WO2023181642A1 - Tôle d'acier à haute résistance et son procédé de production - Google Patents

Tôle d'acier à haute résistance et son procédé de production Download PDF

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WO2023181642A1
WO2023181642A1 PCT/JP2023/002916 JP2023002916W WO2023181642A1 WO 2023181642 A1 WO2023181642 A1 WO 2023181642A1 JP 2023002916 W JP2023002916 W JP 2023002916W WO 2023181642 A1 WO2023181642 A1 WO 2023181642A1
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temperature
steel plate
content
strength steel
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PCT/JP2023/002916
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English (en)
Japanese (ja)
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潤也 戸畑
勇樹 田路
秀和 南
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Ｊｆｅスチール株式会社
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Priority to JP2023528943A priority Critical patent/JP7323096B1/ja
Publication of WO2023181642A1 publication Critical patent/WO2023181642A1/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
    • 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 a high-strength steel plate that is excellent in tensile strength, bendability, flatness in the width direction, and resistance to work embrittlement, and a method for manufacturing the same.
  • the high-strength steel sheet of the present invention can be suitably used as a structural member for automobile parts and the like.
  • High-strength steel sheets used in automobiles are required to have excellent bendability.
  • a part such as a bumper that has a portion bent by roll forming
  • high-strength steel sheets used in automobiles are required to have excellent resistance to mechanical embrittlement from the viewpoint of component performance.
  • frame parts such as automobile bumpers
  • high-strength steel plates that have excellent resistance to mechanical embrittlement and do not become embrittled during press forming.
  • Patent Document 1 describes that warpage of a steel plate adversely affects operational troubles in a forming line and dimensional accuracy of products.
  • the present inventors found that the dimensional accuracy of a product is affected not only by the warpage of the steel plate but also by the flatness in the width direction of the plate, which is evaluated using steepness.
  • the steepness in the width direction is preferably 0.02 or less.
  • Patent Document 2 provides a high-strength steel plate having a tensile strength of 1100 MPa or more and excellent YR, surface texture, and weldability, and a method for manufacturing the same.
  • the technique described in Patent Document 2 does not take into account flatness in the width direction of the plate and resistance to work embrittlement.
  • Patent Document 3 provides a hot-dip galvanized steel sheet with excellent press formability and low-temperature toughness and a tensile strength of 980 MPa or more, and a method for manufacturing the same.
  • the technique described in Patent Document 3 can improve the embrittlement of the steel plate due to a temperature drop, it does not take into account the embrittlement of the steel plate due to processing. Flexibility and flatness in the width direction of the plate are not considered either.
  • Patent Document 4 provides a high-strength steel plate having a tensile strength of 1320 MPa or more and excellent workability and bendability, and a method for manufacturing the same. However, the technique described in Patent Document 4 does not take into consideration flatness in the width direction of the plate and resistance to work embrittlement.
  • the present invention was developed in view of the above circumstances, and aims to provide a high-strength steel plate having a TS of 1180 MPa or more and excellent bendability, flatness in the width direction, and resistance to work embrittlement, and a method for manufacturing the same. purpose.
  • a TS of 1180 MPa or more can be achieved by setting the amount of martensite to 80% or more in terms of area fraction and the total amount of ferrite and bainitic ferrite to 10% or less in terms of area fraction.
  • Excellent bendability can be achieved by setting the amount of retained austenite to 3% or more in volume fraction.
  • the amount of retained austenite is 15% or less in terms of volume fraction
  • the average value of the occupancy of the packet with the maximum occupancy in the prior austenite grains is 70% or less in terms of area fraction
  • the average grain size of prior austenite is By setting the thickness to 20 ⁇ m or less, excellent resistance to work embrittlement can be achieved.
  • the present invention has been made based on the above findings. That is, the gist of the present invention is as follows. [1] In mass%, C: 0.030% or more and 0.500% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more and 5.00% or less, P: 0. Contains 100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities.
  • the amount of martensite is 80% or more in area fraction
  • the amount of retained austenite is 3% or more and 15% or less in volume fraction
  • the amount of ferrite and bainitic ferrite is The total area fraction is 10% or less
  • the average prior austenite grain size is 20 ⁇ m or less
  • the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains is 70% or less in area fraction.
  • the component composition further includes, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.
  • the above processing is carried out 15 times or less, and the average cooling rate in the temperature range of 700 ° C to 600 ° C is 20 ° C / s or more, Cool at an average cooling rate of 20°C/s or more in the temperature range of 499°C to Ms, and bend and unbend a total of 1 to 15 times in the temperature range of 499°C to Ms using a roll with a radius of 800 mm or less.
  • the steel plate is processed and cooled at an average cooling rate of 150°C/s or less in the temperature range from Ms to cooling stop temperature Ta, and the tension applied to the steel plate in the temperature range from Ms to cooling stop temperature Ta is 5 MPa or more and 100 MPa.
  • the cooling stop temperature Ta is 100°C or more (Ms - 80°C) or less
  • Ms is the martensitic transformation start temperature (°C) defined by formula (1)
  • the tempering temperature is Ta A method for producing a high-strength steel sheet, comprising tempering at a temperature of at least 450° C. and a holding time at the tempering temperature of at least 10 seconds and at most 1,000 seconds.
  • Ms 519-474 ⁇ [%C]-30.4 ⁇ [%Mn]-12.1 ⁇ [%Cr]-7.5 ⁇ [%Mo]-17.7 ⁇ [%Ni]...
  • [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni, and do not include In that case, it is set to 0.
  • [5] The method for producing a high-strength steel plate according to [4], which comprises performing a plating treatment.
  • a high-strength steel plate having a TS of 1180 MPa or more and excellent bendability, flatness in the width direction, and resistance to work embrittlement can be obtained. Furthermore, by applying the high-strength steel plate of the present invention to, for example, automobile structural members, it is possible to improve fuel efficiency by reducing the weight of the vehicle body. Therefore, the industrial value is extremely large.
  • FIG. 1 is a diagram showing the structure of a packet having the maximum occupancy in prior austenite grains and a method for calculating the occupancy of the packet according to the present invention.
  • FIG. 2 is a diagram illustrating the concept of the steepness ⁇ in the width direction of a steel plate and the calculation method thereof according to the present invention.
  • C is one of the important basic components of steel, and particularly in the present invention, it is an important element that affects the total amount of martensite, ferrite, and bainitic ferrite.
  • the C content is less than 0.030%, the amount of martensite decreases and the total amount of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1180 MPa or more.
  • the C content exceeds 0.500%, martensite becomes brittle and the work embrittlement resistance deteriorates. Therefore, the content of C is 0.030% or more and 0.500% or less.
  • the lower limit of the C content is preferably 0.050% or more.
  • the upper limit of the C content is preferably 0.400% or less.
  • the lower limit of the C content is more preferably 0.100% or more.
  • the upper limit of the C content is more preferably 0.350% or less.
  • Si 0.50% or more and 2.50% or less
  • Si is one of the important basic components of steel and is an important element that affects TS and the amount of retained austenite. If the Si content is less than 0.50%, the strength of martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, when the Si content exceeds 2.50%, retained austenite increases excessively and the work embrittlement resistance deteriorates. Therefore, the Si content is set to 0.50% or more and 2.50% or less.
  • the lower limit of the Si content is preferably 0.55% or more.
  • the upper limit of the Si content is preferably 2.00% or less.
  • the lower limit of the Si content is more preferably 0.60% or more.
  • the upper limit of the Si content is more preferably 1.80% or less.
  • Mn is one of the important basic components of steel, and is an important element that affects the total amount of martensite, ferrite, and bainitic ferrite.
  • Mn content is set to 1.50% or more and 5.00% or less.
  • the lower limit of the Mn content is preferably 2.00% or more.
  • the upper limit of the Mn content is preferably 4.50% or less.
  • the lower limit of the Mn content is more preferably 2.20% or more.
  • the upper limit of the Mn content is more preferably 4.00% or less.
  • the content of P needs to be 0.100% or less.
  • the lower limit of the P content is not particularly defined, it is preferably 0.001% or more since P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the content of P is 0.100% or less.
  • the lower limit of the P content is preferably 0.001% or more.
  • the upper limit of the P content is preferably 0.070% or less.
  • the lower limit of the S content is not particularly specified, it is preferably 0.0001% or more due to production technology constraints. Therefore, the S content is set to 0.0200% or less.
  • the lower limit of the S content is preferably 0.0001% or more.
  • the upper limit of the S content is preferably 0.0050% or less.
  • Al 1.000% or less
  • Al exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing the work embrittlement resistance. Therefore, the Al content needs to be 1.000% or less.
  • the lower limit of the Al content is not particularly defined, the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. Therefore, the Al content is set to 1.000% or less.
  • the lower limit of the Al content is preferably 0.001% or more.
  • the upper limit of the Al content is preferably 0.500% or less.
  • N 0.0100% or less
  • N exists as a nitride and reduces the ultimate deformability of the steel sheet, thereby reducing the work embrittlement resistance. Therefore, the N content needs to be 0.0100% or less.
  • the lower limit of the N content is not particularly specified, it is preferable that the N content is 0.0001% or more due to constraints on production technology. Therefore, the N content is set to 0.0100% or less.
  • the lower limit of the N content is preferably 0.0001% or more.
  • the upper limit of the N content is preferably 0.0050% or less.
  • O exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing the work embrittlement resistance. Therefore, the content of O needs to be 0.0100% or less.
  • the lower limit of the O content is not particularly defined, it is preferable that the O content is 0.0001% or more due to production technology constraints. Therefore, the O content is set to 0.0100% or less.
  • the lower limit of the O content is preferably 0.0001% or more.
  • the upper limit of the O content is preferably 0.0050% or less.
  • a high-strength steel plate according to an embodiment of the present invention has a composition containing the above-mentioned components, with the remainder containing Fe and inevitable impurities.
  • unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. A total content of these impurities of 0.100% or less is allowed.
  • the high-strength steel plate of the present invention further includes, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.
  • At least one element selected from the following and Bi: 0.200% or less may be contained alone or in combination.
  • the contents of Ti, Nb, and V are each 0.200% or less.
  • the lower limits of the content of Ti, Nb, and V are not particularly specified, but the strength of the steel sheet can be increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, it is more preferable that the contents of Ti, Nb, and V are each 0.001% or more. Therefore, when Ti, Nb, and V are contained, their contents are each 0.200% or less.
  • the lower limit in the case of containing Ti, Nb and V is more preferably 0.001% or more.
  • the upper limit is more preferably 0.100% or less.
  • the contents of Ta and W are each 0.10% or less. Note that there is no particular lower limit to the content of Ta and W, but the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. It is more preferable that the contents of Ta and W are each 0.01% or more. Therefore, when Ta and W are contained, their contents are each 0.10% or less.
  • the lower limit in the case of containing Ta and W is more preferably 0.01% or more.
  • the upper limit when Ta and W are contained is more preferably 0.08% or less.
  • the content of B is preferably 0.0100% or less.
  • the lower limit of the B content is not particularly specified, but since it is an element that segregates at austenite grain boundaries during annealing and improves hardenability, it is preferable that the B content is 0.0003% or more. preferable. Therefore, when B is contained, its content should be 0.0100% or less.
  • the lower limit in the case of containing B is more preferably 0.0003% or more.
  • the upper limit when B is contained is more preferably 0.0080% or less.
  • each of Cr, Mo, and Ni is 1.00% or less, coarse precipitates and inclusions do not increase and the ultimate deformability of the steel sheet does not decrease, so the work embrittlement resistance does not deteriorate. Therefore, it is preferable that the contents of Cr, Mo, and Ni are each 1.00% or less.
  • the lower limit of the content of Cr, Mo, and Ni is not particularly specified, but since they are elements that improve hardenability, it is more preferable that the content of Cr, Mo, and Ni is each 0.01% or more. . Therefore, when Cr, Mo, and Ni are contained, their contents are each 1.00% or less.
  • the lower limit in the case of containing Cr, Mo and Ni is more preferably 0.01% or more.
  • the upper limit when Cr, Mo and Ni are contained is more preferably 0.80% or less.
  • the Co content is preferably 0.010% or less.
  • the lower limit of the Co content is not particularly specified, since it is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, the content should be 0.010% or less.
  • the lower limit in the case of containing Co is more preferably 0.001% or more.
  • the upper limit when Co is contained is more preferably 0.008% or less.
  • the Cu content is preferably 1.00% or less.
  • the lower limit of the Cu content is not particularly specified, since it is an element that improves hardenability, the Cu content is preferably 0.01% or more. Therefore, if Cu is contained, the content should be 1.00% or less.
  • the lower limit in the case of containing Cu is more preferably 0.01% or more.
  • the upper limit when Cu is contained is more preferably 0.80% or less.
  • the content of Sn is preferably 0.200% or less.
  • the lower limit of the Sn content is not particularly specified, but since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content should be 0.001% or more. It is more preferable. Therefore, if Sn is contained, the content should be 0.200% or less.
  • the lower limit in the case of containing Sn is more preferably 0.001% or more.
  • the upper limit when Sn is contained is more preferably 0.100% or less.
  • the content of Sb is preferably 0.200% or less.
  • the lower limit of the Sb content is not particularly defined, it is more preferable that the Sb content is 0.001% or more since it is an element that controls the softening thickness of the surface layer and enables strength adjustment. Therefore, if Sb is contained, the content should be 0.200% or less.
  • the lower limit in the case of containing Sb is more preferably 0.001% or more.
  • the upper limit when Sb is contained is more preferably 0.100% or less.
  • the content of Ca, Mg and REM is preferably 0.0100% or less.
  • the lower limits of the contents of Ca, Mg, and REM are not particularly stipulated, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of steel sheets, the contents of Ca, Mg, and REM are More preferably, each amount is 0.0005% or more. Therefore, when Ca, Mg and REM are contained, their contents are each 0.0100% or less.
  • the lower limit in the case of containing Ca, Mg and REM is more preferably 0.0005% or more.
  • the upper limit when Ca, Mg and REM are contained is more preferably 0.0050% or less.
  • the contents of Zr and Te are preferably 0.100% or less.
  • the lower limits of the contents of Zr and Te are not particularly specified, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the contents of Zr and Te are respectively 0. More preferably, the content is .001% or more. Therefore, when Zr and Te are contained, their contents are each 0.100% or less.
  • the lower limit in the case of containing Zr and Te is more preferably 0.001% or more.
  • the upper limit when Zr and Te are contained is more preferably 0.080% or less.
  • the Hf content is preferably 0.10% or less.
  • the Hf content should be 0.01% or more. It is more preferable to do so. Therefore, if Hf is contained, the content should be 0.10% or less.
  • the lower limit in the case of containing Hf is more preferably 0.01% or more.
  • the upper limit when containing Hf is more preferably 0.08% or less.
  • the Bi content is preferably 0.200% or less.
  • the lower limit of the Bi content is not particularly defined, since it is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, the content should be 0.200% or less.
  • the lower limit in the case of containing Bi is more preferably 0.001% or more.
  • the upper limit in the case of containing Bi is more preferably 0.100% or less.
  • each content of Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi is preferable. If it is less than the lower limit, the effect of the present invention will not be impaired, and therefore it is included as an unavoidable impurity.
  • the area fraction of martensite is 80% or more.
  • the area fraction of martensite is 80% or more.
  • it is 82% or more. More preferably, it is 84% or more.
  • the amount of retained austenite is set to 3% or more and 15% or less.
  • the lower limit of the amount of retained austenite is preferably 5% or more.
  • the upper limit of the amount of retained austenite is preferably 14% or less.
  • the lower limit of the amount of retained austenite is more preferably 7% or more.
  • the upper limit of the amount of retained austenite is more preferably 13% or less.
  • the method for measuring retained austenite is as follows. Retained austenite was determined by polishing the steel plate from 1/4 part of the plate thickness to a surface of 0.1 mm, and then chemically polishing the surface to a further 0.1 mm using an X-ray diffractometer using CoK ⁇ rays. ⁇ , ⁇ 220 ⁇ , ⁇ 311 ⁇ planes and the diffraction peaks of ⁇ 200 ⁇ , ⁇ 211 ⁇ , ⁇ 220 ⁇ planes of BCC iron were measured, and the nine integrated intensity ratios obtained were averaged. Convert and seek.
  • the total amount of ferrite and bainitic ferrite is 10% or less in terms of area fraction] This is an extremely important feature of the invention. If the total amount of ferrite and bainitic ferrite exceeds 10%, it becomes difficult to achieve a TS of 1180 MPa or more. Therefore, the total amount of ferrite and bainitic ferrite is 10% or less. Preferably it is 8% or less. More preferably, it is 5% or less. Note that the lower limit of the total amount of ferrite and bainitic ferrite is not particularly limited. It may be 0%.
  • the method for measuring the total amount of ferrite and bainitic ferrite is as follows. After polishing the L cross section of the steel plate, 3vol. % nital, and 1/4 part of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed in 10 fields at a magnification of 2000 times using an SEM.
  • ferrite and bainitic ferrite have concave portions and a flat structure inside, and have no carbide inside. From the average value of those values, the total amount of ferrite and bainitic ferrite can be determined.
  • the method for measuring the amount of martensite is as follows.
  • the amount of martensite can be determined by measuring the amount of retained austenite, the amount of ferrite, and the amount of bainitic ferrite based on the method described above, and subtracting the total from 100%. Therefore, the amount of martensite in the present invention is an amount that includes both quenched martensite and tempered martensite.
  • the volume ratio of the retained austenite amount is approximately the area ratio, it is subtracted from 100% together with the ferrite amount and the bainitic ferrite amount expressed by the area ratio.
  • Prior austenite average grain size is 20 ⁇ m or less
  • the lower limit of the prior austenite average grain size is not particularly defined, it is preferably 2 ⁇ m or more due to production technology constraints. Therefore, the average crystal grain size of prior austenite is set to 20 ⁇ m or less. Preferably it is 2 ⁇ m or more. Preferably it is 15 ⁇ m or less. More preferably, the thickness is 3 ⁇ m or more. More preferably, the thickness is 10 ⁇ m or less.
  • the method for measuring the average grain size of prior austenite is as follows. After polishing the L cross section of the steel plate, etching it with a mixed solution of picric acid and ferric chloride to expose the prior austenite grain boundaries, 3 to 10 fields of view are photographed using an optical microscope at a magnification of 400x. A total of 20 straight lines (10 vertically x 10 horizontally) are drawn at equal intervals on the obtained image data and determined by a cutting method.
  • the average value of the occupancy of the packets with the maximum occupancy in the prior austenite grains is 70% or less in terms of area fraction] This is an extremely important feature of the invention.
  • the occupancy of the packets having the maximum occupancy within the prior austenite grains influences the flatness in the width direction and the resistance to work embrittlement.
  • the packet with the maximum occupancy rate in the prior austenite grains means that, as shown in Figure 1, there are up to four regions in the prior austenite grains, called packets, that have the same crystal habit plane during transformation. indicates the packet with the largest occupancy rate.
  • the occupancy of one packet within the prior austenite grains is determined by dividing the area of the specified packet by the entire area within the prior austenite grains.
  • the present inventors found that by reducing the occupancy of the packets with the maximum occupancy within the prior austenite grains, the strain between the packets was alleviated and the flatness in the sheet width direction was improved. I discovered that. They also found that by reducing the occupancy of packets, which have the highest occupancy within prior austenite grains, the structure becomes finer and crack propagation can be suppressed, thereby improving the work embrittlement resistance of the steel sheet. . Therefore, the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains is set to be 70% or less. Preferably it is 60% or less. Note that the lower limit of the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is not particularly limited.
  • the occupancy rate of the packet having the maximum occupancy rate in the prior austenite grains is 25%. Therefore, although the lower limit of the average value of the occupancy of packets having the maximum occupancy in prior austenite grains is preferably 25% or more, it is not necessary to be limited to this.
  • the method for measuring the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains is as follows. First, a test piece for microstructural observation is taken from a cold-rolled steel sheet. Next, the sampled test piece is polished by colloidal silica vibration polishing so that the cross section in the rolling direction (L cross section) becomes the observation surface. The observation surface shall be a mirror surface. Next, electron beam backscatter diffraction (EBSD) measurement is performed on a 1/4 part of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) to obtain local crystal orientation data.
  • EBSD electron beam backscatter diffraction
  • the SEM magnification is 1000 times, the step size is 0.2 ⁇ m, the measurement area is 80 ⁇ m square, and the WD is 15 mm.
  • the obtained local orientation data is analyzed using OIM Analysis 7 (OIM), and a color-coded diagram (CP map) for each Close-packed Plane group (CP group) is created using the method described in Non-Patent Document 1.
  • packets are defined as areas to which the same CP group belongs. The area of the packet with the largest occupancy is determined from the obtained CP map and divided by the total area within the prior austenite grains, thereby obtaining the occupancy of the packet with the maximum occupancy within the prior austenite grains. This analysis is performed on ten or more adjacent prior austenite grains, and the average value is taken as the average value of the occupancy of the packets having the maximum occupancy within the prior austenite grain.
  • the method of melting the steel material is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable.
  • the steel slab (slab) is preferably manufactured by a continuous casting method in order to prevent macro segregation.
  • the slab heating temperature, slab soaking time and coiling temperature in hot rolling are not particularly limited.
  • Methods for 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. Examples include.
  • the slab heating temperature, slab soaking time, finish rolling temperature, and coiling temperature in hot rolling are not particularly limited, the lower limit of the slab heating temperature is preferably 1100° C. or higher.
  • the upper limit of the slab heating temperature is preferably 1300°C or less.
  • the lower limit of the slab soaking time is preferably 30 min or more.
  • the upper limit of the slab soaking time is preferably 250 min or less.
  • the lower limit of the finish rolling temperature is preferably equal to or higher than the Ar 3 transformation point.
  • the lower limit of the winding temperature is preferably 350°C or higher.
  • the upper limit of the winding temperature is preferably 650° C. or less.
  • the hot-rolled steel sheet produced in this way is pickled. Since pickling can remove oxides on the surface of the steel sheet, it is important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet. Further, the pickling may be carried out once or may be carried out in multiple steps. Moreover, cold rolling may be performed on the pickled plate after hot rolling, or cold rolling may be performed after heat treatment.
  • the rolling reduction in cold rolling and the plate thickness after rolling are not particularly limited, but the lower limit of the rolling reduction is preferably 30% or more. Further, the upper limit of the rolling reduction ratio is preferably 80% or less. Note that the effects of the present invention can be obtained without any particular limitations on the number of rolling passes and the rolling reduction rate of each pass.
  • the cold rolled steel sheet obtained as described above is annealed.
  • the annealing conditions are as follows.
  • the annealing temperature is 750°C or higher and 950°C or lower.
  • the annealing temperature is set to 750°C or more and 950°C or less.
  • the lower limit of the annealing temperature is preferably 800°C or higher.
  • the upper limit of the annealing temperature is preferably 900°C or less.
  • the holding time at the annealing temperature is 10 seconds or more and 1000 seconds or less.
  • the lower limit of the holding time at the annealing temperature is preferably 50 seconds or more.
  • the upper limit of the holding time at the annealing temperature is preferably 500 seconds or less.
  • the radius of the roll diameter is 600 mm or less.
  • the lower limit of the number of times of bending and unbending is 3 or more times in total.
  • the upper limit of the number of times of bending and unbending is 10 times or less in total.
  • the lower limit of the radius of the roll diameter does not need to be particularly limited, it is preferably 50 mm or more.
  • bending and unbending refers to a process of bending a material in one direction with rolls and then bending it back by the amount of bending in the opposite direction using a known method. The number of times of bending and unbending is counted as one bending and one unbending, rather than one bending and unbending.
  • the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and the work embrittlement resistance deteriorates. Therefore, the average cooling rate from 750°C to 600°C is set to 20°C/s or more. Preferably it is 30°C/s or more. The upper limit does not need to be particularly limited, but is preferably 100° C./s or less.
  • the average cooling rate in the temperature range of 499° C. to Ms affects the total area fraction of the amount of ferrite and bainitic ferrite.
  • the average cooling rate in the temperature range of 499°C to Ms is set to 20°C/s or more. Preferably it is 25°C/s or more.
  • the upper limit does not need to be particularly limited, but is preferably 100° C./s or less.
  • Ms (° C.) is defined by the following equation (1).
  • Ms 519-474 ⁇ [%C]-30.4 ⁇ [%Mn]-12.1 ⁇ [%Cr]-7.5 ⁇ [%Mo]-17.7 ⁇ [%Ni]...
  • [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni, and do not include In that case, it is set to 0.
  • the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and the work embrittlement resistance deteriorates.
  • the average value of the occupancy of the packets having the maximum occupancy in the prior austenite grains exceeds 70%, the flatness in the sheet width direction deteriorates, and the work embrittlement resistance deteriorates.
  • the ultimate deformability of the steel sheet decreases and the work embrittlement resistance decreases. Therefore, bending and unbending is performed a total of 1 to 15 times in a temperature range of 499° C. to Ms with a roll having a radius of 800 mm or less.
  • the radius of the roll diameter is 600 mm or less.
  • the lower limit of the number of times of bending and unbending is 3 or more times in total.
  • the lower limit of the number of times of bending and unbending is 10 times or less in total.
  • the lower limit of the radius of the roll diameter does not need to be particularly limited, it is preferably 50 mm or more.
  • the average cooling rate in the temperature range from Ms to the cooling stop temperature Ta is set to 150° C./s or less. Preferably it is 120°C/s or less.
  • the lower limit does not need to be particularly limited, but is preferably 5° C./s or more.
  • the tension applied to the steel plate in the temperature range from Ms to the cooling stop temperature Ta exceeds 100 MPa, the ultimate deformability of the steel plate is reduced and the work embrittlement resistance is reduced. Therefore, the tension applied to the steel plate in the temperature range from Ms to the cooling stop temperature Ta is set to be 5 MPa or more and 100 MPa or less.
  • the lower limit of the tension applied to the steel plate in the temperature range from Ms to the cooling stop temperature Ta is preferably 6 MPa or more.
  • the upper limit of the tension applied to the steel plate in the temperature range from Ms to the cooling stop temperature Ta is preferably 50 MPa or less.
  • the tension is applied by a known method. As an example, tension may be applied by controlling the speed of the rolls in the furnace.
  • [Cooling stop temperature Ta is 100°C or higher (Ms-80°C) or lower]
  • the cooling stop temperature Ta is set to 100°C or more (Ms-80°C) or less.
  • the lower limit of the cooling stop temperature Ta is preferably 120°C or higher.
  • the upper limit of the cooling stop temperature Ta is preferably (Ms-100°C) or less.
  • Tempeering temperature is Ta or higher and 450°C or lower
  • the remaining austenite is stabilized by holding at that temperature or by reheating and holding at a temperature of 450° C. or lower.
  • the tempering temperature is lower than Ta, the desired retained austenite cannot be obtained, resulting in a decrease in bendability. If the tempering temperature exceeds 450° C., the martensite will be excessively tempered, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the tempering temperature is set to be higher than Ta and lower than 450°C.
  • the lower limit of the tempering temperature is preferably (Ta+10°C) or higher.
  • the upper limit of the tempering temperature is preferably 420°C or less.
  • the holding time at tempering temperature is 10 seconds or more and 1000 seconds or less. If the holding time at the tempering temperature is less than 10 seconds, the austenite will not be sufficiently stabilized and the desired retained austenite will not be obtained, resulting in a decrease in bendability. If the holding time at the tempering temperature exceeds 1000 seconds, the martensite will be excessively tempered, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the holding time at the tempering temperature is 10 seconds or more and 1000 seconds or less.
  • the lower limit of the holding time at the tempering temperature is preferably 50 seconds or more.
  • the upper limit of the holding time at the tempering temperature is preferably 800 seconds or less.
  • Cooling after tempering does not need to be particularly specified, and may be cooled to a desired temperature by any method.
  • the desired temperature is preferably about room temperature.
  • the above-mentioned high-strength steel plate may be processed under conditions that result in an equivalent plastic strain amount of 0.10% or more and 5.00% or less. Further, after processing, reheating may be performed again under the conditions of 100° C. or more and 400° C. or less.
  • the high-strength steel plate may be subjected to plating treatment during or after annealing.
  • Examples of plating treatment during annealing include hot-dip galvanizing treatment during or after cooling at a temperature of 700°C to 600°C after annealing at an average cooling rate of 20°C/s or more, and alloying after hot-dip galvanizing. can.
  • the plating treatment after annealing for example, Zn-Ni electroplating treatment or pure Zn electroplating treatment can be exemplified after tempering.
  • the plating layer may be formed by electroplating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied.
  • the type of plating metal such as Zn plating and Al plating is not particularly limited.
  • Conditions for other manufacturing methods are not particularly limited, but from the viewpoint of productivity, a series of treatments such as annealing, hot-dip galvanizing, and galvanizing alloying are performed on a continuous galvanizing line (CGL), which is a hot-dip galvanizing line. It is preferable to carry out the process using Line). After hot-dip galvanizing, wiping can be performed to adjust the coating weight. Note that the conditions for plating and the like other than the above-mentioned conditions can be based on a conventional method for hot-dip galvanizing.
  • processing may be performed again under conditions that result in an equivalent plastic strain amount of 0.10% or more and 5.00 or less. Further, after processing, reheating may be performed again under the conditions of 100° C. or more and 400° C. or less.
  • Test test For the tensile test, a JIS No. 5 test piece (gauge length 50 mm, parallel part width 25 mm) was taken so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and tested in accordance with JIS Z 2241. A tensile test was conducted at a crosshead speed of 1.67 ⁇ 10 ⁇ 1 mm/sec, and TS was measured. In addition, in this invention, TS of 1180 MPa or more was judged to be a pass.
  • the flatness of the various cold-rolled steel sheets obtained as described above in the sheet width direction was measured by the method shown in FIG. 2. Specifically, a plate with a length of 500 mm in the rolling direction (coil width x 500 mm L x plate thickness) is cut out from the coil, placed on a surface plate so that the warp of the end face faces upward, and the stylus moves over the object to be measured. The height of the steel plate was continuously measured over the entire width direction using a moving contact displacement meter. Based on the results, the steepness, which is an index indicating the flatness of the steel plate shape, was measured according to the method shown in FIG.
  • the resistance to work embrittlement was evaluated by Charpy test.
  • Charpy test piece a plurality of steel plates were stacked together and fastened together with bolts, and after confirming that there were no gaps between the steel plates, a test piece with a V-notch having a depth of 2 mm was prepared.
  • the number of steel plates to be stacked was set so that the thickness of the test piece after stacking was closest to 10 mm. For example, if the plate thickness is 1.2 mm, 8 plates are laminated, resulting in a test piece thickness of 9.6 mm.
  • the laminated Charpy test piece was taken with the width direction of the plate as the longitudinal direction.
  • Tables 5 to 7 The results are summarized in Tables 5 to 7. As shown in Tables 5 to 7, the examples of the present invention have a TS of 1180 MPa or more, and are excellent in bendability, flatness in the width direction, and resistance to work embrittlement. On the other hand, the comparative examples are inferior in any one or more of TS, bendability, flatness in the width direction, or resistance to work embrittlement.

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Abstract

Le but de la présente invention est de fournir : une tôle d'acier à haute résistance qui offre une TS supérieure ou égale à 1 180 MPa, tout en présentant une excellente aptitude au pliage, une excellente planéité dans le sens de la largeur de la tôle et d'excellentes caractéristiques de résistance à la fragilisation due au traitement ; ainsi qu'un procédé de production de cette tôle d'acier à haute résistance.　La présente invention concerne une tôle d'acier à haute résistance qui présente une composition de constituants spécifique, dans laquelle, à une position correspondant à 1/4 de l'épaisseur de tôle : la quantité de martensite est de 80 % ou plus en fractions surfaciques ; la quantité d'austénite résiduelle est de 3 % à 15 % en fractions volumiques ; la quantité totale de ferrite et de ferrite bainitique est inférieure ou égale à 10 % en fractions surfaciques ; la taille moyenne de grains cristallins de l'ancienne austénite est inférieure ou égale à 20 µm ; et la moyenne des occupations de paquets ayant l'occupation maximale dans les anciens grains austénitiques est inférieure ou égale à 70 % en fractions surfaciques.
PCT/JP2023/002916 2022-03-25 2023-01-30 Tôle d'acier à haute résistance et son procédé de production WO2023181642A1 (fr)

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Publication number Priority date Publication date Assignee Title
JP4947176B2 (ja) 2010-03-24 2012-06-06 Ｊｆｅスチール株式会社 超高強度冷延鋼板の製造方法
JP6338025B2 (ja) 2016-02-10 2018-06-06 Ｊｆｅスチール株式会社 高強度鋼板及びその製造方法
WO2018124157A1 (fr) * 2016-12-27 2018-07-05 Ｊｆｅスチール株式会社 Tôle d'acier galvanisée à résistance élevée et son procédé de fabrication
WO2018216522A1 (fr) * 2017-05-24 2018-11-29 株式会社神戸製鋼所 Tôle d'acier plaquée à résistance élevée et son procédé de fabrication
JP6525114B1 (ja) 2017-11-29 2019-06-05 Ｊｆｅスチール株式会社 高強度亜鉛めっき鋼板およびその製造方法
WO2020158066A1 (fr) * 2019-01-30 2020-08-06 Ｊｆｅスチール株式会社 Tôle d'acier haute résistance et procédé de fabrication de celle-ci
JP6777272B1 (ja) 2019-02-06 2020-10-28 日本製鉄株式会社 溶融亜鉛めっき鋼板およびその製造方法
WO2021070925A1 (fr) * 2019-10-09 2021-04-15 日本製鉄株式会社 Tôle d'acier et son procédé de fabrication
WO2022209519A1 (fr) * 2021-03-31 2022-10-06 Ｊｆｅスチール株式会社 Feuille d'acier, élément, procédé de production d'une feuille d'acier et procédé de production d'un élément

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JP4947176B2 (ja) 2010-03-24 2012-06-06 Ｊｆｅスチール株式会社 超高強度冷延鋼板の製造方法
JP6338025B2 (ja) 2016-02-10 2018-06-06 Ｊｆｅスチール株式会社 高強度鋼板及びその製造方法
WO2018124157A1 (fr) * 2016-12-27 2018-07-05 Ｊｆｅスチール株式会社 Tôle d'acier galvanisée à résistance élevée et son procédé de fabrication
WO2018216522A1 (fr) * 2017-05-24 2018-11-29 株式会社神戸製鋼所 Tôle d'acier plaquée à résistance élevée et son procédé de fabrication
JP6525114B1 (ja) 2017-11-29 2019-06-05 Ｊｆｅスチール株式会社 高強度亜鉛めっき鋼板およびその製造方法
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WO2022209519A1 (fr) * 2021-03-31 2022-10-06 Ｊｆｅスチール株式会社 Feuille d'acier, élément, procédé de production d'une feuille d'acier et procédé de production d'un élément

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