Classifications

• • 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
  • C22C38/60—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.
• Hydrogen embrittlement cracking (also known as delayed fracture) can be a problem with high-strength steel plates. Hydrogen embrittlement cracking is a phenomenon in which a steel component that is subjected to high stress during use suddenly breaks due to hydrogen that has penetrated into the steel from the environment.
• Patent Document 1 also teaches that by suppressing the coarsening of carbides in tempered martensite, it is possible to improve the delayed fracture resistance of high-strength steel plates with a TS of 1470 MPa or more.
• the present invention aims to provide a high-strength steel plate with excellent hydrogen embrittlement resistance through a new structure.
• the inventors conducted research with a particular focus on the metal structure of the steel plate. Specifically, the inventors discovered that by forming the metal structure of the steel plate mainly from martensite in order to ensure a tensile strength of 1660 MPa or more, while including a predetermined amount of retained austenite, which has the property of easily absorbing hydrogen, in the metal structure, it is possible to significantly suppress the occurrence of hydrogen embrittlement cracking even when the tensile strength is high, at 1660 MPa or more, and thus completed the present invention.
• the present invention which has achieved the above object, is as follows.
• the tensile strength is 1660 MPa or more;
• the metal structure is, in area percent, Martensite: 85.0% or more, Retained austenite: 1.0 to 7.0%; and remaining structure: 10.0% or less;
• the chemical composition, in mass%, is C: 0.25-0.45%, Si: 0.01 to 1.30%, Mn: 1.00-3.50%, P: 0.0001-0.0200%, S: 0.0001-0.0200%, Al: 0.001-1.000%, N: 0.0001-0.0200%, O: 0.0001 to 0.0200%, Co: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0-1.00%, Cr: 0-2.000%, Ti: 0 to 0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, S
• the chemical composition is, in mass%, Co: 0.01 to 0.50%, Ni: 0.01-1.00%, Mo: 0.01-1.00%, Cr: 0.001-2.000%, Ti: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Nb: 0.001-0.500%, V: 0.001-0.500%, Cu: 0.001 to 0.500%, W: 0.001-0.100%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, As: 0.001 to 0.100%, Mg: 0.0001-0.0500%, Ca: 0.001-0.050%, Y: 0.001-0.050%, Zr: 0.001 to 0.050%, La: 0.001 to 0.050%, and Ce: 0.001 to 0.050%
• a member comprising the steel plate according to any one of (1) to (5) above.
• the present invention makes it possible to provide high-strength steel sheets with excellent resistance to hydrogen embrittlement.
• the steel plate according to the embodiment of the present invention has a tensile strength of 1660 MPa or more,
• the metal structure is, in area percent, Martensite: 85.0% or more, Retained austenite: 1.0 to 7.0%; and remaining structure: 10.0% or less;
• the chemical composition, in mass%, is C: 0.25-0.45%, Si: 0.01 to 1.30%, Mn: 1.00-3.50%, P: 0.0001-0.0200%, S: 0.0001-0.0200%, Al: 0.001-1.000%, N: 0.0001-0.0200%, O: 0.0001 to 0.0200%, Co: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0-1.00%, Cr: 0-2.000%, Ti: 0 to 0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Sn
• the inventors therefore conducted a study focusing particularly on the metal structure of the steel plate in order to improve the hydrogen embrittlement resistance of a steel plate having a very high strength such as a tensile strength of 1660 MPa or more.
• the inventors noted that the residual austenite that can be unavoidably formed when generating martensite from austenite has the property of easily absorbing hydrogen.
• the metal structure of the steel plate is composed mainly of martensite, more specifically, a structure containing 85.0% or more martensite by area percentage, and that the metal structure contains a predetermined amount of retained austenite, more specifically, a retained austenite content of 1.0 to 7.0% by area percentage, thereby significantly suppressing the occurrence of hydrogen embrittlement cracking even when the tensile strength is high, such as 1660 MPa or more.
• the retained austenite has the property of easily absorbing hydrogen as described above, it is believed that by making an appropriate amount of the retained austenite present in the metal structure, the hydrogen that has entered the steel can be appropriately trapped by the retained austenite. In this case, it is possible to suppress the hydrogen that has entered the steel from accumulating at the prior austenite grain boundaries in the metal structure mainly composed of martensite, thereby reducing the bonding strength of the grain boundaries. As a result, it is believed that it is possible to significantly suppress the occurrence of hydrogen embrittlement cracking even in high strengths of 1660 MPa or more.
• the steel plate according to the embodiment of the present invention is particularly useful in the automotive field, where there is a relatively high demand for high strength.
• the steel plate according to the embodiment of the present invention has a tensile strength of 1660 MPa or more.
• the tensile strength is preferably 1700 MPa or more, 1760 MPa or more, 1800 MPa or more, 1900 MPa or more, or 2000 MPa or more.
• the hydrogen embrittlement resistance can be significantly improved by including a predetermined amount of retained austenite in a metal structure mainly composed of martensite.
• the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength may be 2500 MPa or less, 2300 MPa or less, 2200 MPa or less, or 2100 MPa or less.
• the tensile strength is measured by performing a tensile test in accordance with JIS Z 2241:2022 based on a JIS No. 5 test piece taken from a direction in which the longitudinal direction of the test piece is preferably parallel to the rolling transverse direction of the steel plate. Even if the rolling direction of the steel sheet cannot be specified, the effects of the present invention can be enjoyed as long as the steel sheet has the above-mentioned tensile strength in any direction within the plane of the steel sheet.
• the tensile test is performed after removing the coating layer.
• the metal structure of the steel sheet according to the embodiment of the present invention will be described.
• the unit of structure fraction "%" means “area %" unless otherwise specified.
• the metal structure is controlled at the surface layer, the 1/4 thickness position, and the 1/2 thickness position of the steel sheet.
• the surface layer refers to a depth position of 50 ⁇ m from the steel sheet surface in the sheet thickness direction.
• the structure fraction refers to the average value of the structure fraction measured at the surface layer, the 1/4 thickness position, and the 1/2 thickness position.
• the position in the thickness direction is specified for the area excluding the coating layer.
• the metal structure contains, in terms of area%, 85.0% or more of martensite.
• 85.0% or more of martensite it is possible to achieve a tensile strength of 1660 MPa or more.
• the higher the area ratio of martensite the more preferable it is, and it may be, for example, 87.0% or more, 90.0% or more, 92.0% or more, or 95.0% or more.
• the upper limit is not particularly limited, but, for example, the area ratio of martensite may be 99.0% or less or 97.0% or less.
• "martensite” includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.
• the metal structure contains, in terms of area %, 1.0 to 7.0% retained austenite.
• 1.0 to 7.0% retained austenite hydrogen that has penetrated into the steel can be appropriately trapped by the retained austenite, so that even if the tensile strength is high, such as 1660 MPa or more, it is possible to significantly suppress the occurrence of hydrogen embrittlement cracking.
• the retained austenite is contained excessively, it may be impossible to ensure a sufficient area ratio of martensite, and it may be impossible to achieve the desired strength.
• a part of the retained austenite that has trapped hydrogen may be transformed into martensite by cold working or the like, or may be decomposed by aging or the like, and some of the trapped hydrogen may be released. If the retained austenite is contained excessively, the amount of hydrogen released may increase, promoting hydrogen embrittlement. Therefore, the area ratio of the retained austenite is set to 7.0% or less, and may be, for example, 6.0% or less or 5.0% or less.
• the remaining structure other than martensite and retained austenite may be 0% in area ratio, but when the remaining structure is present, the remaining structure is 10.0% or less in area ratio. If the remaining structure is excessively contained, it becomes impossible to control the area ratio of martensite and/or retained austenite within a desired range, and as a result, the desired strength and/or hydrogen embrittlement resistance may not be obtained. Therefore, the area ratio of the remaining structure is 10.0% or less, and may be 8.0% or less, 6.0% or less, 5.0% or less, 4.0% or less, 3.0% or less, or 2.0% or less.
• the total area ratio of martensite and retained austenite is 90.0% or more, and may be 92.0% or more, 94.0% or more, 95.0% or more, 96.0% or more, 97.0% or more, or 98.0% or more.
• the area ratio of the remaining structure may be 0.5% or more or 1.0% or more.
• the sum of the area ratios of martensite and retained austenite may be 99.5% or less or 99.0% or less.
• the remaining structure is not particularly limited, and may include at least one of ferrite, bainite, and pearlite, for example, or may be at least one of them.
• the above sample for EBSD analysis is subjected to electron beam backscattering analysis.
• the EBSD analysis conditions are acceptable as long as they are within the scope of common sense of a person skilled in the art, but detailed conditions are described as an example.
• the EBSD detector may be positioned in any position that provides high detection sensitivity of the electron beam, but it is desirable to bring it as close as possible without colliding with the inside of the FE-SEM chamber, the sample, or the jig or table that holds the sample. After that, adjust the detection sensitivity of the electron beam of the EBSD detector.
• the adjustment of the detection sensitivity varies depending on the performance of the electron gun and EBSD detector of the FE-SEM used, so it is desirable to carry out the adjustment within the scope of common sense of a person skilled in the art. The adjustment should result in a condition in which the EBSD pattern can be clearly observed.
• the electron beam is irradiated to the observation field of view at intervals of Step Size 0.3 ⁇ m, and the EBSD pattern of each measurement point is collected. Based on the EBSD pattern of each measurement point, indexing and crystal orientation calculation are performed. For indexing and crystal orientation calculation, it is preferable to use APEX software manufactured by AMETEK.
• the EBSD data obtained in this way is analyzed using OIM Analysis software (Orientation Imaging Microscopy) version 7 or later, which is EBSD data analysis software manufactured by AMETEK.
• the obtained EBSD data is opened on OIM Analysis, and only the area with a CI value (Confidence Index) of 0.1 or more is extracted.
• the CI value is an index of the reliability of the analysis results of indexing and crystal orientation. Areas with a CI value lower than 0.1 are likely to be areas where the orientations of contamination on the sample surface or grain boundaries overlap during electron beam irradiation, and can be considered to be areas that do not have the original crystal orientation of the metal structure. After that, areas with a GAM value (Grain Average Misorientation) of 0.5° or more are considered to be martensite, and the area ratio is calculated. Note that a grain boundary refers to a boundary between measurement points where the orientation difference between the measurement points is 15° or more.
• the GAM value is the average value of the orientation difference between the measurement points of the crystal orientation in the area surrounded by the grain boundaries, and since a structure formed at low temperatures such as martensite has the characteristic that orientation differences occur within the grains due to transformation strain, etc., it is possible to distinguish by the GAM value.
• Such an investigation is carried out at five or more locations in a field of view of 100 x 100 ⁇ m centered at 1/2 the plate thickness position from the steel plate surface, and the average area ratio is derived.
• the area ratio obtained by the above method is the combined area ratio of martensite and retained austenite.
• the area ratio of martensite at the 1/2 sheet thickness position is obtained by subtracting the area ratio of retained austenite measured using the procedure described below from the area ratio obtained by the above method, that is, by calculating [area ratio obtained by EBSD (area ratio of regions with GAM value of 0.5° or more)] - [area ratio of retained austenite obtained by X-ray diffraction].
• martensite in this application is a structure that includes tempered martensite, and no particular distinction is made regarding the tempered state of martensite.
• the area ratio of martensite at the 1/4 thickness position and at the surface layer is measured in a similar manner.
• the observation area is an area of 50 ⁇ m (in the thickness direction) x 200 ⁇ m (perpendicular to the thickness direction) centered at a depth of 50 ⁇ m from the steel plate surface in the thickness direction.
• the average value of the area ratios measured at the surface layer, the 1/4 thickness position, and the 1/2 thickness position is calculated and determined as the area ratio of martensite.
• the area ratio of the retained austenite is calculated by measurement using X-rays. First, a sample is taken from the same member as the sample for identifying martensite, and then the sample is removed from the plate surface to the 1/2 position of the plate thickness in the plate thickness direction by mechanical polishing and chemical polishing. Next, the structure fraction of the retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of the (200) and (211) planes of the bcc phase and the (200), (220), and (311) planes of the fcc phase obtained by using MoK ⁇ rays as characteristic X-rays for the polished sample, and this is the area ratio of the retained austenite at the 1/2 position of the plate thickness.
• the area ratio of the retained austenite is obtained in the same manner for the surface layer and the 1/4 position of the plate thickness, and finally the average value is calculated and determined as the area ratio of the retained austenite.
• the area ratio of the retained austenite at the 1/2 sheet thickness position corresponds to "RA c " described below, and similarly, the area ratio of the retained austenite in the surface layer portion corresponds to "RA s " described below.
• the area ratio of the remaining structure is determined by subtracting the area ratio of martensite and the area ratio of retained austenite obtained above from 100%.
• the remaining structure may contain at least one of ferrite, bainite, and pearlite, for example, or may be at least one of them, but in the present invention, the specification of these structure types and the determination of the area ratio of each structure are not particularly necessary for achieving the object of the present invention. If there is any need, it is not difficult for a person skilled in the art to specify them using a method normally applied.
• Residual austenite in a metal structure is composed of multiple types of residual austenite with various degrees of stability. Therefore, for example, in a metal structure, there exists residual austenite with relatively low stability, as well as residual austenite with relatively high stability. Both types of residual austenite have hydrogen storage capacity and are therefore capable of trapping hydrogen that penetrates into the steel. However, if there is a large amount of residual austenite with low stability, some of the residual austenite may undergo processing-induced martensite transformation due to cold processing such as cold pressing, or some of the residual austenite may decompose due to aging, etc. In such cases, the hydrogen storage capacity decreases due to partial loss of the residual austenite, and some of the hydrogen trapped in the residual austenite is released.
• the released hydrogen may accumulate at the prior austenite grain boundaries in a metal structure mainly composed of martensite, reducing the bonding strength of the grain boundaries and promoting hydrogen embrittlement. Therefore, from the perspective of further improving hydrogen embrittlement resistance, in addition to controlling the area ratio of the retained austenite to within the range of 1.0 to 7.0%, it is preferable to stabilize the retained austenite, thereby suppressing or reducing the loss of the retained austenite and the associated release of hydrogen.
• the present inventors have studied the relationship between hydrogen embrittlement resistance and stabilization of retained austenite in order to further improve the hydrogen embrittlement resistance of steel sheets. More specifically, the present inventors have conducted studies focusing on the fact that the lattice constant calculated from the X-ray diffraction peak derived from the retained austenite is an index of the stability of the retained austenite.
• the present inventors have found that, when measured by X-ray diffraction, the lattice constants calculated from the diffraction peaks derived from the retained austenite at the surface layer portion, the 1/4 position in the sheet thickness, and the 1/2 position in the sheet thickness, i.e., A s , A q , and A c , are controlled so as to satisfy the above formula 1, thereby making it possible to increase the stability of the retained austenite contained in the metal structure of the steel sheet as a whole.
• the present inventors have found that it is possible to significantly suppress the disappearance of the retained austenite due to the processing-induced martensitic transformation caused by cold working, etc., and the decomposition of the retained austenite due to aging, etc.
• austenite stabilizing elements such as C and/or Mn in the retained austenite
• the value of the lattice constant calculated from the diffraction peak derived from the retained austenite can be increased, thereby improving the stability of the retained austenite.
• austenite stabilizing elements such as Mn can be concentrated in the cementite, making it possible to further improve the stability of the austenite.
• austenite stabilizing elements such as C and Mn in the steel sheet can be concentrated in the retained austenite to sufficiently stabilize the retained austenite.
• the concentrated austenite stabilizing elements such as C and/or Mn, for example, act as an interstitial solid solution element to enter voids where atoms do not exist and expand the lattice in the case of C, and act as a substitutional solid solution element to enter lattice positions where Fe atoms originally exist and expand the lattice, and as a result, the value of the lattice constant is considered to be increased. Therefore, a certain correlation is recognized between the stabilization of the retained austenite caused by the austenite stabilizing elements and the value of the lattice constant.
• the present inventors have found that the stability of the retained austenite contained in the metal structure of the steel sheet can be increased throughout the steel sheet by controlling the lattice constants A s , A q and A c calculated from the diffraction peaks derived from the retained austenite at the surface layer, the 1/4 position of the sheet thickness, and the 1/2 position of the sheet thickness, i.e., A s , A q and A c , so as to satisfy the above formula 1 when measured by X-ray diffraction.
• a s , A q and A c may be 3.5830 ⁇ or more (i.e., A s ⁇ 3.5830 ⁇ , A q ⁇ 3.5830 ⁇ , and A c ⁇ 3.5830 ⁇ , the same applies below), 3.5850 ⁇ or more, 3.5880 ⁇ or more, or 3.5900 ⁇ or more, respectively.
• a s , A q and A c may each be 3.7000 ⁇ or less (i.e., A s ⁇ 3.7000 ⁇ , A q ⁇ 3.7000 ⁇ , and A c ⁇ 3.7000 ⁇ , hereinafter), 3.6500 ⁇ or less, or 3.6000 ⁇ or less.
• the lattice constants A s , A q and A c are calculated as follows. First, the sample is removed from the plate surface to the 1/2 position of the plate thickness in the plate thickness direction by mechanical polishing and chemical polishing.
• the lattice constants A c (200), A c (220) and A c (311) in each plane of the retained austenite are calculated from the diffraction peaks of the (200) , (220) and (311 ) planes of the fcc phase obtained by using MoK ⁇ rays as characteristic X-rays for the polished sample based on the following formulas 3 and 4, and then the lattice constant A c is determined as the average of these based on the following formula 5.
• d c(hkl) is the lattice spacing ( ⁇ ) of the (hkl) plane at the 1/2 position of the plate thickness
• ⁇ is the light source wavelength (wavelength of MoK ⁇ rays) ( ⁇ ) of the X-ray diffraction device
• a c(hkl) is the lattice constant ( ⁇ ) of the (hkl) plane at the 1/2 position of the plate thickness
• 2 ⁇ c(hkl) is the diffraction angle (°) of the diffraction peak of the (hkl) plane at the 1/2 position of the plate thickness.
• 2 ⁇ c(hkl) is calculated from the diffraction intensity and diffraction angle 2 ⁇ obtained by X-ray diffraction.
• the obtained 2 ⁇ value is smoothed by a three-point weighted average using the following formula 6. Then, the 2 ⁇ with the highest diffraction intensity on each diffraction surface is taken as the 2 ⁇ c(hkl) of that diffraction surface.
• 2 ⁇ n (2 ⁇ n-1 ⁇ P n-1 +2 ⁇ n ⁇ P n +2 ⁇ n+1 ⁇ P n+1 )/(P n-1 +P n +P n+1 ).
• Pn refers to the diffraction intensity at the diffraction angle 2 ⁇ n in the data group of "diffraction angle vs.
• diffraction intensity and in addition to Pn , a total of three points of data, namely the diffraction intensities before and after the diffraction angle 2 ⁇ n (i.e., Pn -1 and Pn +1 ), are used to perform a three-point weighted average smoothing process, and this smoothing process is performed on all measurement points.
• Pn -1 and Pn +1 the diffraction intensities before and after the diffraction angle 2 ⁇ n
• the area fraction RA s of the retained austenite in the surface layer portion and the area fraction RA c of the retained austenite at the 1/2 sheet thickness position satisfy RA s /RA c ⁇ 0.75.
• the inventors have found that the hydrogen embrittlement resistance during bending can be significantly improved by controlling the amount of retained austenite in the metal structure of the surface layer of the steel sheet to be less than the amount of retained austenite in the metal structure inside the steel sheet by a predetermined ratio while maintaining the amount of retained austenite in the entire steel sheet within the range of 1.0 to 7.0 %, more specifically, by controlling the area fraction RA s of the retained austenite in the surface layer and the area fraction RA c of the retained austenite at the 1/2 position of the sheet thickness to satisfy RA s /RA c ⁇ 0.75.
• the amount of unstable retained austenite in the surface layer is naturally reduced.
• the amount of unstable retained austenite in the surface layer is reduced in this way, the reduction or disappearance of the retained austenite in the surface layer due to deformation-induced martensitic transformation caused by bending such as cold pressing is suppressed.
• the amount of hydrogen released from the retained austenite in the surface layer due to bending or the like can be reduced, thereby making it possible to further improve the hydrogen embrittlement resistance of the steel plate compared to the case where the amount of retained austenite is simply controlled within the range of 1.0 to 7.0%.
• the area ratio RAs of the retained austenite in the surface layer it is preferable to reduce the amount of hydrogen released from the retained austenite in the surface layer. Therefore, the smaller the ratio of RAs / RAc , the more preferable it is, and it may be, for example, 0.72 or less (i.e., RAs / RAc ⁇ 0.72, the same below), 0.70 or less, 0.68 or less, 0.65 or less, 0.62 or less, or 0.60 or less.
• the lower limit is not particularly limited, and for example, the ratio of RAs / RAc may be more than 0, or 0.10 or more (i.e., RAs / RAc ⁇ 0.10, the same below), 0.20 or more, or 0.30 or more.
• the method of calculating RAs and RAc is as described above in the section [Identification and calculation of metal structure].
• the inventors have found that the hydrogen embrittlement resistance during bending can be significantly improved by controlling the stability of the retained austenite in the metal structure in the region closer to the surface of the steel plate to be relatively high while maintaining the amount of retained austenite in the entire steel plate within the range of 1.0 to 7.0%, more specifically, by controlling the lattice constants calculated from the diffraction peaks derived from the retained austenite in the surface layer, the 1/4 position, and the 1/2 position so as to satisfy the above formula 2.
• the stability of the retained austenite in the surface layer and the 1/4 position it is possible to suppress the reduction or disappearance of the retained austenite in the surface layer and the 1/4 position due to bending such as cold pressing, etc., due to deformation-induced martensitic transformation.
• the amount of hydrogen released from the retained austenite in the surface layer and the 1/4 position due to bending, etc. can be reduced, which makes it possible to further improve the hydrogen embrittlement resistance of the steel plate compared to the case where the amount of retained austenite is simply controlled within the range of 1.0 to 7.0%.
• the upper limit is not particularly limited, but for example, the ratios of A / A and Aq / A may be 1.1000 or less (i.e., Aq / A ⁇ 1.1000, and Aq / A ⁇ 1.1000, the same applies below), 1.0500 or less, or 1.0200 or less.
• the method of calculating A , Aq , and Ac is as described above in the section [Calculation of lattice constants A , Aq , and Ac ].
• the lattice constant calculated from the diffraction peaks derived from the retained austenite at the surface layer portion, the 1/4 sheet thickness position, and the 1/2 sheet thickness position satisfies the above formula 1
• the area fraction RAs of the retained austenite at the surface layer portion and the area fraction RAc of the retained austenite at the 1/2 sheet thickness position satisfy RAs / RAc ⁇ 0.75
• the lattice constant calculated from the diffraction peaks derived from the retained austenite at the surface layer portion, the 1/4 sheet thickness position, and the 1/2 sheet thickness position satisfies the above formula 2.
• the stability of the retained austenite contained in the metal structure of the steel sheet is increased throughout the entire steel sheet, and the amount of unstable retained austenite, particularly in the surface layer portion, is reduced, while the stability of the retained austenite present at the surface layer portion and the 1/4 sheet thickness position can be further increased. Therefore, the amount of hydrogen released from retained austenite due to bending, etc. can be significantly reduced, particularly in the surface layer and 1/4 position of the plate thickness of the steel plate where hydrogen embrittlement is likely to occur during bending, and the hydrogen embrittlement resistance of the steel plate can be more significantly improved.
• the chemical composition of the steel sheet is, in mass%
• the chemical composition, in mass% is C: 0.25-0.45%, Si: 0.01 to 1.30%, Mn: 1.00-3.50%, P: 0.0001-0.0200%, S: 0.0001-0.0200%, Al: 0.001-1.000%, N: 0.0001-0.0200%, O: 0.0001 to 0.0200%, Co: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0-1.00%, Cr: 0-2.000%, Ti: 0 to 0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0 to 0.100%, As: 0 to 0.100%, Mg: 0 to 0.0500%, Ca: 0-0.050%, Y: 0 to 0.0500%, Ca: 0-0.05
• C is an element effective for increasing tensile strength at low cost.
• C is also an element effective for stabilizing austenite.
• the C content is set to 0.25% or more.
• the C content may be 0.26% or more, 0.28% or more, 0.29% or more, or 0.30% or more.
• the C content is set to 0.45% or less.
• the C content may be 0.42% or less, 0.40% or less, or 0.38% or less.
• Silicon acts as a deoxidizer and is an element that affects the morphology of carbides and residual austenite after heat treatment. If silicon is not contained, it may be difficult to suppress the generation of coarse oxides. Therefore, the silicon content is set to 0.01% or more. The silicon content may be 0.05% or more, 0.10% or more, 0.30% or more, or 0.50% or more. On the other hand, if the Si content is excessive, local ductility may decrease. Therefore, the Si content is set to 1.30% or less. The Si content may be 1.20% or less, 1.00% or less, 0.80% or less, or 0.60% or less.
• Mn is an element effective in increasing the hardenability of steel and increasing the strength of steel plate. Mn is also an element effective in stabilizing austenite. In order to fully obtain these effects, the Mn content is set to 1.00% or more. The Mn content may be 1.20% or more, 1.50% or more, 1.80% or more, 2.00% or more, 2.20% or more, 2.40% or more, or 2.50% or more. On the other hand, excessive Mn content not only promotes co-segregation with P and S, but also may deteriorate corrosion resistance. For this reason, the Mn content is set to 3.50% or less. The Mn content may be 3.20% or less, 3.00% or less, 2.80% or less, or 2.60% or less.
• P is an element that embrittles welds and deteriorates platability. Therefore, the P content is set to 0.0200% or less.
• the P content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less.
• the P content is set to 0.0001% or more.
• the P content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.
• S is an element that generates nonmetallic inclusions such as MnS in steel. If S is contained excessively, the generation of nonmetallic inclusions that become the starting point of cracks during cold working becomes significant. For this reason, the S content is set to 0.0200% or less.
• the S content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. Although the lower the S content, the better, reducing the S content to less than 0.0001% requires a long time for refining, which leads to a significant increase in costs. For this reason, the S content is set to 0.0001% or more.
• the S content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.
• Al is an element that acts as a deoxidizer for steel.
• the Al content is set to 0.001% or more.
• the Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.050% or more.
• the Al content is set to 1.000% or less.
• the Al content may be 0.950% or less, 0.900% or less, 0.800% or less, or 0.600% or less.
• N is an element that causes blowholes during welding. Therefore, the N content is set to 0.0200% or less.
• the N content may be 0.0180% or less, 0.0160% or less, 0.0120% or less, or 0.0100% or less.
• reducing the N content to less than 0.0001% would result in a significant increase in manufacturing costs, so the N content is set to 0.0001% or more.
• the N content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.
• O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0200% or less.
• the O content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. The lower the O content, the more preferable. However, reducing the O content to less than 0.0001% leads to a significant increase in manufacturing costs. For this reason, the O content is set to 0.0001% or more.
• the O content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
• the steel plate may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.
• the steel plate may contain at least one selected from the group consisting of Co: 0-0.50%, Ni: 0-1.00%, Mo: 0-1.00%, Cr: 0-2.000%, Ti: 0-0.500%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.100%, and Ta: 0-0.100%.
• the steel plate may also contain at least one selected from the group consisting of Sn: 0-0.100%, Sb: 0-0.100%, and As: 0-0.100%.
• the steel sheet may also contain at least one element selected from the group consisting of Mg: 0-0.0500%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, and Ce: 0-0.050%.
• Co is an element effective for controlling the morphology of carbides and increasing the strength of the steel sheet.
• the Co content may be 0%, but in order to obtain these effects, the Co content is preferably 0.001% or more.
• the Co content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more.
• excessive Co content may cause precipitation of coarse Co carbides. Therefore, the Co content is preferably 0.50% or less.
• the Co content may be 0.40% or less, 0.30% or less, or 0.20% or less.
• Ni is an element effective in increasing the strength of the steel sheet. Ni is also an element effective in improving wettability and promoting alloying reaction.
• the Ni content may be 0%, but in order to obtain these effects, the Ni content is preferably 0.001% or more.
• the Ni content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more.
• the Ni content is preferably 1.00% or less.
• the Ni content may be 0.90% or less, 0.80% or less, 0.60% or less, or 0.30% or less.
• Mo is an element effective in increasing the strength of steel sheets. Mo is also an element that has the effect of suppressing ferrite transformation that occurs during heat treatment in a continuous annealing facility or a continuous hot-dip galvanizing facility.
• the Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more.
• the Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.08% or more.
• the Mo content is preferably 1.00% or less.
• the Mo content may be 0.90% or less, 0.80% or less, 0.60% or less, or 0.30% or less.
• Cr is an element that suppresses pearlite transformation and is effective in increasing the strength of steel.
• the Cr content may be 0%, but in order to obtain such an effect, the Cr content is preferably 0.001% or more.
• the Cr content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.050% or more.
• excessive Cr content may cause coarse Cr carbides to form in the central segregation region, so the Cr content is preferably 2.000% or less.
• the Cr content may be 1.800% or less, 1.500% or less, 1.000% or less, or 0.500% or less.
• Ti is an element that contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by inhibiting the growth of ferrite crystal grains, and strengthening dislocations through inhibiting recrystallization.
• the Ti content may be 0%, but in order to obtain these effects, the Ti content is preferably 0.001% or more.
• the Ti content may be 0.005% or more, 0.010% or more, or 0.050% or more.
• excessive Ti content may increase the precipitation of carbonitrides, so the Ti content is preferably 0.500% or less.
• the Ti content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.
• B is an element that suppresses the formation of ferrite and pearlite during the cooling process from the austenite temperature range and promotes the formation of low-temperature transformation structures such as martensite. B is also an element that is beneficial for increasing the strength of steel.
• the B content may be 0%, but in order to obtain these effects, the B content is preferably 0.0001% or more.
• the B content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more.
• the B content is preferably 0.0100% or less.
• the B content may be 0.0080% or less, 0.0060% or less, 0.0050% or less, or 0.0020% or less.
• Nb is an element effective for controlling the morphology of carbides and is also effective for improving toughness by refining the structure.
• the Nb content may be 0%, but in order to obtain these effects, the Nb content is preferably 0.001% or more.
• the Nb content may be 0.002% or more, 0.010% or more, or 0.020% or more.
• excessive Nb content may generate coarse Nb carbides, and therefore the Nb content is preferably 0.500% or less.
• the Nb content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.
• V is an element that contributes to increasing the strength of the steel sheet through precipitation strengthening, fine grain strengthening by inhibiting the growth of ferrite crystal grains, and dislocation strengthening through inhibiting recrystallization.
• the V content may be 0%, but in order to obtain these effects, the V content is preferably 0.001% or more.
• the V content may be 0.002% or more, 0.010% or more, or 0.020% or more.
• excessive V content may increase the precipitation of carbonitrides, so the V content is preferably 0.500% or less.
• the V content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.
• Cu is an element effective in improving the strength of a steel sheet.
• the Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.001% or more.
• the Cu content may be 0.002% or more, 0.010% or more, or 0.030% or more.
• the Cu content is preferably 0.500% or less.
• the Cu content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.
• W is an element effective in increasing the strength of steel sheet.
• W forms precipitates and crystallized products. Precipitates and crystallized products containing W become hydrogen trapping sites, so W is an element effective in improving hydrogen embrittlement resistance.
• the W content may be 0%, but in order to obtain these effects, the W content is preferably 0.001% or more.
• the W content may be 0.002% or more, 0.005% or more, or 0.010% or more.
• excessive W content may cause the formation of coarse W precipitates or crystallized products. Therefore, the W content is preferably 0.100% or less.
• the W content may be 0.080% or less, 0.060% or less, 0.050% or less, or 0.030% or less.
• Ta is an element effective for controlling the morphology of carbides and increasing the strength of the steel sheet.
• the Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more.
• the Ta content may be 0.002% or more, 0.005% or more, or 0.010% or more.
• the Ta content is preferably 0.100% or less.
• the Ta content may be 0.080% or less, 0.060% or less, 0.050% or less, or 0.020% or less.
• Sn is an element contained in steel when scrap is used as a steel raw material. If the Sn content is high, there is a risk of causing embrittlement of ferrite. Therefore, the Sn content is preferably 0.100% or less.
• the Sn content may be 0.060% or less, 0.030% or less, or 0.020% or less. The lower the Sn content, the better, and even 0% may be used, but reducing the Sn content to less than 0.001% requires a long time for refining, which leads to a significant increase in costs. Therefore, the Sn content may be 0.001% or more.
• the Sn content may be 0.002% or more, 0.005% or more, or 0.010% or more.
• Sb is an element contained when scrap is used as a steel raw material, similar to Sn. Sb is also an element that causes a decrease in ductility. For this reason, the Sb content is preferably 0.100% or less.
• the Sb content may be 0.040% or less, 0.030% or less, or 0.020% or less. The lower the Sb content, the better, and it may be 0%, but reducing the Sb content to less than 0.001% requires a long time for refining, which leads to a significant increase in costs. Therefore, the Sb content may be 0.001% or more.
• the Sb content may be 0.002% or more, 0.005% or more, or 0.008% or more.
• the As content is preferably 0.100% or less.
• the As content may be 0.040% or less, 0.030% or less, or 0.020% or less.
• the As content is preferably as low as possible, and may be 0%, but reducing the As content to less than 0.001% requires a long time for refining, which leads to a significant increase in costs. Therefore, the As content may be 0.001% or more.
• the As content may be 0.002% or more, 0.003% or more, or 0.005% or more.
• Mg is an element that can control the morphology of sulfides when contained in a small amount.
• the Mg content may be 0%, but in order to obtain such an effect, the Mg content is preferably 0.0001% or more.
• the Mg content may be 0.0005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more.
• excessive Mg content may cause the formation of coarse inclusions, and therefore the Mg content is preferably 0.0500% or less.
• the Mg content may be 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0030% or less.
• Ca is useful as a deoxidizing element and is also effective in controlling the morphology of sulfides.
• the Ca content may be 0%, but in order to obtain these effects, the Ca content is preferably 0.0001% or more.
• the Ca content may be 0.0005% or more, 0.001% or more, or 0.002% or more.
• excessive Ca content may cause the formation of coarse inclusions. Therefore, the Ca content is preferably 0.050% or less.
• the Ca content may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.
• Y, Zr, La, and Ce are elements that can control the morphology of sulfides when contained in small amounts, similar to Mg, etc.
• the Y, Zr, La, and Ce contents may be 0%, but in order to obtain such effects, the Y, Zr, La, and Ce contents are preferably 0.0001% or more, and may be 0.0005% or more, 0.001% or more, or 0.002% or more.
• the Y, Zr, La and Ce contents are each preferably 0.050% or less, and may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.
• the remainder other than the above elements consists of Fe and impurities.
• Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel plate is industrially manufactured.
• the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method.
• the measurement may be performed using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for cutting chips in accordance with JIS G 1201:2014.
• ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
• a 35 mm square test piece may be obtained from the vicinity of the 1/2 position of the plate thickness of the steel plate, and the measurement may be performed under conditions based on a calibration curve created in advance using a Shimadzu ICPS-8100 or the like (measuring device).
• C and S which cannot be measured by ICP-AES, may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. If the steel plate has a coating layer on its surface, the coating layer may be removed by mechanical grinding or the like before analyzing the chemical composition.
• the steel sheet according to the embodiment of the present invention generally has a sheet thickness of 0.6 to 6.0 mm.
• the sheet thickness may be 1.0 mm or more, 1.2 mm or more, or 1.4 mm or more, and/or 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.
• the hot-dip plating layer includes, for example, a hot-dip galvanizing layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
• the electroplating layer includes, for example, an electrogalvanizing layer, an electrogalvanizing Zn-Ni alloy plating layer, and the like.
• the coating layer is a hot-dip galvanizing layer, an alloying hot-dip galvanizing layer, or an electrogalvanizing layer.
• the coating layer may have a general coating amount without any particular limitation. As described above, such a coating layer is excluded in determining the tensile strength, the thickness direction position, and the chemical composition in the provisions of the present invention.
• the steel plate according to the embodiment of the present invention has an extremely high tensile strength of 1660 MPa or more, yet has excellent hydrogen embrittlement resistance, making it extremely useful for use as, for example, automotive frame members, bumpers, and other structural and reinforcing members that require strength.
• the method for producing a steel sheet according to an embodiment of the present invention comprises: a hot rolling process including heating a slab having the chemical composition described above in relation to the steel plate to a temperature of 1100-1300°C, then finish rolling the slab under conditions of a final temperature of 850-1050°C, and cooling the finish-rolled steel plate to 500°C or less at an average cooling rate of 20°C/s or more and coiling the steel plate; a pickling step of pickling the obtained hot-rolled steel sheet; a cold rolling process in which the pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 35 to 80%;
• the method includes an annealing step of heating the obtained cold-rolled steel sheet and then holding it at a maximum heating temperature of 830 to 900°C for 20 to 150 seconds, and a cooling step of cooling the cold-rolled steel sheet to the Ms point or lower at an average cooling rate of 1.0°C/second or more, and when a stabilization treatment step and/or a skin-pass rolling step are further included, the
• the slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method.
• the slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to dissolve the alloying elements in the slab. If the heating temperature is less than 1100°C, the alloying elements may not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. For this reason, the heating temperature is preferably 1100°C or higher.
• the upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower from the viewpoint of the capacity and productivity of the heating equipment.
• the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
• the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
• the heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling. Since the slab used as described above contains a relatively large amount of alloy elements, it is necessary to increase the rolling load during hot rolling. For this reason, it is preferable to perform hot rolling at a high temperature.
• the end temperature of the finish rolling is important in terms of controlling the metal structure of the steel sheet. If the end temperature of the finish rolling is low, the metal structure may become non-uniform and the formability may decrease. For this reason, the end temperature of the finish rolling is preferably 850°C or higher. On the other hand, in order to suppress the coarsening of austenite, it is preferable that the end temperature of the finish rolling is 1050°C or lower.
• the finish-rolled steel sheet is cooled to 500°C or less at an average cooling rate of 20°C/s or more and coiled. If the average cooling rate is less than 20°C/s or the coiling temperature is more than 500°C, segregation of P occurs in the hot rolling process, embrittling the hot-rolled steel sheet, and subsequent cold rolling may become difficult.
• the average cooling rate is preferably 25°C/s or more, and the coiling temperature is preferably 480°C or less.
• the average cooling rate is preferably 100°C/s or less, and the coiling temperature is preferably 300°C or more.
• the method further includes a post-hot rolling treatment step of retaining the obtained hot-rolled steel sheet in a temperature range of 400 to 680 ° C. for 1 hour or more and less than 24 hours after the hot rolling step and before the pickling step.
• a post-hot rolling treatment step of retaining the obtained hot-rolled steel sheet in a temperature range of 400 to 680 ° C. for 1 hour or more and less than 24 hours after the hot rolling step and before the pickling step.
• a s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1/4 sheet thickness position and the 1/2 sheet thickness position, respectively.
• the residence temperature is lower than 400°C, the diffusion of elements such as Mn into the cementite is insufficient, and the austenite cannot be sufficiently stabilized in the annealing process.
• the residence temperature is higher than 680°C, the cementite dissolves, and the effect of enriching the cementite with elements cannot be fully obtained.
• the residence time is less than one hour, the formation of cementite is insufficient, and similarly the effect of enriching the cementite with elements cannot be fully obtained. In either case, it becomes difficult to obtain a steel sheet having a metal structure that satisfies the above formula 1.
• the hot-rolled steel sheet after coiling is lower than 400°C, the hot-rolled steel sheet can be heated again as necessary.
• the obtained hot-rolled steel sheet is pickled to remove the oxide scale formed on the surface of the hot-rolled steel sheet.
• the pickling may be performed once or may be performed multiple times to ensure the removal of the oxide scale, as long as the pickling is performed under conditions suitable for removing the oxide scale.
• the pickled hot-rolled steel sheet is cold-rolled at a reduction of 35 to 80% in the cold rolling process.
• the reduction of the cold rolling is preferably 50% or more.
• the reduction of the cold rolling is preferably 70% or less.
• the number of rolling passes and the reduction of each pass are not particularly limited, and may be appropriately set so that the reduction of the entire cold rolling is within the above range.
• the maximum heating temperature is less than 830 ° C or the holding time at the maximum heating temperature is less than 20 seconds, austenitization is likely to be insufficient, making it difficult to obtain the desired area ratio of martensite in the finally obtained steel sheet, and making it difficult to achieve a tensile strength of 1660 MPa or more.
• the maximum heating temperature exceeds 900 ° C or the holding time at the maximum heating temperature exceeds 150 seconds, austenite becomes coarse, hardenability decreases, and ferrite transformation and bainite transformation are likely to occur. As a result, it becomes difficult to obtain the desired structure fraction in the final structure.
• a coating treatment may be applied to the surface of the cold-rolled steel sheet for the purpose of improving corrosion resistance, etc.
• the coating treatment may be a treatment such as hot-dip plating, alloyed hot-dip plating, or electroplating.
• the steel sheet may be subjected to a hot-dip galvanizing treatment as the coating treatment, or an alloying treatment may be performed after the hot-dip galvanizing treatment.
• the coating layer includes, for example, at least one selected from the group consisting of zinc, aluminum, magnesium, and alloys thereof. More specifically, the coating layer may be a hot-dip plating layer or an electroplating layer.
• the hot-dip plating layer includes, for example, a hot-dip galvanizing (GI) layer, an alloyed hot-dip galvanizing (GA) layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
• the electroplating layer includes, for example, an electric galvanizing layer, an electric Zn-Ni alloy plating layer, and the like.
• the coating layer is a hot-dip galvanizing layer, an alloyed hot-dip galvanizing layer, or an electric galvanizing layer.
• the specific conditions for the coating treatment and the alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art.
• the average cooling rate is 10.0°C/s or more and 20.0°C/s or less.
• the stability of the retained austenite contained in the metal structure of the steel sheet can be increased throughout the steel sheet.
• the cold-rolled steel sheet after the annealing step or the covering step is cooled to a controlled temperature T of 100 ° C. or more and Ms point -100 ° C. or less at an average cooling rate of 1.0 ° C./sec or more, and then cooled from the controlled temperature T to room temperature (25 ° C.) at an average cooling rate of less than 1.0 ° C./sec.
• T controlled temperature
• room temperature 25 ° C.
• a s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1/4 sheet thickness position and the 1/2 sheet thickness position, respectively.
• control temperature T is higher than the Ms point -100°C, there is a relatively large amount of untransformed austenite, so even if carbon enrichment occurs in the untransformed austenite, the residual austenite in the final metal structure may not be sufficiently stabilized. In this case, it is difficult to obtain a steel sheet having a metal structure that satisfies the above formula 1.
• control temperature T is less than 100°C or the average cooling rate is less than 1.0°C/sec, it is not possible to promote enrichment of elements such as carbon in austenite, and similarly, the residual austenite in the final metal structure may not be sufficiently stabilized. In this case, it is also difficult to obtain a steel sheet having a metal structure that satisfies the above formula 1.
• the lower limit of the average cooling rate from the control temperature T to room temperature is preferably, for example, 0.1°C/sec or more.
• the cold-rolled steel sheet after the cooling step is subjected to a stabilization treatment.
• the stabilization treatment includes heating the cold-rolled steel sheet cooled to the Ms point or below or room temperature in the cooling step as necessary, and then holding it in a temperature range of 70 ° C. or higher and lower than 280 ° C. for 1 hour to less than 200 hours.
• a s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1/4 sheet thickness position and the 1/2 sheet thickness position, respectively.
• the treatment temperature is less than 70°C or the holding time is less than 1 hour, the stabilization of the retained austenite will be insufficient, making it difficult to obtain a steel sheet having a metal structure that satisfies the above formula 1.
• the stabilization treatment process is carried out and the treatment temperature is 280°C or higher or the holding time is 200 hours or longer, much of the retained austenite will decompose due to excessive heat treatment, and the area ratio of the retained austenite in the final structure may be less than 1.0%, and/or the desired strength may not be achieved due to softening.
• the treatment temperature is 100 to 200°C and the holding time is 10 to 100 hours.
• the cold-rolled steel sheet after the cooling step or the cold-rolled steel sheet after the stabilization step is subjected to skin pass rolling.
• the skin pass rolling includes rolling the cold-rolled steel sheet after the cooling step or the cold-rolled steel sheet after the stabilization step to an elongation rate of more than 0.05% and less than 2.00%.
• the amount of hydrogen released from the retained austenite at the surface layer and at the 1/4 position of the plate thickness due to bending or the like can be significantly reduced, thereby making it possible to further improve the hydrogen embrittlement resistance of the steel plate compared to the case where the amount of retained austenite is simply controlled within the range of 1.0 to 7.0%.
• a s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1/4 sheet thickness position and the 1/2 sheet thickness position, respectively.
• the elongation rate is 0.05% or less, the decomposition of unstable retained austenite present in the surface layer of the steel sheet becomes insufficient, making it difficult to obtain a steel sheet having a metal structure that satisfies RAs / RAc ⁇ 0.75 and/or the above formula 2.
• the elongation rate is 2.00% or more when a skin-pass rolling step is performed, much of the retained austenite may be decomposed due to excessive skin-pass rolling, and the area ratio of the retained austenite in the final structure may be less than 1.0%.
• the elongation rate is 0.10 to 1.00%.
• steel plates according to embodiments of the present invention were manufactured under various conditions, and the tensile strength and hydrogen embrittlement resistance of the resulting steel plates were examined.
• molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1, and these slabs were heated to the heating temperatures shown in Table 2 and hot-rolled.
• Hot rolling was performed by rough rolling and finish rolling, and the end temperatures of the finish rolling were as shown in Table 2.
• the finish-rolled steel sheets were then cooled under the conditions shown in Table 2 and coiled.
• the obtained hot-rolled steel sheets having a thickness of 2.6 mm were appropriately subjected to the post-hot rolling treatment shown in Table 2, then pickled, and cold-rolled at the rolling reduction shown in Table 2 to obtain cold-rolled steel sheets having a thickness of 1.4 mm.
• the obtained cold-rolled steel sheets were heated in the heating furnace and soaking furnace of a continuous annealing line, heated and held under the conditions shown in Table 2, and then appropriately subjected to hot-dip galvanizing (GI) or alloyed hot-dip galvanizing (GA) as a coating treatment.
• GI hot-dip galvanizing
• GA alloyed hot-dip galvanizing
• the cold-rolled steel sheet or plated steel sheet was cooled in the cooling process under the conditions shown in Table 2, and then the stabilization treatment process and/or skin pass rolling process shown in Table 2 were appropriately performed.
• the "average cooling rate to the Ms point or below" was evaluated as the "average cooling rate to the control temperature T" in all examples.
• the properties of the resulting steel plates were measured and evaluated using the following methods.
• the tensile strength was measured by carrying out a tensile test in accordance with JIS Z 2241:2022 on a JIS No. 5 test piece taken in a direction such that the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel plate.
• the hydrogen embrittlement resistance of the bent part of the obtained steel plate was evaluated by the following method. Specifically, first, the steel plate was cut by applying a shear force with a clearance of 12.5% and a shear angle of 0 degrees to obtain a steel plate with a width of 15 mm. At this time, the test piece was taken in a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the rolling direction of the steel plate, that is, the bending ridge line was parallel to the rolling direction of the steel plate. If the rolling direction of the steel plate cannot be specified, the test piece may be taken in any direction within the plate surface of the steel plate. Next, a U-bend test was performed at 8R.
• the U-bend test was performed by the push-bend method described in JIS Z 2248:2022.
• the inner radius r was set to 8 mm, and the distance between the supports L was set to 2r + 2t (thickness of the test piece) ⁇ 1 mm.
• the test piece was pushed until it passed through the supports, and a sample bent at 180° was obtained.
• a strain gauge was attached to the center of the obtained test piece, and stress was applied by tightening both ends of the test piece with bolts. The applied stress was calculated from the strain of the monitored strain gauge.
• the strain gauge used had a gauge length of 1.0 mm, and was attached so that it was parallel to the longitudinal direction of the steel plate at the bend apex, that is, perpendicular to the bend ridge line.
• the load stress was defined as the strain amount at the bend apex measured by the strain gauge ⁇ Young's modulus (constant at 20500 N/ mm2 ), and the load stress was applied at a stress corresponding to 80% of the tensile strength of the test piece. This is because the residual stress introduced during forming is considered to correspond to the tensile strength of the steel plate.
• the obtained U-bend test piece was immersed in an HCl aqueous solution with a pH of 2 at a liquid temperature of 35°C, and then held for 72 hours to check for the presence or absence of cracks.
• High strength steel plates having a tensile strength of 1660 MPa or more and a hydrogen embrittlement resistance rating of A, AA or AAA were evaluated as having excellent hydrogen embrittlement resistance.
• the results are shown in Table 3.
• Table 3 the values of RAs , RAq , As and Aq show only values calculated based on measurements from one surface of the steel plate. However, since all steel plates are manufactured by performing the same treatment on both sides, these values are substantially the same on both sides of the steel plate, and it has been confirmed that in some steel plates, these values are the same on both sides of the steel plate.
• Example A-2 the maximum heating temperature in the annealing process was low, so austenitization was insufficient and the desired martensite area ratio could not be obtained. As a result, the tensile strength decreased.
• Example B-2 the holding time in the annealing process was short, so austenitization was similarly insufficient and the desired martensite area ratio could not be obtained. As a result, the tensile strength decreased.
• Example C-2 the holding time in the annealing process was long, so it is believed that the austenite coarsened and the hardenability decreased.
• Example D-2 the average cooling rate to the Ms point or below in the cooling process was slow, so a relatively large amount of ferrite and bainite was formed during the cooling process. As a result, the desired martensite area ratio could not be obtained, and the tensile strength decreased.
• Example F-2 it is believed that much of the retained austenite decomposed because the processing temperature in the stabilization treatment process was high. As a result, the desired area ratio of retained austenite could not be obtained in the final structure, and hydrogen embrittlement resistance was degraded.
• Example G-2 it is believed that the holding time in the stabilization treatment process was long, which similarly caused much of the retained austenite to decompose. As a result, the desired area ratio of retained austenite could not be obtained in the final structure, and hydrogen embrittlement resistance was degraded.
• Example H-2 it is believed that the elongation rate in the skin pass rolling process was high, which caused much of the retained austenite to decompose. As a result, the desired area ratio of retained austenite could not be obtained in the final structure, and hydrogen embrittlement resistance was degraded.
• the metal structure of the steel plate is mainly composed of martensite, but the metal structure contains a predetermined amount of retained austenite, which has the property of easily absorbing hydrogen; more specifically, the metal structure of the steel plate is controlled to contain, by area percentage, 85.0% or more of martensite and 1.0 to 7.0% of retained austenite. This makes it possible to significantly improve the hydrogen embrittlement resistance, particularly of the bent portion, despite the extremely high tensile strength of 1660 MPa or more.
• a suitable post-hot rolling treatment step was performed after the hot rolling step and before the pickling step, and in the cooling step, the steel was cooled to a controlled temperature T of 100°C or higher and Ms point -100°C or lower at an average cooling rate of 1.0°C/sec or higher, and then cooled from the controlled temperature T to room temperature at an average cooling rate of less than 1.0°C/sec, and a suitable stabilization treatment step and skin pass rolling step were performed, whereby a steel sheet having a metal structure satisfying RA s /RA c ⁇ 0.75, formula 1, and formula 2 in addition to the characteristics of martensite: 85.0% or more and retained austenite: 1.0 to 7.0% could be obtained.
• the hydrogen embrittlement resistance was particularly significantly improved.

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