Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • 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
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • 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
  • 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
  • C22C38/02—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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

• the present invention relates to high strength steel sheets suitable for applications in automobiles.
• the invention relates to cold rolled steel sheets having a tensile strength of at least 980 MPa and an excellent formability.
• Automotive body parts are often stamped out of sheet steels, forming complex structural members of thin sheet.
• such parts cannot be produced from conventional high strength steels, because of a too low formability for complex structural parts.
• multi phase Transformation Induced Plasticity aided steels TRIP steels
• TRIP steels have gained considerable interest in the last years, in particular for use in auto body structural parts and as seat frame materials.
• TRIP steels possess a multi-phase microstructure, which includes a meta-stable retained austenite phase, which is capable of producing the TRIP effect.
• the austenite transforms into martensite, which results in remarkable work hardening. This hardening effect, acts to resist necking in the material and postpone failure in sheet forming operations.
• the microstructure of a TRIP steel can greatly alter its mechanical properties. The most important aspects of the TRIP steel microstructure are the volume percentage, size and morphology of the retained austenite phase, as these properties directly affect the austenite to martensite transformation, when the steel is deformed. There are several ways by which it is possible to chemically stabilize austenite at room temperature.
• the austenite In low alloy TRIP steels the austenite is stabilized through its carbon content and the small size of the austenite grains.
• the carbon content necessary to stabilize austenite is approximately 1 wt. %.
• high carbon content in steel cannot be used in many applications because of impaired weldability.
• a common TRIP steel chemistry also contains small additions of other elements to help in stabilizing the austenite as well as to aid in the creation of microstructures which partition carbon into the austenite.
• the silicon content should be about 1.5 wt. %.
• the most common alloying addition is 1.5 wt. % of both Si and Mn.
• TRIP-aided steel with a Bainitic Ferrite matrix (TBF)-steels have been known for long and attracted a lot of interest, mainly because the bainitic ferrite matrix allows an excellent stretch flangability. Moreover, the TRIP effect ensured by the strain-induced transformation of metastable retained austenite islands into martensite, remarkably improves their drawability.
• TBF Bainitic Ferrite matrix
• TRIP steels The formability of TRIP steels is heavily affected by the transformation characteristics of the retained austenite phase, which is in turn affected by the austenite chemistry, its morphology and other factors.
• ISIJ International Vol. 50(2010) No. 1, p. 162-168 aspects influencing the formability of TBF steels having a tensile strength of at least 980 MPa are discussed.
• the cold rolled materials examined in this document were annealed at 950° C. and austempered at 300-500° C. for 200 s in salt bath. Accordingly, due to the high annealing temperature these materials are not suited for the production in a conventional industrial annealing line.
• WO20131144377 discloses a cold rolled TBF-steel sheet alloyed with Si and Al and having a tensile strength of at least 980 MPa.
• WO2013/144376 discloses a cold rolled TBF-steel sheet alloyed with Si and Cr and a tensile strength of at least 980 MPa.
• the present invention is directed to high strength (TBF) steel sheets having a tensile strength of 980-1100 MPa and an excellent formability, wherein it should be possible to produce the steel sheets on an industrial scale in a Continuous Annealing Line (CAL).
• TBF high strength
• CAL Continuous Annealing Line
• the invention aims at providing a steel composition that can be processed to complicated structural members, where both local elongation and total elongation is of importance, in particular for automobile seat components. However, it is generally considered, that if the total elongation is increased, then the properties governed by the local elongation such as the hole expanding ratio (HER) or ( ⁇ ) is deteriorated.
• the steel sheet has a composition consisting of the following alloying elements (in wt.
• the balance consists of iron and impurities.
• C stabilizes the austenite and is important for obtaining sufficient carbon within the retained austenite phase.
• C is also important for obtaining the desired strength level. Generally, an increase of the tensile strength in the order of 100 MPa per 0.1% C can be expected. When C is lower than 0.07% then it is difficult to attain a tensile strength of 980 MPa. If C exceeds 0.15%, then the weldability is impaired.
• the upper limit may be 0.14, 0.13 or 0.12%.
• the lower limit may be 0.08, 0.09, or 0.10%. A preferred range is 0.08-0.13%.
• Manganese is a solid solution strengthening element, which stabilises the austenite by lowering the M, temperature and prevents ferrite and pearlite to be formed during cooling.
• Mn lowers the A c3 temperature and is important for the austenite stability. At a content of less than 2.3% it might be difficult to obtain the desired amount of retained austenite, a tensile strength of 980 MPa and the austenitizing temperature might be too high for conventional industrial annealing lines. In addition, at lower contents it may be difficult to avoid the formation of polygonal ferrite.
• the upper limit may therefore be 3.1, 3.0, 2.9, 2.8 or 2.7%.
• the lower limit may be 2.3, 2.4, or 2.5%.
• Si acts as a solid solution strengthening element and is important for securing the strength of the thin steel sheet. Si suppresses the cementite precipitation and is essential for austenite stabilization.
• the upper limit is therefore 1.2% and may be restricted to 1.1, 1.05, 1.0 or 0.95%.
• the lower limit may be 0.65, 0.7, 0.75 or 0.80%.
• a preferred range is 0.7-1.0%.
• Cr is effective in increasing the strength of the steel sheet. Cr is an element that forms ferrite and retards the formation of pearlite and bainite. The A c3 temperature and the M % temperature are only slightly lowered with increasing Cr content. Cr results in an increased amount of stabilized retained austenite.
• the amount of Cr is limited to 0.7%. The upper limit may be 0.65, 0.60, 0.55, 0.50, 0.45 or 0.40, 0.35, 0.30 or 0.25%. The lower limit may be 0.10, or 0.15%. A preferred range is 0.1-0.3%.
• the amount of Si+Cr is in the range of 0.9-1.3% because when added in combination Si and Cr have a synergistic effect and result in an increased amount of retained austenite, which, in turn, results in an improved ductility.
• the amount of Si+Cr is preferably limited to the range of 0.9 to 1.2%.
• Al promotes ferrite formation and is also commonly used as a deoxidizer.
• the M, temperature is increased with an increasing Al content.
• a further drawback of Al is that it results in a drastic increase in the A c3 temperature and therefore makes it more difficult to austenitize the steel in the CAL.
• the Al content is preferably limited to less than 0.1%, more preferably to less than 0.08%. It is thus preferred to only use Al for deoxidation.
• the upper level may then be 0.09, 0.08, 0.07 or 0.06%.
• the lower level may set to 0.005, 0.01, 0.02 or 0.03%.
• Nb is commonly used in low alloyed steels for improving strength and toughness, because of its influence on the grain size. Nb increases the strength elongation balance by refining the matrix microstructure and the retained austenite phase due to precipitation of NbC.
• the steel may contain Nb in an amount of ⁇ 0.05%, preferably ⁇ 0.03%. A deliberate addition of Nb is not necessary according to the present invention. The upper limit may therefore be restricted to ⁇ 0.01%.
• the high strength TRIP-assisted bainitic ferrite (TBF) steel sheets of the present invention have microstructure mainly consisting of retained austenite inclusions embedded in the matrix.
• microstructural constituents are in the following expressed in volume % (vol. %).
• the steel comprises a matrix of bainitic ferrite (BF).
• BF bainitic ferrite
• the microstructure may also contain tempered martensite (TM).
• TM tempered martensite
• the constituents BF and TM may be difficult to distinguish from each other. Therefore, the total content of both constituents may be limited to 70-90%. The amount is normally in the range of 80-90%.
• Martensitic may be present in the final microstructure because, depending on its stability, some austenite may transform to martensite during cooling at the end of the overaging step. Martensite may be present in an amount of ⁇ 15%.
• the amount of un-tempered martensite is preferably limited to 10, 9, 8, 7, 6 or 5%. These un-tempered martensite particles are often in close contact with the retained austenite particles and they are therefore often referred to as martensite-austenite (MA) particles.
• MA martensite-austenite
• Retained austenite is a prerequisite for obtaining the desired TRIP effect.
• the amount of retained austenite should therefore be in the range of 2-20%, preferably 5-15%.
• the amount of retained austenite was measured by means of the saturation magnetization method described in detail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, p. 61-64.
• Polygonal ferrite (PF) is not a desired microstructural constituent and is therefore limited to ⁇ 10%, preferably ⁇ 5%, ⁇ 3% or ⁇ 1%. Most preferably, the steel is free from PF.
• the R m . R p0.2 values are derived according to the European norm EN 10002 Part 1, wherein the samples were taken in the longitudinal direction of the strip.
• the total elongation (A 50 ) is derived in accordance with the Japanese Industrial Standard JIS Z 2241: 2011, wherein the samples are taken in the transversal direction of the strip.
• the mechanical properties of the steel sheets of the present invention can be largely adjusted by the alloying composition and the microstructure.
• the microstructure may be adjusted by the heat treatment in the CAL, in particular by the isothermal treatment temperature in the overaging step.
• a high strength cold rolled steel sheet having
• a high strength cold rolled steel sheet having:
• a high strength cold rolled steel sheet fulfilling at least one of the following requirements:
• a high strength cold rolled steel sheet as defined above, wherein the thickness of the cold rolled sheet is 1.0-1.6 mm, preferably 1.1-1.5 mm, more preferably 1.2-1.4 mm.
• a high strength cold rolled steel sheet as defined above, wherein the total content of bainitic ferrite and tempered martensite is 70-90 vol. %, preferably 80-90 vol. %.
• a high strength cold rolled steel sheet as defined above, wherein the product of the tensile strength (R m ) and the total elongation (A 50 ) is ⁇ 13000 MPa %, preferably ⁇ 13500 MPa %.
• Table 1 disclose the composition of the examined steel sheets.
• Example C Si Mn Cr Al Inv. 1 0.105 0.81 2.63 0.195 0.045 lnv. 2 0.106 0.84 2.67 0.197 0.048 Inv. 3 0.106 0.84 2.67 0.197 0.048 Inv. 4 0.105 0.81 2.63 0.195 0.045 Inv. 5 0.118 0.94 2.77 0.17 0.051
• Heats of the steel alloys were produced in a continuous caster.
• the slabs were reheated and subjected to hot rolling to a thickness of about 2.8 mm.
• the hot rolling finishing temperature was about 900° C. and the coiling temperature about 550° C.
• the hot rolled strips were pickled and batch annealed at about 625° C. for a time of 10 hours in order to reduce the tensile strength of the hot rolled strip and thereby reducing the cold rolling forces.
• the strips were thereafter cold rolled in a five stand cold rolling mill to a final thickness of about 1.4 mm and finally subjected to continuous annealing.
• Table 2 discloses the hot and cold rolling parameters. The batch annealing was performed between the hot- and cold rolling steps for about 10 h.
• Hot and cold rolling parameters Hot rolled Batch annealing Cold rolling Cold rolling thickness temperature thickness reduction Example (mm) (° C.) (mm) (%) Inv. 1 2.80 623 1.41 50 Inv. 2 2.79 623 1.41 49 Inv. 3 2.78 625 1.41 49 Inv. 4 2.79 623 1.41 49 lnv. 5 2.79 624 1.42 49
• the annealing cycle consisted of heating to a temperature of about 850° C., soaking for about 120 s, slow gas jet cooling at a rate of about 10° C./s to a temperature of about 750° C., rapid gas cooling at a rate of about 40° C./s to an overaging temperature of about 390-400° C., isothermal holding at the overaging temperature and final cooling to ambient temperature.
• the details of the treatment in the CAL are given in Table 3.
• the material produced according to the invention was found to have excellent mechanical properties as shown in Table 4. All examples had a matrix of bainitic ferrite and contained less than 10% martensite and minimal amounts of ferrite.
• inventive examples disclose a total elongation (A 50 ) of more than 13% at the same time as the hole expansibility ( ⁇ ), as measured by the hole expansion test, exceeded 52% for all inventive examples.
• the R m and R p0 . values are derived according to the European norm EN 10002 Part 1, wherein the samples were taken in the longitudinal direction of the strip.
• the elongation (A 50 ) is derived in accordance with the Japanese Industrial Standard JIS Z 2241: 2011 for samples taken in the transversal direction of the strip.
• the hole expanding ratio ( ⁇ ) is reported as the mean value of three samples subjected to hole expansion tests (HET). It was determined by the hole expanding test method according to ISO/TS16630:2009 (E). In this test a conical punch having an apex of 60° is forced into a 10 mm diameter punched hole made in a steel sheet having the size of 100 ⁇ 100 mm 2 . The test is stopped as soon as the first crack is determined and the hole diameter is measured in two directions orthogonal to each other. The arithmetic mean value is used for the calculation.
• the hole expanding ratio ( ⁇ ) in % is calculated as follows:
• Do is the diameter of the hole at the beginning (10 mm) and Dh is the diameter of the hole after the test.
• the material of the present invention can be widely applied to high strength structural parts in automobiles.
• the high strength steel sheets are particularly well suited for the production of parts having high demands on the total elongation and at the same time a low edge crack sensitivity.

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• 2016
  • 2016-11-25 SE SE1651545A patent/SE1651545A1/en unknown
• 2017
  • 2017-11-24 KR KR1020197018164A patent/KR20190089183A/ko not_active IP Right Cessation
  • 2017-11-24 CN CN201780073115.5A patent/CN110268085A/zh active Pending
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• 2023
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