0001]The present invention relates to a high strength galvanized steel sheet and a manufacturing method thereof.
BACKGROUND
[0002]Recent years, in view improved fuel efficiency from greenhouse gas emissions regulations due to global warming is required, the application of high-strength steel sheet in order to reduce the weight of the vehicle body and crash safety is becoming increasingly larger is there. In particular, in recent years, there is a growing need for more tensile strength 980MPa of ultra-high-strength steel sheet. Moreover, the site is required rust prevention Among vehicle body high strength galvanized steel sheet galvanized on the surface is obtained.
[0003]
　Usually, since the steel sheets for automobiles are processed into member shape by press forming, the steel sheet used, elongation, hole expansion is required. However, when increasing the strength of the general steel, elongation, hole expansion is deteriorated.
[0004]
　Several means to solve this problem have been proposed. For example, Patent Document 1, the metal structure of the steel sheet, by a composite structure including a martensite is ferrite and hard structure is soft tissue, to enhance both the strength and elongation. However, the composite structure disclosed in Patent Document 1 is a combination of soft tissue and hard tissue. Complex structure composed of one another in hardness difference is large organization, poor hole expansibility.
[0005]
　In Patent Document 2, the metal structure of the steel sheet, having an intermediate hardness of ferrite and martensite, by a single tissue of the upper bainite or lower bainite to reduce the hardness difference between the tissues, strength and Anahiroge thereby improving the sex. However, upper bainite and lower bainite, since it is composed of bainitic ferrite and a hard cementite containing many dislocations, poor elongation.
[0006]
　Patent Document 3-5, utilizing deformation-induced transformation of retained austenite to (TRIP), discloses a technique relating to high-strength steel sheet having improved elongation and hole expansion rate. However, in order to generate a residual austenite, because must increase the carbon content in the steel, weldability is degraded.
CITATION
Patent Document
[0007]
Patent Document 1: JP-A-7-11383 JP
Patent Document 2: Japanese Patent No. 2616350
Patent Document 3: WO 2013/51238 Patent
Patent Document 4: JP 2006-104532 Patent Publication
Patent Document 5: JP 2007- 262,494 JP
Summary of the Invention
Problems that the Invention is to Solve
[0008]
　Thus, elongation of high-strength steel sheet by various techniques heretofore, hole expansion, although efforts to improve the bending property have been made, without sacrificing other practical performance, to improve the above properties at the same time It was difficult.
[0009]
　　The present invention has been made in view of the situation described above, an object of the present invention, elongation, hole expansion, excellent balance of flexural resistance and tensile strength of more than 980MPa high strength galvanized steel sheet and to provide a manufacturing method thereof.
Means for Solving the Problems
[0010]
　The gist of the present invention is as follows.
(1) in
　mass%,
　C: 0.050% ~
　0.130% Si: 0.010% ~ 2.00% Mn: 1.50%
　~ 3.50% P: 0.10% or less
　S: 0 .010% or less
　N: 0.010% or less
　O: 0.010% or less
　sol. Al:
　0.001% ~
　1.0% Ti: 0.005% ~ 0.20% B: 0.0005% ~
　0.010% Cr + 2 × Mo: 0.10% ~ 1.50%
the balance being Fe and has a chemical composition consisting of unavoidable impurities,
　the steel structure in the 1/8 thickness 1-3 / 8 thickness range around the 1/4 thickness from the surface, in area%,
　ferrite: 1-50%
　martensite 20-70%
　retained austenite: 0-5%
　perlite: 0-5%
　total particle diameter 0.2μm or more MA and cementite: 0-5%
　bainite: balance
, And the and the number density of grain size 0.2μm or more MA or cementite exists in isolation to ferrite or the bainite grains, 100/1000 .mu.m 2 or less and that the average hardness of the martensite 330Hv characterized in that it is a ~ 500 Hv, a high-strength galvanized steel sheet.
(2) the chemical composition, instead of a part of
　Fe,
　V: 0.001% ~ 1.00% Nb: 0.001% ~ 0.200%
, characterized by containing one or more of , high-strength galvanized steel sheet of the present invention.
(3) the chemical composition, instead of a part of
　Fe,
　Ni: 0.001% ~ 1.00%
　Cu: 0.001% ~ 1.00% Co: 0.001% ~
　1.00% W :
　0.001% ~
　1.00% Sn: 0.001% ~ 1.00% Sb: 0.001% ~ 0.50%
, characterized by containing one or more of the high strength of the present invention hot-dip galvanized steel sheet.
(4) the chemical composition, instead of a part of
　Fe,
　Ca: 0.0001% ~
　0.0100% Mg: 0.0001% ~ 0.0100% Ce: 0.0001% ~ 0.0100%

　Zr:
　0.0001 Pasento ～ 0.0100 Pasento La: 0.0001 Pasento ～ 0.0100
　Pasento Hf: 0.0001 Pasento ～ 0.0100 Pasento Bi: 0.0001 Pasento ～ 0.0100 Pasento
　misch metal: 0.0001 % ~ 0.0100%
, characterized by containing one or more of, the present invention, high-strength galvanized steel sheet.
(5) and satisfies the following steps (A) ~ (C) in this order, the method of producing a high strength galvanized steel sheet of the present invention.
　(A) below (A1) ~ hot-rolling process comprising (A5)
　(A1) Slab heating temperature: 1150 ° C. or higher
　(A2) total rolling reduction at 1150 ~ 1050 ℃: 50% or more
　(A3) less than 1050 ° C. By now, the time-finish rolling final pass total reduction rate of up to before: 66 ~
　95 Pasento (A4) the reduction ratio of finishing the final pass: 10% to 30%, finishing the final pass temperature: 850 ℃ ~ 1000 ℃
　(A5) wound take temperature: 450 ~ 700 ° C.
　(B) step of performing cold rolling 20% to 80%
　galvanized process comprising (C) the following steps　
(C-1) the maximum heating temperature: Ac3-50 ℃ ~ Ac3 + 50 ° C., holding time in the temperature range: 1 second to 500 seconds
　(C-2) the cooling rate between 600 ℃ ~ 720 ℃: 5 ℃ / sec
　(C-3) 480 ℃ ~ 600 ℃ residence time at 5 to 500 seconds
　(C-4) 440 ℃ ~ 480 ℃ residence time at 5 to 60 seconds
　(C-5) after plating, alloying treatment, in the process of cooling to room temperature, the temperature range of Ms ~ 0.99 ° C., the following equation (1), exceeds 30000, cooled to be less than 150,000
[0011]
[Number 1]

[0012]

　However, f M (T) = 1-exp {-0.011 ×
　(Ms-T)} TP (T) = (T + 273) × {Log10 (Delta] t / 3600)
　+6} Ms: martensitic transformation start temperature (℃ )
　T: temperature (° C.)
　Delta] t: T (° C.) from T-1 (° C.) time required to cool the seconds
[0013]
　The present invention, suitable for applications such as automotive steel sheets, elongation, can be obtained hole expansion, a high strength galvanized steel sheet having bendability or more tensile strength 980MPa excellent balance.
DESCRIPTION OF THE INVENTION
[0014]
　The present invention is hot-dip galvanized layer is a steel plate which is provided one side or on both sides, not a galvanized steel plate is before the steel sheet providing a galvanized layer. It described the structure of the present invention below.
"Chemical composition"
　First, the high strength steel sheet according to the present invention (hereinafter, simply referred to as steel sheet) will be described why the chemical composition of the defined as above. Incidentally, "%" for defining the chemical composition herein
is especially all unless otherwise specified, "% by weight".
[0015]
[C: 0.050% ~
　0.130%] C (carbon) are the essential element for the high strength of the steel sheet is added over 0.050%. On the other hand, the addition of excess of more than 0.130% C degrades weldability. Therefore, the C content is 0.050% - 0.130%. Preferably 0.060% to 0.100%, more preferably from 0.065 to 0.090%.
[0016]
[Si: 0.010% ~
　2.00%] Si (silicon) is a solid solution strengthening element, because it is an element effective for increasing the strength of the steel sheet, the addition of 0.010% or more. On the other hand, the addition of excess of more than 2.00% of Si degrades the wettability with molten zinc plating. The addition of excess of more than 2.00% of Si, in order to raise the temper softening resistance of martensite, increasing the hardness of martensite. Accordingly, the content of Si is set to 0.010% to 2.00%. Preferably 0.10 to 1.00%, more preferably from 0.30 to 0.70%.
[0017]
[Mn: 1.50% ~
　3.50%] Mn (manganese) is a powerful austenite stabilizing element, since an element effective in increasing the strength of the steel sheet, the addition 1.50%. On the other hand, the addition of excess of more than 3.50% Mn, the larger the sake of total particle diameter 0.2μm or more cementite and MA fraction, hole expansion is reduced. Accordingly, the content of Mn is set to 1.50 to 3.50%. Preferably from 2.00 to 3.00%, more preferably from 2.20 to 2.80%.
[0018]
[P: 0.10% or less]
　P (phosphorus) is solid-solution strengthening element contained as an unavoidable impurity in steel, is an effective element for increasing the strength of the steel sheet, excessive addition weldability and degrades the toughness. Accordingly, P content is limited 0.10% or less. Preferably 0.05% or less, more preferably 0.02% or less. However, to extremely reduce the P content, since the de-P cost increases, it is preferable to 0.001% the lower limit from the viewpoint of economy.
[0019]
[S: 0.010% or less]
　S (sulfur) is an element contained as an unavoidable impurity in steel, and forms MnS deteriorates the toughness and hole expansion in the steel. Therefore, a range deterioration of toughness and hole expansion is not significant, limiting the S content is 0.010% or less. Preferably 0.006% or below, more preferably 0.003% or less. However, to extremely reduce the S content, since the desulfurization cost is high, it is preferable to 0.0005% the lower limit from the viewpoint of economy.
[0020]
[N: 0.010% or less]
　N (nitrogen) is an element contained as an unavoidable impurity in steel, and forms coarse nitrides in steel when the content is more than 0.010% degrade bendability and hole expansibility. Therefore, N content is limited between 0.010% or less. Preferably 0.006% or below, more preferably 0.003% or less. However, to extremely reduce the N content, for de-N cost increases, it is preferable to 0.0005% the lower limit from the viewpoint of economy.
[0021]
[O: 0.010% or less]
　O (oxygen) is an element contained as an unavoidable impurity in steel, and forms coarse oxides in the steel when the content is more than 0.010% causes spread bending resistance and hole. Therefore, O content is limited between 0.010% or less. Preferably 0.006% or below, more preferably 0.003% or less. However, from the viewpoint of production cost, it is preferable to set the lower and 0.0001%.
[0022]
[Sol. Al: 0.001% ~
　1.0%] Al (aluminum) is at least sol for deoxidation of steel. In Al, the addition of 0.001%. Preferably 0.005% or more. However, not only leads to excessive cost increase in effect is saturated adversaries be added, to increase the transformation temperature of the steel to increase the load during hot rolling. Therefore sol. Al content is set to the upper limit of 1.0%. Preferably not more than 0.50%, more preferably not more than 0.20%. In addition, sol. Al is the amount of Al in the form of an aluminum compound of aluminum nitride or the like are precipitated and dispersed in the steel does not contain, shows the amount of Al in solid solution as an alloying element in the steel.
[0023]
[Ti: 0.005% ~
　0.20%] Ti (titanium) is by fixing N as TiN in the steel, inhibits the formation of BN as a hardenability decrease factor. Also for improving the toughness by refining the austenite grain size at the time of heating, Ti is added at least 0.005%. Preferably 0.010% or more. On the other hand, excessive addition ductility of the steel sheet is lowered. Thus the upper limit of the Ti content is 0.20%. Preferably 0.050% or less.
[0024]
[B: 0.0005% ~
　0.010%] B (boron) is austenite grain boundaries during heating of the steel sheet, or segregated in ferrite / austenite grain boundaries, the hardenability of the steel by stabilizing the grain boundaries enhanced to ensure the amount of martensite by quenching, it is an essential element in the present invention. To obtain the effect sufficiently, B requires the addition of more than 0.0005%. Preferably is 0.0010% or more. On the other hand, excessive addition by forming a boride, resulting in impairing the hardenability of the steel. Thus the upper limit of B content is 0.010%. Preferably 0.0060% or less, more preferably 0.0040%.
[0025]
[Cr + 2 × Mo: 0.10 ~
　1.50%] Cr, Mo none are element for suppressing bainite transformation, in the present invention, Cr + 2 × Mo is added in an amount of 0.10% or more. When Cr + 2 × Mo is below 0.10%, bainite transformation will proceeds excessively, the amount of tempered martensite is reduced. The number density of the MA or cementite is mixed structure isolated by 0.2μm or more martensite and austenite present in the bainite is increased. On the other hand, if the Cr + 2 × Mo is larger than necessary, because it causes the weldability of degradation and cold rolling deterioration, the upper limit 1.50%. The preferred range of Cr + 2 × Mo is 0.20 to 1.0% and more preferably from 0.30 to 0.70%. Although making the Cr + 2 × Mo in the above range is premised, Cr, addition ranges for the respective Mo is effective and for increasing the strength of steel sheets Cr or Mo, in consideration of cost, etc., Cr: 0 .001% - 1.00%, Mo: it is preferably selected suitably from 0.001% to 1.00%.
[0026]
　High-strength cold-rolled steel sheet according to the present invention has the chemical composition, but the balance being Fe and incidental impurities, further, in place of part of the Fe, contain following elements if needed it may be.
[0027]
[V: 0.001% ~ 1.00% , Nb: one or two or more of% ~ 0.200
　0.001%] V (vanadium), Nb (niobium) is a carbide forming element, the steel sheet it may be added as needed because it is an effective element for increasing the strength. However, the effect be excessively added to the cost to the saturated adversaries. Each therefore the content thereof is V: 0.001% ~ 1.00%, Nb: 0.001% - 0.200%. A more preferred content of V is, V: 0.01% 0.200%, the content of more preferred Nb is, Nb: 0.005% - 0.10
0%.
[0028]
[Ni: 0.001% ~ 1.0% , Cu: 0.001% ~ 1.0%, Co: 0.001% ~ 1.0%, W: 0.001% ~ 1.0%, Sn : 0.001% ~ 1.0%, Sb : one or two or more of% ~ 0.50
　0.001%] Ni (nickel), Cu (copper), Co (cobalt), W (tungsten), sn (tin), Sb (antimony) may be added as needed because it is an effective element for increasing the strength of any steel sheet. But the effect the addition of these elements too incurs an increase in cost to the saturated adversaries. Accordingly, the content thereof is Ni: 0.001% ~ 1.0%, Cu: 0.001% ~ 1.0%, Co: 0.001% ~ 1.0%, W: 0.001% ~ 1 .0%, Sn: 0.001% ~ 1.0%, Sb: to 0.001 to 0.50%. More preferably, Ni: 0.1% ~ 0.8% , Cu: 0.1% ~ 0.5%, Co: 0.1% ~ 0.5%, W: 0.1% ~ 0.3 %, Sn: 0.05% ~ 0.2 %, Sb: 0.01 to 0.10%.
[0029]
[Ca: 0.0001% ~ 0.0100% , Mg: 0.0001% ~ 0.0100%, Ce: 0.0001% ~ 0.0100%, Zr: 0.0001% ~ 0.0100%, La : 0.0001% ~ 0.0100%, Hf : 0.0001% ~ 0.0100%, Bi: 0.0001% ~ 0.0100%, mischmetal: an amount ranging from 0.0001% to 0.0100%, consisting of one or two or more from the
　group] Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium), mischmetal in finely dispersed in the steel inclusions an element contributing, Bi (bismuth) is an element to reduce micro-segregation of substitutional alloying elements Mn, Si and the like in the steel, since it contributes to improving the workability of each steel sheet, It is preferably added as needed. To obtain this effect, it requires the addition of more than 0.0001%, respectively. On the other hand, excessive addition causes deterioration of ductility. Therefore the content is the upper limit 0.0100%. Mischmetal is a mixed alloy of mixed rare earth metals to the principal component and lanthanide metals.
[0030]
　High-strength cold-rolled steel sheet of the present invention, except the elements have been described above, consists of Fe and unavoidable impurities, in addition to the respective elements described above, can be contained within a range not to impair the effects of the present invention .
[0031]
"Steel structure"
　will be described the reason for defining the steel structure of the high-strength steel sheet according to the present invention. The Target of the steel structure, the "%" and steel structure, which defines the steel structure herein in 1/8 thickness 1-3 / 8 thickness range around the 1/4 thickness from the surface of the steel sheet is particularly all unless otherwise specified is "area%".
[0032]
[Ferrite: 1-50%]
　ferrite for an organization that is excellent in ductility, improves the elongation of the steel sheet. Meanwhile, since it is soft, it is difficult to secure the excessively to contain strength. Accordingly, the content is 1-50%. Preferably 10% to 40%, more preferably 15 to 35%.
[0033]
: Martensite site 20 to 70%
　steel sheet according to the present invention comprises 20% to 70% of martensite. When martensite is 20% or less, strength of the steel sheet can not be ensured. On the other hand, the elongation of the steel sheet is deteriorated more than 70%. A more preferred content of martensite is 30% to 60%.
　In this specification, the term "martensite", fresh martensite is martensite is not tempered, and the meaning of both tempered martensite. However, in the present invention, as described below, the self-tempering occurs in the martensite after plating alloying process. Therefore, most of the martensite in the present invention is effectively a martensite was tempered least to some extent.
　However, martensite in a mixed structure of martensite and austenite exists in isolation in ferrite or bainite grains described below MA (Martensite-Austenite Constituent) are excluded from the "martensite".
[0034]
Residual Austenite: 0-5%]
　retained austenite, since the transformation into hard martensite by strain-induced transformation during deformation of the steel sheet deteriorates the hole expandability of the steel sheet. Accordingly, the content thereof to 0 to 5%. Preferably from 0 to 3%.
[0035]
: [Perlite 0-5%]
　Since perlite is a metal structure comprising hard and coarse cementite becomes the starting points of voids during widened hole, it deteriorates the hole expandability of the steel sheet. Accordingly, the content thereof to 0 to 3%.
[0036]
Total particle size 0.2μm or more MA and cementite: 0-5%]
　cementite and MA (Martensite-Austenite Constituent) is extremely hard tissue, become starting points of fracture at the time of processing a steel plate, hole expansion to act as a factor of degradation, its content is preferably as small. Further, the larger particles, easily serve as starting points of fracture when processed steel plates. If the particle diameter is small, in particular if it is less than 0.2 [mu] m, the effect is slight. Accordingly, in the present invention, the sum of the particle size 0.2μm or more MA and cementite and 0-5%. Preferably from 0 to 3%. On the other hand, as for the lower limit, it is not particularly limited, 1% or more is preferred in order to improve the strength.
[0037]
　The rest of the organization other than the above, is bainite. Remainder of bainite, upper bainite, be any of the lower bainite, it may be the mixed structure.
[0038]
[Ferrite or the number of particle size 0.2μm or more MA or cementite density present in isolation to bainite grains: 100/1000 .mu.m 2 below]
　MA and cementite is extremely hard tissue destruction during steel processing serving as the starting point. In addition to the area fraction, by limiting the number density per unit area, it is possible to reduce the starting point of fracture, hole expansion property is improved. In the present invention, the number density of grain size 0.2μm or more MA or cementite exists in isolation to ferrite or the bainite grains, 100/1000 .mu.m 2 or less. Preferably 50/1000 .mu.m 2 or less, more preferably 20/1000 .mu.m 2 or less.
　Here, "it exists in isolation to ferrite or the bainite grains" refers to MA or cementite which is not in contact with the orientation difference 15 ° or more grain boundaries. Therefore, misorientation 15 ° less than the grain boundary (e.g., such as infrastructure boundary lath or the like for forming a bainite) contact only, MA or cementite which is not in contact with the orientation difference 15 ° or more grain boundaries, "intragranular it is assumed that exist "in isolation in.
[0039]
Average hardness of martensite: 330Hv ~ 500Hv]
　For the hole expansion improvements, and martensite is a hard tissue, it is important to reduce the hardness difference between ferrite and bainite is soft tissue. Thus, the average hardness of the martensite in accordance with the present invention is not more than 500 Hv. Preferably is less than or equal to 450Hv. On the other hand, the hardness of the martensite is too low, or the tensile strength 980MPa strength can can not be ensured, the average hardness of the martensite and more 330Hv.
　The average hardness of martensite (average hardness of martensite) is determined by measuring the hardness of the martensite in the plate thickness 1/4 parts using Vickers hardness tester. Load, indentation formed in Vickers hardness measurement, determined to be included within the crystal grains of martensite to be measured. Fifty martensite grains hardness was measured, the average value of the hardness, the average hardness of the martensite.
[0040]
　Method for calculating the area percent of the steel structure in the present invention are as follows.
　Ferrite, martensite, pearlite, the area% of the particle diameter of 0.2μm or more MA and cementite are cut rolling direction cross-section of the steel sheet, out after mirror polishing, the steel structure by nital solution current, using a scanning electron microscope to shoot the secondary electron image Te. It has a lower tissue in grains, and determines a region in which carbides are precipitated with a plurality of variants and martensite. Further, a region where cementite is precipitated in the lamellar determines that pearlite. Small luminance, and the region where the infrastructure is not recognized is determined that ferrite. Large brightness and infrastructure to determine areas not to appear by etching as MA or cementite. The area percent of each, by calculating the point counting method, the area% of martensite and pearlite. The point counting method, the total number of grid points to be measured field area 1000Um 2 is preferably a per 1000 points or more.
　Area% of residual austenite, measured by X-ray diffractometry. In 1/8 thickness 1-3 / 8 thickness range around the 1/4 thickness from a thickness of the surface finish a plane parallel mirror to the plate surface, measuring the area ratio of the FCC iron by X-ray diffractometry be it with a the area% residual austenite.
[0041]
　The number density of crystal grain size 0.2μm or more MA or cementite exists in isolation to ferrite or the bainite grains is measured by the following methods. First, cut out the rolling direction cross-section of the steel sheet, the samples were subjected to mechanical polishing and electrolytic polishing, the EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method, the crystal orientation differences draw grain boundaries than 15 ° . Next, the same sample was corroded by nital solution, for the same region as the region was EBSD observation, photographing a secondary electron image with a scanning electron microscope. In the secondary electron image, the brightness is large and infrastructure to determine areas not to appear by etching as MA or cementite. Superimposed secondary electron image and the EBSD grain boundary map, not in contact with the grain boundary, and the number by particle size counts the MA and the number of cementite above 0.2 [mu] m, divided by the area of ​​the measuring field and density.
[0042]
　In the metal material, generally towards the crystal grain size fine strength, since the improved mechanical properties such as ductility, it is preferable that the predetermined particle diameter or less pressure.
[Effective crystal grain size: preferably 5μm or less]
　To a better level of hole expansion, it is preferable that the effective crystal grain size and 5μm or less. Note that the effective crystal grain size means a grain size of the enclosed crystal orientation difference 15 ° or more grain boundaries to be described in later approach area.
　The average effective crystal grain size is measured by EBSP-OIM method. The grain boundaries of the steel, generally defined as recognized by a are high-angle grain boundary thresholds 15 ° as a crystal grain boundary, by visualizing the particle from mapped image misorientation 15 ° or more grain boundaries , to measure the average crystal grain size.
[0043]
"Mechanical properties"
Tensile strength above 980MPa]
　the tensile strength of high-strength steel sheet of the present invention is 980MPa or more.
[0044]
　Then, hot-dip galvanized steel sheet according to the present invention, and the manufacturing method of the galvannealed steel sheet will be described.
[0045]
"Production conditions in the hot rolling step"
[1150 step of heating the slab above ℃]
　to fully dissolve the borides and carbides, the slab heating temperature is defined as above. Incidentally steel slab to be used, it is preferable to cast in a continuous casting method in view of productivity, Zokatamariho may be a thin slab casting process. Further, the cast slab may be once cooled to room temperature, but may be sent directly to the furnace without cooling to room temperature.
[0046]
[In the hot rolling step, the total rolling reduction at 1150 ~ 1050 ℃: 50% or more, and the total rolling reduction to a point-finish rolling final pass before was less than 1050 ℃: 66 ~ 95%]
　rough rolling step , reduction ratio and rolling temperature in the hot rolling process comprising a finish rolling step above conditions are preferred.
　1050 ° C. or more, when the total rolling reduction at 1150 ° C. or less is less than 50%, the recrystallization during hot rolling tends to become insufficient, hot rolled steel sheet microstructure is easily insufficient homogeneity. As a result, Mn segregation zone is not homogenized, MA fraction of the final tissue tends to increase.
　When the total rolling reduction to a point-finishing final pass before becomes less than 1050 ° C. is more than 95%, the texture of the hot-rolled steel sheet develops, anisotropy in the final product plate tends to manifest. On the other hand, if the total rolling reduction to a point-finishing final pass before becomes less than 1050 ° C. is less than 66%, leads to coarsening of the hot-rolled steel sheet structure leads to the coarsening hence workability deterioration of the final product plate tissue Cheap.
　1150 total rolling reduction at ~ 1050 ° C., only rough rolling step, it may be 50% or more, the rough rolling step, even across the finish rolling step, the total rolling reduction at 1150 ~ 1050 ° C. 50% and it may be greater than or equal to. More preferably 60% or more. The total rolling reduction to a point-finish rolling final pass before was less than 1050 ° C. Also, if the rolling process finishing when it becomes less than 1050 ° C., only finishing rolling step, may be from 66 to 95%, 1050 if the time was less than ° C. is rough rolling step, a rough rolling step, even across the finish rolling step, the total rolling reduction to a point-finish rolling final pass before becomes less than 1050 ° C. is at 66 to 95% it is sufficient. More preferably 70% or more.
[0047]
[In the finish rolling step, 10% to 30% rolling reduction finishing final pass, it increases temperature of 850 ° C.-1000 ° C. of finishing final pass]
　reduction allocation and rolling temperature in the finish rolling step above conditions are preferred. If rolling reduction of finish final pass exceeds 30% or finishing final pass of up temperature (rolling completion temperature) is below 850 ° C., since the texture of the hot-rolled steel sheet develops, anisotropy in the final product plate There tends to manifest. On the other hand, reduction ratio of finishing final pass is less than 10%, or the rising temperature of the finishing final pass exceeds 1000 ° C., leads to coarsening of the hot-rolled steel sheet structure, coarsening of the final product plate tissue thus workability occurs It leads easily to.
[0048]
[Coiling temperature: 450 ~ 700 ℃]
　winding temperature is set to 450 ~ 700 ℃. When the coiling temperature is below 450 ° C., hot rolled steel sheet strength becomes excessive, impairs cold rolling property. On the other hand, when the coiling temperature exceeds 700 ° C., the amount of martensite in the final product is below a predetermined amount, it is difficult to obtain a product sheet strength.
　Pickling method of hot-rolled coil may According to the conventional method. It may also be carried out skin pass rolling for shape correction and pickling improvement of hot-rolled coil.
[0049]
"Production conditions in the cold rolling step"
[between 20% to 80% cold rolling]
　cold rolling rate for refining the austenite grain size during heating in a continuous galvanizing line process be 20% or more It is preferred. On the other hand, excessive pressure is for causing an increase in the load of cold rolling mill becomes excessive rolling weighted, the upper limit is 80%. More preferably 30% to 70%.
[0050]
　After cold rolling step is subjected to hot-dip galvanized surface of the steel sheet by continuous galvanizing line process.
"Production conditions in a continuous galvanizing line step"
[600 ° C. ~ maximum heating rate of heating to a temperature: 0.2 ~ 10 ℃ / sec
　heating rate above conditions are preferred. If the heating rate exceeds 10 ° C. / sec, recrystallization of the ferrite is not sufficiently proceed, the elongation of the steel sheet tends to deteriorate. On the other hand, when the heating rate is below 0.2 ° C. / sec, for coarse austenite at the stage of this heating, the finally obtained steel structure is liable to be coarse. More preferably, the 0.5 ° C. / sec or more.
[0051]
[Maximum heating temperature: Ac3-50 ℃ ~ Ac3 + 50 ℃ , 1 second or more in the temperature range, the retention of the following 500 seconds]
　required amount of martensite structure is obtained, in order to proceed austenitized to the extent necessary, the best the heating temperature is at least Ac3-50 ° C. or higher. On the other hand, if excessively raising the heating temperature, it is difficult to ensure the fraction of ferrite. Further, not only leads to toughness degradation and chemical conversion treatment of the deterioration due to the coarsening of austenite grain size, leading to damage to the annealing equipment. Therefore the upper limit of the heating temperature is set to Ac3 + 50 ° C.. Preferably is a Ac3-30 ℃ ~ Ac3. Since the heating time is not austenitization is proceed sufficiently short to the degree necessary, and at least one second or more. More preferably not less than 30 seconds. On the other hand, the upper limit since it inhibits productivity and the heating time is too long and 500 seconds.
[0052]
Cooling Temperature Range: 480 ~ 600 ° C., the cooling rate between 720 ℃ ~ 600 ℃: 5 ℃ / sec]
　after the heating, is cooled to between 480 ~ 600 ° C.. The cooling rate of between 720 ° C. to 600 ° C. is a 5 ° C. / sec or more. If the cooling rate is less than 5 ° C. / sec, it exceeds the upper limit of the ferrite fraction defined in the present invention. Preferably at 10 ° C. / sec or more. The upper limit of the cooling rate need not be particularly specified, a typical continuous annealing furnace cooling capacity is difficult to cooling rate exceeds 100 ° C. / sec.
[0053]
[480 ℃ ~ 600 ℃ residence time at 5 to 500 seconds]
　After cooling to the cooling temperature range, 480 ° C. ~ 600 in a temperature range of ° C. 5 seconds or more, is retained. This process can be suppressed progression of bainite transformation at 440 ~ 480 ° C. vicinity unavoidably generated in the galvanizing line, which will be described later. Although not clear details of the mechanism, while holding at 480 ° C. ~ 600 ° C., intensified segregation ratio of the grain boundary of B atoms in the steel, so that the progress of bainite transformation at 440 ~ 480 ° C. It is considered to be suppressed. By bainite occurs in galvanizing line, ultimately isolated over particle size 0.2μm to exist in MA or cementite number density in ferrite or the bainite grains: 100/1000 .mu.m 2 becomes higher. The residence time is preferably 10 seconds or more, more preferably 30 seconds or more. On the other hand, the residence time is too long, since the pearlite transformation occurs, the following 500 seconds. Preferably not more than 300 seconds. The above residence as long as it satisfies the prescribed conditions, may be held at a constant temperature, such as the case of slow cooling between 480 ° C. ~ 600 ° C., but may be varied.
[0054]
[440 ℃ ~ 480 ℃ residence time at 5 to 60 seconds]
　for applying a galvanized steel sheet, after the step, cooling the steel sheet to 440 ~ 480 ° C.. Here, the residence time in the 440 ° C. ~ 480 ° C. is not more than 60 seconds. Since the temperature range progression bainite transformation is fast temperature range, the residence time at this temperature range is long, bainite transformation proceeds excessively, martensite fraction defined by the present invention can not be obtained. Also within grains of bainite generated in this temperature range, compared to the bainite generated at 480 ° C. ~ 600 ° C., a high content of MA and cementite. Therefore, residence time in the temperature range shorter preferable. However, in the galvanizing line, it is necessary to immerse the steel sheet zinc pot bath, residence time in the temperature range inevitably occurs. Therefore, the residence time in the 440 ~ 480 ° C. is the lower limit 5 seconds.
[0055]
　440 ° C. ~ 480 after 5-60 seconds to dwell at ° C., molten zinc plating may be performed according to a conventional method. For example, the plating bath temperature is 440 ~ 480 ° C., soaking time may at most 5 seconds. Plating bath, but the plating bath containing Al 0.08 ~ 0.2% is preferred, other, unavoidable impurities Fe, Si, Mg, Mn, Cr, Ti, may contain Pb. Also, the basis weight of the plating is preferably controlled by a known method such as gas wiping. The unit weight, per side 25 ~ 75 g / m 2 is preferred.
[0056]
　The high strength galvanized steel sheet to form a galvanized layer may be subjected to alloying treatment if required. In that case, when the alloying treatment temperature is lower than 460 ° C., not only impair the productivity slower alloying speed, since the alloying treatment unevenness occurs, the alloying treatment temperature be 460 ° C. or higher preferable. On the other hand, when the alloying temperature exceeds 600 ° C., since pearlite area ratio of the hot-dip galvanized steel sheet is more than 5%, the alloying treatment temperature is preferably set to 600 ° C. or less. More preferably 580 ° C. or less.
[0057]
[0.99 ° C. ~ residence time in the Ms:]
　After the plating-alloying treatment, the temperature range of Ms ~ 0.99 ° C., the following equation (1), exceeds 30000, cooled to be less than 150,000.
[0058]
[Number 2]

[0059]

　However, f M (T) = 1-exp {-0.011 ×
　(Ms-T)} TP (T) = (T + 273) × {Log10 (Delta] t / 3600)
　+6} Ms is the martensite transformation start temperature, T is the temperature (° C.), Delta] t is T (° C.) from T-1 (° C.) time taken to cool to a (second). In the present invention, martensite transformation occurs during cooling after the plating alloying process. Further, martensite self tempering (autotempered) occurs, martensite softened. Equation (1) is a parameter representing the degree of progression of self-tempered martensite, as Equation (1) is large, self-tempering proceeds, martensite softened. On the other hand, as the equation (1) is small, self-tempering is suppressed, martensite hardening. The average hardness of the martensite in order to 330Hv ~ 500Hv, it is necessary to cool the conditions satisfying the formula (1). F
　in the formula (1) M (T) is the martensite transformation rate at T ° C., as T is small, that the larger the degree of supercooling from the Ms, f M (T) increases. TP (T) is a parameter representing the degree of tempered martensite at T ° C., the larger T is large. Further, as the residence time Δt at T ° C. is large, increases. Also, the higher the Ms of the steel sheet, the value of the expression (1) increases.
　Cooling pattern from Ms to 0.99 ° C. as long as it satisfies the formula (1) may be in any form. For example, it may be linearly cooled at a constant cooling rate from the Ms to room temperature, it may change the cooling rate during cooling. Alternatively, it may be performed holding at a certain temperature.
　Note that the martensitic transformation in the present invention occurs after ferrite transformation and bainite transformation. As the ferrite transformation and bainite transformation, C is distributed to the austenite. Therefore, heating to the austenite single phase, does not coincide with the Ms when quenched. Ms in the present invention, for example, using a thermal expansion measuring apparatus such as a Formaster tester, the galvanizing line of heat cycle (heating temperature, heating rate, heating time, cooling rate, cooling temperature range, the cooling dwell time , plating temperature and same heating temperature, plating time equivalent heating time, when the heat treatment simulating the cooling conditions) after plating to martensitic transformation start to be similar, the thermal expansion of the cooling process by measuring the temperature, it can be obtained.
[0060]
　After the continuous galvanizing line process, flat straightening of the steel sheet, in order to adjust the surface roughness, may be subjected to temper rolling. In this case, in order to avoid the deterioration of the ductility, it is preferable that the elongation 2% or less.
Example
[0061]
　Next, a description will be given of an embodiment of the present invention. Conditions in Examples is an example of conditions adopted for confirming the workability and effects of the present invention. The present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention may employ various conditions.
[0062]
　Steels having the chemical compositions shown in Table 1 were melted to prepare a slab. The slab, by performing hot rolling under the conditions shown in Table 2, were produced hot-rolled steel sheet. Then, the hot-rolled steel sheet was pickled, to remove the scale of the surface. Then cold rolling under the conditions shown in Table 2. Further, the obtained steel sheet, under the conditions shown in Table 3 and heat-treated and continuous galvanizing process.
[0063]
[Table 1]

[0064]
[Table 2]

[0065]
[table 3]

[0066]
　The thus JIS5 No. tensile test specimen was sampled from the perpendicular direction from the obtained steel sheet in the rolling direction, subjected to tissue analysis, subjected to a tensile test according to JIS Z2241, a tensile strength (TS), total elongation (El ) was measured. In addition, complete the "JFS T 1001 hole expansion test method" of the Japan Iron and Steel Federation standard, to measure the hole expansion ratio (λ). Tensile strength above 980 MPa, elongation of 10% or more, the hole expanding ratio mechanical properties of not less than 30% were considered good. The results are shown in Table 4. Incidentally, fine MA and cementite less than 0.2μm is contained in the grains of bainite and martensite, and were counted as bainite or martensite because it is difficult to separate quantitatively.
[0067]
[Table 4]

[0068]
　Experimental Example 10, above the range of Equation (1) defines the present invention. As a result, decreases the hardness of martensite, the strength is below 980 MPa.
　Experimental Example 14, below the range of Formula (1) defines the present invention. As a result, the hardness of the martensite increases, the hole expansion ratio becomes inferior.
　Experimental Example 11, 480 residence time of ~ 600 ° C. is less than the range defined in the present invention. As a result, the number density of the crystal grain size 0.2μm or more MA or cementite exists in isolation to ferrite or the bainite grains is increased, the hole expansion ratio becomes inferior.
　Experimental Example 12, the residence time of 440 ~ 480 ° C. exceeds the range defined in the present invention. As a result, lower than the prescribed area percent present invention martensite, the strength is below 980 MPa, the number density of the crystal grain size 0.2μm or more MA or cementite exists in isolation to ferrite or the bainite grains is increased , the hole expansion rate has become inferior.
　Experimental Example 13, above the range in which the coiling temperature is specified in the present invention. As a result, since the area% of martensite is below the range specified in the present invention, the strength falls below 980 MPa.
　Experimental Example 16, above the range in which alloying treatment temperature is specified in the present invention. As a result, pearlite generates above the provisions of the present invention, the hole expansion ratio becomes inferior.
　Experimental Example 19, the maximum heating temperature is above the range defined in the present invention. As a result, the area% of ferrite is reduced, elongation becomes inferior.
　Experimental Examples 20 and 21, below the range in which cooling rate of 720 ° C. ~ 600 ° C. to define the invention. As a result, the area% of ferrite is increased, either the strength or hole expansion ratio becomes inferior.
[0069]
　Experimental Examples 45-50, the chemical composition deviates from the range defined in the present invention.
　Experimental Example 45 because above the range of C is defined according to the present invention, the greater the hardness of the martensite, the hole expansion ratio becomes inferior.
　Experimental Example 46, since below the range in which B is defined according to the present invention, the martensite fraction is reduced, the tensile strength became inferior.
　Experimental Example 47, since below the range Cr + 2 × Mo defines the present invention, the martensite fraction is reduced, the tensile strength became inferior.
　Experimental Example 49, since exceeding the range Cr + 2 × Mo defines the present invention, the fraction of martensite is increased, the elongation became inferior.
　Experimental Example 48, since exceeding the range of Mn is defined in the present invention, the sum of the particle size 0.2μm or more cementite and MA fraction increases, the hole expansion ratio becomes inferior.
　Experimental Example 50, since exceeding the range of Si is defined in the present invention, the hardness of the martensite increases, the hole expansion ratio becomes inferior.
[0070]
　In contrast, for Examples 1 to 9,15,17,18,22-44,51,52 is the chemical composition and manufacturing conditions is met the range specified in the present invention, it is tissue defining the present invention, good mechanical properties were obtained such.

WE CLAIM

[Requested item 1]
　By
　mass%,
　C: 0.050% ~
　0.130% Si: 0.010% ~ 2.00% Mn: 1.50%
　~ 3.50% P: 0.10% or less
　S: 0.010% The following
　N: 0.010% or less
　O: 0.010% or less
　sol. Al:
　0.001% ~
　1.0% Ti: 0.005% ~ 0.20% B: 0.0005% ~
　0.010% Cr + 2 × Mo: 0.10% ~ 1.50%
the balance being Fe and has a chemical composition consisting of unavoidable impurities,
　the steel structure in the 1/8 thickness 1-3 / 8 thickness range around the 1/4 thickness from the surface, in area%,
　ferrite: 1-50%
　martensite 20-70%
　retained austenite: 0-5%
　perlite: 0-5%
　or more particle size 0.2μm of MA and cementite total: 0-5%
　bainite: balance
is, and isolated into ferrite or the bainite grains the number density of grain size 0.2μm or more MA or cementite present Te is 100/1000 .mu.m 2Less and, and, wherein the average hardness of the martensite is 330Hv ~ 500Hv, high-strength galvanized steel sheet.
[Requested item 2]
　The chemical composition, instead of a part of
　Fe,
　V: 0.001% ~ 1.00% Nb: 0.001% ~ 0.200%
, characterized by containing one or more of, claims high-strength hot-dip galvanized steel sheet according to 1.
[Requested item 3]
　The chemical composition, instead of a part of
　Fe,
　Ni: 0.001% ~
　1.00% Cu: 0.001% ~ 1.00% Co: 0.001% ~
　1.00% W: 0.
　% ~ 1.00 001%
　Sn: 0.001% ~ 1.00% Sb: 0.001% ~ 0.50%
, characterized by containing one or more of, in claim 1 or claim 2 high-strength hot-dip galvanized steel sheet according.
[Requested item 4]
　The chemical composition, instead of a part of
　Fe,
　Ca: 0.0001% ~
　0.0100% Mg: 0.0001% ~ 0.0100% Ce: 0.0001% ~
　0.0100% Zr: 0.
　%
　- 0.0100 0001 Pasento La: 0.0001% - 0.0100
　Pasento Hf: 0.0001% - 0.0100 Pasento Bi: 0.0001% - 0.0100 Pasento
　mischmetal: 0.0001% - 0. 0100%
, characterized in that it contains one or more of, according to any one of claims 1 to 3, the high-strength galvanized steel sheet.
[Requested item 5]
　The following steps (A) ~ (C) and satisfies the order, the method of producing a high strength galvanized steel sheet according to any one of claims 1-4.
　(A) below (A1) ~ hot-rolling process comprising (A5)
　(A1) Slab heating temperature: 1150 ° C. or higher
　(A2) total rolling reduction at 1150 ~ 1050 ℃: 50% or more
　(A3) less than 1050 ° C. By now, the time-finish rolling final pass total reduction rate of up to before: 66 ~
　95 Pasento (A4) the reduction ratio of finishing the final pass: 10% to 30%, finishing the final pass temperature: 850 ℃ ~ 1000 ℃
　(A5) wound take temperature: 450 ~ 700 ° C.
　(B) step of performing cold rolling 20% to 80%
　galvanized process comprising (C) the following steps
(C-1) the maximum heating temperature: Ac3-50 ℃ ~ Ac3 + 50 ° C., holding time in the temperature range: 1 second to 500 seconds
　(C-2) the cooling rate between 600 ℃ ~ 720 ℃: 5 ℃ / sec or more
　(C-3) 480 ℃ ~ residence time at 600 ° C. : 5 to 500 seconds
　(C-4) 440 ℃ ~ 48 ° C. The residence time in the: 5 to 60 seconds
　after (C-5) Plating alloying, in a step of cooling to room temperature, the temperature range of Ms ~ 0.99 ° C., the following equation (1), exceeds 30000, It cooled to be less than 150,000
[Equation 1]

where,
　f M (T) = 1-exp {-0.011 ×
　(Ms-T)} TP (T) = (T + 273) × {Log10 (Delta] t / 3600)
　+6} Ms: martensitic transformation start temperature (°
　C.) T: temperature (° C.)
　Delta] t: T (° C.) from T-1 (° C.) time required to cool the seconds