EP4453265A1

EP4453265A1 - Steel grade for a tube for low internal pressure applications

Info

Publication number: EP4453265A1
Application number: EP22844105.1A
Authority: EP
Inventors: Job Anthonius Van Der Hoeven; Marcel Cruijff; Frenk VAN DEN BERG; Dammes Hans Van Der Weijde
Original assignee: Tata Steel Nederland Technology BV
Current assignee: Tata Steel Nederland Technology BV
Priority date: 2021-12-24
Filing date: 2022-12-22
Publication date: 2024-10-30
Also published as: WO2023118516A1; KR20240128828A

Abstract

A low carbon steel welded tube for low internal pressure applications, wherein the pressure in the tube is, in use, less than 10 kPa, wherein the low carbon steel constituting the welded tube is a hot-rolled steel strip having a thickness of between 8 and 40 mm and wherein the tube provides excellent magnetic shielding, having a composition comprising (in wt.%): - C : 0.001 - 0.100; - Mn: 0.100 - 0.400; - Si: 0 - 0.030 - Alzo: 0.015 - 0.075; - S: 0 - 0.025; - P: 0 - 0.025; - N: 0 - 0.010 optionally one or more of the following - Cr: 0 - 0.100; - Cu: 0 - 0.040; - Ni: 0 - 0.100; - Nb: 0 - 0.010; - V: 0 - 0.010; - B: 0 - 0.0030 - Ti: 0 - 0.010 - Mo: 0 - 0.020 - Sn: 0 - 0.100 - remainder iron and inevitable impurities resulting from the steelmaking process, wherein the average grainsize of the ferrite, as measured at ¼ thickness of the hot-rolled steel strip, in the steel is more than 10 µm, and to the use thereof..

Description

STEEL GRADE FOR A TUBE FOR LOW INTERNAL PRESSURE APPLICATIONS

Field of the invention

This invention relates to a steel grade for a tube for low internal pressure applications.

Background of the invention

Conventional transportation modes via water, land, rail and air result in significant adverse environmental, societal, and economic impacts. This incentivised to find alternative transportation modes that take advantage of the significant improvements in transportation technology, and efficiently move people and materials between locations. High - speed transportation systems such as high - speed trains have been contemplated as a solution to existing transportation challenges while improving safety, decreasing the environmental impact of transportation modes such as airplanes and reducing the overall time commuting between major metropolitan communities.

A high speed, high efficiency transportation system may utilize a low - pressure environment in order to reduce drag on a vehicle at high operating speeds, thus providing the dual benefit of allowing greater speed potential and lowering the energy costs associated with overcoming drag forces. In embodiments, these systems may use a near - vacuum (e.g., low - pressure) environment within a tubular structure.

A hyperloop is a proposed mode of evacuated tube transport (ETT) for passenger and/or freight transportation. Drawing heavily from Robert Goddard's vactrain, a hyperloop comprises a sealed vacuum tube or system of vacuum tubes through which a pod may travel with less or even free of air resistance or friction conveying people or objects at high speed and acceleration. Elon Musk's version of the concept, first publicly mentioned in 2012, incorporates reduced - pressure tubes in which pressurized capsules ride on air bearings driven by linear induction motors and air compressors. The tubes would run above ground on pylons or below ground in tunnels. The concept would allow travel which is considerably faster than current rail or short and medium distance air travel. An ideal hyperloop system will be more energy - efficient, quiet, and autonomous than existing modes of mass transit.

The tubes are an essential part of the ETT-system and beside requirements regarding strength and stiffness, the tubes also must be able to retain the (near - ) vacuum conditions. The tube is the main component of the ETT-system. The main function of the tube is to provide a lasting, evacuated path of travel for a capsule or pod. The tube is the main load bearing member to span distances between supports and resist external pressure from the atmosphere or water. Tubes are usually produced in prefabricated tube sections, which are joined at the construction site by joining means. In the state of the art these joining means comprise bolted flanges or welding the tube sections together.

The ETT-system is often promoted as a sustainable transport system because it can run at high speed but with low drag, due to the low internal pressure in the tube, and is a promising technology to replace air-travel for short and medium distance trips. However, many solutions that are being developed for the tube for this ETT-system suffer from low sustainability. Concrete is often proposed as the material for the tubes but concrete is an energy intensive material with a large CO2 footprint. Between five and ten percent of global CO2 emissions are attributed to this building material. It is also still difficult to recycle used concrete in any economically viable manner. When steel is used as the material for the tubes solutions tend to concentrate on lightness and stiffness and to make such a tube as stiff as required and simultaneously use as little as possible of the metal resource, complicated constructions are needed which require welding or other connecting means which render the tube difficult to repair in case of damage, and the complicated construction also makes it difficult to recycle the material afterwards.

Objectives of the invention

It is an objective of the invention to provide a tube for low internal pressure applications.

It is also an objective of the invention to provide a tube for an ETT system that is easy to produce, easy to repair and easy to recycle.

It is also an objective of the invention to provide a tube for an ETT system that provides excellent damping characteristics.

It is also an objective of the invention to provide a tube for an ETT system that provides excellent shielding of electromagnetic fields.

Description of the invention

One or more of the objectives can be reached with a low carbon steel welded tube for low internal pressure applications, wherein the pressure in the tube is, in use, less than 10 kPa, such as an evacuated tube transport tube, wherein the low carbon steel constituting the welded tube is a hot-rolled steel strip having a thickness of between 8 and 40 mm and wherein the tube provides excellent magnetic shielding, having a composition comprising (in wt.%):

- C : 0.001 - 0.100;

- Mn: 0.100 - 0.400;

- Si: 0 - 0.030

- Alzo: 0.015 - 0.075;

- S: 0 - 0.025;

- P: 0 - 0.025;

- N: 0 - 0.010 optionally one or more of the following

- Cr: 0 - 0.100;

- Cu: 0 - 0.040; - Ni: 0 - 0.100;

- Nb: 0 - 0.010;

- V: 0 - 0.010;

- B: 0 - 0.0030

- Ti: 0 - 0.010

- Mo: 0 - 0.020

- Sn: 0 - 0.100 remainder iron and inevitable impurities resulting from the steelmaking process, wherein the average grainsize of the ferrite, as measured at ¹/4 thickness of the hot-rolled steel strip, in the steel is more than 10 pm.

The welded tube can be produced from welding together cylindrical tube segments or by spiral welding.

Further preferred embodiments are provided in the dependent claims.

Carbon in the steel is kept low to enable the realisation of the required large grains in the as hot-rolled state. A suitable maximum is 0.100 wt. %. Higher levels might lead to grain refinement and the formation of hard microstructural components (such as bainite or martensite) during cooling of the steel during processing or during welding. From the steel making process, a suitable minimum value is 0.010 wt.%. Lower levels can be made, for instance as an ultra-low carbon steel, but this is not cost-effective, because of the vacuum degassing technology that is required to produce the ultra-low carbon content. Very low carbon contents also may cause challenges with maintaining the finish rolling temperature above the Ar3-temperature because to produce a homogeneous microstructure in the as- hot-rolled steel it is important that finish rolling occurs while the steel is austenitic.

In an embodiment the carbon content of the low carbon steel (spiral) welded tube is at least 0.010%, preferably at least 0.015%, more preferably at least 0.020%. The carbon content is preferably at most 0.090%, more preferably at most 0.080% and even more preferably at most 0.075%.

Elements Cr, Sn, Cu and Ni are not needed for the properties of the material but they don't harm the properties either if they do not exceed a certain threshold value. These elements are quite common in scrap, so to meet the targets for sustainability they are allowed in relatively high levels. A maximum of 0.100 wt.% is an allowable maximum, but it is preferred to have as low a value as is economically feasible. Preferably Cr and Ni are at most 0.060 wt.% respectively, and more preferably at most 0.040 wt.% respectively. This is one of the reasons why it is preferable to produce the steel via the BOF-steelmaking process, because that process relies mainly on scrap for cooling purposes, and therefore the amount of scrap in the final melt is much lower (about 20% scrap and 80% blast-furnace pig iron) than in EAF-produced steel which almost entirely consists of melted scrap. However the alternative steelmaking routes using EAF or DRI are possible. Manganese is usually added to obtain the required strength and to improve formability. Strength is not an issue in the tube according to the invention. However, to obtain Mn < 0.100 wt.% specific cost-increasing process measures need to be taken and therefore 0.100 wt.% is considered a suitable and practical minimum value. For Mn > 0.400 wt.% it will become difficult to reach the required big grain size due to the formation of manganese sulphides which will hinder grain growth and by the reduction of Ar3. Transformation from austenite at a lower Ar3-temperature results in a finer grainsize.

In an embodiment the low carbon steel (spiral) welded tube comprises a manganese content of at least 0.125%, preferably of at least 0.150%, more preferably of at least 0.175%. The manganese content is preferably at most 0.350%, more preferably at most 0.325% and most preferably at most 0.300%.

Silicon may be added for deoxidation, solution strengthening and to suppress carbide formation. It also affects welding behaviour, for which the contents must be low enough. For this reason, a maximum amount of silicon is 0.100 wt.%. Silicon deteriorates the ductility of the steel and therefore a maximum amount of silicon is 0.050 wt.% is preferred and even more preferred a maximum of 0.030 wt.% or of at most 0.020 wt.% or at most 0.010 wt.% is preferable.

Sulphur and Phosphorus are considered inevitable elements in Low Carbon Low Alloyed (LCLA) steel which presence cannot be completely prevented.

Phosphorus can be used to improve the strength but at the same time it will reduce the formability and it is detrimental to steelmaking. It also leads to grain refinement. Therefore, phosphorus should be present in low amounts of preferably at most 0.025 wt.% and more preferably at most 0.020 wt.% and even more preferably at most 0.015 wt.%.

Sulphur is detrimental to steelmaking and to the steel itself and therefore has to be present in low amounts, preferably max 0.025 wt.%. More preferably, the maximum amount for S is 0.020 wt.% and even more preferably at most 0.015 wt.%.

Sulphur and Phosphorous are also kept low in low alloyed steels to avoid non-metallic inclusions that may deteriorate formability and fatigue strength.

Nitrogen is an inevitable element in steel. The quantity of nitrogen in steels normally depends on the residual level arising from the steelmaking processes or the amount aimed in case of deliberate addition. Nitrogen is a solid solution strengthening element when present as 'free' nitrogen or it forms precipitates with elements like aluminium or titanium. Both effects of nitrogen are undesired in the context of this invention. Nitrogen levels below 100 ppm (0.010 wt.%) are considered the maximum allowable. Preferably the nitrogen content is at most 70 ppm. More preferably at most 50 ppm.

Nb, V, B, Ti, Mo are strong grain refiners and should therefore be kept low, also in view of the costs. For Nb, V and Ti an allowable maximum is 0.005 wt.%. Mo is common in scrap when high strength steels are involved. To allow for high sustainability (recyclability of steel) the maximum for Mo is set at 0.015 wt.% although Mo < 0.005 wt.% is more preferred. For Sn a maximum of 0.050 wt.% is preferred, and more preferably of 0.025 wt.%.

B is present as interstitial element which lowers the thermal and electrical conductivity. Therefore, it should be kept at very low levels. A suitable maximum level for B is 0.0030 wt.% (30 ppm). Preferably B is at most 0.0010 wt.%

Aluminium is used for deoxidising ("killing") the steel by forming AI2O3. Any excess aluminium is present in solid solution or as aluminium nitrides to reduce the amount of interstitial nitrogen in the steel (Al_sol). For this reason a minimum aluminium amount of 0.015 wt.% is present in the steel. Too high Al_sol is detrimental for weldability. The maximum amount of aluminium is 0.075 wt.% .

Optional elements can be present in the amounts indicated above. Preferably the upper limits for these elements are even lower, such as at most 60% of the amounts indicated above for each additional element, and more preferably these optional elements are not added to the LOLA steel at all, meaning that these elements are only present as unavoidable impurity.

Preferably the sum of S, P, Cr, Cu, Ni, Nb, V, B, Ti, Mo, Sn < 0.150 wt.%, more preferably < 0.100 wt.%. Calcium, if added, is used for inclusion control. The amount of calcium needed for this purposes is between 5 and 150 ppm.

In an embodiment steel for the low carbon steel welded tube comprises one or more of the following elements in the following amount:

- Nb: 0 - 0.005;

- V: 0 - 0.005;

- B: 0 - 0.0010;

- Ti: 0 - 0.005;

- Mo: 0 - 0.015;

- Sn: 0 - 0.025;

In a preferable embodiment the steel for the low carbon steel welded tube comprises one or more of the following elements in the following amount:

- Cr: 0 - 0.060;

- Ni: 0 - 0.060;

In a preferable embodiment the steel for the low carbon steel welded tube has a composition comprising (in wt.%) one or more of the following:

- C: 0.030 - 0.060;

- Mn: 0.175 - 0.275;

- Si: 0 - 0.030, preferably 0 - 0.010;

- Alzo: 0.025 - 0.060;

- S: 0 - 0.020; - P: 0 - 0.020;

- N: 0 - 0.0050 remainder iron and inevitable impurities resulting from the steelmaking process.

According to a second aspect, the invention is also embodied in the use of the LCLA steel tube for low internal pressure applications in a system wherein the pressure in the tube is, in use, less than 10 kPa («0.1 bar).

If the underpressure application or near vacuum application is an evacuated tube transport system tube, then the pressure in the tube is, in use, near vacuum. In the context of this invention wherein the pressure outside the tube is the atmospheric pressure of about 101 kPa (1 bar), near vacuum means that the pressure inside the tube, in use, is less than 10 kPa («0.1 bar). However, to reduce the drag in the tube the internal pressure is preferably less than 1 kPa («0.01 bar or 10 mbar), even more preferably less than 500 Pa (~5 mbar) or even less than 200 Pa («2 mbar), or even less than 100 Pa («1 mbar).

If the underpressure application or near vacuum application is a tube in an Einstein telescope Einstein Telescope (ET) or Einstein Observatory, which is a proposed third- generation ground-based gravitational wave detector, then the pressure in the tube is, in use, a high vacuum of 10'⁷ mbar or lower. The ET is expected to be operating at pressures of around 10'¹⁰ mbar. The LCLA steel tube for low internal pressure applications would preferably be used as an outer tube surrounding an internal tube wherein the pressure between the outer tube and the internal tube is near vacuum as defined herein above, and in the inner tube the internal pressure would be the high vacuum which is needed for the ET to function properly.

For an underpressure application such as an ETT system, a substantially airtight tube is necessary to maintain the vacuum or very low pressure needed for low air friction or drag of the pod during travelling. Because the pod is travelling at high speed, good control of the shape of the tube is very important. Bending of the tube needs to be minimized and buckling must be avoided. These properties are mainly controlled by the design shape and thickness of the tube, and the elastic modulus of the steel, not by strength. The elastic modulus of steel is largely independent of the chemistry, so the resistance against buckling and deflection has to come from the design and the thickness of the tube. Generally speaking for a (spiral) welded single wall thickness tube, a thick-walled tube will be needed to avoid buckling and deflection. The exact thickness will depend on the diameter of the tube and the shape, but in general the wall thickness needs to be above 8 mm for shape considerations and below 40 mm for cost and manufacturing considerations. The common wall thickness for tubes depends on the typical size and shape of the cross-section of the tube. For circular cross-sections of e.g. 2.0 - 2.5 m which may be mostly suitable for cargo, a wall thickness of some 12 - 16 mm is suitable, although design considerations such as the unsupported length between two subsequent supports for the tube and the small deflection of the tube under its own weight may influence the preferable range and lead to using thicker gauges after implementing a safety factor. For passenger transport larger diameters may be preferred of some 3.0 - 4.0 m or even up to 5.0 m, where a larger wall thickness of some 15 - 22 mm is suitable, again depending on the design of the tube and support construction. The inventors have realized that with such a single wall thickness tube, the strength of the steel can be relatively low while still meeting the requirements. Calculations indicate that the stress in the tube will not exceed 150 N/mm².

The resistance against imploding of an unsupported tube (in practice the tube will be supported at its flanges every few meters, providing extra resistance to buckling, so that the actual thickness may be chosen lower) can be calculated by the following formula:

Pcr=(E/(4*(l - v²))*(t/R)³ (1)

Where Per is the critical pressure at which the tube will implode. This critical pressure depends on the E-modulus of the material (Esteei ~ 210 GPa), the Poisson Ratio v (« 0.30), and the wall thickness t of the tube divided by the radius R of the tube. In the formula t/R appears to the power 3, which means that the ratio between the thickness and the radius is the dominating factor. For a steel tube with diameter 2.6 m and Per = 1 bar (= 100 kPa), the formula gives a tube thickness of 15.7 mm. The constructors will use a safety factor, usually a factor 4. With a safety factor of 4, the tube thickness will be 24.9 mm.

The nominal stress in the tube wall is given by o = P*R/t (2)

Where P is the external pressure. For a vacuum tube, the maximum external pressure is 1 bar (0.1 MPa). So, for the tube in the above example, the maximum nominal stress is 0.1*1300/24.9 = 5.2 MPa, making a steel with an Rp>150 MPa a safe choice. There may even be room for some reduction in wall thickness, e.g. in combination with strengthening rings.

Table 1: overview of typical parameters, as reached for different grades (S355JR is a Nb containing HSLA). Clearly, Low Alloy type 1 is a good combination of low CE, low YR and large grains. Types 4 and 5 do also meet these demands, but these are Ultra Low Carbon steel types, which have disadvantages as will be discussed below.

Sustainability is an important property for an ETT system, such as a hyperloop, which is meant to transport goods and/or people, over long distances, in a sustainable way. The carbon footprint is low compared to other transport systems like aviation, trains, or road transport. However, the system needs a vast network of tubes stretching over hundreds, if not thousands of kilometres. Preferably, these tubes are made of a sustainable and recyclable material. Steel has proven it can meet all these requirements. Steel can be recycled practically endlessly and with minimal losses. In a conservative estimate, where the recovery rate of steel is 85%, every 1 kg of steel scrap that is recycled at the end of the products life results in a saving of 1.5 kg CO2 emissions, 13.4 MJ primary energy and 1.4 kg iron ore. This equates to 73, 64 and 90 %, respectively, when compared to 100 % primary production. These numbers go up if the recovery rate increases. For hot-rolled (spiral) welded tube the recovery rate is expected to be much higher because in case of replacement of parts of the ETT-system in case of repair or reconstruction the parts that are taken out of the system can be easily processed into scrap ready for re - use for many future steel applications, also because the steel tube contains little to no other metals or materials. In that respect steel compares favourably with concrete which is currently mainly recycled into road-based aggregate.

In the manufacture of steel, the term 'primary production' refers to the manufacture of iron from iron ore in a blast furnace, which is subsequently processed in the basic oxygen steelmaking process (BOS) to make steel. It is noted that the BOS-route also uses significant amounts (~20%) of scrap in its process, mainly as coolant during the steel refining process. 'Secondary production' refers to the recycling route and typically uses electric arc furnaces (EAF) to convert scrap into new steel by remelting.

Steel is fully recyclable and scrap can be converted to any steel grade depending upon the metallurgy and processing of the required product. Some recycled products require minimal processing, whilst the higher value engineering steels require more metallurgical and process controls to meet tighter specifications. The final economic value of the product is not determined by recycled content, and there are many examples of high value products that contain large amounts of recycled steel. For some steel products the primary route is important because the steel specifications require low residual elements and this can be achieved most cost-effectively using more primary material. So in most cases, scrap with a low amount of residual elements commands a higher market price owing to the ease of processing through the recycling routes. This is one of the reasons why a low carbon low alloy (LCLA) steel is so suitable as a sustainable material for a tube in an ETT-system because as scrap it has higher value, even though the costs of the alloy and the production thereof are lower than the alternatives. Alternatives for the steel according to the invention are titanium and/or niobium stabilised ultra-low carbon steels (ULC) and Ti and/or Nb alloyed HSLA steels. As scrap these alloys are less valuable and versatile because of the presence of Ti and Nb which for some future applications is an undesirable element.

The sustainability of the LCLA steel according to the invention is also reflected in its use of alloying elements and processing requirements. The fact that Ti and/or Nb are not required for the steel according to the invention results in a lower price, but also in a better sustainability of the LCLA steel because whatever element is not added to the steel does not need to be mined, refined and transported. In steel processing HSLA steels need a ladle furnace treatment before casting and are also cast at a considerably lower casting speed than the LCLA steel. The ULC requires a deep vacuum degassing treatment to reduce the carbon content to below 0.010 wt.%. Also, these steels tend to suffer from clogging during casting which is not conducive to a smooth continuous casting process. These ULC steels, due to their low carbon content and thus their high Ar3 temperature, have to be finish rolled at a high temperature to prevent the finish rolling temperature dropping below the Ar3 temperature and causing an undesired microstructure of the hot-rolled strip. Hot rolling of HSLA tends to result in high rolling forces due to the retardation of recrystallisation caused by the presence of Nb and/or Ti, resulting in faster wear of the work rolls, higher energy consumption during rolling and a higher vulnerability to shape defects.

*: only if carbon content < 0.020%

These considerations have led the inventors to the insight that it is possible to select a steel grade that is unconventional for tubes. With a carefully made choice, sustainability can be improved, not only during manufacturing of the steel but also during manufacturing of the tube and the future recycling thereof whilst retaining favourable mechanical and acoustic properties. The inventors have found that a low carbon low carbon steel according to the invention is perfectly suitable. While other developments rely on using high strength steels using costly and ever more scarce alloying elements or rely on complicated designs using e.g. double walled-systems the simplicity of the tube according to the invention is a counterintuitive development. The sustainability of the steel making process of a low carbon alloyed low carbon LCLA steel according to the invention is, compared to alternative steels used for tubes, improved by reducing the alloying costs, simplification of the production process and lower energy consumption and roll wear during production. The rolling forces are lower leading to lower energy requirements, and the sustainability of the tube making process is improved due to improved welding efficiency and better shape control. An additional advantage is that coiling and particularly uncoiling and the optional levelling of the hot-rolled strip is facilitated by the lower yield strength compared to high strength steels.

Another advantage of using a LCLA steel is that it can be supplied with a microstructure comprising relatively large grains such as grains having an average diameter of > 10 pm. Large grains result in better damping of the internal vibrations of the tubes caused by the moving pod and the associated alternating electromagnetic currents. The larger grains will also result in higher magnetic permeability and therefore better shielding of the electromagnetic fields generated inside the tube. To ensure public safety it is very important that the electromagnetic fields outside the tube are very low.

An additional advantage of a large grain size is the improved thermal conductivity. The linear expansion due to a temperature rise, will be intercepted by expansion joints. But high thermal conductivity helps to diffuse the heat from a local heat source, as to homogenise the temperature. A more homogenic temperature avoids "hot spots" and reduces local stress and thermal expansion. And it reduces deformation due to thermal expansion, which leads to shape retention and stable tracks.

Yet another additional advantage of a LCLA steel is the low yield ratio YS/UTS. A low yield ratio ensures that during a landslide or an earthquake, the tube will not break easily but instead it will plastically deform only, thereby potentially preventing a sudden breach of the internal vacuum which can be catastrophic. Other steels with which similar low yield ratios can be obtained are e.g. dual phase steels, but these generally require larger amounts of expensive alloying elements and a much more complicated processing involving high cooling rates. The latter is problematic, particularly with large gauges.

When hot rolling a strip, a crown is formed over the width, which is a difference in thickness with the maximum thickness in the centre and minimum thickness near the edges. It is a well-known fact that the crown that is produced in the hot-rolling process cannot be altered in the subsequent cold rolling without flatness defects in the resulting cold-rolled strip. The relative crown and wedge are measured at a certain distance from the edge due to the edge drop. In case of C40 and W40 this distance is 40 mm from the edge and the crown and wedge are calculated as follows (see also figure 1):

The inventors found that a C40 crown value of the hot-rolled strip of at most 0.045 mm is preferred. More preferably C40 is below 0.040 mm, even more preferably below 0.035 and most preferably below 0.030 mm. This low level of crown produces a strip with a very small thickness differential over the width, which is a benefit for the welding process and reduces the need to trim the edges of the strip. According to the invention it is preferable that the low carbon steel welded tube is produced from a hot-rolled strip which has, prior to the optional width trimming step, a crown C40 of at most 0.045 mm. After the optional width trimming step, or even without the width trimming step, the low crown ensures that the bending and welding of the strip to form the tube goes as smoothly as possible.

Low temperature brittleness is normally not a problem when using LCLA steels. A target value of KV2 at -20°C > 60 J is easily obtainable (EN10025-2:2019 together with ENISO 148-1). The same applies to weldability. As a first approximation of weldability the CE is a good predictor, and any steel with a CE-value below 0.35 is excellently weldable.

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni+Cu)/15 (all in wt.%) (3)

The LCLA steels from the invention easily reach CE < 0.35. In fact, CE < 0.2 can be guaranteed and most chemistries will reach CE < 0.1.

Vibrations will be caused by passage of the pod in the hyperloop tube (acceleration and deceleration, the pressure wave induced by its motion - both in the air and by its mass - and the control loop of its levitation), and by the magneto-mechanical coupling effects (notably the magnetostriction effect [1]) of the linear motors. The latter will generate a 100 Hz frequency when powered by the 50 Hz power network.

In steel, acoustic waves (i.e. vibrations) travel at a speed of ~ 5600 m/s for p-waves (the pressure waves, most common) and ~ 3000 m/s for s-wave (shear waves). A damped vibration travels according to: y(t)=A-e (-at)-cos(ojt+cp) (4) with y(t): the amplitude at time t, A: the initial amplitude, a: decay rate, D: angular frequency (co=2n-f), f: frequency (in Hz), q>: phase offset at t=0.

An often used parameter is the so-called damping ratio less parameter to express the damping: =O/ (O²+ D² )

In this case O« D SO (5) simplifies to: =a/co (6)

If the damping ratio is assumed to be independent of the frequency f, then for the lower frequency f = 100 Hz ( D=600 rad s^-1), the decay rate a is in the range (3.6 - 8) E-2 Hz (see table 2). Table 2.

A decay rate of a=0.6 Hz signifies that the vibration amplitude y (eq. (4)) decreases in a time of 1.6 s ( = 1/ a) by a factor 1/e = 0.37. The decay rates in the table imply that vibrations with frequencies of ~ 1.5 kHz and above will decay and disappear within a couple of seconds. However, low frequencies (100 Hz or below) are damped at a much lower rate, and last for more than half a minute. Frequencies between 5 and 20 Hz cannot be heard but they do pose a risk as they can affect the alignment of the tube. Bad alignment can cause derailing of the pod. So, vibrating of the tube while the next pod arrives needs to be avoided. This makes high damping an important safety issue.

The attenuation of acoustic waves in steel is caused by both absorption and scattering of the waves. These phenomena occur at lattice defects, like dislocations, impurities and grain boundaries. In the actual case, dealing with low frequency acoustic waves (vibrations), the wavelength A, of the waves is in the order of meters, while a typical defect size D in the steel lattice is in the order of 10 pm (i.e. the grain size). Hence, A/D»l , which implies that the scatter is in the so-called Rayleigh regime (Rayleigh, Lord (1899), "On the transmission of light through an atmosphere containing small particles in suspension, and on the origin of the blue of the sky". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 47 (287): 375-384). Typical for Rayleigh scattering is that the attenuation due to scattering is proportional to f⁴D³ [3]. Thus, higher frequencies (i.e. shorter wavelength) scatter more, and defects with large size scatter more. According to the Rayleigh scattering theory, a larger grain size will cause more scattering, and thus more damping. Hence, damping properties are favoured by a large grain size.

The inventors realised that it is beneficial for damping of acoustic waves in steel to have large grains in the microstructure. Large grains result in better damping of the internal vibrations of the tubes caused by the moving pod and the associated alternating electromagnetic currents. The larger grains will also result in higher magnetic permeability and therefore better shielding of the electromagnetic fields generated inside the tube. After hot rolling, i.e. in the 'as supplied' condition, LCLA steels have grain sizes much larger than conventional steels used for tubes and therefore have improved damping properties. A common grain size for a standard tube alloy like S355-JR after hot rolling is 6-8 pm, whereas the LCLA steel according to the invention can easily reach values of more than 15 pm (< 8.5 ASTM).

The improved shielding of electromagnetic fields can be illustrated as follows.

For the ETT-tube, shielding against electromagnetic (EM) fields occurs as follows: 1. Shielding the inside of the tube against EM fields from the outside of the tube (e.g. lightning 1 Hz-300 MHz).

2. Shielding the outside against fields generated in the inside of the tube (50-1500 Hz);

Because of its electrical conductivity and ferromagnetic properties, steel is an ideal tube wall material to shield the radiation between the source and the receiver.

The shielding effectiveness (SE) of different steel grades has been compared in FIG M.

In this figure, the shielding effectiveness is shown as function of frequency in the frequency range of 1 - 5000 Hz. The SE has been calculated according to wherein Pi is the power generated by interference source and P2 is the power passing through the shielding material (httBs /www.matec-

For EM sources inside the tube the reflection component of the shielding has to be set to zero because the reflected components will act again as source on the opposite side of the tube. Thus for internal sources, only the attenuation component contributes, which is evaluated using electromagnetic property values (JMMM_V475_2019_pp 38-43 and JMMM_V473 (2019) 477-483 (Journal of Magnetism and Magnetic Materials)). The figure shows that amongst the various carbon-steels, the softer steels, i.e. the LCLA steels, are favourable for shielding. The difference in SE between the softest and hardest steel is about a factor 2.

For any grade, EM fields with frequencies below 10 Hz are difficult to screen off. On the high frequency edge ( about 1 kHz and higher), shielding is highly effective for wall thickness of 10 mm or thicker. For common 50 (60)Hz EM fields from the power network, the screening becomes critical; Here LCLA grades prove their value for their better EM shielding capabilities. Evidently, the shielding can be improved by increasing the tube wall thickness. The conclusion is therefore the LCLA grades (LA#) perform well compared to the conventional tube grades with regard to electromagnetic shielding

Another aspect of electromagnetic field shielding is pod heating due to dissipation of the EM radiation in the pod shell. Tube material which maximally reduces the field in the tube will minimize pod heating. This essentially implies that the tube material should guide the electric and magnetic fields as well as possible, reducing the field in the interior of the tube. This can be realised by choosing material with a high electrical conductivity and a high magnetic permeability. The inventors found that in a comparison between the alloys of table 1 that the LCLA steels are expected to experience the lowest pod heating.

Large grains result in better damping of the internal vibrations of the tubes caused by the moving pod and the associated alternating electromagnetic currents. The larger grains will also result in higher magnetic permeability and therefore better shielding of the electromagnetic fields generated inside the tube. An overview of these parameters as reached by different grades, is given in Figure 2.

Brief description of the drawings

The invention will now be explained by means of the following, non-limiting figures. Figure 1 shows the principle of the crown of a strip.

Figure 2 shows the shielding as a function of steelgrade

Figure 3 shows the relation between ASTM number and grainsize in mm (according to ASTM El 12).

Figure 4 shows the table with the chemical composition of the tested steels.

Examples

Examples of the steel grades for carbon steel welded tube for low internal pressure applications are provided in figure 4. The shielding performance of these steels is reported in figure 2. Spiral weldability proved to be satisfactory when the crown C40 was at most 0.045 mm. Higher crown values required more prepping of the edges to be welded, and a higher quality weld is obtained with lower C40 values.

Claims

CLAIMS A low carbon steel welded tube for low internal pressure applications, wherein the pressure in the tube is, in use, less than 10 kPa, wherein the low carbon steel constituting the welded tube is a hot-rolled steel strip having a thickness of between 8 and 40 mm and wherein the tube provides excellent magnetic shielding, having a composition comprising (in wt.%):

- C : 0.001 - 0.100;

- Mn: 0.100 - 0.400;

- Si: 0 - 0.030

- Alzo: 0.015 - 0.075;

- S: 0 - 0.025;

- P: 0 - 0.025;

- N: 0 - 0.010 optionally one or more of the following

- Cr: 0 - 0.100;

- Cu: 0 - 0.040;

- Ni: 0 - 0.100;

- Nb: 0 - 0.010;

- V: 0 - 0.010;

- B: 0 - 0.0030

- Ti: 0 - 0.010

- Mo: 0 - 0.020

- Sn: 0 - 0.100 remainder iron and inevitable impurities resulting from the steelmaking process, wherein the average grainsize of the ferrite, as measured at ¹/4 thickness of the hot- rolled steel strip, in the steel is more than 10 pm. The low carbon steel welded tube wherein the tube is a spiral welded tube. The low carbon steel welded tube according to claim 1 or 2 wherein the carbon content is at least 0.010%, preferably at least 0.015%, more preferably at least 0.020% and/or wherein the carbon content is at most 0.090%, preferably at most 0.080%, more preferably at most 0.075%. The low carbon steel welded tube according to any one of claims 1 to 3 wherein the manganese content is at least 0.125%, preferably at least 0.150%, more preferably at least 0.175% and/or wherein the manganese content is at most 0.350%, preferably at most 0.325%, more preferably at most 0.300%. The low carbon steel welded tube according to any one of claims 1 to 4 wherein the steel comprises one or more of the following elements in the following amount:

- Nb: 0 - 0.005;

- V: 0 - 0.005;

- B: 0 - 0.0010;

- Ti: 0 - 0.005;

- Mo: 0 - 0.015;

- Sn: 0 - 0.025; The low carbon steel welded tube according to any one of claims 1 to 5 wherein the steel comprises one or more of the following elements in the following amount:

- Cr: 0 - 0.060;

- Ni: 0 - 0.060; The low carbon steel welded tube according to any one of claims 1 to 6 having a composition comprising (in wt.%) one or more of the following:

- C: 0.030 - 0.060;

- Mn: 0.175 - 0.275;

Si: 0 - 0.030, preferably 0 - 0.010;

- Alzo: 0.025 - 0.060;

- S: 0 - 0.020;

- P: 0 - 0.020;

- N: 0 - 0.0050 remainder iron and inevitable impurities resulting from the steelmaking process. The low carbon steel tube according to any one of claims 1 to 7 having a composition comprising (in wt.%) one or more of the following:

- Cr: 0 - 0.040;

- Ni: 0 - 0.040;

- Nb: 0 - 0.002

- V: 0 - 0.004;

- B: 0 - 0.0005;

- Ti: 0 - 0.002;

- Mo: 0 - 0.010;

- Sn: 0 - 0.010; remainder iron and inevitable impurities resulting from the steelmaking process. The low carbon steel welded tube according to claim 1 to 8 wherein the value of the shielding effectiveness at 50 Hz is at least 50 dB for a 10 mm thick hot-rolled steel 17 strip, and preferably wherein the shielding effectiveness increases with 5 dB per mm increase in thickness of the hot-rolled steel strip.

10. The low carbon steel welded tube according to claim 1 to 9 wherein the steelmaking process to produce the steel for producing the hot-rolled strip is the BOF-steelmaking process.

11. The low carbon steel welded tube according to claim 1 to 10 wherein the hot-rolled strip prior to the optional width trimming step has a crown C40 of at most 0.045 mm.

12. The low carbon steel welded tube according to claim 11 wherein the ratio of the crown and the mid-strip thickness of the slab entering the hot strip mill is equal or smaller than the ratio of the crown and the mid-strip thickness of the hot-rolled strip exiting the hot-rolled strip mill.

13. The low carbon steel welded tube according to any one of claims 1 to 12 wherein the average grainsize of the ferrite, as measured at ¹/4 thickness of the hot-rolled steel strip, in the steel is more than 14 pm.

14. The low carbon steel welded tube wherein the tube is an evacuated tube transport tube wherein the pressure in the tube is, in use, less than 10 kPa («0.1 bar), preferably less than 1 kPa («0.01 bar or 10 mbar), even more preferably less than 500 Pa («5 mbar) or even less than 200 Pa («2 mbar), or even less than 100 Pa («1 mbar).

15. Use of the low carbon steel of the tube according to any one of claims 1 to 14 i) in an evacuated tube transport system wherein the pressure in the tube is, in use, less than 10 kPa («0.1 bar), preferably less than 1 kPa («0.01 bar or 10 mbar), even more preferably less than 500 Pa («5 mbar) or even less than 200 Pa («2 mbar), or even less than 100 Pa («1 mbar), or ii) in an Einstein Telescope wherein the pressure in the tube is, in use, less than 10 kPa («0.1 bar), preferably less than 1 kPa («0.01 bar or 10 mbar), even more preferably less than 500 Pa («5 mbar) or even less than 200 Pa («2 mbar), or even less than 100 Pa («1 mbar).

16. Use of the evacuated tube transport tube according to claim 15 wherein the pressure in the tube is near vacuum i.e. less than 10 kPa («0.1 bar), preferably less than 1 kPa (~0.01 bar or 10 mbar), even more preferably less than 500 Pa («5 mbar) or even less than 200 Pa («2 mbar), or even less than 100 Pa («1 mbar).