RU2549023C1 - Method of production of rolled plates with strength class k65, x80, l555 to manufacture arc welded pipes of main pipelines - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: method of production of rolled plates to manufacture arc welded pipes of main pipelines includes production of steel containing in wt %: C - 0.03-0.08, Si - 0.12-0.35, Mn - 1.65-2.10, Cr - 0.01-0.30, Ni - 0.01-0.40, Cu - 0.01-0.30, Mo - 0.01-0.30, Al - 0.02-0.05, Nb - 0.03-0.09, V - 0.001-0.10, Ti - 0.010-0.035, S - 0.0005-0.003, P - 0.002-0.015, N - 0.001-0.008, iron and inevitable admixtures - rest, at that 0.08<(Mn+Cr+Cu)/20+Si/30+Ni/60+Mo/15+V/10<0.16, -2.7<lg[Nb][C+8N]<-2 and Cr+Ni+Cu+Mo<0.8%. Continuously cast billet is subjected to austenisation at temperature at least by 100°C below temperature Ts (TiN) of dissolution of titanium nitrides according to ratio Ts(TiN)=14400/(5.0-lg[Ti][N]), where Ti and N are content of titanium and nitrogen in steel, wt %, but not below temperature Ts(Nb(C,N)) of dissolution niobium carbonitrides according to ratio: Ts(Nb(C,N))=(10100-27[Mn]+200[Si])/(4.85-lg[Nb][C+8N]), where Mn, Si, Nb, C, N are content of manganese, silicon, niobium, carbon and nitrogen in steel. Time t for holding in holding zone is selected according to equation:where t is holding time, min, T is selected holding temperature, °C. Upon preliminary deformation during last four passes the relative reductions increase as per law: ?=(1.05?1.35)?±2, (%), where ?and ?are reductions in previous and next passes. Temperature T(°C) of accelerated cooling start is: T=977-54Mn-102Ni-20Mo-866C-2.2V±30, where Vis roll cooling rate from rolling completion to start of accelerated cooling, °C/s, and temperature range ?(°C) between temperature Tof rolling completion and temperature Tof accelerated cooling start is determined as follows: ?=-2.5H+92±20, where H is plate thickness, mm.EFFECT: meeting of the requirements for strength, plastic and tough properties characteristic for the rolled plates with strength K65, X80, L555.4 cl, 3 tbl

Description

The invention relates to the field of metallurgy, in particular to the production of sheet metal with a thickness of 15-34 mm at a reversible plate mill for the manufacture of pipes of main pipelines with a diameter of up to 1420 mm.

A known method of producing a strip for pipes of main pipelines (patent RU No. 2397254), including steelmaking, casting into slabs, preliminary rolling of the slab, intermediate undermining of the tack, finishing rolling and cooling, characterized in that the steel of the following composition is melted, wt.%:

carbon 0.03-0.07 silicon 0.16-0.40 manganese 1.75-2.10 nickel 0.040-0.80 copper 0.001-0.50 molybdenum 0.03-0.50 aluminum 0.01-0.10 niobium 0.01-0.10 vanadium 0.001-0.04 titanium 0.01-0.05 sulfur 0.001-0.003 phosphorus 0.003-0.012 calcium 0.001-0.010 iron rest

Preliminary rolling is carried out across the longitudinal axis of the slab with a total degree of deformation of 60-80%, then the rolling is cooled in air to the finish rolling start temperature equal to (Ar3 + 150) ° C, and finish rolling is carried out in the longitudinal axis direction with the end temperature of rolling equal to Ar3 + (20-40) ° C, then cooled to a temperature of 350-450 ° C at a speed of 15-50 ° C / s, and then at a speed of not more than 1 ° C / s, while the ratio of the total degrees of deformation of the preliminary rolling and final rolling is (1: 4) - (1: 8).

The disadvantage of this method is that the obtained metal is characterized by a low level of toughness even at -20 ° C. Also, the relative elongation and the proportion of the viscous component in the fracture (ICE) during falling load tests (IPG) are not guaranteed.

Closest to the manufacturing technology is a method of manufacturing a plate of low-alloy strip (patent RU No. 2393238 - prototype), including austenization of a continuously cast billet, rough rolling, subsequent cooling of the intermediate billet, finishing rolling, accelerated cooling of the obtained strip to a predetermined temperature and its subsequent slow cooling characterized in that the austenization of a continuously cast billet is carried out at a temperature of 1170-1210 ° C for at least 6 hours, rough rolling is carried out pour to a thickness of the intermediate billet equal to 4.0-7.5 thicknesss of the finished strip, while the temperature of the end of the rough rolling is set not lower than 900 ° C, the subsequent cooling of the intermediate billet is carried out to a temperature of 780-820 ° C, then finish rolling with a degree crimping per pass of at least 12%, with the exception of the last three passages, which allow a compression ratio of at least 2%, accelerated cooling of the obtained strip after finishing rolling starts no later than 30 s after the strip leaves the mill stand, etc. plague to a temperature of 320-620 ° C, more sustained cooled to ambient temperature in a stack consisting of at least five sheets. Strips are rolled from low alloy steel with the following ratio of elements, wt.%: C - 0.04-0.1; Mn - 1.60-1.90; Si - 0.15-0.35; (V + Nb + Ti) 0.05-0.25; (Mo + Cr) - 0.20-0.60; (Cu + Ni) - 0.40-0.70; the rest is iron and impurities, with the content of each impurity element less than 0.03%, while the fracture toughness coefficient Rcm does not exceed 0.23, and the strip microstructure contains at least 70 vol.% bainite of rack morphology obtained from unrecrystallized austenite having transverse the average diameter (dY) of austenitic grains is not more than 25 microns.

The disadvantages of this method include low ductility and toughness, the presence in the structure of more than 70 vol.% Rack bainite, which significantly reduces the viscoplastic properties. Also, it is not guaranteed that the share of the viscous component in the fracture during ING is guaranteed. In general, the low viscoplastic properties of rolled products manufactured by the prototype method are due to an incorrect ratio of technological parameters of heating, rough rolling and cooling for a given rolled thickness and the selected chemistry.

The technical result of the invention consists in ensuring the share of the viscous component in the fracture of the samples when tested with a falling load at a test temperature of -20 ° C of at least 85%, impact strength KCV at a test temperature of -40 ° C of at least 250 J / cm ² , while maintaining strength properties at the level of K65, X80, L555.

The technical result is achieved by the fact that in the proposed method for the production of plate products of strength classes K65, X80, L555 for the manufacture of electric-welded pipes of main pipelines, including the production of continuously cast billets from steel, austenitization of continuously cast billets, preliminary deformation, bending of the rolled stock to the temperature of the beginning of the finish rolling, finishing rolling and subsequent regulated accelerated cooling of finished steel with final delayed cooling and / or cooling lowering it in air to ambient temperature, unlike the prototype, the preform is obtained from steel with the following ratio of carbon, silicon, manganese, chromium, nickel, copper, molybdenum, aluminum, niobium, vanadium, titanium, sulfur, phosphorus, nitrogen, iron and unavoidable impurities, wt.%: C - 0.03-0.08; Si - 0.12-0.35; Mn - 1.65-2.10; Cr - 0.01-0.30; Ni - 0.01-0.40; Cu - 0.01-0.30; Mo - 0.01-0.30; Al - 0.02-0.05; Nb - 0.03-0.09; V - 0.001-0.10; Ti - 0.010-0.035; S 0.0005-0.003; P is 0.002-0.015; N - 0.001-0.008; iron and inevitable impurities - the rest, while the following relationships between the content of elements must be fulfilled:

0.08 <(Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 <0.16;

-2.7≤lg [Nb] [C + 8N] <- 2;

Cr + Ni + Cu + Mo <0.8%,

moreover, austenitization is carried out at a temperature of not less than 100 ° C below the temperature of dissolution of titanium nitrides Ts (TiN), in accordance with the ratio:

Ts (TiN) = 14400 / (5.0-log [Ti] [N]),

where Ti and N are the content of titanium and nitrogen in steel, wt.%,

but not lower than the temperature of dissolution of niobium carbonitrides Ts (Nb (C, N)) in accordance with the ratio:

Ts (Nb (C, N)) = (10100-27 [Mn] +200 [Si]) / (4.85-log [Nb] [C + 8N]),

when choosing the exposure time in the languid zone are guided by the equation:

, $$t=10\frac{1314-T}{77}\pm \mathrm{40,}°\text{C}$$

,

where t is the exposure time; T is the selected holding temperature,

the preliminary stage of deformation is carried out so that in its last four passes, the relative compression increases according to the following law:

ε _i = (1.05 ... 1.35) ε _i-1 ± 2,%,

where ε _i and ε _i-1 - compression in the next and previous pass, respectively, accelerated cooling of the finished product is carried out after preliminary editing of the product and is carried out so that its start temperature (T _but , ° C) is determined from the ratio:

_But T = 977-54Mn-102Ni-20Mo-866S-2,2V _OHL ± 30 ° C,

where V _okhl - the cooling rate of the rolling from the end of rolling to the beginning of accelerated cooling, ° C / s,

and the temperature interval Δ between the temperatures of the completion of rolling T _KP and the beginning of accelerated cooling T _but is determined from the ratio:

Δ = -2.5N + 92 ± 20, ° C,

where H is the sheet thickness, mm

The technical result is also achieved by the fact that the preliminary stage of deformation is divided into two, while the second stage is carried out in at least two passes, and the temperature-deformation parameters are chosen so that the degree of accumulated deformation in the inter-deformation pauses was 5-50%, and in the process the pause between the completion of the draft and the beginning of the finishing stage was reduced to 0-10%.

In addition, the technical result is achieved by the fact that accelerated cooling of the rolling is completed at temperatures of 200-320 ° C.

In addition, the technical result is achieved by the fact that accelerated cooling of the rolling is completed below the interval of manifestation of the Leidenfrost effect (Leidenfrost point).

The invention consists in the following.

First, a continuously cast billet is made of steel with a given chemical composition. In general, the given content of elements provides the necessary mechanical properties of the strip during the implementation of the proposed technological regimes.

Below is the justification of the restrictions on the chemical composition of plate products of strength classes K65, X80, L555 for the manufacture of electric-welded pipes of main pipelines.

To obtain the required strength, the C content must be at least 0.03%, but the addition of C more than 0.08% leads to a deterioration in the toughness and weldability of steel.

The addition of Si is necessary for the deoxidation of steel during smelting. To ensure the required level of deoxidation, a minimum of 0.12% Si is added, but when the Si content is more than 0.35%, the toughness of the weld zone near the weld zone deteriorates as a result of an increase in the number of silicate inclusions.

Mn promotes a shift of the γ → α transformation into a region of lower temperatures, which causes a decrease in the size of ferrite grains. As a result of grinding the microstructure, the yield strength is increased with a simultaneous increase in resistance to brittle fracture. With an increase in the Mn content, the transition temperature of brittle fracture decreases up to 2.1%. When the Mn content is over 2.1%, the toughness in the heat affected zone of the weld is reduced. In addition, Mn increases the degree of supersaturation of ferrite with dissolved elements (niobium, titanium, vanadium, carbon, nitrogen), which take part in the precipitation hardening. The minimum required manganese content for the optimal use of precipitation hardening in this steel is 1.65%.

This steel uses the effect of solid solution hardening Cr. The lower limit of influence of Cr is 0.01%. With an increase in Cr concentration, hardenability increases and the possibility of the formation of martensitic structures, leading to a decrease in toughness, appears. Therefore, the upper limit of the Cr content is set at 0.3%.

To increase the stability of austenite, Cu, Ni and Cr are added to the steel. To obtain the desired effect, a minimum of 0.01% Ni is required. It is not economically feasible to add more than 0.4% Ni. To save nickel, steel is alloyed with copper. To obtain the desired effect, a minimum of 0.01% Cu is needed. The addition of more than 0.3% Cu can lead to hot cracks during rolling. The lower limit of influence of Cr is 0.01%. With an increase in Cr concentration, hardenability increases and the possibility of the formation of martensitic structures, leading to a decrease in toughness, appears. Therefore, the upper limit of the Cr content is set at 0.3%.

Mo is an element that increases the hardenability of steel. To obtain this effect, it is necessary to add 0.01% or more of Mo. However, the addition of a large amount of Mo in excess of 0.3% significantly increases the cost of steel and is not economically feasible.

P refers to the number of elements with the greatest propensity for segregation and the formation of segregation along grain boundaries, and, as a result, adversely affects the toughness of steel. Therefore, the upper limit of the phosphorus content is set to 0.015%.

When the S content is more than 0.0030%, the resulting coarse sulfides significantly reduce the toughness.

Nb is necessary for the formation of carbides. Niobium carbides inhibit grain growth during heating, and contribute to the formation of a finely dispersed structure in rolled products due to inhibition of recrystallization during finish rolling. A niobium content of less than 0.03% does not provide sufficient dispersion and grain boundary hardening. The content of niobium in excess of 0.09% is not economically feasible.

Al deoxidizes and modifies steel, binds nitrogen to nitrides. In order to reduce the amount of oxygen in the molten steel, it is necessary to add 0.02% Al or more. When the content is more than 0.05% of aluminum, the ductile properties of the steel decrease.

Ti is a nitride-forming element, which exhibits the effect of grain grinding, with a content of more than 0.010%. However, since the addition of large amounts of Ti leads to a significant deterioration in toughness due to the formation of carbides, the upper limit of its content should be limited to 0.035%.

V is a carbo-nitride-forming element that increases strength. However, the addition of 0.001% or less of V does not produce such an effect. In addition, the addition of more than 0.10% V leads to a deterioration in toughness. Therefore, the vanadium content is set in the range from 0.001% to 0.10%.

N is needed to isolate finely divided TiN in order to reduce the diameter of the austenitic grains. Since the minimum nitrogen content sufficient to form the required amount of TiN is 0.001%, the lower limit of the amount of N is set to 0.001%. In addition, if the amount of N exceeds 0.008%, the amount of dissolved N increases and the low temperature toughness of the starting material deteriorates, therefore, the upper limit of the amount of N is set to 0.008%.

In addition to the content of chemical elements, the most important characteristic of steel is the relationship between the content of certain elements, for example, those that make up the phase and determine both the kinetics of its formation and the morphology of particles, and ultimately determine the effect on the structure and properties of steel.

Due to the containment of the boundaries by niobium carbonitrides, it is possible to avoid intensive grain growth during heating for rolling. In addition, the release of finely dispersed particles of Nb (CN) during reinforcement between rough and finish rolling allows to suppress recrystallization processes during finish rolling, and thereby increase the viscoplastic and strength properties. At the same time, the optimal ratio of niobium, carbon and nitrogen is: -2.7 <log [Nb] [C + 8N] <- 2.

From above, this ratio is limited by the danger of the release of Nb (CN) particles during rough rolling, which can lead to difficulties in the course of static recrystallization and, as a result, to different grain sizes of the austenite structure before finish rolling, which, in turn, will cause a decrease in cold resistance and viscous properties.

The content of Nb, C, N outside the lower value of this range leads to the fact that particles of Nb (CN) do not have time to stand out during the time of stirring. As a result, during finish rolling, the process of static recrystallization of austenite can occur, which has an extremely negative effect on the viscous properties of rolled products.

To prevent the formation of cold cracks in the welded joint, it is necessary that the sum of the following element ratios is less than 0.16: (Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10. But at the same time, if this amount is less than 0.08, then it will not be possible to provide the necessary strength and viscoplastic properties.

The limitation of the total content of Cr + Ni + Cu + Mo <0.8% is necessary to achieve the required weldability of the pipes and reduce the anisotropy of the properties in the sheet.

The combined effect of alloying, microalloying and thermomechanical processing allows you to effectively influence the structure of steel and to obtain the desired ferrite-bainitic structure with a uniformly distributed finely divided carbide phase.

One of the main distinguishing features of the technology is to prevent the formation of a heterogeneous structure at all stages of controlled rolling.

With an increase in temperature during heating for rolling steels containing additives of microalloying elements (MBE) Nb, Ti, and V, in addition to normal (collective recrystallization), abnormal grain growth (secondary recrystallization) is also possible, when a small number of grains grow to very large sizes ( of the order of several millimeters) in a relatively fine-grained matrix. The abnormal growth of austenitic grain upon heating for rolling is associated with the selective solubility of MBE carbonitride phases located at the grain boundaries, which leads to a sharp increase in the mobility of the boundaries of individual grains.

An increase in the heating temperature leads to a decrease in toughness and cold resistance, while the strength properties increase. Such changes are explained by an increase in the austenite grain size upon heating, a more complete solubility of the carbonitride phases and a corresponding increase in the austenite stability upon cooling, as well as an increase in the rough rolling temperature.

Lowering the heating temperature of the slabs in order to grind austenite grain can lead to an increase in the viscous properties and cold resistance of rolled products, but at the same time, the strength properties decrease due to an increase in the number of particles that are practically insoluble during heating and are not involved in hardening.

In this regard, the heating temperature and the exposure time in the languid zone of the furnace must be chosen so as to prevent an abnormal grain growth, but at the same time it is most complete to dissolve the carbonitride phases of MBE.

In order to prevent abnormal grain growth during heating, austenitization is carried out at a temperature of at least 100 ° C below the dissolution temperature of titanium nitrides Ts (TiN) in accordance with the ratio Ts (TiN) = 14400 / (5.0-log [Ti ] [N]), where Ti and N are the contents of titanium and nitrogen in steel, wt.%. In order to most fully dissolve the MBE carbonitride phases, the slabs are heated to a temperature not lower than the dissolution temperature of niobium carbonitrides Ts (Nb (C, N)) in accordance with the ratio Ts (Nb (C, N)) = (10100-27 [Mn] +200 [Si]) / (4.85-log [Nb] [C + 8N]), where Mn, Si, Nb, С, N are the contents of manganese, silicon, niobium, carbon and nitrogen in steel, respectively, wt.%. When choosing the exposure time in the languid zone are guided by the equation:

, $$t=10\frac{1314-T}{77}\pm \mathrm{40,}\left(°\text{C}\right)$$

,

where t is the exposure time, min; T is the selected holding temperature.

If the calculated values for this equation are exceeded, an abnormal grain growth is possible, leading to a decrease in the viscous properties of the rolled product. If a sufficient exposure time is not achieved, the carbonitrides of microalloying elements do not have time to dissolve in the languid zone of the furnace, which has a negative effect on the course of recrystallization processes and reduces the ductile properties of steel.

Hot rolling of the strip, according to the proposed method, is carried out according to regulated temperature-deformation modes in order to form a fine-grained structure in the finished product with an ordered distribution of crystal lattice defects, which provides an increase in the yield strength, impact strength, the share of the viscous component in the fracture (ICE) and the temperature of the viscous-brittle transition.

From the point of view of structure formation, an important stage of controlled rolling is the roughing stage. As a result of this stage, the cast structure of the slab and the initial austenitic grains formed during heating of the slab in the furnace are crushed by sequential static recrystallization of the deformed structure between the rough passages. The dispersion and uniformity of the final structure directly depend on how fully the metal is recrystallized in the rough stage. Incomplete flow of static recrystallization (SR), especially in the last four passes, negatively affects the austenitic structure, because contributes to the formation of heterogeneity, which reduces the ductility of the metal, toughness and ICE with IPG. Therefore, when creating technology for controlled rolling, it is necessary to assign a temperature regime and distribute the reductions in the draft stage in such a way as to ensure complete SR of the metal in the inter-deformation pauses of the last four passes. In this regard, in this invention, the preliminary stage of deformation is carried out so that in its last four passes, the relative compression increases according to the following law: ε _i = (1.05 ... 1.35) ε _i-1 ± 2, (%), where ε _i and ε _i-1 are the reductions in the next and previous pass, respectively.

With this ratio, with a decrease in the temperature of the rolled metal, passage of static recrystallization after each of these passes is ensured.

With large strip thicknesses, preliminary rolling can be divided into two stages in order to prevent excessive grain growth with prolonged curing. In this case, the second stage is carried out in no less than two passes, and the temperature-deformation parameters are chosen so that the degree of accumulated deformation in the inter-deformation pauses was 5-50%, and during the pause between the completion of the draft and the beginning of the finishing stage it decreased to 0-10% due to the flow of static recrystallization of austenite. The essence of the second stage of rough rolling is as follows: in conditions of difficult static recrystallization at low temperature, it becomes possible in several passes to bring the degree of accumulated deformation to a level that allows, during the pause, between the completion of the second stage of rough rolling and the beginning of the finishing stage (which is longer than pauses between passes in the draft stage), completely statically recrystallize the tackle. This treatment allows you to grind the austenite grain, which leads to an increase in viscous properties.

The temperature of the beginning of accelerated cooling and the temperature interval between the temperature of the end of rolling and the temperature of the beginning of accelerated cooling are important structure-forming parameters and therefore must be strictly regulated to exclude the spread in mechanical properties. In this regard, in this invention, accelerated cooling of the finished product is carried out after preliminary editing of the rental, to increase the uniformity of cooling, and is carried out so that its start temperature T _but is determined from the ratio:

_But T = 977-54Mn-102Ni-20Mo-866S-2,2V _OHL ± 30, ° C,

where V _okhl - the cooling rate of rolling from the end of rolling to the beginning of accelerated cooling, ° C / s.

Since the content of these elements in steel and the cooling rate of rolled products determine the kinetics of transformation of austenite, and the temperature interval Δ between the temperatures of completion of rolling T _KP and the beginning of accelerated cooling T _but is determined from the ratio:

Δ (° C) = - 2.5Н + 92 ± 20,

where N is the thickness of the sheet, mm

this allows you to take into account the thickness of the hire and the temperature processes that take place in it before the start of accelerated cooling.

The use of accelerated cooling allows you to create a more dispersed structure of ferrite and intermediate products of transformation. In this case, an increase in the efficiency of dispersion hardening and an increase in the dislocation density are observed. In general, accelerated cooling has a positive effect on the strength and viscoplastic properties. Cooling to a temperature range of 320-200 ° C, due to the creation of finely dispersed martensite-austenitic sections, allows to increase the strength of steel, increase ductility and toughness, and also achieve a lower ratio of σ _T / σ _B.

At the end of accelerated cooling below 200 ° C, the anti-flock treatment does not have time to undergo subsequent delayed cooling, as a result, defects caused by hydrogen can be observed in the sheet. When cooled above a temperature of 320 ° C, the number of formed martensite-austenitic sites is insufficient to obtain the desired effect.

In some cases, with accelerated cooling, a sharp change in the cooling intensity is observed, caused by the Leidenfrost effect. The Leidenfrost effect is a phenomenon in which the liquid in contact with the body is much hotter than the boiling point of this liquid, creates an insulating layer of vapor that protects the liquid from rapid boiling. Due to the so-called film boiling, instability of the temperature of the end of cooling and, accordingly, significant fluctuations in the mechanical properties of rolled products are observed. The temperature range in which the effect of film boiling is manifested depends on many factors, including the initial temperature of the cooling water, the pressure and flow rate of water supplied to the sheet and the amount and composition of the scale on the sheet surface and the design features of the cooling unit and is determined experimentally. In one of the methods according to this invention, accelerated cooling of rolled products is completed below the interval of manifestation of the Leidenfrost effect, which significantly reduces fluctuations in the mechanical properties of the rolled products in length and width, as well as from sheet to sheet.

Examples

For the experiment, slabs of four heats were produced. The chemical composition of the experimental swimming trunks is presented in Table 1.

Swimming trunks 1, 2 and 3 are made in accordance with this invention, melting 4 - according to the prototype.

After austenitizing slabs 312 mm thick to a temperature of 1132-1210 ° C, holding time in the languid zone of the furnace for 58-214 minutes, a preliminary stage of hot rolling was carried out with a total compression of 46-76% at temperatures of 910-1055 ° C. After that, the finishing stage of rolling was carried out with a total compression of 80% at temperatures of 736–833 ° C to a thickness of 15–34 mm. Then, after a regulated temperature drop of 10-42 ° C, accelerated cooling was performed to temperatures of 205-529 ° C, with final delayed cooling and / or in air to ambient temperature.

The technological parameters of rolling and cooling are shown in Table 2. Modes 1-1, 1-2, 1-3, 1-4, 2-1, 2-2, 2-3, 2-4, 2-5, 2-8 , 3-1, 3-2, 3-3, 3-4 - made according to the invention; 4 - in accordance with the prototype; 2-6 and 2-7 - outside the declared range of parameters.

For mode 2-5, rough rolling was carried out in two stages, and the second stage included two reductions at surface temperatures of 920-910 ° C. The pause after the completion of the second stage before the start of finish rolling was 98 s. During the pause, the degree of accumulated deformation decreased to 5%.

For mode 2-8, accelerated cooling was carried out to a temperature of 205 ° C, which is lower than the interval of manifestation of the Leidenfrost effect, which under these conditions manifested itself in the range of 220-350 ° C. For this product, the spread in tensile strength was 20 MPa, with a total level of 40 MPa.

For comparison, mode 2-6 shows the harmful effect of holding too long in the furnace languid zone on ductility, toughness, and mainly on the proportion of the viscous component in the fracture. Mode 2-7 shows how the improperly selected mode of the last four strains in rough rolling negatively affects the viscoplastic properties.

Mechanical properties were determined on transverse samples. Tensile tests were carried out on full-thickness specimens (according to GOST 1497), and on impact bending - on specimens with a V-shaped notch (according to GOST 9454) at a temperature of -40 ° C. The falling load test was performed on full-thickness samples in accordance with API 5L 3 at a test temperature of -20 ° C.

The mechanical properties of the experimental steels are shown in Table 3. It can be seen that when using the prototype, the proportion of the viscous component in the fracture required by the NTD (85%) is not ensured, the impact strength at -40 ° C and ductility are lower, which is caused by non-compliance with technological ratios and chemical composition ratios the present invention.

Proposed in this invention, the technological parameters for the production of rolled products contribute to the formation of a homogeneous ferrite-bainitic structure, providing a high complex of mechanical properties.

The results of the manufacture of prototypes show that the use of technological ratios and chemical composition ratios according to this invention allows achieving the requirements of technical specifications for impact strength, the share of the viscous component in IPG and ductility, while ensuring the stability of obtaining these properties.

An additional economic advantage of this invention is that, in comparison with the prototype, the cost of alloying the experimental steels is significantly lower.

table 2 Technological parameters of rolling and cooling Mode number Thickness mm Heating temperature, ° C The exposure time in the languid zone of the furnace, min The total compression in the preliminary stage of rolling,% Temperature range pre-rolling, ° C Private reduction in the last 4 passes of rough rolling,% The total compression in the finishing stage,% Finish rolling temperature range, ° C The temperature of the beginning of accelerated cooling, ° C Δ, ° C The temperature of the end of accelerated cooling, ° C 1-1 fifteen 1210 58 76 1055-980 19-20-22-25 80 830-788 742 42 415 1-2 17.5 1210 60 72 1048-988 17-18-20-23 80 815-791 757 34 429 1-3 17.5 1210 59 72 1044-987 17-18-20-23 80 780-737 701 36 305 1-4 twenty 1189 79 68 1025-984 16-17-19-22 80 833-771 736 35 418 2-1 23 1213 60 62 1049-1000 12-13-15-18 80 823-770 736 34 529 2-2 23 1145 195 62 1011-963 12-13-15-18 80 830-775 739 36 522 2-3 23 1162 98 62 1010-963 12-13-15-18 80 830-774 738 36 421 2-4 27.7 1170 92 54 1005-975 11-12-14-16 80 815-794 772 22 417 2-5 34 1160 102 46 1015-910 10-11-12-14 80 780-750 740 10 407 2-6 ** 23 1161 180 62 1012-970 12-13-15-18 80 825-773 739 34 417 2-7 ** 23 1159 99 62 1010-962 10-16-10-20 80 820-770 734 36 420 2-8 23 1162 90 62 1015-965 12-13-15-18 80 785-749 714 35 205 3-1 17.5 1134 210 72 1000-943 17-18-20-23 80 781-738 706 32 277 3-2 17.5 1134 214 72 1005-944 17-18-20-23 80 790-745 711 34 349 3-3 17.5 1132 206 72 999-940 17-18-20-23 80 777-736 707 29th 444 3-4 17.5 1136 208 72 1010-949 17-18-20-23 80 785-742 706 36 520 four* 23 1190 150 51 1010-980 11-11-12-12 85 800-775 760 fifteen 370 * - prototype; ** - comparative.

Table 3 Mechanical properties of experimental steels Mode number Tensile strength, MPa Yield Strength, MPa Relative extension, % The ratio of yield strength to tensile strength DVSI at IPG at -20 ° C,% Impact strength (KCV) at -40 ° C, J / cm ² 1-1 730 640 23 0.88 one hundred 450 1-2 720 630 22.5 0.88 one hundred 440 1-3 755 640 23.5 0.85 one hundred 480 1-4 710 630 22 0.89 95 411 2-1 660 610 24.5 0.92 one hundred 441 2-2 680 630 22.5 0.92 95 382 2-3 710 630 21.5 0.89 95 370 2-4 710 630 21 0.89 95 371 2-5 720 640 21 0.89 90 365 2-6 ** 720 640 20.5 0.89 70 260 2-7 ** 705 625 21 0.89 80 290 2-8 740 600 22 0.81 90 364 3-1 730 590 22.5 0.81 90 355 3-2 735 655 22 0.89 90 360 3-3 725 610 23 0.84 95 391 3-4 660 590 25 0.89 95 405 four* 740 630 18.5 0.85 70 325 * - prototype; ** - comparative.

Claims (4)

1. Method for the production of plate products of strength classes K65, X80 and L555 for the manufacture of electric-welded pipes of main pipelines, including the production of continuously cast billets from steel, austenitization of continuously cast billets by heating in a furnace, preliminary deformation, bending rolled to the temperature of the start of finish rolling, finishing rolling and subsequent regulated accelerated cooling of finished steel with final delayed cooling and / or final cooling in air until eratury environment, characterized in that the preform is prepared from a steel with the following ratio of components, wt.%:
carbon 0.03-0.08 silicon 0.12-0.35 manganese 1.65-2.10 chromium 0.01-0.30 nickel 0.01-0.40 copper 0.01-0.30 molybdenum 0.01-0.30 aluminum 0.02-0.05 niobium 0.03-0.09 vanadium 0.001-0.10 titanium 0.010-0.035 sulfur 0.0005-0.003 phosphorus 0.002-0.015 nitrogen 0.001-0.008 iron and inevitable impurities rest
and the ratio between the content of manganese, chromium, copper, silicon, nickel, molybdenum, vanadium, niobium, carbon and nitrogen in accordance with the ratios: 0.08 <(Mn + Cr + Cu) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 <0.16,
-2.7 <log [Nb] [C + 8N] <- 2, Cr + Ni + Cu + Mo <0.8%, and austenitization is carried out at a temperature of at least 100 ° C below the temperature Ts (TiN) of dissolution titanium nitride, in accordance with the ratio:
Ts (TiN) = 14400 / (5.0-log [Ti] [N]),
where Ti and N are the content of titanium and nitrogen in steel, wt.%,
and not lower than the temperature Ts (Nb (C, N)) of dissolution of niobium carbonitrides in accordance with the ratio:
Ts (Nb (C, N)) = (10100-27 [Mn] +200 [Si]) / (4.85-log [Nb] [C + 8N]),
where Nb and C are the content of niobium and carbon in steel, wt.%,
and the choice of time t exposure in the languid zone is carried out in accordance with the equation:

where t is the exposure time, min; T is the selected holding temperature, ° C,
while the preliminary stage of deformation is carried out so that in its last four passes, the relative compression increases in accordance with the ratio:
ε _i = (1.05 ... 1.35) ε _i-1 ± 2,%,
where ε _i and ε _i-1 - compression in the next and previous pass, respectively,
and accelerated cooling of the finished steel is carried out after preliminary editing of the rental, and the temperature of the beginning of cooling T _but is determined from the ratio:
_But T = 977-54Mn-102Ni-20Mo-866S-2,2V _OHL ± 30, ° C,
where V _okhl - the cooling rate of the rolling from the end of rolling to the beginning of accelerated cooling, ° C / s,
and the temperature interval Δ between the temperature of completion of rolling T _KP and the temperature of the beginning of accelerated cooling T _but is determined from the ratio:
Δ = -2.5Н + 92 ± 20, ° С,
where H is the sheet thickness, mm 2. The method according to claim 1, characterized in that the preliminary deformation is divided into a roughing and finishing stages, while the finishing stage is carried out in at least two passes, and the temperature-strain parameters are chosen so that the degree of accumulated strain in the inter-deformation pauses is 5 -50%, and pauses in the process between the completion of the draft and the beginning of the finishing stage are reduced to 0-10%. 3. The method according to claim 1, characterized in that the accelerated cooling of the rolling is completed at temperatures of 200 ÷ 320 ° C. 4. The method according to claim 1, characterized in that the accelerated cooling of the rolling is completed below the interval of manifestation of the Leidenfrost effect.