SE540040C2 - High strength cold rolled steel sheet for automotive use - Google Patents

High strength cold rolled steel sheet for automotive use Download PDF

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SE540040C2
SE540040C2 SE1651545A SE1651545A SE540040C2 SE 540040 C2 SE540040 C2 SE 540040C2 SE 1651545 A SE1651545 A SE 1651545A SE 1651545 A SE1651545 A SE 1651545A SE 540040 C2 SE540040 C2 SE 540040C2
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cold rolled
rpoz
steel sheet
impurities
rolled steel
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SE1651545A
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SE1651545A1 (en
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Thomas Hebesberger Dr
Florian Winkelhofer Dr
SCHWARZENBRUNNER Michael
Ozer Arman Edip
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Priority to SE1651545A priority Critical patent/SE1651545A1/en
Priority to CN201780073115.5A priority patent/CN110268085A/en
Priority to KR1020197018164A priority patent/KR20190089183A/en
Priority to US16/461,689 priority patent/US20190352750A1/en
Priority to EP17808049.5A priority patent/EP3529392B1/en
Priority to PCT/EP2017/080322 priority patent/WO2018096090A1/en
Priority to JP2019528078A priority patent/JP2020509162A/en
Publication of SE540040C2 publication Critical patent/SE540040C2/en
Publication of SE1651545A1 publication Critical patent/SE1651545A1/en
Priority to JP2023065875A priority patent/JP2023099015A/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
    • C22C38/18—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
    • C22C38/18—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
    • 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

Abstract

The invention relates to A high strength cold rolled steel sheet having a composition consisting of the following elements (in wt. %):C 0.07 - 0.15Mn 2.3 - 3.2Si 0.6 - 1.2Cr 0.05 - 0.5A1 ≤ 0.2Nb ≤ 0.1balance Fe apart from impurities, a multiphase microstructure comprising a matrix ofbainitic ferrite and a tensile strength (R) of 980 - 1100 MPa

Description

HIGH STRENGTH COLD ROLLED STEEL SHEET FOR AUTOMOTIVE USE TECHNICAL FIELD The present invention relates to high strength steel sheets suitable for applications inautomobiles. In particular, the invention relates to cold rolled steel sheets having a tensile strength of at least 980 MPa and an excellent forrnability.
BACKGROUND ART For a great variety of applications increased strength levels are a pre-requisite for light-Weight constructions in particular in the automotive industry, since car body mass reduction results in reduced fuel consumption.
Automotive body parts are often stamped out of sheet steels, forrning complex structuralmembers of thin sheet. HoWever, such parts cannot be produced from conventional highstrength steels, because of a too low forrnability for complex structural parts. For thisreason multi phase Transformation Induced Plasticity aided steels (TRIP steels) havegained 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 retainedaustenite phase, Which is capable of producing the TRIP effect. When the steel isdeforrned, the austenite transforrns into martensite, Which results in remarkable Workhardening. This hardening effect, acts to resist necking in the material and postponefailure in sheet forrning operations. The microstructure of a TRIP steel can greatly alterits mechanical properties. The most important aspects of the TRIP steel microstructureare the volume percentage, size and morphology of the retained austenite phase, as theseproperties directly affect the austenite to martensite transformation, When the steel isdeforrned. There are several Ways by Which it is possible to chemically stabilizeaustenite at room temperature. In low alloy TRIP steels the austenite is stabilizedthrough its carbon content and the small size of the austenite grains. The carbon contentnecessary to stabilize austenite is approximately l Wt. %. HoWever, high carbon content in steel cannot be used in many applications because of impaired Weldability.
Specific processing routs are therefore required to concentrate the carbon into theaustenite in order to stabilize it at room temperature. A common TRIP steel chemistryalso contains small additions of other elements to help in stabilizing the austenite asWell as to aid in the creation of microstructures Which partition carbon into theaustenite. In order to inhibit the austenite to decompose during the bainitetransformation it has generally been considered necessary that the silicon content shouldbe 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 longand attracted a lot of interest, mainly because the bainitic ferrite matrix allows anexcellent stretch flangability. Moreover, the TRIP effect ensured by the strain-inducedtransformation of metastable retained austenite islands into martensite, remarkably improves their draWability.
The forrnability of TRIP steels is heavily affected by the transformation characteristicsof the retained austenite phase, Which is in tum affected by the austenite chemistry, itsmorphology and other factors. In ISIJ Intemational Vol. 50(20l0), No. l, p. 162 -168aspects influencing the forrnability of TBF steels having a tensile strength of at least980 MPa are discussed. HoWever, the cold rolled materials examined in this documentWere 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.
HoWever, the high Si-contents generally used in TBF-steels result in the formation ofsilicon oxide layers on the surface of the steel strip, Which may adhere to the rolls in thecontinuous annealing line (CAL) and give rise to surface defects on subsequentlyproduced steel sheets. Therefore, in recent years it has been a strive to reduce the silicon content in TBF steels.
WO2013/ 144377 discloses a cold rolled TBF-steel sheet alloyed With Si and Al andhaving a tensile strength of at least 980 MPa. WO2013/ 144376 discloses a cold rolledTBF-steel sheet alloyed With Si and Cr and a tensile strength of at least 980 MPa.Although these steels disclose several attractive properties there is demand for 980 MPasteel sheets having an improved property profile With respect to advanced forrningoperations, Where both local elongation and total elongation is of importance such as for structural members in automobile seats.
DISCLOSURE OF THE INVENTION The present invention is directed to high strength (TBF) steel sheets having a tensilestrength of 980 - 1100 MPa and an excellent forrnability, Wherein it should be possibleto produce the steel sheets on an industrial scale in a Continuous Annealing Line(CAL). The invention aims at providing a steel composition that can be processed tocomplicated structural members, Where both local elongation and total elongation is ofimportance, in particular for automobile seat components. However, it is generallyconsidered, that if the total elongation is increased, then the properties govemed by the local elongation such as the hole expanding ratio (HER) or (Ä) is deteriorated.
DETAILED DESCRIPTION The invention is described in the claims.
The steel sheet has a composition consisting of the folloWing alloying elements (in Wt.%): C 0.07 - 0.15Mn 2.3 - 3.2Si 0.6 - 1.2Cr 0.05 - 0.5Al S 0.2 Nb S 0.1 the balance consists of iron and impurities.
The importance of the separate elements and their interaction With each other as Well asthe limitations of the chemical ingredients of the claimed alloy are briefly explained inthe folloWing. All percentages for the chemical composition of the steel are given inWeight % (Wt. %) throughout the description. The amount of hard phases is given involume % (vol. %). Upper and lower limits of the individual elements can be freely combined Within the limits set out in the claims.
C: 0.07 - 0.15 % C stabilizes the austenite and is important for obtaining sufficient carbon within theretained 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 beexpected. When C is lower than 0.07 % then it is difficult to attain a tensile strength of980 MPa. If C exceeds 0.15 %, then the weldability is impaired. The upper limit maybe 0.14, 0.13 or 0.12 %. The lower limit may be 0.08, 0.09, or 0.10 %. A preferredrange is 0.08 - 0.13 %.
Mn: 2.3 - 3.2 % Manganese is a solid solution strengthening element, which stabilises the austenite bylowering the MS temperature and prevents ferrite and pearlite to be formed duringcooling. In addition, Mn lowers the A03 temperature and is important for the austenitestability. At a content of less than 2.3 % it might be difficult to obtain the desiredamount of retained austenite, a tensile strength of 980 MPa and the austenitizingtemperature might be too high for conventional industrial annealing lines. In addition, atlower contents it may be difficult to avoid the formation of polygonal ferrite. However,if the amount of Mn is higher than 3.2 %, problems with segregation may occur becauseMn accumulates in the liquid phase and causes banding resulting in a potentiallydeteriorated workability. 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: 0.6 - 1.2 % Si acts as a solid solution strengthening element and is important for securing thestrength of the thin steel sheet. Si suppresses the cementite precipitation and is essential for austenite stabilization.
However, if the content is too high, then to much silicon oxides will form on the stripsurface, which may lead to cladding on the rolls in the CAL and surface defects onsubsequently produced steel sheets. The upper limit is therefore 1.2 % and may berestricted 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: 0.05 - 0.5 % Cr is effective in increasing the strength of the steel sheet. Cr is an element that forrnsferrite and retards the forrnation of pearlite and bainite. The A03 temperature and the Mstemperature are only slightly lowered with increasing Cr content. Cr results in anincreased 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 %. Thelower limit may be 0.10, or 0.15 %. A preferred range is 0.1 - 0.3 %.
Si+Cr: 0.9 - 1.3 % It is preferred that the amount of Si + Cr is in the range of 0.9 - 1.3 % because whenadded in combination Si and Cr have a synergistic effect and result in an increasedamount of retained austenite, which, in tum, results in an improved ductility. For these reasons the amount of Si + Cr is preferably limited to the range of 0.9 to 1.2 %.
Al: S 0.2 % Al promotes ferrite formation and is also commonly used as a deoxidizer. The MStemperature is increased with an increasing Al content. A further drawback of Al is thatit results in a drastic increase in the A03 temperature and therefore makes it moredifficult to austenitize the steel in the CAL. For these reasons the Al content ispreferably limited to less than 0.1 %, more preferably to less than 0.08 %. It is thuspreferred to only use Al for deoxidation. The upper level may then be 0.09, 0.08, 0.07or 0.06 %. For securing a certain effect the lower level may set to 0.005, 0.01, 0.02 or0.03 %.
Nb: < 0.1% 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 balanceby ref1ning the matrix microstructure and the retained austenite phase due toprecipitation of NbC. The steel may contain Nb in an amount of S 0.05 %, preferably S0.03 %. A deliberate addition of Nb is not necessary according to the present invention.
The upper limit may therefore be restricted to S 0.01 %.
The high strength TRIP-assisted bainitic ferrite (TBF) steel sheets of the presentinvention have microstructure mainly consisting of retained austenite inclusions embedded in the matrix.
The microstructural constituents are in the following expressed in Volume % (Vol. %).
The steel comprises a matrix of bainitic ferrite (BF). Hence, the amount of bainiticferrite is generally 2 50 %. The microstructure may also contain tempered martensite(TM). 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 amountis norrnally in the range of 80-90 %.
Martensite may be present in the final microstructure because, depending on itsstability, some austenite may transforrn to martensite during cooling at the end of theoVeraging step. Martensite may be present in an amount of S 15 % preferably S 10 %and more preferably S 8 %. These un-tempered martensite particles are often in closecontact With the retained austenite particles and they are therefore often referred to as martensite-austenite (MA) particles.
Retained austenite is a prerequisite for obtaining the desired TRIP effect. The amount ofretained austenite should therefore be in the range of 2 - 20 %, preferably 5 - 15 %. Theamount of retained austenite Was measured by means of the saturation magnetizationmethod 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 thereforelimited to S 10 %, preferably S 5 %, S 3 % or S 1 %. Most preferably, the steel is freefrom PF.
The mechanical properties of the claimed steel are important and at least one of the following requirements should be fulfilled: tensile strength (Rm) 980 - 1100 MPayield strength (Rpoz) 580 - 920 MPatotal elongation (Aso) 2 13 %hole expansion ratio (Ä) 2 50 %yield ratio (Rpoz/ Rm) 2 0.75 Preferably, all these requirements are fulfilled at the same time.
The Rm, Rpoq values are derived according to the European norrn EN 10002 Part 1,Wherein the samples Were taken in the longitudinal direction of the strip. The totalelongation (Aso) 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 largelyadjusted by the alloying composition and the microstructure. The microstructure may beadjusted by the heat treatment in the CAL, in particular by the isotherrnal treatmenttemperature in the overaging step.
EXAMPLES Table 1 disclose the composition of the examined steel sheets.
Example C Si Mn Cr A1 InV. l 0,105 0,81 2,63 0,195 0,045InV. 2 0,106 0,84 2,67 0,197 0,048InV. 3 0,106 0,84 2,67 0,197 0,048InV. 4 0,105 0,81 2,63 0,195 0,045InV. 5 0,118 0,94 2,77 0,17 0,051 Table 1. Composition of examined steel sheets.
Heats of the steel alloys Were produced in a continuous caster. The slabs Were reheatedand subj ected to hot rolling to a thickness of about 2.8 mm. The hot rolling finishingtemperature Was about 900 °C and the coiling temperature about 550 °C. The hot rolledstrips Were pickled and batch annealed at about 625 °C for a time of 10 hours in order toreduce the tensile strength of the hot rolled strip and thereby reducing the cold rollingforces. 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 Wasperformed between the hot- and cold rolling steps for about 10 h.
Example Hot rolled Batch annealing Cold rolling Cold rollingthickness temperature thickness reduction(mm) (°C) (mm) (%)Inv. 1 2,80 623 1,41 50Inv. 2 2,79 623 1,41 49InV. 3 2,78 625 1,41 49Inv. 4 2,79 623 1,41 49Inv. 5 2,79 624 1,42 49 Table 2. Hot and cold rolling parameters.
The annealing cycle consisted of heating to a temperature of about 850 °C, soaking forabout 120 s, slow gas j et 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 about390 - 400 °C, isotherrnal holding at the oVeraging temperature and final cooling to ambient temperature. The details of the treatment in the CAL are given in Table 3.
Example Annealing Slow Jet Rapid Jet Cooling temp.temp. (°C) Cooling temp. (°C) (°C)InV. l 850 750 393InV. 2 850 750 397InV. 3 846 750 397InV. 4 842 750 394InV. 5 847 750 391 Table 3. Parameters of the treatment in the CAL.
The material produced according to the invention was found to have excellent mechanical properties as shown in Table 4.
In particular, it may be noted that all inVentiVe examples disclose a total elongation(A50) of more than 13 % at the same time as the hole expansibility (Ä), as measured by the hole expansion test, exceeded 52 % for all inVentiVe examples.
Example Yield Tensile Yield ratio Total Emngation, HoleStrength Strength (Rp02/ A50 expandingRp02 Rm Rm) (transversal) ratio Ä(MPa) (MPa) (%) (%)111V- 1 838 1038 0,81 13,4 53,6Inv. 2 806 1018 0,79 13,2 64InV. 3 841 1038 0,81 14,2 71,8Inv. 4 817 1027 0,80 13,4 67,6InV. 5 863 1084 0,80 13,5 52,2 Table 4. Mechanical properties.
The Rm and Rpo. values are derived according to the European norrn EN 10002 Part 1,Wherein the samples Were taken in the longitudinal direction of the strip. The elongation(Aso) is derived in accordance With the Japanese Industrial Standard J IS Z 2241: 2011for samples taken in the transversal direction of the strip.
The hole expanding ratio (Ä) is reported as the mean value of three samples subj ected tohole expansion tests (HET). It Was deterrnined by the hole expanding test methodaccording 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 of100 x 100 mmz. The test is stopped as soon as the first crack is deterrnined and the holediameter is measured in tWo directions orthogonal to each other. The arithmetic mean value is used for the calculation.The hole expanding ratio (Ä) in % is calculated as follows:Ä = (Dh - Do)/Do x 100 Wherein Do is the diameter of the hole at the beginning (10 mm) and Dh is the diameterof the hole after the test.
INDUSTRIAL APPLICABILITY The material of the present invention can be Widely applied to high strength structuralparts in automobiles. The high strength steel sheets are particularly Well suited for theproduction of parts having high demands on the total elongation and at the same time alow edge crack sensitivity.

Claims (10)

1.
2. A high strength cold rolled steel sheet according to claim 1, fulf1lling at least one of A high strength cold rolled steel sheet having a) a composition consisting of the following elements (in Wt. %): C 0.07 - 0.15Mn 2.3 - 3.2Si 0.6 - 1.2Cr 0.05 - 0.5Al S 0.2 Nb S 0.1 balance Fe apart from impurities, b) a multiphase microstructure comprising a matrix of bainitic ferrite, c) a tensile strength (Rm) of 980 - 1100 MPa the following requirements a) a composition fulfilling at least one of the following requirements (in Wt. %): C 0.08 - 0.14Mn 2.4 - 3.1 Si 0.7 - 1.1Cr 0.05 - 0.45Al 0.005 - 0.1Nb S 0.05Wherein the impurities fulfil at least one of the requirements:Ti S 0.05 Mo S 0.05 N S 0.015 B S 0.005 balance Fe apart from impurities, b) a multiphase microstructure comprising at least one of (in Vol. %): 2-20S15 retained austenite martensite 11 bainitic ferrite polygonal ferrite at least one of the following n1echanical properties tensile strength (Rm) 980 - 1100 MPayield strength (Rpoz) 580 - 920 MPatotal elongation (Aso) 2 13 %hole expansion ratio (Ä) 2 50 %yield ratio (Rpoz/ Rm) 2 0.75
3. A high strength cold rolled steel sheet according to claini 1 or 2 having a)%) : b) a con1position fulfilling at least one of the folloWing requirements (in Wt. C 0.08 - 0.13Mn 2.5 - 3.0Si 0.75 - 1.05Cr 0.1 - 0.4Si + Cr 0.9 - 1.3Al 0.01 - 0.08Nb S 0.01Wherein the inipurities fulfil at least one of the requirements:Ti S 0.02 V S 0.02 Mo S 0.03 N S 0.008 B S 0.003 balance Fe apart fron1in1purities, a niultiphase niicrostructure coniprising (in vol. %) retained austenite 5 - 15n1artensite S 10bainitic ferrite 2 60polygonal ferrite S 5 at least one of the folloWing n1echanical propertiesa tensile strength (Rm) 1000 - 1100 MPaa yield strength (Rpoz) 750 - 900 MPa 5 12 2 60 %0.76 - 0.85 a hole expansion ratioa yield ratio (Rpoz/ Rm)
4. A high strength cold rolled steel sheet according to any of the preceding claims fulfilling at least one of the following requirements: %): a composition fulfilling at least one of the following requirements (in wt. C 0.09 - 0.12Mn 2.5 - 2.9Si 0.75 - 1.0Cr 0.1 - 0.3Si + Cr 0.9 - 1.2Al 0.01 - 0.05wherein the impurities fulfil at least one of the requirements:Ti S 0.01 V S 0.02 Mo S 0.03 N S 0.008 B S 0.003 balance Fe apart from impurities, at least one of the following mechanical properties a tensile strength (Rm) 2 1020 MPaa yield strength (Rpoz) 2 800 MPaa yield ratio (Rpoz/ Rm) 2 0.78
5. A high strength cold rolled steel sheet according to claim 1 or 2 fulf1lling the following requirements: a) a composition consisting of (in wt. %): C 0.08 - 0.14Mn 2.4 - 3.1Si 0.7 - 1.1Cr 0.05 - 0.45Al 0.005 - 0.1 Nb S 0.05 b) 13 wherein the impurities fulfil the requirements: Ti S 0.05Mo S 0.05N S 0.015B S 0.005 balance Fe apart from impurities,and/or a multiphase microstructure comprising (in v01. %): retained austenite 2 - 20martensite S 15bainitic ferrite 2 50polygonal ferrite S 10 and/ or the following mechanical properties tensile strength (Rm) 980 - 1100 MPayield strength (Rpoz) 580 - 920 MPatotal elongation (Aso) 2 13 %hole expansion ratio (Ä) 2 50 %yield ratio (Rpoz/ Rm) S 0.84
6. A high strength cold rolled steel sheet according to claim 3 fulf1lling the following requirements: a) a composition fulfilling the following requirements (in wt. %): C 0.08 - 0.13Mn 2.5 - 3.0Si 0.7 - 1.1Cr 0.1 - 0.4Si + Cr 0.9 - 1.3Al 0.01 - 0.08wherein the impurities fulfil the requirements:Ti S 0.02 V S 0.02 Mo S 0.03 N S 0.008 B S 0.003 b) 14 balance Fe apart from impurities, and/ or a multiphase microstructure comprising (in vol. %) retained austenite 5 - 15martensite S 10bainitic ferrite 2 60polygonal ferrite S 5 and/orthe following mechanical propertiesa tensile strength (Rm) 1000 - 1100 MPa a yield strength (Rpoz) 750 - 900 MPaa hole expansion ratio 2 60 %a yield ratio (Rpoz/ Rm) 0.78 - 0.83
7. A high strength cold rolled steel sheet according to any of the preceding claims fulfilling the following requirements: a) a composition fulfilling the following requirements (in wt. %): C 0.09 - 0.12Mn 2.5 - 2.9Si 0.7 - 1.1Cr 0.1 - 0.3Si + Cr 0.9 - 1.2Al 0.01 - 0.05wherein the impurities fulfil the requirements:Ti S 0.01 V S 0.02 Mo S 0.03 N S 0.008 B S 0.003 balance Fe apart from impurities, and/ or the following mechanical propertiestensile strength (Rm) 980 - 1100 MPa yield strength (Rpoz) 750 - 920 MPatotal elongation (Aso) 2 13 %hole expansion ratio (Ä) 2 50 %yield ratio (Rpoz/ Rm) 0.78 - 0.82
8. A high strength cold rolled steel sheet according to any of the preceding clainis,Wherein the thickness of the cold rolled sheet is 1.0 -1. 6 n1n1, preferably 1.1 - 1.5 mm, n1ore preferably 1.2 - 1.4 n1n1. 10 9. A high strength cold rolled steel sheet according to any of the preceding clainis,
9. Wherein the total content of bainitic ferrite and tenipered niartensite is 70 - 90 Vol. %, preferably 80 - 90 Vol. %.
10. A high strength cold rolled steel sheet according to any of the preceding clainis,15 Wherein the product of the tensile strength (Rm) and the total elongation (Aso) is 213000 MPa%, preferably 2 13500 MPa%.
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