EP4139493A1 - Method of producing steel wire rod of round cross-section and steel wire rod of round cross-section - Google Patents

Method of producing steel wire rod of round cross-section and steel wire rod of round cross-section

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Publication number
EP4139493A1
EP4139493A1 EP20853574.0A EP20853574A EP4139493A1 EP 4139493 A1 EP4139493 A1 EP 4139493A1 EP 20853574 A EP20853574 A EP 20853574A EP 4139493 A1 EP4139493 A1 EP 4139493A1
Authority
EP
European Patent Office
Prior art keywords
temperature
rolling
wire rod
steel
band
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP20853574.0A
Other languages
German (de)
French (fr)
Inventor
Zbigniew KUTYLA
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
CMC Poland Sp zoo
Original Assignee
CMC Poland Sp zoo
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by CMC Poland Sp zoo filed Critical CMC Poland Sp zoo
Publication of EP4139493A1 publication Critical patent/EP4139493A1/en
Pending legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the object of the invention is method of producing steel wire rod of round cross- section and steel wire rod of round cross-section.
  • the invention is applied for manufacturing round wire rod within the hot rolling process of billets / blooms of steel with controlled content of Nb, Ti and V elements within the continuous casting process.
  • the invention allows for obtaining round wire rod of strength parameters R m min. 720 MPa and R p o ,2 min. 570 MPa used for production of coupling elements of class 8.8 and wire rod of strength parameters R m min. 820 MPa and R p o ,2 min. 650 MPa for production of coupling elements of class 9.8 with omission of thermal treatment during production.
  • the steel wire rod is a product obtained in the process of hot rolling that has small - in relation to length - usually round cross-section.
  • Round wire rod is usually used for production of high strength coupling elements employed for installation of machines and devices parts and structures made of steel, aluminium or plastics.
  • the new grades of steel used during the production process of coupling elements must characterize with good plasticity during the cold forming of the head as well as should show sufficient level of strength and high susceptibility to strengthening during shaping processes so that it is possible to obtain the required properties in the final product.
  • the mentioned combination of strength and susceptibility for strengthening is possible for example in the mentioned dual-phase steel mostly due to ferritic microstructure, containing also martenzite, bainite and austenite with share of 10 do 20%.
  • Bainitic steels in the form of wire rod of low or very low carbon content are found to be especially well suited for production of coupling elements with omission of thermal treatment processes due to their high strength after rolling which fact in combination with good susceptibility for strengthening within the cold forming process allows for obtaining the required target strength parameters after drawing and forging processes.
  • bainitic steels for cold forming of coupling elements are disclosed in the following patent applications EP0851038A1, EP1565587A1, EP1780293 A2 and FR 2867785.
  • the main process concept disclosed in the aforementioned documents consists in the selection of carbon content and alloying elements in order to obtain the required mechanical parameters.
  • the document EP2199422 A1 discloses the application of carbide-free steel of grained bainite precipitate strengthened structure for production of coupling elements, especially of class 8.8 and with omission of additional thermal treatment processes.
  • the following chemical composition of bainite steel was determined: C within the range 0.04 - 0.15 % by weight, Mn within the range 1.8 - 2.0 % by weight, Si within the range 0.15 — 0.30 % by weight, S below 0.025% by weight, P below 0.025% by weight, Cr below 0.5% by weight, Mo below 0.08% by weight, sum of Ni and Cu content above 0.3% by weight, A1 within the range 0.01 - 0.05% by weight, V below 0.05% by weight, B within the range 10 - 30 ppm, N below 100 ppm and Ti above 0.06% by weight.
  • the production method itself includes conventional hot rolling with the final rolling temperature about ⁇ 1000 ° C, followed by cooling at rate > 3K / s within the temperature range between 800 and 500
  • the document EP2557185 A1 discloses chemical composition of steel and method of manufacturing steel elements of diameter within the range between 6 to 40 mm.
  • Composition of steel according to the said document is selected in order to obtain the highest possible temperature of austenite conversion into bainitic structures therefore the produced bainitic steel is subject to tempering during the production process (in-situ) and is more stable under temperature impact comparing to martensitic steel or lower bainite.
  • tempering during the production process (in-situ) and is more stable under temperature impact comparing to martensitic steel or lower bainite.
  • it characterizes with necessary strength without additional drawing operation - it is plastic enough to enable cold forming through bending or rolling.
  • composition of steel is proposed in the disclosure: C within the range 0.06 - 0.13% by weight, Si within the range 0.05 - 0.50% by weight, Mn within the range 1.70 - 2.50% by weight, Cr within the range 0.05 - 0.90% by weight, Ni within the range 0.05 - 0.40% by weight, Mo within the range 0.01 - 0.50% by weight, Cu within the range 0.05 - 0.30% by weight, S within the range 0.003 - 0.05% by weight, Ti within the range 0.001 - 0.10% by weight, V within the range 0.001 - 0.06% by weight, B up to 0.003% by weight, Nb up to 0.06% by weight and P within the range up to 0.05% by weight, and Fe and contamination to complement to 100%.
  • the process of manufacturing an element of steel of the said chemical composition includes traditional rolling within the range of temperatures 1300 - 900° C and air cooling within the range of temperatures 800 - 500° C at the rate 0,5 - 3 K/sec.
  • Structure of steel obtained as a result of such process - after the final hot rolling stage - characterizes with mean size of austenite grain below 50 pm, and the necessary level of strength is achieved without an additional stage of thermal treatment.
  • the process of rolling the wire rod is performed at the final rolling temperature of 900° C, causing that the obtained microstructure of wire rod still characterizes with low ductility. and impact strength due to too large bainitic ferrite grain.
  • the objective of the invention is to develop a new method of manufacturing steel wire rod for coupling elements using low-temperature thermomechanical rolling as a result of which multi-phase steel structure is obtained.
  • the objective of the invention is to propose a technology wherein properly selected process stages, rolling parameters and the designed chemical composition of steel would completely stop austenite recrystallization within the finishing group.
  • the objective of the invention is to obtain a final product that characterizes with high strength parameters as well as very good ductility, impact resistance and terminal deformability within the cold forming process.
  • charge in the form of ingots obtained in the process of continuous casting is heated up in a heating furnace and then formed in the process of rolling on rolling mill stands, and the ready-made product is subject to cooling.
  • the method characterizes with that the charge heating up stage in the heating furnace is performed at the maximum temperature within the range of 1080 - 1100°C, and then the charge is baked within time not less than 0.5 hour.
  • the stage of forming on rolling mill stands includes preliminary rolling performed within a group of initial stands and finishing rolling performed within a group of finishing stands, wherein the band temperature at the outlet from the last stand of the initial rolling mill stands is within 1020 - 980°C.
  • the band is cooled in water boxes to temperature within the range 875 ⁇ 20°C, and after rolling in the finishing group the band is cooled to temperature 875-750°C before entering the coils piling machine.
  • niobium content in steel does not exceed the maximum content that is subject to dissolving in austenite at the baking temperature of charge given using the following formula:
  • Ti 0.0662- (1.4086*10 4 )*T + 0.61539*C + 5.721*[Ti] - 0.63595*Nb + 1.6769*N + 0.0423*S ⁇ 0.015, where T is the temperature of charge baking in C, and the content of titanium in fixed solution - austenite, [Ti], is within the range 0.03 - 0.07%.
  • the wire rod of round cross-section according to the invention produced within the process of hot rolling followed by cooling in Stelmor line is characterized by that the structure of ready-made wire rod contains polygonal ferrite and irregular bainitic ferrite of grain size below 7 pm and share 80 - 90%, small islands of martensite and bainite of grain size below 7 pm and share 15-20% and particles of residual austenite of size below 3 pm and share 3-5%.
  • the diameter of the ready-made wire-rod is within 8 - 25 mm.
  • the developed method of wire rod manufacturing for coupling elements of strength class 8.8 and 9.8 uses the synergistic effect of impact of the most important process parameters, including: temperature of baking the charge in the heating furnace, temperature of band during the rolling process, accelerated, controlled cooling in Stelmor line and chemical composition of steel, including in particular the controlled content of niobium (Nb), titanium (Ti) and vanadium (V) so that to eliminate the mentioned - unfavourable feature present according to the art, i.e. so that the bainitic ferrite grain size within the wire rod structure according to the invention does not exceed 7 pm.
  • Nb niobium
  • Ti titanium
  • V vanadium
  • fig. 1 presents (a) wire rod microstructure and (b) coupling element formed using it, according to Embodiment 1
  • fig. 2 presents (a) wire rod microstructure and (b) coupling element formed using it, according to Embodiment 2.
  • the developed method of producing applies in particular to a wire rod of round cross-section and of diameter within the range 8 - 25 mm, used for coupling elements of strength classes 8.8 and 9.8.
  • the method according to the invention uses the following principles of the steel chemical composition designing (in % by weight):
  • Nb titanium
  • Nb niobium
  • V vanadium
  • Niobium is the basic steel component according to the invention that strongly affects the austenite recrystallization course in the rolling process.
  • T ra (°C) 835 +129*[Ti] +1790*[Nb] + 80*[V] ⁇ 15 (formula 3)
  • [Nb], [Ti] and [V] is the content of Nb, Ti, V in austenite at the charge baking temperature for rolling resulting from dissolving carbides and nitrides of NbC, TiN, TiC present in the billet / bloom.
  • Titanium is the micro-additive that supports niobium impact. It is to bind the whole nitrogen in the form of carbide-nitride Ti(N,C) and form carbide-sulphide T14C2S2.
  • Titanium is introduced into steel according to the invention at the level so that after binding with nitrogen and sulphur, the content of this element in fixed solution - i.e. austenite (determined as [Ti]) is within the range 0.03 - 0.07 % by weight.
  • Total titanium content in percentage by weight that needs to be introduced to steel is calculated using the following formula (experimentally determined by the inventors):
  • Ti 0.0662- (1.4086*10 4 )*T + 0.61539*C + 5.721*[Ti] - 0.63595*Nb + 1.6769*N + 0.0423*S ⁇ 0.015 (formula 4)
  • T is the temperature of charge baking in °C
  • [Ti] is the content of titanium dissolved in austenite
  • the remaining symbols mean the content of elements in steel.
  • vanadium is added to steel in order to ensure precipitation strengthening of polygonal and bainitic ferrite with VC precipitations.
  • Content of this element in steel is determined at the level so that the sum of all micro additives in steel satisfies the inequality:
  • this element dissolves completely in austenite at the charge baking temperature for rolling.
  • the content of nitrogen (N) is determined so that its share in percentage by weight in steel satisfies the following inequality N ⁇ 0.07*Ti.
  • the essence of the process layer of the method according to the invention is stopping the austenite recrystallization which is possible when the band rolling temperature is less than the recrystallization initiation temperature T m (according to formula 3) and is caused by the dynamic release of niobium dissolved in austenite at the charge baking temperature for rolling in the form of carbide NbC.
  • the charge baking time in the heating furnace is not less than 0.5 h.
  • the rolling process starts that is divided to initial rolling, performed within the group of initial rolling mill stands and finishing rolling, performed in the finishing rolling mill stands group. Moreover, when the wire rod diameter does not exceed 14 mm, between the initial rolling and finishing rolling stages, the intermediate rolling is performed within the intermediate rolling mill stands group. If the wire rod diameter exceeds 14 mm, the intermediate rolling mill stands group is excluded from the production process line.
  • the band Downstream the intermediate rolling mill stand, the band is cooled using a controlled method in two water boxes to the following temperatures:
  • the band Downstream the finishing rolling mill stand, the band is cooled using a controlled method in 3 water boxes.
  • the temperature of entering the band into the coils piling machine is determined within 875 - 750°C.
  • Controlled wire rod cooling according to the invention is performed in Stelmor line of length 120 m covering 9-meter long cooling zones.
  • Cooling in Stelmor line is performed according to the following sequence:
  • cooling is performed using isothermal covers with average cooling rate 1 - 2°C/sec. to temperature 400°C, and then
  • the temperature of the bainitic conversion initiation Bs is calculated according to the following formula (acc. toingtonSteel Forming and Heat Treating Handbook”, A. A. Gorni, December 2002):
  • Bs(C) 637 - 58*C - 35*Mn - 15*Ni - 34*Cr - 41*Mo (formula 5) where C, Mn, Ni, Cr and Mo is the content of elements in steel in % by weight.
  • the non-crystallized austenite structure is optimum for obtaining the required structure of the final wire rod in the cooling process using Stelmor line of the following composition: • polygonal ferrite and irregular bainitic ferrite without iron carbides, of grain size below 7 pm and share 80 - 90%;
  • the obtained steel microstructure does not contain particles of iron carbides. Moreover, the structure does not contain cementite particles which is the result of stopping the austenite recrystallization within the hot rolling process. Structure of wire rod of such characteristics ensures obtaining relationships between the tensile strength and yield strength R m /Rpo , 2 > 1.2.
  • the obtained steel structure is subject to strong strengthening during plastic deformation which is the required result both in terms of technological plasticity of wire rod in the cold forging process and in terms of obtaining high strength of the final product. Additional results can be obtained, comparing to conventional processes, applying the synergistic effect of impact of the above characterized hot rolling process parameters as well as selected content of micro additives and phase conversions in the steel structure within controlled cooling process in Stelmor line.
  • the non-crystallized austenite structure of wire rod obtained as a result of using the method according to the invention, prevents the release of cementite during cooling and phase conversions, which in case of conventional processes is obtained through adding Si or A1 to steel in amount 1-2%.
  • the method according to the invention counteracts the phenomenon of delayed cracking because of MnS, T14C2S2 and V4C3 particles of controlled morphology, present in the bainitic ferrite matrix.
  • Steel structure according to the invention characterizes with high strength parameters necessary to produce coupling elements of the required classes 8.8 and 9.8 as well as very good ductility, impact strength and terminal deformability.
  • Embodiments of strength parameters according to the invention related to the specified quantitative and qualitative chemical composition values of steel and the applied process parameters are described below.
  • Billet of cross-section 160 mm x 160 mm of chemical composition given in table 1 was rolled in to a wire rod of diameter F12.5 mm with baking at temperature 1100°C within lh, and then rolled at the band temperature 825 °C at the entry to the finishing rolling mill stand group. Accelerated, controlled cooling in Stelmor line was performed at rate 9°C/s to temperature 580°C. Then, wire rod was slowly cooled under covers at rate 2°C/s to temperature 450°C.
  • temperature T m for the chemical composition of steel is 860°C. Therefore, in the finishing rolling group, the austenite recrystallization is stopped.
  • wire rod characterized with a structure containing 90% of allotrimorphous and bainitic ferrite of grain size 6 m and containing 10% of martensitic islands of grain size 6 pm. Moreover, the structure contains carbide particles (Ti,Nb)C.
  • Embodiment 2 Billet of cross-section 160 mm x 160 mm of chemical composition given in table 2 was rolled in to a wire rod of diameter F12.5 mm with baking at temperature 1100°C within lh, and then rolled at the band temperature 825 °C at the entry to the finishing rolling mill stand group. Accelerated, controlled cooling in Stelmor line was performed at rate 9°C/s to temperature 570°C. Then, wire rod was slowly cooled under covers at rate 2°C/s to temperature 450°C.
  • temperature T m for the chemical composition of steel is 870C. Therefore, in the finishing rolling group, the austenite recrystallization is stopped. As a result of this, wire rod of structure containing 80% of allotrimorphous and bainitic ferrite of grain size 5 m and containing 17% of martensitic and bainitic islands of grain size 4 pm was obtained. Moreover, the wire rod structure was found to include the presence of residual austenite of share 3% and carbide particles (Ti,Nb)C.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
  • Heat Treatment Of Steel (AREA)
  • Metal Rolling (AREA)

Abstract

Method for producing steel wire rod of round cross-section in the process of hot rolling is disclosed, wherein the charge heating up stage in the heating furnace is performed at the maximum temperature within the range of 1080 - 1100°C, and then the charge is baked within no less than 0.5 hour. The stage of forming in the rolling mill stands includes initial rolling and finishing rolling, wherein the band temperature after completion of the initial rolling is within 1020 - 980°C, then the band is cooled in water boxes to temperature 870±20°C, and after the finishing rolling group before entering the coil piling machine it is cooled to temperature within 875 - 750°C. After completion of the finishing rolling stage, cooling is performed using Stelmor line, wherein firstly the ready-made band is cooled at average cooling rate within 4-10°C/s to temperature of the bainitic conversion initiation BS±20°C, and then the band is cooled using isothermal covers with average cooling rate within 1 - 2°C/s to temperature 400°C, followed by air cooling. The microstructure of steel of the ready-made wire rod made according to the method contains polygonal ferrite and irregular bainitic ferrite of grain size below 7 μm and share 80 - 90%, small islands of martensite and bainite of grain size below 7 μm and share 15-20% and particles of residual austenite of size below 3 μm and share 3-5%.

Description

Method of producing steel wire rod of round cross-section and steel wire rod of round cross-section
The object of the invention is method of producing steel wire rod of round cross- section and steel wire rod of round cross-section. In particular, the invention is applied for manufacturing round wire rod within the hot rolling process of billets / blooms of steel with controlled content of Nb, Ti and V elements within the continuous casting process. The invention allows for obtaining round wire rod of strength parameters Rm min. 720 MPa and Rpo,2 min. 570 MPa used for production of coupling elements of class 8.8 and wire rod of strength parameters Rm min. 820 MPa and Rpo,2 min. 650 MPa for production of coupling elements of class 9.8 with omission of thermal treatment during production.
The steel wire rod is a product obtained in the process of hot rolling that has small - in relation to length - usually round cross-section. Round wire rod is usually used for production of high strength coupling elements employed for installation of machines and devices parts and structures made of steel, aluminium or plastics.
Usually, during production of coupling elements and machine elements two types of thermal treatment are used, namely (i) spheroidizing annealing before cold drawing and forging that leads to soft structure with spherical particles of cementite and (ii) quenching and tempering after the operation of forming a coupling element that includes quenching after austenitization and tempering.
The most expensive and having adverse impact on the environment stage of the coupling elements production technology is the spheroidizing annealing that, depending on the steel grade, may take even up to 24 h. Necessity to use high temperatures and long annealing time has been an important argument in the search for new solutions in order to eliminate thermal treatment in the coupling elements production technology. Corns was one of the first companies trying to develop steel for manufacturing wire rods for high strength coupling elements with omission of thermal treatment processes and introduced a dual-phase steel wire rod (Dupla™) to production.
The new grades of steel used during the production process of coupling elements must characterize with good plasticity during the cold forming of the head as well as should show sufficient level of strength and high susceptibility to strengthening during shaping processes so that it is possible to obtain the required properties in the final product. The mentioned combination of strength and susceptibility for strengthening is possible for example in the mentioned dual-phase steel mostly due to ferritic microstructure, containing also martenzite, bainite and austenite with share of 10 do 20%.
Bainitic steels in the form of wire rod of low or very low carbon content are found to be especially well suited for production of coupling elements with omission of thermal treatment processes due to their high strength after rolling which fact in combination with good susceptibility for strengthening within the cold forming process allows for obtaining the required target strength parameters after drawing and forging processes. For example, bainitic steels for cold forming of coupling elements are disclosed in the following patent applications EP0851038A1, EP1565587A1, EP1780293 A2 and FR 2867785. The main process concept disclosed in the aforementioned documents consists in the selection of carbon content and alloying elements in order to obtain the required mechanical parameters. On the other hand, due to considerable content of hardenability increasing components, such as Mo, Ni or Cr - steels mentioned in the said documents are expensive and in case of bulk or mass production of coupling elements - this reduces the opportunity for their efficient and profitable application within a production process.
To eliminate the problem, the document EP2199422 A1 discloses the application of carbide-free steel of grained bainite precipitate strengthened structure for production of coupling elements, especially of class 8.8 and with omission of additional thermal treatment processes. According to the mentioned document, the following chemical composition of bainite steel was determined: C within the range 0.04 - 0.15 % by weight, Mn within the range 1.8 - 2.0 % by weight, Si within the range 0.15 — 0.30 % by weight, S below 0.025% by weight, P below 0.025% by weight, Cr below 0.5% by weight, Mo below 0.08% by weight, sum of Ni and Cu content above 0.3% by weight, A1 within the range 0.01 - 0.05% by weight, V below 0.05% by weight, B within the range 10 - 30 ppm, N below 100 ppm and Ti above 0.06% by weight. The production method itself includes conventional hot rolling with the final rolling temperature about ~ 1000 ° C, followed by cooling at rate > 3K / s within the temperature range between 800 and 500 °C.
Similarly, the document EP2557185 A1 discloses chemical composition of steel and method of manufacturing steel elements of diameter within the range between 6 to 40 mm. Composition of steel according to the said document is selected in order to obtain the highest possible temperature of austenite conversion into bainitic structures therefore the produced bainitic steel is subject to tempering during the production process (in-situ) and is more stable under temperature impact comparing to martensitic steel or lower bainite. In opposite to conventional ferritic structure, it characterizes with necessary strength without additional drawing operation - it is plastic enough to enable cold forming through bending or rolling. The following composition of steel is proposed in the disclosure: C within the range 0.06 - 0.13% by weight, Si within the range 0.05 - 0.50% by weight, Mn within the range 1.70 - 2.50% by weight, Cr within the range 0.05 - 0.90% by weight, Ni within the range 0.05 - 0.40% by weight, Mo within the range 0.01 - 0.50% by weight, Cu within the range 0.05 - 0.30% by weight, S within the range 0.003 - 0.05% by weight, Ti within the range 0.001 - 0.10% by weight, V within the range 0.001 - 0.06% by weight, B up to 0.003% by weight, Nb up to 0.06% by weight and P within the range up to 0.05% by weight, and Fe and contamination to complement to 100%. The process of manufacturing an element of steel of the said chemical composition includes traditional rolling within the range of temperatures 1300 - 900° C and air cooling within the range of temperatures 800 - 500° C at the rate 0,5 - 3 K/sec. Structure of steel obtained as a result of such process - after the final hot rolling stage - characterizes with mean size of austenite grain below 50 pm, and the necessary level of strength is achieved without an additional stage of thermal treatment.
In case of the above described solutions, the process of rolling the wire rod is performed at the final rolling temperature of 900° C, causing that the obtained microstructure of wire rod still characterizes with low ductility. and impact strength due to too large bainitic ferrite grain. Considering the above, the objective of the invention is to develop a new method of manufacturing steel wire rod for coupling elements using low-temperature thermomechanical rolling as a result of which multi-phase steel structure is obtained. The objective of the invention is to propose a technology wherein properly selected process stages, rolling parameters and the designed chemical composition of steel would completely stop austenite recrystallization within the finishing group. Moreover, the objective of the invention is to obtain a final product that characterizes with high strength parameters as well as very good ductility, impact resistance and terminal deformability within the cold forming process.
According to the invention, in the method of producing steel wire rod of round cross-section within the hot rolling process, charge in the form of ingots obtained in the process of continuous casting is heated up in a heating furnace and then formed in the process of rolling on rolling mill stands, and the ready-made product is subject to cooling. The method characterizes with that the charge heating up stage in the heating furnace is performed at the maximum temperature within the range of 1080 - 1100°C, and then the charge is baked within time not less than 0.5 hour. The stage of forming on rolling mill stands includes preliminary rolling performed within a group of initial stands and finishing rolling performed within a group of finishing stands, wherein the band temperature at the outlet from the last stand of the initial rolling mill stands is within 1020 - 980°C. Then, the band is cooled in water boxes to temperature within the range 875±20°C, and after rolling in the finishing group the band is cooled to temperature 875-750°C before entering the coils piling machine. The stage of cooling after placing the coils is performed on Stelmor line, wherein firstly the ready-made band is cooled at average cooling rate within the range 4-10°C/s to temperature of the bainitic conversion initiation Bs ±20C, calculated according to the formula BS(°C) = 637 - 58*C - 35*Mn - 15*Ni - 34*Cr - 41*Mo, and then the band is cooled using isothermal covers with average cooling rate within the range 1 - 2°C/s to temperature 400°C, and then the coiled band is air cooled.
It is preferable when in case of a wire rod of diameter not more than 14 mm, an additional stage of intermediate rolling within the intermediate rolling mill stands group is performed before the finishing rolling, followed by cooling the band in water boxes to temperature 800-875°C, before entering the finishing unit. It is also preferable, when the charge material is micro-alloying steel where the content of Nb, Ti and V elements is selected so that their sum in steel satisfies the inequality:
%Ti+%Nb+%V < 0.25%, and so that the niobium content in steel does not exceed the maximum content that is subject to dissolving in austenite at the baking temperature of charge given using the following formula:
[Nb]=-0.2488+(2.7834* 104)*T-0.27199*C-0.17492*Ti+0.36984*N+(6.1195*10- 5)*Mn ± 0.003, and so that the total titanium content in steel is:
Ti = 0.0662- (1.4086*104)*T + 0.61539*C + 5.721*[Ti] - 0.63595*Nb + 1.6769*N + 0.0423*S ± 0.015, where T is the temperature of charge baking in C, and the content of titanium in fixed solution - austenite, [Ti], is within the range 0.03 - 0.07%.
It is preferable, when the content of the elements C, Si, S, N, S, P, Al, Ni, Cu, B, Ca and Mn is steel is determined as follows:
C > 0.03%
Si < 0.18%
0.004 % < S < 0.008%
N < 0.1%
P < 0.008%
0.020% < A1 < 0.035%
Ni + Cu < 0.29%
0.0005% < B < 0.001%
0.0016 % < Ca < 0.0035% 0.02% < Mo < 0.12%.
The wire rod of round cross-section according to the invention produced within the process of hot rolling followed by cooling in Stelmor line is characterized by that the structure of ready-made wire rod contains polygonal ferrite and irregular bainitic ferrite of grain size below 7 pm and share 80 - 90%, small islands of martensite and bainite of grain size below 7 pm and share 15-20% and particles of residual austenite of size below 3 pm and share 3-5%.
It is preferable when the diameter of the ready-made wire-rod is within 8 - 25 mm.
The developed method of wire rod manufacturing for coupling elements of strength class 8.8 and 9.8 according to the invention uses the synergistic effect of impact of the most important process parameters, including: temperature of baking the charge in the heating furnace, temperature of band during the rolling process, accelerated, controlled cooling in Stelmor line and chemical composition of steel, including in particular the controlled content of niobium (Nb), titanium (Ti) and vanadium (V) so that to eliminate the mentioned - unfavourable feature present according to the art, i.e. so that the bainitic ferrite grain size within the wire rod structure according to the invention does not exceed 7 pm. This is achieved through the control of band temperature changes during the rolling process so that in combination with niobium (Nb) impact during rolling in the finishing rolling mill stand and in combination with intensive cooling after exiting the stand, the recrystallization is stopped. The phenomenon of austenite recrystallization stoppage occurs in case when the temperature of the rolled band in the finishing group is lower than the temperature characteristic for stopping the recrystallization Tm and is caused by dynamic release of niobium dissolved in austenite at the charge baking temperature for rolling in the form of carbide NbC, causing the stoppage the process of creation and growth of the recrystallization nucleus.
The subject of the invention is presented in the embodiments and figures wherein fig. 1 presents (a) wire rod microstructure and (b) coupling element formed using it, according to Embodiment 1, and fig. 2 presents (a) wire rod microstructure and (b) coupling element formed using it, according to Embodiment 2. The developed method of producing applies in particular to a wire rod of round cross-section and of diameter within the range 8 - 25 mm, used for coupling elements of strength classes 8.8 and 9.8.
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):
According to the invention, the following basic conditions are determined:
C > 0.03%
Si < 0.18%
0.004 % < S < 0.008%
N < 0.01%
P < 0.008%
0.020% < A1 < 0.035%
Ni + Cu < 0.29%
0.0005% < B < 0.001%
0.0016 % < Ca < 0.0035%
0.02% < Mo < 0.12%, wherein Mo content is determined so that the following dependence is met
Mo= 0.02 +0.0059*(D-8) %, where D is the wire rod diameter in mm.
Content of C, Mn, Cr and Ni is determined so that the temperature of initiation of the ferritic conversion Ar3, is within the range:
710°C < Ar3 £ 760°C where temperature A is calculated using the following formula (acc. „ Steel Forming and Heat Treating Handbook”, A.A. Gomi, December 2002): Ar3 (°C) = 910 -310*C - 80*Mn - 20*Cu -15*Cr -55*Ni -80*Mo (formula 1)
An important principle is that together with the increase of wire rod diameter by 1 mm, the temperature of Ar3 should be lowered by about 3°C through modification of steel chemical composition.
One of the most important elements of the method according to the invention is the correct selection of micro-additives content of titanium (Ti), niobium (Nb) and vanadium (V) in the multi-phase steel. Content of these steel components is determined as defined below, wherein in the formulas, Nb, Ti, V mean content of the elements in steel and [Nb], [Ti] and [V] mean the content of elements in austenite - i.e. fixed solution at the charge baking temperature.
Niobium is the basic steel component according to the invention that strongly affects the austenite recrystallization course in the rolling process.
Content of this element in steel in % by weight is determined at the level in order to obtain total dissolution of carbide NbC in a billet / bloom at the temperature of charge heating and it is expressed with the formula (experimentally determined by the inventors):
[Nb]=-0.2488+(2.7834* 104)*T-0.27199*C-0.17492*Ti+0.36984*N+(6.1195*10- 5)*Mn ± 0.003 (formula 2) where T is the temperature of charge heating in °C, and the remaining symbols mean the content of elements in steel.
Value of [Nb] strongly affects the value of recrystallization stoppage temperature Tm expressed with the below dependence (experimentally determined by the inventors):
Tra(°C) = 835 +129*[Ti] +1790*[Nb] + 80*[V] ± 15 (formula 3) where [Nb], [Ti] and [V] is the content of Nb, Ti, V in austenite at the charge baking temperature for rolling resulting from dissolving carbides and nitrides of NbC, TiN, TiC present in the billet / bloom. Titanium is the micro-additive that supports niobium impact. It is to bind the whole nitrogen in the form of carbide-nitride Ti(N,C) and form carbide-sulphide T14C2S2. Titanium is introduced into steel according to the invention at the level so that after binding with nitrogen and sulphur, the content of this element in fixed solution - i.e. austenite (determined as [Ti]) is within the range 0.03 - 0.07 % by weight. Total titanium content in percentage by weight that needs to be introduced to steel is calculated using the following formula (experimentally determined by the inventors):
Ti = 0.0662- (1.4086*104)*T + 0.61539*C + 5.721*[Ti] - 0.63595*Nb + 1.6769*N + 0.0423*S ± 0.015 (formula 4) where T is the temperature of charge baking in °C, [Ti] is the content of titanium dissolved in austenite, and the remaining symbols mean the content of elements in steel.
In contrast to niobium and titanium, vanadium is added to steel in order to ensure precipitation strengthening of polygonal and bainitic ferrite with VC precipitations. Content of this element in steel is determined at the level so that the sum of all micro additives in steel satisfies the inequality:
%Ti+%Nb+%V < 0.25%.
Then, this element dissolves completely in austenite at the charge baking temperature for rolling.
Moreover, the content of nitrogen (N) is determined so that its share in percentage by weight in steel satisfies the following inequality N < 0.07*Ti.
The essence of the process layer of the method according to the invention is stopping the austenite recrystallization which is possible when the band rolling temperature is less than the recrystallization initiation temperature Tm (according to formula 3) and is caused by the dynamic release of niobium dissolved in austenite at the charge baking temperature for rolling in the form of carbide NbC.
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 - 1100 °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. The charge baking time in the heating furnace is not less than 0.5 h.
At the charge heating temperature according to the invention, there are no problems with steel decarbonization. Moreover, low charge baking temperature allows for eliminating the necessity to use too intensive cooling along the rolling line. The most important structural effect is the limitation of austenite grain size growth in the charge that favourably affects the wire rod final structure. Moreover, within the range 1080 - 1100 °C the structure of steel simultaneously contains manganese sulphide MnS and titanium carbide-sulphide T14C2S2. Because it is necessary to ensure thermodynamic balance between these compounds, sulphide and carbide-sulphide particles are small-sized meaning that they represent centres to which hydrogen diffuses and the so called delayed cracking effect is avoided.
After completion of baking, the rolling process starts that is divided to initial rolling, performed within the group of initial rolling mill stands and finishing rolling, performed in the finishing rolling mill stands group. Moreover, when the wire rod diameter does not exceed 14 mm, between the initial rolling and finishing rolling stages, the intermediate rolling is performed within the intermediate rolling mill stands group. If the wire rod diameter exceeds 14 mm, the intermediate rolling mill stands group is excluded from the production process line.
At the moment of rolling - due to low charge temperature - within the initial group, controlled accelerated band cooling is not performed. At the billet temperature 1080 - 1100°C at the furnace exit, temperature of the band at the exit from the last stand of the initial group is within 1020 - 980°C. After the initial group, accelerated, controlled cooling in three water boxes is performed leading to reduction of the band temperature to temperature 875±20°C. This temperature is the temperature of entering the intermediate rolling mill stand when the resulting diameter of wire rod is less than 14 mm. In case of the final wire rod diameter more than 14 mm, the intermediate rolling mill stand is not used.
Downstream the intermediate rolling mill stand, the band is cooled using a controlled method in two water boxes to the following temperatures:
• 800°C for wire rod diameters F8 - F12 mm; • 825°C for diameters F12.5 - F16.5 mm;
• 850°C for diameters F17 - F18.5 mm;
• 875°C for diameters F19 - F25 mm; being the temperature of entering the finishing rolling mill stand.
Downstream the finishing rolling mill stand, the band is cooled using a controlled method in 3 water boxes. When using the controlled cooling, the temperature of entering the band into the coils piling machine is determined within 875 - 750°C.
Controlled wire rod cooling according to the invention is performed in Stelmor line of length 120 m covering 9-meter long cooling zones.
Cooling in Stelmor line is performed according to the following sequence:
• in the first place and depending on the wire rod diameter, cooling is performed with average cooling rate within 4-10°C/sec. to obtain the initiation of the bainitic conversion Bs ±20°C,
• then, cooling is performed using isothermal covers with average cooling rate 1 - 2°C/sec. to temperature 400°C, and then
• the band is naturally cooled in coils.
The temperature of the bainitic conversion initiation Bs is calculated according to the following formula (acc. to „Steel Forming and Heat Treating Handbook”, A. A. Gorni, December 2002):
Bs(C) = 637 - 58*C - 35*Mn - 15*Ni - 34*Cr - 41*Mo (formula 5) where C, Mn, Ni, Cr and Mo is the content of elements in steel in % by weight.
Combination of synergistic impact of the steel chemical composition and controlled temperature within the whole process of producing the wire rod, starting from heating and baking the charge, leads to formation of completely non-crystallized austenite structure at the exit from the finishing rolling group.
The non-crystallized austenite structure is optimum for obtaining the required structure of the final wire rod in the cooling process using Stelmor line of the following composition: • polygonal ferrite and irregular bainitic ferrite without iron carbides, of grain size below 7 pm and share 80 - 90%;
• small islands of martensite and bainite of grain size below 7 pm and share 15-
20%;
• residual austenite particles of size below 3 pm and share 3-5%.
The obtained steel microstructure does not contain particles of iron carbides. Moreover, the structure does not contain cementite particles which is the result of stopping the austenite recrystallization within the hot rolling process. Structure of wire rod of such characteristics ensures obtaining relationships between the tensile strength and yield strength Rm/Rpo,2 > 1.2.
Contrary to solutions according to the art, the obtained steel structure is subject to strong strengthening during plastic deformation which is the required result both in terms of technological plasticity of wire rod in the cold forging process and in terms of obtaining high strength of the final product. Additional results can be obtained, comparing to conventional processes, applying the synergistic effect of impact of the above characterized hot rolling process parameters as well as selected content of micro additives and phase conversions in the steel structure within controlled cooling process in Stelmor line. Firstly, the non-crystallized austenite structure of wire rod, obtained as a result of using the method according to the invention, prevents the release of cementite during cooling and phase conversions, which in case of conventional processes is obtained through adding Si or A1 to steel in amount 1-2%. Moreover, due to low charge baking temperature, there phenomenon of decarbonization does not occur at the wire rod surface and easy galvanization of coupling elements made of wire rod is possible. In addition, the method according to the invention counteracts the phenomenon of delayed cracking because of MnS, T14C2S2 and V4C3 particles of controlled morphology, present in the bainitic ferrite matrix. Steel structure according to the invention characterizes with high strength parameters necessary to produce coupling elements of the required classes 8.8 and 9.8 as well as very good ductility, impact strength and terminal deformability.
Embodiments of strength parameters according to the invention related to the specified quantitative and qualitative chemical composition values of steel and the applied process parameters are described below. Embodiment 1
Billet of cross-section 160 mm x 160 mm of chemical composition given in table 1 was rolled in to a wire rod of diameter F12.5 mm with baking at temperature 1100°C within lh, and then rolled at the band temperature 825 °C at the entry to the finishing rolling mill stand group. Accelerated, controlled cooling in Stelmor line was performed at rate 9°C/s to temperature 580°C. Then, wire rod was slowly cooled under covers at rate 2°C/s to temperature 450°C.
Table 1. Chemical composition (% by weight) of multi -phase steel for manufacturing coupling elements of strength class 8.8.
According to formula 3, temperature Tm for the chemical composition of steel is 860°C. Therefore, in the finishing rolling group, the austenite recrystallization is stopped. As a result of the applied process parameters, wire rod characterized with a structure containing 90% of allotrimorphous and bainitic ferrite of grain size 6 m and containing 10% of martensitic islands of grain size 6 pm. Moreover, the structure contains carbide particles (Ti,Nb)C.
The following mechanical properties of wire rod are obtained:
• Rp02 = 643 MPa,
• Rm = 725 MPa;
• As = 26.5%.
The obtained structure of wire rod of F12.5 mm is presented in fig. la, where FP symbol applies to polyginal ferrite and M is martensite. Coupling element made of the wire rod of strength class 8.8 is presented in fig. lb.
Embodiment 2 Billet of cross-section 160 mm x 160 mm of chemical composition given in table 2 was rolled in to a wire rod of diameter F12.5 mm with baking at temperature 1100°C within lh, and then rolled at the band temperature 825 °C at the entry to the finishing rolling mill stand group. Accelerated, controlled cooling in Stelmor line was performed at rate 9°C/s to temperature 570°C. Then, wire rod was slowly cooled under covers at rate 2°C/s to temperature 450°C.
Table 2. Chemical composition (% by weight) of multi-phase steel for manufacturing coupling elements of strength class 9.8.
According to formula 3, temperature Tm for the chemical composition of steel is 870C. Therefore, in the finishing rolling group, the austenite recrystallization is stopped. As a result of this, wire rod of structure containing 80% of allotrimorphous and bainitic ferrite of grain size 5 m and containing 17% of martensitic and bainitic islands of grain size 4 pm was obtained. Moreover, the wire rod structure was found to include the presence of residual austenite of share 3% and carbide particles (Ti,Nb)C.
The following mechanical properties of wire rod are obtained:
• Rp02 = 723 MPa,
• Rm = 820 MPa;
• As = 24.4%
The obtained structure of wire rod of F12.5 mm is presented in fig. 2a, where FP is polygonal ferrite, and M is martensite and A is residual austenite. Coupling element made of the wire rod of strength class 9.8 is presented in fig. 2b.
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 wire rod of round cross-section within the hot rolling process, wherein charge in the form of ingots obtained in the process of continuous casting is heated up in a heating furnace and then formed in the process of rolling on rolling mill stands, and the ready-made product is subject to cooling, characterized in that the charge heating up stage in the heating furnace is performed at the maximum temperature within the range of 1080 - 1100°C, and then the charge is baked within time not less than 0.5 hour, the stage of forming on rolling mill stands includes preliminary rolling performed within a group of initial stands and finishing rolling performed within a group of finishing stands, wherein the band temperature at the outlet from the last stand of the initial rolling mill stands is within 1020 - 980°C then, the band is cooled in water boxes to temperature within the range 875±20°C, and after rolling in the finishing group the band is cooled to temperature 875-750°C before entering the coils piling machine in that the stage of cooling after placing the coils is performed on Stelmor line, wherein firstly the ready-made band is cooled at average cooling rate within the range 4-10°C/s to temperature of the bainitic conversion initiation BS±20C, calculated according to the formula BS(°C) = 637 - 58*C - 35*Mn - 15*Ni - 34*Cr - 41*Mo, and then the band is cooled using isothermal covers with average cooling rate within the range 1 - 2°C/s to temperature 400°C, and then the coiled band is air cooled.
2. Method according to claim 1, characterized in that in case of a wire rod of diameter not more than 14 mm, an additional stage of intermediate rolling within the intermediate rolling mill stands group is performed before the finishing rolling, followed by cooling the band in water boxes to temperature 800-875°C, before entering the finishing unit.
3. Method according to claim 1 or 2, characterized in that the charge material is micro alloying steel where the content of Nb, Ti and V elements is selected so that their sum in steel satisfies the inequality: %Ti+%Nb+%V < 0.25%, and so that the niobium content in steel does not exceed the maximum content that is subject to dissolving in austenite at the baking temperature of charge given using the following formula:
[Nb]=-0.2488+(2.7834* 104)*T-0.27199*C-0.17492*Ti+0.36984*N+(6.1195*10- 5)*Mn ± 0.003, and so that the total titanium content in steel is:
Ti = 0.0662- (1.4086*104)*T + 0.61539*C + 5.721*[Ti] - 0.63595*Nb + 1.6769*N + 0.0423*S ± 0.015, where T is the temperature of charge baking in C, and the content of titanium in fixed solution - austenite, [Ti], is within the range 0.03 - 0.07%.
4. Method according to claim 1 or 2 or 3, characterized in that the content of the elements C, Si, S, N, S, P, Al, Ni, Cu, B, Ca and Mn is steel is determined as follows:
C > 0.03%
Si < 0.18%
0.004 % < S < 0.008%
N < 0.01%
P < 0.008%
0.020% < Al < 0.035%
Ni + Cu < 0.29%
0.0005% < B < 0.001%
0.0016 % < Ca < 0.0035%
0.02% < Mo < 0.12%.
5. The wire rod of round cross-section produced within the process of hot rolling followed by cooling in Stelmor line characterized in that the structure of ready-made wire rod contains polygonal ferrite and irregular bainitic ferrite of grain size below 7 pm and share 80 - 90%, small islands of martensite and bainite of grain size below 7 pm and share 15-20% and particles of residual austenite of size below 3 pm and share 3- 5%.
6. Wire rod according to claim 5, characterized in that the diameter of the ready-made wire-rod is within 8 - 25 mm.
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