High-strength cold-rolled and annealed steel sheet and method of producing such steel sheet

Abstract

The invention relates to high strength cold rolled and annealed 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 and annealed steel sheet having an ultimate tensile strength of at least 1180 MPa and to a method for producing such steel sheet.

Description

[0001] HIGH-STRENGTH COLD-ROLLED AND ANNEALED STEEL SHEET AND METHOD OF PRODUCING

[0002] SUCH STEEL SHEET

[0003] FIELD OF THE INVENTION

[0004] The invention relates to high strength cold rolled and annealed 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 and annealed steel sheet having an ultimate tensile strength of at least 1180 MPa and to a method for producing such steel sheet.

[0005] BACKGROUND TO THE INVENTION

[0006] For a wide variety of applications increased strength levels are a pre-requisite for lightweight 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 automotive part cannot be produced from conventional high strength steels because of too low a formability for complex structural parts .

[0007] There is a demand for high-strength cold-rolled and annealed steel strips that, beside sufficient strength for downgauging and weightsaving, offer a suitable combination of global formability, viz. tensile elongation, and local formability, viz. hole -expans ion capacity, for component geometry optimisation and adequate stiffness.

[0008] DESCRIPTION OF THE INVENTION

[0009] As will be appreciated herein, for any description of steel compositions or preferred steel compositions, all references to percentages are by weight percent unless otherwise indicated.

[0010] As used herein, the term "about" when used to describe a compositional range or amount of an alloying addition means that the actual amount of the alloying addition may vary from the nominal intended amount due to factors such as standard processing variations as understood by those skilled in the art.

[0011] As used herein, the term “up to” as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.20% of Sn may include a steel having no Sn. It is an object of the invention to provide a high-strength cold-rolled and annealed steel strip having a high strength in combination with a good formability as represented by the tensile elongation and hole expansion capacity.

[0012] It is another object of the invention to provide a method of manufacturing such a high- strength cold-rolled and annealed steel strip.

[0013] These and other objects and further advantages are met or exceeded by the present invention providing a steel sheet according to independent claim 1 and a method of manufacturing a steel sheet according to independent claim 9, and with preferred embodiments in the dependent claims and this description.

[0014] In order to achieve these objects, the present invention proposes, in a first aspect, a high strength cold-rolled and annealed steel sheet with an ultimate tensile strength greater than 1180 MPa, the composition comprising, in wt.% 0.130 to 0.240% of C; 1.50 to 3.00%of Mn; 0.10 to 0.45% of Si; 0.010 to 0.50% of Al; 0.010 to 0.15% of Ti; 0.0003 to 0.0050% of B; up to 0.050% of P; up to 0.015% of S; up to 0.020% of N; up to 0.20% of Sn; up to 0.010% of Ca; optionally one or more elements selected from the group consisting of: up to 0.50% of V, up to 0.50% of Cu, up to 0.50% of Ni , up to 0.60% of Mo, up to 0.35 wt.% of G, up to 0.10% of Nb; the balance is made by iron and inevitable impurities, viz. resulting from the ironmaking and steelmaking process; a microstructure having (in vol.%): tempered martensite (TM): 5-64% bainite (B): 35- 90% transformation ferrite (F): S 5% retained austenite (RA): S 10% fresh martensite (FM): < 14% sum of pearlite, cementite and precipitates S 3% and the balance is adding up to 100% and the following mechanical properties: a tensile strength (Rm) > 1180 MPa, a tensile elongation (JIS5) > 6.5% and a hole expansion capacity (HEQ S 25%

[0015] In accordance with the invention it has been found that the high-strength cold-rolled and annealed steel strip having these narrow alloy compositional ranges in combination with the defined microstructure provides for an improved balance of strength and formability as represented by the tensile elongation combined with the hole expansion capacity (HEQ. The steel sheet has the following mechanical properties: an ultimate tensile strength (UIS or Rm) > 1180 MPa, preferably > 1200 MPa; a tensile elongation (JIS5) > 6.5% preferably S 7.5% and a hole expansion capacity (HEQ > 25% preferably S 40% In an embodiment the high-strength cold-rolled and annealed steel strip has in the Lr direction a yield strength (YS or Rp0.2) prior to temper rolling of at least 820 MPa, and preferably of at least 900 MPa, and most preferably of at least 950 MPa.

[0016] In an embodiment the mechanical properties are further characterised by: a ratio Rp0.2 / Rm > 0.70, and preferably > 0.75, and a ratio HEC / (RpO.2 / Rm) > 35% and more preferably > 40% and most preferably > 50% These ratios are indicative also for the good formability of the cold-rolled and annealed steel sheet according to this invention.

[0017] The high-strength cold-rolled and annealed steel strip according to this invention is distinct from and forms an alternative to TBF (TRIP-assisted Bainitic Ferrite) sheet strip commonly characterised by a relatively low yield strength (YS) values and inadequate local formability properties, i.e. low HEC values. Typically, TBF steel strip has a yield strength of less than 850 MPa and HEC values in a range of 5 to 25% This appears to be the result of an excess of hard martensite in the final microstructure.

[0018] The required microstructure for this high-strength cold-rolled and annealed steel strip can be obtained by the narrow compositional ranges in combination with the careful control of the manufacturing process, in particular in a continuous hot-dip galvanizing (HDG or CGL) line the control of the cooling in the rapid cool section (RCS) with a cool-stop or quench-stop in a critical temperature range below the Ms temperature and forming the required fraction of bainite in the overage section (OAS) of a continuous galvanizing line (CGL). An advantage of the method is that it does not require a heat-up apparatus, e.g. an inductor, to be positioned at the exit of the RCS leading to considerable CAPEXand OPEXsavings.

[0019] The limitation of the elements C, Mn, Si, Al, Ti, and B is essential to the invention for the reasons set out below:

[0020] Carbon (Q should be present in a range of 0.130 to 0.240 wt.% and is an element which stabilizes austenite and is important for obtaining the desired strength level. When C is lower than 0.130 wt.% then it is difficult to attain a tensile strength of at least 1180 MPa, and preferably of at least 1200 MPa. If C exceeds 0.240 wt.% then formability and weldability are impaired. For these reasons, a preferred range is 0.150 to 0.220 wt.% and more preferably in a range of 0.170 to 0.200 wt.% depending on the desired strength level and formability.

[0021] Manganese (Mn) should be present in a range of 1.50 to 3.00 wt.% and 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 of the steel. At a content of less than 1.50 wt.% it might be difficult to obtain a tensile strength of 1200 MPa and the austenitizing temperature might be too high for conventional industrial annealing lines. However, if the amount of Mn is higher than 3.00 wt.% problems with segregation may occur and the workability may deteriorate. Preferred ranges are therefore 1.80 to 2.70 wt.%

[0022] Silicon (Si) should be in a range of 0.10 to 0.45 wt.% and acts as a solid solution strengthening element and is important for securing the strength and formability of the steel sheet. Furthermore, Si is beneficial to suppress cementite formation. Si is insoluble in cementite and will therefore act to greatly de by 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. In a preferred embodiment the Si content is at least 0.15 wt.% preferable at least 0.20 wt.% and more preferably at least 0.25 wt.% In an embodiment the Si-content does not exceed 0.390 wt.% and preferably does not exceed 0.34 wt.% Amongst others too high a Si- content may lead to LME cracks occurring after spot welding of zinc or zinc alloy coated steel sheet.

[0023] Aluminium (Al) should be present in a range of 0.010 to 0.50 wt.% and behaves comparable to Si in the steel according to the invention. It slows down the carbide precipitation kinetics and suppresses the formation of cementite. When Al is less than about 0.010 wt.% the effect of suppression of carbide formation is negligible. Values of aluminium lower than 0.010 wt.% are deemed to be residuals from the deoxidation step during steelmaking, and therefore a minimum value of about 0.010 wt.%is required. On the other hand, when Al is above about 0.50 wt.% there can be excessive oxide formation during thermo-mechanical processing (sbb reheating, hot rolling, coiling, etc.) of the steel, which can deteriorate formability and toughness. Also, with increasing Al content a higher amount of surface oxidation can occur at higher temperatures. These oxide scales are detrimental for hot rolling, pickling, coating and overall surface appearance. Also, the rolling forces during hot rolling increase when the Al increases in combination with the presence of Si to such a level making the steel very brittle and more difficult to hot roll. Therefore, Al in the present invention is present in an amount of up to 0.50 wt.% preferably in a range of 0.010 to 0.30 wt.% and more preferably in the range of 0.010 to 0.20 wt.% and most preferably in a range of 0.010 to 0.080 wt.%

[0024] Titanium (Ti) is an alloying element that can be present in a range of 0.010 to 0.15 wt.% and it provides precipitation strengthening as it acts as a carbide forming element suppressing the formation of cementite while providing precipitation strengthening via the formation of small Ti-based carbides. However. Ti also combines with N, S and C to form nitrides, and carbo- sulphides, depending on the specific chemical composition of the steel. Therefore, preferably at least 0.010 wt.% Ti is present to bind substantially all the N in the steel and to have sufficient free B in the steel to increase the hardenability. In a preferred embodiment at least about 0.020 wt.% Ti is present. When more than 0.15 wt.% Ti is present, coarse Ti nitrides, carbo-nitrides, and carbides may form which are difficult to dissolve during reheating of the slab prior to hot rolling. Furthermore, these coarse Ti nitrides, carbo-nitrides, and carbides can lead to a deterioration of the hole expansion capacity and fracture toughness of the steel. In an embodiment the Ti content does not exceed 0.08 wt.% and more preferably it does not exceed 0.040 wt.%

[0025] Boron (B) should be present in a range of 0.0003 to 0.0050 wt.% Boron enhances the hardenability of steel by inhibiting the formation of ferrite, pearlite, and bainite during cooling in both the SCS and RCS processes, from the soak furnace temperature to the cool-stop temperature (Tq). Additionally, Boron reduces the kinetics of bainite formation in a TBF process when Tq is higher than the martensite start temperature (Ms). However, Boron does not affect martensite formation, which is crucial for this invention which involves a cool-stop or quench- stop in a critical temperature range below the Afc temperature and thereby forming a specific fraction of martensite. This martensite stimulates the nucleation and growth of bainite in the overage section (OAS) of a continuous galvanizing line (CGL). Research leading to this invention has revealed that even a small fraction of martensite, as little as 10 vol.% can significantly influence bainite formation in the microstructure, effectively negating the impact of B.

[0026] In an embodiment, when Si and Al are added in combination have a synergistic and a completely unforeseen effect, resulting in an increased surface oxidation. For these reasons the total amount or sum is preferably limited to Si+Al < 0.69 wt.% and more preferably to Si+Al < 0.49 wt.%

[0027] In addition to the C, Mn, Si, Al, and B the steel may optionally contain one or more of the following elements in order to influence the transformation kinetics and thereby adjust the microstructure, and / or to fine tune one or more of the mechanical properties of the steel sheet, viz. up to 0.30 wt.%ofV, up to 0.50 wt.% of Cu, up to 0.50 wt.%ofNi , up to 0.60 wt.% of o, up to 0.35 wt.% of Cr, and up to 0.10 wt.% of Nb. There may be an absence of one or more of V, Cu, hfi, Mo, G, and Nb.

[0028] Niobium (Nb) can be present in a range of up to 0. 10 wt.% and is commonly used in low alloyed steels for improving the strength of the steel partly by precipitation hardening but foremost by grain refinement. Nb increases the strength elongation balance by refining the matrix microstructure due to precipitation of NbC. In an embodiment is Nb is present in a range of up to 0.10 wt.%

[0029] Molybdenum (Mo) can be present up to 0.60 wt.% 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. In an embodiment the Mo content does not exceed 0.40 wt.% and when added is preferably present in a range of 0.10 to 0.40 wt.% more preferably in a range of 0.10 to 0.30 wt.%

[0030] Chromium (Cr) can be present up to 0.35% and retards the formation of pearlite and bainite and is effective to increase the strength of the steel sheet. When added, Cr can be present in a range of up to 0.35 wt.% and when added is preferably present in a range of 0.030 to 0.25 wt.% and more preferably in a range of 0.030 to 0. 15 wt.%

[0031] Each of Qi, V, and Ni are solid solution strengthening elements and may have a positive effect on the corrosion resistance.

[0032] When added, copper (Qi) can be present in a range of up to 0.50 wt.% and is preferably present in a range of 0.03 to 0.50 wt.% and more preferably in a range of 0.03 to 0.20 wt.%

[0033] When added, vanadium (V) can be present in a range of up to 0.50% and is preferably present in a range of 0.03 to 0.50 wt.% and more preferably in a range of 0.03 to 0.20 wt.%

[0034] When added, nickel (Ni) can be present in a range of up to 0.50 wt.% and is preferably present in a range of 0.03 to 0.50 wt.% and more preferably in a range of 0.03 to 0.20 wt.%

[0035] In an embodiment one or more, or all, of V, Qi, Ni, Mo, and Nb, are not purposively added and only present as an inevitable impurity resulting from the ironmaking and steelmaking process.

[0036] Tin (Sn) is an impurity and can be present up to 0.20 wt.% and preferably is limited to up to 0.050 wt.%

[0037] Calcium (Ca) up to 0.010 wt.%can be present to control the morphology of the inclusions in the steel during the steelmaking process and thereby improve the hole expansibility. Preferably it does not exceed 0.0050%

[0038] Nitrogen (N), sulphur (S) and phosphorus (P) are residual elements present in the steel as a result of steel making and refining process. Their amounts are limited to up to 0.015 wt.% of S, up to 0.050 wt.% of P, and up to 0.020 wt.% of N. Amounts higher than these are detrimental for mechanical properties, formability, toughness, fatigue, and weldability. In an embodiment P is present only up to 0.015 wt.% In an embodiment S is present only up to 0.005 wt.% (50 ppm) and more preferably only up to 0.0025 wt.% (25 ppm), and most preferably up to 0.0012 wt.% (12 ppm). Preferably the N content is limited to up to 0.0075 wt.% (75 ppm), and more preferably up to 0.0065 wt.% (65 ppm).

[0039] In an embodiment the steel strip has a composition comprising as its main alloying elements C, Mn, Si, AL, Ti, and B, in wt.%

[0040] 0.150 to 0.220 wt.%ofC;

[0041] 1.80 to 2.70 wt.%ofMn;

[0042] 0.15 to 0.390 wt.%of Si;

[0043] 0.010 to 0.30 wt.%ofAl;

[0044] 0.010 to 0.08 wt.%ofTi;

[0045] 0.0003 to 0.0050 wt.%ofB; and with more preferred ranges as here described.

[0046] In an embodiment the steel strip has a composition comprising as its main alloying elements C, Mn, Si, Mo, AL, Ti, and B, in wt.%

[0047] 0.130 to 0.240 wt.%ofC;

[0048] 1.50 to 3.00 wt.%ofMn;

[0049] 0.10 to 0.40 wt.%ofMo, and preferably 0.10 to 0.30 wt.%

[0050] 0.10 to 0.45 wt.%of Si, and preferably 0.10 to 0.390 wt.%

[0051] 0.010 to 0.50 wt.%of Al, and preferably 0.010 to 0.30 wt.%

[0052] 0.010 to 0.15 wt.%of Ti, and preferably 0.010 to 0.08 wt.%

[0053] 0.0003 to 0.0050 wt.% of B; and with more preferred ranges as here described.

[0054] In an embodiment the steel strip has a composition comprising as its main alloying elements C, Mn, Si, Cr, AL, Ti, and B, in wt.%

[0055] 0.130 to 0.240 wt.%ofC;

[0056] 1.50 to 3.00 wt.%ofMn;

[0057] 0.030 to 0.35 wt.% of Cr, and preferably 0.030 to 0.25 wt.%

[0058] 0.10 to 0.45 wt.%of Si, and preferably 0.10 to 0.390 wt.%

[0059] 0.010 to 0.50 wt.%of AL, and preferably 0.010 to 0.30 wt.%

[0060] 0.010 to 0.15 wt.%ofTi, and preferably 0.010 to 0.08 wt.%

[0061] 0.0003 to 0.0050 wt.%ofB; and with more preferred ranges as here described.

[0062] In embodiment the steel strip has a composition consisting of, in wt.% 0.130 to 0.240% of C; 1.50 to 3.00% of Mn; 0.10 to 0.45% of Si; 0.010 to 0.50% of Al; 0.010 to 0.15% of Ti; 0.0003 to 0.0050%of B; up to 0.050%of P; up to 0.015%of S; up to 0.020%ofN; up to 0.20% of Sn; up to 0.010%of Ca; optionally one or more elements selected from the group consisting of: up to 0.50%ofV, up to 0.50%of Cu, up to 0.50%ofNi , up to 0.60%ofMo, up to 0.35 wt.% of Cr, up to 0.10% of Nb; the balance is made by iron and inevitable impurities, viz. resulting from the ironmaking and steelmaking process, and with preferred narrower ranges are herein described.

[0063] The 0.2% offset proof strength or yield strength (Rp0.2), ultimate tensile strength (Rm), uniform elongation (Ag). and tensile elongation (JIS5) were determined from quasistatic (strain rate 3 x IO-4s’1) tensile tests at room temperature with JIS5 specimen geometry with tensile testing parallel to the rolling direction (Indirection) according to EN 10002-1 / 150 6892-1. The geometry of the tensile specimens consisted in 50 mm gauge length in the rolling direction, 25 mm in width and a thickness depending on the final gauge. The strength of the steel at 0.2% offset strain is measured as the yield strength (Rp0.2 or YS).

[0064] The stretch-flange ability of the steel strip or the hole expansion capacity (HEQ was determined by hole expansion tests. Specimens of dimension 90 mm by 90 mm by final thickness of the steel sheet were cut from the as-coiled steel. A hole of 10 mm diameter was punched in the middle of the specimens, and the hole expansion tests were carried out according to ISO / IS 16630:2003(E) standard. Hole expansion testing of the samples was done with upper burring. A conical punch of 60° was pushed up from below and the hole diameter df was measured when a through-thickness crack formed. The hole expansion ratio I was calculated using the formula below with do= 10 mm:

[0065] For all the above mechanical tests, at least three specimens were tested for each condition and the average values are reported herein.

[0066] It is an important feature that the microstructure of the high-strength cold-rolled and annealed steel sheet in accordance with the invention is consisting of (in vol.%): tempered martensite (TM): 5 to 64% preferably 5 to 49% and more preferably 7 to 39% bainite (B): 35 to 90% preferably 50 to 90% and more preferably 60 to 90% transformation ferrite (F): < 5% preferably S 2% retained austenite (RA): < 10% preferably S 7% more preferably S 4% fresh martensite (FM): < 14% preferably S 10% more preferably S 7% sum of pearlite, cementite and precipitates S 3% and the balance is adding up to 100%

[0067] A low fraction of tempered martensite is preferred to ensure a high fraction of bainite. A higher bainite content leads to better stabilization of retained austenite (RA). Aiditionally, maximizing the temperature increase due to bainite (B) formation is desirable, and this temperature rise, caused by the latent heat, is proportional to the fraction of bainite (B). The fraction of retained austenite (RA) should be limited to ensure its stability. Unstable austenite can more easily transform into martensite, driven not only by a decrease in temperature (i.e. thermal driving force) but also by external loads (i.e. mechanical driving force). This latter phenomenon is known as the TRIP effect. While the TRIP effect can enhance tensile elongation during a tensile test, the early transformation of austenite to hard martensite during HEC testing can deteriorate HEC performance. Those who are skilled in the art know how to determine the microstructure and phase analysis of the steel sheet using Scanning Electron Microscopy (SEM), X-Ray Diffraction Spectroscopy (XRD) and Electron Back Scatter Diffraction (EBSD). EBSB is a technique well known in the art which also allows the quantification of the volume fraction of the various components.

[0068] In a preferred embodiment the high-strength cold-rolled and annealed steel strip is provided on one or both of its main surfaces with a thin metallic coating layer, typically up to about 100 g / m2per side of the steel strip, and preferably up to about 60 g / m2per side, preferably being applied by means of hot-dip coating. The metallic coating is a zinc coating or a zinc alloy coating, i.e . Zn-Al alloy, Zn-Mg alloy, Zn-Fe alloy, Zn- Al- Mg alloy, or Zn-Mg- Al alloy.

[0069] The composition of the zinc or zinc alloy coating layer is not limited. Although the coating layer can be applied in various ways, hot-dip galvanising is preferred using a G1 coating bath. The Zn based coating layer may comprise a Zn alloy containing Al as an alloying element. A preferred zinc bath composition contains about 0.10-0.35 wt.%Al, the remainder being zinc and unavoidable impurities.

[0070] In an embodiment of the high-strength cold-rolled and annealed steel strip it has a thickness in a range of 0.7 to 3.0 mm, and preferably in a range of 0.8 to 2.50 mm, and more preferably in a range of 0.9 to 2.0 mm.

[0071] In another aspect of the invention it relates to a method of manufacturing a high-strength cold-rolled and annealed steel strip as herein described and claimed, the method comprising the steps of, in that order, a. providing a cold-rolled steel sheet having a composition as set out in this description and any of the claims; b. annealing the cold-rolled steel sheet at an annealing temperature, T_an, above the Ac3 temperature in order to fully austenitize the steel; c. cooling of the cold-rolled and annealed steel sheet from the annealing temperature T an to a cool-stop or quench-stop temperature, Tq, that is in a range of Ms-70°C to Ms-15°C, with Ms being the temperature at which the transformation from austenite to martensite starts; In a preferred embodiment the Tq is not lower than Ms-55°C, and more preferably not lower than Ms-45°C d. austempering of the cold-rolled and annealed steel sheet at a temperature, Toa, that is between Tq and Tq+100°C, and preferably for a time, t_oa, in a range of 20 to 700 sec.; e. cooling the steel sheet from Toa to ambient temperature, typically cooling to room temperature.

[0072] The method according to the invention results in an uncoated high-strength cold-rolled and annealed steel strip having the narrow alloy compositional ranges in combination with the defined microstructure and providing for an improved balance of strength and formability as represented by the tensile elongation combined with the hole expansion capacity and making the steel sheet an ideal candidate for use of manufacturing shaped automotive parts.

[0073] In a preferred embodiment of the method, after step d.) and prior to step e.) the temperature of the cold-rolled steel is increased from the Toa to a temperature Tcoat in a range of about 420 to 470°C such that it can be galvanised in a zinc or zinc alloy bath. When the steel sheet is at temperature Tcoat the steel sheet is also galvanised by means of hot-dip zinc coating by immersing the steel sheet in molten zinc or zinc alloy at a temperature in a range of about 445 to 460°C Preferably the holding time, t_coat, of the steel sheet in the zinc bath is in a range of 2 to 30 sec., and preferably in a range of 2 to 20 sec.

[0074] In an embodiment of the method, increasing the temperature of the cold-rolled steel sheet from Toa to a temperature Tcoat in a range of 420 to 470°C is at a heat-up rate in a range of 5 to 150 °(7sec, and preferably performed using an inductor.

[0075] In a preferred embodiment the galvanizing is performed in a continuous galvanizing line (CGL). A continuous galvanizing line may operate at a line speed of up to about 200 m / min, and is preferably operated at a line speed in a range of 60 to 140 m / min.

[0076] The method according to the invention results in a galvanised high-strength cold-rolled and annealed steel strip having the narrow alloy compositional ranges in combination with the defined microstructure and providing an improved balance of strength and formability as represented by the tensile elongation combined with the hole expansion capacity. Some reasons, preferred embodiments and advantages of the various method steps are set out below:

[0077] In the method hot-rolled strip is pickled and / or brushed according to methods commonly used in the art to give a surface finish suitable for cold rolling. Subsequently, in accordance with the method of the invention the provided cold-rolled sheet having the composition in accordance with the invention is cold rolled under standard conditions, for example by reducing the thickness of the hot-rolled strip from about 2.5 to 5 mm to arrive at a cold-rolled sheet thickness of about 0.7 to 3.0 mm. Next, in method step b.) an annealing treatment is carried out, preferably by continuous annealing.

[0078] The annealing treatment of the cold-rolled steel sheet is at an annealing temperature, T_an, above the Ac3 temperature of the steel to fully austenitize the structure in the steel. In an embodiment the annealing temperature does not exceed Ac3+90°C in order not to coarsen the grain size of the austenite too much. The steel sheet is held at the annealing temperature typically for 50 to 180 sec.

[0079] Next in method step c.) the annealed steel sheet is cooled from the annealing temperature, Tan, to a cool-stop or quench-stop temperature, Tq, that is between Ms-70°C to Ms-15°C In a preferred embodiment the Tq is between Ms-55°C to Ms-15°C and preferably between Ms-45°Cto Ms-15°C

[0080] Ms being the martensite start temperature and can be calculated using an empirical model according to S.M.C. van Bohemen, “Bainite and martensite start temperature calculated with exponential carbon dependence”, in Materials Science and Technology, 28, issue 4, (2012), pages 487-495.

[0081] Preferably the cooling rate of the annealed steel sheet from Tan to Tq is in a range of about 20 to 120 °(7sec, preferably in a range of about 20 to 100°(7sec, and should be sufficiently high enough to prevent austenite decomposition to ferrite. Rapid cooling of the annealed strip to Tq is in particular performed from a temperature in the range of about 650- 770°C, more preferably of about 690-750°C

[0082] During cooling of the steel sheet, preferably in the RCS ofa CG to Ms-70°Cto Ms-15°C a martensite fraction between 5% and 64% is formed. This martensite fraction will accelerate the formation of bainite in the subsequent OAS of a CGL. The amount of martensite formed is carefully controlled by the cool-stop temperature and is an important feature of the invention. When the martensite fraction is <5% it is too low a fraction to positively affect the increase in bainite kinetics. And when the martensite fraction is >64% the bainite fraction that can form in the remaining austenite is too low, and the corresponding latent heat is insufficient to increase the temperature. When Tq is at Ms or higher the formation is bainite is too slow. When Tq is lower than Ms-70°C then there is not sufficient bainite formation and the latent heat of bainite formation will not be enough the increase the temperature.

[0083] During method step d.) there is austempering of the cold-rolled and annealed steel sheet, e.g. in an OAS of a CGL the temperature of the cold-rolled and annealed steel sheet is gradually increased from Tq to an overageing temperature or austempering temperature, Toa, that is between Tq and Tq+100°C The temperature of the steel sheet increases due to latent heat accompanying the bainite formation. This increase is roughly proportional to the fraction (f) of bainite (B) according to the relation delta T [°C] = 1.1 x f B [vol%], as is schematically illustrated in Fig. 2. The holding time, t_oa, of austempering of the steel sheet is preferably for a time in a range of 20 to 700 sec., and is dependent on the line speed of the installation. This time t_oa should be long enough for most of the remaining austenite to transform to bainite. When the method is performed in a CGLt_oa is preferably in a range of 20 to 100 sec.

[0084] An important advantage of the method, in particular when applied in a continuous galvanizing line, is that it does not require a heat-up apparatus, e.g. an inductor, to be positioned at the exit of the rapid cool section (RCS) of a continuous hot-dip galvanizing line leading to considerable CAPEX and OPEX savings. Thus no external heating is being applied to the steel sheet between step c) and d).

[0085] In method step e.) the steel sheet is cooled from Toa to ambient temperature, preferably cooled to room temperature. Next the cold-rolled and annealed steel sheet may be skin pass rolled or temper rolled to improve sheet flatness, and typically this leads to a small cold rolling reduction of less than 1%

[0086] In an aspect of the invention it relates to a galvanized steel strip obtained by hot dip galvanizing the steel strip according to this invention.

[0087] In another aspect of the invention it relates to an automotive component, in particular an automotive chassis part, made from or incorporating the high-strength cold-rolled and annealed steel strip according to this invention and taking benefit from amongst others improved balance of strength and formability. The steel strip can be shaped into an automotive component in a cold forming operation or warm forming operation as are known in the art. The automotive component includes, but is not limited to, a suspension arm, a reinforcement member, body-in- white frame member, as side member, a seat frame, a seat rail, bumper beam, battery boxes for electrical vehicles, battery box lids, all having an intricate shape. By using the high-strength cold-rolled and annealed steel strip, these components can be fabricated with high quality, cost efficiently and with high yields. The high-strength cold-rolled and annealed steel product according to this invention can be used also for engineering applications.

[0088] DESCRIPTION OF THE FIGURES

[0089] The invention shall also be illustrated with reference to the appended non-limiting figures, in which:

[0090] Fig. 1 shows schematically the annealing schedule applied in accordance with the invention.

[0091] Fig. 2 shows schematically a typical dependency between temperature Tq and the fraction of martensite formed for steel alloys comprising 0.1-0.3 wt.%carbon as can be determined with dilatometry.

[0092] Fig. 1 shows schematically the temperature-time cycle of the steel sheet processing according to the invention carried out in the preferred embodiment of continuous galvanizing line (CGL). In step b.) the cold-rolled steel sheet is heated to annealing temperature T an above the Ac3 temperature of the steel to fully austenitize the steel. In step c.) the cold-rolled and annealed steel sheet is cooled from T_an to a cool-stop temperature, Tq, that is between Ms- 70°C to Ms-15°C As is often used in the art, the steel sheet is first cooled at a cooling rate of about 0.1-10 “O'sec from T_an to a temperature 13 in a range of about 690-770°C in the Slow Cooling Section (SCS), if present, of a CGL and next fast cooled in the Rapid Cool Section (RCS) of a CGL. The rapid cooling of the annealed strip to Tq is in particular performed from a temperature in the range of about 650-770°C, more preferably of about 690-750°C The cooling rate is in a range of about 20 to 120 °(7sec, preferably in a range of about 20 to 100 °(7sec, and should be sufficiently high enough to prevent austenite decomposition to ferrite.

[0093] Next in method step d.) austempering of the cold-rolled and annealed steel sheet at a temperature, Toa, that is between Tq and Tq+100°C, and preferably for a time, t_oa, in a range of 20-700 sec. The austempering is preferably performed in the OverAge Section (OAS) of a CGL.

[0094] In Fig. 1 with step d.) line 1 represents a temperature increase not according to the invention as the temperature Tq is above Ms. line 3 represents a temperature increase also not according to the invention as the temperature Tq is below Ms-70°C And line 2 represents a temperature increase according to the invention.

[0095] Preferably, in a next method step the temperature of the cold-rolled steel sheet is increased, e.g. using an induction furnace (IF), from the Toa to a temperature Tcoat in a range of 420-470°C such that subsequently it can be galvanised in a zinc or zinc alloy bath, preferably by means of hot-dip zinc coating by immersing the steel sheet moving in the CGL in molten zinc or zinc alloy at a temperature in a range of 445-460°C Following the galvanizing step, the galvanized steel sheet is cooled in step e.) to ambient temperature.

[0096] In an embodiment of the invention (not shown in Fig. 1), the cold-rolled and annealed sheet is not galvanised and following step d.) wherein the cold-rolled and annealed steel sheet is austempered at a temperature, Toa between Tq and Tq+100°C, for a time, t_oa in a range of 20 to 700 sec. to arrive at the defined microstructure in accordance with the invention, the steel sheet is cooled in step e.) to ambient temperature, preferably cooled to room temperature.

[0097] The invention will now be illustrated with reference to non-limiting comparative and examples according to the invention.

[0098] EXAMPLES

[0099] Cold-rolled steel sheet having a thickness of 1 mm and of six different chemical compositions, in wt.%, as set of in Table 1 have been provided. For the steels the Ac3 and Ms temperature are listed. Steel 1 to 3 have a composition according to the invention, steel 4 is comparative in view of too high a Si content, steel 5 because of too low a Mn content and steel 6 because of too high a C content.

[0100] The cold-rolled steel sheets have been annealed at either 835°C or 845°C using a laboratory continuous annealing simulator to obtain a fully austenitize the steel. Next the annealed steel sheets 1 to 4 have been subjected to different heat-treatment cycles by rapid cooling from 13, either 700°C or 730°C, to a temperature Tq and austempering for time t_oa (in seconds), at the end of the austempering the sheet is at temperature T4, and subsequently subjected to a simulated hot-dip temperature cycle by heating the steel sheet from T4 (is the temperature increase due to the latent heat) to a T_coat of 455°C using a heat-up rate of about 20 °C7sec and soaked t_coat at this temperature for either 14 or 17 sec. and thereafter cooled to room temperature with a cooling rate in a range of 2-40 °(7sec. Thus the steel sheets 1 to 4 have not been provided with a hot-dip galvanizing layer. Steels 5 and 6 have not been subjected to a simulated hot-dip temperature cycle, but after reaching temperature T4 cooled to room temperature with a cooling rate in a range of 2-40oC7scc. The various cycles applied are listed in Table 2.

[0101] The mechanical properties of the steels for the various heat-treatment cycles have been measured and are listed in Table 3 and the microstructures in Table 4. In Table 3 “YR” stands for the ratio of Rp0.2 / Rm and should preferably > 0.70. Table 3 also lists the ratio HEC7YR and is preferably > 0.35. These ratios are indicative also for the formability of the cold-rolled and annealed steel sheet.

[0102] Table 1. The chemical composition of the steels, in wt.% The balance is made by iron and unavoidable impurities.

[0103] Table 2. The heat-treatment cycles applied for steels 1 to 6. Table 3. Mechanical properties of steel 1 to 4 resulting from the various heat-treatments applied, “-“means not measured. Table 4. Microstructures of steel 1 and 2 resulting from the various heat-treatments applied. From the results of Table 3 it can be seen that the HEC performance for steel 1 and 2 after treatments according to cycle A is too low (< 25%), which can be attributed to a too a large fraction of fresh martensite (FM) in the microstructure (see Table 4). This leads to local hardness differences in the microstructure which deteriorates the local formability of the steel which is measured as a low HEC value. In the other cycles according to the invention the fraction of martensite formed at Tq at the end of the RCS is sufficiently high to significantly enhance the kinetics of the bainite formation in the OAS. This results in a better stabilization of austenite at the end of the austempering with time t_oa, and therefore much less FM will be formed during the final cooling to room temperature .

[0104] Steel 3 has too high a Si content leading to a low ultimate tensile strength when processed according to the invention (cycle H). Steel 4 has too low a Ma content leading to a low tensile yield strength and a low ultimate tensile strength when processed according to the invention (cycle I). Steel 5 has too high a C content leading to a low HEC performance when processed according to the invention (cycle J).

[0105] Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as herein described.

Claims

CLAIMS1. A high-strength cold-rolled and annealed steel sheet with an ultimate tensile strength greater than 1180 MPa, the composition comprising, in wt.%0.130 to 0.240% of C;1.50 to 3.00%ofMn;0.10 to 0.45%of Si;0.010 to 0.50%ofAl;0.010 to 0.15%ofTi;0.0003 to 0.0050%ofB; up to 0.050%ofP; up to 0.015%of S; up to 0.020% of N; up to 0.20% of Sn; up to 0.010%ofCa; optionally one or more elements selected from the group consisting of: up to 0.50% V, up to 0.50% Cu, up to 0.50%Ni , up to 0.60% o, up to 0.35 wt.%of Cr, up to 0.10% Nb; balance iron and inevitable impurities; a microstructure consisting of (in vol.%): tempered martensite: 5-64% bainite: 35-90% transformation ferrite: S 5% retained austenite: S 10% fresh martensite: S 14% sum of pearlite, cementite and precipitates S 3% and the balance is adding up to 100% and the following mechanical properties: an ultimate tensile strength (Rm) > 1180 MPa, a tensile elongation (JIS5) > 6.5 % and a hole expansion capacity (HEQ > 25%2. A high-strength cold-rolled and annealed steel sheet according to claim 1, wherein the composition has at least one of the following elements, in wt.%0.150 to 0.220% of C;1.80 to 2.70%ofMn;0.15 to 0.390%of Si;0.010 to 0.30%ofAl.

3. A high-strength cold-rolled and annealed steel sheet according to claim 1 or 2, wherein the composition has at least the following elements, in wt.%0.130 to 0.240% of C;1.50 to 3.00%ofMn;0.10 to 0.40%of Mo, and preferably 0.10 to 0.30%0.10 to 0.45%of Si, and preferably 0.10 to 0.390%0.010 to 0.50%of Al, and preferably 0.010 to 0.30%0.010 to 0.15%of Ti; and preferably 0.010 to 0.08%0.0003 to 0.0050%ofB.

4. A high-strength cold-rolled and annealed steel sheet according to claim 1 or 2, wherein the composition has at least the following elements, in wt.%0.130 to 0.240% of C;1.50 to 3.00%ofMn;0.030 to 0.35% of G, and preferably 0.030 to 0.25%0.10 to 0.45%of Si, and preferably 0.10 to 0.390%0.010 to 0.50%of Al, and preferably 0.010 to 0.30%0.010 to 0.15%of Ti; and preferably 0.010 to 0.08%0.0003 to 0.0050%ofB.

5. A high-strength cold-rolled and annealed steel sheet according to any one of claims 1 to4, wherein the sum of Si+Al < 0.69 wt.% and preferably Si+Al < 0.49 wt.%6. A high-strength cold-rolled and annealed steel sheet according to any one of claims 1 to5, wherein the microstructure consists of, in vol.% tempered martensite: 5-49% preferably 7-39% bainite: 50-90% preferably 60-90% transformation ferrite: S 5% preferably S 2% retained austenite: S 7% preferably S 4% fresh martensite: S 14% preferably S 10% sum of pearlite, cementite and precipitates S 3% and the balance is adding up to 100%7. A high-strength cold-rolled and annealed steel sheet according to any one of claims 1 to 6, wherein the steel sheet fulfils the following requirement: an ultimate tensile strength (Rm) S 1200 MPa, a tensile elongation (JIS5) > 7.5% a HEC > 40% yield strength (YS) prior to temper rolling > 820 MPa, preferably > 900 MPa.

8. A steel sheet according to any one of claims 1 to 7, wherein the steel sheet is provided on one or both of its faces with a zinc or zinc -alloy layer, preferably a hot-dip galvanized layer.

9. A method of producing a high-strength cold-rolled and annealed steel sheet according to any one of claims 1 to 8, comprising the steps of: a. providing a cold-rolled steel sheet having a composition as set out in any of the preceding claims; b. annealing the cold-rolled steel sheet at an annealing temperature, T_an, above the Ac3 temperature in order to fully austenitize the steel; c. cooling of the cold-rolled and annealed steel sheet from T an to a cool-stop temperature, Tq, that is between Ms-70°C to Ms-15°C, with Ms being the temperature at which the transformation from austenite to martensite starts; d. austempering of the cold-rolled and annealed steel sheet at a temperature, Toa, between Tq and Tq+100°C, and preferably for a time, t_oa, in a range of 20 to 700 sec.; e. cooling the steel sheet from Toa to ambient temperature.

10. A method according to claim 9, wherein during step c.) cooling of the cold-rolled and annealed steel sheet from Tan to a cool-stop temperature, Tq, that is between Ms-55°Cto Ms-15°C, and preferably is between Ms-45°Cto Ms-15°C11. A method according to claim 9 or 10, wherein after step d.) the temperature of the cold- rolled steel is increased from Toa to a temperature Tcoat in a range of 420 to 470°C12. A method according to claim 11, wherein when the steel sheet is at temperature Tcoat the steel sheet is galvanised by means of hot-dip zinc coating by immersing the steel sheet in molten zinc or zinc alloy at a temperature in a range of 445 to 460°C 13. A method according to claim 12, wherein the galvanizing is performed in a continuous galvanizing line (CGL).

14. A method according to any one of claims 9 to 13, wherein step d.) is performed in the overage section (OAS) of a continuous galvanizing line (CGL).

15. An automotive component made from the high-strength cold-rolled and annealed steel strip according to any one of claims 1 to 8 or made from the high-strength cold-rolled and annealed steel strip obtainable by the method according to any one of claims 9 to 14.

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