[0001]The present invention, automotive, building, suitable as a structural member, such as home appliances, steel tensile strength excellent in delayed fracture resistance of more than 780 MPa, galvanized steel sheet, and galvannealed steel sheet, as well as their a method for manufacturing.
Background technique
[0002]
　Recently, automobiles, buildings, the steel sheet used as a structural member, such as home appliances, other required strength and formability is required to have excellent delayed fracture resistance. Delayed fracture is hydrogen which has entered into the steel material by integrating the stress concentration portion is a phenomenon of destroying the steel.
[0003]
　Delayed fracture, high strength bolts, can occur in the high-strength steel material PC steel wire, and line pipes and the like have been conventionally known. On these high strength steels, measures to improve the various delayed fracture resistance have been proposed.
[0004]
　For example, Non-Patent Document 1, Cr, discloses that Mo, and V element such as is effective in improving the delayed fracture resistance. This, Cr, Mo, and carbide of V such precipitated in crystal grains, these carbides, by utilizing as the site for trapping hydrogen (hydrogen trapping sites), in a technique for suppressing embrittlement of the grain boundaries is there.
[0005]
　High strength materials, resistance to plastic deformation, so difficult to further break, often high stress is used in an environment that act. Also, as steel sheets for automobiles, in the steel material used as a member after molding, the residual stress is generated after the molding. This residual stress as well, because the more increases the strength of the steel sheet is high, increases the concern of delayed fracture in a high-strength steel sheet.
[0006]
　Therefore, in order to apply the high strength steel sheet for automotive parts increases the moldability of the steel sheet in order to obtain the component by molding steel, yet, in order to withstand use in environments where high stress is applied, the steel plate delayed fracture characteristics it is essential to increase.
[0007]
　Further, the above-described Cr, Mo, and carbide of an element of V, etc., function as a hydrogen trap site, derived from the consistency of the interface between the matrix and the carbide (coherent strain), the function, cold reducing and through the rolling and heat treatment. Therefore, Cr, Mo, and the use of carbide of elements V such as hydrogen trap sites is not applicable to the type of steel sheet cold rolling and heat treatment are required.
[0008]
　Patent Document 1, is mainly Ti, and oxides of Mg are disclosed to be effective for the suppression of hydrogen defects (improvement in delayed fracture resistance). The disclosures of hydrogen embrittlement measures in Patent Document 1, in particular, but is intended to improve the hydrogen embrittlement after high heat input welding, the subject patent document 1 is a steel plate, high required for steel sheet molding no consideration is given to compatibility between sex and delayed fracture resistance.
[0009]
　For the hydrogen embrittlement of the steel sheet, for example, in Non-Patent Document 2, hydrogen embrittlement of due to steel sheet to strain induced transformation of retained austenite has been disclosed to be promoted. That is, in the thin steel sheet, in order not to degrade the delayed fracture resistance is disclosed that it is necessary to regulate the amount of residual austenite.
[0010]
　However, the delayed fracture resistance improvement measures disclosed in Non-Patent Document 2 relates to a high strength thin steel sheet having a specific tissue, in which it can not be said fundamental delayed fracture resistance improvement measures.
[0011]
　The improvement of both delayed fracture resistance and formability as a thin steel sheet for the purpose, in Patent Document 2, excellent enamel container steel sheet resistance Tsumatobi properties is disclosed. The steel sheet is then trapped hydrogen entering the steel sheet during the time of manufacture with oxide in the steel sheet, it is to suppress the "Tsumatobi" (surface defect) occurring after the cliff enamel.
[0012]
　Therefore, the steel sheet disclosed in Patent Document 2, includes a large amount of oxides in the interior. However, the high density is dispersed oxides in the steel plate, moldability is deteriorated. Thus, the technique disclosed in Patent Document 2, can not be applied to automotive steel sheets which require high moldability.
[0013]
　On the other hand, as a technique for improving the moldability of the steel sheet, to disperse the residual austenite in the steel sheet, during processing of a steel sheet (during molding), a technique of using a transformation-induced plasticity to transform the residual austenite into martensite (TRIP effect) known (Patent documents 3 and 4, reference). However, since increases generation of the generated martensite delayed fracture after molding, the steel sheet to improve the moldability and the delayed fracture resistance, it is difficult to utilize the TRIP effect (Non-Patent Document 2, see). Thus, in the steel plate, it is difficult to increase both the moldability and the delayed fracture resistance.
CITATION
Patent Literature
[0014]
Patent Document 1: Japanese Laid-Open Patent Publication No. 11-293383
Patent Document 2: Japanese Unexamined Patent Publication No. 11-100638
Patent Document 3: Japanese Laid-Open Patent Publication No. 01-230715
Patent Document 4: Japanese Unexamined Patent Application 02-217425 Bulletin No.
Non-Patent Document
[0015]
Non-Patent Document 1: "New Development of delayed fracture elucidation" (Japan Iron and Steel Institute, issued January 1997)
Non-Patent Document 2: CAMP-ISIJ Vol. 5 NO. 6 1839-1842 pages, Yamazaki et al., 10 May 1992, issued Japan Iron and Steel Institute
Summary of the Invention
Problems that the Invention is to Solve
[0016]
　As described above, in the steel sheet, it is difficult to increase both the moldability and the delayed fracture resistance. The present invention is more tensile strength 780MPa of the steel sheet, galvanized steel sheet, and, in galvannealed steel sheet, while ensuring the moldability, it is an object to improve the delayed fracture resistance. The present invention is a steel sheet to solve the problem, hot-dip galvanized steel sheet, and an object of the invention to provide a galvannealed steel sheet, and a process for their preparation.
Means for Solving the Problems
[0017]
　The present inventors have intensively studied a technique for solving the above problems. As a result, a C content of 0.05 to .40 percent, a tensile strength in the steel sheet is more than 780 MPa, containing a main phase tempered martensite in the required volume fraction, preferably the second phase contain one or two of ferrite and bainite is at the required volume fraction, and to form a tissue volume fraction is limited in the other phases, the required number of iron-based carbide in tempered martensite precipitated with a density above and by 20% or more of iron-based carbide and ε carbides, while ensuring the moldability of the steel sheet, the present inventors have found that it is possible to improve the delayed fracture resistance is found It was.
[0018]
　The present invention has been made based on the above findings and has as its gist is as follows.
[0019]
(1) steel sheet according to one embodiment of the present invention, chemical components, by mass%, C: 0.05 ~ 0.40% , Si: 0.05 ~ 3.00%, Mn: 1.50% or more less than 3.50%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, O: 0.006% or less, Al: 0 ~ 2.00%, Cr: 0 ~ 1.00%, Mo: 0 ~ 1.00%, Ni: 0 ~ 1.00%, Cu: 0 ~ 1.00% Nb: 0 ~ 0.30%, Ti: 0 ~ 0.30%, V: 0 ~ 0.50% B: 0 ~ 0.01% Ca: 0 ~ 0.04%, Mg: 0 ~ 0.04%, and, REM: 0 contains ~ 0.04%, the balance being Fe and impurities, the sheet thickness 1/4 parts of tissue, volume fraction, tempered martensite: 70% or more, and ferrite and one or bainite: containing less than 20% in total In tissue of the plate thickness 1/4 parts by volume fraction, is less than 10% residual austenite, and 10% or less fresh martensite, pearlite is not more than 10%, and the residual austenite, the fresh martensite, and the total volume fraction of the pearlite is not more than 15%, in said tempered martensite in the plate thickness 1/4 parts, the number density of more iron-based carbide diameter 5nm is 5 × 10 7 cells / mm 2 or more, with respect to the number of the iron-based carbides or major axis 5nm in the thickness 1/4 parts, ratio of the number of ε carbide is 20% or more, a tensile strength of not less than 780 MPa. (2) In the steel sheet according to the above (1), the chemical composition, in mass%, Cr: 0.05 ~ 1.00% , Mo: 0.01 ~ 1.00%, Ni: 0.05 ~ 1.00%, and, Cu: may contain one or more of from 0.05 to 1.00%. In steel sheet according to (3) above (1) or (2), the chemical composition, by mass%, Nb: 0.005 ~ 0.30% , Ti: 0.005 ~ 0.30%, and, V: it may contain one or more 0.005 to 0.50 percent.
(4) In the steel sheet according to any one of the above (1) to (3), the chemical composition, by mass%, B: may contain 0.0001 to 0.01%.
In the steel sheet according to any one of (5) above (1) to (4), the chemical composition, by mass%, Ca: 0.0005 ~ 0.04% , Mg: 0.0005 ~ 0. 04%, and, REM: may contain one or more 0.0005 to 0.04%.
(6) In the steel sheet according to any one of the above (1) to (5), the average major axis of the iron-based carbide may be 350nm or less.
(7) In the hot-dip galvanized steel sheet according to another aspect of the present invention, the surface of the steel sheet according to any one of the above (1) to (6), with Fe is 15 wt% or less, the balance being Zn, al, and hot dip galvanized layer is formed consisting of impurities.
The galvannealed steel sheet according to another aspect of the (8) The present invention, on the surface of the steel sheet according to any one of the above (1) ~ (6), Fe is 15 wt% or less, the balance being Zn, Al, and galvannealed layer formed of impurity is formed.
Effect of the invention
[0020]
　According to the present invention, automotive, building, suitable as a structural member, such as home appliances, steel tensile strength excellent in delayed fracture resistance of more than 780 MPa, galvanized steel sheet, and a galvannealed steel sheet , it is possible to provide a process for their preparation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
[1] iron-based number of carbide density (number / mm in tempered martensite 2 is a graph showing the relationship)
2 is a diagram showing the relationship between the ratio and the delayed fracture resistance of ε carbide ferrous carbide.
DESCRIPTION OF THE INVENTION
[0022]
　Cr, Mo, and carbide such as V functions as a hydrogen trap sites, it is known to improve the delayed fracture resistance due to hydrogen embrittlement (Non-Patent Document 1, reference). However, Cr, with Mo, and it takes a long time to heat treatment for precipitating carbides of V, etc., the production line must be performed in a short time heat treatment (such as continuous annealing line or a continuous plating line) production in the steel plate needs to be, it is difficult to use Cr, Mo, and carbide precipitation of V etc. in order to improve the delayed fracture resistance.
[0023]
　Function of trapping hydrogen is derived from the consistency of the interface between the base material and the carbide (coherent strain), Cr was precipitated during hot rolling, Mo, and the hydrogen trapping ability of carbides V such since reduced by passing through a cold rolling and heat treatment, the type of steel sheet cold rolling and heat treatment is required, Cr in order to improve the delayed fracture resistance, Mo, and carbide precipitation V such it is difficult to use.
[0024]
　The present inventors have found that in the tensile strength of the steel sheet above 780 MPa, tissue and iron-based carbides (especially, epsilon carbide) by suitably controlling, while maintaining moldability, is to improve the delayed fracture resistance It was found that.
[0025]
　The following describes steel sheet excellent in delayed fracture resistance according to an embodiment of the present invention (hereinafter sometimes referred to as "steel sheet according to the present embodiment").
[0026]
　Steel sheet according to the present embodiment, the tensile strength and excellent delayed fracture resistance of more than 780MPa steel, molten plated steel plate, and, in galvannealed steel sheets, the main phase of the tissue as a tempered martensite tempered martensite to thereby precipitate a fine iron-based carbide (cementite, and ε carbides), by taking advantage of these iron-based carbides as hydrogen trap sites, while maintaining moldability, is to improve the delayed fracture resistance the basic idea.
[0027]
　Specifically, steel sheet excellent in delayed fracture resistance according to an embodiment of the present invention (hereinafter sometimes referred to as "steel sheet according to the present embodiment"), the chemical composition, in mass%, C: 0.05 ~ 0.40%, Si: 0.05 ~ 3.00%, Mn: less than 1.50% or more 3.50%, P: 0.04% or less, S: 0.01% or less, N: 0. 0.1% or less, O: 0.006% or less, Al: 0 ~ 2.00%, Cr: 0 ~ 1.00%, Mo: 0 ~ 1.00%, Ni: 0 ~ 1.00%, Cu: 0 ~ 1.00% Nb: 0 ~ 0.30%, Ti: 0 ~ 0.30%, V: 0 ~ 0.50% B: 0 ~ 0.01% Ca: 0 ~ 0.04%, Mg 0 to 0.04%, and, REM: 0 contains ~ 0.04%, the balance being Fe and impurities, the sheet thickness 1/4 parts of tissue, volume fraction, tempered martensite: 7 % Or more, and ferrite and one or bainite: it contains less than 20% in total, in a tissue of the thickness 1/4 parts by volume fraction, a residual austenite is less than 10%, fresh martensite site is 10% pearlite is not more than 10%, and the residual austenite, the fresh martensite, and the total volume fraction of the pearlite is not more than 15%, the in the plate thickness 1/4 parts in tempered martensite, the number density of the major diameter 5nm or more iron-based carbides 5 × 10 7 cells / mm 2 or more, with respect to the number of the iron-based carbides or major axis 5nm in the thickness 1/4 parts, epsilon ratio of the number of system carbides is 20% or more, a tensile strength of not less than 780 MPa.
[0028]
　Excellent hot-dip galvanized steel sheet in delayed fracture resistance according to the present embodiment (hereinafter sometimes referred to as "hot-dip galvanized steel sheet according to the present embodiment".) Is, on the surface of the steel sheet according to the present embodiment, Fe is 15 or less by mass%, the balance being Zn, Al, and galvanized layer composed of impurity is formed.
[0029]
　Excellent galvannealed steel sheet delayed fracture resistance according to the present embodiment (hereinafter sometimes referred to as "galvannealed steel sheet according to the present embodiment".) Is, on the surface of the steel sheet according to the embodiment , Fe is 15 wt% or less, the balance being Zn, Al, and, galvannealed layer formed of impurity is formed.
[0030]
　First described chemical components of the steel sheet according to the present embodiment. Unit "mass%" for the content of each element contained in the chemical composition, hereinafter referred to as "%".
[0031]
　C: 0.05 to 0.40 percent
　steel sheet according to the present embodiment contains 0.05 to 0.40 percent of C, tensile strength of the steel sheet of more than 780 MPa. C, the strength increase of the steel sheet, and is an element necessary for the precipitation of iron-based carbide functioning as hydrogen trap sites (cementite, epsilon carbide or the like). When C content is less than 0.05%, it is difficult to obtain a higher tensile strength 780 MPa. Further, when the C content is less than 0.05%, the amount of iron-based carbides precipitated is insufficient, the delayed fracture resistance is not improved.
[0032]
　On the other hand, the C content exceeds 0.40%, the martensite transformation start temperature is lowered, it is impossible to secure a sufficient amount of martensite, thus ensuring 70% or more by volume of tempered martensite it becomes difficult.
[0033]
　Therefore, C content of the steel sheet according to the present embodiment is set to 0.05 to 0.40 percent. Preferred lower limit of the C content is 0.10%. Preferred upper limit of the C content is 0.25%.
[0034]
　Si: 0.05 ~
　3.00% Si is an element effective for improving the strength. Furthermore, Si suppresses the action of the precipitation of iron-based carbides in the austenite, and is an element having an effect of suppressing coarsening of iron-based carbides formed during martensite. More iron-based carbides in martensite is fine, since the delayed fracture resistance is improved, Si has the effect of improving the delayed fracture resistance.
[0035]
　The Si content is less than 0.05%, the effect described above can not be obtained sufficiently, Si content should be 0.05% or more. Preferably, Si content is 0.10% or more. On the other hand, when the Si content exceeds 3.00%, excessively increases the strength of the steel sheet, because moldability of the steel sheet is lowered, Si content should be less 3.00%. Si content is preferably 2.00% or less.
[0036]
　Mn: less than 1.50 ~ 3.50%
　Mn is an element effective for improving the strength of the steel sheet. Moreover, Mn, during the heat treatment for annealing or galvanizing, is an element having an effect of suppressing ferrite transformation that occurs in the middle cooling. This action is required to the tempered martensite of the steel sheet according to the present embodiment within a predetermined range.
[0037]
　The Mn content is less than 1.50%, the effect described above can not be obtained sufficiently, tempered martensite required volume fraction can not be obtained. Therefore, Mn content should be 1.50% or more. Preferably, Mn content is 1.70% or more. On the other hand, when the Mn content is 3.50%, the strength of the slab and hot-rolled sheet is excessively increased, the manufacturing of the steel sheet is degraded, the Mn content is required to be less than 3.50% . Preferably, Mn content is less 3.00%.
[0038]
　P: 0.04% or less
　P is an impurity element, inhibit the toughness segregated in the thickness center portion of the steel plate, also an element which embrittle the weld. When the P content exceeds 0.04%, since the brittleness of the welded portion with reduced toughness becomes significant, it is necessary to set the P content is 0.04% or less. Preferably, P content is 0.02% or less. P content, since the less preferred, the lower limit of the P content is not particularly limited, so that the P content is less than 0.0001% is economically disadvantageous, 0.0001% is P content it is a substantial lower limit of.
[0039]
　S: 0.01% or less
　S is an impurity element, inhibit weldability, also, is an element that inhibits production of casting and during hot rolling. Further, S is, to form a coarse MnS, is an element that inhibits hole expandability. When S content exceeds 0.01%, a reduction in weldability, decreased productivity, and, since the decrease in the hole expandability becomes remarkable, S content should be 0.01% or less. Preferably, S content is 0.005% or less. S content, since the less preferred, the lower limit of the S content is not particularly limited, so that the S content is less than 0.0001% is economically disadvantageous, S content is 0.0001% it is a substantial lower limit of the amount.
[0040]
　N: 0.01% or less
　N, by forming a coarse nitride is an element inhibiting the bendability and hole expandability, also, an element that cause the occurrence of blowholes during welding. When N content exceeds 0.01%, a reduction in bending resistance and hole expandability, as well as the generation of blowholes is remarkable, N content should be 0.01% or less. N content, since the less preferred, the lower limit of the N content is not particularly limited, to the N content to less than 0.0005%, so leads to a significant increase in production costs, is 0.0005% it is a substantial lower limit of the N content.
[0041]
　O: 0.006% or less
　O forms an oxide, which is an element that inhibits formability. When O content exceeds 0.006%, the decrease in the moldability becomes remarkable, the O content should be 0.006% or less. O content, since the less preferred, the lower limit is not particularly limited, to the O content is less than 0.001%, since not economically preferable leads to excessive cost, is 0.001% O it is a substantial lower limit of the content.
[0042]
　Steel sheet according to the present embodiment, in addition to the above elements, Al, Cr, Mo, Ni, and one or more Cu, Nb, Ti, one or more of V, B, and / or , Ca, Mg, and, one or more of REM, and may contain appropriate. However, since it is not essential to the steel sheet according to the present embodiment contains these elements, the lower limit of the content of these elements is 0%.
[0043]
　Al: 0 ~ 2.00% Al
　is an effective element as a deoxidizer, also, like Si, is an element having an effect of inhibiting the precipitation of iron-based carbides in the austenite. Furthermore, Al oxide is an element contributing to the improvement of the delayed fracture resistance, it may be contained in the steel sheet according to Al to the present embodiment. However, if the Al content exceeds 2.00%, Al oxide is excessively formed, so production is deteriorated, the Al content is required to be 2.00% or less. Preferably, Al content is 1.00% or less. Since Al is necessary not included in the steel sheet according to the present embodiment, the lower limit of the Al content is 0%. However, since it is difficult to completely remove the Al contained as impurities in the raw material of the steel sheet, the lower limit of the Al content may be 0.001%.
[0044]
　Cr: 0 ~ 1.00%
　Cr, together with an element which improves the tensile strength of the steel plate or the like, during cooling after annealing at annealing equipment or galvanizing, suppresses ferrite transformation, thereby tempered martensite is an element having an effect of increasing the amount of. Since Cr is necessary not included in the steel sheet according to the present embodiment, the lower limit of the Cr content is 0%. However, in order to obtain the effect described above, the Cr content may be 0.05% or more. More preferably, Cr content is 0.10% or more. On the other hand, when the Cr content exceeds 1.00%, since inhibit production time and hot rolling of manufacturability, Cr content is preferably 1.00% or less. More preferably, Cr content is 0.70%.
[0045]
　Mo: 0 ~ 1.00%
　Mo, as well as an element that improves the tensile strength of the steel plate or the like, during cooling after annealing at annealing equipment or continuous galvanizing line, suppresses ferrite transformation, thereby tempered martensite is an element having an effect of increasing the amount of sites. Since Mo is necessary not included in the steel sheet according to the present embodiment, the lower limit of the Mo content is 0%. However, in order to obtain the above-described effects, Mo content may be 0.01% or more. Mo content is more preferably 0.05% or more. On the other hand, when the Mo content exceeds 1.00%, since inhibit production time and hot rolling of manufacturability, the content of Mo is preferably 1.00% or less. Mo content is more preferably 0.70%.
[0046]
　Ni: 0 ~ 1.00%
　Ni, as well as an element that improves the tensile strength of the steel plate or the like, during cooling after annealing at annealing equipment or continuous galvanizing line, suppresses ferrite transformation, thereby tempered martensite is an element having an effect of increasing the amount of sites. Since Ni is necessary not included in the steel sheet according to the present embodiment, the lower limit of the Ni content is 0%. However, in order to obtain the effect described above, Ni content may be 0.05% or more. Ni content is more preferably 0.10% or more. On the other hand, when the Ni content exceeds 1.00%, since inhibit production time and hot rolling of manufacturability, the Ni content is preferably 1.00% or less. Ni content is more preferably 0.70%.
[0047]
　Cu: 0 ~ 1.00%
　Cu, as well as an element that improves the tensile strength of the steel plate or the like, during cooling after annealing at annealing equipment or continuous galvanizing line, suppresses ferrite transformation, thereby tempered martensite is an element having an effect of increasing the amount of sites. Since Cu is necessary not included in the steel sheet according to the present embodiment, the lower limit of the Cu content is 0%. However, in order to obtain the effect described above, Cu content may be 0.05% or more. Cu content is more preferably 0.10% or more. On the other hand, when the Cu content exceeds 1.00%, since inhibit production time and hot rolling of manufacturability, Cu content is preferably 1.00% or less. Cu content is more preferably 0.70%.
[0048]
　Nb: 0 ~
　0.30% Nb, the precipitation strengthening, fine reinforced, and the dislocation strengthening, is an element contributing to the increase in the strength of the steel sheet. Since Nb is necessary not included in the steel sheet according to the present embodiment, the lower limit of the Nb content is 0%. However, in order to obtain the effects described above, Nb content may be 0.005% or more. Nb content is more preferably 0.010% or more. On the other hand, when the Nb content exceeds 0.30%, since the moldability is increasing the amount of precipitated carbonitrides degrades, Nb content is preferably 0.30% or less. Nb content is more preferably 0.20% or less.
[0049]
　Ti: 0 ~
　0.30% Ti, the precipitates strengthening, fine reinforced, and the dislocation strengthening, is an element contributing to the increase in the strength of the steel sheet. Since Ti is necessary not included in the steel sheet according to the present embodiment, the lower limit of the Ti content is 0%. However, in order to obtain the above-described effects, Ti content may be 0.005% or more. Ti content is more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.30%, since the moldability is increasing the amount of precipitated carbonitrides degrades, Ti content is preferably 0.30% or less. Ti content is more preferably 0.15% or less.
[0050]
　V: 0 ~ 0.50%
　V is precipitation strengthening, fine reinforced, and the dislocation strengthening, is an element contributing to the increase in the strength of the steel sheet. Because V is necessary not included in the steel sheet according to the present embodiment, the lower limit of the V content is 0%. However, in order to obtain the effect described above, the V content may be 0.005% or more. V content is more preferably 0.10% or more. On the other hand, when the V content exceeds 0.50%, since the moldability is increasing the amount of precipitated carbonitrides degrades, V content is preferably 0.50% or less. V content is more preferably not more than 0.35%.
[0051]
　B: 0 ~ 0.01% B
　is an element to strengthen grain boundaries, also during cooling after annealing at annealing equipment or continuous galvanizing line, suppresses ferrite transformation, thereby the tempered martensite is an element having an effect of increasing the amount. Since it is not necessary to be included in the steel sheet according to the embodiment B, the lower limit of the B content is 0%. However, in order to obtain the effect described above, B content may be used as 0.0001% or more. B content is more preferably 0.0005% or more. On the other hand, when the B content exceeds 0.01%, the productivity of hot rolling is lowered, the B content is preferably 0.01% or less. B content is more preferably 0.005% or less.
[0052]
　Ca:
　0 ~
　0.04% Mg: 0 ~ 0.04% REM: 0 ~
　0.04% Ca, Mg and,, REM controls the form of oxides and sulfides, the steel sheet hole-expandability of an element which contributes to the improvement. Ca, Mg, and, since REM need not be included in the steel sheet according to the present embodiment, Ca content, Mg content, and the respective lower REM content is 0%. However, in order to obtain the effect described above, Ca content, Mg content, and REM content, respectively may be 0.0005% or more. Ca content, Mg content, and REM content is respectively, more preferably 0.0010% or more. On the other hand, Ca content, Mg content, and the REM content each exceeds 0.04%, the castability deteriorates, Ca content, Mg content, and REM content more than 0.04% each It is preferred. Ca content, Mg content, and REM content is respectively, and more preferably 0.01% or less.
[0053]
　Note that the "REM", Sc, refers to a total of 17 elements consisting of Y and lanthanoid, the "REM content" means the total content of these 17 elements. When using a lanthanoid as REM, the industrial, REM is often added in the form of misch metal. Again, the steel sheet according to the present embodiment exhibits the effect of the steel sheet according to the present embodiment. Further, also contain a metal REM such as metal La and metal Ce, steel sheet according to the present embodiment exhibits the effect of the steel sheet according to the present embodiment.
[0054]
　Steel sheet according to the present embodiment, in addition to the above elements, with the balance being iron and impurities. The impurities, in producing the steel industrially, a component mixed by various factors, such raw materials or manufacturing processes as ores or scraps are allowed in a range in the present invention does not adversely affect It means a thing.
[0055]
　Tensile strength: more than 780MPa
　tensile strength of the steel sheet according to the present embodiment shall not be less than 780MPa. The tensile strength, by controlling the chemical composition of the steel sheet within the range described above, and obtained by the embodiment as described the structure of the steel sheet below.
[0056]
　Then, tissue thickness 1/4 parts of the steel sheet according to the present embodiment (hereinafter sometimes abbreviated as "tissue") will be described. The thickness 1/4 parts, and 1/8 of the depth of the surface of the steel sheet thickness t from the surface of the steel sheet (upper and lower surfaces of the steel plate), a surface of the 3/8 depth of the steel sheet thickness t from the surface of the steel sheet it is a region between. 1/4 of the depth of the surface of the steel sheet thickness t from the surface of the steel sheet is the center plane of the sheet thickness 1/4 parts. ¼ of the sheet thickness parts are so positioned in the middle between the central plane and the surface of the plate of the steel plate, and has an average tissue. Therefore, the steel sheet according to the present embodiment, the tissue in the sheet thickness 1/4 parts is defined.
[0057]
　Tissue thickness 1/4 parts of the steel sheet according to the present embodiment, a volume fraction,
(tissue A) tempered martensite: 70% or more,
(tissue B) 1 or ferrite and bainite or two: in total less than 20%, and
(tissue C) residual austenite, fresh martensite and pearlite,: each less than 10%
is defined as. Tissue A is most greatly affected tissue tensile strength and delayed fracture resistance of the steel sheet according to the present embodiment by having the ε carbides, i.e. the main phase. Organization B has the function of improving the properties of the steel sheet according to the present embodiment, since the steel sheet according to the present embodiment even if not contain tissue B can solve the problem, tissue B the lower limit of the content of 0% by volume. Tissue C, because not have the function of improving the properties of the steel sheet according to the present embodiment, the tissue need not be included, the lower limit of its content is 0% by volume.
[0058]
　(Organizations A) thickness 1/4 parts of tempered martensite (major phase): 70%
　in tissue, tempered martensite is an important organization in ensuring the strength and delayed fracture resistance of the steel sheet.
[0059]
　Tempered martensite is a collection of lath-like grains, containing iron-based carbides therein. As iron-based carbides, it belongs to a plurality of iron-based carbides group extending in different directions, which functions as a hydrogen trap sites. The major axis of the iron-based carbide is, for example, 5nm or more. Some of the iron-based carbide in tempered martensite by heat treatment carried out under appropriate conditions, can be ε carbide to be described later.
[0060]
　By performing tempering in hardened martensite, tempered martensite is obtained. When the volume fraction of the tempered martensite is 70% or more, since it is possible to make the tensile strength of the steel sheet reliably 780MPa or more, the volume fraction of tempered martensite is 70% or more. The volume fraction of tempered martensite is preferably 75% or more.
[0061]
　The upper limit of the volume fraction of tempered martensite, because it is not particularly limited, may be 100%. However, when the volume fraction of tempered martensite is more than 90%, excessively increases the tensile strength of the steel sheet, because it may moldability of the steel sheet is decreased, the volume fraction of tempered martensite is 90% or less it is preferable that the. The volume fraction of tempered martensite is more preferably 85% or less.
[0062]
　One or two of (tissue B) ferrite and bainite (second phase): 20% less than total
　in the steel sheet according to the present embodiment, tissue other than the above-mentioned tempered martensite mainly one ferrite and bainite or tissue B composed of two.
[0063]
　Ferrite is a soft tissue, reduced strength of the steel sheet. When the amount of ferrite is excessive, there is a case where the tensile strength of the steel sheet is less than 780 MPa. Therefore, the steel sheet according to the present embodiment may not contain ferrite. However, soft ferrite than tempered martensite, when was contained instead of a part of the tempered martensite, because it has an effect of enhancing the moldability of the steel sheet, the steel sheet according to the present embodiment also contain ferrite good.
[0064]
　Bainite, like the martensite, a collection of lath-like grains, the interior, for example, the above major axis 5 nm, a tissue containing the iron-based carbide. The iron-based carbides function as hydrogen trap sites, thereby improving the delayed fracture resistance of the steel sheet. On the other hand, the tensile strength improvement of bainite is less than the tensile strength enhancing effect of martensite. Steel sheet according to the present embodiment has a tensile strength of at least 780MPa with the martensite volume fraction is 70% or more. If the amount of bainite is excessive, there is a case where the tensile strength of the steel sheet is less than 780 MPa. Therefore steel sheet according to the present embodiment does not need to contain bainite. However, tempered martensite softer bainite than in the case of containing instead of a part of the tempered martensite, because it has an effect of enhancing the moldability of the steel sheet, as long as the tempered martensite amount described above is not impaired, the steel sheet according to the embodiment may contain bainite.
[0065]
　Incidentally, bainite containing iron-based carbides that contribute tissue to the improvement of the delayed fracture resistance. However bainite, unlike the martensite can be controlled precipitation of carbides by heat treatment after martensite, since the organization that generates and holds a long time to the required temperature, the ε carbide portion of the iron-based carbide it is not possible to keep Mom.
[0066]
　The present inventors have found that the tissue contained in the steel sheet according to the present embodiment, the essential organization A comprising ε carbides (i.e. tempered martensite), not including ε carbides and essential steel sheet according to the embodiment but not, classified into can lead to positive effects such as moldability improving tissue B (i.e. ferrite and bainite), and tissue C need not be contained in the steel sheet according to the present embodiment, the content of each group it is to define the amount, the delayed fracture resistance was determined to be necessary to control preferably all moldability, and tensile strength. Therefore, the steel sheet according to the present embodiment, the volume fraction of the sum of ferrite and bainite is defined. The steel sheet according to the present embodiment, in order to tensile strength of the steel sheet reliably 780MPa or more, the total volume fraction of ferrite and bainite is less than 20%. Total volume fraction of ferrite and bainite is preferably 10% or less.
[0067]
　Although the lower limit of the total volume fraction of ferrite and bainite is 0%, in order to improve the moldability of the steel sheet with ferrite and bainite, the lower limit of the total volume fraction of ferrite and bainite more than 5% it may be.
[0068]
　(Organizations C) retained austenite: less than 10 vol%
　(tissue C) fresh martensite: 10% or less by volume
　(tissue C) perlite: 10 vol% or less
　(tissue C) residual austenite, the total amount of fresh martensite and pearlite,: 15 vol% or less
　steel sheet according to the present embodiment, the tempered martensite, ferrite, and in addition to the bainite, residual austenite, fresh martensite, and which may contain pearlite.
[0069]
　Retained austenite, contributes to improvement in moldability by TRIP effect. However, when the volume fraction of residual austenite is increased, and transformation into hard fresh martensite during molding as automotive parts, machining characteristics are be deteriorated.
[0070]
　The present inventors have found that, when the volume fraction of residual austenite in the structure of the steel sheet is 10% or more, the processing characteristics were confirmed by experiments that the degradation. Therefore, in the steel sheet according to the present embodiment, the volume fraction of retained austenite and less than 10%. The volume fraction of residual austenite is preferably not more than 7%. On the other hand, be 0% of the volume fraction of retained austenite, the steel sheet according to the present embodiment has a sufficient moldability. Therefore, the steel sheet according to the present embodiment since there is no need to contain residual austenite, the lower limit of the volume fraction of residual austenite is 0%.
[0071]
　Fresh martensite, a martensite that does not contain Fe carbide. Steel sheet containing fresh martensite is a high strength, since processing characteristics inferior, to limit the volume fraction of fresh martensite of the steel sheet according to the present embodiment to less than 10%. On the other hand, the volume fraction of fresh martensite is 0% steel sheet according to the present embodiment has a sufficient strength. Therefore, the steel sheet according to the present embodiment since there is no need to include fresh martensite, the lower limit of the volume fraction of fresh martensite is 0%.
[0072]
　Perlite reduces the processing properties of the steel sheet. Thus limiting the volume fraction of pearlite of the steel sheet according to the present embodiment to less than 10%. Meanwhile, perlite is a tissue containing the cementite is Fe carbides, it is not possible to vary the cementite ε carbide, pearlite sufficiently no effect of improving the delayed fracture resistance. Therefore, the steel sheet according to the present embodiment since there is no need to include a pearlite, the lower limit of the volume fraction of pearlite is 0%.
[0073]
　Moreover, the steel sheet according to the present embodiment, the residual austenite volume fraction of the total fresh martensite and pearlite, must be 15% or less, it is preferably set to 12% or less. It retained austenite volume fraction of the total is 15 percent, fresh martensite, and pearlite, may impair the processability characteristics of the steel sheet.
[0074]
　Tempered martensite, ferrite, bainite, and residual austenite, further fresh martensite, pearlite, and, the identification of other tissues, confirmation of the location and measurement of the volume fraction, nital reagent, and, JP using a reagent disclosed in Japanese Sho 59-219473, and corrode the steel plate rolling direction cross-section or rolling direction perpendicular cross section, carried out by observing the cross section with 1,000 to 100,000 times the scanning electron microscope and a transmission electron microscope be able to.
[0075]
　Also, FE-SEM (field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope) to the supplied EBSD: Electron Back-Scatter Diffraction crystal orientation analysis method using) crystal orientation analysis by, or micro-Vickers from the hardness measurement of micro regions such as the hardness measurement, it is possible to determine the tissue.
[0076]
　For example, as described above, the tempered martensite and bainite, because carbide formation site and the crystal orientation relationship (extending direction) and the like are different, using FE-SEM, elongation of iron-based carbides within the lath-shaped crystal grains by observing the direction, it is possible to easily distinguish the tempered martensite and bainite.
[0077]
　Tempered martensite in the plate thickness 1/4 parts of the steel sheet, ferrite, and the volume fraction of bainite, and / or volume fraction of pearlite is, a sample taken as an observation plane parallel plate thickness cross section in the rolling direction of the steel sheet and, polishing the observation surface, and etched with nital solution, ¼ of the sheet thickness parts (range thickness of 1 / 8-3 / 8 around the 1/4 point of the thickness), FE- obtained by observing with SEM, the area fraction of each tissue was measured and obtained by regarding these area fraction and volume fraction. Incidentally, the area fraction of each tissue at a magnification of 5,000, an average value of the area fraction of each organization in each field obtained by 10 perimetry.
[0078]
　Fresh martensite and residual austenite, the cross section of the steel sheet was etched with Repera solution, by observing the thickness 1/4 parts by FE-SEM, clearly distinguishes the tissue (tempered martensite, ferrite, bainite) and be able to. Therefore, the volume fraction of fresh martensite can be obtained as a difference between the area fraction of a region which is not corroded was observed by FE-SEM, the area fraction of retained austenite was measured by X-ray.
[0079]
　Next, the number density of the iron-based carbide in tempered martensite 5 × 10 7 (pieces / mm 2 defined as)
[0080]
　In tempered martensite in the sheet thickness 1/4 parts, the number density of the major diameter 5nm or more iron-based carbide: 5 × 10 7 (pieces / mm 2 ) or more
　in a steel sheet according to the present embodiment, the delayed fracture resistance and formability to increase both the, in the main phase tempered martensite in the tissues with a thickness of 1/4 parts, 5 × the number density of more iron-based carbide diameter 5 nm 10 7 (pieces / mm 2 ) or more and regulatory. In the present embodiment, "number density of iron-based carbides in the tempered martensite", the number of iron-based carbide containing tempered martensite in the observation plane, dividing the area of tempered martensite in the observation plane is a value obtained by.
[0081]
　Martensite after quenching is a high strength, since delayed fracture resistance is low, there is a need for improvement. Therefore, the tempered martensite tempered martensite, in the sheet thickness 1/4 parts, this in tempered martensite, the above iron-based carbide diameter 5 nm 5 × 10 7 (pieces / mm 2 is deposited) Delayed fracture resistance of the tempered martensite (major phase) is superior martensite is not tempered.
[0082]
　The present inventors have investigated the delayed fracture resistance, the relationship between the number density of the iron-based carbide in tempered martensite in the sheet thickness 1/4 parts. The results are shown in Figure 1.
[0083]
　The number density of the iron-based carbides, samples were taken as an observation plane parallel plate thickness cross section in the rolling direction of the steel sheet, grinding the observation surface, and etched with nital solution, the 10 fields in the sheet thickness 1/4 parts FE in-SEM, observed at 5000 magnification, included in the tempered martensite in each field, the number of major diameter 5nm or more iron-based carbide, a value obtained by dividing by the area of ​​the tempered martensite in the field of view It was measured by averaging. Incidentally, the major axis is the number of iron-based carbide having smaller than 5nm were not measured. Iron-based carbide having smaller than the major diameter is 5nm is because not significantly affect the delayed fracture resistance of the steel sheet. Later, there is a case in which the above-mentioned iron-based carbide diameter 5nm simply referred to as "iron-based carbide".
[0084]
　Delayed fracture resistance of the steel sheet, the rolling direction to the length of 100mm was cut out at a right angle, width 30 mm, and a thickness of 1.3mm or three-point bending a strip specimen of 1.6mm were processed in the steel plate, the rectangular test after mounting the water resistance of the strain gauges on the surface of the strip, by immersing the strip specimen during thiocyanate aqueous ammonium current density 0.1 mA / cm 2 strip specimen in by electrolyzing thiocyanate aqueous solution of ammonium in infested with hydrogen, after 2 hours, it was assessed by confirming the presence or absence of cracks.
[0085]
　Bending radius of the strip specimen was 10 mm. Load stress applied to the strip-shaped test piece having a thickness of 1.3mm is to 60% of the tensile strength of the steel sheet (TS), a load stress applied to the strip-shaped test piece having a thickness of 1.6mm, the tensile strength of the steel sheet (TS ) was 90 percent. The strip specimen was broken at 60% of the applied stress of the tensile strength (TS) "VERY BAD", without breaking a 60% load stress tensile strength (TS), 90% of the load of the tensile strength (TS) "BAD" a broken strip-shaped test piece in the stress, and the strip-shaped test piece was not broken in both of load stress was evaluated as "GOOD".
[0086]
　The present inventors, as shown in FIG. 1, the number density of the iron-based carbide in tempered martensite in the sheet thickness 1/4 parts of at least 5 × 10 7 (pieces / mm 2 When the)
[0087]
　Therefore, the number density of the iron-based carbide in tempered martensite in the sheet thickness 1/4 parts 5 × 10 7 (pieces / mm 2 was defined as) The number density of the iron-based carbide in tempered martensite in the sheet thickness 1/4 parts, preferably 1 × 10 8 (pieces / mm 2 is at) or more, more preferably 3 × 10 8 (pieces / mm 2 ) or more it is.
[0088]
　Delayed fracture resistance improving effect by the iron-based carbide in tempered martensite is more conspicuous as the iron-based carbide is small. Then, most of the iron-based carbide, since the deposited within lath martensite, does not inhibit the mechanical properties required for steel sheets such as ductility and formability. Therefore, as the diameter of the iron-based carbide in tempered martensite preferably small, preferably not more than 350 nm. Major axis of the iron-based carbide in tempered martensite is more preferably 250nm or less, further more preferably 200nm or less. On the other hand, since the iron-based carbide diameter is too small has no fracture resistance improving effect delayed, in steel sheet according to the present embodiment, the iron-based carbide diameter is less than 5nm is not considered.
[0089]
　As described above, the sheet thickness 1/4 parts, so positioned in the middle between the central plane and the surface of the plate of the steel plate, and it has an average tissue. Therefore, the steel sheet according to the present embodiment, if the number density of the iron-based carbide in tempered martensite in the sheet thickness 1/4 parts of the preferred range, excellent characteristics can be obtained over the entire steel sheet.
[0090]
　Ratio of the number of ε carbide to the number of all the iron-based carbide: 20% or more
　for all of the number of iron-based carbides in the tempered martensite in the present embodiment, the ratio occupied by the number of ε-based carbides (hereinafter , and if there is) of 20% or more abbreviated as "ratio of ε carbide". Accordingly, moldability, without particular inhibit hole expandability, it is possible to improve the delayed fracture resistance.
[0091]
　Iron-based carbides in the tempered martensite is mainly cementite (Fe 3 is C). Usually, the iron of the matrix phase (bcc structure) cementite (Fe 3 interface with C) is thought to function as trap sites for trapping hydrogen. Therefore, the presence of cementite is said to contribute to the improvement of the delayed fracture resistance.
[0092]
　However, cementite, since the starting point of the ductile fracture, to improve both the moldability and the delayed fracture resistance by using only cementite is difficult.
[0093]
　The present inventors have intensive studies results, epsilon carbide (Fe of various iron-based carbide 2.4 The use of C),
[0094]
　The iron-based carbides consisting of Fe and C, and different ε carbide crystal structure, chi-based carbides, and cementite (theta carbide) or the like is present. These iron-based carbides in martensite, which precipitates in a state having a specific crystal orientation relationship between iron bcc structure of the matrix phase.
[0095]
　Among the various iron-based carbide described above, epsilon carbide (Fe 2.4 and C), and the iron bcc structure, matched interface (Coherent in interface, the interface of the two phases, in each phase for all atoms outermost forming an interface close to the interface) the relationship of the adjacent atoms are satisfied. ε carbides (Fe 2.4 interface with C) and iron (bcc structure), the interface between the cementite and iron (bcc structure), so is excellent consistency, hydrogen trapping capability, is higher than cementite It is estimated. Furthermore, epsilon carbides are the finer than cementite, hardly becomes a start point of the ductile fracture.
[0096]
　Accordingly, the present inventors have, epsilon carbides (Fe 2.4 focused on C), The results are shown in Figure 2.
[0097]
　The ε-based carbides (hexagonal) and cementite (orthorhombic), the crystal structure is different, different diffraction pattern of X-ray diffraction or electron beam diffraction, can be easily distinguished. The present inventors, by observing the thin film sample with an electron microscope to identify the type of iron-based carbide. Irradiating an electron beam to the iron-based carbides, and analyzes the resulting diffraction pattern, epsilon carbide (Fe 2.4 were identified C).
[0098]
　Epsilon carbides of iron-based carbide in each sample (Fe 2.4 ratio of C), as the 10000-fold magnification, epsilon-containing carbide according to each field obtained by measuring at 10 fields (Fe 2.4 It was calculated by averaging the proportion of C). Evaluation of delayed fracture resistance were conducted in the evaluation method described above.
[0099]
　From Figure 2, epsilon carbides of iron-based carbide (Fe 2.4 the percentage of C) by 20% or more, it can be seen that can ensure excellent processing properties and delayed fracture resistance. To processing properties and delayed fracture properties further improved, epsilon carbides of iron-based carbide (Fe 2.4 ratio of C) is preferably at least 30%, more preferably 40% or more.
[0100]
　Incidentally, the proportion of ε carbide ferrous carbide is less than 20%, not only the delayed fracture resistance is inferior, not good processing characteristics.
[0101]
　As described above, the sheet thickness 1/4 parts, so positioned in the middle between the central plane and the surface of the plate of the steel plate, and it has an average tissue. Therefore, the steel sheet according to the present embodiment, if the proportion of ε carbide ferrous carbide in tempered martensite in the sheet thickness 1/4 parts of the preferred range, good characteristics over the entire steel sheet is obtained .
[0102]
　Galvanized steel sheet according to the present embodiment, the surface of the steel sheet according to the present embodiment, Fe is 15 wt% or less, the balance being Zn, Al, and that the galvanized layer composed of impurity is formed and features. Usually, Fe concentration of galvanized layer is often less than 7% by weight. The lower limit of the Fe concentration in the galvanizing is not particularly limited, preferably 1.0 wt%.
[0103]
　Galvannealed steel sheet according to the present embodiment, the surface of the steel sheet according to the present embodiment, Fe is 15 wt% or less, the balance being Zn, Al, and, galvannealed layer formed of impurity is formed characterized in that it is. The lower limit of the Fe concentration in the galvannealed is not particularly limited, is often 7 mass%.
[0104]
　Then, the steel sheet according to the present embodiment, galvanized steel sheet, and a method for manufacturing a galvannealed steel sheet will be described.
[0105]
　First, a method for manufacturing the steel sheet of the present invention.
[0106]
　The production method of the steel sheet of the present invention,
a cast slab having the same composition as the steel sheet according to the embodiment (a), (a1) directly subjected to hot rolling, then winding, or after once cooled (a2) heated, subjected to hot rolling, then winding,
(b) after pickling, subjected to cold rolling, then annealing, then
cooled (c) annealed steel sheet, then subjected to tempering , then
to two-stage cooling (d) it is baked returned steel sheet
, characterized in that. (D) is the proportion of ε carbide ferrous carbide is an important step to 20% or more.
[0107]
　Cast slab subjected to hot rolling may be a cast slab is not limited to a particular cast slab. For example, the continuous cast slab may be a slab produced by a thin slab caster. Casting slab, subjected to hot rolling. In this case, after casting the cast slab may be directly subjected to hot rolling it may be subjected to hot rolling after reheating after once cooled.
[0108]
　The cast slab, directly continuous casting - direct rolling (CC-DR) or subjected to hot rolling, Ar 3 to complete the hot rolling at (temperature ferrite transformation begins when cooled steel) transformation point or higher temperature region it to a temperature at which it is, it is necessary to heat the cast slab during the hot rolling start. When the finish rolling temperature is in the 2-phase temperature region (austenite + ferrite), tissue inhomogeneity of the hot-rolled steel sheet is increased, moldability of the finally obtained steel sheet is because deteriorated.
[0109]
　Maximum tensile strength is not less than 780 MPa, the steel sheet according to the present embodiment may contains a large amount of alloy elements. In this case, since the cast slab rolling load during hot rolling increases, it is preferable to hot rolling at high temperatures. From the above, the finish rolling temperature, Ar 3 and transformation point or more.
[0110]
　The present inventors have conducted experiments, for example, the heating temperature before hot rolling to about 1150 ° C., when the finish rolling temperature and 920 ° C., the finally obtained steel sheet has good formability It was confirmed.
[0111]
　Incidentally, during hot rolling, rough rolled sheet to each other may be subjected to a continuous hot rolling by joining, also taking once winding the rough rolled sheet may be subjected during the next hot rolling.
[0112]
　Coiling temperature after completion of hot rolling is increased excessively the thickness of the oxide produced on the surface of the steel sheet, pickling needs a temperature does not decrease. Further, the coiling temperature after the hot rolling is completed, generates coarse ferrite and pearlite in the hot rolling tissue, tissue heterogeneity after annealing becomes large, moldability of the final product is not deteriorated temperature there needs to be.
[0113]
　The present inventors have conducted experiments, for example, when the coiling temperature of about 590 ° C., and refining the structure after annealing, the intensity - to improve ductility balance, further, to uniformly disperse the second phase by, it was confirmed that for improving the moldability of the finally obtained steel sheet.
[0114]
　Winding the hot-rolled steel sheet rewinding took, subjected to pickling, subjected to cold rolling, thereby obtaining a cold rolled steel sheet. In pickling, by removing the oxides on the surface of the hot-rolled steel sheet, thereby improving the chemical conversion properties and plating properties of the cold rolled steel sheet. Pickling, may be a one-time, it may be divided into multiple times.
[0115]
　Pickled hot-rolled steel sheet, flat keep the shape of the cold-rolled steel sheet, also need to be cold-rolled at a high reduction ratio enough to provide sufficient ductility to the final product. On the other hand, if the rolling reduction is too high, the rolling load becomes excessively large, rolling becomes difficult. The present inventors have made experiments, for example, when the cumulative rolling reduction in cold rolling (cold rolling rate) was 50%, it was confirmed that favorable results are obtained. On the other hand, for example, the cumulative rolling reduction in cold rolling as 90%, if the slab having the chemical composition of the steel sheet according to the present embodiment is cold rolled, cracks on the steel sheet occurs. Incidentally, the number of rolling passes, rolling reduction per pass is not particularly limited.
[0116]
　Next, annealing the cold-rolled steel sheet. Annealing, in order to enhance the manufacturability, preferably continuous annealing.
[0117]
　If the annealing temperature is insufficient (e.g., 750 ° C.), it is not possible to generate sufficient martensite cold-rolled steel sheet after annealing, the volume fraction of tempered martensite of the finally obtained steel sheet 70% it is difficult to be greater than or equal to.
[0118]
　On the other hand, if the annealing temperature is excessive, leading to increase in manufacturing cost, economically unfavorable, further steel sheet shape becomes poor, for example, troubles such as lowering the life of the roll carrying the steel sheet in a continuous annealing equipment the induce.
[0119]
　Further, if the annealing time is insufficient (for example, about 1 second), it is impossible to dissolve the iron-based carbides generated in hot rolling, also insufficient martensite contained in the cold-rolled steel sheet after annealing since, not the volume fraction of tempered martensite of the finally obtained steel sheet can be 70% or more. On the other hand, if the annealing time is excessive, leading to increase in manufacturing cost, economically undesirable.
[0120]
　The present inventors have conducted experiments, for example, an annealing temperature of about 880 ° C., when the annealing time of about 100 seconds, to be tempered martensite amount of the finally obtained steel sheet within the appropriate range possible was confirmed to be.
[0121]
　Cold-rolled steel sheet after the continuous annealing termination is cooled. Thereafter, an annealing post and tempering prior to cooling, may be referred to as primary cooling.
[0122]
　If the cooling stop temperature in the primary cooling (primary cooling stop temperature) is too low (e.g., about 80 ° C.), since the fresh martensite containing much distortion is likely to remain after tempering, which will be described later, of 20% or more ε-based carbide in tempered It can not be ensured. On the other hand, when the primary cooling stop temperature is too high (e.g., about 550 ° C.), the amount of bainite 20% ultra next, can not be secured a tensile strength of at least 780 MPa.
[0123]
　The cooling method, roll cooling, air cooling, water cooling, and may be any of these in combination.
[0124]
　The present inventors have conducted experiments, for example in a temperature range from the annealing temperature to 400 ° C., when performing cooling at a cooling rate of about 2 ° C. / sec, the amount of ε carbide and bainite within the appropriate range It was confirmed to be a.
[0125]
　Following the above cooling is carried out tempering, so the microstructure control the cold-rolled steel sheet. This tempering, tempering the martensite contained in the cold rolled steel sheet, the number density of the iron-based carbides in the tempered martensite 5 × 10 7 (pieces / mm 2 and) above.
[0126]
　In tempering, the steel plate temperature is maintained for the predetermined holding temperature (isothermal hold temperature) for a predetermined time (isothermal hold time). If the holding temperature in the tempering is too low (e.g., about 0.99 ° C.), 5 × 10 7 (pieces / mm 2 can not be obtained number density)
[0127]
　On the other hand, when the holding temperature in the tempering is excessive (e.g., about 550 ° C.), martensite is excessively tempered, tensile strength of the finally obtained steel sheet is less than 780 MPa. Further, when the holding temperature in the tempering is excessive, it precipitated iron-based carbide is coarsened, delayed fracture resistance is not improved.
[0128]
　If insufficient retention time in the tempering (for example, about 1 second), tempered martensite is insufficient, and the number density of the iron-based carbides 5 × 10 7 (pieces / mm 2 is difficult to)
[0129]
　The present inventors have conducted experiments, for example, the holding temperature in the tempering was about 400 ° C., and if the retention time in the tempering was about 280 seconds, tempered martensite amount of the finally obtained steel sheet, and an iron-based it is possible to make the number density of carbides within the appropriate range was confirmed.
[0130]
　After the temperature hold, 20% or more of iron-based carbides contained in the tempered martensite to the ε carbide, for cooling.
[0131]
　As described above, the iron-based carbides, different ε carbide crystal structure, a χ-based carbides, and cementite (theta carbide) or the like. Of these various iron-based carbides, epsilon carbide (Fe 2.4 C), the iron of the bcc structure, the forming an interface close to the matched interface, a high hydrogen trapping capacity. Moreover, since the ε carbide is finer than cementite, hardly becomes a start point of the ductile fracture.
[0132]
　The present inventors have found that the amount of ε-based inclusions, not only the cooling conditions in the tempering, C content, cooling temperature of the annealing, the holding temperature in the tempering, is estimated to be also affected by the holding time at the tempering . To obtain ε-based inclusions are required, taking into account the interaction of regulator of these ε based inclusion amount, it is necessary to determine the production conditions.
[0133]
　The present inventors have made various experiments, to produce a large number of ε carbide, in order to improve the delayed fracture resistance has a temperature range of holding temperature to about 360 ° C., about 360 ° C.-100 ° C. it has knowledge that it is necessary to perform a temperature range in a two-stage cooling to vary the cooling rate. Hereinafter, referred to cool the temperature range of holding temperature to about 360 ° C. and the secondary cooling, sometimes referred to as tertiary cooling the cooling temperature range of from about 360 ° C.-100 ° C..
[0134]
　According to the experiments of the present inventors, a cooling in a range where the cooling rate is too low (e.g., about 1 ° C. / sec) or when too high (e.g., about 75 ° C. / sec), or tertiary cooling in the range of the secondary cooling rate is too low (e.g., about 1 ° C. / sec) or when too high (e.g., about 65 ° C. / sec) where the amount of ε-based carbide is insufficient.
　According to the experiments of the present inventors, if the temperature to vary the cooling rate 360 ° C. ± 10 ° C., it was found that the required effect can be obtained. On the other hand, if the temperature of changing the cooling rate is too low (e.g., about 200 ° C.), the amount of ε-based carbide is insufficient. Furthermore, even if the end temperature of the two-stage cooling is too high (e.g., about 200 ° C.), the amount of ε-based carbide is insufficient.
[0135]
　The present inventors, for example, the C content is set to 0.06%, the retention time and holding temperature in the annealing conditions and tempering as exemplary values above and the cooling rate of the temperature range of holding temperature ~ 360 ° C. to about 11 ° C. / a sec, 360 ~ and the cooling rate of the temperature range of 100 ° C. to about 15 ° C. / sec, and two stages by a 100 ° C. or less the end temperature of the cooling, the number density of the iron-based carbide 5 × 10 7 pieces / mm 2 or more, the production of steel plate a rate of ε carbide ferrous carbide is 20% or more in the sheet thickness 1/4 parts was found that feasible.
[0136]
　Then, as described above, epsilon carbide (Fe 2.4 C) is finer than cementite, since hardly become a starting point of the ductile fracture, while maintaining moldability, can significantly increase the delayed fracture resistance.
[0137]
　Mechanisms delayed fracture resistance is remarkably improved is not clear, during temperature retention described above, to produce the nuclei in the tempered martensite of fine ε carbides, thereafter the two-stage cooling, fine ε carbide is presumed to be generated.
[0138]
　Next, a method of manufacturing a galvanized steel sheet according to the present embodiment, and method of manufacturing a galvannealed steel sheet according to the present embodiment will be described.
[0139]
　Method for manufacturing a galvanized steel sheet according to the present embodiment,
a cast slab having the same composition as the steel sheet according to (a) the present embodiment, (a1) directly subjected to hot rolling, then winding, or, (a2 ) once heated after cooling, subjected to hot rolling, then winding,
(b) after pickling, subjected to cold rolling, then annealing, then
cooled to (c1) the annealed steel sheet , galvanized after the temperature of the steel sheet in the vicinity of hot-dip galvanizing bath temperature, or,
(c2) and the annealed steel sheet was cooled, further cooled to room temperature and then heated to near galvanizing bath temperature and, galvanized,
for two-stage cooling (d) it is galvanized steel sheet
, characterized in that.
[0140]
　Galvanizing, Fe is 15 wt% or less, the balance is Zn, Al, and, galvanized consisting impurities.
[0141]
　A steel sheet, if the Fe were formed plating layer of less than 7% by weight, usually not subjected to alloying treatment to the plated layer, it is often used as a galvanized steel sheet. On the other hand, the steel sheet, if the Fe were formed 7 mass% or more of the plated layers, usually subjected to alloying treatment to the plated layer, is often used as a galvannealed steel sheet.
[0142]
　The method for producing a galvanized steel sheet according to the embodiment (a) and (b) is the same as the manufacturing method of the steel sheet according to the embodiment (a) and (b). Further, in (d) of the method of manufacturing the galvanized steel sheet according to the present embodiment, similarly to the manufacturing method of the steel sheet according to the present embodiment, it is necessary to perform a two-stage cooling.
[0143]
　In the production method of galvanized steel sheet according to the present embodiment, after annealing, cooling the steel sheet, the temperature of the steel sheet after the near galvanizing bath temperature, or subjected to hot-dip galvanizing, or, after annealing, the steel plate cooling was further cooled to room temperature after, then it was heated to near galvanizing bath temperature, subjected to hot-dip galvanizing. And annealing, cooling is performed between the galvanizing is carried out in the same manner as cooling included in (c) of the method of manufacturing the steel sheet according to the present embodiment described above.
[0144]
　Galvanizing, the steel sheet temperature from the vicinity of the galvanizing bath temperature, carried out by immersing in the plating bath. By immersion in a plating bath temperature of the steel strip after the near galvanizing bath temperature, the galvanized layer on the surface of the steel sheet, good adhesion can be formed uniformly.
[0145]
　If the temperature at the time of immersing the steel sheet in a molten zinc plating bath is too low, upon entering the plating bath of the steel sheet, large heat extraction, part of the molten zinc is solidified, which may deteriorate the coating appearance. On the other hand, if the temperature at the time of immersing the steel sheet in a molten zinc plating bath is too high, the plating bath temperature to induce operation troubles increased. Incidentally, the plating bath in addition to pure zinc, Fe, Al, Mg, Mn, Si, may contain such Cr.
[0146]
　In the method of manufacturing the galvanized steel sheet according to the present embodiment, by immersing the steel sheet in a molten zinc plating bath, the same tissue control and tempering the steel sheet according to the present embodiment. Thermal history of the steel plate at the time of immersion, be the same as the thermal history of the tempering of the steel sheet according to the present embodiment described above, soaking the steel sheet in a molten zinc plating bath does not destroy properties of the steel sheet.
[0147]
　After forming a galvanized layer on the surface of the steel sheet, in (d) of the method of manufacturing the galvanized steel sheet according to the present embodiment, similarly to the manufacturing method of the steel sheet according to the embodiment (d), a two-stage cooling There is a need to do.
[0148]
　And held in the plating bath, by a combination of the two-stage cooling after plating, with the number density of fine iron-based carbide in tempered martensite of the main phase of the required tissue 5 × 10 7 (pieces / mm 2 ) precipitating or more and the proportion of ε carbide ferrous carbides is 20% or more, while maintaining moldability, it is possible to increase significantly the delayed fracture resistance.
[0149]
　Method for manufacturing a galvannealed steel sheet according to the present embodiment,
a cast slab having the same composition as the steel sheet according to (a) the present embodiment, (a1) directly subjected to hot rolling, then winding, or, (a2) once heated after cooling, subjected to hot rolling and then coiling,
subjecting (b) after pickling, cold rolling, then annealing, thereafter,
(c-1) the annealed steel sheet the cooled, galvanized a temperature of the steel sheet from the near galvanizing bath temperature and then subjected to alloying treatment, or,
(c-2) the annealed steel sheet was cooled, and further cooled to room temperature , then heated to near galvanizing bath temperature, galvanized, and then subjected to alloying treatment,
to two-stage cooling steel plate that has been processed (d) alloying
, characterized in that.
[0150]
　Furthermore, the manufacturing method of alloyed galvanized steel sheet according to the present embodiment, following (d), the reheated (e), after heat treatment, cooling to room temperature
may include that.
[0151]
　The galvannealed is, Fe is 15 wt% or less, the balance is Zn, Al, and, galvannealed consisting impurities.
[0152]
　Method for manufacturing a galvannealed steel sheet according to the present embodiment, the production method of the present invention galvanized steel sheet, in which the hot-dip galvanizing layer was applied the step of alloying. When alloying temperature is insufficient, not formed good adhesion alloyed layer, while when it is excessive, the alloy layer is too thick, moldability of the plating layer decreases.
[0153]
　The present inventors have conducted experiments, for example, if the alloying temperature was about 480 ° C., was confirmed that the galvannealed steel sheet having a good alloy layer can be obtained.
[0154]
　Method for manufacturing a galvannealed steel sheet according to the present embodiment, after the alloying and two-stage cooling, to increase the proportion of ε carbide ferrous carbide to form a high interfacial of hydrogen trapping capability , heat treatment may be carried out again.
Example
[0155]
　Next, a description will be given of an embodiment of the present invention, conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is, in this single condition example the present invention is not limited. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0156]
　Method of Example steel sheets, a cast slab having a composition disclosed in (a) table, (a1) directly subjected to hot rolling, then winding, or by heating after once cooled (a2) Te, subjected to hot rolling, then winding, (b) after pickling, subjected to cold rolling, then annealing, then cooled (c) annealed steel sheet, then subjected to tempering, then was equipped to two-stage cooling (d) is baked returned steel.
　Method of Example of galvanized steel sheet, a cast slab having a composition disclosed in (a) table, (a1) directly subjected to hot rolling, then winding, or were once cooled (a2) and post-heating, subjected to hot rolling, then winding, (b) after pickling, subjected to cold rolling, then annealing, then cooled (c1) annealed steel sheet, the temperature of the steel sheet the galvanized after near hot-dip galvanizing bath temperature, or, (c2) and the annealed steel sheet was cooled, further cooled to room temperature, then heated to near galvanizing bath temperature, the molten galvanized, it was equipped to two-stage cooling (d) is galvanized steel sheet.
　Method of Example galvannealed steel sheet, a cast slab having a composition disclosed in (a) table, (a1) directly subjected to hot rolling, then winding, or, (a2) once heated after cooling, subjected to hot rolling, then winding, (b) after pickling, subjected to cold rolling, then annealing, then cooled (c-1) the annealed steel sheet , galvanized a temperature of the steel sheet from the near galvanizing bath temperature and then subjected to alloying treatment, or, (c-2) the annealed steel sheet was cooled, and further cooled to room temperature, then, zinc heated to near the plating bath temperature, galvanized, and then subjected to alloying treatment, it was equipped to two-stage cooling steel plate that has been processed (d) alloying.
　Conventional method, all the hot-rolled steel sheet was pickled. Thickness after hot rolling in all Examples and Comparative Examples (excluding those cracks during hot rolling or cold rolling occurs) is 3.2 mm, the primary cooling rate is 2 ° C. / sec met It was. Other production conditions were as shown in Table. The symbol "* 1" in the table, since cracking during hot rolling is found, indicates that the production is stopped, the symbols in the table, "* 2", was discovered cracks during cold rolling since, indicating that production was discontinued. Examples of symbol "* 1" or "* 2" is attached, the characteristic evaluation is not performed. Examples which is described as "NO" relates plating, plating is not performed. Marked "YES" relates plating and galvanizing is performed to the examples as "NO" relates alloying, plating and to the examples as "YES" relates both alloying alloyed molten zinc plating was carried out.
[0157]
[Table 1]

[0158]
[Table 2]

[0159]
[table 3]

[0160]
[Table 4]

[0161]
[table 5]

[0162]
[Table 6]

[0163]
　In the resulting steel sheet, the volume fraction of tempered martensite (Organization A volume fraction), one or two of the total volume fraction of ferrite and bainite (tissue B volume fraction), residual austenite, fresh martensite, and total volume fraction of pearlite (structure C volume fraction), the number density (carbides number density) of iron-based carbides in the tempered martensite, and the proportion of the number of epsilon carbide to the number of iron-based carbides (epsilon system the proportion of carbide) was determined. Furthermore, the tensile strength of the resulting steel sheet (TS), total elongation (EL), and to measure the hole expandability (lambda), was evaluated the delayed fracture resistance of the resulting steel sheet.
[0164]
　The tensile strength and elongation, a right angle JIS5 test piece No. in the rolling direction of the steel sheet was taken, subjected to a tensile test according to JIS Z 2242, tensile strength and (TS) was measured total elongation (El). For hole expandability, hole expansion ratio in accordance with the Japan Iron and Steel Federation standard JFS T1001 (λ (%)) was measured.
[0165]
　Delayed fracture resistance of the steel sheet, the rolling direction to the length of 100mm was cut out at a right angle, width 30 mm, and a thickness of 1.3mm or three-point bending a strip specimen of 1.6mm were processed in the steel plate, the rectangular test after mounting the water resistance of the strain gauges on the surface of the strip, by immersing the strip specimen during thiocyanate aqueous ammonium current density 0.1 mA / cm 2 strip specimen in by electrolyzing thiocyanate aqueous solution of ammonium in infested with hydrogen, after 2 hours, it was assessed by confirming the presence or absence of cracks.
[0166]
　Bending radius of the strip specimen was 10 mm. Load stress applied to the strip-shaped test piece having a thickness of 1.3mm is to 60% of the tensile strength of the steel sheet (TS), a load stress applied to the strip-shaped test piece having a thickness of 1.6mm, the tensile strength of the steel sheet (TS ) was 90 percent. The strip specimen was broken at 60% of the applied stress of the tensile strength (TS) "VERY BAD", without breaking a 60% load stress tensile strength (TS), 90% of the load of the tensile strength (TS) "BAD" a broken strip-shaped test piece in the stress, and the strip-shaped test piece was not broken in both of load stress was evaluated as "GOOD". Steel plate of evaluation, "GOOD" is a steel sheet excellent in delayed fracture resistance.
[0167]
[Table 7]

[0168]
[Table 8]

[0169]
　As shown in Table, in the steel sheets of Examples of the present invention, the delayed fracture resistance large amount of iron-based carbides precipitated is functioning as hydrogen trapping sites remarkably excellent, and the moldability in the phase structure of the tissue it can be seen that are also excellent. In the steel sheet of the comparative example, it can be seen that inferior strength, the delayed fracture resistance, at least one of moldability.
Industrial Applicability
[0170]
　As described above, according to the present invention, automotive, building, suitable as a structural member, such as home appliances, tensile strength and excellent delayed fracture resistance of more than 780MPa steel, galvanized steel sheet, and alloying it is possible to provide a galvanized steel sheet, a method for their preparation. Accordingly, the present invention has high applicability in structural member manufacturing and use industries.

claims

[Claim 1]Ga chemical composition,でmass%,
C: 0.05 ~ 0.40%, Si: 0.05
~ 3.00%, Mn: not more than 1.50% 3.50% the
Man, P: 0.04% or less ,
S: 0.01% or
less, N: 0.01% or
less, O: 0.006% or
less,
of Al: 0 ~ 2.00%, of Cr:
0 ~ 1.00%, of Mo: 1.00 ~ 0
%,
of
Ni: 0 ~ 1.00%, a
Cu: 0 ~ 1.00% of Nb: 0 ~ 0.30%, of
Ti: 0 ~ 0.30%, V: 0 ~
0.50% B: 0 ~ 0
% .01 of
Ca: 0 ~ 0.04%, of Mg: 0 ~ 0.04%, and
び, the REM: 0.04% 0 ~
wo shi containing, remnants ga yoびおFe impurities kara na ri,
　thickness 1/4 portionのtissue ga, volume fractionで,
firing Ti shi ma Hikaru Tensilon Cytec Suites: 70%,おyoび
one kind fu S Factory LITE andびBakeliteイnaイSuitesのand ha two kinds: 20% not the Man totalで
wo containing shi,
　before the note thickness 1/4 portionの Tissue niお~ te, the volume fractionでresidue Bioーsu Te naイSuites ga 10% not the Manでthou ri, fu RayonッSiliconeュma Hikaru Tensilon Cytec Suites ga 10%でthou ri, paーLITE ga 10%でthou ri, and zu former referred residue Bioーsu Te naイSuites, front note fu RayonッSiliconeュma Hikaru Tensilon Cytec Suites,おfront yoびreferred ParaloidーLITEのTotal volume fraction ga 15% or lessでthou ri,
　before the former referred to the thickness niおke ru 1/4 portion referred firing Ti shi ma Hikaru Tensilon Cytec Suites inの, a long diameter of at least 5nmのTie-based carbideのnumber density ga. 5 × 10 . 7 th / mm 2 or moreでthou ri,
　front mind plate than 5nm frontのthick 1/4 portion niおke ru long diameter referred Tie-based carbide numberのni Dui suru, [epsilon] based carbideのnumberのcutting engagement ga 20%でthou ri,
　tensile strength ga 780MPa or moreでthou ru
koとwo special Zhiとsuru plate.
[Claim 2]
　The chemical composition of the steel sheet contains, by
mass%,
Cr: 0.05 ~ 1.00%,
Mo: 0.01 ~ 1.00%, Ni: 0.05 ~ 1.00%,
and, Cu: 0 containing one or more .05 ~ 1.00%
steel sheet according to claim 1, characterized in that.
[Claim 3]
　The chemical composition of the steel sheet contains, by
mass%,
Nb: 0.005 ~ 0.30%, Ti: 0.005 ~ 0.30%,
and, V: 0.005 ~ 0.50% of one or containing two or more
steel sheet according to claim 1 or 2, characterized in that.
[Claim 4]
　The chemical composition of the steel sheet contains, by
mass%, B: containing 0.0001 to 0.01%
steel sheet according to any one of claims 1 to 3, wherein the.
[Claim 5]
　The chemical composition of the steel sheet contains, by
mass%,
Ca: 0.0005 ~ 0.04%, Mg: 0.0005 ~ 0.04%,
and, REM: 0.0005 ~ 0.04% of one or containing two or more
steel sheet according to any one of claims 1 to 4, characterized in that.
[Claim 6]
　Steel sheet according to any one of claims 1 to 5, wherein the average length of the iron-based carbide is 350nm or less.
[Claim 7]
　On the surface of the steel sheet according to any one of claims 1 ~ 6, Fe is 15 wt% or less, the balance being Zn, Al, and galvanized layer composed of impurity is formed
, characterized in that hot-dip galvanized steel sheet.
[8.]
　On the surface of the steel sheet according to any one of claims 1 ~ 6, Fe is 15 wt% or less, the balance being Zn, Al, and galvannealed layer formed of impurity are formed
, characterized in that galvannealed steel sheet according to.