WO2020256010A1 - Tôle d'acier à haute résistance - Google Patents

Tôle d'acier à haute résistance Download PDF

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Publication number
WO2020256010A1
WO2020256010A1 PCT/JP2020/023742 JP2020023742W WO2020256010A1 WO 2020256010 A1 WO2020256010 A1 WO 2020256010A1 JP 2020023742 W JP2020023742 W JP 2020023742W WO 2020256010 A1 WO2020256010 A1 WO 2020256010A1
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steel sheet
ferrite
strength
steel
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PCT/JP2020/023742
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English (en)
Japanese (ja)
Inventor
亜梨紗 池田
正春 亀田
上西　朗弘
裕之 川田
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日本製鉄株式会社
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Priority to JP2021526837A priority Critical patent/JP7092264B2/ja
Publication of WO2020256010A1 publication Critical patent/WO2020256010A1/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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper

Definitions

  • the present invention relates to high-strength steel sheets.
  • the present application claims priority based on Japanese Patent Application No. 2019-112081 filed in Japan on June 17, 2019, the contents of which are incorporated herein by reference.
  • the steel sheet When a steel sheet having a high yield strength and a low ductility is used for an automobile part, the steel sheet may break due to insufficient elongation at the time of an automobile collision. When the steel sheet is broken in this way, the steel sheet cannot sufficiently absorb the impact, and the performance due to the high yield strength may not be sufficiently exhibited.
  • Auto parts may be manufactured by press forming a steel plate.
  • seizure hardening occurs on the steel sheet.
  • Seizure hardening is a phenomenon in which the yield strength of a steel sheet increases due to strain aging, and is also called strain aging hardening.
  • Baking hardening is caused by intrusive elements such as solid solution C and solid solution N in steel. Since DP steel and TRIP steel contain a large amount of C, the amount of seizure hardening (the amount of increase in the yield strength of the steel sheet due to strain aging) is large.
  • the ductility of conventional automobile parts made of DP steel or TRIP steel may decrease even if the yield strength increases due to seizure hardening.
  • a technique for reducing hard phases such as martensite and bainite in the steel structure while ensuring the strength of the steel sheet is being studied.
  • Patent Document 1 discloses a steel sheet in which a small amount of hard phase such as bainite and martensite is suppressed.
  • the steel sheet described in Patent Document 1 contains ferrite as the main phase, contains 2 to 12% of pearlite and 3% or less of martensite in terms of volume fraction, and the balance is composed of a low temperature generated phase.
  • Patent Document 1 has a tensile strength of 440 MPa class, and is insufficient in strength to be applied to a shock absorbing member used in recent automobiles and the like. Further, Patent Document 1 does not mention the ductility of a steel sheet when paint baking is performed after press molding.
  • an object of the present invention is to provide a high-strength steel sheet in which a decrease in ductility due to baking hardening is suppressed.
  • a predetermined amount of Ni and Al are simultaneously contained to form a columnar precipitation phase of Ni—Al (an intermetallic compound of Ni—Al), and a structure composed of ferrite, cementite and / or pearlite is the main phase.
  • the steel plate containing a small amount of the hard phase of bainite and / or martensite has a tensile strength of 590 MPa or more, and the amount of change in elongation in the tensile test is BH-treated (after 2% prestrain is applied, 20 at 170 ° C.). It becomes 4% or less before and after (heat treatment for minutes).
  • B In order to obtain a steel sheet having a steel structure as described above, it is important to control the cooling rate after hot rolling and the subsequent quenching conditions and winding temperature.
  • strain aging hardening is a phenomenon in which carbon adheres to dislocations introduced by processing to stabilize dislocations and increase resistance to dislocation motion, thereby increasing yield strength.
  • the phase transformation from austenite to martensite or bainite involves the formation of dislocations. Therefore, when compared with steel sheets of similar strength, the dislocation density of the steel sheet according to the present invention containing a small amount of bainite and / or martensite is lower than the dislocation density of a conventional steel sheet containing a large amount of martensite and / or bainite. ..
  • the dislocation density of the steel sheet according to the present invention is lower than the dislocation density of the conventional steel sheet.
  • the steel sheet according to the present invention having a low structure fraction of bainite and / or martensite has a small amount of dislocations stabilized by strain age hardening and suppresses a decrease in ductility.
  • the present invention has been made based on the above findings, and the gist thereof is as follows.
  • the high-strength steel sheet according to one aspect of the present invention has a component composition of mass%.
  • C 0.0150% or more and 0.3000% or less
  • Si More than 0% and less than 3.000%
  • Mn 0.050% or more and 3.600% or less
  • P More than 0% and less than 0.030%
  • S More than 0% and less than 0.0200%
  • Al More than 0% and less than 0.0200%
  • Al More500% or more and 5.000% or less
  • N More than 0% and less than 0.0100%
  • Cu 0% or more and 4.800% or less
  • Mo 0% or more and 2.500% or less
  • Ca 0% or more and 0.0200% or less
  • Mg 0% or more and 0.0200% or less
  • REM 0% or more and 0.0200% or less
  • the balance is composed of
  • the high-strength steel sheet according to the above [1] has a component composition of mass%. Cu: 0.050% or more and 4.800% or less, Mo: 0.030% or more and 2.500% or less, Ca: 0.0001% or more and 0.0200% or less, It may contain at least one selected from Mg: 0.0001% or more and 0.0200% or less, and REM: 0.0001% or more and 0.0200% or less.
  • the high-strength steel sheet according to the above [1] or [2] may have a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface.
  • the above aspect of the present invention it is possible to provide a high-strength steel sheet in which a decrease in ductility due to baking hardening is suppressed.
  • the high-strength steel sheet according to the above aspect is suitable for a shock absorbing member that requires collision resistance.
  • a high-strength steel sheet according to an embodiment of the present invention (hereinafter, may be referred to as a high-strength steel sheet or simply a steel sheet according to the present embodiment) will be described.
  • the high-strength steel sheet according to this embodiment has a component composition (chemical composition) and a metal structure described below.
  • the "high-strength steel sheet” means a steel sheet having a tensile strength (TS) of 590 MPa or more.
  • C 0.0150% or more and 0.3000% or less
  • C is an essential element for forming carbides in steel to obtain bainite, martensite and retained austenite. If the C content is less than 0.0150%, the total area ratio of martensite, bainite and retained austenite cannot be 1.0% or more. Therefore, the C content is set to 0.0150% or more. On the other hand, when the C content exceeds 0.3000%, seizure hardening is excessively developed as the amount of solid solution carbon in the steel increases, and the ductility of the steel sheet after baking hardening is significantly deteriorated. Therefore, the C content is set to 0.3000% or less.
  • Si: more than 0% and less than 3.000% Si is an element that thermodynamically stabilizes ferrite, and is a useful element that dissolves in ferrite to develop solid solution hardening.
  • Si is an element that promotes the formation of martensite by inhibiting the formation of carbides. In order to obtain these effects, the Si content is set to more than 0%.
  • Si is also an element that enhances the hardenability of steel. Therefore, if the Si content is too high, the transformation from austenite to ferrite is delayed in the cooling process after hot rolling, and bainite and / or martensite is likely to be formed.
  • the Si content is set to 3.000% or less in order to generate a desired amount of ferrite in the cooling process after hot rolling.
  • Mn 0.050% or more and 3.600% or less
  • Mn is an element that thermodynamically stabilizes austenite. Further, Mn is an element useful for reducing surface defects during hot rolling by S. In order to obtain these effects, the Mn content is set to 0.050% or more.
  • Mn enhances hardenability, if the Mn content is too large, the transformation from austenite to ferrite is delayed in the cooling process after hot rolling, and bainite and / or martensite is likely to be formed.
  • the Mn content is set to 3.600% or less in order to suppress an increase in the area ratio of bainite and / or martensite to obtain a desired steel structure.
  • P More than 0% and less than 0.030%
  • P is an element that easily segregates at grain boundaries. If the P content is too high, embrittlement may be promoted in the high-strength steel sheet and the workability may be lowered. Therefore, the P content is set to 0.030% or less.
  • P is also an element that enhances the strength of the steel sheet by hardening the solid solution. Further, if the P content is excessively reduced, the refining cost increases. Therefore, the P content may be more than 0% or 0.001% or more.
  • S More than 0% and less than 0.0200% S exists as an inclusion in the steel. If the S content exceeds 0.0200%, the amount of inclusions may increase, and the inclusions may become the starting point of fracture during processing of the steel sheet and the formability of the steel sheet may decrease. Therefore, while the S content is 0.0200% or less, if the S content is excessively reduced, the refining cost increases. Therefore, the S content may be more than 0% or 0.0001% or more.
  • Al 0.500% or more and 5.000% or less
  • Al is an element that thermodynamically stabilizes ferrite and also an element that inhibits the formation of carbides in steel.
  • Al is an element that increases the strength of the steel sheet when it is contained at the same time as a predetermined amount of Ni. It is considered that the increase in strength of the steel sheet by simultaneously containing Al and Ni is due to the formation of the columnar precipitation phase (Ni-Al intermetallic compound) of Ni—Al and the solid solution cluster. Further, in order to obtain the effect, it is considered necessary to promote the ferrite transformation after hot rolling. If the Al content is less than 0.500%, a sufficient amount of ferrite is not produced and the above effect cannot be obtained.
  • the Al content is set to 0.500% or more.
  • the Al content exceeds 5.000%, ferrite will be formed during hot rolling.
  • processed austenite and processed ferrite are formed during hot rolling, so that a mixed structure consisting of ferrite formed by transformation of processed austenite during cooling and ferrite formed by recovery and recrystallization of processed ferrite is formed. Will be done.
  • the mechanical properties of such mixed structures fluctuate sharply with their formation rate. Therefore, when the steel structure of the steel sheet has a mixed structure as described above, industrially stable mechanical properties cannot be obtained. Therefore, the Al content is set to 5.000% or less.
  • N More than 0% and less than 0.0100%
  • N is an penetrating element and is an element that contributes to strain age hardening.
  • Al aluminum nitride
  • the N content is set to 0.0100% or less.
  • the N content may be more than 0% or 0.0001% or more.
  • Ni 1.000% or more and 12.300% or less
  • Ni is an element that thermodynamically stabilizes austenite and is also an element that inhibits the formation of carbides in steel.
  • Ni is an element that increases the strength of the steel sheet when it is contained at the same time as a predetermined amount of Al. The reason for this is not clear, but it is presumed that a columnar precipitation phase (Ni-Al intermetallic compound) of Ni and Al and a solid solution cluster are formed. In order to obtain the effect of increasing the strength of the steel sheet by simultaneously containing Ni and Al, the Ni content is set to 1.000% or more.
  • the Ni content exceeds 12.300%, the transformation from austenite to ferrite is delayed in the cooling process after hot rolling, and a large amount of bainite and / or martensite is formed to obtain a desired steel structure. Absent. Therefore, the Ni content is set to 12.300% or less.
  • elements other than the above-mentioned elements that is, the balance is Fe and impurities.
  • Impurities are elements that are inevitably mixed from steel raw materials or scrap, or elements that are inevitably mixed in the steelmaking process, and the high-strength steel sheet according to the present embodiment has the effect of the high-strength steel sheet according to the present embodiment. Elements that are acceptable within the playable range can be exemplified.
  • the total content of impurities is preferably 0.100% or less. Of the impurities, the content of O is preferably 0.010% or less.
  • the high-strength steel sheet according to the present embodiment may contain one or more of the optional elements shown below in place of a part of the remaining Fe.
  • the lower limit of the content of the optional elements is 0%.
  • Cu: 0% or more and 4.800% or less Cu, like Mn and Ni, is an element that thermodynamically stabilizes the austenite phase. In order to surely obtain this effect, it is preferable that the Cu content is 0.050% or more. On the other hand, if the Cu content is too high, a desired amount of ferrite cannot be produced in the cooling process after hot rolling. Therefore, even when it is contained, the Cu content is set to 4.800% or less.
  • Mo: 0% or more and 2.500% or less Mo, like Si and Al, is an element that stabilizes the ferrite phase. In order to surely obtain this effect, the Mo content is preferably 0.030% or more. On the other hand, if the Mo content is too high, a desired amount of ferrite cannot be produced in the cooling process after hot rolling. Therefore, even when it is contained, the Mo content is 2.500% or less.
  • Ca, Mg and REM are all elements that control the shape of inclusions such as oxides and sulfides. Specifically, it is an element that finely disperses inclusions, reduces the factors of the starting point of fracture during processing, and contributes to the improvement of workability of the steel sheet. In order to surely exhibit these effects, it is preferable that the content of even one of Ca, Mg and REM is 0.0001% or more.
  • the contents of Ca, Mg and REM are set to 0.0001% or more and 0.0200% or less, respectively.
  • REM refers to 17 elements from scandium, yttrium, lanthanum to lutetium, and is generally called a rare earth element or rare earth.
  • the above-mentioned content of REM means the total content of these elements.
  • C%, Si%, Mn%, Al%, Ni%, Cu% and Mo% are the contents of C, Si, Mn, Al, Ni, Cu and Mo in mass%, respectively. If not, substitute 0.
  • the above equation (1) is a conditional equation for hot rolling in the single-phase region of austenite without forming ferrite during hot rolling. Further, the formula (1) is also a conditional formula showing an index for improving the strength-ductility balance of the steel sheet by the columnar precipitation phase of Ni—Al and suppressing the decrease in ductility after baking and curing.
  • the former influence on the steel sheet characteristics will be described. If the content of the element that stabilizes ferrite such as Si, Al and Mo is higher than the content of the element that stabilizes austenite such as C, Mn, Ni and Cu, not only austenite but also ferrite will be generated during hot rolling. Will be generated.
  • austenite and ferrite mixed during hot rolling become processed austenite and processed ferrite by hot rolling, respectively, the former is the ferrite generated by transformation during cooling, and the latter is recovery / recrystallization. It becomes ferrite formed by crystals and forms a mixed structure. Since the mechanical properties of such a mixed structure vary greatly depending on their formation ratio, it is not possible to obtain industrially stable mechanical properties. Next, the latter effect on the steel sheet characteristics will be described.
  • the Ni atom that stabilizes austenite is uniformly dispersed in the austenite phase, but the Al atom that stabilizes ferrite segregates at the grain boundaries of austenite rather than being dissolved in the austenite phase.
  • the strength-ductility balance and the decrease in ductility after seizure hardening are suppressed by forming a columnar precipitation phase at the ferrite grain boundaries (including the vicinity) in the steel after hot rolling.
  • a row-like precipitation phase is formed at the ferrite grain boundaries, the deformation resistance near the ferrite grain boundaries increases, and when the steel plate before the seizure hardening treatment is deformed, it is inside the ferrite grains of the steel. It is considered that the growth of dislocations is less likely to accumulate at the ferrite grain boundaries, so that the formation of voids due to deformation is suppressed and the strength-ductility balance is improved.
  • the reason why the ductility after the baking hardening treatment is deteriorated in the conventional steel is that the dislocation mobility is lowered by performing the baking heat treatment in the state where the dislocations are excessively accumulated at the ferrite grain boundaries, and the ferrite grain boundaries are reduced. It is understood that it is possible to suppress the decrease in ductility after the baking hardening treatment by suppressing the excessive accumulation of dislocations on the surface. Therefore, in the high-strength steel sheet according to the present embodiment, in addition to the content of each element being in a predetermined range in the component composition, it is necessary to satisfy the above equation (1).
  • Equation (2) above is a conditional equation for generating ferrite during cooling after hot rolling.
  • the present inventors have shown that the development of high strength by simultaneously containing Ni and Al occurs when a columnar precipitation phase is formed at the ferrite grain boundary after the formation of a ferrite structure having a bcc crystal structure. Is getting.
  • the amount of elements that enhance the hardenability exceeds a certain amount, ferrite is not sufficiently generated during cooling after hot rolling, and the formation of a row-precipitated phase at the ferrite grain boundaries is suppressed, so that the strength of the steel sheet is increased. High ductility cannot be achieved. Therefore, in the component composition of the high-strength steel sheet according to the present embodiment, in addition to the content of each element being in a predetermined range, it is necessary to satisfy the above equation (2).
  • the above formula (3) is a conditional formula that defines the required Ni content from the viewpoint of preventing hot brittleness due to the molten phase of Cu. From the viewpoint of preventing hot brittleness due to Cu, the Cu content is allowed up to 2.0 times the Ni content. When the Ni content is half (0.5 times) or more of the Cu content, hot brittleness can be suppressed. Therefore, the composition of the high-strength steel sheet according to the present embodiment needs to satisfy the above equation (3).
  • the steel structure of the high-strength steel plate according to the present embodiment has a total area ratio of 85.0% or more and 99.0% or less of ferrite, cementite and pearlite, and a total of 1.0% or more and 15.0% or less. Consists of martensite, bainite and retained austenite.
  • the area ratio of each tissue can be measured by the following method. Samples are taken so that the cross section of the thickness parallel to the rolling direction of the steel sheet is the observation surface. A region of 100 ⁇ m ⁇ 100 ⁇ m in the range of t / 8 to 3 t / 8 in the plate thickness direction centered on the position of 1/4 of the plate thickness t (position of t / 4) from the surface of the steel plate on the observation surface. The observation area. This observation area is corroded by repeller etching, and the tissue image of the corroded surface is analyzed using an optical microscope to calculate the area ratio of each tissue.
  • the steel structure of the high-strength steel plate according to the present embodiment is composed of a combination of a plurality of structures among ferrite, cementite, pearlite, bainite, martensite and retained austenite.
  • the captured tissue image can be distinguished into a black part, a gray part, and a white part.
  • the gray part contains ferrite and bainite
  • the black part contains cementite and pearlite
  • the white part contains martensite and retained austenite. Therefore, the white part is regarded as martensite and retained austenite.
  • the lath-like structure is regarded as bainite
  • the structure other than the lath-like structure is regarded as ferrite.
  • the tissue arranged in dots in the black portion is regarded as cementite
  • the tissue in which the black portions are arranged in rows or exist as a mass having a diameter of more than 1.0 ⁇ m is regarded as pearlite.
  • the area ratio of each tissue is obtained by analyzing the tissue images of at least three observation regions and calculating the average value of the area ratio of each tissue.
  • the area ratios of ferrite, cementite and pearlite are 85.0% or more and 99.0% or less in total.
  • the area ratio of ferrite alone in the above structure is preferably 40.0% or more.
  • the structure other than ferrite, cementite and pearlite is one or more of martensite, bainite and retained austenite.
  • the deformed portion and the undeformed portion coexist, and the deformation is locally concentrated in the thin portion where the deformation has progressed, and the plate fracture occurs at an early stage. That is, the ductility of the steel sheet becomes low. Therefore, adding 1.0% or more of martensite, bainite and retained austenite suppresses an excessive increase in yield elongation.
  • these structures contribute to the increase in strength, but as the area ratio increases, the amount of baking hardening also increases, and the ductility of the steel sheet after the coating baking treatment deteriorates. Therefore, the area ratio of martensite, bainite and retained austenite shall be 15.0% or less.
  • a row-like precipitation phase of Ni—Al exists on the grain boundaries of ferrite, and the coverage of the ferrite grain boundaries by the row-like precipitation phase is 5.0% or more of the total grain boundary length. Is present on the ferrite grain boundaries (covers the ferrite grain boundaries), and the coverage of the ferrite grain boundaries by the columnar precipitation phase of Ni—Al is 5.0% or more, which is a high value according to the present embodiment. With a strong steel plate, high strength of 590 MPa or more can be obtained even if the area ratio of structures such as bainite and martensite is small.
  • the upper limit of the coverage is not particularly limited, but is preferably 90.0% or less in terms of suppressing an excessive increase in yield elongation.
  • Ni-Al intermetallic compounds having a rod-like shape having a diameter of 20 nm or less and a length of 500 nm or less are precipitated in rows or dots, and the distance between adjacent Ni-Al intermetallic compounds.
  • the massive structure dispersed so that the thickness is within 100 nm is defined as one row-like precipitation phase.
  • a material for observation is cut out from a steel plate using a precision cutting machine, and this is cut and polished with emery paper to a thickness of 1/4 of the plate thickness direction, which is the observation position, and the material thickness is adjusted to 0.1 mm.
  • a sample for TEM observation is prepared by performing double-sided jet electropolishing on a sample punched from a material having a diameter of 3 mm by a punching punch.
  • a region of 4.0 ⁇ m ⁇ 4.0 ⁇ m was randomly selected, and Ni-Al was analyzed by elemental analysis by EDS and crystal structure analysis by nanobeam diffraction (NBD / Nano Beam Diffraction) method.
  • the location of the metallographic compound in the metal is identified, and a TEM photograph is taken so that the location is included.
  • the grain boundary length of ferrite in contact with the columnar precipitation phase of Ni—Al is divided by the total grain boundary length of ferrite to cover the ferrite grain boundary. Find the rate.
  • the length of the ferrite grain boundaries in contact with the columnar precipitation phase is determined by the following procedure by image analysis. In the columnar precipitation phase (region where the intermetallic compound is dispersed and precipitated) existing in one ferrite grain, a circle with a radius of 100 nm is drawn around each of the Ni—Al intermetallic compounds located at the outermost periphery thereof.
  • the range obtained by connecting these circles is defined as the boundary region of the columnar precipitation phase, and the length of the ferrite grain boundaries contained in this boundary region is defined as the boundary region of the columnar precipitation phase.
  • the ferrite grain boundary is a boundary in which the crystal orientation difference is 15 ° or more in the ferrite phase having a body-centered cubic lattice crystal structure, and the crystal is crystallized by the EBSD (Electron Backscattered Diffraction Pattern) method or TEM electron diffraction pattern analysis. It can be obtained from the identification of the orientation.
  • the high-strength steel sheet according to the present embodiment may be provided with a plating layer on its surface for the purpose of improving corrosion resistance and the like. That is, the high-strength steel plate according to the present embodiment may include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
  • the amount of adhesion of the plating layer is not particularly limited, and may be a general amount of adhesion.
  • a steel piece having the above-mentioned composition is produced.
  • the method for producing this steel piece is not particularly limited.
  • the steel pieces may be produced by a general method such as continuous casting or thin slab casters.
  • the obtained steel piece is used as it is, or once the temperature of the steel sheet is lowered to room temperature, the steel piece is heated.
  • the heating temperature at this time is preferably 900 ° C. or higher and 1300 ° C. or lower. If the heating temperature is less than 900 ° C., the austenite transformation cannot be sufficiently performed, the solution of the precipitate becomes insufficient, and the desired strength may not be obtained. On the other hand, when the heating temperature exceeds 1300 ° C., the amount of scale generated increases, and the surface flaws associated therewith may increase.
  • the heating time in the temperature range of 900 ° C. or higher and 1300 ° C. or lower is preferably 60 minutes or longer in order to sufficiently transform austenite. Moreover, the heating time is preferably 240 minutes or less from the viewpoint of suppressing scale formation.
  • the final rolling temperature is set to 750 ° C. or higher.
  • the processed ferrite is in a state where high-density dislocations are left in the ferrite grains, and the above-mentioned columnar precipitation phase of Ni—Al is preferentially generated on the dislocations in the ferrite grains rather than the ferrite grain boundaries. Therefore, when processed ferrite is generated, the formation of a columnar precipitation phase of Ni—Al at the ferrite grain boundaries is suppressed. Therefore, the final rolling temperature is set to 750 ° C. or higher. The final rolling temperature is the surface temperature of the steel sheet on the exit side of the finish rolling mill.
  • This cooling step includes a first cooling step of cooling so that the average cooling rate in a temperature range of 550 ° C. or higher and 750 ° C. or lower is 1 ° C./sec or higher and 20 ° C./sec or lower, and a temperature range of 550 ° C. or lower and 300 ° C. or higher. It consists of a second cooling step of cooling to 300 ° C. or lower so that the average cooling rate in the above is 30 ° C./sec or more and 300 ° C./sec or less.
  • the average cooling rate in the first cooling step is a value obtained by dividing the temperature drop width from 750 ° C. to 550 ° C. by the time required for cooling from 750 ° C.
  • the average cooling rate in the second cooling step is a value obtained by dividing the temperature drop width from 550 ° C. to 300 ° C. by the time required for cooling from 550 ° C. to 300 ° C.
  • the average cooling rate in the first cooling step and the second cooling step is adjusted, for example, by adjusting the amount of water injected from the nozzle toward the steel plate on the cooling floor.
  • the average cooling rate in the temperature range of 550 ° C. or higher and 750 ° C. or lower is 1 ° C./sec or more and 20 ° C./sec or lower.
  • the average cooling rate in the above temperature range is preferably 1 ° C./sec or more.
  • the steel sheet is cooled to 300 ° C. or lower so that the average cooling rate in the temperature range of 300 ° C. or higher and 550 ° C. or lower is 30 ° C./sec or more and 300 ° C./sec or lower. Then, the steel sheet is wound at 300 ° C. or lower.
  • austenite that has not been transformed up to 550 ° C. is transformed into bainite and / or martensite.
  • the average cooling rate in the temperature range of 300 ° C. or higher and 550 ° C. or lower is set to 30 ° C./sec or higher. If the average cooling rate in the above temperature range exceeds 300 ° C., uneven cooling is likely to occur, and the shape of the steel sheet may deteriorate. Therefore, the average cooling rate in the above temperature range is preferably 300 ° C./sec or less.
  • the winding temperature of the steel sheet is preferably 50 ° C. or higher and 300 ° C. or lower.
  • the reason for this is that it is possible to generate fine Ni—Al columnar precipitation phases at the ferrite grain boundaries, improve the strength-ductility balance of the hot-rolled steel sheet, and remarkably obtain the effect of suppressing deterioration of ductility after baking and curing. Because it can be done.
  • the grain size of the ferrite grains generated in the austenite grains in the above-mentioned first cooling step increases during the first cooling step, the movement of the austenite / ferrite interface at this time segregates Al atoms and the like. Pinned at the austenite grain boundary.
  • a hot-dip galvanized steel sheet is obtained by forming a hot-dip galvanized layer on the high-strength steel sheet according to the present embodiment, the steel sheet wound as described above is rewound, then descaled, and then hot-dip galvanized. Should be applied. Further, when a hot-dip galvanized steel sheet is obtained by forming an alloyed hot-dip galvanized layer, the hot-dip galvanized steel sheet may be alloyed.
  • the linear precipitation phase of Ni—Al produced by hot rolling has high thermal stability, and the columnar precipitation phase formed at the ferrite grain boundaries is not completely melted unless it is heated to 1000 ° C. or higher. When heated to a temperature of 1000 ° C.
  • the austenite grains become abnormally coarse, and even if the steel sheet is subsequently given a cooling history within the range shown in the hot spreading conditions of the present invention, the structure morphology shown in the present invention cannot be obtained. Therefore, in order to obtain the structure of the present invention even after the plating treatment, when the steel sheet before the plating treatment is heated, the temperature is limited to the temperature range of 400 ° C. or higher and 730 ° C. or lower, which is the ferrite region. There is a need to.
  • the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention.
  • the present invention is not limited to this one-condition example.
  • the present invention may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
  • Example 1 Steels having the component compositions shown in Tables 1A and 1B were melted, cast, and solidified to obtain steel pieces. Then, the steel piece is hot-rolled as it is, or once cooled to room temperature, heated at 1050 ° C or higher and 1300 ° C or lower for 60 minutes or longer and 240 minutes or shorter, and hot-rolled under the conditions shown in Tables 2A and 2B. Manufactured steel plate. The thickness of the hot-rolled steel sheet was 3.2 mm. In Tables 1A and 1B, the values on the left sides of the above equations (1), (2) and (3) are also shown.
  • Yield strength (YP) and tensile (maximum) strength (TS) are obtained by collecting test pieces from the manufactured hot-rolled steel sheet so that the direction perpendicular to the rolling direction is the longitudinal direction and conducting a tensile test. , Elongation (El) was measured.
  • the yield strength, tensile strength and elongation were measured using the No. 5 test piece described in JIS Z 2241: 2011 according to the method described in JIS Z 2241: 2011.
  • TS tensile strength
  • TS ⁇ El (MPa ⁇ %) was determined from the obtained tensile strength (TS) and elongation (El), and the strength-ductility balance was evaluated. When TS ⁇ El was 15,000 MPa ⁇ % or more, it was judged that the strength-ductility balance was excellent. Further, after applying 2% tensile deformation to the No. 5 test piece collected from the hot-rolled steel sheet so that the direction perpendicular to the rolling direction is the longitudinal direction, a strain aging hardening treatment at 170 ° C. for 20 minutes (hereinafter, hereinafter, BH treatment) was applied. This strain age hardening treatment was performed to simulate the coating baking treatment.
  • BH treatment strain aging hardening treatment
  • the tensile strength (TS), yield strength (YP) and elongation (El) after the BH treatment were measured by carrying out a tensile test according to the method described in JIS Z 2241: 2011.
  • TS tensile strength
  • YiP yield strength
  • El elongation
  • the composition of the steel was in a preferable range, but the average cooling rate at 550 to 300 ° C. was small, so that the desired amounts of martensite, bainite and retained austenite could not be obtained, and the ductile deterioration was large.
  • the composition of the steel was in a preferable range, but the winding temperature was low, the proportion of ferrite in the steel structure decreased, and at the same time, the coverage of ferrite grain boundaries due to the columnar precipitation phase of Ni—Al decreased. Therefore, the ductile deterioration was large. No. In No. In No. 31, the composition of the steel was in a preferable range, but the average cooling rate at 550 to 300 ° C. was small, so that the desired amounts of martensite, bainite and retained austenite could not be obtained, and the ductile deterioration was large.
  • the composition of the steel was in a preferable range, but the winding temperature was low, the proportion of ferrite in the steel structure decreased, and at the
  • Example 2 The hot-dip galvanized steel sheets of sample codes 1 and 8 produced in Example 1 were heated to 660 to 720 ° C. and subjected to hot-dip galvanizing treatment to obtain hot-dip galvanized steel sheets.
  • Sample code 8 was obtained as an alloyed hot-dip galvanized steel sheet by performing an alloying treatment at 540 to 580 ° C. after the hot-dip galvanizing treatment.
  • the obtained hot-dip galvanized steel sheet (sample code 1) and alloyed hot-dip galvanized steel sheet (sample code 8) were subjected to microstructure observation and evaluation of mechanical properties in the same manner as in Example 1. The results are shown in Table 3.
  • the high-strength steel sheet according to the above aspect is suitable for a shock absorbing member that requires collision resistance.
  • fuel efficiency is improved by reducing the weight of the vehicle body and collision safety is achieved. Since it can contribute to the further improvement of the industrial value, it has extremely high industrial utility value.

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Abstract

La présente invention concerne une tôle d'acier à haute résistance comprenant une composition spécifiée de composants, les teneurs, en termes de % en masse, de C, Si, Mn, Al, Ni, Cu et Mo satisfaisant à une relation spécifiée, la structure d'acier étant composée, en termes de pourcentages de superficie, de ferrite, de cémentite et de perlite dans une superficie totale de 85,0 à 99,0 % inclus, et de martensite, de bainite, et d'austénite résiduelle, dans une superficie totale de 1,0 à 15,0 % inclus, une phase de précipité de rangée Ni-Al étant présente sur un joint de grains de ferrite, le rapport de couverture du joint de grains de ferrite et de la phase de précipité de rangées étant de 5,0 % ou plus de la longueur totale de joint de grains, et la résistance à la traction de la tôle d'acier est de 590 MPa ou plus.
PCT/JP2020/023742 2019-06-17 2020-06-17 Tôle d'acier à haute résistance WO2020256010A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000050658A1 (fr) * 1999-02-22 2000-08-31 Nippon Steel Corporation Plaque d'acier galvanise a haute resistance, d'excellent comportement pour l'adhesion des placages de metal et la mise en forme sous presse, et plaque d'acier allie galvanise a haute resistance, et procede de production correspondant
JP2006193789A (ja) * 2005-01-14 2006-07-27 Nisshin Steel Co Ltd 熱処理強化型高強度フェライト系ステンレス鋼及びその製造方法
JP2016028174A (ja) * 2014-07-08 2016-02-25 新日鐵住金株式会社 伸びフランジ性と打ち抜き性に優れた高強度熱延鋼板及び溶融亜鉛めっき高強度熱延鋼板とそれらの製造方法

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000050658A1 (fr) * 1999-02-22 2000-08-31 Nippon Steel Corporation Plaque d'acier galvanise a haute resistance, d'excellent comportement pour l'adhesion des placages de metal et la mise en forme sous presse, et plaque d'acier allie galvanise a haute resistance, et procede de production correspondant
JP2006193789A (ja) * 2005-01-14 2006-07-27 Nisshin Steel Co Ltd 熱処理強化型高強度フェライト系ステンレス鋼及びその製造方法
JP2016028174A (ja) * 2014-07-08 2016-02-25 新日鐵住金株式会社 伸びフランジ性と打ち抜き性に優れた高強度熱延鋼板及び溶融亜鉛めっき高強度熱延鋼板とそれらの製造方法

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