EP2831299B2 - High strength cold rolled steel sheet and method of producing such steel sheet - Google Patents
High strength cold rolled steel sheet and method of producing such steel sheet Download PDFInfo
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- EP2831299B2 EP2831299B2 EP13717208.6A EP13717208A EP2831299B2 EP 2831299 B2 EP2831299 B2 EP 2831299B2 EP 13717208 A EP13717208 A EP 13717208A EP 2831299 B2 EP2831299 B2 EP 2831299B2
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- 229910000831 Steel Inorganic materials 0.000 title claims description 102
- 239000010959 steel Substances 0.000 title claims description 102
- 239000010960 cold rolled steel Substances 0.000 title claims description 34
- 238000000034 method Methods 0.000 title claims description 9
- 238000001816 cooling Methods 0.000 claims description 45
- 229910001566 austenite Inorganic materials 0.000 claims description 40
- 238000000137 annealing Methods 0.000 claims description 25
- 229910000734 martensite Inorganic materials 0.000 claims description 22
- 230000000717 retained effect Effects 0.000 claims description 20
- 229910000859 α-Fe Inorganic materials 0.000 claims description 17
- 238000005279 austempering Methods 0.000 claims description 14
- 229910001563 bainite Inorganic materials 0.000 claims description 14
- 229910001568 polygonal ferrite Inorganic materials 0.000 claims description 12
- 229910052799 carbon Inorganic materials 0.000 claims description 11
- 239000000470 constituent Substances 0.000 claims description 10
- 239000000203 mixture Substances 0.000 claims description 9
- 230000015572 biosynthetic process Effects 0.000 claims description 7
- 238000010438 heat treatment Methods 0.000 claims description 4
- 239000012535 impurity Substances 0.000 claims description 3
- 238000005246 galvanizing Methods 0.000 claims 1
- 230000000052 comparative effect Effects 0.000 description 14
- 239000011159 matrix material Substances 0.000 description 13
- 230000009466 transformation Effects 0.000 description 12
- 229910052710 silicon Inorganic materials 0.000 description 11
- 230000000694 effects Effects 0.000 description 10
- 239000011572 manganese Substances 0.000 description 10
- 229910000794 TRIP steel Inorganic materials 0.000 description 8
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 6
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 6
- 229910052782 aluminium Inorganic materials 0.000 description 6
- 229910001567 cementite Inorganic materials 0.000 description 6
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 6
- 239000010703 silicon Substances 0.000 description 6
- 238000012360 testing method Methods 0.000 description 5
- 238000007792 addition Methods 0.000 description 4
- 229910045601 alloy Inorganic materials 0.000 description 4
- 239000000956 alloy Substances 0.000 description 4
- 238000005530 etching Methods 0.000 description 4
- 229910052748 manganese Inorganic materials 0.000 description 4
- 238000004626 scanning electron microscopy Methods 0.000 description 4
- 229910052804 chromium Inorganic materials 0.000 description 3
- 230000001276 controlling effect Effects 0.000 description 3
- 238000013461 design Methods 0.000 description 3
- 238000010191 image analysis Methods 0.000 description 3
- 238000004519 manufacturing process Methods 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 238000001556 precipitation Methods 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- 241001387976 Pera Species 0.000 description 2
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 2
- 239000004411 aluminium Substances 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 229910001562 pearlite Inorganic materials 0.000 description 2
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- 229910002796 Si–Al Inorganic materials 0.000 description 1
- 238000002441 X-ray diffraction Methods 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- 238000004364 calculation method Methods 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 239000012141 concentrate Substances 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 239000004035 construction material Substances 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 238000005088 metallography Methods 0.000 description 1
- 238000005192 partition Methods 0.000 description 1
- 230000008092 positive effect Effects 0.000 description 1
- 238000004881 precipitation hardening Methods 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 230000001105 regulatory effect Effects 0.000 description 1
- 150000003839 salts Chemical class 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 239000002436 steel type Substances 0.000 description 1
- 238000005482 strain hardening Methods 0.000 description 1
- 230000002195 synergetic effect Effects 0.000 description 1
Classifications
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- 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
- 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
- C21D1/20—Isothermal quenching, e.g. bainitic hardening
-
- 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
- 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
- C22C38/04—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
- 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 cold rolled steel sheet suitable for applications in automobiles, construction materials and the like, specifically high strength steel sheet excellent in formability.
- the invention relates to a cold rolled steel sheet having a tensile strength of at least 980 MPa.
- 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 in which 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 most common additions are 1.5 wt. % of both Si and Mn.
- the silicon content should be at least 1 wt. %.
- the silicon content of the steel is important as silicon is insoluble in cementite. US 2009/0238713 discloses such a TRIP steel.
- TRIP steels The formability of TRIP steels is mainly 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 on 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 the 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.
- the present invention is directed to a high strength cold rolled steel sheet having a tensile strength of at least 980 MPa and having an excellent formability and a method of producing the same on an industrial scale.
- the invention relates to a cold rolled TBF steel sheet having properties adapted for the production in a conventional industrial annealing -line. Accordingly, the steel shall not only possess good formability properties but at the same time be optimized with respect to A c3 - temperature, M s - temperature, austempering time and temperature and other factors such as sticky scale influencing the surface quality of the hot rolled steel sheet and the processability of the steel sheet in the industrial annealing line.
- the cold rolled high strength TBF steel sheet has a steel composition consisting of the following elements (in wt. %): C 0.15 - 0.18 Mn 2.2 - 2.4 Si 0.7 - 0.9 Cr 0.1 - 0.9 Si + 0.8 Al +Cr 0.5 - 1.8 Al 0.2 - 0.6 Nb ⁇ 0.1 Mo ⁇ 0.3 Ti ⁇ 0.2 V ⁇ 0.2 Cu ⁇ 0.5 Ni ⁇ 0.5 S ⁇ 0.01 P ⁇ 0.02 N ⁇ 0.02 B ⁇ 0.005 Ca ⁇ 0.005 Mg ⁇ 0.005 REM ⁇ 0.005 balance Fe apart from impurities.
- C is an element which stabilizes 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 0.1 % then it is difficult to attain a tensile strength of 980 MPa. If C exceeds 0.3 % then weldability is impaired. For this reasons, the preferred range is 0.15 - 0.18 %, depending on the desired strength level.
- Manganese is a solid solution strengthening element, which stabilises the austenite by lowering the Ms temperature and prevents ferrite and pearlite to be formed during cooling.
- Mn lowers the Ac3 temperature. At a content of less than 2% it might be difficult to obtain a tensile strength of 980 MPa and the austenitizing temperature might be too high for conventional industrial annealing lines. However, if the amount of Mn is higher than 3 % problems with segregation may occur and the workability may be deteriorated. The preferred range is therefore 2.2 - 2.4%.
- Si acts as a solid solution strengthening element and is important for securing the strength of the thin steel sheet.
- Si is insoluble in cementite and will therefore act to greatly delay the formation of carbides during the bainite transformation as time must be given to Si to diffuse away from the bainite grain boundaries before cementite can form.
- the preferred range is therefore 0.7 - 0.9 %.
- 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 Ac3 temperature and the Ms temperature are only slightly lowered with increasing Cr content.
- the amount of Cr is preferably limited to 0.6 %.
- the preferred range is 0.1-0.35
- Si, Al and Cr when added in combination have a synergistic and completely unforeseen effect, resulting in an increased amount of residual austenite, which, in turn, results in an improved ductility.
- the amount of Si + 0.8 Al + Cr is limited to the range 1.4 - 1.8 %.
- Al promotes ferrite formation and is also commonly used as a deoxidizer.
- Al like Si, is not soluble in the cementite and therefore diffuses away from the bainite grain boundaries before cementite can form.
- the Ms temperature is increased with increasing Al content.
- a further drawback of Al is that it results in a drastic increase in the Ac3 temperature such that the austenitizing temperature might be too high for conventional industrial annealing lines.
- the contents of Al refers to acid soluble Al.
- the steel may optionally contain one or more of the following elements in order to adjust the microstructure, influence on transformation kinetics and/or to fine tune one or more of the mechanical properties of the steel sheet.
- Nb is commonly used in low alloyed steels for improving strength and toughness because of its remarkable influence on the grain size development. Nb increases the strength elongation balance by refining the matrix microstructure and the retained austenite phase due to precipitation of NbC. At contents above 0.1 % the effect is saturated.
- Preferred ranges are therefore 0.02-0.08 %, 0.02 - 0.04 % and 0.02 - 0.03 %.
- Mo can be added in order to improve the strength of the steel sheet. Addition of Mo together with Nb results in precipitation of fine NbMoC which results in a further improvement in the combination of strength and ductility.
- Ti may be added in preferred amounts of ⁇ 0.01 - 0.1 %, 0.02 - 0.08 % or 0.02 - 0.05 %.
- V may be added in preferred amounts of 0.01 - 0.1 % or 0.02 - 0.08 %.
- These elements are solid solution strengthening elements and may have a positive effect on the corrosion resistance.
- The may be added in amounts of 0.05 - 0.5 % or 0.1 - 0.3 % if needed.
- S preferably ⁇ 0.003 P preferably ⁇ 0.01 N preferably ⁇ 0.003
- B suppresses the formation of ferrite and improves the weldability of the steel sheet. For having a noticeable effect at least 0.0002 % should be added. However, excessive amounts of deteriorate the workability. Preferred ranges are ⁇ 0.004 %, 0.0005- 0.003 % and 0.0008 -0.0017 %.
- These elements may be added in order to control the morphology of the inclusions in the steel and thereby improve the hole expansibility and the stretch flangeability of the steel sheet.
- Preferred ranges are 0.0005 -0.005 % and 0.001-0.003 %.
- the high strength cold rolled steel sheet according to the invention has a silicon aluminium based design, i.e. the cementite precipitation during the bainitic transformation is accomplished by Si and Al.
- Si silicon aluminium based design
- the amount of Si is reduced is preferably that it is larger than the amount of Al, preferably Si > 1.1 Al, more preferably Si > 1.3 Al or even Si > 2 Al.
- the amount of Si is preferred to be larger than the amount of Cr and to restrict the amount of Cr in order to retard the bainite transformation too much. For this reason it preferred to keep Si > Cr, preferably Si > 1.5 Cr, more preferably Si > 2 Cr, most preferably Si > 3 Cr.
- the cold rolled high strength TBF steel sheet has a multiphase microstructure comprising (in vol. %) retained austenite 5 - 20 bainite + bainitic ferrite + tempered martensite ⁇ 80 polygonal ferrite ⁇ 10
- the amount of retained austenite is 5-20%, preferably from 5 - 16 %, most preferably from 5 - 10 %. Because of the TRIP effect retained austenite is a prerequisite when high elongation is necessary. High amount of residual austenite decreases the stretch flangeability.
- the polygonal ferrite is replace by bainitic ferrite (BF) and the microstructure generally contains more than 50 % BF.
- the matrix consists of BF laths strengthened by a high dislocation density and between the laths the retained austenite is contained.
- MA (martensite/austenite) constituent represents the individual islands in the microstructure consisting of retained austenite and/or martensite. These two microstructural compounds are difficult to be distinguished by common etching technique for advanced high strength steels (AHSS) - Le Pera etching and also by investigations with scanning electron microscopy (SEM). Le Pera etching, which is very common to the person skilled in the art can be found eg in "F.S. LePera, Improved etching technique for the determination of percent martensite in high-strength dual-phase steels Metallography, Volume 12, Issue 3, September 1979, Pages 263-268". Furthermore, for properties such as hole expansion the amount and size of MA constituent plays an important role. Therefore, in an industrial practice the fraction and size of MA constituent are often used by AHSS for the correlations in terms of their mechanical properties and formability.
- the size of the martensite-austenite (MA) shall be max 5 ⁇ m, preferably 3 ⁇ m. Minor amounts of martensite may be present in the structure.
- the amount of MA shall be max 20 %, preferably max 16 %, most preferably below 10 %.
- the cold rolled high strength TBF steel sheet preferably has the following mechanical properties tensile strength (Rm) ⁇ 980 - 1200 MPa total elongation (A80) ⁇ 11 % hole expanding ratio ( ⁇ ) ⁇ 45 %, preferably ⁇ 50 %.
- the R m and A 80 values were derived according to the European norm EN 10002 Part 1, wherein the samples were taken in the longitudinal direction of the strip.
- the hole expanding ratio ( ⁇ ) was determined by the hole expanding test according to ISO/WD 16630. 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 x 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 formability properties of the steel sheet were further assessed by the parameters: strength-elongation balance (R m x A 80 ) and stretch-flangeability (R m x ⁇ ).
- An elongation type steel sheet has a high strength-elongation balance and a high hole expansibility type steel sheet has a high stretch flangeability.
- the steel sheet of the present invention fulfils at least one of the following conditions: R m ⁇ A 80 ⁇ 13 000 MPa ⁇ % R m ⁇ ⁇ ⁇ 50 000 MPa ⁇ %
- the mechanical properties of the steel sheet of the present invention can be largely adjusted by the alloying composition and the microstructure.
- a comparative chemical composition may comprise 0.19 C, 2.6 Mn, 0.82 Si, 0.3-0.7 Al, 0.10 Mo, rest Fe apart from impurities.
- the steel sheets of the present invention can be produced using a conventional CA-line.
- the processing comprises the steps of:
- the process shall preferably further comprise the steps of:
- the amount of polygonal ferrite in the steel sheet can be controlled. If the annealing temperature, T an , is below the temperature at which the steel is fully austenitic, A c3 , there is a risk that the amount of polygonal ferrite in the steel sheet will exceed 10%. Too much polygonal ferrite gives larger size of the MA constituent.
- the size of MA constituent in the steel sheet can be controlled. If the cooling stop temperature of rapid cooling, T RC , exceeds the martensite start temperature, T MS , the size of MA constituent becomes larger which lowers the R m x ⁇ product under the value necessary for a high hole expansion type steel sheet. In the case of a high elongation type steel sheet the cooling stop temperature, T RC might be above the martensite start temperature, T MS .
- the size of MA constituent and the amount of retained austenite, RA can be controlled.
- a lower austempering temperature, T 0A will lower the amount of RA.
- a higher austempering temperature, T 0A will lower the amount of RA and increase the size of MA constituent. In both cases, this will lower the uniform elongation, Ag, and total elongation, A 80 , of the steel sheet.
- the amount of polygonal ferrite can be controlled. Lowering the cooling rates will increase the amount of polygonal ferrite to more than 10%.
- the steel sheet is a high elongation type steel having strength-elongation balance R m x A 80 ⁇ 13 000 MPa%, preferably ⁇ 15 000 MPa.
- the steel sheet is a high hole expansibility type steel having stretch-flangeability R m x ⁇ ⁇ 50 000 MPa%, preferably ⁇ 55 000 MPa.
- test alloys A-M were manufactured having chemical compositions according to table I. Steel sheets were manufactured and subjected to heat treatment in a conventional CA-line according to the parameters specified in Table II. The microstructure of the steel sheets were examined along with a number of mechanical properties and the result is presented in Table II
- Amount of retained austenite was measured by X ray analysis at a 1/4 position of the sheet thickness.
- a photograph of a microstructure taken by the SEM was subjected to image analysis to measure each of a volume-% of a MA, volume-% of matrix phase (bainitic ferrite + bainite + tempered martensite), volume-% of retained austenite and volume-% of polygonal ferrite.
- Bainitic ferrite + bainite + tempered martensite
- the present invention can be widely applied to high strength steel sheets having excellent formability for vehicles such as automobiles.
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Description
- The present invention relates to high strength cold rolled steel sheet suitable for applications in automobiles, construction materials and the like, specifically high strength steel sheet excellent in formability. In particular, the invention relates to a cold rolled steel sheet having a tensile strength of at least 980 MPa.
- 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, forming complex structural members of thin sheet. However, such part cannot be produced from conventional high strength steels because of a too low formability for complex structural parts. For this reason multi phase Transformation Induced Plasticity aided steels (TRIP steels) have gained considerable interest in the last years.
- TRIP steels possess a multi-phase microstructure, which includes a meta-stable retained austenite phase, which is capable of producing the TRIP effect. When the steel is deformed, 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 in which to chemically stabilize austenite at room temperature. 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. %. 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 the austenite in order to stabilize it at room temperature. 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 most common additions are 1.5 wt. % of both Si and Mn. In order to inhibit the austenite to decompose during the bainite transformation it is generally considered necessary that the silicon content should be at least 1 wt. %. The silicon content of the steel is important as silicon is insoluble in cementite.
US 2009/0238713 discloses such a TRIP steel. However, high silicon content can be responsible for a poor surface quality of hot rolled steel and a poor coatability of cold rolled steel. Accordingly, partial or complete replacement of silicon by other elements has been investigated and promising results have been reported for Al-based alloy design. However, a disadvantage with the use of aluminium is the rise of the transformation temperature (Ac3) which makes full austenitizing in conventional industrial annealing lines very difficult or impossible. - Depending on the matrix phase the following main types of TRIP steels are cited:
- TPF TRIP steel with matrix of polygonal ferrite
TPF steels, as already mentioned before-hand, contain the matrix from relatively soft polygonal ferrite with inclusions from bainite and retained austenite. Retained austenite transforms to martensite upon deformation, resulting in a desirable TRIP effect, which allows the steel to achieve an excellent combination of strength and drawability. Their stretch flangeability is however lower compared to TBF, TMF and TAM steels with more homogeneous microstructure and stronger matrix. - TBF TRIP steel with matrix of bainitic ferrite
TBF steels have been known for long and attracted a lot of interest because the bainitic ferrite matrix allows an excellent stretch flangeability. Moreover, similarly to TPF steels, the TRIP effect, ensured by the strain-induced transformation of metastable retained austenite islands into martensite, remarkably improves their drawability. - TMF TRIP steel with matrix of martensitic ferrite
TMF steels also contain small islands of metastable retained austenite embedded into strong martensitic matrix, which enables these steels to achieve even better stretch flangeability compared to TBF steels. Although these steels also exhibit the TRIP effect, their drawability is lower compared to TBF steels. - TAM TRIP steel with matrix of annealed martensite
TAM steels contain the matrix from needle-like ferrite obtained by re-annealing of fresh martensite. A pronounced TRIP effect is again enabled by the transformation of metastable retained austenite inclusions into martensite upon straining. Despite their promising combination of strength, drawability and stretch flangeability, these steels have not gained a remarkable industrial interest due to their complicated and expensive double-heat cycle. - The formability of TRIP steels is mainly affected by the transformation characteristics of the retained austenite phase, which is in turn affected by the austenite chemistry, its morphology and other factors. In ISIJ International Vol. 50(2010), No. 1, p. 162 -168 aspects influencing on the formability of TBF steels having a tensile strength of at least 980 MPa are discussed. However, the cold rolled materials examined in this document were annealed at 950 °C and the 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.
- The present invention is directed to a high strength cold rolled steel sheet having a tensile strength of at least 980 MPa and having an excellent formability and a method of producing the same on an industrial scale. In particular, the invention relates to a cold rolled TBF steel sheet having properties adapted for the production in a conventional industrial annealing -line. Accordingly, the steel shall not only possess good formability properties but at the same time be optimized with respect to Ac3- temperature, Ms- temperature, austempering time and temperature and other factors such as sticky scale influencing the surface quality of the hot rolled steel sheet and the processability of the steel sheet in the industrial annealing line.
- The invention is described in the claims.
- The cold rolled high strength TBF steel sheet has a steel composition consisting of the following elements (in wt. %):
C 0.15 - 0.18 Mn 2.2 - 2.4 Si 0.7 - 0.9 Cr 0.1 - 0.9 Si + 0.8 Al +Cr 0.5 - 1.8 Al 0.2 - 0.6 Nb < 0.1 Mo < 0.3 Ti < 0.2 V < 0.2 Cu < 0.5 Ni < 0.5 S ≤ 0.01 P ≤ 0.02 N ≤ 0.02 B < 0.005 Ca < 0.005 Mg < 0.005 REM < 0.005 - The limitation of the elements is explained below.
- The limitation of the elements C, Mn, Si, Al and Cr is essential to the invention for the reasons set out below:
- C is an element which stabilizes 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 0.1 % then it is difficult to attain a tensile strength of 980 MPa. If C exceeds 0.3 % then weldability is impaired. For this reasons, the preferred range is 0.15 - 0.18 %, depending on the desired strength level.
- Manganese is a solid solution strengthening element, which stabilises the austenite by lowering the Ms temperature and prevents ferrite and pearlite to be formed during cooling. In addition, Mn lowers the Ac3 temperature. At a content of less than 2% it might be difficult to obtain a tensile strength of 980 MPa and the austenitizing temperature might be too high for conventional industrial annealing lines. However, if the amount of Mn is higher than 3 % problems with segregation may occur and the workability may be deteriorated. The preferred range is therefore 2.2 - 2.4%.
- Si acts as a solid solution strengthening element and is important for securing the strength of the thin steel sheet. Si is insoluble in cementite and will therefore act to greatly delay the formation of carbides during the bainite transformation as time must be given to Si to diffuse away from the bainite grain boundaries before cementite can form. The preferred range is therefore 0.7 - 0.9 %.
- 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 Ac3 temperature and the Ms temperature are only slightly lowered with increasing Cr content. However, due to the retardation of the bainite transformation longer holding times are required such that the processing on a conventional industrial annealing line is made difficult or impossible, when using normal line speeds. For this reason the amount of Cr is preferably limited to 0.6 %. The preferred range is 0.1-0.35
- Si, Al and Cr when added in combination have a synergistic and completely unforeseen effect, resulting in an increased amount of residual austenite, which, in turn, results in an improved ductility. For these reasons the amount of Si + 0.8 Al + Cr is limited to the range 1.4 - 1.8 %.
- Al promotes ferrite formation and is also commonly used as a deoxidizer. Al, like Si, is not soluble in the cementite and therefore diffuses away from the bainite grain boundaries before cementite can form. The Ms temperature is increased with increasing Al content. A further drawback of Al is that it results in a drastic increase in the Ac3 temperature such that the austenitizing temperature might be too high for conventional industrial annealing lines. The contents of Al refers to acid soluble Al.
- In addition to C, Mn, Si, Al and Cr the steel may optionally contain one or more of the following elements in order to adjust the microstructure, influence on transformation kinetics and/or to fine tune one or more of the mechanical properties of the steel sheet.
- Nb is commonly used in low alloyed steels for improving strength and toughness because of its remarkable influence on the grain size development. Nb increases the strength elongation balance by refining the matrix microstructure and the retained austenite phase due to precipitation of NbC. At contents above 0.1 % the effect is saturated.
- Preferred ranges are therefore 0.02-0.08 %, 0.02 - 0.04 % and 0.02 - 0.03 %.
- Mo can be added in order to improve the strength of the steel sheet. Addition of Mo together with Nb results in precipitation of fine NbMoC which results in a further improvement in the combination of strength and ductility.
- These elements are effective for precipitation hardening. Ti may be added in preferred amounts of ≤ 0.01 - 0.1 %, 0.02 - 0.08 % or 0.02 - 0.05 %. V may be added in preferred amounts of 0.01 - 0.1 % or 0.02 - 0.08 %.
- These elements are solid solution strengthening elements and may have a positive effect on the corrosion resistance. The may be added in amounts of 0.05 - 0.5 % or 0.1 - 0.3 % if needed.
- These elements are not desired in this type of steel and should therefore be limited.
S preferably ≤ 0.003
P preferably ≤ 0.01
N preferably ≤ 0.003 - B suppresses the formation of ferrite and improves the weldability of the steel sheet. For having a noticeable effect at least 0.0002 % should be added. However, excessive amounts of deteriorate the workability. Preferred ranges are < 0.004 %, 0.0005- 0.003 % and 0.0008 -0.0017 %.
- These elements may be added in order to control the morphology of the inclusions in the steel and thereby improve the hole expansibility and the stretch flangeability of the steel sheet. Preferred ranges are 0.0005 -0.005 % and 0.001-0.003 %.
- The high strength cold rolled steel sheet according to the invention has a silicon aluminium based design, i.e. the cementite precipitation during the bainitic transformation is accomplished by Si and Al. Although the amount of Si is reduced is preferably that it is larger than the amount of Al, preferably Si > 1.1 Al, more preferably Si > 1.3 Al or even Si > 2 Al.
- In the steel sheet of the present invention it is preferred to control the amount of Si to be larger than the amount of Cr and to restrict the amount of Cr in order to retard the bainite transformation too much. For this reason it preferred to keep Si > Cr, preferably Si > 1.5 Cr, more preferably Si > 2 Cr, most preferably Si > 3 Cr.
- The cold rolled high strength TBF steel sheet has a multiphase microstructure comprising (in vol. %)
retained austenite 5 - 20 bainite + bainitic ferrite + tempered martensite ≥ 80 polygonal ferrite ≤ 10 - The amount of retained austenite is 5-20%, preferably from 5 - 16 %, most preferably from 5 - 10 %. Because of the TRIP effect retained austenite is a prerequisite when high elongation is necessary. High amount of residual austenite decreases the stretch flangeability. In these steel sheet the polygonal ferrite is replace by bainitic ferrite (BF) and the microstructure generally contains more than 50 % BF. The matrix consists of BF laths strengthened by a high dislocation density and between the laths the retained austenite is contained.
- MA (martensite/austenite) constituent represents the individual islands in the microstructure consisting of retained austenite and/or martensite. These two microstructural compounds are difficult to be distinguished by common etching technique for advanced high strength steels (AHSS) - Le Pera etching and also by investigations with scanning electron microscopy (SEM). Le Pera etching, which is very common to the person skilled in the art can be found eg in "F.S. LePera, Improved etching technique for the determination of percent martensite in high-strength dual-phase steels Metallography, Volume 12, Issue 3, September 1979, Pages 263-268". Furthermore, for properties such as hole expansion the amount and size of MA constituent plays an important role. Therefore, in an industrial practice the fraction and size of MA constituent are often used by AHSS for the correlations in terms of their mechanical properties and formability.
- The size of the martensite-austenite (MA) shall be max 5 µm, preferably 3 µm. Minor amounts of martensite may be present in the structure. The amount of MA shall be max 20 %, preferably max 16 %, most preferably below 10 %.
- The cold rolled high strength TBF steel sheet preferably has the following mechanical properties
tensile strength (Rm) ≥ 980 - 1200 MPa total elongation (A80) ≥ 11 % hole expanding ratio (λ) ≥ 45 %, preferably ≥ 50 %. - The Rm and A80 values were derived according to the European norm EN 10002 Part 1, wherein the samples were taken in the longitudinal direction of the strip. The hole expanding ratio (λ) was determined by the hole expanding test according to ISO/WD 16630. 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 x 100 mm2. 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 formability properties of the steel sheet were further assessed by the parameters: strength-elongation balance (Rm x A80) and stretch-flangeability (Rm x λ).
- An elongation type steel sheet has a high strength-elongation balance and a high hole expansibility type steel sheet has a high stretch flangeability.
-
- The mechanical properties of the steel sheet of the present invention can be largely adjusted by the alloying composition and the microstructure.
- A comparative chemical composition may comprise 0.19 C, 2.6 Mn, 0.82 Si, 0.3-0.7 Al, 0.10 Mo, rest Fe apart from impurities.
- The steel sheets of the present invention can be produced using a conventional CA-line. The processing comprises the steps of:
- a) providing a cold rolled steel steel strip having a composition as set out above,
- b) annealing the cold rolled steel steel strip at an annealing temperature, Tan, above the Ac3 temperature in order to fully austenitize the steel, followed by
- c) cooling the cold rolled steel steel strip from the annealing temperature, Tan, to a cooling stop temperature of rapid cooling, TRC, at a cooling rate sufficient to avoid the ferrite formation, the cooling rate being 20 - 100 °C/s, while:
- for a high hole expansion type steel sheet the cooling stop temperature, TRC, being lower than the martensite start temperature, TMS, TMS being between 300 and 400 °C, preferably between340 and 370 °C,
- for a high elongation type steel sheet the cooling stop temperature, TRC, being between 360 and 460 °C, preferably between 380 and 420 °C, followed by
- d) austempering the cold rolled steel strip at an overageing/austempering temperature, T0A, that is between 360 and 460 °C, preferably between 380 and 420 °C, and
- e) cooling the cold rolled steel strip to ambient temperature.
- The process shall preferably further comprise the steps of:
- in step b) the annealing being performed at an annealing temperature, Tan, that is between 910 and 930 °C, during an annealing holding time, tan, which is between 150-200 s, preferably 180 s,
- in step c) the cooling being performed according to a cooling pattern having two separate cooling rates; a first cooling rate, CR1, of 80 - 100 °C/s, preferably of 85 - 95 °C/s, preferably about 90 °C/s to a temperature which is between 530 to 570 °C, preferably 550 °C, and a second cooling rate, CR2, of 35 - 45 °C, preferably about 40 °C/s to the stop temperature of rapid cooling, TRC, and
- in step d) the austempering being performed at an overageing/austempering holding time, t0A, which is between 150 and 600 s, preferably 180 and 540 s.
- Preferably, no external heating is applied to the steel strip between step c) and d).
- The reasons for regulating the heat treatment conditions are set out below:
- Annealing temperature, Tan, > Ac3 temperature:
- By fully austenitizing the steel the amount of polygonal ferrite in the steel sheet can be controlled. If the annealing temperature, Tan, is below the temperature at which the steel is fully austenitic, Ac3, there is a risk that the amount of polygonal ferrite in the steel sheet will exceed 10%. Too much polygonal ferrite gives larger size of the MA constituent.
- By controlling the cooling stop temperature of rapid cooling, TRC, the size of MA constituent in the steel sheet can be controlled. If the cooling stop temperature of rapid cooling, TRC, exceeds the martensite start temperature, TMS, the size of MA constituent becomes larger which lowers the Rm x λ product under the value necessary for a high hole expansion type steel sheet. In the case of a high elongation type steel sheet the cooling stop temperature, TRC might be above the martensite start temperature, TMS.
- By controlling the austempering temperature, T0A, to a temperature between 360 and 460 °C, preferably between 380 and 420 °C, the size of MA constituent and the amount of retained austenite, RA, can be controlled. A lower austempering temperature, T0A, will lower the amount of RA. A higher austempering temperature, T0A, will lower the amount of RA and increase the size of MA constituent. In both cases, this will lower the uniform elongation, Ag, and total elongation, A80, of the steel sheet.
- By controlling the first cooling rate, CR1, of 80 - 100 °C/s, preferably of 85 - 95 °C/s, preferably about 90 °C/s to a temperature which is between 530 to 570 °C, preferably 550 °C, and a second cooling rate, CR2, of 35 - 45 °C preferably about 40 °C/s to the stop temperature of rapid cooling, TRC, the amount of polygonal ferrite can be controlled. Lowering the cooling rates will increase the amount of polygonal ferrite to more than 10%.
- In one embodiment of the invention the steel sheet is a high elongation type steel having strength-elongation balance Rm x A80 ≥ 13 000 MPa%, preferably ≥ 15 000 MPa.
- In another embodiment of the invention the steel sheet is a high hole expansibility type steel having stretch-flangeability Rm x λ ≥ 50 000 MPa%, preferably ≥ 55 000 MPa.
- A number of test alloys A-M were manufactured having chemical compositions according to table I. Steel sheets were manufactured and subjected to heat treatment in a conventional CA-line according to the parameters specified in Table II. The microstructure of the steel sheets were examined along with a number of mechanical properties and the result is presented in Table II
- A completely different behaviour is found for the inventive steel sheets. Partly based on these results the claimed TBF steel sheet having a Si-Al based alloy design, optionally with additions of Cr having a high stretch flangeability and an improved processability for the production in a continuous annealing line was developed.
- Amount of retained austenite was measured by X ray analysis at a 1/4 position of the sheet thickness. A photograph of a microstructure taken by the SEM was subjected to image analysis to measure each of a volume-% of a MA, volume-% of matrix phase (bainitic ferrite + bainite + tempered martensite), volume-% of retained austenite and volume-% of polygonal ferrite.
- A crystal grain in which a white point (or white line composed of a linear array of continuously connected white point) was observed in the image analysis of the SEM photograph.
- A crystal grain in which no white point (or no white line) was observed in the image analysis of the SEM photograph.
Table I Chemical composition in wt. % Steel type No. C Si Mn P S sol-Al Cr Mo Nb sol-Ti B N Si+Cr Si+Cr+0.8Al Ms point Ac3* A 0.192 0.82 2.55 0.008 0.0022 0.70 0.101 0.0040 0.83 1.39 386 902 comparative steel B 0.187 0.83 2.56 0.007 0.0020 0.70 0.01 0.030 0.0029 0.84 1.40 388 904 comparative steel C 0.196 0.82 2.58 0.008 0.0020 0.69 0.01 0.10 0.0033 0.83 1.38 381 904 comparative steel D 0.192 0.82 2.58 0.008 0.0023 0.69 0.01 0.10 0.030 0.0032 0.83 1.38 383 903 comparative steel E 0.205 0.78 2.57 0.008 0.0022 0.70 0.31 0.050 0.0033 1.09 1.65 374 903 comparative steel F 0.175 0.81 2.28 0.008 0.0024 0.290 0.0045 0.81 1.04 403 870 comparative steel G 0.172 0.79 2.27 0.009 0.0026 0.588 0.0043 0.79 1.26 405 903 comparative steel H 0.171 0.79 2.25 0.008 0.0026 0.291 0.0005 0.0045 0.79 1.02 406 870 comparative steel I 0.177 0.79 2.24 0.008 0.0027 0.590 0.0006 0.0048 0.79 1.26 403 902 comparative steel J 0.195 0.56 2.26 0.0065 0.0025 0.85 0.038 0.005 0.002 0.005 0.0003 0.0025 0.598 1.28 393 951 comparative steel K 0.198 0.62 1.74 0.008 0.0024 0.6 0.013 0.004 0.002 0.005 0.0004 0.0028 0.633 1.11 409 884 comparative steel L 0.168 0.81 2.49 0.007 0.0025 0.57 0.01 0.10 0.002 0.006 0.0003 0.0042 0.82 1.28 397 910 comparative steel M 0.130 0.4 2.41 0.013 0.002 0.045 0.004 0.4 0.44 420 830 comparative steel Ms = 561-474C%-33Mn-17Cr-21Mo
Ac3 : Measured by dilatometer - The present invention can be widely applied to high strength steel sheets having excellent formability for vehicles such as automobiles.
Claims (13)
- A high strength cold rolled steel sheet havinga) a composition consisting of the following elements (in wt. %):
C 0.15 - 0.18 Mn 2.2 - 2.4 Si 0.7 - 0.9 Cr 0.1 - 0.9 Al 0.2 - 0.6 Si + 0.8 Al +Cr 1.4-1.8 Nb < 0.1 Mo < 0.3 Ti < 0.2 V < 0.2 Cu < 0.5 Ni < 0.5 S ≤ 0.01 P ≤ 0.02 N ≤ 0.02 B < 0.005 Ca < 0.005 Mg < 0.005 REM < 0.005 b) a multiphase microstructure comprising (in vol. %)retained austenite 5-20 bainite + bainitic ferrite + tempered martensite ≥ 80 polygonal ferrite ≤ 10 c) the following mechanical propertiesa tensile strength (Rm) 980 - 1200 MPa an elongation (A80) ≥ 11 % a hole expanding ratio (λ) ≥ 45 % Rm x A80 ≥ 13 000 MPa% Rm x λ ≥ 50 000 MPa% - A high strength cold rolled steel sheet according to any of the preceding claims fulfilling at least one of:
Nb 0.02 - 0.08 Mo 0.05 - 0.3 Ti 0.02 - 0.08 V 0.02 - 0.1 Cu 0.05 - 0.4 Ni 0.05 - 0.4 B 0.0005 - 0.003 Ca 0.0005 - 0.005 Mg 0.0005 - 0.005 REM 0.0005 - 0.005 - A high strength cold rolled steel sheet according to any of the preceding claims fulfilling at least one of:
S ≤ 0.01 preferably ≤ 0.003 P ≤ 0.02 preferably ≤ 0.01 N ≤ 0.02 preferably ≤ 0.003 Ti > 3.4N - A high strength cold rolled steel sheet according to according to any of the preceding claims wherein the maximum size of the martensite-austenite constituent (MA) is ≤ 5 µm, preferably ≤ 3 µm.
- A high strength cold rolled steel sheet according to any of the preceding claims wherein the multiphase microstructure comprising (in vol. %)
retained austenite 5 - 16, preferably below 10 % bainite + bainite ferrite + tempered martensite ≥ 80 polygonal ferrite ≤ 10 martensite-austenite constituent (MA) ≤ 20 %, preferably ≤ 16 %, most preferably below 10 % - A high strength cold rolled steel sheet according to any of the preceding claims wherein the steel comprises
Nb 0.02- 0.03 - A high strength cold rolled steel sheet any of the preceding claims wherein the ratio (Mn+Cr)/(Si+Al) ≥ 1.6.
- A high strength cold rolled steel sheet according to any of the preceding claims wherein the amount of Si is on the order of the amount of Al or larger than the amount of Al, preferably Si > 1.1 Al, more preferably Si > 1.3 Al or even Si > 2 Al.
- A high strength cold rolled steel sheet according to any of the preceding claims which is not provided with a hot dip galvanizing layer.
- A method of producing a high strength cold rolled steel sheet according to any of the preceding claims comprising the steps of:a) providing a cold rolled steel strip having a composition as set out in any of the preceding claims,b) annealing the cold rolled steel strip at a temperature above the Ac3 temperature in order to fully austenitize the steel, followed byc) cooling the cold rolled steel strip from the annealing temperature, Tan, to a cooling stop temperature of rapid cooling, TRC, that is between 360 and 460 °C, preferably between 380 and 420 °C, at cooling rate sufficient to avoid the ferrite formation, the cooling rate being 20 -100 °C/s, followed byd) austempering the cold rolled steel strip at an overageing/austempering temperature, TOA, that is between 360 and 460 °C, preferably 380 and 420 °C, ande) cooling the cold rolled steel strip to ambient temperature,
wherein the steel sheet is a high elongation type steel sheet having strength-elongation balance Rm x A80 ≥ 13 000 MPa%, preferably ≥ 15 000 MPa%. - A method of producing a high strength cold rolled steel sheet according to any of claims 1-9 comprising the steps of:a) providing a cold rolled steel strip having a composition as set out in any of the preceding claims,b) annealing the cold rolled steel strip at a temperature above the Ac3 temperature in to fully austenitizing the steel, followed byc) cooling the cold rolled steel strip from the annealing temperature, Tan, to a cooling stop temperature of rapid cooling TRC < TMS, TMS being between300 and 400 °C, preferably between 340 and 370 °C, at cooling rate sufficient to avoid the formation ferrite, the cooling rate being 20 -100 °C/s, followed byd) austempering the cold rolled steel strip at an overageing/austempering temperature, TOA that is between360 and 460 °C, preferably between 380 and 420 °C, preferably TOA > TRC, ande) cooling the cold rolled steel strip to ambient temperature,
wherein the steel sheet is a high hole expansibility type steel sheet having stretch-flangeability Rm x λ ≥ 50 000 MPa% preferably ≥ 55 000 MPa. - A method of producing a high strength cold rolled steel sheet according to claim 10 and 11 wherein:in step b) the annealing being performed at an annealing temperature, Tan, that is between 910 and 930 °C, during an annealing holding time, tan, which is between 150 and 200 s, preferably 180 s,in step c) the cooling being performed according to a cooling pattern having two separate cooling rates; a first cooling rate, CR1, of 80 - 100 °C/s, preferably of 85 - 95 °C/s, preferably about 90 °C/s to a temperature which is between 530 to 570 °C, preferably 550 °C, and a second cooling rate, CR2, of 35 - 45 °C, preferably about 40 °C/s to the stop temperature of rapid cooling, TRC, andin step d) the austempering of the steel being performed at a time interval of 150-600 s, preferably 180 - 540 s.
- A method of producing a high strength cold rolled steel sheet according to claims 10 and 11 wherein no external heating being applied to the steel sheet between step c) and d).
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WO2013144377A1 (en) † | 2012-03-30 | 2013-10-03 | Voestalpine Stahl Gmbh | High strength cold rolled steel sheet and method of producing such steel sheet |
WO2013144376A1 (en) † | 2012-03-30 | 2013-10-03 | Voestalpine Stahl Gmbh | High strength cold rolled steel sheet and method of producing such steel sheet |
WO2013144373A1 (en) † | 2012-03-30 | 2013-10-03 | Voestalpine Stahl Gmbh | High strength cold rolled steel sheet and method of producing such steel sheet |
Cited By (4)
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DE102022102418A1 (en) | 2022-02-02 | 2023-08-03 | Salzgitter Flachstahl Gmbh | High-strength, hot-dip coated steel strip having structural transformation-induced plasticity and method of making same |
WO2023148199A1 (en) | 2022-02-02 | 2023-08-10 | Salzgitter Flachstahl Gmbh | High-strength hot dip-coated steel strip with plasticity brought about by microstructural transformation and method for production thereof |
DE102022132188A1 (en) | 2022-12-05 | 2024-06-06 | Salzgitter Flachstahl Gmbh | Process for producing a high-strength steel flat product with a multiphase structure and corresponding high-strength steel flat product |
WO2024120962A1 (en) | 2022-12-05 | 2024-06-13 | Salzgitter Flachstahl Gmbh | Method for producing a high-strength flat steel product having a multiphase microstructure, and corresponding high-strength flat steel product |
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EP2831299B1 (en) | 2017-09-13 |
EP2831299A1 (en) | 2015-02-04 |
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