HEMMILÄ, Mikko (Järvitie 6, Vihanti, FI-86400, FI)
STEEN, Petteri (Lukiokatu 9 D 13, Hämeenlinna, FI-13100, FI)
RAJALA, Juha (Onkijärventie 76, Eteläinen, FI-14770, FI)
MINKKINEN, Jussi (Ortelantie 6 E 31, Hämeenlinna, FI-13210, FI)
TIHINEN, Sakari (Rautiontie 16 C 19, Kempele, FI-90440, FI)
HÄMEENKORPI, Tanja (Pesäpolku 2, Saloinen, FI-92160, FI)
PEKOLA, Pasi (Viertotie 14, Iittala, FI-14500, FI)
LIIMATAINEN, Tommi (Niemenkaari 3, Raahe, FI-92130, FI)
HEMMILÄ, Mikko (Järvitie 6, Vihanti, FI-86400, FI)
STEEN, Petteri (Lukiokatu 9 D 13, Hämeenlinna, FI-13100, FI)
RAJALA, Juha (Onkijärventie 76, Eteläinen, FI-14770, FI)
MINKKINEN, Jussi (Ortelantie 6 E 31, Hämeenlinna, FI-13210, FI)
TIHINEN, Sakari (Rautiontie 16 C 19, Kempele, FI-90440, FI)
HÄMEENKORPI, Tanja (Pesäpolku 2, Saloinen, FI-92160, FI)
PEKOLA, Pasi (Viertotie 14, Iittala, FI-14500, FI)
| Claims 1. A method for producing high-strength hot-dip galvanized steel product (16, 18) of a steel slab, the composition of the steel slab comprising: C: 0.04-0.10% Si: <0.03% Mn: 0.3-1.2% Nb: 0.015-0.10% N: <0.01% P: <0.02% S: <0.02% Al: 0.01-0.08% V: <0.05%, the remaining being iron, unavoidable impurities or residual contents, in which method the steel slab is rolled (5) in a strip mill into a steel strip so that the rolling temperature of the steel slab in the last pass is 760-960 °C, characterized in that the method comprises the stage: the steel strip is direct quenched (8) after the last pass performed in the strip mill at a cooling speed of 30-150 °C/s to a temperature of maximum 300°C, which direct quenching (8) is performed at the latest 15s after the last pass. 2. The method of claim 1, characterized in that the steel slab is hot rolled into a steel strip in a strip mill (5). 3. The method of claim 1 or 2, characterized in that in the method the steel strip is cold formed (12) into a steel product (16, 18) and the steel product (16, 18) is hot-dip galvanized (14). 4. The method of any of the preceding claims 1-3, characterized in that the steel strip is coiled (10) at a temperature of 20-300°C before cold forming (12) of the steel strip. 5. The method of any of the preceding claims 1-4, characterized in that the final temperature in the direct quenching is maximum 100 °C. 6. The method of any of the preceding claims 1-5, characterized in that the steel strip is treated thermo-mechanically, whereupon tempering is not performed after the direct quenching (8). 7. The method of any of the preceding claims 3-6, characterized in that external rounding to a value of R=2-4T of at least one corner is cold formed. 8. The method of any of the preceding claims 3-7, characterized in that roll forming is used as cold forming method (12). 9. The method of claim 8, characterized in that there is roll formed a closed section, wherein the closing of the section is achieved by lane joint welding. 10. The method of any of the preceding claims 3-9, characterized in that in the hot-dip galvanizing (14) in the zinc bath are used the additive contents - lead Pb<0.02% and tin Sn<0.02%. 11. The method of any of the preceding claims 1-10, characterized in that the content of the steel slab in addition comprises the titanium content Ti: <0.06%. 12. A hot-dip galvanized steel product (16, 18), produced by the method of any of the claims 1-11, characterized in that the composition of the hot-dip galvanized steel product (16, 18) has been chosen in such a way that its yield strength is 460 - 600 MPa. 13. The hot-dip galvanized steel product (16, 18) of claim 12, characterized in that the composition of the hot-dip galvanized steel product (16, 18) is chosen in such a way that its yield strength is 500 - 600 MPa or preferably 550 - 600 MPa. 14. A steel product (16, 18) with a material strength of 2 - 14mm, and the yield strength of which steel product is minimum 460 MPa, characterized in that the composition of the steel product (16, 18) in percentages by weight comprises: C: 0.04-0.10% Si: <0.03% Mn: 0.3-1.2% Nb: 0.015-0.10% N: <0.01% P: <0.02% S: <0.02% Al: 0.01-0.08% V: <0.05%, the remaining being iron, unavoidable impurities or residual contents. 15. The steel product (16, 18) of claim 14, characterized in that the impact strength of the steel product is minimum 120J/cm2 (measured Charpy V-60). 16. The steel product (16, 18) of claim 14 or 15, characterized in that the steel product is hot-dip galvanized. 17. The steel product (16, 18) of any of the preceding claims 14 - 16, characterized in that the carbon content of the steel product (16, 18) in percentages by weight is C: 0.04 - 0.07%. 18. The steel product (16, 18) of any of the preceding claims 14 - 17, characterized in that the composition of the steel product in addition in percentages by weight comprises the titanium content Ti: <0.06%. 19. The steel product (16, 18) of any of the preceding claims 14 - 18, characterized in that the steel product (16, 18) has an external rounding =2-4T on one or several cor- ners, or that the steel product (16, 18) has one or several bended angles with a magnitude of <120\ 20. The steel product (16, 18) of any of the preceding claims 14 - 19, characterized in that the steel product (16, 18) is a closed section, or that the steel product (16, 18) is a closed section, which is a square or rectangle formed structural hollow section. 21. The steel product (16, 18) of any of the preceding claims 14 - 20, characterized in that the microstructure of the steel product (16, 18) essentially is ferritic. 22. The steel product (16, 18) of any of the preceding claims 14 - 21, characterized in that the material strength of the steel product (16, 18) is 4 -12.5 mm. 23. The steel product (16, 18) of any of the preceding claims 14 - 22, characterized in that the yield strength of the steel product (16, 18) is minimum 460 MPa, preferably 460 - 600 MPa, more preferably 500 - 600 MPa, or even more preferably 550 - 590 MPa. 24. The steel product (16, 18) of any of the preceding claims 14 - 23, characterized in that the steel product (16, 18) is produced of a steel strip, direct quenched (8) in a strip milling process (5). 25. The use of steel that in percentages by weight comprises: C: 0.04-0.10% Si: <0.03% Mn: 0.3-1.2% Nb: 0.015-0.10% N: <0.01% P: <0.02% S: <0.02% Al: 0.01-0.08% V: <0.05%, the remaining being iron, unavoidable impurities or residual contents, for producing a hot-dip galvanized steel product. |
The invention relates to a hot-dip galvanized steel product. More specifically, the inven- tion relates to the method for producing a hot-dip galvanized steel product according to the preamble of claim 1. In addition, the invention relates to the hot-dip galvanized steel product according to the preamble of claim 12 and the steel product according to the preamble of claim 14 as well as the use according to claim 25. The invention relates especially to producing a high-strength galvanized product, the composition of which product in percentage by weight comprises
C: 0.04 - 0.10%,
Si: <0.03%
Mn: 0.3 - 1.2%
Nb: 0.015 - 0.10%
N: <0.01%
P: <0.02%
S: <0.02%
Al: 0.01 - 0.08%
V: <0.05%, the remaining being iron, unavoidable contaminants and residues. The invention also relates to a high-strength product, the composition of which is the aforementioned and it may excellently be hot-dip galvanized.
For instance in the construction, transport and lifting equipment industries the trend is towards using stronger steel qualities, since by using strong steels one may obtain saving in weight and thus among other things reduce manufacturing, fuel and transport costs of the products. In addition, steel structures should be protected against corrosion, for which purpose hot-dip galvanizing is widely used as method, because it is an effi- cient and cost-effective way to create a corrosion protective zinc coating on the steel surface. The zinc coating protects the steel even for hundreds of years depending on the environment and the thickness of the layer.
l The use of hot-dip galvanizing for coating strong structural hollow sections is, however, problematic because of different kinds of embrittlement phenomena, such as liquid metal embrittlement in corner points of cold formed products, in which residual stresses have remained after the cold forming. Exposing a cold formed product to a hot zinc pot causes a crack in the inside corner of a bend in traditional high-strength steel products. The springback after bending causes a local tensile stress peak in the inside corner of the bend, which also increases the risk of liquid metal embrittlement.
Residual stresses in a cold formed steel product can be eliminated by heat treatments after the forming or by surface modifying, but in production at an industrial scale it is not preferred as it calls for an additional work stage.
In addition to residual stress, other matters that influence liquid metal embrittlement are the composition of the molten zinc bath and the properties of the steel. At its worst, a wrong composition of the steel causes rupturing in the corner points of the product during the hot-dip galvanizing stage. Thus producing high-strength steel products that shall be galvanized has been problematic and yield strengths have normally remained in the 355 MPa class.
One may, as well known, improve the strength of steel by raising carbon and alloy contents, but raising the carbon or alloy content level increases liquid metal embrittlement of the steel in hot-dip galvanizing. One way to estimate steel durability in hot-dip gal- vanizing is to calculate the LME index of the steel: LME=201-370C-22Si-51Mn- 35P+33S-28Cu-22Ni-87Cr-123Mo-275V-182Nb-82Ti-24Al+1700N-1550 00B, where a smaller index value indicates an increased tendency to liquid metal embrittlement. From the index, one observes that high alloying increases the tendency to liquid metal embrittlement. In addition, a high carbon equivalent of the steel increases liquid metal embrit- tlement.
The Japanese patent specification JP8232041 presents a steel composition for forming a thick-walled, high-strength and galvanized structural hollow section, where the carbon content C is 0.04 - 0.12%, <0.05% Si, 0.2-2.0% Mn, and 2 or several of the following alloys <0.08% Nb, <0.08% V and <0.05% Ti. According to the specification, <0.05% Si improves resistance to the embrittlement phenomenon. Further, according to Euronorm 10025-2, steel with a silicon content of 0.03 - 0.05% reacts poorly to galvanization, because then one moves in the so-called Sandell zone, where the thickness of the zinc layer grows out of control. The yield strength of the galvanized structural hollow section according to the referred specification remains at a level of maximum 460MPa and the high strength classes (over 460MPa) and high impact strength according to this invention are not reached using the teachings of JP8232041. Further, the specification does not teach how to control additive contents in order to achieve better response to galvanization, whereupon it is uncertain, whether hot-dip galvanization of a structural hollow section will succeed. Brief description of the invention
The object of this invention is to resolve a product to be hot-dip galvanized to a yield strength of 460 MPa - 600M Pa, and which excellently withstands the stress caused by the liquid metal in hot-dip galvanization. Another object of the invention is to provide a new method for producing a high-strength and galvanized steel product, in which steel there for improving strength are needed no significant amounts of alloys that impede galvanizing and, in addition, increase costs. In addition, there is not necessarily any need for performing stress relief annealing after cold forming or any other finishing stages.
According to the invention, it has surprisingly been discovered that by direct quenching steel of the mentioned composition one obtains a high-strength steel with good impact strength properties, the hot-dip galvanizing properties of which are excellent.
In order to implement this, characteristic for the method according to the invention is what is mentioned in the characterizing part of claim 1. Characteristic for a product according to the invention to be hot-dip galvanized is what is defined in the characterizing part of claim 12. Characteristic for a steel product according to the invention is furthermore what has been defined in the characterizing part of claim 14.
Preferred embodiments of the invention are presented in the dependent claims. The biggest advantages of the method according to the invention are that it enables hot- dip galvanized high-strength steel with excellent impact strength. In addition, direct quenching speeds up production turnaround, since there is no need to wait for the coil to cool. The biggest advantages with a steel product according to the invention are that it has high strength combined with good impact strength and it may excellently be hot-dip galvanized. The aims of the invention are reached by choosing the composition of the steel of the steel product according to the invention and by direct quenching of the steel slab with the chosen composition in connection with the producing. In other words, the present invention is implemented by combining steel composition and direct quenching according to the invention.
Brief description of the drawings
Hereinafter, the present invention is described in more detail by reference to the accompanying drawings, wherein:
Figure 1 shows the main stages of the method according to the invention;
Figure 2 is a more detailed presentation of the stages of the method;
Figure 3 shows a product according to the invention, which is a square hollow structural section; and
Figure 4 shows a product according to the invention, which is a rectangular hollow structural section.
Description of the reference numbers
Alloying 2
Roughing 4
Rolling 5
Direct quenching 8
Coiling 10 Cold forming 12
Hot-dip galvanizing 14
Structural hollow section 16, 18 Detailed description of the invention
Hereinafter there are described in detail the stages of the method according to the invention, of which the essential steps from the invention point of view are shown in figures 1 and 2.
In figure 1 there are shown the main stages of the production of a hot-dip steel product according to the present invention. The steel slab is rolled at the rolling stage 5, from which the resulting steel strip is directed to the direct quenching stage 8. The direct quenched steel strip is further directed to cold forming 12, where the steel product is achieved. The produced steel product is directed on to hot-dip galvanizing 14.
In figure 2 there is more closely shown the production of a steel product according to the invention. After the alloying stage 2 and austenizing, the steel slab is rolled according to stage 4 in figure 2. The rolling 4 is, for instance, performed in such a way that hot rolling at stage 4 is performed at a temperature of 950 - 1280 °C to a thickness of typically 25 - 50 mm, wherefrom it immediately is forwarded to the strip mill at stage 5, where it is rolled to a strip with a final thickness of 2 - 14 mm. Advisably the final thickness of the steel strip is minimum 4 mm. It is also advisable that the final thickness of the steel strip is maximum 12.5 mm.
The number of passes in the strip mill is typically 5 - 7. In the strip mill the last pass is performed within the temperature range 760 - 960 °C, advisably within the temperature range 850 - 920 °C, especially if the strip is relatively thin, whereupon the rolling forces are lower.
After the last pass in the rolling 5, direct quenching 8 of the steel strip is started within 15 seconds. As direct quenching 8 commences, the temperature of the steel strip shall be minimum 700 °C. Direct quenching 8 is performed as water quenching in such a way that the quenching speed is 30 - 150 °C/s, advisably the upper limit is maximum 120 °C/s. Direct quenching 8 is performed to a temperature of maximum 300 °C, advisably maximum 100 °C. Immediately after direct quenching 8 the steel strip can be coiled at stage 10. Coiling may thus be performed at a temperature range of 30 - 300 °C. Advisably the initial temperature in the coiling 10 is maximum 100 °C, since during coiling of steel at a temperature exceeding 100 °C there may develop a steam mattress, which complicates the process.
As a result of the mentioned thermo-mechanical treatment, the microstructure is homogenous and is formed by a main phase, which advisably is low-carbon ferrite and/or low-carbon bainite. The main phase is typically over 90 %. There are thus very small amounts of high-carbon bainite and/or residual austenite and/or martensite as very small high-carbon islets. It is also essential that the microstructure does not contain any big grains at all, thanks to which the ductility properties of the steel are especially good taking the strength of the steel into consideration.
Preferably direct quenching speed 8 is maximum 120 °C /s, because then one obtains such a microstructure of the steel that gives the steel particularly good mechanical properties, good impact strength included.
Preferably the final temperature in the direct quenching is maximum 100 °C, because then there is after quenching obtained a flat strip, where also the edges are even and flat.
Preferably the steel strip is direct quenched 8 directly to coiling temperature and coiled. The treatment of the steel strip is preferably thermo-mechanic, whereupon no post heat treatment after direct quenching 8 is performed, such as tempering, where the steel is heated, after which it is left to cool. The mechanical properties of a steel product produced according to the method are good without any need to undertake any cost-adding post heat treatments. Tempering does not significantly improve the mechanical proper- ties of a steel product and it complicates the process.
After coiling 10 the steel strip is cold formed 12 to the wished product. In other words, the product is produced from the steel strip direct quenched 8 in the strip process 5. Before cold forming 12, the coiled steel strip is coiled open and in case of need the steel strip can be slit and/or cut to suitable dimensions. Cold forming 12 is carried out by well-known section and/or hollow section production methods, preferably cold forming 12 is, however, carried out by roll forming from standard cross sections between several twinrolls.
In the roll forming process, the form of the product is deformed at each twinrolls stage. In the case of a closed section, lane joint welding is carried out on the section. The last stage is typically cutting off a continuous product to a wished dimension. Alternatively, the product may be a piece that is cold formed 12 by beveling. As a result of the cold forming 12 there remain residual stresses in the product.
After cold forming 12 before hot-dip galvanizing 14 necessary intermediate stages can be performed on the steel, such as cleaning and pickling in order to accomplish successful galvanizing.
In the hot-dip galvanizing 14 the piece of steel is hoisted into a zinc pot, wherein the temperature of the liquid zinc is app. 450°C. The additive contents of the zinc bath according to the invention shall preferably be limited in the case of lead Pb<0.02%, tin Sn<0.02%, and bismuth Bi, because it has been observed that these increase the tendency to liquid metal embrittlement. In other words, in hot-dip galvanization 14 the additive contents used in the zinc pot are lead Pb<0.02% and tin Sn<0.02%.
Zinc reacts with steel, forming a coat that offers protection against corrosion. After a sufficient dipping time the high-strength galvanized product is lifted out of the pot and cooled. Cooling can be achieved in well-known ways, for instance in the air.
For professionals in the industry it stands clear that at least a part of the advantages that direct quenching accomplishes for the product may be non-preferably realized also without the direct relationship between quenching 8 and strip rolling 5. Thus, characteristic for a product according to the invention is what is defined in the independent claim 12 or 14.
Hereinafter the geometry and contents of a product according to the invention is described more closely. Comer points of galvanized products were studied from images captured using a metal microscope and it was surprisingly discovered that the increased strength of the steel achieved by direct quenching 8 enables hot-dip galvanizing 14 of high-strength products without comer cracks. In chart 1 below are shown test examples (steels 1, 2, and 3.1) of strength values measured in cold formed structural hollow sections 16, 18, as well as their geometrical parameters. In figures 2 and 3 the shapes of the structural hollow sections are shown graphically. Chart 1. Stren th and im act stren th ro erties of steel roducts
In chart 1 it can be seen that in the invention there has been accomplished a product to be galvanized, the yield strength Rp 0,2 of which even exceeds 460 MPa, 460 - 600 Mpa, preferably 500 - 600 MPa, more preferably 550 - 590 MPa and the impact strength of which is minimum 120J/cm 2 also at low, -60 ° C temperatures. The yield strength Rp 0,2 of a steel product according to the invention is thus over 470 MPa, more preferably over 490 MPa, in other words, the yield strength is preferably 470 - 600 MPa, more preferably 490 - 600 MPa and most preferably 500-590 MPa.
The achieved high strength has been accomplished by the lowish alloying 2 and direct quenching 8 of the steel product according to the invention.
The yield relationship of a product according to the invention (Rp 0,2 / Rm) is over 0.85, preferably over 0.9. A fairly high yield relationship is important from the point of view of the use of the product for instance as a structural hollow section.
In chart 1 there has also been shown measuring results of steel products after hot-dip galvanizing tests, which were carried out by metal microscope. In the corners of galvanized structural hollow sections (steels 1, 2, and 3.1) according to the invention, no cracks whatsoever caused by liquid metal embrittlement were discovered. In certain cases, small aberrations in surface quality, generated during the cold forming of the product, may occur on the inside surface of the corner. Rl and R2 are reference exam- pies of structural hollow sections of corresponding strength level, the strength of which has been enhanced by alloys without direct quenching 8. The reference tests Rl and R2 caused a visually discernible crack in the inside corner points of the structural hollow section, as they were hot-dip galvanized 14.
The steel compositions of the hot-dip galvanizing tests are shown in chart 2 below, where the chemical elements (Cu, Cr, Ni, Mo) that have been residuals contents in the examples, have been marked in italic in the last columns.
Chart 2. Com ositions of the tested steels
In chart 2 it can be discerned that in the invention, important alloys are C, Si, Mn, Al and Nb. In addition, it is important from the point of view of the invention to limit the contents of the impurities P and S. In addition, one can use vanadinium in V<0.05% or titanium alloy Ti<0.06%, preferably TI<0.03%, contents. The general alloy content level is low, which supports a successful accomplishment of hot-dip galvanization. In order to achieve the steel quality according to the invention, especially silicon Si, vanadinium V, and manganese Mn, contents have been decreased significantly. The contents are shown as percentages by weight in chart 2 as well as in the descriptions.
The steel has a low carbon content C: 0.04 - 0.10% which is advantageous from the point of view of impact strength, beveling and welding properties of the material. The low carbon equivalent of the steel (C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15) has also a positive effect on good welding properties. Preferably the carbon content C of the steel C is 0.04 - 0.07%, which improves the hot-dip galvanization ability and the aforementioned properties even further. The silicon content of the steel is chosen in the range Si <0.03%, the steel is thus a so called low-silicon steel. Then the zinc layer will become thin (<90μηι) and it has good adhesive properties. Still, with low-silicon steel one achieves a fine lustre in the zinc surface when hot-dip galvanizing, in addition to which a low silicon content prevents tendency to liquid metal embrittlement. A low silicon content is thus apt to improve the hot-dip galvanization abilities of the steel.
Manganese Mn, is used 0.3 - 1.2%. 0.3% is needed at least in order to achieve the necessary strength of the steel, on the other hand it has been established that a too high content impairs impact strength in the case of direct quenched steels. In order to secure impact strength, manganese content is preferably 0.4 - 0.9%. Niobium Nb is a potent alloy that increases strength and it has to be alloyed 0.015- 0.10%. Preferably niobium Nb is alloyed maximum 0.08%.
Titanium content is limited to Ti<0.06%, preferably Ti<0.03%, because high Ti contents increase the amount of hard titanium nitrides (TiN), which may have a harmful ef- feet on among other things impact strength, formability properties and elongation. Preferably titanium content really is Ti <0.02%.
The content of nitrogen N is limited to N: <0.01%, preferably N: <0.005%.
Most preferably the content of both titanium Ti and nitrogen N is limited to Ti<0.06 %, more preferably Ti<0.03%, and even more preferably Ti <0.02% and N<0.005% in order to ensure excellent formability properties.
Vanadinium V is added as an alloy that enhances strength, but its content must be limited to V<0.05% contents, because it has been established that vanadinium weakens weldability and impact strength. Preferably vanadinium content is V<0.01%. Phosphorus P content must be limited to P<0.02%, in order to keep up good hot-dip galvanization abilities. Preferably phosphorus content is limited together with silicon Si, then preferably P(%)+Si(%)<0.04%.
Sulphur S is found in steel in small contents and because of its detrimental properties one tries to limit its content to S<0.02%. Aluminium Al is used for condensing steel in contents of 0.01 - 0.08%.
It is not necessary to alloy copper Cu, chromium Cr, nickel Ni and molybdenium Mo into steel according to the invention and their contents stay as unavoidable residual contents.
According to the aforementioned the content of the steel used for the production of the steel product according to the present invention is:
C: 0.04-0.10%
Si: <0.03%
Mn: 0.3-1.2%
Nb: 0.015-0.10%
N: <0.01%
P: <0.02%
S: <0.02% Al: 0.01-0.08%
V: <0.05%, the remaining being iron, unavoidable impurities or residual contents.
Preferably the composition of the steel used for producing the steel product consists only of the above mentioned alloys, because additional alloys and/or too big contents impair hot-dip galvanization abilities. On the other hand, the aims of the invention are not necessarily reached by too low alloying.
By direct quenching 8, high strength has been reached in relation to alloying 2, even if the microstructure of the steel mainly is low-carbon ferrite and/or bainite, without containing significant amounts of high-carbon martensite or high-carbon bainite. Advisably, the main phase is ferrite in such a way that the microstructure advisably is nearly totally ferritic, in addition to which there are small amounts of bainite and/or martensite and/or residual austenite in the form of very small islets with richened carbon content. In other words, direct quenching 8 means accelerated cooling immediately after hot rolling.
The invention offers a solution for example for producing high-strength galvanized structural hollow sections 16, 18 according to figures 3 and 4, which in connection with hot-dip galvanization 14 are prone to liquid metal embrittlement in the corner points, in which the external radius of curvature R has been marked. Liquid metal embrittlement phenomena occur especially at small external corner radiuses of curvature R, when R<5T, where T is the material strength of the product 16, 18. Liquid metal embrittlement may occur also at angle magnitudes >90°, but preferably at angle magnitudes <120°. In chart 1 one can observe that the invention works even with R=2T external corner radiuses of curvature; it can therefore be stated that the invention works in high strengths also with R=2-4T external corner radiuses of curvature. The external radiuses of curvature of a corner are measured from the external corner of the section according to figures 2 and 3.
Enhancing of the material strength T may increase the degree of strain hardening of the product 16, 18, which increases liquid metal embrittlement. In the case of angular struc- rural hollow sections, the degree of strain hardening of the product can be estimated by the formula (B+H)/2T, where a smaller numerical value means a higher degree of strain hardening. In addition, enhancing of the material strength complicates rapid quenching of the steel in connection with the direct quenching 8, when one cannot obtain a uni- formly strong material out of too thick a strip, as the interior remains softer than the surface. Of this reason, material thickness of the product 16, 18 according to the invention is 2 - 14mm, preferably, however 4 - 12.5mm.
By a product 16, 18 according to the invention is meant a cold formed steel product, in which there is cold formed at least one corner. Preferably the product comprises at least one corner, the external corner radius of which is <5T and preferably, in addition, one or several angles with a magnitude of <120°. Preferably the steel product 16, 18 according to the invention is an elongated product and most preferably thereby is meant a closed section, such as a structural hollow section.
Previously, the invention has been elucidated by examples. Accordingly, it is to be un- derstood that details of the invention may be implemented in several ways within the scope of the accompanying claims.