EP4416312A1 - High hardness low alloyed hot rolled steel and method of manufacturing thereof - Google Patents

High hardness low alloyed hot rolled steel and method of manufacturing thereof

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Publication number
EP4416312A1
EP4416312A1 EP22800790.2A EP22800790A EP4416312A1 EP 4416312 A1 EP4416312 A1 EP 4416312A1 EP 22800790 A EP22800790 A EP 22800790A EP 4416312 A1 EP4416312 A1 EP 4416312A1
Authority
EP
European Patent Office
Prior art keywords
hot rolled
rolled steel
high hardness
alloyed hot
low alloyed
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
EP22800790.2A
Other languages
German (de)
French (fr)
Inventor
Chiradeep Ghosh
Rahul Kumar
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.)
Tata Steel Ltd
Original Assignee
Tata Steel Ltd
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 Tata Steel Ltd filed Critical Tata Steel Ltd
Publication of EP4416312A1 publication Critical patent/EP4416312A1/en
Pending legal-status Critical Current

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Classifications

    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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 present invention relates to a high hardness low alloyed hot rolled steel, and more particularly to the high hardness wear resistant hot rolled steel having improved abrasion resistance and impact toughness, and method of manufacturing the high hardness low alloyed hot rolled steel.
  • Wear involves gradual decay of material from any surface due to relative motion between the active and counter body species. This can either happen due to mechanical action or chemical reaction.
  • abrasive wear resistant material is of high demand. Surprisingly, the abrasive wear alone contributes as high as 63% of the total wear loss, which is undesirable. With better design or by selecting some alternative materials, this can be reduced to a great extent.
  • a new design of a component is often coupled with other associated risks, and also does not always make an economically viable alternative. In contrary, selecting a better material sometime considered as a better option.
  • Another objective of the present invention is to provide a high hardness low alloyed hot rolled steel sheet, having the following composition in weight%: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45- 0.55, Al: 0.15-0.25, carbon equivalent: 0.34-0.69, and the balance being Iron (Fe) and unavoidable impurities and a microstructure consisting of ferrite, austenite, pearlite, bainite and martensite.
  • Another objective of present invention is to produce a method to manufacture the high hardness low alloyed hot rolled steel possessing sufficient amount of martensite by controlling cooling rate after finish rolling in the run-outtable (ROT) during hot rolling operation.
  • ROT run-outtable
  • Another objective of present invention is to estimate the saturation cooling rate to obtain the maximum possible hardness in the said composition range.
  • It is yet another objective of the present invention, is to provide new easier manufacturing method combining thermomechanical, and heat treatment processes for the proposed chemical composition to manufacture the high hardness low alloyed hot rolled steel having improved abrasion resistance, impact toughness, hardness, and weldability.
  • the high hardness low alloyed hot rolled steel comprises the following composition expressed in weight %: Carbon (C): 0.10% - 0.25%, Manganese (Mn): 1.05% - 1.5%, Chromium (Cr): 0.15-0.25%, Silicon (Si): 0.25%- 0.35%, Nickel (Ni): 0.45%-0.55%, Aluminium (Al): 0.15-0.25%, Molybdenum (Mo): 0.05%-0.15%, Vanadium: maximum 0.03%, carbon equivalent: 0.34 - 0.69 and the remaining being substantially iron and incidental impurities.
  • the high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
  • the high hardness low alloyed hot rolled steel has a hardness (HV 1 kg) > 200 Hv. In an embodiment, the high hardness low alloyed hot rolled steel has hardness (HV 1 kg) in the range of 200 - 460 Hv.
  • the high hardness low alloyed hot rolled steel has a tensile strength > 600 MPa. In an embodiment, the high hardness low alloyed hot rolled steel has a yield strength > 500 MPa. In an embodiment, the high hardness low alloyed hot rolled steel has total elongation in range 10 to 20%.
  • the Mn content of the high hardness low alloyed hot rolled steel is kept below 1.8 wt.% to avoid center-line segregation.
  • the C content in the high hardness low alloyed hot rolled steel helps in stabilizing the austenite at room temperature.
  • the Si content of the high hardness low alloyed hot rolled steel is kept in the range between 0.25- 0.35 wt.% to partially retard cementite formation.
  • the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.15, Mn - 1.1, Si - 0.3, Cr - 0.2, V - 0.03, Ni - 0.5, Al - 0.2, Mo - 0.1, carbon equivalent 0.43 and the balance being Iron (Fe) and unavoidable impurities.
  • the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.21, Mn - 1.47, Si - 0.3, Cr - 0.2, Ni - 0.5, Mo - 0.1, carbon equivalent 0.55, and the balance being Iron (Fe) and unavoidable impurities.
  • the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 220-380 Hv.
  • the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 300-460 Hv.
  • a method for manufacturing high hardness low alloyed hot rolled steel sheet comprises casting steel slab having a composition expressed in weight %: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45-0.55, Al: 0.15-0.25, carbon equivalent: 0.34-0.69, and the balance being Iron (Fe) and unavoidable impurities.
  • the method also comprises reheating the steel slab to a temperature greater than 1150°C.
  • the method further comprises hot rolling the steel slab to about 70% deformation to produce a steel sheet such that finish rolling is done at a temperature (TERT).
  • the TFRT varies in the range 830°C to 890°C.
  • the method comprises cooling at a cooling rate in the range of 5°C/s - 60°C/s till a room temperature is reached to obtain the high hardness low alloyed hot rolled steel sheet.
  • the high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
  • the high hardness low alloyed hot rolled steel has a tensile strength > 600 MPa and yield strength > 500 MPa, total elongation in range 10 to 20%.
  • the TFRT is 870°C.
  • the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.15, Mn - 1.1, Si - 0.3, Cr - 0.2, V - 0.03, Ni - 0.5, Al - 0.2, Mo - 0.1, carbon equivalent 0.43, and the balance being Iron (Fe) and unavoidable impurities.
  • the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.21, Mn - 1.47, Si - 0.3, Cr - 0.2, Ni - 0.5, Mo - 0.1, carbon equivalent 0.55, and the balance being Iron (Fe) and unavoidable impurities.
  • the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 220-380 Hv.
  • the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 300-460 Hv.
  • the saturation cooling rate exists for the high hardness low alloyed hot rolled steel in the range of 20-30 °C/s.
  • the maximum hardness (HV 1kg) high hardness low alloyed hot rolled steel is in the range of 360-380 Hv.
  • the maximum hardness (HV 1kg) of the high hardness low alloyed hot rolled steel is in the range of 450-460 Hv.
  • the steel slab is reheated to a temperature between 1150°C - 1250°C for a duration of 20 minutes to 2 hours depending on the slab thickness.
  • the thickness of the steel sheet is in the range of 4 mm - 8 mm.
  • a component produced from the high hardness low alloyed hot rolled steel is used in structural as well as wear resistant applications of the L&E equipment, particularly used in mining and earthmoving applications.
  • Figure 1 illustrates a flowchart of a method of manufacturing a high hardness low alloyed hot rolled steel sheet, according to an embodiment of the present invention
  • Figure 2 illustrates a schematic diagram of cooling technique followed after rolling (in the run-out table of the hot-strip mill) during manufacture of the high hardness low alloyed hot rolled steel sheet, according to the embodiment of the present invention
  • Figure 3 illustrates a graphical representation of variation of hardness versus cooling rates of examples (steel 1 and steel 2) of the high hardness low alloyed hot rolled steel, according to the embodiments of present invention.
  • the high hardness low alloyed hot rolled steel having a minimum tensile strength of 600 MPa comprises the following composition expressed in weight %: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45-0.55, Al: 0.15-0.25, carbon equivalent: 0.34- 0.69, and the balance being Iron (Fe) and unavoidable impurities.
  • the high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
  • the high hardness low alloyed hot rolled steel has a hardness > 200 Hv.
  • the high hardness low alloyed hot rolled steel has a hardness in the range of 200-460 Hv.
  • the high hardness low alloyed hot rolled steel has a yield strength > 500 MPa, and total elongation in the range between 10- 20%.
  • the high hardness low alloyed hot rolled steel has improved abrasion resistance, impact toughness, hardness, and weldability.
  • the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.15, Mn - 1.1, Si - 0.3, Cr - 0.2, V - 0.03, Ni - 0.5, Al - 0.2, Mo - 0.1, carbon equivalent 0.43 and the balance being Iron (Fe) and unavoidable impurities.
  • the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 220-380 Hv.
  • the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.21, Mn - 1.47, Si - 0.3, Cr - 0.2, Ni - 0.5, Mo - 0.1, carbon equivalent 0.55, and the balance being Iron (Fe) and unavoidable impurities.
  • the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 300-460 Hv.
  • the martensite in the microstructure provides excellent resistance to abrasive wear and improves the impact toughness.
  • the high hardness low alloyed hot rolled steel is suitable for producing components to be used in structural as well as wear resistant applications of the Lifting & Excavating equipment, particularly used in mining and earthmoving applications.
  • the method (100) of manufacturing the high hardness low alloyed hot rolled steel sheet of the desired composition comprises casting molten steel having composition expressed in weight %: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45-0.55, Al: 0.15-0.25, carbon equivalent: 0.34-0.69, and the balance being Iron (Fe) and unavoidable impurities.
  • the molten steel is cast in a casting apparatus to obtain steel slabs (cast ingots). In the illustrated example, the steel is cast either in a conventional continuous caster or thin-slab caster.
  • the method (100) comprises reheating the steel slab (steel casting) to a temperature greater than 1150°C.
  • the slab is reheated to temperature ranging between 1150 to 1250°C for a duration of 20 minutes to 2 hours depending on the slab thickness.
  • a reheating temperature greater than 1250°C is also undesirable because it may lead to excessive grain coarsening of austenite and/or scale loss.
  • the method (100) comprises hot rolling the steel slab to about 70% deformation to produce a steel sheet such that finish rolling is done at a temperature (TFRT).
  • TFRT temperature
  • the TFRT is varied in the range 830°C to 890°C, more preferably 870°C.
  • the slabs of higher thicknesses are rough rolled in roughing stands in a conventional hot-rolling mill.
  • the rough rolling is done above the recrystallization temperature.
  • the finish rolling is carried out in a 6 or 7 stands tandem rolling mill.
  • the rolling is finished at the finish rolling temperature, TFRT given by such that 830 ⁇ TFRT ⁇ 890°C.
  • the above range of the finish rolling temperature (TFRT) is chosen to finish the hot rolling in the austenitic range.
  • the TFRT is kept below T m to exploit the thermo-mechanical controlled rolling.
  • the method (100) comprises cooling at a cooling rate in the range of 5°C/s - 60°C/s till a room temperature is reached to obtain the high hardness low alloyed hot rolled steel sheet.
  • the cooling rate is kept between 5°C/s - 60°C/s so as to ensure sufficient amount of martensite formation in the microstructure.
  • the steel is cooled to allow development of higher amount of martensite in the final microstructure, which will give rise to the improved abrasion resistance, impact toughness, hardness, and weldability.
  • the obtained high hardness low alloyed hot rolled steel sheet has microstructure represented by, in area%, maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
  • the high hardness low alloyed hot rolled steel has a hardness > 200 Hv. More particularly, in the range of 200-460 Hv.
  • the high hardness low alloyed hot rolled steel sheet exhibits tensile strength greater than 600 MPa, total elongation in the range of 10-20%, and yield strength greater than 500 MPa.
  • the thickness of the steel sheet is in the range of 4 mm - 8 mm.
  • the disclosed invention method (100) it is possible to manufacture the high hardness hot-rolled steel sheet with a minimum 600 MPa tensile strength consisting of ferrite + pearlite + bainite + austenite + martensite microstructure which has improved abrasion resistance, impact toughness, hardness, and weldability.
  • a steel sheet is suited for producing components to be used in structural as well as wear resistant applications of the L&E equipment, particularly used in mining and earthmoving applications where good abrasive wear resistance and impact toughness is a requirement along with the high hardness.
  • C 0.10-0.25%: The presence of carbon is inevitable in any commercial steel. Carbon is added to control the strength of the steel. Depending on the final requirement, the weight percentage of the carbon addition changes. After quenching from the austenitic region to the partitioning temperature, the microstructure mainly constitutes of austenite and martensite. Depending on the partitioning temperature and composition of the base steel, carbon gets rejected from the supersaturated martensite. This carbon migrates from martensite to austenite. As a result of the carbon enrichment of the austenite, the stability of this phase increases. Afterwards, at room temperature the presence of austenite in the microstructure can also be visible. This austenite improves the mechanical property of the steel.
  • Si 0.25-0.35%: Si is added during steel making process as an effective deoxidizer. Additionally, Si is a very efficient solid solution strengthener. Furthermore, cementite precipitation can be retarded to a large extent by Si. This is attributed to the build-up of Si in front of the advancing cementite/matrix interface. Since Si has little solubility in cementite, it is rejected by a growing cementite particle causing an enrichment of Si at the interface. Now for the continuous precipitation, Si should diffuse away from the interface. This does not happen. As a consequence, this creates a favourable situation for carbon partitioning to take place instead of carbide precipitation. However, higher amount of Si in steel creates some operational issue in terms of surface appearance of the steel.
  • Al 0.15-0.25%: Al is also added in molten steel as a deoxidizer. The presence of nitrogen in steel creates an environment to form AIN precipitate. This precipitate limits the grain growth during reheating of the steel slab before hot rolling. Furthermore, like Si, Al also exhibits to suppress the carbide precipitation in steel. However, it does not create any operational hindrance during hot rolling. Therefore, in the current context Al is added between 0.15-0.25 wt.%.
  • Mn 1.05-1.50%: Mn is a mild solid solution strengthener. It decreases the ductile to brittle transition temperature. More importantly Mn is an austenite stabilizer. It delays the austenite decomposition during accelerated cooling. As a consequence, finish rolling can be performed more effectively in the austenitic region and more amount of deformation can be imparted. This creates a finer ferrite grain size which in turn increases the strength of the steel. However, at higher Mn content there is always a chance for centerline segregation in the steel due to low diffusivity of Mn. Therefore, in the present context Mn content is restricted between 1.05-1.50 wt.%.
  • Cr 0.15-0.25%: Cr improves the hardenability of the steel to a large extent. It retards the formation of pearlite during continuous cooling from the austenite region. Thus, helps in formation of the martensite event at a nominal cooling rate. Therefore, the Cr content is restricted between 0.15-0.25 wt.%.
  • Mo 0.05-0.15%: Mo is also added to primarily to improve the hardenability of the steel. Although Mo impairs the bainite formation to a certain extent, it retards the pearlite formation to a much greater degree. Considering this, Mo is added in the range of 0.05-0.15 wt.%.
  • Ni 0.45-0.55%: Ni is primarily added in steel to improve the fracture toughness. Ni induces grain refinement by reducing the transformation temperature. This in turn increases the number of grain boundaries which is responsible for better fracture toughness properties. Keeping this in mind Ni has been added in the range 0.45-0.55 wt.% in the current context.
  • V 0.03% or less: V forms VN/VC in the ferrite region thereby increases the strength of the material. This is the reason for V addition in the current steel.
  • Microstructure The final set of desired properties in the high hardness low alloyed hot-rolled steel is achieved by the presence of ferrite, bainite, pearlite, austenite and martensite. All the hot-rolling, controlled cooling and coiling conditions have significance in achieving the final microstructure and properties. The contribution of the each of the phases i.e. ferrite, pearlite, austenite, bainite and martensite are described below.
  • Ferrite The final hot-rolled microstructure contains a small amount 10% ferrite, which is strengthened by the contributions from the alloying elements mentioned above.
  • Pearlite A small amount of fine pearlite (20% maximum) is observed in the final microstructure. A small amount of fine pearlite imparts strength in the final microstructure.
  • Austenite A small amount of austenite (10% maximum) is observed in the final microstructure. This is essential to impart toughness in the final product.
  • Bainite A small amount of bainite (20% maximum) is observed in the final microstructure. This contributes towards the final strength and toughness of the product.
  • Martensite The final hot-rolled microstructure contains 60-100% martensite.
  • the martensite in the microstructure ensures the hardness of 200-460 Hv is achieved in the final hot-rolled steel sheet.
  • Table -2 Variation in hardness with cooling rate.
  • the present invention provides the high hardness low alloyed hot rolled steel having improved abrasion resistance, impact toughness, hardness, and weldability.
  • the martensite present in the microstructure provides higher hardness for the produced steel.
  • the disclosed steel is suitable for producing components to be used in structural as well as wear resistant applications of the L&E equipment, particularly used in mining and earthmoving applications.
  • the invention also provides a method to manufacture the high hardness low alloyed hot rolled steel having higher amount of martensite by controlling cooling rate after finish rolling in the run-out-table (ROT) during hot rolling operation.
  • the high hardness low alloyed hot rolled steel makes an important contribution towards durable, cost effective, futuristic and strategic application of steel with greater factor of safety. Further the steel having high hardness and good wear resistance may be used in the sectors of engineering, mining, construction, agriculture, port, electric power and metallurgy etc.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

A high hardness low alloyed hot rolled steel having improved abrasion resistance, impact toughness, hardness, and weldability is provided. The high hardness low alloyed hot rolled steel comprises the following composition expressed in weight %: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45-0.55, Al: 0.15-0.25, carbon equivalent: 0.34-0.69, and the balance being Iron (Fe) and unavoidable impurities. The high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60-100 % martensite. The high hardness low alloyed hot rolled steel exhibits tensile strength ≥ 600 MPa, a total elongation 10-20%, a hardness ≥ 200 Hv, and a yield strength ≥ 500 MPa. A method (100) for manufacturing high hardness low alloyed hot rolled steel strip of the above-mentioned composition is also provided.

Description

HIGH HARDNESS LOW ALLOYED HOT ROLLED STEEL AND METHOD OF MANUFACTURING THEREOF
FIELD OF INVENTION
[0001] The present invention relates to a high hardness low alloyed hot rolled steel, and more particularly to the high hardness wear resistant hot rolled steel having improved abrasion resistance and impact toughness, and method of manufacturing the high hardness low alloyed hot rolled steel.
BACKGROUND
[0002] Wear involves gradual decay of material from any surface due to relative motion between the active and counter body species. This can either happen due to mechanical action or chemical reaction. For specific engineering applications, like in L&E or mining sectors, abrasive wear resistant material is of high demand. Surprisingly, the abrasive wear alone contributes as high as 63% of the total wear loss, which is undesirable. With better design or by selecting some alternative materials, this can be reduced to a great extent. However, a new design of a component is often coupled with other associated risks, and also does not always make an economically viable alternative. In contrary, selecting a better material sometime considered as a better option.
[0003] In recent times, medium to high carbon steels containing martensite or tempered martensitic microstructure are being used for most of the wear resistant applications. The wear resistivity of steel is greatly improved with surface hardness, which is directly proportional to the amount of martensite that can be generated in steel matrix. In other words, higher the amount of martensite in the microstructure, higher is the hardness of the steel. This is the reason why there is a unidirectional effort all across the globe to manufacture martensitic steel for wear resistant application. However, to produce martensite is a challenging task especially in low alloy steel.
[0004] The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts. OBJECTIVE OF INVENTION
[0005] It is an object of the invention to solve the problems of the prior art and to develop a high hardness low alloyed hot rolled wear resistant steel for L&E and mining applications.
[0006] Another objective of the present invention is to provide a high hardness low alloyed hot rolled steel sheet, having the following composition in weight%: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45- 0.55, Al: 0.15-0.25, carbon equivalent: 0.34-0.69, and the balance being Iron (Fe) and unavoidable impurities and a microstructure consisting of ferrite, austenite, pearlite, bainite and martensite.
[0007] Another objective of present invention is to produce a method to manufacture the high hardness low alloyed hot rolled steel possessing sufficient amount of martensite by controlling cooling rate after finish rolling in the run-outtable (ROT) during hot rolling operation.
[0008] Another objective of present invention is to estimate the saturation cooling rate to obtain the maximum possible hardness in the said composition range.
[0009] It is yet another objective of the present invention, is to provide new easier manufacturing method combining thermomechanical, and heat treatment processes for the proposed chemical composition to manufacture the high hardness low alloyed hot rolled steel having improved abrasion resistance, impact toughness, hardness, and weldability.
SUMMARY OF INVENTION
[0010] This summary is provided to introduce concepts related to a high hardness low alloyed hot rolled steel having improved abrasion resistance, impact toughness, hardness, and weldability, and a method of manufacturing the high hardness low alloyed hot rolled steel sheet. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. [0011] In one aspect of the present invention, a high hardness low alloyed hot rolled steel is provided. The high hardness low alloyed hot rolled steel comprises the following composition expressed in weight %: Carbon (C): 0.10% - 0.25%, Manganese (Mn): 1.05% - 1.5%, Chromium (Cr): 0.15-0.25%, Silicon (Si): 0.25%- 0.35%, Nickel (Ni): 0.45%-0.55%, Aluminium (Al): 0.15-0.25%, Molybdenum (Mo): 0.05%-0.15%, Vanadium: maximum 0.03%, carbon equivalent: 0.34 - 0.69 and the remaining being substantially iron and incidental impurities. The high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
[0012] In an embodiment, the high hardness low alloyed hot rolled steel has a hardness (HV 1 kg) > 200 Hv. In an embodiment, the high hardness low alloyed hot rolled steel has hardness (HV 1 kg) in the range of 200 - 460 Hv.
[0013] In an embodiment, the high hardness low alloyed hot rolled steel has a tensile strength > 600 MPa. In an embodiment, the high hardness low alloyed hot rolled steel has a yield strength > 500 MPa. In an embodiment, the high hardness low alloyed hot rolled steel has total elongation in range 10 to 20%.
[0014] In an embodiment, the Mn content of the high hardness low alloyed hot rolled steel is kept below 1.8 wt.% to avoid center-line segregation. In an embodiment, the C content in the high hardness low alloyed hot rolled steel helps in stabilizing the austenite at room temperature. In an embodiment, the Si content of the high hardness low alloyed hot rolled steel is kept in the range between 0.25- 0.35 wt.% to partially retard cementite formation.
[0015] In an embodiment, the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.15, Mn - 1.1, Si - 0.3, Cr - 0.2, V - 0.03, Ni - 0.5, Al - 0.2, Mo - 0.1, carbon equivalent 0.43 and the balance being Iron (Fe) and unavoidable impurities.
[0016] In an embodiment, the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.21, Mn - 1.47, Si - 0.3, Cr - 0.2, Ni - 0.5, Mo - 0.1, carbon equivalent 0.55, and the balance being Iron (Fe) and unavoidable impurities. [0017] In an embodiment, the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 220-380 Hv.
[0018] In an embodiment, the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 300-460 Hv.
[0019] In another aspect of the present invention, a method for manufacturing high hardness low alloyed hot rolled steel sheet is provided. The method comprises casting steel slab having a composition expressed in weight %: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45-0.55, Al: 0.15-0.25, carbon equivalent: 0.34-0.69, and the balance being Iron (Fe) and unavoidable impurities. The method also comprises reheating the steel slab to a temperature greater than 1150°C. The method further comprises hot rolling the steel slab to about 70% deformation to produce a steel sheet such that finish rolling is done at a temperature (TERT). The TFRT varies in the range 830°C to 890°C. The method comprises cooling at a cooling rate in the range of 5°C/s - 60°C/s till a room temperature is reached to obtain the high hardness low alloyed hot rolled steel sheet.
[0020] In an embodiment, the high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
[0021] In an embodiment, the high hardness low alloyed hot rolled steel has a tensile strength > 600 MPa and yield strength > 500 MPa, total elongation in range 10 to 20%. In an embodiment, the TFRT is 870°C.
[0022] In an embodiment, the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.15, Mn - 1.1, Si - 0.3, Cr - 0.2, V - 0.03, Ni - 0.5, Al - 0.2, Mo - 0.1, carbon equivalent 0.43, and the balance being Iron (Fe) and unavoidable impurities.
[0023] In an embodiment, the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.21, Mn - 1.47, Si - 0.3, Cr - 0.2, Ni - 0.5, Mo - 0.1, carbon equivalent 0.55, and the balance being Iron (Fe) and unavoidable impurities. [0024] In an embodiment, the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 220-380 Hv.
[0025] In an embodiment, the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 300-460 Hv.
[0026] In an embodiment, the saturation cooling rate exists for the high hardness low alloyed hot rolled steel in the range of 20-30 °C/s.
[0027] In an embodiment, the maximum hardness (HV 1kg) high hardness low alloyed hot rolled steel is in the range of 360-380 Hv.
[0028] In an embodiment, the maximum hardness (HV 1kg) of the high hardness low alloyed hot rolled steel is in the range of 450-460 Hv.
[0029] In an embodiment, the steel slab is reheated to a temperature between 1150°C - 1250°C for a duration of 20 minutes to 2 hours depending on the slab thickness.
[0030] In an embodiment, the thickness of the steel sheet is in the range of 4 mm - 8 mm.
[0031] In an embodiment, a component produced from the high hardness low alloyed hot rolled steel. In an embodiment, the component is used in structural as well as wear resistant applications of the L&E equipment, particularly used in mining and earthmoving applications.
[0032] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Figure 1 illustrates a flowchart of a method of manufacturing a high hardness low alloyed hot rolled steel sheet, according to an embodiment of the present invention;
[0034] Figure 2 illustrates a schematic diagram of cooling technique followed after rolling (in the run-out table of the hot-strip mill) during manufacture of the high hardness low alloyed hot rolled steel sheet, according to the embodiment of the present invention; and [0035] Figure 3 illustrates a graphical representation of variation of hardness versus cooling rates of examples (steel 1 and steel 2) of the high hardness low alloyed hot rolled steel, according to the embodiments of present invention.
[0036] The drawings referred to in this description are not to be understood as being drawn to scale except if specifically noted, and such drawings are only exemplary in nature.
DETAILED DESCRIPTION
[0037] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0038] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0039] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise . It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. [0040] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0041] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0042] The high hardness low alloyed hot rolled steel having a minimum tensile strength of 600 MPa according to the present invention comprises the following composition expressed in weight %: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45-0.55, Al: 0.15-0.25, carbon equivalent: 0.34- 0.69, and the balance being Iron (Fe) and unavoidable impurities. The high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
[0043] The high hardness low alloyed hot rolled steel has a hardness > 200 Hv. In the illustrated example, the high hardness low alloyed hot rolled steel has a hardness in the range of 200-460 Hv. The high hardness low alloyed hot rolled steel has a yield strength > 500 MPa, and total elongation in the range between 10- 20%. The high hardness low alloyed hot rolled steel has improved abrasion resistance, impact toughness, hardness, and weldability.
[0044] In one illustrated example (Steel 1), the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.15, Mn - 1.1, Si - 0.3, Cr - 0.2, V - 0.03, Ni - 0.5, Al - 0.2, Mo - 0.1, carbon equivalent 0.43 and the balance being Iron (Fe) and unavoidable impurities. The high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 220-380 Hv. [0045] In another illustrated example (Steel 2), the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.21, Mn - 1.47, Si - 0.3, Cr - 0.2, Ni - 0.5, Mo - 0.1, carbon equivalent 0.55, and the balance being Iron (Fe) and unavoidable impurities. The high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 300-460 Hv.
[0046] The high hardness low alloyed hot-rolled steel sheet with a minimum 600 MPa tensile strength consisting of ferrite + pearlite + austenite + bainite + martensite. The martensite in the microstructure provides excellent resistance to abrasive wear and improves the impact toughness. The high hardness low alloyed hot rolled steel is suitable for producing components to be used in structural as well as wear resistant applications of the Lifting & Excavating equipment, particularly used in mining and earthmoving applications.
[0047] Referring to Figures 1 and 2, the method (100) of manufacturing the high hardness low alloyed hot rolled steel sheet of the desired composition is illustrated. At step (102), the method (100) comprises casting molten steel having composition expressed in weight %: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45-0.55, Al: 0.15-0.25, carbon equivalent: 0.34-0.69, and the balance being Iron (Fe) and unavoidable impurities. The molten steel is cast in a casting apparatus to obtain steel slabs (cast ingots). In the illustrated example, the steel is cast either in a conventional continuous caster or thin-slab caster.
[0048] At step (104), the method (100) comprises reheating the steel slab (steel casting) to a temperature greater than 1150°C. In the illustrated example, the slab is reheated to temperature ranging between 1150 to 1250°C for a duration of 20 minutes to 2 hours depending on the slab thickness. A reheating temperature greater than 1250°C is also undesirable because it may lead to excessive grain coarsening of austenite and/or scale loss.
[0049] At step (106), the method (100) comprises hot rolling the steel slab to about 70% deformation to produce a steel sheet such that finish rolling is done at a temperature (TFRT). In the illustrate example, the TFRT is varied in the range 830°C to 890°C, more preferably 870°C. After the steel slab is cast in the specified composition and reheated, it is hot-rolled. The slabs of higher thicknesses are rough rolled in roughing stands in a conventional hot-rolling mill. The rough rolling is done above the recrystallization temperature. Then the finish rolling is carried out in a 6 or 7 stands tandem rolling mill. The rolling is finished at the finish rolling temperature, TFRT given by such that 830 < TFRT < 890°C. The above range of the finish rolling temperature (TFRT) is chosen to finish the hot rolling in the austenitic range. Here the TFRT is kept below Tm to exploit the thermo-mechanical controlled rolling.
[0050] At step (108), the method (100) comprises cooling at a cooling rate in the range of 5°C/s - 60°C/s till a room temperature is reached to obtain the high hardness low alloyed hot rolled steel sheet. The cooling rate is kept between 5°C/s - 60°C/s so as to ensure sufficient amount of martensite formation in the microstructure.
[0051] The steel is cooled to allow development of higher amount of martensite in the final microstructure, which will give rise to the improved abrasion resistance, impact toughness, hardness, and weldability. The obtained high hardness low alloyed hot rolled steel sheet has microstructure represented by, in area%, maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite. In the illustrated example, the high hardness low alloyed hot rolled steel has a hardness > 200 Hv. More particularly, in the range of 200-460 Hv. The high hardness low alloyed hot rolled steel sheet exhibits tensile strength greater than 600 MPa, total elongation in the range of 10-20%, and yield strength greater than 500 MPa. In the illustrated example, the thickness of the steel sheet is in the range of 4 mm - 8 mm.
[0052] According to the disclosed invention method (100), it is possible to manufacture the high hardness hot-rolled steel sheet with a minimum 600 MPa tensile strength consisting of ferrite + pearlite + bainite + austenite + martensite microstructure which has improved abrasion resistance, impact toughness, hardness, and weldability. Such a steel sheet is suited for producing components to be used in structural as well as wear resistant applications of the L&E equipment, particularly used in mining and earthmoving applications where good abrasive wear resistance and impact toughness is a requirement along with the high hardness.
[0053] Following portions of the present disclosure, provides details about the proportion of each element in a composition of the high hardness low alloyed hot rolled steel sheet and their role in enhancing properties.
[0054] C: 0.10-0.25%: The presence of carbon is inevitable in any commercial steel. Carbon is added to control the strength of the steel. Depending on the final requirement, the weight percentage of the carbon addition changes. After quenching from the austenitic region to the partitioning temperature, the microstructure mainly constitutes of austenite and martensite. Depending on the partitioning temperature and composition of the base steel, carbon gets rejected from the supersaturated martensite. This carbon migrates from martensite to austenite. As a result of the carbon enrichment of the austenite, the stability of this phase increases. Afterwards, at room temperature the presence of austenite in the microstructure can also be visible. This austenite improves the mechanical property of the steel. Therefore, in other words, the presence of carbon in the steel helps in stabilizing the austenite at room temperature. However, carbon cannot be increased to a large extent in steel as it impairs the weldability of the steel. Thus, in the current steel carbon content is restricted between 0. 10 - 0.25 wt.%.
[0055] Si: 0.25-0.35%: Si is added during steel making process as an effective deoxidizer. Additionally, Si is a very efficient solid solution strengthener. Furthermore, cementite precipitation can be retarded to a large extent by Si. This is attributed to the build-up of Si in front of the advancing cementite/matrix interface. Since Si has little solubility in cementite, it is rejected by a growing cementite particle causing an enrichment of Si at the interface. Now for the continuous precipitation, Si should diffuse away from the interface. This does not happen. As a consequence, this creates a favourable situation for carbon partitioning to take place instead of carbide precipitation. However, higher amount of Si in steel creates some operational issue in terms of surface appearance of the steel. Therefore, the Si content in the current work is restricted between 0.25-0.35 wt.%. [0056] Al: 0.15-0.25%: Al is also added in molten steel as a deoxidizer. The presence of nitrogen in steel creates an environment to form AIN precipitate. This precipitate limits the grain growth during reheating of the steel slab before hot rolling. Furthermore, like Si, Al also exhibits to suppress the carbide precipitation in steel. However, it does not create any operational hindrance during hot rolling. Therefore, in the current context Al is added between 0.15-0.25 wt.%.
[0057] Mn: 1.05-1.50%: Mn is a mild solid solution strengthener. It decreases the ductile to brittle transition temperature. More importantly Mn is an austenite stabilizer. It delays the austenite decomposition during accelerated cooling. As a consequence, finish rolling can be performed more effectively in the austenitic region and more amount of deformation can be imparted. This creates a finer ferrite grain size which in turn increases the strength of the steel. However, at higher Mn content there is always a chance for centerline segregation in the steel due to low diffusivity of Mn. Therefore, in the present context Mn content is restricted between 1.05-1.50 wt.%.
[0058] Cr: 0.15-0.25%: Cr improves the hardenability of the steel to a large extent. It retards the formation of pearlite during continuous cooling from the austenite region. Thus, helps in formation of the martensite event at a nominal cooling rate. Therefore, the Cr content is restricted between 0.15-0.25 wt.%.
[0059] Mo: 0.05-0.15%: Mo is also added to primarily to improve the hardenability of the steel. Although Mo impairs the bainite formation to a certain extent, it retards the pearlite formation to a much greater degree. Considering this, Mo is added in the range of 0.05-0.15 wt.%.
[0060] Ni: 0.45-0.55%: Ni is primarily added in steel to improve the fracture toughness. Ni induces grain refinement by reducing the transformation temperature. This in turn increases the number of grain boundaries which is responsible for better fracture toughness properties. Keeping this in mind Ni has been added in the range 0.45-0.55 wt.% in the current context.
[0061] V: 0.03% or less: V forms VN/VC in the ferrite region thereby increases the strength of the material. This is the reason for V addition in the current steel. [0062] Microstructure: The final set of desired properties in the high hardness low alloyed hot-rolled steel is achieved by the presence of ferrite, bainite, pearlite, austenite and martensite. All the hot-rolling, controlled cooling and coiling conditions have significance in achieving the final microstructure and properties. The contribution of the each of the phases i.e. ferrite, pearlite, austenite, bainite and martensite are described below.
[0063] Ferrite: The final hot-rolled microstructure contains a small amount 10% ferrite, which is strengthened by the contributions from the alloying elements mentioned above.
[0064] Pearlite: A small amount of fine pearlite (20% maximum) is observed in the final microstructure. A small amount of fine pearlite imparts strength in the final microstructure.
[0065] Austenite: A small amount of austenite (10% maximum) is observed in the final microstructure. This is essential to impart toughness in the final product.
[0066] Bainite: A small amount of bainite (20% maximum) is observed in the final microstructure. This contributes towards the final strength and toughness of the product.
[0067] Martensite: The final hot-rolled microstructure contains 60-100% martensite. The martensite in the microstructure ensures the hardness of 200-460 Hv is achieved in the final hot-rolled steel sheet.
Examples
[0068] Further embodiments of the present disclosure will be now described with examples of compositions of the high hardness low alloyed hot rolled steel, which are illustrated in Table 1. Various experiments and tests were conducted on a laboratory scale in order to evaluate various conditions.
[0069] Two alloys were designed considering the following factors:
• Higher Ms temperature.
• Low carbon equivalent to eliminate the problem of weldability and low carbon content to improve impact toughness. • Higher hardenability to eliminate bainite or pearlite formation during quenching.
[0070] The chemical composition of the proposed alloys, their carbon equivalent (CE) values and Ms temperature calculated empirically as well as through dilatometry experiment are shown in Table 1.
Table -1: Chemical composition, carbon equivalent (CE) and Ms temperatures of Alloy 1 & 2
[0071] Two different alloys were designed and cast at 40 kg scale which were subsequently hot forged. In order to ensure the complete homogenization, the forged steel plates were first austenitized at 1200°C for 2 hours. After that the plates were 70% hot rolled. The finish rolling temperature (FRT) was maintained at ~870°C. After this the hot rolled plates were cooled to room temperature at different cooling rates starting from 5 °C/s to 60 °C/s. In the actual industrial setup this is achieved in the ROT section. The detailed hot rolling scheme is depicted in Figure 2. [0072] After cooling to room temperature, standard metallography technique was followed to prepare the samples for hardness measurements. This is characterized in Vickers hardness testing machine at 1 kg scale with a dwell time of 10 s. The measured hardness values for both the steels are depicted in Table 2.
Table -2: Variation in hardness with cooling rate.
[0073] This variation in hardness values with cooling rate in pictorially depicted in Figure 3 for both the steels. From the figure it appears that the hardness value increases with the increase in cooling rate. However, after a certain range there exists a saturation cooling rate, beyond which the hardness does not change. This is also shown in Figure 3. The saturation cooling rate exists for the high hardness low alloyed hot rolled steel in the range of 20-30 °C/s. It appears from the figure that beyond the cooling rate of 30 °C/s, hardness value does not change. Furthermore, the maximum hardness (HV 1kg) of the steel 1 is in the range of 360- 380 Hv. The maximum hardness (HV 1kg) of the steel 2 is in the range of 450-460 Hv. By controlling the cooling rate that is being experienced by the two alloys, the maximum attainable hardness values of these steels can be exploited, which is the prerequisite property for wear resistant plates used in L&E and mining application.
[0074] The present invention provides the high hardness low alloyed hot rolled steel having improved abrasion resistance, impact toughness, hardness, and weldability. The martensite present in the microstructure provides higher hardness for the produced steel. The disclosed steel is suitable for producing components to be used in structural as well as wear resistant applications of the L&E equipment, particularly used in mining and earthmoving applications. The invention also provides a method to manufacture the high hardness low alloyed hot rolled steel having higher amount of martensite by controlling cooling rate after finish rolling in the run-out-table (ROT) during hot rolling operation. The high hardness low alloyed hot rolled steel makes an important contribution towards durable, cost effective, futuristic and strategic application of steel with greater factor of safety. Further the steel having high hardness and good wear resistance may be used in the sectors of engineering, mining, construction, agriculture, port, electric power and metallurgy etc.
[0075] It should be understood that the experiments are carried out for particular compositions of the high hardness low alloyed hot rolled steel sheet and the results brought out in the previous paragraphs are for the composition shown in Table 1. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the high hardness low alloyed hot rolled steel strip, as well.
[0076] Furthermore, the terminology used herein is for describing embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[0077] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[0078] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

Claims

1. A high hardness low alloyed hot rolled steel comprising the following composition expressed in weight %:
Carbon (C): 0.10% - 0.25%,
Manganese (Mn): 1.05% - 1.5%,
Chromium (Cr): 0.15-0.25%,
Silicon (Si): 0.25%-0.35%,
Nickel (Ni): 0.45%-0.55%,
Aluminium (Al): 0.15-0.25%,
Molybdenum (Mo): 0.05%-0.15%, Vanadium: maximum 0.03%, carbon equivalent: 0.34 - 0.69 and the remaining being substantially iron and incidental impurities, wherein the high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
2. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the high hardness low alloyed hot rolled steel has a hardness (HV 1 kg) > 200 Hv.
3. The high hardness low alloyed hot rolled steel as claimed in the claim 2, wherein the high hardness low alloyed hot rolled steel has hardness (HV 1 kg) in the range of 200 - 460 Hv.
4. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the high hardness low alloyed hot rolled steel has a tensile strength > 600 MPa.
5. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the high hardness low alloyed hot rolled steel has a yield strength > 500 MPa.
6. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the high hardness low alloyed hot rolled steel has total elongation in range 10 to 20%. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the Mn content of the high hardness low alloyed hot rolled steel is kept below 1.8 wt.% to avoid center-line segregation. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the C content in the high hardness low alloyed hot rolled steel helps in stabilizing the austenite at room temperature. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the Si content of the high hardness low alloyed hot rolled steel is kept in the range between 0.25-0.35 wt.% to partially retard cementite formation. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.15, Mn - 1.1, Si - 0.3, Cr - 0.2, V - 0.03, Ni - 0.5, Al - 0.2, Mo - 0.1, carbon equivalent 0.43 and the balance being Iron (Fe) and unavoidable impurities. The high hardness low alloyed hot rolled steel as claimed in the claim 1, wherein the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.21, Mn - 1.47, Si - 0.3, Cr - 0.2, Ni - 0.5, Mo - 0.1, carbon equivalent 0.55, and the balance being Iron (Fe) and unavoidable impurities. The high hardness low alloyed hot rolled steel as claimed in the claim 10, wherein the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 220-380 Hv. The high hardness low alloyed hot rolled steel as claimed in the claim 11, wherein the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 300-460 Hv. A method (100) for manufacturing high hardness low alloyed hot rolled steel sheet, the method (100) comprising: casting steel slab having a composition expressed in weight %: C: 0.10-0.25, Mn: 1.05-1.5, Si: 0.25-0.35, Cr: 0.15-0.25, Mo: 0.05-0.15, Ni: 0.45-0.55, Al: 0.15-0.25, carbon equivalent: 0.34-0.69, and the balance being Iron (Fe) and unavoidable impurities; reheating the steel slab to a temperature greater than 1150°C; hot rolling the steel slab to about 70% deformation to produce a steel sheet such that finish rolling is done at a temperature (TFRT), wherein TFRT varies in the range 830°C to 890°C; and cooling at a cooling rate in the range of 5°C/s - 60°C/s till a room temperature is reached to obtain the high hardness low alloyed hot rolled steel sheet.
15. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claim 14, wherein the high hardness low alloyed hot rolled steel comprises a microstructure of maximum 10 % ferrite, maximum 10% austenite, maximum 20% pearlite, maximum 20% bainite and 60 -100 % martensite.
16. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claims 14 and 15, wherein the high hardness low alloyed hot rolled steel has a tensile strength > 600 MPa and yield strength > 500 MPa, total elongation in range 10 to 20%.
17. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claim 14, wherein TFRT is 870°C.
18. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claim 14, wherein the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.15, Mn - 1.1, Si - 0.3, Cr - 0.2, V - 0.03, Ni - 0.5, Al - 0.2, Mo - 0.1, carbon equivalent 0.43, and the balance being Iron (Fe) and unavoidable impurities.
19. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claim 14, wherein the high hardness low alloyed hot rolled steel comprises the composition expressed in weight %: C - 0.21, Mn - 1.47, Si - 0.3, Cr - 0.2, Ni - 0.5, Mo - 0.1, carbon equivalent 0.55, and the balance being Iron (Fe) and unavoidable impurities.
20. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claim 18, wherein the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 220-380 Hv.
18
21. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claim 19, wherein the high hardness low alloyed hot rolled steel has hardness (HV 1kg) in the range of 300-460 Hv.
22. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claim 18 and 19, wherein the saturation cooling rate exists for the high hardness low alloyed hot rolled steel in the range of 20-30 °C/s.
23. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claims 18 and 22, wherein the maximum hardness (HV 1kg) high hardness low alloyed hot rolled steel is in the range of 360-380 Hv.
24. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claims 19 and 22, wherein the maximum hardness (HV 1kg) of the high hardness low alloyed hot rolled steel is in the range of 450-460 Hv.
25. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claim 14, wherein the steel slab is reheated to a temperature betweenl 150°C - 1250°C for a duration of 20 minutes to 2 hours depending on the slab thickness.
26. The method (100) for manufacturing high hardness low alloyed hot rolled steel sheet as claimed in the claims 15 to 20, wherein the thickness of the steel sheet is in the range of 4 mm - 8 mm.
27. A component produced from the high hardness low alloyed hot rolled steel as claimed in the claims 1 to 26.
28. The component as claimed in the claim 27, wherein the component is used in structural as well as wear resistant applications of the L&E equipment, particularly used in mining and earthmoving applications.
EP22800790.2A 2021-10-20 2022-10-20 High hardness low alloyed hot rolled steel and method of manufacturing thereof Pending EP4416312A1 (en)

Applications Claiming Priority (2)

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IN202131047704 2021-10-20
PCT/IB2022/060094 WO2023067544A1 (en) 2021-10-20 2022-10-20 High hardness low alloyed hot rolled steel and method of manufacturing thereof

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Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
SI2789699T1 (en) * 2013-08-30 2017-06-30 Rautaruukki Oyj A high-hardness hot-rolled steel product, and a method of manufacturing the same
PL3653736T3 (en) * 2018-11-14 2021-05-17 Ssab Technology Ab Hot-rolled steel strip and manufacturing method
SI3719148T1 (en) * 2019-04-05 2023-06-30 Ssab Technology Ab High-hardness steel product and method of manufacturing the same

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