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
• • 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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • 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
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

• the invention relates to a method for producing weather resistant hot-rolled high strength structural steel product as defined in the preamble of independent claim 1.
• the invention also relates to a weather resistant hot-rolled high strength structural steel product having bainitic-martensitic or pure martensitic microstructure as defined in the preamble of independent claim 18.
• Weather resistant steels are commonly utilized in steel structures with as well as without paint coating.
• the weather resistant performance of weather resistant steels is obtained by a tight oxide layer on the surface of the steel which tight oxide layer hinders atmospheric corrosion from advancing on the steel surface. This property is usually achieved by alloying high amount of copper Cu, silicon Si, nickel Ni and/or phosphor P.
• stainless steels are typically produced by alloying very high amount of chromium Cr to the steel. The improving effect on weather resistant performance of those chemical elements is present also when paint is applied on the surface. In such cases an oxide layer that is formed on possible scratches hinders corrosion from advancing under the paint.
• Patent publication FI 47908 presents a low-carbon high strength structural forge steel having good weldability containing not more than 0.08 % carbon, from 0.5 to 2.2 % manganese, up to 1 % silicon, from 3 to 5 % chromium, up to 0.05 % aluminum, and up to 0.1 % niobium.
• a problem with this known steel is however the presence of niobium which has been alloyed to control the grain size during forging process and the following heat treatment.
• the object of the invention is to provide a method for producing a weather resistant hot-rolled high strength structural steel product and to provide a weather resistant hot-rolled high strength structural steel product comprising bainitic-martensitic or pure martensitic microstructure which provides for both improved surface quality and for good mechanical cold working properties such as good flangeability.
• the great impact toughness properties are also desired.
• the weather resistant hot-rolled high strength structural steel product of the invention is correspondingly characterized by the definitions of independent claim 18.
• the weather resistant steel according to the present invention has a relatively high chromium content combined with the absence, or alternatively, only a small Nb content.
• the relatively high Cr content hinders thick scale formation during hot-rolling by preventing oxidation of the surface. This reduces the need of after-treatments. Together with the absence, or alternatively the small amount of Nb, an improved flanging performance combined with good surface quality is obtained.
• the present invention provides an excellent weather resistant hot-rolled high strength structural steel product in terms of strength, plastic deformation capability, weather resistant performance and surface quality.
• This kind of steel is especially suitable for thin shell constructions. Especially after hot-rolling the surface quality of a weather resistant hot-rolled high strength structural steel product according to the invention is fine taking into account the weather resistant performance.
• Figure 1 shows a flow sheet of an embodiment of a method for producing a weather resistant hot-rolled high strength structural steel product in the form of a steel strip
• Figure 2 shows a flow sheet of another embodiment of a method for producing a weather resistant hot-rolled high strength structural steel product in the form of a steel strip that additionally comprises a tempering step
• Figure 3 shows a flow sheet of still another embodiment of a method for producing a weather resistant hot-rolled high strength structural steel product in the form of a steel strip
• Figure 4 shows a flow sheet of still another embodiment of a method for producing a weather resistant hot-rolled high strength structural steel product in the form of a steel strip that additionally comprises a tempering step
• Figure 5 is a diagram showing surface hardness of steels 1367 and 1368 as measured as the distance from the surface.
• the invention relates to a method for producing a weather resistant hot-rolled high strength structural steel product and to a weather resistant hot-rolled high strength structural steel product.
• the method for producing a weather resistant hot-rolled high strength structural steel product comprises a providing step for providing a steel slab containing in percentages of mass,
• V less than 0.2%>
• Al 0.01 to 0.15 %
• the providing step can include a step for continuous casting a steel slab having the above- specified composition.
• a heating step (not shown) for austenitizing the steel slab in 900 to 1350 °C can be performed. This may be performed for instance in a walking beam type furnace.
• the method comprises also a hot-rolling step for hot rolling said steel slab such that the final rolling temperature is between 750 and 950 °C to obtain a hot-rolled steel product.
• the method comprises hot rolling said steel slab in the hot-rolling step such that the final rolling temperature is between 780 and 930 °C. It is even preferred that the final rolling temperature is above A r 3 inorder to ensure avoiding excessive formation of ferrite to the microstructure. According to one embodiment the final rolling temperature is above recrystallization stop temperature, in order to avoid flattened austenite grains prior to direct quenching. If the final rolling temperature is too high, the austenite may become too coarse grained prior to direct quenching.
• the method comprises also a direct quenching step for quenching the hot-rolled steel from a temperature range between 700 and 940 °C to a temperature below 500°C, preferably below 250 °C so that the cooling rate in the quenching step is between 10 and 150 °C/s, preferably between 30 and 150 °C/s to obtain the weather resistant hot-rolled high strength structural steel product.
• Direct quenching from the temperature between 700 and 940 °C, preferably 750-940 °C provides austenite required for hardening.
• Direct quenching is performed to a temperature below 500°C, because otherwise the formed microstructure is not necessarily hard and strong enough.
• this quenching stop temperature is below 250 °C, because otherwise toughness may suffer, especially in case of coiled steel material. Also this ensures that martensitic phases are formed enough.
• the quenching stop temperature is below Mf - temperature (temperature at which the martensite formation is finished), such as in the range 20 to 100 °C, to obtain a microstructure comprising high content of martensitic phases.
• Given cooling speeds are defined to provide desired microstructure after cooling. Range 30 to 150 °C/s is especially suitable in case of steel strip.
• the hot-rolling step may be performed in a strip mill to obtain a hot-rolled steel product in the form of a hot-rolled steel strip.
• the thickness of the hot-rolled steel strip can be between 2 and 12 mm, preferably between 2 and 10 mm, more preferably between 2 and 6 mm.
• the method comprises preferably, as shown in figures 1 to 4, a coiling step for coiling the hot-rolled steel strip, which coiling step is performed after the quenching step.
• the method may additionally, as shown in figure 2 and 4, comprise a tempering step, that is performed after the coiling step, if improved elongation properties are desired.
• the hot-rolling step may be performed in a plate mill to obtain a hot-rolled steel product in the form of a hot-rolled steel plate.
• the thickness of the hot-rolled steel plate can be between 10 and 40 mm, preferably between 12 and 30 mm.
• the hot-rolling step may include a step or steps for turning the steel slab horizontally 90 degrees in the horizontal plane of the steel slab during the hot-rolling step.
• the method may additionally comprise a tempering step that is performed after the hot-rolling step, if good elongation properties are desired.
• Table 1 shows the composition of two test steels 1368 and 1367.
• Table 1 Compositions of the test steels 1368 and 1367
• the upper limit for C is in percentages of mass set to 0.16 % and the lower limit for C is in percentages of mass set to 0.04 %.
• the upper limit is set because the otherwise (with high Cr) the steel is not ductile enough to be subjected for demanding flanging for instance.
• it is preferred that the upper limit for carbon content is in percentages of mass set to 0.10%. This is because under 0.10% also impact toughness and weldability can be ensured. At least 0.04 %> of carbon must be present for the hardening of the steel and in order to achieve good impact strength.
• the method comprises most preferably providing a steel slab containing in percentages of mass C 0.05 to 0.08 %>, because flangebility, impact strength and weldability has been found to be especially good in this range.
• the steel according to this embodiment having a low carbon content benefits on good weldability and ductile martensite in the microstructure.
• the carbon equivalent, CEV, as calculated by formula (1) below can be however high despite the carbon content being low.
• a high carbon equivalent has a positive effect on achieving a combination of good strength and toughness in a welding joint between two structural steel parts made of structural steel produced by the method.
• the carbon equivalent, CEV, as calculated by formula (1) preferably is between 0.5 and 1.2, more preferably between 0.90 and 1.20.
• the CEV of the two example steels 1368 and 1367 was 1.2 and correspondingly 1.1.
• the silicon content is less than 0.8%. This means that silicon is not necessarily added. Due to the high Cr content, the present invention also enables the weather resistant steel with very low Si-content such as Si ⁇ 0.1%, especially if best possible surface quality is desired.
• the upper limit for Si is in percentages of mass set to 0.8 % and the lower limit for Si is in percentages of mass set to 0.1 %.
• a Si content of below 0.1 % is not preferably used, because sulfur removal from the steel and control of inclusions in the steel is easier when the steel contains at least some Si.
• Si also improves the weather resistance performance of the steel and increases strength.
• Si is known to form a red-scale layer on the surface of hot-rolled steel which impairs the surface quality if not removed properly.
• a high Cr content can also reduce this undesired red-scale tendency of Si.
• Even though the steel contains relatively high amounth of Cr, a too high Si content may however have a negative effect on surface quality, whereby the upper limit for Si is set to 0.8 %.
• the method comprises therefore preferably, but not necessarily, providing a steel slab containing in percentages of mass Si 0.1 to 0.4 % and most preferably 0.15 to 0.25 % in order to ensure both good impact strength and good surface quality.
• the upper limit for Mn is in percentages of mass set to 2.0 % and the lower limit for Mn is in percentages of mass set to 0.2 %.
• Mn is needed at least for desulphuration but is useful for hardening also, therefore at least 0.2% is needed. Contents more than 2.0% (with high Cr) debilitates formability.
• the upper limit for Mn is in percentages of mass set to 1.5 % and the lower limit for Mn is in percentages of mass set to 0.7 %.
• This preferred lower limit 0.7% is set because Mn improves the hardenability of the steel.
• Upper limit 1.5% is set because Mn can segregate during continuous casting and can have a negative effect on the elongation properties and on fracture mechanics of the steel product. Because of this the method comprises most preferably, but not necessarily, providing step for providing a steel slab containing in percentages of mass Mn 0.9 to 1.2 %. A Mn content, more than 0.9 % may be needed if the thickness of the structural steel product is over 6 mm.
• Chromium, Cr The upper limit for Cr is in percentages of mass set to 5.0 % and the lower limit for Cr is in percentages of mass set to 2.0 %>.
• the Cr content is at least 2.0 %>, because Cr improves the weather resistant performance and improves the surface quality by preventing oxidation of the steel surface during hot-rolling. Cr additionally compensates the reduced hardenability caused by low carbon content.
• the Cr content is at the most 5.0 %, because a too high Cr content may increase tendency to pit corrosion by localizing the corrosion mechanism too much. Also too high Cr content has a negative effect on weldability and it also increases unnecessarily the alloying costs. Addition of Cr also raises the carbon equivalent (CEV) of the steel as calculated using formula (1). Because of these reasons, the method comprises preferably, but not necessarily, providing a steel slab containing in percentages of mass Cr 3.0 to 4.5 %.
• the upper limit for B is in percentages of mass set to 0.03 % and the lower limit for B is in percentages of mass set to 0 %.
• the upper limit for Ti is in percentages of mass set to 0.04 % and the lower limit for Ti is in percentages of mass set to 0%. This means that Ti is not necessarily actively added. Upper limit 0.04%) is added because otherwise excessive coarsening of TiN can occur. It is however preferable that Ti is alloyed 0.005-0.02% in order to control the grain size during heating. This way toughness of a weld joint can be improved.
• Ti may also be needed in order to prevent B from reacting with nitrogen, N, to form boron nitride, BN, and this way to ensure the function of B.
• the method comprises in one embodiment of the method providing a steel slab containing in percentages of mass Cr 2.5 to 5.0 %, preferable 3.0 to 4.5 % and in percentages of mass B less than 0.0005 %, ie. B not alloyed purposefully to have a function to increase hardenability, and Ti less than 0.02 %. Because of low Ti content, coarse TiN particles does not form in the steel. Such coarse TiN particles may weaken the impact strength, elongation and the fracture behaviour of the steel product. In this embodiment, the low carbon steel with high Cr content makes possible the omission of B and this way its possible to keep the Ti content low enough to ensure the impact strength.
• the method comprises in another embodiment of the method providing a steel slab containing in percentages of mass Cr 2.0 to 2.5 %, and in percentages of mass B 0.0005 to 0.003 %>, and Ti 0.02 - 0.04%.
• boron is used to compensate reduced hardening caused by lower Cr content.
• the suitable amount of Ti is alloyed to secure the functioning of B as described earlier.
• the method comprises in yet another embodiment of the method providing a steel slab containing in percentages of mass Cr 2.0 to 5.0 %, and in percentages of mass B 0.0005 to 0.003 %, and Ti 0.02 - 0.04%.
• the suitable amount of Ti is alloyed to secure the functioning of B as described above.
• the method comprises in still another embodiment of the method providing a steel slab containing in percentages of mass Cr 2.5 to 5.0 %, preferably Cr 3.0 to 4.5 %, and in percentages of mass B 0.0005 to 0.003 %, and Ti 0.02 - 0.04%.
• the suitable amount of Ti is alloyed to secure the functioning of B as described above.
• the upper limit for Mo is in percentages of mass set to 0.8 % and the lower limit for Mo is in percentages of mass set to 0 %. This means that Mo is not necessarily actively added. Mo improves strength, but the upper limit for Mo is set to 0.8 %>, highly preferable 0.4%>, because a too high Mo content has a negative effect on cold working properties of the steel such as on flangeability. Addition of Mo also increases alloying costs and raises the carbon equivalent (CEV) of the steel as calculated using formula ( 1 )
• the method may comprise providing a steel slab containing in percentages of mass Mo 0.1 to 0.2 %, because Mo effectively prevents recrystallization during hot-rolling, ie. this way resulting easier the flattened austenite and fine microstructure of the steel product.
• Mo effectively prevents recrystallization during hot-rolling, ie. this way resulting easier the flattened austenite and fine microstructure of the steel product.
• the upper limit for Nb is in percentages of mass set to 0.05 % and the lower limit for Nb is in percentages of mass set to 0 %. This means that Nb is not necessarily actively added or at least its content is very low. Preferably, the upper limit for Nb is in percentages of mass set to 0.03 %
• the method comprises most preferably providing a steel slab containing in percentages of mass Nb less than 0.01 %, such as less than 0.005 %. It has been found that Nb can have harmful effect on flangeability to the steel according to the invention. The reason for this is not exhaustively solved, but one reason for this might be the flattened prior austenite grains that are formed easily during hot-rolling in case of niobium alloying. These may impair, but not necessarily impair, flangeability of this kind of weathering steel if surface quality is not good.
• the upper limit for V is in percentages of mass set to 0.2 % and the lower limit for V is in percentages of mass set to 0 %>. This means that V is not necessarily actively added.
• the upper limit for V is set to 0.2 %>, because V reduces impact strength, weldability and flangeability.
• Preferably upper limit for V is in percentages of mass set to 0.1 %, due to the reasons above.
• the method comprises most preferably providing a steel slab containing in percentages of mass V less than 0.05 %>, due to the reason above. Aluminium, Al
• the upper limit for Al is in percentages of mass set to 0.15 % and the lower limit for Al is in percentages of mass set to 0.01 %.
• Al is used for de-oxidation (killing) the steel.
• the upper limit for Ni is in percentages of mass set to 2.0 % and the lower limit for Ni is in percentages of mass set to 0 %. This means that Ni is not necessarily actively added. Ni improves weather resistance, but because of the relatively high Cr content, Ni is not necessarily needed for additionally improving weather resistance. Addition of Ni raises the carbon equivalent (CEV) of the steel as calculated using formula (1).
• the method comprises preferably providing a steel slab containing in percentages of mass Ni less than 0.2 %. Ni improves weather resistance, but because of the relatively high Cr content, Ni is not necessarily needed. Copper, Cu
• the upper limit for Cu is in percentages of mass set to 2.0 % and the lower limit for Cu is in percentages of mass set to 0 %. This means that Cu is not necessarily actively added. Cu improves weather resistance, but because of the relatively high Cr content, Cu is not necessarily needed for additionally improving weather resistance. Addition of Cu raises the carbon equivalent (CEV) of the steel as calculated using formula (1).
• the method comprises preferably providing a steel slab containing in percentages of mass Cu less than 0.2 %.
• Calsium Ca may be used in Ca or CaSi smelt processing, resulting in contents up to 0.01%.
• the upper limit for Ca is in percentages of mass set to 0.005 % and the lower limit for Ca is in percentages of mass set to 0.0005 %.
• Sulfur can be considered to be an unavoidable impurity.
• the upper limit for S is in percentages of mass set to 0.01 %.
• the S content is as low as possible.
• the method comprises preferably providing a steel slab containing in percentages of mass S less than 0.005 %, or even less than 0.004%, in order to further improve flangeability and ensure good impact strength. Phosphorus, P
• Phosphorus can be considered to be an unavoidable impurity.
• the upper limit for P is in percentages of mass set to 0.025 %>.
• the P content is as low as possible.
• the method comprises preferably providing a steel slab containing in percentages of mass P less than 0.015 % in order to further improve flangeability and ensure good impact strength.
• Nitrogen, N also nitrogen can be considered to be an unavoidable impurity.
• the upper limit for N is in percentages of mass set to 0.01 %.
• the N content is as low as possible.
• Weather resistant steel is generally and also in this invention understood as steel, which prevents the atmospheric corrosion from advancing in the steel by creating an oxide layer on the steel surface when exposed to atmosphere for a long period of time.
• Weather resistant steel can be in a form hot-rolled steel product meaning that the steel product is hot-rolled but not treated by cold- rolling including thickness reductions more than 10% (preferably more than 5%). This means that said levelling or skin pass rolling type secondary rolling in cold state may be performed for hot- rolled steel.
• weather resistant steel can be in a form of cold-rolled steel product meaning that steel is treated by cold-rolling having reductions more than 10% (preferably more than 5%).
• the present invention relates to hot-rolled steel products and excludes defined cold-rolled products. This is for instance because desired thickness and properties can be achieved with hot-rolled steel product.
• Hot rolled high strength structural steel product can be considered as structural steel product or as wear resistant product. Therefore, the term structural is not necessarily restrictive.
• a high strength steel is in this context meant a steel having a yield strength of at least 840 MPa and a yield ratio (R p o.2 / R m ) of more than 0.7, preferable more than 0.8.
• the hot rolled steel product comprising bainitic-martensitic or pure martensitic microstructure,contains in percentages of mass percentages
• V less than 0.2 %
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass C 0.04 to 0.10 and more preferable 0.05 to 0.08 %.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass Si 0.1 to 0.8 %, more preferably Si 0.1 to 0.4 %, most preferable 0.15 to 0.25 %. Due to the high Cr content, the present invention also enables the weather resistant steel with very low Si-content such as Si ⁇ 0.1%.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass Mn 0.7 to 1.5% and more preferably 0.9 to 1.2 %.
• the hot-rolled steel product may contain in percentages of mass Cr 2.5 to 5.0 %, preferable 3.0 to 4.5 %. If the hot-rolled steel product contains in percentages of mass Cr 2.5 to 5.0 %>, the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass B less than 0.0005 %>, i.e. not B alloyed purposefully to have a function to increase hardenability, and Ti less than 0.02 %. Because of low Ti content, coarse TiN particles to form in the steel. Such coarse TiN particles may weaken the impact strength, elongation and the fracture behaviour of the steel product. In this embodiment, the low carbon steel with high Cr content makes possible the omission of B and this way its possible to keep the Ti content low enough to ensure the impact strength.
• the hot-rolled steel product may contain percentages of mass Cr 2.0 to 2.5 %. If the hot-rolled steel product contains in percentages of mass Cr 2.0 to 2.5 %>, the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass, B 0.0005 to 0.003 %>, and Ti 0.02 - 0.04 %> in order to ensure function of boron as described above. In this embodiment, the boron is used to compensate reduced hardening caused by lower Cr content. Further, the suitable amount of Ti is alloyed to secure the functioning of B
• the hot-rolled steel product may however in some embodiments contain in percentages of mass Cr 2.0 to 5.0 %>, and in percentages of mass, B 0.0005 to 0.003 %>, and Ti 0.02 - 0.04 %> in order to ensure function of boron as described above.
• Such alloying results in high surface hardness which may be of an advantage especially in thin steel products in the form of a weather resistant hot-rolled high strength structural steel strip having a thickness in the range of 2 to 6 mm.
• the hot-rolled steel product may however in some embodiments contain in percentages of mass Cr 2.5 to 5.0 %>, preferably Cr 3.0 to 4.5 %>, and in percentages of mass, B 0.0005 to 0.003 %>, and Ti 0.02 - 0.04 %> in order to ensure function of boron as described above.
• Such alloying results in high surface hardness which may be of an advantage especially in thin steel products in the form of a weather resistant hot-rolled high strength structural steel strip having a thickness in the range of 2 to 6 mm.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass less than 0.4% of Mo such as 0.1 to 0.2 %.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass Nb less than 0.03%, more preferably 0.01 %, most preferable less than 0.005 %.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass V less than 0,10% more preferably less than 0.05 %.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass Ni less than 0.2 %.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass Cu less than 0.2 %.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass S less than 0.005 %, more preferably less than 0.004%,
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass P less than 0.015 %.
• the hot-rolled steel product contains preferable, but not necessarily, in percentages of mass Ca 0.0005-0.005%
• the microstructure of the weather resistant hot-rolled high strength structural steel product comprises bainitic-martensitic or pure martensitic microstructure.
• bainitic-martensitic or pure martensitic microstructure is preferable, in terms on volyme percentages, more than 90 %, preferable more than 95 %. This means that the main phase of the microstructure is bainitic-martensitic or pure martensitic.
• bainite of said bainitic-martensitic microstructure is upper bainite and preferably additionally lath type bainite.
• the microstructure of the weather resistant hot-rolled high strength structural steel product may comprise martensite and upper bainite.
• Martensite may also be self tempered during cooling after hot-rolling.
• the microstructure may comprise retained austenite and quasi-polygonal ferrite, preferably in terms of volyme percentages, less than or equal to 5%.
• the microstructure of the weather resistant hot-rolled high strength structural steel product may comprise retained austenite and quasi-polygonal ferrite and in addition to bainitic-martensitic structures, which martensitic structures may be self tempered.
• pure martensitic microstructure may be more advantageous since homogenous microstructure affects positively on flangeability. Pure martensitic microstructure may also be tempered or self-tempered.
• the aspect ratio of the prior austenite grain structure is at least 1.5 and the mean linear intercept (MLI) of the prior austenite grain structure is under 20 micrometer. This means that prior austenite grains are at least partly flattened in this embodiment. This embodiment may improve impact toughness.
• the aspect ratio of the prior austenite grain structure is less than 1.5 and the mean linear intercept (MLI) of the prior austenite grain structure is under 20 micrometer. This means that prior austenite grains are not substantially flattened in this embodiment. This embodiment may still improve the flangeability.
• MLI mean linear intercept
• R/t value in which R is the bending radius measured from the outer side of the steel strip/plate and t is the thickness of the steel strip/plate.
• Good numerical values for flangeability are usually dependent on the thickness area and can be such as: strip/plate thickness below 4mm: R/t ⁇ 3.0
• the product is obtained by the method according to this invention.
• the invention relates also to a method for producing a weather resistant hot-rolled high strength structural steel product. Next few embodiments of the method are disclosed.
• the method is a method for producing a weather resistant hot- rolled high strength structural steel strip, and the method comprises a providing step for providing a steel slab having the following composition:
• C 0.04 to 0.16 %, preferable C 0.04 - 0.10 %, and more preferable C 0.05 to 0.08 Si: less than 0.8%, preferable Si 0.1 to 0.4 %, more preferable Si 0.15 to 0.25 %, Mn: 0.2 to 2.0%, preferable Mn 0.7 to 1.5 %, more preferable Mn 0.9 to 1.2 %, Cr: 2.5 to 5.0 %, more preferable Cr 3.0 to 4.5 %,
• Mo less than 0.8 %, preferable Mo less than 0.4%, more preferable Mo 0.1 to 0.2 %
• Nb less than 0.05 %, preferable less than 0.03 %, more preferable Nb less than 0.01 %, most preferable Nb less than 0.005 %.
• V less than 0.2 %, preferable less than 0.1 %, more preferable V less than 0.05 %.
• Ni less than 2.0 %, preferable Ni less than 0.2 %.
• Cu less than 2.0 %, preferable Cu less than 0.2 %.
• the providing step may include a step for continuous casting a steel slab having the above- specified composition.
• This first embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip comprises a hot-rolling step for hot rolling said steel slab such that the final rolling temperature is between 750 and 950 °C, preferable between 780 and 930 °C, to obtain a hot-rolled steel having especially a thickness of 2 to 12 mm, preferably between 5 and 12 mm, more preferably between 6 and 10 mm.
• a heating step may be performed for example in a walking beam type furnace for austenitizing the steel slab in 1200 to 1350 °C and a pre-rolling step may be performed in a roughing mill after such heating step for pre-hot-rolling the steel slab in 950 to 1280 °C to a thickness of 25 to 50 mm.
• This first embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip comprises also a quenching step for direct quenching the hot-rolled steel within 15 seconds, preferably within between 8 and 15 seconds, from a last rolling pass of the hot-rolling step from a temperature range between 700 and 940 °C to a temperature below 500 °C, preferable below 250 °C, more preferable to a temperature in the range 20 to 100 °C, and so that the cooling rate in the direct quenching step is between 30 and 150 °C/s, preferably between 30 and 120 °C/s to obtain the weather resistant hot-rolled high strength structural steel strip.
• This first embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip may comprise a coiling step following the quenching step.
• This first embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip may also comprise a tempering step following the coiling step.
• a such tempering step can for example be performed in a temperature range from 500 to 600 °C
• the method is a method for producing a weather resistant hot- rolled high strength structural steel strip, and the method comprises a providing step for providing a steel slab having the following composition:
• Si less than 0.8%, preferable Si 0.1 to 0.4 %, more preferable Si 0.15 to 0.25 %,
• Mn 0.2 to 2.0%, preferable Mn 0.7 to 1.5 %, more preferable Mn 0.9 to 1.2 %,
• Nb less than 0.05 %>, preferable less than 0.03 %>, more preferable Nb less than 0.01 %>, more preferable Nb less than 0.005 %>.
• V less than 0.2 %>, preferable less than 0.1 %>, more preferable V less than 0.05 %>.
• Ni less than 2.0 %, preferable Ni less than 0.2 %.
• Cu less than 2.0 %>, preferable Cu less than 0.2 %>.
• the providing step may include a step for continuous casting a steel slab having the above- specified composition.
• This second embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip comprises a hot-rolling step for hot rolling said steel slab such that the final rolling temperature is between 750 to 950 °C, preferable between 780 to 930 °C, to obtain a hot-rolled steel having especially a thickness of 2 to 12 mm, preferably between 5 and 12 mm, more preferably between 6 and 10 mm.
• a heating step may be performed for example in a walking beam type furnace for austenitizing the steel slab in 1200 to 1350 °C.
• a heating step may be performed in a walking beam type furnace for austenitizing the steel slab in 1200 to 1350 °C and a pre-rolling step may be performed in a roughing mill after such heating step for pre-hot-rolling the steel slab in 950 to 1280 °C to a thickness of 25 to 50 mm.
• This second embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip comprises a quenching step for direct quenching the hot-rolled steel within 15 seconds, preferably within between 8 and 15 seconds, from a last rolling pass of the hot- rolling step from a temperature range between 700 and 940 °C to a temperature below 500 °C , preferable below 250 °C, more preferable to a temperature in the range 20 to 100 °C, and so that the cooling rate of the in the direct quenching step is between 30 and 150 °C/s, preferably between 30 and 120 °C/s to obtain the weather resistant hot-rolled high strength structural steel strip.
• This second embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip may comprise a coiling step following the quenching step.
• This second embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip may also comprise a tempering step following the coiling step.
• a such tempering step can for example be performed in a temperature range from 500 to 600 °C
• the method is a method for producing a weather resistant hot- rolled high strength structural steel plate, and the method comprises a providing step for providing a steel slab having the following composition:
• Si less than 0.8%, preferable Si 0.1 to 0.4 %, more preferable Si 0.15 to 0.25 %, Mn: 0.2 to 2.0%, preferable Mn 0.7 to 1.5 %, more preferable Mn 0.9 to 1.2 %, Cr: 2.5 to 5.0 %, more preferable Cr 3.0 to 4.5 %,
• Nb less than 0.05 %>, preferable less than 0.03 %>, more preferable Nb less than 0.01 %>, most preferable Nb less than 0.005 %>.
• V less than 0.2 %>, preferable less than 0.1 %>, more preferable V less than 0.05 %>.
• Ni less than 2.0 %, preferable Ni less than 0.2 %.
• Cu less than 2.0 %>, preferable Cu less than 0.2 %>.
• the providing step may include a step for continuous casting a steel slab having the above- specified composition.
• This third embodiment of the method for producing a weather resistant hot-rolled high strength structural plate comprises a hot-rolling step for hot rolling said steel slab such that the final rolling temperature is between 750 and 950 °C to obtain a hot-rolled steel having a thickness of 10 to 40 mm, preferably between 12 and 30 mm.
• a heating step may be performed for example in a pusher type furnace for austenitizing the steel slab in 1000 to 1300 °C.
• This third embodiment of the method for producing a hot-rolled steel plate comprises a quenching step for quenching the hot-rolled steel from a temperature range between 700 and 940 °C to a temperature below 500 °C, preferable below 250 °C, more preferable to a temperature in the range 20 to 100 °C, and so that the cooling rate of the in the quenching step is between 10 and 150 °C/s, preferably between 30 and 150 °C/s to obtain the weather resistant hot-rolled high strength structural plate.
• This quenching step may be performed as direct quenching within 15 and 30 seconds from a last rolling pass of the hot-rolling step.
• This third embodiment of the method for producing a hot-rolled plate may comprise a tempering step following the quenching step.
• the method is a method for producing a weather resistant hot- rolled high strength structural steel plate, and the method comprises a providing step for providing a steel slab having the following composition:
• Si less than 0.8%, preferable Si 0.1 to 0.4 %, more preferable Si 0.15 to 0.25 %,
• Mn 0.2 to 2.0%, preferable Mn 0.7 to 1.5 %, more preferable Mn 0.9 to 1.2 %,
• Mo less than 0.8 %>, preferable Mo less than 0.4%>, more preferable Mo 0.1 to 0.2 %>
• Nb less than 0.05 %>, preferable less than 0.03 %>, more preferable Nb less than 0.01 %>, most preferable Nb less than 0.005 %>.
• V less than 0.2 %>, preferable less than 0.1 %>, more preferable V less than 0.05 %>.
• Ni less than 2.0 %>, preferable Ni less than 0.2 %>.
• Cu less than 2.0 %>, preferable Cu less than 0.2 %>.
• the providing step may include a step for continuous casting a steel slab having the above- specified composition.
• This fourth embodiment of the method for producing a weather resistant hot-rolled high strength structural steel plate comprises a hot-rolling step for hot rolling said steel slab such that the final rolling temperature is between 750 and 950 °C to obtain a hot-rolled steel having a thickness of 10 to 40 mm, preferably between 12 and 30 mm.
• a heating step may be performed for example in a pusher type furnace for austenitizing the steel slab in 1000 to 1300 °C.
• This fourth embodiment of the method for producing a weather resistant hot-rolled high strength structural steel plate comprises a quenching step for quenching the hot-rolled steel from a temperature range between 700 and 940 °C to a temperature below 500 °C , preferable below 250 °C, more preferable to a temperature in the range 20 to 100 °C, and so that the cooling rate of the in the quenching step is between 10 and 150 °C/s, preferably between 30 and 150 °C/s to obtain the weather resistant hot-rolled high strength structural plate.
• This quenching step may be performed as direct quenching within 15 and 30 seconds from a last rolling pass of the hot- rolling step.
• This fourth embodiment of the method for producing a hot-rolled plate may comprise a tempering step following the quenching step.
• the method is a method for producing a weather resistant hot- rolled high strength structural steel strip, and the method comprises a providing step for providing a steel slab having the following composition:
• Si less than 0.8%, preferable Si 0.1 to 0.4 %, more preferable Si 0.15 to 0.25 %,
• Mn 0.2 to 2.0%, preferable Mn 0.7 to 1.5 %, more preferable Mn 0.9 to 1.2 %,
• Mo less than 0.8 %, preferable less than 0.4 %, more preferable Mo 0.1 to 0.2 % Nb: less than 0.05 %>, preferable less than 0.03 %>, more preferable Nb less than 0.01 %>, most preferable Nb less than 0.005 %>.
• V less than 0.2 %>, preferable less than 0.1 %>, more preferable V less than 0.05 %>.
• Ni less than 2.0 %, preferable Ni less than 0.2 %.
• Cu less than 2.0 %>, preferable Cu less than 0.2 %>.
• the providing step may include a step for continuous casting a steel slab having the above- specified composition.
• This fifth embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip comprises a hot-rolling step for hot rolling said steel slab such that the final rolling temperature is between 750 and 950 °C, preferable between 780 and 930 °C, to obtain a hot-rolled steel having a thickness of 2 to 12 mm, preferably between 2 and 6 mm.
• a heating step may be performed for example in a walking beam type furnace for austenitizing the steel slab in 1200 to 1350 °C and a pre-rolling step may be performed in a roughing mill after such heating step for pre-hot-rolling the steel slab in 950 to 1280 °C to a thickness of 25 to 50 mm.
• This fifth embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip comprises also a quenching step for direct quenching the hot-rolled steel within 15 seconds, preferably within between 8 and 15 seconds, from a last rolling pass of the hot-rolling step from a temperature range between 700 and 940 °C to a temperature below 500 °C , preferable below 250 °C, more preferable to a temperature in the range 20 to 100 °C, and so that the cooling rate of the in the direct quenching step is between 30 and 150 °C/s, preferably between 30 and 120 °C/s to obtain the weather resistant hot-rolled high strength structural steel strip.
• This fifth embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip may comprise a coiling step following the quenching step.
• This fifth embodiment of the method for producing a weather resistant hot-rolled high strength structural steel strip may comprise a tempering step following the coiling step.
• a such tempering step can for example be performed in a temperature range from 500 to 600 °C.
• weather resistant hot-rolled high strength structural steel product will be illustrated through tests made in laboratory conditions which in some aspects may vary from full-scale production conditions in steel factories. It is known in the art that steels produced in full-scale production conditions in steel factories have a higher strength and impact toughness than steels made in laboratories. This is a result of a greater degree of deformation resulting in prior austenite having a smaller grain size in steels produced in full-scale production conditions in steel factories than in steels made in laboratories. For this reason, the weather resistant hot-rolled high strength structural steel product will have even better properties, especially higher yield strength, than presented here when produced in full-scale production conditions in steel factories.
• Steel slabs having the compositions shown in table 1 and having a thickness of 55 mm were casted.
• the steel slabs were heated in a furnace to 1225 °C.
• the steel slabs were hot-rolled to a thickness 5mm, and a quenching was performed after the last rolling pass, i.e. direct quenching was performed.
• the final dimensions of the hot rolled steels were 1120 x 95 x 5 mm.
• Table 2 shows that both steels 1368 and 1367 in both test had a yield strength Rp 0 .2 of over 840 MPa. Preferable the yield strength Rp 0 . 2 is 900 to 1100 MPa.
• Table 2 shows furthermore that both steels 1368 and 1367 in both test had a tensile strength R m of over 1000 MPa. Preferable the tensile strength R m is 1000 to 1300 MPa.
• Table 2 shows furthermore that both steels 1368 and 1367 in both test had an excellent elongation at fracture A5 (%) > 10 %. Preferable the elongation at fracture A5 (%) is more than 10 %.
• Table 2 shows however that in the steel having high Cr-content, the addition of boron, B and higher titanium, Ti, content has a detrimental effect on impact toughness. This can be seen in the results of steel 1367 on table 2. In other words, the absence of boron and low titanium results in improved toughness.
• the Charpy V impact strength of the steel according to the invention measured parallel to rolling direction (i.e. in the longitudinal direction), in -40°C,can reach values more than 40 J/cm , preferable values more than 60 J/cm .

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• 2012
  • 2012-01-19 FI FI20125063A patent/FI20125063L/fi not_active Application Discontinuation
• 2013
  • 2013-01-18 WO PCT/EP2013/050948 patent/WO2013107864A1/en active Application Filing

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