WO2011154831A1 - Procédé pour la production d'un produit de type acier laminé à chaud et acier laminé à chaud - Google Patents

Procédé pour la production d'un produit de type acier laminé à chaud et acier laminé à chaud Download PDF

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Publication number
WO2011154831A1
WO2011154831A1 PCT/IB2011/001436 IB2011001436W WO2011154831A1 WO 2011154831 A1 WO2011154831 A1 WO 2011154831A1 IB 2011001436 W IB2011001436 W IB 2011001436W WO 2011154831 A1 WO2011154831 A1 WO 2011154831A1
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Prior art keywords
steel
hot
percentage
weight
content
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PCT/IB2011/001436
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WO2011154831A4 (fr
Inventor
Jarkko Vimpari
Tommi Liimatainen
Mikko HEMMILÄ
Ari Hirvi
Jaakko Savola
Juha Kuoppala
Sakari Tihinen
Päivi TAMMINEN
Teemu Peltonen
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Rautaruukki Oyj
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Application filed by Rautaruukki Oyj filed Critical Rautaruukki Oyj
Priority to CN201180039212.5A priority Critical patent/CN103097556B/zh
Priority to EP11743330.0A priority patent/EP2576848B1/fr
Publication of WO2011154831A1 publication Critical patent/WO2011154831A1/fr
Publication of WO2011154831A4 publication Critical patent/WO2011154831A4/fr

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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
    • C21D8/0221—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
    • 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
    • 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
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following 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
    • C22C38/42—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
    • C22C38/50—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 invention relates in particular to direct quenched martensitic sheet-like steels, on which temper annealing is conducted, i.e. tempering steels and their production.
  • the object of the invention is a method for producing a hot-rolled steel according to claim 1.
  • the object of the invention is also a hot-rolled steel according to claim 19.
  • From EP1860205 Al is known a martensitic hot-rolled steel with a tensile strength greater than 980MPa, which is very capable of being mechanically cut.
  • the composition of the steel as percentages by weight is: 0.03 - 0.10% of carbon, C; 0.2 - 2.0% of silicon, Si; 0.5 - 2.5% of manganese, Mn; 0.02 - 0.10% of aluminium, Al; 0.20 - 1.5% of chromium, Cr; 0.1 - 0.5% of molybdenum, Mo; and to which can further be added 0.0005 - 0.005% of boron, B; 0.1 - 2.0% of nickel, Ni; and 0.0005- 0.0050% of calcium, Ca.
  • the steel is produced by direct quenching at a temperature of less than 400°C, such as, for example, at a temperature of 250 - 300°C. Temper annealing is not conducted on the steel.
  • the purpose of the publication is to achieve mechanical proper- ties without precipitation hardening alloying elements, such as titanium Ti, niobium Nb or vanadium V, as well as by decreasing the carbon C and increasing the molybdenum Mo content. According to the teaching, the effect of molybdenum Mo ends at an upper limit of 0.5%> Mo, after which alloying it pointlessly increases costs. Additionally, the publication teaches that nickel can be added 0.1 - 2.0%.
  • the disadvantage of this known steel composition and method is that the steel presented therein it is not suitable for use as a structural steel in application sites, because its elongation and impact toughness are not remarkably good. Elongation and impact toughness are difficult to improve in the steel in question, because it is not particularly temper-resistant. In addition, a disadvantage is that it is not well suited for steel products that during usage will have to be for long periods of time in the temperature range of 450 - 600°C, which is a dangerous temperature range due to the upper temper brittleness.
  • Steel can be subjected to this temperature range in different usage situations, such as in heat treatments, or in a situation, in which steel structures are reworked hot (in shape corrections by heating) or during bell furnace annealing, in which annealing occurs a slow cooling in said temperature range.
  • a higher temper brittleness the steel becomes fragile at room temperature and thus quite use- less. Temper brittleness causes, among other things, atomic segregations forming at the grain boundaries, which weaken the structure.
  • tempering steels whose carbon content is high, such as on the level of C 0.12 - 0.18% and/or into these is alloyed more nickel Ni, copper Cu or niobium Nb than in a hot-rolled steel according to the invention.
  • tempering steels particularly direct quenched tempering steels, all important properties, such as yield strength, elongation, impact toughness, flangeability and temper resistance, are difficult to achieve at good levels simultaneously in the same steel.
  • the object of the invention is to eliminate the disadvantages related to known art and to achieve a high-strength hot-rolled steel that is very temper-resistant after the direct quenching process, wherein by tempering it is made further high- strength (Rp0.2 > 890MPa) combined at the same time with good impact toughness (Charpy V (-20°C) > 37J/cm ) and flangeability as well as good weldability.
  • Another object of the invention is to provide a hot-rolled steel production method that is as easy as possible in relation to tempering treatment, i.e. a hot-rolled steel according to the invention must be as robust as possible in relation to tempering, or easily tempered, wherein it is preferable to implement a tempering treatment.
  • Steel is not critical, for example, in relation to tempering temperature and the time used for tempering and its tendency for upper temper brittleness is low.
  • the method according to the invention is characterized in that it is arranged a steel billet, whose composition as percentages by weight is
  • the microstructure of a hot-rolled steel is preferably tempering martensitic, i.e. in the steel has, as a result of direct quenching, formed an essentially martensitic microstructure, after which the steel is subjected to temper annealing, wherein the final result is a hot-rolled steel product, whose impact toughness and strength are of the desired level.
  • temper annealing which substantially improves the impact toughness and elongation of the steel product, is simple to implement on a hot-rolled steel according to the invention.
  • the strength and impact toughness properties of the steel are not sensitive to changes in tempering temperature and time nor to the cooling rate of the sheet after tempering.
  • direct quenching it is also achieved a good flangeability for the steel, which is typically more difficult to achieve for the direct quenched tempering steel in comparison to traditionally furnace-quenched steel.
  • the composition of a hot- rolled steel according to the invention is in particular characterized in that the carbon C and manganese Mn contents are low, being in the range presented, and, additionally, into the steel is always alloyed the presented contents of boron B, vanadium V and titanium Ti, in order that the objects of the invention can be achieved. It is not absolutely necessary to alloy Niobium Nb and if it is alloyed, its content is limited. Additionally, the nickel Ni and copper Cu contents can be quite low, being even at the level of impurities. The significance and effects of the alloying elements are described further in the detailed section of the description.
  • the properties of a hot-rolled steel are presented in independent claim 19.
  • the preferred embodiments of a hot-rolled steel are presented in claims 20 - 36.
  • a hot-rolled steel according to the invention is exceptionally temper- resistant, because its composition enables that high-strength martensitic steel can be tempered, for example, in a bell furnace and, additionally, at the same time efficiently limit the detrimental effects of upper temper brittleness.
  • the impact toughness proper- ties of the steel are, indeed, excellent also as measured from HAZ (heat affected zone) area of the welding seam, which is exceptionally important for building steel use.
  • HAZ heat affected zone
  • the steel is also quite suitable for use particularly in the welded boom structures of cranes. Additionally, the steel possesses excellent usability due to good weldability and flangeability.
  • Fig. 2 shows a welding test arrangement, which shows the measuring point of the fusion line FL
  • Fig. 3 shows the microstructure of a hot-rolled steel according to the inven- tion in a tempering martensitic state
  • composition of a hot-rolled steel according to the invention centages by weight is:
  • the rest being iron, Fe, and unavoidable impurities.
  • composition of a hot- rolled steel according to the invention and, by way of example, the properties achieved by each composition with the most important production parameters. Additionally, preferred embodiments and their advantages are presented. The contents are percentages by weight.
  • All the steels of the tables are produced by the method according to the invention, i.e. by direct quenching to a low temperature, wherein the coiling temperature has been below 300 °C and by the subsequent tempering treatment, which is performed, for example, in a Bell-type type of furnace.
  • Impact toughness tests are per- formed as Charpy V tests using a 6 mm thick test material.
  • Flanging is implemented by a known method as a V-bending between an upper-lower tool. Free flanging is used as the manner of flanging.
  • Limiting the maximum carbon and manganese contents is particularly important, when temper annealing occurs below a temperature of 600°C or the steel cools down slowly after tempering through the temperature range in question.
  • a high impact toughness is pro- quizd for both the base material and the welding HAZ area, particularly such that the Charpy V impact toughness of the base material is at least 37J/cm2 as measured longitudinally in relation to the direction of rolling and at a temperature of -20°C.
  • the impact toughness of the base material is at least 33J/cm2 as measured transver- sally in relation to the direction of rolling and at a temperature of at least -20°C.
  • said impact toughness requirements are also achieved as measured at a temperature of -40°C.
  • the impact toughness of the steel was defined as a Charpy V test using three welding HAZ (heat affected zone) areas, forming a notch in the following sites: 1. In the fusion line, where impact toughness was measured from the site FL, where a segment placed in the direction of the sheet and mid-way in the thickness of the sheet cuts the fusion line formed while welding,
  • ICHAZ partially austenitized zone
  • the temperature is at the maximum 700 - 850 °C
  • impact toughness is retained in the steel while welding as better than in the higher- carbonous tempering steel produced in a typical traditional manner.
  • austenitizing occurs only there, where nucleation of the austenite has been easy, i.e. mainly there, where carbon content has been high.
  • the high-carbonous austenitized part changes upon cooling to martensite and bainite.
  • the high- carbonous local austenite area can form as a hard MA island, weakening the impact toughness of the zone, wherein the lower carbon content of the developed steel is of advantage, because the formation of hard and more fragile microstructures is lesser in the area of the ICHAZ.
  • composition of a hot-rolled steel according to the invention achieves exceptionally good impact toughness particularly in the area of the partially austenitized zone (ICHAZ), which is measured from the site FL+3.
  • ICHAZ partially austenitized zone
  • the hot-rolled steel according to the invention is thus quite weldable also without expensive alloying of nickel, when the steel is achieved alloyed with vanadium, wherein impact toughness in the HAZ zone is at least on the level of typical tempering steels or better.
  • Table 4 shows typical impact toughness values with different heat inputs for the composition K, which is presented in table 1.
  • Table 4 shows typical impact toughness values with different heat inputs for the composition K, which is presented in table 1.
  • Table 4 shows typical impact toughness values with different heat inputs for the composition K, which is presented in table 1.
  • MAG welding in the flat position without preheating and a 50 ° V-groove as the groove shape.
  • Carbon content as a percentage by weight 0.075 - 0.12% is low in com- parison to typical tempering steels, wherein impact toughness remains at a good level. If the carbon content of the steel as a percentage by weight is less than 0.075%, then it is difficult to get the steel strong and impact tough enough, because, in this case, enough martensite is not formed as a consequence of direct quenching. If the carbon content as a percentage by weight is above 0.12%, then impact toughness is weakened too much and the objects of the invention are not achieved.
  • the carbon content of the steel as a percentage by weight is 0.08 - 0.11%), more preferably 0.09 - 0.11%, wherein, in welding, also the HAZ zone achieves adequate uniform strength with the base material while at the same time the impact toughness of the base material is adequate.
  • Silicon content as a percentage by weight is 0.1 - 0.8%.
  • silicon content as a percentage by weight is 0.1 - 0.4%, more preferably 0.1 - 0.3%>.
  • too high a silicon content, such as a 0.5% content as a percentage by weight can detrimentally effect on the impact toughness of the steel. This can be clearly seen in steel F from graph 2.
  • silicon content as a percentage by weight is preferably at the most 0.4%>. Silicon contents less than 0.1 % are not recommended, because desul- phurisation of the steel and form control of inclusions are easier, when the steel contains some silicon.
  • silicon increases the strength of the steel without a rise in carbon equivalent, which is an advantage especially if carbon content is close to the carbon content upper limit 0.11 - 0.12% of a hot-rolled steel according to the invention.
  • Manganese content as a percentage by weight is 0.8 - 1.7%.
  • manganese content as a percentage by weight is 0.8 - 1.4%, more preferably 1.0 - 1.2%.
  • manganese content as a percentage by weight must be at least 0.8%, preferably at least 1%.
  • unfavourable infiltration of manganese is less, when manganese content as a percentage by weight is limited to at the most 1.4%, preferably at the most 1.2%.
  • graph 3 the detriment of high manganese content to the base material steel G, whose content is shown in table 1 as well as rolling and temper annealing parameters, and mechanical properties in table 2.
  • Chromium content as a percentage by weight is 0.2 - 1.3%, more prefera- bly 0.5 - 1.3%, in order that the high strength steel is achieved and hardenability is good.
  • chromium content as a percentage by weight is 0.8 - 1.2%.
  • chromium content is 0.8 - 1.2%.
  • Boron content as a percentage by weight is 0.0005 - 0.003%), because alloying with boron is a preferred means to assure the good hardenability of the steel. At contents above 0.003%, the hardenability-increasing effect of boron weakens and, additionally, too much boron weakens the weldability of the steel.
  • boron is alloyed 0.0008 - 0.002% as a percentage by weight both to retain good impact toughness of the weld and to assure adequate hardenability.
  • Nickel content must be limited to a content of less than 1% as a percentage by weight, because nickel can, under some circumstances, even decrease somewhat the impact toughness of the tempered steel or its effect is slight. Additionally, nickel is an expensive alloying element. Preferably, the content of nickel is to be limited to a content of less than 0.1% as a percentage by weight, more preferably less than 0.05%>, wherein the alloying costs of the steel can be kept as low as possible.
  • the composition of a nickel-alloyed steel B after tempering treatment is of modest impact toughness, transversal impact toughness results in particular are modest, which is observed from graph 4. Tempering treatment is performed in a Bell-type furnace for 24 hours at the most and at a temperature below 500 °C.
  • Molybdenum content as a percentage by weight is 0.15 - 0.80%.
  • molybdenum content as a percentage by weight is 0.30 - 0.80%>, because, with a molybdenum content of less than 0.30% in a steel according to the invention, adequate strength is not achieved without the needing to alloy into the steel large contents of other alloying elements, such as carbon C, silicon Si, nickel Ni or manganese Mn, the detrimental effects of which are described earlier and also later in connection with the indexes TBI and UTBI presented in the description.
  • Molybdenum precipitates in temper annealing, which decreases the lowering of strength caused by tempering treatment and thus helps in achieving high strength. Additionally, molybdenum is used i.a. to prevent the upper temper brittleness of steel by slowing infiltration of i.a. phosphorus, P, to the grain boundaries during temper annealing at the critical temperature range of 450 - 600 °C. Molybdenum also efficiently increases the hardenability of steel.
  • molyb- denum is alloyed 0.50 - 0.70% as a percentage by weight. Contents exceeding a 0.8%> molybdenum content increase the carbon equivalent value and increase excessively alloying element costs, because molybdenum is an expensive alloying element. On the other hand, at a Mo content less than 0.15%, as in steel M, whose composition is shown in table 1 and test results in table 2, show that strength remains low in temper annealing of 500 - 600 °C already for a relatively short 1 hour temper annealing time. For this reason, i.e. to achieve adequate strength, molybdenum must be alloyed at least 0.15%) as a percentage by weight, preferably at least 0.30%> or even at least 0.50%>.
  • niobium alloy is used in many conventionally produced, well- flangeable tempering steels
  • the flange- ability of direct quenched steel is not achieved at a good level, neither in the hardened nor in the tempered state, if the steel contains large amounts of niobium, Nb.
  • steel H in table 3 As an example of this is steel H in table 3.
  • niobium can crucially weaken steel flangeability in a hot-rolled steel according to the invention, especially at large contents.
  • niobium content is to be limited to 0.03%) Nb at the most, because, at the 0.05%> niobium content of steel H, it was observed a clear weakening of flangeability. More preferably, niobium content is lim- ited to less than 0.005%), wherein the best possible flangeability properties for the steel are assured.
  • Vanadium content must be 0.02 - 0.1% as a percentage by weight. In order to assure strength, vanadium, V, is to be alloyed at least 0.02%> as a percentage by weight. As vanadium content increases, weldability can weaken and, for this reason, the vanadium content maximum value as a percentage by weight is 0.1% at the most. According to the preferred embodiment, the vanadium content must be 0.04 - 0.1% as a percentage by weight, when niobium, Nb, is not alloyed, i.e. when Nb is less than 0.005%. Vanadium is thus alloyed in particular without alloying of niobium, in order that flangeability would be as good as possible.
  • vanadium contents and niobium contents are selected as follows: V 0.04 - 0.10% as a percentage by weight and Nb 0.008 - 0.03%> as a percentage by weight, wherein it is achieved a good combination of impact toughness and strength while flangeability still remains reasonable.
  • vanadium contents and niobium contents are selected as follows: V 0.02 - 0.03%> as a percentage by weight and Nb 0.008 - 0.03%> as a percentage by weight, wherein it is achieved, above all, a combination of HAZ zone strength and impact toughness in the highest possible quality, particularly by severely limiting the content of vanadium, but by, however, still reasonably alloying niobium. Alloying of niobium is of advantage particularly in achieving adequate strength and impact toughness in the base material.
  • Copper content is limited to less than 0.5%> as a percentage by weight. It is not absolutely necessary to alloy copper, but it can be used in a small amount as needed to increase strength or improve weather resistance of the steel. If copper, Cu, is alloyed more than 0.3%, nickel must be alloyed at least 0.33 * Cu content, in order that the surface quality of the steel remains good in hot-rolling.
  • copper content as a percentage by weight is less than 0.05%>, wherein its content is on the level of impurities, and adequate strength can be attained less expensively in terms of costs and properties without alloying copper.
  • Aluminium content as a percentage by weight is 0.015 - 0.08%. Aluminium, Al, is used to condense steel, i.e. to bind oxygen from the steel. Preferably, alu- minium content is 0.02 - 0.06% as a percentage by weight.
  • Titanium content is 0.01 - 0.05%> as a percentage by weight, because titanium is required for binding nitrogen, N, in the steel, in order that boron, B, functions efficiently as an improver of hardenability and does not form boron nitrides. Titanium is used, because it works more reliably with direct quenched steel than aluminium, Al.
  • higher contents increase amounts of the relatively large-sized TiN, which is detrimental in terms of impact toughness.
  • Ti/N ratio is preferably 3-4.
  • Phosphorus content as a percentage by weight must be limited to P less than 0.012%, because phosphorus weakens impact toughness.
  • phosphorus content as a percentage by weight is limited to less than 0.008%.
  • Sulphur content is limited as an impurity to a level of less than 0.005% as a percentage by weight to assure good impact toughness and formability.
  • graph 5 presents as an example (steel K of table 1) the excellent effect achieved by the composition of a hot-rolled steel according to the invention on the impact toughness of the steel, which is excellent both transversally and longitudinally.
  • a hot-rolled steel means a steel hot-rolled to be sheet-like, such as a hot- rolled heavy plate or hot-rolled strip steel.
  • the hot-rolled steel is a hot-rolled strip steel, because it is most easily achieved as excellent in terms of production efficiency, costs, surface quality and measurement tol- erances.
  • the thickness of the strip steel can be 2 - 10 mm, however, preferably in the range of 4 - 8 mm.
  • a hot-rolled steel means in particular a direct quenched steel, whose microstructure is essentially martensitic. Most preferably, after direct quenching, temper- ing treatment is conducted on the hot-rolled steel, wherein it is a question of a direct quenched tempering steel, whose microstructure is essentially tempering martensitic.
  • the microstructure of the steel before tempering treatment is preferably as perfectly composed as possible (above 90%) of martensite and self-tempered marten- site. In any event, the majority of the microstructure must be like this, wherein bainite may appear in the structure to some degree.
  • the content of ferrite and perlite before tempering must generally be in total less than 10%.
  • the austenite of a hot-rolled steel is flattened before direct quenching.
  • the flattening ratio of the grain is the numeric ratio of average grain height (H) / width (W) defined from the microsection. Grain is measured from the section, the section surface of which is in the direction of rolling and in the direction of sheet thickness as well as at the inspection site of a depth about 1 ⁇ 4 the thickness of the sheet.
  • the flattening ratio of the grain must preferably be greater than 2.0, which is formed, when steel is direct quenched directly from the hot-rolling occurring in the austenite area and the steel does not have time to re-crystallize. In traditional furnace- tempered steels, the ratio is less than 2.0. Most preferably, the average flattening ratio of the grain structure of a hot-rolled steel according to the invention is greater than 4.0.
  • Fig. 3 shows a picture of the microstructure of a steel product produced by the method according to the invention, in which is shown the height (H) and width (W) of the grain.
  • the figure shows thus the preferred embodiment of a hot-rolled steel ac- cording to the invention in a direct quenched and tempered state, i.e. as tempering martensitic, in which the flattening of the microstructure is still recognizable.
  • the flattening ratio of the grain structure Wl/Hl is approx. 16 and W2/H2 is approx. 28.
  • the flattening of the grain structure is significantly affected by the rolling temperature used, which, in the method according to the invention, is at the last rolling pass in the range of 760 - 960 °C.
  • the yield strength of a hot-rolled steel according to the invention is 890 - 1200 MPa, most preferably 960 - 1100 MPa. This is achieved by immediate direct quenching after the rolling of hot-rolling, after which tempering treatment is conducted. Tempering treatment can be conducted either immediately or later. Elongation at break (A5) is at least 8%, most preferably more than 10%. Yield ratio is typically somewhat high in structural steels and the yield ratio (yield strength/breaking strength) of a hot-rolled steel according to the invention is above 0.85.
  • the method according to the invention is characterized in that it is ar- ranged a steel billet, whose composition as percentages by weight is
  • the rest being iron, Fe, and unavoidable impurities, in which method the steel billet is
  • Fig. 1 shows the steps of the method according to the invention for produc- ing a hot-rolled steel product.
  • the starting material is a steel billet, whose composition as percentages by weight is C 0.075 - 0.12%
  • the steel billet is heated to the austenitizing temperature of 1200 - 1350 °C.
  • the thickness of the steel billet is, for example, 210 mm and it is heated to the austenitizing temperature of 1280 °C, where it is kept until it is of adequately even warmth and the alloying elements have adequately dissolved into the matrix, in practise for several hours.
  • the thickness of the steel billet can vary from that presented and the austenitizing temperature can be selected differently, but it is recommended that it is in the range of 1200 - 1350 °C. If the austenitizing temperature is below said lower limit, then there is a danger that not all microalloying elements dissolve into the austenite, i.e.
  • the austenite is not made homogenous and, in precipitation, annealing strength may remain low.
  • a higher temperature would lead to exceptionally large grain size of the austenite and increased oxidation of the billet surface.
  • Annealing time can most suitably vary in the range of 2 - 4 hours, but, depending on the selected furnace technology and the thickness of the bil- let, it can also be significantly longer or shorter.
  • hot-rolling 2 is conducted, which comprises pre -rolling step 2 and the subsequent strip rolling step 3.
  • the temperature of hot- rolling at the last pass is 760 - 960 °C.
  • the end temperature at the last pass of the hot-rolling is 800 - 900 °C.
  • the end temperature of hot-rolling is at least 800 °C, in order that rolling forces remain reasonable and at the most 900 °C, wherein i.a. excellent surface quality is assured.
  • the steel is direct quenched, i.e. cooled at an accelerated rate.
  • the speed of direct quenching 4 is at the most 120 °C /s, because, in this case, such a microstructure is achieved for the steel that gives the steel exceptionally good mechanical properties, including good impact toughness, combined with good flangeability. Quenching can be conducted, for example, with water.
  • the end temperature of direct quenching 4 is at the most 130 °C, because, in this case, after quenching, a planar strip is achieved, the edges of which are also even and planar.
  • direct quenching 4 of the steel strip is conducted directly at the coiling temperature and it is coiled 5.
  • the hot-rolled steel product is preferably a steel strip, which, after being direct quenched 4, is coiled and subsequently temper annealed 6.
  • temper annealing treatment 6 is conducted on the steel in the temperature range of 450 - 599 °C, wherein the composition of a low-carbon steel ac- cording to the invention can be formed to be inexpensive in terms of both its total amounts of alloying elements and its cost.
  • tempering treatment 6 of the steel can be conducted in the temperature range of 200 - 449 °C or 600 - 650 °C.
  • temper annealing treatment 6 of the method according to the invention can be implemented for the strip sheet cut from the coil or for a sheet continuously unwinding from the coil.
  • temper annealing treatment can alternatively be implemented also for a whole coil, for example, in a bell furnace, in which the temperature rises and falls slowly. Temperature variation between the midpoint and the surface specific to tem- pering of the coil is not a problem, because a hot-rolled steel according to the invention is exceptionally robust in terms of tempering. Robust means in this connection that for the steel homogenous mechanical properties are achieved in every part of the coil regardless of how the steel is tempered.
  • the method can very well be implemented also for sheet-rolled sheets of different thicknesses and strip sheets cut from the coil without the tempering furnace technology needing to be adjustable for exceptionally exact tempering temperature and time. This, in turn, enables the use of inexpensive and simple furnace technology and decreases the risk of rejection of material.
  • the hot- rolled steel, on which direct quenching 4 is conducted is cut as sheets, after which the sheets are straightened and only at the end is tempering treatment conducted.
  • temper annealing treatment 6 for straightened sheets in the straightening of which could have formed detrimental stresses.
  • the final result is an exceptionally even steel sheet of exceptionally even quality and 2 - 12 mm in thickness, in which elongation and impact toughness are somewhat better than with other embodiments.
  • brittleness or toughness of the steel in tempering was examined by annealing test steels in different types of furnaces (bell furnace and conventional), using different tempering times (0.5 - 24 h), and temperatures (200 - 650 °C).
  • TBI temper brittleness index
  • UTBI upper temper brittleness index
  • TBI describes a measured impact energy value in a Charpy V test, when the steel is annealed in the non-critical area for upper temper brittleness, i.e. above or below the temperature range of 450 - 599 °C (at temperature T below 450 °C or T above 599 °C).
  • TBI is defined on the basis of the tensile strength of the steel, from the di- rection of the impact test bar in relation to the direction of rolling, the measuring temperature of the impact test and the composition of the alloying elements according to the following equation:
  • TBI temper brittleness index
  • Rm is the tensile strength of the sample (MPa)
  • Test temperature is the testing temperature of the Charpy V test (°C)
  • UTBI is defined on the basis of the tensile strength of the steel, from the direction of the impact test bar in relation to the direction of rolling, the measuring temperature of the impact test and the composition of the alloying elements according to the following equation:
  • Rm is the tensile strength of the sample (MPa)
  • Test temperature is the testing temperature of the Charpy V test (°C)
  • TBI and UTBI are dependent on temperature such that, as the testing temperature rises, the index value also rises.
  • TBI which describes achievable impact toughness after tempering treatment (at temperature T below 450°C or T above 599 °C)
  • detrimental alloying elements for tempering steel are Si, Mn and Ni, but surprisingly the effects of Nb and V are the opposite.
  • the composi- tion of a hot-rolled steel according to the invention is limited on the part of these alloying elements to the limits presented earlier.
  • the TBI index describing impact toughness is for the longitudinal impact test bar at least 120, as defined at a tempera- ture of -40 °C.
  • the UTBI index describing impact toughness is for the longitudinal impact test bar at least 100, as defined at a temperature of -40 °C.
  • UTBI differs from TBI mainly in that the multipliers of the factors are different, but the alloying elements effect in the same direction, so according to the invention it is possible to optimise the steel such that the values of both indexes UTBI and TBI are high, wherein, in accordance with the invention, steel can be produced with such a composition that it retains its impact toughness in over a wide tempering temperature range as well as in a upper temper brittleness range.
  • An exam- pie of this is in table 6.
  • composition of a steel according to the invention (steels I, L, F and H of table 1), in particular the compositions of steels according to the preferred embodiments of the invention (steels I and L of table 1) achieve considerably better impact toughness properties, as measured in both longitudinal and transversal directions, than the comparison steels (steels B, C of table 1).
  • the conventional furnace type of table 6 (conventional) describes a manner, in which the steel is tempered in the conventional manner one sheet at a time in a furnace, wherein the sheet cools down slowly.
  • Furnace type (Bell-type) means a furnace, in which the steel is annealed as a coil, where the temperature falls slowly, particularly the core of the steel coil cools down slowly.
  • example steels K and L see table 1
  • the composition achieves uniform mechanical properties and good impact toughness regardless of at how high a temperature tempering treatment is conducted, of which in table 6 is example steel L in comparison to example steel K.

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Abstract

L'invention porte sur un procédé pour la production d'un acier laminé à chaud à partir d'un acier, dont la composition, sous forme de pourcentages en poids, est de 0,075 - 0,12 % de C, 0,1 - 0,8 % de Si, 0,8 - 1,7 % de Mn, 0,015 - 0,08 % d'Al, moins de 0,012 % de P, moins de 0,005 % de S, 0,2 - 1,3 % de Cr, 0,15 - 0,80 % de Mo, 0,01 - 0,05 % de Ti, 0,0005 - 0,003 % de B, 0,02 - 0,10 % de V, moins de 0,3 % de Nb, moins de 1 % de Ni, moins de 0,5 % de Cu, le reste étant du fer et des impuretés inévitables. L'invention porte en particulier sur des aciers en forme de tôle martensitiques ayant subi une trempe directe, sur lesquels un recuit de revenu est effectué, c'est-à-dire des aciers de revenu et sur leur production. L'acier laminé à chaud selon l'invention est exceptionnellement résistant au revenu après le procédé de trempe directe, le revenu permettant de lui conférer une résistance encore plus élevée (Rp 0,2 ≥ 890 MPa) combinée en même temps avec une bonne résistance au choc (V Charpy (-20°C) ≥ 37 J/cm2) et une bonne aptitude au bordage ainsi qu'une bonne aptitude au soudage.
PCT/IB2011/001436 2010-06-07 2011-06-07 Procédé pour la production d'un produit de type acier laminé à chaud et acier laminé à chaud WO2011154831A1 (fr)

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RU2549807C1 (ru) * 2013-12-30 2015-04-27 Публичное акционерное общество "Северсталь" (ПАО "Северсталь") Способ производства рулонного проката из высокопрочной хладостойкой стали
RU2551324C1 (ru) * 2013-12-30 2015-05-20 Публичное акционерное общество "Северсталь" (ПАО "Северсталь") Способ производства полос из низколегированной свариваемой стали
WO2015110585A1 (fr) * 2014-01-24 2015-07-30 Rautaruukki Oyj Produit de bande d'acier très haute résistance laminé à chaud
EP2729590B1 (fr) 2011-07-10 2015-10-28 Tata Steel IJmuiden BV Bande d'acier haute résistance laminée à chaud avec résistance élevée au ramollissement haz et son procédé de production
CN105143493A (zh) * 2013-02-22 2015-12-09 罗奇钢铁公司 用于制造金属涂覆和热成型的钢构件及金属涂覆钢带产品的方法
RU2625861C1 (ru) * 2016-05-23 2017-07-19 Открытое акционерное общество "Магнитогорский металлургический комбинат" Способ производства листовой стали с высокой износостойкостью
US9988700B2 (en) 2011-07-29 2018-06-05 Nippon Steel & Sumitomo Metal Corporation High-strength steel sheet and high-strength galvanized steel sheet excellent in shape fixability, and manufacturing method thereof
RU2674797C1 (ru) * 2018-06-07 2018-12-13 Публичное акционерное общество "Магнитогорский металлургический комбинат" Способ производства высокопрочного хладостойкого листового проката из низколегированной стали
WO2019031773A1 (fr) 2017-08-09 2019-02-14 주식회사 포스코 Tôle d'acier plaquée ayant une excellente qualité de surface, une excellente résistance et une excellente ductilité

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DE102018122901A1 (de) * 2018-09-18 2020-03-19 Voestalpine Stahl Gmbh Verfahren zur Herstellung ultrahochfester Stahlbleche und Stahlblech hierfür
US20230323493A1 (en) * 2020-09-07 2023-10-12 Arcelormittal Forged part of steel and a method of manufacturing thereof
EP3964591A1 (fr) 2020-09-07 2022-03-09 ThyssenKrupp Steel Europe AG Produit en acier plat laminé à chaud et procédé de fabrication d'un produit en acier plat laminé à chaud
EP4047105A1 (fr) * 2021-02-17 2022-08-24 ThyssenKrupp Steel Europe AG Produit en acier plat laminé à chaud et procédé de fabrication d'un produit en acier plat laminé à chaud
CN116254483B (zh) * 2023-02-01 2024-06-14 桂林理工大学 一种具有优异低温冲击韧性的高强钢板及其制造方法

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Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2729590B1 (fr) 2011-07-10 2015-10-28 Tata Steel IJmuiden BV Bande d'acier haute résistance laminée à chaud avec résistance élevée au ramollissement haz et son procédé de production
US9988700B2 (en) 2011-07-29 2018-06-05 Nippon Steel & Sumitomo Metal Corporation High-strength steel sheet and high-strength galvanized steel sheet excellent in shape fixability, and manufacturing method thereof
RU2519720C2 (ru) * 2012-08-15 2014-06-20 Октрытое Акционерное Общество "Магнитогорский металлургический комбинат" Способ производства штрипсов из низколегированной стали
CN105143493A (zh) * 2013-02-22 2015-12-09 罗奇钢铁公司 用于制造金属涂覆和热成型的钢构件及金属涂覆钢带产品的方法
RU2549807C1 (ru) * 2013-12-30 2015-04-27 Публичное акционерное общество "Северсталь" (ПАО "Северсталь") Способ производства рулонного проката из высокопрочной хладостойкой стали
RU2551324C1 (ru) * 2013-12-30 2015-05-20 Публичное акционерное общество "Северсталь" (ПАО "Северсталь") Способ производства полос из низколегированной свариваемой стали
WO2015110585A1 (fr) * 2014-01-24 2015-07-30 Rautaruukki Oyj Produit de bande d'acier très haute résistance laminé à chaud
US10837079B2 (en) 2014-01-24 2020-11-17 Rautaruukki Oyj Hot-rolled ultrahigh strength steel strip product
RU2625861C1 (ru) * 2016-05-23 2017-07-19 Открытое акционерное общество "Магнитогорский металлургический комбинат" Способ производства листовой стали с высокой износостойкостью
WO2019031773A1 (fr) 2017-08-09 2019-02-14 주식회사 포스코 Tôle d'acier plaquée ayant une excellente qualité de surface, une excellente résistance et une excellente ductilité
RU2674797C1 (ru) * 2018-06-07 2018-12-13 Публичное акционерное общество "Магнитогорский металлургический комбинат" Способ производства высокопрочного хладостойкого листового проката из низколегированной стали

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WO2011154831A4 (fr) 2012-03-08
EP2576848A1 (fr) 2013-04-10
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FI122313B (fi) 2011-11-30
CN103097556A (zh) 2013-05-08
CN103097556B (zh) 2016-01-20

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