EP4090780A1

EP4090780A1 - Method of producing steel bar of non-round cross-section and steel bar of non-round cross section

Info

Publication number: EP4090780A1
Application number: EP20853573.2A
Authority: EP
Inventors: Zbigniew KUTYLA
Original assignee: CMC Poland Sp zoo
Current assignee: CMC Poland Sp zoo
Priority date: 2020-01-17
Filing date: 2020-12-22
Publication date: 2022-11-23
Also published as: PL432599A1; PL239419B1; WO2021144643A1

Abstract

A method of producing steel bar of non-round cross-section using the hot rolling process is disclosed, wherein the charge in the form of ingots obtained in the process of continuous casting is heated in a furnace and then shaped in the rolling process in rolling mill stand followed by cooling down the ready-made product. The stage of heating up the charge in a furnace is performed up to maximum temperature within the range of 1080 - 1180°C. The stage of shaping using the rolling mill stands includes initial rolling performed in a group of initial stands and finishing rolling performed in a group of finishing stands, wherein the time interval between the last rolling reduction in the initial group and the first rolling reduction in the finishing group is at least 20 seconds, wherein the total relative rolling reduction in the finishing group is within the range of 60 - 80%. Steel microstructure of ready-made bar manufactured according to the method, produced includes fine-grained polygonal ferrite and irregular bainitic ferrite of grain size 4-7 μm and share 75-85% and martensitic and bainitic islands of size less than 10 μm and share 15-25%.

Description

Method of producing steel bar of non-round cross-section and steel bar of nonround cross section

The subject of the invention is method of producing steel bar of non-round cross-section and steel bar of non-round cross section. In particular, the invention is used during manufacturing long products in the form of flat bars in the process of hot- rolling of rectangular or square billets and blooms of steel containing controlled content of micro-additives of Nb, Ti, V and Mo. The invention allows for producing flat bars characterized by high strength parameters, i.e. of minimum yield strength at the level between 460 MPa and 700 MPa and impact energy kV (-20°C) at the level minimum 47 J.

Steel long products in the form of flat bars of high mechanical parameters are mostly used in the process of manufacturing semi-trailers for trucks as well as other structural elements, for example construction and mining machines, bridges, building and crane structures. Due to high strength and ductility, steel bars provide perfect utility parameters, including high mechanical fatigue resistance and impact load with simultaneous maintenance of very good weldability.

Typically, within the processes of manufacturing flat products in the form of metal sheets, yield strength at the level of 700 MPa and higher are obtained as a result of thermo-mechanical rolling wherein fine-grained structure is formed under the influence of temperature and plastic deformation, leading to high strength and ductility of the product. Within the process, firstly the charge is heated up to high temperature, usually in the range of 1200°C - 1300°C that ensures complete solubility of micro additives Nb, Ti and V, initially present in the charge in the form of NbC, TiC and VC. Then, a controlled scheme of deformations (so called passes) is performed under a defined temperature regime and within defined intervals with application of accelerated and controlled cooling along the rolling line before the final passes as well as after the last pass (typically at rate 10-40°C/sec. and in case of thin metal sheets even up to 100°C/sec.), followed by rolling into a coil and its very slow cooling at rate about 0.4°C/min.

Despite the above described characteristics, the thermo-mechanical metal sheet rolling ensures high homogeneity of the deformation and temperature distribution within a band which favourably translates to the condition of the obtained steel structure and its uniformity after the process completion. This facilitates determination of the deformation scheme optimum for the process that allows to control its course, therefore - high strength parameters obtained within its result.

For example, due to application of high temperature of charge heating, carbides NbC, TiC and VC previously dissolved in austenite re-release during the rolling process decelerating the process of austenite recrystallization and growth of the grain after recrystallization. The growth of dynamically released carbides particles in austenite is limited by high rate of rolling within the finishing section and application of water cooling therefore the particles have significant contribution to the precipitation hardening. The said intense and controlled water cooling favours the structure fragmentation and as a result of lowering the temperature under which the coils are wound, it is possible to obtain different, required ferritic and pearlitic structures, ferritic and bainitic structures, ferritic and martensitic structures or different combinations of the aforementioned phases. Finally, slow cooling of the coil after winding favourably affects the increase of yield strength related to precipitation hardening.

In case of long products in the form of flat bars rolled using conventional method, the rolling charge, typically in the form of rectangular or square billets and blooms, is also heated to high temperature close to 1300°C, and then formed in a rolling mill stand within about 15 - 30 passes, followed by natural (i.e. not forced) cooling of the semi-finished product and final cutting to sections of defined length. Contrary to the process of metal sheet rolling, rolling of long products in the form of flat bars is characterized by high non-homogeneity of deformation and temperature distribution within a band cross-section, in particular within the initial passes, which fact strongly differentiates the austenite structure condition. Within areas of higher deformation and higher temperature, austenite recrystallization is faster comparing to areas where deformation and temperature during the process were lower. As a result, this leads to significant differentiation of the band cross-section structure, which fact negatively affects the final strength parameters, the impact strength in particular. Moreover, in contrast to rolling metal sheets, it is much more difficult to control value of deformation when rolling long products in the form of flat bars at the process level, what leads to technological limitations within the finally obtained properties.

Due to this fact, there are works pending concerning further enhancement of flat bars rolling method. Except for improving the process stages, the methods also include proper selection of quantitative and qualitative composition of the alloying additives used in steel that affect the hardening of the obtained material structure and final values of strength parameters.

For example, DE3434744 A1 discloses a method of hot rolling the bars used in the process of machine elements production that might be dynamically and / or statically loaded. According to this method, bars are rolled at the temperature of the process completion within the range between 800°C and 1150°C or they are subject to special thermal treatment from the temperature at which the obtained ferritic and pearlitic structure is heated to temperature between 800°C and 1000°C. In both cases, the bars are then cooled down using gaseous, liquid or sprayed coolant or using fluid bed at rate 1.5°C/sec. to 10°C/sec., which fact affects precipitation hardening and / or formation of small grains and produces ferritic and pearlitic micro-structure in the bar material, avoiding formation of bainite structure. Cooling proceeds to temperature at least 50°C below the temperature where the conversion to ferrite and perlite is completed. During the process, bars of microscopic steel are produced that contain carbon within the range from 0.3 to 0.65% by weight, silicon 1.2% by weight, manganese within the range from 0.3 to 0.8% by weight, sulphur below 0.065% by weight, in total 0 to 0.7% by weight of chromium and / or nickel and / or copper and / or molybdenum, nitrogen within the range from 0.005 to 0.025% by weight and as precipitation hardening and / or fine-grained elements in total 0.05 to 0.20% by weight of vanadium and / or niobium and / or titanium and / or aluminium and / or zirconium as well as boron within the range of 0.0005 to 0.005% by weight. The remaining part is iron and contamination caused by melting, wherein the total content of chromium and manganese does not exceed 1.0% by weight.

Similarly, EP 1700925 A1 discloses the method of producing hot rolled ferritic and pearlitic steel bars of high yield strength, high fatigue strength and good machinability as well as steel alloy which when hot treated has ferritic and pearlitic microstructure and austenite grain size higher than ASTM 10 (less than 10 pm). Chemical composition of steel is as follows: carbon within the range of 0.15 ... 0.6 % by weight, silicon 1.25...2.0 % by weight, manganese 0.5 ... 1.6 % by weight, sulphur 0 ... 0.2 % by weight, chromium 0 ... 1.5 % by weight, molybdenum 0.02 ... 0.1 % by weight, aluminium 0 ... 0.11% by weight, vanadium 0 ... 0.2 % by weight, nitrogen 0 ... 0,04 % wag., niobium within the range from 0 ... 0.1 % by weight and titanium 0 ... 0.05 % by weight. According to the disclosed invention, the manufacturing process includes heating a billet at temperature higher than 800°C followed by plastic working that includes hot rolling followed by immediate and controlled cooling of the product under steady or flowing gaseous medium or air and water mixture.

The above discussed solutions disclose the methods of producing high strength flat elements, including bars, wherein in order to improve strength, the process parameters are combined with selection of alloying elements in steel. Finally, the obtained product is still characterized by perlite content in its microstructure. Moreover, during the processes, the growth of austenite grains during rolling still cannot be decelerated and the recrystallization process cannot be controlled that for example affect the achievement of lower yield strength.

Due to these facts, the objective of the invention is to propose an improved method of manufacturing long products in the form of flat bars wherein properly selected process stages and rolling parameters lead to complete stopping of austenite recrystallization during the final passes of the initial group. The objective of the invention is to obtain the final product that can be characterized by high strength parameters as well as very good weldability.

According to the invention concerning the method of producing steel bar of non round cross-section using the hot rolling process, the charge in the form of ingots obtained in the process of continuous casting is heated in a furnace and then shaped in the rolling process in rolling mill stand and then the ready-made product is cooled down. The method is characterized by that the stage of heating up the charge in a furnace is performed up to maximum temperature within the range of f080 - f f80°C. The stage of shaping using the rolling mill stands includes initial rolling performed in a group of initial stands and finishing rolling performed in a group of finishing stands, wherein the time interval between the last rolling reduction in the initial group and the first rolling reduction in the finishing group is at least 20 seconds, wherein the total relative rolling reduction in the finishing set, expressed with the formula {[Pp - Pk ]/ Pp } * 100%, where (Pp) is the cross-section area of a band after the last stand of the initial stands group and (Pk) is the cross-section area of the ready-made product, is within the range of 60 - 80%. The cooling stage of the ready made bar is performed using air.

It is preferable, when low-alloy steel is the charge material wherein content of elements C, Mn, Ni, Cu, Cr, Mo and V is selected in order to meet the condition:

0.30% < C_e < 0.40%, where C_e is carbon equivalent of value expressed with the formula:

It is preferable, when the Ti, Nb and V elements content is selected in order to meet the condition Ti+Nb+V < 0.30%, wherein the maximum content of Nb in % by weight is expressed with the following formula:

Log[Nb]*[C+12/14N] = 2.26 - 6770/T, and the Ti content is determined so that content of this element in austenite is within 0.020 - 0.070%.

It is preferable, when content of the elements C, Si, Mn, N, S, Mo, Cr, Ni, Cu, B and A1 in steel is as follows:

C < 0.10%; Si < 0.20%; 1.35% < Mn < 1.95%; N < 0.010%; 0.005% < S < 0.010%; 0.02% < Mo < 0.25%; Cr + Ni + Cu < 0.80%; 0.0004% < B < 0.0010%; A1 > 0.020%.

Moreover it is preferable, when steel bar of non-round cross-section represents long product in the form of flat bar. Steel bar of non-round cross-section according to the invention, produced in the process of hot rolling is characterized by the fact that the microstructure of steel of the ready-made bar includes fine-grained polygonal ferrite and irregular bainitic ferrite of grain size 4-7 pm and share 75-85% and martensitic and bainitic islands of size less than 10 pm and share 15-25%.

It is preferable when the steel bar of non-round cross-section represents a long product in the form of flat bar of thickness up to 20 mm and width up to 250 mm of maximum value of the minimum yield strength R_e amounting 700 MPa.

The developed production method uses the effect of synergistic impact of the following key process parameters: heating temperature of the charge for rolling in the heating furnace, final temperature of rolling controlled by the time interval between the last pass in the initial group and the first pass in the finishing group, size of the band cross-section reduction (relative rolling reduction), rate of deformation and chemical composition of steel including in particular the content of niobium (Nb), titanium (Ti) and molybdenum (Mo). The method according to the invention does not use accelerated and controlled cooling of the band with water of mixture of water and air mist. Temperature changes of the rolled band are caused by heat transfer phenomena into rollers and atmosphere however the basic parameters affecting the achievement of the assumed temperature at the end the rolling are as follows: temperature of heating up the billets / blooms in a heating furnace, rate of deformation and time interval between rolling in the initial group and the finishing group. As a result of the selected parameters and chemical composition of steel according to the invention, long products in the form of flat bars of minimum yield strength 460 - 700 MPa are obtained, wherein the impact energy within the impact test KV(-20°C) is minimum 47J. Undoubted and additional benefit of the long products in the form of flat bars obtained according to the method of the invention is their good weldability resulting mostly from limitation imposed on the content of the following elements: C, Mn, Cr, Ni, Cu and V.

The subject of the invention is presented in the embodiments and figures wherein fig. 1 presents microstructure of the flat bar, according to Embodiment 1 and fig. 2 presents microstructure of the flat bar, according to Embodiment 2.

The developed method particularly applies to long products in the form of flat bars of thickness up to 20 mm and width up to 250 mm. In order to achieve the assumed rolling effect, the method according to the invention uses the following principles of the steel chemical composition designing (in % by weight):

Carbon content in steel meets the condition: C < 0.10%.

Content of Mn, Ni, Cu, Cr, Mo and V is determined in order to meet the condition:

0,30% < C_s £ 0,40% where C_e is carbon equivalent expressed using the formula (according to annex C of the standard PN-EN 1011-2:2004+Al:2005 Welding - Recommendation of welding metallic materials - Part 2: Arc welding of ferritic steel):

¾Mn %Ni + %Cu %Cr + %Mo + %V

C, = %C f

6 ⁺ IS 5

(formula 1)

Total content of Cr, Ni and Cu meets the condition Cr+Ni+Cu < 0.80%.

Content of molybdenum (Mo) that increases steel hardenability thus causes that the high strength component - instead of perlite - is bainite and martensite in the long product structure in the form of a flat bar, is determined so that the following dependence is satisfied 0.02% < Mo < 0.25%. The impact of the cooling rate reduction of the long product in the form of flat bar in a cold room after rolling related to the increase of its thickness on the temperature of the ferritic conversion initiation and kinematics of the bainitic conversion is compensated by proportionally increasing the content of boron (B) in steel that especially effectively increases steel hardenability in combination with molybdenum (Mo), from the content of 0.0004% for the long product in the form of a flat bar of thickness 10 mm to the content of 0.001% for the long product in the form of flat bar of thickness 20 mm.

Content of aluminium (Al), nitrogen (N) and sulphur (S) in steel is specified in the following ranges: 0.020 % £ Al < 0.040%, N < 0.010% and 0.005% < S < 0.010%.

Aluminium protects boron against oxygen and together with titanium against nitrogen. Nitrogen content is limited because together with the increase of content of this element in steel the content of Nb and Ti reduces that can be dissolved in the austenite matrix at the temperature of the charge heating. However, sulphur content is controlled so that carbide-sulphide (Ti,Nb)4C2S2, that is present in steel, is gradually dissolved during rolling therefore introducing niobium and titanium into fixed solution released during ferritic conversion in the form of fine particles of TiC and NbC, strengthening the steel matrix. This is achieved by determining the sulphur (S) content within the range of 0.005% - 0.010%. Then, this compound is stable within the ingot heating temperature range for rolling however it is unstable below 1050°C. Therefore, it dissolves during flat bar rolling and complements fixed solution (austenite) with Ti and Nb.

In case of titanium (Ti), niobium (Nb) and vanadium (V), the sum of their percentage content in steel meets the dependence Ti+Nb+V < 0.30% , however the below mentioned conditions must be met. The content of Nb introduced to steel is determined optimally at the level so that carbide NbC - present in ingot - completely dissolves in austenite at the charge heating temperature. Maximum Nb content (in % by weight) that dissolves in austenite is expressed with the formula (according to Irvine solubility product):

Log[Nb]*[C+12/14N] = 2.26 - 6770/T, (formula 2) where: T - temperature of charge heating °C, [Nb] - niobium content in austenite at charge heating temperature, C - carbon content in steel, N - free nitrogen content in steel in the form of TiN and T14C2S2_.

Content of Nb, according to formula 2, is introduced to steel, when the required yield strength of a flat bar is min. 700 MPa. In case of bars of lower yield strength, content of Nb in steel is reduced by value 0.008% x (700 - R_e)/50, where R_e is the minimum required yield strength;

Content of Ti introduced to steel is determined so that the content of this element in austenite [Ti] at the charge heating temperature is optimally within the range of 0.020% - 0.070%. Content of titanium in austenite is expressed with the formula (Applicant mass balance):

[Ti] = Ti - 3.43*N - 3*S, (formula 3) where: value of 3.43 refers to the part of titanium content bound in nitride TiN, and value 3*S refers to content of titanium bound in titanium carbide-sulphide T14C2S2_.

In the method according to the invention, firstly the charge in the form of ingots obtained during continuous casting process is heated in the heating furnace. Heating the charge in the heating furnace is performed up to maximum temperature within the range of 1080 - 1180 °C. During heating the charge in the heating furnace, carbides and nitrides of type MX (M=Nb,Ti,V) dissolve to the assumed content of Nb, Ti and V in the form of fixed solution in austenite. Content of Ti is selected so that nitrogen in bound in the form of TiN and so that sulphide-carbide (Ti,Nb)4C2S2is present in steel after heating.

After completion of heating the rolling process begins that is divided to initial rolling performed in the initial rolling mill stands and finishing rolling performed in the finishing rolling mill stands. The rolling line configuration is selected so that the time interval between the last rolling reduction in the initial groups and the first rolling reduction in the finishing group is at least 20 seconds. According to the above description - rolling in the first stand of the finishing stands group starts only after expiration of the said at least 20 seconds after completion of the initial rolling in the last stand of the initial stands group. Moreover, the condition according to which the total relative rolling reduction in the finishing group (i.e. obtained after the last stand of the finishing group) expressed with the following formula must be met:

{ [ Pp - Pk ] / Pp } * 100% (formula 4), where (Pp) is the cross-section area of the band after the lasts stand of the initial stands group, and (Pk) is the cross-section area of the ready-made product, is within the range of 60 - 80%.

At the stage of hot rolling, the process kinetics of the released carbide NbC is decelerated through introduction of Mo to steel within the weight range 0.02% - 0.25%. Therefore, austenite recrystallization takes place within the first passes and is stopped within the last passes of the group. Total recrystallization of austenite and formation of fine and homogeneous grain takes place within time interval between completion of the initial rolling and starting the finishing rolling.

After completion of rolling, natural cooling of the ready-made product in the form of flat bard is performed in air to ambient temperature. It is assumed that cooling takes place at rate within the range 0.5 -2.0 °C/sec. depending on the bar thickness. The designed chemical composition of steel causes that the difference between the final rolling temperature (within the range of ca. 830 - 790°C) and the temperature of the ferritic conversion initiation (within the range of ca. 790 - 770°C) is small. Under these conditions, before conversion, the fixed solution (austenite) contains ca. 0.015% Nb and up to 0.04% Ti depending on the content of these elements introduced to steel. Therefore, very fine particles of (Nb, Ti)C are released during conversion that strengthen the ferrite matrix. The contribution of the precipitation hardening into yield strength can be increased by addition of V to steel that forms carbides (Nb,V)C and (Ti,V)C. Phase conversion of autenite gives fine ferrite grain of size within the range of 4-6 pm. Fragmentation of the grain leads both to the growth of strength and ductility of steel. The used Mn content in steel and synergistic impact of small amounts of Cr, Ni, Cu (from scram), Mo in amount of 0.02-0.25% and boron in amount of 0.0004-0.0010% cause deceleration of the pearlitic conversion. Instead of perlite in the steel structure, there are small martensite and bainite islands.

Inhibition of austenite recrystallization according to the inventive method is implemented by introduction of niobium into steel in such an amount so that NbC carbide present in the charge is completely dissolved in austenite at the rolling heating temperature not higher than 1180 °C. Value of billet / bloom heating temperature is determined at the level so that as a result of synergistic effect of dissolved niobium and parameters of plastic working such as band cross-section reduction, rate of deformation and temperature of the rolling band, the austenite recrystallization is stopped within the last passes of rolling in the initial group as a result of dynamic release of NbC particles and its complete recrystallization within at least 20 seconds after the last pass of the initial group of the rolling line. This is the minimum time interval between rolling in the initial group and finishing group applied according to the invention. The number of rolling mill stands in the initial group is selected so that the final dimension of the product can be achieved in the finishing group with total relative deformation within the range 60 - 80% and the rolling completion temperature not to exceed the range of 790 - 830°C. During the initial rolling, austenite recrystallization is gradually decelerated as a result of intensive dynamic release of carbide particles NbC, and below 850+15°C it is completely stopped. In combination with steel hardenability determined by its chemical composition, in particular by controlled content of molybdenum (Mo) and boron (B) as well as natural cooling condition causing that the cooling rate of long products in the form of flat bars is within 0.5 - 2.0°C/sec depending on the flat bar thickness, the temperature of the austenite conversion initiation into ferrite in the flat bar is within the range of 790 -770°C and the sequence and range of temperature of further phase conversions of austenite into bainite and martensite causes that the ready-made product structure contains the following main components:

• fine-grained polygonal ferrite and irregular bainitic ferrite of dislocation density and grain size within the range of 4-7 pm and share 75-85%;

• martensitic and bainitic islands of size below 10 pm and share 15-25%.

Polygonal ferrite and bainite matrix also contains fine particles of (Nb,Ti)C of size below 10 nm and volumetric share within the range of 0.0005 - 0.0015 that are released from austenite during ferritic conversion.

Moreover, the structure of flat bars contains large particles of (Ti,Nb)(N,C) of size over 10 nm serving different functions in the process of manufacturing this product. First and foremost, they bind nitrogen (N) that unfavourably affects the mechanical properties of flat bars in the form of nitrides (Ti,Nb)N. Then, this is NbC carbide particles dynamically released during the process of rolling, inhibiting the austenite recrystallization. The result of synergistic impact of the aforementioned hot rolling process parameters and the release processes performed in the describe temperature regime by the qualitatively and quantitatively determined micro-additives and the phase conversion processes taking place in the steel structure, additional effects can be obtained, comparing to conventional processes. In the first place, the structure of long products in the form of flat bars, obtained as a result of application of the method according to the invention, significantly differs from the structure of flat products in the form of steel sheets with micro-additives of Nb, Ti and V manufactured in the process of thermo-mechanical rolling. The structure of flat products in the form of metal sheets contains ferrite and mostly perlite as the second component. Moreover, characteristic feature of thermo-mechanically rolled sheets is very strong banding of perlite, bainite and martenzite that form elongated bands parallel to the sheets rolling direction. This unfavourably affects their ductility and first and foremost impact strength at lower temperature. In the structure of long products in the form of flat bars produced according to the invention, bainite and martenzite are present in the form of small particles (islands) homogeneously distributed within ferritic matrix. As a result of this, despite high strength and ductility, flat bars characterize with high value of impact energy KV(-20°C) > 47J. An important feature related to morphology of bainite and martenzite particles in the flat bar structure is its strong strengthening during deformation after reaching yield strength. In case of ferritic and pearlitic structures of thermo-mechanically rolled sheets, this phenomenon does not occur however together with the increase of strength - caused by fragmentation of ferrite grain - value of yield strength approaches the value of tensile strength.

Below are examples of strength parameters obtained according to the invention, for the specific quantitative and qualitative values of steel composition and the applied process parameters. Example 1

Steel of chemical composition given in table 1 (parameters in % by weight) was rolled to a flat bar of width 140 x thickness 10 mm according to the developed technology. Temperature of the charge heating in the heating furnace was 1180 °C. The final rolling temperature was 820°C. Value of yield strength R_e min. = 700 MPa. Industrial trials have been performed according to the description herein.

Table 1.

Mechanical properties and characteristics of the flat bar structure after rolling and cooling in a cold room are given in table 2. The obtained bar structure with very fine carbide particles (Nb,Ti)C is given in fig. 1.

Table 2.

F_m - share of martenzite with bainite; D_a = ferrite grain size

Example 2

Steel of chemical composition given in table 3 (parameters in % by weight) was rolled to a flat bar of width 140 x thickness 10 mm according to the developed technology. Temperature of the charge heating in the heating furnace was 1080 °C. The final rolling temperature was 780°C. Value of yeild strength R_e min. = 460 MPa.

Table 3. Mechanical properties and characteristics of the flat bar structure after rolling and cooling in a cold room are given in table 4. The obtained bar structure with very fine carbide particles (Nb,Ti)C is given in fig. 2.

Table 4.

F_m - share of martenzite with bainite; D_a = ferrite grain size

Of course, the invention in question is not limited to the presented embodiments - its different modifications and extensions are possible within the scope of the enclosed claims without departing from the claims.

Claims

Patent claims

1. Method of producing steel bar of non-round cross-section using the hot rolling process, wherein the charge in the form of ingots obtained in the process of continuous casting is heated in a furnace and then shaped in the rolling process in rolling mill stand followed by cooling down the ready-made product, characterized in that stage of heating in the heating furnace is performed up to maximum temperature within the range of 1080 - 1180°C, stage of shaping using the rolling mill stands includes initial rolling performed in a group of initial stands and finishing rolling performed in a group of finishing stands, wherein the time interval between the last rolling reduction in the initial group and the first rolling reduction in the finishing group is at least 20 seconds, wherein the total relative rolling reduction in the finishing group, expressed with the formula {[Pp - Pk ]/ Pp } * 100%, where (Pp) is the cross-section area of a band after the last stand of the initial stands group and (Pk) is the cross-section area of the ready-made product, is within the range of 60 - 80%, and cooling stage of the ready made bar is performed using air.

2. Method according to claim 1, characterized in that low-alloy steel is the charge material wherein content of elements C, Mn, Ni, Cu, Cr, Mo and V is selected in order to meet the condition:

3. Method according to claim 1 or 2, characterized in that the Ti, Nb and V elements content is selected in order to meet the condition Ti+Nb+V < 0.30%, wherein the maximum content of Nb in % by weight is expressed with the following formula: Log[Nb]*[C+12/14N] = 2.26 - 6770/T, and the Ti content is determined so that content of this element in austenite is within 0.020 - 0.070%.

4. Method according to claim 2 or 3, characterized in that content of the elements C, Si, Mn, N, S, Mo, Cr, Ni, Cu, B and A1 in steel is as follows:

C < 0,10%;

Si < 0.20%;

1.35% <Mn < 1.95%;

N < 0.010%;

0.005% < S < 0.010%;

0.02% < Mo < 0.25%;

Cr + Ni + Cu < 0.80%

0.0004% <B < 0.0010%;

A1 > 0.020%.

5. Method according to any of the claims 1-4, characterized in that the steel bar of non-round cross-section represents long product in the form of flat bar.

6. Steel bar of non-round cross-section, produced in the process of hot rolling characterized in that the microstructure of steel of the ready-made bar includes fine-grained polygonal ferrite and irregular bainitic ferrite of grain size 4-7 pm and share 75-85% and martensitic and bainitic islands of size less than 10 pm and share 15-25%.

7. Bar according to claim 6, characterized in that the long product in the form of flat bar of thickness up to 20 mm and width up to 250 mm of maximum value of the minimum yield strength R_e is 700 MPa.