WO2024057669A1 - Tôle d'acier, élément, et procédés de fabrication associés - Google Patents

Tôle d'acier, élément, et procédés de fabrication associés Download PDF

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WO2024057669A1
WO2024057669A1 PCT/JP2023/024254 JP2023024254W WO2024057669A1 WO 2024057669 A1 WO2024057669 A1 WO 2024057669A1 JP 2023024254 W JP2023024254 W JP 2023024254W WO 2024057669 A1 WO2024057669 A1 WO 2024057669A1
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temperature
steel
cooling
content
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PCT/JP2023/024254
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Japanese (ja)
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三周 知場
英之 木村
佑馬 本田
洋一郎 松井
琴未 野口
秀斗 尾園
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Ｊｆｅスチール株式会社
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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
    • 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
    • C22C38/06—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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to steel plates, members, and methods of manufacturing them. More specifically, the present invention relates to steel plates and members having a tensile strength (TS) of 780 MPa or more and excellent formability and material stability, and methods for manufacturing them. The present invention relates to steel plates and members used in various applications such as automobiles, and methods of manufacturing them.
  • TS tensile strength
  • TRIP steel sheets have been developed in which retained austenite is dispersed in the structure.
  • steel containing C: 0.04 to 0.12%, Si: 0.8 to 2.5%, and Mn: 0.5 to 2.0% is annealed at 300 to 500°C.
  • Austempering carbon distribution accompanying bainite transformation held for 10 to 900 seconds generates 2 to 10% residual ⁇ , resulting in high ductility of TS ⁇ El ⁇ 21000MPa ⁇ % and high stretch flange formability of 70% or more. It is disclosed that a steel plate having the following properties can be obtained.
  • DP steel (Dual Phase steel) has been developed as a steel plate with low YR and excellent ductility that is effective in reducing springback.
  • General DP steel is a multi-phase steel in which martensite is dispersed in a ferrite structure which is the main phase.
  • DP steel has the disadvantage of poor stretch flange formability because cracks are likely to occur due to stress concentration at the interface between ferrite and martensite.
  • Techniques for improving the stretch flange formability of DP steel include, for example, Patent Document 2 and Patent Document 3.
  • the space factor of ferrite is controlled to 5 to 30% and the space factor of martensite to the entire structure is controlled to 50 to 95%, and the average grain size is fine ferrite with an equivalent circle diameter of 3 ⁇ m or less. It is disclosed that ductility and stretch flange formability can be improved by controlling the grain size to martensite having a circular equivalent diameter of 6 ⁇ m or less.
  • the space factor of ferrite is controlled to be 50% or more and the space factor of martensite is controlled to 3 to 30% with respect to the entire structure, and the average crystal grain size of ferrite is controlled to be 10 ⁇ m or less, and the average grain size of martensite is controlled to be 10 ⁇ m or less.
  • a technique has been disclosed in which deterioration in stretch flange formability is suppressed by controlling the crystal grain size to 5 ⁇ m or less.
  • Patent No. 5515623 Japanese Patent Application Publication No. 2008-297609 Patent No. 3936440
  • Patent Document 1 and Patent Document 2 disclose a method for producing a steel plate with excellent ductility and stretch-flange formability, it is necessary to form a large amount of soft phase ferrite, so that the tensile strength is 780 MPa, for example. It is difficult to further increase the strength.
  • Patent Document 3 discloses a method for manufacturing a DP steel sheet that has low YR and excellent ductility and stretch-flange formability, but since it has a DP structure, the area ratio of martensitic structure is As ductility increases, ductility decreases, and fractures were observed at bending ridges where ductility is required for forming difficult-to-form parts such as center pillars, making it clear that ductility is not always sufficient.
  • the present invention provides a steel plate and member having a tensile strength (TS) of 780 MPa or more, excellent press formability, ductility and stretch flange formability, and excellent material stability in the width direction.
  • TS tensile strength
  • the tensile strength refers to tensile strength (TS) obtained in accordance with JIS Z2241 (2011).
  • "Excellent press formability” means that the yield ratio YR obtained according to JIS Z2241 (2011) is 0.8 or less.
  • Excellent ductility means that the total elongation EL obtained in accordance with JIS Z2241 (2011) satisfies any of the following (A) to (C).
  • the positions in the board width direction are W/24, 2W/24, 3W/24, 4W/24, 5W/24, 6W/24, 7W/24, 8W/24.
  • measurement positions X are defined as measurement positions X.
  • the present inventors investigated various factors affecting press formability, ductility, stretch-flange formability, and material stability for various thin steel sheets having a tensile strength of 780 MPa or more.
  • the area ratio of polygonal ferrite is 10% or more and 80% or less, upper bainite, tempered martensite, and lower part.
  • the total area ratio of bainite is 10% or more and 70% or less, the area ratio of retained austenite (residual ⁇ ) is 3% or more and 15% or less, and the area ratio of quenched martensite is 15% or less (including 0%). Then, with respect to the total area of quenched martensite and residual ⁇ , the total area ratio (space factor) of quenched martensite and residual ⁇ with an aspect ratio of 3 or less and an equivalent circle diameter of 2.0 ⁇ m or more is 20 % or less, and by creating a steel structure in which the area ratio of the C-enriched region (SC ⁇ 0.5 ) with respect to the entire structure is 20% or less with respect to the entire structure, excellent press formability is achieved. It has been found that a high-strength cold-rolled steel sheet having good ductility and stretch-flange formability and excellent material stability in the sheet width direction (with small material variations) can be obtained.
  • the present invention was made based on the above findings, and the gist thereof is as follows. [1] In mass%, C: 0.05-0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol.
  • Al 1.00% or less
  • N Contains less than 0.015%
  • it has a steel structure consisting of a residual structure, The total area ratio of hardened martensite and retained austenite having an aspect ratio of 3 or less and an equivalent circle diameter of 2.0 ⁇ m or more is 20% or less with respect to the total area ratio of hardened martensite and retained austenite, A steel plate in which the area ratio of C-enriched regions (SC ⁇ 0.5 ) in which the C concentration is 0.5 mass% or more relative to the entire structure is 20% or less.
  • the component composition further includes, in mass%, Ti: 0.1% or less, B: 0.01% or less, The steel plate according to [1], containing one or two selected from among the above.
  • the component composition further includes, in mass%, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, The steel plate according to [1] or [2], containing one or more selected from the following.
  • the component composition further includes, in mass%, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less,
  • [6] A member using the steel plate according to any one of [1] to [5].
  • the obtained cold rolled steel plate is A method of manufacturing a steel plate that undergoes annealing,
  • the annealing is A holding step of heating the cold rolled steel plate to an annealing temperature of 750 to 880°C and holding at the annealing temperature for 10 to 500 seconds;
  • a method for producing a steel sheet comprising: a third cooling step in which cooling is performed in a temperature range from the second cooling stop temperature to 50° C. at a third average cooling rate: 0.05 to 1.0° C./s.
  • a steel plate can be obtained that has a high tensile strength TS of 780 MPa or more, has excellent press formability, ductility, and stretch flange formability, and has excellent material stability in the width direction.
  • TS tensile strength
  • ductility ductility
  • stretch flange formability a steel plate
  • material stability in the width direction.
  • the steel plate of the present invention has C: 0.05 to 0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, and P: 0.02% or less in mass %. , S: 0.01% or less, sol.
  • the component composition contains Al: 1.00% or less, N: less than 0.015%, the balance is iron and inevitable impurities, and the area ratio of polygonal ferrite is 10% or more and 80% or less, and upper bainite.
  • the total area ratio of tempered martensite and lower bainite is 10% or more and 70% or less, the volume ratio of retained austenite is 3% or more and 15% or less, and the area ratio of hardened martensite is 15% or less (0%). ), and further has a steel structure consisting of a residual structure, and has an aspect ratio of 3 or less with respect to the total area ratio of hardened martensite and retained austenite, and an equivalent circle diameter of 2.0 ⁇ m or more.
  • the total area ratio of certain quenched martensite and retained austenite is 20% or less, and the area ratio of the C-enriched region (S C ⁇ 0.5 ) where the C concentration is 0.5 mass% or more with respect to the entire structure is 20%. % or less.
  • the steel plate of the present invention will be described below in the order of its component composition and steel structure. First, the reason for limiting the component composition of the present invention will be explained. In the following description, all percentages indicating the components of steel are percentages by mass unless otherwise specified.
  • C is contained from the viewpoint of securing a predetermined strength through transformation strengthening, and from the viewpoint of securing a predetermined amount of retained austenite (residual ⁇ ) to improve ductility. If the C content is less than 0.05%, these effects cannot be sufficiently ensured. On the other hand, when the C content exceeds 0.20%, the martensitic transformation start temperature (Ms point) decreases. As a result, in the third cooling process in which the temperature range from the second cooling stop temperature to 50°C is cooled at a third average cooling rate: 0.05 to 1.0°C/s, martensite transformation and subsequent martensite transformation occur. Tempering is not performed sufficiently.
  • the C content is set to 0.05% or more and 0.20% or less.
  • the C content is preferably 0.08% or more. Further, the C content is preferably 0.18% or less.
  • Si is contained from the viewpoint of strengthening the ferrite and increasing its strength, and from the viewpoint of suppressing the formation of carbides in martensite and bainite to ensure a predetermined amount of residual ⁇ and improving ductility. If the Si content is less than 0.40%, these effects cannot be sufficiently ensured. On the other hand, when the Si content exceeds 1.50%, carbon distribution to untransformed austenite is excessively promoted, and the formation of a C-enriched region of 0.5 mass% or more ( SC ⁇ 0.5 ) is promoted. , stretch flange formability and material stability in the plate width direction decrease. Therefore, the Si content is set to 0.40% or more and 1.50% or less. The Si content is preferably 0.60% or more. Further, the Si content is preferably 1.20% or less.
  • Mn improves the hardenability of steel sheets, promoting high strength through transformation strengthening, and, like Si, suppresses the formation of carbides in bainite and promotes the formation of retained austenite, which contributes to ductility. Contains from the viewpoint of improving. In order to obtain these effects, the Mn content needs to be 1.9% or more. On the other hand, when the Mn content exceeds 3.5%, bainite transformation is significantly delayed, a predetermined amount of retained austenite cannot be secured, and ductility decreases. Moreover, when the Mn content exceeds 3.5%, it becomes difficult to suppress the formation of coarse quenched martensite, and stretch flange formability deteriorates. Therefore, the Mn content is set to 1.9% or more and 3.5% or less. The Mn content is preferably 2.1% or more. Further, the Mn content is preferably 3.3% or less.
  • P is an element that strengthens steel, but if its content is large, it deteriorates spot weldability. Therefore, the P content is 0.02% or less, preferably 0.01% or less. Note that although it is not necessary to contain P, it is preferable that the P content is 0.001% or more because reducing it to less than 0.001% requires a great deal of cost.
  • S has the effect of improving scale peelability during hot rolling and suppressing nitridation during annealing, but is an element that has an adverse effect on spot weldability, bendability, and hole expandability.
  • the S content is at least 0.01% or less, preferably 0.0020% or less.
  • the S content is preferably 0.0001% or more from the viewpoint of manufacturing costs.
  • the S content is more preferably 0.0005% or more, and still more preferably 0.0015% or more.
  • Al 1.00% or less> Al is contained for the purpose of deoxidizing or obtaining residual ⁇ . sol. Although the lower limit of Al is not particularly specified, in order to perform deoxidation stably, sol.
  • the Al content is preferably 0.005% or more. sol.
  • the Al content is more preferably 0.010% or more, and still more preferably 0.020% or more.
  • sol. If the Al content exceeds 1.00%, a large amount of Al-based coarse inclusions will increase, and stretch flange formability will deteriorate. For this reason, sol. Al content shall be 1.00% or less.
  • N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces stretch flange formability, so it is necessary to limit its content. Therefore, the N content should be less than 0.015%.
  • the N content is preferably 0.010% or less, more preferably 0.005% or less. Note that although it is not necessary to contain N, reducing the N content to less than 0.0001% requires a great deal of cost, so the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs.
  • the N content is more preferably 0.0010% or more, and still more preferably 0.0020% or more.
  • the component composition of the steel sheet in the present invention contains the above-mentioned component elements as basic components, and the remainder includes iron (Fe) and inevitable impurities.
  • the component composition of the steel plate in the present invention has a component composition in which the balance consists of Fe and unavoidable impurities.
  • the composition of the steel sheet of the present invention can appropriately contain one or more optional elements selected from the following (A) to (C).
  • Ti fixes N in steel as TiN, and has the effect of improving hot ductility and the effect of B on improving hardenability. Further, the precipitation of TiC has the effect of making the structure finer. In order to obtain these effects, it is desirable that the Ti content be 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. The Ti content is more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.1%, the rolling load will increase and the ductility will decrease due to an increase in the amount of precipitation strengthening, so if Ti is contained, the Ti content should be 0.1% or less. Preferably, the Ti content is 0.05% or less, more preferably 0.03% or less.
  • B is an element that improves the hardenability of steel, and has the advantage of easily producing tempered martensite and/or bainite with a predetermined area ratio. Therefore, it is preferable that the B content is 0.0005% or more. Further, the B content is more preferably 0.0010% or more. On the other hand, when the B content exceeds 0.01%, the effect not only becomes saturated, but also causes a significant decrease in hot ductility and causes surface defects. Therefore, when B is contained, the B content is set to 0.01% or less. Preferably, the B content is 0.005% or less, more preferably 0.003% or less.
  • Cu improves corrosion resistance in the automotive environment. Further, the corrosion products of Cu coat the surface of the steel sheet, which has the effect of suppressing hydrogen intrusion into the steel sheet.
  • Cu is an element that is mixed in when scrap is used as a raw material, and by allowing Cu to be mixed in, recycled materials can be used as raw materials and manufacturing costs can be reduced. From such a viewpoint, it is preferable to contain Cu in an amount of 0.005% or more, and from the viewpoint of improving delayed fracture resistance, it is more preferable to contain Cu in an amount of 0.05% or more.
  • the Cu content is more preferably 0.10% or more. More preferably, the Cu content is 0.25% or more, even more preferably 0.50% or more. However, if the Cu content becomes too large, surface defects will occur, so when Cu is contained, the Cu content is set to 1% or less.
  • Ni is also an element that has the effect of improving corrosion resistance. Further, Ni has the effect of suppressing the occurrence of surface defects that are likely to occur when Cu is included. For this reason, it is desirable to contain Ni in an amount of 0.01% or more.
  • the Ni content is more preferably 0.04% or more, and even more preferably 0.06% or more.
  • the Ni content is set to 1% or less. Preferably, the Ni content is 0.5% or less, more preferably 0.3% or less.
  • ⁇ Cr 1% or less> Cr can be contained because of its effect of improving the hardenability of steel and suppressing the formation of carbides in martensite and upper/lower bainite.
  • the Cr content is preferably 0.01% or more.
  • the Cr content is more preferably 0.03% or more, and still more preferably 0.06% or more.
  • the Cr content is set to 1% or less.
  • Mo can be contained because of its effect of improving the hardenability of steel and suppressing the formation of carbides in martensite and upper/lower bainite.
  • the Mo content is preferably 0.01% or more.
  • the Mo content is more preferably 0.03% or more, and still more preferably 0.06% or more. More preferably, the Mo content is 0.1% or more, even more preferably 0.2% or more.
  • the Mo content is set to 0.5% or less.
  • V 0.5% or less> V is included because it has the effect of improving the hardenability of steel, suppressing the formation of carbides in martensite and upper/lower bainite, refining the structure, and precipitating carbides to improve delayed fracture resistance. be able to.
  • the V content is preferably 0.003% or more.
  • the V content is more preferably 0.005% or more, and still more preferably 0.010% or more. Even more preferably, the V content is 0.020% or more, even more preferably 0.050% or more. However, if a large amount of V is contained, the castability will be significantly deteriorated, so when V is contained, the V content should be 0.5% or less.
  • the V content is 0.3% or less, more preferably 0.2% or less.
  • Nb can be contained because it has the effect of refining the steel structure and increasing its strength, promoting bainite transformation through grain refinement, improving bendability, and improving delayed fracture resistance.
  • the Nb content is preferably 0.002% or more.
  • the Nb content is more preferably 0.004% or more, and still more preferably 0.010% or more.
  • the Nb content is set to 0.1% or less.
  • the Nb content is 0.05% or less, more preferably 0.03% or less.
  • Mg fixes O as MgO and contributes to improving formability such as bendability. Therefore, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0004% or more, and further preferably 0.0006% or more. On the other hand, if a large amount of Mg is added, the surface quality and bendability are deteriorated, so when Mg is contained, the Mg content is set to 0.0050% or less, and preferably, the Mg content is set to 0.0040% or less.
  • Ca fixes S as CaS and contributes to improving bendability and delayed fracture resistance.
  • the Ca content is preferably 0.0002% or more.
  • the Ca content is more preferably 0.0005% or more, and still more preferably 0.0010% or more.
  • the Ca content should be 0.0050% or less.
  • the Ca content is 0.0040% or less.
  • the Sn content is preferably 0.002% or more.
  • the Sn content is more preferably 0.004% or more, and still more preferably 0.006% or more. More preferably, the Sn content is 0.008% or more, even more preferably 0.010% or more.
  • the Sn content is preferably 0.030% or more, more preferably 0.060% or more.
  • the Sb content is preferably 0.002% or more.
  • the Sb content is more preferably 0.004% or more, and still more preferably 0.006% or more. More preferably, the Sb content is 0.008% or more, even more preferably 0.010% or more.
  • the Sb content is preferably 0.025% or more, more preferably 0.050% or more.
  • REM is an element that suppresses the adverse effects of sulfide on stretch flange formability and improves stretch flange formability by making the shape of sulfide spheroidal.
  • the REM content is preferably 0.0005% or more.
  • the REM content is more preferably 0.0010% or more, and still more preferably 0.0020% or more.
  • the REM content exceeds 0.0050%, the effect of improving stretch flange formability will be saturated, so when REM is contained, the REM content should be 0.0050% or less.
  • REM as used in the present invention refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
  • the REM concentration in the present invention is the total content of one or more elements selected from the above-mentioned REMs.
  • REM is not particularly limited, but preferably contains Sc, Y, Ce, and La.
  • the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. Therefore, when the above-mentioned arbitrary element is included in an amount less than the lower limit value, the above-mentioned arbitrary element is included as an unavoidable impurity.
  • the steel plate of the present invention has a tensile strength (TS) of 780 MPa or more.
  • TS tensile strength
  • the upper limit of the tensile strength is not particularly limited, from the viewpoint of coexistence with other properties, the tensile strength is preferably 1300 MPa or less.
  • the total elongation EL is 16.0% or more when TS: 780 MPa or more, 14.0% or more when TS: 980 MPa or more, and 12.0% or more when TS: 1180 MPa or more is secured. Stability is greatly improved. Since cracking during press molding can be suppressed by ensuring a hole expansion rate ⁇ of 30% or more, ⁇ is set to 30% or more.
  • the measurement position contact points for each width are W/24, 2W/24, 3W/24, 4W/24, 5W/24, 6W/24, 7W/24, 8W/24, 9W/24, 10W/24, 11W.
  • the region A has a length in the sheet width direction of 80% or more of the total sheet width.
  • the deviation of EL in the board width direction is 10% or less with respect to the measured value at the board width center position
  • the deviation of ⁇ in the board width direction is 10% or less with respect to the measured value at the board width center position.
  • the area should be 80% or more of the entire board width area.
  • the range of the unsteady portion is allowed to be up to 20% in total at both ends in the width direction. Because the end of the steel plate comes into contact with other structures during transportation and work processes, the end is not used to ensure quality. Therefore, the usable effective plate width does not reach 100%. Therefore, the effective plate width is preferably less than 100%.
  • the area where the deviation of EL in the sheet width direction is 10% or less of the measured value at the center of the sheet width and the deviation of ⁇ is 10% or less to 80% or more of the entire sheet width, yields are significantly improved. Therefore, in the present invention, the area where the deviation of EL in the board width direction is 10% or less of the measured value at the center of the board width and the deviation of ⁇ is 10% or less is set to be 80% or more of the entire board width region. . Preferably it is 85% or more.
  • a steel plate having a tensile strength of 780 MPa or more is defined as a high-strength steel plate.
  • a steel plate having a yield ratio YR of 0.8 or less is a steel plate having excellent press formability.
  • a steel plate with excellent ductility has a total elongation EL of 16.0% or more when TS: 780 MPa or more, 14.0% or more when TS: 980 MPa or more, and 12.0% or more when TS: 1180 MPa or more.
  • d 0 is the initial hole diameter (mm)
  • d is the hole diameter at the time of crack occurrence (mm)
  • the hole expansion rate ⁇ (%) ⁇ (d - d 0 )/d 0 ⁇ 100
  • the average value of the three points obtained during the test is evaluated as ⁇ .
  • Steel having a ⁇ of 30% or more is judged to have excellent hole expandability and stretch flangeability. Preferably it is 40% or more.
  • the plate width of the steel plate in the present invention is preferably 600 mm or more. Moreover, the plate width of the steel plate in the present invention is preferably 1700 mm or less.
  • the area ratio of polygonal ferrite is 10% or more, and in order to obtain higher ductility, it is preferably 20% or more.
  • the area ratio of the polygonal ferrite should be 80% or less, preferably 75% or less, and more preferably 70%.
  • Total area ratio of upper bainite, tempered martensite, and lower bainite 10% or more and 70% or less>
  • the total area ratio of upper bainite, tempered martensite, and lower bainite is set to 10% or more, and in order to obtain higher strength, it is preferably set to 15% or more.
  • the area ratio is set to 70% or less. More preferably it is 65% or less, still more preferably 60% or less.
  • volume fraction of retained austenite (retained ⁇ ): 3% or more and 15% or less>
  • volume fraction of retained austenite is 3% or more, preferably 5% or more.
  • the retained austenite is set to 15% or less. More preferably it is 13% or less.
  • ⁇ Area ratio of quenched martensite 15% or less (including 0%)> Since the hard quenched martensitic structure lowers ⁇ , it is necessary to suppress its area ratio. In order to obtain the desired ⁇ , the area ratio of quenched martensite is set to 15% or less. In order to obtain ⁇ more stably, the area ratio of hardened martensite is preferably 12% or less, more preferably 10% or less.
  • the steel structure other than the above, it consists of the remainder structure.
  • the area ratio of the remaining tissue is preferably 5% or less.
  • the remaining structure may be carbide or pearlite. These tissues may be determined by SEM observation as described later.
  • the retained austenite becomes a hard martensitic structure due to the TRIP effect during press molding, tensile processing, etc. Therefore, in the present invention, from the viewpoint of stretch flangeability, quenched martensite and retained austenite are controlled together.
  • hardened martensite or retained austenite with a circular equivalent diameter of 2.0 ⁇ m or more is formed, voids are formed at stress concentration areas at the interface with other structures, which may deteriorate stretch flange formability.
  • the total area ratio of hardened martensite and retained austenite with an aspect ratio of 3 or less and an equivalent circle diameter of 2.0 ⁇ m or more shall be 20% or less of the total area ratio of hardened martensite and retained austenite.
  • This total area ratio is preferably 18% or less, more preferably 16% or less.
  • the hardness of quenched martensite is determined by the amount of C dissolved in the quenched martensite.
  • the structures in which a large amount of solid solute C exists are quenched martensite and retained austenite.
  • Retained austenite is a structure that contributes to high ductility, and the C concentration is 0.5 mass% or more, but the area ratio of the structure with a C concentration of 0.5 mass% or more is 20% or less of all constituent structures.
  • the area ratio (occupation ratio) of the C-enriched region (SC ⁇ 0.5 ) where the C concentration is 0.5 mass% or more is 20% or less.
  • This area ratio is preferably 15% or less, more preferably 12% or less.
  • this area ratio is preferably 3% or more, more preferably 5% or more.
  • polygonal ferrite To measure the area ratio of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and hardened martensite (fresh martensite), cut out a cross section parallel to the rolling direction, mirror polish it, and then use 1 vol% nital. Corroded, 10 fields of view were observed at 1/4 thickness using SEM at 5000x magnification, and the photographed tissue photographs were quantified by image analysis.
  • Polygonal ferrite is a relatively equiaxed ferrite with almost no carbide inside. This is the area that appears blackest in the SEM.
  • Upper bainite is a ferritic structure with the formation of carbides or retained austenite that appear white under SEM.
  • the area of ferrite with an aspect ratio ⁇ 2.0 is classified as polygonal ferrite, and the area with an aspect ratio >2.0 is classified as upper bainite, and the area ratio is calculated.
  • the aspect ratio is determined by determining the major axis length a where the particle length is the longest, and setting the particle length that crosses the particle longest in the direction perpendicular to it to be the minor axis length b, and a/b is the aspect ratio. Take the ratio.
  • the tempered martensite and lower bainite are regions with a lath-like substructure and carbide precipitation in the SEM.
  • Quenched martensite (fresh martensite) is a massive region that appears white with no underlying structure visible in the SEM.
  • the residual structure is a carbide and/or pearlite structure, and is a structure that can be confirmed by white contrast in SEM.
  • Carbide has a structure with a particle size of 1 ⁇ m or less, and pearlite has a lamellar structure, so they can be distinguished from each other.
  • the quantitative evaluation of the structure described above and the measurement of the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be performed using image analysis software such as Image J (Fiji).
  • image analysis software such as Image J (Fiji).
  • a cross-section of the plate parallel to the rolling direction was cut out, polished to a mirror surface, corroded with 1 vol% nital, and observed at 1/4 thickness position with an SEM at 5000x magnification for 10 fields of view, and machine learning using Image J (Fiji) was performed.
  • Each tissue can be identified and quantitatively evaluated using the Trainable Weka segmentation method that allows area identification.
  • the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be measured using a particle analysis program that is also a function of Image J, and only the quenched martensite and retained austenite identified as above can be extracted and measured. do.
  • the volume fraction of retained austenite is determined by chemically polishing a 1/4 thickness position from the surface layer and using X-ray diffraction.
  • a Co-K ⁇ ray source is used for incident X-rays, and the volume of retained austenite is determined from the intensity ratio of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite. Calculate the rate.
  • the volume fraction of the retained austenite determined by X-ray diffraction can be taken as the area fraction of the retained austenite.
  • the area ratio of the C-enriched region where the C concentration is 0.5 mass% or more is measured using a JEOL field emission electron A line microanalyzer (FE-EPMA) JXA-8500F is used. Then, the C concentration distribution is measured by mapping analysis using an accelerating voltage of 6 kV, an irradiation current of 7 ⁇ 10 ⁇ 8 A, and a beam diameter of the minimum, and an area ratio at which the C concentration is 0.5 mass% or more is calculated. However, in order to eliminate the influence of contamination, background components are subtracted so that the average value of C obtained in the analysis is equal to the carbon content of the base material.
  • FE-EPMA JEOL field emission electron A line microanalyzer
  • the increased amount is considered to be contamination, and the true value at each location is calculated by uniformly subtracting that increased amount from the analysis value at each location. Let the amount of C be .
  • the method for producing a steel plate of the present invention involves hot rolling, pickling and cold rolling a steel slab having the above-mentioned composition, and then annealing the obtained cold rolled steel plate.
  • This is a manufacturing method, and the annealing includes a holding step of heating the cold rolled steel sheet to an annealing temperature of 750 to 880°C and holding at the annealing temperature for 10 to 500 seconds, and a holding step of heating the cold rolled steel plate to an annealing temperature of 350 to 550°C from the annealing temperature.
  • a first cooling step in which the temperature range up to the first cooling stop temperature is set at a first average cooling rate of 2 to 50°C/s to the above first cooling stop temperature, and a residence temperature of 350 to 550°C for 10 seconds or more for 60 seconds.
  • a second cooling step in which cooling is performed at a second average cooling rate of 3 to 50°C/s to a second cooling stop temperature of 150 to 360°C, and and a third cooling step in which the temperature range is cooled at a third average cooling rate: 0.05 to 1.0° C./s.
  • Hot rolling steel slabs include rolling the slab after heating, directly rolling the slab after continuous casting without heating it, and rolling after subjecting the slab after continuous casting to a short heat treatment. and so on.
  • Hot rolling may be carried out according to a conventional method, for example, the slab heating temperature is 1100 to 1300°C, the soaking temperature is 20 to 300 min, the finish rolling temperature is Ar 3 transformation point to Ar 3 transformation point + 200°C, and rolling The temperature may be 400 to 720°C.
  • the winding temperature is preferably 430 to 530° C. from the viewpoint of suppressing plate thickness variations and stably ensuring high strength.
  • the Ar 3 transformation point can be calculated from the composition of the steel plate and the following empirical formula (A).
  • ⁇ Acid washing> Pickling may be carried out according to a conventional method.
  • Cold rolling may be carried out according to a conventional method, and the cumulative rolling ratio may be 30 to 85%. From the viewpoint of stably securing high strength and reducing anisotropy, the rolling ratio is preferably 35 to 85%. Note that when the rolling load is high, it is possible to perform softening annealing treatment at 450 to 730° C. in a CAL (continuous annealing line) or BAF (box annealing furnace).
  • CAL continuous annealing line
  • BAF box annealing furnace
  • a cold rolled steel plate (cold rolled steel plate) manufactured according to a conventional method is annealed under the following conditions.
  • the annealing equipment is not particularly limited, it is preferable to use a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL) from the viewpoint of productivity and ensuring desired heating and cooling rates.
  • CAL continuous annealing line
  • CGL continuous hot-dip galvanizing line
  • the annealing temperature (soaking temperature) exceeds 880°C, the temperature becomes an austenite single phase temperature, the desired polygonal ferrite cannot be obtained, and the YR increases and the ductility decreases. Therefore, the annealing temperature (soaking temperature) is set to 880° C. or lower.
  • the annealing temperature (soaking temperature) is preferably 850°C or lower, more preferably 830°C or lower.
  • the time for holding at the above annealing temperature is less than 10 seconds, austenite will not be formed sufficiently at the above annealing temperature (soaking temperature), and polygonal ferrite will increase, resulting in less than the specified amount.
  • Upper bainite, tempered martensite, and lower bainite cannot be obtained, and not only the desired strength cannot be obtained, but also sufficient residual austenite cannot be obtained, and desired ductility cannot be secured.
  • the time for holding at the above annealing temperature (soaking time) exceeds 500 seconds, the structure will significantly coarsen, making it impossible to secure the desired strength. Therefore, the time for holding at the above annealing temperature (soaking time) is set to 10 to 500 seconds.
  • the time for holding at the annealing temperature is preferably 80 seconds or more, more preferably 100 seconds or more. Further, the time for holding at the annealing temperature (soaking time) is preferably 400 seconds or less, more preferably 300 seconds or less.
  • First cooling step cooling the temperature range from the annealing temperature to the first cooling stop temperature of 350 to 550°C to the first cooling stop temperature at a first average cooling rate of 2 to 50°C/s]
  • the temperature range from the above annealing temperature to the first cooling stop temperature of 350 to 550°C is set at a first average cooling rate of 2 to 50°C/s. Cool it down. If the cooling rate is less than 2°C/s, operability will deteriorate, so the first average cooling rate is set to 2°C/s or more.
  • the first average cooling rate is preferably 5°C/s or more.
  • the first average cooling rate becomes too high, the plate shape will deteriorate, so it is set to 50° C./s or less.
  • the first average cooling rate is preferably 40°C/s or less, more preferably less than 30°C/s.
  • the first average cooling rate is "(annealing temperature (°C) - first cooling stop temperature (°C))/cooling time (seconds) from the annealing temperature to the first cooling stop temperature.”
  • a temperature range (retention temperature) below the first cooling stop temperature and from 350° C. to 550° C. upper bainite is formed, a predetermined residual austenite can be obtained, and desired ductility can be obtained.
  • the residence time exceeds 60 seconds, the concentration of C from bainite to lumpy untransformed ⁇ progresses, leading to an increase in the amount of remaining lumpy structure, making it impossible to obtain the desired ⁇ . Therefore, the residence time is set to 10 seconds or more and 60 seconds or less.
  • the second average cooling rate is preferably 5°C/s or more, more preferably 8°C/s or more. If the cooling rate in this temperature range becomes too high, the plate shape will deteriorate, so the cooling rate in this temperature range (second average cooling rate) is set to 50° C./s or less. Preferably it is 40°C/s or less.
  • the second cooling stop temperature exceeds 360° C., the area ratio of tempered martensite or lower bainite does not reach the predetermined area ratio, the area ratio of hardened martensite after annealing increases, and stretch flange formability deteriorates. Therefore, the second cooling stop temperature is set to 360° C. or lower.
  • the second cooling stop temperature is set to 150°C or higher.
  • the second average cooling rate is "retention end temperature (°C) - second cooling stop temperature (°C)/cooling time (seconds) from the residence end temperature to the second cooling stop temperature".
  • the cooling rate (third average cooling rate) in the temperature range from the second cooling stop temperature is set to 0.05°C/s or more.
  • the third average cooling rate is preferably 0.08°C/s or more, more preferably 0.10°C/s or more.
  • the third average cooling rate is preferably 0.80°C/s or less, more preferably 0.60°C/s or less.
  • the third average cooling rate is "second cooling stop temperature (°C) - 50°C/cooling time (seconds) from the second cooling stop temperature (°C) to 50°C".
  • the surface of the steel sheet may be galvanized to obtain a steel sheet having a galvanized layer on the surface.
  • the type of plating treatment is not particularly limited, and may be either hot-dip galvanizing or electrogalvanizing.
  • the alloying hot-dip galvanizing treatment a plating treatment in which alloying is performed after hot-dip galvanizing may be performed. Hot-dip galvanizing is used for automobile steel sheets and the like.
  • the steel sheet When applying hot-dip galvanizing, the steel sheet is immersed in a hot-dip galvanizing bath in a continuous annealing furnace at the front stage of the continuous hot-dip galvanizing line after the above-mentioned annealing holding step and first cooling step to form a hot-dip galvanized layer on the surface of the steel sheet. It is sufficient to form an alloyed galvanized steel sheet by subsequently performing an alloying treatment.
  • hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment can be performed on the surface of the steel sheet.
  • the soaking and cooling steps and the plating step described above may be performed in separate lines.
  • electrogalvanizing can be performed after annealing, that is, after the third cooling step.
  • the thickness of the steel plate of the present invention obtained as described above is preferably 0.5 mm or more. Further, the thickness of the steel plate of the present invention is preferably 2.0 mm or less. Further, the plate width is preferably 600 mm or more. Moreover, it is preferable that the plate width of the steel plate of the present invention is 1700 mm or less.
  • the member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and bonding. Further, the method for manufacturing the member of the present invention includes the step of subjecting the steel plate of the present invention to at least one of forming and bonding to produce a member.
  • the steel sheet of the present invention has a tensile strength of 780 MPa or more, excellent press formability, ductility, and stretch flange formability, and excellent material stability in the sheet width direction. Therefore, members obtained using the steel sheet of the present invention also have high strength, excellent press formability, ductility, and stretch flange formability, and excellent material stability in the sheet width direction. Furthermore, by using the member of the present invention, it is possible to reduce the weight. Therefore, the member of the present invention can be suitably used for, for example, vehicle body frame parts.
  • the members of the invention also include welded joints.
  • general processing methods such as press working can be used without restriction.
  • general welding such as spot welding and arc welding, rivet joining, caulking joining, etc. can be used without limitation.
  • a slab manufactured by continuous casting having the composition shown in Table 1 was heated to 1200°C, and the soaking time was 200 min.
  • Table 2 shows a cold-rolled steel sheet with a thickness of 1.4 mm manufactured by cold rolling at a rolling ratio of 50% after a hot rolling process with a finish rolling temperature of 900°C and a coiling temperature of 550°C.
  • a steel plate of the present invention and a steel plate of a comparative example were manufactured by processing under the annealing conditions shown in . The width of all the obtained steel plates was 1500 mm.
  • a steel plate was immersed in a galvanizing bath at a temperature of 440° C. or higher and 500° C. or lower to perform hot-dip galvanizing treatment, and then the amount of plating deposited was adjusted by gas wiping or the like.
  • a galvanizing bath having an Al content of 0.10% or more and 0.22% or less was used for the hot-dip galvanizing.
  • hot-dip galvanized steel sheets were subjected to alloying treatment after the hot-dip galvanizing treatment to obtain alloyed hot-dip galvanized steel sheets (GA).
  • alloying treatment was performed in a temperature range of 460° C. or higher and 550° C. or lower.
  • steel plates cold rolled steel plates: CR
  • EG electrogalvanized steel plates
  • the steel structure was measured using the following method. The measurement results are shown in Table 3. To measure the area ratio of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and hardened martensite (fresh martensite), cut out a cross section parallel to the rolling direction, mirror polish it, and then use 1 vol% nital. Corroded, 10 fields of view were observed at 1/4 thickness using SEM at 5000x magnification, and the photographed tissue photographs were quantified by image analysis. Polygonal ferrite is a relatively equiaxed ferrite with almost no carbide inside. This is the area that appears blackest in the SEM. Upper bainite is a ferritic structure with the formation of carbides or retained austenite that appear white under SEM.
  • the area of ferrite with an aspect ratio ⁇ 2.0 is classified as polygonal ferrite, and the area with an aspect ratio >2.0 is classified as upper bainite, and the area ratio is calculated.
  • the aspect ratio is determined by determining the major axis length a where the particle length is the longest, and setting the particle length that crosses the particle longest in the direction perpendicular to it to be the minor axis length b, and a/b is the aspect ratio. It was compared.
  • the tempered martensite and lower bainite are regions with a lath-like substructure and carbide precipitation in the SEM.
  • Quenched martensite (fresh martensite) is a massive region that appears white with no underlying structure visible in the SEM.
  • the residual structure is a carbide and/or pearlite structure, and is a structure that can be confirmed by white contrast in SEM.
  • Carbide has a structure with a particle size of 1 ⁇ m or less, and pearlite has a lamellar structure, so they can be distinguished from each other.
  • the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be measured using a particle analysis program that is also a function of Image J, and only the quenched martensite and retained austenite identified as above can be extracted and measured. did.
  • the volume fraction of retained austenite was determined by X-ray diffraction after chemically polishing a 1/4 thickness position from the surface layer.
  • a Co-K ⁇ ray source is used for incident X-rays, and the volume of retained austenite is determined from the intensity ratio of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite. calculated the rate.
  • the area ratio of the C-enriched region where the C concentration is 0.5 mass% or more is measured using a JEOL field emission electron A line microanalyzer (FE-EPMA) JXA-8500F was used. Then, the C concentration distribution was measured by mapping analysis using an accelerating voltage of 6 kV, an irradiation current of 7 ⁇ 10 ⁇ 8 A, and a minimum beam diameter, and an area ratio at which the C concentration was 0.5 mass% or more was calculated. However, in order to eliminate the influence of contamination, background components were subtracted so that the average value of C obtained in the analysis was equal to the carbon content of the base material.
  • the increased amount is considered to be contamination, and the true value at each location is calculated by uniformly subtracting that increased amount from the analysis value at each location.
  • the amount of C was set to .
  • d 0 is the initial hole diameter (mm)
  • d is the hole diameter at the time of crack occurrence (mm)
  • the hole expansion rate ⁇ (%) ⁇ (d - d 0 )/d 0 ⁇ 100
  • the material stability evaluation in the board width direction 23 points were evaluated from both board width directions at intervals of 100 mm or less from the board width center position (12W/24 position (W: board width)). (including the width center position), and the EL and ⁇ at each position (measurement position X) were determined. Then, the material stability in the board width direction was evaluated by determining the ratio of the difference between the measured value at the board width center position and each position to the measured value at the center position. Using EL and ⁇ at the center of the board width as a reference, consecutive measurement groups where the difference in EL and ⁇ is 10% or less are defined as areas where the difference in EL and ⁇ is 10% or less, and this area is defined for the entire board width.
  • members obtained by forming, joining, and forming and joining the steel sheets of the invention examples have a high quality. It has high strength, excellent press formability, ductility, stretch flange formability, and material stability in the sheet width direction. It was found that it has excellent stretch flange formability and material stability in the width direction of the plate.

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Abstract

L'invention fournit une tôle d'acier, un élément et des procédés de fabrication associés. La tôle d'acier de l'invention présente une résistance à la traction supérieure ou égale à ７８０ＭＰａ, est dotée d'excellentes propriétés de moulage sous pression, d'endurance et de moulage de bord tombé, et se révèle excellente en termes de stabilité de matériau dans une direction largeur de tôle.　Plus précisément, la tôle d'acier de l'invention présente une composition et une structure selon des plages prédéfinies, le rapport surfacique total d'une martensite trempée et d'une austénite résiduelle de rapport d'aspect inférieur ou égal à 3 et de diamètre de cercle équivalent supérieur ou égal à ２,０μｍ, est inférieur ou égal à 20% pour le rapport surfacique total de martensite trempée et d'austénite résiduelle, et le rapport surfacique d'une région concentrée en C (ＳＣ≧０,５) de concentration en C supérieure ou égale à ０,５% en masse est inférieur ou égal à 20% pour l'ensemble de la structure.
PCT/JP2023/024254 2022-09-15 2023-06-29 Tôle d'acier, élément, et procédés de fabrication associés WO2024057669A1 (fr)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2009096596A1 (fr) * 2008-01-31 2009-08-06 Jfe Steel Corporation Tôle d'acier haute résistance et son procédé de production
WO2017002883A1 (fr) * 2015-06-30 2017-01-05 新日鐵住金株式会社 Tôle d'acier laminée à froid de haute résistance, tôle d'acier galvanisée de haute résistance et tôle d'acier recuite après galvanisation de haute résistance
WO2017150117A1 (fr) * 2016-02-29 2017-09-08 株式会社神戸製鋼所 Tôle en acier à haute résistance et son procédé de fabrication
JP2020100894A (ja) * 2018-12-21 2020-07-02 Ｊｆｅスチール株式会社 薄鋼板およびその製造方法
WO2022019209A1 (fr) * 2020-07-20 2022-01-27 日本製鉄株式会社 Tôle d'acier et son procédé de production

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2009096596A1 (fr) * 2008-01-31 2009-08-06 Jfe Steel Corporation Tôle d'acier haute résistance et son procédé de production
WO2017002883A1 (fr) * 2015-06-30 2017-01-05 新日鐵住金株式会社 Tôle d'acier laminée à froid de haute résistance, tôle d'acier galvanisée de haute résistance et tôle d'acier recuite après galvanisation de haute résistance
WO2017150117A1 (fr) * 2016-02-29 2017-09-08 株式会社神戸製鋼所 Tôle en acier à haute résistance et son procédé de fabrication
JP2020100894A (ja) * 2018-12-21 2020-07-02 Ｊｆｅスチール株式会社 薄鋼板およびその製造方法
WO2022019209A1 (fr) * 2020-07-20 2022-01-27 日本製鉄株式会社 Tôle d'acier et son procédé de production

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