[0001]The present invention is a high yield ratio and excellent in resistance to hydrogen embrittlement and to a galvanized steel sheet ultrahigh strength. Specifically, primarily molded into various shapes by press working or the like, to a galvanized steel sheet high yield ratio and ultra-high strength with excellent resistance to hydrogen embrittlement of automobile having excellent workability.
BACKGROUND
[0002]
　Recently, from the viewpoint of regulating the emission of greenhouse gases as global warming, improved fuel economy of automobiles it has been required. Therefore, in order to ensure crashworthiness with the weight of the vehicle body, while the application of high-strength steel sheet is increasingly expanded there. In addition, the site where rust resistance is required, an ultra high-strength steel sheet is required which galvanized.
[0003]
　In particular, recently, there is a growing need for tensile strength 1300MPa or more ultra-high strength steel plate and ultra high-strength galvanized steel sheet. Further, the member which is required to suppress deformation at the time of collision, ultra high-strength steel sheet is required to have a high yield ratio.
[0004]
　However, when the application of ultra-high-strength steel sheet tensile strength exceeding 1300 MPa, it is necessary to solve the hydrogen embrittlement of the steel sheet. The hydrogen embrittlement is a phenomenon in which the steel member having a high stress under use conditions is acts, due to the hydrogen entering from the environment, destroying at maximum tensile stress following additional stress.
[0005]
　Generally, hydrogen embrittlement resistance of the steel sheet is degraded as the tensile strength of the steel sheet is increased, the mechanism itself is not in a still uncertain.
[0006]
　Up to now, various attempts to improve the hydrogen embrittlement of the steel sheet have been made. It shows the study cases below.
[0007]
　Patent Document 1, the steel sheet surface layer is softened by an increase in the ferrite volume fraction of the steel sheet surface by decarburization treatment, and the tissues inside the steel sheet and ferrite-based, further, a small amount of martensite with a fine block by dispersing discloses a technique relating to high-strength steel sheet having both a high strength and resistance to hydrogen embrittlement. However, the steel sheet described in Patent Document 1, since the substantial amount containing ferrite is soft tissue, not preferable for obtaining a high yield ratio.
[0008]
　Patent Document 2, by appropriately controlling the average particle size and the aspect ratio in the form of ferrite, discloses a technique relating to high-strength galvanized steel sheet having both the processability and resistance to hydrogen embrittlement. However, even in the steel sheet described in Patent Document 2, since it contains a certain amount of ferrite is soft tissue, are expected to undesirable to obtain a high yield ratio.
[0009]
　Patent Document 3, a steel structure and martensite mainly tissue further, Nb, V, Cr, Ti, and, by precipitating carbides such as Mo, by the hydrogen trapping sites, and improved resistance to hydrogen embrittlement It discloses a technique relating to high-strength galvanized steel sheet. However, even in the steel sheet described in Patent Document 3, a high yield ratio is not considered.
[0010]
　Patent Document 4, a steel structure and bainite main tissue, further limiting the residual austenite to less than 4%, discloses a technique relating to high-strength galvanized steel sheet with improved resistance to hydrogen embrittlement.
[0011]
　However, bainite generated in hot-dip galvanizing process, from the holding temperature range, is often upper bainite. Upper bainite, as compared to tempered martensite and lower bainite, since the organization having poor toughness, the steel sheet of the upper bainite mainly tissue, decrease in toughness is concerned.
CITATION
Patent Document
[0012]
Patent Document 1: WO 2011/065591
Patent Document 2: JP 2010-126787 Patent Publication
Patent Document 3: JP 2004-323951 Patent Publication
Patent Document 4: JP-A 06-145893 JP
Patent Document 5: JP 2013-144830 JP
Patent Document 6: JP 2009-203549 Patent Publication
Patent Document 7: WO 2013/047821 Patent
Patent Document 8: WO 2013/047755
Patent Document 9: WO 2011/065591
Patent Document 10: JP-a-10-001740
Patent Document 11: JP-a 09-111398 JP
-Patent Document 12: JP-a-06-145891
Patent Document 13: International Publication No. 2011/105385
Patent Document 14: JP open Patent Publication No. 2007-197819
Non-patent literature
[0013]
Non-Patent Document 1: CAMP-ISIJ Vol. 17 (2004) p. 396
Non-Patent Document 2: Iron and Steel, vol. 74 (1988), p. 2353
Summary of the Invention
Problems that the Invention is to Solve
[0014]
　The present invention is excellent in resistance to hydrogen embrittlement, and an object thereof is to provide a galvanized steel sheet which can obtain a high tensile strength and yield ratio.
Means for Solving the Problems
[0015]
　The present inventors have found that excellent resistance to hydrogen embrittlement, intensive studies for high tensile strength, e.g. 1300MPa or more tensile strength, and high yield ratio, a method of obtaining a hot-dip galvanized steel sheet can be obtained, for example 75% or more yield ratio As a result, it led to obtain the following findings.
[0016]
　The area ratio of (a) ferrite and upper bainite is limited to below a predetermined area ratio, and martensite mainly.
[0017]
　For (b) hydrogen embrittlement cracks are prevented from being progress along the prior austenite grain boundaries, with the inclusion of B is a grain boundary strengthening element a certain amount or more, the average effective crystal grain size, such as martensite, a predetermined It is under the control of the grain diameter or less in.
[0018]
　(C) the area ratio of martensite having a predetermined number density or more Fe carbide to the total martensite is 50% or more.
[0019]
　(A), we found that it is possible to achieve and (b), and, if satisfied all (c), the mechanical properties and the desired embrittlement.
[0020]
　The present invention has been made based on the above findings, its gist the following.
[0021]
　(1)
　in
　mass%, C:
　0.14 ~
　0.3%, Si: 0.001 ~ 2.0%, Mn: 2.0 ~
　4.0%, P: 0.05% or
　less, S: 0.01% or
　less, N: 0.01% or
　less,
　Al: 0.001 ~ 1.0%,
　Ti: 0.001 ~ 0.10%, B: 0.0001 ~
　0.01%, Mo: 0 　0.50%
~, 　Cr: 0 ~ 0.80%, Ni: 0 ~ 　1.00%, Cu: 0 ~ 1.00%, V: 　0 ~ 0.50%, Nb: 0.0 ~ 0. 　% 　10, 　Ca: 0.00 ~ 0.01%, Mg: 0.00 ~ 0.01%, 　REM: 0.00 ~ 0.01%, Bi: 0.00 ~ 0.01%, and 　balance: Fe and impurities, 　have in a chemical composition represented, 　the area ratio, 　polygonal ferrite: 10% or less,

　Upper bainite: 20% or less,
　residual austenite: 5% or less,
　martensite: 70% or more,
　1 × 10 6 / mm 2 or more number density in martensite has a Fe carbide: 50% or more relative to the total martensite, and
　average effective crystal grain size: 5.0 .mu.m or less,
　in galvanized steel sheet characterized by having a steel structure represented.
[0022]
　(2)
　the amount of solid solution B is 0.0010% to 0.0100% by weight, galvanized according to, characterized in that prior austenite grain size of 1.0 .mu.m-7.0 .mu.m (1) steel sheet.
[0023]
　(3)
　galvanized steel sheet according to the product of solid solute B amount and the prior austenite grain size is equal to or is 0.0010 wt% · [mu] m or more (2).
[0024]
　(4)
　In the above chemical
　composition, Mo: 0.001 - 0.50%
galvanized steel sheet according to any one of wherein the holds (1) to (3).
[0025]
　(5)
　In the above chemical
　composition,
　Cr: 0.001 ~ 0.80%, Ni: 0.001 ~ 1.00%, or
　Cu: 0.001 ~ 1.00%
　, or that any combination of these is true hot-dip galvanized steel sheet according to any one of the features (1) to (4).
[0026]
　(6)
　In the above chemical
　composition, V: 0.001 - 0.50%, or
　Nb: 0.001 - 0.10%
　, or, characterized in that they are both true (1) either - (5) hot-dip galvanized steel plate of crab described.
[0027]
　(7)
　In the above chemical
　composition,
　Ca: 0.0001 ~
　0.01%, Mg: 0.0001 ~ 0.01%, REM: 0.0001 ~ 0.01%, or
　Bi: 0.0001 ~ 0. 0.1%
　or hot-dip galvanized steel sheet according to any one of wherein the arbitrary combination of these is true (1) to (6).
Effect of the invention
[0028]
　According to the present invention, excellent resistance to hydrogen embrittlement, it is possible to obtain a high tensile strength and yield ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029]
[1] Figure 1 is a diagram showing the steps of the plating and heat treatment employed in Example 1 schematically.
FIG. 2 is a diagram showing the steps of the plating and heat treatment employed in Example 2 schematically.
DESCRIPTION OF THE INVENTION
[0030]
　First described the chemical composition of the slab used in the galvanized steel sheet and a manufacturing according to an embodiment of the present invention. Although details will be described later, hot-dip galvanized steel sheet according to the embodiment of the present invention, hot rolling of the slab, cold rolling, continuous annealing, is manufactured through a galvanizing treatment and tempering or the like. Therefore, the chemical composition of the molten zinc plated steel sheet and slab, not only the characteristics of the hot-dip galvanized steel sheet, is taken into consideration these processes. In the following description, "%" is a unit of content of each element contained in the hot-dip galvanized steel sheet, in particular means "% by mass" unless otherwise specified. Galvanized steel sheet according to the embodiment of the present invention, in mass%, C: 0.14 ~ 0.3%, Si: 0.001 ~ 2.0%, Mn: 2.0 ~ 4.0%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Al: 0.001 ~ 1.0%, Ti: 0.001 ~ 0.10%, B: 0. 0001 ~ 0.01%, Mo: 0 ~ 0.50%, Cr: 0 ~ 0.80%, Ni: 0 ~ 1.00%, Cu: 0 ~ 1.00%, V: 0 ~ 0.50 %, Nb: 0.0 ~ 0.10%, Ca: 0.00 ~ 0.01%, Mg: 0.00 ~ 0.01%, REM (rare earth metals: rare earth metal): 0.00 ~ 0 .01%, Bi: 0.00 ~ 0.01%, and the balance having a chemical composition expressed by Fe and impurities. Here, as the impurity, which is contained in raw materials such as ores and scrap, intended to be included in the manufacturing process, it is exemplified.
[0031]
　(C: 0.14 ~ 0.3%)
　C is an essential element in order to obtain the desired tensile strength. If it is less than 0.14%, since the desired tensile strength can not be obtained, and 0.14% or more. Preferably 0.17% or more. On the other hand, when it exceeds 0.3%, the resistance to hydrogen embrittlement and weldability is lowered, to 0.3% or less. Preferably not more than 0.25%, more preferably not more than 0.22%.
[0032]
　(Si: 0.001 ~
　2.0%) Si is an element effective for increasing the strength of the steel sheet. If it is less than 0.001%, because the addition effect is not exhibited, and 0.001% or more. Preferably 0.010% or more. On the other hand, when it exceeds 2.0%, the wettability and alloying rate of galvanized decreases. Further, since Si is a ferrite forming element, the Si content of 2.0 percent, comprised an area ratio of polygonal ferrite is difficult to below 10%. Therefore, Si content is 2.0% or less. Preferably 1.50% or less, more preferably 0.90% or less, still more preferably not more than 0.50%.
[0033]
　(Mn: 2.0 ~
　4.0%) Mn is a strong austenite stabilizer, is a valid element hardenability improvement of the steel sheet. If it is less than 2.0%, since the effect of addition is not exhibited sufficiently, and 2.0% or more. Preferably it is greater than or equal to 2.2%. On the other hand, when it exceeds 4.0%, since the resistance to hydrogen embrittlement is reduced to 4.0% or less. Preferably 3.5% or less, more preferably 3.0% or less.
[0034]
　(P: 0.05% or less)
　P is a solid solution strengthening element and is an effective element for high strength of the steel sheet. However, when it exceeds 0.05%, the weldability and the toughness is lowered, and 0.05% or less. Preferably not more than 0.02%. The lower limit is not particularly limited, but is practically 0.001% are substantially the lower.
[0035]
　(S: 0.01% or less)
　S is an impurity element, it is a preferred element as small. It exceeds 0.01%, since the forms MnS in steel to degrade the toughness and hole expansion, and 0.01% or less. Preferably 0.005% or less, more preferably 0.002% or less. The lower limit is not particularly limited, but is practically about 0.0001% is substantially the lower.
[0036]
　(N: 0.01% or less)
　N is an impurity element, is a preferred element as small. Exceeds 0.01%, the hole expansion generates coarse nitrides in the steel is reduced to 0.01% or less. Preferably not more than 0.005%. The lower limit is not particularly limited, but is practically 0.001% are substantially the lower.
[0037]
　(Al: 0.001 ~
　1.00%) Al is an element added for deoxidation. If it is less than 0.001%, because the addition effect is not exhibited, and 0.001% or more. Preferably 0.010% or more. On the other hand, when it exceeds 1.00%, the effect of addition is saturated, in addition to cost increases, the load at the time of transformation temperature of the steel rises hot rolling is increased, and 1.00% or less. Preferably 0.50% or less, more preferably 0.20% or less.
[0038]
　(Ti: 0.001 ~
　0.10%) Ti is the N fixed by forming a TiN in steel, with forming the effect of inhibiting the generation of BN as a reduction factor of hardenability, during heating the austenite grain size is an element contributing to the improvement of miniaturization and toughness and resistance to hydrogen embrittlement. If it is less than 0.001%, because the addition effect is not exhibited, and 0.001% or more. Preferably 0.010% or more. On the other hand, when it exceeds 0.10%, to produce coarse Ti carbide, since toughness and resistance to hydrogen embrittlement of the steel sheet is reduced to 0.10% or less. Preferably not more than 0.07%.
[0039]
　(B: 0.0001 ~ 0.01%) B
　segregates at the austenite grain boundaries during heating of the steel sheet, with an action of increasing the hardenability of steel to stabilize the austenite grain boundaries, increasing the grain boundary strength Te is an element contributing to the improvement of toughness and resistance to hydrogen embrittlement of the steel sheet. If it is less than 0.0001%, the addition effect is not exhibited, and 0.0001% or more. Preferably 0.0006% or more, and more preferably not less than 0.0011%.
[0040]
　On the other hand, when it exceeds 0.01%, to produce borides, since hardenability of the steel is inhibited, and 0.01% or less. Preferably 0.005% or less, more preferably 0.004% or less.
[0041]
　Mo, Cr, Ni, Cu, V, Nb, Ca, Mg and REM is not an essential element, a steel plate and may optionally elements be appropriately contained in the limits of the predetermined amount to the steel.
[0042]
　(Mo: 0 - 0.50%) Mo
　serves to contribute to improvement in hardenability of the steel sheet, delays the bainite transformation occurring during cooling-plating immersion after heating in the annealing step, contribute to the formation of the desired tissue it is an element. Further, Mo is an element contributing to the improvement of the particle size of the austenite is miniaturized toughness and resistance to hydrogen embrittlement during heating. Therefore, Mo may be contained. The Mo content is less than 0.001%, the addition effect is not exhibited, Mo content is preferably 0.001% or more, more preferably 0.050% or more. On the other hand, when the Mo content exceeds 0.50%, the addition along with the effect is saturated, so the production cost is increased, the Mo content 0.50% or less, preferably 0.30% or less. In other words: it is preferred that the "Mo 0.001 ~ 0.50%" is true.
[0043]
　(Cr: 0 ~ 0.80%, Ni: 0 ~ 1.00%, Cu: 0
　~ 1.00%) Cr, Ni, Cu are each an element effective for increasing the strength of the steel sheet. Therefore, Cr, Ni, or Cu, or any combination thereof may be contained. Cr, Ni, none of Cu, when the content is less than 0.001%, the addition effect is not exhibited, respectively, preferably 0.001% or more, more preferably 0.010% or more. On the other hand, if Cr content exceeds 0.80% or the Ni content exceeds 1.00%, or when the Cu content exceeds 1.00%, the effect of addition is saturated, the manufacturing cost increases to. Therefore, Cr content 0.80% is less, Ni content is 1.00% or less, Cu content is not more than 1.00%, preferably, Cr content 0.50% is less, Ni content 0.50% or less, Cu content is 0.50% or less. In other words, "Cr: 0.001 ~ 0.80%," "Ni: 0.001 ~ 1.00%", or "Cu: 0.001 ~ 1.00%", or holds any combination thereof is it is preferable.
[0044]
　(V: 0 ~ 0.50%, Nb: 0.0 ~ 0.10%)
　V and Nb carbides to form an element which contributes to the strength of a steel sheet. Therefore, V or Nb or both may be contained. Each element is also, when the content is less than 0.001%, the addition effect is not exhibited, either V content and the Nb content is preferably 0.001% or more, more preferably, V-containing the amount 0.030% or more, Nb content is 0.005% or more. On the other hand, the V content exceeds 0.50%, or if Nb content exceeds 0.10%, the effect of addition is saturated, the cost is increased, the V content 0.50% is less, Nb content is set to 0.10% or less, preferably V content 0.30% is less, the Nb content is 0.05% or less. In other words, "V: 0.001 ~ 0.50%", or "Nb: 0.001 ~ 0.10%", or it is preferable that they are both satisfied.
[0045]
　(Ca: 0.00 ~ 0.01%, Mg: 0.00 ~ 0.01%, REM: 0.00 ~ 0.01%, Bi: 0.00
　~ 0.01%) Ca, Mg, REM contributes to fine dispersion of inclusions in the steel. Further, Bi can reduce the micro-segregation of substitutional alloying elements Mn, Si and the like in the steel. Both an element which contributes to the improvement of the toughness and workability of the steel sheet. Therefore, Ca, Mg, REM, or Bi, or any combination thereof may be contained. Each element is also the content is less than 0.0001%, the addition effect is not exhibited, it is preferably 0.0001% or more, more preferably 0.0010% or more. On the other hand, each element also exceeds 0.01%, the inhibiting ductility, and 0.01% or less, preferably 0.005% or less. In other words, "Ca: 0.0001 ~ 0.01%", "Mg: 0.0001 ~ 0.01%", "REM: 0.0001 ~ 0.01%", or "Bi: 0.0001 ~ 0 .01% ", or it is preferable that any combination of these holds.
[0046]
　Next, a description will be given reasons for limiting the steel structure of the hot-dip galvanized steel sheet according to the embodiment of the present invention. In the following description, the unit of the proportion of phases or tissues constituting the steel structure "%" is especially meant the area ratio in an arbitrary cross-section unless otherwise specified (percent). Galvanized steel sheet according to the embodiment of the present invention, an area ratio, polygonal ferrite: 10% or less, upper bainite: 20% or less, residual austenite: 5% or less, martensite: 70% or more, 1 × 10 6 / mm 2 or more number density in martensite has a Fe carbide: 50% or more relative to the total martensite, and average effective crystal grain size: 5.0 .mu.m with a steel structure represented by the following.
[0047]
　(Polygonal ferrite: 10% or less, upper bainite: 20% or less)
　polygonal ferrite exceeds 10% or if the upper bainite exceeds 20% that the steel sheet to obtain a softened by 75% or more yield ratio since it is difficult, polygonal ferrite is 10% or less, the upper bainite is 20% or less. Preferably, polygonal ferrite is less than 5%, upper bainite is 10% or less.
[0048]
　(Residual austenite: 5% or less)
　If the residual austenite is more than 5%, the fresh martensite transformed from retained austenite by strain induced transformation after press molding affects the hydrogen embrittlement, obtain excellent hydrogen embrittlement resistance since it is difficult, residual austenite is 5% or less. Preferably 2% or less.
[0049]
　: (Martensite 70% or more)
　when the martensite is less than 70% can not be ensured the required strength, and 70% or more. Preferably 80% or more.
[0050]
　Calculation of the area ratio of the steel structure is performed as follows. Polygonal ferrite, upper bainite, pearlite, cementite, martensite, the area ratio of tempered martensite, cut rolling direction cross-section of the steel sheet, the current out of the steel structure with nital solution, 1/8 in the steel structure was revealing - 3/8 thickness position a scanning electron microscope (magnification: 5000-fold, 10-field) shot by, from the obtained structure photograph, and the area ratio of the average value calculated by the point counting method.
[0051]
　The area ratio of residual austenite, subjected to X-ray diffraction of the surface of the 1/4 thickness of the steel plate as the observation surface, the value calculated from the peak area ratio of the bcc and fcc and area ratio.
[0052]
　(1 × 10 6 / mm 2 : more number density in martensite has a Fe carbide whole martensite 50% or more sites)
　in order to achieve both excellent hydrogen embrittlement resistance and more than 75% of yield ratio, of the martensite contained in the steel structure, 50% or more regions in the area ratio, the number density × 10 1.0 6 / mm 2 and more martensite having a Fe carbide.
[0053]
　Number density 1.0 × 10 6 / mm 2 when the martensite having the above Fe carbides is less than 50% of the total martensite, since it is difficult to obtain more than 75% of the yield ratio, number density 1. × 10 0 6 / mm 2 or more martensite having a Fe carbide is 50% or more. Preferably at least 65%. The number density of the Fe carbides × 10 1.0 6 / mm 2 is less than excellent since hydrogen embrittlement resistance can not be obtained, number density of Fe carbides × 10 1.0 6 / mm 2 and more. Preferably × 10 5.0 6 / mm 2 at least.
[0054]
　The number density of Fe carbides present in martensite, cut rolling direction cross-section of the steel sheet, the current out of the steel structure with nital solution, scanning electron 1 / 8-3 / 8 thickness position in the steel structure was revealing microscope (magnification: 5000-fold, 10-field) shot by, to measure the number of Fe carbides in the resulting structure photograph, to calculate the number density.
[0055]
　(Average effective crystal grain size: 5.0 .mu.m or less)
　effective crystal grain size means the size of the surrounded by crystal orientation difference 10 ° or more grain boundary region (to be described later) (particle diameter). For example, in the martensite, which corresponds to the block diameter.
[0056]
　For good resistance to hydrogen embrittlement, the average effective crystal grain size or less 5.0 .mu.m. If the average effective crystal grain size exceeds 5.0 .mu.m, so hydrogen embrittlement resistance intergranular area is reduced in large angle grain boundaries is reduced, the average effective crystal grain size is not more than 5.0 .mu.m. Preferably is less than or equal to 4.0μm.
[0057]
　The average effective crystal grain size is measured by EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. EBSP-OIM method, irradiated with an electron beam in the high-tilt sample in a scanning electron microscope (SEM), the Kikuchi pattern formed by backscattered imaging with high sensitivity camera. Then, to measure the crystal orientation of the irradiation point in a short waiting in image processing by a computer. Further, it is possible to analyze the measured values ​​using software.
[0058]
　The EBSP-OIM method, it is possible to quantitatively analyze the microstructure and crystal orientation of the steel structure. Resolution in EBSP-OIM method depends on the resolution of the SEM, can be analyzed with minimal 20nm resolution. In the present invention, defines grain boundaries of the steel at the threshold 10 ° for recognizing block boundaries that can be the effective crystal grain boundary, the crystal grains were visualized in the mapping image misorientation 10 ° or more grain boundaries , an average crystal grain size.
[0059]
　(Average dislocation density of the whole steel: 1.0 × 10 15 / m 2 ~ 1.0 × 10 16 / m
　2 ) and tensile strength of at least 1300 MPa, in order to achieve both a good resistance to hydrogen embrittlement, the steel the average dislocation density of the whole × 10 1.0 15 / m 2 ~ 1.0 × 10 16 / m 2 is preferably set to. Dislocations, to contribute to the strengthening of the material, but it is better to include a large amount in terms of strengthening, in terms of hydrogen embrittlement characteristics, the smaller is preferred. The average dislocation density of × 10 1.0 15 / m 2 by weight, since the tensile strength of at least 1300MPa is not obtained, preferably × 10 1.0 15 / m 2 and more, more preferably 5.0 × 10 15 / m 2 is at least.
[0060]
　On the other hand, the average dislocation density of × 10 1.0 16 / m 2 by weight, the interaction between dislocations and hydrogen in the steel, since the hydrogen embrittlement resistance is deteriorated absorbed hydrogen amount in the steel material is increased , preferably × 10 1.0 16 / m 2 and less, more preferably × 10 0.5 16 / m 2 or less.
[0061]
　The average dislocation density of the whole steel, according to the method described in Non-Patent Document "CAMP-ISIJ Vol.17 (2004) p.396" on "Evaluation method of the dislocation density using an X-ray diffraction" (110 ) α, (211) α, and calculates the average dislocation density from the half-width of (220) alpha.
[0062]
　According to galvanized steel sheet according to the embodiment of the present invention constructed as above, for example, tensile strength of at least 1300 MPa, 75% or more of the yield ratio, and excellent resistance to hydrogen embrittlement is obtained. If the tensile strength is less than 1300 MPa, since it may be difficult to secure the weight reduction and crashworthiness, it is preferable that tensile strength of at least 1300 MPa is obtained, that tensile strength of at least 1350MPa is obtained more preferable. If the yield ratio is less than 75%, since it may become difficult to ensure a collision safety, be preferable that 75% or more yield ratio can be obtained, more than 80% of the yield ratio can be obtained preferable.
[0063]
　Preferably the amount of solid solution B is 0.0010% by mass or more, it is preferable prior austenite grain diameter of 1.0 .mu.m ~ 7.0 .mu.m. Solid solute B contributes to the improvement of toughness and resistance to hydrogen embrittlement of the steel sheet by increasing the grain boundary strength of prior austenite grains. However, the amount of solid solution B is less than 0.0010 wt%, it may not sufficient toughness and resistance to hydrogen embrittlement is obtained. Therefore, the amount of solid solution B is preferably 0.0010 mass% or more, more preferably 0.0015 mass% or more. Further, it is less than the prior austenite grain size is 1.0 .mu.m, improving grain boundary strength by solution B, the grain boundary area of ​​prior austenite grains is too large may not become sufficient. Thus, the prior austenite grain size is preferably not less than 1.0 .mu.m, and preferably 2.0μm or more. On the other hand, in the prior austenite grain size is 7.0μm greater, also deteriorates hydrogen embrittlement resistance because the toughness of the material deteriorates. Thus, the prior austenite grain size is preferably not more than 7.0 .mu.m.
[0064]
　Solid solute B amount can be calculated by subtracting the mass of B contained in the precipitate such as boron halides from the total weight of B contained in galvanized steel sheet. Mass of B contained in the precipitates by extraction residue method to measure the mass of B precipitates obtained by converting the mass of B included it in the B precipitates. The method of quantifying B precipitates extraction residue method is described, for example, in Non-Patent Document 2. Prior austenite grain size, cut rolling direction cross-section of the steel sheet, out prior austenite grain boundaries in picric acid alcohol solution current, scanning electron microscope 1 / 8-3 / 8 thickness position in the old austenite grain boundaries and revealing (magnification: 1000 times, 5 fields) captured by, using the average value calculated by the line segment method from the resulting structure photograph.
[0065]
　It is preferred product of solid solute B amount and the prior austenite grain size is 0.0010 wt% · [mu] m or more. As prior austenite grain diameter is small, a large grain boundary area of ​​the prior austenite grains. Therefore, in order to obtain a constant grain boundary strength, as prior austenite grain size is small, are needed many solid solution B. When this point of view the present inventors have carried out an investigation, the product of solid solute B amount and the prior austenite grain size in the case of more than 0.0010 wt% · [mu] m, is particularly excellent hydrogen embrittlement resistance It obtained it was revealed.
[0066]
　Next, a method for manufacturing a galvanized steel sheet according to the embodiment of the present invention. In this production method, hot rolling of a slab having the above chemical composition, cold rolling, performing continuous annealing, galvanizing treatment, an alloying treatment and tempering in that order.
[0067]
　In hot rolling, carried slab heating, rough rolling, finish rolling and cooling.
[0068]
　Slab heating temperature is set to 1180 ° C. or higher. The slab heating temperature is less than 1180 ° C., it is impossible to sufficiently dissolve the boron compound in the slab can not be secured a sufficient amount of solid solution boron. The slabs can be used, for example, a slab obtained by continuous casting, slabs were produced by ingot making method, the slab was cast by thin slab casting. Slab may be subjected to the retained as-hot-rolled facilities 1180 ° C. or more temperature after casting, the temperature of lower than 1180 ° C., for example, may be heated after cooling to room temperature and subjected to hot rolling equipment.
[0069]
　In rough rolling, a temperature of 1050 ° C. or higher 1150 ° C. or less, the total rolling reduction is 50% or more. Cause sufficient recrystallization during hot rolling, in order to tissue hot-rolled steel sheet as homogeneous.
[0070]
　The finish rolling, from the first pass carried out at 1050 ° C. below the temperature from the end of the total rolling reduction of up to the second pass is 95% or more and 60% or less, the rolling reduction of the final pass is 5% or more and 30% or less, the temperature of the final pass to 880 ℃ more than 980 ℃ or less. From the first pass carried out at 1050 ° C. or less of the temperature, or a total rolling reduction is 95% from the last to the second pass, the rolling reduction in the final pass or is 30 percent, boron in the finish rolling products of precipitation is accelerated, can not be secured a sufficient amount of solid solution boron. Be less than the temperature of the final pass 880 ° C., is promoted deposition of boron compound in the finish rolling can not be secured a sufficient amount of solid solution boron. From the first pass carried out at 1050 ° C. or less of the temperature, or a total rolling reduction is less than 60% from the last to the second pass, the rolling reduction in the final pass or less than 10% of the hot rolled steel sheet microstructure become coarse, the desired effective crystal grain size can not be obtained.
[0071]
　Cooling was performed after the elapse of the finish rolling end over one second, to 5 ° C. / sec or higher 50 ° C. / sec or less at a temperature of 450 ° C. or higher 700 ° C. or less at a cooling rate cooling, performing winding at that temperature. When starting for a period shorter cooling from the end of rolling finishing one second or more, the austenite is not sufficiently recrystallization, anisotropy becomes obvious. When the cooling rate is less than 5 ° C. / sec, is promoted ferrite transformation in a high temperature range, tissue coarse hot-rolled steel sheets, not desired effective crystal grain size is obtained. The upper limit of the cooling rate is not particularly provided, it is difficult to substantially 50 ° C. / sec or higher. The coiling temperature is 700 ° C. greater, organized coarsening of the hot-rolled steel sheet, can not be obtained a desired effective crystal grain size, deposition of boron compound is promoted, may not be secured a sufficient amount of solid solution boron to. The coiling temperature is lower than 450 ° C., the strength of the hot rolled steel sheet becomes excessive, it becomes difficult subsequent cold rolling. Coiling temperature is preferably between 500 ° C. or higher 650 ° C. or less.
[0072]
　After the take-up, do the pickling of hot-rolled steel sheet in accordance with a conventional method. The skin pass rolling of hot-rolled steel sheet may be carried out. The skin pass rolling, it is possible to correct the shape or, or improved pickling.
[0073]
　In cold rolling, the reduction ratio is 80% or less than 20%. The rolling reduction is less than 20%, it is impossible to obtain fine austenite grains at annealing. On the other hand, the reduction ratio is 80%, the rolling load is causing an increase in the load of cold rolling mill become excessive. Rolling reduction preferably 70% or less than 30%.
[0074]
　The continuous annealing, heating, holding, and for cooling.
[0075]
　A Atsushi Nobori, 700 ° C. or higher Ac 3 Average heating rate in the points below the temperature range to 0.1 ° C. / sec 10 ° C. / sec or less. The average heating rate With 10 ° C. / sec or less, it is possible to promote the segregation to the austenite grain boundaries of boron element. On the other hand, the average heating rate is less than 0.1 ° C. / sec, it takes a long time to heat the steel plate, the productivity is impaired, which is a substantial lower limit.
[0076]
　After heating, Ac 3 holds only 500 seconds or less time than one second to a temperature range of not lower than point 900 ° C. or less. Holding temperature Ac 3 or less than points, the holding time or less than 1 second, can not be sufficiently austenitized. On the other hand, the holding temperature is 900 ° C. greater, austenite grains are coarsened, resistance to hydrogen embrittlement is deteriorated effective crystal grain size becomes excessively large. The retention time is 500 seconds, greater than the productivity is impaired.
[0077]
　After holding, to cool to a temperature of 450 ° C. or higher 600 ° C. or less from the holding temperature. The average cooling rate from the holding temperature to 650 ° C. is a 0.5 ° C. / sec or more. The average cooling rate is less than 0.5 ° C. / sec, ferrite transformation excessively progresses, the area ratio of polygonal ferrite may exceed 10%. The average cooling rate from 650 ° C. to a temperature of 450 ° C. or higher 600 ° C. or less and 3 ° C. / sec or more. The average cooling rate is less than 3 ° C. / sec, ferrite transformation excessively progresses, the area ratio of polygonal ferrite may exceed 10%. When 3 ° C. / sec or more average cooling rate to a temperature below 450 ° C. to continue the cooling, the product of the upper bainite is promoted, there is the area of ​​the upper bainite exceeds 20%. Cooling at 3 ° C. / sec or more average cooling rate is preferably stopped at 470 ° C. or higher. If you stop the cooling at 3 ° C. / sec or more average cooling rate at 600 ° C. greater than is then generated in the ferrite is promoted, sometimes ferrite area ratio exceeds 10%. The average cooling rate from the holding temperature to a temperature of 450 ° C. or higher 600 ° C. or less may be 3 ° C. / sec or more.
[0078]
　In galvanizing treatment performs immersion in the holding and the plating bath.
[0079]
　Retention time starting from cooling at 3 ° C. / sec or more average cooling speed of the continuous annealing, holding temperature 450 ° C. or higher 600 ° C. or less, the holding time or less 1000 seconds or 1 second. The holding temperature is lower than 450 ° C., formation of upper bainite is promoted, the holding temperature is 600 ° C. greater than ferrite generation is accelerated. The retention time is 1000 seconds, more than upper bainite is excessively formed. Retention time is preferably 500 seconds, more preferably 100 seconds or less. It is a real operation on difficult to make the retention time less than 1 second.
[0080]
　The plating bath, Fe, Si, Al, Mg , Mn, Cr, may contain impurities such as Ti and Pb. For example, the temperature of the plating bath is 420 ° C. or higher 500 ° C. or less, 500 ° C. entering sheet temperature is 420 ° C. or more steel less, the dipping time is less than 5 seconds, the unit weight 25 g / m per side 2 or 75 g / m 2 and less to. Basis weight, for example can be controlled by a known method such as gas wiping.
[0081]
　In the alloying treatment, control and cooling to the processing temperature.
[0082]
　Treatment temperature of alloying treatment to 600 ° C. or less 480 ° C. or higher. If the temperature of the steel sheet plated Yokugo becomes less than 480 ° C., it is heated to a temperature of 600 ° C. or less 480 ° C. or higher. Is less than the processing temperature of 480 ° C., slow progress of alloying, or impaired productivity, sometimes uneven alloying are or occur. The processing temperature is preferably between 500 ° C. or higher. On the other hand, the treatment temperature is 600 ° C. greater, alloying progresses excessively, powdering of the steel sheet is degraded. The processing temperature is preferably between 580 ° C. or less.
[0083]
　Thereafter, the cooling from the processing temperature of the alloying treatment (Ms point -80 ° C.) until a temperature below. The average cooling rate in this cooling and 5 ° C. / sec or more. The average cooling rate is less than 5 ° C. / sec, bainite excessively generated, it may become difficult to obtain the desired microstructure. When 5 ° C. / sec cooling the above average cooling rate (Ms point -80 ° C.) stops at greater than insufficient production of martensite, 1 × 10 6 / mm 2 with a Fe carbide in the above number density the amount of martensite is insufficient. Stop Temperature of cooling at 5 ° C. / sec or more average cooling rate is preferably to (Ms point -120) ° C. or less.
[0084]
　In tempering, for holding the following 5 seconds or more for 500 seconds to a temperature range of 200 ° C. or higher 400 ° C. or less. Or a holding temperature is lower than 200 ° C., the holding time or less than 5 seconds, the tempering is insufficient, as a result, 1 × 10 6 / mm 2 martensite having the above number density in Fe carbides total or is less than 50% of martensite, the average dislocation density of × 10 1.0 16 / m 2 may be or become greater. Holding temperature preferably to 220 ° C. or higher. On the other hand, if a holding temperature is 400 ° C. greater than the holding time or a greater for 500 seconds, the tempering becomes excessive, so that no sufficient tensile strength can not be obtained. Holding temperature preferably to 350 ° C. or less. Tempering may be conducted as a series of heat-treated at galvanizing line in may be performed as a heat treatment using a heat treatment apparatus after winding at room temperature after the galvanizing treatment.
[0085]
　If stop temperature of the cooling at an average cooling rate of more than 5 ° C. / sec in the alloying process is 200 ° C. or higher 400 ° C. or less, and held there 200 ° C. or higher 400 ° C. less than 500 seconds 5 seconds or more to a temperature range it may be. If stop temperature of the cooling at an average cooling rate of more than 5 ° C. / sec in alloying treatment is lower than 200 ° C., it is heated to a temperature of 200 ° C. or higher 400 ° C. or less. Heating rate at this time is preferably from the viewpoint of productivity and 1 ° C. / sec or more.
[0086]
　Alloying treatment may be omitted. In this case, the steel sheet discharged from the plating bath is cooled to 5 ° C. / sec or more average cooling rate (Ms point -80 ° C.) below the temperature, then, 200 ° C. or higher 400 ° C. over 5 seconds the temperature range below 500 performing tempering holding less seconds. If the temperature when discharged from the plating bath in case of omitting the alloying process is 200 ° C. or higher 360 ° C. or less, held as it is 200 ° C. or higher 400 ° C. less than 500 seconds 5 seconds or more to a temperature range for tempering it may be. For temperatures below 200 ° C. when discharged from the plating bath, heated to a temperature of 200 ° C. or higher 400 ° C. or less for tempering. Heating rate at this time is preferably from the viewpoint of productivity and 1 ° C. / sec or more.
[0087]
　It may be subjected to temper rolling after hot-dip galvanizing process. By temper rolling, for example, to correct the flatness of the steel sheet, or can adjust the surface roughness. Elongation by temper rolling, to avoid deterioration of ductility, is preferably 2% or less.
Example
[0088]
　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.
[0089]
　(Example 1)
　and Table 1 shows piece cast by melting a steel having a chemical composition, in the template strip is subjected to hot rolling at a hot-rolling conditions shown in Table 2, hot-rolled steel plate having a thickness of 3mm and the. After applying pickling to the heat-rolled steel sheet is subjected to cold rolling at a cold rolling conditions shown in Table 2 (rolling reduction), and a cold-rolled steel plate having a thickness of 1.2 mm. Blank in Table 1 indicates that the content of the element is less than the detection limit, the balance being Fe and impurities. Underlined in Table 1 indicates that the value is out of range of the present invention.
[0090]
[Table 1]

[0091]
[Table 2]

[0092]
　To the resulting cold-rolled steel sheet, subjected to heat treatment in the heat treatment conditions shown in FIG. 1 and Table 3 were subjected to hot-dip galvanizing plating conditions shown in FIG. 1 and Table 3. Additionally, alloying treatment under the conditions shown in FIG. 1 and Table 3, the secondary cooling, reheating, and performs a tertiary cooled to obtain a galvannealed steel sheet.
[0093]
[table 3]

[0094]
　From the obtained galvannealed steel sheet, were taken JIS5 No. Tensile test pieces in the direction perpendicular to the rolling direction, subjected to a tensile test, tensile strength (TS), to measure the total elongation (EL). According to the "JFS T 1001 hole expansion test method" of the Japan Iron and Steel Federation standard, to measure the hole expansion ratio (λ). According to yet aforementioned method to identify the steel microstructure.
[0095]
　Evaluation of hydrogen embrittlement resistance was carried out by the following test methods.
[0096]
　From the obtained galvannealed steel sheet were taken test piece was punched out into 30mmφ 10% clearance, the punched specimen to pH1 aqueous hydrochloric acid solution was immersed up to 24 hours. Observing the punched end surface of the test piece every 3 hours, and observed the presence or absence of cracks. Those that do not also observed cracking after the immersion of 12 hours was passed.
[0097]
　Table 4 and Table 5 (continuation of Table 4) shows the results obtained. Table 4 or underlined in Table 5 indicates that the number is out of range of the present invention.
[0098]
[Table 4]

[0099]
[table 5]

[0100]
　Chemical composition and manufacturing method, in the invention examples within the scope of the present invention, the range of the steel tissue invention, tensile strength of at least 1300 MPa, 75% or more yield ratio (YR), good resistance to hydrogen embrittlement is It has been obtained. On the other hand, one or both of the chemical composition and the steel structure is, in the comparative example is outside the scope of the present invention, the desired mechanical properties are not obtained.
[0101]
　(Example 2)
　in a part of the steel sheet having a chemical composition shown in Table 1, subjected to hot rolling at hot rolling conditions shown in Table 6, was hot rolled steel plate having a thickness of 3 mm. Was subjected to pickling in this hot rolled steel sheet is subjected to cold rolling at a cold rolling conditions shown in Table 6 (rolling reduction), and a cold-rolled steel plate having a thickness of 1.2 mm.
[0102]
[Table 6]

[0103]
　To the resulting cold-rolled steel sheet, subjected to heat treatment in the heat treatment conditions shown in FIG. 2 and Table 7, it was galvanized plating conditions shown in FIG. 2 and Table 7. Furthermore, the secondary cooling under the conditions shown in FIG. 2 and Table 7, reheated, and performs a tertiary cooled to obtain a galvanized steel sheet.
[0104]
[Table 7]

[0105]
　From the obtained hot-dip galvanized steel sheets were taken JIS5 No. Tensile test pieces in the direction perpendicular to the rolling direction, subjected to a tensile test, tensile strength (TS), to measure the total elongation (EL). According to the "JFS T 1001 hole expansion test method" of the Japan Iron and Steel Federation standard, to measure the hole expansion ratio (λ). In accordance with the above-mentioned method, it was identified steel microstructure.
[0106]
　Evaluation of hydrogen embrittlement resistance was carried out by the following test methods.
[0107]
　From the obtained hot-dip galvanized steel sheets were taken test piece was punched out into 30mmφ 10% clearance, the punched specimen to pH1 aqueous hydrochloric acid solution was immersed up to 24 hours. Observing the punched end surface of the test piece every 3 hours, and observed the presence or absence of cracks. Those that do not also observed cracking after the immersion of 12 hours was passed.
[0108]
　Table 8 shows the results obtained.
[0109]
[Table 8]

[0110]
　In the embodiment shown in Table 8 (invention examples), both, there chemical composition within the scope of the present invention, and, since the steel structure is in the range of the present invention, tensile strength of at least 1300 MPa, at least 75% yield ratio (YR), and good resistance to hydrogen embrittlement is obtained.
Industrial Applicability
[0111]
　The present invention is, for example, can be used in industry in which a suitable steel plate to the body or parts of an automobile.

WE CLAIM

　By
　mass%, C:
　0.14 ~
　0.3%, Si: 0.001 ~ 2.0%, Mn: 2.0 ~
　4.0%, P: 0.05% or
　less, S: 0.01 % or
　less, N: 0.01% or
　less,
　Al: 0.001
　~ 1.0%, Ti: 0.001 ~ 0.10%, B: 0.0001 ~
　0.01%, Mo: 0 ~ 0.
　%
　50,
　Cr: 0 ~ 0.80%,
　Ni: 0 ~ 1.00%, Cu: 0 ~
　1.00%, V: 0 ~ 0.50%, Nb: 0.0 ~ 0.10%, 　Ca:
　0.00 ~ 0.01%, Mg: 0.00 ~ 0.01%, 　REM: 0.00 ~ 0.01%, Bi: 0.00 ~ 0.01%, and 　balance: Fe and impurities , 　in a chemical composition represented, 　the area ratio, 　polygonal ferrite: 10% or less, 　upper bainite: 20% or less,

　Retained austenite: 5% or less,
　martensite: 70% or more,
　1 × 10 6 / mm 2 or more number density in martensite has a Fe carbide: 50% or more relative to the total martensite, and
　average effective crystal grain size : 5.0 .mu.m or less,
　galvanized steel sheet characterized by having a in steel structure represented.
[Requested item 2]
　Amount of solid solution B is 0.0010% to 0.0100% by weight, hot-dip galvanized steel sheet according to claim 1, wherein the prior austenite grain diameter of 1.0 .mu.m-7.0 .mu.m.
[Requested item 3]
　Hot-dip galvanized steel sheet according to claim 2 in which the product of solid solute B amount and the prior austenite grain size is equal to or is 0.0010 wt% · [mu] m or more.
[Requested item 4]
　In the chemical
　composition, Mo: 0.001 ~ 0.50%
galvanized steel sheet according to any one of claims 1 to 3, characterized in that hold.
[Requested item 5]
　In the chemical
　composition,
　Cr: 0.001 ~ 0.80%, Ni: 0.001 ~ 1.00%, or
　Cu: 0.001 ~ 1.00%
　and characterized in that holds true, or any combination thereof galvanized steel sheet according to any one of claims 1 to 4.
[Requested item 6]
　In the chemical
　composition, V: 0.001 ~ 0.50%, or
　Nb: 0.001 ~ 0.10%
　, or according to any one of claims 1 to 5, characterized in that they are both satisfied hot-dip galvanized steel sheet.
[Requested item 7]
　In the chemical
　composition,
　Ca: 0.0001 ~
　0.01%, Mg: 0.0001 ~ 0.01%, REM: 0.0001 ~ 0.01%, or
　Bi: 0.0001 ~ 0.01%
　, or galvanized steel sheet according to any one of claims 1 to 6, characterized in that any combination of these holds.