claimed method meets the condition of patentability "industrial applicability".

Table 1 The chemical composition of the studied steels No. chem. compo
position CHEMICAL COMPOSITION, wt.% FROM Si Mn S R Cr Ni Cu Al N V Ti Nb Mo Fe and inevitable impurities one 0.10 0.418 1,603 0.002 0.010 0,032 0.017 0,041 0,036 0.007 0,040 0,020 0,030 0.002 97,642 2 0.10 0.320 1,650 0.002 0.009 0,030 0.230 0,050 0,035 0.007 0,049 0.024 0,055 0.001 97,438 0.06 0.270 1.510 0.003 , 008 0,030 0.170 0.170 0,036 0.005 0.006 0.018 0,042 0.004 97,668

Table 2 Preliminary calculation of the slab rolling process Phase Passage number Thickness mm Relative compression,% Width mm Length mm Surface temperature, ° C Rolling speed, m / s - 0 300.00 - 2600 3430 1211 - Hydroscale one one 264.73 11.8 2639 3970 1054 1.7 2 242.34 8.5 3929 2902 1031 2,4 3 220.09 9.2 3931 3193 1020 2,4 four 198.37 9.9 3932 3543 1019 2.5 5 177.69 10,4 3934 3951 1002 2.6 6 154.07 13.3 3952 4531 1023 2.5 7 129.40 16,0 3955 5391 1006 2,4 8 109.88 15.1 3957 6343 1028 2.5 Whipping up 2 9 94.22 14.2 3940 7318 848 2.5 10 81.29 13.7 3940 8473 846 3.2 eleven 69.14 15.0 3942 9957 831 3.6 12 58 16.1 3944 11861 859 3.9 13 48.50 16,4 3946 14176 849 4.2 fourteen 40,45 16.6 3948 16987 879 4,5 fifteen 34.06 15.8 3949 20161 867 5.1 16 28.85 15.3 3950 23786 887 5.5 17 24.60 14.7 3950 27877 867 5.9 eighteen 21.13 14.1 3953 32473 872 6.3 19 18.22 13.8 3955 37680 844 6.4 The accelerated cooling with a speed of V = 16 ... 20 ° C / s

Table 3 Actual temperatures Ar ₃ tested steels No. chem. composition Temperature Ar ₃ , ° С one 752.6 2 738.0 3 766.4

Table 4 Technological parameters of hot rolling of sheets 17.8 * 3800 * 11500 mm from complex alloyed structural steels. Chemical compound number
position Mode of production Thickness, mm Temperature Ar ₃ , ° С The temperature of the end of rolling, ° C The temperature of the beginning of accelerated (water) cooling, ° C Yield strength for tensile tests of transverse samples, MPa Temporary resistance
rupture of transverse samples, MPa one proposed 17.8 752.6 790 ... 810 not less than 760 558 663 according to the prototype method 17.8 not known not known not regulations. undefined by the method analogue 1 17.8 not known 710 ... 730 not regulations. 625 769 by the method analogue 2 17.8 not known 945 not regulations. 480 571 2 proposed 17.8 738.0 780 ... 800 not less than 750 577 682 according to the prototype method 17.8 not known not known not regulations. undefined by the method analogue 1 17.8 not known 690 ... 710 not regulations. 651 813 by the method analogue 2 17.8 not known 970 not regulations. 454 557

Continuation of table 4 No. of chemical composition Mode of production Thick
on proc
there M Temperature Ar ₃ , ° С The temperature of the end of rolling, ° C The temperature of the beginning of accelerated (water) cooling, ° C Yield strength for tensile tests of transverse samples, MPa Temporary tensile strength of transverse samples, MPA 3 proposed 17.8 766.4 810 ... 830 not less than 780 534 627 according to the prototype method 17.8 not known not known not regulations. undefined by the method analogue 1 17.8 not known 620 ... 640 not regulations. 703 870 by the method analogue 2 17.8 not known 990 not regulations. 415 539

Claims (1)

A method of manufacturing a sheet of highly alloyed structural steel of increased strength, including steel smelting, continuous casting into slabs, hot rolling with a temperature of the end of rolling above Ar ₃ , accelerated cooling, characterized in that before hot rolling, the phase transformation temperature of Ar _{3 of the} steel is determined by dilatometric tests of a sample made from a slab, and then hot rolling of the slab is carried out with a temperature of the end of rolling equal to above Ar ₃ + 40 ° C to Ar ₃ + 60 ° C, and a minimum temperature of the beginning of accelerated cooling T n.u.o≥Ar ₃ + 10 ° C.