EP4396388A1 - Hot rolled and steel sheet and a method of manufacturing thereof - Google Patents

Hot rolled and steel sheet and a method of manufacturing thereof

Info

Publication number
EP4396388A1
EP4396388A1 EP21766708.8A EP21766708A EP4396388A1 EP 4396388 A1 EP4396388 A1 EP 4396388A1 EP 21766708 A EP21766708 A EP 21766708A EP 4396388 A1 EP4396388 A1 EP 4396388A1
Authority
EP
European Patent Office
Prior art keywords
hot rolled
rolled steel
steel sheet
temperature
cooling
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
EP21766708.8A
Other languages
German (de)
French (fr)
Inventor
Dorien DE KNIJF
Tom Waterschoot
Ulrike Lorenz
Lode Duprez
Lieven Bracke
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 EP4396388A1 publication Critical patent/EP4396388A1/en
Pending legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • C21D1/84—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
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • 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

  • This element is gammagenous and therefore plays an important role in controlling the Residual Austenite fraction as well as enriching the residual austenite with Manganese to impart hardenability to the steel and impact toughness.
  • An amount of at least 3% by weight of Manganese has been found in order to provide the strength and toughness to the steel. But when Manganese content is more than 9 % it produces adverse effects such as it stabilizes the austenite too much and devoid the steel of present invention from TRIP effect. In addition, the Manganese content of above 9% leads to excessive central segregation, hence reducing the formability and also deteriorating the weldability of the present steel.
  • a preferable content for the present invention may be kept from 3.5% to 8.5% and more preferably 4% to 8%.
  • Aluminum is an essential element and is present in the steel from 0.9% to 2.5%.
  • Aluminum is an alphagenous element and a minimum of 0.9% of Aluminum is required to increase the inter-critical temperature range thereby providing strength and toughness to the steel of present invention.
  • Aluminum is also used for removing oxygen from the molten state of the steel to clean steel of present invention and it also prevents oxygen from forming a gas phase. But whenever the Aluminum is more than 2.5% it is difficult to do casting because of the surface defects on the slabs such as breakouts. Therefore, preferable range for the presence of the Aluminum is from 1 % to 2.3% and more preferably from 1 % to 2%.
  • Nitrogen is limited to 0.025% in order to avoid ageing of material and to minimize the precipitation of nitrides during solidification which are detrimental for mechanical properties of the Steel.
  • the preferable upper limit for nitrogen is 0.02% and more preferably 0.005%.
  • Copper may be added as an optional element in an amount of 0% to 1 % to increase the strength of the of Steel of present invention and to improve its corrosion resistance. A minimum of 0.01 % is preferred to get such effects. However, when its content is above 1 %, it can lead to problems such as copper hot shortness during the hot rolling process.
  • Fresh martensite can also be optionally present in the steel of present invention.
  • Fresh martensite may form during cooling after annealing from remaining unstable residual austenite.
  • Fresh martensite can be present from 0% to 15%, preferably from 0 to 10% and even better no fresh martensite is present.
  • the microstructure of the hot rolled steel is free from microstructural components, such as Pearlite and Cementite.
  • Carbides of alloying elements might be present in the steel of present invention in a cumulated amount from 0% to 5% such as of Niobium, Titanium, Vanadium and Iron . These carbides may increase the strength of the steel of present invention by precipitation strengthening, but whenever the presence of carbides is 5% or more, their precipitation consume partly the amount of Carbon, which is detrimental for the stabilization of residual austenite and the steel of present may not have adequate toughness.
  • the slab is reheated to a temperature from Ac3 + 50° C to 1300°C.
  • the temperature of the slab is lower than least Ac3 + 50° C, excessive load is imposed on the rolling mill. Therefore, the temperature of the slab is sufficiently high so that hot rolling can be completed fully in the austenitic range. Reheating at temperatures above 1300°C must be avoided because it causes productivity loss and is also industrially expensive and some segregated parts may melt which may lead to breaking of slabs or cracking of slabs. Therefore, the preferred reheating temperature is from Ac3 + 100° C to 1280°C.
  • the hot rolled steel is then cooled from hot roll finishing temperature to a temperature range from Ms to 20°C at a cooling rate from 1 °C/s to 50°C/s to obtain a hot rolled steel strip.
  • the cooling rate for this step of cooling is from 1 °C/s to 45°C/s and more preferably from 25°C/s to 40°C/s.
  • the hot rolled strip may optionally be coiled wherein coiling temperature is between 20°C and 800°C.
  • the hot rolled steel is being heated from a temperature range from Ms to 20°C up to the first annealing temperature TA1 which is from Ac3 to Ac3 +150°C and preferably from Ac3 to Ac3 +120°C, and more preferably from Ac3 to Ac3 +100°C, such heating being performed at a heating rate HR1 of at least 1 °C/s.
  • the hot rolled steel strip is held at TA1 during 5 seconds to 6000 seconds to ensure the transformation to 100% austenite.
  • the hot rolled steel is cooled wherein the cooling starts from TA1 at a cooling rate CR1 from 0.1 °C/s to 150°C/s, to a cooling stop temperature T1 which is in a range from Ms-10°C to 15°C.
  • the cooling rate CR1 for such cooling is from 0.1 °C/s to 120°C/s.
  • the preferred T1 temperature is from Ms-50°C to 20°C. Cooling rate for cooling after soaking must be sufficiently high to obtain the transformation of Austenite into Martensite.
  • the cooling rate after first annealing is selected in a manner that it ensures at least 80% martensite in hot rolled strip at T1 .
  • the hot rolled steel is held at TA2 during 5 seconds to 6000 seconds to ensure the transformation of the microstructure to form 10% to 25% of austenite .
  • the hot rolled steel is cooled wherein the cooling starts from TA2 at a cooling rate CR2 from 0.1 °C/s to 150°C/s, to a cooling stop temperature T2 which is in a range from Ms-10°C to 15°C.
  • the cooling rate CR2 for such cooling is from 0.1 °C/s to 120°C/s.
  • the preferred T2 temperature is from Ms-20°C to 20°C. Cooling rate after soaking must be sufficiently high to avoid the transformation of Austenite into Bainite so that a sufficient amount of carbon is available for stabilizing the residual austenite during the cooling after annealing. During this cooling the fresh martensite may form from some remaining unstable austenite.
  • Table 1 Steel sheets made of steels with different compositions are gathered in Table 1 , where the steel sheets are produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the steel sheets obtained during the trials and table 4 gathers the result of evaluations of obtained properties. Ac3 and Ms temperature are determined through thermodynamic calculations done with the use of a software like Thermo-Calc®.
  • Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as SEM, EPMA, EBSD, XRD or any other microscope for determining microstructural composition of both the inventive steel and reference trials.
  • the area fractions for the carbides is measured on polished samples after etching them in 2% Nital etching solution for 10 seconds and observed by an SEM.
  • Polygonal Ferrite and tempered martensite are measured using EBSD wherein Electron backscattered diffraction (EBSD) is a SEM based technique to measure crystal orientations with a sub-micron resolution. An electron beam is focused on the 70° tilted specimen in the scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • Electrons that satisfy the Bragg condition for a family of planes are channelled and induce kikuchi bands. Electrons strike a phosphor screen and produce light, which is detected and digitized by a camera. The resulting EBS pattern is analyzed and indexed. This process is realized for each point analysed. For a given steel sample, an EBSD analysis of at least 4 images corresponding to a magnification of 1000 allows to identify the polygonal ferrite and tempered martensite microconstituents, their location and area percentage. The Residual Austenite area fraction is measured using XRD which are demonstrated in table 3.
  • Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels.
  • tensile tests are conducted in accordance of NBN EN ISO6892-1 standards with tensile samples types is having A25.
  • the toughness is tested by a Charpy test performed according to ISO 148-1. All measurements done on the inventive and reference steel are done for steel sheet taken in longitudinal direction (LD). The results of the various mechanical tests conducted in accordance to the standards are gathered

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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

A hot rolled steel sheet having a composition comprising of the elements, 0.02%≤Carbon≤0.2%, 3%≤Manganese≤9%, 0.2%≤Silicon≤1.2%, 0.9%≤Aluminum≤2.5%, 0%≤Phosphorus≤0.03%, 0%≤Sulfur≤ 0.03%, 0%≤Nitrogen≤0.025%, 0%≤Molybdenum≤0.6%, 0%≤Titanium≤0.1%, 0.0001%≤Boron≤0.01%, 0%≤Chromium≤0.5%, 0%≤Niobium≤0.1%, 0%≤Vanadium≤0.15%, 0%≤Nickel≤1%, 0%≤Copper≤1%, 0%≤Calcium≤0.005%, 0%≤Magnesium≤0.0010%, the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, at least 60% of tempered martensite, 15% to 40% residual austenite, 0% to 10% polygonal ferrite, 0% to 5% of bainite, 0 to 15% of fresh martensite and 0% to 5% of carbides of Niobium, Titanium,Vanadium or Iron.

Description

HOT ROLLED AND STEEL SHEET AND A METHOD OF MANUFACTURING THEREOF
The present invention relates to hot rolled steel sheet suitable for use as structural steel or for the manufacturing industrial machinery, yellow goods, green goods and for cryogenic applications.
In recent years, efforts have been actively made to reduce the weight of the equipment and structures by applying high-strength steel far the purpose of improving fuel efficiency as well as reducing the environmental impact. However, when the strength of the steel is increased, the toughness generally deteriorates. Therefore, in the development of high-strength steel, it is an important issue to increase the strength without deteriorating the toughness.
Intense Research and Development endeavors are put in to reduce the amount of material utilized by increasing the strength of material. Conversely, an increase in strength of steel decreases toughness, and thus development of materials having both high strength and good toughness is necessitated.
Earlier Research and Developments in the field of high strength and good toughness steel have resulted in several methods for producing high strength steel, some of which are enumerated herein for conclusive appreciation of the present invention:
EP2392681 discloses thick-walled high-strength hot rolled steel sheet having composition which contains by mass% 0.02 to 0.08% C, 1 .0% or less Si, 0.50 to 1 .85% Mn, 0.03% or less P, 0.005% or less S, 0.1 % or less Al, 0.03 to 0.10% Nb, 0.001 to 0.05% Ti, 0.0005% or less B, optionally one or two kinds or more selected from a group consisting of 0.010% or less Ca, 0.02% or less REM, 0.003% or less Mg, 0.5% or less V, 1.0% or less Mo, 1.0% or less Cr, 4.0% or less Ni, 2.0% or less Cu, other unavoidable impurities and Fe as balance. The steel sheet has the structure formed of bainitic ferrite phase or a bainite phase in which solid-solution C content in ferrite grains is 10ppm or more, and surface layer hardness is 230HV or less in terms of Vickers hardness but the steel of EP2392681 is unable to reach the tensile strength of 700MPa or more. EP2971211 discloses a method for fabricating a high manganese steel component having a composition consisting of: manganese ranging from about 9 to about 20 weight % of the total composition, carbon ranging from about 0.5 to about 2.0 weight % of the total composition, and the balance iron; and optionally: chromium ranging from 0.5 to 30 weight % of the total composition; nickel or cobalt ranging from 0.5 to 20 weight % of the total composition; aluminum ranging from 0.2 to 15 weight % of the total composition; molybdenum, niobium, copper, titanium or vanadium ranging from 0.01 to 10 weight % of the total composition; silicon ranging from 0.1 to 10 weight % of the total composition; nitrogen ranging from 0.001 to 3.0 weight % of the total composition; boron ranging from 0.001 to 0.1 weight % of the total composition; or zirconium or hafnium ranging from 0.2 to 6 weight % of the total composition; heating the composition to at least about 1000°C; cooling the composition at a rate of from about 2°C per second to about 60°C per second, followed by hot rolling the composition at a temperature in a range of about 700°C to about 1000°C; slowly cooling or isothermally holding the composition; and quenching or accelerated cooling or air cooling the composition from a temperature in a range of from 700°C to about 1000°C to a temperature in range of from 0°C to about 500°C at a rate of at least about 10°C per second. But EP2971211 is not able to reach an Impact toughness of 60J/cm2 or more when measured at -40°C
The purpose of the present invention is to solve these problems by making available hot-rolled steel that simultaneously have:
- a yield strength 650 MPa or more,
- a tensile strength of 750 MPa or more and preferably 800MPa or more,
- a total elongation greater than or equal to 15% and more preferably greater than 18%.
- an impact toughness of greater than or equal to 70 J/cm2 when measured at - 40°C and more preferably 90 J/cm2 when measured at -40°C.
In a preferred embodiment, the steel sheets according to the invention may also present a yield strength to tensile strength ratio of 0.5 or more Another object of the present invention is also to make available a method for the manufacturing of these steels that is compatible with conventional industrial processes while being robust towards manufacturing parameters shifts.
The hot rolled steel sheet of the present invention may optionally be coated with zinc or zinc alloys, to improve its corrosion resistance.
Carbon is present in the steel from 0.02% to 0.2%. Carbon is an element necessary for increasing the strength of the steel by assisting in the stabilization of austenite at room temperature. But Carbon content less than 0.02% will not be able to impart the tensile strength to the steel of present invention. On the other hand, at a Carbon content exceeding 0.2%, the steel exhibits poor weldability as well as it is detrimental for the impact toughness which limits its application for the structural parts of yellow or green goods. A preferable content for the present invention may be kept from 0.03% to 0.18%, and more preferably from 0.04% to 0.15%.
Manganese content of the steel of present invention is from 3 % to 9%.
This element is gammagenous and therefore plays an important role in controlling the Residual Austenite fraction as well as enriching the residual austenite with Manganese to impart hardenability to the steel and impact toughness. An amount of at least 3% by weight of Manganese has been found in order to provide the strength and toughness to the steel. But when Manganese content is more than 9 % it produces adverse effects such as it stabilizes the austenite too much and devoid the steel of present invention from TRIP effect. In addition, the Manganese content of above 9% leads to excessive central segregation, hence reducing the formability and also deteriorating the weldability of the present steel. A preferable content for the present invention may be kept from 3.5% to 8.5% and more preferably 4% to 8%.
Silicon content of the steel of present invention is from 0.2% to 1.2%. Silicon is solid solution strengthener for the steel of present invention. In addition, Silicon retards the precipitation of Cementite and also limits the formation of cementite while often cannot completely eliminate cementite formation. Si keeps C in solid solution in austenite, as such lower the Ms temperature to below room temperatue. Hence Si assists in the formation of Residual austenite at room temperature. However, a content of Si more than 1 .2% leads to a problem such as surface defects which adversely effects the steel of present invention. Therefore, the concentration is controlled within an upper limit of 1 .2%. A preferable content for the present invention may be kept from 0.3% to 1 % and more preferably from 0.4% to 0.8%.
Aluminum is an essential element and is present in the steel from 0.9% to 2.5%. Aluminum is an alphagenous element and a minimum of 0.9% of Aluminum is required to increase the inter-critical temperature range thereby providing strength and toughness to the steel of present invention. Aluminum is also used for removing oxygen from the molten state of the steel to clean steel of present invention and it also prevents oxygen from forming a gas phase. But whenever the Aluminum is more than 2.5% it is difficult to do casting because of the surface defects on the slabs such as breakouts. Therefore, preferable range for the presence of the Aluminum is from 1 % to 2.3% and more preferably from 1 % to 2%.
Phosphorus constituent of the steel of present invention is from 0% to 0.03%. Phosphorus reduces the hot ductility and toughness, particularly due to its tendency to segregate at the grain boundaries or co-segregate with Manganese. For these reasons, its content is limited to 0.02% and preferably lower than 0.015%.
Sulfur is not an essential element but may be contained as an impurity in steel and from point of view of the present invention, the Sulfur content is preferably as low as possible, but is 0.03% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese which is detrimental on the steel of present invention, therefore preferred below 0.01 %
Nitrogen is limited to 0.025% in order to avoid ageing of material and to minimize the precipitation of nitrides during solidification which are detrimental for mechanical properties of the Steel. Hence the preferable upper limit for nitrogen is 0.02% and more preferably 0.005%.
Molybdenum is an optional element that constitutes 0% to 0.6% of the steel of present invention. Molybdenum increases the hardenability, as such allowing the steel of present invention to achieve targeted properties for thicker gauges When used in combination with Titanium and Boron it improves the toughness of the steei of present invention. A minimum of 0.1 % of Molybdenum is required to be beneficial in increasing the hardenability. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.6%. Preferable limit for molybdenum is from 0 % to 0.4% and more preferably from 0 % to 0.3%.
Titanium is an optional element and present from 0% to 0.1 % in the steel of present invention. Titanium imparts the steel of present invention with the strength by forming carbide and control the grain size during first annealing. But whenever Titanium is present more than 0.1 % it imparts excess strength and hardness to the steel of present invention which diminishes the toughness beyond the targeted limits. The preferable limit for titanium is from 0 % to 0.09% and more preferred limit is 0% to 0.08%.
Boron is an optional element to the steel of present invention and may be present from 0.0001 % to 0.01 %. Boron imparts toughness to the steel of present invention when added along with Titanium and Molybdenum.
Chromium is an optional element for the present invention. Chromium content may be present in the steel of present invention is from 0% to 0.5%. Chromium is an element that provides hardenability to the steel but content of Chromium higher than 0.5% leads to central co-segregation with Manganese.
Niobium is an optional element for the present invention. Niobium content may be present in the steel of present invention from 0% to 0.1 % and is added in the steel of present invention for forming carbides or carbo-nitrides to impart strength to the steel of present invention by precipitation strengthening. Niobium also controls the grains size during the first annealing. Preferable limit is from 0% and 0.05%
Vanadium is an optional element that may be present from 0% to 0.15% of the steel of present invention. Vanadium is effective in enhancing the strength of steel by forming carbides, nitrides or carbo-nitrides and the upper limit is 0.15% due to economic reasons and even if Vanadium is present above 0.15% it does not bring any considerable benefit to the steel of present invention.
Nickel may be added as an optional element in an amount of 0% to 1 % to increase the strength of the steel present invention and to improve its toughness. A minimum of 0.01 % is preferred to get such effects. However, the content of Nickel is restricted to 1 % due to economic viability.
Copper may be added as an optional element in an amount of 0% to 1 % to increase the strength of the of Steel of present invention and to improve its corrosion resistance. A minimum of 0.01 % is preferred to get such effects. However, when its content is above 1 %, it can lead to problems such as copper hot shortness during the hot rolling process.
Calcium content in the steel of present invention is below 0.005%. Calcium is added to steel of present invention in a preferable amount of 0.0001 to 0.005% as an optional element especially during the inclusion treatment, thereby, retarding the harmful effects of Sulfur.
Other elements such as, Magnesium can be added in the following proportions by weight Magnesium 0.0010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification.
The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing.
The microstructure of the Steels comprises several constituents, in area fraction of total microstructure.
Tempered martensite is present in the steel of present invention in a proportion of at least 60% wherein tempered martensite is the matrix phase for the steel of present invention. The tempered martensite of the steel of present invention preferably has its aspect ratio from 4 to 12 preferably and more preferably from 5 to 11 . The aspect ratio is the ratio between the longest and the shortest dimension within a single grain. Tempered martensite is formed from the martensite which forms during the cooling after the first annealing. Such martensite is then tempered during the annealing process. The tempered martensite of the steel of present invention imparts ductility and strength. t is preferred that; the content of tempered martensite is from 65% to 84% and more preferably from 70% to 80% by area fraction of total microstructure.
Fresh martensite can also be optionally present in the steel of present invention. Fresh martensite may form during cooling after annealing from remaining unstable residual austenite. Fresh martensite can be present from 0% to 15%, preferably from 0 to 10% and even better no fresh martensite is present.
Residual Austenite is an essential microstructural constituent of the steel of present invention and is present from 15% to 40%. Residual Austenite of the present invention imparts toughness to the steel of present invention. Residual Austenite of the present invention can be stabilized at room temperature through enrichment of Manganese and Carbon. The percentage of Carbon inside the Residual Austenite is preferably higher than 0.8wt% and lower than 1 ,1wt%. It is advantageous to have percentage of Manganese in Residual Austenite is preferably more than 5wt% and more preferably more than 5.5wt%. The preferable limit for the presence of Austenite is from 18% to 35% and more preferably from 18% to 30% wherein the preferable Carbon content limit in austenite is preferred from 0.9wt% to 1 .1 wt% and more preferably from 0.95wt% to 1.05wt%.
Polygonal Ferrite constitutes from 0% to 10% of the microstructure by area fraction for the Steel of the present invention. In the present invention, Polygonal Ferrite imparts high strength as well as elongation to the steel of present invention. Polygonal Ferrite may be formed during the soaking and cooling after annealing in steel of present invention. But whenever polygonal ferrite content is present above 10% in steel of present invention, the strength is not achieved.
Bainite may present in the steel of present invention from 0% to 5%. Up to 5%, bainite does not influence the target properties of the steel of present invention.
In addition to the above-mentioned microstructure, the microstructure of the hot rolled steel is free from microstructural components, such as Pearlite and Cementite. Carbides of alloying elements might be present in the steel of present invention in a cumulated amount from 0% to 5% such as of Niobium, Titanium, Vanadium and Iron . These carbides may increase the strength of the steel of present invention by precipitation strengthening, but whenever the presence of carbides is 5% or more, their precipitation consume partly the amount of Carbon, which is detrimental for the stabilization of residual austenite and the steel of present may not have adequate toughness.
A hot rolled steel according to the invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done either into ingots or continuously in form of thick slabs, thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm to 350mm for slabs down to several tens of millimeters for thin strip.
For example, a slab having the above-described chemical composition is manufactured by continuous casting. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.
The slab is reheated to a temperature from Ac3 + 50° C to 1300°C. In case the temperature of the slab is lower than least Ac3 + 50° C, excessive load is imposed on the rolling mill. Therefore, the temperature of the slab is sufficiently high so that hot rolling can be completed fully in the austenitic range. Reheating at temperatures above 1300°C must be avoided because it causes productivity loss and is also industrially expensive and some segregated parts may melt which may lead to breaking of slabs or cracking of slabs. Therefore, the preferred reheating temperature is from Ac3 + 100° C to 1280°C.
Hot rolling finishing temperature for the present invention is at least Ac3 and preferably from Ac3 to Ac3 + 100° C, more preferably from 840°C to 1000°C and even more preferably from 850°C to 990°C.
The hot rolled steel is then cooled from hot roll finishing temperature to a temperature range from Ms to 20°C at a cooling rate from 1 °C/s to 50°C/s to obtain a hot rolled steel strip. In a preferred embodiment, the cooling rate for this step of cooling is from 1 °C/s to 45°C/s and more preferably from 25°C/s to 40°C/s. The hot rolled strip may optionally be coiled wherein coiling temperature is between 20°C and 800°C. The hot rolled steel is being heated from a temperature range from Ms to 20°C up to the first annealing temperature TA1 which is from Ac3 to Ac3 +150°C and preferably from Ac3 to Ac3 +120°C, and more preferably from Ac3 to Ac3 +100°C, such heating being performed at a heating rate HR1 of at least 1 °C/s. The hot rolled steel strip is held at TA1 during 5 seconds to 6000 seconds to ensure the transformation to 100% austenite.
Then, the hot rolled steel is cooled wherein the cooling starts from TA1 at a cooling rate CR1 from 0.1 °C/s to 150°C/s, to a cooling stop temperature T1 which is in a range from Ms-10°C to 15°C. In a preferred embodiment, the cooling rate CR1 for such cooling is from 0.1 °C/s to 120°C/s. The preferred T1 temperature is from Ms-50°C to 20°C. Cooling rate for cooling after soaking must be sufficiently high to obtain the transformation of Austenite into Martensite. The cooling rate after first annealing is selected in a manner that it ensures at least 80% martensite in hot rolled strip at T1 .
The hot rolled steel is being heated from a temperature T1 up to second the annealing temperature TA2 which is from 550°C to Ac3, preferably from 600°C to Ac3 -40°C, such heating being performed at a heating rate HR2 of at least 1 °C/s.
The hot rolled steel is held at TA2 during 5 seconds to 6000 seconds to ensure the transformation of the microstructure to form 10% to 25% of austenite .
Then, the hot rolled steel is cooled wherein the cooling starts from TA2 at a cooling rate CR2 from 0.1 °C/s to 150°C/s, to a cooling stop temperature T2 which is in a range from Ms-10°C to 15°C. In a preferred embodiment, the cooling rate CR2 for such cooling is from 0.1 °C/s to 120°C/s. The preferred T2 temperature is from Ms-20°C to 20°C. Cooling rate after soaking must be sufficiently high to avoid the transformation of Austenite into Bainite so that a sufficient amount of carbon is available for stabilizing the residual austenite during the cooling after annealing. During this cooling the fresh martensite may form from some remaining unstable austenite.
Thereafter cooling the hot rolled steel to room temperature at a cooling rate CR3 from 0.1 °C/s to 150°C/s to obtain a hot rolled steel sheet. The hot rolled steel sheet thus obtained preferably has a thickness from 2mm to 100mm and more preferably from 2 mm to 80 mm and even more preferably from 2 mm to 50 mm.
EXAMPLES
The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only, and will display the advantageous features of the present invention.
Steel sheets made of steels with different compositions are gathered in Table 1 , where the steel sheets are produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the steel sheets obtained during the trials and table 4 gathers the result of evaluations of obtained properties. Ac3 and Ms temperature are determined through thermodynamic calculations done with the use of a software like Thermo-Calc®.
Table 1
I = according to the invention; R = reference; underlined values: not according to the invention.
able 2 able 2 gathers the process parameters implemented on steels of Table 1 .
Table 3
Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as SEM, EPMA, EBSD, XRD or any other microscope for determining microstructural composition of both the inventive steel and reference trials. The area fractions for the carbides is measured on polished samples after etching them in 2% Nital etching solution for 10 seconds and observed by an SEM. Polygonal Ferrite and tempered martensite are measured using EBSD wherein Electron backscattered diffraction (EBSD) is a SEM based technique to measure crystal orientations with a sub-micron resolution. An electron beam is focused on the 70° tilted specimen in the scanning electron microscope (SEM). Electrons that satisfy the Bragg condition for a family of planes are channelled and induce kikuchi bands. Electrons strike a phosphor screen and produce light, which is detected and digitized by a camera. The resulting EBS pattern is analyzed and indexed. This process is realized for each point analysed. For a given steel sample, an EBSD analysis of at least 4 images corresponding to a magnification of 1000 allows to identify the polygonal ferrite and tempered martensite microconstituents, their location and area percentage. The Residual Austenite area fraction is measured using XRD which are demonstrated in table 3.
The results are stipulated herein, in area fractions: 14 sample includes 1 % of niobium carbides and R1 sample includes 2% of iron carbides. No samples were containing any fresh martensite or bainite constituents.
Table 4 Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, yield strength and total elongation, tensile tests are conducted in accordance of NBN EN ISO6892-1 standards with tensile samples types is having A25. The toughness is tested by a Charpy test performed according to ISO 148-1. All measurements done on the inventive and reference steel are done for steel sheet taken in longitudinal direction (LD). The results of the various mechanical tests conducted in accordance to the standards are gathered
I = according to the invention; R = reference; underlined values: not according to the invention.

Claims

1. A hot rolled steel sheet having a composition comprising of the following elements, expressed in percentage by weight:
0.02 % < Carbon < 0.2 %
3 % < Manganese < 9% 0.2% < Silicon <1 .2%
0.9% < Aluminum < 2.5 %
0 % < Phosphorus < 0.03 %
0 % < Sulfur < 0.03%
0 % < Nitrogen < 0.025% and can contain one or more of the following optional elements
0 % < Molybdenum < 0.6% 0% < Titanium < 0.1 %
0.0001 % < Boron < 0.01 %
0% < Chromium < 0.5%
0% < Niobium < 0.1 %
0 % < Vanadium < 0.15%
0% < Nickel < 1 %
0% < Copper < 1 %
0% < Calcium < 0.005%
0 % < Magnesium < 0.0010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, at least 60% of tempered martensite, 15% to 40% residual austenite, 0% to 10% polygonal ferrite, 0% to 5% of bainite, 0 to 15% of fresh martensite and 0% to 5% of carbides of Niobium, Titanium, Vanadium or Iron.
2. Hot rolled steel sheet according to claim 1 , wherein the composition includes 0.3% to 1 % of Silicon.
3. Hot rolled steel sheet according to claim 1 or 2, wherein the composition includes 0.03% to 0.18% of Carbon.
4. Hot rolled steel sheet according to anyone of claims 1 to 3, wherein the composition includes 3.5% to 8.5% of Manganese.
5. Hot rolled steel sheet according to anyone of claim 1 to 4, wherein the composition includes 1 % to 2.3% of Aluminum.
6. Hot rolled steel sheet according to anyone of claim 1 to 5, wherein the amount of Martensite is from 70% to 80%.
7. Hot rolled steel sheet according to anyone of claims 1 to 6, wherein the amount of Residual Austenite from 18% to 35%
8. Hot rolled steel sheet according to anyone of claims 1 to 7, wherein said steel sheet has a tensile strength of 750 MPa or more, and a total elongation of 15% or more.
9. Hot rolled steel sheet according to anyone of claims 1 to 8, wherein the shape ratio of the tempered martensite is from 4 to 12.
10. A method of production of a hot rolled steel sheet comprising the following successive steps:
- providing a steel composition according to anyone of claims 1 to 5;
- reheating said semi-finished product to a temperature from Ac3 +50°C to 1300°C;
- rolling the said semi-finished product in the austenitic range wherein the hot rolling finishing temperature shall be at least Ac3 to obtain a hot rolled steel;
- the hot rolled steel is optionally coiled at a coiling temperature range is from 20°C to 800°C,
- then cooling the said hot rolled steel form hot rolling finishing temperature to a temperature range from Ms to 20°C, with a cooling rate from 1 °C/s to 50°C/s;
- then heating the said hot rolled steel from a temperature range from Ms- to 20°C to a temperature TA1 from Ac3 to Ac3 +150°C, with a heating rate HR1 of at least 1 °C/s, where it is held during 5 to 6000 seconds
- then cooling the said hot rolled steel, wherein cooling starts from TA1 to a cooling stop temperature T1 from Ms-10°C to 15°C, with a cooling rate CR1 from 0.1 °C/s to 150°C/s; 17
- then heating the said hot rolled steel from T1 to a temperature TA2 from 550°C to Ac3, with a heating rate HR2 of at least 1 °C/s, where it is held during 5 to 6000 seconds
- then cooling the said hot rolled steel, wherein cooling starts from TA2 to a cooling stop temperature T2 from Ms-10°C to 15°C, with a cooling rate
CR2 from 0.1 °C/s to 150°C/s;
- thereafter cooling the hot rolled steel to room temperature at a cooling rate CR3 from 0.1 °C/s to 150°C/s to obtain a hot rolled steel sheet. A method according to claim 10, wherein TA2 temperature is from 600°C to Ac3 -40°C. A method according to claims 13 or 14, wherein the T 1 temperature is from Ms- 20°C to 20°C. Use of a steel sheet according to anyone of claims 1 to 9 or of a steel sheet produced according to the method of claims 10 to 12, for the manufacture of parts of industrial machinery or green goods or yellows. Industrial machine comprising a part obtained according to claim 13.
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