High-strength steel strip or sheet and method for manufacturing same

Abstract

The invention relates to a high-strength steel strip or sheet comprising, in wt.%, 0.15 to 0.25% of C, 0.5 to 1.8% of Si, 2.0 to 2.7% of Mn, 0.03 to 1.2% of Al, 0.6 to 2.0% of Si + Al. The strip or sheet has a tensile strength () of 980 to 1110 MPa, comprising a microstructure of 10 to 25 vol% of retained austenite. The retained austenite satisfies: a mechanical stability () of 5-30, a retained austenite carbon content of 0.8-1.2, and > = 200. The invention also relates to a method for producing said strip or sheet comprising a dual annealing cycle.

Description

TECHNICAL FIELD

[0001] The present invention relates to high strength steel strips or sheets suitable for use in automotive applications. In particular, the present invention relates to cold-rolled steel strips or sheets that have been produced by a process comprising a double annealing of the cold-rolled strip. BACKGROUND

[0002] For a wide variety of applications, increased strength levels are a prerequisite for lightweight structures in the automotive industry, as a reduction in the body mass leads to a reduction in fuel consumption.

[0003] Automotive body parts are often stamped from sheet steel, thereby forming complex structural members of thin sheets. However, such parts cannot be produced from conventional high strength steels, as the formability for complex structural parts is too low. For this reason, multi-phase transformation induced plasticity assisted steels (TRIP steels) have gained considerable interest in the past years, especially for applications in automotive body structural parts.

[0004] TRIP steels have a multi-phase microstructure, which includes a metastable residual austenite phase that can generate a TRIP effect. When the steel is deformed, the austenite transforms into martensite, which leads to a significant work hardening. This hardening effect serves to resist necking in the material and to delay failure in sheet forming operations. The microstructure of TRIP steels can be greatly varied to change its mechanical properties.

[0005] Quenching & Partitioning (Q&P) is an annealing cycle consisting of a quenching and a partitioning step. In the quenching step, a fully austenitized steel or an intercritically annealed steel is quenched to a temperature between the martensite start temperature M S and the martensite finish temperature M F to achieve a partial martensitic transformation. The quenched steel is then held at a temperature equal to or higher than the initial quenching temperature. The austenite, which is dominant after quenching, is stabilized by partitioning of carbon from martensite into austenite during the partitioning.

[0006] TRIP-assisted steel sheets with an annealed martensite matrix can be produced by double annealing of cold-rolled strip. The martensite microstructure after the second annealing step results in a mainly lamellar structure and these steels have a large amount of plate-like retained austenite along the lath boundaries of the annealed martensite. The stability of the retained austenite in these steels is generally high due to the lamellar structure of the austenite. The high stability of the retained austenite improves the TRIP effect and the overall ductility. The homogeneous lamellar structure of the austenite is beneficial for the local ductility.

[0007] US2016 / 0177414A1 discloses a cold-rolled steel annealed twice. After the first annealing, the steel is cooled to 320-500°C and held at 320-500°C for 30 seconds or more. After the second annealing, the steel is cooled to 120-320°C and thereafter held at 320-500°C for 30 seconds or more. The steel has a microstructure comprising 3-20% ferrite, 5-20% retained austenite, 5-20% martensite and the balance bainite and / or tempered martensite.

[0008] US2017 / 0327924A1 discloses a cold-rolled steel annealed twice. After the first annealing, the steel is cooled to room temperature or to a controlled temperature above room temperature, preferably below the martensite finish temperature, to achieve a mainly martensitic structure. The second annealing comprises soaking in an intercritical range of 720 to 850°C and thereafter holding at 370 to 430°C. The steel has a microstructure consisting of mainly ferrite (50 up to 80% or more); 5-25% retained austenite; and, 0-15% fresh martensite.

[0009] US2020 / 0392598A1, US2020 / 392610A1, US2020 / 00440421A1, US2019 / 203316A1, JP2004238679A2, WO2022 / 123289A1, US2020 / 354823A1, US2021 / 207236A1 are further examples where multiple annealing has been proposed.

[0010] WO2023 / 001835A1 discloses a steel with TS > 980 MPa. In addition to other elements, the steel comprises 0.3-0.5 Si. According to this document, high Si can reduce the RA stability at elevated temperatures in the later stages of the manufacturing process, for example in hot dip galvanizing and galvannealing, or in postproduction operations, for example welding. High Si can also cause liquid metal embrittlement during welding.

[0011] The object of the present invention is to further improve the steel properties of double annealed steels, preferably properties related to local and / or global ductility. SUMMARY

[0012] The present invention relates to a steel strip or sheet having a tensile strength of 980-1100 MPa. The steel strip or sheet is produced in a double annealing process, wherein the first annealing cycle comprises quenching to a temperature between M S - 20 and M F and partitioning at a temperature higher than the quenching temperature, and the second annealing cycle comprises quenching to a temperature in the range of 150-500 °C and partitioning at a temperature higher than the quenching temperature or isothermal. The properties of the retained austenite result in an improvement of local and global ductility. Thereby an improved formability and crash resistance of safety relevant components manufactured from the steel of the present invention is provided. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 The annealing cycles of the present invention are schematically shown. DETAILED DESCRIPTION

[0014] In a preferred embodiment the strip or sheet has a composition (composition) consisting of the following alloying elements (in wt.%):

[0015]

[0016] The balance, apart from impurities, is Fe.

[0017] The composition (composition) does not include any coating applied to the strip or sheet.

[0018] The importance of each element and their interaction with each other as well as the limits of the chemical composition of the claimed alloy are briefly explained in the following. Throughout the specification all percentages of the chemical composition of the steel are given in weight % (wt.%). The upper and lower limits of the individual elements can be freely combined within the limits stated in the claims. For all values given in this application the arithmetical precision of the numbers can be increased by one or two digits. Thus, a value given as for example 0.1% can also be expressed as 0.10 or 0.100%. The amounts of the microstructural constituents are given in volume % (vol.%).

[0019] C: 0.15-0.25%

[0020] C stabilizes the austenite and is important for obtaining sufficient carbon content in the residual austenite phase. C is also important for obtaining the required strength level. Typically, it can be expected that the tensile strength increases by about 100 MPa per 0.1% of C. In order to achieve sufficient tensile strength, C should be at least 0.10%. If C exceeds 0.25%, the weldability can be impaired. Therefore, the upper limit can be 0.25, 0.24, 0.23, 0.22, 0.21 or 0.20%. The lower limit can be 0.15, 0.16, 0.17 or 0.18%. A preferred range is 0.17-0.23%.

[0021] Si: 0.5-1.8%

[0022] Si acts as a solid solution strengthening element and is important for ensuring the strength of the thin steel strip. Si suppresses the precipitation of cementite and is essential for austenite stabilization. However, if the content is too high, too much silicon oxide will be formed on the strip surface, which can lead to cladding on the rolls in the continuous annealing line (CAL) and thus to surface defects on the subsequently produced steel sheet. The upper limit can be 1.8, 1.7 or 1.6%. The lower limit can be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3 or 1.4%. A preferred range is 0.6-1.6%.

[0023] Mn: 2.0-2.7%

[0024] Manganese is a solid solution strengthening element which stabilizes the austenite by lowering the M s temperature and prevents the formation of ferrite and pearlite during cooling. Furthermore, Mn lowers the A c3 temperature and is important for the austenite stability. At a content of less than 1.4%, it can be difficult to obtain the required amount of residual austenite and sufficient tensile strength. Furthermore, the required austenitizing temperature can be too high for a conventional industrial annealing line. Moreover, at lower contents, it can be difficult to avoid the formation of polygonal ferrite. However, if the amount of Mn is too high, problems related to segregation can occur, as Mn accumulates in the liquid phase and causes banding, leading to possibly deteriorated processability. Therefore, the upper limit can be 2.7, 2.6, 2.5 or 2.4%. The lower limit can be 2.0, 2.1, 2.2 or 2.3%. A preferred range is 2.2-2.6%.

[0025] Al: 0.03-1.2%

[0026] The addition of Al can increase the carbon content in the retained austenite. Al can also be used as a deoxidizer. Al, like Si, is not soluble in cementite, so it can delay the formation of cementite during bainite formation and martensite tempering. The addition of Al can further improve galvanizability and reduce the susceptibility to liquid metal embrittlement. However, M s The temperature also increases with increasing Al content. A further disadvantage of Al is that it leads to an increase in the c3 temperature. The upper limit can be 1.2, 1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4 or 0.3%. The lower limit can be set to 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1%. A preferred range is 0.03-1.0. If Al is used only for deoxidization, the upper limit can be 0.09, 0.08, 0.07 or 0.06%.

[0027] Si + Al: 0.6-2.0%

[0028] Si and Al suppress the precipitation of cementite during bainite formation. Therefore, their combined content is preferably at least 0.6%. The lower limit can be set to 0.6, 0.7, 0.8, 0.9, 1.0, 1.1 or 1.2. A preferred range is 0.7-1.7.

[0029] Optional elements

[0030] Ti: < 0.1%

[0031] Ti is usually used in low-alloy steels to improve strength and toughness, as it influences the grain size by forming carbides, nitrides or carbonitrides. In particular, Ti is a strong nitride former and can be used to bind nitrogen in the steel. However, this effect tends to saturate above 0.1%. The upper limit can be limited to 0.09, 0.07, 0.05, 0.03, 0.01 or 0.005%. According to the present invention, it is not necessary to add Ti intentionally.

[0032] Nb: < 0.1%

[0033] Nb is usually used in low-alloy steels to improve strength and toughness due to its influence on the grain size. Nb refines the base microstructure and the retained austenite phase by precipitation of NbC, thereby improving the strength-elongation balance. The steel can contain Nb in an amount of < 0.1%. The upper limit can be limited to 0.09, 0.07, 0.05, 0.03, 0.01 or 0.005%. According to the present invention, it is not necessary to add Nb intentionally.

[0034] V: < 0.1%

[0035] V functions similarly to Nb in that it contributes to precipitation hardening and grain refinement. The steel can comprise V in an amount of < 0.1 %. The upper limit can be limited to 0.09, 0.07, 0.05, 0.03 or 0.01 %. According to the present invention, V does not necessarily have to be added intentionally.

[0036] Cr: < 0.5%

[0037] Cr is effective for increasing the strength of the steel sheet. Cr is an element that forms ferrite and delays the formation of pearlite and bainite. A c3 Temperature and M s The temperature only slightly decreases with increasing Cr content. Cr leads to an increase in the amount of stabilized retained austenite. When higher than 0.5%, it can impair the surface finish of the steel, and therefore, the amount of Cr is limited to 0.5%. The upper limit can be 0.50 or 0.40, 0.30, 0.20, 0.10 or 0.05 %. The lower limit can be 0.01, 0.03, 0.05, 0.07, 0.10, 0.15, 0.20 or 0.25 %. According to the present invention, Cr does not necessarily have to be added intentionally.

[0038] B: < 0.005%

[0039] B suppresses the formation of ferrite and improves the weldability of the steel sheet. To have a significant effect, at least 0.001 % should be added. However, an excess of B deteriorates the workability. B increases the hardness, but can do so at the expense of reduced bendability, and can make scrap recycling more difficult. According to the present invention, B does not necessarily have to be added intentionally.

[0040] Mo < 0.2%

[0041] Molybdenum is a strong hardenability agent. It can further enhance the benefits of NbC precipitates by reducing carbide coarsening kinetics. Therefore, the steel can comprise Mo in an amount of at most 0.2 %. Mo delays the decomposition of austenite and stabilizes retained austenite. An amount exceeding 0.2 % leads to high costs. The upper limit can be limited to 0.2, 0.1, 0.05, 0.01 %. The minimum amount can be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %. According to the present invention, Mo does not necessarily have to be added intentionally.

[0042] Impurities

[0043] The following impurities can optionally be limited as disclosed below.

[0044] Ca: < 0.05%

[0045] Ca can be used for modification of non-metallic inclusions. The upper limit is 0.05% and can be set to 0.04, 0.03, 0.01 or 0.005%. According to the present application, Ca does not have to be added intentionally.

[0046] Cu: < 0.1%

[0047] Cu is an undesirable impurity element, which is limited to < 0.1% by careful selection of the scrap used. The upper limit is 0.1% and can be further limited to 0.05%.

[0048] Ni: < 0.2%

[0049] Ni is also an undesirable impurity element, which is limited to < 0.2% by careful selection of the scrap used. The upper limit is 0.2% and can be further limited to 0.1 or 0.05%.

[0050] Other impurity elements can be contained in the steel in the amounts normally occurring. However, preferably, the amounts of P, S, As, Zr, Sn are limited to the following optional maximum contents:

[0051] P: < 0.05, < 0.04, < 0.03 or < 0.02%

[0052] S: < 0.05, < 0.03, < 0.01, < 0.005 or < 0.001%

[0053] As: < 0.020 or < 0.010%

[0054] Zr: < 0.010 or < 0.006%

[0055] Sn: < 0.030 or < 0.015%

[0056] Oxygen and hydrogen can be further limited to

[0057] O: < 0.001 or < 0.0003%

[0058] H: < 0.0050 or < 0.0020%

[0059] It is also preferred to control the nitrogen content to the following range:

[0060] N: < 0.015%, preferably 0.001 - 0.008%

[0061] Mechanical properties

[0062] The steel shall meet the following conditions with respect to tensile strength:

[0063]

[0064] Preferably, all these requirements are met simultaneously.

[0065] TS, YS, TE are examples of properties related to the overall ductility. HER is a property related to the local ductility.

[0066] TS (tensile strength) (R m ) can be limited to 1090, 1080, 1070, 1060 or 1050 MPa. The lower limit of TS can be further set to 985, 990, 995 or 1000 MPa.

[0067] R m , R p0.2 values as well as the total elongation and / or the ultimate elongation are determined according to the industrial standard DIN EN ISO 6892-1, wherein samples with a gauge length of 80 mm are extracted in the longitudinal direction of the strip.

[0068] The hole expansion ratio (R ) is determined according to ISO / WD 16630:2009 (E) by the hole expansion test. In this test, a conical punch with a vertex angle of 60° is forced into a 10 mm diameter punched hole made in a steel sheet material with dimensions of 100 x 100 mm 2 . The test is stopped as soon as a first crack is determined and the hole diameter is measured in two directions orthogonal to each other. The arithmetic mean is used for the calculation.

[0069] The hole expansion ratio (R ) in % is calculated as follows:

[0070]

[0071] wherein Do is the diameter of the hole at the beginning (10 mm) and Dh is the diameter of the hole after the test.

[0072] The thickness of the strip or sheet for the final article can be 0.1 - 4 mm, preferably 0.2 - 3 mm. In non-slit conditions, the width of the strip or sheet can be 500 - 2000 mm, preferably 700 - 1750 mm.

[0073] Microstructure

[0074] The microstructural components are expressed in vol.% hereinafter.

[0075]

[0076] Each phase or combination of phases in the above list can be in excess of the microstructure.

[0077] Secondary tempered martensite is the tempered martensite from the first quenching and partitioning (steps i and j) that is tempered again in the second partitioning step (step n). The lower limit of the secondary tempered martensite can be 15, 20, 25 or 30 vol.%. The upper limit can be 60, 55 or 50 vol.%. The preferred range is 30-55 vol.%.

[0078] Tempered martensite is the martensite from the second quenching step that is tempered in the second partitioning step. The lower limit of the tempered martensite can be 0, 5, 10, 15 or 20 vol.%. The upper limit can be 70, 60, 50, 40 or 30 vol.%. The preferred range is 10-40 vol.%.

[0079] After the second quenching and partitioning (steps m and n), new martensite can form upon final cooling. A small amount of carbides can precipitate in the matrix. The upper limit of the new martensite + carbides can be 10 or 5 vol.%. The lower limit can be 0, 1, 2 or 3 vol.%. The preferred range is 0-5 vol.%.

[0080] The steel can comprise bainite. The upper limit of the bainite can be 70, 60, 50, 40, 30, 20 or 10 vol.%. The lower limit can be 0, 5 or 10 vol.%. The preferred range is 5-30 vol.%.

[0081] The amount of tempered martensite + bainite is preferably 15-70 vol.%, more preferably 25-50 vol.%.

[0082] The amount of residual austenite should be in the range of 10-25 vol.%, preferably 15-23 vol.% for obtaining the desired TRIP effect. The residual austenite has a predominantly uniform lamellar structure, which provides the best mechanical stability for improving the TRIP effect, improved overall ductility and improved local ductility.

[0083] The steel can optionally comprise up to 10 vol.% of other phases than the above, such as polygonal ferrite. Preferably, less than 5 vol.% of other phases, most preferably the steel does not comprise any other phases.

[0084] The microstructure (including the amount of each phase) can be identified in a scanning electron microscope (SEM) using a magnification of 2000x. Preferably, a sample is cut from a steel plate and a cross-section of the plate is polished parallel to the rolling direction. The microstructure should be taken from a thickness of The surface can be etched to make the phases more easily identifiable.

[0085] Electron backscatter diffraction (EBSD) can be used for quantitative microstructure analysis in a scanning electron microscope. For example, secondary tempered martensite can be identified by local misorientations from 0° up to and including 0.5°, and tempered martensite can be identified by local misorientations greater than 0.5° up to and including 1.2°.

[0086] The metallurgical reason for this difference stems from a lower lattice distortion, because carbon further diffuses away from the body centered tetragonal (bct) lattice of the martensite during tempering to form carbides or partition into the RA. In addition, annihilation of dislocations during tempering leads to a lower dislocation density, which reduces the local misorientations of the secondary tempered martensite.

[0087] However, the amount of retained austenite is preferably determined by the saturation magnetization method described in detail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, pages 61-64.

[0088] The improved properties of the retained austenite can be determined by one or more of the parameters described below.

[0089] Mechanical stability ) is a parameter describing the mechanical stability of the retained austenite (RA). Factors influencing the value include the chemical composition of the austenite enriched mainly by carbon, the grain size (smaller grain size leads to more stable RA), the morphology of the RA (spheroidized RA is less stable than lath or acicular RA). For these reasons, the chemical composition of the steel as well as the heat treatment parameters are decisive.

[0090] The RA should have a mechanical stability ) in the range of 5-30, preferably 10-25, more preferably 14-23. Values higher than 35 indicate a low stability of the retained austenite (RA) against mechanical loads, if the value is too high, the RA has already transformed during elastic loads (stress assisted) or at very low plastic strains and thus is not sufficient to increase the work hardening behavior of the steel and to enable high elongations. The present invention aims at an optimal stability of the RA. A value in the suggested range improves the stability of the RA against mechanical loads and favors the resistance to decomposition of the RA. The upper limit can be 35, 30, 25 or 20. The lower limit can be 5, 7, 10, 12, 14 or 15.

[0091] Mechanical stability ​​) can be determined using interrupted tensile testing. A tensile sample is deformed to a certain strain that lies between the yield and the necking of the sample. Subsequently the residual austenite content in the undeformed and deformed state is determined.

[0092] The following relationship, which is given by Ludwigson and Berger in J. Iron Steel Inst. 1969, Vol. 207, p. 63, applies:

[0093]

[0094] … initial residual austenite content

[0095] … deformed residual austenite content

[0096] … true strain

[0097] p… constant related to autocatalysis

[0098] … indication for residual austenite stability

[0099] Matsumura et al. in Scr. Metall. 1987, Vol. 21, p. 1301 suggest that in TRIP assisted steels p can be assumed to be 1. Thus, The values can be derived from combined interrupted tensile testing and residual austenite measurements. The true strain is the natural logarithm of the ratio of the instantaneous gauge length over the original gauge length in a tensile test. The residual austenite content can be determined by saturation magnetization measurements. The initial residual austenite content ( ) can be measured in the final heat treated sample. The deformed residual austenite sample ( ) should have the gauge length of the deformed tensile sample extracted. The sample with a gauge length of 80 mm should be extracted in the longitudinal direction of the strip.

[0100] Carbon content in residual austenite Optionally, the following can be met:

[0101] 0.8 - 1.2, preferably 0.9 - 1.10

[0102] Within this range of carbon content in the residual austenite, the best combination of overall formability and local formability can be achieved.

[0103] The carbon content in the retained austenite can be obtained by X-ray diffraction (XRD) measurements using the following equation. Lattice parameters obtained by reflection The calculations, as described in the equations in NH van Dijk, AM Butt, L. Zhao, J. Sietsma, SE Offerman, JP Wright, and S. van der Zwaag, *Thermal stability of retained austenite in TRIP steels studied by synchrotron X-ray diffraction during cooling*, Acta Materialia 53 (2005) 5439-5447, are as follows:

[0104]

[0105] in , and This refers to the content of carbon, manganese, and aluminum in austenite (in wt.%). Assuming the content... and It equals the nominal content of the alloy.

[0106] The amount of retained austenite (RA vol.%), the mechanical stability of austenite ( ) and the carbon content of retained austenite ( The following relationship should be satisfied:

[0107] ≥200, preferably 250-400

[0108] The average aspect ratio (AR) of the retained austenite (RA) and / or the average equivalent circle diameter (ECD) in μm can optionally satisfy the following relationship:

[0109] AR ≥2.0, preferably 2.05-2.30

[0110] ECD ≤0.55, preferably 0.45-0.53

[0111] The equivalent circle diameter (ECD) (in μm) and aspect ratio (AR) of the retained austenite (RA) can be determined by electron backscatter diffraction (EBSD). where A is the measured area of the grain. The aspect ratio = m / n, where m is the average length of the major axis and n is the average length of the minor axis of the grain in the section under investigation. The measurement method is described in Li, M., Wilkinson, D., and Patchigolla, K. (2005) Comparison of Particle Size Distributions Measured Using Different Techniques. Particulate Science and Technology, 23, 265-284.

[0112] Further definitions

[0113] Throughout the specification, temperatures are given in degrees Celsius.

[0114] Ae1 and Ae3 represent the equilibrium transformation temperatures. Austenite is fully stable above Ae3 and partially unstable between Ae3 and Ae1. Ae1 and Ae3 are calculated by ThermoCalc 2022 TCFE 12.

[0115] M S The temperature is calculated using the M S formula for exact microstructural prediction of modern 3rdgeneration AHSS chemistries”, Scr. Mater., Vol. 200, 2021, 113923. S formula calculation.

[0116]

[0117] The M S formula uses the content of each element in weight %.

[0118] M FThe formula is derived from the Koistinen-Marburger equation found in (Koistinen, D. and Marburger, R: "A general equation prescribing the extent of the austenite- martensite transformation in pure iron-carbon alloys and plain carbon steels", Acta Metall, 7, 1959, pp. 59-60):

[0119] ,

[0120] where the fraction of martensite is 0.95.

[0121] Production of cold-rolled strip

[0122] The cold rolled steel strip can be produced by the following steps:

[0123] a) A conventionally metallurgically produced billet is manufactured by converter melting and secondary metallurgy with the above composition.

[0124] b) The billet is hot rolled into a hot rolled strip in the austenite range. Preferably, the billet is reheated to a temperature between 1000 °C and 1280 °C. Preferably, the billet is rolled completely in the austenite range, with the hot rolling finish temperature being greater than or equal to 850 °C, thereby obtaining a hot rolled steel strip.

[0125] c) Thereafter, the hot rolled strip can be coiled at a coiling temperature in the range of 400-700 °C.

[0126] d) Optionally, annealed at a temperature in the range of 500-950 °C. Preferably, batch annealed at 500-700 °C for a duration of 5-30 hours. Alternatively, the strip can be continuously annealed at a temperature in the range of 650-950 °C for 10-200 seconds.

[0127] e) Optionally, the coiled strip is subjected to a descaling process (e.g. pickling) prior to and / or after batch annealing.

[0128] f) Thereafter, the annealed steel strip is cold rolled at a reduction of 20-90%, preferably about 50-70%.

[0129] The steps a) - f) above for manufacturing the cold-rolled strip are examples on how a cold-rolled strip can be manufactured. The present invention can be applied to a cold-rolled strip produced by other known steps.

[0130] According to the present invention, the cold-rolled strip is subjected to a double annealing process disclosed in the steps a) to p) below. Figure 1 The heat cycle of the double annealing process is shown.

[0131] The first annealing cycle described in the steps g) to k) is a quenching and partitioning (Q&P) process, wherein the steel is annealed and thereafter quenched to a temperature between M s - 20°C and M F , followed by partitioning at a temperature above the quenching temperature, and finally cooling to room temperature. The second annealing cycle is described in the steps l) to p).

[0132] The first annealing cycle:

[0133] g) providing a cold-rolled steel strip to a continuous annealing line, said steel strip having a composition as described above.

[0134] h) heating the strip at a rate (HR1) of 1 - 20°C / s to a first annealing temperature (Tan1) between 800°C to 1000°C and soaking for 10 - 300 seconds (tan1), preferably the first annealing temperature (Tan1) is above Ae3.

[0135] i) cooling the strip at a rate (CR1) of 10 - 100°C / s to a first quenching temperature (TQ1) between M F and (M S - 20) °C. The lower limit can be further limited to M F - 20°C and the upper limit to the highest value selected from 150, 160, 170 and 180°C. The upper limit can be further limited to M S - 30, M S - 40, M S - 50, M S - 60, M S - 70, M S - 80, M S - 90 or M S - 100°C.

[0136] j) heating the cooled strip at a rate (HR2) of 1 - 100°C / s to a first partitioning temperature (Toa1) in the range of (TQ1) + 10°C to 500°C and partitioning the strip for 20 - 1000 seconds (toa1). Preferably, the first partitioning temperature (Toa1) is above M SThe upper limit can be limited to 500, 480, 450, 430 or 410℃. The lower limit can be limited to 260, 280, 300, 320, 340, 360 or 380℃.

[0137] k) Cool the strip to a temperature below 50°C at a rate of 1-50°C / second (CR2), preferably to room temperature.

[0138] After the first annealing cycle, the steel matrix consists of tempered martensite with a relatively high amount of carbides and RA. Prior to the second annealing, the higher amount of carbides and RA ensures more nucleation sites for austenite formation. This results in a higher amount of RA in the second annealing cycle, exhibiting optimal stability against strain-induced martensitic transformation (SIMT) and a fine microstructure with a layered morphology that improves both overall and local formability. This combination of overall and local formability surpasses all known dual-annealing concepts.

[0139] The second annealing cycle following the first annealing cycle (steps g to k):

[0140] l) The strip is heated to a second annealing temperature (Tan2) between 700°C and 900°C at a rate of 1-20°C / second (HR3) and homogenized for 10-300 seconds (tan2). Preferably, the second annealing temperature (Tan2) is within the critical range, i.e., between Ae1 and Ae3. However, a second annealing temperature (Tan2) higher than Ae3 is permissible.

[0141] m) Cool the strip to a second quenching temperature (TQ2) in the range of 150-500℃ at a rate of 10-100℃ / s (CR3). The second quenching temperature (TQ2) can be lower than M. S Preferred to be lower than (M) S -20)℃, and must be higher than M F .

[0142] The upper limit can be limited to 500, 450, 400, 350, 300 or 250℃. The lower limit can be limited to 150, 170, 200, 220, 240, 260, 280 or 300℃.

[0143] n) partitioning the strip at a second partitioning temperature (Toa2) in the range of the second quench temperature (TQ2) to 500°C for 20-1000 seconds (toa2). The upper limit can be limited to 500, 480, 450 or 430°C. The lower limit can be limited to 150, 200, 250, 300 or 350°C. The heating rate (HR4) to the second partitioning temperature (Toa1) can be in the range of 1-100°C / second. In the case of isothermal tempering, there is obviously no heating rate from the second quench temperature (TQ2) to the second partitioning temperature (Toa1), i.e. in this case the heating rate (HR4) is 0°C / second.

[0144] o) cooling the strip to a temperature below 50°C, preferably to room temperature, at a rate of 1-50°C / second (CR4).

[0145] p) optionally, manufacturing a sheet from the strip.

[0146] Optional coating

[0147] The steel sheet or strip can optionally be coated and comprise a zinc or zinc alloy coating. The coating can be applied, for example, by:

[0148] - electrogalvanizing (EG), including electroplating.

[0149] - physical vapor deposition (PVD).

[0150] - hot dip galvanizing (HDG) in a hot dip galvanizing line, where the strip is immersed in a molten zinc bath at the end of the final partitioning (step n). The hot dip galvanizing line can be the same line as the continuous annealing line with added hot dip coating.

[0151] - galvannealing (GA) in a galvannealing line, which is similar to a hot dip galvanizing line but with an added annealing step after the hot dip coating. That is, the process is carried out in the same way as a continuous annealing line but including galvannealing at the end of the final partitioning (step n). Galvannealing is a combination of galvanizing and annealing at about 480-560°C in order to promote a higher degree of Fe in the ZnFe coating.

[0152] The zinc alloy coating can comprise, in weight %:

[0153] Mg 0.1-10

[0154] Al 0.1-10

[0155] Optionally, one or more of the following:

[0156] Bi, Pb, Sn, Sb, Si, Ti, Ca, Mn, La, Ce, Cr, Ni and Zr, in a total amount of 0.01 - 1.0

[0157] the balance being Zn and impurities.

[0158] The galvannealed coating can contain 5 - 20 wt.% of diffused Fe.

[0159] Other coating compositions known in the art can be applied.

[0160] Examples

[0161] The alloy L5 was produced by conventional metallurgy by converter smelting and secondary metallurgy. The composition of the alloy (elements in [wt%]) is shown in Table 1, further elements are present only as impurities and below the minimum levels specified in this specification. M S , M F , Ae1 and Ae3 are also shown in Table 1.

[0162] The values of the parameters throughout the examples are determined by the methods given previously in the specification, unless otherwise stated.

[0163] Table 1

[0164]

[0165] The alloy L5 was continuously cast and cut into slabs. The slabs were reheated and hot rolled in the austenitic range to a thickness of about 2.8 mm. The hot rolling finish temperature was about 900 °C. Thereafter the hot rolled strip was coiled at a coiling temperature of 630 °C. The coiled hot rolled strip was pickled and intercritically annealed at about 624 °C for 10 hours to reduce the tensile strength of the hot rolled strip and thereby reduce the cold rolling force. Thereafter the strip was cold rolled in a five stand cold rolling mill to a final thickness of about 1.4 mm.

[0166] All steels were subjected to two final annealing cycles in a continuous annealing line, wherein Table 2a shows the process values for the first annealing cycle and Table 2b shows the process values for the second annealing cycle. The annealing cycles are schematically shown in Figure 1 Reference samples are indicated by Rn, wherein n = 11, 12...14, and samples according to the application are indicated by Sn, wherein n = 14...19.

[0167]

[0168]

[0169] All samples were double annealed except sample R11. Sample R11 was single annealed. After the first anneal, reference samples R12-R14 were quenched to room temperature without any partitioning prior to the second anneal. After the first anneal, inventive samples S14-S19 were quenched to a temperature below M S -20°C but above M F After that, inventive samples S14-S19 were partitioned at a temperature above the quenching temperature prior to the second anneal. After the first anneal, reference sample R11 was quenched to a temperature below M S -20°C but above M F and was partitioned thereafter at a temperature above the quenching temperature.

[0170] After the second anneal, the samples were quenched and subsequently partitioned at a temperature above the quenching temperature or they were quenched and isothermally held after quenching.

[0171] The mechanical properties and details of the retained austenite are shown in Table 3. The inventive steels (S14-S19) have similar tensile strength compared to the reference steels of the same composition (R11-R14). However, the total elongation and the hole expansion ratio of the inventive steels are greater than the reference steels of the same composition. Therefore, the product of the tensile strength, the total elongation and the hole expansion ratio of the inventive samples is significantly greater compared to the reference samples. Based on these findings, the criteria for a steel are determined, for example and It is obvious from the results that none of the reference samples fulfill or .

[0172]

[0173] The microstructure of inventive samples S14 and S15 after the second annealing cycle was examined. The microstructure contained a matrix with lamellar morphology and included secondary tempered martensite, tempered martensite, bainite and a large amount of RA. The microstructure further contained a small amount of fresh martensite and carbides. The amount of RA was determined to be 19, 21 vol%, respectively. The results are presented in Table 4.

[0174]

[0175] The average aspect ratio (AR) and the average equivalent circle diameter (ECD) of the retained austenite (RA) of reference sample R13 were determined and compared to inventive sample S14. The results are shown in Table 5.

[0176]

[0177] From the results shown in Table 5 the following conclusions can be drawn.

[0178] - The amount of RA is higher for the inventive samples.

[0179] - The carbon content of the RA is lower for the inventive samples.

[0180] - The ECD of the RA is smaller for the inventive samples.

[0181] - The AR of the RA is higher for the inventive samples.

[0182] - Only the inventive samples fulfill the criteria .

[0183] Despite the smaller ECD and the higher AR, the lower C content of the RA compared to the reference leads to a lower but optimal stability of the RA of the inventive samples against strain induced martensitic transformation (SIMT). This leads to a better utilization of the TRIP effect and thus to a better overall formability. Furthermore, the lower C content in the RA ensures a lower difference in hardness between the matrix and the RA, which also improves the local formability in the case of the inventive samples.​

Claims

1. High-strength steel strip or sheet, possessing: a) Composition consisting of the following elements, in wt.%: The balance, excluding impurities, is Fe; b) Tensile strength (R) m 980-1100MPa Where R m It is obtained according to European standard EN 10002 Part 1, in which samples are extracted in the longitudinal direction of the strip; c) Contains the following microstructures, in vol.%: 10-25% retained austenite The amount of retained austenite was measured by means of saturation magnetization, which is described in detail in Proc. Int.Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, pp. 61-64, and the other phases were determined by means of scanning electron microscopy (SEM) at 2000x magnification. d) Where the retained austenite satisfies: Mechanical stability ) 5-30, As defined by Ludwigson and Berger in J. Iron Steel Inst. 1969, Vol. 207, 10, p. 63, where p=1: ...initial retained austenite content …residual austenite content after deformation …genuine response; Carbon content of retained austenite (in wt.%): 0.8-1.2， in Determined by the following formula: in and It is the content of manganese and aluminum in austenite (in wt.%); and, where it is assumed that... and The content is equal to the nominal content, and is obtained by X-ray diffraction (XRD) measurement using the following equation. The lattice parameter a is obtained by reflection. γ The equations described are in NH van Dijk, AM Butt, L. Zhao, J. Sietsma, SE Offerman, JP Wright and S. van der Zwaag, Thermal stability of retained austenite in TRIPsteels studied by synchrotron X-ray diffraction during cooling, ActaMaterialia 53 (2005) 5439-5447; and RA*k p / x cγ ≥200。 2. The steel strip or sheet according to claim 1, wherein the microstructure satisfies at least one of the following requirements in vol.%, preferably all of them: 。 3. The steel strip or sheet according to claim 1 or 2, wherein the microstructure satisfies at least one of the following requirements in vol.%, preferably all of them: 。 4. The steel strip or sheet according to any one of the preceding claims, wherein it satisfies one or more of the following mechanical properties: 。 5. The steel strip or sheet according to any one of the preceding claims, wherein, Meets one or more of the following mechanical properties: 。 6. The steel strip or sheet according to any one of the preceding claims, wherein the retained austenite satisfies one or more of the following: 。 7. The high-strength steel strip or sheet according to claim 1, having a composition consisting of the following alloying elements (in wt.%): The balance, excluding impurities, is Fe.

8. A method for producing high-strength steel strip or sheet according to any one of claims 1-7, comprising: g) Providing cold-rolled steel having the composition defined in claim a): h) Heat the strip to a first annealing temperature (Tan1) between 800°C and 1000°C at a rate of 1-20°C / second (HR1), and homogenize for 10-300 seconds (tan1). i) Cool the strip at a rate of 10-100°C / second (CR1) to M F To (M) S The first quenching temperature (TQ1) is between -20℃ and 20℃. j) The cooled strip is heated to a first partitioning temperature (Toa1) within the range of the first quenching temperature (TQ1) + 10°C to 500°C at a rate of 1-100°C / second (HR2), and the strip is partitioned for 20-1000 seconds (toa1). k) Cool the strip to a temperature below 50°C at a rate of 1-50°C / second (CR2). l) Heat the strip to a second annealing temperature (Tan2) between 700°C and 900°C at a rate of 1-20°C / second (HR3) and homogenize for 10-300 seconds (tan2). m) The strip is cooled to a second quenching temperature (TQ2) in the range of 150-500°C at a rate of 10-100°C / second (CR3). n) The strip is divided for 20-1000 seconds (toa2) at a second distribution temperature (Toa2) within the range of the second quenching temperature (TQ2) to 500°C. o) Cool the strip to a temperature below 50°C. p) Optionally, a sheet is manufactured from the strip.

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