EP2646582A1

EP2646582A1 - Ultra high-strength structural steel and method for producing ultra high-strength structural steel

Info

Publication number: EP2646582A1
Application number: EP11815448.3A
Authority: EP
Inventors: Tommi Liimatainen
Original assignee: Rautaruukki Oyj
Current assignee: Rautaruukki Oyj
Priority date: 2010-12-02
Filing date: 2011-12-01
Publication date: 2013-10-09
Anticipated expiration: 2031-12-01
Also published as: FI20106275A0; CN103348020A; RU2013129002A; RU2586953C2; CN103348020B; EP2646582B1; ES2761839T3; FI20106275A; WO2012072884A1; WO2012072884A9

Abstract

The invention relates to an ultra high- strength structural steel, which is produced by hot- rolling as a sheet-like steel product. The invention relates to the production of a direct quenched and ultra high-strength structural steel, whose composition, as percentages by weight, comprises C: 0.07 - 0.12 %, Si: 0.1- 0.7 %, Mn: 0.5 - 2.0 %, Ni : 0.8 - 4.5 %, Cu: 0.25 - 3.0 %, Cr: 0.5 - 1.6 %, Mo: < 0.8 %, and Ti: ▪ 0.04 % as well as iron, unavoidable impurities and residual contents. The invention also relates especially to an ultra high-strength structural steel, whose composition comprises above said elements, wherein, in addition to high strength, the structural steel is also excellently weldable and possesses excellent impact toughness properties.

Description

Ultra high-strength structural steel and method for producing ultra high-strength structural steel

Background of the invention

The invention relates to an ultra high-strength structural steel, which is produced by hot-rolling as a sheet-like steel product.

More specifically, the invention relates to an ultra high-strength structural steel according to the preamble of claim 1 and a method for producing an ultra high-strength structural steel according to the preamble of claim 27. By an ultra high-strength structural steel is meant steel, whose yield strength R_po.₂ is at least 960MPa and yield ratio (Rpo.2 Rm) is more than 0.7, which is typical especially of direct quenched structural steels.

The invention relates to production of an ultra high-strength structural steel, whose composition, as percentages by weight, comprises

C: 0.07 - 0.12 %,

Si: 0.1 - 0.7 %,

Mn: 0.5 - 2.0 %,

Ni : 0.8 - 4.5 %,

Cu: 0.25 - 3.0 %,

Cr: 0.5 -1.6 %,

Mo: < 0.8 %, and

Ti: · 0.04 %

as well as iron, unavoidable impurities and residual contents.

The invention also relates especially to an ultra high-strength struc- tural steel, whose composition comprises above said elements, wherein, in addition to high strength, the ultra high-strength structural steel also has excellent weldability by possessing excellent impact toughness properties in the heat affected zone of the weld.

Structural steels are used in construction and transportation equip- ment industries, in which the trend is to use stronger steels to achieve lighter structures. In this case, steel structures can be lightened, the load-bearing capacity of machines and devices improved and energy consumption decreased. An attempt is made to provide a steel that is as strong as possible, but, as strength increases, usability properties, such as weldability and flangeability as well as other important properties, such as elongation and impact toughness can weaken, as is known. Especially, in such strong steels, the risk related to the hydrogen cracking of welding is great and fracture behaviour is sometimes unsure, which makes the work of the structural engineer and the calculator of strength problematic. In the heat affected zone (HAZ) of the weld, there also can not be during welding any significant softening, or at least the width of the softened zone should remain as narrow as possible, in order that the strength of the welded joint remains good, i.e. as close as possible to the strength of the base metal. Other mechanical properties of the heat affected zone of the weld, such as elongation and especially impact toughness, should also remain as good as possible, in order that the design work of practical structures remain sensible and as simple as possible and the functionality of the structures in practise is assured.

As is known, also the heat input must be significantly limited when welding high and ultra high-strength steels, which can cause an unfavourable form of weld bead for fracture toughness and fatigue resistance, because, at a low heat input, the junction point of the weld metal and base metal does not form smoothly as easily as at a higher heat input. Limitation of the heat input also weakens the productivity of welding work, adding expenses.

Moreover, structural steels must sometimes be weather-resistant, wherein the steel does not necessarily need to be coated because an tight ox- ide layer forms on the surface of the steel, which significantly slows down the progress of corrosion. In demanding sites, the steel must withstand, in addition to normal climatic stress, also the sodium chloride NaCI contained in a maritime climate, wherein ultra high-strength structural steel can excellently be used also in ship and harbour structures even without coating. Also when coated, the du- rability of the paint surface of weather-resistant steel in local damage points of the surface is better, because the progress of rust below the paint surface slows down. Especially by ultra high-strength structural steels, the attempt is for the smallest material thicknesses possible and, therefore, assurance against corrosion damages is further emphasized.

Known is i.a. an ultra high-strength hot-rolled structural steel, the composition of which contains, as percentages by weight, typically C: 0.095 %, Si: 0.20 %, Mn: 1.0 %, Cu: 0.4 %, Cr: 1 %, Ni: 0.20 %, and Ti: 0.03 % as well as iron and unavoidable impurities and residual contents. The steel in question is produced as having a yield strength greater than 960MPa, but the impact toughness of its base metal is only moderate and the steel in question does not withstand the stress of saltwater very well. Likewise, during welding, the heat af- fected zone of the base metal softens significantly and over a relatively wide zone, wherein the load-bearing capacity of the weld seam, such as strength and impact toughness, weakens during typical welding significantly in comparison to the load-bearing capacity of the base metal.

From KR950004775 B1 is known a method for producing a steel having a yield strength of approximately 150KSI (·1034ΜΡ8), in which method a steel of composition C: < 0.12 %, Mn: 0.6 - 0.9 %, Si: 0.2 - 0.35 %, Ni: 4.75 - 5.25 %, Cr: 0.4 - 0.7 %, Mo: 0.3 - 0.65 %, V: 0.05 - 0.10 %, P: < 0.01 %, and S < 0.015 %, the rest being iron and unavoidable impurities, is quenched from the hot-rolling process and, thereafter, temper-treated. However, the problem with this known steel is that the steel is not particularly weather-resistant, because into it was not alloyed copper Cu and also simultaneously just a small amount of chromium Cr. Solution hardening by using nickel Ni without copper Cu leads to excessively high Ni-contents, which is disadvantageous i.e. on the part of alloying element expenses. If the steel is further tempered after direct quenching, this steel has no copper Cu separating, which would further increase the strength of the steel. This known steel is also not particularly weld- able, because into it has not been alloyed titanium Ti to prevent grain formation at a high heat input, wherein, due to small welding energies, it is difficult to get the junction point of the weld bead and the base metal smooth, wherein a tough and strong joint is difficult to achieve.

The combination of excellent strength and impact toughness of known ultra high-strength steels, achieved by thermo-mechanical rolling and direct quenching, weakens in the HAZ due to the heat cycle caused by welding.

Thus, the problem of known solutions is especially that there is not known an ultra high-strength hot-rolled structural steel, in which, at the same time along with high HAZ impact toughness and weather resistance of the base metal, high strength is achieved.

Brief description of the invention

The object of this invention is to solve the problems of known art and to provide an excellent ultra high-strength structural steel, wherein the impact toughness of the HAZ of a weld joint provided in this steel is excellent. In this case, the steel can be used in welded applications requiring impact toughness, such as in welded boom structures.

The invention provides an ultra high-strength structural steel, wherein the impact toughness of the HAZ of a weld provided in this steel, as measured transversely in relation to the direction of rolling at a temperature of -40°C, is more than 34 J/ cm², i.e. expressed differently

Charpy V -40°C T (FL, ICHAZ, CGHAZ) > 34 J/ cm²

The second object of the invention is to provide a new method for producing an ultra high-strength structural steel from a steel, whose composition comprises said element contents.

To implement this, an ultra high-strength structural steel according to the invention is characterized by that what is said in the characterizing part of claim 1.

A method according to the invention is characterized by that which is said in the characterizing part of claim 27.

Advantageous embodiments of the invention are presented in the in- dependent claims as well as in the description.

The objects of the invention are achieved by alloying the composition of the steel according to the invention and, preferably but not necessarily, by direct quenching the steel having the alloyed composition after hot-rolling. In other words, the present invention is implemented, preferably but not necessarily, by using a unique combination of the composition of the steel and direct quenching.

According to the invention, it is surprisingly found that by said composition and, preferably but not necessarily, by direct quenching a steel according to said composition, an ultra high-strength steel is achieved possessing, at the same time, good HAZ impact toughness properties and having the excellent weather resistance and weldability properties. Specifically, in the invention, it is surprisingly found that a highly nickel- and/or copper-alloyed as well as with, at the most, a small amount of titanium-alloyed steel achieves an ultra high- strength steel, whose base metal and weld HAZ strength and impact toughness are at an excellent level.

The greatest advantages of an ultra high-strength structural steel according to the invention are that it has ultra-high strength combined with good impact toughness and it can excellently be welded such that the mechanical properties of the HAZ are excellent. Welding can even be done without preheat- ing. During welding, the problems relating to hydrogen cracking can be avoided and thus can be used, for such a strong direct quenched structural steel, rea- sonably great heat input, wherein the load-bearing capacity of the weld remains at an excellent level and the efficiency of welding work can be kept high. Further, a structural steel according to the invention has good properties relating to fracture toughness, which is advantageous especially in extreme stress situa- tions. As a significant advantage is the high impact toughness of the HAZ, especially of the fusion line, in such a strong structural steel also at unusually low temperatures.

Moreover, the steel is also excellently weather-resistant, wherein it can also be used when a steel surface patinating to dark brown is a design goal. A steel according to the invention is capable of slowing down corrosion progressing under the paint surface, which increases the security of the structure against corrosion, for example, in places, which are difficult to inspect visually or whose coating to prevent corrosion is difficult or impossible. Likewise, structural inspection intervals and intervals between performances of reparative painting can be increased. Moreover, when suitably alloyed, the steel withstands marine conditions longer than normal, even without coating. When coated, the steel withstands excellently even in a relatively strongly corrosive environment, wherein the adhesion of the coating improves and the need to renew the coating is reduced due to a steel according to the invention.

Good security against corrosion and smooth form of the weld seams to prevent fatigue crackings increase the life cycle of the final product to be produced from an ultra high-strength structural steel according to the invention.

The greatest advantages of a method according to the invention are that it enables a structural steel having above said advantages. Due to direct quenching, no separate heating for hardening and hardening need be performed, which means a significant energy savings. Moreover, direct quenching to a low temperature speeds up the throughput of production, when there is no need to wait for the steel product to cool.

Brief description of the figures

In the following, the present invention is described in more detail with reference to the accompanying figures, in which:

Fig. 1 shows the main steps of the method according to the invention,

Fig. 1 shows a method, which further has tempering, Fig. 3 shows in more detail the first preferred embodiment of the method according to the invention, and

Fig. 4 shows in more detail the second preferred embodiment of the method according to the invention.

In the figures, a dashed line means that the next step is a preferred, but not imperative, step of the method.

Description of the reference numbers Alloying 2

Pre-rolling 4

Rolling 5

Direct-quenching 8

Turning the sheet 90· 9

Coiling 10

Tempering 12

Detailed description of the invention The composition of an ultra high-strength structural steel according to the invention comprises, as percentages by weight:

C: 0.07 - 0.12 %,

Si: 0.1 - 0.7 %,

Mn: 0.5 - 2,0 %,

Ni: 0.8 - 4.5 %,

Cu: 0.25 - 3.0 %,

Cr: 0.5 - 1.6 %,

Mo: < 0.8 %, and

Ti: · 0.04 % as well as iron (Fe), unavoidable impurities and residual contents.

Table 1 shows examples of the composition of a steel according to the invention, which composition is described in the following in more detail. The table further has a reference composition R1. Table 1. Examples. Contents as percentages by weight

Carbon C

In relation to its strength level, the steel has a somewhat low carbon content C: 0.07 - 0.12 %, which is useful for the impact toughness and weldabil- ity of the material, wherein the carbon equivalent CEV itself can be somewhat high. Carbon C is needed at least 0.07 %, in order that hardening be succeed and ultra-high strength be provided. Preferably, the carbon content C of the steel is 0.08 - 0.12 %, which further improves said properties. Most preferably, the carbon content is in the range of 0.08 - 0.10 %. A low carbon content also hampers the formation of retained austenite between the martensite laths, wherein the risk of hydrogen crackings is reduced.

Surprisingly, a somewhat high carbon equivalent CEV = (C + Mn/6 +

(Cr + Mo + V)/5 + (Ni + Cu)/ 5) in an ultra high-strength structural steel according to the invention influences positively on the providing a combination of good impact toughness and strength in a weld. From Table 1 , it is observed that, for an example steel according to the invention, the carbon equivalent CEV > 0.50. Thus, during welding, the re-austenitized zone hardens adequately, wherein it is achieved high strength combined with good impact toughness. More preferably, the carbon equivalent of the steel is 0.5 < CEV < 1.2, most preferably 0.65 < CEV < 1.00.

Silicon Si

The Si-content of the steel is 0.1 - 0.7 %, especially to achieve strength. Si-contents of less than 0.1 % are not used, because desulphurization and form control of inclusions of the steel is easier, when the steel contains a little silicon. Moreover, silicon Si contributes to improving the weather resistance of a steel. On the other hand, an excessive Si-content can weaken impact strength and impair surface quality. For this reason, Si-content is preferably 0.15 - 0.4 % and most preferably 0.15 - 0.25 %, wherein the excellent surface quality as well as the excellent impact toughness of a sheet-like steel are assured. Manganese Mn

The Mn-content of the steel is 0.5 - 2.0 %, because, with manganese, the hardenability of the steel is increased. However, the Mn-content of the steel is preferably moderate, because manganese Mn can infiltrate in con- tinuous casting and can weaken unfavourably the elongation of a direct quenched steel as well as weaken also fracture toughness. For this reason, at the thicknesses of structural steel, manganese is used preferably 0.5 - 1.5 %. According to one preferred embodiment, manganese Mn is alloyed at least 0.7 %, especially when the thickness of the structural steel T_h · 5 mm. According to the most preferable embodiment, manganese is alloyed at least 0.9 %, especially when the thickness of the structural steel T_h · 6 mm.

Nickel Ni

The Ni-content of the steel is 0.8 - 4.5 %; preferably 1.5 % - 4.5 %, which is a high content in comparison to a typical structural steel of this strength class. However, by a high Ni-content, it is achieved above all high strength due to increased hardenability without significant risk of hydrogen cracking, wherein the need for preheating during welding can be reduced. Moreover, better impact toughness both of the base metal and of the HAZ is achieved, when it is desired to keep the strength level high in both. Moreover, nickel Ni enables excellent weather resistance properties, which, at contents according to the invention, improve the resistance of the steel even to saltwater corrosion. For this reason, the Ni-content of the steel is most preferably 2.6 - 4.0 %, wherein, for the steel, an excellent saltwater resistance is achieved.

Copper Cu

In the manner of nickel Ni, the Cu-content of the steel increases the strength of the steel. Moreover, copper Cu increases especially the weather resistance of the steel and it is used 0.25 - 3.0 %. In the invention, the low carbon content of the ultra high-strength steel is made possible especially by alloying Cu and Ni, which is preferable for the weldability.

The combined content of copper Cu and nickel Ni is, as percentages by weight, preferably but not necessarily, at least 2.5 %, according to the following condition Cu(%) + Νί(%)· 2.5 %. In this case, for the steel, high strength at a low carbon content is achieved, which assures good weldability and the achievement of a combination of excellent impact toughness as well as strength in the HAZ of a weld. Preferably, Cu(%)+Ni(%) · 3.0 %, more pref- erably Cu(%)+Ni(%) · 3.5 %. However, the sum of nickel and copper is preferably not more than 6 %.

Preferably, Cu-content is 2 - 3 %, wherein, for a weld seam provided in a steel according to the invention, high strength and impact toughness is pro- vided. In this case, especially Ni-content can be kept lower, such as Ni 0.8 - 2 %.

According to one embodiment, Cu-content is 0.25 - 2 %, wherein, at reasonable carbon and nickel contents, good weldability and strength properties in relation to the strength properties of the steel are achieved at reasonable alloying element expenses.

Chromium Cr

The Cr-content of the steel is 0.5 - 1.6 %. Chromium is alloyed at least 0.5 %, because chromium Cr increases the hardenability and strength of the steel and improves the weather resistance. However, an excessively high Cr-content is unnecessary to assure hardenability, when the steel is according to the invention on the part of the other alloying elements. Preferably, Cr- content is 0.7 -1.6 %, most preferably 0.9 - 1.4 %, wherein especially the excellent weather resistance properties of the steel are assured.

For i.a. the weather resistance of the steel, exceptionally essential al- loying elements of a structural steel according to the invention are nickel Ni, copper Cu, chromium Cr and silicon Si. To assure weather resistance, the sum content of these alloying elements, as percentages by weight, is preferably at least 3.0 %, i.e. Cu (%) + Cr (%) + Ni (%) + Si (%) · 3.0 %

Preferably, to assure hardenability, the sum content of chromium Cr and manganese Mn, as percentages by weight, is at least 1.8, i.e. Mn(%) + Cr (%) · 1.8 %

Molybdenum Mo

The Mo-content of the steel is < 0.8 %, because molybdenum Mo increases the strength of steel but, at excessively high contents, it can weaken the cold working properties of a structural steel according to the invention, such as flangeability and, in addition, Mo increases alloying element expenses. Pref- erably, Mo-content is 0.1 -0,8 %, because molybdenum increases the strength and impact toughness by efficiently preventing recrystallization during hot- rolling, wherein austenite grains are flattened and a fine-grained direct quenched microstructure is provided.

Most preferably, Mo-content is 0.1 - 0.25 %, because at least 0.1 % can be needed for the sake of strength, but, on the other hand, Mo-contents of less than 0.25 % contribute to the flangeability of steel.

Titanium Ti

The Ti-content of the steel is limited Ti · 0.04 %, because high Ti- contents can hinder the success of direct quenching and increase the amount of rough titanium nitrides (TiN) in the steel, which can be of detrimental influence on i.a. impact toughness, fracture toughness and elongation. However, titanium Ti is preferably alloyed at least 0.005 %, because titanium Ti improves the welding properties of the steel by hindering the growth of grains in the HAZ area, wherein a higher heat input can be used, which provides a smooth junction point of the weld bead and the base metal. As a result, due to a small alloying of Ti (0.005 - 0.04 %), weld seams become as impact tough as possible, and from a structural steel according to the invention can be produced by weld- ing exceptionally reliable structures, and also the efficiency of welding work can be increased.

Preferably, Ti-content is · 0.02 %, particularly at greater thicknesses Th · 5 mm, to assure the impact toughness. Most preferably, titanium Ti is thus alloyed 0.005 - 0.02 %.

Aluminium Al

Aluminium Al can be used to condense the steel at contents of 0.01 - 0.15 %. A structural steel according to the invention can achieve excellent resistance welding properties, when Al-content is Al · 0.045 %

Calcium Ca

Calcium can be used typically 0.0005 - 0.005 %, for example, for the removal of the detrimental influence of the compounds created in connection with desulphurization and/or in condensing.

Preferably, an ultra high-strength structural steel according to the invention consists of only said elements, the rest being iron, unavoidable impuri- ties and residual contents. Unavoidable impurities can be, for example, nitrogen N, phosphorus P and sulphur S. The content of nitrogen N is limited N: · 0.01 %, preferably N: • 0.005 %. A low nitrogen content also makes it possible to keep the level of Ti low.

Due to their detrimental properties, an attempt is made to keep the contents of phosphorus P and sulphur S as small as possible, for example, such that P < 0.02 % and S < 0.04 %. Preferably, S-content is < 0.005 % to provide the best flangeability and impact toughness. However, although a high content of phosphorus P could be of advantage due to weather resistance properties, its influence in weakening impact toughness is so dramatic in such a strong steel, that it cannot be purposely alloyed and contents as low as possible are desirable.

It is unnecessary to alloy vanadium V into a structural steel according to the invention, because, in a structural steel according to the invention, it can weaken the impact toughness and weldability, especially in multi-run welding. Due to that said, the content of vanadium V is, preferably but not necessarily, limited V < 0.1 %, most preferably V · 0.05 %.

Niobium Nb can, in some cases, be alloyed 0.008 - 0.08 % to increase the toughness. However, the use of niobium is not imperative. Prefera- bly, when the ultra high-strength structural steel is a strip steel, i.e. a steel product produced on a strip rolling line, niobium Nb is not alloyed to assure flangeability properties, wherein its content is less than 0.008 %, most preferably less than 0.005 %.

It is unnecessary to alloy Boron B into a structural steel according to the invention, because adequate hardenability is achieved by other alloying elements. Moreover, leaving out alloying of boron enables lowering of the Ti-level to a level according to the invention, because titanium does not need to be alloyed in an amount adequate to assure the function of the boron. Preferably, the boron content of the steel is thus less than 0.0003 %.

However, especially at greater thicknesses Th 9 - 40 mm, boron B can be alloyed 0.0005 - 0.003 %, if hardenability cannot be adequately assured without it. However, in this case, the Ti-content of the steel must be in the range of 0.02 - 0.04 % or such that Ti (%)>3*N(%) but however, Ti · 0.04 %.

The composition of the steel provided in the alloying step 2 makes the steel hardenable, wherein, in direct quenching 8, the steel hardens substan- tially as martensite. The microstructure of a steel product according to the invention can also consist of self-tempered martensite. There is more than 80 %, preferably more than 90 %, as percentages by volume, of martensite and/or self-tempered martensite. The rest of the microstructure can comprise small amounts of bainite structures, such as upper- or lower bainite.

According to the preferred embodiment, the flatness (aspect ratio) of the prior austenite structure of an ultra high-strength structural steel according to the invention is at least 1.5 and the MLI (mean linear intercept) of the prior austenite structure is less than 20 micrometres.

The definition of MLI is based on the cube root of the product of the cross-section of the three different main directions of prior austenite grain structure. Calculation of the MLI and flatness of the prior austenite structure is described in more detail, for example, in the source: "Worked Examples on Quantitative Metallography, The institute of materials, Minerals and Mining, London, UK (2003), p1, ISBN 978 1 902653 80 8."

Further, it is typical of a steel product according to the invention that flatness (aspect ratio) and preferably also MLI (mean linear intercept), as measured from different sites of the steel product, are substantially of the same value, which is typical for a hot-rolled product unlike, for example, a steel prod- uct having hot-forged shapes. In other words, the deviation of these quantities is low in a steel product according to the invention. As a result, the properties of the steel product are uniform in different points.

According to one embodiment, the steel has been tempered 12 after direct quenching, wherein the steel is tempered martensitic. In this embodiment, it is exceptionally important to alloy copper Cu into the steel, which separates in tempering, increasing the strength of the steel.

In a method according to the invention, according to Fig. 1 , a steel slab is rolled 5 in a mill such that, at the last pass, the rolling temperature of the steel is 720 - 950 · C, in which method, after the last pass performed in the mill, the steel is direct quenched (8) at a cooling speed of 20 - 150 ^»C/s to a temperature of not more than 450 ^«C.

In the following is described the steps of a method according to the invention:

Alloying 2 of the steel is performed by known manners of adding al- loying elements, for example, in the steel handling station of CAS-OB. The alloying elements to be added to the steel and their contents are, for the inven- tion, the most substantial matter of method step 2. In this step, the steel is alloyed 2 such that the composition of the steel comprises, as percentages by weight, the following element contents: C: 0.07 - 0.12 %,

Si: 0.1 - 0.7 %,

Mn: 0.5 - 2.0 %,

Ni : 0.8 - 4.5 %,

Cu: 0.25 - 3.0 %,

Cr: 0.5 - 1.6 %,

Mo: < 0.8 %, and

Ti: · 0.04 %,

as well as iron (Fe), as well as unavoidable impurities and residual contents.

Thereafter, the steel is continuously cast in a known manner as a steel slab, which is further transferred, for example, after austenitizing annealing (900 - 1350 °C) occurring in a walking beam furnace, to be hot-rolled, in which hot-rolling step 5 the steel slab is rolled to the desired thickness as a sheet-like steel product and direct quenched 8 immediately after rolling 5. In other words, after the last pass of rolling 5, direct quenching 8 is performed.

Specifically when in need of great elongation, tempering 12 can be done for the steel, in which the steel is heated and, thereafter, allowed to cool. Tempering can be done, for example, in the temperature range of 500 - 600 °C typically for approximately 0.2 - 2 hours. At higher temperatures, a shorter tem- pering time can be used. The highest tempering temperature is 700 °C, because an ultra high-strength structural steel according to the invention is very difficult to achieve above this temperature, even using a very short tempering temperature.

Preferably, processing of the steel is, however, merely thermo- mechanical, wherein, after direct quenching 8, no heat treatments are performed afterwards, such as tempering 12. By the method, steel products can be produced, whose mechanical properties are good without needing to perform on the product an expense-increasing post-welding heat treatment. Tempering 12 is not imperative to improve the mechanical properties of a structural steel ac- cording to the invention, because, according to the invention, a tough marten- site is achieved. Further, the yield ratio of the structural steel can rise with tern- pering too close to the value 1 , which can be disadvantageous in some applications.

The advantage of a steel direct quenched 8 immediately from hot- rolling is that the alloying elements increasing hardenability are well dissolved and thus efficiently increasing hardenability, because austenitizing annealing has occurred at a high (1000 - 1350 °C) temperature. The grain size of the steel increases at a high reheating temperature, but, in hot-rolling 5, grain size can once again be ground fine through repeated recrystallization of the grain structure. When hot-rolling 5 is continued below the temperature of recrystallization, the austenite can be made to flatten even more, wherein the package size of the martensite decreases and the dislocation density of the martensite rises. In this case, the impact toughness of the martensite formed increases and especially the yield strength increases. Thus, also tensile strengthand hardness can increase slightly. The result is the tough martensitic microstructure of an ultra high-strength structural steel according to the invention.

The present invention enables a tough structure also in a weld joint provided in the steel.

In traditional oven hardening, austenitizing annealing cannot be performed at as high a temperature due to grain growth, because the grain size of the austenite would remain large and the lath package size of the martensite formed as large, which would again weaken the impact toughness of the base metal.

Preferably, with an ultra high-strength structural steel according to the invention to be produced by direct quenching 8, greater strength is achieved at the same chemical composition in comparison to steel produced traditionally by oven hardening. This means that, by a method according to the invention, the amount and contents of the alloying elements can be decreased, which again enables reduction of alloying element expenses.

According to the first embodiment of the method, the structural steel is produced as a strip steel, the specific steps of which, for an invention according to the first embodiment, are shown in Fig. 3.

Fig. 3 shows in more detail the production of an ultra high-strength strip steel according to the first embodiment. After the alloying step 2 and austenitizing annealing (1200 - 1350 °C), the steel slab is rolled according to step 4 of Fig. 3. Rolling 4 is performed, for example, such that, in step 4, hot-rolling is performed at a temperature of 950 - 1280 °C to a thickness of typically 25 - 50 mm, from which it is immediately transferred to the strip mill of step 5, in which it is rolled as a strip, the end thickness of which is 4 - 12 mm. According to recommendations, the end thickness of the strip steel is at least 5 mm. It is also recommended that the end thickness is not more than 10 mm.

The number of passes in the strip mill is typically 5 - 7. The last pass in the strip mill is performed in the temperature range of 760 - 950 °C, according to recommendations in the temperature range of 850 - 920 °C, especially if the strip is relatively thin, wherein the rolling forces remain lower.

After the last pass of rolling 5, direct quenching 8 of the strip steel is begun within 15 seconds. As direct quenching 8 begins, the temperature of the strip steel should be at least 700 °C. Direct-quenching 8 is performed as water quenching such that the speed of quenching is 30 - 150 °C/s, according to recommendations, the upper limit is not more than 120 °C/s. Direct-quenching 8 is performed to a temperature of not more than 300 °C, according to recommen- dations not more than 100 °C. Immediately after direct quenching 8, the strip steel can be reeled in step 10. The temperature of coiling can then occur in the temperature range of 30 - 300 °C. According to recommendations, the start temperature of coiling 10 is not more than 100 °C, because, when coiling a steel at a temperature of more than 100 °C, a discontinuous cushion of steam can form on the surface of the steel, which complicates the process and causes i.e. weakening of flatness.

Preferably, the end temperature of direct quenching 8 is not more than 100 °C, because, in this case, after quenching, a flat strip is obtained, in which the edges are also even and flat.

Preferably, the steel is direct quenched 8 directly to the ambient temperature.

If needed, tempering treatment 12 can be done to the steel, in which the steel is heated and, thereafter, allowed to cool. Tempering 12 can be done, for example, in the temperature range of 500 - 600 °C, for example, for approx- imately less than two hours.

For example, a solution implemented with expedited heating is also conceivable, in which possible tempering 12 is performed before coiling 10.

According to the second embodiment of the method, the ultra high- strength structural steel is produced as a steel sheet, more specifically a so- called quarto plate, wherein the essential steps for the second embodiment are shown in Fig. 4. After the alloying step 2 and austenitizing annealing (1000 - 1300 °C), the steel slab is rolled according to step 5 of Fig. 4. Rolling of the sheet is performed, for example, in a so-called reversible four mill, in which the steel slab is rolled between the mills in back and forth motions at a temperature of 750 - 1300 °C. In step 5, the sheet is rolled either partially or entirely to its final width and, thereafter, a 90· turn 9 is performed in the plane of the sheet. Next, rolling 5 is continued until the desired thickness is achieved. Alternatively, the turn 9 can be performed more than once between the rolling steps 5 and the rolling can be performed in different directions more than once. For the inven- tion, it is essential that, at the last pass of rolling, the temperature in the sheet rolling is less than 950 °C, according to recommendations less than 900 °C. After the last pass of rolling 5, direct quenching 8 of the steel sheet is begun within 30 seconds, preferably within 15 seconds. As direct quenching 8 begins, the temperature of the steel sheet should be at least 700 °C. Direct-quenching 8 is performed as water quenching such that the speed of quenching is 20 - 150 °C/s. Direct-quenching 8 is performed to a temperature of not more than 450 °C, according to recommendations not more than 200 °C.

According to one embodiment, into a structural steel, which is produced in the hot-rolling step of the invention as a steel sheet, is alloyed niobium Nb 0,008 - 0,08 % to increase the toughness.

By a sheet-like steel product is meant such a steel product, whose length and width are noticeably greater than the thickness of rolling, i.e. in other words, by a sheet-like steel product is meant a steel sheet or strip steel. Let it be exemplarily said that the width of a sheet-like steel product can be 1500 mm, while its thickness is 5 mm.

Preferably, the sheet-like steel product is a strip steel, because by strip rolling are achieved the lowest production expenses and the grain structure of the steel can quickly and efficiently be ground fine while hot-rolling 5.

The thickness of an ultra high-strength structural steel according to the invention, Th, is at least 2 mm and preferably at least 4 mm. In the case of strip steels, thickness is preferably Th = 5 - 12 mm, and most preferably Th = 6 - 10 mm, wherein the good impact toughness of the steel is well-utilized in the dimensioning of practical structures and the strip steel is still technically easy to reel as a steel reel.

In the case of quarto plate, the thickness of the steel product can be even 10 - 40 mm, wherein, even at a 40 mm thickness, an adequate depth of hardening according to the invention is achieved. Preferably, the thickness of quarto plate is 12 - 30 mm. If flattened grain structure is, in this case, a specific goal, it is preferable to alloy niobium Nb: 0.008 - 0.08 %. Examples

In the following, the invention is illustrated by means of examples made in a laboratory.

55 mm thick miniature meltings according to that shown in Table 1 1372, 1371 , 1370, and 1369 were rolled 5 to a thickness of 6 mm using a series of six rolling passes. The slabs were heated in an oven to a temperature of 1225 °C. After the last pass, direct quenching was performed on the sheets. The final dimensions of the rolled steel were 1 120x95x6 mm. Tensile test results are shown in accompanying Table 2.

Welding tests were performed using a MAG butt weld as a two-run weld to the groove without root face, in which the groove angle was 50 degrees, root face 0.5 mm and air gap 1.5 mm. The heat input used was, in both the first and the second run welding, approximately 0.6 kJ/mm. In welding, MAG solid wire was used, which is classed as G 89 5 M Mn4Ni2,5CrMo according to the standard EN 12534 and ER120S-G according to standard AWS A 5.28. The weld seams were in the same direction with the rolling direction. The Charpy V impact toughness of the welds was tested using 5X10 mm test bars transverse in relation to the weld seam, and the results are shown in Table 2. The results of the welding tests are comparable when welding is performed according to above said welding test arrangement.

The fusion line FL means the midpoint of the weld joint in the plane of the sheet in the transverse direction in relation to the longitudinal direction of the weld seam. The coarse grained heat affected zone CGHAZ of welding is defined from the site FL+1 mm and ICHAZ from the site FL+3 mm.

The table presents as a reference a full-scale test R1 , which is im- plemented thermo-mechanically by hot-rolling to a thickness of 6 mm and by direct quenching.

As is known, the properties of steels provided in the full scale are higher on the part of strength and impact toughness than the steel properties provided in the laboratory tests, due to the greater degree of deformation and the smaller grain size of prior austenite in the full scale resulting from this. Hence, the properties of a steel product according to the invention are, on an industrial scale, presumably even better than has been demonstrated in this connection. Instead, on the properties of the weld joint in the HAZ this does not have any real influence.

Table 2. Example tests

FL=fusion line

CG HAZ=coarce grained heat affected ICHAZ=inter critical heat affected zone CV=Charpy V

L=Longitudinal in the rolling direction

From Table 2, it is observed that, in examples 1372 and 1371 , an ultra high-strength steel is achieved, whose impact toughness of the HAZ, as measured transversely in relation to the rolling direction at a temperature of - 40°C, is more than 34 J/ cm², i.e. expressed differently

Charpy V -40°C T (FL, ICHAZ, CGHAZ) > 34 J/ cm²

From Table 2, it is also observed that, in examples 1372 and 1371 , an ultra high-strength steel is achieved, whose impact toughness of the coarse grained heat affected zone of the weld, as measured transversely in the rolling direction at a temperature of -40°C, is more than 40 J/ cm², i.e. expressed differently

Charpy V -40 °C T (CGHAZ) > 40 J/ cm².

From Table 2, it is also observed that, in examples 1372 and 1371 , an ultra high-strength steel is achieved, whose impact toughness of the heat affected zone of the weld, as measured from the fusion line transversely in the rolling direction at a temperature of -40°C, is more than 35 J/ cm², i.e. ex- pressed differently

Charpy V -40 °C T (FL) > 35 J/ cm².

Preferably, a steel according to the invention meets corresponding impact toughness requirements also at a temperature of -60 °C.

From Table 2, it is observed especially that the ultra high-strength structural steej R1 , which belongs outside of the invention, is more fragile in the heat affected zone of the weld than ultra high-strength steels 1372 and 1371 according to the invention.

Further, from Table 2, it is observed that, in example 1372, an ultra high-strength steel is achieved, whose yield strength R_p0.2 is approximately 957 MPa and Charpy V -40 °C > 50 J/cm², as measured longitudinally in relation to the rolling direction. Making reference to that said earlier, by doing the same on an industrial scale, a yield strength of at least 960 MPa can assuredly be achieved. Further, from Table 2, it is observed that, in the examples, the yield strength R_p0.2 of the HAZ is at least as great as the yield strength R_p0.2 of the base metal. On an industrial scale, the yield strength R_p0.2 of the HAZ can be achieved substantially as great as the yield strength of the base metal R_p0.2, such as that the yield strength R_p0.2 of the HAZ is at least 85%, preferably at least 90% of the yield strength R_p0.2 of the base metal or greater.

Further, from Table 2, it is observed that, in example 1371 , an ultra high-strength steel is achieved, whose yield strength R_p0.2 is at least 960 MPa and Charpy V of the base metal -40 °C > 50 J/cm², as measured longitudinally in relation to the rolling direction.

By the invention, a structural steel can be provided having the follow- ing superior mechanical properties:

- yield strength R_p0.2 = 960 - 1250 MPa. Such as even R_p0.2 = 1 100 - 1250 MPa. ^* - tensile strengthR_m 980 - 1500 MPa. Such as even 1120 - 1500 MPa *

- yield ratio (R_p0.2 / R_m) > 0.7. Such as (R_p0.₂/ Rm) > 0.8. ^*

- elongation at break A₅ > 7 %. Such as even A₅ > 8 %. *

- when tempering heat-treated 12, elongation at break A₅ > 8 %. Such as even A₅ > 10 %. ^*

- impact toughness of the base metal Charpy V -20 °C > 50 J/cm²,

Such as even Charpy V -40 °C > 50 J/ cm²

Such as especially even Charpy V -60 °C > 50 J/ cm². * ^** ^* as measured by a rod longitudinal in relation to the rolling direction. *^* is measured by a rod having the thickness of the sheet, but by a max. 10 mm rod When welding an ultra high-strength structural steel according to the invention, welding methods typically used in welding high-strength steels can be used without problems and welding can be done from ultra-high strength, without problems, by the normal heat inputs used in welding high-strength steels. Naturally, extremely great welding energies attempt to lower the strength of the weld joint somewhat in relation to the small energies. A steel suitably Ti-alloyed according to the invention is capable of well resisting grain growth in the heat affected zone (HAZ) created during welding, which has an advantageous influence on the impact toughness of the so-called coarse grained zone. Moreover, a structural steel according to the invention hardens efficiently during welding over the re-austenitized zone, wherein the strength of the weld is made high. Due to advantageous grain size of the zone and martensite created due to low carbon content, impact toughness properties are exceptionally good for such a strong structural steel, even though the carbon equivalent is somewhat high. In multi-run welding are also achieved good toughness and strength properties due to a suitable composition, for example, by limiting vanadium content. Preferably, also in welding done in an orthodox manner, yield strength of the weld to be achieved in a transverse tensile test across the weld seam is at least 960 MPa, such as has been demonstrated above.

Moreover, the fracture toughness behaviour of a structural steel produced by the method according to the invention is preferred i.a. due to its low C- content and high Ni-content, i.e. the energy needed for the distortion nucleation and progression is great considering the strength and production manner of the steel and steel fractures persistently especially in the direct quenched 8 state. This is an especially preferred and often imperative property for such a strong structural steel. The property can be roughly evaluated through impact toughness, which, in an ultra high-strength structural steel produced by a method according to the invention, is excellent.

The invention is described in the above by means of the preferred embodiments and it is obvious that the invention can be implemented in its details in many different ways within the scope of the accompanying claims.

Claims

1. A hot-rolled ultra high-strength structural steel, such as a hot-rolled ultra high-strength structural steel product, whose yield strength R_p0.2 is at least 960 MPa, characterized in that the composition of the structural steel com- prises, as percentages by weight:

C: 0.07 - 0.12 %,

Si: 0.1 - 0.7 %,

Mn: 0.5 - 2.0 %,

Ni: 0.8 - 4.5 %,

Cu: 0.25 - 3.0%,

Cr: 0.5 - 1.6 %,

Mo: < 0.8 %, and

Ti: · 0.04 % and iron (Fe), unavoidable impurities and residual contents.

2. A hot-rolled ultra high-strength structural steel according to claim 1 , characterized in that C-content, as percentages by weight, is C: 0.08 - 0.12 %, more preferably 0.08 - 0.10 %.

3. A hot-rolled ultra high-strength structural steel according to claim 1 or 2, characterized in that Ni-content, as percentages by weight, is Ni: 1.5 - 4.5 %, preferably 2.6 - 4.0 %.

4. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 3, characterized in that Cu-content, as percentages by weight, is Cu: 2.0 - 3.0 %.

5. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 4, characterized in that Cr-content, as percentages by weight, is Cr: 0.7 - 1.6 %, most preferably 0.9 - 1 .4 %.

6. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 5, characterized in that Mo-content, as percentages by weight, is Mo: 0.1 - 0.80 %, preferably 0.1 - 0.25 %.

7. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 6, characterized in that Si-content, as percentages by weight, is Si: 0.15 - 0.4 %, more preferably 0.15 - 0.25 %.

8. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 7, characterized in that Mn-content, as percentages by weight, is Mn: 0.5 - 1.5 %, more preferably 0.7 - 1.5 %.

9. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 8, characterized in that Ti-content, as percentages by weight, is

Ti: 0.005 - 0.04 %, more preferably 0.005 - 0.02 %.

10. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 9, characterized in that Ni-content, as percentages by weight, is at least twice as great as Cu-content as percentages by weight, i.e. Ni (%) · 2^*Cu (%).

1 1. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 10, characterized in that the combined content of Cu-content and Ni-content, as percentages by weight, Cu(%) + Ni(%), is at least 2.5 %, more preferably at least 3.0 %, most preferably in the range of 3.5 - 6.0 %.

12. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 11 , characterized in that the combined content of Cu-content, Cr- content, Ni-content and Si-content, as percentages by weight, Cu(%) + Cr(%) + Ni(%) + Si(%), is at least 3.0 %.

13. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 12, characterized in that the combined content of Cr-content and Mn-content, as percentages by weight, Cr(%) + Mn(%), is at least 1.8 %.

14. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 13, characterized in that the carbon equivalent CEV of the structural steel, as calculated by the formula CEV = (C + Mn/6 + (Mo + V)/5 + (Ni + Cu)/15), is more than 0.5, more preferably in the range of 0.5 - 1.2, most prefer- ably in the range of 0.65 - 1.0, even most preferably in the range of 0.65 - 0.9 to assure the hardenability of the re-austenitized zone of the weld. 5. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 14, characterized in that B-content, as percentages by weight, is

B: < 0.0003 %.

16. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 15, characterized in that the ultra high-strength structural steel comprises, as percentages by weight:

C: 0.07 - 0.12 %,

Si: 0.1 - 0.7 %,

Mn: 0.5 - 2.0 %,

Ni: 0.8 - 4.5 %,

Cu: 0.25 - 3.0 %,

Cr: 0.5 - 1.6 %,

Mo: < 0.8 %,

Ti: · 0.04 %,

Ca: 0.0005 - 0.005 %, and

Al: 0.01 - 0.15 %, the rest being iron (Fe), unavoidable impurities and residual contents.

17. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 16, characterized in that the microstructure of the structural steel is substantially martensitic and/or tempered martensitic.

18. A hot-rolled ultra high-strength structural steel according to claim 17, characterized in that the microstructure of the structural steel is, as percent- ages by volume, more than 80 % martensitic and/or tempered martensitic, more preferably more than 90 % martensitic and/or tempered martensitic.

19. A hot-rolled ultra high-strength structural steel according to claim 17 or 18, characterized in that the microstructure of the structural steel is sub- stantially martensitic.

20. A hot-rolled ultra high-strength structural steel according to any one of claims 17 - 19, characterized in that the flatness (aspect ratio) of the prior austenite grain structure of the ultra high-strength structural steel is at least 1.5 and the MLI (mean linear intercept) of the prior austenite structure less than 20 micrometres.

21 . A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 20, characterized in that the ultra high-strength structural steel is a strip steel, whose thickness Th = 5 - 12 mm, preferably 6 - 10 mm.

22. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 21 , characterized in that the ultra high-strength structural steels is a quarto plate, whose thickness Th = 10 - 40 mm, preferably 12 - 30 mm.

23. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 22, characterized in that the ultra high-strength structural steels is produced by direct quenching (8) the steel directly from hot-rolling (5).

24. A hot-rolled ultra high-strength structural steel according to claims 21 and 23, characterized in that the ultra high-strength structural steel is produced by direct quenching (8) the steel directly from strip rolling (5).

25. A hot-rolled ultra high-strength structural steel according to claims 22 and 23, characterized in that the ultra high-strength structural steel is pro- duced by direct quenching the steel directly from plate rolling.

26. A hot-rolled ultra high-strength structural steel according to any one of claims 1 - 25, characterized in that the impact toughness of the HAZ of a weld provided in the ultra high-strength structural steel, as measured trans- versely in relation to the rolling direction at a temperature of -40°C, is more than 34 J/ cm².

27. A method for producing an ultra high-strength structural steel, such as an ultra high-strength structural steel product, wherein the yield strength R_p0.2 of the ultra high-strength structural steel is at least 960 MPa, in which method the steel slab is rolled (5) in a mill such that, at the last pass, the rolling temper- ature of the steel slab is 720 - 950 Ό, in which method, after the last pass performed in the mill, the steel slab is direct quenched (8) at a cooling speed of 20 - 150 ^»C/s to a temperature of not more than 450 ^»C, characterized in that the steel is alloyed (2) for a steel slab such that the composition of the steel com- prises, as percentages by weight, the following element contents:

C: 0.07 - 0.12 %,

Si: 0.1 - 0.7 %,

Mn: 0.5 - 2.0 %,

Ni: 0.8 - 4.5 %,

Cu: 0.25 - 3.0 %,

Cr: 0.5 - 1.6 %,

Mo: < 0.8 %, and

Ti: · 0.04 % and iron (Fe), unavoidable impurities and residual contents.

28. A method according to claim 27, characterized in that the steel is alloyed (2) such that C-content, as percentages by weight, is 0.08 - 0.12 %, more preferably 0.08 - 0.10 %.

29. A method according to claim 27 or 28, characterized in that the steel is alloyed (2) such that Ni-content, as percentages by weight, is Ni: 1.5 - 4.5 %, more preferably 2.6 - 4.0 %.

30. A method according to any one of claims 27 - 29, characterized in that the steel is alloyed (2) such that Cu-content, as percentages by weight, is Cu: 2.0 - 3,0 %.

31. A method according to any one of claims 27 - 30, characterized in that the steel is alloyed (2) such that Cr-content, as percentages by weight, is Cr: 0.7 - 1.6 %, most preferably 0.9 - 1.4 %.

32. A method according to any one of claims 27 - 31 , characterized in that the steel is alloyed (2) such that Mo-content, as percentages by weight, is Mo: 0.1 - 0.8 %, preferably 0.1 - 0.25 %.

33. A method according to any one of claims 27 - 32, characterized in that the steel is alloyed (2) such that Si-content, as percentages by weight, is Si: 0.15 - 0.4 %, more preferably 0.15 - 0.25 %.

34. A method according to any one of claims 27 - 33, characterized in that the steel is alloyed (2) such that Mn-content, as percentages by weight, is Mn: 0.5 - 1 .5 %, more preferably 0.7 - 1 .5 %.

35. A method according to any one of claims 27 - 34, characterized in that the steel is alloyed (2) such that Ti-content, as percentages by weight, is

Ti: 0.005 - 0.04 %, more preferably 0.005 - 0.02 %.

36. A method according to any one of claims 27 - 35, characterized in that the steel is alloyed (2) such that Ni-content, as percentages by weight, is at least twice as great as Cu-content as percentages by weight, i.e.

Ni (%) · 2*Cu (%).

37. A method according to any one of claims 27 - 36, characterized in that the steel is alloyed (2) such that the combined content of Cu-content and Ni-content, as percentages by weight, Cu(%) + Ni(%), is at least 2.5, more preferably at least 3.0 %, most preferably in the range of 3.5 - 6.0 %.

38. A method according to any one of claims 27 - 37, characterized in that the steel is alloyed (2) such that the combined content of Cu-content, Cr- content, Ni-content, and Si-content, as percentages by weight, Cu(%) + Cr(%) +Ni(%) +Si(%), is at least 3.0 %.

39. A method according to any one of claims 27 - 38, characterized in that the steel is alloyed (2) such that the combined content of Cr-content and Mn-content, as percentages by weight, Cr(%) + Mn(%), is at least 1.8 %.

40. A method according to any one of claims 27 - 39, characterized in that the steel is alloyed (2) such that carbon equivalent of the steel, as calculated by the formula CEV = (C + Mn/6 + (Mo + V)/5 + (Ni + Cu)/15), is more than 0.5, more preferably in the range of 0.5 - 1.2, most preferably in the range of

0.65 - 1.0, even most preferably in the range of 0.65 - 0.9 to assure the harden- ability of the re-austenitized zone of the weld.

41 . A method according to any one of claims 27 - 40, characterized in that the steel is alloyed (2) for a steel slab such that the composition of the steel comprises, as percentages by weight, the following element contents:

C: 0.07 - 0.12 %,

Si: 0,1 - 0.7 %,

Mn: 0,5 - 2.0 %,

Ni: 0.8 - 4.5 %,

Cu: 0.25 - 3.0 %,

Cr: 0.5 - 1.6 %,

Mo: < 0.8 %,

ΤΊ: · 0.04 %

Ca: 0.0005 - 0.005 %, and

Al 0.01 - 0.15 %, the rest being iron (Fe), unavoidable impurities and residual contents.

42. A method according to any one of claims 27 - 41, characterized in that the steel is hot-rolled (5) in a strip rolling mill as a strip steel, whose thickness Th = 4 - 12 mm, preferably 6 - 10 mm

43. A method according to any one of claims 27 - 42, characterized in that the steel slab is hot-rolled in a plate rolling mill as a quarto plate, whose thickness Th = 10 - 40 mm, preferably 12 - 30 mm, and that the end temperature of direct quenching is not more than 200 °C.

44. A method according to claim 43, characterized in that the steel is alloyed (2) such that Nb-content, as percentages by weight, is Nb: 0.008 - 0.08 %.

45. A method according to any one of claims 27 - 44, characterized in that, after direct quenching (8), tempering (12) is performed to the steel.

46. A method according to any one of claims 27 - 45, characterized in that the end temperature of direct quenching (8) is not more than 100 °C.