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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • 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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • 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

Definitions

• Manganese content of the steel of the present invention is between 0.5% and 1%. This element is gammagenous. Manganese provides solid solution strengthening and suppresses the ferritic transformation temperature and reduces ferritic transformation rate hence assist in the formation of martensite. An amount of at least 0.5% is required to impart strength as well as to assist the formation of Martensite. But when Manganese content is more than 1% it produces adverse effects such as it retards transformation of Austenite to Martensite during cooling after annealing. Manganese content of above 1% can get excessively segregated in the steel during solidification and homogeneity inside the material is impaired which can cause surface cracks during a hot working process. The preferred limit for the presence of Manganese is between 0.5% and 0.9% and more preferably between 0.6% and 0.8%.
• Chromium content of the composite coil of steel of the present invention is between 0.1% and 1%. Chromium is an essential element that provide strength to the steel by solid solution strengthening and a minimum of 0.1% is required to impart the strength but when used above 1% impairs surface finish of steel.
• the preferred limit for the presence of Chromium is between 0.3% and 0.9% and more preferably between 0.4% and 0.8%.
• the content of the Aluminum is between 0.01% and 1%.
• Aluminum removes Oxygen existing in molten steel to prevent Oxygen from forming a gas phase during solidification process.
• Aluminum also fixes Nitrogen in the steel to form Aluminum nitride to reduce the size of the grains.
• Higher content of Aluminum, above 1%, increases Ac3 point to a high temperature thereby lowering the productivity.
• the preferred limit for the presence of Aluminium is between 0.01% and 0.5%
• 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 0.09% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese and reduces its beneficial impact on the present invention.
• Nitrogen is limited to 0.09% to avoid ageing of material and to minimize the precipitation of Aluminum nitrides during solidification which are detrimental for mechanical properties of the steel.
• Boron is an optional element for the steel of the present invention and may be present between 0% and 0.05%. Boron forms boro-nitirides and impart additional strength to steel of the present invention when added in an amount of at least 0.0001%.
• Calcium can be added to the steel of the present invention in an among between 0.001% and 0.01%%. Calcium is added to steel of the present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the Steel by binding the detrimental Sulfur content in globular form thereby retarding the harmful effect of Sulfur.
• Sn, Pb or Sb can be added individually or in combination in the following proportions: Sn ⁇ 0.1%, Pb ⁇ 0.1% and Sb ⁇ 0.1%. 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.
• Martensite constitutes at least 95% of the microstructure by area fraction.
• the martensite of the present invention can comprise both fresh and tempered martensite.
• fresh martensite is an optional microconstituent which is preferably limited in the steel at an amount of between 0% and 4%, preferably between 0 and 2% and even better equal to 0%.
• Fresh martensite may form during cooling after tempering.
• Tempered martensite is formed from the martensite which forms during the second step of cooling after annealing and particularly after below Ms temperature and more particularly between Ms ⁇ 10° C. and 20° C. Such martensite is then tempered during the holding at a tempering temperature Ttemper between 150° C. and 300° C.
• the martens to of the present invention imparts ductility and strength to such steel.
• the content of martensite is between 96% and 99% and more preferably between 97% and 99%.
• Bainite forms during the reheating before tempering.
• the steel of the present invention contains 1 to 3% of bainite. Bainite can impart formability to the steel but when present in too large an amount, it may adversely impact the tensile strength of the steel.
• Ferrite may form during the first step of cooling after annealing but is not required as a microstructural constituent. Ferrite formation must be kept as low as possible and preferably less than 2% or even less than 1%.
• the microstructure of the cold rolled martensitic steel sheet is free from microstructural components such as pearlite and cementite.
• the steel according to the invention can be manufactured by any suitable methods. It is however preferable to use the method according to the invention that will be detailed, as a non-limitative example.
• a slab having the chemical composition according to the invention is manufactured by continuous casting wherein the slab optionally underwent a direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10.
• the slab provided by the 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, must be at least 1000° C. and must be below 1280° C. In case the temperature of the slab is lower than 1280° 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. Therefore, the temperature of the slab must be high enough so that hot rolling should be completed in the temperature range of Ac3 to Ac3+100° C. Reheating at temperatures above 1280° C. must be avoided because they are industrially expensive.
• the hot rolled steel sheet may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing.
• the hot rolled sheet may then be subjected to an optional hot band annealing.
• such hot band annealing is performed at temperatures between 400° C. and 750° C., preferably for at least 12 hours and not more than 96 hours, the temperature preferably remaining below 750° C. to avoid transforming partially the hot-rolled microstructure and, therefore, possibly losing the microstructure homogeneity.
• an optional scale removal step of this hot rolled steel sheet may be performed through, for example, pickling of such sheet.
• This hot rolled steel sheet is then subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between 35 to 90%.
• the cold rolled steel sheet is then heating in a two step heating process wherein the first step of heating starts from room temperature, the cold rolled steel sheet being heated, at a heating rate HR1 of at least 10° C./s, to a temperature HT1 which is in a range between 550° C. and 750° C.
• the heating rate HR1 for such first step of heating is at least 15° C./s and more preferably at least 18° C./s.
• the preferred HT1 temperature for such first step is between 575° C. and 725° C.
• the cold rolled steel sheet is heated from HT1 to an annealing temperature Tsoak which is between Ac3 and Ac3+100° C., preferably between Ac3 +10° C. and Ac3 +100° C., at a heating rate HR2 which is between 1° C./s and 50° C./s.
• the heating rate HR2 for the second step of heating is between 1° C./s and 25° C./s and more preferably 1° C./s and 20° C./s, wherein Ac3 for the steel sheet is calculated by using the following formula:
• Ms for the steel sheet is calculated by using the following formula:
• the cold rolled steel sheet is reheated to a tempering temperature Ttemper between 150° C. and 300° C. with a heating rate of at least 1° C./s and preferably of at least 2° C./s and more of at least 5° C./s during 100 s and 600 s.
• the preferred temperature range for tempering is between 200° C. and 300° C. and the preferred duration for holding at Ttemper is between 200 s and 500 s.
• the cold rolled steel sheet is cooled down to room temperature to obtain a cold rolled martensitic steel.
• the cold rolled martensitic steel sheet of the present invention may optionally be coated with zinc or zinc alloys, or with aluminum or aluminum alloys to improve its corrosion resistance.
• 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.
• Step 2 Step 1 Step 2 rate to Heating Heating Cooling Cooling coiling CR HR1 HR2 Annealing CR1 CR2 Reheating FRT (° C./ Coiling Reduction HT1 (° C./ Tsoak (° C./ Tsoak Annealing T1 (° C./ T2 (° C./ Trials Steel (° C.) (° C.) s) (° C.) (%) (° C.) s) (° C.) s) (° C.) time(s) (° C.) s) (° C.) s) I1 1 1245 895 30 530 40 630 20 880 2.2 880 226 654 56 20 735 I2 2 1245 895 30 530 40 630 18 887 2.1 887 252 697 40 20 703 I3 3 1245 895 30 530 41 734 20 860 1.8 860 282 640 43 20 577 R1 4 1245 895 30 530 53 550 15
• the table 2 is as follows:
• Table 3 exemplifies the results of the tests conducted in accordance with the standards on different microscopes such as Scanning Electron Microscope for determining the microstructures of both the inventive and reference steels in terms of area fraction. The results are stipulated herein:

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