Hot-rolled ultra-high strength steel strip

Abstract

The invention relates to a hot-rolled ultra-high strength steel strip. The invention also relates to a method of manufacturing such hot-rolled ultra-high strength steel strip.

Description

[0001] HOT-ROLLED ULTRA-HIGH STRENGTH STEEL STRIP

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a hot-rolled ultra-high strength steel strip or sheet, and which is in particular suitable for, but not limited to, use in automotive components. The invention also relates to a method of manufacturing such a hot-rolled ultra-high strength steel strip. Furthermore, the invention relates to an automotive part incorporating the hot-rolled ultra-high strength steel strip.

[0004] BACKGROUND TO THE INVENTION

[0005] It is well-known in the art that as the strength of hot-rolled (HR) steel strip increases, the formability decreases and the fracture toughness, crack susceptibility, and susceptibility to unstable brittle fracture are compromised. A major area of application for HR steels in transport and automotive applications is the chassis and suspension (C&S) domain. Other areas include frame rails of trucks, bumper beams or battery boxes for electrical vehicles. The typical thickness of hot-rolled steels used for these applications is less than 8 mm, and is commonly less than 6 mm or less than 4.5 mm. Thicker gauge HR steel strip such as up to 12 mm can be used in engineering applications such as crane booms or in transport applications for frames of heavy trucks. From the weight reduction perspective it is imperative that higher strength steels should be employed for the above applications in order to be able to reduce the gauge of the steel strip. These applications of the hot-rolled steels demand mechanical properties that are difficult to reconcile. Besides high strength, the steel strip should also have good formability for making the component via e.g. cold-forming because this is an energy efficient manufacturing route in comparison with hot-forming. Furthermore, good impact and fracture toughness or energy absorption capacity is also required for applications like bumper beams, battery housings, crane booms or frame rails. For assembling the components, also a good weldability is required. However, as the tensile strength of the steels increases, the formability parameters decrease. Formability is a generic term for steel sheets which is viewed as a combination of material behaviour during several mechanical operations such as stretching, bending, drawing and flanging. Depending on the component geometry any or a combination of two or more attributes of the material is of importance during sheet metal forming. Next to formability, also the fracture toughness may be compromisedwith an increase of the strength. This can lead to increased edge-crack susceptibility and increased susceptibility to unstable brittle crack propagation under specific mechanical loading conditions, such as local compression in deep-drawing or stamping operations, or alternate loading conditions due to compression followed by springback during deep-drawing or stamping operations.

[0006] Commercially available ultra-high strength steels include martensitic steels (MS) for cold-stamping and roll-forming applications of automotive components. The available tensile strength levels of MS grades is typically between 1200 and 1500 MPa. A drawback of martensitic steels is a low formability and that these steels are susceptible to loss in strength upon a post-rolling heat treatment such as that of a heat-to-coat or hot-dip galvanising cycle.

[0007] Other commercially available ultra-high strength steels are press-hardening steels (PHS) for hot-forming applications for automotive components. The available tensile strength levels of PHS grades is typically about 1500 MPa (PHS1500 or 22MnB5) and 2000 MPa (PHS2000 or 34MnB5) after heat treatment. These steels are intended for hot-forming and not suitable for cold-forming. Most automotive chassis components are produced via cold forming as that is a more cost-efficient method to produce these components. PHS steels have a very limited amount of ductility after forming. Typically these steels have also a relatively low fracture toughness in their final state after postrolling heat treatment.

[0008] There is a demand for hot-rolled ultra-high strength well-drawable steels that, next to sufficient strength for down gauging and weight saving, offer a suitable combination of global (e.g., tensile elongation) and local (e.g., hole-expansion capacity) formability for component geometry optimisation and adequate stiffness while also providing an adequate level of fracture toughness to minimise brittle fracture behaviour, and thereby realising an optimum balance for all these material characteristics, i.e., formability in terms of tensile elongation and hole-expansion capacity, and fracture toughness in terms of edge-crack susceptibility.

[0009] DESCRIPTION OF THE INVENTION

[0010] 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.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] The terms “up to” and “up to about”, as employed herein, explicitly includes, but are not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.20% Cu may include a steel having no Cu.

[0012] As used herein, the term “at ! thickness” when used to describe microstructural features refers to an area between 1 / 8 and 3 / 8 of the steel strip thickness below the surface of the steel strip.

[0013] It is an object of the invention to provide a hot-rolled ultra-high strength deep-drawable steel strip with high edge ductility in terms of hole-expansion capacity and adequate fracture toughness, that is in particular suitable for the manufacture of complex components via cold-stamping or roll-forming for automotive applications.

[0014] It is an object of the invention to provide a hot-rolled ultra-high strength deep-drawable steel strip having a yield strength (Rp0.2) ranging between 950 MPa to 1450 MPa, an ultimate tensile strength (Rm) ranging between 1150 and 1550 MPa, and with a high edge ductility in terms of hole-expansion capacity, and adequate fracture toughness that is in particular suitable for the manufacture of complex components via cold-stamping or roll-forming for automotive applications.

[0015] These and other objects and further advantages are met or exceeded by the present invention providing a hot-rolled ultra-high strength steel strip having a composition, in wt.%,

[0016] C: 0.220 to 0.280%,

[0017] Mn: 1.20 to 2.20%,

[0018] Ti: 0.050 to 0.150%,

[0019] Mo: 0.10 to 0.50%,

[0020] Si: up to 1.20%,

[0021] Cr: up to 1.0%,

[0022] Al: up to 1.0%,

[0023] P: up to 0.02%,S: up to 0.01%,

[0024] N: 0.0020 to 0.020%,

[0025] Nb: up to 0.10%,

[0026] Sn: up to 0.08%,

[0027] B: up to 0.007%;

[0028] optionally one or more elements selected from the group consisting of: Cu: up to 0.20%, V: up to 0.30%, Ni: up to 0.50%;

[0029] and balance Fe and inevitable impurities resulting from the ironmaking and steelmaking process, and wherein parameter Pern is at least 0.30, with

[0030]

[0031] and wherein the steel strip has a microstructure consisting at ! thickness of: at least 95 vol.% bainite, consisting of a mixture of upper and lower bainite, and less than 5 vol.% of secondary phase constituents, including any carbides, cementite, martensite and retained-austenite; and

[0032] an average effective grain size of the bainite matrix of at most 1.5 pm; and a MisOrientation Distribution (MOD) index of at least 1.0;

[0033] and wherein the steel strip has at least the following mechanical properties: A50 tensile elongation of at least 5%, and

[0034] r0-value of at least 0.6, and

[0035] a hole-expansion capacity (HEC) of at least 30%, and

[0036] Rp0.2 of 950-1250 MPa and Rm of 1150-1350 MPa, or

[0037] Rp0.2 of 1150-1450 MPa and Rm 1350-1550 MPa.

[0038] The microstructure, the HEC, and the tensile properties are measured as herein described.

[0039] In accordance with the invention it has been found that the hot-rolled ultra-high strength steel strip having these narrow alloy compositional ranges in combination with the microstructure provides for an improved balance of very high yield strength, very high ultimate tensile strength, an adequate combination of global and local formability (high edge-crack resistance under tensile stress), and high edge-crack resistance under compression or alternate loading conditions with compression followed by tensile stresses due to springback, and an adequate level of fracture toughness. An important advantage is that due to its microstructure the steel strip issignificantly less sensitive to loss of strength when subjected to a heat treatment as a result of applying a metallic coating in a heat-to-coat cycle or hot-dip coating process. Furthermore, and very advantageously, it has been found that the hot-rolled steel strip can be manufactured using a broad hot-rolling process temperature window and thereby is less sensitive for operational scatter when processed on an industrial scale. In particular there appears to be a wider operational window for the hot rolling finish temperature (FRT).

[0040] Based on extensive research, the inventors have found that by careful control of the steel composition, the accelerated cooling in the hot rolling process, and the relatively high coiling temperature (CT) of 350-500°C in particular, and with preferred narrower ranges, a hot-rolled ultra-high strength steel strip is obtained having a finegrained bainite microstructure strengthened with Ti-based and / or Ti Mo-based carbide precipitates that provides sufficient hardenability and the favourable balance of properties. The hot-rolled steel strip is in particular an ideal candidate for manufacturing complex shaped automotive chassis and suspension parts.

[0041] The hot-rolled steel strip steel has a fine-grained bainitic microstructure strengthened with Ti-based and / or TiMo-based carbide precipitates that is produced with an alloy composition and a high enough coiling temperature in a range of 350-450°C that prevents a substantial loss in strength during a heat-to-coat or hot-dip galvanising cycle to apply a zinc coating to the steel strip surface. The fine-grained bainitic microstructure strengthened with Ti-based and / or TiMo-based carbide precipitates is resistant against recovery (i.e., dislocation annihilation) or recrystallisation during a subsequent heat treatment in the form of a heat-to-coat or hot-dip galvanising cycle due to the presence of fine Ti-based and / or TiMo-based carbide precipitates. Furthermore, the composite Ti-based and / or TiMo-based carbide precipitates are thermal resistant and not susceptible to precipitate coarsening during a heat-to-coat or hot-dip galvanising cycle.

[0042] The hot-rolled steel strip according to the invention offers weight saving via down gauging by exploiting the combination of the high strength levels and good formability of said steel, which enables the manufacture of light-weight automotive components via cold-stamping or roll-forming to reduce emissions and extend the drive range of electric vehicles. Furthermore, it enables improving the collision performance or anti-intrusion of components and the overall crash resistance of vehicles and their passengers safety by exploiting the combination of high strengthlevels, good formability and adequate fracture toughness of said steel for the manufacture of cold-stamped or roll-formed automotive components. In addition, and very advantageously, it enables improving the corrosion resistance of the hot-rolled steel strips via the possibility to apply for example a zinc or zinc alloy coating on the steel strip surface via a heat-to-coat or hot-dip galvanising cycle without compromising the strength or other properties of said hot-rolled steel strip.

[0043] The 0.2% offset proof strength or yield strength (Rp0.2), ultimate tensile strength (Rm), uniform elongation (Ag) and tensile elongation (A50) were determined from quasistatic tensile tests at room temperature with A50 specimen geometry with tensile testing parallel to the rolling direction according to ISO-6892 and ISO-10113 using a Zwick tensile testing machine. The geometry of the tensile specimens consisted in 50 mm gauge length in the rolling direction, 12.5 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). The rO or r0value is the plastic strain ratio in the rolling direction with the in-plane plastic strain divided by the plastic strain through-the-thickness, and is based on a regression in the range of 20 to 45% of the expected Rp0.2.

[0044] In an embodiment the hole-expansion capacity (HEC) is at least 40%, preferably of at least 50%, and more preferably of at least 60%.

[0045] The stretch-flangeability of the steel strip or the hole expansion capacity (HEC) was determined by hole expansion tests. Specimens of dimension 90 by 90 mm by final thickness of the strip were cut from the 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 / TS 16630:2009(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 dfwas measured when a through-thickness crack formed. The hole expansion ratio A was calculated using the formula below with d0= 10 mm:

[0046]

[0047] For all the above mechanical tests, at least three specimens were tested for each condition and the average values are reported herein.

[0048] The hot-rolled steel strip has either a tensile yield strength Rp0.2 in a range of 950-1250 MPa in combination with an ultimate tensile strength Rm in a range of 1150-1350 MPa, or a tensile yield strength Rp0.2 in a range of 1150-1450 MPa in combination with an ultimate tensile strength Rm in a range of 1350-1550 MPa.

[0049] In an embodiment the A50 tensile elongation is of at least 6%, preferably of at least 7%, and more preferably of at least 8%.

[0050] In an embodiment the r0-value is at least 0.70, preferably of at least 0.75, and more preferably of at least 0.80.

[0051] The hot-rolled steel strip of this invention has as key feature at !4-thickness a microstructure being precipitation-strengthened bainite and consisting of a mixture of upper and lower bainite. This means that at least 95 vol.% (volume fraction) of the microstructure is formed by the bainite, and more preferably by at least 97 vol.%, and most preferably by at least 98 vol.%. The steel strips have at most 5 vol.% second-phase constituents, including any cementite, pearlite, martensite, and / or retained-austenite, and preferably at most 3 vol.% second-phase constituents, as too high a fraction of hard carbon-rich second-phase constituents will lead to too much damage upon shearing, which is at the expense of the hole-expansion capacity.

[0052] Furthermore, at !4-thickness the microstructure is characterised by an average effective grain size of the bainite matrix of at most 1.5 pm, preferably at most 1.4 pm, and more preferably at most 1.3pm. As known to the skilled person, “effective grain size” refers to grains of which the boundaries are defined as having a misorientation of at least 15 degrees with neighbouring grains. A high-angle grain boundary of 15 degrees or higher is “effective” to retard or even arrest crack propagation. Hence, a smaller effective grain size and hence a higher density of high-angle grain boundaries will improve the toughness of the steel strip. In addition, the microstructure has a MisOrientation Distribution (MOD) index of at least 1.0, and preferably of at least 1.1, and more preferably of at least 1.2. In general, an increase of the MOD index is associated with an increase of the hole-expansion capacity. The MOD index is a measure for the character of the microstructure, which is dominated by the phase constituents making up the matrix. High-temperature phase constituents, for instance polygonal ferrite will typically have a low MOD index. As the transformation temperature drops, the ferrite morphology will shift to quasi-polygonal or irregularshaped ferrite, through acicular and / or bainitic (feathered) ferrite towards upper bainite and eventually lower bainite, before reaching martensite. It is believed that an increased MOD index leads to a more intricate crystallographic arrangement that isbeneficial for improved hole-expansion capacity and that the increased MOD index from the defined coiling temperature (CT) comes with refined cementite particles due to reduced diffusivity.

[0053] The microstructure of the hot-rolled steel strip was analysed by means of Electron Back Scatter Diffraction (EBSD), a technique well known in the art, which in turn also allows the quantification of the area or volume fraction of the various components. The EBSD measurements were conducted on cross sections parallel to the rolling direction (RD-ND plane) mounted in a conductive resin and mechanically polished to 0.25 pm. To obtain a fully deformation free surface, the final polishing step was conducted with colloidal silica (OPS).

[0054] The Scanning Electron Microscope (SEM) used for the EBSD measurements is a Zeiss Ultra Plus machine equipped with a Field Emission Gun (FEG-SEM) and an Oxford instruments Symmetry S2 EBSD system. EBSD scans were collected on the RD-ND plane of the sheets at quarter-thickness. The samples were placed under a 70° angle in the SEM. The acceleration voltage was 15 kV with the high current option switched on. A 120 pm aperture was used and the typically working distance was 17 mm during scanning. To compensate for the high tilt angle of the sample, the dynamic focus correction was used during scanning.

[0055] The EBSD scans were captured using the Oxford instruments AZtec® 6.1 software. Typically, the following data collection settings were used: The Symmetry S2 camera in “speed 2” mode combined with auto background subtraction. The scanned area was in all cases located at ! thickness of the steel and care was taken to avoid as much as possible the inclusions of non-metallic particles in the scanned area. The EBSD scan size was in all cases 200x150 pm, with a step size of 0.1pm, and a scan rate of approximately 1000 frames per second. Fe(a) and Fe(y) were used to index the Kikuchi patterns. The indexing settings used during data collections were: Indexing mode = Optimized-BD; number of bands = 11; Hough resolution is 50; solution refinement = on. After collection the scans were exported to h5oina data format. This resulted in a typical hit rate of >97.5% for the scans.

[0056] The EBSD scans (in h5oina format) were evaluated with Oxford instruments AZtecCrystal 3.1 software. Cleanup procedure: step 1) wild spike removal; step 2) zero solution removal (level 4) step 3) zero solution removal (level 6).The MisOrientation angle Distribution (MOD) index of the Fe(D) partition was calculated using the following method: the normalised misorientation angle distribution (MOD), including all boundaries, ranging from misorientation angles of 5° to 65° with a binning of 1°, was calculated from the partitioned EBSD data set using the TSL OIM Analysis software. Similarly, the normalised theoretically MOD of randomly recrystallized polygonal ferrite was calculated with the same misorientation angle range and binning as the measured curve. In practice this is the so-called “MacKenzie” based MOD included in the TSL OIM Analysis software. Normalisation of the MOD means that the area below the MOD is defined as 1. The MOD index is then defined as the area between the theoretical curve (the dashed line) and the measured curve (the solid line) in Fig. 1 A and Fig. 1 B and can be written as:

[0057]

[0058] with MMOD as the intensity at angle i (ranging from 5° to 65°) of the measured MOD and RMOD as the intensity at angle i of the theoretical or “MacKenzie” based MOD of randomly recrystallized polygonal ferrite.

[0059] Another key feature of the invention is related to the composition (in wt.% or weight percent) of the hot-rolled ultra-high strength steel strip.

[0060] C: 0.220% to 0.280%. Carbon (C) is a necessary element for strength and ductility by forming the required microstructure. When the content of C is less than 0.180%, it is difficult to obtain the required strength. On the other hand, when the C content is more than 0.280%, the formability and weldability are deteriorated. Therefore, the C content is 0.220% to 0.280%, and preferably in a range of 0.220% to 0.250%.

[0061] Mn: 1.20% to 2.20%. Manganese (Mn) is an element which is indispensable for obtaining hardenability of the steel to achieve the desired strength and the formation of upper and lower bainite. Furthermore, Mn assists in grain refinement via the solute drag effect. On the other hand, with too high a Mn content segregation may occur deteriorating formability and sheared-edge quality. Therefore, the Mn content is in a range of 1.20% to 2.20%, and preferably in a range of 1.60% to 2.00%, and more preferably in a range of 1.80% to 2.00%.

[0062] Ti: 0.050 to 0.150%. Titanium (Ti) is beneficial to restrict austenite grain growth during reheating prior to hot-rolling and to control the austenite conditioning during hot-rolling via its effect on the non-recrystallisation temperature (Tnr). In this context, titanium can bring grain refinement of the final microstructure, which increases strength and toughness. Furthermore, titanium brings precipitation strengthening as a joined carbide and nitride forming element. In this context, titanium brings precipitation strengthening via the formation of carbides and carbonitrides. In case of carbides and in the presence of molybdenum, titanium can lead to composite TiMoC precipitates, which are more thermally stable than TiC and help to optimise precipitation strengthening. Additionally, a certain minimum amount of titanium is required to scavenge nitrogen in order to prevent the formation of boron-nitrides (BN) as that would render the required boron addition to increase hardenability useless. On the other hand, too high a Ti content will promote an excessive amount of relatively large TiN cuboid inclusions which impair formability and toughness of the steel strip. Therefore, the Ti content is in a range of 0.050% to 0.150%, and preferably in a range of 0.060% to 0.130%.

[0063] Mo: 0.10 to 0.50%. Molybdenum (Mo) is an important alloying element in this invention. It brings hardenability and can, jointly with titanium, promote the formation of TiMoC precipitates that help to strengthen the bainitic microstructure. The increased thermal resistance to precipitate coarsening of TiMoC compared to TiC is beneficial to maintain strength during a post-rolling heat treatment, e.g. a heat-to-coat or hot-dip galvanisation cycle, as TiMoC precipitates exhibit a low coarsening rate and can help to suppress dislocation annihilation by pinning the movement of dislocations during the recovery stage. This can help to maintain strength after post-rolling heat treatment. Furthermore, molybdenum retards the movement of interfaces like grain boundary and can help to suppress recrystallisation during post-rolling heat treatments, viz. a heat-to-coat or hot-dip galvanisation cycle. On the other hand, Mo is an expensive alloying element and hence it is desirable for economic reasons to limit its use. Therefore, the Mo content is in a range of 0.10% to 0.50%, and preferably in a range of 0.18% to 0.30%, and more preferably of 0.22% to 0.26%.

[0064] Silicon (Si) is an element that can be present up to 1.20% and is effective in increasing the strength of the steel by solid-solution strengthening and is effective to suppress the formation of cementite and helping to keep cementite particles, if any, small. This is beneficial for formability and toughness of the steel strip. When the Si content is higher than 1.20%, the weldability and coatability of the steel are deteriorated. More preferably the Si content does not exceed 1.0%, and preferably does not exceed 0.80%. Preferably a minimum of at least 0.30% is needed to achievesuch effects, and preferably at least 0.35%. At elevated levels, Si may be beneficial to promote retained-austenite in the final microstructure, which can be beneficial for formability via the TRIP-effect. Accordingly, the Si content is preferably in a range of 0.30% to 1.20%, and preferably in a range of 0.35% to 1.0%, and more preferably in a range of 0.35% to 0.80%.

[0065] For the embodiment wherein Rp0.2 is in a range of 1150-1450 MPa and Rm in a range of 1350-1550 MPa, the Si content is preferably in a range of 0.60% to 1.20%, more preferably in a range of 0.70% to 1.20%, and most preferably in a range of 0.70% to 1.0%, while the other alloying elements (C, Mn, Ti, Mo, Si, Cr, Al, and N) are within the ranges and preferred ranges as herein described.

[0066] The primary function of aluminium (Al) is to deoxidise the liquid steel before casting. Furthermore, Al has a similar function as Si to prevent the formation of carbides and to stabilize the retained-austenite and therefore it can be effective in improving the balance between strength and formability. In order to have such effects preferably at least 0.03% Al is required. However, with increasing Al higher amounts of surface oxidation can occur at higher temperatures. These oxide scales are detrimental for hot rolling, pickling, coating and overall surface appearance. In addition, the risk of cracking during casting and hot rolling increases as the Al content is increased. Therefore, the Al content is 0.03% to 1.0%, preferably 0.03% to 0.70%. If the amount of aluminium is to be limited, for instance for, but not limited to, specific surface quality reasons, then the aluminium content is preferably between 0.03% and 0.08%.

[0067] Chromium (Cr) has a strong effect on hardenability and can optionally be useful to obtain more easily a hardened microstructure of upper and lower bainite. However, chromium can have a detrimental effect on corrosion resistance under specific conditions like to those replicated in an SAE J2334 corrosion test. Chromium may be present up to 1.0%, and is preferably limited to up to 0.6%, and more preferably limited to up to 0.3%.

[0068] In an embodiment the Cr content does not exceed 0.03%. More preferably, it is present as an inevitable impurity resulting from the ironmaking and steelmaking process and whereby sufficiently care is taken that any scrap materials used in the steelmaking process have a low presence of Cr.

[0069] Nitrogen: 0.0020 to 0.020%. Nitrogen (N) is also a necessary element in the steel strip. It forms titanium nitrides with Ti (TiN and TiCN) which act as dispersoids foraustenite grain size control during reheating. Titanium nitrides (TiN) and carbo-nitrides (TiCN) have a higher thermal stability compared to titanium carbibes (TiC), and thereby support austenitic grain refinement during the hot-rolling process and increased precipitation strengthening of the final microstructure after phase transformation. The increased thermal stability of TiN and TiCN compared to TiC ensures that TiN and TiCN precipitates are less prone to coarsening during coil cooling and hence are beneficial to suppress a loss of (precipitation) strength due to precipitate coarsening during coil cooling. A more preferred minimum N content is 0.0030% (30 ppm). However, too high N can lead to too many coarse TiN particles impairing hole expansion capacity. Preferably the N content is up to 0.010% (100 ppm), and more preferably up to 0.0080% (80 ppm).

[0070] The assumed mechanism for the beneficial effect of N is that increased N content promotes the formation or presence of TiN, TiCN, and AIN precipitates during reheating and hot rolling, which are capable to pin austenite grain boundaries and delay or even arrest recrystallisation and subsequent austenite refinement during hot rolling. This in turn promotes larger austenite grains, which leads to fewer potential nucleation sites for ferrite formation and delays the austenite-to-ferrite phase transformation. This explains increased hardenability with increased N content under equal process conditions with regard to for instance finish rolling temperature (FRT), time-temperature cooling profile on the run-out-table, and coiling temperature, which widens the industrial process window for manufacturing the hot-rolled ultra-high strength steel strip according to this invention.

[0071] To obtain sufficient hardenability in the hot-rolled ultra-high strength steel strip, it is an important feature of the invention that the metal parameter Pern is at least 0.30, preferably at least 0.32, more preferably at least 0.34, and most preferably at least 0.36. This will ensure that the alloy provides sufficient hardenability to achieve the desired microstructure for an optimum balance of strength, formability, and toughness with realistic run-out-table cooling conditions in the hot-strip mill without compromising the strip shape too much with the use of excessive water. Furthermore, it will allow the use of coiling temperatures in the hot-strip mill that will promote a microstructure associated with final mechanical properties that will not degrade after a heat-to-coat cycle or a hot-dip galvanising cycle.In a preferred embodiment the hot-rolled ultra-high strength steel strip has a composition comprising:

[0072]

[0073] Sulphur (S) and phosphorus (P) are residual elements present in the steel as a result of the steelmaking and refining process. Their amounts are limited to up to about 0.01% S, and up to about 0.02% P. Amounts higher than these are detrimental for mechanical properties, formability, toughness, sheared-edge quality, fatigue, and weldability. In an embodiment P is present only up to 0.015%. In an embodiment S is present only up to 0.005% (50 ppm) and more preferably only up to 0.0025% (25 ppm), and most preferably up to 0.0012% (12 ppm).

[0074] Niobium can be present in the steel up to 0.10%. Nb improves the strength of the steel partly by precipitation hardening but foremost by grain refinement. This grain refinement is also beneficial for increased fracture toughness. However, as Nb is a rather expensive alloying, the Nb content preferably does not exceed 0.080%, more preferably it does not exceed 0.070%, and more preferably does not exceed 0.030%. In an embodiment the Nb content is at least 0.010%, and preferably at least 0.015%. Also, Nb can promote increased anisotropy and additionally has a high tendency to segregate and to form coarse NbC particles due to centre-line segregation, thus the avoidance of or minimum use of Nb results in an improved hole-expansion capacity and sheared-edge quality.

[0075] In another embodiment Nb is not purposively added and only present as an inevitable impurity resulting from the ironmaking and steelmaking process. In practice this means it can be present up to 0.003%.Boron may be added to obtain the desired balance of properties of the steel strip, but can be present up to 0.007% (70 ppm), preferably up to 0.005% (50 ppm), and most preferably up to 0.003% (30 ppm). B is very effective to enhance the hardenability of the steel, which means that lower cooling rates can be used on the run-out table in the hot-rolling process. In an embodiment the B content is at least 0.0002% (2 ppm).

[0076] Tin (Sn) is a residual element present in the steel as a result of the steelmaking and refining process. Its presence is to be limited due to the tendency of Sn to segregate to grain boundaries, resulting in a loss in toughness. Hence, the amount of Sn is to be limited to up to 0.08%, or preferably to 0.050%, or more preferably up to 0.030%.

[0077] The steel strip may have optionally one or more elements selected from the group consisting of: Cu: up to 0.20%, V: up to 0.30%, Ni: up to 0.50%.

[0078] Copper, when present, may increase the strength of the steel strip by both solidsolution strengthening as well as precipitation hardening through copper precipitates. However, the Cu content should not exceed 0.20% as too high a level of Cu can promote hot shortness that can occur during hot working of the steel strip, which leads to embrittlement of the steel. In an embodiment the Cu is not added as a purposive alloying element and can be present up to 0.10 wt.%, more preferably up to 0.060 wt.%, and most preferably up to 0.030%.

[0079] Vanadium can be present in the steel up to 0.30%, more preferably up to 0.20%, and most preferably up to 0.10%. However, V is a relatively costly alloying element that is mostly used to replace Ti for its precipitation strengthening effect and to avoid cementite formation by forming vanadium carbides or composite carbide precipitates with Nb, Ti, and / or Mo.

[0080] Nickel up to 0.50%, and preferably up to 0.30%, increases the impact toughness and counters any hot shortness that can occur during hot working of the steel strip due to the presence of copper. In an embodiment Ni is not added as a purposive alloying element and can be present up to 0.10%, more preferably up to 0.050%, and most preferably only up to 0.030%.

[0081] In a preferred embodiment the hot-rolled ultra-high strength steel strip has a composition comprising:

[0082] Cr: up to 0.03%;

[0083] Nb: up to 0.003%;Sn: up to 0.050%, preferably up to 0.030%;

[0084] Cu: up to 0.10%, preferably up to 0.060%, more preferably up to 0.030%;

[0085] V: up to 0.20%, preferably up to 0.10%;

[0086] Ni: up to 0.10%, preferably up to 0.050%, more preferably up to 0.030%.

[0087] In an embodiment the hot-rolled ultra-high strength steel strip has a composition consisting of, in wt.%, C: 0.220 to 0.280%, Mn: 1.20 to 2.20%, Ti: 0.050 to 0.150%, Mo: 0.10 to 0.50%, Si: up to 1.20%, Cr: up to 1.0%, Al: up to 1.0%, P: up to 0.02%, S: up to 0.01%, N: 0.0020 to 0.020%, Nb: up to 0.10%, Sn: up to 0.08%, B: up to 0.007%, optionally one or more elements selected from the group consisting of: Cu: up to 0.20%, V: up to 0.30%, Ni: up to 0.50%; and balance Fe and inevitable impurities resulting from the ironmaking and steelmaking process, and wherein parameter Pern is at least 0.30, and with more preferred ranges as herein described and claimed.

[0088] In an embodiment the hot-rolled ultra-high strength steel strip has a composition consisting of, in wt.%, C: 0.220% to 0.280%, Mn: 1.20% to 2.20%, Ti: 0.050% to 0.150%, Mo: 0.10% to 0.50%, Si: up to 1.20%, Cr: up to 0.30%, Al: up to 1.0%, P: up to 0.02%, S: up to 0.01%, N: 0.0020 to 0.010%, Nb: up to 0.07%, Sn: up to 0.08%, B: up to 0.007%, Cu: up to 0.030%, V: up to 0.10%, Ni: up to 0.050%, and balance Fe and inevitable impurities resulting from the ironmaking and steelmaking process, and wherein parameter Pern is at least 0.30, and with more preferred ranges as herein described and claimed.

[0089] In an embodiment the hot-rolled ultra-high strength steel strip has a composition consisting of, in wt.%, C: 0.220% to 0.250%, Mn: 1.60% to 2.00%, Ti: 0.060% to 0.130%, Mo: 0.18% to 0.30%, Si: 0.35% to 1.0%, Cr: up to 0.03%, Al: 0.03% to 0.70%, P: up to 0.02%, S: up to 0.01%, N: 0.0020% to 0.010%, Nb: up to 0.003%, Sn: up to 0.050%, B: up to 0.007%, Cu: up to 0.030%, V: up to 0.10%, Ni: up to 0.050%, and balance Fe and inevitable impurities resulting from the ironmaking and steelmaking process, and wherein parameter Pern is at least 0.30, and with more preferred ranges as herein described and claimed.

[0090] In an embodiment the thickness of the hot-rolled ultra-high strength steel strip is in a range of about 1.5 to 8 mm, and more preferably of about 1.8 to 6 mm, and most preferably of about 1.8 to 4.5 mm.

[0091] The hot-rolled ultra-high strength steel strip product can be a bare product or uncoated product or it can be provided on one or both of its main surfaces with a thinmetallic coating layer, typically up to about 100 g / m2per side of the steel strip, and preferably up to about 50 g / m2per side. The metallic coating is preferably selected from the group comprising an aluminium alloy coating (e.g., an Al-Si alloy, or Al-Zn alloy), a magnesium alloy coating, a zinc coating, and a zinc alloy coating (e.g., a Zn-AI alloy, Zn-Mg alloy, Zn-Fe alloy, Zn-AI-Mg alloy, or Zn-Mg-AI alloy).

[0092] In another aspect of the invention it relates to a method of manufacturing a hot-rolled ultra-high strength steel strip as herein described and claimed, the method comprising the steps of, in that order,

[0093] casting a slab having a composition as set out in this description and any of the claims, followed by the step of reheating the solidified slab to a temperature between 1050°C and 1260°C and hot rolling said slab, or casting a slab or strip followed directly by the step of hot rolling said slab or strip;

[0094] hot rolling the steel slab or strip and finishing said hot rolling at a finish rolling temperature (FRT) between 1080°C and Ar3+20°C, preferably between 1000-900°C, more preferably between 1000-920°C, and most preferably between 1000-940°C; and where Ar3 is the temperature at which transformation of austenite to ferrite starts during cooling and can be calculated according to the following equation, elements concentrations are in wt.%:

[0095] Ar3 = 910°C - 203x[C]1 / 2+ 44.7x[Si] + 400x[AI] + 700x[P]- 30x[Mn] +31.5x[Mo] - 11 [Cr] + 400x[Ti] + 104x[V];

[0096] accelerated cooling of the hot-rolled strip with an average cooling rate of between 120 to 30 °C / s to a cooling stop temperature (TexitROT) in a range of 300°C to 500°C, preferably in a range of 350°C to 480°C, and more preferably of 400°C to 450°C;

[0097] coiling the hot-rolled and cooled strip at a coiling temperature (CT) in a range of 350°C and 500°C, preferably in a range of 350°C to 480°C, and more preferably in a range of 350°C to 450°C;

[0098] allowing the coiled hot-rolled strip to further cool down to ambient temperature, preferably to room temperature;

[0099] optionally pickling of the hot-rolled steel strip; andoptionally providing the hot-rolled steel strip with a metallic coating layer, preferably selected from the group comprising: a Zn-layer, Zn-based alloy layer, a Mg-alloy layer, an Al-based alloy layer, and preferably being applied by means of heat-to-coat, hot-dip coating, or electrolytic deposition. In a preferred embodiment the hot-rolled steel strip is provided with a Zn-layer or Zn-based alloy layer by means of a heat-to-coat or hot-dip coating process.

[0100] The method of manufacturing herein described and claimed results in the desired microstructure providing for the aimed improved balance of high yield strength, high ultimate tensile strength, with an adequate combination of global and local formability (high edge-crack resistance under tensile stress) together with high edge-crack resistance under compression, and an adequate level of fracture toughness. An important advantage is also that due to the microstructure obtained the steel strip is significantly less sensitive to loss of strength when subjected to a heat treatment as a result of applying a metallic coating in a heat-to-coat cycle or hot-dip coating process.

[0101] Furthermore, and very advantageously, it has been found that the method offers a fairly broad hot-rolling process temperature window and thereby has proven to be less sensitive for operational scatter when applied on an industrial scale for manufacturing the hot-rolled ultra-high strength steel strip according to this invention.

[0102] The invention is also embodied in a steel strip manufactured by the method described herein and claimed having said composition, microstructure and improved balance of mechanical properties.

[0103] Some reasons, preferred embodiments, and advantages of the various method steps are set out below:

[0104] The invention is not limited by the casting method. The steel can be cast as a conventional thick-slab having a cast thickness of between 150-350 mm, and typically of 225-250 mm, as well as a thin-slab having a cast thickness of between 50-150 mm in a direct strip plant. For conventional thick-slab casting, reheating of the slab is necessary to reheat the slab from ambient temperatures (usually the thick cast slabs have cooled down from the casting temperature to ambient temperatures in a slab yard) and to homogenise the slab with respect to composition, and therefore the reheating temperature should be above 1050°C also to dissolve any precipitates when micro-alloying elements are present and to bring the slab to such a temperature thatthe final hot rolling in the finishing mill can still be performed such that the finish rolling temperature is between 1080°C and Ar3+20°C, and with preferred narrower ranges as herein described. Often this requires a (slab) reheating temperature of between about 1050-1260°C. For thin-slab casting the cast slab is subjected to a homogenisation treatment in a homogenising furnace immediately after casting the thin slab wherein the homogenisation temperature should be above 1050°C, and is typically about 1100-1160°C. This would also prevent any precipitates from forming when micro-alloying elements, if any, are present.

[0105] The hot rolling of the steel must be carried out in the austenitic phase to control the final microstructure. A high finish rolling temperature (FRT) is beneficial to suppress anisotropy and to reduce planar anisotropy with optimised minimum r-value (for hot-rolled steels the r0or r90). A lower degree of anisotropy, planar anisotropy and higher rmin promotes a higher hole-expansion capacity. However, a high finish rolling temperature results in less grain refinement, which in turn impairs fracture toughness. To optimize the balance between hole-expansion capacity and fracture toughness, it is desirable to have a high enough finish rolling temperature combined with fast cooling after hot-rolling and low enough coiling temperature to promote grain refinement. On the other hand, the transformation temperatures should be high enough to promote a bainitic microstructure that is still sufficiently strengthened with TiC and / or TiMoC precipitates.

[0106] The run-out-table cooling process and coiling temperature need to be as herein stated to promote the formation of a bainitic microstructure and some degree of TiC and / or TiMoC precipitation to strengthen that matrix. The run-out-table cooling process comprises accelerated cooling of the hot-rolled strip with an average cooling rate of between 120-30 °C / s to a cooling stop temperature (TexitROT) in a range of 300-500°C, preferably in a range of 350-480°C, and more preferably of 400-450°C. The cooling stop temperature should be low enough to promote bainite without an excessive formation of martensite and be high enough to promote diffusion and provide some kinetics for the formation of precipitates. Once active water cooling has stopped on the run-out-table, residual latent heat from the phase transformation can be used to provide thermal energy to promote the formation of TiC and / or TiMoC precipitates to further strengthen the final microstructure and to reach the desired final coiling temperature.

[0107] It is an important feature of the invention that the coiling temperature of the hot-rolled strip is in a range of 350-500°C, preferably in a range of 350-480°C, and morepreferably in a range of 350-450°C. The relatively high coiling temperature in combination with the accelerated cooling, the cooling stop temperature and the steel composition ensures that the required bainitic microstructure is achieved. Thus the coiling temperature is in the bainitic region of the subject steel, a relatively high coiling temperature is desirable to have sufficient kinetics for diffusion and subsequently precipitation to further strengthen the final microstructure. A relatively high coiling temperature in the bainitic region will also help to ensure that the influence of a postrolling heat-treatment such as heat-to-coat or hot-dip coating process will have less influence on the properties, in particular the strength of the steel strip. This contrary to martensitic steels coiled at substantially lower coiling temperatures and which are very sensitive to loss in strength upon a heat-to-coat cycle or hot-dip coating process due to martensitic tempering or even recrystallidation.

[0108] After the steel strip has cooled to room temperature, the oxides (scale) on the hot-rolled steel strip are removed either by pickling in an acid solution (e.g., HCI) at warm temperatures (about 80-120°C) or by a combination of pickling and mechanical brushing of the strip surface. This step is necessary for rendering the steel strip surface suitable for direct use as uncoated hot-rolled steel or making it amenable to the coating process, when optionally needed for improved corrosion resistance.

[0109] In an embodiment the thickness of the hot-rolled ultra-high strength steel strip is in a range of about 1.5 to 8 mm, and more preferably of about 1.8 to 6 mm, and most preferably of about 1.8 to 4.5 mm.

[0110] It is an aspect of the invention that the hot-rolled steel strip is subsequently not subjected to a cold rolling operation having a thickness reduction of more than 1.5%. Thus optionally, the hot-rolled and optionally coated 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 1.5%.

[0111] The hot-rolled ultra-high strength steel strip product can be a bare product or uncoated product or it can be 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 50 g / m2per side. The metallic coating is preferably selected from the group comprising an aluminium alloy coating (e.g., an Al-Si alloy, or Al-Zn alloy), a magnesium alloy coating, a zinc coating, and a zinc alloy coating (e.g., a Zn-AI alloy, Zn-Mg alloy, Zn-Fe alloy, Zn-AI-Mg alloy, or Zn-Mg-AI alloy). More preferably the metallic coating is a zinc or zinc-alloy coating.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 standard Gl 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% Al, the remainder being zinc and unavoidable impurities.

[0112] In another embodiment the metallic coating comprises a (commercially pure) aluminium layer or an aluminium alloy layer. A typical metal bath for hot-dip coating such an aluminium layer comprises aluminium alloyed with silicon e.g. aluminium alloyed with about 8-11% of silicon and at most about 4% of iron, optionally at most 0.2% of one or more additional elements such as calcium, unavoidable impurities, the remainder being aluminium. Silicon is present in order to prevent the formation of a thick iron-metallic intermetallic layer which reduces adherence and formability. Iron is preferably present in amounts between about 1% and 4%, more preferably at about least 2%.

[0113] In an aspect of the invention it relates to a galvanized steel strip obtained by hot dip galvanizing the hot-rolled ultra-high strength steel strip according to this invention.

[0114] In another aspect of the invention it relates to an automotive component, in particular an automotive chassis part, made from or incorporating the hot-rolled ultra-high strength steel strip according to this invention and taking benefit from amongst others the improved balance of high yield strength, high ultimate tensile strength, with an adequate combination of global and local formability (high edge crack resistance under tensile stress) together with high edge-crack resistance under compression, and an adequate level of fracture toughness. The steel strip can be shaped into an automotive component in a cold forming operation as known in the art, in particular via cold-stamping, roll-forming or warm-forming. 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 and lids for electrical vehicles, all having an intricate shape. By using the hot-rolled high-strength steel strip, these components can be fabricated with high quality, cost efficiently and with high yields. The hot-rolled ultra-high strength steel product according to this invention can be used also for engineering applications.In yet another aspect of the invention it relates to an automotive chassis part, preferably being provided with a zinc or zinc-alloy coating, using the hot-rolled ultra-high strength steel strip according to and / or produced according to the invention, and preferably being manufactured via cold-stamping or roll-forming.

[0115] DESCRIPTION OF THE FIGURES

[0116] The invention shall also be described with reference to the appended non-limiting figures, in which:

[0117] Fig. 1A and Fig. 1B show examples of MisOrientation Distribution (MOD) profiles used to explain the physical meaning and definition of the MOD index to characterise the matrix of the microstructure in a quantitative manner.

[0118] The invention will now be illustrated with reference to non-limiting comparative and examples according to the invention.

[0119] EXAMPLES

[0120] Ingots of ten steels A to I of dimensions 320x100x100 mm were cast by melting charges in a vacuum induction furnace. All the ingots were reheated for 1 hour at 1240°C and rough-rolled to 35 mm thickness and left to air cool to ambient temperature before sawing into smaller blocks of 75x105x35 mm. Then, these blocks were reheated again to 1240°C for 40 minutes, and hot rolled to their final thickness of about 3.0 to 3.7 mm in 4 rolling passes. The chemical compositions of these steels having a thickness (t) of about 3 mm are given in Table 1. Steel compositions G and H are comparative compositions due to their too low a carbon content, a corresponding low Pern, and too low a N content. Steel I is comparative in view of too low a N content, and Steel A to F are according to the invention.

[0121] The hot rolling starts at temperature Tentry, and after the final rolling pass at a finish rolling temperature (FRT), the hot rolled steels were transferred to the run-out table with a start temperature run-out table (TentryROT) and actively cooled from the austenitic phase field with a mixture of water and air to an end temperature in the ferritic phase field at the run-out table (T6XitR0T) at a cooling rate in the range of 30 to 70°C / s. Next, the steels were transferred to a furnace to replicate slow coil cooling. This was done with furnace temperatures (CT - coiling temperature) of 400 and 450°C. The process settings are listed in Table 2.In this experimental set up it is possible that the exit temperature at the run-out table (TexitRoi) is lower than the coiling temperature (CT). It is envisaged that this can also occur in industrial practice whereby the hot rolled steel increases in temperature due to the release of latent heat of the phase transformation on the run-out-table prior to coiling, resulting in a higher coiling temperature.

[0122] Table 1. Compositions (in wt.%) and hardenability parameter Pern. Balance Fe and impurities.

[0123]

[0124] For all hot-rolled steels the mechanical properties and microstructural features have been determined using the methods as herein described.

[0125] Table 2 lists the mechanical properties (Rp0.2, Rm, A50, r0, and the HEC) and Table 3 lists the microstructure (type, average effective grain size of the bainite matrix GSN.15, and the MOD). In Table 3 “LB” stands for lower bainite with carbide precipitation inside laths, and “UB” stands for upper bainite with carbide precipitation on lath boundaries. All samples showed a combined LB+UB of more than 97 vol.%.

[0126] From the results of Table 2 it can been seen that Alloys G and H having a low Pern while processed according to the invention provide too low a yield strength and ultimate tensile strength.

[0127] Alloy E having a high Pern due to a high Si content provides a very high yield strength and ultimate tensile strength and is a good representative of the embodiment wherein Rp0.2 is in a range of 1150-1450 MPa and Rm in a range of 1350-1550 MPa. The increase in Rm is surprisingly significantly higher than expected from the addition of about 0.55% Si compared to alloy A. The skilled person would expect an increase of about 40-45 MPa for the Rm, but compared to alloy A the increase is about 80-90 MPa for the Rm when processed in about the same manner. For this embodiment the Sicontent is preferably in a range of 0.60 to 1.20%, more preferably in a range of 0.70 to 1.20%, and most preferably in a range of 0.70 to 1.0%.

[0128] From a comparison of Alloy A and B it can be seen that lowering the Ti content lowers the yield strength and the ultimate tensile strength, but in general increases the revalue and HEC. Too low a Ti content would results in too low mechanical strength.

[0129] From a comparison of Alloy A and C it can be seen that increasing the Cr content in general has a modest beneficial effect on the hole-expansion capacity, while providing steels that meet Rp0.2 of 950-1250 MPa and Rm of 1150-1350 MPa. Since Cr, like Mn, increases hardenability but acts as a ferrite stabiliser rather than an austenite stabiliser like Mn, adding Cr helps improve microstructural homogeneity. This enhanced homogeneity can, in turn, increase the steel's hole-expansion capacity.

[0130] From a comparison of Alloy A and D it can be seen that increasing the Mo content in general has a modest beneficial effect on the hole-expansion capacity, while providing steels that meet Rp0.2 of 950-1250 MPa and Rm of 1150-1350 MPa. Since Mo, like Mn, increases hardenability but acts as a ferrite stabiliser rather than an austenite stabiliser like Mn, adding Mo helps improve microstructural homogeneity. This enhanced homogeneity can, in turn, increase the steel's hole-expansion capacity.

[0131] From a comparison of Alloy A and D it can be seen that increasing the Mo content allows the use of a lower C content while still producing steels that meet Rp0.2 of 950-1250 MPa and Rm of 1150-1350 MPa with in some cases an increase in holeexpansion capacity.

[0132] From a comparison of Alloy A and G it can be seen that increasing the Al content has no significant effect on the yield and tensile strength though Al is known to reduce the hardenability. Even with a relatively high Al addition of 0.585% compared to alloy A, alloy G is still able to provide steels that meet Rp0.2 of 950-1250 MPa and Rm of 1150-1350 MPa.

[0133] For Alloy I it can be seen that a high FRT of 970°C and a low CT of 400°C does provide the required strength levels (see steel 28). A low FRT of 875°C in combination with a low CT of 400°C does not provide the required Rm (steel 29). A high FRT of 980°C in combination with a high CT of 450°C does not provide the required Rp0.2 and Rm (steel 30). A low FRT of 895°C in combination with a high CT of 450°C does not provide the required Rp0.2 and Rm (steel 31). Whereas alloy A having a higher N content compared to steel J provides the required strength levels irrespective of highor low FRT or high or low CT (steel 1 to 4). This illustrates that the alloy A when processed according to the invention provides the required strength levels and very advantageously that the hot-rolled steel strip can be manufactured using a broad hot-rolling process temperature window and thereby is less sensitive for operational scatter when processed on an industrial scale. Although not fully understood, it believed that the increased N content results in more hardenability, also at relatively low finish hot roll temperature (FRT).

[0134] The hypothetical mechanism to explain this observation is that an increased N content, promotes the formation or presence of TiN, TiCN, and AIN precipitates during reheating and hot rolling, which are capable to pin austenite grain boundaries and delay or even arrest recrystallisation and subsequent austenite refinement during hot rolling. This in turn promotes larger austenite grains, which leads to fewer potential nucleation sites for ferrite formation and delays the austenite-to-ferrite phase transformation. This may explain increased hardenability with increased N content under equal process conditions with regard to e.g. finish rolling temperature (FRT), time-temperature cooling profile on the run-out-table, and coiling temperature.

[0135] The degree of hardenability is also influenced by process conditions, of which the FRT is a major element. A lower FRT will in general promote more nucleation sites for ferrite formation and lead to a decrease in hardenability and hence a decrease in (transformation) strength. In contrast, a higher FRT will in general lead to less nucleation sites for ferrite formation and lead to an increase in hardenability and hence an increase in (transformation) strength. By using an increased N content to promote the formation or presence of TiN, TiCN, and AIN precipitates during reheating and hot rolling, increased hardenability apparently is also realised for lower FRT, which favourably widens the industrial hot rolling process window for manufacturing the hot-rolled ultra-high strength steel strip according to this invention.

[0136] From these results it can be seen that the hot-rolled ultra-high strength steel strip according to the invention is an ideal candidate to manufacture an automotive component, in particular an automotive chassis part as it benefit from amongst others the improved balance of high yield strength, high ultimate tensile strength, with an adequate combination of global and local formability (high edge crack resistance under tensile stress) together with high edge-crack resistance under compression, and an adequate level of fracture toughness. The steel strip can be shaped into an automotivecomponent in a cold forming operation as known in the art, in particular via coldstamping, roll-forming or warm-forming.

[0137] Table 2. Process settings and mechanical properties.

[0138]

[0139] Table 3. Process settings and microstructural features.

[0140]

Claims

CLAIMS1. Hot-rolled ultra-high strength steel strip having a composition, in wt.%,C: 0.220 to 0.280%,Mn: 1.20 to 2.20%,Ti: 0.050 to 0.150%,Mo: 0.10 to 0.50%,Si: up to 1.20%,Cr: up to 1.0%,Al: up to 1.0%,P: up to 0.02%,S: up to 0.01%,N: 0.0020 to 0.020%,Nb: up to 0.10%,Sn: up to 0.08%,B: up to 0.007%;optionally one or more elements selected from the group consisting of: Cu: up to 0.20%, V: up to 0.30%, Ni: up to 0.50%;and balance Fe and inevitable impurities resulting from the ironmaking and steelmaking process, and wherein parameter Pern is at least 0.30, withand wherein the steel strip has a microstructure consisting at ! thickness of: at least 95 vol.% bainite, consisting of a mixture of upper and lower bainite, and less than 5 vol.% of secondary phase constituents, including any carbides, cementite, martensite and retained-austenite, andan average effective grain size of the bainite matrix of at most 1.5 pm, and a MisOrientation Distribution (MOD) index of at least 1.0;and wherein the steel strip has at least the following mechanical properties: A50 tensile elongation of at least 5%, andrevalue of at least 0.6, anda hole-expansion capacity (HEC) of at least 30%, and2. Hot-rolled steel strip according to claim 1, wherein Pern is at least 0.32, preferably at least 0.34, and more preferably at least 0.36.

3. Hot-rolled steel strip according to claim 1 or 2, wherein the hot-rolled steel strip has one or more of, and preferably all of:A50 tensile elongation of at least 6%, preferably of at least 7%;r0-value of at least 0.7, preferably of at least 0.75;a hole-expansion capacity (HEC) of at least 40%, preferably of at least 50%.

4. Hot-rolled steel strip according to any one of claims 1 to 3, wherein the microstructure has an average effective grain size of the bainite matrix of at most 1.4 pm, and preferably the microstructure has a MisOrientation Distribution (MOD) index of at least 1.1.

5. Hot-rolled steel strip according to any one of claims 1 to 4, wherein the Si- content is in a range of 0.35% to 1.0%, and preferably of 0.35% to 0.80%.

6. Hot-rolled steel strip according to any one of claims 1 to 5, wherein the Ti- content is in a range of 0.060% to 0.130%.

7. Hot-rolled steel strip according to any one of claims 1 to 6, wherein the Mo- content is in a range of 0.18% to 0.30%, and preferably of 0.22% to 0.26%.

8. Hot-rolled steel strip according to any one of claims 1 to 7, wherein the steel composition comprises:Cr: up to 0.03%;Nb: up to 0.003%;Sn: up to 0.050%, preferably up to 0.030%;Cu: up to 0.030%;V: up to 0.10%;Ni: up to 0.050%, preferably up to 0.030%.

9. Hot-rolled steel strip according to any one of claims 1 to 8, wherein the steel composition comprises, in wt.%:C: 0.220% to 0.250%;Mn: 1.60% to 2.00%, preferably 1.80% to 2.00%;Ti: 0.060% to 0.130%;Mo: 0.13% to 0.35%, preferably 0.18% to 0.30%;Si: 0.30% to 1.20%, preferably 0.35% to 0.80%;Cr: up to 0.3%;Al: 0.03% to 1.40%, preferably 0.03% to 1.20%;N: 0.0020% to 0.010%, preferably 0.0030% to 0.020%.

10. Hot-rolled steel strip according to any one of claims 1 to 9, wherein the hot-rolled steel strip is provided with a metallic coating layer, preferably selected from the group comprising: a Zn-layer, a Zn-based alloy layer, a Mg-based alloy layer, an Al-based alloy layer.

11. Method of manufacturing a hot-rolled ultra-high strength steel strip according to any one of claims 1 to 10, the method comprising the steps of:casting a slab having a composition according to any one of claims 1, 2, or 6-9, followed by the step of reheating the solidified slab to a temperature between 1050°C and 1260°C and hot rolling said slab, or casting a slab or strip followed directly by the step of hot rolling said slab or strip; hot rolling the steel slab or strip and finishing said hot rolling at a finish rolling temperature between 1080°C and Ar3+20°C;accelerated cooling of the hot-rolled strip with an average cooling rate of between 120 to 30 °C / s to a cooling stop temperature (TexitROT) in a range of 300°C to 500°C, preferably in a range of 350°C to 480°C; coiling the hot-rolled and cooled strip at a coiling temperature between 350°C and 450°C;allowing the coiled hot-rolled steel strip to further cool to ambient temperature;optionally pickling of the hot-rolled steel strip; andoptionally providing of the hot-rolled steel strip with a metallic coating layer, preferably selected from the group comprising: a Zn-layer, Zn-based alloy layer, a Mg-alloy layer, an Al-based alloy layer, and preferably being applied by means of heat-to-coat, hot-dip coating, or electrolytic deposition.

12. Method according to claim 11, wherein the coiling temperature is in a range of 350°C to 480°C, and preferably of 350°C to 450°C.

13. Method according to claim 11 or 12, wherein the cooling stop temperature (TexitROT) in a range of 350°C to 480°C, preferably in a range of 400°C to 450°C.

14. Method according to any one of claims 11 to 13, wherein the hot-rolled steel strip is provided a metallic coating layer selected from the group comprising: a Zn- layer, Zn-based alloy layer, an Al-based alloy layer, and being applied by via heat-to-coat or hot-dip coating.

15. An automotive component made from the hot-rolled ultra-high strength steel strip according to any one of claims 1 to 10 or made from the hot-rolled ultra-high strength steel strip obtainable by the method according to any one of claim 11 to

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