Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• Ultra high-strength structural steel and method for producing ultra high-strength structural steel are provided.
• 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 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.
• an ultra high-strength structural steel is meant steel, whose yield strength R p o. 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
• 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.
• 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.
• usability properties such as weldability and flangeability as well as other important properties, such as elongation and impact toughness can weaken, as is known.
• 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.
• 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.
• 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.
• the steel 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.
• 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.
• ultra high-strength structural steels the attempt is for the smallest material thicknesses possible and, therefore, assurance against corrosion damages is further emphasized.
• 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.
• 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.
• KR950004775 B1 From KR950004775 B1 is known a method for producing a steel having a yield strength of approximately 150KSI ( ⁇ 1034 ⁇ 8), 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.
• 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 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.
• 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.
• 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/ cm 2 , i.e. expressed differently
• 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.
• 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.
• 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.
• the present invention is implemented, preferably but not necessarily, by using a unique combination of the composition of the steel and direct quenching.
• 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.
• 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.
• an ultra high-strength structural steel according to the invention 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.
• 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.
• structural inspection intervals and intervals between performances of reparative painting can be increased.
• the steel 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.
• 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.
• 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
• Fig. 4 shows in more detail the second preferred embodiment of the method according to the invention.
• a dashed line means that the next step is a preferred, but not imperative, step of the method.
• composition of an ultra high-strength structural steel according to the invention comprises, as percentages by weight:
• 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.
• 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.
• the carbon content C of the steel is 0.08 - 0.12 %, which further improves said properties.
• 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.
• 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.
• 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.
• manganese is used preferably 0.5 - 1.5 %.
• manganese Mn is alloyed at least 0.7 %, especially when the thickness of the structural steel T h ⁇ 5 mm.
• manganese is alloyed at least 0.9 %, especially when the thickness of the structural steel T h ⁇ 6 mm.
• 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.
• 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.
• 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.
• 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.
• the Cu-content of the steel increases the strength of the steel.
• copper Cu increases especially the weather resistance of the steel and it is used 0.25 - 3.0 %.
• 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(%) + ⁇ (%) ⁇ 2.5 %.
• 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.
• the sum of nickel and copper is preferably not more than 6 %.
• 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.
• Ni-content can be kept lower, such as Ni 0.8 - 2 %.
• 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.
• 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.
• 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.
• exceptionally essential al- loying elements of a structural steel according to the invention are nickel Ni, copper Cu, chromium Cr and silicon Si.
• 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 %
• the sum content of chromium Cr and manganese Mn is at least 1.8, i.e. Mn(%) + Cr (%) ⁇ 1.8 %
• 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.
• 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.
• 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.
• 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.
• TiN titanium nitrides
• 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.
• 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.
• Ti-content is ⁇ 0.02 %, particularly at greater thicknesses Th ⁇ 5 mm, to assure the impact toughness.
• titanium Ti is thus alloyed 0.005 - 0.02 %.
• 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 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.
• 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.
• phosphorus P and sulphur S are small as possible, for example, such that P ⁇ 0.02 % and S ⁇ 0.04 %.
• S-content is ⁇ 0.005 % to provide the best flangeability and impact toughness.
• 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.
• 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 %.
• the boron content of the steel is thus less than 0.0003 %.
• boron B can be alloyed 0.0005 - 0.003 %, if hardenability cannot be adequately assured without it.
• 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.
• 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.
• 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.”
• flatness 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.
• MLI mean linear intercept
• the steel has been tempered 12 after direct quenching, wherein the steel is tempered martensitic.
• it is exceptionally important to alloy copper Cu into the steel, which separates in tempering, increasing the strength of the steel.
• 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.
• 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.
• 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 %,
• 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.
• austenitizing annealing 900 - 1350 °C
• 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.
• processing of the steel is, however, merely thermo- mechanical, wherein, after direct quenching 8, no heat treatments are performed afterwards, such as tempering 12.
• 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.
• 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.
• the impact toughness of the martensite formed increases and especially the yield strength increases.
• 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.
• 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.
• 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.
• the amount and contents of the alloying elements can be decreased, which again enables reduction of alloying element expenses.
• 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.
• 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.
• 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.
• direct quenching 8 of the strip steel is begun within 15 seconds.
• 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.
• the strip steel can be reeled in step 10. The temperature of coiling can then occur in the temperature range of 30 - 300 °C.
• 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.
• 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.
• the steel is direct quenched 8 directly to the ambient temperature.
• 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.
• 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.
• 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.
• 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.
• 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.
• the temperature in the sheet rolling is less than 950 °C, according to recommendations less than 900 °C.
• direct quenching 8 of the steel sheet is begun within 30 seconds, preferably within 15 seconds.
• 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.
• niobium Nb 0,008 - 0,08 % is alloyed into a structural steel, which is produced in the hot-rolling step of the invention as a steel sheet, to increase the toughness.
• a sheet-like steel product 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.
• the width of a sheet-like steel product can be 1500 mm, while its thickness is 5 mm.
• 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.
• Th is at least 2 mm and preferably at least 4 mm.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• a steel according to the invention meets corresponding impact toughness requirements also at a temperature of -60 °C.
• 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.
• a structural steel can be provided having the follow- ing superior mechanical properties:
• 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.
• 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.
• multi-run welding are also achieved good toughness and strength properties due to a suitable composition, for example, by limiting vanadium content.
• 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.
• 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.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Steel (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (2)

Family

ID=43414971

Family Applications (1)

Country Status (6)

Cited By (8)

Families Citing this family (9)

Citations (6)

Family Cites Families (2)

• 2010
  • 2010-12-02 FI FI20106275A patent/FI20106275A/fi not_active Application Discontinuation
• 2011
  • 2011-12-01 EP EP11815448.3A patent/EP2646582B1/en active Active
  • 2011-12-01 WO PCT/FI2011/051066 patent/WO2012072884A1/en active Application Filing
  • 2011-12-01 RU RU2013129002/02A patent/RU2586953C2/ru active
  • 2011-12-01 ES ES11815448T patent/ES2761839T3/es active Active
  • 2011-12-01 CN CN201180066512.2A patent/CN103348020B/zh active Active

Patent Citations (6)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events