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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • 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
  • C22C38/04—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
  • C22C38/12—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
  • 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 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.
• 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.
• 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.
• 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.
• 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.
• passes 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.
• 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.
• slow cooling of the coil after coiling promotes the increase of yield strength due to the precipitation strengthening.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ m).
• 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.
• 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.
• 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 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 ⁇ m and volume fraction of 75-85% and martensitic and bainitic islands of size less than 10 ⁇ m and volume fraction of 15-25%.
• 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.
• 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.
• Mo molybdenum
• Al aluminium
• N nitrogen
• S sulphur
• 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.
• 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.
• Phase transformation of austenite gives fine ferrite grain of size in the range of 4-6 ⁇ m. 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.
• 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:
• 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.
• 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.
• they bind nitrogen (N) that unfavourably affects the mechanical properties of flat bars in the form of nitrides (Ti,Nb)N.
• NbC carbide particles dynamically released during the process of rolling, inhibiting the austenite recrystallization.
• 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.
• thermo-mechanically rolled sheets 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.
• 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.

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

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• 2020
  • 2020-01-17 PL PL432599A patent/PL239419B1/pl unknown
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  • 2020-12-22 WO PCT/IB2020/062318 patent/WO2021144643A1/en not_active Ceased
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