WO2010030021A1 - 高強度鋼板およびその製造方法 - Google Patents

高強度鋼板およびその製造方法 Download PDF

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Publication number
WO2010030021A1
WO2010030021A1 PCT/JP2009/065981 JP2009065981W WO2010030021A1 WO 2010030021 A1 WO2010030021 A1 WO 2010030021A1 JP 2009065981 W JP2009065981 W JP 2009065981W WO 2010030021 A1 WO2010030021 A1 WO 2010030021A1
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steel sheet
martensite
strength
seconds
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PCT/JP2009/065981
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English (en)
French (fr)
Japanese (ja)
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松田広志
船川義正
田中靖
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Ｊｆｅスチール株式会社
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First worldwide family litigation filed litigation Critical https://patents.darts-ip.com/?family=42005270&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=WO2010030021(A1) "Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to EP09813166.7A priority Critical patent/EP2327810B1/en
Priority to CN2009801355751A priority patent/CN102149841B/zh
Priority to KR1020117006084A priority patent/KR101341731B1/ko
Priority to CA2734978A priority patent/CA2734978C/en
Priority to US13/062,574 priority patent/US20110162762A1/en
Publication of WO2010030021A1 publication Critical patent/WO2010030021A1/ja

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high-strength steel sheet having a tensile strength (TS) of 980 MPa or more excellent in workability, particularly ductility and stretch flangeability, used in industrial fields such as automobiles and electrical equipment, and a method for producing the same.
  • TS tensile strength
  • the workability of the steel plate is strongly influenced by the workability of the hard phase. This is because when the ratio of hard phase is small and soft polygonal ferrite is large, the deformability of polygonal ferrite dominates the workability of the steel sheet, and even when the hard phase has insufficient workability. Workability such as ductility was ensured, but when the ratio of the hard phase is large, the deformability of the hard phase itself directly affects the formability of the steel sheet, not the deformation of polygonal ferrite, and the hard phase itself This is because if the workability is insufficient, the workability of the steel sheet is significantly deteriorated.
  • steel plates having a hard phase other than martensite there are steel plates in which the main phase is polygonal ferrite, the hard phase is bainite or pearlite, and carbides are generated in these hard phases bainite or pearlite.
  • This steel sheet is a steel sheet that not only improves the workability with polygonal ferrite alone, but also improves the workability of the hard phase itself by generating carbides in the hard phase, and in particular, improves the stretch flangeability. .
  • the main phase is polygonal ferrite, it is difficult to achieve both high strength and workability of 980 MPa or higher in tensile strength (TS).
  • Patent Document 1 proposes a high-tensile steel plate that is excellent in bending workability and impact properties by defining alloy components and making the steel structure fine and uniform bainite having retained austenite.
  • Patent Document 2 proposes a composite structure steel plate having excellent bake hardenability by defining predetermined alloy components, making the steel structure bainite having retained austenite, and defining the amount of retained austenite in bainite. ing.
  • Patent Document 3 a predetermined alloy component is defined, the steel structure is 90% or more in area ratio of bainite having retained austenite, the amount of retained austenite in bainite is 1% or more and 15% or less, and the hardness of bainite.
  • HV a composite structure steel plate excellent in impact resistance
  • the above-described steel sheet has the following problems.
  • it is difficult to ensure a stable amount of retained austenite that exhibits the TRIP effect in a high strain region when strain is applied to the steel sheet, and bendability is obtained.
  • the ductility until plastic instability occurs is low, and the stretchability is inferior.
  • Patent Document 2 has bake hardenability, even when trying to increase the tensile strength (TS) to 980 MPa or higher, or even 1050 MPa or higher, bainite or martensite mainly composed of ferrite is suppressed as much as possible. Therefore, it is difficult to ensure workability such as ductility and stretch flangeability at the time of securing strength or increasing strength.
  • TS tensile strength
  • the main purpose of the steel sheet described in Patent Document 3 is to improve impact resistance, and since it has a main phase of bainite having a hardness of HV 250 or less, specifically, it has a structure containing more than 90%. It is difficult to set the tensile strength (TS) to 980 MPa or more.
  • the present invention advantageously solves the above-mentioned problems, and provides a high-strength steel sheet having a tensile strength (TS) of 980 MPa or more, which is excellent in workability, particularly ductility and stretch flangeability, together with its advantageous production method.
  • the high-strength steel sheet of the present invention includes a steel sheet obtained by subjecting the surface of the steel sheet to hot dip galvanization or galvannealing.
  • excellent workability means that TS ⁇ T.
  • the value of EL satisfies 20000 MPa ⁇ % or more and the value of TS ⁇ ⁇ satisfies 25000 MPa ⁇ %.
  • TS is tensile strength (MPa)
  • T.I. EL is the total elongation (%)
  • ⁇ is the critical hole expansion rate (%).
  • the inventors have made extensive studies on the component composition and microstructure of the steel sheet in order to solve the above problems.
  • the lower bainite structure and / or martensite structure is utilized to increase the strength, and the C content in the steel sheet is increased to 0.17% or more and the C content is increased, and then the upper bainite transformation is utilized.
  • stable retained austenite advantageous for obtaining the TRIP effect can be secured, and by making a part of the martensite tempered martensite, workability, in particular, balance between strength and ductility, and strength and stretch flange It was found that a high-strength steel sheet having a tensile strength of 980 MPa or more that is excellent in both the balance of properties can be obtained.
  • the present invention is based on the above findings, and the gist of the present invention is as follows. 1. C: 0.17% to 0.73% by mass%, Si: 3.0% or less, Mn: 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less and N: 0.010% or less, and Si + Al satisfies 0.7% or more, the balance is the composition of Fe and inevitable impurities, As the steel sheet structure, the area ratio of the total amount of lower bainite and all martensite is 10% or more and 90% or less, the amount of retained austenite is 5% or more and 50% or less, and the steel structure of bainitic ferrite in the upper bainite.
  • the area ratio with respect to the whole is 5% or more, of the total amount of the lower bainite and all martensite, 75% or less of the as-quenched martensite, and the area ratio of the polygonal ferrite with respect to the entire steel sheet structure is 10% or less (0% A high-strength steel sheet characterized in that the average C content in the retained austenite is 0.70% or more and the tensile strength is 980 MPa or more.
  • the steel sheet is further in mass%, Cr: 0.05% or more and 5.0% or less, 2.
  • the steel sheet is further in mass%, 1 or 2 above, which contains one or two elements selected from Ti: 0.01% to 0.1% and Nb: 0.01% to 0.1%. High strength steel sheet as described.
  • the steel sheet is further in mass%
  • B The high-strength steel sheet according to any one of 1 to 3 above, which contains 0.0003% or more and 0.0050% or less.
  • the steel sheet is further in mass%, 1 to 4 above, which contains one or two elements selected from Ni: 0.05% to 2.0% and Cu: 0.05% to 2.0%
  • the high-strength steel sheet according to any one of the items.
  • the steel sheet is further in mass%, 1 to 5 above, which contains one or two elements selected from Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.
  • the high-strength steel sheet according to any one of the items.
  • a high-strength steel sheet comprising a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the steel sheet according to any one of 1 to 6 above.
  • the steel slab having the composition described in any one of 1 to 6 above is hot-rolled to form a cold-rolled steel sheet by cold rolling, and then the cold-rolled steel sheet is 600 seconds or more in the austenite single-phase region.
  • cooling stop temperature determined in the first temperature range of 350 ° C. or higher and 490 ° C. or lower: when cooling to T ° C., the average cooling rate is controlled to 5 ° C./s or higher until at least 550 °
  • the high-strength steel sheet is cooled and then held in the first temperature range for 15 seconds to 1000 seconds and then held in the second temperature range of 200 ° C. to 350 ° C. for 15 seconds to 1000 seconds. Manufacturing method.
  • TS tensile strength
  • the utility value is extremely large, and is particularly useful for reducing the weight of automobile bodies.
  • the area ratio is the area ratio relative to the entire steel sheet structure.
  • Area ratio of total amount of lower bainite and all martensite 10% or more and 90% or less
  • Lower bainite and martensite are structures necessary for increasing the strength of a steel sheet. If the area ratio of the total amount of lower bainite and all martensite is less than 10%, the tensile strength (TS) of the steel sheet does not satisfy 980 MPa. On the other hand, if the area ratio of the total amount of the lower bainite and all martensite exceeds 90%, the upper bainite decreases, and as a result, stable retained austenite enriched in C cannot be secured, so workability such as ductility is reduced. Decreasing becomes a problem. Therefore, the area ratio of the total amount of lower bainite and all martensite is set to 10% or more and 90% or less. Preferably, it is in the range of 20% to 80%. More preferably, it is in the range of 30% to 70%.
  • ratio of as-quenched martensite 75% or less
  • the ratio of as-quenched martensite is the sum of lower bainite and all martensite present in the steel sheet. If it exceeds 75% of the amount, the tensile strength becomes 980 MPa or more, but the stretch flangeability is inferior.
  • the as-quenched martensite is extremely hard and the deformability of the as-quenched martensite itself is extremely low, so that the workability of the steel sheet, particularly the stretch flangeability, is significantly deteriorated.
  • the ratio of martensite as quenched in martensite is 75% or less with respect to the total amount of lower bainite and all martensite present in the steel sheet. Preferably it is 50% or less.
  • the as-quenched martensite is a structure in which carbides are not recognized in the martensite and can be observed by SEM.
  • Residual austenite amount 5% or more and 50% or less Residual austenite undergoes martensitic transformation by the TRIP effect during processing, and improves ductility by increasing strain dispersibility.
  • Residual austenite amount 5% or more and 50% or less Residual austenite undergoes martensitic transformation by the TRIP effect during processing, and improves ductility by increasing strain dispersibility.
  • utilizing the upper bainite transformation in particular, retained austenite with an increased amount of C concentration is formed in the upper bainite.
  • retained austenite that can exhibit the TRIP effect even in a high strain region during processing can be obtained.
  • good workability can be obtained even in a high strength region where the tensile strength (TS) is 980 MPa or more, specifically, TS ⁇ T.
  • the value of El can be set to 20000 MPa or more, and a steel sheet having an excellent balance between strength and ductility can be obtained.
  • the retained austenite in the upper bainite is formed between the laths of the bainitic ferrite in the upper bainite and is finely distributed. Is necessary and accurate quantification is difficult.
  • the amount of retained austenite formed between the laths of the bainitic ferrite is a certain amount commensurate with the amount of bainitic ferrite formed.
  • the area ratio of bainitic ferrite in the upper bainite is 5% or more
  • X-ray diffraction is a method for measuring the amount of retained austenite that has been conventionally performed. If the amount of retained austenite obtained from the strength measurement, specifically the X-ray diffraction intensity ratio of ferrite and austenite is 5% or more, a sufficient TRIP effect can be obtained, and the tensile strength (TS) is 980 MPa or more. TS ⁇ T. It was found that El can achieve 20000 MPa ⁇ % or more.
  • the amount of retained austenite obtained by a conventional method for measuring the amount of retained austenite is equivalent to the area ratio of retained austenite to the entire steel sheet structure.
  • the amount of retained austenite is in the range of 5% to 50%.
  • it is in the range of more than 5%, more preferably 10% or more and 45% or less. More preferably, it is the range of 15% or more and 40% or less.
  • Average C content in retained austenite 0.70% or more
  • TS tensile strength
  • the amount of C in the austenite is important.
  • C is concentrated in the retained austenite formed between the laths of bainitic ferrite in the upper bainite.
  • the conventional austenite in the retained austenite If the average C content in the retained austenite obtained from the shift amount of the diffraction peak in X-ray diffraction (XRD), which is a method for measuring the average C content (average of the C content in the retained austenite) is 0.70% or more It was found that excellent processability can be obtained. When the average C content in the retained austenite is less than 0.70%, martensitic transformation occurs in the low strain region during processing, and the TRIP effect in the high strain region that improves workability cannot be obtained.
  • XRD X-ray diffraction
  • the average amount of C in the retained austenite is 0.70% or more. Preferably it is 0.90% or more.
  • the average C content in the retained austenite is preferably 2.00% or less. More preferably, it is 1.50% or less.
  • the area ratio of bainitic ferrite in the upper bainite 5% or more
  • the formation of bainitic ferrite by the upper bainite transformation concentrates C in the untransformed austenite and exhibits the TRIP effect in the high strain region during processing. It is necessary to obtain retained austenite that enhances strain resolution.
  • the transformation from austenite to bainite occurs over a wide temperature range of approximately 150 to 550 ° C., and various types of bainite are produced within this temperature range. In the prior art, such various bainite was often simply defined as bainite, but in order to obtain the target workability in the present invention, it is necessary to clearly define the bainite structure.
  • the bainite and lower bainite are defined as follows.
  • the upper bainite is composed of lath-like bainitic ferrite and residual austenite and / or carbide existing between bainitic ferrite, and there is no fine carbide regularly arranged in lath-like bainitic ferrite. It is a feature.
  • the lower bainite is composed of the lath-shaped bainitic ferrite and the residual austenite and / or carbide existing between the bainitic ferrites in common with the upper bainite. It is characterized by the presence of fine carbides regularly arranged in the bainitic ferrite. That is, the upper bainite and the lower bainite are distinguished by the presence or absence of regularly arranged fine carbides in bainitic ferrite.
  • Such a difference in the state of carbide formation in bainitic ferrite has a great influence on the concentration of C in the retained austenite. That is, when the area ratio of the bainitic ferrite of the upper bainite is less than 5%, even when the bainite transformation is advanced, the amount of C generated as carbides in the bainitic ferrite increases, resulting in a The amount of C enriched in the residual austenite present in the steel decreases, and the amount of residual austenite that exhibits the TRIP effect in the high strain region during processing decreases. Therefore, the area ratio of bainitic ferrite in the upper bainite needs to be 5% or more in terms of the area ratio with respect to the entire steel sheet structure. On the other hand, if the area ratio of the bainitic ferrite of the upper bainite to the entire steel sheet structure exceeds 85%, it may be difficult to ensure the strength.
  • Polygonal ferrite area ratio 10% or less (including 0%)
  • TS tensile strength
  • the area ratio of polygonal ferrite is 10% or less, even if polygonal ferrite is present, a small amount of polygonal ferrite is isolated and dispersed in the hard phase, and strain concentration can be suppressed. Degradation of workability can be avoided. Therefore, the area ratio of polygonal ferrite is 10% or less. Preferably it is 5% or less, More preferably, it is 3% or less, and 0% may be sufficient.
  • the hardness of the hardest structure in the steel sheet structure is HV ⁇ 800. That is, in the steel sheet of the present invention, when there is no as-quenched martensite, either the tempered martensite or the lower bainite or the upper bainite is the hardest phase, but these structures all have HV ⁇ 800. It is a phase. In addition, when there is as-quenched martensite, the as-quenched martensite becomes the hardest structure, in the steel sheet of the present invention, even if it is as-quenched martensite, the hardness is HV ⁇ 800, There is no extremely hard martensite such that HV> 800, and good stretch flangeability can be secured.
  • the steel sheet of the present invention may contain pearlite, Widmanstatten ferrite, or lower bainite as the remaining structure.
  • the allowable content of the remaining tissue is preferably 20% or less in terms of area ratio. More preferably, it is 10% or less.
  • C 0.17% or more and 0.73% or less
  • C is an element indispensable for increasing the strength of a steel sheet and ensuring a stable retained austenite amount, and for ensuring the amount of martensite and allowing austenite to remain at room temperature. It is a necessary element. If the C content is less than 0.17%, it is difficult to ensure the strength and workability of the steel sheet. On the other hand, if the amount of C exceeds 0.73%, the welded part and the heat-affected zone are hardened and the weldability deteriorates. Accordingly, the C content is in the range of 0.17% to 0.73%. Preferably, it is 0.20% or more and 0.48% or less of range, More preferably, it is 0.25% or more.
  • Si 3.0% or less (including 0%) Si is a useful element that contributes to improving the strength of steel by solid solution strengthening. However, if the amount of Si exceeds 3.0%, the workability and toughness deteriorate due to the increase in the amount of solid solution in polygonal ferrite and bainitic ferrite, and the surface properties due to the occurrence of red scale, etc. In the case of deterioration or hot dipping, it causes deterioration of plating adhesion and adhesion. Therefore, the Si content is 3.0% or less. Preferably it is 2.6% or less. More preferably, it is 2.2% or less. Si is an element useful for suppressing the formation of carbides and promoting the formation of retained austenite. Therefore, the Si content is preferably 0.5% or more, but the formation of carbides is only Al. In the case of suppressing by Si, Si does not need to be added, and the Si amount may be 0%.
  • Mn 0.5% or more and 3.0% or less Mn is an element effective for strengthening steel. If the amount of Mn is less than 0.5%, carbide precipitates in a temperature range higher than the temperature at which bainite and martensite are generated during cooling after annealing, so ensure the amount of hard phase that contributes to strengthening of steel. I can't. On the other hand, when the amount of Mn exceeds 3.0%, castability is deteriorated. Accordingly, the amount of Mn is set in the range of 0.5% to 3.0%. Preferably, the range is 1.5% or more and 2.5% or less.
  • P 0.1% or less
  • P is an element useful for strengthening steel, but if the P content exceeds 0.1%, the impact resistance deteriorates due to embrittlement due to grain boundary segregation, and the steel is alloyed. In the case of applying hot dip galvanizing, the alloying speed is greatly delayed. Therefore, the P content is 0.1% or less. Preferably it is 0.05% or less.
  • the amount of P is preferably reduced, but if it is less than 0.005%, it causes a significant increase in cost, so the lower limit is preferably about 0.005%.
  • S 0.07% or less Since S generates MnS and becomes inclusions, which causes deterioration of impact resistance and cracks along the metal flow of the weld, it is preferable to reduce the amount of S as much as possible. However, excessively reducing the amount of S causes an increase in manufacturing cost, so the amount of S is set to 0.07% or less. Preferably it is 0.05% or less, More preferably, it is 0.01% or less. In addition, since it is accompanied by a big increase in manufacturing cost to make S less than 0.0005%, the lower limit is about 0.0005% from the point of manufacturing cost.
  • Al 3.0% or less
  • Al is a useful element to be added as a deoxidizer in the steel making process, as well as a useful element for strengthening steel.
  • the Al content is 3.0% or less.
  • Al is an element useful for suppressing the formation of carbides and promoting the formation of retained austenite.
  • the Al content should be 0.001% or more.
  • it is more preferably 0.005% or more.
  • the amount of Al in the present invention is the amount of Al contained in the steel sheet after deoxidation.
  • N 0.010% or less
  • N is an element that greatly deteriorates the aging resistance of steel, and is preferably reduced as much as possible.
  • the N content exceeds 0.010%, deterioration of aging resistance becomes remarkable, so the N content is set to 0.010% or less. Note that, if N is less than 0.001%, a large increase in manufacturing cost is caused, so that the lower limit is about 0.001% from the viewpoint of manufacturing cost.
  • the basic component has been described above. However, in the present invention, it is not sufficient to satisfy the above component range, and it is necessary to satisfy the following equation. Si + Al ⁇ 0.7% As described above, both Si and Al are useful elements for suppressing the formation of carbides and promoting the formation of retained austenite. Although suppression of the formation of carbides is effective even if Si or Al is contained alone, it is necessary to satisfy 0.7% or more in total of the Si amount and the Al amount.
  • the amount of Al in the above formula is the amount of Al contained in the steel sheet after deoxidation.
  • the component described below other than the above-mentioned basic component can be contained appropriately.
  • One or more selected from Cr: 0.05% to 5.0%, V: 0.005% to 1.0% and Mo: 0.005% to 0.5% , V and Mo are elements having an action of suppressing the formation of pearlite during cooling from the annealing temperature. The effect is obtained when Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more.
  • Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more if it exceeds Cr: 5.0%, V: 1.0% and Mo: 0.5%, the amount of hard martensite becomes excessive, and the strength becomes higher than necessary. Accordingly, when Cr, V and Mo are contained, Cr: 0.05% to 5.0%, V: 0.005% to 1.0% and Mo: 0.005% to 0.5% % Or less.
  • Ti and Nb are useful for the precipitation strengthening of steel.
  • Each content is 0.01% or more.
  • the workability and the shape freezing property are lowered. Therefore, when Ti and Nb are contained, the range is Ti: 0.01% to 0.1% and Nb: 0.01% to 0.1%.
  • B 0.0003% or more and 0.0050% or less B is an element useful for suppressing the formation and growth of ferrite from the austenite grain boundary. The effect is obtained when the content is 0.0003% or more. On the other hand, if the content exceeds 0.0050%, the workability decreases. Therefore, when it contains B, it is set as B: 0.0003% or more and 0.0050% or less of range.
  • Ni and Cu are effective elements for strengthening steel. Moreover, when performing hot dip galvanization or alloying hot dip galvanization to a steel plate, the internal oxidation of a steel plate surface layer part is accelerated
  • Ca and REM spheroidize the shape of the sulfide, and stretch flange Useful to improve the negative effects of sulfides on sex.
  • the effect is obtained when each content is 0.001% or more.
  • the respective contents exceed 0.005%, inclusions and the like increase, causing surface defects and internal defects. Therefore, when Ca and REM are contained, the range is Ca: 0.001% to 0.005% and REM: 0.001% to 0.005%.
  • components other than the above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.
  • a steel slab adjusted to the above preferred component composition is manufactured, then hot-rolled, and then cold-rolled to obtain a cold-rolled steel sheet.
  • these treatments are not particularly limited, and may be performed according to ordinary methods.
  • the preferred production conditions are as follows. After heating the steel slab to a temperature range of 1000 ° C. or higher and 1300 ° C. or lower, hot rolling is finished in a temperature range of 870 ° C. or higher and 950 ° C. or lower, and the obtained hot rolled steel sheet is heated to a temperature of 350 ° C. or higher and 720 ° C. or lower. Take up in the area.
  • the hot-rolled steel sheet is pickled and then cold-rolled at a rolling reduction in the range of 40% to 90% to obtain a cold-rolled steel sheet.
  • the steel sheet is manufactured through normal steelmaking, casting, hot rolling, pickling and cold rolling processes.
  • the steel plate is heated by thin slab casting or strip casting. You may manufacture by omitting a part or all of a hot rolling process.
  • the obtained cold-rolled steel sheet is subjected to the heat treatment shown in FIG.
  • Annealing is performed for 15 seconds to 600 seconds in an austenite single phase region.
  • the steel sheet of the present invention is mainly composed of upper bainite, lower bainite and martensite which are transformed from untransformed austenite in a relatively low temperature range of 350 ° C. or more and 490 ° C. or less.
  • annealing in the austenite single phase region is necessary.
  • the annealing temperature is not particularly limited as long as it is in the austenite single phase region, but if the annealing temperature exceeds 1000 ° C., the growth of austenite grains is remarkable, which causes coarsening of the constituent phases caused by subsequent cooling, and deteriorates toughness and the like.
  • the annealing temperature is of less than A 3 point (austenitic transformation point) is already generated by the polygonal ferrite in the annealing step, in order to suppress the growth of polygonal ferrite during cooling than 500 ° C. It becomes necessary to cool the temperature range very rapidly. Accordingly, the annealing temperature needs to be 3 points or more, and is preferably 1000 ° C. or less.
  • the annealing time is in the range of 15 seconds to 600 seconds. Preferably, it is the range of 60 seconds or more and 500 seconds or less.
  • [X%] is defined as mass% of the component element X of the steel sheet.
  • the cold-rolled steel sheet after annealing is cooled to a cooling stop temperature: T ° C. determined in a first temperature range of 350 ° C. or more and 490 ° C. or less, but at least up to 550 ° C., the average cooling rate is controlled to 5 ° C./s or more. And cooled.
  • the average cooling rate from the annealing temperature to the first temperature range is set to 5 ° C./s or more. Preferably, it is 10 ° C./s or more.
  • the upper limit of the average cooling rate is not particularly limited as long as the cooling stop temperature does not vary, but in general equipment, when the average cooling rate exceeds 100 ° C./s, the structure in the longitudinal direction and the sheet width direction of the steel plate
  • the dispersion is significantly increased, so that it is preferably 100 ° C./s or less.
  • the steel sheet cooled to 550 ° C. is continuously cooled to a cooling stop temperature: T ° C.
  • the speed at which the steel sheet is cooled in the temperature range of T ° C. or more and 550 ° C. or less is not particularly limited except that the holding time in the first holding temperature range is 15 seconds or more and 1000 seconds or less, but the steel sheet is excessively slow.
  • the steel plate is preferably cooled at an average rate of 1 ° C./s or higher.
  • Cooling stop temperature The steel sheet cooled to T ° C is held for a period of 15 seconds to 1000 seconds in a first temperature range of 350 ° C to 490 ° C.
  • the upper limit of the first temperature range exceeds 490 ° C.
  • carbide precipitates from untransformed austenite and a desired structure cannot be obtained.
  • the lower limit of the first temperature range is less than 350 ° C.
  • lower bainite is generated instead of upper bainite, and there is a problem that the amount of C concentration in austenite decreases. Therefore, the range of the first temperature range is 350 ° C. or more and 490 ° C. or less.
  • the holding time in the first temperature range is less than 15 seconds, the amount of upper bainite transformation is reduced, and the amount of C enrichment in untransformed austenite is problematic.
  • the retention time in the first temperature range exceeds 1000 seconds, stable retained austenite in which C is concentrated by precipitation of carbides from untransformed austenite which becomes retained austenite as the final structure of the steel sheet cannot be obtained.
  • the desired processability cannot be obtained. Therefore, the holding time is 15 seconds or more and 1000 seconds or less. Preferably, it is the range of 30 seconds or more and 600 seconds or less.
  • the steel sheet that has been held in the first temperature range is cooled to a second temperature range of 200 ° C. or higher and 350 ° C. or lower at an arbitrary rate, and held in the second temperature range for 15 seconds to 1000 seconds.
  • the upper limit of the second temperature range exceeds 350 ° C.
  • the lower bainite transformation does not proceed, resulting in a problem that the amount of martensite as quenched is increased.
  • the lower limit of the second temperature range is less than 200 ° C., similarly, the lower bainite transformation does not proceed and the amount of martensite as quenched is increased. Therefore, the range of the second temperature range is 200 ° C. or more and 350 ° C. or less.
  • the holding time is in the range of 15 seconds to 1000 seconds. Preferably, it is the range of 30 seconds or more and 600 seconds or less.
  • the holding temperature does not need to be constant as long as it is within the predetermined temperature range described above, and even if it fluctuates within the predetermined temperature range, the gist of the present invention is not impaired.
  • the cooling rate As long as the thermal history is satisfied, the steel sheet may be heat-treated with any equipment.
  • the method for producing a high-strength steel sheet of the present invention can be further subjected to hot dip galvanizing treatment or galvannealing treatment obtained by adding alloying treatment to hot dip galvanizing treatment.
  • the hot dip galvanizing process or the alloying hot dip galvanizing process may be performed during the cooling to the first temperature range or in the first temperature range.
  • the holding time in the first temperature range is 15 seconds or more and 1000 seconds or less including the processing time in the first temperature range of the hot dip galvanizing process or the alloying galvanizing process.
  • the hot dip galvanizing treatment or alloying hot dip galvanizing treatment is preferably performed in a continuous hot dip galvanizing line.
  • the hot-dip galvanizing treatment or the alloying hot-dip galvanizing treatment is performed again. You can add that. Further, according to the production method of the present invention, the hot dip galvanizing treatment or the alloying hot dip galvanizing treatment can be performed after the holding in the second temperature range.
  • the method of performing hot dip galvanizing treatment or alloying hot dip galvanizing treatment on a steel sheet is as follows.
  • the steel sheet is infiltrated into the plating bath and the amount of adhesion is adjusted by gas wiping.
  • the amount of dissolved Al in the plating bath ranges from 0.12% to 0.22% in the case of hot dip galvanizing, and ranges from 0.08% to 0.18% in the case of galvannealed alloying. It is preferable that In the case of hot dip galvanizing, the temperature of the plating bath may be in the range of 450 ° C. or higher and 500 ° C. or lower. When further alloying is performed, the temperature during alloying is 550 ° C. or lower. It is preferable.
  • alloying temperature exceeds 550 ° C.
  • carbide precipitates from untransformed austenite or pearlite is generated in some cases, so that strength and workability or both cannot be obtained, and the powdering property of the plating layer is also low. to degrade.
  • the temperature during alloying is less than 450 ° C.
  • Coating weight is preferably in a per side 20 g / m 2 or more 150 g / m 2 or less. If the plating adhesion amount is less than 20 g / m 2 , the corrosion resistance is insufficient. On the other hand, if it exceeds 150 g / m 2 g, the corrosion resistance effect is saturated and only the cost is increased.
  • the alloying degree (Fe mass% (Fe content)) of the plating layer is preferably in the range of 7 mass% to 15 mass%. If the degree of alloying of the plating layer is less than 7% by mass, unevenness in alloying occurs and the appearance quality deteriorates, or the so-called ⁇ phase is generated in the plating layer and the slidability of the steel sheet deteriorates. On the other hand, when the degree of alloying of the plating layer exceeds 15% by mass, a large amount of hard and brittle ⁇ phase is formed, and the plating adhesion deteriorates.
  • the slab obtained by melting the steel having the composition shown in Table 1 is heated to 1200 ° C, the hot-rolled steel sheet finished by hot rolling at 870 ° C is wound up at 650 ° C, and then the hot-rolled steel sheet is pickled. Thereafter, it was cold-rolled at a rolling rate of 65% to obtain a cold-rolled steel sheet having a sheet thickness of 1.2 mm.
  • the obtained cold-rolled steel sheet was heat-treated under the conditions shown in Table 2.
  • the cooling stop temperature: T in Table 2 is a temperature at which the cooling of the steel sheet is stopped when the steel sheet is cooled from the annealing temperature. Further, some cold-rolled steel sheets were subjected to hot dip galvanizing treatment or alloying hot dip galvanizing treatment.
  • the hot dip galvanizing treatment double-side plating was performed so that the plating bath temperature was 463 ° C. and the basis weight (per one side) was 50 g / m 2 .
  • the alloying hot dip galvanizing treatment is performed by adjusting the alloying conditions so that the weight per unit area (per one side): 50 g / m 2 and the degree of alloying (Fe mass% (Fe content)) is 9 mass%. Double-sided plating was applied.
  • the hot dip galvanizing treatment and the alloying hot galvannealing hot dip galvanizing treatment were performed after cooling to T ° C shown in Table 2 once.
  • the amount of retained austenite was determined by measuring the X-ray diffraction intensity after grinding and polishing the steel plate to 1 ⁇ 4 of the plate thickness in the plate thickness direction. For incident X-rays, Co—K ⁇ is used, and from the intensity ratio of each surface of austenite (200), (220), (311) to the diffraction intensity of each surface of ferrite (200), (211), (220). The average value was calculated for the amount of retained austenite.
  • the average amount of C in the retained austenite is obtained by calculating the lattice constant from the intensity peaks of the (200), (220), and (311) surfaces of austenite in the X-ray diffraction intensity measurement.
  • C amount (mass%) was calculated
  • a0 0.3580 + 0.0033 ⁇ [C%] + 0.00095 ⁇ [Mn%] + 0.0056 ⁇ [Al%] + 0.022 ⁇ [N%]
  • [X%] Mass% of element X
  • mass% of elements other than C was mass% with respect to the whole steel plate.
  • TS tensile strength
  • T.P. El total elongation
  • TS ⁇ T.El product of strength and total elongation
  • the stretch flangeability was evaluated in accordance with Japan Iron and Steel Federation standard JFST1001.
  • Each steel plate obtained was cut to 100 mm ⁇ 100 mm, a hole with a clearance of 12% of the plate thickness and a diameter of 10 mm was punched out, and then pressed with a wrinkle holding force of 88.2 kN using a die with an inner diameter of 75 mm.
  • a 60 ° conical punch was pushed into the hole, the hole diameter at the crack initiation limit was measured, and the critical hole expansion ratio ⁇ (%) was obtained from the equation (1).
  • Limit hole expansion ratio ⁇ (%) ⁇ (Df ⁇ D0) / D0 ⁇ ⁇ 100 (1)
  • Df is a hole diameter (mm) at the time of crack occurrence
  • D0 is an initial hole diameter (mm).
  • the product of strength and limit hole expansion rate (TS ⁇ ⁇ ) was calculated using ⁇ measured in this manner, and the balance between strength and stretch flangeability was evaluated. In the present invention, when TS ⁇ ⁇ ⁇ 25000 MPa ⁇ %, the stretch flangeability is good.
  • the hardness of the hardest structure in the steel sheet structure was judged by the following method. That is, when martensite is observed as-quenched as a result of structure observation, these martensite as-quenched is measured at 10 points at a load of 0.02N with ultra micro Vickers, and the average value thereof is measured in the steel sheet structure. The hardness of the hardest tissue.
  • any of the structures of tempered martensite, upper bainite or lower bainite is the hardest phase in the steel sheet of the present invention. These hardest phases were HV ⁇ 800 in the case of the steel sheet of the present invention.
  • Table 3 shows the above evaluation results.
  • all the steel plates of the present invention have a tensile strength of 980 MPa or more and TS ⁇ T. Since the value of El satisfies 20000 MPa ⁇ % or more and TS ⁇ ⁇ ⁇ 25000 MPa ⁇ %, it was confirmed that both the high strength and excellent workability, particularly excellent stretch flangeability were obtained.
  • sample no. No. 1 has an average cooling rate of up to 550 ° C. that is outside the proper range, so that a desired steel sheet structure cannot be obtained and TS ⁇ ⁇ ⁇ 25000 MPa ⁇ % is satisfied, but tensile strength (TS) ⁇ 980 MPa and TS ⁇ T. EL ⁇ 20000 MPa ⁇ % was not satisfied.
  • Sample No. No. 2 is a sample No. 2 because the holding time in the first temperature range is outside the proper range. 5, since the annealing temperature is below A 3 point ° C., Sample No. 6 is the cooling stop temperature: T is outside the first temperature range. Since the holding temperature in the second temperature range is outside the proper range, No.
  • TS 11 has a holding time in the second temperature range that is outside the appropriate range, so that a desired steel sheet structure cannot be obtained, and although tensile strength (TS) ⁇ 980 MPa is satisfied, TS ⁇ T. Either EL ⁇ 20000 MPa ⁇ % or TS ⁇ ⁇ ⁇ 25000 MPa ⁇ % was not satisfied. Sample No. 30 to 34, since the component composition is outside the proper range, the desired steel sheet structure cannot be obtained, and the tensile strength (TS) ⁇ 980 MPa, TS ⁇ T. Any one or more of EL ⁇ 20000 MPa ⁇ % and TS ⁇ ⁇ ⁇ 25000 MPa ⁇ % were not satisfied.

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