WO2021116976A1 - Heat treated cold rolled steel sheet and a method of manufacturing thereof - Google Patents

Heat treated cold rolled steel sheet and a method of manufacturing thereof Download PDF

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Publication number
WO2021116976A1
WO2021116976A1 PCT/IB2020/061765 IB2020061765W WO2021116976A1 WO 2021116976 A1 WO2021116976 A1 WO 2021116976A1 IB 2020061765 W IB2020061765 W IB 2020061765W WO 2021116976 A1 WO2021116976 A1 WO 2021116976A1
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Prior art keywords
steel sheet
rolled steel
cold rolled
heat treated
temperature
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PCT/IB2020/061765
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French (fr)
Inventor
Matthieu SIEBENTRITT
Julien LAMOUCHE
Dorien DE KNIJF
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Arcelormittal
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Publication date
Application filed by Arcelormittal filed Critical Arcelormittal
Priority to CN202080084937.5A priority Critical patent/CN114761584A/en
Priority to EP20823957.4A priority patent/EP4073280A1/en
Priority to KR1020227019565A priority patent/KR20220095240A/en
Priority to JP2022535560A priority patent/JP2023505693A/en
Priority to CA3163313A priority patent/CA3163313C/en
Priority to MX2022007054A priority patent/MX2022007054A/en
Priority to US17/783,409 priority patent/US20230036084A1/en
Publication of WO2021116976A1 publication Critical patent/WO2021116976A1/en
Priority to ZA2022/05649A priority patent/ZA202205649B/en

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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
    • C22C38/04—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
    • 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
    • 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
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold 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
    • 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
    • C21D8/0273—Final recrystallisation 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
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing 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
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation 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
    • 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
    • C22C38/06—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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum

Definitions

  • the present invention relates to cold rolled steel sheet with high strength and high formability having tensile strength of 950 MPa or more and a hole expansion ratio of more than 56% which is suitable for use as a steel sheet for vehicles.
  • Automotive parts are required to satisfy two inconsistent necessities, viz. ease of forming and strength but in recent years a third requirement of improvement in fuel consumption is also bestowed upon automobiles in view of global environment concerns.
  • automotive parts must be made of material having high formability in order that to fit in the criteria of ease of fit in the intricate automobile assembly and at same time have to improve strength for vehicle crashworthiness and durability while reducing weight of vehicle to improve fuel efficiency. Therefore, intense Research and development endeavors are put in to reduce the amount of material utilized in car by increasing the strength of material. Conversely, an increase in strength of steel sheets decreases formability, and thus development of materials having both high strength and high formability is necessitated.
  • Earlier research and developments in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are enumerated herein for conclusive appreciation of the present invention:
  • JP2012111978 is a patent application having a composition C: 0.05-0.3%, Si: 0.01-3.0%, Mn:0.5-3%, Al: 0.01-0.1%, and the balance Fe and incidental impurities and having a component composition consisting of, ferrite and tempered martensite as a main component of the high-strength cold rolled steel sheet but such steel is not able to reach more than 50% of hole expansion ratio.
  • EP2971209 is patent that relates to a high strength hot dip galvanised complex phase steel strip having improved formability to be used in the automotive industry having an mandatory elemental composition C: 0.13 - 0.19 %, Mn :1.70 - 2.50 % Si: 0- 0.15 %, Al : 0.40 - 1.00 %, Cr : 0.05 - 0.25 %, Nb : 0.01 - 0.05 %,P : 0- 0.10 %, Ca: 0-0.004 %, S : 0- 0.05 %, N : 0- 0.007
  • the known prior art related to the manufacture of high strength and high formability steel sheets is inflicted by one or the other lacuna: hence there lies a need for a cold rolled steel sheet having high strength and high formability and a method of manufacturing the same.
  • the purpose of the present invention is to solve these problems by making available cold-rolled steel sheets that simultaneously have:
  • - a total elongation greater than or equal to 8%.
  • - a hole expansion ratio of 56% or more and preferably 57% or more.
  • the steel sheet according to the invention may have a yield strength value greater than or above 750 MPa.
  • such steel can also have a good suitability for forming, in particular for rolling with good weldability and coatability.
  • Another object of the present invention is also to make available a method for the manufacturing of these sheets that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts.
  • Carbon is present in the steel between 0.09% and 0.15%. Carbon is an element necessary for increasing the strength of a steel sheet by producing a low- temperature transformation phase such as martensite. A content less than 0.09% would not allow securing adequate amount of martensite, thereby decreasing strength as well as ductility. On the other hand, at a carbon content exceeding 0.15%, a weld zone and a heat-affected zone are significantly hardened, and thus the mechanical properties of the weld zone are impaired.
  • the preferred limit for Carbon is between 0.1% and 0.14%, more preferably 0.1% and 0.13%.
  • Manganese content of the steel of present invention is between 1.8% and 2.5%.
  • Manganese is an element that imparts strength by solid solution strengthening. An amount of at least about 1.8 % by weight of manganese has been found in order to provide the strength and hardenability of the steel sheet. Thus, a higher percentage of Manganese such as 1.9% to 2.4% is preferred and more preferred limit is between 2.0% and 2.3%. But when manganese is more than 2.5%, this produces adverse effects such as slowing down the transformation of austenite to martensite during the cooling after annealing, leading to a reduction of strength. Moreover, a manganese content above 2.5% would also reduce the weldability of the present steel.
  • Silicon is an essential element for the steel of present invention, Silicon is present between 0.2% and 0.7%. Silicon is added to the steel of present invention to impart strength by solid solution strengthening. Silicon plays a part in the formation of the microstructure by preventing the precipitation of carbides and by promoting the formation of martensite. But whenever the silicon content is more than 0.7%, surface properties and weldability of steel is deteriorated, therefore the Silicon content is preferred between 0.3% and 0.7% and more preferably between 0.4% and 0.6%.
  • Aluminum content of the present invention is between 0.01% and 0.1%. Aluminum is added to de-oxidise the steel of present invention. Aluminum is an alphageneous element and also retarding the formation of carbides. This can increase the formability and ductility of steel.
  • Aluminum content is required at 0.01% or more.
  • Ac3 point increases beyond acceptable, austenite single phase is very difficult to achieve industrially hence hot rolling in complete austenite region cannot be performed. Therefore, Aluminum content must not be more than 0.1%.
  • the preferable limit for the presence of Aluminum is between 0.01% and 0.08% and more preferably 0.01 % and 0.05%.
  • Phosphorus content of the steel of present invention is limited to 0.09%.
  • Phosphorus is an element which hardens in solid solution and also interferes with formation of carbides. Therefore a small amount of phosphorus, of at least 0.002% can be advantageous, but phosphorus has its adverse effects also, such as a reduction of the spot weldability and the hot ductility, particularly due to its tendency to segregation at the grain boundaries or co-segregation with manganese. For these reasons, its content is preferably limited a maximum of 0.02%.
  • Sulfur is not an essential element but may be contained as an impurity in steel.
  • the sulfur content is preferably as low as possible, but is 0.09% or less and preferably at most 0.01%, from the viewpoint of manufacturing cost. Further if higher sulfur is present in steel it combine to form sulfide especially with Mn and Ti and reduces their beneficial impact on the present invention.
  • Nitrogen is limited to 0.09% in order to avoid ageing of material, nitrogen forms the nitrides which impart strength to the steel of present invention by precipitation strengthening with Vanadium and Niobium but whenever the presence of nitrogen is more than 0.09% it can form high amount of Aluminum Nitrides which are detrimental for the present invention hence the preferable upper limit for nitrogen is 0.01%.
  • Niobium is an optional element that can be added to the steel up to 0.1%, preferably between 0.001% and 0.1%. It is suitable for forming carbonitrides to impart strength to the steel according to the invention by precipitation hardening. Because niobium delays the recrystallization during the heating, the microstructure formed at the end of the holding temperature and as a consequence after the complete annealing is finer, this leads to the hardening of the product. But, when the niobium content is above 0.1% the amount of carbo-nitrides is not favorable for the present invention as large amount of carbo-nitrides tend to reduce the ductility of the steel.
  • Titanium is an optional element which may be added to the steel of the present invention up to 0.1%, preferably between 0.001% and 0.1%. As niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form TiN appearing during solidification of the cast product. The amount of Ti is so limited to 0.1% to avoid coarse TiN detrimental for hole expansion. In case the titanium content is below 0.001% it does not impart any effect on the steel of present invention. Chromium content of the steel of present invention is between 0 % and 1%.
  • Chromium is an optional element that provide strength and hardening to the steel, but when used above 1% impairs surface finish of the steel.
  • Molybdenum is an optional element that constitutes between 0% and 1% of the Steel of present invention; Molybdenum increases the hardenability of the steel of present invention and influences the transformation of austenite to Ferrite and Bainite during cooling after annealing. 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 1%.
  • Vanadium is an optional element which may be added to the steel of the present invention up to 0.1%, preferably between 0.001% and 0.01%. As niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form VN appearing during solidification of the cast product.
  • V is so limited to 0.1% to avoid coarse VN detrimental for hole expansion.
  • vanadium content is below 0.001% it does not impart any effect on the steel of present invention.
  • Calcium is an optional element which may be added to the steel of present invention up to 0.005%, preferably between 0.001% and 0.005%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the steel by arresting the detrimental sulphur content in globularizing it.
  • Ce ⁇ 0.1%, B ⁇ 0.01%, Mg ⁇ 0.05% and Zr ⁇ 0.05% can be added individually or in combination in the following proportions: Ce ⁇ 0.1%, B ⁇ 0.01%, Mg ⁇ 0.05% and Zr ⁇ 0.05%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. Present invention does not intend to add Copper and Nickel but these elements may be present as residuals up to 0.1% either severaly or cumulatively.
  • the remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.
  • the microstructure of the steel sheet according to the invention comprises in area fractions 65% to 85% of Tempered Martensite, 0% and 5% of residual austenite and cumulative amount of bainite and ferrite between 15% and 35%.
  • Tempered martensite constitutes the matrix phase for the steel of present invention
  • Tempered Martensite constitutes between 65% and 85% of the microstructure by area fraction. Tempered martensite is formed from the martensite which forms during the second step of cooling after annealing and particularly below Ms temperature and more particularly between Ms-50°C and 20°C. Such martensite is then tempered during the holding at a tempering temperature Temper between 150°C and 400°C. Tempered martensite of the present invention does not contain coarse Iron Carbides specifically iron based carbide having a grain size aspect ratio of 3.5 or more because these iron carbides inhibits the present invention to reach the target hole expansion ratio. The martensite of the present invention imparts ductility and strength to such steel.
  • the content of martensite is between 65% and 80% and more preferably between 68% and 78%.
  • Bainite and Ferrite are cumulatively present in the steel between 15% and 35%.
  • the range for cumulated amount of ferrite and bainite is between 20% and 35% and more preferably between 22% and 32%.
  • Ferrite constituent improves the properties of the steel of the present invention, in particular regarding elongation and hole expansion ratio as ferrite is a soft and intrinsically ductile constituent.
  • This ferrite is mainly formed during the first step of cooling after annealing.
  • ferrite can be present at least in an amount of 15%.
  • Bainite can impart strength to the steel but when present in a large amount it may adversely impact the hole expansion ratio and elongation of the steel. Bainite forms during the reheating before tempering. In a preferred embodiment, the bainite content is kept between 0% and 10%more preferably below 8% and even more preferably below 5%. Residual Austenite is an optional phase that can be present between 0% and 5% in the steel, but is preferably not present.
  • the steel sheet according to the invention may be obtained by any appropriate method. It is however preferred to use the process according to the preferred embodiments of the invention, which comprises the following successive steps:
  • Such process includes providing a semi-finished product of steel with a chemical composition according to the invention.
  • the semi-finished product can be casted either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm for slabs up to several tens of millimeters for thin strip, for example.
  • a slab will be considered as a semi-finished product.
  • a slab having the above-described chemical composition is manufactured by continuous casting wherein the slab preferably underwent a direct soft reduction during casting to ensure the elimination of central segregation and porosity reduction.
  • 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 temperature of the slab which is subjected to hot rolling is at least 1000°C, preferably above 1100°C and must be below 1250°C.
  • the temperature of the slab is lower than 1000° C, excessive load is imposed on a rolling mill, and further, the temperature of the steel may decrease to a ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed ferrite contained in the structure.
  • the temperature must not be above 1250°C as there would be a risk of formation of rough ferrite grains resulting in coarse ferrite grain which decreases the capacity of these grains to re-crystallize during hot rolling.
  • the temperature of the slab is preferably sufficiently high so that hot rolling can be completed entirely in the austenitic range, the finishing hot rolling temperature remaining above Ac3 and preferably above Ac3 + 50°C. It is necessary that the final rolling be performed above Ac3, because below this temperature the steel sheet exhibits a significant drop in rollability.
  • a final rolling temperature is preferably above Ac3 +50° C to have a structure that is favorable to recrystallization and rolling.
  • the sheet obtained in this manner is then cooled down at a cooling rate of at least 30°C/s to the coiling temperature which is below 600°C.
  • the cooling rate will be less than or equal to 65°C/s and above 35°C/s.
  • the coiling temperature is preferably of at least 350°C to avoid the transformation of austenite into ferrite and pearlite and to contribute in forming an homogenous bainite and martensite microstructure.
  • the coiled hot rolled steel sheet may be cooled down to room temperature before subjecting it to an optional hot band annealing or may be send to an optional hot band annealing directly.
  • Hot rolled steel sheet may be subjected to an optional pickling to remove the scale formed during the hot rolling, if needed.
  • the hot rolled sheet is then subjected to an optional hot band annealing at a temperature between 400°C and 750°C, preferably during 1 to 96 hours.
  • pickling of this hot rolled steel sheet may be performed if necessary to remove the scale.
  • the hot rolled steel sheets are then cold rolled with a thickness reduction between 35 to 90%.
  • the cold rolled steel sheet is then subjected to annealing to impart the steel of present invention with targeted microstructure and mechanical properties.
  • the cold rolled steel sheet is heated in a two- step heating process, in step one the cold rolled sheets is heated to a temperature HT1 between 600°C and 650°C at a heating rate HR1 of at least10°C/s. Then, in step two, the cold rolled sheet is heated from HT1 to an annealing temperature between Ac3 and Ac3 +200°C at a heating rate HR2 of at leastl °C/s and preferably at least 2.0°C/sHR1 is always higher than HR2.
  • the preferred HR1 is at least 15°C/s and the preferred HT1 temperature range is between 600°C and 630°C.
  • the preferred range for annealing temperature is between Ac3 +10°C and Ac3 +150°C and more preferably between Ac3 +20°C and Ac3 +100°C.
  • the cold rolled steel sheet is held at the annealing temperature during at least 5s and not more than 1000s.
  • the temperature and time are selected to ensure 100% re-crystallization i.e. to obtain a percentage of 100% austenite at the end of the annealing.
  • step one the cold rolled sheet is cooled from the annealing temperature to a temperature CT1 between 675°C and 725°C at a cooling rate CR1 of 10°C/s or less.
  • step two the cold rolled sheet is cooled from CT1 to CT2 between 450°C and 550°C at a cooling rate CR2 of at least 30°C/s.
  • step three the cold rolled sheet is cooled from CT2 to CT3 between Ms-50°C and 20°C at a cooling rate CR3 which is at least 200°C/s.
  • the cooling rate CR1 is 5° C/s or less and CT1 is preferably between 685°C and 720°C and more preferably 685°C and 700°C.
  • the preferred range for CR2 is at least 40°C/s and the preferred range for CT2 is between 450°C and 525°C and more preferably between 460°C and 510°C.
  • the preferred range for CR 3 is at least 300°C/s and more preferably at least 400°C/s.
  • Preferred limit for CT3 is between Ms-80°C and 20°C and more preferably between Ms-100°C and 20°C.
  • the cold rolled steel sheet at a heating rate of at least 10°C/s, or better of at least 20°C/s and to a tempering temperature between 300°C and 380°C and held at tempering temperature during at least 100 s but not more than 1000 s. to obtain tempered martensite, conferring the steel of present invention with good mechanical properties.
  • the preferred tempering temperature range is between 320°C and 360°C and more preferably is 330°C and 350°C.
  • the cold rolled steel sheet is then cooled to room temperature, preferably at a cooling rate of 200°C/s or less.
  • An optional skin pass operation with a reduction rate below 1% may be performed at that stage or an optional tension leveling operation.
  • the heat treated cold rolled sheet may then be optionally coated by electrodeposition or vacuum coating or any other suitable process.
  • An optional post batch annealing preferably done at 170 to 210°C during 12h to 30h can be done optionally after annealing on uncoated product or after coating on coated product in order to reduce hardness gradient between phases and ensure degasing for coated products.
  • Table 2 gathers the annealing process parameters implemented on steels of Table 1.
  • Table 2 also shows Ac3 and Martensite transformation Ms temperatures of the steel samples.
  • the calculation of Ac3 and Ms is done by using following formulas:
  • samples were heated to a temperature between 1000° C and 1250°C and then subjected to hot rolling with finish temperature 890° C and thereafter were coiled at a temperature below 600°C.
  • the hot rolled coils were then processed as claimed and cold rolled with a thickness reduction between 35 to 90%.
  • Table 2a underlined values: not according to the invention.
  • Table 2b
  • Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials.
  • Table 3 microstructures of the trials underlined values: not according to the invention.
  • Table 4 gathers the mechanical properties of both the inventive steel and reference steel.
  • the tensile strength, yield strength and total elongation test are conducted in accordance with ISO 6892 standards, whereas to estimate hole expansion, a test called hole expansion is applied according the standard IS016630:2009.
  • HER% 100*(Df-Di)/Di Table 4 : mechanical properties of the trials underlined values: not according to the invention.

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Abstract

A heat treated and cold rolled steel sheet having a composition comprising of the following elements 0.09% ≤ Carbon ≤ 0.15 %,1.8% ≤ Manganese ≤ 2.5%, 0.2 % ≤ Silicon ≤ 0.7 %, 0.01% ≤ Aluminum ≤ 0.1%, 0 % ≤ Phosphorus ≤ 0.09 %, 0 % ≤ Sulfur ≤ 0.09 %, 0 % ≤ Nitrogen ≤ 0.09%, 0 % ≤ Niobium ≤ 0.1%, 0 % ≤ Titanium ≤ 0.1%, 0 % ≤ Chromium ≤ 1%, 0 % ≤ Molybdenum ≤ 1%, 0 % ≤ Vanadium ≤ 0.1%, 0 % ≤ Calcium ≤ 0.005%, 0 % ≤ Boron ≤ 0.01%, 0 % ≤ Cerium ≤ 0.1%, 0 % ≤ Magnesium ≦ 0.05%, 0 % ≤ Zirconium ≦ 0.05% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 65 to 85% Tempered Martensite, 0% to 5% Residual Austenite and a cumulative presence of Ferrite and Bainite between 15 and 35%.

Description

HEAT TREATED COLD ROLLED STEEL SHEET AND A METHOD OF
MANUFACTURING THEREOF
The present invention relates to cold rolled steel sheet with high strength and high formability having tensile strength of 950 MPa or more and a hole expansion ratio of more than 56% which is suitable for use as a steel sheet for vehicles.
Automotive parts are required to satisfy two inconsistent necessities, viz. ease of forming and strength but in recent years a third requirement of improvement in fuel consumption is also bestowed upon automobiles in view of global environment concerns. Thus, now automotive parts must be made of material having high formability in order that to fit in the criteria of ease of fit in the intricate automobile assembly and at same time have to improve strength for vehicle crashworthiness and durability while reducing weight of vehicle to improve fuel efficiency. Therefore, intense Research and development endeavors are put in to reduce the amount of material utilized in car by increasing the strength of material. Conversely, an increase in strength of steel sheets decreases formability, and thus development of materials having both high strength and high formability is necessitated. Earlier research and developments in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are enumerated herein for conclusive appreciation of the present invention:
JP2012111978 is a patent application having a composition C: 0.05-0.3%, Si: 0.01-3.0%, Mn:0.5-3%, Al: 0.01-0.1%, and the balance Fe and incidental impurities and having a component composition consisting of, ferrite and tempered martensite as a main component of the high-strength cold rolled steel sheet but such steel is not able to reach more than 50% of hole expansion ratio. EP2971209 is patent that relates to a high strength hot dip galvanised complex phase steel strip having improved formability to be used in the automotive industry having an mandatory elemental composition C: 0.13 - 0.19 %, Mn :1.70 - 2.50 % Si: 0- 0.15 %, Al : 0.40 - 1.00 %, Cr : 0.05 - 0.25 %, Nb : 0.01 - 0.05 %,P : 0- 0.10 %, Ca: 0-0.004 %, S : 0- 0.05 %, N : 0- 0.007
% the balance being Fe and inevitable impurities, wherein 0.40 % < Al + Si < 1.05 % and Mn + Cr > 1.90 %, and having a complex phase microstructure, in volume percent, comprising 8-12 % retained austenite, 20 - 50 % bainite, less than 10 % martensite, the remainder being ferrite but the granted patent is unable to reach the tensile strength beyond 900MPa.
The known prior art related to the manufacture of high strength and high formability steel sheets is inflicted by one or the other lacuna: hence there lies a need for a cold rolled steel sheet having high strength and high formability and a method of manufacturing the same. The purpose of the present invention is to solve these problems by making available cold-rolled steel sheets that simultaneously have:
- an ultimate tensile strength greater than or equal to 950 MPa and preferably above 980 MPa, or even above 1000 MPa,
- a total elongation greater than or equal to 8%. - a hole expansion ratio of 56% or more and preferably 57% or more.
In a preferred embodiment, the steel sheet according to the invention may have a yield strength value greater than or above 750 MPa.
Preferably, such steel can also have a good suitability for forming, in particular for rolling with good weldability and coatability. Another object of the present invention is also to make available a method for the manufacturing of these sheets that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts. Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.
Carbon is present in the steel between 0.09% and 0.15%. Carbon is an element necessary for increasing the strength of a steel sheet by producing a low- temperature transformation phase such as martensite. A content less than 0.09% would not allow securing adequate amount of martensite, thereby decreasing strength as well as ductility. On the other hand, at a carbon content exceeding 0.15%, a weld zone and a heat-affected zone are significantly hardened, and thus the mechanical properties of the weld zone are impaired. The preferred limit for Carbon is between 0.1% and 0.14%, more preferably 0.1% and 0.13%.
Manganese content of the steel of present invention is between 1.8% and 2.5%. Manganese is an element that imparts strength by solid solution strengthening. An amount of at least about 1.8 % by weight of manganese has been found in order to provide the strength and hardenability of the steel sheet. Thus, a higher percentage of Manganese such as 1.9% to 2.4% is preferred and more preferred limit is between 2.0% and 2.3%. But when manganese is more than 2.5%, this produces adverse effects such as slowing down the transformation of austenite to martensite during the cooling after annealing, leading to a reduction of strength. Moreover, a manganese content above 2.5% would also reduce the weldability of the present steel.
Silicon is an essential element for the steel of present invention, Silicon is present between 0.2% and 0.7%. Silicon is added to the steel of present invention to impart strength by solid solution strengthening. Silicon plays a part in the formation of the microstructure by preventing the precipitation of carbides and by promoting the formation of martensite. But whenever the silicon content is more than 0.7%, surface properties and weldability of steel is deteriorated, therefore the Silicon content is preferred between 0.3% and 0.7% and more preferably between 0.4% and 0.6%. Aluminum content of the present invention is between 0.01% and 0.1%. Aluminum is added to de-oxidise the steel of present invention. Aluminum is an alphageneous element and also retarding the formation of carbides. This can increase the formability and ductility of steel. In order to obtain such an effect, Aluminum content is required at 0.01% or more. However, when the Aluminum content exceeds 0.1%, Ac3 point increases beyond acceptable, austenite single phase is very difficult to achieve industrially hence hot rolling in complete austenite region cannot be performed. Therefore, Aluminum content must not be more than 0.1%. The preferable limit for the presence of Aluminum is between 0.01% and 0.08% and more preferably 0.01 % and 0.05%.
Phosphorus content of the steel of present invention is limited to 0.09%. Phosphorus is an element which hardens in solid solution and also interferes with formation of carbides. Therefore a small amount of phosphorus, of at least 0.002% can be advantageous, but phosphorus has its adverse effects also, such as a reduction of the spot weldability and the hot ductility, particularly due to its tendency to segregation at the grain boundaries or co-segregation with manganese. For these reasons, its content is preferably limited a maximum of 0.02%.
Sulfur is not an essential element but may be contained as an impurity in steel. The sulfur content is preferably as low as possible, but is 0.09% or less and preferably at most 0.01%, from the viewpoint of manufacturing cost. Further if higher sulfur is present in steel it combine to form sulfide especially with Mn and Ti and reduces their beneficial impact on the present invention.
Nitrogen is limited to 0.09% in order to avoid ageing of material, nitrogen forms the nitrides which impart strength to the steel of present invention by precipitation strengthening with Vanadium and Niobium but whenever the presence of nitrogen is more than 0.09% it can form high amount of Aluminum Nitrides which are detrimental for the present invention hence the preferable upper limit for nitrogen is 0.01%.
Niobium is an optional element that can be added to the steel up to 0.1%, preferably between 0.001% and 0.1%. It is suitable for forming carbonitrides to impart strength to the steel according to the invention by precipitation hardening. Because niobium delays the recrystallization during the heating, the microstructure formed at the end of the holding temperature and as a consequence after the complete annealing is finer, this leads to the hardening of the product. But, when the niobium content is above 0.1% the amount of carbo-nitrides is not favorable for the present invention as large amount of carbo-nitrides tend to reduce the ductility of the steel.
Titanium is an optional element which may be added to the steel of the present invention up to 0.1%, preferably between 0.001% and 0.1%. As niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form TiN appearing during solidification of the cast product. The amount of Ti is so limited to 0.1% to avoid coarse TiN detrimental for hole expansion. In case the titanium content is below 0.001% it does not impart any effect on the steel of present invention. Chromium content of the steel of present invention is between 0 % and 1%.
Chromium is an optional element that provide strength and hardening to the steel, but when used above 1% impairs surface finish of the steel.
Molybdenum is an optional element that constitutes between 0% and 1% of the Steel of present invention; Molybdenum increases the hardenability of the steel of present invention and influences the transformation of austenite to Ferrite and Bainite during cooling after annealing. 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 1%. Vanadium is an optional element which may be added to the steel of the present invention up to 0.1%, preferably between 0.001% and 0.01%. As niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form VN appearing during solidification of the cast product. The amount of V is so limited to 0.1% to avoid coarse VN detrimental for hole expansion. In case the vanadium content is below 0.001% it does not impart any effect on the steel of present invention. Calcium is an optional element which may be added to the steel of present invention up to 0.005%, preferably between 0.001% and 0.005%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the steel by arresting the detrimental sulphur content in globularizing it.
Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following proportions: Ce < 0.1%, B < 0.01%, Mg < 0.05% and Zr < 0.05%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. Present invention does not intend to add Copper and Nickel but these elements may be present as residuals up to 0.1% either severaly or cumulatively.
The remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.
The microstructure of the steel sheet according to the invention comprises in area fractions 65% to 85% of Tempered Martensite, 0% and 5% of residual austenite and cumulative amount of bainite and ferrite between 15% and 35%. Tempered martensite constitutes the matrix phase for the steel of present invention
Tempered Martensite constitutes between 65% and 85% of the microstructure by area fraction. Tempered martensite is formed from the martensite which forms during the second step of cooling after annealing and particularly below Ms temperature and more particularly between Ms-50°C and 20°C. Such martensite is then tempered during the holding at a tempering temperature Temper between 150°C and 400°C. Tempered martensite of the present invention does not contain coarse Iron Carbides specifically iron based carbide having a grain size aspect ratio of 3.5 or more because these iron carbides inhibits the present invention to reach the target hole expansion ratio. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of martensite is between 65% and 80% and more preferably between 68% and 78%. Bainite and Ferrite are cumulatively present in the steel between 15% and 35%. In a preferred embodiment, the range for cumulated amount of ferrite and bainite is between 20% and 35% and more preferably between 22% and 32%.
Ferrite constituent improves the properties of the steel of the present invention, in particular regarding elongation and hole expansion ratio as ferrite is a soft and intrinsically ductile constituent. This ferrite is mainly formed during the first step of cooling after annealing. In a preferred embodiment, ferrite can be present at least in an amount of 15%.
Bainite can impart strength to the steel but when present in a large amount it may adversely impact the hole expansion ratio and elongation of the steel. Bainite forms during the reheating before tempering. In a preferred embodiment, the bainite content is kept between 0% and 10%more preferably below 8% and even more preferably below 5%. Residual Austenite is an optional phase that can be present between 0% and 5% in the steel, but is preferably not present.
In a preferred embodiment the steel sheet according to the invention may be obtained by any appropriate method. It is however preferred to use the process according to the preferred embodiments of the invention, which comprises the following successive steps:
Such process includes providing a semi-finished product of steel with a chemical composition according to the invention. The semi-finished product can be casted either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm for slabs up to several tens of millimeters for thin strip, for example.
For the purpose of simplification of the present invention, a slab will be considered as a semi-finished product. A slab having the above-described chemical composition is manufactured by continuous casting wherein the slab preferably underwent a direct soft reduction during casting to ensure the elimination of central segregation and porosity reduction. 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 temperature of the slab which is subjected to hot rolling is at least 1000°C, preferably above 1100°C and must be below 1250°C. In case the temperature of the slab is lower than 1000° C, excessive load is imposed on a rolling mill, and further, the temperature of the steel may decrease to a ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed ferrite contained in the structure. Further, the temperature must not be above 1250°C as there would be a risk of formation of rough ferrite grains resulting in coarse ferrite grain which decreases the capacity of these grains to re-crystallize during hot rolling. The larger the initial ferrite grain size, the less easily it re-crystallizes, which means that reheat temperatures above 1250°C must be avoided because they are industrially expensive and unfavorable in terms of the recrystallization of ferrite. The temperature of the slab is preferably sufficiently high so that hot rolling can be completed entirely in the austenitic range, the finishing hot rolling temperature remaining above Ac3 and preferably above Ac3 + 50°C. It is necessary that the final rolling be performed above Ac3, because below this temperature the steel sheet exhibits a significant drop in rollability. A final rolling temperature is preferably above Ac3 +50° C to have a structure that is favorable to recrystallization and rolling.
The sheet obtained in this manner is then cooled down at a cooling rate of at least 30°C/s to the coiling temperature which is below 600°C. Preferably, the cooling rate will be less than or equal to 65°C/s and above 35°C/s. The coiling temperature is preferably of at least 350°C to avoid the transformation of austenite into ferrite and pearlite and to contribute in forming an homogenous bainite and martensite microstructure.
The coiled hot rolled steel sheet may be cooled down to room temperature before subjecting it to an optional hot band annealing or may be send to an optional hot band annealing directly.
Hot rolled steel sheet may be subjected to an optional pickling to remove the scale formed during the hot rolling, if needed. The hot rolled sheet is then subjected to an optional hot band annealing at a temperature between 400°C and 750°C, preferably during 1 to 96 hours.
Thereafter, pickling of this hot rolled steel sheet may be performed if necessary to remove the scale.
The hot rolled steel sheets are then cold rolled with a thickness reduction between 35 to 90%. The cold rolled steel sheet is then subjected to annealing to impart the steel of present invention with targeted microstructure and mechanical properties.
To anneal the cold rolled steel sheet, the cold rolled steel sheet is heated in a two- step heating process, in step one the cold rolled sheets is heated to a temperature HT1 between 600°C and 650°C at a heating rate HR1 of at least10°C/s. Then, in step two, the cold rolled sheet is heated from HT1 to an annealing temperature between Ac3 and Ac3 +200°C at a heating rate HR2 of at leastl °C/s and preferably at least 2.0°C/sHR1 is always higher than HR2.
The preferred HR1 is at least 15°C/s and the preferred HT1 temperature range is between 600°C and 630°C. The preferred range for annealing temperature is between Ac3 +10°C and Ac3 +150°C and more preferably between Ac3 +20°C and Ac3 +100°C.
Then the cold rolled steel sheet is held at the annealing temperature during at least 5s and not more than 1000s. The temperature and time are selected to ensure 100% re-crystallization i.e. to obtain a percentage of 100% austenite at the end of the annealing.
The sheet is then cooled in a three-step cooling process. In step one, the cold rolled sheet is cooled from the annealing temperature to a temperature CT1 between 675°C and 725°C at a cooling rate CR1 of 10°C/s or less. Then, in step two, the cold rolled sheet is cooled from CT1 to CT2 between 450°C and 550°C at a cooling rate CR2 of at least 30°C/s. Then, in step three, the cold rolled sheet is cooled from CT2 to CT3 between Ms-50°C and 20°C at a cooling rate CR3 which is at least 200°C/s.
In a preferred embodiment, the cooling rate CR1 is 5° C/s or less and CT1 is preferably between 685°C and 720°C and more preferably 685°C and 700°C. The preferred range for CR2 is at least 40°C/s and the preferred range for CT2 is between 450°C and 525°C and more preferably between 460°C and 510°C. The preferred range for CR 3 is at least 300°C/s and more preferably at least 400°C/s. Preferred limit for CT3 is between Ms-80°C and 20°C and more preferably between Ms-100°C and 20°C.
Then the cold rolled steel sheet at a heating rate of at least 10°C/s, or better of at least 20°C/s and to a tempering temperature between 300°C and 380°C and held at tempering temperature during at least 100 s but not more than 1000 s. to obtain tempered martensite, conferring the steel of present invention with good mechanical properties. The preferred tempering temperature range is between 320°C and 360°C and more preferably is 330°C and 350°C.
The cold rolled steel sheet is then cooled to room temperature, preferably at a cooling rate of 200°C/s or less. An optional skin pass operation with a reduction rate below 1% may be performed at that stage or an optional tension leveling operation.
The heat treated cold rolled sheet may then be optionally coated by electrodeposition or vacuum coating or any other suitable process.
An optional post batch annealing, preferably done at 170 to 210°C during 12h to 30h can be done optionally after annealing on uncoated product or after coating on coated product in order to reduce hardness gradient between phases and ensure degasing for coated products. EXAMPLES
The following tests and examples 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 and expound the significance of the parameters chosen by inventors after extensive experiments and further establish the properties that can be achieved by the steel according to the invention. Samples of the steel sheets according to the invention and to some comparative grades were prepared with the compositions gathered in table 1 and the processing parameters gathered in table 2. The corresponding microstructures of those steel sheets were gathered in table 3 and the properties in table 4. Table 1 depicts the steels with the compositions expressed in percentages by weight.
Table 1 : composition of the trials
Figure imgf000012_0001
Table 2 gathers the annealing process parameters implemented on steels of Table 1.
Table 2 also shows Ac3 and Martensite transformation Ms temperatures of the steel samples. The calculation of Ac3 and Ms is done by using following formulas:
Figure imgf000012_0002
Further, the samples were heated to a temperature between 1000° C and 1250°C and then subjected to hot rolling with finish temperature 890° C and thereafter were coiled at a temperature below 600°C. The hot rolled coils were then processed as claimed and cold rolled with a thickness reduction between 35 to 90%.
Table 2 : process parameters of the trials
Table 2a
Figure imgf000014_0001
underlined values: not according to the invention. Table 2b
Figure imgf000014_0002
Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials. Table 3 : microstructures of the trials
Figure imgf000014_0003
underlined values: not according to the invention. Table 4 gathers the mechanical properties of both the inventive steel and reference steel. The tensile strength, yield strength and total elongation test are conducted in accordance with ISO 6892 standards, whereas to estimate hole expansion, a test called hole expansion is applied according the standard IS016630:2009. In this test, sample is subjected to punching to form a hole of 10mm (=Di) and deformed. After deformation, the hole diameter Df was measured and the hole expansion ratio (HER) is calculated using the under formula:
HER%= 100*(Df-Di)/Di Table 4 : mechanical properties of the trials
Figure imgf000015_0001
underlined values: not according to the invention.
The examples show that the steel sheets according to the invention are the only one to show all the targeted properties thanks to their specific composition and microstructures.

Claims

1. A heat treated and cold rolled steel sheet having a composition comprising of the following elements, expressed in percentage by weight:
0.09% < Carbon < 0.15 %
1.8% < Manganese < 2.5%
0.2 % < Silicon < 0.7 %
0.01% < Aluminum < 0.1%
0 % < Phosphorus < 0.09 %
0 % < Sulfur < 0.09 %.
0 % < Nitrogen < 0.09% and can contain one or more of the following optional elements 0 % < Niobium < 0.1%
0 % < Titanium < 0.1 %
0 % < Chromium < 1%
0 % < Molybdenum < 1%
0 % < Vanadium < 0.1 %
0 % £ Calcium < 0.005%
0 % < Boron < 0.01%
0 % £ Cerium < 0.1%
0 % < Magnesium £ 0.05%
0 % < Zirconium £ 0.05% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 65 to 85% Tempered Martensite, 0% to 5% Residual Austenite and a cumulative presence of Ferrite and Bainite between 15 and 35%.
2. Heat treated and cold rolled steel sheet according to claim 1 , wherein the composition includes 0.3% to 0.7% of Silicon.
3. Heat treated and cold rolled steel sheet according to anyone of claims 1 or 2, wherein the composition includes 0.01% to 0.08% of Aluminum.
4. Heat treated and cold rolled steel sheet according to anyone of claims 1 to 3, wherein the composition includes 1.9% to 2.4% of Manganese.
5. Heat treated and cold rolled steel sheet according to anyone of claims 1 to 4, wherein the composition includes 0.1 % to 0.13% of Carbon.
6. Heat treated and cold rolled steel sheet according to anyone of claims 1 to 5, wherein, the cumulated amounts of Silicon and Aluminum is between 0.3% and 0.8%.
7. Heat treated and cold rolled steel sheet according to anyone of claims 1 to 6, wherein, the cumulated amounts of Ferrite and Bainite is between 22% and 35% wherein the percentage of Ferrite is at least 15% of total area fraction of steel.
8. Heat treated and cold rolled steel sheet according to anyone of claims 1 to 7, wherein the carbon content of residual austenite is between 0.7% and 0.9%.
9. Heat treated and cold rolled steel sheet according to anyone of claims 1 to 8, wherein the tempered martensite is between 65% and 80%.
10. Heat treated and cold rolled steel sheet according to anyone of claims 1 to 9, wherein bainite is between 0% and 10%.
11. Heat treated and cold rolled steel sheet according to anyone of claims 1 to 10, wherein said steel sheet has an ultimate tensile strength of 950 MPa or more, and a total elongation of 8% or more.
12. Cold rolled and coated steel sheet according to claim 11 , wherein said steel sheet has an ultimate tensile strength of 1000 MPa or more and a hole expansion ratio of greater of at least 55%.
13. A method of production of heat treated and cold rolled steel sheet comprising the following successive steps: - providing a steel composition according to anyone of claims 1 to 6;
- reheating said semi-finished product to a temperature between 1000°C and 1250°C;
- rolling the said semi-finished product in the temperature range between Ac3 and Ac3 +100°C wherein the hot rolling finishing temperature shall be above Ac3 to obtain a hot rolled steel;
- cooling the hot rolled steel at a cooling rate of at least 30°C/s to a coiling temperature which is below 600°C; and coiling the said hot rolled steel;
- cooling the said hot rolled steel to room temperature;
- optionally performing scale removal process on said hot rolled steel sheet;
- optionally annealing is performed on hot rolled steel sheet between 400°C and 750°C;
- optionally performing scale removal process on said hot rolled steel sheet;
- cold rolling the said hot rolled steel sheet with a reduction rate between 35 and 90% to obtain a cold rolled steel sheet;
- annealing the said cold rolled steel sheet in two steps heating wherein: o the first step starts from heating the steel sheet to a temperature HT1 between 600°C and 650°C, with a heating rate HR1 of at least 10°C/s, o the second step starts from heating further the steel sheet from HT1 to an annealing temperature range between Ac3 and Ac3 +200°C, with a heating rate HR2 of 1°C/s or more, HR2 being lower than HR1 ,
- then perform annealing at annealing temperature during 5 to 1000 seconds, then cooling the cold rolled steel sheet in three steps cooling wherein : o the first step starts from cooling the steel sheet from the annealing temperature to a temperature CT1 between 675°C and 725°C, with a cooling rate CR1 of 10°C/s or less, o the second step starts from cooling further the steel sheet from CT1 to CT2 between 450°C and 550°C, with a cooling rate CR2 of 30°C/s or more, o the third step starts from cooling further the steel sheet from CT2 to CT3 between Ms-50°C and 20°C, with a cooling rate CR2 of 200°C/s or more,
- then the said cold rolled steel sheet is heated at a heating rate of at least 10°C/s to a tempering temperature between 300°C and 380°C and tempered during 100s to 1000 seconds, and then cooled to room temperature range to obtain a heat treated and cold rolled steel sheet.
14. A method according to claim 13, wherein the coiling temperature is between 350°C and 600°C.
15. A method according to claim 13 or 14, wherein the HT1 temperature is between 600°C and 630°C with a HR1 heating rate of at least 15°C/s.
16. A method according to anyone of claims 13 to 15, wherein annealing soaking temperature is between Ac3+10°C and Ac3 +150°C.
17. A method according to anyone of claims 13 to 16, wherein the tempering temperature range is between 320°C and 360°C.
18. Use of a steel sheet according to anyone of claims 1 to 12 or of a steel sheet produced according to the method of claims 13 to 17, for the manufacture of structural or safety parts of a vehicle.
19. Vehicle comprising a part obtained according to claim 18.
PCT/IB2020/061765 2019-12-13 2020-12-10 Heat treated cold rolled steel sheet and a method of manufacturing thereof WO2021116976A1 (en)

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CN202080084937.5A CN114761584A (en) 2019-12-13 2020-12-10 Heat-treated cold-rolled steel sheet and method for producing same
EP20823957.4A EP4073280A1 (en) 2019-12-13 2020-12-10 Heat treated cold rolled steel sheet and a method of manufacturing thereof
KR1020227019565A KR20220095240A (en) 2019-12-13 2020-12-10 Heat treatment cold rolled steel sheet and manufacturing method thereof
JP2022535560A JP2023505693A (en) 2019-12-13 2020-12-10 Heat-treated cold-rolled steel sheet and manufacturing method thereof
CA3163313A CA3163313C (en) 2019-12-13 2020-12-10 Heat treated cold rolled steel sheet and a method of manufacturing thereof
MX2022007054A MX2022007054A (en) 2019-12-13 2020-12-10 Heat treated cold rolled steel sheet and a method of manufacturing thereof.
US17/783,409 US20230036084A1 (en) 2019-12-13 2020-12-10 Heat treated cold rolled steel sheet and a method of manufacturing thereof
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