EP4180544A1 - Warmgewalztes stahlbandprodukt und verfahren zu dessen herstellung - Google Patents

Warmgewalztes stahlbandprodukt und verfahren zu dessen herstellung Download PDF

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EP4180544A1
EP4180544A1 EP21207843.0A EP21207843A EP4180544A1 EP 4180544 A1 EP4180544 A1 EP 4180544A1 EP 21207843 A EP21207843 A EP 21207843A EP 4180544 A1 EP4180544 A1 EP 4180544A1
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European Patent Office
Prior art keywords
hot
steel strip
rolled steel
strip product
product according
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EP21207843.0A
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English (en)
French (fr)
Inventor
Robin Nilsson
Magnus Gladh
Erik Nymann
Martin Lind
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SSAB Technology AB
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SSAB Technology AB
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Priority to EP21207843.0A priority Critical patent/EP4180544A1/de
Priority to PCT/EP2022/081342 priority patent/WO2023083898A1/en
Priority to EP22818208.5A priority patent/EP4430223A1/de
Publication of EP4180544A1 publication Critical patent/EP4180544A1/de
Withdrawn legal-status Critical Current

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    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D6/02—Hardening by precipitation
    • 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
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/18—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
    • C22C38/50—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
    • 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

  • the present disclosure relates in general to a hot-rolled steel strip product, in particular to a hot-rolled steel strip product having a tensile strength of above 1400 MPa and having a microstructure comprising at least 90 % by volume of martensite.
  • the present disclosure further relates in general to a method for producing such a hot-rolled steel strip product.
  • the present disclosure also relates in general to an automotive component produced from the hot-rolled steel strip product.
  • Hydrogen embrittlement is a possible phenomenon occurring when a steel is exposed to hydrogen dissolving and diffusing into the steel in combination with stresses, which in turn may result in loss of ductility and/or toughness and reduction in load bearing capability, even at stress levels less that the yield strength. This degradation could be potentially dangerous since the fracture is often unpredictable and preceded without any clear warnings except possibly optically visible cracks, which in turn could be hard to notice once the part is mounted in a larger unit and/or during service/usage. In general, the phenomenon is more common or prone to develop in high-strength steel grades having a tensile strength of above 1200 MPa, and becomes more pronounced as the tensile strength increases.
  • the object of the present invention is to provide a steel strip product, suitable for automotive applications, with a good balance between the properties strength, formability, toughness and hydrogen embrittlement resistance.
  • a hot-rolled steel strip product has a tensile strength (R m ) above 1400 MPa and a microstructure comprising at least 90 % by volume of martensite.
  • the steel has a composition comprising, in percent by weight (wt.-%): C 0.20 - 0.26, Si 0.05 - 0.5, Mn 0.2 - 0.8, Cr 0.2 - 0.6, Ni 0.2 - 0.5, Al 0.015 - 0.065, Ti 0.005 - 0.02, B 0.001- 0.005,
  • the hot-rolled steel strip product of the present disclosure it is possible to obtain an excellent combination of, inter alia , high tensile strength, formability, toughness as well as resistance to hydrogen embrittlement. This makes the hot-rolled steel strip product particularly useful for producing various components in the automotive industry.
  • the hot-rolled steel strip product may have a microstructure comprising at least 95 % by volume of martensite, preferably at least 98 % by volume of martensite.
  • the hot-rolled steel strip product may have a thickness of from 1.5 mm to 6 mm.
  • the hot-rolled steel strip product has a thickness suitable for producing components within the automotive industry.
  • the hot-rolled steel strip product has a thickness of from 2 mm to 4 mm.
  • the hot-rolled steel strip product may have a tensile strength (R m ) of at least 1475 MPa, a yield strength (R p0.2 ) of at least 1100 MPa, and an elongation (A 50 ) of at least 4%.
  • the hot-rolled steel strip product has a yield strength (R p0.2 ) of at least 1200 MPa.
  • the hot-rolled steel strip product may have a time to fracture of at least 130 minutes (preferably at least 140 minutes, more preferably at least 150 minutes) when subjected to a constant load test, as described in the detailed description, at a load of 80% of the tensile strength of the hot-rolled steel strip.
  • the present disclosure further relates to a method for producing the hot-rolled steel strip product as described above.
  • the method comprises the following steps: providing a steel slab of a steel with the following composition, in percent by weight (wt.-%): C 0.20 - 0.26, Si 0.05 - 0.5, Mn 0.2 - 0.8, Cr 0.2 - 0.6, Ni 0.2 - 0.5, Al 0.015 - 0.065, Ti 0.005 - 0.02, B 0.001- 0.005,
  • the present disclosure further provides an automotive component produced from the hot-rolled steel strip product as described above.
  • the automotive component may be a reinforcement beam (such as a reinforcement beam for a battery pack), a chassis part, or a bumper beam, but is not limited thereto.
  • the present disclosure provides a hot-rolled steel strip product which is suitable for use in the manufacture of automotive components.
  • the hot-rolled steel strip product has a tensile strength (R m ) above 1400 MPa and a microstructure comprising at least 90 % by volume of martensite.
  • the steel has a composition comprising, in percent by weight (wt.-%): C 0.20 - 0.26, Si 0.05 - 0.5, Mn 0.2 - 0.8, Cr 0.2 - 0.6, Ni 0.2 - 0.5, Al 0.015 - 0.065, Ti 0.005 - 0.02, B 0.001- 0.005,
  • microstructure of the hot-rolled steel strip product is described below.
  • the amount of phase(s) in the microstructure is given in volume-% (vol.-%) throughout the present disclosure. It should here be recognized that a certain volume percentage is generally determined within this technical field by considering an area percentage of the relevant constituent component (phase) in a sample, said area percentage considered to correspond to the volume percentage.
  • Carbon increases strength by solid solution strengthening, and thus directly affect the strength level in martensitic structures. Carbon is also an austenite stabilizing element. Therefore, in the present steel, carbon is present in an amount of at least 0.20 wt.-% to reach the desired strength level for the steel strip product. Suitably, the steel comprises at least 0.21 wt.-% of carbon.
  • the steel comprises at most 0.26 wt.-% carbon.
  • the carbon content may be 0.25 wt.-% or less.
  • the composition of the present steel comprises at least 0.05 wt.-% of silicon.
  • silicon may be present in an amount of at least 0.10 wt.-%, preferably at least 0.15 wt.-%.
  • silicon is present in an amount of 0.5 wt.-% or less.
  • silicon may be present in an amount of 0.45 wt.-% or less, preferably 0.40 wt.-% or less.
  • Manganese has the beneficial effect of suppressing the ferrite transformation temperature as well as the ferrite transformation rate. Alloying with manganese lowers the martensite start temperature (Ms) and martensite finish temperature (Mf), which may suppress autotempering of martensite during quenching. Reduced autotempering of martensite leads to higher internal stresses that may enhance the risk for quench-induced cracking. However, a lower degree of autotempered martensite is beneficial for achieving higher hardness. Manganese also increases austenite hardenability and therefore enhances hardness. Therefore, manganese is present in an amount of at least 0.2 wt.-% to achieve desired hardenability. Suitably, manganese is present in an amount of at least 0.3 wt.-%, preferably at least 0.4 wt.-%.
  • manganese is present in an amount of at most 0.8 wt.-%.
  • the steel comprises manganese in an amount of 0.65 wt.-% or less.
  • Chromium 0.2 - 0.6 wt.-%
  • Chromium in solid solution enhances strength and hardness by increasing austenite hardenability. Chromium also suppresses ferrite formation, in a manner similar to manganese. Therefore, chromium is present in an amount of at least 0.2 wt.-%.
  • chromium is present in an amount of 0.6 wt.-% or less.
  • chromium is present in an amount of 0.5 wt.-% or less .
  • Nickel 0.2 - 0.6 wt.%
  • Nickel is an alloying element that improves austenite hardenability, thereby increasing strength. Nickel also improves the ductility and is generally considered to increase toughness in martensitic steels. Therefore, nickel is present in an amount of at least 0.2 wt.-%.
  • nickel contents of above 0.5 wt.-% would unduly increase the alloying costs without significant technical improvement.
  • nickel is present in an amount of 0.4 wt.-% or less.
  • Aluminum is an element frequently used in the steel industry as deoxidizing or killing agent that can remove oxygen from the melt during the steelmaking process. Moreover, aluminum removes nitrogen by forming stable aluminum nitride particles and provides grain refinement, which is beneficial for toughness. Therefore, aluminum is present in an amount of at least 0.015 wt.-%.
  • aluminum is present in an amount of 0.065 wt.-% or less.
  • Titanium 0.005 - 0.02 wt.-%
  • Titanium is used for binding free nitrogen by formation of titanium nitrides. Thereby, the risk for a reduction in toughness is reduced. Moreover, such titanium nitride particles can efficiently prevent austenite grain growth when the steel slab, during the manufacturing process, is heated to the austenitizing temperature. Thereby, titanium may reduce the risk for abnormal grain growth of the austenite grains. Moreover, formation of titanium nitrides suppresses precipitation of boron nitride, thereby leaving boron free to make its contribution to hardenability. Therefore, titanium is present in an amount of at least 0.005 wt.-%. Suitably, titanium is present in an amount of at least 0.006 wt.-%.
  • titanium is present in an amount of 0.02 wt.-% or less.
  • titanium is present in an amount of 0.015 wt.-% or less.
  • Boron is a micro-alloying element used to increase hardenability.
  • boron also has a tendency of forming boron nitrides. Therefore, in order to achieve effective alloying with boron, it is required that the steel comprises nitrogen stabilizing elements to prevent formation of boron nitride. As described above, titanium may be used for this purpose.
  • the present steel comprises at least 10 ppm of boron to achieve desired hardenability.
  • boron is present in an amount of 50 ppm or less.
  • boron is present in an amount of 40 ppm or less. More preferably, boron is present in an amount of 30 ppm or less.
  • Copper optionally up to 0.3 wt.-%
  • Copper may be added, for example for solid solution strengthening purposes, if desired. Copper may alternatively be present as a result of the raw material used for making the steel, in which case it may be considered as an unavoidable impurity. However, high contents of copper may lead to brittleness and are therefore not desired. For said reason, the copper content (if present) is limited to less than 0.3 wt.-%.
  • Molybdenum optionally up to 0.1 wt-%
  • Molybdenum may, if desired, be added to increase strength. However, addition of molybdenum is not essential to the present steel product and may increase the alloying costs. Therefore, the content of molybdenum (if present) is less than 0.10 wt.%.
  • Niobium optionally up to 0.01 wt.-%
  • Niobium may, if desired, be used for the purpose of increasing strength by grain refinement. However, niobium is not essential to the present steel product and may increase the alloying costs. Moreover, niobium may reduce formability and bendability and should therefore be restricted. Hence, the content of niobium (if present) is less than 0.01 wt.-%.
  • Vanadium optionally up to 0.04 wt.-%
  • Vanadium may, if desired, be added for the purpose of increasing strength. However, vanadium is not essential to the steel product and may increase the alloying costs. Therefore, content of vanadium (if present) is in the present steel less than 0.04 wt.-%.
  • unavoidable impurities are considered to be impurities resulting from the manufacturing process and/or the raw material used.
  • an unavoidable impurity is phosphorus (P).
  • Low levels of phosphorous is beneficial for toughness and formability. Therefore, the phosphorus content is limited to at most 0.02 wt.-%.
  • Another example of an unavoidable impurity is sulfur, which should also be present in low amounts to avoid deterioration of toughness and formability. Sulphur may therefore be present in an amount of 0.005 wt.-% or less.
  • the sulfur content is equal to or less than 0.045 wt.-%.
  • Nitrogen is another example of an unavoidable impurity. Nitrogen may also cause a reduction of toughness and formability if present in too high contents. Therefore, nitrogen is present in an amount of equal to or less than 0.0055 wt.-%.
  • the nitrogen content may suitably be 40 ppm or less, preferably equal to or less than 35 ppm.
  • an unavoidable impurity is calcium (Ca) which may result from the manufacturing process since it is commonly added during the steelmaking process for refining, deoxidation, and desulphurization.
  • An excessive amount of Ca should however be avoided to achieve a clean steel and avoid deterioration of the mechanical properties, in particular toughness and bendability.
  • the calcium content is suitably limited to 0.005 wt.-% or less, preferably 0.003 wt.-% or less.
  • unavoidable elements include hydrogen (H) and oxygen (O), where the latter may contribute to formation of impurities.
  • the hydrogen content should preferably be limited to maximally 0.5 ppm.
  • the oxygen content should preferably be limited to maximally 25 ppm.
  • the microstructure of the hot-rolled steel strip may be assessed by Scanning Electron Microscopy (SEM), combined with electron backscatter diffraction (EBSD) where applicable, as known in the art.
  • SEM Scanning Electron Microscopy
  • EBSD electron backscatter diffraction
  • an image obtained via SEM of a cross-section of a representative sample is considered.
  • the image analysis may be made manually by the skilled person and/or by usage of computer analysis as known in the art.
  • the hot-rolled steel strip product has a microstructure comprising at least 90 vol.-% of martensite .
  • the microstructure comprises at least 95 vol.-%, more preferably at least 98 vol.-%, of martensite.
  • a representative fraction of martensite may therefore suitably be measured at 1 ⁇ 4 thickness of the steel strip product.
  • the martensitic structure may be non-tempered, autotempered or tempered depending on the manufacturing process and/or other conditions that the hot-rolled steel strip product may be subjected to.
  • the microstructure comprises tempered martensite. It is within the normal routine of the person skilled in the art to, by investigating the microstructure, determine whether the martensite is non-tempered, autotempered or tempered. The presence of tempered martensite can be identified, during investigation of the microstructure, mainly by evenly dispersed formation of different carbides depending on the time-temperature pattern of the tempering. Thereby, tempered martensite may be identified in the microstructure of a sample by comparison with images of previous samples, tempered according to known time-temperature patterns.
  • the microstructure may comprise small amounts of bainite, retained austenite and/or carbides.
  • the average prior austenite grain size may be 50 ⁇ m or less.
  • the average prior austenite grain size of the strip steel product is 30 ⁇ m or less or even 20 ⁇ m or less.
  • the average prior austenite grain size is here determined according to ASTM E 112. Local variations in surface zone and center region may be present as a result of the manufacturing process. Therefore, a representative average prior austenite grain size may suitably be determined at 1 ⁇ 4 thickness of the steel strip product.
  • the first step in the production of the hot-rolled steel strip product is to provide a steel slab of a steel with the above-described composition.
  • the steel slab may for example be provided casting a melt of the composition by means of continuous casting, also known as strand casting.
  • the steel slab is heated to an austenitizing temperature of from 1150 to 1300 °C (including the end values of the range).
  • the temperature to which the steel slab is heated is important for controlling the austenite grain growth. An increase in the temperature to which the steel slab is heated can cause dissolution of alloy precipitates and result in grain coarsening and/or abnormal grain growth.
  • the steel slab is, after the above-described heating step, hot-rolled to the desired thickness of the finished product.
  • the desired thickness may be within the range of 1.5 mm to 6 mm, preferably 2 mm to 4 mm, but is not limited thereto.
  • the temperature of the steel slab is in the range of Ar3 to 1280 °C.
  • Ar3 is the temperature at which austenite begins to transform into ferrite during cooling. It should also be noted that Ar3 is dependent of the actual chemical composition of the steel. The skilled person is well aware of methods for determining Ar3 for a particular composition, and this will therefore not be further described in the present disclosure.
  • the finish rolling temperature if the hot-rolling step is in the range of from 800 °C to 960 °C, preferably 860 to 950 °C.
  • the hot-rolled strip is thereafter quenched to a temperature at which it is, or at least may be, coiled. In other words, it is quenched to a coiling temperature.
  • the coiling temperature is250 °C or less.
  • the hot-rolled strip is quenched to a coiling temperature of 150 °C or less.
  • the average cooling rate during said quenching is at least 30 °C/seconds. Said average cooling rate is determined by considering the temperature range from finish rolling temperature to the coiling temperature and the total time for said quenching between these temperatures, if following an arbitrary point of the hot-rolled strip in the longitudinal direction of the strip.
  • the cooling rate during said quenching will vary over the quenching step.
  • the cooling rate is larger at the start of the quenching step (i.e. at higher temperatures) than at the end of the quenching step.
  • the cooling rate may be as high as about 300 °C/second in the quenched region down to 500 °C.
  • quenching of the hot-rolled strip is suitably initiated shortly after hot-rolling to ensure desired microstructure and mechanical properties.
  • quenching is initiated within about 10 seconds from the hot-rolling step, although longer times may also be plausible.
  • the steel strip product may after the quenching step be coiled, and allowed to cool down to room temperature.
  • the hot-rolled steel strip product may thereafter be subjected to one or more further processing steps.
  • the hot-rolled steel strip product may be galvanized whereby it is provided with a corrosion protective layer on the surface of the hot-rolled steel strip. This may for example be performed through electro-galvanizing or a similar process that does not substantially alter the properties of the hot-rolled steel strip product in a way that the properties are not met afterwards.
  • the hot-rolled steel strip product may be subjected to low temperature temper annealing. More specifically, the hot-rolled steel strip product may be subjected to temper annealing at a temperature of from 100 °C to 250 °C.
  • the appropriate duration of such a low temperature annealing depends on various factors, such as whether it is performed on the hot-rolled steel strip in coiled or un-coiled form. As an example, in case the low temperature temper annealing is performed on a coiled hot-rolled steel strip, the duration may suitably be at least 2 hours to ensure that all the material reaches the intended temperature, whereas in uncoiled condition the duration may be considerably shorter, such as in the order of a few minutes or less.
  • the low temperature temper annealing is beneficial since it may further improve the resistance of the hot-rolled steel strip product to hydrogen embrittlement. This is due to the low temperature temper annealing reducing defects in the crystal lattice of the hot-rolled steel strip product. Furthermore, although the low temperature temper annealing may lead to a slight reduction in the tensile strength, it may also result in an increase in yield strength which is advantageous.
  • the hot-rolled steel strip product according to the present disclosure has a tensile strength, R m , above 1400 MPa. More specifically, the hot-rolled steel strip may have a tensile strength of at least 1475 MPa. The tensile strength is however typically less than 1750 MPa. Moreover, the hot-rolled steel strip product may have a yield strength, R p0.2 of at least 1100 MPa. Preferably, the yield strength is at least 1200 MPa. The yield strength is however typically less than 1550 MPa.
  • the hot rolled steel strip product may also have an elongation to fracture, A50, of at least 4%, preferably at least 5% or even 6% or higher. The tensile strength, yield strength and elongation may be determined according to SS-EN ISO 6892-1. These tensile properties make the hot-rolled steel strip highly suitable for used in automotive applications.
  • the hot-rolled steel strip product also has good formability, which is important when seeking to produce for example various automotive components.
  • the formability may for example be quantified by the bendability of the hot-rolled steel strip product.
  • the hot-rolled steel strip product according to the present disclosure may have a bendability Ri/t, determined by subjecting a test piece of the hot-rolled steel strip product to plastic deformation by three-point bending (with one single stroke) until a specified angle 90° of the bend is reached after unloading, of 4.0 or less, wherein Ri is the inner radius and t is the thickness of the hot-rolled steel strip product.
  • the hot-rolled steel strip product also has high toughness. More specifically, the toughness measured on a sub sized specimen (1/4 sized specimen) in longitudinal direction is at least 15 J at -40 °C, preferably at least 20 J at -40 °C, when determined according to SS-EN ISO 148-1.
  • Hydrogen embrittlement is a phenomenon that may occur when a steel is exposed to hydrogen dissolving and diffusing into the steel in combination with stresses. Hydrogen embrittlement may lead to loss in ductility and/or toughness. Moreover, hydrogen embrittlement may lead to a reduction in load bearing capacity even below the yield strength. Hydrogen could be introduced to the steel during the steelmaking process, further processes such as electro galvanizing processes or the like, or during use of the steel. Moreover, hydrogen embrittlement is more common for high strength grades with tensile strength above 1200 MPa and the risk generally increases with increasing strength due to higher amount of defects in the crystal lattice. Hydrogen embrittlement is often a concern within the automotive industry, primarily since it may lead to fracture without any reasonably detectable warning. The hot-rolled steel strip product according to the present disclosure has good resistance to hydrogen embrittlement despite the fact that it also has high strength.
  • Resistance to hydrogen embrittlement may be investigated by using a Constant Load Test (CLT), wherein a test specimen is subjected to a constant tensile load in the range of 40-80% of tensile strength while introducing hydrogen to the specimen and measuring time to fracture.
  • CLT Constant Load Test
  • a test specimen is subjected to a constant tensile load in the range of 40-80% of tensile strength while introducing hydrogen to the specimen and measuring time to fracture.
  • the hot-rolled steel strip product according to the present disclosure may have a resistance to hydrogen embrittlement, if tested according to the Constant Load Test as described above, of at least 130 minutes when subjected to a load of 80% of the tensile strength.
  • the resistance to hydrogen embrittlement is at least 140 minutes, and more preferably at least 150 minutes, if tested according to the Constant Load Test as described above.
  • the alloys were produced in full scale charges of 100-200 tonnes and cast by continuous casting to slabs.
  • the slabs were reheated to a reheating temperature (i.e. austenitizing temperature) with a holding time of at least 30 minutes at said temperature.
  • the slabs were thereafter subjected to hot rolling, to a final sheet thickness, from the reheating temperature to a finishing rolling temperature, followed by quenching to a coiling temperature of lower than 150 °C.
  • Some of the hot-rolled strips were subjected to tempering. Where tempering was performed , this was made with the hot-rolled strip in coiled condition except for Alloy 3 (sample L), where tempering was performed on a lab scale.
  • the parameters of the processing steps for the respective samples are specified in Table 2.
  • the average cooling rate specified in Table 2 was calculated by dividing the temperature range from the finish rolling temperature to the coiling temperature with the duration for the material to pass these measured temperatures.
  • Yield strength (R p0.2 ), tensile strength (R m ) and total elongation (A50) were determined through tensile testing according to SS-EN ISO 6892-1. The samples were longitudinal to rolling direction.
  • Bendability was tested by subjecting a test piece to plastic deformation by three-point bending, with one single stroke, until a specified angle 90 ° of the bend is reached after unloading.
  • the inspection and assessment of the bends is a continuous process during the whole test series. This is to be able to decide if the punch radius (R) should be increased, maintained or decreased.
  • Bendability was tested with the bend parallel with rolling direction, and the result is given as inner radius (Ri) divided by sample thickness (t).
  • the limit of bendability (Ri/t) was identified in a test series when a consistent bend radius, without any defects, is fulfilled with the same punch radius (R).
  • cracks, surface necking marks and flat bends were considered as defects.
  • the bend test was performed according to SS-EN ISO 7438.
  • CLT Constant Load Test
  • Metallographic cross sections were extracted from material in tempered condition, hot mounted in conductive phenolic resin, ground and polished to a 1 ⁇ m surface finish, either etched or further polished using colloidal silica.
  • the etched and further polished sample were examined using a scanning electron microscope (SEM) equipped for electron backscatter diffraction (EBSD) respectively.
  • Figure 1 represents an example of a SEM image of a cross section of Sample No. B at about 1 ⁇ 4 of the thickness. The cross-section was taken essentially parallel to the rolling direction.
  • Figure 2 represents Inverse Pole Figure from EBSD measurement of Sample No. D, at about 1 ⁇ 4 of the thickness, also taken with the cross-section essentially parallel to the rolling direction.
  • Prior austenite grain size was estimated according to ASTM E 112, giving an average size below 50 ⁇ m for all samples. For example, an average size of 14.6 ⁇ m was obtained for Sample No. B, and 16.4 ⁇ m was obtained for Sample No. D.

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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
EP21207843.0A 2021-11-11 2021-11-11 Warmgewalztes stahlbandprodukt und verfahren zu dessen herstellung Withdrawn EP4180544A1 (de)

Priority Applications (3)

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EP21207843.0A EP4180544A1 (de) 2021-11-11 2021-11-11 Warmgewalztes stahlbandprodukt und verfahren zu dessen herstellung
PCT/EP2022/081342 WO2023083898A1 (en) 2021-11-11 2022-11-09 A hot-rolled steel strip product and method for its production
EP22818208.5A EP4430223A1 (de) 2021-11-11 2022-11-09 Warmgewalztes stahlbandprodukt und verfahren zu seiner herstellung

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JP2009030093A (ja) * 2007-07-26 2009-02-12 Jfe Steel Kk 耐低温焼戻し脆化割れ特性に優れた耐磨耗鋼板
JP4874434B1 (ja) * 2010-02-15 2012-02-15 新日本製鐵株式会社 厚鋼板の製造方法
EP2647730A2 (de) * 2012-04-03 2013-10-09 Rautaruukki Oy Verfahren zur Herstellung eines hochfesten formbaren durchlaufgeglühten Stahlstreifens, hochfestes formbares durchlaufgeglühtes Produkt und Stahlspule
US20150140358A1 (en) * 2012-04-06 2015-05-21 Nippon Steel & Sumitomo Metal Corporation Hot-dip galvannealed hot-rolled steel sheet and process for producing same
EP3719149A1 (de) * 2019-04-05 2020-10-07 SSAB Technology AB Hochhartes stahlprodukt und verfahren zur herstellung davon

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CN104513936B (zh) * 2014-12-19 2019-04-23 宝山钢铁股份有限公司 一种屈服强度1100MPa级调质高强钢及其生产方法
CN106086657B (zh) * 2016-08-24 2019-01-08 东北大学 一种屈服强度大于1300MPa的超高强度结构钢板及其制备方法
WO2018215600A1 (en) * 2017-05-24 2018-11-29 Tata Steel Uk Limited High-strength, hot rolled abrasive wear resistant steel strip
KR102209555B1 (ko) * 2018-12-19 2021-01-29 주식회사 포스코 강도 편차가 적은 열연 소둔 강판, 부재 및 이들의 제조방법

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009030093A (ja) * 2007-07-26 2009-02-12 Jfe Steel Kk 耐低温焼戻し脆化割れ特性に優れた耐磨耗鋼板
JP4874434B1 (ja) * 2010-02-15 2012-02-15 新日本製鐵株式会社 厚鋼板の製造方法
EP2647730A2 (de) * 2012-04-03 2013-10-09 Rautaruukki Oy Verfahren zur Herstellung eines hochfesten formbaren durchlaufgeglühten Stahlstreifens, hochfestes formbares durchlaufgeglühtes Produkt und Stahlspule
US20150140358A1 (en) * 2012-04-06 2015-05-21 Nippon Steel & Sumitomo Metal Corporation Hot-dip galvannealed hot-rolled steel sheet and process for producing same
EP3719149A1 (de) * 2019-04-05 2020-10-07 SSAB Technology AB Hochhartes stahlprodukt und verfahren zur herstellung davon

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