EP4634427A1 - Hot rolled steel plate with high wear resistance and method of manufacturing the same - Google Patents

Hot rolled steel plate with high wear resistance and method of manufacturing the same

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Publication number
EP4634427A1
EP4634427A1 EP23821369.8A EP23821369A EP4634427A1 EP 4634427 A1 EP4634427 A1 EP 4634427A1 EP 23821369 A EP23821369 A EP 23821369A EP 4634427 A1 EP4634427 A1 EP 4634427A1
Authority
EP
European Patent Office
Prior art keywords
steel plate
temperature
steel
hot rolled
rolled steel
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
EP23821369.8A
Other languages
German (de)
French (fr)
Inventor
David QUIDORT
Céline KNAFOU
Alexandre GIORGI
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.)
ArcelorMittal SA
Original Assignee
ArcelorMittal SA
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 ArcelorMittal SA filed Critical ArcelorMittal SA
Publication of EP4634427A1 publication Critical patent/EP4634427A1/en
Pending legal-status Critical Current

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    • 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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • 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
    • 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
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22—Martempering
    • 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
    • C21D1/26—Methods of annealing
    • 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
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
    • 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
    • C21D1/84—Controlled slow cooling
    • 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
    • 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
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the 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
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys 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
    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/02—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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
    • C22C38/40—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
    • 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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a hot rolled steel plate having high wear resistance, high strength and high toughness, and to a method to obtain such steel plate.
  • the purpose of the invention therefore is to solve the above-mentioned problem and to provide a steel plate having high hardness above or equal to 400HB and preferably to 440HB at mid thickness of the plate and above or equal to 430HB and preferably to 450 HB at the surface of the plate, and high toughness with Charpy impact energy at -40°C above or equal to 25 J, and easily processable on conventional process route.
  • the hot rolled steel plate has a yield strength YS above or equal to 970MPa and preferably above or equal to 980 MPa.
  • the hot rolled steel plate has a tensile strength TS above or equal to 1400MPa.
  • the object of the present invention is achieved by providing a steel plate according to claim 1 .
  • Another object is achieved by providing the method according to claim 2.
  • composition of the steel according to the invention will now be described, the content being expressed in weight percent.
  • the carbon content is from 0.10% to 0.25 %. If the carbon content is too high, the weldability of the steel is insufficient. If the carbon content is lower than 0.10%, the austenite fraction is not stabilized enough to obtain targeted properties. In a preferred embodiment of the invention, the carbon content is from 0.15% and 0.20%.
  • the manganese content is from 3.0% to 5.0 %. Above 5.0% of addition, the risk of central segregation increases to the detriment of the toughness. Below 3.0%, the final structure comprises an insufficient retained austenite fraction to obtain the desired properties. In a preferred embodiment of the invention the manganese content is between 3.5% and 4.5%.
  • the silicon content is from 0.80% to 1.60%.
  • a silicon addition of at least 0.80% helps to stabilize a sufficient amount of retained austenite.
  • silicon is detrimental for toughness.
  • silicon oxides form at the surface, which impairs the coatability of the steel.
  • the silicon content is from 1.00% to 1.60%.
  • the aluminium content is from 0.10% to 0.60 %, as it is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Moreover, aluminium improves weldability of the steel. The aluminium content is lower than 0.60% to avoid the occurrence of inclusions and to avoid oxidation problems.
  • the boron content can be from 0.0003% and 0.004%.
  • the presence of boron can increase the toughness.
  • boron improves weldability of the steel. Above 0.004%, the formation of borocarbides at the prior austenite grain boundaries is promoted, making the steel more brittle. Below 0.0003%, there is not a sufficient concentration of free B that segregates at the prior austenite grain boundaries to increase toughness of the steel.
  • Titanium can be added optionally up to 0.06% to provide precipitation strengthening.
  • a minimum of 0.01 % of titanium is added in addition of boron to protect boron against the formation of BN.
  • Niobium can be added up to 0.05% to refine the austenite grains during hot-rolling and to provide precipitation strengthening.
  • the minimum amount of niobium added is 0.0010%.
  • Molybdenum can optionally be added, in a limit of maximum 0.3 %. Molybdenum stabilizes the austenite and increases toughness of the steel. Moreover, molybdenum improves weldability of the steel. Above 0.3%, the addition of molybdenum is costly and ineffective in view of the properties which are required. Preferably, the minimum amount of molybdenum is 0.0010%.
  • chromium A maximum of 0.80% of chromium is allowed. Above, a saturation effect is noted, and adding chromium is both useless and expensive. Preferably, the minimum amount of chromium is 0.0010%.
  • Copper can be added up to 0.2% in order to increase the toughness of the steel.
  • Nickel can be added up to 0.30% to limit the risk of delayed fracture due to hydrogen embrittlement.
  • the remainder of the composition of the steel is iron and impurities resulting from the smelting.
  • P, S and N at least are considered as residual elements which are unavoidable impurities.
  • Their content is below or equal to 0.010 % for S, below or equal to 0.020 % for P and below or equal to 0.008 % for N.
  • the hot rolled steel plate has a microstructure consisting of, in surface fraction, from 5% to 10% of retained austenite, the balance being auto tempered martensite.
  • the steel plate is hot rolled with a final rolling temperature FRT above Ac1 .
  • the FRT is below Ac3 and the microstructure consists of ferrite and austenite. During the subsequent cooling, ferrite and a part of austenite are transformed in martensite.
  • the FRT is above or equal to Ac3
  • the microstructure is fully austenitic.
  • austenite is transformed in martensite.
  • the martensite is auto tempered, because of the slow cooling rate of the subsequent cooling, which leads to obtain the targeted mechanical properties.
  • the steel plate can have a thickness above 25mm, and below 40mm.
  • a heating step up to a temperature TH from 850°C to 950°C is necessary to obtain a homogeneous microstructure of austenite in the thickness of the plate.
  • a temperature TQ below 400°C a part of this austenite is transformed in martensite. This martensite is then auto-tempered during the subsequent air cooling.
  • a part of the austenite remains in the final microstructure.
  • This retained austenite contributes to obtain a great wear resistance and is beneficial for toughness.
  • the retained austenite will be transformed to martensite inducing a volume expansion from the fee to bcc structure, which can fill impact cracks and stop their propagation.
  • the TRIP effect induces also a great workhardening in service, which largely contribute to improve the wear resistance of the bare material.
  • the steel plate according to the invention can be produced by any appropriate manufacturing method and the man skilled in the art can define one. It is however preferred to use the method according to the invention comprising the following steps:
  • a semi-product able to be further hot-rolled is provided with the steel composition described above.
  • the semi product is heated to a temperature of 1100°C to 1300°C, so to make it possible to ease hot rolling, with a final hot rolling temperature FRT above Ac1.
  • FRT is above Ac1 +100°C, more preferably above Ac1 +150°C.
  • the microstructure will not contain enough austenite to ensure the hardness and toughness of the plate.
  • the FRT is below Ac3.
  • the FRT is above or equal to Ac3.
  • the hot-rolled steel plate having a thickness above or equal to 5mm and below 40 mm is then air cooled.
  • the cooling rate at mid thickness of the plate is below 5°C/s.
  • the steel plate is reheated to a temperature TH of 850°C to 950°C and maintained at said temperature for a holding time below or equal to 30 minutes, in order to obtain an homogeneous microstructure in all the thickness of the plate.
  • the plate is then quenched at a cooling rate higher than 1 °C/s to a temperature TQ below 400°C, in order to obtain at mid thickness of the steel plate a temperature of 300°C to 400°C at the end of the quenching, before being air cooled to room temperature.
  • Calculations done with the use of a software like Forge® allow to determine the duration of the quenching to obtain a temperature of 300°C to 400°C at mid thickness of the steel plate at the end of the quenching.
  • the quenching temperature TQ is above or equal to 200°C, more preferably above or equal to 250°C.
  • the tested compositions are gathered in the following table wherein the element contents are expressed in weight percent.
  • phase percentages of the microstructure of the obtained hot rolled steel plate were determined at quarter thickness d/4 of the steel plate.
  • the surface fractions of phases in the microstructure are determined through the following method: a specimen is cut from the hot rolled plate, polished and etched with a reagent known per se, to reveal the microstructure. The section is afterwards examined through scanning electron microscope, for example with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification greater than 5000x, in secondary electron mode.
  • FEG-SEM Field Emission Gun
  • the steel part of trials 4-6 have a chemical composition with lower manganese.
  • the quenching step in trial 4 is performed down to room temperature at a slow cooling rate. A part of austenite is then transformed in martensite, being auto tempered because of the slow cooling but also in bainite, which is detrimental for hardness.
  • the quenching step is done down to room temperature, at a higher cooling rate. Austenite is thus totally transformed into martensite. The absence of austenite is detrimental for toughness.
  • Trials 2 and 6 undergo the same process, the difference being in the chemical composition.
  • the low level of manganese combined to the absence of boron in trial 6 lead to low toughness.
  • the high thickness steel plate of trial 7 is reheated and quenched.
  • This quenching step is interrupted at a temperature of 350°C before being air cooled to room temperature.
  • Austenite formed during the heating step is then partly transformed into martensite.
  • This martensite is auto-tempered during the air cooling step, leading to the high hardness and toughness level.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • 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)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

The invention deals with a hot rolled steel plate having a composition comprising,by weight percent: C: 0.10 - 0.25 %, Mn: 3.0 – 5.0 %, Si: 0.80 – 1.60 %, Al : 0.10% - 0.60%, S ≤ 0.010 %, P ≤ 0.020 %, N ≤ 0.008 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure consisting of, in surface fraction: from 5 to 10% of residual austenite, the rest being auto-tempered martensite.

Description

Hot rolled steel plate with high wear resistance and method of manufacturing the same
The present invention relates to a hot rolled steel plate having high wear resistance, high strength and high toughness, and to a method to obtain such steel plate.
In the mineral industry, water quenched martensitic steel are used today to manufacture wear components, for applications in mines and quarries, cement and steelmaking industries, public works and agricultural machinery. In those fields, wear damage produced by heavy impacts and scratches is a major issue requiring repetitive machine downtown and maintenance costs to change worn components such as liners, buckets and jaw crushers. Considerable effort is paid to prolong the lifetime of wear parts and a large variety of materials and designs is commonplace. To obtain a significant increase in wear resistance, according to the applications, hardness should be higher than 440 HB. Knowing the impact toughness is closely related to the wear resistance, this property must be the highest, but a classical water quench martensitic steel presents a low impact toughness.
Moreover, in view of the global environmental conservation, the steel making industry is looking for a less-energy-consuming fabrication route.
The purpose of the invention therefore is to solve the above-mentioned problem and to provide a steel plate having high hardness above or equal to 400HB and preferably to 440HB at mid thickness of the plate and above or equal to 430HB and preferably to 450 HB at the surface of the plate, and high toughness with Charpy impact energy at -40°C above or equal to 25 J, and easily processable on conventional process route.
Preferably the hot rolled steel plate has a yield strength YS above or equal to 970MPa and preferably above or equal to 980 MPa.
Preferably the hot rolled steel plate has a tensile strength TS above or equal to 1400MPa. The object of the present invention is achieved by providing a steel plate according to claim 1 . Another object is achieved by providing the method according to claim 2.
The invention will now be described in detail and illustrated by examples without introducing limitations.
The composition of the steel according to the invention will now be described, the content being expressed in weight percent.
The carbon content is from 0.10% to 0.25 %. If the carbon content is too high, the weldability of the steel is insufficient. If the carbon content is lower than 0.10%, the austenite fraction is not stabilized enough to obtain targeted properties. In a preferred embodiment of the invention, the carbon content is from 0.15% and 0.20%.
The manganese content is from 3.0% to 5.0 %. Above 5.0% of addition, the risk of central segregation increases to the detriment of the toughness. Below 3.0%, the final structure comprises an insufficient retained austenite fraction to obtain the desired properties. In a preferred embodiment of the invention the manganese content is between 3.5% and 4.5%.
According to the invention, the silicon content is from 0.80% to 1.60%. A silicon addition of at least 0.80% helps to stabilize a sufficient amount of retained austenite. Above 1.60%, silicon is detrimental for toughness. Moreover, silicon oxides form at the surface, which impairs the coatability of the steel. In a preferred embodiment of the invention, the silicon content is from 1.00% to 1.60%.
The aluminium content is from 0.10% to 0.60 %, as it is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Moreover, aluminium improves weldability of the steel. The aluminium content is lower than 0.60% to avoid the occurrence of inclusions and to avoid oxidation problems.
Optionally some elements can be added to the composition of the steel according to the invention.
The boron content can be from 0.0003% and 0.004%. The presence of boron can increase the toughness. Moreover, boron improves weldability of the steel. Above 0.004%, the formation of borocarbides at the prior austenite grain boundaries is promoted, making the steel more brittle. Below 0.0003%, there is not a sufficient concentration of free B that segregates at the prior austenite grain boundaries to increase toughness of the steel.
Titanium can be added optionally up to 0.06% to provide precipitation strengthening. Preferably a minimum of 0.01 % of titanium is added in addition of boron to protect boron against the formation of BN.
Niobium can be added up to 0.05% to refine the austenite grains during hot-rolling and to provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%.
Molybdenum can optionally be added, in a limit of maximum 0.3 %. Molybdenum stabilizes the austenite and increases toughness of the steel. Moreover, molybdenum improves weldability of the steel. Above 0.3%, the addition of molybdenum is costly and ineffective in view of the properties which are required. Preferably, the minimum amount of molybdenum is 0.0010%.
A maximum of 0.80% of chromium is allowed. Above, a saturation effect is noted, and adding chromium is both useless and expensive. Preferably, the minimum amount of chromium is 0.0010%.
Copper can be added up to 0.2% in order to increase the toughness of the steel. Nickel can be added up to 0.30% to limit the risk of delayed fracture due to hydrogen embrittlement.
The remainder of the composition of the steel is iron and impurities resulting from the smelting. In this respect, P, S and N at least are considered as residual elements which are unavoidable impurities. Their content is below or equal to 0.010 % for S, below or equal to 0.020 % for P and below or equal to 0.008 % for N.
The microstructure of the hot rolled steel plate according to the invention will now be described.
The hot rolled steel plate has a microstructure consisting of, in surface fraction, from 5% to 10% of retained austenite, the balance being auto tempered martensite.
The steel plate is hot rolled with a final rolling temperature FRT above Ac1 . In a preferred embodiment of the invention, the FRT is below Ac3 and the microstructure consists of ferrite and austenite. During the subsequent cooling, ferrite and a part of austenite are transformed in martensite.
In another preferred embodiment of the invention, the FRT is above or equal to Ac3, and the microstructure is fully austenitic. During the subsequent cooling, a part of austenite is transformed in martensite.
In a preferred embodiment of the invention, in which the steel plate has a thickness above or equal to 5mm and below 40mm, the martensite is auto tempered, because of the slow cooling rate of the subsequent cooling, which leads to obtain the targeted mechanical properties. Preferably, the steel plate can have a thickness above 25mm, and below 40mm.
In another preferred embodiment of the invention, in which the steel plate has a thickness above or equal to 40mm and below or equal to 100 mm, a heating step up to a temperature TH from 850°C to 950°C is necessary to obtain a homogeneous microstructure of austenite in the thickness of the plate. During the quenching step down to a temperature TQ below 400°C, a part of this austenite is transformed in martensite. This martensite is then auto-tempered during the subsequent air cooling.
A part of the austenite remains in the final microstructure. This retained austenite contributes to obtain a great wear resistance and is beneficial for toughness. During wear damage and impacts, the retained austenite will be transformed to martensite inducing a volume expansion from the fee to bcc structure, which can fill impact cracks and stop their propagation. The TRIP effect induces also a great workhardening in service, which largely contribute to improve the wear resistance of the bare material.
The steel plate according to the invention can be produced by any appropriate manufacturing method and the man skilled in the art can define one. It is however preferred to use the method according to the invention comprising the following steps:
A semi-product able to be further hot-rolled, is provided with the steel composition described above. The semi product is heated to a temperature of 1100°C to 1300°C, so to make it possible to ease hot rolling, with a final hot rolling temperature FRT above Ac1. Preferably, the FRT is above Ac1 +100°C, more preferably above Ac1 +150°C. When FRT is lower than Ac1 , the microstructure will not contain enough austenite to ensure the hardness and toughness of the plate.
In a preferred embodiment of the invention, the FRT is below Ac3.
In another preferred embodiment of the invention, the FRT is above or equal to Ac3. The hot-rolled steel plate having a thickness above or equal to 5mm and below 40 mm is then air cooled. Preferably, the cooling rate at mid thickness of the plate is below 5°C/s.
From 40mm of thickness, and up to 100 mm of thickness, the steel plate is reheated to a temperature TH of 850°C to 950°C and maintained at said temperature for a holding time below or equal to 30 minutes, in order to obtain an homogeneous microstructure in all the thickness of the plate. The plate is then quenched at a cooling rate higher than 1 °C/s to a temperature TQ below 400°C, in order to obtain at mid thickness of the steel plate a temperature of 300°C to 400°C at the end of the quenching, before being air cooled to room temperature.
Calculations done with the use of a software like Forge® allow to determine the duration of the quenching to obtain a temperature of 300°C to 400°C at mid thickness of the steel plate at the end of the quenching. Preferably the quenching temperature TQ is above or equal to 200°C, more preferably above or equal to 250°C.
The invention will be now illustrated by the following examples, which are by no way limitative.
Example
Four grades, whose compositions are gathered in table 1 , were cast in semiproducts and processed into steel plates
Table 1 - Compositions
The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent.
Steel A is according to the invention, B - D out of the invention.
Ac1 and Ac3 of steel A have been determined through dilatometry measures.
Ac1 and Ac3 of steels B-D have been determined through a formula, based on the weight percent of the corresponding elements:
5 Ac3 = 925 - 219 (%C) - 7 %Mn + 39 %Si - 16 %Ni + 13 %Mo, Ac1 = 742 - 29%C - 14%Mn + 13%Si + 16%Cr - 17%Ni - 16%Mo + 36%Cu.
Table 2 - Process parameters
10 Steel ingots were heated at 1250°C, and hot rolled with a final rolling temperature FRT to obtain a hot rolled steel plate with a thickness d. The hot rolled steel plate of trials 1 -3, 6 and 8 are then air cooled. For trials 4 and 5, the steel plates are then heated to a temperature TH and guenched to room temperature (RT) at various cooling rate. In trial 7, the steel plate having a thickness of 40mm is heated
15 up to a temperature TH and guenched to a temperature TQ higher than RT, in order to obtain at mid thickness of the steel plate a temperature of 300°C to 400°C at the end of the guenching, before being air cooled. The following specific conditions were applied:
Underline values: not corresponding to the invention
The hot rolled plates were then analyzed and the corresponding microstructure elements and mechanical properties were respectively gathered in tables 3 and 4.
Table 3 - Microstructure of the hot rolled steel plate
The phase percentages of the microstructure of the obtained hot rolled steel plate were determined at quarter thickness d/4 of the steel plate.
Underlined values: not corresponding to the invention
The surface fractions of phases in the microstructure are determined through the following method: a specimen is cut from the hot rolled plate, polished and etched with a reagent known per se, to reveal the microstructure. The section is afterwards examined through scanning electron microscope, for example with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification greater than 5000x, in secondary electron mode. The determination of the surface fraction of retained austenite and auto tempered martensite is performed thanks to SEM observations after Nital or Picral/Nital reagent etching and thanks to X-ray diffraction analysis.
Table 4 - Mechanical properties of the hot rolled steel plate
Mechanical properties of the tested samples were determined and gathered in the following table. The hardness has been determined at the surface of the steel plate (down to 2mm below the top surface) and at mid thickness (d/2) of the steel plate. Hardness is measured according to Standard ASTM E10. Charpy impact energy at -40°C is measured according to Standard ISO 148-1 :2006 (F) and ISO 148-1 :2017(F). YS and TS are measured according to ISO standard ISO 6892-1.
Underlined values: do not match the targeted values
The examples show that the steel plates according to the invention, namely trials 1 to 3 and 7-8 are the only ones to show the targeted property, thanks to their specific compositions and microstructures.
The steel part of trials 4-6 have a chemical composition with lower manganese. The quenching step in trial 4 is performed down to room temperature at a slow cooling rate. A part of austenite is then transformed in martensite, being auto tempered because of the slow cooling but also in bainite, which is detrimental for hardness. In trial 5, the quenching step is done down to room temperature, at a higher cooling rate. Austenite is thus totally transformed into martensite. The absence of austenite is detrimental for toughness.
Moreover, Trials 2 and 6 undergo the same process, the difference being in the chemical composition. The low level of manganese combined to the absence of boron in trial 6 lead to low toughness.
The high thickness steel plate of trial 7 is reheated and quenched. This quenching step is interrupted at a temperature of 350°C before being air cooled to room temperature. Austenite formed during the heating step is then partly transformed into martensite. This martensite is auto-tempered during the air cooling step, leading to the high hardness and toughness level.

Claims

1. Hot-rolled steel plate, made of a steel having a composition comprising, by weight percent:
C : 0.10 - 0.25 %
Mn : 3.0 - 5.0 %
Si : 0.80 - 1.60 %
Al : 0.10% - 0.60%
S < 0.010 %
P < 0.020 %
N < 0.008 % and comprising optionally one or more of the following elements, in weight percentage:
B: 0.0003 - 0.004 %
Ti < 0.06 %
Nb < 0.05 %
Mo < 0.3 %
Cr < 0.80 %
Cu < 0.2%
Ni < 0.30% the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel plate having a microstructure comprising, in surface fraction,
- from 5 to 10% of residual austenite,
- the rest being auto-tempered martensite.
2. A method for manufacturing a hot-rolled steel plate, comprising the following successive steps:
- casting a steel to obtain a semi-product, said semi product having a composition according to claim 1 ,
- reheating the semi-product to a temperature Treheat of 1100°C to 1300°C, - hot rolling the semi-product with a finish hot rolling temperature FRT above Ac1 ,
- optionally heating the steel plate to a temperature TH of 850°C to 950°C, and maintained at said temperature for a holding time below or equal to 30 minutes.
- optionally quenching the steel plate from TH to a temperature TQ below 400°C, in order to obtain at mid thickness of the steel plate a temperature of 300°C to 400°C at the end of the quenching,
- and air cooling the steel plate to room temperature.
EP23821369.8A 2022-12-12 2023-12-07 Hot rolled steel plate with high wear resistance and method of manufacturing the same Pending EP4634427A1 (en)

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