EP1990430A1 - Plaque d'acier laminée à chaud haute résistance et son procédé de fabrication - Google Patents

Plaque d'acier laminée à chaud haute résistance et son procédé de fabrication Download PDF

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Publication number
EP1990430A1
EP1990430A1 EP08154615A EP08154615A EP1990430A1 EP 1990430 A1 EP1990430 A1 EP 1990430A1 EP 08154615 A EP08154615 A EP 08154615A EP 08154615 A EP08154615 A EP 08154615A EP 1990430 A1 EP1990430 A1 EP 1990430A1
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Prior art keywords
steel plate
rolling
strength
phase
hot rolled
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German (de)
English (en)
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EP1990430B1 (fr
Inventor
Kazuaki Hakomori
Yuuji Kusumoto
Fuyuki Yoshida
Ichiro Chikushi
Takashi Ohtani
Ryurou Kurahashi
Masahiko Oda
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Nakayama Steel Works Ltd
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Nakayama Steel Works Ltd
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    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • C22C38/04—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • 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
    • C21D2211/001—Austenite
    • 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
    • C21D2211/002—Bainite
    • 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 invention relates to a high-strength hot rolled steel plate having high tension strength and superior workability, and also relates to a manufacturing method thereof.
  • Hot pressing is a hot press working process and usually generates a quite small amount of spring back, thus exhibiting preferable shape freezing properties.
  • this method can present parts having significantly higher strength, with high accuracy.
  • this process requires heating the steel plate prior to subjecting it to the hot press working, and also requires reduction of the manufacturing scale after the hot press working. Thus, this process has possibility to significantly deteriorate the working efficiency.
  • shorter life of the mold, which should contact with a heated steel plate inevitably increase the manufacturing cost.
  • the dual phase steel plate is formed by finely dispersing the hard martensite phase in the ferrite phase. Due to the highly hard martensite phase, significant work hardening is caused upon transformation, thus providing higher ductility to the steel plate.
  • TRIP steel plate examples include steel plate of this type containing the retained austenite phase exhibits highly excellent ductility and moldability both attributed to working induced transformation, depending on the amount the retained austenite phase and the stability to the deformation.
  • delayed fracture means a phenomenon wherein while cracking and/or fracture is not generated upon working and assembly for respective members, it appears suddenly during use of them.
  • a high-strength steel plate disclosed in Patent Document 3 is intended to provide more preferable anti-delayed fracture properties, by reducing a soft phase, such as the ferrite phase, as much as possible, and by controlling the volume fraction of the retained austenite phase to be less than 4%, relative to the low-temperature transforming phase, such as the bainite phase and/or the tempered martensite phase.
  • Patent Document 1 JP-A-No. 60-43425
  • Patent Document 2 JP-A-No. 9-104947
  • Patent Document 3 JP-B-No. 3247908
  • the TRIP steel plate exhibits higher ductility and has more excellent deep drawing properties. Therefore, this material is suitable for providing a part or member for which a complicated shape, higher workability and more enhanced strength are required.
  • the TRIP steel plate described in the Patent Document 1 is manufactured by a method comprising: creating the ferrite phase in the austenite phase by holding a raw material at 450 to 650°C for 4 to 20 seconds in a cooling step after rolling, cooling it to a temperature lower than 350°C, and coiling it around a rod material.
  • a raw material in order to promote formation of the ferrite phase in the austenite phase in a cooling process after the rolling, a raw material is gently cooled at Ar3 to Ar1 or subjected to a rolling completion temperature of approximately Ar3 then cooled to a temperature within a range of 350 to 500°C, and is wound around a rod material.
  • Such a TRIP steel plate has a structure in which the martensite phase, retained austenite phase and/or bainite phase is dispersed in the ferrite parent phase, and exhibits excellent strength and elongation properties.
  • the hot rolled steel plate described in the Patent Document 1 exhibits lower rolling workability and has a metallic structure in which coarse ferrite particles and retained austenite particles are present contiguously because of the temporary stopping for the cooling at a point of approximately A1.
  • Hydrogen dissolved in the steel plate which is likely to be a cause of the delayed fracture, is a factor of determining the crystal phase, and is trapped preferentially in the retained austenite phase.
  • the interface between the martensite phase and ferrite phase having been subjected to the impact or working, i.e., the working induced transformation site is considered to be a highly possible trapping site for hydrogen.
  • Coarser retained austenite particles will provide a more reduced ratio of the area of the interface between the martensite phase and ferrite phase having been experienced the working induced transformation, as compared with the volume of the retained austenite particles. Consequently, the concentration of hydrogen to be trapped is increased, as such presenting a greater risk of the delayed fracture. If the martensite phase and the retained austenite phase coexist contiguously (in an M-A state), propagation of the fracture is likely to be promoted, thus providing a further increased risk of the fracture.
  • the high-strength steel plate described in the Patent Document 3 is intended to enhance the anti-delayed fracture properties by limiting the amount of the retained austenite.
  • utilization of the retained austenite is substantially effective. Accordingly, it is desirable if the presence of the retained austenite will not detrimentally affect the anti-delayed fracture properties without providing any limitation as described above.
  • the present inventors have developed a new low-alloy and higher-strength steel plate and a method of manufacturing thereof, the steel plate having a bainite phase in which seven or more of the retained austenite particles having a particle size of 1 ⁇ m or less are finely dispersed per 10 ⁇ m 2 (the volume fraction is within the range of from 5% to 20%), thereby exhibiting higher strength as well as more preferred workability and secure anti-delayed fracture properties.
  • a preferably high-strength steel plate can be obtained by employing appropriate rolling conditions and selecting a proper composition of components for the steel plate. Namely, higher strength and excellent ductility as well as secure anti-delayed fracture properties can be provided to a low-alloy steel plate, by subjecting a slab having a proper composition of components to rough hot rolling under high pressure conditions, completing rear-stage higher strain rolling in a finish rolling process under high temperature conditions, starting a cooling process after air-cooling for several seconds, and coiling the processed material at an appropriate temperature.
  • a high-strength hot rolled steel plate of the present invention comprises: a retained austenite phase in a volume fraction of 5% to 20%; a martensite phase in a volume fraction of 0% to 10%; and a bainite phase in the remaining volume fraction, wherein particles constituting the retained austenite phase have a particle size of 1 ⁇ m or less. More preferably, the particle size of prior austenite is 10 ⁇ m or less, and the average aspect ratio of the particles is 2.0 or less.
  • the lath structure of the bainite phase can be made fine.
  • the retained austenite particles having a particle size of 1 ⁇ m or less can be finely and effectively dispersed in the phase with the density of seven or more particles per 10 ⁇ m 2 ( Fig. 8 ).
  • anisotropy of the material which is drawn in both of the rolling direction and the direction vertical to the rolling direction, can be reduced, as such enhancing the workability ( Fig. 4 ).
  • the high-strength hot rolled steel plate of the present invention has a composition comprising: C (0.13 to 0.21 (% by weight)), Si (0.5 to 2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4), P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and the remaining components including Fe and other inevitable impurities.
  • Such a chemical composition comprising proper types and amounts of the selected components can facilitate formation of the high-strength steel plate which can include the phases described above and exhibit desired mechanical properties.
  • the alloy elements described above can constitute the desired steel plate structure of the present invention in the steps of cooling after hot rolling, and coiling the cooled material, Cr and Si having greater influence on the bainite transformation are included as major elements. With controlling of amounts of these elements, the bainite transformation can be promoted, and formation of the martensite phase can be suppressed, thereby to control the entire phase to have an aimed strength.
  • the high-strength hot rolled steel plate has the structure as described above, and also a plate thickness of 1.0 to 3.0mm, tensile strength (TS) of 1200MPa or greater, and elongation of 13% or greater (JIS No. 5 test piece).
  • TS tensile strength
  • this steel plate can possess the structure described above, and hence exhibit greater strength and more excellent elongation properties.
  • a method for manufacturing a high-strength hot rolled steel plate according to the present invention comprises the steps of:
  • a temperature history for maintaining the temperature
  • a uniform phase of baitnite in which martensite and retained austenite are finely dispersed, can be obtained, by adding Cr and Si as major alloy elements and selecting a composition containing lower Mn and Ni ( Fig. 7(b) ).
  • the austenite having carbon density of 0.8% or higher can be retained in a greater amount. In this way, a steel plate having enhanced strength and more excellent workability can be obtained ( Fig. 11 ).
  • the hot rolling finish temperature By controlling the hot rolling finish temperature to be 950°C or higher, the aspect ratio of the prior austenite particles can be controlled at 2.0 or less ( Fig. 3 ).
  • a reduction amount of a topmost portion of the steel material is reduced, as needed, as compared with an expected reduction amount (or reduction amount originally set for a predetermined rolling), in first to fifth rolling mills (in the case of using six stages of finish rolling mills, while first to sixth rolling mills are used in the case of using seven stages of finish rolling mills), wherein the reduction amount is increased by 10% or less, as compared with the expected amount of each rolling mill. It is also preferred that a length to be rolled in the increased reduction amount is within 5m as measured from a biting position of the topmost portion of the rolling material.
  • a special high-grip roll is used as a working roll for each of finish first to third rolling mills including the final rolling mill.
  • the retained austenite is incorporated in the baitnite phase in a volume fraction of 5% to 20% such that it is finely dispersed with the density of seven or more particles per 10 ⁇ m 2 . Therefore, both strength and workability, which are contrary to each other, can be provided to the steel plate, and excellent anti-delayed fracture properties can also be provided thereto.
  • the high-strength steel plate described above can be readily and securely manufactured.
  • the steel plate has a composition containing the following components: C (0.13 to 0.21 (% by weight)), Si (0.5 to 2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4), P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and the remaining components include Fe and other inevitable impurities.
  • sheet plate means a steel plate having a thickness of from 1.0mm to 3.0mm.
  • the steel plate to be manufactured under the above compositional conditions can be mainly used as parts for cars, consumer electrical appliances, electronic equipment and the like, which require higher workability and strength.
  • the steel plate can also be applied to materials for steel pipes.
  • the amount of carbon (C) should be within the range of 0.13 to 0.21%.
  • C is the most important component for stabilizing the retained austenite. If the amount of C is less than 0.13%, sufficient stability can not be obtained, thus an amount of C of 0.13% or greater should be required. However, if it exceeds 0.21%, a welded portion becomes too hard and is likely to be broken. Such a situation provides some limitation of use to the sheet steel to be formed. Therefore, the upper limit described above is provided to the amount of C. Namely, by setting the amount of C within the range of 0.13 to 0.21%, it has been found that a composite structure which accords with an intention of the present invention can be obtained.
  • the amount of silicon (Si) should be within the range of 0.5 to 2.0%. Si also serves to stabilize the retained austenite. In addition, Si enhances the strength to be obtained by reinforcement due to solid solution. If the amount of Si is 0.5% or greater, a preferred composite structure and material quality can be obtained. A greater amount of Si can increase more retained austenite as well as enhance the stability. However, if the amount of Si exceeds 2.0%, properties for balancing the strength and the ductility will be saturated, thus the upper limit of the Si amount should be set at 2.0% in view of reduction of the cost.
  • the amount of chromium (Cr) should be within the range of 1.0 to 4.0%. Cr can create the bainite phase, and enhance the strength of the steel plate to be formed therewith.
  • the Cr amount should be 1.0% or greater. However, if it exceeds 4.0%, the martensite phase is likely to be produced, thus making the steel plate strength too high and hence rendering the anti-delayed fracture properties insufficient. Therefore, 4.0% is set as the upper limit.
  • the amount of manganese (Mn) should be within the range of 0.2 to 1.0%. If the Mn amount is less than 0.2%, the manufacture of the steel plate will be difficult. Therefore, it should be 0.2% or greater.
  • the upper limit of the Mn amount should be set at 1.0%.
  • Ni nickel
  • the amount of nickel (Ni) should be within the range of 0.02 to 1.0%. Ni can enhance the strength of the steel plate by reinforcement due to solid solution. However, if the amount of Ni is too increased, the martensite phase is likely to be produced. Moreover, inadvertent addition would lead to increase of the production cost. Thus, the upper limit should be set at 1.0%.
  • Molybdenum can create the bainaite phase as is similar to Cr, and enhance the strength of the steel plate to be formed therewith.
  • Mo carbides can create the bainaite phase as is similar to Cr, and enhance the strength of the steel plate to be formed therewith.
  • a hydrogen trapping effect due to Mo carbides is useful for providing anti-delayed fracture properties to the steel plate.
  • the Mo amount should be set within the range of from 0.05 to 0.40%.
  • the upper limit of this element should be set at 0.010%.
  • the upper limit of this element should be set at 0.003%.
  • the amount of nitrogen (N) should be within the range of 0.005 to 0.015%. As is similar to carbon, nitrogen is useful to stabilize the austenite phase. However, its excessive existence will cause degradation of the weldability. Thus, the range of this amount should be set at a value of from 0.005 to 0.015%.
  • a slab produced to have the composition as described above is then subjected to hot rolling after heated again or subjected to hot rolling immediately after casting.
  • Fig. 1 is a graph for schematically showing a temperature history of hot rolling in a manufacturing process of one embodiment of the present invention, in which particles sizes of prior austenite are also designated.
  • the horizontal axis denotes the elapsed time and the vertical axis denotes the temperature.
  • the extraction temperature of the reheating furnace was set at 1250°C. This temperature was selected to preferentially secure the surface temperature of 950°C after the finish, even though some inevitable growth of austenite particles would be caused in the reheating furnace due to such a high temperature condition.
  • the size or diameter of the austenite particles will be lessened in the following rolling process. Therefore, it is necessary to reduce the particle size of the prior austenite as finely as possible before subjecting it to a finishing rolling mill.
  • the crystal particle size is reduced in advance to 35 ⁇ m or less, by setting the reduction ratio of each of final three passes for roughly rolling at 30% or greater, at a discharging-side temperature of 1030°C or higher on the discharging side of the roughly rolling mills.
  • Fig. 2 shows a particle size of prior austenite after subjected to the roughly rolling, wherein the processed material was cut by a pre-finish crop shearing machine.
  • the reduction ratio per mill is set at 40% or higher.
  • Accumulated strain in the pressed state for three rolling mills of a finish rear-stage is set at 0.5 or greater, and the finishing rolling mill discharging-side temperature is securely set at 950°C or higher, so as to render the austenite particle size equal to or less than 10 ⁇ m.
  • air-cooling is provided for 2 to 6 seconds after the finish rolling, followed by water-cooling.
  • a coiling temperature is set at 550°C to 650°C.
  • the size of the austenite particles is also controlled. Namely, during the hot rolling step, the particle size of the prior austenite is controlled to be 10 ⁇ m or less before post-hot-rolling hot run cooling is started, so as to control the size of the prior austenite particles to eliminate working strain.
  • Fig. 3 shows a result of observation for the prior austenite particles of the steel plate according to the present invention by using an SEM phase observation.
  • An average particle size of the prior austenite particles is 9.3 ⁇ m, presenting a uniformly granulated structure.
  • An average aspect ratio of the major axis/the short axis is 1.7.
  • Fig. 4 shows a relationship between the finish rolling mill discharging-side temperature (FDT) and anisotropy of elongation. As is seen from Fig. 4 , the anisotropy of elongation appears when the FDT is 950°C or lower.
  • This anisotropy is defined by an equation of
  • Hot rolling is completed at a temperature of 950°C or higher, and the material is then subjected to air-cooling for 2 to 6 seconds without undergoing the post-hot-rolling hot run cooling, so as to reduce the dislocation density in the crystal particles.
  • Fig. 6 changes in the austenite particle size and changes in the dislocation density are illustrated, wherein these data are obtained by calculation over a period from a finishing F1 rolling mill to starting the hot run cooling, in the case of changing the rolling temperatures for the same type of steel. From the drawing, it can be seen that the dislocation density is significantly influenced by the rolling temperature.
  • the austenite particle size will be smaller under lower temperature conditions, provided that the processing temperature is equal to or higher than that required for Ar3 transformation.
  • the dislocation density will be higher, thus providing a material with further increased anisotropy.
  • setting the aspect ratio at 2.0 or less can be translated into controlling the dislocation density to be at least 2.50E + 10 ( ⁇ /cm 2 ) or less (this was confirmed from the results of comparison between actual data and the simulation model).
  • the reduction of the dislocation density leads to increase of the size of the prior austenite particle.
  • the aforementioned rolling conditions rolling temperature: 950°C or higher, and cooling time: 2 to 6 seconds.
  • Fig. 7 sectional phases of three-types of high-strength steel plates are shown.
  • Fig. 7(b) shows a structure obtained according to the present invention.
  • austenite is retained in each interface between the prior austenite particles as well as in each packet boundary and each block boundary, i.e., in the prior austenite particles themselves.
  • the retained austenite can be closely and uniformly dispersed into a parent phase such that seven or more of the retained austenite particles having a very fine particle size, such as 1 ⁇ m or less, are dispersed per 10 ⁇ m 2 , by employing the bainite phase as the parent phase and setting the size of prior austenite particles before transformation at 10 ⁇ m or less.
  • the retained austenite phase shown by a bright color constitutes a structure in which seven or more of the retained austenite particles having a particle size of 1 ⁇ m or less are finely and uniformly dispersed per 10 ⁇ m 2 .
  • the bainite phase can be obtained, in which the retained austenite particles are finely and uniformly dispersed.
  • the binding failure at the topmost portion of the rolling material will be likely to frequently occur (rate of occurrence: 50%), at the final rolling mill and the first to second rolling mills of the front-stage rolling mills, if using a material of TS greater than 1000MPa.
  • rate of occurrence 50%
  • securely effective results could not be obtained.
  • Fig. 9 we have attempted to render the topmost portion of the rolling material thinner, over a place within the range of 5m from the discharging side of the rolling mill, so as to make a thinner plate thickness (by 10% of a thickness finally expected). Thereafter, an inclination up to the expected plate thickness was provided to the plate material.
  • the setting reduction amount employed in a range from the finish front-stage rolling mills to the rolling mills located before the finish final rolling mill was set at a value to be obtained by adding 10% or less of an expected set value thereto.
  • the reduction setting time is set within two seconds from a biting site of the topmost portion of the plate material into the rolling mill.
  • Fig. 11 is a graph showing a relationship between the volume fraction (V ⁇ ) of the retained austenite in the heat rolled steel plate produced by the manufacturing process shown in Fig. 1 and data obtained by the tensile test.
  • Fig. 11(a) shows a relationship between the volume fraction V ⁇ and (the tensile strength ⁇ elongation).
  • Fig. 11(b) shows a relationship between the volume fraction V ⁇ and the elongation.
  • the metallic phase corresponding to the data can be considered as the lower-martensite fine bainite phase as shown in Fig. 7(b) .
  • the present invention was made on the above empirical basis.
  • Slab materials (rolling materials) were prepared from melted steel having each chemical composition shown in Table 1 by using a forging method or continuous casting method. Subsequently, these slab materials were heated again, and subjected to hot rolling, so as to obtain hot rolled steel plates, respectively.
  • Table 2 shows respective conditions of the hot rolling and properties of the materials.
  • A, B, C designate steel plates prepared in accordance with the present invention, while D, E, F, G, H are provided as comparative examples.
  • the steel type D as one comparative example contains significantly lower Si and is excessively rich in Ni, thus departing from the preferred range of the present invention.
  • the steel type E contains significantly lower Si, thus also departing from the range defined according to the present invention.
  • the steel types F and G contains lower C, as such departing from the preferred range of the present invention, and the steel type I exhibits an unduely high content of C, thus also departing from the desired range of the present invention.
  • the steel type H is excessively rich in Cr, as such departing from the preferred range of the present invention.
  • Nos. 1 to 6 in Table 2 are examples in which the steel types A, B, C in Table 1, respectively satisfying the preferred range of the present invention, are subjected to rolling under various conditions.
  • No. 1 was prepared by using the steel type A containing 0.51% Si and by employing the hot rolling coiling temperature of 655°C.
  • the data of TS (tensile strength) ⁇ EL (elongation) is quite preferable, but the tensile strength is 778MPa, which is undesirably low.
  • No. 2 was prepared by using the steel type A and employing the hot rolling coiling temperature of 630°C. This example shows the tensile strength of 1200MPa and the elongation of 13%, thus exhibiting excellent properties.
  • No. 3 was prepared by using the steel type B containing 1.00% Si and by employing the hot rolling coiling temperature of 595°C. As shown in Table 2, this example is excellent in both of the strength and the elongation. Additionally, this example shows more enhanced properties in both of the strength and the elongation, as compared with the No. 2 example.
  • No. 4 and No. 5 were prepared under unsatisfied reduction-ratio conditions during the hot rolling, as such these examples exhibit negative delayed fracture while satisfying the strength and the elongation.
  • No. 6 was prepared by using the steel type C containing 1.44% Si and by employing the hot rolling coiling temperature of 610°C. This example exhibits excellent properties in both of the strength and the elongation, and is superior to the No. 3 example in both of the strength and the elongation.
  • Nos. 7 to 12 were respectively prepared by carrying out hot rolling, using steel types of comparative examples departing from the desired range of the composition used in the present invention.
  • No. 7 was prepared by rolling, using the steel type D containing lower Si and higher Ni.
  • This comparative example is insufficient in the spot welding properties (S/W properties) as well as in the delayed fracture properties.
  • No. 8 was prepared by using the steel type E containing lower Si, thus exhibiting insufficient strength and poor balance of strength/ductility.
  • No. 9 and No. 10 were prepared by using the steel types F and G both containing lower C, respectively, as such exhibiting unduely lower strength and poor balance of strength/ductility.
  • No. 11 and No. 12 were prepared by using the steel types H and I both containing excessively high C, thus exhibiting properly higher strength and good balance of strength/ductility.
  • these comparative examples are insufficient in the spot welding properties as well as in delayed fracture properties.
  • the volume fraction of the ferrite particles was measured by observation using an optical microscope, after polishing a section cut along the rolling direction of each steel plate and then subjecting the polished surface to nital corrosion. The measurement also used a commercially available image analyzer.
  • the volume fraction of the martensite was obtained by measuring the martensite phase expressed by a white color in an image analysis process during observation using an optical microscope for a position directed to 1/4 of the plate thickness direction, after polishing a section cut along the rolling direction of each steel plate and then etching the polished surface by using a liquid formed by mixing 1:1 of 4% picric acid-alcohol and 2% sodium pyrophosphate.
  • the measurement of the retained austenite was carried out by employing the X-ray diffraction by using Ka ray of Cu.
  • the volume fraction was determined as an average of the volume fraction of the retained austenite to be calculated from a combination of data obtained by respectively measuring integrated intensities of (200), (220) and (311) faces of the austenite phase and those of (200), (211) faces of the ferrite phase, after electrolytic polishing for a position directed to 1/2t of the plate thickness direction.
  • tensile strength (TS) and elongation (EL) were measured by subjecting each sample to a tensile test, the sample being formed into the shape in accordance with the JIS No. 5 test piece.
  • the delayed fracture properties was assessed by observation of each sample dipped in a 1N hydrochloric acid solution for a predetermined period of time, after forming ⁇ 10mm punch holes with a 12.2% clearance in a central portion subjected to the tensile test, onto which 8% or more of strain had been loaded.
  • the high-strength steel plates obtained by the examples which exhibit high strength and high ductility properties in a lower alloy composition are suitable for use as components for manufacturing car structures.
  • the high-strength steel plates according to the present invention can be used as quite preferred materials, such as center pillars for cars, which require highly excellent properties, including sufficient tensile strength for supporting doors and preventing deformation upon collision or the like, bendability for press molding, deep drawability, hole extending workability for forming an attachment hole to be used for associated equipment, and weldability for welding the material to another car component.

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  • Chemical & Material Sciences (AREA)
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  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Metal Rolling (AREA)
  • Reduction Rolling/Reduction Stand/Operation Of Reduction Machine (AREA)
  • Heat Treatment Of Steel (AREA)
EP08154615A 2007-04-17 2008-04-16 Plaque d'acier laminée à chaud haute résistance et son procédé de fabrication Active EP1990430B1 (fr)

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EP3012341A4 (fr) * 2013-06-19 2017-02-22 Baoshan Iron & Steel Co., Ltd. Tôle d'acier présentant une résistance à la fissuration induite par le zinc et son procédé de production
EP2690183B1 (fr) 2012-07-27 2017-06-28 ThyssenKrupp Steel Europe AG Produit plat en acier laminé à chaud et son procédé de fabrication

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EP2599887B1 (fr) 2010-07-28 2021-12-01 Nippon Steel Corporation Tôle en acier laminée à chaud, tôle en acier laminée à froid et tôle en acier galvanisée
ES2665982T3 (es) * 2011-03-28 2018-04-30 Nippon Steel & Sumitomo Metal Corporation Lámina de acero laminada en frío y su procedimiento de producción
JP5459441B2 (ja) * 2011-04-13 2014-04-02 新日鐵住金株式会社 熱延鋼板及びその製造方法
KR101540877B1 (ko) 2011-04-13 2015-07-30 신닛테츠스미킨 카부시키카이샤 가스 연질화용 열연 강판 및 그 제조 방법
US9631265B2 (en) 2011-05-25 2017-04-25 Nippon Steel Hot-rolled steel sheet and method for producing same
JP5440672B2 (ja) * 2011-09-16 2014-03-12 Ｊｆｅスチール株式会社 加工性に優れた高強度鋼板およびその製造方法
JP6132017B2 (ja) 2013-05-14 2017-05-24 新日鐵住金株式会社 熱延鋼板およびその製造方法
WO2016079565A1 (fr) * 2014-11-18 2016-05-26 Arcelormittal Procédé de fabrication d'un produit en acier haute résistance et produit en acier ainsi obtenu
JP7217274B2 (ja) * 2018-06-29 2023-02-02 東洋鋼鈑株式会社 熱延鋼板、高強度冷延鋼板およびそれらの製造方法
KR102164078B1 (ko) * 2018-12-18 2020-10-13 주식회사 포스코 성형성이 우수한 고강도 열연강판 및 그 제조방법
JP7226458B2 (ja) * 2020-01-23 2023-02-21 Ｊｆｅスチール株式会社 高強度熱延鋼板の製造方法
CN111621624B (zh) * 2020-05-11 2021-10-22 北京交通大学 提高中锰钢耐氢致延迟断裂性能的工艺方法

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Publication number Priority date Publication date Assignee Title
EP2690183B1 (fr) 2012-07-27 2017-06-28 ThyssenKrupp Steel Europe AG Produit plat en acier laminé à chaud et son procédé de fabrication
EP3012341A4 (fr) * 2013-06-19 2017-02-22 Baoshan Iron & Steel Co., Ltd. Tôle d'acier présentant une résistance à la fissuration induite par le zinc et son procédé de production

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JP2008266695A (ja) 2008-11-06
KR20080093883A (ko) 2008-10-22
EP1990430B1 (fr) 2011-06-15
JP5214905B2 (ja) 2013-06-19
CN101319295A (zh) 2008-12-10
CN101319295B (zh) 2012-05-30
US20090223609A1 (en) 2009-09-10
KR101446354B1 (ko) 2014-10-01

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