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

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

[0001] The subject of the invention is the method of producing a steel flat bar up to 20 mm thick and up to 250 mm wide and a steel flat bar up to 20 mm thick and up to 250 mm wide that is produced vis said method. 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 enables production of 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 47J.

[0002] Steel long products in the form of flat bars of high mechanical parameters are mostly used in the process of manufacturing of 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 toughness with simultaneous maintenance of very good weldability.

[0003] 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.

[0004] 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 homogeneity after the process completion. This facilitates determination of the deformation scheme optimum for the process that allows to control its performance , therefore - high strength parameters obtained within its result.

[0005] For example, due to application of high temperature of charge heating in the rolling process, carbides NbC, TiC and VC previously dissolved in austenite precipitate during the rolling process retarding the process of austenite recrystallization and grain growth after recrystallization. The growth of dynamically precipitated carbides particles in austenite is limited by high rate of rolling within the finishing mill and application of water cooling therefore the particles have significant contribution to the precipitation strengthening. The said accelerated and controlled water cooling favours the structure refinement and as a result of lowering the coiling temperature, it is possible to obtain varied structures including ferritic - pearlitic, ferritic -bainitic, ferritic -martensitic or different combinations of the structural constituents. Finally, slow cooling of the coil after coiling promotes the increase of yield strength due to the precipitation strengthening.

[0006] 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 applying typically 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 in a band cross-section, in -particular within the initial passes, which strongly differentiates the austenite structure condition. In areas of higher deformation and higher temperature, austenite recrystallization is faster comparing to areas where deformation and temperature during the process are lower. As a result, this leads to significant variation of the band cross-section structure, which unfavourably affects the final strength properties, in particular the impact toughness. Moreover, in contrast to rolling metal sheets, it is much more difficult to control value of deformation while rolling long products in the form of flat bars at the process level, what leads to technological limitations within the finally obtained properties.

[0007] 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 of alloying elements used in steel that affect the strengthening of the obtained material structure and final mechanical properties.

[0008] 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 to ambient temperature using gaseous, liquid or sprayed coolant or using fluid bed at rate 1.5°C / sec. to 10°C / sec., which fact affects precipitation strengthening and / or formation of small grains and produces ferritic and pearlitic microstructure in the bar material, avoiding formation of bainite structure. Cooling proceeds to temperature at least 50°C below the temperature where the transformation to ferrite and perlite is completed. During the process, bars of microalloyed steel are produced that contain carbon in the range from 0.3 to 0.65% by weight, silicon 1.2% by weight, manganese in 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 strengthening and / or elements causing refinement 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 in the range of 0.0005 to 0.005% by weight. The remaining part is iron and residual elements introduced during melting, wherein the total content of chromium and manganese does not exceed 1.0% by weight.

[0009] Similarly, EP1700925 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 µm). 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 % by weight, niobium in 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 mist.

[0010] Additionally, in EP0792379 A1 a dual-phase steel and the method for preparing a high strength dual phase steel of at least 110 ksi (758MPa) yield strength after 1-3 percent deformation is disclosed, where steel has a ferrite phase and 40 to 80 vol percent of a martensite / bainite phase of which bainite is no more than 50 vol percent. The method comprising (a) heating a steel billet, which contains vanadium and niobium in concentrations totalling from 0.1 to 0.27 wt percent, to a temperature sufficient to dissolve substantially all vanadium carbonitrides and niobium carbonitrides; (b) rolling the billet, and forming plate, in one or more passes to a first reduction in a temperature range in which austenite re-crystallizes; (c) rolling the plate in one or more passes to a second reduction in a temperature range below the austenite recrystallization temperature and above the Ar 3 transformation point; (d) cooling the further reduced plate to a temperature between the Ar 3 and Ar 1 transformation points; (e) finish rolling the cooled plate in one or more passes in a third rolling reduction at a temperature between the Ar 3 and Ar 1 transformation points; and (f) water cooling / quenching, at a rate of at least 25 degrees centigrade per second, the finished rolled plate to a temperature less than or equal to 400 degrees centigrade.

[0011] The above presented solutions disclose the methods of producing high strength flat products, including bars, wherein in order to improve strength properties, 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.

[0012] Due to these facts, the subject of the invention is to propose an improved method of manufacturing of long products in the form of flat bars wherein properly selected process stages and rolling parameters lead to complete suppression of austenite recrystallization during the final passes of the roughing group. The subject of the invention is to obtain the final product that is characterized by high strength parameters as well as very good weldability.

[0013] The invention is as stated in the attached claims.

[0014] According to the invention the method of producing steel flat bar of thickness up to 20 mm and width up to 250 mm, using the hot rolling process, wherein the charge in the form of billets obtained in the process of continuous casting is heated in a furnace and then formed in the rolling process in rolling mill stands followed by cooling down the flat bar to ambient temperature. The method is characterized in that in the stage of heating in the furnace is performed up to maximum temperature within the range of 1080°C - 1180°C. The stage of shaping using the rolling mill stands includes roughing rolling performed in a group of roughing stands and finishing rolling performed in a group of finishing stands where the finish rolling temperature is between 790°C and 830°C. The minimum acceptable time interval between the last rolling reduction in the roughing group and the first rolling reduction in the finishing group is 20 seconds, wherein the total relative rolling reduction in the finishing group, expressed with the formula {[Pr - Pf] / Pr }*100%, where (Pr) is the cross-section area of a band after the last stand of the roughing stands group and (Pf) is the cross-section area of the flat bar is within the range of 60 - 80%. The cooling stage of the flat bar is performed using air by natural cooling condition at rate from 0.5°C / sec. to 2.0°C / s. The charge material is low-alloy steel, 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: C e = %C + %Mn 6 + %Ni + %Cu 15 + %Cr + %Mo + %V 5 . . The Ti, Nb and V elements content is fixed 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, where T is the billet reheating temperature in K, [Nb] is niobium content by weight % dissolved in austenite at the reheating temperature, C is carbon content in the steel, N is free nitrogen content, wherein the Ti content is determined so that content of this element in austenite is within 0.020 - 0.070%. The content of Ti introduced into the steel is determined so that the content of this element dissolved in austenite [Ti] at the billet heating temperature is within the range of 0.020% - 0.070%. Content of titanium in austenite is expressed with the formula: [Ti] = Ti - 3.43*N t - 3*S, where: N t is total nitrogen content in the steel, and value of 3.43*N c refers to the part of titanium content bound in nitride TiN, and value 3*S refers to the content of titanium bound in titanium carbide-sulphide Ti 4 C 2 S 2 , wherein the steel contains Mn in the amount of 1.35 to 1.95%, small amounts of Cr, Ni and Cu originating from the scrap, Mo in the amount from 0.02 to 0.25% and B in the amount from 0.0004 to 0.0010%, where S is in amount from 0.005 to 0.010%, Al content is in the range from 0.02 to 0.04%, C content is not greater than 0.10%, Si content is not greater than 0.20%, and N content is not greater than 0.010 %, wherein Cr + Ni + Cu ≤ 0.80%, wherein the balance being Fe and impurities.

[0015] The another aspect of the invention is steel flat bar of thickness up to 20 mm and width up to 250 mm and the minimum yield strength R e in the maximum value of 700 MPa according to the invention, produced in the process of hot rolling is characterized by the fact that its microstructure is composed of fine-grained polygonal ferrite and irregular bainitic ferrite of grain size 4-7 µm and volume fraction of 75-85% and martensitic and bainitic islands of size less than 10 µm and volume fraction of 15-25%.

[0016] The developed production method uses the synergistic effect of the following crucial process parameters: heating temperature of the charge for rolling in the furnace, final temperature of rolling controlled by the time interval between the last pass in the roughing 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 and air mist. Temperature changes of the rolled band are caused by heat transfer phenomena into rolls 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 furnace, rate of deformation and time interval between rolling in the roughing group and the finishing group. As a result of the adjusted rolling parameters and chemical composition of the steel according to the invention, long products in the form of flat bars of minimum yield strength in the range of 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.

[0017] The subject of the invention is presented in the embodiments and figures wherein fig. 1 presents microstructure of the flat bar, according to Example 1 and fig. 2 presents microstructure of the flat bar, according to Example 2.

[0018] 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 rules_of the steel chemical composition designing (in % by weight): Carbon content in the steel meets the condition: C ≤ 0.10%.

[0019] Content of Mn, Ni, Cu, Cr, Mo and V is determined in order to meet the condition: 0 , 30 % ≤ C θ ≤ 0 , 40 % , where C e is carbon equivalent expressed using the formula (according to annex C of the standard PN-EN 1011-2:2004+A1:2005 Welding - Recommendation of welding of metallic materials - Part 2: Arc welding of ferritic steel): C e = %C + %Mn 6 + %Ni + %Cu 15 + %Cr + %Mo + %V 5 Total content of Cr, Ni and Cu meets the condition Cr+Ni+Cu ≤ 0.80%.

[0020] 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 fixed in the range: 0.02% ≤ Mo ≤ 0.25%.

[0021] The effect of the cooling rate reduction of the long product in the form of flat bar in a cooling bed after rolling related to the increase of its thickness on the temperature of the ferritic transformation initiation and kinetics of the bainitic transformation is compensated by proportionally increasing the content of boron (B) in the 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.

[0022] Content of aluminium (Al), nitrogen (N) and sulphur (S) in steel is fixed in the following ranges: 0.020 % ≤ Al ≤ 0.040%, N ≤ 0.010% and 0.005% ≤ S ≤ 0.010%.

[0023] Aluminium protects boron against oxygen and simultaneously with titanium against nitrogen. Nitrogen content is limited because along with the increase of content of this element in the 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) 4 C 2 S 2 , that is present in steel, is gradually dissolved during rolling therefore introducing niobium and titanium into solid solution released during ferritic transformation 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 solid solution (austenite) with Ti and Nb.

[0024] 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.

[0025] The content of Nb introduced into the steel is determined optimally at the level so that carbide NbC - present in billet - completely dissolves in austenite at the billet 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 / 14 N = 2.26 − 6770 / T , 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 the steel.

[0026] 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 the steel is reduced by the value 0.008% x (700 - R e ) / 50, where R e is the minimum required yield strength; Content of Ti introduced into the steel is determined so that the content of this element dissolved in austenite [Ti] at the billet heating temperature is optimally within the range of 0.020% - 0.070%. Content of titanium in austenite is expressed with the formula: Ti = Ti − 3.43 * N − 3 * S , where: value of 3.43*N 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 Ti 4 C 2 S 2 .

[0027] In the method according to the invention, firstly the charge in the form of billet obtained during continuous casting process is heated in the furnace. Heating of the billet in the heating furnace is performed up to maximum temperature within the range of 1080 - 1180 °C. During reheating of the billet in the 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 solid 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) 4 C 2 S 2 is present in steel after heating.

[0028] After reheating completion the rolling process begins that is divided to roughing rolling performed in the roughing 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 roughing 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 elapsing of the said at least 20 seconds after completion of the roughing rolling in the last stand of the roughing 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: Pr − Pf / Pr * 100 % where (Pr) is the cross-section area of the band after the lasts stand of the roughing stands group, and (Pf) is the cross-section area of the flat bar, is within the range of 60 - 80%.

[0029] At the stage of hot rolling, the precipitation kinetics of the NbC carbide 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 after the last passes of the group. Total recrystallization of austenite and formation of fine and homogeneous grain size distribution takes place within time interval between the completion of the roughing rolling and start of the finishing rolling.

[0030] After completion of rolling, natural cooling of the flat bar is performed in air to ambient temperature. It is assumed that cooling takes place at a rate within the range 0.5 -2.0 °C / sec., depending on the bar thickness.

[0031] 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 transformation initiation (within the range of ca. 790 - 770°C) is small. Under these conditions, before transformation, the solid 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 transformation that strengthen the ferrite matrix. The contribution of the precipitation hardening to yield strength can be increased by addition of V to steel that forms carbides (Nb,V)C and (Ti,V)C. Phase transformation of austenite gives fine ferrite grain of size in the range of 4-6 µm. Refinement of the grains leads both to the growth of strength and ductility of the steel. The used Mn content in the steel and synergistic effect of small amounts of Cr, Ni, Cu (from scrap), Mo in the amount of 0.02-0.25% and boron in the amount of 0.0004-0.0010% cause deceleration of the pearlitic transformation. Instead of perlite in the steel structure, there are small martensite and bainite islands.

[0032] Inhibition of austenite recrystallization according to the inventive method is implemented by introduction of niobium into steel in such an amount so that NbC carbides present in the billet is completely dissolved in austenite at the reheating temperature for rolling not higher than 1180°C. Value of billet / bloom reheating 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 after the last passes of rolling in the roughing initial group as a result of dynamic precipitation of NbC particles and its complete recrystallization within at least 20 seconds after the last pass of the roughing group of the rolling line. This is the minimum time interval between roughing rolling in the roughing group and finishing group applied according to the invention. The number of rolling mill stands in the roughing 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 finish rolling temperature not to exceed the range of 790 - 830°C. During the roughing 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 in the range of 0.5 - 2.0°C / sec. depending on the flat bar thickness, the temperature of the austenite transformation 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 transformations of austenite into bainite and martensite causes that the flat bar structure contains the following main constituents: fine-grained polygonal ferrite and irregular bainitic ferrite of dislocation density and grain size within the range of 4-7 µm and volume fraction 75-85%; martensitic and bainitic islands of size below 10 µm and volume fraction 15-25%.

[0033] Polygonal ferrite and bainite matrix also contains fine particles of (Nb, Ti)C of size below 10 nm and volume fraction in the range of 0.0005 - 0.0015 that are precipitated from austenite during ferritic transformation.

[0034] 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.

[0035] The result of synergistic effect of the aforementioned hot rolling process parameters and the precipitation processes occurring in the described temperature regime by the qualitatively and quantitatively determined micro-additives and the phase transformations taking place in the steel structure, additional effects can be obtained, compared 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 toughness at lower temperature. In the structure of long products in the form of flat bars produced according to the invention, bainite and martensite 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 in the Charpy test KV(-20°C) is greater than 47J. An important feature related to morphology of bainite and martensite islands in the flat bar structure is its subject to strong work hardening 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 refinement of ferrite grain - value of yield strength approaches the value of tensile strength.

[0036] 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

[0037] Steel of chemical composition given in table 1 (in % by weight) was rolled to a flat bar of width 140 mm x thickness 10 mm according to the developed technology. Temperature of the billet heating in the furnace was 1180 °C. The finish rolling temperature was 820°C. Value of yield strength R e min.=700 MPa. Following rolling, the flat bars were cooled in the cooling bed in still air. Table 1.CMnSiPsCrNiAlCuMovNbBTiN0.051.750.050.0110.0070.060.060.0350.180.0310.010.0660.00040.130.0094

[0038] Mechanical properties and characteristics of the flat bars structure after rolling and cooling in a cooling bed in still air are given in table 2. The obtained bar structure with very fine carbide particles (Nb,Ti)C is shown in fig. 1. The meaning of the descriptions in the figure is as follows: F - ferrite, B - bainite, M - martensite. Table 2.PropertiesStructureR eH ,MPaR m , MPaR eH / R m A 5 ,%KV -20 ,JD α , µmF m 7157740.9222.1136.74.20.21F m --volume fraction of martensite with bainite; D α = ferrite grain size Example 2 (reference)

[0039] Steel of chemical composition given in table 3 (in % by weight) was rolled to a flat bar of width 140 mm x thickness 10 mm according to the developed technology. Temperature of the charge heating in the furnace was 1080 °C. The final rolling temperature was 780°C. Following rolling, the flat bars was cooled in the cooling bed in still air. Table 3.CMnSiPsCrNiAlCuMovNbBTiN0.041.610.190.0100.0060.050.080.0290.150.0210.0050.030.00040.060.0099

[0040] Mechanical properties and characteristics of the flat bars structure after rolling and cooling in a cooling bed in still air are given in table 4. The obtained bar structure with very fine carbide particles (Nb, Ti)C is shown in fig. 2. Table 4.PropertiesStructureR eH ,MPaR m , MPaR eH / R m A 5 ,%KV -20 ,JD α , µmF m 4685570.8428.52076.00.17F m - volume fraction of martensite with bainite; D α = ferrite grain size

[0041] 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

1. Method of producing steel flat bar of thickness up to 20 mm and width up to 250 mm, using the hot rolling process, wherein the charge in the form of billets obtained in the process of continuous casting is heated in a furnace and then formed in the rolling process in rolling mill stand followed by cooling down to ambient temperature, characterized in that stage of heating in the furnace is performed up to maximum temperature within the range of 1080°C - 1180°C, stage of shaping using the rolling mill stands includes roughing rolling performed in a group of roughing stands and finishing rolling performed in a group of finishing stands where the finish rolling temperature is between 790°C and 830°C, wherein the minimum allowable time interval between the last rolling reduction in the roughing group and the first rolling reduction in the finishing group is 20 seconds, wherein the total relative rolling reduction in the finishing group, expressed with the formula {[Pr -Pf ] / Pr }*100%, where (Pr) is the cross-section area of a band after the last stand of the roughing stands group and (Pf) is the cross-section area of the flat bar, is within the range of 60 - 80%, and cooling stage of the flat bar is performed using air by natural cooling condition at rate from 0.5°C to 2.0°C / sec., and in that the charge material is low-alloy steel, wherein content of elements C, Mn, Ni, Cu, Cr, Mo and V is selected in order to meet the condition: 0.30 % ≤ C θ ≤ 0.40 % , where Ce is carbon equivalent of value expressed with the formula: C e = %C + %Mn 6 + %Ni + %Cu 15 + %Cr + %Mo + %V 5 , and that Ti, Nb and V elements content is fixed 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, where T is the billet reheating temperature, [Nb] is niobium content by weight % dissolved in austenite at the reheating temperature, C, N, respectively, carbon and nitrogen content remained in austenite - after TiN formation, wherein the Ti content is determined so that content of this element in austenite is within 0.020 - 0.070%, and wherein the content of Ti introduced into the steel is determined so that the content of this element dissolved in austenite [Ti] at the billet heating temperature is within the range of 0.020% - 0.070% content of titanium in austenite is expressed with the formula: [Ti] = Ti - 3.43*Nt - 3*S, where: Nt is total nitrogen content in the steel, and value of 3.43*Nc refers to the part of titanium content bound in nitride TiN, and value 3*S refers to the content of titanium bound in titanium carbide-sulphide Ti4C2S2, wherein the steel contains Mn in the amount from 1.35 to 1.95%, small amounts of Cr, Ni and Cu originating from the scrap, Mo in the amount from 0.02 to 0.25% and B in the amount from 0.0004 to 0.0010%, , where S is in amount from 0.005 to 0.010%, Al content is in the range from 0.020 to 0.040%, C content is not greater than 0.10%, Si content is not greater than 0.20%, and N content is not greater than 0 010 %, wherein Cr + Ni + Cu ≤ 0.80%, wherein the balance being Fe and impurities.

2. Steel flat bar of thickness up to 20 mm and width up to 250 and the minimum yield strength Re in the maximum value of 700 MPa, produced in the process of hot rolling according to claim 1, characterized in that the microstructure of steel of the flat bar includes fine-grained polygonal ferrite and irregular bainitic ferrite of grain size 4-7 µm and volume fraction of 75-85% and martensitic and bainitic islands of size less than 10 µm and volume fraction of 15-25%.

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