HOT-ROLLED HIGH-TENSILE STRENGTH STEEL SHEET WITH EXCELLENT LOW-TEMPERATURE IMPACT TOUGHNESS AND METHOD FOR MANUFACTURING THE SAME FIELD OF THE INVENTION This invention relates to a hot-rolled high-strength low-alloy steel (HSLA) with excellent low-temperature impact toughness. The invention relates also to a method of manufacturing such a thick hot-rolled high-strength low-alloy steel (HSLA) with excellent low-temperature impact toughness. BACKGROUND TO THE INVENTION A general trend in steel development for engineering applications is towards higher strength and higher low-temperature impact toughness combined with good weldability. Conventional and standard heavy plate steels have been traditionally produced with relative high carbon levels to obtain sufficient strength. Due to the high carbon content these steels have deteriorated weldability, poor toughness, in particular impact toughness at low temperature, e.g. at minus 20 ^o C or lower. High-strength low-alloy steel (HSLA) is a type of steel that provides better mechanical properties while maintaining good weldability than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather specific mechanical properties. The mechanical properties are strongly dependent on the thickness of the steel sheet or steel plate. For thicknesses greater than 20 mm, cooling in the centre of the sheet is slower than near the surface, thus favouring grain growth and resiling in decreasing low temperature toughness values. Commonly these HSLA steels have a carbon content in a broad range of about 0.02-0.25 wt.% to retain formability and weldability. Other alloying elements include up to about 2.0 wt.% manganese together with one or more elements selected from the group comprising: copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare-earth elements, and zirconium. Copper, titanium, vanadium, and niobium are added for strengthening purposes. These elements are intended to alter the microstructure of carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of carbides which increases the material's strength by refining the grain size and by precipitation strengthening. Their yield strengths are generally in a range of about 250 to 600 MPa. HSLA steels are used in cars, trucks, cranes, bridges, hydraulic tools, mining and earth-moving equipment, load-handling equipment, and other structures that are designed to handle large amounts of stress or need a good strength-to-weight ratio. Patent document CN114457282-A discloses a hot-rolled steel plate having a thickness of 6.0 to 12.0 mm for the longitudinally-cut welded pipe with the yield strength of 415 MPa and comprising, wt.%: 0.04%-0.06% of C, 0.09%-0.16% of Si, 0.90%-1.00% of Mn, smaller than or equal to 0.015% of P, smaller than or equal to 0.004% of S, smaller than or equal to 0.0060% of N, 0.010%-0.045% of Al, 0.01%-0.02% of Ti, 0.045%-0.055% of Nb, and the Mn/Si ratio is 5-10, the balance of iron and inevitable impurities. Patent document KR20210153330-A discloses a hot rolled steel material having excellent low-temperature toughness and a low yield ratio and a manufacturing method thereof. The hot rolled steel material having a low yield ratio comprises, in wt.%: 0.04-0.07% of carbon (C); 0.15- 0.25% of silicon (Si); 1.10-1.50% of manganese (Mn); 0.01-0.05% of soluble aluminum (S_Al); 0.02-0.03% of niobium (Nb); 0.20-0.30% of chromium (Cr); 0.001-0.003% of calcium (Ca); more than 0 to 0.018% of phosphorus (P); more than 0 to 0.003% of sulfur (S); and the remainder consisting of iron (Fe) and inevitable impurities, wherein the yield ratio (YR) satisfies 0.80-0.90 and the value of a drop weight tear test (DWTT) at the temperature of 0-20°C satisfies 85-100%. Patent document US10,584,405-B2 discloses an electric resistance welded steel pipe having a composition comprising, in wt.%, C: 0.025 to 0.168%, Si: 0.10 to 0.30%, Mn: 0.60 to 1.90%, P: 0.001 to 0.018%, S: 0.0001 to 0.0029%, Al: 0.010 to 0.10%, Ca: 0.0001 to 0.0035%, N: 0.0050% or less, O: 0.0030% or less, Nb: 0.001 to 0.070%, and Ti: 0.001 to 0.033%, with the balance being Fe and unavoidable impurities, wherein Pcm defined by formula (1) is 0.20 or less, and having a structure, in each of a base material portion and an electric resistance weld zone, that includes a quasi-polygonal ferrite phase having an average grain size of 10 μm or less and serving as a primary phase at a volume fraction of 90% or more, with the balance being a secondary phase at a volume fraction of 10% or less, wherein the base material portion has a yield strength YS of 400 MPa or more in an axial direction of the electric resistance-welded steel pipe, wherein electric resistance weld zone toughness of the electric resistance welded steel pipe is such that an absorbed energy vE−60 in a Charpy impact test performed at a test temperature of −60°C according to specifications of JIS-Z-2242 is 110 J or more and that a CTOD value in a CTOD test performed at a test temperature of 0°C according to specifications of BS 7448-1995 is 0.80 mm or more, and wherein no leakage occurs in an internal pressure test performed under conditions of a test temperature of 0°C and an internal pressure of 0.95×(yield strength at room temperature σyRT): Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, where C, Si, Mn, Cu, Ni, Cr, Mo, V, and B represent contents (in wt.%) of respective elements, which is made zero for an element not contained. There is a demand for thicker gauge steel sheets suitable for engineering applications having high strength, high impact toughness at low temperatures and which can be produced in an economical favourable manner. It is an object of the invention to provide a hot-rolled high-strength low-alloy steel (HSLA) with excellent low-temperature impact toughness. It is another object of the invention to provide a method of manufacturing such a hot- rolled high-strength low-alloy steel (HSLA) with excellent low-temperature impact toughness. DESCRIPTION OF THE INVENTION As will be appreciated herein, for any description of steel compositions or preferred steel compositions, all references to percentages are by weight percent (wt.%) 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. The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular steel component to which it refers. For example, up to 0.02% V may include a steel strip composition having no V. This and other objects and further advantages are met or exceeded by the present invention providing a hot-rolled high-tensile strength steel sheet (HSLA) having a composition consisting of, in wt.%: C 0.020-0.070%, Mn 0.80-1.40%, Nb 0.010-0.080%, Ti up to about 0.05%, Si 0.005-0.10%, Al up to about 0.10%, N up to about 0.010%, P up to about 0.02%, S up to about 0.01%, B up to about 0.0005%, V up to about 0.05%, preferably up to 0.02%, Mo up to about 0.05%, preferably up to 0.02%, Cr up to about 0.05%, preferably up to 0.02%, Co up to about 0.05%, preferably up to 0.02%, balance Fe and unavoidable impurities; and further optionally one or two alloying elements selected from the group consisting of: 0.05-0.50% Cu, and 0.05-0.50% Ni; wherein the steel sheet between t/10 and 9t/10, wherein t is the sheet thickness, has a microstructure, in vol.%: ferrite > 95 vol.%, and at most 5 vol.% of second-phase constituents including any martensite, cementite, pearlite, and bainite; and an average Feret grain diameter between 5 µm and 20 µm, preferably between 5 µm to 15 µm, at position between t/4 and 3t/4-sheet thickness; and the steel sheet has at least the following properties: - yield strength (Rp0.2) of more than 370 MPa in the L-direction, - ultimate tensile strength (Rm) of more than 450 MPa in the L-direction, - elongation at fracture (A50) > 30% in the L-direction, - Charpy V-notch impact toughness of more than 250 J/cm ² when measured in the L- and T-directions at -20 ^o C. In accordance with the invention it has been found that the combination of the narrow compositional ranges with the method of manufacturing by carefully controlling the temperatures of the hot rolling process and the coiling process produces a hot-rolled steel sheet (HSLA) providing a metallographic microstructure, in particular in its centre, offering a favourable balance of mechanical properties, including a high impact toughness at low- temperatures, both at -20 ^o C and also at -40 ^o C and illustrating the low anisotropy in the mechanical properties. The steel sheet composition avoids the need for expensive alloying elements, in particular V, Mo, Cr, Co, and rare-earth elements, making the steel sheet economically more cost effective. Also, the need for alloying elements like Ni and Cu can be avoided, making the steel sheet even more cost effective. In an embodiment of the hot-rolled steel sheet it has a thickness of 22 mm or more, and preferably of 23 mm or more. In an embodiment the thickness of the hot-rolled steel sheet is not more than about 40 mm, and preferably of not more than about 30 mm. In a preferred embodiment the hot-rolled steel sheet has a thickness in a range of about 23 to 28 mm. The L-direction is the longitudinal or rolling direction of the steel sheet, and the T- direction represents the transverse direction or transverse to the rolling direction. In an embodiment of the hot-rolled steel sheet it has a yield strength of more than 380 MPa in the L-direction. In an embodiment it has a yield strength of more than 370 MPa in the T- direction, and preferably more than 380 MPa in the T-direction. In an embodiment of the hot-rolled steel sheet it has an ultimate tensile strength of more than 460 MPa in the L-direction, and more preferably more than 470 MPa in the L-direction. In an embodiment it has an ultimate tensile strength of more than 450 MPa in the T-direction, and preferably also more than 460 MPa in the T-direction. In an embodiment of the hot-rolled steel sheet it has an elongation at fracture (A50) of more than 33% in the L-direction, and preferably of more than 35%. In an embodiment of the hot-rolled steel sheet it has a Charpy V-notch impact toughness of more than 300 J/cm ² , and preferably of more than 340 J/cm ² , in the L-direction and when measured at -20 ^o C. In an embodiment of the hot-rolled steel sheet it has a Charpy V-notch impact toughness of more than 300 J/cm ² , and preferably of more than 340 J/cm ² , in the T-direction and when measured at -20 ^o C. The hot-rolled steel sheet according to the invention has very low anisotropy for the Charpy V-notch impact toughness values. In an embodiment of the hot-rolled steel sheet it has a Charpy V-notch impact toughness of more than 250 J/cm ² , and preferably of more than 300 J/cm ² , and more preferably of more than 340 J/cm ² , in the L-direction and when measured at -40 ^o C. The 0.2% proof strength (Rp0.2) or yield strength (Rp), ultimate tensile strength (Rm), uniform elongation (Ag) and tensile elongation or elongation at fracture (A50) are determined from tensile tests at room temperature with A50 specimen geometry with tensile testing parallel to the rolling direction and to the transverse direction according to ISO 6892-1 B-method. The toughness or impact toughness at low-temperatures is determined by Charpy V- notch impact toughness tests according to the standard ASTM A370, where specimen dimensions are 55x10x10 mm (L,H,W). The sampling is made at 2 mm from the sheet surface and the samples have a width of 10 mm, such that for a sheet thickness of >10 mm at least 75% of the microstructure of the test bars correspond to the microstructure developed at 3t/4 and t/2, depending on the sheet thickness, and hence illustrates the effect of the microstructure on low-temperature impact toughness. An important feature of steel strip according to this invention is its microstructure consisting of, in volume percentages or vol.%, at the position between t/10 and 9t/10, wherein t is the sheet thickness: ferrite >95 vol.% and at most 5 vo.% of second-phase constituents, including any martensite, cementite, pearlite, and bainite. In a preferred embodiment the amount of ferrite >97 vo.% and at most 3% of second-phase constituents, including any martensite, cementite, pearlite. In a more preferred embodiment the amount of ferrite >98 vol.% and at most 2 vol.% of second-phase constituents, including any martensite, cementite, pearlite. Another important feature of the microstructure of the steel sheet is also that it has an average Feret grain diameter between 5 µm and 20 µm at position between t/4- and 3t/4-sheet thickness. In an embodiment the average Feret grain diameter is in a range of 5 µm and 15 µm, and preferably in a range of 7 µm to 12 µm. The small grain size near the centre of the steel sheet is favourable for the balance in strength and impact toughness at very low temperatures. The hot-rolled steel sheet is low-alloyed with cost-efficient alloying elements such as C, Mn, Al, Nb, Ti, and Si. Expensive alloying elements as V, Mo, Cr, and Co, are not added to the steel sheet composition while still achieving high impact toughness properties at very low temperatures in the thick gauge steel sheet. In practice this means that each of these elements, i.e., V, Mo, Cr and Co can be tolerated up to about 0.05%, and preferably these are present, if present, to a level not exceeding 0.02% each, and more preferably not exceeding 0.010% each. Alloying elements Ni and Cu may be present, viz. each in a range of 0.05-0.50% Cu and 0.05-0.50% Ni. Ni is an element that may both improve strength and toughness in the steel sheet. Too high amounts adds to the costs of the steel as it is a rather expensive alloying element, and too high amount has an adverse effect on weldability. Cu is an element that may improve strength, while minimizing a decrease in toughness of the steel sheet. Too high a Cu content may have an adverse effect on the surface quality of the steel sheet. However, in a preferred embodiment each of Ni and Cu are not purposively added to the steel composition and can be tolerated up to about 0.05% each, and preferably these can be tolerated, if present, to a level not exceeding 0.02% each, and more preferably not exceeding 0.01% each. Carbon (C) should be present in the steel sheet in a range of 0.020% to 0.070% as it determines to a large extent the required strength level. A C content of less than 0.02% may lead to insufficient strength. In an embodiment, the lower limit for the C content is 0.030%, and more preferably it is 0.040%. However, C has detrimental effects on the weldability and impact toughness of the steel sheet. In an embodiment, the C content does not exceed 0.060%, and preferably does not exceed 0.055%. Manganese (Mn) in a range of about 0.80% to 1.40% is an important alloying element of the steel sheet and improves the balance between the strength and low-temperature impact toughness. Although a low Mn content promotes the formation of ferrite nucleus during the austenite to ferrite transformation in the two phases regions, i.e. between the A3 and A1 temperature, too low a Mn-content does not provide sufficient strength by solid strengthening to the steel sheet. In an embodiment the steel sheet has at least 1.05% Mn, and more preferably at least 1.10% Mn, to provide sufficient strength. In an embodiment the steel sheet has not more than 1.30% Mn to further improve the balance in strength and low-temperature impact toughness. Niobium (Nb) is an important alloying element of the steel sheet and should be in a range of about 0.010% to 0.080%. Nb forms carbides NbC and Nb(C,N) nano-precipitates that pin grain boundaries, and is considered to be a major grain refining element and thereby contributing to increasing the impact toughness. Nb contributes to the strength with precipitation strengthening too. In an embodiment the steel sheet has at least 0.015% Nb. In an embodiment the Nb content does not exceed 0.050%. Too high a Nb-content will not improve grain refinement any further. Too high a Nb-content will increase also the rolling load making hot rolling more difficult. In an embodiment of the steel sheet composition R-ratio and defined as C/(Nb+Ti+V) is less than 2.0, and more preferably less than 1.5. Too high a R-ratio results in a delay in Nb, Ti, and V carbides precipitation affecting the final microstructure. In an embodiment of the steel sheet composition it has a CEQ <280, and preferably <250, and wherein CEQ = C + Mn/6 + (C+V+S)/80 + (Nb+Ti)/150, with element concentrations in m.wt.%. CEQ stands for Carbon EQuivalent. These alloying elements adversely affect the impact toughness of the steel according to this invention. Too high a CEQ, e.g., by increasing the C or Mn content, will affect the microstructure by adversely increasing the volume fraction of second phase constituents and the Mn-segregation line at the strip mid-thickness. It has been found that by controlling the R-ratio and the CEQ, the steel sheet has become significantly less sensitive for fluctuations in the method of manufacturing resulting in an important reduction of the scatter between the values of Charpy V-notch impact toughness measured in the L-direction at a temperature of -20 ^o C for a plurality of steel sheets belonging to the same inspection lot. Titanium (Ti) is present in the steel sheet in a range of up to about 0.05%. Ti suppresses growth of grains during reheating and significantly improves low-temperature impact toughness. Ti is a strong carbide forming element which decreases C in solid solution and improves low- temperature impact toughness. Excessive addition of Ti may cause problems such as clogging of a nozzle during continuous casting or reduce the low-temperature impact toughness due to crystallization in the core area of the steel sheet. In an embodiment Ti is present in a range of 0.001% to 0.05%. Silicon (Si) should be present in the steel sheet in a narrow range of about 0.005% to 0.10%. Si is an element used as a deoxidizer and is an element contributing to improving strength via solution strengthening. However, excessive addition of Si may deteriorate low- temperature impact toughness and weldability, and thus, for the present invention the upper- limit is carefully controlled.For that reason the Si content does not exceed 0.10%, and more preferably does not exceed about 0.050%. Too high a Si content will concentrate C in the austenite phase and favours on cooling the formation of bainite/martensite which is detrimental to low-temperature impact toughness. Furthermore, too high a Si content will increase the risk of poor surface quality of the hot-rolled steel sheet. Aluminium (Al) in a range of up to 0.10%, and preferably in a range of about 0.005% to 0.10%, is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process. Al also removes N by forming stable AlN particles and provides grain refinement, which effects promote high impact toughness, especially at low temperatures. However, excess Al may increase non-metallic inclusions thereby deteriorating metal cleanliness. It may also cause clogging of a nozzle during continuous casting. In an embodiment the Al content is in a range of about 0.01% to 0.05%. Nitrogen, sulphur and phosphorus are residual elements present in the steel sheet as a result of steel making and refining process. Their amounts are limited to up to 0.01 wt.% S, up to 0.02 wt.% P, and up to 0.010 wt.% N (100 ppm). Amounts higher than these are detrimental for mechanical properties, formability, toughness, 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.% and more preferably only up to 0.0025 wt.%. In an embodiment the N content is up to 0.0060 wt.% (60 ppm). In an embodiment the hot-rolled steel sheet has a composition consisting of, in wt.%: C 0.020-0.070%, preferably in the range of 0.030-0.060%, Mn 0.80-1.40%, preferably in the range of 1.05-1.30%, Nb 0.010-0.080%, preferably in the range of 0.015-0.050%, Ti up to 0.05%, Si 0.005-0.10%, Al up to 0.10%, N up to 0.010%, P up to 0.02%, S up to 0.01%, B up to 0.0005%, elements selected from the group consisting of: V, Mo, Co, Cr, Cu and Ni, each up to 0.05%, preferably each up to 0.02%, and the balance being Fe and inevitable impurities resulting from the ironmaking and steelmaking process; and with preferred compositional ranges as herein described and claimed. In an embodiment the hot-rolled steel sheet has a composition consisting of, in wt.%: C 0.020-0.070%, preferably in the range of 0.030-0.060% C, Mn 0.80-1.40%, preferably in the range of 1.05-1.30% Mn, Nb 0.010-0.080%, preferably in the range of 0.015-0.050% Nb, Ti up to 0.05%, Si 0.005-0.10%, Al up to 0.10%, N up to 0.010%, P up to 0.02%, S up to 0.01%, B up to 0.0005%, and balance being Fe and inevitable impurities resulting from the ironmaking and steelmaking process; and with preferred compositional ranges as herein described and claimed. The thick hot-rolled steel sheet can be a bare product or it can be provided with a thin metallic coating layer to enhance corrosion protection. A thin metallic coating layer can be present typically up to about 250 g/m ² per side of the steel sheet, and preferably up to about 150 g/m ² . In an embodiment 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 zinc coating, and a zinc alloy coating, e.g., a Zn-Al alloy, Zn-Mg alloy, Zn-Fe alloy, Zn-Al-Mg alloy, or Zn-Mg-Al alloy. The metallic coating layer is preferably applied by means heat-to-coat or hot-dip coating. The invention is also embodied in a method of manufacturing a hot-rolled steel sheet as herein described and claimed, the method comprising the steps of, in that order, - casting a slab, followed by the step of reheating the solidified slab to a temperature between about 950-1280 ^o C, preferably for a time of about 30 minutes or more, and more preferably of about 60 minutes or more, and hot rolling said slab, or casting a slab or strip followed by the step of hot rolling said slab or strip; - hot rolling the steel slab or strip into a hot-rolled steel sheet, having preferably a thickness of at least 22 mm, and finishing said hot rolling at a finish rolling temperature between about 830 ^o C and 940 ^o C, preferably between about 840 ^o C and 940 ^o C, and more preferably between 850 ^o C and 920 ^o C. The finish hot-rolling temperature (FRT) is above the Ar3 temperature of the steel, where Ar3 is the temperature at which transformation of austenite to ferrite starts during cooling. As known in the art the Ar3 temperature can be calculated according to the following equation: Ar3 = 910 ^o C - 203 x [C] ^1/2 + 44.7 x [Si] - 30 x [Mn] ; - accelerated cooling the hot rolled steel sheet on a run-out table with a run-out table cooling rate between 3 to 80 ^o C/s, and preferably between 3 to 50 ^o C/s; - coiling of the hot-rolled and cooled sheet at a coiling temperature (CT) between 550 ^o C and 660 ^o C, preferably between 580 ^o C and 630 ^o C, and more preferably between 600-630 ^o C; - further cooling of the coiled hot-rolled steel sheet to ambient temperature; - optionally pickling the hot-rolled steel sheet; and - optionally providing the hot-rolled steel sheet with a metallic coating layer. Preferably selected from the group comprising: a Zn-layer, Zn-based alloy layer, an Al-based alloy layer, to provide improved corrosion resistance in service. The metallic coating layer is preferably applied by means of heat-to-coat or hot-dip coating. The method of manufacturing herein described and claimed results in the desired microstructure providing for the aimed improved balance of mechanical properties. The invention is also embodied in a hot-rolled steel sheet manufactured by the method described herein and claimed having said microstructure and improved balance of mechanical properties. 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 mm and 350 mm, and typically of 225 mm to 250 mm, as well as a thin-slab having a cast thickness of between 50 mm and 150 mm in 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 about 950°C also to dissolve any precipitates when microalloying elements are present and to bring the slab to such a temperature that the final hot rolling in the finishing mill can still be performed at FRT>Ar3. Often this requires a slab reheating temperature of between 950 ^o C up to about 1280°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 about 950 ^o C, and is typically about 1050 ^o C to 1160°C. This would also prevent any precipitates from forming when microalloying elements, if any, are present and also bring the thin slab to such a temperature that the final hot rolling in the finishing mill can still be performed at FRT>Ar3. According to the invention the reheating or the homogenisation time for the thin slab casting route is preferably about 30 minutes or more. The hot rolling of the steel must be carried out in the austenitic phase to control the final microstructure in the hot-rolled steel sheet. On an industrial scale of rolling the FRT should be kept above the Ar3 temperature. In a preferred embodiment the FRT is above Ar3+30 ^o C, e.g. typically above about 850 ^o C, to avoid hot rolling locally below Ar3 at colder edges or the tail of the strip. The FRT should not be too high in the austenite region. A FRT of above about 940 ^o C will result in amongst others increased edge crack sensitivity. Furthermore, a not too high FRT will also increase grain size in the microstructure. A lower FRT will promote more austenite deformation and hence contribute to pancake grain formation which adversely enhances the mechanical properties anisotropy. After hot rolling, the steel strip is cooled accelerated on a run-out table (ROT). An accelerated cooling rate is desired to suppress recovery and loss of internal stored energy in the austenite in order to promote grain refinement of the final microstructure. There is no critical run-out table cooling rate (ROT-CR) as long as the herein mentioned cooling rate is not exceeded through-thickness of the steel strip from a microstructure point of view. However, an unnecessarily high ROT-CR may affect the flatness of the strip after cooling and cause control problems to stop at the correct cooling step temperature and therefore a suitable maximum ROT-CR is about 80°C/s, preferably about 50°C/s and more preferably about 30°C/s. A practical ROT-CR range is 3-50 ^o C/s, and more preferably 3-30 ^o C/s, depending on steel sheet thickness, as this is achievable through air cooling, laminar cooling or water jet cooling depending on the thickness of the steel sheet. For practical reasons the run-out table cooling rate (ROT-CR) is defined as the average cooling rate of the surface of the steel sheet. Next, it is important that the hot-rolled steel sheet is coiled at a temperature (CT) between 550-660 ^o C, preferably between 580-630 ^o C, and more preferably between 600-630 ^o C. The coiling temperature (CT) of the steel sheet is a key process parameter to arrive at the required microstructure of the steel sheet providing for the improved balance in mechanical properties and impact toughness as herein described for the claimed steel sheet composition. The temperature difference between FRT and CT should exceed 180 ^o C, and more preferably it should exceed 200 ^o C, and more preferably it should exceed 210 ^o C. This achieves the effect that prior to coiling of the hot-rolled steel sheet that in the microstructure the percentage austenite transformed to ferrite is more than 40 vol.%, and preferably more than 60 vol.%. It is an important aspect of the invention to promote the formation of a high density of ferrite nuclei before the coiler positioned after the run-out table. The higher the ferrite nuclei density, the finer the resultant microstructure. It is preferred to have first the nuclei formation and then the growth thereof. It is assumed that the growth will occur predominantly during the coiling and subsequent cooling. If there is too large an overlap between nuclei formation and grain growth, there is an increased probability that the grains will grow extensively at the expense of the nuclei. It is for this reason that the temperature difference between FRT and CT is controlled to >180 ^o C , and preferably >200°C, and more preferably >210 ^o C, to allow the formation of a high nuclei density. It is also for that reason that the narrow compositional ranges for the alloying elements are relevant, in particular for the alloying elements that affect the austenite to ferrite transition, e.g., Mn, Si, and also Nb and Ti. Too high an amount of Mn, Si or Nb may retard the austenite to ferrite transition between the run out table and the coiler, and hence the ferrite nuclei formation. Ti in solid solution will promote the austenite to ferrite transition. The amount of C and Nb in the defined ranges has a favourable effect on amongst others the low- temperature impact toughness as it promotes the NbC precipitate formation at high temperature facilitating grain refinement, either austenite or ferrite, and whereby finer austenite grains will transform earlier to ferrite. Furthermore, the Nb removes C from solid solution thereby decreasing the formation of second-phase constituents, e.g., pearlite/bainite, which are very detrimental for impact toughness. When the coiling temperature is too low there is insufficient kinetics for precipitation and consequently low strength levels will be achieved. When the coiling temperature is too high there is insufficient grain refinement leading to reduced fracture toughness and increased edge- crack susceptibility. Following the coiling of the hot-rolled steel sheet, the coil is allowed to cool to below about 200 ^o C. It has been found that the coil cooling rate is not critical and can be performed as is usual in the art., which is usually cooling in still air. After the steel strip has cooled to below about 200 ^o C, preferably cooled to ambient temperature, the oxides or scale on the hot-rolled steel sheets are removed either by pickling in an acid solution, e.g. HCl, at warm temperatures, typically at 80-120 ^o C, or by mechanical brushing of the steel sheet surface. This step is necessary for rendering the steel strip surface suitable for direct use as uncoated hot-rolled steel sheet or making it amenable to a coating process, when optionally needed for enhanced corrosion resistance. Uncoated hot-rolled steel sheet is commonly uncoiled and cut-to-length. The method employed to manufacture the hot-rolled steel sheets with the narrow compositional ranges provides for the desirable balance of mechanical properties and very high impact toughness at low temperatures. It provides guaranteed high impact toughness values at -20 ^o C, but also at -40 ^o C, and even lower temperatures. It delivers high Charpy V-notch impact toughness values of more than 250 J/cm ² at -20 ^o C in both the longitudinal and transverse direction, and preferably of more than 300 J/cm ² at -20 ^o C, and having low anisotropy in this engineering property. It has been found also that the method provides for considerable consistency in product properties and reduces significantly the scatter of in particular the impact toughness values within the various steel sheet lots, i.e. steel sheets for different coils. The method according to the invention when manufacturing thick steel sheets avoids the need for hot rolling steps at temperature below the austenite non-recrystallization temperature of the steel sheet which adversely would lead to considerable anisotropy in the final mechanical properties. It also avoids the need for asymmetric hot rolling. It is an aspect of the invention that the hot-rolled steel sheet is not subsequently subjected to a cold deformation operation, e.g. stretching or levelling, having a thickness reduction of more than 2%. The present invention also makes it possible to reduce the scatter between the values of Charpy V-notch impact toughness measured both in the L-direction and in the T-direction at a temperature of -20 ^o C for a plurality of hot-rolled steel sheets belonging to the same inspection lot, such that all the steel sheets have a Charpy V-notch impact toughness value measured in the L-direction at a temperature of -20 ^o C displaying a standard deviation less than or equal to 45 J/cm ² , and preferably less than or equal to 35 J/cm ² , around an average value of said Charpy-V toughness values of the steel sheets in the inspection lot. In an embodiment the average value is about 300 J/cm ² or more, and preferably and advantageously is about 340 J/cm ² or more. The standard deviation between the measurements of the Charpy-V notch impact toughness of the different hot-rolled steel sheets in a batch or over different coils may depend on the number of hot-rolled steel sheets contained in the batch or inspection lot. In particular, a standard deviation obtained on two measurements is not significant and may be very high or very low. From 5 hot-rolled steel sheets originating from 5 different coils, i.e. one sheet from each coil, the standard deviation of the measurements may be considered, but preferably, the quality control batches used within the scope of the present invention contains at least 10 hot-rolled steel sheets from 10 different coils, i.e. at least one hot-rolled steel sheet from each coil. The invention is also embodied in a welded structure having two or more components welded to each other, at least one component being a hot-rolled steel sheet according to this invention as herein described and claimed. The invention is also embodied in a welded pipe comprising a base metal portion and a weld zone, wherein the base metal portion comprises a hot-rolled steel sheet according to this invention as herein described and claimed. The welded pipe has been made in a forming operation by forming the hot-rolled steel sheet into a pipe shape, e.g. by means of bending or roll forming, and welding together abutted portions thereof. In an embodiment the welding is done by means of high-frequency electric resistance welding, gas metal arc welding, or submerged arc welding. The cross-section of the pipe can be substantially circular, oval, elliptic, or rectangular. The invention will now be illustrated with reference to comparative and non-limiting embodiments according to the invention. EXAMPLE. Steel slabs of three chemistries A, B and C, were produced industrially via basic oxygen steelmaking and continuous casting. The steel compositions are listed in Table 1. In Table 1 also the CEQ and the ratio R = C/(Nb+Ti+V) have been presented for each steel composition. Steel A is according to the invention. Steel B is outside the invention at least due to too high a CEQ. Steel C is outside the invention at least due to too high a C content and too high a value for the R ratio. Table 1. Steel composition, in wt.%, balance process impurities and Fe. The slabs have been processed on an industrial scale and the range of the relevant processing parameters FRT and CT used are given in Table 2. FRT means finish hot-rolling temperature and CT means coiling temperature. The slabs has been processed into HSLA hot- rolled steel sheets having a final thickness of 25 mm. The run-out table cooling rates were in a range of 5 to 12 ^o C/s. In Table 2 also the strength levels in L-direction at room temperature and the Charpy V- notch impact toughness measured at -20 ^o C in the L-direction are shown. For the Charpy V- notch impact toughness both the range and the average over the given number of steel sheets are listed. The average over a given numbers of steel sheets is based on 3 or 4 sheets tested per coil such that a number of for example 62 sheet tested relates to steel sheets originating from about 20 different coils. Also typical microstructures of plates A to C have been determined at three locations in a plate and the results are listed in Table 3. The subject steel sheets had a Charpy V-notch impact toughness measured at -20 ^o C in the L-direction: steel A 390 J/cm ² , steel B 18 J/cm ² , and steel C 16 J/cm ² . The microstructure has been revealed after Nital etching as is well-known in the art. The average Feret grain size in µm and the volume percentage of second phase constituents have been determined using ImageJ software. Micrographs have been loaded in segmentation editor mode to select grain boundaries based on pixels, and the results have been treated using the analyse particles command. Table 2. Range of the relevant process parameters and the mechanical properties (in L- direction) and Charpy V-notch impact toughness (in L-direction). Table 3. Typical microstructure of steels A, B, and C. ( t = sheet thickness ) From the results of Table 3 it can be seen that Steel A has the smallest average grain size and the lowest presence of undesirable second phase constituents. Considering the FRT and CT temperature fluctuations inherent to an industrial scale process of manufacturing hot-rolled steel sheets, from Table 2 it can be seen that steel A has the highest average value in a Charpy V-notch impact toughness measured at -20 ^o C in the L-direction and a very narrow scatter in this engineering property compared to steel B and steel C. It is a key aspect of the invention that the combination of the narrow compositional ranged and the method employed provides for the desirable balance of mechanical properties and very high impact toughness at low temperatures and low anisotropy. It provides guaranteed high impact toughness values at -20 ^o C, but also at -40 ^o C, and even lower temperatures. It delivers high Charpy V-notch impact toughness values of more than 250 J/cm ² at -20 ^o C in both the longitudinal and transverse direction, and preferably of more than 300 J/cm ² at -20 ^o C. It has been found also that the method provides for considerable consistency in product properties and reduces significantly the scatter of in particular the impact toughness values within the various steel sheet lots. 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.