EP3872194A1 - Procédé de fabrication d'un produit plat en acier laminé à chaud et produit plat en acier - Google Patents

Procédé de fabrication d'un produit plat en acier laminé à chaud et produit plat en acier Download PDF

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Publication number
EP3872194A1
EP3872194A1 EP20159609.5A EP20159609A EP3872194A1 EP 3872194 A1 EP3872194 A1 EP 3872194A1 EP 20159609 A EP20159609 A EP 20159609A EP 3872194 A1 EP3872194 A1 EP 3872194A1
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EP
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Prior art keywords
temperature
hot
martensite
flat steel
steel product
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EP20159609.5A
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German (de)
English (en)
Inventor
Frank Dr. Hisker
Uwe Dr. Herwig
Roger Dr. Kost
Bettina Röttgers
Anja Fahning
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ThyssenKrupp Steel Europe AG
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ThyssenKrupp Steel Europe AG
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Priority to EP20159609.5A priority Critical patent/EP3872194A1/fr
Publication of EP3872194A1 publication Critical patent/EP3872194A1/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
    • 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
    • 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
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • 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
    • C22C38/02—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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • C21D2211/005—Ferrite
    • 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
    • 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
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium 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

Definitions

  • the invention relates to a method for producing a hot-rolled flat steel product with a structure whose main components are tempered or freshly formed martensite and ferrite, the remainder of the structure being filled with retained austenite, bainite and / or cementite.
  • the invention also relates to a flat steel product with a corresponding structure, the flat steel product being able to be produced in particular by the method according to the invention.
  • the image analysis for the quantitative determination of the structure is carried out optically by means of light microscopy ("LOM”) with a resolution of 1000 times and with a field emission scanning electron microscope (“FE-SEM”) with a resolution of 20,000 times.
  • LOM light microscopy
  • FE-SEM field emission scanning electron microscope
  • the strength and elongation properties mentioned here such as tensile strength Rm, uniform elongation Ag, elongation at break A50 of flat steel products were determined in the tensile test according to DIN-EN 6892-1 specimen form 1 transverse to the rolling direction (WR), unless otherwise noted.
  • the elongation at break A80 was calculated in accordance with DIN EN 2566-1 (Sept 1999 section 9.3).
  • the hole expansion behavior or the achievable hole expansion HER of the flat steel products were determined on 100 ⁇ 100 mm 2 samples according to ISO 16630.
  • a hot-rolled flat steel product which, in% by mass, consists of C: 0.10-0.60%, Si: 0.4-2.0%, Al: up to 2.0%, Mn: 0.4 - 2.5%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Ti: up to 0.2%, Nb: up to to 0.2%, V: up to 0.5% and the remainder of iron and unavoidable impurities.
  • the structure of the flat steel product consists of optionally available proportions of up to 5 vol .-% ferrite and up to 10 vol .-% martensite to at least 60 vol .-% of bainite and the remainder of retained austenite, with at least part of the retained austenite in blocky shape and the blocks of the retained austenite present in blocky form have a mean diameter of at least 98% of less than 5 ⁇ m.
  • Such a flat steel product can be produced by producing a pre-product in the form of a slab, thin slab or a cast strip from a melt composed in the specified manner, which is then hot-rolled into a hot strip in one or more rolling passes, the hot strip obtained at Leaving the last rolling pass one Has a final hot rolling temperature of at least 880 ° C.
  • the hot-rolled flat steel product obtained in this way is accelerated and cooled at a cooling rate of at least 5 ° C./s to a coiling temperature which is between the martensite start temperature MS and 600 ° C., and at this temperature it is wound into a coil.
  • the flat steel product is then cooled in the coil, with the temperature of the coil being kept in a temperature range during the cooling for the formation of bainite, the upper limit of which is equal to the baihit start temperature BS, from which bainite is formed in the structure of the hot strip, and the lower limit of which is equal to the martensite start temperature MS from which martensite is formed in the structure of the hot strip until at least 60% by volume of the structure of the hot strip consists of bainite.
  • a hot-rolled flat steel product produced in this way regularly has tensile strengths Rm of more than 1000 MPa, in particular at least 1200 MPa, with elongations A80 which are also regularly above 17%, in particular above 19%. Accordingly, the quality Rm * A80 of the known flat steel products is regularly in the range of 18,000 - 30,000 MPa *%.
  • the task has been to specify a method for producing a flat steel product which is further improved with regard to its mechanical properties and is characterized in particular by a favorable hole expansion behavior.
  • a hot-rolled flat steel product should be specified with a property spectrum that has an optimized combination of high strength and good deformability, in particular good hole expansion behavior.
  • the invention has achieved this object in that at least the work steps specified in claim 1 are carried out in the production of a hot-rolled flat steel product.
  • a hot-rolled flat steel product that solves the above-mentioned object has at least the features specified in claim 7.
  • the section or sections of the edge region of the martensite islands in which or in which a higher C content is present than in the central region of the respective martensite island, in total, takes up at least 30-70% of the circumference of the martensite island in question FIG 2 evident.
  • the martensite islands of the structure of a flat steel product according to the invention are at least over part of their circumference surrounded by a hem consisting of retained austenite. Its width is typically 10 nm to 1 ⁇ m, but can also be up to a third of the diameter of the respective martensite island.
  • the improved tensile strength and elongation are achieved in a flat steel product according to the invention through the presence of several phases and the associated high level of solidification, and the good hole expansion through the reduction of the shear stresses compared to pure dual-phase structures.
  • a flat steel product according to the invention thus achieves tensile strengths Rm, hole widenings HER and uniform elongations Ag, the product of which Rm x HER x Ag is regularly at least 200,000 MPa% 2 , in particular at least 300,000 MPa% 2 .
  • the tensile strength Rm of a hot-rolled flat steel product according to the invention regularly reaches values of at least 530 MPa, the hole expansion HER regularly values of at least 20% and the uniform elongation Ag regularly values of at least 5%, in particular at least 8%.
  • the alloy of the melt produced for the production of a flat steel product according to the invention and the associated steel substrate of a flat steel product according to the invention has been selected as follows: Carbon ("C") is present in the flat steel product according to the invention in contents of 0.05-0.15 mass% in order to achieve the required level of strength. For this, at least 0.05 mass% C is required. The effects of the presence of C used according to the invention are particularly certain reached when the C content is at least 0.065% by mass.
  • the C content By limiting the C content to at most 0.15% by mass, in particular less than 0.15% by mass, it is ensured that a sufficient amount of ferrite is formed in the structure of a flat steel product according to the invention and that the martensite formed is in Can deform partial areas at all and thus shear stresses can be reduced. This effect can be achieved in particular when the C content is limited to a maximum of 0.14% by mass, in particular a maximum of 0.12% by mass.
  • Si Silicon
  • Si can be present in the steel of a steel flat product according to the invention in order to strengthen the steel. This effect can be achieved reliably with Si contents of at least 0.01% by mass, in particular 0.04% by mass. However, too high Si contents would increase the Ar3 temperature. This would make the hot rolling aimed for according to the invention more difficult in a temperature range in which the flat steel product has a completely austenitic structure.
  • the invention avoids this in that the Si content is limited to less than 0.5% by mass, in particular less than 0.4% by mass.
  • Manganese is present in the steel of a flat steel product according to the invention in contents of 0.7-2.1% by mass in order to minimize the concentration of C in the structure and the associated formation of undesirable hard martensite. This effect is achieved particularly reliably with Mn contents of at least 0.7% by mass. If the content is more than 2.1% by mass, there is a risk of Mn segregations occurring in the structure of the flat steel product according to the invention, which would impair the mechanical properties. This negative influence of the presence of Mn can be excluded with particular certainty that the Mn content is limited to a maximum of 2.0 mass%.
  • the aluminum (“Al”) content in the steel of a flat steel product according to the invention is limited to less than 0.1% by mass in order to avoid effects of this alloying element on the Ar3 temperature and to ensure optimized castability of the steel melt.
  • Al can be used for deoxidation in the course of steel production. This typically requires Al contents of at least 0.02% by mass. Negative effects triggered by the presence of Al can in particular be avoided by limiting the Al content to less than 0.05% by mass.
  • Chromium is present in the steel of a flat steel product according to the invention in contents of 0.2-1% by mass in order to increase the hardenability and suppress the formation of pearlite.
  • Cr Chromium
  • contents of 0.2-1% by mass in order to increase the hardenability and suppress the formation of pearlite.
  • at least 0.2% by mass of Cr are required, whereby the favorable effects of the presence of Cr can be used particularly reliably with Cr contents of at least 0.25% by mass.
  • the Cr content is at most 1% by mass in order to enable the formation of ferrite in the structure of the flat steel product according to the invention, which is aimed at according to the invention. This can be ensured particularly reliably by limiting the Cr content to a maximum of 0.9% by mass.
  • the steel of a flat steel product according to the invention contains at least one of the microalloying elements niobium (“Nb”), vanadium (“V”) and titanium (“Ti”) in order to increase the fine grain size and strength.
  • the sum of the contents of these elements is 0.01-0.1% by mass.
  • the respectively envisaged content of the micro-alloy elements can be taken up by one of the micro-elements alone or two or three of the mentioned micro-alloy elements can be present in combination.
  • the positive influences of the micro-alloy elements on the mechanical properties of a flat steel product according to the invention can be used particularly reliably if the sum of their contents is at least 0.01% by mass.
  • the contents of the Micro-alloy elements limited to a maximum of 0.1 mass%, in particular a maximum of 0.05 mass%, in order to avoid precipitations and to enable accelerated recrystallization.
  • B Boron
  • B can optionally be present in the steel of a flat steel product according to the invention in contents of up to 0.0015% by mass. It increases the hardenability particularly strongly. However, this must not be too high in order to enable the formation of sufficient quantities of ferrite in the structure of a flat steel product according to the invention. Negative effects of the presence of B can be avoided particularly reliably by limiting the B content to a maximum of 0.0008% by mass.
  • Molybdenum can optionally also be added to the steel of a flat steel product according to the invention in contents of less than 0.2% by mass, in particular less than 0.20% by mass, in order to increase the hardenability. For this purpose, at least 0.01 mass% Mo can be provided in practice.
  • An alloy of a steel according to the invention which is particularly balanced in terms of cost / benefit aspects, contains up to 0.18% by mass of Mo or up to 0.1% by mass of Mo, in particular up to 0.05% by mass of Mo or up to 0.018% by mass % Mon
  • Copper can optionally also be added to the steel of a flat steel product according to the invention in contents of less than 0.2% by mass in order to further increase the strength (precipitation and mixed crystal strengthening).
  • the positive effect of the presence of Cu at contents of at least 0.1 mass% Cu can be used reliably.
  • Nickel can optionally also be added to the steel of a flat steel product according to the invention in contents of less than 0.2% by mass in order to increase the strength through precipitation and mixed crystal strengthening to increase further.
  • the positive effect of the presence of Ni with contents of at least 0.1 mass% Ni can be used reliably.
  • Phosphorus (“P”) can also optionally be present in the steel according to the invention in contents of less than 0.05% by mass in order to further increase the strength and to control the transformation behavior.
  • the positive effect of the presence of P can be used safely with contents of at least 0.002 mass% P.
  • N Nitrogen
  • contents of less than 0.01% by mass are permitted to be harmless for the properties. Higher concentrations would lead to coarse precipitations, which could have a negative effect on the forming behavior.
  • the intermediate product which is composed of a melt in accordance with the above remarks, is cast in an otherwise conventional manner and is heated through at a temperature of 1150-1380 ° C. for a period of typically 60-960 minutes.
  • the maximum temperature and the duration of the heating must be measured in such a way that all the carbides contained in the preliminary product are dissolved.
  • the heating temperature is preferably below 1380 ° C.
  • a heating period of at least 60 minutes has proven to be particularly effective, with heating for a maximum of 8 hours being sufficient in practice for conventional slab dimensions for heating.
  • the lower limit of the range of the through-heating temperature specified according to the invention is at least 1150 ° C., preferably more than 1200 ° C., in order to prevent the formation of precipitates and other undesirable phases in the structure of the preliminary product.
  • the pre-product can optionally be descaled before it is fed into the hot rolling process.
  • the pre-product in the event that the pre-product is a slab, the pre-product is pre-rolled at temperatures of 1020-1150 ° C to a thickness of 30-50 mm.
  • the pre-rolling compacts the cast structure of the slab so that the best conditions are created for the subsequent finish hot rolling. If the pre-product is a thin slab or a cast strip, pre-rolling can be dispensed with.
  • the hot rolling of the optionally pre-rolled preliminary product to a thickness of 1.5 - 6.4 mm can be carried out in a conventional manner in one or more steps.
  • the only decisive factor here is that the hot rolling end temperature ET, at which hot rolling is ended, is at least equal to the Ar 3 temperature of the steel and at most 200 ° C higher than the Ar 3 temperature of the steel, with hot rolling end temperatures of 820-900 ° C are particularly practical.
  • the hot rolling end temperature is selected so that hot rolling is carried out as exclusively as possible in a temperature range in which im hot-rolled flat steel product has an austenitic structure.
  • the hot rolling end temperature can be set to at least 820 ° C.
  • the final hot-rolling temperature is at most 200 ° C., in particular less than 200 ° C., above the Ar 3 temperature in order to support the development of a fine-grained austenite structure in which as many nuclei as possible for the subsequent ferrite formation are present.
  • Particularly suitable hot rolling end temperatures are accordingly in the range of 820-900 ° C.
  • the cooling rate Td1 between the hot rolling end temperature and the intermediate temperature of Ar1 -100 ° C must be at least 20 K / s, so that a concentration profile of C in the austenite arises during the ferrite formation, which is later converted to martensite. Cooling speeds Td1 of at least 30 K / s are particularly suitable here. In practice, the upper cooling rate Td1 is limited to 90 K / s for reasons of efficiency.
  • the cooling rate is controlled in such a way that, on the one hand, sufficient ferrite is formed and a sufficiently high diffusion of carbon from the ferrite into the adjacent austenite is made possible, through which the residual austenite, which later forms the fringing area of the martensite islands, with carbon is enriched.
  • the "higher C content” is defined in such a way that it is at least 0.05% by weight, with higher C contents of at least 0.1% by weight, in particular at least 0.15% by weight have proven to be particularly advantageous in practice.
  • the concentration of the C content is determined with an FE microprobe within a range of 300 ⁇ 300 nm 2 .
  • island-like martensite is obtained in the structure of a flat steel product according to the invention, which mostly has an inhomogeneous distribution of the carbon content over its volume.
  • retained austenite remains on the martensite edges, which forms a border surrounding the respective martensite island.
  • This typically has a width of 10 nm - 1 ⁇ m, whereby its width can also be up to 1/3 of the island diameter.
  • the carbon concentration increasing towards the edge area extends in the case of a flat steel product according to the invention by at least 30% of the circumference of the martensite islands (see, for example, the martensite islands shown in FIG Fig. 1 shown) and is included at least 70% of all martensite islands are present.
  • the C gradient generated according to the invention in the martensite islands of the structure increases the hole expansion HER, since the formation of large martensite islands homogeneous with regard to the carbon distribution is prevented, which would increase the shear stress in a ferritic matrix and thus minimize the hole expansion.
  • the residual austenite present according to the invention between the ferrite matrix and the respective martensite island achieves smoother transitions between the soft ferrite matrix and the hard martensite islands or facilitates the deformation in partial areas of the martensite island. Due to the C distribution, areas in the martensite can be deformed earlier in the event of an external load, which reduces steep hardness jumps that are harmful to the hole expansion HER. Nevertheless, there is still a high degree of consolidation of the structure due to the differences in hardness. This results in good elongation in combination with good hole expansion values HER with high strength values.
  • the further cooling strategy promotes the advantageous product properties to a subordinate extent: After the intermediate temperature Tz has been reached, the cooling rate in the temperature range up to the martensite start temperature T MS is controlled in a second cooling section in such a way that the diffusion length of C in austenite remains as limited as possible.
  • cooling is carried out at a cooling rate Td2 'of at least 5 K / s, in particular more than 5 K / s or at least 20 K / s, until the range between the martensite start temperature TMS and room temperature is reached so that the carbon diffusion does not homogenize all of the adjacent retained austenite with carbon.
  • the cooling rate in this variant is limited to a maximum of 100 K / s in order to ensure that diffusion of carbon from the previously formed ferrite into the adjacent austenite can take place. This can be ensured particularly reliably by limiting the cooling rate Td2 'to a maximum of 70 K / s. It is particularly practical when the cooling rate Td2 'is 20 - 70 K / s.
  • the cooling up to the martensite start temperature TMS is completed at a cooling rate Td2 ′′ of 10-130 K / s.
  • a cooling rate Td2 ′′ of at least 10 K / s, in particular at least 30 K / s, is also limited the carbon diffusion from the ferrite into the austenite. The diffusion of a sufficient amount of carbon can be supported by interrupting the cooling at a cooling stop temperature of 550 - 700 ° C for up to 5 s. A break of at least 1 s is particularly practical here.
  • the cooling rate Td2 ′′ should be at most 130 K / s, in particular less than 100 K / s, in order to even allow a sufficient C diffusion length in the austenite limited to a maximum of 80 K / s will. It is particularly practical when the cooling rate Td2 "is 30 - 80 K / s.
  • the third section of cooling, in which the hot-rolled flat steel product reaches the coiling temperature HT, is not critical and can take place at a cooling rate in still air.
  • the coiling temperature HT is lower than the martensite start temperature and can reach room temperature.
  • the reel temperature HT is typically 20 - 80 ° C.
  • the hot-rolled flat steel product cooled in this way is wound into a coil.
  • the coiling temperature HT is above room temperature
  • the flat steel product is finally cooled to room temperature in the coil.
  • melts E1-E4 composed in accordance with the requirements of the invention and a comparative melt V1 not composed according to the invention, the compositions of which are given in Table 1, were melted.
  • Table 1 shows the martensite start temperatures Tmst, Ar 3 temperatures and Ar 1 temperatures estimated in accordance with the above-explained formulas (1) - (3) for the melts E1-E4 and V1.
  • the melts E1-E4 and V1 have been cast in the conventional manner to form slabs, which were each heated through at 1150-1380 ° for a period of 60-240 minutes.
  • the slabs heated through in this way have been subjected to rough rolling, in which they have been hot rough rolled in the temperature range from 1020 to 1150 ° C. to form a rough strip with a thickness of 30 to 50 mm.
  • hot strip hot-rolled strips
  • Twe hot-rolling end temperature
  • the hot strips W1-W11, WV obtained were cooled, starting from their respective final hot-rolling temperature Twe, at a cooling rate dT1 to an intermediate temperature Tz which was 50 ° C. below the Ar 1 temperature of the steel E1-E4 and V1 from which the hot strips W1 - W11, WV each passed.
  • the hot strips W1-W11, WV have been cooled at a cooling rate dT2 'down to the martensite start temperature Tmst of the steel.
  • the hot strips W1-W11, WV have been cooled down to the respective coiling temperature HT at a cooling rate dT3 at which they have been coiled into a coil. Finally, cooling to room temperature took place in the coil.
  • the tensile strength Rm, the yield point Re, the uniform elongation Ag, the elongation A50, the elongation A80 and the hole expansion HER have been determined for the hot strips W1 - W11, WV.
  • the relevant properties as well as the product Rm x HER x Ag and the ratio Re / Rm are listed in Table 3 for the hot strips W1 - W11, WV.
  • the martensite, ferrite, bainite, pearlite and retained austenite components of the structure have been determined for the hot strips W1 - W11, WV.
  • the hot strips W1-W3 and W6-W11 produced according to the invention and alloyed according to the invention reliably have high mechanical parameters Rm, Re, Ag, A50, A80 and HER, which are too high values for the product Rm x HER x Lead ag.
  • the hot strip WV produced from the comparative steel V1 not alloyed according to the invention in a manner not according to the invention does not reach the minimum limit specified according to the invention for the product Rm x HER x Ag.
  • Fig. 1 the illustration of a martensite island M which is present in the structure of the hot strip W1 and is embedded in a ferritic structure F is shown.
  • the one around the martensite island M is clearly visible Residual austenite border RAS, by which the martensite island M is separated from the surrounding ferrite F.
  • the central area MMB of the martensite island M is delimited by an edge area MRB, around which the retained austenite border RAS in turn runs.
  • Table 1 stolen Figures in% by mass, remainder iron and unavoidable impurities ° C C. Si Mn P. S. Al Cr Mon N Ti + Nb + V B.
EP20159609.5A 2020-02-26 2020-02-26 Procédé de fabrication d'un produit plat en acier laminé à chaud et produit plat en acier Pending EP3872194A1 (fr)

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CN114535294A (zh) * 2022-03-14 2022-05-27 武汉钢铁有限公司 一种采用CSP产线生产厚度1.0mm热轧带钢的方法

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CN114535294A (zh) * 2022-03-14 2022-05-27 武汉钢铁有限公司 一种采用CSP产线生产厚度1.0mm热轧带钢的方法

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