EP3084030B1

EP3084030B1 - High strength hot-finished steel hollow sections with low carbon equivalent for improved welding

Info

Publication number: EP3084030B1
Application number: EP14823925.4A
Authority: EP
Inventors: David Crowther; Edwin William JACKSON; William Arthur SIMMONS; David Evans; Stewart Matthew JEFFREY
Original assignee: Tata Steel UK Ltd
Current assignee: Tata Steel UK Ltd
Priority date: 2013-12-18
Filing date: 2014-12-11
Publication date: 2018-02-14
Anticipated expiration: 2034-12-11
Also published as: PL3084030T3; EP3084030A1; NO3084030T3; SG11201604636QA; ES2666738T3; WO2015091216A1

Description

This invention relates to a high-strength hot-finished steel hollow section with low carbon equivalent for improved welding and a method for producing said section.
Hot finished steel hollow sections are used in structural applications owing to their stable and uniform properties over the whole section. Hollow sections are e.g. known from JP 2001-262275A . Increasing the strength of the section allows either less material to be used in the same application, or more complicated design problems to be solved. Increasing the strength of steel typically leads to increasing the carbon equivalent level, which in turn leads to reduced performance in and upon welding, and can require special and more costly welding practices to be employed.
The excellent properties of the hollow sections have been recognised for a long time. An outstanding example of bridge design is the Firth of Forth Bridge in Scotland (1890) with a free span of 521 m, which has been built up from tubular sections made of rolled plates which have been riveted together. Welded tubular sections have become available after the development of the continuous welding process in the 1930's. In 1952 the rectangular hollow section was developed by Stewarts and Lloyds (now Tata Steel). Rectangular hollow sections are made by deforming circular hollow sections through forming rollers. Generally, the design of constructions using hollow sections is based on yield, since post yield the deformation under loads becomes excessive. The mechanical properties are given in standards. For hot-finished structural hollow sections of non-alloy and fine grain structural steels the relevant European standard is EN10210-1:2006. Usually hollow sections can be supplied with a square, rectangular, circular or elliptical cross-section. The wall thickness varies as well as the external dimensions of the section.
It is an object of the present invention to offer a hot-finished steel hollow section that meets the S420 (EN10210-1:2006) industry standard for mechanical properties whilst meeting the maximum carbon equivalent of the S355 standard.
One or more of the objects is reached by a high strength hot-finished steel hollow section with low CEV wherein $CEV = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Cu + Ni}{15}$
and wherein (in wt.%)

C: 0.12-0.18;
Si: <0.60;
Mn: 1.2-1.6;
P: <0.035;
S: <0.015;
V: 0.13-0.20;
Al: < 0.04;
N: 0.008-0.025;
Ti: <0.01;
Cr: <0.15; preferably < 0.05%
Ni: <0.20; preferably < 0.05%
Mo: <0.04;
Cu: <0.20; preferably < 0.05%
Nb: < 0.05%
Optionally Ca in an amount consistent with a calcium treatment for inclusion control;
Remainder iron and inevitable impurities;

The section according to the invention offers a hot-finished steel hollow section that meets the S420 (EN10210-1:2006) industry standard for mechanical properties whilst also meeting the carbon equivalent maximum of the hot finished S355 standard which represents an 18% increased minimum yield strength level for the same carbon equivalent level. The invention achieves the required strength level in the hot formed condition by careful control of steel composition and processing steps.
Alternatives in the marketplace with the same minimum guaranteed yield strength are either cold formed hollow sections, cold formed and heat treated hollow sections, or hot finished hollow sections with higher carbon equivalents. Cold formed hollow sections do not have uniform grain sizes and have greater brittleness in the corners or seam welded zones, leading to reduced performance in like-for-like hollow sections. Moreover, cold formed hollow sections have a need for larger corner radii than hot-formed sections to prevent corner cracking. Cold formed and heat treated hollow sections offer improved performance over solely cold formed, but still do not offer the uniform grain size, hardness and tensile properties of a hot formed tube. Other hot finished hollow sections, of which there are few in the market, offer a higher carbon equivalent level, which means reduced performance in and upon welding, and will require special and more costly welding practices to be employed.
To achieve the required strength after hot forming whilst maintaining a low CEV for good weldability requires the final hot formed microstructure to have a fine ferrite grain size, a harder second phase such as pearlite or bainite, and fine precipitates such as vanadium carbonitrides (V(CN)) to give additional strength. The fine precipitates are vital, the required minimum yield strength of 420 MPa cannot be achieved by grain refinement alone if low CEV is to be maintained. The fine ferrite grain size is also vital in achieving good Charpy toughness.
It is further noted that the section according to the invention offers a hot-finished steel hollow section that meets the S420NH as well as the S420NLH standard, which indicates that the section has excellent impact properties at -20 and -50 °C.
To achieve a fine ferrite grain size in the final product requires a fine austenite grain size during the hot forming process. Second phase particles are required to pin the austenite grain boundaries at the hot forming temperature. VN particles are effective in achieving this, as they retain a small size, and small particles are most efficient at pinning grain boundaries. It has been found that AlN particles are not as effective as VN, since AlN particles are larger. To obtain the appropriate fraction of VN particles requires the appropriate amount of vanadium and nitrogen and levels required are significantly higher than those used for conventional grades. The fine austenite grains whose boundaries are pinned by VN particles subsequently transform to fine ferrite grains on cooling through the transformation.
As well as a fine grain size, additional precipitation strengthening is required by fine particles. The VN particles which pin the austenite grain boundaries are too large to make a significant contribution, and will have lost coherency with the matrix due to the transformation and their size. Fine particles based on V(CN) are suitable to give this additional precipitation strengthening, also because these are (semi-)coherent precipitates. At the hot forming temperature, some vanadium combines with nitrogen to form VN for austenite grain boundary pinning, but if the appropriate level of vanadium is chosen, some vanadium remains in solution at the hot forming temperature, and then precipitates as fine V(CN) precipitates during cooling, which make an important contribution to final strength. An appropriate carbon content is required to form sufficient V(CN) precipitates.
As well as reacting with vanadium to form V(CN), carbon also makes a contribution to strength via the formation of other second phases such as pearlite. However, if carbon is too high, the CEV will increase and reduce weldability. Similarly, increasing carbon decreases Charpy toughness, and there is a requirement for high Charpy toughness in the final product. The carbon content is therefore limited between 0.12 and 0.18 % (all compositional percentages are given in weight percent (wt.%) unless indicated otherwise). Preferably the carbon content is at least 0.13%. A suitable maximum carbon content was found to be 0.16%.
In order to enable the formation of sufficient quantities of V-containing precipitates, a vanadium content of 0.13 to 0.20% is necessary. A preferable minimum vanadium content was found to be at least 0.15% or even at least 0.16%. A suitable maximum vanadium content was found to be at most 0.19% or even at most 0.18%. Although the decreasing width of the vanadium range may pose challenges in the preparation of the steel melt and slab, so that it is not preferable from that perspective to take the range too narrow, the degree of precipitation control is increased with the more narrow range of vanadium.
To achieve the correct combination of precipitates, other elements as well as vanadium, carbon and nitrogen must be controlled. If the aluminium content is too high, AlN forms in preference to VN, and AlN is not as efficient at pinning austenite grain boundaries as VN due to its larger size. Consequently the aluminium soluble (Al_sol) content is below 0.04%. The total aluminium content may be slightly higher due to the presence of e.g. alumina or aluminates. Preferably the aluminium content is at most 0.035%. A suitable minimum aluminium content was found to be 0.005%.
Titanium content must be kept low, as titanium preferentially reacts with N, to form TiN rather than VN. These TiN-precipitates are not as effective as VN in pinning austenite grain boundaries. Moreover, TiN-precipitates are cuboids which may act as stress enhancers which may be undesirable in constructions which are stressed under conditions favourable to induce fatigue cracking. Consequently the titanium content is below 0.01%.
Silicon adds strength without having a detrimental effect on weldability, but high silicon levels can have a detrimental effect on the ability to galvanise the steel, so the preference is for low silicon (<0.25%). High silicon additions result in thick layers of Fe-Zn compounds after galvanising which can be brittle. A suitable minimum silicon addition is 0.10%. Preferably the silicon content is at least 0.15% and/or at most 0.25%.
Manganese is a useful strengthening element, and also contributes to grain refinement by lowering the austenite to ferrite transformation temperature. However, high levels of manganese do result in high CEV levels, which reduces weldability. For these reasons the manganese content is limited between 1.2 and 1.6%. Preferably the minimum content for manganese is 1.3%, more preferably at least 1.35%. A suitable maximum manganese content is 1.5%.
Phosphorus and sulphur must be controlled to low levels to allow good Charpy toughness and weldability to be achieved, and to allow defect free slabs to be produced for rolling to strip. Phosphorus is therefore limited to at most 0.035% and sulphur to at most 0.015%. Preferably Phosphorus is therefore limited to at most 0.025% and/or sulphur to at most 0.008%.
Nitrogen is an important element in that it participates in the reduction of the austenite grain size, and thus in that of the ferrite grain size, as well as in the precipitation hardening of the ferrite. A nitrogen content between 0.008 and 0.025% (i.e. 80 to 250 ppm) is needed for this purpose. Lower nitrogen levels result in an insufficient degree of precipitation and grain size control and higher levels require unfeasibly high slab reheating temperatures. Preferably the minimum content is 0.010 (100 ppm). A suitable maximum nitrogen content is 0.022 (220 ppm), preferably 0.020 % (200 ppm).
CEV is defined as: $CEV = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Cu + Ni}{15}$
The CEV must be kept low and below 0.45, so that weldability is equivalent to (i.e. just as good as) that of lower strength hollow sections. A low CEV means that additional weld processing steps such as pre-heating can be avoided, thus reducing fabrication costs. Preferably CEV ≤ 0.44%, more preferably CEV ≤ 0.435%.
During reheating prior to hot rolling, a sufficiently high reheating temperature is required to dissolve the majority of precipitates such as AlN and V(CN) formed during and after solidification of the steel. If these precipitates are not dissolved prior to the start of rolling, they will be too large to have any useful metallurgical effect later in processing.
If niobium is added in amounts of below 0.05% Nb(CN) particles will be present during the hot forming operation which will help to pin austenite grain boundaries, and hence promote a fine grain size in the final transformed ferritic product. However, the solubility of Nb(CN) is less than that of V(CN), and insufficient niobium is in solution at the hot forming temperature to form fine precipitates during cooling which could contribute to strength. The addition of niobium enhances the susceptibility to the formation of cracks during continuous casting, so the addition must be done selectively.
Chromium, nickel, molybdenum and copper may be used in the steel, provided the CEV stays below the threshold value. Since these elements affect the CEV as defined above directly, it is preferred to keep the amounts for these elements low. Chromium should be below 0.15, nickel and copper both below 0.20, and Molybdenum below 0.04. Preferably chromium and/or nickel and/or copper are below 0.05%. Preferably chromium, nickel and copper are each below 0.05%, and/or at most 0.10% jointly.
The calcium treatment of aluminium-killed steels leads to the modification of non-metallic inclusions and the change of their chemical composition and plastic deformability. Calcium treatment has the benefit of modifying inclusion composition, and the shape and size of these inclusions are also adjusted. Two of the main advantages of calcium treatment are not only the improvement of the castability (prevention of clogging) but also the improvement of the final properties of the steels' machinability, toughness and surface quality. The effects of calcium are mainly based on its strong ability to form sulphides and oxides. In aluminium deoxidised steels, the inclusion population will generally include alumina inclusions and maybe some silicates and manganese sulphides. After calcium treatment, the inclusions are restricted mainly to calcium aluminates (CaO-Al₂O₃) and the sulphur in the steel is associated with these inclusions as calcium sulphide. The calcium levels in the final steel are low, and preferably below Ca<0.015%, more preferably below 0.005. However, if the steel has been subjected to a calcium treatment, then calcium is present in the steel as an alloying element (i.e. deliberately added), even though the amounts are minute, and not as an impurity (i.e. unavoidably present).
In an embodiment the section comprises

C: 0.13-0.16% and/or
Si: 0.15-0.25% and/or
Mn: 1.3-1.5% and/or
P: <0.025% and/or
S: <0.008% and/or
V: 0.16-0.18% and/or
Al: 0.005 - 0.035% and/or
N:0.008-0.020%.

In an embodiment the wall thickness of the section is at most 40 mm.
The thickness range over which the section according to the invention satisfies the S420 industry standard is very wide. The EN10210 prescribes a yield strength of at least 400 MPa at a thickness above 16 mm. This value of at least 400 MPa is also met with the section according to the invention.
Preferably the wall thickness of the section is at most 16 mm. For these thickness values the EN10210 requirement for yield strength of at least 420 MPa is met.
Although the above is directed primarily to the steel hollow section as produced by hot rolling, forming, welding and heat treating, the inventors found that the same chemistry can be advantageously used to produce steel hollow sections by means of a seamless production route wherein the steel hollow section is produced seamlessly and heat treated. Also for these sections the weldability is of great importance, as well as the mechanical properties.
According to a second aspect, a process is provided for producing a section according to any one of the preceding claims comprising the steps of:

casting a steel slab having a composition according to the invention;
(re)heating the steel slab to a temperature of at least 1150 °C;
hot-rolling the steel slab to a hot-rolled strip with a finish rolling temperature in the range 800-950 °C;
cooling the hot-rolled strip with a cooling rate between 2 and 50 °C/s;
coiling the hot-rolled strip at a coiling temperature in the range 550-720 °C;
cold forming and welding the strip to form a tube blank;
hot-forming the tube blank at temperatures in the range 800-1050 °C to a final hollow section with its final dimensions;
cooling the final hollow section to ambient temperature.

producing a steel billet having a composition according to the invention;
(re)heating the steel billet to a temperature of at least 1150 °C;
Piercing the hot billet to produce a hollow tube shell followed by rolling the hollow tube shell in a plug mill, pilger mill or mandrel mill to produce a tube;
Optionally passing the tube through a reeling mill to reduce the wall thickness and/or a sizer to produce the desired outer diameter;
Optionally reheating the tube to a temperature in the range of 800-950 °C, preferably by means of induction, and rolling in a stretch-reducing mill to further reduce the outside diameter and/or the wall thickness;
Normalising the final tube;
cooling the final tube to ambient temperature.

The process for producing the steel hollow section is directed primarily to the hollow section as produced by hot rolling, forming, welding and heat treating, the inventors surprisingly found that the same chemistry can be advantageously used to produce steel hollow sections by means of a seamless production route wherein the hollow section is produced seamlessly and heat treated. Also for these sections the weldability is of great importance, as well as the mechanical properties.
The slab casting process can be the thick slab casting process, resulting in slabs having a thickness above 150 mm, or the thin slab casting or thin slab casting and direct rolling process, resulting in slabs having a thickness below 150 mm, and generally having a thickness between 50 and 100 mm.
During hot strip rolling, it is advantageous to achieve a fine grain size in the hot rolled strip, as this makes the achievement of a fine austenite grains size during hot-forming the tube blank easier. Hot-rolling is therefore performed by rolling the steel slab to a hot-rolled strip with a finish rolling temperature in the range 800-950 °C, i.e. when the hot-rolling process is executed while the steel is fully austenitic; The fine ferrite grain size in the hot rolled strip is achieved by control of finish rolling temperature (FRT) in the austenite region, the use of water sprays to increase the cooling rates on the run out table between finish rolling and coiling, and by the selection of an appropriate coiling temperature. If the coiling temperature is too low, then the hot rolled strip is hard and difficult to cold form. If the coiling temperature is too high, a large ferrite grain size can result, and coarse AlN and V(CN) precipitates, which make achieving the required fine austenite grain size during hot forming difficult.
The hot-forming temperature of the tube blank has to be carefully controlled. If the temperature is too low, then the tube blank is not completely transformed to austenite and vanadium is not dissolved to the appropriate extent. If the hot forming temperature is too high, then a large grain size will be formed, together with undesirable microstructures such as Widmannstätten ferrite, which will result in poor strength and toughness.
In an embodiment the coiling temperature of the hot rolled strip is at least 560 °C and/or at most 650 °C. The upper boundary helps in achieving a fine microstructure, and the lower boundary helps to avoid harder microstructural components such as bainitic or even martensitic components.
In an embodiment the cooling of the final hollow section to ambient temperature occurs by still air cooling, or by forced air cooling. The maximum cooling rate depends mainly on the steel grade. The occurrence of (small) islands of martensite in the final microstructure needs to be prevented to prevent a drop in yield strength. The average cooling rate between 850 and 550 °C is preferably at most 10 °C/s. Preferably the average cooling rate is at most 7.5 °C/s and more preferably at most 5 °C/s.
In the tube forming process coiled strip is slit to the required width, and the front end of the new coil is joined to the back end of the previous coil (e.g.) by a flash butt weld or MIG welding to produce a continuous length. The continuous strip is then passed through the forming mill, where a series of rolls form the strip into the required shape. High frequency electric resistance induction welding completes the formation of the tube. Immediately after welding the external weld bead is removed, and the weld area is water cooled. The tube then passes through in line non-destructive testing, and is then sized to produce the required diameter and cut to length.
In Electric Weld Stretch Reduction (EWSR) the welded tube forms the feedstock for the stretch reduction process. Lengths of tube up to 120m long are heated to 900-1050 °C depending on the steel grade. The tubes are then passed through a series of roll stands in which they are stretch reduced to the required size and thickness. The sections are then cut to length, and placed on cooling racks, where continuous turning ensures uniform cooling.
In Electric Weld (EW) tubes that are to be hot finished and are not stretch reduced to achieve the final size are passed into a furnace where they are heated to a temperature between 850-1000 °C, and then hot rolled into their final profile after which they are allowed to cool.
During the production of seamless tube, billets are reheated typically to temperatures in the range 1150-1250 °C, and then pierced. The pierced billet is then rolled to reduce the outside diameter and wall thickness. The tube may then pass through a final sizing mill, or in some processes be reheated again prior to passing through a stretch-reduction mill to achieve the final dimensions. The seamless tube may now be given further heat treatments, including heating in the range 850-1000 °C, to achieve the required final mechanical properties.
The invention will now be described with reference to the following non-limiting examples.
An overview of the chemical analysis of the casts used for the examples is given in Figure 2.

In Table 1 the results of steels 19 to 27 are presented, wherein steels 23 and 24 are comparative examples.

Table 1 - results of steels 19 to 27

ID	Type	YS (MPa)	TS (MPa)	Elongation A (%)	Charpy at - 20°C (J)
23	Comparison	414	554	31	214
24	Comparison	400	567	30	163
19	Invention	441	570	29	192
20	Invention	453	597	29	208
21	Invention	423	572	30	165
25	Invention	430	560	32	208
26	Invention	455	585	29	190
27	Invention	441	567	30	204

These examples were produced by reheating to 1250 °C, followed by hot rolling to 12.5 mm with a finish rolling temperature of 850 °C. A cooling of about 10 °C/s was used to cool the finish rolled steel to the coiling temperature of 600 °C. After cooling to ambient temperatures the samples were reheated to a normalising temperature in the austenitic region (i.e. to about 900 °C) and formed to a section after which the samples were allowed to cool to ambient temperatures in still air. Samples were taken from these samples. The elongation (A) of the tensile samples was measured over a proportional gauge length L₀. L₀ is defined as 5.651·√(S₀), where S₀ is the surface of the cross section of the tensile specimen. E.g. for a round tensile specimen with a diameter of 8 mm, the gauge length is 40 mm.
The TEM examination of carbon extraction replicas taken from samples quenched from the normalising temperature indicates the presence of VN precipitates and AlN precipitates. The VN precipitates mostly had a spherical or cuboid shape, but occasionally a larger, plate-like morphology was observed. The spherical and cuboid VN precipitates were mostly distributed in a random fashion, but occasionally short rows of precipitates were observed. The AlN precipitates were usually in the shape of rods or thin, angular prisms, and were much larger in size than the VN precipitates (typical AlN precipitates are about 100 nm in diameter, whereas the VN-precipitates are about 10nm in diameter) . They were usually arranged in short rows or clusters. Typical examples of precipitation in the base steel are shown in Figure 3a (AlN-precipitates) and b (VN-precipitates).

In Table 2 the results for different types of stretch reduced (SR) or hot- finished (HF) section are presented. Rt0.5 is the yield point (in MPa) at 0.5% elongation.

Table 2 - Results for SR and HF sections.

Mill	ID	Size (mm³)	Cond.	Wall Thickness (mm)	ReH (MPa)	Rt0.5 (MPa)	UTS (MPa)	A (%)	IE*** (J)
EWSR	69425-1	60x40x6.3	SR*	6,3	476	473	593	30	110
EWSR	69425-2	60x40x6.3	SR	6,3	460	461	583	29	111
EWSR	63226-1	80x80x8	SR	8	481	461	610	27	63
EWSR	63226-2	114.3x6.3	SR	6,3	500	493	593	30	96
EWSR	63226-3	139.7x6.3	SR	6,3	475	474	574	30	82
EWSR	63226-4	139.7x8.0	SR	8	453	445	581	28	69
EW	63225-1	150x100x6.	HF**	6,3	434	429	547	31	88
EW	69424-1	150x100x6.	HF	6,3	451	434	561	30	77
EW	63225-2	150x100x6.	HF	6,3	439	425	548	31	76
EW	63225-2	120x120x1	HF	10	435	432	550	30	150
EW	69424-2	120x120x1	HF	12,5	433	427	549	31	231
EW	69424-3	160x80x12.	HF	12,5	429	424	548	30	228

*SR: Stretch Reduced; **HF: Hot Finished; *** IE: Impact energy at -20 °C

The rolling conditions to produce the hot-rolled strip to be processed in the hollow section mills for the results presented in Table 2 were similar to those in table 1. The hot-rolled strip is processed into hollow sections in Tata Steel's Electric Weld Stretch Reduction mill (EWSR), and in Tata Steel's Electric Weld mill (EW).
Figure 4 shows a schematic image of the production process.
In Table 3 results are given for steel 28, which has been processed as seamless tube of 12mm wall thickness, and then heat treated at temperatures in the range 880-1000 °C. Table 3 - results of steels 28

ID Normalising temperature, °C YS (MPa) UTS (MPa) A (%)

A9 880 438 544 30.7

A11 920 439 559 31.7

A13 960 476 563 30.7

A14 980 478 571 30.3

A15 1000 494 584 30.0

In Table 4 results are given for steel 69424, which has been hot formed into a hollow section with 5.6 mm wall thickness at temperatures in the range 880-1000 °C and cooled at different cooling rates. The average cooling rate between 850 and 550 °C was measured, and it is noted that the yield strength and tensile strength increase with increasing average cooling rate up to an average cooling rate of ∼ 4 °C/s. If the cooling rate increases further there is a risk that the yield strength starts to fall due to the onset of the formation of small islands of martensite which causes a dual phase like effect. The UTS continued to rise as cooling rate increased.

Table 4 - results of accelerated cooling of the hot formed hollow section.

QA	Average cooling rate 850 - 550 °C (°C/s)	Reh (MPa)	Rm (MPa)	A (%)
13JJ82 A	0.45	463	563	26
13JJ82 B	2.14	468	590	28
13JJ82 C	2.77	483	587	26
13JJ82 D	2.97	501	602	28
13JJ82 E	3.90	483	606	27

Claims

High strength hot-finished steel hollow section with low CEV wherein CEV= C+(Mn/6)+((Cr+Mo+V)/5)+((Cu+Ni)/15), and wherein (in wt.%)
• C: 0.12-0.18;

• Si: <0.60;

• Mn: 1.2-1.6;

• P: <0.035;

• S: <0.015;

• V: 0.13-0.20;

• Al: < 0.04;

• N: 0.008-0.025;

• Ti: <0.01;

• Cr: <0.15;

• Ni: <0.20;

• Mo: <0.04;

• Cu: <0.20;

• Nb: < 0.05%

• Optionally Ca<0.015%, more preferably below 0.005, in an amount consistent with a calcium treatment for inclusion control;

• Remainder iron and inevitable impurities;
wherein CEV ≤ 0.45%; and wherein the mechanical properties meet the S420NH and S420NLH industry standard according to EN10210-1:2006.
Section according to claim 1 wherein Si ≤ 0.25%.
Section according to any one of the preceding claims wherein Si ≥ 0.10%.
Section according to any one of the preceding claims wherein V ≥ 0.15%.
Section according to any one of the preceding claims wherein V ≤ 0.19%.
Section according to any one of the preceding claims wherein one or more of Cr, Ni and Cu are at most 0.05% and/or at most 0.10% jointly.
Section according to any one of the preceding claims wherein CEV ≤ 0.44%, preferably ≤ 0.435%.
Section according to any one of the preceding claims wherein
• C: 0.13-0.16% and/or

• Si: 0.15-0.25% and/or

• Mn: 1.3-1.5% and/or

• P: <0.025% and/or

• S: <0.008% and/or

• V: 0.16-0.18% and/or

• Al: 0.005 - 0.035% and/or

• N: 0.008-0.020%.
Section according to any one of the preceding claims wherein Mn ≥ 1.35%.
Section according to any one of the preceding claims wherein the wall thickness of the section is at most 40 mm.
Section according to any one of the preceding claims wherein the wall thickness of the section is at most 16 mm.
Section according to any one of the preceding claims wherein the section is a welded section or a seamless section.
Process for producing a high strength hot-finished steel hollow section according to any one of the preceding claims comprising the steps of:
• casting a steel slab having a composition in accordance with any one of claim 1 to 9;

• (re)heating the steel slab to a temperature of at least 1150 °C;

• hot-rolling the steel slab to a hot-rolled strip with a finish rolling temperature in the range 800-950 °C;

• cooling the hot-rolled strip with a cooling rate between 2 and 50 °C/s;

• coiling the hot-rolled strip at a coiling temperature in the range 550-720 °C;

• cold forming and welding the strip to form a tube blank;

• hot-forming the tube blank at temperatures in the range 800-1050 °C to a final hollow section with its final dimensions;

• cooling the final hollow section to ambient temperature.
or comprising the steps of:
• producing a steel billet having a composition in accordance with any one of claim 1 to 9;

• (re)heating the steel billet to a temperature of at least 1150 °C;

• Piercing the hot billet to produce a hollow tube shell followed by rolling the hollow tube shell in a plug mill, pilger mill or mandrel mill to produce a tube;

• Optionally passing the tube through a reeling mill to reduce the wall thickness and/or a sizer to produce the desired outer diameter;

• Optionally reheating the tube to a temperature in the range of 800-950 °C, preferably by means of induction, and rolling in a stretch-reducing mill to further reduce the outside diameter and/or the wall thickness;

• Normalising the final tube;

• cooling the final tube to ambient temperature.
Process according to claim 13 wherein the cooling of the final hollow section to ambient temperature occurs by still air cooling, or by forced air cooling.
Process according to claim 13 or 14 wherein the coiling temperature of the hot rolled strip is at least 560 °C and/or at most 650 °C.