LU102322B1 - Green production route for low carbon, low nitrogen steel - Google Patents

Abstract

The invention relates to a method of manufacturing extra and ultra-low carbon and/or in combination with, lowest, low nitrogen steel grades. The method comprises: charging in an EAF a predetermined, nominal charge of solid metal comprising scrap and direct reduced iron, DRI, wherein the DRI has a carbon content of less than 0.1 wt.%, operating the EAF to melt the nominal solid metal charge and form a liquid metal bath with predetermined composition targets and tapping temperature; tapping the liquid metal from the EAF, the tapped liquid metal having the predetermined tapping temperature as well as a target carbon content of no more than 0.1 wt.% and a target nitrogen content of no more than 50 ppm.Operating the EAF includes denitriding the metal bath by implementing one or more of the following measures: (a) injecting oxygen in the metal bath to react with a further charge of solid or liquid iron containing material with a carbon content above 1 wt.%, thereby causing CO bubbling; (b) injecting oxygen in the metal bath to react with a further charge of non-DRI-bound carbon material containing less than 50 ppm of nitrogen, thereby causing CO bubbling; (c) injecting a washing gas from below the metal bath

Description

Green production route for low carbon, low nitrogen steel

FIELD OF THE INVENTION The present invention relates to the field of iron metallurgy and more specifically to the production of extra and ultra-low carbon and/or in combination with lowest and low nitrogen steel.

BACKGROUND OF THE INVENTION With the Paris Agreement and near-global consensus on the need for action on emissions, it is imperative that each industrial sector looks into the development of solutions towards improving energy efficiency and decreasing CO; output.

In this context, players in the field of iron metallurgy have developed new approaches in order to reduce the environmental footprint of both the blast furnace iron making route and the electrical steel making route.

One technology developed to reduce the carbon footprint during steel production is the iron ore direct reduction process. Although annual direct reduction iron production remains small compared to the production of blast furnace pig iron, it is indeed very attractive for its considerably lower CO, emissions, which are 40 to 60% lower for the direct reduction electric-arc furnace (EAF) route, compared to the blast furnace - basic oxygen (BF — BOF )route.

In a direct reduction shaft furnace, a charge of pelletized or lump iron ore is loaded into the top of the furnace and is allowed to descend, by gravity, through a reducing gas. The reducing gas, mainly comprised of hydrogen and carbon monoxide (syngas), flows upwards, through the ore bed. Reduction of the iron oxides occurs in the upper section of the furnace, conventionally at temperatures up to 950 °C and even higher. The solid product, called direct reduced iron {DRI} is typically charged hot into Electric Arc Furnaces, or is hot briquetted (MBI).

In most of the existing applications of DR! the above-mentioned syngas is generated via reforming of natural gas; in some cases, a suitable gas is already available, whereby natural gas is not required. The well-known MIDREX® NG process is based on reforming of natural gas. The MIDREX plant comprises a reformer that cost effectively makes reducing gas for the iron ore reduction reactions that take piace in the MIDREX® Shaft Furnace. The reformer externally generates reducing gas and further optimizes the shaft furnace performance by converting recycled gas (from the iron reduction reactions) along with fresh natural gas into Hz and CO to produce additional reducing gas.

Such natural gas based DRI production, when paired with an EAF, has lower CO» emission than the conventional BF-BOF route.

The electric steelmaking route, ie. with EAF, is also popular for its high flexibility in operation and raw materials, and the lower investment costs compared to integrated BF plants.

The main limitation of electric steelmaking is however the presence of tramp elements in scrap as main charging material and higher nitrogen content compared to blast furnace - BOF route. Conventionally, tramp elements are reduced by mixing the scrap with virgin iron material such as hot metal/pig iron and/or addition of HBI/DRI (hot briquetted iron/ Direct reduced iron). The adsorption of nitrogen in the furnace is reduced by the application of slag foaming and carbon boiling control. The installation of vacuum degassers in the EAF steel making route improves the quality of steels products, allowing production of steels with low carbon and low gas contents.

Still with the aim of further decreasing emissions, initiatives are being developed that use hydrogen as a fue! and chemical reactant in the process. Although the possibility of producing DRI by direct reduction from hydrogen has been disclosed decades ago in the literature, industrialization of such process only starts now.

Indeed, the industrial vision to further reduce the environmental impact of steel production for now and the near future is to use green hydrogen to produce DRI

FLERE ASOSLS 3 LU102322 as fredstiock for sipeeimaking. In this process, hydrogen is obtained by sieciraiysis or from biomass. Against these recent approaches, the Blast Furnace — Basie oxygen fumace remains the most common route for steel manufectaing allowing for the largest versaîiity of steel grades, The BF is conventionally charged with iron ore ani carbon k needed ax reducing agent, wharshy CO: missions are unavoidable. European sipsimakers have minimised the amount of carbon used as far as its possible within the thermodyriamit frites of the process The BF roule permis manufacturing of any steel nuances. The pig ban produced by the blast furnace has à relatively high carbon (4-5 wi 9h) and sulfur content itis tharefors processed to reduce the carbon and sulphur content and produce various grades of steel used for construction materials, automobiles. ships and machinery, Desuiphurisaion ustslly takes place during he fransnort of the figuid hol metal fo the steeheorks, In a further sten, in the basic oxygen 15 furmnace, he carbon Is vaidired by blowing oxygen onto the guid sig on io form crude steel i should be noted that some of the most demanding steel grades are foday only manufacturad vis the BF route, such as 6.9. sulomolive exposed siseïs and non grain crented (NGO) electrical steels There, the classical roule is BF desulphurization, BOF, vacuum degassing with oxygen Mowing ic lower OL For these grades of steal, very low nitrogen contents are quired on the final product obtained, sg a8 low as 15 to 25 ppm for certain steed sheets intended for automobile construction 2.9, 2xposed applications ar for siesis for packing, af from 40 to 50 ppm for iyre-reinforoing wires, sie.

For hs reason, the BAF mule is considered noi to be desirable, due fo cormaminetion of the metal with nitrogen, which is dus fo He cracking of the molecule of nitrogen of the air in the heat zone of the electric arc which facilitates ts transfer to the liquid metal This phenomenon is Kaown to an important factor whith prevents production by the “electrical procedure” of a 32 part of the grades produced today by the BE-BOF route yy whith lower ratrogen contents, of the order of 20 ppm, are currently obtained,

Nevertheless, the production of low nitrogen steel via electric steelmaking has been reported in mini and midi steel! plants, see e.g. S. K. Dutta and A. B. Lele in Production of Quality (low nitrogen) steels by using sponge iron in EAF, IRON & STEEL REVIEW, VOL. 55 NO.3, August 2011. The disclosed method uses sponge iron, i.e. DRI, as charging material, mixed with scrap or alone. The use of DRI with high carbon content, i.e. 2.4 wt. % and above) is recommended to promote vigorous carbon boil due to CO gas formation. The CO gas formation helps removing nitrogen from the molten bath and also promotes slag foaming which protects the metal bath from air infiltration (providing a barrier between molten bath and atmosphere).

OBJECT OF THE INVENTION The object of the present invention is to provide an improved approach for the production of low C and low N steel, which takes into account industrial requirements and environmental issues.

This object is achieved by a method as claimed in claim 1.

SUMMARY OF THE INVENTION The present invention proposes a green production route for low C and low N steel grades, based on the use of green DRI.

As used herein, the term ‘green DRI’ means direct reduced iron produced using hydrogen as reducing gas, itself originating most preferably from a green hydrogen source, i.e. a source involving low or zero CO» emission. Green hydrogen may be produced using a water electrolysis plant operated with renewable electricity, namely photovoltaic or windturbines. Less preferably nuclear power, being non-sustainable but having very low CO; emissions can be used for the electrolysis. Depending on the production route, the green DRI has, in the context of the invention, a carbon content of less than 0.1 wt. %, in particular less than 0.05 wt.%, and typically less than 0.01 wt.%. Ideally, green

P-PWU-804/.U 5 LU102322 DRI is reduced without carbonaceous material and has a carbon content of 0% (except for unavoidable impurities).

The term green DRI here also encompasses any suitable form of the DRI, be it free flowing powder or granules, or preferably lumpy form, balls, briquettes, agglomerates of any shape.

In the context of the invention, the green DRI can be charged to the EAF in cold or hot state. Accordingly, the language hot DRI designates DRI that is at a temperature well above the room temperature, typically several hundred °C, for example between 500 to 700°C, in particular about 600°C. The DRI can be charged hot from a direct reduction plant, or heated up in a dedicated furnace before introduction in the EAF.

According to the present invention, a method of manufacturing low carbon, low nitrogen steel comprises: charging in an EAF a predetermined, nominal charge of solid metal comprising scrap and direct reduced iron, DRI, wherein the DRI has a carbon content of less than 0.1 wt. %, operating the EAF to melt the nominal solid metal charge and form a liquid metal bath with predetermined composition targets and tapping temperature; tapping the liquid metal from the EAF, the tapped liquid metal having the predetermined tapping temperatures as well as a target carbon content of no more than 0.1 wt.%, preferably no more than 0.05 wt%, and a target nitrogen content of no more than 50 ppm; wherein operating the EAF includes denitriding the metal bath by implementing one or more of the following measures: (a) injecting oxygen in the metal bath to react with a further charge of solid or liquid iron containing material with a carbon content above 1 wt. %, thereby causing CO bubbling;

(b) injecting oxygen in the metal bath to react with a further charge of non- DRI-bound carbon material with less than 50 ppm of nitrogen, thereby causing CO bubbling; (c) injecting a washing gas from below the metal bath.

‘Nominal charge’ designates the total amount of scrap and green DRI that is actually loaded in the EAF to reach a design hot metal volume in one tapping cycle of the EAF.

Scrap conventionally means iron bearing metallic scrap, in particular steel scrap. The average carbon content of the scrap used for the nominal charge may be between 0.1 to 0.3 wt.%, on particular about 0.15 to 0.2 wt.%. Concentrations of materials used in the EAF are, unless specified otherwise, expressed in percentage or ppm are with respect to weight. À particularly original aspect of the invention is the use of green DRI in electric steelmaking for the production of extra and ultra-low carbon and/or in combination with, lowest and low nitrogen steel grades. Indeed, these steel grades are typically manufactured via the BF-BOF route that allows a better control on nitrogen levels. In this context, it may be noted that in the conventional EAF production, where the metal charge consists of scrap and conventional DRI (i.e. >1. wt.% carbon content), the nitrogen content at tapping is around20 ppm when using only DRI and 80 ppm when using only scrap, and about 0.05-0.1 wt.% for carbon. As is known in the art, in the conventional EAF practice a level of around 50 ppm nitrogen can only be obtained by addition of carbon containing DRI for the purpose of diluting the tramp elements of the scrap charge, and for introducing carbon in the bath that will be helpful for heating the bath and removing nitrogen - via Oxygen lancing and formation of CO. The general understanding of the person skilled in the art of electric steelmaking is that carbon is useful for reducing the electric heating energy and for denitriding, the carbon being brought by pig iron (solid or liquid) and/or via addition of carbon containing DRI, ie. obtained from conventional direct reduction processes with syngas (from natural gas).

in the past, electric steelmaking has been mainly used for long products, where nitrogen levels and impurities are less critical.

The present invention goes against conventional wisdom by selecting the electric steelmaking route for the manufacture of extra and ultra-low carbon and/or in combination with, lowest and low nitrogen steel grades, since these steels are traditionally manufactured via the BF-BOF route. Furthermore, denitriding in the EAF has been conventionally obtained by addition of pig iron or carbon containing DRI, permitting nitrogen removal via CO gas evolution and resulting in a dilution effect. DRI from the conventional MIDREX process has carbon content between 1.5 and 4 wt.%. Feedback for EAF production of low carbon low nitrogen steel grades insist on the need for high carbon content DRI. Hence, the selection of the EAF steelmaking route to produce extra and ultra- low carbon and/or in combination with, lowest and low nitrogen steel grades from green DRI (ie. with essentially no carbon content) would, from the outset, appear as inappropriate. However, the present inventors have found that this production route can actually be envisaged at industrial scale. The low carbon content is not an issue for the target nuance, but is problematic for reducing the nitrogen content. It is indeed considered that for ultra low carbon / IF steel grades for automotive exposed, i.e. with typically less than 15 ppm carbon and 30 ppm nitrogen, it will not be possible to achieve the grade composition requirements in the secondary metallurgy. Indeed nitrogen —in secondary metallurgy- can only be removed if the Sulphur has been lowered to extremely low levels, which requires long treatment times. Despite all precautions (carbon lean ladle linings, carbon lean additives...) taken to avoid carbon pick-up during the desulphurization process, it is likely that the carbon content will increase above target levels.

The lack of carbon from the charge material can be compensated by addition of carbon containing material. From the process perspective, carbon addition is required for slag foaming, which forms a protective cover maintaining heat and reducing nitrogen adsorption. The addition of carbon to the metal bath also allows CO bubbling by injection of oxygen in the bath (typically via lances), and thus removing nitrogen from the metal bath. The lack of high carbon from the charge material thus implies the use of comparatively higher additions of carbon containing material to the EAF as compared to a situation using conventional DR! with about 2 wt.% carbon. Hence, the skilled person would a priori avoid such process, where carbon addition would be higher, since the goal is to develop a more environmentally friendly approach with reduced CO; emissions.

Against this background, and in a surprising manner, the present inventors have found that despite the need for additional carbon containing material, the present method requires overall less energy than the conventional operation with carbon containing DRI (this includes the chemical energy of the carbon in the DRI). Based on first simulation results, it has been found that the present method requires globally less energy consumption that conventional operation. Furthermore, the inventors have calculated that the direct CO, emissions can be reduced by as much as 30%.

The charging, operating and tapping of the EAF, in terms of operational steps, may be carried out according to conventional practice. In particular, the scrap is normally loaded into the open EAF, e.g. in baskets. The EAF may typically contain a hot heel of liquid metal. The charge of green DRI, or a part thereof could be charged together with the scrap metal. But it is thermally more efficient to introduce at least part of the green DR! amount in a progressive manner, as the furnace is operated (arcing). This allows dissolving the green DRI in the hot metal bath. The further addition of carbon containing material to the metal bath for the purpose of CO bubbling can be done (in whole or in part) while the furnace is open, concurrently with the scrap. Further carbon addition can be done during the operation of the EAF, in particular in a progressive manner. The further addition of carbon containing material to the metal bath under measures a) and b) will permit, by oxygen injection/lancing, forming CO bubbling and hence removing nitrogen from the metal bath.

The further charge of solid or liquid iron containing material with a carbon content above 1 wt.% (or preferably above 2%) may include one or more of liquid hot metal, solidified hot metal, conventional DRI (ie. with at least 1 wt.% carbon). These metal charges preferably consist of desulphurized metal. The pig iron may be green if produced by charcoal blast furnace.

The further charge of non-DRI-bound carbon material having a nitrogen content of less than 50 ppm can be introduced in the EAF before arcing, but is preferably introduced progressively into the furnace during operation, so that it is added to the moiten metal bath. Injection of this non-DRI-bound carbon material can be done through injectors in the upper furnace region and/or lower furnace region. In embodiments, such non-DRi-bound carbon material with of less than 50 ppm nitrogen may e.g. comprise carbon black and/or carbon from CH, pyrolysis. Preferably the non-DRI-bound carbon material contains less than 40, or most preferably less than 30, 20 or 10 ppm nitrogen.

The further charge of solid or liquid iron containing material and/or the further charge of non-DRI-bound carbon material is/are added in quantities sufficient to reach a carbon content in the liquid metal bath of about 1 to 1.5 wt.%, in particular about 1.3 to 1.5 wt.%, before injection of oxygen as per measure a) or b).

In order to reach a carbon content in the liquid metal bath of about 1.3 to 1.5 wt.%, the following carbon additions can be envisaged: - for a nominal solid metal charge comprising between 50 and 60 wt. % DR! of less than 0.1 wt.%, the further charge of solid or liquid iron carbon may be added at a rate of 8 to 16 kg per ton of nominal solid metal charge.

- for a nominal solid metal charge comprising between 50 and 60 wt. % DRI of less than 0.1 wt. %, the further charge of solid or liquid iron carbon may be added as pig iron at a rate of 150 to 350kg, in particular 300 kg, per ton of nominal solid metal charge.

As regards the use of washing gas, any appropriate gas capable of capturing the nitrogen in the bath and having, before introduction into the bath, a low nitrogen content (typically less than 50 ppm and preferably below 40, 30 or 20 ppm). Washing gas introduction is preferably achieved via purging elements well distributed through the bottom wall of the EAF.

The purging elements may be formed multi hole purging bricks fed with the washing gas.

Suitable gases for use as washing gas are argon, helium, CO, or a combination thereof.

Less preferably CO» could be used if for example captured downstream.

The feed temperature of these gases is preferably ambient.

The operation of the EAF may conventionally further include slag foaming by injecting, e.g., devolatilized coal (i.e coke) out of lignite or hard coal or synthetic coke such as petroleum Coke with a grain size distribution between 0.1 and 2mm.

Once removed (tapped) from the EAF. the tapped liquid steel may be conventionally treated via secondary metallurgy to adjust the composition to the desired steel grade.

The present invention is particularly suitable for manufacturing the following steels grades: NGO, automotive exposed, Interstitial Free Steel (in particular White goods, Packing, or DWI cans). Such operation is also beneficial for the production of grain oriented (GO) electrical steel and high quality sour gas resistant grades (also known as HIC grades). Examples of such steel grades that can be obtained by secondary metallurgy from the liquid metal tapped from the EAF according to the present invention are given in table 1 below.

D

VU =

S Co oO

A = Grades (ppm / %) c s 500 - 600 ppm 0,2-0,3% 3.20% 150 ppm 76- 90 ppm 70- 90 ppm 260 - 310 ppm 7 i ü " " Cu or i * i > wr NGO high quality 40 ppm 0,2-0,5% 2% - 2,5% 400 ppm 40 ppm 40 ppm 200 ppm 6° 4 F | FF | , + 1 ’ | Fr | Fr I Fr | =r | | Ï | | i i | Automotive exposed / bake | 15 ppm | 0,2%-0,45% 200 (600) ppm | 200-550 ppm | 100 (120) ppm 30 ppm | - | hardening (ppm) | | | | | | | | IFZULC White market | 35 ppm i 0,15-0,4% | 004-0,12% | 250-380ppm (50) 100 (120) ppmj 43 ppm | 200-450ppm | | | | | | i | | _ IF/ULC Packing | 40 ppm | 035-045% | 01-013% | 380-600ppm [{50) 100-200 ppm 80 ppm | 200-800ppm | > | | | | | ! | | IF/ULC DWI cans | 80 ppm | 0,17-0,25% 0,04 - 0,12% | 250 - 650 ppm | 120 (50) ppm | 50 ppm | 400 - 700 ppm | | | | | | 1 | | X 80 | 0,05 - 0,07% | 1,5- 1,9% | 0,3-0,4% 80 ppm | 10 (5-8) ppm | 60 ppm | 0,03 - 0,04 % | | | | | | ! | | X 70 | 0,05 - 0,07% | 13-18% | 02-04% | 90-150ppm | 20(5-8)ppm | 60(80}ppm | 0,02-004% | | | | | | | i |

À D

T @ — | | | | | | | | | | | | | | | |

Examples Simulations have been carried out for two examples in order to determine the involved energy consumption and related CO, emissions for a nominal charge containing 50 wt.% scrap and 50 % of DRI.

The simulation involves estimating the energy consumption for several similar operational parameters to achieve a same bath target composition with a target carbon content of no more than 0.05 wt% with a tapping temperature of 1650°C. The DRI charging temperature is 600°C.

For creating the foaming slag, a dedicated carbon (petroleum coke) injection into the slag layer of 15 kg/t was considered for a nominal charge with green DRI (0% Carbon), whereas an injection rate of 10 kg/t was considered for conventional DRI (2.5% C). in case of green DRI, metal bath denitriding is carried out by adding carbon to the metal bath and subsequent oxygen injection, to perform CO bubbling. in the example, carbon containing material is added to reach a carbon addition of 5 kg/t of metal bath. The energy related to the following aspects has been estimated: electrical energy for operating the EAF, DRI sensible heat, operating EAF burners in the initial stage, carbon injection, bath exothermal energy and electrode consumption. The values are summarized in table 2. The “bath exothermal energy” includes, for conventional DRI, the heat due to combustion already present in the bath, whereas in the case of green DRI it includes the heat due to the combustion of the additional carbon added to the metal bath. As can be seen, although a sensibly greater carbon injection is required, operation with green DRI leads to lower total energy consumption and substantially reduced CO; emissions.

~~] Counter-example | Invention Energy consumption in kWh/t DRI [C] 2.5% DRI! [C] 0% 465 58 58 oo injected carbon 110 __ 140 bath exothermal reaction _ 21 _ Electrode consumption 0 18 _ Totalemergy me | 717 CO; emissions | LU... [CO2(kg) 9 80

Table 2

Claims (20)

Claims

1. A method of manufacturing low carbon and/or low nitrogen steel grades, in particular extra and ultra-low carbon and/or in combination with, lowest, low nitrogen steel grades, comprises: charging in an EAF a predetermined, nominal charge of solid metal comprising scrap and direct reduced iron, DRI, wherein the DRI has a carbon content of less than 0.1 wt.%, operating the EAF to melt the nominal solid metal charge and form a liquid metal bath with predetermined composition targets and tapping temperature; tapping the liquid metal from the EAF, the tapped liquid metal having the predetermined tapping temperature as well as a target carbon content of no more than 0.1 wt.% and a target nitrogen content of no more than 50 ppm; wherein operating the EAF includes denitriding the metal bath by implementing one or more of the following measures: (a) injecting oxygen in the metal bath to react with a further charge of solid or liquid iron containing material with a carbon content above 1 wt.%, thereby causing CO bubbling; (b) injecting oxygen in the metal bath to react with a further charge of non-DRI-bound carbon material containing less than 50 ppm of nitrogen, thereby causing CO bubbling; (c} injecting a washing gas from below the metal bath.

2. The method according to claim 1, wherein said nominal solid metal charge comprises between 15 and 75 wt.%, preferably between 40 and 70 wt.%, in particular about 50 to 60 wt.% of DRI with a carbon content of less than

0.1 wt.%.

3. The method according to claim 1 or 2, wherein the DRI in said nominal solid metal charge has a carbon content of less than 0.01 wt. %.

4 The method according to claim 1, 2 or 3, wherein the DRI of said nominal metal charge is charged hot in the furnace.

5. The method according to claim 4, wherein the scrap of the nominal metal charge is loaded in the open furnace, and at least part of the DRI of the nominal metal charge being introduced progressively into the operating furnace.

6. The method according to any one of the preceding claims, wherein said further charge of solid or liquid iron containing material includes pig iron, in particular pig iron produced from a charcoal operated blast furnace.

7. The method according to any one of the preceding claims, wherein said further charge of solid or liquid iron containing material includes solidified hot metal.

8. The method according to claim 6, wherein said solidified hot metal has a sulfur content of less than 200 ppm.

9. The method according to any one of the preceding claims, wherein further said charge of solid or liquid iron containing material includes direct reduced iron with at least 1 wt.% carbon.

10.The method according to any one of the preceding claims, wherein said further charge of solid or liquid iron containing material is introduced together with said nominal sotid metal charge.

11.The method according to any one of the preceding claims, wherein said further charge of non DRI bound carbon material comprises carbon black and/or carbon from CH4 pyrolysis.

12. The method according to any one of the preceding claims, wherein said charge of carbon material is introduced from above or from below the metal bath.

13.The method according to any one of the preceding claims, wherein said further charge of solid or liquid iron containing material and/or said further charge of non-DRI-bound carbon material are added in quantities sufficient to reach a carbon content in the liquid metal bath of about 1 to 1.5 wt.%, in particular about 1.3 to 1.5 wt.%, before injection of oxygen as per measure a) or b).

14. The method according to claim 13, wherein the nominal solid metal charge comprises between 50 and 60 wt.% DRI of less than 0.1 wt%; and the further charge of solid or liquid iron carbon is added at a rate of 8 to 16 kg per ton of nominal solid metal charge.

15. The method according to claim 13, wherein the nominal solid metal charge comprises between 50 and 60 wt.% DRI of less than 0.1 wt.%; and the further charge of solid or liquid iron carbon is added as pig iron at a rate of 150 to 350kg, in particular 300 kg, per ton of nominal solid metal charge.

16. The method according to any one of the preceding claims, wherein said washing gas includes argon, helium, CO and/or CO».

17. The method according to any one of the preceding claims, wherein operating the EAF includes performing slag foaming.

18. The method according to any one of the preceding claims, wherein the tapping target carbon content is less than 0.01 wt.%.

19. The method according to any one of the preceding claims, wherein the tapped steel is treated via secondary metallurgy to adjust the composition to the desired steel grade.

20. A low carbon, low nitrogen steel obtained from the method according to any one of the preceding claims, said steel comprising iron with: carbon: 15 to 80 ppm; manganese: 0.2 to 0.5 wt.%; silicon: 0.02 to 3.2 wt.% phosphorous: up to 650 ppm; sulphur: up to 30, 100 or 200 ppm nitrogen: up to 100 ppm, in particular up to 80 or 50 ppm; and aluminum: 0.03 to 1 wt.%; optionally, copper up to 0.10 wt.%; and unavoidable impurities.